Angiostatins

Abstract: Proteolytic enzymes are involved in generation of a number of endogenous angiogenesis inhibitors. Previously, we reported that angiostatin, a potent angiogenesis inhibitor, is a proteolytic fragment containing the first four kringle modules of plasminogen. In this report, we demonstrate that urokinase-activated plasmin can process plasminogen to release an angiogenesis inhibitor, K1–5 (protease-activated kringles 1–5). K1–5 inhibits endothelial-cell proliferation with a half-maximal concentration of approximately 50 pM. This inhibitory effect is endothelial-cell-specific and appears to be at least approximately 50-fold greater than that of angiostatin. A synergistic efficacy of endothelial inhibition was observed when angiostatin and kringle 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 factor-induced corneal neovascularization, whereas angiostatin had no effect at the same dose. K1–5 also suppressed angiogenesis in chicken embryos. Systemic administration of K1–5 at a low dose at which angiostatin was ineffective 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 involved in the negative switch of angiogenesis.

Abstract: The main physiological function of plasmin is blood clot fibrinolysis and restoration of normal blood flow. To date, however, it became apparent that in addition to thrombolysis, the plasminogen/plasmin system plays an important physiological and pathological role in a number of other essential processes: degradation of the extracellular matrix, embryogenesis, cell migration, tissue remodeling, wound healing, angiogenesis, inflammation, and tumor cell migration. This review focuses on structural features of plasminogen, regulation of its activation by physiological plasminogen activators, inhibitors of plasmin, and plasminogen activators, and the role of plasminogen binding to fibrin, cellular receptors, and extracellular ligands in various functions performed by plasmin thus formed.