Brain tumour stem cells: possibilities of new therapeutic strategies

MALDI Imaging Mass Spectrometry of High-Grade Gliomas: A Review of Recent Progress and Future Perspective

Glioblastoma (GBM) is the most common malignancy of the brain with a relatively short median survival and high mortality. Advanced age, high socioeconomic status, exposure to ionizing radiation, and other factors have been correlated with an increased incidence of GBM, while female sex hormones, history of allergies, and frequent use of specific drugs might exert protective effects against this disease. However, none of these explain the pathogenesis of GBM. The most recent WHO classification of CNS tumors classifies neoplasms based on their histopathological and molecular characteristics. Modern laboratory techniques, such as matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry, enable the comprehensive metabolic analysis of the tissue sample. MALDI imaging is able to characterize the spatial distribution of a wide array of biomolecules in a sample, in combination with histological features, without sacrificing the tissue integrity. In this review, we first provide an overview of GBM epidemiology, risk, and protective factors, as well as the recent WHO classification of CNS tumors. We then provide an overview of mass spectrometry workflow, with a focus on MALDI imaging, and recent advances in cancer research. Finally, we conclude the review with studies of GBM that utilized MALDI imaging and offer our perspective on future research.

LncRNA as potential biomarker and therapeutic target in glioma

Glioma is the most frequent type of malignant tumor in the central nervous system, accounting for about 80% of primary malignant brain tumors, usually with a poor prognosis. A number of studies have been conducted on the molecular abnormalities in glioma to further understand its pathogenesis, and it has been found that lncRNAs (long non-coding RNA) play a key role in angiogenesis, tumor growth, infiltration and metastasis of glioma. Since specific lncRNAs have an aberrant expression in brain tissue, cerebrospinal fluid as well as peripheral circulation of glioma patients, they are considered to be potential biomarkers. This review focuses on the biological characteristics of lncRNA and its value as a biomarker for glioma diagnosis and prognosis. Moreover, in view of the role of lncRNAs in glioma proliferation and chemoradiotherapy resistance, we discussed the feasibility for lncRNAs as therapeutic targets. Finally, the persisting deficiencies and future prospects of using lncRNAs as clinical biomarkers and therapeutic targets were concluded.

The Importance of Tumor Stem Cells in Glioblastoma Resistance to Therapy

Glioblastoma (GBM) is known to be the most common and lethal primary malignant brain tumor. Therapies against this neoplasia have a high percentage of failure, associated with the survival of self-renewing glioblastoma stem cells (GSCs), which repopulate treated tumors. In addition, despite new radical surgery protocols and the introduction of new anticancer drugs, protocols for treatment, and technical advances in radiotherapy, no significant improvement in the survival rate for GBMs has been realized. Thus, novel antitarget therapies could be used in conjunction with standard radiochemotherapy approaches. Targeted therapy, indeed, may address specific targets that play an essential role in the proliferation, survival, and invasiveness of GBM cells, including numerous molecules involved in signal transduction pathways. Significant cellular heterogeneity and the hierarchy with GSCs showing a therapy-resistant phenotype could explain tumor recurrence and local invasiveness and, therefore, may be a target for new therapies. Therefore, the forced differentiation of GSCs may be a promising new approach in GBM treatment. This article provides an updated review of the current standard and experimental therapies for GBM, as well as an overview of the molecular characteristics of GSCs, the mechanisms that activate resistance to current treatments, and a new antitumor strategy for treating GSCs for use as therapy.

Steroid receptor coactivator-1 enhances the stemness of glioblastoma by activating long noncoding RNA XIST/miR-152/KLF4 pathway

