Negative regulation of the glycoprotein hormone alpha gene promoter by thyroid hormone: mutagenesis of a proximal receptor binding site preserves transcriptional repression

Reversing thyroid-hormone-mediated repression of a HSV-1 promoter via computationally guided mutagenesis

Thyroid hormones (THs) and their DNA-binding nuclear receptors (TRs) direct transcriptional regulation in diverse ways depending on the host cell environment and specific promoter characteristics of TH-sensitive genes. This study sought to elucidate the impact on transcriptional repression of nucleotide sequence or orientation within TR binding sites - the TH response elements (TREs) of TH-sensitive promoters - to better understand ligand-dependent transcriptional repression of wild-type promoters. Computational analysis of the HSV-1 thymidine kinase (TK) gene TRE bound by TR and retinoid X receptor (RXR) revealed a single TRE point mutation sufficient to reverse the TRE orientation. In vitro experiments showed that the TRE point mutation had distinct impacts on promoter activity, sufficient to reverse the TH-dependent negative regulation in neuroendocrine differentiated cells. This point mutation altered the promoter's regulatory mechanism by discrete changes in transcription factor TR occupancy and altered enrichment of the repressive chromatin modification of histone-3-lysine-9-trimethyl (H3K9Me3). Insights relating to this negative TRE (nTRE) mechanism aids our understanding of other nTREs and TRE mutations associated with TH and herpes diseases.

Liganded thyroid hormone receptor inhibits phorbol 12-O-tetradecanoate-13-acetate-induced enhancer activity via firefly luciferase cDNA

Thyroid hormone receptor (TR) belongs to the nuclear hormone receptor (NHR) superfamily and regulates the transcription of its target genes in a thyroid hormone (T3)-dependent manner. While the detail of transcriptional activation by T3 (positive regulation) has been clarified, the mechanism of T3-dependent repression (negative regulation) remains to be determined. In addition to naturally occurring negative regulations typically found for the thyrotropin β gene, T3-bound TR (T3/TR) is known to cause artificial negative regulation in reporter assays with cultured cells. For example, T3/TR inhibits the transcriptional activity of the reporter plasmids harboring AP-1 site derived from pUC/pBR322-related plasmid (pUC/AP-1). Artificial negative regulation has also been suggested in the reporter assay with firefly luciferase (FFL) gene. However, identification of the DNA sequence of the FFL gene using deletion analysis was not performed because negative regulation was evaluated by measuring the enzymatic activity of FFL protein. Thus, there remains the possibility that the inhibition by T3 is mediated via a DNA sequence other than FFL cDNA, for instance, pUC/AP-1 site in plasmid backbone. To investigate the function of FFL cDNA as a transcriptional regulatory sequence, we generated pBL-FFL-CAT5 by ligating FFL cDNA in the 5' upstream region to heterologous thymidine kinase promoter in pBL-CAT5, a chloramphenicol acetyl transferase (CAT)-based reporter gene, which lacks pUC/AP-1 site. In kidney-derived CV1 and choriocarcinoma-derived JEG3 cells, pBL-FFL-CAT5, but not pBL-CAT5, was strongly activated by a protein kinase C activator, phorbol 12-O-tetradecanoate-13-acetate (TPA). TPA-induced activity of pBL-FFL-CAT5 was negatively regulated by T3/TR. Mutation of nt. 626/640 in FFL cDNA attenuated the TPA-induced activation and concomitantly abolished the T3-dependent repression. Our data demonstrate that FFL cDNA sequence mediates the TPA-induced transcriptional activity, which is inhibited by T3/TR.

