Identifying cancer stem cells in solid tumors: case not proven

Cancer stem cell: implications in cancer biology and therapy with special reference to lung cancer

The cancer stem cell (CSC) theory is currently central to the field of cancer research, because it is not only a matter of academic interest but also crucial in cancer therapy. CSCs share a variety of biological properties with normal somatic stem cells in terms of self-renewal, the propagation of differentiated progeny, the expression of specific cell markers and stem cell genes, and the utilization of common signaling pathways and the stem cell niche. However, CSCs differ from normal stem cells in their tumorigenic activity. Thus, CSCs are also termed cancer initiating cells. In this paper, we briefly review hitherto described study results and refer to some excellent review articles to understand the basic properties of CSCs. In addition, we focus upon CSCs of lung cancers, since lung cancer is still increasing in incidence worldwide and remains the leading cause of cancer deaths. Understanding the properties of, and exploring cell markers and signaling pathways specific to, CSCs of lung cancers, will lead to progress in therapy, intervention, and improvement of the prognosis of patients with lung cancer. In the near future, the evaluation of CSCs may be a routine part of practical diagnostic pathology.

Cancer stem cells and tumor response to therapy: current problems and future prospects

The presence of a subpopulation of cells within tumors, so-called cancer stemlike cells, that is uniquely capable of reestablishing the tumor during and after definitive radio(chemo)therapy and must be effectively controlled for a long-term cure is being increasingly appreciated. The existence and physiology of a rare cancer cell population, termed cancer cell clonogens, with similar properties has been extensively described in the radiobiology literature for several decades based on studies using tumor cells transplanted into syngeneic or immunodeficient animals. The earlier studies have identified important features that govern tumor establishment; tumor growth and homeostasis; and therapeutic resistance, including clonogen number, tumor type, vascular status, hypoxia, repopulation dynamics during treatment, and immunologic and microenvironmental status. These discoveries led to therapeutic strategies, some of which have shown efficacy and have become current standard clinical practice (eg, concomitant boost and concurrent radio chemotherapy). Although the identity of cancer stemlike cells and cancer cell clonogens has not been definitively shown, recent characterization of molecular signaling pathways controlling stem cells and their microenvironmental niche combined with the earlier findings on clonogen physiology may now lead to the development of molecularly targeted strategies to overcome therapeutic resistance of this rare subpopulation of tumor cells. Along these lines, we describe 3 unique treatment settings (ie, before, during, and after definitive radio[chemo]therapy) in which molecularly targeted approaches might specifically counteract cancer stemlike cell resistance mechanisms and enhance the curative efficiency of radio(chemo)therapy.

Adult human mesenchymal stem cells enhance breast tumorigenesis and promote hormone independence

Adult human mesenchymal stem cells (hMSCs) have been shown to home to sites of breast cancer and integrate into the tumor stroma. We demonstrate here the effect of hMSCs on primary breast tumor growth and the progression of these tumors to hormone independence. Co-injection of bone marrow-derived hMSCs enhances primary tumor growth of the estrogen receptor-positive, hormone-dependent breast carcinoma cell line MCF-7 in the presence or absence of estrogen in SCID/beige mice. We also show hormone-independent growth of MCF-7 cells when co-injected with hMSCs. These effects were found in conjunction with increased immunohistochemical staining of the progesterone receptor in the MCF-7/hMSC tumors as compared to MCF-7 control tumors. This increase in PgR expression indicates a link between MCF-7 cells and MSCs through ER-mediated signaling. Taken together, our data reveal the relationship between tumor microenvironment and tumor growth and the progression to hormone independence. This tumor stroma-cell interaction may provide a novel target for the treatment of estrogen receptor-positive, hormone-independent, and endocrine-resistant breast carcinoma.

Statins, stem cells, and cancer

The statins (3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors) were proven to be effective antilipid agents against cardiovascular disease. Recent reports demonstrate an anticancer effect induced by the statins through inhibition of cell proliferation, induction of apoptosis, or inhibition of angiogenesis. These effects are due to suppression of the mevalonate pathway leading to depletion of various downstream products that play an essential role in cell cycle progression, cell signaling, and membrane integrity. Recent evidence suggests a shared genomic fingerprint between embryonic stem cells, cancer cells, and cancer stem cells. Activation targets of NANOG, OCT4, SOX2, and c-MYC are more frequently overexpressed in certain tumors. In the absence of bona fide cancer stem cell lines, human embryonic stem cells, which have similar properties to cancer and cancer stem cells, have been an excellent model throwing light on the anticancer affects of various putative anticancer agents. It was shown that key cellular functions in karyotypically abnormal colorectal and ovarian cancer cells and human embryonic stem cells are inhibited by the statins and this is mediated via a suppression of this stemness pathway. The strategy for treatment of cancers may thus be the targeting of a putative cancer stem cell within the tumor with specific agents such as the statins with or without chemotherapy. The statins may thus play a dual prophylactic role as a lipid-lowering drug for the prevention of heart disease and as an anticancer agent to prevent certain cancers. This review examines the relationship between the statins, stem cells, and certain cancers.

