Antiestrogen can establish nonproductive receptor complexes and alter chromatin structure at target enhancers

Systems Structural Biology Analysis of Ligand Effects on ERα Predicts Cellular Response to Environmental Estrogens and Anti-hormone Therapies

Environmental estrogens and anti-hormone therapies for breast cancer have diverse tissue- and signaling-pathway-selective outcomes, but how estrogen receptor alpha (ERα) mediates this phenotypic diversity is poorly understood. We implemented a statistical approach to allow unbiased, parallel analyses of multiple crystal structures, and identified subtle perturbations of ERα structure by different synthetic and environmental estrogens. Many of these perturbations were in the sub-Å range, within the noise of the individual structures, but contributed significantly to the activities of synthetic and environmental estrogens. Combining structural perturbation data from many structures with quantitative cellular activity profiles of the ligands enabled identification of structural rules for ligand-specific allosteric signaling-predicting activity from structure. This approach provides a framework for understanding the diverse effects of environmental estrogens and for guiding iterative medicinal chemistry efforts to generate improved breast cancer therapies, an approach that can be applied to understanding other ligand-regulated allosteric signaling pathways.

Ligand-binding dynamics rewire cellular signaling via estrogen receptor-α

Ligand-binding dynamics control allosteric signaling through the estrogen receptor-α (ERα), but the biological consequences of such dynamic binding orientations are unknown. Here, we compare a set of ER ligands having dynamic binding orientation (dynamic ligands) with a control set of isomers that are constrained to bind in a single orientation (constrained ligands). Proliferation of breast cancer cells directed by constrained ligands is associated with DNA binding, coactivator recruitment and activation of the estrogen-induced gene GREB1, reflecting a highly interconnected signaling network. In contrast, proliferation driven by dynamic ligands is associated with induction of ERα-mediated transcription in a DNA-binding domain (DBD)-dependent manner. Further, dynamic ligands displayed enhanced anti-inflammatory activity. The DBD-dependent profile was predictive of these signaling patterns in a larger diverse set of natural and synthetic ligands. Thus, ligand dynamics directs unique signaling pathways, and reveals a novel role of the DBD in allosteric control of ERα-mediated signaling.

Bacterial biosensors for screening isoform-selective ligands for human thyroid receptors α-1 and β-1

Subtype-selective thyromimetics have potential as new pharmaceuticals for the prevention or treatment of heart disease, high LDL cholesterol and obesity, but there are only a few methods that can detect agonistic behavior of TR-active compounds. Among these are the rat pituitary GH₃ cell assay and transcriptional activation assays in engineered yeast and mammalian cells. We report the construction and validation of a newly designed TRα-1 bacterial biosensor, which indicates the presence of thyroid active compounds through their impacts on the growth of an engineered Escherichia coli strain in a simple defined medium. This biosensor couples the configuration of a hormone receptor ligand-binding domain to the activity of a thymidylate synthase reporter enzyme through an engineered allosteric fusion protein. The result is a hormone-dependent growth phenotype in the expressing E. coli cells. This sensor can be combined with our previously published TRβ-1 biosensor to detect potentially therapeutic subtype-selective compounds such as GC-1 and KB-141. To demonstrate this capability, we determined the half-maximal effective concentration (EC₅₀) for the compounds T₃, Triac, GC-1 and KB-141 using our biosensors, and determined their relative potency in each biosensor strain. Our results are similar to those reported by mammalian cell reporter gene assays, confirming the utility of our assay in identifying TR subtype-selective therapeutics. This biosensor thus provides a high-throughput, receptor-specific, and economical method (less than US$ 0.10 per well at laboratory scale) for identifying important therapeutics against these targets.

Cell cycle and anti-estrogen effects synergize to regulate cell proliferation and ER target gene expression

Antiestrogens are designed to antagonize hormone induced proliferation and ERalpha target gene expression in mammary tumor cells. Commonly used drugs such as OH-Tamoxifen and ICI 182780 (Fulvestrant) block cell cycle progression in G0/G1. Inversely, the effect of cell cycle stage on ER regulated gene expression has not been tested directly. We show that in ERalpha-positive breast cancer cells (MCF-7) the estrogen receptor gene and downstream target genes are cell cycle regulated with expression levels varying as much as three-fold between phases of the cell cycle. Steroid free culture conditions commonly used to assess the effect of hormones or antiestrogens on gene expression also block MCF-7 cells in G1-phase when several ERalpha target genes are overexpressed. Thus, cell cycle effects have to be taken into account when analyzing the impact of hormonal treatments on gene transcription. We found that antiestrogens repress transcription of several ERalpha target genes specifically in S phase. This observation corroborates the more rapid and strong impact of antiestrogen treatments on cell proliferation in thymidine, hydroxyurea or aphidicolin arrested cells and correlates with an increase of apoptosis compared to similar treatments in lovastatin or nocodazol treated cells. Hence, cell cycle effects synergize with the action of antiestrogens. An interesting therapeutic perspective could be to enhance the action of anti-estrogens by associating hormone-therapy with specific cell cycle drugs.

A Bacterial Biosensor of Endocrine Modulators

The nuclear hormone receptors comprise one of the largest classes of protein targets for drug discovery, as their function has been linked to a variety of serious diseases, including several forms of cancer. Identifying novel compounds with the ability to modulate the function of these targets could lead to the development of effective therapeutics. In vivo sensors of ligand binding have emerged as tools that can greatly accelerate the lead identification process, allowing new drugs to be discovered more rapidly and cheaply. In this work, a novel sensor of nuclear hormone binding has been developed in Escherichia coli by constructing a fusion of the ligand-binding domain of the human estrogen receptor with a thymidylate synthase enzyme (TS). Expression of this fusion protein in TS-deficient bacterial cells resulted in growth phenotypes that were dependent on the presence of estrogen. Subsequent replacement of the estrogen receptor with the ligand-binding domain of the human thyroid hormone receptor led to specific thyroid hormone-enhanced growth that was insensitive to estrogen. This biosensor was then challenged with a small library of estrogen and thyroid hormone analogues, and it was observed that levels of cell growth correlate well with ligand-binding affinity. Remarkably, this simple biosensor was able to discriminate between agonistic and antagonistic activities, as combinations of estrogen agonists had an additive impact on cell growth, whereas known estrogen antagonists were found to neutralize agonist effects. This system constitutes a technique for facile selection of lead compounds with potential medical applications.

Dominant-negative nuclear receptor corepressor relieves transcriptional inhibition of retinoic acid receptor but does not alter the agonist/antagonist activities of the tamoxifen-bound estrogen receptor

Repression of the transcriptional activities of the estrogen receptor (ER) is a main goal in the treatment of breast cancer. The antiestrogen tamoxifen is an effective therapy for breast cancer patients because it inhibits estrogen-stimulated gene expression and cell proliferation. Previous studies have implicated a complex containing the nuclear receptor corepressor (N-CoR) in the mechanism by which tamoxifen represses ER-mediated transcriptional activity. In the present study a truncated N-CoR construct was used to inhibit endogenous N-CoR activity in an ER-positive breast cancer cell line. This dominant-negative N-CoR was successful in relieving repression conferred by the unliganded retinoic acid receptor, but it failed to affect the transcriptional activity of the ER in the presence of tamoxifen. Correspondingly, the histone acetylation levels of nucleosomes on endogenous estrogen-responsive genes were unaltered in cells expressing the N-CoR dominant-negative, regardless of ligand. In addition, in vitro cell proliferation and in vivo tumor growth were unchanged in cells that express dominant-negative N-CoR. In conclusion, these results may reveal that N-CoR affects tamoxifen-liganded ER in a manner distinct from its influence on retinoic acid receptor-mediated transcriptional activity or that corepressors other than N-CoR may be involved in the ability of tamoxifen to repress estrogen-responsive transcription and tumor growth.

