Strategies to eliminate cancer stem cells: clinical implications

Hydrodynamic shear-based purification of cancer cells with enhanced tumorigenic potential

Tumor-initiating cells (TICs), a subpopulation of cancerous cells with high tumorigenic potential and stem-cell-like properties, drive tumor progression and are resistant to conventional therapies. Identification and isolation of TICs are limited by their low frequency and lack of robust markers. Here, we characterize the heterogeneous adhesive properties of a panel of human and murine cancer cells and demonstrate differences in adhesion strength among cells, which exhibit TIC properties and those that do not. These differences in adhesion strength were exploited to rapidly (~10 min) and efficiently isolate cancerous cells with increased tumorigenic potential in a label-free manner by use of a microfluidic technology. Isolated murine and human cancer cells gave rise to larger tumors with increased growth rate and higher frequency in both immunocompetent and immunocompromised mice, respectively. This rapid and label-free TIC isolation technology has the potential to be a valuable tool for facilitating research into TIC biology and the development of more efficient diagnostics and cancer therapies.

Eco-oncology: Applying ecological principles to understand and manage cancer

Cancer is a disease of single cells that expresses itself at the population level. The striking similarities between initiation and growth of tumors and dynamics of biological populations, and between metastasis and ecological invasion and community dynamics suggest that oncology can benefit from an ecological perspective to improve our understanding of cancer biology. Tumors can be viewed as complex, adaptive, and evolving systems as they are spatially and temporally heterogeneous, continually interacting with each other and with the microenvironment and evolving to increase the fitness of the cancer cells. We argue that an eco-evolutionary perspective is essential to understand cancer biology better. Furthermore, we suggest that ecologically informed therapeutic approaches that combine standard of care treatments with strategies aimed at decreasing the evolutionary potential and fitness of neoplastic cells, such as disrupting cell-to-cell communication and cooperation, and preventing successful colonization of distant organs by migrating cancer cells, may be effective in managing cancer as a chronic condition.

Directing Stem Cell Fate: The Synthetic Natural Product Connection

Stem cells possess remarkable potential for the treatment of a broad array of diseases including many that lack therapeutic options. However, the use of cell-based products derived from stem cells as therapeutics has limitations including rejection, sufficient availability, and lack of appropriate engraftment. Chemical control of stem cells provides potential solutions for overcoming many of the current limitations in cell-based therapeutics. The development of exogenous molecules to control stem cell self-renewal or differentiation has arrived at natural product-based agents as an important class of modulators. The ex vivo production of cryopreserved cellular products for use in tissue repair is a relatively new area of medicine in which the conventional hurdles to implementing chemicals to effect human health are changed. Translational challenges centered on chemistry, such as pharmacokinetics, are reduced. Importantly, in many cases the desired human tissues can be evaluated against new chemicals, and approaches to cellular regulation can be validated in the clinically applicable system. As a result linking new and existing laboratory syntheses of natural products with findings of the compounds' unique abilities to regulate stem cell fate provides opportunities for developing improved methods for tissue manufacture, accessing probe compounds, and generating new leads that yield manufactured cells with improved properties. This review provides a summary of natural products that have shown promise in controlling stem cell fate and which have also been fully synthesized thereby providing chemistry platforms for further development.

Targeting acute myeloid leukemia stem cell signaling by natural products

Acute myeloid leukemia (AML) is the most commonly diagnosed leukemia in adults (25%) and comprises 15-20% in children. It is a genetically heterogeneous aggressive disease characterized by the accumulation of somatically acquired genetic changes, altering self-renewal, proliferation, and differentiation of hematopoietic progenitor cells, resulting in uncontrolled clonal proliferation of malignant progenitor myeloid cells in the bone marrow, peripheral blood, and occasionally in other body tissues. Treatment with modern chemotherapy regimen (cytarabine and daunorubicin) usually achieves high remission rates, still majority of patients are found to relapse, resulting in only 40-45% overall 5 year survival in young patients and less than 10% in the elderly AML patients. The leukemia stem cells (LSCs) are characterized by their unlimited self-renewal, repopulating potential and long residence in a quiescent state of G0/G1 phase. LSCs are considered to have a pivotal role in the relapse and refractory of AML. Therefore, new therapeutic strategies to target LSCs with limited toxicity towards the normal hematopoietic population is critical for the ultimate curing of AML. Ongoing research works with natural products like parthenolide (a natural plant extract derived compound) and its derivatives, that have the ability to target multiple pathways that regulate the self-renewal, growth and survival of LSCs point to ways for a possible complete remission in AML. In this review article, we will update and discuss various natural products that can target LSCs in AML.

Update on acute myeloid leukemia stem cells: New discoveries and therapeutic opportunities

The existence of cancer stem cells has been well established in acute myeloid leukemia. Initial proof of the existence of leukemia stem cells (LSCs) was accomplished by functional studies in xenograft models making use of the key features shared with normal hematopoietic stem cells (HSCs) such as the capacity of self-renewal and the ability to initiate and sustain growth of progenitors in vivo. Significant progress has also been made in identifying the phenotype and signaling pathways specific for LSCs. Therapeutically, a multitude of drugs targeting LSCs are in different phases of preclinical and clinical development. This review focuses on recent discoveries which have advanced our understanding of LSC biology and provided rational targets for development of novel therapeutic agents. One of the major challenges is how to target the self-renewal pathways of LSCs without affecting normal HSCs significantly therefore providing an acceptable therapeutic window. Important issues pertinent to the successful design and conduct of clinical trials evaluating drugs targeting LSCs will be discussed as well.

