Tissue Factor-Deficiency and Protease Activated Receptor-1-Deficiency Reduce Inflammation Elicited by Diet-Induced Steatohepatitis in Mice

Abstract

Altered hepatic lipid homeostasis , hepatocellular injury , and inflammation are features of nonalcoholic steatohepatitis , which contributes significantly to liver-related morbidity and mortality in the Western population. A collection of inflammatory mediators have been implicated in the pathogenesis of steatohepatitis in mouse models. However , the pathways essential for coordination and amplification of hepatic inflammation and injury caused by steatosis are not completely understood. We tested the hypothesis that tissue factor (TF)-dependent thrombin generation and the thrombin receptor protease activated receptor-1 (PAR-1) contribute to liver inflammation induced by steatosis in mice. Wild-type C57Bl/6J mice fed a diet deficient in methionine and choline for 2 weeks manifested steatohepatitis characterized by increased serum alanine aminotransferase activity, macrovesicular hepatic steatosis , hepatic inflammatory gene expression , and lobular inflammation. Steatohepatitis progression was associated with thrombin generation and hepatic fibrin deposition. Coagulation cascade activation was significantly reduced in low TF mice , which express 1% of normal TF levels. Non-alcoholic fatty liver disease (NAFLD) is increasingly appreciated as a hepatic feature of the metabolic syndrome. NAFLD may occur in 25% of the Western population and altered hepatic function increases the risk for developing diseases including diabetes and atherosclerosis.1,2 The progression of simple hepatic steatosis to the more severe nonalcoholic steatohepatitis (NASH) contributes significantly to liver-related morbidity and mortality.3 Requisite histological features of NASH include macrovesicular hepatic steatosis, evidence of hepatocellular injury, and lobular inflammation. 4 In a subset of patients with chronic steatohepatitis, stellate cell activation coordinates a fibrogenic response causing fibrosis and cirrhosis.5 Of importance, the mechanisms required for the progression of hepatic inflammation during steatohepatitis are not completely understood.Animal models used to define mechanisms of steatohepatitis have used genetic and dietary modification to induce various features of the disease.2 In particular, feeding mice a diet deficient in methionine and choline (MCD diet) is an established model to study the progression of steatohepatitis and has been extensively used to study mechanisms of hepatic inflammation and fibrosis. Rodents fed an MCD diet for 2 weeks manifest a defect in hepatic ␤ oxidation resulting in accumulation of triglyceride and the induction of steatohepatitis.2,6,7 Prolonged feeding (Ͼ4 weeks) of the MCD diet activates hepatic stellate cells and increases collagen expression and deposition in the liver. Utilization of the MCD diet model has revealed the contribution of hepatic triglyceride, 8 various inflammatory mediators, 9,10

Stimulation of fat accumulation in hepatocytes by PGE2-dependent repression of hepatic lipolysis, β-oxidation and VLDL-synthesis

“…Similarly, COX2 was induced in livers of mice with methionine-choline-deficient diet-induced steatosis. 30,31 In these NASH models, inflammatory cells recruited into the liver might contribute to prostaglandin E 2 in addition to resident macrophages. Inhibition of prostanoid synthesis in a NASH mouse model with a selective COX2 inhibitor not only attenuated the liver damage and signs of inflammation but, most notably, also improved steatosis.…”

From the Cover: Coagulation-Driven Hepatic Fibrosis Requires Protease Activated Receptor-1 (PAR-1) in a Mouse Model of TCDD-Elicited Steatohepatitis

“…Thrombin also triggers intracellular signaling in numerous hepatic nonparenchymal cells (eg, macrophages, stellate cells) through activation of the protease activated receptor-1 (PAR-1; F2r). Thrombin activation of PAR-1 has been shown to drive multiple NAFLD-associated pathologies, including hepatic lipid accumulation, inflammation and hepatocellular injury, depending on the experimental setting (Kassel et al, , 2012Kopec et al, 2014;Luyendyk et al, 2010). Notably, PAR-1 activation promotes stellate cell activation in vitro (Fiorucci et al, 2004;Gaca et al, 2002), and hepatic fibrosis is reduced in PAR-1 À/À mice challenged chronically with hepatocellular or biliary toxicants .…”

Thrombin promotes diet-induced obesity through fibrin-driven inflammation

“…Hepatic triglyceride determination. Hepatic triglyceride content was determined as described previously (78). Briefly, 100 mg of frozen liver tissue was homogenized, mixed with chloroform/methanol (2:1), and incubated overnight at room temperature.…”