Novel facets of glioma invasion

Malignant gliomas including Glioblastoma (GBM) are characterized by extensive diffuse tumor cell infiltration throughout the brain, which represents a major challenge in clinical disease management. While surgical resection is beneficial for patient outcome, it is well recognized that tumor cells at the invasive front or beyond stay behind and constitute a major source of tumor recurrence. Invasive glioma cells also represent a difficult therapeutic target since they are localized within normal functional brain areas with an intact blood brain barrier (BBB), thereby excluding most systemic drug treatments. Cell movement is mediated via the actin cytoskeleton where corresponding membrane protrusions play essential roles. This review provides an overview of the various paths of glioma cell invasion and underlines the specific aspects of the brain microenvironment. We highlight recent insight into tumor microtubes, neuro-glioma synapses and tumor metabolism which can regulate collective invasion processes. We also focus on the deregulation of actin cytoskeleton-related components in the context of glioma invasion, a deregulation that may be controlled by genomic alterations in tumor cells as well as by various external factors, including extracellular matrix (ECM) components and non-malignant stromal cells. Finally we critically assess the challenges and opportunities for therapeutically targeting glioma cell invasion.

Long-term treatment with valganciclovir improves lentiviral suicide gene therapy of glioblastoma

BACKGROUND

Suicide gene therapy for malignant gliomas has shown encouraging results in the latest clinical trials. However, prodrug application was most often restricted to short-term treatment (14 days), especially when replication-defective vectors were used. We previously showed that a substantial fraction of herpes simplex virus thymidine kinase (HSV-TK) transduced tumor cells survive ganciclovir (GCV) treatment in an orthotopic glioblastoma (GBM) xenograft model. Here we analyzed whether these TK+ tumor cells are still sensitive to prodrug treatment and whether prolonged prodrug treatment can enhance treatment efficacy.

METHODS

Glioma cells positive for TK and green fluorescent protein (GFP) were sorted from xenograft tumors recurring after suicide gene therapy, and their sensitivity to GCV was tested in vitro. GBM xenografts were treated with HSV-TK/GCV, HSV-TK/valganciclovir (valGCV), or HSV-TK/valGCV + erlotinib. Tumor growth was analyzed by MRI, and survival as well as morphological and molecular changes were assessed.

RESULTS

TK-GFP+ tumor cells from recurrent xenograft tumors retained sensitivity to GCV in vitro. Importantly, a prolonged period (3 mo) of prodrug administration with valganciclovir (valGCV) resulted in a significant survival advantage compared with short-term (3 wk) application of GCV. Recurrent tumors from the treatment groups were more invasive and less angiogenic compared with primary tumors and showed significant upregulation of epidermal growth factor receptor (EGFR) expression. However, double treatment with the EGFR inhibitor erlotinib did not increase therapeutic efficacy.

CONCLUSION

Long-term treatment with valGCV should be considered as a replacement for short-term treatment with GCV in clinical trials of HSV-TK mediated suicide gene therapy.

Interfering with long non-coding RNA MIR22HG processing inhibits glioblastoma progression through suppression of Wnt/β-catenin signalling

Long non-coding RNAs play critical roles in tumour progression. Through analysis of publicly available genomic datasets, we found that MIR22HG, the host gene of microRNAs miR-22-3p and miR-22-5p, is ranked among the most dysregulated long non-coding RNAs in glioblastoma. The main purpose of this work was to determine the impact of MIR22HG on glioblastoma growth and invasion and to elucidate its mechanistic function. The MIR22HG/miR-22 axis was highly expressed in glioblastoma as well as in glioma stem-like cells compared to normal neural stem cells. In glioblastoma, increased expression of MIR22HG is associated with poor prognosis. Through a number of functional studies, we show that MIR22HG silencing inhibits the Wnt/β-catenin signalling pathway through loss of miR-22-3p and -5p. This leads to attenuated cell proliferation, invasion and in vivo tumour growth. We further show that two genes, SFRP2 and PCDH15, are direct targets of miR-22-3p and -5p and inhibit Wnt signalling in glioblastoma. Finally, based on the 3D structure of the pre-miR-22, we identified a specific small-molecule inhibitor, AC1L6JTK, that inhibits the enzyme Dicer to block processing of pre-miR-22 into mature miR-22. AC1L6JTK treatment caused an inhibition of tumour growth in vivo. Our findings show that MIR22HG is a critical inducer of the Wnt/β-catenin signalling pathway, and that its targeting may represent a novel therapeutic strategy in glioblastoma patients.

