Acetylation of estrogen receptor alpha by p300 at lysines 266 and 268 enhances the deoxyribonucleic acid binding and transactivation activities of the receptor

Control of progesterone receptor transcriptional synergy by SUMOylation and deSUMOylation

Background

Covalent modification of nuclear receptors by the Small Ubiquitin-like Modifier (SUMO) is dynamically regulated by competing conjugation/deconjugation steps that modulate their overall transcriptional activity. SUMO conjugation of progesterone receptors (PRs) at the N-terminal lysine (K) 388 residue of PR-B is hormone-dependent and suppresses PR-dependent transcription. Mutation of the SUMOylation motif promotes transcriptional synergy.

Results

The present studies address mechanisms underlying this transcriptional synergy by using SUMOylation deficient PR mutants and PR specifically deSUMOylated by Sentrin-specific proteases (SENPs). We show that deSUMOylation of a small pool of receptors by catalytically competent SENPs globally modulates the cooperativity-driven transcriptional synergy between PR observed on exogenous promoters containing at least two progesterone-response elements (PRE₂). This occurs in part by raising PR sensitivity to ligands. The C-terminal ligand binding domain of PR is required for the transcriptional stimulatory effects of N-terminal deSUMOylation, but neither a functional PR dimerization interface, nor a DNA binding domain exhibiting PR specificity, are required.

Conclusion

We conclude that direct and reversible SUMOylation of a minor PR protein subpopulation tightly controls the overall transcriptional activity of the receptors at complex synthetic promoters. Transcriptional synergism controlled by SENP-dependent PR deSUMOylation is dissociable from MAPK-catalyzed receptor phosphorylation, from SRC-1 coactivation and from recruitment of histone deacetylases to promoters. This will provide more information for targeting PR as a part of hormonal therapy of breast cancer. Taken together, these data demonstrate that the SUMOylation/deSUMOylation pathway is an interesting target for therapeutic treatment of breast cancer.

New Neurons in Aging Brains: Molecular Control by Small Non-Coding RNAs

Adult neurogenesis generates functional neurons from neural stem cells present in specific brain regions. It is largely confined to two main regions: the subventricular zone of the lateral ventricle, and the subgranular zone of the dentate gyrus (DG), in the hippocampus. With age, the function of the hippocampus and particularly the DG is impaired. For instance, adult neurogenesis is decreased with aging, in both proliferating and differentiation of newborn cells, while in parallel an age-associated decline in cognitive performance is often seen. Surprisingly, the synaptogenic potential of adult-born neurons is only marginally influenced by aging. Therefore, although proliferation, differentiation, and synaptogenesis of adult-born new neurons in the DG are closely related to each other, they are differentially affected by aging. In this review we discuss the crucial roles of a novel class of recently discovered modulators of gene expression, the small non-coding RNAs, in the regulation of adult neurogenesis. Multiple small non-coding RNAs are differentially expressed in the hippocampus. In particular a subgroup of the small non-coding RNAs, the microRNAs, fine-tune the progression of adult neurogenesis. This makes small non-coding RNAs appealing candidates to orchestrate the functional alterations in adult neurogenesis and cognition associated with aging. Finally, we summarize observations that link changes in circulating levels of steroid hormones with alterations in adult neurogenesis, cognitive decline, and vulnerability to psychopathology in advanced age, and discuss a potential interplay between steroid hormone receptors and microRNAs in cognitive decline in aging individuals.

Estrogen receptor dependent gene expression by osteoblasts - direct, indirect, circumspect, and speculative effects

Hormone activated estrogen receptors (ERs) have long been appreciated as potent mediators of gene expression in female reproductive tissues. These highly targeted responses likely evolved from more elemental roles in lower organisms, in agreement with their widespread effects in the cardiovascular, immunological, central nervous, and skeletal tissue systems. Still, despite intense investigation, the multiple and often perplexing roles of ERs retain significant attention. In the skeleton, this in part derives from apparently opposing effects by ER agonists on bone growth versus bone remodeling, and in younger versus older individuals. The complexity associated with ER activation can also derive from their interactions with other hormone and growth factor systems, and their direct and indirect effects on gene expression. We propose that part of this complexity results from essential interactions between ERs and other transcription factors, each with their own biochemical and molecular intricacies. Solving some of the many questions that persist may help to achieve better, or better directed, use of agents that can drive ER activation in focused and possibly tissue restricted ways.

Structural and functional analysis of domains of the progesterone receptor

Steroid hormone receptors are multi-domain proteins composed of conserved well-structured regions, such as ligand (LBD) and DNA binding domains (DBD), plus other naturally unstructured regions including the amino-terminal domain (NTD) and the hinge region between the LBD and DBD. The hinge is more than just a flexible region between the DBD and LBD and is capable of binding co-regulatory proteins and the minor groove of DNA flanking hormone response elements. Because the hinge can directly participate in DNA binding it has also been termed the carboxyl terminal extension (CTE) of the DNA binding domain. The CTE and NTD are dynamic regions of the receptor that can adopt multiple conformations depending on the environment of interacting proteins and DNA. Both regions have important regulatory roles for multiple receptor functions that are related to the ability of the CTE and NTD to form multiple active conformations. This review focuses on studies of the CTE and NTD of progesterone receptor (PR), as well as related work with other steroid/nuclear receptors.

Activation of estrogen receptor α by raloxifene through an activating protein-1-dependent tethering mechanism in human cervical epithelial cancer cells: a role for c-Jun N-terminal kinase

Nuclear estrogen receptor α (ERα) regulates target gene expression in response to ligands through two distinct mechanisms: direct binding to DNA and indirect tethering through other DNA-bound transcription factors, such as AP-1. In the studies described herein, we examined the molecular mechanisms underlying the activation of ERα in the AP-1 tethering pathway by the selective estrogen receptor modulator (SERM) raloxifene (Ral). Our results with the MMP1 and PRUNE genes indicate that the c-Fos component of the AP-1 tethering factor and the c-Jun N-terminal kinase 1 (JNK1) are constitutively bound at the promoter regions prior to Ral exposure. Ral then promotes the binding of ERα at the promoter in a c-Fos-dependent manner. Interestingly, we found that JNK1 enzymatic activity is required for Ral-dependent gene activation through ERα. Our results suggest that one role for Ral-dependent recruitment of ERα to the AP-1 binding site is to stimulate JNK1 enzymatic activity. Alternatively, Ral-occupied ERα might recruit protein substrates to promoter-bound JNK1 without any change in JNK1 activity. Collectively, our studies have revealed a new role for JNK1 in determining gene regulatory outcomes by ERα.

