ARE TYROSINE PHOSPHORYLATION OF P125(FAK) AND PAXILLIN OR THE SMALL GTP-BINDING PROTEIN, RHO, NEEDED FOR CCK-STIMULATED PANCREATIC AMYLASE SECRETION

Studies of a possible role of tyrosine phosphorylation in the secretor y process in rat pancreatic acinar cells provide conflicting conclusio ns. Recent studies show that tyrosine phosphorylation of the focal adh esion kinase, p125(FAK) and the cytoskeletal protein, paxillin. may me diate a number of cellular changes and this phosphorylation is depende nt on the activation of the small GTP binding protein, p21(Rho) (Rho). In this work we have investigated the role of tyrosine phosphorylatio n of each of these proteins and of the activation of Rho in pancreatic enzyme secretion. Pretreatment with genistein, a tyrosine kinase inhi bitor, decreased CCK-8-stimulated tyrosine phosphorylation of p125(FAK ) and paxillin and CCK-8-stimulated amylase secretion by more than 60% , raising the possibility that tyrosine phosphorylation of these two p roteins could be important in the ability of CCK-8 to stimulate amylas e release. However, genistein did not alter the amylase release stimul ated by TPA but inhibited TPA-stimulated p125(FAK) and paxillin tyrosi ne phosphorylation by 70%. Pretreatment with C3 transferase, which spe cifically inactivates Rho, causes a decrease in CCK-8-induced maximal amylase release by 33%. Moreover, C3 transferase pretreatment causes a 48% and a 38% decrease in the tyrosine phosphorylation of p125(FAK) a nd paxillin by CCR-8, respectively. Pretreat-ment with different conce ntrations of cytochalasin D, an actin cytoskeleton assembly inhibitor, completely inhibited CCK-8-stimulated tyrosine phosphorylation of p12 5(FAK) and paxillin without having any effect on either the potency or efficacy of CCK-8 at stimulating amylase release. Furthermore, cytoch alasin D completely inhibited TPA-stimulated tyrosine phosphorylation of both proteins without affecting TPA-stimulated amylase release. The se results show that tyrosine phosphorylation of p125(FAK) and paxilli n is not required for CCK-8 stimulation of enzyme secretion. However, our results suggest Rho is involved in the CCK-S stimulation of amylas e release by a parallel pathway to its involvement in the CCK-8-stimul ated tyrosine phosphorylation of p125(FAK) and paxillin. 0167-4889/98/ $ - see front matter (C) 1998 Elsevier Science B.V. All rights reserve d.