A novel retinoid X receptor-independent thyroid hormone response element is present in the human type 1 deiodinase gene

Cross-talk between thyroid hormone receptor and liver X receptor regulatory pathways is revealed in a thyroid hormone resistance mouse model

Hypercholesterolemia is found in patients with hypothyroidism and resistance to thyroid hormone. In this study, we examined cholesterol metabolism in a thyroid hormone receptor beta (TR-beta) mutant mouse model of resistance to thyroid hormone. Whereas studies of cholesterol metabolism have been reported in TR-beta knock-out mice, generalized expression of a non-ligand binding TR-beta protein in this knock-in model more fully recapitulates the hypothyroid state, because the hypothyroid effect of TRs is mediated by the unliganded receptor. In the hypothyroid state, a high cholesterol diet increased serum cholesterol levels in wild-type animals (WT) but either did not change or reduced levels in mutant (MUT) mice relative to hypothyroidism alone. 7alpha-Hydroxylase (CYP7A1) is the rate-limiting enzyme in cholesterol metabolism and mRNA levels were undetectable in the hypothyroid state in all animals. triiodothyronine replacement restored CYP7A1 mRNA levels in WT mice but had minimal effect in MUT mice. In contrast, a high cholesterol diet markedly induced CYP7A1 levels in MUT but not WT mice in the hypothyroid state. Elevation of CYP7A1 mRNA levels and reduced hepatic cholesterol content in MUT animals are likely because of cross-talk between TR-beta and liver X receptor alpha (LXR-alpha), which both bind to a direct repeat + 4 (DR+4) element in the CYP7A1 promoter. In transfection studies, WT but not MUT TR-beta antagonized induction of this promoter by LXR-alpha. Electromobility shift analysis revealed that LXR/RXR heterodimers bound to the DR+4 element in the presence of MUT but not WT TR-beta. A mechanism for cross-talk, and potential antagonism, between TR-beta and LXR-alpha is proposed.

A 'Swinging Cradle' model for in vitro classification of different types of response elements of a nuclear receptor

Nuclear receptors are hormone-activated transcription factors that bind to specific target sequences termed hormone-response element (HRE). A HRE usually consists of two half-sites (5'-AGGTCA-3' consensus sequence) arranged as a direct, everted or inverted repeat with variable spacer region. Assignment of a HRE as a direct, everted or inverted repeat is based on its homology to the consensus half-site, but minor variations can make such an assignment confusing. We hypothesize a 'Swinging Cradle' model for HRE classification, whereby the core HRE functions as the "sitting platform" for the NR, and the extra nucleotides at either end act as the "sling" of the Cradle. We show that in vitro binding of the thyroid hormone receptor and 9-cis retinoic acid receptor heterodimer to an everted repeat TRE follows the 'Swinging Cradle' model, whereas the other TREs do not. We also show that among these TREs, the everted repeat mediates the highest biological activity.

Type 2 iodothyronine deiodinase is the major source of plasma T3 in euthyroid humans

The relative roles of the types 1 and 2 iodothyronine deiodinases (D1 and D2) in extrathyroidal 3,5,3'-triiodothyronine (T3) production in humans are unknown. We calculated the rate of thyroxine (T4) to T3 conversion by intact cells transiently expressing D1 or D2 at low (2 pM), normal (20 pM), and high (200 pM) free T4 concentrations. Deiodinase activities were then assayed in cell sonicates. The ratio of T3 production in cell sonicates (catalytic efficiency) was multiplied by the tissue activities reported in human liver (D1) and skeletal muscle (D2). From these calculations, we predict that in euthyroid humans, D2-generated T3 is 29 nmol/d, while that of D1-generated T3 is 15 nmol/d, from these major deiodinase-expressing tissues. The total estimated extrathyroidal T3 production, 44 nmol/d, is in close agreement with the 40 nmol T3/d based on previous kinetic studies. D2-generated T3 production accounts for approximately 71% of the peripheral T3 production in hypothyroidism, but D1 for approximately 67% in thyrotoxic patients. We also show that the intracellular D2-generated T3 has a greater effect on T3-dependent gene transcription than that from D1, which indicates that generation of nuclear T3 is an intrinsic property of the D2 protein. We suggest that impairment of D2-generated T3 is the major cause of the reduced T3 production in the euthyroid sick syndrome.

Type 2 iodothyronine deiodinase is the major source of plasma T3 in euthyroid humans

The relative roles of the types 1 and 2 iodothyronine deiodinases (D1 and D2) in extrathyroidal 3,5,3'-triiodothyronine (T3) production in humans are unknown. We calculated the rate of thyroxine (T4) to T3 conversion by intact cells transiently expressing D1 or D2 at low (2 pM), normal (20 pM), and high (200 pM) free T4 concentrations. Deiodinase activities were then assayed in cell sonicates. The ratio of T3 production in cell sonicates (catalytic efficiency) was multiplied by the tissue activities reported in human liver (D1) and skeletal muscle (D2). From these calculations, we predict that in euthyroid humans, D2-generated T3 is 29 nmol/d, while that of D1-generated T3 is 15 nmol/d, from these major deiodinase-expressing tissues. The total estimated extrathyroidal T3 production, 44 nmol/d, is in close agreement with the 40 nmol T3/d based on previous kinetic studies. D2-generated T3 production accounts for approximately 71% of the peripheral T3 production in hypothyroidism, but D1 for approximately 67% in thyrotoxic patients. We also show that the intracellular D2-generated T3 has a greater effect on T3-dependent gene transcription than that from D1, which indicates that generation of nuclear T3 is an intrinsic property of the D2 protein. We suggest that impairment of D2-generated T3 is the major cause of the reduced T3 production in the euthyroid sick syndrome.

Regulation of type 1 iodothyronine deiodinase in health and disease

The major physiologic function of type 1 iodothyronine deiodinase (D1) is to produce triiodothyronine (T(3)) for the plasma. D1 activity is regulated by numerous factors, perhaps the most important of which in human pathophysiology is T(3). T(3) induces D1 expression, contributing to the T(3) excess commonly found in hyperthyroidism. Cytokines, nutritional status, sex steroids, and other factors also regulate D1 activity, although different organs often show different responses. Numerous homeostatic mechanisms can counterbalance isolated changes in D1 expression, such as the genetically decreased expression in C3H/He mice. Two relatively commonly used drugs, propylthiouracil and amiodarone, inhibit D1, which can have substantial effects on circulating thyroid hormone levels. Overall, many factors interact in complex ways to establish D1 levels, contributing to the circulating concentrations of thyroxine (T(4)) and T(3).

Decreased type 1 iodothyronine deiodinase expression might be an early and discrete event in thyroid cell dedifferentation towards papillary carcinoma

OBJECTIVE

Type I iodothyronine deiodinase (D1) catalyses the 5' monodeiodination of T4 and is highly expressed in normal human thyroid gland. We have investigated D1 expression in a series of benign and malignant differentiated thyroid neoplasias.

DESIGN

Surgically isolated thyroid tumour fragments were used. D1 expression was determined by reverse transcription polymerase chain reaction (RT-PCR) and enzymatic assay.

PATIENTS

Tumours and adjacent normal tissues were obtained from 28 unselected patients (papillary carcinoma, n = 14; follicular adenoma, n = 7; follicular carcinoma, n = 6; anaplastic carcinoma, n = 1).

MEASUREMENTS

D1 mRNA levels were determined using specific primers for the human D1 gene and enzymatic assays were performed using T4 as substrate.

