The transcriptional coactivator TAZ regulates mesenchymal differentiation in malignant glioma.

Recent molecular classification of glioblastoma (GBM) has shown that patients with a mesenchymal (MES) gene expression signature exhibit poor overall survival and treatment resistance. Using regulatory network analysis of available expression microarray data sets of GBM, including The Cancer Genome Atlas (TCGA), we identified the transcriptional coactivator with PDZ-binding motif (TAZ), to be highly associated with the MES network. TAZ expression was lower in proneural (PN) GBMs and lower-grade gliomas, which correlated with CpG island hypermethylation of the TAZ promoter compared with MES GBMs. Silencing of TAZ in MES glioma stem cells (GSCs) decreased expression of MES markers, invasion, self-renewal, and tumor formation. Conversely, overexpression of TAZ in PN GSCs as well as murine neural stem cells (NSCs) induced MES marker expression and aberrant osteoblastic and chondrocytic differentiation in a TEAD-dependent fashion. Using chromatin immunoprecipitation (ChIP), we show that TAZ is directly recruited to a majority of MES gene promoters in a complex with TEAD2. The coexpression of TAZ, but not a mutated form of TAZ that lacks TEAD binding, with platelet-derived growth factor-B (PDGF-B) resulted in high-grade tumors with MES features in a murine model of glioma. Our studies uncover a direct role for TAZ and TEAD in driving the MES differentiation of malignant glioma.

Suppression of RAF/MEK or PI3K synergizes cytotoxicity of receptor tyrosine kinase inhibitors in glioma tumor-initiating cells.

BACKGROUND: The majority of glioblastomas have aberrant receptor tyrosine kinase (RTK)/RAS/phosphoinositide 3 kinase (PI3K) signaling pathways and malignant glioma cells are thought to be addicted to these signaling pathways for their survival and proliferation. However, recent studies suggest that monotherapies or inappropriate combination therapies using the molecular targeted drugs have limited efficacy possibly because of tumor heterogeneities, signaling redundancy and crosstalk in intracellular signaling network, indicating necessity of rationale and methods for efficient personalized combination treatments. Here, we evaluated the growth of colonies obtained from glioma tumor-initiating cells (GICs) derived from glioma sphere culture (GSC) in agarose and examined the effects of combination treatments on GICs using targeted drugs that affect the signaling pathways to which most glioma cells are addicted. METHODS: Human GICs were cultured in agarose and treated with inhibitors of RTKs, non-receptor kinases or transcription factors. The colony number and volume were analyzed using a colony counter, and Chou-Talalay combination indices were evaluated. Autophagy and apoptosis were also analyzed. Phosphorylation of proteins was evaluated by reverse phase protein array and immunoblotting. RESULTS: Increases of colony number and volume in agarose correlated with the Gompertz function. GICs showed diverse drug sensitivity, but inhibitions of RTK and RAF/MEK or PI3K by combinations such as EGFR inhibitor and MEK inhibitor, sorafenib and U0126, erlotinib and BKM120, and EGFR inhibitor and sorafenib showed synergy in different subtypes of GICs. Combination of erlotinib and sorafenib, synergistic in GSC11, induced apoptosis and autophagic cell death associated with suppressed Akt and ERK signaling pathways and decreased nuclear PKM2 and ß-catenin in vitro, and tended to improve survival of nude mice bearing GSC11 brain tumor. Reverse phase protein array analysis of the synergistic treatment indicated involvement of not only MEK and PI3K signaling pathways but also others associated with glucose metabolism, fatty acid metabolism, gene transcription, histone methylation, iron transport, stress response, cell cycle, and apoptosis. CONCLUSION: Inhibiting RTK and RAF/MEK or PI3K could induce synergistic cytotoxicity but personalization is necessary. Examining colonies in agarose initiated by GICs from each patient may be useful for drug sensitivity testing in personalized cancer therapy.

Signal transducer and activator of transcription 5b drives malignant progression in a PDGFB-dependent proneural glioma model by suppressing apoptosis.

