Distinct steady-state nuclear receptor coregulator complexes exist in vivo

The multifaceted therapeutic value of targeting steroid receptor coactivator-1 in tumorigenesis

Steroid receptor coactivator-1 (SRC-1, also known as NCOA1) frequently functions as a transcriptional coactivator by directly binding to transcription factors and recruiting to the target gene promoters to promote gene transcription by increasing chromatin accessibility and promoting the formation of transcriptional complexes. In recent decades, various biological and pathological functions of SRC-1 have been reported, especially in the context of tumorigenesis. SRC-1 is a facilitator of the progression of multiple cancers, including breast cancer, prostate cancer, gastrointestinal cancer, neurological cancer, and female genital system cancer. The emerging multiorgan oncogenic role of SRC-1 is still being studied and may not be limited to only steroid hormone-producing tissues. Growing evidence suggests that SRC-1 promotes target gene expression by directly binding to transcription factors, which may constitute a novel coactivation pattern independent of AR or ER. In addition, the antitumour effect of pharmacological inhibition of SRC-1 with agents including various small molecules or naturally active compounds has been reported, but their practical application in clinical cancer therapy is very limited. For this review, we gathered typical evidence on the oncogenic role of SRC-1, highlighted its major collaborators and regulatory genes, and mapped the potential mechanisms by which SRC-1 promotes primary tumour progression.

Functional Relationships between Long Non-Coding RNAs and Estrogen Receptor Alpha: A New Frontier in Hormone-Responsive Breast Cancer Management

In the complex and articulated machinery of the human genome, less than 2% of the transcriptome encodes for proteins, while at least 75% is actively transcribed into non-coding RNAs (ncRNAs). Among the non-coding transcripts, those ≥200 nucleotides long (lncRNAs) are receiving growing attention for their involvement in human diseases, particularly cancer. Genomic studies have revealed the multiplicity of processes, including neoplastic transformation and tumor progression, in which lncRNAs are involved by regulating gene expression at epigenetic, transcriptional, and post-transcriptional levels by mechanism(s) that still need to be clarified. In breast cancer, several lncRNAs were identified and demonstrated to have either oncogenic or tumor-suppressive roles. The functional understanding of the mechanisms of lncRNA action in this disease could represent a potential for translational applications, as these molecules may serve as novel biomarkers of clinical use and potential therapeutic targets. This review highlights the relationship between lncRNAs and the principal hallmark of the luminal breast cancer phenotype, estrogen receptor α (ERα), providing an overview of new potential ways to inhibit estrogenic signaling via this nuclear receptor toward escaping resistance to endocrine therapy.

Transcriptional regulatory networks of circulating immune cells in type 1 diabetes: A community knowledgebase

Investigator-generated transcriptomic datasets interrogating circulating immune cell (CIC) gene expression in clinical type 1 diabetes (T1D) have underappreciated re-use value. Here, we repurposed these datasets to create an open science environment for the generation of hypotheses around CIC signaling pathways whose gain or loss of function contributes to T1D pathogenesis. We firstly computed sets of genes that were preferentially induced or repressed in T1D CICs and validated these against community benchmarks. We then inferred and validated signaling node networks regulating expression of these gene sets, as well as differentially expressed genes in the original underlying T1D case:control datasets. In a set of three use cases, we demonstrated how informed integration of these networks with complementary digital resources supports substantive, actionable hypotheses around signaling pathway dysfunction in T1D CICs. Finally, we developed a federated, cloud-based web resource that exposes the entire data matrix for unrestricted access and re-use by the research community.

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Re-use of transcriptomic type 1 diabetes (T1D) circulating immune cells (CICs) datasets

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We generated transcriptional regulatory networks for T1D CICs

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Use cases generate substantive hypotheses around signaling pathway dysfunction in T1D CICs

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Networks are freely accessible on the web for re-use by the research community

Nuclear Receptor Coregulators in Hormone-Dependent Cancers

Nuclear receptors (NRs) function collectively as a transcriptional signaling network that mediates gene regulatory actions to either maintain cellular homeostasis in response to hormonal, dietary and other environmental factors, or act as orphan receptors with no known ligand. NR complexes are large and interact with multiple protein partners, collectively termed coregulators. Coregulators are essential for regulating NR activity and can dictate whether a target gene is activated or repressed by a variety of mechanisms including the regulation of chromatin accessibility. Altered expression of coregulators contributes to a variety of hormone-dependent cancers including breast and prostate cancers. Therefore, understanding the mechanisms by which coregulators interact with and modulate the activity of NRs provides opportunities to develop better prognostic and diagnostic approaches, as well as novel therapeutic targets. This review aims to gather and summarize recent studies, techniques and bioinformatics methods used to identify distorted NR coregulator interactions that contribute as cancer drivers in hormone-dependent cancers.

The Landscape of AhR Regulators and Coregulators to Fine-Tune AhR Functions

The aryl-hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates numerous cellular responses. Originally investigated in toxicology because of its ability to bind environmental contaminants, AhR has attracted enormous attention in the field of immunology in the last 20 years. In addition, the discovery of endogenous and plant-derived ligands points to AhR also having a crucial role in normal cell physiology. Thus, AhR is emerging as a promiscuous receptor that can mediate either toxic or physiologic effects upon sensing multiple exogenous and endogenous molecules. Within this scenario, several factors appear to contribute to the outcome of gene transcriptional regulation by AhR, including the nature of the ligand as such and its further metabolism by AhR-induced enzymes, the local tissue microenvironment, and the presence of coregulators or specific transcription factors in the cell. Here, we review the current knowledge on the array of transcription factors and coregulators that, by interacting with AhR, tune its transcriptional activity in response to endogenous and exogenous ligands.

Molecular Functions of Thyroid Hormone Signaling in Regulation of Cancer Progression and Anti-Apoptosis

Several physiological processes, including cellular growth, embryonic development, differentiation, metabolism and proliferation, are modulated by genomic and nongenomic actions of thyroid hormones (TH). Several intracellular and extracellular candidate proteins are regulated by THs. 3,3,5-Triiodo-L-thyronine (T₃) can interact with nuclear thyroid hormone receptors (TR) to modulate transcriptional activities via thyroid hormone response elements (TRE) in the regulatory regions of target genes or bind receptor molecules showing no structural homology to TRs, such as the cell surface receptor site on integrin αvβ3. Additionally, L-thyroxine (T₄) binding to integrin αvβ3 is reported to induce gene expression through initiating non-genomic actions, further influencing angiogenesis and cell proliferation. Notably, thyroid hormones not only regulate the physiological processes of normal cells but also stimulate cancer cell proliferation via dysregulation of molecular and signaling pathways. Clinical hypothyroidism is associated with delayed cancer growth. Conversely, hyperthyroidism is correlated with cancer prevalence in various tumor types, including breast, thyroid, lung, brain, liver and colorectal cancer. In specific types of cancer, both nuclear thyroid hormone receptor isoforms and those on the extracellular domain of integrin αvβ3 are high risk factors and considered potential therapeutic targets. In addition, thyroid hormone analogs showing substantial thyromimetic activity, including triiodothyroacetic acid (Triac), an acetic acid metabolite of T₃, and tetraiodothyroacetic acid (Tetrac), a derivative of T₄, have been shown to reduce risk of cancer progression, enhance therapeutic effects and suppress cancer recurrence. Here, we have reviewed recent studies focusing on the roles of THs and TRs in five cancer types and further discussed the potential therapeutic applications and underlying molecular mechanisms of THs.

