Characterization of the Interactions of Estrogen Receptor and MNAR in the Activation of cSrc

FGF1 supports glycolytic metabolism through the estrogen receptor in endocrine-resistant and obesity-associated breast cancer

BACKGROUND

Obesity increases breast cancer risk and breast cancer-specific mortality, particularly for people with estrogen receptor (ER)-positive tumors. Body mass index (BMI) is used to define obesity, but it may not be the best predictor of breast cancer risk or prognosis on an individual level. Adult weight gain is an independent indicator of breast cancer risk. Our previous work described a murine model of obesity, ER-positive breast cancer, and weight gain and identified fibroblast growth factor receptor (FGFR) as a potential driver of tumor progression. During adipose tissue expansion, the FGF1 ligand is produced by hypertrophic adipocytes as a stimulus to stromal preadipocytes that proliferate and differentiate to provide additional lipid storage capacity. In breast adipose tissue, FGF1 production may stimulate cancer cell proliferation and tumor progression.

METHODS

We explored the effects of FGF1 on ER-positive endocrine-sensitive and resistant breast cancer and compared that to the effects of the canonical ER ligand, estradiol. We used untargeted proteomics, specific immunoblot assays, gene expression profiling, and functional metabolic assessments of breast cancer cells. The results were validated in tumors from obese mice and breast cancer datasets from women with obesity.

RESULTS

FGF1 stimulated ER phosphorylation independently of estradiol in cells that grow in obese female mice after estrogen deprivation treatment. Phospho- and total proteomic, genomic, and functional analyses of endocrine-sensitive and resistant breast cancer cells show that FGF1 promoted a cellular phenotype characterized by glycolytic metabolism. In endocrine-sensitive but not endocrine-resistant breast cancer cells, mitochondrial metabolism was also regulated by FGF1. Comparison of gene expression profiles indicated that tumors from women with obesity shared hallmarks with endocrine-resistant breast cancer cells.

CONCLUSIONS

Collectively, our data suggest that one mechanism by which obesity and weight gain promote breast cancer progression is through estrogen-independent ER activation and cancer cell metabolic reprogramming, partly driven by FGF/FGFR. The first-line treatment for many patients with ER-positive breast cancer is inhibition of estrogen synthesis using aromatase inhibitors. In women with obesity who are experiencing weight gain, locally produced FGF1 may activate ER to promote cancer cell metabolic reprogramming and tumor progression independently of estrogen.

The emerging role of estrogen's non-nuclear signaling in the cardiovascular disease

Sexual dimorphism exists in the epidemiology of cardiovascular disease (CVD), which indicates the involvement of sexual hormones in the pathophysiology of CVD. In particular, ample evidence has demonstrated estrogen's protective effect on the cardiovascular system. While estrogen receptors, bound to estrogen, act as a transcription factor which regulates gene expressions by binding to the specific DNA sequence, a subpopulation of estrogen receptors localized at the plasma membrane induces activation of intracellular signaling, called "non-nuclear signaling" or "membrane-initiated steroid signaling of estrogen". Although the precise molecular mechanism of non-nuclear signaling as well as its physiological impact was unclear for a long time, recent development of genetically modified animal models and pathway-selective estrogen receptor stimulant bring new insights into this pathway. We review the published experimental studies on non-nuclear signaling of estrogen, and summarize its role in cardiovascular system, especially focusing on: (1) the molecular mechanism of non-nuclear signaling; (2) the design of genetically modified animals and pathway-selective stimulant of estrogen receptor.

Expression of estrogen receptors, PELP1, and SRC in human spermatozoa and their associations with semen quality

Sperm cells are target cells for both estrogens and xenoestrogens. Due to the specific structure of spermatozoa, these hormonal compounds may act on sperm in a non-genomic mechanism only. However, the ESR-mediated signaling pathways are still poorly understood. In this study, we obtained 119 samples from male participants of Caucasian descent who donated semen for standard analysis. We analyzed gene expression of estrogen receptors (ESR1 and ESR2) and their coregulators-proline-, glutamic acid-, and leucine-rich protein 1 (PELP1), and cellular kinase c-Src (SRC). RNA level was established using reverse-transcribed RNA as a template, followed by a polymerase chain reaction. Proteins' presence was confirmed by western blot and immunocytochemistry techniques. "Normal" values of semen parameters were defined as follows: > 32% sperm with progressive motility, > 4% sperm cells with normal morphology, > 15 × 10⁶ sperm per mL, > 58% live spermatozoa and leukocyte amount < 10⁶ cells per mL, according to WHO 2010 reference. Semen parameters that deviated from these "normal" values were labeled as "abnormal". Gene expression ratios revealed significant, moderate, and negative correlations for ESR1/ESR2 and weak, negative ESR2/PELP1 correlations in the subgroup of patients with abnormal values of semen parameters. In addition, SRC/PELP1 was moderately and positively correlated in the subgroup with parameters within the reference values established by WHO 2010. Our study showed that both PELP1 scaffolding protein and SRC kinase might influence semen quality via ESRs. It seems that not the expression of a single gene may affect the sperm quality, but more gene-to-gene mutual ratio. Characterization of estrogen-signaling pathway-related genes' modulated expression in sperm cells could aid in better understanding sperm biology and quality.

Cryo-EM reveals the architecture of the PELP1-WDR18 molecular scaffold

PELP1 (Proline-, Glutamic acid-, Leucine-rich protein 1) is a large scaffolding protein that functions in many cellular pathways including steroid receptor (SR) coactivation, heterochromatin maintenance, and ribosome biogenesis. PELP1 is a proto-oncogene whose expression is upregulated in many human cancers, but how the PELP1 scaffold coordinates its diverse cellular functions is poorly understood. Here we show that PELP1 serves as the central scaffold for the human Rix1 complex whose members include WDR18, TEX10, and SENP3. We reconstitute the mammalian Rix1 complex and identified a stable sub-complex comprised of the conserved PELP1 Rix1 domain and WDR18. We determine a 2.7 Å cryo-EM structure of the subcomplex revealing an interconnected tetrameric assembly and the architecture of PELP1's signaling motifs, including eleven LxxLL motifs previously implicated in SR signaling and coactivation of Estrogen Receptor alpha (ERα) mediated transcription. However, the structure shows that none of these motifs is in a conformation that would support SR binding. Together this work establishes that PELP1 scaffolds the Rix1 complex, and association with WDR18 may direct PELP1's activity away from SR coactivation.