RESULTS

In papillary thyroid carcinoma (PTC), D1 mRNA and activity levels were decreased compared with the surrounding tissue (0.25 +/- 0.24 vs. 1.09 +/- 0.54 arbitrary units (AU), P < 0.001 and 0.08 +/- 0.07 vs. 0.24 +/- 0.15 pmol T4/min/mg protein, P = 0.045, respectively). Decreased D1 expression was consistent and was observed in all histological subtypes and clinical stages analysed, including microcarcinomas. By contrast, significantly higher D1 mRNA levels and enzyme activity were present in follicular adenoma (1.9 +/- 1.5 vs. 0.83 +/- 0.58 AU, P = 0.028 and 2.67 +/- 1.42 vs. 0.22 +/- 0.06 pmol T4/min/mg protein, P = 0.044, respectively) and in follicular thyroid carcinoma (FTC) than in surrounding normal tissue (1.2 +/- 0.46 vs. 0.67 +/- 0.18 AU, P = 0.038 and 1.20 +/- 0.58 vs. 0.20 +/- 0.10 pmol T4/min/mg protein, P < 0.001, respectively). Type II iodothyronine deiodinase (D2) activity was also significantly higher in metastatic FTC samples than in normal thyroid tissues (5.20 +/- 0.81 vs. 0.30 +/- 0.27 fmol T4/min/mg protein, P < 0.001).

CONCLUSIONS

These findings suggest that thyroid cell dedifferentiation promotes changes in D1 gene expression by pretranscriptional mechanisms and indicate that decreased D1 expression might be an early and discrete event in thyroid cell dedifferentiation towards papillary thyroid carcinoma.

A Novel Mechanism of Thyroid Hormone-dependent Negative Regulation by Thyroid Hormone Receptor, Nuclear Receptor Corepressor (NCoR), and GAGA-binding Factor on the Rat CD44 Promoter

CD44 is an adhesion molecule in the extracellular matrix that shows various functions, including tumor genesis and metastasis. A recent study showed that CD44 expression level was strongly correlated with the generation of papillary thyroid carcinomas, the most prevalent malignancy of the thyroid gland. We report here that CD44 is negatively regulated by thyroid hormone (T(3)) through a novel mechanism. We demonstrate that nuclear receptor corepressor (NCoR) enhances thyroid hormone receptor (TR)-mediated basal transactivation by a weak TR.DNA interaction in the absence of T(3), which is repressed by T(3) through a transient TR .DNA interaction. Initially, we identified that CD44 was negatively directly transcriptionally T(3) -responsive. Deletion and mutation analysis indicated that both a weak TR and a GAGA-binding factor (GAF) binding sites on the CD44 promoter were required for negative regulation by T(3). The weak TR.DNA interaction was further confirmed by electrophoretic gel mobility shift assay, chromatin immunoprecipitation, and transfection assays using a non-DNA-binding TRalpha1 mutant. More interestingly, NCoR acted as a co-activator to enhance TR-mediated basal transactivation in the absence of T(3). This effect was eliminated by removal of TR or NCoR binding. Most strikingly, T(3) induced a remarkable increase in TR.DNA binding at 40-60 min after T(3) exposure that rapidly returned to basal levels, suggesting a T(3)-induced remodeling of chromatin structure at the early stage of T(3) stimulation resulting in repression. Therefore, we propose a mechanism by which NCoR, GAF, and TR interact with the CD44 negative T(3)-responsive element to enhance basal transactivation, whereas T(3) induces the remodeling of chromatin structure for repression.

Regulation of regional expression in rat brain PC2 by thyroid hormone/characterization of novel negative thyroid hormone response elements in the PC2 promoter

The prohormone convertases (PCs) PC1 and PC2 are involved in the tissue-specific endoproteolytic processing of neuropeptide precursors within the secretory pathway. We previously showed that changes in thyroid status altered pituitary PC2 mRNA and that this regulation was due to triiodothyronine-dependent interaction of the thyroid hormone receptor (TR) with negative thyroid hormone response elements (nTREs) contained in a large proximal region of the human PC2 promoter. In the current study, we examined the in vivo regulation of brain PC2 mRNA by thyroid status and found that 6-n-propyl-2-thiouracil-induced hypothyroidism stimulated, whereas thyroxine-induced hyperthyroidism suppressed, PC2 mRNA levels in the rat hypothalamus and cerebral cortex. To address the mechanism of T3 regulation of the PC2 gene, we used human PC2 (hPC2) promoter constructs transiently transfected into GH3 cells and found that triiodothyronine negatively and 9-cis-retinoic acid positively regulated hPC2 promoter activity. EMSAs, using purified TRalpha1 and retinoid X receptor-beta (RXRbeta) proteins demonstrated that TRalpha bound the distal putative nTRE-containing oligonucleotide in the PC2 promoter, and RXR bound to both nTRE-containing oligonucleotides. EMSAs with oligonucleotides containing deletion mutations of the nTREs demonstrated that the binding to TR and RXR separately is reduced, but specific binding to TR and RXR together persists even with deletion of each putative nTRE. We conclude that there are two novel TRE-like sequences in the hPC2 promoter and that these regions act in concert in a unique manner to facilitate the effects of thyroid hormone and 9-cis-retinoic acid on PC2.

Characteristics and thyroid state-dependent regulation of iodothyronine deiodinases in pigs

Three iodothyronine deiodinases (D1, D2, and D3) regulate local and systemic availability of thyroid hormone. D1 and D2 activate the prohormone T4 to the thyromimetic T3, and D3 inactivates T4 and T3 to rT3 and 3,3'-diiodothyronine, respectively. The expression of the three deiodinases is tightly regulated with regard to developmental stage and cell type to provide fine tuning of T3 supply to target cells. Most studies regarding distribution and regulation of deiodinases have been carried out in rodents. However, in different respects, rodents do not seem to be the optimal experimental model for human thyroid hormone physiology. For instance, D2 expression has been observed in human thyroid and skeletal muscle but not in these tissues in rodents. In this study, we have explored the pig as an alternative model. Porcine D1, D2, and D3 were cloned by RT-PCR, and their catalytic properties were shown to be virtually identical to those reported for human and rodent deiodinases. The tissue distribution of deiodinases was studied in normal pigs and in pigs made hypothyroid by methimazole treatment or in pigs made hyperthyroid by T4 treatment. D1 activity in liver and kidney was increased in T4-treated pigs. D2 activities in cerebrum and pituitary were decreased after T4 treatment and strongly increased after methimazole treatment. Remarkably, D2 activity in thyroid and skeletal muscle was induced in hypothyroid pigs. Significant expression of D3 was observed in cerebrum and was positively regulated by thyroid state. In conclusion, the pig appears to be a valuable model for human thyroid hormone physiology. The expression of D2 activity in thyroid and skeletal muscle is of particular interest for studies on the importance of this enzyme in (hypothyroid) humans.

A novel cell type-specific mechanism for thyroid hormone-dependent negative regulation of the human type 1 deiodinase gene

We have identified a cell type-specific, negative thyroid hormone-responsive element in the human type 1 iodothyronine deiodinase (hdio1) gene. This fragment, termed a JEG response element, bound tightly to a JEG-cell nuclear protein [JEG cell-specific transcription factor (JTF)] also present in placenta but not in COS-7, HeLa, or human embryonic kidney-293 cells. In JEG-3 cells, three copies of the JEG response element conferred a more than 40-fold transcriptional stimulation to the heterologous rat GH promoter which was further increased 2-fold by apo-thyroid hormone receptor (TR) and reduced 3-fold by T(3). Dimethyl sulfide footprinting showed overlapping contact sites for the high-affinity interaction of JTF and low-affinity binding of TR-retinoid X receptor. Expression of the same construct was unaffected by TR or T(3) in COS cells, indicating JTF was required for negative regulation by T(3)-TR. Mutations of the critical thyroid hormone responsive element binding P box amino acids EG to GS in TRalpha1 or TRbeta2 eliminated the apo-TR and T(3)-TR effects. These studies identify a novel mechanism for cell type-specific, promoter-independent negative regulation by T(3).

