Glycogen synthase kinase-3 interacts with and phosphorylates estrogen receptor alpha and is involved in the regulation of receptor activity

Insulin-like growth factor binding protein-4 and -5 modulate ligand-dependent estrogen receptor-α activation in breast cancer cells in an IGF-independent manner

Insulin-like growth factor binding proteins (IGFBPs) are modulators of numerous cellular processes including cell proliferation. Although IGFBPs classically act by sequestration of extracellular insulin-like growth factors (IGFs), thereby contributing to the fine-tuning of growth factor signals, IGF-independent actions of IGFBPs have also been described. In the breast, growth factor signaling in association with estradiol (E2)-stimulated estrogen receptor function is organized in a complex cross-talk. The importance of phosphatidylinositol 3-kinase/protein kinase B (Akt/PKB) pathway components for the E2-induced activation of estrogen receptor-alpha (ERα) is well accepted. Here we show that in the absence of IGFs, IGFBP-4 or IGFBP-5, either overexpressed in MCF-7 breast cancer cells or added exogenously, decreased the capability of E2 to induce ERα transcriptional activity. In addition, overexpression or addition of recombinant IGFBP-4 or IGFBP-5 resulted in reduction of E2-induced phosphorylation of Akt/PKB, GSK-3α/β and ERα in MCF-7 cells. The activation of the Akt/PKB-pathway describes a non-genomic effect of E2, which did not involve activation/phosphorylation of the IGF-I receptor (IGF-IR). Furthermore, knockdown of the IGF-IR did not affect the inhibition of E2-induced ERα phosphorylation by IGFBP-4 and 5. Moreover, IGFBP-4 and IGFBP-5 strongly decreased E2-triggered growth of MCF-7 cells. Our data suggest that IGFBPs interfere with the E2-induced activation of the Akt/PKB-pathway and prevent full hormone-dependent activation of ERα and breast cancer cell growth in an IGF- and IGF-IR-independent manner.

Derailed estrogen signaling and breast cancer: an authentic couple

Estrogen or 17β-estradiol, a steroid hormone, plays a critical role in the development of mammary gland via acting through specific receptors. In particular, estrogen receptor-α (ERα) acts as a transcription factor and/or a signal transducer while participating in the development of mammary gland and breast cancer. Accumulating evidence suggests that the transcriptional activity of ERα is altered by the action of nuclear receptor coregulators and might be responsible, at least in part, for the development of breast cancer. In addition, this process is driven by various posttranslational modifications of ERα, implicating active participation of the upstream receptor modifying enzymes in breast cancer progression. Emerging studies suggest that the biological outcome of breast cancer cells is also influenced by the cross talk between microRNA and ERα signaling, as well as by breast cancer stem cells. Thus, multiple regulatory controls of ERα render mammary epithelium at risk for transformation upon deregulation of normal homeostasis. Given the importance that ERα signaling has in breast cancer development, here we will highlight how the activity of ERα is controlled by various regulators in a spatial and temporal manner, impacting the progression of the disease. We will also discuss the possible therapeutic value of ERα modulators as alternative drug targets to retard the progression of breast cancer.

High glucose and insulin differentially modulates proliferation in MCF-7 and MDA-MB-231 cells

Various preclinical and clinical studies have linked diabetes and breast cancer, but little is known regarding the molecular mechanism involved. This study aimed to investigate the effect of high glucose and insulin in breast cancer cells (MCF-7: non-invasive, hormone dependent, and MDA-MB-231: invasive, hormone independent). In contrast to MCF-7 cells, high glucose augmented proliferation of MDA-MB-231 cells as observed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and bromodeoxyuridine assays. The high-glucose condition led to increased expression of cyclin D1, de-phosphorylation of p38, and increased phosphorylation of ERK in MDA-MB-231 cells but not in MCF-7 cells. Interestingly, we observed increased phosphorylation of GSK-3β, NF-κB, and ERα only in MCF-7 cells, highlighting their role as potential targets in prevention of progression of breast cancer under a high-glucose and insulin condition. Furthermore, insulin treatment under a high-glucose condition resulted in increased histone H3 phosphorylation and de-acetylation only in MDA-MB-231 cells. Taken together, we provide the first evidence that high glucose and insulin promotes proliferation of MDA-MB-231 cells by differential alteration of GSK-3β, NF-κB, and ERα expression and histone H3 modifications, which may directly or indirectly modulate the expression of genes involved in its proliferation.

Sequential activation of classic PKC and estrogen receptor α is involved in estradiol 17ß-D-glucuronide-induced cholestasis

Estradiol 17ß-d-glucuronide (E17G) induces acute cholestasis in rat with endocytic internalization of the canalicular transporters bile salt export pump (Abcb11) and multidrug resistance-associated protein 2 (Abcc2). Classical protein kinase C (cPKC) and PI3K pathways play complementary roles in E17G cholestasis. Since non-conjugated estradiol is capable of activating these pathways via estrogen receptor alpha (ERα), we assessed the participation of this receptor in the cholestatic manifestations of estradiol glucuronidated-metabolite E17G in perfused rat liver (PRL) and in isolated rat hepatocyte couplets (IRHC). In both models, E17G activated ERα. In PRL, E17G maximally decreased bile flow, and the excretions of dinitrophenyl-glutathione, and taurocholate (Abcc2 and Abcb11 substrates, respectively) by 60% approximately; preadministration of ICI 182,780 (ICI, ERα inhibitor) almost totally prevented these decreases. In IRHC, E17G decreased the canalicular vacuolar accumulation of cholyl-glycylamido-fluorescein (Abcb11 substrate) with an IC50 of 91±1 µM. ICI increased the IC50 to 184±1 µM, and similarly prevented the decrease in the canalicular vacuolar accumulation of the Abcc2 substrate, glutathione-methylfluorescein. ICI also completely prevented E17G-induced delocalization of Abcb11 and Abcc2 from the canalicular membrane, both in PRL and IRHC. The role of ERα in canalicular transporter internalization induced by E17G was confirmed in ERα-knocked-down hepatocytes cultured in collagen sandwich. In IRHC, the protection of ICI was additive to that produced by PI3K inhibitor wortmannin but not with that produced by cPKC inhibitor Gö6976, suggesting that ERα shared the signaling pathway of cPKC but not that of PI3K. Further analysis of ERα and cPKC activations induced by E17G, demonstrated that ICI did not affect cPKC activation whereas Gö6976 prevented that of ERα, indicating that cPKC activation precedes that of ERα. Conclusion: ERα is involved in the biliary secretory failure induced by E17G and its activation follows that of cPKC.

