Triiodothyronine (T3) causes a 30- to 50-fold increase in transcription of the malic enzyme gene in chick-embryo hepatocytes in culture; mediumchain fatty acids (MCFA's) inhibit this increase. The action of T3 is mediated via thyroid hormone receptors (TR's) which bind to thyroid hormone response elements (T3RE's) in the 5′-flanking DNA of the malic enzyme gene. In previous work from our laboratory, sequences which confer this response on the malic enzyme gene were localized to a T3 response unit between -3903 and -3768 bp upstream of the start site for transcription of the malic enzyme gene. The sequences between -3883 and -3858 bp contain the major functional T3RE of the chicken malic enzyme gene, and this T3RE is complemented by at least six weak T3RE half-sites. Sequence elements that confer inhibition by MCFA co-localized with the TSRE's. When the T3 response unit was linked to a minimal promoter from Herpes Simplex Virus thymidine klnase (HSVTK) and introduced into chick-embryo hepatocytes in culture by transient transfection, T3 caused a 60- to 90-fold increase in chloramphenicol acetyltransferase (CAT) activity. This T3-induced increase in CAT activity was inhibited by 50 to 80% by 1 mM hexanoate. Point mutations and block deletions in each of the T3RE half-sites inhibited promoter activity and resulted in T3-stimulation that varied from 3.5- to 90-fold. Nevertheless, the MCFA-inhibition with each of these constructs was still 50 to 80%. Artificial TSRE's also exhibited MCFA-inhibition. MCFA's inhibited T3-stimulated transcription even when TR was overexpressed, suggesting that the protein which MCFA regulates may not compete with TR for T3RE occupancy. These results are consistent with the hypothesis that MCFA represses T3-induced transcription by inhibiting a function of TR. Supported by Grant DK21594 from the NIH.
|Original language||English (US)|
|State||Published - Dec 1 1996|
ASJC Scopus subject areas
- Molecular Biology