Signal transducer and activator of transcription 5b (STAT5b) is likely the relevant STAT5 isoform with respect to the process of malignant progression in gliomas. STAT5b is a latent cytoplasmic protein involved in cell signaling through the modulation of growth factors, apoptosis, and angiogenesis. Previous in vitro studies have shown increased STAT5b expression in glioblastomas relative to low-grade tumors and normal brain. We recently demonstrated that phosphorylated STAT5b associates with delta epidermal growth factor receptor in the nucleus and subsequently binds the promoters of downstream effector molecules, including aurora kinase A. Analysis of TCGA dataset reveals that STAT5b is predominantly expressed in proneural (PN) gliomas relative to mesenchymal and neural gliomas. Here, we modeled ectopic expression of STAT5b in vivo using a platelet-derived growth factor subunit B (PDGFB)-dependent mouse model of PN glioma to determine its effect on tumor formation and progression. We showed that coexpression of STAT5b and PDGFB in mice yielded a significantly higher rate of high-grade gliomas than PDGFB expression alone. We also observed shorter survival in the combined expression set. High-grade tumors from the STAT5b + PDGFB expression set were found to have a lower rate of apoptosis than those from PDGFB alone. Furthermore, we showed that increased expression of STAT5b + PDGFB led to increased expression of downstream STAT5b targets, including Bcl-xL, cyclin D1 and aurora kinase A in high-grade tumors when compared to tumors derived from PDGFB alone. Our findings show that STAT5b promotes the malignant transformation of gliomas, particularly the PN subtype, and is a potential therapeutic target.

Bcl-2 promotes malignant progression in a PDGF-B-dependent murine model of oligodendroglioma.

A significant subset of gliomas arises after activation of the proproliferative platelet-derived growth factor (PDGF) pathway. The progression of low-grade gliomas to more malignant tumors may be due to oncogenic cellular programs combining with those suppressing apoptosis. Antiapoptotic genes are overexpressed in a variety of cancers, and the antiapoptotic gene, BCL2, is associated with treatment resistance and tumor recurrence in gliomas. However, the impact of antiapoptotic gene expression to tumor formation and progression is unclear. We overexpressed Bcl-2 in a PDGFB-dependent mouse model of oligodendroglioma, a common glioma subtype, to assess its effect in vivo. We hypothesized that the antiapoptotic effect would complement the proproliferative effect of PDGFB to promote tumor formation and progression to anaplastic oligodendroglioma (AO). Here, we show that coexpression of PDGFB and Bcl-2 results in a higher overall tumor formation rate compared to PDGFB alone. Coexpression of PDGFB and Bcl-2 promotes progression to AO with prominent foci of necrosis, a feature of high-grade gliomas. Median tumor latency was shorter in mice injected with PDGFB and Bcl-2 compared to those injected with PDGFB alone. Although independent expression of Bcl-2 was insufficient to induce tumors, suppression of apoptosis (detected by cleaved caspase-3 expression) was more pronounced in AOs induced by PDGFB and Bcl-2 compared to those induced by PDGFB alone. Tumor cell proliferation (detected by phosphohistone H3 activity) was also more robust in high-grade tumors induced by PDGFB and Bcl-2. Our results indicate that suppressed apoptosis enhances oligodendroglioma formation and engenders a more malignant phenotype.

Anti-PD-1 Induces M1 Polarization in the Glioma Microenvironment and Exerts Therapeutic Efficacy in the Absence of CD8 Cytotoxic T Cells.

