Differential SERM effects on corepressor binding dictate ERalpha activity in vivo

Palazestrant (OP-1250), A Complete Estrogen Receptor Antagonist, Inhibits Wild-type and Mutant ER-positive Breast Cancer Models as Monotherapy and in Combination

The estrogen receptor (ER) is a well-established target for the treatment of breast cancer, with the majority of patients presenting as ER-positive (ER+). Endocrine therapy is a mainstay of breast cancer treatment but the development of resistance mutations in response to aromatase inhibitors, poor pharmacokinetic properties of fulvestrant, agonist activity of tamoxifen, and limited benefit for elacestrant leave unmet needs for patients with or without resistance mutations in ESR1, the gene that encodes the ER protein. Here we describe palazestrant (OP-1250), a novel, orally bioavailable complete ER antagonist and selective ER degrader. OP-1250, like fulvestrant, has no agonist activity on the ER and completely blocks estrogen-induced transcriptional activity. In addition, OP-1250 demonstrates favorable biochemical binding affinity, ER degradation, and antiproliferative activity in ER+ breast cancer models that is comparable or superior to other agents of interest. OP-1250 has superior pharmacokinetic properties relative to fulvestrant, including oral bioavailability and brain penetrance, as well as superior performance in wild-type and ESR1-mutant breast cancer xenograft studies. OP-1250 combines well with cyclin-dependent kinase 4 and 6 inhibitors in xenograft studies of ER+ breast cancer models and effectively shrinks intracranially implanted tumors, resulting in prolonged animal survival. With demonstrated preclinical efficacy exceeding fulvestrant in wild-type models, elacestrant in ESR1-mutant models, and tamoxifen in intracranial xenografts, OP-1250 has the potential to benefit patients with ER+ breast cancer.

The beneficial effects of tamoxifen on arteries: a key player for cardiovascular health of breast cancer patient

Breast cancer is the most common cancer in women. Over the past few decades, advances in cancer detection and treatment have significantly improved survival rate of breast cancer patients. However, due to the cardiovascular toxicity of cancer treatments (chemotherapy, anti-HER2 antibodies and radiotherapy), cardiovascular diseases (CVD) have become an increasingly important cause of long-term morbidity and mortality in breast cancer survivors. Endocrine therapies are prescribed to reduce the risk of recurrence and specific death in estrogen receptor-positive (ER+) early breast cancer patients, but their impact on CVD is a matter of debate. Whereas aromatase inhibitors and luteinizing hormone-releasing hormone (LHRH) analogs inhibit estrogen synthesis, tamoxifen acts as a selective estrogen receptor modulator (SERM), opposing estrogen action in the breast but mimicking their actions in other tissues, including arteries. This review aims to summarize the main clinical and experimental studies reporting the effects of tamoxifen on CVD. In addition, we will discuss how recent findings on the mechanisms of action of these therapies may contribute to a better understanding and anticipation of CVD risk in breast cancer patients.

Unconventional isoquinoline-based SERMs elicit fulvestrant-like transcriptional programs in ER+ breast cancer cells

Estrogen receptor alpha (ERα) is a ligand-dependent master transcriptional regulator and key driver of breast cancer pathology. Small molecule hormones and competitive antagonists favor unique ERα conformational ensembles that elicit ligand-specific transcriptional programs in breast cancer and other hormone-responsive tissues. By affecting disparate ligand binding domain structural features, unconventional ligand scaffolds can redirect ERα genomic binding patterns to engage novel therapeutic transcriptional programs. To improve our understanding of these ERα structure-transcriptional relationships, we develop a series of chemically unconventional antagonists based on the antiestrogens elacestrant and lasofoxifene. High-resolution x-ray co-crystal structures show that these molecules affect both classical and unique structural motifs within the ERα ligand binding pocket. They show moderately reduced antagonistic potencies on ERα genomic activities but are effective anti-proliferative agents in luminal breast cancer cells. Interestingly, they favor a 4-hydroxytamoxifen-like accumulation of ERα in breast cancer cells but lack uterotrophic activities in an endometrial cell line. Importantly, RNA sequencing shows that the lead molecules engage transcriptional pathways similar to the selective estrogen receptor degrader fulvestrant. This advance shows that fulvestrant-like genomic activities can be achieved without affecting ERα accumulation in breast cancer cells.

Brain region- and sex-specific transcriptional profiles of microglia

Microglia are resident macrophages of the brain, performing roles related to brain homeostasis, including modulation of synapses, trophic support, phagocytosis of apoptotic cells and debris, as well as brain protection and repair. Studies assessing morphological and transcriptional features of microglia found regional differences as well as sex differences in some investigated brain regions. However, markers used to isolate microglia in many previous studies are not expressed exclusively by microglia or cannot be used to identify and isolate microglia in all contexts. Here, fluorescent activated cell sorting was used to isolate cells expressing the microglia-specific marker TMEM119 from prefrontal cortex (PFC), striatum, and midbrain in mice. RNA-sequencing was used to assess the transcriptional profile of microglia, focusing on brain region and sex differences. We found striking brain region differences in microglia-specific transcript expression. Most notable was the distinct transcriptional profile of midbrain microglia, with enrichment for pathways related to immune function; these midbrain microglia exhibited a profile similar to disease-associated or immune-surveillant microglia. Transcripts more highly expressed in PFC isolated microglia were enriched for synapse-related pathways while microglia isolated from the striatum were enriched for pathways related to microtubule polymerization. We also found evidence for a gradient of expression of microglia-specific transcripts across the rostral-to-caudal axes of the brain, with microglia extracted from the striatum exhibiting a transcriptional profile intermediate between that of the PFC and midbrain. We also found sex differences in expression of microglia-specific transcripts in all 3 brain regions, with many selenium-related transcripts more highly expressed in females across brain regions. These results suggest that the transcriptional profile of microglia varies between brain regions under homeostatic conditions, suggesting that microglia perform diverse roles in different brain regions and even based on sex.

Stereospecific lasofoxifene derivatives reveal the interplay between estrogen receptor alpha stability and antagonistic activity in ESR1 mutant breast cancer cells

Chemical manipulation of estrogen receptor alpha ligand binding domain structural mobility tunes receptor lifetime and influences breast cancer therapeutic activities. Selective estrogen receptor modulators (SERMs) extend estrogen receptor alpha (ERα) cellular lifetime/accumulation. They are antagonists in the breast but agonists in the uterine epithelium and/or in bone. Selective estrogen receptor degraders/downregulators (SERDs) reduce ERα cellular lifetime/accumulation and are pure antagonists. Activating somatic ESR1 mutations Y537S and D538G enable resistance to first-line endocrine therapies. SERDs have shown significant activities in ESR1 mutant setting while few SERMs have been studied. To understand whether chemical manipulation of ERα cellular lifetime and accumulation influences antagonistic activity, we studied a series of methylpyrollidine lasofoxifene (Laso) derivatives that maintained the drug’s antagonistic activities while uniquely tuning ERα cellular accumulation. These molecules were examined alongside a panel of antiestrogens in live cell assays of ERα cellular accumulation, lifetime, SUMOylation, and transcriptional antagonism. High-resolution x-ray crystal structures of WT and Y537S ERα ligand binding domain in complex with the methylated Laso derivatives or representative SERMs and SERDs show that molecules that favor a highly buried helix 12 antagonist conformation achieve the greatest transcriptional suppression activities in breast cancer cells harboring WT/Y537S ESR1. Together these results show that chemical reduction of ERα cellular lifetime is not necessarily the most crucial parameter for transcriptional antagonism in ESR1 mutated breast cancer cells. Importantly, our studies show how small chemical differences within a scaffold series can provide compounds with similar antagonistic activities, but with greatly different effects of the cellular lifetime of the ERα, which is crucial for achieving desired SERM or SERD profiles.