An RNA-binding Domain in the Thyroid Hormone Receptor Enhances Transcriptional Activation

Thyroid hormone plays important roles in development, differentiation, and metabolic homeostasis by binding to nuclear thyroid hormone receptors, which regulate target gene expression by interacting with DNA response elements and coregulatory proteins. We show that thyroid hormone receptors also are single-stranded RNA binding proteins and that this binding is functionally significant. By using a series of deletion mutants, a novel RNA-binding domain was localized to a 41-amino acid segment of thyroid hormone receptor alpha1 between the second zinc finger and the ligand-binding domain. This RNA-binding domain was necessary and sufficient for thyroid hormone receptor binding to the steroid receptor RNA activator (SRA). Although SRA does not bind directly to steroid receptors, it has been identified as a steroid receptor coactivator, and was thought not to be a coactivator for thyroid hormone receptors. However, transfection studies revealed that SRA enhances thyroid hormone induction of appropriate reporter genes and that the thyroid hormone receptor RNA-binding domain is important for this enhancement. We conclude that thyroid hormone receptors bind RNA through a novel domain and that the interaction of this domain with SRA, and perhaps other RNAs, enhances thyroid hormone receptor function.

Deubiquitination of type 2 iodothyronine deiodinase by von Hippel-Lindau protein-interacting deubiquitinating enzymes regulates thyroid hormone activation

The type 2 iodothyronine deiodinase (D2) is an integral membrane ER-resident selenoenzyme that activates the pro-hormone thyroxine (T4) and supplies most of the 3,5,3'-triiodothyronine (T3) that is essential for brain development. D2 is inactivated by selective conjugation to ubiquitin, a process accelerated by T4 catalysis and essential for the maintenance of T3 homeostasis. A yeast two-hybrid screen of a human-brain library with D2 as bait identified von Hippel-Lindau protein-interacting deubiquitinating enzyme-1 (VDU1). D2 interaction with VDU1 and VDU2, a closely related deubiquitinase, was confirmed in mammalian cells. Both VDU proteins colocalize with D2 in the ER, and their coexpression prolongs D2 half-life and activity by D2 deubiquitination. VDU1, but not VDU2, is markedly increased in brown adipocytes by norepinephrine or cold exposure, further amplifying the increase in D2 activity that results from catecholamine-stimulated de novo synthesis. Thus, deubiquitination regulates the supply of active thyroid hormone to brown adipocytes and other D2-expressing cells.

Regulation of iodothyronine deiodinases in the Pax8-/- mouse model of congenital hypothyroidism

Thyroid hormones are essential for a variety of developmental and metabolic processes. Congenital hypothyroidism (CHT) results in severe defects in the development of different tissues, in particular brain. As an animal model for CHT, we studied Pax8(-/-) mice, which are born without a thyroid gland. We determined the expression of iodothyronine deiodinase D1 in liver and kidney, D2 in brain and pituitary, and D3 in brain, as well as serum T(4), T(3), and rT(3) levels in Pax8(-/-) vs. control mice during the first 3 wk of life. In control mice, serum T(4) and T(3) were undetectable on the day of birth (d 0) and increased to maximum levels on d 15. In Pax8(-/-) mice, serum T(4) and T(3) remained below detection limits. Serum rT(3) was high on d 0 in both groups and rapidly decreased in Pax8(-/-), but not in control mice. Hepatic and renal D1 activities and mRNA levels were low on d 0 and increased in control mice roughly parallel to serum T(4) and T(3) levels. In Pax8(-/-) mice, tissue D1 activities and mRNA levels remained low. Cerebral D2 activities were low on d 0 and increased to maximum levels on d 15, which were approximately 10-fold higher in Pax8(-/-) than in control mice. D2 mRNA levels were higher in Pax8(-/-) than in control mice only on d 21. Cerebral D3 activities and mRNA levels were high on d 0 and showed a moderate decrease between d 3 and 15, with values slightly lower in Pax8(-/-) than in control mice. One day after the injection of 200 ng T(4) or 20 ng T(3)/g body weight, tissue deiodinase activities and mRNA levels were at least partially restored toward control levels, with the exception of cerebral D3 activity. In conclusion, these findings show dramatic age and thyroid state-dependent changes in the expression of deiodinases in central and peripheral tissues of mice during the first 3 wk of life.

In vivo dimerization of types 1, 2, and 3 iodothyronine selenodeiodinases

The goal of the present investigation was to test the hypothesis that types 1, 2, and 3 iodothyronine selenodeiodinases (D1, D2, and D3) can form homodimers. The strategy included transient coexpression of wild-type (wt) deiodinases (target), and FLAG-tagged alanine or cysteine mutants (bait) in human embryonic kidney epithelial cells. SDS-PAGE of the immunoprecipitation pellet of (75)Se-labeled cell lysates using anti-FLAG antibody revealed bands of the correct sizes for the respective wt enzymes, which corresponded to approximately 2-5% of the total deiodinase protein in the cell lysate. Western blot analysis with anti-FLAG antibody of lysates of cells transiently expressing individual FLAG-tagged-cysteine deiodinases revealed specific monomeric bands for each deiodinase and additional minor bands of relative molecular mass (M(r)) of 55,000 for D1, M(r) 62,000 for D2, and M(r) 65,000 for D3, which were eliminated by 100 mM dithiothreitol at 100 C. Anti-FLAG antibody immunodepleted 10% of D1 and 38% of D2 activity from lysates of cells coexpressing inactive FLAG-tagged Ala mutants and the respective wt enzymes (D1 or D2) but failed to immunodeplete wtD3 activity. D1 or D2 activities were present in these respective pellets. We conclude 1) that overexpressed selenodeiodinases can homodimerize probably through disulfide bridges; and 2) at least for D1 and D2, monomeric forms are catalytically active, demonstrating that only one wt monomer partner is required for catalytic activity of these two deiodinases.

Direct interactions between corepressors and coactivators permit the integration of nuclear receptor-mediated repression and activation

The unliganded thyroid hormone receptor beta (TRbeta) represses the basal transcriptional activity of target genes, in part through interactions with the nuclear receptor corepressor (N-CoR). In this study we have identified a rather unexpected interaction between N-CoR and the nuclear receptor coactivator ACTR. We have demonstrated in vitro and in intact cells that N-CoR directly associates with ACTR and that the interaction surfaces on N-CoR and ACTR are distinct from those required for TR binding. The significance of this finding was demonstrated by showing that N-CoR facilitates an interaction between unliganded-TRbeta and ACTR. One possible consequence of the formation of the trimeric complex of N-CoR/ACTR/unliganded-TR is that N-CoR may raise the local concentration of ACTR at target gene promoters. In support of this hypothesis it was demonstrated that the presence of N-CoR can enhance TRbeta-mediated transcriptional activation. It is proposed, therefore, that TRbeta- mediated activation and repression are integrally linked in a manner that is not predicted by the current models of nuclear receptor action.

