The yeast Ada complex mediates the ligand-dependent activation function AF-2 of retinoid X and estrogen receptors

The nuclear receptor 4A family members: mediators in human disease and autophagy

The Nuclear receptor 4A (NR4A) subfamily, which belongs to the nuclear receptor (NR) superfamily, has three members: NR4A1 (Nur77), NR4A2 (Nurr1) and NR4A3 (Nor1). They are gene regulators with broad involvement in various signaling pathways and human disease responses, including autophagy. Here, we provide a concise overview of the current understanding of the role of the NR4A subfamily members in human diseases and review the research into their regulation of cell autophagy. A deeper understanding of these mechanisms has potential to improve drug development processes and disease therapy.

Overexpression of Alteration/Deficiency in Activation 3 correlates with poor prognosis in non-small cell lung cancer

Alteration/Deficiency in Activation 3 (ADA3), the human homologue of yeast ADA3, is involved in a variety of cell biological processes and plays an important role in tumorigenesis. Here, western blotting and reverse-transcription quantitative PCR (RT-qPCR) were conducted to explore the expression pattern of ADA3 in non-small cell lung cancer (NSCLC) patients. It was found that ADA3 protein expression in cancerous tissues was significantly higher than that in adjacent normal lung tissues, but there were no differences in mRNA levels. Tissue microarray immunohistochemical assay (TMA-IHC) was performed and we investigated the prognostic significance of ADA3 expression in 84 cases of NSCLC. Survival analyses showed that high expression of ADA3 was an independent prognostic factor for unfavorable overall survival (OS) in patients with NSCLC. In summary, the ADA3 expression level elevates in NSCLC and correlates with poor OS in NSCLC patients.

Tale of a multifaceted co-activator, hADA3: from embryogenesis to cancer and beyond

Human ADA3, the evolutionarily conserved transcriptional co-activator, remains the unified part of multiple cellular functions, including regulation of nuclear receptor functions, cell proliferation, apoptosis, senescence, chromatin remodelling, genomic stability and chromosomal maintenance. The past decade has witnessed exciting findings leading to considerable expansion in research related to the biology and regulation of hADA3. Embryonic lethality in homozygous knockout Ada3 mouse signifies the importance of this gene product during early embryonic development. Moreover, the fact that it is a novel target of Human Papillomavirus E6 oncoprotein, one of the most prevalent causal agents behind cervical cancer, helps highlight some of the crucial aspects of HPV-mediated oncogenesis. These findings imply the central involvement of hADA3 in regulation of various cellular functional losses accountable for the genesis of malignancy and viral infections. Recent reports also provide evidence for post-translational modifications of hADA3 leading to its instability and contributing to the malignant phenotype of cervical cancer cells. Furthermore, its association with poor prognosis of breast cancer suggests intimate association in the pathogenesis of the disease. Here, we present the first review on hADA3 with a comprehensive outlook on the molecular and functional roles of hADA3 to provoke further interest for more elegant and intensive studies exploring assorted aspects of this protein.

Retinoic acid receptors: from molecular mechanisms to cancer therapy

Retinoic acid (RA), the major bioactive metabolite of retinol or vitamin A, induces a spectrum of pleiotropic effects in cell growth and differentiation that are relevant for embryonic development and adult physiology. The RA activity is mediated primarily by members of the retinoic acid receptor (RAR) subfamily, namely RARα, RARβ and RARγ, which belong to the nuclear receptor (NR) superfamily of transcription factors. RARs form heterodimers with members of the retinoid X receptor (RXR) subfamily and act as ligand-regulated transcription factors through binding specific RA response elements (RAREs) located in target genes promoters. RARs also have non-genomic effects and activate kinase signaling pathways, which fine-tune the transcription of the RA target genes. The disruption of RA signaling pathways is thought to underlie the etiology of a number of hematological and non-hematological malignancies, including leukemias, skin cancer, head/neck cancer, lung cancer, breast cancer, ovarian cancer, prostate cancer, renal cell carcinoma, pancreatic cancer, liver cancer, glioblastoma and neuroblastoma. Of note, RA and its derivatives (retinoids) are employed as potential chemotherapeutic or chemopreventive agents because of their differentiation, anti-proliferative, pro-apoptotic, and anti-oxidant effects. In humans, retinoids reverse premalignant epithelial lesions, induce the differentiation of myeloid normal and leukemic cells, and prevent lung, liver, and breast cancer. Here, we provide an overview of the biochemical and molecular mechanisms that regulate the RA and retinoid signaling pathways. Moreover, mechanisms through which deregulation of RA signaling pathways ultimately impact on cancer are examined. Finally, the therapeutic effects of retinoids are reported.

Altered nuclear cofactor switching in retinoic-resistant variants of the PML-RARα oncoprotein of acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) results from a reciprocal translocation that fuses the gene for the PML tumor suppressor to that encoding the retinoic acid receptor alpha (RARα). The resulting PML-RARα oncogene product interferes with multiple regulatory pathways associated with myeloid differentiation, including normal PML and RARα functions. The standard treatment for APL includes anthracycline-based chemotherapeutic agents plus the RARα agonist all-trans retinoic acid (ATRA). Relapse, which is often accompanied by ATRA resistance, occurs in an appreciable frequency of treated patients. One potential mechanism suggested by model experiments featuring the selection of ATRA-resistant APL cell lines involves ATRA-resistant versions of the PML-RARα oncogene, where the relevant mutations localize to the RARα ligand-binding domain (LBD). Such mutations may act by compromising agonist binding, but other mechanisms are possible. Here, we studied the molecular consequence of ATRA resistance by use of circular dichroism, protease resistance, and fluorescence anisotropy assays employing peptides derived from the NCOR nuclear corepressor and the ACTR nuclear coactivator. The consequences of the mutations on global structure and cofactor interaction functions were assessed quantitatively, providing insights into the basis of agonist resistance. Attenuated cofactor switching and increased protease resistance represent features of the LBDs of ATRA-resistant PML-RARα, and these properties may be recapitulated in the full-length oncoproteins.

Human ADA3 regulates RARalpha transcriptional activity through direct contact between LxxLL motifs and the receptor coactivator pocket

The alternation/deficiency in activation-3 (ADA3) is an essential component of the human p300/CBP-associated factor (PCAF) and yeast Spt-Ada-Gcn5-acetyltransferase (SAGA) histone acetyltransferase complexes. These complexes facilitate transactivation of target genes by association with transcription factors and modification of local chromatin structure. It is known that the yeast ADA3 is required for nuclear receptor (NR)-mediated transactivation in yeast cells; however, the role of mammalian ADA3 in NR signaling remains elusive. In this study, we have investigated how the human (h) ADA3 regulates retinoic acid receptor (RAR) α-mediated transactivation. We show that hADA3 interacts directly with RARα in a hormone-dependent manner and this interaction contributes to RARα transactivation. Intriguingly, this interaction involves classical LxxLL motifs in hADA3, as demonstrated by both ‘loss’ and ‘gain’ of function mutations, as well as a functional coactivator pocket of the receptor. Additionally, we show that hADA3 associates with RARα target gene promoter in a hormone-dependent manner and ADA3 knockdown impairs RARβ2 expression. Furthermore, a structural model was established to illustrate an interaction network within the ADA3/RARα complex. These results suggest that hADA3 is a bona fide transcriptional coactivator for RARα, acting through a conserved mechanism involving direct contacts between NR boxes and the receptor’s co-activator pocket.

