William L. McGuire Memorial Lecture. Antiestrogens: mechanisms of action and resistance in breast cancer

Role of estrogen receptor ligand and estrogen response element sequence on interaction with chicken ovalbumin upstream promoter transcription factor (COUP-TF)

Estrogen-responsive genes are regulated by altering the balance of estrogen receptor (ER) interaction with transcription activators and inhibitors. Here we examined the role of ER ligand on ER interaction with the Chicken Ovalbumin Upstream Promoter Transcription Factor (COUP-TF) orphan nuclear receptor. COUP-TF binding to half-site estrogen response elements (EREs) was increased by the addition of estradiol (E2) -liganded ER (E2-ER), but not by ER liganded with the antiestrogens 4-hydroxytamoxifen (4-OHT-ER) or tamoxifen aziridine (TAz-ER). ER did not bind to single half-sites. Conversely, COUP-TF enhanced the ERE binding of purified E2-ER, but did not affect TAz-ER-ERE binding. In contrast, only antiestrogens enhanced direct interaction between ER and COUP-TF as assessed by GST pull-down assays. Identical results were obtained using either purified bovine or recombinant human ERalpha. Co-immunoprecipitation assays showed that ER and COUP-TF interact in extracts from MCF-7 and ERalpha-transfected MDA-MB-231 cells. Here we document that ER ligand impacts COUP-TF-ER interaction. COUP-TF interaction is mediated by the DNA binding and ligand-binding domains of ER. We suggest that changes in ER conformation induced by DNA binding reduce ER-COUP-TF interaction. Transient transfection of human MCF-7 breast cancer cells with a COUP-TFI expression vector repressed E2-induced luciferase reporter gene expression from single or multiple tandem copies of a consensus ERE. COUP-TFI stimulated 4-OHT-induced luciferase activity from a minimal ERE. Alone, COUP-TFI increased transcription from ERE half-sites or a single ERE in a sequence-dependent manner. These data provide evidence that the ERE sequence and its immediate flanking regions influence whether COUP-TF enhances, inhibits, or has no effect on ER ligand-induced ERE reporter gene expression and that COUP-TFI activates gene transcription from ERE half-sites. We suggest that COUP-TFI plays a role in mitigating estrogen-responsive gene expression.

Current Status and Controversial Issues concerning Endocrine Therapy for Patients with Recurrent Breast Cancer in Japan

BACKGROUND: Four different endocrine therapeutic agents have been used in Japan since 1996. However, a consensus regarding proper use of these agents has notyet been established. Therefore, a questionaire survey of Japanese breast cancer authorities on endocrine therapy and a multi-institute survey to investigate the efficacy of a single first-line endocrine therapy for recurrent breast cancerwere conducted. MATERIALS AND PATIENTS: A total of 279 questionaires were sent to the Councilors of the Japanese Breast Cancer Society. The clinico-pathological data of 77 breast cancer patients who underwent a single first-line endocrine therapy were collected from five institutes. RESULTS: The response rate to this questionaire survey was 67.4%. The resultsshow that many authorities consider that: 1) both ER and PgR in primary tumors should be measured, 2) patient age, the disease-free interval and postoperative adjuvant therapy do not provide enough information for the selection of endocrine therapies, 3) antiestrogen and LH-RH agonists should be used as first-line endocrine therapies, 4) combined endocrine therapies, such as an antiestrogen plus an LH-RH agonist, should be used, 5) the optimal sequence of use of endocrine therapeutic agents is most controversial. The objective response rate to first-line endocrine therapies was 40.3% and the duration of response was over 15 months.The objective response rate to second-line endocrine therapies was 42.1%. A multiple regression analysis of predictive factors for the efficacy of first-line endocrine therapies indicated two factors, the disease-free interval and dominantsite of metastasis, to be significant. Conclusions: This questionaire survey suggests that clinical trials to investigate the optimal sequence of use of endocrine therapies and to clarify the usefulness of combined endocrine therapies should be conducted. Single first- or second-line endocrine therapies for recurrent breast cancer are effective and should be carried out by general clinicians.