Tyrosine kinase inhibitor-induced CD70 expression mediates drug resistance in leukemia stem cells by activating Wnt signaling

In chronic myelogenous leukemia (CML), oncogenic BCR-ABL1 activates the Wnt pathway, which is fundamental for leukemia stem cell (LSC) maintenance. Tyrosine kinase inhibitor (TKI) treatment reduces Wnt signaling in LSCs and often results in molecular remission of CML; however, LSCs persist long term despite BCR-ABL1 inhibition, ultimately causing disease relapse. We demonstrate that TKIs induce the expression of the tumor necrosis factor (TNF) family ligand CD70 in LSCs by down-regulating microRNA-29, resulting in reduced CD70 promoter DNA methylation and up-regulation of the transcription factor specificity protein 1. The resulting increase in CD70 triggered CD27 signaling and compensatory Wnt pathway activation. Combining TKIs with CD70 blockade effectively eliminated human CD34(+) CML stem/progenitor cells in xenografts and LSCs in a murine CML model. Therefore, targeting TKI-induced expression of CD70 and compensatory Wnt signaling resulting from the CD70/CD27 interaction is a promising approach to overcoming treatment resistance in CML LSCs.

Targeting leukemia stem cells: which pathways drive self-renewal activity in T-cell acute lymphoblastic leukemia?

T-Cell acute lymphoblastic leukemia (t-all) is a malignancy of white blood cells, characterized by an uncontrolled accumulation of T-cell progenitors. During leukemic progression, immature T cells grow abnormally and crowd into the bone marrow, preventing it from making normal blood cells and spilling out into the bloodstream. Recent studies suggest that only discrete cell populations that possess the ability to recreate the entire tumour might be responsible for the initiation and propagation of t-all. Those unique cells are commonly called "cancer stem cells" or, in the case of hematopoietic malignancies, "leukemia stem cells" (lscs). Like normal hematopoietic stem cells, lscs are thought to be capable of self-renewal, during which, by asymmetrical division, they give rise to an identical copy of themselves as well as to a daughter cell that is no longer capable of self-renewal activity and represents a more "differentiated" progeny. Here, we review the main pathways of self-renewal activity in lscs, focusing on their involvement in the maintenance and development of t-all. New stem cell-directed therapies and lsc-targeted agents are also discussed.

Targeting acute myeloid leukemia stem cells: a review and principles for the development of clinical trials

Despite an increasingly rich understanding of its pathogenesis, acute myeloid leukemia remains a disease with poor outcomes, overwhelmingly due to disease relapse. In recent years, work to characterize the leukemia stem cell population, the disease compartment most difficult to eliminate with conventional therapy and most responsible for relapse, has been undertaken. This, in conjunction with advances in drug development that have allowed for increasingly targeted therapies to be engineered, raises the hope that we are entering an era in which the leukemia stem cell population can be eliminated, resulting in therapeutic cures for acute myeloid leukemia patients. For these therapies to become available, they must be tested in the setting of clinical trials. A long-established clinical trials infrastructure has been employed to shepherd new therapies from proof-of-concept to approval. However, due to the unique features of leukemia stem cells, drugs that are designed to specifically eliminate this population may not be adequately tested when applied to this model. Therefore, in this review article, we seek to identify the relevant features of acute myeloid leukemia stem cells for clinical trialists, discuss potential strategies to target leukemia stem cells, and propose a set of guidelines outlining the necessary elements of clinical trials to allow for the successful testing of stem cell-directed therapies.

Advances in patient-derived tumor xenografts: from target identification to predicting clinical response rates in oncology

Most oncology compounds entering clinical development have passed stringent preclinical pharmacology evaluation criteria. However, only a small fraction of experimental agents induce meaningful antitumor activities in the clinic. Low predictability of conventional preclinical pharmacology models is frequently cited as a main reason for the unusually high clinical attrition rates of therapeutic compounds in oncology. Therefore, improvement in the predictive values of preclinical efficacy models for clinical outcome holds great promise to reduce the clinical attrition rates of experimental compounds. Recent reports suggest that pharmacology studies conducted with patient derived xenograft (PDX) tumors are more predictive for clinical outcome compared to conventional, cell line derived xenograft (CDX) models, in particular when therapeutic compounds were tested at clinically relevant doses (CRDs). Moreover, the study of the most malignant cell types within tumors, the tumor initiating cells (TICs), relies on the availability of preclinical models that mimic the lineage hierarchy of cells within tumors. PDX models were shown to more closely recapitulate the heterogeneity of patient tumors and maintain the molecular, genetic, and histological complexity of human tumors during early stages of sequential passaging in mice, rendering them ideal tools to study the responses of TICs, tumor- and stromal cells to therapeutic intervention. In this commentary, we review the progress made in the development of PDX models in key areas of oncology research, including target identification and validation, tumor indication search and the development of a biomarker hypothesis that can be tested in the clinic to identify patients that will benefit most from therapeutic intervention.

