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

Ligand-dependent switching of ubiquitin-proteasome pathways for estrogen receptor

Recent evidence indicates that the transactivation of estrogen receptor alpha (ERalpha) requires estrogen-dependent receptor ubiquitination and degradation. Here we show that estrogen-unbound (unliganded) ERalpha is also ubiquitinated and degraded through a ubiquitin-proteasome pathway. To investigate this ubiquitin-proteasome pathway, we purified the ubiquitin ligase complex for unliganded ERalpha and identified a protein complex containing the carboxyl terminus of Hsc70-interacting protein (CHIP). CHIP preferentially bound to misfolded ERalpha and ubiquitinated it to induce degradation. Ligand binding to the receptor induced the dissociation of CHIP from ERalpha. In CHIP-/- cells, the degradation of unliganded ERalpha was abrogated; however, estrogen-induced degradation was observed to the same extent as in CHIP+/+ cells. Our findings suggest that ERalpha is regulated by two independent ubiquitin-proteasome pathways, which are switched by ligand binding to ERalpha. One pathway is necessary for the transactivation of the receptor and the other is involved in the quality control of the receptor.

Intranuclear organization and function of the androgen receptor

We have entered an exciting era for androgen-receptor (AR) research that should provide a detailed description of how the AR functions as a ligand-regulated transcription factor. That AR activity is regulated by subcellular compartmentalization was first established a decade ago with the finding that binding of androgen to the AR induces its translocation from the cytoplasm to the nucleus. The contribution of compartmentalization to AR activity is, however, likely to extend beyond simple delivery to the nucleus. Defects in AR and coregulator compartmentalization in the nucleus have been demonstrated in prostate cancer, androgen-insensitivity syndrome, and spinal and bulbar muscular atrophy. A complete understanding of AR function and dysfunction in disease requires integrating transcription with the spatial and temporal regulation imposed by subnuclear organization and nuclear transport.

Quantitative sequential chromatin immunoprecipitation, a method for analyzing co-occupancy of proteins at genomic regions in vivo

Sequential chromatin immunoprecipitation (SeqChIP) is a procedure in which formaldehyde-crosslinked, protein-DNA complexes from living cells are subjected to two sequential immunoprecipitations with antibodies of different specificity. SeqChIP has been used to address, in a qualitative manner, whether two proteins can simultaneously co-occupy a stretch of DNA in vivo. Here, we expand on our earlier work and describe theoretical and practical considerations for performing and interpreting SeqChIP experiments in a quantitative manner. We provide a detailed experimental procedure for designing and performing SeqChIP experiments as well as experimental examples of the three possible outcomes: full co-occupancy, no co-occupancy and partial co-occupancy. In some cases of partial co-occupancy, the order of immunoprecipitations in SeqChIP can strongly influence the outcome. We experimentally confirm a quantitative parameter that provides a measure of co-occupancy of two proteins on a given region of DNA and provide information on how to interpret the results of SeqChIP experiments. Our quantitative treatment of SeqChIP data substantially expands the usefulness of the technique for elucidating molecular mechanisms in vivo.

Phenotypic anchoring of gene expression changes during estrogen-induced uterine growth

A major challenge in the emerging field of toxicogenomics is to define the relationships between chemically induced changes in gene expression and alterations in conventional toxicologic parameters such as clinical chemistry and histopathology. We have explored these relationships in detail using the rodent uterotrophic assay as a model system. Gene expression levels, uterine weights, and histologic parameters were analyzed 1, 2, 4, 8, 24, 48, and 72 hr after exposure to the reference physiologic estrogen 17 beta-estradiol (E2). A multistep analysis method, involving unsupervised hierarchical clustering followed by supervised gene ontology-driven clustering, was used to define the transcriptional program associated with E2-induced uterine growth and to identify groups of genes that may drive specific histologic changes in the uterus. This revealed that uterine growth and maturation are preceded and accompanied by a complex, multistage molecular program. The program begins with the induction of genes involved in transcriptional regulation and signal transduction and is followed, sequentially, by the regulation of genes involved in protein biosynthesis, cell proliferation, and epithelial cell differentiation. Furthermore, we have identified genes with common molecular functions that may drive fluid uptake, coordinated cell division, and remodeling of luminal epithelial cells. These data define the mechanism by which an estrogen induces organ growth and tissue maturation, and demonstrate that comparison of temporal changes in gene expression and conventional toxicology end points can facilitate the phenotypic anchoring of toxicogenomic data.

Altered SMRT levels disrupt vitamin D3 receptor signalling in prostate cancer cells

We hypothesized that key antiproliferative target genes for the vitamin D receptor (VDR) were repressed by an epigenetic mechanism in prostate cancer cells resulting in apparent hormonal insensitivity. To explore this possibility, we examined nuclear receptor corepressor expression in a panel of nonmalignant and malignant cell lines and primary cultures, and found frequently elevated SMRT corepressor mRNA expression often associated with reduced sensitivity to 1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)2D3). For example, PC-3 and DU-145 prostate cancer cell lines had 1.8-fold and twofold increases in SMRT mRNA relative to normal PrEC cells (P<0.05). Similarly, 10/15 primary tumour cultures (including three matched to normal cells from the same donors) had elevated SMRT mRNA levels; generally NCoR1 and Alien were not as commonly elevated. Corepressor proteins often have associated histone deacetylases (HDAC) and reflectively the antiproliferative action of 1alpha,25(OH)2D3 can be 'restored' by cotreatment with low doses of HDAC inhibitors such as trichostatin A (TSA, 15 nM) to induce apoptosis in prostate cancer cell lines. To decipher the transcriptional events that lead to these cellular responses, we undertook gene expression studies in PC-3 cells after cotreatment of 1alpha,25(OH)2D3 plus TSA after 6 h. Examination of known VDR target genes and cDNA microarray analyses revealed cotreatment of 1alpha,25(OH)2D3 plus TSA cooperatively upregulated eight (out of 1176) genes, including MAPK-APK2 and GADD45alpha. MRNA and protein time courses and inhibitor studies confirmed these patterns of regulation. Subsequently, we knocked down SMRT levels in PC-3 cells using a small interfering RNA (siRNA) approach and found that GADD45alpha induction by 1alpha,25(OH)2D3 alone became very significantly enhanced. The same distortion of gene responsiveness, with repressed induction of GADD45alpha was found in primary tumour cultures compared and to matched peripheral zone (normal) cultures from the same donor. These data demonstrate that elevated SMRT levels are common in prostate cancer cells, resulting in suppression of target genes associated with antiproliferative action and apparent 1alpha,25(OH)2D3-insensitivity. This can be targeted therapeutically by combination treatments with HDAC inhibitors.

