Previous studies suggest a role for the plasminogen or fibrinolytic system in the activation of latent-transforming growth beta (L-TGFbeta) into active TGFbeta. In the present study, the anti-apoptotic activity of TGFbeta on cultured vascular smooth muscle cells (SMC) isolated from the aorta of transgenic mice with single inactivation of genes encoding the tissue-type plasminogen activator (t-PA(-/-)), urokinase-type plasminogen activator (u-PA(-/-)), urokinase receptor (u-PAR(-/-)) or plasminogen (Plg(-/-)) genes was examined. Latent-TGFbeta inhibited serum deprivation-induced apoptosis of SMC isolated from wild-type and t-PA(-/-) mice but failed to reduce apoptosis of SMC isolated from u-PA(-/-), u-PAR(-/-) or Plg(-/-) mice. Active TGFbeta, however, was able to inhibit serum deprivation-induced apoptosis of these 5 cell types, indicating that u-PA and/or plasmin were involved in the activation of L-TGFbeta. The anti-apoptotic effect of L-TGFbeta could not be evoked by addition of exogenous t-PA to u-PA(-/-) cells, but was revealed by addition of exogenous u-PA or plasmin. This effect was dependent on the catalytic activity of plasmin as revealed by the dose-dependent inhibition of aprotinin or epsilon aminocaproic acid (EACA). These results therefore indicate that, at least in vitro, u-PA-mediated plasmin, through the generation of active TGFbeta from L-TGFbeta, is required for the anti-apoptotic activity of TGFbeta on SMC.