CD133+ and nestin+ tumor-initiating cells dominate in N29 and N32 experimental gliomas

The current study was designed to critically evaluate the notion that cancer stem cell (CSC)-like cells constitute a subpopulation of cells within experimental gliomas. Virtually all cells within the N29 and N32 rat glioma models homogenously expressed CD133, the stem/progenitor marker nestin as well as the neural lineage markers glial fibrillary acidic protein, betaIII-tubulin, and CNPase in vitro. The phenotype was largely retained on exposure to conditions promoting differentiation in vitro and after intracranial implantation of tumor cells into syngeneic hosts. Unsorted adherently grown cells displayed very high clonogenicity in vitro and robust tumorigenicity in vivo. Single N29 and N32 tumor cells invariably formed clones in vitro, and intracerebral inoculation of as few as 10 adherently growing N29 and N32 tumor cells, respectively, gave rise to a tumor. These results provide an alternative view on CSC-like cells in glioma models: sphere-formation is not a prerequisite for accumulation of tumorigenic cells, and CSC-like cells do not reside within a rare subpopulation of cells in these glioma models. N29 and N32 gliomas may accordingly be used for the development of treatment strategies directed specifically against a practically pure population of brain tumor-initiating CSC-like cells.

The controversial clinicobiological role of breast cancer stem cells

Breast cancer remains a leading cause of morbidity and mortality in women mainly because of the propensity of primary breast tumors to metastasize. Growing experimental evidence suggests that cancer stem cells (CSCs) may contribute to tumor progression and metastasis spread. However, despite the tremendous clinical potential of such cells and their possible therapeutic management, the real nature of CSCs remains to be elucidated. Starting from what is currently known about normal mammary stem/progenitor cells, to better define the cell that originates a tumor or is responsible for metastatic spread, this review will discuss experimental evidence of breast cancer stem cells and speculate about the clinical importance and implications of their evaluation.

Progress on potential strategies to target brain tumor stem cells

The identification of brain tumor stem cells (BTSCs) leads to promising progress on brain tumor treatment. For some brain tumors, BTSCs are the driving force of tumor growth and the culprits that make tumor revive and resistant to radiotherapy and chemotherapy. Therefore, it is specifically significant to eliminate BTSCs for treatment of brain tumors. There are considerable similarities between BTSCs and normal neural stem cells (NSCs), and diverse aspects of BTSCs have been studied to find potential targets that can be manipulated to specifically eradicate BTSCs without damaging normal NSCs, including their surface makers, surrounding niche, and aberrant signaling pathways. Many strategies have been designed to kill BTSCs, and some of them have reached, or are approaching, effective therapeutic results. Here, we will focus on advantages in the issue of BTSCs and emphasize on potential therapeutic strategies targeting BTSCs.

Detection and analysis of mammary gland stem cells

Emerging evidence from a variety of tissue types, including the mammary gland, suggests that normal stem and progenitor cells are the likely targets for malignant transformation, and that these transformed cells can function as cancer stem cells that drive tumour growth. In order to develop therapies that target these cancer stem cells, it is essential to determine the molecular mechanisms that regulate the growth and differentiation of these cells and their normal counterparts. To this end, a number of quantitative robust clonal assays have been developed that can detect the presence of human and mouse mammary stem and progenitor cells. These assays, when used in conjunction with cell-sorting strategies, have permitted the prospective isolation and characterization of a variety of cell types, including stem cells. Evidence to date indicates that these stem cells exhibit properties of basal mammary cells, possess extensive self-renewal properties, and are capable of generating a large number of phenotypically-distinct progenitor cells, many of which display characteristics of luminal cells. This review article will focus on the assays used to detect mammary stem and progenitor cells, some of the properties of these cells and their progeny and how they relate to the cancer stem cells that drive breast tumour growth.

Die hard: are cancer stem cells the Bruce Willises of tumor biology?

In recent years, an exponentially growing number of studies have focused on identifying cancer stem cells (CSC) in human malignancies. The rare CSCs could be crucial in controlling and curing cancer: through asymmetric division CSCs supposedly drive tumor growth and evade therapy with the help of traits shared with normal stem cells such as quiescence, self-renewal ability, and multidrug resistance pump activity. Here, we give a brief overview of techniques used to confirm the stem cell-like behavior of putative CSCs and discuss markers and methods for identifying, isolating, and culturing them. We touch on the limitations of each marker and why the combined use of CSC markers, in vitro and in vivo assays may still fail to identify all relevant CSC populations. Finally, the various experimental findings supporting and contradicting the CSC hypothesis are summarized. The large number of tumor types thus far with a subpopulation of uniquely tumorigenic and therapy resistant cells suggests that despite the unanswered questions and inconsistencies, the CSC hypothesis has a legitimate role to play in tumor biology. At the same time, experimental evidence supporting the established alternative theory of clonal evolution can be found as well. Therefore, a model that describes cancer initiation and progression should combine elements of clonal evolution and CSC theory.

Cancer research meets evolutionary biology

There is increasing evidence that Darwin's theory of evolution by natural selection provides insights into the etiology and treatment of cancer. On a microscopic scale, neoplastic cells meet the conditions for evolution by Darwinian selection: cell reproduction with heritable variability that affects cell survival and replication. This suggests that, like other areas of biological and biomedical research, Darwinian theory can provide a general framework for understanding many aspects of cancer, including problems of great clinical importance. With the availability of raw molecular data increasing rapidly, this theory may provide guidance in translating data into understanding and progress. Several conceptual and analytical tools from evolutionary biology can be applied to cancer biology. Two clinical problems may benefit most from the application of Darwinian theory: neoplastic progression and acquired therapeutic resistance. The Darwinian theory of cancer has especially profound implications for drug development, both in terms of explaining past difficulties, and pointing the way toward new approaches. Because cancer involves complex evolutionary processes, research should incorporate both tractable (simplified) experimental systems, and also longitudinal observational studies of the evolutionary dynamics of cancer in laboratory animals and in human patients. Cancer biology will require new tools to control the evolution of neoplastic cells.

