Mechanisms of β-cell dedifferentiation in diabetes: recent findings and future research directions

in Journal of Endocrinology

Like all the cells of an organism, pancreatic β-cells originate from embryonic stem cells through a complex cellular process termed differentiation. Differentiation involves the coordinated and tightly controlled activation/repression of specific effectors and gene clusters in a time-dependent fashion thereby giving rise to particular morphological and functional cellular features. Interestingly, cellular differentiation is not a unidirectional process. Indeed, growing evidence suggests that under certain conditions, mature β-cells can lose, to various degrees, their differentiated phenotype and cellular identity and regress to a less differentiated or a precursor-like state. This concept is termed dedifferentiation and has been proposed, besides cell death, as a contributing factor to the loss of functional β-cell mass in diabetes. β-cell dedifferentiation involves: (1) the downregulation of β-cell-enriched genes, including key transcription factors, insulin, glucose metabolism genes, protein processing and secretory pathway genes; (2) the concomitant upregulation of genes suppressed or expressed at very low levels in normal β-cells, the β-cell forbidden genes; and (3) the likely upregulation of progenitor cell genes. These alterations lead to phenotypic reconfiguration of β-cells and ultimately defective insulin secretion. While the major role of glucotoxicity in β-cell dedifferentiation is well established, the precise mechanisms involved are still under investigation. This review highlights the identified molecular mechanisms implicated in β-cell dedifferentiation including oxidative stress, endoplasmic reticulum (ER) stress, inflammation and hypoxia. It discusses the role of Foxo1, Myc and inhibitor of differentiation proteins and underscores the emerging role of non-coding RNAs. Finally, it proposes a novel hypothesis of β-cell dedifferentiation as a potential adaptive mechanism to escape cell death under stress conditions.

Abstract

Like all the cells of an organism, pancreatic β-cells originate from embryonic stem cells through a complex cellular process termed differentiation. Differentiation involves the coordinated and tightly controlled activation/repression of specific effectors and gene clusters in a time-dependent fashion thereby giving rise to particular morphological and functional cellular features. Interestingly, cellular differentiation is not a unidirectional process. Indeed, growing evidence suggests that under certain conditions, mature β-cells can lose, to various degrees, their differentiated phenotype and cellular identity and regress to a less differentiated or a precursor-like state. This concept is termed dedifferentiation and has been proposed, besides cell death, as a contributing factor to the loss of functional β-cell mass in diabetes. β-cell dedifferentiation involves: (1) the downregulation of β-cell-enriched genes, including key transcription factors, insulin, glucose metabolism genes, protein processing and secretory pathway genes; (2) the concomitant upregulation of genes suppressed or expressed at very low levels in normal β-cells, the β-cell forbidden genes; and (3) the likely upregulation of progenitor cell genes. These alterations lead to phenotypic reconfiguration of β-cells and ultimately defective insulin secretion. While the major role of glucotoxicity in β-cell dedifferentiation is well established, the precise mechanisms involved are still under investigation. This review highlights the identified molecular mechanisms implicated in β-cell dedifferentiation including oxidative stress, endoplasmic reticulum (ER) stress, inflammation and hypoxia. It discusses the role of Foxo1, Myc and inhibitor of differentiation proteins and underscores the emerging role of non-coding RNAs. Finally, it proposes a novel hypothesis of β-cell dedifferentiation as a potential adaptive mechanism to escape cell death under stress conditions.

Introduction

All the cells of an organism, including pancreatic β-cells, originate from embryonic stem cells through a complex cellular process termed differentiation (Keller 2005). β-cell differentiation involves the coordinated and tightly controlled activation/repression of specific transcription factors and downstream gene clusters in a time-dependent manner. These complex signalling events drive the transition from definitive endoderm cells to mature insulin-secreting β-cells with the progressive acquisition of specific morphological and functional cellular features (Puri & Hebrok 2010, Seymour & Sander 2011, Stanger & Hebrok 2013) (Fig. 1).

Figure 1

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Figure 1

Schematic representation of the process of β-cell differentiation. This process involves coordinated and tightly controlled activation/repression of specific transcription factors that drive the transition from a stem cell to a mature β-cell during embryonic development and the postnatal period until weaning. Evidence from animal and human studies suggest that mature β-cells could regress backwards to a precursor-like stage and/or transdifferentiate into other islet cells in type 2 diabetes.

Citation: Journal of Endocrinology 236, 2; 10.1530/JOE-17-0516

Interestingly, cellular differentiation is not unidirectional. Indeed, growing evidence suggests that under specific conditions, mature β-cells can lose, to various degrees, their differentiated phenotype and cellular identity and regress to a less differentiated or even a precursor-like state (Fig. 1). This concept, termed dedifferentiation, has been implicated in the pathogenesis of diabetes (Weir et al. 2013). Several in vivo studies in animal models and in humans over the last two decades suggest that, in addition to cell death (Butler et al. 2003, Jurgens et al. 2011), dedifferentiation (Jonas et al. 1999, Kjorholt et al. 2005, Talchai et al. 2012, Marselli et al. 2014, Cinti et al. 2016) may be an important contributing factor to the loss of functional β-cell mass in type 2 diabetes (Sakuraba et al. 2002, Yoon et al. 2003, Deng et al. 2004, Rahier et al. 2008).

β-cell dedifferentiation is manifested by reduced expression of β-cell-enriched genes, including key transcription factors, insulin, glucose metabolism genes, protein processing and secretory pathway genes and the concomitant upregulation of genes suppressed or expressed at very low levels in normal β-cells; the so-called β-cell disallowed or forbidden genes (Bensellam et al. 2012b, Schuit et al. 2012, Rutter et al. 2015, Lemaire et al. 2016, 2017). Moreover, the presence of progenitor cell markers has been observed in the dedifferentiated islets of diabetic animals (Talchai et al. 2012, Wang et al. 2014, Kim-Muller et al. 2016) and even markers of α-cells have been detected suggesting the transdifferentiation of β-cells to α-cells in diabetic animals and humans (Spijker et al. 2013, 2015, Brereton et al. 2014, Cinti et al. 2016). These alterations lead to metabolic and structural reconfiguration of β-cells and ultimately defective insulin secretion. While earlier and recent reports have well established the major role of hyperglycemia and the ensuing glucotoxicity in β-cell dedifferentiation (Bensellam et al. 2012b), the precise molecular mechanisms involved are still under investigation. Understanding these mechanisms opens novel therapeutic horizons to preserve/restore the functional β-cell mass in diabetes.

In this extensive review, we first highlight the close relationship between the degree of β-cell differentiation and insulin secretory function. We then describe the known molecular mechanisms involved in β-cell dedifferentiation in diabetes with a focus on the role of glucotoxicity and its downstream pathways including oxidative stress, endoplasmic reticulum (ER) stress, inflammation and hypoxia. We depict the roles of the transcriptional regulators such as Foxo1, Myc and the inhibitors of differentiation and discuss the role of the failure of adaptive unfolded protein response (UPR). We also discuss the mounting role of non-coding RNAs in β-cell dedifferentiation. Finally, based on our and others emerging evidence, we propose a novel view of dedifferentiation as a potential adaptive mechanism to escape cell death under stress conditions.

Relationship of β-cell differentiation to insulin secretory function

Pancreatic β-cells play a fundamental role in the maintenance of glucose homeostasis in mammalians. They synthesize and secrete insulin after meals to keep blood glucose levels within a tight range (between 4 and 6 mM in fasting adult humans). Glucose is the major physiological modulator of β-cell function and undeniable evidence indicates that glucose must be metabolized by β-cells in order to stimulate insulin secretion (Malaisse et al. 1979, Newgard & McGarry 1995, Jitrapakdee et al. 2010). Thus, upon a rise in its plasma concentration, glucose rapidly equilibrates across the β-cell plasma membrane via the high-capacity, low-affinity glucose transporter GLUT2 (also known as SLC2A2) (Thorens 2015). In humans, the high-affinity glucose transporter GLUT1 prevails instead of GLUT2 (De Vos et al. 1995, Ferrer et al. 1995). Nevertheless, in both human and rodent β-cells, glucose transport exceeds glucose utilization and is therefore not limiting (Newgard 1996). In the cytosol, glucose is then phosphorylated by glucokinase (GCK); a low-affinity type IV hexokinase. GCK regulates the rate of glucose utilization and is therefore considered as the glucose sensor of β-cells (Matschinsky 2009). Interestingly, β-cells present the remarkable feature of an almost complete repression of high-affinity type I–III hexokinases to avoid insulin release at low plasma glucose concentrations in the fasted state (Sekine et al. 1994, Schuit et al. 1999). Glucose-6-phosphate is then oxidized by glycolysis to generate ATP, NADH and pyruvate. Because β-cells present another remarkable feature of expressing very low levels of lactate dehydrogenase (LDH) and monocarboxylate transporter (MCT) (Sekine et al. 1994, Ishihara et al. 1999, Zhao et al. 2001, Thorrez et al. 2011), almost all the pyruvate enters mitochondria to provide a substrate for the Krebs cycle (Fig. 2). This particular design also avoids exercise-induced insulin release (Otonkoski et al. 2007). To compensate for the low LDH activity and maintain the glycolytic flux, β-cells present a third notable characteristic of two highly active mitochondrial membrane electron shuttles: the glycerol-3-phosphate and the malate-aspartate shuttles that couple the regeneration of cytosolic NAD+ with the production of NADH or FADH2 in the mitochondria (MacDonald 1981, 1982, Sekine et al. 1994, Eto et al. 1999). This NADH shuttle system allows tight coupling of glycolysis to mitochondrial metabolism (Fig. 2). In the mitochondria, pyruvate is either oxidized to acetyl-CoA (~50%) by pyruvate dehydrogenase (PDH) or carboxylated to oxaloacetate (~50%) by pyruvate carboxylase (PC) (MacDonald 1993, Khan et al. 1996). The latter enzyme replenishes Krebs cycle intermediates (anaplerosis) to compensate for their removal for biosynthetic purposes (cataplerosis) (Brun et al. 1996, Schuit et al. 1997, Farfari et al. 2000). Pyruvate metabolism in the Krebs cycle generates reducing equivalent NADH and FADH2 which together with those generated by the mitochondrial shuttles transfer electrons to the respiratory chain to generate ATP (Duchen et al. 1993, Eto et al. 1999) (Fig. 2). The resulting atypical rise in ATP/ADP ratio (Detimary et al. 1996, 1998) leads to the closure of ATP-gated K+ (KATP) channels, plasma membrane depolarization, opening of voltage-gated Ca2+ channels (VDCC), Ca2+ influx and the elevation of cytosolic free Ca2+ concentration ([Ca2+]i), which is the triggering signal for insulin secretion (Fig. 2). This sequence of events is referred to as the triggering pathway of glucose-stimulated insulin secretion (GSIS) (Henquin 2011). Besides, glucose can modulate insulin secretion independently from its action on KATP channels by generating signals that amplify the action of Ca2+ on insulin granule exocytosis, provided that Ca2+ influx is already stimulated and [Ca2+]i is high. It is called the metabolic amplifying pathway of GSIS (Henquin 2011). This pathway requires glucose metabolism but the coupling factors have not been identified with certainty with a probable implication of NADPH (Ivarsson et al. 2005, Ferdaoussi et al. 2015) and other effectors (Fig. 2). Moreover, other nutrients (such as free fatty acids (Gravena et al. 2002, Itoh et al. 2003)), hormones (such as GLP1 (Campbell & Drucker 2013)) and neurotransmitters (such as acetylcholine (Gautam et al. 2007, Molina et al. 2014)) can also augment the action of Ca2+ on exocytosis (Fig. 2). The effect of hormones and neurotransmitters is known as the neurohormonal amplification of insulin secretion (Henquin 2011) (Fig. 2).

