Brain cancer stem-like cells

Both stem cells and cancer cells are thought to be capable of unlimited proliferation. Moreover, many tumours and cancer cell lines express stem cell markers, including adenosine triphosphate (ATP)-binding cassette transporters, by which the cells pump out specific fluorescent dyes as well as anti-cancer drugs, suggesting either that cancer cells resemble stem cells or that cancers contain stem-like cells. Using the common characteristics of brain tumour cells and neural stem cells, several research groups have succeeded in identifying stem-like cells (cancer stem-like cells) in brain tumours and brain cancer cell lines. The purified cancer stem-like cells, but not the other cancer cells, self-renew and form tumours when transplanted in vivo. Thus, cancer stem-like cells in brain tumours might be a crucial target for anti-brain tumour therapy.     

Maintenance of HCT116 colon cancer cell line conforms to a stochastic model but not a cancer stem cell model

The cancer stem cell (CSC) model, in which a small population of cells within a tumor possesses the ability to self-renew and reconstitute the phenotype of primary tumor, has gained wide acceptance based on evidence over the past decade. It has also been reported that cancer cell lines contain a CSC subpopulation. However, phenotypic differences between CSCs and non-CSCs in cancer cell lines are not better defined than in primary tumors. Furthermore, some cell lines do not have a CSC population, revealed as a side population and expression of CD133. Thus, the identification of CSCs in cancer cell lines remains elusive. Here, we investigated the CSC hierarchy within HCT116 colon cancer cells, which do not have a CD133-positive subpopulation. We examined the expression of alternative CSC markers epithelial specific antigen (ESA) and CD44 in floating-sphere-derived cells, which are known to be the cells of enriching CSCs. Sphere-derived HCT116 cells exhibited heterogeneous expression of ESA and CD44. The two major subpopulations of HCT116 sphere cells (ESA^lowCD44^−/low and ESA^highCD44^high) exhibited a biological/proliferative hierarchy of sphere-forming and soft agar colony-forming activity. However, there was no difference between the two subpopulations in the incidence of xenograft tumors. When ESA^lowCD44^−/low cells were allowed to aggregate and re-form floating-spheres, the biological/proliferative hierarchy of parental HCT116 spheres was reconstituted, in terms of ESA and CD44 expression. Thus, HCT116 cells have plasticity when they are set in floating-spheres, suggesting that maintenance of the HCT116 cell line conforms to a stochastic model, not a CSC model. ( Cancer Sci 2009; 100: 2275–2282)     

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.     

Implications of cancer stem cells in the treatment of cancer

Identification of cancer stem cells (CSCs) in both hematological and solid malignancies suggests that CSCs may be a common phenomenon for most malignancies. Similarly to normal stem cells, CSCs can self-renew and differentiate into progeny cancer cells. Almost all current therapy against cancer targets differentiated cancer cells. CSCs are more resistant to therapy secondary to quiescence, increased expression of antiapoptotic proteins and drug efflux transporters. In this article, we review the current status of CSC research and propose the targeting of CSC cell-surface molecules, signal transduction pathways, the stem cell niche, stem cell differentiation and drug resistance.     

The telomerase inhibitor imetelstat depletes cancer stem cells in breast and pancreatic cancer cell lines

Cancer stem cells (CSC) are rare drug-resistant cancer cell subsets proposed to be responsible for the maintenance and recurrence of cancer and metastasis. Telomerase is constitutively active in both bulk tumor cell and CSC populations but has only limited expression in normal tissues. Thus, inhibition of telomerase has been shown to be a viable approach in controlling cancer growth in nonclinical studies and is currently in phase II clinical trials. In this study, we investigated the effects of imetelstat (GRN163L), a potent telomerase inhibitor, on both the bulk cancer cells and putative CSCs. When breast and pancreatic cancer cell lines were treated with imetelstat in vitro, telomerase activity in the bulk tumor cells and CSC subpopulations were inhibited. Additionally, imetelstat treatment reduced the CSC fractions present in the breast and pancreatic cell lines. In vitro treatment with imetelstat, but not control oligonucleotides, also reduced the proliferation and self-renewal potential of MCF7 mammospheres and resulted in cell death after <4 weeks of treatment. In vitro treatment of PANC1 cells showed reduced tumor engraftment in nude mice, concomitant with a reduction in the CSC levels. Differences between telomerase activity expression levels or telomere length of CSCs and bulk tumor cells in these cell lines did not correlate with the increased sensitivity of CSCs to imetelstat, suggesting a mechanism of action independent of telomere shortening for the effects of imetelstat on the CSC subpopulations. Our results suggest that imetelstat-mediated depletion of CSCs may offer an alternative mechanism by which telomerase inhibition may be exploited for cancer therapy.     

