The thyroid hormone receptors, TRα and TRβ, are ligand-dependent transcription factors that regulate gene expression by recruitment of coactivators and corepressors. These receptors play an important role in normal and malignant cell proliferation. Particularly, we have shown that TRs antagonize ras-dependent proliferation, transformation and tumorigenesis in fibroblasts. Furthermore, expression of TRβ in human cancer cells retards tumor growth and inhibits invasiveness, extravasation and metastasis formation in euthyroid nude mice. When cells are inoculated into hypothyroid hosts, tumor growth is retarded, but tumors that do grow are more invasive and metastatic growth is enhanced. Increased malignancy of skin tumors is found in mice lacking TRs, further demonstrating the role of these receptors as inhibitors of tumor progression and suggesting that they represent a potential therapeutic target in cancer. TRs have a dual effect on proliferation, because they are required for proliferation of normal hepatocytes or keratinocytes, while acting as inhibitors of tumor progression.

Keywords

Coactivators; Corepressors; Metastasis; Proliferation; Thyroid hormone receptors; Tumor progression

Thyroid Hormone Action

The important physiological actions of the thyroid hormones (THs) are mediated by binding to the nuclear thyroid hormone receptors (TRs). The thyroid gland produces predominantly thyroxine (T4), but triiodothyronine (T3) is the most active TH, since it has a higher affinity by the receptors.1 THs are released by the thyroid gland to the bloodstream and they enter the cells through the adenosine triphosphate (ATP)-dependent monocarboxylate transporters MCT8 and MCT10 and the organic anion transporter proteins (OATPs).2 The amount of T3 available for binding to the nuclear receptors is regulated by cell-specific expression of selenoenzymes deiodinases (DIOs). DIO1 and DIO2 catalyze the conversion of T4 to T3 in target tissues, increasing intracellular levels of the active hormone, while DIO3 causes hormone inactivation since it converts T4 and T3 by inner ring deiodination to the inactive metabolites reverse T3 (rT3) and T2, respectively.

TRs belong to the superfamily of nuclear receptors and act as ligand-dependent transcription factors.3 Several TR protein isoforms are generated by promoter use or alternative splicing of the primary transcripts of the TRα and TRβ genes. The TRα1, TRβ1 and TRβ2 are the main hormone-binding isoforms and their relative levels of expression vary among cell types and at different developmental stages, suggesting that they could have organ-specific functions. In the case of TRβ, TRβ1 is more widely expressed, while the expression of TRβ2 is restricted to the anterior pituitary, and some neural cells.4,5 Studies with genetically modified mice have shown that TRα and TRβ can substitute for each other to mediate some actions of the thyroid hormones but they can also mediate isoform-specific functions.6

As shown in Figure 1.1, TRs are composed of several functional domains. The N-terminal region (A/B) contains a constitutive ligand-independent transcriptional activation domain, the autonomous activation function 1 (AF-1). This region is followed by the DNA-binding domain (DBD), or region C. The DBD is the most conserved region among the nuclear receptors and is composed of two zinc fingers. In each zinc finger, four invariable cysteines coordinate tetrahedrically with one zinc ion. Amino acids required for discrimination of the thyroid hormone response element (TRE) are present at the base of the first finger in a region termed the “P box,” and other residues of the second zinc finger that form the so called “D box” are involved in dimerization. Through the DBD the receptors interact with the major groove of DNA. A hinge domain, or D region, connects the DBD with the E region or ligand-binding domain (LBD), also responsible for dimerization. This hinge domain contains residues essential for interaction with corepressors. Crystallographic analysis has shown that the LBDs are formed by 12 α-helices, and the C-terminal helix (H12) encompasses the ligand-dependent transcriptional activation function, or AF-2.

TRs regulate gene transcription by binding, preferentially as heterodimers with retinoid X receptors (RXRs), to short DNA binding motifs, called thyroid hormone response elements or TREs, which are located in regulatory regions of target genes.7 TREs are composed of two copies of the AGG/TTCA motif. They can be configured as palindromes (Pal), inverted palindromes (IPs), or direct repeats spaced preferably by four non-conserved nucleotides (DR4). Although TRs can bind to their response elements as monomers or homodimers, heterodimerization with RXR strongly increases the affinity for DNA and transcriptional activity.

Transcriptional regulation by these receptors is mediated by the recruitment of coactivators and corepressors.3,8,9 In the absence of ligand, TRs can act as constitutive repressors when bound to TREs, due to their association with corepressors such as NCoR (nuclear receptor corepressor) or SMRT (silencing mediator of retinoic and thyroid receptor). NCoR and SMRT belong to multicomponent repressor complexes that contain histone deacetylases (HDACs) and cause chromatin compaction and consequently transcriptional inhibition.10 NCoR and SMRT are related both structurally and functionally. They contain three autonomous repressor domains (RD) and a receptor interacting domain (the CoRNR motif) located toward the carboxyl terminus. Transcriptional repression by the corepressor-bound receptors appears to be mediated by the recruitment of HDACs to the target gene. HDAC1 or 2 (class I deacetylases) are recruited to the first RD of the corepressors via the adaptor mSin3 protein, and the RD3 has been demonstrated to repress transcription by directly interacting with class II deacetylases (HDACs 4, 5 and 7). In addition, a repressor complex containing the corepressors, HDAC3 and transducin beta-like proteins (TBL1 or TBL1R) appears to be required for repression by TR. Although a receptor CoR box, located within the hinge region, is essential for interaction of receptors with the corepressors, the CoRNR motif does not interact directly with residues in this region, but docks to a hydrophobic groove in the surface of the LBD at H3 and 4.

Hormone binding induces a conformational change in the receptor that allows the release of corepressors and allows the recruitment in a sequential manner of coactivator complexes. The stronger change observed in the receptors upon ligand binding is the position of H12.11 This helix projects away from the body of the LBD in the absence of ligand. However, upon hormone binding H12 moves in a “mouse-trap” model being tightly packed against H3 or 4 and making direct contacts with the ligand. This change generates a hydrophobic cleft responsible for interaction with coactivators.12 A glutamic acid residue in H12 and a lysine residue in H3, which are conserved throughout the superfamily of nuclear receptors, interact directly with the coactivator and form a charge clamp that stabilizes binding. Consequently, mutation of these residues abolishes coactivator binding and causes the loss of thyroid hormone-dependent transcriptional activation.13 Since the coactivator binding surface overlaps with that involved in corepressors interaction, coactivator and corepressor binding is mutually exclusive. Some coactivators belong to ATP-dependent chromatin-remodeling complexes, others are part of complexes that induce post-translational modifications of histones, such as acetylation or arginine methylation, and others interact with the basic transcriptional machinery causing the recruitment of RNA polymerase II...