Targeting PRMT5 enhances the radiosensitivity of tumor cells grown in vitro and in vivo.

PRMT5 is a widely expressed arginine methyltransferase that regulates processes involved in tumor cell proliferation and survival. In the study described here, we investigated whether PRMT5 provides a target for tumor radiosensitization. Knockdown of PRMT5 using siRNA enhanced the radiosensitivity of a panel of cell lines corresponding to tumor types typically treated with radiotherapy. To extend these studies to an experimental therapeutic setting, the PRMT5 inhibitor LLY-283 was used. Exposure of the tumor cell lines to LLY-283 decreased PRMT5 activity and enhanced their radiosensitivity. This increase in radiosensitivity was accompanied by an inhibition of DNA double-strand break repair as determined by γH2AX foci and neutral comet analyses. For a normal fibroblast cell line, although LLY-283 reduced PRMT5 activity, it had no effect on their radiosensitivity. Transcriptome analysis of U251 cells showed that LLY-283 treatment reduced the expression of genes and altered the mRNA splicing pattern of genes involved in the DNA damage response. Subcutaneous xenografts were then used to evaluate the in vivo response to LLY-283 and radiation. Treatment of mice with LLY-283 decreased tumor PRMT5 activity and significantly enhanced the radiation-induced growth delay. These results suggest that PRMT5 is a tumor selective target for radiosensitization.

MGMT inhibition regulates radioresponse in GBM, GSC, and melanoma.

Radiotherapy is the standard treatment for glioblastoma (GBM), but the overall survival rate for radiotherapy treated GBM patients is poor. The use of adjuvant and concomitant temozolomide (TMZ) improves the outcome; however, the effectiveness of this treatment varies according to MGMT levels. Herein, we evaluated whether MGMT expression affected the radioresponse of human GBM, GBM stem-like cells (GSCs), and melanoma. Our results indicated a correlation between MGMT promoter methylation status and MGMT expression. MGMT-producing cell lines ACPK1, GBMJ1, A375, and MM415 displayed enhanced radiosensitivity when MGMT was silenced using siRNA or when inhibited by lomeguatrib, whereas the OSU61, NSC11, WM852, and WM266-4 cell lines, which do not normally produce MGMT, displayed reduced radiosensitivity when MGMT was overexpressed. Mechanistically lomeguatrib prolonged radiation-induced γH2AX retention in MGMT-producing cells without specific cell cycle changes, suggesting that lomeguatrib-induced radiosensitization in these cells is due to radiation-induced DNA double-stranded break (DSB) repair inhibition. The DNA-DSB repair inhibition resulted in cell death via mitotic catastrophe in MGMT-producing cells. Overall, our results demonstrate that MGMT expression regulates radioresponse in GBM, GSC, and melanoma, implying a role for MGMT as a target for radiosensitization.

Revisiting Concurrent Radiation Therapy, Temozolomide, and the Histone Deacetylase Inhibitor Valproic Acid for Patients with Glioblastoma-Proteomic Alteration and Comparison Analysis with the Standard-of-Care Chemoirradiation.

