Responsiveness of a Xenopus laevis cell line to the aryl hydrocarbon receptor ligands 6-formylindolo[3,2-b]carbazole (FICZ) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

The aryl hydrocarbon receptor (AHR) mediates the toxic effects of environmental contaminants, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Frogs are very insensitive to TCDD toxicity, and AHRs from Xenopus laevis (African clawed frog) bind TCDD with >20-fold lower affinity than mouse AHRb-1. Frog AHRs may nonetheless be highly responsive to structurally distinct compounds, especially putative endogenous ligands. We sought to determine the responsiveness of an X. laevis cell line, XLK-WG, to the candidate endogenous AHR ligand 6-formylindolo[3,2-b]carbazole (FICZ), a tryptophan photoproduct that exhibits high potency in mammalian systems. FICZ readily induced mRNAs for CYP1A6 and CYP1A7. Cells exposed to FICZ for 3 h expressed up to 5-fold greater quantities of CYP1A6/7 mRNAs than those exposed for 24 h, suggesting FICZ is metabolized following rapid enzyme induction. FICZ appeared more potent than TCDD. Following a 3-h exposure, the EC50 for CYP1A6 mRNA induction by FICZ was ∼6 nM, while the TCDD response was greater than 174 nM. These potencies were lower than those determined for mouse hepatoma cells (Hepa1c1c7; EC50 = ∼0.06 nM each). The difference in ligand potency between cell lines was confirmed by induction of ethoxyresorufin-O-deethylase (EROD) activity. mRNA from XLK-WG cells treated with 100 nM FICZ, 100 nM TCDD, or vehicle was also analyzed on expression microarrays. FICZ altered the expression of 105 more transcripts than TCDD, and common targets were altered more dramatically by FICZ. Overall, these studies demonstrate that although FICZ is a less potent CYP1A inducer in frog cells than in mouse cells, the reduction is much less than for TCDD. Relative conservation of the FICZ response in a TCDD-insensitive species suggests its physiological importance as an AHR ligand.