PRIMING OF PLATELET ALPHA(IIB)BETA(3) BY OXIDANTS IS ASSOCIATED WITH TYROSINE PHOSPHORYLATION OF BETA(3)

Reactive oxygen species play an important role at the site of vascular injuries and arterial thromboses. We studied the mechanism mediating platelet aggregation induced by H2O2, a major cellular oxidant, Exposu re to H2O2 triggered platelet aggregation, but only when the platelets were stirred. Strong platelet aggregation induced by H2O2 required th e presence of the tyrosine phosphatase inhibitor sodium orthovanadate (NaVO4) and was dependent on the participation of integrin alpha(IIb)b eta(3)(glycoprotein IIb-IIIa). A specific inhibitor of alpha(IIb)beta( 3) blocked platelet aggregation induced by H2O2 and NaVO4, thus confir ming that aggregation requires this receptor. In the presence of H2O2 and NaVO4, multiple platelet substrates were phosphorylated on tyrosin e, Such tyrosine kinase response was necessary but not sufficient to a ctivate alpha(IIb)beta(3), as detected by binding of soluble fibrinoge n to platelets, Stirring of the platelets exposed to H2O2 and NaVO4 wa s also needed to allow for binding of fibrinogen to alpha(IIb)beta(3). The tyrosine kinase inhibitor genistein was able to block platelet ag gregation induced by H2O2 and NaVO4, thus confirming that tyrosine kin ase activity was needed to trigger alpha(IIb)beta(3) activation on sti rring. N-Acetyl-L-cysteine, a cell-permeant antioxidant, blocked the t yrosine phosphorylation of platelet substrates and also the platelet a ggregation induced by H2O2 and NaVO4. We found that beta(3) was phosph orylated on tyrosine in platelets exposed to H2O2 and NaVO4, even in t he absence of aggregation, Hence, tyrosine phosphorylation of beta(3) might contribute to the ''priming'' of alpha(IIb)beta(3) induced by H2 O2 and NaVO4, whereby the receptor can become activated on stirring of the platelets.