The role of S6K1 in ER-positive breast cancer

The 40S ribosomal S6 kinase 1 (S6K1) is a conserved serine/threonine protein kinase that belongs to the AGC family of protein kinases, which also includes Akt and many others. S6K1 is the principal kinase effector downstream of the mammalian target of rapamycin complex 1 (mTORC1). S6K1 is sensitive to a wide range of signaling inputs, including growth factors, amino acids, energy levels and hypoxia. S6K1 relays these signals to regulate a growing list of substrates and interacting proteins in control of oncogenic processes, such as cell growth and proliferation, cell survival and apoptosis and cell migration and invasion. Several lines of evidence suggest an important role for S6K1 in estrogen receptor (ER)-positive breast cancer. S6K1 directly phosphorylates and activates ERα. Furthermore, S6K1 expression is estrogenically regulated. Therefore, hyperactivation of mTORC1/S6K1 signaling may be closely related to ER-positive status in breast cancer and may be utilized as a marker for prognosis and a therapeutic target.

Glycogen synthase kinase-3β is involved in ligand-dependent activation of transcription and cellular localization of the glucocorticoid receptor

Glucocorticoids (GC) induce cell cycle arrest and apoptosis in different cell types and therefore are widely used to treat a variety of diseases including autoimmune disorders and cancer. This effect is mediated by the GC receptor (GR), a ligand-activated transcription factor that translocates into the nucleus where it modulates transcription of target genes in a promoter-specific manner. Glycogen synthase kinase-3 (GSK3) regulates GR response by genomic and nongenomic mechanisms, although the specific role of each isoform is not well defined. We used GSK3 pharmacological inhibitors and isoform-specific small interfering RNA to evaluate the role of GSK3 in the genomic regulation induced by GC. GSK3 inhibition resulted in the reduction of GC-induced mRNA expression of GC-induced genes such as BIM, HIAP1, and GILZ. Knockdown of GSK3β but not GSK3α reduced endogenous GILZ induction in response to dexamethasone and GR-dependent reporter gene activity. Chromatin immunoprecipitation experiments revealed that GSK3 inhibition impaired the dexamethasone-mediated binding of GR and RNA polymerase II to endogenous GILZ promoter. These results indicate that GSK3β is important for GR transactivation activity and that GSK3β inhibition suppresses GC-stimulated gene expression. Furthermore, we show that genomic regulation by the GR is independent of known GSK3β phosphorylation sites. We propose that GC-dependent transcriptional activation requires functional GSK3β signaling and that altered GSK3β activity influences cell response to GC.

Large scale phosphoproteome analysis of LNCaP human prostate cancer cells

Prostate cancer is the most frequently diagnosed cancer among men in the western world. The androgen receptor, a phosphoprotein, is suspected to be involved in all stages of the prostate cancer. Androgen receptor activity can be modulated by various kinases such as PKA, MAPK, AKT, and Src. Phosphorylation is an important post-translational modification and serves as a molecular on-off switch to regulate signaling. Disruptions of cellular phosphorylation are associated with various diseases such as cancer and kinases provide important drug targets. Here we present an analysis of the phosphoproteome in LNCaP human prostate cancer cells. The analytical strategy employed here used proteomics based methodologies with a combination of detergents and chaotropic reagents during trypsin digestion followed by titanium dioxide enrichment of phosphopeptides. Over the course of multiple analyses by mass spectrometry we identified a total of 746 phosphorylation sites in 540 phosphopeptides corresponding to 116 phosphoproteins, of which 56 had not been previously reported. Phosphoproteins identified included transcription factors, co-regulators of the androgen receptor, and cancer-related proteins that include β-catenin, USP10, and histone deacetylase-2. The information of signaling pathways, motifs of phosphorylated peptides, biological processes, molecular functions, cellular components, and protein interactions from the identified phosphoproteins established a map of phosphoproteome and signaling pathways in LNCaP cells.

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

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

Regulation of estrogen receptor α N-terminus conformation and function by peptidyl prolyl isomerase Pin1

Estrogen receptor alpha (ERα), a key driver of growth in the majority of breast cancers, contains an unstructured transactivation domain (AF1) in its N terminus that is a convergence point for growth factor and hormonal activation. This domain is controlled by phosphorylation, but how phosphorylation impacts AF1 structure and function is unclear. We found that serine 118 (S118) phosphorylation of the ERα AF1 region in response to estrogen (agonist), tamoxifen (antagonist), and growth factors results in recruitment of the peptidyl prolyl cis/trans isomerase Pin1. Phosphorylation of S118 is critical for Pin1 binding, and mutation of S118 to alanine prevents this association. Importantly, Pin1 isomerizes the serine118-proline119 bond from a cis to trans isomer, with a concomitant increase in AF1 transcriptional activity. Pin1 overexpression promotes ligand-independent and tamoxifen-inducible activity of ERα and growth of tamoxifen-resistant breast cancer cells. Pin1 expression correlates with proliferation in ERα-positive rat mammary tumors. These results establish phosphorylation-coupled proline isomerization as a mechanism modulating AF1 functional activity and provide insight into the role of a conformational switch in the functional regulation of the intrinsically disordered transactivation domain of ERα.

Cracking the estrogen receptor's posttranslational code in breast tumors

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

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

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

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

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

The Effects of 17β-estradiol in Cancer are Mediated by Estrogen Receptor Signaling at the Plasma Membrane

Two different isoforms of the estrogen receptors (i.e., ERα and ERβ) mediate pleiotropic 17β-estradiol (E2)-induced cellular effects. The ERs are principally localized in the nucleus where they act by globally modifying the expression of the E2-target genes. The premise that E2 effects are exclusively mediated through the nuclear localized ERs has been rendered obsolete by research over the last 15 years demonstrating that ERα and ERβ proteins are also localized at the plasma membranes and in other extra-nuclear organelles. The E2 modulation of cancer cell proliferation represents a good example of the impact of membrane-initiated signals on E2 effects. In fact, E2 via ERα elicits rapid signals driving cancer cells to proliferation (e.g., in breast cancer cells), while E2-induced ERβ rapid signaling inhibits proliferation (e.g., in colon cancer cells). In this review we provide with an overview of the complex system of E2-induced signal transduction pathways, their impact on E2-induced cancer cell proliferation, and the participation of E2-induced membrane-initiated signals in tumor environment.

