Glioblastoma: Microenvironment and Niche Concept

Theranostic Design of Angiopep-2 Conjugated Hyaluronic Acid Nanoparticles (Thera-ANG-cHANPs) for Dual Targeting and Boosted Imaging of Glioma Cells

Simple Summary

Glioblastoma multiforme is the most aggressive malignant brain tumor with poor patient prognosis. The presence of the blood-brain barrier and the complex tumor microenvironment impair the efficient accumulation of drugs and contrast agents, causing late diagnosis, inefficient treatment and monitoring. Functionalized theranostic nanoparticles are a valuable tool to modulate biodistribution of active agents, promoting their active delivery and selective accumulation for an earlier diagnosis and effective treatment, and provide simultaneous therapy and imaging for improved evaluation of treatment efficacy. In this work, we developed angiopep-2 functionalized crosslinked hyaluronic acid nanoparticles encapsulating gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) and irinotecan (Thera-ANG-cHANPs) that were shown to boost relaxometric properties of Gd-DTPA by the effect of Hydrodenticity, improve the uptake of nanoparticles by the exploitation of angiopep-2 improved transport properties, and accelerate the therapeutic effect of Irinotecan.

Abstract

Glioblastoma multiforme (GBM) has a mean survival of only 15 months. Tumour heterogeneity and blood-brain barrier (BBB) mainly hinder the transport of active agents, leading to late diagnosis, ineffective therapy and inaccurate follow-up. The use of hydrogel nanoparticles, particularly hyaluronic acid as naturally occurring polymer of the extracellular matrix (ECM), has great potential in improving the transport of drug molecules and, furthermore, in facilitatating the early diagnosis by the effect of hydrodenticity enabling the T₁ boosting of Gadolinium chelates for MRI. Here, crosslinked hyaluronic acid nanoparticles encapsulating gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) and the chemotherapeutic agent irinotecan (Thera-cHANPs) are proposed as theranostic nanovectors, with improved MRI capacities. Irinotecan was selected since currently repurposed as an alternative compound to the poorly effective temozolomide (TMZ), generally approved as the gold standard in GBM clinical care. Also, active crossing and targeting are achieved by theranostic cHANPs decorated with angiopep-2 (Thera-ANG-cHANPs), a dual-targeting peptide interacting with low density lipoprotein receptor related protein-1(LRP-1) receptors overexpressed by both endothelial cells of the BBB and glioma cells. Results showed preserving the hydrodenticity effect in the advanced formulation and internalization by the active peptide-mediated uptake of Thera-cHANPs in U87 and GS-102 cells. Moreover, Thera-ANG-cHANPs proved to reduce ironotecan time response, showing a significant cytotoxic effect in 24 h instead of 48 h.

Drug Resistance in Glioblastoma: The Two Faces of Oxidative Stress

Glioblastomas (GBM) are the most common primary brain tumor with a median survival of 15 months. A population of cells with stem cell properties (glioblastoma stem cells, GSCs) drives the initiation and progression of GBM and is localized in specialized microenvironments which support their behavior. GBM are characterized as extremely resistant to therapy, resulting in tumor recurrence. Reactive oxygen species (ROS) control the cellular stability by influencing different signaling pathways. Normally, redox systems prevent cell oxidative damage; however, in gliomagenesis, the cellular redox mechanisms are highly impaired. Herein we review the dual nature of the redox status in drug resistance. ROS generation in tumor cells affects the cell cycle and is involved in tumor progression and drug resistance in GBM. However, excess ROS production has been found to induce cell death programs such as apoptosis and autophagy. Since GBM cells have a high metabolic rate and produce high levels of ROS, metabolic adaptation in these cells plays an essential role in resistance to oxidative stress-induced cell death. Finally, the microenvironment with the stromal components participates in the enhancement of the oxidative stress to promote tumor progression and drug resistance.

The Subventricular Zone, a Hideout for Adult and Pediatric High-Grade Glioma Stem Cells

Both in adult and children, high-grade gliomas (WHO grades III and IV) account for a high proportion of death due to cancer. This poor prognosis is a direct consequence of tumor recurrences occurring within few months despite a multimodal therapy consisting of a surgical resection followed by chemotherapy and radiotherapy. There is increasing evidence that glioma stem cells (GSCs) contribute to tumor recurrences. In fact, GSCs can migrate out of the tumor mass and reach the subventricular zone (SVZ), a neurogenic niche persisting after birth. Once nested in the SVZ, GSCs can escape a surgical intervention and resist to treatments. The present review will define GSCs and describe their similarities with neural stem cells, residents of the SVZ. The architectural organization of the SVZ will be described both for humans and rodents. The migratory routes taken by GSCs to reach the SVZ and the signaling pathways involved in their migration will also be described hereafter. In addition, we will debate the advantages of the microenvironment provided by the SVZ for GSCs and how this could contribute to tumor recurrences. Finally, we will discuss the clinical relevance of the SVZ in adult GBM and pediatric HGG and the therapeutic advantages of targeting that neurogenic region in both clinical situations.

The impact of initial tumor microenvironment on imaging phenotype

Models of human cancer, to be useful, must replicate human disease with high fidelity. Our focus in this study is rat xenograft brain tumors as a model of human embedded cerebral tumors. A distinguishing signature of such tumors in humans, that of contrast-enhancement on imaging, is often not present when the human cells grow in rodents, despite the xenografts having nearly identical DNA signatures to the original tumor specimen. Although contrast enhancement was uniformly evident in all the human tumors from which the xenografts’ cells were derived, we show that long-term contrast enhancement in the model tumors may be determined conditionally by the tumor microenvironment at the time of cell implantation. We demonstrate this phenomenon in one of two patient-derived orthotopic xenograft (PDOX) models using cancer stem-like cell (CSC)-enriched neurospheres from human tumor resection specimens, transplanted to groups of immune-compromised rats in the presence or absence of a collagen/fibrin scaffolding matrix, Matrigel. The rats were imaged by dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and their brains were examined by histopathology. Targeted proteomics of the PDOX tumor specimens grown from CSC implanted with and without Matrigel showed that while the levels of the majority of proteins and post-translational modifications were comparable between contrast-enhancing and non-enhancing tumors, phosphorylation of Fox038 showed a differential expression. The results suggest key proteins determine contrast enhancement and suggest a path toward the development of better animal models of human glioma. Future work is needed to elucidate fully the molecular determinants of contrast-enhancement.