BACKGROUND: Glioblastoma (GBM) is the most common brain tumor with an overall survival (OS) of less than 30% at two years. Valproic acid (VPA) demonstrated survival benefits documented in retrospective and prospective trials, when used in combination with chemo-radiotherapy (CRT). PURPOSE: The primary goal of this study was to examine if the differential alteration in proteomic expression pre vs. post-completion of concurrent chemoirradiation (CRT) is present with the addition of VPA as compared to standard-of-care CRT. The second goal was to explore the associations between the proteomic alterations in response to VPA/RT/TMZ correlated to patient outcomes. The third goal was to use the proteomic profile to determine the mechanism of action of VPA in this setting. MATERIALS AND METHODS: Serum obtained pre- and post-CRT was analyzed using an aptamer-based SOMAScan® proteomic assay. Twenty-nine patients received CRT plus VPA, and 53 patients received CRT alone. Clinical data were obtained via a database and chart review. Tests for differences in protein expression changes between radiation therapy (RT) with or without VPA were conducted for individual proteins using two-sided t-tests, considering p-values of <0.05 as significant. Adjustment for age, sex, and other clinical covariates and hierarchical clustering of significant differentially expressed proteins was carried out, and Gene Set Enrichment analyses were performed using the Hallmark gene sets. Univariate Cox proportional hazards models were used to test the individual protein expression changes for an association with survival. The lasso Cox regression method and 10-fold cross-validation were employed to test the combinations of expression changes of proteins that could predict survival. Predictiveness curves were plotted for significant proteins for VPA response (p-value < 0.005) to show the survival probability vs. the protein expression percentiles. RESULTS: A total of 124 proteins were identified pre- vs. post-CRT that were differentially expressed between the cohorts who received CRT plus VPA and those who received CRT alone. Clinical factors did not confound the results, and distinct proteomic clustering in the VPA-treated population was identified. Time-dependent ROC curves for OS and PFS for landmark times of 20 months and 6 months, respectively, revealed AUC of 0.531, 0.756, 0.774 for OS and 0.535, 0.723, 0.806 for PFS for protein expression, clinical factors, and the combination of protein expression and clinical factors, respectively, indicating that the proteome can provide additional survival risk discrimination to that already provided by the standard clinical factors with a greater impact on PFS. Several proteins of interest were identified. Alterations in GALNT14 (increased) and CCL17 (decreased) (p = 0.003 and 0.003, respectively, FDR 0.198 for both) were associated with an improvement in both OS and PFS. The pre-CRT protein expression revealed 480 proteins predictive for OS and 212 for PFS (p < 0.05), of which 112 overlapped between OS and PFS. However, FDR-adjusted p values were high, with OS (the smallest p value of 0.586) and PFS (the smallest p value of 0.998). The protein PLCD3 had the lowest p-value (p = 0.002 and 0.0004 for OS and PFS, respectively), and its elevation prior to CRT predicted superior OS and PFS with VPA administration. Cancer hallmark genesets associated with proteomic alteration observed with the administration of VPA aligned with known signal transduction pathways of this agent in malignancy and non-malignancy settings, and GBM signaling, and included epithelial-mesenchymal transition, hedgehog signaling, Il6/JAK/STAT3, coagulation, NOTCH, apical junction, xenobiotic metabolism, and complement signaling. CONCLUSIONS: Differential alteration in proteomic expression pre- vs. post-completion of concurrent chemoirradiation (CRT) is present with the addition of VPA. Using pre- vs. post-data, prognostic proteins emerged in the analysis. Using pre-CRT data, potentially predictive proteins were identified. The protein signals and hallmark gene sets associated with the alteration in the proteome identified between patients who received VPA and those who did not, align with known biological mechanisms of action of VPA and may allow for the identification of novel biomarkers associated with outcomes that can help advance the study of VPA in future prospective trials.

Glioblastoma cells have increased capacity to repair radiation-induced DNA damage after migration to the olfactory bulb.

BACKGROUND: The invasive nature of GBM combined with the diversity of brain microenvironments creates the potential for a topographic heterogeneity in GBM radioresponse. Investigating the mechanisms responsible for a microenvironment-induced differential GBM response to radiation may provide insights into the molecules and processes mediating GBM radioresistance. METHODS: Using a model system in which human GBM stem-like cells implanted into the right striatum of nude mice migrate throughout the right hemisphere (RH) to the olfactory bulb (OB), the radiation-induced DNA damage response was evaluated in each location according to γH2AX and 53BP1 foci and cell cycle phase distribution as determined by flow cytometry and immunohistochemistry. RNAseq was used to compare transcriptomes of tumor cells growing in the OB and the RH. Protein expression and neuron-tumor interaction were defined by immunohistochemistry and confocal microscopy. RESULTS: After irradiation, there was a more rapid dispersal of γH2AX and 53BP1 foci in the OB versus in the RH, indicative of increased double strand break repair capacity in the OB and consistent with the OB providing a radioprotective niche. With respect to the cell cycle, by 6 h after irradiation there was a significant loss of mitotic tumor cells in both locations suggesting a similar activation of the G2/M checkpoint. However, by 24 h post-irradiation there was an accumulation of G2 phase cells in the OB, which continued out to at least 96 h. Transcriptome analysis showed that tumor cells in the OB had higher expression levels of DNA repair genes involved in non-homologous end joining and genes related to the spindle assembly checkpoint. Tumor cells in the OB were also found to have an increased frequency of soma-soma contact with neurons. CONCLUSION: GBM cells that have migrated to the OB have an increased capacity to repair radiation-induced double strand breaks and altered cell cycle regulation. These results correspond to an upregulation of genes involved in DNA damage repair and cell cycle control. Because the murine OB provides a source of radioresistant tumor cells not evident in other experimental systems, it may serve as a model for investigating the mechanisms mediating GBM radioresistance.

