Role of the proteasome in the regulation of estrogen receptor alpha turnover and function in MCF-7 breast carcinoma cells

Azobenzene-Tagged Photopeptides Exhibiting Excellent Selectivity and Light-Induced Cytotoxicity in MCF-7 Cells over HeLa and A549

The precise regulation of proteasome activity has become a focal point in current research, particularly its implications in cancer treatment. Bortezomib is used for treating multiple myeloma and is found to be ineffective against solid tumors. A spatiotemporal control over the proteasome is one of the solutions to resolve these issues using external stimuli, such as light. Thus, we designed and synthesized azobenzene-containing tripeptide vinyl sulfones 1, 2, 3, and 4, as the azobenzene moiety can impart E↔Z isomerism upon exposure to UV light. Further, the hydrophobicity of these peptides was fine-tuned by systematically varying the size of hydrophobic amino acids at the P1, P2, and P3 positions. The light-induced Z isomers of these photopeptides showed excellent cellular potency in HeLa, MCF-7, and A549 cell lines. Photopeptide 4 with valine at the proximal position, phenylalanine at P2, and leucine at the P1 positions exhibited 19.3- and 6.6-fold cellular potency in MCF-7 and A549 cells, respectively.

Selective impact of ALK and MELK inhibition on ERα stability and cell proliferation in cell lines representing distinct molecular phenotypes of breast cancer

Breast cancer (BC) is a leading cause of global cancer-related mortality in women, necessitating accurate tumor classification for timely intervention. Molecular and histological factors, including PAM50 classification, estrogen receptor α (ERα), breast cancer type 1 susceptibility protein (BRCA1), progesterone receptor (PR), and HER2 expression, contribute to intricate BC subtyping. In this work, through a combination of bioinformatic and wet lab screenings, followed by classical signal transduction and cell proliferation methods, and employing multiple BC cell lines, we identified enhanced sensitivity of ERα-positive BC cell lines to ALK and MELK inhibitors, inducing ERα degradation and diminishing proliferation in specific BC subtypes. MELK inhibition attenuated ERα transcriptional activity, impeding E2-induced gene expression, and hampering proliferation in MCF-7 cells. Synergies between MELK inhibition with 4OH-tamoxifen (Tam) and ALK inhibition with HER2 inhibitors revealed potential therapeutic avenues for ERα-positive/PR-positive/HER2-negative and ERα-positive/PR-negative/HER2-positive tumors, respectively. Our findings propose MELK as a promising target for ERα-positive/PR-positive/HER2-negative BC and highlight ALK as a potential focus for ERα-positive/PR-negative/HER2-positive BC. The synergistic anti-proliferative effects of MELK with Tam and ALK with HER2 inhibitors underscore kinase inhibitors' potential for selective treatment in diverse BC subtypes, paving the way for personalized and effective therapeutic strategies in BC management.

Glyphosate mimics 17β-estradiol effects promoting estrogen receptor alpha activity in breast cancer cells

Glyphosate, the active ingredient in several broad-spectrum herbicide formulations, has been validated and widely used throughout the world. Recent reports have questioned its safety, showing that glyphosate may act as an endocrine disruptor by promoting estrogenic activity. However, the molecular mechanism involved in this phenomenon remains unclear. Therefore, here we aimed to elucidate the mechanism by which glyphosate induces estrogenic activity using estrogen-sensitive breast cancer cell line models. Our results show that glyphosate mimics the cell effects of 17β-estradiol (E2), promoting estrogen receptor α (ERα) phosphorylation, its degradation, and transcriptional activity at high concentrations. The molecular mechanism seems involved in the ERα ligand-binding domain (LBD). Molecular simulations suggest a plausible interaction between glyphosate and the LBD through a coordinated complex involving divalent cations such as Zn (II). In addition, glyphosate exposure alters the level of Cyclin-dependent kinase 7 that contribute to ERα phosphorylation. Finally, glyphosate increases cell proliferation rate and levels of cell cycle regulators, accompanied by an increase in anchorage-independent growth capacity. These findings suggest that glyphosate at high concentrations, induces estrogen-like effects through an ERα ligand binding site-dependent mechanism, leading to cellular responses resulting from a complex interplay of genomic and non-genomic events.

A New Anti-Estrogen Discovery Platform Identifies FDA-Approved Imidazole Anti-Fungal Drugs as Bioactive Compounds against ERα Expressing Breast Cancer Cells

17β-estradiol (E2) exerts its physiological effects through the estrogen receptor α (i.e., ERα). The E2:ERα signaling allows the regulation of cell proliferation. Indeed, E2 sustains the progression of ERα positive (ERα+) breast cancers (BCs). The presence of ERα at the BC diagnosis drives their therapeutic treatment with the endocrine therapy (ET), which restrains BC progression. Nonetheless, many patients develop metastatic BCs (MBC) for which a treatment is not available. Consequently, the actual challenge is to complement the drugs available to fight ERα+ primary and MBC. Here we exploited a novel anti-estrogen discovery platform to identify new Food and Drug Administration (FDA)-approved drugs inhibiting E2:ERα signaling to cell proliferation in cellular models of primary and MBC cells. We report that the anti-fungal drugs clotrimazole (Clo) and fenticonazole (Fenti) induce ERα degradation and prevent ERα transcriptional signaling and proliferation in cells modeling primary and metastatic BC. The anti-proliferative effects of Clo and Fenti occur also in 3D cancer models (i.e., tumor spheroids) and in a synergic manner with the CDK4/CDK6 inhibitors palbociclib and abemaciclib. Therefore, Clo and Fenti behave as “anti-estrogens”-like drugs. Remarkably, the present “anti-estrogen” discovery platform represents a valuable method to rapidly identify bioactive compounds with anti-estrogenic activity.

Ligand Binding Prolongs Androgen Receptor Protein Half-Life by Reducing its Degradation

Androgens are important in female reproduction, but the molecular actions of androgens in female reproductive tissues are not fully understood. We investigated the androgen-responsive transcriptome in human and mouse granulosa cells (GCs) and surprisingly found that the gene-regulation activity of androgen receptor (AR) in these cells is negligible. We then investigated extranuclear actions of AR and found that in human and mouse GCs, as well as in prostate cancer cells, dihydrotestosterone (DHT) dramatically increases the half-life of its own receptor protein. Using the human granulosa-like KGN cells, we show that this effect is not the result of increased AR gene transcription or protein synthesis, nor is it fully abrogated by proteasome inhibition. Knockdown of PTEN, which contributes to degradation of cytoplasmic AR, did not diminish AR accumulation in the presence of DHT. Using immunofluorescence cellular localization studies, we show that nuclear AR is selectively protected from degradation in the presence of DHT. Knockdown of importin 7 expression, a potential regulator of AR nuclear import, does not affect DHT-mediated nuclear accumulation of AR, suggesting importin 7-independent nuclear import of AR in GCs. Further, DNA binding is not required for this protective mechanism. In summary, we show that ligand binding sequesters AR in the nucleus through enhanced nuclear localization independent of DNA binding, thereby protecting it from proteasome degradation in the cytoplasm. This phenomenon distinguishes AR from other sex steroid receptors and may have physiological significance through a positive feedback loop in which androgen induces its own activity in male and female reproductive tissues.

