Function of estrogen receptor tyrosine 537 in hormone binding, DNA binding, and transactivation

Molecular Mechanisms of Anti-Estrogen Therapy Resistance and Novel Targeted Therapies

Breast cancer (BC) is the most commonly diagnosed cancer in women, constituting one-third of all cancers in women, and it is the second leading cause of cancer-related deaths in the United States. Anti-estrogen therapies, such as selective estrogen receptor modulators, significantly improve survival in estrogen receptor-positive (ER+) BC patients, which represents about 70% of cases. However, about 60% of patients inevitably experience intrinsic or acquired resistance to anti-estrogen therapies, representing a major clinical problem that leads to relapse, metastasis, and patient deaths. The resistance mechanisms involve mutations of the direct targets of anti-estrogen therapies, compensatory survival pathways, as well as alterations in the expression of non-coding RNAs (e.g., microRNA) that regulate the activity of survival and signaling pathways. Although cyclin-dependent kinase 4/6 and phosphatidylinositol 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) inhibitors have significantly improved survival, the efficacy of these therapies alone and in combination with anti-estrogen therapy for advanced ER+ BC, are not curative in advanced and metastatic disease. Therefore, understanding the molecular mechanisms causing treatment resistance is critical for developing highly effective therapies and improving patient survival. This review focuses on the key mechanisms that contribute to anti-estrogen therapy resistance and potential new treatment strategies alone and in combination with anti-estrogen drugs to improve the survival of BC patients.

In Silico Studies of Tumor Targeted Peptide-Conjugated Natural Products for Targeting Over-Expressed Receptors in Breast Cancer Cells Using Molecular Docking, Molecular Dynamics and MMGBSA Calculations

In this work, in silico studies were carried out for the design of diterpene and polyphenol-peptide conjugates to potentially target over-expressed breast tumor cell receptors. Four point mutations were induced into the known tumor-targeting peptide sequence YHWYGYTPQN at positions 1, 2, 8 and 10, resulting in four mutated peptides. Each peptide was separately conjugated with either chlorogenate, carnosate, gallate, or rosmarinate given their known anti-tumor activities, creating dual targeting compounds. Molecular docking studies were conducted with the epidermal growth factor receptor (EGFR), to which the original peptide sequence is known to bind, as well as the estrogen receptor (ERα) and peroxisome proliferator-activated receptor (PPARα) using both Autodock Vina and FireDock. Based on docking results, peptide conjugates and peptides were selected and subjected to molecular dynamics simulations. MMGBSA calculations were used to further probe the binding energies. ADME studies revealed that the compounds were not CYP substrates, though most were Pgp substrates. Additionally, most of the peptides and conjugates showed MDCK permeability. Our results indicated that several of the peptide conjugates enhanced binding interactions with the receptors and resulted in stable receptor-ligand complexes; Furthermore, they may successfully target ERα and PPARα in addition to EGFR and may be further explored for synthesis and biological studies for therapeutic applications.

Molecular Mechanisms of Endocrine Resistance in Estrogen-Positive Breast Cancer

The estrogen receptor is a vital receptor for therapeutic targets in estrogen receptor-positive breast cancer. The main strategy for the treatment of estrogen receptor-positive breast cancers is blocking the estrogen action on estrogen receptors by endocrine therapy but this can be restricted via endocrine resistance. Endocrine resistance occurs due to both de novo and acquired resistance. This review focuses on the mechanisms of the ligand-dependent and ligand-independent pathways and other coregulators, which are responsible for endocrine resistance. It concludes that combinatorial drugs that target different signaling pathways and coregulatory proteins together with endocrine therapy could be a novel therapeutic modality to stop endocrine resistance.

Chlorpyrifos subthreshold exposure induces epithelial-mesenchymal transition in breast cancer cells

Chlorpyrifos (CPF) is one of the most frequently used pesticide in extensive agriculture around the world and can be incorporated by humans and animals with possible consequences on health. The effects of this pesticide on carcinogenesis are not clear and there is no consensus concerning the risks of this compound. In previous work, we demonstrated that CPF induces proliferation of breast cancer cells both in vivo and in vitro. In this work we investigate whether CPF promotes the epithelial-mesenchymal transition (EMT) in breast cancer cells. Herein, we demonstrate that 50 μM CFP induces invasion in MCF-7 and MDA-MB-231 cells. In addition, 0.05 and 50 μM CPF increases migration in both cell lines. In MCF-7 cells, 0.05 and 50 μM CPF increase the metalloprotease MMP2 expression and decrease E-Cadherin and β-Catenin expression diminishing their membrane location. Furthermore, 50 μM CPF induces Vimentin expression and Slug nuclear translocation in MCF-7 cells. 0.05 and 50 μM CPF increase MMP2 gelatinolytic activity and expression, decrease β-Catenin expression and increase Vimentin expression in MDA-MB-231 cells. Inhibition of the oncoprotein c-Src reverses all the effects induced by CPF in MDA-MB-231 but not in MCF-7 indicating that c-Src is a kinase with a crucial role in the cells which grow in an estrogen-independent way. In MCF-7 cells both c-Src and estrogen receptor alpha must be blocked to completly inhibit the CPF-mediated effects. Our results show for the first time that the exposure to subthreshold concentrations of CPF promotes the modulation of EMT-molecular markers and pathways. These results, together with the ubiquitous distribution of the pesticide CPF, make it of utmost importance to take measures to minimize the risk of exposure to this compound.