Inhibition of the Translation Initiation Factor eIF4A Enhances Tumor Cell Radiosensitivity.

A fundamental component of cellular radioresponse is the translational control of gene expression. Because a critical regulator of translational control is the eukaryotic translation initiation factor 4F (eIF4F) cap binding complex, we investigated whether eIF4A, the RNA helicase component of eIF4F, can serve as a target for radiosensitization. Knockdown of eIF4A using siRNA reduced translational efficiency, as determined from polysome profiles, and enhanced tumor cell radiosensitivity as determined by clonogenic survival. The increased radiosensitivity was accompanied by a delayed dispersion of radiation-induced γH2AX foci, suggestive of an inhibition of DNA double-strand break repair. Studies were then extended to (-)-SDS-1-021, a pharmacologic inhibitor of eIF4A. Treatment of cells with the rocaglate (-)-SDS-1-021 resulted in a decrease in translational efficiency as well as protein synthesis. (-)-SDS-1-021 treatment also enhanced the radiosensitivity of tumor cell lines. This (-)-SDS-1-021-induced radiosensitization was accompanied by a delay in radiation-induced γH2AX foci dispersal, consistent with a causative role for the inhibition of double-strand break repair. In contrast, although (-)-SDS-1-021 inhibited translation and protein synthesis in a normal fibroblast cell line, it had no effect on radiosensitivity of normal cells. Subcutaneous xenografts were then used to evaluate the in vivo response to (-)-SDS-1-021 and radiation. Treatment of mice bearing subcutaneous xenografts with (-)-SDS-1-021 decreased tumor translational efficiency as determined by polysome profiles. Although (-)-SDS-1-021 treatment alone had no effect on tumor growth, it significantly enhanced the radiation-induced growth delay. These results suggest that eIF4A is a tumor-selective target for radiosensitization.

CX-5461 induces radiosensitization through modification of the DNA damage response and not inhibition of RNA polymerase I.

Increased ribosome biogenesis is a distinguishing feature of cancer cells, and small molecule inhibitors of ribosome biogenesis are currently in clinical trials as single agent therapy. It has been previously shown that inhibiting ribosome biogenesis through the inhibition of nuclear export of ribosomal subunits sensitizes tumor cells to radiotherapy. In this study, the radiosensitizing potential of CX-5461, a small molecule inhibitor of RNA polymerase I, was tested. Radiosensitization was measured by clonogenic survival assay in a panel of four tumor cell lines derived from three different tumor types commonly treated with radiation. 50 nM CX-5461 radiosensitized PANC-1, U251, HeLa, and PSN1 cells with dose enhancement factors in the range of 1.2-1.3. However, 50 nM CX-5461 was not sufficient to inhibit 45S transcription alone or in combination with radiation. The mechanism of cell death with the combination of CX-5461 and radiation occurred through mitotic catastrophe and not apoptosis. CX-5461 inhibited the repair and/or enhanced the initial levels of radiation-induced DNA double strand breaks. Understanding the mechanism of CX-5461-induced radiosensitization should be of value in the potential application of the CX-5461/radiotherapy combination in cancer treatment.

Translation Initiation Machinery as a Tumor Selective Target for Radiosensitization.

Towards improving the efficacy of radiotherapy, one approach is to target the molecules and processes mediating cellular radioresponse. Along these lines, translational control of gene expression has been established as a fundamental component of cellular radioresponse, which suggests that the molecules participating in this process (i.e., the translational machinery) can serve as determinants of radiosensitivity. Moreover, the proteins comprising the translational machinery are often overexpressed in tumor cells suggesting the potential for tumor specific radiosensitization. Studies to date have shown that inhibiting proteins involved in translation initiation, the rate-limiting step in translation, specifically the three members of the eIF4F cap binding complex eIF4E, eIF4G, and eIF4A as well as the cap binding regulatory kinases mTOR and Mnk1/2, results in the radiosensitization of tumor cells. Because ribosomes are required for translation initiation, inhibiting ribosome biogenesis also appears to be a strategy for radiosensitization. In general, the radiosensitization induced by targeting the translation initiation machinery involves inhibition of DNA repair, which appears to be the consequence of a reduced expression of proteins critical to radioresponse. The availability of clinically relevant inhibitors of this component of the translational machinery suggests opportunities to extend this approach to radiosensitization to patient care.

