Isolation and characterization of stem cell-like precursor cells from primary human anaplastic oligoastrocytoma

Expression of pluripotency-related genes in human glioblastoma

Background

Cancer is a group of heterogeneous diseases characterized by several disruptions of the genetic and epigenetic components of cell biology. Some types of cancer have been shown to be constituted by a mosaic of cells with variable differentiation states, with more aggressive tumors being more undifferentiated. In most cases, undifferentiated tumor cells express associated embryonic markers such as the OCT4, NANOG, SOX2, and CARM1 genes. The ectopic or reminiscent expression of some master regulator genes of pluripotency has been indicated as the cause of the poorly differentiated state of tumors, and based on the evidence of some reports, can be used as a possible therapeutic target. Considering this information, a more detailed investigation of the expression of pluripotency-associated genes is necessary to evaluate the roles of these genes in the etiology of some tumors and their use targets of therapy.

Methods

The expression of four pluripotency-related genes was investigated (OCT4, NANOG, SOX2, and CARM1) in the most malignant primary human brain tumor, glioblastoma (GBM).

Results and Conclusion

The results demonstrated a signature of OCT4/SOX2/CARM1 genes and a significant increase of CARM1 expression in GBM cases.

Stat3-positive tumor cells contribute to vessels neoformation in primary central nervous system lymphoma

With the aim of elucidating the relationship between Stat3 expression and tumor vessels abnormalities in the PCNLs, in this study we evaluated Stat3 and pStat3 expression by Real-time PCR and by immunohistochemistry in biopsy sections from PCNSL patients. Correlations of the expression levels with the presence of aberrant vessels were analyzed by confocal laser microscopy analysis, using FVIII as endothelial cell marker, CD133 and nestin as cancer stem cell (CSC) marker, CD20 as tumor cell marker, and Stat3. In addition, we investigated Stat3 mutations in lymphoma cells to clarify the role of the constitutive expression of Stat3 and of its phosphorylated forms. Results showed that in PCNSL, putative endothelial cells lining the vessels are heterogeneous, expressing FVIII/ pStat3/CD133 (presumably originally they are vascular progenitor cells), as well as FVIII/CD20/CD133 (presumably originally they are tumor cells). Finally, we detected a fraction of the FVIII⁺ endothelial cell that co-expressed Stat3 bearing a tetraploid karyotype, while no amplification signal for the Stat3 gene was detected.

A three-dimensional collagen scaffold cell culture system for screening anti-glioma therapeutics

Three-dimensional (3D) culture, which can simulate in vivo microenvironments, has been increasingly used to study tumor cell biology. Since most preclinical anti-glioma drug tests still rely on conventional 2D cell culture, we established a collagen scaffold for 3D glioma cell culture. Glioma cells cultured on these 3D scaffolds showed greater degree of dedifferentiation and quiescence than cells in 2D culture. 3D-cultured cells also exhibited enhanced resistance to chemotherapeutic alkylating agents, with a much higher proportion of glioma stem cells and upregulation of O6-methylguanine DNA methyltransferase (MGMT). Importantly, tumor cells in 3D culture showed chemotherapy resistance patterns similar to those observed in glioma patients. Our results suggest that 3D collagen scaffolds are promising in vitro research platforms for screening new anti-glioma therapeutics.

Resistance of glioma cells to nutrient-deprived microenvironment can be enhanced by CD133-mediated autophagy

CD133 is a pentaspan transmembrane protein that can serve as a biomarker for cancer stem cells, although its biochemical mechanism remains unclear. Here we report that CD133 expression enhances glioma cell tolerance of a nutrient-deprived microenvironment. Under starvation conditions, CD133-positive cells exhibited higher survival and decreased levels of apoptosis. These changes were dependent on activation of autophagy-associated gene signaling and were impaired by the autophagic inhibitor chloroquine. Furthermore, rapamycin up-regulated the level of autophagy and inversely reduced CD133 expression. Immunofluorescence confirmed that starvation promoted release of CD133 from the plasma membrane to the cytoplasm, with CD133 also partially co-localizing with LC3 upon starvation. Additionally, CD133 partially co-localized with Beclin1, Atg5, and lysosomes, indicating that CD133 directly participates in the autophagosome membrane fusion process and ultimately undergoes lysosomal degradation. Collectively, our results demonstrate that CD133 contributes to cell survival by regulating autophagy, and that targeting CD133-linked signaling and autophagy may be useful in improving anti-cancer treatments.

Reprogramming Factors Remodel Melanoma Cell Phenotype by Changing Stat3 Expression

The limited availability of melanoma stem cells is a major challenge for therapeutic reagent screening and study of molecular mechanisms. It has been shown that induced expression of four stem cell factors (Oct4, Sox2, Klf4, and c-Myc) changes the phenotype of osteosarcoma and breast cancer cells to osteosarcoma stem cells and breast cancer stem cells, respectively. The present study aimed to explore whether these four factors might change the phenotype of melanoma cells to melanoma stem cells and, if so, to examine the possible molecular signal involved. Melanoma B16-F10 cells were transfected with the plasmid TetO-FUW-OSKM which contains cDNA expressing four factors, driven by the Tet-On element. We found that expression of the four transcription factors was highly induced by DOX in the stable melanoma cell clones. Further studies confirmed that induced expression of these factors remodeled the phenotype of the melanoma cells to melanoma stem cells (MSCs). This conclusion was supported by the evidence that induced expression of these factors increased the numbers of tumor-initiating cells, (namely MSCs), both in an in vitro cell culture system and in a mouse in vivo model. The conclusion was further supported by the observation that the induction of these factors exclusively increased the mRNA of signal transducer and activator of transcription 3 which has been reported to play a crucial role in stem cell maintenance. Thus, phenotypic remodeling of melanoma cells following the induction of these four factors provided a simple and optimal means to constantly obtain MSCs for screening new therapeutic reagents. The result also reveals that Stat3 may be a crucial link between the induction of the four factors and the cell remodeling, suggesting its potential role as a target to fight melanoma.

Glioblastoma vasculogenic mimicry: signaling pathways progression and potential anti-angiogenesis targets

Glioblastoma (GBM) is a highly angiogenic malignancy that is resistant to standard therapy; neo-formed vessels of this aggressive malignancy are thought to arise by sprouting of pre-existing brain capillaries. However, the conventional anti-angiogenic therapy, which seemed promising initially, shows transitory and incomplete efficacy. The discovery of vasculogenic mimicry (VM) has offered a new horizon for understanding tumor vascularization. VM is a tumor cell-constituted, matrix-embedded fluid-conducting meshwork that is independent of endothelial cells and is positively correlated with poor prognosis. Therefore, a better understanding of GBM vasculature is needed to optimize anti-angiogenic therapy. This review focuses on the signaling molecules and cascades involved in VM in relation to ongoing glioma research, as well as the clinical translational advances in GBM that have been offered by the development of optimized anti-angiogenesis treatment modalities.