Extranuclear signaling by sex steroid receptors and clinical implications in breast cancer

Estrogen and progesterone play essential roles in the development and progression of breast cancer. Over 70% of breast cancers express estrogen receptors (ER) and progesterone receptors (PR), emphasizing the need for better understanding of ER and PR signaling. ER and PR are traditionally viewed as transcription factors that directly bind DNA to regulate gene networks. In addition to nuclear signaling, ER and PR mediate hormone-induced, rapid extranuclear signaling at the cell membrane or in the cytoplasm which triggers downstream signaling to regulate rapid or extended cellular responses. Specialized membrane and cytoplasmic proteins may also initiate hormone-induced extranuclear signaling. Rapid extranuclear signaling converges with its nuclear counterpart to amplify ER/PR transcription and specify gene regulatory networks. This review summarizes current understanding and updates on ER and PR extranuclear signaling. Further investigation of ER/PR extranuclear signaling may lead to development of novel targeted therapeutics for breast cancer management.

Genomic modelling of the ESR1 Y537S mutation for evaluating function and new therapeutic approaches for metastatic breast cancer

Drugs that inhibit estrogen receptor-α (ER) activity have been highly successful in treating and reducing breast cancer progression in ER-positive disease. However, resistance to these therapies presents a major clinical problem. Recent genetic studies have shown that mutations in the ER gene are found in >20% of tumours that progress on endocrine therapies. Remarkably, the great majority of these mutations localize to just a few amino acids within or near the critical helix 12 region of the ER hormone binding domain, where they are likely to be single allele mutations. Understanding how these mutations impact on ER function is a prerequisite for identifying methods to treat breast cancer patients featuring such mutations. Towards this end, we used CRISPR-Cas9 genome editing to make a single allele knock-in of the most commonly mutated amino acid residue, tyrosine 537, in the estrogen-responsive MCF7 breast cancer cell line. Genomic analyses using RNA-seq and ER ChIP-seq demonstrated that the Y537S mutation promotes constitutive ER activity globally, resulting in estrogen-independent growth. MCF7-Y537S cells were resistant to the anti-estrogen tamoxifen and fulvestrant. Further, we show that the basal transcription factor TFIIH is constitutively recruited by ER-Y537S, resulting in ligand-independent phosphorylation of Serine 118 (Ser118) by the TFIIH kinase, cyclin-dependent kinase (CDK)7. The CDK7 inhibitor, THZ1 prevented Ser118 phosphorylation and inhibited growth of MCF7-Y537S cells. These studies confirm the functional importance of ER mutations in endocrine resistance, demonstrate the utility of knock-in mutational models for investigating alternative therapeutic approaches and highlight CDK7 inhibition as a potential therapy for endocrine-resistant breast cancer mediated by ER mutations.

Greater Reduction of Balance as a Result of Increased Plantar Fascia Elasticity at Ovulation during the Menstrual Cycle

One of the sexual hormones, estrogen, increases elasticity of human connective tissue such as the anterior cruciate ligament during the menstrual cycle in women. In the present investigation, the plantar fascia was investigated to see if there is a difference in elasticity with the menstrual cycle. Fifteen young healthy females in the age range of 18-35 years old with a regular menstrual cycle were tested twice throughout one full menstrual cycle; once during the early follicular phases and once at ovulation. Foot length, while standing on both feet and one foot were used to assess plantar fascia elasticity, ultrasound measured plantar fascia thickness while lying and standing, and posture sway and tremor using a balance platform during 8 different balance tests were assessed to see the impact of elasticity changes. Foot length increased significantly at ovulation compared to menstruation when standing on two feet (p = 0.03) and standing on one foot (p < 0.001). There was also a significant increase in plantar fascia in thinning per kilogram weight applied to the foot at ovulation compared to menstruation (p = 0.014). Associated with this increase in elasticity at ovulation, there was a reduction in balance in the most difficult balance tasks and an increase in tremor during ovulation (p < 0.05). Plantar fascia elasticity change during the menstrual cycle might have effects on posture sway and tremor, which could have a potential risk of falling. Therefore, healthy professionals working with young female adults should recognize these physiological effects.

Estrogen Receptor Folding Modulates cSrc Kinase SH2 Interaction via a Helical Binding Mode

The estrogen receptors (ERs) feature, next to their transcriptional role, important nongenomic signaling actions, with emerging clinical relevance. The Src Homology 2 (SH2) domain mediated interaction between cSrc kinase and ER plays a key role in this; however the molecular determinants of this interaction have not been elucidated. Here, we used phosphorylated ER peptide and semisynthetic protein constructs in a combined biochemical and structural study to, for the first time, provide a quantitative and structural characterization of the cSrc SH2-ER interaction. Fluorescence polarization experiments delineated the SH2 binding motif in the ER sequence. Chemical shift perturbation analysis by nuclear magnetic resonance (NMR) together with molecular dynamics (MD) simulations allowed us to put forward a 3D model of the ER-SH2 interaction. The structural basis of this protein-protein interaction has been compared with that of the high affinity SH2 binding sequence GpYEEI. The ER features a different binding mode from that of the "two-pronged plug two-hole socket" model in the so-called specificity determining region. This alternative binding mode is modulated via the folding of ER helix 12, a structural element directly C-terminal of the key phosphorylated tyrosine. The present findings provide novel molecular entries for understanding nongenomic ER signaling and targeting the corresponding disease states.