Liganded and unliganded activation of estrogen receptor and hormone replacement therapies

Over the past two decades, our understanding of estrogen receptor physiology in mammals widened considerably as we acquired a deeper appreciation of the roles of estrogen receptor alpha and beta (ERα and ERβ) in reproduction as well as in bone and metabolic homeostasis, depression, vascular disorders, neurodegenerative diseases and cancer. In addition, our insights on ER transcriptional functions in cells increased considerably with the demonstration that ER activity is not strictly dependent on ligand availability. Indeed, unliganded ERs may be transcriptionally active and post-translational modifications play a major role in this context. The finding that several intracellular transduction molecules may regulate ER transcriptional programs indicates that ERs may act as a hub where several molecular pathways converge: this allows to maintain ER transcriptional activity in tune with all cell functions. Likely, the biological relevant role of ER was favored by evolution as a mean of integration between reproductive and metabolic functions. We here review the post-translational modifications modulating ER transcriptional activity in the presence or in the absence of estrogens and underline their potential role for ER tissue-specific activities. In our opinion, a better comprehension of the variety of molecular events that control ER activity in reproductive and non-reproductive organs is the foundation for the design of safer and more efficacious hormone-based therapies, particularly for menopause. This article is part of a Special Issue entitled: Translating Nuclear receptors from health to disease.

Estrogen receptor β signaling regulates the progression of Chinese non-small cell lung cancer

Prospective studies have found that the risk of non-small cell lung cancer (NSCLC) has close relationship with estrogen. The effects of estrogens are mediated via two estrogen receptor (ER) isoforms, that is, ER alpha (ERα) and ER beta (ERβ). ERα in NSCLC has been evaluated mostly by immunohistochemistry. However, our previous study showed that ERβ was also highly expressed in Chinese NSCLC. But the roles of ERβ in Chinese NSCLC have not been clarified as yet. So in the present study, two Chinese lung adenocarcinoma cell lines, SPC-A1 and LTEP-a2, were used and the role of ERβ in lung tumorigenesis was focused to be investigated by in vitro and in vivo experiments. The results showed that over-expressed ERβ can promote the development of NSCLC, while siRNAs targeting ERβ gene can inhibit growth of NSCLC cells and induce apoptosis of these cells via mitochondrial depolarization and caspase-3 activation. These results indicated that ERβ plays an important role in development of Chinese NSCLC. This suggests that ERβ deactivation or down-regulation may possess potential therapeutic utility for the treatment of lung cancer.

17β-Estradiol-induced cell proliferation requires estrogen receptor (ER) α monoubiquitination

Protein monoubiquitination (monoUbq) (i.e., the attachment of one single ubiquitin to the substrate) is a non-proteolytic reversible modification that controls protein functions. Among other proteins, the estrogen receptor α (ERα), which mediates the pleiotropic effects of the cognate hormone 17β-estradiol (E2), is a monoubiquitinated protein. Although it has been demonstrated that E2 rapidly reduces ERα monoUbq in breast cancer cells, the impact of monoUbq in the regulation of the ERα activities is poorly appreciated. Here, we show that mutation of the ERα monoUbq sites prevents the E2-induced ERα phosphorylation in the serine residue 118 (S118), reduces ERα transcriptional activity, and precludes the ERα-mediated extranuclear activation of signaling pathways (i.e., AKT activation) thus impeding the E2-induced cyclin D1 promoter activation and consequently cell proliferation. In addition, the interference with ERα monoUbq deregulates E2-induced association of ERα to the insulin like growth factor receptor (IGF-1-R). Altogether these data demonstrate an inherent role for monoUbq in ERα signaling and point to the physiological function of ERα monoUbq in the regulation of E2-induced cell proliferation.

The IGF pathway regulates ERα through a S6K1-dependent mechanism in breast cancer cells

The IGF pathway stimulates malignant behavior of breast cancer cells. Herein we identify the mammalian target of rapamycin (mTOR)/S6 kinase 1 (S6K1) axis as a critical component of IGF and estrogen receptor (ER)α cross talk. The insulin receptor substrate (IRS) adaptor molecules function downstream of IGF-I receptor and dictate a specific biological response, in which IRS-1 drives proliferation and IRS-2 is linked to motility. Although rapamycin-induced mTOR inhibition has been shown to block IGF-induced IRS degradation, we reveal differential effects on motility (up-regulation) and proliferation (down-regulation). Because a positive correlation between IRS-1 and ERα expression is thought to play a central role in the IGF growth response, we investigated the potential role of ERα as a downstream mTOR target. Small molecule inhibition and targeted knockdown of S6K1 blocked the IGF-induced ERα(S167) phosphorylation and did not influence ligand-dependent ERα(S118) phosphorylation. Inhibition of S6K1 kinase activity consequently ablated IGF-stimulated S6K1/ERα association, estrogen response element promoter binding and ERα target gene transcription. Moreover, site-specific ERα(S167) mutation reduced ERα target gene transcription and blocked IGF-induced colony formation. These findings support a novel link between the IGF pathway and ERα, in which the translation factor S6K1 affects transcription of ERα-regulated genes.

Nuclear receptor coactivators: structural and functional biochemistry

Transcription of eukaryotic cell is a multistep process tightly controlled by concerted action of macromolecules. Nuclear receptors are ligand-activated sequence-specific transcription factors that bind DNA and activate (or repress) transcription of specific sets of nuclear target genes. Successful activation of transcription by nuclear receptors and most other transcription factors requires "coregulators" of transcription. Coregulators make up a diverse family of proteins that physically interact with and modulate the activity of transcription factors and other components of the gene expression machinery via multiple biochemical mechanisms. The coregulators include coactivators that accomplish reactions required for activation of transcription and corepressors that suppress transcription. This review summarizes our current knowledge of nuclear receptor coactivators with an emphasis on their biochemical mechanisms of action and means of regulation.

Epigenetically deregulated microRNA-375 is involved in a positive feedback loop with estrogen receptor alpha in breast cancer cells

Estrogen receptor α (ERα) upregulation causes abnormal cell proliferation in about two thirds of breast cancers, yet understanding of the underlying mechanisms remains incomplete. Here, we show that high expression of the microRNA miR-375 in ERα-positive breast cell lines is a key driver of their proliferation. miR-375 overexpression was caused by loss of epigenetic marks including H3K9me2 and local DNA hypomethylation, dissociation of the transcriptional repressor CTCF from the miR-375 promoter, and interactions of ERα with regulatory regions of miR-375. Inhibiting miR-375 in ERα-positive MCF-7 cells resulted in reduced ERα activation and cell proliferation. A combination of expression profiling from tumor samples and miRNA target prediction identified RASD1 as a potential miR-375 target. Mechanistic investigations revealed that miR-375 regulates RASD1 by targeting the 3' untranslated region in RASD1 mRNA. Additionally, we found that RASD1 negatively regulates ERα expression. Our findings define a forward feedback pathway in control of ERα expression, highlighting new strategies to treat ERα-positive invasive breast tumors.