Neurotensin signaling regulates stem-like traits of glioblastoma stem cells through activation of IL-8/CXCR1/STAT3 pathway

We recently found that neurotensin (NTS) and its primary receptor NTSR1 play a crucial role in glioblastoma cell proliferation and invasion. However, very little is known regarding the functional role of NTS/NTSR1 signaling in glioblastoma stem cells (GSCs). Here, we showed that NTSR1 is highly expressed in GSCs than its non-GSC counterparts. Pharmacological blockade with SR48692 or lentivirus mediated knockdown of NTSR1 efficiently reduced the sphere-forming ability and expression of stem cell markers such as nestin and Sox2 in GSCs isolated from glioblastoma cell line and glioblastoma tissues. Conversely, treated GSCs with NTS led to increase of tumor sphere formation. Mechanistically, we demonstrated that EGFR-dependent enhancement of IL-8 secretion is responsible for the effect of NTS signaling in the regulation of stem-like traits. Finally, we showed that NTSR1 or IL-8 knockdown decreased the phosphorylation of transcriptional factor STAT3 at Tyr705, which is a major transcription factor implicated in the regulation of GSC stem-like traits. Although both CXCR1 and CXCR2 inhibition reduced the tumor sphere formation, we found that CXCR1, but not CXCR2, is primarily responsible for STAT3 phosphorylation. Taken together, our findings suggest that NTS/IL-8/CXCR1/STAT3 signaling is crucial for the maintenance of stem-like traits in GSCs and provides a potential therapeutic target for glioblastoma therapy.

Human adult stem cells as the target cells for the initiation of carcinogenesis and for the generation of "cancer stem cells"

The inference to stem cells has been found in ancient myths and the concept of stem cells has existed in the fields of plant biology, developmental biology and embryology for decades. In the field of cancer research, the stem cell theory was one of the earliest hypotheses on the origin of a cancer from a single cell. However, an opposing hypothesis had it that an adult differentiated somatic cell could "de-differentiate" to become a cancer cell. Only within the last decade, via the "cloning" of Dolly, the sheep, did the field of stem cell biology really trigger an exciting revolution in biological research. The isolation of human embryonic stem cells has created a true revolution in the life sciences that has led to the hope that these human stem cells could lead to (a) basic science understanding of gene regulation during differentiation and development; (b) stem cell therapy; (c) gene therapy via stem cells; (d) the use of stem cells for drug discovery; (e) screening for toxic effects of chemicals; and (f) understand the aging and diseases of aging processes.

High expression of leptin receptor leads to temozolomide resistance with exhibiting stem/progenitor cell features in gliobalastoma

Glioblastoma is a highly aggressive malignant disease with notable resistance to chemotherapy. In this study, we found that leptin receptor (ObR)-positive glioblastoma cells were resistant to temozolomide (TMZ), and TMZ-resistant cells exhibited high expression of ObR. ObR can serve as a marker to enrich glioblastoma cells with some stem/progenitor cell traits, which explained the reason for TMZ resistance of ObR+ cells. STAT3-mediated SOX2/OCT4 signaling axis maintained the stem/progenitor cell properties of ObR+ cells, which indirectly regulated glioblastoma TMZ resistance. These findings gain insight into the molecular link between obesity and glioblastoma, and better understanding of this drug-resistant population may lead to the development of more effective therapeutic interventions for glioblastoma.

Implantation of GL261 neurospheres into C57/BL6 mice: a more reliable syngeneic graft model for research on glioma-initiating cells

Recent studies have demonstrated that inflammatory cells and inflammatory mediators are indispensable components of the tumor-initiating cell (TIC) niche and regulate the malignant behavior of TICs. However, conventional animal models for glioma-initiating cell (GIC) studies are based on the implantation of GICs from human glioblastoma (GBM) into immunodeficient mice without the regulation of immune system. Whether animal models can mimic the cellular microenvironment of malignancy and evaluate the biological features of GICs accurately is unclear. Here, we detected the biological features of neurosphere-like tumor cells derived from the murine GBM cell line GL261 (GL261-NS) and from primary human GBM (PGBM-NS) in vitro, injected GL261-NS into syngeneic C57/BL6 mouse brain and injected PGBM-NS into NOD/SCID mouse brain, respectively. The tumorigenic characteristics of the two different orthotopic transplantation models were analyzed and the histological discrepancy between grafts and human primary GBM was compared. We found that GICs enriched in GL261-NS, GL261-NS and PGBM-NS exhibited increased GIC potential and enhanced chemoresistance in vitro. GL261-NS was significantly more aggressive compared to GL261 adhesive cells (GL261-AC) in vivo and the enhanced aggression was more significant in syngeneic mice compared to immunodeficient mice. The discrepancy of tumorigenicity between GL261-NS and GL261-AC in C57/BL6 mice was also larger compared to that between PGBM-NS and PGBM-AC in immunodeficient mice. Syngrafts derived from GL261-NS in C57/BL6 mice corresponded to the human GBM histologically better, compared with xenografts derived from PGBM-NS in NOD/SCID mice, which lack inflammatory cells and inflammatory mediators. We conclude that the inflammatory niche is involved in the tumorigenicity of GICs and implantation of GL261-NS into C57/BL6 mice is a more reliable syngeneic graft model for in vivo study on GICs relative to the immunodeficiency model.

Vascular endothelial growth factor receptor 2 (VEGFR-2) plays a key role in vasculogenic mimicry formation, neovascularization and tumor initiation by Glioma stem-like cells

Human glioblastomas (GBM) are thought to be initiated by glioma stem-like cells (GSLCs). GSLCs also participate in tumor neovascularization by transdifferentiating into vascular endothelial cells. Here, we report a critical role of GSLCs in the formation of vasculogenic mimicry (VM), which defines channels lined by tumor cells to supply nutrients to early growing tumors and tumor initiation. GSLCs preferentially expressed vascular endothelial growth factor receptor-2 (VEGFR-2) that upon activation by VEGF, mediated chemotaxis, tubule formation and increased expression of critical VM markers by GSLCs. Knockdown of VEGFR-2 in GSLCs by shRNA markedly reduced their capacity of self-renewal, forming tubules, initiating xenograft tumors, promoting vascularization and the establishment of VM. Our study demonstrates VEGFR-2 as an essential molecule to sustain the "stemness" of GSLCs, their capacity to initiate tumor vasculature, and direct initiation of tumor.

Hypoxia enhances stemness of cancer stem cells in glioblastoma: an in vitro study

OBJECTIVE

To investigate the relationship between hypoxia and in vitro "stemness" of cancer stem cells (CSCs).

