Background: Chronic allograft nephropathy (CAN), the major cause of renal graft failure, frequently displays extensive interstitial fibrin deposition. Little is known in regard to the cause of the altered coagulation/fibrinolysis balance and its relevance in the pathogenesis of CAN. Thrombin, present within the fibrin clots, can interact with a specific receptor, protease-activated receptor 1 (PAR-1), and modulate a variety of cell functions. On the other hand, the derangement of the fibrinolytic system may directly affect extracellular matrix (ECM) degradation.
Methods: In the present study, we investigated, by in situ hybridization, PAR-1 gene expression and the mRNA levels for tissue factor and plasminogen activator inhibitor 1 (PAI-1), two key regulatory molecules of coagulation and fibrinolysis, in 16 CAN biopsies and in 10 normal human kidney grafts. The thrombin-induced transforming growth factor beta (TGF-beta) gene and protein expression in proximal tubular cells (PTC) was investigated by Northern blotting and ELISA, respectively.
Results: Fibrin deposits, absent in normal grafts, were observed in the interstitial space and arterial wall of CAN. Tissue factor gene expression was not increased either at the vascular or at the interstitial level in CAN. On the contrary, PAI-1 gene expression, barely detectable in control tissue, was strikingly increased in CAN, with a distribution resembling the pattern of fibrin deposition. Note that PAI-1 gene expression was directly correlated with the degree of interstitial fibrosis. In addition, fibrin deposits were strictly associated with a marked increase of PAR-1 gene expression in endothelial cells and PTC. The tubular expression of PAR-1 was significantly higher in Banff grade II-III than in grade I. In vitro, incubation of PTC with thrombin caused a significant up-regulation of TGF-beta gene expression, followed by an increased TGF-beta release into the supernatant. Interestingly, urine from CAN patients contained significantly higher levels of TGF-beta.
Conclusions: Fibrin deposits in CAN may result from the increased expression of PAI-1 and the subsequent inhibition of fibrinolysis. The reduced fibrinolysis may cause, in turn, a decreased ECM turnover. Finally, thrombin, preserved in the active form within the fibrin clots, may interact with PAR-1 highly expressed on PTC and induce an up-regulation of ECM deposition in a TGF-beta-dependent manner.