In vivo effect of anti-estrogens on the localisation and replenishment of estrogen receptor

Transcriptional control by steroid hormones: the role of chromatin

The mouse mammary tumour virus (MMTV) promoter contains a complex hormone-responsive unit composed of four hormone-responsive elements, a nuclear factor I (NFI) binding site and two octamer motifs. All these sites are required for optimal hormonal induction. Although synergism has been found between hormone receptors and octamer transcription factor 1 (Oct-1/OTF-1), we were unable to detect a positive interaction between receptors and NFI in vitro. In chromatin, the MMTV hormone-responsive unit is contained in a phased nucleosome. The precise positioning of the DNA double helix on the surface of the histone octamer precludes binding of NFI and Oct-1/OTF-1 to their cognate sequences, while still allowing recognition of two hormone-responsive elements by the hormone receptors. Hormone treatment leads to a characteristic change in chromatin structure that makes the centre of the nucleosome more accessible to digestion by DNase I and facilitates binding of receptors, NFI and Oct-1/OTF-1 to the nucleosomally organized promoter. The MMTV promoter functions in yeast in a hormone receptor-dependent and NFI-dependent fashion. Depletion of nucleosomes activates hormone-independent transcription from the MMTV promoter. These results imply that nucleosome positioning not only represses hormone-independent transcription, but also enables binding of a full complement of transcription factors to the hormone-responsive unit after hormone induction.

A new interpretation of antiestrogen action

Despite differences in their pharmacological behavior, type I and type II antiestrogens have certain important properties in common. Both differ from estradiol in that they enhance the immunoreactivity of estrogen receptors, apparently by inducing conformational change that exposes an additional epitope for a particular monoclonal antibody. Moreover, both types of antihormones not only compete with estradiol for its binding to the receptor but they also react with another domain not recognized by the hormone. The binding capacity for either type of antiestrogen is nearly twice that for estradiol, providing definitive evidence for the existence of specific antiestrogen-binding sites that are postulated to be important in antagonist action. These findings suggest a unified two-site model which helps explain how the same substance can be both an agonist and an antagonist; why there may be species variations in the agonist/antagonist relationship of type I compounds; and why type II agents show only antagonistic properties. It is suggested that interaction with secondary, antagonist-specific binding sites may provide a useful screen in the search for new and improved antihormonal agents.

Nuclear acceptor sites for steroid hormone receptors: comparisons of steroids and antisteroids

The respective chromatin binding sites (acceptor sites) for both the avian oviduct progesterone receptor (PR) and the rabbit uterine estrogen receptor (ER) reported by two separate laboratories are compared. Support for a saturable, high affinity binding to the chromatin acceptor sites by both receptors is described. Nonradiolabelled PR or ER compete with their homologous radiolabelled receptors for binding. However, there is no competition between the heterologous ER and PR for the nuclear binding sites. In both receptor systems, evidence for a receptor dependent, receptor specific binding, which mimics the binding measured in vivo, is reported. With both the PR and ER, evidence for extensive masking in chromatin of many of the acceptor sites is described. The PR and ER acceptor sites appear to be composed of specific acceptor proteins bound to DNA. The dissociation of these proteins from DNA causes a loss of specific binding. Reannealing of these same chromatin protein fractions, but not other protein fractions, back to the DNA reinstates the specific PR binding. Antibodies against the PR acceptors proteins are described. These antibodies block PR but not ER binding to the chromatin acceptor sites, supporting the steroid receptor specific acceptor sites. In the rabbit uterine system, the ER acceptor proteins dissociate from chromatin in three distinct fractions according to their affinity for the DNA. Each of these fractions contains acceptor activity in that each can be reannealed to the DNA to reconstitute specific ER binding sites, i.e. those which are saturable, high affinity, and receptor dependent. Interestingly, the antiestrogen receptor complexes do not bind to one fraction of acceptor proteins which does bind the native estrogen receptor complex. This ER acceptor protein fraction contains two regions of acceptor activity in the molecular mass range of 50,000 and 12,000 daltons. The latter species is in the size range of the PR acceptor proteins. This difference in binding to chromatin acceptor sites by the estrogen receptor, complexed with an estrogen or antiestrogen, may explain the differential regulation of gene expression and the differential biological response to antiestrogens compared to the native estrogens.