Glioblastoma (GBM) recurrence is attributed to the presence of therapy-resistant glioblastoma stem cells. Steroid receptor coactivator-1 (SRC-1) acts as an oncogenic regulator in many human tumors. The relationship between SRC-1 and GBM has not yet been studied. Herein, we investigate the role of SRC-1 in GBM. In this study, we found that SRC-1 expression is positively correlated with grades of glioma and inversely correlated with glioma patient's prognosis. Steroid receptor coactivator-1 promotes the proliferation, migration, and tumor growth of GBM cells. Notably, SRC-1 knockdown suppresses the stemness of GBM cells. Mechanistically, long noncoding RNA X-inactive specific transcript (XIST) is regulated by SRC-1 at the posttranscriptional level and mediates the function of SRC-1 in promoting stemness-like properties of GBM. Steroid receptor coactivator-1 can promote the expression of Kruppel-like factor 4 (KLF4) through the XIST/microRNA (miR)-152 axis. Additionally, arenobufagin and bufalin, SRC small molecule inhibitors, can reduce the proliferation and stemness of GBM cells. This study reveals SRC-1 promotes the stemness of GBM by activating the long noncoding RNA XIST/miR-152/KLF4 pathway and provides novel markers for diagnosis and therapy of GBM.

The Protein Arginine Methyltransferases 1 and 5 affect Myc properties in glioblastoma stem cells

Protein Arginine (R) methylation is the most common post-translational methylation in mammalian cells. Protein Arginine Methyltransferases (PRMT) 1 and 5 dimethylate their substrates on R residues, asymmetrically and symmetrically, respectively. They are ubiquitously expressed and play fundamental roles in tumour malignancies, including glioblastoma multiforme (GBM) which presents largely deregulated Myc activity. Previously, we demonstrated that PRMT5 associates with Myc in GBM cells, modulating, at least in part, its transcriptional properties. Here we show that Myc/PRMT5 protein complex includes PRMT1, in both HEK293T and glioblastoma stem cells (GSCs). We demonstrate that Myc is both asymmetrically and symmetrically dimethylated by PRMT1 and PRMT5, respectively, and that these modifications differentially regulate its stability. Moreover, we show that the ratio between symmetrically and asymmetrically dimethylated Myc changes in GSCs grown in stem versus differentiating conditions. Finally, both PRMT1 and PRMT5 activity modulate Myc binding at its specific target promoters. To our knowledge, this is the first work reporting R asymmetrical and symmetrical dimethylation as novel Myc post-translational modifications, with different functional properties. This opens a completely unexplored field of investigation in Myc biology and suggests symmetrically dimethylated Myc species as novel diagnostic and prognostic markers and druggable therapeutic targets for GBM.

Inhibition of autophagy increases susceptibility of glioblastoma stem cells to temozolomide by igniting ferroptosis

The role of autophagy in cancer onset and progression appears still controversial. On one hand, autophagy allows cancer cell to survive in unfavorable environmental conditions, on the other hand, once internal energy resources are exhausted, it leads to cell death. In addition, autophagy interpheres with cell cycle progression, de facto exerting a cytostatic activity. Hence, it represents an important target for anticancer therapy. For example, temozolomide (TMZ), of use for glioblastoma (GBM) treatment, appears as capable of inducing autophagy partially inhibiting cancer cell proliferation. However, GBM, a very aggressive brain tumor with poor prognosis even after surgery and radio-chemotherapy, invariably recurs and leads to patient death. Since cancer stem cells have been hypothesized to play a role in refractory/relapsing cancers, in the present work we investigated if autophagy could represent a constitutive cytoprotection mechanism for glioblastoma stem-like cells (GSCs) and if the modulation of autophagic process could affect GBM growth and survival. Thus, in the present study we first evaluated the relevance of autophagy in GBM tumor specimens, then its occurrence in GSCs and, finally, if modulation of autophagy could influence GSC response to TMZ. Our results suggested that, in vitro, the impairing autophagic process with quinacrine, a compound able to cross the blood-brain barrier, increased GSC susceptibility to TMZ. Death of GSCs was apparently due to the iron dependent form of programmed cell death characterized by the accumulation of lipid peroxides called ferroptosis. These results underscore the relevance of the modulation of autophagy in the GSC survival and death and suggest that triggering of ferroptosis in GSCs could represent a novel and important target for the management of glioblastoma.