Identification of a Blood-Based Protein Biomarker Panel for Lung Cancer Detection

Lung cancer is the deadliest cancer worldwide, mainly due to its advanced stage at the time of diagnosis. A non-invasive method for its early detection remains mandatory to improve patients’ survival. Plasma levels of 351 proteins were quantified by Liquid Chromatography-Parallel Reaction Monitoring (LC-PRM)-based mass spectrometry in 128 lung cancer patients and 93 healthy donors. Bootstrap sampling and least absolute shrinkage and selection operator (LASSO) penalization were used to find the best protein combination for outcome prediction. The PanelomiX platform was used to select the optimal biomarker thresholds. The panel was validated in 48 patients and 49 healthy volunteers. A 6-protein panel clearly distinguished lung cancer from healthy individuals. The panel displayed excellent performance: area under the receiver operating characteristic curve (AUC) = 0.999, positive predictive value (PPV) = 0.992, negative predictive value (NPV) = 0.989, specificity = 0.989 and sensitivity = 0.992. The panel detected lung cancer independently of the disease stage. The 6-protein panel and other sub-combinations displayed excellent results in the validation dataset. In conclusion, we identified a blood-based 6-protein panel as a diagnostic tool in lung cancer. Used as a routine test for high- and average-risk individuals, it may complement currently adopted techniques in lung cancer screening.

Tumor-penetrating peptide for systemic targeting of Tenascin-C

Extracellular matrix in solid tumors has emerged as a specific, stable, and abundant target for affinity-guided delivery of anticancer drugs. Here we describe the homing peptide that interacts with the C-isoform of Tenascin-C (TNC-C) upregulated in malignant tissues. TNC-C binding PL3 peptide (amino acid sequence: AGRGRLVR) was identified by in vitro biopanning on recombinant TNC-C. Besides TNC-C, PL3 interacts via its C-end Rule (CendR) motif with cell-and tissue penetration receptor neuropilin-1 (NRP-1). Functionalization of iron oxide nanoworms (NWs) and metallic silver nanoparticles (AgNPs) with PL3 peptide increased tropism of systemic nanoparticles towards glioblastoma (GBM) and prostate carcinoma xenograft lesions in nude mice (eight and five-fold respectively). Treatment of glioma-bearing mice with proapoptotic PL3-guided NWs improved the survival of the mice, whereas treatment with untargeted particles had no effect. PL3-coated nanoparticles were found to accumulate in TNC-C and NRP-1-positive areas in clinical tumor samples, suggesting a translational relevance. The systemic tumor-targeting properties and binding of PL3-NPs to the clinical tumor sections, suggest that the PL3 peptide may have applications as a targeting moiety for the selective delivery of imaging and therapeutic agents to solid tumors.

Suicide gene therapy for the treatment of high-grade glioma: past lessons, present trends, and future prospects

Suicide gene therapy has represented an experimental cancer treatment modality for nearly 40 years. Among the various cancers experimentally treated by suicide gene therapy, high-grade gliomas have been the most prominent both in preclinical and clinical settings. Failure of a number of promising suicide gene therapy strategies in the clinic pointed toward a bleak future of this approach for the treatment of high-grade gliomas. Nevertheless, the development of new vectors and suicide genes, better prodrugs, more efficient delivery systems, and new combinatorial strategies represent active research areas that may eventually lead to better efficacy of suicide gene therapy. These trends are evident by the current increasing focus on suicide gene therapy for high-grade glioma treatment both in the laboratory and in the clinic. In this review, we give an overview of different suicide gene therapy approaches for glioma treatment and discuss clinical trials, delivery issues, and immune responses.

Therapeutic implications of altered cholesterol homeostasis mediated by loss of CYP46A1 in human glioblastoma

Dysregulated cholesterol metabolism is a hallmark of many cancers, including glioblastoma (GBM), but its role in disease progression is not well understood. Here, we identified cholesterol 24-hydroxylase (CYP46A1), a brain-specific enzyme responsible for the elimination of cholesterol through the conversion of cholesterol into 24(S)-hydroxycholesterol (24OHC), as one of the most dramatically dysregulated cholesterol metabolism genes in GBM. CYP46A1 was significantly decreased in GBM samples compared with normal brain tissue. A reduction in CYP46A1 expression was associated with increasing tumour grade and poor prognosis in human gliomas. Ectopic expression of CYP46A1 suppressed cell proliferation and in vivo tumour growth by increasing 24OHC levels. RNA-seq revealed that treatment of GBM cells with 24OHC suppressed tumour growth through regulation of LXR and SREBP signalling. Efavirenz, an activator of CYP46A1 that is known to penetrate the blood-brain barrier, inhibited GBM growth in vivo. Our findings demonstrate that CYP46A1 is a critical regulator of cellular cholesterol in GBM and that the CYP46A1/24OHC axis is a potential therapeutic target.

