Time‐dependent relative potency factors for polycyclic aromatic hydrocarbons and their derivatives in the H4IIE‐luc bioassay

Abstract

The H4IIE-luc transactivation bioassay for aryl hydrocarbon receptor (AhR) agonists was used to investigate the relative potency factors (REPs) of 22 individual polycyclic aromatic hydrocarbons (PAHs) and their oxygenated-, methylated-, and N-containing derivatives (azaarenes), which are often present in PAH-contaminated soils. Naphthacene and dibenz[ah]acridine exhibited greater AhR-mediated potency, whereas lesser molecular-weight azaarenes were less potent AhR agonists. Six oxygenated PAHs had calculable REPs, but their potencies were less than their parent PAHs. Unlike the parent, unsubstituted PAHs, oxidation of methylated PAHs seemed to increase the AhR-mediated potency of the compounds, with 2-methylanthracene-9,10-dione being almost 2 times more potent than 2-methylanthracene. Both bioassay and gas chromatography-mass spectrometry analysis were used to examine the exposure time-dependent effects on the REPs at 24 h, 48 h, and 72 h of exposure in the H4IIE-luc transactivation bioassay. Changes in concentrations of 5 compounds including the model reference 2,3,7,8-tetrachlorodibenzo-p-dioxin in the cell culture wells were measured, and the amounts in the cell medium, in the cells, and adsorbed to the wells was determined and the influence on the REPs was studied. Declining REP values with increased duration of exposure were shown for all compounds, which we concluded were a consequence of the metabolism of PAHs and PAH derivatives in H4IIe-luc cells. The present study provides new knowledge regarding the degradation and distribution of compounds in the wells during exposure.

Methylated PACs are more potent than their parent compounds: A study of aryl hydrocarbon receptor–mediated activity, degradability, and mixture interactions in the H4IIE‐luc assay

“…The decline in REPs or luciferase activity with increased exposure duration is assumed to be a consequence of metabolic degradation of the PAHs in the cells, resulting from the induction of CYP1A1. This assumption is supported by the findings of previous studies (Jones et al ; Larsson et al ). However, declining REPs can also result from differences in cell density in the wells, potential changes in cell activity because of crowding, or a loss of PACs due to evaporation or adsorption to the plastic wells.…”

“…However, the other O ‐heterocyclic compound tested in the present study (benzo[ b ]naphtho[2,1‐ d ]furan) was more than 10 4 times less potent than was dinaphtho[1,2‐ b ;1′,2′‐ d ]furan at all exposure durations. Comparison of benzo[ b ]naphtho[2,1‐ d ]furan with the parent compound benzo[ a ]fluorene (Larsson et al ) showed that benzo[ b ]naphtho[2,1‐ d ]furan was significantly less potent than its parent PAH and the oxygenated PAH benzo[ a ]fluorenone (Larsson et al ) after 24 h of exposure ( p < 0.5, Student's t test). Dibenzothiophene and its methylated derivatives were also weak AhR inducers.…”

“…Multiple point estimates over the range of effect concentrations of EC20 to EC80 can be used to identify uncertainties in the REP estimates (Villeneuve et al ). The REPs of derivatives were compared with the REPs of parent compounds and derivatives from previous studies (Larsson et al , ).…”

“…It has been reported to be an accurate reference model to predict toxicities of mixtures of chemicals that have the same or a similar mode of action (Faust et al 2001;Backhaus et al 2004). Additive behavior among PAHs has been reported previously (Larsson et al 2012(Larsson et al , 2014b.…”

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

“…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).…”