Stem -like cancer cells in human cell lines and xenografts: Identification and characterization

Recently, various human tumors have been shown to contain stem-like cells called cancer stem cells. These cells, although rare, appear to be important in tumor maintenance and metastasis. Cancer stem cells have been identified in human leukemia, colon, breast, and brain cancers. Importantly, stem-like cells are also found to be present in cell lines and xenografts, which were previously thought to be homogenous. In this study, we demonstrate that cells within a tumor are organized as a hierarchy comprising a spectrum of cells at various stages of differentiation. Cells in the side population (SP) isolated from prostate cancer (PCa) and other cell lines are about a 1000 times more tumorigenic than the corresponding non-SP cells. Moreover, SP cells can generate non-SP cells, be serially transplanted, and preferentially express genes known to regulate critical stem cell-properties. The SP phenotype is most likely mediated by a combination of ABC transporters since the use of ABCG2 as a marker by itself appears to identify fast-cycling tumor progenitors. We next focus on the identification of stem-like cells in PCa using CD44 as a marker. The CD44+ cells are 10-100 times more tumorigenic than CD44 - cells, suggesting that the CD44+ population is enriched in tumorigenic stem/progenitor cells. CD44+ cells are also more invasive and metastatic, and have certain intrinsic properties of stem/progenitor cells. We then use α2β1, a normal prostate stem cell marker, in an attempt to further enrich for PCa stem/progenitor cells. Like ABCG2, α2β1 does not identify more primitive cells but marks highly proliferative progenitor cells. In combination with CD44, however, we begin to notice a subtle but distinct hierarchy, with CD44+α2β1+/hi cells showing the highest tumorigenicity. Taken together, our data show that solid tumors like PCa contain multiple populations of tumorigenic cells, each of which can initiate tumors with varying efficiencies. This information can serve as groundwork for the identification, and eventual eradication of tumor-initiating PCa stem/progenitor cells in primary patient tumors.