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-1-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 33342 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.