HPV16 E6-induced and E6AP-dependent inhibition of the transcriptional coactivator hADA3 in human cervical carcinoma cells

To determine whether there exists an additional pathway of E6 that is independent of direct P53 degradation and whether hADA3, a transcriptional coactivator, is involved in this process. We investigated the association between E6 and hADA3, as well as E6-associated protein (E6AP) and hADA3, in SiHa cells via RNA interference technique. Our results showed that the expression of hADA3 protein was significantly increased, cellular proliferation was decreased and apoptotic rate was increased in SiHa cells treated by E6 siRNA and E6AP siRNA respectively. Our results suggested that oncoprotein E6 inhibits hADA3 in cervical cancer cells and this process is E6AP-dependent.

Ankyrin repeats-containing cofactors interact with ADA3 and modulate its co-activator function

ANCO (ankyrin repeats-containing cofactor)-1 and ANCO-2 are a family of unique transcriptional co-regulators with dual properties: they interact with both the co-activators and the co-repressors [Zhang, Yeung, Li, Tsai, Dinh, Wu, Li and Chen (2004) J. Biol. Chem. 279, 33799-33805]. Specifically, ANCO-1 is thought to recruit HDACs (histone deacetylases) to the p160 co-activator to repress transcriptional activation by nuclear receptors. In the present study, we provide new evidence to suggest further that ANCO-1 and ANCO-2 also interact with the co-activator ADA3 (alteration/deficiency in activation 3). The interaction occurs between the conserved C-terminal domain of ANCO-1 and the N-terminal transactivation domain of ADA3. Several subunits of the P/CAF {p300/CBP [CREB (cAMP-response-element-binding protein)-binding protein]-associated factor} complex, including ADA3, ADA2alpha/beta and P/CAF, showed co-localization with ANCO-1 nuclear dots, indicating an in vivo association of ANCO-1 with the P/CAF complex. Furthermore, a transient reporter assay revealed that both ANCO-1 and ANCO-2 repress ADA3-mediated transcriptional co-activation on nuclear receptors, whereas ANCO-1 stimulated p53-mediated transactivation. These data suggest that ADA3 is a newly identified target of the ANCO proteins, which may modulate co-activator function in a transcription-factor-specific manner.

A TFTC/STAGA module mediates histone H2A and H2B deubiquitination, coactivates nuclear receptors, and counteracts heterochromatin silencing

Transcriptional activators, several different coactivators, and general transcription factors are necessary to access specific loci in the dense chromatin structure to allow precise initiation of RNA polymerase II (Pol II) transcription. Histone acetyltransferase (HAT) complexes were implicated in loosening the chromatin around promoters and thus in gene activation. Here we demonstrate that the 2 MDa GCN5 HAT-containing metazoan TFTC/STAGA complexes contain a histone H2A and H2B deubiquitinase activity. We have identified three additional subunits of TFTC/STAGA (ATXN7L3, USP22, and ENY2) that form the deubiquitination module. Importantly, we found that this module is an enhancer of position effect variegation in Drosophila. Furthermore, we demonstrate that ATXN7L3, USP22, and ENY2 are required as cofactors for the full transcriptional activity by nuclear receptors. Thus, the deubiquitinase activity of the TFTC/STAGA HAT complex is necessary to counteract heterochromatin silencing and acts as a positive cofactor for activation by nuclear receptors in vivo.

Ada3 requirement for HAT recruitment to estrogen receptors and estrogen-dependent breast cancer cell proliferation

We have previously shown that evolutionarily conserved alteration/deficiency in activation (Ada) protein associates with and promotes estrogen receptor (ER)-mediated target gene expression. Here, we examined the role of endogenous Ada3 to recruit histone acetyl transferases (HAT) to an ER-responsive promoter and its role in estrogen-dependent cell proliferation and malignant phenotype. Using a combination of glycerol gradient cosedimentation and immunoprecipitation analyses, we show that Ada3, ER, and three distinct HATs [p300, (p300/CBP-associated factor) PCAF, and general control nonrepressed 5 (Gcn5)] are present in a complex. Using chromatin immunoprecipitation analysis, we show that short hairpin RNA (shRNA)-mediated knockdown of Ada3 in ER-positive breast cancer cells significantly reduced the ligand-dependent recruitment of p300, PCAF, and Gcn5 to the ER-responsive pS2 promoter. Finally, we use shRNA knockdown to show that Ada3 is critical for estrogen-dependent proliferation of ER-positive breast cancer cell lines in two-dimensional, as well as three-dimensional, culture. Knockdown of Ada3 in ER-positive MCF-7 cells induced reversion of the transformed phenotype in three-dimensional culture. Thus, our results show an important role of Ada3 in HAT recruitment to estrogen-responsive target gene promoters and for estrogen-dependent proliferation of breast cancer cells.

Modulation of aryl hydrocarbon receptor transactivation by carbaryl, a nonconventional ligand

Carbaryl (1-naphthyl-N-methylcarbamate), a widely used carbamate insecticide, induces cytochrome P450 1A gene expression in mammalian cells. This activity is usually mediated by the interaction of the compound with the aryl hydrocarbon receptor. However, it has been proposed that this mechanism does not apply to carbaryl because its structure differs from that of typical aryl hydrocarbon receptor ligands. We show here that carbaryl promotes activation of target genes in a yeast-based bioassay expressing both aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator. By contrast, carbaryl acted as a competitive inhibitor, rather than as an agonist, in a simplified yeast system, in which aryl hydrocarbon receptor nuclear translocator function is bypassed by fusing aryl hydrocarbon receptor to a heterologous DNA binding domain. This dual action of carbaryl, agonist and partial antagonist, was also observed by comparing carbaryl response in two vertebrate cell lines. A yeast two-hybrid assay showed that the mammalian coactivator cAMP response element-binding protein readily interacts with aryl hydrocarbon receptor bound to its canonical ligand beta-naphthoflavone, but not with the carbaryl-aryl hydrocarbon receptor complex. We propose that carbaryl interacts with aryl hydrocarbon receptor, but that its peculiar structure imposes a substandard configuration on the aryl hydrocarbon receptor ligand-binding domain that prevents interaction with key coactivators and activates transcription without the need for aryl hydrocarbon receptor nuclear translocator. This effect may be relevant in explaining its physiological effects in exposed animals, and may help to predict its effects, and that of similar compounds, in humans. Our data also identify the aryl hydrocarbon receptor/cAMP response element-binding protein interaction as a molecular target for the identification and development of new aryl hydrocarbon receptor antagonists.

