16 alpha-substituted analogs of the antiprogestin RU486 induce a unique conformation in the human progesterone receptor resulting in mixed agonist activity

The Selective Progesterone Receptor Modulator Ulipristal Acetate Inhibits the Activity of the Glucocorticoid Receptor

CONTEXT

The selective progesterone modulator ulipristal acetate (ulipristal) offers a much-needed therapeutic option for the clinical management of uterine fibroids. Although ulipristal initially passed safety evaluations in Europe, postmarketing analysis identified cases of hepatic injury and failure, leading to restrictions on the long-term use of ulipristal. One of the factors potentially contributing to significant side effects with the selective progesterone modulators is cross-reactivity with other steroid receptors.

OBJECTIVE

To determine whether ulipristal can alter the activity of the endogenous glucocorticoid receptor (GR) in relevant cell types.

DESIGN

Immortalized human uterine fibroid cells (UtLM) and hepatocytes (HepG2) were treated with the synthetic glucocorticoid dexamethasone and/or ulipristal. Primary uterine fibroid tissue was isolated from patients undergoing elective gynecological surgery and treated ex vivo with dexamethasone and/or ulipristal. In vivo ulipristal exposure was performed in C57Bl/6 mice to measure the effect on basal gene expression in target tissues throughout the body.

RESULTS

Dexamethasone induced the expression of established glucocorticoid-target genes period 1 (PER1), FK506 binding protein 51 (FKBP5), and glucocorticoid-induced leucine zipper (GILZ) in UtLM and HepG2 cells, whereas cotreatment with ulipristal blocked the transcriptional response to glucocorticoids in a dose-dependent manner. Ulipristal inhibited glucocorticoid-mediated phosphorylation, nuclear translocation, and DNA interactions of GR. Glucocorticoid stimulation of PER1, FKBP5, and GILZ was abolished by cotreatment with ulipristal in primary uterine fibroid tissue. The expression of glucocorticoid-responsive genes was decreased in the lung, liver, and uterus of mice exposed to 2 mg/kg ulipristal. Interestingly, transcript levels of Fkbp5 and Gilz were increased in the hippocampus and pituitary.

CONCLUSIONS

These studies demonstrate that ulipristal inhibits endogenous glucocorticoid signaling in human fibroid and liver cells, which is an important consideration for its use as a long-term therapeutic agent.

Ulipristal acetate for Japanese women with symptomatic uterine fibroids: A double-blind, randomized, phase II dose-finding study

PURPOSE

A multicenter, randomized, double-blind, placebo-controlled trial was conducted to evaluate the efficacy, safety, and appropriate dose of ulipristal acetate (UPA) in Japanese women with symptomatic uterine fibroids (UFs).

METHODS

A total of 121 premenopausal women with UFs were enrolled to receive either placebo, UPA-2.5 mg, UPA-5 mg, UPA-10 mg, or leuprorelin acetate (LEU), a reference drug, for 12 weeks. The primary end point was the rate of patients having achieved amenorrhea for 35 days at Week 12.

RESULTS

The rates for amenorrhea were 4.5%, 60.0%, 72.7%, 88.0%, and 76.2% in the placebo, UPA-2.5 mg, UPA-5 mg, UPA-10 mg, and LEU groups, respectively. The median times to amenorrhea were 20.0, 5.0, 5.0, and 23.0 days for treatment with UPA-2.5 mg, UPA-5 mg, UPA-10 mg, and LEU, respectively. A significant dose-response of UPA for the rate of amenorrhea was observed. The overall incidence rates of adverse events were 45.8% in the placebo group, 56.5%-80.0% in the UPA groups, and 100.0% in the LEU group. There were no notable safety issues with UPA.

CONCLUSIONS

Ulipristal acetate was effective and well tolerated in Japanese women with UFs. The recommended dose of UPA is considered to be 10 mg.

Novel RU486 (mifepristone) analogues with increased activity against Venezuelan Equine Encephalitis Virus but reduced progesterone receptor antagonistic activity

There are currently no therapeutics to treat infection with the alphavirus Venezuelan equine encephalitis virus (VEEV), which causes flu-like symptoms leading to neurological symptoms in up to 14% of cases. Large outbreaks of VEEV can result in 10,000 s of human cases and mass equine death. We previously showed that mifepristone (RU486) has anti-VEEV activity (EC50 = 20 μM) and only limited cytotoxicity (CC50 > 100 μM), but a limitation in its use is its abortifacient activity resulting from its ability to antagonize the progesterone receptor (PR). Here we generate a suite of new mifepristone analogues with enhanced antiviral properties, succeeding in achieving >11-fold improvement in anti-VEEV activity with no detectable increase in toxicity. Importantly, we were able to derive a lead compound with an EC50 of 7.2 µM and no detectable PR antagonism activity. Finally, based on our SAR analysis we propose avenues for the further development of these analogues as safe and effective anti-VEEV agents.

Understanding how long-acting β2 -adrenoceptor agonists enhance the clinical efficacy of inhaled corticosteroids in asthma - an update

In moderate-to-severe asthma, adding an inhaled long-acting β₂ -adenoceptor agonist (LABA) to an inhaled corticosteroid (ICS) provides better disease control than simply increasing the dose of ICS. Acting on the glucocorticoid receptor (GR, gene NR3C1), ICSs promote anti-inflammatory/anti-asthma gene expression. In vitro, LABAs synergistically enhance the maximal expression of many glucocorticoid-induced genes. Other genes, including dual-specificity phosphatase 1(DUSP1) in human airways smooth muscle (ASM) and epithelial cells, are up-regulated additively by both drug classes. Synergy may also occur for LABA-induced genes, as illustrated by the bronchoprotective gene, regulator of G-protein signalling 2 (RGS2) in ASM. Such effects cannot be produced by either drug alone and may explain the therapeutic efficacy of ICS/LABA combination therapies. While the molecular basis of synergy remains unclear, mechanistic interpretations must accommodate gene-specific regulation. We explore the concept that each glucocorticoid-induced gene is an independent signal transducer optimally activated by a specific, ligand-directed, GR conformation. In addition to explaining partial agonism, this realization provides opportunities to identify novel GR ligands that exhibit gene expression bias. Translating this into improved therapeutic ratios requires consideration of GR density in target tissues and further understanding of gene function. Similarly, the ability of a LABA to interact with a glucocorticoid may be suboptimal due to low β₂ -adrenoceptor density or biased β₂ -adrenoceptor signalling. Strategies to overcome these limitations include adding-on a phosphodiesterase inhibitor and using agonists of other Gs-coupled receptors. In all cases, the rational design of ICS/LABA, and derivative, combination therapies requires functional knowledge of induced (and repressed) genes for therapeutic benefit to be maximized.

