Protein phosphatases in pancreatic islets

in Journal of Endocrinology

Correspondence should be addressed to Å Sjöholm; Email: ake.sjoholm@sodertaljesjukhus.se

The prevalence of diabetes is increasing rapidly worldwide. A cardinal feature of most forms of diabetes is the lack of insulin-producing capability, due to the loss of insulin-producing β-cells, impaired glucose-sensitive insulin secretion from the β-cell, or a combination thereof, the reasons for which largely remain elusive. Reversible phosphorylation is an important and versatile mechanism for regulating the biological activity of many intracellular proteins, which, in turn, controls a variety of cellular functions. For instance, significant changes in protein kinase activities and in protein phosphorylation patterns occur subsequent to the stimulation of insulin release by glucose. Therefore, the molecular mechanisms regulating the phosphorylation of proteins involved in the insulin secretory process by the β-cell have been extensively investigated. However, far less is known about the role and regulation of protein dephosphorylation by various protein phosphatases. Herein, we review extant data implicating serine/threonine and tyrosine phosphatases in various aspects of healthy and diabetic islet biology, ranging from control of hormonal stimulus–secretion coupling to mitogenesis and apoptosis.

Abstract

The prevalence of diabetes is increasing rapidly worldwide. A cardinal feature of most forms of diabetes is the lack of insulin-producing capability, due to the loss of insulin-producing β-cells, impaired glucose-sensitive insulin secretion from the β-cell, or a combination thereof, the reasons for which largely remain elusive. Reversible phosphorylation is an important and versatile mechanism for regulating the biological activity of many intracellular proteins, which, in turn, controls a variety of cellular functions. For instance, significant changes in protein kinase activities and in protein phosphorylation patterns occur subsequent to the stimulation of insulin release by glucose. Therefore, the molecular mechanisms regulating the phosphorylation of proteins involved in the insulin secretory process by the β-cell have been extensively investigated. However, far less is known about the role and regulation of protein dephosphorylation by various protein phosphatases. Herein, we review extant data implicating serine/threonine and tyrosine phosphatases in various aspects of healthy and diabetic islet biology, ranging from control of hormonal stimulus–secretion coupling to mitogenesis and apoptosis.

Type 2 diabetes: a growing epidemic

Type 2 diabetes (T2D) is a syndrome characterized by disordered metabolism, resulting in hyperglycemia. The most common and dreaded long-term complication of diabetes is cardiovascular disease, which accounts for 75–80% of all diabetes-related deaths (Meetoo et al. 2007). Diabetes is widespread and it is the fourth leading cause of death in the USA (Meetoo et al. 2007). The expenses by diabetes have been shown to be a major drain on health- and productivity-related resources for healthcare systems and governments. In the USA alone, the annual cost for diabetes amounts to the considerable sum of $245 billion, of which ∼97% is targeted to T2D (American Diabetes Association 2013). Improved glycemia is a main focus of T2D therapy and HbA1c levels of 5–6% (DCCT standard; corresponding to 31–42 mmol/mol by IFCC standard) are recommended treatment goals. However, more than 50% of patients with T2D have a HbA1c level of >7% (53 mmol/mol by IFCC standard) and are thus inadequately controlled (Koro et al. 2004).

Loss of glucose-sensitive insulin secretion of the pancreatic β-cell is an early pathogenic event and contributes significantly to the development of the diabetic state (Ward et al. 1984, Bell & Polonsky 2001, Grimsby et al. 2003). The changes in β-cell function in diabetes include decline in glucose-sensitive insulin secretory output (Ward et al. 1984), disturbances in pulsatile insulin release (Tengholm & Gylfe 2009), and impaired insulin synthesis (Kahn & Halban 1997). Thus, improvement of β-cell function is a major goal in the clinical management of the disease.

Inadequacy of the pancreatic β-cell also results from a combination of impaired secretory function and insufficient β-cell mass. The ability of the β-cell to expand its proliferative capacity in response to an increased insulin demand may be of critical regulatory significance for the development of diabetes (Sjöholm 1996, Lee & Nielsen 2009). T2D patients exhibit a reduced β-cell mass, possibly due to increased rates of apoptosis (Butler et al. 2003). Maintaining islet β-cell mass and adequate insulin secretion to meet metabolic demands is crucial to avoid glucose intolerance and the development of T2D.

There is a progressive and relentless deterioration in β-cell function over time in T2D, regardless of therapy allocation, such as insulin, glibenclamide, or metformin treatment (Group 1998a,b), eventually leaving many patients reliant on exogenous insulin replacement therapy.

The role of declining β-cell mass and function in the development of T2D has drawn attention to the need for agents that can halt this process. Moreover, in individuals with established T2D, inhibition of the increased apoptosis may lead to restoration of β-cell mass and it may also prevent pre-diabetic subjects to progress into overt T2D.

Regulation of insulin secretion

Pancreatic β-cells are equipped to rapidly sense ambient glycemia. In order for the cells to respond appropriately with insulin secretion, glucose must be metabolized within the β-cells (Hedeskov 1980, Ashcroft & Rorsman 2012). Glucose rapidly enters the cells via the efficient glucose transporter 2 (GLUT2 (GLUT1 in human islets)) that enables a balance between the extracellular and intracellular concentration of glucose (Meglasson & Matschinsky 1986, Newgard & McGarry 1995). Following entry, glucose is phosphorylated by glucokinase, which acts as a glucose sensor by controlling the amount of glucose that traverses through the glycolytic pathway (Matschinsky et al. 1998). Glucose metabolism results, among other things, in increased production of ATP, leading to an increased ATP:ADP ratio (Detimary et al. 1995), which (such as sulfonylurea drugs) closes the ATP-sensitive K+ (KATP) channels (Ashcroft et al. 1984, Cook & Hales 1984). This causes depolarization of the plasma membrane, opening of voltage-dependent Ca2+ channels, and influx of extracellular Ca2+. Elevation of cytosolic Ca2+ is the main trigger for granule translocation and insulin exocytosis (Jonas et al. 1998). However, experiments indicate that glucose retains an excellent ability to secrete insulin even in the presence of maximally effective concentrations of K+ and diazoxide, which acts by opening K+ channels (Gembal et al. 1992, Komatsu et al. 1997). Thus, although signaling molecules other than ATP and Ca2+ must be involved in glucose sensing in the β-cell, the precise nature by which these complementary signals promote secretion and the KATP-independent signaling pathways activated by glucose have remained elusive. Insulin secretion is a complex process, tuned by many mechanisms, and has been the topic of excellent reviews (Ashcroft & Rorsman 2012, Rorsman & Braun 2013).

Introduction to reversible protein phosphorylation and protein phosphatases

In 1992, the Nobel Prize in Physiology or Medicine was awarded jointly to Edmond H Fischer and Edwin G Krebs, for their earlier discoveries revealing that the reversible covalent attachment of phosphate to a protein functions as a mechanism to regulate biological activity. The protein that was reversibly phosphorylated was glycogen phosphorylase, and the proteins that catalyzed phosphorylation and dephosphorylation were termed phosphorylase kinase and phosphorylase phosphatase respectively (Sutherland & Wosilait 1955, Fischer et al. 1959, Krebs et al. 1959). Today, this simple reaction, in which a kinase catalyzes the transfer of phosphate from the gamma position of a high energy phosphonucleotide (usually ATP) to the side-chain hydroxyl of a protein (usually serine, threonine, or tyrosine) and a phosphatase catalyzes phosphate hydrolysis, is established as a fundamental, if not paramount, mechanism by which eukaryotic cells regulate virtually all aspects of cell biology. Accordingly, there has been an intensive global effort to identify and characterize the biological roles of these important regulatory enzymes.

Sequence data from the human genome indicate humans express ∼520 protein kinases, with ∼90 acting as tyr-kinases and 428 acting as ser/thr kinases (Johnson & Hunter 2005). Many kinases are highly conserved in nature. However, the tyrosine kinases appear to have evolved more recently, with the evolution of multicellular eukaryote organisms (Alonso et al. 2004, Johnson & Hunter 2005). To counter these kinases, humans have a nearly equal number (∼107) of phospho-tyr-phosphatases, suggesting that each tyr-kinase is countered by a single tyr-phosphatase (Alonso et al. 2004). In contrast, the number of genes encoding proteins capable of catalyzing the hydrolysis of phospho-ser/thr residues is more limited (∼40 genes). Estimates of total protein phosphate from studies using [32P] labeling (Hunter & Sefton 1980, Hunter et al. 1980) or proteomic analysis of phosphorylation sites of human proteins (Olsen et al. 2006) are in agreement, indicating that ∼98% of the phosphate in proteins is attached to ser/thr residues, with <2% being attached to tyrosine. Thus, the tyrosine kinases/phosphatases may be viewed as ‘thoroughbreds’ acting extensively only in the restricted arena of multicellular eukaryotes. Ser/thr kinases/phosphatases are more primitive and are more likely to act as the ‘work horses’ of reversible phosphorylation in both single and multicellular eukaryotes. Clearly, both families are important and several excellent reviews focus on ser/thr and tyrosine kinases (Manning et al. 2002, Nolen et al. 2004, Taylor & Kornev 2011, Endicott et al. 2012).

When compared with their kinase counterparts, less is known about the biological roles played by protein phosphatases (PPs). This is due, in part, to technical difficulties associated with accurately measuring protein dephosphorylation, and to early and lingering misconceptions that phosphatases act as simple ‘housekeeping’ or pleiotropic enzymes. Today we know that phosphatases are not simple housekeeping enzymes, rather they play specific, active, and sometimes even dominant roles in controlling both the levels of phosphorylation in cells and the regulation of physiological processes (Alonso et al. 2004). In this review, we will focus on PPs, emphasizing their roles in pancreatic islets.

Phosphotyrosine phosphatases (PTPs) have received the most recognition for playing precise and regulated roles in cell signaling, and the 107 genes-encoding PTPs can be divided into four families (Alonso et al. 2004; Fig. 1). The largest family (class I Cys-base PTPs) contains 99 genes. Class I PTPs share a common catalytic mechanism (Fig. 2). In this reaction, during the cleavage of the scissile P–O bond, a covalent phospho-cysteine intermediate is produced at the catalytic site. Hydrolysis of the cysteinyl-phosphate intermediate is then facilitated by the protonation of phenolic oxygen by a conserved aspartic acid and the positioning of an activated water molecule by a conserved active site, glutamine or serine. Mutation of the conserved aspartic acid to alanine can aid the identification of substrates, by producing substrate-trapping mutants that retain the covalent attachment at the catalytic cysteine (Flint et al. 1997, Blanchetot et al. 2005).

Figure 1
Figure 1

Family tree of PTPs.

Citation: Journal of Endocrinology 221, 3; 10.1530/JOE-14-0002

Figure 2
Figure 2

Comparison of PTPase and PPP catalytic mechanism. (A) Schematic representation of PTP-Cys-mediated hydrolysis of substrate derived from the crystal structure of PTP1B (data from Barford et al. 1994). (B) Schematic representation of metal ion-mediated hydrolysis of substrate derived from the crystal structure of PP5C (data from Swingle et al. 2004). The attacking hydroxide W1 is shown in blue and the leaving group of the substrate is in green. The substrate, the planar PO3 moiety of the transition state, and the phosphate product are all shown in red. Solid lines to the metal ions denote metal–ligand bonds, and solid or dashed wedges indicate metal–ligand bonds directed above or below the plane of the page respectively. Wavy lines to the metal ions indicate strained contacts with poor coordination geometry. Dotted lines indicate hydrogen bonds, and the nearly dissociated axial bonds in the transition state are shown by half-dotted double lines.

Citation: Journal of Endocrinology 221, 3; 10.1530/JOE-14-0002

The class I PTPs can be further divided into four subgroups, with the most studied subgroup referred to as the classical PTPs (38 genes). Classical PTPs are strictly tyrosine specific and come in two forms, transmembrane PTPases (21 genes) and non-receptor PTPase (17 genes). The transmembrane PTPs have external ‘ligand-binding’ domains, a membrane-spanning domain, and a cytosolic catalytic domain(s). In many ways, these PTPs mimic the general characteristics of receptor-tyrosine kinases, the enzymes that they commonly counter in a cell. The non-receptor PTPs lack the extracellular and transmembrane domains. They are generally cytosolic proteins, some of which are anchored to membranes by prenylation.

The largest and most diverse family (in terms of substrate specificity) of class-I cys-based PTPs are called dual-specificity phosphatases (DSPs or DUSPs; 69 genes). DSPs share the cys-based catalytic mechanism, and as their name implies can act on phosphotyrosine and phosphothreonine residues. Eleven DSPs have MAPK-targeting motifs and may act exclusively at specific phosphotyrosine or phosphothreonine sites on MAPKs. Nineteen are considered atypical DSPs and they represent a poorly characterized family of enzymes that lack MAPK-targeting motifs (Alonso et al. 2004). PTENS (five genes) and myotubularins (16 genes) are present in the DSP family, but they appear to have evolved to specifically dephosphorylate the D3-phosphate of inositol phospholipids (Wishart & Dixon 2002).

The human genome has only one gene encoding a class II cysteine-based PTPs (ACP1), which encodes a low (18 kDa) molecular weight protein. Humans express three class III cysteine-based PTPs, which encode CDC25A, CDC25B, and CDC25C. The CDC25 PTPs are well-characterized phosphatases that function to dephosphorylate cyclin-dependent kinases at their inhibitory dually phosphorylated thr/tyr motifs. For further details on the PTPs, see the excellent review by Alonso et al. (2004).

Phospho-ser/thr-phosphatases (PSPs) are divided into three major families based on different catalytic mechanisms (PPPs, phosphoprotein phosphatases; PPMs, metal-dependent PPs; and FCP/SCP, aspartate-based phosphatases (Shi 2009); Fig. 3). Although the nomenclature may suggest otherwise, the catalytic mechanism employed by both PPPs and PPMs requires two metal ions (Fig. 2B). All PPP family members share a common catalytic domain, with ten absolutely conserved amino acids at the active site (Swingle et al. 2004). Six act to coordinate two metal ions (Fe/Zn) needed for the activation of a water molecule, which functions as the critical nucleophile during catalysis. The others position the incoming substrate for near perfect inline nucleophilic attack by the activated water (Swingle et al. 2004). PPMs are Mn2+/Mg2+-dependent phosphatases. PPMs evolved a different folding strategy to produce a similar catalytic mechanism that also utilizes metal ions in the activation of a water molecule for the dephosphorylation reaction (Shi 2009). Unlike PTPs, a covalent intermediate is not produced during the reaction. The aspartate-based catalysis mechanism utilized by FCP/SCP is different and may be limited to a limited number of substrates that contain random repeats of SYPTSPS (for review see Shi (2009)).

Figure 3
Figure 3

Family tree of PSPs.

Citation: Journal of Endocrinology 221, 3; 10.1530/JOE-14-0002

Based on the number of genes encoding proteins with phosphatase catalytic activity, PPMs represent the largest family of human PSPs. The PPM family included pyruvate dehydrogenase phosphatase and ∼16 genes encoding >20 isoforms of the PP2C (Lammers & Lavi 2007). These enzymes are insensitive to natural inhibitors (i.e. okadaic acid, microcystin, cantharidin, and calyculin A), and the actions of most PPMs are poorly understood. However, due to their unique expression and subcellular localization patterns, most are predicted to act on a single substrate or limited substrates (Lammers & Lavi 2007).

The PPP family contains seven subfamilies (PPP1CA--PPP1C7B; Fig. 3), which are encoded by only 13 human genes yet together catalyze over 90% of all protein dephosphorylation occurring in eukaryotic cells (Moorhead et al. 2007, Virshup & Shenolikar 2009). Humans have three genes encoding four isoforms of PP1 (PPP1CA, PPP1CB, PPP1CC) with the two PPP1CC isoforms (called PP1Cγ1 and γ2) produced by alternate splicing of the PPP1CC gene). Two human genes encode nearly identical (98%) isoforms of PP2A (PPP2CA, PPP2CB). PP4 (PPP4C) and PP6 (PPP6C) share 65% identity with PP2AC, but are encoded by distinct genes (Honkanen & Golden 2002). Humans express three highly homologous isoforms of PP2B/calcineurin (PP2Bα, PP2Bβ, and PP2Bγ) and two genes encode isoforms of PP7 (also called PPEF (PPEF1)). PP5 is unique in the respect that humans only express a single isoform of PP5. All PPP-family members are highly conserved in nature (e.g. the ortholog of PP2Aα in Neurospora crassa (bread mold) shares 87% amino acid identity with human PP2Aα). Figure 4 shows a structural comparison of PP1-MYTP1, PP2Ac-A-B, and PP5.

Figure 4
Figure 4

Structural comparison of PP1-MYTP1, PP2Ac-A–B, and PP5. (A) PP1 (green) in complex with myosin phosphatase targeting subunit MYPT1 (blue). (B) PP2A holoenzyme: PP2A catalytic subunit (green) in complex with the PP2A scaffold A (blue) and a B55-regulatory targeting subunit (yellow). (C) PP5 in an inactive conformation. The catalytic domain is shown in green, N-terminal inhibitory/TPR-targeting domain in yellow, and a unique C-terminal inhibitory domain in blue. The images were generated using PyMol based on protein data bank accession number 1S70 (Terrak et al. 2004; PP1-MYTP1), 3DW8 (Xu et al. 2008; PP2Ac/A/B), and 1WA0 (Yang et al. 2005; PP5). Arrows indicate the catalytic site with metal ions shown as red spheres.