Isolating single cells in a neurosphere assay using inertial microfluidics

Sphere forming assays are routinely used for in vitro propagation and differentiation of stem cells. Because the stem cell clusters can become heterogeneous and polyclonal, they must first be dissociated into a single cell suspension for further clonal analysis or differentiation studies. The dissociated population is marred by the presence of doublets, triplets and semi-cleaved/intact clusters which makes identification and further analysis of differentiation pathways difficult. In this work, we use inertial microfluidics to separate the single cells and clusters in a population of chemically dissociated neurospheres. In contrast to previous microfluidic sorting technologies which operated at high flow rates, we implement the spiral microfluidic channel in a novel focusing regime that occurs at lower flow rates. In this regime, the curvature-induced Dean's force focuses the smaller, single cells towards the inner wall and the larger clusters towards the center. We further demonstrate that sorting in this low flow rate (and hence low shear stress) regime yields a high percentage (>90%) of viable cells and preserves multipotency by differentiating the sorted neural stem cell population into neurons and astrocytes. The modularity of the device allows easy integration with other lab-on-a-chip devices for upstream mechanical dissociation and downstream high-throughput clonal analysis, localized electroporation and sampling. Although demonstrated in the case of the neurosphere assay, the method is equally applicable to other sphere forming assays.

Myc and Omomyc functionally associate with the Protein Arginine Methyltransferase 5 (PRMT5) in glioblastoma cells

The c-Myc protein is dysregulated in many human cancers and its function has not been fully elucitated yet. The c-Myc inhibitor Omomyc displays potent anticancer properties in animal models. It perturbs the c-Myc protein network, impairs c-Myc binding to the E-boxes, retaining transrepressive properties and inducing histone deacetylation. Here we have employed Omomyc to further analyse c-Myc activity at the epigenetic level. We show that both Myc and Omomyc stimulate histone H4 symmetric dimethylation of arginine (R) 3 (H4R3me2s), in human glioblastoma and HEK293T cells. Consistently, both associated with protein Arginine Methyltransferase 5 (PRMT5)—the catalyst of the reaction—and its co-factor Methylosome Protein 50 (MEP50). Confocal experiments showed that Omomyc co-localized with c-Myc, PRMT5 and H4R3me2s-enriched chromatin domains. Finally, interfering with PRMT5 activity impaired target gene activation by Myc whereas it restrained Omomyc-dependent repression. The identification of a histone-modifying complex associated with Omomyc represents the first demonstration of an active role of this miniprotein in modifying chromatin structure and adds new information regarding its action on c-Myc targets. More importantly, the observation that c-Myc may recruit PRMT5-MEP50, inducing H4R3 symmetric di-methylation, suggests previously unpredictable roles for c-Myc in gene expression regulation and new potential targets for therapy.

EGFR Amplification and Glioblastoma Stem-Like Cells

Glioblastoma (GBM), the most common malignant brain tumor in adults, contains a subpopulation of cells with a stem-like phenotype (GS-cells). GS-cells can be maintained in vitro using serum-free medium supplemented with epidermal growth factor, basic fibroblast growth factor-2, and heparin. However, this method does not conserve amplification of the Epidermal Growth Factor Receptor (EGFR) gene, which is present in over 50% of all newly diagnosed GBM cases. GS-cells with retained EGFR amplification could overcome the limitations of current in vitro model systems and contribute significantly to preclinical research on EGFR-targeted therapy. This review recapitulates recent methodological approaches to expand stem-like cells from GBM with different EGFR status in order to maintain EGFR-dependent intratumoral heterogeneity in vitro. Further, it will summarize the current knowledge about the impact of EGFR amplification and overexpression on the stem-like phenotype of GBM-derived GS-cells and different approaches to target the EGFR-dependent GS-cell compartment of GBM.

Silencing of the TPM1 gene induces radioresistance of glioma U251 cells

The present study was designed to investigate the relationship between tropomyosin 1 (TPM1) and radioresistance in human U251 cells. Radioresistant U251 (RR-U251) cells were established by repeated small irradiating injury. TPM1 levels in the U251 and RR-U251 cells were inhibited by transfection with TPM1-short hairpin RNA (shRNA) while overexpression was induced by treatment with pcDNA3.1‑TPM1. The radiosensitivity of the U251 and RR-U251 cells and the plasmid-transfected cells was evaluated by cell viability, migration and invasion assays. Cell apoptosis was also examined in vitro. The radiosensitivity of U251 xenografts was observed by tumor growth curve after radiotherapy in an in vivo experiment. Western blotting and immunohistochemistry were used to detect the level of TPM1 in vivo. The expression of TPM1 was significantly decreased in the RR-U251 cells, which may be correlated with the radioresistance of the glioma U251 cells. In the TPM1-silenced RR-U251 and TPM1-silenced U251 cells, cell viability, migration and invasion ability were significantly increased, and the rate of cell apoptosis was decreased. Consistent with these results, in the TPM1-overexpressing U251 and RR-U251 cells, cell viability, migration and invasion abilities were markedly decreased, and increased apoptosis was noted when compared to the control group. Tumor growth of the U251 xenografts was significantly inhibited following treatment with pcDNA3.1‑TPM1 combined with radiotherapy. Taken together, these results indicate that TPM1 may be one mechanism underlying radiation resistance, and TPM1 may be a potential target for overcoming the radiation resistance in glioma.