Catalase, a therapeutic target in the reversal of estrogen-mediated aging

Despite increasing interest in the reversal of age-related processes, there is a paucity of data regarding the effects of post-menopausal-associated estrogen loss on cellular function. We studied human adipose-derived mesenchymal stem cells (hASCs) isolated from women younger than 45 years old (pre-menopause, pre-hASC) or older than 55 years old (post-menopause, post-hASC). In this study, we provide proof of concept that the age-related ineffective functionality of ASCs can be reversed to improve their ability in promoting tissue repair. We found reduced estrogen receptor expression, decreased estrogen receptor activation, and reduced sensitivity to 17β-estradiol in post-hASCs. This correlated with decreased antioxidants (catalase and superoxide dismutase [SOD] expression) and increased oxidative stress compared with pre-hASCs. Increasing catalase expression in post-hASCs restored estrogen receptor (ER) expression and their functional capacity to promote tissue repair as shown in human skin ex vivo wound healing and in vivo mouse model of lung injury. Our results suggest that the consequences of 17β-estradiol decline on the function of hASCs may be reversible by changing the oxidative stress/antioxidant composition.

Dual-mechanism estrogen receptor inhibitors

Efforts to improve estrogen receptor-α (ER)-targeted therapies in breast cancer have relied upon a single mechanism, with ligands having a single side chain on the ligand core that extends outward to determine antagonism of breast cancer growth. Here, we describe inhibitors with two ER-targeting moieties, one of which uses an alternate structural mechanism to generate full antagonism, freeing the side chain to independently determine other critical properties of the ligands. By combining two molecular targeting approaches into a single ER ligand, we have generated antiestrogens that function through new mechanisms and structural paradigms to achieve antagonism. These dual-mechanism ER inhibitors (DMERIs) cause alternate, noncanonical structural perturbations of the receptor ligand-binding domain (LBD) to antagonize proliferation in ER-positive breast cancer cells and in allele-specific resistance models. Our structural analyses with DMERIs highlight marked differences from current standard-of-care, single-mechanism antiestrogens. These findings uncover an enhanced flexibility of the ER LBD through which it can access nonconsensus conformational modes in response to DMERI binding, broadly and effectively suppressing ER activity.

The Potential Role of Nutraceuticals as an Adjuvant in Breast Cancer Patients to Prevent Hair Loss Induced by Endocrine Therapy

Nutraceuticals, natural dietary and botanical supplements offering health benefits, provide a basis for complementary and alternative medicine (CAM). Use of CAM by healthy individuals and patients with medical conditions is rapidly increasing. For the majority of breast cancer patients, treatment plans involve 5–10 yrs of endocrine therapy, but hair loss/thinning is a common side effect. Many women consider this significant, severely impacting on quality of life, even leading to non-compliance of therapy. Therefore, nutraceuticals that stimulate/maintain hair growth can be proposed. Although nutraceuticals are often available without prescription and taken at the discretion of patients, physicians can be reluctant to recommend them, even as adjuvants, since potential interactions with endocrine therapy have not been fully elucidated. It is, therefore, important to understand the modus operandi of ingredients to be confident that their use will not interfere/interact with therapy. The aim is to improve clinical/healthcare outcomes by combining specific nutraceuticals with conventional care whilst avoiding detrimental interactions. This review presents the current understanding of nutraceuticals beneficial to hair wellness and outcomes concerning efficacy/safety in breast cancer patients. We will focus on describing endocrine therapy and the role of estrogens in cancer and hair growth before evaluating the effects of natural ingredients on breast cancer and hair growth.

In silico study of molecular mechanisms of action: Estrogenic disruptors among phthalate esters

Phthalate esters (PAEs), as widely used plasticizers, have been concerned for their possible disruption of estrogen functions via binding to and activating the transcription of estrogen receptors (ERs). Nevertheless, the computational interpretation of the mechanism of ERs activities modulated by PAEs at the molecular level is still insufficient, which hinders the reliable screening of the ERs-active PAEs with high speed and high throughput. To bridge the gap, the in silico simulations considering the effects of coactivators were accomplished to explore the molecular mechanism of action for the purpose of predicting the estrogenic potencies of PAEs. The transcriptional activation functions of human ERα (hERα) modulated by PAEs is predicted via the simulations including binding interaction of PAEs and hERα, conformational changes of PAEs-hERα complexes and recruitment of coactivators. Molecular insight into the diverse estrogen mechanism of action among PAEs with regard to hERα agonists and selective estrogen receptor modulators (SERMs) is provided. Agonist-modulated conformational change of hERα leads to the optimal exposure of its Activation Function 2 (AF-2) surface which, in turn, facilitates the recruitment of coactivators, therefore promoting the transcriptional activation functions of hERα. Conversely, binding interaction of hERα with SERMs among PAEs leads to the conformational change with blocked AF-2 surface, thus preventing the recruitment of coactivators and consequently inhibiting the AF-2 activity. The two-hybrid recombinant yeast is experimentally used for verification. The established in silico evaluation methodology exhibits great promise to speed up the prediction of chemicals which work as hERα agonist or SERMs.

Tamoxifen induction of Cre recombinase does not cause long-lasting or sexually divergent responses in the CNS epigenome or transcriptome: implications for the design of aging studies

The systemic delivery of tamoxifen (Tam) to activate inducible CreERT2-loxP transgenic mouse systems is now widely used in neuroscience studies. This critical technological advancement allows temporal control of DNA-cre recombination, avoidance of embryonically lethal phenotypes, and minimization of residual cell labeling encountered in constitutively active drivers. Despite its advantages, the use of Tam has the potential to cause long-lasting, uncharacterized side effects on the transcriptome and epigenome in the CNS, given its mixed estrogen receptor (ER) agonist/antagonist actions. With the welcome focus on including both sexes in biomedical studies and efforts to understand sex differences, Tam administration could also cause sexually divergent responses that would confound studies. To examine these issues, epigenetic and transcriptomic profiles were compared in C57BL/6 J female and male hippocampus, cortex, and retina 1 month after a 5-day Tam treatment typical for cre induction, or vehicle control (sunflower seed oil). Cytosine methylation and hydroxymethylation levels, in both CG and non-CG contexts, were unchanged as determined by oxidative bisulfite sequencing. Long-lasting Tam transcriptomic effects were also not evident/minimal. Furthermore, there is no evidence of sexually divergent responses with Tam administration and Tam did not alter sex differences evident in controls. Combined with recently reported data that Tam alone does not cause long-lasting changes in behavior and neurogenesis, our findings provide confidence that Tam can be used as a cre-recombinase inducer without introducing significant confounds in transcriptomic and epigenomic neuroscience studies, particularly those focused on genomic and transcriptomic aspects of the aging brain.

