Quantitative analysis of estrogen receptor proteins in rat mammary gland

Estrogen receptor subtypes dictate the proliferative nature of the mammary gland

Estrogen induces proliferation of breast epithelial cells and is responsible for breast development at puberty. This tightly regulated control is lost in estrogen-receptor-positive (ER+) breast cancers, which comprise over 70% of all breast cancers. Currently, breast cancer diagnosis and treatment considers only the α isoform of ER; however, there is a second ER, ERβ. Whilst ERα mediates estrogen-driven proliferation of the normal breast in puberty and breast cancers, ERβ has been shown to exert an anti-proliferative effect on the normal breast. It is not known how the expression of each ER (alone or in combination) correlates with the ability of estrogen to induce proliferation in the breast. We assessed the levels of each ER in normal mouse mammary glands subdivided into proliferative and non-proliferative regions. ERα was most abundant in the proliferative regions of younger mice, with ERβ expressed most abundantly in old mice. We correlated this expression profile with function by showing that the ability of estrogen to induce proliferation was reduced in older mice. To show that the ER profile associated with breast cancer risk, we assessed ER expression in parous mice which are known to have a reduced risk of developing ERα breast cancer. ERα expression was significantly decreased yet co-localization analysis revealed ERβ expression increased with parity. Parous mice had less unopposed nuclear ERα expression and increased levels of ERβ. These changes suggest that the nuclear expression of ERs dictates the proliferative nature of the breast and may explain the decreased breast cancer risk with parity.

Pregnancy and Breast Cancer

Breast cancer is the most commonly diagnosed type of cancer among women worldwide. The majority of breast cancers are sporadic and the etiology is not well understood. Several factors have been attributed to altering the risk of breast cancer. A full-term pregnancy is a crucial factor in altering the risk. Early full-term pregnancy has been shown to reduce the lifetime risk of breast cancer, while a later first full-term pregnancy increases breast cancer risk. Epidemiological and experimental data demonstrate that spontaneous or induced abortions do not significantly alter the risk of breast cancer. In this study, we briefly discuss the different types and stages of breast cancer, various risk factors, and potential mechanisms involved in early full-term pregnancy-induced protection against breast cancer. Understanding how early full-term pregnancy induces protection against breast cancer will help design innovative preventive and therapeutic strategies. This understanding can also help in the development of molecular biomarkers that can be of tremendous help in predicting the risk of breast cancer in the general population.

Silencing Estrogen Receptor-β with siRNA in Cultured Cells

Estrogen receptors α and β (ERα and ERβ) are the two genomic estrogen receptors. ERβ was the second of these receptors to be discovered; its structure is similar to that of ERα but they are different in histological distribution. However, the functions of ERα versus ERβ are still unclear. The ability of small interfering RNAs (siRNAs) to silence gene expression has proven to be invaluable for studying gene function in cultured mammalian cells. This chapter describes the use of siRNA to inhibit the expression of ERβ in renal cell carcinoma (RCC) and to further the understanding of ERβ function in RCC.

Sex hormone receptor repertoire in breast cancer

Classification of breast cancer as endocrine sensitive, hormone dependent, or estrogen receptor (ER) positive refers singularly to ER α . One of the oldest recognized tumor targets, disruption of ER α -mediated signaling, is believed to be the mechanistic mode of action for all hormonal interventions used in treating this disease. Whereas ER α is widely accepted as the single most important predictive factor (for response to endocrine therapy), the presence of the receptor in tumor cells is also of prognostic value. Even though the clinical relevance of the two other sex hormone receptors, namely, ER β and the androgen receptor remains unclear, two discordant phenomena observed in hormone-dependent breast cancers could be causally related to ER β -mediated effects and androgenic actions. Nonetheless, our understanding of regulatory molecules and resistance mechanisms remains incomplete, further compromising our ability to develop novel therapeutic strategies that could improve disease outcomes. This review focuses on the receptor-mediated actions of the sex hormones in breast cancer.

Molecular effects of ER alpha- and beta-selective agonists on regulation of energy homeostasis in obese female Wistar rats

The molecular mechanisms underlying the effects of selective ER subtype activation on lipogenesis, adipogenesis, lipid utilization and storage as well as glucose metabolism are currently largely unknown and were analyzed in female OVX Wistar rats on a high-fat diet. Rats received estradiol (E2), ER subtype-selective agonists (Alpha and Beta), and genistein (Gen) for 10 weeks. In adipose tissue, treatment with E2, Alpha, and Beta significantly decreased lipogenic (SREBP-1c, FAS) and adipogenic genes (LPL, PPAR gamma). In liver and skeletal muscle of E2-, Alpha-, Beta-, and Gen-treated animals, lipogenesis and triglyceride accumulation were significantly reduced. Increased hepatic and muscular PPAR gamma mRNA expression was observed in untreated, Beta- and Gen-treated animals, which correlates with increased hepatic glucose uptake. However, only untreated animals showed impaired insulin sensitivity compared to all other groups. Therefore, PPAR gamma up-regulation in untreated animals suggests a compensatory mechanism, probably due to increased triglyceride accumulation in non-adipose tissues. Beta- and Gen-treated animals may benefit from the anabolic potency of ER beta that ameliorates lipid and glucose utilization in muscle. Activation of either ER subtype reduces fat enrichment and improves insulin sensitivity. Depending on the investigated tissue, different molecular pathways seem to be involved.

Small-animal PET of steroid hormone receptors predicts tumor response to endocrine therapy using a preclinical model of breast cancer

Estrogen receptor-α (ERα) and progesterone receptor (PR) are expressed in most human breast cancers and are important predictive factors for directing therapy. Because of de novo and acquired resistance to endocrine therapy, there remains a need to identify which ERα-positive (ERα(+))/PR-positive (PR(+)) tumors are most likely to respond. The purpose of this study was to use estrogen- and progestin-based radiopharmaceuticals to image ERα and PR in mouse mammary tumors at baseline and after hormonal therapy and to determine whether changes in these imaging biomarkers can serve as an early predictive indicator of therapeutic response.

