P70(S6K) IS ACTIVATED BY CCK IN RAT PANCREATIC ACINI

The expression and activity of p70(s6k)-p85(s6k) in isolated rat pancr eatic acini were revealed by Western blotting, immunoprecipitation, an d kinase assay. Cholecystokinin (CCK) stimulation of p70(s6k) activity was biphasic, with an early phase maximum at 5 min and a late phase m aximum at 60 min. The threshold concentration of CCK to increase p70(s 6k) activity was 3 pM, and the maximal effect was seen at 1 nM CCK. Ca rbachol and bombesin, but not vasoactive intestinal peptide, also acti vated p70(s6k). The protein kinase C (PKC) activator (12-O-tetradecano ylphorbol 13-acetate), the calcium ionophore (ionomycin), and a deriva tive of adenosine 3',5'-cyclic monophosphate induced only a slight inc rease in p70(s6k) activity. Rapamycin potently blocked both the basal and the CCK-stimulated p70(s6K) activity, and this inhibition was reve rsed by an excess of FK-506. The phosphatidylinositol 3-kinase inhibit or, wortmannin, potently inhibited p70(s6k) activation by CCK, whereas the tyrosine kinase inhibitor genistein had only a partial effect. Ne ither rapamycin nor ae wortmannin inhibited amylase release at concent rations that inhibited p70(s6k) activity. Thus the activation pathway of p70(s6k) by CCK is not mediated by PKC or mobilization of intracell ular calcium but seems to be mediated by phosphatidylinositol S-kinase . The effect of rapamycin to inhibit p70(s6k) activity is mediated by binding to the immunophyllin FK-506-binding protein of 12 kDa. The p70 (s6k) is not involved in the secretion of digestive enzymes induced by CCK.