Glioblastoma-initiating cell heterogeneity generated by the cell-of-origin, genetic/epigenetic mutation and microenvironment

Glioblastoma (GBM) and other malignant tumours consist of heterogeneous cancer cells, including GBM-initiating cells (GICs). This heterogeneity is likely to arise from the following: different sets of genetic mutations and epigenetic modifications, which GICs gain in the transformation process; differences in cells of origin, such as stem cells, precursor cells or differentiated cells; and the cancer microenvironment, in which GICs communicate with neural cells, endothelial cells and immune cells. Furthermore, considering that various types of GICs can be generated at different time points of the transformation process, GBM very likely consists of heterogeneous GICs and their progeny. Because cancer cell heterogeneity is responsible for therapy resistance, it is crucial to develop methods of reducing such heterogeneity. Here, I summarize how GIC heterogeneity is generated in the transformation process and present how cell heterogeneity in cancer can be addressed based on recent findings.

Dopamine induces functional extracellular traps in microglia

Dopamine (DA) plays many roles in the brain, especially in movement, motivation, and reinforcement of behavior; however, its role in regulating innate immunity is not clear. Here, we show that DA can induce DNA-based extracellular traps in primary, adult, human microglia and BV2 microglia cell line. These DNA-based extracellular traps are formed independent of reactive oxygen species, actin polymerization, and cell death. These traps are functional and capture fluorescein (FITC)-tagged Escherichia coli even when reactive oxygen species production or actin polymerization is inhibited. We show that microglial extracellular traps are present in Glioblastoma multiforme. This is crucial because Glioblastoma multiforme cells are known to secrete DA. Our findings demonstrate that DA plays a significant role in sterile neuro-inflammation by inducing microglia extracellular traps.

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Dopamine induces ETs in BV2 microglia and primary adult human microglia

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Induced traps are independent of ROS, cell death, and actin polymerization

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Microglia ETs are functional and can trap E. coli

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Microglia ETs are also present in Glioblastoma multiforme

Effect of conditioned medium from neural stem cells on glioma progression and its protein expression profile analysis

BACKGROUND

Emerging evidence suggests that the spread of glioma to the subventricular zone (SVZ) is closely related to glioma recurrence and patient survival. Neural stem cells (NSCs) are the main cell type in the SVZ region and exhibit tumor-homing ability.

AIM

To evaluate the effects of conditioned medium (CM) derived from SVZ NSCs on the cancer-related behaviors of glioma cells.

METHODS

The characteristics of SVZ hNSCs were identified by immunofluorescence. The normoxic-hNSC-CM and hypoxic-hNSC-CM (3% O₂, oxygen-glucose deprived [OGD] culturing) were collected from 80%-90% confluent SVZ NSCs in sterile conditions. The CCK8 and Transwell assays were used to compare and evaluate the effects of normoxic-CM and hypoxic-CM on glioma proliferation and invasion. Then proteins secreted from SVZ NSCs into the CM were investigated by mass spectrometry, and the potential effects of candidate protein NCAN in the regulation of glioma progression were examined by CCK8 and Transwell assays.

RESULTS

The CM from SVZ NSCs significantly increased the proliferation and invasion of glioma cells, particularly the CM from OGD NSCs induced under hypoxic conditions. Furthermore, the secreted protein neurocan (NCAN) in CM from OGD NSCs was identified by proteomic analysis. NCAN was expressed in glioma cells and played regulatory roles in mediating the progression of glioma cells mainly via the Rho/Rho-associated protein kinase pathway.

CONCLUSION

Our study identified a potential interactive mechanism between SVZ NSCs and glioma cells, in which SVZ NSCs promote glioma progression via the secreted protein NCAN. These findings suggested that exploring the CM derived from cells could be a novel strategy for optimizing treatments and that NCAN derived from SVZ NSCs may be a potential new target in glioma progression.

The Molecular and Microenvironmental Landscape of Glioblastomas: Implications for the Novel Treatment Choices

Glioblastoma (GBM) is the most frequent and aggressive primary central nervous system tumor. Surgery followed by radiotherapy and chemotherapy with alkylating agents constitutes standard first-line treatment of GBM. Complete resection of the GBM tumors is generally not possible given its high invasive features. Although this combination therapy can prolong survival, the prognosis is still poor due to several factors including chemoresistance. In recent years, a comprehensive characterization of the GBM-associated molecular signature has been performed. This has allowed the possibility to introduce a more personalized therapeutic approach for GBM, in which novel targeted therapies, including those employing tyrosine kinase inhibitors (TKIs), could be employed. The GBM tumor microenvironment (TME) exerts a key role in GBM tumor progression, in particular by providing an immunosuppressive state with low numbers of tumor-infiltrating lymphocytes (TILs) and other immune effector cell types that contributes to tumor proliferation and growth. The use of immune checkpoint inhibitors (ICIs) has been successfully introduced in numerous advanced cancers as well as promising results have been shown for the use of these antibodies in untreated brain metastases from melanoma and from non-small cell lung carcinoma (NSCLC). Consequently, the use of PD-1/PD-L1 inhibitors has also been proposed in several clinical trials for the treatment of GBM. In the present review, we will outline the main GBM molecular and TME aspects providing also the grounds for novel targeted therapies and immunotherapies using ICIs for GBM.

