Phase II trial evaluating imatinib (I) in patients (pts) with anaplastic thyroid carcinoma (ATC)

6057

Background: There is no standard treatment for ATC. Response to doxorubicin ranges between 5–22%; median survival ranges between 3–6 months. Affymetrix gene chip showed PDGFR overexpression in ATC. In p53 mutated/deficient ATC cell lines, c-Abl is overexpressed, and selective inhibition of c-Abl resulted in cytostatic effect. (I) inhibits tyrosine kinase activity of Bcr-Abl and PDGF. We hypothesize that ATC that overexpress PDGFR or Abl will respond to (I). Methods: Pts ≥ 18 years old with histologically confirmed ATC, overexpressing PDGFR or c-Abl by immunohistochemistry who had measurable disease were eligible. Pts must have Zubrod performance status ≤ 1, and adequate hepatic and renal function. Prior chemotherapy, chemoradiation, radiation therapy, or surgery must have been completed at least 28 days prior to registration. (I) was administered at 400mg orally twice daily without drug holiday. Response to treatment was assessed after every 8 weeks. Pts with complete response (CR)/partial responses (PR)/stable disease (SD) were treated until disease progression. The study was terminated early due to poor accrual. Results: From February 2004 to May 2007, eleven pts from our institution were enrolled and were started on (I) (6 men; 5 women) with a median age of 63 years (ranges 53–80). At baseline, 4/11 pts (36%) had locoregional disease, 5/11 pts (45%) had distant metastases, and 2/11 pts (18%) had both. Nine pts had prior chemoradiation, and 7 pts had thyroidectomy. Out of 11 pts, 8 were evaluable for response; 3 were excluded for lack of follow up radiological evaluation. The overall responses at 8 weeks were CR 0/8; PR 2/8 (25%); SD 4/8 (50%); and PD 2/8 (25%). The median time to follow up was 26 months (ranges 23–30 months). The estimate of 6-month progression free survival was 27% (95% CI, 7–54%). The estimate of 6-month overall survival was 46% (95% CI, 17–71%). The most common G3 toxicity was lymphopenia in 45%; other G3 toxicities included edema (27%), anemia (18%), and hyponatremia (18%). There was no G4 or higher or treatment related death. Conclusions: (I) appears to have activity in advanced ATC and is well tolerated. Due to difficulty of accruing pts with a rare malignancy at a single institution, investigation of (I) in ATC may be warranted in a multi-institution setting.

No significant financial relationships to disclose.

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.

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.

Gene regulation by thyroid hormone

Regulation of gene expression by thyroid hormones (T3, T4) is mediated via thyroid hormone receptors (TRs). TRs are DNA-binding transcription factors that function as molecular switches in response to ligand. TRs can activate or repress gene transcription depending on the promoter context and ligand-binding status. In most cases, in the absence of ligand, TRs interact with a corepressor complex containing histone deacetylase activity, which actively inhibits transcription. The binding of ligand triggers a conformational change in the TR that results in the replacement of the corepressor complex by a coactivator complex containing histone acetyltransferase activity, through which the chromatin structure is remodeled, thereby leading to activation of transcription. In addition, the finding that several TR-interacting coregulators act more directly on the basal transcriptional machinery suggests that mechanisms independent of histone acetylation and deacetylation also are involved in TR action.

2-Methoxyestradiol, an endogenous estrogen metabolite, induces thyroid cell apoptosis

The etiology of autoimmune thyroid diseases is unclear; however, the extreme female predominance suggests that sex hormones may have a pathogenic role. 2-Methoxyestradiol (2-ME) is present in the serum of women during the ovulatory and luteal phases of the menstrual cycle, and during pregnancy. We investigated the actions of 2-ME and estrogen on thyroid follicular cells. 2-ME induced dramatic changes in cell morphology and decreased the viability of the cells, as well as disrupted the structural integrity of cultured thyroid follicles. Flow cytometric analysis showed that 2-ME halted cell proliferation by arresting the cells in the G2/M cell-cycle compartment. Prolonged exposure to 2-ME led to apoptosis and to increased release of the autoantigen thyroid peroxidase (TPO). 17beta-estradiol failed to produce a similar effect even in 40-fold molar excess to 2-ME. Co-treatment with estrogen receptor antagonists did not alter the 2-ME effect, indicating that 2-ME was not operating through a classic nuclear estrogen receptor. In conclusion, this study indicates that 2-ME induces G2/M cycle arrest, apoptosis and the disruption of thyroid follicles. This process results in the release of thyroid antigens that may play a role in high incidence of thyroid autoantibodies and autoimmune thyroid disease in women.

1Alpha,25-dihydroxyvitamin D3 up-regulates Bcl-2 expression and protects normal human thyrocytes from programmed cell death

Apoptosis is thought to play an important role in the pathogenesis of autoimmune thyroid disease. 1alpha,25-dihydroxyvitamin D3 (VD3) has been shown to suppress several autoimmune diseases. However, the mechanism by which VD3 has these effects is not known. We evaluated the alterations in apoptosis, induced by VD3. Thyrocytes were treated with VD3, and the expression of the Bcl-2 family molecules was studied at both the messenger RNA and protein levels. It was found that VD3 significantly induced the expression of Bcl-2 messenger RNA and protein in thyrocytes but had no effect on the expression of Bcl-xl and Bax. The increase in Bcl-2 expression, mediated by VD3, correlated with protection of thyrocytes against the induction of apoptosis by either staurosporine or UV irradiation. VD3-induced increases in the expression of Bcl-2 could be mimicked by VD3 analogs with high nuclear receptor affinity, but not by analogs only with nongenomic actions. These data indicate a role for Bcl-2 in the regulation of apoptosis in thyrocytes and raise the possibility that VD3 or its agonists may have therapeutic benefit in thyroid disorders.