Pathological Maintenance and Evolution of Breast Cancer: The Convergence of Irreversible Biological Actions of ER Alpha

Estrogen receptor alpha (ERα) is a modulator of breast cancer maintenance and evolution. Hence, analysis of underlying mechanisms by which ERα operates is of importance for the improvement of the hormonal therapy of the disease. This review focuses on the irreversible character of the mechanism of action of ERα, which also concerns other members of the steroid hormones receptors family. ERα moves in permanence between targets localized especially at the chromatin level to accomplish gene transcriptions imposed by the estrogenic ligands and specific antagonists. Receptor association as at the plasma membrane, where it interacts with other recruitment sites, extends its regulatory potency to growth factors and related peptides through activation of signal transductions pathways. If the latter procedure is suitable for the transcriptions in which the receptor operates as a coregulator of another transcription factor, it is of marginal influence with regard to the direct estrogenic regulation procedure, especially in the context of the present review. Irreversibility of the successive steps of the underlying transcription cycle guarantees maintenance of homeostasis and evolution according to vital necessities. To justify this statement, reported data are essentially described in a holistic view rather than in the context of exhaustive analysis of a molecular event contributing to a specific function as well as in a complementary perspective to elaborate new therapeutic approaches with antagonistic potencies against those tumors promoting ERα properties.

Unexpected Impact of a Hepatitis C Virus Inhibitor on 17β-Estradiol Signaling in Breast Cancer

17β-Estradiol (E2) controls diverse physiological processes, including cell proliferation, through its binding to estrogen receptor α (ERα). E2:ERα signaling depends on both the receptor subcellular localization (e.g., nucleus, plasma membrane) and intracellular ERα abundance. Indeed, the control of ERα levels is necessary for the effects of E2, and E2 itself induces ERα degradation and cell proliferation in parallel. Thus, the modulation of intracellular ERα levels is a critical parameter for E2-induced cell proliferation. Therefore, we used this parameter as a bait to identify compounds that influence ERα levels and E2-dependent proliferation in breast cancer (BC) cells from a library of Food and Drug Administration (FDA)-approved drugs. We found that telaprevir (Tel) reduces ERα levels and inhibits BC cell proliferation. Tel is an inhibitor of the hepatitis C virus (HCV) NS3/4A serine protease, but its effect on E2:ERα signaling has not been investigated. Here, for the first time, we analyzed the effects of Tel on intracellular ERα levels and E2:ERα signaling to cell proliferation in different ERα-expressing BC cell lines. Overall, our findings demonstrate that Tel reduces intracellular ERα levels, deregulates E2:ERα signaling and inhibits E2-induced proliferation in BC cells and suggest the potential drug repurposing of Tel for the treatment of BC.

An Ethnic Comparison of Arginine Dimethylation and Cardiometabolic Factors in Healthy Black and White Youth: The ASOS and African-PREDICT Studies

Proteinic arginine dimethylation (PADiMe) is a major post-translational modification. Proteolysis of asymmetric and symmetric PADiMe products releases asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA), respectively, two endogenous atherogenic substances. SDMA, ADMA, and its major metabolite dimethylamine (DMA) are eliminated by the kidney. The urinary concentrations of DMA+ADMA, SDMA, and DMA+ADMA+SDMA are useful measures of the whole-body asymmetric and symmetric PADiMe, respectively. Urinary (DMA+ADMA)/SDMA is an index of the asymmetric to symmetric PADiMe balance. In two bi-ethnic studies, the ASOS (39 black boys, 41 white boys) and the African-PREDICT (292 black young men, 281 white young men) studies, we investigated whether ethnicity is a major determinant of PADiMe, and whether PADiMe is associated with blood pressure and ethnicity-dependent growth and inflammatory factors, including HDL. DMA, ADMA, and SDMA were measured in spot urine samples by gas chromatography-mass spectrometry, and their excretion was corrected for creatinine excretion. In black boys, creatinine-corrected DMA, DMA+ADMA, and DMA+ADMA+SDMA concentrations were lower by 11.7%, 9.5%, and 7.6% (all p < 0.05), respectively, compared to the white boys, and 3.4%, 2.0%, and 1.8% lower (all p < 0.05), respectively, in black compared to white men. (DMA+ADMA)/SDMA did not differ between black boys and black men, but was higher in white boys compared to white men. ADMA did not differ between black and white boys, or between black and white men. Creatinine-corrected SDMA excretion was lower in black boys compared to white boys (by 8%) and to white men (by 3.1%). None of the PADiMe indices were associated with blood pressure in either study. IGF-binding protein 3 correlated inversely with all PADiMe indices in the black men only. Our study showed that asymmetric proteinic arginine dimethylation is higher in white boys than in black boys, and that this difference disappears in adulthood. ADMA metabolism and SDMA excretion were lower in the black subjects compared to the white subjects, suggesting ethnicity-dependent hepatic and renal elimination of ADMA and SDMA in the childhood. The results of our study may have clinical relevance beyond atherosclerosis, such as in growth and inflammation, which have not been sufficiently addressed thus far.

Development of bivalent triarylalkene- and cyclofenil-derived dual estrogen receptor antagonists and downregulators

Up to 80% of mammary carcinoma initially exhibit estrogen-dependent growth, which can be treated by aromatase inhibitors or SERMs/SERDs. To increase the options after failure of the hormonal therapy with these drugs, the search for alternatives with a different mode of action to prevent estrogen action is of high relevance. Therefore, this study focused on the inhibition of coactivator recruitment at the estrogen receptor (ER) by targeted attachment of bivalent compounds at the coactivator binding site besides the primary binding at the ligand binding domain. Eight homodimeric 4-[1-(4-hydroxyphenyl)-2-phenyl-1-butenyl]cinnamic acid (GW7604)- or cyclofenilacrylic acid-based ER ligands with diaminoalkane linkers (C2-C5) were synthesized and their effects on the ER subtypes were assessed in vitro. All compounds possessed full antagonistic potency at ERα/β as determined in a transactivation assay. Furthermore, they exerted medium downregulatory effects dependent on the spacer length and did not stimulate the ER expression as observed for 4-hydroxytamoxifen. The cyclofenil-derived dimer with C4 spacer (15b) showed the highest binding affinity to ERα (RBA = 79.2%) and downregulated the ER content in MCF-7 cells with an efficiency of 38% at 1 μM.

Neddylation Inactivation Facilitates FOXO3a Nuclear Export to Suppress Estrogen Receptor Transcription and Improve Fulvestrant Sensitivity

PURPOSE

How the neddylation pathway functions in breast tumor and regulation of estrogen receptor (ER) expression is rarely reported. The purpose of this study was to identify the role of neddylation in breast cancer and ER expression, and further explore the underlying mechanisms.

EXPERIMENTAL DESIGN

Expression patterns of nedd8-activating enzyme (NAE) and nedd8, two key proteins in the neddylation pathway, were examined in human breast specimens. ER-α expression was investigated using animal 18F-FES-PET/CT and immunoblotting upon NAE inhibitor MLN4924 treatment. Chromatin immunoprecipitation assay, luciferase reporter promoter assay, and the CRISPR-Cas9 system were used to elucidate the mechanism of ER-α regulation by MLN4924. The ER-positive breast cancer mouse model was used to determine the synergetic effect of MLN4924 and fulvestrant on tumor growth. All statistical tests were two-sided.

RESULTS

Both NAE1 and nedd8 expressions were higher in the ER-positive subgroup. Higher expressions of NAE1 and nedd8 indicated poorer prognosis. Importantly, ER-α expression was significantly downregulated upon MLN4924 treatment in vitro and in vivo. Mechanistically, MLN4924 treatment delayed serum and glucocorticoid-induced protein kinase (SGK) degradation and induced Forkhead box O3a (FOXO3a) nuclear export as well as decreased binding to the ESR1 promoter. Importantly, MLN4924 single or synergized with fulvestrant significantly suppressed the growth of ER-positive breast cancer in vitro and in vivo.

CONCLUSIONS

Our proof-of-principle study determines the activation of neddylation in breast tumor tissues for the first time and reveals a new ER-α regulatory mechanism, as well as further explores an effective approach to improve fulvestrant sensitivity through a neddylation inactivation combination.