Current status of estrogen receptors

Increasing knowledge on structure and function of estrogen receptors is providing information on the mechanism of action of estrogen agonists, as well as antagonists, and in understanding their tissue-selective action. However, there are still many factors associated with estrogen response which are poorly understood. Therefore, the task of designing a tissue-selective estrogen for use as a pharmaceutical in estrogen-dependent disorders remains an uncertain game. This review provides information on the current status of estrogen receptors for a better understanding.

SKF-82958 is a subtype-selective estrogen receptor-alpha (ERalpha ) agonist that induces functional interactions between ERalpha and AP-1

The transcriptional activity of estrogen receptors (ERs) can be regulated by ligands as well as agents such as dopamine, which stimulate intracellular signaling pathways able to communicate with these receptors. We examined the ability of SKF-82958 (SKF), a previously characterized full dopamine D1 receptor agonist, to stimulate the transcriptional activity of ERalpha and ERbeta. Treatment of HeLa cells with SKF-82958 stimulated robust ERalpha-dependent transcription from an estrogen-response element-E1b-CAT reporter in the absence of estrogen, and this was accompanied by increased receptor phosphorylation. However, induction of ERbeta-directed gene expression under the same conditions was negligible. In our cell model, SKF treatment did not elevate cAMP levels nor enhance transcription from a cAMP-response element-linked reporter. Control studies revealed that SKF-82958, but not dopamine, competes with 17beta-estradiol for binding to ERalpha or ERbeta with comparable relative binding affinities. Therefore, SKF-82958 is an ERalpha-selective agonist. Transcriptional activation of ERalpha by SKF was more potent than expected from its relative binding activity, and further examination revealed that this synthetic compound induced expression of an AP-1 target gene in a tetradecanoylphorbol-13-acetate-response element (TRE)-dependent manner. A putative TRE site upstream of the estrogen-response element and the amino-terminal domain of the receptor contributed to, but were not required for, SKF-induced expression of an ERalpha-dependent reporter gene. Overexpression of the AP-1 protein c-Jun, but not c-Fos, strongly enhanced SKF-induced ERalpha target gene expression but only when the TRE was present. These studies provide information on the ability of a ligand that weakly stimulates ERalpha to yield strong stimulation of ERalpha-dependent gene expression through cross-talk with other intracellular signaling pathways producing a robust combinatorial response within the cell.

N-Myc shares cellular functions wiht c-Myc

N-Myc is a member of the myc family of proto-oncogenes involved in initiation and progression of tumors. While c-MYC, the most characterized member of the family, is well known for its role in cellular proliferation and apoptosis, the function of N-MYC in differentiation and proliferation remains unclear. N-Myc mutant mice present a phenotype more consistent with a role of N-MYC protein in proliferation of precursor populations than in differentiation per se. Recent studies have also shown that N-MYC can enhance apoptosis and shorten the G1 phase of the cell cycle. However, the role of N-MYC in instigating cell-cycle progression has not been clearly demonstrated. Here, we demonstrate that overexpression of N-myc or activation of inducible N-MYC proteins is sufficient to induce apoptosis in serum-starved fibroblast cells, an effect that can be counteracted by overexpression of Bcl-2. Moreover, N-MYC can induce the reentry of quiescent cells into the cell cycle even in the absence of external stimuli. These results indicate that N-MYC and c-MYC share many properties, supporting the model that MYC-specific roles during embryonic development are mediated, at least in part, via their specific profile of expression rather than by their different protein functions.

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

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

Phosphorylation of human estrogen receptor alpha by protein kinase A regulates dimerization

Phosphorylation provides an important mechanism by which transcription factor activity is regulated. Estrogen receptor alpha (ERalpha) is phosphorylated on multiple sites, and stimulation of a number of growth factor receptors and/or protein kinases leads to ligand-independent and/or synergistic increase in transcriptional activation by ERalpha in the presence of estrogen. Here we show that ERalpha is phosphorylated by protein kinase A (PKA) on serine-236 within the DNA binding domain. Mutation of serine-236 to glutamic acid prevents DNA binding by inhibiting dimerization by ERalpha, whereas mutation to alanine has little effect on DNA binding or dimerization. Furthermore, PKA overexpression or activation of endogenous PKA inhibits dimerization in the absence of ligand. This inhibition is overcome by the addition of 17beta-estradiol or the partial agonist 4-hydroxy tamoxifen. Interestingly, treatment with the complete antagonist ICI 182,780 does not overcome the inhibitory effect of PKA activation. Our results indicate that in the absence of ligand ERalpha forms dimers through interaction between DNA binding domains and that dimerization mediated by the ligand binding domain only occurs upon ligand binding but that the complete antagonist ICI 182,780 prevents dimerization through the ligand-binding domain. Heterodimer formation between ERalpha and ERbeta is similarly affected by PKA phosphorylation of serine 236 of ERalpha. However, 4-hydroxytamoxifen is unable to overcome inhibition of dimerization by PKA. Thus, phosphorylation of ERalpha in the DNA binding domain provides a mechanism by which dimerization and thereby DNA binding by the estrogen receptor is regulated.

Direct visualization of the human estrogen receptor alpha reveals a role for ligand in the nuclear distribution of the receptor

The human estrogen receptor alpha (ER alpha) has been tagged at its amino terminus with the S65T variant of the green fluorescent protein (GFP), allowing subcellular trafficking and localization to be observed in living cells by fluorescence microscopy. The tagged receptor, GFP-ER, is functional as a ligand-dependent transcription factor, responds to both agonist and antagonist ligands, and can associate with the nuclear matrix. Its cellular localization was analyzed in four human breast cancer epithelial cell lines, two ER+ (MCF7 and T47D) and two ER- (MDA-MB-231 and MDA-MB-435A), under a variety of ligand conditions. In all cell lines, GFP-ER is observed only in the nucleus in the absence of ligand. Upon the addition of agonist or antagonist ligand, a dramatic redistribution of GFP-ER from a reticular to punctate pattern occurs within the nucleus. In addition, the full antagonist ICI 182780 alters the nucleocytoplasmic compartmentalization of the receptor and causes partial accumulation in the cytoplasm in a process requiring continued protein synthesis. GFP-ER localization varies between cells, despite being cultured and treated in a similar manner. Analysis of the nuclear fluorescence intensity for variation in its frequency distribution helped establish localization patterns characteristic of cell line and ligand. During the course of this study, localization of GFP-ER to the nucleolar region is observed for ER- but not ER+ human breast cancer epithelial cell lines. Finally, our work provides a visual description of the "unoccupied" and ligand-bound receptor and is discussed in the context of the role of ligand in modulating receptor activity.

