Reactome | Coactivators are recruited to liganded PPARG:RXRA heterodimer

Coactivators are recruited to liganded PPARG:RXRA heterodimer

Stable Identifier

R-HSA-9843120

Type

Reaction [uncertain]

Species

Homo sapiens

Compartment

nucleoplasm

ReviewStatus

3/5

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Gene expression (Transcription) (Homo sapiens)

- Epigenetic regulation of gene expression (Homo sapiens)
  - Epigenetic regulation by WDR5-containing histone modifying complexes (Homo sapiens)
    - Epigenetic regulation of gene expression by MLL3 and MLL4 complexes (Homo sapiens)
      - Epigenetic regulation of adipogenesis genes by MLL3 and MLL4 complexes (Homo sapiens)
        
        MLL4 and MLL3 complexes regulate expression of PPARG target genes in adipogenesis and hepatic steatosis (Homo sapiens)
        
        Coactivators are recruited to liganded PPARG:RXRA heterodimer (Homo sapiens)

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Coactivators are recruited to liganded PPARG:RXRA heterodimer

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Transcriptional coactivators recruited to the ligand binding domain (LBD) of the ligand-bound heterodimer of PPARG and RXRA include histone acetyltransferases, such as NCOA1, NCOA3, CREBBP, and EP300, components of the MED1 complex (also known as DRIP or TRAP complex), and PPAR-specific (PPARGC1A, PPARGC1B) and non-specific (NCOA2, NCOA4, NCOA6) coactivators.

NCOA1 (SRC1) binding to PPARG is supported by multiple studies (Zhu et al. 1996: mouse Ncoa1 shown to bind to mouse Pparg1 and act as its coactivator; DiRenzo et al. 1997: NCOA1 from African green monkey cell line CV-1 shown to bind both mouse Pparg and human RXRA; Oberfield et al. 1999: binding to human PPARG2 shown; Yang et al. 2000: ligand-dependent binding of PPARG to NCOA1, and ligand-dependent binding of NCOA1 to the DNA-bound PPARG:RXRA heterodimer, species unspecified - presumably human; Mueller et al. 2002: comparable binding of mouse Ncoa1 to both Pparg1 and Pparg2 mouse isoforms; Fujimura et al. 2005: human recombinant NCOA1 associates with recombinant human PPARG2 in the presence of PPARG agonists; Allen et al. 2006: a mammalian two-hybrid assay using recombinant mouse proteins showed that PPARG agonists induce association of Ncoa1, to Pparg). NCOA1 possesses a single LXXLL motif that binds to LBDs of nuclear receptors so that one molecule of NCOA1 is bound to PPARG and one is bound to RXRA (Gampe et al. 2000). The structural study showed that the LXXLL motif of human NCOA1 interacts with the LBD of human PPARG bound to synthetic agonist rosiglitazone, similarly to previously established interactions between NCOA1 and other nuclear receptors (Nolte et al. 1998). NCOA1 also binds to RXRA in a ligand-dependent manner (Yang et al. 2000: species unspecified).

NCOA3 (ACTR) binding to PPARG is supported by multiple studies (Yang et al. 2000: ligand-dependent binding of PPARG to NCOA3, and ligand-dependent binding of NCOA3 to the DNA-bound PPARG:RXRA heterodimer, species unspecified - presumably human; Hartig et al. 2011: fluorescence resonance energy transfer – FRET – using human NCOA3 and PPARG). NCOA3 also binds to RXRA in a ligand-dependent manner (Yang et al. 2000: species unspecified).

CREBBP (CBP) binding to PPARG is supported by multiple studies (Mizukami and Taniguchi 1997: binding of mouse Crebbp to human PPARG2 in the presence of synthetic PPARG agonist; Oberfield et al. 1999: binding to human PPARG2 shown; Gelman et al. 1999: binding of the N-terminal region of CREBBP to human PPARG2 in the presence of synthetic PPARG agonists; Fujimura et al. 2005: human recombinant CREBBP associates with recombinant human PPARG2 in the presence of PPARG agonists; Allen et al. 2006: a mammalian two-hybrid assay using recombinant mouse proteins showed that PPARG agonists induce association of Crebbp to Pparg; Xie et al. 2015: binding of human CREBBP to PPARG2 in response to synthetic PPARG agonist; Park et al. 2022: a flavonoid, kaempferol, is a PPARG antagonist that prevents rosiglitazone-induced association of Crebbp with Pparg at Pparg target loci in mouse adipocytes).

