Functional analysis of a transactivation domain in the thyroid hormone beta receptor

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

Nuclear receptors regulate transcription by binding to specific DNA response elements of target genes. Herein, we report the molecular cloning and characterization of a novel Xenopus cDNA encoding a transcription coactivator xSRC-3 by using retinoid X receptor (RXR) as a bait in the yeast two-hybrid screening. It belongs to a growing coactivator family that includes a steroid receptor coactivator amplified in breast cancer (AIB1), p300/ CREB-binding protein (CBP)-interacting protein (p/ CIP), and transcriptional intermediate factor 2 (TIF2). It also interacts with a series of nuclear receptors including retinoic acid receptor (RAR), thyroid hormone receptor (TR), and orphan nuclear receptors [hepatocyte nuclear receptor 4 (HNF4) and constitutive androstane receptor (CAR)]. However, it does not interact with small heterodimer partner (SHP), an orphan nuclear receptor known to antagonize ligand-dependent transactivation of other nuclear receptors. In CV-1 cells, cotransfection of xSRC-3 differentially stimulates ligand-induced transactivation of RXR, TR, and RAR in a dose-dependent manner. Interestingly, xSRC-3 is highly expressed in adult liver and early stages of oocyte development, suggesting that studies of xSRC-3 may lead to better understanding of the roles nuclear receptors play in oocyte development as well as liver-specific gene expression.

Regulation of the action of steroid/thyroid hormone receptors by medium-chain fatty acids

Triiodothyronine (T3) causes a 30-fold increase in transcription of the malic enzyme gene in chick embryo hepatocytes; medium-chain fatty acids (MCFAs) inhibit this increase. T3 action is mediated by T3 receptors (TRs) that bind to T3 response elements (T3REs) in this gene's 5'-flanking DNA. In transiently transfected hepatocytes, fragments of 5'-flanking DNA of the malic enzyme gene or artificial T3REs that conferred T3 stimulation also conferred MCFA inhibition to linked reporter genes. Thus, MCFA inhibition may be mediated through cis-acting T3REs and trans-acting TRs, distinguishing MCFA action from that of other fatty acids which act through unique sequence elements. Using binding assays and overexpression of TR, we showed that MCFAs inhibited the transactivating but not the silencing function of TR and did not alter binding of T3 to TR or of TR to T3RE. The C-terminal ligand-binding domain of TR was sufficient to confer stimulation by T3, but not inhibition by MCFA. Inhibition of transactivation by MCFA was specific: ligand-stimulated transcription from T3 or estrogen response elements was inhibited, but that from glucocorticoid or cyclic AMP response elements was not. We propose that MCFAs or metabolites thereof influence the activity of a factor(s) that interacts with the T3 and estrogen receptors to inhibit ligand-stimulated transcription.

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.

Cloning of crustacean ecdysteroid receptor and retinoid-X receptor gene homologs and elevation of retinoid-X receptor mRNA by retinoic acid

We report the cloning and analysis of ecdysteroid receptor (bpEcR) and retinoid-X receptor (UpRXR) cDNA homologs from the fiddler crab Uca pugilator. The deduced amino acid sequence of this crustacean EcR most closely resembles the insect EcRs within the DNA binding and ligand binding domains (LBDs). For UpRXR, the DNA binding domain (DBD) shares greatest identity to the insect USPs. The ligand binding domain, however, is closer to vertebrate RXRs but may have a nonfunctional AF-2 domain. Probes derived from these clones were used to examine transcript levels in blastemas during early limb regeneration. Both UpEcR and UpRXR transcripts were detected in low amounts 1 day after limb loss, but increased during the next 4 days. Immersion of crabs in sea water containing all-trans retinoic acid increased the steady state concentrations of UpRXR transcript and altered the pattern of circulating ecdysteroids. These effects correlate with the disruptive effects of retinoic acid on blastemal differentiation observed in earlier studies.