In vivo interaction of steroid receptor coactivator (SRC)-1 and the activation function-2 domain of the thyroid hormone receptor (TR) beta in TRbeta E457A knock-in and SRC-1 knockout mice

The activation function-2 (AF-2) domain of the thyroid hormone (TH) receptor (TR)-beta is a TH-dependent binding site for nuclear coactivators (NCoA), which modulate TH-dependent gene transcription. In contrast, the putative AF-1 domain is a TH-independent region interacting with NCoA. We determined the specificity of the AF-2 domain and NCoA interaction by evaluating thyroid function in mice with combined disruption of the AF-2 domain in TRbeta, due to a point mutation (E457A), and deletion of one of the NCoAs, steroid receptor coactivator (SRC)-1. The E457A mutation was chosen because it abolishes NCoA recruitment in vitro while preserving normal TH binding and corepressor interactions resulting in resistance to TH. At baseline, disruption of SRC-1 in the homozygous knock-in (TRbeta(E457A/E457A)) mice worsened the degree of resistance to TH, resulting in increased serum T(4) and TSH. During TH deprivation, disruption of AF-2 and SRC-1 resulted in a TSH rise 50% of what was seen when AF-2 alone was removed, suggesting that SRC-1 was interacting outside of the AF-2 domain. Therefore, 1) during TH deprivation, SRC-1 is necessary for activating the hypothalamic-pituitary-thyroid axis; 2) ligand-dependent repression of TSH requires an intact AF-2; and 3) SRC-1 may interact with the another region of the TRbeta or the TRalpha to regulate TH action in the pituitary. This report demonstrates the dual interaction of NCoA in vivo: the TH-independent up-regulation possibly through another domain and TH-dependent down-regulation through the AF-2 domain.

Minireview: Thyrotropin-releasing hormone and the thyroid hormone feedback mechanism

Thyroid hormone (TH) plays a critical role in development, growth, and cellular metabolism. TH production is controlled by a complex mechanism of positive and negative regulation. Hypothalamic TSH-releasing hormone (TRH) stimulates TSH secretion from the anterior pituitary. TSH then initiates TH synthesis and release from the thyroid gland. The synthesis of TRH and TSH subunit genes is inhibited at the transcriptional level by TH, which also inhibits posttranslational modification and release of TSH. Although opposing TRH and TH inputs regulate the hypothalamic-pituitary-thyroid axis, TH negative feedback at the pituitary was thought to be the primary regulator of serum TSH levels. However, study of transgenic animals showed an unexpected, dominant role for TRH in regulating the hypothalamic-pituitary-thyroid axis and an unanticipated involvement of the thyroid hormone receptor ligand-dependent activation function (AF-2) domain in TH negative regulation. These results are summarized in the review.

Negative regulation of TSHalpha target gene by thyroid hormone involves histone acetylation and corepressor complex dissociation

Currently, little is known about histone modifications and molecular mechanisms of negatively regulated transcription. In pituitary cells, thyroid hormone (T(3)) decreased transcription, and surprisingly increased histone acetylation, of TSHalpha promoter. This increase was mediated directly by thyroid hormone receptor. Histone acetylation of H3K9 and H3K18 sites, two modifications usually associated with transcriptional activation, occur in negative regulation of TSHalpha promoter. T(3) also caused release of a corepressor complex composed of histone deacetylase 3 (HDAC3), transducin beta-like protein 1, and nuclear receptor coprepressor (NCoR)/ silencing mediator for retinoic and thyroid hormone receptor from TSHalpha promoter in chromatin immunoprecipitation assays. NCoR and HDAC3 overexpression selectively increased ligand-independent basal transcription. Two histone acetyltransferase inhibitors increased overall transcription but did not abrogate negative regulation or NCoR/HDAC3 complex release by T(3). Chromatin immunoprecipitation analyses of an endogenous positively regulated target gene showed increased histone acetylation and corepressor complex release with T(3) treatment. Finally, microarray analyses suggested there is a subset of negatively regulated genes with increased histone acetylation. These findings demonstrate the critical role of NCoR/HDAC3 complex in negative regulation of TSHalpha gene expression and show that similar complexes and overlapping epigenetic modifications can participate in both negative and positive transcriptional regulation.