Ordered recruitment of histone acetyltransferases and the TRAP/Mediator complex to thyroid hormone-responsive promoters in vivo

Transcriptional coactivators implicated in gene activation by the thyroid hormone receptor (TR) include members of the p160/steroid receptor coactivator (SRC) family of proteins, p300, and the multisubunit TR-associated protein (TRAP)/Mediator complex. We investigated the temporal recruitment of these cofactors to mammalian thyroid hormone (T3)-responsive promoters in vivo. We show that upon T3 treatment, TR recruits all three types of coactivators to specific promoters in at least two sequential steps: p160/SRC proteins and p300 are recruited first and rapidly induce histone acetylation, followed by the recruitment of the TRAP/Mediator complex. Interestingly, inhibition of histone deacetylase activity with trichostatin A elicited a more rapid promoter recruitment of the TRAP/Mediator complex but not p160/SRC proteins. T3-dependent gene expression assays indicate that all three coactivators are targeted to a promoter before significant activation occurs. These findings thus suggest that histone acetylation may be a prerequisite for TRAP/Mediator recruitment and function at specific T3-responsive mammalian promoters.

The promoter of the human sodium/iodide-symporter gene responds to retinoic acid

It was shown previously that hNIS mRNA expression is stimulated by retinoic acid (RA) in human follicular thyroid carcinoma cell lines FTC-133 and FTC-238, and patients with thyroid carcinomas lacking iodide uptake respond to RA treatment with increased radioiodide transport. Here, in transient transfection experiments using FTC-238 cells, hNIS promoter/luciferase reporter constructs showed an up to 2.5-fold increase in transcriptional activity after incubation with 1 microM RA. Stimulation by 10 nM T3 was up to 2.4-fold. Deletion or block mutation of a putative nuclear receptor recognition site, 'DR10', abolished RA and T3 responses. Four copies of the DR10 cloned 5' to the thymidine kinase promoter gave a 2.6-fold and a 1.4-fold increase in transcriptional activity after RA and T3 stimulation, respectively. In electrophoretic mobility shifts, a wildtype DR10 oligonucleotide, but not block mutants of either DR10 halfsite, interacted with nuclear receptors. Thus, RA redifferentiation of advanced thyroid carcinomas may reinduce iodide uptake by stimulating hNIS expression and thereby make tumours accessible for radioiodide therapy again.

Requirement of helix 1 and the AF-2 domain of the thyroid hormone receptor for coactivation by PGC-1

Although PGC-1 (peroxisome proliferator-activated receptor-gamma coactivator-1) has been previously shown to enhance thyroid hormone receptor (TR)/retinoid X receptor-mediated ucp-1 gene expression in a ligand-induced manner in rat fibroblast cells, the precise mechanism of PGC-1 modulation of TR function has yet to be determined. In this study, we show that PGC-1 can potentiate TR-mediated transactivation of reporter genes driven by natural thyroid hormone response elements both in a ligand-dependent and ligand-independent manner and that the extent of coactivation is a function of the thyroid hormone response element examined. Our data also show that PGC-1 stimulation of TR activity in terms of Gal4 DNA-binding domain fusion is strictly ligand-dependent. In addition, an E457A AF-2 mutation had no effect on the ligand-induced PGC-1 enhancement of TR activity, indicating that the conserved charged residue in AF-2 is not essential for this PGC-1 function. Furthermore, GST pull-down and mammalian two-hybrid assays demonstrated that the PGC-1 LXXLL motif is required for ligand-induced PGC-1/TR interaction. This agonist-dependent PGC-1/TR interaction also requires both helix 1 and the AF-2 region of the TR ligand-binding domain. Taken together, these results support the notion that PGC-1 is a bona fide TR coactivator and that PGC-1 modulates TR activity via a mechanism different from that utilized with peroxisome proliferator activator receptor-gamma.

Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases

The goal of this review is to place the exciting advances that have occurred in our understanding of the molecular biology of the types 1, 2, and 3 (D1, D2, and D3, respectively) iodothyronine deiodinases into a biochemical and physiological context. We review new data regarding the mechanism of selenoprotein synthesis, the molecular and cellular biological properties of the individual deiodinases, including gene structure, mRNA and protein characteristics, tissue distribution, subcellular localization and topology, enzymatic properties, structure-activity relationships, and regulation of synthesis, inactivation, and degradation. These provide the background for a discussion of their role in thyroid physiology in humans and other vertebrates, including evidence that D2 plays a significant role in human plasma T(3) production. We discuss the pathological role of D3 overexpression causing "consumptive hypothyroidism" as well as our current understanding of the pathophysiology of iodothyronine deiodination during illness and amiodarone therapy. Finally, we review the new insights from analysis of mice with targeted disruption of the Dio2 gene and overexpression of D2 in the myocardium.

Congenital hypothyroid Pax8(-/-) mutant mice can be rescued by inactivating the TRalpha gene

Mice devoid of all TRs are viable, whereas Pax8(-/-) mice, which lack the follicular cells producing T4 and T3 in the thyroid gland, die during the first weeks of postnatal life. A precise comparison between the two types of mutants reveals that their phenotypes are similar, but the defects in spleen, bone, and small intestine are more pronounced in Pax8(-/-) mice. This is interpreted as the result of a negative effect of the unliganded TR on thyroid hormone target genes expression in the Pax8(-/-) mutants. Pax8/TRalpha compound mutants can survive to adulthood, and the expression of target genes is partially restored. This demonstrates the importance of TRalpha aporeceptor activity in several aspects of postnatal development.

A distinct thyroid hormone response element mediates repression of the human cholesterol 7alpha-hydroxylase (CYP7A1) gene promoter

We examined the molecular basis by which T3 regulates the human cholesterol 7alpha-hydroxylase gene (CYP7A1) promoter. L-T3 decreased chloramphenicol acetyltransferase activity in hepatoma cells cotransfected with a plasmid encoding the T3 receptor (TR) alpha [NR1a1] and a chimeric gene containing nucleotides -372 to +61 of the human CYP7A1 gene fused to the chloramphenicol acetyltransferase structural gene. Deoxyribonuclease I footprinting revealed that recombinant TRalpha protected two regions in this segment of the human CYP7A1 gene promoter. In EMSAs, TRalpha bound to both regions. The binding was competed by oligonucleotides bearing an idealized TRalpha binding motif and abolished by mutation of these elements. In assays of promoter function, mutation of only one of the TRalpha binding sites blocked repression by T3. The results indicate that T3-dependent repression of human CYP7A1 gene expression is mediated via a novel site in the human CYP7A1 gene promoter.

Type 2 iodothyronine deiodinase expression is upregulated by the protein kinase A-dependent pathway and is downregulated by the protein kinase C-dependent pathway in cultured human thyroid cells

Type 1 and 2 iodothyronine deiodinases (D1 and D2) catalyze thyroxine (T4) activation. In human thyroid, unlike rodents', both D1 and D2 are expressed. We have investigated the effects of thyrotropin (TSH), dibutyryl cyclic adenosine monophosphate [(Bu)2cAMP] (an activator of protein kinase A [PKA]), 12-O-tetradecanoylphorbor 13-actate (TPA) (an activator of protein kinase C [PKC]), T4, and triiodothyronine (T3) on the D2 mRNA levels and activity in cultured human thyroid cells. D2 mRNA levels were increased by TSH and (Bu)2cAMP, and the increment was faster and greater than that of D1 mRNA levels. The increment of the maximum velocity (Vmax) value for D2 by (Bu)2cAMP stimulation was similar to that of D2 mRNA levels, suggesting that (Bu)2cAMP enhances D2 activity mainly at the pretranslational level. Cycloheximide, a protein synthesis inhibitor, partially inhibited the increase of D2 mRNA levels by (BU)2cAMP, suggesting that de novo protein synthesis-dependent pathways are involved. TPA suppressed the D2 mRNA levels in the presence of (Bu)2cAMP. However, T3 and T4 did not significantly change the D2 mRNA levels and activity. In conclusion, D2 expression in human thyroid cells is more rapidly and strongly upregulated by the PKA pathway than D1 expression, and is downregulated by the PKC pathway.