Identification of a small TAF complex and its role in the assembly of TAF-containing complexes

TFIID plays a role in nucleating RNA polymerase II preinitiation complex assembly on protein-coding genes. TFIID is a multisubunit complex comprised of the TATA box binding protein (TBP) and 14 TBP-associated factors (TAFs). Another class of multiprotein transcriptional regulatory complexes having histone acetyl transferase (HAT) activity, and containing TAFs, includes TFTC, STAGA and the PCAF/GCN5 complex. Looking for as yet undiscovered subunits by a proteomic approach, we had identified TAF8 and SPT7L in human TFTC preparations. Subsequently, however, we demonstrated that TAF8 was not a stable component of TFTC, but that it is present in a small TAF complex (SMAT), containing TAF8, TAF10 and SPT7L, that co-purified with TFTC. Thus, TAF8 is a subunit of both TFIID and SMAT. The latter has to be involved in a pathway of complex formation distinct from the other known TAF complexes, since these three histone fold (HF)-containing proteins (TAF8, TAF10 and SPT7L) can never be found together either in TFIID or in STAGA/TFTC HAT complexes. Here we show that TAF8 is absolutely necessary for the integration of TAF10 in a higher order TFIID core complex containing seven TAFs. TAF8 forms a heterodimer with TAF10 through its HF and proline rich domains, and also interacts with SPT7L through its C-terminal region, and the three proteins form a complex in vitro and in vivo. Thus, the TAF8-TAF10 and TAF10-SPT7L HF pairs, and also the SMAT complex, seem to be important regulators of the composition of different TFIID and/or STAGA/TFTC complexes in the nucleus and consequently may play a role in gene regulation.

Regulation of human estrogen receptor alpha-mediated gene transactivation in Saccharomyces cerevisiae by human coactivator and corepressor proteins

Human estrogen receptor alpha (ERalpha)-mediated transcription activation was evaluated in the yeast Saccharomyces cerevisiae using both the native ERalpha and a G400V variant. A previous study demonstrated that coexpression of human SRC-1, a potent stimulator of ERalpha function in mammalian cells, potentiated ERalpha-mediated gene expression in yeast over five-fold in an E(2)-dependent manner. In the present study, two additional human coactivator proteins were shown to potentiate ERalpha-mediated gene expression in yeast. SRC2 potentiated transactivation two- to three-fold while SRC3 potentiated transactivation five- to eight-fold. Both human coactivators potentiated both the native ERalpha and the G400V variant in an E(2)-dependent manner. The effect of a human corepressor protein was also evaluated in yeast. Repressor of estrogen receptor activity (REA) did not affect E(2)-induced transactivation by ERalpha (either isoform). However, in a strain that coexpressed human SRC1, REA reduced E(2)-induced transactivation to that observed with ERalpha alone. Furthermore, repression of SRC1 potentiation was specific for the native ERalpha since REA had no effect on SRC1 potentiation of the G400V variant. Additionally, REA repression was specific for SRC1 since potentiation of ERalpha (either isoform) transactivation by SRC2 and SRC3 was unaffected by coexpression of REA. These results support previous observations in mammalian cells that REA does not prevent ERalpha from binding to DNA but does inhibit potentiation of ERalpha-mediated transactivation by SRC1. The results in the present study further characterize REA-mediated repression, and demonstrate the utility of this yeast system for dissecting molecular mechanisms involved in regulating gene transactivation by human ERalpha.

Estrogenic effects of natural and synthetic compounds including tibolone assessed in Saccharomyces cerevisiae expressing the human estrogen alpha and beta receptors

The human estrogen receptors (hER)alpha and hERbeta, differentially expressed and localized in various tissues and cell types, mediate transcriptional activation of target genes. These encode a variety of physiological reproductive and nonreproductive functions involved in energy metabolism, salt balance, immune system, development, and differentiation. As a step toward developing a screening assay for the use in applications where significant numbers of compounds or complex matrices need to be tested for (anti) estrogenic bioactivity, hERalpha and hERbeta were expressed in a genetically modified Saccharomyces cerevisiae strain, devoid of three endogenous xenobiotic transporters (PDR5, SNQ2, and YOR1). By using receptor-mediated transcriptional activation of the green fluorescent protein optimized for expression in yeast (yEGFP) as reporter 17 natural, comprising estrogens and phytoestrogens or synthetic compounds among which tibolone with its metabolites, gestagens, and antiestrogens were investigated. The reporter assay deployed a simple and robust protocol for the rapid detection of estrogenic effects within a 96-well microplate format. Results were expressed as effective concentrations (EC50) and correlated to other yeast based and cell line assays. Tibolone and its metabolites exerted clear estrogenic effects, though considerably less potent than all other natural and synthetic compounds. For the blood serum of two volunteers, considerable higher total estrogenic bioactivity than single estradiol concentrations as determined by immunoassay was found. Visualization of a hERalpha/GFP fusion protein in yeast revealed a sub cellular cytosolic localization. This study demonstrates the versatility of (anti) estrogenic bioactivity determination using sensitized S. cerevisiae cells to assess estrogenic exposure and effects.

GCN5 and p300 share essential functions during early embryogenesis

Previous studies revealed that deletion of genes encoding the histone acetyltransferases GCN5, p300, or CBP results in embryonic lethality in mice. PCAF and GCN5 physically interact with p300 and CBP in vitro. To determine whether these two groups of histone acetyltransferases interact functionally in vivo, we created mice lacking one or more alleles of p300, GCN5, or PCAF. As expected, we found that mice heterozygous for any single null allele are viable. The majority of GCN5(+/-)p300(+/-) mice also survive to adulthood with no apparent abnormalities. However, approximately 25% of these mice die before birth. These embryos are developmentally stunted and exhibit increased apoptosis compared with wild-type or single GCN5(+/-) or p300(+/-) littermates at embryonic day 8.5. In contrast, no abnormalities were observed in PCAF(-/-) p300(+/-) mice. Of interest, we find that p300 protein levels vary in different mouse genetic backgrounds, which likely contributes to the incomplete penetrance of the abnormal phenotype of GCN5(+/-) p300(+/-) mice. Our data indicate that p300 cooperates specifically with GCN5 to provide essential functions during early embryogenesis.

T:G mismatch-specific thymine-DNA glycosylase (TDG) as a coregulator of transcription interacts with SRC1 family members through a novel tyrosine repeat motif

Gene activation involves protein complexes with diverse enzymatic activities, some of which are involved in chromatin modification. We have shown previously that the base excision repair enzyme thymine DNA glycosylase (TDG) acts as a potent coactivator for estrogen receptor-alpha. To further understand how TDG acts in this context, we studied its interaction with known coactivators of nuclear receptors. We find that TDG interacts in vitro and in vivo with the p160 coactivator SRC1, with the interaction being mediated by a previously undescribed motif encoding four equally spaced tyrosine residues in TDG, each tyrosine being separated by three amino acids. This is found to interact with two motifs in SRC1 also containing tyrosine residues separated by three amino acids. Site-directed mutagenesis shows that the tyrosines encoded in these motifs are critical for the interaction. The related p160 protein TIF2 does not interact with TDG and has the altered sequence, F-X-X-X-Y, at the equivalent positions relative to SRC1. Substitution of the phenylalanines to tyrosines is sufficient to bring about interaction of TIF2 with TDG. These findings highlight a new protein-protein interaction motif based on Y-X-X-X-Y and provide new insight into the interaction of diverse proteins in coactivator complexes.

