Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, C53
High-fat diet-fed nuclear factor-erythroid 2-related factor 2-null mice demonstrate increased susceptibility to non-alcoholic steatohepatitis without peripheral insulin resistance.
P. J. Meakin1, S. Chowdhry1, S. V. Walsh2, J. F. Dillon1, J. D. Hayes1, M. L. Ashford1
1. Biomedical Research Institute, University of Dundee, Dundee, United Kingdom. 2. Department of Pathology, University of Dundee, Dundee, United Kingdom.
Non-alcoholic fatty liver disease is associated with obesity and the metabolic syndrome. Many individuals exhibit simple steatosis, but a significant proportion develop non-alcoholic steatohepatitis (NASH), with some progressing to cirrhosis and end-stage liver disease. However, the molecular events underlying this change are not understood. It is postulated that progression of steatosis to NASH is mediated by two metabolic insults: increased delivery of free fatty acids to the liver in association with peripheral insulin resistance and the subsequent increase in oxidative stress through the production of reactive oxygen species. Nuclear factor-erythroid 2-related factor 2 (Nrf2) is a master regulator of cellular adaptive responses to oxidative stress and has been implicated, using a methionine and choline-deficient diet, in the inhibition of liver steatosis (1-3). Thus failure to adapt to metabolic and oxidative stress may be associated with increased susceptibility to NASH. Consequently, we have examined the effects of high-fat diet on hepatic pathology and biomarkers of fatty acid metabolism and inflammation in livers of wild type and Nrf2-null mice, and explored whether these outcomes correlate with peripheral insulin resistance. Wild type and Nrf2-null mice were fed a regular or high-fat diet for 24 weeks, prior to analysis of livers. Both genotypes on regular diet displayed normal glucose homeostasis, whereas on the high-fat diet wild type mice became glucose intolerant, which was resisted by Nrf2-null mice. Independent histopathological scoring of wild type livers showed no evidence of steatosis on regular diet and mild NASH following the high-fat diet. In contrast, Nrf2-null livers displayed mild NASH on the regular diet and severe NASH with cirrhosis on the high-fat diet. Quantitative RT-PCR of wild type mouse liver mRNA showed greater up-regulation of anti-oxidant response genes in response to high-fat diet than Nrf2-null livers. At mRNA level, significantly greater fold increases (n = 11; values are means ± S.E.M. compared by t-test) were noted for the fatty acid metabolism genes, sterol regulatory element-binding protein 1c (1.43 ± 0.09, p < 0.05) and fatty acid synthase (2.20 ± 0.21, p < 0.05) in high-fat diet Nrf2-null, compared to wild type, livers. Nrf2 mouse high-fat diet livers also had significantly greater mRNA levels of pro-inflammatory cytokines: e.g. cyclooxygenase-II (2.25 ± 0.10, p < 0.05), tumour necrosis factor-alpha (2.05 ± 0.13, p < 0.05) and inducible nitric oxide synthase (9.52 ± 0.50) compared to wild type. Thus the absence of Nrf2 markedly accelerates the development of NASH driven by dietary fat excess, but this is not associated with a deterioration of peripheral insulin sensitivity.
Where applicable, experiments conform with Society ethical requirements