High Adenosine Extracellular Levels Induce Glioblastoma Aggressive Traits Modulating the Mesenchymal Stromal Cell Secretome

Glioblastoma is an aggressive, fast-growing brain tumor influenced by the composition of the tumor microenvironment (TME) in which mesenchymal stromal cell (MSCs) play a pivotal role. Adenosine (ADO), a purinergic signal molecule, can reach up to high micromolar concentrations in TME. The activity of specific adenosine receptor subtypes on glioma cells has been widely explored, as have the effects of MSCs on tumor progression. However, the effects of high levels of ADO on glioma aggressive traits are still unclear as is its role in cancer cells-MSC cross-talk. Herein, we first studied the role of extracellular Adenosine (ADO) on isolated human U343MG cells as a glioblastoma cellular model, finding that at high concentrations it was able to prompt the gene expression of Snail and ZEB1, which regulate the epithelial-mesenchymal transition (EMT) process, even if a complete transition was not reached. These effects were mediated by the induction of ERK1/2 phosphorylation. Additionally, ADO affected isolated bone marrow derived MSCs (BM-MSCs) by modifying the pattern of secreted inflammatory cytokines. Then, the conditioned medium (CM) of BM-MSCs stimulated with ADO and a co-culture system were used to investigate the role of extracellular ADO in GBM-MSC cross-talk. The CM promoted the increase of glioma motility and induced a partial phenotypic change of glioblastoma cells. These effects were maintained when U343MG cells and BM-MSCs were co-cultured. In conclusion, ADO may affect glioma biology directly and through the modulation of the paracrine factors released by MSCs overall promoting a more aggressive phenotype. These results point out the importance to deeply investigate the role of extracellular soluble factors in the glioma cross-talk with other cell types of the TME to better understand its pathological mechanisms.

SETDB1 promotes glioblastoma growth via CSF-1-dependent macrophage recruitment by activating the AKT/mTOR signaling pathway

Background

Glioblastoma is a common disease of the central nervous system (CNS), with high morbidity and mortality. In the infiltrate in the tumor microenvironment, tumor-associated macrophages (TAMs) are abundant, which are important factors in glioblastoma progression. However, the exact details of TAMs in glioblastoma progression have yet to be determined.

Methods

The clinical relevance of SET domain bifurcated 1 (SETDB1) was analyzed by immunohistochemistry, real-time PCR and Western blotting of glioblastoma tissues. SETDB1-induced cell proliferation, migration and invasion were investigated by CCK-8 assay, colony formation assay, wound healing and Transwell assay. The relationship between SETDB1 and colony stimulating factor 1 (CSF-1), as well as TAMs recruitment was examined by Western blotting, real-time PCR and syngeneic mouse model.

Results

Our findings showed that SETDB1 upregulated in glioblastoma and relative to poor progression. Gain and loss of function approaches showed the SETDB1 overexpression promotes cell proliferation, migration and invasion in glioblastoma cells. However, knockdown SETDB1 exerted opposite effects in vitro. Moreover, SETDB1 promotes AKT/mTOR-dependent CSF-1 induction and secretion, which leads to macrophage recruitment in the tumor, resulted in tumor growth.

Conclusion

Our research clarified that SETDB1 regulates of tumor microenvironment and hence presents a potential therapeutic target for treating glioblastoma.

Prognostic value and immune cell infiltration of hypoxic phenotype-related gene signatures in glioblastoma microenvironment

Glioblastoma (GBM) is a malignant intracranial tumour with the highest proportion and lethality. It is characterized by invasiveness and heterogeneity. However, the currently available therapies are not curative. As an essential environmental cue that maintains glioma stem cells, hypoxia is considered the cause of tumour resistance to chemotherapy and radiation. Growing evidence shows that immunotherapy focusing on the tumour microenvironment is an effective treatment for GBM; however, the current clinicopathological features cannot predict the response to immunotherapy and provide accurate guidance for immunotherapy. Based on the ESTIMATE algorithm, GBM cases of The Cancer Genome Atlas (TCGA) data set were classified into high- and low-immune/stromal score groups, and a four-gene tumour environment-related model was constructed. This model exhibited good efficiency at forecasting short- and long-term prognosis and could also act as an independent prognostic biomarker. Additionally, this model and four of its genes (CLECL5A, SERPING1, CHI3L1 and C1R) were found to be associated with immune cell infiltration, and further study demonstrated that these four genes might drive the hypoxic phenotype of perinecrotic GBM, which affects hypoxia-induced glioma stemness. Therefore, these might be important candidates for immunotherapy of GBM and deserve further exploration.

β-Elemene Inhibits the Proliferation and Migration of Human Glioblastoma Cell Lines via Suppressing Ring Finger Protein 135

β-Elemene is commonly used as an anti-cancer agent in different types of cancers and its effects on glioblastoma have been studied through different pathways. However, its effect through ring finger protein 135 (RNF135, OMIM 611358) (RNF135), which is upregulated in glioblastomas, has not yet been explored. The current study is focused on the effects of β-elemene on human glioblastoma cell lines U251, U118, A172 and U87 through RNF13 5. A cell counting kit-8 assay and wound healing assay have been utilized to test the proliferation and migration of the cells. Western blot and quantitative real-time-polymerase chain reaction (qRT-PCR) were used to evaluate the level of expression of RNF135. A model of nude mice was used to explore progression of the tumor in vivo. It was observed that increasing treatment time or dose of β-elemene remarkably decreased viability of the cells. The cells that were treated with β-elemene had a much lower speed of moving toward the gap in comparison to untreated cell lines. β-Elemene-treated cells showed a much lower level of expression of RNF135 mRNA than control groups (p <0.05) and the levels of RNF135 protein were lower in the cells treated with β-elemene than in control groups (p <0.05). Moreover, tumor progression in subcutaneous xenograft nude mice was delayed with the injection of β-elemene. Altogether, our findings suggest that β-elemene inhibits proliferation, migration and tumorigenicity of human glioblastoma cells through suppressing RNF135.

