The Nuclear Receptor Corepressors NCoR and SMRT Decrease Peroxisome Proliferator-activated Receptor γ Transcriptional Activity and Repress 3T3-L1 Adipogenesis

Abstract

The peroxisome proliferator-activated receptor ␥ (PPAR␥) is a central regulator of adipogenesis and recruits coactivator proteins in response to ligand. However, the role of another class of nuclear cofactors, the nuclear receptor corepressors, in modulating PPAR␥ transcriptional activity is less clear. Such corepressors include the nuclear receptor corepressor (NCoR) and the silencing mediator of retinoid and thyroid hormone receptors (SMRT). Our data suggest that PPAR␥ recruits SMRT and NCoR in the absence of ligand and that these corepressors are capable of down-regulating PPAR␥-mediated transcriptional activity. The addition of the PPAR␥ ligand pioglitazone results in dissociation of the PPAR␥-corepressor complex. To define the role of SMRT and NCoR in PPAR␥ action, 3T3-L1 cells deficient in SMRT or NCoR were generated by RNA interference. When these cells are exposed to differentiation media, they exhibit increased expression of adipocyte-specific genes and increased production of lipid droplets, as compared with control cells. These data suggest that the nuclear receptor corepressors decrease PPAR␥ transcriptional activity and repress the adipogenic program in 3T3-L1 cells. The thyroid hormone receptors (TRs)1 and retinoic acid receptors (RARs) are nuclear receptors that repress gene transcription in the absence of their respective ligands. This ligandindependent repression is mediated by nuclear receptor corepressors, such as the nuclear receptor corepressor protein (NCoR) and the silencing mediator of retinoid and thyroid hormone receptors (SMRT) (1, 2). NCoR and SMRT are recruited by TR and RAR isoforms in the absence of ligand and are released upon ligand binding. NCoR and SMRT, in turn, recruit a complex with histone deacetylase activity to repress transcription of target genes. More recently, NCoR and SMRT have been shown to be recruited by other nuclear receptors, some of which bind nuclear receptor corepressors in the presence of antagonists. However, it has been controversial whether NCoR and SMRT play a significant role in gene regulation by the peroxisome proliferator-activated receptor ␥ (PPAR␥) isoforms (3).Two types of PPAR␥ isoforms exist, PPAR␥1 and PPAR␥2. These receptors differ only in their amino-terminal A/B domain, such that PPAR␥2 contains an extra 30 amino acids (4). The function of PPAR␥2 has attracted considerable interest because it is specifically expressed in adipocytes and is an essential regulator of adipogenesis. In addition, many important adipocyte-specific genes contain response elements for PPAR␥ in their promoter regions. Although PPAR␥ clearly recruits coactivators in response to exogenous ligands, its ability to recruit corepressors is less certain. In contrast to the TR, PPAR␥ does not appear to be a strong repressor in the absence of its ligand. Such experiments, however, have been limited by the lack of information concerning physiologic endogenous ligands. Early work into PPAR␥ and corepressor recruitment suggested that PPAR␥ might not recruit NCoR or SMRT in the p...

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“…83,84 Similarly, PPARg is bound to its target promoters in association with a number of repressors including HDACs and histone methyltransferases. 57,58,75,76,[80][81][82]97,99 At this stage, most adipogenic genes are characterized by bivalent histone marks, such as methylation of H3K27, H3K9 and H3K4. [35][36][37][38]50 Methylation of DNA is also found 120 as well as recruitment of a still inactive RNA Pol II.…”

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“…Among these are the PGC-1 coactivators that seem to play key roles in activating metabolic gene networks (18 -20). Although the importance of gene activation is well established, it is evident that gene repression is also a regulatory mechanism in adipogenesis (21). Recent studies have determined that gene repression is also a crucial factor in adipocyte function.…”

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