Figure 2

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Figure 2

Schematic summary of the stimulus-secretion coupling mechanism in mature differentiated β-cells with emphasis on key steps and principal components. Upon a rise in its extracellular concentration, glucose rapidly equilibrates across the plasma membrane via the glucose transporter GLUT2 (1) and is phosphorylated by glucokinase (GCK) (2). Glucose-6-phosphate (G-6-P) is further metabolized by glycolysis to generate pyruvate, ATP and NADH (3). The latter requires the mitochondrial shuttles glycerol-3-phosphate (GS) and malate-aspartate (MAS) to regenerate NAD+ to compensate for low lactate dehydrogenase (LDH) activity. Because β-cells express low levels of monocarboxylate transporter (MCT), pyruvate is channelled to mitochondria where it is transformed into acetyl coenzyme A and oxaloacetate by pyruvate dehydrogenase (PDH) and pyruvate carboxylase (PC) respectively (4). Acetyl coenzyme A and oxaloacetate are used by the Krebs cycle (5) to generate NADH and FADH2 that, together with those produced by the shuttles, transfer electrons to the electron transport chain (ETC) to generate ATP (6). The latter, together with ATP generated from glycolysis, increases the ATP/ADP ratio (7). This leads to the closure of ATP-dependent K+ (KATP) channels (8), plasma membrane depolarization (9) and opening of voltage-dependent Ca2+ channels (VDCCs) (10). The ensuing Ca2+ influx and rise in cytosolic free Ca2+ concentration ([Ca2+]) (11) is the triggering signal for insulin secretion (12) and this sequence of events is referred to as the triggering pathway. On the other hand, other metabolic coupling factors generated by glucose metabolism or other nutrient such as free fatty acids in addition to intracellular messengers affected by hormones such as glucagon-like peptide 1 (GLP1) and neurotransmitters such as acetylcholine (Ach) can amplify the effect of Ca2+ on insulin granule exocytosis (13). This is known as the amplifying pathway of insulin secretion. Green arrows: triggering pathway; red arrows: amplifying pathway. Effectors in green: β-cell-enriched genes; effectors in red: β-cell forbidden genes.

Citation: Journal of Endocrinology 236, 2; 10.1530/JOE-17-0516

Adequate β-cell function relies on a specialized pattern of gene expression. Interestingly, the mature metabolic phenotype of pancreatic β-cells is acquired only several weeks after birth (Asplund et al. 1969, Pildes et al. 1969, Lavine et al. 1971, Grill et al. 1981). The immaturity of neonatal β-cells and their poor GSIS have been correlated with low expression of key metabolic genes, high expression of the β-cell forbidden genes Hk2, Ldha and Mct1 and changes in the expression of specific microRNAs (miRNAs) (Rorsman et al. 1989, Tan et al. 2002, Jermendy et al. 2011, Martens et al. 2014, Jacovetti et al. 2015). Dietary differences between neonates and adults, in particular glucose, may drive the maturation of the β-cell phenotype. Indeed, strong evidence indicates that physiological glucose stimulation is a major regulator of the β-cell differentiated phenotype (Table 1). We and others have shown that the function and survival of rat islets and purified β-cells are optimally preserved by culture in the presence of 10 mM glucose (G10), whereas marked alteration accompanies culture at lower (G2–5) or higher glucose concentrations (G30; glucotoxicity will be discussed in ‘The loss of functional β-cell differentiation in diabetes: causes and mechanisms’ section) (Efanova et al. 1998, Flamez et al. 2002, Khaldi et al. 2004, Bensellam et al. 2009). The beneficial physiological glucose stimulation (G10 vs G2–5) is associated with increased mRNA levels of several critical genes including insulin, components of the triggering pathway of insulin secretion, such as Glut2, subunits of the KATP channel Kir6.2 (also known as Kcnj11) and Sur1 (also known as Abcc8) and metabolic genes including glycolysis and Krebs cycle genes, subunits of the mitochondrial electron transport chain, genes implicated in cataplerosis, fatty acid biosynthesis and cholesterol biosynthesis, in parallel with the upregulation of the mRNA levels of the transcription factors Srebp1, Srebp2 and Chrebp (Flamez et al. 2002, Van Lommel et al. 2006, Bensellam et al. 2009) (Table 1). These findings are corroborated by the observation that prolonged fasting (72 h) markedly decreases insulin, Glut2, Gck and voltage-dependent L-type Ca2+ channel α1 subunit mRNA levels. Interestingly, these alterations were rapidly reversed after refeeding in association with the normalization of β-cell function (Iwashima et al. 1994).

Table 1

Physiological glucose stimulation plays a key role in the maintenance of the differentiated β-cell phenotype.

GenesPhysiological glucose stimulationEffectModelsReferences
Islet hormones
PreproinsulinG10 vs G2–5Up (also protein)Cultured male Wistar rat islets (18 h) and purified β cells (3–10 days)Schuit et al. (2002), Van Lommel et al. (2006), Bensellam et al. (2009)
Fed vs fastedUpFasted male C57BL6 mice (overnight) and male Sprague–Dawley rats (72 h)Iwashima et al. (1994), Van Lommel et al. (2006)
IappG10 vs G2–5UpCultured male Wistar rat islets (18 h)Bensellam et al. (2009)
Transcription factors
Pdx1G10 vs G5UpCultured C57BL6J mouse islets (24 h)(Supplementary Fig. 1)
ChrebpG10 vs G2–5UpCultured male Wistar rat islets (18 h)Bensellam et al. (2009)
Srebp1 (Srebf1)G10 vs G2–5UpCultured male Wistar rat islets (18 h) and purified β cells (24 h)Bensellam et al. (2009), Flamez et al. (2002)
Srebp2 (Srebf2)G10 vs G2–5UpCultured male Wistar rat islets (18 h)Bensellam et al. (2009)
Glucose transport and metabolism
Glut2 (Slc2a2)G10 vs G2–5UpCultured male Wistar rat islets (18 h) and purified β cells (24–72 h) and cultured C57BL6J mouse islets (24 h)Bensellam et al. (2009), Flamez et al. (2002), Martens & Pipeleers (2009), (Supplementary Fig. 1)
Fed vs fastedUpFasted male Sprague–Dawley rats (72 h)Iwashima et al. (1994)
Gck (Hk4)G10 vs G2–5UpCultured male Wistar rat islets (18 h) and purified β cells (72 h)Bensellam et al. (2009), Martens & Pipeleers (2009)
Fed vs fastedUpFasted male Sprague–Dawley rats (72 h)Iwashima et al. (1994)
GapdhG10 vs G2–5UpCultured male Wistar rat islets (18 h)Bensellam et al. (2009)
G6pdG10 vs G2–5UpCultured male Wistar rat islets (18 h) and purified β cells (24 h)Bensellam et al. (2009), Flamez et al. (2002)
NADH:ubiquinone oxidoreductase subunits Ndufa1, Ndufa4, Ndufa11, Ndufb4, Ndufs5, Nd3G10 vs G2–5UpCultured male Wistar rat islets (18 h)Bensellam et al. (2009)
CycsG10 vs G2–5UpCultured male Wistar rat islets (18 h)Bensellam et al. (2009)
Cytochrome c oxidase subunits Cox2, Cox3 (Cytb), Cox5b, Cox6b1, Cox7b, Cox17G10 vs G2–5UpCultured male Wistar rat islets (18 h) and purified β cells (24 h)Bensellam et al. (2009), Flamez et al. (2002)
ATP synthase subunits Atp5a1, Atp5e, Atp5g1, Atp5i, Atp5j, Atp5oG10 vs G2–5UpCultured male Wistar rat islets (18 h)Bensellam et al. (2009)
Ant2 (Slc25a5)G10 vs G2–5UpCultured male Wistar rat islets (18 h)Bensellam et al. (2009)
Gpd2G10 vs G5UpCultured C57BL6J mouse islets (24 h)(Supplementary Fig. 1)
Anaplerosis/cataplerosis
AclyG10 vs G2–5UpCultured male Wistar rat islets (18 h) and purified β cells (24 h)Bensellam et al. (2009), Flamez et al. (2002)
Mdh1G10 vs G2–5UpCultured male Wistar rat islets (18 h)Bensellam et al. (2009)
Me1G10 vs G2–5UpCultured male Wistar rat islets (18 h)Bensellam et al. (2009)
Fatty acid metabolism
FasnG10 vs G2–5UpCultured male Wistar rat islets (18 h) and purified β cells (24 h)Bensellam et al. (2009), Flamez et al. (2002)
HadhG10 vs G2–5UpCultured male Wistar rat islets (18 h) and purified β cells (72 h)Bensellam et al. (2009), Martens & Pipeleers (2009)
Scd1G10 vs G2–5UpCultured male Wistar rat islets (18 h)Bensellam et al. (2009)
Scd2G10 vs G2–5UpCultured male Wistar rat islets (18 h) and purified β cells (24 h)Bensellam et al. (2009), Flamez et al. (2002)
Cholesterol metabolism
Hmgcs1, Hmgcr, Mvd, Idi1, Fdps, Fdft1, Sqle, Lss, Sc4mol, Nsdhl, Sc5dl, Dhcr7, Soat1, Scp2, LdlrG10 vs G2–5UpCultured male Wistar rat islets (18 h) and purified β cells (24 h)Bensellam et al. (2009), Flamez et al. (2002)
VldlrG10 vs G2–5DownCultured male Wistar rat islets (18 h) and purified β cells (24 h)Bensellam et al. (2009), Flamez et al. (2002)
Ion channels, pumps and receptors
Kcnj11 (Kir6.2)G10 vs G3UpCultured purified male Wistar rat β cells (24 h)Flamez et al. (2002)
Abcc8 (Sur1)G10 vs G3UpCultured purified male Wistar rat β cells (24 h)Flamez et al. (2002)
Vdccα1DFed vs fastedUpFasted male Sprague–Dawley rats (72 h)Iwashima et al. (1994)
Serca2b (Atp2a2)G10 vs G2–5UpCultured male Wistar rat islets (18 h)Bensellam et al. (2009)
Serca3 (Atp2a3)G10 vs G2–5UpCultured male Wistar rat islets (18 h)Bensellam et al. (2009)
GiprG10 vs G5UpCultured purified male Wistar rat β cells (72 h)Martens & Pipeleers (2009)
Gpr40 (Ffar1)G10 vs G5UpCultured C57BL6J mouse islets (24 h)(Supplementary Fig. 1)
Slc30a1 (ZnT-1)G10 vs G2–5DownCultured male Wistar rat islets (18 h)Bensellam et al. (2009)
Proinsulin processing and insulin granule exocytosis
Pcsk1, Pcsk2, Cpe, Scg3, Syt13, Sytl4, Snap25, Syp, Scfd1, Ykt6G10 vs G2–5UpCultured male Wistar rat islets (18 h) and purified β cells (72 h)Bensellam et al. (2009), Martens & Pipeleers (2009)

