Regulation of glioblastoma multiforme stem-like cells by inhibitor of DNA binding proteins and oligodendroglial lineage-associated transcription factors

DHODH inhibition impedes glioma stem cell proliferation, induces DNA damage, and prolongs survival in orthotopic glioblastoma xenografts

Glioma stem cells (GSCs) promote tumor progression and therapeutic resistance and exhibit remarkable bioenergetic and metabolic plasticity, a phenomenon that has been linked to their ability to escape standard and targeted therapies. However, specific mechanisms that promote therapeutic resistance have been somewhat elusive. We hypothesized that because GSCs proliferate continuously, they may require the salvage and de novo nucleotide synthesis pathways to satisfy their bioenergetic needs. Here, we demonstrate that GSCs lacking EGFR (or EGFRvIII) amplification are exquisitely sensitive to de novo pyrimidine synthesis perturbations, while GSCs that amplify EGFR are utterly resistant. Furthermore, we show that EGFRvIII promotes BAY2402234 resistance in otherwise BAY2402234 responsive GSCs. Remarkably, a novel, orally bioavailable, blood-brain-barrier penetrating, dihydroorotate dehydrogenase (DHODH) inhibitor BAY2402234 was found to abrogate GSC proliferation, block cell-cycle progression, and induce DNA damage and apoptosis. When dosed daily by oral gavage, BAY2402234 significantly impaired the growth of two different intracranial human glioblastoma xenograft models in mice. Given this observed efficacy and the previously established safety profiles in preclinical animal models and human clinical trials, the clinical testing of BAY2402234 in patients with primary glioblastoma that lacks EGFR amplification is warranted.

Monoallelic IDH1 R132H Mutation Mediates Glioma Cell Response to Anticancer Therapies via Induction of Senescence

Heterozygous isocitrate dehydrogenase (IDH) R132H mutation (IDH1^R132H/WT) is an early event during gliomagenesis. Clinically, patients with glioma carrying mutant IDH1 respond better to antitumor therapies. However, the mechanism by which IDH1 mutations contribute to gliomagenesis and therapeutic response remains elusive. Here we report that senescence is involved in the improved therapeutic responses of mutant IDH1 glioma cells. Knocking-in IDH1^R132H/WT in glioma cells significantly enhanced gliomas cell senescence in response to temozolomide and radiation via a DNA-damage mediated mechanism. We further asked if senescence plays a role in IDH1^R132H/WT-induced gliomagenesis. Together with ATRX knockout and p53/RB loss, IDH1^R132H/WT transformed nonneoplastic human astroglial cells to form tumors in mouse brains. In-depth characterization revealed that a subset of these precancerous cells underwent senescence-like phenotypic changes, including flat and enlarged-cell morphology, increased senescence marker expression, decreased cell proliferation, and cell-cycle arrest at the G₂-M phase. Mechanistic studies indicated that the combination of glioma driver genes (p53/RB/IDH1/ATRX) dramatically increased DNA damage and activated DNAdamage response (DDR) pathways ATR/ATR and Chk1/Chk2 in senescent cells. To determine how senescent cells drive tumor formation, we investigated non-cell-autonomous mechanisms such as senescence-associated secretory phenotype (SASP), a panel of proinflammatory and tissue-remodeling factors implicated in a tumor-permissive microenvironment. We found that astroglial cells carrying p53/RB/ATRX loss and IDH1^R132H/WT upregulated key factors in SASP via an epigenetic-mediated mechanism. Our work suggests that drugs that specifically eliminate senescent cells could help kill precancerous cells and senescent tumor cells following antitumor therapies. IMPLICATIONS: The mechanisms by which IDH1 mutations contribute to gliomagenesis and therapeutic responses remain incompletely characterized; this work reveals senescence as a novel mechanism of IDH-mutant-mediated biological impact and describes new therapeutic opportunities concerning IDH1-mutant gliomas.

Experimental glioma with high bHLH expression harbor increased replicative stress and are sensitive toward ATR inhibition

Background

The overexpression of (basic)helix-loop-helix ((b)HLH) transcription factors (TFs) is frequent in malignant glioma. We investigated molecular effects upon disruption of the (b)HLH network by a dominant-negative variant of the E47 protein (dnE47). Our goal was to identify novel molecular subgroup-specific therapeutic strategies.

