Protease-activated receptor 1 and plasminogen activator inhibitor 1 expression in chronic allograft nephropathy - The role of coagulation and fibrinolysis in renal graft fibrosis

Background. Chronic allograft nephropathy (CAN), the major cause of renal g raft failure, frequently displays extensive interstitial fibrin deposition. Little is known in regard to the cause of the altered coagulation/fibrinol ysis balance and its relevance in the pathogenesis of CAN. Thrombin, presen t 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 aff ect extracellular matrix (ECM) degradation. Methods. In the present study, we investigated, by in situ hybridization, P AR-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 pr otein expression in proximal tubular cells (PTC) was investigated by Northe rn blotting and ELISA, respectively. Results. Fibrin deposits, absent in normal grafts, were observed in the int erstitial 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 patte rn of fibrin deposition. Note that PAI-1 gene expression was directly corre lated with the degree of interstitial fibrosis. In addition, fibrin deposit s were strictly associated with a marked increase of PAR-1 gene expression in endothelial cells and PTC. The tubular expression of PAR-1 was significa ntly higher in Banff grade II-III than in grade 1. In vitro, incubation of PTC with thrombin caused a significant up-regulation of TGF-beta gene expre ssion, followed by an increased TGF-beta release into the supernatant. Inte restingly, urine from CAN patients contained significantly higher levels of TGF-beta. Conclusions. Fibrin deposits in CAN may result from the increased expressio n of PAI-1 and the subsequent inhibition of fibrinolysis. The reduced fibri nolysis may cause, in turn, a decreased ECM turnover. Finally, thrombin, pr eserved 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.