The importance of epigenetic changes in the development of hepatic steatosis is largely unknown. The histone variant macroH2A1 under alternative splicing gives rise to macroH2A1.1 and macroH2A1.2. In this study, we show that the macroH2A1 isoforms play an important role in the regulation of lipid accumulation in hepatocytes. Hepatoma cell line and immortalized human hepatocytes transiently transfected or knocked down with macroH2A1 isoforms were used as in vitro model of fat-induced steatosis. Gene expressions were analyzed by quantitative PCR array and Western blot. Chromatin immunoprecipitation analysis was performed to check the association of histone H3 lysine 27 trimethylation (H3K27me3) and histone H3 lysine 4 trimethylation (H3K4me3) with the promoter of lipogenic genes. Livers from knockout mice that are resistant to lipid deposition despite a high-fat diet were used for histopathology. We found that macroH2A1.2 is regulated by fat uptake and that its overexpression caused an increase in lipid uptake, triglycerides, and lipogenic genes compared with macroH2A1.1. This suggests that macroH2A1.2 is important for lipid uptake, whereas macroH2A1.1 was found to be protective. The result was supported by a high positivity for macroH2A1.1 in knockout mice for genes targeted by macroH2A1 (Atp5a1 and Fam73b), that under a high-fat diet presented minimal lipidosis. Moreover, macroH2A1 isoforms differentially regulate the expression of lipogenic genes by modulating the association of the active (H3K4me3) and repressive (H3K27me3) histone marks on their promoters. This study underlines the importance of the replacement of noncanonical histones in the regulation of genes involved in lipid metabolism in the progression of steatosis.
Keywords: bivalent promoter; histone methylation; histone variants; lipid uptake.