Cis-acting elements in the 5'-flanking DNA of the malic enzyme gene regulate tissue-specific T3-responsiveness in chick embryo fibroblasts.

Triiodothyronine (T3) stimulates transcription of the malic enzyme gene in chick embryo hepatocytes (CEH), but not in chick embryo fibroblasts (CEF), even though the two cell types contain similar nuclear T3 binding activities (F. B. Hillgartner, W. Chen, and A. G. Goodridge, J. Biol. Chem. 267, 12299-12306, 1992). Based on Western blot analyses and gel electrophoretic mobility-shift assays, differences in mass of thyroid hormone receptor (TR)alpha or binding of TRalpha to T3 response element (T3RE) are not responsible for tissue-specific T3 responsiveness. Using transfection assays, we show that the primary T3RE in RCAS-TRalpha-CEF, cells that constitutively over-express TRalpha, is located downstream of the T3REs that are primarily responsible for T3 responsiveness in CEH and is only weakly functional in CEH. T3RE 2, the major T3RE of the malic enzyme gene in CEH is active in CEF when the construct does not contain additional malic enzyme DNA, but not in constructs containing DNA from -3858 to -3541 bp. Responsiveness conferred by T3RE 2 is inhibited in CEF and RCAS-TRalpha-CEF by three or more cis-acting elements downstream from T3RE 2. One element each was localized to fragments from -3622 to -3595 and -3561 to -3541 bp. The inhibitory effect of these elements was not observed in CEH and, although they cannot explain all of the difference in responsiveness in the two cell types, may contribute to the tissue-specific T3 responsiveness of the malic enzyme gene.