p70(s6k) is activated by CCK in rat pancreatic acini

The expression and activity of p70(s6k)-p85(s6k) in isolated rat pancreatic acini were revealed by Western blotting, immunoprecipitation, and kinase assay. Cholecystokinin (CCK) stimulation of p70(s6k) activity was biphasic, with an early phase maximum at 5 min and a late phase maximum at 60 min. The threshold concentration of CCK to increase p70(s6k) activity was 3 pM, and the maximal effect was seen at 1 nM CCK. Carbachol and bombesin, but not vasoactive intestinal peptide, also activated p70(s6k). The protein kinase C (PKC) activator (12-O-tetradecanoylphorbol 13-acetate), the calcium ionophore (ionomycin), and a derivative of adenosine 3',5'-cyclic monophosphate induced only a slight increase in p70(s6k) activity. Rapamycin potently blocked both the basal and the CCK-stimulated p70(s6K) activity, and this inhibition was reversed by an excess of FK-506. The phosphatidylinositol 3-kinase inhibitor, wortmannin, potently inhibited p70(s6k) activation by CCK, whereas the tyrosine kinase inhibitor genistein had only a partial effect. Neither rapamycin nor ae wortmannin inhibited amylase release at concentrations that inhibited p70(s6k) activity. Thus the activation pathway of p70(s6k) by CCK is not mediated by PKC or mobilization of intracellular 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.