PMEPA1 isoform a drives progression of glioblastoma by promoting protein degradation of the Hippo pathway kinase LATS1

The Hippo signaling pathway controls organ development and is also known, in cancer, to have a tumor suppressing role. Within the Hippo pathway, we here demonstrate, in human gliomas, a functional interaction of a transmembrane protein, prostate transmembrane protein, androgen induced 1 (PMEPA1) with large tumor suppressor kinase 1 (LATS1). We show that PMEPA1 is upregulated in primary human gliomas. The PMEPA1 isoform PMEPA1a was predominantly expressed in glioma specimens and cell lines, and ectopic expression of the protein promoted glioma growth and invasion in vitro and in an orthotopic xenograft model in nude mice. In co-immunoprecipitation experiments, PMEPA1a associated with the Hippo tumor suppressor kinase LATS1. This interaction led to a proteasomal degradation of LATS1 through recruitment of the ubiquitin ligase, neural precursor cell expressed, developmentally downregulated 4 (NEDD4), which led to silencing of Hippo signaling. Alanine substitution in PMEPA1a at PY motifs resulted in failed LATS1 degradation. Targeting of a downstream component in the Hippo signaling pathway, YAP, with shRNA, interfered with the growth promoting activities of PMEPA1a in vitro and in vivo. In conclusion, the presented work shows that PMEPA1a contributes to glioma progression by a dysregulation of the Hippo signaling pathway and thus represents a promising target for the treatment of gliomas.

Mutant IDH1 Differently Affects Redox State and Metabolism in Glial Cells of Normal and Tumor Origin

IDH1^R132H (isocitrate dehydrogenase 1) mutations play a key role in the development of low-grade gliomas. IDH1^wt converts isocitrate to α-ketoglutarate while reducing nicotinamide adenine dinucleotide phosphate (NADP⁺), whereas IDH1^R132H uses α-ketoglutarate and NADPH to generate the oncometabolite 2-hydroxyglutarate (2-HG). While the effects of 2-HG have been the subject of intense research, the 2-HG independent effects of IDH1^R132H are still ambiguous. The present study demonstrates that IDH1^R132H expression but not 2-HG alone leads to significantly decreased tricarboxylic acid (TCA) cycle metabolites, reduced proliferation, and enhanced sensitivity to irradiation in both glioblastoma cells and astrocytes in vitro. Glioblastoma cells, but not astrocytes, showed decreased NADPH and NAD⁺ levels upon IDH1^R132H transduction. However, in astrocytes IDH1^R132H led to elevated expression of the NAD-synthesizing enzyme nicotinamide phosphoribosyltransferase (NAMPT). These effects were not 2-HG mediated. This suggests that IDH1^R132H cells utilize NAD⁺ to restore NADP pools, which only astrocytes could compensate via induction of NAMPT. We found that the expression of NAMPT is lower in patient-derived IDH1-mutant glioma cells and xenografts compared to IDH1-wildtype models. The Cancer Genome Atlas (TCGA) data analysis confirmed lower NAMPT expression in IDH1-mutant versus IDH1-wildtype gliomas. We show that the IDH1 mutation directly affects the energy homeostasis and redox state in a cell-type dependent manner. Targeting the impairments in metabolism and redox state might open up new avenues for treating IDH1-mutant gliomas.

Reduced expression of proteolipid protein 2 increases ER stress-induced apoptosis and autophagy in glioblastoma

Proteolipid protein 2 (PLP2) is an integral ion channel membrane protein of the endoplasmic reticulum. The protein has been shown to be highly expressed in many cancer types, but its importance in glioma progression is poorly understood. Using publicly available datasets (Rembrandt, TCGA and CGGA), we found that the expression of PLP2 was significantly higher in high‐grade gliomas than in low‐grade gliomas. We confirmed these results at the protein level through IHC staining of high‐grade (n = 56) and low‐grade glioma biopsies (n = 16). Kaplan‐Meier analysis demonstrated that increased PLP2 expression was associated with poorer patient survival. In functional experiments, siRNA and shRNA PLP2 knockdown induced ER stress and increased apoptosis and autophagy in U87 and U251 glioma cell lines. Inhibition of autophagy with chloroquine augmented apoptotic cell death in U87‐ and U251‐siPLP2 cells. Finally, intracranial xenografts derived from U87‐ and U251‐shPLP2 cells revealed that loss of PLP2 reduced glioma growth in vivo. Our results therefore indicate that increased PLP2 expression promotes GBM growth and that PLP2 represents a potential future therapeutic target.

STEM-09. INTRINSIC TUMOR PLASTICITY IN GLIOBLASTOMA ALLOWS FOR RECREATION OF STEM LIKE-STATES AND EFFICIENT TUMOR CELL ADAPTATION TO NEW MICROENVIRONMENTS

BACKGROUND

Cellular heterogeneity is a hallmark of numerous cancer types, including Glioblastoma (GBM). Cancer stem cells (CSC) have been accounted for the generation of phenotypic heterogeneity and tumor progression in GBM. Recent data, however, suggest that CSCs may not represent a stable entity and intrinsic plasticity plays a key role in tumor adaptation to changing microenvironments. The question arises whether CSCs are a defined subpopulation of GBM or whether they represent a cellular state that any cancer cell can adopt.