Interaction of estrogen receptors with insulin-like growth factor-I and Wnt signaling in the nervous system

Estradiol signaling through estrogen receptors in the nervous system involves a variety of rapid membrane/cytoplasm-initiated events that are integrated with different mechanisms of transcriptional regulation. Here we review the role of glycogen synthase kinase 3 (GSK3) and beta-catenin in the coordination of membrane/cytoplasm-initiated and nuclear-initiated estrogen receptor signaling. Estradiol activates in vitro and in vivo the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway in neural cells. By activating this pathway through estrogen receptors, estradiol increases the levels of inactive GSK3beta (phosphorylated in serine 9). In turn, the inhibition of GSK3beta increases the stability of beta-catenin and its nuclear translocation. Then, beta-catenin exerts two different transcriptional effects: (i) regulates beta-catenin/T cell factor (TCF) mediated transcription in a similar but not identical way as Wnt ligands and (ii) regulates estrogen receptor mediated transcription after its association with estrogen receptor alpha. In addition, by the regulation of PI3K/Akt/GSK3/beta-catenin pathway, other factors such as insulin-like growth factor-I (IGF-I) regulate estrogen receptor mediated transcription. Therefore, GSK3 and beta-catenin allow the interaction of membrane/cytoplasm-initiated estrogen receptor signaling, IGF-I signaling, Wnt signaling and nuclear-initiated estrogen receptor signaling in the nervous system.

Glycogen synthase kinase-3beta negatively regulates TGF-beta1 and Angiotensin II-mediated cellular activity through interaction with Smad3

Glycogen synthase kinase-3beta (GSK3beta) is a major negative modulator of cardiac hypertrophy. Here we report that GSK3beta physically and functionally interacts with Smad3. The interaction between GSK3beta and Smad3 may participate in the negative regulation of transforming growth factor beta1 (TGF-beta1) and Angiotensin II-induced transcription and apoptosis. GSK3beta interacted directly with Smad3 to sequester it outside the nucleus and prevent its nuclear translocation. This resulted in the suppression of Smad3-mediated transcriptional activity and gene expression. GSK3beta counteracted the pro-apoptotic effect of Smad3 and attenuated Angiotensin II-induced apoptosis in cardiac myocytes. Furthermore, stimulation of these cells with TGF-beta1 and Angiotensin II led to the endogenous Smad3 disassociating from GSK3beta and inactivating GSK3beta by phosphorylation of its Ser9. These results uncovered a novel mechanism for the GSK3beta negative regulation of TGF-beta1/Smad3 and Angiotensin II/Smad3-mediated transcription and apoptosis by the identification of a crosstalk between GSK3beta and Smad3 signal pathway.

Signal integration through blending, bolstering and bifurcating of intracellular information

A cell's response to its environment is often determined by signaling through the actions of enzyme cascades. The ability to organize these enzymes into multiprotein complexes allows for a high degree of fidelity, efficiency and spatial precision in signaling responses.

Inhibition of glycogen synthase kinase-3beta promotes nuclear export of the androgen receptor through a CRM1-dependent mechanism in prostate cancer cell lines

The androgen receptor (AR) is a ligand-dependent transcription factor belonging to the steroid hormone receptor superfamily. Under normal conditions, in the absence of a ligand, the AR is localized to the cytoplasm and is actively transported into the nucleus upon binding of androgens. In advanced prostate cancer (PCa) cell lines, an increased sensitivity to dihydrotestosterone (DHT), enabling the cells to proliferate under sub-physiological levels of androgens, has been associated with increased stability and nuclear localization of the AR. There is experimental evidence that the glycogen synthase kinase-3beta (GSK-3beta), a multifunctional serine/threonine kinase is involved in estrogen and AR stability. As demonstrated in the following study by immunoprecipitation analysis, GSK-3beta binds to the AR forming complexes in the cytoplasm and in the nucleus. Furthermore, inhibition of GSK-3beta activity by pharmacological inhibitors like the maleimide SB216761, the chloromethyl-thienyl-ketone GSK-3 inhibitor VI or the aminopyrazol GSK-3 inhibitor XIII in cells grown in the presence of DHT triggered a rapid nuclear export of endogenous AR as well as of green fluorescent AR-EosFP. The nuclear export of AR following GSK-3beta inhibition could be blocked by leptomycin B suggesting a CRM1-dependent export mechanism. This assumption is supported by the localization of a putative CRM1 binding site at the C-terminus of the AR protein. The results suggest that GSK-3beta is an important element not only in AR stability but also significantly alters nuclear translocation of the AR, thereby modulating the androgenic response of human PCa cells.

Interaction of the double-strand break repair kinase DNA-PK and estrogen receptor-alpha

Here we show that, upon estrogen stimulation, DNA-dependent protein kinase (DNA-PK) forms a complex with estrogen receptor-α in a breast cancer cell line (MELN). Inhibition of DNA-PK by siRNA technology demonstrated that estrogen-induced ERα activation and cell cycle progression is, at least, partially dependent on DNA-PK.

Estrogens are suggested to play a role in the development and progression of proliferative diseases such as breast cancer. Like other steroid hormone receptors, the estrogen receptor-α (ERα) is a substrate of protein kinases, and phosphorylation has profound effects on its function and activity. Given the importance of DNA-dependent protein kinase (DNA-PK) for DNA repair, cell cycle progression, and survival, we hypothesized that it modulates ERα signaling. Here we show that, upon estrogen stimulation, DNA-PK forms a complex with ERα in a breast cancer cell line (MELN). DNA-PK phosphorylates ERα at Ser-118. Phosphorylation resulted in stabilization of ERα protein as inhibition of DNA-PK resulted in its proteasomal degradation. Activation of DNA-PK by double-strand breaks or its inhibition by siRNA technology demonstrated that estrogen-induced ERα activation and cell cycle progression is, at least, partially dependent on DNA-PK.

Transcriptional activation of DNA-dependent protein kinase catalytic subunit gene expression by oestrogen receptor-alpha

In this report Braun-Dullaeus and co-workers demonstrate that DNA-PK can be directly induced by oestrogen through increased binding of the transcription factor ERα to the DNA-PKcs promoter and identify functional oestrogen responsive DNA elements (EREs) within the DNA-PKcs promoter. Oestrogen-induced DNA-PK transactivation results in an increased ability of the cells to repair DNA double-strand breaks.