Segregation of nuclear and membrane-initiated actions of estrogen receptor using genetically modified animals and pharmacological tools

Estrogen receptor alpha (ERα) and beta (ERβ) are members of the nuclear receptor superfamily, playing widespread functions in reproductive and non-reproductive tissues. Beside the canonical function of ERs as nuclear receptors, in this review, we summarize our current understanding of extra-nuclear, membrane-initiated functions of ERs with a specific focus on ERα. Over the last decade, in vivo evidence has accumulated to demonstrate the physiological relevance of this ERα membrane-initiated-signaling from mouse models to selective pharmacological tools. Finally, we discuss the perspectives and future challenges opened by the integration of extra-nuclear ERα signaling in physiology and pathology of estrogens.

The extra-nuclear interactome of the estrogen receptors: implications for physiological functions

Over the last decades, a great body of evidence has defined a novel view of the cellular mechanism of action of the steroid hormone 17β-estradiol (E2) through its estrogen receptors (i.e., ERα and ERβ). It is now clear that the E2-activated ERs work both as transcription factors and extra-nuclear plasma membrane-localized receptors. The activation of a plethora of signal transduction cascades follows the E2-dependent engagement of plasma membrane-localized ERs and is required for the coordination of gene expression, which ultimately controls the occurrence of the pleiotropic effects of E2. The definition of the molecular mechanisms by which the ERs locate at the cell surface (i.e., palmitoylation and protein association) determined the quest for understanding the specificity of the extra-nuclear E2 signaling. The use of mice models lacking the plasma membrane ERα localization unveiled that the extra-nuclear E2 signaling is operational in vivo but tissue-specific. However, the underlying molecular details for such ERs signaling diversity in the perspective of the E2 physiological functions in the different cellular contexts are still not understood. Therefore, to gain insights into the tissue specificity of the extra-nuclear E2 signaling to physiological functions, here we reviewed the known ERs extra-nuclear interactors and tried to extrapolate from available databases the ERα and ERβ extra-nuclear interactomes. Based on literature data, it is possible to conclude that by specifically binding to extra-nuclear localized proteins in different sub-cellular compartments, the ERs fine-tune their molecular activities. Moreover, we report that the context-dependent diversity of the ERs-mediated extra-nuclear E2 actions can be ascribed to the great flexibility of the physical structures of ERs and the spatial-temporal organization of the logistics of the cells (i.e., the endocytic compartments). Finally, we provide lists of proteins belonging to the potential ERα and ERβ extra-nuclear interactomes and propose that the systematic experimental definition of the ERs extra-nuclear interactomes in different tissues represents the next step for the research in the ERs field. Such characterization will be fundamental for the identification of novel druggable targets for the innovative treatment of ERs-related diseases.

Crosstalk between the muscular estrogen receptor α and BDNF/TrkB signaling alleviates metabolic syndrome via 7,8-dihydroxyflavone in female mice

Objective

7,8-Dihydroxyflavone (7,8-DHF), a small molecular mimetic of brain-derived neurotrophic factor (BDNF), alleviates high-fat diet-induced obesity in female mice in a sex-specific manner by activating muscular tropomyosin-related kinase B (TrkB). However, the underlying molecular mechanism for this sex difference is unknown. Moreover, muscular estrogen receptor α (ERα) plays a critical role in metabolic diseases. Impaired ERα action is often accompanied by metabolic syndrome (MetS) in postmenopausal women. This study investigated whether muscular ERα is involved in the metabolic effects of 7,8-DHF.

Methods

For the in vivo studies, 72 female C57BL/6J mice were given a low-fat diet or high-fat diet, and both received daily intragastric administration of vehicle or 7,8-DHF for 24 weeks. The hypothalamic-pituitary-ovarian (HPO) axis function was assessed by investigating typical sex-related serum hormones and the ovarian reserve. Indicators of menopausal MetS, including lipid metabolism, insulin sensitivity, bone density, and serum inflammatory cytokines, were also evaluated. The expression levels of ERα and other relevant signaling molecules were also examined. In vitro, the molecular mechanism involved in the interplay of ERα and TrkB receptors was verified in differentiated C2C12 myotubes using several inhibitors and a lentivirus short hairpin RNA-knockdown strategy.

Results

Long-term oral administration of 7,8-DHF acted as a protective factor for the female HPO axis function, protecting against ovarian failure, earlier menopause, and sex hormone disorders, which was paralleled by the alleviation of MetS coupled with the production of ERα-rich, TrkB-activated, and uncoupling protein 1 (UCP1) high thermogenic skeletal muscle tissues. 7,8-DHF-stimulated transactivation of ERα at serine 118 (S118) and tyrosine 537 (Y537), which was crucial to activate the BDNF/TrkB signaling cascades. In turn, activation of BDNF/TrkB signaling was also required for the ligand-independent activation of ERα, especially at the Y537 phosphorylation site. In addition, Src family kinases played a core role in the interplay of ERα and TrkB, synergistically activating the signaling pathways related to energy metabolism.

Conclusions

These findings revealed a novel role of 7,8-DHF in protecting the function of the female HPO axis and activating tissue-specific ERα, which improves our understanding of this sex difference in 7,8-DHF-mediated maintenance of metabolic homeostasis and provides new therapeutic strategies for managing MetS in women.

•

7,8-DHF improves hypothalamic-pituitary-ovarian axis function in mature adult female mice.

•

7,8-DHF protects against ovarian failure and onset of earlier menopause.

•

7,8-DHF-induced transactivation of ERα is crucial to activate BDNF/TrkB signaling cascades.

•

7,8-DHF-induced activations of ERα and BDNF/TrkB signaling are interdependent.

•

Src family kinases play a core role in the crosstalk of ERα and BDNF/TrkB signaling pathways.

Src couples estrogen receptor to the anticipatory unfolded protein response and regulates cancer cell fate under stress

Accumulation of unfolded protein, or other stresses, activates the classical reactive unfolded protein response (UPR). In the recently characterized anticipatory UPR, receptor-bound estrogen, progesterone and other mitogenic hormones rapidly elicit phosphorylation of phospholipase C γ (PLCγ), activating the anticipatory UPR. How estrogen and progesterone activating their receptors couples to PLCγ phosphorylation and anticipatory UPR activation was unknown. We show that the oncogene c-Src is a rate-limiting regulator whose tyrosine kinase activity links estrogen and progesterone activating their receptors to anticipatory UPR activation. Supporting Src coupling estrogen and progesterone to anticipatory UPR activation, we identified extranuclear complexes of estrogen receptor α (ERα):Src:PLCγ and progesterone receptor:Src:PLCγ. Moreover, Src inhibition protected cancer cells against cell death. To probe Src's role, we used the preclinical ERα biomodulator, BHPI, which kills cancer cells by inducing lethal anticipatory UPR hyperactivation. Notably, Src inhibition blocked BHPI-mediated anticipatory UPR activation and the resulting rapid increase in intracellular calcium. After unbiased long-term selection for BHPI-resistant human breast cancer cells, 4/11 BHPI-resistant T47D clones, and nearly all MCF-7 clones, exhibited reduced levels of normally growth-stimulating Src. Notably, Src overexpression by virus transduction restored sensitivity to BHPI. Furthermore, in wild type cells, several-fold knockdown of Src, but not of ERα, strongly blocked BHPI-mediated UPR activation and subsequent HMGB1 release and necrotic cell death. Thus, Src plays a previously undescribed pivotal role in activation of the tumor-protective anticipatory UPR, thereby increasing the resilience of breast cancer cells. This is a new role for Src and the anticipatory UPR in breast cancer.