Deregulated proliferation and differentiation in brain tumors

Neurogenesis, the generation of new neurons, is deregulated in neural stem cell (NSC)- and progenitor-derived murine models of malignant medulloblastoma and glioma, the most common brain tumors of children and adults, respectively. Molecular characterization of human malignant brain tumors, and in particular brain tumor stem cells (BTSCs), has identified neurodevelopmental transcription factors, microRNAs, and epigenetic factors known to inhibit neuronal and glial differentiation. We are starting to understand how these factors are regulated by the major oncogenic drivers in malignant brain tumors. In this review, we will focus on the molecular switches that block normal neuronal differentiation and induce brain tumor formation. Genetic or pharmacological manipulation of these switches in BTSCs has been shown to restore the ability of tumor cells to differentiate. We will discuss potential brain tumor therapies that will promote differentiation in order to reduce treatment resistance, suppress tumor growth, and prevent recurrence in patients.

1H NMR detects different metabolic profiles in glioblastoma stem-like cells

The metabolic profiles of glioblastoma stem-like cells (GSCs) growing in neurospheres were examined by (1)H NMR spectroscopy. Spectra of two GSC lines, labelled 1 and 83, from tumours close to the subventricular zone of the temporal lobe were studied in detail and compared with those of neural stem/progenitor cells from the adult olfactory bulb (OB-NPCs) and of the T98G glioblastoma cell line. In both GSCs, signals from myoinositol (Myo-I), UDP-hexosamines (UDP-Hex) and glycine indicated an astrocyte/glioma metabolism. For line 1, the presence of signals from N-acetyl aspartate, GABA and creatine pointed to a neuronal fingerprint. These metabolites were almost absent from line 83 spectra, whereas lipid signals, absent from normal neural lineages, were intense in line 83 spectra and remained low in those of line 1, irrespective of apoptotic fate. Spectra of OB-NPC cells displayed strong similarities with those from line 1, with low lipid signals and clearly detectable neuronal signals. In contrast, the spectral profile of line 83 was more similar to that of T98G, displaying high lipids and nearly complete absence of the neuronal markers. A mixed neural-astrocyte metabolic phenotype with a strong neuronal fingerprint was therefore found in line 1, while an astrocytic/glioma-like metabolism prevailed in line 83. We found a signal assigned to the amide proton of N-acetyl galactosamine in GSC lines and in OB-NPC spectra, whereas it was absent from those of T98G cells. This signal may be related to a stem-cell-specific protein glycosylation pattern and is therefore suggested as a marker of cell multipotency. Other GSC lines from patients with different clinical outcomes were then examined. Unsupervised analysis of spectral data from 13 lines yielded two clusters, with six lines resembling spectral features of line 1 and seven resembling those of line 83, suggesting that distinct metabolic phenotypes may be present in GSC lines.

