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 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.

The vitamin D receptor binds in a transcriptionally inactive form and without a defined polarity on a retinoic acid response element

Heterodimers of the vitamin D receptor (VDR) with the retinoid X receptor (RXR) bind in a transcriptionally unproductive manner to the retinoic acid response element present in the retinoic acid receptor-beta2 promoter. This element is composed of a direct repeat (DR) of the sequence PuGTTCA spaced by five nucleotides. However, the same sequence separated by three nucleotides (DR3) acts as a strong vitamin D response element. Here we show that the polarity of binding of the heterodimers to the DR3 was 5'-RXR-VDR-3', whereas on the DR5, both heterodimeric partners bind indistinctly to the 5' or 3' hemi-sites. These results suggest that the response elements can allosterically regulate the conformation of the receptors to determine positive or negative regulation of gene expression. Despite the altered polarity, the DR5-bound heterodimer was able to recruit the nuclear receptor coactivator ACTR in a vitamin D-dependent fashion. Furthermore, binding of the corepressor SMRT (silencing mediator of retinoid and thyroid hormone receptors) to the RXR/VDR heterodimer on a DR5 was not observed. Binding of RXR/VDR heterodimers to DRs with different transcriptional outcomes may generate selectivity and provide a greater complexity and flexibility to the vitamin D responses.

Composite co-activator ARC mediates chromatin-directed transcriptional activation

Gene activation in eukaryotes is regulated by complex mechanisms in which the recruitment and assembly of the transcriptional machinery is directed by gene- and cell-type-specific DNA-binding proteins. When DNA is packaged into chromatin, the regulation of gene activation requires new classes of chromatin-targeting activity. In humans, a multisubunit cofactor functions in a chromatin-selective manner to potentiate synergistic gene activation by the transcriptional activators SREBP-1a and Sp1. Here we show that this activator-recruited cofactor (ARC) interacts directly with several different activators, including SREBP-1a, VP16 and the p65 subunit of NF-kappaB, and strongly enhances transcription directed by these activators in vitro with chromatin-assembled DNA templates. The ARC complex consists of 16 or more subunits; some of these are novel gene products, whereas others are present in other multisubunit cofactors, such as CRSP, NAT and mammalian Mediator. Detailed analysis indicates that the ARC complex is probably identical to the nuclear hormone-receptor cofactor DRIP. Thus, ARC/DRIP is a large composite co-activator that belongs to a family of related cofactors and is targeted by different classes of activator to mediate transcriptional stimulation.

Ligand-dependent transcription activation by nuclear receptors requires the DRIP complex

Nuclear receptors modulate the transcription of genes in direct response to small lipophilic ligands. Binding to ligands induces conformational changes in the nuclear receptors that enable the receptors to interact with several types of cofactor that are critical for transcription activation (transactivation). We previously described a distinct set of ligand-dependent proteins called DRIPs, which interact with the vitamin D receptor (VDR); together, these proteins constitute a new cofactor complex. DRIPs bind to several nuclear receptors and mediate ligand-dependent enhancement of transcription by VDR and the thyroid-hormone receptor in cell-free transcription assays. Here we report the identities of thirteen DRIPs that constitute this complex, and show that the complex has a central function in hormone-dependent transactivation by VDR on chromatin templates. The DRIPs are almost indistinguishable from components of another new cofactor complex called ARC, which is recruited by other types of transcription activators to mediate transactivation on chromatin-assembled templates. Several DRIP/ARC subunits are also components of other potentially related cofactors, such as CRSP, NAT, SMCC and the mouse Mediator, indicating that unique classes of activators may share common sets or subsets of cofactors. The role of nuclear-receptor ligands may, in part, be to recruit such a cofactor complex to the receptor and, in doing so, to enhance transcription of target genes.

