Chicken ovalbumin upstream-promoter transcription factor and E-box-binding proteins enhance thyroid-hormone responsiveness of the malic enzyme gene in avian hepatocytes

Influence of neonatal hypothyroidism on hepatic gene expression and lipid metabolism in adulthood

Thyroid hormones are required for normal growth and development in mammals. Congenital-neonatal hypothyroidism (CH) has a profound impact on physiology, but its specific influence in liver is less understood. Here, we studied how CH influences the liver gene expression program in adulthood. Pregnant rats were given the antithyroid drug methimazole (MMI) from GD12 until PND30 to induce CH in male offspring. Growth defects due to CH were evident as reductions in body weight and tail length from the second week of life. Once the MMI treatment was discontinued, the feed efficiency increased in CH, and this was accompanied by significant catch-up growth. On PND80, significant reductions in body mass, tail length, and circulating IGF-I levels remained in CH rats. Conversely, the mRNA levels of known GH target genes were significantly upregulated. The serum levels of thyroid hormones, cholesterol, and triglycerides showed no significant differences. In contrast, CH rats showed significant changes in the expression of hepatic genes involved in lipid metabolism, including an increased transcription of PPARα and a reduced expression of genes involved in fatty acid and cholesterol uptake, cellular sterol efflux, triglyceride assembly, bile acid synthesis, and lipogenesis. These changes were associated with a decrease of intrahepatic lipids. Finally, CH rats responded to the onset of hypothyroidism in adulthood with a reduction of serum fatty acids and hepatic cholesteryl esters and to T3 replacement with an enhanced activation of malic enzyme. In summary, we provide in vivo evidence that neonatal hypothyroidism influences the hepatic transcriptional program and tissue sensitivity to hormone treatment in adulthood. This highlights the critical role that a euthyroid state during development plays on normal liver physiology in adulthood.

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.

A thyroid hormone response unit formed between the promoter and first intron of the carnitine palmitoyltransferase-Ialpha gene mediates the liver-specific induction by thyroid hormone

Carnitine palmitoyltransferase-I (CPT-I) catalyzes the rate-controlling step of fatty acid oxidation. CPT-I converts long-chain fatty acyl-CoAs to acylcarnitines for translocation across the mitochondrial membrane. The mRNA levels and enzyme activity of the liver isoform, CPT-Ialpha, are greatly increased in the liver of hyperthyroid animals. Thyroid hormone (T3) stimulates CPT-Ialpha transcription far more robustly in the liver than in non-hepatic tissues. We have shown that the thyroid hormone receptor (TR) binds to a thyroid hormone response element (TRE) located in the CPT-Ialpha promoter. In addition, elements in the first intron participate in the T3 induction of CPT-Ialpha gene expression, but the CPT-Ialpha intron alone cannot confer a T3 response. We found that deletion of sequences in the first intron between +653 and +744 decreased the T3 induction of CPT-Ialpha. Upstream stimulatory factor (USF) and CCAAT enhancer binding proteins (C/EBPs) bind to elements within this region, and these factors are required for the T3 response. The binding of TR and C/EBP to the CPT-Ialpha gene in vivo was shown by the chromatin immunoprecipitation assay. We determined that TR can physically interact with USF-1, USF-2, and C/EBPalpha. Transgenic mice were created that carry CPT-Ialpha-luciferase transgenes with or without the first intron of the CPT-Ialpha gene. In these mouse lines, the first intron is required for T3 induction as well as high levels of hepatic expression. Our data indicate that the T3 stimulates CPT-Ialpha gene expression in the liver through a T3 response unit consisting of the TRE in the promoter and additional factors, C/EBP and USF, bound in the first intron.

Sterol regulatory element-binding protein-1 interacts with the nuclear thyroid hormone receptor to enhance acetyl-CoA carboxylase-alpha transcription in hepatocytes

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.