Brain tumour stem cells: the undercurrents of human brain cancer and their relationship to neural stem cells

Conceptual and technical advances in neural stem cell biology are being applied to the study of human brain tumours. These studies suggest that human brain tumours are organized as a hierarchy and are maintained by a small number of tumour cells that have stem cell properties. Most of the bulk population of human brain tumours comprise cells that have lost the ability to initiate and maintain tumour growth. Although the cell of origin for human brain tumours is uncertain, recent evidence points towards the brain's known proliferative zones. The identification of brain tumour stem cells has important implications for understanding brain tumour biology and these cells may be critical cellular targets for curative therapy.     

Neural stem cells, tumour stem cells and brain tumours: dangerous relationships?

Neural stem cells (NSC) have been implicated not only in brain development and neurogenesis but also in tumourigenesis. Brain tumour stem cells (BTSC) have been isolated from several paediatric or adult human brain tumours, however their origin is still disputed. This review discusses the normal role of NSC in the adult mammalian brain and their anatomical location. It compares the molecular characteristics and the biological behaviour of NSC/BTSC, and describes the molecular pathways involved in controlling self-renewal and maintenance of adult NSC/BTSC and brain tumour development. It also assesses the current hypotheses about the origin of BTSC and the clinical consequences.     

Comparison of intracerebral transplantation effects of different stem cells on rodent stroke models

In the present study, induced pluripotent stem cells (iPSCs), induced neural stem cells (iNSCs), mesenchymal stem cells (MSCs) and an immortalized cell line (RMNE6), representing different characteristics of stem cells, were transplanted into normal and/or injured brain areas of rodent stroke models, and their effects were compared to select suitable stem cells for cell replacement stroke therapy. The rat and mice ischaemic models were constructed using the middle cerebral artery occlusion technique. Both electrocoagulation of the artery and the intraluminal filament technique were used. The behaviour changes and fates of grafted stem cells were determined mainly by behaviour testing and immunocytochemistry. Following iPSC transplantation into the corpora striata of normal mice, a tumour developed in the brain. The iNSCs survived well and migrated towards the injured area without differentiation. Although there was no tumourigenesis in the brain of normal or ischaemic mice after the iNSCs were transplanted in the cortices, the behaviour in ischaemic mice was not improved. Upon transplanting MSC and RMNE6 cells into ischaemic rat brains, results similar to iNSCs in mice were seen. However, transplantation of RMNE6 caused a brain tumour. Thus, tumourigenesis and indeterminate improvement of behaviour are challenging problems encountered in stem cell therapy for stroke, and the intrinsic characteristics of stem cells should be remodelled before transplantation. Copyright © 2015 John Wiley & Sons, Ltd.     

CD133: holy of grail of neuro‐oncology or promiscuous red‐herring?

The CD133 glycoprotein is a controversial cancer stem cell marker in the field of neuro‐oncology, based largely on the now considerable experimental evidence for the existence of both CD133+ve and CD133?ve populations as tumour‐initiating cells. It is thought that decreasing oxygen tension enhances the complex regulation and phenotype of CD133 in glioma. In light of these ideologies, establishing the precise functional role of CD133 is becoming increasingly critical. In this article, we review the complex regulation of CD133 and its extracellular epitope AC133, and associated alterations, to tumour cell behaviour by hypoxia. Furthermore, its role in functional modulation of tumours, rather than determination of a specific stem cell type is therefore alluded to, while evidence for and against its ability as a cancer stem cell marker in primary brain tumours, is critically evaluated. Thus, the suggestion that CD133 may be a central ‘holy grail’ in identifying core cells for propagation of malignant glial neoplasms seems increasingly less convincing. It remains to be seen, however, whether CD133 is randomly expressed on such brain tumour cell populations or whether it is of major significance to brain biological behaviour.     