METHODS

We interrogated intra-tumoral phenotypic heterogeneity at the single cell transcriptomic and proteomic level in GBM biopsies, patient-derived stem-like cultures and orthotopic xenografts (PDOXs). Tumor cell subpopulations, classified based on their expression of four proposed stem cell markers (CD133, CD15, A2B5 and CD44), were FACS isolated and functionally characterized under various microenvironmental conditions. Mathematical Markov modelling was applied to calculate state transitions.

RESULTS

GBM patient biopsies, PDOXs and stem-like cell cultures displayed remarkable stem cell-associated intra-tumoral heterogeneity. However independent of marker expression, all analysed tumor subpopulations carried stem-cell properties and recreated phenotypic heterogeneity. Mathematical modeling revealed a different propensity in reforming heterogeneity over time, which was independent of the proliferation index but linked to in vivo tumorigenic potential. Although GBM subpopulations varied in their potential to adapt to new environments, all were able to reach a steady state microenvironment-specific equilibrium.

CONCLUSIONS

Our results suggest that phenotypic heterogeneity in GBM results from intrinsic plasticity allowing tumor cells to adapt to changing microenvironmental conditions. Cellular states are non-hierarchical, reversible and occur via stochastic state transitions, striving towards a microenvironment-instructed equilibrium. Our data provides evidence that CSCs do not represent a defined clonal entity, but rather a cellular state determined by environmental conditions, which has implications for the design of treatment strategies targeting CSC-like states.

ANGI-03. PHARMACOLOGICAL TARGETING OF APELIN/APLNR SIGNALING BLUNTS THERAPY RESISTANCE TO VEGFA/VEGFR2 ANTI-ANGIOGENIC TREATMENT IN GLIOBLASTOMA

Anti-angiogenic therapy of glioblastoma with bevacizumab, a vascular endothelial growth factor-A (VEGFA) blocking antibody, may accelerate tumor cell invasion and induce alternative angiogenic pathways. We investigated the roles of the pro-angiogenic receptor APLNR and its cognate ligand apelin in VEGFA/VEGFR2 anti-angiogenic therapy against distinct subtypes of glioblastoma. In proneural glioblastoma, apelin levels were downregulated by VEGFA or VEGFR2 blockade by use of bevacizumab or ramucirumab, respectively. A central role for apelin/APLNR in controlling glioblastoma vascularization was corroborated in a serial implantation model of the angiogenic switch that occurs in human glioblastoma. Apelin and APLNR are broadly expressed in human glioblastoma, and knockdown or knockout of APLN in orthotopic models of proneural or classical glioblastoma subtypes massively reduced glioblastoma vascularization as compared with controls. What is more, direct infusion of the bioactive peptide apelin-13 was able to rescue this vascular loss-of-function phenotype, demonstrating the specific control of tumor vascularization by apelin/APLNR signaling. While high levels of apelin correlated with reduced tumor cell invasiveness, the reduction in apelin expression led to accelerated glioblastoma cell invasion. Analysis of stereotactic glioblastoma biopsies from patients as well as from in vitro and in vivo experiments revealed increased dissemination of APLNR-positive tumor cells when apelin levels were reduced. Most interestingly, application of apelin-F13A, a mutant APLNR ligand, blocked both tumor angiogenesis and glioblastoma cell invasion. Furthermore, co-targeting VEGFR2 and APLNR synergistically improved survival of mice bearing proneural glioblastoma. In summary, we show that apelin/APLNR signaling controls glioblastoma angiogenesis and invasion directly, and that both pathological features are blunted by apelin-F13A. We suggest that apelin-F13A can improve the efficiency and reduce the side effects of established anti-angiogenic treatments for distinct glioblastoma subtypes.

GENE-02. ESTABLISHING PERSONALIZED TREATMENT OPTIONS FOR RECURRENT HIGH-GRADE GLIOMAS

BACKGROUND

High grade glioma (HGG) patients develop resistance to standard treatment leading to disease progression and limited life expectancy. Recent advances in the molecular characterisation of treatment-naïve HGGs based on next generation sequencing and DNA methylation analyses have led to a better delineation of HGG-subtypes and identification of distinct genomic abnormalities opening opportunities for personalized treatment strategies.

METHODS

We collected 300 fresh glioma specimen with approximately 100 longitudinal samples of initial and recurrent tumors from 43 matched patients. We succeeded in generating a live-biobank of HGG patient-derived orthotopic xenografts (PDOX) and 3D tumor organoids that neatly recapitulates the mutational spectrum including structural DNA variations and methylation-based subtypes of gliomas. A highlight is the generation of 19 PDOXs of paired initial and relapse HGGs from 9 glioma patients, enabling high-throughput drug screens. We performed comprehensive molecular profiling using arrayCGH, DNA-methylation and targeted DNA sequencing on patient specimen and their derivatives, 3D tumor organoids and PDOXs.