Next-Generation ERα Inhibitors for Endocrine-Resistant ER+ Breast Cancer

One in eight women will be diagnosed with breast cancer in their lifetime. Because estrogen receptor-α (ERα) is expressed in ~70% of patients, therapeutic intervention by ERα-targeted endocrine therapies remains the leading strategy to prevent progression and/or metastasis in the adjuvant setting. However, the efficacy of these therapies will be diminished by the development of acquired resistance after prolonged treatment regimens. In preclinical models of endocrine-resistant metastatic breast cancers that retain ERα expression, antiestrogens with improved efficacy and potency can overcome resistance to shrink tumors and prevent metastasis. In particular, selective ER degraders or downregulators, which both antagonize ERα actions and induce its degradation, have demonstrated substantial antitumor efficacy in this setting. In the present review, we have discussed the mechanisms of acquired endocrine resistance in luminal breast cancers and the strategies used by next-generation endocrine therapies to antagonize ERα.

Antagonists for Constitutively Active Mutant Estrogen Receptors: Insights into the Roles of Antiestrogen-Core and Side-Chain

A major risk for patients having estrogen receptor α (ERα)-positive breast cancer is the recurrence of drug-resistant metastases after initial successful treatment with endocrine therapies. Recent studies have implicated a number of activating mutations in the ligand-binding domain of ERα that stabilize the agonist conformation as a prominent mechanism for this acquired resistance. There are several critical gaps in our knowledge regarding the specific pharmacophore requirements of an antagonist that could effectively inhibit all or most of the different mutant ERs. To address this, we screened various chemotypes for blocking mutant ER-mediated transcriptional signaling and identified RU58668 as a model compound that contains structural elements that support potent ligand-induced inhibition of mutant ERs. We designed and synthesized a focused library of novel antagonists and probed how small and large perturbations in different ligand structural regions influenced inhibitory activity on individual mutant ERs in breast cancer cells. Effective inhibition derives from both nonpolar and moderately polar motifs in a multifunctional side chain of the antagonists, with the nature of the ligand core making important contributions by increasing the potency of ligands possessing similar types of side chains. Some of our new antagonists potently blocked the transcriptional activity of the three most common mutant ERs (L536R, Y537S, D538G) and inhibited mutant ER-mediated cell proliferation. Supported by our molecular modeling, these studies provide new insights into the role of specific components, involving both the ligand core and multifunctional side chain, in suppressing wild-type and mutant ER-mediated transcription and breast cancer cell proliferation.

Role of SUMOylation in differential ERα transcriptional repression by tamoxifen and fulvestrant in breast cancer cells

Antiestrogens (AEs) are widely used for treatment of estrogen receptor alpha (ERα)-positive breast cancer, but display variable degrees of partial agonism in estrogen target tissues and breast cancer (BC) cells. The fact that BC cells resistant to selective ER modulators (SERMs) like tamoxifen (Tam) can still be sensitive to pure AEs, also called selective ER downregulators, suggests different mechanisms of action, some of which may contribute to the more complete suppression of estrogen target genes by pure AEs. We report herein that pure AEs such as fulvestrant induce transient binding of ERα to DNA, followed by rapid release after 30–40 min without loss of nuclear localization. Loss of DNA binding preceded receptor degradation and was not prevented by proteasome inhibition. Chromatin was less accessible in the presence of fulvestrant than with estradiol or Tam as early as 20 min following treatment, suggesting that chromatin remodeling by pure AEs at ERα target regions prevents transcription in spite of receptor binding. SUMO2/3 marks were detected on chromatin at the peak of ERα binding in cells treated with pure AEs, but not SERMs. Furthermore, decreasing SUMOylation by overexpressing the deSUMOylase SENP1 significantly delayed receptor release from DNA and de-repressed expression of estrogen target genes in the presence of fulvestrant, both in ERα-expressing MCF-7 cells and in transiently transfected ER-negative SK-BR-3 cells. Finally, mutation V534E, identified in a breast metastasis resistant to hormonal therapies, prevented ERα modification and resulted in increased transcriptional activity of estrogen target genes in the presence of fulvestrant in SK-BR-3 cells. Together, our results establish a role for SUMOylation in achieving a more complete transcriptional shut-off of estrogen target genes by pure AEs vs. SERMs in BC cells.

Specific stereochemistry of OP-1074 disrupts estrogen receptor alpha helix 12 and confers pure antiestrogenic activity

Complex tissue-specific and cell-specific signaling by the estrogen receptor (ER) frequently leads to the development of resistance to endocrine therapy for breast cancer. Pure ER antagonists, which completely lack tissue-specific agonist activity, hold promise for preventing and treating endocrine resistance, however an absence of structural information hinders the development of novel candidates. Here we synthesize a small panel of benzopyrans with variable side chains to identify pure antiestrogens in a uterotrophic assay. We identify OP-1074 as a pure antiestrogen and a selective ER degrader (PA-SERD) that is efficacious in shrinking tumors in a tamoxifen-resistant xenograft model. Biochemical and crystal structure analyses reveal a structure activity relationship implicating the importance of a stereospecific methyl on the pyrrolidine side chain of OP-1074, particularly on helix 12.

Estrogen receptor alpha (ERα) plays critical roles in the etiology and treatment of breast cancer. Here the authors synthesize benzopyrans with variable side chains to identify antiestrogenic compounds and characterize OP-1074, a compound that exhibits pure antiestrogenic activity by inducing the degradation of ERα and possesses greater potency than tamoxifen or fulvestrant in a xenograft model.

