Protein C inhibits endocytosis of thrombin-thrombomodulin complexes in A549 lung cancer cells and human umbilical vein endothelial cells

We investigated the effect of protein C on the endocytosis of thrombin- thrombomodulin complexes. We previously showed that exposure of umbilical vein endothelial cells to thrombin stimulated the internalization and degradation of thrombin. A similar internalization was stimulated by a monoclonal antithrombomodulin antibody. We have repeated these studies in the presence of protein C and found that endocytosis of 125I-thrombin-thrombomodulin complexes, but not 125I- antithrombomodulin-thrombomodulin complexes, is inhibited. Activated protein C did not inhibit endocytosis of thrombin-thrombomodulin complexes. Protein C inhibited both internalization and degradation of 125I-thrombin and diisopropylphosphoryl (DIP) 125I-thrombin in human lung cancer cells (A549). These effects were observed at protein C concentrations found in human plasma. Protein S had no effect on the inhibition of endocytosis of thrombin-thrombomodulin complexes by protein C. We propose that protein C may regulate the rate of endocytosis of thrombin-thrombomodulin complexes in vivo and thereby control the capacity for endothelium to activate protein C.