In vitro models of cancer stem cells and clinical applications

Cancer cells, stem cells and cancer stem cells have for a long time played a significant role in the biomedical sciences. Though cancer therapy is more effective than it was a few years ago, the truth is that still none of the current non-surgical treatments can cure cancer effectively. The reason could be due to the subpopulation called “cancer stem cells” (CSCs), being defined as those cells within a tumour that have properties of stem cells: self-renewal and the ability for differentiation into multiple cell types that occur in tumours.The phenomenon of CSCs is based on their resistance to many of the current cancer therapies, which results in tumour relapse. Although further investigation regarding CSCs is still needed, there is already evidence that these cells may play an important role in the prognosis of cancer, progression and therapeutic strategy. Therefore, long-term patient survival may depend on the elimination of CSCs. Consequently, isolation of pure CSC populations or reprogramming of cancer cells into CSCs, from cancer cell lines or primary tumours, would be a useful tool to gain an in-depth knowledge about heterogeneity and plasticity of CSC phenotypes and therefore carcinogenesis. Herein, we will discuss current CSC models, methods used to characterize CSCs, candidate markers, characteristic signalling pathways and clinical applications of CSCs. Some examples of CSC-specific treatments that are currently in early clinical phases will also be presented in this review.     

Human breast and melanoma cancer stem cells biomarkers

Cancer progression in humans is difficult to infer because we do not routinely sample patients at multiple stages of their disease. The identification cancer stem cell (CSC) subpopulations inside tumor opens a new view of cancer development, since it implies that tumors can only be eradicated by targeting CSCs. Several markers have been proposed in the literature to identify CSCs both in breast and melanoma but no consensus has been reached, leading to the hypothesis that the CSC phenotype might be dynamically switched. Herein we provide a critical discussion of the biological markers described in the literature for breast cancer and melanoma. Due to its complexity the field would benefit from an interdisciplinary approach to investigate tumor heterogeneity and its progression. Similar considerations could also be relevant for normal tissue stem cells.     

靶向肿瘤干细胞治疗肿瘤

具有独特的分子表达、表面标志物、干性相关信号通路和代谢模式等方面特征的肿瘤干细胞（cancer stem cell, CSC） 因其具有高致瘤、高转移、高治疗抵抗能力,可能是多种类型恶性肿瘤生长、转移、治疗抵抗的关键因素,也是肿瘤发生和复发的 重要根源。正常干细胞在产生了第一个致癌突变之后将逐步发展成为癌前干细胞和CSC,随后在突变和微环境的共同作用下进 一步积累突变增加异质性,并与CSC可塑性转变交织在一起推动肿瘤的发生和进展,促进肿瘤的复发、转移及治疗抵抗。为了更 好地治疗肿瘤,现已研发了多种类型的靶向CSC的治疗策略,包括靶向CSC的细胞表面标志物、信号转导途径、微环境、代谢模式 等,以及促CSC分化、靶向CSC的免疫治疗等其他策略。多个靶向CSC治疗肿瘤的新药在临床试验中已经展现出良好的治疗效 果,然而,也有一些抗肿瘤新药的失败为未来研发提供了值得注意的教训。未来肿瘤治疗中,特异地靶向患者肿瘤中所有异质性 的CSC,并同时清除癌前干细胞和子代肿瘤细胞,将会更好地抑制肿瘤生长、转移和复发,从而为治愈肿瘤带来新的希望。     

Identification of cancer stem cells in the "side population"

Both normal somatic stem cells and cancer cells are thought to be capable of unlimited proliferation. Paradoxically, however, some cancers seem to contain stem-like cells (cancer stem cells). There is increasing evidence that cancers might contain their own stem cells. Many cancers, like normal organs, seem to be maintained by a hierarchical organization that includes slowly dividing stem cells,rapidly dividing transit amplifying cells (precursor cells), and differentiated cells. Malignant gliomas, for example,often contain both undifferentiated and differentiated cells and sometimes contain cells that express neuronal markers as well as cells that express glial markers, suggesting that they may contain multipotent neural stem cell-like cells. We have shown that some cancer cell lines contain a small side population (SP), which, in many normal tissues, is thought to contain the stem cells of the tissue. We provide evidence that SP cells in the C6 glioma cell line can produce both neurons and glial cells and thus have cancer stem cell property. Taken together with studies on normal neural stem cells, studies on cancer stem cells will help us to understand a link between normal stem cells and cancer stem cells.     

