Mutations of tyrosine 537 in the human estrogen receptor-alpha selectively alter the receptor's affinity for estradiol and the kinetics of the interaction

ERα/PR crosstalk is altered in the context of the ERα Y537S mutation and contributes to endocrine therapy-resistant tumor proliferation

The constitutively active ESR1 Y537S mutation is associated with endocrine therapy (ET) resistance and progression of metastatic breast cancer through its effects on estrogen receptor (ERα) gene regulatory functions. However, the complex relationship between ERα and the progesterone receptor (PR), known as ERα/PR crosstalk, has yet to be characterized in the context of the ERα Y537S mutation. Using proximity ligation assays, we identify an increased physical interaction of ERα and PR in the context of the ERα Y537S mutation, including in the nucleus where this interaction may translate to altered gene expression. As such, more than 30 genes were differentially expressed in both patient tumor and cell line data (MCF7 and/or T47D cells) in the context of the ERα Y537S mutation compared to ERα WT. Of these, IRS1 stood out as a gene of interest, and ERα and PR occupancy at chromatin binding sites along IRS1 were uniquely altered in the context of ERα Y537S. Furthermore, siRNA knockdown of IRS1 or treatment with the IRS1 inhibitor NT-157 had a significant anti-proliferative effect in ERα Y537S cell lines, implicating IRS1 as a potential therapeutic target for restoring treatment sensitivity to patients with breast cancers harboring ERα Y537S mutations.

Ribociclib Induces Broad Chemotherapy Resistance and EGFR Dependency in ESR1 Wildtype and Mutant Breast Cancer

While endocrine therapy is highly effective for the treatment of oestrogen receptor-α (ERα)-positive breast cancer, a significant number of patients will eventually experience disease progression and develop treatment-resistant, metastatic cancer. The majority of resistant tumours remain dependent on ERα-action, with activating ESR1 gene mutations occurring in 15-40% of advanced cancers. Therefore, there is an urgent need to discover novel effective therapies that can eradicate cancer cells with aberrant ERα and to understand the cellular response underlying their action. Here, we evaluate the response of MCF7-derived, CRISPR-Cas9-generated cell lines expressing mutant ERα (Y537S) to a large number of drugs. We report sensitivity to numerous clinically approved inhibitors, including CDK4/6 inhibitor ribociclib, which is a standard-of-care therapy in the treatment of metastatic ERα-positive breast cancer and currently under evaluation in the neoadjuvant setting. Ribociclib treatment induces senescence in both wildtype and mutant ERα breast cancer models and leads to a broad-range drug tolerance. Strikingly, viability of cells undergoing ribociclib-induced cellular senescence is maintained via engagement of EGFR signalling, which may be therapeutically exploited in both wildtype and mutant ERα-positive breast cancer. Our study highlights a wide-spread reduction in sensitivity to anti-cancer drugs accompanied with an acquired vulnerability to EGFR inhibitors following CDK4/6 inhibitor treatment.

Ligand Pathways in Nuclear Receptors

Nuclear receptors (NRs) are ligand-inducible transcription factors that play an essential role in a multitude of physiological processes as well as diseases, rendering them attractive drug targets. Crystal structures revealed the binding site of NRs to be buried in the core of the protein, with no obvious route for ligands to access this cavity. The process of ligand binding is known to be an often-neglected contributor to the efficacy of drug candidates and is thought to influence the selectivity and specificity of NRs. While experimental methods generally fail to highlight the dynamic processes of ligand access or egress on the atomistic scale, computational methods have provided fundamental insight into the pathways connecting the buried binding pocket to the surrounding environment. Methods based on molecular dynamics (MD) and Monte Carlo simulations have been applied to identify pathways and quantify their capability to transport ligands. Here, we systematically review findings of more than 20 years of research in the field, including the applied methodology and controversies. Further, we establish a unified nomenclature to describe the pathways with respect to their location relative to protein secondary structure elements and summarize findings relevant to drug design. Lastly, we discuss the effect of NR interaction partners such as coactivators and corepressors, as well as mutations on the pathways.

