The anticoagulant factor, protein S, is produced by cultured human vascular smooth muscle cells and its expression is up-regulated by thrombin

The anticoagulant factor protein S is a secreted vitamin K-dependent γ-carboxylated protein that is mainly made in the liver. Protein S is homologous to the growth arrest specific protein, Gas6, the expression of which is up-regulated in cultured fibroblasts upon serum withdrawal. We report here the synthesis and secretion of protein S by cultured human vascular smooth muscle cells (HVSMCs). Western blot analysis revealed that similar amounts of protein S are secreted by both growing and growth-arrested HVSMCs. HVSMC-derived protein S was found to be γ-carboxylated as it was precipitated by barium citrate and was shown to possess protein C cofactor activity. Treatment with the vitamin K antagonist warfarin led to the accumulation of intracellular undercarboxylated protein S forms that were rapidly secreted upon the reintroduction of vitamin K. Northern blotting analysis showed that cultured HVSMCs express a protein S transcript. The expression of protein S messenger RNA was unaffected by either warfarin, growth arrest, or various VSMC mitogens, such as platelet-derived growth factor-BB, basic fibroblast growth factor, transforming growth factor-β, or hepatocyte growth factor. Thrombin, however, induced an up-regulation of protein S expression at both messenger RNA and protein levels. The evidence we provide for protein S secretion by cultured HVSMCs and its up-regulation by thrombin, together with earlier reports showing that protein S acts as a mitogen for these cells, suggests that, in addition to its known role in regulating blood clotting, protein S may also be an important autocrine factor in the pathophysiology of the vasculature.