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

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.

Hormone-dependent interaction between the amino- and carboxyl-terminal domains of progesterone receptor in vitro and in vivo

Full transcriptional activation by steroid hormone receptors requires functional synergy between two transcriptional activation domains (AF) located in the amino (AF-1) and carboxyl (AF-2) terminal regions. One possible mechanism for achieving this functional synergy is a physical intramolecular association between amino (N-) and carboxyl (C-) domains of the receptor. Human progesterone receptor (PR) is expressed in two forms that have distinct functional activities: full-length PR-B and the amino-terminally truncated PR-A. PR-B is generally a stronger activator than PR-A, whereas under certain conditions PR-A can act as a repressor in trans of other steroid receptors. We have analyzed whether separately expressed N- (PR-A and PR-B) and C-domains [hinge plus ligand-binding domain (hLBD)] of PR can functionally interact within cells by mammalian two-hybrid assay and whether this involves direct protein contact as determined in vitro with purified expressed domains of PR. A hormone agonist-dependent interaction between N-domains and the hLBD was observed functionally by mammalian two-hybrid assay and by direct protein-protein interaction assay in vitro. With both experimental approaches, N-C domain interactions were not induced by the progestin antagonist RU486. However, in the presence of the progestin agonist R5020, the N-domain of PR-B interacted more efficiently with the hLBD than the N-domain of PR-A. Coexpression of steroid receptor coactivator-1 (SRC-1) and the CREB binding protein (CBP), enhanced functional interaction between N- and C-domains by mammalian two-hybrid assay. However, addition of SRC-1 and CBP in vitro had no influence on direct interaction between purified N- and C-domains. These results suggest that the interaction between N- and C-domains of PR is direct and requires a hormone agonist-induced conformational change in the LBD that is not allowed by antagonists. Additionally, coactivators are not required for physical association between the N- and C-domains but are capable of enhancing a functionally productive interaction. In addition, the more efficient interaction of the hLBD with the N-domain of PR-B, compared with that of PR-A, suggests that distinct interactions between N- and C-terminal regions contribute to functional differences between PR-A and PR-B.

Transcriptional control of the murine polymeric IgA receptor promoter by glucocorticoids

Glucocorticoids have been implicated as an important regulator of intestinal epithelial cell ontogeny. The polymeric IgA receptor (pIgR) is expressed in the intestinal epithelial layer and is regulated by several mediators, including glucocorticoids. The mechanism of how corticosteroids alter the transcriptional regulation of pIgR expression has not been defined. In this study, we demonstrated that glucocorticoids upregulate steady-state pIgR mRNA levels in the proximal intestine of suckling rats and in the IEC-6 intestinal cell line. We performed functional analysis of the 5'-flanking region in the presence of glucocorticoids and its receptor using the intestinal cell line Caco-2. We screened 4.7 kb of the upstream region of the murine gene and identified the most potent steroid response element to reside between nt -215 and -163 relative to the start of transcription. Substitution mutation analysis of this region identified the location of the putative steroid response element to be between nt -195 and -176. In vitro DNase I footprint analysis using the recombinant glucocorticoid receptor DNA binding domain confirmed a single area of protection that spans the nt identified by mutagenesis analysis. Electrophoretic mobility shift assays of the putative element confirmed the binding of both recombinant and cell synthesized glucocorticoid receptor in a specific manner. In summary, we report the identification and characterization of the glucocorticoid-DNA response element located in the immediate 5'-upstream region of the murine pIgR gene.

Transcriptional regulation of the StAR gene

The steroidogenic acute regulatory (StAR) protein regulates the rate-limiting step of steroidogenesis. In steroidogenic tissues, the StAR gene is regulated acutely by trophic hormone through a cAMP second messenger pathway. Thus, the gene encoding StAR must be finely regulated so that it is expressed in steroidogenic tissues at the proper time in development, and must be rapidly induced in response to cAMP stimulation. We have summarized the available information concerning the regulation of StAR mRNA levels including promoter mapping and transactivation studies. We also discuss the various transcription factors which have been implicated in the regulation of the StAR gene thus far, and propose models of how StAR transcription may be regulated.

Cooperation between phosphorylation and acetylation processes in transcriptional control

We previously reported that the activation of the M promoter of the human choline acetyltransferase (ChAT) gene by butyrate and trapoxin in transfected CHP126 cells is blocked by PD98059, a specific mitogen-activated protein kinase kinase (MEK) inhibitor (E. Espinos and M. J. Weber, Mol. Brain Res. 56:118-124, 1998). We now report that the transcriptional effects of histone deacetylase inhibitors are mediated by an H7-sensitive serine/threonine protein kinase. Activation of the ChAT promoter by butyrate and trapoxin was blocked by 50 microM H7 in both transient- and stable-transfection assays. Overexpression of p300, a coactivator protein endowed with histone acetyltransferase activity, stimulated the ChAT promoter and had a synergistic effect on butyrate treatment. These effects were blocked by H7 and by overexpressed adenovirus E1A 12S protein. Moreover, both H7 and PD98059 suppressed the activation of the Rous sarcoma virus (RSV) and simian virus 40 promoters by butyrate in transfection experiments. Similarly, the induction of the cellular histone H1(0) gene by butyrate in CHP126 cells was blocked by H7 and by PD98059. Previous data (L. Cuisset, L. Tichonicky, P. Jaffray, and M. Delpech, J. Biol. Chem. 272:24148-24153, 1997) showed that the induction of the H1(0) gene by butyrate is blocked by okadaic acid, an inhibitor of protein phosphatases. We now show that the activation of the ChAT and RSV promoters by butyrate in transfected CHP126 cells is also blocked by 200 nM okadaic acid. Western blotting and in vivo metabolic labeling experiments showed that butyrate has a biphasic effect on histone H3 phosphorylation, i.e., depression for up to 16 h followed by stimulation. The data thus strongly suggest that the transcriptional effects of histone deacetylase inhibitors are mediated through the activation of MEK1 and of an H7-sensitive protein kinase in addition to protein phosphatases.

Glucocorticoid induction of epithelial sodium channel expression in lung and renal epithelia occurs via trans-activation of a hormone response element in the 5'-flanking region of the human epithelial sodium channel alpha subunit gene

In airway and renal epithelia, the glucocorticoid-mediated stimulation of amiloride-sensitive Na+ transport is associated with increased expression of the epithelial Na+ channel alpha subunit (alphaENaC). In H441 lung cells, 100 nM dexamethasone increases amiloride-sensitive short-circuit current (3.3 microA/cm2 to 7.5 microA/cm2), correlating with a 5-fold increase in alphaENaC mRNA expression that could be blocked by actinomycin D. To explore transcriptional regulation of alphaENaC, the human alphaENaC 5'-flanking region was cloned and tested in H441 cells. By deletion analysis, a approximately 150-base pair region 5' to the upstream promoter was identified that, when stimulated with 100 nM dexamethasone, increased luciferase expression 15-fold. This region, which contains two imperfect GREs, also functioned when coupled to a heterologous promoter. When individually tested, only the downstream GRE functioned in cis and bound GR in a gel mobility shift assay. In the M-1 collecting duct line Na+ transport, malphaENaC expression and luciferase expression from alphaENaC genomic fragments were also increased by 100 nM dexamethasone. In a colonic cell line, HT29, trans-activation via a heterologously expressed glucocorticoid receptor restored glucocorticoid-stimulated alphaENaC gene transcription. We conclude that glucocorticoids stimulate alphaENaC expression in kidney and lung via activation of a hormone response element in the 5'-flanking region of halphaENaC and this response, in part, is the likely basis for the up-regulation of Na+ transport in these sites.

