Alterations in retinoid X receptor-alpha expression contribute to cell-type dependent differences in thyroid hormone regulation of malic enzyme transcription

Trichoplax adhaerens reveals a network of nuclear receptors sensitive to 9-cis-retinoic acid at the base of metazoan evolution

Trichoplax adhaerens, the only known species of Placozoa is likely to be closely related to an early metazoan that preceded branching of Cnidaria and Bilateria. This animal species is surprisingly well adapted to free life in the World Ocean inhabiting tidal costal zones of oceans and seas with warm to moderate temperatures and shallow waters. The genome of T. adhaerens (sp. Grell) includes four nuclear receptors, namely orthologue of RXR (NR2B), HNF4 (NR2A), COUP-TF (NR2F) and ERR (NR3B) that show a high degree of similarity with human orthologues. In the case of RXR, the sequence identity to human RXR alpha reaches 81% in the DNA binding domain and 70% in the ligand binding domain. We show that T. adhaerens RXR (TaRXR) binds 9-cis retinoic acid (9-cis-RA) with high affinity, as well as high specificity and that exposure of T. adhaerens to 9-cis-RA regulates the expression of the putative T. adhaerens orthologue of vertebrate L-malate-NADP⁺ oxidoreductase (EC 1.1.1.40) which in vertebrates is regulated by a heterodimer of RXR and thyroid hormone receptor. Treatment by 9-cis-RA alters the relative expression profile of T. adhaerens nuclear receptors, suggesting the existence of natural ligands. Keeping with this, algal food composition has a profound effect on T. adhaerens growth and appearance. We show that nanomolar concentrations of 9-cis-RA interfere with T. adhaerens growth response to specific algal food and causes growth arrest. Our results uncover an endocrine-like network of nuclear receptors sensitive to 9-cis-RA in T. adhaerens and support the existence of a ligand-sensitive network of nuclear receptors at the base of metazoan evolution.

Action of Thyroid Hormones, T3 and T2, on Hepatic Fatty Acids: Differences in Metabolic Effects and Molecular Mechanisms

The thyroid hormones (THs) 3,3′,5,5′-tetraiodo-l-thyronine (T4) and 3,5,3′-triiodo-l-thyronine (T3) influence many metabolic pathways. The major physiological function of THs is to sustain basal energy expenditure, by acting primarily on carbohydrate and lipid catabolism. Beyond the mobilization and degradation of lipids, at the hepatic level THs stimulate the de novo fatty acid synthesis (de novo lipogenesis, DNL), through both the modulation of gene expression and the rapid activation of cell signalling pathways. 3,5-Diiodo-l-thyronine (T2), previously considered only a T3 catabolite, has been shown to mimic some of T3 effects on lipid catabolism. However, T2 action is more rapid than that of T3, and seems to be independent of protein synthesis. An inhibitory effect on DNL has been documented for T2. Here, we give an overview of the mechanisms of THs action on liver fatty acid metabolism, focusing on the different effects exerted by T2 and T3 on the regulation of the DNL. The inhibitory action on DNL exerted by T2 makes this compound a potential and attractive drug for the treatment of some metabolic diseases and cancer.

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.

Retinoid-X-receptor alpha (RXRalpha) expression during laryngeal carcinogenesis: detrimental or beneficial event?

RXRalpha is an obligatory heterodimerization partner in many signal transducing pathways. To evaluate RXRalpha expression during laryngeal carcinogenesis, immunohistochemistry was performed on laryngeal epithelial specimens of 154 patients with normal-appearing, hyperplastic, dysplastic laryngeal epithelium and squamous cell carcinoma. RXRalpha up-regulation was detected from the early stages of laryngeal carcinogenesis compared with normal epithelium. Statistical analysis and correlation of the intensity of nuclear immunostaining among the various histologic entities revealed statistically significant results. We pose that RXRalpha overexpression might represent an early protective effect in the process of laryngeal carcinogenesis that might be overwhelmed by an epigenetic disability in forming heterodimers with PPARgamma.

