Molecular heterogeneity in a patient-derived glioblastoma xenoline is regulated by different cancer stem cell populations

Aberrant Notch signaling in gliomas: a potential landscape of actionable converging targets for combination approach in therapies resistance

The current therapeutic protocols and prognosis of gliomas still depend on clinicopathologic and radiographic characteristics. For high-grade gliomas, the standard of care is resection followed by radiotherapy plus temozolomide chemotherapy. However, treatment resistance develops due to different mechanisms, among which is the dynamic interplay between the tumor and its microenvironment. Different signaling pathways cause the proliferation of so-called glioma stem cells, a minor cancer cell population with stem cell-like characteristics and aggressive phenotype. In the last decades, numerous studies have indicated that Notch is a crucial pathway that maintains the characteristics of resistant glioma stem cells. Data obtained from preclinical models indicate that downregulation of the Notch pathway could induce multifaceted drug sensitivity, acting on the expression of drug-transporter proteins, inducing epithelial-mesenchymal transition, and shaping the tumor microenvironment. This review provides a brief overview of the published data supporting the roles of Notch in drug resistance and demonstrates how potential novel strategies targeting Notch could become an efficacious action to improve the therapy of high-grade glioma to overcome drug resistance.

Assessing Nordihydroguaiaretic Acid Therapeutic Effect for Glioblastoma Multiforme

In this study, we demonstrate that Raman microscopy combined with computational analysis is a useful approach to discriminating accurately between brain tumor bio-specimens and to identifying structural changes in glioblastoma (GBM) bio-signatures after nordihydroguaiaretic acid (NDGA) administration. NDGA phenolic lignan was selected as a potential therapeutic agent because of its reported beneficial effects in alleviating and inhibiting the formation of multi-organ malignant tumors. The current analysis of NDGA's impact on GBM human cells demonstrates a reduction in the quantity of altered protein content and of reactive oxygen species (ROS)-damaged phenylalanine; results that correlate with the ROS scavenger and anti-oxidant properties of NDGA. A novel outcome presented here is the use of phenylalanine as a biomarker for differentiating between samples and assessing drug efficacy. Treatment with a low NDGA dose shows a decline in abnormal lipid-protein metabolism, which is inferred by the formation of lipid droplets and a decrease in altered protein content. A very high dose results in cell structural and membrane damage that favors transformed protein overexpression. The information gained through this work is of substantial value for understanding NDGA's beneficial as well as detrimental bio-effects as a potential therapeutic drug for brain cancer.

STAT3 induces breast cancer growth via ANGPTL4, MMP13 and STC1 secretion by cancer associated fibroblasts

In the tumor microenvironment, Cancer Associated Fibroblasts (CAFs) become activated by cancer cells and increase their secretory activity to produce soluble factors that contribute to tumor cells proliferation, invasion and dissemination to distant organs. The pro-tumorigenic transcription factor STAT3 and its canonical inducer, the pro-inflammatory cytokine IL-6, act conjunctly in a positive feedback loop that maintains high levels of IL-6 secretion and STAT3 activation in both tumor and stromal cells. Here, we demonstrate that STAT3 is essential for the pro-tumorigenic functions of murine breast cancer CAFs both in vitro and in vivo, and identify a STAT3 signature significantly enriched for genes encoding for secreted proteins. Among these, ANGPTL4, MMP13 and STC-1 were functionally validated as STAT3-dependent mediators of CAF pro-tumorigenic functions by different approaches. Both in vitro and in vivo CAFs activities were moreover impaired by MMP13 inhibition, supporting the feasibility of a therapeutic approach based on inhibiting STAT3-induced CAF-secreted proteins. The clinical potential of such an approach is supported by the observation that an equivalent CAF-STAT3 signature in humans is expressed at high levels in breast cancer stromal cells and characterizes patients with a shorter disease specific survival, including those with basal-like disease.