Competitive Agonists and Antagonists of Steroid Nuclear Receptors: Evolution of the Concept or Its Reversal

The mechanisms displaying pure and mixed steroid agonist/antagonist activity as well as principles underlying in vivo action of selective steroid receptor modulators dependent on tissue or cell type including interaction with various types of nuclear receptors are analyzed in this work. Mechanisms of in vitro action for mixed agonist/antagonist steroids are discussed depending on: specific features of their interaction with receptor hormone-binding pocket; steroid-dependent allosteric modulation of interaction between hormone-receptor complex and hormone response DNA elements; features of interacting hormone-receptor complex with protein transcriptional coregulators; level and tissue-specific composition of transcriptional coregulators. A novel understanding regarding context-selective modulators replacing the concept of steroid agonists and antagonists is discussed.

An analysis of glucocorticoid receptor-mediated gene expression in BEAS-2B human airway epithelial cells identifies distinct, ligand-directed, transcription profiles with implications for asthma therapeutics

BACKGROUND AND PURPOSE

International asthma guidelines recommend that inhaled glucocorticoids be used as a monotherapy in all patients with mild to moderate disease because of their ability to suppress airways inflammation. Current evidence suggests that the therapeutic benefit of glucocorticoids is due to the transactivation and transrepression of anti-inflammatory and pro-inflammatory genes respectively. However, the extent to which clinically relevant glucocorticoids are equivalent in their ability to modulate gene expression is unclear.

EXPERIMENTAL APPROACH

A pharmacodynamics investigation of glucocorticoid receptor (GR)-mediated gene transactivation in BEAS-2B human airway epithelial cells was performed using a glucocorticoid response element luciferase reporter coupled with an analysis of glucocorticoid-inducible genes encoding proteins with anti-inflammatory and adverse-effect potential.

KEY RESULTS

Using transactivation as a functionally relevant output, a given glucocorticoid displayed a unique, gene expression 'fingerprint' where intrinsic efficacy and GR density were essential determinants. We showed that depending on the gene selected for analysis, a given glucocorticoid can behave as an antagonist, partial agonist, full agonist or even 'super agonist'. In the likely event that different, tissue-dependent gene expression profiles are reproduced in vivo, then the anti-inflammatory and adverse-effect potential of many glucocorticoids currently available as asthma therapeutics may not be equivalent.

CONCLUSIONS AND IMPLICATIONS

The generation of gene expression 'fingerprints' in target and off-target human tissues could assist the rational design of GR agonists with improved therapeutic ratios. This approach could identify compounds that are useful in the management of severe asthma and other inflammatory disorders where systemic exposure is desirable.

Ulipristal acetate does not impact human normal breast tissue

BACKGROUND

Antiprogestins are of growing interest for the development of new treatments in the gynecological field. Ulipristal acetate (UPA) is a progesterone receptor (PR) modulator considered for long-term administration in contraception and is currently being registered for the treatment of uterine fibroids. In light of the influences of hormonal dysfunction in breast pathologies, the secondary consequences of chronic UPA therapy need to be established. The aim of this study was to determine UPA actions mediated by PR and glucocorticoid receptor (GR) in normal and transformed breast.

METHODS

UPA, progesterone (P) and dexamethasone (DEX) effects were observed on PR and GR responsive genes and on proliferation and apoptosis of normal human breast epithelial (HBE) and breast cancer cells. Human normal breast tissue samples were xenografted in athymic mice and treated with estradiol (E2), or E2 + P, or E2 + P + UPA.

RESULTS

Analysis of PR and GR reporter gene transactivation and their respective endogenous target genes indicated that UPA exerted anti-progestational and anti-glucocorticoid activity in both types of cells with a more pronounced effect in cancer cells. When combined with P or DEX, UPA limits the proliferation of HBE cells but increases growth in breast cancer cell lines. UPA administration had no impact on the mitotic index on xenografted human breast tissue exposed to gonadal hormones at similar concentrations to those present in normal women.

CONCLUSIONS

Although further clinical trials are required to confirm that the results from our experimental models can be extrapolated to women treated with UPA, they suggest that such treatment would not be deleterious to normal breast tissue at least for a cycle (28 days) of continuous administration.

X-ray structures of progesterone receptor ligand binding domain in its agonist state reveal differing mechanisms for mixed profiles of 11β-substituted steroids

We present here the x-ray structures of the progesterone receptor (PR) in complex with two mixed profile PR modulators whose functional activity results from two differing molecular mechanisms. The structure of Asoprisnil bound to the agonist state of PR demonstrates the contribution of the ligand to increasing stability of the agonist conformation of helix-12 via a specific hydrogen-bond network including Glu(723). This interaction is absent when the full antagonist, RU486, binds to PR. Combined with a previously reported structure of Asoprisnil bound to the antagonist state of the receptor, this structure extends our understanding of the complex molecular interactions underlying the mixed agonist/antagonist profile of the compound. In addition, we present the structure of PR in its agonist conformation bound to the mixed profile compound Org3H whose reduced antagonistic activity and increased agonistic activity compared with reference antagonists is due to an induced fit around Trp(755), resulting in a decreased steric clash with Met(909) but inducing a new internal clash with Val(912) in helix-12. This structure also explains the previously published observation that 16α attachments to RU486 analogs induce mixed profiles by altering the binding of 11β substituents. Together these structures further our understanding of the steric and electrostatic factors that contribute to the function of steroid receptor modulators, providing valuable insight for future compound design.