Essential role of GATA2 in the negative regulation of thyrotropin beta gene by thyroid hormone and its receptors

Previously we reported that the negative regulation of the TSHbeta gene by T(3) and its receptor [thyroid hormone receptor (TR)] is observed in CV1 cells when GATA2 and Pit1 are introduced. Using this system, we further studied the mechanism of TSHbeta inhibition. The negative regulatory element (NRE), which had been reported to mediate T(3)-bound TR (T(3)-TR)-dependent inhibition, is dispensable, because deletion or mutation of NRE did not impair suppression. The reporter construct, TSHbeta-D4-chloramphenicol acetyltransferase, which possesses only the binding sites for Pit1 and GATA2, was activated by GATA2 alone, and this transactivation was specifically inhibited by T(3)-TR. The Zn finger region of GATA2 interacts with the DNA-binding domain of TR in a T(3)-independent manner. The suppression by T(3)-TR was impaired by overexpression of a dominant-negative type TR-associated protein (TRAP) 220, an N- and C-terminal deletion construct, indicating the participation of TRAP220. Chromatin immunoprecipitation assays with a thyrotroph cell line, TalphaT1, revealed that T(3) treatment recruited histone deacetylase 3, reduced the acetylation of histone H4, and caused the dissociation of TRAP220 within 15-30 min. The reduction of histone H4 acetylation was transient, whereas the dissociation of TRAP220 persisted for a longer period. In the negative regulation of the TSHbeta gene by T(3)-TR we report that 1) GATA2 is the major transcriptional activator of the TSHbeta gene, 2) the putative NRE previously reported is not required, 3) TR-DNA-binding domain directly interacts with the Zn finger region of GATA2, and 4) histone deacetylation and TRAP220 dissociation are important.

Negative regulation by thyroid hormone receptor requires an intact coactivator-binding surface

Thyroid hormone (TH) action is mediated by TH receptors (TRs), which are members of the nuclear hormone receptor superfamily. In vitro studies have demonstrated that TR activity is regulated by interactions with corepressor and coactivator proteins (CoRs and CoAs, respectively). TH stimulation is thought to involve dissociation of CoRs and recruitment of CoAs to the liganded TR. In contrast, negative regulation by TH is thought to occur via recruitment of CoRs to the liganded TR. The physiological role of CoAs bound to TRs, however, has yet to be defined. In this study, we used gene-targeting techniques to mutate the TR-beta locus within its activation function-2 (AF-2) domain (E457A). This mutation was chosen because it completely abolished CoA recruitment in vitro, while preserving normal triiodothyronine (T3) binding and CoR interactions. As expected, TH-stimulated gene expression was reduced in homozygous E457A mice. However, these animals also displayed abnormal regulation of the hypothalamic-pituitary-thyroid axis. Serum thyroxine, T3, and thyroid-stimulating hormone (TSH) levels and pituitary Tshb mRNA levels were inappropriately elevated compared with those of WT animals, and L-T3 treatment failed to suppress serum TSH and pituitary Tshb mRNA levels. Therefore, the AF-2 domain of TR-beta is required for positive and, paradoxically, for negative regulation by TH in vivo.

A Response Unit in the First Exon of the β-Amyloid Precursor Protein Gene Containing Thyroid Hormone Receptor and Sp1 Binding Sites Mediates Negative Regulation by 3,5,3′-Triiodothyronine

Thyroid hormones repress expression of APP (beta-amyloid precursor protein) in cultured cells of neuronal origin. The effect involves binding to the nuclear thyroid hormone receptor (TR) and is mediated by DNA sequences located within the first exon of the gene. These sequences contain a thyroid hormone response element that is necessary, but not sufficient, to mediate the inhibitory effect of the thyroid hormone T(3). In this report, we show that repression by T(3) is mediated by a response unit composed by the thyroid hormone response element and 5'-flanking sequences that bind Sp1 and mediate stimulation by this transcription factor. In that unit, binding sites for TR and Sp1 overlap and a complex mechanism appears to account for the TR-mediated regulation of APP. Unliganded TR does not bind to DNA and allows Sp1 to bind to DNA and stimulate APP basal expression. Binding of ligand T(3), which increases affinity of TR by DNA, precludes binding of Sp1 to DNA and decreases the Sp1-dependent expression of APP.

