Agonist-antagonist activity of anti-estrogens in the human breast cancer cell line MCF-7: an hypothesis for the interaction with a site distinct from the estrogen binding site

Endocrine Disruptors: A Critical Review of In Vitro and In Vivo Testing Strategies for Assessing Their Toxic Hazard to Humans

Currently, there is much concern that a wide range of both synthetic and naturally occurring environmental chemicals can act as endocrine disruptors (EDs), and can adversely affect humans and wildlife. Many in vivo and in vitro tests have been proposed for screening EDs, and several regulatory agencies, including the US Environmental Protection Agency (EPA), have recommended tier-testing schemes. Unfortunately, most of the proposed toxicity tests have substantial problems, including non-specificity and lack of reproducibility. There is also uncertainty concerning their relevance for generating useful hazard data for risk assessment purposes, in view of the diversity of the possible ED mechanisms of action (for example, receptor binding, steroidogenesis and modulation of the homeostatic processes which regulate endogenous responses to hormones). Moreover, most of the suggested test methods have yet to be validated according to internationally accepted criteria, although the OECD and the US EPA have defined tests for validation, and an interlaboratory “prevalidation” exercise has been initiated by the OECD. All this is compounded by the lack of information regarding human exposure levels to EDs, and a lack of direct evidence for a causal link between exposure and the development of adverse human health effects. In addition, the regulatory testing of EDs has important negative implications for animal welfare, as some of the proposed in vivo tests require large group sizes of animals and stressful procedures. From a detailed analysis of the available published literature, it is concluded that it is impossible to assess the relative values of currently available in vitro and in vivo toxicity tests for EDs, or to recommend any test or test battery. Any plans for the widespread testing of EDs are therefore premature and might be unnecessary, at least for detecting possible human effects. Several recommendations are made for rectifying this unsatisfactory situation, including the postponement of screening programmes pending: a) more information on human exposure; b) further details of the mechanisms of action of EDs; and c) the development of improved tests, followed by their proper scientific validation.

Fatty-acylation target sequence in the ligand-binding domain of vertebrate steroid receptors demarcates evolution from estrogen-related receptors

Present-day nuclear receptors (NRs) responding to adrenal and sex steroids are key regulators of reproduction and growth in mammals, and are thought to have evolved from an ancestral NR most closely related to extant estrogen-related receptors (ERRs). The molecular events (and ligands) that distinguish steroid-activated NRs (SRs) from their inferred ancestor, that gave rise to both the ERRs and SRs, remain unknown. We report that target sequences for fatty-acylation (palmitoylation) at a key cysteine residue (corresponding to Cys447 in human estrogen receptor ERα) in helix 8 of the ligand-binding domain accurately demarcate SRs from ERRs. Docking studies are consistent with the hypothesis that palmitate embeds into a key groove in the receptor surface. The implications of lipidation, and of potential alternative ligands for the key cysteine residue, for receptor function and the evolution of SRs are discussed.

Natural Anti-Estrogen Receptor Alpha Antibodies Able to Induce Estrogenic Responses in Breast Cancer Cells: Hypotheses Concerning Their Mechanisms of Action and Emergence

The detection of human anti-estrogen receptor α antibodies (ERαABs) inducing estrogenic responses in MCF-7 mammary tumor cells suggests their implication in breast cancer emergence and/or evolution. A recent report revealing a correlation between the titer of such antibodies in sera from patients suffering from this disease and the percentage of proliferative cells in samples taken from their tumors supports this concept. Complementary evidence of the ability of ERαABs to interact with an epitope localized within the estradiol-binding core of ERα also argues in its favor. This epitope is indeed inserted in a regulatory platform implicated in ERα-initiated signal transduction pathways and transcriptions. According to some experimental observations, two auto-immune reactions may already be advocated to explain the emergence of ERαABs: one involving probably the idiotypic network to produce antibodies acting as estrogenic secretions and the other based on antibodies able to abrogate the action of a natural ERα inhibitor or to prevent the competitive inhibitory potency of released receptor degradation products able to entrap circulating estrogens and co-activators. All of this information, the aspect of which is mainly fundamental, may open new ways in the current tendency to combine immunological and endocrine approaches for the management of breast cancer.

