Ligand induction of a transcriptionally active thyroid hormone receptor coactivator complex

Thyroid hormone receptor-associated proteins and general positive cofactors mediate thyroid hormone receptor function in the absence of the TATA box-binding protein-associated factors of TFIID

Coactivators previously implicated in ligand-dependent activation functions by thyroid hormone receptor (TR) include p300 and CREB-binding protein (CBP), the steroid receptor coactivator-1 (SRC-1)-related family of proteins, and the multicomponent TR-associated protein (TRAP) complex. Here we show that two positive cofactors (PC2 and PC4) derived from the upstream stimulatory activity (USA) cofactor fraction act synergistically to mediate thyroid hormone (T3)-dependent activation either by TR or by a TR-TRAP complex in an in vitro system reconstituted with purified factors and DNA templates. Significantly, the TRAP-mediated enhancement of activation by TR does not require the TATA box-binding protein-associated factors of TFIID. Furthermore, neither the pleiotropic coactivators CBP and p300 nor members of the SRC-1 family were detected in either the TR-TRAP complex or the other components of the in vitro assay system. These results show that activation by TR at the level of naked DNA templates is enhanced by cooperative functions of the TRAP coactivators and the general coactivators PC2 and PC4, and they further indicate a potential functional redundancy between TRAPs and TATA box-binding protein-associated factors in TFIID. In conjunction with earlier studies on other nuclear receptor-interacting cofactors, the present study also suggests a multistep pathway, involving distinct sets of cofactors, for activation of hormone responsive genes.

Identity between TRAP and SMCC complexes indicates novel pathways for the function of nuclear receptors and diverse mammalian activators

The human thyroid hormone receptor-associated protein (TRAP) complex, an earlier described coactivator for nuclear receptors, and an SRB- and MED-containing cofactor complex (SMCC) that mediates activation by Gal4-p53 are shown to be virtually the same with respect to specific polypeptide subunits, coactivator functions, and mechanisms of action (activator interactions). In parallel with ligand-dependent interactions of nuclear receptors with the TRAP220 subunit, p53 and VP16 activation domains interact directly with a newly cloned TRAP80 subunit. These results indicate novel pathways for the function of nuclear receptors and other activators (p53 and VP16) through a common coactivator complex that is likely to target RNA polymerase II. Identification of the TRAP230 subunit as a previously predicted gene product also suggests a coactivator-related transcription defect in certain disease states.

Regulation of embryonic/fetal globin genes by nuclear hormone receptors: a novel perspective on hemoglobin switching

The CCAAT box is one of the conserved motifs found in globin promoters. It binds the CP1 protein. We noticed that the CCAAT-box region of embryonic/fetal, but not adult, globin promoters also contains one or two direct repeats of a short motif analogous to DR-1 binding sites for non-steroid nuclear hormone receptors. We show that a complex previously named NF-E3 binds to these repeats. In transgenic mice, destruction of the CCAAT motif within the human epsilon-globin promoter leads to substantial reduction in epsilon expression in embryonic erythroid cells, indicating that CP1 activates epsilon expression; in contrast, destruction of the DR-1 elements yields striking epsilon expression in definitive erythropoiesis, indicating that the NF-E3 complex acts as a developmental repressor of the epsilon gene. We also show that NF-E3 is immunologically related to COUP-TF orphan nuclear receptors. One of these, COUP-TF II, is expressed in embryonic/fetal erythroid cell lines, murine yolk sac, intra-embryonic splanchnopleura and fetal liver. In addition, the structure and abundance of NF-E3/COUP-TF complexes vary during fetal liver development. These results elucidate the structure as well as the role of NF-E3 in globin gene expression and provide evidence that nuclear hormone receptors are involved in the control of globin gene switching.

A novel human SRB/MED-containing cofactor complex, SMCC, involved in transcription regulation

A novel human complex that can either repress activator-dependent transcription mediated by PC4, or, at limiting TFIIH, act synergistically with PC4 to enhance activator-dependent transcription has been purified. This complex contains homologs of a subset of yeast mediator/holoenzyme components (including SRB7, SRB10, SRB11, MED6, and RGR1), homologs of other yeast transcriptional regulatory factors (SOH1 and NUT2), and, significantly, some components (TRAP220, TRAP170/hRGR1, and TRAP100) of a human thyroid hormone receptor-associated coactivator complex. The complex shows direct activator interactions but, unlike yeast mediator, can act independently of the RNA polymerase II CTD. These findings demonstrate both positive and negative functional capabilities for the human complex, emphasize novel (CTD-independent) regulatory mechanisms, and link the complex to other human coactivator complexes.

