Rho-kinase regulates human platelet activation induced by thromboxane A₂ independently of p38 MAP kinase

We have previously demonstrated that ristocetin, an activator of GPIb/IX/V, induces the release of soluble CD40 ligand (sCD40L) via thromboxane A₂ production in human platelets. It has been shown that thromboxane A₂ induces the activation of Rho-kinase, a downstream effector of Rho, in human platelets. In the present study, we investigated the exact roles of Rho-kinase in thromboxane A₂-induced platelet activation. We found that U46619, a thromboxane receptor (TP) agonist, induced the phosphorylation of cofilin, a target of Rho-kinase signaling, and that the cofilin phosphorylation by U46619 was suppressed by Y27632 or fasudil, specific inhibitors of Rho-kinase. Y27632 and fasudil markedly decreased large platelet aggregate formation by U46619. The release of sCD40L and secretion of platelet-derived growth factor (PDGF)-AB stimulated by U46619 were inhibited by Y27632 and fasudil. SB203580, a specific inhibitor of p38 mitogen-activated protein (MAP) kinase, reduced the sCD40L release and PDGF-AB secretion. Y27632 and fasudil failed to affect the phosphorylation of p38 MAP kinase whereas SB203580 had little effect on the phosphorylation of cofilin induced by U46619. In conclusion, our results strongly suggest that Rho-kinase regulates thromboxane A₂-induced human platelet activation independently of p38 MAP kinase.