Citation: Journal of Endocrinology 221, 3; 10.1530/JOE-14-0002

The ability of <15 gene products to counter ∼90% of all cellular protein phosphorylation produced the lingering misconception that PPP family enzymes act as pleiotropic or simple housekeeping enzymes. More recently, this popular, yet erroneous, belief has given way to overwhelming data that indicate the actions of most PPPs are dynamic and highly regulated. What the early studies failed to reveal was that, although PPPs share a structurally related catalytic core and identical catalytic mechanisms, they function in the cell as multi-subunit protein complexes. In cells, each PPP family member can achieve many specific functions, because the protein encoded by a PPP gene represents a catalytic subunit that can interact with a distinct set of substrates and interaction proteins. PP1 and PP2A are the most studied, and to date nearly 200 PP1-interacting proteins have been validated (Heroes et al. 2013). These PP1-interacting proteins share little or no structural similarity beyond their PP1-interacting domains and many are only expressed in differentiated or highly specialized cells (Virshup & Shenolikar 2009, Heroes et al. 2013). Therefore, PP1 actually represents a vast array of PP1c-containing holoenzymes, in which the structurally unrelated binding partners control the subcellular localization, activity, and substrate specificity of PP1 (Bollen et al. 2010, Heroes et al. 2013).

PP2A, PP4, and PP6 also gain regulation and substrate specificity by assembling into a number of different multi-subunit holoenzymes that share a common catalytic subunit. For this family, PP2A is best studied. PP2A commonly functions as a three-protein holoenzyme. Most human cells express both the catalytic subunit (PP2Ac) and an A-subunit that functions as a scaffold to tether PP2Ac to a number of different regulatory/targeting B-subunit. In humans, there are 15 genes encoding four families of B-subunits that produce >21 B-isoforms, many of which are expressed only in certain types of cells or during different stages of development (Shi 2009, Virshup & Shenolikar 2009). The final composition of the PP2A-holoenzyme is then derived from the combinatorial assembly of one of the two isoforms of PP2Ac, one of the two isoforms of PP2A-A, and one of the >20 B-subunits (Virshup & Shenolikar 2009, Sents et al. 2013). Therefore, substrate specificity, subcellular targeting, and control of PP2A holoenzyme activity is usually regulated by assembly and mainly determined by the regulatory B-subunits (Virshup & Shenolikar 2009, Lambrecht et al. 2013, Sents et al. 2013). Similar regulatory, targeting, and control mechanisms are starting to emerge from studies of PP4 and PP6, which have their own scaffold and regulatory proteins (Chen et al. 2008, Couzens et al. 2013). In addition, there are a few examples (i.e. α4) in which interaction of certain B-type regulatory proteins are shared by PP2A, PP4, and PP6 (Chen et al. 1998, Kloeker et al. 2003, Breitkreutz et al. 2010).

PP2B, more commonly called calcineurin, is the target of cyclosporin A, which is useful in a clinical setting as a strong immunosuppressive agent. Both calcineurin and PP7 are insensitive to okadaic acid and microcystin (Huang & Honkanen 1998, Honkanen & Golden 2002), and both calcineurin and PP7 are regulated by calcium. For calcineurin, the catalytic-A subunit is maintained in an inactive/inhibited state by the binding of an inhibitory protein, commonly call calcineurin B. Calcineurin only becomes active upon the calcium-mediated association with Ca2+-bound calmodulin (Shi 2009). PP7 is also activated by calcium; however, the C-terminal domain of PP7 contains calmodulin-like and EF-hand-like domains that appear to directly bind Ca2+ (Huang & Honkanen 1998, Huang et al. 2000, Honkanen & Golden 2002). Calcineurin expression is high in brain, while the expression of PP7 is limited, mostly to retina (Huang & Honkanen 1998, Shi 2009).

PP5 is unique in the respect that the catalytic, regulatory, and substrate-targeting domains of PP5 are encoded by a single gene and expressed as a single polypeptide (Honkanen & Golden 2002, Golden et al. 2008). The catalytic core of PP5 is similar in structure to that of the catalytic subunit of PP1, PP2A, PP4, and PP6, and like these PSPs, PP5 is sensitive to inhibition by okadaic acid, calyculin A, cantharidin, and microcystins (Honkanen & Golden 2002, Swingle et al. 2004, 2007, Virshup & Shenolikar 2009). Indeed, the vast majority of the studies that use these natural inhibitors to study PPP actions in cells draw conclusions that implicate PP1 and PP2A in the processes that are being studied, failing to acknowledge that PP4, PP5, and PP6 are also widely expressed in human tissues and potently inhibited by these natural compounds (Swingle et al. 2004, 2007, Virshup & Shenolikar 2009). Unlike PP1 and PP2A, the catalytic activity of PP5 is minimal when PP5 is not associated in a complex with other proteins (Golden et al. 2008). This is because when PP5 is alone, the N-terminal domain of PP5 folds back over the catalytic site, producing an auto-inhibitory complex that blocks substrate access to the catalytic site (Golden et al. 2008). The N-terminal region of PP5 also contains three tetratricopeptide repeat (TPR) domains, which mediate the binding of PP5 to proteins that contain TPR-docking sites. The best studied interaction for PP5 is the interaction with heat shock protein 90 (HSP90; Skarra et al. 2011). PP5 binds HSP90 via interactions between the N-terminal TPR domains of PP5 and a C-terminal TPR-docking domain in HSP90 (Silverstein et al. 1997, Ramsey et al. 2000, Russell et al. 2006, Skarra et al. 2011). Upon binding, PP5 appears to undergo a conformational change allowing substrate access to the active site (Yang et al. 2005). To date, ∼110 proteins have been identified with TPR-docking domains, suggesting that PP5 could also play many unique cellular roles. Currently, PP5 is known to affect stress, hormone- (i.e. glucocorticoid receptor (GR)) and metabolic-mediated signaling cascade (Amable et al. 2011, Skarra et al. 2011, Grankvist et al. 2012, 2013). However, the mechanism controlling PP5 interactions and activity remains largely unexplained.

PPs in β-cell proliferation and apoptosis

Protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPs) are a superfamily of enzymes which oppose the roles of their protein tyrosine kinase counterparts (Andersen et al. 2001). In relation to β-cell apoptosis, PTPN2 (also known as TC-PTP or PTP-S2; a member of the first nontransmembrane subfamily of PTPs), has attracted interest. PTPN2 was identified a candidate gene for T1D, which is expressed in pancreatic β-cells (Ylipaasto et al. 2005, Todd et al. 2007). Furthermore, PTPN2 expression was regulated by cytokines (Cardozo et al. 2001, Moore et al. 2009). Transfection with PTPN2 siRNAs inhibited basal- and cytokine-induced PTPN2 expression in rat β-cells and dispersed human islets cells. Decreased PTPN2 expression exacerbated interleukin β (ILβ)+interferon γ (IFNγ)-induced β-cell apoptosis and turned IFNγ alone into a proapoptotic signal (Moore et al. 2009). Inhibition of PTPN2 amplified IFNγ-induced STAT1 phosphorylation, whereas double knockdown of both PTPN2 and STAT1 protected β-cells against cytokine-induced apoptosis, suggesting that STAT1 hyperactivation is responsible for the aggravation of cytokine-induced β-cell death in PTPN2-deficient cells (Moore et al. 2009). Further studies have shown that PTPN2 modulates pancreatic β-cell apoptosis via regulation of the BH3-only protein BIM (Santin et al. 2011). PTPN2 knockdown exacerbated type 1 IFN-induced apoptosis in INS1E, primary rat, and human β-cells. PTPN2 silencing and exposure to type 1 and 2 IFNs induced BAX translocation to the mitochondria, cytochrome c release, and caspase 3 activation. There was also an increase in BIM phosphorylation that was at least in part regulated by JNK1. From these data, it can be concluded that PTPN2 confers cytoprotective effects to pancreatic β-cells. However, such an anti-apoptotic role of PTPN2 cannot be generalized to other PTPs.

In contrast to the anti-apoptotic role played by PTPN2, ablation of PTP1B increases β-cell proliferation in vivo (Fernandez-Ruiz et al. 2014). Morphometric analysis of pancreatic islets from Ptp1b−/− mice showed a higher β-cell area, concomitantly with higher β-cell proliferation and a lower β-cell apoptosis when compared with islets from their respective WT cognates (Fernandez-Ruiz 2014, #249). At a functional level, isolated islets from Ptp1b−/− mice exhibited enhanced glucose-stimulated insulin secretion. Moreover, Ptp1b−/− mice were able to partially reverse streptozotocin-induced β-cell loss, all indicating that inhibition of PTP1B activity in islet cells may be a therapeutic avenue to promote islet function.

PTP-BL is a nonreceptor PTP that is expressed in β-cells under the control of the MODY5 gene product, HNF1β (Lee et al. 1999, Thomas et al. 2004). In mature β-cells HNF1β expression is low, but forced induction of HNF1β leads to enhanced rates of apoptosis, altered regulation of the cell cycle, and inhibition of stimulated insulin secretion in β-cells, suggesting that control of HNF1β expression may be important for the regulation of β-cell viability and function (Welters et al. 2006). Stably transfected insulin-producing cells, expressing the WT form of PTP-BL, display compromised cell proliferation but with no change in the rate of cell apoptosis (Welters et al. 2008). Furthermore, cells overexpressing PTP-BL were less responsive toward mitogenic stimulation by Wnt3a. Although only performed in a β-cell line, these data suggest that PTP-BL may play a role in the regulation of cell-cycle progression in β-cells, and that it interacts functionally with the components of the Wnt signaling pathway. Future studies are needed to determine whether PTP-BL plays a regulatory role during β-cell proliferation in vivo. For example, it will be interesting to determine whether forced expression of PTP-BL inhibits adaptive β-cell proliferation in response to reduced insulin sensitivity.

Ser/thr PP1

PP1 is regulated by its interaction with a variety of protein subunits that target the catalytic subunit (PP1C) to specific subcellular compartments and determines its localization, activity, and substrate selectivity (Cohen 2002). In the field of β-cell research, the PP1 regulatory subunit PPP1R15A has attracted special interest. PPP1R15A targets PP1 to the endoplasmic reticulum (ER) and is induced under conditions of ER stress (Rutkowski et al. 2006). The physiological response to ER stress is a collection of cellular events aiming to alleviate ER stress by decreasing overall protein synthesis through phosphorylation of the eukaryotic initiation factor 2α (eIF2α), through enhanced protein folding capacity by increased expression of chaperones and through activation of mechanisms for protein degradation (Ortsäter & Sjöholm 2007). The function of PPP1R15A is to serve as the regulatory subunit of the PP1A catalytic domain. Through their interaction, PPP1R15A and PP1C dephosphorylate eIF2α and thus exert a regulatory feedback that can allow for re-initiation of protein synthesis and thereby allows for the expression of stress-induced genes (Novoa et al. 2003). Inhibition of PP1-mediated dephosphorylation of eIF2α by the compound salubrinal was found to be cytotoxic by itself in β-cells and in isolated islets of Langerhans. In addition, salubrinal potentiated fatty acid-induced ER stress and apoptosis (Cnop et al. 2007, Ladriere et al. 2010). Besides being a part of ER-stress signaling, PP1 plays a pivotal role in glucose-induced stimulation of overall translation in β-cells, which depends on a PP1-mediated decrease in Ser51 phosphorylation of eIF2α (Vander Mierde et al. 2007). Thus, the steady-state level of eIF2α phosphorylation in β-cells is the result of a balance between folding-load-induced phosphorylation and PP1-dependent dephosphorylation. Because defects in the pancreatic ER kinase–eIF2α signaling system lead to β-cell failure and diabetes, deregulation of the PP1 system could likewise lead to cellular dysfunction and disease.

Ser/thr PP2A

The PP2A family of enzymes is a major class of ser/thr PPs. They are also one of the most abundant cellular proteins, accounting for ∼1% of total cellular protein and some 80% of all cellular ser/thr PP activity (Janssens & Goris 2001, Shi 2009). Evidence suggests that PP2A activation can be linked to apoptosis, e.g. activation of caspase-3 causes cleavage of the regulatory A subunit of PP2A, which in turn increases PP2A activity (Santoro et al. 1998). As discussed below, PP2A may have a critical role in β-cell survival and demise. Exposure (for at least 24 h) of insulin-secreting cells to the phosphatase inhibitor, okadaic acid, at concentrations inhibiting PP1, PP2A, PP4, PP5, and PP6, reduces cell proliferation and insulin secretion. The reduced proliferation was found to be related to the induction of apoptosis as evident by morphological criteria and the occurrence of DNA fragmentation (Krautheim et al. 1999). Of particular interest is that PP2A is hyper-activated by chronic exposure to high glucose (Arora et al. 2013) and ceramide (Kowluru & Metz 1997), which are both well-known inducers of β-cell apoptosis. In the case of glucose, it was found that siRNA-mediated knockdown of the catalytic subunit of PP2A (PP2Ac) markedly attenuates glucose-induced activation of PP2A (Arora et al. 2013). Moreover, metabolizable – but not non-metabolizable – glucose derivatives induce Leu309 methylation of the catalytic subunit of PP2A. As a consequence, knockdown of the cytosolic leucine carboxymethyl transferase 1 (LCMT1), which carboxymethylates PP2Ac, significantly attenuates PP2A activation induced by high glucose. It was also found that glucose exposure induced LCMT1 expression, as well as the PP2A regulatory subunit B55α. Taken together, the data indicate that high glucose exposure hyperactivates PP2A via the induction of the methylating enzyme LCMT1 and the regulatory subunit B55α. Recent experiments have established a link between glucose-induced activation of PP2A and nuclear import of forkhead box O1 (FOXO1) in β-cells (Yan et al. 2012). Under conditions of oxidative stress evoked by high glucose stimulation, FOXO1 associates with the PP2A holoenzyme composed of the catalytic C, structural A, and B55α regulatory subunits. Knockdown of B55α in INS1 cells reduced FOXO1 dephosphorylation, inhibited FOXO1 nuclear translocation, and attenuated oxidative stress-induced cell death (Yan et al. 2012). This mechanism may be relevant also in vivo because both B55α and nuclear Foxo1 levels were increased under hyperglycemic conditions in db/db mouse islets, an animal model of T2D (Yan et al. 2012). Taken together, these data tell us that PP2A may play a role for glucotoxicity in β-cells via dephosphorylation of FOXO1 and that prevention of PP2A hyperactivation may confer protection against glucotoxicity.

Ser/thr PP2B/calcineurin

PP2B or calcineurin is a two-subunit enzyme, with a 58- to 64-kDa catalytic and calmodulin-binding subunit – calcineurin A – that is tightly bound to a regulatory 19-kDa calcium-binding regulatory subunit – calcineurin B (Klee et al. 1988).

Calcineurin is a Ca2+-activated cytosolic phosphatase that is critical for antigen-stimulated T lymphocyte activation (Crabtree & Olson 2002). Therefore, pharmacologic calcineurin inhibition is highly effective in preventing allograft rejection. However, calcineurin is also expressed in β-cells (Tamura et al. 1995, Ebihara et al. 1996, Redmon et al. 1996), where it has two well-described molecular targets, the nuclear factor of activated T cell 2 family of transcription factors (Lawrence et al. 2001) and the cAMP-responsive element-binding protein (CREB) transcriptional co-activator, transducer of regulated CREB activity 2 (TORC2) (Screaton et al. 2004). Through dephosphorylation-mediated nuclear localization of these targets, calcineurin integrates Ca2+ and cAMP signals generated by physiologic stimuli, such as hyperglycemia and incretin receptor activation, to alter gene expression (Lawrence et al. 2001, 2002, Screaton et al. 2004). CREB is a cAMP- and Ca2+-responsive transcriptional activator that is required for β-cell proliferation and survival (Jhala et al. 2003, Inada et al. 2004, Hussain et al. 2006). Glucose and incretin hormones promote synergistic CREB activity by inducing the nuclear re-localization of TORC2, a co-activator for CREB (Screaton et al. 2004, Koo et al. 2005, Shaw et al. 2005). In islet cells, under basal conditions, when CREB activity is low, TORC2 is phosphorylated and sequestered in the cytoplasm by 14-3-3 proteins (Screaton et al. 2004). In response to feeding stimuli, TORC2 is dephosphorylated, enters the nucleus, and binds to CREB located at target gene promoters (Bittinger et al. 2004, Screaton et al. 2004, Koo et al. 2005). Ser275 of TORC2 is a 14-3-3 binding site that is phosphorylated under low-glucose conditions and which becomes dephosphorylated by calcineurin in response to glucose influx (Jansson et al. 2008). Dephosphorylation of Ser275 is essential for both glucose- and cAMP-mediated activation of CREB in β-cells and islets, demonstrating the essential role of calcineurin activity in β-cell physiology.

Given this role of calcineurin in β-cell biology, it is not surprising that pharmacologic calcineurin inhibition – necessary to prevent rejection in the setting of islet transplantation – is associated with post-transplant β-cell failure. New-onset diabetes mellitus after transplantation is a frequent complication after kidney transplantation, with an incidence of 15–30% (Cosio et al. 2002, Kasiske et al. 2003).

