Oral Administration of Apple Procyanidins Ameliorates Insulin Resistance via Suppression of Pro-Inflammatory Cytokine Expression in Liver of Diabetic ob/ob Mice.

Procyanidins, the main ingredient of apple polyphenols, are known to possess antioxidative and anti-inflammatory effects associated closely with the pathophysiology of insulin resistance and type 2 diabetes. We investigated the effects of orally administered apple procyanidins (APCs) on glucose metabolism using diabetic ob/ob mice. We found no difference in body weight or body composition between mice treated with APCs and untreated mice. A 4 week oral administration of APCs containing water [0.5% (w/v)] ameliorated glucose tolerance, insulin resistance, and hepatic gluconeogenesis in ob/ob mice. APCs also suppressed the increase in the level of the pancreatic ß-cell. Insulin-stimulated Akt phosphorylation was significantly enhanced; pro-inflammatory cytokine expression levels were significantly decreased, and c-Jun N-terminal kinase phosphorylation was downregulated in the liver of those mice treated with APCs. In conclusion, APCs ameliorate insulin resistance by improving hepatic insulin signaling through suppression of hepatic inflammation in ob/ob mice, which may be a mechanism with possible beneficial health effects of APCs in disturbed glucose metabolism.

Nardilysin Is Required for Maintaining Pancreatic ß-Cell Function.

Type 2 diabetes (T2D) is associated with pancreatic ß-cell dysfunction, manifested by reduced glucose-stimulated insulin secretion (GSIS). Several transcription factors enriched in ß-cells, such as MafA, control ß-cell function by organizing genes involved in GSIS. Here we demonstrate that nardilysin (N-arginine dibasic convertase; Nrd1 and NRDc) critically regulates ß-cell function through MafA. Nrd1(-/-) mice showed glucose intolerance and severely decreased GSIS. Islets isolated from Nrd1(-/-) mice exhibited reduced insulin content and impaired GSIS in vitro. Moreover, ß-cell-specific NRDc-deficient (Nrd1(delß)) mice showed a diabetic phenotype with markedly reduced GSIS. MafA was specifically downregulated in islets from Nrd1(delß) mice, whereas overexpression of NRDc upregulated MafA and insulin expression in INS832/13 cells. Chromatin immunoprecipitation assay revealed that NRDc is associated with Islet-1 in the enhancer region of MafA, where NRDc controls the recruitment of Islet-1 and MafA transcription. Our findings demonstrate that NRDc controls ß-cell function via regulation of the Islet-1-MafA pathway.

Src regulates insulin secretion and glucose metabolism by influencing subcellular localization of glucokinase in pancreatic ß-cells.

AIMS/INTRODUCTION: Src, a non-receptor tyrosine kinase, regulates a wide range of cellular functions, and hyperactivity of Src is involved in impaired glucose metabolism in pancreatic ß-cells. However, the physiological role of Src in glucose metabolism in normal, unstressed ß-cells remains unclear. In the present study, we investigated the role of Src in insulin secretion and glucose metabolism. MATERIALS AND METHODS: Src was downregulated using small interfering ribonucleic acid in INS-1 cells, and glucose-induced insulin secretion, adenosine triphosphate content, intracellular calcium concentration, glucose utilization and glucokinase activity were measured. Expression levels of messenger ribonucleic acid and protein of glucokinase were examined by semiquantitative real-time polymerase chain reaction and immunoblotting, respectively. Cells were fractionated by digitonin treatment, and subcellular localization of glucokinase was examined by immunoblotting. Interaction between glucokinase and neuronal nitric oxide synthase was estimated by immunoprecipitation. RESULTS: In Src downregulated INS-1 cells, glucose-induced insulin secretion was impaired, whereas insulin secretion induced by high K(+) was not affected. Intracellular adenosine triphosphate content and elevation of intracellular calcium concentration by glucose stimulation were suppressed by Src downregulation. Src downregulation reduced glucose utilization in the presence of high glucose, which was accompanied by a reduction in glucokinase activity without affecting its expression. However, Src downregulation reduced glucokinase in soluble, cytoplasmic fraction, and increased it in pellet containing intaracellular organelles. In addition, interaction between glucokinase and neuronal nitric oxide synthase was facilitated by Src downregulation. CONCLUSIONS: Src plays an important role in glucose-induced insulin secretion in pancreatic ß-cells through maintaining subcellular localization and activity of glucokinase.

Comparison of incretin immunoassays with or without plasma extraction: Incretin secretion in Japanese patients with type 2 diabetes.

UNLABELLED: Aims/Introduction: The effectiveness of incretin-based therapies in Asian type 2 diabetes requires investigation of the secretion and metabolism of glucose-dependent insulinotropic polypepide (GIP) and glucagon-like peptide 1 (GLP-1). Plasma extractions have been suggested to reduce variability in intact GLP-1 levels among individuals by removing interference that affects immunoassays, although no direct demonstration of this method has been reported. We have evaluated the effects of ethanol and solid-phase extractions on incretin immunoassays. We determined incretin levels during meal tolerance tests in Japanese patients with type 2 diabetes and characterized predictors for incretin secretion. MATERIALS AND METHODS: Japanese patients with type 2 diabetes (23 anti-diabetic drug-naïve and 18 treated with sulfonylurea [SU] alone) were subjected to meal tolerance tests, and incretin levels were determined by immunoassays with or without extraction. RESULTS: Intact GLP-1 levels determined by an intact GLP-1 immunoassay with ethanol and solid-phase extractions were lower than those determined without extraction. Intact GLP-1 levels determined by the extractions were highly correlated with each other, much more so than the levels with and without extraction. Total GLP-1 was unaffected by extractions, showing that extractions remove interference only in the case of intact GLP-1. Incretin secretion after meal ingestion was similar between drug-naïve and SU-treated patients. Fasting and postprandial GLP-1 levels were correlated positively with fasting free fatty acids and negatively with dipeptidyl peptidase-4 activity. CONCLUSIONS: Ethanol and solid-phase extractions remove interference for intact GLP-1 immunoassay. SU showed little effect on incretin secretion. GLP-1 and GIP secretion were predicted by different factors. (J Diabetes Invest, doi: 10.1111/j.2040-1124.2011.00141.x, 2012).

Predicting efficacy of dipeptidyl peptidase-4 inhibitors in patients with type 2 diabetes: Association of glycated hemoglobin reduction with serum eicosapentaenoic acid and docosahexaenoic acid levels.

This study was initiated to identify clinical and dietary parameters that predict efficacy of dipeptidyl peptidase-4 inhibitors. A total of 72 untreated Japanese patients with type 2 diabetes who received DPP-4 inhibitors (sitagliptin, alogliptin or vildagliptin) for 4 months were examined for changes of glycated hemoglobin (HbA1c) and body mass index (BMI), and self-administered 3-day food records, as well as serum levels of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). DPP-4 inhibitors significantly reduced HbA1c (before initiation of DPP-4 inhibitors 7.2 ± 0.7%, 4 months after initiation of DPP-4 inhibitors 6.7 ± 0.6% [paired t-test, P < 0.01 vs before]). Multiple regression analysis showed that changes of HbA1c were significantly correlated with baseline HbA1c, as well as estimated intake of fish. Furthermore, changes of HbA1c were significantly correlated with serum levels of EPA (r = -0.624, P < 0.01) and DHA (r = -0.577, P < 0.01). HbA1c reduction by DPP-4 inhibitors is significantly correlated with estimated intake of fish and serum levels of EPA and DHA. (J Diabetes Invest, doi: 10.1111/j.2040-1124.2012.00214.x, 2012).