Sirtuin 1 (SIRT1): the misunderstood HDAC

The sirtuin family of NAD-dependent histone deacetylases (HDACs) consists of seven mammalian proteins, SIRT1-7. Many of the sirtuin isoforms also deacetylate nonhistone substrates, such as p53 (SIRT1) and α-tubulin (SIRT2). The sirtuin literature focuses on pharmacological activators of SIRT1 (e.g., resveratrol, SRT1720), proposed as therapeutics for diabetes, neurodegeneration, inflammation, and others. However, many of the SIRT1 activator results may have been due to artifacts in the assay methodology (i.e., use of fluorescently tagged substrates). A biological role for SIRT1 in cancer has been given less scrutiny but is no less equivocal. Although proposed initially as an oncogene, we present herein compelling data suggesting that SIRT1 is indeed a context-specific tumor suppressor. For oncology, SIRT1 inhibitors (dual SIRT1/2) are indicated as potential therapeutics. A number of sirtuin inhibitors have been developed but with mixed results in cellular systems and animal models. It is unclear whether this has been due to poorly understood model systems, signalling redundancy, and/or inadequately potent and selective tool compounds. This review provides an overview of recent developments in the field of SIRT1 function. While focusing on oncology, it aims to shed light on new concepts of expanding the selectivity spectrum, including other sirtuins such as SIRT2.

CBP mediates NF-κB-dependent histone acetylation and estrogen receptor recruitment to an estrogen response element in the BIRC3 promoter

Estrogen receptor (ER) and NF-κB are transcription factors with profound effects on breast cancer cell proliferation and survival. While many studies demonstrate that ER and NF-κB can repress each other, we previously identified a gene signature that is synergistically upregulated by these two factors in more aggressive luminal B breast tumors. Herein, we examine a novel mechanism of cross talk between ER and NF-κB that results in the upregulation of the antiapoptotic gene BIRC3 (also known as cIAP2). We demonstrate that NF-κB, acting through two response elements, is required for ER recruitment to an adjacent estrogen response element (ERE) in the BIRC3 promoter. This effect is accompanied by a major increase in NF-κB-dependent histone acetylation around the ERE. Interestingly, CBP, a histone acetyltransferase previously implicated in repressive interactions between ER and NF-κB, plays a permissive role by promoting histone acetylation and ER recruitment, as well as enhanced expression of BIRC3. These findings suggest a new gene regulatory mechanism by which inflammation and NF-κB activation can influence ER recruitment to inherently inactive ER binding sites. This fine-tuning mechanism may explain how two factors that generally repress each other's activity may work together on certain genes to promote breast cancer cell survival and tumor progression.

Estrogen signaling and cardiovascular disease

Estrogen has pleiotropic effects on the cardiovascular system. The mechanisms by which estrogen confers these pleiotropic effects are undergoing active investigation. Until a decade ago, all estrogen signaling was thought to occur by estrogen binding to nuclear estrogen receptors (estrogen receptor-α and estrogen receptor-β), which bind to DNA and function as ligand-activated transcription factors. Estrogen binding to the receptor alters gene expression, thereby altering cell function. Estrogen also binds to nuclear estrogen receptors that are tethered to the plasma membrane, resulting in acute activation of signaling kinases such as PI3K. An orphan G-protein-coupled receptor, G-protein-coupled receptor 30, can also bind estrogen and activate acute signaling pathways. Thus, estrogen can alter cell function by binding to different estrogen receptors. This article reviews the different estrogen receptors and their signaling mechanisms, discusses mechanisms that regulate estrogen receptor levels and locations, and considers the cardiovascular effects of estrogen signaling.

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.

SIRT1 is essential for oncogenic signaling by estrogen/estrogen receptor α in breast cancer

The NAD-dependent histone deacetylase silent information regulator 1 (SIRT1) is overexpressed and catalytically activated in a number of human cancers, but recent studies have actually suggested that it may function as a tumor suppressor and metastasis inhibitor in vivo. In breast cancer, SIRT1 stabilization has been suggested to contribute to the oncogenic potential of the estrogen receptor α (ERα), but SIRT1 activity has also been associated with ERα deacetylation and inactivation. In this study, we show that SIRT1 is critical for estrogen to promote breast cancer. ERα physically interacted and functionally cooperated with SIRT1 in breast cancer cells. ERα also bound to the promoter for SIRT1 and increased its transcription. SIRT1 expression induced by ERα was sufficient to activate antioxidant and prosurvival genes in breast cancer cells, such as catalase and glutathione peroxidase, and to inactivate tumor suppressor genes such as cyclin G2 (CCNG2) and p53. Moreover, SIRT1 inactivation eliminated estrogen/ERα-induced cell growth and tumor development, triggering apoptosis. Taken together, these results indicated that SIRT1 is required for estrogen-induced breast cancer growth. Our findings imply that the combination of SIRT1 inhibitors and antiestrogen compounds may offer more effective treatment strategies for breast cancer.

Crosstalk of TGF-β and estrogen receptor signaling in breast cancer

Estrogen receptor-α (ERα) and transforming growth factor (TGF)-β signaling pathways are major regulators during mammary gland development, function and tumorigenesis. Predominantly, they have opposing roles in proliferation and apoptosis. While ERα signaling supports growth and differentiation and is antiapoptotic, mammary gland epithelia cells are very sensitive to TGF-β-induced cell cycle arrest and apoptosis. Their regulatory pathways intersect, and ERα blocks TGF-β pathway by multiple means, including direct interactions of its signaling components, Smads. However, relatively little is known of the dysfunction of their interactions in cancer. A better understanding would help to develop new strategies for breast cancer treatment.

Reciprocal roles of DBC1 and SIRT1 in regulating estrogen receptor α activity and co-activator synergy

Estrogen receptor α (ERα) plays critical roles in development and progression of breast cancer. Because ERα activity is strictly dependent upon the interaction with coregulators, coregulators are also believed to contribute to breast tumorigenesis. Cell Cycle and Apoptosis Regulator 1 (CCAR1) is an important co-activator for estrogen-induced gene expression and estrogen-dependent growth of breast cancer cells. Here, we identified Deleted in Breast Cancer 1 (DBC1) as a CCAR1 binding protein. DBC1 was recently shown to function as a negative regulator of the NAD-dependent protein deacetylase SIRT1. DBC1 associates directly with ERα and cooperates synergistically with CCAR1 to enhance ERα function. DBC1 is required for estrogen-induced expression of a subset of ERα target genes as well as breast cancer cell proliferation and for estrogen-induced recruitment of ERα to the target promoters in a gene-specific manner. The mechanism of DBC1 action involves inhibition of SIRT1 interaction with ERα and of SIRT1-mediated deacetylation of ERα. SIRT1 also represses the co-activator synergy between DBC1 and CCAR1 by binding to DBC1 and disrupting its interaction with CCAR1. Our results indicate that DBC1 and SIRT1 play reciprocal roles as major regulators of ERα activity, by regulating DNA binding by ERα and by regulating co-activator synergy.