Several studies show that calcineurin inhibitors can target β-cells directly. Tacrolimus (FK506), a calcineurin inhibitor used in clinical practice to suppress islet graft rejection, induces β-cell apoptosis as evident by TUNEL staining in cultured human islets within 48 h of exposure (Soleimanpour et al. 2010). This study identified insulin receptor substrate 2, a known CREB target and upstream regulator of the PI3K/Akt pathway, as a calcineurin target in β-cells. It was found that tacrolimus decreased Akt phosphorylation, suggesting that calcineurin could regulate replication and survival via the PI3K/Akt pathway (Soleimanpour et al. 2010). Similarly, rapamycin and cyclosporin A (also calcineurin inhibitors) decrease cell viability in human and rat pancreatic islets (Ozbay et al. 2011, Barlow et al. 2012) and in clonal insulin-producing cells (Plaumann et al. 2008). Mechanistically, calcineurin inhibition activates the dual leucine-zipper-bearing kinase (DLK), which in turn activates apoptotic MAPK signaling (Merritt et al. 1999, Plaumann et al. 2008). Human β-cell proliferation decreases exponentially with increasing age (Meier et al. 2008). Thus, studies of human β-cells, which are often carried out on islets from elderly donors, often fail to detect β-cell proliferation. Therefore, studies of β-cell proliferation are often carried out on β-cells obtained from rodent donors. In such studies, tacrolimus decreased β-cell proliferation by 72% in C57Bl/6 mice compared with vehicle-treated controls (Goodyer et al. 2012). These results lend support to experiments carried out in mice lacking calcineurin in β-cells (Heit et al. 2006). Mice with a β-cell-specific deletion of the calcineurin phosphatase regulatory subunit b1 develop age-dependent diabetes, characterized by decreased β-cell proliferation and mass, reduced pancreatic insulin content, and hypoinsulinemia. Moreover, β-cells lacking calcineurin activity have a reduced expression of established regulators of β-cell proliferation, such as MafA, Beta2, and Pdx1 (Heit et al. 2006). The impact of calcineurin on β-cell function is complex because, transgenic overexpression of active calcineurin in β-cells phenocopied mice with a β-cell-specific deletion of the calcineurin and resulted in decreased β-cell mass and hyperglycemia (Bernal-Mizrachi et al. 2010). These mice, which express a constitutively active form of calcineurin under the insulin gene promoter, exhibit glucose intolerance (Bernal-Mizrachi et al. 2010). In vitro studies of islets isolated from such mice demonstrated that decreased β-cell mass was accompanied by decreased proliferation and enhanced apoptosis (Bernal-Mizrachi et al. 2010). Taken together, these results demonstrate that pharmacological inhibition of calcineurin and genetic calcineurin deletion markedly inhibit rodent β-cell proliferation and promote β-cell apoptosis, which should be taken into account when treating patients in the need of immunosuppression. This may be especially important with patients displaying insulin resistance as the diabetogenic effect of tacrolimus, and cyclosporin A is more pronounced in insulin resistant obese rats (Rodriguez-Rodriguez et al. 2013).

While the vast majority of data suggest that calcineurin inhibition reduces β-cell viability and may cause a diabetes phenotype, the situation is different when it comes to β-cell death induced by either proinflammatory cytokines (Grunnet et al. 2009) or glucocorticoids (Ranta et al. 2006, 2008).

Treatment of isolated rats or human islets with cytokines promotes β-cell apoptosis by the intrinsic apoptotic pathway along with dephosphorylation of the proapoptotic protein BAD at ser136 (Grunnet et al. 2009). This particular serine residue is a target for calcineurin (Wang et al. 1999). In concordance, supplementation of tacrolimus to the cytokine-containing cell media prevented BAD dephosphorylation and cytokine-induced cytotoxicity (Grunnet et al. 2009), showing that – under these circumstances – calcineurin inhibition is favorable for β-cell viability.

The situation is similar under conditions of glucocorticoid exposure in culture. Although glucocorticoid-induced β-cell apoptosis has not been demonstrated in vivo, it is clear that these steroid hormones are cytotoxic to β-cells during ex vivo culture conditions (Ranta et al. 2006, 2008, Avram et al. 2008, Reich et al. 2012, Fransson et al. 2013). Glucocorticoids activate calcineurin, which in turn dephosphorylates the apoptotic protein BAD (Tumlin et al. 1997). Such a mechanism has also been demonstrated in insulin-producing cells (Ranta et al. 2006). Inhibition of calcineurin activity by tacrolimus and deltamethrin in insulin-secreting INS1 cells reduced apoptosis provoked by the synthetic glucocorticoid analog dexamethasone (Ranta et al. 2008). Thus, direct inhibition of calcineurin activity in β-cells decreases cell viability and reduces β-cell function. Of note, the situation is different for β-cell death induced by cytokines and glucocorticoids. In such cases, inhibition of calcineurin counteracts the cytotoxic effect of cytokines and glucocorticoids. Pharmacological inhibitors are never 100% specific, so these seemingly contradictory findings may be, at least partly, explained by effects that are independent of calcineurin. For example, tacrolimus can inhibit NF-κB activity, leading to the inhibition of NO formation (Tunon et al. 2003).

Ser/thr PP2C

Identification of PP2C isoforms traces back to the 1980s (Hiraga et al. 1981, Pato & Adelstein 1983). PP2C enzymes act on a variety of substrate classes, e.g. kinases, receptors, channels, and transcription factors, thereby affecting quite diverse physiological effects, e.g. stress response, metabolism, and cell cycle (Klumpp et al. 2006). In contrast to most other ser/thr PPs, inhibitors such as okadaic acid, microcystin, tautomycin, or inhibitor proteins I1 and I2 have no effect on PP2C isoenzymes. Hitherto PP2C isoforms have not been implicated in β-cell apoptosis. However, they are sensitive to stimulation by unsaturated fatty acids (Krieglstein et al. 2008). In this aspect, PP2C isoforms have been linked with fatty acid-induced apoptosis in neural and endothelial cells (Schwarz et al. 2006) and similar mechanisms may be operative in pancreatic β-cells.

Ser/thr PPs 4, 6, and 7

The ser/thr PPs 4, 6, and 7 have not been studied with regards to their possible implications in β-cell apoptosis. The catalytic subunit of PP4 is expressed in islets of Langerhans as evident by immunohistochemistry (http://www.proteinatlas.org), where it is located in the nucleus (Veluthakal et al. 2006). Neither PP6 nor PP7 catalytic subunit expression has been documented in β-cells.

Ser/thr PP5

PP5 is another member of the PPP family (Andreeva & Kutuzov 1999, Swingle et al. 2004) that is highly conserved among species and expressed in most, if not all, mammalian cells. However, the roles of PP5 in biology and disease are only beginning to emerge (Yong et al. 2007, Amable et al. 2011, Hinds et al. 2011), and the influence of PP5 on β-cell function is still unknown. The human gene encoding PP5 (PPP5C) is localized on chromosome 19 (Xu et al. 1996). PP5 has been reported to be present both in the nucleus and cytosol (Chinkers 2001). It has been proven difficult to study the biological role of PP5, partly because until recently, only a few physiological substrates have been identified. The polyunsaturated fatty acid, arachidonic acid, and the structural component of caveolae, caveolin-1, have both been shown to activate PP5 (Ramsey & Chinkers 2002, Taira & Higashimoto 2013). A high-throughput screening effort identified chaulmoogric acid as a compound that activate PP5 at fairly high concentrations (Cher et al. 2010). Suramin was identified as a novel PP5 activator by its competitively binding to a domain of PP5 and thereby causing its activation (Yamaguchi et al. 2013). During standard conditions, PP5 is predominately in an inactive state (Sinclair et al. 1999), causing a very low basal activity that represent <1% of the total measurable phosphatase activity. A unique characteristic of PP5 is that it is expressed as a single polypeptide, which consists of a phosphatase catalytic domain near its C-terminus and a regulatory domain at the N-terminus (Becker et al. 1994, Golden & Honkanen 2003). An additional feature, unique for PP5 among its family members, is the extended N-terminal region containing multiple TPR domains, by which PP5 mediates protein–protein interactions (Das et al. 1998). PP5 is associated with numerous proteins involved in diverse signaling networks, including Hsp90 in complex with the GR (Chen et al. 1996, Silverstein et al. 1997), the cell division cycle (CDC16/CDC27/CDC37) subunits of the anaphase-promoting complex (Ollendorff & Donoghue 1997, Vaughan et al. 2008), cryptochrome 2 (Zhao & Sancar 1997), ataxia–telangiectasia and Rad3-related (Zhang et al. 2005) ataxia–telangiectasia and Rad3-mutated (Ali et al. 2004) DNA-dependent protein kinase catalytic subunit (Wechsler et al. 2004), apoptosis signal regulating kinase 1 (ASK1; Morita et al. 2001), Hsp90-dependent heme-regulated eIF2α kinase (Shao et al. 2002), Rac GTP-binding protein (Gentile et al. 2006), the A-regulatory subunit of PP2A (Lubert et al. 2001), Raf proto-oncogene ser/thr protein kinase (von Kriegsheim et al. 2006), stress-induced phosphoprotein 1 (Skarra et al. 2011), and the Gα12/Gα13 subunits of heterotrimeric GTP-binding proteins (Yamaguchi et al. 2002).

PP5 has been recently shown for the first time to play a role in the β-cells (Grankvist et al. 2012). During the progression toward T2D, β-cells are often exposed to a combination of high levels of glucose and fatty acids, resulting in the production of the so-called glucolipotoxicity, which is associated with increased production of reactive oxygen species (ROS; Oprescu et al. 2007). In turn, increased levels of ROS cause initiation of apoptosis, resulting in a reduced β-cell mass (Butler et al. 2003). Several studies have indicated that PP5 is acting in the regulation of signaling cascades activated by oxidative stress.

Elevated levels of ROS can induce the association of PP5 with ASK1, leading to reduced ASK1 phosphorylation at Thr845, and thereby causing ASK1 inactivation (Morita et al. 2001, Huang et al. 2004, Zhou et al. 2004, Kutuzov et al. 2005). This suggests that PP5 can suppress the oxidative stress-induced apoptosis by averting sustained activation of ASK1 and its downstream target, JNK. PP5 may accordingly act as a negative regulator of the ASK1/JNK signaling pathway and in so doing protect cells from apoptosis (Morita et al. 2001, Kutuzov et al. 2005, Mkaddem et al. 2009). This concept was supported by the recent publication (Grankvist et al. 2012) indicating that islets from mice lacking PP5 were more susceptible toward stress-induced apoptosis than WT cognates. In addition, PP5-deficient mice had lower fasting glycemia and improved glucose tolerance compared with the WT mice, suggesting a novel role for PP5 in the regulation of glucose homeostasis. These findings cannot be explained by a difference in islet mass between the PP5-deficient and WT mice, because no difference was observed (Grankvist et al. 2012). Furthermore, a high-fat diet treatment for 10 weeks revealed that the mice lacking PP5 gained markedly less weight, did not accumulate visceral fat, and displayed enhanced insulin sensitivity compared with the WT littermates (Grankvist et al. 2013). Another group (Hinds et al. 2011) also recently published studies indicating that embryonic fibroblasts from PP5 knockout mice did not accumulate lipids after adipogenic stimuli. Together, these studies suggest that PP5 may play a previously unrecognized role in both glucose and lipid metabolism. Nevertheless, additional studies are necessary to further address the role of PP5 in glucose homeostasis and β-cell function. Table 1 presents the summarized view on the role of different PSPs in β-cell biology.

Table 1

Summary of the protein phosphatases' effects on pancreatic β-cells

Protein nameInterventionBiological materialEffect on β-cellsReferences
Protein tyrosine phosphatases
 PTPN2Downregulation with siRNAINS1E cells, rat and human isletsPromotes cytokine-induced apoptosisMoore et al. (2009) and Santin et al. (2011)
 PTP1BGlobal genetic deletionMicePromotion of β-cell proliferation and reduced islet cell apoptosisFernandez-Ruiz (2014, #249)
 PTP-BLStable over expressionINS1 cellsCompromised cell proliferation but no change in the rate of cell apoptosisWelters et al. (2008)
Se/thr protein phosphatases
 Ppp1R15APharmacological inhibition by salubrinalINS1E cells, rat, and human isletsInduce apoptosis and augment fatty acid-induced apoptosis and controls glucose-mediated translationCnop et al. (2007), Vander Mierde et al. (2007) and Ladriere et al. (2010)
 PP2AHigh glucose or ceramide exposureINS1 832/13 cells and rat isletsEnhanced PP2A activity and loss of GSISKowluru & Metz (1997) and Arora et al. (2013)
 PP2AHigh glucose exposureINS1 and βTC-3 cellsFOXO1 activationYan et al. (2012)
 PP2B (calcineurin)Pharmacological inhibition by tacrolimus, rapamycin, and cyclosporin AHuman and rat islets, MIN6 cellsIncreased apoptosisPlaumann et al. (2008), Soleimanpour et al. (2010), Ozbay et al. (2011) and Barlow et al. (2012)
 PP2B (calcineurin)Pharmacological inhibition by tacrolimusC57Bl/6j mice in vivoInhibition of β-cell proliferationGoodyer et al. (2012)
 PP2B (calcineurin)β-cell-specific genetic deletionMiceDevelops age-dependent diabetes alongside loss of β-cell massHeit et al. (2006)
 PP2B (calcineurin)β-cell-specific transgenic overexpressionMiceGlucose intolerance and loss of β-cell massBernal-Mizrachi et al. (2010)
 PP2B (calcineurin)Pharmacological inhibition by tacrolimus or deltamethrinHuman and rat isletsAttenuation of cytokine- and glucocorticoid-induced apoptosisRanta et al. (2008) and Grunnet et al. (2009)
 PP5Downregulation with siRNA or use of islets isolated from Ppp5c−/− miceMIN6 cells and mouse isletsPromotes glucocorticoid- and palmitate-induced apoptosisGrankvist et al. (2012) and Fransson et al. (2013)
 PP5Global genetic deletionMIN6 cells and miceImproves glucose toleranceGrankvist et al. (2012, 2013)

PPs and islet hormone secretion

Significant changes in protein kinase activities and in protein phosphorylation patterns occur subsequent to the stimulation of β-cell insulin release by nutrients (Newgard & McGarry 1995, Jones & Persaud 1998, Sjöholm 1998). Therefore, the molecular mechanisms regulating phosphorylation by protein kinases of proteins involved in the insulin secretory process by the β-cell have been extensively investigated. However, far less is known about the role and regulation of protein dephosphorylation by various PPs.

While early investigators reported the presence of phosphatase activity in pancreatic islets (Taljedal 1967, Lipson et al. 1979, Lernmark et al. 1980, Colca et al. 1984), the identity of these enzymes was unknown at the time. More contemporary studies have established that i) the β-cell contains ser/thr and tyrosine PP activity (Gagliardino et al. 1991, Sjöholm et al. 1993b, Chen & Ostenson 2005); ii) stimulation of protein phosphorylation by direct activation of PKA and PKC with forskolin or phorbol ester, respectively, results in a stimulated insulin secretion (Sjöholm 1991, Arkhammar et al. 1994, Hisatomi et al. 1996); iii) physiological stimuli of insulin secretion increase β-cell phosphorylation state (Jones & Persaud 1998); and iv) short-term treatment of β-cells or permeabilized rat pancreatic islets with the specific PP inhibitor okadaic acid promotes Ca2+ entry and insulin exocytosis (Ämmälä et al. 1994, Haby et al. 1994, Hisatomi et al. 1996, Larsson et al. 1997). These combined findings point to an important functional role for protein (de)phosphorylation in regulation of the stimulus–secretion coupling in the β-cell. The role of PPs in β-cell function and insulin secretion is nonetheless poorly understood.

Ser/Thr-PPases

Identification and characterization

PPP types 1 and 2A were identified in crude RINm5F β-cell homogenates by both enzymatic assay and western blot analysis (Sjöholm et al. 1993b). They were also characterized in terms of their sensitivity to the inhibitory actions of several compounds isolated from cyanobacteria, marine dinoflagellates, and marine sponges, (viz. okadaic acid, microcystin-LR, calyculin-A, and nodularin). It was found that okadaic acid was the least potent inhibitor (IC50 ∼10−9 M and IC100 ∼10−6 M), while the other compounds exhibited IC50 values of ∼5×10−10 M and IC100 ∼5×10−9 M (Sjöholm et al. 1993b).

Role in insulin stimulus–secretion coupling

The mechanisms that regulate insulin secretion were electrophysiologically investigated in single β-cells (Haby et al. 1994). The secretory responses were substantially increased by conditions that promote protein phosphorylation, such as activation of protein kinases A and C or inhibition of PPP family members (PP1, PP2A, and PP4–PP6) by okadaic acid. These results suggest that, although Ca2+ is required for the initiation of exocytosis, modulation of exocytosis by protein kinases and phosphatases is of much greater quantitative importance. Similar findings were reported by other groups (Mayer et al. 1994). It should be noted, however, that not all investigators have arrived at this conclusion (Tamagawa et al. 1992, Ammon et al. 1996, Murphy & Jones 1996, Sato et al. 1998, Krautheim et al. 1999). In some of these studies, PP inhibitors (e.g. okadaic acid) were added to intact cells, sometimes for long periods of time. It is important to keep in mind that inhibitors such as okadaic acid may exert non-specific effects and toxicity when added to intact cells. The agent is known to interfere with membrane integrity by non-specific mechanisms. Loss of membrane integrity will disrupt, for instance the Ca2+ gradient over the membrane, causing massive uncontrolled Ca2+ influx that may cause apoptosis. A more physiologic way of studying the roles of PP by using inhibitors is to apply these acutely to permeabilized cells or in patch clamp settings, in which Ca2+ gradients are not operative. Indeed, the inhibitory effect of okadaic acid on GSIS in intact cells was mimicked by the inactive analog 1-nor-okadaone; in contrast, okadaic acid stimulated insulin secretion from permeabilized cells (Ratcliff & Jones 1993).

