Glioma Radioresistance Research Articles

Abstract A45: Semaphorin 3C mediates glioma stem cell tumorigenicity and radioresistance

Abstract Purpose: Different cancer cell compartments often communicate through soluble factors to facilitate tumor growth. Glioma stem cells (GSCs) are a subset of tumor cells that resist standard therapy to contribute to disease progression. How GSCs employ a distinct secretory program to communicate with and nurture each other over the non-stem tumor cell (NSTC) population is not well defined. Methods/Materials: GSCs were enriched from patient specimens and functionally validated. We used a candidate approach to screen for secreted factors that were preferentially used by GSCs but not NSTCs. Expression was assessed by Western blot, and confirmed by immunohistochemistry and immunofluorescence in vitro and in human and mouse GBM tissues. Ligand-receptor interactions were confirmed by co-immunoprecipitation. Rac activity was assessed by Rac-GTP pulldown. Co-culture epistasis experiments were performed using knockdown and recombinant protein approaches. Glioma stem cell marker analysis (CD133, Sox2, Olig2), cell viability (ATP-dependent assays), apoptosis (TUNEL, PI/AnnexinV, cleaved caspase 3/7), self-renewal (tumorsphere formation and limiting dilution assay) and invasion (transwell migration assay) were performed. GSCs were separated in PlexinD1 hi and lo populations by FACS. Radiation (3 Gy) was delivered by Cs-137 irradiator. GSCs were used to establish orthotopic xenograft models of glioblastoma and tumor growth and animal survival were assessed. Results: We have found that GSCs preferentially secrete Sema3C and coordinately express PlexinA2/D1 receptors to activate Rac1/NFkB signaling in an autocrine and paracrine loop to promote their own survival. Importantly, Sema3C is not expressed in neural progenitor cells (NPCs) or NSTCs. Disruption of Sema3C induced apoptosis of GSCs, but not NPCs or NSTCs, and suppressed tumor growth in orthotopic models of glioblastoma. Introduction of activated Rac1 rescued the Sema3C knockdown phenotype in vivo. Furthermore, GSCs that expressed high levels of PlexinD1 receptors were more resistant to radiation, and Sema3C and PlexinD1 were upregulated after irradiation. Together, these data support that Sema3C signaling contributes to radiation resistance. Conclusions: Our study supports targeting Sema3C to break this GSC-specific autocrine/paracrine loop to improve glioblastoma treatment, potentially with a high therapeutic index. Citation Format: Jennifer Yu, Jianghong Man, Shoemake Jocelyn, Wu Qiulian, Jeremy Rich, Shideng Bao. Semaphorin 3C mediates glioma stem cell tumorigenicity and radioresistance. [abstract]. In: Proceedings of the AACR Special Conference: Advances in Brain Cancer Research; May 27-30, 2015; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2015;75(23 Suppl):Abstract nr A45.

The NG2 Proteoglycan Protects Oligodendrocyte Precursor Cells against Oxidative Stress via Interaction with OMI/HtrA2.

The NG2 proteoglycan is characteristically expressed by oligodendrocyte progenitor cells (OPC) and also by aggressive brain tumours highly resistant to chemo- and radiation therapy. Oligodendrocyte-lineage cells are particularly sensitive to stress resulting in cell death in white matter after hypoxic or ischemic insults of premature infants and destruction of OPC in some types of Multiple Sclerosis lesions. Here we show that the NG2 proteoglycan binds OMI/HtrA2, a mitochondrial serine protease which is released from damaged mitochondria into the cytosol in response to stress. In the cytosol, OMI/HtrA2 initiates apoptosis by proteolytic degradation of anti-apoptotic factors. OPC in which NG2 has been downregulated by siRNA, or OPC from the NG2-knockout mouse show an increased sensitivity to oxidative stress evidenced by increased cell death. The proapoptotic protease activity of OMI/HtrA2 in the cytosol can be reduced by the interaction with NG2. Human glioma expressing high levels of NG2 are less sensitive to oxidative stress than those with lower NG2 expression and reducing NG2 expression by siRNA increases cell death in response to oxidative stress. Binding of NG2 to OMI/HtrA2 may thus help protect cells against oxidative stress-induced cell death. This interaction is likely to contribute to the high chemo- and radioresistance of glioma.

Wnt/β-catenin pathway involvement in ionizing radiation-induced invasion of U87 glioblastoma cells.

