Effector protease receptor 1 mediates the mitogenic activity of factor Xa for vascular smooth muscle cells in vitro and in vivo

The binding of I-125-factor Xa to human aortic smooth muscle cell (SMC) monolayers was studied, At 4 degrees C, (125)-factor Xa bound to a single class of binding sites with a dissociation constant value of 3.6 +/- 0.7 nM and a binding site density of 11,720 +/- 1,240 sites/cell (n = 9), I-125-factor Xa binding was not affected by factor X, thrombin, or by DX9065, a direct inhibitor of factor Xa, but was inhibited by factor Xa (IC50 = 5.4 +/- 0.2 nM; n = 9) and by antibodies specific for the effector cell protease receptor 1 (EPR-1), a well-known receptor of factor Xa on various cell types, A factor X peptide duplicating the inter-EGF sequence Leu(83)-Leu(88)-(Gly) blocked the binding of I-125-factor Xa to these cells in a dose-dependent manner (IC50 = 110 +/- 21 nM), Factor Xa increased phosphoinositide turnover in SMCs and when added to SMCs in culture was a potent mitogen, These effects were inhibited by DX9065 and by antibodies directed against EPR-1 and PDGF, Increased expression of EPR-1 was identified immunohistochemically on SMCs growing in culture and in SMCs from the rabbit carotid artery after vascular injury, When applied locally to air-injured rabbit carotid arteries, antibodies directed against EPR-1 (100 mu g/artery) strongly reduced myointimal proliferation 14 d after vascular injury (65-71% inhibition, P < 0.01), DX9065 (10 mg/kg, subcutaneous) inhibited myointimal proliferation significantly (43% inhibition, P < 0.05), These findings indicate that SMCs express functional high affinity receptors for factor Xa related to EPR-1, which may be of importance in the regulation of homeostasis of the vascular wall and after vascular injury.