In one study, it was shown that the inhibitory effect of leptin on insulin secretion in rat and human islets is associated with decreased expression and activation of a PP1-like enzyme (Kuehnen et al. 2011).

In another study (Sjöholm et al. 1995), the effects of known insulin secretagogues and intracellular second messengers on the activities of cation-independent ser/thr PPs in insulin-secreting RINm5F β-cells were investigated. The stimulation of intact RINm5F cells with the insulin secretagogues, l-arginine, l-glutamine, K+, or extracellular ATP elicited time-dependent changes in PPP activities with an early decrease in type 1-like and/or type 2A-like PPP activity that gradually returned to normal levels. Addition of cAMP, cGMP, or prostaglandins E2 and F1α at widely different concentrations to RINm5F cell homogenates failed to affect PPP activities. In contrast, addition of physiological concentrations of adenine nucleotides, known to increase upon nutrient stimulation, to RINm5F β-cell homogenates inhibited PP2A-like and, to a lesser extent, PP1-like PPP activity. It was concluded that insulin secretagogues cause time- and concentration-dependent inhibitory effects on RINm5F β-cell PPP activities, which may contribute to the increase in the phosphorylation state that occurs after stimulation of insulin release (Sjöholm et al. 1995). Thus, inhibition of protein dephosphorylation may be a regulatory mechanism controlling the stimulus–secretion coupling in insulin-producing cells. However, there are also contradictory findings: Murphy & Jones (1996) reported that PP1/2A was stimulated by glucose and required for GSIS in rat islets. The reasons for these discrepancies remain unknown at this time, but may relate to different models used.

In another study (Sjöholm et al. 2002), it was demonstrated that glycolytic and Krebs cycle intermediates, whose concentrations increase upon glucose stimulation, not only dose dependently inhibited ser/thr PP enzymatic activities but also directly promote insulin exocytosis from permeabilized β-cells. Thus, fructose-1,6-bisphosphate, phosphoenolpyruvate, 3-phosphoglycerate, citrate, and oxaloacetate inhibited PPPs and significantly enhanced insulin exocytosis, non-additive to that of okadaic acid, at micromolar Ca2+ concentrations. In contrast, the effect of GTP was potentiated by okadaic acid, suggesting that the action of GTP does not require PPP inactivation. It was concluded that specific glucose metabolites and GTP inhibit β-cell PP activities and directly stimulate Ca2+-independent insulin exocytosis. The glucose metabolites, but not GTP, seem to require PP inactivation for their stimulatory effect on exocytosis. Thus, an increase in phosphorylation state, through inhibition of protein dephosphorylation by metabolic intermediates, may link glucose sensing to insulin exocytosis in the β-cell.

Although disputed (MacDonald & Fahien 2000), a messenger role has been postulated for l-glutamate in linking glucose stimulation to sustained insulin exocytosis in the β-cell (Maechler & Wollheim 1999), but the precise nature by which l-glutamate controls insulin secretion remains elusive. Effects of l-glutamate on the activities of PPPs and Ca2+-regulated insulin exocytosis in INS1E β-cells were investigated (Lehtihet et al. 2005). Glucose was found to increase l-glutamate contents and promote insulin secretion from INS1E cells. l-glutamate also dose dependently inhibited PP enzyme activities, mimicking the specific PPP inhibitor, okadaic acid. l-glutamate and okadaic acid directly and non-additively promoted insulin exocytosis from permeabilized INS1E cells in a Ca2+-independent manner. Thus, an inhibition of protein dephosphorylation by glucose-derived l-glutamate may link glucose sensing to sustained insulin exocytosis.

It has been additionally demonstrated that inositol hexakisphosphate (InsP6), whose concentration in β-cells transiently increases upon glucose stimulation (Larsson et al. 1997), dose dependently and differentially inhibits enzyme activities of ser/thr PPPs in physiologically relevant concentrations (Lehtihet et al. 2004). However, and in contrast to previous findings in rat islets (Gagliardino et al. 1997), none of the hypoglycemic sulfonylureas tested (glipizide, glibenclamide, and tolbutamide) affected PP1 or PP2A activity at clinically relevant concentrations in RINm5F cells. The reasons for this discrepancy remain elusive at this time; however, in part they may be due to different cell subclones, experimental conditions, and phosphoprotein substrates used.

The insulin secretagogue l-arginine, an immediate metabolic precursor to polyamines, was reported to cause a rapid and transient decrease in PP1 activity in RINm5F β-cells (Sjöholm et al. 1995). It was previously reported that polyamines dose dependently suppress PP1-like activity when added to RINm5F cell homogenates at physiologic concentrations, while having minor and inconsistent effects on PP2A-like activity (Sjöholm & Honkanen 2000). The IC50 value for spermine on PP1-like activity was ∼4 mM. The inhibitory effect was reproduced and of comparable magnitude on purified PPs types 1A and 2A. On the other hand, when endogenous polyamine pools were exhausted by 4 days of exposure to the specific l-ornithine decarboxylase inhibitor d,l-α-difluoromethylornithine (Sjöholm et al. 1993a), there was an increase in PP2A-like activity. Quantitative western analysis revealed that the amount of PP2A protein did not change after this treatment. It was concluded that polyamines cause time-and concentration-dependent inhibitory effects on the PPP activities of RINm5F β-cell, which may contribute to the increase in phosphorylation state that occurs after secretory stimulation. Figure 5 shows the proposed model of PPP regulation of insulin stimulus–secretion coupling.

Figure 5
Figure 5

Regulation of β-cell PP activities and their effects on the insulin stimulus–secretion coupling. Glucose, the β-cell's main stimulus, is taken up across the plasma membrane by the facilitative GLUT (GLUT2). The sugar is further metabolized in the glycolytic pathway and TCA cycle to yield coupling factors suppressing PP activity, thereby activating influx of Ca2+ that sets in motion the exocytotic release of insulin. The ATP generated during glucose catabolism also serves to close K+ channels, causing depolarization, and as a substrate for cAMP formation. Receptor-operated, G protein-coupled signaling pathways through phospholipase C–PKC and AC are also depicted. See text for details. AC, adenylyl cyclase; DAG, diacylglycerol; ER, endoplasmic reticulum; G, GTP-binding protein; Gln, glutamine; Glu, glutamate; GLUT2, glucose transporter 2; GTP, guanosine trisphosphate; InsP3, inositol trisphosphate; InsP6, inositol hexakisphosphate; KATP, ATP-dependent K+ channel; OA, okadaic acid; PIP2, phosphatidylinositol bisphosphate; PKC, protein kinase C; PLC, phospholipase C; PP, protein phosphatase; R, receptor; TPA, 12-O-tetradecanoyl phorbolacetate; VGCC, voltage-gated Ca2+ channel.

Citation: Journal of Endocrinology 221, 3; 10.1530/JOE-14-0002

Elegant work by the Kowluru laboratory has elucidated in great detail how PP2A is regulated (Kowluru 2005). The catalytic subunit of PP2A is subject to several means of post-translational modification: i) reversible carboxylmethylation at Leu309, catalyzed by a PP methyltransferase, results in activation of the enzyme, holoenzyme assembly, and substrate association (Kowluru et al. 1996). As ebelactone, an inhibitor of methyl esterases, not only delayed demethylation of PP2A but also decreased GSIS, a negative role was suggested for PP2A in normal rat islet GSIS (Kowluru et al. 1996). On the other hand, genetic silencing of the catalytic subunit of PP2A in INS1 832/13 insulinoma cells by siRNA was found to abrogate GSIS (Jangati et al. 2007). Carboxylmethylation of the catalytic subunit of PP2A was inhibited by certain glucose metabolites and by increased cytosolic Ca2+ (Palanivel et al. 2004), leading to inactivation of the enzyme. It was suggested that this mechanism facilitates hyperphosphorylation of exocytotic proteins, thereby augmenting insulin secretion. ii) Phosphorylation at Tyr307 has been shown to inhibit PP2A catalytic activity, whereas nitration of Tyr307 alleviates the enzyme from inactivation by phosphorylation (Kowluru & Matti 2012). iii) Phosphorylation at Thr304 results in inactivation of PP2A (Kowluru & Matti 2012).

Not only secretion, but also protein synthesis, may also be translationally regulated by reversible phosphorylation. It was suggested that glucose-stimulated translation in the β-cell requires a PP1-mediated decrease in Ser51 phosphorylation of eIF2α, an important factor controlling translational fidelity (Vander Mierde et al. 2007). In addition, in INS1 832/13 cells, glucose dephosphorylates elongation factor 2, probably through activation of PP2A (Yan et al. 2003). This suggests that INS1 832/13 cell protein translation rates are controlled by glucose-induced reversible phosphorylation of elongation factor 2.

Control of transcription factors may also be regulated by reversible phosphorylation: in INS1E cells, high glucose downregulates the expression of PPARα, leading to decreased fatty acid oxidation, through activation of PP2A and inactivation of AMPK, a cellular energy gauge (Ravnskjaer et al. 2006). Also other important enzymes of critical regulatory role in GSIS appear regulated by phosphorylation. For instance, acetyl-CoA-carboxylase – which catalyzes malonyl-CoA formation – was found to be activated by magnesium and glutamate probably through an okadaic acid-sensitive PP2A-like enzyme (Kowluru et al. 2001), an effect that may stimulate β-cell anaplerosis through provision of long-chain fatty acids.

PP2B (calcineurin) has also been implicated in the control of islet hormone secretion: Renstrom et al. (1996) showed that the inhibitory effect of several neurotransmittors known to inhibit insulin secretion (viz. somatostatin, galanin, and epinephrine) was associated with an activation of PP2B. Conversely, this inhibition of secretion was prevented by PP2B inhibitors. As PP2B inhibitors (e.g. cyclosporine-A) are used in islet transplantation for immunosuppressive purposes, the physiological role of the enzyme in islets is clinically very relevant. While short-term PP2B inhibition stimulates insulin secretion (Ebihara et al. 1996), long-term inhibition of the enzyme – or overexpression of its inhibitory regulators (Peiris et al. 2012) – may cause β-cell functional suppression and demise (Sjöholm 1994). PP2B is also required for proper cAMP-stimulated gene transcription in HIT β-cells (Schwaninger et al. 1995).

Phosphotyrosine phosphatases

Vanadate inhibits most PTPs and has been shown to exert direct glucose-dependent insulinotropic effects in isolated rodent islets by mechanisms involving phosphoinositide hydrolysis and Ca2+ handling (Fagin et al. 1987, Zhang et al. 1991). Interestingly, vanadium salts have also been found to exert anti-diabetic and islet-protective effects in various and widely different diabetic animal models, such as streptozotocin (Pederson et al. 1989), 90% pancreatectomy (Nakamura et al. 1995), and Zucker diabetic fatty rats (Winter et al. 2005), adding further credence to PTPs as inhibitors of insulin secretion also in vivo.

The PTPs IA-2 (ICA-512) and IA-2β (phogrin) are major autoantigens in T1D (Torii 2009), located in secretory granules, but developmentally differentially regulated. Whereas expression of phogrin appears insensitive to factors that influence β-cell function, IA-2 expression seems regulated by glucose, cAMP, and autocrine insulin (Lobner et al. 2002). In vivo, genetic disruption of IA-2 or phogrin results in glucose intolerance due to impaired insulin secretion (Henquin et al. 2008). However, it is likely that both enzymes are regulating the stability and/or loading of secretory granules, rather than influencing the exocytotic process per se. Thus, the main effects of PTPs on insulin secretion seem inhibitory.

PPs and diabetes

Surprisingly, little is known regarding the role of different PP in islets in diabetic states, in spite of the fact that PTPs IA-2 and phogrin are important autoantigens in T1D. Nonetheless, work from the Östenson laboratory (Östenson et al. 2002) reported a 60% overexpression of PTP σ in the islets of Goto–Kakizaki rats, a lean genetic model of T2D. Importantly, downregulation of PTPσ led to increased GSIS in these normally ‘glucose-blind’ islets. The authors concluded that increased expression of PTPσ may be of pathogenetic significance for the defective insulin secretion in GK rat islets. Interestingly, the same group reported that genetic variation in receptor PTPσ is associated with T2D in Swedish Caucasians (Langberg et al. 2007).

Another connection to both T1D and T2D may be ceramide, which is formed during sphingomyelin breakdown by proinflammatory cytokines such as IL1 (Mullen et al. 2012). Ceramide may inhibit β-cell mitogenesis and insulin production (Sjöholm 1995), possibly through the activation of JNK and the transcription factor ATF2 (Welsh 1996). Many effects of ceramide are believed to be mediated by a ceramide-activated PPP (CAPP), a PP2A-like enzyme expressed in islets (Kowluru & Metz 1997). The genetic silencing of the α isoform of the PP2A catalytic subunit, achieved through siRNA knockdown, was found to significantly reduce CAPP enzymatic activity in INS 832/13 cells (Jangati et al. 2006).

The Kowluru lab also reported that the catalytic subunit of PP4, present in INS1 cell nuclei, can be regulated by IL1 the following: exposure of the INS1 cells to IL1 led to the expected increase in NO formation, but also reduced the expression, carboxylmethylation, and enzymatic activity of PP4 (Veluthakal et al. 2006). PP4 catalytic subunit was found to form complex with nuclear lamin-B, which regulates nuclear envelope assembly. The authors proposed that IL1/NO-induced inhibition of PP4 expression and enzymatic activity may aid keeping lamin-B phosphorylated and thereby make it amenable for pro-apoptotic caspases that may lead to β-cell death (Veluthakal et al. 2006).

This effector system may also be relevant in T2D, as studies have shown that cytokines such as IL1 are produced by islet cells and increased by glucotoxicity (Maedler et al. 2002).

Another connection between T2D and ceramide is the lipotoxicity prevailing in T2D. While IL1-induced ceramide is formed from sphingolipids, islet ceramide accumulating under conditions of hyperlipidemia appears to be derived from de novo synthesis from free fatty acids (FFAs; Shimabukuro et al. 1998). Thus, islet ceramide and CAPP may be increased by two different mechanisms in T2D: a glucotoxic pathway involving paracrine/autocrine IL1 promoting sphingomyelin breakdown and a lipotoxic pathway in which ceramide is generated from FFAs. These two pathways, which normally potentiate each other's toxicity, may thus additively or synergistically activate islet CAPP.

In islets of T2D Goto–Kakizaki rats, the magnesium and glutamate-sensitive PP2A-like enzyme mentioned earlier (Kowluru et al. 2001) appears to be dysregulated, in that the activation of ACC by magnesium and glutamate seems to be markedly reduced (Palanivel et al. 2005). The pathophysiological relevance of this derangement is unclear at this time, but could conceivably result in reduced formation of long-chain fatty acids and contribute to the loss of GSIS in this widely used animal model.

Elegant studies from the Kowluru laboratory have provided in-depth mechanistic insights into the role of PP2A in islets under diabetes-like glucotoxic conditions (Arora et al. 2013). During chronic hyperglycemia, mimicked by high glucose in vitro, PP2A becomes hyperactivated – an effect coupled to loss of GSIS. Knockdown by siRNA of the PP2A catalytic subunit prevented this hyperactivation. Also, glucose, but not non-metabolizable sugars, augmented the carboxylmethylation of Leu307 of the catalytic subunit (Arora et al. 2013). High glucose increased the expression of a regulatory subunit of PP2A, which has been implicated in islet dysfunction during glucotoxicity. No clear role for ER stress in glucose-induced activation of PP2A could be found. Authors proposed that exposure of the β-cell to high glucose results in exaggerated PP2A activity and subsequent loss of GSIS.

Future prospects

To understand how protein dephosphorylation is regulated within the islet and how this controls hormone secretion, apoptosis, and proliferation, thorough and deep mechanistic studies are clearly needed. With the exception of PP5, which is the only member of the PPP family with the catalytic and regulatory subunit encoded by one gene, knockdown strategies targeting the catalytic subunits will not probably be a fruitful avenue to follow in order to explore the function of ser/thr PPs. Elimination of the regulatory subunits is likely to be more exact in targeting specific cellular functions. Knockdown experiments should be followed by an investigation of changes in the phoshoproteome to further pinpoint which proteins that are targeted. Furthermore, characterization of the different PSPs/PTPs expressed in the various islet cell types may prove important not only from a diabetes pathogenic perspective but may also offer clues to development of novel antidiabetic drugs.

Declaration of interest

The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the review.

Funding

Work from the authors' laboratories cited in this paper was supported by grants from the Swedish Diabetes Foundation (Diabetesfonden), Folksam Research Foundation, the Diabetes Research Wellness Foundation, the Tore Nilsson Foundation, the Lars Hierta's memorial foundation, Tornspiran Foundation, Berth von Kantzow's Foundation, Golje's Memorial Foundation, Eva and Oscar Ahrén's Foundation, and NIH CA 60750.