METHODS

Mammary adenocarcinomas that spontaneously develop in aged female mice deficient in signal transducer and activator of transcription-1 (STAT1) were used. Imaging of ERα and PR in primary tumor-bearing mice and mice implanted with mammary cell lines (SSM1, SSM2, and SSM3) derived from primary STAT1-deficient (STAT1(-/-)) tumors was performed. Hormonal treatments consisted of estradiol, an ER agonist; letrozole, an aromatase inhibitor; and fulvestrant, a pure ER antagonist. Small-animal PET/CT was performed using (18)F-fluoroestradiol ((18)F-FES) for ER, (18)F-fluoro furanyl norprogesterone ((18)F-FFNP) for PR, and (18)F-FDG for glucose uptake. Tracer uptake in the tumor was quantified and compared with receptor concentration determined by in vitro assays of resected tumors.

RESULTS

Primary STAT1(-/-) mammary tumors and implanted SSM2 and SSM3 tumors showed high (18)F-FES and (18)F-FFNP uptake and were confirmed to be ERα(+)/PR(+). Classic estrogen-induced regulation of the progesterone receptor gene was demonstrated by increased (18)F-FFNP uptake of estradiol-treated SSM3 tumors. Treatment with fulvestrant decreased (18)F-FFNP, (18)F-FES, and (18)F-FDG uptake and inhibited growth of SSM3 tumors but decreased only (18)F-FES uptake in SSM2 tumors, with no effect on growth, despite both tumors being ERα(+)/PR(+). Decreased (18)F-FFNP uptake by SSM3 tumors occurred early after initiation of treatment, before measurable tumor growth inhibition.

CONCLUSION

Using small-animal PET, a profile was identified that distinguished fulvestrant-sensitive from fulvestrant-resistant ERα(+)/PR(+) tumors before changes in tumor size. This work demonstrates that imaging baseline tumoral (18)F-FES uptake and initial changes in (18)F-FFNP uptake in a noninvasive manner is a potentially useful strategy to identify responders and nonresponders to endocrine therapy at an early stage.

Serum estradiol levels associated with specific gene expression patterns in normal breast tissue and in breast carcinomas

BACKGROUND

High serum levels of estradiol are associated with increased risk of postmenopausal breast cancer. Little is known about the gene expression in normal breast tissue in relation to levels of circulating serum estradiol.

METHODS

We compared whole genome expression data of breast tissue samples with serum hormone levels using data from 79 healthy women and 64 breast cancer patients. Significance analysis of microarrays (SAM) was used to identify differentially expressed genes and multivariate linear regression was used to identify independent associations.

RESULTS

Six genes (SCGB3A1, RSPO1, TLN2, SLITRK4, DCLK1, PTGS1) were found differentially expressed according to serum estradiol levels (FDR = 0). Three of these independently predicted estradiol levels in a multivariate model, as SCGB3A1 (HIN1) and TLN2 were up-regulated and PTGS1 (COX1) was down-regulated in breast samples from women with high serum estradiol. Serum estradiol, but none of the differentially expressed genes were significantly associated with mammographic density, another strong breast cancer risk factor. In breast carcinomas, expression of GREB1 and AREG was associated with serum estradiol in all cancers and in the subgroup of estrogen receptor positive cases.

CONCLUSIONS

We have identified genes associated with serum estradiol levels in normal breast tissue and in breast carcinomas. SCGB3A1 is a suggested tumor suppressor gene that inhibits cell growth and invasion and is methylated and down-regulated in many epithelial cancers. Our findings indicate this gene as an important inhibitor of breast cell proliferation in healthy women with high estradiol levels. In the breast, this gene is expressed in luminal cells only and is methylated in non-BRCA-related breast cancers. The possibility of a carcinogenic contribution of silencing of this gene for luminal, but not basal-like cancers should be further explored. PTGS1 induces prostaglandin E2 (PGE2) production which in turn stimulates aromatase expression and hence increases the local production of estradiol. This is the first report studying such associations in normal breast tissue in humans.

Effects of raloxifene on the mammary epithelium of female rats in persistent estrus

OBJECTIVE

The aim of this study was to evaluate the morphological and morphometric alterations produced by raloxifene in the mammary epithelium of female rats in persistent estrus.

MATERIALS AND METHODS

Twenty-one female Wistar-Hannover rats in persistent estrus induced by 1.25 mg of testosterone propionate were randomly divided into two groups: Group I (n = 10), receiving only water and used as control; Group II (experimental, n = 11), treated with 3 mg of raloxifene daily for 21 days. The first abdominoinguinal pair of mammary glands was extirpated and processed for morphological and morphometric evaluation. The data were statistically analysed using Student's t-test (p < 0.05).

RESULTS

Morphology revealed signs of epithelial atrophy, and morphometry showed a significant reduction in the mean number of ducts and alveoli in the experimental group (12.82 +/- 0.42 and 2.91 +/- 0.53, respectively) when compared with the control group (28.70 +/- 1.15 and 7.20 +/- 0.57, respectively). This difference was statistically significant, both for the ducts and for the alveoli (p < 0.001).

CONCLUSION

The present results indicate that, at the dose and during the time of treatment used, raloxifene induced atrophy of the mammary epithelium of rats in persistent estrus.