Phosphatidylinositol-3 Kinase Inhibitors, Buparlisib and Alpelisib, Sensitize Estrogen Receptor-positive Breast Cancer Cells to Tamoxifen

Tamoxifen is the standard first-line hormonal therapy for premenopausal women with estrogen receptor (ER)-positive metastatic breast cancer (BC). One of the crucial mechanisms underlying hormonal therapy resistance is the collateral activation of the phosphatidylinositol-3 kinase (PI3K)/AKT pathway. We explored whether PI3K inhibitors, buparlisib and alpelisib, enhance the efficacy of tamoxifen against ER-positive BC cells. We have observed a synergism between alpelisib or buparlisib and tamoxifen in the treatment for ER-positive BC cell lines harboring different PI3K alterations. Immunoblotting analysis showed alpelisib, buparlisib, or either drug in combination with tamoxifen downregulated the PI3K downstream targets in the MCF-7 and ZR75-1 cells. In the MCF-7 cells transfected with a constitutive active (myristoylated) AKT1 construct or mutant ER, the synergistic effect between alpelisib and tamoxifen was markedly attenuated, indicating that synergism depends on AKT inhibition or normally functioning ER. Combining alpelisib or buparlisib with tamoxifen also attenuated MCF-7 tumor growth in Balb/c nude mice. Our data suggest that additional PI3K blockade might be effective in enhancing the therapeutic effect of tamoxifen in ER-positive BC and support the rationale combination in clinical trials.

An alternative conformation of ERβ bound to estradiol reveals H12 in a stable antagonist position

The natural ligand 17β-estradiol (E2) is so far believed to induce a unique agonist-bound active conformation in the ligand binding domain (LBD) of the estrogen receptors (ERs). Both subtypes, ERα and ERβ, are transcriptionally activated in the presence of E2 with ERβ being somewhat less active than ERα under similar conditions. The molecular bases for this intriguing behavior are mainly attributed to subtype differences in the amino-terminal domain of these receptors. However, structural details that confer differences in the molecular response of ER LBDs to E2 still remain elusive. In this study, we present a new crystallographic structure of the ERβ LBD bound to E2 in which H12 assumes an alternative conformation that resembles antagonist ERs structures. Structural observations and molecular dynamics simulations jointly provide evidence that alternative ERβ H12 position could correspond to a stable conformation of the receptor under physiological pH conditions. Our findings shed light on the unexpected role of LBD in the lower functional response of ERβ subtype.

ESR1 mutations: Pièce de résistance

Estrogen and estrogen receptor-alpha (ER) signaling are important factors in the pathogenesis and treatment of ER-positive breast cancers. Therefore targeted therapies against ER, known as endocrine therapies, are widely used in the treatment of ER-positive breast cancers. While these therapies have shown great success, de novo and acquired resistance are common. The approach to the problem of endocrine therapy resistance is two-fold: identify the mechanisms of resistance and develop alternative treatments to overcome these mechanisms. This review focuses on the progress and integration of these two aspects of resolving endocrine therapy resistance in ER-positive breast cancer patients.

Subtype-specific modulation of estrogen receptor-coactivator interaction by phosphorylation

The estrogen receptor (ER) is the number one target for the treatment of endocrine responsive breast cancer and remains a highly attractive target for new drug development. Despite considerable efforts to understand the role of ER post-translational modifications (PTMs), the complexity of these modifications and their impact, at the molecular level, are poorly understood. Using a chemical biology approach, fundamentally rooted in an efficient protein semisynthesis of tyrosine phosphorylated ER constructs, the complex role of the ER tyrosine phosphorylation is addressed here for the first time on a molecular level. The semisynthetic approach allows for the site-specific introduction of PTMs as well as biophysical probes. A combination of biophysical techniques, including NMR, with molecular dynamics studies reveals the role of the phosphorylation of the clinically relevant tyrosine 537 (Y537) in ERα and the analogous tyrosine (Y488) in ERβ. Phosphorylation has important effects on the dynamics of the ER Helix 12, which is centrally involved in receptor activity regulation, and on its interplay with ligand and cofactor binding, but with differential regulatory effects of the analogous PTMs on the two ER subtypes. Combined, the results bring forward a novel molecular model of a phosphorylation-induced subtype specific ER modulatory mechanism, alternative to the widely accepted ligand-induced activation mechanism.

