Molecular cloning of xSRC-3, a novel transcription coactivator from Xenopus, that is related to AIB1, p/CIP, and TIF2

Lipid-sensors, enigmatic-orphan and orphan nuclear receptors as therapeutic targets in breast-cancer

Breast-cancer is heterogeneous and consists of various groups with different biological characteristics. Innovative pharmacological approaches accounting for this heterogeneity are needed. The forty eight human Nuclear-Hormone-Receptors are ligand-dependent transcription-factors and are classified into Endocrine-Receptors, Adopted-Orphan-Receptors (Lipid-sensors and Enigmatic-Orphans) and Orphan-receptors. Nuclear-Receptors represent ideal targets for the design/synthesis of pharmacological ligands. We provide an overview of the literature available on the expression and potential role played by Lipid-sensors, Enigmatic-Orphans and Orphan-Receptors in breast-cancer. The data are complemented by an analysis of the expression levels of each selected Nuclear-Receptor in the PAM50 breast-cancer groups, following re-elaboration of the data publicly available. The major aim is to support the idea that some of the Nuclear-Receptors represent largely unexploited therapeutic-targets in breast-cancer treatment/chemo-prevention. On the basis of our analysis, we conclude that the Lipid-Sensors, NR1C3, NR1H2 and NR1H3 are likely to be onco-suppressors in breast-cancer. The Enigmatic-Orphans, NR1F1 NR2A1 and NR3B3 as well as the Orphan-Receptors, NR0B1, NR0B2, NR1D1, NR2F1, NR2F2 and NR4A3 exert a similar action. These Nuclear-Receptors represent candidates for the development of therapeutic strategies aimed at increasing their expression or activating them in tumor cells. The group of Nuclear-Receptors endowed with potential oncogenic properties consists of the Lipid-Sensors, NR1C2 and NR1I2, the Enigmatic-Orphans, NR1F3, NR3B1 and NR5A2, as well as the Orphan-Receptors, NR2E1, NR2E3 and NR6A1. These oncogenic Nuclear-Receptors should be targeted with selective antagonists, reverse-agonists or agents/strategies capable of reducing their expression in breast-cancer cells.

Modulation of xenobiotic receptors by steroids

Nuclear receptors (NRs) are ligand-activated transcription factors that regulate the expression of their target genes. NRs play important roles in many human diseases, including metabolic diseases and cancer, and are therefore a key class of therapeutic targets. Steroids play important roles in regulating nuclear receptors; in addition to being ligands of steroid receptors, steroids (and their metabolites) also regulate other NRs, such as the pregnane X receptor and constitutive androstane receptor (termed xenobiotic receptors), which participate in steroid metabolism. Xenobiotic receptors have promiscuous ligand-binding properties, and their structurally diverse ligands include steroids and their metabolites. Therefore, steroids, their metabolism and metabolites, xenobiotic receptors, steroid receptors, and the respective signaling pathways they regulate have functional interactions. This review discusses these functional interactions and their implications for activities mediated by steroid receptors and xenobiotic receptors, focusing on steroids that modulate pathways involving the pregnane X receptor and constitutive androstane receptor. The emphasis of the review is on structure-function studies of xenobiotic receptors bound to steroid ligands.

The orphan nuclear receptor DAX-1 functions as a potent corepressor of the constitutive androstane receptor (NR1I3)

Regulation of gene transcription is controlled in part by nuclear receptors that function coordinately with coregulator proteins. The human constitutive androstane receptor (CAR; NR1I3) is expressed primarily in liver and regulates the expression of genes involved in xenobiotic metabolism as well as hormone, energy, and lipid homeostasis. In this report, DAX-1, a nuclear receptor family member with corepressor properties, was identified as a potent CAR regulator. Results of transaction and mutational studies demonstrated that both DAX-1's downstream LXXLL and its PCFQVLP motifs were critical contributors to DAX-1's corepression activities, although two other LXXM/LL motifs located nearer the N terminus had no impact on the CAR functional interaction. Deletion of DAX-1's C-terminal transcription silencing domain restored CAR1 transactivation activity in reporter assays to approximately 90% of control, demonstrating its critical function in mediating the CAR repression activities. Furthermore, results obtained from mammalian two-hybrid experiments assessing various domain configurations of the respective receptors showed that full-length DAX-1 inhibited the CAR-SRC1 interaction by approximately 50%, whereas the same interaction was restored to 90% of control when the DAX-1 transcription silencing domain was deleted. Direct interaction between CAR and DAX-1 was demonstrated with both alpha-screen and coimmunoprecipitation experiments, and this interaction was enhanced in the presence of the CAR activator 6-(4-chlorophenyl)imidazo[2,1-b]thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime (CITCO). Results obtained in primary human hepatocytes further demonstrated DAX-1 inhibition of CAR-mediated CITCO induction of the CYP2B6 target gene. The results of this investigation identify DAX-1 as a novel and potent CAR corepressor and suggest that DAX-1 functions as a coordinate hepatic regulator of CAR's biological function.

