title: The partial agonist activity of antagonist-occupied steroid receptors is controlled by a novel hinge domain-binding coactivator L7/SPA and the corepressors N-CoR or SMRT.
abstract: Steroid receptor antagonists, such as the antiestrogen tamoxifen or the antiprogestin RU486, can have inappropriate agonist-like effects in tissues and tumors. To explain this paradox we postulated that coactivators are inadvertently brought to the promoters of DNA-bound, antagonist-occupied receptors. The human (h) progesterone receptor (PR) hinge-hormone binding domain (H-HBD) was used as bait in a two-hybrid screen of a HeLa cDNA library, in which the yeast cells were treated with RU486. We have isolated and characterized two interesting steroid receptor-interacting proteins that regulate transcription in opposite directions. The first is L7/SPA, a previously described 27-kDa protein containing a basic region leucine zipper domain, having no known nuclear function. When coexpressed with tamoxifen-occupied estrogen receptors (hER) or RU486-occupied hPR or glucocorticoid receptors (hGR), L7/SPA increases the partial agonist activity of the antagonists by 3- to 10-fold, but it has no effect on agonist-mediated transcription. The interaction of L7/SPA with hPR maps to the hinge region, and indeed, the hPR hinge region squelches L7/SPA-dependent induction of antagonist-mediated transcription. Interestingly, pure antagonists that lack partial agonist effects, such as the antiestrogen ICI164,384 or the antiprogestin ZK98299, cannot be up-regulated by L7/SPA. We also isolated, cloned, and sequenced the human homolog (hN-CoR) of the 270-kDa mouse (m) thyroid/retinoic acid receptor corepressor. Binding of hN-CoR maps to the hPR-HBD. mN-CoR, and a related human corepressor, SMRT, suppress RU486 or tamoxifen-mediated partial agonist activity by more than 90%. This suppression is completely squelched by overexpression of the hPR H-HBD. Additionally, both corepressors reverse the antagonist-dependent transcriptional up-regulation produced by L7/SPA. Our data suggest that the direction of transcription by antagonist-occupied steroid receptors can be controlled by the ratio of coactivators to corepressors recruited to the transcription complex by promoter-bound receptors. In normal tissues and in hormone-resistant breast cancers in which the agonist activity of mixed antagonists predominates, steroid receptors may be preferentially bound by coactivators. This suggests a strategy by which such partial agonist activity can be eliminated and by which candidate receptor ligands can be screened for this activity.
authors.affiliation: Department of Medicine, University of Colorado Health Sciences Center, Denver 80262, USA.
authors.forename: T A
authors.initials: TA
authors.lastname: Jackson
authors.forename: J K
authors.initials: JK
authors.lastname: Richer
authors.forename: D L
authors.initials: DL
authors.lastname: Bain
authors.forename: G S
authors.initials: GS
authors.lastname: Takimoto
authors.forename: L
authors.initials: L
authors.lastname: Tung
authors.forename: K B
authors.initials: KB
authors.lastname: Horwitz
mesh_terms: D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D019556:COS Cells; D018076:DNA, Complementary; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology*; D005786:Gene Expression Regulation / Q000187:drug effects; D006367:HeLa Cells; D006801:Humans; D016350:Leucine Zippers; D051379:Mice; D015735:Mifepristone / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000235:genetics / Q000502:physiology*; D056971:Nuclear Receptor Co-Repressor 1; D056985:Nuclear Receptor Co-Repressor 2; D011980:Receptors, Progesterone / Q000819:agonists* / Q000037:antagonists & inhibitors* / Q000235:genetics; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000494:pharmacology; D012097:Repressor Proteins / Q000235:genetics / Q000502:physiology*; D012269:Ribosomal Proteins / Q000302:isolation & purification / Q000502:physiology*; D015003:Yeasts
chemical_list: D018076:DNA, Complementary; D004268:DNA-Binding Proteins; C539127:NCOR1 protein, human; C539130:NCOR2 protein, human; C539128:Ncor1 protein, mouse; C539131:Ncor2 protein, mouse; D009687:Nuclear Proteins; D056971:Nuclear Receptor Co-Repressor 1; D056985:Nuclear Receptor Co-Repressor 2; C053634:RPL7 protein, human; D011980:Receptors, Progesterone; D011993:Recombinant Fusion Proteins; D012097:Repressor Proteins; D012269:Ribosomal Proteins; C512845:Rpl7 protein, mouse; D015735:Mifepristone