Extracellular matrix degradation by cultured mesangial cells: mediators and modulators

Decreased degradation of the glomerular extracellular matrix (ECM) is thought to contribute to the accumulation of glomerular ECM that occurs in diabetic nephropathy and other chronic renal diseases. Several lines of evidence indicate a key role for the plasminogen activator/plasminogen/plasmin system in glomerular ECM degradation. However, which of the two plasminogen activators (PAs) present in renal tissue, tissue plasminogen activator (tPA) or urokinase-type plasminogen activator (uPA), is responsible for plasmin generation and those factors that modulate the activity of this system remain unclear. This study utilized mesangial cells isolated from mice with gene deletions for tPA, uPA, and plasminogen activator inhibitor 1 (PAI-1) to further delineate the role of the PA/plasminogen/plasmin system in ECM accumulation. ECM degradation by uPA-null mesangial cells was not significantly different from controls (92% +/- 1%, n = 12). In contrast, ECM degradation by tPA-null mesangial cells was markedly reduced (-78 +/- 1%, n = 12, P < 0.05) compared with controls, whereas tPA/uPA double-null mesangial cells degraded virtually no ECM. Previous studies from this laboratory have established that transforming growth factor-beta1 (TGFbeta1) inhibits ECM degradation by cultured mesangial cells by increasing the production of PAI-1, the major physiological PA inhibitor. In keeping with this observation, TGFbeta1 (1 ng/ml) had no effect on ECM degradation by PAI-1-null MC. High glucose levels (30 mM) in the presence or absence of insulin (0.1 mM) caused a moderate increase in ECM degradation by normal human mesangial cells. In contrast, glycated albumin, whose concentration is known to increase in diabetes, produced a dose-dependent (0.2-0.5 mg/ml) inhibition of ECM degradation by normal human mesangial cells. Taken together, these results document the importance of tPA versus uPA in renal plasmin production and indicate that in contrast to elevated glucose, glycated albumin may contribute to ECM accumulation in diabetic nephropathy.