The brain tumor microenvironment

In vivo models of brain tumors: roles of genetically engineered mouse models in understanding tumor biology and use in preclinical studies

Although our knowledge of the biology of brain tumors has increased tremendously over the past decade, progress in treatment of these deadly diseases remains modest. Developing in vivo models that faithfully mirror human diseases is essential for the validation of new therapeutic approaches. Genetically engineered mouse models (GEMMs) provide elaborate temporally and genetically controlled systems to investigate the cellular origins of brain tumors and gene function in tumorigenesis. Furthermore, they can prove to be valuable tools for testing targeted therapies. In this review, we discuss GEMMs of brain tumors, focusing on gliomas and medulloblastomas. We describe how they provide critical insights into the molecular and cellular events involved in the initiation and maintenance of brain tumors, and illustrate their use in preclinical drug testing.

Loss of PDCD4 contributes to enhanced chemoresistance in Glioblastoma multiforme through de-repression of Bcl-xL translation

Glioblastoma multiforme (GBM) is the most common and aggressive form of tumor of the central nervous system. Despite significant efforts to improve treatments, patient survival rarely exceeds 18 months largely due to the highly chemoresistant nature of these tumors. Importantly, misregulation of the apoptotic machinery plays a key role in the development of drug resistance. We previously demonstrated that Bcl-xL, an important anti-apoptotic protein, is regulated at the level of translation by the tumor suppressor programmed cell death 4 (PDCD4). We report here a strong correlation between low expression of PDCD4 and high expression of Bcl-xL in adult de novo GBM, GBM tumor initiating cells, and established GBM cell lines. Importantly, high Bcl-xL expression correlated significantly with poor progression and patient survival. We demonstrate that re-expression of PDCD4 in GBM cells down-regulated Bcl-xL expression and decreased cell viability. Finally, we show that direct inhibition of Bcl-xL by small molecule antagonist ABT-737 sensitizes GBM cells to doxorubicin. Our results identify Bcl-xL as a novel marker of GBM chemoresistance and advocate for the combined use of Bcl-xL antagonists and existing chemotherapeutics as a treatment option for this aggressive tumor.

Increased expression of stress inducible protein 1 in glioma-associated microglia/macrophages

Factors released by glioma-associated microglia/macrophages (GAMs) play an important role in the growth and infiltration of tumors. We have previously demonstrated that the co-chaperone stress-inducible protein 1 (STI1) secreted by microglia promotes proliferation and migration of human glioblastoma (GBM) cell lines in vitro. In the present study, in order to investigate the role of STI1 in a physiological context, we used a glioma model to evaluate STI1 expression in vivo. Here, we demonstrate that STI1 expression in both the tumor and in the infiltrating GAMs and lymphocytes significantly increased with tumor progression. Interestingly, high expression of STI1 was observed in macrophages and lymphocytes that infiltrated brain tumors, whereas STI1 expression in the circulating blood monocytes and lymphocytes remained unchanged. Our results correlate, for the first time, the expression of STI1 and glioma progression, and suggest that STI1 expression in GAMs and infiltrating lymphocytes is modulated by the brain tumor microenvironment.

Stem cell niches in glioblastoma: a neuropathological view

Glioblastoma (GBM) stem cells (GSCs), responsible for tumor growth, recurrence, and resistance to therapies, are considered the real therapeutic target, if they had no molecular mechanisms of resistance, in comparison with the mass of more differentiated cells which are insensitive to therapies just because of being differentiated and nonproliferating. GSCs occur in tumor niches where both stemness status and angiogenesis are conditioned by the microenvironment. In both perivascular and perinecrotic niches, hypoxia plays a fundamental role. Fifteen glioblastomas have been studied by immunohistochemistry and immunofluorescence for stemness and differentiation antigens. It has been found that circumscribed necroses develop inside hyperproliferating areas that are characterized by high expression of stemness antigens. Necrosis developed inside them because of the imbalance between the proliferation of tumor cells and endothelial cells; it reduces the number of GSCs to a thin ring around the former hyperproliferating area. The perinecrotic GSCs are nothing else that the survivors remnants of those populating hyperproliferating areas. In the tumor, GSCs coincide with malignant areas so that the need to detect where they are located is not so urgent.

The taxonomy of brain cancer stem cells: what's in a name?

With the increasing recognition that stem cells play vital roles in the formation, maintenance, and potential targeted treatment of brain tumors, there has been an exponential increase in basic laboratory and translational research on these cell types. However, there are several different classes of stem cells germane to brain cancer, each with distinct capabilities and functions. In this perspective, we discuss the types of stem cells relevant to brain tumor pathogenesis, and suggest a nomenclature for future preclinical and clinical investigation.

Reciprocal Supportive Interplay between Glioblastoma and Tumor-Associated Macrophages

Glioblastoma multiforme (GBM) is the most lethal and aggressive type of primary brain malignancy. Failures of the traditional therapies in treating GBMs raise the urgent requirement to develop new approaches with more responsive targets. The phenomenon of the high infiltration of tumor-associated macrophages (TAMs) into GBMs has been observed for a long time. Regardless of the limited knowledge about TAMs, the high percentage of supportive TAM in GBM tumor mass makes it possible to be a good target for GBM treatment. In this review, we discussed the unique features of TAMs in GBMs, including their origin, the tumor-supportive properties, the secreted cytokines, and the relevant mechanisms. In addition, we tried to interpret the current understandings about the interplay between GBM cancer cells and TAMs. Finally, the translational studies of targeting TAMs were also described.

Alpha-enolase as a potential cancer prognostic marker promotes cell growth, migration, and invasion in glioma

BACKGROUND

The success of using glycolytic inhibitors for cancer treatment relies on better understanding the roles of each frequently deregulated glycolytic genes in cancer. This report analyzed the involvement of a key glycolytic enzyme, alpha-enolase (ENO1), in tumor progression and prognosis of human glioma.

METHODS

ENO1 expression levels were examined in glioma tissues and normal brain (NB) tissues. The molecular mechanisms of ENO1 expression and its effects on cell growth, migration and invasion were also explored by 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide (MTT) assay, Transwell chamber assay, Boyden chamber assay, Western blot and in vivo tumorigenesis in nude mice.

RESULTS

ENO1 mRNA and protein levels were upregulated in glioma tissues compared to NB. In addition, increased ENO1 was associated disease progression in glioma samples. Knocking down ENO1 expression not only significantly decreased cell proliferation, but also markedly inhibited cell migration and invasion as well as in vivo tumorigenesis. Mechanistic analyses revealed that Cyclin D1, Cyclin E1, pRb, and NF-κB were downregulated after stable ENO1 knockdown in glioma U251 and U87 cells. Conversely, knockdown of ENO1 resulted in restoration of E-cadherin expression and suppression of mesenchymal cell markers, such as Vimentin, Snail, N-Cadherin, β-Catenin and Slug. Furthermore, ENO1 suppression inactivated PI3K/Akt pathway regulating the cell growth and epithelial-mesenchymal transition (EMT) progression.

CONCLUSION

Overexpression of ENO1 is associated with glioma progression. Knockdown of ENO1 expression led to suppressed cell growth, migration and invasion progression by inactivating the PI3K/Akt pathway in glioma cells.