The highly conserved and multifunctional NuA4 HAT complex

Histone acetyltransferase complexes have been shown to be key regulators of gene expression. Among these, the NuA4 complex, first characterized in yeast, stands out as it controls multiple key nuclear functions in eukaryotic cells. Many subunits of this protein assembly have been directly linked to global and targeted acetylation of histone H4 tails in vivo, regulation of transcription, cell-cycle progression as well as to the process of DNA repair. Recent studies presented here have established its remarkable structural conservation from yeast to human cells and contributed to the understanding of its diverse functions.

Dynamic control of nuclear receptor transcription

This Perspective highlights experiments that probe the complexity and regulation underlying cyclical association of nuclear receptor transcriptional complexes with DNA. Using advanced in vivo and in vitro techniques, these studies emphasize the importance of chromatin remodeling and histone modification in defining the timing and nature of the cycles. In addition, they reveal the multiplicity of receptor-coregulator complexes that reside on a single promoter. These conceptual and technical achievements integrate kinetic and combinatorial regulation into a new dynamic model of nuclear receptor-mediated transcription.

Coregulators and chromatin remodeling in transcriptional control

Despite many years of investigation by numerous investigators, transcriptional regulatory control remains an intensely investigated and continuously evolving field of research. Transcriptional regulation is dependent not only on transcription factor activation and chromatin remodeling, but also on a host of transcription factor coregulators-coactivators and corepressors. In addition to transcription factor activation and chromatin changes, there is an expanding array of additional modifications that titrate transcriptional regulation for the specific conditions of a particular cell type, organ system, and developmental stage, and such events are likely to be greatly influenced by upstream signaling cascades. Here, we will briefly review the highlights and perspectives of chromatin remodeling and transcription controls as affected by cofactor availability, cellular energy state, relative ratios of reducing equivalents, and upstream signaling. We also present the C-terminal binding protein (CtBP) as a novel nuclear receptor (NR) coregulator, which exemplifies the integration of a number of transcriptional regulatory controls.

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

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

Nature of the accessible chromatin at a glucocorticoid-responsive enhancer

To gain a better understanding of the nature of active chromatin in mammals, we have characterized in living cells the various chromatin modification events triggered by the glucocorticoid receptor (GR) at the rat tyrosine aminotransferase gene. GR promotes a local remodeling at a glucocorticoid-responsive unit (GRU) located 2.5 kb upstream of the transcription start site, creating nuclease hypersensitivity that encompasses 450 bp of DNA. Nucleosomes at the GRU occupy multiple frames that are remodeled without nucleosome repositioning, showing that nucleosome positioning is not the key determinant of chromatin accessibility at this locus. Remodeling affects nucleosomes and adjacent linker sequences, enhancing accessibility at both regions. This is associated with decreased interaction of both the linker histone H1 and the core histone H3 with DNA. Thus, our results indicate that nucleosome and linker histone removal rather than nucleosome repositioning is associated with GR-triggered accessibility. Interestingly, GR induces hyperacetylation of histones H3 and H4, but this is not sufficient either for remodeling or for transcriptional activation. Finally, our data favor the coexistence of several chromatin states within the population, which may account for the previously encountered difficulties in characterizing unambiguously the active chromatin structure in living cells.

Histone deimination antagonizes arginine methylation

Methylation of arginine residues within histone H3 has been linked to active transcription. This modification appears on the estrogen-regulated pS2 promoter when the CARM1 methyltransferase is recruited during transcriptional activation. Here we describe a process, deimination, that converts histone arginine to citrulline and antagonizes arginine methylation. We show that peptidyl arginine deiminase 4 (PADI4) specifically deiminates, arginine residues R2, R8, R17, and R26 in the H3 tail. Deimination by PADI4 prevents arginine methylation by CARM1. Dimethylation of arginines prevents deimination by PADI4 although monomethylation still allows deimination to take place. In vivo targeting experiments on an endogenous promoter demonstrate that PADI4 can repress hormone receptor-mediated gene induction. Consistent with a repressive role for PADI4, this enzyme is recruited to the pS2 promoter following hormone induction when the gene is transcriptionally downregulated. The recruitment of PADI4 coincides with deimination of the histone H3 N-terminal tail. These results define deimination as a novel mechanism for antagonizing the transcriptional induction mediated by arginine methylation.

Retracted: In vivopotentiation of human oestrogen receptor α by Cdk7-mediated phosphorylation

Phosphorylation of the Ser(118) residue in the N-terminal A/B domain of the human oestrogen receptor alpha (hERalpha) by mitogen-activated protein kinase (MAPK), stimulated via growth factor signalling pathways, is known to potentiate ERalpha ligand-induced transactivation function. Besides MAPK, cyclin dependent kinase 7 (Cdk7) in the TFIIH complex has also been found to potentiate hERalpha transactivation in vitro through Ser(118) phosphorylation. To investigate an impact of Cdk7 on hERalpha transactivation in vivo, we assessed activity of hERalpha in a wild-type and cdk7 inactive mutant Drosophila that ectopically expressed hERalpha in the eye disc. Ectopic expression of the wild-type or mutant receptors, together with a green fluorescent protein (GFP) reporter gene, allowed us to demonstrate that hERalpha expressed in the fly tissues was transcriptionally functional and adequately responded to hERalpha ligands in the patterns similar to those observed in mammalian cells. Replacement of Ser(118) with alanine in hERalpha (S118A mutant) significantly reduced the ligand-induced hERalpha transactivation function. Importantly, while in cdk7 inactive mutant Drosophila the wild-type hERalpha exhibited reduced response to the ligand; levels of transactivation by the hERalpha S118A mutant were not affected in these inactive cdk7 mutant flies. Furthermore, phosphorylation of hERalpha at Ser(118) has been observed in vitro by both human and Drosophila Cdk7. Our findings demonstrate that Cdk7 is involved in regulation of the ligand-induced transactivation function of hERalphain vivo via Ser(118) phosphorylation.