Progesterone inhibits the growth of human neuroblastoma: in vitro and in vivo evidence

We investigated the antitumorogenic effects of progesterone (P4) in a human neuroblastoma (SK-N-AS) cell line in vitro and in a mouse xenograft model of neuroblastoma. The safety of P4 was tested in rat primary cortical neurons and human foreskin fibroblasts (HFF-1). At high doses, P4 significantly (P < 0.05) decreased SK-N-AS cell viability in vitro, and this effect was not blocked either by 5α-reductase inhibitor, finasteride or the P4 receptor antagonist RU486. Even at very high doses, P4 did not induce any cell death in healthy primary cortical neurons or HFF-1. The bioavailability of P4 24 h after the last injection in the serum of treated animals was significantly (P < 0.05) higher (10-33 μg/mL) than in untreated animals. In nude mice, P4 (50 and 100 mg/kg) inhibited neuroblastoma growth by ~50% over 8 d of treatment. No drug toxicity was observed in the mice, as measured by body weight and activity. P4 suppressed the expression of vascular endothelial growth factor (VEGF) and matrix metalloproteinases (MMP-9, MMP-2), which are involved in tumor vascular development. High-dose P4 inhibited tumor growth by suppressing cell proliferation and inducing apoptosis, as evidenced by the expression of proliferating cell nuclear antigen and cleaved caspase-3. P4 significantly increased the expression of P4 receptor isoform-A and suppressed phospho-Akt (Ser437) expression. In conclusion, at high doses, P4 effectively inhibits the growth of solid neuroblastoma tumor and has high bioavailability, selective toxicity and a high margin of safety, making it a possible candidate for further study as a potential clinical treatment of neuroblastoma.

Distinct temporal and spatial activities of RU486 on progesterone receptor function in reproductive organs of ovariectomized mice

RU486 is an incomplete progesterone receptor (PR) antagonist due to its partial agonist activity. To investigate the tissue-specific effects of RU486 on PR function in an ovariectomized mouse model, we used the progesterone receptor activity indicator mouse and evaluated the key determinants of progesterone-dependent gene activity: PR, coregulators, and kinases. As might be expected, acute RU486 treatment (6 h after injection) reduced PR transcriptional activity in the uterus, compared with vehicle or progesterone (P4) treatments. Chronic RU486 treatment (3 d) had a distinctly different effect on PR-mediated transcription, elevating PR activity in both the uterus and mammary gland, whereas chronic P4 treatment reduced PR activity in both tissues. This elevated uterine PR activity was associated with increased modified forms of PR and total protein levels of steroid receptor coactivator (SRC)-1 without affecting SRC-2 or SRC-3 protein levels. In addition to increased levels of coactivators, chronic RU486 treatment activated the ERK-1/2 and c-Jun N-terminal kinase signaling pathways in the uterus in a manner comparable with P4 treatment. In contrast to our observations in the uterus, chronic RU486 treatment increased modified forms of PR and the SRC-3 protein levels (but not SRC-1 and SRC-2 levels) in luminal epithelial cells of the mammary gland. Chronic RU486 also activated the c-Jun N-terminal kinase, but not ERK-1/2, signaling pathways in mammary luminal epithelial cells. This report suggests that in comparison with chronic natural hormone (P4), a mixed antagonist/agonist (RU486) induces a distinct temporal and spatial pattern of cellular genetic regulators that accompany ligand-specific gene expression.

Determination of conformational changes in the progesterone receptor using ELISA-like assays

The conformation of proteins often influences their functional activity. The effect of progesterone receptor ligands on the C-terminal conformation of the progesterone receptor affects the recruitment of transcriptional cofactors. These conformations can be studied by differential sensitivity to proteolytic cleavage or immunoprecipitation with a conformation-specific antibody. This study describes an ELISA-like method using conformation-specific antibodies to the C-terminal or an area adjacent to the DNA binding region. Progesterone receptor ligands are shown to influence how the progesterone receptor interacts with these antibodies in a concentration dependent manner. This method allows for quick determination of the potency of agonists as well as mechanistic studies of antagonism. The conformation inducing activity of several standard agonist and antagonist compounds were compared to their binding affinity and ability to induce alkaline phosphatase in T47D cells. This method is useful for screening compounds for functional activity at the progesterone receptor and demonstrates that J867 induces an antagonist conformation of the progesterone receptor similar to the antagonist RU486.

Role of nonhuman primate models in the discovery and clinical development of selective progesterone receptor modulators (SPRMs)

Selective progesterone receptor modulators (SPRMs) represent a new class of progesterone receptor ligands that exert clinically relevant tissue-selective progesterone agonist, antagonist, partial, or mixed agonist/antagonist effects on various progesterone target tissues in an in vivo situation depending on the biological action studied. The SPRM asoprisnil is being studied in women with symptomatic uterine leiomyomata and endometriosis. Asoprisnil shows a high degree of uterine selectivity as compared to effects on ovulation or ovarian hormone secretion in humans. It induces amenorrhea and decreases leiomyoma volume in a dose-dependent manner in the presence of follicular phase estrogen concentrations. It also has endometrial antiproliferative effects. In pregnant animals, the myometrial, i.e. labor-inducing, effects of asoprisnil are blunted or absent. Studies in non-human primates played a key role during the preclinical development of selective progesterone receptor modulators. These studies provided the first evidence of uterus-selective effects of asoprisnil and structurally related compounds, and the rationale for clinical development of asoprisnil.

