Distinct spectrum of mutations induced by crocidolite asbestos: clue for 8-hydroxydeoxyguanosine-dependent mutagenesis in vivo.

DNA damage due to reactive oxygen or nitrogen species is proposed to be involved in the molecular mechanism of asbestos-induced carcinogenicity. However, indications for this hypothesis came mainly from in vitro assays using cultured cells or cell-free systems. In the present study, the mutagenicity of crocidolite fibers and the underlying molecular mechanisms were investigated in vivo. Mutation frequencies were determined in DNA of omenta, a relevant target tissue for mesothelioma carcinogenesis, using lacI transgenic rats. The mutagenic effect of 2 and 5 mg of crocidolite asbestos was demonstrated, with a maximal relative increase in mutation frequency of 3.4 compared with the control group. The molecular analysis of the mutations revealed striking differences according to mutation types between asbestos-induced mutations and spontaneous mutations. Therefore, a specific molecular mechanism induced by crocidolite that differs from that induced by the generation of spontaneous mutations can be proposed. G to T transversions, which are known to be induced by the premutagenic DNA adduct 8-hydroxydeoxyguanosine (8-OHdG), were the most prominent mutation type (29%) within crocidolite-induced mutations. In additional experiments, 8-OHdG in DNA of omenta from rats treated with 1 or 2 mg of crocidolite asbestos was determined. Levels of 8-OHdG in animals treated with crocidolite were significantly increased compared with negative controls. These data give strong evidence for the involvement of reactive oxygen or nitrogen species in crocidolite-induced mutagenesis in vivo.