Strategies to degrade estrogen receptor α in primary and ESR1 mutant-expressing metastatic breast cancer

With the advent of omic technologies, our understanding of the molecular mechanisms underlying estrogen receptor α (ERα)-expressing breast cancer (BC) progression has grown exponentially. Nevertheless, the most widely used therapy for inhibiting this disease is endocrine therapy (ET) (i.e., aromatase inhibitors, tamoxifen - Tam, faslodex/fulvestrant - FUL). However, in a considerable number of cases, prolonged patient treatment with ET generates the development of resistant tumor cells and, consequently, tumor relapse, which manifests as metastatic disease that is extremely difficult to manage, especially because such metastatic BCs (MBCs) often express ERα mutations (e.g., Y537S, D538G) that confer pronounced growth advantages to tumor cells. Interestingly, ET continues to be the therapy of choice for this neoplasia, which underscores the need to identify novel drugs that could work in primary and MBCs. In this study, we review the approaches that have been undertaken to discover these new anti-ERα compounds, especially considering those focused on evaluating ERα degradation. A literature analysis demonstrated that current strategies for discovering new anti-BC drugs are focusing on the identification either of novel ERα inhibitors, of compounds that inhibit ERα-related pathways or of drugs that influence ERα-unrelated cellular pathways. Several lines of evidence suggest that all of these molecules alter the ERα content and block the proliferation of both primary and MBCs. In turn, we propose to rationalize all these discoveries into the definition of e.m.eral.d.s (i.e., selective modulators of ERα levels and degradation) as a novel supercategory of anti-ERα drugs that function both as modulators of ERα levels and inhibitors of BC cell proliferation.

Estrogen receptor alpha isoform ERdelta7 in myometrium modulates uterine quiescence during pregnancy

BACKGROUND

Circulating estrogen (E2) levels are high throughout pregnancy and increase towards term, however its local tissue specific actions vary across gestation. For example, myometrial E2 regulated uterotonic action is disabled until term, whereas it's proliferative function is maintained in the breast. We have identified gestationally regulated splicing events, mediated by hnRNPG and modulated by E2 that generate alternatively spliced estrogen receptor alpha (ERα) variants (ERΔ7 and ERα46) in the myometrium. These variants allow for differential, gestationally regulated, modulation of the uterotonic action of E2.

METHODS

Human myometrium isolated from preterm and term non-laboring and laboring pregnant women were analyzed for ERα isoforms and splice factor levels. Lentiviral mediated shRNA knockdown of hnRNPG and overexpression of ERΔ7 were performed in human myometrial (hTERT-HM) cells. Functional 3D collagen contraction assays were executed.

FINDINGS

ERΔ7 acts as a dominant negative repressor of the uterotonic action of ERα66 and ERα46 isoforms through the regulation of the myometrial gap junction protein GJA1. Elimination of hnRNPG inhibits the generation of ERΔ7 while overexpression of ERΔ7 inhibited GJA1 expression. Moreover in vivo human myometrial hnRNPG levels decline at term in an E2 dependent manner resulting in a withdrawal of ERΔ7 levels and its tocolytic action at term.

INTERPRETATION

Our findings implicate the unique role of ERΔ7 as a modulator of myometrial quiescence and define the mechanism of ERΔ7 generation, through hormonally regulated splicing events. FUND: This study was supported by NIH OPRU U01 supplement (HD047905), University of Pittsburgh and Wayne State University Perinatal Research Initiative (USA).

Sialic Acid-Binding Lectin from Bullfrog Eggs Exhibits an Anti-Tumor Effect Against Breast Cancer Cells Including Triple-Negative Phenotype Cells

Sialic acid-binding lectin from Rana catesbeiana eggs (cSBL) is a multifunctional protein that has lectin and ribonuclease activity. In this study, the anti-tumor activities of cSBL were assessed using a panel of breast cancer cell lines. cSBL suppressed the cell growth of all cancer cell lines tested here at a concentration that is less toxic, or not toxic at all, to normal cells. The growth suppressive effect was attributed to the cancer-selective induction of apoptosis. We assessed the expressions of several key molecules associated with the breast cancer phenotype after cSBL treatment by western blotting. cSBL decreased the expression level of estrogen receptor (ER) α, while it increased the phosphorylation level of p38 mitogen-activated protein kinase (MAPK). cSBL also suppressed the expression of the progesterone receptor (PgR) and human epidermal growth factor receptor type 2 (HER2). Furthermore, it was revealed that cSBL decreases the expression of the epidermal growth factor receptor (EGFR/HER1) in triple-negative breast cancer cells. These results indicate that cSBL induces apoptosis with decreasing ErbB family proteins and may have great potential for breast cancer chemotherapy, particularly in triple-negative phenotype cells.

Gender Bias in Human Systemic Lupus Erythematosus: A Problem of Steroid Receptor Action?

Systemic lupus erythematosus (SLE) is a chronic systemic autoimmune disease resulting from abnormal interactions between T and B cells. The acquisition of SLE is linked to genetic susceptibility, and diverse environmental agents can trigger disease onset in genetically susceptible individuals. However, the strongest risk factor for developing SLE is being female (9:1 female to male ratio). The female sex steroid, estradiol, working through its receptors, contributes to the gender bias in SLE although the mechanisms remain enigmatic. In a small clinical trial, monthly administration of the estrogen receptor (ERα) antagonist, ICI182,780 (fulvestrant), significantly reduced disease indicators in SLE patients. In order to identify changes that could account for improved disease status, the present study utilized fulvestrant (Faslodex) to block ERα action in cultured SLE T cells that were purified from blood samples collected from SLE patients (n = 18, median age 42 years) and healthy control females (n = 25, median age 46 years). The effects of ERα antagonism on estradiol-dependent gene expression and canonical signaling pathways were analyzed. Pathways that were significantly altered by addition of Faslodex included T helper (Th) cell differentiation, steroid receptor signaling [glucocorticoid receptor (GR), ESR1 (ERα)], ubiquitination, and sumoylation pathways. ERα protein expression was significantly lower (p < 0.018) in freshly isolated, resting SLE T cells suggesting ERα turnover is inherently faster in SLE T cells. In contrast, ERα/ERβ mRNA and ERβ protein levels were not significantly different between SLE and normal control T cell samples. Plasma estradiol levels did not differ (p > 0.05) between SLE patients and controls. A previously undetected interaction between GR and ERα signaling pathways suggests posttranslational modification of steroid receptors in SLE T cells may alter ERα/GR actions and contribute to the strong gender bias of this autoimmune disorder.

ERE-dependent transcription and cell proliferation: Independency of these two processes mediated by the introduction of a sulfone function into the weak estrogen estrothiazine

The synthetic coumestrol derivative 6,12-dihydro-3-methoxy-1-benzopyrano[3,4-b][1,4]benzothiazin-6-one (estrothiazine, ESTZ) has been identified as a weak estrogen receptor α (ERα) ligand unable to compete with tritiated estradiol. The biological activity of this compound, supported by a methoxy group in position 3, seems mainly to result from its capacity to activate ERα dimerization without any participation of coactivators. In support of this view and referring to conventional estrogens, an ESTZ metabolism study conducted with hepatic human microsomes failed to provide any argument in favour of an estrogenic activity dependent on a metabolic conversion of the compound into hydroxylated metabolites with strong receptor activation ability. Interestingly, we failed to detect any oxidation of the sulfur atom of the compound. In the light of pharmacological literature data concerning sulfonylation, we assessed ERα-mediated activities generated by two sulfonylated ESTZ derivatives in which the methoxy group that plays a key role in its mechanism of action was maintained or removed. Sulfonylated ESTZ, even in its demethoxylated form, induced ERE-mediated transcriptions in MCF-7 breast cancer cells, without affecting the ERα turnover rate. In contrast to ESTZ, this compound failed to enhance the proliferation of ERα-positive breast cancer cells, suggesting that its sulfone function confers upon the receptor a capacity to elicit some of the known characteristics associated with estrogenic responses. Moreover, we demonstrated that this sulfone may contribute to ERα dimerization without any requirement of the methoxy group. Nevertheless, it seems to cooperate with this group, as reflected by a weak ability of the sulfonylated form of ESTZ to compete with tritiated estradiol for ERα-binding. Assessment of the docking of this compound within the ligand-binding domain of the receptor by molecular dynamics provided an explanation for this observation since the sulfone is engulfed in a small hydrophobic pocket involving the residues Leu-346, Leu-349, Ala-350 and Leu-384, also known to recruit coactivators. This work not only reports the sulfone functional group as a pharmacophore for estrogenic activity, but also opens new perspectives for the development of estrogenic molecules with therapeutic purpose and devoid of proliferative side effects.

