GPR40 protein levels are crucial to the regulation of stimulated hormone secretion in pancreatic islets. Lessons from spontaneous obesity-prone and non-obese type 2 diabetes in rats.

The role of islet GPR40 protein in the pathogenesis of diabetes is unclear. We explored the influence of GPR40 protein levels on hormone secretion in islets from two rat models of spontaneous type 2 diabetes displaying either hyperlipidaemia or hyperglycaemia. GPR40 expression was analysed by confocal microscopy, Western blot and qPCR in islets from preobese Zucker (fa/fa) rats, diabetic Goto-Kakizaki (GK) rats, and controls. Confocal microscopy of control islets showed expression of GPR40 protein in insulin, glucagon and somatostatin cells. GPR40 expression was strongly increased in islets of hyperlipidaemic fa/fa rats and coincided with a concentration-related increase in palmitate-induced release of insulin and glucagon and its inhibition of somatostatin release. Conversely, hyperglycaemic GK islets displayed an extremely faint expression of GPR40 as did high-glucose-cultured control islets. This was reflected in abolished palmitate-induced hormone response in GK islets and high-glucose-cultured control islets. The palmitate antagonist rosiglitazone promoted reappearance of GPR40 in high-glucose-cultured islets and served as partial agonist in glucose-stimulated insulin release. GPR40 protein is abundantly expressed in pancreatic islets and modulates stimulated hormone secretion. Mild hyperlipidaemia in obesity-prone diabetes creates increased GPR40 expression and increased risk for an exaggerated palmitate-induced insulin response and lipotoxicity, a metabolic situation suitable for GPR40 antagonist treatment. Chronic hyperglycaemia creates abrogated GPR40 expression and downregulated insulin release, a metabolic situation suitable for GPR40 agonist treatment to avoid glucotoxicity. GPR40 protein is interactively modulated by both free fatty acids and glucose and is a promising target for pharmacotherapy in different variants of type 2 diabetes.

Antidiabetic actions of an estrogen receptor ß selective agonist.

The estrogen receptor ß (ERß) is emerging as an important player in the physiology of the endocrine pancreas. We evaluated the role and antidiabetic actions of the ERß selective agonist WAY200070 as an insulinotropic molecule. We demonstrate that WAY200070 enhances glucose-stimulated insulin secretion both in mouse and human islets. In vivo experiments showed that a single administration of WAY200070 leads to an increase in plasma insulin levels with a concomitant improved response to a glucose load. Two-week treatment administration increased glucose-induced insulin release and pancreatic ß-cell mass and improved glucose and insulin sensitivity. In addition, streptozotocin-nicotinamide-induced diabetic mice treated with WAY200070 exhibited a significant improvement in plasma insulin levels and glucose tolerance as well as a regeneration of pancreatic ß-cell mass. Studies performed in db/db mice demonstrated that this compound restored first-phase insulin secretion and enhanced pancreatic ß-cell mass. We conclude that ERß agonists should be considered as new targets for the treatment of diabetes.

Secreted frizzled-related protein 4 reduces insulin secretion and is overexpressed in type 2 diabetes.

A plethora of candidate genes have been identified for complex polygenic disorders, but the underlying disease mechanisms remain largely unknown. We explored the pathophysiology of type 2 diabetes (T2D) by analyzing global gene expression in human pancreatic islets. A group of coexpressed genes (module), enriched for interleukin-1-related genes, was associated with T2D and reduced insulin secretion. One of the module genes that was highly overexpressed in islets from T2D patients is SFRP4, which encodes secreted frizzled-related protein 4. SFRP4 expression correlated with inflammatory markers, and its release from islets was stimulated by interleukin-1ß. Elevated systemic SFRP4 caused reduced glucose tolerance through decreased islet expression of Ca(2+) channels and suppressed insulin exocytosis. SFRP4 thus provides a link between islet inflammation and impaired insulin secretion. Moreover, the protein was increased in serum from T2D patients several years before the diagnosis, suggesting that SFRP4 could be a potential biomarker for islet dysfunction in T2D.

The insulinogenic effect of whey protein is partially mediated by a direct effect of amino acids and GIP on ß-cells.