肿瘤干细胞的临床意义

肿瘤干细胞(cancer stem cell, CSC)是近年来在许多肿瘤组织中发现的一类特殊干细胞。肿瘤干细胞具有自我更新和分化的能力,可以通过不断分化肿瘤细胞使新的肿瘤产生;肿瘤干细胞具有很强的耐药性和放射抗拒,这可以用来解释肿瘤的复发和转移。肿瘤干细胞可用于对肿瘤的诊断和治疗：通过对肿瘤干细胞标志物的鉴定可实现对一些肿瘤的早期诊断;一些新的治疗手段则通过作用于肿瘤干细胞的信号转导途径、表面标记和其生存的微环境,以及诱导其分化,从而达到靶向治疗肿瘤的目的。深入研究肿瘤干细胞的耐药性以及确定更多的肿瘤干细胞标志物,可为肿瘤治疗提供新途径。     

Ovarian cancer stem cells: Working towards the root of stemness

Despite medical advances made over the past decade, ovarian cancer remains one of the more lethal gynecologic cancers in the United States. While current therapeutic strategies are relatively effective, there is a high incidence of recurrent chemoresistant disease. This has been attributed, in part, to a regenerative tumor cell sub-population that has acquired stem cell properties which allows these cells to escape standard chemotherapeutics and drive recurrent disease. To date, a number of laboratories have identified these cancer stem cell (CSC) sub-populations in ovarian cancer cell lines, tumors or ascites and the collective findings suggest ovarian CSCs are likely to be as heterogeneous as the disease itself. Moreover, the multiple ovarian histophenotypes and possible sites of disease origin together with the potential for differential hierarchal contributions of multiple CSCs populations represent significant challenges to the identification, functional characterization and therapeutic targeting of ovarian CSC. This review will highlight the markers and methodology currently used to identify and isolate these cells. We will discuss some of the underlying ovarian CSC biology, the signaling pathways implicated in their survival, replication and differentiation and potential therapeutic targeting strategies.     

胃癌干细胞研究进展

肿瘤干细胞(CSC)是一群具有自我更新能力和多向分化潜能的肿瘤细胞,许多研究已证实在多种实体瘤中存在CSC.虽然CSC在肿瘤细胞总数中只占很小比例,但在肿瘤的起源、发展、转移及复发等方面均有重要的作用.胃癌CSC的研究尚处于探索阶段,仍未发现胃癌CSC特异性标记物,但对其存在和来源均开展了一系列的研究.     

Aldehyde dehydrogenase in combination with CD133 defines angiogenic ovarian cancer stem cells that portend poor patient survival

Markers that reliably identify cancer stem cells (CSC) in ovarian cancer could assist prognosis and improve strategies for therapy. CD133 is a reported marker of ovarian CSC. Aldehyde dehydrogenase (ALDH) activity is a reported CSC marker in several solid tumors, but it has not been studied in ovarian CSC. Here we report that dual positivity of CD133 and ALDH defines a compelling marker set in ovarian CSC. All human ovarian tumors and cell lines displayed ALDH activity. ALDH(+) cells isolated from ovarian cancer cell lines were chemoresistant and preferentially grew tumors, compared with ALDH(-) cells, validating ALDH as a marker of ovarian CSC in cell lines. Notably, as few as 1,000 ALDH(+) cells isolated directly from CD133(-) human ovarian tumors were sufficient to generate tumors in immunocompromised mice, whereas 50,000 ALDH(-) cells were unable to initiate tumors. Using ALDH in combination with CD133 to analyze ovarian cancer cell lines, we observed even greater growth in the ALDH(+)CD133(+) cells compared with ALDH(+)CD133(-) cells, suggesting a further enrichment of ovarian CSC in ALDH(+)CD133(+) cells. Strikingly, as few as 11 ALDH(+)CD133(+) cells isolated directly from human tumors were sufficient to initiate tumors in mice. Like other CSC, ovarian CSC exhibited increased angiogenic capacity compared with bulk tumor cells. Finally, the presence of ALDH(+)CD133(+) cells in debulked primary tumor specimens correlated with reduced disease-free and overall survival in ovarian cancer patients. Taken together, our findings define ALDH and CD133 as a functionally significant set of markers to identify ovarian CSCs.     