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.

Acetylation-mediated epigenetic regulation of glucocorticoid receptor activity: circadian rhythm-associated alterations of glucocorticoid actions in target tissues

Glucocorticoids influence organ functions through the glucocorticoid receptor, a protein acetylated and deacetylated by several histone acetyltransferases and deacetylases. We reported that the circadian rhythm-related transcription factor "Clock", a key component of the biological CLOCK with inherent histone acetyltransferase activity, acetylates glucocorticoid receptor lysines within its hinge region--a "lysine cluster" containing a KXKK motif--and represses its transcriptional activity. This Clock-induced repression of the glucocorticoid receptor activity is inversely phased to the diurnally circulating glucocorticoids and may act as a local counter regulatory mechanism to the actions of these hormones. Importantly, uncoupling of the central CLOCK-regulated hypothalamic-pituitary-adrenal axis and peripheral CLOCK-mediated alterations of glucocorticoid action, such as chronic stress and frequent trans-time zone travel or night-shift work, may cause functional hypercortisolism and contribute to various pathologies. Thus, acetylation-mediated epigenetic regulation of the glucocorticoid receptor may be essential for the maintenance of proper time-integrated glucocorticoid action, significantly influencing human well-being and longevity.

Selective mutations in estrogen receptor alpha D-domain alters nuclear translocation and non-estrogen response element gene regulatory mechanisms

The three main mechanisms of ERα action are: 1) nuclear, genomic, direct DNA binding, 2) nuclear, genomic, "tethered"-mediated, protein-protein interactions, and 3) non-nuclear, non-genomic, rapid action responses. Reports suggest the D-domain or hinge region of ERα plays an important role in mechanisms 1 and 2 above. Studies demonstrating the functionality of the ERα hinge region have resected the full D-domain; therefore, site directed mutations were made to attribute precise sequence functionality to this domain. This study focuses on the characterization and properties of three novel site directed ERα- D-domain mutants. The Hinge 1 (H1) ERα mutant has disrupted nuclear localization, can no longer perform tethered mediated responses and has lost interaction with c-Jun, but retains estrogen response element (ERE)-mediated functions as demonstrated by confocal microscopy, reporter assays, endogenous gene expression and co-immunoprecipitation. The H2 ERα mutant is non-nuclear, but translocates to the nucleus with estradiol (E2) treatment and maintains ERE-mediated functionality. The H2+NES ERα mutant does not maintain nuclear translocation with hormone binding, no longer activates ERE-target genes, functions in ERE- or tethered-mediated luciferase assays, but does retain the non-genomic, non-nuclear, rapid action response. These studies reveal the sequence(s) in the ERα hinge region that are involved in tethered-mediated actions as well as nuclear localization and attribute important functionality to this region of the receptor. In addition, the properties of these ERα mutants will allow future studies to further dissect and characterize the three main ERα mechanisms of action and determine the mechanistic role each action has in estrogen hormone regulation.

ERα-associated protein networks

Estrogen receptor α (ERα) is a ligand-activated transcription factor that, upon binding hormone, interacts with specific recognition sequences in DNA. An extensive body of literature has documented the association of individual regulatory proteins with ERα. It has recently become apparent that, instead of simply recruiting individual proteins, ERα recruits interconnected networks of proteins with discrete activities that play crucial roles in maintaining the structure and function of the receptor, stabilizing the receptor-DNA interaction, influencing estrogen-responsive gene expression, and repairing misfolded proteins and damaged DNA. Together these studies suggest that the DNA-bound ERα serves as a nucleating factor for the recruitment of protein complexes involved in key processes including the oxidative stress response, DNA repair, and transcription regulation.

Multiple sequence-specific DNA-binding proteins mediate estrogen receptor signaling through a tethering pathway

The indirect recruitment (tethering) of estrogen receptors (ERs) to DNA through other DNA-bound transcription factors (e.g. activator protein 1) is an important component of estrogen-signaling pathways, but our understanding of the mechanisms of ligand-dependent activation in this pathway is limited. Using proteomic, genomic, and gene-specific analyses, we demonstrate that a large repertoire of DNA-binding transcription factors contribute to estrogen signaling through the tethering pathway. In addition, we define a set of endogenous genes for which ERα tethering through activator protein 1 (e.g. c-Fos) and cAMP response element-binding protein family members mediates estrogen responsiveness. Finally, we show that functional interplay between c-Fos and cAMP response element-binding protein 1 contributes to estrogen-dependent regulation through the tethering pathway. Based on our results, we conclude that ERα recruitment in the tethering pathway is dependent on the ligand-induced formation of transcription factor complexes that involves interplay between the transcription factors from different protein families.

Acetylation and nuclear receptor action

Acetylation is an essential post-translational modification featuring an acetyl group that is covalently conjugated to a protein substrate. Histone acetylation was first proposed nearly half a century ago by Dr. Vincent Allfrey. Subsequent studies have shown that the acetylated core histones are often associated with transcriptionally active chromatin. Acetylation at lysine residues of histone tails neutralizes the positive charge, which decreases their binding ability to DNA and increases the accessibility of transcription factors and coactivators to the chromatin template. In addition to histones, a number of non-histone substrates are acetylated. Acetylation of non-histone proteins governs biological processes, such as cellular proliferation and survival, transcriptional activity, and intracellular trafficking. We demonstrated that acetylation of transcription factors can regulate cellular growth. Furthermore, we showed that nuclear receptors (NRs) are acetylated at a phylogenetically conserved motif. Since our initial observations with the estrogen and androgen receptors, more than a dozen NRs have been shown to function as substrates for acetyltransferases with diverse functional consequences. This review focuses on the acetylation of NRs and the effect of acetylation on NR function. We discuss the potential role of acetylation in disease initiation and progression with an emphasis on tumorigenesis.

Evolution of ligand specificity in vertebrate corticosteroid receptors

Background

Corticosteroid receptors include mineralocorticoid (MR) and glucocorticoid (GR) receptors. Teleost fishes have a single MR and duplicate GRs that show variable sensitivities to mineralocorticoids and glucocorticoids. How these receptors compare functionally to tetrapod MR and GR, and the evolutionary significance of maintaining two GRs, remains unclear.