Ets-dependent regulation of target gene expression during megakaryopoiesis

Megakaryopoiesis is the process by which hematopoietic stem cells in the bone marrow differentiate into mature megakaryocytes. The expression of megakaryocytic genes during megakaryopoiesis is controlled by specific transcription factors. Fli-1 and GATA-1 transcription factors are required for development of megakaryocytes and promoter analysis has defined in vitro functional binding sites for these factors in several megakaryocytic genes, including GPIIb, GPIX, and C-MPL. Herein, we utilize chromatin immunoprecipitation to examine the presence of Ets-1, Fli-1, and GATA-1 on these promoters in vivo. Fli-1 and Ets-1 occupy the promoters of GPIIb, GPIX, and C-MPL genes in both Meg-01 and CMK11-5 cells. Whereas GPIIb is expressed in both Meg-01 and CMK11-5 cells, GPIX and C-MPL are only expressed in the more differentiated CMK11-5 cells. Thus, in vivo occupancy by an Ets factor is not sufficient to promote transcription of some megakaryocytic genes. GATA-1 and Fli-1 are both expressed in CMK11-5 cells and co-occupy the GPIX and C-MPL promoters. Transcription of all three megakaryocytic genes is correlated with the presence of acetylated histone H3 and phosphorylated RNA polymerase II on their promoters. We also show that exogenous expression of GATA-1 in Meg-01 cells leads to the expression of endogenous c-mpl and gpIX mRNA. Whereas GPIIb, GPIX, and C-MPL are direct target genes for Fli-1, both Fli-1 and GATA-1 are required for formation of an active transcriptional complex on the C-MPL and GPIX promoters in vivo. In contrast, GPIIb expression appears to be independent of GATA-1 in Meg-01 cells.

Vitamin D-interacting protein 205 (DRIP205) coactivation of estrogen receptor alpha (ERalpha) involves multiple domains of both proteins

Vitamin D-interacting protein 205 (DRIP205) is a mediator complex protein that anchors the complex to the estrogen receptor (ER) and other nuclear receptors (NRs). In ZR-75 breast cancer cells treated with 17beta-estradiol (E2) and transfected with a construct containing three tandem estrogen responsive elements (pERE(3)), DRIP205 coactivates ERalpha-mediated transactivation. DRIP205Delta587-636 is a DRIP205 mutant in which both NR boxes within amino acids 587-636 have been deleted and, in parallel transfection studies, DRIP205Delta587-636 also coactivates ERalpha. Moreover, both wild-type and variant DRIP205 also colocalize with ERalpha in the nuclei of transfected cells. Extensive deletion analysis of DRIP205 shows that multiple domains of this protein play a role in coactivation of ERalpha and in interactions with ERalpha. Coactivation of ERalpha by DRIP205 does not require NR boxes, and variants with deletion of N-terminal (amino acids 1-639) and C-terminal (amino acids 576-1566) significantly coactivate ERalpha. DRIP205 resembles p160 coactivators that also interact with multiple regions of ERalpha; however, unlike p160 coactivators, DRIP205 coactivation of ERalpha does not require NR boxes.

A contemporary understanding of progesterone receptor function

Following the discovery of coactivators for nuclear receptors and the identification of a significant number of progesterone receptor (PR) target genes, our understanding of PR action has extended from its normally recognized functions to a wide variety of seemingly unlinked biological processes. Recent advances in defining the action of PR coactivators has revealed important mechanisms involving multiple layers of regulation in PR-mediated transcription. In this review, we will discuss the current understanding of PR physiology and the molecular basis of coactivator function in PR signaling events.

Differential gene regulation by the SRC family of coactivators

SRCs (steroid receptor coactivators) are required for nuclear receptor-mediated transcription and are also implicated in the transcription initiation by other transcription factors, such as STATs and NFkappaB. Despite phenotypic manifestations in gene knockout mice for SRC-1, GRIP1, and AIB1 of the SRC (Steroid Receptor Coactivator) family indicating their differential roles in animal physiology, there is no clear evidence, at the molecular level, to support a functional specificity for these proteins. We demonstrated in this report that two species of SRC coactivators, either as AIB1:GRIP1 or as AIB1:SRC-1 are recruited, possibly through heterodimerization, on the promoter of genes that contain a classical hormone responsive element (HRE). In contrast, on non-HRE-containing gene promoters, on which steroid receptors bind indirectly, either GRIP1 or SRC-1 is recruited as a monomer, depending on the cellular abundance of the protein. Typically, non-HRE-containing genes are early genes activated by steroid receptors, whereas HRE-containing genes are activated later. Our results also showed that SRC proteins contribute to the temporal regulation of gene transcription. In addition, our experiments revealed a positive correlation between AIB1/c-myc overexpression in ER+ breast carcinoma samples, suggesting a possible mechanism for AIB1 in breast cancer carcinogenesis.

Potential Role of a Novel Transcriptional Coactivator PELP1 in Histone H1 Displacement in Cancer Cells

The estrogen receptor plays an important role in breast cancer progression. Proline-, glutamic acid-, and leucine-rich protein 1 (PELP1), also called modulator of nongenomic activity of estrogen receptor (MNAR), a novel coactivator of estrogen receptor, modulates estrogen receptor transactivation functions. The mechanisms by which PELP1 modulates estrogen receptor genomic functions is not known. Here, using biochemical and scanning confocal microscopic analysis, we have demonstrated nuclear localization and functional implications of PELP1. Subnuclear fractionation showed PELP1 association with chromatin and nuclear matrix fractions. Ligand stimulation promoted recruitment of PELP1 to 17beta-estradiol responsive promoters, its colocalization with acetylated H3, and increased PELP1-associated histone acetyltransferase enzymatic activity. Far Western analysis revealed that PELP1 interacts with histone 1 and 3, with more preference toward histone 1. Using deletion analysis, we have identified the PELP1 COOH-terminal region as the histone 1 binding site. The PELP1 mutant lacking histone 1-binding domain acts as a dominant-negative and blocks estrogen receptor alpha-mediated transcription. Chromatin immunoprecipitation analysis showed a cyclic association and dissociation of PELP1 with the promoter, with recruitment of histone 1 and PELP1 occurring in opposite phases. PELP1 overexpression increased the micrococcal nuclease sensitivity of estrogen response element-containing nucleosomes. Our results provide novel insights about the transcription regulation of PELP1 and suggest that PELP1 participates in chromatin remodeling activity via displacement of histone 1 in cancer cells.

The co-activator CREB-binding protein participates in enhancer-dependent activities of bicoid

Bicoid (Bcd) is a transcriptional activator required for early embryonic patterning in Drosophila. Despite extensive studies, it currently remains unclear how Bcd activates transcription and what proteins participate in its activation process. In this report, we describe experiments to analyze the role of the Drosophila co-activator dCBP in Bcd-mediated activation. In Drosophila S2 cells, the Bcd activity is increased by the co-transfection of plasmids expressing dCBP and reduced by double-stranded RNA-mediated interference against dCBP. We further show that Bcd and dCBP can interact with each other and that Bcd-interacting domains of dCBP can cause dominant negative effects on Bcd activity in S2 cells. Our comparison of two Bcd-responsive enhancers, hunchback (hb) and knirps (kni), reveals a differential role of dCBP in facilitating Bcd activation. A dCBP mutant defective in its histone acetyltransferase activity exhibits a reduced, but not abolished, co-activator function for Bcd. Our chromatin immunoprecipitation experiments show that dCBP can increase not only the occupancy of Bcd itself at the enhancers but also the recruitment of general transcription factors to the promoter. Together, these experiments suggest that dCBP is an enhancer-dependent co-activator of Bcd, facilitating its activation through multiple mechanisms.

