We tested effects of long-term exposure of pancreatic islets to free fatty acids (FFA) in vitro on B cell function. Islets isolated from male Sprague-Dawley rats were exposed to palmitate (0.125 or 0.25 mM), oleate (0.125 mM), or octanoate (2.0 mM) during culture. Insulin responses were subsequently tested in the absence of FFA. After a 48-h exposure to FFA, insulin secretion during basal glucose (3.3 mM) was severalfold increased. However, during stimulation with 27 mM glucose, secretion was inhibited by 30-50% and proinsulin biosynthesis by 3040%. Total protein synthesis was similarly affected. Conversely, previous palmitate did not impair a-ketoisocaproic acid (5 mM)-induced insulin release. Induction and reversibility of the inhibitory effect on glucose-induced insulin secretion required between 6 and 24 h. Addition of the carnitine palmitoyltransferase I inhibitor etomoxir (1 ,uM) partially reversed (by > 50%) FFA-associated decrease in secretory as well as proinsulin biosynthetic responses to 27 mM glucose. The inhibitory effect of previous palmitate was similar when co-culture was performed with 5.5, 11, or 27 mM glucose. Exposure to palmitate or oleate reduced the production of 14CO2 from D-[U-14C1 glucose, and of "4CO2 from D-13,4-"CIglucose, both effects being reversed by etomoxir. Conclusions: long-term exposure to FFA inhibits glucose-induced insulin secretion and biosynthesis probably through a glucose fatty acid cycle. (J.

We evaluated antihyperglycemic and anti-obese effects of Panax ginseng berry extract and its major constituent, ginsenoside Re, in obese diabetic C57BL/6J ob/ ob mice and their lean littermates. Animals received daily intraperitoneal injections of Panax ginseng berry extract for 12 days. On day 12, 150 mg/kg extract-treated ob/ob mice became normoglycemic (137 +/- 6.7 mg/dl) and had significantly improved glucose tolerance. The overall glucose excursion during the 2-h intraperitoneal glucose tolerance test decreased by 46% (P < 0.01) compared with vehicle-treated ob/ob mice. The improvement in blood glucose levels in the extract-treated ob/ ob mice was associated with a significant reduction in serum insulin levels in fed and fasting mice. A hyperinsulinemic-euglycemic clamp study revealed a more than twofold increase in the rate of insulin-stimulated glucose disposal in treated ob/ ob mice (112 +/- 19.1 vs. 52 +/- 11.8 micromol x kg(-1) x min(-1) for the vehicle group, P < 0.01). In addition, the extract-treated ob/ob mice lost a significant amount of weight (from 51.7 +/- 1.9 g on day 0 to 45.7 +/- 1.2 on day 12, P < 0.01 vs. vehicle-treated ob/ob mice), associated with a significant reduction in food intake (P < 0.05) and a very significant increase in energy expenditure (P < 0.01) and body temperature (P < 0.01). Treatment with the extract also significantly reduced plasma cholesterol levels in ob/ob mice. Additional studies demonstrated that ginsenoside Re plays a significant role in antihyperglycemic action. This antidiabetic effect of ginsenoside Re was not associated with body weight changes, suggesting that other constituents in the extract have distinct pharmacological mechanisms on energy metabolism.

Inhibitors of dipeptidyl peptidase-4 (DPP-4), a key regulator of the actions of incretin hormones, exert antihyperglycemic effects in type 2 diabetic patients. A major unanswered question concerns the potential ability of DPP-4 inhibition to have beneficial disease-modifying effects, specifically to attenuate loss of pancreatic ␤-cell mass and function. Here, we investigated the effects of a potent and selective DPP-4 inhibitor, an analog of sitagliptin (des-fluoro-sitagliptin), on glycemic control and pancreatic ␤-cell mass and function in a mouse model with defects in insulin sensitivity and secretion, namely high-fat diet (HFD) streptozotocin (STZ)-induced diabetic mice. Significant and dose-dependent correction of postprandial and fasting hyperglycemia, HbA 1c , and plasma triglyceride and free fatty acid levels were observed in HFD/STZ mice following 2-3 months of chronic therapy. Treatment with des-fluoro-sitagliptin dose dependently increased the number of insulin-positive ␤-cells in islets, leading to the normalization of ␤-cell mass and ␤-cell-to-␣-cell ratio. In addition, treatment of mice with des-fluoro-sitagliptin, but not glipizide, significantly increased islet insulin content and improved glucose-stimulated insulin secretion in isolated islets. These findings suggest that DPP-4 inhibitors may offer long-lasting efficacy in the treatment of type 2 diabetes by modifying the courses of the disease. Diabetes 55: [1695][1696][1697][1698][1699][1700][1701][1702][1703][1704] 2006 I n patients with type 2 diabetes, several key pathogenic abnormalities contribute to increased blood glucose levels, including abnormal insulin secretion caused by impaired ␤-cell function and insulin resistance in target tissues (1). Insulin resistance appears to be an important early lesion that is accompanied by compensatory increases in pancreatic ␤-cell insulin release (2). In patients destined to develop type 2 diabetes, however, ␤-cell function deteriorates progressively and Ͼ50% of ␤-cell function is typically lost by the time hyperglycemia is diagnosed (2-5). This loss of ␤-cell function and/or mass leads to rising blood glucose levels and to frank diabetes.The incretin hormone glucagon-like peptide-1 (GLP-1) plays a key role in the regulation of insulin secretion and glucose homeostasis. Administration of GLP-1 and its analogs or the GLP-1 mimetic exendin-4 has shown remarkable glucose-lowering efficacy in diabetic subjects (6 -8). Importantly, these agents have demonstrated beneficial effects on increasing islet neogenesis and differentiation as well as modulating ␤-cell mass in part by reducing apoptosis in animal models of diabetes (9 -12). These findings have engendered significant interest in the potential of GLP-1-based therapeutics to enhance ␤-cell function and thereby modify the course of disease in subjects with type 2 diabetes.Orally administered dipeptidyl peptidase-4 (DPP-4; or CD26) inhibitors have emerged as a new class of antidiabetic agents owing to their ability to extend the biological effects o...