Type I 5'-iodothyronine deiodinase activity and mRNA are remarkably reduced in renal clear cell carcinoma

The purpose of this study was to compare thyroid hormone metabolism between non-cancerous tumor-surrounding human kidney tissues and renal clear cell carcinomas (RCCC). The material consisted of samples taken from 10 RCCC patients of both sexes and three grades of differentiation, G1 to G3. We showed that, similar to rat tissue, type I 5' monodeiodinase (5'DI) expression is heterogeneous within the human kidney. We also found a poor correlation between 5'DI activity and mRNA level in non-cancerous tumor-surrounding tissue suggesting significant post-transcriptional regulation of 5'DI expression by an unidentified process in the human kidney. In all RCCC tissues both 5'DI activity and mRNA levels were undetectable. This suggests either loss of human 5'DI gene expression during neoplastic transformation or the origination of RCCC from a tubular cell type that does not express 5'DI.

Distinct tissue-specific roles for thyroid hormone receptors beta and alpha1 in regulation of type 1 deiodinase expression

Type 1 deiodinase (D1) metabolizes different forms of thyroid hormones to control levels of T3, the active ligand for thyroid hormone receptors (TR). The D1 gene is itself T3-inducible and here, the regulation of D1 expression by TRalpha1 and TRbeta, which act as T3-dependent transcription factors, was investigated in receptor-deficient mice. Liver and kidney D1 mRNA and activity levels were reduced in TRbeta(-/-) but not TRalpha1(-/-) mice. Liver D1 remained weakly T3 inducible in TRbeta(-/-) mice whereas induction was abolished in double mutant TRalpha1(-/-)TRbeta(-/-) mice. This indicates that TRbeta is primarily responsible for regulating D1 expression whereas TRalpha1 has only a minor role. In kidney, despite the expression of both TRalpha1 and TRbeta, regulation relied solely on TRbeta, thus revealing a marked tissue restriction in TR isotype utilization. Although TRbeta and TRalpha1 mediate similar functions in vitro, these results demonstrate differential roles in regulating D1 expression in vivo and suggest that tissue-specific factors and structural distinctions between TR isotypes contribute to functional specificity. Remarkably, there was an obligatory requirement for a TR, whether TRbeta or TRalpha1, for any detectable D1 expression in liver. This suggests a novel paradigm of gene regulation in which the TR sets both basal expression and the spectrum of induced states. Physiologically, these findings suggest a critical role for TRbeta in regulating the thyroid hormone status through D1-mediated metabolism.

Thyroid hormone response element sequence and the recruitment of retinoid X receptors for thyroid hormone responsiveness

Thyroid hormone receptors (TRs) are transcription factors that bind to thyroid hormone response elements (TREs) in the regulatory regions of target genes. TRs are thought to activate transcription primarily as heterodimers with retinoid X receptors (RXRs), with RXR binding upstream to the two directly repeated half-sites in a typical TRE. However, given that TRs and RXRs prefer to bind to different DNA sequences (T(A/G)AGGTCA and GGGGTCA), we postulate that only certain TREs require RXR-TR heterodimerization, depending on the TRE sequence. We have tested this hypothesis by comparing in Saccharomyces cerevisiae the functional activity of TR +/- RXR on 10 naturally occurring mammalian TREs. S. cerevisiae was used as a model system because yeast lack endogenous nuclear receptors and thus can be manipulated to express TRs and/or RXRs. We first studied ligand-independent reporter gene activation, which reflects the activity of the activator function 1 (AF-1) domain. The 10 TREs formed a continuous spectrum from being fully dependent on RXR for TR AF-1 activity to being essentially independent of RXR. Relative independence of RXR generally was seen when the TRE upstream half-site has a TA or TG 5' to the core hexamer. Gel mobility shift assays revealed that functional independence of RXR correlates with the strong binding of TR alone, whereas more RXR dependence correlates with higher binding of RXR-TR heterodimers. Restoration of ligand-dependent (AF-2 domain) reporter gene activation was achieved by expression of the coactivator TIF2. This ligand-induced stimulation was stronger in the presence of TR alone than with RXR plus TR, suggesting a preference for TIF2 activation of TR homodimers. Overall the data support the notion that the TRE sequence plays an important role in determining the nuclear hormone receptor and coactivator requirements for TR action.

The human, but not rat, dio2 gene is stimulated by thyroid transcription factor-1 (TTF-1)

Types 1 and 2 iodothyronine deiodinases (D1 and D2) catalyze the production of T(3) from T(4). D2 mRNA is abundant in the human thyroid but very low in adult rat thyroid, whereas D1 activity is high in both. To understand the molecular regulation of these genes in thyroid cells, the effect of thyroid transcription factor 1 (TTF-1) and the paired domain-containing protein 8 (Pax-8) on the transcriptional activity of the deiodinase promoters were studied. Both the approximately 6.5-kb hdio2 sequence and its most 3' 633 bp were activated 10-fold by transiently expressed TTF-1 in COS-7 cells, but the hdio1 was unaffected. Surprisingly, the response of the rdio2 gene to TTF-1 was only 3-fold despite the 73% identity with the proximal 633-bp region of hdio2 including complete conservation of a functional cAMP response element at -90. Neither human nor rat dio2 nor human dio1 was induced by Pax-8. The binding affinity of four putative TTF-1 binding sites in hdio2 were compared by a semiquantitative gel retardation assay using in vitro expressed TTF-1 homeodomain protein. Only two sites, D and C1 (both of which are absent in rdio2), had significant affinity. Functional analyses showed that both sites are required for the full response to TTF-1. These results can explain the differential expression of dio2 in thyroid and potentially other tissues in humans and rats.

Regulation of prohormone convertase 1 (PC1) by thyroid hormone

The prohormone convertases (PCs) PC1 and PC2 are key enzymes capable of processing a variety of prohormones to their bioactive forms. In this study, we demonstrated that 6-n-propyl-2-thiouracil (PTU)-induced hypothyroidism stimulated, whereas triido-L-thyronine (T(3))-induced hyperthyroidism suppressed, PC1 mRNA levels in the rat anterior pituitary. Using 5' deletions of the human PC1 (hPC1) promoter transiently transfected into GH3 (a somatotroph cell line) cells, we found that T(3) negatively regulated hPC1 promoter activity and that this regulation required the region from -82 to +19 bp relative to the transcription start site. Electrophoretic mobility shift assays (EMSAs) using purified thyroid hormone receptor-alpha1 (TR alpha 1) and retinoid X receptor-beta (RXRbeta) proteins and GH3 nuclear extracts demonstrated that the region from -10 to +19 bp of the hPC1 promoter bound TR alpha 1 as both a monomer and a homodimer and bound TR alpha 1/RXR beta as a heterodimer and multimer. EMSAs with oligonucleotides containing point mutations of the putative negative thyroid response elements (TREs) exhibited diminished homodimer and loss of multimer binding. We conclude that there are multiple novel TRE-like sequences in the hPC1 promoter located from -10 to +19 bp.

