Glucose Promotes Pancreatic Islet Beta-cell Survival Through a PI 3-kinase/Akt-signaling Pathway

The concentration of glucose in plasma is an important determinant of pancreatic beta-cell mass, whereas the relative contributions of hypertrophy, proliferation, and cell survival to this process are unclear. Glucose results in depolarization and subsequent calcium influx into islet beta-cells. Because depolarization and calcium (Ca(2+)) influx promote survival of neuronal cells, we hypothesized that glucose might alter survival of islet beta-cells through a similar mechanism. In the present studies, cultured mouse islet beta-cells showed a threefold decrease in apoptosis under conditions of 15 mM glucose compared with 2 mM glucose (P < 0.05). MIN6 insulinoma cells incubated in 25 mM glucose for 24 h showed a threefold decrease in apoptosis compared with cells in 5 mM glucose (1.7 +/- 0.2 vs. 6.3 +/- 1%, respectively, P < 0.001). High glucose (25 mM) enhanced survival-required depolarization and Ca(2+) influx and was blocked by phosphatidylinositol (PI) 3-kinase inhibitors. Glucose activation of the protein kinase Akt was demonstrated in both insulinoma cells and cultured mouse islets by means of an antibody specific for Ser(473) phospho-Akt and by an in vitro Akt kinase assay. Akt phosphorylation was dependent on PI 3-kinase but not on MAPK. Transfection of insulinoma cells with an Akt kinase-dead plasmid (Akt-K179M) resulted in loss of glucose-mediated protection, whereas transfection with a constitutively active Akt enhanced survival in glucose-deprived insulinoma cells. The results of these studies defined a novel pathway for glucose-mediated activation of a PI 3-kinase/Akt survival-signaling pathway in islet beta-cells. This pathway may provide important targets for therapeutic intervention.