Survey of phosphorylation near drug binding sites in the Protein Data Bank (PDB) and their effects

While it is currently estimated that 40 to 50% of eukaryotic proteins are phosphorylated, little is known about the frequency and local effects of phosphorylation near pharmaceutical inhibitor binding sites. In this study, we investigated how frequently phosphorylation may affect the binding of drug inhibitors to target proteins. We examined the 453 non-redundant structures of soluble mammalian drug target proteins bound to inhibitors currently available in the Protein Data Bank (PDB). We cross-referenced these structures with phosphorylation data available from the PhosphoSitePlus database. Three hundred twenty-two of 453 (71%) of drug targets have evidence of phosphorylation that has been validated by multiple methods or labs. For 132 of 453 (29%) of those, the phosphorylation site is within 12 Å of the small molecule-binding site, where it would likely alter small molecule binding affinity. We propose a framework for distinguishing between drug-phosphorylation site interactions that are likely to alter the efficacy of drugs versus those that are not. In addition we highlight examples of well-established drug targets, such as estrogen receptor alpha, for which phosphorylation may affect drug affinity and clinical efficacy. Our data suggest that phosphorylation may affect drug binding and efficacy for a significant fraction of drug target proteins.

Nuclear receptor full-length architectures: confronting myth and illusion with high resolution

The crystal structures of three nuclear receptor (NR) complexes have emerged to reveal their multidomain architectures on DNA. These pictures provide unprecedented views of interfacial couplings between the DNA-binding domains (DBDs) and ligand-binding domains (LBDs). The detailed pictures contrast with previous interpretations of low-resolution electron microscopy (EM) and small angle X-ray scattering (SAXS) data, which had suggested a common architecture with noninteracting DBDs and LBDs. Revisiting both historical and recent interpretations of NR architecture, we invoke new principles underlying higher-order quaternary organization and the allosteric transmission of signals between domains. We also discuss how NR architectures are being probed in living cells to understand dimerization and DNA-binding events in real time.

Leptin induces CYP1B1 expression in MCF-7 cells through ligand-independent activation of the ERα pathway

Leptin, a hormone with multiple biological actions, is produced predominantly by adipose tissue. Among its functions, leptin can stimulate tumour cell growth. Oestrogen receptor α (ERα), which plays an essential role in breast cancer development, can be transcriptionally activated in a ligand-independent manner. In this study, we investigated the effect of leptin on CYP1B1 expression and its mechanism in breast cancer cells. Leptin induced CYP1B1 protein, messenger RNA expression and promoter activity in ERα-positive MCF-7 cells but not in ERα-negative MDA-MB-231 cells. Additionally, leptin increased 4-hydroxyoestradiol in MCF-7 cells. Also, ERα knockdown by siRNA significantly blocked the induction of CYP1B1 expression by leptin, indicating that leptin induced CYP1B1 expression via an ERα-dependent mechanism. Transient transfection with CYP1B1 deletion promoter constructs revealed that the oestrogen response element (ERE) plays important role in the up-regulation of CYP1B1 by leptin. Furthermore, leptin stimulated phosphorylation of ERα at serine residues 118 and 167 and increased ERE-luciferase activity, indicating that leptin induced CYP1B1 expression by ERα activation. Finally, we found that leptin activated ERK and Akt signalling pathways, which are upstream kinases related to ERα phosphorylation induced by leptin. Taken together, our results indicate that leptin-induced CYP1B1 expression is mediated by ligand-independent activation of the ERα pathway as a result of the activation of ERK and Akt in MCF-7 cells.

Estrogen receptor (ER) α mutations in breast cancer: hidden in plain sight

The idea that somatic ERα mutations could play an important role in the evolution of hormone-dependent breast cancers was proposed some years ago (Fuqua J Mammary Gland Biol Neoplasia 6(4):407-417, 2001; Dasgupta et al. Annu Rev Med 65:279-292, 2013), but has remained controversial until recently. A significant amount of new data has confirmed these initial observations and shown their significance, along with much additional work relevant to the treatment of breast cancer. Thus, it is the purpose of this review to summarize the research to date on the existence and clinical consequences of ERα mutations in primary and metastatic breast cancer.