Dendritic cell vaccination for glioblastoma multiforme: review with focus on predictive factors for treatment response

Glioblastoma multiforme (GBM) is the most common and most aggressive type of primary brain cancer. Since median overall survival with multimodal standard therapy is only 15 months, there is a clear need for additional effective and long-lasting treatments. Dendritic cell (DC) vaccination is an experimental immunotherapy being tested in several Phase I and Phase II clinical trials. In these trials, safety and feasibility have been proven, and promising clinical results have been reported. On the other hand, it is becoming clear that not every GBM patient will benefit from this highly personalized treatment. Defining the subgroup of patients likely to respond to DC vaccination will position this option correctly amongst other new GBM treatment modalities, and pave the way to incorporation in standard therapy. This review provides an overview of GBM treatment options and focuses on the currently known prognostic and predictive factors for response to DC vaccination. In this way, it will provide the clinician with the theoretical background to refer patients who might benefit from this treatment.

Six-color intravital two-photon imaging of brain tumors and their dynamic microenvironment

The majority of intravital studies on brain tumor in living animal so far rely on dual color imaging. We describe here a multiphoton imaging protocol to dynamically characterize the interactions between six cellular components in a living mouse. We applied this methodology to a clinically relevant glioblastoma multiforme (GBM) model designed in reporter mice with targeted cell populations labeled by fluorescent proteins of different colors. This model permitted us to make non-invasive longitudinal and multi-scale observations of cell-to-cell interactions. We provide examples of such 5D (x,y,z,t,color) images acquired on a daily basis from volumes of interest, covering most of the mouse parietal cortex at subcellular resolution. Spectral deconvolution allowed us to accurately separate each cell population as well as some components of the extracellular matrix. The technique represents a powerful tool for investigating how tumor progression is influenced by the interactions of tumor cells with host cells and the extracellular matrix micro-environment. It will be especially valuable for evaluating neuro-oncological drug efficacy and target specificity. The imaging protocol provided here can be easily translated to other mouse models of neuropathologies, and should also be of fundamental interest for investigations in other areas of systems biology.

Interleukin-1-induced changes in the glioblastoma secretome suggest its role in tumor progression

The tumor microenvironment including glial cells and their inflammatory products regulates brain tumor development and progression. We have previously established that human glioma cells are exquisitely sensitive to IL-1 stimulation leading us to undertake a comparative analysis of the secretome of unstimulated and cytokine (IL-1)-stimulated glioblastoma cells. We performed label-free quantitative proteomic analysis and detected 190 proteins which included cytokines, chemokines, growth factors, proteases, cell adhesion molecules, extracellular matrix (ECM) and related proteins. Measuring area under the curve (AUC) of peptides for quantitation, the IL-1-induced secretome contained 13 upregulated and 5 downregulated extracellular proteins (p<0.05) compared to controls. Of these, IL-8, CCL2, TNC, Gal-1 and PTX3 were validated as upregulated and SERPINE1, STC2, CTGF and COL4A2 were validated as downregulated factors by immunochemical methods. A major representation of the ECM and related proteins in the glioblastoma secretome and their modulation by IL-1 suggested that IL-1 induces its effect in part by altering TGFβ expression, activity and signaling. These findings enhance our understanding of IL-1-induced modulation of glioma microenvironment, with implications for increased tumor invasion, migration and angiogenesis. They further provide novel targets for the glioblastoma intervention.

BIOLOGICAL SIGNIFICANCE

Present study is on an unbiased screening of the glioblastoma secretome stimulated by IL-1 which triggers neuroinflammatory cascades in the central nervous system. Network of secreted proteins were shown to be regulated revealing their possible contribution to glioma progression. Label free quantitative proteomics has provided unique novel targets for potential glioblastoma intervention.

Smac mimetic promotes glioblastoma cancer stem-like cell differentiation by activating NF-κB

Recently, a broader role of inhibitor of apoptosis (IAP) proteins besides their antiapoptotic functions has been described. Therefore, we investigated the effect of non-toxic concentrations of the small-molecule Smac mimetic BV6, which antagonizes IAP proteins, on differentiation of cancer stem-like cells (CSLCs) derived from primary glioblastoma (GBM) specimens. Here, we identify a novel function of BV6 in regulating differentiation of GBM CSLCs by activating NF-κB. BV6 at non-lethal doses stimulates morphological changes associated with the differentiation of GBM CSLCs. BV6 increases transcriptional activity, mRNA and protein levels of the astrocytic marker GFAP without altering expression of the neuronal marker β-III-tubulin, indicating that BV6 induces astrocytic differentiation of GBM CSLCs. Molecular studies reveal that BV6 triggers processing of the NF-κB subunit p100 to p52, nuclear translocation of p52 and p50 and increased NF-κB DNA-binding. Intriguingly, inhibition of NF-κB by overexpression of dominant-negative IκBα super-repressor (IκBα-SR) blocks the BV6-stimulated increase in GFAP and differentiation. Interestingly, this BV6-stimulated differentiation is associated with reduced expression of stemness markers such as CD133, Nanog and Sox2 in GBM CSLCs. In contrast, BV6 does not alter cell morphology, differentiation and expression of stemness markers in non-malignant neural stem cells. Importantly, BV6 treatment reduces clonogenicity of GBM CSLCs in vitro and in vivo, suppresses their tumorigenicity in orthotopic and subcutaneous mouse models and significantly increases the survival of mice. By identifying a novel role of BV6 in promoting differentiation of GBM CSLCs, these findings provide new insights into Smac mimetic-regulated non-apoptotic functions with important implications for targeting GBM CSLCs.

Proteolysis of MOB1 by the ubiquitin ligase praja2 attenuates Hippo signalling and supports glioblastoma growth

Human glioblastoma is the most frequent and aggressive form of brain tumour in the adult population. Proteolytic turnover of tumour suppressors by the ubiquitin–proteasome system is a mechanism that tumour cells can adopt to sustain their growth and invasiveness. However, the identity of ubiquitin–proteasome targets and regulators in glioblastoma are still unknown. Here we report that the RING ligase praja2 ubiquitylates and degrades Mob, a core component of NDR/LATS kinase and a positive regulator of the tumour-suppressor Hippo cascade. Degradation of Mob through the ubiquitin–proteasome system attenuates the Hippo cascade and sustains glioblastoma growth in vivo. Accordingly, accumulation of praja2 during the transition from low- to high-grade glioma is associated with significant downregulation of the Hippo pathway. These findings identify praja2 as a novel upstream regulator of the Hippo cascade, linking the ubiquitin proteasome system to deregulated glioblastoma growth.

Tumour suppressors can be inactivated in cancer not only as a result of mutation, but also by proteolytic degradation. Here the authors show that, during glioma development, the accumulation of the ubiquitin ligase praja2 sustains tumour growth by degrading MOB1—a core component of the Hippo pathway.

