The TEF system for dioxin-like compounds has included assignment of TEF values for mono-ortho polychlorinated biphenyls (MO-PCBs). Small traces of aryl hydrocarbon receptor (AhR)-active impurities could result in artifactually higher relative potency (REP) values. MO-PCBs -105, -118, -156, and -167 were purified on an active charcoal column to remove AhR agonists that could be present as impurities. Activation or inhibition of AhR-dependent gene expression by purified MO-PCBs was studied in stably transfected cell lines (H1G1.1c3 mouse, H4G1.1c2 rat hepatoma), containing an AhR-responsive (AhR-EGFP) reporter gene. In addition, EROD activity was used as marker for CYP1A1 activity in these cell lines. MO-PCBs -105, -118, -156 induced AhR-EGFP expression in both rodent cell lines, with PCB-156 (10 μM) being most effectively; inducing gene expression to ∼27% of TCDD (mouse cells) and 62.5 ± 3.4% (rat cells) of TCDD. This concurred with increased EROD activity in both cell lines to maxima of 20.5 ± 1.5% and 68 ± 3.2% of TCDD, respectively. No induction was observed for PCB-167. In the H1G1.1c3 mouse cells, PCB-105, -118 and -156 (10 μM) significantly reduced TCDD-induced AhR-EGFP expression to 50.9 ± 2.9%, 58.3 ± 2.2% and 70.8 ± 1.3% of TCDD. Reduced EROD activity was also observed, of 39.3 ± 2.8%, 67 ± 5% and 48.3 ± 4% compared to TCDD. PCB-167 did not result in significant reduction. In rat cells, only PCB-156 resulted in significant decrease in TCDD-induced AhR-EGFP expression of 35%, suggesting species differences play a role. Our results suggest that purification of MO-PCBs is an essential step in determining accurate REP values, and could very likely lead to lower TEF values than those presently assigned by the WHO.
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