Ligand binding is without effect on complex formation of the ligand binding domain of the ecdysone receptor (EcR)

The ligand-binding domain (LBD) encompassing the C-terminal parts of the D- and the complete E-domains of the ecdysteroid receptor (EcR) fused to Gal4(AD) is present in two high molecular weight complexes (600 and 150 kDa) in yeast extracts according to size exclusion chromatography (Superdex 200 HR 10/30). Hormone binding is mainly associated with 150-kDa complexes. Complex formation is not influenced by hormone, but the ligand stabilizes the complexes at elevated salt concentrations. Mutational analysis of Gal4(AD)-EcR(LBD) revealed that formation of 600-kDa, but not 150-kDa, complexes depends on dimerization mediated by the EcR(LBD). Deletion of helix 12 is without effect. Mutation of K497 in helix 4, known to be essential for comodulator binding, abolishes 600-KDa complexes, but does not interfere with the formation of 150-kDa complexes. In contrast, the DE-domains of USP fused to Gal4(DBD) elute as monomer after elimination of the dimerization capacity of the ligand-binding domains by mutation of P463 in helix 10. The data presented here reveal that the complex formation of ligand-binding domains EcR and USP ligand is different.

Vinexin beta interacts with the non-phosphorylated AF-1 domain of retinoid receptor gamma (RARgamma) and represses RARgamma-mediated transcription

Nuclear retinoic acid receptors (RARs) are ligand-dependent transcription factors that regulate the expression of retinoic acid target genes. Although the importance of RAR phosphorylation in their N-terminal domain is clearly established, the underlying mechanism for the phosphorylation-dependent transcriptional activity of the receptors had not been elucidated yet. Here, using a yeast two-hybrid system, we report the isolation of vinexin beta as a new cofactor that interacts with the N-terminal A/B domain of the RARgamma isotype. Vinexin beta is a multiple SH3 motif-containing protein associated with the cytoskeleton and also present in the nucleus. We demonstrate that vinexin beta colocalizes with RARgamma in the nucleus and interacts with the non-phosphorylated form of the AF-1 domain of RARgamma. We also show that this interaction is prevented upon phosphorylation of the AF-1 domain. Using F9 cells stably overexpressing vinexin beta or vinexin knockdown by RNA interference, we demonstrate that vinexin beta is an inhibitor of RARgamma-mediated transcription. We propose a model in which phosphorylation of the AF-1 domain controls RARgamma-mediated transcription through triggering the dissociation of vinexin beta.

Schizophrenia, epigenetics and ligand-activated nuclear receptors: a framework for chromatin therapeutics

Covalent modifications of DNA and its surrounding chromatin constitute an essential and powerful regulatory mechanism for gene transcription. Epigenetics is the study of this regulatory system. There is now strong albeit indirect evidence that epigenetic mechanisms contribute to the pathophysiology of schizophrenia. Furthermore, the discovery that valproic acid, a widely used psychotropic, has powerful epigenetic effects in clinically relevant concentrations suggests new therapeutic possibilities, i.e., drugs that act on chromatin structure. Fortunately, many proteins engaged in these processes, particularly chromatin remodeling, are accessible to pharmacological agents that have a high likelihood of crossing the blood brain barrier. This review will first summarize the essentials of the epigenetic regulatory system, then address the molecular evidence for altered epigenetic mechanisms in schizophrenia, and finally focus on the retinoic acid family of ligand-activated nuclear transcription factors as a likely system for new drug development in the management of schizophrenia-related symptoms.

Human ADA3 binds to estrogen receptor (ER) and functions as a coactivator for ER-mediated transactivation

We have recently identified the hADA3 protein, the human homologue of yeast transcriptional coactivator yADA3, as a novel HPV16 E6 target. Using ectopic expression approaches, we further demonstrated that hADA3 directly binds to the 9-cis retinoic acid receptors alpha and beta, and functions as a coactivator for retinoid receptor-mediated transcriptional activation. Here, we examined the role of endogenous hADA3 as a coactivator for estrogen receptor (ER), an important member of the nuclear hormone receptor superfamily. We show that ADA3 directly interacts with ER alpha and ER beta. Using the chromatin immunoprecipitation assay, we also show that hADA3 is a component of the activator complexes bound to the native ER response element within the promoter of the estrogen-responsive gene pS2. Furthermore, using an ER response element-luciferase reporter, we show that overexpression of ADA3 enhances the ER alpha- and ER beta-mediated sequence-specific transactivation. Reverse transcription-PCR analysis showed an ADA3-mediated increase in estrogen-induced expression of the endogenous pS2 gene. More importantly, using RNA interference against hADA3, we demonstrate that inhibition of endogenous hADA3 inhibited ER-mediated transactivation and the estrogen-induced increase in the expression of pS2, cathepsin D, and progesterone receptor, three widely known ER-responsive genes. The HPV E6 protein, by targeting hADA3 for degradation, inhibited the ER alpha-mediated transactivation and the protein expression of ER target genes. Thus, our results demonstrate that ADA3 directly binds to human estrogen receptor and enhances the transcription of ER-responsive genes, suggesting a broader role of mammalian hADA3 as a coactivator of nuclear hormone receptors and the potential role of these pathways in HPV oncogenesis.

Functional studies on the ligand-binding domain of Ultraspiracle from Drosophila melanogaster

The functional insect ecdysteroid receptor is comprised of the ecdysone receptor (EcR) and Ultraspiracle (USP). The ligand-binding domain (LBD) of USP was fused to the GAL4 DNA-binding domain (GAL4-DBD) and characterized by analyzing the effect of site-directed mutations in the LBD. Normal and mutant proteins were tested for ligand and DNA binding, dimerization, and their ability to induce gene expression. The presence of helix 12 proved to be essential for DNA binding and was necessary to confer efficient ecdysteroid binding to the heterodimer with the EcR (LBD), but did not influence dimerization. The antagonistic position of helix 12 is indispensible for interaction between the fusion protein and DNA, whereas hormone binding to the EcR (LBD) was only partially reduced if fixation of helix 12 was disturbed. The mutation of amino acids, which presumably bind to a fatty acid evoked a profound negative influence on transactivation ability, although enhanced transactivation potency and ligand binding to the ecdysteroid receptor was impaired to varying degrees by mutation of these residues. Mutations of one fatty acidbinding residue within the ligand-binding pocket, I323, however, evoked enhanced transactivation. The results confirmed that the LBD of Ultraspiracle modifies ecdysteroid receptor function through intermolecular interactions and demonstrated that the ligand-binding pocket of USP modifies the DNA-binding and transactivation abilities of the fusion protein.

Retinoid receptors and their coregulators

Retinoids regulate gene transcription by binding to the nuclear receptors, the retinoic acid (RA) receptors (RARs), and the retinoid X receptors (RXRs). RARs and RXRs are ligand-activated transcription factors for the regulation of RA-responsive genes. The actions of RARs and RXRs on gene transcription require a highly coordinated interaction with a large number of coactivators and corepressors. This review focuses on our current understanding of these coregulators known to act in concert with RARs and RXRs. The mechanisms of action of these coregulators are beginning to be uncovered and include the modification of chromatin and the recruitment of basal transcription factors. Challenges remain to understand the specificity of action of RARs and RXRs and the formation of specific transcription complexes consisting of the receptors, coregulators, and other unknown factors.

