Human endothelial cells synthesize protein S

Human umbilical vein endothelial cells were analyzed for the presence of prothrombin, factor VII, protein C, and protein S in culture supernatants and cell extracts using specific radioimmunoassays. Only protein S was detected. Conditioned medium from 24-hour cultures and cell lysates contained 21.7 ng/mL and 88.8 ng/10(7) cells of protein S, respectively. Intrinsic labeling and immunoprecipitation indicated that protein S was synthesized and secreted as a 75,000 molecular weight protein. Vitamin K, phorbol myristate acetate, and thrombin increased the production or specific activity (determined from activity/antigen ratios of 0.99 to 1.07, 0.93 to 1.04, and 0.90 to 1.04, respectively) of protein S. While untreated cells secreted a partially active protein S (activity/antigen = 0.40), warfarin greatly decreased the specific activity (less than 0.10) of this molecule, suggesting that endothelial cells contain the enzymes required for the carboxylation of selected glutamic acid residues. The production of protein S by these cells supports the hypothesis that cofactor production and expression by the endothelial cells may play a significant regulatory role in the initiation, propagation, and suppression of hemostasis and thrombosis.