Ubiquitylation of nuclear receptors: new linkages and therapeutic implications

The nuclear receptor (NR) superfamily is a group of transcriptional regulators that control multiple aspects of both physiology and pathology and are broadly recognized as viable therapeutic targets. While receptor-modulating drugs have been successful in many cases, the discovery of new drug targets is still an active area of research, because resistance to NR-targeting therapies remains a significant clinical challenge. Many successful targeted therapies have harnessed the control of receptor activity by targeting events within the NR signaling pathway. In this review, we explore the role of NR ubiquitylation and discuss how the expanding roles of ubiquitin could be leveraged to identify additional entry points to control receptor function for future therapeutic development.

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.

Links between oestrogen receptor activation and proteolysis: relevance to hormone-regulated cancer therapy

Oestrogen receptor-α (ERα) is a master transcription factor that regulates cell proliferation and homeostasis in many tissues. Despite beneficial ERα functions, sustained oestrogenic exposure increases the risk and/or the progression of various cancers, including those of the breast, endometrium and ovary. Oestrogen–ERα interaction can trigger post-translational ERα modifications through crosstalk with signalling pathways to promote transcriptional activation and ubiquitin-mediated ERα proteolysis, with co-activators that have dual roles as ubiquitin ligases. These processes are reviewed herein. The elucidation of mechanisms whereby oestrogen drives both ERα transactivation and receptor proteolysis might have important therapeutic implications not only for breast cancer but also potentially for other hormone-regulated cancers.

Extranuclear ERα is associated with regression of T47D PKCα-overexpressing, tamoxifen-resistant breast cancer

Background

Prior to the introduction of tamoxifen, high dose estradiol was used to treat breast cancer patients with similar efficacy as tamoxifen, albeit with some undesirable side effects. There is renewed interest to utilize estradiol to treat endocrine resistant breast cancers, especially since findings from several preclinical models and clinical trials indicate that estradiol may be a rational second-line therapy in patients exhibiting resistance to tamoxifen and/or aromatase inhibitors. We and others reported that breast cancer patients bearing protein kinase C alpha (PKCα)- expressing tumors exhibit endocrine resistance and tumor aggressiveness. Our T47D:A18/PKCα preclinical model is tamoxifen-resistant, hormone-independent, yet is inhibited by 17β-estradiol (E2) in vivo. We previously reported that E2-induced T47D:A18/PKCα tumor regression requires extranuclear ERα and interaction with the extracellular matrix.

Methods

T47D:A18/PKCα cells were grown in vitro using two-dimensional (2D) cell culture, three-dimensional (3D) Matrigel and in vivo by establishing xenografts in athymic mice. Immunofluoresence confocal microscopy and co-localization were applied to determine estrogen receptor alpha (ERα) subcellular localization. Co-immunoprecipitation and western blot were used to examine interaction of ERα with caveolin-1.

Results

We report that although T47D:A18/PKCα cells are cross-resistant to raloxifene in cell culture and in Matrigel, raloxifene induces regression of tamoxifen-resistant tumors. ERα rapidly translocates to extranuclear sites during T47D:A18/PKCα tumor regression in response to both raloxifene and E2, whereas ERα is primarily localized in the nucleus in proliferating tumors. E2 treatment induced complete tumor regression whereas cessation of raloxifene treatment resulted in tumor regrowth accompanied by re-localization of ERα to the nucleus. T47D:A18/neo tumors that do not overexpress PKCα maintain ERα in the nucleus during tamoxifen-mediated regression. An association between ERα and caveolin-1 increases in tumors regressing in response to E2.

Conclusions

Extranuclear ERα plays a role in the regression of PKCα-overexpressing tamoxifen-resistant tumors. These studies underline the unique role of extranuclear ERα in E2- and raloxifene-induced tumor regression that may have implications for treatment of endocrine-resistant PKCα-expressing tumors encountered in the clinic.

Ecdysone receptor and ultraspiracle proteins are tyrosine phosphorylated during adult development of silkmoths

20-hydroxy ecdysone (20E) is essential to promote adult development in diapausing silkmoth pupae. Increases in protein tyrosine/serine-phosphorylations observed soon after 20E administration supported the initial hypothesis that activation of receptor tyrosine kinase-ras-MAPK pathway could be responsible for the growth promoting effects of 20E. This report pertains to the high levels of protein tyrosine phosphorylations (PTP) that occurred later during the growth to differentiation transition because of its novelty and relevance to 20E dependence of adult development. Further analyses demonstrated that both ecdysone receptor (EcR) and ultraspiracle (USP), the two dimerizing partners of the functional ecdysone receptor, are tyrosine phosphorylated coincidental with high PTP. Enhanced PTP during growth to differentiation transition and concomitant tyrosine phosphorylation of EcR and USP was shown to occur in another silkmoth species pointing to the necessity of similar protein tyrosine phosphorylation pathways for adult development. Properly timed increases in tissue protein tyrosine kinase (PTK) activity could explain the enhancement of PTP in the wing epidermis of both the silkmoths. Thymidine incorporation measurements showed that cessation of DNA synthesis preceded the increase in PTK activity thus emphasizing a role for PTP in aspects of tissue physiology related to differentiative events rather than cell proliferation. Phosphatase and tyrosine kinase inhibitors (Tyrphostins) had minimal effects on adult wing development in vivo. However, the escape of the adult from the pupal case was blocked by a tyrphostin indicating the importance of PTKs in eclosion.

