TOR: a new orphan receptor expressed in the thymus that can modulate retinoid and thyroid hormone signals

Vitamin A and other fat-soluble hormones and vitamins have important roles as modulators of essential biological processes such as homeostasis, development, differentiation, and oncogenesis and also as regulators of the immune system. The active form of vitamin A, retinoic acid, as well as vitamin D3 and thyroid hormones exert their actions by binding to specific nuclear receptors that represent one subfamily of the steroid/thyroid hormone receptor superfamily. To identify new members of the retinoid/thyroid hormone receptor subfamily that could play a role in the immune system, a screening of a T cell cDNA library was performed using a retinoid X receptor probe. A clone was isolated encoding a novel nuclear receptor expressed mainly in the thymus and T cell lines. This new receptor, TOR (thymus orphan receptor), is most closely related in both its DNA-binding domain and ligand-binding domain, 90% and 53%, respectively, to ROR alpha/RZR alpha and clusters with these two receptors and RZR beta in a phylogenetic tree, when both the DNA-binding domain and the ligand-binding domain sequences of nuclear receptors are compared. Thus, TOR is part of a subgroup of receptors, one of which has recently been reported to be activated by melatonin. TOR binds specifically to a direct repeat of the half-site sequence 5'-AGGTCA-3' with a four- or five-nucleotide spacer, DNA sequences that also serve as binding sites for thyroid hormone (TR), and retinoic acid receptors (RAR). In transient transfection experiments TOR does not activate a reporter gene carrying these sequences in the absence or the presence of any known nuclear receptor ligands. TOR, however, is able to repress TR and RAR activity on DR-4-TREs or DR-5-RAREs, respectively. Therefore, our data suggest that TOR, similar to COUP-TF, can negatively regulate retinoic acid and thyroid hormone signals. However, the response elements recognized by TOR and COUP-TF differ as do the expression patterns of these receptors. Thus, one important role of TOR could be to modulate retinoid and thyroid hormone signals in the thymus.     

Lack of coactivator interaction can be a mechanism for dominant negative activity by mutant thyroid hormone receptors

We studied the interactions of two natural thyroid hormone receptor (TR) mutants from patients with resistance to thyroid hormone (RTH) and an artificial TR mutant with a nuclear receptor corepressor, N-CoR, and a steroid receptor coactivator, SRC-1. In electrophoretic mobility shift assays, wild-type TRbeta-1 interacted with N-CoR in the absence of ligand, whereas T3 caused dissociation of the TRbeta-1/N-CoR complex and formation of TRbeta-1/SRC-1 complex. In contrast, a natural mutant (G345R) with poor T3-binding affinity formed TRbeta-1/N-CoR complex, both in the absence and presence of T3, but could not form TRbeta-1/SRC-1 complex. Another TR mutant, which bound T3 with normal affinity and containing a mutation in the AF-2 region (E457D), had normal interactions with N-CoR but could not bind SRC-1. Both these mutants had strong dominant negative activity on wild-type TR transactivation. Studies with a TR mutant that had slightly decreased T3-binding affinity (R320H) showed a T3-dependent decrease in binding to N-CoR and increase in binding to SRC-1 that reflected its decreased ligand binding affinity. Additionally, when N-CoR and SRC-1 were added to these receptors at various T3 concentrations in electrophoretic mobility shift assays, TR/N-CoR and TR/SRC-1 complexes, but not intermediate complexes were observed, suggesting that N-CoR release is necessary before SRC-1 binding to TR. Our data provide new insight on the molecular mechanisms of dominant negative activity in RTH and suggest that the inability of mutant TRs to interact with coactivators such as SRC-1, which results from reduced T3-binding affinity, is a determinant of dominant negative activity.     

Identification of thyroid hormone response elements in the human fatty acid synthase promoter

To investigate the regulation of the human fatty acid synthase gene by the thyroid hormone triiodothyronine, various constructs of the human fatty acid synthase promoter and the luciferase reporter gene were transfected in combination with plasmids expressing the thyroid hormone and the retinoid X receptors in HepG2 cells. The reporter gene was activated 25-fold by the thyroid hormone in the presence of the thyroid hormone receptor. When both the thyroid hormone and the retinoid X receptors were expressed in HepG2 cells, there was about a 100-fold increase in reporter gene expression. 5'-Deletion analysis disclosed two thyroid hormone response elements, TRE1 (nucleotides -870 to -650) and TRE2 (nucleotides -272 to -40), in the human fatty acid synthase promoter. The presence of thyroid hormone response elements in these two regions of the promoter was confirmed by cloning various fragments of these two regions in the minimal thymidine kinase promoter-luciferase reporter gene plasmid construct and determining reporter gene expression. The results of this cloning procedure and those of electrophoretic mobility shift assays indicated that the sequence GGGTTAcgtcCGGTCA (nucleotides -716 to -731) represents TRE1 and that the sequence GGGTCC (nucleotides -117 to -112) represents TRE2. The sequence of TRE1 is very similar to the consensus sequence of the thyroid hormone response element, whereas the sequence of TRE2 contains only a half-site of the thyroid hormone response element consensus motif because it lacks the direct repeat. The sequences on either side of TRE2 seem to influence its response to the thyroid hormone and retinoid X receptors.