Dominant negative activity of thyroid hormone receptor variant alpha2 and interaction with nuclear corepressors

The splicing variant of the thyroid hormone receptor alpha (TRalpha) gene, TR variant alpha2 (TRv alpha2), lacks the second half of the ninth heptad, a domain thought to be important for heterodimerization with retinoid X receptors (RXRs). In transient transfection studies, TRv alpha2 exhibits weak dominant negative inhibition of TRalpha1-mediated transcription. In contrast, a TRv alpha2 mutant in which the ninth heptad was restored (alpha2 + 9H), exhibits very strong dominant negative activity. We have examined the role of nuclear corepressors (CoRs) in the dominant negative activity of TRv alpha2 and alpha2 + 9H. Glutathione S-transferase pull down experiments revealed that TRv alpha2 barely interacts with CoRs, whereas alpha2 + 9H interaction with CoRs is as strong as that of TRalpha1. A P160R CoR box mutation was introduced in the context of TRv alpha2 and alpha2 + 9H, which nearly abolishes the ability of these receptors to interact with CoRs. In transient transfection the dominant negative activity of TRv alpha2 was only marginally impaired by the P160R mutation. In contrast, alpha2 + 9H-P160R had approximately 66% less dominant negative activity than alpha2 + 9H. These results suggest that the weak dominant negative activity of TRv alpha2 is due in part to its lack of interaction with CoRs, and that restoration of the ninth heptad restores CoR interaction and strong dominant negative activity. Further, the data reveal aspects of the dominant negative action that are dependent on the orientation of the TRE.

Protein-protein interaction domains and the heterodimerization of thyroid hormone receptor variant alpha2 with retinoid X receptors

Heterodimerization between thyroid hormone receptors (TRs) and retinoid X receptors (RXRs) is mediated by a weak dimerization interface within the DNA- binding domains (DBDs) and a strong interface within the C-terminal ligand- binding domains of the receptors. Previous studies have shown that the conserved ninth heptad in the TR ligand-binding domain appears to play a critical role in heterodimerization with RXR. However, despite lacking the full ninth heptad, TR variant alpha2 (TRv alpha2) can heterodimerize with RXR on specific direct repeat response elements, but not on palindromic elements or in solution. Two possibilities may account for TRv alpha2-RXR heterodimerization on direct repeats. First, the DBD of TRv alpha2 may play a critical role in heterodimerization with RXR. Second, a specific sequence within the unique C terminus of TRv alpha2 may promote the formation of TRv alpha2-RXR heterodimers. In this study, we used receptor chimeras in which the DBD of RXR was replaced by either the TR DBD or an unrelated DBD from the metalloregulatory transcription factor AMT1 to address the role of the DBD dimerization interface in TRv alpha2-RXR heterodimerization. Gel mobility shift analyses showed that whereas TR alpha1 formed heterodimers with these chimeras, TRv alpha2 failed to do so. Deletion of the unique C terminus of TRv alpha2 had only a marginal effect on heterodimerization with RXR. Mutations within the DBD dimerization interface abolished heterodimerization of full-length TRv alpha2 with RXR but only marginally affected heterodimerization of full-length TR alpha1 with RXR. These data support the hypothesis that the TR-RXR DBD dimerization interface plays a critical role in TRv alpha2-RXR heterodimerization. Additional data show that the amino acid residues that make direct TR-RXR contacts within the DBDs also may play a role in receptor monomer binding to DNA, since mutations within these residues severely impair this interaction.

Characterization of the DNA-binding and dominant negative activity of v-erbA homodimers

The oncoprotein v-erbA is a mutated form of thyroid hormone receptor alpha1 that is virtually incapable of binding T3. V-erbA is a dominant repressor of transcription induced by thyroid hormone receptors and retinoic acid receptors; however, the genetic targets of v-erbA that lead to oncogenesis are not known. Although v-erbA can bind as monomers and dimers to DNA containing the consensus sequence AGGTCA arranged as direct, inverted, or everted repeats, it is not known which sequence represents the optimal v-erbA-binding site. Determination of the DNA recognition properties of v-erbA would allow a better understanding of the repressor activity of this oncoprotein. The current studies, by using a random DNA selection strategy, have determined that the imperfect everted repeat 5'-TGACC(T/C)NT(A/G)AGGTCAC is the optimal v-erbA homodimer-binding site, where N represents any di- or trinucleotide. Functional studies show that everted repeats containing this sequence are substantially more potent v-erbA response elements than direct or inverted repeats, even though many classic T3 response elements are direct repeats. Thus, v-erbA represses only a subset of T3 response elements. In a similar fashion, v-erbA was found to repress a subset of vitamin D response elements. Of general interest, the data indicate that the two molecules of a transcription factor homodimer do not necessarily have identical DNA-binding specificities.

Thyroid hormone receptor coactivators and corepressors

In the absence of triiodothyronine (T3), thyroid hormone receptors (TRs) repress transcription of many genes; in the presence of T3, TRs activate transcription of those same genes. Both of these events are dependent on interactions between TRs and other nuclear proteins. TRs bind to specific DNA sequences, generally found in the 5' flanking regions of target genes. In the unliganded state, TRs interact with one of several corepressor proteins. These proteins, in turn, interact with a series of other proteins, which includes histone deacetylases. Histone deacetylation tightens chromatin structure, thus impairing access of critical transcription factors and thereby repressing transcription. In addition, corepressors may invoke mechanisms of gene repression independent of histone deacetylation. The binding of T3 causes a conformational change in the TR that results in release of the corepressor and recruitment of coactivator proteins. Several coactivator proteins appear to bind the ligand-occupied TR as a multiprotein complex. Opposite to corepressors, coactivators acetylate histones, thereby loosening chromatin structure and facilitating access of key transcription factors. Again, mechanisms independent of histone acetylation also may be involved. Overall, gene activation by T3 is a two-step process; removal of active repression, and induction of transcription to levels above the "neutral" state.

Thyroid hormone response element architecture affects corepressor release from thyroid hormone receptor dimers

Thyroid hormone receptors are ligand-modulated transcription factors that can repress or activate transcription depending upon the absence or presence of thyroid hormone and the nature of the hormone response element to which the receptors are bound. The ability of thyroid hormone receptors to repress transcription in the absence of ligand is thought to be due to associations with nuclear hormone receptor corepressors. Ligand binding by the thyroid hormone receptor is believed to dissociate these corepressors and recruit coactivators to promote transcription from target promoters. We hypothesize that variations in response element architecture may influence both the association and dissociation of corepressors from DNA-bound thyroid hormone receptors. Using a chimeric corepressor, we find that ligand alone does not fully relieve corepressor-mediated repression, particularly in the presence of thyroid hormone receptor and its heterodimerization partner, the retinoid X receptor. Interestingly, the steroid receptor coactivator 1 together with ligand is able to mediate full release of corepression, but this relief is dependent upon the architecture of the response element to which the nuclear receptor dimer-corepressor complex is bound. These studies suggest that other cellular factors in addition to ligand may be required for the release of corepressors from thyroid hormone receptor dimers.

