OBJECTIVEReactive oxygen species (ROS) is one of most important factors in impaired metabolism secretion coupling in pancreatic β-cells. We recently reported that elevated ROS production and impaired ATP production at high glucose in diabetic Goto-Kakizaki (GK) rat islets are effectively ameliorated by Src inhibition, suggesting that Src activity is upregulated. In the present study, we investigated whether the glucagon-like peptide-1 signal regulates Src activity and ameliorates endogenous ROS production and ATP production in GK islets using exendin-4.RESEARCH DESIGN AND METHODSIsolated islets from GK and control Wistar rats were used for immunoblotting analyses and measurements of ROS production and ATP content. Src activity was examined by immunoprecipitation of islet lysates followed by immunoblotting. ROS production was measured with a fluorescent probe using dispersed islet cells.RESULTSExendin-4 significantly decreased phosphorylation of Src Tyr416, which indicates Src activation, in GK islets under 16.7 mmol/l glucose exposure. Glucose-induced ROS production (16.7 mmol/l) in GK islet cells was significantly decreased by coexposure of exendin-4 as well as PP2, a Src inhibitor. The Src kinase–negative mutant expression in GK islets significantly decreased ROS production induced by high glucose. Exendin-4, as well as PP2, significantly increased impaired ATP elevation by high glucose in GK islets. The decrease in ROS production by exendin-4 was not affected by H-89, a PKA inhibitor, and an Epac-specific cAMP analog (8CPT-2Me-cAMP) significantly decreased Src Tyr416 phosphorylation and ROS production.CONCLUSIONSExendin-4 decreases endogenous ROS production and increases ATP production in diabetic GK rat islets through suppression of Src activation, dependently on Epac.

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Gastric inhibitory polypeptide (GIP) is an incretin that potentiates insulin secretion from pancreatic ␤-cells by binding to GIP receptor (GIPR) and subsequently increasing the level of intracellular adenosine 3Ј,5Ј-cyclic monophosphate (cAMP). We have identified a novel GIPR splice variant in mouse ␤-cells that retains intron 8, resulting in a COOH-terminal truncated form (truncated GIPR). This isoform was coexpressed with full-length GIPR (wild-type GIPR) in normal GIPR-expressing tissues. In an experiment using cells transfected with both GIPRs, truncated GIPR did not lead to cAMP production induced by GIP but inhibited GIP-induced cAMP production through wild-type GIPR (n ϭ 3-4, P Ͻ 0.05). Wild-type GIPR was normally located on the cell surface, but its expression was decreased in the presence of truncated GIPR, suggesting a dominant negative effect of truncated GIPR against wild-type GIPR. The functional relevance of truncated GIPR in vivo was investigated. In high-fat diet-fed obese mice (HFD mice), blood glucose levels were maintained by compensatory increased insulin secretion (n ϭ 8, P Ͻ 0.05), and cAMP production (n ϭ 6, P Ͻ 0.01) and insulin secretion (n ϭ 10, P Ͻ 0.05) induced by GIP were significantly increased in isolated islets, suggesting hypersensitivity of the GIPR. Total GIPR mRNA expression was not increased in the islets of HFD mice, but the expression ratio of truncated GIPR to total GIPR was reduced by 32% compared with that of control mice (n ϭ 6, P Ͻ 0.05). These results indicate that a relative reduction of truncated GIPR expression may be involved in hypersensitivity of GIPR and hyperinsulinemia in diet-induced obese mice.gastric inhibitory polypeptide; gastric inhibitory polypeptide receptor; alternative splicing; dominant negative effect; obesity OBESITY LEADS TO INSULIN RESISTANCE, characterized by fasting hyperinsulinemia and excessive insulin secretion after meal ingestion in the attempt to maintain euglycemia (25). Obesity is an important risk factor in progression to type 2 diabetes mellitus (14) and also in cardiovascular disease (16), and reduction of obesity can normalize hyperinsulinemia and impede the progression of diabetes and arteriosclerosis.Incretins are a group of peptide hormones released from the gastrointestinal tract into the circulation in response to meal ingestion that potentiate glucose-stimulated insulin secretion and include gastric inhibitory polypeptide (GIP), also called glucose-dependent insulinotropic polypeptide (24). GIP is secreted from the K cells of the duodenum and proximal jejunum upon meal ingestion and binds to the GIP receptor (GIPR) on the surface of pancreatic ␤-cells, adipose tissues, and osteoblasts to stimulate insulin secretion (21), fat accumulation (20), and bone formation (30) by increasing the level of intracellular adenosine 3Ј,5Ј-cyclic monophosphate (cAMP).Previously, we found that GIPR-deficient mice exhibit insufficient compensatory insulin secretion upon high-fat loading (21), indicating that GIP plays a critical role in mainta...