Molecular characterization and tissue-specific expression of the aryl hydrocarbon receptor in the Beluga, Delphinapterus leucas

The aryl hydrocarbon receptor (AHR) mediates the toxic and biochemical effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related planar halogenated aromatic hydrocarbons (PHAH). Differences in AHR expression or function may contribute to species-specific and tissue-specific differences in susceptibility to PHAH. Odontocetes (toothed whales) bioaccumulate some of the highest PHAH tissue concentrations reported. To ascertain the potential for PHAH effect at the biochemical level, we cloned, sequenced, and expressed an AHR from beluga liver. The 3.2 kb cDNA encodes an 845 amino acid protein with a predicted size of 95.5 kDa. The beluga AHR expressed by in vitro transcription and translation exhibits specific, high-affinity binding of [3H]-TCDD. We used several techniques to measure tissue-specific expression of the beluga AHR. Western blotting revealed a band of expected size in cytosols prepared from beluga liver, lung, thymus, and spleen, but not heart, kidney, or testis. Semi-quantitative analysis using RT-PCR detected expression of AHR transcripts in all tissues tested. Analysis of [3H]-TCDD binding by velocity sedimentation on sucrose gradients detected specific binding in liver and lung cytosols, and a lesser amount in heart. cDNA cloning of susceptibility genes coupled with in vitro expression and functional analysis of the encoded proteins may serve as a valuable approach to characterize the susceptibility of protected species to PHAH-induced toxicity. (NIEHS Grant ES06272 and Sea Grant NA46RG0470 (R/B-137)).