Side population in MDA-MB-231 human breast cancer cells exhibits cancer stem cell-like properties without higher bone-metastatic potential.

An increasing body of evidence suggests that cancers contain a small subset of their own stem-like cells called cancer stem cells (CSCs), which play critical roles in the initiation, maintenance and relapse of tumors. However, the role of CSCs in cancer metastasis, especially in metastasis to bone, has not been extensively studied. Side population (SP) has been shown to enrich CSCs in several types of cancer, including breast cancer. In the present study, we characterized the SP cells isolated from the human breast cancer cell line MDA-MB-231 in comparison to non-SP (NSP). Fluorescence-activated cell sorter analysis demonstrated the existence of SP in MDA-MB-231 cells, which was markedly reduced in the presence of fumitremorgin C, a specific inhibitor of ATP-binding cassette sub-family G member 2 (ABCG2). Quantitative RT-PCR analysis showed that ABCG2 mRNA expression was significantly higher in SP cells than in NSP cells. SP cells formed increased numbers of tumor-spheres in suspension culture. Furthermore, the tumor growth in the orthotopic mammary fat pad in nude mice was significantly accelerated in SP cells. On the other hand, the development of bone metastases determined by intracardiac injection into nude mice showed no difference between SP and NSP cells. SP abundance in the tumor cells isolated from the bone metastases was not increased either compared with that from the mammary tumors. These results suggest that the SP in MDA-MB-231 cells possesses some of the CSC-like properties but does not have higher metastatic potential to bone.