The nuclear receptor corepressor (NCoR) was isolated as a peroxisome-proliferator-activated receptor (PPAR) δ interacting protein using the yeast two-hybrid system. NCoR interacted strongly with the ligand-binding domain of PPARδ, whereas interactions with the ligand-binding domains of PPARγ and PPARα were significantly weaker. PPAR—NCoR interactions were antagonized by ligands in the two-hybrid system, but were ligand-insensitive in in vitro pull-down assays. Interaction between PPARδ and NCoR was unaffected by coexpression of retinoid X receptor (RXR) α. The PPARδ—RXRα heterodimer bound to an acyl-CoA oxidase (ACO)-type peroxisome-proliferator response element recruited a glutathione S-transferase—NCoR fusion protein in a ligand-independent manner. Contrasting with most other nuclear receptors, PPARδ was found to interact equally well with interaction domains I and II of NCoR. In transient transfection experiments, NCoR and the related silencing mediator for retinoid and thyroid hormone receptor (SMRT) were shown to exert a marked dose-dependent repression of ligand-induced PPARδ-mediated transactivation; in addition, transactivation induced by the cAMP-elevating agent forskolin was efficiently reduced to basal levels by NCoR as well as SMRT coexpression. Our results suggest that the transactivation potential of liganded PPARδ can be fine-tuned by interaction with NCoR and SMRT in a manner determined by the expression levels of corepressors and coactivators.
- transcriptional repression
↵1 These authors have contributed equally to this work.
Abbreviations used: PPAR, peroxisome-proliferator-activated receptor; RXR, retinoid X receptor; RAR, retinoic acid receptor; TR, thyroid hormone receptor; HLBD, hinge and ligand-binding domain; AF2, activator function 2; DBD, DNA-binding domain; AD, activator domain; RID, receptor-interacting domain; DR1, direct repeat with 1bp spacing; ACO, acyl-CoA oxidase; PPRE, peroxisome-proliferator response element; GST, glutathione S-transferase; NCoR, nuclear receptor corepressor; SMRT, silencing mediator for retinoid and thyroid hormone receptor; TBP, TATA-binding protein; cPGI2, cyclic prostaglandin I2; T3, thyroid hormone; ID-I/II, interaction domain I/II; LBD, ligand-binding domain.
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