Systems mapping of genes controlling chemotherapeutic drug efficiency for cancer stem cells

Cancer can be controlled effectively by using chemotherapeutic drugs to inhibit cancer stem cells, but there is considerable inter-patient variability regarding how these cells respond to drug intervention. Here, we describe a statistical framework for mapping genes that control tumor responses to chemotherapeutic drugs as well as the efficacy of treatments in arresting tumor growth. The framework integrates the mathematical aspects of the cancer stem cell hypothesis into genetic association studies, equipped with a capacity to quantify the magnitude and pattern of genetic effects on the kinetic decline of cancer stem cells in response to therapy. By quantifying how specific genes and their interactions govern drug response, the model provides essential information to tailor personalized drugs for individual patients.

Patient-derived xenografts, the cancer stem cell paradigm, and cancer pathobiology in the 21st century

Cancer is a heterogeneous disease manifest in many forms. Tumor histopathology can differ significantly among patients and cellular heterogeneity within tumors is common. A primary goal of cancer biologists is to better understand tumorigenesis and cancer progression; however, the complex nature of tumors has posed a substantial challenge to unlocking cancer's secrets. The cancer stem cell (CSC) paradigm for the pathobiology of solid tumors appropriately acknowledges phenotypic and functional tumor cell heterogeneity observed in solid tumors and accounts for the disconnect between drug approval based on response and the general inability of approved therapies to meaningfully impact survival due to their failure to eradicate these most important of cellular targets. First proposed to exist decades ago, CSC have only recently begun to be precisely identified due to technical advancements that facilitate identification, isolation, and interrogation of distinct tumor cell subpopulations with differing ability to form and perpetuate tumors. Precise identification of CSC populations and the complete hierarchy of cells within solid tumors will facilitate more accurate characterization of patient subtypes and ultimately contribute to more personalized and effective therapies. Rapid advancement in the understanding of tumor biology as it exists in patients requires cooperation among institutions, surgeons, pathologists, cancer biologists and patients alike, primarily because this translational research is best done with patient-derived tissue grown in the xenograft setting as patient-derived xenografts. This review calls for a broader change in the approaches taken to study cancer pathobiology, highlights what implications the CSC paradigm has for pathologists and cancer biologists alike, and calls for greater collaboration between institutions, physicians and scientists in order to more rapidly advance our collective understanding of cancer.

Prognostic value of changes in the expression of stem cell markers in the peripheral blood of patients with colon cancer

Cancer stem cells play an important role in carcinogenesis and resistance to treatment and may lead to metastasis. The isolation of circulating stem cells involves cell sorting based on the presence of cell surface markers. Many surface markers such as CD133, c-Kit, SOX, OCT4 and TWIST have been reported. In the present study, we determined the expression of different stem cell markers and their variation in expression at different stages of the treatment process. Samples of EDTA blood were collected from metastatic colorectal cancer patients, and circulating cancer stem cells were isolated for the analysis of the expression of stem cell markers using RT-PCR. These findings were correlated with the response to therapy. All statistical analyses were performed using the GraphPad Prism 5.03 software. Significant differences were found in the expression levels of the markers CD133, SOX2, OCT4 and TWIST1. No differences were found in c-Kit expression. Correlation in the expression levels of most of the markers was observed. Expression of CD133, OCT4, SOX2 and TWIST1 had a predictive value for colon cancer behavior. Evaluation of this stem cell gene expression panel may be useful for predicting the response during the process of treatment, and the relative easy access to samples facilitates this method. Moreover the correlation between CD133 and TWIST1 expression may be associated with tumor regrowth and metastatic relapse.

Tumor-initiating cells and FZD8 play a major role in drug resistance in triple-negative breast cancer

Triple-negative breast cancer (TNBC) studies have shown that neoadjuvant chemotherapy before surgery was effective in the minority of women, whereas the majority who had residual tumor had a relatively poor outcome. To identify the mechanism by which residual cancer cells survive chemotherapy, we initially conducted gene expression profiling using the CRL2335 TNBC cell line derived from a squamous breast carcinoma before and after treatment with cisplatin plus TRAIL. We found a significant increase in the expression of FZD8, one of Wnt receptors, and its downstream targets LEF1 and TCF in residual CRL2335 tumor cells after treatment with cisplatin plus TRAIL. Increased FZD8 levels were further confirmed in other TNBC cell lines. Inhibition of FZD8 by siRNA in CRL2335 cells in the presence of cisplatin plus TRAIL reduced β-catenin and survivin levels and increased apoptosis compared with scrambled siRNA-treated cells. In vivo data show that cisplatin plus TRAIL treatment significantly reduces tumor volume in NOD/SCID mice. However, we found that cisplatin plus TRAIL treatment predominantly eliminated non-tumor-initiating cells, as shown by whole-body fluorescent imaging of mice injected with mammosphere-forming CRL2335 cells stably transfected with DsRed. This led to TIC enrichment in residual tumors, as confirmed by immunostaining for TIC markers. Moreover, an increase in FZD8 expression was observed in residual tumors treated with cisplatin and TRAIL. Taken together, our findings suggest that FZD8-mediated Wnt signaling may play a major role in mediating resistance to chemotherapy, making it a potential target to enhance chemotherapeutic efficacy in patients with TNBCs.

