A2B5 surface ganglioside binding distinguishes between two GFAP-positive clones from a human glioma-derived cell line

Phenotyping clonal populations of glioma stem cell reveals a high degree of plasticity in response to changes of microenvironment

The phenotype of glioma-initiating cells (GIC) is modulated by cell-intrinsic and cell-extrinsic factors. Phenotypic heterogeneity and plasticity of GIC is an important limitation to therapeutic approaches targeting cancer stem cells. Plasticity also presents a challenge to the identification, isolation, and propagation of purified cancer stem cells. Here we use a barcode labelling approach of GIC to generate clonal populations over a number of passages, in combination with phenotyping using the established stem cell markers CD133, CD15, CD44, and A2B5. Using two cell lines derived from isocitrate dehydrogenase (IDH)-wildtype glioblastoma, we identify a remarkable heterogeneity of the phenotypes between the cell lines. During passaging, clonal expansion manifests as the emergence of a limited number of barcoded clones and a decrease in the overall number of clones. Dual-labelled GIC are capable of forming traceable clonal populations which emerge after as few as two passages from mixed cultures and through analyses of similarity of relative proportions of 16 surface markers we were able to pinpoint the fate of such populations. By generating tumour organoids we observed a remarkable persistence of dominant clones but also a significant plasticity of stemness marker expression. Our study presents an experimental approach to simultaneously barcode and phenotype glioma-initiating cells to assess their functional properties, for example to screen newly established GIC for tumour-specific therapeutic vulnerabilities.

Deregulated microRNAs identified in isolated glioblastoma stem cells: an overview

Glioblastoma multiforme (GBM), the most common and aggressive primary brain tumor, is extremely resistant to current treatment paradigms and has a high rate of tumor recurrence. Recent progress in the field of tumor-initiating cells suggests that GBM stem cells (GBMSCs) may be responsible for tumor progression, resistance to treatment, and tumor relapse. Therefore, understanding the biologically significant pathways involved in modulating GBMSC-specific characteristics offers great promise for development of novel therapeutics, which may improve therapeutic efficacy and overcome present drug resistance. In addition, targeting deregulated microRNA (miRNA) has arisen as a new therapeutic strategy in treating malignant gliomas. In GBMSCs, miRNAs regulate a wide variety of tumorigenic processes including cellular proliferation, stemness maintenance, migration/invasion, apoptosis, and tumorigenicity. Nevertheless, the latest progress with GBMSCs and subsequent miRNA profiling is limited by the identification and isolation of GBMSCs. In this review, we thus summarize current markers and known features for isolation as well as the aberrant miRNAs that have been identified in GBM and GBMSCs.

Identification of A2B5+CD133- tumor-initiating cells in adult human gliomas

OBJECTIVE

Several studies have shown that human gliomas contain a small population of cells with stem cell-like features. It has been proposed that these "cancer stem cells" may be uniquely responsible for glioma formation and recurrence. However, human gliomas also contain an abundance of cells that closely resemble more differentiated glial progenitors. Animal model studies have shown that these cells also possess the capacity to form malignant gliomas.

METHODS

To investigate the contributions of stem-like and progenitor-like cells in human gliomas, we used flow cytometry to characterize the expression of a cancer stem cell marker (CD133) and a glial progenitor marker (A2B5) in 25 tumors. We found that human gliomas consistently express A2B5 in a large percentage of cells (61.7 +/- 3.8%, standard error of the mean). In contrast, CD133 expression was less abundant and less consistent (14.8 +/- 3.6%, standard error of the mean), with several glioblastomas containing very few or no detectable CD133+ cells. When present, the CD133+ population was almost entirely contained within the A2B5+ population. Thus, most gliomas could be divided into three distinct populations on the basis of these markers (A2B5+CD133+, A2B5+CD133-, and A2B5-CD133-). To test the tumorigenic potential of these populations, we separated cells from six tumors by fluorescence-activated cell sorting and reinjected them into nude rats.