Bench to bedside radiosensitizer development strategy for newly diagnosed glioblastoma.

Glioblastoma is the most common primary brain malignancy and carries with it a poor prognosis. New agents are urgently needed, however nearly all Phase III trials of GBM patients of the past 25 years have failed to demonstrate improvement in outcomes. In 2019, the National Cancer Institute Clinical Trials and Translational Research Advisory Committee (CTAC) Glioblastoma Working Group (GBM WG) identified 5 broad areas of research thought to be important in the development of new herapeutics for GBM. Among those was optimizing radioresponse for GBM in situ. One such strategy to increase radiation efficacy is the addition of a radiosensitizer to improve the therapeutic ratio by enhancing tumor sensitivity while ideally having minimal to no effect on normal tissue. Historically the majority of trials using radiosensitizers have been unsuccessful, but they provide important guidance in what is required to develop agents more efficiently. Improved target selection is essential for a drug to provide maximal benefit, and once that target is identified it must be validated through pre-clinical studies. Careful selection of appropriate in vitro and in vivo models to demonstrate increased radiosensitivity and suitable bioavailability are then necessary to prove that a drug warrants advancement to clinical investigation. Once investigational agents are validated pre-clinically, patient trials require consistency both in terms of planning study design as well as reporting efficacy and toxicity in order to assess the potential benefit of the drug. Through this paper we hope to outline strategies for developing effective radiosensitizers against GBM using as models the examples of XPO1 inhibitors and HDAC inhibitors developed from our own lab.

The Radiosensitizing Effect of AZD0530 in Glioblastoma and Glioblastoma Stem-Like Cells.

AZD0530, a potent small-molecule inhibitor of the Src kinase family, is an anticancer drug used in the treatment of various cancers. In the case of glioblastoma (GBM), where resistance to radiotherapy frequently occurs, Src kinase is known as one of the molecules responsible for imparting radioresistance to GBM. Thus, we evaluated the effect of AZD0530 on the radiosensitivity of human GBM cells and human glioblastoma stem-like cells (GSCs). We show that Src activity of GBM and GSC is increased by radiation and inhibited by AZD0530, and using clonogenic assays, AZD0530 enhances the radiosensitivity of GBM and GSCs. Also, AZD0530 induced a prolongation of radiation-induced γH2AX without specific cell cycle and mitotic index changes, suggesting that AZD0530-induced radiosensitization in GBM cells and GSCs results from the inhibition of DNA repair. In addition, AZD0530 was shown to inhibit the radiation-induced EGFR/PI3K/AKT pathway, which is known to promote and regulate radioresistance and survival of GBM cells by radiation. Finally, mice bearing orthotopic xenografts initiated from GBM cells were then used to evaluate the in vivo response to AZD0530 and radiation. The combination of AZD0530 and radiation showed the longest median survival compared with any single modality. Thus, these results show that AZD0530 enhances the radiosensitivity of GBM cells and GSCs and suggest the possibility of AZD0530 as a clinical radiosensitizer for treatment of GBM.

Detection of metabolic change in glioblastoma cells after radiotherapy using hyperpolarized 13 C-MRI.

Dynamic nuclear polarization (DNP) of 13 C-labeled substrates enables the use of magnetic resonance imaging (MRI) to monitor specific enzymatic reactions in tumors and offers an opportunity to investigate these differences. In this study, DNP-MRI chemical shift imaging with hyperpolarized [1-13 C] pyruvate was conducted to evaluate the metabolic change in glycolytic profiles after radiation of two glioma stem-like cell-derived gliomas (GBMJ1 and NSC11) and an adherent human glioblastoma cell line (U251) in an orthotopic xenograft mouse model. The DNP-MRI showed an increase in Lac/Pyr at 6 and 16 h after irradiation (18% ± 4% and 14% ± 3%, respectively; mean ± SEM) compared with unirradiated controls in GBMJ1 tumors, whereas no significant change was observed in U251 and NSC11 tumors. Metabolomic analysis likewise showed a significant increase in lactate in GBMJ1 tumors at 16 h. An immunoblot assay showed upregulation of lactate dehydrogenase-A expression in GBMJ1 following radiation exposure, consistent with DNP-MRI and metabolomic analysis. In conclusion, our preclinical study demonstrates that the DNP-MRI technique has the potential to be a powerful diagnostic method with which to evaluate GBM tumor metabolism before and after radiation in the clinical setting.