Radiotherapy has been reported to promote the invasion of glioblastoma cells; however, the underlying mechanisms remain unclear. Here, we investigated the role of the Wnt/β-catenin pathway in radiation-induced invasion of glioblastoma cells. U87 cells were irradiated with 3 Gy or sham irradiated in the presence or absence of the Wnt/β-catenin pathway inhibitor XAV 939. Cell invasion was determined by an xCELLigence real-time cell analyser and matrigel invasion assays. The intracellular distribution of β-catenin in U87 cells with or without irradiation was examined by immunofluorescence and Western blotting of nuclear fractions. We next investigated the effect of irradiation on Wnt/β-catenin pathway activity using TOP/FOP flash luciferase assays and quantitative polymerase chain reaction analysis of β-catenin target genes. The expression levels and activities of two target genes, matrix metalloproteinase (MMP)-2 and MMP-9, were examined further by Western blotting and zymography. U87 cell invasiveness was increased significantly by ionizing radiation. Interestingly, ionizing radiation induced nuclear translocation and accumulation of β-catenin. Moreover, we found increased β-catenin/TCF transcriptional activities, followed by up-regulation of downstream genes in the Wnt/β-catenin pathway in irradiated U87 cells. Importantly, inhibition of the Wnt/β-catenin pathway by XAV 939, which promotes degradation of β-catenin, significantly abrogated the pro-invasion effects of irradiation. Mechanistically, XAV 939 suppressed ionizing radiation-triggered up-regulation of MMP-2 and MMP-9, and inhibited the activities of these gelatinases. Our data demonstrate a pivotal role of the Wnt/β-catenin pathway in ionizing radiation-induced invasion of glioblastoma cells, and suggest that targeting β-catenin is a promising therapeutic approach to overcoming glioma radioresistance.

STAT3 Serine 727 Phosphorylation: A Relevant Target to Radiosensitize Human Glioblastoma.

Radiotherapy is an essential component of glioma standard treatment. Glioblastomas (GBM), however, display an important radioresistance leading to tumor recurrence. To improve patient prognosis, there is a need to radiosensitize GBM cells and to circumvent the mechanisms of resistance caused by interactions between tumor cells and their microenvironment. STAT3 has been identified as a therapeutic target in glioma because of its involvement in mechanisms sustaining tumor escape to both standard treatment and immune control. Here, we studied the role of STAT3 activation on tyrosine 705 (Y705) and serine 727 (S727) in glioma radioresistance. This study explored STAT3 phosphorylation on Y705 (pSTAT3-Y705) and S727 (pSTAT3-S727) in glioma cell lines and in clinical samples. Radiosensitizing effect of STAT3 activation down-modulation by Gö6976 was explored. In a panel of 15 human glioma cell lines, we found that the level of pSTAT3-S727 was correlated to intrinsic radioresistance. Moreover, treating GBM cells with Gö6976 resulted in a highly significant radiosensitization associated to a concomitant pSTAT3-S727 down-modulation only in GBM cell lines that exhibited no or weak pSTAT3-Y705. We report the constitutive activation of STAT3-S727 in all GBM clinical samples. Targeting pSTAT3-S727 mainly in pSTAT3-Y705-negative GBM could be a relevant approach to improve radiation therapy.

Silencing of R-Spondin1 increases radiosensitivity of glioma cells

Although radiation therapy is the most effective postoperative adjuvant treatment, it does not substantially improve the long-term outcomes of glioma patients because of the characteristic radioresistance of glioma. We found that R-Spondin1 (Rspo1) expression was elevated in high-grade gliomas and was associated with worse overall survival and disease-free survival. Rspo1 expression was also associated with reduced survival rates in glioma patients after treatment with radiotherapy and temozolomide (RT-TMZ). Importantly, Rspo1 was dramatically upregulated after radiation treatment in patients with glioma. Rspo1 silencing by shRNA potentiated glioma cell death upon radiation treatment. In a xenograft nude mouse model, combining radiation and silencing of Rspo1 potentiated tumor growth inhibition. Thus, combining radiotherapy with silencing of Rspo1 is a potential therapeutic approach.

