Targeting cancer stem cells

Recent evidence has demonstrated the existence of a small subset of the tumour mass that is wholly responsible for the sustained growth and propagation of the tumour. This cancer stem cell (CSC) compartment is also likely to be responsible both for disease relapse and the resistance to therapy that often accompanies relapse. The evidence for CSCs in various malignancies is presented. The failure of existing therapeutics to eradicate CSCs suggests that they are relatively resistant to present cancer treatments. This resistance may reflect the preservation of normal stem cell protective mechanisms, such as an increased expression of drug efflux pumps or alterations in apoptotic, cell cycle and DNA repair mechanisms. Targeting these mechanisms, and taking advantage of potential differences in the biology of normal stem cells and CSCs, such as differences in surface phenotype, self renewal/quiescence and stem cell–niche interactions are discussed and preliminary preclinical or clinical data are presented. Finally, the authors give their opinion of the direction in which one must travel to successfully target the CSC and improve treatment outcomes in malignant disease.     

Targeting cancer stem cells

Recent evidence has demonstrated the existence of a small subset of the tumour mass that is wholly responsible for the sustained growth and propagation of the tumour. This cancer stem cell (CSC) compartment is also likely to be responsible both for disease relapse and the resistance to therapy that often accompanies relapse. The evidence for CSCs in various malignancies is presented. The failure of existing therapeutics to eradicate CSCs suggests that they are relatively resistant to present cancer treatments. This resistance may reflect the preservation of normal stem cell protective mechanisms, such as an increased expression of drug efflux pumps or alterations in apoptotic, cell cycle and DNA repair mechanisms. Targeting these mechanisms, and taking advantage of potential differences in the biology of normal stem cells and CSCs, such as differences in surface phenotype, self renewal/quiescence and stem cell-niche interactions are discussed and preliminary preclinical or clinical data are presented. Finally, the authors give their opinion of the direction in which one must travel to successfully target the CSC and improve treatment outcomes in malignant disease.     

Targeting the phosphatidylinositol 3-kinase/Akt/mammalian target of rapamycin signaling network in cancer stem cells

Cancer stem cells (CSCs) comprise a subset of hierarchically organized, rare cancer cells with the ability to initiate cancer in xenografts of genetically modified murine models. CSCs are thought to be responsible for tumor onset, self-renewal/maintenance, mutation accumulation, and metastasis. The existence of CSCs could explain the high frequency of neoplasia relapse and resistance to all of currently available therapies, including chemotherapy. The phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) signaling pathway is a key regulator of physiological cell processes which include proliferation, differentiation, apoptosis, motility, metabolism, and autophagy. Nevertheless, aberrantly upregulated PI3K/Akt/mTOR signaling characterizes many types of cancers where it negatively influences prognosis. Several lines of evidence indicate that this signaling system plays a key role also in CSC biology. Of note, CSCs are more sensitive to pathway inhibition with small molecules when compared to healthy stem cells. This observation provides the proof-of-principle that functional differences in signaling transduction pathways between CSCs and healthy stem cells can be identified. Here, we review the evidence which links the signals deriving from the PI3K/Akt/mTOR network with CSC biology, both in hematological and solid tumors. We then highlight how therapeutic targeting of PI3K/Akt/mTOR signaling with small molecule inhibitors could improve cancer patient outcome, by eliminating CSCs.     

Understanding and targeting cancer stem cells: therapeutic implications and challenges

Cancer stem cells (CSCs) have been identified as rare cell populations in many cancers, including leukemia and solid tumors. Accumulating evidence has suggested that CSCs are capable of self-renewal and differentiation into various types of cancer cells. Aberrant regulation of gene expression and some signaling pathways has been observed in CSCs compared to other tumor cells. CSCs are thought to be responsible for cancer initiation, progression, metastasis, recurrence and drug resistance. The CSC hypothesis has recently attracted much attention due to the potential for discovery and development of CSC-related therapies and the identification of key molecules involved in controlling the unique properties of CSC populations. Over the past several years, a tremendous amount of effort has been invested in the development of new drugs, such as nanomedicines, that can take advantage of the “Achilles'' heel” of CSCs by targeting cell-surface molecular markers or various signaling pathways. Novel compounds and therapeutic strategies that selectively target CSCs have been identified, some of which have been evaluated in preclinical and clinical studies. In this article, we review new findings related to the investigation of the CSC hypothesis, and discuss the crucial pathways involved in regulating the development of CSC populations and the advances in studies of drug resistance. In addition, we review new CSC-targeted therapeutic strategies aiming to eradicate malignancies.     