Enhancement of cancer stem-like and epithelial-mesenchymal transdifferentiation property in oral epithelial cells with long-term nicotine exposure: reversal by targeting SNAIL

Cigarette smoking is one of the major risk factors in the development and further progression of tumorigenesis, including oral squamous cell carcinoma (OSCC). Recent studies suggest that interplay cancer stem-like cells (CSCs) and epithelial-mesenchymal transdifferentiation (EMT) properties are responsible for the tumor maintenance and metastasis in OSCC. The aim of the present study was to investigate the effects of long-term exposure with nicotine, a major component in cigarette, on CSCs and EMT characteristics. The possible reversal regulators were further explored in nicotine-induced CSCs and EMT properties in human oral epithelial (OE) cells. Long-term exposure with nicotine was demonstrated to up-regulate ALDH1 population in normal gingival and primary OSCC OE cells dose-dependently. Moreover, long-term nicotine treatment was found to enhance the self-renewal sphere-forming ability and stemness gene signatures expression and EMT regulators in OE cells. The migration/cell invasiveness/anchorage independent growth and in vivo tumor growth by nude mice xenotransplantation assay was enhanced in long-term nicotine-stimulated OE cells. Knockdown of Snail in long-term nicotine-treated OE cells was found to reduce their CSCs properties. Therapeutic delivery of Si-Snail significantly blocked the xenograft tumorigenesis of long-term nicotine-treated OSCC cells and largely significantly improved the recipient survival. The present study demonstrated that the enrichment of CSCs coupled EMT property in oral epithelial cells induced by nicotine is critical for the development of OSCC tumorigenesis. Targeting Snail might offer a new strategy for the treatment of OSCC patients with smoking habit.

Cancer stem cells, microRNAs, and therapeutic strategies including natural products

Embryonic stem cells divide continuously and differentiate into organs through the expression of specific transcription factors at specific time periods. Differentiated adult stem cells on the other hand remain in quiescent state and divide by receiving cues from the environment (extracellular matrix or niche), as in the case of wound healing from tissue injury or inflammation. Similarly, it is believed that cancer stem cells (CSCs), forming a smaller fraction of the tumor bulk, also remain in a quiescent state. These cells are capable of initiating and propagating neoplastic growth upon receiving environmental cues, such as overexpression of growth factors, cytokines, and chemokines. Candidate CSCs express distinct biomarkers that can be utilized for their identification and isolation. This review focuses on the known and candidate cancer stem cell markers identified in various solid tumors and the promising future of disease management and therapy targeted at these markers. The review also provides details on the differential expression of microRNAs (miRNAs), and the miRNA- and natural product-based therapies that could be applied for the treatment of cancer stem cells.

Differential expression of CD133 based on microsatellite instability status in human colorectal cancer

The association between the types of genomic instability and cancer stem cell (CSC) has not been elucidated. We aimed to investigate the expressions of CSC markers with respect to microsatellite instability (MSI) status in human colorectal cancer (CRC). Immunostainings for CD133, CD44, and CD166, and K-ras mutation analysis were performed on 50 MSI-high (MSI-H), and 50 microsatellite stable (MSS) CRC tissues. In 11 MSS and MSI-H CRC cell lines, CD133 expression and DNA methylation statuses of the CD133 promoter were determined. The proportion of CD133 positive cells and the ability of colosphere formation were compared between HCT116 cells and HCT116 + Chr3 cells (hMLH1-restored HCT116 cells). Immunohistochemistry for CSC markers revealed that high CD133 expression was more frequent in MSS cancers than in MSI-H (P < 0.001, 74.0% vs. 28.0%, respectively), and related with short disease-free survival. Neither CD44 nor CD166 expression differed significantly with respect to MSI status. K-ras mutation showed no association with expressions of CD133, CD44, or CD166. CD133 expression was relatively high in the MSS cell lines compared to those in MSI-H, and showed a reverse correlation with DNA methylation of the CD133 promoter. hMLH1-restored HCT116 cells increased proportions of CD133 positive cells and colosphere forming ability, compared to those in HCT116 cells. In conclusion, high levels of CD133 expression were observed more frequently in MSS CRC than in MSI-H, suggesting that differential expression of colon CSC markers may be linked to tumor characteristics dependent on MSI status.

Breast cancer stem cells: obstacles to therapy

Only a fraction of the cells in a breast tumour are able to seed new tumour growth. These so-called breast cancer stem cells (bCSCs) are characterised by a number of discrete functional properties, some of which impact on therapeutic strategies aimed at eliminating these cells from tumours. Here we discuss how recent experimental evidence indicates that phenotypic plasticity is a central feature of tumour cell heterogeneity and drug resistance, traits that must be overcome in order to efficiently target bCSCs as a therapy for breast cancer. We propose that a better understanding of this fundamental property of breast cancer stem cells, over and above their identification in tumours, is a priority for improvement of patient survival.