Targeting the Bromodomain of BRG-1/BRM Subunit of the SWI/SNF Complex Increases the Anticancer Activity of Temozolomide in Glioblastoma

Glioblastoma (GBM) is a deadly and incurable brain cancer with limited therapeutic options. PFI-3 is a small-molecule bromodomain (BRD) inhibitor of the BRM/BRG1 subunits of the SWI/SNF chromatin remodeling complex. The objective of this study is to determine the efficacy of PFI-3 as a potential GBM therapy. We report that PFI-3 binds to these BRDs when expressed in GBM cells. PFI-3 markedly enhanced the antiproliferative and cell death-inducing effects of temozolomide (TMZ) in TMZ-sensitive GBM cells as well as overcame the chemoresistance of highly TMZ-resistant GBM cells. PFI-3 also altered gene expression in GBM and enhanced the basal and interferon-induced expression of a subset of interferon-responsive genes. Besides the effects of PFI-3 on GBM cells in vitro, we found that PFI-3 markedly potentiated the anticancer effect of TMZ in an intracranial GBM animal model, resulting in a marked increase in survival of animals bearing GBM tumors. Taken together, we identified the BRG1 and BRM subunits of SWI/SNF as novel targets in GBM and revealed the therapeutic potential of applying small molecule inhibitors of SWI/SNF to improve the clinical outcome in GBM using standard-of-care chemotherapy.

Design of 18 nm Doxorubicin-Loaded 3-Helix Micelles: Cellular Uptake and Cytotoxicity in Patient-Derived GBM6 Cells

The fate of nanocarrier materials at the cellular level constitutes a critical checkpoint in the development of effective nanomedicines, determining whether tissue level accumulation results in therapeutic benefit. The cytotoxicity and cell internalization of ∼18 nm 3-helix micelle (3HM) loaded with doxorubicin (DOX) were analyzed in patient-derived glioblastoma (GBM) cells in vitro. The half-maximal inhibitory concentration (IC₅₀) of 3HM-DOX increased to 6.2 μg/mL from <0.5 μg/mL for free DOX in patient-derived GBM6 cells, to 15.0 μg/mL from 6.5 μg/mL in U87MG cells, and to 21.5 μg/mL from ∼0.5 μg/mL in LN229 cells. Modeling analysis of previous 3HM biodistribution results predicts that these cytotoxic concentrations are achievable with intravenous injection in rodent GBM models. 3HM-DOX formulations were internalized intact and underwent intracellular trafficking distinct from free DOX. 3HM was quantified to have an internalization half-life of 12.6 h in GBM6 cells, significantly longer than that reported for some liposome and polymer systems. 3HM was found to traffic through active endocytic processes, with clathrin-mediated endocytosis being the most involved of the pathways studied. Inhibition studies suggest substantial involvement of receptor recognition in 3HM uptake. As the 3HM surface is PEG-ylated with no targeting functionalities, protein corona-cell surface interactions, such as the apolipoprotein-low-density lipoprotein receptor, are expected to initiate internalization. The present work gives insights into the cytotoxicity, pharmacodynamics, and cellular interactions of 3HM and 3HM-DOX relevant for ongoing preclinical studies. This work also contributes to efforts to develop predictive mathematical models tracking the accumulation and biodistribution kinetics at a systemic level.

Identification of Molecular Signatures in Neural Differentiation and Neurological Diseases Using Digital Color-Coded Molecular Barcoding

Human pluripotent stem cells (PSCs), including embryonic stem cells and induced pluripotent stem cells, represent powerful tools for disease modeling and for therapeutic applications. PSCs are particularly useful for the study of development and diseases of the nervous system. However, generating in vitro models that recapitulate the architecture and the full variety of subtypes of cells that make the complexity of our brain remains a challenge. In order to fully exploit the potential of PSCs, advanced methods that facilitate the identification of molecular signatures in neural differentiation and neurological diseases are highly demanded. Here, we review the literature on the development and application of digital color-coded molecular barcoding as a potential tool for standardizing PSC research and applications in neuroscience. We will also describe relevant examples of the use of this technique for the characterization of the heterogeneous composition of the brain tumor glioblastoma multiforme.

