INSULIN MEDIATES GLUCOSE-STIMULATED PHOSPHORYLATION OF PHAS-I BY PANCREATIC BETA-CELLS - AN INSULIN-RECEPTOR MECHANISM FOR AUTOREGULATION OF PROTEIN-SYNTHESIS BY TRANSLATION

Although glucose regulates the biosynthesis of a variety of beta cell proteins at the level of translation, the mechanism responsible for th is effect is unknown, We demonstrate that incubation of pancreatic isl ets with elevated glucose levels results in rapid and concentration-de pendent phosphorylation of PHAS-I, an inhibitor of mRNA cap-binding pr otein, eukaryotic initiation factor (eIF)-4E. Our initial approach was to determine if this effect is mediated by the metabolism of glucose and activation of islet cell protein kinases, or whether insulin secre ted from the beta cell stimulates phosphorylation of PHAS-I via an ins ulin-receptor mechanism as described for insulin sensitive cells, In s upport of the latter mechanism, inhibitors of islet cell protein kinas es A and C exert no effect on glucose-stimulated phosphorylation of PH AS-I, whereas the phosphatidylinositol 3-kinase inhibitor, wortmannin, the immunosuppressant, rapamycin, and theophylline, a phosphodiestera se inhibitor, promote marked dephosphorylation of PHAS-I. In addition, exogenous insulin and endogenous insulin secreted by the beta cell li ne, beta TC6-F7, increase phosphorylation of PHAS-I, suggesting that b eta cells of the islet, in part, mediate this effect, Studies with bet a cell lines and islets indicate that amino acids are required for glu cose or exogenous insulin to stimulate the phosphorylation of PHAS-I, and amino acids alone dose-dependently stimulate the phosphorylation o f PHAS-I, which is further enhanced by insulin, Furthermore, rapamycin inhibits by similar to 62% the increase in total protein synthesis st imulated by high glucose concentrations. These results indicate that g lucose stimulates PHAS-I phosphorylation via insulin interacting with its own receptor on the beta cell which may serve as an important mech anism for autoregulation of protein synthesis by translation.