MULTIPLE INHIBITORY EFFECTS OF GENISTEIN ON STIMULUS-SECRETION COUPLING IN RAT PANCREATIC ACINI

Genistein, a tyrosine kinase inhibitor, inhibited cholecystokinin (CCK )-induced maximal amylase release from rat pancreatic acini by 18, 31, and 46% at concentrations of 100, 300, and 750 mu M, respectively, af ter 30 min preincubation. Genistein similarly decreased amylase releas e stimulated by bombesin but not that stimulated by secretin or vasoac tive intestinal peptide. The steps of stimulus-secretion coupling affe cted by genistein were further evaluated. We found genistein dose depe ndently suppressed CCK- as well as NaF-induced polyphosphoinositide hy drolysis with a 50% inhibitory concentration of 380 and 400 mu M, resp ectively, but only slightly suppressed the increase of intracellular C a2+ concentration in response to either low or high concentrations of CCK. Genistein at 300 mu M also decreased incorporation of [H-3]inosit ol into phosphatidylinositol 4,5-bisphosphate. Most strikingly, 300 mu M genistein inhibited Ca2+-stimulated amylase release by 85% in strep tolysin O-permeabilized acini and thapsigargin-stimulated amylase rele ase by 69% in intact acini. Daidzein, the inactive analogue of geniste in, had no effect on any of the above parameters. Genistein, up to 750 mu M, did not affect amylase release in response to phorbol ester. To relate these inhibitory effects of genistein to its inhibition of tyr osine phosphorylation, Western blotting was performed with an anti-pho sphotyrosine monoclonal antibody. Genistein at 100 mu M partly and at 300 mu M completely inhibited CCK-induced tyrosine phosphorylation. In conclusion, genistein inhibits amylase release at multiple stages of stimulus-secretion coupling. These effects most likely involve both ty rosine kinase-dependent and -independent mechanisms.