Role of xenobiotic metabolism in cancer: involvement of transcriptional and miRNA regulation of P450s

Cytochrome P450 enzymes (P450s) are important targets in cancer, due to their role in xenobiotic metabolism. Since P450s are the "bridges" between the environment and our body, their function can be linked in many ways to carcinogenesis: they activate dietary and environmental components to ultimate carcinogens (i), the cancer tissue maintains its drug resistance with altered expression of P450s (ii), P450s metabolize (sometimes activate) drugs used for cancer treatment (iii) and they are potential targets for anticancer therapy (iiii). These highly polymorphic enzymes are regulated at multiple molecular levels. Regulation is as important as genetic difference in the existing individual variability in P450 activity. In this review, examples of the transcriptional (DNA methylation, histone modification, modulation by xenosensors) and post-transcriptional (miRNA) regulation will be presented and thereby introduce potential molecular targets at which the metabolism of anticancer drugs, the elimination of cancerogenes or the progress of carcinogenesis could be affected.

Proteomic analysis of native hepatocyte nuclear factor-4α (HNF4α) isoforms, phosphorylation status, and interactive cofactors

Hepatocyte nuclear factor-4α (HNF4α, NR2A1) is a nuclear receptor that has a critical role in hepatocyte differentiation and the maintenance of homeostasis in the adult liver. However, a detailed understanding of native HNF4α in the steady-state remains to be elucidated. Here we report the native HNF4α isoform, phosphorylation status, and complexes in the steady-state, as shown by shotgun proteomics in HepG2 hepatocarcinoma cells. Shotgun proteomic analysis revealed the complexity of native HNF4α, including multiple phosphorylation sites and inter-isoform heterodimerization. The associating complexes identified by label-free semiquantitative proteomic analysis include the following: the DNA-dependent protein kinase catalytic subunit, histone acetyltransferase complexes, mRNA splicing complex, other nuclear receptor coactivator complexes, the chromatin remodeling complex, and the nucleosome remodeling and histone deacetylation complex. Among the associating proteins, GRB10 interacting GYF protein 2 (GIGYF2, PERQ2) is a new candidate cofactor in metabolic regulation. Moreover, an unexpected heterodimerization of HNF4α and hepatocyte nuclear factor-4γ was found. A biochemical and genomewide analysis of transcriptional regulation showed that this heterodimerization activates gene transcription. The genes thus transcribed include the cell death-inducing DEF45-like effector b (CIDEB) gene, which is an important regulator of lipid metabolism in the liver. This suggests that the analysis of the distinctive stoichiometric balance of native HNF4α and its cofactor complexes described here are important for an accurate understanding of transcriptional regulation.

A microarray screen for direct targets of Zic1 identifies an aquaporin gene, aqp-3b, expressed in the neural folds

The Zic1 transcription factor plays multiple roles during early development, for example, in patterning the early neural plate and formation of the neural crest, somites, and cerebellum. To identify direct downstream target genes of Zic1, a microarray screen was conducted in Xenopus laevis that identified 85 genes upregulated twofold or more. These include transcription factors, receptors, enzymes, proteins involved in retinoic acid signaling, and an aquaglyceroporin (aqp-3b), but surprisingly no genes known to be involved in cell proliferation. We show that both aqp-3 and aqp-3b were expressed in adult tissues, while during early embryonic development, only aqp-3b was transcribed. During neurula stages, aqp-3b was expressed specifically in the neural folds. This pattern of aqp-3b expression closely resembled that of NF-protocadherin (NFPC), which is involved in cell adhesion and neural tube closure. Aqp-3b may also be involved in neural tube closure, since mammalian Aqp-3 promotes cell migration and proliferation.