Estrogen receptor binding to estrogen response elements slows ligand dissociation and synergistically activates reporter gene expression

Estradiol (E2)-liganded estrogen receptor (ER) bound to three or four tandem copies of a consensus ERE (EREc38) in a cooperative manner. E2-ER binding to one or two EREs was non-cooperative. When ER was liganded by the antiestrogen 4-hydroxytamoxifen (4-OHT), ER-ERE binding was not cooperative, regardless of the number of EREs. Here we evaluated how binding to EREc38 affects ER conformation in the ligand binding domain (LBD) as reflected in the dissociation kinetics of [3H]ligand from the ER. Binding of ER to EREc38 slowed the rate of dissociation of either E2 or 4-OHT, indicating that DNA allosterically modulates the LBD conformation creating a tighter fit between the ligand and the ER. Conformational differences in ER induced by E2 versus antiestrogen were not reflected in differences in E2 or 4-OHT dissociation parameters under these conditions. No difference in the association rate of E2- versus 4-OHT-liganded ER binding to EREc38 was detected in electrophoretic mobility shift assay (EMSA). Synergistic, E2-dependent activation of a reporter gene was detected from three and four, but not one or two, tandem copies of EREc38. These observations suggest that cooperative binding of E2-ER to multiple copies of EREc38 is likely responsible for transcriptional synergy and that cooperativity may not involve direct interaction between the LBDs of ERE-bound ER. Since the number of copies of EREc38 did not alter E2 dissociation kinetics, functional synergy must involve cellular factors in addition to the ER ligand.

Specific androgen receptor activation by an artificial coactivator

Transcription activation of steroid receptors, such as the androgen receptor (AR), is mediated by coactivators, which bridge the receptor to the preinitiation complex. To develop a tool for studying the role of the AR in normal development and disease, we constructed artificial coactivators consisting of the transcription activation domains of VP16 or p65/RelA and the AR hinge and ligand-binding domain (ARLBD), which has been shown to interact with the AR N-terminal domain. The artificial VP16-ARLBD and ARLBD-p65 coactivators interacted with the AR N terminus and wild-type AR in an androgen-dependent and androgen-specific manner. VP16-ARLBD and ARLBD-p65 enhanced the AR transactivity up to 4- and 13-fold, respectively, without affecting the expression of the AR protein. The coactivators did not enhance the transcription activity of the progesterone receptor (PR) or the glucocorticoid receptor (GR), showing their specificity for the AR. In addition, to construct PR- and GR-specific coactivators, the VP16 activation domain was fused to the PR and GR hinge/ligand-binding domain. Although VP16-PRLBD and VP16-GRLBD interacted with the C-terminal portion of steroid receptor coactivator-1, they did not enhance the transcription activity of their receptor. The presented strategy of directing activation domains or other protein activities into the DNA-bound AR complex provides a novel means of manipulating AR function in vitro and in vivo.

Interactions in the transcriptional regulation exerted by Stat5 and by members of the steroid hormone receptor family

The pathways which connect extracellular signals with the regulation of the activity of transcription factors are being investigated in molecular detail. Extensive progress has been made in the description of the mode of action of steroid hormones and of cytokines. Steroid hormones associate intracellularly with latent receptor molecules, cause the dissociation of masking proteins, the dimerization of receptors, and their binding to specific hormone response elements in the promoters of target genes. Cytokines also activate latent transcription factors (Stats--signal transducers and activators of transcription), but act through an enzymatic mechanism. Tyrosine kinases associated with the transmembrane cytokine receptors phosphorylate Stat molecules. The phosphorylated monomers dimerize and assume specific DNA binding ability. Both classes of transcription factors bind to different response elements and regulate different target genes and both signals, cytokines and steroid hormones, can affect growth differentiation and homeostasis of different cell types. Here, we describe that Stat5, a molecule activated by several essential cytokines, functionally interacts with members of the steroid receptor family. We find that glucocorticoid receptor, mineralocorticoid receptor and progesterone receptor synergize with Stat5 in the induction of the transcription from the beta-casein gene promoter. The estrogen receptor diminishes Stat5 mediated induction and the androgen receptor has no effect. Conversely, Stat5 negatively interferes with glucocorticoid receptor, mineralocorticoid receptor and progesterone receptor induced transcription from the MMTV LTR and the estrogen receptor induced transcription from an ERE-containing promoter.

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.

Cloning and characterization of androgen receptor coactivator, ARA55, in human prostate

Androgen receptor (AR) is a hormone-activated transcriptional factor that can bind to androgen response elements and that regulates the transcription of target genes via a mechanism that presumably involves cofactors. We report here the cloning of a novel AR coactivator ARA55 using a yeast two-hybrid system. ARA55 consists of 444 amino acids with the predicted molecular mass of 55 kDa and its sequence shows very high homology to mouse hic5, a TGF-beta1-inducible gene. Yeast and mammalian two-hybrid systems and co-immunoprecipitation assays all prove ARA55 can bind to AR in a ligand-dependent manner. Transient transfection assay in prostate cancer DU145 cells further demonstrates that ARA55 can enhance AR transcriptional activity in the presence of 1 nM dihydrotestosterone or its antagonists such as 100 nM 17beta-estradiol or 1 microM hydroxyflutamide. Our data also suggest the C-terminal half of ARA55, which includes three LIM motifs, is sufficient to interact with AR. Northern blot and polymerase chain reaction quantitation showed ARA55 can be expressed differently in normal prostate and prostate tumor cells. Together, our data suggests that ARA55 may play very important roles in the progression of prostate cancer by the modulation of AR transactivation.

