Mouse Estrogen Receptor β Isoforms Exhibit Differences in Ligand Selectivity and Coactivator Recruitment

Divergent features of ERβ isoforms in triple negative breast cancer: progress and implications for further research

Estrogen receptor β (ERβ) was discovered more than 20 years ago. However, the extent and role of ERβ expression in breast cancer remain controversial, especially in the context of triple-negative breast cancer (TNBC). ERβ exists as multiple isoforms, and a series of studies has revealed an inconsistent role of ERβ isoforms in TNBC. Our recent results demonstrated contrasting functions of ERβ1 and ERβ2/β5 in TNBC. Additional research should be conducted to explore the functions of individual ERβ isoforms and develop targeted drugs according to the relevant mechanisms. Consequently, a systematic review of ERβ isoforms is necessary. In this review, we overview the structure of ERβ isoforms and detail what is known about the function of ERβ isoforms in normal mammary tissue and breast cancer. Moreover, this review highlights the divergent features of ERβ isoforms in TNBC. This review also provides insights into the implications of targeting ERβ isoforms for clinical treatment. In conclusion, this review provides a framework delineating the roles and mechanisms of different ERβ isoforms in TNBC and sheds light on future directions for basic and clinical research.

Differential Activation of a Mouse Estrogen Receptor β Isoform (mERβ2) with Endocrine-Disrupting Chemicals (EDCs)

BACKGROUND

Endocrine-disrupting chemicals (EDCs) are suspected of altering estrogenic signaling through estrogen receptor (ER) α or β (mERβ1 in mice). Several EDC effects have been reported in animal studies and extrapolated to human studies. Unlike humans, rodents express a novel isoform of ERβ (mERβ2) with a modified ligand-binding domain sequence. EDC activity through this isoform remains uncharacterized.

OBJECTIVES

We identified the expression pattern of mERβ2 in mouse tissues and assessed the estrogenic activity of EDCs through mERβ2.

METHODS

mERβ2 mRNA expression was measured in mouse tissues. HepG2 cells were used to assess the transactivation activity of mERβ isoforms with EDCs and ER co-activators. 293A cells transiently transfected with mER isoforms were used to detect EDC-mediated changes in endogenous ER target gene expression.

RESULTS

Expression of mERβ2 mRNA was detected in mouse reproductive tissues (ovary, testis, and prostate) and lung and colon tissues from both female and male mice. Five (E2, DES, DPN, BPAF, Coum, 1-BP) of 16 compounds tested by reporter assay had estrogenic activity through mERβ2. mERβ2 had a compound-specific negative effect on ERβ/ligand-mediated activity and ER target genes when co-expressed with mERβ1. mERβ2 recruited coactivators SRC2 or SRC3 in the presence of EDCs, but showed less recruitment than mERβ1.

CONCLUSION

mERβ2 showed weaker estrogenic activity than mERβ1 in our in vitro system, and can dampen mERβ1 activity. In vivo models of EDC activity and ER-mediated toxicity should consider the role of mERβ2, as rodent tissue responses involving mERβ2 may not be reproduced in human biology.

Estrogen Receptor β as a Pharmaceutical Target

A major issue in clinical endocrinology today is how to use hormones to achieve the health benefits that they clearly can provide but avoid the negative side effects, that is, how to develop more precise medicines. This problem of how to use hormones is pervasive in clinical endocrinology. It is true for estrogen, progesterone, androgen, vitamin D, and thyroid hormone, and the problem is amplified in the case of new ligands for the more recently discovered nuclear receptors. Selective targeting of hormone receptor subtypes is one attractive way to harness the beneficial effects of hormones while reducing unwanted side effects. Here, we focus on estrogen receptor (ER)β, which has promise as a selective target in hormone replacement therapy, and in breast and prostate cancers.

