The conserved ninth C-terminal heptad in thyroid hormone and retinoic acid receptors mediates diverse responses by affecting heterodimer but not homodimer formation

1993 ◽  
Vol 13 (9) ◽  
pp. 5725-5737
Author(s):  
M Au-Fliegner ◽  
E Helmer ◽  
J Casanova ◽  
B M Raaka ◽  
H H Samuels
Keyword(s):  
Retinoic Acid ◽  
Thyroid Hormone ◽  
Dominant Negative ◽  
Heptad Repeat ◽  
Specific Response ◽  
Wild Type ◽  
Negative Effects ◽  
Related Factors ◽  
Response Element ◽  
Response Elements

The receptors for thyroid hormone (T3R), all-trans-retinoic acid (RAR), and 9-cis-retinoic acid (RXR) bind DNA response elements as homo- and heterodimers. The ligand-binding domains of these receptors contain nine conserved heptads proposed to play a role in dimerization. Mutant receptors with changes in the first or last hydrophobic amino acids in the highly conserved ninth heptad of chick T3R alpha [cT3R alpha(L365R) and cT3R(L372R)] and human RAR alpha (hRAR alpha) [hRAR(M377R) and hRAR(L384R)] reveal that this heptad is essential for certain heterodimeric interactions and for diverse functional activities. Without ligands, wild-type receptors form both homodimers and heterodimers, while these mutants form only homodimers. Surprisingly, the cognate ligand for each mutant enables heterodimer formation between cT3R(L365R) and RAR or RXR and between hRAR(M377R) and T3R or RXR. Both cT3R(L365R) and hRAR(M377R) mediate ligand-dependent transcriptional regulation. However, unlike the wild-type receptor, non-ligand-associated cT3R(L365R) does not suppress the basal activity of certain promoters containing thyroid hormone response elements, suggesting that this silencing effect of T3R is mediated by unliganded heterodimers of T3R and endogenous RXR or related factors. Heterodimerization is also necessary for the strong ligand-independent inhibition between T3R and RAR on a common response element, since the ninth-heptad mutants function as poor inhibitors. However, with a T3R-specific response element, hRAR(M377R) acts as a retinoic acid-dependent inhibitor of cT3R, indicating the importance of heterodimerization for this inhibition. Our studies also suggest that the ninth heptad is necessary for the dominant inhibition of wild-type T3Rs by mutant T3Rs, as has been found for the thyroid hormone-resistant syndrome in humans. Thus, the ninth heptad repeat is required for heterodimerization, suppression of basal promoter activity, and dominant negative effects of T3R and RAR. Lastly, the finding that cT3R(L365R) and hRAR(M377R) require ligands for heterodimer formation also raises the possibility that heterodimeric interactions are mediated by the ninth heptad without ligands but by a second region of these receptors with ligands.

scholarly journals The conserved ninth C-terminal heptad in thyroid hormone and retinoic acid receptors mediates diverse responses by affecting heterodimer but not homodimer formation.

1993 ◽  
Vol 13 (9) ◽  
pp. 5725-5737 ◽  
Author(s):  
M Au-Fliegner ◽  
E Helmer ◽  
J Casanova ◽  
B M Raaka ◽  
H H Samuels
Keyword(s):  
Retinoic Acid ◽  
Thyroid Hormone ◽  
Dominant Negative ◽  
Heptad Repeat ◽  
Specific Response ◽  
Wild Type ◽  
Negative Effects ◽  
Related Factors ◽  
Response Element ◽  
Response Elements

The receptors for thyroid hormone (T3R), all-trans-retinoic acid (RAR), and 9-cis-retinoic acid (RXR) bind DNA response elements as homo- and heterodimers. The ligand-binding domains of these receptors contain nine conserved heptads proposed to play a role in dimerization. Mutant receptors with changes in the first or last hydrophobic amino acids in the highly conserved ninth heptad of chick T3R alpha [cT3R alpha(L365R) and cT3R(L372R)] and human RAR alpha (hRAR alpha) [hRAR(M377R) and hRAR(L384R)] reveal that this heptad is essential for certain heterodimeric interactions and for diverse functional activities. Without ligands, wild-type receptors form both homodimers and heterodimers, while these mutants form only homodimers. Surprisingly, the cognate ligand for each mutant enables heterodimer formation between cT3R(L365R) and RAR or RXR and between hRAR(M377R) and T3R or RXR. Both cT3R(L365R) and hRAR(M377R) mediate ligand-dependent transcriptional regulation. However, unlike the wild-type receptor, non-ligand-associated cT3R(L365R) does not suppress the basal activity of certain promoters containing thyroid hormone response elements, suggesting that this silencing effect of T3R is mediated by unliganded heterodimers of T3R and endogenous RXR or related factors. Heterodimerization is also necessary for the strong ligand-independent inhibition between T3R and RAR on a common response element, since the ninth-heptad mutants function as poor inhibitors. However, with a T3R-specific response element, hRAR(M377R) acts as a retinoic acid-dependent inhibitor of cT3R, indicating the importance of heterodimerization for this inhibition. Our studies also suggest that the ninth heptad is necessary for the dominant inhibition of wild-type T3Rs by mutant T3Rs, as has been found for the thyroid hormone-resistant syndrome in humans. Thus, the ninth heptad repeat is required for heterodimerization, suppression of basal promoter activity, and dominant negative effects of T3R and RAR. Lastly, the finding that cT3R(L365R) and hRAR(M377R) require ligands for heterodimer formation also raises the possibility that heterodimeric interactions are mediated by the ninth heptad without ligands but by a second region of these receptors with ligands.