TLS (translocated-in-liposarcoma) is a high-affinity interactor for steroid, thyroid hormone, and retinoid receptors

Nuclear receptors for steroid hormones, thyroid hormone, retinoids, and vitamin D are thought to mediate their transcriptional effects in concert with coregulator proteins that modulate receptor interactions with components of the basal transcription complex. In an effort to identify potential coregulators, receptor fusions with glutathione-S-transferase were used to isolate proteins in nuclear extracts capable of binding nuclear hormone receptors. Glutathione-S-transferase fusions with mouse retinoid X receptor-alpha enabled the selective isolation of a 65-kDa protein (p65) from nuclear extracts of rat and human cells. Binding of p65 to mouse retinoid X receptor-alpha was centered around the DNA-binding domain. p65 also bound regions encompassing the DNA-binding domain in estrogen, thyroid hormone, and glucocorticoid receptors. p65 was identified as TLS (translocated-in-liposarcoma), a recently identified member of the RNP family of nuclear RNA-binding proteins whose members are thought to function in RNA processing. The N-terminal half of TLS bound to thyroid hormone receptor with high affinity while the receptor was bound to appropriate DNA target sites. Functional studies indicated that the N-terminal half of TLS can interact with thyroid hormone receptor in vivo. TLS was originally discovered as part of a fusion protein arising from a chromosomal translocation causing human myxoid liposarcomas. TLS contains a potent transactivation domain whose translocation-induced fusion with a DNA-binding protein (CHOP) yields a powerful transforming oncogene and transcription factor. The transactivation and RNA-binding properties of TLS and the nature of its interaction with nuclear receptors suggest a novel role in nuclear receptor function.

Nucleosome positioning and transcription-associated chromatin alterations on the human estrogen-responsive pS2 promoter

The positioning of nucleosomes on a promoter is a significant determinant in its responsiveness to inducing signals. We have mapped the chromatin structure of the human, estrogen-responsive pS2 promoter at nucleotide level resolution within the context of its normal genomic location in human mammary epithelial cells. In vivo digestion by nucleases followed by ligation-mediated polymerase chain reaction analysis revealed two rotationally phased and translationally positioned nucleosomes within the promoter between nucleotide positions -450 and +7. The estrogen response elements at -400 and TATAA box at -35 are each located at the edge of a nucleosome. The two precisely positioned nucleosomes exist in both transformed and nontransformed human mammary epithelial cells, regardless of estrogen receptor status or transcriptional activity of the gene. However, two structural alterations correlate with the transcriptional potential of the promoter. In MCF-7 cells, in which the pS2 promoter is inducible, the chromatin exhibits an increased sensitivity to DNase I in a region of DNA adjacent to the TATAA box and an additional micrococcal nuclease-hypersensitive site in the linker DNA between the two positioned nucleosomes. We were also able to demonstrate that nucleotides -1100 to +10 of the pS2 promoter are sufficient to determine the positioning of these two nucleosomes. Our results establish the structural features of the chromatin covering the pS2 promoter as well as transcriptionally associated alterations, suggesting how the nucleosomal template influences transcriptional regulation by estrogen receptor.

Chromatin remodeling regulated by steroid and nuclear receptors

Coactivators and corepressors regulate transcription by controlling interactions between sequence-specific transcription factors, the basal transcriptional machinery and the chromatin environment. This review consider the access of nuclear and steroid receptors to chromatin, their use of corepressors and coactivators to modify chromatin structure and the implications for transcriptional control. The assembly of specific nucleoprotein architectures and targeted histone modification emerge as central controlling elements for gene expression.

Hydroxylated polychlorinated biphenyls (PCBs) as estrogens and antiestrogens: structure-activity relationships

The effects of structure on the estrogenicity and antiestrogenicity of hydroxylated polychlorinated biphenyls were investigated using the following estrogen-sensitive assays: competitive binding to the rat and mouse cytosolic estrogen receptor (ER); immature rat and mouse uterine wet weight, peroxidase and progesterone receptor (PR) levels; induction of luciferase activity in HeLa cells stably transfected with a Gal4:human ER chimera and a 17mer-regulated luciferase reporter gene; proliferation of MCF-7 human breast cancer cells; induction of chloramphenicol acetyl transferase (CAT) activity in MCF-7 cells transiently transfected with a full-length human ER expression plasmid and a plasmid containing an estrogen-responsive vitellogenin A2 promoter linked to a CAT reporter gene. The chemicals synthesized for this study contained a 4-hydroxy group in one ring, a 2- or 3-chloro substituent meta or ortho to the hydroxyl group, and variable substitution (2',3',4',5'-, 2',3',4',6'-, 2',3',5',6'-tetrachloro and 2',4',6'-trichloro) in the chlorophenyl ring. The compounds included: 2,2',3',4',5'- (A), 2,2',3',4',6'- (B), and 2,2',3',5',6'-pentachloro- (C); 2,2',4',6'-tetrachloro-4-biphenylol (D); 2',3,3',4',5'- (E), 2',3,3',4',6'- (F), and 2',3,3',5',6'-pentachloro (G); and 2',3,4',6'-tetrachloro-4-biphenylol (H). With the exception of 2',3,4',6'-tetrachloro-4-biphenylol (H), all of the compounds competitively bound to the mouse and rat ER with relative binding affinities [compared to 17beta-estradiol (E2)] varying from 1.4 x 10(-3) to 5.3 x 10(-5). The structure-ER binding relationships for the hydroxy-PCB congeners were different in the rat and mouse, and no dose-dependent estrogenic activities were observed in the mouse or rat uterus. Several hydroxy-PCB congeners exhibited antiestrogenic activity (primarily in the mouse uterus) and two compounds, 2,2',3',5',6- and 2,2',3',4',6'-pentachloro-4-biphenylol, inhibited E2-induced uterine wet weight, PR binding, and peroxidase activity in the mouse uterus. 2,2',3',4',5'- and 2,2',3',4',6'-Pentachloro-4-biphenylol induced CAT activity in MCF-7 cells transiently transfected with the Vit-CAT plasmid; the remaining congeners did not induce CAT activity but exhibited antiestrogenic activity in MCF-7 cells cotreated with 10(-9) E2 plus 10(-5) M hydroxy-PCBs. Complementary structure-estrogenicity relationships were observed utilizing the HeLa cell luciferase induction and MCF-7 cell proliferation assays. The placement of the 2- or 3-chloro groups in the phenolic ring had minimal effects on estrogenic activity, whereas 2,4,6-trichloro- and 2,3,4,6-tetrachloro substitution in the chlorophenyl ring (B, D, F, and H) were required for this response. Substitution in the phenolic ring was also not important for structure-antiestrogenicity relationships, and the most active compounds (A, C, E, and G) contained 2',3',4',5'- and 2',3',5',6'-tetrachlorophenyl groups. Thus, structure-estrogenicity/antiestrogenicity relationships for this series of hydroxy-PCBs were complex and response-specific.

