First among equals: the cancer cell hierarchy

Primary cancer cells exhibit heterogeneity in their proliferative ability. The cancer stem cell (CSC) model accounts for this heterogeneity by proposing that each cancer consists of a small population of CSCs that are capable of unlimited growth and self-renewal and a much larger population of cells, descendants of the CSCs, that have lost self-renewal capacity. The CSC model has important implications for cancer therapy. Eradication of CSCs, the cells responsible for maintenance of the neoplasm, would be necessary and sufficient to achieve cure. By extension, both the frequency of stem cells in a tumor and their propensity to undergo self-renewal (P_sr) would have a direct impact on the curability of that tumor. The P_sr is a critical biological characteristic of CSCs—small differences in P_sr have enormous impact on the probability of success in cancer therapy. Differentiation therapy, defined as treatment that reduces the P_sr of CSCs, is one approach to targeting CSCs.     

Current status and issues in cancer stem cell study

Cancer stem cells (CSCs) have been positively identified and successfully isolated from some but not all cancers. The studies on CSCs to date suggest that these cells are rare among the tumor cell population, and they are capable of self-renewing and maintaining tumor growth and heterogeneity. Therapies aimed at CSCs have shown some promise, but their further development will require a more thorough understanding of the biology of CSCs and methods for identifying and isolating this cell subpopulation. This review examines what is known to date regarding the similarities and differences between cancer and somatic stem cells: CSC surface marker development and cell isolation (including a model isolation from our lab), the frequency, potential origin, and signal transduction of CSCs, and the current state of CSC-targeting therapeutic strategies.     

Sequencing of breast cancer stem cell populations indicates a dynamic conversion between differentiation states in vivo

Introduction

The cancer stem cell model implies a hierarchical organization within breast tumors maintained by cancer stem-like cells (CSCs). Accordingly, CSCs are a subpopulation of cancer cells with capacity for self-renewal, differentiation and tumor initiation. These cells can be isolated through the phenotypic markers CD44+/CD24-, expression of ALDH1 and an ability to form nonadherent, multicellular spheres in vitro. However, controversies to describe the stem cell model exist; it is unclear whether the tumorigenicity of CSCs in vivo is solely a proxy for a certain genotype. Moreover, in vivo evidence is lacking to fully define the reversibility of CSC differentiation.

Methods

In order to answer these questions, we undertook exome sequencing of CSCs from 12 breast cancer patients, along with paired primary tumor samples. As suggested by stem classical cell biology, we assumed that the number of mutations in the CSC subpopulation should be lower and distinct compared to the differentiated tumor cells with higher proliferation.

Results

Our analysis revealed that the majority of somatic mutations are shared between CSCs and bulk primary tumor, with similar frequencies in the two.

Conclusions

The data presented here exclude the possibility that CSCs are only a phenotypic consequence of certain somatic mutations, that is a distinct and non-reversible population of cells. In addition, our results imply that CSCs must be a population of cells that can dynamically switch from differentiated tumor cells, and vice versa. This finding increases our understanding of CSC function in tumor heterogeneity and the importance of identifying drugs to counter de-differentiation rather than targeting CSCs.     

Cancer stem cell hypothesis and gastric carcinogenesis: Experimental evidence and unsolved questions

Traditionally, the clonal evolution model has been used to explain gastric cancer (GC) growth dynamics. According to this model, GC cells result from multiple mutations over time resulting in a population of continually diversifying cells. This heterogeneity enables the survival of different clones under particular conditions allowing growth at metastatic locations or resistance to chemotherapeutics. Cancer stem cell (CSC) theory completely overturns this traditional understanding of cancer suggesting that only CSCs can self-renew and promote tumor growth. CSCs are relatively refractory to conventional therapies, thus explaining why anti-cancer therapies are far from curative and why relapses of cancer are frequent. The identification of the CSC component of a tumor might, thus, open new therapeutic perspective based on the selective targeting of this small population of cells. In this review we examine the current scientific evidence supporting the existence of CSC in gastric tumors and analyze the main unsolved questions of this difficult field of cancer research.     