Delayed-rectifier K؉ currents (I DR ) in pancreatic ␤-cells are thought to contribute to action potential repolarization and thereby modulate insulin secretion. The voltagegated K ؉ channel, K V 2.1, is expressed in ␤-cells, and the biophysical characteristics of heterologously expressed channels are similar to those of I DR in rodent ␤-cells. A novel peptidyl inhibitor of K V 2.1/K V 2.2 channels, guangxitoxin (GxTX)-1 (half-maximal concentration ϳ1 nmol/l), has been purified, characterized, and used to probe the contribution of these channels to ␤-cell physiology. In mouse ␤-cells, GxTX-1 inhibits 90% of I DR and, as for K V 2.1, shifts the voltage dependence of channel activation to more depolarized potentials, a characteristic of gating-modifier peptides. GxTX-1 broadens the ␤-cell action potential, enhances glucose-stimulated intracellular calcium oscillations, and enhances insulin secretion from mouse pancreatic islets in a glucose-dependent manner. These data point to a mechanism for specific enhancement of glucose-dependent insulin secretion by applying blockers of the ␤-cell I DR , which may provide advantages over currently used therapies for the treatment of type 2 diabetes.

Mutations in the hepatocyte nuclear factor-1␣ (HNF-1␣] i , and corrected the insulin secretion defect. NAD(P)H responses to glucose were substantially reduced, and inhibitors of glycolytic NADH generation reproduced the mutant phenotype in normal islets. Flux of glucose through glycolysis in islets from mutant mice was reduced, as a result of which ATP generation in response to glucose was impaired. We conclude that hepatocyte nuclear factor-1␣ diabetes results from defective ␤-cell glycolytic signaling, which is potentially correctable using substrates that bypass the defect.

To study effects of Bcl-x(L) in the pancreatic beta-cell, two transgenic lines were produced using different forms of the rat insulin promoter. Bcl-x(L) expression in beta-cells was increased 2- to 3-fold in founder (Fd) 1 and over 10-fold in Fd 2 compared with littermate controls. After exposure to thapsigargin (10 microM for 48 h), losses of cell viability in islets of Fd 1 and Fd 2 Bcl-x(L) transgenic mice were significantly lower than in islets of wild-type mice. Unexpectedly, severe glucose intolerance was observed in Fd 2 but not Fd 1 Bcl-x(L) mice. Pancreatic insulin content and islet morphology were not different from control in either transgenic line. However, Fd 2 Bcl-x(L) islets had impaired insulin secretory and intracellular free Ca(2+) ([Ca(2+)](i)) responses to glucose and KCl. Furthermore, insulin and [Ca(2+)](i) responses to pyruvate methyl ester (PME) were similarly reduced as glucose in Fd 2 Bcl-x(L) islets. Consistent with a mitochondrial defect, glucose oxidation, but not glycolysis, was significantly lower in Fd 2 Bcl-x(L) islets than in wild-type islets. Glucose-, PME-, and alpha-ketoisocaproate-induced hyperpolarization of mitochondrial membrane potential, NAD(P)H, and ATP production were also significantly reduced in Fd 2 Bcl-x(L) islets. Thus, although Bcl-x(L) promotes beta-cell survival, high levels of expression of Bcl-x(L) result in reduced glucose-induced insulin secretion and hyperglycemia due to a defect in mitochondrial nutrient metabolism and signaling for insulin secretion.

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