Effective combination therapies in preclinical endocrine resistant breast cancer models harboring ER mutations

Although endocrine therapy is successfully used to treat patients with estrogen receptor (ER) positive breast cancer, a substantial proportion of this population will relapse. Several mechanisms of acquired resistance have been described including activation of the mTOR pathway, increased activity of CDK4 and activating mutations in ER. Using a patient derived xenograft model harboring a common activating ER ligand binding domain mutation (D538G), we evaluated several combinatorial strategies using the selective estrogen receptor degrader (SERD) fulvestrant in combination with chromatin modifying agents, and CDK4/6 and mTOR inhibitors. In this model, fulvestrant binds WT and MT ER, reduces ER protein levels, and downregulated ER target gene expression. Addition of JQ1 or vorinostat to fulvestrant resulted in tumor regression (41% and 22% regression, respectively) though no efficacy was seen when either agent was given alone. Interestingly, although the CDK4/6 inhibitor palbociclib and mTOR inhibitor everolimus were efficacious as monotherapies, long-term delayed tumor growth was only observed when co-administered with fulvestrant. This observation was consistent with a greater inhibition of compensatory signaling when palbociclib and everolimus were co-dosed with fulvestrant. The addition of fulvestrant to JQ1, vorinostat, everolimus and palbociclib also significantly reduced lung metastatic burden as compared to monotherapy. The combination potential of fulvestrant with palbociclib or everolimus were confirmed in an MCF7 CRISPR model harboring the Y537S ER activating mutation. Taken together, these data suggest that fulvestrant may have an important role in the treatment of ER positive breast cancer with acquired ER mutations.

Skeletal and Uterotrophic Effects of Endoxifen in Female Rats

Endoxifen, the primary active metabolite of tamoxifen, is currently being investigated as a novel endocrine therapy for the treatment of breast cancer. Tamoxifen is a selective estrogen receptor modulator that elicits potent anti-breast cancer effects. However, long-term use of tamoxifen also induces bone loss in premenopausal women and is associated with an increased risk of endometrial cancer in postmenopausal women. For these reasons, we have used a rat model system to comprehensively characterize the impact of endoxifen on the skeleton and uterus. Our results demonstrate that endoxifen elicits beneficial effects on bone in ovary-intact rats and protects against bone loss following ovariectomy. Endoxifen is also shown to reduce bone turnover in both ovary-intact and ovariectomized rats at the cellular and biochemical levels. With regard to the uterus, endoxifen decreased uterine weight but maintained luminal epithelial cell height in ovariectomized animals. Within luminal epithelial cells, endoxifen resulted in differential effects on the expression levels of estrogen receptors α and β as well as multiple other genes previously implicated in regulating epithelial cell proliferation and hypertrophy. These studies analyze the impact of extended endoxifen exposure on both bone and uterus using a Food and Drug Administration-recommended animal model. Although endoxifen is a more potent breast cancer agent than tamoxifen, the results of the present study demonstrate that endoxifen does not induce bone loss in ovary-intact rats and that it elicits partial agonistic effects on the uterus and skeleton in ovariectomized animals.

Flexible small molecular anti-estrogens with N,N-dialkylated-2,5-diethoxy-4-morpholinoaniline scaffold targets multiple estrogen receptor conformations

Estrogen mediates various cellular processes including cell proliferation, differentiation, growth and mammary gland function. Estrogen Receptors (ERs) are expressed in 70% of breast cancers. Consequently, estrogen mediated ER signaling plays a critical role in breast cancer diagnosis, prognosis, and treatment. ERs are ligand-triggered transcription factors. However, in the absence of a cognate estrogenic ligand, ERs can be activated by a variety of other extracellular signals. Tamoxifen, an anti-estrogen that selectively targets ER, induces substantial regression of breast tumors and an increase in disease-free survival. Tamoxifen mimics estrogen effects in other tissues thereby providing some beneficial effects including reduced risk of osteoporosis. However, breast cancers that initially respond well to tamoxifen tend to develop resistance and resume growth despite the continued presence of the antagonist. Library of compounds with substituted morpholinoaniline scaffold, a set of structurally divergent potential ER antagonists that fit the tamoxifen pharmacophore, were designed to target ER Ligand Binding Domain (LBD) and to recruit co-regulator proteins including BRCA1 over a range of conformational changes. Two of the lead compounds in the library, BR46 and BR47, were found to inhibit estrogen induced cell proliferation and cell viability. Discovery of novel lead molecules targeting ligand binding pockets of hER has provided structural clues toward the development of new breed of small molecule therapeutics for tamoxifen-resistant breast cancers and would complement already existent anti-estrogen therapy.

The effect of selective estrogen receptor modulators on type 2 diabetes onset in women: Basic and clinical insights

Selective estrogen receptor modulators (SERMs) are a class of compounds that interact with estrogen receptors (ERs) and exert agonist or antagonist effects on ERs in a tissue-specific manner. Tamoxifen, a first generation SERM, is used for treatment of ER positive breast cancer. Raloxifene, a second generation SERM, was used to prevent postmenopausal osteoporosis. The third-generation SERM bazedoxifene (BZA) effectively prevents osteoporosis while preventing estrogenic stimulation of breast and uterus. Notably, BZA combined with conjugated estrogens (CE) is a new menopausal treatment. The menopausal state predisposes to metabolic syndrome and type 2 diabetes, and therefore the effects of SERMs on metabolic homeostasis are gaining attention. Here, we summarize knowledge of SERMs' impacts on metabolic, homeostasis, obesity and diabetes in rodent models and postmenopausal women.

Antiestrogens: structure-activity relationships and use in breast cancer treatment

About 70% of breast tumors express estrogen receptor alpha (ERα), which mediates the proliferative effects of estrogens on breast epithelial cells, and are candidates for treatment with antiestrogens, steroidal or non-steroidal molecules designed to compete with estrogens and antagonize ERs. The variable patterns of activity of antiestrogens (AEs) in estrogen target tissues and the lack of systematic cross-resistance between different types of molecules have provided evidence for different mechanisms of action. AEs are typically classified as selective estrogen receptor modulators (SERMs), which display tissue-specific partial agonist activity (e.g. tamoxifen and raloxifene), or as pure AEs (e.g. fulvestrant), which enhance ERα post-translational modification by ubiquitin-like molecules and accelerate its proteasomal degradation. Characterization of second- and third-generation AEs, however, suggests the induction of diverse ERα structural conformations, resulting in variable degrees of receptor downregulation and different patterns of systemic properties in animal models and in the clinic.

Effect of selective estrogen receptor modulators on metabolic homeostasis

Selective estrogen receptor modulators (SERMs) are estrogen receptor (ER) ligands that exhibit either estrogen agonistic or antagonistic activity in a tissue-specific manner. The first and second generation SERMs, tamoxifen and raloxifene, are used for treatment of ER positive breast cancer and postmenopausal osteoporosis respectively. The third-generation SERM, bazedoxifene (BZA), effectively prevents osteoporosis while blocking the estrogenic stimulation in breast and uterus. Notably, BZA combined with conjugated estrogens (CE) in a tissue-selective estrogen complex (TSEC) is a new menopausal treatment. Postmenopausal estrogen deficiency predisposes to metabolic syndrome and type 2 diabetes, and therefore the effects of SERMs and TSECs on metabolic homeostasis are gaining attention. In this article, we summarize current knowledge about the impact of SERMs on metabolic homeostasis and metabolic disorders in animal models and postmenopausal women.