Insights into the cell of origin in breast cancer and breast cancer stem cells

The precise cell types that give rise to tumors and mechanisms that underpin tumor heterogeneity are poorly understood. There is increasing evidence to suggest that diverse solid tumors are hierarchically organized and may be sustained by a distinct subpopulation of cancer stem cells (CSCs). The CSC hypothesis provides an attractive cellular mechanism that can account for the therapeutic refractoriness and dormant behavior exhibited by many tumor types. Breast cancer was the first solid malignancy from which CSCs were identified and isolated. Direct evidence for the CSC hypothesis has also recently emerged from mouse models of mammary tumorigenesis, although alternative models to explain heterogeneity also seem to apply. Our group has found that the luminal epithelial progenitor marker CD61/β3 integrin identified a CSC population in mammary tumors from MMTV‐wnt‐1 mice. However, no CSCs could be identified in the more homogeneous MMTV‐neu/erbB2 model, suggesting an alternate (clonal evolution or stochastic) model of tumorigenesis. It seems likely that both paradigms of tumor propagation exist in human cancer. From a clinical perspective, the CSC concept has significant implications. Quiescent CSCs are thought to be more resistant to chemotherapy and targeted therapy. Enrichment of putative CSCs has been noted in studies of chemotherapy‐treated patients, lending support to the CSC hypothesis and their potential role in chemoresistance. Although many unresolved questions on CSCs remain, ongoing efforts to identify and characterize CSCs continue to be an important area of investigation, with the potential to identify novel tumor targeting strategies.     

Cancer stem cells: A product of clonal evolution?

The cancer stem cell (CSC) model has emerged as a prominent paradigm for explaining tumour heterogeneity. CSCs in tumour recurrence and drug resistance have also been implicated in a number of studies. In fact, CSCs are often identified by their expression of drug‐efflux proteins which are also highly expressed in normal stem cells. Similarly, pro‐survival or proliferation signalling often exhibited by stem cells is regularly reported as being upregulated by CSC. Here we review evidence suggesting that many aspects of CSCs are more readily described by clonal evolution. As an example, cancer cells often exhibit copy number gains of genes involved in drug‐efflux proteins and pro‐survival signalling. Consequently, clonal selection for stem cell traits may result in cancer cells developing “stemness” traits which impart a fitness advantage, without strictly following a CSC model. Finally, since symmetric cell division would give rise to more cells than asymmetric division, it is expected that more advanced tumours would depart from a CSC. Collectively, these observations suggest clonal evolution may explain many aspects of the CSC.     

Differentiation and transdifferentiation potentials of cancer stem cells

Tumor cells actively contribute to constructing their own microenvironment during tumorigenesis and tumor progression. The tumor microenvironment contains multiple types of stromal cells that work together with the extracellular matrix and local and systemic factors to coordinately contribute to tumor initiation and progression. Tumor cells and their stromal compartments acquire many genetic and/or epigenetic alternations to facilitate tumor growth and metastasis. The cancer stem cell (CSC) concept has been widely applied to interpreting tumor initiation, growth, metastasis, dormancy and relapse. CSCs have differentiation abilities to generate the original lineage cells that are similar to their normal stem cell counterparts. Interestingly, recent evidence demonstrates that CSCs also have the potential to transdifferentiate into vascular endothelial cells and pericytes, indicating that CSCs can transdifferentiate into other lineage cells for promoting tumor growth and metastasis in some tissue contexts instead of only recruiting stromal cells from local or distant tissues. Although the transdifferentiation of CSCs into tumor stromal cells provides a new dimension that explains tumor heterogeneity, many aspects of CSC transdifferentiation remain elusive. In this review, we summarize the multi-lineage differentiation and transdifferentiation potentials of CSCs as well as discuss their potential contributions to tumor heterogeneity and tumor microenvironment in tumor progression.     