RESULTS

We found that the capacity for these different populations to form tumors varied depending on the human tumor specimen from which they were isolated. Of the six human gliomas tested, four contained A2B5+/CD133- cells that formed tumors when transplanted into nude rats, three contained A2B5+/CD133+ cells that formed tumors, and only one glioma contained A2B5-/CD133- cells with the capacity to form tumors.

CONCLUSION

Together, these results demonstrate that human gliomas contain multiple populations of cells with the capacity to form tumors and specifically identify a population of tumorigenic A2B5+ cells that are phenotypically distinct from CD133+ cells.

Approaches to mitochondrially mediated cancer therapy

For some malignant cancers even combined surgical, radiotherapeutic and chemotherapeutic approaches are not curative, indeed, in certain tumour types even a modest survival benefit is difficult to achieve. There are various biological reasons which underlie this profound resistance but the propensity of cancer cells to repair breaks caused by DNA-damaging radiation and cytotoxic drugs is of major significance in this context. Such highly resistant tumours include the malignant gliomas which are intrinsic to and directly affect the brain and spinal cord. In evaluating approaches which do not elicit tumour cell death directly by DNA damage, it is intriguing to consider mitochondrially mediated apoptosis as a potentially effective alternative. Since the mitochondrial membrane potentials in cancer cells are frequently reduced in comparison with those of non-neoplastic cells this allows a window of opportunity for small molecule agents to enter the tumour cell mitochondria and reduce oxygen consumption with subsequent release of cytochrome c and activation of a caspase pathway to apoptosis which is cancer cell specific. In the quest for agents which can selectively destroy neoplastic cells in this manner, whilst leaving normal adjacent cells intact, various tricyclic drugs have come under scrutiny. In a range of laboratory assays we, and others, have established that certain cancers and, in particular, malignant glioma, are intrinsically sensitive to this approach. We have also established the cellular, molecular and biochemical mechanisms underlying this process. While such archival tricyclics as the antidepressants, clomipramine and amitriptyline, have been used in these experiments their commercial development in cancer therapy has not been forthcoming and their clinical use in glioma has been confined to anecdotal cases. In addition, the dose-dependant role of agents such as anticonvulsants and steroids commonly used in glioma patients in modulating efficacy of the tricyclics is a matter for continued investigation. Other ways of targeting the mitochondrion for cancer therapy include exploitation of the 18kDa translocator protein (peripheral-type benzodiazepine receptor) within the mitochondrial permeability transition pore and enzymatic or molecular modification of a species of ganglioside (GD3/GD3(A)) expressed on the surface of neoplastic cells which are determinants of mitochondrially mediated apoptosis. It is hoped that such approaches may lead to clinical programmes which will improve the prognosis for patients suffering from highly resistant neoplasms.

Induction of membrane-type-1 matrix metalloproteinase by epidermal growth factor-mediated signaling in gliomas

Increased expression of membrane-type matrix metalloproteinases (MT-MMPs) has previously been reported to correlate with increasing grade of malignancy in gliomas, a relationship shared with alterations in epidermal growth factor receptor (EGFR) signaling. To investigate the possibility of a causative role for EGFR signaling in increasing MT-MMP expression and subsequent peritumoral proteolysis, we characterized glioma cell lines for expression of MT1-MMP, MT2-MMP, MT3-MMP, and MT5-MMP by Western blotting and by quantitative real-time polymerase chain reaction analysis, and for MMP-2 activity following epidermal growth factor (EGF) stimulation. EGF stimulation of glioma cell lines resulted in a 2- to 4-fold increase in MT1-MMP mRNA levels. Although there were slight differences in MT2-, MT3-, and MT5-MMP mRNA expression following EGF stimulation, none of these demonstrated an increase similar to that of MT1-MMP expression. Treatment of high-grade glioma cell lines U251MG and IPSB-18 with EGF for 24 h resulted in a several-fold increase in MT1-MMP protein (2.5- and 5.1-fold, respectively) and in cyclin D1 (2.9-fold), as compared to untreated controls. No significant increase was detected in other MT-MMPs at the protein level. Although there was no detectable increase in proMMP-2 protein, there was an increase in MMP-2 activity. Furthermore, the MT1-MMP induction by EGF was prevented by pretreatment with the EGFR-specific tyrphostin inhibitor AG1478. Similarly, treatment with the phosphatidylinositol 3-kinase inhibitor LY294002 prevented the induction of MT1-MMP protein by EGF stimulation. These compounds additionally inhibited EGF-stimulated invasion in Matrigel Transwell assays. Our results indicate that one mechanism of EGFR-mediated invasiveness in gliomas may involve the induction of MT1-MMP.