Role of the conserved C-terminal region of thyroid hormone receptor-alpha in ligand-dependent transcriptional activation

The ligand binding domain (LBD) of thyroid hormone (T3) receptors contains subdomains that participate in transcriptional activation, hormone-relieved repression and dimerization. A sequence conserved within the nuclear receptor superfamily is found at positions 397-405 of the 408-amino acid chicken T3 receptor-alpha (cTR alpha) and is deleted in the related avian v-erbA. Since v-erbA exhibits compromised ligand binding and transcriptional activation, this conserved region may play a role in ligand-dependent transcriptional activation. Transfections reveal that cTR alpha(1-392) and site-directed mutants cTR alpha(L398R) and cTR alpha(F399E) are inactive, while cTR alpha(1-403) displays reduced ligand-dependent transcriptional activity. The loss of transcriptional activity in cTR alpha(1-392) is not caused by impaired DNA binding or receptor dimer formation. Proteolytic protection assays reveal that both transcriptionally active and inactive cTR alpha derivatives undergo T3-mediated conformational changes. Gal4 chimeras containing the final 16, 35 or 44 amino acids of cTR alpha indicate that the conserved C-terminal region does not function as an independent transactivation domain. Our results are consistent with a model in which ligand plays a structural role to position the conserved C-terminal regions of cTR alpha and related receptors in a transcriptionally active conformation.

Nuclear corepressors enhance the dominant negative activity of mutant receptors that cause resistance to thyroid hormone

The syndrome of resistance to thyroid hormone (RTH) is caused by multiple distinct mutations in the ligand-binding domain of the thyroid hormone receptor-beta (TRbeta). Although the mutant receptors are transcriptionally inactive, they inhibit normal receptor function in a dominant negative manner to cause hormone resistance. Recently, a group of transcriptional cofactors, referred to as corepressors (CoRs), was shown to induce ligand-independent silencing of genes that contain positive T3 response elements. CoRs also play a role in the ligand-independent basal activation of genes that are negatively regulated in response to T3. We hypothesized that CoR might play a role in the dominant negative inhibition by TRbeta mutants that cause RTH. In gel mobility shift assays, RTH mutants retained interactions with CoRs even in the presence of T3, whereas the ligand dissociated CoR from wild-type TRbeta. Using Gal4-TR chimeric receptors and a VP16-CoR fusion protein in an interaction assay, a strong positive correlation was found between mutant receptor interactions with CoR and transcriptional silencing activity. A mutation (P214R) that impairs CoR interactions with TR was introduced into the RTH mutants to assess the role of CoR in dominant negative activity. In transient transfection assays, introduction of the P214R CoR mutation decreased RTH mutant silencing of positively regulated genes and basal activation of negatively regulated genes. The dominant negative activity of several different RTH mutants, studied by cotransfection with wild-type receptor, was greatly diminished by the CoR mutation, and this effect was seen with both positively and negatively regulated genes. These results suggest that CoR interactions play a critical role in the dominant negative effect of RTH mutants and support the idea that these proteins are involved in the regulation of genes that are positively as well as negatively regulated by T3.

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.

Weak Interactions in Ternary Copper(II) Complexes with Iodotyrosinates. Biological Significance of the Iodines in Thyroid Hormones