Nuclear receptors CAR and PXR: Molecular, functional, and biomedical aspects

Nuclear receptors (NRs) are ligand-activated transcription factors sharing a common evolutionary history and having similar sequence features at the protein level. Selective ligand(s) for some NRs is not known, therefore these NRs have been named "orphan receptors". Whenever ligands have been recognized for any of the orphan receptor, it has been categorized and grouped as "adopted" orphan receptor. This group includes the constitutive androstane receptor (CAR) and the pregnane X receptor (PXR). They function as sensors of toxic byproducts derived from endogenous metabolites and of exogenous chemicals, in order to enhance their elimination. This unique function of CAR and PXR sets them apart from the steroid hormone receptors. The broad response profile has established that CAR and PXR are xenobiotic sensors that coordinately regulate xenobiotic clearance in the liver and intestine via induction of genes involved in drug and xenobiotic metabolism. In the past few years, research has revealed new and mostly unsuspected roles for CAR and PXR in modulating hormone, lipid, and energy homeostasis as well as cancer and liver steatosis. The purpose of this review is to highlight the structural and molecular bases of CAR and PXR impact on human health, providing information on mechanisms through which diet, chemical exposure, and environment ultimately impact health and disease.

Regulation of hepatocyte nuclear factor 4 alpha-mediated transcription

Hepatocyte nuclear factor 4alpha (HNF4alpha, NR2A1) is required for development of the liver and for controlling the expression of many genes specifically expressed in the liver and associated with a number of critical metabolic pathways. Among the genes regulated by HNF4alpha are the xenobiotic-metabolizing cytochromes P450, UDP-glucuronosyltransferases and sulfotransferases thus making this transcription factor critical in the control of drug metabolism. HNF4alpha, a member of the nuclear receptor superfamily, binds as a homodimer to direct repeat elements upstream of target genes. However, in contrast to many other nuclear receptors, there is no convincing evidence that HNF4alpha is activated by exogenous ligands, at least in the classic mechanism used by other steroid and metabolic nuclear receptors. X-ray crystallographic studies revealed that HNF4alpha has a fatty acid embedded in its putative ligand binding site that may not be easily released or displaced by exogenous ligands. HNF4alpha, as a general rule, controls constitutive expression of many hepatic genes but under certain circumstances can be subjected to regulation by differential co-activator recruitment, by phosphorylation and by interaction with other nuclear receptors. The ability of HNF4alpha to be regulated offers hope that it could be a drug target.

CAR and PXR: the xenobiotic-sensing receptors

The xenobiotic receptors CAR and PXR constitute two important members of the NR1I nuclear receptor family. They function as sensors of toxic byproducts derived from endogenous metabolism and of exogenous chemicals, in order to enhance their elimination. This unique function of CAR and PXR sets them apart from the steroid hormone receptors. In contrast, the steroid receptors, exemplified by the estrogen receptor (ER) and glucocorticoid receptor (GR), are the sensors that tightly monitor and respond to changes in circulating steroid hormone levels to maintain body homeostasis. This divergence of the chemical- and steroid-sensing functions has evolved to ensure the fidelity of the steroid hormone endocrine regulation while allowing development of metabolic elimination pathways for xenobiotics. The development of the xenobiotic receptors CAR and PXR also reflect the increasing complexity of metabolism in higher organisms, which necessitate novel mechanisms for handling and eliminating metabolic by-products and foreign compounds from the body. The purpose of this review is to discuss similarities and differences between the xenobiotic receptors CAR and PXR with the prototypical steroid hormone receptors ER and GR. Interesting differences in structure explain in part the divergence in function and activation mechanisms of CAR/PXR from ER/GR. In addition, the physiological roles of CAR and PXR will be reviewed, with discussion of interactions of CAR and PXR with endocrine signaling pathways.

Global analysis of RAR-responsive genes in the Xenopus neurula using cDNA microarrays

Retinoid signaling is important for patterning the vertebrate hindbrain and midaxial regions. We recently showed that signaling through retinoic acid receptors (RARs) is essential for anteroposterior patterning along the entire body axis. To further investigate the mechanisms through which RARs act, we used microarray analysis to investigate the effects of modulating RAR activity on target gene expression. We identified 334 up-regulated genes (92% of which were validated), including known RA-responsive genes, known genes that have never been proposed as RA targets and many hypothetical and unidentified genes (n = 166). Sixty-seven validated down-regulated genes were identified, including known RA-responsive genes and anterior marker genes. The expression patterns of selected up-regulated genes (n = 45) were examined at neurula stages using whole-mount in situ hybridization. We found that most of these genes were expressed in the neural tube and many were expressed in anterior tissues such as neural crest, brain, eye anlagen, and cement gland. Some were expressed in tissues such as notochord, somites, pronephros, and blood islands, where retinoic acid (RA) plays established roles in organogenesis. Members of this set of newly identified RAR target genes are likely to play important roles in neural patterning and organogenesis under the control of RAR signaling pathways, and their further characterization will expand our understanding of RA signaling during development.