In addition to metabolic genes, physiological glucose stimulation improves and maintains the β-cell differentiated phenotype by modulating several stress responses and prosurvival/proapototic effectors and pathways (Van de Casteele et al. 2003, Martens et al. 2005, Hou et al. 2008a, Bensellam et al. 2009, Roma et al. 2012, Sarre et al. 2012). Among them, the UPR occupies a central place (Elouil et al. 2007, Bensellam et al. 2009). β-cells are highly specialized secretory cells that synthesize large amounts of proinsulin in response to glucose stimulation (Schuit et al. 1988, Ling & Pipeleers 1996). To cope with this heavy ER load, β-cells are endowed with a fundamental adaptive response to prevent accumulation of misfolded or unfolded proteins in the ER when client protein load overwhelms the ER folding capacity. UPR activity is orchestrated by three sensors located in the membrane of the ER, namely the pancreatic ER kinase (PERK also known as EIF2AK3), the endoribonuclease/kinase inositol requiring 1 (IRE1 also known as ERN1) and the activating transcription factor 6 (ATF6). Active PERK phosphorylates the α-subunit of the translation initiation factor 2 (eIF2α) thereby leading to a transient and global attenuation of protein translation to decrease ER load in parallel with a paradoxical increase in the translation of special transcripts such as ATF4. This transcription factor upregulates the expression of chaperones and antioxidant genes as well as proapoptotic genes like Ddit3 (also known as Chop/Gadd153), Atf3 and Trb3 (also known as Trib3) that contribute to β-cell death under prolonged or unresolved ER stress. Because eIF2α can be phosphorylated by kinases other than PERK in response to other types of stress, we will refer to the eIF2α-ATF4 arm of the UPR by Integrated Stress Response (ISR) if PERK activation has not been demonstrated. Activation of IRE1 leads to the unconventional splicing of Xbp1 pre-mRNA and the ensuing upregulation of active X-Box-binding protein 1 (XBP1s). This key transcription factor, together with active ATF6, stimulates the expression of Xbp1, chaperones, foldases and genes of the ER-associated degradation (ERAD) pathway thereby leading to improved ER folding capacity and enhanced clearance of unfolded proteins from the ER (Schroder & Kaufman 2005). Interestingly, glucose stimulation exerts opposite effects on the PERK-eIF2α-ATF4 and IRE1-XBP1s/ATF6 arms of the UPR. Thus, the PERK-eIF2α-ATF4 arm is maximally activated at low glucose concentrations (PERK and eIF2α phosphorylation, upregulation of ATF4 and downstream target genes such as Ddit3), where protein synthesis, ATP levels and [Ca2+]ER are low, while the IRE1-XBP1 arm is maintained at basal level. Glucose stimulation rapidly repressed the ISR and triggered Xbp1 pre-mRNA splicing leading subsequently to the upregulation of XBP1s and a wide array of downstream target genes to face the ER synthetic load (Elouil et al. 2007, Vander Mierde et al. 2007, Gomez et al. 2008, Bensellam et al. 2009, Jonas et al. 2009, Moore et al. 2011). Of note, an inverse relationship between blood glucose levels and eIF2α phosphorylation in whole pancreatic extracts has been reported in the models of fasting mice and mice receiving an intraperitoneal glucose load (Zhang et al. 2002a). The fine-tuning of this adaptive response is vital for the preservation of the β-cell differentiated phenotype. Indeed, our and others recent evidence indicated that the failure of adaptive UPR is associated with the progression to diabetes and altered β-cell differentiation (Herbert & Laybutt 2016) (see ‘The loss of β-cell differentiation in diabetes: causes and mechanisms’ section).

Besides the role of nutrient-induced stimulation of gene expression, the acquisition and maintenance of the differentiated β-cell phenotype also relies on gene repression. The latter involves particular mechanisms including epigenetic and miRNA regulation. Thus, histone and DNA methylation are implicated in the metabolic maturation of β-cells in the postnatal period by repressing the expression of Hk1, Mct1 and Ldha (Thorrez et al. 2011, Dhawan et al. 2015). Moreover, a growing body of data suggests that miRNAs are implicated not only in the repression of β-cell forbidden genes (Pullen et al. 2011, Martinez-Sanchez et al. 2015), but also in the modulation of insulin gene expression (El Ouaamari et al. 2008, Tang et al. 2009, Melkman-Zehavi et al. 2011, Zhao et al. 2012), the metabolic reconfiguration of neonatal β-cells (Jacovetti et al. 2015) and insulin granule exocytosis (Poy et al. 2004, Lovis et al. 2008, Latreille et al. 2014). The switch from high-fat milk in newborns to carbohydrate-rich diet post-weaning has been suggested as an important driver of changes in miRNA expression that paralleled the maturation of the β-cell phenotype (Jacovetti et al. 2015). In agreement, glucose stimulation has been shown to regulate the expression of several miRNAs in β-cell lines and isolated islets (El Ouaamari et al. 2008, Esguerra et al. 2011). Recent reports have revealed another mechanism of gene repression whereby the transcription factors NK2 homeobox 2 (NKX2.2), paired box 6 (PAX6) and Regulatory Factor X, 6 (RFX6) play a dual role by activating the expression of various β-cell-enriched genes and also by directly binding to and repressing the expression of non-β-cell endocrine genes and other islet forbidden genes such as Ldha (Piccand et al. 2014, Gutierrez et al. 2017, Swisa et al. 2017). Of note, Nkx2.2 mRNA levels were upregulated in rat islets cultured in the presence of G10 instead of G2–5 (Bensellam et al. 2009).

However, while physiological glucose stimulation plays a key role in the maintenance of the β-cell differentiated phenotype, exposure to supraphysiological glucose levels, such as in the context of diabetes, leads to β-cell dedifferentiation. This may contribute to the progressive loss of functional β-cell mass and the aggravation of the disease over time. Our understanding of this phenomenon has markedly expanded in the last two decades with the identification of various mechanisms and effectors.

The loss of β-cell differentiation in diabetes: causes and mechanisms

The loss of functional β-cell mass does not only result from apoptosis but also result from a loss of differentiation

It is now well recognized that the β-cell mass is reduced by 25–60% in T2D patients in comparison to weight-matched normal subjects (Sakuraba et al. 2002, Butler et al. 2003, Yoon et al. 2003, Deng et al. 2004, Rahier et al. 2008). The observation that β-cell mass decline correlated with the duration of T2D (Rahier et al. 2008) supports a role of chronic hyperglycemia and glucotoxicity. Indeed, prolonged or intermittent exposure of rodent and human islets to elevated glucose concentrations in vitro has been shown to increase β-cell apoptosis (Ling et al. 1994, Efanova et al. 1998, Piro et al. 2002, Khaldi et al. 2004, DelGuerra et al. 2007, Hou et al. 2008b, Bensellam et al. 2009, Jonas et al. 2009). These results are strongly supported by the observation of increased β-cell apoptosis in different animal models of T2D (Koyama et al. 1998, Pick et al. 1998, Donath et al. 1999, Finegood et al. 2001, Huang et al. 2007, Song et al. 2008) and in human patients (Butler et al. 2003, Jurgens et al. 2011). However, the rate of β-cell apoptosis is relatively low and may not fully explain the loss of β-cell mass in T2D. Thus, alternative mechanisms are possible. The concept of the loss of differentiation, introduced in the late nineties (Jonas et al. 1999), has been recently proposed as an important contributor to the reduced β-cell mass in T2D (Talchai et al. 2012, Marselli et al. 2014, Spijker et al. 2015, Cinti et al. 2016). Similar to apoptosis, chronic hyperglycemia has been shown to be a major inducer of β-cell dedifferentiation.

Role of hyperglycemia and glucotoxicity

In T2D, the complex interaction of genetic and environmental factors leads ultimately to defective insulin secretion. The latter is a consequence of the failure of β-cells to cope with the important metabolic demand imposed on them by insulin resistance. The ensuing chronic hyperglycemia exerts further damage on the β-cell phenotype thereby leading to the progressive decline of functional β-cell mass and deterioration of the disease over time. This is the concept of glucotoxicity (Bensellam et al. 2012b). Glucotoxicity-related loss of β-cell identity is characterised by alterations in the expression of many genes, with much initial and recent attention focused on the reduced expression of β-cell-enriched genes, in particular insulin.

Downregulation of insulin gene expression

Earlier studies in β-cell lines and isolated rodent and human islets have shown that prolonged exposure to elevated glucose concentrations markedly reduced the mRNA levels of insulin (Robertson et al. 1992, Olson et al. 1995, 1998, Briaud et al. 1999, Marshak et al. 1999). In agreement, a similar reduction was observed in the islets of Zucker diabetic fatty (ZDF) rats (Harmon et al. 2001) and 90% pancreatectomized (Px) rats (Jonas et al. 1999). Interestingly, in these animal models, insulin mRNA levels were restored by phlorizin treatment (an inhibitor of renal glucose reabsorption) and normalization of glycaemia (Jonas et al. 1999, Harmon et al. 2001). Reduced insulin mRNA levels have also been confirmed in β-cells of human T2D subjects (Marchetti et al. 2004, DelGuerra et al. 2005, Marselli et al. 2010, Guo et al. 2013). This reduction has been attributed to reduced expression and binding of the key transcription factors, pancreatic and duodenal homeobox 1 (PDX-1 also called IPF-1, STF-1, IUF-1, IDX-1 or GSF) and V-maf musculoaponeurotic fibrosarcoma oncogene homolog A (MAFA also known as RIPE3b1), to the insulin gene promoter (Olson et al. 1995, Sharma et al. 1995, Poitout et al. 1996, Moran et al. 1997, Harmon et al. 1998, Marshak et al. 1999, Gleason et al. 2000, Pino et al. 2005, Matsuoka et al. 2010) (Fig. 3 and Supplementary Fig. 1, see section on supplementary data given at the end of this article). Reduced mRNA levels of Pdx1 and Mafa and nuclear expression of their proteins has also been reported in the islets of T2D patients (Guo et al. 2013). The reduction of the mRNA levels of Pdx1 in the islets of Px rats and Neurod1 in the islets of db/db mice was prevented by phlorizin treatment, thereby demonstrating the role of glucotoxicity (Jonas et al. 1999, Kjorholt et al. 2005). These results are further corroborated by a study showing that treatment of db/db mice with the sodium glucose cotransporter 2 (SGLT2) inhibitor luseogliflozin upregulated islet mRNA levels of insulin, Pdx1 and Mafa (Okauchi et al. 2016).

Figure 3

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Figure 3

Schematic representation of the key effectors and mechanisms implicated in the loss of insulin gene expression in type 2 diabetes. Please see the text for details. Direction of the vertical black arrows close to proteins, mRNAs, miRNAs or stresses indicate the effect of glucotoxicity. C/EBPβ, CCAAT/enhancer-binding protein β; ChREBP, carbohydrate response element binding protein; ER, endoplasmic reticulum; JNK, c-Jun N-terminal kinase; ROS, reactive oxygen species; ETS1, V-ets avian erythroblastosis virus E26 oncogene homolog 1; IRE1α, inositol requiring 1α; MAFA, V-maf musculoaponeurotic fibrosarcoma oncogene homolog A; MYC, myelocytomatosis viral oncogene homolog (avian); NEUROD1, neurogenic differentiation 1; PDX-1, pancreatic and duodenal homeobox 1; PTB, polypyrimidine tract binding protein; STAT3, signal transducer and activator of transcription 3; TXN, thioredoxin; TXNIP, thioredoxin-interacting protein.

