Cross-talk between ERs and signal transducer and activator of transcription 5 is E2 dependent and involves two functionally separate mechanisms

Sexual hormones and diabetes: The impact of estradiol in pancreatic β cell

Diabetes is one of the most prevalent metabolic diseases and its incidence is increasing throughout the world. Data from World Health Organization (WHO) point-out that diabetes is a major cause of blindness, kidney failure, heart attacks, stroke and lower limb amputation and estimated 1.6 million deaths were directly caused by it in 2016. Population studies show that the incidence of this disease increases in women after menopause, when the production of estrogen is decreasing in them. Knowing the impact that estrogenic signaling has on insulin-secreting β cells is key to prevention and design of new therapeutic targets. This chapter explores the role of estrogen and their receptors in the regulation of insulin secretion and biosynthesis, proliferation, regeneration and survival in pancreatic β cells. In addition, delves into the genetic animal models developed and its application for the specific study of the different estrogen signaling pathways. Finally, discusses the impact of menopause and hormone replacement therapy on pancreatic β cell function.

Regulation of the JAK2-STAT5 Pathway by Signaling Molecules in the Mammary Gland

Janus kinase 2 (JAK2) and signal transducers and activators of transcription 5 (STAT5) are involved in the proliferation, differentiation, and survival of mammary gland epithelial cells. Dysregulation of JAK2-STAT5 activity invariably leads to mammary gland developmental defects and/or diseases, including breast cancer. Proper functioning of the JAK2-STAT5 signaling pathway relies on crosstalk with other signaling pathways (synergistically or antagonistically), which leads to normal biological performance. This review highlights recent progress regarding the critical components of the JAK2-STAT5 pathway and its crosstalk with G-protein coupled receptor (GPCR) signaling, PI3K-Akt signaling, growth factors, inflammatory cytokines, hormone receptors, and cell adhesion.

Repression of insulin gene transcription by indirect genomic signaling via the estrogen receptor in pancreatic beta cells

The mechanism whereby 17β-estradiol (E2) mediates insulin gene transcription has not been fully elucidated. In this study, exposure of hamster insulinoma (HIT-T15) cells to 5 × 10-9 to 1 × 10-7 M E2 led to a concentration-dependent decrease of insulin mRNA levels. Transient expression of the estrogen receptor (ER) in HIT-T15 cells revealed that estrogen receptor α (ERα) repressed transcription of the rat insulin II promoter in both ligand-dependent and ligand-independent manners. The N-terminal A/B domain of ERα was not required for either activity. However, the repression was absent with mutated ER lacking the DNA-binding domain. Moreover, introducing mutations in the D-box and P-box of the zinc finger of ER (C227S, C202L) also abolished the repression. Deletion of the insulin promoter region revealed that nucleotide positions - 238 to - 144 (relative to the transcriptional start site) were needed for ER repression of the rat insulin II gene. PDX1- and BETA2-binding sites were required for the repression, but an estrogen response element-like sequence or an AP1 site in the promoter was not involved. In conclusion, we found that estrogen repressed insulin mRNA expression in a beta cell line. In addition, the ER suppressed insulin gene transcription in a ligand-independent matter. These observations suggest ER may regulate insulin transcription by indirect genomic signaling.

17β-Estradiol and ICI182,780 Differentially Regulate STAT5 Isoforms in Female Mammary Epithelium, With Distinct Outcomes

Prolactin (PRL) and estrogen cooperate in lobuloalveolar development of the mammary gland and jointly regulate gene expression in breast cancer cells in vitro. Canonical PRL signaling activates STAT5A/B, homologous proteins that have different target genes and functions. Although STAT5A/B are important for physiological mammary function and tumor pathophysiology, little is known about regulation of their expression, particularly of STAT5B, and the consequences for hormone action. In this study, we examined the effect of two estrogenic ligands, 17β-estradiol (E2) and the clinical antiestrogen, ICI182,780 (ICI, fulvestrant) on expression of STAT5 isoforms and resulting crosstalk with PRL in normal and tumor murine mammary epithelial cell lines. In all cell lines, E2 and ICI significantly increased protein and corresponding nascent and mature transcripts for STAT5A and STAT5B, respectively. Transcriptional regulation of STAT5A and STAT5B by E2 and ICI, respectively, is associated with recruitment of estrogen receptor alpha and increased H3K27Ac at a common intronic enhancer 10 kb downstream of the Stat5a transcription start site. Further, E2 and ICI induced different transcripts associated with differentiation and tumor behavior. In tumor cells, E2 also significantly increased proliferation, invasion, and stem cell-like activity, whereas ICI had no effect. To evaluate the role of STAT5B in these responses, we reduced STAT5B expression using short hairpin (sh) RNA. shSTAT5B blocked ICI-induced transcripts associated with metastasis and the epithelial mesenchymal transition in both cell types. shSTAT5B also blocked E2-induced invasion of tumor epithelium without altering E2-induced transcripts. Together, these studies indicate that STAT5B mediates a subset of protumorigenic responses to both E2 and ICI, underscoring the need to understand regulation of its expression and suggesting exploration as a possible therapeutic target in breast cancer.

Impact of Triclosan on Female Reproduction through Reducing Thyroid Hormones to Suppress Hypothalamic Kisspeptin Neurons in Mice

Triclosan (TCS), a broad-spectrum antimicrobial agent, is widely used in clinical settings and various personal care products. The aim of this study was to evaluate the influence of TCS on reproductive endocrine and function. Here, we show that the exposure of adult female mice to 10 or 100 mg/kg/day TCS caused prolongation of diestrus, and decreases in antral follicles and corpora lutea within 2 weeks. TCS mice showed decreases in the levels of serum luteinizing hormone (LH), follicle-stimulating hormone (FSH) and progesterone, and gonadotrophin-releasing hormone (GnRH) mRNA with the lack of LH surge and elevation of prolactin (PRL). TCS mice had lower kisspeptin immunoreactivity and kiss1 mRNA in anteroventral periventricular nucleus (AVPV) and arcuate nucleus (ARC). Moreover, the estrogen (E2)-enhanced AVPV-kisspeptin expression was reduced in TCS mice. In addition, the serum thyroid hormones (triiodothyronine (T3) and thyroxine (T4)) in TCS mice were reduced with increases in levels of thyroid stimulating hormone (TSH) and thyroid releasing hormone (TRH). In TCS mice, the treatment with Levothyroxine (L-T4) corrected the increases in PRL, TSH and TRH; the administration of L-T4 or type-2 dopamine receptors agonist quinpirole inhibiting PRL release could rescue the decline of kisspeptin expression in AVPV and ARC; the treatment with L-T4, quinpirole or the GPR45 agonist kisspeptin-10 recovered the levels of serum LH and FSH and progesterone, and GnRH mRNA. Furthermore, TCS mice treated with L-T4 or quinpirole resumed regular estrous cycling, follicular development and ovulation. Together, these results indicate that exposing adult female mice to TCS (≥10 mg/kg) reduces thyroid hormones causing hyperprolactinemia that then suppresses hypothalamic kisspeptin expression, leading to deficits in reproductive endocrine and function.

Oestrogen receptor β regulates epigenetic patterns at specific genomic loci through interaction with thymine DNA glycosylase

Background

DNA methylation is one way to encode epigenetic information and plays a crucial role in regulating gene expression during embryonic development. DNA methylation marks are established by the DNA methyltransferases and, recently, a mechanism for active DNA demethylation has emerged involving the ten-eleven translocator proteins and thymine DNA glycosylase (TDG). However, so far it is not clear how these enzymes are recruited to, and regulate DNA methylation at, specific genomic loci. A number of studies imply that sequence-specific transcription factors are involved in targeting DNA methylation and demethylation processes. Oestrogen receptor beta (ERβ) is a ligand-inducible transcription factor regulating gene expression in response to the female sex hormone oestrogen. Previously, we found that ERβ deficiency results in changes in DNA methylation patterns at two gene promoters, implicating an involvement of ERβ in DNA methylation. In this study, we set out to explore this involvement on a genome-wide level, and to investigate the underlying mechanisms of this function.

Results

Using reduced representation bisulfite sequencing, we compared genome-wide DNA methylation in mouse embryonic fibroblasts derived from wildtype and ERβ knock-out mice, and identified around 8000 differentially methylated positions (DMPs). Validation and further characterisation of selected DMPs showed that differences in methylation correlated with changes in expression of the nearest gene. Additionally, re-introduction of ERβ into the knock-out cells could reverse hypermethylation and reactivate expression of some of the genes. We also show that ERβ is recruited to regions around hypermethylated DMPs. Finally, we demonstrate here that ERβ interacts with TDG and that TDG binds ERβ-dependently to hypermethylated DMPs.

Conclusion

We provide evidence that ERβ plays a role in regulating DNA methylation at specific genomic loci, likely as the result of its interaction with TDG at these regions. Our findings imply a novel function of ERβ, beyond direct transcriptional control, in regulating DNA methylation at target genes. Further, they shed light on the question how DNA methylation is regulated at specific genomic loci by supporting a concept in which sequence-specific transcription factors can target factors that regulate DNA methylation patterns.

Electronic supplementary material

The online version of this article (doi:10.1186/s13072-016-0055-7) contains supplementary material, which is available to authorised users.

Modeling prolactin actions in breast cancer in vivo: insights from the NRL-PRL mouse

Elevated exposure to prolactin (PRL) is epidemiologically associated with an increased risk of aggressive ER+ breast cancer. To understand the underlying mechanisms and crosstalk with other oncogenic factors, we developed the NRL-PRL mouse. In this model, mammary expression of a rat prolactin transgene raises local exposure to PRL without altering estrous cycling. Nulliparous females develop metastatic, histotypically diverse mammary carcinomas independent from ovarian steroids, and most are ER+. These characteristics resemble the human clinical disease, facilitating study of tumorigenesis, and identification of novel preventive and therapeutic approaches.