Parallel solid-phase synthesis of a model library of 7alpha-alkylamide estradiol derivatives as potential estrogen receptor antagonists

The C17-THP derivative of 7alpha-(11-azidoundecanyl)-estradiol (4) was synthesized and coupled to an aminomethyl resin via a photolabile o-nitrobenzyl linker. Reduction of the azide by the Staudinger reaction to its corresponding amine followed by acylation using four activated NFmoc protected amino acids gave a first level of diversity. Subsequent deprotection of the Fmoc followed by a second acylation with five activated carboxylic acids produced, after photocleavage, a model library of twenty antiestrogen-related 7alpha-alkylamide estradiol derivatives in acceptable overall yields and very good purities.

The 1999 James Ewing Lecture: in pursuit of molecules of oncogenesis and neoplastic therapy

Breast cancer is a progressive, phenotypic, genetic and epigenetic alteration, induced by various stimulants of which the principal steroid hormone, estrogen, initiates and promotes neoplastic transformation of normal ductal mammary epithelium to acquire distinct biochemical and molecular characteristics. The molecular events that characterize oncogenesis with dysregulated physiologic states represent an area of intense translational research. The progress of the last century suggests that the targeted responses initiated by this steroid hormone can be attenuated and modified with anti-estrogen therapies or through applied intervention with selective estrogen receptor modulation. New pharmacological, genetic, and biological agents will undoubtedly afford clinical investigators of the next millennium the opportunity to provide major impact on the disease progression of this neoplasm, through these targeted endocrine responses for estrogen.

Theoretical basis for the benefit of postmenopausal estrogen substitution

Women are being presented with an increasing number of choices for health care management as they move through the aging process. Estrogen has positive effects on mood, sexual function, target end organs and cognitive function, and may play an important role in the etiology of Alzheimer's Disease by acting to prevent amyloid plaque formation, oxidative stress, or deterioration of the cholinergic neurotransmitter system. The benefits of estrogen therapy for osteoporosis, the cardiovascular system, and lipid metabolism are far reaching, but the possibility of developing breast cancer later in life is also relevant. Understanding the mechanisms for the action of the estrogens, anti-estrogens, and the selective estrogen receptor modulators, and possible alternative routes of symptom management for some menopausal events is important to make appropriate decisions on choice of therapy. This review discusses the theoretical basis for estrogen's actions in the management of the postmenopausal stage of the life cycle.

DNA methylation analysis at distal and proximal promoter regions of the oestrogen receptor gene in breast cancers

Oestrogen receptor alpha (ER-alpha) gene has two specific promoters, distal (P0) and proximal (P1), which induce almost identical transcripts in size due to different splicing. We examined the methylation at both promoter regions of the ER-alpha gene using HpaII, a methylation-sensitive restriction enzyme, prior to polymerase chain reaction (PCR) amplification. To confirm the results of PCR-based methylation analysis, Southern hybridization was also performed. Twenty of 29 patients with ER-alpha-positive tumours and five of 27 with ER-alpha-negative tumours were unmethylated at the P1 promoter region of the ER-alpha gene. The incidence of methylation was highly negatively correlated with ER-alpha expression (P = 0.0002). A similarly negative correlation was observed at the P0 promoter region of the ER-alpha gene (P = 0.0154). Additionally, the tumours with the ER-alpha gene hypermethylated at both promoter regions had definitely negative ER-alpha values. It was suggested that this epigenetic change might control ER-alpha expression, and might play an important role in the loss of hormone-dependence in breast cancer.

Peptide antagonists of the human estrogen receptor

Estrogen receptor alpha transcriptional activity is regulated by distinct conformational states that are the result of ligand binding. Phage display was used to identify peptides that interact specifically with either estradiol- or tamoxifen-activated estrogen receptor alpha. When these peptides were coexpressed with estrogen receptor alpha in cells, they functioned as ligand-specific antagonists, indicating that estradiol-agonist and tamoxifen-partial agonist activities do not occur by the same mechanism. The ability to regulate estrogen receptor alpha transcriptional activity by targeting sites outside of the ligand-binding pocket has implications for the development of estrogen receptor alpha antagonists for the treatment of tamoxifen-refractory breast cancers.

