The presence of procoagulants and fibrin deposition have been demonstrated in malignant tumors. Although thrombin, a key enzyme in coagulation, has other various biological functions, the significance of its presence in tumors is not known. We studied the effects of thrombin on the expression of urokinase-type plasminogen activator (uPA) which is known to play a role in tumor invasion, using a human prostate cancer cell line PC-3. Human α-thrombin added to cultures of PC-3 produced a dose-dependent and time-dependent increased secretion of uPA that was greatest at 3–6 h after exposure to thrombin. Increase in uPA antigen paralleled the increase in mRNA level, which reached a maximum at 4 h. Thrombin showed the maximum effect on uPA expression at a concentration 1–2 units/ml. Zymography showed that transient exposure to thrombin induced an increase in fibrinolytic activity which could be quenched by anti-uPA antibody. The thrombin receptor-activating peptide also caused an increase in uPA protein and mRNA level, indicating the presence of the same thrombin specific receptor on PC-3 cells as on platelets and endothelial cells. Thrombin did not affect the expression of other components of the plasminogen activation system, tissue-type plasminogen activator and type-1 plasminogen activator inhibitor, and uPA receptor. These results indicate that thrombin increases uPA expression selectively by the stimulation of a functional thrombin receptor on PC-3 cells. Since uPA is known to play a role in pericellular proteolysis of extracellular matrix, thrombin may be involved in the regulation of tumor invasion and metastasis.

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This work is supported in part by a VA Merit Review Research Grant.

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