Thyroid function in Rubinstein-Taybi syndrome

Rubinstein-Taybi syndrome (RTS) is a genetic syndrome characterized by broad thumbs and halluces, growth retardation, mental retardation, and craniofacial abnormalities. This condition recently was found to be caused by mutations in the gene encoding cAMP response element-binding protein (CREB)-binding protein. As CREB-binding protein has been shown to be a critical coactivator for thyroid hormone receptors, it is plausible that RTS would be characterized by thyroid hormone resistance. In fact, features of RTS, such as mental retardation and short stature, are consistent with thyroid hormone deficiency or resistance. To assess the function of the thyroid axis in RTS, free T4 and TSH were measured in 12 subjects with this syndrome. The free T4 level was normal in all 12 (mean +/- SD, 0.97 +/- 0.20 ng/dL; normal range, 0.73-1.79), as was the TSH level (2.24 +/- 0.87 microU/mL; normal range, 0.3-6.5). Thus, overt thyroid hormone resistance does not appear to be a typical feature of RTS.

Effect of glyoxylate on the function of the calcitriol receptor and vitamin D metabolism

The biological action of calcitriol is mostly mediated through the interaction of the calcitriol receptor (VDR) with vitamin D response elements (VDREs) of target genes. These interactions produce special proteins that carry out the biological activities of calcitriol. Recently, we showed that the interaction of VDRs with VDREs is inhibited by uremic toxins. We hypothesize that uremic toxins that contain aldehyde or ketone groups potentially could form Schiff bases with lysine residues of the VDR DNA binding domain and inhibit VDR interaction with VDREs. We therefore chose glyoxylate, a compound which has an aldehyde group, to test this hypothesis. In vitro glyoxylate inhibited VDR binding to the osteocalcin and osteopontin VDREs as assessed by electrophoretic mobility shift assay and the inhibition was reversed when glyoxylate was preincubated with lysine. Further, this chemical compound also blocked the induction of chloramphenicol acetyltransferase (CAT) enzyme induced by calcitriol in cells transfected with a calcitriol responsive CAT reporter gene. Since induction of 24-hydroxylase synthesis is a VDR regulated process, we also studied the effect of glyoxylate on the activity of intestinal 24-hydroxylase in rats. This enzyme activity was suppressed in rats infused with glyoxylate. Taken together, our study suggests that glyoxylate could inhibit the interaction of VDR with VDREs and alter the biological action of calcitriol.

Effect of glucose on the function of the calcitriol receptor and vitamin D metabolism

The genomic action of calcitriol is mediated through the interaction of the calcitriol receptor (VDR) with vitamin D response elements (VDREs) of the target genes. It has been proposed that chemicals capable of Schiff base formation with the VDR potentially could alter the physiological function of VDR and calcitriol metabolism. Since glucose has been shown to form Schiff bases with proteins, we tested the hypothesis that glucose could influence the function of VDR and thereby alter calcitriol metabolism. Glucose 6-phosphate inhibited VDR binding to the osteocalcin VDRE and chemically modified the DNA binding domain or the dimerization domain of the VDR in vitro. Further, glucose also blocked the production of chloramphenicol acetyltransferase (CAT) enzyme induced by calcitriol in cells transfected with a constructed VDRE attached to a CAT reporter gene. Hyperglycemia induced by glucose infusion or by streptozotocin in normal rats significantly reduced intestinal 1 alpha, 25-dihydroxyvitamin D-24-hydroxylase activity. Taken together, these findings are consistent with the hypothesis that glucose could interact with the VDR to impair its DNA binding and function within cells.

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).

Thyroid hormone receptor variant alpha2. Role of the ninth heptad in dna binding, heterodimerization with retinoid X receptors, and dominant negative activity

Thyroid hormone receptors bind DNA with highest affinity as heterodimers with retinoid X receptors, and such heterodimers generally are thought to be the biological mediators of thyroid hormone action. An alternative splice product of the thyroid hormone receptor alpha gene, thyroid hormone receptor variant alpha2, does not bind thyroid hormone and functions as a weak dominant negative inhibitor of thyroid hormone action. Thyroid hormone receptor variant alpha2 is missing one-half of the ninth heptad, a region of the bona fide receptor thought to be important for heterodimerization with retinoid X receptors. The role of the ninth heptad in heterodimerization has been evaluated further. Thyroid hormone receptor variant alpha2-retinoid X receptor heterodimers form on a subset of direct repeat response elements but not on palindromic or inverted palindromic elements. Restoration of the missing ninth heptad sequence is critical for restoring heterodimerization on the palindromic DNA, but either the ninth heptad amino acids or a stretch of alanines is equally able to restore heterodimerization on the inverted palindrome. Thus, the role of the ninth heptad in heterodimerization differs on direct repeat, palindromic, and inverted palindromic response elements, suggesting that the protein-protein interactions differ on each of these elements. The dominant negative activity of thyroid hormone receptor variant alpha2 requires DNA binding, but the relatively weak nature of the dominant negative activity is only partially explained by the weak DNA binding.

Effect of Schiff base formation on the function of the calcitriol receptor

The genomic action of calcitriol is mediated through the interaction of the calcitriol receptor (VDR) with vitamin D response elements (VDREs) of the target genes. We have shown that the interaction of VDRs with VDREs is inhibited by uremic toxins. We hypothesize that uremic toxins form Schiff bases with the lysine residues of the VDR DNA binding domain and inhibit the VDR interaction with the VDRE. In this study, pyridoxal 5'-phosphate was used as a probe to test Schiff base formation as the inhibitory mechanism, since it forms Schiff bases with steroid receptors. Pyridoxal 5'-phosphate inhibited the VDR binding to the VDREs and chemically modified the DNA binding domain of the VDR in vitro. The inhibition was reversed when pyridoxal 5'-phosphate was preincubated with lysine. Further, this chemical agent also blocked the production of chloramphenicol acetyltransferase (CAT) enzyme induced by calcitriol in cells transfected with a constructed VDRE attached to a CAT reporter gene. This finding is consistent with the hypothesis that pyridoxal 5'-phosphate could interact with the VDR and impair its DNA binding within cells. Since induction of 24-hydroxylase synthesis is a receptor mediated process, we studied the effect of pyridoxal 5'-phosphate on the synthesis of renal 24-hydroxylase in rats. When pyridoxal 5'-phosphate was infused to rats, renal 24-hydroxylase activity was suppressed, consequently, degradation of calcitriol was also reduced in these animals. Thus, chemicals capable of Schiff base formation potentially could alter the physiological function of VDR and calcitriol.