Determinants of coactivator LXXLL motif specificity in nuclear receptor transcriptional activation

Ligand-dependent activation of gene transcription by nuclear receptors is dependent on the recruitment of coactivators, including a family of related NCoA/SRC factors, via a region containing three helical domains sharing an LXXLL core consensus sequence, referred to as LXDs. In this manuscript, we report receptor-specific differential utilization of LXXLL-containing motifs of the NCoA-1/SRC-1 coactivator. Whereas a single LXD is sufficient for activation by the estrogen receptor, different combinations of two, appropriately spaced, LXDs are required for actions of the thyroid hormone, retinoic acid, peroxisome proliferator-activated, or progesterone receptors. The specificity of LXD usage in the cell appears to be dictated, at least in part, by specific amino acids carboxy-terminal to the core LXXLL motif that may make differential contacts with helices 1 and 3 (or 3') in receptor ligand-binding domains. Intriguingly, distinct carboxy-terminal amino acids are required for PPARgamma activation in response to different ligands. Related LXXLL-containing motifs in NCoA-1/SRC-1 are also required for a functional interaction with CBP, potentially interacting with a hydrophobic binding pocket. Together, these data suggest that the LXXLL-containing motifs have evolved to serve overlapping roles that are likely to permit both receptor-specific and ligand-specific assembly of a coactivator complex, and that these recognition motifs underlie the recruitment of coactivator complexes required for nuclear receptor function.

Activation of the prolactin gene by peroxisome proliferator-activated receptor-alpha appears to be DNA binding-independent

Although the effects of the peroxisome proliferator-activated receptors (PPARs) have been studied primarily in adipocytes and liver, the wide distribution of these receptors suggests that they might also play a role in other cell types. We present evidence that PPAR activators stimulate the expression of the prolactin gene in pituitary GH4C1 cells. Transfection assays in non-pituitary HeLa cells showed that stimulation of the prolactin promoter by PPARalpha requires the presence of the transcription factor GHF-1 (or Pit-1). Proximal promoter sequences confer responsiveness to PPARalpha, and activation by this receptor is lost concomitantly with the response to GHF-1. Surprisingly, expression of the retinoid X receptor (RXR) abolishes stimulation by PPARalpha. Furthermore, the promoter region that confers PPARalpha responsiveness does not contain a PPAR response element. This suggests that the transcriptional effect of PPARalpha might be mediated by protein-protein interactions rather than by binding of PPAR/RXR to the promoter. A direct interaction between PPARalpha and GHF-1 was confirmed by in vitro binding studies. Expression of the coactivators SRC-1 and CREB-binding protein, which bind to PPAR, also enhanced the responsiveness of the prolactin promoter to PPARalpha. Furthermore, CREB-binding protein also significantly increased activation by GHF-1, and both proteins associated in vitro. Thus, PPARalpha, a receptor that normally acts as a ligand-dependent transcription factor by binding to specific DNA sequences in one context, can also stimulate the prolactin promoter by association with GHF-1 and coactivator proteins.

Alteration of nucleosome structure as a mechanism of transcriptional regulation

The nucleosome, which is the primary building block of chromatin, is not a static structure: It can adopt alternative conformations. Changes in solution conditions or changes in histone acetylation state cause nucleosomes and nucleosomal arrays to behave with altered biophysical properties. Distinct subpopulations of nucleosomes isolated from cells have chromatographic properties and nuclease sensitivity different from those of bulk nucleosomes. Recently, proteins that were initially identified as necessary for transcriptional regulation have been shown to alter nucleosomal structure. These proteins are found in three types of multiprotein complexes that can acetylate nucleosomes, deacetylate nucleosomes, or alter nucleosome structure in an ATP-dependent manner. The direct modification of nucleosome structure by these complexes is likely to play a central role in appropriate regulation of eukaryotic genes.