Coactivator recruitment is essential for liganded thyroid hormone receptor to initiate amphibian metamorphosis

Thyroid hormone receptors (TRs) can repress or activate target genes depending on the absence or presence of thyroid hormone (T3), respectively. This hormone-dependent gene regulation is mediated by recruitment of co-repressors in the absence of T3 and coactivators in its presence. Many TR-interacting coactivators have been characterized in vitro. In comparison, few studies have addressed the developmental roles of these cofactors in vivo. We have investigated the role of coactivators in transcriptional activation by TR during postembryonic tissue remodeling by using amphibian metamorphosis as a model system. We have previously shown that steroid receptor coactivator 3 (SRC3) is expressed and upregulated during metamorphosis, suggesting a role in gene regulation by liganded TR. Here, we have generated transgenic tadpoles expressing a dominant negative form of SRC3 (F-dnSRC3). The transgenic tadpoles exhibited normal growth and development throughout embryogenesis and premetamorphic stages. However, transgenic expression of F-dnSRC3 inhibits essentially all aspects of T3-induced metamorphosis, as well as natural metamorphosis, leading to delayed or arrested metamorphosis or the formation of tailed frogs. Molecular analysis revealed that F-dnSRC3 functioned by blocking the recruitment of endogenous coactivators to T3 target genes without affecting corepressor release, thereby preventing the T3-dependent gene regulation program responsible for tissue transformations during metamorphosis. Our studies thus demonstrate that coactivator recruitment, aside from corepressor release, is required for T3 function in development and further provide the first example where a specific coactivator-dependent gene regulation pathway by a nuclear receptor has been shown to underlie specific developmental events.

Tissue- and Gene-specific Recruitment of Steroid Receptor Coactivator-3 by Thyroid Hormone Receptor during Development

Numerous coactivators that bind nuclear hormone receptors have been isolated and characterized in vitro. Relatively few studies have addressed the developmental roles of these cofactors in vivo. By using the total dependence of amphibian metamorphosis on thyroid hormone (T3) as a model, we have investigated the role of steroid receptor coactivator 3 (SRC3) in gene activation by thyroid hormone receptor (TR) in vivo. First, expression analysis showed that SRC3 was expressed in all tadpole organs analyzed. In addition, during natural as well as T3-induced metamorphosis, SRC3 was up-regulated in both the tail and intestine, two organs that undergo extensive transformations during metamorphosis and the focus of the current study. We then performed chromatin immunoprecipitation assays to investigate whether SRC3 is recruited to endogenous T3 target genes in vivo in developing tadpoles. Surprisingly, we found that SRC3 was recruited in a gene- and tissue-dependent manner to target genes by TR, both upon T3 treatment of premetamorphic tadpoles and during natural metamorphosis. In particular, in the tail, SRC3 was not recruited in a T3-dependent manner to the target TRbetaA promoter, suggesting either no recruitment or constitutive association. Finally, by using transgenic tadpoles expressing a dominant negative SRC3 (F-dnSRC3), we demonstrated that F-dnSRC3 was recruited in a T3-dependent manner in both the intestine and tail, blocking the recruitment of endogenous coactivators and histone acetylation. These results suggest that SRC3 is utilized in a gene- and tissue-specific manner by TR during development.

CAR, Driving into the Future

The nuclear orphan receptor CAR is active in the absence of ligand with the unique capability to be further regulated by activators. A number of these activators, including phenobarbital, do not directly bind to the receptor. Considered a xenobiotic sensing receptor, CAR transcriptionally modifies the expression of genes involved in the metabolism and elimination of xenobiotics and steroids in response to these compounds and other cellular metabolites. Its hepatic expression pattern endows the liver with the ability to protect against not only exogenous but also endogenous insults. The mechanism of CAR activation is complex, involving translocation from the cytoplasm to the nucleus in the presence of activators, followed by further activation steps in the nucleus. Although this mechanism remains under investigation, we have summarized here the cellular signaling pathways elucidated so far and speculate on the mechanism by which CAR activators regulate gene expression through this network.