Unique autoantibody epitopes in an immunodominant region of thyroid peroxidase

To define the autoantibody epitopes in amino acids 513-633 of thyroid peroxidase (TPO), a region frequently recognized in thyroiditis, cDNA sequences coding for peptide fragments of this region were amplified and ligated into pMalcRI and pGEX vectors for expression as recombinant fusion proteins. Western blots and enzyme-linked immunosorbent assay were then used to examine the reactivity in sera from 45 Hashimoto's and 47 Graves' disease patients. Two autoantibody epitopes within TPO amino acids 589-633 were identified; 16 of 35 patients reactive to TPO513-633 recognized the epitope of TPO592-613, while 6 patients recognized the epitope of TPO607-633. Eleven other patients with thyroiditis and two with Graves' disease recognized only the whole 589-633 fragment, and this response accounted for the Hashimoto's disease specificity. An amino acid sequence comparison of TPO592-613 with analogous regions of other peroxidase enzymes revealed significant differences in this area, and the substitution of even a single amino acid in one of the epitopes markedly decreased the binding affinity of autoantibodies. Additionally, the exclusive recognition by patients of only one of the epitopes within this region suggests a genetic restriction of the autoantibody response.

Retinoid X receptor alpha binds with the highest affinity to an imperfect direct repeat response element

The regulation of gene expression by retinoids is mediated by two classes of receptors, retinoic acid receptors and retinoid X receptors (RXR). RXR can bind to specific target genes as homodimers, and these homodimers can activate gene expression in the presence of the ligand 9-cis-retinoic acid. A direct repeat of AGGTCA with a 1 base pair spacer (DR1) acts as a RXR homodimer response element in the presence of 9-cis-retinoic acid. However, it is not known if this represents the highest affinity binding site for the RXR homodimer. To investigate this question, we used a nonbiased strategy to isolate from a pool of random DNA those sequences that have the highest affinity for RXR alpha homodimers. The imperfect DR1 sequence 5'-GGGGTCAAAGGTCA displayed the highest in vitro binding affinity for RXR alpha homodimers. Transient transfection studies confirmed that this sequence is a more potent response element than is a perfect DR1 of either AGGTCA or GGGGTCA. The results also indicate that for RXR alpha homodimers, the receptor bound to the 5' half-site dislays different DNA binding specificity than that bound to the 3' half-site. Thus, DNA binding specificity is determined not only by the amino acid sequence of the protein but also by its protein-protein interactions and its position on the response element (5' vs. 3').

Inhibition of calcitriol receptor binding to vitamin D response elements by uremic toxins

The genomic action of calcitriol (1,25-dihydroxy-vitamin D3) is mediated through the interaction of the calcitriol receptor (VDR) with vitamin D response elements (VDREs). Although renal failure is associated with resistance to the action of calcitriol, the mechanism of this resistance is not well understood. Therefore, we used the electrophoretic mobility shift assay to compare the ability of VDRs from normal and renal failure rats to bind to the osteocalcin gene VDRE. The results indicate that VDRs from renal failure rats have only half the DNA binding capacity as VDRs from control rats, despite identical calcitriol binding. Furthermore, incubation of normal VDRs with a uremic plasma ultrafiltrate resulted in a loss of > 50% of the binding sites for the osteocalcin VDRE. When VDRs bound to DNA as heterodimers with retinoid X receptors, the inhibitory effect of the uremic ultrafiltrate was due to a specific interaction with the VDR, not retinoid X receptors. In addition, uremic ultrafiltrate blocked calcitriol-induced reporter gene activity in transfected JEG-3 cells. Taken together, the results indicate that an inhibitory effect of a uremic toxin(s) on VDR-VDRE binding could underlie the calcitriol resistance of renal failure.

Comparison of the DNA binding specificity and function of v-ErbA and thyroid hormone receptor alpha 1

The oncoprotein v-ErbA is a mutated version of thyroid hormone receptor alpha 1. Although the basis for the oncogenic action of v-ErbA is unknown, expression of this protein is known to inhibit thyroid hormone and retinoic acid induction of target genes. The DNA binding domain of v-ErbA differs from that of thyroid hormone receptor alpha 1 in two amino acids felt to be crucial for determining the specificity of DNA binding. However, the DNA binding properties of v-ErbA have not been examined independent of a comparison of binding to already known thyroid hormone response elements. In the current studies a non-biased strategy was used to select from a pool of random DNA those sequences that bind v-ErbA with high affinity. The highest affinity binding sequence was identified as the decamer 5'-T(A/G)AGGTCACG, which is closely related to the optimal thyroid hormone receptor alpha 1 binding sequence, TAAG-GTCA. Transfection studies demonstrate that among equal thyroid hormone responsive elements, those that contain the optimal v-ErbA consensus will be repressed by v-ErbA in preference to those that do not. These studies indicate that v-ErbA and thyroid hormone receptor alpha 1 regulate overlapping sets of response elements, and that all sequences that are highly responsive to thyroid hormone are not necessarily responsive to v-ErbA.

Pyroglutamyl peptidase-II ("thyroliberinase") activity in human serum: influence of weight and thyroid status

The tripeptide hormone, TRH, is metabolized by three enzymes, the most specific of which is pyroglutamyl peptide hydrolase-II (also termed thyroliberinase), a metalloenzyme present in serum and brain. Because pyroglutamyl peptidase-II activity in rat serum is regulated by thyroid hormone levels, we tested the hypothesis that this activity is similarly altered in humans. We studied serum pyroglutamyl peptidase-II activity in 6 patients with hyperthyroidism, 18 patients with hypothyroidism, and 31 euthyroid, normal weight volunteers. Because TRH [or its metabolite cyclo(His-Pro)] is believed to be an important hormone regulating appetite and metabolism, we also evaluated pyroglutamyl peptidase-II activity in 27 euthyroid patients with obesity. Serum pyroglutamyl peptidase-II activity was elevated in patients with hypothyroidism (mean +/- SEM, 33.9 +/- 3.7 nmol/mL.h) compared to that in euthyroid, normal weight volunteers (24.5 +/- 2.8 nmol/mL.h; P < 0.05), but not that in patients with hyperthyroidism (28.3 +/- 4.1 nmol/mL.h; P = NS). Euthyroid obese patients had the highest pyroglutamyl peptidase-II activity (43.6 +/- 2.8 nmol/mL.h; P < 0.0001 vs. normal weight volunteers). Pyroglutamyl peptidase-II activity was positively correlated with body mass index (r2 = 0.30; P < 0.0001). After correction for body mass index, there were no difference in pyroglutamyl peptidase-II activity in hypothyroid, hyperthyroid, and euthyroid individuals. We conclude that serum pyroglutamyl peptidase-II activity is regulated by, or regulates, body weight.