Citation: Journal of Endocrinology 236, 2; 10.1530/JOE-17-0516

The observation that the alteration of insulin gene expression was prevented by antioxidant treatment suggested an implication of oxidative stress (Tanaka et al. 1999, Robertson & Harmon 2006). In support of this idea, hydrogen peroxide treatment of β-cell lines and isolated islets has been shown to alter the nuclear expression and DNA-binding activity of MAFA and PDX1 (Kaneto et al. 2002b, Harmon et al. 2005, Guo et al. 2013). Moreover, reduced nuclear expression of MAFA in the islets of diabetic db/db mice was restored by transgenic expression of the antioxidant gene Gpx1 (Harmon et al. 2009, Guo et al. 2013). Similarly, altered nuclear expression of PDX1 and MAFA in the islets of diabetic ZDF rats was markedly prevented by treatment with the glutathione peroxidase (GPX) mimetic ebselen (Mahadevan et al. 2013). Oxidative stress-mediated alteration of PDX1 nuclear localization and DNA-binding activity has been proposed to involve the activation of c-Jun N-terminal kinase (JNK) (Kaneto et al. 2002b, Kawamori et al. 2003) (Fig. 3). Besides oxidative stress, it has been proposed that chronic ER stress could affect the specificity of the endoribonuclease activity of IRE1, thus leading to the cleavage of ER-associated RNAs including that of insulin (Hollien & Weissman 2006, Pirot et al. 2007, Lipson et al. 2008, Han et al. 2009, Hollien et al. 2009) (Fig. 3). Moreover, other mechanisms have been identified implicating novel effectors, in particular miRNAs. Thus, it has been demonstrated that the high glucose-induced redox protein thioredoxin-interacting protein (TXNIP) (Shalev et al. 2002, Saxena et al. 2010), via a STAT3-dependent pathway, upregulated miR-204 thereby leading to reduced Mafa mRNA levels (Xu et al. 2013) (Fig. 3). In addition, upregulation of miR-30a-5p by high glucose levels in INS1 cells and rat islets has been shown to inhibit Neurod1 expression (Kim et al. 2013) (Fig. 3). Furthermore, in human islets exposed to high glucose, the induction of miR-133a targeted the polypyrimidine tract-binding protein and led to reduced insulin biosynthesis, although insulin mRNA levels were not significantly affected in this study (Fred et al. 2010) (Fig. 3). Another report has also shown that miR-124a is markedly upregulated in the islets of human T2D subjects and targeted Foxa2 and Neurod1 in MIN6 cells. Interestingly, overexpression of miR-124a blunted GSIS in MIN6 cells (Sebastiani et al. 2015). However, it is unclear whether miR-124a is regulated by glucotoxicity or another mechanism in T2D. On the other hand, the alteration of insulin gene expression under glucotoxic conditions also involves transcriptional repression by transcriptional repressor proteins. Thus, upregulation of CCAAT/enhancer-binding protein β (C/EBPβ) in β-cell lines and islets of ZDF and Px rats has been implicated in the downregulation of insulin gene expression (Lu et al. 1997, Seufert et al. 1998). C/EBPβ interacted with the basic helix-loop-helix transcription factor E47 thereby inhibiting its DNA binding to the insulin gene promoter and its transactivation ability (Fig. 3). High glucose also affected insulin gene transcription by upregulating the expression of myelocytomatosis viral oncogene homolog (avian) (MYC). MYC has been shown to inhibit neurogenic differentiation 1 (NEUROD1, also known as BETA2)-mediated transcriptional activation of the insulin gene promoter in β-cell lines and isolated rat islets (Kaneto et al. 2002a) (Fig. 3). Of note, Myc is also an oxidative stress-responsive gene (Elouil et al. 2005, Jonas et al. 2009) that has been implicated in β-cell dedifferentiation (Jonas et al. 2001, Laybutt et al. 2002c, Pascal et al. 2008, Robson et al. 2011). On the other hand, it has been proposed that Myc expression is negatively regulated by PDX1 (Chen et al. 2007). More recently, V-ets avian erythroblastosis virus E26 oncogene homolog 1 (ETS1) has been shown to repress the expression of Pdx1 under glucotoxicity by directly binding to its promoter and inhibiting its transcription in rodent β-cells (Chen et al. 2016) (Fig. 3). This study also suggested that ETS1 could interact with the transcription factor forkhead box O1 (FOXO1) and increase its binding to the Pdx1 promoter (Chen et al. 2016). FOXO1 has previously been proposed to repress FOXA2-dependent Pdx1 transcription and to be involved in the nucleocytoplasmic translocation of PDX1 under oxidative stress (Kitamura et al. 2002, Kawamori et al. 2006). However, findings in the last decade revealed that the loss of FOXO1 under severe hyperglycemia is implicated in β-cell dedifferentiation thereby indicating a complex role of this transcription factor in β-cell pathophysiology (discussed in the next section).

Downregulation of β-cell-enriched genes

Besides the insulin gene and key upstream transcription factors, glucotoxicity alters the expression of additional genes that maintain the differentiated β-cell phenotype including other transcription factors such as Nkx6.1, Hnf1α (also known as Tcf1) and Hnf4α, glucose sensing and metabolism genes such as Glut2, Gck and Gpd2 (Supplementary Fig. 1), stimulus-secretion coupling genes such as Kcnj11 and Abcc8, amplification of insulin secretion genes such as Glp1r, Chrm3 and Gpr40 (Supplementary Fig. 1) and insulin granule exocytosis genes such as N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex proteins. A comprehensive and up to date list of the most important genes and the models in which they are affected are presented in Table 2. Many of these alterations, but not all, have also been confirmed in islets of human T2D subjects (Table 2). The normalization of these alterations in diabetic animals using phlorizin treatment, sulfonylureas or insulin therapy, but not bezafibrate treatment, strongly supports the implication of glucotoxicity rather than lipotoxicity (Jonas et al. 1999, Harmon et al. 2001, Gaisano et al. 2002, Kjorholt et al. 2005, Xu et al. 2007, Brereton et al. 2014, Wang et al. 2014). Several pathways downstream of glucotoxicity (Bensellam et al. 2012b) have been implicated in the loss of β-cell-enriched genes including oxidative stress (Tanaka et al. 1999, Kaneto et al. 2002b, Harmon et al. 2005, Robertson & Harmon 2006, Guo et al. 2013), ER stress (Hollien & Weissman 2006, Pirot et al. 2007, Lipson et al. 2008, Han et al. 2009, Szabat et al. 2011, Lombardi et al. 2012), the hexosamine pathway (Kaneto et al. 2001, Yoshikawa et al. 2002), inflammation (Nordmann et al. 2017) and hypoxia (Puri et al. 2013, Sato et al. 2014, 2017).

Table 2

Downregulation of β-cell-enriched genes under glucotoxicity.

GenesGlucotoxicity modelConfirmation in human T2DGlucotoxic effect
In vitro/ex vivoIn vivo
Islet hormones
PreproinsulinHIT-T15 cells (Robertson et al. 1992, Sharma et al. 1995)

β-TC6 cells (Poitout et al. 1996)

INS1 cells (Olson et al. 1998)

Wistar rat islets (Briaud et al. 1999)

Human islets (Marshak et al. 1999)
Px rat islets (Zangen et al. 1997)

ZDF rat islets (Tokuyama et al. 1995)

Rat islets transplanted into diabetic rats (Laybutt et al. 2007)

Psammomys obesus islets (Leibowitz et al. 2001)

MKR mouse islets (Lu et al. 2008)
Yes: (Marchetti et al. 2004, DelGuerra et al. 2005, Masini et al. 2012, Guo et al. 2013)mRNA, protein and volume density of insulin granules
IappPx rat islets (Jonas et al. 1999, Laybutt et al. 2003)

Rat islets transplanted into diabetic rats (Laybutt et al. 2007)
Yes: (Marselli et al. 2010, Bugliani et al. 2013)mRNA
Transcription factors
Pdx1INS1 cells (Wang et al. 2005)

INS 832/13 (Yang et al. 2012)

Sprague–Dawley rat islets (Hou et al. 2008b)

C57BL6J mouse islets (Supplementary Fig. 1)
Px rat islets (Zangen et al. 1997, Jonas et al. 1999)

Rat islets transplanted into diabetic rats (Laybutt et al. 2007)

db/db mouse islets (Kjorholt et al. 2005, Chan et al. 2013)

NZO mouse islets (Kluth et al. 2011)
ControversialYes: (Ostenson et al. 2006, Yang et al. 2012, Guo et al. 2013)

Upregulated: (DelGuerra et al. 2005)
DNA methylation, mRNA, protein, and subcellular localization
MafaHIT-T15 cells (Sharma et al. 1995)db/db mouse islets (Harmon et al. 2009, Chan et al. 2013, Guo et al. 2013)

NZO mouse islets (Kluth et al. 2011)βV59M mouse islets (Brereton et al. 2014)
Yes: (Guo et al. 2013, Cinti et al. 2016)mRNA, protein and subcellular localization
MafbYes: (Bugliani et al. 2013, Guo et al. 2013)mRNA, protein and subcellular localization
Neurod1 (Beta2)Px rat islets (Jonas et al. 1999, Laybutt et al. 2003)

Rat islets transplanted into diabetic rats (Laybutt et al. 2007)

db/db mouse islets (Kjorholt et al. 2005, Chan et al. 2013)
mRNA
Nkx6.1Px rat islets (Jonas et al. 1999, Laybutt et al. 2003)

Rat islets transplanted into diabetic rats (Laybutt et al. 2007)

db/db mouse islets (Kjorholt et al. 2005, Chan et al. 2013, Guo et al. 2013)

NZO mouse islets (Kluth et al. 2011)βV59M mouse islets (Brereton et al. 2014)
Yes: (Guo et al. 2013, Cinti et al. 2016)mRNA, protein and subcellular localization
Hnf1α (Tcf1)Px rat islets (Jonas et al. 1999, Laybutt et al. 2003)Yes: (Marselli et al. 2010)mRNA
Hnf4αPx rat islets (Jonas et al. 1999, Laybutt et al. 2003)Yes: (Gunton et al. 2005)mRNA
Hnf3β (Foxa2)Px rat islets (Jonas et al. 1999)mRNA
Pax6Rat islets transplanted into diabetic rats (Laybutt et al. 2007)

db/db mouse islets (Kjorholt et al. 2005, Swisa et al. 2017)
mRNA
Foxo1GIRKO mouse islets (Talchai et al. 2012)

db/db mouse islets (Kim-Muller et al. 2014)
ControversialYes: (Marselli et al. 2010, Cinti et al. 2016)

Upregulated: (DelGuerra et al. 2005)
mRNA and protein
Tcf7l2 (Tcf4)Vancouver Diabetic Fatty (VDF) Zucker rat islets (Shu et al. 2009)

db/db mouse islets (Shu et al. 2009)
Yes: (Shu et al. 2009)Protein
PparαINS 832/13 (Roduit et al. 2000)

Wistar rat islets (Roduit et al. 2000)
Px rat islets (Laybutt et al. 2002b)

ZDF rat islets (Wang et al. 1998)
mRNA and protein
Glucose transport and metabolism
Glut1Yes: (DelGuerra et al. 2005, Guo et al. 2013)mRNA
Glut2INS1 cells (Wang et al. 2005)

C57BL6J mouse islets (Supplementary Fig. 1)
Px rat islets (Jonas et al. 1999, Laybutt et al. 2003)

Rat islets transplanted into diabetic rats (Laybutt et al. 2007)

db/db mouse islets (Thorens et al. 1992, Kjorholt et al. 2005, Chan et al. 2013, Guo et al. 2013)βV59M mouse islets (Brereton et al. 2014)
Yes: (DelGuerra et al. 2005, Ostenson et al. 2006, Marselli et al. 2010, Taneera et al. 2012, Guo et al. 2013)mRNA and protein
GckINS1 cells (Wang et al. 2005)Px rat islets (Jonas et al. 1999, Laybutt et al. 2003)

ZDF rat islets (Tokuyama et al. 1995)

Rat islets transplanted into diabetic rats (Laybutt et al. 2007)

db/db mouse islets (Kjorholt et al. 2005, Chan et al. 2013)
Yes: (DelGuerra et al. 2005)mRNA
PcPx rat islets (Jonas et al. 1999, Laybutt et al. 2003)

ZDF rat islets (MacDonald et al. 1996)

db/db mouse islets (Kjorholt et al. 2005, Chan et al. 2013)

MKR mouse islets (Lu et al. 2008)
Yes: (MacDonald et al. 2009)mRNA, protein and enzyme activity
Gpd2 (mGpdh)C57BL6J mouse islets (Supplementary Fig. 1)Px rat islets (Jonas et al. 1999, Laybutt et al. 2003)

ZDF rat islets (Tokuyama et al. 1995, MacDonald et al. 1996)

db/db mouse islets (Kjorholt et al. 2005, Chan et al. 2013)

MKR mouse islets (Lu et al. 2008)
Yes: (MacDonald et al. 2009, Marselli et al. 2010)mRNA and enzyme activity
AclyGK rat islets (Homo-Delarche et al. 2006, Hasan et al. 2010)Yes: (MacDonald et al. 2009, Marselli et al. 2010, Bugliani et al. 2013)mRNA and enzyme activity
Receptors, ion channels and pumps
Glp1rSprague–Dawley rat islets (Xu et al. 2007)Px rat islets (Xu et al. 2007)

GK rat islets (Homo-Delarche et al. 2006)

Glucose-infused rat islets (Xu et al. 2007)

db/db mouse islets (Chan et al. 2013)
Yes: (Shu et al. 2009, Taneera et al. 2012, Guo et al. 2013)mRNA and protein
GiprPx rat islets (Xu et al. 2007)

db/db mouse islets (Chan et al. 2013)
Yes: (Shu et al. 2009)mRNA and protein
Gpr40C57BL6J mouse islets (Supplementary Fig. 1)db/db mouse islets (Chan et al. 2013)mRNA
Chrm3ICR mouse islets (Hauge-Evans et al. 2014)ob/ob mouse islets (Hauge-Evans et al. 2014)mRNA and protein
Kcnj11 (Kir6.2)Px rat islets (Jonas et al. 1999, Laybutt et al. 2003)