The estrogen receptor-alpha S118P variant does not affect breast cancer incidence or response to endocrine therapies

PURPOSE

Estrogen receptor-alpha (ER) is a therapeutic target of ER-positive (ER+) breast cancers. Although ER signaling is complex, many mediators of this pathway have been identified. Specifically, phosphorylation of ER at serine 118 affects responses to estrogen and therapeutic ligands and has been correlated with clinical outcomes in ER+ breast cancer patients. We hypothesized that a newly described germline variant (S118P) at this residue would drive cellular changes consistent with breast cancer development and/or hormone resistance.

METHODS

Isogenic human breast epithelial cell line models harboring ER S118P were developed via genome editing and characterized to determine the functional effects of this variant. We also examined the frequency of ER S118P in a case-control study (N = 536) of women with and without breast cancer with a familial risk.

RESULTS

In heterozygous knock-in models, the S118P variant demonstrated no significant change in proliferation, migration, MAP Kinase pathway signaling, or response to the endocrine therapies tamoxifen and fulvestrant. Further, there was no difference in the prevalence of S118P between women with and without cancer relative to population registry databases.

CONCLUSIONS

This study suggests that the ER S118P variant does not affect risk for breast cancer or hormone therapy resistance. Germline screening and modification of treatments for patients harboring this variant are likely not warranted.

Peroxisome proliferator-activated receptor γ coactivator family members competitively regulate hepatitis b virus biosynthesis

Transcriptional coactivators represent critical components of the transcriptional pre-initiation complex and are required for efficient gene activation. Members of the peroxisome proliferator-activated receptor gamma coactivator 1 (PGC1) family differentially regulate hepatitis b virus (HBV) biosynthesis. Whereas PGC1α has been shown to be a potent activator of HBV biosynthesis, PGC1β only very poorly activates HBV RNA and DNA synthesis in human hepatoma (HepG2) and embryonic kidney (HEK293T) cells. Furthermore, PGC1β inhibits PGC1α-mediated HBV biosynthesis. These observations suggest that a potential competition between human hepatoma (HepG2) and embryonic kidney (HEK293T) cells PGC1α and PGC1β for common transcription factor target(s) may regulate HBV transcription and replication in a context and signal transduction pathway dependent manner.

Nuclear receptor coactivator SRC-1 interacts with the Q-rich subdomain of the AhR and modulates its transactivation potential

The aryl hydrocarbon receptor (AhR), a soluble cytosolic protein, mediates many of the toxic effects of TCDD and related chemicals. The toxic effects are largely cell, tissue, and promoter context dependent. Although many details of the overall dioxin signal transduction have been elucidated, the transcriptional regulation of dioxin-induced genes like cyp1A1 is not yet completely understood. Previously, we have shown that the co-regulator RIP140 is a potential AhR coactivator. In this report, the role of coactivator, SRC-1, in AhR-mediated transcriptional regulation was examined. SRC-1 increased AhR-mediated, TCDD-dependent reporter gene activity three-fold in Hepa-1 and COS-1 cells. In in vitro interaction assays, SRC-1 was found to interact with AhR but not with ARNT. SRC-1 interacted weakly with AhR in the absence of TCDD and the addition of ligand further increased SRC-1 binding to AhR. Deletional mapping studies of the AhR revealed that SRC-1 binds to the AhR transactivation domain. Finer mapping of the SRC-1-interacting subdomains in the AhR transactivation domain suggested that the Q-rich subdomain was necessary and sufficient for interaction, similar to that seen with RIP140. Using GFP-tagged constructs, SRC-1 was shown to interact with AhR in cells. Unlike RIP140, LXXLL motifs in SRC-1 were necessary for interaction with AhR in vitro and for coactivation in Hepa-1 cells. The recruitment of certain coactivators by a variety of receptors suggests possible common coactivator pools and competition among receptors for limiting coactivators. Examination of the role of SRC-1 in AhR/ARNT transactivation in ARNT-deficient mutant Hepa-1 c4 cells demonstrates that the AhR transactivation domain is sufficient for enhanced coactivation mediated by SRC-1 in the presence of a transactivation domain deleted ARNT protein.

Hormone-induced repression of genes requires BRG1-mediated H1.2 deposition at target promoters

Eukaryotic gene regulation is associated with changes in chromatin compaction that modulate access to DNA regulatory sequences relevant for transcriptional activation or repression. Although much is known about the mechanism of chromatin remodeling in hormonal gene activation, how repression is accomplished is much less understood. Here we report that in breast cancer cells, ligand-activated progesterone receptor (PR) is directly recruited to transcriptionally repressed genes involved in cell proliferation along with the kinases ERK1/2 and MSK1. PR recruits BRG1 associated with the HP1γ-LSD1 complex repressor complex, which is further anchored via binding of HP1γ to the H3K9me3 signal deposited by SUV39H2. In contrast to what is observed during gene activation, only BRG1 and not the BAF complex is recruited to repressed promoters, likely due to local enrichment of the pioneer factor FOXA1. BRG1 participates in gene repression by interacting with H1.2, facilitating its deposition and stabilizing nucleosome positioning around the transcription start site. Our results uncover a mechanism of hormone-dependent transcriptional repression and a novel role for BRG1 in progestin regulation of breast cancer cell growth.

CACUL1 functions as a negative regulator of androgen receptor in prostate cancer cells

The androgen receptor (AR) plays a critical role in the initiation and progression of prostate cancer (PCa), and thus its regulation is an important tool in PCa therapy. Here, we report that CDK2-associated cullin 1 (CACUL1) directly associates with AR and suppresses AR transcriptional activity. In addition, CACUL1 represses histone demethylase LSD1-mediated AR transactivation by competing with LSD1 for AR binding. Depletion of CACUL1 enhances the LSD1 occupancy of the AR-target promoter, accompanied by decreased accumulation of H3K9me2, a repressive transcriptional marker. CACUL1 and LSD1 oppositely regulate CDX-induced cell death in AR-positive LNCaP and metastatic castrate-resistant LNCaP-LN3 cells. These data suggest that CACUL1 impairs LSD1-mediated activation of AR, thereby implicating it as a potential antitumor target in PCa.

Aging and Loss of Circulating 17β-Estradiol Alters the Alternative Splicing of ERβ in the Female Rat Brain

Loss of circulating 17β-estradiol (E2) that occurs during menopause can have detrimental effects on cognitive function. The efficacy of hormone replacement therapy declines as women become farther removed from the menopausal transition, yet the molecular mechanisms underlying this age-related switch in E2 efficacy are unknown. We hypothesized that aging and varying lengths of E2 deprivation alters the ratio of alternatively spliced estrogen receptor (ER)β isoforms in the brain of female rats. Further, we tested whether changes in global transcriptional activity and splicing kinetics regulate the alternative splicing of ERβ. Our results revealed brain region-specific changes in ERβ alternative splicing in both aging and E2-deprivation paradigms and showed that ERβ could mediate E2-induced alternative splicing. Global transcriptional activity, as measured by phosphorylated RNA polymerase II, was also regulated by age and E2 in specific brain regions. Finally, we show that inhibition of topoisomerase I resulted in increased ERβ2 splice variant expression.