Study of the action of tamoxifen on the mammary gland epithelium of premenopausal patients by lysosome quantification

Tamoxifen is an antiestrogen drug widely utilized for the adjuvant hormonal treatment of breast carcinoma. Its use in the primary prophylaxis of this disease is currently being proposed. Although the drug has few side effects, its precise action on breast tissue that has not undergone neoplastic transformation has not been fully elucidated. This prospective, randomized study assessed the estrogen activity of tamoxifen on the mammary gland epithelium of premenopausal patients using a quantitative analysis of mammary epithelium lysosome identified by the cytochemical technique of GOMORI for acid phosphatase and by light microscopy. Tamoxifen significantly increased the number of lysosomes only during the secretory phase of the menstrual cycle. We concluded that the early effect of the drug on normal mammary tissue is synergistic with the effect of estrogen during the premenopausal period.

Regulation of tyrosine aminotransferase gene expression by glucocorticoids in quiescent and regenerating liver

Following 70% hepatectomy, the induction of tyrosine amino-transferase mRNA by glucocorticoids was marginal at 1.5 h, significantly impaired between 3 and 8 h and, at 16 h post-hepatectomy, reached a value approx. 5-fold the basal level, similar to the level observed in quiescent liver. The fold induction of the mRNA was accounted for by a similar fold activation of transcription of the gene by glucocorticoids in regenerating but not in quiescent liver; in the latter, activation of transcription was marginal in spite of glucocorticoid-induced hypersensitivity to cleavage by DNase I at the glucocorticoid-dependent enhancer of the gene. The possibility that in quiescent liver glucocorticoids act at a transcriptional step beyond initiation, increasing the rate of elongation or overcoming a blockage in elongation, was excluded. However, a similar fold induction was determined for total and nuclear tyrosine aminotransferase mRNA in the presence of glucocorticoids, suggesting that in quiescent liver glucocorticoids promote efficient maturation of the tyrosine aminotransferase primary transcript. Thus a glucocorticoid-induced nuclear post-transcriptional up-regulation apparently compensates for impaired activation of transcription of the tyrosine aminotransferase gene by glucocorticoids in quiescent liver.

Direct study of DNA-protein interactions in repressed and active chromatin in living cells

Current methods for analysis of chromatin architecture are invasive, utilizing chemicals or nucleases that damage DNA, making detection of labile constituents and conclusions about true in vivo structure problematic. We describe a sensitive assay of chromatin structure which is performed in intact, living yeast. The approach utilizes expression of SssI DNA methyltransferase (MTase) in Saccharomyces cerevisiae to provide an order-of-magnitude increase in resolution over previously introduced MTases. Combining this resolution increase with the novel application of a PCR-based, positive chemical display of modified cytosines provides a significant advance in the direct study of DNA-protein interactions in growing cells that enables quantitative footprinting. The validity and efficacy of the strategy are demonstrated in mini-chromosomes, where positioned nucleosomes and a labile, operator-bound repressor are detected. Also, using a heterologous system to study gene activation, we show that in vivo hormone occupancy of the estrogen receptor is required for maximal site-specific DNA binding, whereas, at very high receptor-expression levels, hormone-independent partial occupancy of an estrogen-responsive element was observed. Receptor binding to a palindromic estrogen-responsive element leads to a footprint with strand-specific asymmetry, which is explicable by known structural information.

The future of antihormone therapy: innovations based on an established principle

Endocrine therapy of mammary and prostate cancer has been established for decades. The therapies available to block sex-hormone-receptor-mediated tumor growth are based on two principles: (i) ligand depletion, which can be achieved surgically, by use of luteinizing-hormone-releasing hormone analogues or inhibitors of enzymes involved in steroid biosynthesis or by interfering with the feedback mechanisms of sex hormone synthesis at the pituitary/hypothalamic level; (ii) blockade of sex hormone receptor function by use of antihormones. The antiestrogen tamoxifen, which is the compound of choice for the treatment of mammary carcinoma, has the drawback of being a partial agonist. A complete blockade of estrogen receptor (ER) function can be achieved by a new class of compounds, pure antiestrogens. In contrast to aromatase inhibitors, pure antiestrogens are able to block ER activation by ligands other than estradiol and can also interfere with ligand-independent ER activation. In addition to estradiol, progesterone has a strong proliferative effect in mammary carcinomas. Antiprogestins are promising new tools for clinical breast cancer therapy. These compounds clearly need a functionally expressed progesterone receptor to block tumor growth, but there is strong experimental evidence that their tumor inhibition is based on more than just progesterone antagonism. The ability of these compounds to induce tumor cell differentiation that leads to apoptosis is unique among all other endocrine therapeutics. In prostate tumors that have relapsed from current androgen-ablation therapies the androgen receptor (AR) is still expressed and, compared to the primary tumors, its level is often even enhanced. Mutated AR that can be activated by other compounds such as adrenal steroids, estrogens, progestins and even antiandrogens have been detected in recurrent tumors. Thus, relapse of tumors under the selective pressure of common androgen-ablation therapies can be caused by acquired androgen hypersensitivity and AR activation by ligands other than (dihydro-)testosterone. There is a clinical need for future compounds that produce a complete blockade of AR activity even in recurrent tumors. Preclinical experiments indicate that combination therapy as well as the extension of endocrine treatments to several other tumor entities are promising approaches for further developments. Examples are the combination of antiestrogens and antiprogestins for breast cancer treatment, or the treatment of prostate carcinomas with antiprogestins.

NF-M (chicken C/EBP beta) induces eosinophilic differentiation and apoptosis in a hematopoietic progenitor cell line

CAAT/enhancer binding proteins (C/EBPs) are transcriptional activators implicated in the differentiation processes of various cell lineages. We have shown earlier that NF-M, the chicken homolog of C/EBP beta, is specifically expressed in myelomonocytic and eosinophilic cells of the hematopoietic system. To investigate the role of NF-M in hematopoietic cell lineage commitment, we constructed a conditional form of the protein by fusing it to the hormone binding domain of the human estrogen receptor. This construct was stably expressed in a multipotent progenitor cell line transformed by the Myb-Ets oncoprotein. We report here that both NF-M-dependent promoter constructs and resident genes could be activated by addition of beta-estradiol to the NF-M-estrogen receptor expressing progenitors. At the same time, we observed a down-regulation of progenitor-specific surface markers and the up-regulation of differentiation markers restricted to the eosinophil and myeloid lineages. In addition to the onset of differentiation, cell death was induced with typical apoptotic features. Our results suggest that NF-M plays an important role in commitment along the eosinophil lineage and in the induction of apoptosis.