Structures and spectroscopic studies have been carried out on ternary copper(II) complexes, Cu(DA)(AA), with aromatic amino acids (AA = 3,5-diiodo-L-tyrosinate (L-I(2)tyr), 3-iodo-L-tyrosinate (L-Ityr), L-tyrosinate) and diamines (DA = 1,10-phenanthroline, 2,2'-bipyridine (bpy), 2-(aminomethyl)pyridine, histamine (hista), or ethylenediamine). The charge transfer (CT) absorption bands observed in the near UV region and the CD spectral magnitude anomaly observed in the d-d region indicated an effective aromatic ring stacking interaction between the side-chain phenol ring of AA and the aromatic diamine DA in the complexes in solution. Two complexes, [Cu(bpy)(L-Ityr)(H(2)O)].NO(3).CH(3)OH.H(2)O (1) and [Cu(hista)(L-I(2)tyrO(-))(H(2)O)](2).2H(2)O (2), where O(-) represents the deprotonated form of the phenol hydroxyl group, were isolated as single crystals, and their structures were determined by X-ray analysis. Both 1 and 2 crystallized in the monoclinic space group P2(1), with a = 7.549(1) Å, b = 11.431(1) Å, c = 14.292(2) Å, beta = 100.08(1) degrees, and Z = 2 for 1 and a = 9.9642(9) Å, b = 15.825(1) Å, c = 12.451(1) Å, beta = 91.565(7) degrees, and Z = 2 for 2. The molecular structures of 1 and 2 revealed the intramolecular aromatic ring stacking between DA and the iodinated phenol ring of Ityr and I(2)tyrO(-), respectively, in correspondence with the solution spectral observations. The stacking with hista was found to be weaker than that with bpy from the interplanar distances in 1 and 2 and the CT band intensities in solution. The molecular and crystal structures revealed some intermolecular iodine-aromatic ring and iodine-oxygen interactions as well as some hydrogen bonds involving the phenol hydroxyl group.

Isolation and characterization of a novel ligand-dependent thyroid hormone receptor-coactivating protein

The thyroid hormone receptor (TR) regulates the expression of target genes upon binding to triiodothyronine (T3) response elements. In the presence of T3, the TR recruits coactivating proteins that both modulate and integrate the ligand response. We report here the cloning of a novel protein using the TR ligand-binding domain as bait in the yeast two-hybrid system. Analysis of a putative full-length clone demonstrates a cDNA sequence that encodes a protein of 920 amino acids with a size of 120 kDa (p120). Alignment with known sequences shows homology to a previously identified protein of unknown function, termed skeletal muscle abundant protein. Interaction studies demonstrate that p120 interacts with the TR AF-2 domain in the presence of ligand through a 111-amino acid region. Northern analysis demonstrates widespread expression in human tissues. Cotransfection assays in CV-1 cells demonstrate that p120 enhances TR-mediated transactivation on multiple T3 response elements in the presence of T3. In addition, CREB-binding protein synergizes with p120 to enhance this effect. When linked to the GAL4 DNA-binding domain, p120 is an activator of transcription alone. Thus, p120 satisfies a number of important criteria as a nuclear receptor coactivator.

The CCAAT box binding factor, NF-Y, is required for thyroid hormone regulation of rat liver S14 gene transcription

Triiodothyronine (T3) activates rat liver S14 gene transcription through T3 receptors (TRbeta) binding distal thyroid hormone response elements located between -2.8 and -2.5 kilobase pairs upstream from the transcription start site. Previous studies suggested that proximal promoter elements located between -220 to -80 base pairs upstream from the 5' end of the S14 gene were involved in hormone activation of the S14 gene. This report identifies an inverted CCAAT box (or Y box) at -104ATTGG-100 as a core cis-regulatory element. Gel shift studies using rat liver nuclear proteins show that at least three CCAAT-binding factors interact with this region as follows: NF-Y and c/EBP-related proteins formed major complexes, whereas NF-1/CTF forms a minor complex in gel shift assay. Mutation of the Y box indicated that loss of NF-Y binding, but not c/EBP or NF-1, correlated closely with a decline in basal activity and a loss of T3-mediated transactivation. Substitution of the S14 Y box in reporter genes with elements binding only NF-Y elevated basal activity and T3-mediated transactivation, whereas substitution with elements binding c/EBP-related proteins or SP1 displayed low basal activity and T3-mediated transactivation. These studies indicate that NF-Y and TRbeta functionally interact to confer T3 control to the S14 gene.

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.

