Frontiers in targeting glioma stem cells

Towards an advanced cell-based in vitro glioma model system

The modulation of tumor growth and development in vitro has always been one of the key factors in the research of the malignant transformation, including gliomas, prevalent and most deadly cancers of the brain. Indeed, cellular and molecular biology research employing in vitro model cell-based systems have great potential to advance both the mechanistic understanding and the treatment of human glial tumors, as it facilitates not only the understanding of glioma biology and its regulatory mechanisms Additionally they promise to afford the screening of the putative anti-tumor agents and alternative treatment approaches in a personalized manner, i.e. by virtue of using the patient-derived tumor material for such tests. However, in order to become reliable and representative, glioma model systems need to move towards including most inherent cancer features such as local hypoxia, specific genetic aberrations, native tumor microenvironment, and the three-dimensional extracellular matrix. This review starts with a brief introduction on the general epidemiological and molecular characteristics of gliomas followed by an overview of the cell-based in vitro models currently used in glioma research. As a conclusion, we suggest approaches to move to innovative cell-based in vitro glioma models. We consider that main criteria for selecting these approaches should include the adequate resemblance to the key in vivo characteristics, robustness, cost-effectiveness and ease to use, as well as the amenability to high throughput handling to allow the standardized drug screening.

Chemotherapeutic Drugs: DNA Damage and Repair in Glioblastoma

Despite improvements in therapeutic strategies, glioblastoma (GB) remains one of the most lethal cancers. The presence of the blood-brain barrier, the infiltrative nature of the tumor and several resistance mechanisms account for the failure of current treatments. Distinct DNA repair pathways can neutralize the cytotoxicity of chemo- and radio-therapeutic agents, driving resistance and tumor relapse. It seems that a subpopulation of stem-like cells, indicated as glioma stem cells (GSCs), is responsible for tumor initiation, maintenance and recurrence and they appear to be more resistant owing to their enhanced DNA repair capacity. Recently, attention has been focused on the pivotal role of the DNA damage response (DDR) in tumorigenesis and in the modulation of therapeutic treatment effects. In this review, we try to summarize the knowledge concerning the main molecular mechanisms involved in the removal of genotoxic lesions caused by alkylating agents, emphasizing the role of GSCs. Beside their increased DNA repair capacity in comparison with non-stem tumor cells, GSCs show a constitutive checkpoint expression that enables them to survive to treatments in a quiescent, non-proliferative state. The targeted inhibition of checkpoint/repair factors of DDR can contribute to eradicate the GSC population and can have a great potential therapeutic impact aiming at sensitizing malignant gliomas to treatments, improving the overall survival of patients.

Cancer Stem Cells in Glioblastoma Multiforme

Aim

To identify and characterize cancer stem cells (CSC) in glioblastoma multiforme (GBM).

Methods

Four-micrometer thick formalin-fixed paraffin-embedded GBM samples from six patients underwent 3,3-diaminobenzidine (DAB) and immunofluorescent (IF) immunohistochemical (IHC) staining for the embryonic stem cell (ESC) markers NANOG, OCT4, SALL4, SOX2, and pSTAT3. IF IHC staining was performed to demonstrate co-expression of these markers with GFAP. The protein expression and the transcriptional activities of the genes encoding NANOG, OCT4, SOX2, SALL4, and STAT3 were investigated using Western blotting (WB) and NanoString gene expression analysis, respectively.

Results

DAB and IF IHC staining demonstrated the presence of a CSC population expressing NANOG, OCT4, SOX2, SALL4, and pSTAT3 with the almost ubiquitous presence of SOX2 and a relatively low abundance of OCT4, within GBM. The expression of NANOG, SOX2 and, pSTAT3 but, not OCT and SALL4, was confirmed by WB. NanoString gene analysis demonstrated transcriptional activation of NANOG, OCT4, SALL4, STAT3, and SOX2 in GBM.

Conclusion

This study demonstrated a population of CSCs within GBM characterized by the expression of the CSC markers NANOG, SALL4, SOX2, pSTAT3 and OCT4 at the protein and mRNA levels. The almost ubiquitous presence of SOX2 and a relatively low abundance of OCT4 would support the putative existence of a stem cell hierarchy within GBM.