Potentiation of human estrogen receptor alpha-mediated gene expression by steroid receptor coactivator-1 (SRC-1) in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae was used to reconstruct a human estrogen receptor alpha (ERalpha)-mediated transcription activation system. The level of reporter gene activation was dependent on both the position of the estrogen response element (ERE) relative to the translation start site and the number of EREs in the hybrid promoter. A G400V amino acid alteration in the ERalpha polypeptide decreased sensitivity to 17beta-estradiol (E(2)), demonstrating the hormone responsiveness of ERalpha to be qualitatively and quantitatively similar in yeast and mammalian cells. Coexpression of SRC-1a, a potent stimulator of ERalpha function in mammalian cells, potentiated ERalpha-mediated gene expression over fivefold in a E(2)-dependent manner. Deletion of 56 amino acids at the C-terminal end of SRC-1a resulted in a protein with enhanced ability to potentiate ERalpha-mediated gene expression, which mimics the activity of the same truncation in human SRC-1a as well as the SRC-1e isoform that has the 56 C-terminal residues replaced with a different 14 amino acid peptide. The selective estrogen receptor modulator tamoxifen acted as a weak agonist of ERalpha-mediated gene expression and this weak activity was potentiated by SRC-1. Tamoxifen had no effect on E(2)-induced gene activation in either the presence or absence of SRC-1. In contrast to previously reported yeast-based ERalpha-transactivation systems, the system reported here in which SRC-1 functions as a bona fide coactivator should permit a more thorough dissection of the factors involved in ERalpha-mediated transcriptional activation.

The role of transcriptional corepressor Nif3l1 in early stage of neural differentiation via cooperation with Trip15/CSN2

Mouse Nif3l1 gene is highly conserved from bacteria to human. Even though this gene is expressed throughout embryonic development, its biological function is still obscure. Here, we show that Nif3l1 participates in retinoic acid-primed neural differentiation of P19 embryonic carcinoma cells through cooperation with Trip15/CSN2, a transcriptional corepressor/component of COP9 signalosome. We isolated Nif3l1 cDNA from P19 cell cDNA library by a yeast two-hybrid screening using Trip15/CSN2 as a bait. This interaction was confirmed by a pull-down assay and an epitope-tagged coimmunoprecipitation. Although Nif3l1 was mainly detected in the cytoplasm, the translocation of Nif3l1 into the nuclei was observed in retinoic acid-primed neural differentiation of P19 cells and enhanced by the enforced expression of Trip15/CSN2. Furthermore, enforced expression of sense Nif3l1 RNA, but not antisense RNA, enhanced the neural differentiation of P19 cells accompanying the intense down-regulation of Oct-3/4 mRNA expression and the rapid induction of Mash-1 mRNA expression. Luciferase reporter assay showed that Nif3l1 could act as a transcriptional repressor and synergized the transcriptional repression by Trip15/CSN2. These results indicate that Nif3l1 implicates in neural differentiation through the cooperation with Trip15/CSN2.

Two different Drosophila ADA2 homologues are present in distinct GCN5 histone acetyltransferase-containing complexes

We have isolated a novel Drosophila (d) gene coding for two distinct proteins via alternative splicing: a homologue of the yeast adaptor protein ADA2, dADA2a, and a subunit of RNA polymerase II (Pol II), dRPB4. Moreover, we have identified another gene in the Drosophila genome encoding a second ADA2 homologue (dADA2b). The two dADA2 homologues, as well as many putative ADA2 homologues from different species, all contain, in addition to the ZZ and SANT domains, several evolutionarily conserved domains. The dada2a/rpb4 and dada2b genes are differentially expressed at various stages of Drosophila development. Both dADA2a and dADA2b interacted with the GCN5 histone acetyltransferase (HAT) in a yeast two-hybrid assay, and dADA2b, but not dADA2a, also interacted with Drosophila ADA3. Both dADA2s further potentiate transcriptional activation in insect and mammalian cells. Antibodies raised either against dADA2a or dADA2b both immunoprecipitated GCN5 as well as several Drosophila TATA binding protein-associated factors (TAFs). Moreover, following glycerol gradient sedimentation or chromatographic purification combined with gel filtration of Drosophila nuclear extracts, dADA2a and dGCN5 were detected in fractions with an apparent molecular mass of about 0.8 MDa whereas dADA2b was found in fractions corresponding to masses of at least 2 MDa, together with GCN5 and several Drosophila TAFs. Furthermore, in vivo the two dADA2 proteins showed different localizations on polytene X chromosomes. These results, taken together, suggest that the two Drosophila ADA2 homologues are present in distinct GCN5-containing HAT complexes.

Two different Drosophila ADA2 homologues are present in distinct GCN5 histone acetyltransferase-containing complexes

We have isolated a novel Drosophila (d) gene coding for two distinct proteins via alternative splicing: a homologue of the yeast adaptor protein ADA2, dADA2a, and a subunit of RNA polymerase II (Pol II), dRPB4. Moreover, we have identified another gene in the Drosophila genome encoding a second ADA2 homologue (dADA2b). The two dADA2 homologues, as well as many putative ADA2 homologues from different species, all contain, in addition to the ZZ and SANT domains, several evolutionarily conserved domains. The dada2a/rpb4 and dada2b genes are differentially expressed at various stages of Drosophila development. Both dADA2a and dADA2b interacted with the GCN5 histone acetyltransferase (HAT) in a yeast two-hybrid assay, and dADA2b, but not dADA2a, also interacted with Drosophila ADA3. Both dADA2s further potentiate transcriptional activation in insect and mammalian cells. Antibodies raised either against dADA2a or dADA2b both immunoprecipitated GCN5 as well as several Drosophila TATA binding protein-associated factors (TAFs). Moreover, following glycerol gradient sedimentation or chromatographic purification combined with gel filtration of Drosophila nuclear extracts, dADA2a and dGCN5 were detected in fractions with an apparent molecular mass of about 0.8 MDa whereas dADA2b was found in fractions corresponding to masses of at least 2 MDa, together with GCN5 and several Drosophila TAFs. Furthermore, in vivo the two dADA2 proteins showed different localizations on polytene X chromosomes. These results, taken together, suggest that the two Drosophila ADA2 homologues are present in distinct GCN5-containing HAT complexes.