Estrogen receptor regulation of the Na+/H+ exchange regulatory factor

To better understand the actions of estrogens and antiestrogens in estrogen target cells, we have searched for estrogen-regulated genes in human breast cancer cells, in which the number of genes known to be directly activated by estrogen is quite small. Using differential display RNA methods, we have identified the human homolog of the Na+ -H+ exchanger regulatory factor (NHE-RF), an approximately 50-kDa protein that is also an ezrin-radixin-moesin-binding phosphoprotein, as being under rapid and direct regulation by estrogen in estrogen receptor (ER)-containing breast cancer cells. Stimulation by estrogen of NHE-RF RNA is rapid, being near maximal (approximately 6-fold) by 1 h, and is not blocked by cycloheximide, indicating that it is a primary response. Stimulation is selective for estrogen ligands, with no stimulation by other classes of steroid hormones, and stimulation by estrogen is suppressed by the antiestrogens tamoxifen and ICI 182,780. Induction is shown to require an active ER through several approaches, including the use of ER-negative breast cancer cells containing a stably integrated ER. NHE-RF protein levels, monitored using antibodies specific for this protein, increase after estrogen and reach maximal levels at 24-48 h. Interestingly, NHE-RF is a PDZ domain-containing protein that is enriched in polarized epithelia, where it is known to be localized in microvilli. Among various human tissues we have examined, we found that NHE-RF is expressed at a fairly high level in mammary tissue. NHE-RF regulates protein kinase A inhibition of the Na+ -H+ exchanger and may serve as a scaffold adaptor protein that contributes to the specificity of signal transduction events. Our findings suggest that the early, known effects of estrogen on cell cytoarchitecture (e.g. increasing microvilli on breast cancer cells) and on some cell signaling pathways (e.g. those involving cAMP) may involve rapid estrogen-mediated changes in the production of NHE-RF.

An estrogen receptor-selective coregulator that potentiates the effectiveness of antiestrogens and represses the activity of estrogens

The action of nuclear hormone receptors is tripartite, involving the receptor, its ligands, and its coregulator proteins. The estrogen receptor (ER), a member of this superfamily, is a hormone-activated transcription factor that mediates the stimulatory effects of estrogens and the inhibitory effects of antiestrogens such as tamoxifen in breast cancer and other estrogen target cells. To understand how antiestrogens and dominant negative ERs suppress ER activity, we used a dominant negative ER as bait in two-hybrid screening assays from which we isolated a clone from breast cancer cells that potentiates the inhibitory activities of dominant negative ERs and antiestrogen-liganded ER. At higher concentrations, it also represses the transcriptional activity of the estradiol-liganded ER, while having no effect on other nuclear hormone receptors. This clone, denoted REA for "repressor of estrogen receptor activity," encodes a 37-kDa protein that is an ER-selective coregulator. Its competitive reversal of steroid receptor coactivator 1 enhancement of ER activity and its direct interaction with liganded ER suggest that it may play an important role in determining the sensitivity of estrogen target cells, including breast cancer cells, to antiestrogens and estrogens.

Adenovirus-mediated delivery of a dominant negative estrogen receptor gene abrogates estrogen-stimulated gene expression and breast cancer cell proliferation

Dominant negative estrogen receptors are transcriptionally inactive, altered forms of the estrogen receptor (ER) that can dimerize with the ER and have the potential to inactivate the biological functions of this receptor. Here, we provide the first report that adenoviral delivery of a dominant negative ER to ER-positive breast cancer cells is able to effectively suppress estrogen-stimulated cell proliferation and the hormonal induction of endogenous genes. We constructed recombinant adenoviral vectors expressing a dominant negative ER (S554 fs, Ad-fs) or, for comparison, antisense ER (Ad-AS), or the sense wild-type ER (Ad-WT). Expression of the dominant negative ER or antisense ER, but not wild-type ER, blocked estradiol stimulation of the estrogen-responsive genes pS2 and c-myc. The dominant negative ER also fully abolished the estradiol-induced increase in proliferation of MCF-7 breast cancer cells, as did the antisense ER. The antiproliferative effects of the dominant negative and antisense ERs are explained by an increase in the number of cells in the G0/G1 stage of the cell cycle and decrease in the number of cells in G2/M as determined by flow cytometry, and also by a significant increase in the percentage of cells undergoing apoptosis. Our data strongly support the idea that targeting ER action using recombinant viral delivery of dominant negative ERs is an effective way to suppress ER-positive breast cancer cell proliferation and suggests the potential attractiveness of dominant negative gene therapy approaches targeted to the ER for the treatment of hormone-responsive breast cancer.