Results

We used up to seven steroids (including aldosterone, cortisol and 11-deoxycorticosterone [DOC]) to compare the ligand specificity of the ligand binding domains of corticosteroid receptors between a mammal (Mus musculus) and the midshipman fish (Porichthys notatus), a teleost model for steroid regulation of neural and behavioral plasticity. Variation in mineralocorticoid sensitivity was considered in a broader phylogenetic context by examining the aldosterone sensitivity of MR and GRs from the distantly related daffodil cichlid (Neolamprologus pulcher), another teleost model for neurobehavioral plasticity. Both teleost species had a single MR and duplicate GRs. All MRs were sensitive to DOC, consistent with the hypothesis that DOC was the initial ligand of the ancestral MR. Variation in GR steroid-specificity corresponds to nine identified amino acid residue substitutions rather than phylogenetic relationships based on receptor sequences.

Conclusion

The mineralocorticoid sensitivity of duplicate GRs in teleosts is highly labile in the context of their evolutionary phylogeny, a property that likely led to neo-functionalization and maintenance of two GRs.

Nε-lysine acetylation of a bacterial transcription factor inhibits Its DNA-binding activity

Evidence suggesting that eukaryotes and archaea use reversible N(ε)-lysine (N(ε)-Lys) acetylation to modulate gene expression has been reported, but evidence for bacterial use of N(ε)-Lys acetylation for this purpose is lacking. Here, we report data in support of the notion that bacteria can control gene expression by modulating the acetylation state of transcription factors (TFs). We screened the E. coli proteome for substrates of the bacterial Gcn5-like protein acetyltransferase (Pat). Pat acetylated four TFs, including the RcsB global regulatory protein, which controls cell division, and capsule and flagellum biosynthesis in many bacteria. Pat acetylated residue Lys180 of RcsB, and the NAD(+)-dependent Sir2 (sirtuin)-like protein deacetylase (CobB) deacetylated acetylated RcsB (RcsB(Ac)), demonstrating that N(ε)-Lys acetylation of RcsB is reversible. Analysis of RcsB(Ac) and variant RcsB proteins carrying substitutions at Lys180 provided biochemical and physiological evidence implicating Lys180 as a critical residue for RcsB DNA-binding activity. These findings further the likelihood that reversible N(ε)-Lys acetylation of transcription factors is a mode of regulation of gene expression used by all cells.

Manipulating protein acetylation in breast cancer: a promising approach in combination with hormonal therapies?

Estrogens play an essential role in the normal physiology of the breast as well as in mammary tumorigenesis. Their effects are mediated by two nuclear estrogen receptors, ERα and β, which regulate transcription of specific genes by interacting with multiprotein complexes, including histone deacetylases (HDACs). During the past few years, HDACs have raised great interest as therapeutic targets in the field of cancer therapy. In breast cancer, several experimental arguments suggest that HDACs are involved at multiple levels in mammary tumorigenesis: their expression is deregulated in breast tumors; they interfere with ER signaling in intricate ways, restoring hormone sensitivity in models of estrogen resistance, and they clinically represent new potential targets for HDACs inhibitors (HDIs) in combination with hormonal therapies. In this paper, we will describe these different aspects and underline the clinical interest of HDIs in the context of breast cancer resistance to hormone therapies (HTs).

The progesterone receptor hinge region regulates the kinetics of transcriptional responses through acetylation, phosphorylation, and nuclear retention

Progesterone receptors (PRs) are critical regulators of mammary gland development and contributors to breast cancer progression. Posttranslational modifications of PR have been shown to alter hormone responsiveness. Site-directed mutagenesis demonstrated that upon hormone binding, PR is acetylated at the consensus sequence, KXKK (amino acids 638-641), located within the hinge region. We created an acetylation-deficient (K-A) mutant as well as acetylation mimics (K-Q or K-T). Interestingly, similar to K-A PR, PR acetylation mimics (K-Q or K-T) displayed delayed phosphorylation and nuclear entry relative to wild-type (wt) PR-B, indicative of disruption of PR nuclear-cytoplasmic shuttling. Wt PR-B, but not K-mutant PRs, induced c-myc at 1 h of progestin treatment. However, at 6 h of treatment, c-myc induction was comparable with levels induced by wt PR-B, suggesting that the precise timing of PR phosphorylation and nuclear retention are critical for cells to rapidly initiate robust transcriptional programs. In contrast to c-myc, progestin-induced serum- and glucocorticoid-regulated kinase (SGK) expression displayed sensitivity to PR acetylation but not nuclear entry. Namely, in the presence of progestin, acetylation-deficient (K-A) mutant PR-B up-regulated SGK mRNA relative to wt PR; progesterone response element-luciferase assays confirmed this result. However, K-Q and K-T acetylation mimics only weakly induced SGK expression independently of nuclear retention. These data reveal the ability of PR acetylation to alter the magnitude of transcriptional response at selected (slow response) promoters (SGK), whereas the hinge region dictates the kinetics of the transcriptional response to hormone at other (rapid response) promoters (c-myc). In sum, the PR hinge region is multifunctional. Understanding the ability of this region to couple acetylation, phosphorylation, and nuclear entry may provide clues to mechanisms of altered hormone responsiveness.

NAD: a master regulator of transcription

Cellular processes such as proliferation, differentiation and death are intrinsically dependent upon the redox status of a cell. Among other indicators of redox flux, cellular NAD(H) levels play a predominant role in transcriptional reprogramming. In addition to this, normal physiological functions of a cell are regulated in response to perturbations in NAD(H) levels (for example, due to alterations in diet/metabolism) to maintain homeostatic conditions. Cells achieve this homeostasis by reprogramming various components that include changes in chromatin structure and function (transcription). The interdependence of changes in gene expression and NAD(H) is evolutionarily conserved and is considered crucial for the survival of a species (by affecting reproductive capacity and longevity). Proteins that bind and/or use NAD(H) as a co-substrate (such as, CtBP and PARPs/Sirtuins respectively) are known to induce changes in chromatin structure and transcriptional profiles. In fact, their ability to sense perturbations in NAD(H) levels has been implicated in their roles in development, stress responses, metabolic homeostasis, reproduction and aging or age-related diseases. It is also becoming increasingly clear that both the levels/activities of these proteins and the availability of NAD(H) are equally important. Here we discuss the pivotal role of NAD(H) in controlling the functions of some of these proteins, the functional interplay between them and physiological implications during calorie restriction, energy homeostasis, circadian rhythm and aging.