A steroid receptor coactivator, SRA, functions as an RNA and is present in an SRC-1 complex

Nuclear receptors play critical roles in the regulation of eukaryotic gene expression. We report the isolation and functional characterization of a novel transcriptional coactivator, termed steroid receptor RNA activator (SRA). SRA is selective for steroid hormone receptors and mediates transactivation via their amino-terminal activation function. We provide functional and mechanistic evidence that SRA acts as an RNA transcript; transfected SRA, unlike other steroid receptor coregulators, functions in the presence of cycloheximide, and SRA mutants containing multiple translational stop signals retain their ability to activate steroid receptor-dependent gene expression. Biochemical fractionation shows that SRA exists in distinct ribonucleoprotein complexes, one of which contains the nuclear receptor coactivator steroid receptor coactivator 1. We suggest that SRA may act to confer functional specificity upon multiprotein complexes recruited by liganded receptors during transcriptional activation.

Coactivator and corepressor complexes in nuclear receptor function

The nuclear hormone receptors constitute a large family of transcription factors. The binding of the hormonal ligands induces nuclear receptors to assume a configuration that leads to transcriptional activation. Recent studies of retinoic acid and thyroid hormone receptors revealed that, upon ligand binding, a histone deacetylase (HDAC)-containing complex is displaced from the nuclear receptor in exchange for a histone acetyltransferase (HAT)-containing complex. These observations suggest that ligand-dependent recruitment of chromatin-remodeling activity serves as a general mechanism underlying the switch of nuclear receptors from being transcriptionally repressive to being transcriptionally active.

Nuclear receptor coactivators: multiple enzymes, multiple complexes, multiple functions

Nuclear receptors are ligand-inducible transcription factors which mediate the physiological effects of steroid, thyroid and retinoid hormones. By regulating the assembly of a transcriptional preinitiation complex at the promoter of target genes, they enhance the expression of these genes in response to hormone. Recent evidence suggests that nuclear receptors act in part by recruiting multiple coregulator proteins which may have specific functions during transcriptional initiation. Liganded receptors recruit members of the SRC family, a group of structurally and functionally related transcriptional coactivators. Receptors also interact with the transcriptional cointegrators p300 and CBP, which are proposed to integrate diverse afferent signals at hormone-regulated promoters. p300/CBP and members of the SRC coactivator family have intrinsic histone acetyltransferase activity which is believed to disrupt the nucleosomal structure at these promoters. Other nuclear receptor coactivators include a member of the SWI/SNF complex, BRG-1, which couples ATP hydrolysis to chromatin remodelling, and the E3 ubiquitin-protein ligases E6-AP and RPF-1. Finally, nuclear receptor coactivators appear to be organized into preformed subcomplexes, an arrangement that may facilitate their efficient assembly into diverse higher order configurations.

Competition between thyroid hormone receptor-associated protein (TRAP) 220 and transcriptional intermediary factor (TIF) 2 for binding to nuclear receptors. Implications for the recruitment of TRAP and p160 coactivator complexes

Transcriptional activation by nuclear receptors (NRs) involves the concerted action of coactivators, chromatin components, and the basal transcription machinery. Crucial NR coactivators, which target primarily the conserved ligand-regulated activation (AF-2) domain, include p160 family members, such as TIF2, as well as p160-associated coactivators, such as CBP/p300. Because these coactivators possess intrinsic histone acetyltransferase activity, they are believed to function mainly by regulating chromatin-dependent transcriptional activation. Recent evidence suggests the existence of an additional NR coactivator complex, referred to as the thyroid hormone receptor-associated protein (TRAP) complex, which may function more directly as a bridging complex to the basal transcription machinery. TRAP220, the 220-kDa NR-binding subunit of the complex, has been identified in independent studies using both biochemical and genetic approaches. In light of the functional differences identified between p160 and TRAP coactivator complexes in NR activation, we have attempted to compare interaction and functional characteristics of TIF 2 and TRAP220. Our findings imply that competition between the NR-binding subunits of distinct coactivator complexes may act as a putative regulatory step in establishing either a sequential activation cascade or the formation of independent coactivator complexes.

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.