References

  • AliAZhangJBaoSLiuIOtternessDDeanNMAbrahamRTWangXF2004Requirement of protein phosphatase 5 in DNA-damage-induced ATM activation. Genes and Development18249254. (doi:10.1101/gad.1176004)

    • Search Google Scholar
    • Export Citation
  • AlonsoASasinJBottiniNFriedbergIFriedbergIOstermanAGodzikAHunterTDixonJMustelinT2004Protein tyrosine phosphatases in the human genome. Cell117699711. (doi:10.1016/j.cell.2004.05.018)

    • Search Google Scholar
    • Export Citation
  • AmableLGrankvistNLargenJWOrtsaterHSjoholmAHonkanenRE2011Disruption of serine/threonine protein phosphatase 5 (PP5:PPP5c) in mice reveals a novel role for PP5 in the regulation of ultraviolet light-induced phosphorylation of serine/threonine protein kinase Chk1 (CHEK1). Journal of Biological Chemistry2864041340422. (doi:10.1074/jbc.M111.244053)

    • Search Google Scholar
    • Export Citation
  • American Diabetes AssociationEconomic costs of diabetes in the U.S. in 2012Diabetes Care36201310331046. (doi:10.2337/dc12-2625)

  • ÄmmäläCEliassonLBokvistKBerggrenPOHonkanenRESjöholmARorsmanP1994Activation of protein kinases and inhibition of protein phosphatases play a central role in the regulation of exocytosis in mouse pancreatic β cells. PNAS9143434347. (doi:10.1073/pnas.91.10.4343)

    • Search Google Scholar
    • Export Citation
  • AmmonHPHeurichROKolbHALangFSchaichRDrewsGLeiersT1996The phosphatase inhibitor okadaic acid blocks KCl-depolarization-induced rise of cytosolic calcium of rat insulinoma cells (RINm5F). Naunyn-Schmiedeberg's Archives of Pharmacology35495101. (doi:10.1007/BF00178708)

    • Search Google Scholar
    • Export Citation
  • AndersenJNMortensenOHPetersGHDrakePGIversenLFOlsenOHJansenPGAndersenHSTonksNKMollerNP2001Structural and evolutionary relationships among protein tyrosine phosphatase domains. Molecular and Cellular Biology2171177136. (doi:10.1128/MCB.21.21.7117-7136.2001)

    • Search Google Scholar
    • Export Citation
  • AndreevaAVKutuzovMA1999RdgC/PP5-related phosphatases: novel components in signal transduction. Cellular Signalling11555562. (doi:10.1016/S0898-6568(99)00032-7)

    • Search Google Scholar
    • Export Citation
  • ArkhammarPJuntti-BerggrenLLarssonOWelshMNanbergESjoholmAKohlerMBerggrenPO1994Protein kinase C modulates the insulin secretory process by maintaining a proper function of the β-cell voltage-activated Ca2+ channels. Journal of Biological Chemistry26927432749.

    • Search Google Scholar
    • Export Citation
  • AroraDKMachhadiehBMattiAWadzinskiBERamanadhamSKowluruA2013High glucose exposure promotes activation of protein phosphatase 2A in rodent islets and INS-1 832/13 β-cells by increasing the posttranslational carboxylmethylation of its catalytic subunit. Endocrinology155380391. (doi:10.1210/en.2013-1773)

    • Search Google Scholar
    • Export Citation
  • AshcroftFMRorsmanP2012Diabetes mellitus and the β cell: the last ten years. Cell14811601171. (doi:10.1016/j.cell.2012.02.010)

  • AshcroftFMHarrisonDEAshcroftSJ1984Glucose induces closure of single potassium channels in isolated rat pancreatic β-cells. Nature312446448. (doi:10.1038/312446a0)

    • Search Google Scholar
    • Export Citation
  • AvramDRantaFHennigeAMBerchtoldSHoppSHaringHULangFUllrichS2008IGF-1 protects against dexamethasone-induced cell death in insulin secreting INS-1 cells independent of AKT/PKB phosphorylation. Cellular Physiology and Biochemistry21455462. (doi:10.1159/000129638)

    • Search Google Scholar
    • Export Citation
  • BarfordDFlintAJTonksNK1994Crystal structure of human protein tyrosine phosphatase 1B. Science26313971404. (doi:10.1126/science.8128219)

    • Search Google Scholar
    • Export Citation
  • BarlowADXieJMooreCECampbellSCShawJANicholsonMLHerbertTP2012Rapamycin toxicity in MIN6 cells and rat and human islets is mediated by the inhibition of mTOR complex 2 (mTORC2). Diabetologia5513551365. (doi:10.1007/s00125-012-2475-7)

    • Search Google Scholar
    • Export Citation
  • BeckerWKentrupHKlumppSSchultzJEJoostHG1994Molecular cloning of a protein serine/threonine phosphatase containing a putative regulatory tetratricopeptide repeat domain. Journal of Biological Chemistry2692258622592.

    • Search Google Scholar
    • Export Citation
  • BellGIPolonskyKS2001Diabetes mellitus and genetically programmed defects in β-cell function. Nature414788791. (doi:10.1038/414788a)

  • Bernal-MizrachiECras-MeneurCYeBRJohnsonJDPermuttMA2010Transgenic overexpression of active calcineurin in β-cells results in decreased β-cell mass and hyperglycemia. PLoS ONE5e11969. (doi:10.1371/journal.pone.0011969)

    • Search Google Scholar
    • Export Citation
  • BittingerMAMcWhinnieEMeltzerJIourgenkoVLatarioBLiuXChenCHSongCGarzaDLabowM2004Activation of cAMP response element-mediated gene expression by regulated nuclear transport of TORC proteins. Current Biology1421562161. (doi:10.1016/j.cub.2004.11.002)

    • Search Google Scholar
    • Export Citation
  • BlanchetotCChagnonMDubeNHalleMTremblayML2005Substrate-trapping techniques in the identification of cellular PTP targets. Methods354453. (doi:10.1016/j.ymeth.2004.07.007)

    • Search Google Scholar
    • Export Citation
  • BollenMPetiWRagusaMJBeullensM2010The extended PP1 toolkit: designed to create specificity. Trends in Biochemical Sciences35450458. (doi:10.1016/j.tibs.2010.03.002)

    • Search Google Scholar
    • Export Citation
  • BreitkreutzAChoiHSharomJRBoucherLNeduvaVLarsenBLinZYBreitkreutzBJStarkCLiuG2010A global protein kinase and phosphatase interaction network in yeast. Science32810431046. (doi:10.1126/science.1176495)

    • Search Google Scholar
    • Export Citation
  • ButlerAEJansonJBonner-WeirSRitzelRRizzaRAButlerPC2003β-cell deficit and increased β-cell apoptosis in humans with type 2 diabetes. Diabetes52102110. (doi:10.2337/diabetes.52.1.102)

    • Search Google Scholar
    • Export Citation
  • CardozoAKKruhofferMLeemanROrntoftTEizirikDL2001Identification of novel cytokine-induced genes in pancreatic β-cells by high-density oligonucleotide arrays. Diabetes50909920. (doi:10.2337/diabetes.50.5.909)

    • Search Google Scholar
    • Export Citation
  • ChenJOstensonCG2005Inhibition of protein-tyrosine phosphatases stimulates insulin secretion in pancreatic islets of diabetic Goto–Kakizaki rats. Pancreas30314317. (doi:10.1097/01.mpa.0000161887.25115.6c)

    • Search Google Scholar
    • Export Citation
  • ChenMSSilversteinAMPrattWBChinkersM1996The tetratricopeptide repeat domain of protein phosphatase 5 mediates binding to glucocorticoid receptor heterocomplexes and acts as a dominant negative mutant. Journal of Biological Chemistry2713231532320. (doi:10.1074/jbc.271.50.32315)

    • Search Google Scholar
    • Export Citation
  • ChenJPetersonRTSchreiberSL1998α4 associates with protein phosphatases 2A, 4, and 6. Biochemical and Biophysical Research Communications247827832. (doi:10.1006/bbrc.1998.8792)

    • Search Google Scholar
    • Export Citation
  • ChenGITisayakornSJorgensenCD'AmbrosioLMGoudreaultMGingrasAC2008PP4R4/KIAA1622 forms a novel stable cytosolic complex with phosphoprotein phosphatase 4. Journal of Biological Chemistry2832927329284. (doi:10.1074/jbc.M803443200)

    • Search Google Scholar
    • Export Citation
  • CherCTremblayMHBarberJRChung NgSZhangB2010Identification of chaulmoogric acid as a small molecule activator of protein phosphatase 5. Applied Biochemistry and Biotechnology16014501459. (doi:10.1007/s12010-009-8647-3)

    • Search Google Scholar
    • Export Citation
  • ChinkersM2001Protein phosphatase 5 in signal transduction. Trends in Endocrinology and Metabolism122832. (doi:10.1016/S1043-2760(00)00335-0)

    • Search Google Scholar
    • Export Citation
  • CnopMLadriereLHekermanPOrtisFCardozoAKDogusanZFlamezDBoyceMYuanJEizirikDL2007Selective inhibition of eukaryotic translation initiation factor 2α dephosphorylation potentiates fatty acid-induced endoplasmic reticulum stress and causes pancreatic β-cell dysfunction and apoptosis. Journal of Biological Chemistry28239893997. (doi:10.1074/jbc.M607627200)

    • Search Google Scholar
    • Export Citation
  • CohenPT2002Protein phosphatase 1 – targeted in many directions. Journal of Cell Science115241256.

  • ColcaJRKotagalNLacyPEBrooksCLNorlingLLandtMMcDanielML1984Glucose-stimulated protein phosphorylation in the pancreatic islet. Biochemical Journal220529537.

    • Search Google Scholar
    • Export Citation
  • CookDLHalesCN1984Intracellular ATP directly blocks K+ channels in pancreatic B-cells. Nature311271273. (doi:10.1038/311271a0)

  • CosioFGPesaventoTEKimSOseiKHenryMFergusonRM2002Patient survival after renal transplantation: IV. Impact of post-transplant diabetes. Kidney International6214401446. (doi:10.1111/j.1523-1755.2002.kid582.x)

    • Search Google Scholar
    • Export Citation
  • CouzensALKnightJDKeanMJTeoGWeissADunhamWHLinZYBagshawRDSicheriFPawsonT2013Protein interaction network of the Mammalian hippo pathway reveals mechanisms of kinase–phosphatase interactions. Science Signaling6rs15. (doi:10.1126/scisignal.2004712)

    • Search Google Scholar
    • Export Citation
  • CrabtreeGROlsonEN2002NFAT signaling: choreographing the social lives of cells. Cell109 (Suppl) S67S79. (doi:10.1016/S0092-8674(02)00699-2)

    • Search Google Scholar
    • Export Citation
  • DasAKCohenPWBarfordD1998The structure of the tetratricopeptide repeats of protein phosphatase 5: implications for TPR-mediated protein–protein interactions. EMBO Journal1711921199. (doi:10.1093/emboj/17.5.1192)

    • Search Google Scholar
    • Export Citation
  • DetimaryPJonasJCHenquinJC1995Possible links between glucose-induced changes in the energy state of pancreatic B cells and insulin release. Unmasking by decreasing a stable pool of adenine nucleotides in mouse islets. Journal of Clinical Investigation9617381745. (doi:10.1172/JCI118219)

    • Search Google Scholar
    • Export Citation
  • EbiharaKFukunagaKMatsumotoKShichiriMMiyamotoE1996Cyclosporin A stimulation of glucose-induced insulin secretion in MIN6 cells. Endocrinology13752555263.

    • Search Google Scholar
    • Export Citation
  • EndicottJANobleMEJohnsonLN2012The structural basis for control of eukaryotic protein kinases. Annual Review of Biochemistry81587613. (doi:10.1146/annurev-biochem-052410-090317)

    • Search Google Scholar
    • Export Citation
  • FaginJAIkejiriKLevinSR1987Insulinotropic effects of vanadate. Diabetes3614481452. (doi:10.2337/diab.36.12.1448)

  • Fernandez-RuizRVieiraEGarcia-RovesPMGomisR2014Protein tyrosine phosphatase-1B modulates pancreatic β-cell mass. PLoS One9e90344. (doi:10.1371/journal.pone.0090344)

    • Search Google Scholar
    • Export Citation
  • FischerEHGravesDJCrittendenERKrebsEG1959Structure of the site phosphorylated in the phosphorylase b to a reaction. Journal of Biological Chemistry23416981704.

    • Search Google Scholar
    • Export Citation
  • FlintAJTiganisTBarfordDTonksNK1997Development of “substrate-trapping” mutants to identify physiological substrates of protein tyrosine phosphatases. PNAS9416801685. (doi:10.1073/pnas.94.5.1680)

    • Search Google Scholar
    • Export Citation
  • FranssonLRosengrenVSahaTKGrankvistNIslamTHonkanenRESjöholmÅOrtsäterH2013Mitogen-activated protein kinases and protein phosphatase 5 mediate glucocorticoid-induced cytotoxicity in pancreatic islets and β-cells. Molecular and Cellular Endocrinology383126136. (doi:10.1016/j.mce.2013.12.010)

    • Search Google Scholar
    • Export Citation
  • GagliardinoJJKrinksMHGagliardinoEE1991Identification of the calmodulin-regulated protein phosphatase, calcineurin, in rat pancreatic islets. Biochimica et Biophysica Acta1091370373. (doi:10.1016/0167-4889(91)90202-9)

    • Search Google Scholar
    • Export Citation
  • GagliardinoJJRossiPFGarciaME1997Inhibitory effect of sulfonylureas on protein phosphatase activity in rat pancreatic islets. Acta Diabetologia3469. (doi:10.1007/s005920050057)

    • Search Google Scholar
    • Export Citation
  • GembalMGilonPHenquinJC1992Evidence that glucose can control insulin release independently from its action on ATP-sensitive K+ channels in mouse B cells. Journal of Clinical Investigation8912881295. (doi:10.1172/JCI115714)

    • Search Google Scholar
    • Export Citation
  • GentileSDardenTErxlebenCRomeoCRussoAMartinNRossieSArmstrongDL2006Rac GTPase signaling through the PP5 protein phosphatase. PNAS10352025206. (doi:10.1073/pnas.0600080103)

    • Search Google Scholar
    • Export Citation
  • GoldenTAHonkanenRE2003Regulating the expression of protein phosphatase type 5. Methods in Enzymology366372390.

  • GoldenTSwingleMHonkanenRE2008The role of serine/threonine protein phosphatase type 5 (PP5) in the regulation of stress-induced signaling networks and cancer. Cancer Metastasis Reviews27169178. (doi:10.1007/s10555-008-9125-z)

    • Search Google Scholar
    • Export Citation
  • GoodyerWRGuXLiuYBottinoRCrabtreeGRKimSK2012Neonatal β cell development in mice and humans is regulated by calcineurin/NFAT. Developmental Cell232134. (doi:10.1016/j.devcel.2012.05.014)

    • Search Google Scholar
    • Export Citation
  • GrankvistNAmableLHonkanenRESjöholmÅOrtsäterH2012Serine/threonine protein phosphatase 5 regulates glucose homeostasis in vivo and apoptosis signalling in mouse pancreatic islets and clonal MIN6 cells. Diabetologia5520052015. (doi:10.1007/s00125-012-2541-1)

    • Search Google Scholar
    • Export Citation
  • GrankvistNHonkanenRESjöholmÅOrtsäterH2013Genetic disruption of protein phosphatase 5 in mice prevents high-fat diet feeding-induced weight gain. FEBS Letters58738693874. (doi:10.1016/j.febslet.2013.10.022)

    • Search Google Scholar
    • Export Citation
  • GrimsbyJSarabuRCorbettWLHaynesNEBizzarroFTCoffeyJWGuertinKRHilliardDWKesterRFMahaneyPE2003Allosteric activators of glucokinase: potential role in diabetes therapy. Science301370373. (doi:10.1126/science.1084073)

    • Search Google Scholar
    • Export Citation
  • GroupUS1998aEffect of intensive blood–glucose control with metformin on complications in overweight patients with type 2 diabetes (UKPDS 34). UK Prospective Diabetes Study (UKPDS) Group. Lancet352854865. (doi:10.1016/S0140-6736(05)61359-1)

    • Search Google Scholar
    • Export Citation
  • GroupUS1998bIntensive blood–glucose control with sulphonylureas or insulin compared with conventional treatment and risk of complications in patients with type 2 diabetes (UKPDS 33). UK Prospective Diabetes Study (UKPDS) Group. Lancet352837853. (doi:10.1016/S0140-6736(05)61359-1)

    • Search Google Scholar
    • Export Citation
  • GrunnetLGAikinRTonnesenMFParaskevasSBlaabjergLStorlingJRosenbergLBillestrupNMaysingerDMandrup-PoulsenT2009Proinflammatory cytokines activate the intrinsic apoptotic pathway in β-cells. Diabetes5818071815. (doi:10.2337/db08-0178)

    • Search Google Scholar
    • Export Citation
  • HabyCLarssonOIslamMSAunisDBerggrenPOZwillerJ1994Inhibition of serine/threonine protein phosphatases promotes opening of voltage-activated L-type Ca2+ channels in insulin-secreting cells. Biochemical Journal298341346.

    • Search Google Scholar
    • Export Citation
  • HedeskovCJ1980Mechanism of glucose-induced insulin secretion. Physiological Reviews60442509.

