Phenotypic Screening Reveals Topoisomerase I as a Breast Cancer Stem Cell Therapeutic Target

Cancer stem cells (CSCs) are a subpopulation generally thought to be responsible for cancer initiation and progression. Because CSCs are often rare in the total tumor cell population and differentiate rapidly when grown in culture, it has been challenging to uncover compounds that selectively target CSCs. We previously described CSC-emulating cells derived from breast cancer cell lines that maintained a stable undifferentiated state. We optimized a phenotypic assay with these cells and screened 1,280-bioactive compounds, identifying five that preferentially inhibited CSC-like cell proliferation. Using a compound-guided target identification approach, we found high topoisomerase I (Topo I) expression levels in breast CSC-like cells and primary breast CSCs. Structurally unrelated small molecules targeting Topo I preferentially inhibited CSC-like cells. These results illustrate the substantial power of this CSC phenotypic screening platform and promote Topo I as a potential molecular therapeutic target for therapies aimed at expunging CSCs.     

Targeting cancer stem‐like cells in glioblastoma and colorectal cancer through metabolic pathways

Cancer stem‐like cells (CSCs) are thought to be the main cause of tumor occurrence, progression and therapeutic resistance. Strong research efforts in the last decade have led to the development of several tailored approaches to target CSCs with some very promising clinical trials underway; however, until now no anti‐CSC therapy has been approved for clinical use. Given the recent improvement in our understanding of how onco‐proteins can manipulate cellular metabolic networks to promote tumorigenesis, cancer metabolism research may well lead to innovative strategies to identify novel regulators and downstream mediators of CSC maintenance. Interfering with distinct stages of CSC‐associated metabolics may elucidate novel, more efficient strategies to target this highly malignant cell population. Here recent discoveries regarding the metabolic properties attributed to CSCs in glioblastoma (GBM) and malignant colorectal cancer (CRC) were summarized. The association between stem cell markers, the response to hypoxia and other environmental stresses including therapeutic insults as well as developmentally conserved signaling pathways with alterations in cellular bioenergetic networks were also discussed. The recent developments in metabolic imaging to identify CSCs were also summarized. This summary should comprehensively update basic and clinical scientists on the metabolic traits of CSCs in GBM and malignant CRC.     

Targeting breast cancer stem cells

The cancer stem cell (CSC) hypothesis postulates that tumors are maintained by a self‐renewing CSC population that is also capable of differentiating into non‐self‐renewing cell populations that constitute the bulk of the tumor. Although, the CSC hypothesis does not directly address the cell of origin of cancer, it is postulated that tissue‐resident stem or progenitor cells are the most common targets of transformation. Clinically, CSCs are predicted to mediate tumor recurrence after chemo‐ and radiation‐therapy due to the relative inability of these modalities to effectively target CSCs. If this is the case, then CSC must be efficiently targeted to achieve a true cure. Similarities between normal and malignant stem cells, at the levels of cell‐surface proteins, molecular pathways, cell cycle quiescence, and microRNA signaling present challenges in developing CSC‐specific therapeutics. Approaches to targeting CSCs include the development of agents targeting known stem cell regulatory pathways as well as unbiased high‐throughput siRNA or small molecule screening. Based on studies of pathways present in normal stem cells, recent work has identified potential “Achilles heals” of CSC, whereas unbiased screening provides opportunities to identify new pathways utilized by CSC as well as develop potential therapeutic agents. Here, we review both approaches and their potential to effectively target breast CSC.     

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。     

Stem cell-like cancer cells in cancer cell lines

Both stem cells and cancer cells are thought to be capable of unlimited proliferation. Moreover, a small number of cancer cells express stem cell markers, including CD133 and ATP-binding cassette transporters, by which the cells can pump out specific fluorescence dyes, such as Hoechst33342, as well as anti-cancer drugs, suggesting that either cancer cells resemble stem cells or cancers contain stem cell-like cancer cells, called "cancer stem cells (CSCs)". Using the common characteristics of tissue-specific stem cells, it was demonstrated that many types of tumors and cancer cell lines contain CSCs, which self-renew, express stem cell markers, and are tumorigenic. It was also shown that CSCs are resistant to anti-cancer drugs and irradiation. Thus CSCs might be a crucial target for the therapy. Because tumors contain CSCs and recruited normal stem cells, both of which contribute to tumorigenesis, it is difficult to separate CSCs from tumors. By contrast, cancer cell lines do not have any contaminating normal stem cells that quickly loose mulitpotentiality and differentiate in normal culture condition, suggesting that cancer cell lines could be an attractive alternative source of cells for CSC research. In this review I summarize the recent progress in CSC research using cancer cell lines.     

