Are estrogen receptors cytoplasmic or nuclear? Some immunocytochemical and biochemical studies

Kinesins Modify ERR1-Dependent Transcription Using a Conserved Nuclear Receptor Box Motif

Kinesin family motors are microtubule (MT)-stimulated ATPases known best as transporters of cellular cargoes through the cytoplasm, regulators of MT dynamics, organizers of the mitotic spindle, and for insuring equal division of DNA during mitosis. Several kinesins have also been shown to regulate transcription by interacting with transcriptional cofactors and regulators, nuclear receptors, or with specific promotor elements on DNA. We previously showed that an LxxLL nuclear receptor box motif in the kinesin-2 family motor KIF17 mediates binding to the orphan nuclear receptor estrogen related receptor alpha (ERR1) and is responsible for the suppression of ERR1-dependent transcription by KIF17. Analysis of all kinesin family proteins revealed that multiple kinesins contain this LxxLL motif, raising the question as to whether additional kinesin motors contribute to the regulation of ERR1. In this study, we interrogate the effects of multiple kinesins with LxxLL motifs on ERR1-mediated transcription. We demonstrate that the kinesin-3 family motor KIF1B contains two LxxLL motifs, one of which binds to ERR1. In addition, we show that expression of a KIF1B fragment containing this LxxLL motif inhibits ERR1-dependent transcription by regulating nuclear entry of ERR1. We also provide evidence that the effects of expressing the KIF1B-LxxLL fragment on ERR1 activity are mediated by a mechanism distinct from that of KIF17. Since LxxLL domains are found in many kinesins, our data suggest an expanded role for kinesins in nuclear receptor mediated transcriptional regulation.

The race to develop oral SERDs and other novel estrogen receptor inhibitors: recent clinical trial results and impact on treatment options

Hormonal therapy plays a vital part in the treatment of estrogen receptor-positive (ER +) breast cancer. ER can be activated in a ligand-dependent and independent manner. Currently available ER-targeting agents include selective estrogen receptor modulators (SERMs), selective estrogen receptor degraders (SERDs), and aromatase inhibitors (AIs). Estrogen receptor mutation (ESR1 mutation) is one of the common mechanisms by which breast cancer becomes resistant to additional therapies from SERMs or AIs. These tumors remain sensitive to SERDs such as fulvestrant. Fulvestrant is limited in clinical utilization by its intramuscular formulation and once-monthly injection in large volumes. Oral SERDs are being rapidly developed to replace fulvestrant with the potential of higher efficacy and lower toxicities. Elacestrant is the first oral SERD that went through a randomized phase III trial showing increased efficacy, especially in tumors bearing ESR1 mutation, and good tolerability. Two other oral SERDs recently failed to achieve the primary endpoints of longer progression-free survival (PFS). They targeted tumors previously treated with several lines of prior therapies untested for ESR1 mutation. Initial clinical trial data demonstrated that tumors without the ESR1 mutation are less likely to benefit from the SERDs and may still respond to SERMs or AIs, including tumors previously exposed to hormonal therapy. Testing for ESR1 mutation in ongoing clinical trials and in hormonal therapy for breast cancer is highly recommended. Novel protein degradation technologies such as proteolysis-targeting chimera (PROTACS), molecular glue degrader (MGD), and lysosome-targeting chimeras (LYTACS) may result in more efficient ER degradation, while ribonuclease-targeting chimeras (RIBOTAC) and small interfering RNA (siRNA) may inhibit the production of ER protein.

Zearalenone alters the excitability of rat neuronal networks after acute in vitro exposure

Zearalenone (ZEA) is a mycotoxin produced by Fusarium species, detectable in various cereals and processed food products worldwide. ZEA displays a significant estrogenic activity, thus its main health risk is the interference with sexual maturation and reproduction processes. However, in addition to being key hormonal regulators of reproductive function, estrogenic compounds have a widespread role in brain, as neurotrophic and neuroprotective factors, and they may influence the activity of several brain areas not directly linked to reproduction, as well. Therefore, in the present study, acute effects of ZEA were studied on certain neuronal functions in rats. Experiments were performed on rat brain slices or live rats. Slices were incubated in ZEA-containing (10-100 μM) solution for 30 min. Electrically evoked and spontaneous field potentials were studied in the neocortex and in the hippocampus. At higher concentrations, ZEA incubation of the slices altered excitability and the pattern of epileptiform activity in neocortex and inhibited the development of LTP in hippocampus. For the verification of these in vitro results, in vivo electrophysiological and immunohistochemical investigations were also performed. ZEA was administered systemically (5 mg/kg, i.p.) to male rats and somatosensory evoked potentials and neuronal activation studied by c-fos expression were analyzed. No neuronal activation could be demonstrated in the hippocampus within 2 h of the injection. In the somatosensory cortex, ZEA did not change in vivo evoked potential parameters, but the activation of a small neuronal population could be demonstrated with the c-fos technique in this brain area. This result could be associated with the ZEA-induced alteration of epileptiform activity observed in vitro. Altogether, the toxin altered the excitability and plasticity of neuronal networks after direct treatment in slices, but the effects were less prominent on the given brain areas after systemic treatment in vivo. A probable explanation for the partial lack of in vivo effects may be that after a single injection, ZEA did not cross the blood-brain barrier at sufficient rate to allow the build-up of comparable concentrations in the investigated brain areas. However, in case of compromised blood-brain barrier functions or long-term repeated exposure, alterations in cortical and hippocampal functions cannot be ruled out.

