Oestrogen receptor facilitates the formation of preinitiation complex assembly: involvement of the general transcription factor TFIIB

Estrogen signaling and the aging brain: context-dependent considerations for postmenopausal hormone therapy

Recent clinical studies have spurred rigorous debate about the benefits of hormone therapy (HT) for postmenopausal women. Controversy first emerged based on a sharp increase in the risk of cardiovascular disease in participants of the Women's Health Initiative (WHI) studies, suggesting that decades of empirical research in animal models was not necessarily applicable to humans. However, a reexamination of the data from the WHI studies suggests that the timing of HT might be a critical factor and that advanced age and/or length of estrogen deprivation might alter the body's ability to respond to estrogens. Dichotomous estrogenic effects are mediated primarily by the actions of two high-affinity estrogen receptors alpha and beta (ER α & ER β ). The expression of the ERs can be overlapping or distinct, dependent upon brain region, sex, age, and exposure to hormone, and, during the time of menopause, there may be changes in receptor expression profiles, post-translational modifications, and protein:protein interactions that could lead to a completely different environment for E2 to exert its effects. In this review, factors affecting estrogen-signaling processes will be discussed with particular attention paid to the expression and transcriptional actions of ER β in brain regions that regulate cognition and affect.

Identification of a hormone-regulated dynamic nuclear actin network associated with estrogen receptor alpha in human breast cancer cell nuclei

Estrogen receptor alpha (ERalpha) is a modular protein of the steroid/nuclear receptor family of transcriptional regulators that upon binding to the hormone undergoes structural changes, resulting in its nuclear translocation and docking to specific chromatin sites. In the nucleus, ERalpha assembles in multiprotein complexes that act as final effectors of estrogen signaling to the genome through chromatin remodeling and epigenetic modifications, leading to dynamic and coordinated regulation of hormone-responsive genes. Identification of the molecular partners of ERalpha and understanding their combinatory interactions within functional complexes is a prerequisite to define the molecular basis of estrogen control of cell functions. To this end, affinity purification was applied to map and characterize the ERalpha interactome in hormone-responsive human breast cancer cell nuclei. MCF-7 cell clones expressing human ERalpha fused to a tandem affinity purification tag were generated and used to purify native nuclear ER-containing complexes by IgG-Sepharose affinity chromatography and glycerol gradient centrifugation. Purified complexes were analyzed by two-dimensional DIGE and mass spectrometry, leading to the identification of a ligand-dependent multiprotein complex comprising beta-actin, myosins, and several proteins involved in actin filament organization and dynamics and/or known to participate in actin-mediated regulation of gene transcription, chromatin dynamics, and ribosome biogenesis. Time course analyses indicated that complexes containing ERalpha and actin are assembled in the nucleus early after receptor activation by ligands, and gene knockdown experiments showed that gelsolin and the nuclear isoform of myosin 1c are key determinants for assembly and/or stability of these complexes. Based on these results, we propose that the actin network plays a role in nuclear ERalpha actions in breast cancer cells, including coordinated regulation of target gene activity, spatial and functional reorganization of chromatin, and ribosome biogenesis.

A novel estradiol/estrogen receptor alpha-dependent transcriptional mechanism controls expression of the human prolactin receptor

Prolactin exerts diverse functions in target tissues through its membrane receptors, and is a potent mitogen in normal and neoplastic breast cells. Estradiol (E(2)) induces human prolactin receptor (hPRLR) gene expression through stimulation of its generic promoter (PIII). This study identifies a novel E(2)-regulated non-estrogen responsive element-dependent transcriptional mechanism that mediates E(2)-induced hPRLR expression. E(2) stimulated transcriptional activity in MCF7A(2) cells transfected with PIII lacking an estrogen responsive element, and increased hPRLR mRNA and protein. The abolition of the E(2) effect by mutation of Sp1 or C/EBP elements that bind Sp1/Sp3 and C/EBPbeta within PIII indicated the cooperation of these transfactors in E(2)-induced transcription of the hPRLR. DNA affinity protein assay showed that E(2) induced estrogen receptor alpha (ERalpha) binding to Sp1/Sp3 and C/EBPbeta DNA-protein complexes. The ligand-binding domain of ERalpha was essential for its physical interaction with C/EBPbeta, and E(2) promoted this association, and its DNA binding domain was required for transactivation of PIII. Co-immunoprecipitation studies revealed tethering of C/EBPbeta to Sp1 by E(2)-activated ERalpha. Chromatin immunoprecipitation analysis showed that E(2) induced recruitment of C/EBPbeta, ERalpha, SRC1, p300, pCAF, TFIIB, and Pol II, with no change in Sp1/Sp3. E(2) also induced promoter-associated acetylation of H3 and H4. These findings demonstrate that an E(2)/ERalpha, Sp1, and C/EBPbeta complex with recruitment of coactivators and TFIIB and Pol II are required for E(2)-activated transcriptional expression of the hPRLR through PIII. Estradiol produced in breast stroma and adipose tissue, which are major sources of estrogen in post-menopausal women, could up-regulate hPRLR gene expression and stimulate breast tumor growth.