Distinct steady-state nuclear receptor coregulator complexes exist in vivo

Transcriptional regulation by members of the nuclear hormone receptor superfamily is a modular process requiring the mediation of distinct subclasses of coregulators. These subclasses include members of the steroid receptor coactivator-1 (SRC-1) coactivator family, p300/CBP and their associated proteins, such as p300/CBP-associated factor, human homologs of SWI/SNF proteins such as BRG-1, and the less well-characterized E3 ubiquitin-protein ligases E6 papillomavirus protein-associated protein and receptor-potentiating factor-1. Because functional studies indicate that these coregulators may form higher order complexes, we analyzed steady-state complexes of different coregulator subclasses in vivo. T47D and HeLa cell lysates were subjected to biochemical fractionation and screened by immunoblotting using coregulator-specific antibodies. We show that different subclasses of nuclear receptor coregulators exhibit distinct fractionation profiles. Furthermore, evidence is provided that SRC-1 family members may exist in vivo in heteromultimeric forms with each other. In addition, we demonstrate that liganded PR is present in stable complexes containing SRC-1 and transcription intermediary factor 2 (TIF2) in vivo. Our results suggest that the assembly of large, modular transcriptional complexes by recruitment of distinct subclasses of preformed coregulator subcomplexes may be involved in transcriptional regulation by activated nuclear receptors.

Coactivators and corepressors as mediators of nuclear receptor function: an update

The past 3 years have been an exciting time in the field of hormone receptor research because of the discovery and characterization of novel groups of proteins that mediate the transcriptional activity of steroid receptors. These classes of proteins, called coactivators and corepressors, have greatly enhanced our understanding of how steroid receptors activate or inhibit transcription of their target genes. Multiple coactivators have been identified that fit the definition of a protein that connects or bridges the DNA-bound receptor to proteins in the preinitiation complex and thereby enhance transcription. Besides this bridging function, some coactivators can modify chromatin by histone acetylation and make promoters more accessible for the binding of other transcription factors. This finding explains old data concerning steroid receptor-induced nucleosome displacement and indicates a dual role for coactivators as bridging factors and chromatin remodeling proteins. The opposites of coactivators are corepressors, which are recruited into the receptor-DNA-bound complex in the absence of ligand and actively inhibit transcription of the target gene. Although unliganded steroid receptors are associated with heat shock proteins and do not bind to their response elements, the binding of antagonists to these receptors can result in the recruitment of corepressors. The expression level and repertoire of coactivators and corepressors have become important determinants in the functional activity of steroid hormones and their receptors.

The thyroid hormone receptor functions as a ligand-operated developmental switch between proliferation and differentiation of erythroid progenitors

The avian erythroblastosis virus (AEV) oncoprotein v-ErbA represents a mutated, oncogenic thyroid hormone receptor alpha (c-ErbA/ TRalpha). v-ErbA cooperates with the stem cell factor-activated, endogenous receptor tyrosine kinase c-Kit to induce self-renewal and to arrest differentiation of primary avian erythroblasts, the AEV transformation target cells. In this cooperation, v-ErbA substitutes for endogenous steroid hormone receptor function required for sustained proliferation of non-transformed erythroid progenitors. In this paper, we propose a novel concept of how v-ErbA transforms erythroblasts. Using culture media strictly depleted from thyroid hormone (T3) and retinoids, the ligands for c-ErbA/TRalpha and its co-receptor RXR, we show that overexpressed, unliganded c-ErbA/ TRalpha closely resembles v-ErbA in its activity on primary erythroblasts. In cooperation with ligand-activated c-Kit, c-ErbA/ TRalpha causes steroid-independent, long-term proliferation and tightly blocks differentiation. Activation of c-ErbA/ TRalpha by physiological T3 levels causes the loss of self-renewal capacity and induces synchronous, terminal differentiation under otherwise identical conditions. This T3-induced switch in erythroid progenitor development is correlated with a decrease of c-ErbA-associated histone deacetylase activity. Our results suggest that the crucial role of the mutations activating v-erbA as an oncogene is to 'freeze' c-ErbA/ TRalpha in its non-liganded, repressive conformation and to facilitate its overexpression.

Therapeutic potential of selective modulators of nuclear receptor action

Nuclear receptors belong to a superfamily of ligand-inducible transcription factors that, in addition to directly regulating their cognate gene programs, can also mutually interfere with other signaling pathways. The recent identification of selective agonists/antagonists of the glucocorticoid, retinoid and estrogen receptors suggests that it might be possible to selectively elicit only a subset of the nuclear receptor functions that are induced by the natural ligand, with the aim of increasing the functional and, perhaps, tissue selectivity of nuclear receptor ligands and reducing unwanted side effects.

