Metabolites of chemicals can often be ultimate genotoxic species; thus, in vitro routine testing requires the use of rat liver S9. However, there is a question as to whether this represents an appropriate surrogate for human metabolism. We have previously demonstrated the usefulness of HepG2 transformants expressing major human cytochrome P450 (CYP) isoforms to assess the genotoxicity of metabolites. We further assessed the advantages of these transformants from the following three aspects. First, the sensitivity of these transformants was confirmed with micronucleus (MN) induction by 7,12-dimethylbenz[a]anthracene or ifosfamide in transformants expressing the corresponding CYP1A1 or CYP2B6 and CYP2C9, respectively. Second, by using these transformants, beta-endosulfan, a chemical for which the CYP isoforms contributing to its genotoxicity are unknown, was found to induce MN through the CYP3A4-mediated pathway. This result was confirmed by the facts that the decreased CYP3A4 activity using a inhibitor or short interfering RNA (siRNA) repressed MN induction by beta-endosulfan and that endosulfan sulfate, one of the metabolites produced by CYP3A4, induced MN in the transformants harboring an empty vector. Third, the interaction between phase I and II drug-metabolizing enzymes was demonstrated by MN induction with inhibitors of uridine diphosphate (UDP)-glucuronosyltransferases in tamoxifen-treated transformants harboring the corresponding CYP3A4 or with inhibitors of glutathione S-transferase in safrole-treated transformants harboring the corresponding CYP2D6, whereas neither tamoxifen nor safrole alone induced MN in any transformant. These advantages provide the benefits of newly established transformants for in vitro genotoxicity testing that reflects comprehensive metabolic pathways including not only human CYP isoforms but also the phase II enzymes.
We previously reported that the proportion of large-size micronuclei (MN) can be a reliable parameter to discriminate aneugens from clastogens in the in vitro MN assay using Chinese hamster lung cells. The frequencies of polynuclear (PN) and mitotic (M) cells are also supposed to be useful parameters for the same purpose since they are known to be increased by aneugens. In the present study, we investigated whether morphological observations of the cell nucleus can be applied for the in vitro MN assay using the p53-competent human lymphoblastoid cell line, TK6 cells. Our present MN assay with six clastogens and six aneugens revealed that the frequencies of large-size MN or PN cells cannot distinguish aneugens from clastogens, while the frequencies of M cells can distinguish them, suggesting that the M-cell frequency is a recommended parameter to determine a mode of action for MN induction in the in vitro MN assay using TK6 cells. Our further investigation using p53-null mutant NH32 cells showed that the frequencies of large-size MN or PN cells induced by aneugen treatments were higher than those in TK6 cells but not by clastogen treatments. These findings suggest that p53 abrogation promotes the susceptibility for morphological changes in the nucleus to aneugens and that morphological observation of the cell nucleus including size-classifying MN counting could distinguish aneugens from clastogens in the MN assay using NH32 cells.
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