Characterization of brain cancer stem cells: a mathematical approach

Objective:  In recent years, support has increased for the notion that a subpopulation of brain tumour cells in possession of properties typically characteristic of stem cells is responsible for initiating and maintaining the tumour. Unravelling details of the brain tumour stem cell (BTSC) hierarchy, as well as interactions of these cells with various therapies, will be essential in the design of optimal treatment strategies.
Materials and methods:  Motivated by this, we have developed a mathematical model of the BTSC hypothesis that may aid in characterization of brain tumours, as well as in prediction of effective therapeutic strategies, which can be further validated in experimental and clinical studies. At the level of a small number of cells, the model developed herein is stochastic. For larger populations of cancer cells, the model is handled from a deterministic approach.
Results and conclusions:  In the stochastic regime, importance of a relationship between the likelihoods of two distinct types of symmetric BTSC divisions in determining BTSC survival rates becomes apparent, consequently emphasizing the need for a set of biomarkers that are able to better characterize the BTSC hierarchy. At the large scale, we predict the importance of the aforementioned symmetric division rates in dictating brain tumour composition. Furthermore, we demonstrate possible therapeutic benefits of considering combination treatments of radiotherapy and putative BTSC inhibitors, such as bone morphogenetic proteins, while reinforcing the importance of developing novel treatment strategies that specifically target the BTSC subpopulation.     

Mesenchymal stem cells overexpressing PEDF decrease the angiogenesis of gliomas

The present study is an exploration of a novel strategy to target a therapeutic gene to brain tumour tissues. In the present study, we evaluated the feasibility of using hMSCs (human mesenchymal stem cells) to deliver PEDF (pigment epithelium-derived factor), a potent inhibitor of tumour angiogenesis, in a model of intracranial gliomas. To assess its potential of tracking gliomas, MSCs (mesenchymal stem cells) were injected into the cerebral hemisphere and it showed that MSCs infiltrated into the vessel beds and scattered throughout the tumour. In vitro migration assay showed that the VEGF (vascular endothelial growth factor) enhanced MSC migration. In contrast, the migratory activity of MSCs was significantly inhibited with the presence of PEDF. Systematic delivery of AAV (adeno-associated virus)–PEDF to established glioma xenografts resulted in increased apoptosis of gliomas. In addition, MSC–PEDF treatment prolonged the survival of mice bearing U87 gliomas. Taken together, these data validate that MSCs–PEDF can migrate and deliver PEDF to target glioma cells, which may be a novel and promising therapeutic approach for refractory brain tumour.     

Cellular immortality in brain tumours: An integration of the cancer stem cell paradigm

Brain tumours are a diverse group of neoplasms that continue to present a formidable challenge in our attempt to achieve curable intervention. Our conceptual framework of human brain cancer has been redrawn in the current decade. There is a gathering acceptance that brain tumour formation is a phenotypic outcome of dysregulated neurogenesis, with tumours viewed as abnormally differentiated neural tissue. In relation, there is accumulating evidence that brain tumours, similar to leukaemia and many solid tumours, are organized as a developmental hierarchy which is maintained by a small fraction of cells endowed with many shared properties of tissue stem cells. Proof that neurogenesis persists throughout adult life, compliments this concept. Although the cancer cell of origin is unclear, the proliferative zones that harbour stem cells in the embryonic, post-natal and adult brain are attractive candidates within which tumour-initiation may ensue. Dysregulated, unlimited proliferation and an ability to bypass senescence are acquired capabilities of cancerous cells. These abilities in part require the establishment of a telomere maintenance mechanism for counteracting the shortening of chromosomal termini. A strategy based upon the synthesis of telomeric repeat sequences by the ribonucleoprotein telomerase, is prevalent in ～ 90% of human tumours studied, including the majority of brain tumours. This review will provide a developmental perspective with respect to normal (neurogenesis) and aberrant (tumourigenesis) cellular turnover, differentiation and function. Within this context our current knowledge of brain tumour telomere/telomerase biology will be discussed with respect to both its developmental and therapeutic relevance to the hierarchical model of brain tumourigenesis presented by the cancer stem cell paradigm.     