Zearalenone and 17 β-estradiol induced damages in male rats reproduction potential; evidence for ERα and ERβ receptors expression and steroidogenesis

The estrogen receptors (ERs)-dependent effects of Zearalenone (ZEA) on structure and function of the testis as well as sperm parameters were compared with 17-β estradiol as endogenous substance. For this purpose, 30 mature male rats were assigned into five groups as; control (appropriate volume of normal saline, i. p.), ZEA-received (1, 2 and 4 mg/kg, b. w., i. p.) and 17 β-estradiol (E2)-received (appropriate dose of 0.1 mg/kg, i. p.). Following 28 days, the mRNA levels of estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ) in the testis and sperms and the expression of them at protein levels in testicles were estimated. Mitochondrial content of germinal epithelium, Leydig cells steroid foci, sperm quality parameters and serum level of testosterone were assessed. Fluorescent techniques were used for analyzing apoptosis and mRNA damage in necrotic cells. ZEA reduced the mRNA and protein levels of ERα in testicles while up-regulated the ERβ expression. The mRNA level of ERα decreased in sperms of ZEA and E2-received animals. No remarkable changes were found for ERβ expression in sperms from ZEA and E2-received animals. ZEA reduced the Leydig cells steroidogenesis, mitochondrial content of germinal cells and elevated cellular apoptosis and necrosis dose-dependently. E2 reduced the testosterone concentration, enhanced the apoptosis and reduced sperm quality. Our data suggest that ZEA-induced detrimental effects in the structure and function of testis, may attribute to changing the ERs expression at mRNA and translational level.

Implications of estrogen receptor alpha and estrogen receptor beta for adipose tissue functions and cardiometabolic complications

There is growing evidence that estrogen signaling regulates energy metabolism and exerts important functions in maintaining adipose tissue metabolism, including controlling the distribution of body fat. Changes in the physiological functions of adipose tissue, particularly the white adipose tissue, have been strongly connected to obesity and the development of related cardiometabolic complications. In this review, we will focus on discussing the role of estrogen signaling in regulating adipocyte differentiation, metabolism and its endocrine function with a focus on the possible underlying molecular mechanisms mediated by estrogen receptor α and estrogen receptor β.

Deficiency of ERβ and prostate tumorigenesis in FGF8b transgenic mice

Estrogens contribute to the development and growth of the prostate and are implicated in prostate tumorigenesis. In their target tissues, estrogens mediate their effects via estrogen receptor α (ERα (ESR1)) and β (ERβ (ESR2)). Hyperplasia and decreased differentiation of epithelial cells in the prostate have been reported in ERβ knockout (BERKO) mice. Herein, we studied the effect of ERβ deficiency on prostate tumorigenesis by crossing BERKOFVB mice with prostate-targeted human fibroblast growth factor 8b transgenic (FGF8b-Tg) mice. Consistent with results described in our previous report, the prostates of 1-year-old FGF8b-Tg mice displayed stromal aberrations, prostatic intraepithelial neoplasia (mPIN) lesions, inflammation, and occasionally cancer. The prostates of BERKOFVB mice exhibited mild epithelial hypercellularity and inflammation. The prostate phenotypes of FGF8b-Tg-BERKOFVB mice closely resembled those of FGF8b-Tg mice. However, mucinous metaplasia, indicated by Goblet-like cells in the epithelium, was significantly more frequent in the prostates of FGF8b-Tg-BERKOFVB mice when compared with FGF8b-Tg mice. Furthermore, compared with FGF8b-Tg mice, there was a tendency for increased frequency of inflammation but milder hyperplasias in the prostate stroma of FGF8b-Tg-BERKOFVB mice. The expression levels of mRNAs for FGF8b-regulated genes including osteopontin (Spp1), connective tissue growth factor (Ctgf), fibroblast growth factor receptors (Fgfrs), and steroid hormone receptors and cytokines were similar in the prostates of FGF8b-Tg and FGF8b-Tg-BERKOFVB mice. Our results indicate that ERβ plays a role in the differentiation of the prostatic epithelium and, potentially, in the defensive mechanism required for protection against inflammation but do not support a direct tumor-suppressive function of ERβ in the prostate of FGF8b-Tg mice.

Nuclear receptor coactivators: regulators of steroid action in brain and behaviour

Steroid hormones act in specific regions of the brain to alter behaviour and physiology. Although it has been well established that the bioavailability of the steroid and the expression of its receptor is critical for understanding steroid action in the brain, the importance of nuclear receptor coactivators in the brain is becoming more apparent. The present review focuses on the function of the p160 family of coactivators, which includes steroid receptor coactivator-1 (SRC-1), SRC-2 and SRC-3, in steroid receptor action in the brain. The expression, regulation and function of these coactivators in steroid-dependent gene expression in both brain and behaviour are discussed.