Selective progesterone receptor modulator development and use in the treatment of leiomyomata and endometriosis

Selective progesterone receptor modulators (SPRMs) represent a new class of progesterone receptor ligands. SPRMs exert clinically relevant tissue-selective progesterone agonist, antagonist, or mixed agonist/antagonist effects on various progesterone target tissues in vivo. Asoprisnil (J867) is the first SPRM to reach an advanced stage of clinical development for the treatment of symptomatic uterine fibroids and endometriosis. Asoprisnil belongs to the class of 11beta-benzaldoxime-substituted estratrienes that exhibit partial progesterone agonist/antagonist effects with high progesterone receptor specificity in animals and humans. Asoprisnil has no antiglucocorticoid activity in humans at therapeutic doses. It exhibits endometrial antiproliferative effects on the endometrium and breast in primates. Unlike progesterone antagonists, asoprisnil does not induce labor in relevant models of pregnancy and parturition. It induces amenorrhea primarily by targeting the endometrium. In human subjects with uterine fibroids, asoprisnil suppressed both the duration and intensity of uterine bleeding in a dose-dependent manner and reduced tumor volume in the absence of estrogen deprivation. In subjects with endometriosis, asoprisnil was effective in reducing nonmenstrual pain and dysmenorrhea. Asoprisnil may, therefore, provide a novel, tissue-selective approach to control endometriosis-related pain. SPRMs have the potential to become a novel treatment of uterine fibroids and endometriosis.

Selective progesterone receptor modulators and progesterone antagonists: mechanisms of action and clinical applications

Since the discovery of the antiprogestin mifepristone, hundreds of similar compounds have been synthesized, which can be grouped in a large family of progesterone receptor ligands. This family includes pure agonists such as progesterone itself or progestins and, at the other end of the biological spectrum, pure progesterone receptor antagonists (PA). Selective progesterone receptor modulators (SPRM) have mixed agonist-antagonist properties, and occupy an intermediate position of the spectrum. These compounds have numerous applications in female health care. Mifepristone is used to terminate pregnancy, and as such is commercially available in many countries. The negative abortion-related image of mifepristone has clearly limited the involvement of the major pharmaceutical companies in the development of PA and SPRM. Many PA and SPRM display direct antiproliferative effects in the endometrium, although with variable actions which seem product- and dose-dependent. This property justifies their use in the treatment of myomas and endometriosis. PA also suppress late follicular development, block the LH surge and retard endometrial maturation, which renders them potential estrogen-free contraceptive drugs. SPRM such as asoprisnil are not as effective in blocking the LH surge and appear to target the endometrium directly and produce amenorrhoea. Interestingly, clinical data show that treatment with these compounds is not associated with hypo-estrogenism and bone loss. The potential clinical applications of these compounds cover a broad field and are very promising in major public health areas. These include emergency contraception, long-term estrogen-free contraception (administered alone, or in association with a progestin-only pill to improve bleeding patterns), myomas (where they induce a marked reduction in tumour volume and produce amenorrhoea) and endometriosis. Further developments might also include hormone replacement therapy in post-menopausal women, as well as the treatment of hormone-dependent tumours.

Progesterone antagonists and progesterone receptor modulators

Since the discovery of the antiprogestin RU 486 (mifepristone), other compounds have been synthesised that function as pure progesterone antagonists or progesterone receptor modulators. The latter are mixed agonists-antagonists. Mifepristone is usually used to terminate pregnancy but these compounds have numerous other applications in female healthcare. Mifepristone is an excellent agent for emergency contraception. Many progesterone antagonists and progesterone receptor modulators display antiproliferative effects on the endometrium and thus have application in the treatment of endometriosis and uterine myoma without being associated with hypoestrogenism and bone loss. They also have contraceptive potential by suppressing follicular development, blocking the luteinising hormone surge and retarding endometrial maturation. Finally, they have clinical application in GeneSwitch system, a plasmid-based method enabling controlled expression of specific genes (e.g., erythropoietin) using a progesterone antagonist as the inducer.

Progesterone antagonists and progesterone receptor modulators: an overview

Since the original description of the structure of the antiprogestin, mifepristone, was published, numerous related compounds have been synthesized which may function as progesterone antagonists (PAs) or progesterone receptor modulators (PRMs). The latter are mixed agonists-antagonists. Both PAs and PRMs have therapeutic applications in female health care. Mifepristone is predominantly a PA and displays only minimum agonist activity in certain systems. Together with a prostaglandin, mifepristone can terminate pregnancies of less than 9 weeks duration, and it may also be used at later gestational ages. Mifepristone causes expulsion of the uterine contents following intrauterine fetal death. A mifepristone-prostaglandin combination has been shown to be very effective treatment in women with menses delay of 11 days or less. Many PAs and PRMs display antiproliferative effects in the endometrium. Serum estradiol levels however remain in the early to mid-follicular phase range. For this reason, they have application in the treatment of endometriosis and myoma without being associated with bone loss and hypoestrogenism. PRMs may also find application in the treatment of dysfunctional bleeding as well as an adjunct to estrogens in hormone replacement therapy in postmenopausal women. Many PAs have contraceptive potential by suppressing follicular development and blocking the LH surge. Low doses may also be potential contraceptives by retarding endometrial maturation without affecting ovulation or inducing bleeding. Mifepristone is an excellent agent for use as an emergency "postcoital" contraceptive. PAs may also be useful in IVF programs to prevent a premature LH surge and to delay the emergence of the implantation window. In addition to their use in women's health care, mifepristone and several other PAs are potent antiglucocorticoid agents and may be used to treat ACTH-independent Cushing's syndrome. They may also be used in the treatment of tumors containing steroid receptors and in other situations which require suppression of the ACTH-cortisol axis.

Progesterone receptor transcription and non-transcription signaling mechanisms

The diverse effects of progesterone on female reproductive tissues are mediated by the progesterone receptor (PR), a member of the nuclear receptor family of ligand-dependent transcription factors. Thus, PR is an important therapeutic target in female reproduction and in certain endocrine dependent cancers. This paper reviews our understanding of the mechanism of action of the most widely used PR antagonist RU486. Although RU486 is a competitive steroidal antagonist that can displace the natural hormone for PR, it's potency derives from additional "active antagonism" that involves inhibiting the activity of PR hormone agonist complexes in trans through heterodimerization and competition for binding to progesterone response elements on target DNA, and by recruitment of corepressors that have the potential to actively repress gene transcription. An additional functional role for PR has recently been defined whereby a subpopulation of PR in the cytoplasm or cell membrane is capable of mediating rapid progesterone induced activation of certain signal transduction pathways in the absence of gene transcription. This paper also reviews recent results on the mechanism of the extra-nuclear action of PR and the potential biological roles and implications of this novel PR signaling pathway.