Targeting cancer stem cells for treatment of glioblastoma multiforme

Cancer stem cells (CSCs) in glioblastoma multiforme (GBM) are radioresistant and chemoresistant, which eventually results in tumor recurrence. Targeting CSCs for treatment is the most crucial issue. There are five methods for targeting the CSCs of GBM. One is to develop a new chemotherapeutic agent specific to CSCs. A second is to use a radiosensitizer to enhance the radiotherapy effect on CSCs. A third is to use immune cells to attack the CSCs. In a fourth method, an agent is used to promote CSCs to differentiate into normal cells. Finally, ongoing gene therapy may be helpful. New therapeutic agents for targeting a signal pathway, such as epidermal growth factor (EGF) and vascular epidermal growth factor (VEGF) or protein kinase inhibitors, have been used for GBM but for CSCs the effects still require further evaluation. Nonsteroidal anti-inflammatory drugs (NSAIDs) such as cyclooxygenase-2 (Cox-2) inhibitors have proven to be effective for increasing radiation sensitivity of CSCs in culture. Autologous dendritic cells (DCs) are one of the promising immunotherapeutic agents in clinical trials and may provide another innovative method for eradication of CSCs. Bone-morphogenetic protein 4 (BMP4) is an agent used to induce CSCs to differentiate into normal glial cells. Research on gene therapy by viral vector is also being carried out in clinical trials. Targeting CSCs by eliminating the GBM tumor may provide an innovative way to reduce tumor recurrence by providing a synergistic effect with conventional treatment. The combination of conventional surgery, chemotherapy, and radiotherapy with stem cell-orientated therapy may provide a new promising treatment for reducing GBM recurrence and improving the survival rate.

Isolation and enrichment of PC-3 prostate cancer stem-like cells using MACS and serum-free medium

Prostate cancer stem-like cells (PCSLCs) are considered to be the ‘seed’ of prostate cancer. The aim of this study was to confirm that the PC-3 cells, which we isolated and enriched from PC-3 cells through magnetic bead cell sorting (MACS) and serum-free medium (SFM) culture, were PCSLCs. Combinations of MACS, flow cytometry (FCM), SFM and immunocytochemistry (ICC) were used to ensure the positive expression of CD133 and CD44 on PC-3 and sphere-forming cell membranes. Self-renewal, multi-potential differentiation, unlimited proliferation and permanency assays were also applied to indentify whether the PC-3 cells exhibited the characteristics of cancer stem cells (CSCs). As a result, there was a low proportion of PCSLCs in the PC-3 cells. In the FCM assay, the proportion of cells expressing CD133 or CD44 in the PC-3 cells was 0.51 and 0.31%, respectively. In addition, we found that the proportion of PC-3 cells sorted by MACS that expressed CD133 was significantly increased compared with that of the sphere-forming cells cultured in SFM (99.09 vs. 84.80%, P<0.05), while no difference was observed in the proportion of cells expressing CD44 between them (99.88 vs. 99.82%, P>0.05). The expression of PAP and AR as detected by western blot analysis of induced PCSLCs was significantly increased compared with that of uninduced PCSLCs (P<0.05); the proliferation capacity of PCSLCs was significantly higher than that of both the PC-3 cells (P<0.05) and induced PCSLCs (P<0.05). Furthermore, the PCSLCs that were isolated from SFM and MACS both demonstrated certain characteristics of stem cells and should be considered as stem cell-like. These data may hold potential for further exploring the role of PCSLCs.

miR-125b regulates the proliferation of glioblastoma stem cells by targeting E2F2. FEBS Lett. 2012;586:3831-3839. [PMID: 22999819 DOI: 10.1016/j.febslet.2012.08.023]

microRNAs (miRNAs) play important role in regulating cancer stem cell self-renewal and differentiation, but the expression prolife of miRNAs in glioma stem cells (GSCs) has not been addressed. Here, we found that CD133 positive GSCs possess a unique miRNAs profile compared to CD133 negative glioblastoma cells. miR-125b, as one of neuronal miRNAs, is the most significantly down-regulated miRNAs and overexpression of miR-125b inhibits the proliferation of CD133 positive GSCs and reduces the expression of "stem" marker. Furthermore, two binding sites for miR-125b are identified in the 3'UTR of E2F2 and overexpression of miR-125b in CD133 positive GSCs represses the endogenous level of E2F2 protein. This study demonstrated that miR-125b plays important roles in regulating the proliferation of GSCs by directly targeting E2F2.