The TRAP220 component of a thyroid hormone receptor- associated protein (TRAP) coactivator complex interacts directly with nuclear receptors in a ligand-dependent fashion

Cognate cDNAs are described for 2 of the 10 thyroid hormone receptor-associated proteins (TRAPs) that are immunopurified with thyroid hormone receptor alpha (TRalpha) from ligand-treated HeLa (alpha-2) cells. Both TRAP220 and TRAP100 contain LXXLL domains found in other nuclear receptor-interacting proteins and both appear to reside in a single complex with other TRAPs (in the absence of TR). However, only TRAP220 shows a direct ligand-dependent interaction with TRalpha, and these interactions are mediated through the C terminus of TRalpha and (at least in part) the LXXLL domains of TRAP220. TRAP220 also interacts with other nuclear receptors [vitamin D receptor, retinoic acid receptor alpha, retinoid X receptor alpha, peroxisome proliferation-activated receptor (PPAR) alpha, PPARgamma and, to a lesser extent, estrogen receptor] in a ligand-dependent manner, whereas TRAP100 shows only marginal interactions with estrogen receptor, retinoid X receptor alpha, PPARalpha, and PPARgamma. Consistent with these results, TRAP220 moderately stimulates human TRalpha-mediated transcription in transfected cells, whereas a fragment containing the LXXLL motifs acts as a dominant negative inhibitor of nuclear receptor-mediated transcription both in transfected cells (TRalpha) and in cell free transcription systems (TRalpha and vitamin D receptor). These studies indicate that TRAP220 plays a major role in anchoring other TRAPs to TRalpha during the function of the TRalpha-TRAP complex and, further, that TRAP220 (possibly along with other TRAPs) may be a global coactivator for the nuclear receptor superfamily.

A novel protein complex that interacts with the vitamin D3 receptor in a ligand-dependent manner and enhances VDR transactivation in a cell-free system

Nuclear receptors transduce hormonal signals by binding directly to DNA target sites in promoters and modulating the transcription of linked genes. Receptor-mediated transactivation appears to be potentiated in response to ligand by a number of coactivators that may provide key interactions with components of the transcription preinitiation complex and/or alter chromatin structure. Here, we use the vitamin D3 receptor ligand-binding domain (VDR LBD) as an affinity matrix to identify components of a transcriptionally active nuclear extract that interact with VDR in response to ligand. We describe the purification of a complex of at least 10 VDR interacting proteins (DRIPs) ranging from 65 to 250 kD that associate with the receptor in a strictly 1,25-dihydroxyvitamin D3-dependent manner. These proteins also appear to interact with other, but not all, nuclear receptors, such as the thyroid hormone receptor. The DRIPs are distinct from known nuclear receptor coactivators, although like these coactivators, their interaction also requires the AF-2 transactivation motif of VDR. In addition, the DRIP complex contains histone acetyltransferase activity, indicating that at least one or more of the DRIPs may function at the level of nucleosomal modification. However, we show that the DRIPs selectively enhance the transcriptional activity of VDR on a naked DNA template utilizing a cell-free, ligand-dependent transcription assay. Moreover, this activity can be specifically depleted from the extract by liganded, but not unliganded, VDR-LBD. Overexpression of DRIP100 in vivo resulted in a strong squelching of VDR transactivation, suggesting the sequestration of other limiting factors, including components of the DRIP complex. These results demonstrate the existence of a new complex of novel functional nuclear receptor coactivators.

Molecular genetics of the RNA polymerase II general transcriptional machinery

Transcription initiation by RNA polymerase II (RNA pol II) requires interaction between cis-acting promoter elements and trans-acting factors. The eukaryotic promoter consists of core elements, which include the TATA box and other DNA sequences that define transcription start sites, and regulatory elements, which either enhance or repress transcription in a gene-specific manner. The core promoter is the site for assembly of the transcription preinitiation complex, which includes RNA pol II and the general transcription fctors TBP, TFIIB, TFIIE, TFIIF, and TFIIH. Regulatory elements bind gene-specific factors, which affect the rate of transcription by interacting, either directly or indirectly, with components of the general transcriptional machinery. A third class of transcription factors, termed coactivators, is not required for basal transcription in vitro but often mediates activation by a broad spectrum of activators. Accordingly, coactivators are neither gene-specific nor general transcription factors, although gene-specific coactivators have been described in metazoan systems. Transcriptional repressors include both gene-specific and general factors. Similar to coactivators, general transcriptional repressors affect the expression of a broad spectrum of genes yet do not repress all genes. General repressors either act through the core transcriptional machinery or are histone related and presumably affect chromatin function. This review focuses on the global effectors of RNA polymerase II transcription in yeast, including the general transcription factors, the coactivators, and the general repressors. Emphasis is placed on the role that yeast genetics has played in identifying these factors and their associated functions.