Nuclear factor I-mediated repression of the mouse mammary tumor virus promoter is abrogated by the coactivators p300/CBP and SRC-1

To better understand the function of nuclear factor I (NFI) proteins in transcription, we have used transient transfection assays to assess transcriptional modulation by NFI proteins on the NFI-dependent mouse mammary tumor virus (MMTV) promoter. Expression of NFI-C or NFI-X, but not NFI-A or NFI-B proteins, represses glucocorticoid induction of the MMTV promoter in HeLa cells. Repression is DNA binding-independent as a deletion construct expressing the NH2-terminal 160 residues of NFI-C represses but does not bind DNA. Repression by NFI-C is cell type-dependent and occurs in HeLa and COS-1 cells but not 293 or JEG-3 cells. NFI-C does not repress progesterone induction of the MMTV promoter in HeLa cells, suggesting that progesterone induction of the promoter differs mechanistically from glucocorticoid induction. NFI-C-mediated repression is alleviated by overexpression of glucocorticoid receptor (GR), suggesting that NFI-C represses the MMTV promoter by preventing GR function. However, repression by NFI-C occurs with only a subset of glucocorticoid-responsive promoters, as the chimeric NFIGREbeta-gal promoter that is activated by GR is not repressed by NFI-C. Since the coactivator proteins p300/CBP, SRC-1A, and RAC3 had previously been shown to function at steroid hormone-responsive promoters, we asked whether they could influence NFI-C-mediated repression of MMTV expression. Expression of p300/CBP or SRC-1A alleviates repression by NFI-C, whereas RAC3 has no effect. This abrogation of NFI-C-mediated repression by p300/CBP and SRC-1A suggests that repression by NFI-C may occur by interference with coactivator function at the MMTV promoter.

Ligand-activated retinoic acid receptor inhibits AP-1 transactivation by disrupting c-Jun/c-Fos dimerization

In the presence of retinoic acid (RA), the retinoid receptors, retinoic acid receptor (RAR) and retinoid X receptor (RXR), are able to up-regulate transcription directly by binding to RA-responsive elements on the promoters of responsive genes. Liganded RARs and RXRs are also capable of down-regulating transcription, but, by contrast, this is an indirect effect, mediated by the interaction of these nuclear receptors not with DNA but the transcription factor activating protein-1 (AP-1). AP-1 is a dimeric complex of the protooncoproteins c-Jun and c-Fos and directly regulates transcription of genes important for cellular growth. Previous in vitro results have suggested that RARs can block AP-1 DNA binding. Using a mammalian two-hybrid system, we report here that human RARalpha (hRARalpha) can disrupt in a RA-dependent manner the homo- and heterodimerization properties of c-Jun and c-Fos. This inhibition of dimerization is cell specific, occurring only in those cells that exhibit RA-induced repression of AP-1 transcriptional activity. Furthermore, this mechanism appears to be specific for the RARs, since another potent inhibitor of AP-1 activity, the glucocorticoid receptor, does not affect AP-1 dimerization. Our data argue for a novel mechanism by which RARs can repress AP-1 DNA binding, in which liganded RARs are able to interfere with c-Jun/c-Jun homodimerization and c-Jun/c-Fos heterodimerization and, in this way, may prevent the formation of AP-1 complexes capable of DNA binding.

CBP-dependent and independent enhancing activity of steroid receptor coactivator-1 in thyroid hormone receptor-mediated transactivation

Full-length of steroid receptor coactivator-1 (F-SRC-1) has been shown to interact with thyroid hormone receptors (TRs) in a ligand-dependent manner and to stimulate receptor-dependent transcription. To identify functional domains of F-SRC-1, several internal deletion mutants of F-SRC-1 were constructed. Although in vitro pull down assay with TR showed interaction of all of these mutants with TR, lack of mid legion (amino acids 398-1172) lost enhancing activity of TR-mediated transcription in a transient transfection assay. However, F-SRC-1 mutant lacking CBP-interacting domain still preserved enhancing activity. Surprisingly, F-SRC-1 mutants also increased basal level of viral promoter activity depending upon their deleted region. Yeast activation function assay revealed that these F-SRC-1 mutants had intrinsic activation function when bound to DNA. Analyses of small fragments of F-SRC-1 identified three separable activation domains. In vitro binding assay showed that TBP and TFIIB bound to C-terminal half of F-SRC-1. These results suggest that F-SRC-1 can function via both CBP-dependent and independent manners using various sets of activation domains and that direct interactions between F-SRC-1 and TBP or TFIIB may not be important for CBP-independent transcription.

Segregation of steroid receptor coactivator-1 from steroid receptors in mammary epithelium

Steroid receptor coactivator-1 (SRC-1) family members interact with steroid receptors, including estrogen receptor alpha (ERalpha) and progesterone receptor (PR), to enhance ligand-dependent transcription. However, the expression of ERalpha and SRC-1 was found to be segregated in distinct subsets of cells within the epithelium of the estrogen-responsive rat mammary gland. This finding was in contrast to the finding for the stroma, where significant numbers of cells coexpressed ERalpha and SRC-1. Treatment of animals with estrogen induced PR expression in the ERalpha-expressing mammary epithelial cells in the absence of detectable SRC-1 and did not affect the segregated pattern of SRC-1 and ERalpha expression. PR was neither expressed nor induced by estrogen treatment in stroma, despite the coexpression of ERalpha and SRC-1. These results suggest that SRC-1 is not necessary for ERalpha-mediated induction of PR in mammary epithelial cells and is also not sufficient for PR induction in stromal cells expressing both ERalpha and SRC-1. Furthermore, the expression of SRC-1 in a subpopulation of mammary epithelial cells distinct from those expressing ERalpha or PR raises the possibility that SRC-1 has cell type-specific functions other than simply to act as coactivator for ERalpha or PR in the mammary epithelium.

Thymus-derived glucocorticoids and the regulation of antigen-specific T-cell development

Bidirectional interactions of both a stimulatory and inhibitory nature occur between the neuroendocrine and the immune systems, and these interactions play an important modulatory role during T-cell ontogeny. Specifically, glucocorticoids potently induce apoptosis in thymocytes and activated T cells, but can also rescue these cells from activation-induced cell death. The objective of this review is to discuss current data on the interactions of the immune system with steroid hormones in the thymus and to describe a model that includes glucocorticoids in the shaping of the peripheral T-cell antigen-specific repertoire and deals with their potential role in the generation of autoimmune disease.

Interaction of the putative androgen receptor-specific coactivator ARA70/ELE1alpha with multiple steroid receptors and identification of an internally deleted ELE1beta isoform

