Increased expression of c-jun in nonalcoholic fatty liver disease

Abstract

The lack of animal models which closely mimic the human situation is one main reason why the molecular mechanisms causing progression from nonalcoholic fatty liver to (NASH) still are elusive. In addition to the quantity of consumed calories, certain dietary components, namely saturated fatty acids, fructose, and cholesterol are increasingly recognized as dietary factors affecting NAFLD progression. The aim of this study was to developed and characterize a NASH inducing diet (ND) containing these nutrients in a composition which mimics Western food. Methods and Results: Feeding ND to male C57Bl6/N mice for 12 weeks induced significant body weight gain, an impaired glucose tolerance and elevated fasting blood glucose levels as well as decreased adiponectin and increased leptin serum levels. Furthermore, ND-feeding led to marked hepatic steatosis and distinct signs of hepatic oxidative stress and lipid peroxidation. In line with this, ND-fed mice exhibited more apoptotic hepatocytes and elevated transaminase serum levels as well as hepatic inflammation, activation of hepatic stellate cells, and beginning fibrosis, as assessed by histology and hydroxyprolin analysis. Transcriptome-wide gene expression analysis and classification in Gene Ontology categories revealed main alterations in the categories „lipid metabolic process”, “innate immune response” and “inflammatory response“ in the liver of HD-fed mice, which closely resembled NASH patients. Search for over-represented transcription factor target sites among the differenzially expressed genes identified AP-1 as the most likely factor to cause the transcriptional changes. Interestingly, protein-protein interaction network analysis identified c-JUN (a component if the AP-1 complex) as hub. Westernblot and immunohistochemical analysis confirmed c-jun-phosphorylation and nuclear translocation in murine ND-livers as well as NASH patients. Unexpectedly, also expression of (unphosphorylated) c-jun was significantly enhanced not only in NASH, but also in fatty liver without inflammation. Immunohistochemical analysis of a tissue micro array containing 80 human NAFLD tissues revealed a significant correlation between c-jun expression and the extent of hepatic steatosis. Conclusions: Our novel murine NASH-model induces important pathomechanisms also found in human NASH and led to the identification of increased c-jun already in simple steatosis. It is likely that higher abundance of this crucial pro-inflammatory regulator facilitates NASH upon additional stimuli. The identification of the latter as well as c-jun itself appear as attractive prognostic and therapeutic targets of NAFLD progression.