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Thomas H Claus, Clark Q Pan, Joanne M Buxton, Ling Yang, Jennifer C Reynolds, Nicole Barucci, Michael Burns, Astrid A Ortiz, Steve Roczniak, James N Livingston, Kevin B Clairmont, and James P Whelan

Type 2 diabetes is characterized by reduced insulin secretion from the pancreas and overproduction of glucose by the liver. Glucagon-like peptide-1 (GLP-1) promotes glucose-dependent insulin secretion from the pancreas, while glucagon promotes glucose output from the liver. Taking advantage of the homology between GLP-1 and glucagon, a GLP-1/glucagon hybrid peptide, dual-acting peptide for diabetes (DAPD), was identified with combined GLP-1 receptor agonist and glucagon receptor antagonist activity. To overcome its short plasma half-life DAPD was PEGylated, resulting in dramatically prolonged activity in vivo. PEGylated DAPD (PEG-DAPD) increases insulin and decreases glucose in a glucose tolerance test, evidence of GLP-1 receptor agonism. It also reduces blood glucose following a glucagon challenge and elevates fasting glucagon levels in mice, evidence of glucagon receptor antagonism. The PEG-DAPD effects on glucose tolerance are also observed in the presence of the GLP-1 antagonist peptide, exendin(9–39). An antidiabetic effect of PEG-DAPD is observed in db/db mice. Furthermore, PEGylation of DAPD eliminates the inhibition of gastrointestinal motility observed with GLP-1 and its analogues. Thus, PEG-DAPD has the potential to be developed as a novel dual-acting peptide to treat type 2 diabetes, with prolonged in vivo activity, and without the GI side-effects.

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Linda Ahlkvist, Bilal Omar, Anders Valeur, Keld Fosgerau, and Bo Ahrén

Stimulation of insulin secretion by short-term glucagon receptor (GCGR) activation is well characterized; however, the effect of long-term GCGR activation on β-cell function is not known, but of interest, since hyperglucagonemia occurs early during development of type 2 diabetes. Therefore, we examined whether chronic GCGR activation affects insulin secretion in glucose intolerant mice. To induce chronic GCGR activation, high-fat diet fed mice were continuously (2 weeks) infused with the stable glucagon analog ZP-GA-1 and challenged with oral glucose and intravenous glucose±glucagon-like peptide 1 (GLP1). Islets were isolated to evaluate the insulin secretory response to glucose±GLP1 and their pancreas were collected for immunohistochemical analysis. Two weeks of ZP-GA-1 infusion reduced insulin secretion both after oral and intravenous glucose challenges in vivo and in isolated islets. These inhibitory effects were corrected for by GLP1. Also, we observed increased β-cell area and islet size. We conclude that induction of chronic ZP-GA-1 levels in glucose intolerant mice markedly reduces insulin secretion, and thus, we suggest that chronic activation of the GCGR may contribute to the failure of β-cell function during development of type 2 diabetes.

Free access

N M Whalley, L E Pritchard, D M Smith, and A White

Proglucagon is cleaved to glucagon by prohormone convertase 2 (PC2) in pancreatic α-cells, but is cleaved to glucagon-like peptide-1 (GLP-1) by PC1 in intestinal L-cells. The aim of this study was to identify mechanisms which switch processing of proglucagon to generate GLP-1 in the pancreas, given that GLP-1 can increase insulin secretion and β-cell mass. The α-cell line, αTC1-6, expressed PC1 at low levels and GLP-1 was detected in cells and in culture media. GLP-1 was also found in isolated human islets and in rat islets cultured for 7 days. High glucose concentrations increased Pc1 gene expression and PC1 protein in rat islets. High glucose (25 mM) also increased GLP-1 but decreased glucagon secretion from αTC1-6 cells suggesting a switch in processing to favour GLP-1. Three G protein-coupled receptors, GPR120, TGR5 and GPR119, implicated in the release of GLP-1 from L-cells are expressed in αTC1-6 cells. Incubation of these cells with an agonist of TGR5 increased PC1 promoter activity and GLP-1 secretion suggesting that this is a mechanism for switching processing to GLP-1 in the pancreas. Treatment of isolated rat islets with streptozotocin caused β-cell toxicity as evidenced by decreased glucose-stimulated insulin secretion. This increased GLP-1 but not glucagon in the islets. In summary, proglucagon can be processed to GLP-1 in pancreatic cells. This process is upregulated by elevated glucose, activation of TGR5 and β-cell destruction. Understanding this phenomenon may lead to advances in therapies to protect β-cell mass, and thereby slow progression from insulin resistance to type 2 diabetes.