Roles of Brahma and Brahma/SWI2-related gene 1 in hypoxic induction of the erythropoietin gene

Upon hypoxia, the human erythropoietin (EPO) gene is transactivated by the heterodimeric hypoxia-inducible factor 1 (HIF-1). Mammalian SWI/SNF is a chromatin-remodeling complex involved in the modulation of gene expression. We demonstrate that Brahma (Brm) and Brahma/SWI2-related gene 1 (Brg-1), alternative ATPase subunits of SWI/SNF, potentiate reporter gene activation mediated by HIF-1 in an ATPase-dependent manner. Brm was more potent than Brg-1 in the reporter gene assays. Simultaneous depletion of both Brm and Brg-1 by small interfering RNAs significantly compromised the transcription of the endogenous EPO gene triggered by hypoxia. Whereas knocking down Brm alone resulted in a moderate reduction in transcription of the EPO gene, depletion of Brg-1 resulted in an augmentation of transcription of both the EPO gene and the Brm gene, indicating that Brm can compensate for loss of Brg-1. Chromatin immunoprecipitation (ChIP) and sequential ChIP (re-ChIP) analysis showed that both Brm and Brg-1 associate with the enhancer region of the EPO gene in vivo in a hypoxia-dependent fashion and that each is present in a complex with HIF-1. Brm and Brg-1 were also recruited to the promoter of the vascular endothelial growth factor (VEGF) gene in a hypoxia-dependent fashion, although hypoxic induction of VEGF transcription was not affected by depletions of either or both Brm and Brg-1. Together these studies reveal a novel role for SWI/SNF in the activation of transcription of the EPO gene, indicate an important communication and compensation between Brm and Brg-1, and suggest that the requirement for SWI/SNF during hypoxic induction is gene-specific.

cAMP-dependent protein kinase regulates ubiquitin-proteasome-mediated degradation and subcellular localization of the nuclear receptor coactivator GRIP1

Nuclear receptors and their coactivators are key regulators of numerous physiological functions. GRIP1 (glucocorticoid receptor-interacting protein) is a member of the steroid receptor coactivator family. Here, we show that GRIP1 is regulated by cAMP-dependent protein kinase (PKA) that induces its degradation through the ubiquitin-proteasome pathway. GRIP1 was down-regulated in transiently transfected COS-1 cells after treatment with 8-para-chlorophenylthio-cAMP or forskolin and 3-isobutyl-1-methylxanthine and in adrenocortical Y1 cells after incubation with adrenocorticotropic hormone. Pulse-chase experiments with transiently transfected COS-1 cells demonstrated that the half-life of GRIP1 was markedly reduced in cells overexpressing the PKA catalytic subunit, suggesting that activation of PKA increases the turnover of GRIP1 protein. The proteasome inhibitors MG132 and lactacystin abolished the PKA-mediated degradation of GRIP1. Using ts20 cells, a temperature-sensitive cell line that contains a thermolabile ubiquitin-activating E1 enzyme, it was confirmed that PKA-mediated degradation of GRIP1 is dependent upon the ubiquitin-proteasome pathway. Coimmunoprecipitation studies of COS-1 cells transfected with expression vectors encoding GRIP1 and ubiquitin using anti-GRIP1 and anti-ubiquitin antibodies showed that the ubiquitination of GRIP1 was increased by overexpression of PKA. Finally, we show that PKA regulates the intracellular distribution pattern of green fluorescent protein-GRIP1 and stimulates recruitment of GRIP1 to subnuclear foci that are colocalized with the proteasome. Taken together, these data demonstrate that GRIP1 is ubiquitinated and degraded through activation of the PKA pathway. This may represent a novel regulatory mechanism whereby hormones down-regulate a nuclear receptor coactivator.

Transcriptional complexes engaged by apo-estrogen receptor-alpha isoforms have divergent outcomes

Unliganded (apo-) estrogen receptor alpha (ERalpha, NR3A1) is classically considered as transcriptionally unproductive. Reassessing this paradigm demonstrated that apo-human ERalpha (ERalpha66) and its N-terminally truncated isoform (ERalpha46) are both predominantly nuclear transcription factors that cycle on the endogenous estrogen-responsive pS2 gene promoter in vivo. Importantly, isoform-specific consequences occur in terms of poising the promoter for transcription, as evaluated by determining (i) the engagement of several cofactors and the resulting nucleosomal organization; and (ii) the CpG methylation state of the pS2 promoter. Although transcriptionally unproductive, cycling of apo-ERalpha66 prepares the promoter to respond to ligand, through sequentially targeting chromatin remodeling complexes and general transcription factors. Additionally, apo-ERalpha46 recruits corepressors, following engagement of cofactors identical to those recruited by apo-ERalpha66. Together, these data describe differential activities of ERalpha isoforms. Furthermore, they depict the maintenance of a promoter in a repressed state as a cyclical process that is intrinsically dependent on initial poising of the promoter.

Human PAD4 regulates histone arginine methylation levels via demethylimination

Methylation of arginine (Arg) and lysine residues in histones has been correlated with epigenetic forms of gene regulation. Although histone methyltransferases are known, enzymes that demethylate histones have not been identified. Here, we demonstrate that human peptidylarginine deiminase 4 (PAD4) regulates histone Arg methylation by converting methyl-Arg to citrulline and releasing methylamine. PAD4 targets multiple sites in histones H3 and H4, including those sites methylated by coactivators CARM1 (H3 Arg17) and PRMT1 (H4 Arg3). A decrease of histone Arg methylation, with a concomitant increase of citrullination, requires PAD4 activity in human HL-60 granulocytes. Moreover, PAD4 activity is linked with the transcriptional regulation of estrogen-responsive genes in MCF-7 cells. These data suggest that PAD4 mediates gene expression by regulating Arg methylation and citrullination in histones.

Rational design of RAR-selective ligands revealed by RARbeta crystal stucture

The crystal structure of the ligand-binding domain of RARbeta, a suspect tumour suppressor, reveals important features that distinguish it from the two other RAR isotypes. The most striking difference is an extra cavity allowing RARbeta to bind more bulky agonists. Accordingly, we identified a ligand that shows RARbeta selectivity with a 100-fold higher affinity to RARbeta than to alpha or gamma isotypes. The structural differences between the three RAR ligand-binding pockets revealed a rationale explaining how a single retinoid can be at the same time an RARalpha, gamma antagonist and an RARbeta agonist. In addition, we demonstrate how to generate an RARbeta antagonist by gradually modifying the bulkiness of a single substitution. Together, our results provide structural guidelines for the synthesis of RARbeta-selective agonists and antagonists, allowing for the first time to address pharmacologically the tumour suppressor role of RARbeta in vitro and in animal models.