COUP orphan receptors are negative regulators of retinoic acid response pathways

1992 ◽  
Vol 12 (10) ◽  
pp. 4666-4676
Author(s):  
P Tran ◽  
X K Zhang ◽  
G Salbert ◽  
T Hermann ◽  
J M Lehmann ◽  
...  
Keyword(s):  
Retinoic Acid ◽  
Thyroid Hormone ◽  
Nuclear Receptors ◽  
Thyroid Hormone Receptor ◽  
Cdna Probe ◽  
Specific Response ◽  
Retinoid Receptor ◽  
Orphan Receptors ◽  
Response Element ◽  
Response Elements

The vitamin hormone retinoic acid (RA) regulates many complex biological programs. The hormonal signals are mediated at the level of transcription by multiple nuclear receptors. These receptors belong to the steroid/thyroid hormone receptor superfamily that also includes a large number of orphan receptors whose biological roles have not yet been determined. Although much has been learned in recent years about RA receptor (RAR) functions, little is known about how specific RA response programs are restricted to certain tissues and cell types during development and in the adult. It has been recently shown that RAR activities are regulated by retinoid X receptors (RXR) through heterodimer formation. In an effort to isolate and further characterize nuclear receptors that modulate RAR and/or RXR activities, we have screened cDNA libraries by using a RXR alpha cDNA probe. Two clones, COUP alpha and COUP beta, identical and closely related to the orphan receptor COUP-TF, were obtained. We show that COUP proteins dramatically inhibit retinoid receptor activities on certain response elements that are activated by RAR/RXR heterodimers or RXR homodimers. COUP alpha and -beta bind strongly to these response elements, including a palindromic thyroid hormone response element and a direct repeat RA response element as well as an RXR-specific response element. In addition, we found that the previously identified COUP-TF binding site in the ovalbumin gene functions in vitro as an RA response element that is repressed in the presence of COUP. Our data suggest that the COUP receptors are a novel class of RAR and RXR regulators that can restrict RA signaling to certain elements. The COUP orphan receptors may thus play an important role in cell- or tissue-specific repression of subsets of RA-sensitive programs during development and in the adult.

scholarly journals COUP orphan receptors are negative regulators of retinoic acid response pathways.

1992 ◽  
Vol 12 (10) ◽  
pp. 4666-4676 ◽  
Author(s):  
P Tran ◽  
X K Zhang ◽  
G Salbert ◽  
T Hermann ◽  
J M Lehmann ◽  
...  
Keyword(s):  
Retinoic Acid ◽  
Thyroid Hormone ◽  
Nuclear Receptors ◽  
Thyroid Hormone Receptor ◽  
Cdna Probe ◽  
Specific Response ◽  
Retinoid Receptor ◽  
Orphan Receptors ◽  
Response Element ◽  
Response Elements

The vitamin hormone retinoic acid (RA) regulates many complex biological programs. The hormonal signals are mediated at the level of transcription by multiple nuclear receptors. These receptors belong to the steroid/thyroid hormone receptor superfamily that also includes a large number of orphan receptors whose biological roles have not yet been determined. Although much has been learned in recent years about RA receptor (RAR) functions, little is known about how specific RA response programs are restricted to certain tissues and cell types during development and in the adult. It has been recently shown that RAR activities are regulated by retinoid X receptors (RXR) through heterodimer formation. In an effort to isolate and further characterize nuclear receptors that modulate RAR and/or RXR activities, we have screened cDNA libraries by using a RXR alpha cDNA probe. Two clones, COUP alpha and COUP beta, identical and closely related to the orphan receptor COUP-TF, were obtained. We show that COUP proteins dramatically inhibit retinoid receptor activities on certain response elements that are activated by RAR/RXR heterodimers or RXR homodimers. COUP alpha and -beta bind strongly to these response elements, including a palindromic thyroid hormone response element and a direct repeat RA response element as well as an RXR-specific response element. In addition, we found that the previously identified COUP-TF binding site in the ovalbumin gene functions in vitro as an RA response element that is repressed in the presence of COUP. Our data suggest that the COUP receptors are a novel class of RAR and RXR regulators that can restrict RA signaling to certain elements. The COUP orphan receptors may thus play an important role in cell- or tissue-specific repression of subsets of RA-sensitive programs during development and in the adult.

