Novel antiprogestins Org 31806 and 31710: interaction with mammalian progesterone receptor and DNA binding of antisteroid receptor complexes

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.

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.

Time-dependent ovulation inhibition of a selective progesterone-receptor antagonist (Org 31710) and effects on ovulatory mediators in the in vitro perfused rat ovary

Progesterone (P) is one of several local mediators in the ovulatory cascade in the rat. The precise mechanisms of action for P in ovulation and in what phase of the ovulatory process P is critical, however, need to be clarified. The present study used a selective P-receptor antagonist, Org 31710, in the in vitro perfused rat ovary model to examine the local role of P and possible effects on prostaglandin (PG) and plasminogen-activator (PA) release in ovulation. Ovaries from eCG (15 IU)-primed rats were perfused for 20 h with LH (0.2 microg/ml) and 3-isobutyl-1-methylxanthine (IBMX, 200 microM) to induce ovulation (median = 10.0, 25%-75% range = 8.5-13). Org 31710 was added at either 0, 3.5, 7, or 9 h after LH+IBMX, resulting in significant suppression of ovulation after addition at 0 and 3.5 h (1.0, 1-5.5; and 5.0, 2.5-7.75 ovulations, respectively) but no suppressive effect when added at later time points. Progesterone and estradiol levels in the perfusion media were increased after LH+IBMX but were not affected by the presence of Org 31710. Ovarian tissue levels of PGE(2), PGF(2 alpha), and PA activity were measured in ovaries that had been perfused for 10 h, a time that was 2 to 5 h before anticipated ovulation. The presence of Org 31710 significantly decreased the levels of PGE(2), PGF(2 alpha), and PA activity. These results suggest that P is essential in ovulation during the initial stages of the ovulatory process. The effect of P to facilitate ovulation seems to relate to stimulation of the PG- and PA-mediator systems.

Selective activation of the glucocorticoid receptor by steroid antagonists in human breast cancer and osteosarcoma cells

Steroid hormones regulate the transcription of numerous genes via high affinity receptors that act in concert with chromatin remodeling complexes, coactivators and corepressors. We have compared the activities of a variety of glucocorticoid receptor (GR) antagonists in breast cancer and osteosarcoma cell lines engineered to stably maintain the mouse mammary tumor virus promoter. In both cell types, GR activation by dexamethasone occurs via the disruption of mouse mammary tumor virus chromatin structure and the recruitment of receptor coactivator proteins. However, when challenged with a variety of antagonists the GR displays differential ability to activate transcription within the two cell types. For the breast cancer cells, the antagonists fail to activate the promoter and do not promote the association of the GR with either remodeling or coactivator proteins. In contrast, in osteosarcoma cells, the antiglucocorticoids, RU486 and RU43044, exhibit partial agonist activity. The capacity of these antagonists to stimulate transcription in the osteosarcoma cells is reflected in the ability of the RU486-bound receptor to remodel chromatin and associate with chromatin-remodeling proteins. Similarly, the observation that the RU486-bound receptor does not fully activate transcription is consistent with its inability to recruit receptor coactivator proteins.

Single monthly administration of the anti-progestagen Org 31710 in users of the 75 microg desogestrel progestagen-only pill: effects on pituitary-ovarian activity

Endocrine and ultrasound effects were studied of an intermittent (every 28 days) oral administration of 150 mg of the anti-progestagen Org 31710 during the continued daily use of 75 microg desogestrel (DSG) for progestagen-only contraception. A randomized, double-blind, placebo-controlled two-centre study was conducted in 50 healthy volunteers. Serum luteinizing hormone (LH), follicle stimulating hormone (FSH), oestradiol and progesterone concentrations, and follicle number and size were studied, as well as endometrial thickness, which was assessed by transvaginal sonography at least twice weekly during a single medication cycle (cycle 3-5). Forty-eight women were evaluated (Org 31710, n = 25; placebo, n = 23). Seven ovulations were observed in the treated group versus none in the placebo group. LH concentrations were higher on days 9 and 11 and oestradiol concentrations lower on day 3 in the treated group, irrespective of whether ovulation occurred. No parameter could predict ovulation. Endometrial thickness was greater on cycle days 7-13 and 19 in the treated group. However, within the Org 31710 group, no significant differences were found in volunteers who did or did not ovulate. Observed differences may be attributed to a competitive effect of Org 31710 with progestagen-induced suppression of the pituitary-ovarian axis, altered oestradiol feedback mechanisms, and/or altered receptor availability.