Altered target gene regulation controlled by estrogen receptor-alpha concentration

Estrogen receptor-alpha (ERalpha) is a transcriptional activator whose concentration is tightly regulated by the cellular environment. In breast tumors of postmenopausal women, elevated receptor concentrations can be associated with negative clinical outcomes, yet it remains poorly understood how such high levels impact ERalpha function. We previously demonstrated that high nuclear concentrations of ERalpha in breast cancer cells bypass the requirement for ligand and are sufficient to activate transcription and accelerate proliferation. Here, we extended those studies and asked whether the transcriptional targets and activation mechanism are similar or different from that of estrogen-stimulated ERalpha. We found that at elevated levels, ERalpha activated, but could not repress, known estrogen-responsive genes. Moreover, the set of activated genes was expanded to include the uterine-restricted target gene, complement component 3. The activation mechanism of ERalpha under these conditions depends both on activation function-1 and residues in the proximal region of the ligand-binding domain. Mutations of aspartate 351 and leucine 372 can inhibit ERalpha transcriptional activity gained at high concentrations and discriminate concentration-inducible ERalpha function from that induced by estrogen. Moreover, we demonstrate that at high levels, ERalpha stimulates transcription without recruiting steroid receptor coactivator-3 and without interference by a Gal4-receptor interaction domain box fusion protein containing LxxLL motifs, further distinguishing this mode of regulation from known activation mechanisms. Together these results demonstrate that the concentration of receptor in breast cancer cells can influence the pattern of target gene expression through a noncanonical activation mechanism.

p21WAF1/CIP1 selectively controls the transcriptional activity of estrogen receptor alpha

Estrogen receptors (ER) are ligand-dependent transcription factors that regulate growth, differentiation, and maintenance of cellular functions in a wide variety of tissues. We report here that p21WAF1/CIP1, a cyclin-dependent kinase (Cdk) inhibitor, cooperates with CBP to regulate the ERalpha-mediated transcription of endogenous target genes in a promoter-specific manner. The estrogen-induced expression of the progesterone receptor and WISP-2 mRNA transcripts in MCF-7 cells was enhanced by p21WAF1/CIP1, whereas that of the cyclin D1 mRNA was reduced and the pS2 mRNA was not affected. Chromatin immunoprecipitation assays revealed that p21WAF1/CIP1 was recruited simultaneously with ERalpha and CBP to the endogenous progesterone receptor gene promoter in an estrogen-dependent manner. Experiments in which the p21WAF1/CIP1 protein was knocked down by RNA interference showed that the induction of the expression of the gene encoding the progesterone receptor required p21WAF1/CIP1, in contrast with that of the cyclin D1 and pS2 genes. p21WAF1/CIP1 induced not only cell cycle arrest in breast cancer cells but also milk fat globule protein and lipid droplets, indicators of the differentiated phenotype, as well as cell flattening and increase of the volume of the cytoplasm. These results indicate that p21WAF1/CIP1, in addition to its Cdk-regulatory role, behaves as a transcriptional coactivator in a gene-specific manner implicated in cell differentiation.