The cellular response to DNA double-strand break (DSB) occurs through an integrated sensing and signalling network that maintains genomic stability. Oestrogen (E2), among its many functions, is known to have a positive effect on global genomic DNA repair; however, the mechanism by which it functions is unclear. A central enzyme involved in DNA DSB repair in mammalian cells is the DNA-dependent protein kinase (DNA-PK). Here, we show that E2 enhances DNA-PK catalytic subunit (DNA-PKcs) promoter activity with subsequent transcriptional and translational upregulation of DNA-PKcs in a breast cancer cell line. We identify two potential E2 receptor-α (ERα)-binding sites in a region upstream from the DNA-PKcs initiation site. By using small interfering RNA and the specific E2 receptor antagonist ICI 182,780, we demonstrate that ERα knockdown reduces E2-induced upregulation of DNA-PKcs expression and activity in breast carcinoma cells. E2-induced DNA-PK transactivation results in an increased ability of the cells to repair DNA DSB. This previously unknown mechanism of DNA-PK regulation sheds new light on tumour biology and reveals new possibilities for the prevention and therapy of E2-sensitive proliferative diseases.

c-Abl regulates estrogen receptor alpha transcription activity through its stabilization by phosphorylation

Estrogen receptors are members of the steroid hormone superfamily of nuclear receptors that act as ligand-activated transcription factors. Similar to other steroid hormone receptors, estrogen receptor alpha (ERalpha) is a substrate for protein kinases, and phosphorylation has profound effects on the function of this receptor. In this study, we show that ERalpha associates with c-Abl nonreceptor tyrosine kinase. The direct interaction is mediated by two PXXP motifs of ERalpha and the c-Abl SH3 domain. Mutational analysis and in vitro kinase assays show that ERalpha can be phosphorylated on two sites, tyrosine 52 (Y-52) and tyrosine 219 (Y-219). ERalpha phosphorylation by c-Abl stabilizes ERalpha, resulting in enhanced ERalpha transcriptional activity and increased expression of endogenous ERalpha target genes. Furthermore, ERalpha phosphorylation at the Y-219 site affects DNA binding and dimerization by ERalpha. Both the c-Abl inhibitor and the c-Abl kinase dead mutation abolish the c-Abl-induced accumulation of ERalpha and enhancement of ERalpha transcriptional activity, indicating that c-Abl kinase activity is required for regulation of the ERalpha function. Moreover, the ERalpha (Y52,219F) mutant shows reduced breast cancer cell growth and invasion. Taken together, these results show that c-Abl is a novel kinase that upregulates ERalpha expression and promotes breast cancer cell proliferation, suggesting a great potential for this kinase to function as a therapeutic target for breast cancer.

ERalpha signaling through slug regulates E-cadherin and EMT

The ERalpha signaling pathway is one of the most important and most studied pathways in human breast cancer, yet numerous questions still exist such as how hormonally responsive cancers progress to a more aggressive and hormonally independent phenotype. We have noted that human breast cancers exhibit a strong direct correlation between ERalpha and E-cadherin expression by immunohistochemistry, suggesting that ERalpha signaling might regulate E-cadherin and implying that this regulation might influence epithelial-mesenchymal transition (EMT) and tumor progression. To investigate this hypothesis and the mechanisms behind it, we studied the effects of ERalpha signaling in ERalpha-transfected ERalpha-negative breast carcinoma cell lines, the MDA-MB-468 and the MDA-MB-231 and the effects of ERalpha knockdown in naturally expressing ERalpha-positive lines, MCF-7 and T47D. When ERalpha was overexpressed in the ERalpha-negative lines, 17beta-estradiol (E2) decreased slug and increased E-cadherin. Clones maximally exhibiting these changes grew more in clumps and became less invasive in Matrigel. When ERalpha was knocked down in the ERalpha-positive lines, slug increased, E-cadherin decreased, cells became spindly and exhibited increased Matrigel invasion. ERalpha signaling decreased slug expression by two different mechanisms: directly, by repression of slug transcription by the formation of a corepressor complex of ligand-activated ERalpha, HDAC inhibitor (HDAC1), and nuclear receptor corepressor (N-CoR) that bound the slug promoter in three half-site estrogen response elements (EREs); indirectly by phosphorylation and inactivation of GSK-3beta through phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt). The GSK-3beta inactivation, in turn, repressed slug expression and increased E-cadherin. In human breast cancer cases, there was a strong inverse correlation between slug and ERalpha and E-cadherin immunoreactivity. Our findings indicate that ERalpha signaling through slug regulates E-cadherin and EMT.

Identification of four novel phosphorylation sites in estrogen receptor alpha: impact on receptor-dependent gene expression and phosphorylation by protein kinase CK2

Background

Estrogen receptor α (ERα) phosphorylation is important for estrogen-dependent transcription of ER-dependent genes, ligand-independent receptor activation and endocrine therapy response in breast cancer. However ERα phosphorylation at the previously identified sites does not fully account for these receptor functions. To determine if additional ERα phosphorylation sites exist, COS-1 cells expressing human ERα were labeled with [³²P]H₃PO₄ in vivo and ERα tryptic phosphopeptides were isolated to identify phosphorylation sites.

Results

Previously uncharacterized phosphorylation sites at serines 46/47, 282, 294, and 559 were identified by manual Edman degradation and phosphoamino acid analysis and confirmed by mutagenesis and phospho-specific antibodies. Antibodies detected phosphorylation of endogenous ERα in MCF-7, MCF-7-LCC2, and Ishikawa cancer cell lines by immunoblot. Mutation of Ser-282 and Ser-559 to alanine (S282A, S559A) resulted in ligand independent activation of ERα as determined by both ERE-driven reporter gene assays and endogenous pS2 gene expression in transiently transfected HeLa cells. Mutation of Ser-46/47 or Ser-294 to alanine markedly reduced estradiol dependent reporter activation. Additionally protein kinase CK2 was identified as a kinase that phosphorylated ERα at S282 and S559 using motif analysis, in vitro kinase assays, and incubation of cells with CK2 kinase inhibitor.

Conclusion

These novel ERα phosphorylation sites represent new means for modulation of ERα activity. S559 represents the first phosphorylation site identified in the extreme C-terminus (F domain) of a steroid receptor.

Specific involvement of glycogen synthase kinase-3 in the function and activity of sex steroid hormone receptors reveals the complexity of their regulation

Protein kinases represent key nodes for the integration of multiple intracellular signalling pathways, resulting in modulation of both ligand-dependent and ligand-independent mechanisms of sex steroid receptor (sSR) signalling cascades. The proline-directed Ser/Thr kinases including mitogen-activated protein kinases and cyclin dependent kinases were especially reported to contribute to the function and activity of sSRs. The relevant effects of these kinases are well-documented but the impact of glycogen synthase kinase-3 (GSK-3), another member of this kinase family, has been underestimated. Indeed, the specific role of GSK-3 regarding the different sSRs will help to understand further the complexity of sSR signalling. So far, AR and ERalpha were identified as GSK-3 substrates. Additionally, the docking properties of GSK-3 were demonstrated to play a crucial role in sSR signal transduction. Reciprocally, GSK-3 was described as a potential target of non-genomic effects of sSRs. Therefore, GSK-3 regulates and is regulated by sSRs. This review focuses on the emerging and promising involvements of GSK-3 regarding the signalling cascade of the respective sSRs. This review represents a necessary complement of information to highlight the importance of GSK-3 regarding sSR function and activity.