The interplay of half-site sequence and spacing on the activity of direct repeat thyroid hormone response elements

Direct repeats of the hexamer AGGTCA can serve as response elements for vitamin D, thyroid hormone, or retinoic acid. The specificity of the response appears to reside in the spacing between the hexamers, with response elements for vitamin D restricted to direct repeats separated by a 3-base pair (bp) spacer, thyroid hormone a 4-bp spacer, and retinoic acid a 5-bp spacer (3-4-5 rule). Recently we have shown that the optimum thyroid hormone receptor binding site consists of an 8-bp sequence (TAAGGTCA), not a hexamer. Therefore we tested whether the 3-4-5 rule is valid for octamer sequence direct repeats. In transfection experiments octamer direct repeats with 3-, 4-, or 5-bp spacers conferred equivalently strong thyroid hormone responses, although a repeat with a 9-bp spacer was substantially weaker. For the 4- and 5-bp spacer constructs, the 5' half-site octamer had as strong an influence on thyroid hormone induction as did the 3' half-site octamer, although for the 3-bp spacer construct the 5' octamer was marginally less potent than the 3' octamer. Transfection and gel shift experiments did not suggest a simple correlation between the binding of thyroid hormone receptor-retinoid X receptor heterodimers and thyroid hormone induction from these response elements. We conclude that half-site sequence can override the effect of spacing in determining the hormone responsiveness of a direct repeat response element. In addition, the thyroid hormone response may not be due simply to the binding of thyroid hormone receptor-retinoid X receptor heterodimers to the DNA.

DNA binding specificity and function of retinoid X receptor alpha

Retinoid X receptors are members of the erbA superfamily of ligand-inducible transcription factors. Similar to several other members of this gene family, retinoid X receptors are known to bind to the hexameric DNA sequence AGGTCA. After binding to a direct repeat of this hexamer with a one-base pair spacer, retinoid X receptor homodimers are able to activate transcription in the presence of the ligand 9-cis-retinoic acid. However, it is not known if AGGTCA represents the highest affinity binding site for retinoid X receptors. A combination of the electrophoretic mobility shift assay and polymerase chain reaction was used to isolate from a pool of random DNA those sequences that bind retinoid X receptors with highest affinity. This approach, combined with mutational analysis and DNA footprinting, led to the identification of the seven-base pair sequence GGGGTCA as the highest affinity retinoid X receptor binding site. A direct repeat of this sequence is substantially more active than a direct repeat of AGGTCA as a retinoid X response element.

Specificity and mechanism of thyroid hormone induction from an octamer response element

Thyroid hormone response elements are specific DNA sequences that allow thyroid hormone receptors to confer ligand-dependent regulation of gene expression. The response elements characterized to date have been composed of varying arrangements of multiple copies of a conserved hexameric sequence. The traditional consensus half-site of these response elements is the sequence 5'-AGGTCA, although we have demonstrated recently that the optimal thyroid hormone receptor monomer binding site is 2 base pairs larger, 5'-TAAGGTCA. Since other members of this family of nuclear receptors also have been shown to use varying arrangements of the traditional hexamer sequence as response elements, we examined whether the octamer sequence was specific as a thyroid hormone response element. The studies reported here demonstrate that only thyroid hormone receptors confer ligand responsiveness to a reporter gene containing a single copy of the octamer sequence as a response element and that qualitative and quantitative differences in the binding of related nuclear receptors to this sequence can account for this functional specificity. We also have shown that thyroid hormone induction from the octamer response element occurs independently of retinoid X receptors, in contrast to the induction from traditional complex thyroid hormone response elements.

Nucleotide substitutions differentially affect direct repeat and palindromic thyroid hormone response elements

Thyroid hormone receptors are transcription factors that bind to specific DNA sequences and regulate gene expression in a ligand-dependent manner. Thyroid hormone response elements can be composed of direct repeat or palindromic arrangements of half-sites. The traditional half-site has been considered to be the sequence 5'-AGGTCA, although we recently demonstrated that the optimal monomer-binding site is 2 base pairs larger, 5'-TAAGGTCA. It has generally been presumed that alterations in half-site sequence have commensurate effects on gene expression from direct repeat and palindromic response elements. However, in the present studies we demonstrate that palindromic elements are highly dependent on the presence of the two 5'-nucleotides (TA) for function, whereas this portion of the response element has minimal influence on hormone induction from direct repeat elements. Hormone induction correlates poorly with binding affinity for thyroid hormone receptor monomers, homodimers, or heterodimers with retinoid X receptors, either in the absence or presence of ligands. We speculate that the magnitude of hormone induction may reflect protein conformation differences induced by a combination of the presence of the appropriate 5'-nucleotides plus half-site orientation.

Differential autoantibody responses to thyroid peroxidase in patients with Graves' disease and Hashimoto's thyroiditis

In order to examine the specificity of the autoantibody response to thyroid peroxidase (TPO, previously identified as thyroid microsomal antigen) in autoimmune thyroid disease, we examined reactivity of sera from 45 Hashimoto's and 48 Graves' patients to native thyroid microsomes, denatured and reduced human TPO and several recombinant fragments of human TPO corresponding to amino acids 457-933 of the native protein. Both Graves' and Hashimoto's sera bound native, denatured and reduced TPO at significantly greater rate than normal controls, and no differences were noted between the two disorders in binding to these forms of the autoantigen. Binding was also noted to two recombinant fragments of TPO, corresponding to amino acids 513-633 and 633-933 in TPO. The frequency of autoantibodies to the TPO AA(633-933) region was not significantly different in Hashimoto's vs. Graves' disease patients (58% vs. 65% respectively), and appeared to relate to evidence of glandular inflammation in the Graves' patients (presence of anti-thyroglobulin antibodies and elevated anti-microsomal antibody levels). In contrast, antibodies to the TPO AA(513-633) fragment were significantly more common and of higher titer in Hashimoto's vs. Graves' disease patients, and did not correlate with any measure of glandular inflammation. These results identify two specific regions of TPO autoantibody binding and indicate that there are differences in the autoantibody response to TPO in Hashimoto's and Graves' diseases.