The effects of turmeric (curcumin) on tumor suppressor protein (p53) and estrogen receptor (ERα) in breast cancer cells

Curcumin (CUR) is a compound that has antibacterial, antiviral, anti-inflammatory, and anticancer properties. In this study, we have analyzed the effects of CUR on the expression of ERα and p53 in the presence of hormones and anti-hormones in breast cancer cells. Cells were cultured in a medium containing charcoal-stripped fetal bovine serum to deplete any endogenous steroids and treated with CUR at varying concentrations or in combination with hormones and anti-hormones. Protein analysis revealed a relative decrease in the levels of p53 and ERα upon treatment with 5–60 µM CUR. In cell proliferation studies, CUR alone caused a 10-fold decrease compared with the treatment with estrogen, which suggests its antiproliferative effects. Delineating the role of CUR in the regulation of p53, ERα, and their mechanisms of action may be important in understanding the influence of CUR on tumor suppressors and hormone receptors in breast cancer.

ERα dimerization: a key factor for the weak estrogenic activity of an ERα modulator unable to compete with estradiol in binding assays

Estrothiazine (ESTZ) is a weak estrogen sharing structural similarities with coumestrol. ESTZ failed to compete with [³H]17β-estradiol ([³H]17β-E₂) for binding to the estrogen receptor α (ERα), questioning its ability to interact with the receptor. However, detection by atomic force spectroscopy (AFS) of an ESTZ-induced ERα dimerization has eliminated any remaining doubts. The effect of the compound on the proliferation of ERα-positive and negative breast cancer cells confirmed the requirement of the receptor. The efficiency of ESTZ in MCF-7 cells was weak without any potency to modify the proliferation profile of estradiol and coumestrol. Growth enhancement was associated with a proteasomal degradation of ERα without substantial recruitment of LxxLL coactivators. This may be related to an unusual delay between the acquisition by the receptor of an ERE-binding capacity and the subsequent estrogen-dependent transcription. A complementary ability to enhance TPA-induced AP-1 transcription was observed, even at concentrations insufficient to activate the ERα, suggesting a partly independent mechanism. ESTZ also rapidly and transiently activated ERK1/2 likely through membrane estrogenic pathways provoking a reorganization of the actin network. Finally, the systematic absence of biological responses with an ESTZ derivative unable to induce ERα dimerization stresses the importance of this step in the action of the compound, as reported for conventional estrogens. In view of the existence of many other ERα modulators (endocrine disruptors such as, for example, pesticides, environmental contaminants or phytoestrogens) with extremely weak or similar apparent lack of binding ability, our work may appear as pilot investigation for assessing their mechanism of action.

Clomiphene citrate down-regulates estrogen receptor-α through the ubiquitin-proteasome pathway in a human endometrial cancer cell line

We examined how clomiphene citrate (CC) reduces estrogen receptor-α (ERα) in a human endometrial cancer cell line. Ishikawa human endometrial cancer cells were treated with ERα ligands such as 17β-estradiol (E2), CC, and the pure antiestrogen, ICI 182,780 (ICI). Thereafter, the expression levels of ERα protein and mRNA were analyzed by western blot and real-time quantitative PCR, respectively, and those of ubiquitinated ERα were analyzed by immunoprecipitation of ERα followed by immunoblotting with an anti-ubiquitin antibody. The expression levels of ERα protein after treatment with E2, CC, and ICI were significantly decreased compared to pre-treatment levels without a corresponding decrease in ERα mRNA. These ligands significantly increased the levels of ubiquitinated ERα compared to vehicle treatment. Co-treatment with the proteasome inhibitor, MG132, abrogated the decrease in ERα levels caused by treatment with the ligands only. We demonstrated, for the first time, a CC-induced decrease in ERα mediated by the ubiquitin-proteasome pathway in human endometrial cancer cells.

Dynamin II is required for 17β-estradiol signaling and autophagy-based ERα degradation

17β-estradiol (E2) regulates diverse physiological effects, including cell proliferation, by binding to estrogen receptor α (ERα). ERα is both a transcription factor that drives E2-sensitive gene expression and an extra-nuclear localized receptor that triggers the activation of diverse kinase cascades. While E2 triggers cell proliferation, it also induces ERα degradation in a typical hormone-dependent feedback loop. Although ERα breakdown proceeds through the 26S proteasome, a role for lysosomes and for some endocytic proteins in controlling ERα degradation has been reported. Here, we studied the role of the endocytic protein dynamin II in E2-dependent ERα signaling and degradation. The results indicate that dynamin II siRNA-mediated knock-down partially prevents E2-induced ERα degradation through the inhibition of an autophagy-based pathway and impairs E2-induced cell proliferation signaling. Altogether, these data demonstrate that dynamin II is required for the E2:ERα signaling of physiological functions and uncovers a role for autophagy in the control of ERα turnover.

Tetrahydro-iso-alpha Acids Antagonize Estrogen Receptor Alpha Activity in MCF-7 Breast Cancer Cells

Tetrahydro-iso-alpha acids commonly called THIAA or Tetra are modified hop acids extracted from hop (Humulus lupulus L.) which are frequently used in brewing industry mainly in order to provide beer bitterness and foam stability. Interestingly, molecular structure of tetrahydro-iso-alpha acids is close to a new type of estrogen receptor alpha (ERα) antagonists aimed at disrupting the binding of coactivators containing an LxxLL motif (NR-box). In this work we show that THIAA decreases estradiol-stimulated proliferation of MCF-7 (ERα-positive breast cancer cells). Besides, we show that it inhibits ERα transcriptional activity. Interestingly, this extract fails to compete with estradiol for ERα binding and does not significantly impact the receptor turnover rate in MCF-7 cells, suggesting that it does not act like classical antiestrogens. Hence, we demonstrate that THIAA is able to antagonize ERα estradiol-induced recruitment of the LxxLL binding motif.