ANGPTL4 Induces TMZ Resistance of Glioblastoma by Promoting Cancer Stemness Enrichment via the EGFR/AKT/4E-BP1 Cascade

Glioblastoma (GBM) is the most aggressive type of brain tumor, with strong invasiveness and a high tolerance to chemotherapy. Despite the current standard treatment combining temozolomide (TMZ) and radiotherapy, glioblastoma can be incurable due to drug resistance. The existence of glioma stem-like cells (GSCs) is considered the major reason for drug resistance. However, the mechanism of GSC enrichment remains unclear. Herein, we found that the expression and secretion of angiopoietin-like 4 protein (ANGPTL4) were clearly increased in GSCs. The overexpression of ANGPTL4 induced GSC enrichment that was characterized by polycomb complex protein BMI-1 and SRY (sex determining region Y)-box 2 (SOX2) expression, resulting in TMZ resistance in GBM. Furthermore, epidermal growth factor receptor (EGFR) phosphorylation induced 4E-BP1 phosphorylation that was required for ANGPTL4-induced GSC enrichment. In particular, ANGPTL4 induced 4E-BP1 phosphorylation by activating phosphoinositide 3-kinase (PI3K)/AKT and extracellular signal–regulated kinase (ERK) cascades for inducing stemness. To elucidate the mechanism contributing to ANGPTL4 upregulation in GSCs, chromatin immunoprecipitation coupled with sequencing (ChIP-Seq) revealed that specificity protein 4 (Sp4) was associated with the promoter region, −979 to −606, and the luciferase reporter assay revealed that Sp4 positively regulated activity of the ANGPTL4 promoter. Moreover, both ANGPTL4 and Sp4 were highly expressed in GBM and resulted in a poor prognosis. Taken together, Sp4-mediated ANGPTL4 upregulation induces GSC enrichment through the EGFR/AKT/4E-BP1 cascade.

Multi-omics profiling reveals key signaling pathways in ovarian cancer controlled by STAT3

Inhibiting STAT3 signaling reduces tumor progression, metastasis and chemoresistance, however the precise molecular mechanism has not been fully delineated in ovarian cancer.

Methods: In this study, we generated STAT3 knockout (KO) ovarian cancer cell lines. Effects of STAT3 KO on cell proliferation, migration and spheroid formation were assessed in vitro and effects on in vivo tumor growth were tested using several tumor xenograft models. We used multi-omic genome-wide profiling to identify multi-level (Bru-Seq, RNA-Seq, and MS Proteomic) expression signatures of STAT3 KO ovarian cancer cells.

Results: We observed that deletion of STAT3 blocked cell proliferation and migration in vitro and suppressed tumor growth in mice. Deletion of STAT3 transcriptionally suppressed key genes involved in EMT, cell cycle progression, E2F signaling, and altered stemness markers. Notably, KO of STAT3 resulted in modulation of the expression of other STAT family members.

Conclusion: Our study presents a rich, multi-faceted summary of the molecular mechanisms impacted by STAT3 deletion and provides new insight for STAT3's potential as a therapeutic target in ovarian cancer.

APELA Expression in Glioma, and Its Association with Patient Survival and Tumor Grade

Glioblastoma (GBM) is the most common and deadliest primary adult brain tumor. Invasion, resistance to therapy, and tumor recurrence in GBM can be attributed in part to brain tumor-initiating cells (BTICs). BTICs isolated from various patient-derived xenografts showed high expression of the poorly characterized Apelin early ligand A (APELA) gene. Although originally considered to be a non-coding gene, the APELA gene encodes a protein that binds to the Apelin receptor and promotes the growth of human embryonic stem cells and the formation of the embryonic vasculature. We found that both APELA mRNA and protein are expressed at high levels in a subset of brain tumor patients, and that APELA is also expressed in putative stem cell niche in GBM tumor tissue. Analysis of APELA and the Apelin receptor gene expression in brain tumor datasets showed that high APELA expression was associated with poor patient survival in both glioma and glioblastoma, and APELA expression correlated with glioma grade. In contrast, gene expression of the Apelin receptor or Apelin was not found to be associated with patient survival, or glioma grade. Consequently, APELA may play an important role in glioblastoma tumorigenesis and may be a future therapeutic target.