EP300 (p300) binding to PPARG is supported by multiple studies (Gelman et al. 1999: EP300 stimulates transcription from PPAR response elements - PPAREs - in the presence of synthetic PPARG ligands, and the N-terminus of EP300 binds to human PPARG2; Jang and Jung 2015: binding of Pparg to Ep300 in mouse white preadipocyte line 3T3-L1 is negatively regulated by Atf3). EP300 was reported to acetylate PPARG, which increases PPARG transcriptional activity (Han et al. 2010). Levels of acetylated PPARG are significantly lower in young than in senescent human embryonic lung diploid fibroblast cell lines (Han et al. 2010). EP300 was reported to directly bind to RXRA in a ligand-dependent manner and acetylate it on lysine residue K145, which increases RXRA transcriptional activity (Zhao et al. 2007: recombinant and endogenous human RXRA and EP300 were used). The interaction between EP300 and RXRA is negatively regulated by NR4A1 (also known as TR3), an orphan nuclear receptor, which can sequester RXRA to mitochondria, triggering apoptosis (Zhao et al. 2007).

The A/B domain of PPARG (residues 1-136 in mouse Pparg2) is needed for recruitment of CREBBP and EP300 to PPARG target genes (Bugge et al. 2009).

PCAF is another histone acetyltransferase that may associate with PPARG (Puigserver et al. 1999).

The entire MED1 complex (also known as DRIP or TRAP complex) co-immunoprecipitates (coIPs) with the ligand-bound PPARG (Yang et al. 2000: ligand-dependent coIP of the MED1 complex associated with PPARG, ligand-dependent association of MED1 with the DNA-bound PPARG:RXRA heterodimer, species unspecified; Ge et al. 2002: recombinant mouse Pparg2 was used to coIP proteins from mouse embryonic fibroblasts; Allen et al. 2006: a mammalian two-hybrid assay using recombinant mouse proteins showed that PPARG agonists induce association of Med1, also known as TRAP220, to Pparg (Allen et al. 2006). Ge et al. 2008: recombinant mouse Pparg was used to coIP proteins from HeLa cell nuclear extracts). The interaction between MED1 and PPARG involves the LXXLL motifs in MED1 that are known to associate with nuclear receptors (Ge et al. 2008: mouse Pparg2 and human MED1 were used). Mouse Med1 binds with similar affinity to mouse Pparg1 and Pparg2 isoforms (Mueller et al. 2002). Epigenetic readers, such as BRD4, which can bind to epigenetic marks deposited at the PPARG target genes, may enable recruitment of the MED1 complex during adipogenesis (Lee et al. 2017). Mouse Med1 knockout fibroblasts were reported to be refractory to Pparg2-stimulated adipogenesis but not to Myod-stimulated myogenesis (Ge et al. 2002), but this does not depend on direct interaction between MED1 and PPARG (Ge et al. 2008). Knockout of another Mediator complex subunit, Med24, also impairs the adipogenic program in mouse fibroblasts (Ge et al. 2008). In mouse 3T3-L1 preadipocyte line, it was shown that the Mediator complex component Med14 directly interacts with the N-terminus of Pparg2 and facilitates the recruitment of the Mediator complex to PPARG targets (Grontved et al. 2010). Med14 knockdown impairs adipogenesis in 3T3-L1 cells (Grontved et al. 2010). Mouse Med14 binds with similar affinity to mouse Pparg1 and Pparg2 isoforms (Mueller et al. 2002). A structural study of the RXRA heterodimer with VDR (Vitamin D3 receptor) found that a single molecule of MED1 was bound to the heterodimer, interacting with both RXRA and VDR (Belorusova et al. 2020: human proteins were used), which implies that a single MED1 complex associates with one PPARG:RXRA heterodimer. In an earlier study, RXRA was shown to have a lower affinity for MED1 compared to PPARG2 (Yang et al. 2000: species unspecified).