ERβ1 represses FOXM1 expression through targeting ERα to control cell proliferation in breast cancer

In this study, we investigated the effects of ectopic estrogen receptor (ER)β1 expression in breast cancer cell lines and nude mice xenografts and observed that ERβ1 expression suppresses tumor growth and represses FOXM1 mRNA and protein expression in ERα-positive but not ERα-negative breast cancer cells. Furthermore, a significant inverse correlation exists between ERβ1 and FOXM1 expression at both protein and mRNA transcript levels in ERα-positive breast cancer patient samples. Ectopic ERβ1 expression resulted in decreased FOXM1 protein and mRNA expression only in ERα-positive but not ERα-negative breast carcinoma cell lines, suggesting that ERβ1 represses ERα-dependent FOXM1 transcription. Reporter gene assays showed that ERβ1 represses FOXM1 transcription through an estrogen-response element located within the proximal promoter region that is also targeted by ERα. The direct binding of ERβ1 to the FOXM1 promoter was confirmed by chromatin immunoprecipitation analysis, which also showed that ectopic expression of ERβ1 displaces ERα from the endogenous FOXM1 promoter. Forced expression of ERβ1 promoted growth suppression in MCF-7 cells, but the anti-proliferative effects of ERβ1 could be overridden by overexpression of FOXM1, indicating that FOXM1 is an important downstream target of ERβ1 signaling. Together, these findings define a key anti-proliferative role for ERβ1 in breast cancer development through negatively regulating FOXM1 expression.

Steroid receptor coactivator-2 expression in brain and physical associations with steroid receptors

Estradiol and progesterone bind to their respective receptors in the hypothalamus and hippocampus to influence a variety of behavioral and physiological functions, including reproduction and cognition. Work from our lab and others has shown that the nuclear receptor coactivators, steroid receptor coactivator-1 (SRC-1) and SRC-2, are essential for efficient estrogen receptor (ER) and progestin receptor (PR) transcriptional activity in brain and for hormone-dependent behaviors. While the expression of SRC-1 in brain has been studied extensively, little is known about the expression of SRC-2 in brain. In the present studies, we found that SRC-2 was highly expressed throughout the hippocampus, amygdala and hypothalamus, including the medial preoptic area (MPOA), ventral medial nucleus (VMN), arcuate nucleus (ARC), bed nucleus of the stria terminalis, supraoptic nucleus and suprachiasmatic nucleus. In order for coactivators to function with steroid receptors, they must be expressed in the same cells. Indeed, SRC-2 and ER(alpha) were coexpressed in many cells in the MPOA, VMN and ARC, all brain regions known to be involved in female reproductive behavior and physiology. While in vitro studies indicate that SRC-2 physically associates with ER and PR, very little is known about receptor-coactivator interactions in brain. Therefore, we used pull-down assays to test the hypotheses that SRC-2 from hypothalamic and hippocampal tissue physically associate with ER and PR subtypes in a ligand-dependent manner. SRC-2 from both brain regions interacted with ER(alpha) bound to agonist, but not in the absence of ligand or in the presence of the selective ER modulator, tamoxifen. Analysis by mass spectrometry confirmed these ligand-dependent interactions between ER(alpha) and SRC-2 from brain. In dramatic contrast, SRC-2 from brain showed little to no interaction with ERbeta. Interestingly, SRC-2 from both brain regions interacted with PR-B, but not PR-A, in a ligand-dependent manner. Taken together, these findings reveal that SRC-2 is expressed in brain regions known to mediate a variety of steroid-dependent functions. Furthermore, SRC-2 is expressed in many ER(alpha) containing cells in the hypothalamus. Finally, SRC-2 from brain interacts with ER and PR in a subtype-specific manner, which may contribute to the functional differences of these steroid receptor subtypes in brain.