Mechanism of action of progesterone antagonists

The effects of progesterone on target tissues are mediated by progesterone receptors (PRs), which belong to a family of nuclear receptors and function as ligand-activated transcription factors to regulate the expression of specific sets of target genes. Progesterone antagonists repress the biological actions of progesterone by "actively" inhibiting PR activation. This work discusses the first clinically used progesterone antagonist RU486 and closely related compounds in terms of how these compounds inhibit progesterone action through heterodimerization and competition for DNA binding and by the recruitment of corepressors to promoters of target genes to repress transcription. We discuss cellular factors that may influence the activity of these compounds, such as the availability of coactivators and corepressors and the context of specific target promoters in any given cell type. We also discuss steroidal and nonsteroidal antagonist selectivity for PR versus other steroid hormone receptors and suggest that it may be possible to develop tissue/cell specific modulators of PR.

Jun dimerization protein 2 functions as a progesterone receptor N-terminal domain coactivator

The progesterone receptor (PR) contains two transcription activation function (AF) domains, constitutive AF-1 in the N terminus and AF-2 in the C terminus. AF-2 activity is mediated by a hormone-dependent interaction with a family of steroid receptor coactivators (SRCs). SRC-1 can also stimulate AF-1 activity through a secondary domain that interacts simultaneously with the primary AF-2 interaction site. Other protein interactions and mechanisms that mediate AF-1 activity are not well defined. By interaction cloning, we identified an AP-1 family member, Jun dimerization protein 2 (JDP-2), as a novel PR-interacting protein. JDP-2 was first defined as a c-Jun interacting protein that functions as an AP-1 repressor. PR and JDP-2 interact directly in vitro through the DNA binding domain (DBD) of PR and the basic leucine zipper (bZIP) region of JDP-2. The two proteins also physically associate in mammalian cells, as detected by coimmunoprecipitation, and are recruited in vivo to a progesterone-inducible target gene promoter, as detected by a chromatin immunoprecipitation (ChIP) assay. In cell transfection assays, JDP-2 substantially increased hormone-dependent PR-mediated transactivation and worked primarily by stimulating AF-1 activity. JDP-2 is a substantially stronger coactivator of AF-1 than SRC-1 and stimulates AF-1 independent of SRC-1 pathways. The PR DBD is necessary but not sufficient for JDP-2 stimulation of PR activity; the DBD and AF-1 are required together. JDP-2 lacks an intrinsic activation domain and makes direct protein interactions with other coactivators, including CBP and p300 CBP-associated factor (pCAF), but not with SRCs. These results indicate that JDP-2 stimulates AF-1 activity by the novel mechanism of docking to the DBD and recruiting or stabilizing N-terminal PR interactions with other general coactivators. JDP-2 has preferential activity on PR among the nuclear receptors tested and is expressed in progesterone target cells and tissues, suggesting that it has a physiological role in PR function.

Identification and characterization of novel estrogen receptor-beta-sparing antiprogestins

The steroid hormones estrogen and progesterone together regulate the development and maintenance of the female reproductive system. The actions of these two hormones are mediated by their respective nuclear receptors located within overlapping cell populations in target organs. The molecular mechanism of action of these two hormones has been defined to a large extent using estrogen receptor (ER) and progesterone receptor (PR) antagonists. In the case of ER, the available antagonists are highly receptor selective. With respect to PR, however, the available antiprogestins also interact with the receptors for glucocorticoids, mineralocorticoids, and androgens. Whereas these cross-reactivities can usually be managed in studies of female reproductive function, it is the recent demonstration that RU486 is an effective antagonist of the beta-isoform of ER that suggested the need for more selective antiprogestins. In this study, we used cell-based transcriptional assays combined with screens using coactivator peptide analogs to identify two novel classes of antiprogestins that distinguish themselves from the antiprogestin RU486 in the manner they interact with PR. One class exhibits the characteristics of a pure antiprogestin in that its members bind to the receptor and induce a conformational change that prevents the presentation of two potential coactivator binding surfaces on the protein. The second class of compounds distinguish themselves from RU486 in that they are ERbeta sparing. When tested in vivo the ER-sparing antiprogestins were as effective as RU486 in suppressing superovulation. It is anticipated that the availability of these new antiprogestins will advance the studies of PR pharmacology in a manner similar to how the availability of selective ER modulators has helped the study of ER action.

Coactivator/corepressor ratios modulate PR-mediated transcription by the selective receptor modulator RU486

Selective receptor modulators, such as the antiprogestin RU486, are known to exhibit partial agonist activities in a cell-type-dependent manner. Employing an in vitro chromatin transcription system that recapitulates progesterone receptor (PR)-mediated transcription in vivo, we have investigated the molecular basis by which the antiprogestin RU486 regulates transcription in a cell-type-specific manner. We have compared the effects of RU486 on PR-dependent transcription in vitro using T47D and HeLa cell nuclear extracts. RU486 exhibits a differential ability to activate transcription within these two cell types. The differential effect on transcription correlates with different ratios of endogenous coactivators/corepressors in these cells. Unlike agonist-bound PR that interacts only with coactivators such as steroid receptor coactivator-1 (SRC-1), RU486-bound PR binds to both coactivator SRC-1 and corepressor silencing mediator for retinoid and thyroid hormone receptor (SMRT) in vitro. Both SRC-1 and SMRT have the capacity to modulate RU486-dependent activity. Moreover, a change in the relative levels of SRC-1 and SMRT contained in our chromatin transcription system modulates agonist/antagonist effects of RU486 on transcription by PR. Our data indicate that the ability of RU486 to activate transcription is modulated by the ratio of coactivators to corepressors and substantiate the important roles of coregulators in the regulation of steroid receptor mediated transactivation in response to selective receptor modulators.