The role of G-protein-coupled membrane estrogen receptor in mouse Leydig cell function-in vivo and in vitro evaluation

In this study, G-coupled estrogen receptor (GPER) was inactivated, by treatment with antagonist (G-15), in testes of C57BL/6 mice: immature (3 weeks old), mature (3 months old) and aged (1.5 years old) (50 μg/kg bw), as well as MA-10 mouse Leydig cells (10 nM/24 h) alone or in combination with 17β-estradiol or antiestrogen (ICI 182,780). In G-15-treated mice, overgrowth of interstitial tissue was found in both mature and aged testes. Depending on age, differences in structure and distribution of various Leydig cell organelles were observed. Concomitantly, modulation of activity of the mitochondria and tubulin microfibers was revealed. Diverse and complex GPER regulation at the mRNA level and protein of estrogen signaling molecules (estrogen receptor α and β; ERα, ERβ and cytochrome P450 aromatase; P450arom) in G-15 Leydig cells was found in relation to age and the experimental system utilized (in vivo and in vitro). Changes in expression patterns of ERs and P450arom, as well as steroid secretion, reflected Leydig cell heterogeneity to estrogen regulation throughout male life including cell physiological status.We show, for the first time, GPER with ERs and P450arom work in tandem to maintain Leydig cell architecture and supervise its steroidogenic function by estrogen during male life. Full set of estrogen signaling molecules, with involvement of GPER, is crucial for proper Leydig cell function where each molecule acts in a specific and/or complementary manner. Further understanding of the mechanisms by which GPER controls Leydig cells with special regard to male age, cell of origin and experimental system used is critical for predicting and preventing testis steroidogenic disorders based on perturbations in estrogen signaling.

Cerebral hypoperfusion increases estrogen receptor abundance in the ovine fetal brain and pituitary

BACKGROUND/AIMS

Estrogen is an important component of fetal neuroendocrine function in late-gestation fetal sheep; however, little is known about the regulation of estrogen receptor abundance in the brain and pituitary of fetuses. The present study was performed to test the hypotheses that estrogen receptor abundance in the fetal brain and pituitary are influenced by circulating estradiol concentrations and that they are acutely regulated after cerebral hypoperfusion.

METHODS

We studied 16 time-dated fetal sheep (124-128 days gestation) that were chronically catheterized and instrumented at least 5 days before study. Four groups (n = 4 each) were studied in which fetuses received estradiol (0.25 mg/day, producing physiological increases in fetal plasma estradiol concentrations) or placebo implants, and in which fetuses received a 10-min period of brachiocephalic occlusion (BCO) or sham-BCO. One hour after BCO or sham-BCO, fetuses were euthanized and tissues rapidly removed for analysis of estrogen receptors (ER)-alpha and -beta at the mRNA and protein levels.

RESULTS

Both BCO and estradiol treatment were effective in changing ER expression, although the effects were region-specific. BCO dramatically increased ER-alpha in the pituitary and both ER-alpha and ER-beta in the brainstem, while decreasing ER-alpha expression in the hypothalamus. Estradiol treatment decreased ER-alpha expression in the hypothalamus, whereas it increased ER-alpha expression in the brainstem, cerebral cortex and hippocampus.

CONCLUSIONS

We conclude that the expression of ER-alpha and ER-beta in the brain and pituitary of fetal sheep are influenced by circulating estrogen concentrations and acutely regulated in response to cerebral hypoperfusion.

Estradiol stimulates progenitor cell division in the ventricular and subventricular zones of the embryonic neocortex

Two distinct populations of cerebral cortical progenitor cells that generate neurons during embryogenesis have been identified: radial glial cells and intermediate progenitor cells. Despite advances in our understanding of progenitor cell populations, we know relatively little about factors that regulate their proliferative behaviour. 17-beta-Estradiol (E2) is present in the adult and developing mammalian brain, and plays an important role in central nervous system processes such as neuronal differentiation, survival and plasticity. E2 also stimulates neurogenesis in the adult dentate gyrus. We examined the role of E2 during embryonic cortical neurogenesis through immunohistochemistry, in situ hybridization, functional enzyme assay, organotypic culture and in utero administration of estradiol-blocking agents in mice. We show that aromatase, the E2 synthesizing enzyme, is present in the embryonic neocortex, that estrogen receptor-alpha is present in progenitor cells during cortical neurogenesis, that in vitro E2 administration rapidly promotes proliferation, and that in utero blockade of estrogen receptors decreases proliferation of embryonic cortical progenitor cells. Furthermore, the E2 inhibitor alpha-fetoprotein is expressed at high levels by radial glial cells but at lower levels by intermediate progenitor cells, suggesting that E2 differentially influences the proliferation of these cortical progenitor cell types. These findings demonstrate a new functional role for E2 as a proliferative agent during critical stages of cerebral cortex development.