RESULTS

Detailed analysis of the paired longitudinal samples indicated that PDOXs closely recapitulate the evolutionary trajectory of the parental tumors. Furthermore, targeted genomic sequencing of paired HGGs suggests that relapse tumors also accumulate somatic mutations in epigenetic effectors. Based on patient-derived material we carried out drug response screening on 3D tumor organoids using a compound library matching the majority of genes that were assessed with targeted sequencing. Differential drug responses between initial and recurrent tumors were observed and the prevailing primary drug response profiles were essentially recapitulad in the relapse setting.

CONCLUSIONS

Response assessment of treatment-naïve gliomas and their recurrences provides crucial information on the differential sensitivity between initial and relapsed HGGs and offers novel personalized therapeutic options for the relapse setting. Furthermore, in depth correlation of the profiled somatic molecular landscape with drug response will enable pharmacogenomic predictions of potential inhibitors in the clinical setting.

The invasive proteome of glioblastoma revealed by laser-capture microdissection

Background

Glioblastomas are heterogeneous tumors composed of a necrotic and tumor core and an invasive periphery.

Methods

Here, we performed a proteomics analysis of laser-capture micro-dissected glioblastoma core and invasive areas of patient-derived xenografts.

Results

Bioinformatics analysis identified enriched proteins in central and invasive tumor areas. Novel markers of invasion were identified, the genes proteolipid protein 1 (PLP1) and Dynamin-1 (DNM1), which were subsequently validated in tumors and by functional assays.

Conclusions

In summary, our results identify new networks and molecules that may play an important role in glioblastoma development and may constitute potential novel therapeutic targets.

OS6.3 The complex role of lactate dehydrogenases in glioblastoma development

BACKGROUND

Glioblastomas are among the most malignant primary brain tumors. GBMs are highly angiogenic, exhibit invasive growth, and elevated glycolysis. Under glycolytic conditions, glucose from the blood is metabolized in astrocytes into lactate by LDHA, and exported by MCT4 into the extracellular compartment, inducing a concomitant acidification of the microenvironment. LDHB, generally expressed in oligodendrocytes or neurons, metabolizes lactate into pyruvate for generating ATP in mitochondria. LDH expression was reported to be linked to phenotypic modifications in vitro in GBMs but the mechanisms and the precise role in vivo have not yet been investigated.

MATERIAL AND METHODS

We designed LDHA and LDHB Crispr-Cas9 constructs for infecting glioblastoma stem-like cells. Results: In vitro tumor cell invasion was not significantly impaired in sgLDHA glioblastoma cells, even under extreme hypoxic conditions. Tumor development was moderately impacted in terms of invasion or vascular density. We then explored the role of LDHB in these processes. LDHB knock-out cells had decreased invasive properties in vitro but surprisingly tumors were highly hemorrhagic and angiogenic, supporting a role of tumor-derived LDHB in blood vessel development. We furthermore evaluated the consequences of a double LDHA and LDHB knock-out in the glioma cells. Under hypoxic conditions, sgLDHA/B cell invasion was dramatically decreased in comparison to control cells, and apoptosis was also increased. Tumor development was dramatically impaired for LDHA/LDHB knockout tumors.

CONCLUSION

These results indicate the complex role of LDH enzymes in glioblastoma development. It constitutes the basis for further mechanistical studies linking lactate metabolism to brain tumor development and perturbation of the neuro-vascular microenvironment.

OS12.2 Targeting epigenetic pathways in the treatment of recurrent high-grade glioma

BACKGROUND

High grade glioma (HGG) patients develop resistance to standard treatment leading to disease progression and limited life expectancy. Advances in the molecular characterisation of treatment-naïve HGGs based on next-generation sequencing and DNA methylation analyses have led to a better delineation of HGG subtypes and the identification of distinct genomic abnormalities. Furthermore, using large patient cohorts of longitudinal tumor samples, comprehensive genomic profiling studies emerged to investigate therapy-associated evolution of gliomas. All together, those studies point out the need for personalised treatment strategies, where applied drugs will be adapted to the unique patient-specific genetic abnormalities.

MATERIAL AND METHODS

We collected fresh samples of more than 800 brain tumors containing almost 300 glioma specimen with approximately 100 longitudinal samples of initial and recurrent tumors from 43 matched patients. By now, we have successfully established 34 patient-derived orthotopic xenografts (PDOXs) in mice. We performed comprehensive molecular profiling using array comparative genomic hybridisation, DNA methylation analysis and targeted DNA sequencing on patient specimen and their derivatives such as 3D tumor organoids and PDOXs. The custom-design sequencing panel comprises 234 genes that reflect both established genetic identifiers for individual glioma subtype classification and novel genes encoding mainly epigenetic effector genes. Based on patient-derived material we carried out drug response screening on 3D tumor organoids using a compound library matching the majority of genes that were assessed by targeted sequencing.