Nonbiased identification of DNA sequences that bind thyroid hormone receptor alpha 1 with high affinity

Thyroid hormone receptors are transcription factors that bind to specific DNA sequences and regulate gene expression in a ligand-dependent manner. Although thyroid hormone receptors are known to bind to the hexamer 5'-AGGTCA, it is not known if this represents the optimal binding site. Therefore, a nonbiased strategy was used to identify DNA sequences which bind thyroid hormone receptor alpha 1 with high affinity. Such DNA sequences were isolated from a pool of random sequences using a strategy combining an electrophoretic mobility shift assay with the polymerase chain reaction. It was found that thyroid hormone receptor alpha 1 binds with highest affinity to the octamer 5'-TAAGGTCA. Mutation of the two 5'-nucleotides decreased the affinity of thyroid hormone receptor alpha 1 for this DNA sequence approximately 5-fold, and the importance of those nucleotides in receptor binding was confirmed by DNA footprinting. A single copy of the octamer sequence (but not the hexamer AGGTCA) could impart T3 responsiveness to a heterologous promoter in a transient transfection assay. The results indicate that the optimal binding site for thyroid hormone receptor alpha 1 is 2 base pairs larger than previously thought, and that a single binding site can function as a response element. In addition, we speculate that the optimal binding sites for thyroid hormone, vitamin D, and retinoic acid receptors may not be identical, as had previously been thought.

Tissue specificity and serologic reactivity of an autoantigen associated with autoimmune thyroid disease

A recent report has identified a new autoantigen called D1 that appears to be associated with Graves' ophthalmopathy and is expressed in the thyroid and eye muscle. To better characterize the tissue specificity and disease relevance of this antigen, we evaluated the expression of D1 RNA in various human tissues using a reverse transcriptase polymerase chain reaction assay. These studies indicate a wide tissue distribution of the messenger RNA for this antigen, including the thyroid, eye muscle, parathyroid, spleen, skeletal muscle, and uterus. There were variations in the relative amounts of specific message for D1 in the different tissues, with the uterus, thyroid, and eye muscle having the greatest amount of product per microgram of total RNA. A maltose binding protein-D1 fusion protein was expressed in Escherichia coli, purified, and used to assess serologic reactivity to D1 by Western blot. Autoantibodies to this antigen were noted in 19 of 24 (78%) of Hashimoto's disease patients, 26 of 41 (63%) of Graves' disease patients, and in 9 of 17 (53%) of normal controls. Sixty percent of Graves' disease patients with clinical ophthalmopathy had antibodies to D1, as did 63% of Graves' patients without signs or symptoms of clinical ophthalmopathy. There was no correlation between reactivity to D1 and either clinical measures of hyperthyroidism or antibody titers to thyroid peroxidase or thyroglobulin. The presence of autoantibodies to this antigen in patients with Hashimoto's disease, in Graves' disease patients without ophthalmopathy and in normal controls indicate that serologic recognition of this antigen is not restricted to patients with ophthalmopathy. In addition, the expression of messenger RNA for this antigen in multiple types of cells questions the tissue specificity of this autoantigen.

Dimerization interfaces of thyroid hormone, retinoic acid, vitamin D, and retinoid X receptors

A subclass of erbA-related nuclear receptors has been shown to require interaction with an auxiliary protein(s) from nuclear extract in order to achieve high affinity DNA binding in vitro. The retinoid X receptor recently has been demonstrated to be such an auxiliary protein as it enhances specific DNA binding by thyroid hormone receptors, retinoic acid receptors, and the vitamin D receptor. Mutation of a highly conserved 20-amino acid region within the ligand-binding domain of thyroid hormone receptor beta disrupts its physical association with auxiliary protein from JEG-3 cells as well as with recombinant retinoid X receptor beta. The homologous 20-amino acid regions from retinoic acid receptor alpha and the vitamin D receptor also are critical determinants of the heterodimeric interaction between these receptors and JEG-3 cell auxiliary protein as well as retinoid X receptor beta. However, the same region of retinoid X receptor beta appears to play a minor, if any, role in heterodimerization. In addition, transfection studies indicate that disruption of heterodimerization impairs the ability of these receptors to function as ligand-dependent transcriptional activators.

Ligand-dependent synergy of thyroid hormone and retinoid X receptors

The binding of thyroid hormone receptors to DNA is enhanced by heterodimerization with nuclear proteins. One such heterodimerization partner has recently been characterized as the retinoid X receptor. 9-cis-Retinoic acid has been identified as a natural ligand for retinoid X receptors, suggesting a potential receptor-mediated interaction between thyroid hormone and 9-cis-retinoic acid in the regulation of thyroid hormone-responsive genes. A transient cotransfection assay was used to test for such an interaction. When a complex thyroid hormone response element composed of both direct and inverted repeat hexamers was tested, these two ligands activated gene expression synergistically. In contrast, when the response element consisted only of directly repeated hexamers, unliganded retinoid X receptors enhanced thyroid hormone responsiveness, but 9-cis-retinoic acid induced no additional activation. The results suggest a unique mechanism to achieve differential suggest a unique mechanism to achieve differential thyroid hormone sensitivity of thyroid hormone-responsive genes within a cell. Genes with appropriate response elements will show amplification of the thyroid hormone response by 9-cis-retinoic acid in the presence of retinoid X receptors; other thyroid hormone-responsive genes will be influenced by retinoid X receptors, but not 9-cis-retinoic acid.

Identification of localized autoantibody epitopes in thyroid peroxidase

Recent reports have disagreed on the nature of the autoantibody epitopes in thyroid peroxidase (TPO). We used immunoprecipitation of recombinant human TPO constructs to determine if localized autoantibody binding sites exist in this autoantigen. In vitro transcription and translation of TPO cDNA fragments yielded 35S-labeled products consisting of either full-length protein (933 amino acids) or N-terminal peptides of 631, 455, and 120 amino acids. Immunoprecipitates analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography revealed that the Hashimoto's sera consistently precipitated the full-length and the 631 amino acid products, but not the shorter N-terminal peptides. An additional construct resulting in a full-length TPO peptide with an internal deletion of amino acids 4-455 was also made, and this product was also precipitated by the Hashimoto's sera. A fusion protein consisting of maltose binding protein followed by amino acids 456-933 of human TPO was produced in Escherichia coli and subjected to Western blot analysis using the Hashimoto's sera. The Hashimoto's sera reacted with the MalTose binding protein TPO (MBP/TPO) fusion protein, but not a control fusion protein (MBP/LacZ alpha). Together, these results indicate the presence of localized autoantibody epitopes in the portion of the human TPO molecule from amino acids 456 to 933, with at least one binding site located between amino acids 456 and 631.