METHODS

U87 cells, U251 cells and primary glioma cells (n=3) experienced hypoxia. Transmission electron microscopy was done to detect the ultrastructure of these cancer cells; MTT assay to detect the cell growth; flow cytometry to detect cell cycle and CD133 expression; Transwell chamber assay was carried out to detect the cell migration; colony-forming assay to detect the colony-forming efficiency; real-time quantitative PCR and Western blot were carried out to detect the mRNA and protein expression of markers of stem cells and their differentiation, respectively.

RESULTS

Hypoxia maintained the undifferentiated state of primary glioma cells, slowed down the growth of glioma cells which were in a relatively quiescent stage, increased the colony forming efficiency and migration of glioma cells, and elevated the expression of markers of stem cells, but the expression of markers for stem cell differentiation was reduced after hypoxia treatment.

CONCLUSION

Hypoxia may induce the "dedifferentiation" of differentiated glioma cells which then acquire the stemness.

Human skin cancer stem cells: a tale of mice and men

Carcinomas, cancers of epithelial tissues, are the commonest malignancies and cause the greatest cancer mortality worldwide. Among these, the incidence of keratinocyte-derived non-melanoma skin cancers (NMSC), by far the greatest, is increasing rapidly. Yet despite access to tumor tissue, acceptance of human NMSC as a model carcinoma has been hindered by the lack of a reliable xenograft model. Instead, we have relied on the murine two-step carcinogenesis protocol as a reproducible squamous cell carcinoma (SCC) model, but this differs from their human counterpart in cause, site, genetic basis and biological behaviour. By xeno-engraftment of primary human SCC, we were recently successful in demonstrating the presence of primary human SCC cancer stem cells or tumor-initiating cells. These findings once more align the study human SCC as the archetypal carcinoma model. In this review, we describe the evidence for the existence of tumor-initiating cells, with emphasis on skin cancer, limiting our discussions to primary human cancer studies where possible.

Biology, genetics and imaging of glial cell tumours

Despite advances in therapy, gliomas remain associated with poor prognosis. Clinical advances will be achieved through molecularly targeted biological therapies, for which knowledge of molecular genetic and gene expression characteristics in relation to histopathology and in vivo imaging are essential. Recent research supports the molecular classification of gliomas based on genetic alterations or gene expression profiles, and imaging data supports the concept that molecular subtypes of glioma may be distinguished through non-invasive anatomical, physiological and metabolic imaging techniques, suggesting differences in the baseline biology of genetic subtypes of infiltrating glioma. Furthermore, MRI signatures are now being associated with complex gene expression profiles and cellular signalling pathways through genome-wide microarray studies using samples obtained by image guidance which may be co-registered with clinical imaging. In this review we describe the pathobiology, molecular pathogenesis, stem cells and imaging characteristics of gliomas with emphasis on astrocytomas and oligodendroglial neoplasms.

Embryonic retinal tumors in SV40 T-Ag transgenic mice contain CD133+ tumor-initiating cells

PURPOSE

Human retinoblastomas form during the proliferative phase of retina development and are caused by mutations that result in absent or functionally defective Rb protein. Similar tumors occur in mice only when multiple Rb gene family members are absent. We asked if retinal tumors can arise from an undifferentiated retinal cell. The tumor-initiating cells isolated from these tumors that formed in early embryonic murine retinas were characterized.

METHODS

Transgenic mice were created using a Pax6 promoter to target expression of SV40 large T-antigen (T-Ag) in the undifferentiated murine embryonic retina. T-Ag, which sequesters all Rb family proteins and p53, is expressed in the retina and lens by murine embryonic day 10 (E10) and tumors are observed by E12.5. A cell line that is adherent in serum-containing media and forms neurospheres in supplemented serum-free media was developed from retinal tumors isolated on postnatal day 7.

RESULTS

In all, 1.5% of attached cells form neurospheres when transferred to serum-free medium. All cultured cells express T-Ag, confirming that they derive from the original tumors; 0.5% of adherent cells express detectable levels of CD133. CD133+ FACS-sorted cells cultured in serum-free medium form 3-fold more neurospheres than do CD133- cells. Six of seven mice injected with CD133+ cells and one of seven mice injected with CD133- cells formed tumors during a 6-month period. Unlike primary adherent cells, primary and secondary tumors heterogeneously express markers of stem cells and differentiation similar to human retinoblastoma.

CONCLUSIONS

CD133+ tumor-initiating cells can originate from proliferating undifferentiated precursor cells.

Tumor-associated microglia/macrophages enhance the invasion of glioma stem-like cells via TGF-β1 signaling pathway

The invasion of malignant glioma cells into the surrounding normal brain tissues is crucial for causing the poor outcome of this tumor type. Recent studies suggest that glioma stem-like cells (GSLCs) mediate tumor invasion. However, it is not clear whether microenvironment factors, such as tumor-associated microglia/macrophages (TAM/Ms), also play important roles in promoting GSLC invasion. In this study, we found that in primary human gliomas and orthotopical transplanted syngeneic glioma, the number of TAM/Ms at the invasive front was correlated with the presence of CD133(+) GSLCs, and these TAM/Ms produced high levels of TGF-β1. CD133(+) GSLCs isolated from murine transplanted gliomas exhibited higher invasive potential after being cocultured with TAM/Ms, and the invasiveness was inhibited by neutralization of TGF-β1. We also found that human glioma-derived CD133(+) GSLCs became more invasive upon treatment with TGF-β1. In addition, compared with CD133(-) committed tumor cells, CD133(+) GSLCs expressed higher levels of type II TGF-β receptor (TGFBR2) mRNA and protein, and downregulation of TGFBR2 with short hairpin RNA inhibited the invasiveness of GSLCs. Mechanism studies revealed that TGF-β1 released by TAM/Ms promoted the expression of MMP-9 by GSLCs, and TGFBR2 knockdown reduced the invasiveness of these cells in vivo. These results demonstrate that TAM/Ms enhance the invasiveness of CD133(+) GSLCs via the release of TGF-β1, which increases the production of MMP-9 by GSLCs. Therefore, the TGF-β1 signaling pathway is a potential therapeutic target for limiting the invasiveness of GSLCs.

Connexin 43 reverses malignant phenotypes of glioma stem cells by modulating E-cadherin

Malfunctioned gap junctional intercellular communication (GJIC) has been thought associated with malignant transformation of normal cells. However, the role of GJIC-related proteins such as connexins in sustaining the malignant behavior of cancer stem cells remains unclear. In this study, we obtained tumorspheres formed by glioma stem cells (GSCs) and adherent GSCs and then examined their GJIC. All GSCs showed reduced GJIC, and differentiated glioma cells had more gap junction-like structures than GSCs. GSCs expressed very low level of connexins, Cx43 in particular, which are key components of gap junction. We observed hypermethylation in the promoter of gap junction protein α1, which encodes Cx43 in GSCs. Reconstitution of Cx43 in GSCs inhibited their capacity of self-renewal, invasiveness, and tumorigenicity via influencing E-cadherin and its coding protein, which leads to changes in the expression of Wnt/β-catenin targeting genes. Our results suggest that GSCs require the low expression of Cx43 for maintaining their malignant phenotype, and upregulation of Cx43 might be a potential strategy for treatment of malignant glioma.