Thyroid hormone receptor DNA binding is required for both positive and negative gene regulation

The beta isoform of thyroid hormone receptor (TR-beta) has a key role in the feedback regulation of the hypothalamic-pituitary-thyroid (H-P-T) axis. The mechanism of trans-repression of the hypothalamic thyrotropin-releasing hormone (TRH) and pituitary thyroid-stimulating hormone (TSH) subunit genes, however, remains poorly understood. A number of distinct mechanisms for TR-beta-mediated negative regulation by thyroid hormone have been proposed, including those that require and do not require DNA binding. To clarify the importance of DNA binding in negative regulation, we constructed a DNA-binding mutant of TR-beta in which two amino acids within the P box were altered (GSG for EGG) to resemble that found in the glucocorticoid receptor (GR). We termed this mutant GS125, and as expected, it displayed low binding affinities for positive and negative thyroid hormone-response element (pTRE and nTRE, respectively) in gel-mobility shift assays. In transient transfection assays, the GS125 mutant abolished transactivation on three classic pTREs (DR+4, LAP, and PAL) and all negatively regulated promoters in the H-P-T axis (TRH, TSH-beta, and TSH-alpha). However, GS125 TR-beta bound to a composite TR/GR-response element and was fully functional on this hybrid TR/GR-response element. Moreover, the GS125 TR-beta mutant displayed normal interactions with transcriptional cofactors in mammalian two-hybrid assays. These data do not support a DNA-binding independent mechanism for thyroid hormone negative regulation in the H-P-T axis.

Hormone-dependent repression of the E2F-1 gene by thyroid hormone receptors

Thyroid hormone induces differentiation of many different tissues in mammals, birds, and amphibians. The different tissues all differentiate from proliferating precursor cells, and the normal cell cycle is suspended while cells undergo differentiation. We have investigated how thyroid hormone affects the expression of the E2F-1 protein, a key transcription factor that controls G1- to S-phase transition. We show that during thyroid hormone-induced differentiation of embryonic carcinoma cells and of oligodendrocyte precursor cells, the levels of E2F-1 mRNA and E2F-1 protein decrease. This is caused by the thyroid hormone receptor (TR) regulating the transcription of the E2F-1 gene. The TR binds directly to a negative thyroid hormone response element, called the Z-element, in the E2F-1 promoter. When bound, the TR activates transcription in the absence of ligand but represses transcription in the presence of ligand. In addition, liganded TR represses transcription of the S-phase-specific DNA polymerase alpha, thymidine kinase, and dihydropholate reductase genes. These results suggest that thyroid hormone-induced withdrawal from the cell cycle takes place through the repression of S-phase genes. We suggest that this is an initial and crucial step in thyroid hormone-induced differentiation of precursor cells.

Thyroid hormone is a critical determinant for the regulation of the cochlear motor protein prestin

The most impressive property of outer hair cells (OHCs) is their ability to change their length at high acoustic frequencies, thus providing the exquisite sensitivity and frequency-resolving capacity of the mammalian hearing organ. Prestin, a protein related to a sulfate/anion transport protein, recently has been identified and proposed as the OHC motor molecule. Homology searches of 1.5 kb of genomic DNA 5' of the coding region of the prestin gene allowed the identification of a thyroid hormone (TH) response element (TRE) in the first intron upstream of the prestin ATG codon. Prestin(TRE) bound TH receptors as a monomer or presumptive heterodimer and mediated a triiodothyronine-dependent transactivation of a heterologous promotor in response to triiodothyronine receptors alpha and beta. Retinoid X receptor-alpha had an additive effect. Expression of prestin mRNA and prestin protein was reduced strongly in the absence of TH. Although prestin protein typically was redistributed to the lateral membrane before the onset of hearing, an immature pattern of prestin protein distribution across the entire OHC membrane was noted in hypothyroid rats. The data suggest TH as a first transcriptional regulator of the motor protein prestin and as a direct or indirect modulator of subcellular prestin distribution.

Physiological and molecular basis of thyroid hormone action

Thyroid hormones (THs) play critical roles in the differentiation, growth, metabolism, and physiological function of virtually all tissues. TH binds to receptors that are ligand-regulatable transcription factors belonging to the nuclear hormone receptor superfamily. Tremendous progress has been made recently in our understanding of the molecular mechanisms that underlie TH action. In this review, we present the major advances in our knowledge of the molecular mechanisms of TH action and their implications for TH action in specific tissues, resistance to thyroid hormone syndrome, and genetically engineered mouse models.

