PROTEIN-TYROSINE KINASE P72(SYK) IS ACTIVATED BY PLATELET-ACTIVATING-FACTOR IN PLATELETS

It has been demonstrated that activation of platelets by platelet-acti vating factor (PAF) results in a dramatic increase in tyrosine phospho rylation of several cellular proteins. We report here that p(72syk) is a potential candidate for the protein-tyrosine phosphorylation follow ing PAF stimulation in porcine platelets. Immunoprecipitation kinase a ssay revealed that PAF stimulation resulted in a rapid activation of p 72(syk) which peaked at 10 s. The level of activation was found to be dose dependent and could be completely inhibited by the PAF receptor a ntagonist, CV3988. Phosphorylation at the tyrosine residues of p72(syk ) coincided with activation of p72(syk). Pretreatment of platelets wit h aspirin and apyrase did not affect PAF induced activation of p72(syk ). Furthermore, genistein, a potent protein-tyrosine-kinase inhibitor, diminished PAF-induced p72(syk) activation and Ca2+ mobilization as w ell as platelet aggregation. These results suggest that p72(syk) may p lay a critical role in PAF-induced aggregation, possibly through regul ation of Ca2+ mobilization.