Free access

L M McShane, N Irwin, D O’Flynn, Z J Franklin, C M Hewage, and F P M O’Harte

Ablation of glucagon receptor signaling represents a potential treatment option for type 2 diabetes (T2DM). Additionally, activation of glucose-dependent insulinotropic polypeptide (GIP) receptor signaling also holds therapeutic promise for T2DM. Therefore, this study examined both independent and combined metabolic actions of desHis1Pro4Glu9(Lys12PAL)-glucagon (glucagon receptor antagonist) and d-Ala2GIP (GIP receptor agonist) in diet-induced obese mice. Glucagon receptor binding has been linked to alpha-helical structure and desHis1Pro4Glu9(Lys12PAL)-glucagon displayed enhanced alpha-helical content compared with native glucagon. In clonal pancreatic BRIN-BD11 beta-cells, desHis1Pro4Glu9(Lys12PAL)-glucagon was devoid of any insulinotropic or cAMP-generating actions, and did not impede d-Ala2GIP-mediated (P<0.01 to P<0.001) effects on insulin and cAMP production. Twice-daily injection of desHis1Pro4Glu9(Lys12PAL)-glucagon or d-Ala2GIP alone, and in combination, in high-fat-fed mice failed to affect body weight or energy intake. Circulating blood glucose levels were significantly (P<0.05 to P<0.01) decreased by all treatments regimens, with plasma and pancreatic insulin elevated (P<0.05 to P<0.001) in all mice receiving d-Ala2GIP. Interestingly, plasma glucagon concentrations were decreased (P<0.05) by sustained glucagon inhibition (day 28), but increased (P<0.05) by d-Ala2GIP therapy, with a combined treatment resulting in glucagon concentration similar to saline controls. All treatments improved (P<0.01) intraperitoneal and oral glucose tolerance, and peripheral insulin sensitivity. d-Ala2GIP-treated mice showed increased glucose-induced insulin secretion in response to intraperitoneal and oral glucose. Metabolic rate and ambulatory locomotor activity were increased (P<0.05 to P<0.001) in all desHis1Pro4Glu9(Lys12PAL)-glucagon-treated mice. These studies highlight the potential of glucagon receptor inhibition alone, and in combination with GIP receptor activation, for T2DM treatment.

Full access

Olena A Fedorenko, Pawitra Pulbutr, Elin Banke, Nneoma E Akaniro-Ejim, Donna C Bentley, Charlotta S Olofsson, Sue Chan, and Paul A Smith

L-type channel antagonists are of therapeutic benefit in the treatment of hyperlipidaemia and insulin resistance. Our aim was to identify L-type voltage-gated Ca2+ channels in white fat adipocytes, and determine if they affect intracellular Ca2+, lipolysis and lipogenesis. We used a multidisciplinary approach of molecular biology, confocal microscopy, Ca2+ imaging and metabolic assays to explore this problem using adipocytes isolated from adult rat epididymal fat pads. CaV1.2, CaV1.3 and CaV1.1 alpha1, beta and alpha2delta subunits were detected at the gene expression level. The CaV1.2 and CaV1.3 alpha1 subunits were identified in the plasma membrane at the protein level. Confocal microscopy with fluorescent antibodies labelled CaV1.2 in the plasma membrane. Ca2+ imaging revealed that the intracellular Ca2+ concentration, [Ca2 +]i was reversibly decreased by removal of extracellular Ca2+, an effect mimicked by verapamil, nifedipine and Co2+, all blockers of L-type channels, whereas the Ca2+ channel agonist BAY-K8644 increased [Ca2+]i. The finding that the magnitude of these effects correlated with basal [Ca2+]i suggests that adipocyte [Ca2+]i is controlled by L-type Ca2+ channels that are constitutively active at the adipocyte depolarized membrane potential. Pharmacological manipulation of L-type channel activity modulated both basal and catecholamine-stimulated lipolysis but not insulin-induced glucose uptake or lipogenesis. We conclude that white adipocytes have constitutively active L-type Ca2+ channels which explains their sensitivity of lipolysis to Ca2+ channel modulators. Our data suggest CaV1.2 as a potential novel therapeutic target in the treatment of obesity.