In Vitro Methods for the Study of Glioblastoma Stem-Like Cell Radiosensitivity.

Ionizing radiation is a critical component of glioblastoma (GBM) therapy. Recent data have implicated glioblastoma stem-like cells (GSCs) as determinants of GBM development, maintenance, and treatment response. Understanding the response of GSCs to radiation should thus provide insight into the development of improved GBM treatment strategies. Towards this end, in vitro techniques for the analysis of GSC radiosensitivity are an essential starting point. One such method, the clonogenic survival assay has been adapted to assessing the intrinsic radiosensitivity of GSCs and is described here. As an alternative method, the limiting dilution assay is presented for defining the radiosensitivity of GSC lines that do not form colonies or only grow as neurospheres. In addition to these cellular strategies, we describe γH2AX foci analysis, which provides a surrogate marker for radiosensitivity at the molecular level. Taken together, the in vitro methods presented here provide tools for defining intrinsic radiosensitivity of GSCs and for testing agents that may enhance GBM radioresponse.

Improving Radiation Response in Glioblastoma Using ECO/siRNA Nanoparticles Targeting DNA Damage Repair.

Radiation therapy is a mainstay in the standard of care for glioblastoma (GBM), thus inhibiting the DNA damage response (DDR) is a major strategy to improve radiation response and therapeutic outcomes. Small interfering RNA (siRNA) therapy holds immeasurable potential for the treatment of GBM, however delivery of the siRNA payload remains the largest obstacle for clinical implementation. Here we demonstrate the effectiveness of the novel nanomaterial, ECO (1-aminoethylimino[bis(N-oleoylcysteinylaminoethyl) propionamide]), to deliver siRNA targeting DDR proteins ataxia telangiectasia mutated and DNA-dependent protein kinase (DNApk-cs) for the radiosensitzation of GBM in vitro and in vivo. ECO nanoparticles (NPs) were shown to efficiently deliver siRNA and silence target protein expression in glioma (U251) and glioma stem cell lines (NSC11, GBMJ1). Importantly, ECO NPs displayed no cytotoxicity and minimal silencing of genes in normal astrocytes. Treatment with ECO/siRNA NPs and radiation resulted in the prolonged presence of γH2AX foci, indicators of DNA damage, and increased radiosensitivity in all tumor cell lines. In vivo, intratumoral injection of ECO/siDNApk-cs NPs with radiation resulted in a significant increase in survival compared with injection of NPs alone. These data suggest the ECO nanomaterial can effectively deliver siRNA to more selectively target and radiosensitize tumor cells to improve therapeutic outcomes in GBM.

Detection of glioblastoma intratumor heterogeneity in radiosensitivity using patient-derived neurosphere cultures.

PURPOSE: Glioblastoma (GBM) is characterized by extensive clonal diversity suggesting the presence of tumor cells with varying degrees of treatment sensitivity. Radiotherapy is an integral part of glioblastoma treatment. Whether GBMs are comprised of spatially distinct cellular populations with uniform or varying degrees of radiosensitivity has not been established. METHODS: Spatially distinct regions of three GBMs (J3, J7 and J14) were resected and unique cell lines were derived from each region. DNA from cell lines, corresponding tumor fragments, and patient blood was extracted for whole exome sequencing. Variants, clonal composition, and functional implications were compared and analyzed with superFreq and IPA. Limiting dilution assays were performed on cell lines to measure intrinsic radiosensitivity. RESULTS: Based on WES, cell lines generated from different regions of the same tumor were more closely correlated with their tumor of origin than the other GBMs. Variant and clonal composition comparisons showed that cell lines from distinct tumors displayed increasing levels of ITH with J3 and J14 having the lowest and highest, respectively. The radiosensitivities of the cell lines generated from the J3 tumor were similar as were those generated from the J7 tumor. However, the radiosensitivities of the 2 cell lines generated from the J14 tumor (J14T3 and J14T6) were significantly different with J14T6 being more sensitive than J14T3. CONCLUSION: Data suggest a tumor dependent ITH in radiosensitivity. The existence of ITH in radiosensitivity may impact not only the initial therapeutic response but also the effectiveness of retreatment protocols.

