ANTITHROMBIN-III INHIBITS THROMBIN-INDUCED PROLIFERATION IN HUMAN ARTERIAL SMOOTH-MUSCLE CELLS

Thrombin has attracted increasing attention as a possible mitogen for vascular smooth muscle cells in lesion development both after vascular injury and in atherogenesis. In this study, the ability of antithromb in III to inhibit alpha-thrombin-induced DNA synthesis and cell prolif eration in human arterial smooth muscle cells was analyzed. We demonst rate a concentration-dependent initiation of DNA synthesis and cell pr oliferation by alpha-thrombin. This effect was abolished when complex formation with antithrombin III was allowed before thrombin was added to the cell cultures. Addition of alpha-thrombin and antithrombin III simultaneously at the beginning of the incubation period also resulted in an inhibition of thrombin-induced DNA synthesis, but to a lower de gree. The inhibitory activity of antithrombin III was enhanced in the presence of heparin, which on its own had no inhibitory effect on thro mbin-induced DNA synthesis. In contrast, the mitogenic activity of alp ha-thrombin could be inhibited by heparin in the presence of low conce ntrations of serum. This inhibition was dependent on the presence of a ntithrombin III in serum, since heparin lacked effect if antithrombin III was depleted from serum by immunoaffinity chromatography. Analysis of the enzymatic activity of thrombin showed that the influence on ca talytic activity of thrombin corresponded to the mitogenic activity of thrombin in the presence of heparin, antithrombin III, and serum. The results suggest that the mitogenic activity of thrombin is regulated by antithrombin III. Therefore, antithrombin III may serve dual functi ons by inhibiting thrombin in the coagulation cascade and by neutraliz ing its growth-promoting effects on vascular smooth muscle cells.