Lysosomal function is involved in 17β-estradiol-induced estrogen receptor α degradation and cell proliferation

The homeostatic control of the cellular proteome steady-state is dependent either on the 26S proteasome activity or on the lysosome function. The sex hormone 17β-estradiol (E2) controls a plethora of biological functions by binding to the estrogen receptor α (ERα), which is both a nuclear ligand-activated transcription factor and also an extrinsic plasma membrane receptor. Regulation of E2-induced physiological functions (e.g., cell proliferation) requires the synergistic activation of both transcription of estrogen responsive element (ERE)-containing genes and rapid extra-nuclear phosphorylation of many different signalling kinases (e.g., ERK/MAPK; PI3K/AKT). Although E2 controls ERα intracellular content and activity via the 26S proteasome-mediated degradation, biochemical and microscopy-based evidence suggests a possible cross-talk among lysosomes and ERα activities. Here, we studied the putative localization of endogenous ERα to lysosomes and the role played by lysosomal function in ERα signalling. By using confocal microscopy and biochemical assays, we report that ERα localizes to lysosomes and to endosomes in an E2-dependent manner. Moreover, the inhibition of lysosomal function obtained by chloroquine demonstrates that, in addition to 26S proteasome-mediated receptor elimination, lysosome-based degradation also contributes to the E2-dependent ERα breakdown. Remarkably, the lysosome function is further involved in those ERα activities required for E2-dependent cell proliferation while it is dispensable for ERα-mediated ERE-containing gene transcription. Our discoveries reveal a novel lysosome-dependent degradation pathway for ERα and show a novel biological mechanism by which E2 regulates ERα cellular content and, as a consequence, cellular functions.

pRb controls estrogen receptor alpha protein stability and activity

A cross talk between the Estrogen Receptor (ESR1) and the Retinoblastoma (pRb) pathway has been demonstrated to influence the therapeutic response of breast cancer patients but the full mechanism remains poorly understood. Here we show that the N-terminal domain of pRb interacts with the CD domain of ESR1 to allow for the assembly of intermediate complex chaperone proteins HSP90 and p23. We demonstrated that a loss of pRb in human/mouse breast cells decreases the expression of the ESR1 protein through the proteasome pathway. Our work reveals a novel regulatory mechanism of ESR1 basal turnover and activity and an unanticipated relationship with the pRb tumor suppressor.

Whole transcriptome analysis of the ERα synthetic fragment P295-T311 (ERα17p) identifies specific ERα-isoform (ERα, ERα36)-dependent and -independent actions in breast cancer cells

ERα17p is a peptide corresponding to the sequence P295LMIKRSKKNSLALSLT311 of the estrogen receptor alpha (ERα) and initially found to interfere with ERα-related calmodulin binding. ERα17p was subsequently found to elicit estrogenic responses in E2-deprived ERα-positive breast cancer cells, increasing proliferation and ERE-dependent gene transcription. Surprisingly, in E2-supplemented media, ERα17p-induced apoptosis and modified the actin network, influencing cell motility. Here, we report that ERα17p internalizes in breast cancer cells (T47D, MDA-MB-231, SKBR3) and induces a massive early (3 h) transcriptional activity. Remarkably, about 75% of significantly modified transcripts were also modified by E2, confirming the pro-estrogenic profile of ERα17p. The different ER spectra of the used cell lines allowed us to identify a specific ERα17p signature related to ERα as well as its variant ERα36. With respect to ERα, the peptide activates nuclear (cell cycle, cell proliferation, nucleic acid and protein synthesis) and extranuclear signaling pathways. In contrast, through ERα36, it mainly triggers inhibitory actions on inflammation. This is the first work reporting a detailed ERα36-specific transcriptional signature. In addition, we report that ERα17p-induced transcripts related to apoptosis and actin modifying effects of the peptide are independent from its estrogen receptor(s)-related actions. We discuss our findings in view of the potential use of ERα17p as a selective peptidomimetic estrogen receptor modulator (PERM).

Phosphorylation by p38 mitogen-activated protein kinase promotes estrogen receptor α turnover and functional activity via the SCF(Skp2) proteasomal complex

The nuclear hormone receptor estrogen receptor α (ERα) mediates the actions of estrogens in target cells and is a master regulator of the gene expression and proliferative programs of breast cancer cells. The presence of ERα in breast cancer cells is crucial for the effectiveness of endocrine therapies, and its loss is a hallmark of endocrine-insensitive breast tumors. However, the molecular mechanisms underlying the regulation of the cellular levels of ERα are not fully understood. Our findings reveal a unique cellular pathway involving the p38 mitogen-activated protein kinase (p38MAPK)-mediated phosphorylation of ERα at Ser-294 that specifies its turnover by the SCF(Skp2) proteasome complex. Consistently, we observed an inverse relationship between ERα and Skp2 or active p38MAPK in breast cancer cell lines and human tumors. ERα regulation by Skp2 was cell cycle stage dependent and critical for promoting the mitogenic effects of estradiol via ERα. Interestingly, by the knockdown of Skp2 or the inhibition of p38MAPK, we restored functional ERα protein levels and the control of gene expression and proliferation by estrogen and antiestrogen in ERα-negative breast cancer cells. Our findings highlight a novel pathway with therapeutic potential for restoring ERα and the responsiveness to endocrine therapy in some endocrine-insensitive ERα-negative breast cancers.

Palmitoylation regulates 17β-estradiol-induced estrogen receptor-α degradation and transcriptional activity

The estrogen receptor-α (ERα) is a transcription factor that regulates gene expression through the binding to its cognate hormone 17β-estradiol (E2). ERα transcriptional activity is regulated by E2-evoked 26S proteasome-mediated ERα degradation and ERα serine (S) residue 118 phosphorylation. Furthermore, ERα mediates fast cell responses to E2 through the activation of signaling cascades such as the MAPK/ERK and phosphoinositide-3-kinase/v-akt murine thymoma viral oncogene homolog 1 pathways. These E2 rapid effects require a population of the ERα located at the cell plasma membrane through palmitoylation, a dynamic enzymatic modification mediated by palmitoyl-acyl-transferases. However, whether membrane-initiated and transcriptional ERα activities integrate in a unique picture or represent parallel pathways still remains to be firmly clarified. Hence, we evaluated here the impact of ERα palmitoylation on E2-induced ERα degradation and S118 phosphorylation. The lack of palmitoylation renders ERα more susceptible to E2-dependent degradation, blocks ERα S118 phosphorylation and prevents E2-induced ERα estrogen-responsive element-containing promoter occupancy. Consequently, ERα transcriptional activity is prevented and the receptor addressed to the nuclear matrix subnuclear compartment. These data uncover a circuitry in which receptor palmitoylation links E2-dependent ERα degradation, S118 phosphorylation, and transcriptional activity in a unique molecular mechanism. We propose that rapid E2-dependent signaling could be considered as a prerequisite for ERα transcriptional activity and suggest an integrated model of ERα intracellular signaling where E2-dependent early extranuclear effects control late receptor-dependent nuclear actions.

Cracking the estrogen receptor's posttranslational code in breast tumors

Estrogen signaling pathways, because of their central role in regulating the growth and survival of breast tumor cells, have been identified as suitable and efficient targets for cancer therapies. Agents blocking estrogen activity are already widely used clinically, and many new molecules have entered clinical trials, but intrinsic or acquired resistance to treatment limits their efficacy. The basic molecular studies underlying estrogen signaling have defined the critical role of estrogen receptors (ER) in many aspects of breast tumorigenesis. However, important knowledge gaps remain about the role of posttranslational modifications (PTM) of ER in initiation and progression of breast carcinogenesis. Whereas major attention has been focused on the phosphorylation of ER, many other PTM (such as acetylation, ubiquitination, sumoylation, methylation, and palmitoylation) have been identified as events modifying ER expression and stability, subcellular localization, and sensitivity to hormonal response. This article will provide an overview of the current and emerging knowledge on ER PTM, with a particular focus on their deregulation in breast cancer. We also discuss their clinical relevance and the functional relationship between PTM. A thorough understanding of the complete picture of these modifications in ER carcinogenesis might not only open new avenues for identifying new markers for prognosis or prediction of response to endocrine therapy but also could promote the development of novel therapeutic strategies.