Suppressing H19 Modulates Tumorigenicity and Stemness in U251 and U87MG Glioma Cells

Glioblastoma multiforme (GBM) is a type of malignant carcinoma found in the brain. Its high frequency of occurrence and poor survival rate have garnered much research attention in recent years. Long non-coding RNAs (lncRNAs) are known to be related to the formation and progression of several cancer types by both promoting and suppressing tumor transformation. H19 is one such lncRNA and has been shown to be upregulated in a few types of cancer. In this study, we discovered that the expression of H19 increased in GBM cell lines. H19 knocked down GBM cells also displayed decreased cellular proliferation and a higher apoptosis rate when induced by temozolomide. Interestingly, the GBM cell lines U87MG and U251 were found to express cancer stem cell markers CD133, NANOG, Oct4 and Sox2. Expression of these markers was downregulated in H19-deficient cells. Collectively, these data suggest a role for H19 in contributing to GBM malignancy and the maintenance of its stem cell properties.

CCAAT/enhancer binding protein β induces motility and invasion of glioblastoma cells through transcriptional regulation of the calcium binding protein S100A4

We have previously shown that decreased expression of CCAAT/enhancer binding protein β (C/EBPβ) inhibits the growth of glioblastoma cells and diminishes their transformation capacity and migration. In agreement with this, we showed that C/EBPβ depletion decreases the mRNA levels of different genes involved in metastasis and invasion. Among these, we found S100 calcium binding protein A4 (S100A4) to be almost undetectable in glioblastoma cells deficient in C/EBPβ. Here, we have evaluated the possible role of S100A4 in the observed effects of C/EBPβ in glioblastoma cells and the mechanism through which S100A4 levels are controlled by C/EBPβ. Our results show that C/EBPβ suppression significantly reduced the levels of S100A4 in murine GL261 and human T98G glioblastoma cells. By employing an S100A4-promoter reporter, we observed a significant induction in the transcriptional activation of the S100A4 gene by C/EBPβ. Furthermore, overexpression of S100A4 in C/EBPβ-depleted glioblastoma cells reverses the enhanced migration and motility induced by this transcription factor. Our data also point to a role of S100A4 in glioblastoma cell invasion and suggest that the C/EBPβ gene controls the invasive potential of GL261 and T98G cells through direct regulation of S100A4. Finally, this study indicates a role of C/EBPβ on the maintenance of the stem cell population present in GL261 glioblastoma cells.

Chemical Library Screening and Structure-Function Relationship Studies Identify Bisacodyl as a Potent and Selective Cytotoxic Agent Towards Quiescent Human Glioblastoma Tumor Stem-Like Cells

Cancer stem-like cells reside in hypoxic and slightly acidic tumor niches. Such microenvironments favor more aggressive undifferentiated phenotypes and a slow growing "quiescent state" which preserves them from chemotherapeutic agents that essentially target proliferating cells. Our objective was to identify compounds active on glioblastoma stem-like cells, including under conditions that mimick those found in vivo within this most severe and incurable form of brain malignancy. We screened the Prestwick Library to identify cytotoxic compounds towards glioblastoma stem-like cells, either in a proliferating state or in more slow-growing "quiescent" phenotype resulting from non-renewal of the culture medium in vitro. Compound effects were assessed by ATP-level determination using a cell-based assay. Twenty active molecules belonging to different pharmacological classes have thus been identified. Among those, the stimulant laxative drug bisacodyl was the sole to inhibit in a potent and specific manner the survival of quiescent glioblastoma stem-like cells. Subsequent structure-function relationship studies led to identification of 4,4'-dihydroxydiphenyl-2-pyridyl-methane (DDPM), the deacetylated form of bisacodyl, as the pharmacophore. To our knowledge, bisacodyl is currently the only known compound targeting glioblastoma cancer stem-like cells in their quiescent, more resistant state. Due to its known non-toxicity in humans, bisacodyl appears as a new potential anti-tumor agent that may, in association with classical chemotherapeutic compounds, participate in tumor eradication.

Long Non-Coding RNAs Dysregulation and Function in Glioblastoma Stem Cells

Glioblastoma multiforme (GBM), the most common form of primary brain tumor, is highly resistant to current treatment paradigms and has a high rate of recurrence. Recent advances in the field of tumor-initiating cells suggest that glioblastoma stem cells (GSCs) may be responsible for GBM's rapid progression, treatment resistance, tumor recurrence and ultimately poor clinical prognosis. Understanding the biologically significant pathways that mediate GSC-specific characteristics offers promises in the development of novel biomarkers and therapeutics. Long non-coding RNAs (lncRNAs) have been increasingly implicated in the regulation of cancer cell biological behavior through various mechanisms. Initial studies strongly suggested that lncRNA expressions are highly dysregulated in GSCs and may play important roles in determining malignant phenotypes in GBM. Here, we review available evidence on aberrantly expressed lncRNAs identified by high throughput microarray profiling studies in GSCs. We also explore the potential functional pathways by analyzing their interactive proteins and miRNAs, with a view to shed lights on how this novel class of molecular candidates may mediate GSC maintenance and differentiation.