Antagonistic actions of estradiol and tamoxifen upon forskolin-dependent meiotic arrest, intercellular coupling, and the cyclic AMP content of hamster oocyte-cumulus complexes

The effects of estradiol (E2) and the anti-E2 tamoxifen (tam) on forskolin (F)-dependent meiotic arrest in hamster oocytes were investigated. The hypotheses tested were that 1) the arresting action of F is enhanced by E2 and suppressed by tam and 2) the extent of heterologous metabolic coupling and the concomitant transfer of cumulus cell cAMP into the oocyte is increased and decreased by E2 and tam, respectively. E2 was tested with the ID25 F (where ID25 is the dose of F previously shown to arrest the meiosis of 25% cultured oocytes; intact, 3 microM; denuded, 10 microM) and tam was tested with the ID75 F (intact, 10 microM; denuded, 100 microM). E2 induced reversible dose-dependent increases in the percent germinal vesicle (%GV; determined cytogenetically) of both intact and denuded oocytes in the presence of ID25 F (intact: ID50 E2 = 18.0 microM; denuded: ID50 E2 = 17.2 microM) but, in contrast to intact oocytes, E2 also exerted a dose-dependent action on denuded oocytes in the absence of F (ID50 = 26.1 microM). E2 induced dose-dependent increases in the cAMP content (determined by RIA) of intact oocytes (cAMP-oo) and of cumulus masses (cAMP-cm) and in the ratio of cAMPooo:cAMP-cm but failed to elevate F-stimulated cAMP in denuded oocytes. Heterologous metabolic coupling, as assessed by determination of the fraction of radiolabeled uridine marker that was transferred from the cumulus cells to the oocyte, was not significantly enhanced by E2. In contrast to denuded oocytes, tam induced dose-dependent decreases in the %GV and cAMP content of intact oocytes in the presence of ID75 F and significantly depressed heterologous metabolic coupling. While tam failed to antagonize the E2 action on denuded oocytes in the presence of ID25 F, in intact oocytes cultured with E2 and the ID25 F, the anti-E2 significantly decreased the %GV, the cAMP-oo and cAMP-cm, and the extent of heterologous metabolic coupling. These data show that while E2 can directly arrest the maturation of denuded hamster oocytes with no associated elevation of cAMP-oo, E2-enhancement of arrest in intact oocytes is correlated with both elevation of cAMP within the oocyte-cumulus complex and maintenance of heterologous metabolic coupling and is accompanied by an increase in cAMP-oo.

RU 16117, an orally active estriol-like weak estrogen

RU 16117, the 11 alpha-methoxy derivative of ethynyl estradiol, is an orally active weak estrogen potentially effective in the treatment of estrogen-deficiency in postmenopausal women (climacteric symptoms and severe osteoporosis). Biochemical studies have shown that RU 16117, like estriol, possesses the properties characteristic of a partial estrogen agonist/antagonist. RU 16117 binds to the cytosol estrogen receptor (ER) to form a complex which dissociates much faster than the estradiol complex. This explains its lower nuclear uptake. Furthermore, the nuclear RU 16117 complex also dissociates faster than the estradiol complex. Consequently, although low doses of RU 16117 can induce the majority of the effects of estradiol (increased polymerase A and B activities, cytosol ER replenishment, progestin receptor induction, increased uterine weight), these effects are long-lived only if the dose is considerably increased or if the compound is administered repeatedly or continuously. Since RU 16117 transiently occupies available estrogen binding sites, it can prevent the full response of estradiol. Thus, under appropriate kinetic conditions, it acts as an estrogen antagonist on the above parameters and also on DMBA-induced mammary tumors in the rat. At a daily dose of 24 micrograms for a period of 4 weeks RU 16117 led to 65% reduction in the number of already-established tumors. RU 16117 inhibits basal gonadotropin secretion and decreases the LH response to LHRH. Injection of 5 micrograms s.c. to the rat in estrus markedly inhibited the spontaneous peaks of LH, FSH and PRL measured on the afternoon of expected proestrus. Low doses which block ovulation by 100% had no detectable effect on vaginal cornification, thus suggesting a greater sensitivity at the hypothalamo-pituitary level.(ABSTRACT TRUNCATED AT 400 WORDS)