Cancer stem cells and chemoradiation resistance

Cancer is a disease of genetic and epigenetic alterations, which are emphasized as the central mechanisms of tumor progression in the multistepwise model. Discovery of rare subpopulations of cancer stem cells (CSCs) has created a new focus in cancer research. The heterogeneity of tumors can be explained with the help of CSCs supported by antiapoptotic signaling. CSCs mimic normal adult stem cells by demonstrating resistance to toxic injuries and chemoradiation therapy. Moreover, they might be responsible for tumor relapse following apparent beneficial treatments. Compared with hematopoietic malignancies, conventional therapy regimes in solid tumors have improved the overall survival marginally, illustrating the profound impact of treatment resistance. This implies that the present therapies, which follow total elimination of rapidly dividing and differentiated tumor cells, need to be modified to target CSCs that repopulate the tumor. In this review article, we report on recent findings regarding the involvement of CSCs in chemoradiation resistance and provide new insights into their therapeutic implications in cancer. (Cancer Sci 2008; 99: 1871–1877)     

胃癌干细胞与微环境的研究进展

胃癌目前是仅次于肺癌的第二大致死性肿瘤,目前胃癌的发病机制还不是很清楚。近年来随着对肿瘤干细胞（CSC）和肿瘤生物学的研究,目前已经在多种实体瘤中发现CSC,但是由于胃癌干细胞缺乏特异性的标志物,因此还有很多空白待探究。虽然已发现一些胃癌干细胞表面标志物如CD44、CD133等,但缺乏特异性,仍需进一步探究更具特异性的胃癌干细胞标志物。CSC生存的环境在肿瘤的进程中也起重要的作用。文章对胃癌干细胞和微环境进行研究将有助于胃癌的诊断和治疗。     

The Role of Cancer Stem Cells in Breast Cancer Initiation and Progression: Potential Cancer Stem Cell‐Directed Therapies

Recent studies have identified a small population of highly tumorigenic cells with stem cell properties in human breast and other solid tumors that are considered to be the source of tumor initiation and maintenance; these cells are referred to as cancer stem cells (CSCs). Preclinical data suggest that current breast cancer treatment strategies lead to CSC enrichment, contributing to chemotherapy and radiotherapy resistance, although a strong correlation with clinical parameters and prognosis is yet to be established. Importantly, overcoming treatment failure by effective targeting of CSCs may be an appealing approach, potentially leading to improved clinical outcomes for patients with breast cancer. Several preclinical studies provide promising results that support this hypothesis. The purpose of this review is to summarize the role of CSCs in breast cancer recurrence and resistance and to discuss current attempts of CSC targeting.     

Pathobiology of pancreatic cancer: implications on therapy

Although the concept of tumor heterogeneity was established several decades ago, the interest in this topic is still unbroken. With the identification of inter- and intratumoral genomic rearrangements and the detection of cancer stem cells (CSCs) through phenotypic variations of cancer cells there are increasing options for pancreatic cancer therapy. Indeed, some pre-clinical studies have shown promising results in the treatment of drug-resistant CSCs, whereby a few strategies were already tested in clinical trials. Basically, CSCs are influenced by the tumor microenvironment and an epigenetic reprogramming to gain stem cell-like characteristics. Targeting options inhibiting the epithelial-mesenchymal crosstalk or promoting epigenetic-driven differentiation of CSCs to a less aggressive phenotype raised the possibilities of further therapeutic applications, which will be discussed in this review.     

The mammary progenitor marker CD61/beta3 integrin identifies cancer stem cells in mouse models of mammary tumorigenesis

The cells of origin and mechanisms that underpin tumor heterogeneity in breast cancer are poorly understood. Here, we have examined three mouse models of mammary tumorigenesis (MMTV-wnt-1, MMTV-neu, and p53(+/-)) for changes in their epithelial cell hierarchy during the preneoplastic and neoplastic stages of tumor progression. In preneoplastic tissue, only MMTV-wnt-1 mice showed a perturbation in their epithelial subpopulations. In addition to an expanded mammary stem cell pool, repopulating cells capable of yielding extensive mammary outgrowths in vivo were revealed in the committed luminal progenitor population. These findings indicate that wnt-1 activation induces the appearance of aberrant progenitor cells, and suggest that both mammary stem and progenitor cells can serve as the cellular targets of wnt-1-induced tumorigenesis. In tumors arising in MMTV-wnt-1 tumors, the luminal epithelial progenitor marker CD61/beta3 integrin identified a cancer stem cell (CSC) population that was highly enriched for tumorigenic capability relative to the CD61(-) subset. CD61 expression also defined a CSC subset in 50% of p53(+/-)-derived tumors. No CSCs, however, could be identified in the more homogeneous MMTV-neu/erbB2 model, suggesting an alternative model of tumorigenesis. Overall, our findings show the utility of the progenitor marker CD61 in the identification of CSCs that sustain specific mammary tumors.     