Expression and intracellular localization of progesterone receptors in cultured human gingival fibroblasts

OBJECTIVE

The aim of this immunocytochemical study was to characterize the expression and distribution of the progesterone receptor (PR) and estrogen receptor (ER) in gingival fibroblasts using culture cells derived from people at various ages.

BACKGROUND

The reaction of female hormones is tissue or cell specific, and receptor availability in the cell is one of the major causes for the different reactions. Gingiva is a target tissue for female hormones; however, the characteristics of PR and ER in both the fibroblasts and the other component cells remain largely unknown.

MATERIALS

Gingival tissue was obtained from six people at various ages and culture fibroblasts were established. At least three passages of each cell line were strained for PR and ER with monoclonal antibodies (Clone 1A6, Clone 1D5, respectively).

RESULTS

PR positive cells were detected in all six cell lines through early passages to late ones, but ER were only observed in two of six samples with faint reactions. The staining intensity for PR was greater than for ER, but less than that shown in the MCF-7 breast cancer cells, positive control. In every positive control test, ER reactivity was equal to or higher than that of PR. During the interphase, significantly fewer positive fibroblasts occurred compared with negative fibroblasts, and positive nuclei were even fewer. Meanwhile, most of the mitotic cells were PR positive, showing intense localization around chromosomes and on microtubules. These findings suggest that gingival fibroblasts are fundamentally capable of expressing PR and transmit the signal to target genes.

CONCLUSIONS

The present study may conclude that in either gender or at any age, gingival fibroblasts express PR rather low in level and do not necessarily localize PR in a nuclear dominant fashion, which is an essential feature for reproductive organ cells. The poor ER reactivity shown in the gingival fibroblasts was discussed in view of the receptor subtype.

A rat cell line derived from DMBA-induced mammary carcinoma

A new cell line, designated UHKBR-01, was successfully established from a 7,12-dimethylbenz[a]anthracene (DMBA)-induced rat mammary tumour. DMBA was administered orally at a dose of 4 mg/ml per rat on the first day of the experiment and thereafter at weekly intervals of same dosage, until the rats have reached a weight of around 150-200 g. The tumours grew rapidly after the injection, and were transplanted into nude mice one the harvest size (2.5 x 2 x 1 mm(3)) was reached, it was transplanted onto nude mice. We have developed a cell line from a portion of the DMBA-induced carcinoma of the nude mice. The UHKBR-01 cell exhibited a slow increase in growth rate during the time of culture and was highly tumourigenic in nude mice. The cells have been grown in culture for over 40 passages. Characterization of the cell line was performed. This included morphology by light and transmission electron microscopy, karyotype, growth rate, tumour antigen expression and xenograft implantation into nude mice. These cells exhibit ultrastructural and immunohistochemical features of epithelial cells of mammary origin. The above analyses also demonstrated that UHKBR-01 cells were oestrogen- and progesterone-receptor positive, in likeness to other established breast cancer cell lines such as MDA-MB-231 and MCF-7. The cell line grows as monolayers of oval-shaped cells with large folded nuclei accompanied by a rich supply of mitochondria. This report describes the first in vitro cell line from transplantable DMBA-induced mammary carcinoma of nude mice, which presents unique characteristics that may prove to be a good experimental model for investigating breast cancer biology.

Membrane fluidity effects of estratrienes

Estrogens have demonstrable neuroprotective effects. This fact has lead to the proposed use of estrogens for the prevention and/or treatment of Alzheimer's disease. The exact protective mechanism estrogens provide is not fully understood. In this report, a potential non-genomic mechanism for estratrienes involving alterations in membrane fluidity was studied. Steroids, such as estrogen, are known to be membrane-active and can alter the lipid packing. In this study we used fluorescent methodologies to address the effect of naturally occurring steroids (17alpha and 17beta-estradiol, testosterone, and progesterone) and new estratriene analogs on membrane fluidity using liposomes and HT-22 hippocampal cells. The study's results indicate steroids, based on the estratriene nucleus, can modulate lipid packing as evidenced by (1) decreased membrane fusion events and (2) decreased membrane fluidity. The effects on the membrane were both time and concentration dependent. It was also demonstrated through rational design estratriene analogs can be synthesized with enhanced membrane effects. Finally, in a glutamate-induced toxicity HT-22 model, we also demonstrated cellular protection with the estratriene-based molecules and analogs. The data suggest the plethora of cellular actions of estrogens may relate to or be influenced by membrane effects of the steroid.