Roles of histone H3-lysine 4 methyltransferase complexes in NR-mediated gene transcription

Transcriptional regulation by nuclear hormone receptors (NRs) requires multiple coregulators that modulate chromatin structures by catalyzing a diverse array of posttranslational modifications of histones. Different combinations of these modifications yield dynamic functional outcomes, constituting an epigenetic histone code. This code is inscribed by histone-modifying enzymes and decoded by effector proteins that recognize specific covalent marks. One important modification associated with active chromatin structures is methylation of histone H3-lysine 4 (H3K4). Crucial roles for this modification in NR transactivation have been recently highlighted through our purification and subsequent characterization of a steady-state complex associated with ASC-2, a coactivator of NRs and other transcription factors. This complex, designated ASCOM for ASC-2 complex, contains H3K4-methyltransferase MLL3/HALR or its paralogue MLL4/ALR and represents the first Set1-like H3K4-methyltransferase complex to be reported in vertebrates. This review focuses on recent progress in our understanding of how ASCOM-MLL3 and ASCOM-MLL4 influence NR-mediated gene transcription and of their physiological function.

Estrogen receptor-beta agonist diarylpropionitrile: biological activities of R- and S-enantiomers on behavior and hormonal response to stress

Estrogens have been shown to have positive and negative effects on anxiety and depressive-like behaviors, perhaps explained by the existence of two distinct estrogen receptor (ER) systems, ERalpha and ERbeta. The ERbeta agonist, diarylpropionitrile (DPN) has been shown to have anxiolytic properties in rats. DPN exists as a racemic mixture of two enantiomers, R-DPN and S-DPN. In this study, we compared R-DPN and S-DPN for their in vitro binding affinity, ability to activate transcription in vitro at an estrogen response element, and in vivo endocrine and behavioral responses. In vitro binding studies using recombinant rat ERbeta revealed that S-DPN has a severalfold greater relative binding affinity for ERbeta than does R-DPN. Furthermore, cotransfection of N-38 immortalized hypothalamic cells with an estrogen response element-luc reporter and ERbeta revealed that S-DPN is a potent activator of transcription in vitro, whereas R-DPN is not. Subsequently, we examined anxiety-like behaviors using the open-field test and elevated plus maze or depressive-like behaviors, using the forced swim test. Ovariectomized young adult female Sprague Dawley rats treated with racemic DPN, S-DPN, and the ERbeta agonist, WAY-200070, showed significantly decreased anxiety-like behaviors in both the open-field and elevated plus maze and significantly less depressive-like behaviors in the forced swim test compared with vehicle-, R-DPN-, or propylpyrazoletriol (ERalpha agonist)-treated animals. In concordance with the relative binding affinity and transcriptional potency, these results demonstrate that the S-enantiomer is the biologically active form of DPN. These studies also indicate that estrogen's positive effects on mood, including its anxiolytic and antidepressive actions, are due to its actions at ERbeta.

Ligand induced interaction of thyroid hormone receptor beta with its coregulators

Thyroid hormones exert most of their physiological effects through two thyroid hormone receptor (TR) subtypes, TRalpha and TRbeta, which associate with many transcriptional coregulators to mediate activation or repression of target genes. The search for selective TRbeta ligands has been stimulated by the finding that several pharmacological actions mediated by TRbeta might be beneficial in medical conditions such as obesity, hypercholesterolemia and diabetes. Here, we present a new methodology which employs surface plasmon resonance to investigate the interactions between TRbeta ligand binding domain (LBD) complexes and peptides derived from the nuclear receptor interaction motifs of two of its coregulators, SRC2 and DAX1. The effect of several TRbeta ligands, including the TRbeta selective agonist GC-1 and the TRbeta selective antagonist NH-3, were investigated. We also determined the kinetic rate constants for the interaction of TRbeta-T3 with both coregulators, and accessed the thermodynamic parameters for the interaction with DAX1. Our findings suggest that flexibility plays an important role in the interaction between the receptor and its coregulators, and point out important aspects of experimental design that should be addressed when using TRbeta LBD and its agonists. Furthermore, the methodology described here may be useful for the identification of new TRbeta ligands.

Identification and characterization of PNRC splicing variants

Nuclear receptor (NR) dependent transcriptional action requires recruitment of diverse factors characterized as coregulators. PNRC (proline-rich nuclear receptor coregulatory protein) is a member of coregulators that are capable of potentiating the transcriptional activity of NRs. Here we identified three human PNRC splicing variants designated PNRC1c, PNRC1d and PNRC1f. PNRC1c and PNRC1f are generated through alternative recognition of the 3'-splice site in exon 1, leading to in-frame deletion of 79 amino acids (aa) and an altered reading frame, respectively. PNRC1d is generated through the alternate promoter usage and forms a truncated protein containing C-terminus 142 aa of full-length PNRC. These isoforms differ in their abilities to bind NRs and potentiate NR mediated transcriptions. Moreover, PNRC1d can modulate the activity of full-length PNRC in enhancing ER mediated transcription. Our results suggest that PNRC exists as functionally distinct isoforms and alternative splicing serves as a regulatory mechanism of PNRC coactivator activity.