Chromatin Remodeling Factor BRG1 Regulates Stemness and Chemosensitivity of Glioma Initiating Cells

Glioblastoma multiforme (GBM) is a highly aggressive and malignant brain tumor that is refractory to existing therapeutic regimens, which reflects the presence of stem-like cells, termed glioma-initiating cells (GICs). The complex interactions between different signaling pathways and epigenetic regulation of key genes may be critical in the maintaining GICs in their stem-like state. Although several signaling pathways have been identified as being dysregulated in GBM, the prognosis of GBM patients remains miserable despite improvements in targeted therapies. In this report, we identified that BRG1, the catalytic subunit of the SWI/SNF chromatin remodeling complex, plays a fundamental role in maintaining GICs in their stem-like state. In addition, we identified a novel mechanism by which BRG1 regulates glycolysis genes critical for GICs. BRG1 downregulates the expression of TXNIP, a negative regulator of glycolysis. BRG1 knockdown also triggered the STAT3 pathway, which led to TXNIP activation. We further identified that TXNIP is an STAT3-regulated gene. Moreover, BRG1 suppressed the expression of interferon-stimulated genes, which are negatively regulated by STAT3 and regulate tumorigenesis. We further demonstrate that BRG1 plays a critical role in the drug resistance of GICs and in GIC-induced tumorigenesis. By genetic and pharmacological means, we found that inhibiting BRG1 can sensitize GICs to chemotherapeutic drugs, temozolomide and carmustine. Our studies suggest that BRG1 may be a novel therapeutic target in GBM. The identification of the critical role that BRG1 plays in GIC stemness and chemosensitivity will inform the development of better targeted therapies in GBM and possibly other cancers. Stem Cells 2018;36:1806-12.

The critical role that STAT3 plays in glioma-initiating cells: STAT3 addiction in glioma

Glioma-Initiating Cells (GICs) are thought to be responsible for tumor initiation, progression and recurrence in glioblastoma (GBM). In previous studies, we reported the constitutive phosphorylation of the STAT3 transcription factor in GICs derived from GBM patient-derived xenografts, and that STAT3 played a critical role in GBM tumorigenesis. In this study, we show that CRISPR/Cas9-mediated deletion of STAT3 in an established GBM cell line markedly inhibited tumorigenesis by intracranial injection but had little effect on cell proliferation in vitro. Tumorigenesis was rescued by the enforced expression of wild-type STAT3 in cells lacking STAT3. In contrast, GICs were highly addicted to STAT3 and upon STAT3 deletion GICs were non-viable. Moreover, we found that STAT3 was constitutively activated in GICs by phosphorylation on both tyrosine (Y705) and serine (S727) residues. Therefore, to study STAT3 function in GICs we established an inducible system to knockdown STAT3 expression (iSTAT3-KD). Using this approach, we demonstrated that Y705-STAT3 phosphorylation was critical and indispensable for GIC-induced tumor formation. Both phosphorylation sites in STAT3 promoted GIC proliferation in vitro. We further showed that S727-STAT3 phosphorylation was Y705-dependent. Targeted microarray and RNA sequencing revealed that STAT3 activated cell-cycle regulator genes, and downregulated genes involved in the interferon response, the hypoxia response, the TGFβ pathway, and remodeling of the extracellular matrix. Since STAT3 is an important oncogenic driver of GBM, the identification of these STAT3 regulated pathways in GICs will inform the development of better targeted therapies against STAT3 in GBM and other cancers.