Correlation between low-level expression of the tumor suppressor gene TAp73 and the chemoresistance of human glioma stem cells

OBJECTIVE

Glioma stem cells (GSCs) are regarded as the root of glioma growth and recurrence. Chemoresistance is one of the characteristics of GSCs that increases the difficulties in eradicating the cells by anticancer drugs.

PURPOSE

The objective of this study is to investigate the correlation between expression of the tumor suppressor gene TAp73 and the chemoresistance of human GSCs.

METHODS

MTT and tumor sphere formation assays were used to analyze the chemoresistance phenotype of GSCs derived from primary human glioma specimens under cisplatin exposure. Reverse transcription real-time PCR was applied for assaying mRNA levels of TAp73. Protein levels of TAp73, p21, Bax, and cleared caspase 3 were assayed by western blot. Cell apoptosis was analyzed by flow cytometry after the annexin V fluorescence staining.

RESULTS

GSCs exhibited increased chemoresistance compared to differentiated glioma cells (DGCs) derived from the same tumor specimen. The expression of TAp73 was lower in GSCs and was not sensitive to cisplatin treatment as compared to DGCs. Overexpression of TAp73 by transfection increased the apoptosis of GSCs in the presence of cisplatin and reduced the chemoresistance of GSC. TAp73 knockdown by siRNA in DGCs reduced cisplatin-induced apoptosis and increased the resistance to cisplatin.

CONCLUSION

These findings indicate that TAp73 silencing is hallmark of GSC to maintain their chemoresistance phenotype. Thus, targeting TAp73 may provide a novel strategy to eradicating GSCs.

Telomestatin impairs glioma stem cell survival and growth through the disruption of telomeric G-quadruplex and inhibition of the proto-oncogene, c-Myb

PURPOSE

Glioma stem cells (GSC) are a critical therapeutic target of glioblastoma multiforme (GBM).

EXPERIMENTAL DESIGN

The effects of a G-quadruplex ligand, telomestatin, were evaluated using patient-derived GSCs, non-stem tumor cells (non-GSC), and normal fetal neural precursors in vitro and in vivo. The molecular targets of telomestatin were determined by immunofluorescence in situ hybridization (iFISH) and cDNA microarray. The data were then validated by in vitro and in vivo functional assays, as well as by immunohistochemistry against 90 clinical samples.

RESULTS

Telomestatin impaired the maintenance of GSC stem cell state by inducing apoptosis in vitro and in vivo. The migration potential of GSCs was also impaired by telomestatin treatment. In contrast, both normal neural precursors and non-GSCs were relatively resistant to telomestatin. Treatment of GSC-derived mouse intracranial tumors reduced tumor sizes in vivo without a noticeable cell death in normal brains. iFISH revealed both telomeric and non-telomeric DNA damage by telomestatin in GSCs but not in non-GSCs. cDNA microarray identified a proto-oncogene, c-Myb, as a novel molecular target of telomestatin in GSCs, and pharmacodynamic analysis in telomestatin-treated tumor-bearing mouse brains showed a reduction of c-Myb in tumors in vivo. Knockdown of c-Myb phenocopied telomestatin-treated GSCs both in vitro and in vivo, and restoring c-Myb by overexpression partially rescued the phenotype. Finally, c-Myb expression was markedly elevated in surgical specimens of GBMs compared with normal tissues.

CONCLUSIONS

These data indicate that telomestatin potently eradicates GSCs through telomere disruption and c-Myb inhibition, and this study suggests a novel GSC-directed therapeutic strategy for GBMs.

Chemoattractant receptors as pharmacological targets for elimination of glioma stem-like cells

Malignant tumors are thought to be initiated by a small population of cells that display stem cell properties, including the capacity of self-renewal, multipotent differentiation, initiation of tumor tissues and resistance to therapy. Cancer stem cells (CSCs) have also been identified in gliomas in which they are named as glioma stem-like cells (GSLCs), or glioma stem cells. In xenograft transplantation models, GSLCs propagate tumor and promote tumor progression. The tumorigenesis of GSLCs depends not only on their autonomous proliferation but also on interaction with microenvironment components. Among these components, G protein coupled chemoattractant receptors (GPCRs) and their agonists have attracted much attention for their capacity to mediate leukocyte infiltration, angiogenesis, tumor invasion and metastasis. Chemoattractant GPCRs are widely expressed by tumor cells and stromal cells and recognize agonists present in the tumor microenvironment. Such GPCRs have been found to be expressed also by CSCs including GSLCs. In this brief review, we will summarize the recent development in the studies of the function, regulation and signal transduction of chemoattractant GPCRs in GSLCs in hope to promote a better understanding of the mechanistic basis of the progression of gliomas and the identification of molecular targets for the novel anti-glioma therapy.

Identification and characterization of tumor-initiating cells in human primary cutaneous squamous cell carcinoma

Primary human squamous cell carcinoma (SCCa) are heterogeneous invasive tumors with proliferating outer layers and inner differentiating cell masses. To determine if tumor initiating cells (TIC) are present in SCCa, we utilized newly developed reliable in vitro and in vivo xenograft assays that propagate human SCCa, and demonstrated that a small subset of SCCa cells (~1%) expressing Prominin-1 (CD133) in the outer layers of SCCa were highly enriched for TIC (~1/400) compared to unsorted SCCa cells (TIC ~1/10⁶). Xenografts of CD133+ SCCa recreated the original SCCa tumor histology and organizational hierarchy, while CD133- cells did not, and only CD133+ cells demonstrated the capacity for self-renewal in serial transplantation studies. We present a model of human SCCa in which tumor projections expand with outer leading edges that contain CD133+ TIC. Successful cancer treatment will likely require that the TIC identified in cancers be targeted therapeutically. The demonstration that TIC are present in SCCa and are enriched in a CD133-expressing subpopulation to our knowledge has not previously been reported.

