Inhibition of platelet activation by tyrosine kinase inhibitors

Protein tyrosine kinase (PTK) blockers (tyrphostins) inhibit in a dose-dependent fashion thrombin-induced aggregation and serotonin release with ic₅₀ values in the 10-35 μM concentration range. The inhibition of thrombin-induced aggregation correlates with their potency in inhibiting phosphorylation of proteins on tyrosine residues. Using metabolically ³²P-labelled human platelets, it was found that the tyrphostins have no effect on the decrease in [³²P]phosphatidylinositol bisphosphate but prevent the replenishment of [³²P]polyphosphoinositide. Tyrphostins decreased [³²P]phosphatidic acid production induced by thrombin, although never by more than 50%, and only delayed the peak of diacylglycerol, suggesting that phospholipase C was still activated. Tyrphostins inhibited the thrombinelicited early phosphorylation of p43 and p20, substrates for protein kinase C (PKC) and myosin light chain kinase, respectively, at short times of activation. This inhibition, however, was overcome after 1 min of stimulation with thrombin. Tyrphostin AG213 also inhibited platelet aggregation and tyrosine protein phosphorylation induced by phorbol myristate acetate (PMA), but did not inhibit pleckstrin phosphorylation. These results suggest that thrombin induces the phosphorylation of proteins on tyrosine residues which most probably results in the activation of phosphoinositide kinases. The ability of tyrphostins to inhibit phosphorylation of p43 and p20 when induced by thrombin but not when induced by PMA confirms that PTKs may be involved subsequent to PKC activation.