Distinctly different dynamics and kinetics of two steroid receptors at the same response elements in living cells

Closely related transcription factors (TFs) can bind to the same response elements (REs) with similar affinities and activate transcription. However, it is unknown whether transcription is similarly orchestrated by different TFs bound at the same RE. Here we have compared the recovery half time (t_1/2), binding site occupancy and the resulting temporal changes in transcription upon binding of two closely related steroid receptors, the androgen and glucocorticoid receptors (AR and GR), to their common hormone REs (HREs). We show that there are significant differences at all of these levels between AR and GR at the MMTV HRE when activated by their ligands. These data show that two TFs bound at the same RE can have significantly different modes of action that can affect their responses to environmental cues.

Modulation of testosterone-dependent male sexual behavior and the associated neuroplasticity

Steroids modulate the transcription of a multitude of genes and ultimately influence numerous aspects of reproductive behaviors. Our research investigates how one single steroid, testosterone, is able to trigger this vast number of physiological and behavioral responses. Testosterone potency can be changed locally via aromatization into 17β-estradiol which then activates estrogen receptors of the alpha and beta sub-types. We demonstrated that the independent activation of either receptor activates different aspects of male sexual behavior in Japanese quail. In addition, several studies suggest that the specificity of testosterone action on target genes transcription is related to the recruitment of specific steroid receptor coactivators. We demonstrated that the specific down-regulation of the coactivators SRC-1 or SRC-2 in the medial preoptic nucleus by antisense techniques significantly inhibits steroid-dependent male-typical copulatory behavior and the underlying neuroplasticity. In conclusion, our results demonstrate that the interaction between several steroid metabolizing enzymes, steroid receptors and their coactivators plays a key role in the control of steroid-dependent male sexual behavior and the associated neuroplasticity in quail.

A conserved surface on the ligand binding domain of nuclear receptors for allosteric control

Nuclear receptors (NRs) form a large superfamily of transcription factors that participate in virtually every key biological process. They control development, fertility, gametogenesis and are misregulated in many cancers. Their enormous functional plasticity as transcription factors relates in part to NR-mediated interactions with hundreds of coregulatory proteins upon ligand (e.g., hormone) binding to their ligand binding domains (LBD), or following covalent modification. Some coregulator association relates to the distinct residues that shape a coactivator binding pocket termed AF-2, a surface groove that primarily determines the preference and specificity of protein-protein interactions. However, the highly conserved AF-2 pocket in the NR superfamily appears to be insufficient to account for NR subtype specificity leading to fine transcriptional modulation in certain settings. Additional protein-protein interaction surfaces, most notably on their LBD, may contribute to modulating NR function. NR coregulators and chaperones, normally much larger than the NR itself, may also bind to such interfaces. In the case of the androgen receptor (AR) LBD surface, structural and functional data highlighted the presence of another site named BF-3, which lies at a distinct but topographically adjacent surface to AF-2. AR BF-3 is a hot spot for mutations involved in prostate cancer and androgen insensitivity syndromes, and some FDA-approved drugs bind at this site. Structural studies suggested an allosteric relationship between AF-2 and BF-3, as occupancy of the latter affected coactivator recruitment to AF-2. Physiological relevant partners of AR BF-3 have not been described as yet. The newly discovered site is highly conserved among the steroid receptors subclass, but is also present in other NRs. Several missense mutations in the BF-3 regions of these human NRs are implicated in pathology and affect their function in vitro. The fact that AR BF-3 pocket is a druggable site evidences its pharmacological potential. Compounds that may affect allosterically NR function by binding to BF-3 open promising avenues to develop type-specific NR modulators.

Association of polymorphisms for nuclear receptor coactivator 1 gene with egg production traits in the maternal line of Shaobo hens

1. The objectives of the study were to find polymorphic sites and elucidate the association between SNPs in the nuclear receptor coactivator 1 (NCOA1) gene and reproductive traits. 2. SNPs were detected by PCR-SSCP and DNA sequencing. Four SNPs were detected, including T10155007A, T10125838C, G10118492A and G10109315T. Three polymorphisms were associated with total egg production at the age of 300 d and the G10109315T polymorphism was associated with age at first egg. 3. In conclusion, the NCOA1 gene can be used as a molecular marker for reproductive traits in hens.

Phosphorylation of Estrogen Receptor α at serine 118 directs recruitment of promoter complexes and gene-specific transcription

Phosphorylation of estrogen receptor α (ERα) is important for receptor function, although the role of specific ERα phosphorylation sites in ERα-mediated transcription remains to be fully evaluated. Transcriptional activation by ERα involves dynamic, coordinate interactions with coregulators at promoter enhancer elements to effect gene expression. To determine whether ERα phosphorylation affects recruitment of unique protein complexes at gene-specific promoters, changes in ERα Ser118 phosphorylation were assessed for effects on receptor and coregulator recruitment and transcription of ERα-regulated genes. Chromatin immunoprecipitation assays to measure promoter association found a 17β-estradiol (E2)-dependent recruitment of ERα at 150 min to ERα-regulated promoters, whereas ERα phosphorylated at Ser118 was dissociated from promoters after E2 treatment. Mutation of Ser118 to alanine (S118A) altered unliganded and ligand-induced association of ERα and p160 coregulators with ERα target promoters when compared with wild-type (WT)-ERα transfection. S118A and WT-ERα exhibited a similar level of recruitment to the estrogen response element-driven pS2 promoter and induced pS2 mRNA after E2 treatment. Although WT-ERα was recruited to c-myc and cyclin D1 promoters after E2 treatment and induced mRNA expression, S118A exhibited reduced interaction with c-myc and cyclin D1 promoters, and E2 did not induce c-myc and cyclin D1 mRNA. In addition, S118A resulted in increased recruitment of steroid receptor coactivator-1, glucocorticoid receptor interacting protein-1, and activated in breast cancer-1 to pS2, c-myc, and cyclin D1 irrespective of the presence of E2. Together, these data indicate that site specific phosphorylation of ERα directs gene-specific recruitment of ERα and transcriptional coregulators to ERα target gene promoters.

