New Antiprogestins with Partial Agonist Activity: Potential Selective Progesterone Receptor Modulators (SPRMs) and Probes for Receptor- and Coregulator-Induced Changes in Progesterone Receptor Induction Properties

New Antiprogestins with Partial Agonist Activity: Potential Selective Progesterone Receptor Modulators (SPRMs) and Probes for Receptorand Coregulator-Induced Changes in Progesterone Receptor Induction Properties Georgia Giannoukos*, Daniele Szapary, Catharine L. Smith, James E. W. Meeker†, and S. Stoney Simons, Jr. Steroid Hormones Section (G.G., D.S., J.E.W.M., S.S.S.) Laboratory of Molecular and Cellular Biology National Institute of Diabetes, Digestive and Kidney Diseases and the Laboratory of Receptor Biology and Gene Expression (C.L.S.) National Cancer Institute/Division of Basic Sciences National Institutes of Health Bethesda, Maryland 20892-0805 A pharmacologically relevant property of steroid hormone-regulated gene induction is the partial agonist activity of antisteroid complexes. We now report that dexamethasone-mesylate (Dex-Mes) and dexamethasone-oxetanone (Dex-Ox), each a derivative of the glucocorticoid-selective steroid dexamethasone (Dex), are two new antiprogestins with significant amounts of agonist activity with both the A and B isoforms of progesterone receptor (PR), for different progesterone-responsive elements, and in several cell lines. These compounds continue to display activity under conditions where another partial antiprogestin (RTI-020) is inactive. These new antiprogestins were used to determine whether the partial agonist activity of PR complexes can be modified by changing concentrations of receptor or coregulator, as we have recently demonstrated for glucocorticoid receptors (GRs). Because GR and coregulator concentrations simultaneously altered the position of the physiologically relevant dose-response curve, and associated EC50, of GR-agonist complexes, we also examined this phenomenon with PR. We find that elevated PR or transcriptional intermediary factor 2 (TIF2) concentrations increase the partial agonist activity of Dex-Mes and Dex-Ox, and the EC50 of agonists, independently of changes in total gene transactivation. Furthermore, the corepressors SMRT (silencing mediator for retinoid and thy- roid receptors) and NCoR (nuclear receptor corepressor) each suppresses gene induction but NCoR acts opposite to SMRT and, like the coactivator TIF2, reduces the EC50 and increases the partial agonist activity of antiprogestins. These comparable responses of GR and PR suggest that variations in receptor and coregulator concentrations may be a general mechanism for altering the induction properties of other steroid receptors. Finally, the magnitude of coregulator effects on PR induction properties are often not identical for agonists and the new antagonists, suggesting subtle mechanistic differences. These properties of DexMes and Dex-Ox, plus the sensitivity of their activity to cellular differences in PR and coregulator concentrations, make these steroids potential new SPRMs (selective progesterone receptor modulators) that should prove useful as probes of PR induction properties. (Molecular Endocrinology 15: 255–270, 2001) INTRODUCTION A common use of endocrine therapies in the clinical setting is to block the action of an endogenous hormone in both normal and malignant tissues. The inhibition of progesterone action in women is used to prevent conception at a variety of stages (1, 2). The growth of many breast cancer tumors is retarded by suppressing the action of endogenous estrogens (3, 4). Pure antisteroids, which display no agonist activity, 0888-8809/01/$3.00/0 Molecular Endocrinology 15(2): 255–270 Copyright © 2001 by The Endocrine Society Printed in U.S.A. 255 MOL ENDO · 2001 256 represent one of the most straightforward methods of blocking the actions of steroid hormones. The antisteroid undergoes most of the same steps as agonist steroids, including steroid binding to the cognate receptor, activation, nuclear translocation, and DNA binding (5, 6). It is predominantly in the association with coregulators and the transcriptional machinery that the antisteroid complexes appear to differ and prevent the changes in rates of gene transcription seen with agonist complexes (7–9). Unfortunately, the side effects of antisteroid therapies can be quite severe. A pure antagonist will block all of the actions of a particular receptor in addition to the one that is targeted. Furthermore, some antisteroids, like RU 486 (10), cross-react with other receptors to block the actions of multiple classes of steroids (11), thereby magnifying the number of undesired actions. An exciting new approach to endocrine therapies has emerged with the concept of SRMs, or selective receptor modulators. These are compounds that are antagonists for some genes/tissues but agonists for others. Thus, they are not pure antagonists. This ability to display partial agonist activity with selected reporter genes can be clinically very beneficial. For example, the agonist activity of raloxifene and tamoxifen in bone coupled with their antagonist activity in breast (Ref. 12 and references therein) suggests that their use in the treatment of breast cancer might not be accompanied by the osteoporosis seen with other antiestrogens (13). While it is not yet possible to predict the genes for which a particular SRM will display antagonist vs. agonist activity, an absolute requirement is that the steroid possesses partial agonist activity for at least one gene. Surprisingly, there are very few antiprogestins with partial agonist activity for any genes and thus are candidate-selective progesterone receptor modulators, or SPRMs. RU 486 is the most commonly used antiprogestin and displays partial agonist activity only under selected conditions (14, 15). More recently, a closely related derivative of RU 486, called RTI 3021– 020 (RTI-020), was found to be a partial agonist in T47D cells but was not active in CV-1 cells (16). Therefore, it would be extremely helpful for theoretical and clinical studies to identify new antiprogestins with partial agonist activity. Another approach for modifying the activities of antisteroids involves the coregulators that appear to be recruited by DNA-bound receptors to help modify the rates of target gene transcription. Ligand-free nuclear receptors, and some steroid receptors bound by antagonists (9, 15, 17), are usually associated with the corepressors SMRT (signal mediator and repressor of transcription) or NCoR (nuclear receptor corepressor) (18, 19). Agonist binding is thought to cause the release of corepressors and allow the association of coactivators (20–24). While corepressors are known to influence the partial agonist activity of antisteroid complexes (8, 9, 15, 17, 25, 26), an effect of coactivators on the activity of receptor-antisteroid complexes has Vol. 15 No. 2 only lately been seen with GRs (26, 27). Interestingly, different concentrations in coregulators also alter the concentration of the glucocorticoid dexamethasone (Dex) required for half-maximal induction, or EC50 (26, 27). Variations in the EC50 are highly significan (...truncated)