Estrogen receptor binding to estrogen response elements slows ligand dissociation and synergistically activates reporter gene expression

Estradiol (E2)-liganded estrogen receptor (ER) bound to three or four tandem copies of a consensus ERE (EREc38) in a cooperative manner. E2-ER binding to one or two EREs was non-cooperative. When ER was liganded by the antiestrogen 4-hydroxytamoxifen (4-OHT), ER-ERE binding was not cooperative, regardless of the number of EREs. Here we evaluated how binding to EREc38 affects ER conformation in the ligand binding domain (LBD) as reflected in the dissociation kinetics of [3H]ligand from the ER. Binding of ER to EREc38 slowed the rate of dissociation of either E2 or 4-OHT, indicating that DNA allosterically modulates the LBD conformation creating a tighter fit between the ligand and the ER. Conformational differences in ER induced by E2 versus antiestrogen were not reflected in differences in E2 or 4-OHT dissociation parameters under these conditions. No difference in the association rate of E2- versus 4-OHT-liganded ER binding to EREc38 was detected in electrophoretic mobility shift assay (EMSA). Synergistic, E2-dependent activation of a reporter gene was detected from three and four, but not one or two, tandem copies of EREc38. These observations suggest that cooperative binding of E2-ER to multiple copies of EREc38 is likely responsible for transcriptional synergy and that cooperativity may not involve direct interaction between the LBDs of ERE-bound ER. Since the number of copies of EREc38 did not alter E2 dissociation kinetics, functional synergy must involve cellular factors in addition to the ER ligand.

A yeast-based bioassay for the determination of functional and non-functional estrogen receptors

The response to endocrine therapy of breast cancer is not entirely predictable from hormone receptor status alone since some point mutated or splicing variants of the estrogen receptor (ER) show altered biological activities. In order to characterize the activities of all forms of ER in a heterogeneous breast tumor, a functional assay in Saccharomyces cerevisiae was developed. Total RNA isolated from breast cancer cells and one breast cancer specimen was reverse transcribed and the ER cDNA was amplified by PCR. The products were then cloned into an expression vector by in vivo homologous recombination in yeast. The yeast strain carries a reporter gene ( ADE2 ) coupled to an estrogen response element. Activation of the reporter by ER yielded white colonies whereas lack of ER activity produced red colonies. This permitted the testing for functionality of individual ER molecules and subsequent analysis by rescuing of the ER expression plasmids and complete DNA sequencing. This simple visual test allows discrimination between wild-type ER, constitutively active ER and inactive ER.

Estrogen receptor (ER) modulators each induce distinct conformational changes in ER alpha and ER beta

Estrogen receptor (ER) modulators produce distinct tissue-specific biological effects, but within the confines of the established models of ER action it is difficult to understand why. Previous studies have suggested that there might be a relationship between ER structure and activity. Different ER modulators may induce conformational changes in the receptor that result in a specific biological activity. To investigate the possibility of modulator-specific conformational changes, we have applied affinity selection of peptides to identify binding surfaces that are exposed on the apo-ERs alpha and beta and on each receptor complexed with estradiol or 4-OH tamoxifen. These peptides are sensitive probes of receptor conformation. We show here that ER ligands, known to produce distinct biological effects, induce distinct conformational changes in the receptors, providing a strong correlation between ER conformation and biological activity. Furthermore, the ability of some of the peptides to discriminate between different ER alpha and ER beta ligand complexes suggests that the biological effects of ER agonists and antagonists acting through these receptors are likely to be different.