Cytoplasmic ERα and NFκB Promote Cell Survival in Mouse Mammary Cancer Cell Lines

There is a desperate need in the field for mouse mammary tumors and cell lines that faithfully mimic estrogen receptor (ER) expression and activity found in human breast cancers. We found that several mouse mammary cancer cell lines express ER but fail to demonstrate classical estrogen-driven proliferation or transcriptional activity. We investigated whether these cell lines may be used to model tamoxifen resistance by using small molecule inhibitors to signaling pathways known to contribute to resistance. We found that the combination of NFκB inhibition and ER antagonists significantly reduced cell proliferation in vitro, as well as growth of syngeneic tumors. Surprisingly, we found that ER was localized to the cytoplasm, regardless of any type of treatment. Based on this, we probed extra-nuclear functions of ER and found that co-inhibition of ER and NFκB led to an increase in oxidative stress and apoptosis. Together, these findings suggest that cytoplasmic ER and NFκB may play redundant roles in protecting mammary cancer cells from oxidative stress and cell death. Although this study has not identified a mouse model with classical ER activity, cytoplasmic ER has been described in a small subset of human breast tumors, suggesting that these findings may be relevant for some breast cancer patients.

Sex differences in Hippocampal Memory and Learning following Neonatal Brain Injury: Is There a Role for Estrogen Receptor-α?

Neonatal encephalopathy due to hypoxia-ischemia (HI) leads to severe, life-long morbidities in thousands of neonates born in the USA and worldwide each year. Varying capacities of long-term episodic memory, verbal working memory, and learning can present without cerebral palsy and have been associated with the severity of neonatal encephalopathy sustained at birth. Among children who sustain a moderate degree of HI at birth, girls have larger hippocampal volumes compared to boys. Clinical studies indicate that female neonatal brains are more resistant to the effects of neonatal HI, resulting in better long-term cognitive outcomes compared to males with comparable brain injury. Our most recent mechanistic studies have addressed the origins and cellular basis of sex differences in hippocampal neuroprotection following neonatal HI-related brain injury and implicate estrogen receptor-α (ERα) in the neurotrophin receptor-mediated hippocampal neuroprotection in female mice. This review summarizes the recent findings on ERα-dependent, neurotrophin-mediated hippocampal neuroprotection and weighs the evidence that this mechanism plays an important role in preservation of long-term memory and learning following HI in females.

Effects of female sex hormones on folic acid-induced anti-angiogenesis

AIM

Pregnant women have been recommended to take FA daily to prevent birth defects in the brain and spinal cord. We previously showed that folic acid (FA) exerts an anti-angiogenic activity. As angiogenesis is important for endometrial reorganization and embryonic development, there should be some mechanisms to allow the pregnant mother and the foetus to escape from the FA-induced anti-angiogenesis. This study was designed to investigate the effect of female sex hormones on the FA-induced anti-angiogenic activity.

METHODS

The protein levels and protein-protein interaction were examined by Western blot analysis and immunoprecipitation assay respectively. The cell proliferation and migration were examined by MTT assay and wound healing assay respectively. The in vivo angiogenesis was evaluated by Matrigel angiogenesis assay.

RESULTS

In human umbilical venous endothelial cells (HUVEC), FA receptor (FR) formed a complex with progesterone receptor (PR), oestradiol receptor (ER) and cSrc. Pregnancy levels of progesterone (P4) or oestradiol (E2) prevented FA-induced inhibitions of proliferation and migration in HUVEC. Both E2 and P4 prevented the FA-induced anti-angiogenesis in vivo. Moreover, cotreatment with FA and P4 or E2 inhibited the signalling pathways involved in FA-induced inhibitions of proliferation and migration in HUVEC.

CONCLUSION

Female sex hormones interrupt the FA-induced anti-angiogenic action through receptor-receptor interaction.

Ginsenoside Rg1 activates ligand-independent estrogenic effects via rapid estrogen receptor signaling pathway

Background

Ginsenoside Rg1 was shown to exert ligand-independent activation of estrogen receptor (ER) via mitogen-activated protein kinase–mediated pathway. Our study aimed to delineate the mechanisms by which Rg1 activates the rapid ER signaling pathways.

Methods

ER-positive human breast cancer MCF-7 cells and ER-negative human embryonic kidney HEK293 cells were treated with Rg1 (10⁻¹²M, 10⁻⁸M), 17ß-estradiol (10⁻⁸M), or vehicle. Immunoprecipitation was conducted to investigate the interactions between signaling protein and ER in MCF-7 cells. To determine the roles of these signaling proteins in the actions of Rg1, small interfering RNA or their inhibitors were applied.

Results

Rg1 rapidly induced ERα translocation to plasma membrane via caveolin-1 and the formation of signaling complex involving linker protein (Shc), insulin-like growth factor-I receptor, modulator of nongenomic activity of ER (MNAR), ERα, and cellular nonreceptor tyrosine kinase (c-Src) in MCF-7 cells. The induction of extracellular signal-regulated protein kinase and mitogen-activated protein kinase kinase (MEK) phosphorylation in MCF-7 cells by Rg1 was suppressed by cotreatment with small interfering RNA against these signaling proteins. The stimulatory effects of Rg1 on MEK phosphorylation in these cells were suppressed by both PP2 (Src kinase inhibitor) and AG1478 [epidermal growth factor receptor (EGFR) inhibitor]. In addition, Rg1-induced estrogenic activities, EGFR and MEK phosphorylation in MCF-7 cells were abolished by cotreatment with G15 (G protein-coupled estrogen receptor-1 antagonist). The increase in intracellular cyclic AMP accumulation, but not Ca mobilization, in MCF-7 cells by Rg1 could be abolished by G15.

