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 thrombosi s suggesting that factor XII is an antithrombotic protein. The biochemical mechanism leading to this clinical observation is unknown. We have previous ly reported high molecular weight kininogen (HK) inhibition of thrombin-ind uced 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 effe ct on SFLLRN-induced platelet aggregation. Moreover, an antibody to the thr ombin site on protease-activated receptor-1 failed to block factor XII bind ing to platelets. Inhibition of thrombin-induced platelet aggregation was d emonstrated with factor Wa but not with factor XII zymogen or factor XIIa i ndicating that the conformational exposure of the heavy chain following pro teolytic activation is required for inhibition. However, inactivation of th e catalytic activity of factor XIIa did not affect the inhibition of thromb in-induced platelet aggregation. Factor XII showed displacement of biotin-l abeled 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 o f the GP Ib complex. Antibodies to GP Ib and GP IX, which inhibited HK bind ing to platelets, did not block factor XII binding. However, using a biosen sor, which monitors protein-protein interactions, both HX and factor XII bi nd to GP IB alpha. Factor XIII may serve to regulate thrombin binding to th e GP Ib receptor by colocalizing with HK, to control the extent of platelet aggregation in vivo.