4220D. Ribatti and othersin the CAM, as suggested by the residual <strong>angiogenic</strong> <strong>activity</strong>exerted by PMSF-treated uPA. Relevant to this point, previousstudies have shown that purified uPA exerts an uPARdependentmitogenic <strong>activity</strong> on human fibroblasts that cannotbe mimicked by ATF alone, suggesting that both receptoroccupancy and catalytic <strong>activity</strong> may play a <strong>role</strong> in thebiological <strong>activity</strong> <strong>of</strong> uPA (De Petro et al., 1994).Transgenic mice lacking uPA and/or tissue-type PA(Carmeliet et al., 1994) or plasminogen (Bugge et al., 1995)develop normally, demonstrating that the proteolytic cascadetriggered by plasminogen activation is not a fundamentalcomponent for neovascularization during development. On theother hand, uPA/plasmin inhibitors affect angiogenesis in vitroand in <strong>vivo</strong> in different experimental models (Mignatti et al.,1989; Yasunaga et al., 1989; Avery et al., 1990; Bastaki et al.,1997). Also, the reduced rate <strong>of</strong> growth <strong>of</strong> murine experimentaltumors by anti-fibrinolytic drug-treatment may depend on areduced neovascularization (Astedt and Tropè, 1980; Corasantiet al., 1982; Teuscher and Pester, 1984). Finally, an alteredfibrinolytic status affects polyoma middle T-induced vasculartumor formation (Sabapathy et al., 1997). Our observationsindicate for the first time that uPA/plasmin-dependentdegradation <strong>of</strong> ECM, with the consequent mobilization <strong>of</strong>stored angiogenesis factors, may represent one <strong>of</strong> themechanisms by which the fibrinolytic system modulatesangiogenesis in <strong>vivo</strong>.We thank A. 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