Ubiquitous receptor: a receptor that modulates gene activation by retinoic acid and thyroid hormone receptors.

The cDNA for a member of the nuclear receptor family was cloned and named ubiquitous receptor (UR), since UR protein and mRNA are detected in many cell types. Rat UR/human retinoid X receptor alpha (hRXR alpha) heterodimers bound preferentially to double-stranded oligonucleotide direct repeats having the consensus half-site sequence AGGTCA and 4-nt spacing (DR-4). Coexpression of UR in COS-1 cells inhibited the stimulation of chloramphenicol acetyltransferase (CAT) reporter gene expression by hRXR alpha and human retinoic acid receptor alpha in the presence of all-trans-retinoic acid when DR-4 (but not DR-5) was present upstream of the promoter of a CAT reporter gene (DR-4-CAT). UR expression also inhibited the activation of a DR-4-CAT reporter gene by hRXR alpha and 9-cis-retinoic acid or by thyroid hormone receptor beta in the presence of thyroid hormone. However, in the absence of 9-cis-retinoic acid, UR in combination with hRXR alpha stimulation DR-4-CAT expression. Coexpression of thyroid hormone receptor markedly reduced this stimulation in the absence of thyroid hormone. UR may play an important role in normal growth and differentiation by modulating gene activation in retinoic acid and thyroid hormone signaling pathways.     

Regulation of retinoid and thyroid hormone action through homodimeric and heterodimeric receptors

Biologic responses to retinoids and thyroid hormones are mediated by their intracellular receptor proteins. Many exciting advances have been made recently in understanding the molecular mechanism by which these receptor proteins operate. In contrast to the steroid hormone receptors that function predominantly as homodimers, thyroid hormone receptors (TRs)and retinoic acid receptors (RARs) require interaction with the retinoid X receptors (RXRs) for efficient DNA binding and transactivation. In addition, RXRs, in the presence of their specific ligands such as 9-cis RA, can form homodimers that recognize a subset of retinoic acid responsive elements (RAREs). The retinoid responses mediated by RXR homodimers and RAR-RXR heterodimers can be restricted by the COUP-TF orphan receptors that bind strongly to certain RAREs as homodimers. Thus, a complex network of receptor interaction has been unraveled that promises a better understanding of thyroid and retinoid hormone regulation of fundamental biologic processes and diseases.     

The oligomeric state of thyroid receptor regulates hormone binding kinetics

We previously reported that mutations in the thyroid hormone receptor (TR) surface that mediates dimer and heterodimer formation do not alter affinity for cognate hormone (triiodothyronine (T(3))) yet dramatically enhance T(3) association and dissociation rates. This study aimed to show that TR oligomeric state influences binding and dissociation kinetics. We performed binding assays using marked hormone ((125)I-T(3)) and TRs expressed and purified by different methods. We find that T(3) associates with TRs with biphasic kinetics in solution; a rapid step (half-life ±0.1 h) followed by a slower second step (half-life ±5 h) and that purification of monomers suggests that biphasic kinetics are due to the presence of monomers and dimers in our preparations. In support of this idea, incubation of TR ligand binding domain monomers with corepressor peptide induces dimer formation and decreases association rates and T(3) binds to, and dissociates from, a TRβ mutant that only forms dimers (TRβD355R) with slow single-phase kinetics. In addition, heterodimer formation with retinoid X receptors also influences ligand binding kinetics. Together, these results suggest that the dimer/heterodimer surface is allosterically coupled to the hormone binding pocket and that different interactions at this surface exert different effects on ligand binding that may be relevant for TR actions in the cell.     

Fatty acids and retinoids control lipid metabolism through activation of peroxisome proliferator-activated receptor-retinoid X receptor heterodimers.

The nuclear hormone receptors called PPARs (peroxisome proliferator-activated receptors alpha, beta, and gamma) regulate the peroxisomal beta-oxidation of fatty acids by induction of the acyl-CoA oxidase gene that encodes the rate-limiting enzyme of the pathway. Gel retardation and cotransfection assays revealed that PPAR alpha heterodimerizes with retinoid X receptor beta (RXR beta; RXR is the receptor for 9-cis-retinoic acid) and that the two receptors cooperate for the activation of the acyl-CoA oxidase gene promoter. The strongest stimulation of this promoter was obtained when both receptors were exposed simultaneously to their cognate activators. Furthermore, we show that natural fatty acids, and especially polyunsaturated fatty acids, activate PPARs as potently as does the hypolipidemic drug Wy 14,643, the most effective activator known so far. Moreover, we discovered that the synthetic arachidonic acid analogue 5,8,11,14-eicosatetraynoic acid is 100 times more effective than Wy 14,643 in the activation of PPAR alpha. In conclusion, our data demonstrate a convergence of the PPAR and RXR signaling pathways in the regulation of the peroxisomal beta-oxidation of fatty acids by fatty acids and retinoids.     

Multimeric complexes of the PML-retinoic acid receptor alpha fusion protein in acute promyelocytic leukemia cells and interference with retinoid and peroxisome-proliferator signaling pathways.

The t(15;17) chromosomal translocation, specific for acute promyelocytic leukemia (APL), fuses the PML gene to the retinoic acid receptor alpha (RAR alpha) gene, resulting in expression of a PML-RAR alpha hybrid protein. In this report, we analyzed the nature of PML-RAR alpha-containing complexes in nuclear protein extracts of t(15;17)-positive cells. We show that endogenous PML-RAR alpha can bind to DNA as a homodimer, in contrast to RAR alpha that requires the retinoid X receptor (RXR) dimerization partner. In addition, these cells contain oligomeric complexes of PML-RAR alpha and endogenous RXR. Treatment with retinoic acid results in a decrease of PML-RAR alpha protein levels and, as a consequence, of DNA binding by the different complexes. Using responsive elements from various hormone signaling pathways, we show that PML-RAR alpha homodimers have altered DNA-binding characteristics when compared to RAR alpha-RXR alpha heterodimers. In transfected Drosophila SL-3 cells that are devoid of endogenous retinoid receptors PML-RAR alpha inhibits transactivation by RAR alpha-RXR alpha heterodimers in a dominant fashion. In addition, we show that both normal retinoid receptors and the PML-RAR alpha hybrid bind and activate the peroxisome proliferator-activated receptor responsive element from the Acyl-CoA oxidase gene, indicating that retinoids and peroxisome proliferator receptors may share common target genes. These properties of PML-RAR alpha may contribute to the transformed phenotype of APL cells.