α-Thrombin inhibits DNA synthesis in rat hepatocytes but not in hepatoma cells by receptor activation and proteolysis

Prothrombin is a plasma protein, which after tissue injury is converted to α-thrombin and is mainly involved in blood clot formation. It has also been shown to have a mitogenic effect on primary endothelial cells, vascular smooth muscle cells, fibroblasts and some tumor cells, but is an inhibitor of rat hepatocyte DNA synthesis on fibronectin matrix in cell culture. We now report that prothrombin is converted to α-thrombin by primary cultures of normal adult rat hepatocytes and α-thrombin is also a potent inhibitor of hepatocytes DNA synthesis. In contrast, rat hepatoma cells cultured under similar conditions were resistant to α-thrombin mediated DNA synthesis inhibition. The inhibitory effect of α-thrombin on DNA synthesis was antagonized by hirudin and antithrombin, two specific α-thrombin inhibitors or by the presence of collagen-I matrix. A thrombin receptor activating peptide (TRAP6) also inhibited EGF-mediated rat hepatocyte DNA synthesis, suggesting a role of the thrombin receptors in this process. Matrix fibronectin was degraded by α-thrombin. However, no appreciable cell detachment was observed. These results suggest a role of α-thrombin as a potent growth inhibitor of normal hepatocytes, possibly through control of fibronectin or other matrix protein(s).