Nuclear receptor modulation--role of coregulators in selective estrogen receptor modulator (SERM) actions

Selective estrogen receptor modulators (SERMs) are a class of small-molecule chemical compounds that bind to estrogen receptor (ER) ligand binding domain (LBD) with high affinity and selectively modulate ER transcriptional activity in a cell- and tissue-dependent manner. The prototype of SERMs is tamoxifen, which has agonist activity in bone, but has antagonist activity in breast. Tamoxifen can reduce the risk of breast cancer and, at same time, prevent osteoporosis in postmenopausal women. Tamoxifen is widely prescribed for treatment and prevention of breast cancer. Mechanistically the activity of SERMs is determined by the selective recruitment of coactivators and corepressors in different cell types and tissues. Therefore, understanding the coregulator function is the key to understanding the tissue selective activity of SERMs.

Estrogens and aging skin

Estrogen deficiency following menopause results in atrophic skin changes and acceleration of skin aging. Estrogens significantly modulate skin physiology, targeting keratinocytes, fibroblasts, melanocytes, hair follicles and sebaceous glands, and improve angiogenesis, wound healing and immune responses. Estrogen insufficiency decreases defense against oxidative stress; skin becomes thinner with less collagen, decreased elasticity, increased wrinkling, increased dryness and reduced vascularity. Its protective function becomes compromised and aging is associated with impaired wound healing, hair loss, pigmentary changes and skin cancer.

 

Skin aging can be significantly delayed by the administration of estrogen. This paper reviews estrogen effects on human skin and the mechanisms by which estrogens can alleviate the changes due to aging. The relevance of estrogen replacement, selective estrogen receptor modulators (SERMs) and phytoestrogens as therapies for diminishing skin aging is highlighted. Understanding estrogen signaling in skin will provide a basis for interventions in aging pathologies.

Sex hormone receptor repertoire in breast cancer

Classification of breast cancer as endocrine sensitive, hormone dependent, or estrogen receptor (ER) positive refers singularly to ER α . One of the oldest recognized tumor targets, disruption of ER α -mediated signaling, is believed to be the mechanistic mode of action for all hormonal interventions used in treating this disease. Whereas ER α is widely accepted as the single most important predictive factor (for response to endocrine therapy), the presence of the receptor in tumor cells is also of prognostic value. Even though the clinical relevance of the two other sex hormone receptors, namely, ER β and the androgen receptor remains unclear, two discordant phenomena observed in hormone-dependent breast cancers could be causally related to ER β -mediated effects and androgenic actions. Nonetheless, our understanding of regulatory molecules and resistance mechanisms remains incomplete, further compromising our ability to develop novel therapeutic strategies that could improve disease outcomes. This review focuses on the receptor-mediated actions of the sex hormones in breast cancer.

The role of activation functions 1 and 2 of estrogen receptor-α for the effects of estradiol and selective estrogen receptor modulators in male mice

Estradiol (E2) is important for male skeletal health and the effect of E2 is mediated via estrogen receptor (ER)-α. This was demonstrated by the findings that men with an inactivating mutation in aromatase or a nonfunctional ERα had osteopenia and continued longitudinal growth after sexual maturation. The aim of the present study was to evaluate the role of different domains of ERα for the effects of E2 and selective estrogen receptor modulators (SERMs) on bone mass in males. Three mouse models lacking either ERαAF-1 (ERαAF-1(0)), ERαAF-2 (ERαAF-2(0)), or the total ERα (ERα(-/-)) were orchidectomized (orx) and treated with E2 or placebo. E2 treatment increased the trabecular and cortical bone mass and bone strength, whereas it reduced the thymus weight and bone marrow cellularity in orx wild type (WT) mice. These parameters did not respond to E2 treatment in orx ERα(-/-) or ERαAF-2(0). However, the effects of E2 in orx ERαAF-1(0) [corrected] were tissue-dependent, with a clear response in cortical bone parameters and bone marrow cellularity, but no response in trabecular bone. To determine the role of ERαAF-1 for the effects of SERMs, we treated orx WT and ERαAF-1(0) mice with raloxifene (Ral), lasofoxifene (Las), bazedoxifene (Bza), or vehicle. These SERMs increased total body areal bone mineral density (BMD) and trabecular volumetric BMD to a similar extent in orx WT mice. Furthermore, only Las increased cortical thickness significantly and only Bza increased bone strength significantly. However, all SERMs showed a tendency toward increased cortical bone parameters. Importantly, all SERM effects were absent in the orx ERαAF-1(0) mice. In conclusion, ERαAF-2 is required for the estrogenic effects on all evaluated parameters, whereas the role of ERαAF-1 is tissue-specific. All evaluated effects of Ral, Las and Bza are dependent on a functional ERαAF-1. Our findings might contribute to the development of bone-specific SERMs in males.

Role of SUMOylation in full antiestrogenicity

The selective estrogen receptor downregulator (SERD) fulvestrant can be used as second-line treatment for patients relapsing after treatment with tamoxifen, a selective estrogen receptor modulator (SERM). Unlike tamoxifen, SERDs are devoid of partial agonist activity. While the full antiestrogenicity of SERDs may result in part from their capacity to downregulate levels of estrogen receptor alpha (ERα) through proteasome-mediated degradation, SERDs are also fully antiestrogenic in the absence of increased receptor turnover in HepG2 cells. Here we report that SERDs induce the rapid and strong SUMOylation of ERα in ERα-positive and -negative cell lines, including HepG2 cells. Four sites of SUMOylation were identified by mass spectrometry analysis. In derivatives of the SERD ICI164,384, SUMOylation was dependent on the length of the side chain and correlated with full antiestrogenicity. Preventing SUMOylation by the overexpression of a SUMO-specific protease (SENP) deSUMOylase partially derepressed transcription in the presence of full antiestrogens in HepG2 cells without a corresponding increase in activity in the presence of agonists or of the SERM tamoxifen. Mutations increasing transcriptional activity in the presence of full antiestrogens reduced SUMOylation levels and suppressed stimulation by SENP1. Our results indicate that ERα SUMOylation contributes to full antiestrogenicity in the absence of accelerated receptor turnover.

The terminal substituents of 7α, 6-hexanyl derivatives of estradiol determine their selective estrogen receptor modulator versus agonist activities

Pure antiestrogens were clinically developed as alternative therapies for estrogen receptor (ER) positive breast cancers. Unlike the selective estrogen receptor modulators (SERMs), these antiestrogens are devoid of tissue-specific ER agonist activity. Many of these compounds are steroidal in nature, containing an estradiol (E2) structural core with long alkyl side chains at the C-7α position. Two novel 7α-substituted E2 derivatives were evaluated that retain high binding affinity for ER. Compared to known pure antiestrogens, these compounds, referred to as compound 13 (C13) and C14, contain shorter 7α alkyl side chains and differ only in their terminal substituent: a hydroxyl moiety versus a benzyloxy group, respectively. Herein we assessed the effects of these compounds on ER transcriptional activity and report that despite their similar overall structure, C13 and C14 produce distinct cell type-specific responses. Of note, C13 functions as a mixed agonist/antagonist in Hela cells, inducing only weak ER transcriptional activity while preventing coactivator recruitment and stabilizing ER expression. However, this compound effectively stimulates ER activity in MCF-7 cells, does not increase ER levels and promotes cell proliferation on par with E2. Conversely, C14 stimulates transcriptional activity in both cell types and enhances ER-coactivator interactions. The activities of both compounds were inhibited by the pure antiestrogen ICI 182,780. Taken together, these results reveal that C13 is a SERM while C14 is an ER agonist, and indicate that the terminal modification of the C-7α hexanyl side chain of these estradiol derivatives is an important determinant of the biocharacter of these ER ligands.

