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New Findings What is the central question of this study? Recent studies have demonstrated strong correlations between circulating branched-chain amino acid (AA) levels and insulin resistance, a predictor of susceptibility to type 2 diabetes. This study investigated how elevations in various AA concentrations over a chronic incubation period affect rat pancreatic islet cell viability and insulin secretion., What is the main finding and its importance? We report that chronic exposure to specific AAs provoked islet cell toxic effects, particularly the insulinotropic AA l-arginine. Dose-dependent decreases in insulin secretion and cell proliferation were observed in addition to activation of the cell endoplasmic reticulum stress response. These results show, for the first time, that elevated l-arginine levels may impact negatively on islet function, providing further insights into β-cell dysfunction in type 2 diabetes., Recent metabolic profiling studies have identified a correlation between branched-chain amino acid levels, insulin resistance associated with prediabetes and susceptibility to type 2 diabetes. Glucose and lipids in chronic excess have been reported to induce toxic effects in pancreatic β-cells, but the effect of elevated amino acid concentrations on primary islet cell function has not been investigated to date. The aim of this study was to investigate the effect of chronic exposure to various amino acids on islet cell function in vitro. Isolated rat islets were incubated over periods of 48 h with a range of concentrations of individual amino acids (0.1 μ m to 10 m m). After 48 h, islets were assessed for glucose-dependent insulin secretion capacity, proliferation or islet cell apoptosis. We report that elevated levels of branched-chain amino acids have little effect on pancreatic islet cell function or viability; however, increased levels of the amino acid l-arginine were found to be β-cell toxic, causing a dose-dependent decrease in insulin secretion accompanied by a decrease in islet cell proliferation and an increase in islet cell apoptosis. These effects were not due to l-arginine-dependent increases in production of nitric oxide but arose through elicitation of the islet cell endoplasmic reticulum stress response. This novel finding indicates, for the first time, that the l-arginine concentration in vitro may impact negatively on islet cell function, thus indicating further complexity in relationship to in vivo susceptibility of β-cells to nutrient-induced dysfunction. [ABSTRACT FROM AUTHOR]

BACKGROUND AND PURPOSE β-cells express a range of fatty acid-responsive G protein-coupled receptors, including GPR119, which regulates insulin secretion and is seen as a potential therapeutic target in type 2 diabetes. The long-chain unsaturated fatty acid derivative oleoylethanolamide (OEA) is an endogenous agonist of GPR119 and, under certain conditions, some long-chain unsaturated fatty acids can promote β-cell cytoprotection. It is not known, however, if OEA is cytoprotective in β-cells. The present study has examined this and determined whether GPR119 is involved. METHODS Clonal rat insulin-secreting cell lines, BRIN-BD11 or INS-1E, were exposed to fatty acids complexed with BSA. cAMP levels, insulin release and cell viability were measured. Protein expression was studied by Western blotting and receptor expression by RT-PCR. KEY RESULTS GPR119 was expressed in both BRIN-BD11 and INS-1E cells and OEA was cytoprotective in these cells. However, cytoprotection was not reproduced by any of a range of selective, synthetic ligands of GPR119. The cytoprotective response to OEA was lost during exposure to inhibitors of fatty acid amide hydrolase (FAAH) suggesting that OEA per se is not the cytoprotective species but that release of free oleate is required. Similar data were obtained with anandamide, which was cytoprotective only under conditions favouring release of free arachidonate. CONCLUSIONS AND IMPLICATIONS Activation of GPR119 is not required to mediate the cytoprotective actions of OEA in BRIN-BD11 or INS-1E cells. Rather, OEA is internalised and subjected to hydrolysis by FAAH to release free oleate, which then mediates the cytoprotection. [ABSTRACT FROM AUTHOR]

Glucokinase (GK) plays a key role in maintaining glucose homeostasis by stimulating insulin secretion from pancreatic β-cells and increasing hepatic glucose uptake. In this study we have investigated the effects of a novel, orally active glucokinase activator (GKA71) on insulin secretion and glucose tolerance in mice with high-fat diet (HFD) induced glucose intolerance. Mice were fed the HFD (58% fat by energy) for 8 weeks prior to administration of GKA71, which was provided in the diet (150 mg GKA71/kg food) giving a daily dose of ∼300 µg GKA71/mouse. Control mice were fed HFD or a normal diet (ND, 11% fat by energy). After 1 and 4 weeks with GKA71, glucose tolerance was studied in intravenous glucose tolerance test (IVGTT). Blood samples were taken just before and 1, 5, 10, 20, 50 and 75 min after the glucose injection (0.75g/kg). After 4-weeks treatment islets and liver were isolated. The GKA treatment significantly reduced basal plasma glucose level (5.3±0.3 vs. 8.3±0.3 mM in HFD, P<0.001, and 7.4±0.3 mM in ND, P<0.001). Basal insulin levels were also reduced by GKA71 (210±20 vs. 520±100 pM in HFD, P<0.01). The IVGTTs showed that GKA71 significantly improved glucose tolerance compared to HFD control mice (K[sub G] 4.0±0.5 vs. 2.5±0.2 %/min in HFD, P<0.01) being similar to ND fed mice (K[sub G] 3.8±0.2 %/min), although without increasing insulin secretion (AIR 485±120 vs. 670±85 pM in HFD, P>0.05). The AIR was 890±74 pM in the ND fed mice. There was no difference between the IVGTT results after 1 or 4 weeks with GKA71, suggesting full effect already after l-weeks treatment. Isolated islets from GKA treated mice demonstrated augmented glucose stimulated insulin secretion. Liver glycogen and triglyceride levels were similar in HFD control and HFD-GKA mice. In conclusion, activation of GK in HFD-fed mice potently reduces glycemia and normalizes glucose tolerance, demonstrating GKA as a potential therapeutic agent for treatment of type 2 diabetes. [ABSTRACT FROM AUTHOR]