D538G mutation in estrogen receptor-α: A novel mechanism for acquired endocrine resistance in breast cancer

Resistance to endocrine therapy occurs in virtually all patients with estrogen receptor α (ERα)-positive metastatic breast cancer, and is attributed to various mechanisms including loss of ERα expression, altered activity of coregulators, and cross-talk between the ERα and growth factor signaling pathways. To our knowledge, acquired mutations of the ERα have not been described as mediating endocrine resistance. Samples of 13 patients with metastatic breast cancer were analyzed for mutations in cancer-related genes. In five patients who developed resistance to hormonal therapy, a mutation of A to G at position 1,613 of ERα, resulting in a substitution of aspartic acid at position 538 to glycine (D538G), was identified in liver metastases. Importantly, the mutation was not detected in the primary tumors obtained prior to endocrine treatment. Structural modeling indicated that D538G substitution leads to a conformational change in the ligand-binding domain, which mimics the conformation of activated ligand-bound receptor and alters binding of tamoxifen. Indeed, experiments in breast cancer cells indicated constitutive, ligand-independent transcriptional activity of the D538G receptor, and overexpression of it enhanced proliferation and conferred resistance to tamoxifen. These data indicate a novel mechanism of acquired endocrine resistance in breast cancer. Further studies are needed to assess the frequency of D538G-ERα among patients with breast cancer and explore ways to inhibit its activity and restore endocrine sensitivity.

Ligand unbinding from the estrogen receptor: a computational study of pathways and ligand specificity

The estrogen receptor (ER) belongs to the nuclear receptor superfamily, whose members regulate important cellular events like development and metabolism. The ER functions as a transcription regulator and can be activated on ligand binding. Consequently, ligand binding and unbinding constitute fundamental processes in the regulation of genes. Even though both biochemical and structural data of ER are available, the actual mechanism of the ligand binding/unbinding remains elusive. We have performed computational studies on the unbinding mechanism of ERalpha and ERbeta, in the presence of cofactors and with ligands ranging from agonist to a full antagonist. Our results show that agonists or selective ER modulators can dissociate from the receptor through multiple pathways with minor effect on the receptor structure, whereas an antagonist requires larger conformational changes. Furthermore, a specific receptor/ligand combination can exhibit a pathway preference depending on character and conformation of both parts. Hence, it is possible that the binding/unbinding mechanism can explain ligand subtype specificity and thus have an impact in drug discovery.

Cloning and expression pattern of peroxisome proliferator-activated receptors, estrogen receptor alpha and retinoid X receptor alpha in the thicklip grey mullet Chelon labrosus

Aquatic organisms are exposed to diverse xenobiotics that cause peroxisome proliferation and/or endocrine disruption, both modulated in vertebrates by transcription factors of the nuclear receptor (NR) superfamily. Peroxisome proliferators are agonists of peroxisome proliferator-activated receptors (PPARs) that heterodimerize with the retinoid X receptor (RXR). Many xenoestrogens activate the estrogen receptor (ER). Here, 1090 bp of PPARalpha, 1255 bp of PPARgamma, 278 bp of RXRalpha, and 578 bp of ERalpha of thicklip grey mullet Chelon labrosus were cloned. Sequences were highly conserved, although relevant changes with respect to mammalian homologs were identified in PPARgamma and ERalpha. Semi-quantitative RT-PCR was used to determine if these NRs were expressed in different tissues of male, female and undifferentiated mullets captured in January and June. Expression of PPARs was highest in liver and lowest in muscle. RXRalpha expression was homogeneous excepting a low expression in male and female gill in January and brain and heart of undifferentiated fish in January and June. ERalpha expression predominated in liver and female gonad in June. The expression level of PPARs and ERalpha was significantly higher in liver in January than in gills in January or June. The present results show tissue-dependent modulation of expression of NRs in mullets.

