Amino Acid Sequence of the Ligand-Binding Domain of the Aryl Hydrocarbon Receptor 1 Predicts Sensitivity of Wild Birds to Effects of Dioxin-Like Compounds

Abstract

The sensitivity of avian species to the toxic effects of dioxin-like compounds (DLCs) varies up to 1000-fold among species, and this variability has been associated with interspecies differences in aryl hydrocarbon receptor 1 ligand-binding domain (AHR1 LBD) sequence. We previously showed that LD(50) values, based on in ovo exposures to DLCs, were significantly correlated with in vitro EC(50) values obtained with a luciferase reporter gene (LRG) assay that measures AHR1-mediated induction of cytochrome P4501A in COS-7 cells transfected with avian AHR1 constructs. Those findings suggest that the AHR1 LBD sequence and the LRG assay can be used to predict avian species sensitivity to DLCs. In the present study, the AHR1 LBD sequences of 86 avian species were studied, and differences at amino acid sites 256, 257, 297, 324, 337, and 380 were identified. Site-directed mutagenesis, the LRG assay, and homology modeling highlighted the importance of each amino acid site in AHR1 sensitivity to 2,3,7,8-tetrachlorodibenzo-p-dioxin and other DLCs. The results of the study revealed that (1) only amino acids at sites 324 and 380 affect the sensitivity of AHR1 expression constructs of the 86 avian species to DLCs and (2) in vitro luciferase activity of AHR1 constructs containing only the LBD of the species of interest is significantly correlated (r (2) = 0.93, p < 0.0001) with in ovo toxicity data for those species. These results indicate promise for the use of AHR1 LBD amino acid sequences independently, or combined with the LRG assay, to predict avian species sensitivity to DLCs.

When evolution is the solution to pollution: Key principles, and lessons from rapid repeated adaptation of killifish (Fundulus heteroclitus) populations

“…Among HAHs, ligand affinity is a good predictor of toxicity; compounds that bind with greatest affinity are the most potent at altering gene expression and causing toxic effects (Safe, 1994). Differences in AHR properties, including affinity for ligand binding, explain differences in sensitivity to DLCs in several vertebrate systems, including strains of rodents (Moffat, Roblin, Harper, Okey, & Pohjanvirta, 2007; Poland, Palen, & Glover, 1994), species of birds (Farmahin et al., 2013; Karchner, Franks, Kennedy, & Hahn, 2006), and populations of some fish (Atlantic tomcod) (Wirgin et al., 2011). (At the extreme end of this spectrum are invertebrates, which have AHR proteins that lack the ability to bind DLCs [Powell‐Coffman, Bradfield, & Wood, 1998; Butler, Kelley, Powell, Hahn, & Van Beneden, 2001] and also are largely insensitive to DLC toxicity [Hahn, 1998a]. )…”

Measured and predicted affinities of binding and relative potencies to activate the AhR of PAHs and their alkylated analogues

“…However, key binding domains of AhR could be different by species. For avian species, amino acids sites at 324 and 380 in AhR are sensitively responsive to dioxin-like compounds (Farmahin et al, 2013).…”

“…Relative potencies of ligands to activate the AhR are directly proportional to the affinity with which they bind to the AhR (Kramer and Giesy, 1999). The greater the proportion of occupancy on the receptor, the greater probability of the transformed receptor-ligand complex interacting with the dioxin response enhancer (DRE) on DNA (Farmahin et al, 2013;Lee et al, 2013;Larsson et al, 2014).…”

Subfunctionalization of Paralogous Aryl Hydrocarbon Receptors from the FrogXenopus Laevis: Distinct Target Genes and Differential Responses to Specific Agonists in a Single Cell Type

“…AHR1 sequence signatures predict toxicity of dioxin-like compounds in birds (Farmahin et al, 2013). In contrast, AHR2 mediates TCDD toxicity in developing zebrafish (Prasch et al, 2003), even though TCDD is an in vitro AHR1B agonist (Karchner et al, 2005).…”