Ligand- and nuclear factor-dependent change in hydrophobicity of thyroid hormone beta1 receptor

An aqueous two-phase partitioning assay was performed using in vitro translated human thyroid hormone beta1 receptor (TRbeta1). Wild-type TRbeta1 was less hydrophobic in the presence of both triiodothyronine (T3) and nuclear extract. This reflects a conformational change, or change in electrostatic properties, of the TRbeta1-nuclear factor complex as a result of T3 binding. Mutant TRbeta1s with reduced T3 binding affinity required a higher concentration of T3 for the shift of hydrophobicity, and a mutant without T3 binding activity did not show any shift, even in the presence of 1 mM T3. The unique mutant receptor, R243Q, has impaired transcriptional function despite virtually normal binding affinity for T3. When this mutant was examined in this assay, the shift of hydrophobicity was significantly impaired even in the presence of both nuclear extract and a high concentration of T3. Nuclear extract of COS1 cells did not affect the T3-binding affinity of R243Q. These results indicate that the R243Q mutant has impaired a ligand-dependent conformational change and interaction with nuclear factor(s). Inability of R243Q to interact normally with nuclear factor(s) may explain, in part, the molecular mechanism of discordance between ligand binding and transactivation function of this mutant.

The receptor-associated coactivator 3 activates transcription through CREB-binding protein recruitment and autoregulation

Transcriptional coactivators are involved in gene activation by nuclear hormone receptors. The receptor-associated coactivator 3 (RAC3) was recently identified to be highly related to the steroid receptor coactivator-1 and transcriptional intermediate factor 2, thereby establishing a novel family of nuclear receptor coactivators. In this study, we identified a RAC3 fragment containing three LXXLL motifs conserved among this family, which is sufficient to mediate nuclear receptor interaction in vivo and in vitro. Point mutations that disrupt ligand-dependent activation function of the receptor inhibited the interaction. We found that a 162-amino acid fragment of RAC3 conferred transcriptional activation and recruited the CREB-binding protein and that three distinct LXXLL motifs mediated the transcriptional activation. A trimeric far Western analysis demonstrated the formation of a ternary complex containing CREB-binding protein, RAC3, and the receptor. In addition, we showed that RAC3, transcriptional intermediate factor 2, and steroid receptor coactivator-1 are expressed in specific tissues and cancer cells and that RAC3 transcript is directly up-regulated by retinoid treatment. These results suggest that RAC3 may contribute to amplified transcriptional responses through both recruitment of additional coactivators and autoregulation by the receptor-coactivator complex.

Distinct requirements for chromatin assembly in transcriptional repression by thyroid hormone receptor and histone deacetylase

Histone deacetylase and chromatin assembly contribute to the control of transcription of the Xenopus TRbetaA gene promoter by the heterodimer of Xenopus thyroid hormone receptor and 9-cis retinoic acid receptor (TR-RXR). Addition of the histone deacetylase inhibitor Trichostatin A (TSA) relieves repression of transcription due to chromatin assembly following microinjection of templates into Xenopus oocyte nuclei, and eliminates regulation of transcription by TR-RXR. Expression of Xenopus RPD3p, the catalytic subunit of histone deacetylase, represses the TRbetaA promoter, but only after efficient assembly of the template into nucleosomes. In contrast, the unliganded TR-RXR represses templates only partially assembled into nucleosomes; addition of TSA also relieves this transcriptional repression. This result indicates the distinct requirements for chromatin assembly in mediating transcriptional repression by the deacetylase alone, compared with those needed in the presence of unliganded TR-RXR. In addition, whereas hormone-bound TR-RXR targets chromatin disruption as assayed through changes in minichromosome topology and loss of a regular nucleosomal ladder on micrococcal nuclease digestion, addition of TSA relieves transcriptional repression but does not disrupt chromatin. Thus, TR-RXR can facilitate transcriptional repression in the absence of hormone through mechanisms in addition to recruitment of deacetylase, and disrupts chromatin structure through mechanisms in addition to the inhibition or release of deacetylase.

