Perhaps the most challenging issue facing the field is the fact that CSCs in primary tumors do not always display the properties classically used to define normal stem cells, cells with the ability to self-renew and -differentiate into multiple cell types. The cell-surface immunophenotype of primary tumors, as well as the frequency of functionally defined CSCs, can vary dramatically among different patients. In some cases, CSCs are relatively rare, whereas in others CSCs can constitute a substantial proportion of the tumor mass (12
). So, why are tumor CSCs so variable? In normal steady-state systems, one can expect reasonably well-conserved development behavior, but upon any kind of substantial genetic or epigenetic perturbation, the rules that define cell and tissue behavior are not easily predicted and must be defined empirically. Thus, in the context of inherently unstable conditions such as cancer, the fact that CSCs display varying behaviors is no surprise. Moreover, the properties of CSCs appear to be influenced by both the specific genetic aberrancies in a given tumor as well as the stage of disease progression and the types of drugs used to challenge tumor growth (). Consequently, for any particular type of cancer the patient-to-patient variability of CSCs may be quite substantial. Taken together, these issues make any consistent definition of CSC properties difficult and suggest that being overly rigid in how CSCs are defined is not realistic. Furthermore, the variability in CSC properties introduces problems when developing new therapies.
Fig. 3 Stages of CSC evolution. For many tumor types, the de novo mutations leading to primary CSC are varied. Thus, one would expect that the biology of primary CSCs may also be heterogeneous. Properties such as CSC frequency, cell-surface phenotype, and drug (more ...)
A genetic program that might account in part for the diversity of abundance of CSCs in solid tumors is the ability of cells to undergo an epithelial-to-mesenchymal transition (EMT). Recent studies have suggested that induction of EMT in immortalized human mammary epithelia cells results in cells with stem-like properties, such as “ an increased ability to form mammo-spheres, a property associated with mammary epithelial stem cells” (15
). A number of studies have suggested that cells at the leading invasive edge of solid cancers, such as colon, breast, and pancreatic tumors, exhibit more mesenchymal features and are characterized by the expression of CSC markers (16
). Thus, there has been a convergence of the well-established concept that EMT is associated with tumor progression with the more recent hypothesis of migratory cancer stem cells (19
). So how might this process be regulated in CSCs? Recent studies have implicated transcription factors, such as Zeb1 and −2 and Twist, which are known inducers of EMT (21
), and a negative feedback pathway involving transforming growth factor–β (TGFβ) (16
). So, changes in the surrounding microenvironment that influence expression of TGFβ family members as well as other cytokines expressed by mesenchymal stem cells or other cells in the microenvironment may influence both EMT and the reverse process of mesenchymal-to-epithelial transition, which is most likely critical for metastasis and colonization at distant sites (23
). If true, then the CSC state may be transitory in certain circumstances, with tumor cells acquiring more of a stem-like phenotype upon stimulation with the appropriate environmental cues.
Studies of putative tumor stem cells have also served to highlight the potential clinical importance of the relationship between EMT and CSCs. For example, using the same sub-population of tumorigenic breast cancer cells identified in (10
), we reported the intrinsic resistance of these cells to chemotherapy studied in paired breast cancer biopsies (24
). We also identified a TGFβ-like tumorigenic gene signature in a molecular subtype of human breast tumors characterized by expression of many mesenchymal-associated genes. Tumors resistant to conventional treatments were enriched for cells bearing this signature, and increased mesenchymal markers were observed in the posttreatment specimens. These data support a growing body of evidence for a mesenchymal-like phenotype in breast and possibly other solid tumors that may be responsible for invasion, metastases, and even treatment resistance.
To understand the relationship between CSCs and normal stem cells, bioinformatic studies compared the transcriptional programs in embryonic stem cells (ESCs) with adult tissue stem cells and human cancers (21
). The ESC-like transcriptional program shown to be activated in diverse human epithelial cancers was a predictor of metastasis and death. The oncogene c-Myc, but not other oncogenes, appeared to be sufficient to activate this ESC-like program and could increase the fraction of CSCs. These authors concluded that “Activation of an ESC-like transcriptional program in differentiated adult cells may induce pathologic self-renewal characteristic of cancer stem cells.” Further, Weinberg and colleagues have reported that a subset of ESC-associated transcriptional regulators are more frequently overexpressed in poorly differentiated tumors (25
). These authors concluded “that these genes contribute to stem cell-like phenotypes shown by many tumors.” Such data imply that some poor-prognosis tumors may possess higher frequencies of CSCs.