Suppression of angiogenesis and tumor growth by the inhibitor K1-5 generated by plasmin-mediated proteolysis

Proteolytic enzymes are involved in generation of a number of endogenous an giogenesis inhibitors. Previously, we reported that angiostatin, a potent a ngiogenesis inhibitor, is a proteolytic fragment containing the first four kringle modules of plasminogen. In this report, we demonstrate that urokina se-activated plasmin can process plasminogen to release an angiogenesis inh ibitor, K1-5 (protease-activated kringles 1-5). K1-5 inhibits endothelial-c ell proliferation with a half-maximal concentration of approximately 50 pM. This inhibitory effect is endothelial-cell-specific and appears to be at l east approximately 50-fold greater than that of angiostatin. A synergistic efficacy of endothelial inhibition was observed when angiostatin and kringl e 5 (K5) were coincubated with capillary endothelial cells. The synergistic effect is comparable to that produced by K1-5 alone. Systemic treatment of mice with K1-5 at a low dose significantly blocked the fibroblast growth f actor-induced corneal neovascularization, whereas angiostatin had no effect at the same dose. K1-5 also suppressed angiogenesis in chicken embryos. Sy stemic administration of K1-5 at a low dose at which angiostatin was ineffe ctive significantly suppressed the growth of a murine T241 fibrosarcoma in mice. The antitumor effect correlates with the reduced neovascularization. These findings suggest that the plasmin-mediated proteolysis may be involve d in the negative switch of angiogenesis.