Steroid receptor phosphorylation: Assigning function to site-specific phosphorylation

Steroid receptors (SRs) are hormone-activated transcription factors important for a wide variety of cellular functions. Post-translational modifications of SRs, including phosphorylation, ubiquitination, acetylation, and sumoylation regulate their expression and function. The remarkable number of phosphorylation sites in these receptors and the wide variety of kinases shown to modulate phosphorylation influence the integration between cell-signaling pathways and SR action. These phosphorylation sites have been identified in all of the functional domains with the majority being located within the amino-terminal portions of the receptors. The regulation of function is receptor specific, site specific, and often dependent on the cellular context. Numerous roles for site-specific phosphorylation have been elucidated including sensitivity of hormone response, DNA binding, expression, stability, subcellular localization, dimerization, and protein-protein interactions that can determine the regulation of specific target genes. This review summarizes the current knowledge regarding receptor site-specific phosphorylation and regulation of function. As functional assays become more sophisticated, it is likely that additional roles for phosphorylation in receptor function will be identified.

The DEAD-box protein p72 regulates ERalpha-/oestrogen-dependent transcription and cell growth, and is associated with improved survival in ERalpha-positive breast cancer

The DEAD-box RNA helicases p68 (DDX5) and p72 (DDX17) have been shown to act as transcriptional co-activators for a diverse range of transcription factors, including estrogen receptor α (ERα). Here, we show that, although both proteins interact with and co-activate ERα in reporter gene assays, siRNA-mediated knockdown of p72, but not p68, results in a significant inhibition of estrogen-dependent transcription of endogenous ERα-responsive genes and estrogen-dependent growth of MCF-7 and ZR75-1 breast cancer cells. Furthermore, immunohistochemical staining of ERα-positive primary breast cancers for p68 and p72 indicate that p72 expression is associated with an increased period of relapse-free and overall survival (p=0.006 and p=0.016 respectively), as well as being inversely associated with Her2 expression (p=0.008). Conversely, p68 shows no association with relapse-free period, or overall, survival but it is associated with an increased expression of Her2 (p=0.001), AIB-1 (p<0.001) and higher tumour grade (p=0.044). Our data thus highlight a crucial role for p72 in ERα co-activation and estrogen-dependent cell growth and provide evidence in support of distinct but important roles for both p68 and p72 in regulating ERα activity in breast cancer.

Proliferative effect of Apidra (insulin glulisine), a rapid-acting insulin analogue on mammary epithelial cells

The structural modification of insulin results in the generation of insulin analogues that show altered binding affinities to the insulin receptor and/or the IGF-I receptor, and as a consequence insulin analogues may have altered mitogenic potency. We analysed the proliferative effect of the rapid-acting insulin analogue Apidra (insulin glulisine) on mammary epithelial cells. We show that Apidra and Actrapid (recombinant human insulin) have similar proliferative effects on benign MCF10A and tumorigenic MCF7 cells and on epithelial cells of mouse mammary gland. Whereas Apidra and Actrapid induced similar activation of Erk1/2, activation of Akt/PKB by Apidra was significantly weaker compared to regular insulin. As AKT/PKB, an effector of the phosphoinositide 3-kinase pathway, mediates metabolic effects of insulin, we studied induction of hexokinase-2 in MCF7 cells and hexokinase-2 and hexokinase-4 in HepG2 cells by Actrapid and Apidra. Both genes were not significantly induced by Actrapid and Apidra in these cell lines.

Prolactin and estradiol utilize distinct mechanisms to increase serine-118 phosphorylation and decrease levels of estrogen receptor alpha in T47D breast cancer cells

Potential interactions between prolactin (PRL) and estradiol (E2) in breast cancer cells were explored by examining the effect of PRL on estrogen receptor (ER) serine-118 phosphorylation, ER down-regulation, and E2-stimulated cell proliferation. Both E2 and PRL resulted in prolonged ERalpha serine-118 phosphorylation, but used different signaling pathways to achieve this end. Both hormones also decreased the amount of ERalpha, but the mechanisms were different: for E2, the decrease was rapid and resulted from proteasomic degradation, whereas for PRL the decrease was slow and resulted from an effect on levels of ERalpha mRNA. PRL alone had no effect on cell number, but enhanced the increase in number in response to E2. These results are the first to demonstrate similar effects of PRL and E2 on parameters considered key to E2's effects. This suggests heretofore unrecognized and potentially important interactions between these two hormones in the natural history of breast cancer.

CK1delta modulates the transcriptional activity of ERalpha via AIB1 in an estrogen-dependent manner and regulates ERalpha-AIB1 interactions

Oncogenesis in breast cancer often requires the overexpression of the nuclear receptor coactivator AIB1/SRC-3 acting in conjunction with estrogen receptor-α (ERα). Phosphorylation of both ERα and AIB1 has been shown to have profound effects on their functions. In addition, proteasome-mediated degradation plays a major role by regulating their stability and activity. CK1δ, a member of the ubiquitous casein kinase-1 family, is implicated in the progression of breast cancer. In this study, we show that both ERα and AIB1 are substrates for CK1δ in vitro, and identify a novel AIB1 phosphorylation site (S601) targeted by CK1δ, significant for the co-activator function of AIB1. CK1δ is able to interact with ERα and AIB1 in vivo, while overexpression of CK1δ in breast cancer cells results in an increased association of ERα with AIB1 as confirmed by co-immunoprecipitation assays from cell lysates. Using an siRNA-based approach, luciferase reporter assays and qRT-PCR, we observe that silencing of CK1δ leads to reduced ERα transcriptional activity, despite increased ERα levels, similarly to proteasome inhibition. We provide evidence that AIB1 protein levels are reduced by CK1δ silencing, in an estradiol-dependent manner; such destabilization can be inhibited by pre-treatment with the proteasome inhibitor MG132. We propose that differing activities adopted by ERα and AIB1 as a consequence of their interactions with and phosphorylation by CK1δ, particularly AIB1 stabilization, influence the transcriptional activity of ERα, and therefore have a role in breast cancer development.