Cancer Stem Cells and Response to Therapy

The cancer stem cell (CSC) model states that cancers are organized in cellular hierarchies, which explains the functional heterogeneity often seen in tumors. Like normal tissue stem cells, CSCs are capable of self-renewal,either by symmetric or asymmetric cell division, and have the exclusive ability to reproduce malignant tumors indefinitely. Current systemic cancer therapies frequently fail to eliminate advanced tumors, which may be dueto their inability to effectively target CSC populations. It has been shown that embryonic pathways such as Wnt, Hedgehog, and Notch control self-renewal and cell fate decisions of stem cells and progenitor cells. These are evolutionary conserved pathways, involved in CSC maintenance. Targeting these pathways may be effective in eradicating CSCs and preventing chemotherapy or radiotherapy resistance.     

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.     

Cancer stem cells and tumor angiogenesis

Cancer stem cells (CSCs) have been identified in several human solid and hematological tumors. They are able to initiate tumor formation and metastasis and express specific cell surface markers. CSC tend to be more resistant to chemotherapeutic agents and radiation therapy than more mature cell types from the same tissue because of increased expression of antiapoptotic proteins. In this context, the development of agents that eliminate or control CSC may be an effective strategy for cancer prevention.     

A robust culture method for maintaining tumorigenic cancer stem cells in the hepatocellular carcinoma cell line Li‐7

Cancer tissues contain small populations of highly tumorigenic cells termed cancer stem cells (CSCs). Immortalized cell lines containing CSCs are valuable and powerful experimental tools for research into the characteristics of these stem cells. We previously reported that the hepatocellular carcinoma cell line Li‐7 includes abundant CD13⁺CD166^? CSCs; however, the number of these cells decreases after long‐term culture as a result of differentiation to non‐CSC populations. To ensure consistent and reproducible results in experiments using Li‐7 cells, it is important that the CSC population is maintained stably regardless of culture duration and passage. In the present study, we found that a commercially available culture medium for maintenance of embryonic stem cells and induced pluripotent stem cells, mTeSR1, effectively prevented spontaneous differentiation by CD13⁺CD166^? cells to CD13^?CD166⁺ cells and therefore maintained the CSC population in Li‐7 cell cultures. CD13⁺CD166^? CSCs maintained using this culture medium retained high tumorigenicity after transplantation into mice; they also showed the ability to differentiate in vitro into non‐CSC populations in RPMI‐1640 with 10% FBS medium. We analyzed gene expression profiles of CSC and non‐CSC populations in Li‐7 cultures using an RNA sequencing method. Genes such as FGFR, NOTCH1, and JAG1, that are associated with tumorigenicity and stemness, were upregulated in the CSC population. Our results suggest that CSCs can be maintained in immortalized cancer cell lines cultured over an extended period using a medium developed for culture of embryonic/induced pluripotent stem cells.     

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。     

小细胞肺癌干细胞信号通路的研究进展

小细胞肺癌是具有高度侵袭性的肺肿瘤,其主要临床特征是化疗有效率高但易在短时间内复发转移,这一特点可能与肿瘤干细胞的存在有关。肿瘤干细胞被认为是恶性肿瘤发生发展、耐药、复发及转移的根源。目前多认为肿瘤干细胞与正常干细胞有着相同的信号通路,如Hedgehog、Notch、Wnt等通路。本文就这几条信号通路在小细胞肺癌干细胞中所起的作用以及针对这几条信号通路治疗药物的研究进展和可能的信号通路交互作用等方面进行综述。     

Targeting cancer stem cell-specific markers and/or associated signaling pathways for overcoming cancer drug resistance

Cancer stem cells (CSCs) are a small subpopulation of tumor cells with capabilities of self-renewal, dedifferentiation, tumorigenicity, and inherent chemo-and-radio therapy resistance. Tumor resistance is believed to be caused by CSCs that are intrinsically challenging to common treatments. A number of CSC markers including CD44, CD133, receptor tyrosine kinase, aldehyde dehydrogenases, epithelial cell adhesion molecule/epithelial specific antigen, and ATP-binding cassette subfamily G member 2 have been proved as the useful targets for defining CSC population in solid tumors. Furthermore, targeting CSC markers through new therapeutic strategies will ultimately improve treatments and overcome cancer drug resistance. Therefore, the identification of novel strategies to increase sensitivity of CSC markers has major clinical implications. This review will focus on the innovative treatment methods such as nano-, immuno-, gene-, and chemotherapy approaches for targeting CSC-specific markers and/or their associated signaling pathways.