Regulation of hepatocyte thyroxine 5'-deiodinase by T3 and nuclear receptor coactivators as a model of the sick euthyroid syndrome

The syndrome of nonthyroidal illness, also known as the sick euthyroid syndrome, is characterized by a low plasma T3 and an "inappropriately normal" plasma thyrotropin in the absence of intrinsic disease of the hypothalamic-pituitary-thyroid axis. The syndrome is due in part to decreased activity of type I iodothyronine 5'-deiodinase (5' D-I), the hepatic enzyme that converts thyroxine to T3 and that is induced at the transcriptional level by T3. The hypothesis tested is that cytokines decrease T3 induction of 5' D-I, resulting in decreased T3 production and hence a further decrease in 5' D-I. The proposed mechanism is competition for limiting amounts of nuclear receptor coactivators between the 5' D-I promoter and the promoters of cytokine-induced genes. Using primary cultures of rat hepatocytes, we demonstrate that interleukins 1 and 6 inhibit the T3 induction of 5' D-I RNA and enzyme activity. This effect is at the level of transcription and can be partially overcome by exogenous steroid receptor coactivator-1 (SRC-1). The physical mass of endogenous SRC-1 is not affected by cytokine exposure, and exogenous SRC-1 does not affect 5' D-I in the absence of cytokines. The data support the hypothesis that cytokine-induced competition for limiting amounts of coactivators decreases hepatic 5' D-I expression, contributing to the etiology of the sick euthyroid syndrome.

Temporal formation of distinct thyroid hormone receptor coactivator complexes in HeLa cells

Thyroid hormone receptors (TRs) regulate transcription by recruiting distinct coregulatory complexes to target gene promoters. Coactivators implicated in ligand-dependent activation by TR include p300, the CREB-binding protein (CBP), members of the p160/SRC family, and the multisubunit TR-associated protein (TRAP) complex. Using a stable TR-expressing HeLa cell line, we show that interaction of TR with members of the p160/SRC family, CBP, and the p300/CBP-associated factor (PCAF) occurs rapidly (approximately 10 min) following addition of thyroid hormone (T3). In close agreement with these observations, we find that TR is associated with potent histone acetyltransferase activity rapidly following T3-treatment. By contrast, we observe that formation of TR-TRAP complexes occurs significantly later (approximately 3 h) post T3 treatment. An examination of the kinetics of T3-induced gene expression in HeLa cells reveals bimodal or delayed activation on specific T3-responsive promoters. Taken together, our data are consistent with the hypothesis that T3-dependent activation at specific target promoters may involve the regulated action of multiple TR-coactivator complexes.

Interactions between the prohormone convertase 2 promoter and the thyroid hormone receptor

The majority of prohormones are cleaved at paired basic residues to generate bioactive hormones by prohormone convertases (PCs). As PC1 and PC2, two neuroendocrine-specific PCs, appear to be the key enzymes capable of processing a variety of prohormones, alterations of PC2 and/or PC1 levels will probably have a profound effect on hormonal homeostasis. We investigated the regulation of PC2 messenger RNA (mRNA) by thyroid hormone using GH3 cells to demonstrate that T3 negatively regulated PC2 mRNA levels in a dose- and time-dependent fashion. Functional analysis of progressive 5'-deletions of the human (h) PC2 promoter luciferase constructs in GH3 cells demonstrated that the regulation probably occurs at the transcriptional level, and that putative negative thyroid hormone response elements were located within the region from -44 to + 137 bp relative to the transcriptional start site. Transient transfections in JEG-3 cells and COS-1 cells showed that the suppressive effect of T3 was equally mediated by the thyroid hormone receptor (TR) isoforms TRalpha1 and TRbeta1. Electrophoretic mobility shift assays using purified TRal and retinoid X receptor-beta protein as well as GH3 nuclear extracts showed that regions from +51 to +71 bp and from +118 to +137 bp of the hPC2 promoter bind to TRalpha1 as both a monomer and a homodimer and with TRalpha1/retinoid X receptor-beta as a heterodimer. Finally, the in vivo regulation of pituitary PC2 mRNA by thyroid status was demonstrated in rats. These results demonstrate that T3 negatively regulates PC2 expression at the transcriptional level and that functional negative thyroid hormone response elements exist in the hPC2 promoter. We postulate that the alterations of PC2 activity may mediate some of the pathophysiological consequences of hypo- or hyperthyroidism.

A potential role of activated NF-kappa B in the pathogenesis of euthyroid sick syndrome

Euthyroid sick syndrome, characterized by low serum 3,5, 3'-triiodothyronine (T(3)) with normal L-thyroxine levels, is associated with a wide variety of disorders including sepsis, malignancy, and AIDS. The degree of low T(3) in circulation has been shown to correlate with the severity of the underlying disorders and with the prognosis. Elevated TNF-alpha levels, which accompany severe illness, are associated with decreased activity of type I 5'-deiodinase (5'-DI) in liver, leading us to speculate that high levels of this factor contribute to euthyroid sick syndrome. Here we demonstrate that the activation of NF-kappa B by TNF-alpha interferes with thyroid-hormone action as demonstrated by impairment of T(3)-dependent induction of 5'-DI gene expression in HepG2 cells. Inhibition of NF-kappa B action by a dominant-negative NF-kappa B reversed this effect and allowed T(3) induction of 5'-DI. Furthermore, we show that an inhibitor of NF-kappa B activation, clarithromycin (CAM), can inhibit TNF-alpha-induced activation of NF-kappa B and restore T(3)-dependent induction of 5'-DI mRNA and enzyme activity. These results suggest that NF-kappa B activation by TNF-alpha is involved in the pathogenesis of euthyroid sick syndrome and that CAM could help prevent a decrease in serum T(3) levels and thus ameliorate euthyroid sick syndrome.

Retinoic acid redifferentiation therapy for thyroid cancer

For the treatment of differentiated thyroid cancer, surgery, radioiodide therapy, and thyrotropin-suppressive thyroxine application represent established therapeutic measures of proven efficiency, affording a good prognosis for this disease. However, in up to 30% of the cases, dedifferentiation is observed, giving rise to tumors that are refractory to conventional treatment. Eventually, this may lead to the most malignant human tumor, anaplastic thyroid carcinoma, with a life expectancy of only a few months after diagnosis. Among novel approaches for the treatment of dedifferentiated thyroid carcinomas, retinoic acid redifferentiation therapy was evaluated in several in vitro and in vivo studies. Cell culture experiments in thyroid carcinoma lines show that RA treatment affects thyroid specific functions (type I 5'-deiodinase, sodium/iodide-symporter), cell-cell or cell-matrix interaction (intercellular adhesion molecule-1, E-cadherin), differentiation markers (alkaline phosphatase, CD97), growth, and tumorigenicity. The observed changes, which involve multiple parameters that characterize a mature, functional thyrocyte, may be interpreted as partial redifferentiation. In clinical pilot studies, about 40% of the patients responded to RA application with an increased radioiodide uptake. In an evaluation of 20 RA-treated patients with well-documented data sets, 8 exhibited a decrease (4) or stabilization (4) in tumor size and/or in serum thyroglobulin levels in addition to enhanced radioiodide transport. This indicates that these patients with a long history of unresponsiveness to other treatment may have experienced an actual therapeutic benefit. These data suggest that RA redifferentiation therapy, considering especially its comparatively mild side effects, may soon represent an alternative therapeutic approach to otherwise untreatable thyroid tumors.

Association of some potential hormone response elements in human genes with the Alu family repeats

Short interspersed repeats of the Alu family located in promoters of some human genes contain high-affinity binding sites for thyroid hormone receptor, retinoic acid receptor and estrogen receptor. The standard binding sites for the receptors represent variants of duplicated AGGTCA motif with different spacing and orientation (direct, DR, or inverted, IR), and Alu sequences were found to have functional DR-4, DR-2 or variant IR-3/IR-17 elements. In this study we analyzed distribution and abundance of the elements in a set of human genomic sequences from GenBank and their association with Alu repeats. Our results indicate that a major fraction of potentially active DR-4, DR-2 and variant IR-3/IR-17 elements in the genes is located within Alu repeats. Alu-associated DR-2 elements are conserved in primate evolution. However, very few Alu have potential DR-3 glucocorticoid-response elements. Gel-shift experiments with the probe (AUB) corresponding to the consensus Alu sequence just upstream of the RNA polymerase III promoter B-box and containing duplicated AGGTCA motif indicate that the probe interacts in a sequence-specific manner with human nuclear proteins which bind to standard IR-0, DR-1, DR-4 or DR-5 elements. The AUB sequence was also able to promote thyroid hormone-dependent trans-activation of a reporter gene. The results support the view that Alu retroposons played an important role in evolution of regulation of the primate gene expression by nuclear hormone receptors.