Estradiol and lithium chloride specifically alter NMDA receptor subunit NR1 mRNA and excitotoxicity in primary cultures

Glutamate facilitates calcium influx via NMDAR, and excess calcium influx increases excitotoxicity--a pathological characteristic of neurological diseases. Both 17beta-estradiol (E2) and lithium influence NMDAR expression/signaling and excitotoxicity. This led us to hypothesize that combined E2 and lithium will alter NMDAR expression and excitotoxicity. We tested this hypothesis using primary cell cultures from the cortex and hippocampus of C57BL/6J fetal mice pretreated with E2, lithium chloride (LiCl) and combined E2/LiCl for 12, 24 or 48 h. We examined cultures for brain cell type and changes in cell type caused by experimental procedures using glia and neuron gene specific primers. These cultures expressed increased glial fibrillary acidic protein (GFAP) mRNA with low neurofilament-heavy chain (NF-H) mRNA expression. Subsequent analysis of cortical cell cultures indicated that combined E2/LiCl decreased NR1 mRNA expression after a 12 and 48 h treatment period. Combined E2/LiCl also reduced NR1 mRNA expression in hippocampal cultures but only after a 48 h treatment period. LiCl-treated hippocampal cultures also reduced NR1 mRNA expression after a 24 and 48 h treatment. We next examined the response of 48 h pretreated cultures to a toxic level of glutamate. Excitotoxicity was measured using fluorescein diacetate/propidium iodide (FDA/PI) cell viability assay. Results from FDA/PI assay revealed that LiCl pretreatment increased viability for cortical cultures while E2 and combined E2/LiCl reduced viability. All pretreatments for hippocampal cultures failed to increase viability. Our results showed combined E2/LiCl reduced NR1 mRNA and prevented protection against glutamate excitotoxicity in glial primary cultures.

The transactivating function 1 of estrogen receptor alpha is dispensable for the vasculoprotective actions of 17beta-estradiol

Full-length 66-kDa estrogen receptor alpha (ERalpha) stimulates target gene transcription through two activation functions (AFs), AF-1 in the N-terminal domain and AF-2 in the ligand binding domain. Another physiologically expressed 46-kDa ERalpha isoform lacks the N-terminal A/B domains and is consequently devoid of AF-1. Previous studies in cultured endothelial cells showed that the N-terminal A/B domain might not be required for estradiol (E2)-elicited NO production. To evaluate the involvement of ERalpha AF-1 in the vasculoprotective actions of E2, we generated a targeted deletion of the ERalpha A/B domain in the mouse. In these ERalphaAF-1(0) mice, both basal endothelial NO production and reendothelialization process were increased by E2 administration to a similar extent than in control mice. Furthermore, exogenous E2 similarly decreased fatty streak deposits at the aortic root from both ovariectomized 18-week-old ERalphaAF-1(+/+) LDLr(-/-) (low-density lipoprotein receptor) and ERalphaAF-1(0) LDLr (-/-) mice fed with a hypercholesterolemic diet. In addition, quantification of lesion size on en face preparations of the aortic tree of 8-month-old ovariectomized or intact female mice revealed that ERalpha AF-1 is dispensable for the atheroprotective action of endogenous estrogens. We conclude that ERalpha AF-1 is not required for three major vasculoprotective actions of E2, whereas it is necessary for the effects of E2 on its reproductive targets. Thus, selective ER modulators stimulating ERalpha with minimal activation of ERalpha AF-1 could retain beneficial vascular actions, while minimizing the sexual effects.

Activation of the unliganded estrogen receptor by prolactin in breast cancer cells

Both prolactin (PRL) and estrogen (E2) are involved in the pathogenesis and progression of mammary neoplasia, but the mechanisms by which these hormones interact to exert their effects in breast cancer cells are not well understood. We show here that PRL is able to activate the unliganded estrogen receptor (ER). In breast cancer cells, PRL activates a reporter plasmid containing estrogen response elements (EREs) and induces the ER target gene pS2. These actions are blocked by the antagonist ICI 182,780, showing that ER is required for the PRL-mediated effect. Moreover, PRL leads to phosphorylation of ERalpha in serine-118 (P-ERalpha), a modification related to the potentiation of ligand-independent transcriptional activation. In addition, PRL mimics the effect of E2 on target gene expression by inducing cyclical recruitment of ERalpha and P-ERalpha to ERE-containing promoters, resulting in recruitment of co-activators and acetylation of histone H3. Finally, PRL induces expression of c-Myc and Cyclin D1 and leads to increased cell proliferation, which is specifically antagonized by ICI 182,780 or ERalpha depletion. These results show that ligand-independent ERalpha activation appears to be an important component of the proliferative and transcriptional actions of PRL in breast cancer cells.

ERalpha suppresses slug expression directly by transcriptional repression

Two of the most common signalling pathways in breast cancer are the ER (oestrogen receptor) ligand activation pathway and the E-cadherin snai1 slug EMT (epithelial-mesenchymal transition) pathway. Although these pathways have been thought to interact indirectly, the present study is the first to observe direct interactions between these pathways that involves the regulation of slug expression. Specifically we report that ligand-activated ERalpha suppressed slug expression directly by repression of transcription and that knockdown of ERalpha with RNA interference increased slug expression. More specifically, slug expression was down-regulated in ERalpha-negative MDA-MB-468 cells transfected with ERalpha after treatment with E2 (17beta-oestradiol). The down-regulation of slug in the ERalpha-positive MCF-7 cell line was mediated by direct repression of slug transcription by the formation of a co-repressor complex involving ligand-activated ERalpha protein, HDAC1 (histone deacetylase 1) and N-CoR (nuclear receptor co-repressor). This finding was confirmed by sequential ChIP (chromatin immunoprecipitation) studies. In the MCF-7 cell line, slug expression normally was low. In addition, knockdown of ERalpha with RNA interference in this cell line increased slug expression. This effect could be partially reversed by treatment of the cells with E2. The efficacy of the effect of ERalpha on slug repression was dependent on the overall level of ERalpha. These observations confirmed that slug was an E2-responsive gene.

Systematic mapping of posttranslational modifications in human estrogen receptor-alpha with emphasis on novel phosphorylation sites

A systematic study of posttranslational modifications of the estrogen receptor isolated from the MCF-7 human breast cancer cell line is reported. Proteolysis with multiple enzymes, mass spectrometry, and tandem mass spectrometry achieved very high sequence coverage for the full-length 66-kDa endogenous protein from estradiol-treated cell cultures. Nine phosphorylated serine residues were identified, three of which were previously unreported and none of which were previously observed by mass spectrometry by any other laboratory. Two additional modified serine residues were identified in recombinant protein, one previously reported but not observed here in endogenous protein and the other previously unknown. Although major emphasis was placed on identifying new phosphorylation sites, N-terminal loss of methionine accompanied by amino acetylation and a lysine side chain acetylation (or possibly trimethylation) were also detected. The use of both HPLC-ESI and MALDI interfaced to different mass analyzers gave higher sequence coverage and identified more sites than could be achieved by either method alone. The estrogen receptor is critical in the development and progression of breast cancer. One previously unreported phosphorylation site identified here was shown to be strongly dependent on estradiol, confirming its potential significance to breast cancer. Greater knowledge of this array of posttranslational modifications of estrogen receptor, particularly phosphorylation, will increase our understanding of the processes that lead to estradiol-induced activation of this protein and may aid the development of therapeutic strategies for management of hormone-dependent breast cancer.