Human papilloma virus 16 E6 oncoprotein inhibits retinoic X receptor-mediated transactivation by targeting human ADA3 coactivator

The expression of human papillomavirus (HPV) E6 oncoprotein is causally linked to high-risk HPV-associated human cancers. We have recently isolated hADA3, the human homologue of yeast transcriptional co-activator yADA3, as a novel E6 target. Human ADA3 binds to the high-risk (cancer-associated) but not the low-risk HPV E6 proteins and to immortalization-competent but not to immortalization-defective HPV16 E6 mutants, suggesting a role for the perturbation of hADA3 function in E6-mediated oncogenesis. We demonstrate here that hADA3 directly binds to the retinoic X receptor (RXR)alpha in vitro and in vivo. Using chromatin immunoprecipitation, we show that hADA3 is part of activator complexes bound to the native RXR response elements within the promoter of the cyclin-dependent kinase inhibitor gene p21. We show that hADA3 enhances the RXR(alpha)-mediated sequence-specific transactivation of retinoid target genes, cellular retinoic acid-binding protein II and p21. Significantly, we demonstrate that E6 inhibits the RXR(alpha)-mediated transactivation of target genes, implying that perturbation of RXR-mediated transactivation by E6 could contribute to HPV oncogenesis.

TIF2 mediates the synergy between RARalpha 1 activation functions AF-1 and AF-2

Nuclear receptors recruit coregulator complexes through both their AF-1 and AF-2 activation domains. Here we demonstrate that TIF2, a p160 coactivator, is able to bridge the two activation domains of the retinoic acid (RA) receptor isotype RARalpha1, resulting in synergistic activation of transcription. Bridging requires the presence of motifs in region A of RARalpha1 and in the activation domain AD1 of TIF2. Notably, only RARalpha1 exerted this interaction, which requires additional unknown factors. This is the first observation of a RAR isotype-selective coactivator interaction. Because another p160 coactivator, SRC-1, has no effect, this is also the first demonstration of a difference between the members of this coactivator family.

Human papillomavirus oncoprotein E6 inactivates the transcriptional coactivator human ADA3

High-risk human papillomaviruses (HPVs) are associated with carcinomas of the cervix and other genital tumors. The HPV oncoprotein E6 is essential for oncogenic transformation. We identify here hADA3, human homologue of the yeast transcriptional coactivator yADA3, as a novel E6-interacting protein and a target of E6-induced degradation. hADA3 binds selectively to the high-risk HPV E6 proteins and only to immortalization-competent E6 mutants. hADA3 functions as a coactivator for p53-mediated transactivation by stabilizing p53 protein. Notably, three immortalizing E6 mutants that do not induce direct p53 degradation but do interact with hADA3 induced the abrogation of p53-mediated transactivation and G(1) cell cycle arrest after DNA damage, comparable to wild-type E6. These findings reveal a novel strategy of HPV E6-induced loss of p53 function that is independent of direct p53 degradation. Given the likely role of the evolutionarily conserved hADA3 in multiple coactivator complexes, inactivation of its function may allow E6 to perturb numerous cellular pathways during HPV oncogenesis.

ADA3-containing complexes associate with estrogen receptor alpha

Transcriptional repression and activation by nuclear receptors (NRs) are brought about by coregulator complexes. These complexes modify the chromatin environment of target genes and affect the activity of the basal transcription machinery. We have previously implicated the yeast ADA3 protein in transcriptional activation by estrogen and retinoid X receptors in yeast and mammalian cells. Here we report the cloning of the mouse homolog of ADA3 and its characterization with respect to the estrogen receptor alpha (ERalpha) function. Mouse mADA3 is 23% identical and 47% similar to yeast yADA3, and mADA3 in contrast to yADA3 does not interact with NRs directly even though it contains two LxxLL NR boxes. However, the ADA3-containing TBP-free-TAF-containing complex (TFTC) can interact with ERalpha in a ligand-independent manner, indicating that other subunits of the complex are sufficient to mediate interaction with NRs.

Distinct mutations in yeast TAF(II)25 differentially affect the composition of TFIID and SAGA complexes as well as global gene expression patterns

The RNA polymerase II transcription factor TFIID, composed of the TATA-binding protein (TBP) and TBP-associated factors (TAF(II)s), nucleates preinitiation complex formation at protein-coding gene promoters. SAGA, a second TAF(II)-containing multiprotein complex, is involved in transcription regulation in Saccharomyces cerevisiae. One of the essential protein components common to SAGA and TFIID is yTAF(II)25. We define a minimal evolutionarily conserved 91-amino-acid region of TAF(II)25 containing a histone fold domain that is necessary and sufficient for growth in vivo. Different temperature-sensitive mutations of yTAF(II)25 or chimeras with the human homologue TAF(II)30 arrested cell growth at either the G(1) or G(2)/M cell cycle phase and displayed distinct phenotypic changes and gene expression patterns. Immunoprecipitation studies revealed that TAF(II)25 mutation-dependent gene expression and phenotypic changes correlated at least partially with the integrity of SAGA and TFIID. Genome-wide expression analysis revealed that the five TAF(II)25 temperature-sensitive mutant alleles individually affect the expression of between 18 and 33% of genes, whereas taken together they affect 64% of all class II genes. Thus, different yTAF(II)25 mutations induce distinct phenotypes and affect the regulation of different subsets of genes, demonstrating that no individual TAF(II) mutant allele reflects the full range of its normal functions.

Mammalian transcription factors in yeast: strangers in a familiar land

Many transcription factors in human cells have functional orthologues in yeast, and a common experimental theme has been to define the function of the yeast protein and then test whether the mammalian version behaves similarly. Although, at first glance, this approach does not seem feasible for factors that do not have yeast counterparts, mammalian transcriptional activators or repressors can be expressed directly in yeast. Often, the mammalian factor retains function in yeast, and this allows investigators to exploit the experimental tractability of yeast to ask a diverse set of questions.

Bakers yeast rises to the challenge: reconstitution of mammalian steroid receptor signalling in S. cerevisiae

Steroid hormones are an important class of signalling molecule, regulating a diverse range of processes in metazoan eukaryotes. The actions of these hormones are mediated by intracellular receptor proteins that act as ligand-activated transcription factors. The ability to reconstitute steroid receptor signalling in the budding yeast, Saccharomyces cerevisiae, provides a genetically tractable model system in which to investigate steroid receptor structure and function. Through targeted disruption and genetic screening, an increasing number of genes have been identified that are likely to have a role in steroid receptor action.

Ligand- and coactivator-mediated transactivation function (AF2) of the androgen receptor ligand-binding domain is inhibited by the cognate hinge region

Transactivation functions (AF2) in the ligand-binding domains (LBD) of many steroid receptors are well characterized, but there is little evidence to support such a function for the LBD of the androgen receptor (AR). We report a mutant AR, with residues 628-646 in the hinge region deleted, which exhibited transactivation activity that was more than double that of the wild type (WT) AR. Although no androgen-dependent AF2 activity could be observed for the WT ARLBD fused to a heterologous DNA-binding domain, the mutant ARLBD(Delta628-646) was 30-40 times more active than the WT ARLBD. In the presence of the p160 coactivator TIF2, AR(Delta628-646) was significantly more active than similarly treated WT AR. Deletion of residues 628-646 also enhanced TIF2-ARLBD activity 8-fold, an effect not present when the LBD-interacting LXXLL motifs of TIF2 were mutated, suggesting that the negative modulatory activity of residues 628-646 were exerted via coactivator pathways. Although the AP-1 (c-Jun/c-Fos) system and NcoR have been reported to interact with and repress the activity of some steroid receptors, c-Jun, c-Fos, c-Jun/c-Fos, nor NcoR function was consistently affected by the absence or presence of residues 628-646, implying that the AR hinge region exerts its silencing effects in a manner independent of these corepressors. Our data provide evidence for the novel finding that strong androgen-dependent AF2 exists in the ARLBD and is the first report of a negative regulatory domain in the AR. Because mutations in this region are commonly associated with prostate cancer, it is important to characterize the mechanisms by which the hinge region exerts its repressor effect on ligand-activated and coactivator-mediated AF2 activity of the ARLBD.