Estrogen receptor translocation and replenishment in rat hypothalamus and pituitary gland after the application of catecholestrogen or nonsteroidal antiestrogen

The time course of nuclear translocation of estrogen receptors and the replenishment of cytosolic receptor concentrations in the pituitary gland and hypothalamus of ovariectomized/adrenalectomized rats after the application of a high dose (0.1 mg/animal) of the catecholestrogens, 2-hydroxyethynylestradiol or 4-hydroxyethynylestradiol, was identical to that seen after the application of the monophenolic estrogen 17 alpha-ethynylestradiol. A distinctly different pattern of receptor distribution between these cell compartments was observed after the application of the nonsteroidal antiestrogen monohydroxytamoxifen (ICI 79,280) resulting in a prolonged depletion of cytosolic receptor concentrations. It therefore seems unlikely that the proposed antiestrogenic properties of 2-hydroxyestrogens are the result of a blocking of cytosolic receptor replenishment but may be explained by a reduced intrinsic nuclear activity of estrogen receptors complexed with 2-hydroxyestrogens.

Mode of action of LN 1643 (a triphenylbromoethylene antiestrogen): probable mediation by the estrogen receptor and high affinity metabolite

After in vivo administration of [3H]LN 1643, a triphenylbromoethylene antiestrogen, to immature female rats, polar metabolites were selectively accumulated in uterine nuclear fractions which contained most of the estrogen receptor. One metabolite comigrated with the 4-hydroxylated derivative (LN 2839) of LN 1643. LN 1643 and LN 2839 inhibited competitively and reversibly the binding of estradiol to the estrogen receptor, and the affinity of LN 2839 for the estrogen receptor was about 150-fold higher than that of LN 1643. Both compounds prevented the growth of the MCF7 human breast cancer cells and LN 2839 was about 10-fold more efficient than LN 1643. These results and previous data obtained with tamoxifen (a parent triphenylethylene antiestrogen) and its 4-hydroxylated metabolite, suggest that the antiestrogenic action of LN 1643 is mediated by the estrogen receptor as for the other synthetic antiestrogens, and that LN 1643 acts at least partly via its 4-hydroxy metabolite.

Dephosphorylation of oestradiol nuclear receptor in vitro. A hypothesis on the mechanism of action of non-steroidal anti-oestrogens

We show that a mouse uterus nuclear phosphatase exists that is capable of inactivating nuclear oestrogen receptor complexed to oestradiol-17 beta in vitro but is ineffective when the receptor is complexed with the two non-steroidal anti-oestrogens, nafoxidine and tamoxifen. We suggest that the long half-life of the tamoxifen-receptor complex versus the short half-life of the oestradiol-receptor complex in uterine nuclei in vivo is the result of the ineffectiveness of the phosphatase in dephosphorylating the anti-oestrogen-receptor complex.

Nuclear association states of rat uterine oestrogen receptors as probed by nuclease digestion

The solubilization of oestrogen receptors from uterine nuclei by micrococcal nuclease and deoxyribonuclease I was examined after the injection of oestradiol or Nafoxidine into castrated female rats. At 1h after an injection of oestradiol, 30% (0.18pmol/mg of DNA) of the nuclear oestrogen receptors was solubilized by 5 min of mild digestion with either nuclease. No further receptor release occurred, although DNA hydrolysis continued throughout a 20min interval. The limitation in receptor solubilization was not due to an artifact of digestion conditions or insufficient nuclease concentrations. Similar patterns of receptor solubilization and DNA hydrolysis were obtained with both nucleases whether the animals had been injected with oestradiol 1h before death or if the uteri from uninjected animals were incubated with [(3)H]oestradiol for 1h in vitro. When uterine nuclei were digested with these enzymes 12h after the animal was injected with oestradiol there was little change in the quantity of nuclease-sensitive sites (0.11pmol/mg of DNA); however, the quantity of nuclease-resistant sites decreased 10-fold. These values correspond quantitatively to the changes in salt-resistant and salt-extractable sites observed over a 12h interval after oestradiol treatment. Nuclease digestion of uterine nuclei obtained 16h after Nafoxidine treatment gave a pattern qualitatively and quantitatively similar to that observed 1h after oestradiol treatment, a result consistent with the agonist/antagonist action of this compound. An analysis by sucrose-density-gradient centrifugation of the time course of nuclease-dependent receptor solubilization indicated that the solubilized receptors were not associated with discrete nucleosomal fragments. We believe that these data indicate that only a portion of the receptors translocated to the nucleus become associated with chromatin, and this association may occur on regions of chromatin that are preferentially susceptible to nucleolytic cleavage.

