t-PA promotes glomerular plasmin generation and matrix degradation in experimental glomerulonephritis

Background. In addition to its well-known role in degrading fibrin, recent evidence suggests that plasmin degrades matrix proteins and activates prome talloproteinases. Plasmin is generated from plasminogen by tissue plasminog en activator (t-PA). We hypothesized that t-PA treatment increases plasmin generation in nephritic glomeruli and degrades pathological matrix leading to a therapeutic reduction in matrix accumulation. Methods. Anti-Thy-1 nephritis was induced by injection of OX-7 antibody. Ra ts were given twice daily intravenous injections of saline (disease control group) or human recombinant t-PA (rt-PA; 1 mg/kg body weight) on days 3 th rough 5. Proteinuria, glomerular matrix protein staining, and glomerular mR NA levels for transforming growth factor-beta1 (TGF-beta1), fibronectin, an d plasminogen activator inhibitor type 1 (PAI-1) were evaluated at day 6. L ocalization of rt-PA, plasmin generation by glomeruli in vitro, and glomeru lar production and content of active TGF-beta1 were also investigated. Results. Compared with disease control animals, proteinuria and staining sc ore for periodic acid-Schiff (2.75 +/- 0.17 vs. 1.41 +/- 0.09), fibronectin -EDA+ (19 +/- 2 vs. 14 +/- 1), laminin (35 +/- 2 vs. 25 +/- 2), type I coll agen (33 +/- 1 vs. 21 +/- 3), and type IV collagen (27 +/- 2 vs. 23 +/- 1) were significantly reduced in treated rats (P < 0.01). Glomerular TGF-<beta >1. fibronectin, and PAI-I mRNA levels were unchanged, rt-PA colocalized wi th fibrin along glomerular capillary walls and in the mesangium. Nephritic glomeruli in vitro had decreased plasmin activity, which was elevated by an in vivo presacrifice injection of rt-PA. Glomerular production and content of active TGF-beta1 were unchanged by the rt-PA injection. Conclusions. These results show that injected rt-PA binds to fibrin in neph ritic glomeruli, thus increasing plasmin generation and promoting pathologi cal matrix degradation without activating latent TGF-beta. Agents that incr ease plasmin generation. such as t-PA, may have potential as antifibrotic t herapies.