Expression of stem cell marker ABCG2 and its significance in hepatocellular carcinoma tissue and cell lines

AIM: To investigate ABCG2 expression and its significance in hepatocellular carcinoma (HCC) tissue and HCC cell lines.

METHODS: The expression of ABCG2 and the distribution of ABCG2-positive cells of 30 HCC cases and 8 cases of paraneoplastic liver cirrhosis were examined using immunohistochemistry SP methods. Dual-fluorescent immunostaining was used to detect ABCG2 positive cells in two HCC cell lines, and flow cytometry was used to figure out the percentages of ABCG2-positive cells in the two HCC cell lines.

RESULTS: The present study confirmed the predominant localization of ABCG2 transporter in plasma membrane. Some of ABCG2-positive tumors showed mixed membranous and cytoplasmic staining. ABCG2 expression was detected in 63.33% (19/30) of HCC and 25% (2/8) of paraneoplastic liver cirrhosis. ABCG2 expression was observed both in the two HCC cell lines, ABCG2 expression in HepG2 was only observed in the cytoplasm, whereas ABCG2 expression was observed on the cell membrane in addition to the cytoplasm in PLC/PRF/5; ABCG2-positive cells can be detected by flow cytometry in PLC/PRF/5 (P< 0.05), but not in HepG2.

CONCLUSION: The expression of ABCG2 found in HCC tissue and HCC cell lines may be closely related to oncogenesis and progress of HCC, and the ABCG2 may be regarded as a potential maker for liver cancer stem cells.

Cancer stem cells: a model in the making

Cancer stem cells and their potential roles in tumor heterogeneity are currently subjects of intense investigation. Studies suggest that these cells may develop from any normal cell and have begun to elucidate their molecular profiles. The percentage of a tumor composed of cancer stem cells varies greatly, and researchers believe that multiple types of these cells may exist in a single neoplasm. Cancer stem cells may be formed by epithelial-mesenchymal transition and seem to be less prevalent in metastases than in corresponding primary tumors. These cells appear to have therapeutic sensitivities different from those of cancer cells with more differentiated features. Looking into the many questions that remain about the cancer stem cells model might lead to more effective cancer prevention, diagnosis, and treatment.

Cancer stem cells are enriched in the side population cells in a mouse model of glioma

The recent identification of cancer stem cells (CSCs) in multiple human cancers provides a new inroad to understanding tumorigenesis at the cellular level. CSCs are defined by their characteristics of self-renewal, multipotentiality, and tumor initiation upon transplantation. By testing for these defining characteristics, we provide evidence for the existence of CSCs in a transgenic mouse model of glioma, S100beta-verbB;Trp53. In this glioma model, CSCs are enriched in the side population (SP) cells. These SP cells have enhanced tumor-initiating capacity, self-renewal, and multipotentiality compared with non-SP cells from the same tumors. Furthermore, gene expression analysis comparing fluorescence-activated cell sorting-sorted cancer SP cells to non-SP cancer cells and normal neural SP cells identified 45 candidate genes that are differentially expressed in glioma stem cells. We validated the expression of two genes from this list (S100a4 and S100a6) in primary mouse gliomas and human glioma samples. Analyses of xenografted human glioblastoma multiforme cell lines and primary human glioma tissues show that S100A4 and S100A6 are expressed in a small subset of cancer cells and that their abundance is positively correlated to tumor grade. In conclusion, this study shows that CSCs exist in a mouse glioma model, suggesting that this model can be used to study the molecular and cellular characteristics of CSCs in vivo and to further test the CSC hypothesis.

Human cancer cell lines: Experimental models for cancer cells in situ? For cancer stem cells?

Established human cancer cell lines are routinely used as experimental models for human cancers. Their validity for such use is analyzed and discussed, with particular focus on thyroid tumors. Although cell lines retain some properties of the cells of origin, from the points of view of their genetics, epigenetics and gene expression, they show clear differences in these properties compared to in vivo tumors. This can be explained by a prior selection of initiating cells and a Darwinian evolution in vitro. The properties of the cell lines are compared to those of the postulated cancer stem cells and their use as models in this regard are discussed. Furthermore, other proper and possible uses of the cell lines are discussed.

Brain tumor stem cells

The dogma that solid tumors are composed of tumor cells that all share the same ability to produce proliferating daughter cells has been challenged in recent years. There is growing evidence that many adult tissues contain a set of tissue stem cells, which might undergo malignant transformation while retaining their stem cell characteristics. These include the ability of indefinite self-renewal and the capability to differentiate into daughter cells of tissue-specific lineages. Brain tumors such as medulloblastomas or glioblastomas often contain areas of divergent differentiation, which raises the intriguing question of whether these tumors could derive from neural stem cells (NSCs).This chapter reviews the current knowledge of NSCs and relates them to brain tumor pathology. Current therapy protocols for malignant brain tumors are targeted toward the reduction of bulk tumor mass. The concept of brain-tumor stem cells could provide new insights for future therapies, if the capacity for self-renewal of tumor cells and growth of the tumor mass would reside within a small subset of cancer cells.