Comparison of the effects of antiprogestins RU38486, ZK98299 and ORG31710 on periovulatory hypophysial, ovarian and adrenal hormone secretion in the rat

The antiprogestin (AP) RU38486 (RU) blocks progesterone (P) and glucocorticoid (G) actions. Administration of 4 mg RU on proestrous morning to cyclic rats dissociates LH and FSH secretion on proestrous afternoon, early estrus and on estrous afternoon. In order to ascertain which action blocked by RU is predominant in the control of periovulatory LH and FSH secretion, a study was made on the effects of: a) 1 or 4 mg of ZK98299 (ZK) (type I P antagonist; Schering), b) 2 or 8 mg of Org31710 (OR) (type II P antagonist lacking anti-G actions; Organon) or c) 1 or 4 mg of RU (type II P antagonist; Exelgyn) to 4-day cyclic rats on proestrous morning on serum concentrations of LH, FSH, inhibin-alpha (I), estradiol-17beta (E), progesterone (P) and corticosterone (B) at 18:30 h on proestrus and at 02:00 and 18:30 h on estrus. Controls, receiving 0.2 ml oil, had elevated serum concentrations of all six hormones on proestrous afternoon; at early estrus, only serum concentrations of FSH and P remained elevated, and, on estrous afternoon, all hormones but I and B, that peaked again, had reached their lowest serum levels. All AP treatments except 1 mg ZK had the same effects. On proestrous afternoon serum LH concentrations were reduced and serum FSH concentrations were suppressed whereas serum levels of I, E, P and B were unaffected. At early estrus, basal serum concentrations of LH and E increased while FSH secretion was abolished. Serum levels of I, P and B did not differ from controls. AP treatments increased basal LH concentration, hyperstimulated FSH secretion and reduced serum I concentration on the afternoon of estrus. E, P and B serum levels did not differ from controls at this stage. Treatment with 1 mg ZK was less effective in reducing serum FSH on proestrous afternoon and at early estrus, and had no effect on serum concentrations of any hormone on estrous afternoon. These results indicate that blockade of P receptor activation by P is, predominantly, the mechanism of AP action on periovulatory gonadotropin secretion in rats.

Effects of antiprogestins on the rate of proliferation of breast cancer cells

We have examined the influence of progestins (progesterone, R5020) and antiprogestins (RU486, ZK98299, Org 31710 and Org 31806) on the rate of proliferation of wild type T47D cells cultured in whole fetal bovine serum (FBS) or in single charcoal stripped fetal bovine serum (SSFBS). All of the progesterone antagonists RU486, ZK98299 and two novel antiprogestins Org 31710 and Org 31806 inhibited cell proliferation when cells were cultured in FBS. In contrast, all of the antiprogestins with the exception of ZK98299 enhanced cell growth when cells were cultured in SSFBS. This stimulatory effect of RU486 was observed only at a high concentration of the ligand (1 microM). The effect of R5020, however, was concentration independent. The number of cells in the presence of RU486 was approximately 600% followed by R5020 approximately 400% above control values after a 28 day culturing period. In contrast, when the cells were grown in the presence of medium containing non-stripped whole serum, RU486 inhibited the extent of cell proliferation by 45%. Estradiol (E2) stimulated the rate of proliferation in cells cultured in SSFBS. Similar to when cells were cultured in whole serum, the antiprogestins inhibited cell growth in E2-supplemented SSFBS. Detection of the growth enhancement effects of progesterone receptor (PR) ligands such as RU486 and R5020 on the cells grown in charcoal-stripped medium appear to require the removal of E2 by charcoal stripping of the serum.

Chromatin remodelling by the glucocorticoid receptor requires the BRG1 complex

The assembly of transcriptional regulatory DNA sequences into chromatin plays a fundamental role in modulating gene expression. The promoter of the mouse mammary-tumour virus (MMTV) is packaged into a regular array of nucleosomes when it becomes stably integrated into mammalian chromosomes, and has been used to investigate the relationship between chromatin architecture and transcriptional activation by the hormone-bound glucocorticoid and progesterone receptors. In mammalian cells that express both of these receptors, the progesterone receptor activates transcription from transiently transfected MMTV DNA but not from organized chromatin templates. Moreover, the activated progesterone receptor inhibits the chromatin remodelling and consequent transcriptional stimulation that is mediated by the glucocorticoid receptor. Here we investigate the mechanism of this inhibition by characterizing the interaction of the glucocorticoid receptor with transcriptional co-activator and chromatin remodelling protein complexes. We show that when this receptor is prevented from interacting with the hBRG1/BAF chromatin remodelling complex, it can activate transcription from transiently transfected DNA but not from organized chromatin templates. Our results indicate that it may be possible to separate the transcriptional activation and chromatin remodelling activities of proteins that interact with hormone receptors.

Antiprogestins mediate differential effects on glucocorticoid receptor remodeling of chromatin structure

We examined the mechanism(s) by which the progesterone receptor (PR) is able to inhibit glucocorticoid receptor (GR) activation from the mouse mammary tumor virus (MMTV) promoter in vivo. Using specific hormone antagonists, we demonstrate that the PR complexed with an type II antiprogestin blocks glucocorticoid-induced activation of the MMTV promoter. However, when complexed with a type I antiprogestin the PR is unable to block glucocorticoid-induced activation. PR repression of GR activity results from the inhibition of the ability of the GR to remodel chromatin such that the antiprogestin-occupied/PR prevents the glucocorticoid induced assembly of a preinitiation complex at MMTV promoter. These experiments suggest that the specific chromatin organization of the MMTV promoter provides a mechanism for regulating cross-talk between the GR and PR in vivo.