Immunoreactivity to Yersinia enterocolitica antigens in patients with autoimmune thyroid disease

Recent reports have suggested that Yersinia enterocolitica proteins encoded by a 72-kilobase virulence plasmid (known as release proteins and now identified as YOP2-5) are antigens recognized specifically by patients with Graves' disease and of potential etiological importance in this disorder. To examine this hypothesis, we evaluated immune responses to YOP in patients with autoimmune thyroid disease and in normal controls. Humoral responses to Yersinia were assessed using Western blots of crude Y. enterocolitica membrane proteins, Yersinia release proteins (YOP2-5), and human thyrocyte membranes. Twenty-four of 25 Graves' and 10 of 18 Hashimoto's patients showed reactivity with the release proteins, primarily the 67-, 46-, 36-, and 25-kilodalton bands. However, 17 of 24 normal subjects also demonstrated serological reactivity to the release proteins, and the pattern of reactivity of these sera was similar to that in the thyroid patients. No correlation was noted between serological reactivity to the release proteins and thyroid hormone levels. Patients and controls with serological reactivity to YOP also showed reactivity with Yersinia membranes. In addition to the serological studies, cellular immune responses were determined by peripheral blood mononuclear cell proliferation assays. Cellular reactivity to the release proteins was present in four of five Graves' and both Hashimoto's patients tested, but also in two of six nonthyroid illness patients with serological immunity to the release proteins. Intrathyroidal lymphocytes obtained from two Graves' patients demonstrated marked proliferation in response to the release proteins. These results indicate that there is no unique pattern of serological reactivity against Yersinia membranes or the release proteins in patients with autoimmune thyroid diseases and suggest that any causal relationship between Yersinia infection and Graves' disease may be related to T-cell immunity.

Expression of thyroid hormone receptor beta 2 in rat hypothalamus

A polymerase chain reaction based assay was used to evaluate expression of thyroid hormone receptor beta 2 mRNA in rat hypothalamus. Expression was detected in the arcuate, ventromedial and paraventricular nuclei, as well as the median eminence. Trace expression was found in the dorsomedial nucleus, but no expression of thyroid hormone receptor beta 2 was detected in the lateral hypothalamus or the preoptic region. The results indicate that, contrary to previous belief, expression of thyroid hormone receptor beta 2 is not confined to the anterior pituitary.

Thyroid hormone receptor mutations that interfere with transcriptional activation also interfere with receptor interaction with a nuclear protein

A 20 amino acid segment within the ligand-binding domain of the rat beta 1-thyroid hormone receptor (T3R; amino acids 286-305) is conserved in all members of the erbA superfamily. Mutations in this region impair T3 induction of reporter gene expression in a transfection system without impairing T3 binding or nuclear localization of the receptors. We postulate that a nuclear protein may need to interact with this domain for full transcriptional activity and provide evidence for the existence of this putative protein. Specifically, a JEG-3 cell protein (denoted TRAP, T3R auxiliary protein) interacts with wild-type T3R-beta 1 to increase binding of the receptor to T3 response elements in vitro. Mutations within amino acids 286-305 impair the ability of TRAP to enhance T3R binding to hormone response elements. Cross-linking studies indicate that TRAP has an apparent mol wt of 63 kDa. Surprisingly, TRAP does not enhance binding of erbA-alpha 2 and v-erbA to DNA, even though these proteins contain highly conserved versions of the 20-amino acid beta 1 T3R sequence under study. TRAP or a similar protein, however, does enhance binding of retinoic acid receptor-beta to a hormone response element. We conclude that the inability of T3Rs with mutations in this domain to fully activate transcription may be due to an inability of these proteins to fully interact with TRAP. We postulate that TRAP or related proteins may be important regulators of ligand-dependent transcriptional activation for other members of the erbA family in addition to the T3R.

Mutational analysis identifies a new functional domain of the thyroid hormone receptor

We studied the functional significance of a region of the thyroid hormone receptor ligand binding domain which is conserved in all members of the erbA superfamily. The homologous region of the glucocorticoid receptor has been implicated in the binding of heat shock protein 90, and deletions of the glucocorticoid receptor that include this region result in constitutive activity. Both deletion and point mutations were made in this area of the rat beta 1 thyroid hormone receptor (amino acids 286-305), and the functional consequences of these mutations were analyzed in JEG cells by transient cotransfection along with a T3 responsive reporter gene. All mutations studied resulted in significant inhibition of ligand-dependent transcriptional regulation without inducing significant constitutive activity. For some mutations, the lack of transcriptional response correlated with a diminished ability to bind ligand. However, point mutations of amino acids 288, 290, and 300 resulted in impaired transcriptional regulation despite wild type T3 binding affinity. In addition, mutations of these three amino acids failed to impair localization of the receptor to the nucleus or binding to DNA in vitro. Cotransfection of plasmids expressing the wild type and mutant T3 receptor proteins resulted in inhibition of the wild type T3 receptor function. We conclude that this region of the rat beta 1 T3 receptor is essential for full transcriptional activity, but this is not due to a role in T3 binding, DNA binding, or nuclear localization. We postulate that nuclear factors may need to bind to this region for full transcriptional activity.

Expression of erbA alpha and beta mRNAs in regions of adult rat brain

The proto-oncogenes erbA alpha and erbA beta together encode three functional thyroid hormone receptors (erbA alpha 1, beta 1, and beta 2), as well as two proteins (erbA alpha 2 and alpha 3) that do not bind T3. The erbA alpha 2 protein has been shown to inhibit the T3 inductive effects of functional receptors, and alpha 2 mRNA is expressed at high levels in adult rat brain. Thus, expression of erbA alpha 2 may explain the observation that adult rat brain is not a T3 responsive organ, despite the presence of T3 receptors. However, expression of the different erbA mRNAs has not been studied within distinct regions of rat brain. To gain further insight into the roles of these molecules, we have used polymerase chain reaction to investigate the expression of all five erbA mRNAs within discrete regions of adult rat brain. The results indicate that all three erbA alpha mRNAs are expressed in all regions studied (brainstem, cerebellum, cortex, hippocampus, pituitary, quadrigeminal plate, striatum, and thalamus). All regions contained less erbA alpha 3 RNA than either alpha 1 or alpha 2. Expression of alpha 2 exceeded that of alpha 1 in all regions except striatum. ErbA beta 1 was expressed in all brain regions, whereas erbA beta 2 was confined to the pituitary.

