Liver carcinogen aflatoxin B1 as an inducer of mitotic recombination in a human cell line

The mycotoxin aflatoxin B1 (AFB1) is one of the most potent rodent and human liver carcinogens. Upon cytochrome P450-specific metabolism, it induces mutations as well as mitotic recombination events in in vitro systems. We have found that in the lower eukaryote yeast, the recombinagenic activity of AFB1 surpasses its mutagenic activity, and we speculated on possible consequences in terms of the mechanism of liver carcinogenesis. In this study we investigated whether the recombinagenic activity of AFB1 also would be identified in human cells. To address this question, we followed the fate of a heterozygous thymidine kinase (tk) allele in the human lymphoblastoid cell line TK6 upon exposure to AFB1. Individual mutants that had lost tk activity were subjected to loss of heterozygosity analysis of the tk locus and its flanking markers. Fluorescence in situ hybridization analysis on chromosome 17 also was performed. In parallel, a similar analysis was performed on TK6 cells exposed to the alkylating agent N-nitrosomethylurea, a well-known classic point mutagen. Our analysis showed a difference in the molecular mechanism leading to inactivation of the tk allele upon exposure to these two mutagens. In AFB1-exposed cells the fraction of recombination-derived mutants predominated, whereas in N-nitrosomethylurea-exposed cells the fraction of point mutants was higher. Thus, the recombinagenic activity of AFB1 previously identified in a lower eukaryote also was found in the human cell line TK6. Our data support the hypothesis that mitotic recombination represents a central mechanism of action in AFB1-induced liver carcinogenesis.