Demonstration of estrogen receptors and of estrogen responsiveness in the HKT-1097 cell line derived from diethylstilbestrol-induced kidney tumors

This study was undertaken in order to examine the estrogen sensitivity of HKT-1097, an established cell line recently derived from diethylstilbestrol (DES)-induced kidney tumors in Syrian hamsters. Estrogen receptor (ER) level in HKT-1097, determined by enzyme-linked immunoassay, was 67 fmol/mg protein, i.e., a value approx. 30% lower than that found in Syrian hamster kidney tumors. ER immunostaining in cells fixed with Carnoy's mixture, as well as ER demonstration by Western blotting, suggested DES-induced nuclear translocation or stabilization of the receptor within the nucleus. Kinetic parameters of estrogen binding to ER in HKT-1097 cells were 8.4 x 10(-11) M and 60.8 fmol/mg protein for Kd and Bmax, respectively. The Kd of estrogen binding to ER in HKT-1097 was close to that evaluated for the receptor in breast cancer-derived MCF-7 cell line, whereas the Bmax value was approx. seven times lower in HKT-1097 as compared to MCF-7. In HKT-1097 cells, antiestrogens ICI 182,780 and RU 58,668 induced ER downregulation and competed with estrogen binding to the receptor. As demonstrated by Western blot analysis, DES exposure led to an increased expression of progesterone receptor (PgR) in HKT-1097 cells. Addition of DES to estrogen-free medium produced a stimulation of growth in both HKT-1097 and MCF-7 cells, but the mitogenic effect was less marked for HKT-1097. Despite the fact that ICI 182,780 and RU 58,668 clearly interact with HKT-1097 cell ER, they appeared unable to suppress DES-induced stimulation of growth and increase of PgR expression.

A hormone pulse induces transient changes in the subcellular distribution and leads to a lysosomal accumulation of the estradiol receptor alpha in target tissues

An intrauterine pulse-stimulation with estradiol induced changes in the subcellular localization of estrogen receptor alpha in porcine endometrium, as detected with F(ab') fragments of various anti-receptor antibodies covalently linked to nanogold. The low-sterically hindered immunoreagents--recognizing different epitopes within the hormone binding domain--allowed for an efficient immunolabeling of estradiol receptor alpha, detecting it both in the cytoplasm and the nucleus of nonstimulated epithelium cells. In the cytoplasm, the receptor often seemed to be associated with actin filaments and the endoplasmatic reticulum. After the stimulation with estradiol, a predominantly nuclear localization and a labeling of nucleoli was observed. Our immunoelectron microscopy study demonstrates a localization of the receptor in cytoplasmic organelles that increased after the hormone pulse. These organelles exhibited the morphological properties of lysosomes and relocated to the perinuclear area. In analogous cytoplasmic organelles, the presence of cathepsin D was detected via indirect immunogold labeling, justifying their classification as lysosomes. Quantitative examinations revealed that not only the number of lysosomes in the proximity of the nucleus but also their immunostaining for estradiol receptor alpha increased significantly after the hormone pulse. Thus, estradiol induces both the rapid shift of receptor into the nucleus, a slower perinuclear accumulation of lysosomes and an increase of lysosomal ERalpha-immunoreactivity. These results suggest a role for lysosomes in the degradation of receptor shuttling out of the nucleus. This could serve as termination of the estradiol receptor alpha-dependent activation of target cells. This hypothesis is strengthened by the fact that the receptor content in uterine tissue declined drastically few hours after the hormone pulse.

Retrieval of estradiol receptor in paraffin sections of resting porcine uteri by microwave treatment. Immunostaining patterns obtained with different primary antibodies

The unmasking of estradiol receptor in paraffin sections of Bouin's-fixed uterine tissue from ovariectomized gilts was attained with microwave treatment. Immunocytochemistry of the receptor was performed using a polyclonal or five monoclonal antibodies, two of which are commercially available, reacting with different domains of the protein and an amplified-peroxidase system for detection. With five of the antibodies, a predominance of nuclear staining was observed in cells of endometrial glands, while one monoclonal antibody (13H2), reacting with the receptor's domain E, showed a preference for the cytoplasmic receptor. In stroma, all antibodies detected more receptor in nuclei than in cytoplasm. In epithelium, the commercially available antibody H222, our monoclonals 13H2 and HT65, and the polyclonal antibody 402 demonstrated more receptor in cytoplasmic than in nuclear areas. In myometrium, the nuclei from longitudinal and ring muscles were definitely stained with the antibodies. We conclude that the accessibilities of the antibody epitopes of the receptor differ according to the functional uterine cell type.