Distinct expression profiles of transcriptional coactivators for thyroid hormone receptors during Xenopus laevis metamorphosis

The biological effects of thyroid hormone (T3) are mediated by the thyroid hormone receptor (TR). Amphibian metamorphosis is one of the most dramatic processes that are dependent on T3. T3 regulates a series of orchestrated developmental changes, which ultimately result in the conversion of an aquatic herbivorous tadpole to a terrestrial carnivorous frog. T3 is presumed to bind to TRs, which in turn recruit coactivators, leading to gene activation. The best-studied coactivators belong to the p160 or SRC family. Members of this family include SRC1/NCoA-1, SRC2/TIF2/GRIP1, and SRC3/pCIP/ACTR/AIB-1/RAC-3/TRAM-1. These SRCs interact directly with liganded TR and function as adapter molecules to recruit other coactivators such as p300/CBP. Here, we studied the expression patterns of these coactivators during various stages of development. Amongst the coactivators cloned in Xenopus laevis, SRC3 was found to be dramatically upregulated during natural and T3-induced metamorphosis, and SRC2 and p300 are expressed throughout postembryonic development with little change in their expression levels. These results support the view that these coactivators participate in gene regulation by TR during metamorphosis.

Metamorphic T3-response genes have specific co-regulator requirements

Thyroid hormone receptors (TRs) have several regulatory functions in vertebrates. In the absence of thyroid hormone (T3; tri-iodothyronine), apo-TRs associate with co-repressors to repress transcription, whereas in the presence of T3, holo-TRs engage transcriptional coactivators. Although many studies have addressed the molecular mechanisms of T3 action, it is not known how specific physiological responses arise. We used T3-dependent amphibian metamorphosis to analyse how TRs interact with particular co-regulators to differentially regulate gene expression during development. Using chromatin immuno-precipitation to study tissue from pre-metamorphic tadpoles, we found that TRs are physically associated with T3-responsive promoters, whether or not T3 is present. Addition of T3 results in histone H4 acetylation specifically on T3-response genes. Most importantly, we show that individual T3-response genes have distinct co-regulator requirements, the T3-dependent co-repressor-to-coactivator switch being gene-specific for both co-regulator categories.

Review of the in vivo functions of the p160 steroid receptor coactivator family

The p160 steroid receptor coactivator (SRC) gene family contains three homologous members, which serve as transcriptional coactivators for nuclear receptors and certain other transcription factors. These coactivators interact with ligand-bound nuclear receptors to recruit histone acetyltransferases and methyltransferases to specific enhancer/promotor regions, which facilitates chromatin remodeling, assembly of general transcription factors, and transcription of target genes. This minireview summarizes our current knowledge about the molecular structures, molecular mechanisms, temporal and spatial expression patterns, and biological functions of the SRC family. In particular, this article highlights the roles of SRC-1 (NCoA-1), SRC-2 (GRIP1, TIF2, or NCoA-2) and SRC-3 (p/CIP, RAC3, ACTR, AIB1, or TRAM-1) in development, organ function, endocrine regulation, and nuclear receptor function, which are defined by characterization of the genetically manipulated animal models. Furthermore, this article also reviews our current understanding of the role of SRC-3 in breast cancer and discusses possible mechanisms for functional specificity and redundancy among SRC family members.

Dynamic inhibition of nuclear receptor activation by corepressor binding

Nuclear receptors adopt dramatically different conformations in the presence or absence of ligand, and such liganded (holo) and unliganded (apo) receptors are specifically recognized by transcriptional coactivators and corepressors, respectively. These two states likely exist in dynamic equilibrium, contrary to the conventional model of static off and on conformations. First, corepressor SMRT [for silencing mediator of thyroid hormone receptor (TR) and retinoic acid receptor (RAR)] inhibits the interaction of coactivator steroid receptor coactivator-1 with liganded TR/RAR. Second, SMRT enables receptors to adopt apo-form even in the presence of ligand, as demonstrated with limited proteolyses and decreased binding of radiolabeled retinoid to RAR. Finally, chromatin immunoprecipitation results indicate that SMRT and steroid receptor coactivator-1 dynamically compete for receptor bindings in vivo in the presence of ligand. These results suggest that corepressor binding can drive receptors to adopt the apo-state, even in the presence of ligand, and inhibit activated liganded (holo) nuclear receptors in vivo.