ZDF rat islets (Tokuyama et al. 1995)

db/db mouse islets (Kjorholt et al. 2005, Chan et al. 2013)
Yes: (Ostenson et al. 2006, Taneera et al. 2012)mRNA
Abcc8 (Sur1)Yes: (Ostenson et al. 2006, Marselli et al. 2010)mRNA
Vdccα1DPx rat islets (Jonas et al. 1999)

ZDF rat islets (Tokuyama et al. 1995)
Yes: (Marselli et al. 2010)mRNA
VdccβPx rat islets (Laybutt et al. 2003)mRNA
Serca2b (Atp2a2b)Px rat islets (Jonas et al. 1999)

db/db mouse islets (Kjorholt et al. 2005)
Yes: (Bugliani et al. 2013)mRNA
Serca3 (Atp2a3)Px rat islets (Jonas et al. 1999)

db/db mouse islets (Kjorholt et al. 2005)
mRNA
Hormone processing and insulin granule exocytosis
Pcsk1 (Pc1)MKR mouse islets (Lu et al. 2008)Yes: (Masini et al. 2012)mRNA and protein
Pcsk2 (Pc2)MKR mouse islets (Lu et al. 2008)Protein
CpeMKR mouse islets (Lu et al. 2008)Yes: (Masini et al. 2012)mRNA and protein
Stx1aINS1E cells (Dubois et al. 2007)

Sprague–Dawley rat islets (Torrejon-Escribano et al. 2011)
GK rat islets (Nagamatsu et al. 1999, Gaisano et al. 2002, Zhang et al. 2002b)

db/db mouse islets (Do et al. 2014)
Yes: (Ostenson et al. 2006)mRNA and protein
Stxbp1 (Munc18-1)GK rat islets (Zhang et al. 2002b)Yes: (Ostenson et al. 2006)mRNA and protein
SNAP-25GK rat islets (Nagamatsu et al. 1999, Gaisano et al. 2002, Zhang et al. 2002b)Yes: (Ostenson et al. 2006)mRNA and protein
Vamp2INS1E cells (Dubois et al. 2007)

Wistar (Gaisano et al. 2002) and Sprague–Dawley rat islets (Torrejon-Escribano et al. 2011)

Human islets (Dubois et al. 2007)
Px rat islets (Torrejon-Escribano et al. 2011)

GK rat islets (Gaisano et al. 2002, Zhang et al. 2002b)

Rat islets transplanted into diabetic rats (Torrejon-Escribano et al. 2011)

MKR mouse islets (Lu et al. 2008)
Yes: (Ostenson et al. 2006)mRNA and protein
Vamp3Sprague–Dawley rat islets (Torrejon-Escribano et al. 2011)Px rat islets (Torrejon-Escribano et al. 2011)

Rat islets transplanted into diabetic rats (Torrejon-Escribano et al. 2011)
Protein
SypYes: (Ostenson et al. 2006)mRNA and protein
Sytl4MKR mouse islets (Lu et al. 2008)mRNA and protein

Downregulation of the β-cell-enriched genes is linked with altered expression of key transcription factors. Indeed, above and beyond the regulation of insulin gene expression, the β-cell key transcription factors PDX1, MAFA and NEUROD1, in addition to hepatocyte nuclear factors (HNF1α, HNF4α and HNF3β/FOXA2) and PAX6, are implicated in a complex network of inter-regulation (Ben-Shushan et al. 2001, Samaras et al. 2002, 2003, Shih et al. 2002, Pedersen et al. 2005) and regulation of other β-cell-enriched genes including key metabolic genes such as Glut2, Gck and Pc, ion channel genes such as Abcc8 and receptor genes such as Glp1r (Waeber et al. 1996, Cha et al. 2000, Shih et al. 2001, Sund et al. 2001, Kim et al. 2002, Moates et al. 2003, Zhang et al. 2005, Wang et al. 2007). Interestingly, PDX1, NKX6.1 and PAX6 have also been shown to play a key role in preserving β-cell identity by repressing the expression of other islet cell type genes (Schaffer et al. 2013, Taylor et al. 2013, Gao et al. 2014, Mitchell et al. 2017, Swisa et al. 2017). Besides these canonical transcription factors, recent evidence has suggested that reduced expression of sex-determining region Y-box 5 (SOX5) may also contribute to the downregulation of β-cell-enriched genes in T2D (Axelsson et al. 2017). Adequate regulation of β-cell-enriched gene expression by these transcription factors depends on correct binding to specific promoters, but also on promoter-enhancer looping of chromatin and formation of multimeric transcriptional complexes. This has been shown to involve LIM domain-binding protein 1 (LDB1). LDB1, together with its binding partner islet 1 (ISL1), was enriched at DNA regions occupied by PDX1, NKX2.2, NKX6.1 and FOXA2 and LDB deletion in adult murine β-cells markedly affected the expression of β-cell-enriched genes including Mafa, Pdx1, Nkx6.1, Glut2, Kcnj11, Abcc8 and Glp1r. Interestingly, reduced LDB1 enrichment in a putative active enhancer nearby GLUT2 gene has been observed in islets from T2D donors (Ediger et al. 2017). But whether dysregulation of LDB1 is involved in downregulation of other β-cell-enriched genes in human T2D requires further exploration.

Gene inhibition/overexpression strategies in vitro and in vivo have allowed a better understanding of the implication of these genes in the alteration of β-cell identity and function. The numerous forms of the maturity-onset diabetes of the young (MODY) best illustrate this in humans. These monogenic disorders result from mutations in transcription factor genes such as Hnf4α (MODY1), Hnf1α (MODY3), Pdx1 (MODY4), Hnf1β (MODY5) and Neurod1 (MODY6), and mutations in ion channel and metabolic genes such as Gck (MODY2), Abcc8 (MODY12) and Kcnj11 (MODY13) (Hattersley & Patel 2017). However, despite important advances in this field, there are still significant gaps of knowledge with regards to the precise targets of these transcription factors, their complex interaction network and the identification of novel transcription factors implicated in the loss of β-cell identity.

In recent years, FOXO1 has received much attention as a potential transcription factor linking metabolic stress with β-cell dedifferentiation in T2D. FOXO1 is a multifunctional transcription factor that has been shown to play complex adaptive/deleterious roles in β-cells under stress depending on the context. Thus, evidence showed that FOXO1 translocates to the nucleus under oxidative stress and promotes the expression of Neurod1 and Mafa (Kitamura et al. 2005). In transgenic mouse models, FOXO1 has been shown to play a role in β-cell compensation during insulin resistance via increasing β-cell proliferation, mass, function and antioxidant gene expression (Okamoto et al. 2006, Zhang et al. 2016). Interestingly, Foxo1 somatic deletion in β-cells resulted in β-cell dedifferentiation in ageing male mice and multiparous female mice in parallel with the upregulation of progenitor and pluripotency markers (Talchai et al. 2012). Furthermore, loss of FOXO1 has been observed in the islets of severely hyperglycemic db/db mice and insulin-resistant GIRKO mice (Talchai et al. 2012, Kim-Muller et al. 2014). This observation was confirmed by the same group in human T2D islets and was associated with altered expression and nuclear localization of MAFA and NKX6.1 (Cinti et al. 2016). In contrast, FOXO1 nuclear translocation in response to glucose starvation or palmitate treatment has been implicated in β-cell apoptosis, at least in part, via the stimulation of Ddit3 gene expression (Martinez et al. 2006, 2008). FOXO1 subcellular localization and activity are modulated by posttranslational modifications including phosphorylation, ubiquitination, OGlcNacylation and acetylation (Kitamura 2013). In the NZO mouse model, a dietary carbohydrate challenge and the ensuing hyperglycemia were associated with rapid and marked dephosphorylation of FOXO1 (activation) followed by reduced GLUT2, PDX1, MAFA and NKX6.1 protein levels and increased apoptosis (Kluth et al. 2011), in agreement with the previously reported role of FOXO1 in the inhibition of Pdx1 transcription (Kitamura et al. 2002). However, in this model, it is unclear whether nuclear expression of active FOXO1 is upregulated and whether it declines thereafter or not (Kluth et al. 2011). Another study has shown increased nuclear expression of FOXO1 in β-cells of diabetic db/db mice in association with downregulation of PDX1, MAFA and NKX6.1. Interestingly, caloric restriction for three months markedly reversed these alterations and normalized glycaemia and β-cell function (Sheng et al. 2016). Furthermore, increased mRNA levels of Foxo1 have been reported in the islets of T2D subjects (DelGuerra et al. 2005). All in all, although undeniable evidence supports an important adaptive role of FOXO1 in the preservation of β-cell identity, especially under hyperglycemia and subsequent oxidative stress, additional work is required to further elucidate the mechanisms and timing of activation/inhibition of FOXO1 in diabetes and identify its precise targets and interacting network. The use of models of inducible expression/deletion in adult animals could be useful.

Besides altered expression and subcellular localization of transcription factors, downregulation of β-cell-enriched genes also involves gene repression by epigenetic mechanisms and miRNAs. Thus, in the islets of human T2D patients, increased DNA methylation in the promoter and enhancer regions of Pdx1 has been proposed to be involved in its downregulation under hyperglycemia (Yang et al. 2012). Similarly, increased DNA methylation in the Glp1r promoter has been observed in the islets of human T2D subjects (Hall et al. 2013). We have highlighted in the previous section the implication of several miRNAs in the inhibition of insulin gene expression and upstream transcription factors (Fred et al. 2010, Kim et al. 2013, Xu et al. 2013, Sebastiani et al. 2015). In addition, various miRNAs upregulated in the islets of GK rats have been shown to target transport and secretory pathway genes (Esguerra et al. 2011). Moreover, it has recently been shown that upregulation of miR-130a-3p, miR-130b-3p and miR-152-3p in the islets of GK rats was associated with reduced protein expression of GCK and pyruvate dehydrogenase E1-α and reduced intracellular ATP levels. Upregulation of these miRNAs has also been confirmed in the islets of hyperglycemic human donors (Ofori et al. 2017). Furthermore, upregulation of miR-187 in the islets of human T2D subjects was associated with altered GSIS and has been proposed to target Hipk3 (Locke et al. 2014), a gene encoding for a serine-threonine kinase implicated in the regulation of β-cell function and survival (Shojima et al. 2012). However, transfection of INS1 cells with miR187 mimetic had only a minor effect on Hipk3 mRNA levels and promoter activity, thereby suggesting the implication of other effectors (Locke et al. 2014). Thus, growing evidence supports an important role of miRNAs in the alteration of β-cell identity. Future studies are needed in this emerging field of research to identify which miRNAs are directly implicated in the loss of β-cell identity in T2D, map their precise targets and determine the upstream mechanisms of induction. A special attention in the in vitro studies must be paid to the duration of exposure to elevated glucose levels when evaluating the effects of glucotoxicity. Indeed, acute and sub-acute/chronic exposure may have different/opposite effects on the expression of some miRNAs (El Ouaamari et al. 2008, Esguerra et al. 2011, Sun et al. 2011). Altogether, these findings show the multiple levels at which glucotoxicity affects the β-cell phenotype as well as the various potential mechanisms of β-cell dedifferentiation in T2D. More large-scale studies, especially using human islets, human cell lines and/or human β-cells derived from induced pluripotent stem cells, are needed to validate and extend the findings in rodents and to understand the pathophysiological role of β-cell dedifferentiation in T2D.

In addition to the loss of insulin, transcription factor, metabolism and secretory pathway genes, a growing body of data indicates that the loss of the adaptive UPR is a key event that accompanies β-cell decompensation and progression towards diabetes.