MicroRNA-30c expression level is an independent predictor of clinical benefit of endocrine therapy in advanced estrogen receptor positive breast cancer

MicroRNAs (miRNAs) are small RNA molecules that modulate gene expression and which have been implicated in cancer. We evaluated whether five candidate predictive miRNAs, derived from a pilot study in which 249 miRNAs were assayed, were associated with clinical benefit of tamoxifen therapy in advanced breast cancer. These five miRNAs were measured in an independent series of 246 estrogen receptor (ER)-positive primary breast tumors of patients who received tamoxifen for advanced disease by quantitative Real Time PCR. Univariate analysis showed that higher expression levels of hsa-miR-30a-3p, hsa-miR-30c, and hsa-miR-182 were significantly associated with benefit of tamoxifen treatment and with longer PFS (all P-values <0.01). In multivariate analysis, corrected for the traditional predictive factors, only hsa-miRNA-30c was an independent predictor (P-value <0.01). Finally, in an attempt to understand the biology connected to this miRNA, Global testing pathway analysis showed an association of hsa-miRNA-30c expression with HER and RAC1 signaling pathways. We identified hsa-miRNA-30c as an independent predictor for clinical benefit of tamoxifen therapy in patients with advanced breast cancer. Assessment of tumor levels and connected pathways could be helpful to improve treatment strategies.

Steroid receptor phosphorylation: Assigning function to site-specific phosphorylation

Steroid receptors (SRs) are hormone-activated transcription factors important for a wide variety of cellular functions. Post-translational modifications of SRs, including phosphorylation, ubiquitination, acetylation, and sumoylation regulate their expression and function. The remarkable number of phosphorylation sites in these receptors and the wide variety of kinases shown to modulate phosphorylation influence the integration between cell-signaling pathways and SR action. These phosphorylation sites have been identified in all of the functional domains with the majority being located within the amino-terminal portions of the receptors. The regulation of function is receptor specific, site specific, and often dependent on the cellular context. Numerous roles for site-specific phosphorylation have been elucidated including sensitivity of hormone response, DNA binding, expression, stability, subcellular localization, dimerization, and protein-protein interactions that can determine the regulation of specific target genes. This review summarizes the current knowledge regarding receptor site-specific phosphorylation and regulation of function. As functional assays become more sophisticated, it is likely that additional roles for phosphorylation in receptor function will be identified.

Phosphorylation of estrogen receptor alpha, serine residue 305 enhances activity

Upon ligand binding the estrogen receptor alters its conformation, dimerizes, binds to estrogen response elements (EREs), recruits cofactors and initiates the formation of a transcriptional complex. In addition to estradiol binding, hormone receptor activity is modulated by phosphorylation at several key residues. Previous studies have shown that p21-activated kinase-1 (Pak1) and cyclic-AMP dependent protein kinase (PKA) can phosphorylate ERalpha at serine residue 305. However, the effects of serine 305 phosphorylation on ERalpha activity have not been fully characterized. To study these effects, ERalpha S305E and S305A mutants were created to mimic constitutively phosphorylated or un-phosphorylated states, respectively. Using yeast two-hybrid assays we showed that dimerization of ERalpha S305E was still ligand dependent. However, the capability of dimerization in the presence of estradiol was significantly higher in S305E compared to wild-type ERalpha. Transactivation assays demonstrated that phospho-mimetic ERalpha S305E is active in the absence of ligand. Chromatin immunoprecipitation (ChIP) analysis shows a change of in vivo DNA binding in which S305E mutant binds to ERalpha DNA target sequences and exhibits increased residency in the absence of ligand. We also observed increased cell growth in cells stably transfected with S305E ERalpha. Thus, we suggest that phosphorylation of S305 does not trigger ERalpha dimerization but increases binding to target gene promoters, which can lead to increased cell growth in the absence of estradiol. This implies a shift from hormone-induced activation of ERalpha to activation through phosphorylation, which could confer resistance to hormone based therapies for breast cancer.

Calmodulin, a regulatory partner of the estrogen receptor alpha in breast cancer cells

Although calmodulin (CaM) interaction with estrogen receptor alpha (ERalpha) has been known for more than two decades, it is only recently that the molecular mechanism of CaM-mediated regulation of ERalpha is beginning to emerge. Others and we have identified a putative calmodulin binding site (P(295)LMIKRSKKNSLALSTADQMVS(317)) in ERalpha, at the boundary between the hinge and the ligand binding domain. ERalpha mutations affecting its association with CaM have been reported to generate high basal, estrogen-independent transactivation activity, indicating that the P(295)-T(317) sequence has an inhibitory function. Moreover, we found that a synthetic peptide (ERalpha17p: P(295)-T(311)) containing residues crucial for CaM binding exerts estrogenic effects on breast carcinoma cells. Finally, computer-aided conformational studies revealed that the CaM binding site might associate with a region located downstream in ERalpha (the beta turn/H4 region), this association likely resulting in an auto-inhibitory folding of the receptor. Thus, we propose as a hypothesis that CaM acts as a positive regulator by relieving this ERalpha auto-inhibition.

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.

