Cancer stem cell model and hypothesis has greatly changed the biological and clinical views of cancer [1
]. The molecular profiles of purified CD133+ GBM stem cells derived from the previously treated recurrent tumors characterized dormant-like cells and therefore support the hypothesis that quiescent nature of CD133+ GBM stem cells may underline the treatment resistant to the conventional therapy. The quiescent nature of cancer stem cells has been described in chronic myeloid leukemia (CML), where CML stem cells remain viable in a quiescent state even in the presence of growth factors and tyrosine kinase inhibitor [55
]. Indeed, while the genetic changes and tumorigenic potential were demonstrated in purified CD133+ GBM cells sorted from the CD133+ GBM spheres, the molecular profile characterized an antiproliferative nature of CD133+ GBM stem cells, suggesting the pathologic effects of molecular changes to be manifested primarily in a more differentiated progeny. Indeed, the molecular profiles of CD133+ GBM spheres (contain majority of CD133− daughter cells) initiated by the purified CD133+ GBM cells express “proliferative tumor markers” as that of CD133− GBM daughter cells, which distinctive to the quiescent CD133+ GBM daughter cells. Thus, the more differentiated CD133− GBM progeny should be considered as the true effector cells characterized fast-growing and highly angiogenic GBM tumors. The mechanisms and pathways underlying the spontaneous re-entry into active cell cycle from the quiescent state in cultures and in animal experiments remain to be elucidated [57
]. In contrast to that fast-growing CD133− daughter cells, the predicted slow-cycling, non-inflammatory, and non-angiogenic nature of CD133+ GBM cells (based on the molecular profiles) may explain that GBM tumor can not be eradicated by the anti-cell cycle-based radiochemotherapy, anti-inflammatory drugs, or antiangiogenic agents.
A hallmark of all stem cells is the ability to simultaneously make identical copies of themselves (e.g. CD133+ GBM daughter cells) and give rise to a hierarchy of more differentiated progeny (e.g. CD133− GBM daughter cells). Indeed, CD133+ GBM cells fulfill this definition and are capable of undergoing cell division that give rise to a malignant tumor tissue. GBM spheres initiated with one single CD133+ GBM cell contain heterogeneous population that showed differences in cell size and proliferative potential. By RT-PCR analysis, we were able to amplify both Numb and Numb-like signals in CD133+ GBM cells (data not shown), suggesting CD133+ GBM cells may possess normal neuroepithelial-like properties capable of undergoing asymmetric cell divisions, thereby maintaining a tumor-suppressor-like phenotype [58
]. Prominin/CD133 is selectively localized in protrusions of the apical membrane in neuroepithelial cells, and it was suggested that CD133 plays an important role in the maintenance of apical-basal polarity [59
]. Therefore, loss of CD133 (or with other genes) may restrict CD133− GBM daughter cells to the symmetric mode of cell division and act like proliferative intermediate progenitor cells [60
]. Consequently, increasing the number of CD133+ GBM cells within the tumor would reflect a fast generation of proliferative and angiogenic CD133− daughter cells to form the bulk tumor [5
]. While our data indicated that GBM tumor growth in these two study cases depends on the CD133+ GBM cells, GBM tumor growth seemed to depend on CD133− GBM tumor-initiating cells for other cases [63
]. The isolation of CD133+ GBM cells from CD133− GBM cell-initiating tumor was also reported [65
], indicating the CD133 is not an obligated marker for GBM stem cells.
We previously showed that GBM tumor lines established from recurrent tumors possess mesenchymal differentiation potential [29
]. A recent study further showed that GBM stem cells formed tumors capable of undergoing mesenchymal differentiation [64
]. The expression of mesenchymal developmental genes in CD133+ GBM stem cells may therefore provide a potential explanation for the chondrogenic/mesenchymal differentiation in tumors [66
] and the shifting toward the mesenchymal phenotype upon tumor recurrence [49
]. Moreover, based on the molecular profiles of purified CD133+ GBM cells, we hypothesize that the cell-of-origin of GBM tumors may be the migratory neural crest-like cell or radial glial-like cell. Overexpression of radial glial cell (RGC) marker, FABP7, in CD133+ GBM cells supports the recent finding that RGC can give rise to adult subventricular zone stem cells [67
]. On the other hand, although overexpression of MYCN is associated with a childhood malignant tumor of neural crest origin, it was recently found to be one of the most frequently amplified oncogenes in GBM tumors (42%) [68
]. More samples should be analyzed in order to generalize these observations. Moreover, whether these molecular properties also applied in the general properties of CD133+ GBM cells derived from the non-treated tumor remain to be investigated. Gene expression profile analyses of GBM tumors by DNA microarrays support the notion that tumor development may indeed via distinct oncogenic mechanisms among the GBM subtypes [26
]. The thought of heterogeneity in the pathway of GBM tumor development is further supported by the recent finding in studies of expression profiles of GBM sphere cultures, which showed distinct molecular properties among the GBM stem cell lines [51
]. Likewise, although CD133+ GBM cells sorted from CD133+ D431 and CD133+ S496 spheres express shared molecular properties, unsupervised gene and sample clustering segregated two GBM sphere lines by the genes that are associated with mesenchymal developmental pathway versus neural developmental pathway, thus implying the distinct cellular-origin of these two recurrent tumors.
In summary, we characterized CD133+ GBM stem cells isolated from two tumors that are recurrent and had previous treatment. Our in vitro and in vivo data suggest that the tumorigenic CD133+ GBM stem cells are maintained at dormant-like stage state but are able to spontaneously enter the proliferative cell cycle to generate highly proliferative and angiogenic CD133− GBM daughter cells (animal data). This observation implies that tumorigenesis may be initiated through asymmetric cell division of CD133+ GBM cells [71
]. Thus, identifying the genes and pathways that promote the CD133+ GBM cells entering proliferative cell division cycle may facilitate the development and design of more effective therapies that specifically target the tumorigenic potential of CD133+ GBM stem cells.