The localization and non-genomic function of the membrane-associated estrogen receptor in oligodendrocytes

There is general acceptance that the estrogen receptor can act as a transcription factor. However, estrogens can also bind to receptors that are located at the plasma membrane and stimulate rapid intracellular signaling processes. We recently showed that a membrane-associated estrogen receptor (mER) is present within myelin and at the oligodendrocyte (OL) plasma membrane. To understand the physiological function of mER in OLs, we investigated its cellular localization and involvement in rapid signaling in CG4 cells and OL primary cultures. An ERalpha was expressed along the lacy network of veins in the membrane sheets and in the perikaryon and nucleus in OLs. ERbeta was located in the nucleus, and to a lesser extent along the veins. The expression of ERalpha and ERbeta in OL membranes was confirmed by Western analysis of isolated membranes. OL membranes mainly had truncated forms of ERalpha, 53 and 50 kDa, in addition to some 65 kDa form, whereas ERbeta was a 54 kDa form. CG4 cells and OLs were pulsed with 17alpha- and 17beta-estradiol for various times and the total lysates were analyzed for phosphorylated kinases. Both 17alpha- and 17beta-estradiol elicited rapid phosphorylation of p42/44MAPK, Akt, and GSK-3beta within 8 min. This rapid signaling is consistent with estradiol ligation of a membrane form of ER. Since 17alpha-estradiol is produced at higher concentrations than 17beta-estradiol in the brain of both sexes, signaling via 17alpha-estradiol-liganded mER may have an important function in OLs.

A comparison of the effects of soya isoflavonoids and fish oil on cell proliferation, apoptosis and the expression of oestrogen receptors alpha and beta in the mammary gland and colon of the rat

Isoflavonoids and fish oil may be protective against colorectal cancer, but the evidence in relation to breast cancer risk is ambiguous. In the present study, we have investigated the impact of soya-derived isoflavonoids and n-3 fatty acids from fish oil, both individually and in combination, on apoptosis, cell proliferation and oestrogen receptor (ER) expression in the colon and mammary gland of the rat. Female rats were fed diets high in n-3 fatty acids (80 g/kg diet) or soya protein (765 mg/kg diet isoflavones) for 2 weeks, and then killed before the removal of the colon and mammary glands. Cell proliferation and apoptosis were quantified morphologically in whole crypts and terminal end buds. The expressions of ERalpha and ERbeta were measured in colon tissue scrapes and the mammary gland. Fish oil significantly increased apoptosis and decreased mitosis in both tissues, an effect associated with a decrease in the expressions of ERalpha and ERbeta. Soya had no effect on apoptosis in either tissue, but reduced mitosis in the colon (P < 0.001) while increasing it in the mammary gland (P = 0.001). The changes in proliferation were associated with contrasting changes in the ER expression such that fish oil significantly decreased both ERbeta and ERalpha, while soya increased ERalpha and decreased ERbeta. The results may provide a novel mechanism by which n-3 fatty acids could reduce cancer risk, but the interpretation of the results in relation to soya consumption and breast cancer risk requires further investigation.

Artemisinin selectively decreases functional levels of estrogen receptor-alpha and ablates estrogen-induced proliferation in human breast cancer cells

MCF7 cells are an estrogen-responsive human breast cancer cell line that expresses both estrogen receptor (ER) alpha and ERbeta. Treatment of MCF7 cells with artemisinin, an antimalarial phytochemical from the sweet wormwood plant, effectively blocked estrogen-stimulated cell cycle progression induced by either 17beta-estradiol (E(2)), an agonist for both ERs, or by propyl pyrazole triol (PPT), a selective ERalpha agonist. Artemisinin strongly downregulated ERalpha protein and transcripts without altering expression or activity of ERbeta. Transfection of MCF7 cells with ERalpha promoter-linked luciferase reporter plasmids revealed that the artemisinin downregulation of ERalpha promoter activity accounted for the loss of ERalpha expression. Artemisinin treatment ablated the estrogenic induction of endogenous progesterone receptor (PR) transcripts by either E(2) or PPT and inhibited the estrogenic stimulation of a luciferase reporter plasmid driven by consensus estrogen response elements (EREs). Chromatin immunoprecipitation assays revealed that artemisinin significantly downregulated the level of endogeneous ERalpha bound to the PR promoter, whereas the level of bound endogeneous ERbeta was not altered. Treatment of MCF7 cells with artemisinin and the pure antiestrogen fulvestrant resulted in a cooperative reduction of ERalpha protein levels and enhanced G(1) cell cycle arrest compared with the effects of either compound alone. Our results show that artemisinin switches proliferative human breast cancer cells from expressing a high ERalpha:ERbeta ratio to a condition in which ERbeta predominates, which parallels the physiological state linked to antiproliferative events in normal mammary epithelium.

17beta-estradiol induces the translocation of the estrogen receptors ESR1 and ESR2 to the cell membrane, MAPK3/1 phosphorylation and proliferation of cultured immature rat Sertoli cells

The aim of the present study was to determine the mechanisms involved in estrogen actions in cultured rat Sertoli cells. RT-PCR detected transcripts for the estrogen receptors ESR1 and ESR2 in cultured immature Sertoli cells and in the testis of 15-, 28-, and 120-day-old rats. The expression of ESR1 and ESR2 was confirmed in Sertoli cells by immunofluorescence and Western blot. Immunohistochemistry with cryosections of testes from immature and adult rats revealed that ESR1 is present in Sertoli, Leydig, and some peritubular myoid cells, and ESR2 is present in multiple cell types, including germ cells. Treatment of Sertoli cells with 17beta-estradiol (E(2)) induced a translocation of ESR1 and ESR2 to the plasma membrane and a concomitant phosphorylation of MAPK3/1. Both effects reached a maximum after 10 min and were blocked by PP2, an inhibitor of the SRC family of protein tyrosine kinases, and by the antiestrogen ICI 182,780 (ICI). MAPK3/1 phosphorylation was also decreased in the presence of AG 1478, an inhibitor of the epidermal growth factor receptor (EGFR) kinase, and in the presence of MAP2K1/2 inhibitor UO126. Treatment with E(2) for 24 h increased the incorporation of [methyl-(3)H]thymidine, which was blocked by ICI. These results indicate that E(2) activates an SRC-mediated translocation of estrogen receptors to the plasma membrane, which results in the activation of EGFR and the mitogen-activated protein kinase signaling pathway. In addition, activation of ESR1 and/or ESR2 by E(2) is involved in proliferation of immature Sertoli cells. The estrogen actions in Sertoli cells might be a key step mediating cellular events important for spermatogenesis and fertility.