Tumor stem cells: A new approach for tumor therapy (Review)

Recent studies have demonstrated the existence of a minority of tumor cells possessing the stem cell properties of self-renewal and differentiation in leukemia and several solid tumors. However, these cells do not possess the normal regulatory mechanisms of stem cells. Following transplantation, they are capable of initiating tumorigenesis and are therefore known as 'tumor stem cells'. Cellular origin analysis of tumor stem cells has resulted in three hypotheses: Embryonal rest hypothesis, anaplasia and maturation arrest. Several signaling pathways which are involved in carcinogenesis, including Wnt/β-catenin, Notch and Oct-4 signaling pathways are crucial in normal stem cell self-renewal decisions, suggesting that breakdown in the regulation of self-renewal may be a key event in the development of tumors. Thus, tumors can be regarded as an abnormal organ in which stem cells have escaped from the normal constraints on self-renewal, thus, leading to abnormally differentiated tumor cells that lose the ability to form tumors. This new model for maligancies has significance for clinical research and treatment.

Knockdown of checkpoint kinase 1 is associated with the increased radiosensitivity of glioblastoma stem-like cells

Glioblastoma multiforme is an aggressive brain tumor with a poor prognosis. The glioblastoma stem-like cells (GSCs) represent a rare fraction of human glioblastoma cells with the capacity for multi-lineage differentiation, self-renewal and exact recapitulation of the original tumor. Interestingly, GSCs are more radioresistant compared with other tumor cells. In addition, the remarkable radioresistance of GSCs has been known to promote radiotherapy failure and therefore is associated with a significantly higher risk of a local tumor recurrence. Moreover, the hyperactive cell cycle checkpoint kinase (Chk) 1 and 2 play a pivotal role in the DNA damage response including radiation and chemical therapy. Based on aforementioned, we hypothesized that knockdown of Chk1 or Chk2 might confer radiosensitivity on GSCs and thereby increases the efficiency of radiotherapy. In this study, we knocked down the expression of Chk1 or Chk2 in human GSCs using lentivirus-delivered short hairpin RNA (shRNA) to examine its effect on the radiosensitivity. After radiation, the apoptosis rate and the cell cycle of GSCs were measured with Flow Cytometry. Compared with control GSCs (apoptosis, 7.82 ± 0.38%; G2/M arrest, 60.20 ± 1.28%), Chk1 knockdown in GSCs increased the apoptosis rate (37.87 ± 0.32%) and decreased the degree of the G2/M arrest (22.37 ± 2.01%). In contrast, the radiosensitivity was not enhanced by Chk2 knockdown in GSCs. These results suggest that depletion of Chk1 may improve the radio-sensitivity of GSCs via inducing cell apoptosis. In summary, the therapy targeting Chk1 gene in the GSCs may be a novel way to treat glioblastoma.

Regulation of glioblastoma stem cells by retinoic acid: role for Notch pathway inhibition. Oncogene. 2011;30:3454-3467. [PMID: 21383690 DOI: 10.1038/onc.2011.58]

It is necessary to understand mechanisms by which differentiating agents influence tumor-initiating cancer stem cells. Toward this end, we investigated the cellular and molecular responses of glioblastoma stem-like cells (GBM-SCs) to all-trans retinoic acid (RA). GBM-SCs were grown as non-adherent neurospheres in growth factor supplemented serum-free medium. RA treatment rapidly induced morphology changes, induced growth arrest at G1/G0 to S transition, decreased cyclin D1 expression and increased p27 expression. Immunofluorescence and western blot analysis indicated that RA induced the expression of lineage-specific differentiation markers Tuj1 and GFAP and reduced the expression of neural stem cell markers such as CD133, Msi-1, nestin and Sox-2. RA treatment dramatically decreased neurosphere-forming capacity, inhibited the ability of neurospheres to form colonies in soft agar and inhibited their capacity to propagate subcutaneous and intracranial xenografts. Expression microarray analysis identified ∼350 genes that were altered within 48 h of RA treatment. Affected pathways included retinoid signaling and metabolism, cell-cycle regulation, lineage determination, cell adhesion, cell-matrix interaction and cytoskeleton remodeling. Notch signaling was the most prominent of these RA-responsive pathways. Notch pathway downregulation was confirmed based on the downregulation of HES and HEY family members. Constitutive activation of Notch signaling with the Notch intracellular domain rescued GBM neurospheres from the RA-induced differentiation and stem cell depletion. Our findings identify mechanisms by which RA targets GBM-derived stem-like tumor-initiating cells and novel targets applicable to differentiation therapies for glioblastoma.