Differential transactivation by two isoforms of the orphan nuclear hormone receptor CAR

We have identified a new murine orphan member of the nuclear hormone receptor superfamily, termed mCAR, that is closely related to the previously described human orphan MB67, referred to here as hCAR. Like hCAR, mCAR expression is highest in liver. In addition to the most abundant mCAR1 isoform, the mCAR gene expresses a truncated mCAR2 variant that is missing the C-terminal portion of the ligand binding/dimerization domain. The mCAR gene has 8 introns, and this mCAR2 variant is generated by a splicing event that skips the 8th exon. mCAR1, like hCAR, binds as a heterodimer with the retinoid X receptor to the retinoic acid response element from the promoter of the retinoic acid receptor beta2 isoform. Consistent with its lack of a critical heterodimerization interface, the mCAR2 variant does not bind this site. Both mCAR1 and hCAR are apparently constitutive transcriptional activators. This activity is dependent on the presence of the conserved C-terminal AF-2 transcriptional activation motif. As expected from its inability to bind DNA, the mCAR2 variant neither transactivates by itself nor inhibits transactivation by hCAR or mCAR1.

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.

Mutations in the 1,25-dihydroxyvitamin D3 receptor identifying C-terminal amino acids required for transcriptional activation that are functionally dissociated from hormone binding, heterodimeric DNA binding, and interaction with basal transcription factor IIB, in vitro

To investigate a potential ligand-dependent transcriptional activation domain (AF-2) in the C-terminal region of the human vitamin D receptor (hVDR), two conserved residues, Leu-417 and Glu-420, were replaced with alanines by site-directed mutagenesis (L417A and E420A). Transcriptional activation in response to 1, 25-dihydroxyvitamin D3 (1,25-(OH)2D3) was virtually eliminated when either point mutant was transfected into several mammalian cell lines. Furthermore, both mutants exhibited a dominant negative phenotype when expressed in COS-7 cells. Scatchard analysis at 4 degrees C and a ligand-dependent DNA binding assay at 25 degrees C revealed essentially normal 1,25-(OH)2D3 binding for the mutant hVDRs, which were also equivalent to native receptor in associating with the rat osteocalcin vitamin D responsive element as a presumed heterodimer with retinoid X receptor. Glutathione S-transferase-human transcription factor IIB (TFIIB) fusion protein linked to Sepharose equally coprecipitated the wild-type hVDR and the AF-2 mutants. These data implicate amino acids Leu-417 and Glu-420, residing in a putative alpha-helical region at the extreme C terminus of hVDR, as critical in the mechanism of 1, 25-(OH)2D3-stimulated transcription, likely mediating an interaction with a coactivator(s) or a component of the basal transcriptional machinery distinct from TFIIB.

Thyroid hormone-mediated enhancement of heterodimer formation between thyroid hormone receptor beta and retinoid X receptor

A subset of nuclear receptors, including those for thyroid hormone (TR), retinoic acid, vitamin D3, and eicosanoids, can form heterodimers with the retinoid X receptor (RXR) on DNA regulatory elements in the absence of their cognate ligands. In a mammalian two-hybrid assay, we have found that recruitment of a VP16-RXR chimera by a Gal4-TRbeta ligand-binding domain fusion is enhanced up to 50-fold by thyroid hormone (T3). This was also observed with a mutant fusion, Gal4-TR(L454A), lacking ligand-inducible activation function (AF-2) and unable to interact with putative coactivators, suggesting that the AF-2 activity of TR or intermediary cofactors is not involved in this effect. The wild-type and mutant Gal4-TR fusions also exhibited hormone-dependent recruitment of RXR in yeast. Hormone-dependent recruitment of RXR was also evident with another Gal4-TR mutant, AHTm, which does not interact with the nuclear receptor corepressor N-CoR, suggesting that ligand-enhanced dimerization is not a result of T3-induced corepressor release. Finally, we have shown that the interaction between RXR and TR is augmented by T3 in vitro, arguing against altered expression of either partner in vivo mediating this effect. We propose that ligand-dependent heterodimerization of TR and RXR in solution may provide a further level of control in nuclear receptor signaling.

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.

