Insulin inhibits glucagon secretion by the activation of PI3-kinase in In-R1-G9 cells.

Intracellular mechanisms through which insulin inhibits glucagon secretion remain to be elucidated in glucagon secreting cells. In this study, we confirmed that, in In-R1-G9 cells, a pancreatic alpha cell line, insulin stimulated phosphorylation of insulin receptor substrate-1 (IRS-1) and activated phosphatidylinositol 3-kinase (PI3-kinase). We further studied, using wortmannin, an inhibitor of PI3-kinase, whether the inhibitory effect of insulin on glucagon secretion was mediated through PI3-kinase pathway in these cells. In static incubation studies, insulin significantly inhibited glucagon secretion at 2, 6 and 12 h, which was completely abolished by pretreatment with wortmannin. In perifusion studies, insulin significantly suppressed glucagon secretion after 10 min, which was also blocked by wortmannin. Insulin also reduced glucagon mRNA at 6 and 12 h but not at 2 h. Wortmannin also abolished insulin-induced reduction of glucagon mRNA. Insulin increased the amount of 85 kDa subunit of PI3-kinase in plasma membrane fraction (PM), with a reciprocal decrease of the kinase in cytosol fraction (CY). Insulin also increased PI3-kinase activity in PM, but not in CY. Our results suggest that insulin suppressed glucagon secretion by inhibiting glucagon release and gene expression. Both actions were mediated by activation of PI3-kinase. Recruitment and activation of PI3-kinase in plasma membrane might be relevant at least in part to insulin-induced inhibition of glucagon release.