The transcriptional cofactor complex CRSP is required for activity of the enhancer-binding protein Sp1

Activation of gene transcription in metazoans is a multistep process that is triggered by factors that recognize transcriptional enhancer sites in DNA. These factors work with co-activators to direct transcriptional initiation by the RNA polymerase II apparatus. One class of co-activator, the TAF(II) subunits of transcription factor TFIID, can serve as targets of activators and as proteins that recognize core promoter sequences necessary for transcription initiation. Transcriptional activation by enhancer-binding factors such as Sp1 requires TFIID, but the identity of other necessary cofactors has remained unknown. Here we describe a new human factor, CRSP, that is required together with the TAF(II)s for transcriptional activation by Sp1. Purification of CRSP identifies a complex of approximate relative molecular mass 700,000 (M(r) approximately 700K) that contains nine subunits with M(r) values ranging from 33K to 200K. Cloning of genes encoding CRSP subunits reveals that CRSP33 is a homologue of the yeast mediator subunit Med7, whereas CRSP150 contains a domain conserved in yeast mediator subunit Rgr1. CRSP p200 is identical to the nuclear hormone-receptor co-activator subunit TRIP2/PBP. CRSPs 34, 77 and 130 are new proteins, but the amino terminus of CRSP70 is homologous to elongation factor TFIIS. Immunodepletion studies confirm that these subunits have an essential cofactor function. The presence of common subunits in distinct cofactor complexes suggests a combinatorial mechanism of co-activator assembly during transcriptional activation.

Transcriptional targets of the vitamin D3 receptor-mediating cell cycle arrest and differentiation

We are exploring the mechanism of action of the hormonal form of the nutrient vitamin D, 1,25(OH)2D3, and its cognate nuclear receptor at the level of gene control. In doing so, we have focused on a dual track as follows: 1) to define the vitamin D3 receptor (VDR) function and structure by examining its various actions at the molecular level; and 2) to isolate and characterize VDR target genes that might be playing key roles in mediating vitamin D growth suppression and differentiation in responsive cells, specifically, the elucidation of vitamin D target genes as they relate to myeloid differentiation. Here, we will summarize some of our recent results from both tracks because a detailed understanding of how VDR functions as a ligand-regulated transcription factor will allow us to study its actions on these newly discovered genes more effectively.

Selective interaction of vitamin D receptor with transcriptional coactivators by a vitamin D analog

The nuclear vitamin D receptor (VDR) is a member of a nuclear receptor superfamily and acts as a ligand-dependent transcription factor. A family of cotranscriptional activators (SRC-1, TIF2, and AIB-1) interacts with and activates the transactivation function of nuclear receptors in a ligand-dependent way. We examined interaction of VDR with these coactivators that was induced by several vitamin D analogs, since they exert differential subsets of the biological action of vitamin D through unknown mechanisms. Unlike other vitamin D analogs tested, OCT (22-oxa-1alpha,25-dihydroxyvitamin D3) induced interaction of VDR with TIF2 but not with SRC-1 or AIB-1. Consistent with these interactions, only TIF2 was able to potentiate the transactivation function of VDR bound to OCT. Thus, the present findings suggest that the structure of VDR is altered in a vitamin D analog-specific way, resulting in selective interactions of VDR with coactivators. Such selective interaction of coactivators with VDR may specify the array of biological actions of a vitamin D analog like OCT, possibly through activating a particular set of target gene promoters.

Redox regulation of the glucocorticoid receptor

Redox regulation is currently considered as a mode of signal transduction for coordinated regulation of a variety of cellular processes. The transcriptional regulation of gene expression is also influenced by cellular redox state, most possibly through the oxido-reductive modification of transcription factors. The glucocorticoid receptor belongs to a nuclear receptor superfamily and acts as a ligand-dependent transcription factor. We demonstrate that the glucocorticoid receptor function is regulated via redox-dependent mechanisms at multiple levels. Moreover, it is suggested that redox regulation of the receptor function is one of dynamic cellular responses to environmental stimuli and plays an important role in orchestrated crosstalk between central and peripheral stress responses.

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.