Regulation of gene expression in the bovine mammary gland by ovarian steroids

It is well established that estrogen is required for mammary epithelial cell proliferation and ductal development in the growing animal, and that lobuloalveolar development during gestation is dependent on progesterone. The effects of these steroid hormones on gene expression in the mammary gland are mediated primarily by their respective nuclear hormone receptors, which function as hormone-bound transcription factors. To gain insight into how estrogen and progesterone regulate mammary gland growth and function in cattle, we and others have characterized the expression patterns of their cognate nuclear hormone receptors in the bovine mammary gland throughout development, pregnancy, and lactation. This work has identified a lack of expression of estrogen receptor beta and a greater abundance of progesterone receptor during lactation in the bovine mammary gland, compared with the rodent gland. We speculate that interactions among the estrogen receptor isoforms that regulate progesterone receptor expression may contribute to these species differences. Further, demonstrated expression of substantial quantities of estrogen receptor within the prepubertal bovine mammary fat pad, along with coordinated insulin-like growth factor-I expression, suggests that this tissue may stimulate parenchymal growth via an estrogen-responsive paracrine mechanism. In addition, the recent availability of bovine genomic sequence information and microarray technologies has permitted the study of global gene expression in the mammary gland in response to the steroid environment. We have identified more than 100 estrogen-responsive genes, of which the majority are novel estrogen gene targets. Estrogen-induced changes in gene expression were consistent with increased mammary epithelial cell proliferation, increased extracellular matrix turnover in parenchyma, and increased extracellular matrix deposition in the fat pad. A comparison of estrogen-responsive genes in the mammary glands of humans, mice, and cattle suggests considerable variation among species, as well as potential differences in regulatory elements in common estrogen receptor gene targets. Continuing studies using advanced molecular techniques should assist in elucidating the complex regulation of mammary function at the transcript level.

Indole-3-Carbinol Selectively Uncouples Expression and Activity of Estrogen Receptor Subtypes in Human Breast Cancer Cells

Estrogen-responsive breast cancer cells, such as MCF7 and T47D cells, express both estrogen receptor (ER)-α (ERα) and ERβ. Indole-3-carbinol (I3C) strongly down-regulated ERα protein and transcript levels, without altering the level of ERβ protein, in both cell lines. In cells transfected with the ERα promoter linked to a luciferase gene reporter, I3C ablated ERα promoter activity. Propyl pyrazole triol (PPT) is a highly selective ERα agonist, whereas, 17β-estradiol activates both ERα and ERβ. I3C treatment inhibited the PPT- and 17β-estradiol-induced proliferation of breast cancer cells, disrupted the PPT and 17β-estradiol stimulation of estrogen response element (ERE)-driven reporter plasmid activity as well as of endogenous progesterone receptor transcripts. Using an in vitro ERE binding assay, I3C was shown to inhibit the level of functional ERα and stimulated the level of ERE binding ERβ even though the protein levels of this receptor remained constant. In ERα−/ERβ+ MDA-MB-231 breast cancer cells, I3C treatment stimulated a 6-fold increase in binding of ERβ to the ERE. I3C also induced ERE- and activator protein 1-driven reporter plasmid activities in the absence of an ER agonist, suggesting that ERβ is activated in indole-treated cells. Taken together, our results demonstrate that the expression and function of ERα and ERβ can be uncoupled by I3C with a key cellular consequence being a significantly higher ERβ:ERα ratio that is generally highly associated with antiproliferative status of human breast cancer cells.

Characterization and regulation of the gene expression of amino acid transport system A (SNAT2) in rat mammary gland

Amino acid transport via system A plays an important role during lactation, promoting the uptake of small neutral amino acids, mainly alanine and glutamine. However, the regulation of gene expression of system A [sodium-coupled neutral amino acid transporter (SNAT)2] in mammary gland has not been studied. The aim of the present work was to understand the possible mechanisms of regulation of SNAT2 in the rat mammary gland. Incubation of gland explants in amino acid-free medium induced the expression of SNAT2, and this response was repressed by the presence of small neutral amino acids or by actinomycin D but not by large neutral or cationic amino acids. The half-life of SNAT2 mRNA was 67 min, indicating a rapid turnover. In addition, SNAT2 expression in the mammary gland was induced by forskolin and PMA, inducers of PKA and PKC signaling pathways, respectively. Inhibitors of PKA and PKC pathways partially prevented the upregulation of SNAT2 mRNA during adaptive regulation. Interestingly, SNAT2 mRNA was induced during pregnancy and to a lesser extent at peak lactation. beta-Estradiol stimulated the expression of SNAT2 in mammary gland explants; this stimulation was prevented by the estrogen receptor inhibitor ICI-182780. Our findings clearly demonstrated that the SNAT2 gene is regulated by multiple pathways, indicating that the expression of this amino acid transport system is tightly controlled due to its importance for the mammary gland during pregnancy and lactation to prepare the gland for the transport of amino acids during lactation.