Failure of the adaptive UPR and progression towards diabetes

β-cells are vulnerable to ER stress due to their elevated rate of insulin biosynthesis in response to glucose stimulation. Therefore, an operational and fine-tuned UPR is fundamental for the preservation of β-cell function and survival under physiological conditions, and even more under conditions of high insulin demand, such as obesity and pregnancy. Indeed, genetic disruption of several UPR components, including Perk, eIF2α, Ire1, Xbp1 and Dnajc3 leads to β-cell demise and the development of insulin-dependent diabetes in man and rodents (Delepine et al. 2000, Harding et al. 2001, Zhang et al. 2002a, Ladiges et al. 2005, Back et al. 2009, Iwawaki et al. 2010, Julier & Nicolino 2010, Lee et al. 2011, Synofzik et al. 2014, Xu et al. 2014, Han et al. 2015, Hassler et al. 2015). The UPR is thus an important trait of differentiated β-cells. Until recently, the accepted concept was that excessive stimulation of the UPR under chronic/unresolved ER stress plays an important role in β-cell decompensation and failure. This view was supported by several studies showing increased activation of ER stress sensors and downstream effectors and target genes in different in vitro and in vivo models in addition to human T2D (reviewed in Bensellam et al. 2012b). Furthermore, ablation of the ISR gene Ddit3 in different animal models of diabetes has been shown to be protective (Oyadomari et al. 2002, Song et al. 2008), while overexpression of Trib3 altered glucose tolerance and GSIS and increased β-cell apoptosis (Qian et al. 2008, Liew et al. 2010, Fang et al. 2014). Moreover, adenovirus-mediated overexpression of Atf6 in INS1 cells was associated with altered GSIS and reduced expression of insulin, Pdx1 and Mafa (Seo et al. 2008). Likewise, overexpression of Xbp1 in primary rat β-cells was correlated with impaired function, increased apoptosis and reduced expression of insulin, Pdx1, Mafa and Glut2 (Allagnat et al. 2010). As described earlier, chronic IRE1 activation has also been shown to alter β-cell differentiation via the degradation of ER-localized insulin mRNA (Pirot et al. 2007, Lipson et al. 2008, Han et al. 2009).

However, emerging evidence has challenged this concept by showing in different models of obesity and insulin resistance that the loss of β-cell differentiation and progression towards diabetes is accompanied by failure of the adaptive UPR. This led to the hypothesis that β-cell dysfunction in T2D may result from a failure of the UPR to adequately adapt rather than from a maladaptive UPR (Herbert & Laybutt 2016). By comparing time-dependent gene expression changes in islets of diabetes-prone (db/db mice on C57BL/KsJ genetic background) and diabetes-resistant (ob/ob mice on C57BL/6J genetic background) mouse models of obesity, we observed in ob/ob mice a progressive upregulation of adaptive UPR genes, including Hspa5, Fkbp11 and Dnajc3, in parallel with the maintenance of increased spliced/total Xbp1 (Xbp1s/t) mRNA ratio. In contrast, adaptive UPR genes were upregulated in prediabetic db/db mouse islets (6 weeks of age) but declined in diabetic mice (16 weeks of age) in parallel with a similar decline in Xbp1s/t mRNA ratio. Moreover, despite partial downregulation of some β-cell key transcription factor genes and other β-cell-enriched genes in prediabetic db/db mice and age-matched ob/ob mice, the β-cell phenotype recovered in 16-week-old ob/ob mice. On the contrary, there was a further loss of these genes in 16-week-old diabetic db/db mice. Interestingly, downregulation of β-cell-enriched genes in db/db mouse islets was partially restored with the chemical chaperone 4-phenylbutyrate suggesting that the inactivation of the adaptive UPR plays a role in β-cell dedifferentiation (Chan et al. 2013). Downregulation of adaptive UPR gene expression is unlikely to result from reduced proinsulin biosynthesis since the latter has been shown to be increased in diabetic db/db mice (Alarcon et al. 2016). These results were corroborated by similar observations in the islets of female ZDF (fZDF) rats. Thus, adaptive UPR genes were upregulated in the islets of obese, prediabetic fZDF rats fed a chow diet (model of β-cell compensation) in comparison to age-matched lean controls. However, UPR gene expression was downregulated in diabetic fZDF rats fed a high-fat diet (model of β-cell decompensation) in comparison to age-matched obese, prediabetic fZDF rats fed a chow diet, in parallel with altered β-cell differentiation (Omikorede et al. 2013). Noteworthy, in vitro, proteomic analysis of glucose-responsive (low passage; more differentiated) vs glucose non-responsive (high passage; less differentiated) MIN6 cells also revealed downregulation of HSPA5, HSP90B1, PDI and ERP29 (Dowling et al. 2006). In humans, a previous microarray study has revealed a significant downregulation of Hsp90b1 in the islet of T2D donors (Gunton et al. 2005). A more recent study has confirmed and extended this finding by showing that numerous UPR, ER-Golgi (retrograde)transport, ER quality control, ERAD and (retro)translocon-related genes were significantly downregulated in islets of T2D donors vs BMI-matched non-diabetic subjects, including Perk, Hsp90b1, Hspa8, Hspa13, Ero1β, Pdia6, Ppib, Erp44, Dnaja2, Dnajb9, Dnajc6, Dnajc9, Dnajc10, Dnajc12, Serp1, Man1a1, Cnx, Herpud1, Ssr1, Ssr3, Sec11a, Sec13, Sec23a, Sec23b, Sec23ip, Sec24a, Sec24b, Sec24d, Sec62, Sec63, Copb1, Copb2 and Copz1, in addition to downregulation of the ubiquitin-proteasome system genes and reduced proteasome activity (Bugliani et al. 2013). A following report examining protein expression of XBP1s, ATF6 and phosphorylated-eIF2α in pancreatic sections from non-BMI-matched normal, prediabetic and diabetic women with different durations of diabetes has shown an overall decreased staining in the islets of diabetic vs control and prediabetic women for the three markers (Engin et al. 2014). Collectively, these observations support the notion that adaptive UPR inactivation occurs during the progression towards diabetes in rodents and humans and may play an important role in β-cell dedifferentiation. But what could be the mechanism(s) behind this inactivation?

Hyperglycemia and the ensuing glucotoxicity are primary candidates. Indeed, downregulation of adaptive UPR and ISR gene expression has been observed in mouse islets transplanted into diabetic mice with a non-sufficient islet number, in comparison to those transplanted into non-diabetic control mice. These alterations were paralleled by similar changes in the expression of β-cell key transcription factors and other β-cell-enriched genes, thereby further supporting a link between β-cell dedifferentiation and failure of the UPR. Interestingly, these alterations were prevented in mouse islets transplanted into diabetic mice with a sufficient number to restore normoglycaemia, thereby suggesting that hyperglycaemia contributes to the downregulation of the UPR (Walters et al. 2013). Glucotoxicity exerts its deleterious effects via myriad mechanisms, including hypoxia (Sato et al. 2011, 2014, Bensellam et al. 2012a,b). We have recently provided evidence that hypoxia could be an important cellular mechanism for the inactivation of the adaptive UPR in T2D. Thus, we have shown that hypoxia-response genes were upregulated in the islets of diabetic, but not prediabetic, db/db mice in an inverse relationship with UPR gene expression (Fig. 4). We have demonstrated that hypoxia inhibits the adaptive UPR specifically in β-cells and that this inhibition was associated with impaired ER-to-Golgi protein trafficking and was implicated in increased apoptosis under hypoxic stress. These effects were mediated by the activation of JNK and Ddit3, but were independent of hypoxia-inducible factor 1 α (HIF1α) (Bensellam et al. 2016) (Fig. 4). Interestingly, JNK activation has also been observed in the islets of diabetic db/db mice and its inhibition increased adaptive UPR gene expression and reduced apoptosis (Chan et al. 2015).

Figure 4

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Figure 4

Schematic representation of the role of hypoxia and inflammation in the loss of adaptive UPR under chronic hyperglycemia. Under chronic hyperglycemia, the acceleration of mitochondrial metabolism and the stimulation of ATP consuming cellular processes, increases β-cell O2 consumption. On the other hand, chronic hyperglycemia negatively affects islet vasculature thereby resulting in reduced O2 supply. These events lead to β-cell hypoxia and activation of downstream pathways including c-Jun N-terminal kinase (JNK), double-stranded RNA-dependent protein kinase (PKR) and downstream DNA damage inducible transcript 3 (DDIT3). Activation of these effectors plays an important role in the inhibition of the adaptive unfolded protein response (UPR) under hypoxia. Inflammation induced by chronic hyperglycemia, but also by hypoxia, can also increase JNK phosphorylation and has been proposed to play a role in the failure of adaptive UPR. The timing of these complex events is represented in the insert. In the prediabetic phase (β-cell compensation), the UPR is markedly upregulated in parallel with a moderate activation of inflammatory signaling in the absence of signs of islet hypoxia. However, in the diabetic phase (β-cell decompensation), the adaptive UPR declines concomitantly with strong upregulation of inflammatory and hypoxia-response genes.

Citation: Journal of Endocrinology 236, 2; 10.1530/JOE-17-0516

In addition to hypoxia, inflammation is another potential mechanism that may be involved in UPR inactivation (Fig. 4). T2D is characterized by a low-grade inflammation and proinflammatory cytokines released by immune cells, endothelial cells and adipose tissue under chronic hyperglycemia have been proposed to contribute to islet inflammation and β-cell dedifferentiation (Donath & Shoelson 2011, Nordmann et al. 2017). Interestingly, cytokine treatment of mouse islets downregulated the mRNA levels of several adaptive UPR genes including Hspa5, Hsp90b1, Fkbp11 and Dnajc3 (Chan et al. 2011). Importantly, the progression towards diabetes in db/db mice was accompanied by a progressive upregulation of inflammatory genes including Il6, Il1β, Cxcl1, Tnfα, Ccl2 and the macrophages marker gene Cd68 (Fig. 4). This inflammatory signature was however absent in the islets of ob/ob mice (Chan et al. 2013). Moreover, assessment of time-dependent changes in XBP1s and ATF6 protein expression in the islets of mouse models of type 1 diabetes (T1D), which is an immune-mediated disease, revealed a progressive downregulation. These alterations were also observed on pancreatic section of humans T1D donors (Engin et al. 2013). Of note, hypoxia also upregulated inflammatory genes in mouse islets (Bensellam et al. 2016) (Fig. 4). Altogether, these observations suggest a role for inflammatory signalling in the downregulation of the adaptive UPR in T2D.

Very interestingly, in rat models of hypertrophic and failing heart, myocardial XBP1s and HSPA5 protein levels were upregulated in the early phase of the disease (adaptive phase) but declined in the late phase (maladaptive phase), very similarly to what has been observed in db/db mouse islets. In the cardiac context, it has been shown that miR-30-p3 family and miR-214 were implicated in XBP1s downregulation (Duan et al. 2015). Likewise, miRNAs may also be involved in UPR inactivation in β-cells. The expression of various miRNAs is affected in prediabetic and diabetic db/db mouse islets in comparison to their age-matched lean controls (Nesca et al. 2013), and miR-200 has been shown to negatively regulate Dnajc3 expression (Belgardt et al. 2015). Studies comparing the time-dependent changes in miRNA expression in obese models of β-cell compensation and decompensation are needed to shed more light on this possibility.

In sum, there is growing evidence that failure of the adaptive UPR in T2D is linked with β-cell dedifferentiation and dysfunction. Our evidence strongly supports the implication of hypoxia and very likely inflammatory signalling in UPR inactivation. But how the inactivation of the UPR could affect the differentiated β-cell phenotype is unclear at this stage. One possibility is that failure of the adaptive UPR exacerbates hyperglycemia-induced oxidative stress. Not only is the role of the latter in β-cell dedifferentiation well established, but oxidative stress is also linked with ER stress. Indeed, defective ER protein folding machinery and subsequent accumulation of unfolded proteins in the ER has been shown to trigger oxidative stress and damage in association with altered β-cell differentiation (Back et al. 2009, Han et al. 2015). Moreover, expression of a mutant form of proinsulin that is prone to misfolding in Akita mouse islets has also been associated with increased oxidative stress and damage (Yuan et al. 2012). Furthermore, treatment of glucose-infused rats with chemical chaperones reduced islet superoxide anion generation and preserved β-cell function (Tang et al. 2012). Thus, it is postulated that the inactivation of the UPR in T2D leads to progressive accumulation of unfolded proteins in the ER; this potentiates hyperglycemia-induced reactive oxygen species (ROS) production and oxidative stress, which leads to a gradual loss of β-cell identity. Future work in this field is needed to verify this hypothesis and explore the mechanism(s) by which protein misfolding in the ER induces/aggravates oxidative stress and whether a crosstalk between the ER and mitochondria may be involved.