Cancer stemness contributes to cluster formation of colon cancer cells and high metastatic potentials

The ability of cancer cells to form clusters is a characteristic feature in the development of metastatic tumours with drug resistance. Several studies demonstrated that clusters of circulating tumour cells (CTCs) have a greater metastatic potential to establish new tumours at secondary sites than single CTCs. However, the mechanism of cluster formation is not well understood. In this study, we investigated whether cancer stemness would contribute to cluster formation. We used a tumour sphere culture method to enrich cancer stem cells (CSCs) from colon cancer cells and found that during the second generation of sphere culture, clusters (between 3 and 5 cells) formed within the first 24 hours, whereas the rest remained as single cells. The clusters were analysed for stemness and metastatic potential, including gene expressions for cancer stemness (CD133 and Lgr5), epithelial-mesenchymal transition (E-cadherin and TGF-β 1-3) and hypoxia-induced factors (HIF-1α and HIF-2α). The results showed that the clusters expressed higher levels of these genes and colon CSC surface markers (including CD24, CD44 and CD133) than the single cells. Among these markers, CD24 seemed the major contributor linking the cells into the clusters. These clusters also showed a stronger ability to both form colonies and migrate. Our data collectively suggest that colon cancer stemness contributes to cluster formation and that clustered cells exhibit a great metastatic potential. Our study thus provides a method to study the CTC clusters and derive insight into oncogenesis and metastasis.     

Drug resistance and cancer stem cells: the shared but distinct roles of hypoxia‐inducible factors HIF1α and HIF2α

Chemotherapy resistance is a major contributor to poor treatment responses and tumour relapse, the development of which has been strongly linked to the action of cancer stem cells (CSCs). Mounting evidence suggests that CSCs are reliant on low oxygen conditions and hypoxia‐inducible factors 1α and 2α (HIF1α and HIF2α) to maintain their stem cell features. Research in the last decade has begun to clarify the functional differences between the two HIFα subtypes (HIFαs). Here, we review and discuss these differences in relation to CSC‐associated drug resistance. Both HIFαs contribute to CSC survival but play different roles –HIF1α being more responsible for survival functions and HIF2α for stemness traits such as self‐renewal – and are sensitive to different degrees of hypoxia. Failure to account for physiologically relevant oxygen concentrations in many studies may influence the current understanding of the roles of HIFαs. We also discuss how hypoxia and HIFαs contribute to CSC drug resistance via promotion of ABC drug transporters Breast cancer resistance protein (BCRP), MDR1, and MRP1 and through maintenance of quiescence. Additionally, we explore the PI3K/AKT cell survival pathway that may support refractory cancer by promoting CSCs and activating both HIF1α and HIF2α. Accordingly, HIF1α and HIF2α inhibition, potentially via PI3K/AKT inhibitors, could reduce chemotherapy resistance and prevent cancer relapse.     

CD133 as a target for colon cancer

INTRODUCTION: Recent evidence based on cancer stem cell (CSC) models, is boosting the progress of translational research and providing relevant clinical implications in many tumour types, including colorectal cancer. The current failure of standard therapies is attributed to a small fraction of the primary cell population with stem-like characteristics, such as self-renewal and differentiation. Identification of CSCs is based on two different criteria of selection: stemness-selective conditions and direct isolation based on putative stem cell markers expression. CD133, a transmembrane glycoprotein, was associated with tumor-initiating cells derived from several histological variants of tumors, including colon. AREAS COVERED: In this review the current understandings about CD133 as putative marker of tumour-initiating cells in colorectal cancer (CRC) is described. The focus of the discussion is on the need for additional markers to better identify the cell population able to recapitulate the parental tumor in immunocompromised mice. EXPERT OPINION: Identification and characterization of CSCs represents a relevant issue to define innovative therapeutic approaches, overcoming the emergence of cancer cell clones capable of evading standard therapy.     

Targeting cancer stem cells with natural products

The cancer stem cell (CSC) hypothesis presents a fundamentally different paradigm for cancer treatment. CSCs reflect a small fraction of tumor initiating cells capable of sustained self-renewal and differentiation to form the heterogeneous tumor bulk. In order to cure cancer, it is necessary to eliminate cancer stem cells in addition to differentiated cancer cells to decrease metastasis, reduce recurrence, and improve patient survival. In this article, we review cancer stem cell signaling pathways, including Wnt, Hedgehog, and Notch, as well as interactions of CSCs with the tumor microenvironment. We also review methods to isolate CSCs and demonstrate therapeutic efficacy of natural products to modulate these signaling pathways for eliminating CSCs.     

