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 carbachol and glucose causes increased tyrosine phosphorylation of a 125-kDa protein concurrently with increased insulin secretion. The effect was observed in two different insulin-secreting cell lines and in rat pancreatic islets. Tyrosine phosphorylation was largely calcium independent and occurred within 2 min after stimulation of beta cells with glucose and the muscarinic agonist carbachol. In islets, the effect of glucose was greatly diminished by the addition of mannoheptulose, a seven-carbon sugar that inhibits glucokinase, suggesting that glucose metabolism is required for tyrosine phosphorylation of the protein to occur. Neither insulin nor insulin-like growth factor I significantly increased tyrosine phosphorylation of the 125-kDa protein, suggesting that it was not an autocrine effect. Depolarization of beta cells with glyburide or 50 m potassium dramatically increased insulin secretion but had no significant effect on tyrosine phosphorylation. Addition of phorbol ester caused a less than 2-fold increase in tyrosine phosphorylation, whereas the calcium ionophore A23187 had no effect. Among the various fuel secretagogues tested, only -glucose stimulated tyrosine phosphorylation, both alone and in combination with carbachol. Finally, the tyrosine kinase inhibitor AG879 inhibited both tyrosine phosphorylation and insulin secretion in a dose-dependent manner. Taken together, these data demonstrate the presence of a novel signaling pathway in glucose-induced insulin secretion: tyrosine phosphorylation of beta cell p125, which is a proximal step in insulin secretion. Our current working hypothesis is that glucose stimulation of beta cell p125 tyrosine phosphorylation is an essential step for insulin secretion.
Konrad
R J
RJ
Department of Pathology and Laboratory Medicine, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104, USA.
Dean
R M
RM
Young
R A
RA
Billings
P C
PC
Wolf
B A
BA
D000818:Animals; D015153:Blotting, Western; D002118:Calcium / Q000502:physiology; D002217:Carbachol / Q000494:pharmacology; D002460:Cell Line; D005947:Glucose / Q000494:pharmacology*; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000502:physiology*; D010750:Phosphoproteins / Q000502:physiology*; D010766:Phosphorylation; D019000:Phosphotyrosine / Q000378:metabolism; D051381:Rats; D012636:Secretory Rate / Q000187:drug effects; D015398:Signal Transduction
D007328:Insulin; D010750:Phosphoproteins; D019000:Phosphotyrosine; D002217:Carbachol; D005947:Glucose; D002118:Calcium