Steroid-regulated gene transcription requires the coordinate physical and functional interaction of hormone receptors, basal transcription factors, and transcriptional coactivators. In this context ARA70, previously called RFG and ELE1, has been described as a putative coactivator that specifically enhances the activity of the androgen receptor (AR) but not that of the glucocorticoid receptor (GR), the progesterone receptor, or the estrogen receptor (ER). Here we describe the cloning of the cDNA for ELE1/ARA70 by RT-PCR from RNA derived from different cell lines (HeLa, DU-145, and LNCaP). In accordance with the previously described sequence, we obtained a 1845-bp PCR product for the HeLa and the LNCaP RNA. Starting from T-47D RNA, however, an 860-bp PCR product was obtained. This shorter variant results from an internal 985-bp deletion and is called ELE1beta; accordingly, the longer isoform is referred to as ELE1alpha. The deduced amino acid sequence of ELE1alpha, but not that of ELE1beta, differs at specific positions from the one previously published by others, suggesting that these two proteins are encoded by different nonallelic genes. ELE1alpha is expressed in the three prostate-derived cell lines examined (PC-3, DU-145, and LNCaP), and this expression is not altered by androgen treatment. Of all rat tissues examined, ELE1alpha expression is highest in the testis. This is also the only tissue in which we could demonstrate ELE1beta expression. Both ELE1alpha and ELE1beta interact in vitro with the AR, but also with the GR and the ER, in a ligand-independent way. Overexpression of either ELE1 isoform in DU-145, HeLa, or COS cells had only minor effects on the transcriptional activity of the human AR. ELE1alpha has no intrinsic transcription activation domain or histone acetyltransferase activity, but it does interact with another histone acetyltransferase, p/CAF, and the basal transcription factor TFIIB. The interaction with the AR occurs through the ligand-binding domain and involves the region corresponding to the predicted helix 3. Mutation in this domain of leucine 712 to arginine greatly reduces the affinity of the AR for ELE1alpha but has only moderate effects on its transcriptional activity. Taken together, we have identified two isoforms of the putative coactivator ARA70/ELE1 that may act as a bridging factor between steroid receptors and components of the transcription initiation complex but which lack some fundamental properties of a classic nuclear receptor coactivator. Further experiments will be required to highlight the in vivo role of ELE1 in nuclear receptor functioning.

The coactivator CBP stimulates human T-cell lymphotrophic virus type I Tax transactivation in vitro

Tax interacts with the cellular cyclic AMP-responsive element binding protein (CREB) and facilitates the binding of the coactivator CREB binding protein (CBP), forming a multimeric complex on the cyclic AMP-responsive element (CRE)-like sites in the human T-cell lymphotrophic virus type I (HTLV-I) promoter. The trimeric complex is believed to recruit additional regulatory proteins to the HTLV-I long terminal repeat, but there has been no direct evidence that CBP is required for Tax-mediated transactivation. We present evidence that Tax and CBP activate transcription from the HTLV-I 21 base pair repeats on naked DNA templates. Transcriptional activation of the HTLV-I sequences required both Tax and CBP and could be mediated by either the N-terminal activation domain of CBP or the full-length protein. Fluorescence polarization binding assays indicated that CBP does not markedly enhance the affinity of Tax for the trimeric complex. Transcription analyses suggest that CBP activates Tax-dependent transcription by promoting transcriptional initiation and reinitiation. The ability of CBP to activate the HTLV-I promoter does not involve the stabilization of Tax binding, but rather depends upon gene activation properties of the co-activator that function in the context of a naked DNA template.

SRC-1 and GRIP1 coactivate transcription with hepatocyte nuclear factor 4

Hepatocyte nuclear factor-4 (HNF4), a member of the nuclear receptor superfamily, plays an important role in tissue-specific gene expression, including genes involved in hepatic glucose metabolism. In this study, we show that SRC-1 and GRIP1, which act as coactivators for various nuclear receptors, associate with HNF4 in vivo and enhance its transactivation potential. The AF-2 domain of HNF4 is required for this interaction and for the potentiation of transcriptional activity by these coactivators. p300 can also serve as a coactivator with HNF4, and it synergizes with SRC-1 to further augment the activity of HNF4. HNF4 is also a key regulator of the expression of hepatocyte nuclear factor-1 (HNF1). The overexpression of SRC-1 or GRIP1 enhances expression from a HNF1 gene promoter-reporter in HepG2 hepatoma cells, and this requires an intact HNF4-binding site in the HNF1 gene promoter. Type 1 maturity onset diabetes of young (MODY), which is characterized by abnormal glucose-mediated insulin secretion, is caused by mutations of the HNF4 gene. A mutation of the HNF4-binding site in the HNF1 gene promoter has also been associated with MODY. Thus, HNF4 is involved in the regulation of glucose homeostasis at several levels and along with the SRC-1, GRIP1, and p300 may play an important role in the pathophysiology of non-insulin-dependent diabetes mellitus.

Ligand-independent recruitment of steroid receptor coactivators to estrogen receptor by cyclin D1

The estrogen receptor (ER) is an important regulator of growth and differentiation of breast epithelium. Transactivation by ER depends on a leucine-rich motif, which constitutes a ligand-regulated binding site for steroid receptor coactivators (SRCs). Cyclin D1 is frequently amplified in breast cancer and can activate ER through direct binding. We show here that cyclin D1 also interacts in a ligand-independent fashion with coactivators of the SRC-1 family through a motif that resembles the leucine-rich coactivator binding motif of nuclear receptors. By acting as a bridging factor between ER and SRCs, cyclin D1 can recruit SRC-family coactivators to ER in the absence of ligand. A cyclin D1 mutant that binds to ER but fails to recruit coactivators preferentially interferes with ER activation in breast cancer cells that have high levels of cyclin D1. These data support that cyclin D1 contributes significantly to ER activation in breast cancers in which the protein is overexpressed. Our present results reveal a novel route of coactivator recruitment to ER and establish a direct role for cyclin D1 in regulation of transcription.

Cloning and characterization of mouse RIP140, a corepressor for nuclear orphan receptor TR2

The mouse homologue of the human receptor-interacting protein 140 (RIP140) was isolated from a mouse embryonic cDNA library in yeast two-hybrid screening experiments by using the ligand binding domain (LBD) of nuclear orphan receptor TR2 as the bait. The receptor-interacting domains of mouse RIP140 were mapped to the regions containing the LXXLL motif (where L is leucine and X is any amino acid), and the RIP140-interacting domain of TR2 was mapped to its C-terminal 10- to 20-amino-acid sequence, a putative activation function 2 (AF-2) region. In a GAL4 reporter system and a reporter driven by the proximal region of the TR2 promoter, RIP140 functioned as a corepressor for both a GAL4 DNA binding domain (BD)-TR2 fusion and the wild-type receptor. When tethered to the BD of GAL4, RIP140 exerted a trans-repressive effect on the GAL4 reporter. In addition, RIP140 suppressed the retinoic acid (RA) receptor-mediated RA induction in a dose-dependent manner. Finally, it was demonstrated that in the presence of RIP140, a cytosolic, green fluorescent protein-tagged TR2 LBD translocated into the nucleus, and TR2 and RIP140 were coimmunoprecipitated from the cell extract, indicating that the interaction between RIP140 and the LBD of TR2 occurred in vivo. The potential biological role of RIP140 in TR2-modulated transcriptional activity is discussed.