Inhibitory cross-talk between estrogen receptor (ER) and constitutively activated androstane receptor (CAR). CAR inhibits ER-mediated signaling pathway by squelching p160 coactivators

Estrogen receptor (ER) activity can be modulated by the action of other nuclear receptors. To study whether ER activity is altered by orphan nuclear receptors that mediate the cellular response to xenobiotics, cross-talk between ER and constitutive androstane receptor (CAR), steroid and xenobiotic receptor, or peroxisome proliferator-activated receptor gamma was examined in HepG2 cells. Of these receptors, CAR substantially inhibited ER-mediated transcriptional activity of the vitellogenin B1 promoter as well as a synthetic estrogen responsive element (ERE)-containing promoter. Treatment with an agonist of CAR, 1,4-bis-(2-(3,5-dichloropyridoxyl))benzene, potentiated CAR-mediated transcriptional repression. In contrast, an antagonist of CAR, androstenol, alleviated the repression effect. Although CAR interacted with the ER in solution, CAR did not interact with the ER bound to the ERE. CAR/retinoid X receptor bound to the ERE but with much lower affinity than ER. Incremental amounts of CAR elicited a progressive reduction of the ER activity induced by the p160 coactivator glucocorticoid receptor interacting protein 1 (GRIP-1). In turn, increasing amounts of GRIP-1 progressively reversed the depression of ER activity by CAR. An agonist or antagonist of CAR potentiated or alleviated, respectively, the CAR-mediated repression of the GRIP-1-enhanced ER activity, which is consistent with the ability of theses ligands to increase or decrease, respectively, the interaction of CAR with GRIP-1. A CAR mutant that did not interact with GRIP-1 did not inhibit ER-mediated transactivation. Our data demonstrate that xenobiotic nuclear receptor CAR antagonizes ER-mediated transcriptional activity by squelching limiting amounts of p160 coactivator and imply that xenobiotics may influence ER function of female reproductive physiology, cell differentiation, tumorigenesis, and lipid metabolism.

Glucocorticoid receptor-interacting protein 1 mediates ligand-independent nuclear translocation and activation of constitutive androstane receptor in vivo

Phenobarbital (PB) induction of CYP2B genes is mediated by translocation of the constitutively active androstane receptor (CAR) to the nucleus. Interaction of CAR with p160 coactivators and enhancement of CAR transactivation by the coactivators have been shown in cultured cells. In the present studies, the interaction of CAR with the p160 coactivator glucocorticoid receptor-interacting protein 1 (GRIP1) was examined in vitro and in vivo. Binding of GRIP1 to CAR was shown by glutathione S-transferase (GST) pull-down and affinity DNA binding. N- or C-terminal fragments of GRIP1 that contained the central receptor-interacting domain bound to GST-CAR, but the presence of ligand increased the binding to GST-CAR of only the fragments containing the C-terminal region. In gel shift analysis, binding to CAR was observed only with GRIP1 fragments containing the C-terminal region, and the binding was increased by a CAR agonist and decreased by a CAR antagonist. Expression of GRIP1 enhanced CAR-mediated transactivation in cultured hepatic-derived cells 2-3-fold. In hepatocytes transfected in vivo, expression of exogenous GRIP1 alone induced transactivation of the CYP2B1 PB-dependent enhancer 15-fold, whereas CAR expression alone resulted in only a 3-fold enhancement in untreated mice. Remarkably, CAR and GRIP1 together synergistically transactivated the enhancer about 150-fold, which is approximately equal to activation by PB treatment. In PB-treated mice, expression of exogenous CAR alone had little effect, expression of GRIP1 increased transactivation about 2-fold, and with CAR and GRIP, a 4-fold activation was observed. In untreated mice, expression of GRIP resulted in nuclear translocation of green fluorescent protein-CAR. These results strongly suggest that a p160 coactivator functions in CAR-mediated transactivation in vivo in response to PB treatment and that the synergistic activation of CAR by GRIP in untreated animals results from both nuclear translocation and activation of CAR.

p300 requires its histone acetyltransferase activity and SRC-1 interaction domain to facilitate thyroid hormone receptor activation in chromatin