Prolactin-sensitive neurons express estrogen receptor-α and depend on sex hormones for normal responsiveness to prolactin

Estrogens and prolactin share important target tissues, including the gonads, brain, liver, kidneys and some types of cancer cells. Herein, we sought anatomical and functional evidence of possible crosstalk between prolactin and estrogens in the mouse brain. First, we determined the distribution of prolactin-responsive neurons that express the estrogen receptor α (ERα). A large number of prolactin-induced pSTAT5-immunoreactive neurons expressing ERα mRNA were observed in several brain areas, including the anteroventral periventricular nucleus, medial preoptic nucleus, arcuate nucleus of the hypothalamus, ventrolateral subdivision of the ventromedial nucleus of the hypothalamus (VMH), medial nucleus of the amygdala and nucleus of the solitary tract. However, although the medial preoptic area, periventricular nucleus of the hypothalamus, paraventricular nucleus of the hypothalamus, retrochiasmatic area, dorsomedial subdivision of the VMH, lateral hypothalamic area, dorsomedial nucleus of the hypothalamus and ventral premammillary nucleus contained significant numbers of prolactin-responsive neurons, these areas showed very few pSTAT5-immunoreactive cells expressing ERα mRNA. Second, we evaluated prolactin sensitivity in ovariectomized mice and observed that sex hormones are required for a normal responsiveness to prolactin as ovariectomized mice showed a lower number of prolactin-induced pSTAT5 immunoreactive neurons in all analyzed brain nuclei compared to gonad-intact females. In addition, we performed hypothalamic gene expression analyses to determine possible post-ovariectomy changes in components of prolactin signaling. We observed no significant changes in the mRNA expression of prolactin receptor, STAT5a or STAT5b. In summary, sex hormones exert a permissive role in maintaining the brain's prolactin sensitivity, most likely through post-transcriptional mechanisms.

STAT signaling in mammary gland differentiation, cell survival and tumorigenesis

The mammary gland is a unique organ that undergoes extensive and profound changes during puberty, menstruation, pregnancy, lactation and involution. The changes that take place during puberty involve large-scale proliferation and invasion of the fat-pad. During pregnancy and lactation, the mammary cells are exposed to signaling pathways that inhibit apoptosis, induce proliferation and invoke terminal differentiation. Finally, during involution the mammary gland is exposed to milk stasis, programmed cell death and stromal reorganization to clear the differentiated milk-producing cells. Not surprisingly, the signaling pathways responsible for bringing about these changes in breast cells are often subverted during the process of tumorigenesis. The STAT family of proteins is involved in every stage of mammary gland development, and is also frequently implicated in breast tumorigenesis. While the roles of STAT3 and STAT5 during mammary gland development and tumorigenesis are well studied, others members, e.g. STAT1 and STAT6, have only recently been observed to play a role in mammary gland biology. Continued investigation into the STAT protein network in the mammary gland will likely yield new biomarkers and risk factors for breast cancer, and may also lead to novel prophylactic or therapeutic strategies against breast cancer.

An estrogen receptor dependent mechanism of Oroxylin A in the repression of inflammatory response

Oroxylin A, a natural flavonoid, is one of the main bioactive compounds that underlie the anti-inflammatory effect of the medicinal herb Scutellaria baicalensis Georgi widely used in southeastern Asia; however, the molecular mechanisms for the therapeutic benefits remain largely unclear. In this study, we found that Oroxylin A induces estrogen-responsive gene expression and promoter activity. In macrophages, Oroxylin A treatment significantly attenuates lipopolysaccharide (LPS)-induced but not basal inflammatory response, including nitric oxide (NO) production and the expression of inflammatory mediators (i.e., iNOS and COX-2) and cytokines (i.e., TNF-α, IL-1β, and IL-6), in an estrogen receptor (ER)-dependent manner. Oroxylin A treatment also dramatically decreases LPS-induced secretion of pro-inflammatory cytokines. Furthermore, the downregulation of all these inflammatory parameters by Oroxylin A was abolished when cells were pretreated with specific ER antagonist. Thus, Oroxylin A is a novel phytoestrogen and exhibits anti-inflammatory effects that are mediated by ER activity.

Hormone-sensing cells require Wip1 for paracrine stimulation in normal and premalignant mammary epithelium

INTRODUCTION

The molecular circuitry of different cell types dictates their normal function as well as their response to oncogene activation. For instance, mice lacking the Wip1 phosphatase (also known as PPM1D; protein phosphatase magnesium-dependent 1D) have a delay in HER2/neu (human epidermal growth factor 2), but not Wnt1-induced mammary tumor formation. This suggests a cell type-specific reliance on Wip1 for tumorigenesis, because alveolar progenitor cells are the likely target for transformation in the MMTV(mouse mammary tumor virus)-neu but not MMTV-wnt1 breast cancer model.

METHODS

In this study, we used the Wip1-knockout mouse to identify the cell types that are dependent on Wip1 expression and therefore may be involved in the early stages of HER2/neu-induced tumorigenesis.

RESULTS

We found that alveolar development during pregnancy was reduced in Wip1-knockout mice; however, this was not attributable to changes in alveolar cells themselves. Unexpectedly, Wip1 allows steroid hormone-receptor-positive cells but not alveolar progenitors to activate STAT5 (signal transducer and activator of transcription 5) in the virgin state. In the absence of Wip1, hormone-receptor-positive cells have significantly reduced transcription of RANKL (receptor activator of nuclear factor kappa-B ligand) and IGF2 (insulin-like growth factor 2), paracrine stimulators of alveolar development. In the MMTV-neu model, HER2/neu activates STAT5 in alveolar progenitor cells independent of Wip1, but HER2/neu does not override the defect in STAT5 activation in Wip1-deficient hormone-sensing cells, and paracrine stimulation remains attenuated. Moreover, ERK (extracellular signal-regulated kinase) activation by HER2/neu in hormone-sensing cells is also Wip1 dependent.

CONCLUSIONS

We identified Wip1 as a potentiator of prolactin and HER2/neu signaling strictly in the molecular context of hormone-sensing cells. Furthermore, our findings highlight that hormone-sensing cells convert not only estrogen and progesterone but also prolactin signals into paracrine instructions for mammary gland development. The instructive role of hormone-sensing cells in premalignant development suggests targeting Wip1 or prolactin signaling as an orthogonal strategy for inhibiting breast cancer development or relapse.

Estrogens regulate the hepatic effects of growth hormone, a hormonal interplay with multiple fates

The liver responds to estrogens and growth hormone (GH) which are critical regulators of body growth, gender-related hepatic functions, and intermediate metabolism. The effects of estrogens on liver can be direct, through the direct actions of hepatic ER, or indirect, which include the crosstalk with endocrine, metabolic, and sex-differentiated functions of GH. Most previous studies have been focused on the influence of estrogens on pituitary GH secretion, which has a great impact on hepatic transcriptional regulation. However, there is strong evidence that estrogens can influence the GH-regulated endocrine and metabolic functions in the human liver by acting at the level of GHR-STAT5 signaling pathway. This crosstalk is relevant because the widespread exposition of estrogen or estrogen-related compounds in human. Therefore, GH or estrogen signaling deficiency as well as the influence of estrogens on GH biology can cause a dramatic impact in liver physiology during mammalian development and in adulthood. In this review, we will summarize the current status of the influence of estrogen on GH actions in liver. A better understanding of estrogen-GH interplay in liver will lead to improved therapy of children with growth disorders and of adults with GH deficiency.

The influence of estrogens on the biological and therapeutic actions of growth hormone in the liver

GH is main regulator of body growth and composition, somatic development, intermediate metabolism and gender-dependent dimorphism in mammals. The liver is a direct target of estrogens because it expresses estrogen receptors which are connected with development, lipid metabolism and insulin sensitivity, hepatic carcinogenesis, protection from drug-induced toxicity and fertility. In addition, estrogens can modulate GH actions in liver by acting centrally, regulating pituitary GH secretion, and, peripherally, by modulating GHR-JAK2-STAT5 signalling pathway. Therefore, the interactions of estrogens with GH actions in liver are biologically and clinically relevant because disruption of GH signaling may cause alterations of its endocrine, metabolic, and gender differentiated functions and it could be linked to dramatic impact in liver physiology during development as well as in adulthood. Finally, the interplay of estrogens with GH is relevant because physiological roles these hormones have in human, and the widespread exposition of estrogen or estrogen-related compounds in human. This review highlights the importance of these hormones in liver physiology as well as how estrogens modulate GH actions in liver which will help to improve the clinical use of these hormones.

The biology of progesterone receptor in the normal mammary gland and in breast cancer

This paper reviews work on progesterone and the progesterone receptor (PR) in the mouse mammary gland that has been used extensively as an experimental model. Studies have led to the concept that progesterone controls proliferation and morphogenesis of the luminal epithelium in a tightly orchestrated manner at distinct stages of development by paracrine signaling pathways, including receptor activator of nuclear factor κB ligand (RANKL) as a major paracrine factor. Progesterone also drives expansion of stem cells by paracrine signals to generate progenitors required for alveologenesis. During mid-to-late pregnancy, progesterone has another role to suppress secretory activation until parturition mediated in part by crosstalk between PR and prolactin/Stat5 signaling to inhibit induction of milk protein gene expression, and by inhibiting tight junction closure. In models of hormone-dependent mouse mammary tumors, the progesterone/PR signaling axis enhances pre-neoplastic progression by a switch from a paracrine to an autocrine mode of proliferation and dysregulation of the RANKL signaling pathway. Limited experiments with normal human breast show that progesterone/PR signaling also stimulates epithelial cell proliferation by a paracrine mechanism; however, the signaling pathways and whether RANKL is a major mediator remains unknown. Work with human breast cancer cell lines, patient tumor samples and clinical studies indicates that progesterone is a risk factor for breast cancer and that alteration in progesterone/PR signaling pathways contributes to early stage human breast cancer progression. However, loss of PR expression in primary tumors is associated with a less differentiated more invasive phenotype and worse prognosis, suggesting that PR may limit later stages of tumor progression.