Coactivators necessary for transcriptional output of the hypoxia inducible factor, HIF, are directly recruited by ARNT PAS-B

Hypoxia-inducible factor (HIF) is the key transcriptional effector of the hypoxia response in eukaryotes, coordinating the expression of genes involved in oxygen transport, glycolysis, and angiogenesis to promote adaptation to low oxygen levels. HIF is a basic helix-loop-helix (bHLH)-PAS (PER-ARNT-SIM) heterodimer composed of an oxygen-labile HIF-α subunit and a constitutively expressed aryl hydrocarbon receptor nuclear translocator (ARNT) subunit, which dimerize via basic helix-loop-helix and PAS domains, and recruit coactivators via HIF-α C-terminal transactivation domains. Here we demonstrate that the ARNT PAS-B domain provides an additional recruitment site by binding the coactivator transforming acidic coiled-coil 3 (TACC3) in a step necessary for transcriptional responses to hypoxia. Structural insights from NMR spectroscopy illustrate how this PAS domain simultaneously mediates interactions with HIF-α and TACC3. Finally, mutations on ARNT PAS-B modulate coactivator selectivity and target gene induction by HIF in vivo, demonstrating a bifunctional role for transcriptional regulation by PAS domains within bHLH-PAS transcription factors.

Nuclear receptor coregulators merge transcriptional coregulation with epigenetic regulation

Members of the nuclear steroid/thyroid hormone receptor (NR) gene superfamily are DNA-binding transcription factors that regulate target genes in a spatiotemporal manner, depending on the promoter context. In vivo observations of ligand responses in NR-mediated gene regulation led to the identification of ligand-dependent coregulators that directly interact with NRs. Functional dissection of NR coregulators revealed that their transcriptional coregulation was linked to histone acetylation. However, recent work in the fields of reversible histone modification and chromatin remodeling indicates that histone-modifying enzymes, including histone methylases and chromatin remodelers, are potential transcriptional coregulators that interact directly and indirectly with NRs.

Homeostatic levels of SRC-2 and SRC-3 promote early human adipogenesis

The related coactivators SRC-2 and SRC-3 interact with peroxisome proliferator activated receptor γ (PPARγ) to coordinate transcriptional circuits to promote adipogenesis. To identify potential coactivator redundancy during human adipogenesis at single cell resolution, we used high content analysis to quantify links between PPARγ, SRC-2, SRC-3, and lipogenesis. Because we detected robust increases and significant cell-cell heterogeneity in PPARγ and lipogenesis, without changes in SRC-2 or SRC-3, we hypothesized that permissive coregulator levels comprise a necessary adipogenic equilibrium. We probed this equilibrium by down-regulating SRC-2 and SRC-3 while simultaneously quantifying PPARγ. Individual or joint knockdown equally inhibits lipid accumulation by preventing lipogenic gene engagement, without affecting PPARγ protein levels. Supporting dominant, pro-adipogenic roles for SRC-2 and SRC-3, SRC-1 knockdown does not affect adipogenesis. SRC-2 and SRC-3 knockdown increases the proportion of cells in a PPARγ(hi)/lipid(lo) state while increasing phospho-PPARγ-S114, an inhibitor of PPARγ transcriptional activity and adipogenesis. Together, we demonstrate that SRC-2 and SRC-3 concomitantly promote human adipocyte differentiation by attenuating phospho-PPARγ-S114 and modulating PPARγ cellular heterogeneity.

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.

Androgen receptor signalling in prostate cancer: the functional consequences of acetylation

The androgen receptor (AR) is a ligand activated transcription factor and member of the steroid hormone receptor (SHR) subfamily of nuclear receptors. In the early stages of prostate carcinogenesis, tumour growth is dependent on androgens, and AR directly mediates these effects by modulating gene expression. During transcriptional regulation, the AR recruits numerous cofactors with acetylation-modifying enzymatic activity, the best studied include p300/CBP and the p160/SRC family of coactivators. It is known that recruitment of histone acetyltransferases (HATs) and histone deacetylases (HDACs) is key in fine-tuning responses to androgens and is thus likely to play a role in prostate cancer progression. Further, these proteins can also modify the AR itself. The functional consequences of AR acetylation, the role of modifying enzymes in relation to AR transcriptional response, and prostate cancer will be discussed.

Steroid receptor coactivator (SRC) family: masters of systems biology

The three members of the p160 family of steroid receptor coactivators (SRC-1, SRC-2, and SRC-3) steer the functional output of numerous genetic programs and serve as pleiotropic rheostats for diverse physiological processes. Since their discovery ∼15 years ago, the extraordinary sum of examination of SRC function has shaped the foundation of our knowledge for the now 350+ coregulators that have been identified to date. In this perspective, we retrace our steps into the field of coregulators and provide a summary of selected seminal work that helped define the SRCs as masters of systems biology.

Isoform-specific degradation of PR-B by E6-AP is critical for normal mammary gland development

E6-associated protein (E6-AP), which was originally identified as an ubiquitin-protein ligase, also functions as a coactivator of estrogen (ER-α) and progesterone (PR) receptors. To investigate the in vivo role of E6-AP in mammary gland development, we generated transgenic mouse lines that either overexpress wild-type (WT) human E6-AP (E6-AP(WT)) or ubiquitin-protein ligase-defective E6-AP (E6-AP(C833S)) in the mammary gland. Here we show that overexpression of E6-AP(WT) results in impaired mammary gland development. In contrast, overexpression of E6-AP(C833S) or loss of E6-AP (E6-AP(KO)) increases lateral branching and alveolus-like protuberances in the mammary gland. We also show that the mammary phenotypes observed in the E6-AP transgenic and knockout mice are due, in large part, to the alteration of PR-B protein levels. We also observed alteration in ER-α protein level, which might contribute to the observed mammary phenotype by regulating PR expression. Furthermore, E6-AP regulates PR-B protein levels via the ubiquitin-proteasome pathway. Additionally, we also show that E6-AP impairs progesterone-induced Wnt-4 expression by decreasing the steady state level of PR-B in both mice and in human breast cancer cells. In conclusion, we present the novel observation that E6-AP controls mammary gland development by regulating PR-B protein turnover via the ubiquitin proteasome pathway. For the first time, we show that the E3-ligase activity rather than the coactivation function of E6-AP plays an important role in the mammary gland development, and the ubiquitin-dependent PR-B degradation is not required for its transactivation functions. This mechanism appears to regulate normal mammogenesis, and dysregulation of this process may be an important contributor to mammary cancer development and progression.

Streamlined analysis schema for high-throughput identification of endogenous protein complexes

Immunoprecipitation followed by mass spectrometry (IP/MS) has recently emerged as a preferred method in the analysis of protein complex components and cellular protein networks. Targeting endogenous protein complexes of higher eukaryotes, particularly in large-scale efforts, has been challenging due to cellular heterogeneity, high proteome complexity, and, compared to lower organisms, lack of efficient in-locus epitope-tagging techniques. It is further complicated by variability in nonspecific identifications and cross-reactivity of primary antibodies. Still, the study of endogenous human protein networks is highly desired despite its challenges. Here we describe a streamlined IP/MS protocol for the purification and identification of extended endogenous protein complexes. We investigate the sources of nonspecific protein binding and develop semiquantitative specificity filters that are based on peptide spectral count measurements. We also outline logical constraints for the derivation of accurate complex composition from IP/MS data and demonstrate the effectiveness of this approach by presenting our analyses of different transcriptional coregulator complexes. We show consistent purification of novel components for the Integrator complex, analyze the composition of the Mediator complex solely from our data to demonstrate the wide usability of spectral counts, and deconvolute heterogeneous HDAC1/2 networks into core complex modules and several novel subcomplex interactions.

