title: An aryl hydrocarbon receptor repressor from Xenopus laevis: function, expression, and role in dioxin responsiveness during frog development.
abstract: Xenopus laevis and other frogs are extremely insensitive to the toxicity of xenobiotic ligands of the aryl hydrocarbon receptor (AHR), including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Premetamorphic life stages are especially insensitive, and they are reported to be refractory to induction of Cytochrome P4501As, which are readily induced in older animals. The AHR repressor (AHRR) is a member of the AHR gene family. AHRR expression is induced by TCDD; it then represses AHR in an apparent negative feedback loop. In this study, we sought to test the hypothesis that constitutive AHRR expression underlies the lack of TCDD responsiveness in frog early life stages. We determined the sequence of an AHRR complimentary DNA encoding an 85.3-kDa protein sharing 52-55% identity with the bHLH/PAS domains of other AHRRs. In transient transfection assays, X. laevis AHRR inhibited TCDD-induced reporter gene expression mediated by either X. laevis AHR paralog, AHR1alpha or AHR1beta. AHRR messenger RNA was expressed at low levels in embryos (Nieuwkoop-Faber stage 33-38; approximately 52 h.p.f.) and was induced approximately twofold following TCDD exposure (42 ng/g wet weight). In contrast, AHRR exhibited higher constitutive expression and was induced more than threefold in tadpoles at stage 52-55 (prometamorphic; approximately 4 weeks postfertilization) and in isolated viscera of stage 62 tadpoles (in the metamorphic climax; approximately 7 weeks postfertilization). Although the magnitude of induction was smaller, the temporal pattern of AHRR expression and inducibility resembled that of CYP1A6. Thus, attenuated transcriptional activation of AHR target genes and low TCDD toxicity in X. laevis embryos cannot be explained by constitutive, high-level expression of AHRR.
authors.affiliation: Biology Department, Kenyon College, Gambier, Ohio 43022, USA.
authors.forename: Anna L
authors.initials: AL
authors.lastname: Zimmermann
authors.forename: Elizabeth A
authors.initials: EA
authors.lastname: King
authors.forename: Emelyne
authors.initials: E
authors.lastname: Dengler
authors.forename: Shana R
authors.initials: SR
authors.lastname: Scogin
authors.forename: Wade H
authors.initials: WH
authors.lastname: Powell
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D051792:Basic Helix-Loop-Helix Proteins; D019556:COS Cells; D002522:Chlorocebus aethiops; D019363:Cytochrome P-450 CYP1A1 / Q000235:genetics; D018076:DNA, Complementary / Q000235:genetics; D004625:Embryo, Nonmammalian / Q000187:drug effects* / Q000378:metabolism; D004785:Environmental Pollutants / Q000633:toxicity*; D008969:Molecular Sequence Data; D010802:Phylogeny; D000072317:Polychlorinated Dibenzodioxins / Q000633:toxicity*; D012333:RNA, Messenger / Q000378:metabolism; D018336:Receptors, Aryl Hydrocarbon / Q000378:metabolism*; D012097:Repressor Proteins / Q000235:genetics*; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA; D029867:Xenopus Proteins / Q000378:metabolism*; D014982:Xenopus laevis / Q000235:genetics*
chemical_list: D051792:Basic Helix-Loop-Helix Proteins; D019363:Cytochrome P-450 CYP1A1; D018076:DNA, Complementary; D004785:Environmental Pollutants; D000072317:Polychlorinated Dibenzodioxins; D012333:RNA, Messenger; D018336:Receptors, Aryl Hydrocarbon; D012097:Repressor Proteins; D029867:Xenopus Proteins; C476769:aryl hydrocarbon receptor, Xenopus