Nuclear receptors CAR and PXR cross talk with FOXO1 to regulate genes that encode drug-metabolizing and gluconeogenic enzymes.

Pharmacogenetics Section, Laboratory of Reproductive and Developmental Toxicology, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, NC 27709, USA.
Molecular and Cellular Biology (Impact Factor: 5.04). 10/2004; 24(18):7931-40. DOI: 10.1128/MCB.24.18.7931-7940.2004
Source: PubMed

ABSTRACT The nuclear receptors CAR and PXR activate hepatic genes in response to therapeutic drugs and xenobiotics, leading to the induction of drug-metabolizing enzymes, such as cytochrome P450. Insulin inhibits the ability of FOXO1 to express genes encoding gluconeogenic enzymes. Induction by drugs is known to be decreased by insulin, whereas gluconeogenic activity is often repressed by treatment with certain drugs, such as phenobarbital (PB). Performing cell-based transfection assays with drug-responsive and insulin-responsive enhancers, glutathione S-transferase pull down, RNA interference (RNAi), and mouse primary hepatocytes, we examined the molecular mechanism by which nuclear receptors and FOXO1 could coordinately regulate both enzyme pathways. FOXO1 was found to be a coactivator to CAR- and PXR-mediated transcription. In contrast, CAR and PXR, acting as corepressors, downregulated FOXO1-mediated transcription in the presence of their activators, such as 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) and pregnenolone 16alpha-carbonitrile, respectively. A constitutively active mutant of the insulin-responsive protein kinase Akt, but not the kinase-negative mutant, effectively blocked FOXO1 activity in cell-based assays. Thus, insulin could repress the receptors by activating the Akt-FOXO1 signal, whereas drugs could interfere with FOXO1-mediated transcription by activating CAR and/or PXR. Treatment with TCPOBOP or PB decreased the levels of phosphoenolpyruvate carboxykinase 1 mRNA in mice but not in Car(-/-) mice. We conclude that FOXO1 and the nuclear receptors reciprocally coregulate their target genes, modulating both drug metabolism and gluconeogenesis.

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    ABSTRACT: Numerous experimental, clinical and epidemiological studies show that exposure to environmental contaminants may disrupt endocrine and metabolic functions of our organism. This would contribute to the development of obesity and associated metabolic disorders such as nonalcoholic hepatic steatosis, characterized by an excessive accumulation of triglycerides in the liver, which may lead to more severe forms of NAFLD (Non-Alcoholic Fatty Liver Diseases) such as inflammation, fibrosis, cirrhosis and hepatocellular carcinoma. In this work, in vivo studies have highlighted that chronic exposure to the xenoestrogen BPA, widely used in plastic food packaging industry, affects hepatic energy metabolism. It promotes the storage of triglycerides and cholesterol ester in the liver, in association with the induction of the hepatic transcriptome, more particularly of genes involved in lipid, carbohydrates and cholesterol synthesis. These effects, which could contribute to promote hepatic steatosis, follow an inverted U shape non-monotonic dose-response curve and were observed below the reference dose in regulatory toxicology: the tolerable daily intake. These results strengthen the idea that BPA act as a metabolic disruptor, particularly at low doses. Nuclear receptors are targets through which metabolic disruptors may influence gene expression. Recent studies showed that the nuclear receptors CAR (Constitutive Receptor Androstane) and PXR (Pregnane X Receptor), initially identified as key receptors of the detoxification process, are also involved in the regulation of energy metabolism. We identified the gene coding for adiponutrin/PNPLA3 (Patatin-like phospholipase domain-containing) as a new target of these xenosensors. We have shown with transgenic animal models and with hepatocyte cell lines that the CAR and PXR receptors regulate the Pnpla3 gene expression. This protein has a central role in hepatic lipid metabolism through its dual transacylase and lipase activity. In human, a variant of the Pnpla3 (SNP I148M) gene has been identified as a new marker of hepatic steatosis and is associated with an increased risk of NAFLD. Since the xenosensors CAR and PXR are known to be activated by many drugs and environmental pollutants, our results highlight the risk of a development of hepatic steatosis after their activation. Taken together, these results highlight the risk of metabolic disruptions after exposure to various environmental contaminants such as endocrine disruptors and CAR activators. RÉSUMÉ De nombreuses études expérimentales, cliniques et épidémiologiques récentes montrent que l’exposition à des contaminants de notre environnement pourrait perturber les fonctions métaboliques et endocriniennes des organismes. Ceci contribuerait au développement de l'obésité et des pathologies métaboliques associées telles que la stéatose hépatique non alcoolique, caractérisée par une accumulation massive de triglycérides dans le foie, et qui est susceptible d’évoluer vers des pathologies plus sévères (inflammation, fibrose, cirrhose, carcinome hépatocellulaire), regroupées sous le terme de NAFLD (Non- Alcoholic Fatty Liver Diseases). Nos études réalisées chez le rongeur nous ont permis de mettre en évidence un impact sur le métabolisme hépatique suite à une exposition chronique au Bisphénol A (BPA), un contaminant oestrogéno-mimétique largement exploité dans l’industrie des emballages alimentaires plastiques. Il modifie l’expression des gènes impliqués dans la synthèse des lipides, des glucides et du cholestérol et favorise l’accumulation de triglycérides et d’esters de cholestérol au niveau hépatique. Ces effets, qui pourraient contribuer à l’émergence de la stéatose hépatique, ont été observés en deçà de la dose de référence en toxicologie réglementaire (la dose journalière admissible) et suivent une courbe dose-réponse non monotone en U inversé. Ces résultats renforcent l’idée que le BPA est un perturbateur métabolique, surtout lors d’expositions à faibles doses. Les récepteurs nucléaires représentent des cibles potentielles des perturbateurs métaboliques. Plusieurs études récentes montrent que les récepteurs nucléaires CAR (Constitutive Androstane Receptor) et PXR (Pregnane X Receptor), initialement identifiés comme des récepteurs clés du système de détoxification, sont également impliqués dans la régulation du métabolisme énergétique. Nos travaux ont permis d’identifier une nouvelle cible de ces xénosenseurs : le gène codant pour l’adiponutrine/PNPLA3 (Patatinlike phospholipase domain-containing). Nous avons montré, in vivo et sur des lignées d'hépatocytes en culture, que les récepteurs CAR et PXR régulent l’expression du gène Pnpla3. Cette protéine présente une activité à la fois transacylase et lipase qui lui confère un rôle central dans la régulation du métabolisme lipidique hépatique. Chez l’Homme, un variant du gène Pnpla3 (SNP I148M) a été identifié comme un nouveau marqueur de la stéatose hépatique et est associé à un risque accru de développement de NAFLD. Les xénosenseurs CAR et PXR étant activés par de nombreux médicaments et polluants environnementaux, nos résultats mettent en exergue le risque de développement de stéatoses hépatiques suite à leur activation. L’ensemble de ces résultats renforce l’idée d’un risque de développement de pathologies métaboliques suite à l’exposition à différents contaminants environnementaux, qu’il s’agisse de perturbateurs endocriniens de type métaboliques ou d’activateurs des xénosenseurs CAR et PXR.
    12/2012, Degree: PhD, Supervisor: Dr Laila MSELLI-LAKHAL ; Dr Thierry PINEAU


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