Pterostilbene suppressed irradiation-resistant glioma stem cells by modulating GRP78/miR-205 axis

Glioblastoma multiforme (GBM) is the most aggressive type characterized by relapse and resistance even with the combination of radio- and chemotherapy. The presence of glioma stem cells (GSCs) has been shown to contribute to tumorigenesis, recurrence and treatment resistance. Particularly, CD133-positive glioma cells have been shown to represent the subpopulation that confers glioma radioresistance and suggested to be the source of tumor recurrence after radiation. Thus, a better understanding and the development of agents which target GSCs could potentially lead to a significant improvement in treating GBM patients. Here, we demonstrated that GRP78 (an antistress protein) was highly expressed in GBM cells along with β-catenin and Notch and correlated to the development of GSCs. CD133+ GSCs exhibited enhanced migration/invasion and self-renewal abilities. When GRP78 was silenced, GSC properties were suppressed and the sensitivity towards irradiation increased. In addition, the level of microRNA 205 appeared to be negatively associated with GRP78 expression. Our previous study indicated that pterostilbene (PT) possessed anticancer stem cell properties in hepatocellular carcinoma. Thus, we examined whether PT is also effective against GSCs. We found that PT-treated GSCs exhibited suppressed self-renewal and irradiation-resistant abilities. PT-mediated effects were associated with an increase of miR-205. Finally, we showed that PT treatment suppressed tumorigenesis in GSC xenograft mice. In conclusion, we provided evidence that GRP78/miR-205 axis played an important role in GSC maintenance and irradiation resistance. PT treatment suppressed GSC development via negatively modulating GRP78 signaling. PT may be considered for combined therapeutic agent to enhance irradiation efficacy in GBM patients.

A functional screen identifies miRs that induce radioresistance in glioblastomas.

The efficacy of radiotherapy in many tumor types is limited by normal tissue toxicity and by intrinsic or acquired radioresistance. Therefore, it is essential to understand the molecular network responsible for regulating radiosensitivity/resistance. Here, an unbiased functional screen identified four microRNAs (miR1, miR125a, miR150, and miR425) that induce radioresistance. Considering the clinical importance of radiotherapy for patients with glioblastoma, the impact of these miRNAs on glioblastoma radioresistance was investigated. Overexpression of miR1, miR125a, miR150, and/or miR425 in glioblastoma promotes radioresistance through upregulation of the cell-cycle checkpoint response. Conversely, antagonizing with antagomiRs sensitizes glioblastoma cells to irradiation, suggesting their potential as targets for inhibiting therapeutic resistance. Analysis of glioblastoma datasets from The Cancer Genome Atlas (TCGA) revealed that these miRNAs are expressed in glioblastoma patient specimens and correlate with TGFβ signaling. Finally, it is demonstrated that expression of miR1 and miR125a can be induced by TGFβ and antagonized by a TGFβ receptor inhibitor. Together, these results identify and characterize a new role for miR425, miR1, miR125, and miR150 in promoting radioresistance in glioblastomas and provide insight into the therapeutic application of TGFβ inhibitors in radiotherapy. Systematic identification of miRs that cause radioresistance in gliomas is important for uncovering predictive markers for radiotherapy or targets for overcoming radioresistance.

Abstract 3944: Caveolin-mediated Tie2 nuclear translocation results in enhanced NHEJ repair and glioma radioresistance