Single-Cell Mechanophenotyping in Microfluidics to Evaluate Behavior of U87 Glioma Cells

Integration of microfabricated, single-cell resolution and traditional, population-level biological assays will be the future of modern techniques in biology that will enroll in the evolution of biology into a precision scientific discipline. In this study, we developed a microfabricated cell culture platform to investigate the indirect influence of macrophages on glioma cell behavior. We quantified proliferation, morphology, motility, migration, and deformation properties of glioma cells at single-cell level and compared these results with population-level data. Our results showed that glioma cells obtained slightly slower proliferation, higher motility, and extremely significant deformation capability when cultured with 50% regular growth medium and 50% macrophage-depleted medium. When the expression levels of E-cadherin and Vimentin proteins were measured, it was verified that observed mechanophenotypic alterations in glioma cells were not due to epithelium to mesenchymal transition. Our results were consistent with previously reported enormous heterogeneity of U87 glioma cell line. Herein, for the first time, we quantified the change of deformation indexes of U87 glioma cells using microfluidic devices for single-cells analysis.

New hints towards a precision medicine strategy for IDH wild-type glioblastoma

Glioblastoma represents the most common primary malignancy of the central nervous system in adults and remains a largely incurable disease. The elucidation of disease subtypes based on mutational profiling, gene expression and DNA methylation has so far failed to translate into improved clinical outcomes. However, new knowledge emerging from the subtyping effort in the IDH-wild-type setting may provide directions for future precision therapies. Here, we review recent learnings in the field, and further consider how tumour microenvironment differences across subtypes may reveal novel contexts of vulnerability. We discuss recent treatment approaches and ongoing trials in the IDH-wild-type glioblastoma setting, and propose an integrated discovery stratagem incorporating multi-omics, single-cell technologies and computational approaches.

Frequency of mismatch repair protein deficiency and PD-L1 in high-grade gliomas in adolescents and young adults (AYA)

Central nervous system tumors in adolescents and young adults (AYA) are rarely reported in the literature. The association with cancer predisposition syndrome is not established. Programmed death ligand 1 (PD-L1) can predict the potential response of patients to immunotherapy. A link between mismatch repair protein deficiency (MMRP-D) and response to immunotherapy is established. P53 is reported to be positive in MMRD-D cases. We aim to investigate the frequency of MMRP-D in AYA with high-grade glioma and any potential association with PD-L1. A total of 96 cases were tested including 49 (51.0%) cases of glioblastoma. Six cases (6.25%) were MMRP-D, 17 (17.7%) were PD-L1 positive, mostly in grade IV tumors (8.7% in grade III compared to 26% in grade IV, p value = 0.027), and 69 (71.9%) were P3 positive. None of the MMRP-D cases expressed PD-L1. P53-positive cases were mostly MMRP proficient (n = 67; 74.4%, p value 0.051). Fourteen cases (28.7%) were positive for both PD-L1 and P53, while p53-positive grade IV tumors were mostly associated with negative PD-L1 (n = 29, 58%, p value = 0.043). MMRP deficiency does not appear to be prevalent in high-grade glioma in AYA. Expression of PD-L1 in a quarter of cases might suggest a role for immunotherapy in high-grade glioma.

Radioresistance in Glioblastoma and the Development of Radiosensitizers

Ionizing radiation is a common and effective therapeutic option for the treatment of glioblastoma (GBM). Unfortunately, some GBMs are relatively radioresistant and patients have worse outcomes after radiation treatment. The mechanisms underlying intrinsic radioresistance in GBM has been rigorously investigated over the past several years, but the complex interaction of the cellular molecules and signaling pathways involved in radioresistance remains incompletely defined. A clinically effective radiosensitizer that overcomes radioresistance has yet to be identified. In this review, we discuss the current status of radiation treatment in GBM, including advances in imaging techniques that have facilitated more accurate diagnosis, and the identified mechanisms of GBM radioresistance. In addition, we provide a summary of the candidate GBM radiosensitizers being investigated, including an update of subjects enrolled in clinical trials. Overall, this review highlights the importance of understanding the mechanisms of GBM radioresistance to facilitate the development of effective radiosensitizers.

Transcriptional regulatory networks of tumor-associated macrophages that drive malignancy in mesenchymal glioblastoma

BACKGROUND

Glioblastoma (GBM) is a complex disease with extensive molecular and transcriptional heterogeneity. GBM can be subcategorized into four distinct subtypes; tumors that shift towards the mesenchymal phenotype upon recurrence are generally associated with treatment resistance, unfavorable prognosis, and the infiltration of pro-tumorigenic macrophages.

RESULTS

We explore the transcriptional regulatory networks of mesenchymal-associated tumor-associated macrophages (MA-TAMs), which drive the malignant phenotypic state of GBM, and identify macrophage receptor with collagenous structure (MARCO) as the most highly differentially expressed gene. MARCO^high TAMs induce a phenotypic shift towards mesenchymal cellular state of glioma stem cells, promoting both invasive and proliferative activities, as well as therapeutic resistance to irradiation. MARCO^high TAMs also significantly accelerate tumor engraftment and growth in vivo. Moreover, both MA-TAM master regulators and their target genes are significantly correlated with poor clinical outcomes and are often associated with genomic aberrations in neurofibromin 1 (NF1) and phosphoinositide 3-kinases/mammalian target of rapamycin/Akt pathway (PI3K-mTOR-AKT)-related genes. We further demonstrate the origination of MA-TAMs from peripheral blood, as well as their potential association with tumor-induced polarization states and immunosuppressive environments.

CONCLUSIONS

Collectively, our study characterizes the global transcriptional profile of TAMs driving mesenchymal GBM pathogenesis, providing potential therapeutic targets for improving the effectiveness of GBM immunotherapy.