The clinical and therapeutic implications of cancer stem cell biology

Cancer stem cells (CSCs) have provided new insights into the tumorigenesis and metastatic potential of cancer. The discovery of CSCs has provided many new insights into the complexities of cancer therapy: tumor initiation, treatment resistance, metastasis, recurrence, assessment of prognosis and prediction of clinical course. Recent rapid advances in molecular analysis have contributed to the better understanding of the molecular attributes and pathways that give CSCs their unique attributes. Use of these molecular techniques has facilitated elucidation of specific surface markers and pathways that favor propagation of CSCs - allowing for targeted therapy. Furthermore, it has been discovered that a specific microenvironment, or niche, is essential for the genesis of tumors from CSCs. Therapeutic strategies that alter these microenvironments compromise CSC proliferation and constitute another method of targeted cancer therapy. We review the clinical and therapeutic implications of CSCs, with a focus on treatment resistance and metastasis, and the emerging approaches to target CSCs and their microenvironments in order to attain improved outcomes in cancer. It is noteworthy that CSCs are the only cells capable of sustaining tumorigenesis; however, the cell of origin of cancer, in which tumorigenesis is initiated, may be distinct from CSCs that propagate the tumor.     

MicroRNA regulation of cancer stem cells

Cancer stem cells (CSC), or cancer cells with stem cell properties, have been reported in many human tumors and are thought to be responsible for tumor initiation, therapy resistance, progression, relapse, and metastasis. Despite their potential clinical importance, how CSCs are regulated at the molecular level is not well understood. MicroRNAs (miRNA), small noncoding RNAs that play critical roles in normal stem cell functions during development, have emerged as important regulators of CSCs as well. In this review, we summarize the current major findings of miRNA regulation of various CSCs and discuss our recent findings that miR-34a suppresses prostate CSCs and metastasis by directly repressing CD44. This recent progress has important implications for understanding how CSCs are intricately regulated by networks of miRNAs and for developing novel mechanism-based miRNA therapeutics that specifically target CSCs.     

The clinical and therapeutic implications of cancer stem cell biology

Cancer stem cells (CSCs) have provided new insights into the tumorigenesis and metastatic potential of cancer. The discovery of CSCs has provided many new insights into the complexities of cancer therapy: tumor initiation, treatment resistance, metastasis, recurrence, assessment of prognosis and prediction of clinical course. Recent rapid advances in molecular analysis have contributed to the better understanding of the molecular attributes and pathways that give CSCs their unique attributes. Use of these molecular techniques has facilitated elucidation of specific surface markers and pathways that favor propagation of CSCs – allowing for targeted therapy. Furthermore, it has been discovered that a specific microenvironment, or niche, is essential for the genesis of tumors from CSCs. Therapeutic strategies that alter these microenvironments compromise CSC proliferation and constitute another method of targeted cancer therapy. We review the clinical and therapeutic implications of CSCs, with a focus on treatment resistance and metastasis, and the emerging approaches to target CSCs and their microenvironments in order to attain improved outcomes in cancer. It is noteworthy that CSCs are the only cells capable of sustaining tumorigenesis; however, the cell of origin of cancer, in which tumorigenesis is initiated, may be distinct from CSCs that propagate the tumor.     