Mathematical model for the cancer stem cell hypothesis

Recent research on the origin of brain cancer has implicated a subpopulation of self-renewing brain cancer stem cells for malignant tumour growth. Various genes that regulate self-renewal in normal stem cells are also found in cancer stem cells. This implies that cancers can occur because of mutations in normal stem cells and early progenitor cells. A predictive mathematical model based on the cell compartment method is presented here to pose and validate non-intuitive scenarios proposed through the neural cancer stem cell hypothesis. The growths of abnormal (stem and early progenitor) cells from their normal counterparts are ascribed with separate mutation probabilities. Stem cell mutations are found to be more significant for the development of cancer than a similar mutation in the early progenitor cells. The model also predicts that, as previously hypothesized, repeated insult to mature cells increases the formation of abnormal progeny, and hence the risk of cancer.     

Long non‐coding RNAs in brain tumours: Focus on recent epigenetic findings in glioma

Glioma biology is a major focus in tumour research, primarily due to the aggressiveness and high mortality rate of its most aggressive form, glioblastoma. Progress in understanding the molecular mechanisms behind poor prognosis of glioblastoma, regardless of treatment approaches, has changed the classification of brain tumours after nearly 100 years of relying on anatomopathological criteria. Expanding knowledge in genetic, epigenetic and translational medicine is also beginning to contribute to further elucidating molecular dysregulation in glioma. Long non‐coding RNAs (lncRNAs) and their main representatives, large intergenic non‐coding RNAs (lincRNAs), have recently been under scrutiny in glioma research, revealing novel mechanisms of pathogenesis and reinforcing others. Among those confirmed was the reactivation of events significant for foetal brain development and neuronal commitment. Novel mechanisms of tumour suppression and activation of stem‐like behaviour in tumour cells have also been examined. Interestingly, these processes involve lncRNAs that are present both during normal brain development and in brain malignancies and their reactivation might be explained by epigenetic mechanisms, which we discuss in detail in the present review. In addition, the review discusses the lncRNAs‐induced changes, as well as epigenetic changes that are consequential for tumour formation, affecting, in turn, the expression of various types of lncRNAs.     

The neurobiology of gliomas: from cell biology to the development of therapeutic approaches

Gliomas are the most common type of primary brain tumour and are often fast growing with a poor prognosis for the patient. Their complex cellular composition, diffuse invasiveness and capacity to escape therapies has challenged researchers for decades and hampered progress towards an effective treatment. Recent molecular characterization of tumour cells combined with new insights into cellular diversification that occurs during development, and the modelling of these processes in transgenic animals have enabled a more detailed understanding of the events that underlie gliomagenesis. Combining this enhanced understanding of the relationship between neural stem cell biology and the cell lineage relationships of tumour cells with model systems offers new opportunities to develop specific and effective therapies.     

Hypothesis: are neoplastic macrophages/microglia present in glioblastoma multiforme?

Most malignant brain tumours contain various numbers of cells with characteristics of activated or dysmorphic macrophages/microglia. These cells are generally considered part of the tumour stroma and are often described as TAM (tumour-associated macrophages). These types of cells are thought to either enhance or inhibit brain tumour progression. Recent evidence indicates that neoplastic cells with macrophage characteristics are found in numerous metastatic cancers of non-CNS (central nervous system) origin. Evidence is presented here suggesting that subpopulations of cells within human gliomas, specifically GBM (glioblastoma multiforme), are neoplastic macrophages/microglia. These cells are thought to arise following mitochondrial damage in fusion hybrids between neoplastic stem cells and macrophages/microglia.     