Ligand-induced conformational alterations of the androgen receptor analyzed by limited trypsinization. Studies on the mechanism of antiandrogen action

Limited proteolysis of in vitro produced human androgen receptor was used to probe the different conformations of the receptor after binding of androgens and several antiandrogens. The results provide evidence for five different conformations of the receptor, as detected by the formation of proteolysis resisting fragments: 1) an initial conformation of the unoccupied receptor not resisting proteolytic attack; and receptor conformations characterized by 2) a 35-kDa proteolysis resisting fragment spanning the ligand binding domain and part of the hinge region, obtained with most antagonists, and in an initial step after agonist binding; 3) a 29-kDa proteolysis resisting fragment spanning the ligand binding domain, obtained in the presence of agonists after an activation process; 4 and 5) 30- and 25-kDa fragments, derived from 2 and 3, but missing part of the C terminus, obtained with RU486 (RU486 has antiandrogenic properties, besides its effects as an antiprogestagen/antiglucocorticoid). Concomitantly with the change from 2 to 3 (and of 4 to 5 for RU486), dissociation of the 8 S complex of receptor with associated proteins occurred. With a mutant receptor (LNCaP cell mutation in C-terminal region), some antagonists activated transcription analogous to agonists, and induced the activated receptor conformation 3. A mutant lacking the C-terminal 12 amino acids bound RU486 but not androgens, and formed with RU486 conformation 5. These data imply that, after the initial rapid binding of ligand, androgens induce a conformational change of the receptor, a process that also involves release of associated proteins. RU486 induces an inappropriate conformation of the C-terminal end, similar as found for its effect on the progesterone receptor. In contrast, the other antiandrogens act at a different step in the mechanism of action: they do not induce an abnormal conformation, but act earlier and prevent a conformation change by stabilizing a complex with associated proteins.

Differential functional activities of rainbow trout and human estrogen receptors expressed in the yeast Saccharomyces cerevisiae

The cDNA of rainbow trout estrogen receptor (rtER), highly and stably expressed in yeast, Saccharomyces cerevisiae, was used to analyse the biological activity of the receptor. The rtER mRNA encoded a 65-kDa protein which was immunorevealed by a specific antibody and migrated with the authentic rtER major protein form detected in trout liver. Yeast rtER bound estradiol with high affinity and the dissociation constant (Kd = 1.35 nM) was very similar to the value measured from trout liver extracts but 3-5-fold higher than the Kd found for human estrogen receptor (hER). This indicates therefore that the rtER has a lower estradiol affinity compared to the human receptor. While the hER Kd remained unchanged at both 4 degrees C or 22 degrees C, it was slightly modified at 30 degrees C. The Kd measured for rtER at 22 degrees C and 30 degrees C were about 2-fold, and 12-fold higher, respectively, than the Kd obtained at 4 degrees C suggesting an alteration of the rtER affinity for its ligand at elevated temperature. To examine the estrogen-receptor-mediated activation of transcription in yeast, reporter plasmids integrated or not in the yeast genome were used. The reporter genes consist of one, two, or three copies of estrogen-responsive elements (ERE) upstream of the yeast proximal CYC1 or URA3 promoters fused to the lacZ gene of Escherichia coli coding for beta-galactosidase. The induction of beta-galactosidase activity for all reporter genes was strictly dependent on the presence of rtER and estrogens. The activation of transcription mediated by rtER responded in an estradiol-dose-dependent manner as in animal cells. However, compared to hER, the estradiol concentration necessary to achieve maximal activation was 10-fold higher. This is probably a consequence of the lower estradiol-affinity for rtER compared to hER. The levels of induction of the reporter genes containing two or three ERE were strongly enhanced compared to the one ERE construct. This is in agreement with the synergistic effect previously described for multiple ERE. The magnitudes of transcriptional induction mediated by rtER and hER were similar when the reporter gene containing three ERE was used but changed when the one ERE construct was used. In this case transcriptional activation indicated by rtER was 10-20 fold lower. This suggests that rtER requires protein/protein interaction for its stabilization on DNA. Antiestrogens were able to bind rtER and promote gene transcription. However, to produce effects comparable to those obtained with estrogens, much higher concentrations were required. This may imply nonetheless that antihormones were capable of provoking efficient interactions of rtER with the transcriptional machinery.

Tissue-specific regulation of the insulin gene by a novel basic helix-loop-helix transcription factor

The insulin gene is one of the best paradigms of tissue-specific gene expression. It is developmentally regulated and is expressed exclusively in the pancreatic beta-cell. This restricted expression is directed by a tissue-specific enhancer, within the promoter, which contains an E-box sequence. The insulin E-box binds an islet-specific protein complex, termed 3a1. E-boxes bind proteins belonging to the basic helix-loop-helix (bHLH) family of transcription factors. The bHLH proteins function as potent transcriptional activators of tissue-specific genes by forming heterodimers between ubiquitous and cell-restricted family members. In addition, the cell-restricted bHLH members play an important role in specifying cell fate. To isolate the tissue-specific bHLH factor controlling insulin gene expression and study its role in islet cell differentiation, a modified yeast two-hybrid system was utilized to clone a novel bHLH factor, BETA2 (beta-cell E-box trans-activator 2), from a hamster insulin tumor (HIT) cell cDNA library. Northern analysis demonstrates that high-level expression of the BETA2 gene is restricted to pancreatic alpha- and beta-cell lines. As expected of tissue-specific bHLH members, BETA2 binds to the insulin E-box sequence with high affinity as a heterodimer with the ubiquitous bHLH factor E47. More importantly, antibody supershift experiments clearly show that BETA2 is a component of the native insulin E-box-binding complex. Transient transfection assays demonstrate that the BETA2/E47 heterodimer synergistically interacts with a neighboring beta-cell-specific complex to activate an insulin enhancer. In contrast, other bHLH factors such as MyoD and E47, which can bind to the insulin E-box with high affinity, fail to do so. Thus, a unique, cooperative interaction is the basis by which the insulin E-box enhancer discriminates between various bHLH factors to achieve tissue-specific activation of the insulin gene.

A-ring nitro- and amino-substituted estradiol analogs produce a negative cooperative or noncooperative [3H]estradiol-estrogen receptor binding mechanism

We have investigated the relation between ligand structure and binding mechanism between the calf uterine estrogen receptor. A series of structurally altered estradiol analogs was used in which either an amino- or a nitro group had been added to the 2 or 4 position on the phenolic A-ring. The binding affinity of both amino analogs and the 4-nitro analog for the estrogen receptor was reduced relative to that of estradiol, as measured by competitive binding assay; the values were between 0.008% and 8% of estradiol's affinity. The slope of the displacement curve for the 4-nitro analog was also significantly different from that of estradiol (p < 0.05), indicating that the binding mechanism of these two ligands was different. The affinity of the 2-nitroestradiol ligand for the receptor was too low to be measured. The binding mechanism was then further investigated by measuring the Hill coefficient of [3H]estradiol binding in the presence of the analog. The presence of a nitro group on C4 eliminated the positive cooperativity of the [3H]estradiol-estrogen receptor interaction; the Hill coefficient of [3H]estradiol binding in the presence of the analog was 0.99 compared with 1.7 for [3H]estradiol alone. Most interestingly, the presence of an amino group on either C2 or C4 brought about a switch from a positive to a negative cooperative binding interaction; the Hill coefficients of [3H]estradiol binding in the presence of the analogs were between 0.6 and 0.7. These results provide additional support for an induced-fit mechanism of ligand-estrogen receptor interactions.