Methods

Glioma cell lines LN229, LNZ308, and GS-2/GS-9 were lentivirally transduced. Functional characterization included immunocytochemistry, immunoblots, cytotoxic, and clonogenic survival assays in vitro, and latency until neurological symptoms in vivo. Results of cap analysis gene expression and RNA-sequencing were further validated by immunoblot, flow cytometry, and functional assays in vitro.

Results

The induction of dnE47-RFP led to cytoplasmic sequestration of (b)HLH TFs and antiglioma activity in vitro and in vivo. Downstream molecular events, ie, alterations in transcription start site usage and in the transcriptome revealed enrichment of cancer-relevant pathways, particularly of the DNA damage response (DDR) pathway. Pharmacologic validation of this result using ataxia telangiectasia and Rad3 related (ATR) inhibition led to a significantly enhanced early and late apoptotic effect compared with temozolomide alone.

Conclusions

Gliomas overexpressing (b)HLH TFs are sensitive toward inhibition of the ATR kinase. The combination of ATR inhibition plus temozolomide or radiation therapy in this molecular subgroup are warranted.

All-trans retinoic acid therapy induces asymmetric division of glioma stem cells from the U87MG cell line

The poor therapeutic effect of the current treatments for malignant glioma may be attributed to glioma stem cells (GSCs), which have been demonstrated to divide symmetrically. All-trans retinoic acid (ATRA)-induced differentiation is considered to target GSCs and has been reported to have the capability of eradicating cancer stem cells in specific malignancies. The aim of the present study was to investigate the effects of ATRA on the division mode of GSCs isolated from the U87MG glioblastoma cell line of unknown origin. The expressions of the GSC markers CD133 and nestin were detected using immunocytochemistry to identify GSCs. In addition, the differentiation potency of these GSCs was observed by detecting the expression of glial fibrillary acidic protein, β-tubulin III and galactosylceramidase using immunofluorescent staining. The Numb protein distribution was analyzed in two daughter cells following a GSC division. The results of the present study demonstrated that Numb protein is symmetrically segregated into two daughter cells during GSC division. Furthermore, the present study demonstrated that treatment with ATRA increased the asymmetric cell division of GSCs. In conclusion, these results suggest a therapeutic effect from ATRA-induced asymmetric division of GSCs from the U87MG cell line.

Extracellular Matrix Protein Tenascin C Increases Phagocytosis Mediated by CD47 Loss of Function in Glioblastoma

Glioblastomas (GBM) are highly infiltrated by myeloid-derived innate immune cells that contribute to the immunosuppressive nature of the brain tumor microenvironment (TME). CD47 has been shown to mediate immune evasion, as the CD47-SIRPα axis prevents phagocytosis of tumor cells by macrophages and other myeloid cells. In this study, we established CD47 homozygous deletion (CD47^-/-) in human and mouse GBM cells and investigated the impact of eliminating the "don't eat me" signal on tumor growth and tumor-TME interactions. CD47 knockout (KO) did not significantly alter tumor cell proliferation in vitro but significantly increased phagocytosis of tumor cells by macrophages in cocultures. Compared with CD47 wild-type xenografts, orthotopic xenografts derived from CD47^-/- tumor cells grew significantly slower with enhanced tumor cell phagocytosis and increased recruitment of M2-like tumor-associated microglia/macrophages (TAM). CD47 KO increased tumor-associated extracellular matrix protein tenascin C (TNC) in xenografts, which was further examined in vitro. CD47 loss of function upregulated TNC expression in tumor cells via a Notch pathway-mediated mechanism. Depletion of TNC in tumor cells enhanced the growth of CD47^-/- xenografts in vivo and decreased the number of TAM. TNC knockdown also inhibited phagocytosis of CD47^-/- tumor cells in cocultures. Furthermore, TNC stimulated release of proinflammatory factors including TNFα via a Toll-like receptor 4 and STAT3-dependent mechanism in human macrophage cells. These results reveal a vital role for TNC in immunomodulation in brain tumor biology and demonstrate the prominence of the TME extracellular matrix in affecting the antitumor function of brain innate immune cells. SIGNIFICANCE: These findings link TNC to CD47-driven phagocytosis and demonstrate that TNC affects the antitumor function of brain TAM, facilitating the development of novel innate immune system-based therapies for brain tumors.