Methodologies in assaying prostate cancer stem cells

The cancer stem cell (CSC) theory posits that only a small population of tumor cells within the tumor has the ability to reinitiate tumor development and is responsible for tumor homeostasis and progression. Tumor initiation is a defining property of putative CSCs, which have been reported in both blood malignancies and solid tumors. In order to test whether any given human tumor cell population has CSC properties, the relatively enriched single cells have to be put into a foreign microenvironment in a recipient animal to test their tumorigenic potential. Furthermore, various in vitro assays need be performed to demonstrate that the presumed CSCs have certain biological properties normally associated with the stem cells (SCs). Herein, we present a comprehensive review of the experimental methodologies that our lab has been using in assaying putative prostate cancer (PCa) SCs in culture, xenograft tumors, and primary tumor samples.

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.

Epithelial stem cells and malignancy

The renewal of normal epithelia depends on a small sub-population of cells, termed somatic stem cells, whose primary characteristic is an ability for indefinite self-renewal. Evidence is accumulating that the growth of tumours similarly depends on a sub-population of malignant stem cells, often termed tumour-initiating cells. Tumour-initiating sub-populations within solid tumours have been identified by their cell surface expression of various phenotypic markers and by their ability to regenerate tumours in immune-deficient mice. Cells with such clonogenic abilities differ consistently from the remainder of the cell population in cellular properties such as size, adhesiveness, dye exclusion, and patterns of gene expression. Sub-populations of malignant cells freshly isolated from tumours also show differing patterns of expression of molecules related to stem cell maintenance and asymmetric division. As the cells ultimately responsible for tumour renewal, malignant stem cells appear to form the necessary target of therapy but some findings indicate greater resistance of these cells to the induction of apoptotic cell death and their potential failure to respond effectively to standard therapeutic procedures. Of particular interest, cells with clonogenic properties and expression patterns similar to those of tumour-initiating cells in vivo persist in malignant cell lines and show similar apoptotic resistance. Cell lines may thus provide a model for analysis of malignant stem cell properties and may be useful for the development of appropriate methods for their elimination.

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.

Cancer stem cells: beyond Koch's postulates

Until the last century, infectious diseases were the leading cause of human mortality. Therefore, our current medical reasoning is profoundly influenced by views that originated from medical microbiology. The notion that cancer growth is sustained by a sub-population of particular cells, the cancer stem cells, is highly reminiscent of the germ theory of disease as exemplified by Koch's postulates in the XIXth century. However, accumulating data underscore the importance of cell-cell interactions and tumor environment. Hence it is essential to critically review the basic tenets of the cancer stem cell concept on the light of their relationships with Koch's postulates. Shifting the pathogenic element from a special cellular entity (cancer stem cell or microorganism) to a "pathogenic field" could be critical for curing both cancer and drug-resistant infectious diseases.

Transcription factors, chromatin and cancer

Transcription factors, chromatin and chromatin-modifying enzymes are key components in a complex network through which the genome interacts with its environment. For many transcription factors, binding motifs are found adjacent to the promoter regions of a large proportion of genes, requiring mechanisms that confer binding specificity in any given cell type. These include association of the factor with other proteins and packaging of DNA, as chromatin, at the binding sequence so as to inhibit or facilitate binding. Recent evidence suggests that specific post-translational modifications of the histones packaging promoter DNA can help guide transcription factors to selected sites. The enzymes that put such modifications in place are dependent on metabolic components (e.g. acetyl CoA, S-adenosyl methionine) and susceptible to inhibition or activation by environmental factors. Local patterns of histone modification can be altered or maintained through direct interaction between the transcription factor and histone modifying enzymes. The functional consequences of transcription factor binding are also dependent on protein modifying enzymes, particularly those that alter lysine methylation at selected residues. Remarkably, the role of these enzymes is not limited to promoter-proximal events, but can be linked to changes in the intranuclear location of target genes. In this review we describe results that begin to define how transcription factors, chromatin and environmental variables interact and how these interactions are subverted in cancer. We focus on the nuclear receptor family of transcription factors, where binding of ligands such as steroid hormones and dietary derived factors provides an extra level of environmental input.

Minireview: stem cell contribution to ovarian development, function, and disease

By virtue of the fact that oocytes not only serve to produce embryos after fertilization but also can effectively reprogram adult somatic cell nuclei to a pluripotent state, much of the interest in the role of stem cells in ovarian biology has been focused on the germline. However, very recent studies have revealed that somatic stem cells may also be of considerable relevance to the study of normal ovarian function. Furthermore, stem cell dysfunction may underlie or contribute to disease states such as ovarian cancer and polycystic ovary syndrome. Our objective is to explore these concepts in greater detail, with the hope of stimulating further research efforts into understanding what role stem cells may play in the physiology and pathology of the mammalian female gonads.

Identification and characterization of ovarian cancer-initiating cells from primary human tumors

The objective of this study was to identify and characterize a self-renewing subpopulation of human ovarian tumor cells (ovarian cancer-initiating cells, OCICs) fully capable of serial propagation of their original tumor phenotype in animals. Ovarian serous adenocarcinomas were disaggregated and subjected to growth conditions selective for self-renewing, nonadherent spheroids previously shown to derive from tissue stem cells. To affirm the existence of OCICs, xenoengraftment of as few as 100 dissociated spheroid cells allowed full recapitulation of the original tumor (grade 2/grade 3 serous adenocarcinoma), whereas >10(5) unselected cells remained nontumorigenic. Stemness properties of OCICs (under stem cell-selective conditions) were further established by cell proliferation assays and reverse transcription-PCR, demonstrating enhanced chemoresistance to the ovarian cancer chemotherapeutics cisplatin or paclitaxel and up-regulation of stem cell markers (Bmi-1, stem cell factor, Notch-1, Nanog, nestin, ABCG2, and Oct-4) compared with parental tumor cells or OCICs under differentiating conditions. To identify an OCIC cell surface phenotype, spheroid immunostaining showed significant up-regulation of the hyaluronate receptor CD44 and stem cell factor receptor CD117 (c-kit), a tyrosine kinase oncoprotein. Similar to sphere-forming OCICs, injection of only 100 CD44(+)CD117(+) cells could also serially propagate their original tumors, whereas 10(5) CD44(-)CD117(-) cells remained nontumorigenic. Based on these findings, we assert that epithelial ovarian cancers derive from a subpopulation of CD44(+)CD117(+) cells, thus representing a possible therapeutic target for this devastating disease.