The emerging role of viruses in the treatment of solid tumours

There is increasing optimism for the use of non-pathogenic viruses in the treatment of many cancers. Initial interest in oncolytic virotherapy was based on the observation of an occasional clinical resolution of a lymphoma after a systemic viral infection. In many cancers, by comparison with normal tissues, the competency of the cellular anti-viral mechanism is impaired, thus creating an exploitable difference between the tumour and normal cells, as an unimpeded viral proliferation in cancer cells is eventually cytocidal. In addition to their oncolytic capability, these particular viruses may be engineered to facilitate gene delivery to tumour cells to produce therapeutic effects such as cytokine secretion and anti -tumour immune responses prior to the eventual cytolysis. There is now promising clinical experience with these viral strategies, particularly as part of multimodal studies, and already several clinical trials are in progress. The limitations of standard cancer chemotherapies, including their lack of specificity with consequent collateral toxicity and the development of cross-resistance, do not appear to apply to viral-based therapies. Furthermore, virotherapy frequently restores chemoradiosensitivity to resistant tumours and has also demonstrated efficacy against cancers that historically have a dismal prognosis. While there is cause for optimism, through continued improvements in the efficiency and safety of systemic delivery, through the emergence of alternative viral agents and through favourable clinical experiences, clinical trials as part of multimodal protocols will be necessary to define clinical utility. Significant progress has been made and this is now a major research area with an increasing annual bibliography.

Idiotype vaccine strategies for treatment of follicular lymphoma

Follicular lymphoma is an indolent lymphoma associated with a relapsing course. Immunization with tumor B cell idiotype (Id; a unique variable region of surface B cell immunoglobulin) may induce humoral and cellular immune response against the tumor. Based on promising results from early phase clinical trials with Id vaccine, three Phase III trials were initiated, which, despite failing to meet their primary end points, still provided a glimmer of optimism. This article describes the clinical development of the Id vaccine against follicular lymphoma, outlines the outcomes of clinical trials and delineates the future prospects for the integration of the idiotype vaccine into follicular lymphoma treatment.

Quantitative analyses of CD133 expression facilitate researches on tumor stem cells

CD133 is regarded as a marker of tumor initiating cells in many tumors, including colorectal cancer. O'Brien and Ricci et al. have proved that in primary colorectal tumors there are colorectal tumor stem cells (initiating cells) which are marked by CD133 antigen. Using a genetic knockin lacZ reporter mouse model, Shmelkov et al. challenged this increasingly influential viewpoint and drew two important conclusions that challenge former opinions. First, CD133 is widely distributed throughout the full range of tumor epithelial cells in the colon as opposed to being limited to a few cells. Second, CD133 negative cells of colon tumors are also tumorigenic, and are more inclined to metastasize. Based on these two opinions, we hypothesize that the expression of CD133 is different among tumor cells, and that quantitative but not qualitative analyses of CD133 abundance are necessary to determine the relationship between CD133 expression and tumor stem cell characteristics. To verify this hypothesis, colorectal cancer cell line SW620 was cultured and sorted into CD133(Hi), CD133(Mid) and CD133(Low) subgroups using magnetic microbeads to compare their xenograft biological characteristics. The results showed that the CD133(Hi) subgroup of SW620 is more close to the tumor initiating cells in terms of biological characteristics than CD133(Mid) and CD133(low) subgroups, but the CD133(low) subgroup still maintains the ability of tumorigenicity. It supported that tumor initiating cells are more correlated to the abundance of CD133.

Characteristics of CD133(+) human colon cancer SW620 cells

Worldwide, colorectal cancer is the third most common type of cancer affecting both sexes. It has been proposed that a small subset of cancer cells (cancer stem cells) within each tumor is able to initiate tumor growth. In 2007, two research groups simultaneously identified a colon cancer stem cell population in human tumors by the use of CD133 expression. In the present study, we used a human colon cancer cell line, SW620, to analyze the cancer stem cell-like characteristics of CD133(+) cells in vitro and in vivo. In vitro, CD133(+) SW620 cells had a higher proliferative capacity, were more irradiation- and chemotherapy-resistant, and had a higher expression of β-catenin compared with CD133(-) cells. Injections of either CD133(+) or CD133(-) cells into the skin or rectal mucosa of NOD/SCID mice led to tumors; however, injection of CD133(+) cells resulted in the formation of larger tumors. Tumors derived from injections of CD133(-) cells did not contain any CD133(+) cells, whereas tumors derived from injections of CD133(+) cells did contain CD133(+) cells, suggesting self-renewing capability. However, the proportion of CD133(+) cells in the newly formed tumors in vivo was lower than the proportion of CD133(+) cells in vitro. In conclusion, the human colon cancer cell line, SW620, contains both CD133(+) and CD133(-) phenotypes, and the CD133(+) phenotype has characteristics consistent with those of cancer stem cells.

Progress in stem cell-derived technologies for hepatocellular carcinoma

Primary hepatocellular carcinoma (HCC) is a common malignancy that has a poor prognosis because it is often diagnosed at an advanced stage. HCC normally develops as a consequence of underlying liver disease and is most often associated with cirrhosis. Surgical resection and liver transplantation are the current best options to treat liver cancer. However, problems associated with liver transplantation, such as shortage of donors, risk of immune rejection, and tissue damage following surgery provided the impetus for development of alternative therapies. The emerging field of stem cell therapy has raised hopes for finding curative options for liver cancer. Stem cells have the ability not only to proliferate after transplantation but also to differentiate into most mammalian cell types in vivo. In this review, progress on stem cell-derived technologies for the treatment of liver cancer is discussed.