Abstract Glioblastoma is the most frequent subtype of primary brain tumor in adults and are resistant to current strategies of surgery, irradiation, and chemotherapy. Patients suffering from this disease exhibit a median survival that ranges from 9 to 15 months. Glioblastomas invariably recur after therapy due to the presence of cells exhibiting a multitherapy-resistance phenotype. To decipher the aberrant DNA repair pathways that enable tumor cells to survive DNA damage upon chemotherapy and radiotherapy should guide us to provide our patients with a more efficacious therapy. The abnormal function of tyrosine kinase receptors (TKRs) is a hallmark of malignant gliomas. Our group reported the expression of the TKR Tie2 in in brain tumor stem cells (BTSCs) and in human surgical glioma specimens in relation to malignancy. In in vivo experiments, consisting of ionizing irradiation (IR) of mice bearing intracranial BTSCs-derived xenografts showed unexpected Tie2 nuclear localization. We observed that Tie2 traffics from the cellular membrane to the nucleus upon IR stimuli, as assessed by immunoflurorescence studies using confocal microscope and subcellular fractionation followed by Western blots. This phenomenon is ligand dependent, as increased levels of Tie2 natural ligand, Angiopoietin1 (Ang1), were observed after ionizing radiation, and nuclear Tie2 levels were decreased using soluble Tie2, which blocks the Ang1/Tie2 interaction. Of clinical significance, the presence of Tie2 in the nucleus is significantly associated with radioresistance, as observed by cell viability and clonogenic assays. Tie2 trafficking was associated to Caveolin-1, and this functional complex was disturbed when Caveolin-1 inhibitor or siCaveolin-1 were used, resulting in decreased Tie2 nuclear levels. In addition, we observed that upon IR treatment, nuclear Tie2 bound to DNA/protein complexes, and specifically to the key DNA repair protein γH2AX, as a component of the DNA-repair foci. Based on these results, we hypothesized that Tie2 was involved in DNA damage repair and focused on one of the main pathways involved in double strand break (IR induced), the non-homologous end-joining (NHEJ) repair mechanism. By using a fluorescent reporter construct in which a functional GFP gene is reconstituted following an NHEJ event (gift from Dr. Gorbunova, University of Rochester), we observed that Tie2-expressing cells displayed a more efficient NHEJ repair than Tie2 negative counterparts. Mutation of a Tie2 nuclear localization signal significantly decreased NHEJ efficiency repair and radiosensitization of BTSCs, suggesting a role of nuclear Tie2 in DNA damage repair, specifically in the NHEJ repair mechanism. Collectively, our results should propel the development of preclinical studies of the combination of nuclear Tie2-targeting strategies with radiotherapy for patients with glioblastomas. Citation Format: Mohammad B. Hossain, Nahir Cortes-Santiago, Xuejun Fan, Konrad Gabrusiewicz, Joy Gumin, Erik P. Sulman, Frederick Lang, Raymond Sawaya, W.K.Alfred Yung, Juan Fueyo, Candelaria Gomez-Manzano. Caveolin-mediated Tie2 nuclear translocation results in enhanced NHEJ repair and glioma radioresistance. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3944. doi:10.1158/1538-7445.AM2014-3944

Mitochondrial KATP Channels Control Glioma Radioresistance by Regulating ROS-Induced ERK Activation.

Malignant glioma is the most prevalent form of malignant brain tumor. Although radiotherapy is widely used in glioma treatment, the radioresistance of glioma cells limits the success of the glioma treatment. The lack of effective targets and signaling pathways to reverse glioma radioresistance is the critical obstacle in successful treatment. In this study, we demonstrate that mitochondrial ATP-sensitive potassium channels (mtK(ATP) channels) are overexpressed in glioma cells and are closely related to the malignancy grade and the overall survival of the patients. Importantly, we showed that mtK(ATP) channels could control glioma radioresistance by regulating reactive oxygen species (ROS)-induced ERK activation. The inhibition of mtK(ATP) channels suppresses glioma radioresistance by inhibiting ERK activation both in vitro and in vivo. These findings reveal the important roles of the mitochondria and mtK(ATP) channels as key regulators in the radioresistance of glioma cells, and suggest that mtK(ATP) channel blockers and MAPK/ERK kinase (MEK) inhibitors are potential targets for drug development of glioma treatments.

Transcriptome analysis of glioma cells for the dynamic response to γ-irradiation and dual regulation of apoptosis genes: a new insight into radiotherapy for glioblastomas

Ionizing radiation (IR) is of clinical importance for glioblastoma therapy; however, the recurrence of glioma characterized by radiation resistance remains a therapeutic challenge. Research on irradiation-induced transcription in glioblastomas can contribute to the understanding of radioresistance mechanisms. In this study, by using the total mRNA sequencing (RNA-seq) analysis, we assayed the global gene expression in a human glioma cell line U251 MG at various time points after exposure to a growth arrest dose of γ-rays. We identified 1656 genes with obvious changes at the transcriptional level in response to irradiation, and these genes were dynamically enriched in various biological processes or pathways, including cell cycle arrest, DNA replication, DNA repair and apoptosis. Interestingly, the results showed that cell death was not induced even many proapoptotic molecules, including death receptor 5 (DR5) and caspases were activated after radiation. The RNA-seq data analysis further revealed that both proapoptosis and antiapoptosis genes were affected by irradiation. Namely, most proapoptosis genes were early continually responsive, whereas antiapoptosis genes were responsive at later stages. Moreover, HMGB1, HMGB2 and TOP2A involved in the positive regulation of DNA fragmentation during apoptosis showed early continual downregulation due to irradiation. Furthermore, targeting of the TRAIL/DR5 pathway after irradiation led to significant apoptotic cell death, accompanied by the recovered gene expression of HMGB1, HMGB2 and TOP2A. Taken together, these results revealed that inactivation of proapoptotic signaling molecules in the nucleus and late activation of antiapoptotic genes may contribute to the radioresistance of gliomas. Overall, this study provided novel insights into not only the underlying mechanisms of radioresistance in glioblastomas but also the screening of multiple targets for radiotherapy.