The potential role of ER beta isoforms in the clinical management of breast cancer

The discovery of a second oestrogen receptor, ER beta, was a subject of much interest, as this suggested a means to improve the prognostic stratification of invasive breast cancer, better predict response to endocrine therapy, develop new chemotherapeutic/chemopreventative drugs and perhaps prevent inappropriate treatment. However, this has not proved to be straightforward with the discovery of five ER beta isoforms and numerous exon deletion variants. This review sets out to identify the present state of knowledge regarding the clinicopathological role of ER beta isoforms and discusses possible reasons for conflicting results arising from recent research findings.

Nuclear receptor coactivator/coregulator NCoA6(NRC) is a pleiotropic coregulator involved in transcription, cell survival, growth and development

NCoA6 (also referred to as NRC, ASC-2, TRBP, PRIP and RAP250) was originally isolated as a ligand-dependent nuclear receptor interacting protein. However, NCoA6 is a multifunctional coregulator or coactivator necessary for transcriptional activation of a wide spectrum of target genes. The NCoA6 gene is amplified and overexpressed in breast, colon and lung cancers. NCoA6 is a 250 kDa protein which harbors a potent N-terminal activation domain, AD1; and a second, centrally-located activation domain, AD2, which is necessary for nuclear receptor signaling. The intrinsic activation potential of NCoA6 is regulated by its C-terminal STL regulatory domain. Near AD2 is an LxxLL-1 motif which interacts with a wide spectrum of ligand-bound NRs with high-affinity. A second LxxLL motif (LxxLL-2) located towards the C-terminal region is more restricted in its NR specificity. The potential role of NCoA6 as a co-integrator is suggested by its ability to enhance transcriptional activation of a wide variety of transcription factors and from its in vivo association with a number of known cofactors including CBP/p300. NCoA6 has been shown to associate with at least three distinct coactivator complexes containing Set methyltransferases as core polypeptides. The composition of these complexes suggests that NCoA6 may play a fundamental role in transcriptional activation by modulating chromatin structure through histone methylation. Knockout studies in mice suggest that NCoA6 is an essential coactivator. NCoA6-/- embryos die between 8.5-12.5 dpc from general growth retardation coupled with developmental defects in the heart, liver, brain and placenta. NCoA6-/- MEFs grow at a reduced rate compared to WT MEFs and spontaneously undergo apoptosis, indicating the importance of NCoA6 as a prosurvival and anti-apoptotic gene. Studies with NCoA6+/- and conditional knockout mice suggest that NCoA6 is a pleiotropic coregulator involved in growth, development, wound healing and maintenance of energy homeostasis.

AKT regulation of estrogen receptor beta transcriptional activity in breast cancer

Growth factor activation of the phosphatidylinositol 3-kinase (PI3K)-AKT pathway has been shown to activate the estrogen receptor (ER) alpha and to mediate tamoxifen resistance in breast cancer. Here, we investigated the regulation of the transcriptional activity of the newer ER beta by PI3K-AKT signaling. Tissue arrays of breast cancer specimens showed a positive association between the expressions of AKT and ER beta in the clinical setting. Reporter gene assays using pharmacologic and molecular inhibitors of AKT and constitutively active AKT revealed for the first time the ability of AKT to (a) potentiate ER beta activity and (b) target predominantly the activation function-2 (AF2) domain of the receptor, with a requirement for residue K269. Given the importance of coactivators in ER transcriptional activity, we further investigated the possible involvement of steroid receptor coactivator 1 (SRC1) and glucocorticoid receptor-interacting protein 1 (GRIP1) in AKT regulation of ER beta. Mammalian two-hybrid assays revealed that AKT enhanced both SRC1 and GRIP1 recruitment to the ER beta-AF2 domain, and reporter gene analyses revealed that AKT and GRIP1 cooperatively potentiated ER beta-mediated transcription to a level much greater than either factor alone. Investigations into AKT regulation of GRIP with mammalian one-hybrid assays showed that AKT potentiated the activation domains of GRIP1 itself, and in vitro kinase assays revealed that AKT directly phosphorylated GRIP1. The cross-talk between the PI3K-AKT and ER beta pathways, as revealed by the ability of AKT to regulate several components of ER beta-mediated transcription, may represent an important aspect that may influence breast cancer response to endocrine therapy.

