Thrombin‐induced human platelet aggregation is inhibited by protein‐tyrosine kinase inhibitors, ST638 and genistein

Abstract
We have investigated the involvement of protein‐tyrosine kinases in thrombin‐induced aggregation of human platelets, using ST638 and genistein which are known inhibitors of protein‐tyrosine kinase. Preincubation of platelets with 50 μM of ST638 or 25 μg/ml of genistein completely blocked the platelet aggregation induced with 0.05 unit/ml of thrombin. The increase of protein‐tyrosine phosphorylation bands (135‐, 124‐, 76‐, 64‐, and 60‐kDa) induced with thrombin was also inhibited by these inhibitors in a dose‐dependent manner. These inhibitors also blocked the platelet aggregation and protein‐tyrosine phosphorylation induced with thrombin in aspirin‐treated platelets. Increase of the intracellular Ca2+ concentration induced by thrombin was also inhibited by higher concentrations of genistein. The results suggest that the protein‐tyrosine phosphorylation plays a certain role in platelet activation having some relation to the intracellular Ca2+ concentration.

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