Oligodendrocyte progenitor cells promote neovascularization in glioma by disrupting the blood-brain barrier

Enhanced platelet-derived growth factor (PDGF) signaling in glioma drives its development and progression. In this study, we define a unique role for stroma-derived PDGF signaling in maintaining tumor homeostasis within the glioma microenvironment. Large numbers of PDGF receptor-α (PDGFRα)-expressing stromal cells derived from oligodendrocytes progenitor cells (OPC) were discovered at the invasive front of high-grade gliomas, in which they exhibited a unique perivascular distribution. In PDGFRα-deficient host mice, in which orthotopic Gl261 tumors displayed reduced outgrowth, we found that tumor-associated blood vessels displayed smaller lumens and normalized vascular morphology, with tumors in host animals injected with the vascular imaging agent gadolinium also being enhanced less avidly by MRI. Notably, glioma-associated OPC promoted endothelial sprouting and tubule formation, in part by abrogating the inhibitory effect that perivascular astrocytes exert on vascular endothelial conjunctions. Stromal-derived PDGF-CC was crucial for the recruitment and activation of OPC, insofar as mice genetically deficient in PDGF-CC phenocopied the glioma/vascular defects observed in PDGFRα-deficient mice. Clinically, we showed that higher levels of PDGF-CC in glioma specimens were associated with more rapid disease recurrence and poorer overall survival. Our findings define a PDGFRα/PDGF-CC signaling axis within the glioma stromal microenvironment that contributes to vascular remodeling and aberrant tumor angiogenesis in the brain.

Glioblastoma cell enrichment is critical for analysis of phosphorylated drug targets and proteomic-genomic correlations

The quality of cancer genomic and proteomic data relies upon the quality of the clinical specimens examined. Here, we show that data derived from non-microdissected glioblastoma multiforme tumor tissue is either masked or not accurate, producing correlations between genomic and proteomic data that lead to false classifications for therapeutic stratification. We analyzed the level of 133 key signaling proteins and phosphoproteins in laser capture microdissected (LCM) primary tumors from a study set of tissues used for the Cancer Genome Atlas (TCGA) profiling efforts, comparing the results to tissue-matched, nontumor cell-enriched lysates from adjacent sections. Among the analytes, 44%, including targets for clinically important inhibitors, such as phosphorylated mTOR, AKT, STAT1, VEGFR2, or BCL2, differed between matched tumor cell-enriched and nonenriched specimens (even in tumor sections with 90% tumor cell content). While total EGFR protein levels were higher in tumors with EGFR mutations, regardless of tumor cell enrichment, EGFR phosphorylation was increased only in LCM-enriched tumor specimens carrying EGFR mutations. Phosphorylated and total PTEN, which is highly expressed in normal brain, was reduced only in LCM-enriched tumor specimens with either PTEN mutation or loss in PTEN copy number, with no differences observed in non-microdissected samples. These results were confirmed in an independent, non-microdissected, publicly available protein data set from the TCGA database. Our findings highlight the necessity for careful upfront cellular enrichment in biospecimens that form the basis for targeted therapy selection and for molecular characterization efforts such as TCGA.

Evidence for post-translational processing of vascular endothelial (VE)-cadherin in brain tumors: towards a candidate biomarker

Vessel abnormalities are among the most important features in malignant glioma. Vascular endothelial (VE)-cadherin is of major importance for vascular integrity. Upon cytokine challenge, VE-cadherin structural modifications have been described including tyrosine phosphorylation and cleavage. The goal of this study was to examine whether these events occurred in human glioma vessels. We demonstrated that VE-cadherin is highly expressed in human glioma tissue and tyrosine phosphorylated at site Y(685), a site previously found phosphorylated upon VEGF challenge, via Src activation. In vitro experiments showed that VEGF-induced VE-cadherin phosphorylation, preceded the cleavage of its extracellular adhesive domain (sVE, 90 kDa). Interestingly, metalloproteases (MMPs) secreted by glioma cell lines were responsible for sVE release. Because VEGF and MMPs are important components of tumor microenvironment, we hypothesized that VE-cadherin proteolysis might occur in human brain tumors. Analysis of glioma patient sera prior treatment confirmed the presence of sVE in bloodstream. Furthermore, sVE levels studied in a cohort of 53 glioma patients were significantly predictive of the overall survival at three years (HR 0.13 [0.04; 0.40] p ≤ 0.001), irrespective to histopathological grade of tumors. Altogether, these results suggest that VE-cadherin structural modifications should be examined as candidate biomarkers of tumor vessel abnormalities, with promising applications in oncology.

Extracellular vesicles modulate the glioblastoma microenvironment via a tumor suppression signaling network directed by miR-1

Extracellular vesicles have emerged as important mediators of intercellular communication in cancer, including by conveying tumor-promoting microRNAs between cells, but their regulation is poorly understood. In this study, we report the findings of a comparative microRNA profiling and functional analysis in human glioblastoma that identifies miR-1 as an orchestrator of extracellular vesicle function and glioblastoma growth and invasion. Ectopic expression of miR-1 in glioblastoma cells blocked in vivo growth, neovascularization, and invasiveness. These effects were associated with a role for miR-1 in intercellular communication in the microenvironment mediated by extracellular vesicles released by cancer stem-like glioblastoma cells. An extracellular vesicle-dependent phenotype defined by glioblastoma invasion, neurosphere growth, and endothelial tube formation was mitigated by loading miR-1 into glioblastoma-derived extracellular vesicles. Protein cargo in extracellular vesicles was characterized to learn how miR-1 directed extracellular vesicle function. The mRNA encoding Annexin A2 (ANXA2), one of the most abundant proteins in glioblastoma-derived extracellular vesicles, was found to be a direct target of miR-1 control. In addition, extracellular vesicle-derived miR-1 along with other ANXA2 extracellular vesicle networking partners targeted multiple pro-oncogenic signals in cells within the glioblastoma microenvironment. Together, our results showed how extracellular vesicle signaling promotes the malignant character of glioblastoma and how ectopic expression of miR-1 can mitigate this character, with possible implications for how to develop a unique miRNA-based therapy for glioblastoma management.

Establishment of a green fluorescent protein tracing murine model focused on the functions of host components in necrosis repair and the niche of subcutaneously implanted glioma

Due to progress in the research of glioma stem cells and the glioma niche, development of an animal model that facilitates the elucidation of the roles of the host tissue and cells is necessary. The aim of the present study was to develop a subcutaneous xenograft green fluorescent protein nude mouse model and use this model to analyze the roles of host cells in tumor necrosis repair. Tumors derived from the human glioma stem/progenitor cell line SU3 were subcutaneously implanted in green fluorescent protein nude mice. The implanted tumors were then passed from animal to animal for 10 generations. Finally, subcutaneous xenografts were assayed with traditional pathology, immunopathological techniques and fluorescence photography. For each generation, the tumorigenicity rate was 100%. Subcutaneous xenografts were rich in blood vessels, and necrotic and hemorrhagic foci, which highly expressed hypoxia-inducible factor-1α, tumor necrosis factor, Ki-67, CD68 and CD11b. In the interstitial tissue, particularly in old hemorrhagic foci, there were numerous cells expressing green fluorescent protein, CD68 and CD11b. Green fluorescent protein nude mouse subcutaneous xenografts not only consistently maintained the high invasiveness and tumorigenicity of glioma stem/progenitor cells, but also consisted of a high concentration of tumor blood vessels and necrotic and hemorrhagic foci. Subcutaneous xenografts also expressed high levels of tumor microenvironment-related proteins and host-derived tumor interstitial molecules. The model has significant potential for further research on tumor tissue remodeling and the tumor microenvironment.