Estrogen regulates JNK1 genomic localization to control gene expression and cell growth in breast cancer cells

Steroid hormone and MAPK signaling pathways functionally intersect, but the molecular mechanisms of this cross talk are unclear. Here, we demonstrate a functional convergence of the estrogen and c-Jun N-terminal kinase 1 (JNK1) signaling pathways at the genomic level in breast cancer cells. We find that JNK1 binds to many promoters across the genome. Although most of the JNK1-binding sites are constitutive, a subset is estrogen regulated (either induced on inhibited). At the estrogen-induced sites, estrogen receptor (ER)α is required for the binding of JNK1 by promoting its recruitment to estrogen response elements or other classes of DNA elements through a tethering mechanism, which in some cases involves activating protein-1. At estrogen-regulated promoters, JNK1 functions as a transcriptional coregulator of ERα in a manner that is dependent on its kinase activity. The convergence of ERα and JNK1 at target gene promoters regulates estrogen-dependent gene expression outcomes, as well as downstream estrogen-dependent cell growth responses. Analysis of existing gene expression profiles from breast cancer biopsies suggests a role for functional interplay between ERα and JNK1 in the progression and clinical outcome of breast cancers.

Activation of estrogen receptor α by raloxifene through an activating protein-1-dependent tethering mechanism in human cervical epithelial cancer cells: a role for c-Jun N-terminal kinase

Nuclear estrogen receptor α (ERα) regulates target gene expression in response to ligands through two distinct mechanisms: direct binding to DNA and indirect tethering through other DNA-bound transcription factors, such as AP-1. In the studies described herein, we examined the molecular mechanisms underlying the activation of ERα in the AP-1 tethering pathway by the selective estrogen receptor modulator (SERM) raloxifene (Ral). Our results with the MMP1 and PRUNE genes indicate that the c-Fos component of the AP-1 tethering factor and the c-Jun N-terminal kinase 1 (JNK1) are constitutively bound at the promoter regions prior to Ral exposure. Ral then promotes the binding of ERα at the promoter in a c-Fos-dependent manner. Interestingly, we found that JNK1 enzymatic activity is required for Ral-dependent gene activation through ERα. Our results suggest that one role for Ral-dependent recruitment of ERα to the AP-1 binding site is to stimulate JNK1 enzymatic activity. Alternatively, Ral-occupied ERα might recruit protein substrates to promoter-bound JNK1 without any change in JNK1 activity. Collectively, our studies have revealed a new role for JNK1 in determining gene regulatory outcomes by ERα.

Helix 12 dynamics and thyroid hormone receptor activity: experimental and molecular dynamics studies of Ile280 mutants

Nuclear hormone receptors (NRs) form a family of transcription factors that mediate cellular responses initiated by hormone binding. It is generally recognized that the structure and dynamics of the C-terminal helix 12 (H12) of NRs' ligand binding domain (LBD) are fundamental to the recognition of coactivators and corepressors that modulate receptor function. Here we study the role of three mutations in the I280 residue of H12 of thyroid hormone receptors using site-directed mutagenesis, functional assays, and molecular dynamics simulations. Although residues at position 280 do not interact with coactivators or with the ligand, we show that its mutations can selectively block coactivator and corepressor binding, and affect hormone binding affinity differently. Molecular dynamics simulations suggest that ligand affinity is reduced by indirectly displacing the ligand in the binding pocket, facilitating water penetration and ligand destabilization. Mutations I280R and I280K link H12 to the LBD by forming salt bridges with E457 in H12, stabilizing H12 in a conformation that blocks both corepressor and coactivator recruitment. The I280M mutation, in turn, blocks corepressor binding, but appears to enhance coactivator affinity, suggesting stabilization of H12 in agonist conformation.

HEXIM1 is a critical determinant of the response to tamoxifen

Tamoxifen resistance is a major problem in the treatment of Estrogen Receptor (ER) positive patients. We have previously reported that Hexamethylene bis-acetamide Inducible Protein 1 (HEXIM1) inhibits ERα activity by competing with ERα for binding to cyclin T1, a subunit of Positive Transcription Elongation b (P-TEFb). This results in the inhibition of the phosphorylation of RNA polymerase II (RNAPII) at serine 2 and the inhibition of transcription elongation of ERα target genes. Since HEXIM1 can inhibit ER activity, we examined whether it plays a critical role in the inhibitory effects of tamoxifen on ER. We observed that tamoxifen induced HEXIM1 recruitment to the promoter region of ER target genes and decreased the recruitment of cyclin T1 and serine 2 phosphorylated RNAPII to the coding regions of these genes. Conversely, in cells wherein HEXIM1 expression has been downregulated we observed attenuation of the inhibitory effects of tamoxifen on estrogen-induced cyclin T1 recruitment to coding regions of ER target genes. As a consequence, downregulation of HEXIM1 resulted in the attenuation of the repressive effects of tamoxifen on estrogen-induced gene expression and proliferation. Conferring clinical relevance to our studies is our analysis of human breast cancer tissue samples that indicated association of lower expression of HEXIM1 with tumor recurrence in patients who received tamoxifen. Our studies provide a better understanding of the mechanistic basis for the inhibitory effect of tamoxifen on ER activity and may suggest new therapeutic targets for the treatment of tamoxifen resistant breast cancer.

Estrogen induces distinct patterns of microRNA expression within the mouse uterus

Control of estrogenic activity within the uterus is evident as unopposed estrogen action is associated with endometrial pathologies such as endometriosis and endometrial carcinoma. MicroRNAs (miRNAs) have emerged as important posttranscriptional regulators, which are postulated to fine-tune the actions of steroids in many systems including the uterus. The objective of the current study was to examine uterine expression of miRNAs in response to estrogen treatment within the mouse uterus using an ovariectomized, steroid-reconstituted mouse model. MicroRNA microarray analysis and subsequent quantitative real-time reverse transcriptase polymerase chain reaction (qRT-PCR) verification revealed that expression of mirn155, mirn429, and mirn451 was significantly increased by estrogen administration whereas mirn181b and mirn204 expression was significantly reduced. Pretreatment with the estrogen receptor (ER) antagonist ICI 182,780 confirmed that estrogen regulation was mediated via the classical ER pathway. This study demonstrates that estrogen regulates specific miRNAs within the murine uterus, which may participate in posttranscriptional regulation of estrogen-regulated genes.