Conclusion

Ginsenoside Rg1 exerted estrogenic actions by rapidly inducing the formation of ER containing signalosome in MCF-7 cells. Additionally, Rg1 could activate EGFR and c-Src ER-independently and exert estrogenic effects via rapid activation of membrane-associated ER and G protein-coupled estrogen receptor.

Role of Scaffold Protein Proline-, Glutamic Acid-, and Leucine-Rich Protein 1 (PELP1) in the Modulation of Adrenocortical Cancer Cell Growth

PELP1 acts as an estrogen receptor (ER) coactivator that exerts an essential role in the ER's functions. ER coregulators have a critical role in the progression and response to hormonal treatment of estrogen-dependent tumors. We previously demonstrated that, in adrenocortical carcinoma (ACC), ERα is upregulated and that estradiol activates the IGF-II/IGF1R signaling pathways defining the role of this functional cross-talk in H295R ACC cell proliferation. The aim of this study was to determine if PELP1 is expressed in ACC and may play a role in promoting the interaction between ERα and IGF1R allowing the activation of pathways important for ACC cell growth. The expression of PELP1 was detected by Western blot analysis in ACC tissues and in H295R cells. H295R cell proliferation decrease was assessed by A3-(4,5-Dimethylthiaoly)-2,5-diphenyltetrazolium bromide (MTT) assay and [3H] thymidine incorporation. PELP1 is expressed in ACC tissues and in H295R cells. Moreover, treatment of H295R with E2 or IGF-II induced a multiprotein complex formation consisting of PELP1, IGF1R, ERα, and Src that is involved in ERK1/2 rapid activation. PELP1/ER/IGF1R/c-Src complex identification as part of E2- and IGF-II-dependent signaling in ACC suggests PELP1 is a novel and more efficient potential target to reduce ACC growth.

Estradiol-induced regulation of GLUT4 in 3T3-L1 cells: involvement of ESR1 and AKT activation

Impaired insulin-stimulated glucose uptake involves reduced expression of the GLUT4 (solute carrier family 2 facilitated glucose transporter member 4, SLC2A4 gene). 17β-estradiol (E₂) modulates SLC2A4/GLUT4 expression, but the involved mechanisms are unclear. Although E₂ exerts biological effects by binding to estrogen receptors 1/2 (ESR1/2), which are nuclear transcriptional factors; extranuclear effects have also been proposed. We hypothesize that E₂ regulates GLUT4 through an extranuclear ESR1 mechanism. Thus, we investigated the effects of E₂ upon (1) subcellular distribution of ESRs and the proto-oncogene tyrosine-protein kinases (SRC) involvement; (2) serine/threonine-protein kinase (AKT) activation; (3) Slc2a4/GLUT4 expression and (4) GLUT4 subcellular distribution and glucose uptake in 3T3-L1 adipocytes. Differentiated 3T3-L1 adipocytes were cultivated or not with E₂ for 24 h, and additionally treated or not with ESR1-selective agonist (PPT), ESR1-selective antagonist (MPP) or selective SRC inhibitor (PP2). Subcellular distribution of ESR1, ESR2 and GLUT4 was analyzed by immunocytochemistry; Slc2a4 mRNA and GLUT4 were quantified by qPCR and Western blotting, respectively; plasma membrane GLUT4 translocation and glucose uptake were analyzed under insulin stimulus for 20 min or not. E₂ induced (1) translocation of ESR1, but not of ESR2, from nucleus to plasma membrane and AKT phosphorylation, effects mimicked by PPT and blocked by MPP and PP2; (2) increased Slc2a4/GLUT4 expression and (3) increased insulin-stimulated GLUT4 translocation and glucose uptake. In conclusion, E₂ treatment promoted a SRC-mediated nucleus-plasma membrane shuttle of ESR1, and increased AKT phosphorylation, Slc2a4/GLUT4 expression and plasma membrane GLUT4 translocation; consequently, improving insulin-stimulated glucose uptake. These results unravel mechanisms through which estrogen improves insulin sensitivity.

Membrane and Nuclear Estrogen Receptor Alpha Actions: From Tissue Specificity to Medical Implications

Estrogen receptor alpha (ERα) has been recognized now for several decades as playing a key role in reproduction and exerting functions in numerous nonreproductive tissues. In this review, we attempt to summarize the in vitro studies that are the basis of our current understanding of the mechanisms of action of ERα as a nuclear receptor and the key roles played by its two activation functions (AFs) in its transcriptional activities. We then depict the consequences of the selective inactivation of these AFs in mouse models, focusing on the prominent roles played by ERα in the reproductive tract and in the vascular system. Evidence has accumulated over the two last decades that ERα is also associated with the plasma membrane and activates non-nuclear signaling from this site. These rapid/nongenomic/membrane-initiated steroid signals (MISS) have been characterized in a variety of cell lines, and in particular in endothelial cells. The development of selective pharmacological tools that specifically activate MISS and the generation of mice expressing an ERα protein impeded for membrane localization have begun to unravel the physiological role of MISS in vivo. Finally, we discuss novel perspectives for the design of tissue-selective ER modulators based on the integration of the physiological and pathophysiological roles of MISS actions of estrogens.

The Amino-terminal Domain of the Androgen Receptor Co-opts Extracellular Signal-regulated Kinase (ERK) Docking Sites in ELK1 Protein to Induce Sustained Gene Activation That Supports Prostate Cancer Cell Growth

The ETS domain transcription factor ELK1 is in a repressive association with growth genes and is transiently activated through phosphorylation by ERK1/2. In prostate cancer (PCa) cells the androgen receptor (AR) is recruited by ELK1, via its amino-terminal domain (A/B), as a transcriptional co-activator, without ELK1 hyper-phosphorylation. Here we elucidate the structural basis of the interaction of AR with ELK1. The ELK1 polypeptide motifs required for co-activation by AR versus those required for activation of ELK1 by ERK were systematically mapped using a mammalian two-hybrid system and confirmed using a co-immunoprecipitation assay. The mapping precisely identified the two ERK-docking sites in ELK1, the D-box and the DEF (docking site for ERK, FXFP) motif, as the essential motifs for its cooperation with AR(A/B) or WTAR. In contrast, the transactivation domain in ELK1 was only required for activation by ERK. ELK1-mediated transcriptional activity of AR(A/B) was optimal in the absence of ELK1 binding partners, ERK1/2 and serum-response factor. Purified ELK1 and AR bound with a dissociation constant of 1.9 × 10⁻⁸m. A purified mutant ELK1 in which the D-box and DEF motifs were disrupted did not bind AR. An ELK1 mutant with deletion of the D-box region had a dominant-negative effect on androgen-dependent growth of PCa cells that were insensitive to MEK inhibition. This novel mechanism in which a nuclear receptor impinges on a signaling pathway by co-opting protein kinase docking sites to constitutively activate growth genes could enable rational design of a new class of targeted drug interventions.

