Src family kinase-mediated and Erk-mediated thromboxane A2 generation are essential for VWF/GPIb-induced fibrinogen receptor activation in human platelets.

The binding of von Willebrand factor (VWF) to the platelet membrane glycoprotein Ib-IX (GPIb-IX) results in platelet activation. In this study, we sought to clarify previous conflicting reports and to elucidate the mechanism of activation and the precise role of extracellular signal-regulated kinase (Erk) in VWF-induced platelet activation. Erk2 is activated in platelets on stimulation with VWF/ristocetin in a time-dependent manner. VWF-induced Erk2 phosphorylation and thromboxane A2 (TXA2) release were completely blocked by PP2, an Src family kinase inhibitor, suggesting that Erk is downstream of Src family kinases. U73122, a phospholipase C inhibitor, also abolished TXA2 generation and Erk phosphorylation. Although VWF fostered the agglutination of platelets regardless of any additional treatment, the inhibition of mitogen-activated protein kinase kinase (MEK) with U0126 abolished VWF-induced platelet aggregation and thromboxane production in non-aspirin-treated washed platelets. However, in platelets treated with aspirin, VWF failed to cause any aggregation. Thus, we conclude that VWF stimulation of platelets results in phospholipase A2 activation through Erk stimulation and that Src family kinases and phospholipase C play essential roles in this event. We further conclude that VWF-induced platelet aggregation does not directly depend on Erk activation but has an absolute requirement for Src/Erk-mediated TXA2 generation.