Epigenetic Reactivation of Estrogen Receptor: Promising Tools for Restoring Response to Endocrine Therapy

Breast tumors expressing estrogen receptor alpha (ER) respond well to therapeutic strategies using SERMs (selective estrogen receptor modulators) such as tamoxifen. However, about thirty percent of invasive breast cancers are hormone independent because they lack ER expression due to hypermethylation of ER promoter. Treatment of ER-negative breast cancer cells with demethylating agents and histone deacetylase inhibitors leads to expression of ER mRNA and functional protein. Additionally, growth factor signaling pathways have also been implicated in ER silencing in ER-negative tumor phenotype. Recently, important role of components of ubiquitin-proteasome pathway has been shown in mediating downregulation of ER. In this article, we will review various mechanisms underlying the silencing of ER in ER negative tumor phenotype and discuss diverse strategies to combat it. Ongoing studies may provide the mechanistic insight to design therapeutic strategies directed towards epigenetic and non-epigenetic mechanisms in the prevention or treatment of ER-negative breast cancer.

Progesterone action in human tissues: regulation by progesterone receptor (PR) isoform expression, nuclear positioning and coregulator expression

Progesterone is a critical regulator of normal female reproductive function, with diverse tissue-specific effects in the human. The effects of progesterone are mediated by its nuclear receptor (PR) that is expressed as two isoforms, PRA and PRB, which are virtually identical except that PRA lacks 164 amino acids that are present at the N-terminus of PRB. Considerable in vitro evidence suggests that the two PRs are functionally distinct and in animals, tissue-specific distribution patterns of PRA and PRB may account for some of the diversity of progesterone effects. In the human, PRA and PRB are equivalently expressed in most target cells, suggesting that alternative mechanisms control the diversity of progesterone actions. PR mediates the effects of progesterone by association with a range of coregulatory proteins and binding to specific target sequences in progesterone-regulated gene promoters. Ligand activation of PR results in redistribution into discrete subnuclear foci that are detectable by immunofluorescence, probably representing aggregates of multiple transcriptionally active PR-coregulator complexes. PR foci are aberrant in cancers, suggesting that the coregulator composition and number of complexes is altered. A large family of coregulators is now described and the range of proteins known to bind PR exceeds the complement required for transcriptional activation, suggesting that in the human, tissue-specific coregulator expression may modulate progesterone response. In this review, we examine the role of nuclear localization of PR, coregulator association and tissue-specific expression in modulating progesterone action in the human.

Importance of steroid receptor coactivators in the modulation of steroid action on brain and behavior

Steroid receptors such as estrogen and androgen receptors are nuclear receptors involved in the transcriptional regulation of a large number of target genes. Steroid-dependent protein expression in the brain controls a large array of biological processes including spatial cognition, copulatory behavior and neuroprotection. The discovery of a competition, or squelching, between two different nuclear receptors introduced the notion that common cofactors may be involved in the modulation of transcriptional activity of nuclear receptors. These cofactors or coregulatory proteins are functionally divided into coactivators and corepressors and are involved in chromatin remodeling and stabilization of the general transcription machinery. Although a large amount of information has been collected about the in vitro function of these coregulatory proteins, relatively little is known regarding their physiological role in vivo, particularly in the brain. Our laboratory and others have demonstrated the importance of SRC-1 in the differentiation and activation of steroid-dependent sexual behaviors and the related neural genes. For example, we report that the inhibition of SRC-1 expression blocks the activating effects of exogenous testosterone on male sexual behaviors and increases the volume of the median preoptic area. Other coactivators are likely to be involved in the modulation in vivo of steroid receptor activity and it seems that the presence of a precise subset of coactivators could help define the phenotype of the cell by modulating a specific downstream pathway after steroid receptor activation. The very large number of coactivators and their association into preformed complexes potentially allows the determination of hundreds of different phenotypes. The study of the expression of the coactivator and their function in vivo is required to fully understand steroid action and specificity in the brain.

Minireview: the SRC family of coactivators: an entrée to understanding a subset of polygenic diseases?

In this perspective, we present the idea that SRC family coactivators are likely agents in human polygenic disease states based upon a number of interlocking aspects of their biology. We argue that their role as key integrators of environmental signals and their ability to regulate the expression of myriad downstream genes makes them likely candidates for strong positive evolutionary selection pressures. Based on the fact that they work as part of multiprotein coactivator complexes, we predict that individual coactivator alleles exist as weakly penetrant disease alleles, each contributing only a fraction of transcriptional activity to the whole coactivator complex. In this way, individual coactivator alleles are free to evolve in the absence of strong negative selection. Emerging genomic and proteomic approaches promise to advance the characterization of coactivator proteins and their physiological functions, allowing us to have a greater appreciation of their roles as master regulators at the nexus between genetics, reproduction, metabolism, cancer, other human diseases, and our environment.

Genome-wide analysis and proteomic studies reveal APE1/Ref-1 multifunctional role in mammalian cells

Apurinic apyrimidinic endonuclease/redox effector factor 1 (APE1/Ref-1) protects cells from oxidative stress by acting as a central enzyme in base excision repair pathways of DNA lesions and through its independent activity as a redox transcriptional co-activator. Dysregulation of this protein has been associated with cancer development. At present, contrasting data have been published regarding the biological relevance of the two functions as well as the molecular mechanisms involved. Here, we combined both mRNA expression profiling and proteomic analysis to determine the molecular changes associated with APE1 loss-of-expression induced by siRNA technology. This approach identified a role of APE1 in cell growth, apoptosis, intracellular redox state, mitochondrial function, and cytoskeletal structure. Overall, our data show that APE1 acts as a hub in coordinating different and vital functions in mammalian cells, highlighting the molecular determinants of the multifunctional nature of APE1 protein.

Who's in charge? Nuclear receptor coactivator and corepressor function in brain and behavior

Steroid hormones act in brain and throughout the body to regulate a variety of functions, including development, reproduction, stress and behavior. Many of these effects of steroid hormones are mediated by their respective receptors, which are members of the steroid/nuclear receptor superfamily of transcriptional activators. A variety of studies in cell lines reveal that nuclear receptor coregulators are critical in modulating steroid receptor-dependent transcription. Thus, in addition to the availability of the hormone and the expression of its receptor, nuclear receptor coregulators are essential for efficient steroid-dependent transactivation of genes. This review will highlight the importance of nuclear receptor coregulators in modulating steroid-dependent gene expression in brain and the regulation of behavior.

