ToxSci Advance Access published online on June 7, 2007
Toxicological Sciences, doi:10.1093/toxsci/kfm139
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Functional Characterization and Evolutionary History of two Aryl Hydrocarbon Receptor Isoforms (AhR1 and AhR2) from Avian Species
* Center for Marine Environmental Studies (CMES), Ehime University, Bunkyo-cho 2-5, Matsuyama 790-8577, Japan
Biology Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
Correspondence to: Eun-Young Kim, Center for Marine Environmental Studies (CMES), Ehime University, Bunkyo-cho 2-5, Matsuyama 790-8577, Japan, Telephone: +81-89-927-8172, Fax: +81-89-927-8172, E-mail: eykim{at}agr.ehime-u.ac.jp
Received March 14, 2007; revision received May 22, 2007; accepted May 23, 2007
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Abstract |
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Dioxins including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induce various toxic effects through the aryl hydrocarbon receptor (AhR) signaling pathway. Here, we investigated the structural and functional characteristics and molecular evolution of multiple AhRs in black-footed albatross (Phoebastria nigripes) and common cormorant (Phalacrocorax carbo). We report the cDNA sequences of two distinct AhRs, designated AhR1 and AhR2, from these species as well as the identification of an AhR2-like gene sequence from the chicken genome database. Phylogenetic analysis reveals that avian AhR1 and AhR2 are orthologous to mammalian AhR1 and fish AhR2, respectively, supporting the hypothesis that an ancestral AhR gene underwent a single duplication prior to the divergence of fish and tetrapod lineages. In vitro-expressed AhR1 and AhR2 isoforms from both albatross and cormorant exhibited specific binding to [3H]TCDD, as assessed by velocity sedimentation. An in vitro reporter gene transactivation assay revealed that both AhR1 and AhR2 are transcriptionally active, but AhR2 appears to have reduced transcriptional efficacy. Hepatic mRNA expression level of cormorant AhR1 was greater than that of AhR2. Together, these results suggest that AhR1 is the dominant form of avian AhRs, in contrast to fish, in which AhR2 is the major form. Comparative analysis of AhR diversity and gene synteny among chicken, zebrafish and human suggests that additional, independent AhR duplications have occurred in the fish and tetrapod lineages following the initial tandem duplication on the ancestral chromosome. The identification and characterization of avian AhR1 and AhR2 provide new insight into the evolution of AhR structure and function in vertebrates.
Key Words: aryl hydrocarbon receptor 2 (AhR2); chromosomal duplication; specific binding; transcriptional activation; black-footed albatross; common cormorant.