We have characterized the mechanism by which coactivator p300 facilitates transcriptional activation mediated by the heterodimer of thyroid hormone (T3) receptor and 9-cis retinoid acid receptor (TR-RXR) in the context of chromatin. We demonstrate that, while p300 can enhance the transcriptional activation mediated by both liganded TR-RXR and GAL4-VP16, its histone acetyltransferase activity (HAT) is required for its ability to facilitate liganded TR-RXR- but not GAL4-VP16-mediated transcriptional activation. To understand how p300 is recruited by liganded TR-RXR, we have analyzed the interactions between TR-RXR and p300 as well as SRC-1 family coactivators. We show that, in contrast to a strong hormone-dependent interaction between TR-RXR and SRC-1 family coactivators, p300 displays minimal, if any, T3-dependent interaction with TR-RXR. However, p300 can be recruited by liganded TR-RXR through its interaction with SRC-1 family coactivators. Consistent with the protein-protein interaction profile described above, we demonstrate that the SRC-1 interaction domain of p300 is important for its ability to facilitate transcriptional activation mediated by TR-RXR, whereas its nuclear receptor interaction domain is dispensable. Collectively, these results reveal the functional significance of the HAT activity of p300 and define an indirect mode for the action of p300 in TR-RXR activation.

XTIF2, a Xenopus homologue of the human transcription intermediary factor, is required for a nuclear receptor pathway that also interacts with CBP to suppress Brachyury and XMyoD

Ligand-bound nuclear receptors (NRs) recruit cofactors such as members of the p160 family and CREB-binding protein (CBP) to activate transcription. We have cloned the Xenopus homologue of the human transcription intermediary factor 2 (TIF2), a member of the p160 family of cofactors. Xenopus TIF2 (XTIF2) mRNA is expressed homogeneously during late blastula-early gastrula stages and later becomes highly expressed in the notochord. To study the function of XTIF2 during development, we have used two dominant negative constructs, one encompassing the NR-binding domain and the other the CBP interacting region of XTIF2. Overexpression of the XTIF2 dominant negative mRNAs causes ectopic expression of Xenopus Brachyury (Xbra) and MyoD in all tissue layers. Moreover, ectopic expression of the dominant negative construct that contains the CBP-binding region produces strong phenotypes at hatching stage such as loss of head structures, shortened trunks and open blastopores, which can be rescued by XTIF2 coexpression. These observed defects are due, at least in part, to repression of dorsal mesoderm and endoderm genes. Our data suggest the existence of a NR pathway that requires XTIF2 and CBP to repress Xbra and XMyoD.

A nuclear factor, ASC-2, as a cancer-amplified transcriptional coactivator essential for ligand-dependent transactivation by nuclear receptors in vivo

Many transcription coactivators interact with nuclear receptors in a ligand- and C-terminal transactivation function (AF2)-dependent manner. We isolated a nuclear factor (designated ASC-2) with such properties by using the ligand-binding domain of retinoid X receptor as a bait in a yeast two-hybrid screening. ASC-2 also interacted with other nuclear receptors, including retinoic acid receptor, thyroid hormone receptor, estrogen receptor alpha, and glucocorticoid receptor, basal factors TFIIA and TBP, and transcription integrators CBP/p300 and SRC-1. In transient cotransfections, ASC-2, either alone or in conjunction with CBP/p300 and SRC-1, stimulated ligand-dependent transactivation by wild type nuclear receptors but not mutant receptors lacking the AF2 domain. Consistent with an idea that ASC-2 is essential for the nuclear receptor function in vivo, microinjection of anti-ASC-2 antibody abrogated the ligand-dependent transactivation of retinoic acid receptor, and this repression was fully relieved by coinjection of ASC-2-expression vector. Surprisingly, ASC-2 was identical to a gene previously identified during a search for genes amplified and overexpressed in breast and other human cancers. From these results, we concluded that ASC-2 is a bona fide transcription coactivator molecule of nuclear receptors, and its altered expression may contribute to the development of cancers.