Phenotypic characterization of mammosphere-forming cells from the human MA-11 breast carcinoma cell line

The phenotypic diversity of breast carcinoma may be explained by the existence of a sub-population of breast cancer cells, endowed with stem cell-like properties and gene expression profiles, able to differentiate along different pathways. A stem cell-like population of CD44(+)CD24(-/low) breast cancer cells was originally identified using cells from metastatic pleural effusions of breast carcinoma patients. We have previously reported that upon in vitro culture as mammospheres under stem cell-like conditions, human MA-11 breast carcinoma cells acquired increased tumorigenicity and lost CD24 expression compared with the parental cell line. We now report that upon passage of MA-11 mammospheres into serum-supplemented cultures, CD24 expression was restored; the rapid increase in CD24 expression was consistent with up-regulation of the antigen, and not with in vitro selection of CD24(+) cells. In tumors derived from subcutaneous injection of MA-11 mammospheres in athymic nude mice, 76.1+/-9.7% of cells expressed CD24, vs. 0.5+/-1% in MA-11 cells dissociated from mammospheres before injection. The tumorigenicity of sorted CD44(+)CD24(-) and CD44(+)CD24(high) MA-11 cells was equal. Single cell-sorted CD24(-) and CD24(high) MA-11 gave rise in vitro to cell populations with heterogeneous CD24 expression. Also, subcutaneous tumors derived from sorted CD24(-) sub-populations and single-cell clones had levels of CD24 expression similar to the unsorted cells. To investigate whether the high expression of CD24 contributed to the tumorigenic potential of MA-11 cells, we silenced CD24 by shRNA. CD24 silencing (95%) resulted in no difference in tumorigenicity upon s.c. injection in athymic nude mice compared with mock-transduced MA-11 cells. Since CD24 silencing was maintained in vivo, our data suggest that the level of expression of CD24 is associated with but does not contribute to tumorigenicity. We then compared the molecular profile of the mammospheres with the adherent cell fraction. Gene expression profiling revealed that the increased tumorigenicity of MA-11 mammospheres was associated with changes in 10 signal transduction pathways, including MAP kinase, Notch and Wnt, and increased expression of aldehyde dehydrogenase, a cancer-initiating cell-associated marker. Our data demonstrate that (i) the level of CD24 expression is neither a stable feature of mammosphere-forming cells nor confers tumorigenic potential to MA-11 cells; (ii) cancer-initiating cell-enriched MA-11 mammospheres have activated specific signal transduction pathways, potential targets for anti-breast cancer therapy.

Tumor biology and cancer therapy - an evolving relationship

The aim of palliative chemotherapy is to increase survival whilst maintaining maximum quality of life for the individual concerned. Although we are still continuing to explore the optimum use of traditional chemotherapy agents, the introduction of targeted therapies has significantly broadened the therapeutic options. Interestingly, the results from current trials put the underlying biological concept often into a new, less favorable perspective. Recent data suggested that altered pathways underlie cancer, and not just altered genes. Thus, an effective therapeutic agent will sometimes have to target downstream parts of a signaling pathway or physiological effects rather than individual genes. In addition, over the past few years increasing evidence has suggested that solid tumors represent a very heterogeneous group of cells with different susceptibility to cancer therapy. Thus, since therapeutic concepts and pathophysiological understanding are continuously evolving a combination of current concepts in tumor therapy and tumor biology is needed. This review aims to present current problems of cancer therapy by highlighting exemplary results from recent clinical trials with colorectal and pancreatic cancer patients and to discuss the current understanding of the underlying reasons.

Possible involvement of brain tumour stem cells in the emergence of a fast-growing malignant meningioma after surgical resection and radiotherapy of high-grade astrocytoma: case report and preliminary laboratory investigation

The case of a 62-year old man diagnosed with radiation-induced meningioma (RIM) after treatment for astrocytoma with an unusually short latency period of 7 months is reported. The patient first presented with a 2-month history of memory decline. Magnetic resonance imaging (MRI) showed a tumour in the left parieto-temporal lobe. Gross total resection was performed and the tumour was confirmed to be an astrocytoma. The patient received cranial radiotherapy 2 weeks later, however 7 months after radiation treatment the patient presented with headache and vomiting. MRI showed massive meningeal enhancement in the left frontal lobe, which progressively enlarged. The patient's clinical condition deteriorated and a second craniotomy was performed with complete removal of the secondary tumour, which was shown to be a malignant meningioma. Immunohistochemical staining identified CD133-positive cells in both tumours. A rare fraction of brain tumour stem cells (BTSC) was isolated from the primary astrocytoma using a serum-free culture system, suggesting that BTSC may have been involved in the rapid emergence of RIM after resection and radiation of the primary astrocytoma.

Cancer stem cell-directed therapies: recent data from the laboratory and clinic

Cancer stem cells (CSCs) are defined by their ability to (i) fully recapitulate the tumor of origin when transplanted into immunodeficient mouse hosts, and (ii) self-renew, demonstrated by their ability to be serially transplanted. These properties suggest that CSCs are required for tumor maintenance and metastasis; thus, it has been predicted that CSC elimination is required for cure. This prediction has profoundly altered paradigms for cancer research, compelling investigators to prospectively isolate CSCs to characterize the molecular pathways regulating their behavior. Many potential strategies for CSC-directed therapy have been proposed, but few studies have rigorously demonstrated their efficacy using in vivo models. Herein, we highlight recent studies that demonstrate the utility of CSC-directed therapies and discuss the implications of the CSC hypothesis to experimental design and therapeutic strategies.