Local activation and inactivation of thyroid hormones: the deiodinase family

Tissue-specific activation and inactivation of ligands of nuclear receptors which belong to the steroid retinoid-thyroid hormone superfamily of transcription factors represents an important principle of development- and tissue-specific local modulation of hormone action. Recently, several enzyme families have been identified which act as 'guardians of the gate' of ligand-activated transcription modulation. Three monodeiodinase isoenzymes which are involved in activation the 'prohormone' L-thyroxine (T4), the main secretory product of the thyroid gland, have been identified, characterized, and cloned. Both, type I and type II 5'-deiodinase generate the thyromimetically active hormone 3,3',5-triiodothyronine (T3) by reductive deiodination of the phenolic ring of T4. Inactivation of T4 and its product T3 occurs by deiodination of iodothyronines at the tyrosyl ring. This reaction is catalyzed both the type III 5-deiodinase and also by the type I enzyme, which has a broader substrate specificity. The three deiodinases appear to constitute a newly discovered family of selenocysteine-containing proteins and the presence of selenocysteine in the protein is critical for enzyme activity. Whereas the selenoenzyme characteristics of the type I and type III deiodinases are definitively established some controversy still exists for the type II 5'-deiodinase in mammals. The mRNA probably encoding the type II 5'-deiodinase subunit is markedly longer than those of the two other deiodinases and its selenocysteine-insertion element is located more than 5 kB downstream of the UGA-codon in the 3'-untranslated region. The three deiodinase isoenzymes show a distinct development- and tissue-specific pattern of expression, operate at individual optimal substrate levels, are differently regulated and modulated by hormones, cytokines, signaling pathways, natural factors, and pharmaceuticals. Whereas circulating T3 mainly originates from hepatic production via the type I 5'-deiodinase, the local cellular thyroid hormone concentration in various tissues including the central nervous system is controlled by complex para-, auto-, and intracrine interactions of all three deiodinases. Local thyroid hormone availability is further modulated by conjugation reactions of the phenolic 4'-OH-group of iodothyronines, which also inactivate the thyroid hormones.

Intracellular proteolytic cleavage of 9-cis-retinoic acid receptor alpha by cathepsin L-type protease is a potential mechanism for modulating thyroid hormone action

We previously reported that the responsiveness of hepatocytes to thyroid hormone is markedly attenuated when they were cultured as monolayers rather than spheroids. To elucidate the mechanisms underlying the altered responsiveness, thyroid hormone receptor auxiliary proteins in the hepatocytes were analyzed by electrophoretic mobility shift assay. The major thyroid hormone receptor auxiliary protein was identified as 9-cis-retinoic acid receptor alpha (RXRalpha) in the hepatocytes regardless of the culture conditions. The cytoplasmic fraction was shown to contain a protease(s) that cleaves RXRalpha at its amino terminus. The presence of the protease in the cytosol, but not in the nucleus, was ascertained by incubating full-length 35S-labeled RXRalpha with each fraction. Using various protease inhibitors, it was shown that cathepsin L-type protease could participate in the cleavage of the RXRalpha. The enzyme activity was much higher in the monolayers than the spheroids. Inhibition of this enzyme activity in the monolayer hepatocyte resulted in the increase of nuclear RXRalpha protein and the augmentation of T3-dependent induction of spot 14 mRNA. These results suggest that the changes in cathepsin L-type protease activity in the cytosol may alter the turnover of RXRalpha in the nucleus and modify the function of steroid receptor superfamilies that heterodimerize with RXRalpha.

Quantitative measurements for type 1 deiodinase messenger ribonucleic acid in human peripheral blood mononuclear cells: mechanism of the preferential increase of T3 in hyperthyroid Graves' disease

To evaluate the regulatory mechanism of human Type 1 iodothyronine deiodinase (D1) gene expression, we measured the D1 mRNA levels in peripheral blood mononuclear cells (PBMC) in normal control subjects and in patients with Graves' disease. We used competitive reverse transcriptase-polymerase chain reaction with the deleted complimentary RNA of D1 as the standard for quantification. The D1 mRNA levels in PBMC were increased significantly in patients with Graves' disease compared with that in normal controls. There was a significant (p < 0.01) positive correlation (r=0.698) between D1 mRNA level and serum T3 concentration. When PBMC from the normal volunteers were cultured with various doses of T3, the quantity of D1 mRNA increased significantly in a dose-dependent manner. These findings indicate that PBMC D1 mRNA is actually up-regulated by T3 in vivo, and we postulate that a vicious spiral of increasing T3 and D1 is responsible for the exacerbation of thyrotoxicosis in hyperthyroid Graves' disease.

Transcriptional regulatory patterns of the myelin basic protein and malic enzyme genes by the thyroid hormone receptors alpha1 and beta1

While there is evidence that the two ubiquitously expressed thyroid hormone (T3) receptors, TRalpha1 and TRbeta1, have distinct functional specificities, the mechanism by which they discriminate potential target genes remains largely unexplained. In this study, we demonstrate that the thyroid hormone response elements (TRE) from the malic enzyme and myelin basic protein genes (METRE and MBPTRE) respectively, are not functionally equivalent. The METRE, which is a direct repeat motif with a 4-base pair gap between the two half-site hexamers binds thyroid hormone receptor as a heterodimer with 9-cis-retinoic acid receptor (RXR) and mediates a high T3-dependent activation in response to TRalpha1 or TRbeta1 in NIH3T3 cells. In contrast, the MBPTRE, which consists of an inverted palindrome formed by two hexamers spaced by 6 base pairs, confers an efficient transactivation by TRbeta1 but a poor transactivation by TRalpha1. While both receptors form heterodimers with RXR on MBPTRE, the poor transactivation by TRalpha1 correlates also with its ability to bind efficiently as a monomer. This monomer, which is only observed with TRalpha1 bound to MBPTRE, interacts neither with N-CoR nor with SRC-1, explaining its functional inefficacy. However, in Xenopus oocytes, in which RXR proteins are not detectable, the transactivation mediated by TRalpha1 and TRbeta1 is equivalent and independent of a RXR supply, raising the question of the identity of the thyroid hormone receptor partner in these cells. Thus, in mammalian cells, the binding characteristics of TRalpha1 to MBPTRE (i.e. high monomer binding efficiency and low transactivation activity) might explain the particular pattern of T3 responsiveness of MBP gene expression during central nervous system development.

Mechanisms of thyroid hormone action. Implications for the clinical manifestation of thyrotoxicosis

Serum thyroid hormone concentrations alone do not explain the variability and severity of the range of symptoms observed in thyrotoxic patients. Despite gaps in our understanding of the links between the clinical manifestations of thyrotoxicosis and the underlying mechanisms, much has been learned. A limited number of markers directly reflect T3 action. The future elucidation of T3 targets that mediate these effects should ultimately lead to additional clinical markers of tissue-specific T3 action. The availability of such tests should allow for more specific treatment of individual patients.