Chondroitin sulfate proteoglycans potently inhibit invasion and serve as a central organizer of the brain tumor microenvironment

Glioblastoma (GBM) remains the most pervasive and lethal of all brain malignancies. One factor that contributes to this poor prognosis is the highly invasive character of the tumor. GBM is characterized by microscopic infiltration of tumor cells throughout the brain, whereas non-neural metastases, as well as select lower grade gliomas, develop as self-contained and clearly delineated lesions. Illustrated by rodent xenograft tumor models as well as pathological human patient specimens, we present evidence that one fundamental switch between these two distinct pathologies--invasion and noninvasion--is mediated through the tumor extracellular matrix. Specifically, noninvasive lesions are associated with a rich matrix containing substantial amounts of glycosylated chondroitin sulfate proteoglycans (CSPGs), whereas glycosylated CSPGs are essentially absent from diffusely infiltrating tumors. CSPGs, acting as central organizers of the tumor microenvironment, dramatically influence resident reactive astrocytes, inducing their exodus from the tumor mass and the resultant encapsulation of noninvasive lesions. Additionally, CSPGs induce activation of tumor-associated microglia. We demonstrate that the astrogliotic capsule can directly inhibit tumor invasion, and its absence from GBM presents an environment favorable to diffuse infiltration. We also identify the leukocyte common antigen-related phosphatase receptor (PTPRF) as a putative intermediary between extracellular glycosylated CSPGs and noninvasive tumor cells. In all, we present CSPGs as critical regulators of brain tumor histopathology and help to clarify the role of the tumor microenvironment in brain tumor invasion.

Involvement of tumor acidification in brain cancer pathophysiology

Gliomas, primary brain cancers, are characterized by remarkable invasiveness and fast growth. While they share many qualities with other solid tumors, gliomas have developed special mechanisms to convert the cramped brain space and other limitations afforded by the privileged central nervous system into pathophysiological advantages. In this review we discuss gliomas and other primary brain cancers in the context of acid-base regulation and interstitial acidification; namely, how the altered proton (H⁺) content surrounding these brain tumors influences tumor development in both autocrine and paracrine manners. As proton movement is directly coupled to movement of other ions, pH serves as both a regulator of cell activity as well as an indirect readout of other cellular functions. In the case of brain tumors, these processes result in pathophysiology unique to the central nervous system. We will highlight what is known about pH-sensitive processes in brain tumors in addition to gleaning insight from other solid tumors.

Identification of U251 glioma stem cells and their heterogeneous stem-like phenotypes

Glioblastoma, the most common and lethal type of intracranial tumor, is characterized by extensive heterogeneity at the cellular and molecular levels. The discovery of glioma stem cells (GSCs) lends support to a new paradigm in tumor biology. In the present study, we aimed to clarify the validity of using U251 glioma cells as a source of GSC culture and critically evaluate the heterogeneous stem-like phenotypes of these cells when grown under various culture conditions. The findings suggested that U251 cells (U251-Adh, U251-SC-Sph and U251-SC-Adh) showed distinctive growth patterns and self-renewal capacity. The U251 glioma cell line is endowed with certain GSC phenotypes that may be moderately enriched in vitro when transferred into stem cell culture conditions, although this is not sustainable and reproducible in vivo. Notably, glioma cells are plastic in response to their environment. The reversible adaptive plasticity contributes to the GSC heterogeneity, which may lead to the heterogeneity of glioblastoma and the differing responses to current therapies. Therefore, an improved understanding of GSC heterogeneity is urgently required for designing more effective therapies against this highly malignant brain tumor.

Coculture system with an organotypic brain slice and 3D spheroid of carcinoma cells

Patients with cerebral metastasis of carcinomas have a poor prognosis. However, the process at the metastatic site has barely been investigated, in particular the role of the resident (stromal) cells. Studies in primary carcinomas demonstrate the influence of the microenvironment on metastasis, even on prognosis^1,2. Especially the tumor associated macrophages (TAM) support migration, invasion and proliferation³. Interestingly, the major target sites of metastasis possess tissue-specific macrophages, such as Kupffer cells in the liver or microglia in the CNS. Moreover, the metastatic sites also possess other tissue-specific cells, like astrocytes. Recently, astrocytes were demonstrated to foster proliferation and persistence of cancer cells^4,5. Therefore, functions of these tissue-specific cell types seem to be very important in the process of brain metastasis^6,7.

Despite these observations, however, up to now there is no suitable in vivo/in vitro model available to directly visualize glial reactions during cerebral metastasis formation, in particular by bright field microscopy. Recent in vivo live imaging of carcinoma cells demonstrated their cerebral colonization behavior⁸. However, this method is very laborious, costly and technically complex. In addition, these kinds of animal experiments are restricted to small series and come with a substantial stress for the animals (by implantation of the glass plate, injection of tumor cells, repetitive anaesthesia and long-term fixation). Furthermore, in vivo imaging is thus far limited to the visualization of the carcinoma cells, whereas interactions with resident cells have not yet been illustrated. Finally, investigations of human carcinoma cells within immunocompetent animals are impossible⁸.

For these reasons, we established a coculture system consisting of an organotypic mouse brain slice and epithelial cells embedded in matrigel (3D cell sphere). The 3D carcinoma cell spheres were placed directly next to the brain slice edge in order to investigate the invasion of the neighboring brain tissue. This enables us to visualize morphological changes and interactions between the glial cells and carcinoma cells by fluorescence and even by bright field microscopy. After the coculture experiment, the brain tissue or the 3D cell spheroids can be collected and used for further molecular analyses (e.g. qRT-PCR, IHC, or immunoblot) as well as for investigations by confocal microscopy. This method can be applied to monitor the events within a living brain tissue for days without deleterious effects to the brain slices. The model also allows selective suppression and replacement of resident cells by cells from a donor tissue to determine the distinct impact of a given genotype. Finally, the coculture model is a practicable alternative to in vivo approaches when testing targeted pharmacological manipulations.

Immature mesenchymal stem cell-like pericytes as mediators of immunosuppression in human malignant glioma

Malignant gliomas are primary brain tumors characterized by profound local immunosuppression. While the remarkable plasticity of perivascular cells - resembling mesenchymal stem cells (MSC) - in malignant gliomas and their contribution to angiogenesis is increasingly recognized, their role as potential mediators of immunosuppression is unknown. Here we demonstrate that FACS-sorted malignant glioma-derived pericytes (HMGP) were characterized by the expression of CD90, CD248, and platelet-derived growth factor receptor-β (PDGFR-β). HMGP shared this expression profile with human brain vascular pericytes (HBVP) and human MSC (HMSC) but not human cerebral microvascular endothelial cells (HCMEC). CD90+PDGFR-β+perivascular cells distinct from CD31+ endothelial cells accumulated in human gliomas with increasing degree of malignancy and negatively correlated with the presence of blood vessel-associated leukocytes and CD8+ T cells. Cultured CD90+PDGFR-β+HBVP were equally capable of suppressing allogeneic or mitogen-activated T cell responses as human MSC. HMGP, HBVP and HMSC expressed prostaglandin E synthase (PGES), inducible nitric oxide synthase (iNOS), human leukocyte antigen-G (HLA-G), hepatocyte growth factor (HGF) and transforming growth factor-β (TGF-β). These factors but not indoleamine 2,3-dioxygenase-mediated conversion of tryptophan to kynurenine functionally contributed to immunosuppression of immature pericytes. Our data provide evidence that human cerebral CD90+ perivascular cells possess T cell inhibitory capability comparable to human MSC and suggest that these cells, besides their critical role in tumor vascularization, also promote local immunosuppression in malignant gliomas and possibly other brain diseases.