Upregulation of β-cell forbidden genes

As well as the downregulation of β-cell-enriched genes (addressed above), β-cell dedifferentiation in T2D features upregulation of genes that are either suppressed or expressed at very low levels in mature β-cells under physiological conditions. These genes that we term as β-cell forbidden genes enclose various types including metabolic genes, transcriptional regulators, signalling effectors and stress response genes (Table 3 and Bensellam et al. 2012b). Upregulation of these genes under hyperglycemia has important consequences on β-cell identity and function.

Table 3

Upregulation of β-cell forbidden genes under glucotoxicity.

GenesGlucotoxicity modelConfirmation in human T2DGlucotoxic effect
In vitro/ex vivoIn vivo
Transcriptional regulators
C/EBPβHIT-T15 cells (Lu et al. 1997)

INS1 cells (Lu et al. 1997)
Px rat islets (Seufert et al. 1998)

ZDF rat islets (Seufert et al. 1998)
mRNA and protein
CremWistar and Sprague–Dawley rat islets (Bensellam et al. 2009, Zhou et al. 2003)mRNA
MycWistar rat islets (Bensellam et al. 2009, Elouil et al. 2005, Kaneto et al. 2002a)Px rat islets (Jonas et al. 2001, Jonas et al. 1999)

Gk rat islets (Lacraz et al. 2010)
mRNA
Id1Human islets (Wice et al. 2001)db/db mouse islets (Kjorholt et al. 2005, Chan et al. 2013, Bensellam et al. 2015)mRNA and protein
Id2GK rat islets (Homo-Delarche et al. 2006)

db/db mouse islets (Bensellam et al. 2015)
mRNA
Id3db/db mouse islets (Bensellam et al. 2015)mRNA and protein
Id4db/db mouse islets (Bensellam et al. 2015)mRNA
Ngn3db/db mouse islets (Talchai et al. 2012)

KATP-GOF mice (Wang et al. 2014)

GIRKO mouse islets (Talchai et al. 2012)

Multiparous and ageing FoxO1-KO mice (Talchai et al. 2012)
Protein
NanogMultiparous FoxO1-KO mice (Talchai et al. 2012)Protein
Oct4db/db mouse islets (Talchai et al. 2012)

GIRKO mouse islets (Talchai et al. 2012)

Multiparous FoxO1-KO mice (Talchai et al. 2012)
Protein
L-Mycdb/db mouse islets (Talchai et al. 2012)

GIRKO mouse islets (Talchai et al. 2012)

Multiparous FoxO1-KO mice (Talchai et al. 2012)
Protein
Sox9Yes: (Marselli et al. 2010, Bugliani et al. 2013)mRNA
Metabolic genes
Hk1Px rat islets (Jonas et al. 1999)

ZDF rat islets (Cockburn et al. 1997)

db/db mouse islets (Kjorholt et al. 2005)

Psammomys obesus (Nesher et al. 1999)
mRNA and enzymatic activity
Hk2Rat islets (Ghanaat-Pour et al. 2007)mRNA
G6pasePx rat islets (Laybutt et al. 2002b)

GK rat islets (Ling et al. 2001)
mRNA and enzymatic activity
Fbp1Px rat islets (Laybutt et al. 2002b)mRNA
Pck1Human islets (Shalev et al. 2002)Yes: (Marselli et al. 2010)mRNA
LdhaWistar rat islets (Bensellam et al. 2009, Bensellam et al. 2012a)Px rat islets (Jonas et al. 1999, Laybutt et al. 2003)

GK rat islets (Homo-Delarche et al. 2006, Sasaki et al. 2013)

ZDF rat islets (Li et al. 2006b)

Rat islets transplanted into diabetic rats (Laybutt et al. 2007)
Yes: (Marselli et al. 2010)mRNA, protein and lactate production
Mct1Px rat islets (Laybutt et al. 2002b)mRNA
Mct2Px rat islets (Laybutt et al. 2002b)mRNA
Mct4Wistar rat islets (Bensellam et al. 2012a)Px rat islets (Laybutt et al. 2002b)mRNA
Ucp2Wistar rat islets (Khaldi et al. 2004)Glucose-infused rats (Kassis et al. 2000)

Px rat islets (Laybutt et al. 2002b)

ZDF rat islets (Oberkofler et al. 2009)

db/db mouse islets (Kjorholt et al. 2005)

ob/ob mouse islets (Zhang et al. 2001)
mRNA and protein
Acot7ZDF rat islets (Parton et al. 2006)Yes: (Marselli et al. 2010)mRNA
Aldh1a3db/db mouse islets (Ishida et al. 2017, Kim-Muller et al. 2016)

Abcc8-KO mice (Stancill et al. 2017)
Yes: (Cinti et al. 2016)mRNA, protein and enzymatic activity

Metabolic genes  A major earlier and recent focus in this field has been on metabolic genes, in particular Ldha and Mct1. Pioneering studies have shown that LDHA activity is markedly lower in β-cells in comparison to other islet cells, exocrine cells and hepatocytes (Hellman & Taljedal 1967, Sekine et al. 1994). Ensuing large-scale studies comparing gene expression patterns of different mouse tissues with pancreatic islets confirmed low expression of Ldha and Mct1 and identified other β-cell forbidden genes (Pullen et al. 2010, Thorrez et al. 2011). Interestingly, Ldha and various Mct isoform genes were upregulated in the islets of rodent models of hyperglycemia and diabetes (Jonas et al. 1999, Laybutt et al. 2002b, 2007, Homo-Delarche et al. 2006, Li et al. 2006b). Upregulation of Ldha has also been reported in β-cells of human T2D subjects (Marselli et al. 2010) (Table 3). The role of hyperglycemia in this induction has been demonstrated by phlorizin treatment experiments in Px rats (Jonas et al. 1999, Laybutt et al. 2002b). In addition, we have previously reported upregulation of Ldha mRNA levels, along with other glycolytic and hypoxia-response genes, in cultured rat islets exposed to elevated glucose levels (Bensellam et al. 2009). The prevention of this induction by hyperoxia suggested an implication of high glucose-induced β-cell hypoxia and activation of HIF1/2α (Bensellam et al. 2012a). Another pathway implicating oxidative stress may be involved in LDHA induction in the islets of GK rats (Sasaki et al. 2013). Evidence has also shown a role of miRNAs in the regulation of Mct1 expression (Pullen et al. 2011, Martinez-Sanchez et al. 2015). But whether altered expression of specific miRNAs in T2D is involved in the upregulation of Mct1 requires further proof. Upregulation of Ldha and Mct genes may alter β-cell function by diverting the glycolytic flux from mitochondrial pyruvate oxidation to lactate production and transport. This could alter glucose homeostasis by sensitising β-cells to secrete insulin in response to non-physiological stimuli (Ishihara et al. 1999, Ainscow et al. 2000, Otonkoski et al. 2007, Pullen et al. 2012).

As explained in ‘Relationship of β-cell differentiation to insulin secretory function’ section, the repression of high-affinity type I–III hexokinases in β-cells prevents insulin release in response to low non-stimulatory glucose levels such as in the fasted state (Sekine et al. 1994, Schuit et al. 1999). In the context of diabetes, increased mRNA levels of Hk1 have been reported in Px rats and db/db mouse islets (Jonas et al. 1999, Kjorholt et al. 2005). In agreement, HK enzymatic activity was upregulated in the islets of ZDF rats (Cockburn et al. 1997), DBA/2 mice (Kooptiwut et al. 2002) and Psammomys obesus on high energy diet (Nesher et al. 1999). Moreover, exposure of rat islets to elevated glucose levels in vitro upregulated the mRNA levels of Hk2 (Ghanaat-Pour et al. 2007) (Table 3). Upregulation of Hks, and likely other glycolytic genes, may explain the reduced threshold glucose concentration for the stimulation of insulin secretion (glucose hypersensitivity) observed in INS1 cells (Roche et al. 1997), rat and mouse islets (Khaldi et al. 2004, Bensellam et al. 2009, Pascal et al. 2010) and purified rat and human β-cells (Ling & Pipeleers 1996, Ling et al. 1996) cultured in the presence of supraphysiological glucose levels, as well as in the islets of animal models of diabetes, including glucose-infused rats (Ammon et al. 1998), Px rats (Leahy et al. 1993, Hosokawa et al. 1995), ZDF rats (Tokuyama et al. 1995), ob/ob mice (Chen et al. 1993) and Psammomys obesus (Pertusa et al. 2002). In support of this hypothesis, overexpression of Hk1 in MIN6 cells and primary rat islets has been shown to increase the basal glycolytic flux and insulin release along with altered GSIS (Becker et al. 1994, Ishihara et al. 1994).

Other metabolic genes presenting low expression under physiological conditions and upregulated in diabetes include the gluconeogenic genes G6pase (Ling et al. 2001, Laybutt et al. 2002b), Fbp1 (Laybutt et al. 2002b) and Pck1 (Marselli et al. 2010). The latter gene was also upregulated in isolated human islets cultured in the presence of high glucose levels (Shalev et al. 2002) (Table 3). Upregulation of G6pase may divert glucose from its classical metabolic route by inducing glucose cycling (GC). GC is a futile cycle where glucose is phosphorylated to glucose-6-phosphate by GCK and then dephosphorylated to glucose by glucose-6-phosphatase (G6Pase) with consumption of an ATP molecule. Indeed, G6pase overexpression in INS1 cells has been shown to increase GC and reduce GSIS (Trinh et al. 1997). Upregulation of G6pase mRNA and enzymatic activity in the islets of Px and GK rats was reversed by phlorizin treatment thereby underscoring the role of hyperglycemia (Ling et al. 2001, Laybutt et al. 2002b).

Another β-cell forbidden gene, Acot7 (Pullen et al. 2010), which encodes for mitochondrial and cytosolic isoforms of acyl-CoA thioesterase 7 (ACOT7), hydrolyses long-chain acyl-CoA esters. Its expression was upregulated in the islets of ZDF rats and β-cells of human T2D patients (Parton et al. 2006, Marselli et al. 2010) (Table 3). Interestingly, repression of Acot7 in mouse has been proposed in a recent study to be necessary for adequate insulin secretion. Indeed, selective overexpression of the mitochondrial Acot7 in β-cells of adult mice impaired glucose tolerance as a consequence of altered insulin secretion (Martinez-Sanchez et al. 2016). However, further work is needed to determine the precise mechanism(s) involved.

Recently, Aldh1a3 has been proposed as a novel β-cell forbidden gene and putative marker of dedifferentiated β-cells (Kim-Muller et al. 2016). Aldehyde dehydrogenase family 1 member A3 (ALDH1A3) expression and activity were upregulated in the islets of various animal models including db/db mice (Kim-Muller et al. 2016, Ishida et al. 2017, Stancill et al. 2017). Increased ALDH1A3 immunostaining has also been confirmed in the islets of diabetic human subjects and was associated in some β-cells with cytoplasmic expression of NKX6.1 (Cinti et al. 2016) (Table 3). However, overexpression of Aldh1a3 in MIN6 cells did not alter the expression of β-cell-enriched genes and was even associated in mouse islets with a slight potentiation of GSIS thereby ruling out a causative role of ALDH1A3 in β-cell dedifferentiation (Kim-Muller et al. 2016). Aldh1a3 is a detoxifying enzyme that catalyses the oxidation of retinal to retinol using NAD+. It is also involved in other metabolic processes including amino acid metabolism and the metabolism of lipid peroxidation products (Duan et al. 2016). Therefore, its upregulation in the islets of diabetic animals and T2D subjects could be a response to oxidative stress and the ensuing damage.