Nuclear and extranuclear-initiated estrogen receptor signaling crosstalk and endocrine resistance in breast cancer

Estrogens regulate function of reproductive and non-reproductive tissues in healthy and diseased states including breast cancer. They mainly work through estrogen receptor alpha (ERα) and/or estrogen receptor beta (ERβ). There are various ERα targeting agents that have been used for treatment of ER (+) breast tumors. The impact of direct nuclear activity of ER is very well characterized in ER (+) breast cancers and development and progression of endocrine resistance. Recent studies also suggested important roles for extranuclear-initiated ERα pathways, which would decrease the potency and efficiency of ERα targeting agents. In this mini-review, we will discuss the role of nuclear and extra-nuclear ER signaling and how they relate to therapy resistance in breast cancer.

The importance of non-nuclear AR signaling in prostate cancer progression and therapeutic resistance

The androgen receptor (AR) remains the major oncogenic driver of prostate cancer, as evidenced by the efficacy of androgen deprivation therapy (ADT) in naïve patients, and the continued effectiveness of second generation ADTs in castration resistant disease. However, current ADTs are limited to interfering with AR ligand binding, either through suppression of androgen production or the use of competitive antagonists. Recent studies demonstrate 1) the expression of constitutively active AR splice variants that no longer depend on androgen, and 2) the ability of AR to signal in the cytoplasm independently of its transcriptional activity (non-genomic); thus highlighting the need to consider other ways to target AR. Herein, we review canonical AR signaling, but focus on AR non-genomic signaling, some of its downstream targets and how these effectors contribute to prostate cancer cell behavior. The goals of this review are to 1) re-highlight the continued importance of AR in prostate cancer as the primary driver, 2) discuss the limitations in continuing to use ligand binding as the sole targeting mechanism, 3) discuss the implications of AR non-genomic signaling in cancer progression and therapeutic resistance, and 4) address the need to consider non-genomic AR signaling mechanisms and pathways as a viable targeting strategy in combination with current therapies.

ERα Signaling Is Required for TrkB-Mediated Hippocampal Neuroprotection in Female Neonatal Mice after Hypoxic Ischemic Encephalopathy(1,2,3)

Male neonate brains are more susceptible to the effects of perinatal asphyxia resulting in hypoxia and ischemia (HI)-related brain injury. The relative resistance of female neonatal brains to adverse consequences of HI suggests that there are sex-specific mechanisms that afford females greater neuroprotection and/or facilitates recovery post-HI. We hypothesized that HI preferentially induces estrogen receptor α (ERα) expression in female neonatal hippocampi and that ERα is coupled to Src family kinase (SFK) activation that in turn augments phosphorylation of the TrkB and thereby results in decreased apoptosis. After inducing the Vannucci’s HI model on P9 (C57BL/6J) mice, female and male ERα wild-type (ERα^+/+) or ERα null mutant (ERα^−/−) mice received vehicle control or the selective TrkB agonist 7,8-dihydroxyflavone (7,8-DHF). Hippocampi were collected for analysis of mRNA of ERα and BDNF, protein levels of ERα, p-TrkB, p-src, and cleaved caspase 3 (c-caspase-3) post-HI. Our results demonstrate that: (1) HI differentially induces ERα expression in the hippocampus of the female versus male neonate, (2) src and TrkB phosphorylation post-HI is greater in females than in males after 7,8-DHF therapy, (3) src and TrkB phosphorylation post-HI depend on the presence of ERα, and (4) TrkB agonist therapy decreases the c-caspase-3 only in ERα^+/+ female mice hippocampus. Together, these observations provide evidence that female-specific induction of ERα expression confers neuroprotection with TrkB agonist therapy via SFK activation and account for improved functional outcomes in female neonates post-HI.

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.

Roles for miRNAs in endocrine resistance in breast cancer

Therapies targeting estrogen receptor alpha (ERα), including selective ER modulators such as tamoxifen, selective ER downregulators such as fulvestrant (ICI 182 780), and aromatase inhibitors such as letrozole, are successfully used in treating breast cancer patients whose initial tumor expresses ERα. Unfortunately, the effectiveness of endocrine therapies is limited by acquired resistance. The role of microRNAs (miRNAs) in the progression of endocrine-resistant breast cancer is of keen interest in developing biomarkers and therapies to counter metastatic disease. This review focuses on miRNAs implicated as disruptors of antiestrogen therapies, their bona fide gene targets and associated pathways promoting endocrine resistance.

Liganded ERα Stimulates the E3 Ubiquitin Ligase Activity of UBE3C to Facilitate Cell Proliferation

Estrogen receptor (ER)α is a well-characterized ligand-dependent transcription factor. However, the global picture of its nongenomic functions remains to be illustrated. Here, we demonstrate a novel function of ERα during mitosis that facilitates estrogen-dependent cell proliferation. An E3 ubiquitin ligase, UBE3C, was identified in an ERα complex from estrogen-treated MCF-7 breast cancer cells arrested at mitosis. UBE3C interacts with ERα during mitosis in an estrogen-dependent manner. In vitro, estrogen dramatically stimulates the E3 activity of UBE3C in the presence of ERα. This effect was inhibited by the estrogen antagonist tamoxifen. Importantly, estrogen enhances the ubiquitination of cyclin B1 (CCNB1) and destabilizes CCNB1 during mitosis in a manner dependent on endogenous UBE3C. ERα, UBE3C, and CCNB1 colocalize in prophase nuclei and at metaphase spindles before CCNB1 is degraded in anaphase. Depletion of UBE3C attenuates estrogen-dependent cell proliferation without affecting the transactivation function of ERα. Collectively, these results demonstrate a novel ligand-dependent action of ERα that stimulates the activity of an E3 ligase. The mitotic role of estrogen may contribute to its effects on proliferation in addition to its roles in target gene expression.