RESULTS

We succeeded in generating a live biobank of HGG patient-derived xenografts and 3D organoids that neatly recapitulates the mutational spectrum including structural DNA variation and methylation-based subtypes of gliomas. A highlight is the generation of 19 PDOXs of paired initial and relapse HGGs from a total of 9 glioma patients. A detailed analysis of the paired longitudinal samples indicated that PDOX models closely recapitulate the evolutionary trajectory of the parental tumors. Targeted sequencing of longitudinal HGG PDOXs suggests that relapse tumors accumulate somatic mutations in epigenetic effectors compared with the Initial. Differential drug responses between initial and relapse tumors were observed after screening of in vitro 3D tumor organoids.

CONCLUSION

Response assessment of naïve initial gliomas and recurrences provides crucial information on the differential sensitivity between initial and relapsed HGGs and offers novel personalised therapeutic options in the relapse setting. Furthermore, in depth correlation of the profiled somatic molecular landscape with drug response will enable pharmacogenomic predictions of potential inhibitors in the clinical setting.

PL3.4 Intrinsic tumor plasticity in Glioblastoma allows for recreation of stem like-states and efficient tumor cell adaptation to new microenvironments

BACKGROUND

Cellular heterogeneity has been well established within numerous cancer types, including malignant brain tumours. Initially, cancer stem cells (CSC) have been accounted for formation of phenotypic heterogeneity and tumor progression in glioblastoma (GBM). Recent data, however, suggest that CSCs may not represent a stable entity and intrinsic plasticity plays a key role in tumor adaptation to changing microenvironments. The question arises whether CSCs are a defined subpopulation of tumor cells or whether they represent a changing entity that any cancer cell can adopt depending on the environmental conditions.

MATERIAL AND METHODS

Intra-tumoral phenotypic heterogeneity was interrogated at the single cell transcriptomic and proteomic level in GBM patient-derived orthotopic xenografts (PDOXs) and stem-like cultures. Tumor cell subpopulations were further classified based on expression of four stem cell-associated membrane markers (CD133, CD15, A2B5 and CD44). The resulting 16 subpopulations were FACS isolated and functionally analyzed. Mathematical Markov modelling was applied to calculate state transitions between cell states.

RESULTS

GBM patient biopsies, PDOXs and stem-like cell cultures display remarkable stem cell-associated intra-tumoral heterogeneity. Independent of marker expression, all analysed tumor subpopulations carried stem-cell properties and had the capacity to recreate phenotypic heterogeneity. Mathematical modeling revealed a different propensity in reforming the original heterogeneity over time, which was independent of the proliferation index but linked to tumorigenic potential. Although subpopulations varied in their potential to adapt to new environments, all were able to reach a steady state microenvironment-specific equilibrium.

CONCLUSION

Our results suggest that phenotypic heterogeneity in GBM results from intrinsic plasticity allowing tumor cells to effectively adapt to new microenvironments. Cellular states are non-hierarchical, reversible and occur via stochastic state transitions of existing populations, striving towards an equilibrium instructed by the microenvironment. Our data provides evidence that CSCs do not represent a clonal entity defined by distinct functional properties and transcriptomic signatures, but rather a cellular state that is determined by environmental conditions, which has implications for the design of treatment strategies targeting CSC-like states.

OS6.2 Loss of CYP46A1 directs altered cholesterol homeostasis and opens therapeutic opportunities for glioblastoma

BACKGROUND

Dysregulated cholesterol metabolism is a hallmark of many cancers, including glioblastoma (GBM), but its role in disease progression is not well understood. Here, we identified cholesterol 24-hydroxylase (CYP46A1), a brain-specific enzyme responsible for elimination of cholesterol through conversion of cholesterol to 24(S)-hydroxycholesterol (24OHC), as one of the most dramatically dysregulated cholesterol metabolism genes in GBM.

MATERIAL AND METHODS

Molecular and clinical data was obtained from publicly genomic databases. Immunohistochemistry was applied to assess protein levels of CYP46A1 in primary GBM samples. Lentiviral constructs expressing CYP46A1 were transduced into LN229, LN18 and primary GBM GSCs for functional assays carried out in vitro and in vivo in an orthotopic xenograft model. RNA-seq was performed to identify downstream targets of 24OHC.