  • HeitJJApelqvistAAGuXWinslowMMNeilsonJRCrabtreeGRKimSK2006Calcineurin/NFAT signalling regulates pancreatic β-cell growth and function. Nature443345349. (doi:10.1038/nature05097)

    • Search Google Scholar
    • Export Citation
  • HenquinJCNenquinMSzollosiAKubosakiANotkinsAL2008Insulin secretion in islets from mice with a double knockout for the dense core vesicle proteins islet antigen-2 (IA-2) and IA-2β. Journal of Endocrinology196573581. (doi:10.1677/JOE-07-0496)

    • Search Google Scholar
    • Export Citation
  • HeroesELesageBGornemannJBeullensMVan MeerveltLBollenM2013The PP1 binding code: a molecular-lego strategy that governs specificity. FEBS Journal280584595. (doi:10.1111/j.1742-4658.2012.08547.x)

    • Search Google Scholar
    • Export Citation
  • HindsTDJrStechschulteLACashHAWhislerDBanerjeeAYongWKhuderSSKawMKShouWNajjarSM2011Protein phosphatase 5 mediates lipid metabolism through reciprocal control of glucocorticoid receptor and peroxisome proliferator-activated receptor-γ (PPARγ). Journal of Biological Chemistry2864291142922. (doi:10.1074/jbc.M111.311662)

    • Search Google Scholar
    • Export Citation
  • HiragaAKikuchiKTamuraSTsuikiS1981Purification and characterization of Mg2+-dependent glycogen synthase phosphatase (phosphoprotein phosphatase IA) from rat liver. European Journal of Biochemistry119503510. (doi:10.1111/j.1432-1033.1981.tb05636.x)

    • Search Google Scholar
    • Export Citation
  • HisatomiMHidakaHNikiI1996Ca2+/calmodulin and cyclic 3,5′ adenosine monophosphate control movement of secretory granules through protein phosphorylation/dephosphorylation in the pancreatic β-cell. Endocrinology13746444649.

    • Search Google Scholar
    • Export Citation
  • HonkanenREGoldenT2002Regulators of serine/threonine protein phosphatases at the dawn of a clinical era?Current Medicinal Chemistry920552075. (doi:10.2174/0929867023368836)

    • Search Google Scholar
    • Export Citation
  • HuangXHonkanenRE1998Molecular cloning, expression, and characterization of a novel human serine/threonine protein phosphatase, PP7, that is homologous to Drosophila retinal degeneration C gene product (rdgC). Journal of Biological Chemistry27314621468. (doi:10.1074/jbc.273.3.1462)

    • Search Google Scholar
    • Export Citation
  • HuangXSwingleMRHonkanenRE2000Photoreceptor serine/threonine protein phosphatase type 7: cloning, expression, and functional analysis. Methods in Enzymology315579593.

    • Search Google Scholar
    • Export Citation
  • HuangSShuLEastonJHarwoodFCGermainGSIchijoHHoughtonPJ2004Inhibition of mammalian target of rapamycin activates apoptosis signal-regulating kinase 1 signaling by suppressing protein phosphatase 5 activity. Journal of Biological Chemistry2793649036496. (doi:10.1074/jbc.M401208200)

    • Search Google Scholar
    • Export Citation
  • HunterTSeftonBM1980Transforming gene product of Rous sarcoma virus phosphorylates tyrosine. PNAS7713111315. (doi:10.1073/pnas.77.3.1311)

    • Search Google Scholar
    • Export Citation
  • HunterTSeftonBMBeemonK1980Studies on the structure and function of the avian sarcoma virus transforming-gene product. Cold Spring Harbor Symposia on Quantitative Biology44931941. (doi:10.1101/SQB.1980.044.01.100)

    • Search Google Scholar
    • Export Citation
  • HussainMAPorrasDLRoweMHWestJRSongWJSchreiberWEWondisfordFE2006Increased pancreatic β-cell proliferation mediated by CREB binding protein gene activation. Molecular and Cellular Biology2677477759. (doi:10.1128/MCB.02353-05)

    • Search Google Scholar
    • Export Citation
  • InadaAHamamotoYTsuuraYMiyazakiJToyokuniSIharaYNagaiKYamadaYBonner-WeirSSeinoY2004Overexpression of inducible cyclic AMP early repressor inhibits transactivation of genes and cell proliferation in pancreatic β cells. Molecular and Cellular Biology2428312841. (doi:10.1128/MCB.24.7.2831-2841.2004)

    • Search Google Scholar
    • Export Citation
  • JangatiGRVeluthakalRKowluruA2006siRNA-mediated depletion of endogenous protein phosphatase 2Acα markedly attenuates ceramide-activated protein phosphatase activity in insulin-secreting INS-832/13 cells. Biochemical and Biophysical Research Communications348649652. (doi:10.1016/j.bbrc.2006.07.100)

    • Search Google Scholar
    • Export Citation
  • JangatiGRVeluthakalRSusickLGruberSAKowluruA2007Depletion of the catalytic subunit of protein phosphatase-2A (PP2Ac) markedly attenuates glucose-stimulated insulin secretion in pancreatic β-cells. Endocrine31248253. (doi:10.1007/s12020-007-0046-3)

    • Search Google Scholar
    • Export Citation
  • JanssensVGorisJ2001Protein phosphatase 2A: a highly regulated family of serine/threonine phosphatases implicated in cell growth and signalling. Biochemical Journal353417439. (doi:10.1042/0264-6021:3530417)

    • Search Google Scholar
    • Export Citation
  • JanssonDNgACFuADepatieCAl AzzabiMScreatonRA2008Glucose controls CREB activity in islet cells via regulated phosphorylation of TORC2. PNAS1051016110166. (doi:10.1073/pnas.0800796105)

    • Search Google Scholar
    • Export Citation
  • JhalaUSCanettieriGScreatonRAKulkarniRNKrajewskiSReedJWalkerJLinXWhiteMMontminyM2003cAMP promotes pancreatic β-cell survival via CREB-mediated induction of IRS2. Genes and Development1715751580. (doi:10.1101/gad.1097103)

    • Search Google Scholar
    • Export Citation
  • JohnsonSAHunterT2005Kinomics: methods for deciphering the kinome. Nature Methods21725. (doi:10.1038/nmeth731)

  • JonasJCGilonPHenquinJC1998Temporal and quantitative correlations between insulin secretion and stably elevated or oscillatory cytoplasmic Ca2+ in mouse pancreatic β-cells. Diabetes4712661273.

    • Search Google Scholar
    • Export Citation
  • JonesPMPersaudSJ1998Protein kinases, protein phosphorylation, and the regulation of insulin secretion from pancreatic β-cells. Endocrine Reviews19429461.

    • Search Google Scholar
    • Export Citation
  • KahnSEHalbanPA1997Release of incompletely processed proinsulin is the cause of the disproportionate proinsulinemia of NIDDM. Diabetes4617251732. (doi:10.2337/diab.46.11.1725)

    • Search Google Scholar
    • Export Citation
  • KasiskeBLSnyderJJGilbertsonDMatasAJ2003Diabetes mellitus after kidney transplantation in the United States. American Journal of Transplantation3178185. (doi:10.1034/j.1600-6143.2003.00010.x)

    • Search Google Scholar
    • Export Citation
  • KleeCBDraettaGFHubbardMJ1988Calcineurin. Advances in Enzymology and Related Areas of Molecular Biology61149200.

  • KloekerSReedRMcConnellJLChangDTranKWestphalRSLawBKColbranRJKamounMCampbellKS2003Parallel purification of three catalytic subunits of the protein serine/threonine phosphatase 2A family (PP2A(C), PP4(C), and PP6(C)) and analysis of the interaction of PP2A(C) with α4 protein. Protein Expression and Purification311933. (doi:10.1016/S1046-5928(03)00141-4)

    • Search Google Scholar
    • Export Citation
  • KlumppSThissenMCKrieglsteinJ2006Protein phosphatases types 2Cα and 2Cβ in apoptosis. Biochemical Society Transactions3413701375. (doi:10.1042/BST0341370)

    • Search Google Scholar
    • Export Citation
  • KomatsuMSchermerhornTNodaMStraubSGAizawaTSharpGW1997Augmentation of insulin release by glucose in the absence of extracellular Ca2+: new insights into stimulus–secretion coupling. Diabetes4619281938. (doi:10.2337/diab.46.12.1928)

    • Search Google Scholar
    • Export Citation
  • KooSHFlechnerLQiLZhangXScreatonRAJeffriesSHedrickSXuWBoussouarFBrindleP2005The CREB coactivator TORC2 is a key regulator of fasting glucose metabolism. Nature43711091111. (doi:10.1038/nature03967)

    • Search Google Scholar
    • Export Citation
  • KoroCEBowlinSJBourgeoisNFedderDO2004Glycemic control from 1988 to 2000 among U.S. adults diagnosed with type 2 diabetes: a preliminary report. Diabetes Care271720. (doi:10.2337/diacare.27.1.17)

    • Search Google Scholar
    • Export Citation
  • KowluruA2005Novel regulatory roles for protein phosphatase-2A in the islet β cell. Biochemical Pharmacology6916811691. (doi:10.1016/j.bcp.2005.03.018)

    • Search Google Scholar
    • Export Citation
  • KowluruAMattiA2012Hyperactivation of protein phosphatase 2A in models of glucolipotoxicity and diabetes: potential mechanisms and functional consequences. Biochemical Pharmacology84591597. (doi:10.1016/j.bcp.2012.05.003)

    • Search Google Scholar
    • Export Citation
  • KowluruAMetzSA1997Ceramide-activated protein phosphatase-2A activity in insulin-secreting cells. FEBS Letters418179182. (doi:10.1016/S0014-5793(97)01379-3)

    • Search Google Scholar
    • Export Citation
  • KowluruASeaveySERabagliaMENesherRMetzSA1996Carboxylmethylation of the catalytic subunit of protein phosphatase 2A in insulin-secreting cells: evidence for functional consequences on enzyme activity and insulin secretion. Endocrinology13723152323.

    • Search Google Scholar
    • Export Citation
  • KowluruAChenHQModrickLMStefanelliC2001Activation of acetyl-CoA carboxylase by a glutamate- and magnesium-sensitive protein phosphatase in the islet β-cell. Diabetes5015801587. (doi:10.2337/diabetes.50.7.1580)

    • Search Google Scholar
    • Export Citation
  • KrautheimARustenbeckISteinfelderHJ1999Phosphatase inhibitors induce defective hormone secretion in insulin-secreting cells and entry into apoptosis. Experimental and Clinical Endocrinology & Diabetes1072934. (doi:10.1055/s-0029-1212069)

    • Search Google Scholar
    • Export Citation
  • KrebsEGGravesDJFischerEH1959Factors affecting the activity of muscle phosphorylase b kinase. Journal of Biological Chemistry23428672873.

    • Search Google Scholar
    • Export Citation
  • KrieglsteinJHufnagelBDworakMSchwarzSKewitzTReinboldMKlumppS2008Influence of various fatty acids on the activity of protein phosphatase type 2C and apoptosis of endothelial cells and macrophages. European Journal of Pharmaceutical Sciences35397403. (doi:10.1016/j.ejps.2008.08.007)

    • Search Google Scholar
    • Export Citation
  • von KriegsheimAPittAGrindlayGJKolchWDhillonAS2006Regulation of the Raf–MEK–ERK pathway by protein phosphatase 5. Nature Cell Biology810111016. (doi:10.1038/ncb1465)

    • Search Google Scholar
    • Export Citation
  • KuehnenPLaubnerKRaileKSchoflCJakobFPilzIPathGSeufertJ2011Protein phosphatase 1 (PP-1)-dependent inhibition of insulin secretion by leptin in INS-1 pancreatic β-cells and human pancreatic islets. Endocrinology15218001808. (doi:10.1210/en.2010-1094)

    • Search Google Scholar
    • Export Citation
  • KutuzovMAAndreevaAVVoyno-YasenetskayaTA2005Regulation of apoptosis signal-regulating kinase 1 (ASK1) by polyamine levels via protein phosphatase 5. Journal of Biological Chemistry2802538825395. (doi:10.1074/jbc.M413202200)

    • Search Google Scholar
    • Export Citation
  • LadriereLIgoillo-EsteveMCunhaDABrionJPBuglianiMMarchettiPEizirikDLCnopM2010Enhanced signaling downstream of ribonucleic acid-activated protein kinase-like endoplasmic reticulum kinase potentiates lipotoxic endoplasmic reticulum stress in human islets. Journal of Clinical Endocrinology and Metabolism9514421449. (doi:10.1210/jc.2009-2322)

    • Search Google Scholar
    • Export Citation
  • LambrechtCHaesenDSentsWIvanovaEJanssensV2013Structure, regulation, and pharmacological modulation of PP2A phosphatases. Methods in Molecular Biology1053283305.

    • Search Google Scholar
    • Export Citation
  • LammersTLaviS2007Role of type 2C protein phosphatases in growth regulation and in cellular stress signaling. Critical Reviews in Biochemistry and Molecular Biology42437461. (doi:10.1080/10409230701693342)

    • Search Google Scholar
    • Export Citation
  • LangbergECGuHFNordmanSEfendicSOstensonCG2007Genetic variation in receptor protein tyrosine phosphatase sigma is associated with type 2 diabetes in Swedish Caucasians. European Journal of Endocrinology157459464. (doi:10.1530/EJE-07-0114)

    • Search Google Scholar
    • Export Citation
  • LarssonOBarkerCJSjoholmACarlqvistHMichellRHBertorelloANilssonTHonkanenREMayrGWZwillerJ1997Inhibition of phosphatases and increased Ca2+ channel activity by inositol hexakisphosphate. Science278471474. (doi:10.1126/science.278.5337.471)

    • Search Google Scholar
    • Export Citation
  • LawrenceMCBhattHSWattersonJMEasomRA2001Regulation of insulin gene transcription by a Ca(2+)-responsive pathway involving calcineurin and nuclear factor of activated T cells. Molecular Endocrinolgy1517581767. (doi:10.1210/mend.15.10.0702)

    • Search Google Scholar
    • Export Citation
  • LawrenceMCBhattHSEasomRA2002NFAT regulates insulin gene promoter activity in response to synergistic pathways induced by glucose and glucagon-like peptide-1. Diabetes51691698. (doi:10.2337/diabetes.51.3.691)

    • Search Google Scholar
    • Export Citation
  • LeeYCNielsenJH2009Regulation of β cell replication. Molecular and Cellular Endocrinology2971827. (doi:10.1016/j.mce.2008.08.033)

  • LeeSHShinMSParkWSKimSYKimHSLeeJHHanSYLeeHKParkJYOhRR1999Immunohistochemical localization of FAP-1, an inhibitor of Fas-mediated apoptosis, in normal and neoplastic human tissues. APMIS10711011108. (doi:10.1111/j.1699-0463.1999.tb01515.x)

    • Search Google Scholar
    • Export Citation
  • LehtihetMHonkanenRESjoholmA2004Inositol hexakisphosphate and sulfonylureas regulate β-cell protein phosphatases. Biochemical and Biophysical Research Communications316893897. (doi:10.1016/j.bbrc.2004.02.144)

    • Search Google Scholar
    • Export Citation
  • LehtihetMWebbDLHonkanenRESjoholmA2005Glutamate inhibits protein phosphatases and promotes insulin exocytosis in pancreatic β-cells. Biochemical and Biophysical Research Communications328601607. (doi:10.1016/j.bbrc.2005.01.024)

    • Search Google Scholar
    • Export Citation
  • LernmarkANielsenDAParmanAUSehlinJSteinerDFTaljedalIBCation-activated phosphatase activities in islet cell plasma membrane preparationsHormone and Metabolic Research. Supplement SeriesSuppl 1019805561.

    • Search Google Scholar
    • Export Citation
  • LipsonLGSiegelEWollheimCBSharpGW1979Insulin release during fasting: studies on adenylate cyclase, phosphodiesterase, protein kinase, and phosphoprotein phosphatase in isolated islets of langerhans of the rat. Endocrinology105702707. (doi:10.1210/endo-105-3-702)

    • Search Google Scholar
    • Export Citation
  • LobnerKSteinbrennerHRobertsGALingZHuangGCPiquerSPipeleersDGSeisslerJChristieMR2002Different regulated expression of the tyrosine phosphatase-like proteins IA-2 and phogrin by glucose and insulin in pancreatic islets: relationship to development of insulin secretory responses in early life. Diabetes5129822988. (doi:10.2337/diabetes.51.10.2982)

    • Search Google Scholar
    • Export Citation
  • LubertEJHongYSargeKD2001Interaction between protein phosphatase 5 and the A subunit of protein phosphatase 2A: evidence for a heterotrimeric form of protein phosphatase 5. Journal of Biological Chemistry2763858238587. (doi:10.1074/jbc.M106906200)

    • Search Google Scholar
    • Export Citation
  • MacDonaldMJFahienLA2000Glutamate is not a messenger in insulin secretion. Journal of Biological Chemistry2753402534027. (doi:10.1074/jbc.C000411200)

    • Search Google Scholar
    • Export Citation
  • MaechlerPWollheimCB1999Mitochondrial glutamate acts as a messenger in glucose-induced insulin exocytosis. Nature402685689. (doi:10.1038/45280)

    • Search Google Scholar
    • Export Citation
  • MaedlerKSergeevPRisFOberholzerJJoller-JemelkaHISpinasGAKaiserNHalbanPADonathMY2002Glucose-induced β cell production of IL-1β contributes to glucotoxicity in human pancreatic islets. Journal of Clinical Investigation110851860. (doi:10.1172/JCI200215318)

    • Search Google Scholar
    • Export Citation
  • ManningGWhyteDBMartinezRHunterTSudarsanamS2002The protein kinase complement of the human genome. Science29819121934. (doi:10.1126/science.1075762)

    • Search Google Scholar
    • Export Citation
  • MatschinskyFMGlaserBMagnusonMA1998Pancreatic β-cell glucokinase: closing the gap between theoretical concepts and experimental realities. Diabetes47307315. (doi:10.2337/diabetes.47.3.307)

    • Search Google Scholar
    • Export Citation
  • MayerPJochumCSchatzHPfeifferA1994Okadaic acid indicates a major function for protein phosphatases in stimulus–response coupling of RINm5F rat insulinoma cells. Experimental and Clinical Endocrinology102313319. (doi:10.1055/s-0029-1211297)

    • Search Google Scholar
    • Export Citation
  • MeetooDMcGovernPSafadiR2007An epidemiological overview of diabetes across the world. British Journal of Nutrition1610021007.