Mesenchymal differentiation mediated by NF-κB promotes radiation resistance in glioblastoma

Despite extensive study, few therapeutic targets have been identified for glioblastoma (GBM). Here we show that patient-derived glioma sphere cultures (GSCs) that resemble either the proneural (PN) or mesenchymal (MES) transcriptomal subtypes differ significantly in their biological characteristics. Moreover, we found that a subset of the PN GSCs undergoes differentiation to a MES state in a TNF-α/NF-κB-dependent manner with an associated enrichment of CD44 subpopulations and radioresistant phenotypes. We present data to suggest that the tumor microenvironment cell types such as macrophages/microglia may play an integral role in this process. We further show that the MES signature, CD44 expression, and NF-κB activation correlate with poor radiation response and shorter survival in patients with GBM.

Toll-like receptor 2 mediates microglia/brain macrophage MT1-MMP expression and glioma expansion

BACKGROUND

Glioblastomas are the most aggressive primary brain tumors in humans. Microglia/brain macrophage accumulation in and around the tumor correlates with malignancy and poor clinical prognosis of these tumors. We have previously shown that microglia promote glioma expansion through upregulation of membrane type 1 matrix metalloprotease (MT1-MMP). This upregulation depends on signaling via the Toll-like receptor (TLR) adaptor molecule myeloid differentiation primary response gene 88 (MyD88).

METHODS

Using in vitro, ex vivo, and in vivo techniques, we identified TLR2 as the main TLR controlling microglial MT1-MMP expression and promoting microglia-assisted glioma expansion.

RESULTS

The implantation of mouse GL261 glioma cells into TLR2 knockout mice resulted in significantly smaller tumors, reduced MT1-MMP expression, and enhanced survival rates compared with wild-type control mice. Tumor expansion studied in organotypic brain slices depended on both parenchymal TLR2 expression and the presence of microglia. Glioma-derived soluble factors and synthetic TLR2 specific ligands induced MT1-MMP expression in microglia from wild-type mice, but no such change in MT1-MMP gene expression was observed in microglia from TLR2 knockout mice. We also found evidence that TLR1 and TLR6 cofunction with TLR2 as heterodimers in regulating MT1-MMP expression in vitro.

CONCLUSIONS

Our results thus show that activation of TLR2 along with TLRs 1 and/or 6 converts microglia into a glioma supportive phenotype.

Tackling the physiological barriers for successful mesenchymal stem cell transplantation into the central nervous system

Over the past decade a lot of research has been performed towards the therapeutic use of mesenchymal stem cells (MSCs) in neurodegenerative and neuroinflammatory diseases. MSCs have shown to be beneficial in different preclinical studies of central nervous system (CNS) disorders due to their immunomodulatory properties and their capacity to secrete various growth factors. Nevertheless, most of the transplanted cells die within the first hours after transplantation and induce a neuroinflammatory response. In order to increase the efficacy of MSC transplantation, it is thus imperative to completely characterise the mechanisms mediating neuroinflammation and cell death following MSC transplantation into the CNS. Consequently, different components of these cell death- and neuroinflammation-inducing pathways can be targeted in an attempt to improve the therapeutic potential of MSCs for CNS disorders.

Aquaporin-4 in glioma and metastatic tissues harboring 5-aminolevulinic acid-induced porphyrin fluorescence

INTRODUCTION

Aquaporin channels (AQPs) are a group of integral membrane proteins that regulate the transport of water through cell membranes. Previous studies have shown that up-regulation of AQP1 and AQP4, two of the predominant AQPs in the human brain, in high grade glial tumors contribute to cerebral edema. Others link AQPs to the regulation of human glioma cell migration and invasion. The aim of this study was to determine AQPs expression in tumor tissue harboring 5-aminolevulinic acid (ALA)-induced porphyrin fluorescence with flow cytometry and compare it to the expression in normal brain tissue.

METHODS

Tissue samples were obtained from fluorescing brain tumors of 26 patients treated with ALA prior to surgery (20 mg/kg b.w.). Expression levels of aquaporin channels were measured in primary tissue cultures using a FACS CANTO I flow cytometer. A control group consisted of four non-fluorescing tissue samples, the C6 and the U87 cell line.

RESULTS

Nineteen gliomas (14 high grade, 5 low grade) and 7 metastases were analyzed. On the 4th post-operative day, expression levels of AQP4 channels, but not of AQP1 channels, were significantly increased in samples from fluorescing tissue compared to non-fluorescing tissue. In addition we could see how ALA induces fluorescence in metastases.

CONCLUSION

Flow cytometry appears to be an auspicious method for the analysis of porphyrins and AQPs in primary brain cell tumor cultures. ALA fluorescing tissue showed higher AQP4 expression compared to normal brain tissue. The demonstrated expression in a context with ALA could open a targeted therapeutic spectrum, for example to selectively target AQP4.

The controversial role of microglia in malignant gliomas

Malignant gliomas contain stroma and a variety of immune cells including abundant activated microglia/macrophages. Mounting evidence indicates that the glioma microenvironment converts the glioma-associated microglia/macrophages (GAMs) into glioma-supportive, immunosuppressive cells; however, GAMs can retain intrinsic anti-tumor properties. Here, we review and discuss this duality and the potential therapeutic strategies that may inhibit their glioma-supportive and propagating functions.

Molecular definition of the pro-tumorigenic phenotype of glioma-activated microglia

Microglia are myeloid cells residing in the central nervous system that participate in inflammatory responses and could promote injury and repair. Gliomas attract microglia and polarize them into tumor-supporting cells that participate in matrix remodeling, invasion, angiogenesis, and suppression of adaptive immunity. Although signaling pathways and critical regulators underlying classical inflammation are well established, signal transduction and transcriptional circuits underlying the alternative activation of microglia are poorly known. Using primary rat microglial cultures exposed to glioma conditioned medium or lipopolysaccharide (LPS), we demonstrate that microglia adapt different fates and polarize into pro-inflammatory or alternatively activated cells. Glioma-derived factors increased cell motility, phagocytosis, and sustained proliferation of microglial cells that was mediated by enhanced focal adhesion kinase and PI-3K/Akt signaling. The signals from glioma cells induced ERK and p38 MAPK but not JNK signaling and failed to activate pro-inflammatory Stat1 and NFκB signaling in microglial cells. Transcriptome analysis of microglial cultures at 6 h after exposure to glioma-conditioned medium or LPS revealed different patterns of gene expression. Glioma-induced activation was associated with induction of genes coding for ID (inhibitor of DNA binding) 1/3 and c-Myc, markers of the alternative phenotype Arg1, MT1-MMP, CXCL14, and numerous cytokines/chemokines implicated in immune cell trafficking. Many classical inflammation-related genes and signaling pathways failed to be induced. Our study indicates for the first time molecular pathways that direct microglia toward the pro-invasive, immunosuppressive phenotype.