An inhibitory region of the DNA-binding domain of thyroid hormone receptor blocks hormone-dependent transactivation

We have employed a chimeric receptor system in which we cotransfected yeast GAL4 DNA-binding domain/retinoid X receptor beta ligand-binding domain chimeric receptor (GAL4RXR), thyroid hormone receptor-beta (TRbeta), and upstream activating sequence-reporter plasmids into CV-1 cells to study repression, derepression, and transcriptional activation. In the absence of T3, unliganded TR repressed transcription to 20% of basal level, and in the presence of T3, liganded TRbeta derepressed transcription to basal level. Using this system and a battery of TRbeta mutants, we found that TRbeta/RXR heterodimer formation is necessary and sufficient for basal repression and derepression in this system. Additionally, an AF-2 domain mutant (E457A) mediated basal repression but not derepression, suggesting that interaction with a putative coactivator at this site may be critical for derepression. Interestingly, a mutant containing only the TRbeta ligand binding domain (LBD) not only mediated derepression, but also stimulated transcriptional activation 10-fold higher than basal level. Studies using deletion and domain swap mutants localized an inhibitory region to the TRbeta DNA-binding domain. Titration studies further suggested that allosteric changes promoting interaction with coactivators may account for enhanced transcriptional activity by LBD. In summary, our findings suggest that TR heterodimer formation with RXR is important for repression and derepression, and coactivator interaction with the AF-2 domain may be needed for derepression in this chimeric system. Additionally, there may be an inhibitory region in the DNA-binding domain, which reduces TR interaction with coactivators, and prevents full-length wild-type TRbeta from achieving transcriptional activation above basal level in this chimeric receptor system.

Chromatin remodeling regulated by steroid and nuclear receptors

Coactivators and corepressors regulate transcription by controlling interactions between sequence-specific transcription factors, the basal transcriptional machinery and the chromatin environment. This review consider the access of nuclear and steroid receptors to chromatin, their use of corepressors and coactivators to modify chromatin structure and the implications for transcriptional control. The assembly of specific nucleoprotein architectures and targeted histone modification emerge as central controlling elements for gene expression.

A thyroid hormone receptor coactivator negatively regulated by the retinoblastoma protein

The retinoblastoma protein (Rb) plays a critical role in cell proliferation, differentiation, and development. To decipher the mechanism of Rb function at the molecular level, we have systematically characterized a number of Rb-interacting proteins, among which is the clone C5 described here, which encodes a protein of 1,978 amino acids with an estimated molecular mass of 230 kDa. The corresponding gene was assigned to chromosome 14q31, the same region where genetic alterations have been associated with several abnormalities of thyroid hormone response. The protein uses two distinct regions to bind Rb and thyroid hormone receptor (TR), respectively, and thus was named Trip230. Trip230 binds to Rb independently of thyroid hormone while it forms a complex with TR in a thyroid hormone-dependent manner. Ectopic expression of the protein Trip230 in cells, but not a mutant form that does not bind to TR, enhances specifically TR-dependent transcriptional activity. Coexpression of wild-type Rb, but not mutant Rb that fails to bind to Trip230, inhibits such activity. These results not only identify a coactivator molecule that modulates TR activity, but also uncover a role for Rb in a pathway that responds to thyroid hormone.

Nuclear receptor coactivator ACTR is a novel histone acetyltransferase and forms a multimeric activation complex with P/CAF and CBP/p300

We report here the identification of a novel cofactor, ACTR, that directly binds nuclear receptors and stimulates their transcriptional activities in a hormone-dependent fashion. ACTR also recruits two other nuclear factors, CBP and P/CAF, and thus plays a central role in creating a multisubunit coactivator complex. In addition, and unexpectedly, we show that purified ACTR is a potent histone acetyltransferase and appears to define a distinct evolutionary branch to this recently described family. Thus, hormonal activation by nuclear receptors involves the mutual recruitment of at least three classes of histone acetyltransferases that may act cooperatively as an enzymatic unit to reverse the effects of histone deacetylase shown to be part of the nuclear receptor corepressor complex.