PURPOSE: Anti-programmed cell death protein 1 (PD-1) therapy has demonstrated inconsistent therapeutic results in patients with glioblastoma (GBM) including those with profound impairments in CD8 T-cell effector responses. EXPERIMENTAL DESIGN: We ablated the CD8α gene in BL6 mice and intercrossed them with Ntv-a mice to determine how CD8 T cells affect malignant progression in forming endogenous gliomas. Tumor-bearing mice were treated with PD-1 to determine the efficacy of this treatment in the absence of T cells. The tumor microenvironment of treated and control mice was analyzed by IHC and FACS. RESULTS: We observed a survival benefit in immunocompetent mice with endogenously arising intracranial glioblastomas after intravenous administration of anti-PD-1. The therapeutic effect of PD-1 administration persisted in mice even after genetic ablation of the CD8 gene (CD8-/-). CD11b+ and Iba1+ monocytes and macrophages were enriched in the glioma microenvironment of the CD8-/- mice. The macrophages and microglia assumed a proinflammatory M1 response signature in the setting of anti-PD-1 blockade through the elimination of PD-1-expressing macrophages and microglia in the tumor microenvironment. Anti-PD-1 can inhibit the proliferation of and induce apoptosis of microglia through antibody-dependent cellular cytotoxicity, as fluorescently labeled anti-PD-1 was shown to gain direct access to the glioma microenvironment. CONCLUSIONS: Our results show that the therapeutic effect of anti-PD-1 blockade in GBM may be mediated by the innate immune system, rather than by CD8 T cells. Anti-PD-1 immunologically modulates innate immunity in the glioma microenvironment-likely a key mode of activity.

Analysis of the inhibitors of apoptosis identifies BIRC3 as a facilitator of malignant progression in glioma.

Gliomas, the most common primary brain tumor in humans, include a spectrum of disease. High-grade gliomas (HGG), such as glioblastoma, may arise from low-grade gliomas (LGG) that have a more indolent course. The process of malignant transformation (MT) of LGG to HGG is poorly understood but likely involves the activation of signaling programs that suppress apoptosis. We previously showed that Survivin (BIRC5) plays a role in malignant progression of glioma. Here, we investigated the role of the remaining members of the Inhibitors of Apoptosis (IAP) family on promoting MT in glioma. Utilizing expression data from the cancer genome atlas (TCGA), we identified BIRC3 as a key facilitator of MT from LGG to HGG. TCGA HGGs with high expression of BIRC 3 demonstrated a survival disadvantage and expression levels of BIRC3 were also significantly higher in TCGA HGG compared to TCGA LGG cases. We validated our findings from TCGA by using matched human specimens to show that BIRC expression is increased in HGG compared to their precursor LGG lesions. Using a unique murine model of glioma, we show that overexpression of BIRC3 promotes higher grade glioma and significantly reduces tumor-free survival in mice.

Survivin transcript variant 2 drives angiogenesis and malignant progression in proneural gliomas.

BACKGROUND: The influence of survivin isoforms on outcome in glioblastoma is poorly understood. We analyzed the dominant anti-apoptotic transcript variants of survivin using expression data and modeled them in vivo to determine their impact on glioma formation and progression. METHODS: Using data from low- and high-grade glioma knowledge bases, we expressed the anti-apoptotic isoforms of survivin (transcript variants 1 and 2) in vivo using the RCAS/Ntv-a model of murine glioma. RESULTS: In low-grade gliomas, survivin RNA expression was increased in 22 of 167 (13.2%) of cases and was associated with shortened survival (P = .005). Survivin RNA was preferentially expressed in proneural (PN) relative to mesenchymal high-grade gliomas (P < .0001). In proneural gliomas, survivin was expressed in 94 of 141 (67%) of cases and was associated with shorter disease-free survival (P = .04). In a platelet-derived growth factor subunit B-dependent murine model of PN glioma, ectopic expression of variant 1 yielded tumors in 28 of 30 (93%) of mice, of which 25% were high-grade tumors, whereas ectopic expression of variant 2 yielded tumors in 27 of 28 (96%), of which 81% were high-grade tumors (P < .0001). Microvascular proliferation was significantly more prominent (P < .0001), and tumor-free survival was shorter in mice with variant 2 than variant 1-derived tumors (P = .01). CONCLUSIONS: Survivin expression in low-grade gliomas is associated with poor survival and is preferentially expressed in PN gliomas. Compared with variant 1, variant 2 was associated with poorer survival and promoted malignant progression, angiogenesis, and shorter tumor-free survival in the PN murine model. Inhibiting survivin transcript variant 2, rather than variant 1 (the common isoform), may be an effective treatment strategy for glioma.