Transcriptional activation by peroxisome proliferator-activated receptor gamma is inhibited by phosphorylation at a consensus mitogen-activated protein kinase site

The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) regulates transcription in response to prostanoid and thiazolidinedione ligands and promotes adipocyte differentiation. The amino-terminal A/B domain of this receptor contains a consensus mitogen-activated protein kinase site in a region common to PPARgamma1 and -gamma2 isoforms. The A/B domain of human PPARgamma1 was phosphorylated in vivo, and this was abolished either by mutation of serine 84 to alanine (S84A) or coexpression of a phosphoprotein phosphatase. In vitro, this domain was phosphorylated by ERK2 and JNK, and this was markedly reduced in the S84A mutant. A wild type Gal4-PPARgamma(A/B) chimera exhibited weak constitutive transcriptional activity. Remarkably, this was significantly enhanced in the S84A mutant fusion. Ligand-dependent activation by full-length mouse PPARgamma2 was also augmented by mutation of the homologous serine in the A/B domain to alanine. The nonphosphorylatable form of PPARgamma was also more adipogenic. Thus, phosphorylation of a mitogen-activated protein kinase site in the A/B region of PPARgamma inhibits both ligand-independent and ligand-dependent transactivation functions. This observation provides a potential mechanism whereby transcriptional activation by PPARgamma may be modulated by growth factor or cytokine-stimulated signal transduction pathways involved in adipogenesis.

Unique requirements for retinoid-dependent transcriptional activation by the orphan receptor LXR

LXR is an orphan nuclear receptor that confers retinoid responsiveness to the retinoid X receptor (RXR) by its interaction on a specific response element called an LXRE. To understand the mechanism of this response, three characteristics were identified that are crucial to activation of the RXR-LXR complex. First, the orientation of the RXR-LXR heterodimer on DNA indicates that as the ligand-binding partner, RXR occupies the 5' half-site of the response element. Next, the sequence specificity of the LXRE was determined in order to identify residues required for retinoid activation of the heterodimer. Remarkably, subtle changes in the nucleotide sequence of the LXRE half-sites that do not substantially alter DNA binding of the RXR-LXR heterodimer have a significant effect on the ability of the complex to be activated by ligand. Finally, we characterized the contributions of the activation domains of each receptor to the trans-activation potential of the RXR-LXR heterodimer. Surprisingly, our results show that only the activation domain of LXR is required for retinoid activation. Taken together, these results demonstrate the existence of a unique form of communication between heterodimer partners in which the activation potential of one receptor (LXR) is enabled by ligand binding to its partner (RXR). Furthermore, we conclude that RXR ligand activation potential is not dictated solely by its position on DNA, but is influenced by other factors such as the receptor partner and sequence of the response element.

A natural transactivation mutation in the thyroid hormone beta receptor: impaired interaction with putative transcriptional mediators

The syndrome of resistance to thyroid hormone is characterized by elevated serum free thyroid hormones, failure to suppress pituitary thyrotropin secretion, and variable peripheral refractoriness to hormone action. Here we describe a novel leucine to valine mutation in codon 454 (L454V) of the thyroid hormone beta receptor (TR beta) in this disorder, resulting in a mutant receptor with unusual functional properties. Although the mutant protein binds ligand comparably to wild-type receptor and forms homo- and heterodimers on direct repeat, everted repeat, or palindromic thyroid response elements, its ability to activate transcription via these elements is markedly impaired. The hydrophobic leucine residue lies within an amphipathic alpha-helix at the carboxyl terminus of TR beta and the position of the homologous residue in the crystal structure of TR alpha indicates that its side chain is solvent-exposed and might interact with other proteins. We find that two putative transcriptional mediators (RIP140 and SRC-1) exhibit hormone-dependent association with wild-type TR. In comparison, the interaction of this natural mutant (L454V) and artificial mutants (L454A, E457A) with RIP140 and SRC-1 is markedly reduced. Furthermore, coexpression of SRC-1 is able to restore the transcriptional activity of the L454V mutant receptor, indicating that the interaction of this residue with accessory proteins is critical for transcriptional activation. Finally, the occurrence of the L454V mutation in resistance to thyroid hormone, together with impaired negative regulation of the thyroid-stimulating hormone alpha promoter by this mutant, suggests that the amphipathic alpha-helix also mediates hormone-dependent transcriptional inhibition, perhaps via interaction with these or other accessory factors.

