title: Ligand-dependent synergy of thyroid hormone and retinoid X receptors.
abstract: The binding of thyroid hormone receptors to DNA is enhanced by heterodimerization with nuclear proteins. One such heterodimerization partner has recently been characterized as the retinoid X receptor. 9-cis-Retinoic acid has been identified as a natural ligand for retinoid X receptors, suggesting a potential receptor-mediated interaction between thyroid hormone and 9-cis-retinoic acid in the regulation of thyroid hormone-responsive genes. A transient cotransfection assay was used to test for such an interaction. When a complex thyroid hormone response element composed of both direct and inverted repeat hexamers was tested, these two ligands activated gene expression synergistically. In contrast, when the response element consisted only of directly repeated hexamers, unliganded retinoid X receptors enhanced thyroid hormone responsiveness, but 9-cis-retinoic acid induced no additional activation. The results suggest a unique mechanism to achieve differential suggest a unique mechanism to achieve differential thyroid hormone sensitivity of thyroid hormone-responsive genes within a cell. Genes with appropriate response elements will show amplification of the thyroid hormone response by 9-cis-retinoic acid in the presence of retinoid X receptors; other thyroid hormone-responsive genes will be influenced by retinoid X receptors, but not 9-cis-retinoic acid.
authors.affiliation: Endocrinology Division, University of Michigan Medical Center, Ann Arbor 48109-0678.
authors.forename: E D
authors.initials: ED
authors.lastname: Rosen
authors.forename: A L
authors.initials: AL
authors.lastname: O'Donnell
authors.forename: R J
authors.initials: RJ
authors.lastname: Koenig
mesh_terms: D000818:Animals; D001483:Base Sequence; D004268:DNA-Binding Proteins / Q000378:metabolism*; D004357:Drug Synergism; D005786:Gene Expression Regulation; D066298:In Vitro Techniques; D008969:Molecular Sequence Data; D011401:Promoter Regions, Genetic; D012333:RNA, Messenger / Q000235:genetics; D051381:Rats; D011956:Receptors, Cell Surface / Q000502:physiology*; D018168:Receptors, Retinoic Acid; D011988:Receptors, Thyroid Hormone / Q000502:physiology*; D047488:Retinoid X Receptors; D014157:Transcription Factors / Q000378:metabolism; D014158:Transcription, Genetic / Q000187:drug effects; D015533:Transcriptional Activation; D014212:Tretinoin / Q000008:administration & dosage*; D014284:Triiodothyronine / Q000008:administration & dosage*
chemical_list: D004268:DNA-Binding Proteins; D012333:RNA, Messenger; D011956:Receptors, Cell Surface; D018168:Receptors, Retinoic Acid; D011988:Receptors, Thyroid Hormone; D047488:Retinoid X Receptors; D014157:Transcription Factors; D014284:Triiodothyronine; D014212:Tretinoin