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

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 EC(50), 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 EC(50) of agonists, independently of changes in total gene transactivation. Furthermore, the corepressors SMRT (silencing mediator for retinoid and thyroid receptors) and NCoR (nuclear receptor corepressor) each suppresses gene induction but NCoR acts opposite to SMRT and, like the coactivator TIF2, reduces the EC(50) 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 Dex-Mes 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.

The size and/or configuration of the cycloalkane D' ring in pentacyclic progesterone derivatives are crucial for their high-affinity binding to a protein in addition to progesterone receptor in rat uterine cytosol

[(3)H]labeled progesterone and a number of its 16alpha, 17alpha-cycloalkano derivatives with an additional three to six-membered D' ring were investigated for mutual competition and equilibrium binding to proteins from rat uterine cytosol. The interaction of all studied [(3)H]ligands with proteins was characterized by comparable affinity (K(d) in nM region) and apparent homogeneity in terms of affinity. At the same time, the concentrations of binding sites for ligands bearing 16alpha,17alpha cyclopentano, cyclohexano, or cyclohexeno substituents were several-fold higher than those for progesterone or 16alpha, 17alpha-cyclopropanoprogesterone. In mutual competition experiments, when [(3)H]progesterone or [(3)H]16alpha, 17alpha-cyclopropanoprogesterone were used, the curves of 'bound radioactivity-log of competitor concentration' for all compounds studied were parallel and corresponded to a model of 'one protein-two ligands.' However, when [(3)H]ligands with bulky 16alpha, 17alpha-substituents (with the possible exception of cyclohexene derivative) were used, competitive curves for various ligands had different appearances and fell into two groups. Parallel curves for derivatives with 5 or 6 carbons in D' ring described by a model of 'one protein-two ligands' formed the 1st group. The 2nd group comprised curves for progesterone or 16alpha, 17alpha-cyclopropanoprogesterone that had lower slopes and could be described by a model of 'two proteins-two ligands.' Taken together, the results suggest the presence in rat uterine cytosol, of a protein in addition to progesterone receptor capable of discriminating between ligands with no or small 16alpha, 17alpha-cycloalkano substituents and ligands with more bulky substituents.

Comparative analyses of mechanistic differences among antiestrogens

Antiestrogens such as tamoxifen are one of the most effective methods of treating estrogen receptor (ERalpha) positive breast cancers; however, the effectiveness of this therapy is limited by the almost universal development of resistance to the drug. If antiestrogens are recognized differently by the cell as it has been suggested, then in disease conditions where tamoxifen fails to function effectively, a mechanistically different antiestrogen might yield successful results. Although many antiestrogens have been developed, a direct comparison of their mechanisms of action is lacking, thus limiting their utility. Therefore, to determine if there are mechanistic differences among available antiestrogens, we have carried out a comprehensive analysis of the molecular mechanisms of action of 4-hydroxy-tamoxifen (40HT), idoxifene, raloxifene, GW7604, and ICI 182,780. Using a novel set of peptides that recognize different surfaces on ERalpha, we have found that following binding to ERalpha, each ligand induces a distinct ERalpha-ligand conformation. Furthermore, transcriptional assays indicate that each ERalpha-ligand complex is recognized distinctly by the transcription machinery, and consequently, antiestrogens vary in their ability to inhibit estradiol- and 40HT-mediated activities. Relative binding assays have shown that the affinity of these ligands for ERalpha is not always representative of their inhibitory activity. Using this assay, we have also shown that the pharmacology of each antiestrogen is influenced differently by hormone binding proteins. Furthermore, GW7604, like ICI 182,780, but unlike the other antiestrogens evaluated, decreases the stability of the receptor. Overall, our results indicate that there are clear mechanistic distinctions among each of the antiestrogens studied. However, GW7604 and ICI 182,780 differ more significantly from tamoxifen than idoxifene and raloxifene. These data, which reveal differences among antiestrogens, should assist in the selection of compounds for the clinical regulation of ERalpha function.

Reporter gene technology: the future looks bright

Reporter gene technology is widely used to monitor the cellular events associated with signal transduction and gene expression. Based upon the splicing of transcriptional control elements to a variety of reporter genes (with easily measurable phenotypes), it "reports" the effects of a cascade of signalling events on gene expression inside cells. The principal advantage of these assays is their high sensitivity, reliability, convenience, and adaptability to large-scale measurements. This review summarises the current status of reporter gene technology including its role in monitoring gene transfer and expression and its development as a biological screen. With the advances in this technology and in detection methods, it is likely that luciferase and green fluorescent protein will become increasingly popular for the non-invasive monitoring of gene expression in living tissues and cells. Such techniques will be important in defining the molecular events associated with gene transcription, which has implications for our understanding of the molecular basis of disease and will influence our approach to gene therapy and drug development.

The novel progesterone receptor antagonists RTI 3021-012 and RTI 3021-022 exhibit complex glucocorticoid receptor antagonist activities: implications for the development of dissociated antiprogestins

We have identified two novel compounds (RTI 3021-012 and RTI 3021-022) that demonstrate similar affinities for human progesterone receptor (PR) and display equivalent antiprogestenic activity. As with most antiprogestins, such as RU486, RTI 3021-012, and RTI 3021-022 also bind to the glucocorticoid receptor (GR) with high affinity. Unexpectedly, when compared with RU486, the RTI antagonists manifest significantly less GR antagonist activity. This finding indicates that, with respect to antiglucocorticoid function, receptor binding affinity is not a good predictor of biological activity. We have determined that the lack of a clear correlation between the GR binding affinity of the RTI compounds and their antagonist activity reflects the unique manner in which they modulate GR signaling. Previously, we proposed a two step "active inhibition" model to explain steroid receptor antagonism: 1) competitive inhibition of agonist binding; and 2) competition of the antagonist bound receptor with that activated by agonists for DNA response elements within target gene promoters. Accordingly, we observed that RU486, RTI 3021-012, and RTI 3021-022, when assayed for PR antagonist activity, accomplished both of these steps. Thus, all three compounds are "active antagonists" of PR function. When assayed on GR, however, RU486 alone functioned as an active antagonist. RTI 3021-012 and RTI 3021-022, on the other hand, functioned solely as "competitive antagonists" since they were capable of high affinity GR binding, but the resulting ligand receptor complex was unable to bind DNA. These results have important pharmaceutical implications supporting the use of mechanism based approaches to identify nuclear receptor modulators. Of equal importance, RTI 3021-012 and RTI 3021-022 are two new antiprogestins that may have clinical utility and are likely to be useful as research reagents with which to separate the effects of antiprogestins and antiglucocorticoids in physiological systems.