An inverse correlation between expression of NCAM-A and the matrix-metalloproteinases gelatinase-A and gelatinase-B in human glioma cells in vitro

Matrix metalloproteinases (MMPs) are an homologous family of proteolytic enzymes capable of degrading components of the extracellular matrix (ECM) and thereby facilitating the invasion of tumour cells into normal tissues. The neural cell adhesion molecules (NCAMs) of neuronal and glial cells provide a Ca2+-independent mechanism for cell-cell and cell-ECM adhesion. NCAMs are downregulated to promote cell disaggregation during cell migration in the developing nervous system whereas MMPs facilitate migration. Recent studies have shown downregulation of MMP secretion in rat glioma cells transfected with an NCAM cDNA, implying an inverse correlation between NCAM and MMP expression. The purpose of this study was to establish whether such a correlation could be demonstrated in a panel of nine human glioma cell-lines, one metastatic carcinoma and one foetal astrocyte derived cell line. The secretion of two MMPs, 72 kDa gelatinase (MMP-2 or gelatinase-A) and 92 kDa gelatinase (MMP-9 or gelatinase-B), was investigated using SDS-PAGE zymography; NCAM-A was assayed by an immunochemiluminescent assay following SDS-PAGE of whole-cell extracts. An inverse correlation was found between the expression of NCAM-A and that of both MMPs studied although the patterns of expression showed no obvious correlation with histological type or grade of the parent tumours. Our results suggest that downregulation of NCAM-A may contribute to tumour invasiveness by promoting both cell disaggregation and protease secretion.

Growth factor modulation of surface ganglioside expression in cloned neoplastic glia

Cloned neoplastic astrocytes from a human glioma-derived cell line (IPSB-18) were grown in fetal calf serum (FCS)-supplemented culture medium in the presence of three growth factors. Basic fibroblast growth factor (bFGF), epidermal growth factor (EGF) but not platelet-derived growth factor (PDGF) induced an increase in the number of cells positive for the ganglioside-recognizing monoclonal antibody, A2B5. No such growth factor-mediated induction could be detected in cells maintained in plasma-derived serum (PDS)-supplemented medium. Small molecules, removed from PDS during dialysis, may, therefore, act synergistically with growth factors in the control of ganglioside synthesis.

Glioma heterogeneity in vitro: the significance of growth factors and gangliosides

Despite renewed attempts by the WHO at updating the system of classification for brain tumours, most of the dynamic biological processes which underlie both the morphological appearances which form the basis for such systems and the malignant potential of gliomas remain an enigma to the neuropathologist. One feature recognized in human gliomas is their phenotypic and genotypic heterogeneity. Such cellular heterogeneity seen in the histological section is retained in vitro, at least during early passage. It is proposed that this heterogeneity is important in the growth and maintenance of the tumour and may be related to the activity of growth factors and gangliosides. Such molecules may not only influence the histoarchitecture of glial neoplasms but may also determine malignant progression and invasive potential. Moreover, there may be an intimate relationship between growth factors and gangliosides constituting an intricate feedback mechanism upon which the biological progression of gliomas depends.