Drug delivery systems for oestrogenic hormones and antagonists: the need for selective targeting in estradiol-dependent cancers

The pleiotropic activity of oestrogens and their mechanism of action via their binding to the two oestrogen receptors alpha (ER alpha) and beta (ER beta) subtypes in the different tissues where oestrogens exert their action have been briefly described. The fate of these compounds trapped into different galenic forms is discussed with regard to their therapeutic applications. Firstly, the advantages and disadvantages of the different forms (pills, i.v. forms and transdermal patches) used in contraception are compared. Secondly, the therapeutic use of formulated oestrogens for the post-menopausal hormone replacement therapy (HRT) is analysed through the various results obtained in different trials. The link between HRT and the risks of breast cancer and cardiovascular disease is underlined. Finally, comparing the activity of selective oestrogen receptor modulators such as tamoxifen and pure anti-oestrogens such as RU58668 and ICI182780, we analysed the reasons leading to the need for a tumor targeting of the latters, but not of the former for the treatment of oestrogen-dependent breast cancer. Different injectable and biodegradable formulations, that lead to a remarkable anti-tumor efficiency in xenografts, have been recently developed and we believe that they may represent promising new administration ways of added therapeutic values for anti-oestrogens. Such devices could be extended to the delivery of other anti-cancer drugs with more aggressive activities than anti-oestrogens.

Attenuation of estrogen receptor alpha-mediated transcription through estrogen-stimulated recruitment of a negative elongation factor

Estrogen receptor alpha (ERalpha) signaling is paramount for normal mammary gland development and function and the repression of breast cancer. ERalpha function in gene regulation is mediated by a number of coactivators and corepressors, most of which are known to modify chromatin structure and/or influence the assembly of the regulatory complexes at the level of transcription initiation. Here we describe a novel mechanism of attenuating the ERalpha activity. We show that cofactor of BRCA1 (COBRA1), an integral subunit of the human negative elongation factor (NELF), directly binds to ERalpha and represses ERalpha-mediated transcription. Reduction of the endogenous NELF proteins in breast cancer cells using small interfering RNA results in elevated ERalpha-mediated transcription and enhanced cell proliferation. Chromatin immunoprecipitation reveals that recruitment of COBRA1 and the other NELF subunits to endogenous ERalpha-responsive promoters is greatly stimulated upon estrogen treatment. Interestingly, COBRA1 does not affect the estrogen-dependent assembly of transcription regulatory complexes at the ERalpha-regulated promoters. Rather, it causes RNA polymerase II (RNAPII) to pause at the promoter-proximal region, which is consistent with its in vitro biochemical activity. Therefore, our in vivo work defines the first corepressor of nuclear receptors that modulates ERalpha-dependent gene expression by stalling RNAPII. We suggest that this new level of regulation may be important to control the duration and magnitude of a rapid and reversible hormonal response.

Histone deacetylase inhibition and estrogen signalling in human breast cancer cells

Estrogens are steroid hormones, which act through specific nuclear estrogen receptors (ERalpha and ERbeta) and are important regulators of breast cancer growth. These receptors control gene expression by recruiting transcriptional cofactors that exhibit various enzymatic activities such as histone acetyltransferase or histone deacetylase (HDAC) which target histone as well as non-histone substrates. The ERalpha itself and some of the transcriptional regulators have been shown to be acetylated proteins. Research performed over the last decade has highlighted the role of HDAC inhibitors (HDACi) as modulators of transcriptional activity and as a new class of therapeutic agents. In human cancer cells, inhibition of HDACs controls the expression of the ERalpha gene and the transcriptional activity in response to partial antiestrogens such as 4-hydroxytamoxifen. Various HDACi strongly inhibit breast cancer cell proliferation and ERalpha-negative (ER-) appear less sensitive than ERalpha-positive (ER+) cell lines. p21WAF1/CIP1 gene expression, in relation with ERalpha levels, could play a role in this differential response of breast cancer cells to hyperacetylating agents.

Interaction networks: coordinating responses to xenobiotic exposure

In the last decade the increased usage of '-omic' technologies, plus the sequencing of over 800 complete genomes has led to a vast increase in the amount of information available to the researcher for examining cellular responses to xenobiotics. Much effort has been put into the identification and analysis of expression profiles associated with pathobiological conditions and/or xenobiotic exposure. These profiles are commonly used in two applications. Firstly, comparative profile experiments are used to classify pathobiological states and for the screening of novel chemical entities to predict their action(s) on the body. Secondly, mechanistic investigations will gain information on the molecular mechanisms underlying toxic responses/pathobiological states. During the course of such analysis it has become increasingly clear that a series of highly refined interaction networks exist within the body, regulating both the sensitivity and selectivity of the body's response to pathobiological states/xenobiotic exposure. These interaction networks exist at several levels: Firstly, within individual cells, the interaction between factors that transmit xenobiotics signals will determine the overall cellular response. Secondly, intraorgan communication occurs between the different cell types/sub-types which makes up an organ, coordinating the overall organ response. Finally, interorgan interactions provide axes of response through the body.

Regulation of nuclear receptor activity by a pseudouridine synthase through posttranscriptional modification of steroid receptor RNA activator

Nuclear receptors (NRs) induce transcription through association with coactivator complexes. We identified a pseudouridine synthase (PUS), mPus1p, as a coactivator for retinoic acid receptor (mRAR)gamma and other NR-dependent transactivation. mPus1p is a member of the truA subfamily of PUSs, a class of enzymes that isomerize uridine to pseudouridine in noncoding RNAs, such as tRNA, to ensure proper folding and function. mPus1p binds the first zinc finger of mRARgamma and also associates with other NRs. Interestingly, mPus1p pseudouridylates coactivator Steroid Receptor RNA Activator (SRA), and when coexpressed, mPus1p and SRA cooperatively enhance mRARgamma-mediated transcription. mPus1p, mRARgamma, and SRA exist in a retinoid-independent, promoter bound complex in the nucleus although mPus1p is also expressed in the nucleolus, where it likely modifies tRNA. Finally, we show that mPus1p-coactivator function required SRA, mPus1p-associated mRARgamma binding, and PUS activities. mPus1p-dependent pseudouridylation of SRA represents an additional type of posttranscriptional modification of a NR-coactivator complex that is important for NR signaling.