Cancer stem cells in squamous cell carcinoma switch between two distinct phenotypes that are preferentially migratory or proliferative

Epithelial-to-mesenchymal transition (EMT) is an important driver of tumor invasion and metastasis, which causes many cancer deaths. Cancer stem cells (CSC) that maintain and initiate tumors have also been implicated in invasion and metastasis, but whether EMT is an important contributor to CSC function is unclear. In this study, we investigated whether a population of CSCs that have undergone EMT (EMT CSCs) exists in squamous cell carcinoma (SCC). We also determined whether a separate population of CSCs that retain epithelial characteristics (non-EMT CSCs) is also present. Our studies revealed that self-renewing CSCs in SCC include two biologically-distinct phenotypes. One phenotype, termed CD44(high)ESA(high), was proliferative and retained epithelial characteristics (non-EMT CSCs), whereas the other phenotype, termed CD44(high)ESA(low), was migratory and had mesenchymal traits characteristic of EMT CSCs. We found that non-EMT and EMT CSCs could switch their epithelial or mesenchymal traits to reconstitute the cellular heterogeneity which was characteristic of CSCs. However, the ability of EMT CSCs to switch to non-EMT character was restricted to cells that were also ALDH1(+), implying that only ALDH1(+) EMT cells had the ability to seed a new epithelial tumor. Taken together, our findings highlight the identification of two distinct CSC phenotypes and suggest a need to define therapeutic targets that can eradicate both of these variants to achieve effective SCC treatment.     

肝癌干细胞研究进展与方向

近年来随着“干细胞”的概念被引入肿瘤学研究,肿瘤干细胞学说逐渐形成。该学说认为,导致肿瘤发生和维持肿瘤生长的是一小群叫做“肿瘤干细胞”的细胞;这些细胞在肿瘤组织中数量极少,具有自我更新、分化及抗治疗能力等干细胞样特性。在过去的数年中,研究者分别通过侧群细胞技术及CD133、CD90、OV6、EpCAM等标志鉴定和分离得到具有很强的体外克隆形成及体内成瘤能力的小群肝癌细胞,为肝癌干细胞的存在提供了有力证据。本文对肿瘤干细胞理论和肝癌干细胞相关研究进展进行了综述,对肝癌干细胞在肝癌诊断和治疗方面的重要性进行了讨论,并对我们面临的有关挑战、机遇和未来的研究方向进行了分析。     

Urothelial carcinoma: Stem cells on the edge

Tumors are heterogeneous collections of cells with highly variable abilities to survive, grow, and metastasize. This variability likely stems from epigenetic and genetic influences, either stochastic or hardwired by cell type-specific lineage programs. That differentiation underlies tumor cell heterogeneity was elegantly demonstrated in hematopoietic tumors, in which rare primitive cells (cancer stem cells (CSCs)) resembling normal hematopoietic stem cells are ultimately responsible for tumor growth and viability. Because of the compelling clinical implications CSCs pose—across the entire spectrum of cancers—investigators applied the CSC model to cancers arising in tissues with crudely understood differentiation programs. Instead of relying on differentiation, these studies used empirically selected markers and statistical arguments to identify CSCs. The empirical approach has stimulated important questions about “stemness” in cancer cells as well as the validity and stoichiometry of CSC assays. The recent identification of urothelial differentiation programs in urothelial carcinomas (UroCas) supports the idea that solid epithelial cancers (carcinomas) develop and differentiate analogously to normal epithelia and provides new insights about the spatial localization and molecular makeup of carcinoma CSCs. Importantly, CSCs from invasive UroCas (UroCSCs) appear well situated to exchange important signals with adjacent stroma, to escape immune surveillance, and to survive cytotoxic therapy. These signals have potential roles in treatment resistance and many participate in druggable cellular pathways. In this review, we discuss the implications of these findings in understanding CSCs and in better understanding how UroCas form, progress, and should be treated.     