The hair follicle as an estrogen target and source

For many decades, androgens have dominated endocrine research in hair growth control. Androgen metabolism and the androgen receptor currently are the key targets for systemic, pharmacological hair growth control in clinical medicine. However, it has long been known that estrogens also profoundly alter hair follicle growth and cycling by binding to locally expressed high-affinity estrogen receptors (ERs). Besides altering the transcription of genes with estrogen-responsive elements, 17beta-estradiol (E2) also modifies androgen metabolism within distinct subunits of the pilosebaceous unit (i.e., hair follicle and sebaceous gland). The latter displays prominent aromatase activity, the key enzyme for androgen conversion to E2, and is both an estrogen source and target. Here, we chart the recent renaissance of estrogen research in hair research; explain why the hair follicle offers an ideal, clinically relevant test system for studying the role of sex steroids, their receptors, and interactions in neuroectodermal-mesodermal interaction systems in general; and illustrate how it can be exploited to identify novel functions and signaling cross talks of ER-mediated signaling. Emphasizing the long-underestimated complexity and species-, gender-, and site-dependence of E2-induced biological effects on the hair follicle, we explore targets for pharmacological intervention in clinically relevant hair cycle manipulation, ranging from androgenetic alopecia and hirsutism via telogen effluvium to chemotherapy-induced alopecia. While defining major open questions, unsolved clinical challenges, and particularly promising research avenues in this area, we argue that the time has come to pay estrogen-mediated signaling the full attention it deserves in future endocrinological therapy of common hair growth disorders.

Progestin Activation of Nongenomic Pathways via Cross Talk of Progesterone Receptor with Estrogen Receptor β Induces Proliferation of Endometrial Stromal Cells

Uterine decidualization is characterized by stromal cell proliferation and differentiation, which are controlled by ovarian hormones estradiol and progesterone. Here we report that the proliferative response of UIII rat uterine stromal cells to a short treatment with progestins requires active progesterone receptor (PR) and estrogen receptor beta (ERbeta) as well as a rapid and transient activation of Erk1-2 and Akt signaling. The optimal R5020 concentration for the proliferative response as well as for activation of the signaling cascades was between 10 and 100 pm. UIII cells are negative for ERalpha and have low levels of ERbeta and PR located mainly in the cytoplasm. Upon progestin treatment PR translocated to the cell nucleus where it colocalized with activated Erk1-2. Neither progestins nor estradiol transactivated the corresponding transfected reporter genes, suggesting that endogenous PR and ERbeta are transcriptionally incompetent. A fraction of endogenous PR and ERbeta form a complex as demonstrated by coimmunoprecipitation. Taken together, our results suggest that the proliferative response of uterine stromal cells to picomolar concentrations of progestins does not require direct transcriptional effects and is mediated by activation of the Erk1-2 and Akt signaling pathways via cross talk between PR and ERbeta.

ERbeta protein expression in female cynomolgus monkey and CF-1 mouse brain: Western analysis

In humans and rodents, multiple ERbeta variants with sizes ranging from 477-549 amino acids (aa) have been described. The identification of these variants in target tissues has important implications for estrogen signaling and cellular responsiveness. Western blot analysis using two anti-ERbeta antibodies specific for mammalian ERbeta sequences (PA1-310B and PA1-311) was employed to examine ERbeta protein expression in neural tissues from ovariectomized (OVX) cynomolgus macaques and CF-1 mice as well as to assess potential regulatory effects of acute and extended estradiol (E(2)) treatment. In hypothalamic extracts from both species, a single ERbeta immunoreactive (ERbeta-ir) band was detected at approximately 54 kDa, corresponding to the expected molecular weight for ERbeta477 and/or 485. In cynomolgus females, oral E(2) administration for 16 weeks had no apparent effect on hypothalamic ERbeta protein expression. In mouse, a single injection of E(2) did not change hypothalamic ERbeta protein levels 1.5, 4, 8, 16, or 24 h after injection. Extending the hormonal treatment to 4 or 21 days in OVX female mice also had no effect on the level of hypothalamic ERbeta protein. Additional regional analyses in female mouse brain with PA1-310B antibody showed that a second, 59 kDa ERbeta-ir band was present in cortex, striatum, hippocampus, and amygdala that could represent one or both of the larger ERbeta variants (530 and 549aa). The expression level of the second ERbeta isoform exhibited regional variation, with the strongest immunoreactivity detected in cortex and amygdala. Elucidating the functions of these ERbeta isoforms in the CNS will facilitate our understanding of the tissue- and promoter-specific actions of estrogen.

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

Estrogens exert their physiological effects through two estrogen receptor (ER) subtypes, ERalpha and ERbeta. In mouse, the cloning of an alternative splice variant of the wild-type ERbeta (mERbeta1), mERbeta2, which contains an 18 amino acid insertion in the ligand binding domain, contributed an additional level of complexity to estrogen signaling. In this study we have assayed the interaction of several known ligands with mERbeta1 and mERbeta2. The binding affinity of estradiol was 14-fold higher for mERbeta1 than for mERbeta2. In contrast, raloxifene was dramatically (8-fold) mERbeta2 selective. The selectivity for mERbeta2 was abolished when the 2-arylbenzothiophene core of the raloxifene molecule was tested for binding affinity, demonstrating that the 3-aroyl side chain of raloxifene plays an important role in contributing to its mERbeta2 selectivity. The opposite isoform selectivity found for estradiol and raloxifene in our ligand binding assay was also reflected in the transactivation assay system. That is, mERbeta2 required 10-fold greater estradiol concentrations for maximal activation compared to mERbeta1, whereas raloxifene was more potent in antagonizing estradiol-induced gene expression via mERbeta2 than mERbeta1. The raloxifene core behaved as a pure agonist. Furthermore, mERbeta2 showed significantly decreased estradiol-induced maximal transcriptional activity as compared to mERbeta1. A pull-down assay indicated that the interactions of TIF2 and RAP250 with mERbeta2 were weaker than with mERbeta1. To assess TIF2 and RAP250 interactions with ERs more quantitatively, we examined the interaction of LXXLL containing peptides derived from TIF2 and RAP250 with mERbeta1 and mERbeta2 using surface plasmon resonance analysis. Our results indicate that mERbeta2 interacts with both coactivators with lower affinity, which may explain its reduced transcriptional activity. Taken together, these results suggest that ligand selectivity and coactivator recruitment of the ERbeta isoforms constitute additional levels of specificity that influence the transcriptional response in estrogen target cells.