Comparison of increased aromatase versus ERα in the generation of mammary hyperplasia and cancer

Factors associated with increased estrogen synthesis increase breast cancer risk. Increased aromatase and estrogen receptor α (ERα) in both normal epithelium and ductal carcinoma in situ lesions are found in conjunction with breast cancer, leading to the idea that altered estrogen signaling pathways predispose the mammary gland to cancer development. Here, we developed a transgenic mouse that conditionally expresses aromatase in the mammary gland, and used it along with a deregulated ERα expression model to investigate the molecular pathways involved in the development of mammary gland preneoplasia and carcinoma. Both increased ERα and aromatase expression led to the development of preneoplasia, but increased preneoplasia, in addition to carcinoma, was found in aromatase overexpressing mice. Increased prevalence of mammary pathologic changes in mice expressing aromatase correlated with increased cyclin E and cyclin-dependent kinase 2 expression. Gain of both ERα and aromatase increased expression of ERα and progesterone receptor, but aromatase produced a higher increase than ERα, accompanied by higher levels of downstream target genes Ccnd1, Myc, and Tnfsf11. In summary, whereas gain of both ERα and aromatase activate abnormal growth pathways in the mammary gland, aromatase induced a wider range of abnormalities that was associated with a higher prevalence of mammary preneoplasia and cancer progression.

Ligand-dependent differences in estrogen receptor beta-interacting proteins identified in lung adenocarcinoma cells corresponds to estrogenic responses

Background

A recent epidemiological study demonstrated a reduced risk of lung cancer mortality in breast cancer patients using antiestrogens. These and other data implicate a role for estrogens in lung cancer, particularly nonsmall cell lung cancer (NSCLC). Approximately 61% of human NSCLC tumors express nuclear estrogen receptor β (ERβ); however, the role of ERβ and estrogens in NSCLC is likely to be multifactorial. Here we tested the hypothesis that proteins interacting with ERβ in human lung adenocarcinoma cells that respond proliferatively to estradiol (E₂) are distinct from those in non-E₂-responsive cells.

Methods

FLAG affinity purification of FLAG-ERβ-interacting proteins was used to isolate ERβ-interacting proteins in whole cell extracts from E₂ proliferative H1793 and non-E₂-proliferative A549 lung adenocarcinoma cell lines. Following trypsin digestion, proteins were identified using liquid chromatography electrospray ionization tandem mass spectrometry (LC-MS/MS). Proteomic data were analyzed using Ingenuity Pathway Analysis. Select results were confirmed by coimmunoprecipitation.

Results

LC-MS/MS identified 27 non-redundant ERβ-interacting proteins. ERβ-interacting proteins included hsp70, hsp60, vimentin, histones and calmodulin. Ingenuity Pathway Analysis of the ERβ-interacting proteins revealed differences in molecular and functional networks between H1793 and A549 lung adenocarcinoma cells. Coimmunoprecipitation experiments in these and other lung adenocarcinoma cells confirmed that ERβ and EGFR interact in a gender-dependent manner and in response to E₂ or EGF. BRCA1 interacted with ERβ in A549 cell lines and in human lung adenocarcinoma tumors, but not normal lung tissue.

Conclusion

Our results identify specific differences in ERβ-interacting proteins in lung adenocarcinoma cells corresponding to ligand-dependent differences in estrogenic responses.

Disrupting actions of bisphenol A and malachite green on growth hormone receptor gene expression and signal transduction in seabream

Environmental estrogen could mimic natural estrogens thereby disrupting the endocrine systems of human and animals. The actions of such endocrine disruptors have been studied mainly on reproduction and development. However, estrogen could also affect the somatotropic axis via multiple targets such as growth hormone (GH). In the present study, two endocrine disruptors were chosen to investigate their effects on the expression level and signal transduction of growth hormone receptor (GHR) in fish. Using real-time PCR, it was found that exposure to both the estrogenic (bisphenol A) and anti-estrogenic (malachite green) compounds could attenuate the expression levels of GHR1 and GHR2 in black seabream (Acanthopagrus schlegeli) hepatocytes. The expression level of IGF-I, the downstream effector of GHR activation in the liver, was decreased by bisphenol A but not by malachite green. Luciferase reporter assay of the beta-casein promoter was used to monitor GHR signaling in transfected cells. In the fish liver cell line Hepa-T1, both GHR1 and GHR2 signaling were attenuated by bisphenol A and malachite green. This attenuation could only occur in the presence of estrogen receptor, indicating that these agents probably produce their actions via the estrogen receptor. Results of the present study demonstrated that estrogenic or anti-estrogenic compounds could down-regulate the somatotropic axis in fish by affecting both the gene expression and signaling of GHR. In view of the increasing prevalence of these compounds in the environment, the impact on fish growth and development both in the wild and in aquaculture would be considerable.

Minireview: Estrogen receptor-beta: mechanistic insights from recent studies

The discovery of estrogen receptor-beta (ERbeta) in 1996 stimulated great interest in the physiological roles and molecular mechanisms of ERbeta action. We now know that ERbeta plays a major role in mediating estrogen action in several tissues and organ systems, including the ovary, cardiovascular system, brain, and the immune system, and that ERbeta and ERalpha generally play distinct physiological roles in the body. Although significant progress has been made toward understanding the molecular mechanisms of ERbeta action, particularly in vitro, there remains a large gap in our understanding of the mechanisms by which ERbeta elicits its biological functions in a true physiological context.

Role of ERalpha in the differential response of Stat5a loss in susceptibility to mammary preneoplasia and DMBA-induced carcinogenesis

Deregulated estrogen signaling is evidently linked to breast cancer pathophysiology, although the role of signal transducer and activator of transcription (Stat)5a, integral to normal mammary gland development, is less clear. A mouse model of mammary epithelial cell-targeted deregulated estrogen receptor alpha (ERalpha) expression [conditional ERalpha in mammary epithelium (CERM)] was crossed with mice carrying a germ line deletion of Stat5a [Stat5a-/-] to investigate interactions between ERalpha and Stat5a in mammary tissue. CERM, CERM/Stat5a+/-, CERM/Stat5a-/-, Stat5a+/-, Stat5a-/- and wild-type (WT) mice were generated to test the roles of ERalpha and Stat5a on pubertal differentiation and cancer progression with and without exposure to the chemical carcinogen 7,12-dimethylbenz[a]anthracene (DMBA). Only CERM/Stat5a-/- mice demonstrated delayed pubertal terminal end bud differentiation. Without DMBA exposure, Stat5a loss abrogated ERalpha-initiated hyperplastic alveolar nodule (HAN) development and, similarly, Stat5a-/- mice did not develop HANs. However, although Stat5a loss still reduced ERalpha-initiated HAN prevalence following DMBA exposure, Stat5a loss without deregulated ERalpha was associated with an increased HAN prevalence compared with WT. Progression to ERalpha(+) and ERalpha(-) adenocarcinoma was found in all CERM-containing genotypes (CERM, CERM/Stat5a+/-, CERM/Stat5a-/-) and ERalpha(+) adenocarcinoma in the Stat5a-/- genotype. The mammary epithelial cell proliferative index was increased only in CERM mice independent of Stat5a loss. No differences in apoptotic indices were found. In summary, Stat5a cooperated with deregulated ERalpha in retarding pubertal mammary differentiation and contributed to ERalpha-initiated preneoplasia, but its loss did not prevent development of invasive cancer. Moreover, in the absence of deregulated ERalpha, Stat5a loss was associated with development of both HANs and invasive cancer following DMBA exposure.

Estrogen-mediated regulation of Igf1 transcription and uterine growth involves direct binding of estrogen receptor alpha to estrogen-responsive elements

Estrogen enables uterine proliferation, which depends on synthesis of the IGF1 growth factor. This proliferation and IGF1 synthesis requires the estrogen receptor (ER), which binds directly to target DNA sequences (estrogen-responsive elements or EREs), or interacts with other transcription factors, such as AP1, to impact transcription. We observe neither uterine growth nor an increase in Igf1 transcript in a mouse with a DNA-binding mutated ER alpha (KIKO), indicating that both Igf1 regulation and uterine proliferation require the DNA binding function of the ER. We identified several potential EREs in the Igf1 gene, and chromatin immunoprecipitation analysis revealed ER alpha binding to these EREs in wild type but not KIKO chromatin. STAT5 is also reported to regulate Igf1; uterine Stat5a transcript is increased by estradiol (E(2)), but not in KIKO or alpha ERKO uteri, indicating ER alpha- and ERE-dependent regulation. ER alpha binds to a potential Stat5a ERE. We hypothesize that E(2) increases Stat5a transcript through ERE binding; that ER alpha, either alone or together with STAT5, then acts to increase Igf1 transcription; and that the resulting lack of IGF1 impairs KIKO uterine growth. Treatment with exogenous IGF1, alone or in combination with E(2), induces proliferation in wild type but not KIKO uteri, indicating that IGF1 replacement does not rescue the KIKO proliferative response. Together, these observations suggest in contrast to previous in vitro studies of IGF-1 regulation involving AP1 motifs that direct ER alpha-DNA interaction is required to increase Igf1 transcription. Additionally, full ER alpha function is needed to mediate other cellular signals of the growth factor for uterine growth.