Ligand binding and nuclear receptor evolution

Nuclear receptors form a superfamily of ligand-activated transcription factors that regulate various physiological functions, from development to homeostasis, in metazoans. The superfamily contains not only receptors for known ligands but also a large number of so-called orphan receptors for which ligands do not exist or have not been identified. The evolution of ligand-binding capacity of nuclear receptors may involve either secondary loss in orphan receptors, or evolutionary acquisition of ligand-binding capacity in liganded receptors. In this review, we present arguments from phylogenetic, functional and structural studies that support the hypothesis that there have been several independent gains of ligand-binding ability of nuclear receptors during metazoan evolution.

GCN5 and ADA adaptor proteins regulate triiodothyronine/GRIP1 and SRC-1 coactivator-dependent gene activation by the human thyroid hormone receptor

We have used yeast genetics and in vitro protein-protein interaction experiments to explore the possibility that GCN5 (general control nonrepressed protein 5) and several other ADA (alteration/deficiency in activation) adaptor proteins of the multimeric SAGA complex can regulate T3/GRIP1 (glucocorticoid receptor interacting protein 1) and SRC-1 (steroid receptor coactivator-1) coactivator-dependent activation of transcription by the human T3 receptor beta1 (hTRbeta1). Here, we show that in vivo activation of a T3/GRIP1 or SRC-1 coactivator-dependent T3 hormone response element by hTRbeta1 is dependent upon the presence of yeast GCN5, ADA2, ADA1, or ADA3 adaptor proteins and that the histone acetyltransferase (HAT) domains and bromodomain (BrD) of yGCN5 must be intact for maximal activation of transcription. We also observed that hTRbeta1 can bind directly to yeast or human GCN5 as well as hADA2, and that the hGCN5(387-837) sequence could bind directly to either GRIP1 or SRC-1 coactivator. Importantly, the T3-dependent binding of hTRbeta1 to hGCN5(387-837) could be markedly increased by the presence of GRIP1 or SRC1. Mutagenesis of GRIP1 nuclear receptor (NR) Box II and III LXXLL motifs also substantially decreased both in vivo activation of transcription and in vitro T3-dependent binding of hTRbeta1 to hGCN5. Taken together, these experiments support a multistep model of transcriptional initiation wherein the binding of T3 to hTRbeta1 initiates the recruitment of p160 coactivators and GCN5 to form a trimeric transcriptional complex that activates target genes through interactions with ADA/SAGA adaptor proteins and nucleosomal histones.

Inactivation of interferon regulatory factor-1 tumor suppressor protein by HPV E7 oncoprotein. Implication for the E7-mediated immune evasion mechanism in cervical carcinogenesis

In studying biological roles of interferon regulatory factor (IRF)-1 tumor suppressor in cervical carcinogenesis, we found that HPV E7 is functionally associated with IRF-1. Binding assays indicate a physical interaction between IRF-1 and HPV E7 in vivo and in vitro. The carboxyl-terminal transactivation domain of IRF-1 was required for the interaction. Transient co-expression of E7 significantly inhibits the IRF-1-mediated activation of IFN-beta promoter in NIH-3T3 cells. Co-transfection of E7 mutants reveals that the pRb-binding portion of E7 is necessary for the E7-mediated inactivation of IRF-1. It was next determined whether histone deacetylase (HDAC) is involved in the inactivation mechanism as recently suggested, where the carboxyl-terminal zinc finger domain of E7 associates with NURD complex containing HDAC. When trichostatin A, an inhibitor of HDAC, was treated, the repressing activity of E7 was released in a dose-dependent manner. Furthermore, the mutation of zinc finger abrogates such activity without effect on the interaction with IRF-1. These results suggest that HPV E7 interferes with the transactivation function of IRF-1 by recruiting HDAC to the promoter. The immune-promoting role of IRF-1 evokes the idea that our novel finding might be important for the elucidation of the E7-mediated immune evading mechanism that is frequently found in cervical cancer.

Transcriptional repression by neuron-restrictive silencer factor is mediated via the Sin3-histone deacetylase complex

A large number of neuron-specific genes characterized to date are under the control of negative transcriptional regulation. Many promoter regions of neuron-specific genes possess the repressor element repressor element 1/neuron-restrictive silencing element (RE1/NRSE). Its cognate binding protein, REST/NRSF, is an essential transcription factor; its null mutations result in embryonic lethality, and its dominant negative mutants produce aberrant expression of neuron-specific genes. REST/NRSF acts as a regulator of neuron-specific gene expression in both nonneuronal tissue and developing neurons. Here, we shown that heterologous expression of REST/NRSF in Saccharomyces cerevisiae is able to repress transcription from yeast promoters engineered to contain RE1/NRSEs. Moreover, we have taken advantage of this observation to show that this repression requires both yeast Sin3p and Rpd3p and that REST/NRSF physically interacts with the product of the yeast SIN3 gene in vivo. Furthermore, we show that REST/NRSF binds mammalian SIN3A and HDAC-2 and requires histone deacetylase activity to repress neuronal gene transcription in both nonneuronal and neuronal cell lines. We show that REST/NRSF binding to RE1/NRSE is accompanied by a decrease in the acetylation of histones around RE1/NRSE and that this decrease requires the N-terminal Sin3p binding domain of REST/NRSF. Taken together, these data suggest that REST/NRSF represses neuronal gene transcription by recruiting the SIN3/HDAC complex.

The essential cofactor TRRAP recruits the histone acetyltransferase hGCN5 to c-Myc

The c-Myc protein functions as a transcription factor to facilitate oncogenic transformation; however, the biochemical and genetic pathways leading to transformation remain undefined. We demonstrate here that the recently described c-Myc cofactor TRRAP recruits histone acetylase activity, which is catalyzed by the human GCN5 protein. Since c-Myc function is inhibited by recruitment of histone deacetylase activity through Mad family proteins, these opposing biochemical activities are likely to be responsible for the antagonistic biological effects of c-Myc and Mad on target genes and ultimately on cellular transformation.

