Recent evidence supports the hypothesis that cancer stem cells are responsible for tumour initiation and formation. Using flow cytometry, we isolated a population of CD44 þ CD24 À prostate cells that display stem cell characteristics as well as gene expression patterns that predict overall survival in prostate cancer patients. CD44 þ CD24 À cells form colonies in soft agar and form tumours in NOD/SCID mice when as few as 100 cells are injected. Furthermore, CD44 þ CD24 À cells express genes known to be important in stem cell maintenance, such as BMI-1 and Oct-3/4. Moreover, we can maintain CD44 þ CD24 À prostate stem-like cells as nonadherent spheres in serum-replacement media without substantially shifting gene expression. Addition of serum results in adherence to plastic and shifts gene expression patterns to resemble the differentiated parental cells. Thus, we propose that CD44 þ CD24 À prostate cells are stem-like cells responsible for tumour initiation and we provide a genomic definition of these cells and the differentiated cells they give rise to. Furthermore, gene expression patterns of CD44 þ CD24 À cells have a genomic signature that is predictive of poor patient prognosis. Therefore, CD44 þ CD24 À LNCaP prostate cells offer an attractive model system to both explore the biology important to the maintenance and differentiation of prostate cancer stem cells as well as to develop the therapeutics, as the gene expression pattern in these cells is consistent with poor survival in prostate cancer patients.

Development of metastasis is a leading cause of cancer-induced death. Acquisition of an invasive tumor cell phenotype suggests loss of cell adhesion and basement membrane breakdown during a process termed epithelial-to-mesenchymal transition (EMT). Recently, cancer stem cells (CSC) were discovered to mediate solid tumor initiation and progression. Prostate CSCs are a subpopulation of CD44 + cells within the tumor that give rise to differentiated tumor cells and also self-renew. Using both primary and established prostate cancer cell lines, we tested the assumption that CSCs are more invasive. The ability of unsorted cells and CD44-positve and -negative subpopulations to undergo Matrigel invasion and EMT was evaluated, and the gene expression profiles of these cells were analyzed by microarray and a subset confirmed using QRT-PCR. Our data reveal that a subpopulation of CD44 + CSC-like cells invade Matrigel through EMT, while in contrast, CD44 -cells are noninvasive. Furthermore, the genomic profile of the invasive cells closely resembles that of CD44 + CD24 -prostate CSCs and shows evidence for increased Hedgehog signaling. Finally, invasive cells from DU145 and primary prostate cancer cells are more tumorigenic in NOD/SCID mice compared with non-invasive cells. Our data strongly suggest that basement membrane invasion, an early and necessary step in metastasis development, is mediated by these potential cancer stem cells.