Expression of human estrogen receptor using an efficient adenoviral gene delivery system is able to restore hormone-dependent features to estrogen receptor-negative breast carcinoma cells

Estrogen receptor (ER)-negative breast carcinomas are often difficult to treat as they do not respond to hormone therapy. In an attempt to determine if expressing the human estrogen receptor in an ectopic manner could restore the hormone responsiveness of these cells, we have expressed the human ER in ER-negative MDA-MB 231 breast cancer cells using a recombinant adenovirus gene delivery system that allows high level expression of ER in essentially all cells. In these cells, the ER was correctly translated, had a wild type hormone binding affinity (Kd = 0.6 nM), bound well to estrogen response element-containing DNA, and showed an activation pattern of estrogen response element-reporter gene activity by estrogen and antiestrogens very similar to that observed in MCF-7 breast cancer cells containing endogenous ER (stimulation by estrogen, no stimulation by the antiestrogens trans-hydroxytamoxifen or ICI 164384, and blockade of estradiol stimulation by trans-hydroxytamoxifen or ICI 164384). Intriguingly, estradiol stimulation of these cells was also able to induce expression of pS2, an estrogen regulated gene considered to be a favorable prognostic marker for endocrine therapy in ER-positive breast cancer cells. Expression of the ER had no effect by itself on the proliferation rate of MDA-MB 231 cells. However, treatment of the ER-containing cells with estradiol or with the pure antiestrogen ICI 164 384 suppressed proliferation of the cells while the antiestrogen trans-hydroxytamoxifen had little effect on proliferation; and cotreatment with trans-hydroxytamoxifen reversed the estradiol- or ICI 164 384-evoked suppression of proliferation. To understand the mechanism underlying the inhibition of proliferation by estradiol, we examined the expression of several growth related endogenous genes. c-Myc protooncogene expression was strongly inhibited by treatment with estradiol as was expression of BRCA1 and BRCA2 genes, which is in agreement with their mitogenic-dependent expression, while expression of beta-actin, a housekeeping gene, was not affected by hormone treatment. Together, these data suggest that reexpressing the human ER in breast cancer cells that no longer express this protein renders them sensitive to hormone treatment. The ability of the antiestrogen ICI 164 384 to suppress the proliferation of ER-negative breast cancer cells that reexpress ER might be useful ultimately as an endocrine gene therapy approach for controlling the growth of ER-negative breast cancer cells. The application of recombinant adenoviruses expressing the human ER presents interesting features which might be used as a basis for designing more powerful and effective treatments for ER-negative breast cancers.

The primary use of endocrine therapies

Primary endocrine therapy is potentially superior to primary chemotherapy in patients with ER-positive tumors. The ability to give endocrine therapy perioperatively may be a better test than chemotherapy of the hypothesis that the events accompanying surgery affect prognosis. However, a major problem in all studies of primary endocrine therapy is that there has been no clear experimental test of preoperative versus postoperative therapy. This is because the major thrust of treatment has been in the elderly with the purpose of determining whether surgery can be avoided altogether. The fact that in ER-positive tumors primary endocrine therapy is associated with similar response rates to chemotherapy make it an attractive therapy for older women. This is the group where adjuvant chemotherapy has not been adequately tested (> or = 70 years of age). In contradistinction, adjuvant endocrine therapy shows marked survival benefits in patients with ER-positive tumors in these age groups (Table 1). It appears likely that primary endocrine therapy will allow breast conservation and prognostic information as is seen with chemotherapy. A major question which requires answering is whether primary endocrine therapy will improve survival more than adjuvant therapy alone. Although ER status is a good marker of responsiveness, its specificity, in particular, is not optimal. The ability to assess the dynamic effects of primary endocrine therapy by sequential biopsy and measurement of biological responses to oestrogen deprivation may allow us to predict precisely the patients likely to benefit from treatment. This clinical scenario allows us to use other potentially useful assessments such as the non-invasive estimation of angiogenesis using quantitative imaging techniques of blood flow. The newer anti-estrogens and aromatase inhibitors appear ideally suited to primary therapy since they have rapid and profound inhibitory activities, few or no agonist effects, and low side effect profiles. A preoperative trial of Faslodex is planned by the EORTC and another with Arimidex is under consideration by the ATAC (Arimidex, tamoxifen and combined) Trialist Group (Fig. 7). The precise design of these studies will require considerable thought.