PPARGC1A (PGC-1alpha) possesses largely independent PPARG, EP300, and MED1 interaction domains (Wallberg et al. 2003). Mouse Ppargc1a binds with equal affinity to mouse Pparg1 and Pparg2 isoforms (Mueller et al. 2002). Human recombinant PPARGC1A associates with recombinant human PPARG2 in the presence of PPARG agonists (Fujimura et al. 2005). Direct interaction between PPARGC1A and MED1 may explain recruitment of MED1 to PPARG even in the absence of LXXLL motifs in MED1 (Ge et al. 2008). Recombinant mouse Ppargc1a coIPs Mediator complex subunits besides MED1 from nuclear extracts of HeLa cells and wild-type, but not Med1 knockout MEFs (Wallberg et al. 2003). In a luciferase reporter assay, transcription from a PPARE is enhanced when PPARGC1A and MED1 are both present in addition to the PPARG:RXRA heterodimer and PPARG natural agonist 15dJ2, and all four proteins form a complex on target DNA based on electrophoretic mobility shift assay (EMSA) (Wallberg et al. 2003). When template DNA is histone-coated, prior EP300-mediated histone acetylation is required for PPARGC1A-stimulated transcription of PPARG:RXRA targets (Wallberg et al. 2003). Besides direct binding to PPARG, the histone acetyltransferases CREBBP, EP300, and NCOA1 also interact directly with PPARGC1A and PPARGC1A, which may aid in recruiting them to PPARG (Puigserver et al. 1999). The more N-terminal of the two PPARGC1A LXXLL motifs is necessary for its interaction with the coactivator-binding domain of PPARG (Li et al. 2008: human recombinant PPARGC1A and PPARG fragments were used for the crystallographic structural study). In mouse, it was shown that Naa10, an N-acetyltransferase, acetylates Ppargc1a, preventing its association with Pparg, and inhibiting beige adipocyte-mediated thermogenesis (Lee et al. 2019). NAA10 is overexpressed in cancer and in white adipose tissue of obese mice and humans (Lee et al. 2019). PPARGC1A was shown to directly associate with RXRA in a ligand-dependent manner and act as an RXRA co-activator, that may, in addition, facilitate recruitment of NCOA2 to RXRA (Delrive et al. 2002: human PPARGC1A and mouse Rxra were used).

PPARGC1B (also known as PGC1beta) can also bind PPARG and act as a transcriptional coactivator (Parham et al. 2015: human PPARG and human PPARGC1B co-immunoprecipitate in human endothelial cells treated with sphingosine 1-phosphate, also known as S1P, a putative PPARG ligand). Ppargc1b is strongly induced during differentiation of mouse brown adipocytes in a Ppargc1a-independent manner (Uldry et al. 2006). The presence of either Ppargc1a or Ppargc1b is needed for the development of mouse brown adipocytes and to increase mitochondrial density in differentiated mouse brown adipocytes in response to cold-induced adrenergic signaling (Uldry et al. 2006). Based on studies in mice, PPARGC1B is likely itself a PPARG target gene, with the PPARG:RXRA heterodimer shown to bind PPAREs in the first intron of the PPARGC1B gene to induce PPARGC1B expression (Deng et al. 2011: Pparg-mediated induction of Ppargc1b was shown in mouse white preadipocyte cell line 3T3-L1 and in the white adipose tissue of obese mice). PPARGC1B and PPARGC1A likely bind to PPARG in a mutually exclusive manner.

