OKADAIC ACID INHIBITS ACTIVATION OF PHOSPHOLIPASE-C IN HUMAN PLATELETS BY MIMICKING THE ACTIONS OF PROTEIN KINASE-A AND KINASE-C

1 The effect of okadaic acid, a potent inhibitor of protein phosphatases 1 and 2A (PP1 and PP2A), on human platelets has been investigated. 2 Okadaic acid exerts a general increase in phosphorylation of platelet proteins but did not induce aggregation or secretion of 5-hydroxytryptamine (5-HT). Okadaic acid, however, did inhibit thrombin-induced functional responses. 3 Maximally effective concentrations of prostacyclin, to elevate adenosine 3':5'-cyclic monophosphate (cyclic AMP), or phorbol dibutyrate, to activate protein kinase C, inhibited the formation of inositol phosphates by thrombin by approximately 60%. When used in combination, prostacyclin and phorbol dibutyrate reduced the levels of inositol phosphates induced by thrombin to 11%. 4 Okadaic acid (1-mu-M) decreased thrombin-induced formation of inositol phosphates by approximately 55% and increased the inhibitory action of prostacyclin or phorbol dibutyrate. Okadaic acid had no further effect when prostacyclin and phorbol dibutyrate were used in combination. 5 These results suggest that protein kinases A and C act to inhibit phospholipase C by distinct mechanisms and that their action is reversed by PP1 and/or PP2A.