The integrative metabolomic-transcriptomic landscape of glioblastome multiforme

The purpose of this study was to map the landscape of metabolic-transcriptional alterations in glioblastoma multiforme. Omic-datasets were acquired by metabolic profiling (1D-NMR spectroscopy n=33 Patient) and transcriptomic profiling (n=48 Patients). Both datasets were analyzed by integrative network modeling. The computed model concluded in four different metabolic-transcriptomic signatures containing: oligodendrocytic differentiation, cell-cycle functions, immune response and hypoxia. These clusters were found being distinguished by individual metabolism and distinct transcriptional programs. The study highlighted the association between metabolism and hallmarks of oncogenic signaling such as cell-cycle alterations, immune escape mechanism and other cancer pathway alterations. In conclusion, this study showed the strong influence of metabolic alterations in the wide scope of oncogenic transcriptional alterations.

Addressing intra-tumoral heterogeneity and therapy resistance

In the last several years, our appreciation of intra-tumoral heterogeneity has greatly increased due to accumulating evidence for the co-existence of genetically and epigenetically divergent cancer cells residing in different microenvironments within a tumor. Herein, we review recent literature discussing intra-tumoral heterogeneity in the context of therapy resistance mechanisms at the genetic, epigenetic and microenvironmental levels. We illustrate the influence of tumor microenvironment on therapy resistance and epigenetic states of cancer cells by highlighting the role of cancer stem cells in therapy resistance. We also summarize different strategies that have been employed to address various resistance mechanisms at genetic, epigenetic, and microenvironmental levels in preclinical and clinical studies. We propose that future personalized cancer therapy design needs to incorporate dynamic and comprehensive analyses of tumor heterogeneity landscape and multi-dimensional mechanisms of therapy resistance.

MiRNA203 suppresses the expression of protumorigenic STAT1 in glioblastoma to inhibit tumorigenesis

MicroRNAs (miRNAs) play critical roles in regulating cancer cell proliferation, migration, survival and sensitivity to chemotherapy. The potential application of using miRNAs for cancer prognosis holds great promise but miRNAs with predictive value remain to be identified and underlying mechanisms of how they promote or suppress tumorigenesis are not completely understood. Here, we show a strong correlation between miR203 expression and brain cancer patient survival. Low miR203 expression is found in subsets of brain cancer patients, especially glioblastoma. Ectopic miR203 expression in glioblastoma cell lines inhibited cell proliferation and migration, increased sensitivity to apoptosis induced by interferon or temozolomide in vitro, and inhibited tumorigenesis in vivo. We further show that STAT1 is a direct functional target of miR203, and miR203 level is negatively correlated with STAT1 expression in brain cancer patients. Knockdown of STAT1 expression mimicked the effect of overexpression of miR203 in glioblastoma cell lines, and inhibited cell proliferation and migration, increased sensitivity to apoptosis induced by IFN or temozolomide in vitro, and inhibited glioblastoma tumorigenesis in vivo. High STAT1 expression significantly correlated with poor survival in brain cancer patients. Mechanistically, we found that enforced miR203 expression in glioblastoma suppressed STAT1 expression directly, as well as that of a number of STAT1 regulated genes. Taken together, our data suggest that miR203 acts as a tumor suppressor in glioblastoma by suppressing the pro-tumorigenic action of STAT1. MiR203 may serve as a predictive biomarker and potential therapeutic target in subsets of cancer patients with low miR203 expression.

Utility of Glioblastoma Patient-Derived Orthotopic Xenografts in Drug Discovery and Personalized Therapy

Despite substantial effort and resources dedicated to drug discovery and development, new anticancer agents often fail in clinical trials. Among many reasons, the lack of reliable predictive preclinical cancer models is a fundamental one. For decades, immortalized cancer cell cultures have been used to lay the groundwork for cancer biology and the quest for therapeutic responses. However, cell lines do not usually recapitulate cancer heterogeneity or reveal therapeutic resistance cues. With the rapidly evolving exploration of cancer “omics,” the scientific community is increasingly investigating whether the employment of short-term patient-derived tumor cell cultures (two- and three-dimensional) and/or patient-derived xenograft models might provide a more representative delineation of the cancer core and its therapeutic response. Patient-derived cancer models allow the integration of genomic with drug sensitivity data on a personalized basis and currently represent the ultimate approach for preclinical drug development and biomarker discovery. The proper use of these patient-derived cancer models might soon influence clinical outcomes and allow the implementation of tailored personalized therapy. When assessing drug efficacy for the treatment of glioblastoma multiforme (GBM), currently, the most reliable models are generated through direct injection of patient-derived cells or more frequently the isolation of glioblastoma cells endowed with stem-like features and orthotopically injecting these cells into the cerebrum of immunodeficient mice. Herein, we present the key strengths, weaknesses, and potential applications of cell- and animal-based models of GBM, highlighting our experience with the glioblastoma stem-like patient cell-derived xenograft model and its utility in drug discovery.