Novel approaches to combat chemoresistance against glioblastomas

The poor prognosis of glioblastoma multiforme (GBM) patients is in part due to resistance to current standard-of-care treatments including chemotherapy [predominantly temozolomide (TMZ; Temodar)], radiation therapy and an anti-angiogenic therapy [an antibody against the vascular endothelial growth factor (bevacizumab; Avastin)], resulting in recurrent tumors. Several recurrent GBM tumors are commonly resistant to either TMZ, radiation or bevacizumab, which contributes to the low survival rate for GBM patients. This review will focus on novel targets and therapeutic approaches that are currently being considered to combat GBM chemoresistance. One of these therapeutic options is a small molecule called OKlahoma Nitrone 007 (OKN-007), which was discovered to inhibit the transforming growth factor β1 pathway, reduce TMZ-resistance and enhance TMZ-sensitivity. OKN-007 is currently an investigational new drug in clinical trials for both newly-diagnosed and recurrent GBM patients. Another novel target is ELTD1 (epidermal growth factor, latrophilin and seven transmembrane domain-containing protein 1; alternatively known as ADGRL4, Adhesion G protein-coupled receptor L4), which we used a monoclonal antibody against, where a therapy against it was found to inhibit Notch 1 in a pre-clinical GBM xenograft model. Notch 1 is known to be associated with chemoresistance in GBM. Other potential therapeutic targets to combat GBM chemoresistance include the phosphoinositide 3-kinase pathway, nuclear factor-κB, the hepatocyte/scatter factor (c-MET), the epidermal growth factor receptor, and the tumor microenvironment.

Modulating microenvironments for treating glioblastoma

Purpose of review

This review focuses on the development and progression of glioblastoma through the brain and glioma microenvironment. Specifically we highlight how the tumor microenvironment contributes to the hallmarks of cancer in hopes of offering novel therapeutic options and tools to target this microenvironment.

Recent findings

The hallmarks of cancer, which represent elements of cancers that contribute to the disease's malignancy, yet elements within the brain tumor microenvironment, such as other cellular types as well as biochemical and biophysical cues that can each uniquely affect tumor cells, have not been well-described in this context and serve as potential targets for modulation.

Summary

Here, we highlight how the brain tumor microenvironment contributes to the progression and therapeutic response of tumor cells. Specifically, we examine these contributions through the lens of Hanahan & Weinberg's Hallmarks of Cancer in order to identify potential novel targets within the brain that may offer a means to treat brain cancers, including the deadliest brain cancer, glioblastoma.

Collective invasion of glioma cells through OCT1 signalling and interaction with reactive astrocytes after surgery

Glioblastoma multiforme (GBM) is the most aggressive form of brain cancer with a short median survival time. GBM is characterized by the hallmarks of aggressive proliferation and cellular infiltration of normal brain tissue. miR-451 and its downstream molecules are known to play a pivotal role in regulation of the balance of proliferation and aggressive invasion in response to metabolic stress in the tumour microenvironment (TME). Surgery-induced transition in reactive astrocyte populations can play a significant role in tumour dynamics. In this work, we develop a multi-scale mathematical model of miR-451-LKB1-AMPK-OCT1-mTOR pathway signalling and individual cell dynamics of the tumour and reactive astrocytes after surgery. We show how the effects of fluctuating glucose on tumour cells need to be reprogrammed by taking into account the recent history of glucose variations and an AMPK/miR-451 reciprocal feedback loop. The model shows how variations in glucose availability significantly affect the activity of signalling molecules and, in turn, lead to critical cell migration. The model also predicts that microsurgery of a primary tumour induces phenotypical changes in reactive astrocytes and stem cell-like astrocytes promoting tumour cell proliferation and migration by Cxcl5. Finally, we investigated a new anti-tumour strategy by Cxcl5-targeting drugs. This article is part of the theme issue 'Multi-scale analysis and modelling of collective migration in biological systems'.

Pericyte-secreted IGF2 promotes breast cancer brain metastasis formation

Brain metastases are life-threatening complications of triple-negative breast cancer, melanoma, and a few other tumor types. Poor outcome of cerebral secondary tumors largely depends on the microenvironment formed by cells of the neurovascular unit, among which pericytes are the least characterized. By using in vivo and in vitro techniques and human samples, here we show that pericytes play crucial role in the development of metastatic brain tumors by directly influencing key steps of the development of the disease. Brain pericytes had a prompt chemoattractant effect on breast cancer cells and established direct contacts with them. By secreting high amounts of extracellular matrix proteins, pericytes enhanced adhesion of both melanoma and triple-negative cancer cells, which might be particularly important in the exclusive perivascular growth of these tumor cells. In addition, pericytes secreted insulin-like growth factor 2 (IGF2), which had a very significant pro-proliferative effect on mammary carcinoma, but not on melanoma cells. By inhibiting IGF2 signaling using silencing or picropodophyllin (PPP), we could block the proliferation-increasing effect of pericytes on breast cancer cells. Administration of PPP (a blood-brain barrier-permeable substance) significantly decreased the size of brain tumors in mice inoculated with triple-negative breast cancer cells. Taken together, our results indicate that brain pericytes have significant pro-metastatic features, especially in breast cancer. Our study underlines the importance of targeting pericytes and the IGF axis as potential strategies in brain metastatic diseases.

Cellular Plasticity and Tumor Microenvironment in Gliomas: The Struggle to Hit a Moving Target

Brain tumors encompass a diverse group of neoplasias arising from different cell lineages. Tumors of glial origin have been the subject of intense research because of their rapid and fatal progression. From a clinical point of view, complete surgical resection of gliomas is highly difficult. Moreover, the remaining tumor cells are resistant to traditional therapies such as radio- or chemotherapy and tumors always recur. Here we have revised the new genetic and epigenetic classification of gliomas and the description of the different transcriptional subtypes. In order to understand the progression of the different gliomas we have focused on the interaction of the plastic tumor cells with their vasculature-rich microenvironment and with their distinct immune system. We believe that a comprehensive characterization of the glioma microenvironment will shed some light into why these tumors behave differently from other cancers. Furthermore, a novel classification of gliomas that could integrate the genetic background and the cellular ecosystems could have profound implications in the efficiency of current therapies as well as in the development of new treatments.

