Second zinc finger mutants of thyroid hormone receptor selectively preserve DNA binding and heterodimerization but eliminate transcriptional activation

The Role of Zinc in Thyroid Hormones Metabolism

Abstract. Thyroid hormones play an important role in body homeostasis by facilitating metabolism of lipids and glucose, regulating metabolic adaptations, responding to changes in energy intake, and controlling thermogenesis. Proper metabolism and action of these hormones requires the participation of various nutrients. Among them is zinc, whose interaction with thyroid hormones is complex. It is known to regulate both the synthesis and mechanism of action of these hormones. In the present review, we aim to shed light on the regulatory effects of zinc on thyroid hormones. Scientific evidence shows that zinc plays a key role in the metabolism of thyroid hormones, specifically by regulating deiodinases enzymes activity, thyrotropin releasing hormone (TRH) and thyroid stimulating hormone (TSH) synthesis, as well as by modulating the structures of essential transcription factors involved in the synthesis of thyroid hormones. Serum concentrations of zinc also appear to influence the levels of serum T3, T4 and TSH. In addition, studies have shown that Zinc transporters (ZnTs) are present in the hypothalamus, pituitary and thyroid, but their functions remain unknown. Therefore, it is important to further investigate the roles of zinc in regulation of thyroid hormones metabolism, and their importance in the treatment of several diseases associated with thyroid gland dysfunction.

Unliganded thyroid hormone receptor α regulates developmental timing via gene repression in Xenopus tropicalis

Thyroid hormone (TH) receptor (TR) expression begins early in development in all vertebrates when circulating TH levels are absent or minimal, yet few developmental roles for unliganded TRs have been established. Unliganded TRs are expected to repress TH-response genes, increase tissue responsivity to TH, and regulate the timing of developmental events. Here we examined the role of unliganded TRα in gene repression and development in Xenopus tropicalis. We used transcription activator-like effector nuclease gene disruption technology to generate founder animals with mutations in the TRα gene and bred them to produce F1 offspring with a normal phenotype and a mutant phenotype, characterized by precocious hind limb development. Offspring with a normal phenotype had zero or one disrupted TRα alleles, and tadpoles with the mutant hind limb phenotype had two truncated TRα alleles with frame shift mutations between the two zinc fingers followed by 40-50 mutant amino acids and then an out-of-frame stop codon. We examined TH-response gene expression and early larval development with and without exogenous TH in F1 offspring. As hypothesized, mutant phenotype tadpoles had increased expression of TH-response genes in the absence of TH and impaired induction of these same genes after exogenous TH treatment, compared with normal phenotype animals. Also, mutant hind limb phenotype animals had reduced hind limb and gill responsivity to exogenous TH. Similar results in methimazole-treated tadpoles showed that increased TH-response gene expression and precocious development were not due to early production of TH. These results indicate that unliganded TRα delays developmental progression by repressing TH-response genes.

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

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

Thyroxine promotes association of mitogen-activated protein kinase and nuclear thyroid hormone receptor (TR) and causes serine phosphorylation of TR

Activated nongenomically by l-thyroxine (T(4)), mitogen-activated protein kinase (MAPK) complexed in 10-20 min with endogenous nuclear thyroid hormone receptor (TRbeta1 or TR) in nuclear fractions of 293T cells, resulting in serine phosphorylation of TR. Treatment of cells with the MAPK kinase inhibitor, PD 98059, prevented both T(4)-induced nuclear MAPK-TR co-immunoprecipitation and serine phosphorylation of TR. T(4) treatment caused dissociation of TR and SMRT (silencing mediator of retinoid and thyroid hormone receptor), an effect also inhibited by PD 98059 and presumptively a result of association of nuclear MAPK with TR. Transfection into CV-1 cells of TR gene constructs in which one or both zinc fingers in the TR DNA-binding domain were replaced with those from the glucocorticoid receptor localized the site of TR phosphorylation by T(4)-activated MAPK to a serine in the second zinc finger of the TR DNA-binding domain. In an in vitro cell- and hormone-free system, purified activated MAPK phosphorylated recombinant human TRbeta1 (). Thus, T(4) activates MAPK and causes MAPK-mediated serine phosphorylation of TRbeta1 and dissociation of TR and the co-repressor SMRT.

A fusion protein of the estrogen receptor (ER) and nuclear receptor corepressor (NCoR) strongly inhibits estrogen-dependent responses in breast cancer cells

Nuclear receptor corepressor (NCoR) mediates repression (silencing) of basal gene transcription by nuclear receptors for thyroid hormone and retinoic acid. The goal of this study was to create novel estrogen receptor (ER) mutants by fusing transferable repressor domains from the N-terminal region of NCoR to a functional ER fragment. Three chimeric NCoR-ER proteins were created and shown to lack transcriptional activity. These fusion proteins silenced basal transcription of the ERE2-tk-Luc reporter gene and inhibited the activity of co-transfected wild-type ER (wtER), indicating that they possess dominant negative activity. One of the fusion proteins (CDE-RD1), containing the ER DNA-binding and ligand-binding domains linked to the NCoR repressor domain (RD1), was selected for detailed examination. Its hormone affinity, intracellular localization, and level of expression in transfected cells were similar to wtER, and it bound to the estrogen response element (ERE) DNA in gel shift assays. Glutathione-S-transferase pull-down assays showed that CDE-RD1 retains the ability to bind to steroid receptor coactivator-1. Introduction of a DNA-binding domain mutation into the CDE-RD1 fusion protein eliminated silencing and dominant negative activity. Thus, the RD1 repressor domain prevents transcriptional activation despite the apparent ability of CDE-RD1 to bind DNA, ligand, and coactivators. Transcriptional silencing was incompletely reversed by trichostatin A, suggesting a histone deacetylase-independent mechanism for repression. CDE-RD1 inhibited ER-mediated transcription in T47D and MDA-MB-231 breast cancer cells and repressed the growth of T47D cells when delivered to the cells by a retroviral vector. These ER-NCoR fusion proteins provide a novel means for inhibiting ER-mediated cellular responses, and analogous strategies could be used to create dominant negative mutants of other transcription factors.