RU486 (mifepristone): mechanisms of action and clinical uses

RU486 (mifepristone) has proved to be a remarkably active antiprogesterone and antiglucocorticosteroid agent in human beings. The mechanism of action involves the intracellular receptors of the antagonized hormones (progesterone and glucocorticosteroids). At the molecular level, the most important features are high binding affinity to the receptor, interaction of the phenylaminodimethyl group in the 11 beta-position with a specific region of the receptor binding pocket, and RU486-induced transconformation differences in the ligand-binding domain. These particularities have consequences at different steps of the receptor function as compared with agonists. However, the reasoning cannot be limited to the RU486-receptor interaction, and, for instance, there is the possibility of a switch from antagonistic property to agonist activity, depending on the intervention of other signaling pathways. It would be desirable to have derivatives with only one of the two antagonistic properties (antiprogestin, antiglucocorticosteroid) in spite of similarities between steroid structures, receptors involved, and responsive machineries in target cells. Clinically, the RU486-plus-prostaglandin method is ready to be used on a large scale and is close to being as convenient and safe as any medical method of abortion may be. The early use of RU486 as a contragestive as soon as a woman fears a pregnancy she does not want will help to defuse the abortion issue. Research should now be conducted to define an efficient and convenient contraceptive method with RU486 or other antiprogestins. The usefulness of RU486 for obstetric indications, including facilitation of difficult delivery, has to be assessed rapidly. Gynecologic trials, particularly in leiomyomata, should be systemically continued. The very preliminary results obtained with tumors, including breast cancers, indicate that further studies are necessary.

Interaction of newly synthesized antiprogesterone ZK98299 with progesterone receptor from human myometrium

We have undertaken characterization of binding of the newly synthesized progesterone receptor (PR) antagonist ZK98299 in the cellular fractions of human myometrium. Specific [3H]progesterone and [3H]ZK98299 binding was observed in the cytosol and the nuclear fractions, and could be competitively replaced by either of the steroids in their radioinert form. Although PR occupied by both steroids exhibited nuclear uptake, the extent of nuclear binding was lower with [3H]ZK98299-receptor complexes. The binding of both ligands to PR was a function of the duration of incubation and the protein concentration: it was saturable at 3-6 nM steroids with a dissociation constant of approximately 2 nM. However, the number of ZK98299 binding sites (72 fmoles/mg protein) was lower compared to that of progesterone (322 fmoles/mg protein). The relative binding affinity (RBA) of ZK98299 for the nuclear PR was about 33% that of progesterone. The results of our study suggest that ZK98299 binds to PR in the cytosol and the nuclear fractions. The antiprogestin effects of ZK98299 reported in the literature are PR-mediated and may result from suboptimal nuclear binding/retention of antiprogestin-receptor complexes. Since this study did not involve isolation and study of individual PR isoforms, PR-A and PR-B, the present data should be viewed as representing an average of contributions by the two receptor forms.

Interaction of cycloalkanoprogesterones with mammalian progesterone receptor: binding of pregna-D'-pentaranes in the calf uterine cytosol

Pregna-D'-pentaranes (pentaranes) are modified progesterones with demonstrable progestational activity and contraceptive effect. We have examined the steroid binding characteristics of the two newly synthesized progesterone analogs, Pentarane A (16 alpha, 17 alpha-cyclohexanoprogesterone) and Pentarane B (6 alpha-methyl, 16 alpha, 17 alpha-cyclohexanoprogesterone), and studied the nature of their interaction with progesterone receptor (PR) from the chicken oviduct and the calf uterine cytosols. Pregna-D'-pentaranes exhibited no affinity for the chick PR but interacted with the calf uterine PR as did R5020. The pentaranes, however, bound PR less tightly. R5020- or pentarane-bound PR sedimented as an 8S moiety in 8-30% linear glycerol gradients. Thermal transformation of receptor resulted in the reduction of the 8S form, and caused an increase in the binding of R5020- and progesterone-bound PR complexes to DNA-cellulose. The pentarane-bound PR bound poorly, if at all, to DNA-cellulose. Our data suggest that pentaranes exhibit both similarities and differences with natural and synthetic progestins with respect to their interaction with calf uterine PR. The lack of pentarane binding to chicken PR is reminiscent of the general phenomenon that antiprogestins (RU486, ZK98299, and Org 31710 and Org 31806) do not interact with chicken PR. Pentaranes, therefore, represent unique steroid analogs to investigate the molecular mechanism of steroid hormone action.