Plasminogen supports tumor growth through a fibrinogen-dependent mechanism linked to vascular patency

The functions of plasminogen in cardiovascular disease

Plasminogen (Plg) and its derivative serine protease, plasmin, together with the activators, inhibitors, modulators, and substrates of the Plg network, are postulated to regulate a wide variety of biologic responses that could influence cardiovascular disease. The development of Plg-deficient mice has provided an incisive approach to test these proposed functions in vivo. Several different models of atherosclerosis, restenosis, aneurysm, and thrombosis have been analyzed in these mice and have demonstrated profound effects of Plg on these events as well as on the inflammatory response, which contributes to these cardiovascular diseases. Plasmin (ogen) may influence the progression of cardiovascular diseases through its degradation of matrix proteins, including fibrin; its activation of matrix metalloproteinases; its regulation of growth factor and chemokine pathways; or its influence on directed cell migration. Dissection of these mechanisms represents a future challenge toward understanding the roles of Plg in vivo.

Platelets and fibrin(ogen) increase metastatic potential by impeding natural killer cell-mediated elimination of tumor cells

To test the hypothesis that platelet activation contributes to tumor dissemination, we studied metastasis in mice lacking Galphaq, a G protein critical for platelet activation. Loss of platelet activation resulted in a profound diminution in both experimental and spontaneous metastases. Analyses of the distribution of radiolabeled tumor cells demonstrated that platelet function, like fibrinogen, significantly improved the survival of circulating tumor cells in the pulmonary vasculature. More detailed studies showed that the increase in metastatic success conferred by either platelets or fibrinogen was linked to natural killer cell function. Specifically, the pronounced reduction in tumor cell survival observed in fibrinogen- and Galphaq-deficient mice relative to control animals was eliminated by the immunologic or genetic depletion of natural killer cells. These studies establish an important link between hemostatic factors and innate immunity and indicate that one mechanism by which the platelet-fibrin(ogen) axis contributes to metastatic potential is by impeding natural killer cell elimination of tumor cells.

Tumor growth and metastasis are not affected in thrombin-activatable fibrinolysis inhibitor-deficient mice

Many studies have indicated that the plasminogen activation system may have a prominent role in cancer. Activation of the zymogen plasminogen into the serine protease plasmin by plasminogen activator is mediated by carboxyterminal basic amino acids in fibrin, including lysines and arginines. Thrombin-activatable fibrinolysis inhibitor (TAFI) is a circulating carboxypeptidase B-type proenzyme that, after activation, removes carboxyterminal lysine or arginine residues in fibrin, resulting in decreased plasminogen activation and attenuated fibrinolysis. To determine directly whether TAFI is involved in primary tumor growth and metastasis formation, we examined the effects of TAFI deficiency on subcutaneous growth and experimentally or spontaneously induced pulmonary metastasis formation of different tumor cell types in mice. In all tumor models TAFI deficiency did not affect the formation and growth of primary and metastasized tumors.