Aromatase inhibitors and other novel agents in breast cancer treatment

Thirty years after the introduction of tamoxifen, which was expanded from palliation of metastatic cancer to recent application for chemoprevention, the primacy of this drug as the mainline pharmacological intervention is currently being challenged by the third generation aromatase inhibitors and inactivators. In contrast to the oestrogen receptor blockade provided by tamoxifen, aromatase inhibitors result in deprivation of oestrogens in postmenopausal women both through paracrine/intracrine and endocrine modulation. Experimental evidence has shown a significant (97-99%) reduction of in vivo aromatase activity and an equal or sometimes better antitumour activity compared with megestrol acetate when these drugs are used as second-line treatment for metastatic breast cancer. Recent pivotal studies in first-line settings comparing tamoxifen for metastatic breast cancer and preliminary results from the neoadjuvant trials demonstrate that third generation aromatase inhibitors are superior to tamoxifen. With a better understanding of local tissue production of oestrogen through oestrone sulfatase, which hydrolyses oestrone sulfate to oestrone, and 17-beta-hydroxysteroid dehydrogenase Type 1, which in turn catalyses the reduction of oestrone to oestradiol, more powerful tactics for oestrogen starvation of cancer may be realised in future.

The expression of estrogen, prolactin, and progesterone receptors in mammary gland and liver of female rats during pregnancy and early postpartum: regulation by thyroid hormones

The aim of this study was to examine, using semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) the changes in mRNA expression of the two estrogen receptor (ER) subtypes, ERalpha and ERbeta, prolactin receptor long and short form, and progesterone (Pg) receptor (PgR), in liver and mammary gland during gestation, early lactation, and weaning in both hyperthyroid (HT) and normal rats. Pregnancy increased long prolactin receptors (PRL-R(L)) and ERalpha mRNAs in liver and PRL-R(I) in mammary gland. Lactation decreased PRL-R(L) in liver and ERbeta and PgR in mammary gland. HT decreased PRL-R(L), at the end of pregnancy (G21), ERalpha (in G21 and L1) in liver and PRL-R(L) in L1 as well as short prolactin receptors (PRL-R(S)) (G7, L1) and ERbeta (G7, G14, L4) in mammary gland. In conclusion, our data indicated that (1) PRL-R1 and ERalpha expression levels are differentially regulated in the liver, and PgR and ERbeta in mammary gland during pregnancy and lactation (2) ERbeta is variably expressed depending on the state of thyroid hormones, however the ERalpha gene expression remained constant in mammary gland. (3) PRL-R1 mRNA expression is highly induced in the mammary gland during late pregnancy and abruptly declines on the first day of lactation for the HT rats.

Estrogen increases calcitonin gene-related peptide-immunoreactive sensory innervation of rat mammary gland

Estrogen plays important roles in preparing mammary tissue for lactation. However, estrogen also influences innervation in some tissues. We examined the effect of estrogen on peripheral innervation of mammary tissues of ovariectomized adult virgin female rats. Seven days after ovariectomy, 17beta-estradiol or placebo pellets were implanted subcutaneously, and tissues were harvested 1 week later. Estrogen treatment decreased mammary gland mass and adipocyte content, while ductal content increased and vascular composition was unaffected. Estrogen increased total areas occupied by nerves in mammary gland sections immunostained for the pan-neuronal marker protein gene product 9.5, and this increase persisted after normalizing for treatment-induced differences in gland mass. Although a significant increase in tyrosine hydroxylase-immunoreactive sympathetic nerve area was observed, no difference was detected following correction for differences in gland size, implying a conserved number of sympathetic nerves in the face of reduced gland volume. Calcitonin gene-related peptide-immunoreactive sensory nerve sectional area was also increased, and corrected nerve area remained 88% greater, indicating nerve proliferation during estrogen treatment. Total, sensory, and sympathetic innervation of the nipple and adjacent dermal tissue were unaffected by estrogen. We conclude that chronic estrogen elevation induces selective proliferation of rat mammary gland calcitonin gene-related peptide-containing nerves, which are associated primarily with blood vessels and are probably nociceptors. Because they are likely to subserve a vasodilatory function, increased innervation may promote increased blood flow necessary for milk formation during suckling. Moreover, these findings may help explain abundant anecdotal reports of increased breast sensitivity in humans under high estrogen conditions.