Anti-implantation effect of 2-[piperidinoethoxyphenyl]-3-[4-hydroxyphenyl]-2H-benzo(b)pyran, a potent antiestrogenic agent in rats

OBJECTIVE

To investigate the anti-implantation effect and hormonal profile of 2-[piperidinoethoxyphenyl]-3-[4-hydroxyphenyl]-2H-benzo(b)pyran (K-1) in rats.

DESIGN

In vivo assays for anti-implantation activity were performed in pregnant rats. Assays for estrogenicity/antiesrogenicity were performed in immature ovariectomized female rats. In vitro competitive binding of K-1 to human recombinant ERα, transient transfection assay using ERE-luciferase reporter, and alkaline phosphatase (ALP) activity as a measure of estrogenicity and/antiestrogenicity in human endometrial carcinoma cells were performed.

SETTING

Research laboratory.

ANIMAL(S)

Adult female rats for anti-implantation activity, immature ovariectomized female rats, and immature castrated/intact male rats.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Number of implantations, uterine growth, luciferase reporter activity, ER binding affinity, and ALP activity.

RESULT(S)

Compound K-1 given orally for 1-7 days post coitum at the dose of 100 μg/kg body weight prevented pregnancy in 100% of rats. K-1 was a potent antiestrogenic, and at 50 μg/kg, it could inhibit the effect of 1 μg E(2) in immature rats. Compound was devoid of uterotrophic, androgenic, or antigonadotropic activity. A high affinity binding to ERα was displayed by K-1, with a relative binding affinity of 5% of E(2). In human endometrial carcinoma cells, K-1 did not induce ERα-mediated transcriptional activation that is measured as luciferase reporter activity. K-1 antagonized the E-induced transcriptional activation significantly. K-1 also antagonized E-induced ALP activity in human endometrial cells.

CONCLUSION(S)

K-1 appeared to exert its antifertility action by virtue of its strong antiestrogenic activity.

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.

Cooperative activation of cyclin D1 and progesterone receptor gene expression by the SRC-3 coactivator and SMRT corepressor

Although the ability of coactivators to enhance the expression of estrogen receptor-alpha (ERalpha) target genes is well established, the role of corepressors in regulating 17beta-estradiol (E2)-induced gene expression is poorly understood. Previous studies revealed that the silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) corepressor is required for full ERalpha transcriptional activity in MCF-7 breast cancer cells, and we report herein the E2-dependent recruitment of SMRT to the regulatory regions of the progesterone receptor (PR) and cyclin D1 genes. Individual depletion of SMRT or steroid receptor coactivator (SRC)-3 modestly decreased E2-induced PR and cyclin D1 expression; however, simultaneous depletion revealed a cooperative effect of this coactivator and corepressor on the expression of these genes. SMRT and SRC-3 bind directly in an ERalpha-independent manner, and this interaction promotes E2-dependent SRC-3 binding to ERalpha measured by co-IP and SRC-3 recruitment to the cyclin D1 gene as measured by chromatin IP assays. Moreover, SMRT stimulates the intrinsic transcriptional activity of all of the SRC family (p160) coactivators. Our data link the SMRT corepressor directly with SRC family coactivators in positive regulation of ERalpha-dependent gene expression and, taken with the positive correlation found for SMRT and SRC-3 in human breast tumors, suggest that SMRT can promote ERalpha- and SRC-3-dependent gene expression in breast cancer.

Estrogen receptors recruit SMRT and N-CoR corepressors through newly recognized contacts between the corepressor N terminus and the receptor DNA binding domain

Estrogen receptors (ERs) are hormone-regulated transcription factors that regulate key aspects of reproduction and development. ERs are unusual in that they do not typically repress transcription in the absence of hormone but instead possess otherwise cryptic repressive functions that are revealed upon binding to certain hormone antagonists. The roles of corepressors in the control of these aspects of ER function are complex and incompletely understood. We report here that ERs recruit SMRT through an unusual mode of interaction involving multiple contact surfaces. Two surfaces of SMRT, located at the N- and C-terminal domains, contribute to the recruitment of the corepressor to ERs in vitro and are crucial for the corepressor modulation of ER transcriptional activity in cells. These corepressor surfaces contact the DNA binding domain of the receptor, rather than the hormone binding domain previously elucidated for other corepressor/nuclear receptor interactions, and are modulated by the ER's recognition of cognate DNA binding sites. Several additional nuclear receptors, and at least one other corepressor, N-CoR, share aspects of this novel mode of corepressor recruitment. Our results highlight a molecular mechanism that helps explain several previously paradoxical aspects of ER-mediated transcriptional antagonism, which may have a broader significance for an understanding of target gene repression by other nuclear receptors.

A novel mutation of thyroid hormone receptor beta (I431V) impairs corepressor release, and induces thyroid hormone resistance syndrome

Resistance to thyroid hormone (RTH) is a rare disorder characterized by variable tissue hyporesponsiveness to thyroid hormone, usually caused by mutations in the thyroid hormone receptor beta (TRbeta). We describe a large Brazilian family harboring a novel mutation affecting TRbeta gene and inducing RTH. A 14-year-old girl was found to have elevated free T4 and free T3 plasma concentrations in coexistence with unsuppressed TSH and a questionable goiter. The diagnosis of RTH was verified by identification of a novel mutation (I431V) in the TRbeta gene. Sixteen asymptomatic relatives of the proposita are also affected by the mutation. Functional studies showed that I431V mutation exerts dominant-negative effect on wild type TRbeta, mainly by impairment of ligand-dependent release of corepressor SMRT. The presence of this mutation reduces potency, but does not affect efficacy of thyroid hormone action, in accordance with the clinical picture of eumetabolism of the affected individuals.

Expression of estrogen receptor co-regulators SRC-1, RIP140 and NCoR and their interaction with estrogen receptor in rat uterus, under the influence of ormeloxifene

Ormeloxifene binds competitively to ERs and antagonizes estrogen-induced gene expression in the uterus. However its detailed molecular mechanisms are not well understood. Present study was aimed to examine the changes in expression pattern of co-regulatory proteins SRC-1 (co-activator), RIP140 and NCoR (co-repressors) and their interaction with ERalpha in rat uterus under the influence of ormeloxifene (Orm) and tamoxifen (Tam). Adult ovariectomized rats were treated with estradiol (E(2)) (5 microg/100g), or Orm or Tam (200 microg/100g, s.c.) alone or along with E(2), for 3 days. RT-PCR analysis of uterine RNA and immunoblotting of uterine extracts revealed that expression of SRC-1, RIP140 and NCoR was insensitive to E(2) or Orm or Tam treatment. Direct protein-protein interaction experiments using co-immunoprecipitation revealed that E(2)-induced the interaction of ERalpha with co-activator SRC-1. In rats given Orm alone or along with E(2), there was a significant reduction in E(2)-induced effect on ERalpha-SRC-1 interaction. In case of ERbeta and SRC-1, Orm reduced interaction only in the absence of E(2). Interaction of RIP140 or NCoR with ERalpha was found to be more in rats treated with Orm along with E(2) as compared to that in E(2)-treated rats whereas no such recruitment was found in Tam treated rats. Interaction of RIP140 with ERbeta was insensitive to Orm or Tam treatment whereas the interaction of NCoR with ERalpha and ERbeta was increased in Orm treated rats. Ormeloxifene also showed inhibitory effects on uterine ER-ERE binding and estrogen-induced expression of progesterone receptor. Taken together, these findings demonstrate that ormeloxifene antagonizes ERalpha-mediated transcription by inhibiting the recruitment of SRC-1 and inducing the recruitment of RIP140 and NCoR.