Estradiol alters transcription factor gene expression in primate prefrontal cortex

Estrogen protects neurons from a variety of experimental insults in vitro, and is thought to protect from acute and chronic neurodegenerative processes in vivo. Estrogen also enhances higher-level cognitive functions that are centered in the dorsolateral prefrontal cortex (DLPFC) in human and non-human primates. To investigate genomic mechanisms involved in estrogenic effects on the primate brain in vivo, we compared transcription factor mRNA and protein expression in the DLPFC of ovariectomized rhesus monkeys treated with either vehicle or estradiol (E2). c-FOS, E2F1, and general transcription factor IIB (TFIIB) mRNA and protein expression were altered significantly by short-term E2 treatment, as shown by DNA array, in situ hybridization, and immunohistochemical and immunoblot evaluations. C-FOS expression was increased significantly whereas E2F1 and TFIIB levels were decreased in the DLPFC of E2-treated animals. These transcription factors were concentrated in cortical pyramids, as were estrogen receptors alpha and beta. These data indicate that estrogen may have direct as well as indirect effects on neuronal gene expression in the primate prefrontal cortex.

Estrogen receptor-alpha directs ordered, cyclical, and combinatorial recruitment of cofactors on a natural target promoter

Transcriptional activation of a gene involves an orchestrated recruitment of components of the basal transcription machinery and intermediate factors, concomitant with an alteration in local chromatin structure generated by posttranslational modifications of histone tails and nucleosome remodeling. We provide here a comprehensive picture of events resulting in transcriptional activation of a gene, through evaluating the estrogen receptor-alpha (NR3A1) target pS2 gene promoter in MCF-7 cells. This description integrates chromatin remodeling with a kinetic evaluation of cyclical networks of association of 46 transcription factors with the promoter, as determined by chromatin immunoprecipitation assays. We define the concept of a "transcriptional clock" that directs and achieves the sequential and combinatorial assembly of a transcriptionally productive complex on a promoter. Furthermore, the unanticipated findings of key roles for histone deacetylases and nucleosome-remodeling complexes in limiting transcription implies that transcriptional activation is a cyclical process that requires both activating and repressive epigenetic processes.

Differential regulation of the human progesterone receptor gene through an estrogen response element half site and Sp1 sites

The progesterone receptor (PR) gene is regulated by estrogen in normal reproductive tissues and in MCF-7 human breast cancer cells. Although it is generally thought that estrogen responsiveness is mediated by interaction of the ligand-occupied estrogen receptor (ER) with estrogen response elements (EREs) in target genes, the human progesterone receptor (PR) gene lacks a palindromic ERE. Promoter A of the PR gene does, however, contain an ERE half site upstream of two adjacent Sp1 sites from +571 to +595, the +571 ERE/Sp1 site. We have examined the individual contributions of the ERE half site and the two Sp1 sites in regulating estrogen responsiveness. Transient transfection assays demonstrated that both Sp1 sites were critical for estrogen-mediated activation of the PR gene. Interestingly, rather than decreasing transcription, mutations in the ERE half site increased transcription substantially suggesting that this site plays a role in limiting transcription. Chromatin immunoprecipitation assays demonstrated that Sp1 was associated with the +571 ERE/Sp1 site in the endogenous PR gene in the absence and in the presence of estrogen, but that ERalpha was only associated with this region of the PR gene after MCF-7 cells had been treated with estrogen. Our studies provide evidence that effective regulation of transcription through the +571 ERE/Sp1 site requires the binding of ERalpha and Sp1 to their respective cis elements and the appropriate interaction of ERalpha and Sp1 with other coregulatory proteins and transcription factors.

Molecular communication between androgen receptor and general transcription machinery

The androgen-androgen receptor (AR) signaling pathway plays a key role in proper development and function of male reproductive organs. Like other transcriptional regulators, AR may communicate with the general transcription machinery on the core promoter to exert its function as a transcriptional modulator. The molecular communication between AR and the general transcription machinery may be achieved either by the direct protein-protein interaction between AR and the general transcription machinery or by the indirect interaction mediated by coregulators. Analyses of AR-mediated transcription suggest that the orchestrated interaction of AR with the transcription factors IIF (TFIIF) and IIH (TFIIH), and positive transcription elongation factor b (P-TEFb), may increase efficiency of transcriptional elongation from the androgen target genes, such as prostate specific antigen (PSA). Based on studies so far, AR may regulate transcription not by enhanced assembly of preinitiation transcription complex but by regulating promoter clearance and elongation stage of transcription.

