A role for p38 MAP kinase in platelet activation by von Willebrand factor

Platelet activation induced by von Willebrand factor (VWF) binding to the membrane GPIb-IX-V receptor involves multiple signal transduction pathways. Among these, recruitment and activation of the FcgammaRIIA and stimulation of phospholipase A(2) represent independent events equally essential to support a complete platelet response. Phospholipase A(2) is activated by calcium and by phosphorylation through MAP kinases. In this work, we found that VWF stimulated the rapid and sustained phosphorylation of p38 MAP kinase (p38MAPK). In vitro kinase assay revealed that VWF-stimulated phosphorylation of p38MAPK was associated with increased kinase activity. Binding ofVVVF to GPlb-IX-V, but not to integrin alpha(llb)beta(3), was required to support phosphorylation of p38MAPK. Neither the blockade of the membrane FcgammaRIIA by a specific monoclonal antibody or the prevention of thromboxane A(2) synthesis by cyclooxygenase inhibitors affected VWF-induced p38MAPK activation. However, phosphorylation of p38MAPK was prevented by the tyrosine kinase Syk inhibitor piceatannol. Treatment of platelets with the p38MAPK inhibitor SB203580 totally prevented VWFstimulated platelet aggregation. Moreover, release of arachidonic acid induced by VWF was strongly impaired by inhibition of p38MAPK. We also found that VWF induced phosphorylation of cytosolic phospholipase A(2), and that this process was prevented by the p38MAPK inhibitor SB203580. These results demonstrate that p38MAPK is a key element in the FcgammaRIIA-independent pathway for VWF-induced platelet activation, and is involved in the stimulation of phospholipase A(2) and arachidonic acid release.