Breast cancer cell resistance to chemotherapeutic agents: Increased presence of stem/progenitor cell subtypes

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Introduction:
 Breast cancers resistant to chemotherapeutic agents pose a major clinical challenge. Breast cancer stem/progenitor cells have been proposed to be involved in chemoresistance. We studied two important characteristics of breast cancer stem/progenitor cells (ability to efflux Hoechst 33342 dye and expression of CD44+ and CD24-) in wild type sensitive and chemoresistant breast cancer cell lines in order to establish a better understanding of chemoresistance.
 Methods:
 MCF-7 breast cancer cells (adriamycin and paclitaxel sensitive and resistant sublines) were stained with Hoechst 33342, in the presence and absence of Verapamil (an ABC transporter inhibitor). Retention of Hoechst 33342 dye was detected by flow cytometry. Cells were also stained with CD44 conjugated with FITC and CD24 conjugated with PE. Expression of CD44 in the absence of CD24 was demonstrated by flow cytometry. All experiments were repeated at least three times.
 Results:
 Cells able to efflux Hoechst 33342 were identified as “side population cells” (SPCs). MCF-7 drug sensitive cells (MCF-7/WT) had 1.24 + 0.27 % of SPCs. With verapamil, SPCs were 0.75 + 0.34 %. MCF-7 adriamycin and paclitaxel resistant cells (MCF-7/ADR and MCF-7/PAC) had 75.70 + 10.04 and 44.95 + 4.80 % of SPCs, 44.79 + 20.11 and 28.50 + 6.45 % with verapamil. The percentage of CD44+ CD24- expressing cells in MCF-7/WT, MCF-7/ADR and MCF-7/PAC, were 0.05 + 0.03, 5.97 + 0.86 and 6.71 + 1.83, respectively.
 Conclusions:
 MCF-7 chemoresistant cell lines demonstrated key characteristics of breast cancer stem/progenitor cells - the ability to efflux Hoechst 33342 dye and expression of CD44 but not CD24. These findings suggest that breast cancer stem/progenitor cells may play an important role in chemoresistance.