Ecdysone receptor-dependent gene regulation mediates histone poly(ADP-ribosyl)ation

While the ecdysone dependency of puff formation in giant polytene chromosomes from fly salivary glands has been well documented, the molecular mechanisms underlying this process remain unknown. However, it does appear to involve chromatin remodeling and modification mediated by ecdysone receptor (EcR). As Drosophila poly(ADP-ribose) polymerase (dPARP) has recently been reported to be involved in ecdysone-induced puff formation, we decided to test the possible role of dPARP in ligand-induced dEcR transactivation in an insect system. dPARP co-activated the ligand-induced transactivation function of EcR in the insect cell line S2, and appeared to physically interact with EcR in a ligand-dependent manner. ChIP analysis of an EcR target gene promoter revealed ligand-dependent recruitment of dPARP with poly(ADP-ribosyl)ation of histones in the EcR binding site and, surprisingly, also in a distal region of the promoter. Our results indicated that EcR-mediated gene regulation may be coupled with chromatin modification through poly(ADP-ribosyl)ation.

Nuclear structure-associated TIF2 recruits glucocorticoid receptor and its target DNA

Assembly of multi-protein complexes on promoter and enhancer elements is a prerequisite for onset of gene transcription. At the beginning of this process, transcription factors are thought to act as nucleating centers for complex formation through the binding of their target DNA sequences, and thereafter recruit coactivators. Here, we investigated this process of assembly by determining the distribution of the glucocorticoid receptor (GR) and its coactivator, TIF2. Both endogenously and ectopically expressed TIF2 were shown to form foci in the nucleus, and GR could be recruited to the TIF2 foci upon GR agonist but not antagonist treatment. Moreover, we show that the coactivators, p300 and PCAF, are also recruited to the TIF2 foci. The TIF2 foci could recruit GR carrying a microinjected GR responsive element. We propose that TIF2 provides a nuclear compartment that allows the assembly of multi-protein complexes required for GR-mediated gene activation.

Discovery of estrogen receptor alpha target genes and response elements in breast tumor cells

Microarray analysis has identified 89 estrogen target genes. The cis-regulatory elements found upstream of those genes are not well conserved in mouse and human.

Background

Estrogens and their receptors are important in human development, physiology and disease. In this study, we utilized an integrated genome-wide molecular and computational approach to characterize the interaction between the activated estrogen receptor (ER) and the regulatory elements of candidate target genes.

Results

Of around 19,000 genes surveyed in this study, we observed 137 ER-regulated genes in T-47D cells, of which only 89 were direct target genes. Meta-analysis of heterogeneous in vitro and in vivo datasets showed that the expression profiles in T-47D and MCF-7 cells are remarkably similar and overlap with genes differentially expressed between ER-positive and ER-negative tumors. Computational analysis revealed a significant enrichment of putative estrogen response elements (EREs) in the cis-regulatory regions of direct target genes. Chromatin immunoprecipitation confirmed ligand-dependent ER binding at the computationally predicted EREs in our highest ranked ER direct target genes, NRIP1, GREB1 and ABCA3. Wider examination of the cis-regulatory regions flanking the transcriptional start sites showed species conservation in mouse-human comparisons in only 6% of predicted EREs.

Conclusions

Only a small core set of human genes, validated across experimental systems and closely associated with ER status in breast tumors, appear to be sufficient to induce ER effects in breast cancer cells. That cis-regulatory regions of these core ER target genes are poorly conserved suggests that different evolutionary mechanisms are operative at transcriptional control elements than at coding regions. These results predict that certain biological effects of estrogen signaling will differ between mouse and human to a larger extent than previously thought.

Coactivator AIB1 links estrogen receptor transcriptional activity and stability

Agonist-mediated degradation of estrogen receptor alpha (ERalpha) has been associated with its transcriptional activity. However, the mechanism by which ERalpha is targeted for degradation and whether there is a direct functional link between ERalpha stability and ERalpha-mediated transactivation have not been elucidated. Here we provide evidence that the p160 coactivator, AIB1, uniquely mediates agonist-induced, but not antagonist-induced, ERalpha degradation. We show that AIB1 recruitment by ERalpha is not only necessary but also sufficient to promote degradation. Suppression of AIB1 levels leads to ERalpha stabilization in the presence of 17beta-estradiol and, despite increased ERalpha levels, reduced recruitment of ERalpha to endogenous target gene promoters. In addition, association of RNA polymerase II with ERalpha target promoters is lost when AIB1 is suppressed, leading to inhibition of target gene transcription. AIB1 thus plays a dual role in regulating ERalpha activity, one in recruiting transcription factors including other coactivators involved in gene activation and the other in regulating ERalpha protein degradation mediated by the ubiquitin-proteosome machinery.

Wnt/beta-catenin and estrogen signaling converge in vivo

Wnt and estrogen signaling represent important regulatory pathways, each controlling a wide range of biological processes. While an increasing number of observations suggest potential convergence between these pathways, no direct evidence of their functional interaction has been reported. Using human colon and breast cancer cells, we found that estrogen receptor (ER) alpha- and beta-catenin precipitated within the same immunocomplexes, reciprocally enhanced the transactivation of cognate reporter genes, and were reciprocally recruited to cognate response elements in the promoters of endogenous target genes. Using transgenic Drosophila that ectopically expressed human ERalpha alone or together with metabolically stable beta-catenin/Armadillo mutants, we demonstrated genetic interaction between these signal transducers in vivo. Thus, we present here the first direct evidence of cross-talk between Wnt and estrogen signaling pathways via functional interaction between beta-catenin and ERalpha.

The p68 and p72 DEAD box RNA helicases interact with HDAC1 and repress transcription in a promoter-specific manner

Background

p68 (Ddx5) and p72 (Ddx17) are highly related members of the DEAD box family and are established RNA helicases. They have been implicated in growth regulation and have been shown to be involved in both pre-mRNA and pre-rRNA processing. More recently, however, these proteins have been reported to act as transcriptional co-activators for estrogen-receptor alpha (ERα). Furthermore these proteins were shown to interact with co-activators p300/CBP and the RNA polymerase II holoenzyme. Taken together these reports suggest a role for p68 and p72 in transcriptional activation.

Results

In this report we show that p68 and p72 can, in some contexts, act as transcriptional repressors. Targeting of p68 or p72 to constitutive promoters leads to repression of transcription; this repression is promoter-specific. Moreover both p68 and p72 associate with histone deacetylase 1 (HDAC1), a well-established transcriptional repression protein.

Conclusions

It is therefore clear that p68 and p72 are important transcriptional regulators, functioning as co-activators and/or co-repressors depending on the context of the promoter and the transcriptional complex in which they exist.