Src promotes estrogen-dependent estrogen receptor alpha proteolysis in human breast cancer

Estrogen drives both transcriptional activation and proteolysis of estrogen receptor alpha (ER alpha; encoded by ESR1). Here we observed variable and overlapping ESR1 mRNA levels in 200 ER alpha-negative and 50 ER alpha-positive primary breast cancers examined, which suggests important posttranscriptional ER alpha regulation. Our results indicate that Src cooperates with estrogen to activate ER alpha proteolysis. Inducible Src stimulated ligand-activated ER alpha transcriptional activity and reduced ER alpha t(1/2). Src and ER alpha levels were inversely correlated in primary breast cancers. ER alpha-negative primary breast cancers and cell lines showed increased Src levels and/or activity compared with ER alpha-positive cancers and cells. ER alpha t(1/2) was reduced in ER alpha-negative cell lines. In both ER alpha-positive and -negative cell lines, both proteasome and Src inhibitors increased ER alpha levels. Src inhibition impaired ligand-activated ER alpha ubiquitylation and increased ER alpha levels. Src siRNA impaired ligand-activated ER alpha loss in BT-20 cells. Pretreatment with Src increased ER alpha ubiquitylation and degradation in vitro. These findings provide what we believe to be a novel link between Src activation and ER alpha proteolysis and support a model whereby crosstalk between liganded ER alpha and Src drives ER alpha transcriptional activity and targets ER alpha for ubiquitin-dependent proteolysis. Oncogenic Src activation may promote not only proliferation, but also estrogen-activated ER alpha loss in a subset of ER alpha-negative breast cancers, altering prognosis and response to therapy.

Kinases and protein phosphorylation as regulators of steroid hormone action

Although the primary signal for the activation of steroid hormone receptors is binding of hormone, there is increasing evidence that the activities of cell signaling pathways and the phosphorylation status of these transcription factors and their coregulators determine the overall response to the hormone. In some cases, enhanced cell signaling is sufficient to cause activation of receptors in medium depleted of steroids. Steroid receptors are targets for multiple kinases. Many of the phosphorylation sites contain Ser/Thr-Pro motifs implicating proline-directed kinases such as the cyclin-dependent kinases and the mitogen-activated kinases (MAPK) in receptor phosphorylation. Although some sites are constitutively phosphorylated, others are phosphorylated in response to hormone. Still others are only phosphorylated in response to specific cell signaling pathways. Phosphorylation of specific sites has been implicated not only in overall transcriptional activity, but also in nuclear localization, protein stability, and DNA binding. The studies of the roles of phosphorylation in coregulator function are more limited, but it is now well established that many of them are highly phosphorylated and that phosphorylation regulates their function. There is good evidence that some of the phosphorylation sites in the receptors and coregulators are targets of multiple signaling pathways. Individual sites have been associated both with functions that enhance the activity of the receptor, as well as with functions that inhibit activity. Thus, the specific combinations of phosphorylations of the steroid receptor combined with the expression levels and phosphorylation status of coregulators will determine the genes regulated and the biological response.

Pathways to tamoxifen resistance

Therapies that target the synthesis of estrogen or the function of estrogen receptor(s) have been developed to treat breast cancer. While these approaches have proven to be beneficial to a large number of patients, both de novo and acquired resistance to these drugs is a significant problem. Recent advances in our understanding of the molecular mechanisms that contribute to resistance have provided a means to begin to predict patient responses to these drugs and develop rational approaches for combining therapeutic agents to circumvent or desensitize the resistant phenotype. Here, we review common mechanisms of antiestrogen resistance and discuss the implications for prediction of response and design of effective combinatorial treatments.

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.

Understanding the human estrogen receptor-alpha using targeted mutagenesis

The estrogen receptor-alpha is a wonderfully complex protein important in normal biology, breast cancer, and as a target for anti-cancer agents. We are using the available structures of the hERalpha as well as secondary structure predictions to guide site-directed mutagenesis in order to test the importance of specific interactions and regions in the ligand-regulated activity of the protein. In one area of interest, we are investigating the role of the F domain in the ligand-stimulated activity of the hERalpha. Results from our laboratory and others suggest that the F domain modulates the activity of the hERalpha. In order to better understand the role of the F domain in the hERalpha, we have constructed mutants within this region. Mutations within a predicted alpha-helical region alter the response of the ER to estradiol (E2), eliminate or impair the agonist activity of 4-hydroxytamoxifen (4-OHT), and alter the ability of E2 to overcome 4-OHT's antagonist activity. Deleting the F domain increases the affinity of the receptor for E2; by contrast, mutating a residue in the middle of the predicted helix to a proline does not alter the affinity for E2, but does change the binding mechanism from a positive cooperative to a noncooperative interaction. These and other results show the F domain exhibits substantial functional complexity, and support the idea that this domain modulates the activity of the hERalpha. In a second area of interest, we are investigating the role of hydrophobic and hydrogen-bonding interactions at the start of helix 12 in the activity of the hERalpha. Leucine-536 (L536) has been proposed to participate in hydrophobic interactions that form part of a capping motif stabilizing the start of helix 12. When mutated, the resulting receptors exhibit a reduced response, or even an inverted response, to E2 and 4-OHT on both ERE-driven and AP-1-driven promoters. Interestingly, these mutated receptors also exhibit altered interactions with probes that recognize the agonist-bound and 4-OHT-bound conformations of the ERalpha. Thus, L536 couples the binding of ligand with the conformation of the receptor. Overall, these results show that combining structure-based hypotheses with functional tests of the ER's activity can identify regions and interactions that are important in the ligand-stimulated activity of the protein.