Cloning and characterization of the mouse histone deacetylase-2 gene

Histone deacetylase-2 (HDAC2) is a component of a complex that mediates transcriptional repression in mammalian cells. A mouse HDAC2 cDNA was used to identify several recombinant clones containing the entire mouse HDAC2 gene. The mouse HDAC2 gene spans over 36 kilobase pairs and is composed of 14 exons (ranging from 58 to 362 nucleotides in length) and 13 introns (ranging from 75 base pairs to 19 kilobase pairs in length). Primer extension analysis with total RNA from NIH3T3 cells revealed a major transcriptional start site at 221 base pairs 5' of the ATG translational start codon. Upstream of the transcriptional start site, no canonical TATA box was found, but binding sites for several known transcription factors were identified. Transient transfection studies with 5' deletion mutants localized the promoter to no more than 76 base pairs upstream from the major transcriptional start site. Fluorescence in situ hybridization mapped mouse HDAC2 to chromosomal location 10B1, which is in close proximity to the growth factor-inducible gene fisp-12. Information concerning the genomic organization and promoter of HDAC2 will be useful in studies of the regulation of histone deacetylase activities, which in turn are important in studies of the regulation of transcriptional repression in mammalian cells.

Different classes of coactivators recognize distinct but overlapping binding sites on the estrogen receptor ligand binding domain

We have analyzed interaction of coactivators with the wild-type estrogen receptor alpha (ER), HEG0, and a mutant, L536P-HEG0, which is constitutively active in several transiently transfected cells and a HeLa line that stably propagates an estrogen-sensitive reporter gene. Different classes of coactivators do not recognize the ER ligand binding domain (LBD) in the same manner. Steroid receptor coactivator-1 (SRC-1), amplified in breast cancer-1 (AIB-1), transcriptional intermediary factor-1 (TIF-1), transcriptional intermediary factor-2 (TIF-2), and receptor interacting protein 140 (RIP140) interacted with HEG0 and L536P-HEG0 in the presence of estradiol, but generally not in the presence of anti-estrogens. However, ICI164,384 stimulated some interaction of RIP140 with LBDs. SRC-1, AIB-1, and RIP140 interacted constitutively with the L536P ER, whereas TIF-1 and TIF-2 interacted only weakly in the absence of hormone. Reciprocal competition for binding to the ER LBD was observed between different classes of coactivators. Moreover, coexpression of RIP140 blocked enhanced transactivation by HEG0 observed in the presence of TIF-2, suggesting that RIP140 may play a negative role in ER signaling. We conclude that constitutive activity of L536P-HEG0 is manifested to similar degrees in different cell types and likely arises from constitutive coactivator binding; different classes of coactivators recognize distinct but overlapping binding sites on the ER LBD. Finally, the observation that L536P-HEG0 interacted constitutively with AIB-1, a coactivator that has been implicated in ER signaling in breast and ovarian cancer, suggests that similar mutations in the ER may contribute to hormone-independent proliferation of breast and ovarian cells.

p300/CREB-binding protein enhances the prolactin-mediated transcriptional induction through direct interaction with the transactivation domain of Stat5, but does not participate in the Stat5-mediated suppression of the glucocorticoid response

Stat5 was discovered as a PRL-induced member of the Stat (signal transducer and activator of transcription) family. Its induction by many other cytokines and interleukins suggests that Stat5 plays a crucial role not only in mammary epithelial, but also in hematopoietic cells. Cell type- and promoter-specific functions of Stat5 are most likely modulated by the interaction with other transcription factors. We recently showed cross-talk between Stat5 and the glucocorticoid receptor. The activated glucocorticoid receptor forms a complex with Stat5 and enhances Stat5-mediated transcriptional induction. Conversely, Stat5 diminishes the induction of glucocorticoid-responsive genes. Here, we investigated the role of p300/CBP(CREB-binding protein), a transcriptional coactivator of several groups of transcription factors, in Stat5-mediated transactivation and in the cross-talk between Stat5 and the glucocorticoid receptor. p300/ CBP acts as a coactivator of Stat5. Its ectopic expression enhances PRL-induced Stat5-mediated transcriptional activation. Consistent with this observation, we find that the adenovirus E1A protein, which binds to p300/CBP, suppresses Stat5-induced transcriptional activation. This inhibition requires the Stat5 transactivation domain and the p300/CBP binding site of E1A. Coimmunoprecipitation and mammalian two-hybrid assays demonstrate a direct interaction between the carboxyl-terminal transactivation domain of Stat5 and p300/CBP. p300/CBP also positively interacts with the glucocorticoid receptor and enhances glucocorticoid receptor-dependent transcriptional activation of the mouse mammary tumor virus-long terminal repeat promoter. Overexpression of p300/CBP does not counteract the Stat5-mediated inhibition of glucocorticoid receptor-dependent transactivation, i.e. the repression of the glucocorticoid response by Stat5 is not a consequence of competition for limiting amounts of p300/CBP.

The estrogen receptor beta subtype: a novel mediator of estrogen action in neuroendocrine systems

The recent discovery that an additional estrogen receptor (ERbeta) subtype is present in many rat, mouse, and human tissues has advanced our understanding of the mechanisms underlying estrogen signalling. Ligand-binding experiments have shown specific binding of 17beta-estradiol by ERbeta with an affinity similar to that of ERalpha. The rat tissue distribution and/or the relative level of ERalpha and ERbeta expression seems to be quite different, i.e., moderate to high expression in uterus, testis, pituitary, ovary, kidney, epididymis, and adrenal for ERalpha and prostate, ovary, lung, bladder, brain, bone, uterus, and testis for ERbeta. Within the same organ it often appears that the ER subtypes are expressed in different cell types, supporting the hypothesis that the ER's may have different biological functions. The cell type-specific expression of ERalpha and ERbeta in rat prostate, testis, uterus, ovary, and brain and the distribution of ERbeta mRNA in the ERalpha knock-out mouse brain are discussed. The discovery of ERbeta suggests the existence of two previously unrecognized pathways of estrogen signalling; via the ERbeta subtype in tissues exclusively expressing this subtype and via the formation of heterodimers in tissues expressing both ER subtypes. The existence of two ER subtypes, their differential expression pattern, and different actions on certain response elements could provide explanations for the striking species-, cell-, and promoter-specific actions of estrogens and antiestrogens. The challenge for the future is to unravel the detailed physiological role of each subtype and to use this knowledge to develop the next generation of ER-targeted drugs with improved therapeutic profiles in the treatment or prevention of osteoporosis, cardiovascular system disorders, Alzheimer's disease, breast cancer, and disorders of the urogenital tract.

Current understanding of the molecular actions of vitamin D

The important reactions that occur to the vitamin D molecule and the important reactions involved in the expression of the final active form of vitamin D are reviewed in a critical manner. After an overview of the metabolism of vitamin D to its active form and to its metabolic degradation products, the molecular understanding of the 1alpha-hydroxylation reaction and the 24-hydroxylation reaction of the vitamin D hormone is presented. Furthermore, the role of vitamin D in maintenance of serum calcium is reviewed at the physiological level and at the molecular level whenever possible. Of particular importance is the regulation of the parathyroid gland by the vitamin D hormone. A third section describes the known molecular events involved in the action of 1alpha,25-dihydroxyvitamin D3 on its target cells. This includes reviewing what is now known concerning the overall mechanism of transcriptional regulation by vitamin D. It describes the vitamin D receptors that have been cloned and identified and describes the coactivators and retinoid X receptors required for the function of vitamin D in its genomic actions. The presence of receptor in previously uncharted target organs of vitamin D action has led to a study of the possible function of vitamin D in these organs. A good example of a new function described for 1alpha,25-dihydroxyvitamin D3 is that found in the parathyroid gland. This is also true for the role of vitamin D hormone in skin, the immune system, a possible role in the pancreas, i.e., in the islet cells, and a possible role in female reproduction. This review also raises the intriguing question of whether vitamin D plays an important role in embryonic development, since vitamin D deficiency does not prohibit development, nor does vitamin D receptor knockout. The final section reviews some interesting analogs of the vitamin D hormone and their possible uses. The review ends with possible ideas with regard to future directions of vitamin D drug design.

