• Epigenetic Regulation of Adipogenic Differentiation And Lipid Metabolism: Role of Enhancer of Zeste Homolog 2 (EZH2)

      YIEW, KAN HUI; Department of Pharmacology and Toxicology (2017)
      Adipose tissue expansion in obesity promotes cardiometabolic disease, for which there is an urgent need to elucidate disease mechanisms and develop novel and effective medical therapies. In obesity, adipose tissues can potentially expand through adipocyte hypertrophy and/or hyperplasia, with the latter being a healthier mechanism of fat expansion. Adipocyte hyperplasia (via adipogenic differentiation) is inexplicably restrained in diet-induced obesity (DIO), however, and hypertrophy ensues, leading to inflammation, insulin resistance, and dysregulated adipose tissue lipid metabolism, which together contribute to cardiometabolic disease. The mechanisms of impaired adipogenic differentiation and lipid metabolism during DIO are unclear, with prior studies suggesting epigenetic dysregulation of histone deacetylase 9 (HDAC9, an endogenous repressor of adipogenic differentiation). The overall goal of my dissertation project is to investigate the role of Enhancer of Zeste Homolog 2 (EZH2), a histone methyltransferase, in the aforementioned processes. In Aim 1, we hypothesized that EZH2 promotes adipogenic differentiation by repressing HDAC9. EZH2 recruitment and histone 3 lysine 27 trimethylation (H3K27me3) modification were elevated at the HDAC9 gene promoter (p<0.05) concurrent with dramatic downregulation of HDAC9 mRNA levels (p<0.05) during adipogenic differentiation of primary human preadipocytes. This suggested a role for EZH2 in silencing HDAC9 gene expression. Counterintuitively, a highly selective EZH2 pharmacological inhibitor (GSK126), at a concentration that effectively blocked H3K27me3, led to increased lipid accumulation (p<0.05) in human adipocytes, without inhibiting adipocyte marker gene expression. Consistently, mice with adipose-specific EZH2 deletion (cKO) displayed significantly elevated body weight, adipose tissue mass, and adipocyte cell size. These phenotypic alterations could not be explained by differences in feeding behavior, locomotor activity, or metabolic energy expenditure, thereby suggesting that EZH2 regulates lipid metabolism. This hypothesis was explored in Aim 2. Human adipocytes treated with either an EZH2 inhibitor or vehicle exhibited comparable rates of de novo lipogenesis (DNL), fatty acid (FA) uptake, and basal/stimulated lipolysis. Consistently, cKO and littermate control mice displayed comparable in vivo and ex vivo basal/stimulated adipose lipolysis. EZH2’s function in other important metabolic pathways such as glycolysis and β-oxidation remain to be investigated. Collectively, our findings suggest a potential role of EZH2 in regulating adipocyte lipid metabolism.