Liver fibrosis is driven by protease‐activated receptor‐1 expressed by hepatic stellate cells in experimental chronic liver injury

BACKGROUND: Blood coagulation protease activity is proposed to drive hepatic fibrosis through activation of protease‐activated receptors (PARs). Whole‐body PAR‐1 deficiency reduces experimental hepatic fibrosis, and in vitro studies suggest a potential contribution by PAR‐1 expressed by hepatic stellate cells. However, owing to a lack of specific tools, the cell‐specific role of PAR‐1 in experimental hepatic fibrosis has never been formally investigated. Using a novel mouse expressing a conditional PAR‐1 allele, we tested the hypothesis that PAR‐1 expressed by hepatic stellate cells contributes to hepatic fibrosis. METHODS: PAR‐1(flox/flox) mice were crossed with mice expressing Cre recombinase controlled by the lecithin retinol acyltransferase (LRAT) promoter, which induces recombination in hepatic stellate cells. Male PAR‐1(flox/flox)/LRATCre and PAR‐1(flox/flox) mice were challenged twice weekly with carbon tetrachloride (CCl(4), 1 mL/kg i.p.) for 6 weeks to induce liver fibrosis. RESULTS: PAR‐1 mRNA levels were reduced (>95%) in hepatic stellate cells isolated from PAR‐1(flox/flox)/LRATCre mice. Hepatic stellate cell activation was evident in CCl(4)‐challenged PAR‐1(flox/flox) mice, indicated by increased α‐smooth muscle actin labeling and induction of several profibrogenic genes. CCl(4)‐challenged PAR‐1(flox/flox) mice displayed robust hepatic collagen deposition, indicated by picrosirius red staining and type I collagen immunolabeling. Notably, stellate cell activation and collagen deposition were significantly reduced (>30%) in PAR‐1(flox/flox)/LRATCre mice. Importantly, the reduction in liver fibrosis was not a consequence of reduced acute CCl(4) hepatotoxicity in PAR‐1(flox/flox)/LRATCre mice. CONCLUSIONS: The results constitute the first direct experimental evidence that PAR‐1 expressed by stellate cells directly promotes their profibrogenic phenotype and hepatic fibrosis in vivo.