  • MeglassonMDMatschinskyFM1986Pancreatic islet glucose metabolism and regulation of insulin secretion. Diabetes/Metabolism Reviews2163214. (doi:10.1002/dmr.5610020301)

    • Search Google Scholar
    • Export Citation
  • MeierJJButlerAESaishoYMonchampTGalassoRBhushanARizzaRAButlerPC2008β-cell replication is the primary mechanism subserving the postnatal expansion of β-cell mass in humans. Diabetes5715841594. (doi:10.2337/db07-1369)

    • Search Google Scholar
    • Export Citation
  • MerrittSEMataMNihalaniDZhuCHuXHolzmanLB1999The mixed lineage kinase DLK utilizes MKK7 and not MKK4 as substrate. Journal of Biological Chemistry2741019510202. (doi:10.1074/jbc.274.15.10195)

    • Search Google Scholar
    • Export Citation
  • MkaddemSBWertsCGoujonJMBensMPedruzziEOgier-DenisEVandewalleA2009Heat shock protein gp96 interacts with protein phosphatase 5 and controls toll-like receptor 2 (TLR2)-mediated activation of extracellular signal-regulated kinase (ERK) 1/2 in post-hypoxic kidney cells. Journal of Biological Chemistry2841254112549. (doi:10.1074/jbc.M808376200)

    • Search Google Scholar
    • Export Citation
  • MooreFColliMLCnopMEsteveMICardozoAKCunhaDABuglianiMMarchettiPEizirikDL2009PTPN2, a candidate gene for type 1 diabetes, modulates interferon-γ-induced pancreatic β-cell apoptosis. Diabetes5812831291. (doi:10.2337/db08-1510)

    • Search Google Scholar
    • Export Citation
  • MoorheadGBTrinkle-MulcahyLUlke-LemeeA2007Emerging roles of nuclear protein phosphatases. Nature Reviews. Molecular Cell Biology8234244. (doi:10.1038/nrm2126)

    • Search Google Scholar
    • Export Citation
  • MoritaKSaitohMTobiumeKMatsuuraHEnomotoSNishitohHIchijoH2001Negative feedback regulation of ASK1 by protein phosphatase 5 (PP5) in response to oxidative stress. EMBO Journal2060286036. (doi:10.1093/emboj/20.21.6028)

    • Search Google Scholar
    • Export Citation
  • MullenTDHannunYAObeidLM2012Ceramide synthases at the centre of sphingolipid metabolism and biology. Biochemical Journal441789802. (doi:10.1042/BJ20111626)

    • Search Google Scholar
    • Export Citation
  • MurphyLIJonesPM1996Phospho-serine/threonine phosphatases in rat islets of Langerhans: identification and effect on insulin secretion. Molecular and Cellular Endocrinology117195202. (doi:10.1016/0303-7207(95)03747-0)

    • Search Google Scholar
    • Export Citation
  • NakamuraSTanigawaKKawaguchiMInoueYXuGNagamiHTeramotoMKatoYTamuraK1995Effect of chronic vanadate administration in partially depancreatized rats. Diabetes Research and Clinical Practice275159. (doi:10.1016/0168-8227(94)01012-O)

    • Search Google Scholar
    • Export Citation
  • NewgardCBMcGarryJD1995Metabolic coupling factors in pancreatic β-cell signal transduction. Annual Review of Biochemistry64689719. (doi:10.1146/annurev.bi.64.070195.003353)

    • Search Google Scholar
    • Export Citation
  • NolenBTaylorSGhoshG2004Regulation of protein kinases; controlling activity through activation segment conformation. Molecular Cell15661675. (doi:10.1016/j.molcel.2004.08.024)

    • Search Google Scholar
    • Export Citation
  • NovoaIZhangYZengHJungreisRHardingHPRonD2003Stress-induced gene expression requires programmed recovery from translational repression. EMBO Journal2211801187. (doi:10.1093/emboj/cdg112)

    • Search Google Scholar
    • Export Citation
  • OllendorffVDonoghueDJ1997The serine/threonine phosphatase PP5 interacts with CDC16 and CDC27, two tetratricopeptide repeat-containing subunits of the anaphase-promoting complex. Journal of Biological Chemistry2723201132018. (doi:10.1074/jbc.272.51.32011)

    • Search Google Scholar
    • Export Citation
  • OlsenJVBlagoevBGnadFMacekBKumarCMortensenPMannM2006Global, in vivo, and site-specific phosphorylation dynamics in signaling networks. Cell127635648. (doi:10.1016/j.cell.2006.09.026)

    • Search Google Scholar
    • Export Citation
  • OprescuAIBikopoulosGNaassanAAllisterEMTangCParkEUchinoHLewisGFFantusIGRozakis-AdcockM2007Free fatty acid-induced reduction in glucose-stimulated insulin secretion: evidence for a role of oxidative stress in vitro and in vivo. Diabetes5629272937. (doi:10.2337/db07-0075)

    • Search Google Scholar
    • Export Citation
  • OrtsäterHSjöholmA2007A busy cell – endoplasmic reticulum stress in the pancreatic β-cell. Molecular and Cellular Endocrinology27715. (doi:10.1016/j.mce.2007.06.006)

    • Search Google Scholar
    • Export Citation
  • ÖstensonCGSandberg-NordqvistACChenJHallbrinkMRotinDLangelUEfendicS2002Overexpression of protein-tyrosine phosphatase PTP sigma is linked to impaired glucose-induced insulin secretion in hereditary diabetic Goto–Kakizaki rats. Biochemical and Biophysical Research Communications291945950. (doi:10.1006/bbrc.2002.6536)

    • Search Google Scholar
    • Export Citation
  • OzbayLASmidtKMortensenDMCarstensJJorgensenKARungbyJ2011Cyclosporin and tacrolimus impair insulin secretion and transcriptional regulation in INS-1E β-cells. British Journal of Pharmacology162136146. (doi:10.1111/j.1476-5381.2010.01018.x)

    • Search Google Scholar
    • Export Citation
  • PalanivelRVeluthakalRKowluruA2004Regulation by glucose and calcium of the carboxylmethylation of the catalytic subunit of protein phosphatase 2A in insulin-secreting INS-1 cells. American Journal of Physiology. Endocrinology and Metabolism286E1032E1041. (doi:10.1152/ajpendo.00587.2003)

    • Search Google Scholar
    • Export Citation
  • PalanivelRVeluthakalRMcDonaldPKowluruA2005Further evidence for the regulation of acetyl-CoA carboxylase activity by a glutamate- and magnesium-activated protein phosphatase in the pancreatic β cell: defective regulation in the diabetic GK rat islet. Endocrine267177. (doi:10.1385/ENDO:26:1:071)

    • Search Google Scholar
    • Export Citation
  • PatoMDAdelsteinRS1983Characterization of a Mg2+-dependent phosphatase from Turkey gizzard smooth muscle. Journal of Biological Chemistry25870557058.

    • Search Google Scholar
    • Export Citation
  • PedersonRARamanadhamSBuchanAMMcNeillJH1989Long-term effects of vanadyl treatment on streptozocin-induced diabetes in rats. Diabetes3813901395. (doi:10.2337/diab.38.11.1390)

    • Search Google Scholar
    • Export Citation
  • PeirisHRaghupathiRJessupCFZaninMPMohanasundaramDMackenzieKDChatawayTClarkeJNBrealeyJCoatesPT2012Increased expression of the glucose-responsive gene, RCAN1, causes hypoinsulinemia, β-cell dysfunction, and diabetes. Endocrinology15352125221. (doi:10.1210/en.2011-2149)

    • Search Google Scholar
    • Export Citation
  • PlaumannSBlumeRBorchersSSteinfelderHJKnepelWOetjenE2008Activation of the dual-leucine-zipper-bearing kinase and induction of β-cell apoptosis by the immunosuppressive drug cyclosporin A. Molecular Pharmacology73652659. (doi:10.1124/mol.107.040782)

    • Search Google Scholar
    • Export Citation
  • RamseyAJChinkersM2002Identification of potential physiological activators of protein phosphatase 5. Biochemistry4156255632. (doi:10.1021/bi016090h)

    • Search Google Scholar
    • Export Citation
  • RamseyAJRussellLCWhittSRChinkersM2000Overlapping sites of tetratricopeptide repeat protein binding and chaperone activity in heat shock protein 90. Journal of Biological Chemistry2751785717862. (doi:10.1074/jbc.M001625200)

    • Search Google Scholar
    • Export Citation
  • RantaFAvramDBerchtoldSDuferMDrewsGLangFUllrichS2006Dexamethasone induces cell death in insulin-secreting cells, an effect reversed by exendin-4. Diabetes5513801390. (doi:10.2337/db05-1220)

    • Search Google Scholar
    • Export Citation
  • RantaFDuferMStorkBWesselborgSDrewsGHaringHULangFUllrichS2008Regulation of calcineurin activity in insulin-secreting cells: stimulation by Hsp90 during glucocorticoid-induced apoptosis. Cellular Signalling2017801786. (doi:10.1016/j.cellsig.2008.06.003)

    • Search Google Scholar
    • Export Citation
  • RatcliffHJonesPM1993Effects of okadaic acid on insulin secretion from rat islets of Langerhans. Biochimica et Biophysica Acta1175188191. (doi:10.1016/0167-4889(93)90022-H)

    • Search Google Scholar
    • Export Citation
  • RavnskjaerKBoergesenMDalgaardLTMandrupS2006Glucose-induced repression of PPARα gene expression in pancreatic β-cells involves PP2A activation and AMPK inactivation. Journal of Molecular Endocrinology36289299. (doi:10.1677/jme.1.01965)

    • Search Google Scholar
    • Export Citation
  • RedmonJBOlsonLKArmstrongMBGreeneMJRobertsonRP1996Effects of tacrolimus (FK506) on human insulin gene expression, insulin mRNA levels, and insulin secretion in HIT-T15 cells. Journal of Clinical Investigation9827862793. (doi:10.1172/JCI119105)

    • Search Google Scholar
    • Export Citation
  • ReichETamaryASionovRVMelloulD2012Involvement of thioredoxin-interacting protein (TXNIP) in glucocorticoid-mediated β cell death. Diabetologia5510481057. (doi:10.1007/s00125-011-2422-z)

    • Search Google Scholar
    • Export Citation
  • RenstromEDingWGBokvistKRorsmanP1996Neurotransmitter-induced inhibition of exocytosis in insulin-secreting β cells by activation of calcineurin. Neuron17513522. (doi:10.1016/S0896-6273(00)80183-X)

    • Search Google Scholar
    • Export Citation
  • Rodriguez-RodriguezAETrinanesJVelazquez-GarciaSPorriniEVega PrietoMJDiez FuentesMLArevaloMSalido RuizETorresA2013The higher diabetogenic risk of tacrolimus depends on pre-existing insulin resistance. A study in obese and lean Zucker rats. American Journal of Transplantation1316651675. (doi:10.1111/ajt.12236)

    • Search Google Scholar
    • Export Citation
  • RorsmanPBraunM2013Regulation of insulin secretion in human pancreatic islets. Annual Review of Physiology75155179. (doi:10.1146/annurev-physiol-030212-183754)

    • Search Google Scholar
    • Export Citation
  • RussellLCWhittSRChenMSChinkersM19992006Identification of conserved residues required for the binding of a tetratricopeptide repeat domain to heat shock protein 90. Journal of Biological Chemistry2742006020063. (doi:10.1074/jbc.274.29.20060)

    • Search Google Scholar
    • Export Citation
  • RutkowskiDTArnoldSMMillerCNWuJLiJGunnisonKMMoriKSadighi AkhaAARadenDKaufmanRJ2006Adaptation to ER stress is mediated by differential stabilities of pro-survival and pro-apoptotic mRNAs and proteins. PLoS Biology4e374. (doi:10.1371/journal.pbio.0040374)

    • Search Google Scholar
    • Export Citation
  • SantinIMooreFColliMLGurzovENMarselliLMarchettiPEizirikDL2011PTPN2, a candidate gene for type 1 diabetes, modulates pancreatic β-cell apoptosis via regulation of the BH3-only protein Bim. Diabetes6032793288. (doi:10.2337/db11-0758)

    • Search Google Scholar
    • Export Citation
  • SantoroMFAnnandRRRobertsonMMPengYWBradyMJMankovichJAHackettMCGhayurTWalterGWongWW1998Regulation of protein phosphatase 2A activity by caspase-3 during apoptosis. Journal of Biological Chemistry2731311913128. (doi:10.1074/jbc.273.21.13119)

    • Search Google Scholar
    • Export Citation
  • SatoYMariotPDetimaryPGilonPHenquinJC1998Okadaic acid-induced decrease in the magnitude and efficacy of the Ca2+ signal in pancreatic β cells and inhibition of insulin secretion. British Journal of Pharmacology12397105. (doi:10.1038/sj.bjp.0701578)

    • Search Google Scholar
    • Export Citation
  • SchwaningerMBlumeRKrugerMLuxGOetjenEKnepelW1995Involvement of the Ca(2+)-dependent phosphatase calcineurin in gene transcription that is stimulated by cAMP through cAMP response elements. Journal of Biological Chemistry27088608866. (doi:10.1074/jbc.270.15.8860)

    • Search Google Scholar
    • Export Citation
  • SchwarzSHufnagelBDworakMKlumppSKrieglsteinJ2006Protein phosphatase type 2Cα and 2Cβ are involved in fatty acid-induced apoptosis of neuronal and endothelial cells. Apoptosis1111111119. (doi:10.1007/s10495-006-6982-1)

    • Search Google Scholar
    • Export Citation
  • ScreatonRAConkrightMDKatohYBestJLCanettieriGJeffriesSGuzmanENiessenSYatesJRIIITakemoriH2004The CREB coactivator TORC2 functions as a calcium- and cAMP-sensitive coincidence detector. Cell1196174. (doi:10.1016/j.cell.2004.09.015)

    • Search Google Scholar
    • Export Citation
  • SentsWIvanovaELambrechtCHaesenDJanssensV2013The biogenesis of active protein phosphatase 2A holoenzymes: a tightly regulated process creating phosphatase specificity. FEBS Journal280644661. (doi:10.1111/j.1742-4658.2012.08579.x)

    • Search Google Scholar
    • Export Citation
  • ShaoJHartsonSDMattsRL2002Evidence that protein phosphatase 5 functions to negatively modulate the maturation of the Hsp90-dependent heme-regulated eIF2α kinase. Biochemistry4167706779. (doi:10.1021/bi025737a)

    • Search Google Scholar
    • Export Citation
  • ShawRJLamiaKAVasquezDKooSHBardeesyNDepinhoRAMontminyMCantleyLC2005The kinase LKB1 mediates glucose homeostasis in liver and therapeutic effects of metformin. Science31016421646. (doi:10.1126/science.1120781)

    • Search Google Scholar
    • Export Citation
  • ShiY2009Serine/threonine phosphatases: mechanism through structure. Cell139468484. (doi:10.1016/j.cell.2009.10.006)

  • ShimabukuroMHigaMZhouYTWangMYNewgardCBUngerRH1998Lipoapoptosis in β-cells of obese prediabetic fa/fa rats. Role of serine palmitoyltransferase overexpression. Journal of Biological Chemistry2733248732490. (doi:10.1074/jbc.273.49.32487)

    • Search Google Scholar
    • Export Citation
  • SilversteinAMGalignianaMDChenMSOwens-GrilloJKChinkersMPrattWB1997Protein phosphatase 5 is a major component of glucocorticoid receptor.hsp90 complexes with properties of an FK506-binding immunophilin. Journal of Biological Chemistry2721622416230. (doi:10.1074/jbc.272.26.16224)

    • Search Google Scholar
    • Export Citation
  • SinclairCBorchersCParkerCTomerKCharbonneauHRossieS1999The tetratricopeptide repeat domain and a C-terminal region control the activity of Ser/Thr protein phosphatase 5. Journal of Biological Chemistry2742366623672. (doi:10.1074/jbc.274.33.23666)

    • Search Google Scholar
    • Export Citation
  • SjöholmÅ1991Phorbol ester stimulation of pancreatic β-cell replication, polyamine content and insulin secretion. FEBS Letters294257260. (doi:10.1016/0014-5793(91)81442-B)

    • Search Google Scholar
    • Export Citation
  • SjöholmÅ1994Inhibitory effects of cyclosporin A on rat insulinoma cell proliferation, polyamine content and insulin secretion. Molecular and Cellular Endocrinology992124. (doi:10.1016/0303-7207(94)90141-4)

    • Search Google Scholar
    • Export Citation
  • SjöholmÅ1995Ceramide inhibits pancreatic β-cell insulin production and mitogenesis and mimics the actions of interleukin-1β. FEBS Letters367283286. (doi:10.1016/0014-5793(95)00470-T)

    • Search Google Scholar
    • Export Citation
  • SjöholmÅ1996Diabetes mellitus and impaired pancreatic β-cell proliferation. Journal of Internal Medicine239211220. (doi:10.1046/j.1365-2796.1996.377740000.x)

    • Search Google Scholar
    • Export Citation
  • SjöholmÅ1998Aspects of novel sites of regulation of the insulin stimulus–secretion coupling in normal and diabetic pancreatic islets. Endocrine9113. (doi:10.1385/ENDO:9:1:1)

    • Search Google Scholar
    • Export Citation
  • SjöholmÅHonkanenRE2000Polyamines regulate serine/threonine protein phosphatases in insulin-secreting cells. Pancreas203237. (doi:10.1097/00006676-200001000-00005)

    • Search Google Scholar
    • Export Citation
  • SjöholmÅArkhammarPWelshNBokvistKRorsmanPHallbergANilssonTWelshMBerggrenPO1993aEnhanced stimulus–secretion coupling in polyamine-depleted rat insulinoma cells. An effect involving increased cytoplasmic Ca2+, inositol phosphate generation, and phorbol ester sensitivity. Journal of Clinical Investigation9219101917. (doi:10.1172/JCI116784)

    • Search Google Scholar
    • Export Citation
  • SjöholmÅHonkanenREBerggrenPO1993bCharacterization of serine/threonine protein phosphatases in RINm5F insulinoma cells. Bioscience Reports13349358. (doi:10.1007/BF01150479)

    • Search Google Scholar
    • Export Citation
  • SjöholmÅHonkanenREBerggrenPO1995Inhibition of serine/threonine protein phosphatases by secretagogues in insulin-secreting cells. Endocrinology13633913397.