High metabolic rate and stem cell characteristics of esophageal cancer stem-like cells depend on the Hsp27–AKT–HK2 pathway

Tumor progression with chemoresistance and local recurrence is commonly happened during treatment of esophageal squamous cell carcinoma (ESCC). Cancer stem cells (CSC) may respond for tumor progression. However, there are few reports regarding metabolism of esophageal CSCs with clinical correlation. In this work, we demonstrated that ESCC cell lines in spheroid culture display CSC phenotypes, including increased ALDH activity, chemoresistance and tumor initiation, which are dependent on Hsp27 activation. Esophageal CSCs also exhibit reprogrammed metabolic features particularly higher glycolysis and oxidative phosphorylation, which are regulated via the Hsp27–AKT–HK2 pathway. Moreover, HK2 is required for maintenance of CSC phenotypes. Inhibition of CSC metabolism reduces cell growth and tumor formation. Clinically, patients who underwent surgical resection for esophageal cancer, and displayed overexpression of both Hsp27 and HK2, had the worst prognosis of all expression types. In conclusion, stem cells features and aberrant metabolic reprogramming of esophageal CSCs depend on the Hsp27–AKT–HK2 pathway. Targeting Hsp27 and HK2 could be novel therapeutic strategy for treating esophageal cancer and warrants further investigation.     

Discovery of the cancer stem cell related determinants of radioresistance

Tumors are known to be heterogeneous containing a dynamic mixture of phenotypically and functionally different tumor cells. The two concepts attempting to explain the origin of intratumor heterogeneity are the cancer stem cell hypothesis and the clonal evolution model. The stochastic model argues that tumors are biologically homogenous and all cancer cells within the tumor have equal ability to propagate the tumor growth depending on continuing mutations and selective pressure. By contrast, the stem cells model suggests that cancer heterogeneity is due to the hierarchy that originates from a small population of cancer stem cells (CSCs) which are biologically distinct from the bulk tumor and possesses self-renewal, tumorigenic and multilineage potential. Although these two hypotheses have been discussed for a long time as mutually exclusive explanations of tumor heterogeneity, they are easily reconciled serving as a driving force of cancer evolution and diversity. Recent discovery of the cancer cell plasticity and heterogeneity makes the CSC population a moving target that could be hard to track and eradicate. Understanding the signaling mechanisms regulating CSCs during the course of cancer treatment can be indispensable for the optimization of current treatment strategies.     

Cellules souches du cancer du sein : prendre le cancer à la racine

In cancer stem cell (CSC) hypothesis, tumors are organized in a hierarchical model with CSC at the top of the hierarchy. CSCs display both stem cell properties (self-renewal and differentiation) and specific tumoral properties (tumorigenicity, metastatic capacity, resistance to conventional therapies). Recent works on breast cancer allow CSCs isolation and help deciphering CSC biology and targeting with specific therapies. In clinical trials, CSC biology has to be taken into account and the criteria to judge therapeutic efficiency have to change.     

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.     

Targeting of pancreatic and prostate cancer stem cell characteristics by Crambe crambe marine sponge extract

Cancer stem cells (CSCs) are suggested as reason for resistance of tumors toward conventional tumor therapy including pancreatic and advanced prostate cancer. New therapeutic agents are urgently needed for targeting of CSCs. Marine sponges harbor novel and undefined compounds with antineoplastic activity but their potential to eliminate CSC characteristics is not examined so far. We collected 10 marine sponges and one freshwater sponge by diving at the seaside and prepared crude methanolic extracts. The effect to established pancreatic and prostate CSC lines was evaluated by analysis of apoptosis, cell cycle, side population, colony and spheroid formation, migratory potential in vitro and tumorigenicity in vivo. While each sponge extract at a 1:10 dilution efficiently diminished viability, Crambe crambe marine sponge extract (CR) still strongly reduced viability of tumor cells at a dilution of 1:1,000 but was less toxic to normal fibroblasts and endothelial cells. CR inhibited self-renewal capacity, apoptosis resistance, and proliferation even in gemcitabine-selected pancreatic cancer cells with acquired therapy resistance and enhanced CSC characteristics. CR pretreatment of tumor cells diminished tumorigenicity of gemcitabine-resistant tumor cells in mice and totally abolished tumor take upon combination with gemcitabine. Our data suggest that CR contains substances, which render standard cancer therapy more effective by targeting of CSC characteristics. Isolation of bioactive metabolites from CR and evaluation in mice are required for development of new CSC-specific chemotherapeutic drugs from a marine sponge.     