A simplified and modified procedure to culture brain glioma stem cells from clinical specimens

In recent years, the theory of brain glioma stem cells (BGSCs) has facilitated the study of gliomas. BGSCs have been accepted as the origin of gliomas and determine their biological features. Numerous efforts have been made to probe into the biological characteristics and behaviors of BGSCs. However, the culturing of target cells remains the essential first step for research on BGSCs. In this study, we established a simplified procedure to culture and isolate BGSCs from samples of clinical glioma patients. Samples of 17 glioma patients were included in the study, and the processed glioma cells were grown in serum-free stem cell media. After the tumor spheres appeared, a proliferation assay, a single-cell-derived colonies formation assay and an induced differentiation assay were carried out, followed by an immunocytochemistry assay. Serial passage was used to purify the target cells, whereas neither animal experiments nor sorting techniques were included. As a result, CD133(+) BGSCs from 8 out of 17 patients were grown and maintained in a serum-free condition combined with EGF, FGF and B-27 supplements. The tumor sphere cells were serially passaged and showed pluripotency in an induced differentiation assay. Immunocytochemistry identified the committed markers (CD133, GFAP and TU-20) and confirmed the cells were BGSCs and their progeny. The results proved that CD133(+) BGSCs from resected glioma tissue may be cultured in serum-free stem cell media, and may also be purified by conditioned culture combining serial passage, which is time-saving and cost-effective, and allows the cells to be used for subsequent research. The cell sorting techniques and animal experiments of tumorigenecity are optional. Thus, this modified procedure is more practical and feasible than other available procedures.

An inhibitor of arachidonate 5-lipoxygenase, Nordy, induces differentiation and inhibits self-renewal of glioma stem-like cells

Recent progress in cancer biology indicates that eradication of cancer stem cells (CSCs) is essential for more effective cancer therapy. Unfortunately, cancer stem cells such as glioma stem-like cells (GSLCs) are often resistant to either radio- or chemotherapy. Therefore, screening and development for novel therapeutic modalities against CSCs has been an important emerging field in cancer research. In this study, we report that a synthetic dl-nordihydroguaiaretic acid compound (dl-NDGA or "Nordy"), inhibited self-renewal and induced differentiation of GSLCs in vitro and in vivo. We found that Nordy inhibited an enzyme known to be involved in leukemia stem cell and leukemia progression, Alox-5, and attenuated the growth of GSLCs in vitro. Nordy reduced the GSLC pool through a decrease in the CD133(+) population and abrogated clonogenicity. Nordy appeared to exert its effect via astrocytic differentiation by up-regulation of GFAP and down-regulation of stemness related genes, rather than by inducing apoptosis of GSLCs. The growth inhibition of xenografted glioma by Nordy was more long-lasting compared with that of the akylating agent BCNU, which exhibited significant relapse on drug discontinuation resulting from an enrichment of GSLCs. Meanwhile, transient exposure to Nordy reduced tumorigenecity of GSLCs and induced differentiation of the xenografts. Taken together, we have identified Alox-5 as a novel target in GSLCs and its inhibition with Nordy exhibits therapeutic implications through inducing GSLC differentiation.

Therapeutic approaches to target cancer stem cells

The clinical relevance of cancer stem cells (CSC) remains a major challenge for current cancer therapies, but preliminary findings indicate that specific targeting may be possible. Recent studies have shown that these tumor subpopulations promote tumor angiogenesis through the increased production of VEGF, whereas the VEGF neutralizing antibody bevacizumab specifically inhibits CSC growth. Moreover, nimotuzumab, a monoclonal antibody against the epidermal growth factor receptor (EGFR) with a potent antiangiogenic activity, has been shown by our group to reduce the frequency of CSC-like subpopulations in mouse models of brain tumors when combined with ionizing radiation. These studies and subsequent reports from other groups support the relevance of approaches based on molecular-targeted therapies to selectively attack CSC. This review discusses the relevance of targeting both the EGFR and angiogenic pathways as valid approaches to this aim. We discuss the relevance of identifying better molecular markers to develop drug screening strategies that selectively target CSC.

Brain Tumor Stem-Like Cells Identified by Neural Stem Cell Marker CD15

In recent years, a small number of cells that have stem cell properties were identified in human gliomas called brain tumor stem cells (BTSCs), which were thought to mainly contribute to the initiation and development of gliomas and could be identified by the surface marker CD133. However, recent studies indicated that the expression of CD133 might be regulated by environmental conditions such as hypoxia and that there might be CD133(-) BTSCs. Genetic mouse models demonstrated that some gliomas originated from transformed neural stem cells (NSCs). Therefore, we investigated the expression of CD15, a surface marker for NSCs, in tumor spheres derived from astrocytoma and ependymoma. CD15(+) cells isolated from these tumor spheres had properties of BTSCs including self-renewal, multidifferentiation, and the ability to recapitulate the phenocopy of primary tumors. CD15 exhibited stable expression in long-term cultured tumor spheres, which sustained BTSCs properties, whereas CD133 expression decreased significantly in late passages. Furthermore, CD15(+)CD133(-) cells isolated from early or late passages of tumor spheres showed similar characteristics of BTSCs. Examination of glioma samples by immunohistochemistry showed that CD15 was expressed in a subset of human brain tumors. Therefore, CD15 can be used as a marker of stem-like cells derived from brain tumors that might contain CD133(-) BTSCs.

Is CD133 a biomarker for cancer stem cells of colorectal cancer and brain tumors? A meta-analysis

BACKGROUND

CD133 has been used to identify normal and cancer stem cells from several different tissues. Nowadays some researchers have reported that CD133 expression was not restricted to cancer stem cells (CSCs) of colorectal cancer and brain tumors, and CD133-negative subsets could also initiate tumors. We therefore performed a meta-analysis to assess the value of CD133 as a biomarker of CSCs for colorectal cancer and brain tumors.

METHODS

A Medline search was performed to identify relevant studies for the analysis. The meta-analysis was done using RevMan 5.0 software. Outcome measures were colony formation rate and xenotransplanted tumor formation rate.

RESULTS

Fifteen identified studies were available for analysis. For in vitro tests, there were no significant differences in the colony formation rates between CD133-positive and CD133-negative cells for colorectal cancer and brain tumors. For in vivo tests, the xenotransplanted tumor formation rate showed a significant difference between CD133-positive cells and CD133-negative cells in colorectal cancer only, corresponding to a risk difference of 0.40 (95%CI: 0.07, 0.73). Samples (cell lines versus tissues), applied biomarkers (combined versus single), and injection site were included as factors in sensitivity analyses, but the results were very inconsistent.

CONCLUSIONS

CD133 may not be suitable as a universe biomarker in identifying CSCs of colorectal cancer and brain tumors. Additional studies are necessary to further delineate its role.