    • Search Google Scholar
    • Export Citation
  • SjöholmÅLehtihetMEfanovAMZaitsevSVBerggrenPOHonkanenRE2002Glucose metabolites inhibit protein phosphatases and directly promote insulin exocytosis in pancreatic β-cells. Endocrinology14345924598. (doi:10.1210/en.2002-220672)

    • Search Google Scholar
    • Export Citation
  • SkarraDVGoudreaultMChoiHMullinMNesvizhskiiAIGingrasACHonkanenRE2011Label-free quantitative proteomics and SAINT analysis enable interactome mapping for the human Ser/Thr protein phosphatase 5. Proteomics1115081516. (doi:10.1002/pmic.201000770)

    • Search Google Scholar
    • Export Citation
  • SoleimanpourSACrutchlowMFFerrariAMRaumJCGroffDNRankinMMLiuCDe LeonDDNajiAKushnerJA2010Calcineurin signaling regulates human islet {β}-cell survival. Journal of Biological Chemistry2854005040059. (doi:10.1074/jbc.M110.154955)

    • Search Google Scholar
    • Export Citation
  • SutherlandEWJrWosilaitWD1955Inactivation and activation of liver phosphorylase. Nature175169170. (doi:10.1038/175169a0)

  • SwingleMRHonkanenRECiszakEM2004Structural basis for the catalytic activity of human serine/threonine protein phosphatase-5. Journal of Biological Chemistry2793399233999. (doi:10.1074/jbc.M402855200)

    • Search Google Scholar
    • Export Citation
  • SwingleMNiLHonkanenRE2007Small-molecule inhibitors of ser/thr protein phosphatases: specificity, use and common forms of abuse. Methods in Molecular Biology3652338.

    • Search Google Scholar
    • Export Citation
  • TairaJHigashimotoY2013Caveolin-1 interacts with protein phosphatase 5 and modulates its activity in prostate cancer cells. Biochemical and Biophysical Research Communications431724728. (doi:10.1016/j.bbrc.2013.01.051)

    • Search Google Scholar
    • Export Citation
  • TaljedalIB1967Electrophoretic studies on phosphatases from the pancreatic islets of obese-hyperglycaemic mice. Acta Endocrinologica55153162.

    • Search Google Scholar
    • Export Citation
  • TamagawaTIguchiAUemuraKMiuraHNonogakiKIshiguroTSakamotoN1992Effects of the protein phosphatase inhibitors okadaic acid and calyculin A on insulin release from rat pancreatic islets. Endocrinologia Japonica39325329. (doi:10.1507/endocrj1954.39.325)

    • Search Google Scholar
    • Export Citation
  • TamuraKFujimuraTTsutsumiTNakamuraKOgawaTAtumaruCHiranoYOharaKOhtsukaKShimomuraK1995Transcriptional inhibition of insulin by FK506 and possible involvement of FK506 binding protein-12 in pancreatic β-cell. Transplantation5916061613. (doi:10.1097/00007890-199506000-00018)

    • Search Google Scholar
    • Export Citation
  • TaylorSSKornevAP2011Protein kinases: evolution of dynamic regulatory proteins. Trends in Biochemical Sciences366577. (doi:10.1016/j.tibs.2010.09.006)

    • Search Google Scholar
    • Export Citation
  • TengholmAGylfeE2009Oscillatory control of insulin secretion. Molecular and Cellular Endocrinology2975872. (doi:10.1016/j.mce.2008.07.009)

    • Search Google Scholar
    • Export Citation
  • TerrakMKerffFLangsetmoKTaoTDominguezR2004Structural basis of protein phosphatase 1 regulation. Nature429780784. (doi:10.1038/nature02582)

    • Search Google Scholar
    • Export Citation
  • ThomasHSenkelSErdmannSArndtTTuranGKlein-HitpassLRyffelGU2004Pattern of genes influenced by conditional expression of the transcription factors HNF6, HNF4α and HNF1β in a pancreatic β-cell line. Nucleic Acids Research32e150. (doi:10.1093/nar/gnh144)

    • Search Google Scholar
    • Export Citation
  • ToddJAWalkerNMCooperJDSmythDJDownesKPlagnolVBaileyRNejentsevSFieldSFPayneF2007Robust associations of four new chromosome regions from genome-wide analyses of type 1 diabetes. Nature Genetics39857864. (doi:10.1038/ng2068)

    • Search Google Scholar
    • Export Citation
  • ToriiS2009Expression and function of IA-2 family proteins, unique neuroendocrine-specific protein-tyrosine phosphatases. Endocrine Journal56639648. (doi:10.1507/endocrj.K09E-157)

    • Search Google Scholar
    • Export Citation
  • TumlinJALeaJPSwansonCESmithCLEdgeSSSomerenJS1997Aldosterone and dexamethasone stimulate calcineurin activity through a transcription-independent mechanism involving steroid receptor-associated heat shock proteins. Journal of Clinical Investigation9912171223. (doi:10.1172/JCI119278)

    • Search Google Scholar
    • Export Citation
  • TunonMJSanchez-CamposSGutierrezBCulebrasJMGonzalez-GallegoJ2003Effects of FK506 and rapamycin on generation of reactive oxygen species, nitric oxide production and nuclear factor kappa B activation in rat hepatocytes. Biochemical Pharmacology66439445. (doi:10.1016/S0006-2952(03)00288-0)

    • Search Google Scholar
    • Export Citation
  • Vander MierdeDScheunerDQuintensRPatelRSongBTsukamotoKBeullensMKaufmanRJBollenMSchuitFC2007Glucose activates a protein phosphatase-1-mediated signaling pathway to enhance overall translation in pancreatic β-cells. Endocrinology148609617. (doi:10.1210/en.2006-1012)

    • Search Google Scholar
    • Export Citation
  • WangHGPathanNEthellIMKrajewskiSYamaguchiYShibasakiFMcKeonFBoboTFrankeTFReedJC1999Ca2+-induced apoptosis through calcineurin dephosphorylation of BAD. Science284339343. (doi:10.1126/science.284.5412.339)

    • Search Google Scholar
    • Export Citation
  • WardWKBolgianoDCMcKnightBHalterJBPorteDJr1984Diminished B cell secretory capacity in patients with noninsulin-dependent diabetes mellitus. Journal of Clinical Investigation7413181328. (doi:10.1172/JCI111542)

    • Search Google Scholar
    • Export Citation
  • VaughanCKMollapourMSmithJRTrumanAHuBGoodVMPanaretouBNeckersLClarkePAWorkmanP2008Hsp90-dependent activation of protein kinases is regulated by chaperone-targeted dephosphorylation of Cdc37. Molecular Cell31886895. (doi:10.1016/j.molcel.2008.07.021)

    • Search Google Scholar
    • Export Citation
  • VeluthakalRWadzinskiBEKowluruA2006Localization of a nuclear serine/threonine protein phosphatase in insulin-secreting INS-1 cells: potential regulation by IL-1β. Apoptosis1114011411. (doi:10.1007/s10495-006-8371-1)

    • Search Google Scholar
    • Export Citation
  • VirshupDMShenolikarS2009From promiscuity to precision: protein phosphatases get a makeover. Molecular Cell33537545. (doi:10.1016/j.molcel.2009.02.015)

    • Search Google Scholar
    • Export Citation
  • WechslerTChenBPHarperRMorotomi-YanoKHuangBCMeekKCleaverJEChenDJWablM2004DNA–PKcs function regulated specifically by protein phosphatase 5. PNAS10112471252. (doi:10.1073/pnas.0307765100)

    • Search Google Scholar
    • Export Citation
  • WelshN1996Interleukin-1β-induced ceramide and diacylglycerol generation may lead to activation of the c-Jun NH2-terminal kinase and the transcription factor ATF2 in the insulin-producing cell line RINm5F. Journal of Biological Chemistry27183078312. (doi:10.1074/jbc.271.4.2121)

    • Search Google Scholar
    • Export Citation
  • WeltersHJSenkelSKlein-HitpassLErdmannSThomasHHarriesLWPearsonERBinghamCHattersleyATRyffelGU2006Conditional expression of hepatocyte nuclear factor-1β, the maturity-onset diabetes of the young-5 gene product, influences the viability and functional competence of pancreatic β-cells. Journal of Endocrinology190171181. (doi:10.1677/joe.1.06768)

    • Search Google Scholar
    • Export Citation
  • WeltersHJOknianskaAErdmannKSRyffelGUMorganNG2008The protein tyrosine phosphatase-BL, modulates pancreatic β-cell proliferation by interaction with the Wnt signalling pathway. Journal of Endocrinology197543552. (doi:10.1677/JOE-07-0262)

    • Search Google Scholar
    • Export Citation
  • WinterCLLangeJSDavisMGGerweGSDownsTRPetersKGKasibhatlaB2005A nonspecific phosphotyrosine phosphatase inhibitor, bis(maltolato)oxovanadium(IV), improves glucose tolerance and prevents diabetes in Zucker diabetic fatty rats. Experimental Biology and Medicine230207216.

    • Search Google Scholar
    • Export Citation
  • WishartMJDixonJE2002PTEN and myotubularin phosphatases: from 3-phosphoinositide dephosphorylation to disease. Trends in Cell Biology12579585. (doi:10.1016/S0962-8924(02)02412-1)

    • Search Google Scholar
    • Export Citation
  • XuXLagercrantzJZickertPBajalica-LagercrantzSZetterbergA1996Chromosomal localization and 5′ sequence of the human protein serine/threonine phosphatase 5′ gene. Biochemical and Biophysical Research Communications218514517. (doi:10.1006/bbrc.1996.0092)

    • Search Google Scholar
    • Export Citation
  • XuYChenYZhangPJeffreyPDShiY2008Structure of a protein phosphatase 2A holoenzyme: insights into B55-mediated Tau dephosphorylation. Molecular Cell31873885. (doi:10.1016/j.molcel.2008.08.006)

    • Search Google Scholar
    • Export Citation
  • YamaguchiYKatohHMoriKNegishiM2002Gα(12) and Gα(13) interact with Ser/Thr protein phosphatase type 5 and stimulate its phosphatase activity. Current Biology1213531358. (doi:10.1016/S0960-9822(02)01034-5)

    • Search Google Scholar
    • Export Citation
  • YamaguchiFYamamuraSShimamotoSTokumitsuHTokudaMKobayashiR2014Suramin is a novel activator of PP5 and biphasically modulates S100-activated PP5 activity. Applied Biochemistry and Biotechnology172237247. (doi:10.1007/s12010-013-0522-6)

    • Search Google Scholar
    • Export Citation
  • YanLNairnACPalfreyHCBradyMJ2003Glucose regulates EF-2 phosphorylation and protein translation by a protein phosphatase-2A-dependent mechanism in INS-1-derived 832/13 cells. Journal of Biological Chemistry2781817718183. (doi:10.1074/jbc.M301116200)

    • Search Google Scholar
    • Export Citation
  • YanLGuoSBraultMHarmonJRobertsonRPHamidRSteinRYangE2012The B55α-containing PP2A holoenzyme dephosphorylates FOXO1 in islet β-cells under oxidative stress. Biochemical Journal444239247. (doi:10.1042/BJ20111606)

    • Search Google Scholar
    • Export Citation
  • YangJRoeSMCliffMJWilliamsMALadburyJECohenPTBarfordD2005Molecular basis for TPR domain-mediated regulation of protein phosphatase 5. EMBO Journal24110. (doi:10.1038/sj.emboj.7600496)

    • Search Google Scholar
    • Export Citation
  • YlipaastoPKutluBRasilainenSRasschaertJSalmelaKTeerijokiHKorsgrenOLahesmaaRHoviTEizirikDL2005Global profiling of coxsackievirus- and cytokine-induced gene expression in human pancreatic islets. Diabetologia4815101522. (doi:10.1007/s00125-005-1839-7)

    • Search Google Scholar
    • Export Citation
  • YongWBaoSChenHLiDSanchezERShouW2007Mice lacking protein phosphatase 5 are defective in ataxia telangiectasia mutated (ATM)-mediated cell cycle arrest. Journal of Biological Chemistry2821469014694. (doi:10.1074/jbc.C700019200)

    • Search Google Scholar
    • Export Citation
  • ZhangAQGaoZYGilonPNenquinMDrewsGHenquinJC1991Vanadate stimulation of insulin release in normal mouse islets. Journal of Biological Chemistry2662164921656.

    • Search Google Scholar
    • Export Citation
  • ZhangJBaoSFurumaiRKuceraKSAliADeanNMWangXF2005Protein phosphatase 5 is required for ATR-mediated checkpoint activation. Molecular and Cellular Biology2599109919. (doi:10.1128/MCB.25.22.9910-9919.2005)

    • Search Google Scholar
    • Export Citation
  • ZhaoSSancarA1997Human blue-light photoreceptor hCRY2 specifically interacts with protein serine/threonine phosphatase 5 and modulates its activity. Photochemistry and Photobiology66727731. (doi:10.1111/j.1751-1097.1997.tb03214.x)

    • Search Google Scholar
    • Export Citation
  • ZhouGGoldenTAragonIVHonkanenRE2004Ser/Thr protein phosphatase 5 inactivates hypoxia-induced activation of an apoptosis signal-regulating kinase 1/MKK-4/JNK signaling cascade. Journal of Biological Chemistry2794659546605. (doi:10.1074/jbc.M408320200)

    • Search Google Scholar
    • Export Citation

If the inline PDF is not rendering correctly, you can download the PDF file here.

 

      Society for Endocrinology

Sept 2018 onwards Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 568 151 9
PDF Downloads 211 104 7
  • View in gallery

    Family tree of PTPs.

  • View in gallery

    Comparison of PTPase and PPP catalytic mechanism. (A) Schematic representation of PTP-Cys-mediated hydrolysis of substrate derived from the crystal structure of PTP1B (data from Barford et al. 1994). (B) Schematic representation of metal ion-mediated hydrolysis of substrate derived from the crystal structure of PP5C (data from Swingle et al. 2004). The attacking hydroxide W1 is shown in blue and the leaving group of the substrate is in green. The substrate, the planar PO3 moiety of the transition state, and the phosphate product are all shown in red. Solid lines to the metal ions denote metal–ligand bonds, and solid or dashed wedges indicate metal–ligand bonds directed above or below the plane of the page respectively. Wavy lines to the metal ions indicate strained contacts with poor coordination geometry. Dotted lines indicate hydrogen bonds, and the nearly dissociated axial bonds in the transition state are shown by half-dotted double lines.

  • View in gallery

    Family tree of PSPs.

  • View in gallery

    Structural comparison of PP1-MYTP1, PP2Ac-A–B, and PP5. (A) PP1 (green) in complex with myosin phosphatase targeting subunit MYPT1 (blue). (B) PP2A holoenzyme: PP2A catalytic subunit (green) in complex with the PP2A scaffold A (blue) and a B55-regulatory targeting subunit (yellow). (C) PP5 in an inactive conformation. The catalytic domain is shown in green, N-terminal inhibitory/TPR-targeting domain in yellow, and a unique C-terminal inhibitory domain in blue. The images were generated using PyMol based on protein data bank accession number 1S70 (Terrak et al. 2004; PP1-MYTP1), 3DW8 (Xu et al. 2008; PP2Ac/A/B), and 1WA0 (Yang et al. 2005; PP5). Arrows indicate the catalytic site with metal ions shown as red spheres.

  • View in gallery

    Regulation of β-cell PP activities and their effects on the insulin stimulus–secretion coupling. Glucose, the β-cell's main stimulus, is taken up across the plasma membrane by the facilitative GLUT (GLUT2). The sugar is further metabolized in the glycolytic pathway and TCA cycle to yield coupling factors suppressing PP activity, thereby activating influx of Ca2+ that sets in motion the exocytotic release of insulin. The ATP generated during glucose catabolism also serves to close K+ channels, causing depolarization, and as a substrate for cAMP formation. Receptor-operated, G protein-coupled signaling pathways through phospholipase C–PKC and AC are also depicted. See text for details. AC, adenylyl cyclase; DAG, diacylglycerol; ER, endoplasmic reticulum; G, GTP-binding protein; Gln, glutamine; Glu, glutamate; GLUT2, glucose transporter 2; GTP, guanosine trisphosphate; InsP3, inositol trisphosphate; InsP6, inositol hexakisphosphate; KATP, ATP-dependent K+ channel; OA, okadaic acid; PIP2, phosphatidylinositol bisphosphate; PKC, protein kinase C; PLC, phospholipase C; PP, protein phosphatase; R, receptor; TPA, 12-O-tetradecanoyl phorbolacetate; VGCC, voltage-gated Ca2+ channel.