Microglia and macrophages in malignant gliomas: recent discoveries and implications for promising therapies

Malignant gliomas are the most common primary brain tumors. Their deadliest manifestation, glioblastoma multiforme (GBM), accounts for 15% of all primary brain tumors and is associated with a median survival of only 15 months even after multimodal therapy. There is substantial presence of microglia and macrophages within and surrounding brain tumors. These immune cells acquire an alternatively activated phenotype with potent tumor-tropic functions that contribute to glioma growth and invasion. In this review, we briefly summarize recent data that has been reported on the interaction of microglia/macrophages with brain tumors and discuss potential application of these findings to the development of future antiglioma therapies.

Multifaceted oncolytic virus therapy for glioblastoma in an immunocompetent cancer stem cell model

Glioblastoma (World Health Organization grade IV) is an aggressive adult brain tumor that is inevitably fatal despite surgery, radiation, and chemotherapy. Treatment failures are attributed to combinations of cellular heterogeneity, including a subpopulation of often-resistant cancer stem cells, aberrant vasculature, and noteworthy immune suppression. Current preclinical models and treatment strategies do not incorporate or address all these features satisfactorily. Herein, we describe a murine glioblastoma stem cell (GSC) model that recapitulates tumor heterogeneity, invasiveness, vascularity, and immunosuppressive microenvironment in syngeneic immunocompetent mice and should prove useful for a range of therapeutic studies. Using this model, we tested a genetically engineered oncolytic herpes simplex virus that is armed with an immunomodulatory cytokine, interleukin 12 (G47-mIL12). G47Δ-mIL12 infects and replicates similarly to its unarmed oncolytic herpes simplex virus counterpart in mouse 005 GSCs in vitro, whereas in vivo, it significantly enhances survival in syngeneic mice bearing intracerebral 005 tumors. Mechanistically, G47-mIL12 targets not only GSCs but also increases IFN-γ release, inhibits angiogenesis, and reduces the number of regulatory T cells in the tumor. The increased efficacy is dependent upon T cells, but not natural killer cells. Taken together, our findings demonstrate that G47Δ-mIL12 provides a multifaceted approach to targeting GSCs, tumor microenvironment, and the immune system, with resultant therapeutic benefit in a stringent glioblastoma model.

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

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

The spectrum of vaccine therapies for patients with glioblastoma multiforme

OPINION STATEMENT

Glioblastoma multiforme (GBM) is the most common primary malignant tumor of the central nervous system (CNS) and one of the most lethal cancers in adults and children. Despite aggressive treatment with surgery, radiation, and chemotherapy, median survival is less than 15 months and overall survival is less than 10 % at 5 years. Development of therapeutics for malignant gliomas has been hampered by their natural complexity as well as protective mechanisms unique to the CNS. Better understanding of the pathogenesis of GBM is opening the path to novel, specific-targeted therapies. Recently, multiple immunotherapy approaches have been acquiring substantial indication of therapeutic efficacy with a very safe profile. Examples of the leading clinical approaches for GBM will be discussed in detail in this review.

Achieving graft-versus-tumor effect in brain tumor patients: from autologous progenitor cell transplant to active immunotherapy

Success in treating aggressive brain tumors like glioblastoma multiforme and medulloblastoma remains challenging, in part because these malignancies overcome CNS immune surveillance. New insights into brain tumor immunology have led to a rational development of immunotherapeutic strategies, including cytotoxic Tlymphocyte therapies and dendritic cell vaccines. However, these therapies are most effective when applied in a setting of minimal residual disease, so require prior use of standard cytotoxic therapies or cytoreduction by surgery. Myeloablative chemotherapy with autologous hematopoietic cell transplantation (autoHCT) can offer a platform upon which different cellular therapies can be effectively instituted. Specifically, this approach provides an inherent 'chemical debulking' through high-dose chemotherapy and a graft-versus-tumor effect through an autologous T-cell replete graft. Furthermore, autoHCT may be beneficial in 'resetting' the body's immune system, potentially 'breaking' tumor tolerance, and in providing a 'boost' of immune effector cells (NK cells or cytotoxic T lymphocytes), which could augment desired anti-tumor effects. As literature on the use of autoHCT in brain tumors is scarce, aspects of immunotherapies applied in non-CNS malignancies are reviewed as potential therapies that could be used in conjunction with autoHCT to eradicate brain tumors.

ATM kinase inhibition preferentially sensitizes p53-mutant glioma to ionizing radiation

PURPOSE

Glioblastoma multiforme (GBM) is the most lethal form of brain cancer with a median survival of only 12 to 15 months. Current standard treatment consists of surgery followed by chemoradiation. The poor survival of patients with GBM is due to aggressive tumor invasiveness, an inability to remove all tumor tissue, and an innate tumor chemo- and radioresistance. Ataxia-telangiectasia mutated (ATM) is an excellent target for radiosensitizing GBM because of its critical role in regulating the DNA damage response and p53, among other cellular processes. As a first step toward this goal, we recently showed that the novel ATM kinase inhibitor KU-60019 reduced migration, invasion, and growth, and potently radiosensitized human glioma cells in vitro.

EXPERIMENTAL DESIGN

Using orthotopic xenograft models of GBM, we now show that KU-60019 is also an effective radiosensitizer in vivo. Human glioma cells expressing reporter genes for monitoring tumor growth and dispersal were grown intracranially, and KU-60019 was administered intratumorally by convection-enhanced delivery or osmotic pump.

RESULTS

Our results show that the combined effect of KU-60019 and radiation significantly increased survival of mice 2- to 3-fold over controls. Importantly, we show that glioma with mutant p53 is much more sensitive to KU-60019 radiosensitization than genetically matched wild-type glioma.

CONCLUSIONS

Taken together, our results suggest that an ATM kinase inhibitor may be an effective radiosensitizer and adjuvant therapy for patients with mutant p53 brain cancers.

Exosomes reflect the hypoxic status of glioma cells and mediate hypoxia-dependent activation of vascular cells during tumor development

Hypoxia, or low oxygen tension, is a major regulator of tumor development and aggressiveness. However, how cancer cells adapt to hypoxia and communicate with their surrounding microenvironment during tumor development remain important questions. Here, we show that secreted vesicles with exosome characteristics mediate hypoxia-dependent intercellular signaling of the highly malignant brain tumor glioblastoma multiforme (GBM). In vitro hypoxia experiments with glioma cells and studies with patient materials reveal the enrichment in exosomes of hypoxia-regulated mRNAs and proteins (e.g., matrix metalloproteinases, IL-8, PDGFs, caveolin 1, and lysyl oxidase), several of which were associated with poor glioma patient prognosis. We show that exosomes derived from GBM cells grown at hypoxic compared with normoxic conditions are potent inducers of angiogenesis ex vivo and in vitro through phenotypic modulation of endothelial cells. Interestingly, endothelial cells were programmed by GBM cell-derived hypoxic exosomes to secrete several potent growth factors and cytokines and to stimulate pericyte PI3K/AKT signaling activation and migration. Moreover, exosomes derived from hypoxic compared with normoxic conditions showed increased autocrine, promigratory activation of GBM cells. These findings were correlated with significantly enhanced induction by hypoxic compared with normoxic exosomes of tumor vascularization, pericyte vessel coverage, GBM cell proliferation, as well as decreased tumor hypoxia in a mouse xenograft model. We conclude that the proteome and mRNA profiles of exosome vesicles closely reflect the oxygenation status of donor glioma cells and patient tumors, and that the exosomal pathway constitutes a potentially targetable driver of hypoxia-dependent intercellular signaling during tumor development.