Free access

Paige V Bauer and Frank A Duca

The rising global rates of type 2 diabetes and obesity present a significant economic and social burden, underscoring the importance for effective and safe therapeutic options. The success of glucagon-like-peptide-1 receptor agonists in the treatment of type 2 diabetes, along with the potent glucose-lowering effects of bariatric surgery, highlight the gastrointestinal tract as a potential target for diabetes treatment. Furthermore, recent evidence suggests that the gut plays a prominent role in the ability of metformin to lower glucose levels. As such, the current review highlights some of the current and potential pathways in the gut that could be targeted to improve glucose homeostasis, such as changes in nutrient sensing, gut peptides, gut microbiota and bile acids. A better understanding of these pathways will lay the groundwork for novel gut-targeted antidiabetic therapies, some of which have already shown initial promise.

Free access

A Shirakami, T Toyonaga, K Tsuruzoe, T Shirotani, K Matsumoto, K Yoshizato, J Kawashima, Y Hirashima, N Miyamura, CR Kahn, and E Araki

Insulin receptor substrate 1 (IRS-1) gene polymorphisms have been identified in type 2 diabetic patients; however, it is unclear how such polymorphisms contribute to the development of diabetes. Here we introduced obesity in heterozygous IRS-1 knockout (IRS-1(+/-)) mice by gold-thioglucose (GTG) injection and studied the impact of reduced IRS-1 expression on obesity-linked insulin resistance. GTG injection resulted in approximately 30% weight gain in IRS-1(+/-) and wild type (WT) mice, compared with saline-injected controls. There was no difference in insulin sensitivity between lean IRS-1(+/-) and lean WT. Elevated fasting insulin levels but no change in fasting glucose were noted in obese IRS-1(+/-) and WT compared with the respective lean controls. Importantly, fasting insulin in obese IRS-1(+/-) was 1.5-fold higher (P<0.05) than in obese WT, and an insulin tolerance test showed a profound insulin resistance in obese IRS-1(+/-) compared with obese WT. The islets of obese IRS-1(+/-) were 1.4-fold larger than those of obese WT. The expression of insulin receptor and IRS-1 and IRS-2 was decreased in obese IRS-1(+/-), which could in part explain the profound insulin resistance in these mice. Our results suggest that IRS-1 is the suspected gene for type 2 diabetes and its polymorphisms could worsen insulin resistance in the presence of other additional factors, such as obesity.

Free access

BD Green, MH Mooney, VA Gault, N Irwin, CJ Bailey, P Harriott, B Greer, FP O'Harte, and PR Flatt

Glucagon-like peptide-1(7-36)amide (GLP-1) possesses several unique and beneficial effects for the potential treatment of type 2 diabetes. However, the rapid inactivation of GLP-1 by dipeptidyl peptidase IV (DPP IV) results in a short half-life in vivo (less than 2 min) hindering therapeutic development. In the present study, a novel His(7)-modified analogue of GLP-1, N-pyroglutamyl-GLP-1, as well as N-acetyl-GLP-1 were synthesised and tested for DPP IV stability and biological activity. Incubation of GLP-1 with either DPP IV or human plasma resulted in rapid degradation of native GLP-1 to GLP-1(9-36)amide, while N-acetyl-GLP-1 and N-pyroglutamyl-GLP-1 were completely resistant to degradation. N-acetyl-GLP-1 and N-pyroglutamyl-GLP-1 bound to the GLP-1 receptor but had reduced affinities (IC(50) values 32.9 and 6.7 nM, respectively) compared with native GLP-1 (IC(50) 0.37 nM). Similarly, both analogues stimulated cAMP production with EC(50) values of 16.3 and 27 nM respectively compared with GLP-1 (EC(50) 4.7 nM). However, N-acetyl-GLP-1 and N-pyroglutamyl-GLP-1 exhibited potent insulinotropic activity in vitro at 5.6 mM glucose (P<0.05 to P<0.001) similar to native GLP-1. Both analogues (25 nM/kg body weight) lowered plasma glucose and increased plasma insulin levels when administered in conjunction with glucose (18 nM/kg body weight) to adult obese diabetic (ob/ob) mice. N-pyroglutamyl-GLP-1 was substantially better at lowering plasma glucose compared with the native peptide, while N-acetyl-GLP-1 was significantly more potent at stimulating insulin secretion. These studies indicate that N-terminal modification of GLP-1 results in DPP IV-resistant and biologically potent forms of GLP-1. The particularly powerful antihyperglycaemic action of N-pyroglutamyl-GLP-1 shows potential for the treatment of type 2 diabetes.