Mammary gland morphogenesis is enhanced by exposure to flaxseed or its major lignan during suckling in rats

The exposure of rats to 10% flaxseed (FS) or an equivalent level of its major lignan, secoisolariciresinol diglucoside (SDG), during suckling enhances mammary gland differentiation, which protects against mammary carcinogenesis at adulthood. We determined whether this diet-induced mammary gland differentiation is mediated through the estrogenic pathway via epidermal growth factor receptor (EGFR) and estrogen receptor (ER) signaling. Rats were fed the AIN-93G basal diet (BD) from day 7 of pregnancy until delivery and then randomized to consume BD, FS, or SDG during lactation. After weaning, female offspring were fed BD throughout the experiment. At postnatal day (PND) 21 and the proestrus phase on PND 49-51, mammary glands of offspring were analyzed for morphology, cell proliferation, and expression of EGFR, epidermal growth factor (EGF), transforming growth factor-alpha, ER-alpha, and ER-beta. At PND 21, compared with the BD control, the number of terminal end buds (TEBs) and terminal ducts were increased by FS, whereas mammary epithelial cell proliferation was increased by both FS and SDG, suggesting that mammary morphogenesis was enhanced. Epithelial EGFR and stromal fibroblast EGF were increased by SDG, whereas epithelial ER-beta was decreased by FS. Conversely, at PND 49-51, a lower number of TEBs but a higher ratio of lobules to TEBs with decreased expression of EGFR or EGF was observed in both treatment groups. EGFR expression was positively associated with EGF expression and cell proliferation in TEB epithelium at PND 21. Urinary lignans of lactating dams were related to their offspring's indices of mammary gland development. In conclusion, exposure to FS or SDG during suckling enhanced mammary gland morphogenesis by modulation of EGFR and ER signaling, which led to more differentiated mammary glands at PND 49-51. The physiological outcomes of FS and SDG were similar, which suggests that SDG is partly responsible for the mammary gland differentiation effect.

Effect of exogenous epidermal-like growth factors on mammary gland development and differentiation in the estrogen receptor-alpha knockout (ERKO) mouse

The development of the mouse mammary gland requires the interaction between several different ovarian and pituitary hormones such as estrogen, progesterone and prolactin as well as several locally-derived growth factors in the mammary gland such as epidermal growth factor (EGF), transforming growth factor alpha (TGFalpha), amphiregulin (AR) and heregulin (HRG). The focus of this study was to investigate the degree of mammary growth and differentiation in the adult, virgin mammary gland of wild type (wt) and estrogen receptor knockout (ERKO) females that lack estrogen receptor alpha (ERalpha) after reciprocal transplantation into the cleared mammary fat pad of virgin wt or ERKO mice. In addition, we assessed the local response of ERKO mammary tissue to TGFalpha or HRGbeta1 delivered from slow release-Elvax pellets. Our initial results indicated that when we transplanted virgin wt mammary tissue into ERKO mammary fat pads, mammary morphogenesis failed to occur. However, when transplanted virgin ERKO mammary tissue was transplanted into fat pads of virgin or pregnant wt mice, the development and differentiation of lobuloalveoli was readily observed. In addition, treatment of the virgin ERKO mammary gland with TGFalpha or HRGbeta1 stimulated ducts to undergo localized branching and growth and both growth factors induced secretory differentiation as evidenced by the production of milk proteins, caseins and/or whey acidic protein (WAP). The results from this study imply that in ERKO mammary tissue. ERKO ductal epithelium has the capacity to proliferate and differentiate in response to non-estrogenic, morphogenic stimuli.

Clinical impact of assay of estrogen receptor beta cx in breast cancer

Since 1996 when estrogen receptor beta(ER beta) was discovered, much effort has been devoted to the question of the value of ER beta as a prognostic and/or predictive factor in breast cancer and its potential as a novel target for pharmacological intervention. When estrogen receptors are applied on sucrose gradients and quantified by ligand binding, we found that in contrast to ER alpha, which has a narrow tissue distribution, ER beta is expressed in many tissues including both normal and malignant breast tissue. Receptor protein levels in tissues can also be measured from the intensities of bands after Western blotting and can be quantified when purified and quantified receptor is used as a standard. With this technique, we found that there were some tumors which had over 600 fmol/mg of ER beta protein but no detectable estradiol binding. In such tumors, RT-PCR analysis revealed that ER beta cx is the only ER beta isoform present. ER beta cx is a splice variant which utilizes an alternative exon 8. This change in the C-terminus results in very poor binding to estradiol (E2) and has a dominant negative effect on ER alpha function. Immunohistochemical analysis with an ER beta cx specific antibody in 115 ER alpha-positive breast cancers revealed that about half of the samples expressed ER beta cx protein. Initial analysis of samples from patients with preoperative tamoxifen treatment revealed that ER alpha-positive tumors expressing ER beta cx and lacking PR seemed to be resistant to the anti-estrogen. We conclude that, in order to better characterize breast cancers and design appropriate therapy for individual patients, assays for ER beta cx must be made available to clinicians.

Colocalization and ligand-dependent discrete distribution of the estrogen receptor (ER)alpha and ERbeta

To investigate the relationships between the loci expressing functions of estrogen receptor (ER)alpha and that of ERbeta, we analyzed the subnuclear distribution of ERalpha and ERbeta in response to ligand in single living cells using fusion proteins labeled with different spectral variants of green fluorescent protein. Upon activation with ligand treatment, fluorescent protein-tagged (FP)-ERbeta redistributed from a diffuse to discrete pattern within the nucleus, showing a similar time course as FP-ERalpha, and colocalized with FP-ERalpha in the same discrete cluster. Analysis using deletion mutants of ERalpha suggested that the ligand-dependent redistribution of ERalpha might occur through a large part of the receptor including at least the latter part of activation function (AF)-1, the DNA binding domain, nuclear matrix binding domain, and AF-2/ligand binding domain. In addition, a single AF-1 region within ERalpha homodimer, or a single DNA binding domain as well as AF-1 region within the ERalpha/ERbeta heterodimer, could be sufficient for the cluster formation. More than half of the discrete clusters of FP-ERalpha and FP-ERbeta were colocalized with hyperacetylated histone H4 and a component of the chromatin remodeling complex, Brg-1, indicating that ERs clusters might be involved in structural changes of chromatin.

