Role of CCAAT/enhancer-binding protein, histone acetylation, and coactivator recruitment in the regulation of malic enzyme transcription by thyroid hormone.

In chick embryo hepatocytes, activation of malic enzyme gene transcription by triiodothyronine (T3) is mediated by a T3 response unit (T3RU) that contains five T3 response elements (T3REs) plus five accessory elements that enhance T3 responsiveness conferred by the T3REs. Results from in vitro binding assays indicate that one of the accessory elements (region F) binds CCAAT/enhancer-binding protein-alpha (C/EBPalpha). Here, we investigated the role of C/EBPalpha in the regulation of malic enzyme transcription by T3. Transfection analyses demonstrated that the stimulation of T3RE function by region F did not require the presence of additional malic enzyme gene promoter sequences. Expression of a dominant negative C/EBP inhibited the ability of region F to stimulate T3 responsiveness. In chromatin immunoprecipitation assays, C/EBPalpha and TR associated with the malic enzyme T3RU in the absence and presence of T3 with the extent of the association being greater in the presence of T3. These observations indicate that C/EBPalpha interacts with TR on the malic enzyme T3RU to enhance T3 regulation of malic enzyme gene transcription. T3 treatment increased the acetylation of histones, decreased the recruitment of nuclear receptor corepressor and increased the recruitment of steroid receptor coactivator-1, CREB binding protein, and the thyroid hormone associated protein/mediator complex at the malic enzyme T3RU. In contrast, T3 treatment had no effect on the acetylation of histones and the recruitment of corepressors and coactivators at the T3RU that mediates the T3 activation of acetyl-CoA carboxylase-alpha gene transcription. We propose that differences between the malic enzyme T3RU and the ACCalpha T3RU in the ability of T3 to modulate histone acetylation and coregulatory protein recruitment are due to differences in the composition of the nuclear receptor complexes that bind these regulatory regions.