Novel actions of estrogen to promote proliferation: integration of cytoplasmic and nuclear pathways

Both steroids and growth factors stimulate proliferation of steroid-dependent tumor cells, and interaction between these signaling pathways occurs at several levels. Steroid receptors are classified as ligand-activated transcription factors, and steps by which they activate target gene transcription are well understood. Several steroid responses have now been functionally linked to other intracellular signaling pathways, including c-Src or tyrosine kinase receptors. Steroids such as 17beta-estradiol (E2), via binding to cytoplasmic or membrane-associated receptors, were also shown to rapidly activate intracellular signaling cascades such as ERK, PI3K and STATs. These E2-stimulated phosphorylations can then contribute to altered tumor cell function. ER-positive breast cancer cells, in which proliferation is stimulated by E2 and suppressed by antiestrogens, have been of particular interest in dissecting nuclear and cytoplasmic roles of estrogen receptors (ER). In some cell contexts, ER interacts directly with the intracellular tyrosine kinase c-Src and other cytoplasmic signaling and adaptor molecules, such as Shc, PI3K, MNAR, and p130 Cas. Although the hierarchy among these associations is not known, it is clear that c-Src plays a fundamental role in both growth factor and E2-stimulated cell growth, and this may also require other growth factor receptors such as those for EGF or IGF-1. STAT transcription factors represent one pathway to integrate E2 cytoplasmic and nuclear signaling. STAT5 is phosphorylated in the cytoplasm at an activating tyrosine in response to E2 or EGF, and then is translocated to the nucleus to stimulate target gene transcription. E2 stimulates recruitment of STAT5 and ER to the promoter of several proliferative genes, and STAT5 knockdown prevents recruitment of either protein to these promoters. STAT5 activation by E2 in breast cancer cells requires c-Src and EGF receptor, and inhibition of c-Src or EGFR, or knockdown of STAT5, prevents E2 stimulation of several genes and breast cancer cell proliferation. Hyperactivation of the growth factor receptor-c-Src pathway can in some contexts decrease growth responses to E2, or render cells and tumors resistant to suppressive actions of endocrine therapies. Crosstalk between growth factors and steroids in both the cytoplasm and nucleus may thus have a profound impact on complex biological processes such as cell growth, and may play a significant role in the treatment of steroid-dependent breast cancers.

Signal transducer and activation of transcription (STAT) 4beta, a shorter isoform of interleukin-12-induced STAT4, is preferentially activated by estrogen

Estrogen, a natural immunomodulatory compound, has been shown to promote the induction of a prototype T helper 1 cytokine, interferon (IFN)-gamma, as well as to up-regulate IFNgamma-mediated proinflammatory molecules (nitric oxide, cyclooxygenase 2, monocyte chemoattractant protein 1). Because IL-12 is a major IFNgamma-inducing cytokine, in this study we investigated whether estrogen treatment of wild-type C57BL/6 mice alters IL-12-mediated signaling pathways. A recent study has shown that IL-12 activates two isoforms of signal transducer and activation of transcription (STAT) 4, a normal-sized (full-length STAT4alpha) and a truncated form (STAT4beta). Interestingly, we found that estrogen treatment preferentially up-regulates the phosphorylation of STAT4beta in splenic lymphoid cells. Time kinetic data showed the differential activation of STAT4beta in splenic lymphoid cells from estrogen-treated mice, but not in cells from placebo controls. The activation of STAT4beta was mediated by IL-12 and not IFNgamma because deliberate addition or neutralization of IL-12, but not IFNgamma, affected the activation of STAT4beta. In contrast to IL-12-induced activation of STAT4beta in cells from estrogen-treated mice, STAT4alpha was not increased, rather it tended to be decreased. In this context, STAT4alpha-induced p27(kip1) protein was decreased in concanavalin A + IL-12-activated lymphocytes from estrogen-treated mice only. By using the in vitro DNA binding assay, we confirmed the ability of pSTAT4beta to bind to the IFNgamma-activated sites (IFNgamma activation sequences)/STAT4-binding sites in estrogen-treated mice. Our data are the first to show that estrogen apparently has selective effects on IL-12-mediated signaling by preferentially activating STAT4beta. These novel findings are likely to provide new knowledge with regard to estrogen regulation of inflammation.

Signal transducer and activator of transcription 5b, c-Src, and epidermal growth factor receptor signaling play integral roles in estrogen-stimulated proliferation of estrogen receptor-positive breast cancer cells

17beta-Estradiol (E2) acts through the estrogen receptor alpha (ERalpha) to stimulate breast cancer proliferation. Here, we investigated the functional relationship between ERalpha and signal transducer and activator of transcription (STAT)5b activity in ER+ MCF-7 and T47D human breast cancer cells after specific knockdown of STAT5b. STAT5b small interfering RNA (siRNA) inhibited E2-induced bromodeoxyuridine (BrdU) incorporation in both cell lines, as well as the E2-induced increase in MCF-7 cell number, cyclin D1 and c-myc mRNA, and cyclin D1 protein expression, indicating that STAT5b is required for E2-stimulated breast cancer proliferation. E2 treatment stimulated STAT5b tyrosine phosphorylation at the activating tyrosine Y699, resulting in increased STAT5-mediated transcriptional activity, which was inhibited by a Y669F STAT5b mutant. E2-induced STAT5-mediated transcriptional activity was inhibited by overexpressing a kinase-defective epidermal growth factor receptor (EGFR), or the EGFR tyrosine kinase inhibitor tyrphostin AG1478, indicating a requirement for EGFR kinase activity. Both E2-induced STAT5b tyrosine phosphorylation and STAT5-mediated transcription were also inhibited by the ER antagonist ICI 182,780 and the c-Src inhibitor PP2, indicating additional requirements for the ER and c-Src kinase activity. EGFR and c-Src kinase activities were also required for E2-induced cyclin D1 and c-myc mRNA. Together, these studies demonstrate positive cross talk between ER, c-Src, EGFR, and STAT5b in ER+ breast cancer cells. Increased EGFR and c-Src signaling is associated with tamoxifen resistance in ER+ breast cancer cells. Here we show that constitutively active STAT5b not only increased basal DNA synthesis, but also conferred tamoxifen resistance. Because STAT5b plays an integral role in E2-stimulated proliferation and tamoxifen resistance, it may be an effective therapeutic target in ER+ breast tumors.

Hypothalamic prolactin receptor messenger ribonucleic acid levels, prolactin signaling, and hyperprolactinemic inhibition of pulsatile luteinizing hormone secretion are dependent on estradiol

Hyperprolactinemia can reduce fertility and libido. Although central prolactin actions are thought to contribute to this, the mechanisms are poorly understood. We first tested whether chronic hyperprolactinemia inhibited two neuroendocrine parameters necessary for female fertility: pulsatile LH secretion and the estrogen-induced LH surge. Chronic hyperprolactinemia induced by the dopamine antagonist sulpiride caused a 40% reduction LH pulse frequency in ovariectomized rats, but only in the presence of chronic low levels of estradiol. Sulpiride did not affect the magnitude of a steroid-induced LH surge or the percentage of GnRH neurons activated during the surge. Estradiol is known to influence expression of the long form of prolactin receptors (PRL-R) and components of prolactin's signaling pathway. To test the hypothesis that estrogen increases PRL-R expression and sensitivity to prolactin, we next demonstrated that estradiol greatly augments prolactin-induced STAT5 activation. Lastly, we measured PRL-R and suppressor of cytokine signaling (SOCS-1 and -3 and CIS, which reflect the level of prolactin signaling) mRNAs in response to sulpiride and estradiol. Sulpiride induced only SOCS-1 in the medial preoptic area, where GnRH neurons are regulated, but in the arcuate nucleus and choroid plexus, PRL-R, SOCS-3, and CIS mRNA levels were also induced. Estradiol enhanced these effects on SOCS-3 and CIS. Interestingly, estradiol also induced PRL-R, SOCS-3, and CIS mRNA levels independently. These data show that GnRH pulse frequency is inhibited by chronic hyperprolactinemia in a steroid-dependent manner. They also provide evidence for estradiol-dependent and brain region-specific regulation of PRL-R expression and signaling responses by prolactin.

JAK-STAT pathway in carcinogenesis: Is it relevant to cholangiocarcinoma progression

The features of JAK-STAT signaling in liver cells are discussed in the current review. The role of this signaling cascade in carcinogenesis is accentuated. The possible involvement of this pathway and alteration of its elements are compared for normal cholangiocytes, cholangiocarcinoma predisposition and development. Prolactin and interleukin-6 are described in detail as the best studied examples. In addition, the non-classical nuclear translocation of cytokine receptors is discussed in terms of its possible implication to cholangiocarcinoma development.