The chemokine CXCL12 and its receptor CXCR4 promote glioma stem cell-mediated VEGF production and tumour angiogenesis via PI3K/AKT signalling

Chemokines and their receptors are actively involved in inflammation, immune responses, and cancer development. Here we report the detection of CD133(+) glioma stem-like cells (GSCs) co-expressing a chemokine receptor CXCR4 in human primary glioma tissues. These GSCs were located in areas adjacent to tumour vascular capillaries, suggesting an association between GSCs and tumour angiogenesis. To test this hypothesis, we isolated CD133(+) GSCs from surgical specimens of human primary gliomas and glioma cell lines. As compared to CD133(-) cells, CD133(+) GSCs expressed significantly higher levels of CXCR4 mRNA and protein, and migrated more efficiently in response to the CXCR4 ligand CXCL12. In addition, CXCL12 induced vascular endothelial growth factor (VEGF) production by CD133(+) GSCs via activation of the PI3K/AKT signalling pathway. Furthermore, knocking down of CXCR4 using RNA interference or inhibition of CXCR4 function by an antagonist AMD3100 not only reduced VEGF production by CD133(+) GSCs in vitro, but also attenuated the growth and angiogenesis of tumour xenografts in vivo formed by CD133(+) GSCs in SCID mice. These results indicate that CXCL12 and its receptor CXCR4 promote GSC-initiated glioma growth and angiogenesis by stimulating VEGF production.

Contribution of cancer stem cells to tumor vasculogenic mimicry

Vasculogenic mimicry (VM), a newly-defined pattern of tumor blood supply, provides a special passage without endothelial cells and is conspicuously different from angiogenesis and vasculogenesis. The biological features of the tumor cells that form VM remain unknown. Cancer stem cells (CSCs) are believed to be tumor-initiating cells, capable of self-renewal and multipotent differentiation, which resemble normal stem cells in phenotype and function. Recently CSCs have been shown to contribute to VM formation as well as angiogenesis. These findings challenge the previous understanding of the cellular basis of VM formation. In this review, we present evidence for participation of CSCs in VM formation. We also discuss the potential mechanisms and possible interaction of CSCs with various elements in tumor microenvironment niche. Based on the importance of VM in tumor progression, it constitutes a novel therapeutic target for cancer.

Sonic hedgehog pathway is essential for maintenance of cancer stem-like cells in human gastric cancer

Abnormal activation of the Sonic hedgehog (SHH) pathway has been described in a wide variety of human cancers and in cancer stem cells (CSCs), however, the role of SHH pathway in gastric CSCs has not been reported. In this study, we investigated the possibility that abnormal activation of the SHH pathway maintained the characteristics of gastric CSCs. First, we identified cancer stem-like cells (CSLCs) from human gastric cancer cell lines (HGC-27, MGC-803 and MKN-45) using tumorsphere culture. Compared with adherent cells, the floating tumorsphere cells had more self-renewing capacity and chemoresistance. The cells expressing CSCs markers (CD44, CD24 and CD133) were also significantly more in tumorsphere cells than in adherent cells. More importantly, in vivo xenograft studies showed that tumors could be generated with 2×10⁴ tumorsphere cells, which was 100-fold less than those required for tumors seeding by adherent cells. Next, RT-PCR and Western blot showed that the expression levels of Ptch and Gli1 (SHH pathway target genes) were significantly higher in tumorsphere cells than in adherent cells. The results of quantitative real-time PCR were similar to those of RT-PCR and Western blot. Further analysis revealed that SHH pathway blocked by cyclopamine or 5E1 caused a higher reduction in self-renewing capacity of HGC-27 tumorsphere cells than that of adherent cells. We also found that SHH pathway blocking strongly enhanced the efficacy of chemotherapeutic drugs in HGC-27 tumorsphere cells in vitro and in vivo but had no significant effect in adherent cells. Finally, we isolated the tumorspheres from gastric cancer specimen, these cells also had chemoresistance and tumorigenic capacity, and SHH pathway maintained the gastric CSLCs characteristics of tumorsphere cells from primary tumor samples. In conclusion, our data suggested that SHH pathway was essential for maintenance of CSLCs in human gastric cancer.

CD133 expression is not selective for tumor-initiating or radioresistant cell populations in the CRC cell line HCT-116

BACKGROUND AND PURPOSE

CD133 is controversially discussed as putative (surrogate) marker for cancer stem/tumor-initiating cell populations (CSC/TIC) in epithelial tumors including colorectal carcinomas (CRCs). We studied CD133 expression in established CRC cell lines and examined in vitro behavior, radioresponse and in vivo tumor formation of CD133+/- subpopulations of one cell line of interest.

MATERIALS AND METHODS

Ten CRC cell lines were analyzed for CD133 expression using flow cytometry and Western blotting. CD133+ and CD133- HCT-116 subpopulations were separated by FACS and studied in 2-D and 3-D culture and colony formation assays after irradiation. Subcutaneous xenograft formation was monitored in NMRI (nu/nu) mice.

RESULTS AND CONCLUSIONS

CRC cell lines could be classified into three groups: (i) CD133-, (ii) CD133+ and (iii) those with two distinct CD133+ and CD133- subpopulations. Isolated CD133+/- HCT-116 subpopulations were studied relative to the original fraction. No difference was found in 2-D growth, spheroid formation or radioresponse in vitro. Also, tumor formation and growth rate did not differ for the sorted subpopulations. However, a subset of xenografts originated from CD133- HCT-116 showed a striking enrichment in the CD133+ fraction. Our data show that CD133 expression is not selective for sphere forming, tumor-initiating or radioresistant subpopulations in the HCT-116 CRC cell line. This implies that CD133 cannot be regarded as a CSC/TIC marker in all CRC cell lines and that functional measurements of tumor formation have to generally accompany CSC/TIC-directed mechanistic or therapeutic studies.

Maintenance of critical properties of brain tumor stem-like cells after cryopreservation

It would be very useful to be able to classify brain tumor stem cells (BTSCs) by certain criteria to afford the design of specific or individualized treatment. Here, we studied two BTSC lines with differing biological and molecular features and whose respective features were well preserved after cryopreservation as single cells in SFM or 90% serum with 10% DMSO, a method not previously reported. The resuscitated BTSCs shared properties indistinguishable from their respective parental cells, including tumor sphere forming potentials, growth and differentiation properties, and tumorigenesis in vivo. The two cell lines also had differing molecule profiles, which can be well preserved after cryopreservation, similar to that of their respective primary tumors. Therefore, BTSCs from different patients, that have their own properties, were well retained by the present cryopreservation method, which might be a useful and reliable method for preserving BTSCs for long-term studies, such as classification and specific therapy design.

Targeting CD133 antigen in cancer

BACKGROUND

Much attention has been focused on CD133 as a marker of cancer cells with stem-cell-like ability. In the cancer stem cells (CSCs) model, only a small proportion of tumour cells are able to self-renew extensively, while the bulk of cells proceed to differentiate into committed heterogeneous clones. On the basis of the involvement of CSCs in tumourigenesis and treatment resistance, it is conceivable that only eradication of CSCs can lead to a cancer cure.