Changes in the subcellular distribution of the rat uterus oestrogen receptor as induced by oestradiol, tamoxifen and ZD 182,780

The aim of this work was to compare the subcellular distribution of the oestrogen receptor from the uteri of rats treated with vehicle alone (control group), oestradiol or one of the anti-oestrogenic drugs tamoxifen and ZD 182,780. The nuclear, microsomal and cytosolic oestrogen receptor contents were evaluated by an immunoenzymatic method ("ER-EIA" kit from Abbott Laboratories) and the results in each fraction were expressed as a percentage of the total number of receptors. Parallel studies were performed to assess the uterotrophic effect of these drugs and to assess that they had reached the uterus. In the control group, we found that the oestrogen receptor was distributed mainly between the microsomal (29.1 +/- 1.3%) and cytosolic (68.1 +/- 0.9%) fractions, with only a small amount located in the nucleus (2.8 +/- 0.5%). When oestradiol was administered, the oestrogen receptor distribution was: nuclear 11.7 +/- 2.0, microsomal 15.5 +/- 1.3 and cytosolic 72.8 +/- 3.3% and, in the tamoxifen group, the results were: nuclear 18.5 +/- 1.5, microsomal 26.0 +/- 3.1 and cytosolic 55.5 +/- 3.4%, which shows a relative shift both to the control and the oestradiol-treated groups. In the uteri of rats treated with ZD 182,780 the results were very similar to those obtained in the control group. Our results indicate that the subcellular distribution of the oestrogen receptor varies according to the drug administered and that this receptor may not be located in a single subcellular compartment. Moreover, the nuclear uptake of the ZD 182,780-oestrogen receptor complex seems to be blocked, possibly due to impaired receptor dimerization. In the case of tamoxifen, the intracellular transport of the receptor also seems to be blocked, probably due to the nuclear retention of the receptor, thus suggesting that tamoxifen must impair the oestrogen receptor function on a step subsequent to the receptor dimerization.

Immunogold labelling of estradiol receptor in MCF7 cells

The distribution of estradiol receptor in serial sections of estradiol-deprived and estradiol-stimulated MCF7 cells was studied by using mouse monoclonal antibodies reacting with different domains of the receptor and goat-antimouse IgG/6 nm gold. In the nucleus and the cytoplasm of estradiol-deprived cells, the receptor was detected by all three monoclonals (13H2, HT 65 and MA1-310). The antibodies 13H2 and MA1-310 detected receptor associated to the microfilament bundles in the cytoplasm. Higher densities of antireceptor attachment to the nuclear areas were accompanied by a reduction in the attachment to the cytoplasm after estradiol stimulation of the cells. The results confirm earlier observations on the presence of cytoplasmic estrogen receptor in estradiol-deprived cells and support the premise of an estradiol-induced translocation of this ligand-dependent transcription regulator.

Characterization of rat uterine estrogen receptors in vivo

In vivo binding of [3H]estradiol ([3H]E2) in the rat uterus was performed by an intraluminal perfusion of the ligand for different time periods. In this way the binding takes place in the intact organ before processing the tissue. In 10 min, with 10 nM [3H]E2 apparent saturation or steady state incorporation of the [3H]E2 was achieved with a similar distribution of the label between cytosol and nuclear fractions. In vitro, the subcellular localization of the estrogen receptor (ER) is influenced by the extent of tissue damage. With the intact organ the ER subcellular distribution approaches that of the in vivo perfusion. With increasing [3H]E2 in the perfusate it was possible to obtain a "saturation" curve and to derive the kinetic parameters. For cytosol: Kd 16 nM; Bmax 235 fmol/mg prot. For nucleus: Kd 2.7 nM; Bmax 103 fmol/mg prot. To follow the time course of the ER movement in vivo, "pulse and wait" experiments were designed. Both uterine horns were perfused for 1 min. One of the horns was immediately processed (0 time) and the other was left in place after the perfusion for different periods. At 0 time 90% of the bound label appeared in the cytosol. At 5, 15 and 30 min, the label in the cytosol decreased and that of the nucleus increased approx. to 50%. Thus, translocation of the bound label from cytosol to nucleus was apparent. The role of the cytoplasm-nucleus ER traffic in the regulation of gene transcription by estrogens is discussed.

Immunogold labelling of the cytoplasmic estradiol receptor in resting porcine endometrium

Serial sections of resting porcine endometrium were analyzed with the monoclonal antibody 13H2 using goat antimouse IgG/5 nm gold as secondary reagent or with either polyclonal antibodies from goat #402 or the rat monoclonal antibody H222, both in combination with protein G/12 nm gold. A modestly higher labelling of nuclei than of cytoplasm was seen only with the monoclonal antibody H222. Polyclonal #402 and monoclonal 13H2 showed fewer attachments over nuclear than over cytoplasmic areas. The highest densities of attachment and of predominantly cytoplasmic labelling were obtained with the monoclonal antibody 13H2. The results confirm the earlier assumption of a restricted accessibility of estradiol receptor in the cytoplasm of resting cells for immunoreagents.