Activation of the phosphorylation of ATM contributes to radioresistance of glioma stem cells

Ionizing radiation (IR) is currently the most efficient therapy available for malignant glioma. Unfortunately, this strategy is palliative due to the characteristics of radioresistance of malignant glioma. The aim of our study was to compare glioma stem cells (GSCs) with glioma cells (GCs) to determine whether GSCs are responsible for the radioresistance phenotype and to elucidate whether cell cycle checkpoint proteins are responsible for the radioresistance of GSCs. In this study, CD133 (a marker of brain cancer stem cells) and nestin were co-expressed in GSCs isolated from GCs. The percent of CD133+ cells in GSCs and GCs were >80 and <2%, respectively. Significantly more GSCs survived following 2, 4, 6 and 8 Gy IR than GCs. IR kills cancer cells primarily through DNA double-strand breaks (DSBs). The neutral comet assay is often used to intuitively show the level of DSBs. Significantly fewer GSCs showed DNA damage than GCs following 2 Gy IR. This demonstrated that GSCs are more resistant to in vitro radiation than GCs. Furthermore, activated ataxia telangiectasia mutated (ATM) is essential for the activation of downstream effector kinases, such as checkpoint kinase 2 (Chk2) and p53 which mainly contribute to the proper regulation of IR-induced arrest in the G1 phase. DNA damage induced by IR potently initiated activation of phosphorylation of the ATM, p53 and Chk2 checkpoint proteins. Activation of the phosphorylation of these checkpoint proteins was significantly higher in the GSCs compared to GCs. We found that inhibition of ATM activation induced cell cycle checkpoint defects and increased the rate of apoptosis of GSCs following IR. Our results suggest that GSCs were more resistant to radiation compared to GCs due to high expression of phosphorylated cell cycle checkpoint proteins, and inhibition of ATM could significantly reduce the radioresistance of GSCs and GCs. ATM may represent a source of radioresistance in GSCs and a target of improved radiosensitivity of GSCs.

Radiation-induced mitotic cell death and glioblastoma radioresistance: A new regulating pathway controlled by integrin-linked kinase, hypoxia-inducible factor 1alpha and survivin in U87 cells

We have previously shown that integrin-linked kinase (ILK) regulates U87 glioblastoma cell radioresistance by modulating the main radiation-induced cell death mechanism in solid tumours, the mitotic cell death. To decipher the biological pathways involved in these mechanisms, we constructed a U87 glioblastoma cell model expressing an inducible shRNA directed against ILK (U87shILK). We then demonstrated that silencing ILK enhanced radiation-induced centrosome overduplication, leading to radiation-induced mitotic cell death. In this model, ionising radiations induce hypoxia-inducible factor 1alpha (HIF-1α) stabilisation which is inhibited by silencing ILK. Moreover, silencing HIF-1α in U87 cells reduced the surviving fraction after 2Gy irradiation by increasing cell sensitivity to radiation-induced mitotic cell death and centrosome amplification. Because it is known that HIF-1α controls survivin expression, we then looked at the ILK silencing effect on survivin expression. We show that survivin expression is decreased in U87shILK cells. Furthermore, treating U87 cells with the specific survivin suppressor YM155 significantly increased the percentage of giant multinucleated cells, centrosomal overduplication and thus U87 cell radiosensitivity.In consequence, we decipher here a new pathway of glioma radioresistance via the regulation of radiation-induced centrosome duplication and therefore mitotic cell death by ILK, HIF-1α and survivin. This work identifies new targets in glioblastoma with the intention of radiosensitising these highly radioresistant tumours.

Abstract 444: Mitochondrial metabolism and phosphorylation of Stat3 in radioresistant cancer stem cells isolated from glioma U87 cells.