Tamoxifen up-regulates oestrogen receptor-alpha, c-fos and glyceraldehyde 3-phosphate-dehydrogenase mRNAs in ovine endometrium

Tamoxifen, the antiestrogen most widely used in medicine, was tested in ewes to determine whether it antagonizes oestradiol up-regulation of ER, PR, and other genes reported to be oestrogen-modulated (c-fos, oxytocin receptor (OTR), glyceraldehyde phosphate dehydrogenase (GAPDH), and apolipoprotein AI (apo AI)) in endometrium and liver. Ovariectomized ewes (n = 6 ewes per group) were injected with 20 mg tamoxifen (Tam) 24 h prior to tissue collection, 50 microg oestradiol (E2) 18 h prior to tissue collection, both drugs (T + E2) or drug vehicle (Con). E2 treatment resulted in 857 +/- 93 pg oestradiol/g endometrium. Gross uterine characteristics of Tam- and T + E2-treated ewes were intermediate to those in Con and E2 groups. In endometrium, Tam treatment mimicked E2 treatment in up-regulating ER, c-fos, and GAPDH mRNAs two- or three-fold. However, neither E2 nor Tam treatments affected concentrations of OTR mRNA in endometrium, or ER, c-fos, GAPDH, OTR and apo AI mRNAs in liver. Like oestradiol, tamoxifen stabilized endometrial ER mRNA more than 3-fold in endometrial explants cultured with the transcription inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB). Thus, tamoxifen acts as an oestradiol agonist in ovine endometrium and shares a posttranscriptional mechanism with oestradiol in the up-regulation of ER gene expression.

Tamoxifen--the treatment of choice. Why look for alternatives?

Tamoxifen is currently established as the endocrine treatment of choice in breast cancer. In advanced breast cancer, response rates of up to 60% in women with oestrogen receptor (ER)-positive tumours have been reported. In early breast cancer, tamoxifen can produce significant benefits, both statistically and clinically, in terms of reduction in relative risk of relapse or death in all patient subgroups (i.e. ER status, aged < or > 50 years) except premenopausal women with ER-negative tumours. The major benefit, however, is seen in women over 50 years old with ER-positive tumours. The results of randomized trials suggest that the optimum duration of tamoxifen therapy is at least 5 years. Two large pragmatic trials (aTTom and ATLAS) are under way to determine whether additional benefit can be gained from continuing tamoxifen treatment beyond 5 years. Recent data also suggest possible synergism between tamoxifen and chemotherapy in the treatment of early breast cancer in post-menopausal women. Other benefits of tamoxifen treatment include reduction in the risk of developing contralateral breast cancer. Included among the non-breast cancer benefits of tamoxifen are reduced risk of cardiovascular disease and protection against bone loss in post-menopausal women. These benefits must be weighed against the possible increased incidence of endometrial cancer. Notwithstanding its undoubted success, there is a need for agents to improve upon tamoxifen. Newer agents, such as the luteinizing hormone-releasing hormone analogue goserelin and the new-generation aromatase inhibitors, such as anastrozole, will add new life to the search for an improved endocrine therapy for early breast cancer.

Predictive value of SRC-1 for tamoxifen response of recurrent breast cancer

Tamoxifen causes an objective response in about one-third of metastatic breast cancer and in only half of the breast cancer patients with estrogen receptor (ER) positive tumors. Steroid-receptor coactivator-1 (SRC-1) appears to be a general coactivator for steroid receptors and rate limiting factor necessary for efficient ER transactivation. We aimed to evaluate whether SRC-1 expression is an additional factor for prediction of response to first-line tamoxifen therapy in patients who developed recurrent disease. Here for the first time, we report on SRC-1 expression using a semi-quantitative RT-PCR in 21 primary breast tumors, seven mammary tumor cell-lines, 12 fibroblast cultures, and six normal breast tissues. The highest levels of SRC-1 were observed in normal tissues, intermediate levels in tumor tissues, and the lowest levels in breast tumor cell-lines. There was no relationship between the levels of SRC-1 in these primary tumors and the proportion of tumor cells within the surgical samples, nor with ER status. The median SRC-1 level was, however, lower in tumors from patients that did not respond to tamoxifen. Our findings suggest that high levels of SRC-1 indicate a favorable response to tamoxifen of patients with recurrent breast cancer.

