Epigenetic regulation of adipogenesis genes by MLL3 and MLL4 complexes

Transgenic mice that express catalytically inactive form of Kmt2c (Lee, Saha et al. 2008; Lee, Lee et al. 2008) or that have a heterozygous loss of Kmt2d are resistant to fatty liver formation induced by high-fat diet. Expression of a large portion of high-fat diet induced genes in mouse liver requires Kmt2d. Among the defined transcription factors that promote high-fat diet-induced hepatic steatosis (ChREBP, SREBP1c, LXRs, and PPARG), ChREBP and SREBP1c do not interact with KMT2C/D.

Gomisin N, a lignan isolated from magnolia-vine (Schisandra chinensis) was shown to inhibit, without cytotoxic effects, differentiation of mouse 3T3-L1 preadipocytes by inhibiting mitotic clonal expansion during early adipogenesis. Gomisin N treatment reduces KMT2D but not KMT2C mRNA levels, and also reduces the mRNA level of adipogenic transcription factors CEBPB, CEBPA, and PPARG, as well as their targets FABP4 (aP2) and FASN (FAS), inhibiting lipid accumulation and formation of lipid droplets in 3T3-L1 preadipocytes. In vivo, Gomisin N treatment in high-fat diet–fed mice reduced body weight and epididymal adipose tissue mass, and inhibited adipocyte enlargement. Moreover, in adipose tissue of high-fat diet–fed mice, Gomisin N alleviated hepatic steatosis and improved overall metabolic parameters.

The epigenomic reader BRD4, essential for both white and brown adipose tissue development but dispensable for adipose tissue maintenance, is enriched on active enhancers during the initial stages of adipogenesis and on promoters that associate with enhancers at terminal stages of adipogenesis. More than 90% of Brd4 genomic binding sites lose Brd4 in mouse preadipocytes that are double knockout for Kmt2c and Kmt2d. The majority of Brd4-bound enhancers are co-occupied by Kmt2d, Ep300 (p300), Cebpa or Cebpb, and Pparg. Brd4 knockout does not affect binding of Cebpb and Kmt2d to adipogenesis target genes nor does it affect deposition of activating epigenetic marks, H3K4me1 and H3K27ac, at these genes, but it reduces binding of transcriptional machinery (e.g. Med1, Tbp, RNA Pol II, Cdk9) and eRNA (enhancer RNA) production. Brd2 and Brd3 may be able to partially compensate for the loss of Brd4. Inhibition of Brd proteins by synthetic inhibitor JQ1 inhibits expression of Pparg targets Cepba, Fabp4 and Adipoq induced by rosiglitazone, a synthetic Pparg agonist. Brd4 was shown to physically associate with Cebpb. Conditional knockout mice in which Brd4 gene is deleted in progenitor cells of brown adipose tissue and muscle lineages die immediately after birth because of breathing impairment, exhibiting severe reduction in muscle mass and brown adipose tissue.

Based on mouse studies, at the onset of white and brown adipogenesis the accessory subunit of MLL3 and MLL4 complexes, PAGR1 (also known as PA1) is needed for induction of CEBPB and CEBPD transcription factors. Based on a study in immortalized mouse brown adipocytes, PAGR1 is recruited to the CEBPB gene promoter by activated CREB1, which is phosphorylated at serine residue S133 (p-S133-CREB1). Based on a study in mouse white preadipocyte cell line 3T3-L1, PAGR1 is recruited to CEBPD gene promoter by ligand-activated glucocorticoid receptor (GR).

Based on a study conducted using the mouse 3T3-L1 white preadipocyte cell line as a model system, dehydroleucodine, a sesquiterpene lactone isolated from Artemisia douglasiana (California mugwort) halts mitotic clonal expansion at the initiation of white adipogenesis. Dehydroleucodine decreases the phosphorylation of ERKs and AKT, as well as levels of CCNA, CCND, CDK2, and CDK4, while increasing the level of p27 (CDKN1B), resulting in G0/G1 cell cycle arrest of mouse white preadipocytes. Dehydroleucodine also downregulates JMJD2B and KMT2D, leading to downregulation of CEBPB and PPARG, respectively.

Expression of the histone 3 mutant H3.3 K4M in mouse brown preadipocytes impairs adipose tissue development, but when H3.3 K4M is expressed in already differentiated mouse brown adipocytes, the maintenance and thermogenic function of brown adipose tissue are not affected.