MicroRNA203a suppresses glioma tumorigenesis through an ATM-dependent interferon response pathway

Glioblastoma (GBM) is a deadly and incurable brain tumor. Although microRNAs (miRNAs) play critical roles in regulating the cancer cell phenotype, the underlying mechanisms of how they regulate tumorigenesis are incompletely understood. We previously showed that miR-203a is expressed at relatively low levels in GBM patients, and ectopic miR-203a expression in GBM cell lines inhibited cell proliferation and migration, increased sensitivity to apoptosis induced by interferon (IFN) or temozolomide in vitro, and inhibited GBM tumorigenesis in vivo. Here we show that ectopic expression of miR-203a in GBM cell lines promotes the IFN response pathway as evidenced by increased IFN production and IFN-stimulated gene (ISG) expression, and high basal tyrosine phosphorylation of multiple STAT proteins. Importantly, we identified that miR-203a directly suppressed the protein levels of ataxia-telangiectasia mutated (ATM) kinase that negatively regulates IFN production. We found that high ATM expression in GBM correlates with poor patient survival and that ATM expression is inversely correlated with miR-203a expression. Knockout of ATM expression and inhibition of ATM function in GBM cell lines inhibited cell proliferation and migration, increased sensitivity to apoptosis induced by therapeutic agents in vitro, and markedly suppressed GBM tumor growth and promoted animal survival. In contrast, restoring ATM levels in GBM cells ectopically expressing miR-203a increased tumorigenicity and decreased animal survival. Our study suggests that low miR-203a expression in GBM suppresses the interferon response through an ATM-dependent pathway.

Comparative Gene Expression Profiling of Primary and Metastatic Renal Cell Carcinoma Stem Cell-Like Cancer Cells

Background

Recent advancement in cancer research has shown that tumors are highly heterogeneous, and multiple phenotypically different cell populations are found in a single tumor. Cancer development and tumor growth are driven by specific types of cells—stem cell-like cancer cells (SCLCCs)—which are also responsible for metastatic spread and drug resistance. This research was designed to verify the presence of SCLCCs in renal cell cancer cell lines. Subsequently, we aimed to characterize phenotype and cell biology of CD105+ cells, defined previously as renal cell carcinoma tumor-initiating cells. The main goal of the project was to describe the gene-expression profile of stem cell-like cancer cells of primary tumor and metastatic origin.

Materials and Methods

Real-time PCR analysis of stemness genes (Oct-4, Nanog and Ncam) and soft agar colony formation assay were conducted to check the stemness properties of renal cell carcinoma (RCC) cell lines. FACS analysis of CD105+ and CD133+ cells was performed on RCC cells. Isolated CD105+ cells were verified for expression of mesenchymal markers—CD24, CD146, CD90, CD73, CD44, CD11b, CD19, CD34, CD45, HLA-DR and alkaline phosphatase. Hanging drop assay was used to investigate CD105+ cell-cell cohesion. Analysis of free-floating 3D spheres formed by isolated CD105+ was verified, as spheres have been hypothesized to contain undifferentiated multipotent progenitor cells. Finally, CD105+ cells were sorted from primary (Caki-2) and metastatic (ACHN) renal cell cancer cell lines. Gene-expression profiling of sorted CD105+ cells was performed with Agilent’s human GE 4x44K v2 microarrays. Differentially expressed genes were further categorized into canonical pathways. Network analysis and downstream analysis were performed with Ingenuity Pathway Analysis.