OBJECTIVE

To highlight the most recent evidence about the role of CD133 as a marker of CSCs in human tumours, and the therapeutic perspectives associated with its specific targeting.

METHODS

A literature search through Medline to locate published full articles using the following key words for selection: 'CD133 and cancer targeting', 'CD133 and chemo resistance', and 'CD133 and molecular pathways'. Only studies in English are considered.

RESULTS/CONCLUSIONS

The role of CD133 as a marker of CSCs has been documented in several human neoplasms; its expression seems to predict unfavourable prognosis. Novel therapeutic strategies aimed at targeting molecular pathways critical for CD133+ CSCs survival are being examined.

CD133 expression is not selective for tumor-initiating or radioresistant cell populations in the CRC cell lines HCT-116

BACKGROUND AND PURPOSE

CD133 is controversially discussed as putative (surrogate) marker for cancer stem/tumor-initiating cell populations (CSC/TIC) in epithelial tumors including colorectal carcinomas (CRCs). We studied CD133 expression in established CRC cell lines and examined in vitro behavior, radioresponse and in vivo tumor formation of CD133(+/-) subpopulations of one cell line of interest.

MATERIALS AND METHODS

Ten CRC cell lines were analyzed for CD133 expression using flow cytometry and Western blotting. CD133+ and CD133(-) HCT-116 subpopulations were separated by FACS and studied in 2-D and 3-D culture and colony formation assays after irradiation. Subcutaneous xenograft formation was monitored in NMRI (nu/nu) mice.

RESULTS AND CONCLUSIONS

CRC cell lines could be classified into three groups: (i) CD133(-), (ii) CD133+ and (iii) those with two distinct CD133+ and CD133(-) subpopulations. Isolated CD133(+/-) HCT-116 subpopulations were studied relative to the original fraction. No difference was found in 2-D growth, spheroid formation or radioresponse in vitro. Also, tumor formation and growth rate did not differ for the sorted subpopulations. However, a subset of xenografts originated from CD133(-) HCT-116 showed a striking enrichment in the CD133+ fraction. Our data show that CD133 expression is not selective for sphere forming, tumor-initiating or radioresistant subpopulations in the HCT-116 CRC cell lines. This implies that CD133 cannot be regarded as a CSC/TIC marker in all CRC cell lines and that functional measurements of tumor formation have to generally accompany CSC/TIC-directed mechanistic or therapeutic studies.

Novel treatment strategies for malignant gliomas using neural stem cells

Recent studies in stem cell biology have refined our understanding of the origin and progression of cancer. Identification and characterization of endogenous neural stem cells (NSCs), especially those in the adult human brain, have inspired new ideas for selectively targeting and destroying malignant gliomas. Gliomas consist of a heterogeneous population of cells, and some of these cells have characteristics of cancer stem cells. These brain tumor stem cells (BTSCs) share certain characteristics with normal NSCs. It is still unclear, however, whether malignant gliomas in human patients originate from these aberrant BTSCs. Nonetheless, the cellular and molecular similarities between BTSCs and normal NSCs suggest a common research landscape underlying both normal and cancer stem cell biology, wherein findings of one field are relevant to the other. Furthermore, the natural tropism of NSCs to gliomas has generated the idea that modified NSCs can deliver modified genes to selectively destroy malignant brain tumor cells, and even BTSCs, while leaving healthy surrounding neurons intact. These studies and others on the basic biology of both BTSCs and NSCs will be crucial to expanding our treatment strategies for malignant gliomas.

Detection of cancer stem cells from the C6 glioma cell line

Various malignant cancers have been found to contain a sub-population of stem cell-like tumour cells, or cancer stem cells (CSCs), however, culture methods for CSCs and the size of the fraction of CSCs in C6, which is a commonly used glioma cell line, remain controversial. In this study, we demonstrated that the C6 cell line contains a fraction of tumour cells that can form tumour spheres in a simplified serum-free neural stem cell medium and express CD133 and nestin, which are widely-used markers for brain CSCs. Immunohistochemistry and immunofluorescence confirmed the existence of CSCs both in the C6 cell line and C6 xenografts. Flow cytometry demonstrated that 4.02% of cells in the C6 cell line and 4.21% in the C6 xenografts presented as CSCs. These results confirm the fraction of CSCs in the C6 cell line and provide a simple and effective method for isolation of CSCs to study the initiation and progression of human glioma and, possibly, other malignant tumours.

Oct4 is expressed in human gliomas and promotes colony formation in glioma cells

There is increasing evidence that self-renewal capacity of cancer cells is critical for carcinogenesis; hence, it is vital to examine the expression and involvement of self-renewal regulatory genes in these cells. Here, we reported that Oct4, a well-known regulator of self-renewal in embryonic stem cells, was highly expressed in human gliomas and glioma cell lines, and the expression levels were increased in parallel with increasing glioma grades. In in vitro cell cultures, Oct4 was only expressed in rat C6 glioma cells and rat neural stem cells but not in rat brain differentiated cells. Downregulation of Oct4 expression by RNA interference in C6 cells was associated with reduced cell proliferation and colony formation. Further analysis revealed that Oct4 could upregulate phosphorylation of Stat3 to promote tumor cell proliferation. Overexpression of Oct4 in C6 cells increased the expression of nestin but decreased the expression of GFAP suggesting that Oct4 might inhibit the differentiation of glioma cells. Our findings may provide further evidence for the stem cell theory of carcinogenesis. In contrast, the results might also imply that Oct4 contributes to the existence of undifferentiated cells in gliomas.

How powerful is CD133 as a cancer stem cell marker in brain tumors?

Cancer stem cells (CSCs) have been identified in a growing number of hematopoietic and solid tissue malignancies and are typically recognized by virtue of the expression of cell surface markers. CD133, a stem cell marker, is now extensively used as a surface marker to identify and isolate brain tumor stem cells (BTSCs) in malignant brain tumors. However, CD133 as the marker to sort BTSCs suffered some controversies. In this review, we reviewed the rise of CD133, analyzed the efficiency of CD133 on identification and isolation of BTSCs, explained some controversial study results and summed up the role of CD133 and other effective CSCs markers in sorting CSCs in other tumors. We analyzed current limited reports and found that the expression of CD133 was correlated with poor clinical prognosis in brain tumors. Finally, we summarized the mechanisms of chemo- and radio- resistance of CD133+ brain tumor cell, especially emphasized that the aberrant activation of development pathways in BTSCs can be potential targets to BTSCs, and outlined current preclinical studies on killing BTSCs or sensitizing BTSCs to chemo- and radio-therapies.

