Cytochrome P4501A1 induction by polychlorinated biphenyls (PCBs) in liver cell lines from rat and trout and the derivation of toxic equivalency factors

Toxic equivalency factors (potency of a compound relative to the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) standard) for polychlorinated biphenyls (PCBs) were derived from cytochrome P4501A1 induction, measured as ethoxyresorufin o-deethylase (EROD) activity, in a rainbow trout (Oncorhynchus mykiss) liver cell line (RTL-W1) and a rat hepatoma cell line (H4IIE). In RTL-W1 under normal growth conditions, EROD activity was induced only by PCBs 77, 81, 118, and 126. In overconfluent RTL-W1 cultures, EROD induction was also detected with PCBs 105, 156, and 169. Immunoblotting with the polyclonal antibody anti-trout CYP1A1(277-294)/KLH confirmed the results of EROD induction. The RTL-W1 toxic equivalency factors were 0.023 for PCB 126, 0.0064 for PCB 81, 0.0034 for PCB 77, 0.000 16 for PCB 169, and 0.000 017 for PCB 118. Toxic equivalency factors for PCBs 105 and 156 were <0.000 03. All seven PCBs clearly induced EROD activity in H4IIE. Generally the H4IIE toxic equivalency factors were higher than the RTL-W1 toxic equivalency factors. Therefore, the toxic impact of PCBs on rainbow trout probably would be overestimated by currently proposed risk assessment toxic equivalency factors and assessed more appropriately with toxic equivalency factors derived from rainbow trout.