Heterogeneous phenotype of human glioblastoma: In vitro study

Glioblastomas (GBMs) are the most lethal primary brain tumours. Increasing evidence shows that brain tumours contain the population of stem cells, so‐called cancer stem cells (CSCs). Stem cell marker CD133 was reported to identify CSC population in GBM. Further studies have indicated that CD133 negative cells exhibiting similar properties and are able to initiate the tumour, self‐renew and undergo multilineage differentiation. GBM is a highly heterogeneous tumour and may contain different stem cell populations with different functional properties. We characterized five GBM cell lines, established from surgical samples, according to the marker expression, proliferation and differentiation potential. CD133 positive cell lines showed increased proliferation rate in neurosphere condition and marked differentiation potential towards neuronal lineages. Whereas two cell lines low‐expressing CD133 marker showed mesenchymal properties in vitro, that is high proliferation rate in serum condition and differentiation in mesenchymal cell types. Further, we compared therapy resistance capacity of GBM cell lines treated with hydroxyurea. Our results suggest that CSC concept is more complex than it was believed before, and CD133 could not define entire stem cell population within GBM. At least two different subtypes of GBM CSCs exist, which may have different biological characteristics and imply different therapeutic strategies. Copyright © 2013 John Wiley & Sons, Ltd.     

Synergism between altered cortical polarity and the PI3K/TOR pathway in the suppression of tumour growth

Loss of function of pins (partner of inscuteable) partially disrupts neuroblast (NB) polarity and asymmetric division, results in fewer and smaller NBs and inhibits Drosophila larval brain growth. Food deprivation also inhibits growth. However, we find that the combination of loss of function of pins and dietary restriction results in loss of NB asymmetry, overproliferation of Miranda-expressing cells, brain overgrowth and increased frequency of tumour growth on allograft transplantation. The same effects are observed in well-fed pins larvae that are mutant for pi3k (phosphatidylinositol 3-kinase) or exposed to the TOR inhibitor rapamycin. Thus, pathways that are sensitive to food deprivation and dependent on PI3K and TOR are essential to suppress tumour growth in Drosophila larval brains with compromised pins function. These results highlight an unexpected crosstalk whereby the normally growth-promoting, nutrient-sensing PI3K/TOR pathway suppresses tumour formation in neural stem cells with compromised cell polarity.     

WATER-SOLUBLE AND PENTANOL-EXTRACTABLE PROTEINS IN HUMAN BRAIN NORMAL TISSUE AND HUMAN BRAIN TUMOURS, WITH SPECIAL REFERENCE TO S-100 PROTEIN

Disc electrophoretic separation of water-soluble and pentanol-extractable protein from normal human brain and human brain tumours (glioblastoma, neurinoma and medulloblastoma) on 10 per cent polyacrylamide gels showed minor differences between tissues. After disc electrophoresis ependymomal tumour cells contained high concentrations of a rapidly migrating anodic protein fraction which was immunologically distinct from S-100 protein. After electrophoresis of normal brain grey matter in a continuous buffer system, a rapidly migrating anodic protein fraction which was immunologically distinct from S-100 protein was found, and this protein fraction had a similar relative mobility to that of ependymomal tumour cells. This protein fraction was present to a low extent in human normal white matter, but absent from neurinoma and glioblastoma. In a continuous buffer system at least two separable protein fractions, immunologically equivalent to S-100 protein, were observed in normal human brain. The more anodic of these two fractions was shown to be present in relatively high amounts in neurinomas, and may be of Schwann cell origin. Additional S-100 protein could be extracted from residual material remaining after removal of water-soluble proteins; 2.8-10 per cent of the water-soluble S-100 in normal material, and 0.1-0.6 per cent of that present in tumour material, was extractable from the water-insoluble residue by pentanol.     

NANOG regulates glioma stem cells and is essential in vivo acting in a cross‐functional network with GLI1 and p53

A cohort of genes associated with embryonic stem (ES) cell behaviour, including NANOG, are expressed in a number of human cancers. They form an ES‐like signature we first described in glioblastoma multiforme (GBM), a highly invasive and incurable brain tumour. We have also shown that HEDGEHOG‐GLI (HH‐GLI) signalling is required for GBM growth, stem cell expansion and the expression of this (ES)‐like stemness signature. Here, we address the function of NANOG in human GBMs and its relationship with HH‐GLI activity. We find that NANOG modulates gliomasphere clonogenicity, CD133⁺ stem cell cell behavior and proliferation, and is regulated by HH‐GLI signalling. However, GLI1 also requires NANOG activity forming a positive loop, which is negatively controlled by p53 and vice versa. NANOG is essential for GBM tumourigenicity in orthotopic xenografts and it is epistatic to HH‐GLI activity. Our data establish NANOG as a novel HH‐GLI mediator essential for GBMs. We propose that this function is conserved and that tumour growth and stem cell behaviour rely on the status of a functional GLI1‐NANOG‐p53 network.     