Inhibitor of DNA binding proteins: implications in human cancer progression and metastasis

Inhibitor of DNA binding (ID) proteins are a class of helix-loop-helix (HLH) transcription regulatory factors that act as dominant-negative antagonists of other basic HLH proteins through the formation of non-functional heterodimers. These proteins have been shown to play critical roles in a wide range of tumor-associated processes, including cell differentiation, cell cycle progression, migration and invasion, epithelial-mesenchymal transition, angiogenesis, stemness, chemoresistance, tumorigenesis, and metastasis. The aberrant expression of ID proteins has not only been detected in many types of human cancers, but is also associated with advanced tumor stages and poor clinical outcome. In this review, we provide an overview of the key biological functions of ID proteins including affiliated signaling pathways. We also describe the regulation of ID proteins in cancer progression and metastasis, and elaborate on expression profiles in cancer and the implications for prognosis. Lastly, we outline strategies for the therapeutic targeting of ID proteins as a promising and effective approach for anticancer therapy.

Krüppel-like factor 9 and histone deacetylase inhibitors synergistically induce cell death in glioblastoma stem-like cells

Background

The dismal prognosis of patients with glioblastoma (GBM) is attributed to a rare subset of cancer stem cells that display characteristics of tumor initiation, growth, and resistance to aggressive treatment involving chemotherapy and concomitant radiation. Recent research on the substantial role of epigenetic mechanisms in the pathogenesis of cancers has prompted the investigation of the enzymatic modifications of histone proteins for therapeutic drug targeting. In this work, we have examined the function of Krüppel-like factor 9 (KLF9), a transcription factor, in chemotherapy sensitization to histone deacetylase inhibitors (HDAC inhibitors).

Methods

Since GBM neurosphere cultures from patient-derived gliomas are enriched for GBM stem-like cells (GSCs) and form highly invasive and proliferative xenografts that recapitulate the features demonstrated in human patients diagnosed with GBM, we established inducible KLF9 expression systems in these GBM neurosphere cells and investigated cell death in the presence of epigenetic modulators such as histone deacetylase (HDAC) inhibitors.

Results

We demonstrated that KLF9 expression combined with HDAC inhibitor panobinostat (LBH589) dramatically induced glioma stem cell death via both apoptosis and necroptosis in a synergistic manner. The combination of KLF9 expression and LBH589 treatment affected cell cycle by substantially decreasing the percentage of cells at S-phase. This phenomenon is further corroborated by the upregulation of cell cycle inhibitors p21 and p27. Further, we determined that KLF9 and LBH589 regulated the expression of pro- and anti- apoptotic proteins, suggesting a mechanism that involves the caspase-dependent apoptotic pathway. In addition, we demonstrated that apoptosis and necrosis inhibitors conferred minimal protective effects against cell death, while inhibitors of the necroptosis pathway significantly blocked cell death.

Conclusions

Our findings suggest a detailed understanding of how KLF9 expression in cancer cells with epigenetic modulators like HDAC inhibitors may promote synergistic cell death through a mechanism involving both apoptosis and necroptosis that will benefit novel combinatory antitumor strategies to treat malignant brain tumors.

Analysis of KLF4 regulated genes in cancer cells reveals a role of DNA methylation in promoter- enhancer interactions

Recent studies have revealed an unexpected role of DNA methylation at promoter regions in transcription activation. However, whether DNA methylation at enhancer regions activates gene expression and influences cellular functions remains to be determined. In this study, by employing the transcription factor krÜppel-like factor 4 (KLF4) that binds to methylated CpGs (mCpGs), we investigated the molecular outcomes of the recruitment of KLF4 to mCpGs at enhancer regions in human glioblastoma cells. First, by integrating KLF4 ChIP-seq, whole-genome bisulfite sequence, and H3K27ac ChIP-seq datasets, we found 1,299 highly methylated (β >0.5) KLF4 binding sites, three-quarters of which were located at putative enhancer regions, including gene bodies and intergenic regions. In the meantime, by proteomics, we identified 16 proteins as putative targets upregulated by KLF4-mCpG binding at enhancer regions. By chromosome conformation capture (3C) analysis, we demonstrated that KLF4 bound to methylated CpGs at the enhancer regions of the B-cell lymphocyte kinase (BLK) and Lim domain only protein 7 (LMO7) genes, and activated their expression via 3D chromatin loop formation with their promoter regions. Expression of mutant KLF4, which lacks KLF4 ability to bind methylated DNA, or removal of DNA methylation in enhancer regions by a DNA methyltransferase inhibitor abolished chromatin loop formation and gene expression, suggesting the essential role of DNA methylation in enhancer-promoter interactions. Finally, we performed functional assays and showed that BLK was involved in glioblastoma cell migration. Together, our study established the concept that DNA methylation at enhancer regions interacts with transcription factors to activate gene expression and influence cellular functions.

