Activation of endogenous Factor V by a homocysteine-induced vascular endothelial cell activator

Abstract

Vascular endothelium possesses multiple procoagulant properties, including synthesis and expression of Factor V. We studied the effects of homocysteine on the regulation of endothelial cell Factor V activity. Elevated levels of homocysteine are associated with the congenital thrombotic disorder homocystinuria. Treatment of cultured endothelial cells with 0.5-10 mM homocysteine had no effect on cell morphology, but did increase Factor V activity and prothrombin activation by Factor X(a). A radioimmunoassay for endothelial cell Factor V demonstrated that homocysteine treatment did not increase Factor V antigen levels. 125I-prothrombin was activated by treated endothelial cells and Factor X(a) in the presence of thrombin inhibitors. Exogenous 125I-Factor V was elevated by homocysteine-treated but not control endothelial cells. 125I-Factor V cleavage products distinct from those generated by thrombin Factor X(a) were identified. These data provide evidence for regulation of endothelial cell Factor V activity, and indicate that increased factor V activity associated with homocysteine-treated vascular endothelium results primarily from induction of an activator of Factor V.