Targeted mutation of key residues at the start of helix 12 in the hERalpha ligand-binding domain identifies the role of hydrogen-bonding and hydrophobic interactions in the activity of the protein

Estradiol (E(2)) and tamoxifen exert their effects through two members of the nuclear receptor superfamily, estrogen receptor (ER)-alpha and -beta. We want to identify the key interactions linking ligand-binding and activity of the ERalpha. Asp-351 and Leu-536 participate in hydrogen bond (Asp-351) and hydrophobic (Leu-536) interactions at the start of helix 12 in the ligand-binding domain (LBD) of the ERalpha. Mutations at each position alter ER activity, but we do not know which is more important. We mutated these residues in combination and individually and assessed the activity of the mutated ERs in the absence and presence of E(2) and 4-OHT on an ERE-driven and an AP-1-driven promoter, as well as their ability to interact with coregulators. On an ERE-driven promoter, the residue at position 351 determined whether E(2) stimulated or reduced the activity of the ER, as well as the level of activity in the presence of 4-OHT. Surprisingly, mutation of both residues generally did not produce cumulative deleterious effects, and they exerted counterbalancing effects on the basal activity on both promoters. Our results identify the contributions of specific interactions to the activity of the hERalpha, and support the concept that this region couples ligand-binding with ER activity.

Estrogen receptor alpha gene polymorphism and systemic lupus erythematosus: a possible risk?

Estrogens and their receptors may play a role in the pathogenesis of systemic lupus erythematosus. Genetic alterations in the exon 8-coding region of the estrogen receptor alpha alter the intracellular signalling of estrogens, leading in enhanced or diminished activity. We investigated whether genetic alterations in exon 8 of ERalpha gene are associated with the occurrence and clinical features of lupus disease. The coding region of ERalpha exon 8 was subjected to mutation analysis using the polymerase chain reaction, denaturing gradient gel electrophoresis and sequence analysis, using DNA isolated from whole blood of 36 female patients and 38 healthy females. Clinical and laboratory parameters were available from the patients' files. We identified the codon 594 polymorphism either in homozygous for the wild type gene (ACG/ACG) or heterozygous (ACG/ACA), both in patients and healthy females. Statistical analysis of the genotype and allele distribution revealed that there was a significant difference (chi2 test, P = 0.02 and P = 0.04, respectively) between patients and healthy women. Odds ratio estimate revealed that carriers of ACG/ACA genotype have three-fold higher risk of developing lupus disease (OR = 3.129, 95% CI 1.181-8.292). Moreover, in patients the heterozygous genotype was associated with rash, mouth ulcers and serositis (Fisher's exact test, P = 0.055, P = 0.083, P = 0.065, respectively). The heterozygous patients were associated significantly with an early age at disease onset (ANOVA test, P < 0.05). We conclude that estrogen receptor alpha codon 594 genotype may influence the development of systemic lupus erythematosus at a younger age, as well as a certain disease clinical pattern.

c-Src-null mice exhibit defects in normal mammary gland development and ERalpha signaling

The c-Src tyrosine kinase has been implicated to play an integral role in modulating growth factor receptor, integrin and steroid receptor function. One class of steroid receptors that c-Src modulates is the estrogen receptor alpha (ERalpha). Although there is strong biochemical evidence supporting a role for c-Src in ERalpha signaling, the consequence of this association is unclear at the biological level. To explore the significance of c-Src in ERalpha signaling, we studied the development of various reproductive organs that are dependent on ERalpha in c-Src-deficient mice. We show that the loss of the c-Src tyrosine kinase correlates with defects in ductal development as well as in uterine and ovarian development. Genetic and biochemical analyses of c-Src-deficient mammary epithelial cells also revealed defects in the ability of mammary epithelial cells to activate a number of signaling pathways in response to exogenous estrogen stimulation. Taken together, these studies demonstrate that c-Src plays a role in ERalpha signaling in vivo.

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.

Therapeutic targeting in the estrogen receptor hormonal pathway

Estrogens work along with genetic changes to promote the development and growth of breast cancers. Because estrogenic hormones act via the estrogen receptors (ERs), ER-alpha and ER-beta, and the ER is present in more than half of breast tumors, this receptor has been the most widely targeted protein in breast cancer therapy. The presence of the ER in breast tumors predicts improved disease-free survival and response to selective ER modulators (SERMs), such as tamoxifen, or other forms of endocrine therapy. Suppression of ER activity by SERMs has proven to be a great benefit in the treatment of breast cancers and also in the prevention of breast cancer in women at high risk for the disease. The Study of Tamoxifen and Raloxifene trial comparing tamoxifen versus raloxifene effectiveness in breast cancer prevention is currently under way. To understand the balance of beneficial and undesirable effects of SERMs and to optimize their effectiveness, current investigations seek to characterize the genes activated or suppressed by these agents. Elucidation of the gene networks and cell signaling pathways under estrogen and SERM regulation and a clearer definition of the respective roles of ER-alpha and ER-beta and their coregulators in the actions of selective ER ligands, should enable the identification of new gene targets for therapeutic intervention and the development of novel drugs for the optimal treatment and prevention of breast cancer.