Targeting UDP-α-D-glucose 6-dehydrogenase inhibits glioblastoma growth and migration

UDP-glucose 6-dehydrogenase (UGDH) produces UDP-α-D-glucuronic acid, the precursors for glycosaminoglycans (GAGs) and proteoglycans of the extracellular matrix. Elevated GAG formation has been implicated in a variety of human diseases, including glioblastoma (GBM). In our previous study, we found that Krüppel-like factor 4 (KLF4) promotes GBM cell migration by binding to methylated DNA, mainly methylated CpGs (mCpG) and transactivating gene expression. We identified UDGH as one of the downstream targets of KLF4-mCpG binding activity. In this study, we show that KLF4 upregulates UGDH expression in a mCpG-dependent manner, and UGDH is required for KLF4-induced cell migration in vitro. UGDH knockdown decreases GAG abundance in GBM cells, as well as cell proliferation and migration in vitro. In intracranial xenografts, reduced UGDH inhibits tumor growth and migration, accompanied by a decrease in the expression of extracellular matrix proteins such as tenascin C, brevican. Our studies demonstrate a novel DNA methylation-dependent UGDH upregulation by KLF4. Developing UGDH antagonists to decrease the synthesis of extracellular matrix components will be a useful strategy for GBM therapy.

The Id-protein family in developmental and cancer-associated pathways

Inhibitors of DNA binding and cell differentiation (Id) proteins are members of the large family of the helix-loop-helix (HLH) transcription factors, but they lack any DNA-binding motif. During development, the Id proteins play a key role in the regulation of cell-cycle progression and cell differentiation by modulating different cell-cycle regulators both by direct and indirect mechanisms. Several Id-protein interacting partners have been identified thus far, which belong to structurally and functionally unrelated families, including, among others, the class I and II bHLH transcription factors, the retinoblastoma protein and related pocket proteins, the paired-box transcription factors, and the S5a subunit of the 26 S proteasome. Although the HLH domain of the Id proteins is involved in most of their protein-protein interaction events, additional motifs located in their N-terminal and C-terminal regions are required for the recognition of diverse protein partners. The ability of the Id proteins to interact with structurally different proteins is likely to arise from their conformational flexibility: indeed, these proteins contain intrinsically disordered regions that, in the case of the HLH region, undergo folding upon self- or heteroassociation. Besides their crucial role for cell-fate determination and cell-cycle progression during development, other important cellular events have been related to the Id-protein expression in a number of pathologies. Dysregulated Id-protein expression has been associated with tumor growth, vascularization, invasiveness, metastasis, chemoresistance and stemness, as well as with various developmental defects and diseases. Herein we provide an overview on the structural properties, mode of action, biological function and therapeutic potential of these regulatory proteins.

The Id-protein family in developmental and cancer-associated pathways

Inhibitors of DNA binding and cell differentiation (Id) proteins are members of the large family of the helix-loop-helix (HLH) transcription factors, but they lack any DNA-binding motif. During development, the Id proteins play a key role in the regulation of cell-cycle progression and cell differentiation by modulating different cell-cycle regulators both by direct and indirect mechanisms. Several Id-protein interacting partners have been identified thus far, which belong to structurally and functionally unrelated families, including, among others, the class I and II bHLH transcription factors, the retinoblastoma protein and related pocket proteins, the paired-box transcription factors, and the S5a subunit of the 26 S proteasome. Although the HLH domain of the Id proteins is involved in most of their protein-protein interaction events, additional motifs located in their N-terminal and C-terminal regions are required for the recognition of diverse protein partners. The ability of the Id proteins to interact with structurally different proteins is likely to arise from their conformational flexibility: indeed, these proteins contain intrinsically disordered regions that, in the case of the HLH region, undergo folding upon self- or heteroassociation. Besides their crucial role for cell-fate determination and cell-cycle progression during development, other important cellular events have been related to the Id-protein expression in a number of pathologies. Dysregulated Id-protein expression has been associated with tumor growth, vascularization, invasiveness, metastasis, chemoresistance and stemness, as well as with various developmental defects and diseases. Herein we provide an overview on the structural properties, mode of action, biological function and therapeutic potential of these regulatory proteins.