Transcriptional regulators  Besides metabolic genes, β-cell forbidden genes also enclose transcriptional regulators. We have described in ‘Down regulation of insulin gene expression’ section the role of C/EBPβ, MYC and ETS1 in the alteration of insulin gene expression. In addition, several transcription factors typically expressed in progenitor cells at the embryonic stage and repressed in adult β-cells were upregulated in β-cells of various diabetic animal models, including neurogenin 3 (NGN3), nanog homeobox (NANOG), octamer-binding transcription factor 4 (OCT4 also known as POU5F1) and v-myc avian myelocytomatosis viral oncogene homolog 1, lung carcinoma derived (L-Myc) (Talchai et al. 2012, Brereton et al. 2014, Wang et al. 2014) (Table 3). Lineage trancing studies confirmed that these dedifferentiated cells originate from cells that expressed insulin and also suggested their transdifferentiation in other islet non-β-cells, including α-cells (Talchai et al. 2012, Wang et al. 2014) (Fig. 2). Interestingly, NGN3 reexpression in the islets of a diabetic mouse model with KATP channel mutation was reversed by insulin therapy (Wang et al. 2014). This observation is of importance. Together with the phlorizin and caloric restriction studies mentioned in previous sections, these reports highlight the astonishing plasticity of the β-cell phenotype and indicate the possibility to reverse β-cell dedifferentiation, at least in rodents. Furthermore, a recent study has also shown the possibility to prevent downregulation of some β-cell key transcription factor genes in vitro under cytokine treatment and in vivo in db/db, ob/ob and Akita mice using small-molecule inhibitors of the tumour growth factor β (TGFβ) pathway that target the TGFβ receptor I (also known as ALK5) (Blum et al. 2014). In humans, to date and to our knowledge, there is no confirmation of such expression of endocrine progenitor transcription factors in β-cells of T2D patients, except for upregulation of POU5F1 mRNA levels (Guo et al. 2013). However, two studies have reported increased mRNA levels of the pre-endocrine gene SOX9 (Marselli et al. 2010, Bugliani et al. 2013). Evidence suggests that SOX9 upregulation in T2D may result from hypoxia and activation of HIF1 and could play a role in the downregulation of β-cell-enriched genes (Puri et al. 2013).

Inhibitor of differentiation (ID) proteins are another class of transcriptional regulators induced in response to glucose stimulation in β-cell lines and human islets (Wice et al. 2001) and upregulated in the islets of diabetic db/db mice and GK rats (Kjorholt et al. 2005, Homo-Delarche et al. 2006, Bensellam et al. 2015) (Table 3). They have been proposed to act as repressors of basic helix-loop-helix (bHLH) transcription factors thereby modulating cell differentiation and proliferation (Lasorella et al. 2014, Nair et al. 2014). Upregulation of Id1 under high-fat diet has been implicated in the inhibition of insulin secretion and alteration of glucose homeostasis (Akerfeldt & Laybutt 2011). We have subsequently demonstrated that IDs are a novel class of oxidative stress-responsive genes in β-cells. Surprisingly, under oxidative stress conditions, deletion of Id1 and/or Id3 resulted in mitochondrial impairment and induced a global attenuation of antioxidant gene expression in association with a further increase in intracellular ROS levels and apoptosis. These effects involved a novel interaction of IDs with the NFE2L2-small Mafs antioxidant pathway (Bensellam et al. 2015). These findings suggest that IDs are key components of the antioxidant response and play an important role in the promotion of β-cell survival under oxidative stress. However, these observations also raise the question of whether dedifferentiation could be an adaptive mechanism to escape cell death under stress conditions (discussed in the next section).

Stress response genes  Chronic hyperglycemia exposes β-cells to higher levels of stress than under physiological conditions. To face such threat, β-cells upregulate endogenous stress responses to uphold homeostasis. Thus, the induction and the regulatory mechanisms of the antioxidant and the ER stress responses have received much attention given the important role of these pathways in β-cell pathophysiology (reviewed in Bensellam et al. 2012b). We have described in ‘Failure of the adaptive UPR and progression towards diabetes’ section the regulation of the UPR in diabetes and the emerging evidence linking the failure of this response with the progression towards diabetes and β-cell dedifferentiation. In the following paragraphs, we will focus on the antioxidant response and its potential implication in β-cell dedifferentiation.

Upregulation of antioxidant gene expression in response to elevated glucose levels has been well documented in vitro and in diabetic animals, and has also been confirmed in human T2D for some antioxidant genes (reviewed in Bensellam et al. 2012b). The primary role of the antioxidant response is to relieve hyperglycemia-induced oxidative stress and promote cell survival. Nevertheless, β-cell mass is reduced by about 60% in GK rats although the mRNA and protein levels of Nfe2l2 and several other antioxidant genes were markedly upregulated (Movassat et al. 1997, Lacraz et al. 2009). Moreover, the β-cell differentiated phenotype is noticeably altered in Px rats despite strong upregulation of the mRNA levels of Hmox1 and Gpx1 (Jonas et al. 1999, Laybutt et al. 2002a). Similarly, β-cell mass is reduced in human T2D patients even with upregulated mRNA levels of several antioxidant genes including various isoforms of metallothioneins (Marchetti et al. 2004, Marselli et al. 2010). These observations suggest that either the endogenous antioxidant response triggered by hyperglycemia is not robust enough to efficiently protect the β-cell differentiated phenotype or its strong and sustained activation under chronic hyperglycemia is deleterious and may play a role in β-cell dedifferentiation.

In support of the first possibility, previous evidence has shown that rat islets express low levels of several antioxidant genes in comparison to other tissues (Lenzen et al. 1996, Tiedge et al. 1997). The H2O2 detoxifying enzyme catalase is specifically repressed in adult mouse β-cells in comparison to other tissues (Thorrez et al. 2011, Pullen et al. 2017). Lower GPX1 expression and activity has also been confirmed in isolated human islets (Tonooka et al. 2007). In highly metabolically active β-cells, such low antioxidant gene expression signature is undoubtedly an Achilles’ heel, especially under prolonged hyperglycemia. Interestingly, β-cell-specific overexpression of Gpx1 in db/db mice prevented the loss of the β-cell-enriched genes MAFA, NKX6.1 and GLUT2 (Harmon et al. 2009, Guo et al. 2013). Similarly, antioxidant supplementation has been proven beneficial, at least in vitro and in rodent models (reviewed in Bensellam et al. 2012b). Why β-cells exhibit lower antioxidant gene expression in comparison to other tissues is unclear. However, some studies have proposed that physiological levels of H2O2 may play the role of a coupling factor for GSIS (Pi et al. 2007, Leloup et al. 2009). In addition, it has been shown that catalase or metallothionein overexpression in β-cells, while reducing cytokine-induced ROS generation, accelerated diabetes onset in male NOD mice in association with increased β-cell death and reduced PDX1 protein levels (Li et al. 2006a). It should be emphasized here that the antioxidant arsenal of β-cells is not limited to the classically studied genes such as Gpx1 and catalase but encloses many other players including those of the glutathione system, the peroxiredoxin system, the thioredoxin system, haem oxygenases and metallothioneins and many of these genes are either expressed at high levels under basal conditions or markedly upregulated under stress (Bensellam et al. 2009). The scenery is even more complex as some antioxidant genes can play other roles and be involved in different cellular processes such as the zinc buffering properties of metallothioneins (Chabosseau & Rutter 2016) or the involvement in iron metabolism for Hmox1 (Gozzelino & Soares 2014). Therefore, one may propose that sustained upregulation of this wide battery of antioxidant genes under chronic hyperglycemia may exert a negative impact on the β-cell differentiated phenotype by yet to be defined mechanisms.

Dedifferentiation as a potential adaptive mechanism

Despite marked alteration of the β-cell differentiated phenotype and reduction of β-cell mass in human T2D, the β-cell apoptosis rate remains relatively low. This phenomenon is also observed in experimental models such as Px rats, db/db mice, Foxo1-KO mice and KATP-GOF mice. An emerging body of data suggests that β-cell dedifferentiation could be an adaptive mechanism to escape cell death under chronic hyperglycemia at the price of altered identity and function.

As mentioned in the previous section, we found that upregulation of IDs (inhibitors of bHLH transcription factors) during oxidative stress played a key role in maintaining the antioxidant response and mitochondrial integrity and thereby promoting β-cell survival (Bensellam et al. 2015). In addition, previous reports have shown that β-cell line maturation from a glucagon-producing phenotype to an insulin-producing phenotype was associated with increased sensitivity to several toxic agents such as cytokines, streptozotocin and alloxan (Nielsen et al. 1999). Further studies revealed an important role of increased expression of Pdx1 and Nkx6.1 in this acquired sensitivity (Nielsen et al. 2004). The potential link between dedifferentiation and adaptation/resistance to stress seems to be a conserved biological mechanism that has also been described in other cell types and models including cancer cells (Del Vecchio et al. 2014, Li et al. 2016, Roesch et al. 2016) and even plant cells (Grafi & Barak 2015). Future work is needed to validate this hypothesis in β-cells and to fully characterize the response of dedifferentiated β-cells to different types of stress.

From a therapeutic perspective, alleviating β-cell stress may provide a novel strategy to reverse the process of dedifferentiation and restore functional β-cell mass. Previous β-cell rest approaches have shown promising results in vitro and in animal models (Grill et al. 2009). Noteworthy, in isolated rat islets, high glucose-induced upregulation of several β-cell forbidden genes and other stress response genes such as Ldha, Crem, Myc, Adm and Hmox1 was markedly prevented by diazoxide treatment (Jonas et al. 2001, 2003, Ma et al. 2007). However, until now, these methods failed to improve glycemic control in T2D patients (reviewed in Bensellam et al. 2012b). Given the key role of hyperglycemia in β-cell dedifferentiation and the demonstrated beneficial effects of the glucose lowering strategies in rodents, SGLT2 inhibition by gliflozins merits further attention in humans (Abdul-Ghani et al. 2017). The use of these drugs in combination with other therapies such as incretins may exert beneficial effects on the β-cell phenotype (Busch & Kane 2017, Kaneto et al. 2017). The potential combination with antioxidants such as GPX mimetics and/or chemical chaperones such as the recently identified small-molecule azoramide (Fu et al. 2015) merits further experimental and clinical testing.

Conclusion

Growing evidence indicates that β-cell dedifferentiation plays a key role in the loss of functional β-cell mass in T2D. Over the last decades, in vitro and animal studies implicated glucotoxicity and downstream pathways in this process. Dysregulation of numerous – and not fully identified – transcriptional regulators, genes, miRNAs, stress responses, metabolic processes and epigenetic mechanisms converge to compromise β-cell identity in T2D. Based on emerging data and observations in other cell types and models, we proposed that β-cell dedifferentiation could be an adaptive mechanism to escape cell death under stress conditions. Interestingly, animal studies suggest the possibility to reverse β-cell dedifferentiation by reducing β-cell stress strategies. More large-scale studies using human β-cells are needed to validate the findings in rodents and shed more light on the different signalling effectors and pathways involved in the alteration of the β-cell differentiated phenotype. A better understanding of the physiological and pathological regulation of these pathways may pave the way to reverse β-cell dedifferentiation and restore the functional β-cell mass in human T2D.

Supplementary data

This is linked to the online version of the paper at https://doi.org/10.1530/JOE-17-0516.

Declaration of interest

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

Funding

M B is supported by a MOVE-in Louvain/EC Marie-Curie incoming postdoctoral fellowship. J C J is Research Director of the Fonds de la Recherche Scientifique-FNRS, Belgium. D R L is supported by an Australian Research Council (ARC) Future Fellowship and grants from the National Health and Medical Research Council (NHMRC) of Australia and the Diabetes Australia Research Program.

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