Transcriptional repression of neurotrophin receptor trkB by thyroid hormone in the developing rat brain

Expression of the neurotrophin receptor trkB is regulated by thyroid hormone (T3) during development of the rat brain. trkB mRNA levels, coding for the full-length and the truncated isoforms, are increased in the cerebral cortex of neonatal experimental hypothyroid animals. Run-on transcription assays with nuclei from postnatal day 15, hypothyroid, and control cerebral cortices demonstrated that an increase in the transcription rate of the trkB gene accounts for the observed effect. Transient transfection experiments using a reporter plasmid containing a 7-kilobase pair DNA fragment upstream of the mouse trkB gene showed that unliganded thyroid hormone receptor (T3R) increases promoter activity, whereas addition of T3 reverses that activity below basal levels. Deletion analysis shows that the T3-dependent repression requires binding of the T3R to a specific region located downstream of the transcription start site. This region, at nucleotide position -465/-432, contains an array of thyroid hormone response half-sites that bind preferentially T3R as heterodimers with retinoid X receptor and whose deletion causes loss of the T3-dependent repression. These half-sites are able to confer negative regulation by T3 to a heterologous promoter, thus indicating the functionality of these sequences. These results demonstrate that, in the developing rat brain, T3 down-regulates the expression of the trkB gene through the active repression of a novel negative response element located downstream of its transcription initiation site.

Negative transcriptional regulation mediated by thyroid hormone response element 144 requires binding of the multivalent factor CTCF to a novel target DNA sequence

DNA target sites for a "multivalent" 11-zinc-finger CCTC-binding factor (CTCF) are unusually long ( approximately 50 base pairs) and remarkably different. In conjunction with the thyroid receptor (TR), CTCF binding to the lysozyme gene transcriptional silencer mediates the thyroid hormone response element (TRE)-dependent transcriptional repression. We tested whether other TREs, which in addition to the presence of a TR binding site require neighboring sequences for transcriptional function, might also contain a previously unrecognized binding site(s) for CTCF. One such candidate DNA region, previously isolated by Bigler and Eisenman (Bigler, J., and Eisenman, R. N. (1995) EMBO J. 14, 5710-5723), is the TRE-containing genomic element 144. We have identified a new CTCF target sequence that is adjacent to the TR binding site within the 144 fragment. Comparison of CTCF recognition nucleotides in the lysozyme silencer and in the 144 sequences revealed both similarities and differences. Several C-terminal CTCF zinc fingers contribute differently to binding each of these sequences. Mutations that eliminate CTCF binding impair 144-mediated negative transcriptional regulation. Thus, the 144 element provides an additional example of a functionally significant composite "TRE plus CTCF binding site" regulatory element suggesting an important role for CTCF in cooperation with the steroid/thyroid superfamily of nuclear receptors to mediate TRE-dependent transcriptional repression.

Mechanisms that mediate negative regulation of the thyroid-stimulating hormone alpha gene by the thyroid hormone receptor

A group of transcriptional cofactors for nuclear hormone receptors, referred to as corepressors (CoRs) and coactivators (CoAs), has been shown to induce transcriptional silencing and hormone-induced activation, respectively, of genes that contain positive hormone response elements. Transcriptional silencing by CoRs involves the recruitment of histone deacetylases (HDACs), whereas ligand-dependent activation is associated with the recruitment of CoAs, which possess or recruit histone acetyltransferases (HATs). In a reciprocal manner, negatively regulated genes are stimulated by nuclear receptors in the absence of ligand and are repressed in response to ligand binding to receptors. We show here that negative regulation of the thyroid-stimulating hormone alpha (TSHalpha) promoter by the thyroid hormone receptor (TR) involves a novel mechanism in which the recruitment of CoRs by TR is associated with transcriptional stimulation and histone acetylation. Expression of excess HDAC reverses the stimulation mediated by the TR.CoR complex, consistent with a pivotal role for acetylation in this event. Addition of the ligand, 3,5,3'-triiodothyronine (T3), induces transcriptional repression of the TSHalpha promoter and is associated with the loss of histone acetylation. T3-dependent repression is blocked by phosphorylation of cAMP response element binding protein, or by inhibition of HDAC, indicating that receptor action is subverted by maneuvers that stimulate histone acetylation of the target gene. We propose that negative regulation of a subset of genes by TR involves the active exchange of CoRs and CoAs with intrinsic promoter regulatory elements that normally strongly induce histone acetylation and transcriptional activation.