Molecular mechanisms of antiestrogen action in breast cancer

The success of antiestrogen therapy to treat all stages of breast cancer, and the evaluation of tamoxifen as a preventive for breast cancer in normal women, have focused attention on the molecular mechanisms of antiestrogen action and mechanisms of drug resistance. The overall goal of research is to enhance current therapies and to develop new approaches for breast cancer treatment and prevention. Recent studies show that tamoxifen and the new pure antiestrogens appear to have different mechanisms of action: tamoxifen and related compounds cause a change in the folding of the steroid binding domain that prevents gene activation whereas the pure antiestrogens cause a reduced interaction at response elements and cause a rapid loss of receptor complexes. Tamoxifen treatment produces changes in the cellular and circulating levels of growth factors that could influence both receptor negative or receptor positive tumor growth and the metastatic potential of a tumor. These events may explain the survival advantage observed with tamoxifen therapy. However, the current therapeutic challenge is to avoid drug resistance during long-term tamoxifen therapy. Numerous explanations for drug resistance to tamoxifen have been suggested, including elevated estrogen levels, increased tumor antiestrogen binding sites, receptor mutations, and impaired signal transduction. However, it is probable that multiple mechanisms evolve to facilitate tumor survival. Most importantly, current research is examining mechanisms responsible for the beneficial actions of tamoxifen on bones and lipids as well as the potentially deleterious effects of tamoxifen on liver and endometrial carcinogenesis and retinopathy. The urgent need to understand antiestrogenic drug mechanisms and toxicity is being facilitated by the application of the technology developed for basic molecular biology.

Antiestrogen regulation of cell cycle progression and cyclin D1 gene expression in MCF-7 human breast cancer cells

The molecular mechanisms by which antiestrogens inhibit breast cancer cell proliferation are not well understood. Using cultured breast cancer cell lines, we studied the effects of antiestrogens on proliferation and cell cycle progression and used this information to select candidate cell cycle regulatory genes that are potential targets for antiestrogens. Under estrogen- and serum-free conditions antiestrogens inhibited proliferation of MCF-7 cells stimulated with insulin. Cells were blocked at a point in G1 phase. These effects are comparable with those in serum- and estrogen-containing medium and were also seen to a lesser degree in nude mice bearing MCF-7 tumors. Similar observations with other peptide mitogens suggest that the process inhibited by antiestrogens is common to estrogen and growth factor activated pathways. Other studies have identified G1 cyclins as potential targets for growth factor and steroid hormone/steroid antagonist regulation of breast epithelial cell proliferation. In MCF-7 cells growing in the presence of fetal calf serum, cyclin D1 mRNA was rapidly down-regulated by steroidal and nonsteroidal antiestrogens by an apparently estrogen receptor mediated mechanism. Cyclin D1 gene expression was maximally inhibited before effects on entry into S phase and inhibition was therefore not merely a consequence of changes in cell cycle progression. Together with data on the effects of antiestrogens in serum-free conditions [1], these results suggest down-regulation of cyclin D1 by antiestrogens may be a general phenomenon in estrogen receptor-positive breast cancer cells, independent of culture conditions and class of antiestrogen. These observations are compatible with the hypothesis that reductions in cyclin D1 levels may mediate in part the action of antiestrogens in blocking entry of cells into S phase.

The mechanism of action of steroid hormones: a new twist to an old tale

Steroid hormones, vitamins, and thyroid hormone are potent chemical messengers that exert dramatic effects on cell differentiation, homeostasis, and morphogenesis. These molecules, though diverse in structure, share a mechanistically similar mode of action. The effector molecules diffuse across cellular membranes and bind to specific high affinity receptors in the target cell nuclei. This interaction results in the conversion of an inactive receptor to one that can interact with the regulatory regions of target genes and modulate the rate of transcription of specific gene sets. The recent cloning and characterization of the functional receptors for these hormones has been enlightening as to the individual steps involved in steroid signal transduction. In addition, emerging evidence suggests that receptor function can be influenced by cell and promoter context indicating that it may be possible to develop tissue specific or tissue-restricted drugs. The concept that a single receptor can modulate gene transcription in a cell-specific manner is of great medical and pharmaceutical importance. The focus of this review is to highlight the recent developments in the steroid receptor field and to illustrate the novel approaches been undertaken to identify novel pharmaceuticals.

Nuclear hormone receptors as targets for new drug discovery

There are two basic types of receptor transducing systems: those which utilize membrane bound receptors and are activated at the cell surface by the appropriate hormone and transmit their signal to the internae of the cell via a second messenger (i.e. cAMP), and those that utilize internal, cytoplasmic or nuclear receptors (intracellular receptors) which upon activation by hormones interact directly with DNA and alter the genetic program of a cell. This review focuses on the mechanism of action of these intracellular receptors and discusses how such an understanding is expected to facilitate the discovery of new therapeutic agents.

Fourteenth Gaddum Memorial Lecture. A current view of tamoxifen for the treatment and prevention of breast cancer

Tamoxifen has been found to be a safe and effective treatment for all stages of breast cancer. Long term tamoxifen therapy is associated with some rare, but potentially serious, side effects so patients should be carefully monitored. However, long term tamoxifen therapy is also associated with a number of physiological benefits over and above its tumouristatic action. These benefits include a decrease in the development of contralateral breast cancer, the maintenance of bone density in postmenopausal women and a decrease in cardiovascular disease. The successful application of tamoxifen to treat breast cancer has increased enthusiasm to test its worth to prevent breast cancer. Although there are individual requests by patients for tamoxifen to prevent breast cancer, individual treatment is inappropriate. Tamoxifen can only be adequately evaluated as a preventive in randomized, double-blind clinical trials. These trials are in place and physicians should encourage women to participate and establish a new therapeutic option as rapidly as possible.

Structure and function of the estrogen receptor

The hormone binding domain of the estrogen receptor is required not only for binding estradiol but also to form stable homodimers of the protein and mediate transcriptional activation by the receptor. Residues that are essential for estrogen binding are also involved in dimerization, suggesting that the hormone-binding pocket is at or near the dimer interface. Distinct hydrophobic and charged residues are essential for hormone-dependent transcriptional activation, and these appear to be conserved by other members of the nuclear receptor family. We have found that the pure antiestrogens ICI 164384 and ICI 182780 increase the turnover of the receptor compared with that in the presence of estradiol. Because it is likely that the pure antiestrogens bind to a similar region of the receptor as that of estradiol, we propose that they inhibit receptor dimerization by means of their 7 alpha alkyl-amide extension. It appears that as a consequence nuclear uptake is inhibited and the receptor more rapidly degraded in the cytoplasm.

Characterization of the interaction of the human mineralocorticosteroid receptor with hormone response elements

Although the mineralocorticosteroid receptor (MR) belongs to the superfamily of hormone-dependent transcription factors, the molecular mechanism by which it regulates gene expression is poorly understood. Binding of the MR to target gene promoters has never been characterized, and specific mineralocorticosteroid response elements (MREs) remain to be identified. The human MR (hMR) was overexpressed in Sf21 insect cells using the baculovirus system. The high degree of similarity between the glucocorticosteroid receptor (GR) and the MR prompted us to examine the DNA-binding properties of the recombinant MR with glucocorticosteroid-regulated genes. Gel shift mobility assays demonstrated that the recombinant receptor interacted with oligonucleotides containing perfect and imperfect palindromic sequences of GRE. A monoclonal anti-hMR antibody (FD4) induced a supershift of protein-DNA complexes and identified the MR in Western blot analysis. In vitro DNAse I protection assays with the hormone-regulated murine mammary tumour virus promoter showed that recombinant hMR generated four footprints whose limits encompassed the GRE motifs. By means of these two complementary approaches, no difference between the interaction of free, agonist- or antagonist-bound MR and DNA was detected. We provide evidence that hMR functions as a sequence-specific DNA-binding protein.

