ToxSci Advance Access published online on April 2, 2008
Toxicological Sciences, doi:10.1093/toxsci/kfn066
An Aryl Hydrocarbon Receptor Repressor from Xenopus laevis: Function, Expression and Role in Dioxin Responsiveness during Frog Development
Biology Department, Kenyon College, Gambier, OH 43022
* To whom correspondence should be addressed:Biology Department, Kenyon College, 302A College Park St., Fischman Wing 202, Gambier, OH 43022, FAX: 740-427-5741, powellw{at}kenyon.edu
Received February 15, 2008; revision received March 14, 2008; accepted March 14, 2008
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Abstract |
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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 aryl hydrocarbon receptor 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 cDNA encoding an 85.3 kDa protein sharing 52-55 percent 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, AHR1
or AHR1β. AHRR mRNA was expressed at low levels in embryos (Nieuwkoop-Faber stage 33-38; approximately 52 h.p.f.) and was induced approximately 2-fold following TCDD exposure (42 ng/g wet weight). In contrast, AHRR exhibited higher constitutive expression and was induced more than 3-fold in tadpoles at stage 52-55 (prometamorphic; 
4 weeks post-fertilization) and in isolated viscera of stage 62 tadpoles (in the metamorphic climax; 7 weeks post-fertilization). 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.