Is tumor growth sustained by rare cancer stem cells or dominant clones?

A key issue for cancer biology and therapy is whether the relentless growth of a tumor is driven by a substantial proportion of its cells or exclusively by a rare subpopulation, commonly termed "cancer stem cells." Support for the cancer stem cell model has been stimulated by experiments in which human tumor cells were transplanted into immunodeficient mice. Most notably, in human acute myeloid leukemia, only a minute proportion of the cells, displaying a defined phenotype, could seed leukemia in mice. Xenotransplantation, however, may fail to reveal many tumor growth-sustaining cells because the foreign microenvironment precludes essential interactions with support cells. In studies that instead have transplanted mouse leukemias and lymphomas into syngeneic animals, most of the tumors seem to be maintained by the dominant cell population, and only a few types of mouse leukemia seem to be sustained by a minor tumor growth-sustaining subpopulation. The collective evidence suggests that various tumors may span the spectrum between the extremes represented by the two models. If tumor growth can indeed be sustained either by rare cancer stem cells or dominant clones or both, as current evidence suggests, curative therapy for many types of tumors will most likely require targeting all the tumor cell populations.

Alpha2beta1 integrin regulates lineage commitment in multipotent human colorectal cancer cells

The human colorectal epithelium is maintained by multipotent stem cells that give rise to absorptive, mucous, and endocrine lineages. Recent evidence suggests that human colorectal cancers are likewise maintained by a minority population of so-called cancer stem cells. We have previously established a human colorectal cancer cell line with multipotent characteristics (HRA-19) and developed a serum-free medium that induces endocrine, mucous and absorptive lineage commitment by HRA-19 cells in vitro. In this study, we investigate the role of the β1 integrin family of cell surface extracellular matrix receptors in multilineage differentiation by these multipotent human colorectal cancer cells. We show that endocrine and mucous lineage commitment is blocked in the presence of function-blocking antibodies to β1 integrin. Function-blocking antibodies to α2 integrin also blocked both HRA-19 endocrine lineage commitment and enterocytic differentiation by Caco-2 human colon cancer cells; both effects being abrogated by the MEK inhibitor, PD98059, suggesting a role for ERK signaling in α2-mediated regulation of colorectal cancer cell differentiation. To further explore the role of α2 integrin in multilineage differentiation, we established multipotent cells expressing high levels of wild-type α2 integrin or a non-signaling chimeric α2 integrin. Overexpression of wild-type α2 integrin in HRA-19 cells significantly enhanced endocrine and mucous lineage commitment, while cells expressing the non-signaling chimeric α2 integrin had negligible ability for either endocrine or mucous lineage commitment. This study indicates that the collagen receptor α2β1 integrin is a regulator of cell fate in human multipotent colorectal cancer cells.

Osteotropic cancers: from primary tumor to bone

It has long been recognized that primary cancers spread to distant organs with characteristic preference. Bone metastases occur in approximately 70% of patients with advanced breast and prostate cancer, causing severe morbidity and hospitalization. In the last decade, we have gained a better understanding of the mechanisms by which certain tumor types tend to metastasize specifically to bone. It appears that the interaction between the organ microenvironment and cancer cells is fundamental for establishing metastatic growth. Accordingly, Stephen Paget's 'seed and soil' hypothesis - stating that circulating cancer cells (the 'seeds') disperse in all directions, but can accomplish metastases only in organs where the microenvironment (the 'soil') is permissive for their growth - still holds forth today. For this reason, this review uses the 'seed and soil' hypothesis as a template to discuss novel insight and developments in the bone metastasis field.

Hypoxia enhances tumor stemness by increasing the invasive and tumorigenic side population fraction

Although advances have been made in understanding the role of hypoxia in the stem cell niche, almost nothing is known about a potentially similar role of hypoxia in maintaining the tumor stem cell (TSC) niche. Here we show that a highly tumorigenic fraction of side population (SP) cells is localized in the hypoxic zones of solid tumors in vivo. We first identified a highly migratory, invasive, and tumorigenic fraction of post-hypoxic side population cells (SPm([hox]) fraction) in a diverse group of solid tumor cell lines, including neuroblastoma, rhabdomyosarcoma, and small-cell lung carcinoma. To identify the SPm((hox)) fraction, we used an "injured conditioned medium" derived from bone marrow stromal cells treated with hypoxia and oxidative stress. We found that a highly tumorigenic SP fraction migrates to the injured conditioned medium in a Boyden chamber. We show that as few as 100 SPm((hox)) cells form rapidly growing tumors in vivo. In vitro exposure to hypoxia increases the SPm((hox)) fraction significantly. Quantitative real-time polymerase chain reaction and immunofluorescence studies showed that SPm((hox)) cells expressed Oct-4, a "stemness" gene having a potential role in TSC maintenance. In nude mice xenografts, SPm((hox)) cells were localized to the hypoxic zones, as demonstrated after quantum dot labeling. These results suggest that a highly tumorigenic SP fraction migrates to the area of hypoxia; this migration is similar to the migration of normal bone marrow SP fraction to the area of injury/hypoxia. Furthermore, the hypoxic microenvironment may serve as a niche for the highly tumorigenic fraction of SP cells.

