Enhancing ecological risk assessment of dioxins in aquatic environments: AHR diversity and species sensitivity differences in tiger puffer (Takifugu rubripes).

Dioxins and dioxin-like compounds (DLCs) exert toxicity through the aryl hydrocarbon receptor (AHR), but species variations in AHR lead to differing sensitivities. Investigating the variation in AHR homolog diversity, expression levels, predominant forms, and AHR sensitivity across species-particularly in fish sensitive to dioxins-is essential for enhancing ecological risk assessment. This study focuses on the tiger puffer (Takifugu rubripes), identifying five AHRs and two ARNTs, with truAHR2a showing the highest expression and the truAHR1 subfamily displaying lower levels. All truAHRs are functional and can be activated by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), with truARNT1 cooperating more efficiently with truAHRs than truARNT2. We determined EC50 values for truAHR1a (0.30 ± 0.10 nM), truAHR1b (0.32 ± 0.20 nM), truAHR2a (0.98 ± 0.63 nM), truAHR2b (2.62 ± 2.48 nM), and truAHR2c (0.43 ± 0.22 nM), with truAHR1a showing the highest sensitivity. The truAHR1 subfamily displayed greater sensitivity than the truAHR2 subfamily, contrasting with medaka and zebrafish, where AHR2 is similar to or more sensitive than AHR1. Comparisons highlighted species- and subform-specific sensitivities in AHRs, differing by one to two orders of magnitude. Ligand-binding assays showed that all truAHRs bound [3H]TCDD specifically. Molecular docking indicated that although TCDD binds AHRs with similar affinities and conserved residues, other subform-specific factors likely contribute to their differential sensitivities. This study provides valuable data on AHR diversity and ligand-sensitivity, contributing to ecological toxicity assessment of dioxin-like compounds.