NCOA2 was reported to bind to PPARG in a ligand-dependent manner (Yang et al. 2000: ligand-dependent binding of NCOA2 to the DNA-bound PPARG:RXRA heterodimer, species unspecified, presumably human; Hartig et al. 2011: interaction between human PPARG and NCOA2 by FRET). While NCOA2 was reported to bind to RXRA in a ligand-dependent manner (Yang et al. 2000: species unspecified), in a structural study in which recombinant human proteins were used, the ligand-bound PPARG:RXRA heterodimer was reported to contain only one molecule of NCOA2, bound to PPARG (Osz et al. 2012). Another study, also using recombinant human proteins confirmed the 1:1 stoichiometry of the ligand-bound PPARX:RXRA heterodimer and NCOA2, showing that a single molecule of NCOA2 interacts with both RXRA (more weakly) and PPARG (more strongly), and this interaction is facilitated by both RXRA and PPARG binding to their respective ligands and to target DNA (de Vera et al. 2017).

NCOA4 (ARA70) was shown to bind directly to both PPARG and RXRA and activate transcription of PPARG target genes (Heinlein et al. 1999: human NCOA4, human RXRA, and mouse Pparg1 were used; binding to RXRA was dependent on the presence of RXRA ligand 9-cis-retinoic acid; binding to Pparg1 was enhanced in the presence of PPARG natural ligand 15dJ2 - 5-deoxy-delta12,14-prostaglandin J2).

Based on a study using mouse proteins, NCOA6 directly binds to both PPARG and RXRA in a ligand-dependent manner and stimulates PPARG:RXRA-mediated transcription (Zhu et al. 2000).

Based on studies in mouse white preadipocyte cell line 3T3-L1, AJUBA was reported to act both as a coactivator of PPARG, aiding in recruitment of EP300 and CREBBP to PPARG (Li et al. 2016), and to act upstream of PPARG, as a coactivator of CEBPB, aiding in recruitment of CREBBP and EP300 to CEBPB at PPARG and CEBPA gene promoters (Yan et al. 2022).

Combinations of coactivators that simultaneously bind to ligand-activated PPARG:RXRA dimers have not been fully elucidated, but can be regulated by cell type- and/or developmental stage-specific gene expression and posttranslational modifications. Also, different coactivators were reported to bind with different affinities to the PPARG:RXRA heterodimer in response to different PPARG and RXRA agonists (Yang et al. 2000: species unspecified). During human adipocyte differentiation, mRNA and protein levels of NCOA2 and NCOA3 were reported to remain constant, while levels of NCOA1 increase; siRNA-mediated knockdown of NCOA1, however, does not affect lipogenesis significantly, while siRNA-mediated knockdown of NCOA2 and NCOA3 does (Hartig et al. 2011). Inhibitory phosphorylation of PPARG on serine residue S112, known to diminish PPARG ligand affinity, interaction with NCOAs, transcriptional activity and adipogenic capacity, was shown to be increased upon NCOA2 and NCOA3 knockdown and decreased after NCOA1 knockdown (Hartig et al. 2011). CDK5-mediated phosphorylation of PPARG2 on serine residue S273 (corresponds to S245 in PPARG1) was reported to decrease the affinity of PPARG2 for the coactivator proteins MED1 (TRAP220), PPARGC1A (PGC 1alpha), and NCOA2 (TIF2) (Dias et al. 2020: human recombinant PPARG2 was used with human recombinant NCOA2, human recombinant MED1, and mouse recombinant PPARGC1A). Binding of NCOA1 and MED1 does not appear to occur simultaneously on each subunit of the PPARG:RXRA heterodimer (Yang et al. 2000). Relative level of NCOA1 to NCOA2 can modulate energy metabolism (Picard et al. 2002). Ncoa2 knockout mice are protected against obesity and exhibit enhanced adaptive thermogenesis, while Ncoa1 knockout mice are prone to obesity due to reduced energy expenditure (Picard et al. 2002). In brown adipose tissue of mice, the lack of NCOA2 facilitates the interaction of PPARGC1A with NCOA1, which promotes transcription of thermogenic genes (Picard et al. 2002).

Literature References

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16314690	FK614, a novel peroxisome proliferator-activated receptor gamma modulator, induces differential transactivation through a unique ligand-specific interaction with transcriptional coactivators Sakuma, H, Kimura, C, Mutoh, S, Aramori, I, Konishi, S, Oe, T, Hosogai, N, Fujimura, T	J Pharmacol Sci	2005
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