A model of estrogen-related gene expression reveals non-linear effects in transcriptional response to tamoxifen

BACKGROUND

Estrogen receptors alpha (ER) are implicated in many types of female cancers, and are the common target for anti-cancer therapy using selective estrogen receptor modulators (SERMs, such as tamoxifen). However, cell-type specific and patient-to-patient variability in response to SERMs (from suppression to stimulation of cancer growth), as well as frequent emergence of drug resistance, represents a serious problem. The molecular processes behind mixed effects of SERMs remain poorly understood, and this strongly motivates application of systems approaches. In this work, we aimed to establish a mathematical model of ER-dependent gene expression to explore potential mechanisms underlying the variable actions of SERMs.

RESULTS

We developed an equilibrium model of ER binding with 17β-estradiol, tamoxifen and DNA, and linked it to a simple ODE model of ER-induced gene expression. The model was parameterised on the broad range of literature available experimental data, and provided a plausible mechanistic explanation for the dual agonism/antagonism action of tamoxifen in the reference cell line used for model calibration. To extend our conclusions to other cell types we ran global sensitivity analysis and explored model behaviour in the wide range of biologically plausible parameter values, including those found in cancer cells. Our findings suggest that transcriptional response to tamoxifen is controlled in a complex non-linear way by several key parameters, including ER expression level, hormone concentration, amount of ER-responsive genes and the capacity of ER-tamoxifen complexes to stimulate transcription (e.g. by recruiting co-regulators of transcription). The model revealed non-monotonic dependence of ER-induced transcriptional response on the expression level of ER, that was confirmed experimentally in four variants of the MCF-7 breast cancer cell line.

CONCLUSIONS

We established a minimal mechanistic model of ER-dependent gene expression, that predicts complex non-linear effects in transcriptional response to tamoxifen in the broad range of biologically plausible parameter values. Our findings suggest that the outcome of a SERM's action is defined by several key components of cellular micro-environment, that may contribute to cell-type-specific effects of SERMs and justify the need for the development of combinatorial biomarkers for more accurate prediction of the efficacy of SERMs in specific cell types.

Hormones and endocrine-disrupting chemicals: low-dose effects and nonmonotonic dose responses

For decades, studies of endocrine-disrupting chemicals (EDCs) have challenged traditional concepts in toxicology, in particular the dogma of "the dose makes the poison," because EDCs can have effects at low doses that are not predicted by effects at higher doses. Here, we review two major concepts in EDC studies: low dose and nonmonotonicity. Low-dose effects were defined by the National Toxicology Program as those that occur in the range of human exposures or effects observed at doses below those used for traditional toxicological studies. We review the mechanistic data for low-dose effects and use a weight-of-evidence approach to analyze five examples from the EDC literature. Additionally, we explore nonmonotonic dose-response curves, defined as a nonlinear relationship between dose and effect where the slope of the curve changes sign somewhere within the range of doses examined. We provide a detailed discussion of the mechanisms responsible for generating these phenomena, plus hundreds of examples from the cell culture, animal, and epidemiology literature. We illustrate that nonmonotonic responses and low-dose effects are remarkably common in studies of natural hormones and EDCs. Whether low doses of EDCs influence certain human disorders is no longer conjecture, because epidemiological studies show that environmental exposures to EDCs are associated with human diseases and disabilities. We conclude that when nonmonotonic dose-response curves occur, the effects of low doses cannot be predicted by the effects observed at high doses. Thus, fundamental changes in chemical testing and safety determination are needed to protect human health.

A two-site model for antiestrogen action

Evidence is presented for a unified model for the reaction of antiestrogens with estrogen receptors that explains much of the unusual pharmacology of these clinically important agents. Agonist activity results from occupancy of the estradiol-binding (primary) site in the receptor and antagonism from the additional interaction with a secondary locus not recognized by hormone. In the case of type I antiestrogens, such as tamoxifen, this is weaker than primary site binding, so these substances are agonists at low concentrations and antagonists at higher levels. With type II antiestrogens, affinities for both sites are comparable, so one never has the agonist situation (only primary site occupied), and these agents are pure antagonists. Reproductive tissues of the mouse and guinea pig contain a small macromolecule that binds hydroxytamoxifen, but not estradiol, keeping the free antiestrogen concentration below that required for secondary binding. Thus, in these species, tamoxifen is a pure agonist.

