Structure and Function of Thyroid Hormone Receptors
Thyroid hormone receptors (TRs) are members of the nuclear hormone receptor (NHR) superfamily of ligand-activated transcription factors. Members of the NHR family include: retinoic acid receptor (RAR), retinoic X receptor (RXR), vitamin D receptor (VDR), liver X receptor (LXR), and ecdysone receptor (EcR). TRs bind non-peptide lipophilic hormones (T3 and T4) and thus regulate the transcription of genes involved in several important physiological functions including differentiation, development and homeostasis. TRs are encoded by THRA (TRα) and THRB (TRβ) genes located on human chromosomes 17 and 3, respectively. As a result of alternative splicing of the primary TR transcripts and different promoter usage, several protein isoforms are generated, among which TRα1, TRβ1, TRβ2 and TRβ3 are the main hormone-binding isoforms.
All thyroid hormone receptors share a similar primary structural organization as that found in all nuclear hormone receptors: a non-conserved N-terminal domain (NTD, also called A/B domain), a highly conserved DNA binding domain (DBD, called C-domain), a hinge region (D domain), a carboxy-terminal ligand binding domain (LBD, E domain), and an F-domain whose function is not yet fully understood.
Thyroid hormone receptors share high sequence homology in the DBD and the LBD regions. As mentioned previously, there are four T3-binding TR isoforms (TRα1, TRβ1, TRβ2, and TRβ3) and two additional TRs (TRα2 and TRα3) which do not bind hormone, but still bind DNA. Although TRα2 and TRα3 do not function as regular TRs, they can compete with other TRs to bind DNA and repress transcription.
The N-terminal domain (A/B domain) is poorly conserved across the NHR superfamily. The N-terminal regions have variable lengths and divergent amino acid sequence among the thyroid hormone receptor isoforms. In the absence of a ligand binding region in the receptor structure, the NTD plays a significant role, as it participates in ligand-independent transcription regulation through a major transactivation domain known as “activation function 1” (AF1).
The DBD, also known as the C-domain, is the most conserved sequence in the NHR family. The DBD recognizes and binds to specific elements in the promoter regions of hormone responsive genes called, hormone response elements (HREs). Several crystal structures for various nuclear receptors have been solved showing their well-defined globular shape. It is important to mention that the C-domain exhibits eight conserved cysteine residues that form two zinc fingers that are involved in homo- and heterodimerization with retinoic X receptor (RXR). Deletion of the zinc fingers or substitution of the cysteine residues disrupts DNA binding and therefore prevents transcriptional activity.
Thyroid hormone receptor complexes (TR/RXR heterodimer) bind to promoter gene regions known as “thyroid response elements” (TREs) through their DBD, with one subunit binding the DNA specifically and the second one non-specifically. These TREs are composed of two sets of five to six nucleotides called “half-sites”. There are only two consensus half-sites: AGGTCA and AGAACA, which can be arranged as direct repeats spaced by four intervening nucleotides (DR4), inverted palindromes (IP6 or F2), or palindromes (PAL). The different orientations, as well as the number of nucleotides between these sites give the receptor its specificity. The ability to heterodimerize with RXR is very important for thyroid hormone receptor binding to the TREs, as dimerization contacts stabilize the DNA binding and determine spacing between half sites.
Nuclear Actions of Thyroid Hormone Receptors
A pivotal issue in understanding the molecular functions of thyroid hormone receptors is how they mediate the effects of the active hormone (T3) at the genomic level. thyroid hormone receptors modulate gene transcription in the absence and presence of hormone (T3), as well as agonists and antagonists. In the cell nucleus TRs bind to TREs preferentially as heterodimers with RXR (TR/RXR), since this provides the highest binding affinity and remains stable in the presence of T3. Nevertheless to a lesser extent TRs can bind to TREs as homodimers (TR/TR) but they associate weakly when ligand is bound.
In the absence of ligand DNA binding domain are associated to specific co-regulators known as co-repressors and mediate basal transcriptional repression in positively regulated target genes. In the presence of T3, co-repressor complexes are released from liganded thyroid hormone receptors that, in turn, associate with co-activator complexes to initiate transactivation of positively regulated target genes.
In contrast to positively regulated target genes, negatively regulated genes can be stimulated in the absence of T3 and repressed in its presence. One of the paradoxes of thyroid hormone receptor function is this ability to act as a T3-dependent inhibitor of negatively regulated target genes such as TSH and TRH49-51. From a physiological perspective, their negative regulation is essential for feedback control of the HPT axis. To date, the molecular mechanisms of negatively regulated transcription are not well understood; in the case of TSH and TRH genes, T3 mediates negative regulation through TRs by interacting with negative TREs on the promoter regions of these genes. Additionally, TRs may contribute to histone modifications on negative TREs that can lead to repression of the TSH genes.
It is important to note that the mechanism of thyroid hormone receptor action and steroid receptor actions (SR) differ. Members of the SR family include: estrogen receptor (ER), androgen receptor (AR), progesterone receptor (PR), mineralocorticoid receptor (MR) and glucocorticoid receptor (GR). SRs are mainly localized in the cytosol and are bound to heat shock proteins (HSP) in the absence of ligand. Once the ligand is bound, the steroid receptors dissociate from HSP and translocate into the nucleus where they bind to their corresponding hormone response elements. Once bound to DNA the receptor recruits co-activators to initiate gene transcription. By contrast TRs are bound to DNA in the nucleus in the absence of ligand and can actively repress transcription.