Inhibition of estrogen signaling activates the NRF2 pathway in breast cancer

Exposure to higher levels of estrogen produces genotoxic metabolites that can stimulate mammary tumorigenesis. Induction of NF-E2-related factor 2 (NRF2)-dependent detoxifying enzymes (e.g., NAD(P)H-quinone oxidoreductase 1 (NQO1)) is considered an important mechanism of protection against estrogen-associated carcinogenesis because they would facilitate removal of toxic estrogens. Here, we studied the impact of estrogen-receptor (ER) signaling on NRF2-dependent gene transcription. In luciferase assay experiments using the 5-flanking region of the human NQO1 gene promoter, we observe that ERα ligand-binding domain (LBD) is required for estrogen inhibition of NQO1 promoter activity in estrogen-dependent breast cancer cells. Chromatin immunoprecipitation (ChIP) assay shows that estrogen recruits ERα and a class III histone deacetylase SIRT1 at the NQO1 promoter, leading to inhibition of NQO1 transcription. Inhibition of ERα expression by the antiestrogen shikonin reverses the inhibitory effect of estrogen on NQO1 expression. As a consequence, a chemoprevention study was undertaken to monitor the impact of shikonin on DNA lesions and tumor growth. Treatment of MCF-7 breast cancer cells with shikonin inhibits estrogen-induced 8-hydroxy-2-deoxyguanosine (8-OHdG), a marker of DNA damage. NQO1 deficiency promotes estrogen-dependent tumor formation, and shikonin inhibits estrogen-dependent tumor growth in an NQO1-dependent manner in MCF-7 xenografts. These results suggest that estrogen-receptor signaling pathway has an inhibitory effect on NRF2-dependent enzymes. Moreover, shikonin reverses the inhibitory effects of estrogen on this pathway and may contribute to breast cancer prevention.

An acetylation switch modulates the transcriptional activity of estrogen-related receptor alpha

Posttranslational modifications are instrumental to achieve gene- and tissue-specific regulatory outcomes by transcription factors. Nuclear receptors are dynamically modulated by several types of posttranslational modifications including phosphorylation, methylation, acetylation, ubiquitination, and sumoylation. The estrogen-related receptor alpha (ERRalpha, NR3B1) is phosphorylated on multiple sites, and sumoylated in the amino-terminal region in a phosphorylation-dependent manner. Here we demonstrate that ERRalpha interacts with and is acetylated by p300 coactivator associated factor (PCAF) in vitro and in mouse liver. Purified PCAF acetylated the DNA-binding domain of ERRalpha on four highly-conserved lysines. In addition, coexpression of PCAF reduced the transcriptional activity of ERRalpha and, reciprocally, a deacetylase screen identified histone deacetylase 8 (HDAC8) and sirtuin 1 homolog (Sirt1) as independent enhancers of ERRalpha transcriptional function. HDAC8 and Sirt1 were also demonstrated to interact directly with ERRalpha in vivo and to deacetylate and increase the DNA binding affinity of ERRalpha in vitro. The removal of PCAF increases the DNA binding of ERRalpha in vivo, whereas the removal of Sirt1 and HDAC8 decreases it as assessed by chromatin immunoprecipitation assay. Altogether, our results show that ERRalpha is an acetylated protein and imply the existence of a dynamic acetylation/deacetylation switch involved in the control of ERRalpha transcriptional activity.

Estrogen receptor mutations and changes in downstream gene expression and signaling

Estrogens play a crucial role in regulating the growth and differentiation of breast cancers, with approximately two thirds of all breast tumors expressing the estrogen receptor alpha (ERalpha). Therefore, therapeutic strategies directed at inhibiting the action of ERalpha by using anti-estrogens such as tamoxifen, or reducing estrogens levels by using aromatase inhibitors, such as letrozole, anastrozole, or exemestane, are the standard treatments offered to women with ERalpha-positive cancer. However, not all patients respond to endocrine therapies (termed de novo resistance), and a large number of patients who do respond will eventually develop disease progression or recurrence while on therapy (acquired resistance). Recently, variant forms of the receptor have been identified owing to alternative splicing or gene mutation. This article reviews these variant receptors and their clinical relevance in resistance to endocrine therapy, by addressing their molecular cross-talk with growth factor receptors and signaling components. Understanding the complexity of receptor-mediated signaling has promise for new combined therapeutic options that focus on more efficient blockade of receptor cross-talk.

The hinge region of the human estrogen receptor determines functional synergy between AF-1 and AF-2 in the quantitative response to estradiol and tamoxifen

Human estrogen receptors α and β (ERα and ERβ) greatly differ in their target genes, transcriptional potency and cofactor-binding capacity, and are differentially expressed in various tissues. In classical estrogen response element (ERE)-mediated transactivation, ERβ has a markedly reduced activation potential compared with ERα; the mechanism underlying this difference is unclear. Here, we report that the binding of steroid receptor coactivator-1 (SRC-1) to the AF-1 domain of ERα is essential but not sufficient to facilitate synergy between the AF-1 and AF-2 domains, which is required for a full agonistic response to estradiol (E2). Complete synergy is achieved through the distinct hinge domain of ERα, which enables combined action of the AF-1 and AF-2 domains. AF-1 of ERβ lacks the capacity to interact with SRC-1, which prevents hinge-mediated synergy between AF-1 and AF-2, thereby explaining the reduced E2-mediated transactivation of ERβ. Transactivation of ERβ by E2 requires only the AF-2 domain. A weak agonistic response to tamoxifen occurs for ERα, but not for ERβ, and depends on AF-1 and the hinge-region domain of ERα.

Trichostatin A enhances acetylation as well as protein stability of ERalpha through induction of p300 protein

Introduction

Trichostatin A (TSA) is a well-characterized histone deacetylase (HDAC) inhibitor. TSA modifies the balance between HDAC and histone acetyltransferase activities that is important in chromatin remodeling and gene expression. Although several previous studies have demonstrated the role of TSA in regulation of estrogen receptor alpha (ERα), the precise mechanism by which TSA affects ERα activity remains unclear.

Methods

Transient transfection was performed using the Welfect-EX™Plus procedure. The mRNA expression was determined using RT-PCR. Protein expression and interaction were determined by western blotting and immunoprecipitation. The transfection of siRNAs was performed using the Oligofectamine™ reagent procedure.

Results

TSA treatment increased acetylation of ERα in a dose-dependent manner. The TSA-induced acetylation of ERα was accompanied by an increased stability of ERα protein. Interestingly, TSA also increased the acetylation and the stability of p300 protein. Overexpression of p300 induced acetylation and stability of ERα by blocking ubiquitination. Knockdown of p300 by RNA interference decreased acetylation as well as the protein level of ERα, indicating that p300 mediated the TSA-induced stabilization of ERα.

