Breast cancer stem-like cells self-renew, survive chemotherapy, and give rise to a hormone/growth factor controlled niche

Understanding dynamic interactions within the cellular hierarchies of carcinomas is key to developing novel therapies. Within many breast cancers, there exists a functional hierarchy of cells, the most primitive of which are thought to drive neoplastic growth by proliferating and differentiating into the diverse tumor cell subtypes. However, studying these primitive 'breast cancer stem-like cells' (BCSCs) has been difficult due to the opinion that freshly isolated cancer cells are the only means to study tumor repopulation. In order to address this problem, we set out to clarify whether functional hierarchies of differentiation are retained in human breast cancer cell lines.;Therefore, eight human breast cell lines (HMECs, MCF10A, MCF7, SUM149, SUM159, SUM1315, MDA.MB.231) were analyzed by flow cytometry for CD44, CD24 and ESA expression. Limiting dilution orthotopic injections were used to evaluate tumor-initiating activity, while serial colony-forming unit, differentiation, and tumorsphere assays were performed to assess self-renewal and differentiation. Pulse-chase BrdU-labeling was used to examine cell cycle kinetics and label-retention of cancer stem-like cells, and cultures were treated with paclitaxel (Taxol) and 5-fluorouracil (5-FU) to assess response to chemotherapy.;We found that cell lines can be classified into luminal, EMT-like, or mixed subtypes on the basis of CD44+/CD24- expression. While the percentage of CD44+/CD24- cells in cell lines did not correlate with tumorigenicity, as few as 100 cells of the CD44+/CD24-/low/ESA+ phenotype can form tumors. Further, we observed that CD44+/CD24 -/ESA+ cells self-renew, differentiate into the heterogeneous cell types of the parental cell line, retain BrdU label, and preferentially survive chemotherapy.;Furthermore, using a combination of cell biologic, proteomic and global transcriptional analyses, we used this cell line based model system to identify a novel FGF9-FGFR3-Tbx3 signaling mechanism that regulates breast cancer stem-like cell expansion in both ERalpha+ (luminal) and ERalpha - (basal) breast cancers. We show that in ERalpha+ breast cancers, estrogen stimulation expands BCSC pools through induction of FGF9-FGFR3-Tbx3 signaling. Further, FGF9-FGFR3-Tbx3 signaling in the non-BCSC populations is required for maintenance of the minority BCSC subpopulations in both ERalpha+ and ERalpha- breast cancers.;These findings demonstrate that BCSC biology is governed by signaling pathways that regulate normal mammary gland development and homeostasis. Furthermore, these data validate the use of cancer cell lines as models for the development and testing of novel therapeutics aimed at eradicating cancer stem-like cells.