Role of stem cell derived exosomes in tumor biology

Exosomes are nano‐scale messengers loaded with bio‐molecular cargo of RNA, DNA, and Proteins. As a master regulator of cellular signaling, stem cell (both normal, and cancer stem cells) secreted exosome orchestrate various autocrine and paracrine functions which alter tumor micro‐environment, growth and progression. Exosomes secreted by one of the two important stem cell phenotypes in cancers a) Mesenchymal stem cells, and b) Cancer stem cells not only promote cancerous growth but also impart therapy resistance in cancer cells. In tumors, normal or mesenchymal stem cell (MSCs) derived exosomes (MSC‐exo) modulate tumor hallmarks by delivering unique miRNA species to neighboring cells and help in tumor progression. Apart from regulating tumor cell fate, MSC‐exo are also capable of inducing physiological processes, for example, angiogenesis, metastasis and so forth. Similarly, cancer stem cells (CSCs) derived exosomes (CSC‐exo) contain stemness‐specific proteins, self‐renewal promoting regulatory miRNAs, and survival factors. CSC‐exo specific cargo maintains tumor heterogeneity and alters tumor progression. In this review we critically discuss the importance of stem cell specific exosomes in tumor cell signaling pathways with their role in tumor biology.     

Complexity of cancer stem cells

Heterogeneity of tumor tissue has been accounted for in recent years by a hierarchy‐based model in which cancer stem cells (CSCs) have the ability both to self‐renew and to give rise to differentiated tumor cells and are responsible for the overall organization of a tumor. Research into CSCs has progressed rapidly and concomitantly with recent advances in the biology of normal tissue stem cells, resulting in the identification of CSCs in a wide range of human tumors. Studies of mouse models of human cancer have provided further insight into the characteristics of CSCs as well as a basis for the development of novel therapies targeted to these cells. However, recent studies have revealed complexities, such as plasticity of stem cell properties and clonal diversity of CSCs, in certain tumor types that have led to revision of the original CSC model. In this review, we summarize the history of the discovery and characterization of CSCs, as well as address recent advances that have revealed the complexity of these cells and their therapeutic implications.     

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.     

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.     

Molecular identification and targeting of colorectal cancer stem cells

Tumor initiating or cancer stem cells (CSCs) are suggested to be responsible for tumor initiation and growth. Moreover, therapy resistance and minimal residual disease are thought to result from selective resistance of CSCs. Isolation of CSCs from colon carcinomas can be accomplished by selection of a subpopulation of tumor cells based on expression of one or multiple cell surface markers associated with cancer stemness, like CD133, CD44, CD24, CD29, CD166 and Lgr5. Identification of colon CSCs will lead to a better rational for new therapies that aim to target this fraction specifically. In this review, we analyze known markers used for selection of colon CSCs and their potential function in CSC biology. Moreover, we discuss potential targeting strategies for eradicating CSCs specifically in order to develop more effective therapeutic strategies as well as to address more fundamental questions like the actual role of CSCs in tumor growth.     

MicroRNA control of epithelial‐mesenchymal transition in cancer stem cells

Cancer stem cells (CSCs) represent a small subset of cancer cell populations that possess characteristics associated with normal stem cells. They have the ability to self‐renew, and are able to generate diverse tumor cells and account for metastases. Therefore, CSCs are widely accepted as potential mediators of therapeutic resistance and novel targets for anti‐cancer treatments. Recent progress has highlighted the significance of epithelial–mesenchymal transition (EMT) process in CSC formation, as well as the crucial role of microRNAs in controlling EMT and cancer metastasis. MicroRNAs are also reported to take part in the control of CSC functions and the regulation of cancer progression by affecting EMT process. Thus, it is highly crucial to develop deeper understanding of the mechanisms that how microRNAs control EMT processes and regulate CSC functions for better therapeutics of cancer disease. Herein we make this review to summarize the current understanding of the regulatory mechanisms of EMT in CSC initiation, with a special focus on the role of microRNAs in EMT control, and discuss the implications of targeting CSCs for cancer therapeutics.