A mitogen-activated protein kinase-dependent signaling pathway in the activation of platelet integrin alpha(IIb)beta(3)

We have recently shown that the platelet integrin alpha (IIb)beta (3) is ac tivated by von Willebrand factor (vWF) binding to its platelet receptor, gl ycoprotein Ib-IX (GPIb-IX, via the protein kinase G (PKG) signaling pathway . Here we show that GPIb-IX-mediated activation of integrin alpha (IIb)beta (3) is inhibited by dominant negative mutants of Raf-1 and MEK1 in a recon stituted integrin activation model in Chinese hamster ovary (CHO) cells and that the integrin-dependent platelet aggregation induced by either vWF or low dose thrombin is inhibited by MEK inhibitors PD98059 and U0126. Thus, m itogen-activated protein kinase MAPK) pathway is important in GPIb-IX-depen dent activation of platelet integrin alpha (IIb)beta (3). Furthermore, vWF binding to GPIb-IX induces phosphorylation of Thr-202/Tyr-204 of extracellu lar signal-regulated kinase 2 (ERK2). GPIb-IX-induced ERK2 phosphorylation is inhibited by PKG inhibitors and enhanced by overexpression of recombinan t PKG. PKG activators also induce ERK phosphorylation, indicating that acti vation of MAPK pathway is downstream from PKG. Thus, our data delineate a n ovel integrin activation pathway in which ligand binding to GPIb-IX activat es PKG that stimulates MAPK pathway, leading to integrin activation.