THE EFFECTS OF GENISTEIN ON PLATELET-FUNCTION ARE DUE TO THROMBOXANE RECEPTOR ANTAGONISM RATHER THAN INHIBITION OF TYROSINE KINASE

Although several previous studies have indicated a role for tyrosine p hosphorylated proteins in platelet function, their precise function an d relationship to other biochemical processes remains elusive. In the present study genistein, an inhibitor of tyrosine kinase activity, was used to address this latter question. Genistein inhibited aggregation of washed human platelets in response to the thromboxane analogue U46 619, to the phorbol ester phorbol myristate acetate, and to the calciu m ionophore A23187. Only in the case of U46619, however, did the conce ntration of genistein required (IC50 of 10 mug/ml) correlate to that r eported to inhibit tyrosine kinases. Likewise, genistein also inhibite d U46619-induced serotonin secretion, elevation of cytosolic calcium, [P-32]-phosphatidic acid production (an index of phospholipase C activ ity) and the phosphorylation of pleckstrin (an index of protein kinase C activity) at similar concentrations (IC50s of 4-9 mug/ml). U46619 c aused the phosphorylation of a phosphoprotein which was insensitive to KOH digestion and therefore presumably a phosphotyrosine. This phosph orylation was also inhibited by genistein (IC50 of 6 mug/ml). However genistein also inhibited [H-3]-U46619 binding to platelets with an IC5 0 of 3 mug/ml. These data suggest that the inhibitory effects of genis tein on platelet activation occurs as a result of antagonism of the th romboxane receptor.