Effects of exon-deleted estrogen receptor β transcript variants on growth, apoptosis and gene expression of human breast cancer cell lines

Estrogen receptor beta gene codes for a variety of transcript isoforms resulting from alternative splicing, which are expressed both in mammary gland and in breast cancer cells. We studied the function of two exon-deleted ERbeta isoforms recently identified by our group in comparison to ERbeta1 in regulation of growth, apoptosis and gene expression of two breast cancer cell lines with different ERalpha status. Overexpression of ERbeta1, but not of the exon-deleted variants exerted strong antitumoral effects both on ERalpha-positive MCF-7 and ERalpha-negative SK-BR-3 cells. ERbeta1 overexpression slowed growth of MCF-7 and SK-BR-3 cells in the absence of E2 and also inhibited E2-triggered growth stimulation of MCF-7 cells, but overexpression of the exon-skipped variants did not affect cell growth. Whereas overexpression of ERbeta1 triggered an increased basal and tamoxifen-induced apoptosis of MCF-7 and SK-BR-3 cells, the isoforms ERbetadelta125 or ERbetadelta1256 did not affect cellular tamoxifen response. The observed lack of function of the exon-deleted variants in terms of regulation of proliferation was accompanied both by their inability to affect expression of cyclins D1 and A2, p21 (WAF1) and PR and their disability to modulate estrogen response element (ERE) activation. In contrast, our results demonstrating antitumoral effects of ERbeta1 on breast cancer cells with different ERalpha-status support the hypothesis that ERbeta is able to exert antitumoral actions both on ERalpha-positive and -negative breast cancer cells.

Phylogenetic conservation of the androgen receptor AR45 variant form in placental mammals

A cDNA coding for a tissue-specific AR45 variant form of the androgen receptor (AR) has recently been identified in humans, with highest expression levels found in heart. The deduced protein comprises the DNA-binding domain, hinge region and ligand-binding domain of the AR, but not the N-terminal domain which is replaced by a unique, short, seven amino-acid-long stretch. This sequence is encoded by the mutually exclusive exon 1B, located between exons 1 and 2 of the human AR gene. As transcript variants of the steroid receptor family have been shown to have important implications for hormone function, we set out to analyse the genomes of different organisms for potential AR45 expression. We found exon 1B to be conserved in the syntenic chromosomal region of non-human primates such as the chimpanzee Pan troglodytes, the orang-utan Pongo pygmaeus, the macaque Macaca mulatta and the marmoset Callithrix jacchus, and of the elephant Loxondonta africana, the pig Sus scrofa and the dog Canis familiaris. Quantification of AR45 transcript levels in heart, skeletal muscle and lung of Macaca fascicularis showed the heart to be the main organ of expression. A complete AR45 cDNA was furthermore isolated from the heart of this species. Comparative analysis of the identified AR45 exon 1B regions and of the deduced amino acids revealed a high conservation among species. The four N-terminal residues were identical in all eight species, whereas a few changes were seen in the other three residues in the marmoset, elephant and pig. In contrast, we observed more divergence in the mouse Mus musculus and rat Rattus norvegicus syntenic regions. Here a stop codon was found downstream of the potential start codon in the putatively deduced protein sequence and it can be inferred that no protein corresponding to AR45 exists in these two species. The existence of AR45 in different placental mammals with the exception of mouse and rat suggests a disappearance in rodents late in evolution, before the separation of the mouse and rat lineages, about 16 million years ago. In view of the potential function of AR45 as a regulator of AR function, and considering the multiple roles of androgens in normal physiology and in several diseases, these findings have important implications with regard to subtle differences in the action of the male sexual hormone in various organisms.

