A new mechanism of mammalian cell resistance to the toxic effects of DNA-binding agents

The phenomenon of active dissociation of the noncovalently binding vital dye Hoechst 33342 from DNA in living cells (DNA clearing) is described. Step-by-step selection with increasing concentrations of the dye resulted in a series of rodent cell lines that were resistant to the toxic action of Hoechst 33342. Some of the lines exhibited enhanced dissociation of the bisbenzimidazole dye-DNA complex. Two cell lines from this group (AA8HoeR-7 and LHoeR-3) were analysed in detail and compared with a Syrian hamster tumour cell line, a typical example of mdr-I-mediated multidrug-resistant cell lines. The markedly enhanced level of DNA clearing in AA8HoeR-7 and LHoeR-3 cells leads to high cellular resistance to the toxic effect of Hoechst 33343 and cross-resistance to mitomycin C, a minor-groove alkylating agent in clinical use. Our results suggest that DNA clearing is one of the mechanisms of multidrug resistance in tumour cells.