Transcriptional activation by the oestrogen receptor alpha is modulated through inhibition of cyclin-dependent kinases

We have investigated the interaction between the expression of p21(WAF1/CIP1/SDI1), a stoichiometric inhibitor of Cdk, and the transcriptional activity of the oestrogen receptor alpha (ER(alpha). Transient transfection experiments demonstrated that the expression of p21(WAF1/CIP1/SDI1) amplified the transcriptional activation by ER(alpha). A dominant negative mutant of Cdk2 also enhanced the ER(alpha) transcriptional activity, indicating that the underlying mechanism relies on the inhibition of Cdk2 activity and cell cycle arrest. In agreement with this conclusion, experiments with p21(WAF1/CIP1/SDI1) mutants demonstrated that the domain involved in the binding of p21(WAF1/CIP1/SDI1) to Cdks was indispensable for the modulation of ER(alpha) activity. In addition, we show that expression of p21(WAF1/CIP1/SDI1) alleviates the block on CBP function mediated by Cdk2 and in turn stimulates transcriptional activation by ER(alpha) in a CBP-histone acetyltransferase (HAT)-dependent manner. These results suggest a novel mechanism by which p21(WAF1/CIP1/SDI1) functions as an enhancer of ER(alpha) activity through the modulation of CBP function.

Alteration of large-scale chromatin structure by estrogen receptor

The estrogen receptor (ER), a member of the nuclear hormone receptor superfamily important in human physiology and disease, recruits coactivators which modify local chromatin structure. Here we describe effects of ER on large-scale chromatin structure as visualized in live cells. We targeted ER to gene-amplified chromosome arms containing large numbers of lac operator sites either directly, through a lac repressor-ER fusion protein (lac rep-ER), or indirectly, by fusing lac repressor with the ER interaction domain of the coactivator steroid receptor coactivator 1. Significant decondensation of large-scale chromatin structure, comparable to that produced by the approximately 150-fold-stronger viral protein 16 (VP16) transcriptional activator, was produced by ER in the absence of estradiol using both approaches. Addition of estradiol induced a partial reversal of this unfolding by green fluorescent protein-lac rep-ER but not by wild-type ER recruited by a lac repressor-SRC570-780 fusion protein. The chromatin decondensation activity did not require transcriptional activation by ER nor did it require ligand-induced coactivator interactions, and unfolding did not correlate with histone hyperacetylation. Ligand-induced coactivator interactions with helix 12 of ER were necessary for the partial refolding of chromatin in response to estradiol using the lac rep-ER tethering system. This work demonstrates that when tethered or recruited to DNA, ER possesses a novel large-scale chromatin unfolding activity.

Cellular effects of purvalanol A: a specific inhibitor of cyclin-dependent kinase activities

We have studied the effects of purvalanol A on the cell cycle progression, proliferation and viability. In synchronized cells, purvalanol A induced a reversible arrest the progression in G1 and G2 phase of the cell cycle, but did not prevent the completion of DNA synthesis in S-phase cells. The specificity of action of the drug was supported by the selective inhibition of the phosphorylation of cyclin-dependent kinase (cdk) substrates such as Rb and cyclin E. The cell contents of cyclins D1 and E were lower in cells incubated with purvalanol A compared to controls, but the level of the cdk inhibitory protein p21(WAF1/CIP1) was increased, indicating that the drug did not cause a general inhibition of gene expression. Purvalanol A did not inhibit transcription under cell-free conditions. This compound, however, caused an inhibition of the estradiol-induced expression of an integrated luciferase gene, suggesting that cdk or related enzymes may participate in the regulation of the activity of certain promoters. When exponentially growing cells, both mouse fibroblasts and human cancer cell lines, were incubated with purvalanol A for prolonged periods of time (24 hr), a lasting inhibition of cell proliferation as well as cell death were observed. In contrast, a 24 hr incubation of quiescent (non-transformed) cells with purvalanol A did not prevent their resumption of cell cycle after removal of the drug.

