Sterol Regulatory Element-binding Protein-1 Interacts with the Nuclear Thyroid Hormone Receptor to Enhance Acetyl-CoA Carboxylase-α Transcription in Hepatocytes

Abstract

In previous work, we characterized a 3,5,3'-triiodothyronine response element (T3RE) in acetyl-CoA carboxylase-alpha (ACCalpha) promoter 2 that mediated 3,5,3'-triiodothyronine (T3) regulation of ACCalpha transcription in chick embryo hepatocytes. Sequence comparison analysis revealed the presence of sterol regulatory element-1 (SRE-1) located 5 bp downstream of the ACCalpha T3RE. Here, we investigated the role of this SRE-1 in modulating T3 regulation of ACCalpha transcription. Transfection analyses demonstrated that the SRE-1 enhanced T3-induced ACCalpha transcription by more than 2-fold in hepatocytes. The effect of the SRE-1 on T3 responsiveness required the presence of the T3RE in its native orientation. In pull-down experiments, the mature form of sterol regulatory element-binding protein-1 (SREBP-1) specifically bound the alpha-isoform of the nuclear T3 receptor (TR), and the presence of T3 enhanced this interaction. A region of TRalpha containing the DNA-binding domain plus flanking sequences (amino acids 21-157) was required for interaction with SREBP-1, and a region of SREBP-1 containing the basic helix-loop-helix-leucine zipper domain (amino acids 300-389) was required for interaction with TRalpha. In gel mobility shift experiments, TRalpha, retinoid X receptor-alpha, and mature SREBP-1 formed a tetrameric complex on a DNA probe containing the ACCalpha T3RE and SRE-1, and the presence of T3 enhanced the formation of this complex. Formation of the tetrameric complex stabilized the binding of SREBP-1 to the SRE-1. These results indicate that SREBP-1 directly interacts with TR-retinoid X receptor in an orientation-specific manner to enhance T3-induced ACCalpha transcription in hepatocytes. T3 regulation of ACCalpha transcription in nonhepatic cell cultures such as chick embryo fibroblasts is markedly reduced compared with that of chick embryo hepatocytes. Here, we also show that alterations in SREBP expression play a role in mediating cell type-dependent differences in T3 regulation of ACCalpha transcription.