Assessing Estrogenic Activity of Pyrethroid Insecticides Using In Vitro Combination Assays

Pyrethroid insecticides are among the most commonly used classes of insecticides worldwide, but their endocrine disrupting activities remain unclear. Therefore, in the present study, we examined the estrogenic activities of pyrethroid insecticides in E-screen and competition binding assays. In addition, we measured estrogen receptor (ER) protein and pS2 mRNA levels in human breast cancer cells (MCF-7 BUS) to clarify the mechanism of their estrogenicity. Seven pyrethroid insecticides (bioallethrine, cypermethrin, deltamethrin, fenvalerate, permethrin, sumithrin, and tetramethrin) were tested because of their worldwide usage. In addition, 17beta-estradiol was tested as a positive control. As expected, 17beta-estradiol significantly increased MCF-7 BUS cell proliferation at concentrations of 10(-11) M and above. Of the pyrethroid insecticides tested, only sumithrin increased MCF-7 BUS cell proliferation in a dose-dependent manner; the maximum induction of cell proliferation was observed at a dose of 10(-5) M. In the anti-estrogenic activity test, bioallethrin, fenvalerate, and permethrin significantly inhibited 17beta-estradiol-induced MCF-7 BUS cell proliferation at 10(-6) M, a concentration comparable to the effective dose (10(-9) M) of ICI 182,780, a pure ER antagonist. However, none of the pyrethroid insecticides competitively inhibited the binding of [(3)H]estradiol to rat uterus ERs in competition binding assays. Both 17beta-estradiol (10(-10) M) and sumithrin (10(-5) M) decreased the levels of cytosolic ERalpha and ERbeta protein expression significantly as compared with the vehicle control. In addition, 17beta-estradiol (10(-10) M) increased pS2 mRNA expression markedly, and sumithrin significantly increased pS2 mRNA levels in a dose-dependent manner. The other six compounds tested in the present study did not affect ER protein levels or pS2 mRNA levels. These results suggest that certain pyrethroid insecticides may be considered to be estrogen-like chemicals that act through pathways other than direct ER binding, and may function as endocrine modulators in both wildlife and humans.

Potential estrogenic effects of bisphenol-A estimated by in vitro and in vivo combination assays

The potential estrogenic activities of bisphenol-A were investigated in vitro (E-screen and estrogen receptor competitive binding bioassays) and in vivo (uterotrophic assay). Uterotrophic responses were evaluated using mature ovariectomized Sprague-Dawley female rats treated subcutaneously with bisphenol A (1, 5, 10, 50, and 100 mg/kg/day), E2 (0.3 microgram/kg), and DES (0.3 microgram/kg) for 3 consecutive days. In a MCF-7 cell proliferation assay, E2 and DES used as positive estrogens induced maximum proliferation of MCF-7 cells at 1.0 nM, whereas BPA slightly induced MCF-7 cell proliferation at a higher level of 0.1 microM and maximum proliferation at 10 microM. In a competitive binding assay, E2 and DES showed inhibition of 17 beta-[3H]estradiol binding to the rat uterus ER with an IC50 of 1.0 nM and 0.5 nM, respectively. However, BPA had an IC50 of 5 microM, which was approximately 5,000 or 10,000-fold greater than the IC50 of E2 and DES. In uterotrophic assays, uterus (wet and blotted) and vagina weights were significantly increased at the dose of BPA 100 mg/kg/day in OVX Sprague-Dawley rats. These studies demonstrate that BPA exhibits weak estrogenic activity in all experimental systems, and thus its migration from epoxy resins or polycarbonate products should be controlled not to exceed a safety levels for humans.

