Liver fibrosis is a common pathologic consequence of persistent liver injury and inflammation and is observed in patients with chronic cholestatic liver diseases such as primary sclerosing cholangitis (PSC). Liver fibrosis, documented by excess expression and accumulation of collagen proteins, disrupts liver function and ultimately may lead to liver failure. Currently, there are no specific treatments for liver fibrosis, nor are antifibrotic strategies available clinically, leaving liver transplantation as the principle treatment alternative. Consequently, a crucial therapeutic goal in patients with liver fibrosis is to limit collagen deposition and promote the resolution of fibrosis. One potential area of research is the hemostatic system, which is activated in PSC patients and in experimental settings of chronic liver injury and fibrosis. Increased coagulation is evident in experimental settings of chronic liver injury and fibrosis. The coagulation protease, thrombin, converts soluble fibrinogen, present in blood, to fibrin monomers that are subsequently deposited in tissue as fibrin polymers. The functional contribution of fibrin deposition has been assumed to be universally pathologic due to its association with areas of cellular injury. Contrary to this untested hypothesis, complete fibrin(ogen) deficiency was found to exacerbate chronic cholestatic liver injury in mice exposed to the biliary toxicant, α-naphthylisothiocyanate (ANIT). These results contradict the assumption that fibrin uniformly supports tissue damage, and instead suggest the novel hypothesis that fibrin exerts protective effects in this model of liver fibrosis. However, the mechanisms mediating this protective effect are not fully understood. The work described in this dissertation reveals the mechanisms mediating the protective effects of fibrin(ogen) in chronic cholestatic liver injury and fibrosis and highlights its potential as a therapeutic target for this difficult to treat disease.A major focus of the work presented here was discovering the molecular pathways that link fibrin(ogen) deposition and its hemostatic function to liver damage during cholestatic liver disease. We found that platelet activation by thrombin-mediated protease activated receptor (PAR)-4 signaling inhibits peribiliary fibrosis in mice exposed chronically to ANIT. Moreover, hepatocellular necrosis and associated peribiliary fibrosis were significantly exacerbated in ANIT-exposed FibγΔ5 mice that express a mutant form of fibrin(ogen) incapable of binding platelet integrin αIIBβ3. Complementing its role in hemostasis, fibrin(ogen) has a non-hemostatic function in that it can engage and activate inflammatory cells by binding the leukocyte integrin αMβ2. Another major focus of this dissertation was to determine if fibrin engages leukocytes via αMβ2 integrin to enhance inflammation and liver fibrosis in ANIT-induced chronic liver disease. The fibrin(ogen)-αMβ2 integrin interaction was found to suppress IFNγ-driven biliary hyperplasia, thereby inhibiting hepatic fibrosis in ANIT-exposed mice. Furthermore, our proof-of-principle studies with the novel small molecule leukadherin-1 suggest that this pathway can be pharmacologically targeted to reduce established liver fibrosis. Overall these studies reveal novel pathways whereby the coagulation protein fibrinogen inhibits experimental chronic cholestatic liver injury and fibrosis. The insight provided by these findings pinpoints unique drug targets (e.g., αMβ2), and informs repurposing of available coagulation-directed therapeutics in chronic liver disease.
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