Conclusions

We report that TSA enhanced acetylation as well as the stability of the ERα protein by modulating stability of p300. These results may provide the molecular basis for pharmacological functions of HDAC inhibitors in the treatment of human breast cancer.

The secret life of NAD+: an old metabolite controlling new metabolic signaling pathways

A century after the identification of a coenzymatic activity for NAD(+), NAD(+) metabolism has come into the spotlight again due to the potential therapeutic relevance of a set of enzymes whose activity is tightly regulated by the balance between the oxidized and reduced forms of this metabolite. In fact, the actions of NAD(+) have been extended from being an oxidoreductase cofactor for single enzymatic activities to acting as substrate for a wide range of proteins. These include NAD(+)-dependent protein deacetylases, poly(ADP-ribose) polymerases, and transcription factors that affect a large array of cellular functions. Through these effects, NAD(+) provides a direct link between the cellular redox status and the control of signaling and transcriptional events. Of particular interest within the metabolic/endocrine arena are the recent results, which indicate that the regulation of these NAD(+)-dependent pathways may have a major contribution to oxidative metabolism and life span extension. In this review, we will provide an integrated view on: 1) the pathways that control NAD(+) production and cycling, as well as its cellular compartmentalization; 2) the signaling and transcriptional pathways controlled by NAD(+); and 3) novel data that show how modulation of NAD(+)-producing and -consuming pathways have a major physiological impact and hold promise for the prevention and treatment of metabolic disease.

Phosphorylation of the mutant K303R estrogen receptor alpha at serine 305 affects aromatase inhibitor sensitivity

We previously identified a lysine to arginine transition at residue 303 (K303R) in ERα in invasive breast cancers, which confers resistance to the aromatase inhibitor (AI) anastrozole (Ana) when expressed in MCF-7 breast cancer cells. Here we show that AI resistance arises through an enhanced cross-talk of the IGF-1R/IRS-1/Akt pathway with ERα, and the serine (S) residue 305 adjacent to the K303R mutation plays a key role in mediating this cross-talk. The ERα S305 residue is an important site that modifies response to tamoxifen; thus, we questioned whether this site could also influence AI response. We generated stable transfectants expressing wild-type (WT), K303R ERα, or a double K303R/S305A mutant receptor, and found that the AI-resistant phenotype associated with expression of the K303R mutation was dependent on activation of S305 within the receptor. Ana significantly reduced growth in K303R/S305A-expressing cells. Preventing S305 phosphorylation with a blocking peptide inhibited IGF-1R/IRS-1/Akt activation, and also restored AI sensitivity. Our data suggest that the K303R mutation and the S305 ERα residue may be a novel determinant of aromatase inhibitor response in breast cancer, and blockade of S305 phosphorylation represents a new therapeutic strategy for treating tumors resistant to hormone therapy.

Modulation of steroid hormone receptor activity

Classical steroid hormones (SHs) - estrogens, androgens, progestins, glucocorticoids and mineralocorticoids - play critical roles in the regulation of reproduction, metabolism and cancer. SHs act via their cognate steroid hormone receptors (SHRs) in multiple target tissues throughout the body, exerting their physiological effects through nuclear receptor (NR)-mediated gene transcription. Since SHRs are the mediators of steroid hormone signalling in cells, regulation of their expression and function is critical for appropriate physiological responses to SHs. Cells regulate SHRs by determining the cellular concentration of SHR proteins in the cell and by tightly regulating their activity through post-translational modifications and interactions with coactivator protein complexes. In this chapter we will examine each of these regulatory mechanisms and assess their functional impact on the activity of SHRs.

BRCA1 regulates acetylation and ubiquitination of estrogen receptor-alpha

Inherited mutations of the breast cancer susceptibility gene BRCA1 confer a high risk for breast cancer development. The (300)RXKK and (266)KXK motifs have been identified previously as sites for acetylation of the estrogen receptor-alpha (ER-alpha), and (302)K was also found to be a site for BRCA1-mediated mono-ubiquitination of ER-alpha in vitro. Here we show that ER-alpha proteins with single or double lysine mutations of these motifs (including K303R, a cancer-associated mutant) are resistant to inhibition by BRCA1, even though the mutant ER-alpha proteins retain the ability to bind to BRCA1. We also found that BRCA1 overexpression reduced and knockdown increased the level of acetylated wild-type ER-alpha, without changing the total ER-alpha protein level. Increased acetylation of ER-alpha due to BRCA1 small interfering RNA was dependent upon phosphatidylinositol 3-kinase/Akt signaling and on up-regulation of the coactivator p300. In addition, using an in vitro acetylation assay, we found that in vitro-translated wild-type BRCA1 but not a cancer-associated point mutant (C61G) inhibited p300-mediated acetylation of ER-alpha. Furthermore, BRCA1 overexpression increased the levels of mono-ubiquitinated ER-alpha protein, and a BRCA1 mutant that is defective for ubiquitin ligase activity but retains other BRCA1 functions (I26A) did not ubiquitinate ER-alpha or repress its activity in vivo. Finally, ER-alpha proteins with mutations of the (300)RXKK or (266)KXK motifs showed modest or no BRCA1-induced ubiquitination. We propose a model in which BRCA1 represses ER-alpha activity, in part, by regulating the relative degree of acetylation vs. ubiquitination of ER-alpha.

Inhibition of SIRT1 deacetylase suppresses estrogen receptor signaling

Estrogen receptor alpha (ERalpha) mediates estrogen-dependent gene transcription, which plays a critical role in mammary gland development, reproduction and homeostasis. Histone acetyltransferases and class I and class II histone deacetylases (HDACs) cause posttranscriptional modification of histone proteins that participate in ERalpha signaling. Here, we report that human SIRT1, a class III HDAC, regulates ERalpha expression. Inhibition of SIRT1 activity by sirtinol suppresses ERalpha expression through disruption of basal transcriptional complexes at the ERalpha promoter. This effect leads to inhibition of estrogen-responsive gene expression. Our in vitro observations were further extended that SIRT1 knockout reduces ERalpha protein in mouse mammary gland. Finally, ERalpha-mediated estrogen response genes are also decreased in mouse embryonic fibroblasts derived from SIRT1-knockout mice. These results suggest that inhibition of SIRT1 deacetylase activity by either pharmacological inhibitors or genetic depletion impairs ERalpha-mediated signaling pathways.