Novel insight from transgenic mice into thyroid hormone resistance and the regulation of thyrotropin

Patients with resistance to thyroid hormone (RTH) exhibit elevated thyroid hormone levels and inappropriate thyrotropin (thyroid-stimulating hormone, or TSH) production. The molecular basis of this disorder resides in the dominant inhibition of endogenous thyroid hormone receptors (TRs) by a mutant receptor. To determine the relative contributions of pituitary versus hypothalamic resistance to the dysregulated production of thyroid hormone in these patients, we developed a transgenic mouse model with pituitary-specific expression of a mutant TR (Delta337T). The equivalent mutation in humans is associated with severe generalized RTH. Transgenic mice developed profound pituitary resistance to thyroid hormone, as demonstrated by markedly elevated baseline and non-triodothyronine (T3)-suppressible serum TSH and pituitary TSH-beta mRNA. Serum thyroxine (T4) levels were only marginally elevated in transgenic mice and thyrotropin-releasing hormone (TRH) gene expression in the paraventricular hypothalamus was downregulated. After TRH administration, T4 concentrations increased markedly in transgenic, but not in wild-type mice. Transgenic mice rendered hypothyroid exhibited a TSH response that was only 30% of the response observed in wild-type animals. These findings indicate that pituitary expression of this mutant TR impairs both T3-mediated suppression and T3-independent activation of TSH production in vivo. The discordance between basal TSH and T4 levels and the reversal with TRH administration demonstrates that resistance at the level of both the thyrotroph and the hypothalamic TRH neurons are required to elevate thyroid hormone levels in patients with RTH.

Thyroid hormone negatively regulates the transcriptional activity of the beta-amyloid precursor protein gene

The expression of the beta-amyloid precursor protein (APP), which plays a key role in the development of Alzheimer's disease, is regulated by a variety of cellular mediators in a cell-dependent manner. In the present study, we present evidence that thyroid hormones negatively regulate the expression of the APP gene in neuroblastoma cells. Transient transfection studies using plasmids that contain progressive deletions of the 5' region of the gene demonstrate that triiodothyronine (T3), the more active form of the thyroid hormones, represses APP promoter activity by a mechanism that requires binding of the nuclear T3 receptor (TR) to a specific sequence located in the first exon. The unliganded receptor increases promoter activity, and T3 reverses that activity to basal levels. The repressive effect of T3 does not exhibit TR isoform specificity, and it is equally mediated by TRalpha and TRbeta. Gel mobility shift assays using in vitro synthesized nuclear receptors and nuclear extracts led to the identification of a negative thyroid hormone response element, at nucleotide position +80/+96, that preferentially binds heterodimers of TR with the retinoid X receptor. Insertion of sequences containing this element confers negative regulation by T3 to a heterologous TK promoter, thus indicating the functionality of the element.

The thyroid hormone down-regulates the mouse alpha-foetoprotein promoter

The thyroid hormone (T3) was shown to down regulate the level of alpha-foetoprotein (AFP) mRNA in hepatoma cells HepG2. Recombinant plasmids containing segments from the mouse AFP gene promoter were transfected in HepG2 cells and transient expression assays showed that the T3 inhibitory effect depends on the sequence limited by positions -80 and -38, upstream from the TATA box. This sequence is able to confer T3 sensitivity to a heterologous promoter and contains a putative T3-responsive element, as well as likely CEBP- and HNF1-responsive elements. These observations suggest that T3 is a good candidate for hormonal control of the AFP gene expression and especially for the neonatal shut off of the gene.

