Role of Coagulation in Xenobiotic-Induced Liver Injury

Abstract

The liver is a common target for xenobiotic-induced toxicity. Of importance, synthesis of soluble coagulation factors by the liver plays an essential role in hemostasis. Blood coagulation cascade activation is evident in both human patients and in animal models of liver injury. Several studies have shown that coagulation is not merely a process reactive to toxicity, but rather a critical determinant of liver disease pathogenesis. Previous studies have utilized global anticoagulation as a strategy to investigate the role of coagulation in liver injury. Currently, our understanding of the mechanisms whereby individual coagulation proteases contribute to hepatotoxicity is inadequate. Blood coagulation is initiated by tissue factor (TF), a transmembrane cellular receptor for the coagulation factor VII/VIIa. The TF:VIIa complex initiates a serine protease cascade, which culminates in the generation of the serine protease thrombin. Thrombin cleaves circulating fibrinogen to initiate fibrin clot formation. Additionally, thrombin signals to multiple cell types through activation of protease activated receptors (PARs), which can promote inflammation and platelet aggregation. The procoagulant response is balanced by several anticoagulant proteins and fibrin clot degradation is catalyzed by plasmin, the main endogenous fibrin degradation enzyme. The aim of this dissertation was to determine the role of several blood coagulation cascade components in the responses elicited by two model hepatotoxicants; &alpha-naphthylisothiocyanate (ANIT) a toxicant that damages bile duct epithelial cells (BDECs), and acetaminophen (APAP), a common analgesic that causes centrilobular hepatocellular necrosis at high doses. We found that TF-dependent generation of thrombin contributed to the progression of chronic ANIT-induced hepatic inflammation and fibrosis by a mechanism requiring PAR-1. PAR-1 activation amplified TGF-&beta1-induced &alphaV&beta6 integrin expression by BDECs, and activation of TGF-&beta signaling in vivo. This suggests a novel feed-forward mechanism whereby coagulation can promote fibrogenesis. In contrast, PAR-1 was not required for the acute hepatotoxic effects of ANIT. Rather, ANIT-induced liver injury occurred by a mechanism involving activation of PAR-4 on platelets and fibrin(ogen). The results highlight a differential contribution of thrombin signaling in acute and chronic cholestatic liver injury. Previous studies identified the primary inhibitor of fibrinolysis, plasminogen activator inhibitor-1 (PAI-1), as a key hepatoprotective factor in APAP-induced liver injury. Indeed, hepatic fibrin deposition is a prominent feature of APAP-induced hepatocellular necrosis. To our surprise, fibrin(ogen) did not contribute to acute APAP-induced liver injury. Of importance, plasminogen deficiency reduced APAP hepatotoxicity. Taken together, the results suggest that plasmin(ogen) promotes APAP-induced liver injury in a fibrin independent manner. Overall, these studies revealed novel pathways whereby elements of the coagulation cascade promote liver injury induced by two classical hepatotoxicants. The results suggest that individual coagulation cascade components may play divergent roles in different models of liver injury and potentially at different time points during injury development. Further studies systematically evaluating individual coagulation factors in animal models and human disease are required to fully understand the dynamic contribution that coagulation cascade proteins play in liver injury.