Ferroptosis in Low-Grade Glioma: A New Marker for Diagnosis and Prognosis

Background

The extent of glioma resection influences the overall survival (OS) and progression-free survival (PFS). Ferroptosis is a newly recognized type of cell death, which may be associated with low-grade glioma border detection and OS. This study is assessed an optimized ferroptosis gene panel for glioma treatment.

Material/Methods

We obtained 45 reports on ferroptosis-related proteins in PubMed and conducted a statistical test of the patients’ overall survival (OS) in the TCGA GBMLGG and CGGA databases. The statistically significant genes were screened for an optimal panel, followed by GO and KEGG analysis and evaluated its correlation with known prognostic factors of glioma, including IDH1 mutation, methylated MGMT, tumor purity, 1p/19q LOH, and methionine cycle.

Results

Eight genes panel (ALOX5, CISD1, FTL, CD44, FANCD2, NFE2L2, SLC1A5, and GOT1) were highly related to OS (P<0.001) and PFS (P<0.001) of low-grade glioma (LGG) patients, out of which 6 genes (ALOX5, CISD1, CD44, FTL, FANCD2, and SLC1A5) were correlated with IDH1_p.R132H (P<0.001) and 5 genes (ALOX5, CD44, FTL, NFE2L2, SLC1A5) showed a correlation with tumor purity (P<0.001). Five genes (ALOX5, CD44, CISD1, FTL, and SLC1A5) were associated with methylated MGMT (P<0.001), out of which 6 genes (ALOX5, CD44, FANCD2, NFE2L2, SLC1A5, and GOT1) had significantly different expression in healthy brain tissue vs. glioma (P<0.001).

Conclusions

Our panel of 8 ferroptosis genes showed a significant correlation with the diagnostic and prognostic factors of low-grade glioma and can be applied in neuroradiology and surgery.

Breaking through the glioblastoma micro-environment via extracellular vesicles

Glioblastoma (GBM) is the most common and most aggressive brain tumour. Prognosis remains poor, despite the combined treatment of radio- and chemotherapy following surgical removal. GBM cells coexist with normal non-neoplastic cells, including endothelial cells, astrocytes and immune cells, constituting a complex and dynamic tumour micro-environment (TME). Extracellular vesicles (EVs) provide a critical means of bidirectional inter-cellular communication in the TME. Through delivery of a diverse range of genomic, lipidomic and proteomic cargo to neighbouring and distant cells, EVs can alter the phenotype and function of the recipient cell. As such, EVs have demonstrated their role in promoting angiogenesis, immune suppression, invasion, migration, drug resistance and GBM recurrence. Moreover, EVs can reflect the phenotype of the cells within the TME. Thus, in conjunction with their accessibility in biofluids, they can potentially serve as a biomarker reservoir for patient prognosis, diagnosis and predictive therapeutic response as well as treatment follow-up. Furthermore, together with the ability of EVs to cross the blood-brain barrier undeterred and through the exploitation of their cargo, EVs may provide an effective mean of drug delivery to the target site. Unveiling the mechanisms by which EVs within the GBM TME are secreted and target recipient cells may offer an indispensable understanding of GBM that holds the potential to provide a better prognosis and overall quality of life for GBM patients.

Genomic copy number variation correlates with survival outcomes in WHO grade IV glioma

Allele-specific copy number analysis of tumors (ASCAT) assesses copy number variations (CNV) while accounting for aberrant cell fraction and tumor ploidy. We evaluated if ASCAT-assessed CNV are associated with survival outcomes in 56 patients with WHO grade IV gliomas. Tumor data analyzed by Affymetrix OncoScan FFPE Assay yielded the log ratio (R) and B-allele frequency (BAF). Input into ASCAT quantified CNV using the segmentation function to measure copy number inflection points throughout the genome. Quantified CNV was reported as log R and BAF segment counts. Results were confirmed on The Cancer Genome Atlas (TCGA) glioblastoma dataset. 25 (44.6%) patients had MGMT hyper-methylated tumors, 6 (10.7%) were IDH1 mutated. Median follow-up was 36.4 months. Higher log R segment counts were associate with longer progression-free survival (PFS) [hazard ratio (HR) 0.32, p < 0.001], and overall survival (OS) [HR 0.45, p = 0.01], and was an independent predictor of PFS and OS on multivariable analysis. Higher BAF segment counts were linked to longer PFS (HR 0.49, p = 0.022) and OS (HR 0.49, p = 0.052). In the TCGA confirmation cohort, longer 12-month OS was seen in patients with higher BAF segment counts (62.3% vs. 51.9%, p = 0.0129) and higher log R (63.6% vs. 55.2%, p = 0.0696). Genomic CNV may be a novel prognostic biomarker for WHO grade IV glioma patient outcomes.

Targeting Tumor Associated Macrophages to Overcome Conventional Treatment Resistance in Glioblastoma

Glioblastoma (GB) is the most common and devastating form of brain cancer. Despite conventional treatments, progression or recurrences are systematic. In recent years, immunotherapies have emerged as an effective treatment in a number of cancers, leaving the question of their usefulness also faced with the particular case of brain tumors. The challenge here is major not only because the brain is the seat of our consciousness but also because of its isolation by the blood-brain barrier and the presence of a unique microenvironment that constitutes the central nervous system (CNS) with very specific constituent or patrolling cells. Much of the microenvironment is made up of immune cells or inflammation. Among these, tumor-associated macrophages (TAMs) are of significant interest as they are often involved in facilitating tumor progression as well as the development of resistance to standard therapies. In this review, the ubiquity of TAMs in GB will be discussed while the specific case of microglia resident in the brain will be also emphasized. In addition, the roles of TAMs as accomplices in the progression of GB and resistance to treatment will be presented. Finally, clinical trials targeting TAMs as a means of treating cancer will be discussed.