Minireview: Evolution of NURSA, the Nuclear Receptor Signaling Atlas

Nuclear receptors and coregulators are multifaceted players in normal metabolic and homeostatic processes in addition to a variety of disease states including cancer, inflammation, diabetes, obesity, and atherosclerosis. Over the past 7 yr, the Nuclear Receptor Signaling Atlas (NURSA) research consortium has worked toward establishing a discovery-driven platform designed to address key questions concerning the expression, organization, and function of these molecules in a variety of experimental model systems. By applying powerful technologies such as quantitative PCR, high-throughput mass spectrometry, and embryonic stem cell manipulation, we are pursuing these questions in a series of transcriptomics-, proteomics-, and metabolomics-based research projects and resources. The consortium's web site (www.nursa.org) integrates NURSA datasets and existing public datasets with the ultimate goal of furnishing the bench scientist with a comprehensive framework for hypothesis generation, modeling, and testing. We place a strong emphasis on community input into the development of this resource and to this end have published datasets from academic and industrial laboratories, established strategic alliances with Endocrine Society journals, and are developing tools to allow web site users to act as data curators. With the ongoing support of the nuclear receptor and coregulator signaling communities, we believe that NURSA can make a lasting contribution to research in this dynamic field.

Molecular basis of coiled coil coactivator recruitment by the aryl hydrocarbon receptor nuclear translocator (ARNT)

The aryl hydrocarbon receptor nuclear translocator (ARNT) serves as the obligate heterodimeric partner for bHLH-PAS proteins involved in sensing and coordinating transcriptional responses to xenobiotics, hypoxia, and developmental pathways. Although its C-terminal transactivation domain is dispensable for transcriptional activation in vivo, ARNT has recently been shown to use its N-terminal bHLH and/or PAS domains to interact with several transcriptional coactivators that are required for transcriptional initiation after xenobiotic or hypoxic cues. Here we show that ARNT uses a single PAS domain to interact with two coiled coil coactivators, TRIP230 and CoCoA. Both coactivators interact with the same interface on the ARNT PAS-B domain, located on the opposite side of the domain used to associate with the analogous PAS domain on its heterodimeric bHLH-PAS partner HIF-2alpha. Using NMR and biochemical studies, we identified the ARNT-interacting motif of one coactivator, TRIP230 as an LXXLL-like nuclear receptor box. Mutation of this motif and proximal sequences disrupts the interaction with ARNT PAS-B. Identification of this ARNT-coactivator interface illustrates how ARNT PAS-B is used to form critical interactions with both bHLH-PAS partners and coactivators that are required for transcriptional responses.

Nuclear receptor coactivators: essential players for steroid hormone action in the brain and in behaviour

Steroid hormones act both in the brain and throughout the body to influence behaviour and physiology. Many of these effects of steroid hormones are elicited by transcriptional events mediated by their respective receptors. A variety of cell culture studies reveal that nuclear receptor coactivators are critical for modulating steroid receptor-dependent transcription. Thus, in addition to the availability of the hormone and the expression of its receptor, nuclear receptor coactivators are essential for steroid-dependent transactivation of genes. This review discusses the mounting evidence indicating that nuclear receptor coactivators are critical for modulating steroid hormone action in the brain and in the regulation of behaviour.

Mechanisms underlying the control of progesterone receptor transcriptional activity by SUMOylation

Posttranslational modification by small ubiquitin-like modifier (SUMO) is a major regulator of transcription. We previously showed that progesterone receptors (PR) have a single consensus psiKXE SUMO-conjugation motif centered at Lys-388 in the N-terminal domain of PR-B and a homologous site of PR-A. SUMOylation of the PR is hormone-dependent and has a suppressive effect on transcription of an exogenous promoter. Here we show that repression of PR activity by SUMOylation at Lys-388 is uncoupled from phosphorylation, involves synergy between tandem progesterone response elements, and is associated with lowered ligand sensitivity and slowed ligand-dependent down-regulation. However, paradoxically, cellular overexpression of SUMO-1 increases PR transcriptional activity even if Lys-388 is mutated, suggesting that the receptors are activated indirectly by other SUMOylated proteins. One of these is the coactivator SRC-1, whose binding to PR and enhancement of agonist-dependent N-/C-terminal interactions is augmented by the presence of SUMO-1. Increased transcription due to SRC-1 is independent of PR SUMOylation based on assays with the Lys-388 mutants and the pure antiprogestin ZK98299, which blocks N-/C-terminal interactions. In summary, SUMOylation tightly regulates the transcriptional activity of PR by repressing the receptors directly while activating them indirectly through augmented SRC-1 coactivation.

BRG1 and LKB1: tales of two tumor suppressor genes on chromosome 19p and lung cancer

Losses of heterozygosity (LOH) of the short arm of chromosome 19 are frequent in lung cancer, suggesting that one or more tumor suppressor genes are present in this region. The LKB1 gene, also called STK11, is somatically inactivated through point mutations and large deletions in lung tumors, demonstrating that LKB1 is a target of the LOH of this chromosomal arm. Data from several independent groups have provided information about the profiles of lung tumors with LKB1 inactivation and it is generally agreed that this alteration strongly predominates in non-small cell lung cancer, in particular adenocarcinomas, in smokers. The LKB1 protein has serine-threonine kinase activity and is involved in the regulation of the cell energetic checkpoint through the phosphorylation and activation of adenosine monophosphate-dependent kinase (AMPK). LKB1 is also involved in other processes such as cell polarization, probably through substrates other than AMPK. Interestingly, another gene on chromosome 19p, BRG1, encoding a component of the SWI/SNF chromatin-remodeling complex, has emerged as a tumor suppressor gene that is altered in lung tumors. Similar to LKB1, BRG1 is somatically inactivated by point mutations or large deletions in lung tumors featuring LOH of chromosome 19p. These observations suggest an important role for BRG1 in lung cancer and highlight the need to further our understanding of the function of Brahma/SWI2-related gene 1 (BRG1) in cancer. Finally, simultaneous mutations at LKB1 and BRG1 are common in lung cancer cells, which exemplifies how a single event, LOH of chromosome 19p in this instance, targets two different tumor suppressors.

P160/SRC/NCoA coactivators form complexes via specific interaction of their PAS-B domain with the CID/AD1 domain

Transcriptional activation involves the ordered recruitment of coactivators via direct interactions between distinct binding domains and recognition motifs. The p160/SRC/NCoA coactivator family comprises three members (NCoA-1, -2 and -3), which are organized in multiprotein coactivator complexes. We had identified the PAS-B domain of NCoA-1 as an LXXLL motif binding domain. Here we show that NCoA family members are able to interact with other full-length NCoA proteins via their PAS-B domain and they specifically interact with the CBP-interaction domain (CID/AD1) of NCoA-1. Peptide competition, binding experiments and mutagenesis of LXXLL motifs point at distinct binding motif specificities of the NCoA PAS-B domains. NMR studies of different NCoA-1-PAS-B/LXXLL peptide complexes revealed similar although not identical binding sites for the CID/AD1 and STAT6 transactivation domain LXXLL motifs. In mechanistic studies, we found that overexpression of the PAS-B domain is able to disturb the binding of NCoA-1 to CBP in cells and that a CID/AD1 peptide competes with STAT6 for NCoA-1 in vitro. Moreover, the expression of an endogenous androgen receptor target gene is affected by the overexpression of the NCoA-1 or NCoA-3 PAS-B domains. Our study discloses a new, complementary mechanism for the current model of coactivator recruitment to target gene promoters.

