Protein tyrosine phosphorylation in pancreatic acini: differential effects of VIP and CCK

Cholecystokinin (CCK) and vasoactive intestinal peptide (VIP) stimulate enzyme secretion from pancreatic acini by binding to heptahelical receptors without intrinsic tyrosine kinase activity. Signal transduction by the CCK receptor involves activation of phospholipase C by Gq proteins and activation of tyrosine kinases, whereas occupation of VIP receptors stimulates adenylyl cyclase through binding to Gs proteins. Here, we use electrophoretic separation of cellular proteins and antiphosphotyrosine immunoblotting to demonstrate a VIP-stimulated rapid and dose-dependent increase in tyrosine phosphorylation of proteins migrating at 130, 115, and 93 kDa in freshly isolated rat pancreatic acini. Phosphorylation of these proteins was increased after direct stimulation of adenylyl cyclase or the adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase with forskolin or dibutyryl cAMP and was inhibited by the tyrosine kinase inhibitors genistein or tyrphostin 23. Compared with VIP, CCK stimulated tyrosine phosphorylation of additional proteins migrating at 60, 66, and 72/78 kDa. Using two-dimensional electrophoretic separation or immunoprecipitation, the 72/78-kDa phosphoprotein was identified as paxillin. We propose that paxillin might be involved in CCK-but not in VIP-induced exocytosis.