Exploring the role of cancer stem cells in radioresistance

Radiobiological research over the past decades has provided evidence that cancer stem cell content and the intrinsic radiosensitivity of cancer stem cells varies between tumours, thereby affecting their radiocurability. Translation of this knowledge into predictive tests for the clinic has so far been hampered by the lack of methods to discriminate between stem cells and non-stem cells. New technologies allow isolation of cells expressing specific surface markers that are differentially expressed in tumour cell subpopulations that are enriched for cancer stem cells. Combining these techniques with functional radiobiological assays holds the potential to elucidate the role of cancer stem cells in radioresistance in individual tumours, and to use this knowledge for the development of predictive markers for optimization of radiotherapy.

Liver cancer stem cells

In an effort to review the evidence that liver cancer stem cells exist, two fundamental questions must be addressed. First, do hepatocellular carcinomas (HCC) arise from liver stem cells? Second, do HCCs contain cells that possess properties of cancer stem cells? For many years the finding of preneoplastic nodules in the liver during experimental induction of HCCs by chemicals was interpreted to support the hypothesis that HCC arose by dedifferentiation of mature liver cells. More recently, recognition of the role of small oval cells in the carcinogenic process led to a new hypothesis that HCC arises by maturation arrest of liver stem cells. Analysis of the cells in HCC supports the presence of cells with stem-cell properties (ie, immortality, transplantability, and resistance to therapy). However, definitive markers for these putative cancer stem cells have not yet been found and a liver cancer stem cell has not been isolated.

The clinicopathologic and prognostic significance of CD44+/CD24(-/low) and CD44-/CD24+ tumor cells in invasive breast carcinomas

Cells with distinct phenotypes and stem cell-like properties have been reported to exist in breast cancer. The aim of the present study was to investigate the clinicopathologic and prognostic significance of the CD44(+)/CD24(-/low) and CD44(-)/CD24(+) tumor phenotypes' prevalence. Double immunohistochemistry was applied on a series of 155 paraffin-embedded breast tissue specimens to detect CD44 and CD24. Evaluation of the phenotypes was performed by image analysis. The prevalence of CD44(+)/CD24(-/low) and CD44(-)/CD24(+) tumor cells was 58.7% and 82.6%, respectively. The dominance of the CD44(+)/CD24(-/low) tumor cells was inversely associated with lymph node metastasis (P = .019) and tended to inversely associate with the stage of the disease (P = .068). Moreover, the prevalence of CD44(+)/CD24(-/low) was found to exert no significant impact on patients' prognosis although it displayed a tendency toward an increase in disease-free survival (P = .074). On the other hand, the prevalence of CD44(-)/CD24(+) tumor cells was found to have no clinicopathologic significance. However, it was found to exert an unfavorable impact on both relapse-free (P = .009) and overall survival (P = .046) of the patients with breast carcinomas of intermediate differentiation (grade 2). In breast tissue, CD44(+)/CD24(-/low) tumor cells seem to be associated with lack of lymph node metastasis and a tendency toward an increase of the relapse-free survival of the patients. On the contrary, tumor cells with the phenotype CD44(-)/CD24(+) seem to identify patients with worse disease-free and overall survival within the group of intermediate-grade differentiation patients whose prognosis is difficult to assess.

Side population cells in human cancers

Cancer stem cells (CSCs) are found in multiple tumor types. While the presence of surface markers selectively expressed on CSCs are used to isolate these cells, no marker or pattern of makers are known to prospectively identify CSCs in many tumor types. In such cases exploitation of stem cell characteristics can be used to identify CSCs and one such characteristic is the capacity to extrude dyes such as Hoechst 33342. Cell that exclude this dye are referred to as side population (SP) cells. These cells share characteristics of CSCs, specifically, they are enriched for tumor initiating capacity, they express stem-like genes, and they are resistant to chemotherapeutic drugs. Dye exclusion is a valuable technique as it identifies a unique population of cells with stem-like characteristics.

Glial progenitor-like phenotype in low-grade glioma and enhanced CD133-expression and neuronal lineage differentiation potential in high-grade glioma

BACKGROUND

While neurosphere- as well as xenograft tumor-initiating cells have been identified in gliomas, the resemblance between glioma cells and neural stem/progenitor cells as well as the prognostic value of stem/progenitor cell marker expression in glioma are poorly clarified.