In vitro transcription system delineates the distinct roles of the coactivators pCAF and p300 during MyoD/E47-dependent transactivation

The transcriptional coactivators p300 and pCAF are necessary for the myogenic factor MyoD to initiate the expression of skeletal muscle genes. In addition to mediating histone acetylation, both of these factors can acetylate MyoD; however, the complexity of cellular systems used to study MyoD has impeded delineation of the specific roles of these two acetyltransferases. Therefore, we established a MyoD-dependent in vitro transcription system that permits us to determine the roles of p300 and pCAF during MyoD-dependent transcriptional activation. Consistent with results from cellular systems, we demonstrate that maximal levels of transactivation in vitro require both p300 and pCAF, as well as the cofactor acetyl CoA. Dissection of the steps leading to transcription initiation revealed that the activities of p300 and pCAF are not redundant. During the initial stages of transactivation, p300 acetylates histone H3 and H4 within the promoter region and then recruits pCAF to MyoD. Once tethered to the promoter, pCAF acetylates MyoD to facilitate the transactivation process. Thus, we have established that pCAF and p300 carry out sequential and functionally distinct events on a promoter leading to transcriptional activation. Further dissection of this in vitro transcription system should be highly useful toward elucidating the mechanism by which coactivators facilitate differential gene expression by MyoD.

Gene regulation by Sp1 and Sp3

The Sp family of transcription factors is united by a particular combination of three conserved Cys2His2 zinc fingers that form the sequence-specific DNA-binding domain. Within the Sp family of transcription factors, Sp1 and Sp3 are ubiquitously expressed in mammalian cells. They can bind and act through GC boxes to regulate gene expression of multiple target genes. Although Sp1 and Sp3 have similar structures and high homology in their DNA binding domains, in vitro and in vivo studies reveal that these transcription factors have strikingly different functions. Sp1 and Sp3 are able to enhance or repress promoter activity. Regulation of the transcriptional activity of Sp1 and Sp3 occurs largely at the post-translational level. In this review, we focus on the roles of Sp1 and Sp3 in the regulation of gene expression.Key words: Sp1, Sp3, gene regulation, sub-cellular localization.

Estrogens and progesterone promote persistent CCND1 gene activation during G1 by inducing transcriptional derepression via c-Jun/c-Fos/estrogen receptor (progesterone receptor) complex assembly to a distal regulatory element and recruitment of cyclin D1 to its own gene promoter

Transcriptional activation of the cyclin D1 gene (CCND1) plays a pivotal role in G(1)-phase progression, which is thereby controlled by multiple regulatory factors, including nuclear receptors (NRs). Appropriate CCND1 gene activity is essential for normal development and physiology of the mammary gland, where it is regulated by ovarian steroids through a mechanism(s) that is not fully elucidated. We report here that CCND1 promoter activation by estrogens in human breast cancer cells is mediated by recruitment of a c-Jun/c-Fos/estrogen receptor alpha complex to the tetradecanoyl phorbol acetate-responsive element of the gene, together with Oct-1 to a site immediately adjacent. This process coincides with the release from the same DNA region of a transcriptional repressor complex including Yin-Yang 1 (YY1) and histone deacetylase 1 and is sufficient to induce the assembly of the basal transcription machinery on the promoter and to lead to initial cyclin D1 accumulation in the cell. Later on in estrogen stimulation, the cyclin D1/Cdk4 holoenzyme associates with the CCND1 promoter, where E2F and pRb can also be found, contributing to the long-lasting gene enhancement required to drive G(1)-phase completion. Interestingly, progesterone triggers similar regulatory events through its own NRs, suggesting that the gene regulation cascade described here represents a crossroad for the transcriptional control of G(1)-phase progression by different classes of NRs.

Elongator interactions with nascent mRNA revealed by RNA immunoprecipitation

The histone acetyltransferase Elongator was originally isolated as a component of the elongating form of RNA polymerase II (RNAPII) and a plethora of data has since supported a role for the factor in transcription. However, recent data has suggested that it is predominantly cytoplasmic and does not associate with the DNA of transcribed genes in vivo. Here, we report that Elongator binds to RNA both in vitro and in vivo. Using a modified chromatin immunoprecipitation procedure, RNA immunoprecipitation (RIP), we show that Elongator is indeed present at several actively transcribed genes and that it associates with the nascent RNA emanating from elongating RNAPII along the entire coding region of a gene. These results strongly support a role for Elongator in transcript elongation.

Selective association of sterol regulatory element-binding protein isoforms with target promoters in vivo

The mRNAs for all three members of the sterol regulatory element-binding protein (SREBP) family are widely expressed, and the proteins are highly similar. They have potential to both hetero- and homodimerize through their bHLHLZ domains, so it has been difficult to definitively study the role of each one apart from the other two. In the current study, we have utilized cell lines that express only one functional SREBP and the chromatin immunoprecipitation technique to analyze individual SREBP binding to three specific target genes: hydroxymethylglutaryl-CoA reductase (Red), fatty acid synthase (FAS), and squalene synthase (SQS). Our studies show that SREBP-2 binds to promoters for all three genes, and in agreement with the original report using these cells, all three mRNAs are also induced. In the line expressing only SREBP-1a, mRNAs for Red and FAS are induced, but SQS is not. Chromatin immunoprecipitation also shows that SREBP-1a is recruited efficiently to Red and FAS promoters but not to SQS. This observation indicates SREBP-2 selectively binds the SQS promoter and is sufficient to explain the lack of SQS mRNA induction in the SREBP-1a-expressing cells. SREBP-1c protein was not stably recruited to any SREBP target promoter despite being fully active in DNA binding when purified from extracts of the corresponding cells. This is also sufficient to explain the lack of SREBP target gene induction by the singular expression of SREBP-1c. We also show that whereas SREBP-1a and -2 proteins interact efficiently with transcriptional co-activators that modify cellular chromatin, SREBP-1c does not. Taken together, our data support a model suggesting that chromatin modification is required during the initial stage of specific site recognition by SREBPs in native chromatin in vivo.