Co-planar 3,3',4,4',5-pentachlorinated biphenyl and non-co-planar 2,2',4,6,6'-pentachlorinated biphenyl differentially induce recruitment of oestrogen receptor alpha to aryl hydrocarbon receptor target genes

In the present study we examined the ability of 3,3',4,4',5-pentachlorinated biphenyl [PCB126 (polychlorinated biphenyl 126)], a prototypical AHR (aryl hydrocarbon receptor) agonist, and 2,2',4,6,6'-PCB (PCB104), which does not activate AHR, to induce the recruitment of ERalpha (oestrogen receptor alpha) to CYP1A1 (cytochrome P4501A1 gene) and CYP1B1 promoters in T-47D human breast cancer cells and other cell lines. PCB126 treatment strongly induced CYP1A1 and CYP1B1 mRNA expression that was unaffected by co-treatment with E2 (17beta-oestradiol). PCB104 failed to induce changes in either CYP1A1 or CYP1B1 expression levels. ChIP (chromatin immunoprecipitation) assays show that PCB126, but not PCB104, increased the promoter occupancy by ERalpha to CYP1A1 and CYP1B1 promoters. Co-treatment with PCB126+E2 significantly enhanced the promoter occupancy of ERalpha at CYP1A1, whereas co-treatment with PCB126+4-hydroxytamoxifen or ICI182,780 did not. Competitive binding studies revealed that neither PCB126 nor PCB104 bound to ERalpha. HEK-293 cells (human embryonic kidney-293 cells) stably transfected with ERalpha showed significantly higher PCB126-induced CYP1A1 expression compared with empty vector controls, whereas no increase was observed in cells stably transfected with ERalpha lacking its N-terminal AF1 (activation function-1) domain (ERalphaDeltaAF1). Despite no increase in AHR-mediated gene expression, ChIP assays revealed that ERalphaDeltaAF1 was present at CYP1A1 and CYP1B1 promoters. HC11 mouse mammary cells stably expressing shRNA (small-hairpin RNA) against ERalpha showed an 8-fold reduction in PCB126-dependent Cyp1a1 expression. Our results provide further evidence that AHR agonists induce ERalpha promoter occupancy at AHR target genes through indirect activation of ERalpha, and support a role for ERalpha in AHR transactivation.

Stat5 activation inhibits prolactin-induced AP-1 activity: distinct prolactin-initiated signals in tumorigenesis dependent on cell context

The essential role of prolactin (PRL) in normal mammary gland growth and differentiation has implicated this hormone in the development and progression of breast cancer. Although Stat5 is the best-characterized mediator of PRL signals, PRL also activates multiple other signals, whose roles in normal and pathologic processes are not well understood. We have shown that PRL stimulates activating protein-1 (AP-1) activity in breast cancer cells, and can cooperate with estradiol in this pathway. AP-1 modulates many processes critical for carcinogenesis, including cell proliferation, survival, transformation, invasion and angiogenesis, and is elevated in many neoplasms, including breast tumors. Here, we investigated the relationship between PRL signals to AP-1 and Stat5. We found that PRL activation of Stat5a and Stat5b, but not Stat1 or Stat3, reduced PRL signals to AP-1, without altering estradiol-induced AP-1 activity. The truncation mutant, Stat5/Delta53C, but not Stat5Y699F, was an effective inhibitor, consistent with a requirement for Stat5 dimerization and nuclear accumulation, but not its C-terminal transactivation activity. The association of Stat5 with AP-1 proteins suggests that this underlies the inhibition. Predictably, the ability of PRL to activate Stat5 and AP-1 was inversely related in mammary cell lines. Further, reduction of Stat5 protein with siRNA in T47D cells, which contain elevated Stat5, increased PRL-induced AP-1 signals, transcripts for the AP-1 target, matrix metalloproteinase-2 and associated invasive behavior. This study points to the importance of cell context in determining the spectrum of PRL-induced actions, which is critical for understanding the contributions of PRL to breast cancer.

SUMO and estrogen receptors in breast cancer

Small ubiquitin-like modifier (SUMO) is a family of proteins structurally similar to ubiquitin that have been found to be covalently attached to certain lysine residues of specific target proteins. By contrast to ubiquitination, however, SUMO proteins do not promote protein degradation but, instead, modulate important functional properties, depending on the protein substrate. These properties include--albeit not limited to--subcellular localization, protein dimerization, DNA binding and/or transactivation of transcription factors, among them estrogen receptors. Moreover, it has been suggested that SUMO proteins might affect transcriptional co-factor complexes of the estrogen receptor signalling cascade. Tissue and/or state specificity seems to be one of their intriguing features. In this regard, elucidation of their contribution to estrogen receptor-mediated transcriptional activity during breast carcinogenesis will offer new insights into the molecular mechanisms governing sensitivity/resistance in currently applied endocrine treatment and/or chemoprevention, and provide novel routes to breast carcinoma therapeutics.

Progesterone Receptor Repression of Prolactin/Signal Transducer and Activator of Transcription 5-Mediated Transcription of the β-Casein Gene in Mammary Epithelial Cells

Prolactin (PRL) and glucocorticoids act synergistically to stimulate transcription of the beta-casein milk protein gene. Signal transducer and activator of transcription 5 (Stat5) mediates PRL-dependent trans-activation, and glucocorticoid potentiation occurs through cross talk between glucocorticoid receptor (GR) and Stat5 at the beta-casein promoter. In the mouse, progesterone withdrawal leads to terminal differentiation and secretory activation of the mammary gland at parturition, indicating progesterone's role in repressing milk protein gene expression during pregnancy. To investigate the mechanism of the inhibitory action of progesterone, experiments were performed with cell culture systems reconstituted to express progesterone receptor (PR), the PRL receptor/Stat5 signaling pathway, and GR, enabling evaluation of PR, GR, and Stat5 interactions at the beta-casein promoter. With COS-1, normal murine mammary gland, HC-11, and primary mammary epithelial cells, progestin-PR directly repressed the PRL receptor/Stat5a signaling pathway's mediation of PRL-induced beta-casein transcription. Progestin-PR also inhibited glucocorticoid-GR enhancement of PRL induced trans-activation of beta-casein. Inhibition depended on a functional PR DNA binding domain and specific PR-DNA interactions at the beta-casein promoter. Chromatin immunoprecipitation assays in HC-11 cells revealed recruitment of PR and Stat5a to the beta-casein promoter by progestin or PRL, respectively. Recruitment was disrupted by cotreatment with progestin and PRL, suggesting a mutual interference between activated PR and Stat5a. Without PRL, progestin-PR also recruited Stat5a to the beta-casein promoter, suggesting that recruitment of an unactivated form of Stat5a may contribute to inhibition of beta-casein by progesterone. These results define a negative cross talk between PR and Stat5a/GR that may contribute to the physiological role of progesterone to repress lactogenic hormone induction of the beta-casein gene in the mammary gland during pregnancy.

ZNF366 is an estrogen receptor corepressor that acts through CtBP and histone deacetylases

The regulation of gene expression by estrogen receptor-alpha (ERalpha) requires the coordinated and temporal recruitment of diverse sets of transcriptional co-regulator complexes, which mediate nucleosome remodelling and histone modification. Using ERalpha as bait in a yeast two-hybrid screen, we have identified a novel ERalpha-interacting protein, ZNF366, which is a potent corepressor of ERalpha activity. The interaction between ZNF366 and ERalpha has been confirmed in vitro and in vivo, and is mediated by the zinc finger domains of the two proteins. Further, we show that ZNF366 acts as a corepressor by interacting with other known ERalpha corepressors, namely RIP140 and CtBP, to inhibit expression of estrogen-responsive genes in vivo. Together, our results indicate that ZNF366 may play an important role in regulating the expression of genes in response to estrogen.

Suppression of prolactin-induced signal transducer and activator of transcription 5b signaling and induction of suppressors of cytokine signaling messenger ribonucleic acid in the hypothalamic arcuate nucleus of the rat during late pregnancy and lactation

During late pregnancy and lactation, the tuberoinfundibular dopamine (TIDA) neurons that regulate prolactin secretion by negative feedback become less able to produce dopamine in response to prolactin, leading to hyperprolactinemia. Because prolactin-induced activation of dopamine synthesis in these neurons requires the Janus kinase/signal transducer and activator of transcription 5b (STAT5b) signaling pathway, we investigated whether prolactin-induced STAT5b signaling is reduced during lactation and whether induction of suppressors of cytokine signaling (SOCS) mRNAs occur at this time and in late pregnancy. During lactation, the ability of exogenous prolactin to induce STAT5 phosphorylation and STAT5b nuclear translocation was markedly reduced when compared with diestrous rats. In nonpregnant female rats, acute treatment with ovine prolactin markedly increased levels of SOCS-1 and -3 and cytokine-inducible SH2-containing protein mRNA in arcuate nucleus micropunches. On gestation d 22, SOCS-1 and SOCS-3 mRNA levels were 10-fold that on G20. SOCS-1 and -3 and cytokine-inducible SH2-containing protein mRNA levels were also elevated on lactation d 7. At these times, dopaminergic activity was decreased and the rats were hyperprolactinemic. The high levels of SOCS mRNA were prevented by bromocriptine pretreatment (gestation d 22) or pup removal (lactation d 7), which suppressed circulating prolactin to basal levels. These results demonstrate that around the end of pregnancy, prolactin loses the ability to activate STAT5b, associated with an increase in SOCS mRNAs. The loss of this stimulating pathway may underlie the reduced tuberoinfundibular dopamine neuron dopamine output and hyperprolactinemia that characterizes late pregnancy and lactation. The high maternal levels of SOCS mRNAs appear to be dependent on prolactin, presumably acting through an alternative signaling pathway to STAT5b.