Involvement of nuclear steroid/thyroid/retinoid receptors and of protein kinases in the regulation of growth and of c-erbB and retinoic acid receptor expression in MCF-7 breast cancer cells

Nuclear steroid/thyroid/retinoid receptors and c-erbB membrane receptor tyrosine kinases control epithelial growth and differentiation. Retinoid receptors can dimerize with the vitamin D receptor, the glucocorticoid receptor or the thyroid receptor. Furthermore, multiple c-erbB receptor dimers have been identified. It has been shown that some of these receptor pathways communicate with each other via cross-connected regulatory networks. Molecular interactions between retinoid receptors or estrogen receptors (ER) and c-erbB-2, and between ER and retinoic acid receptor(RAR)-alpha have been reported. Here, we demonstrate the effects of steroids/thyroids/retinoids and of activators of protein kinase A (forskolin, Forsk) and C (12-O-tetradecanoylphorbol-13-acetate, TPA), on growth and expression of c-erbB and RARs in MCF-7 breast cancer cells, which contain high levels of RAR-alpha and -gamma, and which express significant amounts of c-erbB-2 and -3. All trans-retinoic acid (tRA), the anti-estrogen ICI 182 780 (ICI), Forsk and TPA reduced, whereas triiodothyronine and 17beta-estradiol (E2) stimulated cell growth. Flow cytometry revealed that tRA and E2 reduced c-erbB-2 and -3, whereas tamoxifen, Forsk and TPA up-regulated c-erbB-2. c-erbB-3 was co-regulated with c-erbB-2. Northern analysis demonstrated that RAR-alpha was down-regulated by dexamethasone, ICI, and TPA, whereas vitamin D3 and E2 up-regulated RAR-alpha. RAR-gamma expression was less responsive to such treatment, being reduced only by ICI and Forsk. These data indicate that nuclear receptor and protein kinase signaling communicate with each other and control the expression of RARs and c-erbB receptors. Efficient growth control requires the coordinated interplay of both receptor systems.

The thyroid hormone receptor variant alpha2 is a weak antagonist because it is deficient in interactions with nuclear receptor corepressors

The thyroid hormone receptor splice variant, alpha2, is unable to bind thyroid hormone (T3) and has been proposed to function as an endogenous inhibitor of T3 action. In this report, we examined further the DNA sequence requirements for alpha2 binding to thyroid hormone response elements (TREs) in an attempt to identify response elements that mediate potent inhibition by alpha2. Heterodimers of alpha2 and retinoid X receptor were found to bind to a subset of TREs (DR4, direct repeats spaced by 4 bp) in which selected flanking and spacer sequences enhanced interactions with the AGGTCA core binding sequence. Despite the optimization of the TRE-binding sites, alpha2 remained a weak dominant negative inhibitor of TRE-driven transcription. A promoter interference assay was also developed for testing inhibition by alpha2. In these studies, alpha2 blocked gene transcription, but it required cotransfected retinoid X receptor, and it was not as potent as unliganded thyroid hormone receptors. These results led to the hypothesis that alpha2 might be deficient in interactions with nuclear receptor corepressors. Consistent with this view, alpha2 did not silence basal transcription in its native form or when linked to Gal4. Alpha2 also failed to interact with corepressors (NCoR and SMRT) in both gel shift assays and mammalian two-hybrid assays. We conclude that alpha2 is a weak antagonist of thyroid hormone action because it binds weakly to a limited repertoire of response elements, and it does not interact with corepressors. Thus, alpha2 may be able to compete with thyroid hormone receptors for binding to a limited group of target sites, but it is not able to actively inhibit transcription.

Amino acid residues in both the DNA-binding and ligand-binding domains influence transcriptional activity of the human peroxisome proliferator-activated receptor alpha

We have investigated the basis of the lack of activity of a natural variant human peroxisome proliferator-activated receptor alpha, hPPARalpha6/29. A subcloning approach was used to change the four variant amino acids in the hPPARalpha6/29 sequence, individually and in combination, to those found in an active human PPARalpha. Individual amino acid "back mutations" were unable to confer on hPPARalpha6/29 the ability to be activated by peroxisome proliferators in a transient transfection assay. Although hPPARalpha6/29 was able to bind specifically to DNA in the presence of the retinoid X receptor alpha (RXRalpha), the complete restoration of receptor transcriptional activity required two separate back mutations of the hPPARalpha6/29 sequence, namely amino acid 123 in the DNA binding domain, and amino acid 444 close to the C-terminus. This suggests that sequences in the PPARalpha DNA binding domain influence other receptor functions besides DNA binding.