Role of Neutrophils and Myeloid-Derived Suppressor Cells in Glioma Progression and Treatment Resistance

Recent efforts in brain tumor research have been directed towards the modulation of the immune system for therapeutic interventions. Several human cancers, including gliomas, are infiltrated with immune cell types-including neutrophils and myeloid-derived suppressor cells-that contribute to tumor progression, invasiveness, and treatment resistance. The role of tumor-associated neutrophils and myeloid-derived suppressor cells in cancer biology remains elusive, as these cells can exert a multitude of pro-tumor and antitumor effects. In this review, we provide the current understanding and novel insights on the role of neutrophils and myeloid-derived suppressor cells in glioma progression and treatment resistance, as well as the mechanisms of pleiotropic behaviors in these cells during disease progression, with an emphasis on possible strategies to reprogram these cells towards their antitumor actions.

Tumor Cell Invasion in Glioblastoma

Glioblastoma (GBM) is a particularly devastating tumor with a median survival of about 16 months. Recent research has revealed novel insights into the outstanding heterogeneity of this type of brain cancer. However, all GBM subtypes share the hallmark feature of aggressive invasion into the surrounding tissue. Invasive glioblastoma cells escape surgery and focal therapies and thus represent a major obstacle for curative therapy. This review aims to provide a comprehensive understanding of glioma invasion mechanisms with respect to tumor-cell-intrinsic properties as well as cues provided by the microenvironment. We discuss genetic programs that may influence the dissemination and plasticity of GBM cells as well as their different invasion patterns. We also review how tumor cells shape their microenvironment and how, vice versa, components of the extracellular matrix and factors from non-neoplastic cells influence tumor cell motility. We further discuss different research platforms for modeling invasion. Finally, we highlight the importance of accounting for the complex interplay between tumor cell invasion and treatment resistance in glioblastoma when considering new therapeutic approaches.

To be Wild or Mutant: Role of Isocitrate Dehydrogenase 1 (IDH1) and 2-Hydroxy Glutarate (2-HG) in Gliomagenesis and Treatment Outcome in Glioma

Molecular and clinical research based on isocitrate dehydrogenase (IDH) mutations is much sought after in glioma research since a decade of its discovery in 2008. IDH enzyme normally catalyzes isocitrate to α-keto-glutarate (α-KG), but once the gene is mutated it produces an 'oncometabolite', 2-hydroxyglutarate (2-HG). 2-HG is proposed to inhibit α-KG-dependent dioxygenases and also blocks cellular differentiation. Here, we discuss the role of the IDH1 mutation in gliomagenesis. The review also focuses on the effect of 2-HG on glioma epigenetics, the cellular signaling involved in IDH1 mutant glioma cells and the therapeutic response seen in mutant IDH1(mIDH1) harboring glioma patients in comparison to the patients with wild-type IDH1. The review encompasses the debatable impacts of the mutation on immune microenvironment a propos of various mIDH1 inhibitors in practice or in trials. Recent studies revealing the relation of IDH mutation with the immune microenvironment and inflammatory status in untreated versus treated glioblastoma patients are highlighted with respect to prospective therapeutic targets. Also at the molecular level, the association of mIDH1/2-HG with the intracellular components such as mitochondria and other neighboring cells is discussed.

Oligodendrocyte Progenitor Cells in the Tumor Microenvironment

Glioblastoma (GBM) develops from adult brain white matter and is the most common and lethal primary brain tumor, characterized by rapid growth and invasion. GBM tumors frequently spread into the contralateral hemisphere, including in the beginning of tumor development. However, after complete resection of the tumor mass and chemo-radiotherapy, GBM commonly recurs around the tumor removal site, suggesting that the microenvironment at the tumor border provides therapeutic resistance to GBM cells. To improve patient prognosis, understanding the microenvironment at the tumor border is critical. Several microRNAs (miRNAs) show higher expression at the tumor border, with the top three involved in oligodendrocyte differentiation. Oligodendrocyte progenitor cells (OPCs) may induce stemness and chemo-radioresistance in GBM cells, providing a supportive function to promote GBM. This review describes important features of OPCs and insights into the "border niche," a unique microenvironment that allows GBM cells to survive and recur at the tumor border.

Chondroitin Sulphate Proteoglycans in the Tumour Microenvironment

Proteoglycans are macromolecules that are essential for the development of cells, human diseases and malignancies. In particular, chondroitin sulphate proteoglycans (CSPGs) accumulate in tumour stroma and play a key role in tumour growth and invasion by driving multiple oncogenic pathways in tumour cells and promoting crucial interactions in the tumour microenvironment (TME). These pathways involve receptor tyrosine kinase (RTK) signalling via the mitogen-activated protein kinase (MAPK) cascade and integrin signalling via the activation of focal adhesion kinase (FAK), which sustains the activation of extracellular signal-regulated kinases 1/2 (ERK1/2).Human CSPG4 is a type I transmembrane protein that is associated with the growth and progression of human brain tumours. It regulates cell signalling and migration by interacting with components of the extracellular matrix, extracellular ligands, growth factor receptors, intracellular enzymes and structural proteins. Its overexpression by tumour cells, perivascular cells and precursor/progenitor cells in gliomas suggests that it plays a role in their origin, progression and neo-angiogenesis and its aberrant expression in tumour cells may be a promising biomarker to monitor malignant progression and patient survival.The aim of this chapter is to review and discuss the role of CSPG4 in the TME of human gliomas, including its potential as a druggable therapeutic target.

