Gushiken Francisca C, Patel Vimal, Liu Yan, Pradhan Subhashree, Bergeron Angela L, Peng Yuandong, Vijayan K Vinod
Department of Medicine, Baylor College of Medicine, Houston, Texas 77030, USA.
J Biol Chem. 2008 May 9;283(19):12862-9. doi: 10.1074/jbc.M708804200. Epub 2008 Mar 11.
Integrin alpha(IIb)beta(3) activation is critical for platelet physiology and is controlled by signal transduction through kinases and phosphatases. Compared with kinases, a role for phosphatases in platelet integrin alpha(IIb)beta(3) signaling is less understood. We report that the catalytic subunit of protein phosphatase 2A (PP2Ac) associates constitutively with the integrin alpha(IIb)beta(3) in resting platelets and in human embryonal kidney 293 cells expressing alpha(IIb)beta(3). The membrane proximal KVGFFKR sequence within the cytoplasmic domain of integrin alpha(IIb) is sufficient to support a direct interaction with PP2Ac. Fibrinogen binding to alpha(IIb)beta(3) during platelet adhesion decreased integrin-associated PP2A activity and increased the phosphorylation of a PP2A substrate, vasodilator associated phosphoprotein. Overexpression of PP2Ac(alpha) in 293 cells decreased alpha(IIb)beta(3)-mediated adhesion to immobilized fibrinogen. Conversely, small interference RNA mediated knockdown of endogenous PP2Ac(alpha) expression in 293 cells, enhanced extracellular signal-regulated kinase (ERK1/2) and p38 activation, and accelerated alpha(IIb)beta(3) adhesion to fibrinogen and von Willebrand factor. Inhibition of ERK1/2, but not p38 activation, abolished the increased adhesiveness of PP2Ac (alpha)-depleted 293 cells to fibrinogen. Furthermore, knockdown of PP2A(calpha) expression in bone marrow-derived murine megakaryocytes increased soluble fibrinogen binding induced by protease-activated receptor 4-activating peptide. These studies demonstrate that PP2Ac (alpha) can negatively regulate integrin alpha(IIb)beta(3) signaling by suppressing the ERK1/2 signaling pathway.
整合素α（IIb）β（3）的激活对血小板生理功能至关重要，并受激酶和磷酸酶信号转导的调控。与激酶相比，磷酸酶在血小板整合素α（IIb）β（3）信号传导中的作用尚不清楚。我们报道，蛋白磷酸酶2A（PP2A）的催化亚基在静息血小板以及表达α（IIb）β（3）的人胚肾293细胞中与整合素α（IIb）β（3）组成性结合。整合素α（IIb）胞质域内靠近膜的KVGFFKR序列足以支持与PP2Ac的直接相互作用。血小板黏附过程中纤维蛋白原与α（IIb）β（3）的结合降低了整合素相关的PP2A活性，并增加了PP2A底物血管舒张相关磷蛋白的磷酸化。在293细胞中过表达PP2Ac（α）可降低α（IIb）β（3）介导的对固定化纤维蛋白原黏附。相反，小干扰RNA介导的293细胞内源性PP2Ac（α）表达的敲低增强了细胞外信号调节激酶（ERK1/2）和p38的激活，并加速了α（IIb）β（3）对纤维蛋白原和血管性血友病因子的黏附。抑制ERK1/2而非p38的激活可消除PP2Ac（α）缺失的293细胞对纤维蛋白原增加的黏附性。此外，敲低骨髓来源的小鼠巨核细胞中PP2A（cα）的表达可增加蛋白酶激活受体4激活肽诱导的可溶性纤维蛋白原结合。这些研究表明，PP2Ac（α）可通过抑制ERK1/2信号通路负向调节整合素α（IIb）β（3）信号传导。
