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Polychlorinated biphenyls (PCBs) are persistent organic pollutants with human carcinogenicity. Many lower chlorinated and non-dioxin-like PCBs have been observed to be mutagenic following activation by human CYP2E1, while activation of dioxin-like (DL-) PCBs by this enzyme has never been evidenced. In this study, each DL-PCB was analyzed by molecular docking to human CYP2E1 protein for predicting a substrate interaction. All compounds demonstrated high affinities with the active site of human CYP2E1, binding energy being -8.7 ∼ -9.7 kcal/mol. However, most compounds demonstrated ligand-heme distances as ≥ 6.8 Å, while the values for 2,3,3',4,4'- (PCB 105) and 2,3',4,4',5-pentachlorobiphenyl (PCB 118) were 5.3 and 5.4 Å, respectively (valid for electron transfer). Experimentally, both PCB 105 and 118 induced micronuclei in a V79-derived cell line engineered for expression of human CYP2E1 at low micromolar concentrations, while inactive or weakly positive in V79-Mz control cells; these effects were blocked or reduced by 1-aminobenzotriazole, a suicide CYP inhibitor. However, DL-PCBs 77, 81 and 126 were all negative in both cell lines. In a human hepatoma (C3A) cell line, PCB 105 and 118 induced micronuclei marginally, while with ethanol pretreatment (to stabilize CYP2E1) both compounds induced micronuclei efficiently, and co-exposure to trans-1,2-dichloroethylene (a selective CYP2E1 inhibitor) led to clearly negative results with both compounds. Finally, both PCB 105 and 118 induced PIG-A gene mutations in C3A cells, which was blocked by trans-1,2-dichloroethylene. In summary, in silico and experimental results consistently suggest that DL- PCBs 105 and 118 may be activated by human CYP2E1 for mutagenic activities., Competing Interests: Declaration of Competing Interest The authors declare that they have no conflict of interest., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Polychlorinated biphenyls (PCBs) are persistent organic pollutants and human carcinogens. It was reported that rat CYP1A1 and catfish CYP1A can hydroxylate 3,3',4,4',5-pentachlorobiphenyl (PCB 126) and 3,3',4,4'-tetrachlorobiphenyl (PCB 77), while potential roles of other CYP1 enzymes in the metabolism of dioxin-like (DL) PCBs remain unconfirmed. In this study, three representative DL-PCBs, i.e., PCB 77, PCB 126, and 3,4,4',5-tetrachlorobiphenyl (PCB 81), were investigated on their genotoxicity in Chinese hamster V79-derived cell lines genetically engineered for the expression of human CYP1A1, 1A2 and 1B1, and in the human hepatoma C3A cell line, which endogenously expresses various CYPs. Under both 6 h/18 h and 18 h/6 h (exposure/recovery) regimes, PCB 77 and 81 induced micronuclei in V79-hCYP1B1 cells at micromolar levels, with slightly higher potency in the latter regime, while they were inactive in the parental V79-Mz cells and the V79-derived cell lines expressing human CYP1A1 and 1A2. However, PCB 126 was negative in each cell line. Likewise, PCB 77 and 81 induced micronuclei formation in C3A cells, which expressed CYP1B1. This effect was blocked by co-exposure to tetramethoxystilbene (30 nM), a selective CYP1B1 inhibitor. Immuno-fluorescent staining of centromere protein B in the micronuclei in PCB-treated cultures showed a predominance of centromere-negative micronuclei, which indicated a clastogenic effect. Moreover, all three PCBs elevated the level of γ-H2AX protein (indicating DNA double-strand breaks) in C3A cells, and these effects were blocked by tetramethoxystilbene (10 nM). This study demonstrates that some DL-PCBs are clastogenic in mammalian cells following metabolic activation by human CYP1B1., (Copyright © 2019 Elsevier B.V. All rights reserved.)