Ligand dissociation from estrogen receptor is mediated by receptor dimerization: evidence from molecular dynamics simulations

Estrogen Receptor (ER) is an important target for pharmaceutical design. Like other ligand-dependent transcription factors, hormone binding regulates ER transcriptional activity. Nevertheless, the mechanisms by which ligands enter and leave ERs and other nuclear receptors remain poorly understood. Here, we report results of locally enhanced sampling molecular dynamics simulations to identify dissociation pathways of two ER ligands [the natural hormone 17beta-estradiol (E(2)) and the selective ER modulator raloxifene (RAL)] from the human ERalpha ligand-binding domain in monomeric and dimeric forms. E(2) dissociation occurs via three different pathways in ER monomers. One resembles the mousetrap mechanism (Path I), involving repositioning of helix 12 (H12), others involve the separation of H8 and H11 (Path II), and a variant of this pathway at the bottom of the ligand-binding domain (Path II'). RAL leaves the receptor through Path I and a Path I variant in which the ligand leaves the receptor through the loop region between H11 and H12 (Path I'). Remarkably, ER dimerization strongly suppresses Paths II and II' for E(2) dissociation and modifies RAL escape routes. We propose that differences in ligand release pathways detected in the simulations for ER monomers and dimers provide an explanation for previously observed effects of ER quaternary state on ligand dissociation rates and suggest that dimerization may play an important, and hitherto unexpected, role in regulation of ligand dissociation rates throughout the nuclear receptor family.

An integrated-molecule-format multicolor probe for monitoring multiple activities of a bioactive small molecule

Bioactive small molecules, including steroids, activate multiple signaling pathways in mammalian cells. However, current technologies cannot illuminate such multiple effects of a ligand in mammalian cells. Here, we demonstrate integrated-molecule-format multicolor systems simultaneously visualizing bifacial activities of a ligand, where estrogen receptor alpha (ERalpha) was exemplified to demonstrate the present technology. First, we developed a single-molecule-format probe emitting red bioluminescence for imaging interaction between the phosphorylated ligand binding domain of ERalpha (ER LBD) and the Src homology-2 (SH2) domain of Src. The SH2 domain-linked ER LBD was sandwiched between dissected N- and C-terminal fragments of Pyrophorus plagiophthalamus (click beetle) luciferase emitting red bioluminescence. Second, another single-molecule-format bio-luminescent probe emitting green bioluminescence was constructed to visualize intramolecular interaction between ER LBD and LXXLL motifs. Mammalian cells carrying the two probes emit red and/or green light in response to agonistic and antagonistic activities of a ligand, which correspond to its genomic and nongenomic activities, respectively. Third, the two probes were assembled to make an single-molecule-format multicolor indicator, in which all of the components for ligand sensing and multiple-light emission were integrated. The probe emitted characteristic light spectra in response to various agonists and antagonists. This is the first example where (i) protein phosphorylation was recognized with a single bioluminescent probe and (ii) bifacial activities of a ligand, either agonistic or antagonistic, were simultaneously visualized with multiple colors.

NFkappaB selectivity of estrogen receptor ligands revealed by comparative crystallographic analyses

Our understanding of how steroid hormones regulate physiological functions has been significantly advanced by structural biology approaches. However, progress has been hampered by misfolding of the ligand binding domains in heterologous expression systems and by conformational flexibility that interferes with crystallization. Here, we show that protein folding problems that are common to steroid hormone receptors are circumvented by mutations that stabilize well-characterized conformations of the receptor. We use this approach to present the structure of an apo steroid receptor that reveals a ligand-accessible channel allowing soaking of preformed crystals. Furthermore, crystallization of different pharmacological classes of compounds allowed us to define the structural basis of NFkappaB-selective signaling through the estrogen receptor, thus revealing a unique conformation of the receptor that allows selective suppression of inflammatory gene expression. The ability to crystallize many receptor-ligand complexes with distinct pharmacophores allows one to define structural features of signaling specificity that would not be apparent in a single structure.

