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Heterodimeric receptor complexes determine 3,5,3'-triiodothyronine and retinoid signaling specificities
T Hermann1, B Hoffmann, X K Zhang
1Cancer Research Center, La Jolla Cancer Research Foundation, California 92037.
Abstract:
Thyroid hormone receptors (TRs) and retinoic acid receptors (RARs) have been shown to interact with nuclear auxiliary proteins resulting in heteromeric complexes that bind strongly to their responsive elements. Recently the retinoid X receptors (RXRs) have been identified as one class of these nuclear proteins. RXRs strongly increase binding of TRs and RARs to a synthetic thyroid hormone (and retinoic acid) responsive element. Here results show that the binding of the heteromeric complexes to various natural response elements is highly specific and dictated by the partner of RXR in the complex. TR alpha and TR beta formed complexes with RXR alpha that strongly and selectively bound to natural thyroid hormone responsive elements, i.e. those from the rat alpha-myosin heavy chain gene and the rat malic enzyme gene. RXR alpha complexes with RAR alpha, RAR beta, and RAR gamma bound selectively to retinoic acid responsive elements from the human RAR beta 2 gene (hRAR beta 2), the gene of the rat cellular retinol binding protein I and the human apolipoprotein A1 gene. Under the conditions used here RXR alpha by itself did not bind to any of the responsive elements tested. Although TRs and RARs formed heterodimers with RXR in solution, these complexes were strongly stabilized by specific, high affinity response elements, but not by low affinity response elements. Transfection analyses showed strong synergism between receptors that formed effective heterodimers in transcriptional activation on several but not all response elements. Overall, these data demonstrate that RARs and TRs are unlikely to function as monomers or homodimers on the response elements investigated here and require RXRs or comparable proteins for effective response element activation.
Insights
Thyroid hormone receptors (TRs) and retinoic acid receptors (RARs) require retinoid X receptors (RXRs) to bind effectively to DNA. These complexes show specific binding to natural response elements, highlighting RXRs
Area of Science:
- Molecular Biology
- Endocrinology
- Genetics
Background:
- Nuclear receptors, including thyroid hormone receptors (TRs) and retinoic acid receptors (RARs), form complexes with auxiliary proteins.
- Retinoid X receptors (RXRs) are identified as key nuclear auxiliary proteins that enhance TR and RAR binding.
- The specific partner of RXR dictates the binding specificity of these heteromeric complexes to DNA response elements.
Purpose of the Study:
- To investigate the specificity of heteromeric complexes formed by TRs, RARs, and RXRs binding to natural response elements.
- To determine the role of RXR partners in dictating the selective binding of TRs and RARs to their respective response elements.
- To analyze the functional consequences of these interactions on transcriptional activation.
Main Methods:
- Electrophoretic mobility shift assays (EMSAs) were used to study the binding of receptor complexes to various natural response elements.
- Transfection analyses were performed to assess the synergistic effects of receptor heterodimers on transcriptional activation.
Main Results:
- TR alpha and TR beta formed complexes with RXR alpha that selectively bound to thyroid hormone response elements.
- RXR alpha complexes with RARs selectively bound to retinoic acid response elements.
- RXR alpha alone did not bind to any tested response elements, and heterodimer stabilization was dependent on response element affinity.
- Synergistic transcriptional activation was observed for receptor heterodimers on certain response elements.
Conclusions:
- TRs and RARs require RXRs or similar proteins for effective binding and transcriptional activation of response elements.
- The specific partner of RXR plays a crucial role in determining the DNA-binding specificity of TR-RXR and RAR-RXR complexes.
- TRs and RARs are unlikely to function as monomers or homodimers in the context of the studied response elements.