Cholesterol as a mediator of hepatic injury in non-alcoholic steatohepatitis

Abstract

Non-alcoholic fatty liver disease (NAFLD) is highly prevalent. 20-30% of patients with NAFLD progress to non-alcoholic steatohepatitis (NASH), characterised by steatosis, ballooning degeneration of hepatocytes, inflammation and fibrosis, but the cause(s) of this transition remains unclear. Foz/foz (Alms1 mutant) mice develop hyperphagia and obesity-related NAFLD. High fat (HF)-feeding induces NASH transition by 12 weeks, and fibrosis by 24 weeks. The aims of this research were to investigate the cause and effect of hepatic cholesterol dysregulation in foz/foz mice, its pathogenic significance in NASH, and the potential of dietary and pharmacological approaches to treat NASH. After 24 weeks HF-feeding, hepatic free cholesterol (FC) and cholesteryl ester (CE) content were markedly increased in foz/foz mice. Hepatic cholesterol accumulation was associated with increased expression of low-density lipoprotein receptor (LDLR), but not cholesterol synthesis. Bile acid (BA) biosynthesis genes were suppressed, as was expression of cholesterol and BA export proteins. Transcriptional regulators of cholesterol metabolism were differentially expressed in foz/foz mice with NASH: SREBP-2, a nuclear regulator of LDLR increased significantly, while LXR-a and HNF-4a decreased. Importantly, expression of liver receptor homolog-1 (LRH-1), while significantly elevated in HF-fed WT mice, was unaltered in HF-fed foz/foz mice. To test whether hyperinsulinaemia was responsible for some or all of these changes, experiments were conducted in primary murine hepatocyte cultures. Insulin activated both SREBP-2 and LDLR, while simultaneously decreasing BA export proteins and LRH-1 expression. Removing dietary cholesterol in HF-fed foz/foz mice ameliorated liver injury, whereas increasing dietary cholesterol accentuated injury. Steatohepatitis severity correlated with hepatic FC and CE, but not hepatic triglyceride or free fatty acids. Hepatic cholesterol loading activated nuclear factor (NF)-kB and c-Jun N-terminal kinase (JNK), and increased serum monocyte chemotactic protein (MCP)-1. Finally, quantitative analysis of sirius red staining demonstrated a positive relationship between hepatic cholesterol content and fibrosis severity. To establish whether pharmacological modulation of cholesterol turnover could modulate NASH outcome, HF-fed foz/foz and WT mice were administered atorvastatin and/or ezetimibe after NASH onset. Both agents significantly lowered hepatic CE and FC, serum ALT, total cholesterol and HDL and normalised liver size in foz/foz mice. Hepatocellular apoptosis and expression of inflammatory mediators was suppressed as was hepatic macrophage infiltration. While histological steatosis and ballooning scores were unaltered, lobular inflammation and liver fibrosis were significantly reduced in drug-treated foz/foz mice. In foz/foz mice, NASH is associated with profoundly disordered hepatic cholesterol turnover. Insulin-induced activation of SREBP-2 and suppression of LRH-1 likely result in increased LDLR expression and suppression of genes involved in cholesterol biotransformation and export. Consequently, cholesterol accumulates within the livers of foz/foz mice. The evidence from both dietary and pharmacological intervention experiments is that hepatic cholesterol mediates hepatocellular apoptosis, inflammatory cell recruitment and liver fibrosis in this model. Conversely, inhibition of cholesterol uptake/redistribution and biosynthesis, and dietary cholesterol restriction, reduces liver injury, inflammation, and fibrosis in foz/foz mice with NASH. These findings strongly support the hypothesis that hepatic cholesterol, secondary to insulin resistance, acts as a lipotoxic mediator of pro-fibrotic liver injury and inflammation in experimental NASH. -- provided by Candidate.