Myocyte enhancer factor 2 and class II histone deacetylases control a gender-specific pathway of cardioprotection mediated by the estrogen receptor

RATIONALE

Gender differences in cardiovascular disease have long been recognized and attributed to beneficial cardiovascular actions of estrogen. Class II histone deacetylases (HDACs) act as key modulators of heart disease by repressing the activity of the myocyte enhancer factor (MEF)2 transcription factor, which promotes pathological cardiac remodeling in response to stress. Although it is proposed that HDACs additionally influence nuclear receptor signaling, the effect of class II HDACs on gender differences in cardiovascular disease remains unstudied.

OBJECTIVE

We aimed to examine the effect of class II HDACs on post-myocardial infarction remodeling in male and female mice.

METHODS AND RESULTS

Here we show that the absence of HDAC5 or -9 in female mice protects against maladaptive remodeling following myocardial infarction, during which there is an upregulation of estrogen-responsive genes in the heart. This genetic reprogramming coincides with a pronounced increase in expression of the estrogen receptor (ER)alpha gene, which we show to be a direct MEF2 target gene. ERalpha also directly interacts with class II HDACs. Cardioprotection resulting from the absence of HDAC5 or -9 in female mice can be attributed, at least in part, to enhanced neoangiogenesis in the infarcted region via upregulation of the ER target gene vascular endothelial growth factor-a.

CONCLUSIONS

Our results reveal a novel gender-specific pathway of cardioprotection mediated by ERalpha and its regulation by MEF2 and class II HDACs.

Residues K128, 132, and 134 in the thyroid hormone receptor-alpha are essential for receptor acetylation and activity

The thyroid hormone receptor (TR)-alpha is a nuclear receptor that mediates both transrepression and ligand-dependent transactivation. Here we show that TRalpha is posttranslationally modified by acetylation in response to its own ligand (T(3)). Acetylation increases binding to DNA. Using mutagenesis, we identified three conserved lysine residues in the carboxi-terminal extension (CTE) of the DNA binding domain that are targets of the cAMP-response element-binding protein acetyltransferase. Substitution of these lysines by arginines in TRalpha decreased ligand binding affinity and precluded ligand-dependent release of corepressors and recruitment of coactivators. The acetylation TRalpha mutant lost the ability to transactivate even at high T(3) concentrations and acts as a dominant-negative inhibitor of wild-type TR activity. In addition, whereas native TRalpha interferes with AP-1 function, the mutant is unable to mediate transrepression. Finally, TRalpha suppresses NIH-3T3 fibroblast transformation by the Ras oncogene both in a ligand-dependent and -independent manner, but the CTE mutant is unable to mediate ligand-dependent repression of transformation. These results reveal a key role for the CTE region on acetylation, ligand affinity, transactivation, transrepression, and antitransforming properties of TRalpha.

SIRT1-dependent regulation of chromatin and transcription: linking NAD(+) metabolism and signaling to the control of cellular functions

Sirtuins comprise a family of NAD(+)-dependent protein deacetylases and ADP-ribosyltransferases. Mammalian SIRT1 - a homolog of yeast Sir2, the prototypical member of the sirtuin family - is an important regulator of metabolism, cell differentiation and senescence, stress response, and cancer. As an NAD(+)-dependent enzyme, SIRT1 regulates gene expression programs in response to cellular metabolic status, thereby coordinating metabolic adaptation of the whole organism. Several important mechanisms have emerged for SIRT1-dependent regulation of transcription. First, SIRT1 can modulate chromatin function through direct deacetylation of histones as well as by promoting alterations in the methylation of histones and DNA, leading to the repression of transcription. The latter is accomplished through the recruitment of other nuclear enzymes to chromatin for histone methylation and DNA CpG methylation, suggesting a broader role of SIRT1 in epigenetic regulation. Second, SIRT1 can interact and deacetylate a broad range of transcription factors and coregulators, thereby regulating target gene expression both positively and negatively. Cellular energy state, specifically NAD(+) metabolism, plays a major role in the regulation of SIRT1 activity. Recent studies on the NAD(+) biosynthetic enzymes in the salvage pathway, nicotinamide phosphoribosyltransferase (NAMPT) and nicotinamide mononucleotide adenylyltransferase 1 (NMNAT-1), have revealed important functions for these enzymes in SIRT1-dependent transcription regulation. The collective molecular actions of SIRT1 control specific patterns of gene expression that modulate a wide variety of physiological outcomes.

Biochemical analyses of nuclear receptor-dependent transcription with chromatin templates

Chromatin, the physiological template for transcription, plays important roles in gene regulation by nuclear receptors (NRs). It can (1) restrict the binding of NRs or the transcriptional machinery to their genomic targets, (2) serve as a target of regulatory posttranslational modifications by NR coregulator proteins with histone-directed enzymatic activities, and (3) function as a binding scaffold for a variety of transcription-related proteins. The advent of in vitro or "cell-free" systems that accurately recapitulate ligand-dependent transcription by NRs with chromatin templates has allowed detailed analyses of these processes. Biochemical studies have advanced our understanding of the mechanisms of gene regulation, including the role of ligands, coregulators, and nucleosome remodeling. In addition, they have provided new insights about the dynamics of NR-mediated transcription. This chapter reviews the current methodologies for assembling, transcribing, and analyzing chromatin in vitro, as well as the new information that has been gained from these studies.

Epigenetics meets estrogen receptor: regulation of estrogen receptor by direct lysine methylation

The nuclear hormone receptor estrogen receptor α (ERα) promotes cellular growth through ligand-dependent activation of specific target genes, a process which is targeted in the treatment of ERα-expressing breast cancers. ERα activity is regulated at the protein level by post-translational modifications including phosphorylation and acetylation. A study now shows that ERα can also be directly methylated at lysine 302 (K302) by SET7, a histone methyltransferase that is known to monomethylate H3K4 and is associated with transcriptional activation. It was shown that K302 methylation stabilizes ERα protein and is suggested to increase sensitivity of ERα to estrogens, enhancing transcription of estrogen response elements. Furthermore, SET7 methylation of K302 is enhanced by a breast cancer-associated mutation at K303 (K303R) in vitro. These findings provide an additional mechanism of SET7 mediated transcriptional activation, as well as potential insight into the complex regulation of ERα stability and ligand sensitivity.