Targeting ROR1 inhibits the self‐renewal and invasive ability of glioblastoma stem cells

Glioblastoma is the most malignant of brain tumours and is difficult to cure because of interruption of drug delivery by the blood–brain barrier system, its high metastatic capacity and the existence of cancer stem cells (CSCs). Although CSCs are present as a small population in malignant tumours, CSCs have been studied as they are responsible for causing recurrence, metastasis and resistance to chemotherapy and radiotherapy for cancer. CSCs have self‐renewal characteristics like normal stem cells. The aim of this study was to investigate whether receptor tyrosine kinase‐like orphan receptor 1 (ROR1) is involved in stem cell maintenance and malignant properties in human glioblastoma. Knockdown of ROR1 caused reduction of stemness and sphere formation capacity. Moreover, down‐regulation of ROR1 suppressed the expression of epithelial‐mesenchymal transition‐related genes and the tumour migratory and invasive abilities. The results of this study indicate that targeting ROR1 can induce differentiation of CSCs and inhibit metastasis in glioblastoma. In addition, ROR1 may be used as a potential marker for glioblastoma stem cells as well as a potential target for glioblastoma stem cell therapy. Copyright © 2016 John Wiley & Sons, Ltd.     

Prognostic value of stem cell markers in glioblastoma

Abstract

Background/Context: Glioblastoma (GB) is the most common primary brain tumour in adults and it is associated with a high mortality rate. According to the stem cell theory, the growth, relapse and treatment response of GB is determined by the stem cell subpopulation present in the tumour.

Objective: Our aim is to study the prognostic value of stem cell markers (CD44, Nestin, Olig2 and SOX2) in a series of homogeneously treated GBs.

Material and methods: We study 280 GBs treated with STUPP acheme with a histologican review of the cases and TMA with a máximum of 4 spots for each case. Each slide was immunohistochemically stained and Reading. We compared the immunohistochemical results with survival tme.

Results: Only SOX2 immunoexpression (IE) excedding 10% of the tumour cells was found to be related to good survival (p= 0.037) in univariate analysis. However, amultivariate analysis indicate the age, surgery and MGMT promotes methylation but no SOX2 IE are prognostic factors.

Conclusions: We conclude the immunohistochemical studies of stem cell markers in GB are not useful for predicting prognosis in daily practice.     

Brain tumour stem cells: implications for cancer therapy and regenerative medicine

The cancer relapse and mortality rate suggest that current therapies do not eradicate all malignant cells. Currently, it is accepted that tumorigenesis and organogenesis are similar in many respects, as for example, homeostasis is governed by a distinct sub-population of stem cells in both situations. There is increasing evidence that many types of cancer contain their own stem cells: cancer stem cells (CSC), which are characterized by their self-renewing capacity and differentiation ability. The investigation of solid tumour stem cells has gained momentum particularly in the area of brain tumours. Gliomas are the most common type of primary brain tumours. Nearly two-thirds of gliomas are highly malignant lesions with fast progression and unfortunate prognosis. Despite recent advances, two-year survival for glioblastoma (GBM) with optimal therapy is less than 30%. Even among patients with low-grade gliomas that confer a relatively good prognosis, treatment is almost never curative. Recent studies have demonstrated the existence of a small fraction of glioma cells endowed with features of primitive neural progenitor cells and a tumour-initiating function. In general, this fraction is characterized for forming neurospheres, being endowed with drug resistance properties and often, we can isolate some of them using sorting methods with specific antibodies. The molecular characterization of these stem populations will be critical to developing an effective therapy for these tumours with very dismal prognosis. To achieve this aim, the development of a mouse model which recapitulates the nature of these tumours is essential. This review will focus on glioma stem cell knowledge and discuss future implications in brain cancer therapy and regenerative medicine.