Targeting IL-8 signalling to inhibit breast cancer stem cell activity

Although survival from breast cancer has improved significantly over the past 20 years, disease recurrence remains a significant clinical problem. The concept of stem-like cells in cancer has been gaining currency over the last decade or so, since evidence for stem cell activity in human leukaemia and solid tumours, including breast cancer, was first published. Evidence indicates that this sub-population of cells, known as cancer stem-like cells (CSCs), is responsible for driving tumour formation and disease progression. In breast cancer, there is good evidence that CSCs are intrinsically resistant to conventional chemo-, radio- and endocrine therapies. By evading the effects of these treatments, CSCs are held culpable for disease recurrence. Hence, in order to improve treatment there is a need to develop CSC-targeted therapies. Interleukin-8 (IL-8), an inflammatory cytokine, is upregulated in breast cancer and associated with poor prognostic factors. Accumulating evidence demonstrates that IL-8, through its receptors CXCR1/2, is an important regulator of breast CSC activity. Inhibiting CXCR1/2 signalling has proved efficacious in pre-clinical models of breast cancer providing a good rationale for targeting CXCR1/2 clinically. Here, we discuss the role of IL-8 in breast CSC regulation and development of novel therapies to target CXCR1/2 signalling in breast cancer.     

Targeting CSCs within the tumor microenvironment for cancer therapy: a potential role of mesenchymal stem cells

Introduction: Mesenchymal stem cells (MSCs) are one subgroup of adult stem cells and possess a proliferative potential and ability to differentiate into various ceells. Areas covered: Emerging evidence suggests that MSCs can reprogram toward cancer stem cells (CSCs), due to alterations of intrinsic and extrinsic microenvironments, leading to tumorigenesis. The CSC concept has fundamental clinical implications because of its involvement in cell migration/invasion, metastasis, and treatment resistance. Therefore, targeting CSCs provides a novel therapeutic strategy for cancer treatment. However, the origin of CSCs and its molecular connections are not fully understood. Emerging evidence suggests the existence of an inter-relationship between CSCs and epithelial-to-mesenchymal transition (EMT) phenotypic cells, in the context of inflammation and hypoxia, as well as the potential role of miRNAs. Expert opinion: We suggest that targeting CSC signatures along with EMT, inflammation, and hypoxia will provide a more effective therapeutic approach for the elimination of CSCs. To that end, curcumin especially its synthetic novel analog CDF have been shown to attenuate CSC characteristics along with the deregulation of multiple pathways and miRNAs, leading to the inhibition of human tumor growth in vivo, suggesting the potential role of CDF as an anti-tumor agent for the prevention/treatment of tumor progression.     

Approaches for targeting cancer stem cells drug resistance

ABSTRACT

Introduction: Several reports have suggested that a population of undifferentiated cells known as cancer stem cells (CSCs), is responsible for cancer formation and maintenance. In the last decade, the presence of CSCs in solid cancers have been reported.

Areas covered: This review summarizes the main approaches for targeting CSCs drug resistance. It is indeed known that CSCs may contribute to resistance to conventional chemotherapy, radiotherapy and targeted agents. Among the mechanisms by which CSCs escape anticancer therapies, removal of therapeutic agents by drug efflux pumps, enhanced DNA damage repair, activation of mitogenic/anti-apoptotic pathways; the main features of CSCs, stemness and EMT, are involved, as well as the capability to evade immune response.

Expert opinion: Different approaches are suitable to target CSCs mediated drug resistance. Some of them are currently under clinical evaluation in different cancer types. A better understanding of CSC biology, as well as more accurate study design, may maximize the therapeutic effects of these agents. In this respect, it is important to establish: (i) which molecules should be targeted; (ii) what drug combinations may be suitable; (iii) which patient settings will CSC targeting offer the highest clinical benefit; and (iv) how to integrate therapeutic approaches targeting CSCs with standard cancer therapy.     

Repositioning chloroquine and metformin to eliminate cancer stem cell traits in pre-malignant lesions

Ideal oncology drugs would be curative after a short treatment course if they could eliminate epithelium-originated carcinomas at their non-invasive, pre-malignant stages. Such ideal molecules, which are expected to molecularly abrogate all the instrumental mechanisms acquired by migrating cancer stem cells (CSCs) to by-pass tumour suppressor barriers, might already exist. We here illustrate how system biology strategies for repositioning existing FDA-approved drugs may accelerate our therapeutic capacity to eliminate CSC traits in pre-invasive intraepithelial neoplasias. First, we describe a signalling network signature that overrides bioenergetics stress- and oncogene-induced senescence (OIS) phenomena in CSCs residing at pre-invasive lesions. Second, we functionally map the anti-malarial chloroquine and the anti-diabetic metformin (“old drugs”) to their recently recognized CSC targets (“new uses”) within the network. By discussing the preclinical efficacy of chloroquine and metformin to inhibiting the genesis and self-renewal of CSCs we finally underscore the expected translational impact of the “old drugs–new uses” repurposing strategy to open a new CSC-targeted chemoprevention era.     