The human TFIID components TAF(II)135 and TAF(II)20 and the yeast SAGA components ADA1 and TAF(II)68 heterodimerize to form histone-like pairs

It has been previously proposed that the transcription complexes TFIID and SAGA comprise a histone octamer-like substructure formed from a heterotetramer of H4-like human hTAF(II)80 (or its Drosophila melanogaster dTAF(II)60 and yeast [Saccharomyces cerevisiae] yTAF(II)60 homologues) and H3-like hTAF(II)31 (dTAF(II)40 and yTAF(II)17) along with two homodimers of H2B-like hTAF(II)20 (dTAF(II)30alpha and yTAF(II)61/68). However, it has not been formally shown that hTAF(II)20 heterodimerizes via its histone fold. By two-hybrid analysis with yeast and biochemical characterization of complexes formed by coexpression in Escherichia coli, we showed that hTAF(II)20 does not homodimerize but heterodimerizes with hTAF(II)135. Heterodimerization requires the alpha2 and alpha3 helices of the hTAF(II)20 histone fold and is abolished by mutations in the hydrophobic face of the hTAF(II)20 alpha2 helix. Interaction with hTAF(II)20 requires a domain of hTAF(II)135 which shows sequence homology to H2A. This domain also shows homology to the yeast SAGA component ADA1, and we show that yADA1 heterodimerizes with the histone fold region of yTAF(II)61/68, the yeast hTAF(II)20 homologue. These results are indicative of a histone fold type of interaction between hTAF(II)20-hTAF(II)135 and yTAF(II)68-yADA1, which therefore constitute novel histone-like pairs in the TFIID and SAGA complexes.

Ligand-binding domain of estrogen receptors

Estrogen receptors are multi-domain proteins that interact with other proteins and DNA to fulfil their function: the regulation of transcription. During the past 2-3 years, our understanding of this complex process has increased tremendously as crystal structures of isolated ligand-binding domains in complex with various ligands, as well as co-activator peptides, are now available. The structural information, combined with new data on novel co-activators/co-repressors, muteins and their actions, and novel ligands, allows for the first time the development of detailed theories for the first steps of transcription initiation.

Nuclear receptor cofactors as chromatin remodelers

Nuclear receptors regulate transcription in direct response to their cognate hormonal ligands. Ligand binding leads to the dissociation of corepressors and the recruitment of coactivators. Many of these factors, acting in large complexes, have emerged as chromatin remodelers through intrinsic histone modifying activities or through other novel functions. In addition, other ligand-recruited complexes appear to act more directly on the transcriptional apparatus, suggesting that transcriptional regulation by nuclear receptors may involve a process of both chromatin alterations and direct recruitment of key initiation components at regulated promoters.

Histone acetyltransferase complexes can mediate transcriptional activation by the major glucocorticoid receptor activation domain

Previous studies have shown that the Ada adapter proteins are important for glucocorticoid receptor (GR)-mediated gene activation in yeast. The N-terminal transactivation domain of GR, tau1, is dependent upon Ada2, Ada3, and Gcn5 for transactivation in vitro and in vivo. Using in vitro techniques, we demonstrate that the GR-tau1 interacts directly with the native Ada containing histone acetyltransferase (HAT) complex SAGA but not the related Ada complex. Mutations in tau1 that reduce tau1 transactivation activity in vivo lead to a reduced binding of tau1 to the SAGA complex and conversely, mutations increasing the transactivation activity of tau1 lead to an increased binding of tau1 to SAGA. In addition, the Ada-independent NuA4 HAT complex also interacts with tau1. GAL4-tau1-driven transcription from chromatin templates is stimulated by SAGA and NuA4 in an acetyl coenzyme A-dependent manner. Low-activity tau1 mutants reduce SAGA- and NuA4-stimulated transcription while high-activity tau1 mutants increase transcriptional activation, specifically from chromatin templates. Our results demonstrate that the targeting of native HAT complexes by the GR-tau1 activation domain mediates transcriptional stimulation from chromatin templates.

Investigation of steroid receptor function in the budding yeast Saccharomyces cerevisiae

Steroid hormones are small lipophilic molecules that control a wide range of responses in both the developing and adult organism. The actions of these molecules are mediated by soluble receptor proteins that function as hormone-activated transcription factors. The first steroid receptors were expressed in the yeast Saccharomyces cerevisiae over 10 years ago, and to date virtually all the classical steroid receptors, together with a number of non-steroid members of the nuclear receptor superfamily, have been expressed in yeast. The ability to reconstitute steroid receptor signalling in yeast cells by co-expression of the receptor protein and a reporter gene driven by the appropriate hormone response element has presented researchers with a powerful model system for investigating receptor action. In this review, the use of yeast-based steroid receptor transactivation assays to investigate the roles of molecular chaperones, the mechanisms of DNA binding and gene activation, and the functional properties of hormone mimics will be discussed.

Identification of TATA-binding protein-free TAFII-containing complex subunits suggests a role in nucleosome acetylation and signal transduction

Recently we identified a novel human (h) multiprotein complex, called TATA-binding protein (TBP)-free TAFII-containing complex (TFTC), which is able to nucleate RNA polymerase II transcription and can mediate transcriptional activation. Here we demonstrate that TFTC, similar to other TBP-free TAFII complexes (yeast SAGA, hSTAGA, and hPCAF) contains the acetyltransferase hGCN5 and is able to acetylate histones in both a free and a nucleosomal context. The recently described TRRAP cofactor for oncogenic transcription factor pathways was also characterized as a TFTC subunit. Furthermore, we identified four other previously uncharacterized subunits of TFTC: hADA3, hTAFII150, hSPT3, and hPAF65beta. Thus, the polypeptide composition of TFTC suggests that TFTC is recruited to chromatin templates by activators to acetylate histones and thus may potentiate initiation and activation of transcription.

Nuclear receptor coregulators: cellular and molecular biology

Nuclear receptor coregulators are coactivators or corepressors that are required by nuclear receptors for efficient transcripitonal regulation. In this context, we define coactivators, broadly, as molecules that interact with nuclear receptors and enhance their transactivation. Analogously, we refer to nuclear receptor corepressors as factors that interact with nuclear receptors and lower the transcription rate at their target genes. Most coregulators are, by definition, rate limiting for nuclear receptor activation and repression, but do not significantly alter basal transcription. Recent data have indicated multiple modes of action of coregulators, including direct interactions with basal transcription factors and covalent modification of histones and other proteins. Reflecting this functional diversity, many coregulators exist in distinct steady state precomplexes, which are thought to associate in promoter-specific configurations. In addition, these factors may function as molecular gates to enable integration of diverse signal transduction pathways at nuclear receptor-regulated promoters. This review will summarize selected aspects of our current knowledge of the cellular and molecular biology of nuclear receptor coregulators.

P/CAF associates with cyclin D1 and potentiates its activation of the estrogen receptor

Cyclin D1 is overexpressed in a significant percentage of human breast cancers, particularly in those that also express the estrogen receptor (ER). We and others have demonstrated previously that experimentally overexpressed cyclin D1 can associate with the ER and stimulate its transcriptional functions in the absence of estrogen. This effect is separable from the established function of cyclin D1 as a regulator of cyclin-dependent kinases. Here, we demonstrate that cyclin D1 can also interact with the histone acetyltransferase, p300/CREB-binding protein-associated protein (P/CAF), thereby facilitating an association between P/CAF and the ER. Ectopic expression of P/CAF potentiates cyclin D1-stimulated ER activity in a dose-dependent manner. This effect is largely dependent on the acetyltransferase activity of P/CAF. These results suggest that cyclin D1 may trigger the activation of the ER through the recruitment of P/CAF, by providing histone acetyltransferase activity and, potentially, links to additional P/CAF-associated transcriptional coactivators.