An identification of stem cell-resembling gene expression profiles in high-grade astrocytomas

High-grade astrocytomas are among the most intractable types of cancers and are often fatal. Previous studies have suggested that high-grade astrocytomas may adopt the self-renewal and migration properties of neural stem cells (NSCs) to proliferate and spread by expressing the stem cell-specific genes. However, despite a few common molecules being documented, the molecular basis underlying these similarities remains largely unknown. To have a better understanding of the stem cell characteristics of high-grade astrocytomas, we performed the study to identify the stem cell-resembling gene expression profile in high-grade astrocytomas. cDNA microarray analysis was used to detect the differentially expressed genes of isolated human high-grade astrocytomas versus their peritumoral tissue counterparts, and the identification of stem cell-resembling genes was approached by comparing the high-grade astrocytomas-specific gene expression profile with that of NSCs identified by our previous study and other groups. We identified more than 200 high-grade astrocytomas-specific genes in this study, and near 10% genes or gene families of them exhibited similar up or down expression patterns as in NSCs. Further analysis indicated that these genes were actively involved in cell proliferation, adhesion, migration, and metastasis. This study revealed a list of stem cell-specific genes in high-grade astrocytomas, which was likely to have critical roles in determining the "stem" characteristics of high-grade astrocytomas.

Quantitative mass spectrometry identifies drug targets in cancer stem cell-containing side population

A multifaceted approach is presented as a general strategy to identify new drug targets in a breast cancer stem cell-containing side population. The approach we have utilized combines side population cell sorting and stable isotope labeling by amino acids in cell culture with mass spectrometry to compare and identify proteins with differential expression profiles between side population cells, know to be enriched in cancer stem cells, and nonside population cells, which are depleted in cancer stem cells, for two breast cancer cell lines, MCF7 and MDA-MB231. Almost 900 proteins were quantified, and several important proteins in cell cycle control and differentiation were found to be upregulated in the cancer stem cell-containing side population. Most interestingly, a splice isoform of pyruvate kinase M2 as well as peroxiredoxin 6 were found to be downregulated. The differential levels of three of these proteins, thymosin beta4 (TB4), proliferation-associated protein 2G4, and SIAH-interacting protein, were validated using Western blot. Furthermore, functional validation provided clear evidence that elevated TB4 expression contributes to drug resistance in the stem cell population. Small interfering RNA silencing of TB4 led to a loss of chemoresistance in two separate breast cancer populations. These proteins likely contribute to resistance in the cancer stem cell-containing side population, and their altered expression in a tumor causes clinical resistance to chemotherapy. The ability to perform quantitative mass spectrometry has enabled the identification of a series of proteins that could serve as future therapeutic targets.

Colorectal cancer stem cells are enriched in xenogeneic tumors following chemotherapy

Background

Patients generally die of cancer after the failure of current therapies to eliminate residual disease. A subpopulation of tumor cells, termed cancer stem cells (CSC), appears uniquely able to fuel the growth of phenotypically and histologically diverse tumors. It has been proposed, therefore, that failure to effectively treat cancer may in part be due to preferential resistance of these CSC to chemotherapeutic agents. The subpopulation of human colorectal tumor cells with an ESA⁺CD44⁺ phenotype are uniquely responsible for tumorigenesis and have the capacity to generate heterogeneous tumors in a xenograft setting (i.e. CoCSC). We hypothesized that if non-tumorigenic cells are more susceptible to chemotherapeutic agents, then residual tumors might be expected to contain a higher frequency of CoCSC.

Methods and Findings

Xenogeneic tumors initiated with CoCSC were allowed to reach ∼400 mm³, at which point mice were randomized and chemotherapeutic regimens involving cyclophosphamide or Irinotecan were initiated. Data from individual tumor phenotypic analysis and serial transplants performed in limiting dilution show that residual tumors are enriched for cells with the CoCSC phenotype and have increased tumorigenic cell frequency. Moreover, the inherent ability of residual CoCSC to generate tumors appears preserved. Aldehyde dehydrogenase 1 gene expression and enzymatic activity are elevated in CoCSC and using an in vitro culture system that maintains CoCSC as demonstrated by serial transplants and lentiviral marking of single cell-derived clones, we further show that ALDH1 enzymatic activity is a major mediator of resistance to cyclophosphamide: a classical chemotherapeutic agent.

Conclusions

CoCSC are enriched in colon tumors following chemotherapy and remain capable of rapidly regenerating tumors from which they originated. By focusing on the biology of CoCSC, major resistance mechanisms to specific chemotherapeutic agents can be attributed to specific genes, thereby suggesting avenues for improving cancer therapy.

New hope for cancer treatment: exploring the distinction between normal adult stem cells and cancer stem cells

For decades, intensive studies have attempted to identify the mechanisms underlying malignant tumor growth. Despite significant progress, most therapeutic approaches fail to eliminate all tumor cells. The remaining tumor cells often result in recurrence and metastasis. Recently, the idea of a cancer stem cell was proposed to explain of the origin of cancer cells. According to this hypothesis, a small fraction of tumor cells have the capacity for self-renewal, with unlimited slow proliferation potential. They are often resistant to chemotherapy and radiation and thus are responsible for continuously supplying new cancer cells, which themselves may have a limited life span. In recent years, accumulating experimental evidence supports this hypothesis and provides new possibilities to conquer cancer. This review will focus on the distinction between normal adult stem cells and cancer stem cells and identifies possible key targets for effective therapies of cancer.