Janus kinase 2--a novel negative regulator of estrogen receptor α function

Estrogen receptor α (ERα) functions as a transcription factor to regulate a wide range of cellular activities in response to 17β-estradiol (E2). The regulation of ERα transcriptional activity is highly complex and not yet fully understood. In this respect, recent studies have highlighted the importance of certain cellular protein kinases. To identify novel protein kinases regulating ERα activity, we performed a high-throughput siRNA screening in combination with a luciferase reporter assay in an ERα positive breast cancer cell line. Among the vast majority of potential positive regulators, we found Janus kinase 2 (JAK2), a member of the Janus kinase family of non-receptor tyrosine kinases, to have a negative regulatory effect on E2 induced luciferase activity. In addition, silencing of JAK2 resulted in increased expression of endogenous ERα target genes, pS2 and GREB1. In an attempt to understand the mechanism underlying JAK2 mediated regulation of ERα transcriptional activity, we found that JAK2 negatively regulates ERα protein level. Gene expression analysis revealed no significant influence of JAK2 on ERα mRNA level. Subsequently, a role of JAK2 in regulating ERα protein degradation was analyzed. Inhibition of the lysosome did not alter JAK2 mediated downregulation of ERα. In contrast, using proteasome inhibitors MG132 and lactacystin, we demonstrated that JAK2 governs ERα protein stability via the ubiquitin-proteasome pathway. In contrast to JAK2, the two other members of the JAK family expressed in the breast (JAK1 and TYK2) had no influence on ERα function. In addition, we found that prolonged E2 treatment upregulates JAK2 mRNA and protein levels. These results suggest a novel negative regulation of ERα activity and protein by JAK2 in breast cancer cells and indicate a potential new cross-talk.

β-Catenin Is a Positive Regulator of Estrogen Receptor-α Function in Breast Cancer Cells

Estrogen receptor-alpha (ERα) is a key factor in the development of breast cancer in humans. The expression and activity of ERα is regulated by a multitude of intracellular and extracellular signals. Here we show a cross-talk between β-catenin and ERα in human breast cancer cells. Knockdown of β-catenin by RNAi resulted in significant reduction of ERα mRNA and/or protein levels in MCF-7, T-47D, and BT-474 breast cancer cells and in significant reduction of estradiol-induced expression of the ERα target genes pS2 and GREB1. In addition β-catenin silencing resulted in significant decrease of growth of MCF-7 cells both in the absence and presence of estradiol. β-catenin and ERα could not be co-immunoprecipitated by ERα antibodies from lysates of E2-treated or untreated cells suggesting lack of direct physical interaction. It is concluded that β-catenin is a positive regulator of ERα mRNA and protein expression.

In vivo treatments with fulvestrant and anastrozole differentially affect gene expression in the rat efferent ductules

Estrogen plays a key role in maintaining the morphology and function of the efferent ductules. We previously demonstrated that the antiestrogen fulvestrant markedly affected gene expression in the rat efferent ductules. The mechanism of fulvestrant action to modulate gene expression may involve not only the blockade of ESR1 and ESR2 estrogen receptors, but also the activation of ESR1 and ESR2 when the receptors are tethered to AP-1 or SP1 transcription factors, or the activation of the G protein-coupled estrogen receptor 1. We therefore compared the effects of two strategies to interfere with estrogen action in the rat efferent ductules: treatment with fulvestrant or with the aromatase inhibitor anastrozole. Whereas fulvestrant markedly increased Mmp7 and Spp1, and reduced Nptx1 mRNA levels, no changes were observed with anastrozole. Fulvestrant caused changes in epithelial morphology that were not seen with anastrozole. Fulvestrant shifted MMP7 immunolocalization in the epithelial cells from the supranuclear to the apical region; this effect was less pronounced with anastrozole. In vitro studies of (35)S-methionine incorporation showed that protein release was increased, whereas tissue protein content in the efferent ductules of fulvestrant-treated rats was decreased. Although fulvestrant markedly affected gene expression, no changes were observed on AP-1 and SP1 DNA-binding activity. The blockade of ESRs seems to be the major reason explaining the differences between both treatments. At least some of the effects of fulvestrant appear to result from compensatory mechanisms activated by the dramatic changes caused by ESR1 blockade.

Characterization of the pharmacophore properties of novel selective estrogen receptor downregulators (SERDs)

Selective estrogen receptor (ER) down-regulators (SERDs) reduce ERalpha protein levels as well as block ER activity and therefore are promising therapeutic agents for the treatment of hormone refractory breast cancer. Starting with the triarylethylene acrylic acid SERD 4, we have investigated how alterations in both the ligand core structure and the appended acrylic acid substituent affect SERD activity. The new ligands were based on high affinity, symmetrical cyclofenil or bicyclo[3.3.1]nonane core systems, and in these, the position of the carboxyl group was extended from the ligand core, either retaining the vinylic linkage of the substituent or replacing it with an ether linkage. Although most structural variants showed binding affinities for ERalpha and ERbeta higher than that of 4, only the compounds preserving the acrylic acid side chain retained SERD activity, although they could possess varying core structures. Hence, the acrylic acid moiety of the ligand is crucial for SERD-like blockade of ER activities.

Capacity of type I and II ligands to confer to estrogen receptor alpha an appropriate conformation for the recruitment of coactivators containing a LxxLL motif-Relationship with the regulation of receptor level and ERE-dependent transcription in MCF-7 cells

Estrogen receptor alpha (ERalpha) belongs to the superfamily of nuclear receptors and as such acts as a ligand-modulated transcription factor. Ligands elicit in ERalpha conformational changes leading to the recruitment of coactivators required for the transactivation of target genes via cognate response elements. In many cells, activated ERalpha also undergoes downregulation by proteolysis mediated by the ubiquitin/proteasome system. Although these various molecular processes have been well characterized, little is known as to which extent they are interrelated. In the present study, we used a panel of type I (estradiol derivatives and "linear", non-steroidal ligands) and type II ("angular" ligands) estrogens, in order to identify possible relationships between ligand binding affinity, recruitment of LxxLL-containing coactivators, ERalpha downregulation in MCF-7 cells and related transactivation activity of ligand-bound ERalpha. For type I estrogens, there was a clear-cut relationship between ligand binding affinity, hydrophobicity around C-11 of estradiol and ability of ERalpha to associate with LxxLL motifs, both in cell-free condition and in vivo (MCF-7 cells). Moreover, LxxLL motif recruitment by ERalpha seemed to be a prerequisite for the downregulation of the receptor. By contrast, type II ligands, as well as estradiol derivatives bearing a bulky side chain at 11beta, had much less tendency to promote ERalpha-LxxLL interaction or even behaved as antagonists in this respect, in agreement with the well known partial estrogenicity/antiestrogenicity of some of these compounds. Interestingly, some type II ligands which antagonized LxxLL motif recruitment were nonetheless able to enhance ERalpha-mediated gene transactivation.

OTU Domain-containing ubiquitin aldehyde-binding protein 1 (OTUB1) deubiquitinates estrogen receptor (ER) alpha and affects ERalpha transcriptional activity

Estrogen receptor (ER) alpha is an essential component in human physiology and is a key factor involved in the development of breast and endometrial cancers. ERalpha protein levels and transcriptional activity are tightly controlled by the ubiquitin proteasome system. Deubiquitinating enzymes, a class of proteases capable of removing ubiquitin from proteins, are increasingly being seen as key modulators of the ubiquitin proteasome system, regulating protein stability and other functions by countering the actions of ubiquitin ligases. Using mass spectrometry analysis of an ERalpha protein complex, we identified OTU domain-containing ubiquitin aldehyde-binding protein 1 (OTUB1) as a novel ERalpha-interacting protein capable of deubiquitinating ERalpha in cells and in vitro. We show that OTUB1 negatively regulates transcription mediated by ERalpha in transient reporter gene assays and transcription mediated by endogenous ERalpha in Ishikawa endometrial cancer cells. We also show that OTUB1 regulates the availability and functional activity of ERalpha in Ishikawa cells by affecting the transcription of the ERalpha gene and by stabilizing the ERalpha protein in the chromatin.

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.

