Despite the presence of mutations in APC or beta-catenin, which are believed to activate the Wnt signalling cascade constitutively, most colorectal cancers show cellular heterogeneity when beta-catenin localization is analysed, indicating a more complex regulation of Wnt signalling. We explored this heterogeneity with a Wnt reporter construct and observed that high Wnt activity functionally designates the colon cancer stem cell (CSC) population. In adenocarcinomas, high activity of the Wnt pathway is observed preferentially in tumour cells located close to stromal myofibroblasts, indicating that Wnt activity and cancer stemness may be regulated by extrinsic cues. In agreement with this notion, myofibroblast-secreted factors, specifically hepatocyte growth factor, activate beta-catenin-dependent transcription and subsequently CSC clonogenicity. More significantly, myofibroblast-secreted factors also restore the CSC phenotype in more differentiated tumour cells both in vitro and in vivo. We therefore propose that stemness of colon cancer cells is in part orchestrated by the microenvironment and is a much more dynamic quality than previously expected that can be defined by high Wnt activity.

Glioblastoma is a highly angiogenetic malignancy, the neoformed vessels of which are thought to arise by sprouting of pre-existing brain capillaries. The recent demonstration that a population of glioblastoma stem-like cells (GSCs) maintains glioblastomas indicates that the progeny of these cells may not be confined to the neural lineage. Normal neural stem cells are able to differentiate into functional endothelial cells. The connection between neural stem cells and the endothelial compartment seems to be critical in glioblastoma, where cancer stem cells closely interact with the vascular niche and promote angiogenesis through the release of vascular endothelial growth factor (VEGF) and stromal-derived factor 1 (refs 5-9). Here we show that a variable number (range 20-90%, mean 60.7%) of endothelial cells in glioblastoma carry the same genomic alteration as tumour cells, indicating that a significant portion of the vascular endothelium has a neoplastic origin. The vascular endothelium contained a subset of tumorigenic cells that produced highly vascularized anaplastic tumours with areas of vasculogenic mimicry in immunocompromised mice. In vitro culture of GSCs in endothelial conditions generated progeny with phenotypic and functional features of endothelial cells. Likewise, orthotopic or subcutaneous injection of GSCs in immunocompromised mice produced tumour xenografts, the vessels of which were primarily composed of human endothelial cells. Selective targeting of endothelial cells generated by GSCs in mouse xenografts resulted in tumour reduction and degeneration, indicating the functional relevance of the GSC-derived endothelial vessels. These findings describe a new mechanism for tumour vasculogenesis and may explain the presence of cancer-derived endothelial-like cells in several malignancies.

A novel paradigm in tumor biology suggests that cancer growth is driven by stem-like cells within a tumor. Here, we describe the identification and characterization of such cells from colon carcinomas using the stem cell marker CD133 that accounts around 2% of the cells in human colon cancer. The CD133(+) cells grow in vitro as undifferentiated tumor spheroids, and they are both necessary and sufficient to initiate tumor growth in immunodeficient mice. Xenografts resemble the original human tumor maintaining the rare subpopulation of tumorigenic CD133(+) cells. Further analysis revealed that the CD133(+) cells produce and utilize IL-4 to protect themselves from apoptosis. Consistently, treatment with IL-4Ralpha antagonist or anti-IL-4 neutralizing antibody strongly enhances the antitumor efficacy of standard chemotherapeutic drugs through selective sensitization of CD133(+) cells. Our data suggest that colon tumor growth is dictated by stem-like cells that are treatment resistant due to the autocrine production of IL-4.

Tumors may be initiated and maintained by a cellular subcomponent that displays stem cell properties. We have used the expression of aldehyde dehydrogenase as assessed by the ALDEFLUOR assay to isolate and characterize cancer stem cell (CSC) populations in 33 cell lines derived from normal and malignant mammary tissue. Twenty-three of the 33 cell lines contained an ALDEFLUOR-positive population that displayed stem cell properties in vitro and in NOD/SCID xenografts. Gene expression profiling identified a 413-gene CSC profile that included genes known to play a role in stem cell function, as well as genes such as CXCR1/IL-8RA not previously known to play such a role. Recombinant interleukin-8 (IL-8) increased mammosphere formation and the ALDEFLUORpositive population in breast cancer cell lines. Finally, we show that ALDEFLUOR-positive cells are responsible for mediating metastasis. These studies confirm the hierarchical organization of immortalized cell lines, establish techniques that can facilitate the characterization of regulatory pathways of CSCs, and identify potential stem cell markers and therapeutic targets. [Cancer Res 2009;69(4):1302-13]

Colon carcinoma is one of the leading causes of death from cancer and is characterized by a heterogenic pool of cells with distinct differentiation patterns. Recently, it was reported that a population of undifferentiated cells from a primary tumor, so-called cancer stem cells (CSC), can reconstitute the original tumor on xenotransplantation. Here, we show that spheroid cultures of these colon CSCs contain expression of CD133, CD166, CD44, CD29, CD24, Lgr5, and nuclear β-catenin, which have all been suggested to mark the (cancer) stem cell population. More importantly, by using these spheroid cultures or freshly isolated tumor cells from multiple colon carcinomas, we now provide compelling evidence to indicate that the capacity to propagate a tumor with all differentiated progeny resides in a single CSC. Single-cell-cloned CSCs can form an adenocarcinoma on xenotransplantation but do not generate the stroma within these tumors. Moreover, they can self-renew and are capable of multilineage differentiation. Further analysis indicated that the lineage decision is dictated by phosphoinositide 3-kinase (PI3K) signaling in CSCs. These data support the hypothesis that tumor hierarchy can be traced back to a single CSC that contains multilineage differentiation capacity, and provides clues to the regulation of differentiation in colon cancers in vivo .