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.

Thyroid hormone response elements differentially modulate the interactions of thyroid hormone receptors with two receptor binding domains in the steroid receptor coactivator-1

Ligand-dependent transcriptional activation by nuclear receptors is mediated by interactions with coactivators. Recently, a consensus interaction motif (LXXLL) has been identified in a number of coactivators such as steroid receptor coactivator-1 (SRC-1). SRC-1 contains three such motifs in the central (nuclear receptor binding domain-1, NBD-1) and a single one in the C-terminal (NBD-2) regions. To define the nature and role of the two NBDs in SRC-1, interaction studies between the two NBDs and thyroid hormone receptor (TR) were performed. Although NBD-1 and NBD-2 showed similar ligand- and AF-2-dependent interactions with TR in solution, these two NBDs possessed distinct interaction properties with TR when TR is bound to a thyroid hormone-response element (TRE). Both in vitro and in vivo interaction studies demonstrate that NBD-1, but not NBD-2, exhibits ligand-dependent interaction with TR in the presence of TREs. In addition, a natural isoform of SRC-1, SRC-1E, which lacks NBD-2, preserved TR as well as progesterone receptor-mediated coactivator function on reporter gene expression. Finally, we found that NBD-1 failed to interact with a TR and retinoid X receptor heterodimer complex on a transcriptionally inactive direct repeat +4 TRE in electrophoretic mobility shift assays. These observations indicate that DNA-induced, as well as ligand-induced, conformational change(s) of TR may influence the nature of its binding to SRC-1, and that the two NBDs of SRC-1 may play different roles to regulate ligand-dependent transactivation of TRs.

A composite regulatory element in the first intron of the estrogen-responsive very low density apolipoprotein II gene

During periods of egg laying in the chicken, when circulating levels of estrogen are increased, the liver-specific estrogen-dependent very low density apolipoprotein II (apoVLDLII) gene is expressed. This expression takes place primarily at the level of transcription, driven by two estrogen response elements that reside in the promoter. In transient transfection assays, expression is increased fourfold when the first intron is added to the promoter construct, indicating that 75% of the regulation comes from intron A. Using in vitro DNase I footprinting, six protein-binding sites were revealed throughout the first intron. The functional significance of these binding sites was evaluated by mutation and transient transfection. Two of the protein-binding regions were shown to increase transcription. Site-specific mutations introduced at either the +66 to +86 or +112 to +129 sites disrupted trans-factor binding and reduced the estrogen-dependent expression by 45% and 34%, respectively. A plasmid containing both mutations resulted in a 43% decrease in expression, indicating that the contributions of these regions are not additive. Competition with known sequences in electrophoretic mobility shift assays suggested that the +66 to +86 site binds a chicken member of the nuclear receptor transcription factor family.

Functional interactions of the AF-2 activation domain core region of the human androgen receptor with the amino-terminal domain and with the transcriptional coactivator TIF2 (transcriptional intermediary factor2)

Previous studies in yeast and mammalian cells showed a functional interaction between the amino-terminal domain and the carboxy-terminal, ligand-binding domain (LBD) of the human androgen receptor (AR). In the present study, the AR subdomains involved in this in vivo interaction were determined in more detail. Cotransfection experiments in Chinese hamster ovary (CHO) cells and two-hybrid experiments in yeast revealed that two regions in the NH2-terminal domain are involved in the functional interaction with the LBD: an interacting domain at the very NH2 terminus, located between amino acid residues 3 and 36, and a second domain, essential for transactivation, located between residues 370 and 494. Substitution of glutamic acid by glutamine at position 888 (E888Q) in the AF-2 activation domain (AD) core region in the LBD, markedly decreased the interaction with the NH2-terminal domain. This mutation neither influenced hormone binding nor LBD homodimerization, suggesting a role of the AF-2 AD core region in the functional interaction between the NH2-terminal domain and the LBD. The AF-2 AD core region was also involved in the interaction with the coactivator TIF2 (transcriptional intermediary factor 2), as the E888Q mutation decreased the stimulatory effect of TIF2 on AR AF-2 activity. Cotransfection of TIF2 and the AR NH2-terminal domain expression vectors did not result in synergy between both factors in the induction of AR AF-2 activity. TIF2 highly induced AR AF-2 activity on a complex promoter [mouse mammary tumor virus (MMTV)], but it was hardly active on a minimal promoter (GRE-TATA). In contrast, the AR NH2-terminal domain induced AR AF-2 activity on both promoter constructs. These data indicate that both the AR NH2-terminal domain and the coactivator TIF2 functionally interact, either directly or indirectly, with the AF-2 AD core region in the AR-LBD, but the level of transcriptional response induced by TIF2 depends on the promoter context.

High-mobility group chromatin proteins 1 and 2 functionally interact with steroid hormone receptors to enhance their DNA binding in vitro and transcriptional activity in mammalian cells

We previously reported that the chromatin high-mobility group protein 1 (HMG-1) enhances the sequence-specific DNA binding activity of progesterone receptor (PR) in vitro, thus providing the first evidence that HMG-1 may have a coregulatory role in steroid receptor-mediated gene transcription. Here we show that HMG-1 and the highly related HMG-2 stimulate DNA binding by other steroid receptors, including estrogen, androgen, and glucocorticoid receptors, but have no effect on DNA binding by several nonsteroid nuclear receptors, including retinoid acid receptor (RAR), retinoic X receptor (RXR), and vitamin D receptor (VDR). As highly purified recombinant full-length proteins, all steroid receptors tested exhibited weak binding affinity for their optimal palindromic hormone response elements (HREs), and the addition of purified HMG-1 or -2 substantially increased their affinity for HREs. Purified RAR, RXR, and VDR also exhibited little to no detectable binding to their cognate direct repeat HREs but, in contrast to results with steroid receptors, the addition of HMG-1 or HMG-2 had no stimulatory effect. Instead, the addition of purified RXR enhanced RAR and VDR DNA binding through a heterodimerization mechanism and HMG-1 or HMG-2 had no further effect on DNA binding by RXR-RAR or RXR-VDR heterodimers. HMG-1 and HMG-2 (HMG-1/-2) themselves do not bind to progesterone response elements, but in the presence of PR they were detected as part of an HMG-PR-DNA ternary complex. HMG-1/-2 can also interact transiently in vitro with PR in the absence of DNA; however, no direct protein interaction was detected with VDR. These results, taken together with the fact that PR can bend its target DNA and that HMG-1/-2 are non-sequence-specific DNA binding proteins that recognize DNA structure, suggest that HMG-1/-2 are recruited to the PR-DNA complex by the combined effect of transient protein interaction and DNA bending. In transient-transfection assays, coexpression of HMG-1 or HMG-2 increased PR-mediated transcription in mammalian cells by as much as 7- to 10-fold without altering the basal promoter activity of target reporter genes. This increase in PR-mediated gene activation by coexpression of HMG-1/-2 was observed in different cell types and with different target promoters, suggesting a generality to the functional interaction between HMG-1/-2 and PR in vivo. Cotransfection of HMG-1 also increased reporter gene activation mediated by other steroid receptors, including glucocorticoid and androgen receptors, but it had a minimal influence on VDR-dependent transcription in vivo. These results support the conclusion that HMG-1/-2 are coregulatory proteins that increase the DNA binding and transcriptional activity of the steroid hormone class of receptors but that do not functionally interact with certain nonsteroid classes of nuclear receptors.

