THE PARTIAL AGONIST ACTIVITY OF ANTAGONIST-OCCUPIED STEROID-RECEPTORSIS CONTROLLED BY A NOVEL HINGE DOMAIN-BINDING COACTIVATOR L7 SPA AND THE COREPRESSORS N-COR OR SMRT/

Steroid receptor antagonists, such as the antiestrogen tamoxifen or th e antiprogestin RU486, can have inappropriate agonist-like effects in tissues and tumors. To explain this paradox we postulated that coactiv ators are inadvertently brought to the promoters of DNA-bound, antagon ist-occupied receptors. The human (h) progesterone receptor (PR) hinge -hormone binding domain (H-HBD) was used as bait in a two-hybrid scree n of a HeLa cDNA library, in which the yeast cells were treated with R U486. We have isolated and characterized two interesting steroid recep tor-interacting proteins that regulate transcription in opposite direc tions. The first is L7/SPA, a previously described 27-kDa protein cont aining a basic region leucine zipper domain, having no known nuclear f unction. When coexpressed with tamoxifen-occupied estrogen receptors ( hER) or RU486-occupied hPR or glucocorticoid receptors (hGR), L7/SPA i ncreases the partial agonist activity of the antagonists by 3- to 10-f old, but it has no effect on agonist-mediated transcription. The inter action of L7/SPA with hPR maps to the hinge region, and indeed, the hP R hinge region squelches L7/SPA-dependent induction of antagonist-medi ated transcription. Interestingly, pure antagonists that lack partial agonist effects, such as the antiestrogen ICI164,384 or the antiproges tin ZK98299, cannot be up-regulated by L7/SPA. We also isolated, clone d, and sequenced the human homolog (hN-CoR) of the 270-kDa mouse (m) t hyroid/retinoic acid receptor corepressor. Binding of hN-CoR maps to t he hPR-HBD. mN-CoR, and a related human corepressor, SMRT, suppress RU 486 or tamoxifen-mediated partial agonist activity by more than 90%. T his 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 th at the direction of transcription by antagonist-occupied steroid recep tors can be controlled by the ratio of coactivators to corepressors re cruited to the transcription complex by promoter-bound receptors. In n ormal tissues and in hormone-resistant breast cancers in which the ago nist 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 can didate receptor ligands can be screened for this activity.