Cancer stem cell patents

Background: Cancer stem cells (CSCs) are a small subpopulation of the tumour mass that are capable of sustaining its growth, are resistant to many cancer specific therapies, and can reinitiate disease relapse. CSCs are resistant to current cancer treatments due to specialised transporters, cell cycle changes, DNA repair and antiapoptotic mechanisms. Objective: This paper reviews CSC biology and diagnostic and therapeutic patents as well as those associated with the isolation of CSCs. Methods: Literature and patent searches using the NCBI PubMed, European Patent Office, Scopus, and PATENTSCOPE^® websites were conducted to examine and link CSC biology and therapy development with current patent literature. Conclusion: Development in basic CSC biology research is increasing the patents filed in this area; however, therapeutic patents directly targeting CSCs are more limited, as research in this area remains in its infancy.     

Recent advances in drug delivery systems for targeting cancer stem cells

Cancer stem cells (CSCs) are a subpopulation of cancer cells with functions similar to those of normal stem cells. Although few in number, they are capable of self-renewal, unlimited proliferation, and multi-directional differentiation potential. In addition, CSCs have the ability to escape immune surveillance. Thus, they play an important role in the occurrence and development of tumors, and they are closely related to tumor invasion, metastasis, drug resistance, and recurrence after treatment. Therefore, specific targeting of CSCs may improve the efficiency of cancer therapy. A series of corresponding promising therapeutic strategies based on CSC targeting, such as the targeting of CSC niche, CSC signaling pathways, and CSC mitochondria, are currently under development. Given the rapid progression in this field and nanotechnology, drug delivery systems (DDSs) for CSC targeting are increasingly being developed. In this review, we summarize the advances in CSC-targeted DDSs. Furthermore, we highlight the latest developmental trends through the main line of CSC occurrence and development process; some considerations about the rationale, advantages, and limitations of different DDSs for CSC-targeted therapies were discussed.KEY WORDS: Cancer stem cells, Targeting strategies, Drug delivery systems, Cancer treatment, Niche, Biomarker, Cellular level, Molecular level-lactide); PEG-PCD, poly(ethylene glycol)-block-poly(2-methyl-2-carboxyl-propylene carbonate-graft-dodecanol); PEG-PLA, poly(ethylene glycol)-b-poly(d,l-lactide); PLGA, poly(ethylene glycol)-poly(d,l-lactide-co-glycolide); PTX, paclitaxel; PU-PEI, polyurethane-short branch-polyethylenimine; Sali-ABA, 4-(aminomethyl) benzaldehyde-modified Sali; SLNs, solid lipid nanoparticles; SSCs, somatic stem cells; TNBC, triple negative breast cancer; TPZ, tirapazamine; uPAR, urokinase plasminogen activator receptor     

Targeting CSCs within the tumor microenvironment for cancer therapy: a potential role of mesenchymal stem cells

Introduction: Mesenchymal stem cells (MSCs) are one subgroup of adult stem cells and possess a proliferative potential and ability to differentiate into various ceells.

Areas covered: Emerging evidence suggests that MSCs can reprogram toward cancer stem cells (CSCs), due to alterations of intrinsic and extrinsic microenvironments, leading to tumorigenesis. The CSC concept has fundamental clinical implications because of its involvement in cell migration/invasion, metastasis, and treatment resistance. Therefore, targeting CSCs provides a novel therapeutic strategy for cancer treatment. However, the origin of CSCs and its molecular connections are not fully understood. Emerging evidence suggests the existence of an inter-relationship between CSCs and epithelial-to-mesenchymal transition (EMT) phenotypic cells, in the context of inflammation and hypoxia, as well as the potential role of miRNAs.

Expert opinion: We suggest that targeting CSC signatures along with EMT, inflammation, and hypoxia will provide a more effective therapeutic approach for the elimination of CSCs. To that end, curcumin especially its synthetic novel analog CDF have been shown to attenuate CSC characteristics along with the deregulation of multiple pathways and miRNAs, leading to the inhibition of human tumor growth in vivo, suggesting the potential role of CDF as an anti-tumor agent for the prevention/treatment of tumor progression.     