Signalling cross-talk between hepatocyte nuclear factor 4alpha and growth-hormone-activated STAT5b

In the present study, we have characterized signalling cross-talk between STAT5b (signal transducer and activator of transcription 5b) and HNF4alpha (hepatocyte nuclear factor 4alpha), two major regulators of sex-dependent gene expression in the liver. In a HepG2 liver cell model, HNF4alpha strongly inhibited beta-casein and ntcp (Na+/taurocholate cotransporting polypeptide) promoter activity stimulated by GH (growth hormone)-activated STAT5b, but had no effect on interferon-gamma-stimulated STAT1 transcriptional activity. By contrast, STAT5b synergistically enhanced the transcriptional activity of HNF4alpha towards the ApoCIII (apolipoprotein CIII) promoter. The inhibitory effect of HNF4alpha on STAT5b transcription was associated with the inhibition of GH-stimulated STAT5b tyrosine phosphorylation and nuclear translocation. The short-chain fatty acid, butyrate, reversed STAT5b transcriptional inhibition by HNF4alpha, but did not reverse the inhibition of STAT5b tyrosine phosphorylation. HNF4alpha inhibition of STAT5b tyrosine phosphorylation was not reversed by pervanadate or by dominant-negative phosphotyrosine phosphatase 1B, suggesting that it does not result from an increase in STAT5b dephosphorylation. Rather, HNF4alpha blocked GH-stimulated tyrosine phosphorylation of JAK2 (Janus kinase 2), a STAT5b tyrosine kinase. Thus STAT5b and HNF4alpha exhibit bi-directional cross-talk that may augment HNF4alpha-dependent gene transcription while inhibiting STAT5b transcriptional activity via the inhibitory effects of HNF4alpha on JAK2 phosphorylation, which leads to inhibition of STAT5b signalling initiated by the GH receptor at the cell surface.

STAT3 ser727 phosphorylation and its association with negative estrogen receptor status in breast infiltrating ductal carcinoma

Although it is known that STAT3 transcriptional activity is modulated by phosphorylation at serine residue 727, the role of STAT3 serine phosphorylation in breast cancer remains mostly unexplored. In this study, we examined the expression patterns of serine residue 727-phosphorylated STAT3 (p-ser727-STAT3) in breast infiltrating ductal carcinoma tissues and nearby noncancer tissues by using immunoblotting techniques, and correlated the expression profiles with clinicopathological characteristics. A significantly elevated p-ser727-STAT3 expression was observed in 61.8% (42/68) of breast cancer tissues as compared to corresponding noncancer tissues (p < 0.001). Further, immunohistochemical analysis also showed an increased nuclear p-ser727-STAT3 staining in cancer lesions. The increased p-ser727-STAT3 expression in breast infiltrating ductal carcinoma tissues correlated significantly with negative estrogen receptor (ER) status, increased stage of cancer and increased tumor size (p = 0.001, 0.024 and 0.014, individually). Intriguingly, we noticed that the expression levels of p-ser727-STAT3 in ER-negative breast cancer cell lines were higher than those in ER-positive breast cancer cell lines. In ER-positive MCF7 cells, treatment with ERalpha-specific siRNA increased, whereas treatment with anticancer drug tamoxifen decreased the expression of p-ser727-STAT3, phenomena not observed in ER-negative MDA-MB-231 cells. In conclusion, our results suggest that p-ser727-STAT3 may be involved in the pathogenesis of breast cancer in an ER-dependent manner.

Downregulation of estrogen receptor gene expression by exogenous 17beta-estradiol in the mammary glands of lactating mice

The biological actions of estrogen are mostly conveyed through interaction with the nuclear estrogen receptor (ER). Previous evidence indicated that estrogen participates in self-regulation through the modulation of the expression of its own receptors. However, the self-regulation of estrogen against ER in the mammary gland during established lactation has not yet been investigated. The present study evaluated ER gene expression in the lactating gland activated by large doses of 17beta-estradiol (E(2)). Repeated E(2) treatments dose-dependently decreased the gene expression of ER, especially its subtype ER-alpha mRNA, which was decreased to 10% of the vehicle-injected control by 1 mug E(2) injection, whereas it was decreased by 73% for another subtype, ER-beta. A single injection of 5 mug of E(2) drastically downregulated both ER genes within 12 hrs of injection, and they did not recover to pretreatment level within 48 hrs. Western blot analysis verified that E(2) treatment inhibited the phosphorylation of Stat5, which is a potent transcriptional regulator for ER mRNA. The present findings demonstrate that E(2) treatment decreases the gene expression of its own receptor in the mammary gland during galactopoesis and induces an apparent transition of the ER profile in the mammary gland during lactation into postlactation.

Estrogen stimulates transcription of human immunodeficiency virus type 1 (HIV-1)

Gene expression from human immunodeficiency virus (HIV) provirus is a crucial step for the viral replication. Here we examined a potential role of 17beta-estradiol (E2) in HIV-1 transcription. Transient luciferase expression studies revealed that E2 activated HIV-LTR reporter gene in HEK293 cells when the cells were co-transfected with estrogen receptor alpha (ERalpha) but not ERbeta expression plasmid. This E2 effect was abrogated by a specific antagonist to ER, ICI 182,780, indicating that it was mediated by ERalpha. Mutation analysis revealed that Sp1 binding site but not nuclear factor-kappa B (NF-kappaB) binding site of HIV-1 LTR is critical to the E2 effect. In addition, whereas E2 could not induce DNA-binding activity of NF-kappaB, E2 could augment both Sp1 DNA-binding and transcriptional activity. These findings suggest a contribution of estrogen for HIV-1 replication through ERalpha by augmenting Sp1 DNA-binding and transcriptional activity.

Convergence of protein kinase C and JAK-STAT signaling on transcription factor GATA-4

Angiotensin II (AII), a potent vasoactive hormone, acts on numerous organs via G-protein-coupled receptors and elicits cell-specific responses. At the level of the heart, AII stimulation alters gene transcription and leads to cardiomyocyte hypertrophy. Numerous intracellular signaling pathways are activated in this process; however, which of these directly link receptor activation to transcriptional regulation remains undefined. We used the atrial natriuretic factor (ANF) gene (NPPA) as a marker to elucidate the signaling cascades involved in AII transcriptional responses. We show that ANF transcription is activated directly by the AII type 1 receptor and precedes the development of myocyte hypertrophy. This response maps to STAT and GATA binding sites, and the two elements transcriptionally cooperate to mediate signaling through the JAK-STAT and protein kinase C (PKC)-GATA-4 pathways. PKC phosphorylation enhances GATA-4 DNA binding activity, and STAT-1 functionally and physically interacts with GATA-4 to synergistically activate AII and other growth factor-inducible promoters. Moreover, GATA factors are able to recruit STAT proteins to target promoters via GATA binding sites, which are sufficient to support synergy. Thus, STAT proteins can act as growth factor-inducible coactivators of tissue-specific transcription factors. Interactions between STAT and GATA proteins may provide a general paradigm for understanding cell specificity of cytokine and growth factor signaling.

Expression of mRNA for prolactin receptor (long form) in dopamine and pro-opiomelanocortin neurones in the arcuate nucleus of non-pregnant and lactating rats

Under most conditions, prolactin secretion from the pituitary gland is subject to negative-feedback regulation. Prolactin stimulates dopamine release from tuberoinfundibular (TIDA) neurones in the arcuate nucleus of the hypothalamus, which in turn suppresses the production of prolactin. However, during late pregnancy and continuing into lactation, this feedback mechanism becomes less responsive to prolactin and, as a result, a hyperprolactinaemic state develops. We investigated whether long-form prolactin receptor (PRL-R(L)) mRNA is present on TIDA neurones in nonpregnant and lactating rats. In addition, we examined whether PRL-R(L) mRNA is colocalized on hypothalamic pro-opiomelanocortin (POMC) neurones. Dual-label in situ hybridizations using an (35)S-labelled cRNA probe specific for long-form PRL-R, together with a digoxigenin-labelled RNA probe that encoded either tyrosine hydroxylase (TH) or POMC mRNA, were performed on brain sections. In both nonpregnant and lactating rats, the majority of TH mRNA-positive cells (> 90%) were found to express long-form PRL-R mRNA. In sections from nonpregnant rats, few non-TH positive cells expressed PRL-R(L) mRNA. By contrast, during lactation, the proportion of PRL-R(L) mRNA-positive cells that were not TH mRNA-positive increased to approximately 70%. Only a small number of neurones in this subpopulation of PRL-R(L) mRNA-positive neurones were found to be positive for POMC mRNA. These data show that the loss of responsiveness to prolactin occurring during lactation is not due to down regulation of long-form PRL-R gene expression on TIDA neurones. Moreover, the persistent expression of PRL-R(L) in arcuate neuroendocrine circuits suggests that PRL-R-mediated signalling continues to be important in these neurones during lactation.

Estrogen negatively regulates epidermal growth factor (EGF)-mediated signal transducer and activator of transcription 5 signaling in human EGF family receptor-overexpressing breast cancer cells

Breast cancer cell growth may be stimulated by 17beta-estradiol (E2) or growth factors like epidermal growth factor (EGF). However, tumors typically depend on only one of these pathways and may overexpress either estrogen receptor (ER) or EGF receptor (EGFR) and related family members. Tumors overexpressing EGFR are more aggressive than those expressing ER. Intracellular mediators of these growth-stimulatory pathways are not completely defined, but one potential common mediator of EGF and E2 signaling is the transcription factor signal transducer and activator of transcription 5 (STAT5). To investigate the role of STAT5 in potential crosstalk between E2 and EGF, MDA-MB231 and SKBr3 breast cancer cells, which are ER-negative and overexpress human EGF family receptors, were used. Introduction of ERalpha and treatment with E2 decreased EGF-induced tyrosine phosphorylation of STAT5b, basal and EGF-induced STAT5-mediated transcription, and EGF-stimulated DNA synthesis in these cells. Suppressive effects of E2-EpsilonRalpha were specific for STAT5, as EGF stimulation of MAPK was unaffected. Deletion/mutation analysis of ERalpha demonstrated that the DNA-binding domain was insufficient, and that the ligand-binding domain was required for these responses. ERalpha transcriptional activity was not necessary for suppression of STAT5 activity. Overexpression of c-Src did not prevent suppression of STAT5 activity by E2 and ERalpha. However, ERalpha did prevent basal increases in STAT5 activity with overexpressed c-Src. In the context of human EGF receptor family overexpression, E2-ER opposes EGF signaling by regulating STAT5 activity. STAT5 may be a crucial point of signaling for both E2 and growth factors in breast cancer cells, allowing targeted therapy for many types of breast tumors.