Estrogen receptor and breast cancer

Breast cancer, the most common malignancy in women, was already known to be associated with the steroid hormone estrogen more than a century ago. The discovery of the estrogen receptor (ER) provided us not only with a powerful predictive and prognostic marker, but also an efficient target for the treatment of hormone-dependent breast cancer with antiestrogens. In this paper we will sketch the important role of ER in the development, progression, and treatment of the disease, which is complicated by the receptor's interaction with co-regulatory proteins, its cross-talk with other signal transduction pathways, and its involvement in the development of antiestrogen resistance.

Binding of ligands and activation of transcription by nuclear receptors

Nuclear receptors (NRs) form a superfamily of ligand-inducible transcription factors composed of several domains. Recent structural studies focused on domain E, which harbors the ligand-binding site and the ligand-dependent transcription activation function AF-2. Structures of single representatives in an increasing number of various complexes as well as new structures of further NRs addressed issues such as discrimination of ligands, superagonism, isotype specificity, and partial agonism. Until today, one unique transcriptionally active form of domain E was determined; however, divergent tertiary structures of apo-forms and transcriptionally inactive forms are known. Thus, recent results link the transformation of NRs upon ligand binding to principles of protein folding. Furthermore, the ensemble of NR structures, including those of DNA-binding domains, provides one of the foundations for the understanding of interactions with transcription intermediary factors up to the characterization of the link between NR complexes and the basal transcriptional machinery at the structural level.

Binding of TATA binding protein to a naturally positioned nucleosome is facilitated by histone acetylation

The TATA sequence of the human, estrogen-responsive pS2 promoter is complexed in vivo with a rotationally and translationally positioned nucleosome (NUC T). Using a chromatin immunoprecipitation assay, we demonstrate that TATA binding protein (TBP) does not detectably interact with this genomic binding site in MCF-7 cells in the absence of transcriptional stimuli. Estrogen stimulation of these cells results in hyperacetylation of both histones H3 and H4 within the pS2 chromatin encompassing NUC T and the TATA sequence. Concurrently, TBP becomes associated with the pS2 promoter region. The relationship between histone hyperacetylation and the binding of TBP was assayed in vitro using an in vivo-assembled nucleosomal array over the pS2 promoter. With chromatin in its basal state, the binding of TBP to the pS2 TATA sequence at the edge of NUC T was severely restricted, consistent with our in vivo data. Acetylation of the core histones facilitated the binding of TBP to this nucleosomal TATA sequence. Therefore, we demonstrate that one specific, functional consequence of induced histone acetylation at a native promoter is the alleviation of nucleosome-mediated repression of the binding of TBP. Our data support a fundamental role for histone acetylation at genomic promoters in transcriptional activation by nuclear receptors and provide a general mechanism for rapid and reversible transcriptional activation from a chromatin template.

The polymorphic N terminus in human vitamin D receptor isoforms influences transcriptional activity by modulating interaction with transcription factor IIB

The human vitamin D receptor (hVDR) is a ligand-regulated transcription factor that mediates the actions of the 1,25-dihydroxyvitamin D3 hormone to effect bone mineral homeostasis. Employing mutational analysis, we characterized Arg-18/Arg-22, hVDR residues immediately N-terminal of the first DNA binding zinc finger, as vital for contact with human basal transcription factor IIB (TFIIB). Alteration of either of these basic amino acids to alanine also compromised hVDR transcriptional activity. In contrast, an artificial hVDR truncation devoid of the first 12 residues displayed both enhanced interaction with TFIIB and transactivation. Similarly, a natural polymorphic variant of hVDR, termed F/M4 (missing a FokI restriction site), which lacks only the first three amino acids (including Glu-2), interacted more efficiently with TFIIB and also possessed elevated transcriptional activity compared with the full-length (f/M1) receptor. It is concluded that the functioning of positively charged Arg-18/Arg-22 as part of an hVDR docking site for TFIIB is influenced by the composition of the adjacent polymorphic N terminus. Increased transactivation by the F/M4 neomorphic hVDR is hypothesized to result from its demonstrated enhanced association with TFIIB. This proposal is supported by the observed conversion of f/M1 hVDR activity to that of F/M4 hVDR, either by overexpression of TFIIB or neutralization of the acidic Glu-2 by replacement with alanine in f/M1 hVDR. Because the f VDR genotype has been associated with lower bone mineral density in diverse populations, one factor contributing to a genetic predisposition to osteoporosis may be the F/f polymorphism that dictates VDR isoforms with differential TFIIB interaction.