Estrogens Correlate with PELP1 Expression in ER Positive Breast Cancer

The Proline-, glutamic acid- and leucine-rich protein 1 (PELP1) is an estrogen receptor (ER) coactivator and a proto-oncogene known to be deregulated in endocrine cancers. In breast cancer, PELP1 overexpression has been associated with endocrine therapy resistance. Although PELP1 is known to be regulated by estrogens in vitro, its association with estrogen levels within the tissue of breast cancer patients has not previously been assessed. Here, we determined PELP1 mRNA expression levels in paired samples of normal and malignant breast tissue obtained from 32 postmenopausal and 11 premenopausal women. In the total sample set, PELP1 levels were higher in tumors compared to normal breast tissue (P = 0.041). Among postmenopausal women, PELP1 tumor levels correlated positively with estrone (E₁) and estradiol (E₂) levels in both normal tissue (r = 0.543, P = 0.003 and r = 0.601, P = 0.001, respectively) and plasma (r = 0.392, P = 0.053 and r = 0.403, P = 0.046, respectively). Analyzing all ER+ tumors (n = 26), PELP1 correlated positively with E₁ and E₂ in tumor tissue (r = 0.562, P = 0.003 and r = 0.411, P = 0.037, respectively) and normal tissue (r = 0.461, P = 0.018 and r = 0.427, P = 0.030, respectively) in addition to plasma E₁, E₂ and estrone sulphate (E₁S) concentrations (r = 0.576, P = 0.003, r = 0.456, P = 0.025 and r = 0.406, P = 0.049, respectively). Finally, PELP1 correlated positively with ER mRNA (ESR1) (r = 0.553, P = 0.026) in ER+ tumors, whereas a negative association between PELP1 and ESR1 (r = -0.733, P = 0.010) was observed in ER- breast tumors. Taken together, tumor PELP1 mRNA expression is associated with estrogen levels in breast cancer, suggesting a potentially important role of PELP1 in ER+ breast cancer growth in vivo.

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.

The social network of PELP1 and its implications in breast and prostate cancers

Proline, glutamic acid- and leucine-rich protein 1 (PELP1) is a multi-domain scaffold protein that serves as a platform for various protein-protein interactions between steroid receptors (SRs) and signaling factors and cell cycle, transcriptional, cytoskeletal, and epigenetic remodelers. PELP1 is known to be a coregulator of transcription and participates in the nuclear and extranuclear functions of SRs, ribosome biogenesis, and cell cycle progression. The expression and localization of PELP1 are dysregulated in hormonal cancers including breast and prostate cancers. This review focuses on the interactive functions and therapeutic and prognostic significance of PELP1 in breast and prostate cancers.

Estradiol enhances CIP2A expression by the activation of p70 S6 kinase

Cancerous inhibitor of PP2A (CIP2A) stimulates the proliferation of various cancer cells, and 17β-estradiol (E₂) enhances the proliferation of breast cancer cells. E₂ activates epidermal growth factor receptor (EGFR), stimulating the MEK1/2 and PI3K pathways, and CIP2A expression is increased by the MEK1/2-induced transcription factor ETS1. It is possible for E₂ to increase CIP2A expression. This study examined whether E₂ could increase CIP2A expression and whether CIP2A is highly expressed in estrogen receptor (ER)-positive breast cancer tissues. E₂ increased CIP2A expression at the translational level in a c-MYC-independent manner in MCF-7 cells. E₂-enhanced proliferation was impaired without CIP2A expression. E₂-stimulated EGFR activated the MAPK and PI3K pathways, which converged to activate p70 S6 kinase (S6K). Phosphorylation at all the three phosphorylation sites (S424/T421, T229, and T389) on S6K was required for the phosphorylation of eukaryotic initiation factor 4B (eIF4B), which was responsible for the increase in CIP2A translation. Furthermore, CIP2A expression was higher in ER-positive tissues than in ER-negative tissues. This is the first study, to our knowledge, to demonstrate that CIP2A is a key factor in E₂-enhanced proliferation and that estrogen regulates CIP2A expression by non-genomic action through EGFR.

Recent insights into non-nuclear actions of estrogen receptor alpha

Estrogen receptors (ER) classically function as transcription factors regulating gene expression. More recently, evidence has continued to accumulate that ER additionally serve numerous important functions remote from the nucleus in a variety of cell types, particularly in non-reproductive tissues. The identification of post-translational modifications of ERα and protein-protein interactions with the receptor that are critical to its non-nuclear functions has afforded opportunities to gain greater insights into these novel non-genomic roles of the receptor. The development of a stable ligand that selectively activates non-nuclear ER has also been invaluable. In this review focused on ERα, recent new understanding of the processes underlying non-nuclear ER action and their in vivo consequences will be discussed. Further research into the non-nuclear capacities by which ER modulate cellular behavior is essential to ultimately harnessing these processes for therapeutic gain in numerous disease contexts.

PELP1: a review of PELP1 interactions, signaling, and biology

Proline, glutamic acid, and leucine rich protein 1 (PELP1) is a large multi-domain protein that has been shown to modulate an increasing number of pathways and biological processes. The first reports describing the cloning and characterization of PELP1 showed that it was an estrogen receptor coactivator. PELP1 has now been shown to be a coregulator for a growing number of transcription factors. Furthermore, recent reports have shown that PELP1 is a member of chromatin remodeling complexes. In addition to PELP1 nuclear functions, it has been shown to have cytoplasmic signaling functions as well. In the cytoplasm PELP1 acts as a scaffold molecule and mediates rapid signaling from growth factor and hormone receptors. PELP1 signaling ultimately plays a role in cancer biology by increasing proliferation and metastasis, among other cellular processes. Here we will review (1) the cloning and characterization of PELP1 expression, (2) interacting proteins, (3) PELP1 signaling, and (4) PELP1-mediated biology.

Temporal expression of Pelp1 during proliferation and osteogenic differentiation of rat bone marrow mesenchymal stem cells

Background

Osteogenic induction and bone formation are heavily affected by environmental factors, including estrogen, estrogen receptors, and coregulatory proteins, such as the recently reported proline-, glutamic acid-, and leucine-rich protein 1(Pelp1).

Objective

To investigate Pelp1 expression in rat bone mesenchymal stem cells (rBMSCs) during cell proliferation and osteogenic differentiation.

Methods

rBMSCs were cultured in routine and osteogenic differentiation media. Cell proliferation was assessed at days 1, 3, 5, 7, 9, 11, 14, and 21. Pelp1 protein expression in the nucleus and cytoplasm were detected by immunocytochemical analysis. Real-time RT-PCR and western blot were used to detect mRNA and protein expressions of Pelp1, osteocalcin (OCN), and alkaline phosphatase (ALP).

