Summary

Platelet activation induced by von Willebrand factor (VWF)binding to the membrane GPIb-IX-V receptor involves multiplesignal transduction pathways. Among these, recruitmentand activation of the FcγRIIA and stimulation of phospholipaseA₂ represent independent events equally essential to support acomplete platelet response. Phospholipase A₂ is activated bycalcium and by phosphorylation through MAP kinases. In thiswork, we found that VWF stimulated the rapid and sustainedphosphorylation of p38 MAP kinase (p38MAPK). In vitro kinaseassay revealed that VWF-stimulated phosphorylation ofp38MAPK was associated with increased kinase activity. Bindingof VWF to GPIb-IX-V, but not to integrin α_Iibβ₃, was required tosupport phosphorylation of p38MAPK. Neither the blockade ofthe membrane FcγRIIA by a specific monoclonal antibody or the prevention of thromboxane A₂ synthesis by cyclooxygenaseinhibitors affected VWF-induced p38MAPK activation. How-ever,phosphorylation of p38MAPK was prevented by the tyro-sinekinase Syk inhibitor piceatannol. Treatment of plateletswith the p38MAPK inhibitor SB203580 totally prevented VWF-stimulatedplatelet aggregation. Moreover, release of arachidonicacid induced by VWF was strongly impaired by inhibition ofp38MAPK.We also found that VWF induced phosphorylation ofcytosolic phospholipase A₂, and that this process was preventedby the p38MAPK inhibitor SB203580.These results demonstratethat p38MAPK is a key element in the FcγRIIA-independentpathway for VWF-induced platelet activation, and is involvedin the stimulation of phospholipase A₂ and arachidonic acidrelease.