Activating signal cointegrator 1, a novel transcription coactivator of nuclear receptors, and its cytosolic localization under conditions of serum deprivation

Activating signal cointegrator 1 (ASC-1) harbors an autonomous transactivation domain that contains a putative zinc finger motif which provides binding sites for basal transcription factors TBP and TFIIA, transcription integrators steroid receptor coactivator 1 (SRC-1) and CBP-p300, and nuclear receptors, as demonstrated by the glutathione S-transferase pull-down assays and the yeast two-hybrid tests. The ASC-1 binding sites involve the hinge domain but not the C-terminal AF2 core domain of nuclear receptors. Nonetheless, ASC-1 appears to require the AF2-dependent factors to function (i.e., CBP-p300 and SRC-1), as suggested by the ability of ASC-1 to coactivate nuclear receptors, either alone or in cooperation with SRC-1 and p300, as well as its inability to coactivate a mutant receptor lacking the AF2 core domain. By using indirect immunofluorescence, we further show that ASC-1, a nuclear protein, is localized to the cytoplasm under conditions of serum deprivation but is retained in the nucleus when it is serum starved in the presence of ligand or coexpressed CBP or SRC-1. These results suggest that ASC-1 is a novel coactivator molecule of nuclear receptors which functions in conjunction with CBP-p300 and SRC-1 and may play an important role in establishing distinct coactivator complexes under different cellular conditions.

Bcl3, an IkappaB protein, as a novel transcription coactivator of the retinoid X receptor

We have recently shown that the IkappaB protein IkappaBbeta interacted with the retinoid X receptor (RXR) and inhibited the 9-cis-retinoic acid (RA)-dependent transactivations (Na, S.-Y., Kim, H.-J., Lee, S.-K., Choi, H.-S., Na, D. S., Lee, M.-O., Chung, M., Moore, D. D., and Lee, J. W. (1998) J. Biol. Chem. 6, 3212-3215). Herein, we show that a distinct IkappaB protein Bcl3 also interacts with RXR, as shown in the yeast two-hybrid tests and glutathione S-transferase pull-down assays. The Bcl3 interaction involved two distinct subregions of RXR, i.e. constitutive interactions of the N-terminal ABC domains and 9-cis-RA-dependent interactions of the C-terminal DEF domains. In contrast to IkappaBbeta, Bcl3 did not interact with the AF2 domain of RXR. Bcl3 specifically interacted with the general transcription factors TFIIB, TBP, and TFIIA but not with TFIIEalpha in the GST pull-down assays. TBP and TFIIA, however, were not able to interact with IkappaBbeta. Accordingly, Bcl3 coactivated the 9-cis-RA-induced transactivations of RXR, in contrast to the inhibitory actions of IkappaBbeta. In addition, coexpression of SRC-1 but not p300 further stimulated the Bcl3-mediated enhancement of the 9-cis-RA-induced transactivations of RXR. These results suggest that distinct IkappaB proteins differentially modulate the 9-cis-RA-induced transactivations of RXR in vivo.

Steroid receptor coactivator-1 interacts with serum response factor and coactivates serum response element-mediated transactivations

Steroid receptor coactivator-1 (SRC-1) specifically bound to serum response factor (SRF), as demonstrated by glutathione S-transferase pull down assays, and the yeast and mammalian two-hybrid tests. In mammalian cells, SRC-1 potentiated serum response element (SRE)-mediated transactivations in a dose-dependent manner. Coexpression of p300 synergistically enhanced this SRC-1-potentiated level of transactivations, consistent with the recent finding (Ramirez, S., Ali, S. A. S., Robin, P., Trouche, D., and Harel-Bellan, A. (1997) J. Biol. Chem. 272, 31016-31021) in which the p300 homologue CREB-binding protein was shown to be a transcription coactivator of SRF. Thus, we concluded that at least two distinct classes of coactivator molecules may cooperate to regulate SRF-dependent transactivations in vivo.

Estrogen receptor, a common interaction partner for a subset of nuclear receptors

Nuclear receptors regulate transcription by binding to specific DNA response elements as homodimers or heterodimers. Herein, the yeast and mammalian two-hybrid tests as well as glutathione-S-transferase pull-down assays were exploited to demonstrate that estrogen receptor (ER) directly binds to a subset of nuclear receptors through protein-protein interactions between ligand-binding domains. These receptors include hepatocyte nuclear factor 4, thyroid hormone receptor (TR), retinoic acid receptor (RAR), ERbeta, and retinoid X receptor (RXR). In yeast cells, a LexA fusion protein to the human ER ligand-binding domain (LexA/ER-LBD) was an inert transactivator of a LacZ reporter gene controlled by upstream LexA-binding sites. However, LexA/ER-LBD differentially modulated the LacZ reporter gene expression when coexpressed with native TRs, RARs, or RXRs. Similarly, cotransfection of these receptors in CV1 cells up- or down-regulated transactivations by ER. From these results, we propose that ER is a common interaction partner for a subset of receptors, and these interactions should mediate novel signaling pathways in vivo.