Effect of estrogens on ontogenetic expression of progesterone receptor in the fetal female rat reproductive tract

Ontogenetic expression of progesterone receptor (PR) and effect of estrogens on PR expression in the fetal female rat reproductive tract were investigated. To evaluate ontogenetic PR expression, female reproductive tract from untreated fetuses was examined on gestational days (GD) 15.5, 17.5, 19.5 and 21.5. To evaluate estrogen effects, pregnant rats were injected once per day with oil, 17beta-estradiol (E(2)) or diethylstilbestrol (DES) from GD 15 through 21. Female fetuses were prepared for real-time reverse-transcription polymerase chain reaction (RT-PCR) or immunohistochemistry for PR. Increase in PR mRNA expression was detected in the Müllerian duct on GD 21.5 compared to that on GDs 15.5 and 17.5 in untreated fetuses (P<0.05). Prenatal administration of E(2) or DES increased Müllerian PR mRNA levels by GD 21.5 compared with oil controls (P<0.01). To identify cell and region in which PR was expressed and up-regulated by E(2) and DES, localization was evaluated within three regions along the Müllerian duct axis which differentiate into oviduct, uterus and upper vagina in immunohistochemistry. In untreated fetuses, Müllerian epithelial PR immunoreactivity was weak on GDs 15.5 and 17.5, but then became moderate on GDs 19.5 and 21.5 in all three regions. These fetuses exhibited faint signals in Müllerian mesenchymal PR immunoreactivity during gestational monitoring. Critically, Müllerian mesenchymal PR staining became intense after E(2) exposure in all three regions by GD 21.5, but no change was observed in Müllerian epithelial PR. Similarly, DES dramatically induced Müllerian mesenchymal PR in all regions by GD 21.5, and also enhanced proximal epithelial PR. On the other hand, middle and caudal epithelial PRs were reduced by DES. These affected mesenchymal and epithelial cells by DES were ER alpha immunopositive in the Müllerian duct, except for middle Müllerian epithelium. These findings clearly demonstrate cell-specific PR localization and region-specific effect of DES on PR in the developing rat Müllerian duct, and provide fundamental information critical for investigating the tissue-specific mechanisms underlying the prenatal response to estrogen receptor agonists.

Concurrent pregnancy retards mammary involution: effects on apoptosis and proliferation of the mammary epithelium after forced weaning of mice

The effect of pregnancy on postweaning mammary gland involution was investigated in mice. On the third day after forced weaning at Lactation Day 10, the apoptotic index was 56% lower in mammary tissue of mice that were pregnant at the time of weaning than in nonpregnant mice. Conversely, the bromodeoxyuridine-labeling index was increased sevenfold in pregnant mice compared to nonpregnant controls (3.5% vs. 0.5%, respectively). Structure of mammary alveoli was largely maintained in postweaning pregnant mice. The effect of pregnancy on three specific mammary epithelial cell survival pathways was also examined. First, pregnancy blocked the loss of Stat5a phosphorylation during involution. Significantly, loss of Stat5a phosphorylation during involution was not correlated with loss of Stat5a nuclear localization. Second, pregnancy maintained nuclear-localized progesterone receptor during lactation. Third, pregnancy was associated with increased expression of bfl-1 during involution but had little effect on the expression of other bcl-2 family members. The data indicate that pregnancy inhibits mammary cell apoptosis after weaning while permitting proliferation of the mammary epithelium, and they support the hypothesis that Stat5a and progesterone-signaling pathways act in concert to mediate this effect.

Differential expression of estrogen receptors alpha and beta in the reproductive tracts of adult male dogs and cats

Expression of estrogen receptors (ERs) in the reproductive tracts of adult male dogs and cats has not been reported. In the present study, ERalpha and ERbeta were localized by immunohistochemistry using ER-specific antibodies. ERalpha was found in interstitial cells and peritubular myoid cells in the dog testis, but only in interstitial cells of the cat. In rete testis of the dog, epithelial cells were positive for ERalpha staining, but in the cat, rete testis epithelium was only weakly positive. In efferent ductules of the dog, both ciliated and nonciliated cells stained intensely positive. In the cat, ciliated epithelial cells were less stained than nonciliated epithelial cells. Epithelial cells in dog epididymis and vas deferens were negative for ERalpha. In the cat, except for the initial region of caput epididymis, ERalpha staining was positive in the epithelial cells of epididymis and vas deferens. Multiple cell types of dog and cat testes stained positive for ERbeta. In rete testis and efferent ductules, epithelial cells were weakly positive for ERbeta. Most epithelial cells of the epididymis and vas deferens exhibited a strong positive staining in both species. In addition, double staining was used to demonstrate colocalization of both ERalpha and ERbeta in efferent ductules of both species. The specificity of antibodies was demonstrated by Western blot analysis. This study reveals a differential localization of ERalpha and ERbeta in male dog and cat reproductive tracts, demonstrating more intensive expression of ERbeta than ERalpha. However, as in other species, the efferent ductules remained the region of highest concentration of ERalpha.

Mechanisms of estrogen action

Our appreciation of the physiological functions of estrogens and the mechanisms through which estrogens bring about these functions has changed during the past decade. Just as transgenic mice were produced in which estrogen receptors had been inactivated and we thought that we were about to understand the role of estrogen receptors in physiology and pathology, it was found that there was not one but two distinct and functional estrogen receptors, now called ER alpha and ER beta. Transgenic mice in which each of the receptors or both the receptors are inactive have revealed a much broader role for estrogens in the body than was previously thought. This decade also saw the description of a male patient who had no functional ER alpha and whose continued bone growth clearly revealed an important function of estrogen in men. The importance of estrogen in both males and females was also demonstrated in the laboratory in transgenic mice in which the aromatase gene was inactivated. Finally, crystal structures of the estrogen receptors with agonists and antagonists have revealed much about how ligand binding influences receptor conformation and how this conformation influences interaction of the receptor with coactivators or corepressors and hence determines cellular response to ligands.