GDNF mediates glioblastoma-induced microglia attraction but not astrogliosis

High-grade gliomas are the most common primary brain tumors. Their malignancy is promoted by the complex crosstalk between different cell types in the central nervous system. Microglia/brain macrophages infiltrate high-grade gliomas and contribute to their progression. To identify factors that mediate the attraction of microglia/macrophages to malignant brain tumors, we established a glioma cell encapsulation model that was applied in vivo. Mouse GL261 glioma cell line and human high-grade glioma cells were seeded into hollow fibers (HF) that allow the passage of soluble molecules but not cells. The glioma cell containing HF were implanted into one brain hemisphere and simultaneously HF with non-transformed fibroblasts (controls) were introduced into the contralateral hemisphere. Implanted mouse and human glioma- but not fibroblast-containing HF attracted microglia and up-regulated immunoreactivity for GFAP, which is a marker of astrogliosis. In this study, we identified GDNF as an important factor for microglial attraction: (1) GL261 and human glioma cells secret GDNF, (2) reduced GDNF production by siRNA in GL261 in mouse glioma cells diminished attraction of microglia, (3) over-expression of GDNF in fibroblasts promoted microglia attraction in our HF assay. In vitro migration assays also showed that GDNF is a strong chemoattractant for microglia. While GDNF release from human or mouse glioma had a profound effect on microglial attraction, the glioma-induced astrogliosis was not affected. Finally, we could show that injection of GL261 mouse glioma cells with GDNF knockdown by shRNA into mouse brains resulted in reduced tumor expansion and improved survival as compared to injection of control cells.

On the role of 25-hydroxycholesterol synthesis by glioblastoma cell lines. Implications for chemotactic monocyte recruitment

Glioblastoma multiforme (GBM) is the most common malignant primary brain tumor and is invariably fatal to affected patients. Oxysterols belong to a class of bioactive lipids that are implicated in neurological disease and are associated with various types of cancer. Here, we investigated expression and transcriptional regulation of cholesterol 25-hydroxylase (CH25H) in human U87MG and GM133 glioblastoma cell lines. We demonstrate that in both cell lines transcription and translation of CH25H are increased in response to TNFα and IL1β. In parallel, both cell lines upregulate 25-hydroxycholesterol (25-OHC) synthesis and secretion to levels comparable to bone marrow-derived mouse macrophages under inflammatory conditions. To determine whether 25-OHC acts as chemoattractant for tumor-associated macrophages, the human THP-1 monoblastic leukemia cell line was treated with varying amounts of the oxysterol. Experiments revealed that 25-OHC and lipid extracts isolated from GM133-conditioned medium (containing 7-fold higher 25-OHC concentrations than U87MG medium) induce chemotactic migration of THP-1 cells. Of note, 25-OHC also induced the migration of primary human peripheral blood monocytes. In response to exogenously added 25-OHC, THP-1 cells reorganized intermediate filament-associated vimentin to more cortical and polarized structures. Chemotactic migration of monocytes in response to 25-OHC was pertussis toxin-sensitive, indicating the involvement of G protein-coupled receptors. Using RNA interference we demonstrated that G protein-coupled receptor 183 (EBI2) contributes to 25-OHC-mediated chemotactic migration of THP-1 cells. These in vitro data indicate that GBM-derived and secreted 25-OHC may be involved in the recruitment of immune-competent cells to a tumor via EBI2.

Interferon regulatory factor 3 alters glioma inflammatory and invasive properties

Glioblastoma multiforme (GBM) is the most common, highly malignant primary tumor of the brain with poor prognosis. Even with the improved therapy regimen including temozolomide, the average survival rate is less than 2 years. Additional approaches to therapy targeting multiple aspects of glioma progression are in need. In the present work, we have tested the possibility that upregulation of the transcription factor interferon regulatory factor 3 (IRF3) can inhibit glioma invasiveness, proliferation and production of pro-inflammatory and pro-angiogenic factors in cultures of malignant glioma cell lines (U271, U87 and SNB-19). IRF3 is an essential transcription factor involved in TLR3/4-mediated signaling and generation of type I interferons. Although IRF3 has been suggested as a potential tumor suppressor gene, its role in glioma remains uninvestigated. In this study, we find that human glioma immune activation is potently elicited by a cytokine combination, IL-1/IFNγ (or poly IC), but not by bacterial lipopolysaccharide (LPS), similar to primary human astrocytes. GBM biopsy specimens show little detectable IRF3 immunoreactivity, and in vitro adenovirus-mediated IRF3 gene transfer in glioma cells modulates IL-1/IFNγ-induced cytokine and chemokine genes, resulting in upregulation of IFNβ and IP-10 (IRF3-stimulated genes) and downregulation of proinflammatory and angiogenic genes including IL-8, TNFα and VEGF (IRF3-represssed genes). Cytokines (IL-1β and TNFα) also induce the expression of miR-155 and miR-155*, the microRNAs crucial in immunity and inflammation-induced oncogenesis and this is dose-dependently suppressed by IRF3. Importantly, IRF3 also inhibits glioma proliferation, migration and invasion. Together, these data suggest that IRF3 can suppress glioma progression. Agents that promote IRF3 activation and expression (such as IRF3 gene transfer) could be explored as potential future therapy.

Hitting Them Where They Live: Targeting the Glioblastoma Perivascular Stem Cell Niche

Glioblastoma growth potential and resistance to therapy is currently largely attributed to a subset of tumor cells with stem-like properties. If correct, this means that cure will not be possible without eradication of the stem cell fraction and abrogation of those mechanisms through which stem cell activity is induced and maintained. Glioblastoma stem cell functions appear to be non-cell autonomous and the consequence of tumor cell residence within specialized domains such as the perivascular stem cell niche. In this review we consider the multiple cellular constituents of the perivascular niche, the molecular mechanisms that support niche structure and function and the implications of the perivascular localization of stem cells for anti-angiogenic approaches to cure.

Patient-specific orthotopic glioblastoma xenograft models recapitulate the histopathology and biology of human glioblastomas in situ

Frequent discrepancies between preclinical and clinical results of anticancer agents demand a reliable translational platform that can precisely recapitulate the biology of human cancers. Another critical unmet need is the ability to predict therapeutic responses for individual patients. Toward this goal, we have established a library of orthotopic glioblastoma (GBM) xenograft models using surgical samples of GBM patients. These patient-specific GBM xenograft tumors recapitulate histopathological properties and maintain genomic characteristics of parental GBMs in situ. Furthermore, in vivo irradiation, chemotherapy, and targeted therapy of these xenograft tumors mimic the treatment response of parental GBMs. We also found that establishment of orthotopic xenograft models portends poor prognosis of GBM patients and identified the gene signatures and pathways signatures associated with the clinical aggressiveness of GBMs. Together, the patient-specific orthotopic GBM xenograft library represent the preclinically and clinically valuable "patient tumor's phenocopy" that represents molecular and functional heterogeneity of GBMs.

