GLUCOSE-INDUCED TYROSINE PHOSPHORYLATION OF P125 IN BETA-CELLS AND PANCREATIC-ISLETS - A NOVEL PROXIMAL SIGNAL IN INSULIN-SECRETION

In this study, we demonstrate that stimulation of beta cells with carb achol and glucose causes increased tyrosine phosphorylation of a 125-k Da protein concurrently with increased insulin secretion. The effect w as observed in two different insulin-secreting cell lines and in rat p ancreatic islets. Tyrosine phosphorylation was largely calcium indepen dent and occurred within 2 min after stimulation of beta cells with gl ucose and the muscarinic agonist carbachol. In islets, the effect of g lucose was greatly diminished by the addition of mannoheptulose, a sev en-carbon sugar that inhibits glucokinase, suggesting that glucose met abolism is required for tyrosine phosphorylation of the protein to occ ur. Neither insulin nor insulin-like growth factor I significantly inc reased tyrosine phosphorylation of the 125-kDa protein, suggesting tha t it was not an autocrine effect. Depolarization of beta cells with gl yburide or 50 mM potassium dramatically increased insulin secretion bu t had no significant effect on tyrosine phosphorylation. Addition of p horbol ester caused a less than 2-fold increase in tyrosine phosphoryl ation, whereas the calcium ionophore A23187 had no effect. Among the v arious fuel secretagogues tested, only D-glucose stimulated tyrosine p hosphorylation, both alone and in combination with carbachol. Finally, the tyrosine kinase inhibitor AG879 inhibited both tyrosine phosphory lation and insulin secretion in a dose-dependent manner. Taken togethe r, these data demonstrate the presence of a novel signaling pathway in glucose-induced insulin secretion: tyrosine phosphorylation of beta c ell p125, which is a proximal step in insulin secretion. Our current w orking hypothesis is that glucose stimulation of beta cell p125 tyrosi ne phosphorylation is an essential step for insulin secretion.