Inhibition of glycogen synthase kinase-3 in androgen-responsive prostate cancer cell lines: are GSK inhibitors therapeutically useful?

The glycogen synthase kinase 3 (GSK-3) is a serine/threonine kinase widely expressed in mammalian tissues. Initially identified by its ability to modulate glycogen synthesis, GSK-3 turned out to be a multifunctional enzyme, able to phosphorylate many proteins, including members of the steroid receptor superfamily. Although GSK-3 was shown to phosphorylate the androgen receptor (AR), its effects on AR transcriptional activity remain controversial. Analysis of short hairpin RNA (shRNA)-mediated downmodulation of GSK-3 proteins in prostate cancer cells showed a reduction in AR transcriptional activity and AR protein levels. Pharmacological GSK-3 inhibitors such as the maleimide SB216763 or the aminopyrazole GSK inhibitor XIII inhibited AR-dependent reporter gene activity and AR expression in vitro. Analysis of androgen-induced nuclear translocation of the AR was performed in PC3 cells transfected with pAR-t1EosFP coding for EosAR, a green fluorescent AR fusion protein. When grown in presence of androgens, EosAR was predominantly nuclear. Incubation with SB216763 before and after androgen treatment almost completely reduced nuclear EosAR. In contrast, the thiazole-containing urea compound AR-A014418 increased rather than decreased AR-expression/function. Although not all GSK inhibitors affected AR-stability/function, our observations suggest a potential new therapeutic application for some of these compounds in prostate cancer.

Gender differences and estrogen effects in parkin null mice

Estrogens are considered neurotrophic for dopamine neurons. Parkinson's disease is more frequent in males than in females, and more prevalent in females with short reproductive life. Estrogens are neuroprotective against neurotoxic agents for dopamine neurons in vivo and in vitro. Here, we have investigated the role of estrogens in wild-type (WT) and parkin null mice (PK-/-). WT mice present sexual dimorphisms in neuroprotective mechanisms (Bcl-2/Bax, chaperones, and GSH), but some of these inter-sex differences disappear in PK-/-. Tyrosine hydroxylase (TH) protein and TH+ cells decreased earlier and more severely in female than in male PK-/- mice. Neuronal cultures from midbrain of WT and PK-/- mice were treated with estradiol from 10 min to 48 h. Short-term treatments activated the mitogen-activated protein kinase pathway of WT and PK-/- neurons and the phosphatidylinositol 3'-kinase/AKT/glycogen synthase kinase-3 pathway of WT but not of PK-/- cultures. Long-term treatments with estradiol increased the number of TH+ neurons, the TH expression, and the extension of neurites, and decreased the level of apoptosis, the expression of glial fibrillary acidic protein, and the number of microglial cells in WT but not in PK-/- cultures. The levels of estrogen receptor-alpha were elevated in midbrain cultures and in the striatum of adult PK-/- male mice, suggesting that suppression of parkin changes the estrogen receptor-alpha turnover. From our data, it appears that parkin participates in the cellular estrogen response which could be of interest in the management of parkin-related Parkinson's disease patients.

Phosphorylation at serines 104 and 106 by Erk1/2 MAPK is important for estrogen receptor-alpha activity

Phosphorylation of estrogen receptor-alpha (ERalpha) at specific residues in transcription activation function 1 (AF-1) can stimulate ERalpha activity in a ligand-independent manner. This has led to the proposal that AF-1 phosphorylation and the consequent increase in ERalpha activity could contribute to resistance to endocrine therapies in breast cancer patients. Previous studies have shown that serine 118 (S118) in AF-1 is phosphorylated by extracellular signal-regulated kinases 1 and 2 (Erk1/2) mitogen-activated protein kinase (MAPK) in a ligand-independent manner. Here, we show that serines 104 (S104) and 106 (S106) are also phosphorylated by MAPK in vitro and upon stimulation of MAPK activity in vivo. Phosphorylation of S104 and S106 can be inhibited by the MAP-erk kinase (MEK)1/2 inhibitor U0126 and by expression of kinase-dead Raf1. Further, we show that, although S118 is important for the stimulation of ERalpha activity by the selective ER modulator 4-hydroxytamoxifen (OHT), S104 and S106 are also required for the agonist activity of OHT. Acidic amino acid substitution of S104 or S106 stimulates ERalpha activity to a greater extent than the equivalent substitution at S118, suggesting that phosphorylation at S104 and S106 is important for ERalpha activity. Collectively, these data indicate that the MAPK stimulation of ERalpha activity involves the phosphorylation not only of S118 but also of S104 and S106, and that MAPK-mediated hyperphosphorylation of ERalpha at these sites may contribute to resistance to tamoxifen in breast cancer.

Estradiol up-regulates AUF1p45 binding to stabilizing regions within the 3'-untranslated region of estrogen receptor alpha mRNA

Estradiol up-regulates expression of the estrogen receptor alpha gene in the uterus by stabilizing estrogen receptor alpha mRNA. Previously, we defined two discrete minimal estradiol-modulated stability sequences (MEMSS) within the extensive 3'-untranslated region of estrogen receptor alpha mRNA with an in vitro stability assay using cytosolic extracts from sheep uterus. We report here that excess MEMSS RNA inhibited the enhanced stability of estrogen receptor alpha mRNA in extracts from estradiol-treated ewes compared with those from control ewes. Several estradiol-induced MEMSS-binding proteins were characterized by UV cross-linking in uterine extracts from ewes in a time course study (0, 8, 16, and 24 h after estradiol injection). The pattern of binding proteins changed at 16 h post-injection, concurrent with enhanced estrogen receptor alpha mRNA stability and the highest rate of accumulation of estrogen receptor alpha mRNA. The predominant MEMSS-binding protein induced by estradiol treatment was identified as AUF1 (A + U-rich RNA-binding factor 1) protein isoform p45 (a product of the heterogeneous nuclear ribonucleoprotein D gene). Immunoblot analysis indicated that only two of four AUF1 protein isoforms were present in the uterine cytosolic extracts and that estradiol treatment strongly increased the ratio of AUF1 isoforms p45 to p37. Nonphosphorylated recombinant AUF1p45 protected estrogen receptor alpha mRNA in vitro in a dose-dependent manner. These studies describe estrogenic induction of AUF1p45 binding to the estrogen receptor alpha mRNA as a molecular mechanism for post-transcriptional up-regulation of gene expression.