Different DNA elements can modulate the conformation of thyroid hormone receptor heterodimer and its transcriptional activity

Thyroid-hormone receptors (TRs) form heterodimers with retinoid-X receptors (RXRs) on thyroid-hormone-response elements (TREs). However, it is not known whether the formation of liganded TR/RXR heterodimer on a TRE alone is sufficient to dictate transcriptional activity. We designed several mutated DR4s (half-sites arranged as direct repeats with a nucleotide gap of 4) that bound TR/RXR heterodimers preferentially, and employed them to characterize functional and biochemical properties of the heterodimers on DNA. Although TR/RXR heterodimer binding was similar on some of the mutated DR4s, transient transfection assays showed that TRalpha failed to support triiodothyronine (T3)-stimulated transcription on "inactive" DR4s but mediated basal repression on both "active" and inactive mutated DR4. T3 binding assays showed that the mutated DR4s did not affect T3 binding to the heterodimer. Finally, partial proteolysis studies revealed that binding of active DR4 elements and T3 to the heterodimer synergistically enhanced heterodimerization-induced protease resistance of TR, but not RXR, in the heterodimer. These results suggest that: 1) liganded TR/RXR heterodimer binding to a DR4 is not sufficient for transcriptional activation of the target gene, and 2) DNA sequences in specific TREs may modify T3-mediated transcription by affecting the conformation of the liganded heterodimer.

A structural role for hormone in the thyroid hormone receptor

The crystal structure of the rat alpha 1 thyroid hormone receptor ligand-binding domain bound with a thyroid hormone agonist reveals that ligand is completely buried within the domain as part of the hydrophobic core. In addition, the carboxy-terminal activation domain forms an amphipathic helix, with its hydrophobic face constituting part of the hormone binding cavity. These observations suggest a structural role for ligand, in establishing the active conformation of the receptor, that is likely to underlie hormonal regulation of gene expression for the nuclear receptors.

Two consecutive nucleotide substitutions resulting in the T3 receptor beta gene resulting in an 11-amino acid truncation in a patient with generalized resistance to thyroid hormone

We identified unusual mutations in the T3 receptor (TR) beta gene in a 6-year-old Japanese girl with generalized resistance to thyroid hormone. Two consecutive base substitutions, T to A and C to A at nucleotide positions 1637 and 1638, respectively, changed the 451st codon coding for Phe(TTC) to stop codon (TAA), resulting in an 11-amino acid carboxyl(C)-terminus truncation. The patient was a heterozygote. Western blotting using an anti-TR antibody demonstrated the truncated receptor protein. The patient showed severe mental retardation (IQ41), disturbance in speech development, and attention deficit hyperactivity disorder. Thyroid functional status by clinical evaluation was considered within the normal range in spite of high serum thyroid hormone levels (T4 725.9 nmol/l, T3 12.7 nmol/l, FT4 166.0 pmol/l). TSH increased from 0.6 to 24 mU/L after TRH (150 micrograms) injection. TSH secretion as well as 123I-uptake was suppressed only partially by T3 (75 micrograms/day for a week). Close examination of thyroid functions and TR beta gene analysis were not possible in the family, except for paternal grandmother and one of her two sisters who showed no abnormality. The patient's truncated TR beta showed very low T3 binding activity (Ka = 0.1 x 10-10 M), transcriptional activity, and a very strong dominant negative effect. When co-expressed with wild-type TR beta at the molar ratio 1:1 in CV-1 cells, the mutant receptor inhibited the wild-type TR beta transcriptional activity by 74% at 10 nM T3. Even 1 microM T3 could not normalize these impaired functions.