Abstract Abstract Radiation therapy continues to play a central role in the management of brain tumors. The radioresistance of high-grade gliomas remains one of the reasons for the failure of treatment, which would otherwise require tumoricidal doses beyond the normal tissue tolerance and beyond safe clinical practice. Thus, determining the cellular, molecular and biochemical mechanisms underlying radiation resistance becomes critical for successful treatment of these tumors. For this purpose, glioma cell line U87 were treated with fractionated doses of radiation and the resistant clones isolated from the surviving tumor cells were characterized. The radioresistant U87 cells showed enhanced aggressiveness and clonogenic survival with an increased expression of HER2 and CD133, glioma cancer stem cell surface markers, as well as elevated mitochondrial membrane potential but lower oxygen consumption and ATP generation. In addition, compared with the wild type U87 cells, the phosphorylation of mitochondrial Stat3 at Serine 727 was decreased in the radioresistant U87 cells, indicating a role for mitochondrial Stat3 in the radioresistance mechanisms. Additionally, mutation of Serine 727 of Stat3 into Alanine resulted in increased radiosensitivity. These results demonstrate that the radioresistant glioma cells surviving routine radiotherapy are enriched with cancer stem cells expressing CD133 and HER2. The radioresistant phenotype is linked with a lowered mitochondrial metabolism and increased anti-apoptotic function due to inhibition of mitochondrial Stat3 phosphorylation. Thus, improving the mitochondrial metabolism by enhancing mitochondrial Stat3 phosphorylation may serve as an effective approach to re-sensitize the radioresistant glioma tumors. Citation Format: Ming Fan, Yan Shi, Jian Jian Li. Mitochondrial metabolism and phosphorylation of Stat3 in radioresistant cancer stem cells isolated from glioma U87 cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 444. doi:10.1158/1538-7445.AM2013-444

MicroRNAs involved in chemo- and radioresistance of high-grade gliomas

High-grade gliomas (HGGs) are malignant primary brain tumors of glial cell origin. Despite optimal course of treatment, including maximal surgical resection followed by adjuvant chemo- and/or radiotherapy, the prognosis still remains poor. The main reason is the commonly occurring chemo- and radioresistance of these tumors. In recent years, several signaling pathways, especially PI3K/AKT and ATM/CHK2/p53, have been linked to the resistance of gliomas. Moreover, additional studies have shown that these pathways are significantly regulated by microRNAs (miRNAs), short endogenous RNA molecules that modulate gene expression and control many biological processes including apoptosis, proliferation, cell cycle, invasivity, and angiogenesis. MiRNAs are not only highly deregulated in gliomas, their expression signatures have also been shown to predict prognosis and therapy response. Therefore, they present promising biomarkers and therapeutic targets that might overcome the resistance to treatment and improve prognosis of glioma patients. In this review, we summarize the current knowledge of the functional role of miRNAs in gliomas resistance to chemo- and radiotherapy.

Expression of TIP-1 Confers Radioresistance of Malignant Glioma Cells

BackgroundMalignant gliomas represent one group of tumors that poorly respond to ionizing radiation (IR) alone or combined with chemotherapeutic agents because of the intrinsic or acquired resistance. In this study, TIP-1 was identified as one novel protein that confers resistance of glioma cells to IR.Methodology/Principal FindingsMeta-analysis indicated that high TIP-1 expression levels correlate with the poor prognosis of human malignant gliomas after radiotherapy. Studies with established human glioma cell lines demonstrated that TIP-1 depletion with specific shRNAs sensitized the cells to IR, whereas an ectopic expression of TIP-1 protected the glioma cells from the IR-induced DNA damage and cell death. Biochemical studies indicated that TIP-1 protein promoted p53 ubiquitination and resulted in a reduced p53 protein level. Furthermore, p53 and its ubiquitination are required for the TIP-1 regulated cellular response to IR. A yeast two-hybrid screening identified that TIP-1, through its single PDZ domain, binds to the carboxyl terminus of LZAP that has been studied as one tumor suppressor functioning through ARF binding and p53 activation. It was revealed that the presence of TIP-1 enhances the protein association between LZAP and ARF and modulates the functionality of ARF/HDM2 toward multi-ubiquitination of p53, while depleting TIP-1 rescued p53 from polyubiquitination and degradation in the irradiated glioma cells. Studies with a mouse xenograft model indicated that depleting TIP-1 within D54 cells improved the tumor growth control with IR.Conclusions/SignificanceThis study provided the first evidence showing that TIP-1 modulates p53 protein stability and is involved in the radioresistance of malignant gliomas, suggesting that antagonizing TIP-1 might be one novel approach to sensitize malignant gliomas to radiotherapy.