Effect of miR-142-3p on the M2 macrophage and therapeutic efficacy against murine glioblastoma.

BACKGROUND: The immune therapeutic potential of microRNAs (miRNAs) in the context of tumor-mediated immune suppression has not been previously described for monocyte-derived glioma-associated macrophages, which are the largest infiltrating immune cell population in glioblastomas and facilitate gliomagenesis. METHODS: An miRNA microarray was used to compare expression profiles between human glioblastoma-infiltrating macrophages and matched peripheral monocytes. The effects of miR-142-3p on phenotype and function of proinflammatory M1 and immunosuppressive M2 macrophages were determined. The therapeutic effect of miR-142-3p was ascertained in immune-competent C57BL/6J mice harboring intracerebral GL261 gliomas and in genetically engineered Ntv-a mice bearing high-grade gliomas. Student t test was used to evaluate the differences between ex vivo datasets. Survival was analyzed with the log-rank test and tumor sizes with linear mixed models and F test. All statistical tests were two-sided. RESULTS: miR-142-3p was the most downregulated miRNA (approximately 4.95-fold) in glioblastoma-infiltrating macrophages. M2 macrophages had lower miR-142-3p expression relative to M1 macrophages (P = .03). Overexpression of miR-142-3p in M2 macrophages induced selective modulation of transforming growth factor beta receptor 1, which led to subsequent preferential apoptosis in the M2 subset (P = .01). In vivo miR-142-3p administration resulted in glioma growth inhibition (P = .03, n = 5) and extended median survival (miR-142-3p-treated C57BL/6J mice vs scramble control: 31 days vs 23.5 days, P = .03, n = 10; miR-142-3p treated Ntv-a mice vs scramble control: 32 days vs 24 days, P = .03, n = 9), with an associated decrease in infiltrating macrophages (R (2) = .303). CONCLUSIONS: These data indicate a unique role of miR-142-3p in glioma immunity by modulating M2 macrophages through the transforming growth factor beta signaling pathway.

Immune heterogeneity of glioblastoma subtypes: extrapolation from the cancer genome atlas.

PURPOSE: The molecular heterogeneity of glioblastoma has been well recognized and has resulted in the generation of molecularly defined subtypes. These subtypes (classical, neural, mesenchymal, and proneural) are associated with particular signaling pathways and differential patient survival. Less understood is the correlation between these glioblastoma subtypes with immune system effector responses, immune suppression and tumor-associated and tumor-specific antigens. The role of the immune system is becoming increasingly relevant to treatment as new agents are being developed to target mediators of tumor-induced immune suppression which is well documented in glioblastoma. EXPERIMENTAL DESIGN: To ascertain the association of antigen expression, immune suppression, and effector response genes within glioblastoma subtypes, we analyzed the Cancer Genome Atlas (TCGA) glioblastoma database. RESULTS: We found an enrichment of genes within the mesenchymal subtype that are reflective of anti-tumor proinflammatory responses, including both adaptive and innate immunity and immune suppression. CONCLUSIONS: These results indicate that distinct glioma antigens and immune genes demonstrate differential expression between glioblastoma subtypes and this may influence responses to immune therapeutic strategies in patients depending on the subtype of glioblastoma they harbor.

miR-124 inhibits STAT3 signaling to enhance T cell-mediated immune clearance of glioma.