Disruption of vitamin D receptor-retinoid X receptor heterodimer formation following ras transformation of human keratinocytes

A partial resistance to the growth inhibitory influence of 1, 25-dihydroxyvitamin D3 is apparent when immortalized keratinocytes are transformed by the ras oncogene. The vitamin D receptor (VDR) was isolated, analyzed, and found to be identical in normal, immortalized, and ras-transformed keratinocytes. Subsequently, nuclear extracts from immortalized and ras-transformed keratinocytes were analyzed in gel mobility shift assays utilizing labeled vitamin D response elements or thyroid hormone response elements. A specific protein.DNA complex that was shown to contain VDR using an anti-VDR antibody was identified in both types of extracts; however, the addition of an anti-retinoid X receptor (RXR) antibody identified RXR in the complex of both normal and immortalized keratinocyte cell extracts, but not in ras-transformed keratinocytes. Furthermore, transfection of ras-transformed keratinocytes with wild-type human RXRalpha rescued VDR.RXR and thyroid hormone receptor.RXR complexes as demonstrated by a supershift in the presence of the anti-RXR antibody. Both cell lines were found to express RXRalpha message in equal amounts. Western blot analysis revealed that RXRalpha protein from ras-transformed keratinocytes was indistinguishable from that from immortalized keratinocytes and from control cells. These results suggest a causal relationship between resistance to the growth inhibitory influences of 1,25-dihydroxyvitamin D3 and disruption of the VDR.RXR complex in malignant keratinocytes.

Estrogen response elements can mediate agonist activity of anti-estrogens in human endometrial Ishikawa cells

Anti-estrogens like hydroxytamoxifen (OHT) have mixed agonist/antagonist activities, leading to tissue-specific stimulation of cellular proliferation. Partial agonist activity of OHT can be observed in vitro in endometrial carcinoma cells like Ishikawa. Here, we have compared several anti-estrogens (including extensively characterized OHT and pure anti-estrogens such as ICI164, 384 and RU58,668, which are devoid of uterotrophic activity) for their capacity to stimulate promoters containing estrogen response elements (EREs) or AP1-binding sites (12-O-tetradecanoylphorbol-13-acetate response elements, TREs), the two types of DNA motifs known to mediate transcriptional stimulation by estrogen receptors. Assays were performed in Ishikawa cells either by transient transfection or by using cell lines with stably propagated reporter vectors. In transient transfection experiments, none of the anti-estrogens displayed agonist activity on the promoters tested. In contrast, significant transcriptional stimulation was observed with low concentrations of OHT and RU39,411 in Ishikawa cells stably propagating reporter constructs containing a minimal ERE3-TATA promoter. In addition, micromolar concentrations of OHT, but not of RU39,411, stimulated stably propagated AP1-responsive reporter constructs. No transcriptional stimulation of ERE- or TRE-containing promoters was observed with the pure anti-estrogens ICI164,384 and RU58,668. These results indicate that the presence of estrogen response elements in promoters is sufficient to mediate cell-specific agonism of anti-estrogens at the transcriptional level, and that stimulation of AP1 activity may be restricted to a subset of anti-estrogens possessing agonist activity on EREs. In addition, our results suggest that transient transfections do not fully recapitulate in vivo conditions required to observe agonist activity of anti-estrogens.

Vitamin D and regulation of gene expression

The function of 1,25-dihydroxyvitamin D3, the biologically active form of vitamin D, extends from bone and mineral homeostasis to the control of cell growth and differentiation in a variety of tissues. Most of these actions are mediated by activation of the nuclear vitamin D receptor, which regulates the transcription of vitamin D target genes. Considerable progress has been made in the understanding of vitamin D receptor function (especially regarding its interaction with coactivators), as well as in the identification of novel vitamin D responsive genes related to cell growth, differentiation and cytokine production.

Isolation and characterization of a novel coactivator protein, NCoA-62, involved in vitamin D-mediated transcription

The vitamin D receptor (VDR) forms a heterodimeric complex with retinoid X receptor (RXR) and binds to vitamin D-responsive promoter elements to regulate the transcription of specific genes or gene networks. The precise mechanism of transcriptional regulation by the VDR.RXR heterodimer is not well understood, but it may involve interactions of VDR.RXR with transcriptional coactivator or corepressor proteins. Here, a yeast two-hybrid strategy was used to isolate proteins that selectively interacted with VDR and other nuclear receptors. One cDNA clone designated NCoA-62, encoded a 62, 000-Da protein that is highly related to BX42, a Drosophila melanogaster nuclear protein involved in ecdysone-stimulated gene expression. Yeast two-hybrid studies and in vitro protein-protein interaction assays using glutathione S-transferase fusion proteins demonstrated that NCoA-62 formed a direct protein-protein contact with the ligand binding domain of VDR. Coexpression of NCoA-62 in a vitamin D-responsive transient gene expression system augmented 1, 25-dihydroxyvitamin D3-activated transcription, but it had little or no effect on basal transcription or gal4-VP16-activated transcription. NCoA-62 also interacted with retinoid receptors, and its expression enhanced retinoic acid-, estrogen-, and glucocorticoid-mediated gene expression. These data indicate that NCoA-62 may be classified into an emerging set of transcriptional coactivator proteins that function to facilitate vitamin D- and other nuclear receptor-mediated transcriptional pathways.

