NFATc1 driven ER-stress responses in the progression of non-alcoholic fatty liver disease

Abstract

Non-alcoholic fatty liver disease (NAFLD) mirrors the hepatic manifestation of the metabolic syndrome and is defined by progressive fat accumulation in hepatocytes (steatosis). Uncomplicated NAFLD may progress to a severe form of non-alcoholic steatohepatitis (NASH), characterized by marked organ inflammation and fibrosis. Progression of the disease can lead to distinctive cirrhosis and even the development of hepatocellular carcinoma. Not surprisingly, NAFLD has now emerged as the second most common cause of liver transplantations in the USA and has been predicted to become one of the leading causes of hepatocellular carcinoma in few years (Wong et al., 2015). Unfortunately, however, despite great efforts from multiple research groups, the mechanisms underlying the progression from uncomplicated NAFLD to NASH and beyond, are still largely unknown. Here, we describe a key role of NFATc1 in NAFLD progression. NFATc1, apart from its well-known function in T-cell activation, has been reported to play a major role in various inflammatory diseases (i.e., skin inflammation) and inflammation-associated tumors as well (i.e., colon cancer, pancreatic cancer). However, its role in inflammation associated liver inflammatory diseases has not been comprehensively studied. This project aimed to characterize the functional implication of NFATc1 and its therapeutic potential in NAFLD development and progression. To address this, we utilized primary hepatocytes, AML12 cells and genetically modified mouse model with hepatocyte-specific genetic alteration in NFATc1. Initially, we confirmed the robust expression and nuclear localization of NFATc1 in hepatocytes of human NAFLD patients. We also demonstrated fat-induced activation of the transcription factor in hepatocytes and provided experimental evidence that NFATc1 activation is dispensable for the development of steatosis but is strictly required for the progression of the disease. In fact, in-depth biochemical, molecular, and functional analysis revealed that NFATc1 activation promotes inflammation and fibrosis, thus culminating in the development of steatohepatitis. Using transgenic mice with hepatocytes-specific differential expression of NFATc1, we were able to demonstrate that NFATc1 deficiency protects mice from fat-induced inflammation and fibrosis. Mechanistically, SUMMARY 2 NFATc1 promotes NAFLD through terminal ER stress and subsequent activation of the PERK unfolded protein response pathway in hepatocytes to promote apoptosis and activation of the inflammasome. Lastly, this study highlighted the therapeutic potential of ER stress relief by treatment with TUDCA. In fact, TUDCA application prevented hepatocytes from NFATc1 driven ER stress and UPR pathway activation and hence, resulted in strikingly lower hepatocyte damage (apoptosis and inflammasome activation), inflammation and fibrosis in mouse models of NAFLD.