RESULTS

CYP46A1 was significantly decreased in GBM samples compared to normal brain tissue. Reduced CYP46A1 expression was associated with increasing tumour grade and poor prognosis in GBM patients. Ectopic expression of CYP46A1 suppressed cell proliferation and in vivo tumour growth by increasing 24OHC levels. Treatment of GBM cells with 24OHC suppressed tumour growth through regulation of LXR and SREBP signalling. Efavirenz (EFV), an activator of CYP46A1 with BBB penetration, inhibited GBM growth in vivo.

CONCLUSION

Our findings demonstrate that CYP46A1 is a critical regulator of cellular cholesterol in GBM and that the CYP46A1/24OHC axis is a potential therapeutic target.

Long Noncoding RNA SChLAP1 Forms a Growth-Promoting Complex with HNRNPL in Human Glioblastoma through Stabilization of ACTN4 and Activation of NF-κB Signaling

PURPOSE

Long noncoding RNAs (lncRNA) have essential roles in diverse cellular processes, both in normal and diseased cell types, and thus have emerged as potential therapeutic targets. A specific member of this family, the SWI/SNF complex antagonist associated with prostate cancer 1 (SChLAP1), has been shown to promote aggressive prostate cancer growth by antagonizing the SWI/SNF complex and therefore serves as a biomarker for poor prognosis. Here, we investigated whether SChLAP1 plays a potential role in the development of human glioblastoma (GBM).

EXPERIMENTAL DESIGN

RNA-ISH and IHC were performed on a tissue microarray to assess expression of SChLAP1 and associated proteins in human gliomas. Proteins complexed with SChLAP1 were identified using RNA pull-down and mass spectrometry. Lentiviral constructs were used for functional analysis in vitro and in vivo.

RESULTS

SChLAP1 was increased in primary GBM samples and cell lines, and knockdown of the lncRNA suppressed growth. SChLAP1 was found to bind heterogeneous nuclear ribonucleoprotein L (HNRNPL), which stabilized the lncRNA and led to an enhanced interaction with the protein actinin alpha 4 (ACTN4). ACTN4 was also highly expressed in primary GBM samples and was associated with poorer overall survival in glioma patients. The SChLAP1-HNRNPL complex led to stabilization of ACTN4 through suppression of proteasomal degradation, which resulted in increased nuclear localization of the p65 subunit of NF-κB and activation of NF-κB signaling, a pathway associated with cancer development.

CONCLUSIONS

Our results implicated SChLAP1 as a driver of GBM growth as well as a potential therapeutic target in treatment of the disease.

P11.52 Reduced expression of proteolipid protein 2 increases ER-stress-induced apoptosis and autophagy in glioblastoma

BACKGROUND

The PLP2 gene encodes for the Proteolipid protein 2 (PLP2) which is an integral ion channel membrane protein of the endoplasmic reticulum. The protein has been shown to be involved in several human cancers, but the importance of PLP2 in gliomas is poorly understood. In the present study, we therefore investigated the role of PLP2 in human glioma development.

MATERIAL AND METHODS

Immunohistochemistry was carried out for paraffin-embedded glioma samples, and changes in protein level were detected by western blot analysis. Small interfering RNA transfections were used for knockdown of specific genes. Cell viability and proliferation was then assessed by CCK-8 assay and EdU assay. Transmission electron microscopy was used to observe the ultrastructure of the cells and flow cytometry for the evaluation of apoptosis. Finally, U87 and U251 cells were treated with lentiviral transduction to obtain stably PLP2-knockdown cell lines, which were used for an in vivo study.

RESULTS

Data from publicly available datasets (Rembrandt, TCGA and CGGA) showed a correlation between up-regulation of PLP2 levels and increased malignancy. This was confirmed by IHC staining of sections from our own clinical glioma samples. Mechanistically, down-regulation of PLP2 in U87 and U251 glioma cell lines decreased the proliferation and increased apoptosis and autophagy, mediated by ER-stress. In PLP2 knockdown U87 and U251 cells, autophagy inhibition by chloroquine (CQ) augmented apoptotic cell death. Finally, orthotopic U87-shPLP2 and U251-shPLP2 intracranial xenograft models revealed that down-regulating of PLP2 inhibited glioma development in vivo.

CONCLUSION

In conclusion, the results indicate that PLP2 expression is related to glioma progression, and could be a potential target for future treatment strategies.

P11.57 A 3D brain organoid coculture system delineates the invasive cell components in glioblastoma

BACKGROUND

Glioblastoma (GBM) cell infiltration into the surrounding normal brain tissue where the blood brain barrier is intact, represents a major problem for clinical management and therapy. There is a vital need to understand the molecular mechanism that drives tumor cell invasion into the surrounding brain. We have previously developed a 3D coculture model where mature brain organoids are confronted with patient-derived glioblastoma stem-like cells (GSCs). In such a coculture system, single cell invasion into the normal brain tissue can be studied in detail. Here, we first describe in detail, by RNA-seq and proteomics, the differentiation of various neural cell lineages into mature brain organoids as well as their cellular organization. By real-time confocal microscopy and imaging analyses we also determine the speed of tumor cell invasion into the brain. Finally, we used this coculture system to delineate in detail the cellular heterogeneity within the invasive compartment and their gene expression.