Immunohistochemical labeling of androgen receptors in the brain of rat and monkey

Androgen receptor antibodies have recently been developed using fusion proteins containing fragments of human prostatic androgen receptor. We have used a polyclonal antibody raised in rabbits to label androgen receptors in brain sections from male and female rats and monkeys. Free-floating frozen sections were incubated in primary antibody, and processed by the peroxidase-avidin-biotin complex method using biotinylated anti-rabbit IgG. Nickel intensified diaminobenzidine was used as the chromagen, and neurons were labeled in the amygdala, hippocampus, bed nucleus of stria terminalis, septum, preoptic area, in several hypothalamic nuclei including the supraoptic and paraventricular nuclei, in several brain stem motor nuclei and in cerebral cortex. Staining was most intense in cell nuclei but also occurred in cytoplasm and in some neuronal processes. Labeling was more restricted in monkey than in rat brain. Omitting the primary antibody or pre-incubating the primary antibody with rat prostatic cytosol for control purposes demonstrated the specificity of staining.

Characterization of estrogen and antiestrogen binding to the cytosol and microsomes of breast tumors

The binding of [3H]estradiol and [3H]hydroxytamoxifen to the cytosol and microsomal fractions of several human breast tumors was investigated. By washing microsomal membranes with a KCl-free or a KCl-containing medium we could distinguish between intrinsic, extrinsic and contaminant estradiol binding sites in these membranes. We observed that treatment of the microsomes with low salt medium removes about 80% of the total estradiol binding sites, whereas 20% are not extractable. The concentration of unextractable [3H]estradiol binding sites in the microsomes varies in proportion to the level of cytosolic estrogen receptors (ER). About 10% of the total extranuclear specific estrogen binding sites was consistently found tightly associated to the microsomal fraction, which displays an affinity for estradiol (Kd = 0.1-0.6 nM) similar to that of the cytosolic ER. The displacement of [3H]estradiol with unlabeled hormone or with the antiestrogens, nafoxidine, enclomiphene and tamoxifen (TAM) exhibits identical IC50 values either in the cytosol or in the microsomal membranes. On the other hand, the microsomal fraction of breast tumors also binds [3H]hydroxyTAM, but with higher capacity and lower affinity than those of the cytosolic fraction. Furthermore, we did not observe correlation between the concentrations of ER and of antiestrogen binding sites (AEBS) in the tumors. These results indicate that microsomal membranes of human breast tumors contain estrogen binding sites which may be related to the cytosol ER recycling and that specific AEBS are predominantly localized in this membrane system. Furthermore, it is shown that the magnitude of estradiol binding to microsomes depends on the ER positive degree of the tumors, whereas the magnitude of the antiestrogen binding to the microsomes is independent of the ER status of the tumors.

Effect of age on physico-chemical properties of the uterine nuclear estrogen receptors of albino rats

Estrogen receptors (ER) were extracted with high ionic strength buffer from the nuclei of uteri of young (21 weeks) and old (89 weeks) rats. Following the analysis of these receptors on a sucrose gradient and a Sephadex column, two peaks representing the two forms of receptors were obtained. The minor peak sedimented at 6.8 S with Stokes radius (RS) = 7.2 nm, molecular weight (Mr) = 204 K and frictional ratio (f/f0) = 1.71. On the other hand, the major peak sedimented at 4.1 S with RS = 3.3 nm, Mr = 57 K and f/f0 = 1.18. These properties of nuclear ER were similar in both ages. Also, the half life of ER complexes from both ages at 25 degrees C and 37 degrees C were 135 min and 30 min, respectively. However, these complexes were retained for longer periods in the nuclei of young than old rats. Furthermore, the dissociation constant of the binding of nuclear receptors to estrogen remained constant, but the number of binding sites decreased from 1.56 in young to 1.05 pmol/mg DNA in the old. In young rats, about 61% of nuclear receptors bound to DNA-cellulose. Out of this 2/5 was eluted with 0.15 M and the remaining 3/5 with 0.5 M KCl. On contrary, only 37% of total receptors bound to DNA-cellulose in the old. Out of this 3/5 was eluted with 0.15 M and the remaining 2/5 with 0.5 M KCl. These data suggested that despite the similarity in different physicochemical properties, the number of estrogen binding sites and the retention time of ER complexes in nuclei and the ability of these complexes to bind to DNA decrease in the uterus of old age.