Inhibition of growth-factor-activated proliferation by anti-estrogens and effects on early gene expression of MCF-7 cells

Recently, it was reported that the anti-estrogen tamoxifen not only inhibits estradiol-stimulated growth of MCF-7 cells but also significantly reduces the proliferation rate of cells stimulated by growth factors. We have confirmed this finding and also shown that the new anti-estrogen droloxifene inhibits the proliferation of epidermal growth factor (EGF) and insulin-like growth factor-I (IGF-I)-stimulated MCF-7 cells. The growth-factor-induced proliferation was inhibited in a dose-dependent manner by the anti-estrogens in the complete absence of estrogen and FCS. Of the anti-estrogens, droloxifene was considerably more potent than tamoxifen. Because the expression of the proto-oncogenes c-fos and c-myc has been considered a key event in development of the mitogenic response, we examined the effects of anti-estrogens on c-myc and c-fos gene expression. We included in these investigations the steroidal anti-estrogen ICI 164,384 because this compound has no or very little estrogenic activity. The studies revealed that all 3 anti-estrogens transiently induced c-myc mRNA expression. However, the anti-estrogens inhibited estradiol-induced c-myc mRNA expression, although with different potencies. Pre-incubation of MCF-7 cells with droloxifene and tamoxifen resulted in elevated levels of growth-factor-induced c-myc mRNA expression. In contrast, the anti-estrogens did not induce c-fos mRNA or affect the expression of c-fos mRNA induced by growth factors. In conclusion, non-steroidal anti-estrogens inhibit growth-factor-stimulated proliferation of MCF-7 cells without inhibiting growth-factor-induced c-myc or c-fos mRNA expression.

Action of "pure" antiestrogens in inhibiting estrogen receptor action

The mechanism of action of the pure antiestrogens ICI 164384 and ICI 182780 has been investigated. Both antagonists are steroidal antiestrogens with 7 alpha-alkylamide side-chains. The antiestrogens reduce the cellular content of the estrogen receptor by reducing the half-life of the protein. A potential mechanism for this effect is suggested by the observation that the DNA binding activity of receptors which have been over-expressed in cells was inhibited in vitro. The inhibitory activity of analogues of ICI 164384 with different side chain lengths correlates with their ability to function as pure antiestrogens in vivo. Since the estrogen binding site overlaps with residues involved in dimerisation, the antiestrogens are likely to bind to a similar site and may therefore with receptor dimerisation in the hormone binding domain by means of the 7 alpha side-chain. We propose that the increased turnover of the receptor in the presence of ICI 164384 and ICI 182380 is a consequence of impaired dimerisation of the proteins.

A model to describe how a point mutation of the estrogen receptor alters the structure-function relationship of antiestrogens

The antiestrogen tamoxifen [(Z)-1(p-beta-dimethylamino-ethoxyphenyl)-1,2- diphenylbut-1-ene] is an effective anticancer agent for the treatment of hormone responsive breast cancer. Previous studies have demonstrated that a point mutation in the estrogen receptor (ER) resulted in an alteration of the pharmacology of 4-hydroxytamoxifen, the active metabolite of tamoxifen (Jiang et al, Mol Endocrinol 6:2167-2174, 1992). We have extended our studies to evaluate the effect of a point mutation, a Val substitution for Gly at amino acid 400 in the ligand binding domain of ER, on the pharmacology of other antiestrogens in ER stable transfectants derived from the ER-negative breast cancer cell line MDA-MB-231 CL10A. The compounds were tested with or without estradiol-17 beta (E2) for their effects on cell growth in cells expressing the wild type ER (S30) or the mutant ER (ML alpha 2H) or in control antisense ER transfectant AS23 which does not express ER protein. MCF-7 cells, which express the wild type ER, were also used as a control. The growth of AS23 cells was not affected by any of the compounds at a concentration of 1 microM. E2 stimulated the growth of MCF-7 cells but inhibited the growth of ER transfectants S30 and ML alpha 2H. The ML alpha 2H cells were about 10 to 100-fold less sensitive to E2 and antiestrogens than S30 and MCF-7 cells. Keoxifene, an antiestrogen with a high affinity for the ER, maintained antiestrogenic activities in both ER transfectants and MCF-7 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Transcription activation by nuclear receptors

Nuclear receptors constitute a superfamily of ligand-inducible transcription factors which respond to endocrine, paracrine and, possibly, autocrine signals. Multiple regulatory mechanisms assure that signal transduction results in an accurate regulation of the respective gene networks. Apart from selective expression of the cognate receptor and its binding to specific hormone response elements of target genes, additional mechanisms are responsible for the cell- and promoter-specific transcription activation. They are based on the "interpretation" of the signal by the multiple functional modules of a given receptor and involve a specific interplay with various factors binding to complex target gene promoters and cell-specific intermediary transcription factors that mediate the activity of the two receptor transcription activation functions, as well as homo- and heterodimerization, and interference with other signalling pathways. Moreover, a single ligand may initiate different gene programs due to the differential target gene specificities of nuclear receptor isoforms. Thus, signal transduction by nuclear receptors involves a multitude of interactive elements, as could have been expected from the central role of these signals in homeostasis, embryonic development and differentiation. Two distinct mechanisms are involved in anti-hormone action. Type I anti-hormones impair the activity of the transcription activation function, while type II antagonists impair DNA binding. Experiments aimed at an understanding of these mechanisms are discussed.

Functions of estrogens and anti-estrogens in the rat endometrial adenocarcinoma cell lines RUCA-I and RUCA-II

Inbred rats of the DA/Han and BDII/Han strains have been proposed as suitable model systems for studying hormonal carcinogenesis, because they die mainly from hormone-dependent endometrial adenocarcinoma. Here we characterize the RUCA-I cell line derived from an endometrial adenocarcinoma of an inbred DA/Han rat and the RUCA-II cell line derived from an endometrial adenocarcinoma of an inbred BDII/Han rat. The RUCA-I cell line, if transplanted to the neck of female DA/Han rats, gives rise to endometrial adenocarcinomas at the ectopic site. The morphology of these ectopically grown tumors is predominantly of the moderately differentiated sub-class. In contrast, ectopic tumor growth of the RUCA-II cell line can be observed only if cells are transplanted to athymic nude mice. Biochemically, both cell lines are characterized by the stable expression of estrogen receptors. However, no statistically significant mitotic response of RUCA-I and RUCA-II cells to estradiol was measurable, and no induction of expression of the progesterone receptor by estradiol was detectable, although estradiol transformed the estrogen receptor into its stable DNA-binding state. In contrast, the rate of proliferation of RUCA-I but not of RUCA-II cells was reduced in the presence of 10(-6) M tamoxifen. From these results we conclude that (i) both cell lines, RUCA-I and RUCA-II, represent a new and promising endometrial tumor model; (ii) the mechanism of the hormone-dependent growth regulation of RUCA-I and RUCA-II cells is obviously impaired; (iii) the RUCA-I cell line appears to be a suitable model system for the study of molecular aspects of estrogen- and tamoxifen-dependent gene expression.