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

In chick embryo hepatocytes (CEH), stimulation of malic enzyme transcription by 3,3',5-tri-iodothyronine (T3) is mediated by a liver-specific and T3-inducible DNase I hypersensitive region (-3910 to -3640 bp) in the malic enzyme gene. Previous studies have shown that this region contains a cluster of five T3 response elements (T3REs), referred to as a T3 response unit (T3RU), plus three accessory elements that enhance T3 responsiveness conferred by the T3RU. Here we report the identification of two additional accessory elements within the -3910 to -3640 bp region. Each element augments T3 regulation of malic enzyme transcription in CEH. One element, designated region G (-3681/-3666 bp), contains a single nuclear-hormone-receptor half-site that binds the orphan receptor chicken ovalbumin upstream-promoter transcription factor. The other element, designated region H (-3655/-3646 bp), contains an E-box motif that binds proteins of unknown identity. Stimulation of T3RE function by region G or region H does not require the presence of additional malic enzyme sequences. In contrast with the stimulatory effects of regions G and H on T3 responsiveness in CEH, neither of these elements is effective in modulating T3 responsiveness in chick embryo fibroblasts (CEF). Instead, region H functions as a T3-insensitive repressor of transcription in CEF. These results indicate that chicken ovalbumin upstream-promoter transcription factor and E-box-binding proteins interact with nuclear T3 receptors to enhance T3 regulation of malic enzyme transcription in CEH and that alterations in region G and region H activities contribute to diminished T3 regulation of malic enzyme transcription in CEF relative to CEH. As the pattern of protein binding to regions G and H varies substantially between CEH and CEF, the mechanism for cell-type-dependent differences in region G and region H activity may involve alterations in protein binding to these T3 accessory elements.

The homeodomain proteins PBX and MEIS1 are accessory factors that enhance thyroid hormone regulation of the malic enzyme gene in hepatocytes

Triiodothyronine (T3) stimulates a robust increase (>40-fold) in transcription of the malic enzyme gene in chick embryo hepatocytes. Previous work has shown that optimal T3 regulation of malic enzyme transcription is dependent on the presence of an accessory element (designated as region E) that immediately flanks a cluster of five T3 response elements in the malic enzyme gene. Here, we have analyzed the binding of nuclear proteins to region E and investigated the mechanism by which region E enhances T3 responsiveness. In nuclear extracts from hepatocytes, region E binds heterodimeric complexes consisting of the homeodomain proteins PBX and MEIS1. Region E contains four consecutive PBX/MEIS1 half-sites. PBX-MEIS1 heterodimers bind the first and second half-sites, the third and fourth half-sites, and the first and fourth half-sites. The configuration conferring the greatest increase in T3 responsiveness consists of the first and fourth half-sites that are separated by 7 nucleotides. Stimulation of T3 response element functions by region E does not require the presence of additional malic enzyme sequences. In pull-down experiments, PBX1a and PBX1b specifically bind the nuclear T3 receptor-alpha, and this interaction is enhanced by the presence of T3. A T3 receptor-alpha region containing the DNA binding domain plus flanking sequences (amino acids 21-157) is necessary and sufficient for binding to PBX1a and PBX1b. These results indicate that PBX-MEIS1 complexes interact with nuclear T3 receptors to enhance T3 regulation of malic enzyme transcription in hepatocytes.

Thyroid hormone stimulates acetyl-coA carboxylase-alpha transcription in hepatocytes by modulating the composition of nuclear receptor complexes bound to a thyroid hormone response element

Triiodothyronine (T3) stimulates a 7-fold increase in transcription of the acetyl-CoA carboxylase-alpha (ACCalpha) gene in chick embryo hepatocytes. Here, we characterized an ACCalpha T3 response element (ACCalpha-T3RE) with unique functional and protein binding properties. ACCalpha-T3RE activated transcription both in the absence and presence of T3, with a greater activation observed in the presence of T3. In nuclear extracts from hepatocytes incubated in the absence of T3, ACCalpha-T3RE bound protein complexes (complexes 1 and 2) containing the liver X receptor (LXR) and the retinoid X receptor (RXR). In nuclear extracts from hepatocytes incubated in the presence of T3 for 24 h, ACCalpha-T3RE bound a different set of complexes. One complex contained LXR and RXR (complex 3) and another contained the nuclear T3 receptor (TR) and RXR (complex 4). Mutations of ACCalpha-T3RE that inhibited the binding of complexes 1 and 2 decreased transcriptional activation in the absence of T3, and mutations of ACCalpha-T3RE that inhibited the binding of complexes 3 and 4 decreased transcriptional activation in the presence of T3. The stimulation of ACCalpha transcription caused by T3 was closely associated with changes in the binding of complexes 1-4 to ACCalpha-T3RE. These data suggest that T3 regulates ACCalpha transcription by a novel mechanism involving changes in the composition of nuclear receptor complexes bound to ACCalpha-T3RE. We propose that complexes containing LXR/RXR ensure a basal level of ACCalpha expression for the synthesis of structural lipids in cell membranes and that complexes containing LXR/RXR and TR/RXR mediate the stimulation of ACCalpha expression caused by T3.