High-affinity binding to the estrogen receptor of [3H]4-hydroxytamoxifen, an active antiestrogen metabolite

The tritiated 4-hydroxytamoxifen (OHT), an active metabolite of tamoxifen found in estrogen target nuclei, was prepared in vitro and its interaction with the cytosol estrogen receptor (R) of uterus and chick oviduct was specified. OHT bound to the 8S R with the same affinity and the same kinetic parameters as estradiol (E2). Its ability to protect the E2-binding sites and to stabilize the R--DNA interaction was also similar to that of E2. We conclude that the binding characteristics in vitro of ligands on the cytosol R do not presently allow us to discriminate between agonist and antagonist ligands.

Interaction of the anti-estrogen CI-628 and estradiol on plasma LH and hypothalamic LH-RH in the female rat

The effects of the anti-estrogen Ci-628 have been tested on both plasma LH and hypothalamic LH-RH content in ovariectomized or ovariectomized, estradiol (E2)-implanted rats, in order to correlate those parameters with the [3H]E2 retention in the hypothalamus and the pituitary. Incresing doses of CI-628 induced a dose-dependent inhibition of [3H]E2 retention in both cytosolic and nuclear fractions of the pituitary. In contrast, hypothalamic retention of [3H]E2 is only inhibited significantly with higher doses of CI-628 (2.4 and 24 mg/kg). In ovariectomized rats, only a high dose of CI-628 (24 mg/kg) is able to decrease elevated LH levels observed following castration. In the presence of E2, CI-628 has both estrogenic and anti-estrogenic properties on LH secretion. CI-628 acts at the pituitary level to decrease the tissue sensitivity to LH-RH, but has no effect on mediobasal hypothalamic (MBH) LH-RH content.

The binding of [3H]-oestradiol-17 beta in the immature rat uterus during the sequential administration of non-steroidal anti-oestrogens

1 The binding of [(3)H]-oestradiol-17beta (0.08 mug) in the uterus, vagina, liver and heart of immature female rats has been studied in vivo and the effect of daily administrations of the non-steroidal anti-oestrogens, tamoxifen and monohydroxytamoxifen, on the 2 h accumulation of [(3)H]-oestradiol-17beta in the uterus has been determined.2 Doses of tamoxifen (8 mug daily) or monohydroxytamoxifen (1.28 mug daily), which have previously been found to antagonize completely the uterotrophic activity of oestradiol-17beta (0.08 mug daily), did not significantly reduce the total uterine binding of 0.08 mug [(3)H]-oestradiol-17beta administered on day 4 of a 3-day uterine weight test.3 The simultaneous administration of tamoxifen (8 mug) or monohydroxytamoxifen (1.28 mug) with [(3)H]-oestradiol-17beta (0.08 mug) on day 3 of a uterine weight test did not significantly reduce the total uterine binding of oestradiol-17beta. The binding of [(3)H]-oestradiol-17beta was reduced if monohydroxytamoxifen or tamoxifen was administered 4 h before the oestradiol.4 Tamoxifen (8 mug daily) or monohydroxytamoxifen (1.28 mug daily) did not prevent the translocation of [(3)H]-oestradiol (0.08 mug) to the uterine cell nucleus on day 3 of a 3-day uterine weight test.5 The measurement of total nuclear oestrogen receptors by an exchange assay technique demonstrated a higher concentration of oestrogen receptors in anti-oestrogen-treated animals compared with controls.6 Since the administration of anti-oestrogenic doses of non-steroidal anti-oestrogens during a 3-day uterine weight test did not inhibit the total binding of oestradiol in the uterus, or affect the translocation of the steroid to the nucleus, the mechanism of action of non-steroidal anti-oestrogens over the range of the partial agonist dose-response curve must involve an interaction, or competition of oestradiol-17beta- and anti-oestrogen-oestrogen receptor complexes for sites within the nucleus.

