Molecular Basis for Differential Dioxin Sensitivity in Birds: Characterization of Avian AHR Isoforms

Our previous study demonstrated that avian species possesses two distinct aryl hydrocarbon receptors (AHR1 and AHR2). To elucidate the functional characterization of avian AHRs, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) binding affinities and in vitro DRC-induced transcriptional activities of AHR isoforms from chicken (Gallus gallus), black-footed albatross (Phoebastria nigripes), common cormorant (Phalacrocorax carbo) and jungle crow (Corvus macrorhynchos) were investigated. All avian AHR isoforms exhibit specific binding affinity to [3H]TCDD, as assessed by velocity sedimentation. An in vitro reporter gene transactivation assay revealed that AHR2s from chicken, albatross and cormorant had reduced transcriptional efficiencies to TCDD as compared to AHR1s from the corresponding species. Particularly, chicken AHR2 exhibited dramatically reduced transactivation. In contrast, the transcriptional ability of crow AHR2 was almost comparable to that of AHR1. One possible reason accounting for this difference in transactivation may be the structural difference in C-terminal halves. Furthermore, tissue expression profiles of AHR isoform mRNAs in cormorant and crow revealed that AHR1s were expressed in most tissues, whereas AHR2s were detectable in limited tissues. However, comparison of hepatic mRNA expression levels of two AHR isoforms showed a species difference; AHR1 is dominant than AHR2 in cormorant, in contrast to crow, in which AHR2 is the major form. These results suggest that function of AHR isoforms mediating the response to DRC exposure is diverse even within avian species.