Triphenylethylene antiestrogens induce uterine vascular endothelial growth factor expression via their partial estrogen agonist activity

Estradiol induces vascular endothelial growth factor (VEGF) expression in the rat uterus and this may contribute to the hyperemia and increased vascularity produced by estrogens in this target tissue. Triphenylethylene antiestrogens such as tamoxifen have mixed agonist/antagonist activity and their specific effects are tissue and gene specific. These drugs exhibit primarily antiestrogenic actions in mammary tissue and are thus used for the treatment of breast cancer. These drugs are also suggested to be inhibitors of angiogenesis. However, uterine side effects of tamoxifen are thought to stem largely from the agonist activity of the drug in this tissue. Since side effects of tamoxifen such as uterine bleeding and endometrial cancer seem likely to have an angiogenic component, we have examined the effects of this drug, its metabolite, 4-hydroxy-tamoxifen and two additional triphenylethylene antiestrogens, nafoxidine and clomiphene, on the expression of VEGF and another estrogen regulated gene, c-fos, using the rat uterus as an experimental system. All four compounds increase uterine VEGF and c-fos mRNA levels indicating that the triphenylethylene class of antiestrogens are predominantly agonists for the induction of these genes in the uterus.

Induction of the estrogen specific mitogenic response of MCF-7 cells by selected analogues of estradiol-17 beta: a 3D QSAR study

Analogues of estradiol-17 beta (E2) have been evaluated for estrogen receptor (ER) binding affinity and mitogenic potential in the human breast cancer cell line MCF-7. These 42 compounds represent subtle modifications of the natural estrogen structure through the placement of hydroxyl, amino, nitro, or iodo groups around the ring system in addition to, or as replacement of, the 3- and 17 beta-hydroxyls of E2. The mitogenic activity of the analogues was found to be related to ER binding only to a limited extent. In order to elucidate structural features that are uniquely responsible for receptor binding affinity or mitogen potential of estrogens, the three-dimensional quantitative structure-activity (QSAR) method Comparative Molecular Field Analysis (CoMFA) was employed. Separate CoMFA models for receptor binding and cell growth stimulation were optimized through the use of various alignment rules and region step size. Whereas the CoMFA contour plots did outline the shared structural requirements for the two measured biological properties, specific topological features in this set of estrogens were delineated that distinguish mitogenic potential from ER binding ability. In particular, steric interference zones which affected growth extend in a band from above the A-ring to position 4 and below, whereas the ER binding steric interference zones are limited to isolated polyhedra in the 1, 2 and 4 positions and the alpha face of the B-ring. In addition, electronegative features located around the A-, B-, or C-rings contribute to receptor affinity. However, growth is dependent only on electronegative and electropositive properties near the 3-position. In a final QSAR model for the mitogenic response, the value of ER binding was included along with structural features as a descriptor in CoMFA. The resulting 3D-QSAR has the most predictive potential of the models in this study and can be considered a prototype model for the general evaluation of a steroidal estrogen's growth stimulating ability in MCF-7 cells. For example, the location of D-ring contours illustrate the model's preference for 17 beta-hydroxy steroids over the less mitogenic 17 alpha- and 16 alpha-hydroxy compounds. In addition, the enhanced mitogenic effect of steric bulk in the 11 alpha-position is also evident. The QSAR studies in this report illustrate the fact that while ER binding may be a required factor of the estrogen dependent growth response in MCF-7 cells, particular structural characteristics, in addition to those responsible for tight receptor binding, must be present to induce an optimal mitogenic response. Therefore, this report demonstrates that the CoMFA QSAR method can be utilized to characterize structural features of test compounds that account for different types of estrogenic responses.