Differences in immunoreactivity of estrogen receptor (ER) in tamoxifen-sensitive and -resistant breast carcinomas: preclinical and first clinical investigations

Inherited or acquired tamoxifen resistance is a major constraint in the endocrinological treatment of breast carcinomas. We developed an enzyme-immunoassay that discriminates between tamoxifen-sensitive and -resistant tumors. The procedure was established and standardized using two xenografted breast carcinomas--3366 (highly sensitive to tamoxifen) and 3366/TAM (acquired tamoxifen resistance). The latter model was developed by treatment of 3366 tumor-bearing nude mice during serial passaging over 3 years with tamoxifen. Both lines were estrogen receptor (ER) positive (101 or 82 fmol/mg protein), and revealed no differences in the nucleotide sequences of the hormone binding domain of the ER protein. However, while in the sensitive tumors an upregulation of ER levels was registered after estradiol treatment of tumor bearing nude mice, the ER expression in the resistant line remained unchanged. The tamoxifen sensitive and -resistant breast carcinoma 3366 differed, additionally, in their immunoreactivity of ER to mAB H222. While an incubation with estradiol or tamoxifen of immobilized ER prepared from cytosols of the sensitive tumors 3366 led to a significant increase in immunoreactivity, samples of resistant tumors failed in the exposition of additional immunologically reactive epitopes. These results were the basis for the development of an assay for determination of the tamoxifen response in patients. Our retrospective results with 38 breast tumors from a tumor bank indicated that patients with an increase of immunoreactivity of ER more rarely had a recurrence while under going tamoxifen therapy compared with patients expecting no increase. However, the data indicate interesting changes occurring with the ER of tam-resistant tumors that are to be explained by further mutational or protein-chemical analysis.

The ligand insertion hypothesis in the genomic action of steroid hormones

Gene regulation by steroids is tightly coupled to hormone concentration and stereochemistry. A key step is binding of hormones to receptors which interact with consensus DNA sequences known as hormone response elements (HREs). The specificity and strength of hormone binding do not correlate well with hormonal activity suggesting an additional step involving recognition of ligand by the gene. Stereospecific fit of hormones between base pairs and correlation of fit with hormonal activity led to the proposal that such recognition involves insertion of hormone into DNA. Here, the feasibility of insertion was investigated using computer models of the glucocorticoid receptor DNA binding domain bound to its HRE. The site reported to accommodate glucocorticoids was found in the HRE and was exposed to permit unwinding at this locus. The resulting cavity in the unwound DNA/receptor interface fit cortisol remarkably well; cortisol formed hydrogen bonds to both the receptor and DNA. Current experimental evidence is generally consistent with ligand binding domains of receptors undergoing a conformational change which facilitates transfer of the ligand into the unwound DNA/receptor interface. We propose this step is rate limiting and alterations in receptor, DNA or hormone which attenuate insertion impair hormonal regulation of gene function.

The novel estrogen receptor-beta subtype: potential role in the cell- and promoter-specific actions of estrogens and anti-estrogens

The recent discovery that an additional estrogen receptor (ER) subtype is present in various rat, mouse and human tissues has advanced our understanding of the mechanisms underlying estrogen signalling. The discovery of a second ER subtype (ERbeta) suggests the existence of two previously unrecognised pathways of estrogen signalling: via the ERbeta subtype in tissues exclusively expressing this subtype and via the formation of heterodimers in tissues expressing both ER subtypes. Various models have been suggested as explanations for the striking cell- and promoter-specific effects of estrogens and anti-estrogens, all on the basis of the assumption that only a single ER gene exists. This minireview describes several of these models and focuses on the potential role which the novel ERbeta subtype might have in this regard.

Antagonism of estrogen receptor and calmodulin association by antiestrogens is not dependent on an interaction with calmodulin

Previously, two antiestrogens estradiol derivatives (3 and 4) bearing the basic side chain of tamoxifen were shown to impede the binding of the estrogen receptor (ER) to calmodulin (CaM)-Sepharose. In this study, the interaction of these and related compounds with calmodulin was examined using the cyclic AMP phosphodiesterase (cAMP-PDE) assay. Neither of the steroids gave any significant inhibition of the calmodulin dependent cAMP-PDE activity up to a final concentration of 20 microM. For comparison, tamoxifen and nafoxidine produced IC50 values of 6.7 microM +/- 1.0 and 7.4 microM +/- 1.1, respectively. In addition, a comparison was made of the activity of some triphenylethylene derivatives against CaM dependent cAMP-PDE and the ER-CaM Sepharose assays, but no relationship was observed. Overall, these results demonstrate that inhibition of the ER-CaM association by various steroidal and triphenylethylene antiestrogens does not relate to antagonism of calmodulin function or their binding affinity for the estrogen receptor.

