ABSTRACT: Background: Cancer stem cells are presumed to have virtually unlimited proliferative and self-renewal abilities and to be highly resistant to chemotherapy, a feature that is associated with overexpression of ATP-binding cassette transporters. We investigated whether prolonged continuous selection of cells for drug resistance enriches cultures for cancer stem-like cells. Methods: Cancer stem cells were defined as CD44+/CD24– cells that could self-renew (ie, generate cells with the tumorigenic CD44+/CD24– phenotype), differentiate, invade, and form tumors in vivo. We used doxorubicin-selected MCF-7/ADR cells, weakly tumorigenic parental MCF-7 cells, and MCF-7/MDR, an MCF-7 subline with forced expression of ABCB1 protein. Cells were examined for cell surface markers and side-population fractions by microarray and flow cytometry, with in vitro invasion assays, and for ability to form mammospheres. Xenograft tumors were generated in mice to examine tumorigenicity (n = 52). The mRNA expression of multidrug resistance genes was examined in putative cancer stem cells and pathway analysis of statistically significantly differentially expressed genes was performed. All statistical tests were two-sided. Results: Pathway analysis showed that MCF-7/ADR cells express mRNAs from ABCB1 and other genes also found in breast cancer stem cells (eg, CD44, TGFB1, and SNAI1). MCF-7/ADR cells were highly invasive, formed mammospheres, and were tumorigenic in mice. In contrast to parental MCF-7 cells, more than 30% of MCF-7/ADR cells had a CD44+/CD24– phenotype, could self-renew, and differentiate (ie, produce CD44+/CD24– and CD44+/CD24+ cells), and overexpressed various multidrug resistance-linked genes (including ABCB1, CCNE1, and MMP9). MCF-7/ADR cells were statistically significantly more invasive in Matrigel than parental MCF-7 cells (MCF-7 cells = 0.82 cell per field and MCF-7/ADR = 7.51 cells per field, difference = 6.69 cells per field, 95% confidence interval = 4.82 to 8.55 cells per field, P<.001). No enrichment in the CD44+/CD24– or CD133+ population was detected in MCF-7/MDR. Conclusion: The cell population with cancer stem cell characteristics increased after prolonged continuous selection for doxorubicin resistance. PARALLEL study design with 4 samples Parental MCF-7 cell line versus Doxorubicin Resistant MCF-7 cell sublines Biological replicates: 2 parental controls, 2 drug resistant, independently grown and harvested. agent:Selection agent is multi-step doxorubicin selection: MCF7226ng, MCF7262ng biological replicate: MCF71, MCF72 biological replicate: MCF226ng, MCF7262ng
