Plasminogen-dependent and -independent proteolytic activity of murine endothelioma (End) cells that were derived from mice with targeted inactivation of the tissue-type plasminogen activator (t-PA ^-/-), urokinase-type plasminogen activator (u-PA^-/-) or plasminogen activator inhibitor-1 (PAI-1 ^-/- genes was studied with the use of fibrin and extracellular matrix degradation assays. In a buffer milieu, the activation rate of plasminogen (final concentration 0.25 µM) with wild-type and t-PA^-/- End cells (3 X 10⁴ to 4 X 10⁶ cells/ml) was comparable, but it was about 4-fold reduced with u-PA ^-/- End cells and 3-fold enhanced with PAI-1End cells. Plasminogen activation was markedly reduced by addition of amiloride or of anti-murine u-PA antibodies but not by addition of anti-murine t-PA antibodies, and it was not stimulated by addition of fibrin. Lysis of¹²⁵I-fibrin labeled matrix in the presence of plasminogen was comparable with wild-type, t-PA^-/- and PAI-1^-/- End cells (50% lysis in 3 h with 0.7 to 1.5 X 10⁶ cells/ml), but was significantly reduced with u-PA^-/- End cells (50% lysis in 20 h with 0.87 X 10⁶ cells/ml). Lysis of³H-proline labeled extracellular matrix in the presence of plasminogen with wild-type, t-PA^-/- and PAI-1^-/- End cells (20% lysis in 48 h with 3 to 5 X 10⁶ cells/ml) was comparable, but it was virtually abolished with u-PA^-/- End cells. In the absence of plasminogen, lysis of both the fibrin and the extracellular matrix by all four cell types was drastically reduced and was virtually abolished by addition of phenylmethylsulfonylfluoride or 1,10 phenanthroline.

These data indicate that the proteolytic activity of the transformed murine endothelioma cells, measured in plasminogen activation or matrix degradation assays, is essentially u-PA-related and largely plasminogen-dependent.