Characterization of Thyroid Hormone Response Elements in the Gene for Chicken Malic Enzyme

Transcription of the gene for malic enzyme in chick embryo hepatocytes is stimulated about 30-fold by triiodothyronine (T3). T3 responsiveness is mediated by seven direct repeat hexamers that resemble T3 response elements (T3REs); these elements are located far upstream in the 5′-flanking DNA (Hodnett, D. W., Fantozzzi, D. A., Thurmond, D. C., Klautky, S. A., MacPhee, K. G., Estrem, S. T., Xu, G., and Goodridge, A. G. (1996) Arch. Biochem. Biophys. 334, 309–324). In transiently transfected hepatocytes, single copies of six of these elements conferred varying degrees of T3 responsiveness to linked reporter genes. In gel electrophoretic mobility shift analyses, the T3REs bound retinoid X receptor (RXR)-T3 receptor (TR) heterodimers and non-RXR/TR factors present in nuclear extracts prepared from hepatocytes. Binding of the non-RXR/TR factors was specific to individual T3REs and was unaffected by antibodies to TR or RXR. Mutagenesis of binding sites for proteins specific for T3REs 2–5 altered binding of the proteins and T3 responsiveness. These factors appear to bind to and alter function of T3REs without binding directly to TR, differentiating their actions from other TR cofactors; they were tentatively characterized as co-repressors, inhibitors, and activators of T3RE function. Together with RXR and TR, they modulate T3 responsiveness of the gene for chicken malic enzyme.