CD24⁺ cells from hierarchically organized ovarian cancer are enriched in cancer stem cells

Cancer stem cells (CSCs) have been identified in solid tumors and cancer cell lines. In this study, we isolated a series of cancer cell clones, which were heterogeneous in growth rate, cell cycle distribution and expression profile of genes and proteins, from ovarian tumor specimens of a patient and identified a sub-population enriched for ovarian CSCs defined by CD24 phenotype. Experiments in vitro demonstrated CD24⁺ sub-population possessed stem cell-like characteristics of remaining quiescence and more chemoresistant compared with CD24^- fraction, as well as a specific capacity for self-renewal and differentiation. In addition, injection of 5 × 10 ³ CD24⁺ cells was able to form tumor xenografts in nude mice, whereas equal number of CD24^- cells remained nontumorigenic. We also found that CD24⁺ cells expressed higher mRNA levels of some stemness genes, including Nestin, Β-catenin, Bmi-1, Oct4, Oct3/4, Notch1 and Notch4 which were involved in modulating many functions of stem cells, and lower E-cadherin mRNA level than CD24 cells. Altogether, these observations suggest human ovarian tumor cells are organized as a hierarchy and CD24 demarcates an ovarian cancer-initiating cell population. These findings will have important clinical applications for developing effective therapeutic strategies to treat ovarian cancer.