Estradiol-estrogen receptor: a key interplay of the expression of syndecan-2 and metalloproteinase-9 in breast cancer cells

Estrogens are related with the growth and development of target tissues and play a critical role in breast cancer progression. The effects of estrogens are mediated by the estrogen receptors ERalpha and ERbeta, which are members of the nuclear steroid receptor superfamily. To date, it is not known how these hormones elicit many of their effects on extracellular matrix molecules and how these effects can be connected with ER expression. For this purpose, the effect of estradiol on ER expression as well as on proteoglycan and metalloproteinase expression was studied. The effect of E2 on extracellular matrix molecule expression has been studied using ERalpha suppression in breast cancer cells. Our studies using ERalpha-positive MCF-7 cells show that estradiol affects the expression of syndecan-2, but not of syndecan-4, through ERalpha. Furthermore, the ability of estradiol to affect MMP-9 and TIMP-1 expression is connected with ERalpha status. Together, these data demonstrate the significant role of ERalpha on mediating the effect of estradiol on extracellular matrix molecules.

Hormone therapy for breast cancer, with an emphasis on the pure antiestrogen fulvestrant: mode of action, antitumor efficacy and effects on bone health

Breast cancer is a major health problem in women of developed Western countries. Whereas estrogen receptor (ER) may be involved in many cases in breast carcinogenesis, its expression in breast tumors may predict a favorable response to hormone therapy. In this review, we report the role played by ER in breast cancer and compare the effects and mechanisms of action of partial (tamoxifen) and pure (fulvestrant) antiestrogens, as well as of aromatase inhibitors. Moreover, as ER also has a critical role in bone metabolism, we review the beneficial and adverse effects of breast cancer hormone therapy on bone health, with a particular emphasis on fulvestrant, the only pure antiestrogen recently approved by the FDA for Phase III clinical trials. We conclude that, because of its therapeutic efficacy and its seemingly minimal effect on bone integrity, fulvestrant represents a new option for the hormonal treatment of breast cancer that deserves further clinical evaluation.

Differential expression of estrogen receptor alpha (ERalpha) protein in MCF-7 breast cancer cells chronically exposed to TCDD

Estrogens play a key role in the development and evolution of breast cancer tumors. Estrogen receptor alpha (ERalpha) mediates many of the biological activities of estrogens, and its expression is associated with low invasiveness and good prognosis. Recent epidemiological reports suggest that long-term exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is implicated in the increased incidence of breast cancer in exposed women. TCDD interferes with the expression of some ERalpha-dependent genes and inhibits estradiol (E2)- dependent growth of breast cancer cells in vitro. However, E2-dependent xenographs of MCF-7 human breast cancer cells resumed growth after a 2-week exposure to TCDD. The mechanisms involved in the resumption of cell growth are not completely understood. In this study, we show that short term-exposure (16 days) to 1 nM TCDD results in the suppression of ERalpha protein expression, while chronic exposure for more than 1 year (LTDX cells) results in the partial re-expression of the receptor. Immunocytochemistry studies showed that re-expression of ERalpha in LTDX cells occurred in some of the cells. Analysis by Western immunoblots indicated that four out of five LTDX clones expressed ERalpha at levels comparable to those in unexposed MCF-7 cells. Removal of TCDD treatment for 16 days restored the expression of ERalpha in the ERalpha-negative clonal cells. These results suggest that MCF-7 cells chronically exposed to TCDD contain at least two cell subpopulations that may respond differently to the ERalpha-mediated effects of TCDD.

Regulatory function of the P295-T311 motif of the estrogen receptor alpha - does proteasomal degradation of the receptor induce emergence of peptides implicated in estrogenic responses?

The way in which estrogen receptor α (ERα) mediates gene transcription and hormone-dependent cancer cell proliferation is now being largely reconsidered in view of several recent discoveries. ERα-mediated transcription appears to be a cyclic and transient process where the proteasome - and thus receptor degradation - plays a pivotal role. In view of our recent investigations, which demonstrate the estrogenic activity of a synthetic peptide corresponding to a regulatory motif of the receptor (ERα17p), we propose that ERα proteasomal degradation could induce the emergence of regulatory peptide(s). The latter would function as a signal and contribute to the ERα activation process, amplifying the initial hormonal stimulation and giving rise to sustained estrogenic response.

Estrogen receptor-alpha hinge-region lysines 302 and 303 regulate receptor degradation by the proteasome

Cellular levels of estrogen receptor-alpha (ERalpha) protein are regulated primarily by the ubiquitin-proteasome pathway. Dynamic interactions between ERalpha and the protein degradation machinery facilitate the down-regulation process by targeting receptor lysine residues for polyubiquitination. To date, the lysines that control receptor degradation have not been identified. Two receptor lysines, K302 and K303, located in the hinge-region of ERalpha, serve multiple regulatory functions, and we examined whether these might also regulate receptor polyubiquitination, turnover, and receptor-protein interactions. We used ERalpha-negative breast cancer C4-12 cells to generate cells stably expressing wild-type (wt)ERalpha or ERalpha with lysine-to-alanine substitutions at K302 and K303 (ERalpha-AA). In the unliganded state, ERalpha-AA displayed rapid polyubiquitination and enhanced basal turnover, as compared with wtERalpha, due to its elevated association with the ubiquitin ligase carboxy terminus of Hsc70-interacting protein (CHIP) and the proteasome-associated cochaperone Bag1. Treatment of C4-12 cells with either 17beta-estradiol (E2) or the pure antiestrogen ICI 182,780 (ICI) induced rapid degradation of wtERalpha via the ubiquitin-proteasome pathway; however, in the presence of these ligands, ERalpha-AA was less efficiently degraded. Furthermore, ERalpha-AA was resistant to ICI-induced polyubiquitination, suggesting that these lysines are polyubiquitinated in response to the antiestrogen and demonstrate a novel role for these two lysines in the mechanism of action of ICI-induced receptor down-regulation. The reduced stability of ERalpha-AA in the unliganded state and the increased stability of ERalpha-AA in the liganded state were concordant with reporter gene assays demonstrating that ERalpha-AA has lower basal activity but higher E2 inducibility than wtERalpha. These data provide the first evidence that K302/303 protect ERalpha from basal degradation and are necessary for efficient E2- and ICI-induced turnover in breast cancer cells.

Development of ER-alpha and ER-beta expression in the developing ovine brain and pituitary

Fetal neuroendocrine development in late gestation is critical for maintenance of fetal homeostasis, growth, and readiness for birth. We designed the present study to identify the regional patterns of expression of the two main isoforms of the estrogen receptor, ER-alpha and ER-beta, in the developing ovine fetal brain. Fetal (80, 100, 120, 130, and 145 days gestation), neonatal (1 and 7 days), and adult sheep were euthanized and the following tissues were collected: pituitary, hypothalamus, hippocampus, cerebral cortex, and brainstem. Both ER's are expressed in the ovine brain as early as 80 days gestation, and the expression of both receptors appears to be developmentally regulated. We conclude that both forms of the estrogen receptor are expressed in fetal brain and pituitary throughout the latter half of gestation.

Trophic effect in MCF-7 cells of ERalpha17p, a peptide corresponding to a platform regulatory motif of the estrogen receptor alpha--underlying mechanisms

As yet, estrogen receptor alpha (ERalpha) inhibitors used in clinical practice target a unique site, i.e. the hormone-binding pocket. With the aim of discovering other potential therapeutic targets in the receptor, we studied its AF-2a domain, a site that proves to be critical for ligand-independent ERalpha activity. Previous studies from our laboratory highlighted an auto-inhibitory action associated with a site included in this domain, i.e. the P295-T311 sequence. Accordingly, a deletion of this sequence produces a constitutively activated receptor mutant. More interestingly, a synthetic peptide with the P295-T311 sequence (ERalpha17p) elicits in breast cancer cell lines estrogenic responses that may be ascribed to a competitive mechanism towards the P295-T311-associated auto-inhibition of ERalpha. In the present study, we show that ERalpha17p sustains MCF-7 cell growth in estrogen-depleted culture medium by inducing molecular events promoting G1/S phase transition. We demonstrate, moreover, that this proliferative activity is associated with receptor down regulation (acceleration of ERalpha degradation and repression of ESR1 gene transcription), similar to that induced by estrogen agonists. Complementary studies suggest that our observations may be, at least in part, relevant to a competitive inhibition affecting ERalpha-Hsp70 association. Hence, the design of drugs able to stabilize ERalpha-Hsp70 complexes - where the receptor is in an inactive conformation - may be of therapeutic value.

