Active dissociation of the fluorescent dye Hoechst 33342 from DNA in a living cell: who could do it?

A new human breast carcinoma cell line resistant to DNA-damaging drugs

To investigate the phenomenon of active dissociation of the vital dye, Hoechst 33342 (Ho342), from DNA (DNA clearing), a new MCF7HoeR-7 human breast carcinoma cell line was isolated from parent MCF7 cells by step-wise selection with increasing concentrations of Ho342. This cell line possesses an enhanced ability for DNA clearing. The MCF7HoeR-7 line is characterised in detail and compared with the parental MCF7 line and a typical P-glycoprotein-mediated multidrug resistant (MDR) cell line, MCF7/Adr. MCF7HoeR-7 cells have an increased population growth rate, a lower DNA content and a reduced number of chromosomes. Enhanced DNA clearing in MCF7HoeR-7 cells is associated with the high resistance of the cells to the toxic effects of Ho342 and cross-resistance to etoposide, a topoisomerase II inhibitor in clinical use. The MCF7HoeR-7 and parent MCF7 cell lines have similar expression levels of transport proteins. The results obtained confirm that DNA clearing is an atypical MDR mechanism in tumour cells.

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.