Radiosensitizers in the temozolomide era for newly diagnosed glioblastoma.

Glioblastoma (GBM) is a challenging diagnosis with almost universally poor prognosis. Though the survival advantage of postoperative radiation (RT) is well established, around 90% of patients will fail in the RT field. The high likelihood of local failure suggests the efficacy of RT needs to be improved to improve clinical outcomes. Radiosensitizers are an established method of enhancing RT cell killing through the addition of a pharmaceutical agent. Though the majority of trials using radiosensitizers have historically been unsuccessful, there continues to be interest with a variety of approaches having been employed. Epidermal growth factor receptor inhibitors, histone deacetylase inhibitors, antiangiogenic agents, and a number of other molecularly targeted agents have all been investigated as potential methods of radiosensitization in the temozolomide era. Outcomes have varied both in terms of toxicity and survival, but some agents such as valproic acid and bortezomib have demonstrated promising results. However, reporting of results in phase 2 trials in newly diagnosed GBM have been inconsistent, with no standard in reporting progression-free survival and toxicity. There is a pressing need for investigation of new agents; however, nearly all phase 3 trials of GBM patients of the past 25 years have demonstrated no improvement in outcomes. One proposed explanation for this is the selection of agents lacking sufficient preclinical data and/or based on poorly designed phase 2 trials. Radiosensitization may represent a viable strategy for improving GBM outcomes in newly diagnosed patients, and further investigation using agents with promising phase 2 data is warranted.

The Olfactory Bulb Provides a Radioresistant Niche for Glioblastoma Cells.

PURPOSE: The various microenvironments that exist within the brain combined with the invasive nature of glioblastoma (GBM) creates the potential for a topographic influence on tumor cell radiosensitivity. The aim of this study was to determine whether specific brain microenvironments differentially influence tumor cell radioresponse. METHODS AND MATERIALS: GBM stem-like cells were implanted into the right striatum of nude mice. To measure radiosensitivity, proliferation status of individual tumor cells was determined according to the incorporation of 5-chloro-2'-deoxyuridine delivered at 4, 12, and 20 days after brain irradiation. As an additional measure of radiosensitivity, the percentage of human cells in the right hemisphere and the olfactory bulb were defined using digital droplet polymerase chain reaction. Targeted gene expression profiling was accomplished using NanoString analysis. RESULTS: Tumor cells were detected throughout the striatum, corpus callosum, and olfactory bulb. After an initial loss of proliferating tumor cells in the corpus callosum and striatum after irradiation, there was only a minor recovery by 20 days. In contrast, the proliferation of tumor cells located in the olfactory bulb began to recover at 4 days and returned to unirradiated levels by day 12 postirradiation. The percentage of human cells in the right hemisphere and the olfactory bulb after irradiation also suggested that the tumor cells in the olfactory bulb were relatively radioresistant. Gene expression profiling identified consistent differences between tumor cells residing in the olfactory bulb and those in the right hemisphere. CONCLUSIONS: These results suggest that the olfactory bulb provides a radioresistant niche for GBM cells.

Radiation Drives the Evolution of Orthotopic Xenografts Initiated from Glioblastoma Stem-like Cells.