Data defining markers of human neural stem cell lineage potential

Neural stem cells (NSCs) and neural progenitor cells (NPCs) are self-renewing and multipotent cells, however, NPCs are considered to be more lineage-restricted with a reduced self-renewing capacity. We present data comparing the expression of 21 markers encompassing pluripotency, self-renewal (NSC) as well as neuronal and glial (astrocyte and oligodendrocyte) lineage specification and 28 extracellular proteoglycan (PG) genes and their regulatory enzymes between embryonic stem cell (ESC)-derived human NSCs (hNSC H9 cells, Thermo Fisher) and human cortex-derived normal human NPCs (nhNPCs, Lonza). The data demonstrates expression differences of multiple lineage and proteoglycan-associated genes between hNSC H9 cells and nhNPCs. Data interpretation of markers and proteoglycans defining NSC and neural cell lineage characterisation can be found in “Cell surface heparan sulfate proteoglycans as novel markers of human neural stem cell fate determination” (Oikari et al. 2015) [1].

All-trans retinoic acid impairs the vasculogenic mimicry formation ability of U87 stem-like cells through promoting differentiation

The poor therapeutic effect of traditional antiangiogenic therapy on glioblastoma multiforme (GBM) may be attributed to vasculogenic mimicry (VM), which was previously reported to be promoted by cancer stem-like cells (SLCs). All-trans retinoic acid (ATRA), a potent reagent which drives differentiation, was reported to be able to eradicate cancer SLCs in certain malignancies. The aim of the present study was to investigate the effects of ATRA on the VM formation ability of U87 glioblastoma SLCs. The expression of cancer SLC markers CD133 and nestin was detected using immunocytochemistry in order to identify U87 SLCs. In addition, the differentiation of these SLCs was observed through detecting the expression of glial fibrillary acidic protein (GFAP), β-tubulin III and galactosylceramidase (Galc) using immunofluorescent staining. The results showed that the expression levels of GFAP, β-tubulin III and Galc were upregulated following treatment with ATRA in a dose-dependent manner. Furthermore, ATRA significantly reduced the proliferation, invasiveness, tube formation and vascular endothelial growth factor (VEGF) secretion of U87 SLCs. In conclusion, the VM formation ability of SLCs was found to be negatively correlated with differentiation. These results therefore suggested that ATRA may serve as a promising novel agent for the treatment of GBM due to its role in reducing VM formation.

Ionizing radiations sustain glioblastoma cell dedifferentiation to a stem-like phenotype through survivin: possible involvement in radioresistance

Glioblastomas (GBM) are some bad prognosis brain tumors despite a conventional treatment associating surgical resection and subsequent radio-chemotherapy. Among these heterogeneous tumors, a subpopulation of chemo- and radioresistant GBM stem-like cells appears to be involved in the systematic GBM recurrence. Moreover, recent studies showed that differentiated tumor cells may have the ability to dedifferentiate and acquire a stem-like phenotype, a phenomenon also called plasticity, in response to microenvironment stresses such as hypoxia. We hypothesized that GBM cells could be subjected to a similar dedifferentiation process after ionizing radiations (IRs), then supporting the GBM rapid recurrence after radiotherapy. In the present study we demonstrated that subtoxic IR exposure of differentiated GBM cells isolated from patient resections potentiated the long-term reacquisition of stem-associated properties such as the ability to generate primary and secondary neurospheres, the expression of stemness markers and an increased tumorigenicity. We also identified during this process an upregulation of the anti-apoptotic protein survivin and we showed that its specific downregulation led to the blockade of the IR-induced plasticity. Altogether, these results demonstrated that irradiation could regulate GBM cell dedifferentiation via a survivin-dependent pathway. Targeting the mechanisms associated with IR-induced plasticity will likely contribute to the development of some innovating pharmacological strategies for an improved radiosensitization of these aggressive brain cancers.

Taking advantage of neural development to treat glioblastoma

Glioblastoma (GBM) is by far the most common and most malignant primary adult brain tumor (World Health Organization grade IV), containing a fraction of stem-like cells that are highly tumorigenic and multipotent. Recent research has revealed that GBM stem-like cells play important roles in GBM pathogenesis. GBM is thought to arise from genetic anomalies in glial development. Over the past decade, a wide range of studies have shown that several signaling pathways involved in neural development, including basic helix-loop-helix, Wnt-β-catenin, bone morphogenetic proteins-Smads, epidermal growth factor-epidermal growth factor receptor, and Notch, play important roles in GBM pathogenesis. In this review, we highlight the significance of these pathways in the context of developing treatments for GBM. Extrapolating knowledge and concepts from neural development will have significant implications for designing better strategies with which to treat GBM.