Interaction of estrogen receptor alpha with protein kinase C alpha and c-Src in osteoblasts during differentiation

In cultured osteoblasts, protein kinase C (PKC) activity increases and estrogen receptor alpha (ERalpha) binding capacity decreases upon confluence. We investigated potential interactions between ERalpha and PKC isoforms and their confluence-induced modulations in clonal ROS.SMER#14 cells and primary osteoblasts. In sub-confluent ROS.SMER#14 cells, which express an exogenous plus small amounts of the endogenous ERalpha gene, the receptor appeared as two main bands of approximately 66 and approximately 46 kDa. In over-confluent, more differentiated cells, the cytosolic approximately 66 kDa ERalpha appeared decreased and the approximately 46 kDa variant increased. Enhanced expression and/or membrane translocation of PKCalpha and PKCepsilon, but not PKCzeta, was evidenced at over-confluence, along with transient increases in expression and kinase activity of c-Src, accompanied by membrane translocation of the kinase-activated enzyme. In contrast, negligible membrane translocation of PKCalpha and/or activated c-Src was observed in parental ROS 17/2.8 cells, which express low levels of full-length ERalpha. PKCalpha from over-confluent cells phosphorylated p60c-Src in vitro, suggesting functional interaction between the two kinases. ERalpha co-immunoprecipitated c-Src and PKCalpha, mostly in its cleaved form (PKMalpha). An analogous interaction was observed in primary osteoblasts. However, in these cells, much more PKCalpha/PKMalpha was ERalpha-co-immunoprecipitated at over-confluence, a condition in which the shorter, approximately 46 kDa ERalpha variant is increased. This interaction was enhanced by estradiol treatment or PKC down-regulation, but was unaffected by c-Src inhibition. These data highlight direct PKCalpha-c-Src-ERalpha interactions, which may be crucial in the modulation of estrogen responsiveness and the differentiation process in osteoblasts.

Interactions between estrogen and growth factor receptors in human breast cancers and the tumor-associated vasculature

Estrogens and growth factors stimulate the proliferation of human breast cancer cells by primary binding and activation of specific receptors that regulate downstream signaling events. Receptors for estrogen are phosphoproteins, and the biologic function of these proteins can be modulated by changes in their phosphorylation state. Signal transduction by growth factor receptors, including HER-2/neu and epidermal growth factor (EGF) receptors, can alter the phosphorylation of estrogen receptor (ER) and the biologic activity of ER-dependent signaling networks both in the presence and in the absence of estrogenic ligands. In addition, both estrogen and growth factor signaling pathways regulate the secretion of vascular endothelial growth factors that stimulate tumor-associated angiogenesis. These molecular interactions significantly impact breast cancer cell growth and survival, and integration of selected signal transduction inhibitors with antiestrogen therapies show promise as a new antitumor treatment strategy that will soon be evaluated in the clinic. Sensitive and reliable assays of estrogen, HER-2/neu, and EGF receptors and tumor-associated angiogenesis will be important biologic factors to consider in the choice of optimal antitumor therapies for patients with breast cancer.

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.

Estrogen receptor phosphorylation

Estrogen receptor alpha (ERalpha) is phosphorylated on multiple amino acid residues. For example, in response to estradiol binding, human ERalpha is predominately phosphorylated on Ser-118 and to a lesser extent on Ser-104 and Ser-106. In response to activation of the mitogen-activated protein kinase pathway, phosphorylation occurs on Ser-118 and Ser-167. These serine residues are all located within the activation function 1 region of the N-terminal domain of ERalpha. In contrast, activation of protein kinase A increases the phosphorylation of Ser-236, which is located in the DNA-binding domain. The in vivo phosphorylation status of Tyr-537, located in the ligand-binding domain, remains controversial. In this review, I present evidence that these phosphorylations occur, and identify the kinases thought to be responsible. Additionally, the functional importance of ERalpha phosphorylation is discussed.

Mutations targeted to a predicted helix in the extreme carboxyl-terminal region of the human estrogen receptor-alpha alter its response to estradiol and 4-hydroxytamoxifen

The human estrogen receptor-alpha, a member of the nuclear receptor superfamily, is a ligand-regulated transcriptional modulator. Because comparatively little is known about the extreme carboxyl-terminal region of the estrogen receptor (F domain), we used secondary structure prediction to design mutations that delete the F domain (S554stop), disrupt a possible turn (G556L/G557L), and alter a predicted helix (S559A/E562A, Q565P), and we evaluated the effects of these mutations on hormone binding and transcription activation in response to estradiol and the mixed agonist/antagonist 4-hydroxytamoxifen. Mutations that deleted the F domain (S554stop) or targeted the predicted helix (S559A/E562A, Q565P) greatly reduced or eliminated the agonist activity of 4-hydroxytamoxifen. Deleting the F domain increased the affinity of the receptor for estradiol and decreased the antagonist activity of 4-hydroxytamoxifen. The Q565P mutant exhibited a non-cooperative hormone-binding mechanism, as well as an impaired response to estradiol and increased antagonist activity of 4-hydroxytamoxifen. Our results show that mutations in the F domain alter not only the response to estradiol, the affinity for hormone, and the interaction between receptor subunits but can uncouple the agonist and antagonist activities of 4-hydroxytamoxifen. These results suggest that the F domain modulates the activity of the estrogen receptor-alpha by multiple mechanisms.