Abstract 2136: Nuclear trafficking of Tie2 is associated with radioresistance of gliomas

Abstract Resistance and relapse are still primary causes that result in poor effectiveness of chemo- and radiotherapy in glioblastoma multiforme (GBMs), the most frequent subtype of primary brain tumors in adults. Patients suffering from this disease exhibit a median survival that ranges from 9 to 15 month, and the disease invariably recur after therapy due to the presence of cells exhibiting a multidrug-resistance phenotype. Aberrant DNA repair pathways can enable tumor cells to survive DNA damage that is induced after chemotherapy and radiotherapy. Therefore, development of new therapeutic strategies requires the identification of key molecular pathways regulating the resistant phenotype of these tumors. Previous work from our laboratory showed that Tie2, a previously considered specific vascular tyrosine kinase receptor, was expressed in glioma tumors and in brain tumor stem cells. The degree of expression of Tie2 was related to the malignancy of these tumors, and played an essential role in the multidrug resistance phenotype of gliomas. Our current research is focused on deciphering the mechanisms underlying this critical function. We found that density of Tie2 receptors in the membrane decreased upon exposure to both ligand (angiopoietin 1, Ang1) and ionizing irradiation (IR). Immunoflurorescence studies using confocal microscope and subcellular fractionation followed by Western blot showed that Tie2 traffics from the cellular membrane to the nucleus upon both stimuli. Western blot analysis using antibodies against N- and C-terminus, and against phosphorylated-Tie2, indicated that the nuclear Tie2 is a full-length protein, and importantly that is phosphorylated at Tyr992. Of biological importance, to find the role of Tie2 in the nucleus, we irradiated the glioma cells, U251.Tie2, and found that ionizing radiation also helps Tie2 to move to the nucleus and importantly it binds with γH2AX one of the key DNA repair protein complex. By using confocal microscopy we found that Tie2 makes foci with γH2AX after DNA damage. We also found that after genotoxic stress like ionizing radiation or DNA-damaging drugs, Tie2-expressing cells were more resistant to DNA damaging effect, as assessed by cell viability assay and comet assay. Collectively, the nuclear Tie2 plays important role in the DNA repair which will result in the design of Tie2-targeting combinational therapies for patients with glioblastomas. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2136. doi:1538-7445.AM2012-2136

Immunotherapy for malignant glioma: preclinical models to improve efficacy of the immune attack

The prognosis of high grade glioma (HGG) is poor in spite of advanced neurosurgery, radio- and chemotherapy. Evidence appears that active specific immunotherapy with mature dendritic cells loaded with tumor lysate (DCm-HGG-L) might induce long term survival. In our preclinical GL261 model, we are now looking for strategies to improve the efficacy of the immune attack. For this, we are approaching two key molecules on the tumor cells. Galectin-1 is known to play a central role in T cell homeostasis and tolerance by inducing T cell apoptosis. Moreover, it induces chemoresistance in tumor cells. Increased expression is found in malignant glioma. We stably knocked down Galectin-1 expression using a lentiviral vector construct, and created KD clones. Current experiments demonstrate that the engraftment of KD GL261 cells is postponed as compared to WT or mock GL261. This is dependent on a spontaneous CD8 T cell-mediated protection, as the effect disappears upon CD8 T cell depletion. DC vaccination results in an improved survival of mice challenged with KD GL261 cells as compared to mice challenged with WT GL261 cells. We are studying currently differences in tumor infiltrating immune cells after tumor challenge with KD or WT GL261. Osteopontin is also an important molecule for chemoresistance, radioresistance, invasion, angiogenesis and immune modulation in malignant glioma. Osteopontin KD tumor cells were created with lentivirus-induced shRNA, and clones were generated. The in vitro responsiveness to irradiation and to doxorubicin was not changed. However, KD cells engrafted slightly slower, and DC vaccination again yielded improved survival as compared to WT cells. We conclude that targeting specific molecules on the malignant glioma cells might induce improved sensitivity to the immune attack after active specific immunotherapy.

Knockdown of the DNA-dependent protein kinase catalytic subunit radiosensitizes glioma-initiating cells by inducing autophagy

