Summary Background The coagulation cascade has been shown to participate in chronic liver injury and fibrosis, but the contribution of various thrombin targets, such as protease activated receptors (PARs) and fibrin(ogen), has not been fully described. Emerging evidence suggests that in some experimental settings of chronic liver injury, platelets can promote liver repair and inhibit liver fibrosis. However, the precise mechanisms linking coagulation and platelet function to hepatic tissue changes following injury remain poorly defined. Objectives To determine the role of PAR-4, a key thrombin receptor on mouse platelets, and fibrin(ogen) engagement of the platelet αIIbβ3 integrin in a model of cholestatic liver injury and fibrosis. Methods Biliary and hepatic injury was characterized following 4 week administration of the bile duct toxicant α-naphthylisothiocyanate (ANIT) (0.025%) in PAR-4-deficient mice (PAR-4−/− mice), mice expressing a mutant form of fibrin(ogen) incapable of binding integrin αIIbβ3 (FibγΔ5), and wild-type mice. Results Elevated plasma thrombin-antithrombin and serotonin levels, hepatic fibrin deposition and platelet accumulation in liver accompanied hepatocellular injury and fibrosis in ANIT-treated wild-type mice. PAR-4 deficiency reduced plasma serotonin levels, increased serum bile acid concentration, and exacerbated ANIT-induced hepatocellular injury and peribiliary fibrosis. Compared to PAR-4-deficient mice, ANIT-treated FibγΔ5 mice displayed more widespread hepatocellular necrosis accompanied by marked inflammation, robust fibroblast activation and extensive liver fibrosis. Conclusions Collectively, the results indicate that PAR-4 and fibrin-αIIbβ3 integrin engagement, pathways coupling coagulation to platelet activation, each exert hepatoprotective effects during chronic cholestasis.
During obstructive cholestasis, increased concentrations of bile acids activate ERK1/2 in hepatocytes, which up-regulates early growth response factor 1, a key regulator of proinflammatory cytokines, such as macrophage inflammatory protein 2 (MIP-2), which, in turn, exacerbates cholestatic liver injury. Recent studies have indicated that IL-17A contributes to hepatic inflammation during obstructive cholestasis, suggesting that bile acids and IL-17A may interact to regulate hepatic inflammatory responses. We treated mice with an IL-17A neutralizing antibody or control IgG and subjected them to bile duct ligation. Neutralization of IL-17A prevented up-regulation of proinflammatory cytokines, hepatic neutrophil accumulation, and liver injury, indicating an important role for IL-17A in neutrophilic inflammation during cholestasis. Treatment of primary mouse hepatocytes with taurocholic acid (TCA) increased the expression of MIP-2. Co-treatment with IL-17A synergistically enhanced up-regulation of MIP-2 by TCA. In contrast to MIP-2, IL-17A did not affect up-regulation of Egr-1 by TCA, indicating that IL-17A does not affect bile acid-induced activation of signaling pathways upstream of early growth response factor 1. In addition, bile acids increased expression of IL-23, a key regulator of IL-17A production in hepatocytes in vitro and in vivo. Collectively, these data identify bile acids as novel triggers of the IL-23/IL-17A axis and suggest that IL-17A promotes hepatic inflammation during cholestasis by synergistically enhancing bile acid-induced production of proinflammatory cytokines by hepatocytes.
Th17 cells are highly pathogenic in a variety of immune-mediated diseases, and a thorough understanding of the mechanisms of cytokine-mediated suppression of Th17 cells has great therapeutic potential. In this report, we characterize the regulation of both in vitro- and in vivo-derived Th17 cells by IL-4. We demonstrate that IL-4 suppresses re-activation of committed Th17 cells, even in the presence of TGFβ, IL-6 and IL-23. Down-regulation of IL-17 by IL-4 is dependent on STAT6 and mediated by inhibition of STAT3 binding at the Il17a promoter. Although Th1 cytokines have been shown to induce IFNγ expression by Th17 cells, IL-4 does not induce a Th2 phenotype in Th17 cells. Suppression by IL-4 is stable and long-lived when applied to immature Th17 cells, but cells that have undergone multiple rounds of stimulation, either in vivo during a Th17-mediated inflammatory disease, or in vitro, become resistant to suppression by IL-4 and lose the ability to signal through the IL-4 receptor. Thus, while IL-4 is a potent suppressor of the Th17 genetic program at early stages after differentiation, prolonged stimulation renders Th17 cells impervious to regulatory cytokines.
Nonalcoholic fatty liver disease (NAFLD) is the hepatic manifestation of obesity and metabolic syndrome. Robust coagulation cascade activation is common in obese patients with NAFLD. We identified a critical temporal relationship between thrombin generation and the manifestation of hepatic steatosis, inflammation, and injury in C57BL/6J mice fed a high-fat diet (HFD) for 1, 2, and 3 months. Mice fed a HFD exhibited dramatic increases in hepatocellular injury and inflammation over time. Hepatic fibrin deposition preceded an increase in serum alanine aminotransferase, and the most dramatic changes in liver histopathology occurred in conjunction with a detectable increase in plasma thrombin-antithrombin levels at 3 months. To directly determine whether thrombin activity promotes NAFLD pathogenesis, mice were fed a HFD and simultaneously treated with the direct thrombin inhibitor dabigatran etexilate for 3 months. Notably, dabigatran treatment significantly reduced hepatic fibrin deposition, hepatic inflammation, hepatocellular injury, and steatosis in mice fed a HFD. Of interest, dabigatran treatment also significantly attenuated HFD-induced body weight gain. Gene expression analysis suggested that thrombin potentially drives NAFLD pathogenesis by altering the expression of genes associated with lipid metabolism and bile acid synthesis. Collectively, the results suggest that thrombin activity is central to HFD-induced body weight gain, liver injury, and inflammation and provide the proof-of-principle evidence that pharmacological thrombin inhibition could be effective in limiting NAFLD and associated pathologies.
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