Estrogen control of progesterone receptor induction in human breast cancer: role of nuclear estrogen receptor

We have provocative data which suggest that the nuclear estrogen-receptor complex interacts with DNA, that this interaction is required for appropriate receptor turnover or processing, and that processing may be essential for induction of a specific protein by estrogen. If the receptor is improperly inserted into DNA, as for instance when it is bound by nafoxidine, processing fails and the biological effect is blunted.

Mechanisms of replenishment of nuclear androgen receptor in rat ventral prostate

1. The concentration of androgen receptor in the nucleus of the prostatic cell is rapidly elevated by the administration in vivo of 2mug of [(3)H]testosterone to 1-day-castrated rats. From a concentration of 2300 receptors/nucleus at 5min after intravenous injection of hormone, there is an increase to 21000 receptors/nucleus at 60min. At the same time, the amount of binding of androgen in the cytoplasm remains constant at a relatively low value. 2. An identical dose of [(3)H]testosterone administered to 7-day-castrated rats produces a much smaller change in the concentration of nuclear receptor, from 700 receptors/nucleus at 5min to only 4300 receptors/nucleus at 60min. Thus the reservoir from which nuclear receptor is replenished is considerably smaller in regressed prostatic cells. Again, the amount of binding of androgen in the cytoplasm remains unchanged at a low value over the experimental time course of 60min. 3. In contrast with the scant labelling of cytoplasmic receptor achieved by injecting animals with [(3)H]testosterone, labelling in vitro, by incubation of tissue slices with radioisotope, indicates that prostate of 1-day-castrated animals actually contains 21400 receptors/cell in the cytoplasmic compartment, and prostate of 7-day-castrated animals 3000 receptors/cell. 4. Owing to the similarity between the concentration of nuclear receptor measured in vivo and the concentration of cytoplasmic receptor measured in vitro, the labelling techniques in vivo and in vitro were used in sequence to demonstrate the movement of most of the cytoplasmic receptor into the nucleus. In the 5-60min interval after the administration of [(3)H]testosterone to 1-day-castrated rats, a decrease of 17400 receptor molecules in the cytoplasm is exactly mirrored by an increase of 17200 receptor molecules in the nucleus. 5. These results imply that, in prostate of 1-day-castrated rats, nuclear receptor is replenished exclusively by translocation of cytoplasmic receptor. However, in the regressed prostate of 7-day-castrated rats, only about 25% of the nuclear receptor is replenished through translocation of existing cytoplasmic receptor. The remainder is ultimately synthesized during new rounds of cell division induced by hormone.

[Mechanisms of action of estrogens physiological and pharmacological aspects of hormonal "receptivity". The anti-estrogens (author's transl)]

Receptivity to estrogens may indeed depend on the concentration of the corresponding receptor in the target cells. This concentration varies physiologically, and would possibly explain, at least in part, certain hormonal interactions and receptivity changes during the physiological cycles. The modifications of receptor concentrations could perhaps be of significance in explaining the effects of "priming" and the negative influence of one steroid on another. The simultaneous presence of different receptors for the same hormone or the competition of different hormones for the same receptor offer interesting pharmacological possibilities. It has already been shown that the characteristics of receptors can explain the differences between a natural hormone like estradiol and diethylstilbestrol, the hormone with the most common synthesis. A series of synthetic derivatives (triphenylethylene) seem to offer the possibility of an antagonist effect without pollution by significant estrogenic activity, and the potential therapeutic consequences are immense. Regulatory proteins, the steroid hormone receptors, provide some of the most advanced models for rational physico-chemical and physiological approaches to pharmacological and therapeutical problems.