Estrogen receptor alpha--identification by a modeling approach of a potential polyproline II recognizing domain within the AF-2 region of the receptor that would play a role of prime importance in its mechanism of action

Estrogen receptors (ERs) behave not only as ligand-dependent transcriptional factors; they can also trigger non-genomic responses involving mitogen activated protein kinases (MAPKs), reported to be crucial in transduction cascades. MAPKs are partially activated by proteins with domains able to interact with polyproline II (PPII) regions. Recent studies have brought up the direct interaction of PPII-containing proteins with the alpha subtype human ER (ERalpha). Such observations suggest that ERalpha may contain a "PPII recognizing domain" (PRD). By sequence alignment, we identified such a potential PRD within the AF-2 region of ERalpha (residues 351-414). According to our modeling studies based on X-ray structural data, this PRD appears to be divided in two sub-regions known to interact with alpha-helix containing coactivators. Our data also reveal the potential existence of intramolecular interactions of this domain with a large PPII-rich region of the receptor (residues 301-330). Implication of these regulatory structural elements in both genomic and non-genomic responses seems likely.

Estrogen receptors in resistance to hormone therapy

Estrogen and its receptors alpha and beta (ERalpha and ERbeta) play a major role in tumor progression and approximately two-thirds of breast cancers express these functional receptors. Thus, the ER is a major target for current and developing therapies. Although most ER-positive tumors initially respond to hormonal therapies such as tamoxifen, many tumors will eventually become resistant to tamoxifen induced growth inhibition. This chapter will discuss molecular mechanisms that contribute to hormonal resistance of current therapies including ERalpha mutations, the roles of proliferation and apoptosis in tumor homeostasis and receptor coregulator proteins. Additionally, the role of nonclassical ERalpha signaling through growth factor receptors and the subsequent downstream-initiated signaling, and the role of the progesterone receptors will be discussed.

Estrogen receptor mutations in human disease

As early as the 1800s, the actions of estrogen have been implicated in the development and progression of breast cancer. The estrogen receptor (ER) was identified in the late 1950s and purified a few years later. However, it was not until the 1980s that the first ER was molecularly cloned, and in the mid 1990s, a second ER was cloned. These two related receptors are now called ERalpha and ERbeta, respectively. Since their discovery, much research has focused on identifying alterations within the coding sequence of these receptors in clinical samples. As a result, a large number of naturally occurring splice variants of both ERalpha and ERbeta have been identified in normal epithelium and diseased or cancerous tissues. In contrast, only a few point mutations have been identified in human patient samples from a variety of disease states, including breast cancer, endometrial cancer, and psychiatric diseases. To elucidate the mechanism of action for these variant isoforms or mutant receptors, experimental mutagenesis has been used to analyze the function of distinct amino acid residues in the ERs. This review will focus on ERalpha and ERbeta alterations in breast cancer.

Dynamic of ligand binding to Drosophila melanogaster ecdysteroid receptor

Ligand binding to ecdysone receptor (EcR) is an autonomous function of the ligand binding domain (LBD) and is not modified by other receptor domains or tags fused to the LBD. Association and dissociation velocity of hormone to EcR was studied in the absence and presence of its main dimerization partner Ultraspiracle (USP). Mutational analysis of the EcR(LBD) revealed that ligand entry and exit is affected differently by the same point mutation, indicating that different pathways are used for association and dissociation of the ligand. Heterodimerization with wild type USP(LBD) increases ligand association to EcR(LBD) about fivefold and reduces dissociation 18-fold. Opposite effects of the same mutation (N626K) on dissociation velocity of ligand in EcR and EcR/USP indicate that not only hormone binding itself, but also the kinetic behaviour of ligand binding is modified by the dimerization partner. A general effect of the point mutations on the 3D architecture seems unlikely due to the highly selective effects on the kinetics of hormone binding.

From ligand structure to biological activity: modified estratrienes and their estrogenic and antiestrogenic effects in MCF-7 cells