Dynamic regulation of cancer stem cells and clinical challenges

A small population of cancer cells referred to as cancer stem cells (CSCs) have received particular attention, as they have been revealed to acquire stem cell-like properties and become the main cause of tumor propagation, metastasis and drug resistance. The CSC theory of tumor formation was believed to follow the hierarchical model initially, and therefore many CSC-targeted therapy methods were expected to cure cancer by eradicating CSCs. However, subsequent CSC research has revealed that rather than a distinct entity, the CSC is a dynamic status that can be continually dedifferentiated from progenitor or differentiated cancer cells. Elucidation of this bidirectional transition mechanism would help perfect the CSC theory and be of great value in the development of more effective anti-cancer drugs. Here, we reviewed the mechanisms of reciprocal conversion between non-CSCs and CSCs. Moreover, several approaches of target CSCs and non-CSCs together with unbiased eradication of all cancer cells are also discussed.     

Breast Cancer Heterogeneity: Need to Review Current Treatment Strategies

Although the heterogeneity of breast cancer has long been recognized, the hierarchical organization and existence of tumor initiating subpopulation within breast tumors was not known until the last decade. These tumor initiating cells called cancer stem cells (CSCs) display features of stem cells such as unlimited ability to self-renew and lineage differentiation. Accumulating evidence now suggests that by virtue of their relative resistance to both radiation and chemotherapy, these cells contribute to resistance and relapse following therapy. Utilizing cell cultures and mouse xenograft models, we and others demonstrated that breast CSCs have far greater invasive and metastatic potential than differentiated tumor cells which comprise the tumor bulk. Altogether, these studies suggest that targeting and elimination of breast CSCs may be required to improve patient outcome. In this review, we will discuss recent developments in breast CSC research and advances in CSC specific targeted therapies that are in preclinical and clinical trials.     

Bufalin inhibits the differentiation and proliferation of human osteosarcoma cell line hMG63-derived cancer stem cells

Cancer stem cells (CSCs) play an important role in drug resistance of tumor and are responsible for high recurrence rates. Agents that can suppress the proliferation and differentiation of CSCs would provide new opportunity to fight against tumor recurrence. In this study, we developed a new strategy to enrich CSCs in human osteosarcoma cell line hMG63. Using these CSCs as model, we tested the effect of bufalin, a traditional Chinese medicine, on the proliferation and differentiation of CSCs. hMG63 cells were cultured in poly-HEMA-treated dish and cancer stem cell-specific medium. In this nonadhesive culture system, hMG63 formed spheres, which were then collected and injected into the immunodeficient mice. Cisplatin was administered every 3 days for five times. The enriched xenograft tumors were cultured in cancer stem cell-specific medium again to form tumor spheres. Expression of cancer stem cell markers of these cells was measured by flow cytometry. These cells were then treated with bufalin, and the proliferation and differentiation ability were indicated by the expression level of molecular markers and the formation of sphere again in vitro. We obtained a low CD133+/CD44 cell population with high-level stem cell marker. When treated with bufalin, the sphere could not get attached to the flask and failed to differentiate, which was indicated by the stable expression of stem cell marker CD133 and OCT-4 in the condition permissive to differentiation. Treatment of bufalin also suppressed the single cells isolated from the sphere to form sphere again in the nonadhesive culture system, and a decreased expression of proliferation marker Ki67 was also detected in these cells. Sphere-formed and chemoresistant colon xenograft tumors in immunodeficient mice could enrich cancer stem cell population. Bufalin could inhibit proliferation and differentiation of CSCs.     

Targeting Cancer Stem Cells by Phytochemicals: a Multimodal Approach to Colorectal Cancer

Cancer stem cells (CSCs) exist within a tumor as a rare subpopulation, with the capacity of self-renewal and the ability to differentiate into heterogeneous population of cancer cells. CSCs are increasingly being implicated in tumor recurrence thereby further augmenting the menace of the malignant disease. Characterization of CSCs has unearthed their pivotal role in all the hallmarks of cancer including tumorigenesis, angiogenesis, metastasis and drug resistance, thereby designating cancer as a “stem cell disease.” Here, we discuss the limitations of current therapeutic strategies that spare CSCs thereby failing to achieve complete cure of colorectal cancer, and elucidate the role of multimodal CSC-targeted treatment strategies, using natural phytochemicals and their derivatives. With emerging evidences identifying the molecular targets of phytochemicals in colorectal CSCs, development of better therapeutic strategies uprooting CSCs, the root of all evils, can be envisaged.