Obesity increases histone H3 lysine 9 and 18 acetylation at Tnfa and Ccl2 genes in mouse liver

Obesity contributes to the development of non-alcoholic fatty liver disease (NAFLD), which is characterized by the upregulated expression of two key inflammatory mediators: tumor necrosis factor (Tnfa) and monocyte chemotactic protein 1 (Mcp1; also known as Ccl2). However, the chromatin make-up at these genes in the liver in obese individuals has not been explored. In this study, to identify obesity-mediated epigenetic changes at Tnfa and Ccl2, we used a murine model of obesity induced by a high-fat diet (HFD) and hyperphagic (ob/ob) mice. Chromatin immunoprecipitation (ChIP) assay was used to determine the abundance of permissive histone marks, namely histone H3 lysine 9 and 18 acetylation (H3K9/K18Ac), H3 lysine 4 trimethylation (H3K4me3) and H3 lysine 36 trimethylation (H3K36me3), in conjunction with polymerase 2 RNA (Pol2) and nuclear factor (NF)-κB recruitment in the liver. Additionally, to correlate the liver tissue-derived ChIP measurements with a robust in vitro transcriptional response at the Tnfa and Ccl2 genes, we used lipopolysaccharide (LPS) treatment to induce an inflammatory response in Hepa1-6 cells, a cell line derived from murine hepatocytes. ChIP revealed increased H3K9/K18Ac at Tnfa and Ccl2 in the obese mice, although the differences were only statistically significant for Tnfa (P<0.05). Unexpectedly, the levels of H3K4me3 and H3K36me3 marks, as well as Pol2 and NF-κB recruitment, did not correspond with the increased expression of these two genes in the obese mice. By contrast, the acute treatment of Hepa1-6 cells with LPS significantly increased the H3K9/K18Ac marks, as well as Pol2 and NF-κB recruitment at both genes, while the levels of H3K4me3 and H3K36me3 marks remained unaltered. These results demonstrate that increased Tnfa and Ccl2 expression in fatty liver at the chromatin level corresponds to changes in the level of histone H3 acetylation.