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.