Female steroid hormones use signal transducers and activators of transcription protein-mediated pathways to modulate the expression of T-bet in epithelial cells: a mechanism for local immune regulation in the human reproductive tract

The transcription factor T-bet promotes the differentiation of inflammatory Th1 T helper cells. T-bet expression in lymphoid cells is regulated by cytoplasmic signaling through Janus kinase phosphorylation, nuclear signaling using signal transducers and activators of transcription (Stat) family proteins, and autocrine/paracrine feedback involving interferon (IFN)-gamma. T-bet is here shown to be present in epithelial cells of the human female reproductive tract. Regulation of T-bet expression was modulated by cytokines and the female reproductive steroids, estrogen, and progesterone. The mechanisms of T-bet regulation in epithelia differ from those in conventional immune cells. During a 15-d exposure to progesterone, T-bet levels in endometrial epithelial cells (EECs) undulated. Prior exposure to estrogen enhanced these effects. More prolonged exposure of EECs to these hormones, singly or in combination, suppressed T-bet production. Stat1 and Stat5 bound to the EEC T-bet regulatory region (TRR) at the IFN-gamma-activated sequence site, but Stat3 and Stat4 did not. Binding of Stat1 and Stat5 to the TRR were modified by progesterone in distinct ways. Estrogen suppressed the binding of Stat1 and Stat5 to the TRR. Mutation of gamma-activated sequence element reduced T-bet promoter activity, binding of Stat proteins to the TRR and regulation of the promoter by cytokines and hormones. In EECs, cytokine exposure caused phosphorylation of Janus kinase 2 and TRR-bound Stat proteins; female steroid hormones altered only phosphorylation of TRR-bound Stat5. Although there is no autocrine IFN-gamma feedback loop in reproductive tract epithelial cells, an IL-15/T-bet positive feedback loop may exist. The implications of hormonally regulated T-bet expression are discussed.

Mechanisms of estrogen receptor signaling: convergence of genomic and nongenomic actions on target genes

Estrogen receptors (ERs) act by regulating transcriptional processes. The classical mechanism of ER action involves estrogen binding to receptors in the nucleus, after which the receptors dimerize and bind to specific response elements known as estrogen response elements (EREs) located in the promoters of target genes. However, ERs can also regulate gene expression without directly binding to DNA. This occurs through protein-protein interactions with other DNA-binding transcription factors in the nucleus. In addition, membrane-associated ERs mediate nongenomic actions of estrogens, which can lead both to altered functions of proteins in the cytoplasm and to regulation of gene expression. The latter two mechanisms of ER action enable a broader range of genes to be regulated than the range that can be regulated by the classical mechanism of ER action alone. This review surveys our knowledge about the molecular mechanism by which ERs regulate the expression of genes that do not contain EREs, and it gives examples of the ways in which the genomic and nongenomic actions of ERs on target genes converge. Genomic and nongenomic actions of ERs that do not depend on EREs influence the physiology of many target tissues, and thus, increasing our understanding of the molecular mechanisms behind these actions is highly relevant for the development of novel drugs that target specific receptor actions.

Transactivating agonists of the EGF receptor require Tyr 845 phosphorylation for induction of DNA synthesis

Signaling networks play important roles in cancer progression. For example, overexpression of the epidermal growth factor receptor (EGFR) is a poor prognostic indicator in multiple tumor types. Recent studies have postulated that the EGFR functions as a central conduit for signaling by different classes of cell surface receptors. In this study, we demonstrated that c-Src-dependent phosphorylation of tyrosine 845 (Tyr 845) on EGFR was required for DNA synthesis induced by the G protein-coupled agonists, endothelin (ET) and lysophosphatidic acid (LPA), and the cytokine, growth hormone (GH), in murine fibroblast and breast cancer model systems. In addition, we showed that a dominant interfering form of signal transducer and activator of transcription (STAT)5b (a downstream effector of phospho-Tyr 845 [pY845] in fibroblasts) abrogates DNA synthesis induced by all agonists in the breast cancer model. To further characterize the role of Tyr 845, a pY845-containing peptide was microinjected into SKBr3 breast cancer cells and murine fibroblasts, and was found to ablate EGF-stimulated S-phase entry in both cell systems. Taken together, these findings suggested that pY845 is critical for DNA synthesis induced by a variety of mitogens and that its signaling effectors may include but are not limited to STAT5b.

Crosstalk between steroid receptors and the c-Src-receptor tyrosine kinase pathways: implications for cell proliferation

Both steroids and growth factors stimulate proliferation of steroid-dependent tumor cells, and interaction between these signaling pathways may occur at several levels. Steroid receptors are typically classified as ligand-activated transcription factors, and steps by which they bind ligand, dimerize, recruit coregulatory molecules, and activate target gene transcription are well understood. Several steroid responses are functionally linked to c-Src or tyrosine kinase receptors, and the physiological impact and the precise molecular pathways involved in these responses are under intensive investigation. Ligand-independent stimulation of steroid receptor-mediated transcription by growth factors is now believed to occur through activated protein kinases that phosphorylate the receptors and receptor coregulators. Recently, steroid hormones themselves have been shown to rapidly activate intracellular signaling cascades, via binding to cognate cytoplasmic or membrane-associated receptors. In some contexts, steroid receptors interact directly with c-Src and other cytoplasmic signaling molecules, such as Shc, PI3K, and p130 Cas. Crosstalk between growth factors and steroids in both the cytoplasm and nucleus could have profound impact on complex biological processes such as cell growth, and play a significant role in the treatment of steroid-dependent cancers. The potential roles of progesterone and estrogen receptors in this crosstalk are discussed in this review.

Mammary tumorigenesis in growth hormone deficient spontaneous dwarf rats; effects of hormonal treatments

This study was carried out to investigate mammary tumorigenesis in growth hormone (GH) deficient spontaneous dwarf rats (SDR). At 50-60 days of age, the rats were divided into five groups. Group 1 received bovine (b) GH (prolonged release formulation) administered at a dose of 40-50 mg/kg body wt. in 50 microl weekly injections; group 2 received recombinant human insulin-like growth factor-I (IGF-I) at a dose of 1 mg/kg body wt./day administered via osmotic pumps; animals in group 3 were fitted with subcutaneous silastic capsule containing 30 microg 17 beta-estradiol (E2) plus 30 mg progesterone (P4), replaced every 2 months; group 4 received both bGH and E2 plus P4 treatments at the same doses as above, and control animals (group 5) received sham treatments (vegetable oil injection, silastic capsules containing cellulose). After 1 week of treatment, all animals were injected intraperitoneally with the carcinogen N-methyl-N-nitrosourea (MNU) at a dose of 50 mg/kg body wt. Other groups of animals, receiving identical hormonal treatment to those exposed to MNU, were treated for 10 days only and then sacrificed for assessment of circulating concentrations of hormones and mammary gland characteristics at the time of carcinogen exposure. The hormonal treatments of the animals exposed to the MNU were continued for an additional 20 weeks and mammary tumor development monitored by weekly palpation and tumors collected as necessary. The rats were weighed weekly. At the end of the treatment period, all animals were sacrificed and remaining tumors were collected. Rats in all groups continued to gain weight throughout the experimental period, but the largest weight gain was see in animals receiving GH either alone or with E2 and P4. Animals treated with IGF-I also gained weight compared to controls, but this weight gain was less than that seen in GH-treated rats. GH treatment alone increased mammary tumor incidence from 4.8% in controls to 100%. Average tumor load and latency in the GH-treated rats were 7.0 +/- 0.8 tumors/tumor-bearing rat (mean +/- SEM) and 57.3 +/- 2.7 days (mean +/- SEM), respectively. As in intact Sprague-Dawley rats, approximately 90% of the tumors that developed in the GH-treated rats were ovarian dependent for growth. IGF-I treatment also increased mammary tumor development to 62.5%. Average tumor load and latency in the IGF-I-treated rats were 1.6 +/- 0.4 tumors/tumor-bearing rat (mean +/- SEM) and 96.2 +/- 14.5 days (mean +/- SEM), respectively. However E2 + P4 treatments did not significantly alter tumorigenesis and, surprisingly, simultaneous treatment with E2 + P4 and GH obliterated the GH-stimulated increase in tumor development. Prolactin (PRL) did not appear to influence mammary tumorigenesis in the SDRs, as untreated SDRs had significantly elevated serum concentration of PRL as compared with normal Sprague-Dawley (SD) rats, whereas GH-treated SDRs had PRL levels similar to that of normal SD rats. No obvious structural characteristics were associated with high or low susceptibility to mammary tumorigenesis, as assessed by mammary gland whole mounts from the different animal groups sacrificed at the time of carcinogen administration. Enhanced expression of the extracellular signal-regulated kinase 1/2 (ERK1/2), and activation (phosphorylation) of ERK1/2 were associated with an increase in mammary tumorigenesis. Similarly, the expression of the estrogen receptor-alpha (ER alpha) was significantly elevated in animal groups with the highest susceptibility to tumorigenesis, whereas the levels of cyclin D1 expression were not related to mammary tumorigenesis.