A consequence of the intratumor heterogeneity (ITH) of glioblastoma (GBM) is the susceptibility to treatment-driven evolution. To determine the potential of radiotherapy to influence GBM evolution, we used orthotopic xenografts initiated from CD133+ GBM stem-like cells (GSC). Toward this end, orthotopic xenografts grown in nude mice were exposed to a fractionated radiation protocol, which resulted in a significant increase in animal survival. Brain tumors from control and irradiated mice were then collected at morbidity and compared in terms of growth pattern, clonal diversity, and genomic architecture. In mice that received fractionated radiation, tumors were less invasive, with more clearly demarcated borders and tumor core hypercellularity as compared with controls, suggesting a fundamental change in tumor biology. Viral integration site analysis indicated a reduction in clonal diversity in the irradiated tumors, implying a decrease in ITH. Changes in clonal diversity were not detected after irradiation of GSCs in vitro, suggesting that the radiation-induced reduction in ITH was dependent on the brain microenvironment. Whole-exome sequencing revealed differences in mutation patterns between control and irradiated tumors, which included modifications in the presence and clonality of driver mutations associated with GBM. Moreover, changes in the distribution of mutations as a function of subpopulation size between control and irradiated tumors were consistent with subclone expansion and contraction, that is, subpopulation evolution. Taken together, these results indicate that radiation drives the evolution of the GSC-initiated orthotopic xenografts and suggest that radiation-driven evolution may have therapeutic implications for recurrent GBM. SIGNIFICANCE: Radiation drives the evolution of glioblastoma orthotopic xenografts; when translated to the clinic, this may have therapeutic implications for recurrent tumors.

4E-BP1 Is a Tumor Suppressor Protein Reactivated by mTOR Inhibition in Head and Neck Cancer.

Aberrant activation of the PI3K-mTOR signaling pathway occurs in >80% of head and neck squamous cell carcinomas (HNSCC), and overreliance on this signaling circuit may in turn represent a cancer-specific vulnerability that can be exploited therapeutically. mTOR inhibitors (mTORi) promote tumor regression in genetically defined and chemically induced HNSCC animal models, and encouraging results have been recently reported. However, the mTOR-regulated targets contributing to the clinical response have not yet been identified. Here, we focused on EIF4E-BP1 (4E-BP1), a direct target of mTOR that serves as key effector for protein synthesis. A systematic analysis of genomic alterations in the PIK3CA-mTOR pathway in HNSCC revealed that 4E-BP1 is rarely mutated, but at least one 4E-BP1 gene copy is lost in over 35% of the patients with HNSCC, correlating with decreased 4E-BP1 protein expression. 4E-BP1 gene copy number loss correlated with poor disease-free and overall survival. Aligned with a tumor-suppressive role, 4e-bp1/2 knockout mice formed larger and more lesions in models of HNSCC carcinogenesis. mTORi treatment or conditional expression of a mutant 4E-BP1 that cannot be phosphorylated by mTOR was sufficient to disrupt the translation-initiation complex and prevent tumor growth. Furthermore, CRISPR/Cas9-targeted 4E-BP1 HNSCC cells resulted in reduced sensitivity to mTORi in vitro and in vivo. Overall, these findings indicate that in HNSCC, mTOR persistently restrains 4E-BP1 via phosphorylation and that mTORi can restore the tumor-suppressive function of 4E-BP1. Our findings also support 4E-BP1 expression and phosphorylation status as a mechanistic biomarker of mTORi sensitivity in patients with HNSCC. SIGNIFICANCE: These findings suggest that EIF4E-BP1 acts as a tumor suppressor in HNSCC and that 4E-BP1 dephosphorylation mediates the therapeutic response to mTORi, providing a mechanistic biomarker for future precision oncology trials.

The Quiescent Metabolic Phenotype of Glioma Stem Cells.

INTRODUCTION: Glioblastoma (GBM) is the most common primary malignant brain tumor in humans and, even with aggressive treatment that includes surgical resection, radiation (IR), and chemotherapy administration, prognosis is poor due to tumor recurrence. There is evidence that within GBMs a small number of glioma stem-like cells (GSLCs) exist, which are thought to be therapy resistant and are thus capable of repopulating a tumor after treatment. Like most cancers, GBMs largely employ aerobic glycolysis to create ATP, a phenomenon known as the Warburg Effect. There is no consensus on the metabolic characteristics of cancer stem cells. GSLCs have been shown to rely more heavily on oxidative phosphorylation, but there is also evidence that cancer stem cells can adapt their metabolism by fluctuating between energy pathways or acquiring intermediate metabolic phenotypes. We hypothesized that the metabolism of GSLCs differs from that of differentiated GBM tumor cell lines, and that the steady state metabolism would be differentially altered following radiation treatment. MATERIALS AND METHODS: We evaluated the oxygen consumption rate, extracellular acidification rate, and metabolic enzyme levels of GBM cell lines and GSLCs before and after irradiation using extracellular flux assays. We also measured absolute metabolite levels in these cells via mass spectroscopy with and without radiation treatment. RESULTS: GSLCs were found to be significantly more quiescent in comparison to adherent GBM cell lines, highlighted by lower glycolytic and maximal respiratory capacities as well as lower oxygen consumption and extracellular acidification rates. Analysis of individual metabolite concentrations revealed lower total metabolite concentrations overall but also elevated levels of metabolites in different energy pathways for GSLCs compared to GBM cell lines. Additionally, the metabolism of both GSLCs and GBM cell lines were found to be altered by IR. CONCLUSIONS: While there is not one metabolic alteration that distinguishes irradiated GSLC metabolism from that of GBM cell lines, therapies targeting more metabolically quiescent tumor cells and thus the resistant GSLC population may increase a cancer's sensitivity to radiotherapy.