The Oncogenic Potential of Mesenchymal Stem Cells in the Treatment of Cancer: Directions for Future Research

Mesenchymal stem cells (MSCs) represent a promising new approach to the treatment of several diseases that are associated with dismal outcomes. These include myocardial damage, graft versus host disease, and possibly cancer. Although the potential therapeutic aspects of MSCs continue to be well-researched, the possible hazards of MSCs, and in particular their oncogenic capacity are poorly understood. This review addresses the oncogenic and tumor-supporting potential of MSCs within the context of cancer treatment. The risk for malignant transformation is discussed for each stage of the clinical lifecycle of MSCs. This includes malignant transformation in vitro during production phases, during insertion of potentially therapeutic transgenes, and finally in vivo via interactions with tumor stroma. The immunosuppressive qualities of MSCs, which may facilitate evasion of the immune system by a tumor, are also addressed. Limitations of the methods employed in clinical trials to date are reviewed, including the absence of long term follow-up and lack of adequate screening methods to detect formation of new tumors. Through discussions of the possible oncogenic and tumor-supporting mechanisms of MSCs, directions for future research are identified which may eventually facilitate the future clinical translation of MSCs for the treatment of cancer and other diseases.

Emerging strategies for the identification and targeting of cancer stem cells

The hypothesis of cancer stem cells (CSCs) is receiving increasing interest and has become the subject of considerable debate among cancer researchers. Recent rapid progress in CSC research has encountered increasing difficulties and challenges. Understanding the biologic characteristic of CSCs is crucial to start with better identification and diagnosis based on CSC markers and eventually targeting to CSCs will undoubtedly result in improved prevention and treatment of many types of CSCs. We discuss here some of the approaching strategies that include establishing special methods of identifying CSCs and targeting therapies of CSCs.

DNER, an epigenetically modulated gene, regulates glioblastoma-derived neurosphere cell differentiation and tumor propagation

Neurospheres derived from glioblastoma (GBM) and other solid malignancies contain neoplastic stem-like cells that efficiently propagate tumor growth and resist cytotoxic therapeutics. The primary objective of this study was to use histone-modifying agents to elucidate mechanisms by which the phenotype and tumor-promoting capacity of GBM-derived neoplastic stem-like cells are regulated. Using established GBM-derived neurosphere lines and low passage primary GBM-derived neurospheres, we show that histone deacetylase (HDAC) inhibitors inhibit growth, induce differentiation, and induce apoptosis of neoplastic neurosphere cells. A specific gene product induced by HDAC inhibition, Delta/Notch-like epidermal growth factor-related receptor (DNER), inhibited the growth of GBM-derived neurospheres, induced their differentiation in vivo and in vitro, and inhibited their engraftment and growth as tumor xenografts. The differentiating and tumor suppressive effects of DNER, a noncanonical Notch ligand, contrast with the previously established tumor-promoting effects of canonical Notch signaling in brain cancer stem-like cells. Our findings are the first to implicate noncanonical Notch signaling in the regulation of neoplastic stem-like cells and suggest novel neoplastic stem cell targeting treatment strategies for GBM and potentially other solid malignancies.

Brain cancer stem cells

Cancers comprise heterogeneous cells, ranging from highly proliferative immature precursors to more differentiated cell lineages. In the last decade, several groups have demonstrated the existence of cancer stem cells in both nonsolid solid tumors, including some of the brain: glioblastoma multiforme (GBM), medulloblastoma, and ependymoma. These cells, like their normal counterpart in homologous tissues, are multipotent, undifferentiated, self-sustaining, yet transformed cells. In particular, glioblastoma-stem like cells (GBSCs) self-renew under clonal conditions and differentiate into neuron- and glia-like cells, with aberrant, mixed neuronal/astroglial phenotypes. Remarkably, upon subcutaneous and intracerebral transplantation in immunosuppressed mice, GBSCs are able to form secondary tumors that closely resemble the human pathology, a property retained also throughout serial transplantation. The search is up for the identification of the markers and the molecular mechanisms that underpin the tumorigenic potential of these cells. This is critical if we aim at defining new therapeutic approaches for the treatment of malignant brain tumors. Lately, it has been shown that some key regulatory system that plays pivotal roles in neural stem cell physiology can also regulate the tumorigenic ability of cancer stem cells in GBMs. This suggests that the study of cancer stem cells in brain tumors might help to identify new and more specific therapeutic molecular effectors, with the cancer stem cells themselves representing one of the main targets, in fact the Holy Grail, in cancer cell therapy. This review includes a summary review on brain cancer cells and their usefulness as emerging targets in cancer cell therapy.