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.

Signaling determinants of glioma cell invasion

Tumor cell invasiveness is a critical challenge in the clinical management of glioma patients. In addition, there is accumulating evidence that current therapeutic modalities, including anti-angiogenic therapy and radiotherapy, can enhance glioma invasiveness. Glioma cell invasion is stimulated by both autocrine and paracrine factors that act on a large array of cell surface-bound receptors. Key signaling elements that mediate receptor-initiated signaling in the regulation of glioblastoma invasion are Rho family GTPases, including Rac, RhoA and Cdc42. These GTPases regulate cell morphology and actin dynamics and stimulate cell squeezing through the narrow extracellular spaces that are typical of the brain parenchyma. Transient attachment of cells to the extracellular matrix is also necessary for glioblastoma cell invasion. Interactions with extracellular matrix components are mediated by integrins that initiate diverse intracellular signalling pathways. Key signaling elements stimulated by integrins include PI3K, Akt, mTOR and MAP kinases. In order to detach from the tumor mass, glioma cells secrete proteolytic enzymes that cleave cell surface adhesion molecules, including CD44 and L1. Key proteases produced by glioma cells include uPA, ADAMs and MMPs. Increased understanding of the molecular mechanisms that control glioma cell invasion has led to the identification of molecular targets for therapeutic intervention in this devastating disease.

Contact and encirclement of glioma cells in vitro is an intrinsic behavior of a clonal human neural stem cell line

Pathotropic neural stem and/or progenitor cells (NSCs) can potentially deliver therapeutic agents to otherwise inaccessible cancers. In glioma, NSCs are found in close contact with tumor cells, raising the possibility that specificity of NSC contact with glioma targets originates in the tumor cells themselves. Alternatively, target preferences may originate, at least in part, in the tumor microenvironment. To better understand mechanisms underlying NSC interactions with glioma cells, we examined NSC-target cell contacts in a highly simplified 3-dimensional peptide hydrogel (Puramatrix) in which cell behaviors can be studied in the relative absence of external cues. HB1.F3 is an immortalized clonal human NSC line extensively characterized in preclinical investigations. To study contact formation between HB1.F3 NSCs and glioma cells, we first examined co-cultures of eGFP-expressing HB1.F3 (HB1.F3.eGFP) NSCs and dsRed-expressing U251 glioma (U251.dsRed) cells. Using confocal microscopy, HB1.F3.eGFP cells were observed contacting or encircling U251.dsRed glioma cells, but never the reverse. Next, examining specificity of these contacts, no significant quantitative differences in either percentages of HB1.F3 NSCs contacting targets, or in the extent of target cell encirclement, were observed when HB1.F3.eGFP cells were presented with various potential target cells (human glioma and breast cancer cell lines, patient-derived brain tumor lines, non-tumor fibroblasts, primary mouse and human astroglial cells, and primary adult and newborn human dermal fibroblasts) except that interactions between HB1.F3 cells did not progress beyond establishing contacts. Finally cytoskeletal mechanisms employed by HB1.F3.eGFP cells varied with the substrate. When migrating in Puramatrix, HB1.F3 NSCs exhibited intermittent process extension followed by soma translocation, while during encirclement their movements were more amoeboid. We conclude that formation of contacts and subsequent encirclement of target cells by HB1.F3 NSCs is an intrinsic property of these NSCs, and that preferential contact formation with tumor cells in vivo must therefore be highly dependent on microenvironmental cues.

Phosphoglycerate dehydrogenase induces glioma cells proliferation and invasion by stabilizing forkhead box M1

Phosphoglycerate dehydrogenase (PHGDH) is the first enzyme branching from glycolysis in the three-step serine biosynthetic pathway. Recent evidence has shown that PHGDH is amplified in human breast cancer and melanoma and plays a key role in cancer metabolism. However, PHGDH expression in glioma and a potential non-metabolic role in tumorigenesis have not been reported. We analyzed PHGDH levels in specimens from glioma patients and found that PHGDH, although negative in normal brain tissues, was highly expressed in astrocytic tumors and increasingly expressed in more aggressive cancer types. Inhibition of PHGDH expression in glioma cells downregulated the expression of VEGF, MMP-2, CHK2 and cyclin D1 and reduced glioma cell proliferation, invasion and tumorigenicity in vitro and in vivo. Interestingly, we found that the oncogenic transcription factor FOXM1 was also downregulated in PHDGH-silenced glioma cells. Using LC/LC MS analysis, we identified PHGDH as a novel binding partner of FOXM1. PHGDH interacted with and stabilized FOXM1 at the protein level, promoting the proliferation, invasion and tumorigenicity of glioma cells. Our data identified PHGDH as a potential prognostic marker of glial brain tumors and identified a non-metabolic role for PHGDH in glioma tumorigenesis, providing a novel angle of targeting the PHGDH–FOXM1 axis in future brain tumor therapy.

MEF promotes stemness in the pathogenesis of gliomas

High-grade gliomas are aggressive and uniformly fatal tumors, composed of a heterogeneous population of cells that include many with stem-cell-like properties. The acquisition of stem-like traits might contribute to glioma initiation, growth, and recurrence. Here we investigated the role of the transcription factor myeloid Elf-1 like factor (MEF, also known as ELF4) in gliomas. We found that MEF is highly expressed in both human and mouse glioblastomas and its absence impairs gliomagenesis in a PDGF-driven glioma mouse model. We show that modulation of MEF levels in both mouse neural stem cells and human glioblastoma cells has a significant impact on neurosphere formation. Moreover, we identify Sox2 as a direct downstream target of MEF. Taken together, our studies implicate MEF as a previously unrecognized gatekeeper gene in gliomagenesis that promotes stem cell characteristics through Sox2 activation.

Multifaces of neuropeptide Y in the brain--neuroprotection, neurogenesis and neuroinflammation

Neuropeptide Y (NPY) has been implicated in the modulation of important features of neuronal physiology, including calcium homeostasis, neurotransmitter release and excitability. Moreover, NPY has been involved as an important modulator of hippocampal and thalamic circuits, receiving particular attention as an endogenous antiepileptic peptide and as a potential master regulator of feeding behavior. NPY not only inhibits excessive glutamate release (decreasing circuitry hyperexcitability) but also protects neurons from excitotoxic cell death. Furthermore, NPY has been involved in the modulation of the dynamics of dentate gyrus and subventricular zone neural stem cell niches. In both regions, NPY is part of the chemical resource of the neurogenic niche and acts through NPY Y1 receptors to promote neuronal differentiation. Interestingly, NPY is also considered a neuroimmune messenger. In this review, we highlight recent evidences concerning paracrine/autocrine actions of NPY involved in neuroprotection, neurogenesis and neuroinflammation. In summary, the three faces of NPY, discussed in the present review, may contribute to better understand the dynamics and cell fate decision in the brain parenchyma and in restricted areas of neurogenic niches, in health and disease.