CD133 as a target for colon cancer

Introduction: Recent evidence based on cancer stem cell (CSC) models, is boosting the progress of translational research and providing relevant clinical implications in many tumour types, including colorectal cancer. The current failure of standard therapies is attributed to a small fraction of the primary cell population with stem-like characteristics, such as self-renewal and differentiation. Identification of CSCs is based on two different criteria of selection: stemness-selective conditions and direct isolation based on putative stem cell markers expression. CD133, a transmembrane glycoprotein, was associated with tumor-initiating cells derived from several histological variants of tumors, including colon.

Areas covered: In this review the current understandings about CD133 as putative marker of tumour-initiating cells in colorectal cancer (CRC) is described. The focus of the discussion is on the need for additional markers to better identify the cell population able to recapitulate the parental tumor in immunocompromised mice.

Expert opinion: Identification and characterization of CSCs represents a relevant issue to define innovative therapeutic approaches, overcoming the emergence of cancer cell clones capable of evading standard therapy.     

Rapid phenotyping of cancer stem cells using multichannel nanosensor arrays

Cancer stem cells (CSCs) contribute to multidrug resistance, tumor recurrence and metastasis, making them prime therapeutic targets. Their ability to differentiate and lose stem cell properties makes them challenging to study. Currently, there is no simple assay that can quickly capture and trace the dynamic phenotypic changes on the CSC surface. Here, we report rapid discrimination of breast CSCs from non-CSCs using a nanoparticle-fluorescent-protein based sensor. This nanosensor was employed to discriminate CSCs from non-CSCs, as well as CSCs that had differentiated in vitro in two breast cancer models. Importantly, the sensor platform could also discriminate CSCs from the bulk population of cells in patient-derived xenografts of human breast cancer. Taken together, the results obtained demonstrate the feasibility of using the nanosensor to phenotype CSCs and monitor their fate. Furthermore, this approach provides a novel area for therapeutic interventions against these challenging targets.     

Cancer stem cell targeting: the next generation of cancer therapy and molecular imaging

Cancer stem cells (CSCs) have the capacity to generate the heterogeneous lineages of all cancer cells comprising a tumor and these populations of cells are likely to be more relevant in determining prognosis. However, these cells do not operate in isolation, but instead rely upon signals co-opted from their microenvironment, making the targeting and imaging of CSCs within a cancer mass a daunting task. A better understanding of the molecular cell biology underlying CSC pathology will facilitate the development of new therapeutic targets and novel strategies for the successful eradication of cancer. In addition, the continued investigation of sensitive molecular-imaging modalities will enable more accurate staging, treatment planning and the ability to monitor the effectiveness of CSC-targeted therapies in vivo. In this review, we explore the possibilities and limitations of CSC-directed therapies and molecular imaging modalities.     

Nanog mediates tobacco smoke‐induced enhancement of renal cancer stem cell properties

Nanog plays an important role in the regulation of cancer stem cells (CSCs) which participate in tumorgenesis and progression. In renal cancer, tobacco smoke (TS) is considered a major risk factor. However, the molecular mechanism by which TS induces the development of renal CSC properties remains largely unknown. In this study, we showed that the level of Nanog was elevated in renal cell carcinoma (RCC) patients with a smoking history, and that Nanog overexpression promoted the traits of CSCs in renal cancer. We further demonstrated that a 8‐week exposure of TS enhanced the formation of renal tumorspheres, increased the population of CD133‐positive cells, and stimulated the expression of Nanog and CSC markers. In addition, TS was found to play a role in accelerating the cell growth transition from G1 to S phase in renal CSCs. Finally, we demonstrated that the TS‐induced effects in renal CSCs could be reversed through the downregulation of Nanog. Our results suggested that Nanog plays a role in mediating TS‐induced renal CSC properties. This study may provide new insights into the molecular mechanism of TS‐related renal tumorigenesis, which can contribute to the future development of therapeutics for renal cancer.     

肿瘤干细胞的来源、标志物以及在毒理学研究中的应用

肿瘤干细胞(CSCs)是一类具有自我更新能力并且可以产生异质性肿瘤的细胞,是目前肿瘤研究的热点,在肿瘤的发生、发展和转移的机制探讨,肿瘤的评估、早期诊断和预后以及抗肿瘤药物的筛选方面展现了良好的应用前景。本文综述了CSCs来源的几种形式,归纳了常见恶性肿瘤中的标志物,概括了CSCs在毒理学研究中的应用,最后展望了CSCs研究的发展方向。