EFFECTS OF GENISTEIN, A TYROSINE KINASE INHIBITOR, ON PLATELET FUNCTIONS - GENISTEIN ATTENUATES THROMBIN-INDUCED CA2+ MOBILIZATION IN HUMANPLATELETS BY AFFECTING POLYPHOSPHOINOSITIDE TURNOVER

Genistein, a tyrosine kinase inhibitor, had no or only slight inhibito ry effects on platelet aggregation or serotonin release induced by thr ombin, while intracellular Ca2+ concentration ([Ca2+]i elevation was s ubstantially attenuated. It also inhibited the cyclooxygenase pathway, but this effect was not directly related to the suppressive effect of genistein on [Ca2+]i elevation. In order to clarify the mechanism by which genistein suppresses Ca2+ mobilization, its effect was examined on inositol phospholipid metabolism. The production of inositol-1,4,5- trisphosphate was inhibited by genistein in a dose-dependent manner, w hile 47 kDa protein phosphorylation or phosphatidic acid formation was not affected, suggesting that genistein does not inhibit phospholipas e C activity. Pretreatment of unstimulated platelets with genistein in creased the amount of phosphatidylinositol-4-monophosphate [PI(4)P], w hile that of phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] was red uced. Thrombin stimulation of genistein-pretreated cells intensified t his tendency, i.e. a further increase in the amount of PI(4)P and a de crease in the amount of PI(4,5)P2 in an inversely proportional manner. Taken together, these findings imply that genistein acted at the step of PI(4)P 5-kinase which produces PI(4,5)P2 from PI(4)P. Protein tyro sine phosphorylation induced by thrombin was not affected by genistein , suggesting that the inhibitory effect of genistein on polyphosphoino sitides was unrelated to tyrosine kinase inhibition.