Results

Over 21 days, rBMSCs in routine culture exhibited a 1-2 day lag phase and exponential growth from day 3 to 9, plateauing at day 9, and correlated with temporal mRNA expression of Pelp1, which almost reached baseline levels at day 21. In osteogenic induction cultures, Pelp1 mRNA levels rose at day 9 and steadily increased until day 21, reaching 6.8-fold greater value compared with day 1. Interestingly, Pelp1 mRNA expression in osteogenic cultures exhibited a trend similar to that of OCN expression. Pelp1 knockdown by siRNA transfection inhibited undifferentiated rBMSC proliferation, and bone markers OCN and ALP expressions in rBMSCs cultured in routine and osteogenic differentiation media.

Conclusions

Pelp1 may be a key player in BMSCs proliferation and osteogenic differentiation, meriting further consideration as a target for development of therapies for pathological bone loss conditions, such as menopausal bone loss.

Unliganded estrogen receptor alpha promotes PC12 survival during serum starvation

Many studies have reported proliferative, differentiating or protective effects of estradiol, notably through estrogen receptor alpha (ERα). On the contrary, the ligand-independent action of ERα is currently poorly documented notably in cell protection. The stable transfection of wild type, substituted or truncated form of ERα in PC12 cells (ERα negative cell line) lead the specific study of its ligand-independent action. Hence, we demonstrate here that, in the absence of E2, the expression of ERα prevents cells from apoptosis induced by serum deprivation. This protection is not due to an ERE-mediated transcription and does not require either AF-1 or AF-2 transactivation functions. It is afforded to the Y537 residue of ERα and activation of c-Src/Stat3 signaling pathway.

Point mutations in the ERα Gαi binding domain segregate nonnuclear from nuclear receptor function

A subpopulation of plasma membrane-associated estrogen receptor (ER)α interact directly with G proteins and mediate nonnuclear receptor signaling. This mechanism underlies numerous processes, including important cardiovascular protective actions of estradiol (E(2)), such as the activation of endothelial NO synthase (eNOS) and endothelial cell growth and migration. In the present work we sought a genetic approach to differentiate nonnuclear from nuclear ERα actions. We generated single alanine substitutions within the Gαi-binding domain of ERα (amino acids 251-260) and tested signaling to eNOS or ERK1,2 and activation of luciferase (Luc) reporters signifying transactivation via direct or indirect ERα-DNA binding in HeLa cells. The point mutants ERα-R256A, ERα-K257A, ERα-D258A, and ERα-R260A were all incapable of activating eNOS in response to E(2), and ERα-R256A and ERα-D258A also showed loss of ERK1,2 activation. In contrast, ERα-R256A, ERα-K257A, ERα-D258A, and ERα-R260A all displayed normal capacity to invoke E(2)-induced transactivation of estrogen response element (ERE)-Luc or Sp1-Luc. However, whereas activator protein 1-Luc activation by ERα-R256A and ERα-D258A was intact, ERα-K257A and ERα-R260A were incapable of activator protein 1-Luc activation. In in vitro pull-down assays with the two mutants that lack all nonnuclear functions tested and retain all nuclear functions tested, ERα-R256A and ERα-D258A, there was normal direct interaction between Gαi and ERα-R256A and an absence of interaction between Gαi and ERα-D258A. When expressed in endothelial cells, these two mutants prevented E(2)-induced migration and eNOS activation mediated by endogenous receptor, indicative of dominant-negative action. Thus, the point mutants ERα-R256A and ERα-D258A in the receptor GαI-binding domain provide genetic segregation of nonnuclear from nuclear ERα function.

Activation of rapid oestrogen signalling in aggressive human breast cancers

Oestrogen receptors can mediate rapid activation of cytoplasmic signalling cascades by recruiting Src and PI3K. However, the involvement of this pathway in breast cancer remains poorly defined. We have previously shown that methylation of ERα is required for the formation of the ERα/Src/PI3K complex and that ERα is hypermethylated in a subset of breast cancers. Here, we used Proximity Ligation Assay to demonstrate that this complex is present in the cytoplasm of breast cancer cell lines as well as formalin-fixed, paraffin-embedded tumours. Of particular interest, the analysis of 175 breast tumours showed that overexpression of this complex in a subset of breast tumours correlates to the activation of the downstream effector Akt. Survival analysis revealed that high expression of this complex is an independent marker of poor prognosis and associated with reduced disease-free survival. Our data introduces the new concept that the rapid oestrogen pathway is operative in vivo. It also provides a rationale for patient stratification defined by the activation of this pathway and the identification of target therapies.

Estrogen receptor (ER) expression and function in the pregnant human myometrium: estradiol via ERα activates ERK1/2 signaling in term myometrium

Estrogens are thought to promote labor by increasing the expression of pro-contraction genes in myometrial cells. The specific estrogen receptors ((ERs: ERα and ERβ (also known as ESR1 and ESR2)) and G protein-coupled receptor 30 (GPR30; also known as G protein-coupled estrogen receptor 1)) and signaling pathways that mediate these actions are not clearly understood. In this study, we identified the ERs expressed in the pregnant human myometrium and determined a key extranuclear signaling pathway through which estradiol (E(2)) modulates expression of the gene encoding the oxytocin receptor (OXTR), a major pro-contraction protein. Using quantitative RT-PCR, we found that ERα and GPR30 mRNAs were expressed in the human pregnant myometrium while ERβ mRNA was virtually undetectable. While mRNA encoding ERα was the predominant ER transcript in the pregnant myometrium, ERα protein was largely undetectable in myometrial tissue by immunoblotting. Pharmacological inhibition of 26S proteasome activity increased ERα protein abundance to detectable levels in term myometrial explants, however, indicating rapid turnover of ERα protein by proteasomal processing in the pregnant myometrium. E(2) stimulated rapid extranuclear signaling in myometrial explants, as evidenced by increased extracellularly regulated kinase (ERK1/2) phosphorylation within 10 min. This effect was inhibited by pre-treatment with an ER antagonist, ICI 182 780, indicating the involvement of ERα. Inhibition of ERK signaling abrogated the ability of E(2) to stimulate OXTR gene expression in myometrial explants. We conclude that estrogenic actions in the human myometrium during pregnancy, including the stimulation of contraction-associated gene expression, can be mediated by extranuclear signaling through ERα via activation of the ERK/mitogen-activated protein kinase pathway.