Enzymes in the NAD+ salvage pathway regulate SIRT1 activity at target gene promoters

In mammals, nicotinamide phosphoribosyltransferase (NAMPT) and nicotinamide mononucleotide adenylyltransferase 1 (NMNAT-1) constitute a nuclear NAD(+) salvage pathway which regulates the functions of NAD(+)-dependent enzymes such as the protein deacetylase SIRT1. One of the major functions of SIRT1 is to regulate target gene transcription through modification of chromatin-associated proteins. However, little is known about the molecular mechanisms by which NAD(+) biosynthetic enzymes regulate SIRT1 activity to control gene transcription in the nucleus. In this study we show that stable short hairpin RNA-mediated knockdown of NAMPT or NMNAT-1 in MCF-7 breast cancer cells reduces total cellular NAD(+) levels and alters global patterns of gene expression. Furthermore, we show that SIRT1 plays a key role in mediating the gene regulatory effects of NAMPT and NMNAT-1. Specifically, we found that SIRT1 binds to the promoters of genes commonly regulated by NAMPT, NMNAT-1, and SIRT1 and that SIRT1 histone deacetylase activity is regulated by NAMPT and NMNAT-1 at these promoters. Most significantly, NMNAT-1 interacts with, and is recruited to target gene promoters by SIRT1. Collectively, our results reveal a mechanism for the direct control of SIRT1 deacetylase activity at a set of target gene promoters by NMNAT-1. This mechanism, in collaboration with NAMPT-dependent regulation of nuclear NAD(+) production, establishes an important pathway for transcription regulation by NAD(+).

Minireview: role of protein methylation and demethylation in nuclear hormone signaling

Nuclear hormone receptors (NRs) are transcription factors responsible for mediating the biological effects of hormones during development, metabolism, and homeostasis. Induction of NR target genes is accomplished through the assembly of hormone-bound NR complexes at target promoters and coincides with changes in histone modifications that promote transcription. Some coactivators and corepressors of NR can enhance or inhibit NR function by covalently modifying histones. One such modification is methylation, which plays important roles in transcriptional regulation. Histone methylation is catalyzed by histone methyltransferases and reversed by histone demethylases. Recent studies have uncovered the importance of these enzymes in the regulation of NR target genes. In addition to histones, these enzymes have nonhistone substrates and can methylate and demethylate NRs and coregulatory proteins in order to modulate their function. This review discusses recent progress in our understanding of the role of methylation and demethylation of histones, NRs, and their coregulators in NR-mediated transcription.

Growth factor-induced resistance to tamoxifen is associated with a mutation of estrogen receptor alpha and its phosphorylation at serine 305

Estrogens play a crucial role in breast tumor growth, which is the rationale for the use of antiestrogens, such as tamoxifen, in women with estrogen receptor (ER)-alpha-positive breast cancer. However, hormone resistance is a major clinical problem. Altered growth factor signaling to the ERalpha pathway has been shown to be associated with the development of clinical resistance. We previously have identified a mutation that replaces arginine for lysine at residue 303 (K303R) of ERalpha, which confers hypersensitive growth in low levels of estrogen. To determine if the K303R mutation could participate in the evolution of hormone resistance, we generated MCF-7 breast cancer cells stably transfected with either wild-type (WT) or K303R ERalpha. We found that the mutation confers decreased sensitivity to tamoxifen in the presence of the growth factor heregulin, using anchorage-independent growth assays. K303R ERalpha-expressing cells were hypersensitive to growth factor signals. Our data suggest that phosphorylation of serine 305 within the hinge domain of ERalpha might play a key role in increasing ligand-independent activity of the mutant receptor. We hypothesize that the mutation adapts the receptor for enhanced bidirectional cross-talk with the HER2 growth factor receptor pathway, which then impacts on responsiveness to tamoxifen.

Liver X receptor ligands suppress ubiquitination and degradation of LXRalpha by displacing BARD1/BRCA1

Liver X receptor (LXR) is a ligand-activated transcription factor that plays important roles in cholesterol and lipid homeostasis. However, ligand-induced posttranslational modification of LXR is largely unknown. Here, we show that ligand-free LXRalpha is rapidly degraded by ubiquitination. Without ligand, LXRalpha interacts with an ubiquitin E3-ligase protein complex containing breast and ovarian cancer susceptibility 1 (BRCA1)-associated RING domain 1 (BARD1). Interestingly, LXR ligand represses ubiquitination and degradation of LXRalpha, and the interaction between LXRalpha and BARD1 is inhibited by LXR ligand. Consistently, T0901317, a synthetic LXR ligand, increased the level of LXRalpha protein in liver. Moreover, overexpression of BARD1/BRCA1 promoted the ubiquitination of LXRalpha and reduced the recruitment of LXRalpha to the target gene promoters, whereas BARD1 knockdown reversed such effects. Taken together, these data suggest that LXR ligand prevents LXRalpha from ubiquitination and degradation by detaching BARD1/BRCA1, which might be critical for the early step of transcriptional activation of ligand-stimulated LXRalpha through a stable binding of LXRalpha to the promoters of target genes.

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.

Regulation of estrogen rapid signaling through arginine methylation by PRMT1

Evidence is emerging that estrogen receptor alpha (ERalpha) is central to the rapid transduction of estrogen signaling to the downstream kinase cascades; however, the mechanisms underlying this nongenomic function are not fully understood. Here we report a paradigm of ERalpha regulation through arginine methylation by PRMT1, which transiently methylates arginine 260 within the ERalpha DNA-binding domain. This methylation event is required for mediating the extranuclear function of the receptor by triggering its interaction with the p85 subunit of PI3K and Src. Furthermore, we find that the focal adhesion kinase (FAK), a Src substrate involved in the migration process, is also recruited in this complex. Our data indicate that the methylation of ERalpha is a physiological process occurring in the cytoplasm of normal and malignant epithelial breast cells and that ERalpha is hypermethylated in a subset of breast cancers.

Chromatin context dominates estrogen regulation of pS2 gene expression

Chromatin structure and transcription factor activity collaborate to set the transcription level of a gene. Our understanding of the relative contributions of each of these factors at a specific gene is limited. We studied the effects of an altered chromatin environment on the activity of the estrogen-responsive pS2 promoter. We created stable cell lines with the pS2 promoter situated in an alternative chromatin site in addition to it being in its native site. Both promoters were estrogen-responsive for estrogen receptor alpha (ERalpha) recruitment, but transcription was inducible only at the native site. At the recombinant site, transcription was high and constitutive. Higher histone H3 and H4 acetylation (acH3 and acH4), as well as trimethylated lysine 4 on histone H3 levels, was observed at the recombinant site compared to the native site in vehicle treated cells. Inhibition of histone deacetylases (HDACs) resulted in increased acH4, but only modest increases in acH3, ERalpha binding and basal transcription at the native pS2 site. Inhibiting HDACs had no effect on transcription from the recombinant site. These data suggest that highly active chromatin is not only permissive for transcription, but can override the requirement for the transcription factor at an inducible promoter.