Steroid hormone receptors

In the three decades since the original discovery of receptors for steroid hormones, much has been learned about the biochemical processes by which these regulatory agents exert their effects in target tissues. The intracellular receptor proteins are potential transcription factors, needed for optimal gene expression in hormone-dependent cells. They are present in an inactive form until association with the hormone converts them to a functional state that can react with target genes. Transformation of the receptor protein to the nuclear binding form appears to involve the removal of both macromolecular and micromolecular factors that act to keep the receptor form reacting with DNA. Much of the native receptor is present in the nucleus, loosely bound and readily extractable, but for some and possibly all steroid hormones, some receptor is in the cytoplasm, perhaps in equilibrium with a nuclear pool. Methods have been developed for the stabilization, purification, and characterization of receptor proteins, and through cloning and sequencing of their cDNAs, primary structures for these receptors are now known. This has led to the recognition of structural similarities among the family of receptors for the different steroid hormones and to the identification of regions in the protein molecule responsible for the various aspects of their function. Monoclonal antibodies recognizing specific molecular domains are available for most receptors. Despite the knowledge that has been acquired, many important questions remain unsolved. How does association with the steroid remove factors keeping the receptor protein in its native state, and how does binding of the transformed receptor to the response element in the promoter region enhance gene transcription? Once it has converted the receptor to the nuclear binding state, is there a further role for the steroid in modulating transcription? Still not entirely clear is the involvement of phosphorylation and/or dephosphorylation in hormone binding, receptor transformation, and transcriptional activation. Less vital to basic understanding but important in the overall picture is whether the native receptors for gonadal hormones are entirely confined to the nucleus or whether there is an intracellular distribution equilibrium. With the effort now being devoted to this field, and with the application of new experimental techniques, especially those of molecular biology, our understanding of receptor function is progressing rapidly. The precise mechanism of steroid hormone action should soon be completely established.

Immunocytology with microwave-fixed fibroblasts shows 1 alpha,25-dihydroxyvitamin D3-dependent rapid and estrogen-dependent slow reorganization of vitamin D receptors

Prior studies have given no evidence for regulation of vitamin D receptor (VDR) compartmentalization or subcellular organization. Microwave fixation (9-15 s) and an indirect immunodetection system of avidin-biotin enhancement and phycoerythrin fluorophore resulted in sufficient spatial and temporal resolution to allow analysis of these processes. We studied cultured fibroblasts from normals or from patients with four different types of hereditary defect compromising VDR function (mutant cells). Compartmentalization of VDRs in the absence of 1,25-dihydroxyvitamin D3 (calcitriol) was regulated by serum or estrogen. VDRs were mainly cytoplasmic in cells cultured without serum and phenol red, but VDRs were mainly intranuclear after addition of serum or an estrogen to cells for at least 18 h (slow regulation). Calcitriol initiated a rapid and multistep process (rapid regulation) of reorganization in a portion of VDRs: clumping within 15-45 s, alignment of clumps along fibrils within 30-45 s, perinuclear accumulation of clumps within 45-90 s, and intranuclear accumulation of clumps within 1-3 min. We found similar rapid effects of calcitriol on VDRs in various other types of cultured cells. These sequential VDR pattern changes showed calcitriol dose dependency and calcitriol analogue specificity characteristic for the VDR. In mutant fibroblasts VDR pattern changes after calcitriol were absent or severely disturbed at selected steps. Treatment of normal cells with wheat germ agglutinin, which blocks protein transport through nuclear pores, also blocked calcitriol-dependent translocation of VDRs. We conclude that immunocytology after microwave fixation provides evidence for regulation of VDR organization and localization.

Progesterone and oestradiol-17 beta receptors in the porcine myometrium during the oestrous cycle

The concentrations of the tissue receptors for oestradiol (E) and progesterone (P) in the porcine myometrium at different stages of the oestrous cycle have been investigated by in vitro binding and exchange methods. Both hormones bound to specific cytoplasmic (Rc) and nuclear (Rn) receptors proteins with high affinity. The amount of ERc did not vary significantly throughout the cycle. Significant variations in the amount of myometrial ERn were observed with high concentrations during late pro-oestrus. The amount of PRc reached a top value in early pro-oestrus. A significant increase in PRn levels was observed at early luteal phase, and it remained high during the rest of the luteal phase. The temporal relationships between the variations in levels of oestradiol and progesterone nuclear receptors in myometrial tissue and those of the circulating plasma levels were established. The data obtained in this study suggest a relationship between the changes in levels of oestradiol and progesterone myometrial binding and the uterine motility pattern during the oestrous cycle.

Identification of unoccupied but transformed nuclear estrogen receptor in cultured mouse Leydig cell

The molecular forms of estrogen receptor (ER) in estrogen-responsive mouse Leydig cell line (B-1) have been examined in relation to their biological activity. ER was predominantly recovered in the nuclear fraction upon homogenization even after cells were precultured in the absence of E2 and Phenol Red. This unoccupied nuclear ER (ERn) whose hormone binding ability was extremely thermostable could be extracted with 0.4 M KCl. This stability enabled us to determine hydrodynamic parameters in the ligand-free condition. The Stokes radius and sedimentation constant of this ERn in high salt condition were 5.5 nm and 6.0S, respectively, resulting in its molecular weight of 140,000. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of ER labeled with [3H]tamoxifen aziridine gave a single band of 65,000 Da, indicating that this ERn had a oligomer structure similar to that of transformed nuclear ER complexed with estrogen in the putative target cells. Therefore, we further examined the possibility that this ERn in B-1 cells can activate estrogen-responsive genes without any aid from estrogen. Estrogen responsive element-thymidine kinase promoter-chloramphenicol acetyltransferase fusion gene (ERE-tk-CAT) was transfected into B-1 cells. CAT activity was enhanced only in cells stimulated with estrogen. It may be concluded from these results that transformed ERn can be formed in the absence of estrogen but that binding to estrogen may be required in order to exert its biological activity.