Activation function 1 of retinoic acid receptor beta 2 is an acidic activator resembling VP16

The mechanisms underlying transcriptional activation are not very well understood, and knowledge is based on experiments with a small number of mostly viral activators. We have investigated the mechanism underlying transactivation by the activation domain present in the N-terminal part of retinoic acid receptor (RAR) beta 2 (AF-1). We show that RAR beta 2 phosphorylation is not crucial for its activity although it may modulate AF-1 activity. Sequential mutation of the negatively charged residues (Asp) resulted in a stepwise decrease in activity, while mutation of all aspartic acid residues resulted in complete loss of activity. Comparison of the critical region for activation with other activators revealed moderate homology with the viral activator VP16. The hydrophobic amino acids surrounding the negatively charged residues reported to be critical for activation by VP16 are all conserved in AF-1. The hydrophobic residues are required for AF-1, since mutation of these residues resulted in a decrease in activity. Furthermore, the activity of this activator, VP16 and TA1 of RelA, is squelched by overexpression of an AF-1-containing expression construct, indicating that AF-1 is an acidic activator. Squelching experiments further indicate that AF-1 and AF-2 function by different mechanisms. Comparison of activation functions present in the AB region of other members of the steroid/thyroid hormone receptor family: RAR alpha 2, RAR gamma 2, and GR suggested that also these receptors contain an acidic activation domain. The mechanism underlying activation by AF-1 is discussed.

Polarity-specific activities of retinoic acid receptors determined by a co-repressor

Retinoic acid receptors (RARs) and retinoid-X receptors (RXRs) activate or repress transcription by binding as heterodimers to DNA-response elements that generally consist of two direct repeat half-sites of consensus sequence AGGTCA. On response elements consisting of direct repeats spaced by five base pairs (DR + 5 elements), RAR/RXR heterodimers activate transcription in response to RAR-specific ligands, such as all-trans-retinoic acid (RA). In contrast, on elements consisting of direct repeats spaced by one base pair (DR + 1 elements), RAR/RXR heterodimers exhibit little or no response to activating ligands and repress RXR-dependent transcription. Here we show that ligand-dependent transactivation by RAR on DR + 5 elements requires the dissociation of a new nuclear receptor co-repressor, N-CoR, and recruitment of the putative co-activators p140 and p160. Surprisingly, on DR + 1 elements, N-CoR remains associated with RAR/RXR heterodimers even in the presence of RAR ligands, resulting in constitutive repression. These observations indicate that DNA-response elements can allosterically regulate RAR-co-repressor interactions to determine positive or negative regulation of gene expression.

Structure of the carboxy-terminal region of thyroid hormone nuclear receptors and its possible role in hormone-dependent intermolecular interactions

The thyroid hormone nuclear receptors (TRs) are ligand-dependent transcription factors. To understand the molecular basis of ligand-dependent transactivation, we studied the structure of their carboxy-terminal activation domain. We analyzed the structures of the peptides derived from the C-terminal sequences of human TR subtypes beta 1 (h-TR beta 1) and alpha 1 (h-TR alpha 1) and a human TR mutant, PV, by circular dichroism (CD). Mutant PV has a C-terminal frameshift mutation and does not bind to the thyroid hormone, 3,3',5-triiodo-L-thyronine (T3). Analyses of the secondary structures of the peptides by CD indicate that five amino acids, EVFED, are part of an amphipathic alpha-helix which is required to maintain the structural integrity of the hormone binding domain. A monoclonal antibody, C4 (mAb C4), which recognizes both h-TR beta 1 and h-TR alpha 1 was developed. Using a series of truncated mutants and synthetic peptides, we mapped the epitope of mAb C4 to the conserved C-terminal amino acids, EVFED. Analysis of the binding data indicates that binding of T3 to either h-TR beta 1 or h-TR alpha 1 was competitively inhibited by mAb C4. Deletion of C-terminal amino acids including EVFED led to a total loss of T3 binding activity. Thus, part of the T3 binding site is located in this five amino acid segment. T3 may transduce its hormonal signal to the transcriptional machinery via interaction with EVFED at the C-terminus of TRs.