Specific organization of the negative response elements for retinoic acid and thyroid hormone receptors in keratin gene family

Retinoic acid and thyroid hormone are important regulators of epidermal growth, differentiation, and homeostasis. Retinoic acid is extensively used in the treatment of many epidermal disorders ranging from wrinkles to skin cancers. Retinoic acid and thyroid hormone directly control the transcription of differentiation-specific genes including keratins. Their effect is mediated through nuclear receptors RAR and T3R. We have previously identified the response element in the K14 gene, K14RARE/TRE, to which these receptors bind, and found that it consists of a cluster of five half-sites with variable spacing and orientation. To determine whether this specific structure is found in other keratin genes, we have mapped and analyzed the RARE/TRE elements in three additional epidermal keratin genes: K5, K6, and K17. We used three different approaches to identify these elements: co-transfection of promoter deletion constructs, gel-shift assays, and site-specific mutagenesis. We localized the RARE/TRE elements relatively close to the TATA box in all three promoters. All three RARE/TRE elements have a similar structural organization: they consist of clusters of 3-6 half-sites with variable spacing and orientation. This means that the clustered structure of the RARE/TREs is a common characteristic for keratin genes. RARE and TRE in the K5 promoter are adjacent to each other whereas in the K17 promoter they overlap. All three keratin REs bind specifically both RAR and T3R in gel-shift assays. Interestingly, addition of ligand to the receptor changes the binding pattern ofthe T3R from homodimer to monomer, reflecting the change in regulation from induction to inhibition.

The deiodinase family of selenoproteins

The realization some forty years ago that several iodothyronine compounds are present in the circulation suggested that deiodination occurs in various tissues. Subsequently, deiodination was indeed documented in in vivo studies. Later, using in vitro assay techniques, three deiodinase processes, termed types 1, 2 and 3, were defined that differed in terms of tissue distribution, reaction kinetics, efficiency of substrate utilization and sensitivity to inhibitors. Although purification of the deiodinase enzymes has continued to be problematic, recent molecular cloning studies have identified cDNAs for these three deiodinase isoforms from multiple species. These cDNAs have provided important insights into the structural characteristics of this family of enzymes. Foremost among the structural features has been the demonstration that all three deiodinase isoforms contain at their active site the uncommon amino acid selenocysteine which is of critical importance to their catalytic activity. The availability of cDNAs for these enzymes provides important reagents for pursuing additional studies aimed at defining their biochemical features and roles in thyroid hormone economy.

The promoter of the human type I 5'-deiodinase gene--mapping of the transcription start site and identification of a DR+4 thyroid-hormone-responsive element

The selenoenzyme thyroxine 5'-deiodinase type I deiodinates the prohormone thyroxine to the active thyroid hormone 3,3',5-triiodothyronine. It is thus one of the key enzymes involved in the triiodothyronine-mediated control of growth, differentiation and basal metabolism in vertebrates. We report here the identification of the transcription start site and the cloning of 1500 bases of the upstream regulatory region of the human 5'-deiodinase gene. They contain a complex triiodothyronine-responsive element at nucleotides -696 to -673, consisting of an ideal direct repeat (DR) of two AGGTCA half-sites with a spacing of four nucleotides (DR+4) and a third putative AGTTCA half-site with a spacing of another two nucleotides (DR+2). The whole DR+4+2 specifically bound to thyroid hormone receptor and retinoid X receptor in electrophoretic mobility shift assays. The DR+4+2 mediates triiodothyronine-responsiveness in cotransfection experiments of constructs containing the 5'-deiodinase upstream promoter and enhancer region fused to luciferase or chloramphenicol acetyltransferase reporter genes with expression plasmids of thyroid hormone receptor subtypes. Also, an about 2.5-fold induction of the 5'-deiodinase-promoter-luciferase-reporter construct by all-trans retinoic acid was observed in a cotransfection assay with retinoic acid receptors. Point mutation analysis of the DR+4+2 type hormone-responsive element, however, revealed that it does not alone mediate the retinoic acid effect. The transcription start point of the 5'-deiodinase gene was mapped to nucleotides -23 and -24. No CAAT or TATA box is located within the usual distance to the transcription initiation site. Two GC boxes were found at nucleotides -68 to -63 and -39 to -34. Transfection analysis revealed that the proximal 105 nucleotides in the 5'-flanking region of the 5'-deiodinase gene act as a functional core promoter. This data indicates that triiodothyronine, the end product of thyroid hormone synthesis, positively regulates one of the key enzymes in its production.

5'-flanking sequences in thyroid hormone response element half-sites determine the requirement of retinoid X receptor for receptor-mediated gene expression

Thyroid hormone receptors are ligand-inducible transcription factors that can potentially interact with thyroid hormone response elements as homodimers or heterodimers with the retinoid X receptor. It has generally been felt, however, that the heterodimer is responsible for induction of gene expression. We have demonstrated previously that the optimal thyroid hormone receptor binding sequence is not the consensus hexamer half-site AGGTCA but is an octamer, TAAGGTCA. Based upon these findings, we hypothesize that thyroid hormone response elements composed of optimal half-sites (TAAGGTCA) will bind thyroid hormone receptors readily and activate gene expression independently of the retinoid X receptor. In contrast, response elements composed of suboptimal half-sites (e.g. GCAGGTCA) will require the retinoid X receptor to facilitate thyroid hormone receptor-mediated gene expression. To test this hypothesis, we have reconstituted thyroid hormone receptor-mediated gene expression in yeast. Our studies confirm the hypothesis that the retinoid X receptor is required for gene expression from response elements composed of suboptimal half-sites, whereas thyroid hormone receptors are sufficient to activate gene expression maximally from response elements containing optimal half-sites. Furthermore, coexpression of steroid receptor coactivator-1 is required for ligand-dependent gene activation from single response elements. Surprisingly, however, coexpression of the retinoid X receptor decreases the steroid receptor coactivator-1-dependent thyroid hormone induction. Overall these data demonstrate that the architecture of the thyroid hormone response element dictates the nuclear receptor requirements for gene activation. The studies suggest that different coactivators may be required for gene activation depending upon the response element architecture and the nature of the bound thyroid hormone receptor complex (homo- versus heterodimer).

Characterization of the promoter of the rat sarcoplasmic endoplasmic reticulum Ca2+-ATPase 1 gene and analysis of thyroid hormone responsiveness

Relaxation of skeletal muscle requires the re-uptake of Ca2+, which is mediated by the sarcoplasmic reticulum Ca2+-ATPase (SERCA). Thyroid hormone (T3) stimulates the expression of the SERCA1 isoform, which is essential for fast skeletal muscle fiber phenotype. We have cloned and studied the first 962 base pairs of the 5'-flanking region of the rat SERCA1 gene. This sequence was tested for T3-regulated expression in transient transfection experiments using COS7 cells and for binding of thyroid hormone receptor (TR) alpha in mobility shift assays. A construct of the 5'-flanking region and a reporter gene was unresponsive to T3 in the absence of co-transfected thyroid hormone receptor. In the presence of TRalpha, a T3 induction ratio of almost 4.0 was found, and this induction ratio was doubled with co-transfection of an RXR expression plasmid. Analysis of progressive 5'-deletion fragments of the sequence indicated multiple regions involved in T3 responsiveness. Three regions, R1, R2, and R3, were identified that bound TR complexes in mobility shift assays and conferred T3 responsiveness to a heterologous promoter. The most potent of these thyroid hormone response elements, R3, increased the 2-fold background T3 stimulation of the thymidine kinase promoter to nearly 6-fold. Detailed analysis of this element showed that four TR-binding half-sites, comprising two independent thyroid hormone response elements, interact cooperatively to give the maximal T3 response. T3 regulation of SERCA1 expression is mediated by a complex thyroid hormone response element that may serve to provide a greater range of response in interaction with nuclear receptor partners or cell-specific transcription factors.