Agonist and antagonists induce homodimerization and mixed ligand heterodimerization of human progesterone receptors in vivo by a mammalian two-hybrid assay

This study utilizes the mammalian two-hybrid system to examine the role of ligand in the dimerization of human progesterone receptor (hPR). The GAL4 DNA-binding domain and the herpes simplex virus VP16 transactivation domain were fused to the amino terminus of full-length hPR (both the A and B isoforms) to produce chimeric proteins. PR dimerization was detected by the ability of cotransfected GAL4/PR and VP16/PR chimeras in COS cells to induce expression of a reporter gene under the control of GAL4-binding sites (pG5CAT). Hormone agonist-dependent interactions were observed between the two like isoforms of PR (A-A and B-B) and between PR-A and PR-B (A-B), indicating that hormone can stimulate the formation of the three possible dimeric forms of PR within cells. In contrast, neither type I (ZK98299) nor type II (RU486, ZK112993) progestin antagonists stimulated interaction between these same hybrid PR proteins. However, activation of the VP16/PR chimera by antagonists on a progesterone response element-controlled reporter gene (DHRE-E1b-CAT) was only a fraction (4-13%) of that stimulated by agonist R5020. One possibility for the failure to detect an induction in the two-hybrid assay is antagonist-induced repression of the activity of the VP16/PR fusion protein rather than a failure of antagonists to stimulate interaction between the hybrid proteins. To test this idea, an UP-1 carboxyl-terminal truncation mutant of PR was used to construct the two-hybrid proteins. PR-UP-1 selectively binds antagonists, but not agonists, and is fully activated in response to antagonists. Both types of progestin antagonists stimulated interactions between GAL4/PR(UP-1) and VP16/PR(UP-1) hybrid proteins, indicating that antagonists are capable of stimulating PR dimerization in cells and do not function by disrupting or preventing dimerization. To determine whether PR bound to an antagonist can dimerize in whole cells with PR bound to agonist, GAL4/PR(UP-1) was paired in the two-hybrid assay with a VP16/PR fusion protein harboring a point mutation in PR at amino acid 722 (Gly-Cys) that specifically binds progestin agonist but not antagonist. Neither R5020 nor RU486 alone stimulated interaction between these ligand-specific PR hybrid proteins. However, strong interaction was detected by addition of both agonist and antagonists, indicating the formation of mixed ligand heterodimers and that both PR partners require ligand for dimerization to occur. Based on electrophoretic gel mobility shift assays (EMSAs), these heterodimers appear to have substantially reduced DNA binding activity. Progestin antagonists inhibit agonist activation of PR at concentrations that are too low to be accounted for by a simple competition mechanism for binding to PR. We propose that antiprogestin inactivation of PR in trans by heterodimerization contributes to the biological potency of these compounds.

The antagonists RU486 and ZK98299 stimulate progesterone receptor binding to deoxyribonucleic acid in vitro and in vivo, but have distinct effects on receptor conformation

Three types of transfection experiments were used to detect the abilities of different classes of antagonists to stimulate binding of progesterone receptor (PR) to progesterone response elements (PRE) in intact mammalian cells. These included a promoter interference assay, in which PR binding to PREs positioned between the TATA box and the start of transcription is detected as a reduction of expression of a constitutively active reporter gene, competition of PR antagonist and glucocorticoid receptor agonist for a common glucocorticoid response element/PRE-controlled reporter construct, and activation of a chimeric receptor (PR-VP16) containing the constitutive trans-activation domain derived from the VP16 protein of herpes simplex virus. By each approach, all antagonists tested were equally effective in stimulating PR binding to PREs in the cell. This included previously designated type I (ZK98299) and type II (RU486, ZK98734, and ZK112993) 11beta-aryl substituted steroid analogs. Stimulation of PR binding to PREs in the cell by ZK98299 was of interest because this antagonist has been reported to lack the ability to stimulate PR-DNA binding in vitro by electrophoretic gel mobility shift assay compared with RU486, which promotes efficient binding of PR to PREs. To clarify the apparent discrepancy between intact cell and in vitro results with ZK98299, we altered electrophoretic gel mobility shift assay conditions to allow detection of less stable DNA complexes. Under these conditions, ZK98299 induced the formation of specific PR-PRE complexes. Further analysis of the ZK98299-induced DNA complexes revealed that they exhibited an electrophoretic mobility different from that of the complexes induced by RU486, and the off-rate of PR from DNA was faster than that of the PR bound to agonist. This suggests that ZK98299 promotes a conformational change within PR distinct from that induced by RU486. The present results are consistent with the conclusions that ZK98299 stimulates PR binding to target DNA sequences and that ZK98299 and RU486 represent two mechanistic classes of antagonists based on inducing different conformational changes in PR.