Fusicoccin a, a phytotoxic carbotricyclic diterpene glucoside of fungal origin, reduces proliferation and invasion of glioblastoma cells by targeting multiple tyrosine kinases

Glioblastoma multiforme (GBM) is a deadly cancer that possesses an intrinsic resistance to pro-apoptotic insults, such as conventional chemotherapy and radiotherapy, and diffusely invades the brain parenchyma, which renders it elusive to total surgical resection. We found that fusicoccin A, a fungal metabolite from Fusicoccum amygdali, decreased the proliferation and migration of human GBM cell lines in vitro, including several cell lines that exhibit varying degrees of resistance to pro-apoptotic stimuli. The data demonstrate that fusicoccin A inhibits GBM cell proliferation by decreasing growth rates and increasing the duration of cell division and also decreases two-dimensional (measured by quantitative video microscopy) and three-dimensional (measured by Boyden chamber assays) migration. These effects of fusicoccin A treatment translated into structural changes in actin cytoskeletal organization and a loss of GBM cell adhesion. Therefore, fusicoccin A exerts cytostatic effects but low cytotoxic effects (as demonstrated by flow cytometry). These cytostatic effects can partly be explained by the fact that fusicoccin inhibits the activities of a dozen kinases, including focal adhesion kinase (FAK), that have been implicated in cell proliferation and migration. Overexpression of FAK, a nonreceptor protein tyrosine kinase, directly correlates with the invasive phenotype of aggressive human gliomas because FAK promotes cell proliferation and migration. Fusicoccin A led to the down-regulation of FAK tyrosine phosphorylation, which occurred in both normoxic and hypoxic GBM cell culture conditions. In conclusion, the current study identifies a novel compound that could be used as a chemical template for generating cytostatic compounds designed to combat GBM.

TRAIL and paclitaxel synergize to kill U87 cells and U87-derived stem-like cells in vitro

U87-derived stem-like cells (U87-SLCs) were cultured using serum-free stem cell media and identified by both biological behaviors and markers. Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) and paclitaxel (PX), in combination or alone, was used to treat U87-MG human glioma cells (U87 cells) or U87-SLCs. The results showed that TRAIL/PX cannot only synergistically inhibit U87 cells but also U87-SLCs. We observed a significantly higher apoptotic rate in U87 cells simultaneously treated with TRAIL/PX for 24 h compared to cells treated with either drug alone. Furthermore, there was a remarkably higher apoptosis rate in U87-SLCs induced by the TRAIL/PX combination compared with either drug alone. Unlike the simultaneous treatment in U87 cells, U87-SLCs were pretreated for 24 h with 1 μmol/L of PX followed by 1000 ng/mL of TRAIL. Protein assays revealed that TRAIL/PX synergy was related to DR4, cleaved caspase-8 and cleaved caspase-3 upregulation, whereas the mitochondrial pathway was not involved in TRAIL-induced apoptosis. The present study indicates that PX can sensitize U87 cells and U87-SLCs to TRAIL treatment through an extrinsic pathway of cell apoptosis. The combined treatment of TRAIL and PX may be a promising glioma chemotherapy because of its successful inhibition of U87-SLCs, which are hypothesized to influence chemotherapeutic outcomes of gliomas.

Selective uptake of boronophenylalanine by glioma stem/progenitor cells

The success of boron neutron capture therapy (BNCT) depends on the amount of boron in cells and the tumor/blood and tumor/(normal tissue) boron concentration ratios. For the first time, measurements of boron uptake in both stem/progenitor and differentiated glioma cells were performed along with measurements of boron biodistribution in suitable animal models. In glioma stem/progenitor cells, the selective accumulation of boronophenylalanine (BPA) was lower, and retention of boron after BPA removal was longer than in differentiated glioma cells in vitro. However, boron biodistribution was not statistically significantly different in mice with xenografts.