Immunocytochemical demonstration of mineralocorticoid receptors in rat and human kidney

Using a polyclonal antiserum against the hinge region of the recently cloned human mineralocorticoid receptor (MR) and indirect peroxidase immunohistochemistry, we have shown MR-like immunoreactivity (LI) in superficial nephron segments, including distal convoluted tubule, connecting piece and initial cortical collecting duct. The absence of staining in cells tentatively identified as intercalated cells on light microscopy was confirmed by pre-embedding electron microscopy. Though the intracellular distribution of immunostaining varied with the fixative used, the cellular distribution of MR-LI is in good general agreement with earlier micropuncture and autoradiographic studies.

Concentration and turnover of estradiol in the rat uterus in vivo

The concentrations and turnover of estradiol isolated from cytosolic and nuclear fractions of uteri from ovariectomized rats given estradiol, either in single injections or in continuous infusion, were analyzed by gas chromatography-mass spectrometry. The analytical method was validated for different organs and lower limits of analysis were established. After infusion of 20 ng x h-1 for 18-22 h, mean estradiol levels were 2.0-2.4 fmol x mg-1 uterine wet weight in the nuclear fraction, and 1.2-1.5 fmol x mg-1 in the cytosolic fraction. The concentrations were about five times higher after a single injection of one microgram estradiol but the distribution between nuclear and cytosolic fractions was almost the same. The concentrations of estradiol in nuclei from liver and spleen were 50-200 times lower than those in uterus. Taken together with previous knowledge, the results indicate that the distributions of estradiol and its receptor are not the same and that hormone response cannot be predicted from the concentration of receptors alone. The exchange of estradiol molecules in the uterus was followed after a change of the infusion from unlabelled to [11,12,12-2H3]-labelled estradiol, or vice versa. The uterine uptake of estradiol was calculated to be about 0.7 fmol x h-1 x mg-1 uterine wet weight. The half-life time was calculated to be at least 4 h for estradiol molecules isolated from the nuclear fraction and 3 h (significantly shorter) for those isolated from the cytosolic fraction. The results indicate an uptake of 40-90% of all estradiol passing through the uterus in proestrus with only about 10% of available receptors becoming occupied. When the infusion was changed from estradiol to ethynylestradiol, estradiol disappeared from the uterus at the same rate as in the experiments above. Ethynylestradiol was taken up at a rate of about 0.3-0.4 fmol x h-1 x mg-1 tissue. The percentage of total steroid found in the nuclear fraction was higher for ethynylestradiol, about 70%, than for estradiol, about 60%, indicative of a more stable association of receptor to nuclear binding sites when ethynylestradiol is the ligand.

A monoclonal antibody to a 48 K antigen in oestrogen-dependent human breast cancer cells

A mouse was immunised with an antigen(s) purified by oestradiol-Sepharose affinity chromatography of pooled oestrogen-receptor positive cytosols from human breast cancer tissue. One antibody secreting clone was identified which precipitated labelled antigen and which also stained MCF-7 cells. Culture supernatant and ascites fluid were used for immunofluorescence, SDS-PAGE-Western blotting, photoaffinity labelling and binding studies. The antibody staining of MCF-7 cells was inhibited by preincubation in oestrogen-receptor positive cytosol but was unaffected by oestrogen-receptor negative cytosol. MCF-7 cells stained whether cultured in the presence or absence of oestradiol. The oestrogen-receptor negative cell lines MDA-MB-231 and MDA-MB-330 did not stain. Binding studies with 16-alpha-iodooestradiol using breast cancer tissue cytosols followed by immunoprecipitation showed activity only with oestrogen-receptor positive cytosols with optimal binding activity at 4 degrees C, unaffected by molybdate, but reduced at 25 degrees C or in the presence of 0.4 M KCl. Binding studies with MCF-7, MDA-MB-231 and MDA-MB-330 cytosols and nuclear fractions only showed activity with the MCF-7 cytosol and MCF-7 particulate fractions. The antibody recognised a 48 K species in both MCF-7 cytosol and nuclear fractions but not in the cytosol and nuclear extracts of oestrogen-receptor negative cell lines. Photoaffinity labelling using 16 alpha-iodooestradiol suggests the 48 K antigen does not bind oestradiol directly. The relationship of this antigen to the classical oestrogen-receptor and receptor complex awaits further clarification.