Review Paper: Cancer Stem Cells and Cancer Nonstem Cells: From Adult Stem Cells or from Reprogramming of Differentiated Somatic Cells

Two opposing hypotheses of the origin of cancer have existed for many decades. One hypothesis postulates that the adult stem cell is needed to initiate the carcinogenic process, whereas the other claims a somatic differentiated cell can dedifferentiate or be reprogrammed to regain properties associated with cancer cells. Recent reemergence of the cancer stem cell hypothesis and the isolation of presumptive cancer stem cells from many types of cancer have forced a reexamination of these 2 hypotheses of the origin of cancer. In addition, normal embryonic and adult stem cells have now been isolated and partially characterized. Furthermore, the demonstration of embryonic-like stem cells, being isolated from adult-differentiated skin fibroblast cells of mice, monkey, and human beings, provides a newer opportunity to determine which of these 2 hypotheses might explain the cell type for initiating the carcinogenic process. The goal of this review is to integrate these recent findings, concerning the isolation of normal and cancer stem cells, with several of the classical concepts of carcinogenesis (initiation/promotion/progression; mutation/epigenetic; stem cell theory/dedifferentiation hypotheses; oncogenetumor suppressor theory). Although the weight of the evidence in this review seems to support the stem cell hypothesis, only future studies, probably using comparative animal and human oncologic studies, will determine if targeting the cancer stem cell, with individualized medical approaches, will improve cancer prevention and therapy.

Molecular pathology of oligodendroglial tumors

The term oligodendroglioma was created by Bailey, Cushing, and Bucy based on the observation that these tumors share morphological similarities with oligodendrocytes (Bailey and Cushing 1926; Bailey and Bucy 1929). However, a convincing link between oligodendrocytes and oligodendrogliomas still needs to be shown. Oligoastrocytomas or mixed gliomas are histologically defined by the presence of oligodendroglial and astrocytic components. According to the WHO classification of brain tumors, oligodendroglial tumors are separated into oligodendrogliomas WHO grade II (OII), anaplastic oligodendrogliomas WHO grade III (OIII), oligoastrocytomas WHO grade II (OAII), anaplastic oligoastrocytomas WHO grade III (OAIII), and glioblastomas with oligodendroglioma component WHO grade IV (GBMo) (Louis et al. 2007).The perception of oligodendroglial tumors has changed in recent years. The diagnosis of oligodendroglioma or oligoastrocytomas is made much more frequently than 10 years ago. Treatment modalities have been advanced and novel concepts regarding the origin of oligodendroglial tumors have been developed. This review focuses on recent developments with impact on the diagnosis and understanding of molecular mechanisms in oligodendroglial tumors.

Cancer stem cells and glioma

Despite continued advances in surgical and medical therapies, the outcomes for patients diagnosed with glioblastoma multiforme remain dismal. Recent data suggest that progression of these brain tumors is driven by a small subpopulation of tumor cells, which are termed cancer stem cells (CSCs) because of their capability to self-renew, proliferate and give rise to progeny of multiple neuroepithelial lineages. According to the CSC hypothesis, current therapies that are extremely cytotoxic to the bulk of highly proliferative tumor cells fail to obliterate the relatively quiescent and resistant CSC compartment, thereby allowing these cells to survive and drive tumor recurrence. This Review summarizes current knowledge regarding neural stem cells in the normal adult brain and CSCs in glial tumors and discusses the implications of the CSC hypothesis for the development of future therapies for brain tumors.

Invincible, but Not Invisible: Imaging Approaches Toward In Vivo Detection of Cancer Stem Cells

With evidence emerging in support of a cancer stem-cell model of carcinogenesis, it is of paramount importance to identify and image these elusive cells in their natural environment. The cancer stem-cell hypothesis has the potential to explain unresolved questions of tumorigenesis, tumor heterogeneity, chemotherapeutic and radiation resistance, and even the metastatic phenotype. Intravital imaging of cancer stem cells could be of great value for determining prognosis, as well as monitoring therapeutic efficacy and influencing therapeutic protocols. Cancer stem cells represent a rare population of cells, as low as 0.1% of cells within a human tumor, and the phenotype of isolated cancer stem cells is easily altered when placed under in vitro conditions. This represents a challenge in studying cancer stem cells without manipulation or extraction from their natural environment. Advanced imaging techniques allow for the in vivo observation of physiological events at cellular resolution. Cancer stem-cell studies must take advantage of such technology to promote a better understanding of the cancer stem-cell model in relation to tumor growth and metastasis, as well as to potentially improve on the principles by which cancers are treated. This review examines the opportunities for in vivo imaging of putative cancer stem cells with regard to currently accepted cancer stem-cell characteristics and advanced imaging technologies.

CD133 expression and cancer stem cells predict prognosis in high-grade oligodendroglial tumors

High-grade oligodendroglial tumors, that is, anaplastic oligodendroglial tumors and glioblastomas with oligodendroglial component, differ significantly in terms of prognosis and response to chemotherapy. Differentiation might be difficult because the histological differences are vague and reliable markers are not established. We correlated the presence of putative cancer stem cells (CSC) in high-grade oligodendroglial tumors (WHO grades III and IV) with clinical outcome. Tumors with favorable prognosis neither contained CSC nor did they show CD133 expression. Tumor cells resembled lineage-restricted progenitor cells with limited proliferative capacity and differentiation profile. In contrast, CD133 expression and stem cell-like tumor cells characterized tumors with poor prognosis. They showed neurosphere-like growth, differentiated into cells of all neural lineages, and were tumorigenic in nude mice. In our series, CSC and expression of CD133 predicted the clinical course of disease better than the histological grading. To confirm these results, we retrospectively analyzed 36 high-grade oligodendroglial tumors. Again, CD133 expression indicated shorter survival and predicted clinical outcome more reliable than the histological assessment. Our data show that detection of CSC and expression of CD133 is predictive of prognosis in high-grade oligodendroglial tumors. The presence or absence of CD133(+) CSC might explain the crucial biological difference between WHO grade III and IV oligodendroglial tumors.

Isolation and characterization of cancer stem cells from a human glioblastoma cell line U87

A variety of malignant cancers have been found to contain a subpopulation of stem cell-like tumor cells, or cancer stem cells (CSCs). However, the existence of CSCs in U87, a most commonly used glioma cell line, is still controversial. In this study, we demonstrate that U87 cell line contained a fraction of tumor cells that could form tumor spheres and were enriched by progressively increasing the concentration of serum-free neural stem cell medium with or without low dose vincristine. These cells possessed the ability of self-renewal and multipotency, the defined characteristics of CSCs. Moreover, the tumors formed by the secondary spheres displayed typical histological features of human glioblastoma, including cellular pleomorphism, pseudopalisades surrounding necrosis, hyperchromatic nuclei, high density of microvessels and invasion to the brain parenchyma. These results indicate that gradually increasing the concentration of serum-free neural stem cell culture medium with or without vincristine is a simple and effective method for isolation of CSCs to study the initiation and progression of human glioblastoma.