Characterization of an inducible endothelial cell prothrombin activator

Abstract

In vivo prothrombin activation is thought to occur via a factor X(a)/factor V-dependent mechanism. We investigated whether human venous endothelial cells (EC) could be induced to express a prothrombin activator. EC treated with lipopolysaccharide (LPS) or interleukin-1 activated prothrombin in the absence of exogenous factors X(a) and V. This activity resided in the membrane fraction of EC and was not inhibited by an antibody to factor V. The apparent Km value was 3.3 ± 0.3 μmol/L. Comparative studies of thrombin generation using a model system of phospholipid and factors X(a)/V versus LPS-treated EC were performed to quantitate the effects of known inhibitors to factor X(a). The factor X(a) inhibitor DEGR- chloromethyl ketone and an antibody to factor X inhibited prothrombin activation. However, the EC activator did not hydrolyze a factor X(a) chromogenic substrate, and recombinant tick anticoagulant peptide did not suppress activity of the prothrombin activator. The apparent molecular weight of the EC activator was ~30 kD. Exogenous factor V enhanced the activity of the EC activator, such that in the presence of factor V, the apparent Km value was 1.28 ± 0.10 μmol/L. Additionally, LPS-treated EC activated exogenous factor V. This activator has several characteristics of a previously described inducible murine monocyte prothrombin activator and may contribute to thrombin generation associated with pathologic stimuli.