A variety of compounds, including the selective estrogen receptor (ER) modulators tamoxifen and raloxifene, phytoestrogens such as genistein, and xenoestrogens such as bisphenol, bind to the estrogen receptor and elicit biological responses. Structural studies have linked the altered activity of compounds such as 4-hydroxytamoxifen, raloxifene, genistein, and tetrahydrochrysene, which have substantially different structures from estradiol (E2), to differences in the positioning of the critical "helix 12" within the ligand-binding domain (LBD) of the ER-ligand complex. However, subtle permutations of the E2 molecule would also be expected to modulate the pattern of responses within a cell. Forty-two ligands were constructed by the addition or relocation of double bonds, hydroxyl, keto, amino, and nitro substituents throughout the estra-l,3,5(10)-triene (estratriene) ring system. In this review, we summarize the effects of subtle changes in the estratriene molecule on the ability of the receptor complex to stimulate the growth of MCF-7 cells, or affect the expression of four estrogen-regulated genes (progesterone receptor, pS2 protein, cathepsin D, and tissue plasminogen activator), as well as undergo nuclear processing and downregulate ERalpha mRNA. The affinity of these ligands for, and mechanism of their binding with, the ERalpha have been measured, along with their effect on the conformation of the ER-ERE complex. In particular, two A-ring isomers of E2, 2- and 4-hydroxyestratriene-17beta-ol, display gene selective activity within MCF-7 cells which is dependent on complex endogenous promoters, an intact AF-2 and is sensitive to the level of SRC-1. Both of these A-ring isomers function as antiestrogens. Molecular modeling of these two A-ring isomers complexed with the ER ligand-binding domain supports the idea that the conformation of the LBD is affected by subtle changes in the estratriene structure.

Mutation of Leu-536 in human estrogen receptor-alpha alters the coupling between ligand binding, transcription activation, and receptor conformation

The estrogen receptor (ER), of which there are two forms, ERalpha and ERbeta, is a ligand-modulated transcription factor important in both normal biology and as a target for agents to prevent and treat breast cancer. Crystallographic studies of the ERalpha ligand-binding domain suggest that Leu-536 may be involved in hydrophobic interactions at the start of a helix, "helix 12," that is crucial in the agonist-stimulated activity of ERalpha, as well as in the ability of antagonists to block the activity of ERalpha. We found that certain mutations of Leu-536 increased the ligand-independent activity of ERalpha although greatly reducing or eliminating the agonist activity of 17beta-estradiol (E2) and 4-hydroxytamoxifen (4OHT), on an estrogen response element-driven and an AP-1-driven reporter. The mutations impaired the interaction of the ER ligand-binding domain with the SRC1 receptor-interacting domain in a mammalian two-hybrid system. When tested in the yeast two-hybrid system, mutation of Leu-536 increased the basal reactivity of ERalpha to probes that recognize the agonist-bound conformation but did not significantly alter its reactivity to these probes in the presence of E2. Most interestingly, mutation of Leu-536 reduced the interaction of the 4OHT-bound ERalpha and increased the reactivity of the raloxifene- or ICI 182,780-bound ERalpha, with probes that recognize the 4OHT-bound ERalpha conformation in a yeast two-hybrid system. These results show that Leu-536 is critical in coupling the binding of ligand to the modulation of the conformation and activity of ERalpha.

Plasma membrane localization and function of the estrogen receptor alpha variant (ER46) in human endothelial cells

Estrogen receptor (ER) alpha variants have been identified in an array of nonendothelial cells. We previously demonstrated that estrogen rapidly induces nitric oxide release via a phosphatidylinositol 3-kinase/Akt/endothelial nitric-oxide synthase (eNOS) pathway in EA.hy926 cells (immortalized human endothelial cells), which express a 46-kDa ER. We now confirm that, due to alternative splicing, the 46-kDa endothelial cell protein (ER46) is an amino-terminal truncated product of full-length ER alpha (ER66). ER46 is expressed in the plasma membrane, cytosol, and nucleus of resting, estrogen-deprived cells. Flow cytometric and immunofluorescence microscopic analyses demonstrated that the ER46 C but not N terminus is Ab-accessible in the plasma membrane. Inhibition of palmitoylation with tunicamycin and [(3)H]palmitic acid labeling demonstrated an estrogen-induced, palmitoylation-dependent plasma membrane ER46 recruitment, with reorganization into caveolae. In reconstituted, estrogen-stimulated COS-7 (ER-null) cells, membrane ER46 more efficiently triggered membrane eNOS phosphorylation than ER66. Conversely, ER66 more efficiently mediated estrogen response element reporter-gene transactivation than ER46. These results demonstrate that ER46 is localized and further dynamically targeted to the plasma membrane in a palmitoylation-dependent manner. ER46 more efficiently modulates membrane-initiated estrogen actions, including eNOS activation, than full-length ER66. These findings may have important implications in vascular-specific targeting of estrogen receptor agonists.