Similarities Between Stem Cell Niches in Glioblastoma and Bone Marrow: Rays of Hope for Novel Treatment Strategies

Glioblastoma is the most aggressive primary brain tumor. Slowly dividing and therapy-resistant glioblastoma stem cells (GSCs) reside in protective peri-arteriolar niches and are held responsible for glioblastoma recurrence. Recently, we showed similarities between GSC niches and hematopoietic stem cell (HSC) niches in bone marrow. Acute myeloid leukemia (AML) cells hijack HSC niches and are transformed into therapy-resistant leukemic stem cells (LSCs). Current clinical trials are focussed on removal of LSCs out of HSC niches to differentiate and to become sensitized to chemotherapy. In the present study, we elaborated further on these similarities by immunohistochemical analyses of 17 biomarkers in paraffin sections of human glioblastoma and human bone marrow. We found all 17 biomarkers to be expressed both in hypoxic peri-arteriolar HSC niches in bone marrow and hypoxic peri-arteriolar GSC niches in glioblastoma. Our findings implicate that GSC niches are being formed in glioblastoma as a copy of HSC niches in bone marrow. These similarities between HSC niches and GSC niches provide a theoretic basis for the development of novel strategies to force GSCs out of their niches, in a similar manner as in AML, to induce GSC differentiation and proliferation to render them more sensitive to anti-glioblastoma therapies.

Novel Semi-Replicative Retroviral Vector Mediated Double Suicide Gene Transfer Enhances Antitumor Effects in Patient-Derived Glioblastoma Models

As glioblastomas are mostly localized infiltrative lesions, gene therapy based on the retroviral replicating vector (RRV) system is considered an attractive strategy. Combinations of multiple suicide genes can circumvent the limitations associated with each gene, achieving direct and synergistic cytotoxic effects, along with bystander cell killing. In this study, we constructed a semi-and pseudotyped-RRV (sp-RRV) system harboring two suicide genes—herpes simplex virus type 1 thymidine kinase (TK) and yeast cytosine deaminase (CD)—to verify the dissemination and antitumor efficacy of our sp-RRV system (spRRVe-sEF1α-TK/sRRVgp-sEF1α-CD) in seven patient-derived glioblastoma stem-like cells (GSCs). Flow cytometry and high-content analysis revealed a wide range of transduction efficiency and good correlation between the delivery of therapeutic genes and susceptibility to the prodrugs ganciclovir and 5-fluorocytosine in patient-derived GSCs in vitro. Intra-tumoral delivery of spRRVe-sEF1α-TK/sRRVgp-sEF1α-CD, combined with prodrug treatment, synergistically inhibited cell proliferation and angiogenesis while increasing apoptosis and the depletion of tumor-associated macrophages in orthotopic glioblastoma xenografts. Genomic profiling of patient-derived GSCs revealed that the key genes preventing sp-RRV infection and transmission were associated with cell adhesion, migration, development, differentiation, and proliferation. This is the first report demonstrating that a novel sp-RRV-mediated TK/CD double suicide gene transfer system has high oncolytic power against extremely heterogeneous and treatment-refractory glioblastomas.

Novel concept of the border niche: glioblastoma cells use oligodendrocytes progenitor cells (GAOs) and microglia to acquire stem cell-like features

Glioblastoma (GBM) is a major malignant brain tumor developing in adult brain white matter, characterized by rapid growth and invasion. GBM cells spread into the contralateral hemisphere, even during early tumor development. However, after complete resection of tumor mass, GBM commonly recurs around the tumor removal cavity, suggesting that a microenvironment at the tumor border provides chemo-radioresistance to GBM cells. Thus, clarification of the tumor border microenvironment is critical for improving prognosis in GBM patients. MicroRNA (miRNA) expression in samples from the tumor, tumor border, and peripheral region far from tumor mass was compared, and five miRNAs showing characteristically higher expression in the tumor border were identified, with the top three related to oligodendrocyte differentiation. Pathologically, oligodendrocyte lineage cells increased in the border, but were rare in tumors. Macrophages/microglia also colocalized in the border area. Medium cultured with oligodendrocyte progenitor cells (OPCs) and macrophages induced stemness and chemo-radioresistance in GBM cells, suggesting that OPCs and macrophages/microglia constitute a special microenvironment for GBM cells at the tumor border. The supportive function of OPCs for GBM cells has not been discussed previously. OPCs are indispensable for GBM cells to establish special niches for chemo-radioresistance outside the tumor mass.

Antimicrobial Peptide TP4 Induces ROS-Mediated Necrosis by Triggering Mitochondrial Dysfunction in Wild-Type and Mutant p53 Glioblastoma Cells

Antimicrobial peptide tilapia piscidin 4 (TP4) from Oreochromis niloticus exhibits potent bactericidal and anti-tumorigenic effects. In a variety of cancers, the mutation status of p53 is a decisive factor for therapeutic sensitivity. Therefore, we investigated the impact of p53 status on TP4-induced cytotoxicity in glioblastoma cell lines and the molecular mechanisms that govern cytotoxic effects. Both U87MG (wild-type/WT p53) and U251 (mutant p53) glioblastoma cell lines were sensitive to TP4-induced cytotoxicity. The necrosis inhibitors Necrostatin-1 and GSK’872 attenuated TP4-induced cytotoxicity, and TP4 treatment induced the release of cyclophilin A, a biomarker of necrosis. Moreover, TP4 induced mitochondrial hyperpolarization and dysfunction, which preceded the elevation of intracellular reactive oxygen species, DNA damage, and necrotic cell death in both U87MG and U251 glioblastoma cells. p38 was also activated by TP4, but did not contribute to cytotoxicity. SB202190, a specific p38 inhibitor, enhanced TP4-induced oxidative stress, mitochondrial dysfunction, and cytotoxicity, suggesting a protective role of p38. Furthermore, TP4-induced cytotoxicity, oxidative stress, phosphorylation of p38, and DNA damage were all attenuated by the mitochondrial-targeted reactive oxygen species (ROS) scavenger MitoTEMPO, or the reactive oxygen species scavenger N-acetyl-L-cysteine. Based on these data, we conclude that TP4 induces necrosis in both WT and mutant p53 glioblastoma cells through a mitochondrial ROS-dependent pathway.