Human factor XII binding to the glycoprotein Ib-IX-V complex inhibits thrombin-induced platelet aggregation

Factor XII deficiency has been postulated to be a risk factor for thrombosis suggesting that factor XII is an antithrombotic protein. The biochemical mechanism leading to this clinical observation is unknown. We have previously reported high molecular weight kininogen (HK) inhibition of thrombin-induced platelet aggregation by binding to the platelet glycoprotein (GP) Ib-M-V complex. Although factor XII will bind to the intact platelet through GP Ib alpha (glycocalicin) without activation, we now report that factor XIIa (0.37 mu M), but not factor XII zymogen, is required for the inhibition of thrombin-induced platelet aggregation. Factor XIIa had no significant effect on SFLLRN-induced platelet aggregation. Moreover, an antibody to the thrombin site on protease-activated receptor-1 failed to block factor XII binding to platelets. Inhibition of thrombin-induced platelet aggregation was demonstrated with factor Wa but not with factor XII zymogen or factor XIIa indicating that the conformational exposure of the heavy chain following proteolytic activation is required for inhibition. However, inactivation of the catalytic activity of factor XIIa did not affect the inhibition of thrombin-induced platelet aggregation. Factor XII showed displacement of biotin-labeled HK (30 nM) binding to gel-filtered platelets and, at concentrations of 50 nM, was able to block 50% of the HX binding, suggesting involvement of the GP Ib complex. Antibodies to GP Ib and GP IX, which inhibited HK binding to platelets, did not block factor XII binding. However, using a biosensor, which monitors protein-protein interactions, both HX and factor XII bind to GP IB alpha. Factor XIII may serve to regulate thrombin binding to the GP Ib receptor by colocalizing with HK, to control the extent of platelet aggregation in vivo.