Delayed formation of FancD2 foci in glioma stem cells treated with ionizing radiation

BACKGROUND

Glioblastoma multiforme (WHO grade IV) is a highly lethal brain tumor. Its malignancy is in part due to cell populations refractory to radiotherapy and chemotherapy, which in some patients display stem properties (glioma stem cells-GSC). We and others have recently shown that a major mechanism of resistance of GSC to therapies resides in their slow proliferation that in turn is linked to constitutive activation of the DNA damage response. FancD2, a central player of the Fanconi anemia pathway, is induced when replication forks stall at DNA damage sites.

METHODS

We have analyzed the kinetics of FancD2 induction in two glioma cell lines with pronounced (Borru) and poor (DR177) stem phenotypes, by fluorescence analysis of nuclear foci.

RESULTS

FancD2 activation was significantly delayed in Borru, consistent with the slow replication fork progression in these cells. On the contrary, no significant difference between Borru and DR177 was observed for pH2AX nuclear foci formation that hallmarks a number of DNA structure variations including double-strand breaks.

CONCLUSION

GSC display reduced FancD2 activation following radiation damage, most probably due to their elongated cell cycle.

Characterization of glioma stem cells through multiple stem cell markers and their specific sensitization to double-strand break-inducing agents by pharmacological inhibition of ataxia telangiectasia mutated protein

Previous studies have shown that tumor-driving glioma stem cells (GSC) may promote radio-resistance by constitutive activation of the DNA damage response started by the ataxia telangiectasia mutated (ATM) protein. We have investigated whether GSC may be specifically sensitized to ionizing radiation by inhibiting the DNA damage response. Two grade IV glioma cell lines (BORRU and DR177) were characterized for a number of immunocytochemical, karyotypic, proliferative and differentiative parameters. In particular, the expression of a panel of nine stem cell markers was quantified by reverse transcription-polymerase chain reaction (RT-PCR) and flow cytometry. Overall, BORRU and DR177 displayed pronounced and poor stem phenotypes, respectively. In order to improve the therapeutic efficacy of radiation on GSC, the cells were preincubated with a nontoxic concentration of the ATM inhibitors KU-55933 and KU-60019 and then irradiated. BORRU cells were sensitized to radiation and radio-mimetic chemicals by ATM inhibitors whereas DR177 were protected under the same conditions. No sensitization was observed after cell differentiation or to drugs unable to induce double-strand breaks (DSB), indicating that ATM inhibitors specifically sensitize glioma cells possessing stem phenotype to DSB-inducing agents. In conclusion, pharmacological inhibition of ATM may specifically sensitize GSC to DSB-inducing agents while sparing nonstem cells.

Enhanced antitumor efficacy of low-dose Etoposide with oncolytic herpes simplex virus in human glioblastoma stem cell xenografts

PURPOSE

Glioblastoma (GBM) inevitably recurs despite surgery, radiation, and chemotherapy. A subpopulation of tumor cells, GBM stem cells (GSC), has been implicated in this recurrence. The chemotherapeutic agent etoposide is generally reserved for treating recurrent tumors; however, its effectiveness is limited due to acute and cumulative toxicities to normal tissues. We investigate a novel combinatorial approach of low-dose etoposide with an oncolytic HSV to enhance antitumor activity and limit drug toxicity.

EXPERIMENTAL DESIGN

In vitro, human GBM cell lines and GSCs were treated with etoposide alone, oncolytic herpes simplex virus (oHSV) G47Δ alone, or the combination. Cytotoxic interactions were analyzed using the Chou-Talalay method, and changes in caspase-dependent apoptosis and cell cycle were determined. In vivo, the most etoposide-resistant human GSC, BT74, was implanted intracranially and treated with either treatment alone or the combination. Analysis included effects on survival, therapy-associated adverse events, and histologic detection of apoptosis.

RESULTS

GSCs varied in their sensitivity to etoposide by over 50-fold in vitro, whereas their sensitivity to G47Δ was similar. Combining G47Δ with low-dose etoposide was moderately synergistic in GSCs and GBM cell lines. This combination did not enhance virus replication, but significantly increased apoptosis. In vivo, the combination of a single cycle of low-dose etoposide with G47Δ significantly extended survival of mice-bearing etoposide-insensitive intracranial human GSC-derived tumors.

CONCLUSIONS

The combination of low-dose etoposide with G47Δ increases survival of mice-bearing intracranial human GSC-derived tumors without adverse side effects. These results establish this as a promising combination strategy to treat resistant and recurrent GBM.