miRNAs (miR) have been shown to modulate critical gene transcripts involved in tumorigenesis, but their role in tumor-mediated immunosuppression is largely unknown. On the basis of miRNA gene expression in gliomas using tissue microarrays, in situ hybridization, and molecular modeling, miR-124 was identified as a lead candidate for modulating STAT3 signaling, a key pathway mediating immunosuppression in the tumor microenvironment. miR-124 is absent in all grades and pathologic types of gliomas. Upon upregulating miR-124 in glioma cancer stem cells (gCSC), the STAT3 pathway was inhibited, and miR-124 reversed gCSC-mediated immunosuppression of T-cell proliferation and induction of forkhead box P3 (Foxp3)(+) regulatory T cells (Treg). Treatment of T cells from immunosuppressed glioblastoma patients with miR-124 induced marked effector response including upregulation of interleukin (IL)-2, IFN-γ, and TNF-α. Both systemic administration of miR-124 or adoptive miR-124-transfected T-cell transfers exerted potent anti-glioma therapeutic effects in clonotypic and genetically engineered murine models of glioblastoma and enhanced effector responses in the local tumor microenvironment. These therapeutic effects were ablated in both CD4(+)- and CD8(+)-depleted mice and nude mouse systems, indicating that the therapeutic effect of miR-124 depends on the presence of a T-cell-mediated antitumor immune response. Our findings highlight the potential application of miR-124 as a novel immunotherapeutic agent for neoplasms and serve as a model for identifying miRNAs that can be exploited as immunotherapeutics.

WIP1 enhances tumor formation in a sonic hedgehog-dependent model of medulloblastoma.

BACKGROUND: A significant number of medulloblastomas (MBs) originate from abnormal activation of the sonic hedgehog/patched (SHH/PTC) signaling pathway. Although p53 deficiency enhances tumor formation in mice, inactivation of the p53 gene is seen in a minority of MBs. Wild-type p53-induced phosphatase 1 (WIP1) downregulates p53 expression and has been shown to be overexpressed in MBs. OBJECTIVE: We tested the hypothesis that overexpression of WIP1 enhances tumor formation in an SHH-dependent model of MB. METHODS: We used the RCAS/Ntv-a system to study the effect of WIP1 in vitro and in vivo. We transfected A375-TVA cells with RCAS-WIP1 and then exposed these cells to cisplatin to determine the effect on p53 expression. We modeled ectopic WIP1 expression independently and in combination with SHH in the cerebella of newborn mice to assess the effect on tumor formation. Mice were observed for 12 weeks or until neurological symptoms developed. The brains were examined for tumor formation. RESULTS: A375-TVA cells infected with RCAS-WIP1 demonstrated reduced p53 expression after exposure to cisplatin compared with controls. We detected tumors in 12 of 35 mice (34%) injected with RCAS-WIP1 and RCAS-SHH. Tumors were detected in 3 of 40 mice (8%) injected with RCAS-SHH alone. The difference in tumor formation rates was significant (χ(2) test, P = < .01). Tumors did not form in mice injected with RCAS-WIP1 alone. CONCLUSION: We show that ectopic expression of WIP1 cooperates with SHH to enhance formation of MB, although it is insufficient to induce tumors independently. Our results verify the role of WIP1 in MB formation and provide a crucial link to the inactivation of p53 in MBs.

Signal transducer and activator of transcription 3 promotes angiogenesis and drives malignant progression in glioma.

Signal transducer and activator of transcription (STAT) 3 has been described as a "master regulator" of signaling pathways involved in the transition from low-grade glioma (LGG) to high-grade glioma (HGG). Although STAT3 is overexpressed in HGGs, it remains unclear whether its overexpression is sufficient to induce or promote the malignant progression of glioma. To characterize the effect of STAT3 expression on tumor progression in vivo, we expressed the STAT3 gene in glioneuronal progenitor cells in mice. STAT3 was expressed alone or concurrently with platelet-derived growth factor B (PDGFB), a well-described initiator of LGG. STAT3 alone was insufficient to induce tumor formation; however, coexpression of STAT3 with PDGFB in mice resulted in a significantly higher incidence of HGGs than PDGFB alone. The median symptomatic tumor latency in mice coexpressing STAT3 and PDGFB was significantly shorter, and mice that developed symptomatic tumors demonstrated significantly higher expression of phosphorylated STAT3 intratumorally. In HGGs, expression of STAT3 was associated with suppression of apoptosis and an increase in tumor cell proliferation. HGGs induced by STAT3 and PDGFB also displayed frequent foci of necrosis and microvascular proliferation. The expression of CD31 (a marker of endothelial proliferation) was significantly higher in tumors induced by coexpression of STAT3 and PDGFB. When mice injected with PDGFB and STAT3 were treated with a STAT3 inhibitor, median survival increased and the incidence of HGG and CD31 expression decreased significantly. These results demonstrate that STAT3 promotes the malignant progression of glioma. Inhibiting STAT3 expression mitigates tumor progression and improves survival, validating it as a therapeutic target.

