Spontaneous expulsion of micronuclei by enucleation in the micronucleus assay

The effect of enucleation on the frequency of micronuclei induced by mitomycin C (MMC) and vincristine (VCR) was examined in mouse L-929 cells enucleated with cytochalasin B (Cyt-B). Approximately 30% of the L-929 cells became enucleated cells during the 8-h incubation in medium containing 8 micrograms/ml of Cyt-B. Using this enucleation technique, we estimated the reduction rate of 2 mutagen-induced micronuclei by enucleation. Treatment with MMC caused a dose-dependent induction of micronuclei in L-929 cells, with the reduction rate being 38.6% at the lowest dosage (0.0125 microgram/ml), which induced mostly mono-micronuclei in L-929 cells, and 6.8% at the highest dosage (0.1 microgram/ml), which induced many multi-micronuclei. Furthermore, VCR also induced micronuclei in a dose-dependent way in L-929 cells, and the same tendency for micronucleus reduction as with MMC was observed. The reduction rate of micronucleated cells by enucleation was estimated to be about 31-39% when the micronucleated cells contain mono-micronuclei. Therefore, the rate of reduction is affected by the number of micronuclei per cell, and the reduction depends on the increase in the number of micronuclei per cell.