Estrogen regulation of growth hormone action

GH plays a pivotal role in regulating body growth and development, which is modulated by sex steroids. A close interplay between estrogen and GH leads to attainment of gender-specific body composition during puberty. The physiological basis of the interaction is not well understood. Most previous studies have focused on the effects of estrogen on GH secretion. There is also strong evidence that estrogen modulates GH action independent of secretion. Oral but not transdermal administration of estrogen impairs the metabolic action of GH in the liver, causing a fall in IGF-I production and fat oxidation. This results in a loss of lean tissue and a gain of body fat in postmenopausal women and an impairment of GH effect in hypopituitary women on GH replacement. The negative metabolic sequelae are potentially important because of the widespread use of oral estrogen and estrogen-related compounds. Estrogen affects GH action at the level of receptor expression and signaling. More recently, estrogen has been shown to inhibit Janus kinase/signal transducer and activator of transcription signaling by GH via the induction of suppressor of cytokine signaling-2, a protein inhibitor for cytokine signaling. This represents a novel paradigm of steroid regulation of cytokine receptors and is likely to have significance for a diverse range of cytokine function.

ERalpha and STAT5a cross-talk: interaction through C-terminal portions of the proteins decreases STAT5a phosphorylation, nuclear translocation and DNA-binding

Cross-talk between ERalpha and STAT5a was demonstrated to mediate through a direct physical association between the two proteins. By GST pull-down assays and functional assays with various constructs of ERalpha and STAT5a, it was shown that the C-termini of these two proteins were mainly responsible for this interaction. Furthermore, the interaction between ERalpha and STAT5a was demonstrated to give rise to functional changes in their signaling events. In cell transfection studies, it was shown that ERalpha activation could attenuate PRLR signaling through STAT5a. This ERalpha-mediated attenuation of PRLR signaling was substantiated by observed decreases in the phosphorylation of JAK2 and STAT5a, reduced translocation of STAT5a into the nucleus, and reduced binding of STAT5a onto a GAS-containing nucleotide. Apart from transfected cells, the interaction between ERalpha and STAT5a could also be observed in established breast cancer cell lines of MCF-7 and T-47D in co-immunoprecipitation studies. However, the functional consequence of the interaction in these cancer cells was very different from the transfected HEK293 cells. ER activation could lead to potentiation of PRLR signaling in MCF-7 cells but not in T-47D cells. Conversely, in both MCF-7 and T-47D cells, PRLR activation could lead to attenuation of ER signaling. These data serve to elucidate the mechanisms underlying the ERalpha-STAT5a cross-talk and in demonstrating that the functional consequence of this cross-talk depends on the precise milieus of the intracellular environment.

Transgenic growth hormone mice exposed to lifetime constant illumination: gender-specific effects

Photoperiod affects most of the features altered in transgenic growth hormone (TG) mice, and laboratory rats and mice retain some sensitivity to photoperiod. We examined growth, feeding, longevity, and reproduction of TG mice and normal control mice (Mus musculus L., 1758) in 12 h light : 12 h dark (LD) and 24 h light (LL) photoperiods. Sexual dichotomy in growth and hepatic gene expression are considered to require gender-specific patterns of growth hormone secretion that are absent in TG mice. Regardless, in the LD photoperiod mature TG females were 82.8% (46.8 g) of the mass of TG males (56.5 g, p < 0.05), whereas control mice showed no size dichotomy (≈33 g). Mature masses of TG males and of control mice of either gender were unaffected by the LL photoperiod. TG females, however, reached a mature mass 92% (50.9 g) of that of mature TG males in the LL photoperiod, attenuating the sexual size dichotomy expressed in the LD photoperiod. Growth of females was slower than that of males, even in the control group. TG females in the LL photoperiod expressed faster growth, higher reproduction, and greater mean longevity than TG females in the LD photoperiod. Differences in age-related feeding associated with gender and photoperiod reflected differential growth rates. Females grew more slowly and ate more than males of similar age because they were smaller (i.e., had lower growth efficiencies). The LL photoperiod improved the energy balance of TG females. Possible mechanisms mediating such gender-specific effects are explored.

Estradiol represses prolactin-induced expression of Na+/taurocholate cotransporting polypeptide in liver cells through estrogen receptor-alpha and signal transducers and activators of transcription 5a

Na(+)/taurocholate cotransporting polypeptide (ntcp) mediates the uptake of bile salts from plasma across the basolateral domain of the hepatocyte. We have demonstrated that ntcp expression can be induced by prolactin (PRL) and placental lactogen via the PRL receptor and signal transducers and activators of transcription (Stat)5a pathway. However, elevated levels of placental lactogen do not increase the expression of ntcp in pregnant rats. Because plasma estradiol (E(2)) levels are also elevated in pregnancy, we investigated the inhibitory effects of E(2) on PRL-induced ntcp activation. E(2) treatment inhibited the PRL-induced increase in liver ntcp mRNA to the same levels as in rats treated with E(2) alone. Estrogen receptor-alpha (ERalpha) mRNA and protein expression in liver were increased 2.6-fold and 2.2-fold, respectively, in pregnancy relative to controls. In HepG2 cells, E(2) repressed PRL-induced ntcp reporter gene expression in a dose-dependent manner in the presence of cotransfected ERalpha. The ERalpha antagonist ICI 182,780 reversed E(2)-induced repression, indicating specificity of inhibition by E(2). Overexpression of coactivator p300 did not reverse the inhibitory effects of E(2) and ERalpha. Western and gel shift analysis revealed that E(2)-bound ERalpha decreased the tyrosine phosphorylation and DNA-binding activity of Stat5a, indicating that the inhibitory effect of E(2) was mediated, at least in part, by interfering with PRL-mediated signal transduction. The present studies demonstrate the physiological significance of cross-talk between ERalpha and Stat5a in liver, in which both proteins are expressed. These data also establish a novel mechanism by which expression of ntcp, an important hepatic bile acid transporter, can be regulated by multiple hormones.

NCoA-1/SRC-1 is an essential coactivator of STAT5 that binds to the FDL motif in the alpha-helical region of the STAT5 transactivation domain

Signal transducer and activator of transcription 5 (STAT5) is a transcription factor that activates prolactin (PRL)-dependent gene expression in the mammary gland. For the activation of its target genes, STAT5 recruits coactivators like p300 and the CREB-binding protein (CBP). In this study we analyzed the function of p300/CBP-associated members of the p160/SRC/NCoA-family in STAT5-mediated transactivation of beta-casein expression. We found that only one of them, NCoA-1, acts as a coactivator for both STAT5a and STAT5b. The two coactivators p300/CBP and NCoA-1 cooperatively enhance STAT5a-mediated transactivation. For NCoA-1-dependent coactivation of STAT5, both the activation domain 1 and the amino-terminal bHLH/PAS domain are required. The amino-terminal region mediates the interaction with STAT5a in cells. A motif of three amino acids in an alpha-helical region of the STAT5a-transactivation domain is essential for the binding of NCoA-1 and for the transcriptional activity of STAT5a. Moreover we observed that NCoA-1 is involved in the synergistic action of the glucocorticoid receptor and STAT5a on the beta-casein promoter. These findings support a model in which STAT5, in concert with the glucocorticoid receptor, recruits a multifunctional coactivator complex to initiate the PRL-dependent transcription.

Diabetes undermines estrogen control of inducible nitric oxide synthase function in rat aortic smooth muscle cells through overexpression of estrogen receptor-beta

BACKGROUND

Previous reports from our group have shown that 17beta-estradiol reduces the synthesis and activity of inducible nitric oxide synthase (iNOS) in rat aortic smooth muscle cells (SMC) in response to inflammatory mediators. In this study, we investigated the effect of 17beta-estradiol on iNOS function in aortic SMC from streptozotocin-diabetic rats.

METHODS AND RESULTS

Comparative analysis of NO release and of iNOS mRNA and protein content after 24-hour stimulation with a cytokine mixture revealed milder iNOS activation in diabetic than in control SMC. Furthermore, 17beta-estradiol dose-dependently blocked iNOS synthesis and activity in control but not in diabetic SMC. The defective estrogen response in diabetic SMC at 24 hours could not be attributed to reduced expression of estrogen receptors (ER). In fact, mRNA and protein levels of ERalpha and, to a greater extent, of ERbeta, were increased in diabetic compared with nondiabetic SMC. Cytokines decreased ERalpha and ERbeta expression in both groups. However, 17beta-estradiol dose-dependently restored the expression of ERalpha but further downregulated that of ERbeta, indicating a differential regulation of ER isoforms.

CONCLUSIONS

Estrogenic control of iNOS was impaired in diabetic SMC. This was associated with a larger increase of ERbeta than of ERalpha protein, whereas 17beta-estradiol regulated the two isoforms in an opposite fashion. Thus, modifications in the estrogen modulation of iNOS and in the expression pattern of ER may be involved in diabetic vascular dysfunction.

The role of Stat5 in estrogen receptor-positive breast cancer

Estrogen receptor (ER) has been a successful target for effective prevention and treatment strategies in breast cancer, whereas growth factors and their signaling molecules are proving to be effective treatment targets as well. Understanding the interaction between ER and growth factor signaling pathways should provide clues to optimal treatment approaches and new strategies to overcome and prevent endocrine resistance. Cross-talk between ER and signal transducer and activator of transcription 5 (Stat5) has also been reported. Stat5 regulates growth, differentiation, and survival of mammary and hematopoietic cells. The role of Stat5 in breast cancer has not been established, although Stat5 is critical for some hematopoietic malignancies. We have analyzed the role of Stat5 in the progression of ER-positive breast cancer cells such as T47D and MCF7 in which Stat5b is constitutively activated. Adenoviral-mediated dominant-negative Stat5 induced apoptosis in T47D cells but not in caspase-3 negative MCF7 cells. Our study indicates that targeting Stat5 may represent a new strategy to suppress estrogen receptor activity and induce apoptosis in Stat5-activated, ER-positive breast cancer.