MATERIAL AND METHODS

Immunohistochemistry and immunofluorescence were used to determine the expression and distribution of mature neurons, astrocytes, oligodendrocytes, and microglia within the brain organoids. Proteomics and RNA-seq were used to determine brain development ex-vivo. To assess the clonal composition of the GBM-invasive compartment, we used cellular (RGB) barcoding technology. By advanced imaging, we tracked in real time the invasion of barcoded cells into the brain organoids. Finally, we isolated invasive cells and non-invasive cells from our coculture system and used single cell sequencing to analyze their gene expression profiles and molecular phenotypes.

RESULTS

Immunohistochemistry and immunofluorescence showed that brain organoids, after 21 days of differentiation, display a highly cellular and structural organization. RNA-seq and proteomics, performed at different time points of organoid differentiation, revealed that the brain organoids develop into mature brain structures after 21 days as verified by a comparative analysis to normal rat brain development in vivo. Imaging analyses showed that multiple clones within the GBMs have the capacity to invade into the brain tissue with an average speed of ~ 20 μm/h. RNA-sec analysis of the invasive compartment revealed a strong up-regulation of genes and pathways associated with anaerobic respiration (glycolysis).

CONCLUSION

We describe a highly standardized brain organoid coculture system that can be used to delineate GBM invasion ex-vivo. We demonstrate that this platform can be used to unravel the mechanisms that drive GBM invasion into the normal brain.

Improved Drug Delivery to Brain Metastases by Peptide-Mediated Permeabilization of the Blood-Brain Barrier

Patients with melanoma have a high risk of developing brain metastasis, which is associated with a dismal prognosis. During early stages of metastasis development, the blood-brain barrier (BBB) is likely intact, which inhibits sufficient drug delivery into the metastatic lesions. We investigated the ability of the peptide, K16ApoE, to permeabilize the BBB for improved treatment with targeted therapies preclinically. Dynamic contrast enhanced MRI (DCE-MRI) was carried out on NOD/SCID mice to study the therapeutic window of peptide-mediated BBB permeabilization. Further, both in vivo and in vitro assays were used to determine K16ApoE toxicity and to obtain mechanistic insight into its action on the BBB. The therapeutic impact of K16ApoE on metastases was evaluated combined with the mitogen-activated protein kinase pathway inhibitor dabrafenib, targeting BRAF mutated melanoma cells, which is otherwise known not to cross the intact BBB. Our results from the DCE-MRI experiments showed effective K16ApoE-mediated BBB permeabilization lasting for up to 1 hour. Mechanistic studies showed a dose-dependent effect of K16ApoE caused by induction of endocytosis. At concentrations above IC₅₀, the peptide additionally showed nonspecific disturbances on plasma membranes. Combined treatment with K16ApoE and dabrafenib reduced the brain metastatic burden in mice and increased animal survival, and PET/CT showed that the peptide also facilitated the delivery of compounds with molecular weights as large as 150 kDa into the brain. To conclude, we demonstrate a transient permeabilization of the BBB, caused by K16ApoE, that facilitates enhanced drug delivery into the brain. This improves the efficacy of drugs that otherwise do not cross the intact BBB.

Effective Treatment of Metastatic Melanoma by Combining MAPK and PI3K Signaling Pathway Inhibitors

Malignant melanoma is the most aggressive type of skin cancer and is closely associated with the development of brain metastases. Despite aggressive treatment, the prognosis has traditionally been poor, necessitating improved therapies. In melanoma, the mitogen activated protein kinase and the phosphoinositide 3-kinase signaling pathways are commonly altered, and therapeutically inhibiting one of the pathways often upregulates the other, leading to resistance. Thus, combined treatment targeting both pathways is a promising strategy to overcome this. Here, we studied the in vitro and in vivo effects of the PI3K inhibitor buparlisib and the MEK1/2 inhibitor trametinib, used either as targeted monotherapies or in combination, on patient-derived melanoma brain metastasis cell lines. Scratch wound and trans-well assays were carried out to assess the migratory capacity of the cells upon drug treatment, whereas flow cytometry, apoptosis array and Western blots were used to study apoptosis. Finally, an in vivo treatment experiment was carried out on NOD/SCID mice. We show that combined therapy was more effective than monotherapy. Combined treatment also more effectively increased apoptosis, and inhibited tumor growth in vivo. This suggests a clinical potential of combined treatment to overcome ceased treatment activity which is often seen after monotherapies, and strongly encourages the evaluation of the treatment strategy on melanoma patients with brain metastases.