Results

Metastatic RCC cell lines (ACHN and Caki-1) demonstrated higher colony-forming ability in comparison to primary RCC cell lines. Metastatic RCC cell lines harbor numerous CD105+ cell subpopulations and have higher expression of stemness genes (Oct-4 and Nanog). CD105+ cells adopt 3D grape-like floating structures under handing drop conditions. Sorted CD105+ cells are positive for human mesenchymal stem cell (MSC) markers CD90, CD73, CD44, CD146, and alkaline phosphatase activity, but not for CD24 and hematopoietic lineage markers CD34, CD11b, CD19, CD45, and HLA-DR. 1411 genes are commonly differentially expressed in CD105+ cells (both from primary [Caki-2] and metastatic RCC [ACHN] cells) in comparison to a healthy kidney epithelial cell line (ASE-5063). TGF-β, Wnt/β-catenine, epithelial-mesenchymal transition (EMT), Rap1 signaling, PI3K-Akt signaling, and Hippo signaling pathway are deregulated in CD105+ cells. TGFB1, ERBB2, and TNF are the most significant transcriptional regulators activated in these cells.

Conclusions

All together, RCC-CD105+ cells present stemlike properties. These stem cell-like cancer cells may represent a novel target for therapy. A unique gene-expression profile of CD105+ cells could be used as initial data for subsequent functional studies and drug design.

Cancer stem cell-vascular endothelial cell interactions in glioblastoma

Glioblastoma (GBM), a higher grade glial tumor, is highly aggressive, therapy resistant and often shows poor patient prognosis due to frequent recurrence. These features of GBM are attributed to presence of a significantly smaller proportion of glioma stem cells (GSCs) that are endowed with self-renewal ability, multi-potent nature and show resistance to therapy in patients. GSCs preferably take shelter close to tumor vasculature due to paracrine need of soluble factors secreted by endothelial cells (ECs) of vasculature. The physical proximity of GSCs to ECs creates a localized perivascular niche where mutual GSC-EC interactions regulate GSC stemness, migration, therapy resistance, and cellular kinetics during tumor growth. Together, perivascular niche presents a therapeutically targetable tumor structure for clinical management of GBM. Thus, understanding cellular and non-cellular components in perivascular niche is vital for designing in vitro and in vivo GBM tumor models. Here, we discuss the components and structure of tumor vascular niche and its impact on tumor progression.

Emerging targets for glioblastoma stem cell therapy

Glioblastoma multiforme (GBM), designated as World Health Organization (WHO) grade IV astrocytoma, is a lethal and therapy-resistant brain cancer comprised of several tumor cell subpopulations, including GBM stem cells (GSCs) which are believed to contribute to tumor recurrence following initial response to therapies. Emerging evidence demonstrates that GBM tumors are initiated from GSCs. The development and use of novel therapies including small molecule inhibitors of specific proteins in signaling pathways that regulate stemness, proliferation and migration of GSCs, immunotherapy, and non-coding microRNAs may provide better means of treating GBM. Identification and characterization of GSC-specific signaling pathways would be necessary to identify specific therapeutic targets which may lead to the development of more efficient therapies selectively targeting GSCs. Several signaling pathways including mTOR, AKT, maternal embryonic leucine zipper kinase (MELK), NOTCH1 and Wnt/β-catenin as well as expression of cancer stem cell markers CD133, CD44, Oct4, Sox2, Nanog, and ALDH1A1 maintain GSC properties. Moreover, the data published in the Cancer Genome Atlas (TCGA) specifically demonstrated the activated PI3K/AKT/mTOR pathway in GBM tumorigenesis. Studying such pathways may help to understand GSC biology and lead to the development of potential therapeutic interventions to render them more sensitive to chemotherapy and radiation therapy. Furthemore, recent demonstration of dedifferentiation of GBM cell lines into CSC-like cells prove that any successful therapeutic agent or combination of drugs for GBM therapy must eliminate not only GSCs, but the differentiated GBM cells and the entire bulk of tumor cells.