Overexpression of the alpha-thyroid hormone receptor in avian cell lines. Effects on expression of the malic enzyme gene are selective and cell-specific.
The role of the alpha-thyroid hormone receptor (TR alpha) in regulation of transcription of the gene for chicken malic enzyme was analyzed in fibroblast cell lines normally unresponsive to triiodothyronine (T3). The gene for this transcription factor was introduced stably and overexpressed using a replication-competent retroviral vector. In chick embryo fibroblasts (CEF), overexpression of TR alpha decreased malic enzyme activity by 90% in the absence of T3. Addition of T3 almost completely restored malic enzyme activity to the level of similarly treated control CEF infected with virus lacking TR alpha. These TR alpha-induced changes in malic enzyme activity were mediated by alterations in transcription of the malic enzyme gene. Similar results were obtained when transcriptional activity of TR alpha was analyzed using a transient co-transfection system. Thus, the unliganded TR alpha is a transcriptional repressor of the malic enzyme gene; binding of T3 to the receptor abolishes this repression. In contrast, stable overexpression of TR alpha in QT6 cells had no effect on malic enzyme expression in the absence or presence of T3. Nuclear T3 binding was equally high in CEF and QT6 cells overexpressing TR alpha. These findings suggest that cell-specific factors control the ability of TR alpha to regulate the malic enzyme gene. Overexpression of TR alpha in CEF had no effect on the expression of fatty acid synthase and acetyl-CoA carboxylase, lipogenic enzymes that are stimulated by T3 in hepatocytes in culture. Thus, gene-specific factors also may control the transcriptional activity of TR alpha.
Department of Biochemistry, University of Iowa, Iowa City 52242.
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Hillgartner
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Chen
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Goodridge
D000103:Acetyl-CoA Carboxylase / Q000235:genetics; D000199:Actins / Q000235:genetics; D000818:Animals; D002460:Cell Line; D002642:Chick Embryo; D015342:DNA Probes; D064429:Fatty Acid Synthases / Q000235:genetics; D005347:Fibroblasts / Q000201:enzymology; D015971:Gene Expression Regulation, Enzymologic; D005987:Glyceraldehyde-3-Phosphate Dehydrogenases / Q000235:genetics; D008291:Malate Dehydrogenase / Q000235:genetics*; D011518:Proto-Oncogene Proteins / Q000235:genetics; D012333:RNA, Messenger / Q000378:metabolism; D011988:Receptors, Thyroid Hormone / Q000235:genetics / Q000502:physiology*; D014158:Transcription, Genetic; D014284:Triiodothyronine / Q000378:metabolism
D000199:Actins; D015342:DNA Probes; D011518:Proto-Oncogene Proteins; D012333:RNA, Messenger; D011988:Receptors, Thyroid Hormone; D014284:Triiodothyronine; D008291:Malate Dehydrogenase; D005987:Glyceraldehyde-3-Phosphate Dehydrogenases; D064429:Fatty Acid Synthases; D000103:Acetyl-CoA Carboxylase