METHODOLOGY/PRINCIPAL FINDINGS

Viable glioma cells were characterized for surface marker expression along the glial genesis hierarchy. Six low-grade and 17 high-grade glioma specimens were flow-cytometrically analyzed for markers characteristics of stem cells (CD133); glial progenitors (PDGFRalpha, A2B5, O4, and CD44); and late oligodendrocyte progenitors (O1). In parallel, the expression of glial fibrillary acidic protein (GFAP), synaptophysin and neuron-specific enolase (NSE) was immunohistochemically analyzed in fixed tissue specimens. Irrespective of the grade and morphological diagnosis of gliomas, glioma cells concomitantly expressed PDGFRalpha, A2B5, O4, CD44 and GFAP. In contrast, O1 was weakly expressed in all low-grade and the majority of high-grade glioma specimens analyzed. Co-expression of neuronal markers was observed in all high-grade, but not low-grade, glioma specimens analyzed. The rare CD133 expressing cells in low-grade glioma specimens typically co-expressed vessel endothelial marker CD31. In contrast, distinct CD133 expression profiles in up to 90% of CD45-negative glioma cells were observed in 12 of the 17 high-grade glioma specimens and the majority of these CD133 expressing cells were CD31 negative. The CD133 expression correlates inversely with length of patient survival. Surprisingly, cytogenetic analysis showed that gliomas contained normal and abnormal cell karyotypes with hitherto indistinguishable phenotype.

CONCLUSIONS/SIGNIFICANCE

This study constitutes an important step towards clarification of lineage commitment and differentiation blockage of glioma cells. Our data suggest that glioma cells may resemble expansion of glial lineage progenitor cells with compromised differentiation capacity downstream of A2B5 and O4 expression. The concurrent expression of neuronal markers demonstrates that high-grade glioma cells are endowed with multi-lineage differentiation potential in vivo. Importantly, enhanced CD133 expression marks a poor prognosis in gliomas.

Growth of cancer cell lines under stem cell-like conditions has the potential to unveil therapeutic targets

Malignant tumors comprise a small proportion of cancer-initiating cells (CIC), capable of sustaining tumor formation and growth. CIC are the main potential target for anticancer therapy. However, the identification of molecular therapeutic targets in CIC isolated from primary tumors is an extremely difficult task. Here, we show that after years of passaging under differentiating conditions, glioblastoma, mammary carcinoma, and melanoma cell lines contained a fraction of cells capable of forming spheroids upon in vitro growth under stem cell-like conditions. We found an increased expression of surface markers associated with the stem cell phenotype and of oncogenes in cell lines and clones cultured as spheroids vs. adherent cultures. Also, spheroid-forming cells displayed increased tumorigenicity and an altered pattern of chemosensitivity. Interestingly, also from single retrovirally marked clones, it was possible to isolate cells able to grow as spheroids and associated with increased tumorigenicity. Our findings indicate that short-term selection and propagation of CIC as spheroid cultures from established cancer cell lines, coupled with gene expression profiling, represents a suitable tool to study and therapeutically target CIC: the notion of which genes have been down-regulated during growth under differentiating conditions will help find CIC-associated therapeutic targets.

Brain tumor stem cells and the tumor microenvironment

✓ Recent advances in stem cell research and developmental neurobiology have uncovered new perspectives from which to investigate various forms of cancer. Specifically, the hypothesis that tumors consist of a subpopulation of malignant cells similar to stem cells is of great interest to scientists and clinicians and has been dubbed the “cancer stem cell hypothesis.” The region in which this assertion is most relevant is within the brain. Cancer stem cells have been isolated from brain tumors that exhibit characteristics of differentiation and proliferation normally seen only in neural stem cells. These cancer stem cells may be responsible for tumor origin, survival, and proliferation. Furthermore, these cells must be considered within their immediate microenvironment when investigating mechanisms of tumorigenesis. Evidence of brain tumor stem cells is reviewed along with the role of tumor environment as the context within which these cells should be understood.

Cancer stem cells--old concepts, new insights

Cancer has long been viewed as an exclusively genetic disorder. The model of carcinogenesis, postulated by Nowell and Vogelstein, describes the formation of a tumor by the sequential accumulation of mutations in oncogenes and tumor suppressor genes. In this model, tumors are thought to consist of a heterogeneous population of cells that continue to acquire new mutations, resulting in a highly dynamic process, with clones that out compete others due to increased proliferative or survival capacity. However, novel insights in cancer stem cell research suggest another layer of complexity in the process of malignant transformation and preservation. It has been reported that only a small fraction of the cancer cells in a malignancy have the capacity to propagate the tumor upon transplantation into immuno-compromised mice. Those cells are termed 'cancer stem cells' (CSC) and can be selected based on the expression of cell surface markers associated with immature cell types. In this review, we will critically discuss these novel insights in CSC-related research. Where possible we integrate these results within the genetic model of cancer and illustrate that the CSC model can be considered an extension of the classic genetic model rather than a contradictory theory. Finally, we discuss some of the most controversial issues in this field.

Stem cells as common ancestors in a colorectal cancer ancestral tree

PURPOSE OF REVIEW

Cancer is thought to be an evolutionary process. Modern studies of evolution increasingly rely on genome comparisons, and similar molecular phylogeny approaches could be translated to somatic cell genomes to reconstruct colorectal cancer progression. The purpose of this review is to outline how human somatic cell ancestral trees can organize many old and new observations.

RECENT FINDINGS

A somatic cell tree starts from the zygote and ends with present day normal or neoplastic cells. In between are ancestors and dead ends, which functionally correspond to stem and nonstem cells. Cancer genome projects illustrate that mutations are relatively infrequent, and consistent with normal mutation rates, particularly if mutations begin to accumulate from birth. Therefore, some mutations eventually found in cancers may first occur in normal appearing crypts, which are maintained by niches that allow for stem cell clonal evolution and selection. Although mutations occur too infrequently to function as somatic cell molecular clocks, potentially more labile epigenetic changes in CpG methylation may also record somatic cell ancestry.

SUMMARY

Somatic cell evolution can occur throughout life, and potentially at least some of this unseen past may be reconstructed by 'reading' the lifetime changes that accumulate within our genomes.

