ECM degradation by cultured human mesangial cells is mediated by a PA/plasmin/MMP-2 cascade. We examined the role of the plasminogen activator/plasmin system in extracellular matrix (ECM) degradation by human mesangial cells cultured on thin films of ¹²⁵I-labeled ECM (Matrigel). ECM degradation (release of ¹²⁵I into the medium) was dependent on exogenous plasminogen, proportional to the number of mesangial cells and amount of plasminogen added, and coincident witji the appearance of plasmin in the medium. ECM degradation was completely blocked (P < 0.001) by two plasmin inhibitors, α-2-antiplasmin (40 µg/ml) and aprotinin (216 KIU/ml), and partially reduced (-33 ± 1.8%, P < 0.01) by TIMP-1 (40 µg/ml), a specific inhibitor of matrix metallo-proteinases. Zymography of medium obtained from cells cultured in the absence of plasminogen revealed the presence of latent matrix metallo-proteinase-2 (MMP-2) which was converted to a lower molecular weight, active form in the presence of mesangial cells and plasminogen. Northern analysis of poly A + RNA prepared from cultured human mesangial cells revealed mRNA for tissue-type plasminogen activator (tPA), urokinase-type plasminogen activator (uPA), plasminogen activator inhibitor-1 (PAI-1), and uPA receptor (uPAR). The presence of uPA protein in medium obtained from cultured human mesangial cells was demonstrated by Western blotting and ELISA which revealed a large molar excess of PAI-1 (1.2 ± 0.1 × 10^-9M) over uPA (1.2 ± 0.1 × 10^-12M) and tPA (0.19 ± 0.04 × 10 ^-9M). ECM degradation was reduced by a monoclonal antibody (MAb) against human tPA (-54 ± 8.6%) or human uPA (-39 ± 5.2%) compared to cells treated with identical amounts of non-specific monoclonal IgG (P < 0.01). In contrast, MAb against human PAI-1 increased ECM degradation four-fold (P < 0.001). A MAb against human uPAR had no significant effect on ECM degradation. Taken together, our results indicate that ECM degradation by cultured human mesangial cells is mediated by a proteinase cascade. This cascade is initiated by tPA and generates plasmin and active MMP-2, which together carry out the degradation of the ECM. We postulate that decreased activity of this cascade may represent a final common pathway contributing to glomerular ECM accumulation in progressive renal disease.