Response element selectivity for heterodimerization of vitamin D receptors with retinoic acid and retinoid X receptors

1994 ◽  
Vol 12 (3) ◽  
pp. 327-339 ◽  
Author(s):  
M Schräder ◽  
K M Müller ◽  
M Becker-André ◽  
C Carlberg
Keyword(s):  
Vitamin D ◽  
Retinoic Acid ◽  
Specific Response ◽  
Direct Repeats ◽  
Response Element ◽  
Retinoid X Receptors ◽  
Response Elements ◽  
Vitamin D Receptors ◽  
Receptor Complexes ◽  
X Receptors

ABSTRACT The transcription of vitamin D (VD) responsive genes is regulated by three different nuclear signalling pathways mediated by homodimers of VD receptors (VDRs), heterodimers of VDRs and retinoid X receptors (RXRs) and heterodimers of VDRs with retinoic acid receptors (RARs). Here, the in vitro DNA-binding affinity of all three receptor complexes was shown to be enhanced by the presence of VD. However, the specificity of the three pathways was dictated by the differential affinities of the receptor complexes for VD response elements. Potential response elements were distinguished by the sequence, the separation and the relative orientation of the hexameric core binding motifs. It was found that both VDR-RAR and VDR-RXR heterodimers act functionally on all three response element configurations: direct repeats, palindromes and inverted palindromes. With direct repeats, neither heterodimer type showed a preference for any of the three principal core motifs, (A/G)GGTGA, (A/G)GGTCA and (A/G)GTTCA. However, while they did exhibit preferences for core motifs in palindromes, the spacing requirements were identical for both complexes. Inverted palindromes, however, formed the most specific response elements. A simple model explains a steric link between the optimal spacing of direct repeats and that of inverted palindromes. Taken together, the experimental data and the model provide further criteria for the screening of VD responsive genes.

scholarly journals Vitamin D interferes with transactivation of the growth hormone gene by thyroid hormone and retinoic acid.

1996 ◽  
Vol 16 (1) ◽  
pp. 318-327 ◽  
Author(s):  
P Garcia-Villalba ◽  
A M Jimenez-Lara ◽  
A Aranda
Keyword(s):  
Gene Expression ◽  
Growth Hormone ◽  
Vitamin D ◽  
Retinoic Acid ◽  
Thyroid Hormone ◽  
Vitamin D Receptor ◽  
Growth Hormone Gene ◽  
Dependent Manner ◽  
Transcriptional Responses ◽  
Response Element

The thyroid hormone, retinoic acid (RA), and vitamin D regulate gene expression by binding to similar receptors which act as ligand-inducible transcription factors. Incubation of pituitary GH4C1 cells with nanomolar concentrations of vitamin D markedly reduces the response of the rat growth hormone mRNA to thyroid hormone triiodothyronine (T3) and RA. The stimulation of growth hormone gene expression by both ligands is mediated by a common hormone response element (TREGH) present in the 5'-flanking region of the gene, and the inhibition caused by vitamin D is due to transcriptional interference of the vitamin D receptor on this DNA element. No inhibition of the basal promoter activity by the vitamin was observed. The response to T3 and RA of a heterologous promoter containing this element, the palindromic T3- and RA-responsive sequence TREPAL, or a direct repeat of the same motif is also inhibited by vitamin D. In contrast, vitamin D strongly induces the activity of constructs containing a vitamin D response element, and neither T3 nor RA reduces vitamin D-mediated transactivation. Transfection with an expression vector for the retinoid X receptor alpha (RXR alpha) increases transactivation by T3 and RA but does not abolish the inhibition caused by the vitamin. Gel retardation experiments show that the vitamin D receptor (VDR) as a heterodimer with RXR weakly binds to the T3- and RA-responsive elements. Additionally, VDR displaces binding of T3 and RA receptors in a dose-dependent manner. Our data suggest the formation of TR-VDR and RAR-VDR heterodimers with RXR. The fact that the same response element mediates opposite effects of at least four different nuclear receptors provides a greater complexity and flexibility of the transcriptional responses to their ligands.

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