Mesenchymal stem cells display tumor-specific tropism in an RCAS/Ntv-a glioma model.

Bone marrow-derived mesenchymal stem cells (MSCs) have been shown to localize to gliomas and deliver therapeutic agents. However, the clinical translation of MSCs remains poorly defined because previous studies relied on glioma models with uncertain relevance to human disease, typically xenograft models in immunocompromised mice. To address this shortcoming, we used the RCAS/Ntv-a system, in which endogenous gliomas that recapitulate the tumor and stromal features of human gliomas develop in immunocompetent mice. MSCs were harvested from bone marrow of Ntv-a mice and injected into the carotid artery of Ntv-a mice previously inoculated with RCAS-PDGF-B and RCAS-IGFBP2 to induce malignant gliomas (n = 9). MSCs were labeled with luciferase for in vivo bioluminescence imaging (BLI). After intra-arterial injection, BLI revealed MSCs in the right frontal lobe in seven of nine mice. At necropsy, gliomas were detected within the right frontal lobe in all these mice, correlating with the location of the MSCs. In the two mice without MSCs based on BLI, no tumor was found, indicating that MSC localization was tumor specific. In another cohort of mice (n = 9), MSCs were labeled with SP-DiI, a fluorescent vital dye. After intra-arterial injection, fluorescence microscopy revealed SP-DiI-labeled MSCs throughout tumors 1 to 7 days after injection but not in nontumoral areas of the brain. MSCs injected intravenously did not localize to tumors (n = 12). We conclude that syngeneic MSCs are capable of homing to endogenous gliomas in immunocompetent mice. These findings support the use of MSCs as tumor-specific delivery vehicles for treating gliomas.

Intratumoral mediated immunosuppression is prognostic in genetically engineered murine models of glioma and correlates to immunotherapeutic responses.

PURPOSE: Preclinical murine model systems used for the assessment of therapeutics have not been predictive of human clinical responses, primarily because their clonotypic nature does not recapitulate the heterogeneous biology and immunosuppressive mechanisms of humans. Relevant model systems with mice that are immunologically competent are needed to evaluate the efficacy of therapeutic agents, especially immunotherapeutics. EXPERIMENTAL DESIGN: Using the RCAS/Ntv-a system, mice were engineered to coexpress platelet-derived growth factor B (PDGF-B) receptor + B-cell lymphoma 2 (Bcl-2) under the control of the glioneuronal specific Nestin promoter. The degree and type of tumor-mediated immunosuppression were determined in these endogenously arising gliomas on the basis of the presence of macrophages and regulatory T cells. The immunotherapeutic agent WP1066 was tested in vivo to assess therapeutic efficacy and immunomodulation. RESULTS: Ntv-a mice were injected with RCAS vectors to express PDGF-B + Bcl-2, resulting in both low- and high-grade gliomas. Consistent with observations in human high-grade gliomas, mice with high-grade gliomas also developed a marked intratumoral influx of macrophages that was influenced by tumor signal transducer and activator of transduction 3 (STAT3) expression. The presence of intratumoral F4/80 macrophages was a negative prognosticator for long-term survival. In mice coexpressing PDGF-B + Bcl-2that were treated with WP1066, there was 55.5% increase in median survival time (P < 0.01), with an associated inhibition of intratumoral STAT3 and macrophages. CONCLUSIONS: Although randomization is necessary for including mice in a therapeutic trial, these murine model systems are more suitable for testing therapeutics, especially immunotherapeutics, in the context of translational studies.