Minireview: Recent advances in extranuclear steroid receptor actions

The participation of extranuclear steroid receptor signaling in organ physiology and the impact for pathobiology has increasingly been demonstrated. Important functions of membrane estrogen receptors in the cardiovascular system demonstrate new mechanisms of rapid steroid signaling to gene regulation, preventing cardiovascular disease and maintaining healthy arterial function. In cancer cells, kinase signaling initiated by extranuclear estrogen, progesterone, and androgen receptors modulates transcriptional events in the nucleus, which in turn regulate proliferation, migration, and invasion. Important mediators of cross talk between cytoplasmic and nuclear steroid receptor signaling are the proline-, glutamic acid-, and leucine-rich protein-1 and paxillin proteins, both of which modulate membrane and nuclear receptor pool signaling to promote a variety of cell biological functions.

Understanding extranuclear (nongenomic) androgen signaling: what a frog oocyte can tell us about human biology

Steroids are key factors in a myriad of mammalian biological systems, including the brain, kidney, heart, bones, and gonads. While alternative potential steroid receptors have been described, the majority of biologically relevant steroid responses appear to be mediated by classical steroid receptors that are located in all parts of the cell, from the plasma membrane to the nucleus. Interestingly, these classical steroid receptors modulate different signals depending upon their location. For example, receptors in the plasma membrane interact with membrane signaling molecules, including G proteins and kinases. In contrast, receptors in the nucleus interact with nuclear signaling molecules, including transcriptional co-regulators. These extranuclear and intranuclear signals function together in an integrated fashion to regulate important biological functions. While most studies on extranuclear steroid signaling have focused on estrogens, recent work has demonstrated that nongenomic androgen signaling is equally important and that these two steroids modulate similar signaling pathways. In fact, by taking advantage of a simple model system whereby a physiologically relevant androgen-mediated process is regulated completely independent of transcription (Xenopus laevis oocyte maturation), many novel and conserved concepts in nongenomic steroid signaling have been uncovered and characterized.

Scaffolding proteins mediating membrane-initiated extra-nuclear actions of estrogen receptor

Estrogen mediates biological effects on cell proliferation, differentiation, and homeostasis through estrogen receptor (ER). In addition to functioning as a ligand-activated nuclear transcription factor to directly regulate gene transcription, ER also mediates rapid activation of signaling pathways independent of its transcriptional activity. A subpopulation of ER localized to the cell membrane or cytoplasm has been proposed to mediate ER activation of signaling pathways. This review focuses on recent advances in our understanding of mechanisms responsible for ER cytoplasm/membrane localization, where rapid extra-nuclear signaling is initiated. These mechanisms include lipid modification of the receptor (palmitoylation) and interactions with membrane and cytoplasmic adaptor proteins including caveolins, striatin, p130Cas, Shc, HPIP, MTA-1s, and MNAR/PELP1. While it is clear that ER mediates rapid extra-nuclear signaling resulting in activation of signaling pathways such as Src/MAPK and PI-3 kinase/Akt, how ER extra-nuclear signaling influences overall ER/estrogen physiology is still not well understood. Future studies defining physiological roles of ER extra-nuclear actions and crosstalk with its nuclear counterparts will be important to our overall understanding of estrogen and ER biological functions.

Non-nuclear estrogen receptor signaling in the endothelium

In addition to the classical function of estrogen receptors (ER) as transcription factors, evidence continues to accumulate that they mediate non-nuclear processes in numerous cell types, including the endothelium, in which they activate endothelial NO synthase. Non-nuclear ER signaling entails unique post-translational modifications and protein-protein interactions of the receptor with adaptor molecules, kinases, and G proteins. Recent in vitro and in vivo studies in mice using an estrogen-dendrimer conjugate that is excluded from the nucleus indicate that non-nuclear ER activation underlies the migration and growth responses of endothelial cells to estrogen but not the growth responses of endometrial or breast cancer cells to the hormone. In this minireview, the features of ERα and protein-protein interactions that enable it to invoke extranuclear signaling in the endothelium and the consequences of that signaling are discussed.

Methylation, a key step for nongenomic estrogen signaling in breast tumors

Estrogen receptor alpha (ERalpha) is a member of a large conserved superfamily of steroid hormone nuclear receptors which regulates many physiological pathways by acting as a ligand-dependent transcription factor. Evidence is emerging that estrogens also induce rapid signaling to the downstream kinase cascades; however the mechanisms underlying this nongenomic function remain poorly understood. We have recently shown that ERalpha is methylated specifically by the arginine methyltransferase PRMT1 at arginine 260 in the DNA-binding domain of the receptor. This methylation event is required for mediating the extra-nuclear function of the receptor which would thereby interact with Src/FAK and p85 and propagate the signal to downstream transduction cascades that orchestrate cell proliferation and survival. Of particular interest, a possible role of methylated ERalpha in mammary tumorigenesis is also evident by the fact that, as demonstrated by immunohistochemical studies on a cohort of breast cancer patients, ERalpha is methylated in normal epithelial breast cells and is hypermethylated in a subset of breast cancers. Hypermethylation of ERalpha in breast cancer might cause hyperactivation of cellular kinase signaling, notably of Akt, described as a selective survival advantage for primary tumor cells even in the presence of anti-estrogens. A detailed understanding of the molecular mechanisms that control estrogen signaling in breast cancer is a crucial step in identifying new effective therapies.

Environmental hormones: Multiple pathways for response may lead to multiple disease outcomes

Compounds that mimic vertebrate hormone responses are found throughout the environment, and some are implicated in endocrine disruption. Endocrine disruption has been found in humans, wildlife, and even in the partnership of plants and root symbionts. Most endocrine disruption occurs in estrogenic systems. Estrogens, like other steroid hormones, binds a transcription factor known as a nuclear receptor to regulate gene transcription. Recent research has shown that there are other signaling mechanisms for steroid hormones that involve kinase pathways and G protein-coupled receptors. Mounting evidence suggests estrogen mimics can also act by these pathways which work outside the nucleus. Differential expression of these pathways across cell types, and differential affinity for these pathways by diverse compounds may explain some patterns of endocrine disruption and disease.

PELP1: A novel therapeutic target for hormonal cancers

Recent studies implicate that the estrogen receptor (ER) coregulator proline-, glutamic acid-, and leucine-rich protein (PELP) 1 as playing critical roles in ER-genomic, ER-nongenomic, and ER-signaling cross talk with growth factor signaling pathways. PELP1 expression is deregulated in hormonal cancers and recent studies further elucidated the molecular mechanisms by which PELP1 regulates hormone therapy response. Although PELP1 is important for normal functions of the ER, the possibility to target ER-PELP1 axis appears to be an effective strategy for preventing hormonal carcinogenesis and therapy resistance. Thus, PELP1 may be useful as prognostic marker for hormonal cancers and PELP1 signaling may be useful to generate targeted therapeutics to overcome hormonal therapy resistance.