Late toxicity in long-term survivors from a phase 2 study of concurrent radiation therapy, temozolomide and valproic acid for newly diagnosed glioblastoma.

BACKGROUND: Valproic acid (VPA) is an antiepileptic agent with histone deacetylase inhibitor activity shown to enhance overall survival and progression free survival in patients with newly diagnosed glioblastoma (GBM). This reports on the late toxicity of the VPA/radiotherapy (RT)/temozolomide (TMZ) combination in the long-term survivors of a phase 2 study evaluating this regimen. METHODS: 37 patients with newly diagnosed GBM were initially enrolled on this trial and received combination therapy. VPA/RT/TMZ related late toxicities were evaluated in the 6 patients that lived greater than 3 years using the Cancer Therapy and Evaluation Program Common Toxicity Criteria (CTC) Version 4.0 for toxicity and adverse event reporting as well as the RTOG/EORTC Radiation Morbidity Scoring Scheme. RESULTS: The median duration of follow-up for these 6 patients was 69.5m. In this cohort, the median OS was 73.8m (60.8-103.8m) and median PFS was 53.1m (37.3 - 103.8m). The most common late toxicity of VPA in conjunction with RT/TMZ were the CTC classifications of neurological, pain, and blood/ bone marrow toxicity and most were grade 1/2. There were only two grade 3/4 toxicities. CONCLUSIONS: The addition of VPA to concurrent RT/TMZ in patients with newly diagnosed GBM was well tolerated with little late toxicity. Additionally, VPA may result in improved outcomes as compared to historical data and merits further study.

The XPO1 Inhibitor Selinexor Inhibits Translation and Enhances the Radiosensitivity of Glioblastoma Cells Grown In Vitro and In Vivo.

Analysis of the radiation-induced translatome of glioblastoma stem-like cells (GSC) identified an interacting network in which XPO1 serves as a major hub protein. To determine whether this nuclear export protein provides a target for radiosensitization, we defined the effects of clinically relevant XPO1 inhibitor selinexor on the radiosensitivity of glioblastoma cells. As determined by clonogenic survival analysis, selinexor enhanced the radiosensitivity of GSCs but not normal fibroblast cell lines. On the basis of γH2AX foci and neutral comet analyses, selinexor inhibited the repair of radiation-induced DNA double-strand breaks in GSCs, suggesting that the selinexor-induced radiosensitization is mediated by an inhibition of DNA repair. Consistent with a role for XPO1 in the nuclear to cytoplasm export of rRNA, selinexor reduced 5S and 18S rRNA nuclear export in GSCs, which was accompanied by a decrease in gene translation efficiency, as determined from polysome profiles, as well as in protein synthesis. In contrast, rRNA nuclear export and protein synthesis were not reduced in normal cells treated with selinexor. Orthotopic xenografts initiated from a GSC line were then used to define the in vivo response to selinexor and radiation. Treatment of mice bearing orthotopic xenografts with selinexor decreased tumor translational efficiency as determined from polysome profiles. Although selinexor treatment alone had no effect on the survival of mice with brain tumors, it significantly enhanced the radiation-induced prolongation of survival. These results indicate that selinexor enhances the radiosensitivity of glioblastoma cells and suggest that this effect involves the global inhibition of gene translation. Mol Cancer Ther; 17(8); 1717-26. ©2018 AACR.