Evaluating the World Health Organization Toxic Equivalency Factors of Polychlorinated Dibenzo-p-dioxins, Dibenzofurans, and Biphenyls for Applicability to Amphibians using an In Vitro Ah Receptor Transactivation Assay

The World Health Organization (WHO) coordinated an expert meeting to streamline ecological risk assessments for complex mixtures of dioxin-like compounds (DLCs) by establishing 2,3,7,8-tetrachlorodibenzo-p-dixoin (2,3,7,8-TCDD) equivalency factors (TEFs) for mammals, fishes, and birds. No TEFs exist for amphibians, nor have any studies investigated whether the existing TEFs for mammals, fishes, or birds are suitable estimates of DLC potency for this taxon. Therefore, the objective of the present study was to determine the relative potency (ReP) for DLCs to assess the need for amphibian-specific TEFs. A total of 15 DLCs in addition to 2,3,7,8-TCDD were assessed in a standardized in vitro transactivation assay using COS-7 cells transfected with an expression construct for the aryl hydrocarbon receptor 1 (AHR1) from Xenopus laevis, a model amphibian. Two AHR1 paralogs (AHR1α/ahr.L and AHR1β/ahr.S) are expressed in X. laevis with comparable patterns of expression and comparable sensitivity to activation by DLCs, which suggests that RePs are independent of the specific AHR1 paralog. Compared to the corresponding WHO TEFs, 73% of RePs generated in this study for X. laevis are within 10-fold of the WHO TEFs for birds, which was more similar than comparisons to the WHO TEFs for mammals (33%) and fishes (33%). Three key amino acid residues (N325, A354, and A370) within the ligand binding domain of AHR1 are conserved at homologous positions in all 21 amphibian species with known AHR1 sequences, suggesting that RePs for X. laevis are likely applicable to most, if not all, other amphibians. Overall, this study suggests risk assessors can refer to WHO TEFs for birds when evaluating the toxicity of DLC mixtures to amphibians.