Similar ligand-induced conformational changes of thyroid hormone receptors regulate homo- and heterodimeric functions.
Thyroid hormone receptors (TRs) bind specific thyroid hormone response elements (TREs) as heterodimers with retinoid X receptors (RXRs) and act as transcriptional activators. As homodimers, TRs can bind a distinct set of sequences and function as ligand sensitive repressors. In our study, we compared the natural malic enzyme TRE (ME-TRE) as a model system for the TR/RXR heterodimer pathway to the chicken lysozyme silencer element F2-TRE which is strongly bound and regulated by TR/TR homodimers. Using electrophoretic mobility shift assays, transient transfections with a variety of natural and synthetic triiodothyronine and thyroxine derivatives as well as limited proteolytic analysis, we show that the natural homo- and heterodimeric pathways show similar ligand requirements. Furthermore, we observe that the ligand-induced conformational changes in the receptor proteins that either result in a loss of TR/TR homodimer binding and release of transcriptional repression or in transcriptional activation of TR/RXR heterodimers are indistinguishable. Therefore, we propose that in TR/TR homodimers and TR/RXR heterodimers very similar moieties of the receptors are involved in ligand binding and subsequent conformational changes that lead to loss of gene repression (TR/TR homodimer) and gain of gene activation (TR/RXR heterodimer).
Bendik
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Cancer Center, La Jolla Cancer Research Foundation, California 92037.
Pfahl
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D000818:Animals; D001483:Base Sequence; D002460:Cell Line; D002645:Chickens; D002522:Chlorocebus aethiops; D004305:Dose-Response Relationship, Drug; D015870:Gene Expression / Q000187:drug effects; D007668:Kidney; D007700:Kinetics; D046911:Macromolecular Substances; D008291:Malate Dehydrogenase / Q000096:biosynthesis / Q000378:metabolism; D008969:Molecular Sequence Data; D009113:Muramidase / Q000235:genetics; D009838:Oligodeoxyribonucleotides; D018168:Receptors, Retinoic Acid / Q000737:chemistry / Q000378:metabolism; D011988:Receptors, Thyroid Hormone / Q000737:chemistry* / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000378:metabolism; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D012045:Regulatory Sequences, Nucleic Acid; D047488:Retinoid X Receptors; D013329:Structure-Activity Relationship; D016385:TATA Box; D013974:Thyroxine / Q000031:analogs & derivatives / Q000494:pharmacology; D014157:Transcription Factors / Q000737:chemistry / Q000378:metabolism; D014158:Transcription, Genetic / Q000187:drug effects; D014162:Transfection; D014284:Triiodothyronine / Q000031:analogs & derivatives / Q000494:pharmacology
D046911:Macromolecular Substances; D009838:Oligodeoxyribonucleotides; D018168:Receptors, Retinoic Acid; D011988:Receptors, Thyroid Hormone; D011993:Recombinant Fusion Proteins; D011994:Recombinant Proteins; D047488:Retinoid X Receptors; D014157:Transcription Factors; D014284:Triiodothyronine; D008291:Malate Dehydrogenase; C030574:malate dehydrogenase (decarboxylating); D009113:Muramidase; D013974:Thyroxine