Persistence of side population cells with high drug efflux capacity in pancreatic cancer

AIM: To investigate the persistence of side population (SP) cells in pancreatic cancer and their role and mechanism in the drug resistance.

METHODS: The presentation of side population cells in pancreatic cancer cell line PANC-1 and its proportion change when cultured with Gemcitabine, was detected by Hoechst 33342 staining and FACS analysis. The expression of ABCB1 and ABCG2 was detected by real-time PCR in either SP cells or non-SP cells.

RESULTS: SP cells do exist in PANC-1, with a median of 3.3% and a range of 2.1-8.7%. After cultured with Gemcitabine for 3 d, the proportion of SP cells increased significantly (3.8% ± 1.9%, 10.7% ± 3.7%, t = 4.616, P = 0.001 < 0.05). ABCB1 and ABCG2 expressed at higher concentrations in SP as compared with non-SP cells (ABCB1: 1.15 ± 0.72, 5.82 ± 1.16, t = 10.839, P = 0.000 < 0.05; ABCG2: 1.16 ± 0.75, 5.48 ± 0.94, t = 11.305, P = 0.000 < 0.05), which may contribute to the efflux of fluorescent staining and drug resistance.

CONCLUSION: SP cells with inherently high resistance to chemotherapeutic agents do exist in pancreatic cancers, which may be candidate cancer stem cells contributing to the relapse of the tumor.

Mathematical model for chemotherapeutic drug efficacy in arresting tumour growth based on the cancer stem cell hypothesis

OBJECTIVE

Cancer stem cells have been identified as the growth root for various malignant tumours and are thought to be responsible for cancer recurrence following treatment.

MATERIALS AND METHODS

Here, a predictive mathematical model for the cancer stem cell hypothesis is used to understand tumour responses to chemotherapeutic drugs and judge the efficacy of treatments in arresting tumour growth. The impact of varying drug efficacies on different abnormal cell populations is investigated through the kinetics associated with their decline in response to therapy.

RESULTS AND CONCLUSIONS

The model predicts the clinically established 'dandelion phenomenon' and suggests that the best response to chemotherapy occurs when a drug targets abnormal stem cells. We compare continuous and periodic drug infusion. For the latter, we examine the relative importance of the drug cell-kill rate and the mean time between successive therapies, to identify the key attributes for successful treatment.

Cancer stem cells and oncology therapeutics

PURPOSE OF REVIEW

An overview of the latest developments in the cancer stem cells field and their potential use in the oncology drug discovery process.

RECENT FINDINGS

Recent studies provided evidence of the existence of a subpopulation of cells within a variety of tumor types with a tumorigenic potential that is lacking in the rest of the cells within these tumors. There is mounting evidence that such cells exist in almost all tumor types. Work on the characterization of these cells suggests that deregulation of pathways responsible for stem cell self-renewal is a likely requirement for carcinogenesis and targeting such pathways might be curative. Progress has been made to develop more relevant in-vitro and in-vivo models that incorporate these findings.

SUMMARY

Cancer stem cells have been identified in a variety of tumors. Characterization of these cells, determining how they originate and developing relevant assays is a work in progress. Incorporating these findings in the cancer drug discovery process might lead to better therapeutics.

Tumour-initiating cells vs. cancer 'stem' cells and CD133: what's in the name?

Recent evidence suggests that a subset of cells within a tumour have 'stem-like' characteristics. These tumour-initiating cells, distinct from non-malignant stem cells, show low proliferative rates, high self-renewing capacity, propensity to differentiate into actively proliferating tumour cells, resistance to chemotherapy or radiation, and they are often characterised by elevated expression of the stem cell surface marker CD133. Understanding the molecular biology of the CD133(+) cancer cells is now essential for developing more effective cancer treatments. These may include drugs targeting organelles, such as mitochondria or lysosomes, using highly efficient and selective inducers of apoptosis. Alternatively, agents or treatment regimens that enhance sensitivity of these therapy-resistant "tumour stem cells" to the current or emerging anti-tumour drugs would be of interest as well.

Neoplastic stem cells: A novel therapeutic target in clinical oncology

Cancer is among the leading causes of morbidity and mortality in the Western world. Despite recent advances, most therapeutic approaches fail to eradicate the entire neoplastic clone. The remaining cells often develop metastasis and/or recurrences and therefore may represent attractive targets of therapy. A new exciting concept in this regard suggests that each neoplasm represents a heterogeneous population of cells that pertain to long-term tumor growth both in vivo in the natural host and in experimental animals. This concept postulates the existence of small fractions of 'tumor stem cells' that exhibit a capacity for self-renewal and unlimited growth and therefore are distinct from their progeny. Based on these hypotheses, the targeting of neoplastic stem cells is considered indispensable for eradication of the entire clone and for the development of curative treatment approaches. However, tumor stem cells often may be quiescent cells and may express a different profile of targets compared with 'more mature' tumor cells. Therefore, current efforts have attempted to characterize target expression profiles in cancer stem cells in various malignancies. In the this review, the authors have provided a brief summary of the current knowledge of neoplastic stem cells and the application of respective concepts in translational oncology with the ultimate objective of improving anticancer therapy.