RAC3, a steroid/nuclear receptor-associated coactivator that is related to SRC-1 and TIF2

Steroids, thyroid hormones, vitamin D3, and retinoids are lipophilic small molecules that regulate diverse biological effects such as cell differentiation, development, and homeostasis. The actions of these hormones are mediated by steroid/nuclear receptors which function as ligand-dependent transcriptional regulators. Transcriptional activation by ligand-bound receptors is a complex process requiring dissociation and recruitment of several additional cofactors. We report here the cloning and characterization of receptor-associated coactivator 3 (RAC3), a human transcriptional coactivator for steroid/nuclear receptors. RAC3 interacts with several liganded receptors through a mechanism which requires their respective ligand-dependent activation domains. RAC3 can activate transcription when tethered to a heterologous DNA-binding domain. Overexpression of RAC3 enhances the ligand-dependent transcriptional activation by the receptors in mammalian cells. Sequence analysis reveals that RAC3 is related to steroid receptor coactivator 1 (SRC-1) and transcriptional intermediate factor 2 (TIF2), two of the most potent coactivators for steroid/nuclear receptors. Thus, RAC3 is a member of a growing coactivator network that should be useful as a tool for understanding hormone action and as a target for developing new therapeutic agents that can block hormone-dependent neoplasia.

Steroid receptor induction of gene transcription: a two-step model

Coactivators, such as steroid receptor coactivator 1 (SRC-1A) and CREB (cAMP response element binding protein)-binding protein (CBP), are required for efficient steroid receptor transactivation. Using an in vitro transcription assay, we found that progesterone receptor (PR)-driven transcription is inhibited by a dominant negative PR ligand-binding domain-interacting region of SRC-1A, indicating that SRC-1A is required for actual transcriptional processes. In addition, these coactivators also possess intrinsic histone acetyltransferase (HAT) activity and bind to each other and another HAT, p300/CBP-associated factor. Here we show that the human PR also interacts with p300/CBP-associated factor in vitro. Recruitment of multiple HATs to target promoters suggests an important role for chromatin remodeling in transcriptional activation of genes by steroid receptors. In transient transfection assays, we found that addition of a histone deacetylase inhibitor, trichostatin A, strongly potentiated PR-driven transcription. In contrast, directing histone deacetylase-1 (HD1) to a promoter using the GAL4 DNA binding domain inhibited transcription. Furthermore, PR transactivation was repressed by recruiting HD1 into the PR-DNA complex by fusing HD1 to a PR ligand-binding domain-interacting portion of SRC-1. Collectively, these results suggest that targeted histone acetylation by recruited HAT cofactors and histone deacetylation are important factors affecting PR transactivation. Recruitment of coactivators and HATs by the liganded PR in vivo may result in (i) remodeling of transcriptionally repressed chromatin to facilitate assembly and (ii) enhanced stabilization of the preinitiation complex by the activation functions of coactivators and the liganded PR itself.

Synergistic activation of transcription by CBP and p53

The tumour suppressor p53 is a transcriptional regulator whose ability to inhibit cell growth is dependent upon its transactivation function. Here we demonstrate that the transcription factor CBP, which is also implicated in cell proliferation and differentiation, acts as a p53 coactivator and potentiates its transcriptional activity. The amino-terminal activation domain of p53 interacts with the carboxy-terminal portion of the CBP protein both in vitro and in vivo. In transfected SaoS-2 cells, CBP potentiates activation of the mdm-2 gene by p53 and, reciprocally, p53 potentiates activation of a Gal4-responsive target gene by a Gal4(1-147)-CBP(1678-2441) fusion protein. A double point mutation that destroys the transactivation function of p53 also abolishes its binding to CBP and its synergistic function with CBP. The ability of p53 to interact physically and functionally with a coactivator (CBP) that has histone acetyltransferase activity and with components (TAFs) of the general transcription machinery indicates that it may have different functions in a multistep activation pathway.

The transcriptional co-activator p/CIP binds CBP and mediates nuclear-receptor function

The functionally conserved proteins CBP and p300 act in conjunction with other factors to activate transcription of DNA. A new factor, p/CIP, has been discovered that is present in the cell as a complex with CBP and is required for transcriptional activity of nuclear receptors and other CBP/p300-dependent transcription factors. The highly related nuclear-receptor co-activator protein NCoA-1 is also specifically required for ligand-dependent activation of genes by nuclear receptors. p/CIP, NCoA-1 and CBP all contain related leucine-rich charged helical interaction motifs that are required for receptor-specific mechanisms of gene activation, and allow the selective inhibition of distinct signal-transduction pathways.