MOZ and MOZ-CBP cooperate with NF-kappaB to activate transcription from NF-kappaB-dependent promoters

OBJECTIVE

Monocytic zinc finger (MOZ) maintains hematopoietic stem cells and, upon fusion to the coactivator CREB-binding protein (CBP), induces acute myeloid leukemia (AML). Leukemic stem cells in AML often exhibit excessive signal-dependent activity of the transcription factor nuclear factor (NF)-kappaB. Because aberrant interaction between NF-kappaB and coactivators represents an alternative mechanism for enhancing NF-kappaB activity, we evaluated whether MOZ and MOZ-CBP cooperate with NF-kappaB to activate transcription from NF-kappaB-dependent promoters.

METHODS

The ability of MOZ, MOZ mutants, and MOZ-CBP to enhance expression of NF-kappaB-dependent promoters was tested in reporter studies. The interaction between MOZ and NF-kappaB was evaluated by both coimmunoprecipitation and glutathione S-transferase pulldown assays.

RESULTS

MOZ activates transcription from the NF-kappaB-dependent interleukin-8 promoter; interestingly, this effect is markedly enhanced by CBP. Although MOZ has less potent transcriptional activity than MOZ-CBP, both proteins cooperate with steroid receptor coactivator-1 to activate transcription. MOZ also induces multiple NF-kappaB-dependent viral promoters. Importantly, MOZ associates in a protein complex with the p65 subunit of NF-kappaB and interacts directly with p65 in vitro. Transcriptional activity of MOZ requires its C-terminal domain, which is absent from MOZ-CBP, indicating that the transcriptional activity of MOZ-CBP derives from its CBP sequence.

CONCLUSIONS

MOZ interacts with the p65 subunit of NF-kappaB and enhances expression of NF-kappaB-dependent promoters. The more potent transcriptional activity of MOZ-CBP derives from its CBP sequence. Thus, interaction between NF-kappaB and MOZ-CBP may play an important role in the pathogenesis of certain acute myeloid leukemias.

The p160 steroid receptor coactivator 2, SRC-2, regulates murine endometrial function and regulates progesterone-independent and -dependent gene expression

The role of the p160 steroid receptor coactivator 2 (SRC-2) in the regulation of uterine function and progesterone (P4) signaling was investigated by determining the expression pattern of SRC-2 in the murine uterus during pregnancy and the impact of SRC-2 ablation on uterine function and global uterine gene expression in response to progesterone. SRC-2 is expressed in the endometrial luminal and glandular epithelium from pregnancy d 0.5. SRC-2 is then expressed in the endometrial stroma on pregnancy d 2.5-3.5. Once the embryo is implanted, SRC-2 is expressed in the endometrial stromal cells in the secondary decidual zone. This compartmental expression of SRC-2 can be mimicked by treatment of ovariectomized mice with estrogen and P4. Ablation of SRC-2 in the uterus resulted in a significant reduction in the ability of the uterus to undergo a hormonally induced decidual reaction. Microarray analysis of RNA from uteri of wild-type and SRC-2(-/-) mice treated with vehicle or P4 showed that SRC-2 was involved in the ability of progesterone to repress specific genes. This microarray analysis also revealed that the uteri of SRC-2(-/-) mice showed alterations in genes involved in estrogen receptor, Wnt, and bone morphogenetic protein signaling. This analysis indicates that SRC-2 regulates uterine function by modulating the regulation of developmentally important signaling molecules and the ability of P4 to repress specific genes.

Cells in behaviourally relevant brain regions coexpress nuclear receptor coactivators and ovarian steroid receptors

Oestradiol and progesterone act in the brain to elicit profound effects on behaviour and physiology. One physiological function of oestradiol is the induction of progesterone receptor (PR) expression in a variety of behaviourally relevant brain regions, including the ventromedial nucleus of the hypothalamus (VMN), the medial preoptic nucleus of the preoptic area (MPOA), the arcuate nucleus (ARC) and the medial central grey (MCG). Ligand-dependent transcriptional activity of steroid receptors, including oestrogen receptors (ER) and Pr, is dramatically influenced by nuclear receptor coactivators. In previous studies, we have found that two of these nuclear receptor coactivators, steroid receptor coactivator-1 (SRC-1) and CREB-binding protein (CBP), are important in ER-mediated induction of PR in the VMN and in steroid-dependent behaviours. For nuclear receptor coactivators to function in hormone-dependent transcription in the brain and regulate behaviour, both receptor and coactivator must be expressed in the same cell. In the present study, we used a dual-label immunohistochemical technique to investigate if individual cells in behaviourally relevant brain regions coexpress nuclear receptor coactivators and steroid receptors. Confocal analysis revealed that in oestrogen-primed rats, most of the E-induced PR cells in the VMN (89.6%), MPOA (63%), ARC (82.6%), and many in the MCG (39%), also express SRC-1. In addition, the majority of the cells containing E-induced PR in the VMN (78.3%), MPOA (83.1%), ARC (83.6%), and MCG (60%) also express CBP. These results, taken together with the findings that virtually all oestradiol-induced PR containing cells in the brain express ER, suggest that these neurones represent sites of functional interaction of nuclear receptor coactivators with ovarian steroid receptors in the brain. The present findings provide neuroanatomical evidence that nuclear receptor coactivators are integral in mediating steroid hormone action in behaviourally relevant brain regions.

Stromal-epithelial cell interactions and androgen receptor-coregulator recruitment is altered in the tissue microenvironment of prostate cancer

Tissue recombination experiments show that prostate mesenchyma directs prostate epithelial cell growth and development in an androgen-dependent manner, and that functional differentiation of prostate epithelium requires androgen-driven processes in both epithelia and stroma. The androgen induction of target genes in primary cultures of prostate stromal and epithelial cells was determined using an adenoviral expression system, which employed the MMTV-enhancer driven luciferase reporter as an androgen receptor (AR)-mediated transcription assay. These studies indicate that both cell types contain functional AR. Androgen induction of luciferase reporter activity is 3-fold in stromal cells compared with 10-fold in epithelial cells. AR-mediated transcription activity in stroma cells was enhanced by coculture with epithelial cells or epithelial cell-conditioned media. The elevated AR-mediated transcription activity in stromal cells that were exposed to epithelial factors correlated with increased recruitment of coactivators to the AR transcriptional complex. Epithelial cells facilitated interactions of AR with SRC-1 in an androgen-dependent manner. However, AR-mediated transcriptional activity in stromal cells isolated from prostate cancer was reduced compared with stromal cells isolated from benign prostate and continued to be reduced when cocultured with tumor-derived prostate epithelial cells. The occupancy of AR and coregulators on target genes showed that androgen-bound AR in prostate cancer stromal cells was associated with the corepressor silencing mediator for retinoid and thyroid hormone receptor. Thus, the ability of epithelial cells to modulate coregulator recruitment to the AR transcriptional complex on androgen-responsive genes seems altered in the stromal microenvironment of prostate cancer.

New perspectives into the biological and clinical relevance of oestrogen receptors in the human breast

Oestrogen receptor (ER) is arguably the single most important biological predictive factor that exists today. In the last 10 years or so, however, our understanding of ER biology has undergone a paradigm shift following the identification of a second ER, ERbeta, with the original ER being renamed ERalpha. A number of isoforms have additionally been described, especially for ERbeta. Our knowledge of ER signalling has also increased with the recognition of accessory co-regulatory proteins, which help direct the transcriptional cascade. Here we outline these changes and discuss what biological and clinical implications these could have in the mammary gland.