BMP signaling induces astrocytic differentiation of clinically derived oligodendroglioma propagating cells

Oligodendrogliomas are a type of glioma that lack detailed investigation because of an inability to cultivate oligodendroglioma cells that faithfully recapitulate their salient qualities. We have successfully isolated and propagated glioma stem-like cells from multiple clinical oligodendroglioma specimens. These oligodendroglioma-propagating cells (OligPC) are multipotent and form xenografts with oligodendroglioma features. Bone morphogenetic proteins (BMP) are considered potent inhibitors of oligodendrogliogenesis during development; therefore, the effects of BMP signaling in OligPCs were characterized. BMP pathway components are expressed by OligPCs and canonical signaling via Smad proteins is intact. This signaling potently depletes CD133-positive OligPCs, decreasing proliferation, and inducing astrocytic differentiation. Furthermore, analyses revealed that cytoplasmic sequestration of the oligodendrocyte differentiation factors OLIG1/2 by the BMP signaling effectors ID2 and ID4 is a plausible underlying mechanism. These findings elucidate the molecular pathways that underlie the effects of BMP signaling on oligodendroglioma stem-like cells.

IMPLICATIONS

Stem-like cells are capable of propagating oligodendrogliomas, and BMP signaling potently diminishes their stemness by inducing astrocytic differentiation, suggesting that BMP activation may be effective as a cancer stem cell-targeted therapy.

Mesenchymal high-grade glioma is maintained by the ID-RAP1 axis

High-grade gliomas (HGGs) are incurable brain tumors that are characterized by the presence of glioma-initiating cells (GICs). GICs are essential to tumor aggressiveness and retain the capacity for self-renewal and multilineage differentiation as long as they reside in the perivascular niche. ID proteins are master regulators of stemness and anchorage to the extracellular niche microenvironment, suggesting that they may play a role in maintaining GICs. Here, we modeled the probable therapeutic impact of ID inactivation in HGG by selective ablation of Id in tumor cells and after tumor initiation in a new mouse model of human mesenchymal HGG. Deletion of 3 Id genes induced rapid release of GICs from the perivascular niche, followed by tumor regression. GIC displacement was mediated by derepression of Rap1gap and subsequent inhibition of RAP1, a master regulator of cell adhesion. We identified a signature module of 5 genes in the ID pathway, including RAP1GAP, which segregated 2 subgroups of glioma patients with markedly different clinical outcomes. The model-informed survival analysis together with genetic and functional studies establish that ID activity is required for the maintenance of mesenchymal HGG and suggest that pharmacological inactivation of ID proteins could serve as a therapeutic strategy.

Id-1 is a key transcriptional regulator of glioblastoma aggressiveness and a novel therapeutic target

Glioblastoma is the most common form of primary adult brain tumors. A majority of glioblastomas grow invasively into distant brain tissue, leading to tumor recurrence, which is ultimately incurable. It is, therefore, essential to discover master regulators that control glioblastoma invasiveness and target them therapeutically. We show here that the transcriptional regulator Id-1 plays a critical role in modulating the invasiveness of glioblastoma cell lines and primary glioblastoma cells. Id-1 expression levels positively correlate with glioma cell invasiveness in culture and with histopathologic grades in patient biopsies. Id-1 knockdown dramatically reduces glioblastoma cell invasion that is accompanied by profound morphologic changes and robust reduction in expression levels of "mesenchymal" markers, as well as inhibition of self-renewal potential and downregulation of glioma stem cell markers. Importantly, genetic knockdown of Id-1 leads to a significant increase in survival in an orthotopic model of human glioblastoma. Furthermore, we show that a nontoxic compound, cannabidiol, significantly downregulates Id-1 gene expression and associated glioma cell invasiveness and self-renewal. In addition, cannabidiol significantly inhibits the invasion of glioblastoma cells through an organotypic brain slice and glioma progression in vivo. Our results suggest that Id-1 regulates multiple tumor-promoting pathways in glioblastoma and that drugs targeting Id-1 represent a novel and promising strategy for improving the therapy and outcome of patients with glioblastoma.