Definition of functionally important mechanistic differences among selective estrogen receptor down-regulators

One subclass of antiestrogens, the selective estrogen receptor down-regulators (SERDs), have received considerable attention of late as they competitively inhibit estrogen binding and induce a rapid, proteasome-dependent degradation of the receptor. Contained within this class of molecules is the steroidal antiestrogen ICI182,780 (faslodex), recently approved for the treatment of metastatic cancer, and GW5638/DPC974, a SERD that is currently being evaluated in the clinic. Given that mechanistic differences between different selective estrogen receptor modulators have been translated into important clinical profiles, it was of interest to determine if the SERD subclass of ligands were likewise functionally or mechanistically distinguishable. In this study, we show that although the steroidal and nonsteroidal SERDs target ERalpha for degradation, the underlying mechanism(s) are different. Of note was the identification of a specific protein-protein interaction surface presented on ERalpha in the presence of the ICI182,780-activated receptor which is required for degradation. Interestingly, this surface is also presented on ERalpha in the presence of RU58,668, a SERD that is chemically distinct from ICI182,780. This surface is not required for GW5638-mediated degradation, and thus, this SERD seems to affect ERalpha down-regulation by a different mechanism. These data suggest that sequencing of therapies using drugs of this class is likely to be possible. Finally, because of the unmet need for orally active SERDS that function similarly to ICI182,780, we have used the insights from these mechanistic studies to develop and validate a high-throughput screen for compounds of this class with improved pharmaceutical properties.

Effects of (R,S)/(S,R)-4,5-bis(2-chloro-4-hydroxyphenyl)-2-imidazolines and (R,S)/(S,R)-2,3-bis(2-chloro-4-hydroxyphenyl)piperazines on estrogen receptor alpha level and transcriptional activity in MCF-7 cells

4,5-Diaryl-2-imidazolines (Im(s)) and 2,3-diarylpiperazines (Pip(s)) belong to the type II class of estrogens. These compounds enhance ERalpha-mediated transcription of ERE-driven reporter genes in MCF-7 cells but do not compete with [(3)H]estradiol (E(2)) for receptor binding, because of distinct anchoring modes. The present study examined whether the estrogenic action of Im(s) and Pip(s) is associated with a down regulation of ERalpha, as reported for conventional agonists. Im and Pip derivatives displaying a large spectrum of activities in three distinct ERE-dependent transactivation systems were selected for that purpose. ERalpha immunostaining as well as Western blotting analysis revealed that both classes of compounds down regulated ERalpha with an efficiency closely related to their transactivation potency. MG-132 abrogated this down regulation, pointing to a proteasomal degradation process. Im(s) and Pip(s) with strong transactivation potency also altered [(3)H]E(2) binding parameters, leading to a progressive decrease of cellular estrogen binding capacity. This property occurred largely before ERalpha down regulation and persisted even in presence of MG-132, indicating that it did not result from ERalpha breakdown but rather from a conformational change of the receptor. The additional finding that the most active agonist tested in this study enhanced the capacity of a purified ERalpha recombinant to recruit LxxLL co-activators, while its inactive counterpart failed to do so confirmed this hypothesis. Altogether, our data indicate that the association of Im(s) and Pip(s) with ERalpha elicits similar responses to conventional agonists, even if they interact with distinct residues of the binding pocket.

Ligand-dependent transcription of estrogen receptor alpha is mediated by the ubiquitin ligase EFP

Estrogen-mediated ubiquitylation and subsequent degradation of the estrogen receptor alpha (ERalpha) appears to be involved in the transcriptional activity of ERalpha. We show that the estrogen-responsive finger protein (EFP) interacts with and ubiquitylates ERalpha. EFP promoted the ubiquitylation of ERalphain vitro and in vivo and consequently promoted the degradation of ERalpha. The interaction between EFP and ERalpha was greatly enhanced in the presence of estrogen. The action of EFP on ERalpha in the presence of estrogen resulted in a robust interaction between ERalpha and Tip60, one of the transcriptional coactivators, leading to activation of ERalpha transcriptional activity. However, a dominant negative mutant of EFP lacking the RING domain prolonged the half-life of ERalpha and inhibited the transcription by ERalpha. Our results indicate that EFP functions as a cofactor for ERalpha-mediated transcription, thus suggesting that ERalpha-mediated transcription is closely linked to the ubiquitylation of ERalpha.

Calmodulin-independent, agonistic properties of a peptide containing the calmodulin binding site of estrogen receptor alpha

Calmodulin (CaM) contributes to estrogen receptor alpha (ER)-mediated transcription. In order to study the underlying mechanisms, we synthesized a peptide including the CaM binding site: ERalpha17p (P(295)-T(311)). This peptide inhibited ER-CaM association, unlike two analogs in which two amino acids required for CaM binding were substituted. Exposure of MCF-7 cells to ERalpha17p down regulated ER, stimulated ER-dependent transcription and enhanced the proliferation of ER-positive breast cancer cell lines. Interestingly, ERalpha17p analogs unable to bind to CaM induced similar responses, demonstrating that ERalpha17p-mediated effects are mainly relevant to mechanisms independent of ER-CaM dissociation. The P(295)-T(311) motif is indeed a platform for multiple post-translational modifications not necessarily CaM-dependent. The additional finding that deletion of the P(295)-T(311) sequence in ER produced a constitutive transcriptional activity revealed that this platform motif has autorepressive functions. With regard to cell function, association of CaM to ER would counteract this autorepression, leading thereby to enhanced ER-mediated transactivation.

Synthesis, structure, and estrogenic activity of 4-amino-3-(2-methylbenzyl)coumarins on human breast carcinoma cells

A number of coumarins exhibit interesting pharmacological activities and are therefore of therapeutic use. We report here the synthesis and the structural analysis of new N-substituted 4-amino-3-(2-methylbenzyl)coumarins (compounds 8a-8e) that present structural analogies with estrothiazine and 11- or 7-substituted 17beta-estradiol. These derivatives were tested with respect to estrogenic activity on the estrogen receptor positive (ER+) human MCF-7 breast cancer cell line. Two of the reported compounds (8a and 8b) stimulated specifically the proliferation of MCF-7 cells, but not that of estrogen receptor negative (ER-) human MDA-MB-231 breast cancer cells, suggesting that their mitogenic activity is mediated by ER. Accordingly, the stimulating effect of 8a and 8b was suppressed by the pure antiestrogen fulvestrant. Besides, 8a and 8b induced ER down-regulation similar to that produced by classical ER agonists or pure antagonists. The effects of the compounds under study on ER-mediated transcription were assessed on (ER+) MVLN cells, that is, MCF-7 cells stably transfected with a pVit-tk-Luc reporter plasmid. Derivatives 8a and 8b, and surprisingly compound 8c, enhanced ER-mediated gene transactivation in that model. Finally, no coumarin was able to compete with tritiated 17beta-estradiol ([(3)H]E(2)) for ER binding, suggesting unconventional interactions with the receptor, such as interactions with the second binding pocket or with the coactivator-binding region. To conclude, observations performed in this study on compound 8c reveal that estrogenic activity can be dissociated from enhancement of cell proliferation. Furthermore, ERE-driven transactivation of transcription seems to be a condition necessary, but not sufficient, for estrogen-induced stimulation of cell growth.