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.

Estrogen Receptor β (ERβ1) Transactivation Is Differentially Modulated by the Transcriptional Coregulator Tip60 in a cis-Acting Element-dependent Manner

“…Although ERs interact with the most coactivators and corepressors at either or both N and C termini (50), they also bind to some coregulators at the hinge domain. L7/SPA interacts with the hinge domain of ER␣ and enhances transactivation of antagonist-occupied ER␣ at the ERE site (51). ER␣ also binds to PGC-1 at its hinge domain in a ligand-independent manner (52).…”

HDAC3: taking the SMRT-N-CoRrect road to repression

“…For instance, cyclin D1 can repress both basal as well as ligand-dependent functions of TR and PPARg, by recruiting HDAC3 Fu et al, 2005). In other examples, it is the antagonist-bound receptor that interacts with N-CoR/ SMRT to repress transcription (Jackson et al, 1997;Smith et al, 1997;Lavinsky et al, 1998). In addition to nuclear receptors, other transcription factors, including Mad, BCL6, Pit1 and ETO also use corepressor complexes to establish and maintain the inactive state of their target genes (Heinzel et al, 1997;Huynh and Bardwell, 1998;Lutterbach et al, 1998;Wang et al, 1998;Xu et al, 1998).…”

GCUNC-45 Is a Novel Regulator for the Progesterone Receptor/hsp90 Chaperoning Pathway

“…A construct containing the hinge and hormone-binding domains of human PR was used as bait in a yeast two-hybrid screen in the presence of the mixed progestin antagonist RU486 (21). This revealed a number of antagonist-specific PR-interacting proteins, including a 1.2-kb clone with an open reading frame (ORF) encoding 394 amino acids of a human protein that interacted with the PR bait only when RU486 was present.…”