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.

Menin, a product of the MENI gene, binds to estrogen receptor to enhance its activity in breast cancer cells: possibility of a novel predictive factor for tamoxifen resistance

“…This corepressor, termed RTA, was shown to interact with the N-terminal domain of ERs. With regard to coactivators, in addition to the p160 coactivators, one coactivator that affects tamoxifen activity has been reported, but there is no information on ERa or tamoxifen specificity of this coactivator [52]. In the present study, we demonstrated that menin can bind to AF-2, and tamoxifen inhibits the binding of menin to ERa, but tamoxifen suppressed ERE-luciferase activity only to the levels of nontreated wild-type MCF-7 in menin-overexpressing clones.…”

Transcriptional coregulators of the nuclear receptor superfamily: coactivators and corepressors

“…NCoR/SMRT have also been reported to partner with HDAC4, HDAC5, and HDAC7 [74,75]. Steroid hormone receptors do not appear to interact with NCoR or SMRT in the presence or absence of agonists, whereas both the ER and the progesterone receptor can interact with these corepressors in the presence of their respective antagonists [80][81][82][83]. Interestingly, NCoR/ SMRT are also known to mediate transcriptional repression from a wide variety of other non-receptor-mediated pathways.…”

Receptor density dictates the behavior of a subset of steroid ligands in glucocorticoid receptor-mediated transrepression

“…Extrapolation from this analogy is that the proteins (corepressors and/or HDACs) recruited by cortisol-bound GR are likely to be different, quantitatively and/or qualitatively, from those by RU486-bound GR in inducing GR-mediated transrepression, but similar to those recruited by dexamethasone-bound GR. This explanation has been suggested for the agonist function of RU486 in PR function [15] and for the agonist function of tamoxifen in estrogen receptor function [38]. The recruitment by RU486-bound GR is more sensitive to GR density changes than cortisol-bound GR, suggesting that RU486-bound GR has a greater degree of conformation-dependent functional variability in cell type-dependent agonist action for transrepression.…”