Glioblastoma (GBM) is a highly aggressive brain tumor characterized by increased proliferation and resistance to chemotherapy and radiotherapy. A growing body of evidence suggests that only a small subpopulation of malignant glioma cells, called glioma stem cells or glioma-initiating cells (GICs), have true tumorigenic potential and confer glioma radioresistance. DNA-dependent protein kinase catalytic subunit (DNA-PKcs) plays a major role in the repair of DNA double-strand breaks induced by ionizing radiation (IR). Suppression of one of these components of the DNA-PK complex can inhibit the DNA double-strand break repair and radiosensitize the cells. In general, the cell death induced by IR is considered to be apoptotic. Recently, autophagy, an alternative form of programmed cell death, has been shown to contribute significantly to anti-neoplastic effects of radiation therapy. Autophagy is independent of phagocytes and differs from apoptosis by the presence of autophagosomes, autolysosomes, and an intact nucleus in the cell. Little is known, however, regarding the relationship between DNA-PKcs and IR-induced autophagy in GICs. In the present study, we constructed plasmids encoding short hairpin RNA (shRNA) targeting DNA-PKcs, which were then transfected into GICs. Then, we used GICs and DNA-PKcs-RNAi transfected cells to investigate the role of DNA-PKcs in IR-induced apoptotic and autophagic cell death. IR induced massive autophagic cell death in DNA-PKcs-RNAi transfected cells, but only occasional apoptotic cells were detected among GICs. Specific inhibition of DNA-PKcs in GICs induced autophagy and radiosensitized the cells. Our results suggest that such radiation-induced autophagy may enhance the effect of glioma therapies.

Accumulation of CD133-positive glioma cells after high-dose irradiation by Gamma Knife surgery plus external beam radiation

Recent evidence suggests that a glioma stem cell subpopulation might contribute to radioresistance in malignant gliomas. To investigate this hypothesis, the authors examined recurrent malignant gliomas for histopathological changes after high-dose irradiation with Gamma Knife surgery (GKS) and external beam radiation therapy (EBRT). Thirty-two patients with malignant gliomas (Grade 3 in 8 patients, Grade 4 in 24) underwent GKS in combination with EBRT. Serial MR and L-[methyl-(11)C] methionine PET images were employed to assess remnant or recurrent tumors after GKS. Twelve patients underwent surgical removal after GKS and EBRT. Histological sections were subjected to immunohistochemistry for MIB-1, factor VIII, and stem cell markers, nestin and CD133. The site of GKS treatment failure was local in 16 (76.2%) of 21 patients with glioblastomas showing progression; in 9 of these 16 patients, the recurrence clearly arose within the target lesion of GKS. Histopathological examination after GKS and EBRT showed variable mixtures of viable tumor tissues and necrosis. Viable tumor tissues exhibited high MIB-1 indices but reduced numbers of tumor blood vessels. There was marked accumulation of CD133-positive glioma cells, particularly in remnant tumors within the necrotic areas, in sections obtained after GKS plus EBRT, whereas CD133-positive cells appeared very infrequently in primary sections prior to adjuvant treatment. The results indicate that CD133-positive glioma stemlike cells can survive high-dose irradiation, leading to recurrence, despite prolonged damage to tumor blood vessels. This could be an essential factor limiting the effectiveness of GKS plus EBRT for malignant gliomas.

Abstract 2037: Anti-miRNA-21 confers radiosensitivity through inhibition of PI3K/AKT pathway and enhancing autophagy

Abstract Radiation is a core part of therapy for malignant glioma, which has been given after debulking surgery and prolonged patient's survival. However, the mass of glioma hardly shows response to radiation and frequently become radioresistant after the initial radiation. Among onco-microRNAs, microRNA-21 regulates expression of various molecules involved in anti-apoptosis, invasion, and metastasis in cancer. Yet, there has been known to the relationship between microRNA-21 and radioresistance of glioma. In this study, we investigate the effect of anti-mir-21 on radiosensitivity in glioblastoma and cervical cancer cell lines. Induction of mature microRNA-21 was confirmed by Nothern blotting after irradiation and the transfection of anti-mir-21 resulted in radiosenstivity on colony forming assay. The radiosensitive is thought to be the results of impairment of DNA repair in irradiated cancer cells, which is proved by γ-H2AX staining. In contrast to control group, a significant percentage of anti-mir-21 transfected cells retain DNA foci at 24 hours after irradiation. The suppression of radiation-activated PI3K/Akt pathway is proved on Western blot and thought to be one of the mechanisms for anti-mir-21 induced radiosensitization. Cell cycle analysis shows significant increase of G2/M phase arrest in anti-mir-21 treated cells compared to the control while subG1 population representing apoptotic cells reveals no significant difference between two groups. To investigate further the responsible cell death mechanism other than apoptosis, we measured the degree of autophagy in irradiated cells by both counting acid vesicular organelles using flow cytometry and Western blotting for LC-3 band. The autophagy is significantly increased in anti-mir-21 treated cells after radiation compared to mock treated cells. We suggest that microRNA-21 is a pivotal regulator of mechanisms to evade radiation-induced cell death in cancer cell. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2037.