Side population cells and drug resistance in breast cancer

Several studies have demonstrated that the isolated side population (SP) cells from solid tumors exhibit cancer stem cell‑like properties, and are responsible for drug resistance during chemotherapy and tumor recurrence. In the current study, cancer stem cell‑like SP cells were isolated in the MDU‑22 breast cancer cell line using the Hoechst‑33342 dye exclusion technique. It was observed that SP cells accounted for 3.8% of cells in the MDU‑22 cell line, which was reduced to 0.6% in the presence of verapamil, an inhibitor of the ABC transporter. The sorted SP cells showed an elevated expression of stem cell markers, including ABCG2, OCT‑4 and EpCAM. Furthermore, it was demonstrated that the isolated SP cells undergo rapid proliferation and have a high survival rate. These results indicate that the coexpression of adenosine triphosphatase binding cassette transporters and stem cell surface markers in SP cells may contribute to chemoresistance, tumor recurrence, metastasis and invasion. Therefore, the isolation and characterization of SP cells may provide novel insights for the development of alternative therapeutic agents to target cancer stem cells.