We use adipogenesis and PPARgamma as model system to study the roles of histone methyltransferases and demethylases, and the dynamics of site-specific histone methylation, in regulation of gene expression and cell differentiation. We are interested in methylations on K4, K9, K27 and K36 of histone H3 (H3K4, H3K9, H3K27 and H3K36, respectively). We reported previously that H3K27 methyltransferase Ezh2 constitutively represses Wnt genes to facilitate adipogenesis (Wang L, PNAS 2010), that H3K9 methyltransferase G9a represses PPARgamma expression and adipogenesis (Wang L, EMBO J 2013), and that H3K4me1 methyltransferases MLL3/MLL4 and associated PTIP directly control the induction of PPARgamma and C/EBPa and are essential for adipogenesis (Cho YW, Cell Metab 2009; Lee JE, eLife 2013). Together, these findings indicate that site-specific histone methyltransferases control expression of positive and negative master regulators of adipogenesis. Using conditional knockout mice and preadipocytes, we also show that the epigenomic reader Brd4 controls cell identity gene induction and is essential for adipogenesis (Lee JE, Nat Commun 2017). We have shown recently that depletion of H3K36 methylation by histone H3.3 mutant H3.3K36M in progenitor cells strongly inhibits adipogenesis in culture and in mice. Mechanistically, H3.3K36M targets H3K36me2 methyltransferase Nsd2 to prevent the induction of PPARgamma target genes. In contrast, depletion of H3K36 methylation by H3.3K36M in adipocytes does not affect adipose tissue weight but leads to profound whitening of brown adipose tissue (BAT) and insulin resistance in white adipose tissue (WAT). These mice are resistant to high fat diet-induced WAT hypertrophy. Our results suggest a critical role of Nsd2-mediated H3K36 methylation in adipose tissue development and function (Zhuang L, Nat Commun 2018). We have profiled genomic binding of enhancer epigenomic writers (MLL4, CBP), adipogenic TFs (EBF2, C/EBPa, C/EBPb, PPARgamma), coactivator MED1, RNA Polymerase II, as well as epigenome (H3K4me1/2/3, H3K9me2, H3K27me3, H3K36me3, H3K27ac), transcriptome, and chromatin opening during adipogenesis of immortalized preadipocytes derived from mouse BAT. We show that MLL4 and CBP identify super-enhancers (SEs) of adipogenesis and that MLL3/4 are required for CBP/p300 binding on enhancers and SE formation. In brown adipocytes differentiated in culture, MLL4 identifies primed SEs of genes fully activated in BAT. Our data provide a rich resource for understanding epigenomic regulation of brown adipogenesis (Lai B, NAR 2017). Using conditional knockout mice and preadipocytes, we found surprisingly that although ligand-bound glucocorticoid receptor (GR) accelerates adipogenesis in culture, endogenous GR is dispensable for adipogenesis in culture and in mice (Park Y, MCB 2017a). We also found that KLF4 and Krox20 are dispensable for adipogenesis in culture and in mice (Park Y, MCB 2017b). These unexpected results prompted us to study adipogenesis in vivo.
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