BACKGROUND: Whey protein increases postprandial serum insulin levels. This has been associated with increased serum levels of leucine, isoleucine, valine, lysine, threonine and the incretin hormone glucose-dependent insulinotropic polypeptide (GIP). We have examined the effects of these putative mediators of whey's action on insulin secretion from isolated mouse Langerhans islets. METHODS: Mouse pancreatic islets were incubated with serum drawn from healthy individuals after ingestion of carbohydrate equivalent meals of whey protein (whey serum), or white wheat bread (control serum). In addition the effect of individual amino acid combinations on insulin secretion was also tested. Furthermore, the stimulatory effects of whey serum on insulin secretion was tested in vitro in the absence and presence of a GIP receptor antagonist ((Pro(3))GIP[mPEG]). RESULTS: Postprandial amino acids, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) responses were higher after whey compared to white wheat bread. A stimulatory effect on insulin release from isolated islets was observed with serum after whey obtained at 15 min (+87%, P < 0.05) and 30 min (+139%, P < 0.05) postprandially, compared with control serum. The combination of isoleucine, leucine, valine, lysine and threonine exerted strong stimulatory effect on insulin secretion (+270%, P < 0.05), which was further augmented by GIP (+558% compared to that produced by glucose, P < 0.05). The stimulatory action of whey on insulin secretion was reduced by the GIP-receptor antagonist (Pro(3))GIP[mPEG]) at both 15 and 30 min (-56% and -59%, P < 0.05). CONCLUSIONS: Compared with white wheat bread meal, whey causes an increase of postprandial insulin, plasma amino acids, GIP and GLP-1 responses. The in vitro data suggest that whey protein exerts its insulinogenic effect by preferential elevation of the plasma concentrations of certain amino acids, GIP and GLP-1.

Rapid insulinotropic action of low doses of bisphenol-A on mouse and human islets of Langerhans: role of estrogen receptor ß.

Bisphenol-A (BPA) is a widespread endocrine-disrupting chemical (EDC) used as the base compound in the manufacture of polycarbonate plastics. It alters pancreatic ß-cell function and can be considered a risk factor for type 2 diabetes in rodents. Here we used ERß-/- mice to study whether ERß is involved in the rapid regulation of K(ATP) channel activity, calcium signals and insulin release elicited by environmentally relevant doses of BPA (1 nM). We also investigated these effects of BPA in ß-cells and whole islets of Langerhans from humans. 1 nM BPA rapidly decreased K(ATP) channel activity, increased glucose-induced [Ca(2+)](i) signals and insulin release in ß-cells from WT mice but not in cells from ERß-/- mice. The rapid reduction in the K(ATP) channel activity and the insulinotropic effect was seen in human cells and islets. BPA actions were stronger in human islets compared to mouse islets when the same BPA concentration was used. Our findings suggest that BPA behaves as a strong estrogen via nuclear ERß and indicate that results obtained with BPA in mouse ß-cells may be extrapolated to humans. This supports that BPA should be considered as a risk factor for metabolic disorders in humans.

Mitochondrial proteome analysis reveals altered expression of voltage dependent anion channels in pancreatic ß-cells exposed to high glucose.

Chronic hyperglycemia leads to deterioration of insulin release from pancreatic ß-cells as well as insulin action on peripheral tissues. However, the mechanism underlying ß-cell dysfunction resulting from glucose toxicity has not been fully elucidated. The aim of the present study was to define a set of alterations in mitochondrial protein profiles of pancreatic ß-cell line in response to glucotoxic condition using 2-DE and tandem mass spectrometry. INS1E cells were incubated in the presence of 5.5 and 20 mM glucose for 72 hrs and mitochondria were isolated. Approximately 75 protein spots displayed significant changes (p < 0.05) in relative abundance in the presence of 20 mM glucose compared to controls. Mitochondrial proteins down regulated under glucotoxic conditions includes ATP synthase α chain and δ chain, malate dehydrogenase, aconitase, trifunctional enzyme ß subunit, NADH cytochrome b5 reductase and voltage-dependent anion-selective channel protein (VDAC) 2. VDAC1, 75 kDa glucose-regulated protein, heat shock protein (HSP) 60 and HSP10 were found to be upregulated. The orchestrated changes in expression of VDACs and multiple other proteins involved in nutrient metabolism, ATP synthesis, cellular defense, glycoprotein folding and mitochondrial DNA stability may explain cellular dysfunction in glucotoxicity resulting in altered insulin secretion.