Molecular genetic analysis of FGFR1 signalling reveals distinct roles of MAPK and PLCgamma1 activation for self-renewal of adult neural stem cells

BACKGROUND

Neural stem cells (NSCs) are present in the adult mammalian brain and sustain life-long adult neurogenesis in the dentate gyrus of the hippocampus. In culture, fibroblast growth factor-2 (FGF-2) is sufficient to maintain the self-renewal of adult NSCs derived from the adult rat hippocampus. The underlying signalling mechanism is not fully understood.

RESULTS

In the established adult rat NSC culture, FGF-2 promotes self-renewal by increasing proliferation and inhibiting spontaneous differentiation of adult NSCs, accompanied with activation of MAPK and PLC pathways. Using a molecular genetic approach, we demonstrate that activation of FGF receptor 1 (FGFR1), largely through two key cytoplasmic amino acid residues that are linked to MAPK and PLC activation, suffices to promote adult NSC self-renewal. The canonical MAPK, Erk1/2 activation, is both required and sufficient for the NSC expansion and anti-differentiation effects of FGF-2. In contrast, PLC activation is integral to the maintenance of adult NSC characteristics, including the full capacity for neuronal and oligodendroglial differentiation.

CONCLUSION

These studies reveal two amino acid residues in FGFR1 with linked downstream intracellular signal transduction pathways that are essential for maintaining adult NSC self-renewal. The findings provide novel insights into the molecular mechanism regulating adult NSC self-renewal, and pose implications for using these cells in potential therapeutic applications.

Distinct pools of cancer stem-like cells coexist within human glioblastomas and display different tumorigenicity and independent genomic evolution

Glioblastomas (GBMs) contain transformed, self-maintaining, multipotent, tumour-initiating cancer stem cells, whose identification has radically changed our perspective on the physiology of these tumours. Currently, it is unknown whether multiple types of transformed precursors, which display alternative sets of the complement of properties of true cancer stem cells, can be found in a GBM. If different subsets of such cancer stem-like cells (CSCs) do exist, they might represent distinct cell targets, with a differential therapeutic importance, also depending on their characteristics and lineage relationship. Here, we report the presence of two types of CSCs within different regions of the same human GBM. Cytogenetic and molecular analysis shows that the two types of CSCs bear quite diverse tumorigenic potential and distinct genetic anomalies, and, yet, derive from common ancestor cells. This provides critical information to unravel the development of CSCs and the key molecular/genetic components underpinning tumorigenicity in human GBMs.

Partial biological characterization of cancer stem-like cell line (WJ(2)) of human glioblastoma multiforme

To provide suitable models for human GBM cancer stem cells in vitro and in vivo, and investigate their biological characteristics, a new human GBM cancer stem-like cell line, WJ2, was established in this experiment through serial passages from adherent monolayer culture to nonadherent tumor sphere culture in turns; Its partial biological characteristics were studied through cell proliferation and tumor sphere assay; cell cycle distribution, side population, and CD133 phenotype were analyzed with FCM. The expressions of CD133, Nestin, and GFAP of cancer stem-like cells and xenograft tumor cells were detected with RT-PCR and immunohistochemistry. Biological characterization, side population, CD133 phenotype and CD133 Nestin, BCRP-1, Wnt-1 gene expression revealed the stemness of this cancer stem-like cell line. Tumorigenicity heterotransplanted in nude mice; histopathological characteristics of xenograft tumor, and expressions of CD133, Nestin, and GFAP of xenograft tumor cells indicated that xenograft tumors recapitulated the phenotype and biological characterization of human primary GBM. All findings of this experimental study suggested that WJ(2) cancer stem-like cell line could accurately mimic human GBM cancer stem cell in vitro and in vivo; it would be useful in the cellular and molecular studies as well as in testing novel therapies of CSC-based anti-cancer therapies for human GBM.