Immunohistochemical labelling of steroid receptors in normal and malignant human endometrium

For several years it was generally believed that only a single estrogen receptor (ER) and progesterone receptor (PR) existed. However, the discovery of a new ER (ERbeta) with specificity for estrogens has induced new insights in the estrogen signalling system. Moreover, PR is expressed as two major isoforms, PR-A and PR-B that arise from alternative transcriptional starting sites within the same gene. Although PR-A and PR-B were thought to occur in similar amounts, it is now clear that they are differentially expressed and thus have distinct functions in several human tissues, including human endometrium. The ER and PR expression and distribution pattern might play an important role in normal endometrial function and pathogenesis and the expression and relationship of the two distinct ER's and PR's could be of essential clinical implications. Moreover, the imbalance in ERalpha/ERbeta expression and the PR-A/PR-B ratio might play an important role in endometrial transition and subsequently influence endometrial pathogenesis. The knowledge of the pattern of steroid receptors in human endometrial tissue is of extreme importance, since it might start a new field in hormone therapy of endometrial cancer.

What goes on behind closed doors: physiological versus pharmacological steroid hormone actions

Steroid-hormone-activated receptor proteins are among the best-understood class of factors for altering gene transcription in cells. Steroid receptors are of major importance in maintaining normal human physiology by responding to circulating concentrations of steroid in the nM range. Nonetheless, most studies of steroid receptor action have been conducted using the supra-physiological conditions of saturating concentrations (> or =100 nM) of potent synthetic steroid agonists. Here we summarize the recent developments arising from experiments using two clinically relevant conditions: subsaturating concentrations of agonist (to mimic the circulating concentrations in mammals) and saturating concentrations of antagonists (which are employed in endocrine therapies to block the actions of endogenous steroids). These studies have revealed new facets of steroid hormone action that could not be uncovered by conventional experiments with saturating concentrations of agonist steroids, such as a plethora of factors/conditions for the differential control of gene expression by physiological levels of steroid, a rational approach for examining the gene-specific variations in partial agonist activity of antisteroids, and a dissociation of steroid potency and efficacy that implies the existence of separate, and possibly novel, mechanistic steps and cofactors.

Biological functions and clinical implications of oestrogen receptors alfa and beta in epithelial tissues

For the past 10 years it is known that oestrogen functions through the activation of two oestrogen receptors (ERalpha and ERbeta). To the great surprise of endocrinologists, ERbeta was found to be widely distributed in tissues throughout the body including tissues previously considered as 'oestrogen insensitive'. The epithelium of the ventral prostate and lung as well as ovarian granulosa cells are ERalpha-negative but ERbeta-positive and in these tissues ERbeta seems to be involved in important physiological processes, like differentiation, extracellular matrix organization and stromal-epithelial communication. In tissues where both ERs are expressed, the two receptors seem to counteract each other. In the uterus, mammary gland and immune system, ERalpha promotes proliferation whereas ERbeta has pro-apoptotic and pro-differentiating functions. The challenge of the future will be to develop specific agonists, which can selectively activate/inactivate either ERalpha or ERbeta. These pharmaceuticals are likely to be of clinical importance in the prevention or treatment of various diseases.

Augmentation of estrogen receptor-mediated transcription by steroid and xenobiotic receptor

The estrogen receptor (ER) is a key regulator of proliferation and differentiation in breast cancer cells. In the present study, the effect of steroid and xenobiotic receptor (SXR) on 17/beta-estradiol (E2)-induced transcription through ERalpha was studied. SXR augmented ER-mediated transcription in the presence of E2 in MCF-7 breast cancer-derived cells and CV-1 fibroblast-derived cells. On the other hand, SXR alone did not affect the estrogen response element (ERE)-containing promoter activity in CV-1 cells. SXR did not directly bind to ERalpha or ERE in vitro, indicating that SXR may affect ER-mediated transcription by altering cofactor binding to ER. Although SXR did not alter the binding between ERalpha and p300/CBP interacting protein (p/CIP), it decreased the binding of a specific corepressor, silencing mediator of retinoid and thyroid hormone receptors (SMRT) to liganded ERalpha as assessed by mammalian two-hybrid, glutathione S-transferase pull-down, immunoprecipitation and newly developed Liquid Chemiluminescent DNA Pull-Down Assays. These results indicate that SXR augmented ER-mediated transcription by dissociating SMRT from ERalpha. Thus, the expression of SXR in breast cancer cells may alter the ER signaling, which may play crucial role for growth and differentiation of breast cancer cells.

What are comparative studies telling us about the mechanism of ERbeta action in the ERE-dependent E2 signaling pathway?

Estrogen hormone (E2) signaling is primarily conveyed by the estrogen receptors (ER) alpha and beta. ERs are encoded by two distinct genes and share varying degrees of domain-specific structural/functional similarities. ERs mediate a complex array of nuclear and non-nuclear events critical for the homeodynamic regulation of various tissue functions. The canonical nuclear signaling involves the interaction of ERalpha and ERbeta with specific DNA sequences, the so-called estrogen responsive elements (EREs). This interaction constitutes the initial step in ERE-dependent signaling in which ERbeta is a weaker transcription factor than ERalpha in response to E2. However, it remains unclear why transactivation potencies of ER subtypes differ. Studies suggest that the amino-terminus, the least conserved structural region, of ERbeta, but not that of ERalpha, impairs the ability of the receptor to bind to ERE independent of E2. Although the impaired ERbeta-ERE interaction contributes, it is not sufficient to explain the weak transactivation potency of the receptor. It appears that the lack of transactivation ability and of the capability of the amino-terminus of ERbeta, as opposed to that of ERalpha, to functionally interact with the carboxyl-terminal hormone-dependent activation domain is also critical for the receptor-specific activity. Thus, the structurally distinct amino-termini of ERs are important determinants in defining the function of ER-subtypes in the ERE-dependent pathway. This could differentially affect the physiology and pathophysiology of E2 signaling.