High-affinity binding of the antiestrogen [3H]tamoxifen to the 8S estradiol receptor

The interaction of tamoxifen (ICI 46,474), a synthetic antiestrogen, with uterine cytosol proteins of immature calf and rat has been studied directly using the tritiated compound labeled with a high specific activity. The binding complexes were measured by the dextrancoated charcoal, protamine sulfate and hydroxyapatite assays. Scatchard plots revealed a single class of high-affinity (KD congruent to 1.7 nM) binding sites, with a binding capacity similar to that of estradiol. Competitive experiments showed the same binding specificity for estrogens and antiestrogens. Sucrose gradient analysis revealed an 8S binding protein which could be partially proteolysed by trypsin into a 4S binding protein. Kinetic studies showed that the association rate of tamoxifen was 5 times lower than that of estradiol and reacted according to a second order kinetics. The first-order kinetics of dissociation was considerably higher than that of estradiol, giving a half-dissociation time of 20--40 min at 0--2 degrees C. In some cases tamoxifen displayed two slopes of dissociation, but the proportion of the slow-dissociating complex was always inferior to that found with estradiol. In contrast to estradiol, the kinetic constants ratio (k-/k+) gave a calculated dissociation constant, similar to that determined in equilibrium conditions (KD), agreeing with a simple reactional scheme. We conclude that the antiestrogen tamoxifen binds directly to the 8S cytosol receptor for estrogens and not to another receptor for the antagonists. In contrast to estradiol, the antagonist is rapidly dissociated from the receptor sites and is unable to protect them against thermal inactivation. The affinity of tamoxifen for its receptor sites as determined directly is surprisingly high when compared to its affinity evaluated indirectly by competitive experiments. It is then suggested that the two ligands either bind on two different sites of the same protein or induce a different conformational change of the same binding site.

Effect of the anti-estrogen, Nafoxidine, on NZB/W autoimmune disease

Treatment of female NZB/W F1 mice with Nafoxidine, an anti-estrogen, led to delayed manifestation of autoimmune features. Such treated mice had reduced anti-DNA antibodies, reduced proteinuria, and improved survival. These results support the hypothesis that sex hormones play an important role in the expression of autoimmunity and suggest that estrogens may accelerate autoimmunity in NZB/W F1 mice.

Nonsteroidal antiestrogens: their biological effects and potential mechanisms of action

The uterotropic and antiuterotrapic effects of a variety of structural derivatives of the nonsteroidal antiestrogen tamoxifen have been determined in the rat and the mouse. One derivative, monohydroxytamoxifen, was found to be a potent antiestrogen in the rat, with a high affinity for the estrogen receptor. Various techniques of sucrose density gradient analysis were used to demonstrate that estradiol and tamoxifen bind to the rat uterine cytoplasmic estrogen receptor. Estrogens and antiestrogens provoke the translocation of estrogen receptors to the nucleus and deplete the cytoplasmic estrogen receptor pool for short or long periods depending on the dose administered. Estradiol stimulates endometrial hyperplasia with an increase in total uterine DNA content, whereas tamoxifen stimulates endometrial hypertrophy with only a slight increase in uterine DNA content. It is concluded that the molecular shape of the ligand that binds to the estrogen receptor determines antiestrogenic activity.

Interaction of the anti-oestrogen, nafoxidine hydrochloride, with the soluble nuclear oestradiol-binding protein in chick liver

Nafoxidine hydrochloride (Upjohn, 11100A)injected with oestradiol into immature chicks inhibits the hormone-induced increase in [3H]oestradiol-binding activity in salt extracts of liver nuclei as well as the subsequent production by liver of egg-yolk phosphoprotein. Substantial inhibition of both oestradiol-induced responses is seen when nafoxidine is given in a dose approximately equimolar with that of oestradiol. In vitro nafoxidine competitively inhibits binding of [3H]oestradiol in nuclear extracts. The Ki for the inhibition is 43 nM, which indicates an affinity of nafoxidine for the binding protein about 4% of that of oestradiol. The inhibitory action of nafoxidine in vivo thus is more potent than the relative binding affinity determined in vitro might indicate. One possible explanation is that the primary site of nafoxidine action is at a point proximal to nuclear receptor interaction. Nafoxidine injected alone into the chick does not induce phosphoprotein synthesis, but it does increase [3H]oestradiol-binding activity in extracts of liver nuclei to a limited extent. No differences in the properties of the oestradiol-binding activity in extracts from nafoxidine-treated chicks or from oestradiol-treated chicks were detected. Chick liver cytosol does not contain detectable high-affinity oestradiol-binding activity. A low-affinity oestradiol-binding component with a sedimentation coefficient of 3.5S was found, but it was unaffected by treatment of chicks with earlier nafoxidine or oestradiol. The results suggest a difference in the mechanism of oestradiol action in the chick liver and in the widely studied rat uterus, on which the usual model for oestradiol action is largely based.