In contrast to the glucocorticoid receptor, the thyroid hormone receptor is translated in the DNA binding state and is not associated with hsp90

We have recently reported that the glucocorticoid receptor (GR) becomes bound to the 90-kDa heat shock protein (hsp90) at or near the end of receptor translation in vitro (Dalman, F. C., Bresnick, E. H., Patel, P. D., Perdew, G. H., Watson, S. J., Jr., and Pratt, W. B. (1989) J. Biol. Chem. 264, 19815-19821). In this paper we compare the hsp90 binding and DNA binding activities of the thyroid hormone receptor (TR) to those of the GR after cell-free translation of the two receptors in rabbit reticulocyte lysate. In contrast to the newly translated GR, which is bound to hsp90 and must be transformed to the DNA binding state, the TR is not bound to hsp90 and is translated in its DNA binding form without any requirement for transformation. When the GR is translated in wheat germ extract, which does not contain hsp90, it is translated in its DNA binding form in the same manner as the TR synthesized in reticulocyte lysate. These observations provide direct evidence that binding of GR to hsp90 is associated with repression of its DNA binding function. The fact that the TR does not bind to hsp90 and is translated in its DNA binding form is consistent with the different behavior of this receptor with respect to classic steroid receptors in the intact cell. We propose that binding to hsp90 may account for the fact that most of the steroid receptors are recovered in the cytosolic fraction after lysis of hormone-free cells in low salt buffer whereas the hormone-free TR is recovered in tight association with the nucleus.

Thyroid nodularity after childhood irradiation for lymphoid hyperplasia: a comparison of questionnaire and clinical findings

Ionizing radiation is a well-established cause of thyroid cancer and nodularity, however, important questions relating to the magnitude of the risk following low-dose medical exposures remain unresolved. To address these issues, we conducted a follow-up study of 1590 individuals treated between 1938 and 1969 with X-rays for childhood lymphoid hyperplasia (av. thyroid dose = 24 cGy) and 1499 individuals treated with surgery only. Thyroid nodularity was determined from self-administered questionnaires completed by 1195 irradiated and 1063 surgically-treated subjects and from clinical examinations of 602 irradiated and 457 non-irradiated subjects. A much higher relative risk (RR) for radiation-induced thyroid nodules was estimated from the questionnaire than from the clinical examination data, 15.8 and 2.7, respectively. (The corresponding estimates of excess RR per cGy were 64 and 7%). Analysis of the examination data revealed a strong dose-response relationship, similar excess RR/cGy for males and females, and an inverse relationship with age at exposure. Although the thyroid gland is one of the most sensitive organs to the neoplastic effects of radiation, the radiation-induced risk of thyroid nodularity reported from questionnaire studies may over-estimate the true risk.

Identification of a thyroid hormone receptor that is pituitary-specific

Three cellular homologs of the v-erbA oncogene were previously identified in the rat; two of them encode high affinity receptors for the thyroid hormone triiodothyronine (T3). A rat complementary DNA clone encoding a T3 receptor form of the ErbA protein, called r-ErbA beta-2, was isolated. The r-ErbA beta-2 protein differs at its amino terminus from the previously described rat protein encoded by c-erbA beta and referred to as r-ErbA beta-1. Unlike the other members of the c-erbA proto-oncogene family, which have a wide tissue distribution, r-erbA beta-2 appears to be expressed only in the anterior pituitary gland. In addition, thyroid hormone downregulates r-erbA beta-2 messenger RNA but not r-erbA beta-1 messenger RNA in a pituitary tumor-derived cell line. The presence of a pituitary-specific form of the thyroid hormone receptor that may be selectively regulated by thyroid hormone could be important for the differential regulation of gene expression by T3 in the pituitary gland.

Inhibition of thyroid hormone action by a non-hormone binding c-erbA protein generated by alternative mRNA splicing

Thyroid hormone (T3) binds to a nuclear receptor protein which regulates gene expression by binding to specific DNA sequences near hormone-responsive genes. Proteins encoded by two cellular proto-oncogenes, c-erbA alpha and beta, bind T3 and can act as functional T3 receptors. In rats, alternative splicing of the alpha-gene transcript generates at least two distinct protein products, termed r-erbA alpha 1 and r-erbA alpha 2. Although these proteins bind to the same DNA sequence, r-erbA alpha 2 does not bind T3. We show here that expression of r-erbA alpha 2 inhibits the T3-dependent inductive effect of either r-erbA beta or r-erbA alpha 1 on expression of a T3-responsive test gene. Alternative splicing of the erbA alpha transcript thus generates products with opposing biological activities, suggesting a novel mechanism for the modulation of hormonal responsiveness.

Functional characterization of the rat growth hormone promoter elements required for induction by thyroid hormone with and without a co-transfected beta type thyroid hormone receptor

We have extensively characterized the sequences of the rat growth hormone (rGH) promoter required for induction by T3 (thyroid hormone, 3,5,3'-L-triiodothyronine) in a transient transfection system. Oligonucleotides containing portions of the rGH promoter sequence with various deletions and point mutations were placed upstream of the first 137 base pairs of the rGH promoter or the heterologous herpes virus thymidine kinase promoter in chloramphenicol acetyltransferase expression vectors. The rGH137 and thymidine kinase promoters show no or minimal response to T3 in the basal state. The constructs were tested in GH4C1 rat pituitary cells and COS cells (functionally deficient in thyroid hormone receptor) with and without a co-transfected plasmid expressing a beta type c-erbA gene coding for a functional T3 receptor. Oligonucleotides containing the T3 receptor binding site confer hormone-dependent induction in a manner that is independent of either orientation or variation in position on the helix relative to the promoter. Point mutations in the sequence -189 to -173 result in loss of T3 induction, and bases between -173 and -167 were also required for a full T3 response. The minimal length to confer T3 induction to the rGH promoter was 23 base pairs (-190 to -167). Point mutations creating a perfect duplication of 7 base pairs within the receptor binding site conferred 12-fold T3 response to the rGH137 promoter, 3-fold greater than the wild type rGH237 construct. T3 inductibility was also transferred to the thymidine kinase promoter by an oligonucleotide containing the sequence -200 to -157, demonstrating that cell type specific elements located 3' to 157 of the rGH promoter are not required for thyroid hormone responsiveness.