Beta-trace gene expression is regulated by a core promoter and a distal thyroid hormone response element

We isolated and characterized the human beta-Trace protein (betaTP) gene promoter. betaTP, also known as prostaglandin D2 synthase, is a lipocalin secreted from the choroid plexus and meninges into cerebrospinal fluid. Basal transcription of the betaTP gene is directed from a core promoter found within the first 325 bases of the 5'-flanking sequence. The betaTP gene promoter is responsive to thyroid hormone (3,3',5-triiodothyronine, T3) and efficiently repressed by unliganded human thyroid hormone receptor beta (TRbeta). Functional analysis of the betaTP promoter in TE671 cells revealed that responsiveness to T3 occurs in sequences 2.5 kilobase pairs 5' of the start site. Within the hormone-responsive region we identified a thyroid hormone response element (TRE) located from -2576 to -2562 base pairs relative to the transcription start site. The betaTP TRE is composed of two directly repeated consensus half-sites separated by a 3-base pair space (DR3). The betaTP TRE forms specific complexes with TRbeta. We have shown that a gene active in the choroid plexus and meninges is responsive to T3. T3 may play a role in the regulated transport of substances into the cerebrospinal fluid and ultimately the brain.

Nuclear receptor corepressors activate rather than suppress basal transcription of genes that are negatively regulated by thyroid hormone

A group of transcriptional cofactors referred to as corepressors (CoRs) were recently shown to play a central role in basal silencing of genes that contain positive triiodothyronine (T3) response elements. In a reciprocal manner, negatively regulated genes are stimulated by unliganded thyroid hormone receptor (TR) and repressed upon the addition of T3. We used a TR beta mutant, called P214R, which fails to interact with CoRs, to examine whether CoRs also play a role in the control of genes that are negatively regulated in response to T3. In studies of three negatively regulated genes (the pituitary thyroid-stimulating hormone alpha-subunit [TSH alpha], TSH beta, and hypothalamic thyrotropin-releasing hormone [TRH] genes), stimulation of basal promoter activity by unliganded TR beta was impaired by introducing the P214R CoR mutation. Coexpression of each of the CoRs SMRT (silencing mediator for retinoid receptors and TRs) and NCoR (nuclear receptor CoR) enhanced basal stimulation of the negatively regulated promoters in a TR-dependent manner, but this effect was not seen with the P214R TR mutant. The mechanism of CoR effects on negatively regulated promoters was explored further with a series of GAL4-TR chimeric receptors and mutants that allowed TR effects to be assessed independently of receptor interactions with DNA. These experiments revealed that, like the negative regulation of genes by wild-type TR, basal activation occurred with GAL4-TR, but not with the GAL4-P214R mutant, and was reversed by the addition of T3. These results suggest that TR interactions with negatively regulated genes may be driven through protein-protein interactions. We conclude that a subset of negatively regulated genes are controlled by a novel mechanism that involves TR-mediated recruitment and basal activation by SMRT and NCoR. Addition of T3 reverses basal activation, perhaps by dissociation of CoRs.

The function of retinoid X receptors on negative thyroid hormone response elements