Intact noncovalent dimer of estrogen receptor ligand-binding domain can be detected by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESMS) of the estrogen receptor ligand binding domain (ER LBD) in its estradiol-binding form was performed. A dimeric ER LBD was observed, with a greatly reduced capacity for protonation (major charge state for dimer +16 vs. +23 for a monomer). Peak broadening (probably due to heterogeneity resulting from salt and water adduct formation) adversely affected our ability to distinguish between multiple discreet dimeric species and thus prevented us from establishing an accurate average mass for the dimerized domain. A mixture of species with molecular masses between 57,240 Da and 57,900 Da was observed, which would compare to 57,274 Da, 57,546 Da, and 57,818 Da for the calculated masses of the dimer without estradiol, or with one or two bound ligand molecules, respectively. Hence, nonliganded ER LBD dimer appeared to constitute the major species. The presence of low levels of a singly liganded ER LBD dimer cannot be ruled out, but the data argue against the possibility of the ER LBD dimer carrying two molecules of estradiol. Allowing for current limitations in the technology, our data demonstrate that ESMS on a quadrupole mass spectrometer of limited mass range (4000 Da for singly charged ions) has potential utility for studying ligand-binding proteins. In particular, in future it might be possible to compare spectra obtained from agonist- and antagonist-bound receptors and determine from subtle changes in protonation state possible differences in the higher order structure of those noncovalent protein complexes.

Effects of tamoxifen and ICI 164384 on protein synthesis and vectorial secretion in polarized rat uterine epithelial cells

A polarized, primary cell, bicameral culture system was utilized to test the effects of two types of antiestrogens, tamoxifen (class I) and ICI 164384 (class II) on post-mitotic, uterine epithelial cells. The results demonstrate that in addition to blocking estrogen action in a dose-dependent manner, each of these compounds has independent effects on protein synthesis and secretion in these cells. The effects of both tamoxifen and ICI 164384 on vectorial protein secretion were completely opposite to those of estradiol. Both antiestrogens given alone repressed apical secretion and stimulated basal secretion, whereas estradiol stimulated apical and repressed basal secretion. Furthermore, specific protein bands in both the apical and basal secretory compartments responded differently according to dose to each compound. These experiments using polarized uterine epithelial cell cultures have identified apical and basal protein secretion as two cellular response with increased sensitivity to steroids and antisteroids.

Steroid hormones, receptors, and antagonists

In the thirty-odd years since the first demonstration of estrogen-binding components in reproductive tissues, much has been learned about the molecular details of steroid hormone action. Facts still to be elucidated include the precise mechanism by which interaction with the steroid disrupts the native receptor complex to generate an active transcription factor, just how this activated receptor enhances expression of target genes, and the role of phosphorylation in these events. The concept of receptor-medicated steroid hormone action has provided useful methods for predicting the risk of metastatic recurrence of breast cancers and the response of disseminated disease to endocrine therapy. Application of immunochemical techniques for receptor assay and elucidation of the role of mutant receptors in hormone resistance promise to increase the utility of these diagnostic procedures. A better understanding of the molecular details of steroid hormone antagonism should be helpful in the search for new and more effective agents for the endocrine control of breast cancers.

A new interpretation of antiestrogen action

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

Increasing the number of tandem estrogen response elements increases the estrogenic activity of a tamoxifen analogue

There have been several reports of women who have tumor relapse while on tamoxifen therapy, followed by tumor regression after tamoxifen withdrawal. In such apparently tamoxifen-stimulated tumors, there is likely a genetic change which increases the estrogenicity of tamoxifen. In this study, we determine if increasing the number of estrogen response elements (EREs) in the promoter region of a reporter gene can alter the agonistic activity of fixed-ring 4-hydroxytamoxifen. We show that increasing the number of EREs in the promotor region increases the transcriptional response of the reporter plasmid to estradiol. We also find that while fixed-ring 4-hydroxytamoxifen is unable to stimulate transcription when one ERE is present, transcriptional activation can occur with multiple EREs. These results demonstrate that ERE amplification could explain the agonistic properties of tamoxifen, and suggests a novel mechanism to explain tamoxifen-stimulated breast cancer growth.