Cell cycle regulation of the transcriptional coactivators p300 and CREB binding protein

To respond to changes in its environment, the cell utilizes mechanisms that integrate extracellular signals with specific changes in gene expression. To better understand these critical regulatory mechanisms, research has focused, for the most part, on the identification of sequence-specific DNA-binding proteins, such as the nuclear factor kappaB (NF-kappaB) or activator protein 1 (AP-1) families of transcription factors, that interact with the promoter and enhancer elements of genes induced or repressed during cellular activation. More recently, however, it has become apparent that non-DNA-binding transcriptional coactivators, such as p300 and CREB binding protein (CBP), previously thought to function primarily as "bridging" proteins between DNA-bound transcription factors and the basal transcription complex, play a critical regulatory role as integrators of diverse signalling pathways with the selective induction of gene expression. In this commentary, we shall discuss the implications of a particular aspect of this growing and expanding field: how cell cycle regulation of p300 and CBP impacts our understanding of cellular differentiation, the response to DNA damage, and oncogenesis.

Co-activators and co-repressors in the integration of transcriptional responses

The nuclear hormone receptors are DNA binding transcription factors that are regulated by binding of ligands, switching them from an inactive or repressive state to gene-activating functions. Recent evidence supports the hypothesis that many nuclear receptors switch, in a ligand-dependent manner, between binding of a multicomponent co-repressor complex containing histone deacetyltransferase activity, and binding of a co-activator complex containing factors with histone acetyltransferase activity that are further regulated by diverse signal transduction pathways. The identification of these limiting co-repressor and co-activator complexes and their specific interaction motifs, in concert with solution of the structures of the receptor ligand-binding domain in apo (empty) and ligand bound forms, indicates a common molecular mechanism by which these factors activate and repress gene transcription.

The steroid receptor coactivator-1 contains multiple receptor interacting and activation domains that cooperatively enhance the activation function 1 (AF1) and AF2 domains of steroid receptors

Steroid receptors are ligand-inducible transcription factors, and their association with steroid receptor coactivators (SRCs) upon binding to DNA is necessary for them to achieve full transcriptional potential. To understand the mechanism of SRC-1 action, its ability to interact and enhance the transcriptional activity of steroid receptors was analyzed. First, we show that SRC-1 is a modular coactivator that possesses intrinsic transcriptional activity when tethered to DNA and that it harbors two distinct activation domains, AD1 and AD2, needed for the maximum coactivation function of steroid receptors. We also demonstrate that SRC-1 interacts with both the amino-terminal A/B or AF1-containing domain and the carboxyl-terminal D/E or AF2-containing domain of the steroid receptors. These interactions are carried out by multiple regions of SRC-1, and they are relevant for transactivation. In addition to the inherent histone acetyltransferase activity of SRC-1, the presence of multiple receptor-coactivator interaction sites in SRC-1 and its ability to interact with components of the basic transcriptional machinery appears to be, at least in part, the mechanism by which the individual activation functions of the steroid receptors act cooperatively to achieve full transcriptional activity.

Conjunction dysfunction: CBP/p300 in human disease

CBP and its homolog p300 are large nuclear molecules that coordinate a variety of transcriptional pathways with chromatin remodeling. They interact with transcriptional activators as well as repressors, direct chromatin-mediated transcription, function in TP53-mediated apoptosis, and participate in terminal differentiation of certain tissue types. Recent evidence suggests that the demand for CBP/p300 is greater than the supply, and that competition for CBP/p300 might play an important role in cell growth regulation. Alterations of the human CBP gene have been implicated in hematological malignancies as well as in congenital malformation and mental retardation. Likewise, the p300 gene has been recently implicated in leukemia and mutations in both alleles have been observed in gastric and colorectal carcinomas. The role of these proteins in human disease coupled with biochemical evidence suggests that CBP and p300 are tumor suppressor proteins essential in cell-cycle control, cellular differentiation and human development.

Partial hormone resistance in mice with disruption of the steroid receptor coactivator-1 (SRC-1) gene

The in vivo biological function of a steroid receptor coactivator was assessed in mice in which the SRC-1 gene was inactivated by gene targeting. Although in both sexes the homozygous mutants were viable and fertile, target organs such as uterus, prostate, testis, and mammary gland exhibited decreased growth and development in response to steroid hormones. Expression of RNA encoding TIF2, a member of the SRC-1 family, was increased in the SRC-1 null mutant, perhaps compensating partially for the loss of SRC-1 function in target tissues. The results indicate that SRC-1 mediates steroid hormone responses in vivo and that loss of its coactivator function results in partial resistance to hormone.

Rb interacts with histone deacetylase to repress transcription

Previously, we found that Rb can actively repress transcription of cell cycle genes by binding and inactivating transcription factors at the promoter. Here, we demonstrate that Rb can also repress transcription of endogenous cell cycle genes containing E2F sites through recruitment of histone deacetylase, which deacetylates histones on the promoter, thereby promoting formation of nucleosomes that inhibit transcription. These two mechanisms of repression by Rb are selective-some promoters and transcription factors are blocked by this recruitment of histone deacetylase, whereas others are resistant to histone deacetylase activity and are repressed directly by inhibition of transcription factors.

Isolation and characterization of cDNAs corresponding to an additional member of the human histone deacetylase gene family

Several human cDNAs encoding a histone deacetylase protein, HDAC3, have been isolated. Analysis of the predicted amino acid sequence of HDAC3 revealed an open reading frame of 428 amino acids with a predicted molecular mass of 49 kDa. The HDAC3 protein is 50% identical in DNA sequence and 53% identical in protein sequence compared with the previously cloned human HDAC1. Comparison of the HDAC3 sequence with human HDAC2 also yielded similar results, with 51% identity in DNA sequence and 52% identity in protein sequence. The expressed HDAC3 protein is functionally active because it possesses histone deacetylase activity, represses transcription when tethered to a promoter, and binds transcription factor YY1. Similar to HDAC1 and HDAC2, HDAC3 is ubiquitously expressed in many different cell types.

TRAM-1, A novel 160-kDa thyroid hormone receptor activator molecule, exhibits distinct properties from steroid receptor coactivator-1

Nuclear hormone receptors (NRs) are ligand-dependent transcription factors that regulate target gene transcription. We report the molecular cloning and characterization of a novel human cDNA encoding TRAM-1, a thyroid hormone receptor activator molecule, a approximately 160-kDa protein homologous with SRC-1/TIF2, by far-Western-based expression screening. TRAM-1 binds to thyroid hormone receptor (TR) and other NRs in a ligand-dependent manner and enhances ligand-induced transcriptional activity of TR. The AF-2 region in NRs has been thought to play a critical role in mediating ligand-dependent transactivation by the interaction with coactivators. Surprisingly, TRAM-1 retains strong ligand-dependent interaction with an AF-2 mutant of TR (E457A), while SRC-1 fails to interact with this mutant. Furthermore, we identified a critical TRAM-1 binding site in rat TRbeta1 outside of AF-2, as TRAM-1 shows weak ligand-dependent interaction with a helix 3 ligand binding domain TR mutant (K288A), compared with SRC-1. These results suggest that TRAM-1 is a coactivator that may exhibit its activity by interacting with subdomains of NRs other than the AF-2 region, in contrast to SRC-1/TIF2.