Thrombin down‐regulates the TGF‐β‐mediated synthesis of collagen and fibronectin by human proximal tubule epithelial cells through the EPCR‐dependent activation of PAR‐1

Abstract

Human proximal tubule (HK-2) cells are commonly used as cellular models to understand the mechanism by which inflammatory mediators cause renal injury. It has been observed that thrombin stimulates the expression of TGF-β, extracellular matrix (ECM) proteins and proinflammatory cytokines by HK-2 cells. These in vitro responses correlate well with the pathology of glomerular and tubular diseases observed in acute renal injury. HK-2 cells express PAR-1 and the thrombin activation of this receptor has been reported to up-regulate the TGF-β-mediated expression of ECM proteins, suggesting a possible pathogenic role for PAR-1 signaling by thrombin in acute renal injury. On the other hand, several recent studies have indicated that activated protein C plays a renoprotective role, thus inhibiting the inflammatory responses and attenuating renal injury, presumably by activating the same cell surface receptor. In this study, we show that HK-2 cells express endothelial protein C receptor (EPCR) and that the occupancy of this receptor by protein C switches the signaling specificity of thrombin so that the activation of PAR-1 by thrombin inhibits the TNF-α-mediated synthesis of IL-6 and IL-8 and down-regulates the TGF-β-mediated expression of ECM proteins. These results suggest a possible protective role for EPCR in acute kidney injury.

Low but sustained coagulation activation ameliorates glucose-induced podocyte apoptosis: protective effect of factor V Leiden in diabetic nephropathy

“…26,30,32 The cytoprotective effects of thrombin have also been reported in nonendothelial cells. Thrombin reduces TGF-␤-mediated expression of extracellular matrix proteins in renal tubular cells 28 and thrombin dose dependently regulates proliferation of rat glioma cells. 33 The beneficial effects of low-dose thrombin in endothelial cells and podocytes establish that thrombin can mediate cytoprotection in both cell types constituting the glomerular filtration barrier (endothelial cells and podocytes).…”

“…27 However, recent studies have established that thrombin can mediate cytoprotective effects in endothelial cells in vitro, suggesting that the hemostatic system has a dual function. 26,28 Such a dual function resembles the 2-faced character of reactive oxygen species, also referred to as "redox homeostasis." 29 Given the current and previous results, we propose that the coagulation system has a similar dual function in regulating cellular homeostasis.…”

The role of TGF-β and epithelial-to mesenchymal transition in diabetic nephropathy

“…Glucose-induced TGF-β1 mediated increases in ECM have been reported in cultured mesangial cells [24], podocytes [25] and tubular epithelial cells [26], the cumulative effect of which is destruction of the renal parenchyma. A recent study by Reiniger et al (2010) demonstrated that removal of the AGE receptor in the diabetic OVE26 mouse, attenuated the degree of glomerulosclerosis with a concomitant improvement in renal function [27], whilst over-expression of PKC, has been shown to exert fibrotic effects in human proximal tubular cells [28].…”

Coagulation and non‐coagulation effects of thrombin

“…In secondary hemostasis, thrombin converts fibrinogen into fibrin monomers, finally forming a stable clot that stops bleeding at sites of vascular injury. In addition, thrombin attenuates its own generation through formation of a complex with the endothelial cell (EC) surface receptor thrombomodulin (TM), thereby activating protein C to activated protein C (APC), the main inactivator of activated factor V and activated FVIII [1].…”