Free access

Yoko Yagishita, Akira Uruno, Dionysios V Chartoumpekis, Thomas W Kensler, and Masayuki Yamamoto

The transcription factor Nrf2 (NF-E2-related factor 2) plays a critical role in oxidative stress responses. Although activation of Nrf2 signaling is known to exert anti-inflammatory effects, the function of Nrf2 in inflammation-mediated autoimmune disorders, such as type 1 diabetes, is not well established. To address the roles of Nrf2 in protection against autoreactive T-cell-induced type 1 diabetes, we used non-obese diabetic (NOD) mice, which are a polygenic model of human type 1 diabetes, to generate a genetic model for assessment of the contribution of Nrf2 activation to prevention and/or treatment of type 1 diabetes. Because Keap1 (Kelch-like ECH-associated protein 1) negatively regulates Nrf2, we used Keap1 gene knockdown driven by either hypomorphic or knockout Keap1 alleles, which enhanced Nrf2 signaling to moderate or excess levels, respectively. Nrf2 activation in the NOD::Keap1 FA/ mice inhibited T-cell infiltration within or near the islets, ameliorated impairment of insulin secretion and prevented the development of diabetes mellitus. Notably, Nrf2 activation decreased both the plasma interferon-γ (IFN-γ) levels and the IFN-γ-positive cell numbers in the pancreatic islets. The amelioration of diabetes was also observed in the NOD mice with two hypomorphic Keap1 alleles (Keap1 FA/FA) by intermediate activation of Nrf2. Both NOD::Keap1 FA/ and NOD::Keap1 FA/FA mice had a decreased incidence of diabetes mellitus, demonstrating that activation of Nrf2 signaling prevented the onset of type 1 diabetes mellitus in NOD mice. Thus, Nrf2 appears to be a potential target for the prevention and treatment of type 1 diabetes.

Free access

Ronald Gonzalez, Benjamin K Reingold, Xiaodong Gao, Mandeep P Gaidhu, Robert G Tsushima, and Suraj Unniappan

Nesfatin-1 is a recently discovered multifunctional metabolic hormone abundantly expressed in the pancreatic islets. The main objective of this study is to characterize the direct effects of nesfatin-1 on insulin secretion in vitro using MIN6 cells and islets isolated from C57BL/6 mice. We also examined the expression of the nesfatin-1 precursor protein, nucleobindin 2 (NUCB2) mRNA, and nesfatin-1 immunoreactivity (ir) in the islets of normal mice and in the islets from mice with streptozotocin-induced type 1 diabetes and diet-induced obese (DIO) mice with type 2 diabetes. Nesfatin-1 stimulated glucose-induced insulin release in vitro from mouse islets and MIN6 cells in a dose-dependent manner. No such stimulation in insulin secretion was found when MIN6 cells/islets were incubated with nesfatin-1 in low glucose. In addition, a fourfold increase in nesfatin-1 release from MIN6 cells was observed following incubation in high glucose (16.7 mM) compared to low glucose (2 mM). Furthermore, we observed a significant reduction in both NUCB2 mRNA expression and nesfatin-1-ir in the pancreatic islets of mice with type 1 diabetes, while a significant increase was observed in the islets of DIO mice. Together, our findings indicate that nesfatin-1 is a novel insulinotropic peptide and that the endogenous pancreatic islet NUCB2/nesfatin is altered in diabetes and diet-induced obesity.