Human TAF(II)135 potentiates transcriptional activation by the AF-2s of the retinoic acid, vitamin D3, and thyroid hormone receptors in mammalian cells

We report for the first time the cloning of a complete cDNA encoding the human TFIID subunit hTAF(II)135 (hTAF(II)130). Full-length hTAF(II)135 comprises 1083 amino acids and contains two conserved domains present also in dTAF(II)110 and hTAF(II)105. We show that expression of hTAF(II)135 in mammalian cells strongly and selectively potentiates transcriptional stimulation by the activation function-2 (AF-2) of the retinoic acid, thyroid hormone, and vitamin D3 receptors (RAR, TR, and VDR), but does not affect the AF-2s of the estrogen (ER) or retinoid X (RXR) receptors. The coactivator activity requires an hTAF(II)135 region that is located between the conserved domains but is itself not conserved in dTAF(II)110 and hTAF(II)105. Expression of hTAF(II)135 also stimulates RAR AF-2 activity when a promoter with a low-affinity TATA element (TGTA) is used, indicating that hTAF(II)135 overexpression compensates for the low-affinity of TBP for this promoter and may facilitate the recruitment of TFIID by the RAR AF-2.

Nuclear receptor repression mediated by a complex containing SMRT, mSin3A, and histone deacetylase

The transcriptional corepressors SMRT and N-CoR function as silencing mediators for retinoid and thyroid hormone receptors. Here we show that SMRT and N-CoR directly interact with mSin3A, a corepressor for the Mad-Max heterodimer and a homolog of the yeast global-transcriptional repressor Sin3p. In addition, we demonstrate that the recently characterized histone deacetylase 1 (HDAC1) interacts with Sin3A and SMRT to form a multisubunit repressor complex. Consistent with this model, we find that HDAC inhibitors synergize with retinoic acid to stimulate hormone-responsive genes and differentiation of myeloid leukemia (HL-60) cells. This work establishes a convergence of repression pathways for bHLH-Zip proteins and nuclear receptors and suggests this type of regulation may be more widely conserved than previously suspected.

Nuclear receptor coactivators

Retinoic acid, steroid and thyroid hormones regulate complex programs of gene expression by binding to intracellular receptors that are members of the nuclear receptor superfamily of ligand-dependent transcription factors. Recent studies have led to the identification and cloning of genes encoding coactivator molecules that appear to play important roles in mediating ligand-dependent transcription by members of this family. The identification of these coactivator molecules suggests a point of entry into the general transcriptional machinery that is common to several other classes of regulated transcription factors.

Retinoid X receptor:vitamin D3 receptor heterodimers promote stable preinitiation complex formation and direct 1,25-dihydroxyvitamin D3-dependent cell-free transcription

The numerous members of the steroid/nuclear hormone receptor superfamily act as direct transducers of circulating signals, such as steroids, thyroid hormone, and vitamin or lipid metabolites, and modulate the transcription of specific target genes, primarily as dimeric complexes. The receptors for 9-cis retinoic acid and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], RXR and VDR, respectively, as members of this superfamily, form a heterodimeric complex and bind cooperatively to vitamin D responsive elements (VDREs) to activate or repress the transcription of a multitude of genes which regulate a variety of physiological functions. To directly investigate RXR- and VDR-mediated transactivation, we developed a cell-free transcription system for 1,25(OH)2D3 signaling by utilizing crude nuclear extracts and a G-free cassette-based assay. Transcriptional enhancement in vitro was dependent on purified, exogenous RXR and VDR and was responsive to physiological concentrations of 1,25(OH)2D3. We found that RXR and VDR transactivated selectively from VDRE-linked templates exclusively as a heterodimeric complex, since neither receptor alone enhanced transcription in vitro. By the addition of low concentrations of the anionic detergent Sarkosyl to limit cell-free transcription to a single round and the use of agarose gel mobility shift experiments to assay factor complex assembly, we observed that 1,25(OH)2D3 enhanced RXR:VDR-mediated stabilization or assembly of preinitiation complexes to effect transcriptional enhancement from VDRE-linked promoter-containing DNA.

Mechanism of gene expression by the glucocorticoid receptor: role of protein-protein interactions

The glucocorticoid receptor belongs to an important class of transcription factors that alter the expression of target genes in response to a specific hormone signal. The glucocorticoid receptor can function at least at three levels: (1) recruitment of the general transcription machinery; (2) modulation of transcription factor action, independent of DNA binding, through direct protein-protein interactions; and (3) modulation of chromatin structure to allow the assembly of other gene regulatory proteins and/or the general transcription machinery on the DNA. This review will focus on the multifaceted nature of protein-protein interactions involving the glucocorticoid receptor and basal transcription factors, coactivators and other transcription factors, occurring at these different levels of regulation.