Cancer stem cells as mediators of treatment resistance in brain tumors: status and controversies

Malignant primary brain tumors are characterized by a short median survival and an almost 100% tumor-related mortality. Despite the addition of new chemotherapy regimes, the overall survival has improved marginally, and radiotherapy is only transiently effective, illustrating the profound impact of treatment resistance on prognosis. Recent studies suggest that a small subpopulation of cancer stem cells (CSCs) has the capacity to repopulate tumors and drive malignant progression and mediate radio- and chemoresistance. This implies that future therapies should turn from the elimination of the rapidly dividing, but differentiated tumor cells, to specifically targeting the minority of tumor cells that repopulate the tumor. Although there exists some support for the CSC hypothesis, there remain many uncertainties regarding theoretical, technical, and interpretational aspects of the data supporting it. If correct, the CSC hypothesis could have profound implications for the way tumors are classified and treated. In this review of the literature, we provide original data and hypotheses supporting alternative explanations and outline some of the therapeutic implications that can be derived.

The biology of cancer stem cells

Cancers originally develop from normal cells that gain the ability to proliferate aberrantly and eventually turn malignant. These cancerous cells then grow clonally into tumors and eventually have the potential to metastasize. A central question in cancer biology is, which cells can be transformed to form tumors? Recent studies elucidated the presence of cancer stem cells that have the exclusive ability to regenerate tumors. These cancer stem cells share many characteristics with normal stem cells, including self-renewal and differentiation. With the growing evidence that cancer stem cells exist in a wide array of tumors, it is becoming increasingly important to understand the molecular mechanisms that regulate self-renewal and differentiation because corruption of genes involved in these pathways likely participates in tumor growth. This new paradigm of oncogenesis has been validated in a growing list of tumors. Studies of normal and cancer stem cells from the same tissue have shed light on the ontogeny of tumors. That signaling pathways such as Bmi1 and Wnt have similar effects in normal and cancer stem cell self-renewal suggests that common molecular pathways regulate both populations. Understanding the biology of cancer stem cells will contribute to the identification of molecular targets important for future therapies.

Cancer stem cells: the lessons from pre-cancerous stem cells

How a cancer is initiated and established remains elusive despite all the advances in decades of cancer research. Recently the cancer stem cell (CSC) hypothesis has been revived, challenging the long-standing model of "clonal evolution" for cancer development and implicating the dawning of a potential cure for cancer [1]. The recent identification of precancerous stem cells (pCSCs) in cancer, an early stage of CSC development, however, implicates that the "clonal evolution" is not contradictory to the CSC hypothesis, but is rather an aspect of the process of CSC development [2]. The discovery of pCSC has revealed and will continue to reveal the volatile properties of CSC with respects to their phenotype, differentiation and tumorigenic capacity during initiation and progression. Both pCSC and CSC might also serve as precursors of tumor stromal components such as tumor vasculogenic stem/progenitor cells (TVPCs). Thus, the CSC hypothesis covers the developing process of tumor-initiating cells (TIC) --> pCSC --> CSC --> cancer, a cellular process that should parallel the histological process of hyperplasia/metaplasia (TIC) --> precancerous lesions (pCSC) --> malignant lesions (CSC --> cancer). The embryonic stem (ES) cell and germline stem (GS) cell genes are subverted in pCSCs. Especially the GS cell protein piwil2 may play an important role during the development of TIC --> pCSC --> CSC, and this protein may be used as a common biomarker for early detection, prevention, and treatment of cancer. As cancer stem cell research is yet in its infancy, definitive conclusions regarding the role of pCSC can not be made at this time. However this review will discuss what we have learned from pCSC and how this has led to innovative ideas that may eventually have major impacts on the understanding and treatment of cancer.

Prostate cell cultures as in vitro models for the study of normal stem cells and cancer stem cells

Current existing therapies for prostate cancer eradicate the majority of cells within a tumor. However, most patients with advanced cancer still progress to androgen-independent metastatic disease that remains essentially incurable by current treatment strategies. Recent evidence has shown that cancer stem cells (CSCs) are a subset of the tumor cells that are responsible for initiating and maintaining the disease. Understanding normal stem cells and CSCs may provide insight into the origin of and new therapeutics for prostate cancer. Normal stem cells and CSCs have been identified in prostate tissue by the use of several markers or techniques. Although research on stem cells has been limited by the lack of suitable in vitro systems, recent studies show that not only primary cells but also several established cell lines may exhibit stem cell properties. This review discusses various in vitro culture systems to propagate normal prostate stem cells and prostate CSCs together with molecular markers. These in vitro cell culture models should be useful for elucidating the differentiation of prostatic epithelium and the biological features of prostate cancer.

The fuzzy math of solid tumor stem cells: a perspective

Apparently effective therapeutic agents very often fail to cure cancer patients. It is therefore attractive to wonder whether a specific resistant cell subset should be recognized and separately targeted. In solid tumors, such as carcinomas, a minor population of "cancer stem cells" has been proposed and sought experimentally in human tumors and isolated cell populations. It is often overlooked that the rationale and supportive data are essentially numerical and can be evaluated as such. A reevaluation of the published studies and related claims within awarded U.S. patents suggests that the mathematical support for the concept of therapeutically useful stem cells is weak and may even invalidate the foundations of these publications and patent claims. Mathematical arguments should be used more consistently, because they can serve as a guide for interpreting studies into cancer stem cells of solid tumors.