title: Contribution of the Ah receptor to the phenolic antioxidant-mediated expression of human and rat UDP-glucuronosyltransferase UGT1A6 in Caco-2 and rat hepatoma 5L cells.
abstract: UDP-glucuronosyltransferases (UGTs) represent major phase II enzymes of drug metabolism which are regulated in a tissue-specific manner by endogenous and environmental factors. Among the latter, aryl hydrocarbon receptor (AhR) agonists such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and phenolic antioxidants such as tert-butylhydroquinone (tBHQ) are known to induce the expression of human UGT1A6 in Caco-2 cells. While binding of the TCDD-activated AhR to one xenobiotic response element (XRE) in the 5'-flanking regulatory region of UGT1A6 was characterised previously, the mechanism responsible for tBHQ induction is unknown. Therefore, it was investigated whether antioxidant response elements (AREs) are involved in tBHQ induction of UGT1A6. Transfectants of 3 kb of its regulatory region and its deletion mutants were treated with tBHQ. These studies suggested a region with approximately 2-fold induction, including an ARE-like motif, 15 bp downstream of the previously characterised XRE. Transfectants of the point-mutated ARE-like motif showed marginally reduced response to tBHQ, but surprisingly, loss of response to TCDD, suggesting interference of flanking proteins with the AhR/Arnt complex. Coordinate responses of UGT activity after treatment with TCDD or tBHQ were also observed in rat hepatoma 5L cells, mutants without the AhR and with recomplemented AhR. The results suggest a contribution of the AhR pathway and of proteins binding to the XRE flanking region to the induction of human UGT1A6 by both AhR agonists and phenolic antioxidants.
authors.affiliation: Department of Toxicology, Institute of Pharmacology and Toxicology, University of Tübingen, Tübingen D-72074, Germany.
authors.forename: Peter A
authors.initials: PA
authors.lastname: Münzel
authors.forename: Stephan
authors.initials: S
authors.lastname: Schmohl
authors.forename: Felicitas
authors.initials: F
authors.lastname: Buckler
authors.forename: Jan
authors.initials: J
authors.lastname: Jaehrling
authors.forename: Frank T
authors.initials: FT
authors.lastname: Raschko
authors.forename: Christoph
authors.initials: C
authors.lastname: Köhle
authors.forename: Karl Walter
authors.initials: KW
authors.lastname: Bock
mesh_terms: D024506:5' Flanking Region / Q000187:drug effects; D000818:Animals; D000975:Antioxidants / Q000494:pharmacology*; D018938:Caco-2 Cells; D006528:Carcinoma, Hepatocellular; D004785:Environmental Pollutants / Q000633:toxicity; D015870:Gene Expression / Q000187:drug effects*; D014453:Glucuronosyltransferase / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D006873:Hydroquinones / Q000494:pharmacology*; D009004:Monosaccharide Transport Proteins; D000072317:Polychlorinated Dibenzodioxins / Q000633:toxicity; D051381:Rats; D018336:Receptors, Aryl Hydrocarbon / Q000502:physiology*; D013045:Species Specificity; D014407:Tumor Cells, Cultured; D019324:beta-Naphthoflavone / Q000633:toxicity; D000099237:UDP-Galactose Translocators
chemical_list: D000975:Antioxidants; D004785:Environmental Pollutants; D014453:Glucuronosyltransferase; D006873:Hydroquinones; D009004:Monosaccharide Transport Proteins; D000072317:Polychlorinated Dibenzodioxins; D018336:Receptors, Aryl Hydrocarbon; D019324:beta-Naphthoflavone; D000099237:UDP-Galactose Translocators; C018855:2-tert-butylhydroquinone; C110655:UDP-glucuronosyltransferase, UGT1A6