Cancer stem cells drive tumor formation and metastasis, but how they acquire metastatic traits is not well understood. Here, we show that all colorectal cancer stem cells (CR-CSCs) express CD44v6, which is required for their migration and generation of metastatic tumors. CD44v6 expression is low in primary tumors but demarcated clonogenic CR-CSC populations. Cytokines hepatocyte growth factor (HGF), osteopontin (OPN), and stromal-derived factor 1α (SDF-1), secreted from tumor associated cells, increase CD44v6 expression in CR-CSCs by activating the Wnt/β-catenin pathway, which promotes migration and metastasis. CD44v6(-) progenitor cells do not give rise to metastatic lesions but, when treated with cytokines, acquire CD44v6 expression and metastatic capacity. Importantly, phosphatidylinositol 3-kinase (PI3K) inhibition selectively killed CD44v6 CR-CSCs and reduced metastatic growth. In patient cohorts, low levels of CD44v6 predict increased probability of survival. Thus, the metastatic process in colorectal cancer is initiated by CSCs through the expression of CD44v6, which is both a functional biomarker and therapeutic target.

| The cancer stem cell (CSC) concept has important therapeutic implications, but its investigation has been hampered both by a lack of consistency in the terms used for these cells and by how they are defined. Evidence of their heterogeneous origins, frequencies and their genomic, as well as their phenotypic and functional, properties has added to the confusion and has fuelled new ideas and controversies. Participants in The Year 2011 Working Conference on CSCs met to review these issues and to propose a conceptual and practical framework for CSC terminology. More precise reporting of the parameters that are used to identify CSCs and to attribute responses to them is also recommended as key to accelerating an understanding of their biology and developing more effective methods for their eradication in patients. PERSPECTIVES NATURE REVIEWS | CANCER VOLUME 12 | NOVEMBER 2012 | 767

Purpose: To examine the role of cancer stem cells (CSC) in mediating metastasis in inflammatory breast cancer (IBC) and the association of these cells with patient outcome in this aggressive type of breast cancer.Experimental Design: CSCs were isolated from SUM149 and MARY-X, an IBC cell line and primary xenograft, by virtue of increased aldehyde dehydrogenase (ALDH) activity as assessed by the ALDEFLUOR assay. Invasion and metastasis of CSC populations were assessed by in vitro and mouse xenograft assays. Expression of ALDH1 was determined on a retrospective series of 109 IBC patients and this was correlated with histoclinical data. All statistical tests were two sided. Log-rank tests using Kaplan-Meier analysis were used to determine the correlation of ALDH1 expression with development of metastasis and patient outcome.Results: Both in vitro and xenograft assays showed that invasion and metastasis in IBC are mediated by a cellular component that displays ALDH activity. Furthermore, expression of ALDH1 in IBC was an independent predictive factor for early metastasis and decreased survival in this patient population.Conclusions: These results suggest that the metastatic, aggressive behavior of IBC may be mediated by a CSC component that displays ALDH enzymatic activity. ALDH1 expression represents the first independent prognostic marker to predict metastasis and poor patient outcome in IBC. The results illustrate how stem cell research can translate into clinical practice in the IBC field. Clin Cancer Res; 16(1); 45-55. ©2010 AACR.Inflammatory breast cancer (IBC) is an angioinvasive form of breast cancer associated with a high incidence of early nodal and systemic metastasis. In contrast to the recent decrease in breast cancer incidence in the United States, the annual incidence of IBC continues to increase (1, 2) with an attendant increase in mortality (3). Despite advances in the use of systemic chemotherapy, the prognosis of IBC remains considerably worse than that of other locally advanced breast cancers (1).Several molecular changes have been described in IBC including RHOC overexpression, hypomethylation of caveolin-1 or caveolin-2 promoters, and deletion of the tumor suppressor WISP3 (4-8). In addition, IBCs have been reported to overexpress E-cadherin/α, β-catenin, and angiogenic factors (4,7,(9)(10)(11)(12)(13)(14). Although each of these genetic changes may contribute to the metastatic nature of IBC, no markers have been described that can predict the development of systemic metastasis or survival in IBC patients. Although ERBB2 expression is associated with aggressive behavior in most breast cancers, this is not the case in IBC (15).There is increasing evidence that human breast cancers are driven by a tumor-initiating "cancer stem cell" (CSC) component that may contribute to tumor metastasis and therapeutic resistance (16)(17)(18)(19)(20). Breast CSCs were initially characterized as CD44 + /CD24 − /lin − cells that were capable of serial transplantation in nonobese/severe combined immunodefic...