In vitro fluorescence anisotropy analysis of the interaction of full-length SRC1a with estrogen receptors alpha and beta supports an active displacement model for coregulator utilization

Binding of full-length P160 coactivators to hormone response element-steroid receptor complexes has been difficult to investigate in vitro. Here, we report a new application of our recently described fluorescence anisotropy microplate assay to investigate binding and dissociation of full-length steroid receptor coactivator-1a (SRC1a) from full-length estrogen receptor alpha (ERalpha) or estrogen receptor beta (ERbeta) bound to a fluorescein-labeled (fl) estrogen response element (ERE). SRC1a exhibited slightly higher affinity binding to flERE.ERbeta than to flERE.ERalpha. Binding of SRC1a to flERE.ERalpha and to flERE.ERbeta was 17beta-estradiol (E2)-dependent and was nearly absent when ICI 182,780, raloxifene, or 4-hydroxytamoxifen were bound to the ERs. SRC1a binds to flERE.E2-ERalpha and flERE.E2-ERbeta complexes with a t1/2 of 15-20 s. Short LXXLL-containing nuclear receptor (NR) box peptides from P160 coactivators competed much better for SRC1a binding to flERE.E2-ER than an NR box peptide from TRAP220. However, approximately 40-250-fold molar excess of the P160 NR box peptides was required to inhibit SRC1a binding by 50%. This suggests that whereas the NR box region is a primary site of interaction between SRC1a and ERE.E2-ER, additional contacts between the coactivator and the ligand-receptor-DNA complex make substantial contributions to overall affinity. Increasing amounts of NR box peptides greatly enhanced the rate of dissociation of SRC1a from preformed flERE.E2-ER complexes. The data support a model in which coactivator exchange is facilitated by active displacement and is not simply the result of passive dissociation and replacement. It also shows that an isolated coactivator exhibits an inherent capacity for rapid exchange.

Coregulator exchange and sphingosine-sensitive cooperativity of steroidogenic factor-1, general control nonderepressed 5, p54, and p160 coactivators regulate cyclic adenosine 3',5'-monophosphate-dependent cytochrome P450c17 transcription rate

Transcription of the cytochrome P450 17 (CYP17) gene is regulated by cAMP-dependent binding of steroidogenic factor-1 (SF-1) to its promoter in the adrenal cortex. Using temporal chromatin immunoprecipitation and mammalian two-hybrid experiments, we establish the reciprocal presence of coactivators [general control nonderepressed (GCN5), cAMP response element-binding protein-binding protein, p300, p300/cAMP response element-binding protein-binding protein CBP associated factor, p160s, polypyrimidine tract associated splicing factor, and p54(nrb)], corepressors (class I histone deacetylases, receptor interacting protein, nuclear receptor corepressor, and Sin3A), and SWI/SNF (human homolog of yeast mating type switching/sucrose nonfermenting) and imitation SWI chromatin remodeling ATPases on the CYP17 promoter during transcription cycles in the H295R adrenocortical cell line. A ternary GCN5/SRC-1/SF-1 complex forms on the CYP17 promoter with cAMP-dependence within 30 min of cAMP stimulation, and corresponds with SWI/SNF chromatin remodeling. This complex is sensitive to the SF-1 antagonist sphingosine and results in decreased transcription of CYP17. GCN5 acetyltransferase activity and carboxy terminus binding proteins alternatively mediate disassembly of the complex. This work establishes the temporal order of cAMP-induced events on the promoter of a key steroidogenic gene during SF-1-mediated transcription.

The breast cancer susceptibility gene BRCA1 regulates progesterone receptor signaling in mammary epithelial cells

The progesterone receptor (PR) plays roles in normal mammary development and breast cancer formation, where it may exert both stimulatory and inhibitory actions. Previously, the breast cancer susceptibility gene product BRCA1 was found to interact with and inhibit the transcriptional activity of estrogen receptor-alpha. In this study, we found that exogenous wild-type BRCA1 inhibited the activity of the PR in transient transfection assays utilizing a mouse mammary tumor virus-Luc reporter. Wild-type BRCA1 inhibited the activity of endogenous PR in human breast cancer cells (T47D and MCF-7) and inhibited the activity of exogenous PR-A, PR-B, and [PR-A plus PR-B] isoforms. On the other hand, knockdown of endogenous BRCA1 using small interfering RNA enhanced the progesterone-stimulated activity of the PR by about 4-fold. We documented an in vivo association of the endogenous BRCA1 with PR isoforms A and B and a direct in vitro interaction between BRCA1 and PR, which was partially mapped. Whereas down-regulation of the coactivator p300 contributes to the BRCA1-mediated repression of estrogen receptor-alpha, this mechanism does not contribute to inhibition of PR activity, because exogenous p300 did not rescue the BRCA1 repression of PR activity. The BRCA1-PR interaction has functional consequences. Thus, we showed that BRCA1 inhibits the expression of various endogenous progesterone-responsive genes and inhibits progesterone-stimulated proliferation of T47D cells. Finally, exogenous progesterone caused an exaggerated proliferative response in the mammary glands of mice harboring a mammary-targeted conditional deletion of the full-length isoform of Brca1. These findings suggest that BRCA1 regulates the activity of progesterone, a major hormone of pregnancy that may also participate in mammary carcinogenesis.

Proteomic analysis of steady-state nuclear hormone receptor coactivator complexes

We report our initial efforts in the analysis of endogenous nuclear receptor coactivator complexes as a research bridging strand of the Nuclear Receptor Signaling Atlas (NURSA) (www.NURSA.org). A proteomic approach is used to systematically isolate a variety of coactivator complexes using HeLa cells as a model cell line and to identify the coactivator-associated proteins with mass spectrometry. We have isolated and identified seven coactivator complexes including the p160 steroid receptor coactivator family, cAMP response element binding protein-binding protein, p300, coactivator of activating protein-1 and estrogen receptors, and E6 papillomavirus-associated protein. The newly identified coactivator-associated proteins provide unbiased clues and links for understanding of the endogenous hormone receptor coregulator network and its regulation. We hope that the electronic availability of these data to the general scientific community will facilitate generation and testing of new hypotheses to further our understanding of nuclear receptor signaling and coactivator functions.

Endocrinology and hormone therapy in breast cancer: new insight into estrogen receptor-alpha function and its implication for endocrine therapy resistance in breast cancer

Estrogen and its receptor (ER) are critical for development and progression of breast cancer. This pathway is targeted by endocrine therapies that either block ER functions or deplete ER's estrogen ligand. While endocrine therapies are very effective, de novo and acquired resistance are still common. Laboratory and clinical data now indicate that bidirectional molecular crosstalk between nuclear or membrane ER and growth factor receptor pathways such as HER2/neu is involved in endocrine resistance. Preclinical data suggest that blockade of selected growth factor receptor signaling can overcome this type of resistance, and this strategy is already being tested in clinical trials