The role of PAR-1, PAR-2 and tissue factor in the development of hepatic fibrosis
2017-03-01T03:50:38Z (GMT) by
Hepatic fibrosis and cirrhosis are the common endpoint of a variety of liver diseases and represent a major global health burden. The current model for hepatic fibrosis development is that progressive injurious stimuli lead to dysregulation of extracellular matrix (ECM) turnover. Activation of the hepatic stellate cell (HSC) has been identified as the key cellular event resulting in the accumulation of extracellular matrix (Friedman 2008) and therefore there is considerable interest in factors that regulate HSC activation and collagen expression. There is a strong linkage between inflammation, coagulation and fibrosis (Tacke, Luedde et al. 2009). One proposed mechanism for this linkage is signalling by coagulation factors through their cellular receptors protease-activated receptors (PARs) to activate stellate cells (Anstee, Wright et al. 2009). This thesis has explored the role of PAR-1, PAR-2 and the cytoplasmic domain of tissue factor in the development of hepatic fibrosis. The close relationship between the coagulation cascade and the inflammatory response led to the hypothesis that coagulation factors and their receptors may play an important role in hepatic fibrogenesis. In order to mimic human liver disease processes, a mouse model was studied using carbon tetrachloride administration to generate liver fibrosis. Mice with deletion of the PAR-1 gene, PAR-2 gene, with deletion of the cytoplasmic domain of TF and with dual deletion of PAR-2 gene and TF cytoplasmic domain were individually studied and compared to wildtype. Common fibrosis endpoints were studied in vivo. In vitro experiments were performed with a line of human hepatic stellate cells. Initial experiments demonstrate PAR-1 deficiency protects against liver fibrosis with reduced histological fibrosis, hydroxyproline content, TGF β gene and protein expression seen. This adds evidence to support the view that PAR-1 is involved in hepatic fibrogenesis. PAR-2 deficiency was also found to afford protection from hepatic fibrosis. PAR-1 and PAR-2 activation also induce a profibrogenic phenotype in human hepatic stellate cells in vitro adding weight to the evidence these receptors are important in fibrosis development. In addition to its important role in haemostasis, tissue factor is increasingly recognised as a signalling receptor in a number of non coagulant roles. Deletion of the cytoplasmic domain of tissue factor led to reduction in profibrogenic cytokines, HSC activation and reduced macrophage recruitment and activation which supports the reduced hepatic fibrosis observed. Macrophages play a pivotal role as regulators of fibrosis. They are profibrogenic in fibrosis development but also play a role and are necessary for fibrosis resolution. The reduced macrophage recruitment and activation observed in the PAR-2 and mice with deletion of the cytoplasmic domain of tissue factor may in part explain the amelioration of hepatic fibrosis seen in these mice. A single treatment to completely ameliorate fibrosis may be difficult to achieve given the complex and multiple pathways involved in ECM remodelling. Understanding the mechanisms of fibrosis provide a platform to develop antifibrotic therapies. This thesis has provided further insight into the role of PAR-1 and PAR-2 and the cytoplasmic domain of tissue factor in hepatic fibrogenesis. Both PAR-1 and PAR-2 antagonists are being developed and may represent a novel therapeutic approach in preventing fibrosis in patients with liver disease. The cytoplasmic domain of tissue factor is an attractive therapeutic target as the coagulation is not affected in the host, particularly important in patients with cirrhosis.