Differential DNA binding by monomeric, homodimeric, and potentially heteromeric forms of the thyroid hormone receptor.

Abstract

Binding of the thyroid hormone receptor (TR) to thyroid hormone-responsive elements (TREs) is crucial for regulation of gene expression by thyroid hormone. The TR binds to each half-site of a palindromic TRE separately, as a monomer, or simultaneously, as a homodimer. In addition, the TR monomer interacts with a 42-kDa protein that may be responsible for an increase in the apparent size and stability of the TR-TRE complex after incubation with liver nuclear extract. The multiple DNA-binding forms of the TR contact the TRE differently but compete for binding in a dynamic equilibrium which is highly dependent on the relative concentrations of TR and nuclear protein. Thus, protein-protein interactions are likely to determine the context in which the TR binds to target genes and regulates the transcriptional response to thyroid hormone.Thyroid hormone (T3) is required for normal growth, development, and adult function in a wide range of species, from Xenopus laevis to humans (44). T3 regulates gene transcription by binding to nuclear thyroid hormone receptors (TRs) (47). In mammals, three TRs (axl, ,13, and ,B2) with different tissue-specific, hormonal, and pharmacological regulation have been described (34). The TRs are cellular homologs of the viral oncoprotein v-ErbA (48,54) and are members of a superfamily of ligand-activated transcription factors (15, 24). The most highly conserved region of these receptors, located near their amino termini, is the DNAbinding domain, which contains basic amino acids arranged into two zinc fingers. The carboxyl termini, which are less well conserved, mediate binding of specific ligands. Although T3 is not required for interactions between TRs and T3-responsive elements (TREs) to occur in vitro (33,35,40), its presence can determine whether transfected TRs have repressive or stimulatory effects on gene transcription in cultured cells (6,11,23).The palindromic sequence AGGTCATGACCT (TREp) has been shown to be a strong TRE (21), although some T3-responsive sequences contain direct repeats (7) while others consist of only a single half-site (28). It has been suggested that TRs bind as dimers to the two half-sites of TREp. This view has been supported by the observation of functional repression by non-T3-binding TR mutants which retain heptad repeats of hydrophobic amino acids in their carboxyl termini (18), which have been referred to as interaction domains (22). Such mutant TRs are similarly able to inhibit the action of retinoic acid receptors (RARs), also perhaps through formation of inactive heterodimers. TRs by themselves form multiple TRE-containing complexes, as revealed by the gel electrophoretic mobility shift assay (EMSA) (33), but biochemical identification of monomeric or dimeric TRs in these complexes has not previously been achieved. TR binding to TREs is also affected by nuclear proteins which enhance binding (8,40) and retard the migration of the TR-TRE complexes in the EMSA (33).Since TR-TRE interaction is a key step in T3 action, the existence of a nuclear protein ...

Endogenous retinoid X receptors can function as hormone receptors in pituitary cells.

“…While retinoid-dependent transactivation by RXR is mediated in part by RXR homodimers (66), RXR is also able to form heterodimers with TR and VDR (7,18,36,37,46,48,64,65). In each case, target gene binding by the nuclear hormone receptor is greatly enhanced by its partnership with RXR.…”

Thyroid Hormone-mediated Enhancement of Heterodimer Formation between Thyroid Hormone Receptor β and Retinoid X Receptor

“…The thyroid hormone receptor is also capable of binding to thyroid hormone response elements (TREs) as a monomer or homodimer, but the latter complex is known to dissociate in the presence of thyroid hormone (T 3 ) (8 -11). More recently, it has been shown that TR as well as receptors for vitamin D 3 , retinoic acid (RAR), and eicosanoids form heterodimeric complexes with an auxiliary factor, the retinoid X receptor (RXR) (12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22). The TR⅐RXR heterodimer binds TREs with higher affinity than TR alone and remains stable in the presence of T 3 (8 -11), suggesting that this complex mediates ligand-dependent transcriptional activation.…”

Multiple mutations contribute to repression by the v-Erb A oncoprotein

“…The v-Erb A I-box sequence also contributes to stabilizing receptor homodimer complexes against disruption by T3 Although c-Erb A homodimers assemble with high affinity on many DNA elements in the absence of hormone, these c-Erb A homodimers typically dissociate in the presence of T3, whereas RXR/c-Erb A heterodimers are stable to T3 (Lazar et al, 1991;Forman et al, 1992;Ribeiro et al, 1992;Yen et al, 1992Yen et al, , 1994Miyamoto et al, 1993;Piedrafita et al, 1995;Yoh and Privalsky, 2001). This T3-driven dissociation of homodimers may, in fact, function to drive an exchange of the c-Erb A/c-Erb A homodimers implicated in transcriptional repression for the RXR/c-Erb A heterodimers involved in transcriptional activation.…”

“…A third mode of DNA recognition by c-Erb A is the ability to bind to suitable response elements as a protein monomer (Lazar et al, 1991;Forman et al, 1992;Darling et al, 1993;Glass, 1994;Katz and Koenig, 1994;Koenig, 1995, 1998;Wahlstrom et al, 1996;Subauste, 2002, 2003). These c-Erb A monomers can be observed on DNA elements containing a single half-site, or as part of a mixed population of monomers, homodimers, and heterodimers on response elements containing two or more halfsites.…”

“…The DNA binding domain (DBD), hormone binding domain (HBD), I-box, helices 1-3 (H1-3), and helix 12 regions are indicated for c-Erb A. Retroviral gag sequences, N-and C-terminal deletions, and amino-acid substitutions (asterisks) within the v-Erb A sequence are also shown and are numbered based on the c-Erb A sequence. The locations of the sites used for constructing the various chimeras are indicated by vertical dotted lines; chimera proteins are named based on the origin of swapped fragments (right and bottom of the panel) other members of the nuclear receptor family, such as the retinoid X receptors (RXRs) (Lazar et al, 1991;Forman et al, 1992;Glass, 1994). It has been reported that gag-v-Erb A is impaired in its ability to form heterodimers with RXR compared to c-Erb A (Forman et al, 1989;Selmi and Samuels, 1991;Yen et al, 1994;Wahlstrom et al, 1996;Bauer et al, 1997;Shen and Subauste, 2000;Yoh and Privalsky, 2001;Zubkova and Subauste, 2003).…”