Retinoid X receptors (RXRs) form heterodimers with thyroid hormone receptors (TRs). RXRs increase DNA binding affinity of TRs and T3-mediated transactivation on positive T3 response elements (TREs). However, the role of RXRs on negative TREs, and the relation of RXRs to the dominant negative effect of mutant TRs, are not defined. To clarify the function of RXRs on negative TREs, we performed transient cotransfection studies using the rat glycoprotein hormone alpha promoter fused to luciferase gene (alphaLuc), and human TRH promoter fused to luciferase gene (TRH-Luc) as reporters. We found that the JEG-3 cell-alphaLuc system was very sensitive to TR regulation. Using TRbeta1 wild-type (WT) expression vector, 6.2 ng/well (170 ng/10 cm dish), and 0.2 ng/well (11 ng/10 cm dish) caused maximal, and half maximal, inhibition of Luc activities in the presence of 1 nM T3. A T3 dose dependent inhibition study was also performed. From these studies, we determined that the appropriate conditions in which to study alphaLuc transactivation, in a linear portion of the dose response curve, was using 0.8 ng/well TRbeta1 expression vector and 0.1 nM T3. Under these conditions, TRbeta1 mutant R316H (GH), but not G345R (Mf), showed a weak dominant negative effect at a 1:1 ratio in the presence of 0.1 nM T3 although neither mutant had detectable T3 binding affinity. Moreover this dominant negative effect of R316H on the alphaLuc reporter was enhanced in the presence of RXRgamma. Mutant G345R showed a stronger dominant negative effect than did R316H when using a double palindromic TRE fused to herpes simplex thymidine kinase-Luc reporter as a positive TRE. These results conform to the clinical features of R316H which is associated with apparent pituitary resistance of thyroid hormone (PRTH). Mutant R316H also showed a weak dominant negative effect with TRH-Luc at a 1:1 ratio in the absence or presence of RXRgamma. However RXRgamma did not enhance the dominant negative effect as it did using alphaLuc reporter gene. Electrophoretic gel mobility shift assay (EMSA) showed that RXR alpha augmented the DNA binding affinity of wild type and R316H TRs as heterodimers on the previously reported negative TREs of glycoprotein hormone alpha promoter, suggesting that RXR does not produce its response by removing TRs from these TREs. RXR alpha augmented DNA binding affinity of TRbeta1WT, and R316H showed a weaker heterodimer band than did the wild type in EMSA. Using the TRH-Luc reporter, basal activity was increased by wild type TRbeta1. However a TRbeta1 DNA binding domain mutant, (C127S) which can not bind to DNA, did not increase the basal activity. This indicates that DNA binding of the TR is required for increasing basal activity of TRH promoter. These results indicate that (1) RXR-TR heterodimers play a role in basal transactivation and T3 suppression of negatively regulated genes, and (2) RXRs increase the dominant negative effect of some mutant TRs on specific negative TREs. (3) This effect occurs without removing TRs from the TRE. (4) The differential dominant negative effect of mutant R316H (negative TRE > positive TRE) may explain, at least in part, the presentation of R316H as PRTH. (5) Augmentation of basal activity by wild type TRs on a negative TRE requires DNA binding.

Functional antagonism between CCAAT/Enhancer binding protein-alpha and peroxisome proliferator-activated receptor-gamma on the leptin promoter

The ob gene product, leptin, is a major hormonal regulator of appetite and fat cell mass. Recent work has suggested that the antidiabetic agents, the thiazolidinediones (TZ), which are also high affinity ligands of peroxisome proliferator-activated receptor-gamma (PPARgamma), inhibit leptin expression in rodents. To examine the effects of this class of drug on the leptin gene in adipocytes we performed Northern analysis on primary rat adipocytes cultured in the presence or absence of TZ. TZ reduced leptin mRNA levels by 75%. To determine whether this effect was mediated at the transcriptional level, we isolated 6510 base pairs of 5'-flanking sequence of the leptin promoter and studied reporter constructs in primary rat adipocytes and CV-1 cells. Sequence analysis demonstrated the presence of a consensus direct repeat with a 1-base-pair gap site between -3951 and -3939 as well as a consensus CCAAT/enhancer binding protein (C/EBP) site between -55 and -47. Our functional analysis in transfected primary rat adipocytes demonstrates that, despite the presence of a canonical direct repeat with a 1-base-pair gap site, TZ alone decreases reporter gene expression of leptin promoter constructs ranging from -6510 to +9 to -65 to +9. In CV-1 cells, which contain endogenous PPARgamma, TZ treatment alone had little effect on these constructs. However, TZ treatment did inhibit C/EBPalpha-mediated transactivation of the leptin promoter. This down-regulation of leptin reporter constructs mapped to a -65 to +9 promoter fragment which binds C/EBPalpha in gel-mobility shift assays but does not bind PPARgamma2 alone or as a heterodimer with 9-cis-retinoic acid receptor. Conversely, the promoter (-5400 to +24 base pairs) of the aP2 gene, another adipocyte-specific gene, was induced 7.3-fold by TZ. Co-transfection with C/EBPalpha minimally stimulated the aP2 promoter from basal levels but notably blocked activation by TZ. These data indicate that PPARgamma and C/EBPalpha can functionally antagonize each other on at least two separate promoters and that this mechanism may explain the down-regulation of leptin expression by thiazolidinediones.