Binding of [3-H]estradiol by brain cell nuclei and female rat sexual behavior: inhibition by antiestrogens

The antiestrogens MER-25, CI-628, and nafoxidine inhibit the uptake of [3H]-estradiol in whole homogenates and isolated cell nuclei tissues and the pituitary, and inhibit estradiol-induced female sexual behavior. The antiestrogens were injected intraperitoneally 2 h prior to an intravenous injection of [3H]estradiol, and the animals were killed 2 h after the estradiol. CI-628 reduces radioactivity in whole homogenates and isolated cell nuclei of cerebral cortex, hypothalamus, preoptic area -septum and pituitary. Nafoxidine reduces uptake in cell nuclei of the hypothalamus, preoptic area-septum and pituitary. In this paradigm, MER-25 inhibited uptake only in the pituitary. In the analogous behavioral experiments, with antiestrogens injected 2 h prior to an intravenous injection of unesterified estradiol, CI-628 and nafoxidine totally inhibited lordosis responding. MER-25 shows no inhibition of behavior in this paradigm. However, when MER-25 is injected 12 h prior to the estradiol, it inhibits retention of [3H]estradiol at 2 h in brain and pituitary cell nuclei, and lordosis responding is also inhibited. Additionally, the antiestrogens can apparently displace previously bound [3H]estrdiol. When the antiestrogens are injected 2 h prior to an injection of [3H]estradiol, MER-25, CI-628 and nafoxidine all show greater inhibition of nuclear estradiol retention at 12 h after the [3H]estradiol injection than 2 h. Analogously, when CI-628 is injected 2 h after an intravenous injection of [3H]estradiol, it displaces most of the radioactivity present in hypothalamic-preoptic area nuclei at 12 h after the estradiol injection. These results indicate that antiestrogens can prevent or reverse the nuclear concentration of estradiol in brain cells and are consistent with a role of the cell nucleus in the induction of estrous behavior by estradiol.

Androgen on the estrogen receptor. I - Binding and in vivo nuclear translocation

In the immature rat uterus, high concentrations of androgens competed specifically with estradiol on the estrogen receptor (RE). This competition was stereospecific for C19 steroids bearing a 17beta and/or 3 hydroxyl group. Very low affinity ligands, such as testosterone, could not compete with estradiol at equilibrium but decreased the association rate of estradiol on its receptor. High doses (greater than 0.4mg) of 5dihydrotestosterone provoked in vivo as in vitro the nuclear translocation of RE. The nuclear receptor thus formed displayed the same 5.2 S sedimentation constant as that induced by estradiol. We conclude that the weak affinity binding of androgens to the estrogen receptor is sufficient to induce its nuclear translocation in vivo provided androgen concentration is high enough in uterus to occupy the estradiol binding site. Conversely, progesterone which does not bind RE could not provoke its nuclear translocation.

Induction of uterine ornithine decarboxylase (ODC) by antiestrogens. Inhibition of estradiol-mediated induction of ODC: a possible mechanism of action of antiestrogens

The ability of antiestrogens (tamoxifen and nafoxidine) to affect uterine ornithine decarboxylase (ODC) in the ovariectomized rat was determined. Tamoxifen citrate (1 mg or 10 mg/kg) and nafoxidine (0.5 mg/kg) markedly elevated ODC levels. Tamoxifen (1 mg/kg) given for 4 days totally inhibited the E2 (0.5 mug/kg)-mediated induction of ODC. Similarly nafoxidine (0.5 mug/kg) given once a day for 2 days inhibited the E2-mediated induction of ODC. The relation of the inhibition of ODC induction by antiestrogens to their mechanism of action as antiestrogens is discussed.