Enhancement of estrogen receptor transcriptional activity by the coactivator GRIP-1 highlights the role of activation function 2 in determining estrogen receptor pharmacology

The human estrogen receptor (ER) contains two major activation functions (AFs) responsible for its transcriptional activity. One of these, activation function 2 (AF-2), located within the hormone-binding domain (HBD), has been shown to mediate the ligand-dependent transcriptional activity of ER as well as other members of the nuclear receptor superfamily. Recently, proteins interacting with the HBD of several nuclear receptors have been cloned. One of these proteins, glucocorticoid receptor interacting protein (GRIP-1), has been shown to interact with ER and was originally hypothesized to mediate its transcriptional activity through AF-2. However, we find in this study that the transcriptional activity of ER, containing mutations in the AF-2 core sequence, can be enhanced by coexpression of the coactivator GRIP-1, suggesting that this protein may not rely solely on the AF-2 domain for interaction. We propose, therefore, that the HBD of ER either contains multiple binding sites that are necessary for association with GRIP-1 or, alternatively, that this coactivator contacts the receptor in an undetermined region within the HBD. Importantly, these studies demonstrate also that mutations or deletion of AF-2 alter the ligand pharmacology of the receptor such that ER loses the ability to discriminate between agonists and antagonists. Interestingly, on these mutant receptors GRIP-1 still functions as a coactivator independent of the nature of the bound ligand. It is likely, therefore, that the C-terminal AF-2 domain may function as a molecular switch allowing the wild-type receptor to discriminate between agonists and antagonists as well as providing a surface with which associated proteins can interact.

The nuclear corepressors NCoR and SMRT are key regulators of both ligand- and 8-bromo-cyclic AMP-dependent transcriptional activity of the human progesterone receptor

Previously, we defined a novel class of ligands for the human progesterone receptor (PR) which function as mixed agonists. These compounds induce a conformational change upon binding the receptor that is different from those induced by agonists and antagonists. This establishes a correlation between the structure of a ligand-receptor complex and its transcriptional activity. In an attempt to define the cellular components which distinguish between different ligand-induced PR conformations, we have determined, by using a mammalian two-hybrid assay, that the nuclear receptor corepressor (NCoR) and the silencing mediator for retinoid and thyroid hormone receptor (SMRT) differentially associate with PR depending upon the class of ligand bound to the receptor. Specifically, we observed that the corepressors preferentially associate with antagonist-occupied PR and that overexpression of these corepressors suppresses the partial agonist activity of antagonist-occupied PR. Binding studies performed in vitro, however, reveal that recombinant SMRT can interact with PR in a manner which is not influenced by the nature of the bound ligand. Thus, the inability of SMRT or NCoR to interact with agonist-activated PR when assayed in vivo may relate more to the increased affinity of PR for coactivators, with a subsequent displacement of corepressors, than to an inherent low affinity for the corepressor proteins. Previous work from other groups has shown that 8-bromo-cyclic AMP (8-bromo-cAMP) can convert the PR antagonist RU486 into an agonist and, additionally, can potentiate the transcriptional activity of agonist-bound PR. In this study, we show that exogenous expression of NCoR or SMRT suppresses all 8-bromo-cAMP-mediated potentiation of PR transcriptional activity. Further analysis revealed that 8-bromo-cAMP addition decreases the association of NCoR and SMRT with PR. Thus, we propose that 8-bromo-cAMP-mediated potentiation of PR transcriptional activity is due, at least in part, to a disruption of the interaction between PR and the corepressors NCoR and SMRT. Cumulatively, these results suggest that NCoR and SMRT expression may play a pivotal role in PR pharmacology.

Steroid-induced conformational changes at ends of the hormone-binding domain in the rat glucocorticoid receptor are independent of agonist versus antagonist activity

The underlying molecular mechanism for the expression of agonist versus antagonist activity for a given receptor-steroid complex is still not known. One attractive hypothesis, based on data from progesterone receptors, is that agonist versus antagonist binding induces unique conformations at the C terminus of receptors, which can be detected by the different fragments produced by partial proteolysis. We now report that the determinants of glucocorticoid receptor (GR)-antagonist complex activity are more complex. Steroid binding did cause a conformational change in the GR that was detected by partial trypsin digestion, as described previously (Simons, S. S., Jr., Sistare, F. D., and Chakraborti, P. K. (1989) J. Biol. Chem. 264, 14493-14497). However, there was no uniformity in the digestion patterns of unactivated or activated receptors bound by a series of six structurally different antagonists including the affinity labeling antiglucocorticoid dexamethasone 21-mesylate. A total of four resistant bands were observed on SDS-polyacrylamide gels in the range of 30-27 kDa. Using a series of point mutations and epitope-specific antibodies, it was determined that the 30-kDa species represented the entire C-terminal sequence of amino acids 518-795, whereas the other bands arose from additional N-terminal and/or C-terminal cleavages. Bioassays with GRs containing various point and deletion mutations failed to reveal any C-terminal alterations that could convert antagonists into biologically active agonists. Thus, the presence or absence of C-terminal amino acids of the GR did not uniquely determine either the appearance of smaller trypsin-resistant fragments or the nature of the biological response of receptor-bound antisteroids. When compared with the current model of the ligand-binding domain, which is based on the x-ray structures of the comparable region of thyroid and retinoic acid receptors, the present results suggest that sequences outside of the model structure are relevant for the binding and biological activity of GRs.

Dissection of the molecular mechanism of action of GW5638, a novel estrogen receptor ligand, provides insights into the role of estrogen receptor in bone

The estrogen receptor (ER) mixed agonists tamoxifen and raloxifene have been shown to protect against bone loss in ovariectomized rats. However, the mechanism by which these compounds manifest their activity in bone is unknown. We have used a series of in vitro screens to select for compounds that are mechanistically distinct from tamoxifen and raloxifene in an effort to define the properties of an ER modulator required for bone protection. Using this approach, we identified a novel high affinity ER antagonist, GW5638, which when assayed in vitro functions as an ER antagonist, inhibiting the agonist activity of estrogen, tamoxifen, and raloxifene and reversing the "inverse agonist" activity of the pure antiestrogen ICI182,780. Thus, GW5638 appears to function as an antagonist in these in vitro systems, although in a manner distinct from other known ER modulators. Predictably, therefore, GW5638 alone displays minimal uterotropic activity in ovariectomized rats, but will inhibit the agonist activity of estradiol in this environment. Unexpectedly, however, this compound functions as a full ER agonist in bone and the cardiovascular system. These data suggest that the mechanism by which ER operates in different cells is not identical, and that classical agonist activity is not required for the bone protective activity of ER modulators.