The conserved residues of the ligand-binding domains of steroid receptors are located in the core of the molecules

The relationship between conserved residues and biochemical functions of steroid receptors was investigated. Pairwise three-dimensional (3D) alignment of the ligand-binding domains of the human estrogen (1A52) and progesterone (1A28) receptors revealed two conserved domains; Asn313-Ser456 and Gln471-Lys531 (numbering reflects the sequence in the human estrogen receptor). Alignment of the protein sequences of 39 steroid receptors revealed 36 highly conserved residues (i.e., the residues commonly found in more than 80% of sequences aligned). They were distributed throughout the sequences but formed a contiguous 3D structure. Most of these highly conserved residues were buried in the ligand-binding domain, but several residues were exposed on the surface. The well-known functions commonly associated with the ligand-binding domain of steroid receptors are ligand binding, HSP90 binding, transcriptional activation and dimerization. The relationship between the residues and these functions were checked. To determine the residues involved in dimerization, the differences between the solvent accessibilities of the monomeric and dimeric forms were calculated. These results revealed 32 residues of 1A52 and 15 residues of 1A28 potentially involved in dimerization. Their distribution areas do not overlap greatly. Comparing these putative dimerization sites with highly conserved residues, many of the exposed conserved residues were observed on the side of the domain opposite are the dimerization sites. Some highly conserved residues are located in a steroid-binding site and in transcriptional activation domain. However, few of them were observed in the HSP90 binding site. These results indicate that the core structure made by most of the highly conserved residues among the ligand-binding domains of steroid receptors is important. These conserved residues may be essential for conformational change in the ligand-binding domain from its inactive to active form.

Electrospray ionization mass spectrometry of oligonucleotide complexes with drugs, metals, and proteins

I. Introduction 61 II. Binding of Small Molecules to DNA 62 A. Covalent Binding 62 B. Reversible (Noncovalent) DNA-Binding Agents 65 III. DNA-Metal Ion Complexes 67 A. Platinum Complexes 70 B. Other Metal Ions 73 IV. DNA-Protein Complexes 74 A. Introduction 74 B. ESI-MS of DNA-Protein Complexes 76 C. ESI-MS Analysis of Proteolytic Products of DNA-Protein Complexes 79 D. ESI-MS of Ternary DNA-Protein-Ligand Complexes 80 V. Conclusions 80 Abbreviations 81 References 81 --Interactions of DNA with drugs, metal ions, and proteins are important in a wide variety of biological processes. With the advent of electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI), mass spectrometry (MS) is now a well-established tool for the characterization of the primary structures of biopolymers. The gentle nature of the ESI process, however, means that ESI-MS is also finding application for the study of noncovalent and other fragile biomolecular complexes. We outline here the progress, to date, in the use of ESI-MS for the study of noncovalent drug-DNA and protein-DNA complexes together with strategies that can be employed to examine the binding of small molecules and metal complexes to DNA. In the case of covalent complexes with DNA, sequence information can be derived from ESI-MS used in conjunction with tandem mass spectrometry (MS/MS) and/or enzymatic digestion. MS/MS can also be used to probe the relative binding affinities of drugs that bind to DNA via noncovalent interactions. Overall, the work in this area, to date has demonstrated that ESI-MS and MS/MS will prove to be valuable complements to other structural methods, offering advantages in terms of speed, specificity, and sensitivity. (c) 2001 John Wiley & Sons, Inc.

Nuclear tyrosine phosphorylation: the beginning of a map

Tyrosine phosphorylation is usually associated with cytoplasmic events. Yet, over the years, many reports have accumulated on tyrosine phosphorylation of individual molecules in the nucleus, and several tyrosine kinases and phosphatases have been found to be at least partially nuclear. The question arises as to whether nuclear tyrosine phosphorylation represents a collection of loose ends of events originating in the cytoplasm or if there may be intranuclear signaling circuits relying on tyrosine phosphorylation to regulate specific processes. The recent discovery of a mechanism causing nuclear tyrosine phosphorylation has prompted us to review the cumulative evidence for nuclear tyrosine phosphorylation pathways and their possible role. While we found that no complex nuclear function has yet been shown to rely upon intranuclear tyrosine phosphorylation in an unambiguous fashion, we found a very high number of compelling observations on individual molecules that suggest underlying networks linking individual events. A systematic proteomics approach to nuclear tyrosine phosphorylation should help chart possible interaction pathways.

Estrogen receptor transcription and transactivation: Estrogen receptor alpha and estrogen receptor beta: regulation by selective estrogen receptor modulators and importance in breast cancer

Estrogens display intriguing tissue-selective action that is of great biomedical importance in the development of optimal therapeutics for the prevention and treatment of breast cancer, for menopausal hormone replacement, and for fertility regulation. Certain compounds that act through the estrogen receptor (ER), now referred to as selective estrogen receptor modulators (SERMs), can demonstrate remarkable differences in activity in the various estrogen target tissues, functioning as agonists in some tissues but as antagonists in others. Recent advances elucidating the tripartite nature of the biochemical and molecular actions of estrogens provide a good basis for understanding these tissue-selective actions. As discussed in this thematic review, the development of optimal SERMs should now be viewed in the context of two estrogen receptor subtypes, ERalpha and ERbeta, that have differing affinities and responsiveness to various SERMs, and differing tissue distribution and effectiveness at various gene regulatory sites. Cellular, biochemical, and structural approaches have also shown that the nature of the ligand affects the conformation assumed by the ER-ligand complex, thereby regulating its state of phosphorylation and the recruitment of different coregulator proteins. Growth factors and protein kinases that control the phosphorylation state of the complex also regulate the bioactivity of the ER. These interactions and changes determine the magnitude of the transcriptional response and the potency of different SERMs. As these critical components are becoming increasingly well defined, they provide a sound basis for the development of novel SERMs with optimal profiles of tissue selectivity as medical therapeutic agents.