Identification of a negative regulatory function for steroid receptors

This report describes the identification of a negative regulator of estrogen and progesterone receptor function. Using a reconstituted estrogen-responsive transcription system in Saccharomyces cerevisiae, we have identified a "repressor function," which when mutated, increases the transcriptional activity of the estrogen and progesterone receptors. In the case of the estrogen receptor this mutation increases the sensitivity of estrogen-mediated activation by at least four orders of magnitude. Analysis of derivatives of the estrogen receptor indicated that this repressor specifically affects the transcription activity of the TAF1 activation domain of the estrogen receptor. The repressor was cloned by complementation and identified as SSN6, a previously described mediator of glucose repression in yeast. Our results indicate that SSN6 is likely to be involved also in the repression of other cellular activators. Interestingly, deletion of the SSN6 protein allows the antiestrogens ICI 164384 and nafoxidine to behave as more potent agonists of estrogen receptor function, while RU486 also becomes a more potent activator of progesterone receptor function. These data suggest that in wild-type cells the role of hormone is twofold: it promotes DNA binding of the receptor and it also induces a conformational change in the receptor which overcomes the effects of this repressor function.

Examination of the DNA-binding ability of estrogen receptor in whole cells: implications for hormone-independent transactivation and the actions of antiestrogens

We describe an assay employing the competitive binding of estrogen receptor (ER) with basal transcription factors on a constitutive promoter (cytomegalovirus-hormone response element[s]-chloramphenicol acetyltransferase [CMV-(HRE)n-CAT, containing a hormone response element(s) between the TATA box and the start site of transcription]) to examine the DNA-binding ability of the human ER in whole cells. We used this promoter interference assay to examine the DNA binding of ER in cell lines containing high and low levels of endogenous ER, as well as in CHO cells expressing wild-type and mutant ERs from cotransfected expression vectors. The ER is capable of binding to the promoter interference constructs in the absence of added ligand, and estrogen (estradiol) or antiestrogen (trans-hydroxytamoxifen or ICI 164,384) enhances or stabilizes this interaction. The binding of unoccupied ER to reporter gene activation plasmids results in ligand-independent transactivation, presumably due to the TAF-1 function of the receptor. DNA binding of ER in the absence of ligand is observed in cells containing endogenous ER, or expressed ER, and occurs in cells with high or low receptor contents. Although estrogen- and antiestrogen-occupied ER complexes bind to DNA and reduce the template promoter activity, the extent of suppression achieved by ICI-bound ERs is consistently less than that achieved with the other ligands, presumably caused by the fact that ICI rapidly reduces the level of ER in most of the cells examined. However, the ICI-ER complexes that remain are in sufficient quantity to bind to gene activation reporter constructs, and in these cells, ICI still behaves as a pure antagonist of gene transcription and does not activate reporter genes. Hence, obstruction of ER DNA binding or reduction of ER in target cells may contribute to, but cannot fully explain, the pure antagonist character of the antiestrogen ICI 164,384. In addition, DNA binding by the ER alone is clearly not sufficient for ensuring full activation of transcription and argues for an intermediate in the receptor activation of promoters.

Examination of the DNA-binding ability of estrogen receptor in whole cells: implications for hormone-independent transactivation and the actions of antiestrogens

We describe an assay employing the competitive binding of estrogen receptor (ER) with basal transcription factors on a constitutive promoter (cytomegalovirus-hormone response element[s]-chloramphenicol acetyltransferase [CMV-(HRE)n-CAT, containing a hormone response element(s) between the TATA box and the start site of transcription]) to examine the DNA-binding ability of the human ER in whole cells. We used this promoter interference assay to examine the DNA binding of ER in cell lines containing high and low levels of endogenous ER, as well as in CHO cells expressing wild-type and mutant ERs from cotransfected expression vectors. The ER is capable of binding to the promoter interference constructs in the absence of added ligand, and estrogen (estradiol) or antiestrogen (trans-hydroxytamoxifen or ICI 164,384) enhances or stabilizes this interaction. The binding of unoccupied ER to reporter gene activation plasmids results in ligand-independent transactivation, presumably due to the TAF-1 function of the receptor. DNA binding of ER in the absence of ligand is observed in cells containing endogenous ER, or expressed ER, and occurs in cells with high or low receptor contents. Although estrogen- and antiestrogen-occupied ER complexes bind to DNA and reduce the template promoter activity, the extent of suppression achieved by ICI-bound ERs is consistently less than that achieved with the other ligands, presumably caused by the fact that ICI rapidly reduces the level of ER in most of the cells examined. However, the ICI-ER complexes that remain are in sufficient quantity to bind to gene activation reporter constructs, and in these cells, ICI still behaves as a pure antagonist of gene transcription and does not activate reporter genes. Hence, obstruction of ER DNA binding or reduction of ER in target cells may contribute to, but cannot fully explain, the pure antagonist character of the antiestrogen ICI 164,384. In addition, DNA binding by the ER alone is clearly not sufficient for ensuring full activation of transcription and argues for an intermediate in the receptor activation of promoters.

Ligand dependence of estrogen receptor induced changes in chromatin structure

To determine whether the human estrogen receptor requires ligand to bind to its cognate estrogen receptor element (ERE) in vivo, we have examined the structure of chromatin at a chromosomally integrated ERE-URA3 reporter gene in yeast, and the influence of ligand bound and ligand free estrogen receptors on that structure. Using indirect end-labelling to map DNaseI and micrococcal nuclease sensitive sites, we found that receptor induced alterations in chromatin structure were completely dependent upon the presence of estradiol. These same alterations in chromatin structure were induced by a truncated estrogen receptor with both TAF-1 and TAF-2 transactivation functions deleted, suggesting that DNA binding per se disrupts chromatin structure. These results support models in which the estrogen receptor requires ligand to bind to the ERE in vivo.

Effects of antioestrogens on the DNA binding activity of oestrogen receptors in vitro

We have investigated the effect of a series of steroidal oestrogen antagonists, related to ICI 164,384, on the DNA binding activity of mouse oestrogen receptors expressed in insect cells. The analogues possess different side chains at the 7-position of the B ring in the steroid. Inhibition was observed when the length of the side-chain was 15-16 carbon atoms but not 10 or 20 carbon atoms and only when the 7 alpha isomer was used. The DNA binding activity of receptors expressed in COS-1 cells was also inhibited after extended periods of incubation with antioestrogens but not that of the human receptor in breast cancer cell-extracts. We have proposed that ICI 164,384 might disrupt receptor dimerisation, and therefore the variation in its ability to inhibit DNA binding activity may reflect differences in dimer stability. Since the DNA binding activity of in vitro translated receptors was inhibited when they were translated in the presence of the antioestrogen we suggest that ICI 164,384 might prevent the formation of receptor dimers without necessarily being able to disrupt preformed dimers.