Transcription Coactivator PBP, the Peroxisome Proliferator-activated Receptor (PPAR)-binding Protein, Is Required for PPARα-regulated Gene Expression in Liver

Nuclear receptor coactivator PBP (peroxisome proliferator-activated receptor (PPAR)-binding protein) functions as a coactivator for PPARs and other nuclear receptors. PBP serves as an anchor for TRAP (thyroid hormone receptor-associated proteins)/mediator multisubunit cofactor transcription complex. Disruption of the PBP/TRAP220 gene results in embryonic lethality around embryonic day 11.5 by affecting placental, cardiac, hepatic, and bone marrow development. Because PPAR isoforms alpha, gamma, and beta/delta function as important regulators of lipid homeostasis in mammals, it becomes important to assess the requirement of coactivator PBP in the regulation of PPAR functions in vivo. Sustained activation of PPARalpha by structurally diverse classes of chemicals of biological importance, designated peroxisome proliferators, leads to proliferation of peroxisomes in liver, induction of PPARalpha target genes including those involved in fatty acid oxidation, and the eventual development of liver tumors. Here, we show that targeted deletion of PBP in liver parenchymal cells, using the Cre-loxP system, results in the near abrogation of PPARalpha ligand-induced peroxisome proliferation and liver cell proliferation, as well as the induction of PPARalpha-regulated genes in PBP-deficient liver cells. In contrast, scattered PBP(+/+) hepatocytes in these livers showed DNA synthesis and were markedly hypertrophic with peroxisome proliferation in response to PPARalpha ligands. Chromatin immunoprecipitation data suggest that in PBP conditional null livers, there appears to be reduced association of cofactors, especially of CBP and TRAP150, to the mouse enoyl-CoA hydratase/l-3-hydroxyacyl-CoA dehydrogenase gene promoter. These observations suggest that PBP is required for the stabilization of multiprotein cofactor complexes. In essence, the absence of PBP in hepatocytes in vivo appears to mimic the absence of PPARalpha, indicating that coactivator PBP is essential for PPARalpha-regulated gene expression in liver parenchymal cells.

About GATA3, HNF3A, and XBP1, three genes co-expressed with the oestrogen receptor-alpha gene (ESR1) in breast cancer

In breast tumours and breast cancer cell (BCC) lines, microarray analyses have revealed that a series of genes are expressed in close association with the oestrogen receptor-alpha (ER-alpha) gene, ESR1. Three of them, GATA3, HNF3A (also known as FOXA1), and XBP1 encode transcription factors. Here, we present these factors and we discuss their potential involvement in the ER-alpha-mediated actions in BCC. We notably show the relations that exist, or that might exist, between these factors and the oestrogen-inducible trefoil factor TFF1.

Selective recognition of distinct classes of coactivators by a ligand-inducible activation domain

How nuclear receptors (NRs) coordinate the sequential, ligand-dependent recruitment of multiple coactivator complexes (e.g., SRC complexes and Mediator) that share similar receptor binding determinants is unclear. We show that although the receptor binding subunits of these complexes (i.e., SRCs and Med220, respectively) share overlapping binding sites on estrogen receptor alpha (ERalpha), information contained in the receptor-coactivator interface allows the receptor to distinguish between them. In support of this conclusion, we have identified an ERalpha AF-2 point mutant (L540Q) that selectively binds and recruits Med220, but not SRCs, both in vitro and in vivo. In cells expressing this mutant, the recruitment of Med220 to the pS2 promoter is delayed, and the expression of the vast majority of estrogen target genes is impaired, suggesting a nearly global functional interdependence of these coactivators. Collectively, our results suggest that "facilitated recruitment," rather than competition, drives the sequential recruitment of SRC complexes and Mediator by NRs.

TIP30 interacts with an estrogen receptor alpha-interacting coactivator CIA and regulates c-myc transcription

Deregulation of c-myc expression is implicated in the pathogenesis of many neoplasias. Estrogen receptor alpha (ERalpha) can increase the rate of c-myc transcription through the recruitment of a variety of cofactors to the promoter, yet the precise roles of these cofactors in transcription and tumorigenesis are largely unknown. We show here that a putative tumor suppressor TIP30, also called CC3 or Htatip2, interacts with an ERalpha-interacting coactivator CIA. Using chromatin immunoprecipitation assays, we demonstrate that TIP30 and CIA are distinct cofactors that are dynamically associated with the promoter and downstream regions of the c-myc gene in response to estrogen. Both TIP30 and CIA are recruited to the c-myc gene promoter by liganded ERalpha in the second transcription cycle. TIP30 overexpression represses ERalpha-mediated c-myc transcription, whereas TIP30 deficiency enhances c-myc transcription in both the absence and presence of estrogen. Ectopic CIA cooperates with TIP30 to repress ERalpha-mediated c-myc transcription. Moreover, virgin TIP30 knockout mice exhibit increased c-myc expression in mammary glands. Together, these results reveal an important role for TIP30 in the regulation of ERalpha-mediated c-myc transcription and suggest a mechanism for tumorigenesis promoted by TIP30 deficiency.

Cloning and functional characterization of PELP1/MNAR promoter

Proline-, glutamic acid- and leucine-rich protein 1 (PELP1)/modulator of nongenomic activity of estrogen receptor (MNAR), a novel coactivator of estrogen receptors (ERs; ERalpha and ERbeta), modulates the genomic and nongenomic functions of the ERs. PELP1 expression is developmentally regulated in mammary glands and overexpressed in breast tumors. However, little is known about the regulation of PELP1. In this study, we examined whether PELP1 expression is modulated by steroid hormone 17beta-estradiol (E2)-ER pathway. We found that in MCF-7 breast cancer cells, E2 upregulated PELP1 expression threefold and that this upregulation was reduced by antiestrogen. We also found that E2 modulated PELP1 levels in an actinomycin-D-sensitive manner, suggesting transcriptional regulation. Cloning and analysis of the 2-kb PELP1 promoter region revealed two estrogen-responsive element (ERE) half sites in the PELP1 promoter region. In transient transfection assays, E2 upregulated PELP1 promoter activity in breast, endometrial and osteosarcoma model cancer cell lines in an ICI 182,780-sensitive manner. We demonstrated the recruitment of ER to the PELP1 promoter in vitro using EMSA assays and in vivo using a chromatin immunoprecipitation assay. The PELP1 promoter was similarly upregulated by both ERalpha and ERbeta and differentially regulated by selective estrogen receptor modulators in a cell line-dependent manner. Our results suggest that PELP1 expression is modulated by the E2-ER pathway and that PELP1 is an ER target gene.

The Jensen Symposium

The Jensen Symposium was held at the University of Cincinnati in December 2003 to honor the pioneering contributions of Dr. Elwood Jensen to the field of nuclear hormone action. Those in attendance were treated to an outstanding scientific program that served as an update of recent progress and illustrated the breadth of activity in the nuclear receptor field. Here we highlight recent findings presented at the Symposium that provide new insights into the mechanisms of nuclear receptor action and the diverse roles of members of the nuclear receptor superfamily in development and homeostasis.

Chromatin organization in the mammalian nucleus

Mammalian cells package their DNA into chromatin and arrange it in the nucleus as chromosomes. In interphase cells chromosomes are organized in a radial distribution with the most gene-dense chromosomes toward the center of the nucleus. Gene transcription, replication, and repair are influenced by the underlying chromatin architecture, which in turn is affected by the formation of chromosome territories. This arrangement in the nucleus presumably facilitates cellular functions to occur in an efficient and ordered fashion and exploring the link between transcription and nuclear organization will be an exciting area of further research.