Genetics and pharmacogenetics of estrogen response

Estrogens are a steroid hormone group distributed widely in animals and human beings. Estrogens diffuse across cell phospholipidic membranes and interact with estrogen receptors. Their highest concentration is found in target tissues with reproductive function (breast, ovary, vagina and uterus). High estrogen levels are usually associated with tumor onset and progression, while loss of estrogen or its receptor(s) contributes to development and/or progression of various diseases (osteoporosis, neurodegenerative disease and cardiovascular disease). Despite the numerous efforts to highlight estrogen's mechanism of action, recent discoveries showed an unexpected degree of complexity of estrogenic response.

Estrogen Receptor β2 Negatively Regulates the Transactivation of Estrogen Receptor α in Human Breast Cancer Cells

Estrogens, by binding to and activating two estrogen receptors (ERalpha and ERbeta), are critically involved in the development of the mammary gland and breast cancer. An isoform of ERbeta, ERbeta2 (also called ERbetacx), with an altered COOH-terminal region, is coexpressed with ERalpha in many human breast cancers. In this study, we generated a stable cell line from MCF7 breast cancer cells expressing an inducible version of ERbeta2, along with endogenous ERalpha, and examined the effects of ERbeta2 on the ERalpha protein levels and function. We showed that ERbeta2 inhibited ERalpha-mediated transactivation via estrogen response element and activator protein-1 sites of reporter constructs as well as the endogenous genes pS2 and MMP-1. Chromatin immunoprecipitation assays revealed that ERbeta2 expression caused a significant reduction in the recruitment of ERalpha to both the pS2 and MMP-1 promoters. Furthermore, ERbeta2 expression induced proteasome-dependent degradation of ERalpha. The inhibitory effects of ERbeta2 on ERalpha activity were further confirmed in HEK293 cells that lack functional endogenous ERs. We also showed that ERbeta2 can interact with ERalpha both in vitro and in mammalian cells, which is compatible with a model where ERbeta2/ERalpha heterodimers are targeted to the proteasome. Finally, in human breast cancer samples, we observed that expression of ERbeta2 significantly correlated with ERalpha-negative phenotype. Our data suggest that ERbeta2 could influence ERalpha-mediated effects relevant for breast cancer development, including hormone responsiveness.

Novel estrogen receptor beta transcript variants identified in human breast cancer cells affect cell growth and apoptosis of COS-1 cells

Estrogen receptor (ER) beta gene codes for different transcript variants resulting from alternative splicing. In this study, we report identification of the two novel human exon-skipped ERbeta transcript isoforms ERbetaDelta125 and ERbetaDelta1256 in MDA-MD-231 breast cancer cells. Both transcripts could also be detected in a variety of human tissues. We further report the results of an in vitro attempt to characterize their function in regulation of cell growth, motility, apoptosis and gene expression. COS-1 cells stably transfected with the novel ERbeta transcripts exhibited a notably slower growth even in the absence of estradiol when compared to vector-transfected control cells. Like ERbeta1, both novel ERbeta transcript isoforms raised the basal apoptosis rate of COS-1 cells in a ligand-independent manner. Whereas introduction of ERbetaDelta1256 notably increased the sensitivity of COS-1 cells towards lower concentrations of selective estrogen receptor modulator tamoxifen, presence of ERbeta1 and ERbetaDelta125 transcripts further weakened the growth-inhibitory effect of tamoxifen on this cell line. Furthermore, expression of ERbetaDelta1256 variant was demonstrated to reduce transcript levels of estrogen-responsive genes like cyclin A2, IGFBP-4 and fibulin 1c in COS-1 cells in a ligand-independent manner. Though we were not able to detect the predicted 29 and 34kDa proteins by means of western blot analysis, our data strongly suggest the biological functionality of both isoforms on molecular level. With this report increasing the multitude of existing ERbeta mRNA isoforms, we provide further evidence that their synthesis has to be considered as an important level of estrogen signaling.

Survey of the year 2005 commercial optical biosensor literature

We identified 1113 articles (103 reviews, 1010 primary research articles) published in 2005 that describe experiments performed using commercially available optical biosensors. While this number of publications is impressive, we find that the quality of the biosensor work in these articles is often pretty poor. It is a little disappointing that there appears to be only a small set of researchers who know how to properly perform, analyze, and present biosensor data. To help focus the field, we spotlight work published by 10 research groups that exemplify the quality of data one should expect to see from a biosensor experiment. Also, in an effort to raise awareness of the common problems in the biosensor field, we provide side-by-side examples of good and bad data sets from the 2005 literature.