Mechanism by which the IFN-beta enhanceosome activates transcription

We demonstrate that in contrast to previous findings by using simple synthetic promoters or activators, the natural IFN-beta enhanceosome activates transcription by causing a dramatic increase of the rate by which preinitiation complexes assemble at the promoter. This effect totally depends on the recruitment of the CBP-PolII holoenzyme by the enhanceosome, because its depletion from the extract decelerates the rate of transcription. However, addition of the CBP-PolII holoenzyme back to these extracts fully restores the speed by which the enhanceosome activates transcription. Strikingly, preincubation of the enhanceosome with the CBP-RNA PolII holoenzyme complex results in instant assembly of preinitiation complexes. In contrast, individual IFN-beta gene activators function solely by increasing the number of functional preinitiation complexes and not the rate of their assembly. Thus, fast recruitment of the CBP-RNA PolII holoenzyme complex is critical for the rapid activation of IFN-beta gene expression by virus infection.

Estrogen induction of the cyclin D1 promoter: involvement of a cAMP response-like element

Estrogens induce cell proliferation in target tissues by stimulating progression through the G(1) phase of the cell cycle. Induction of cyclin D1 expression is a critical feature of the mitogenic action of estrogen. We have determined a region between -96 and -29 in the cyclin D1 promoter that confers regulation by estrogens in the human mammary carcinoma cells MCF-7. This region encompasses a unique known transcription factor binding site with a sequence of a potential cAMP response element (CRE-D1). The induction is strictly hormone dependent and requires the DNA binding domain as well as both AF-1 and AF-2 domains of the estrogen receptor (ER) alpha. Destruction of the CRE-D1 motif caused complete loss of estrogen responsiveness. Both c-Jun and ATF-2 transactivated the cyclin D1 promoter in transient transfection experiments, and a clear additional increase was detected when ER was cotransfected with either c-Jun or with c-Jun and ATF-2 but not with ATF-2 alone. Furthermore, the expression of a dominant negative variant of c-Jun, TAM67, completely abolished the induction of the cyclin D1 promoter both in the absence and presence of ER. We show that ATF-2 homodimers and ATF-2/c-Jun heterodimers, but not c-Jun homodimers, were able to bind the CRE of the cyclin D1 promoter. To interpret these results, we propose a mechanism in which ATF-2/c-Jun heterodimers bind to the CRE-D1 element and mediate the activation of cyclin D1 promoter by the ER. This mechanism represents a pathway by which estrogens control the proliferation of target cells.

Isolation of mouse TFIID and functional characterization of TBP and TFIID in mediating estrogen receptor and chromatin transcription

TFIID is a general transcription factor required for the assembly of the transcription machinery on most eukaryotic promoters transcribed by RNA polymerase II. Although the TATA-binding subunit (TBP) of TFIID is able to support core promoter and activator-dependent transcription under some circumstances, the roles of TBP-associated factors (TAF(II)s) in TFIID-mediated activation remain unclear. To define the evolutionarily conserved function of TFIID and to elucidate the roles of TAF(II)s in gene activation, we have cloned the mouse TAF(II)55 subunit of TFIID and further isolated mouse TFIID from a murine FM3A-derived cell line that constitutively expresses FLAG-tagged mouse TAF(II)55. Both mouse and human TFIIDs are capable of mediating transcriptional activation by Gal4 fusions containing different activation domains in a highly purified human cell-free transcription system devoid of TFIIA and Mediator. Although TAF(II)-independent activation by Gal4-VP16 can also be observed in this highly purified human transcription system with either mouse or yeast TBP, TAF(II)s are strictly required for estrogen receptor-mediated activation independently of the core promoter sequence. In addition, TAF(II)s are necessary for transcription from a preassembled chromatin template. These findings clearly demonstrate an essential role of TAF(II)s as a transcriptional coactivator for estrogen receptor and in chromatin transcription.