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PDF - 89K, Functions associated with the top networks for the genes no longer decreased by radiation after INK128 treatment.

PDF - 93K, Genes comprising the DNA replication, recombination and repair biological function whose radiation-induced up-regulation was inhibited by INK128.

Purpose: Radiotherapy remains a primary treatment modality for pancreatic carcinoma, a tumor characterized by aberrant mTOR activity. Given the regulatory role of mTOR in gene translation, in this study, we defined the effects of the clinically relevant, ATP-competitive mTOR inhibitor, INK128 on the radiosensitivity of pancreatic carcinoma cell lines.Experimental Design: Clonogenic survival was used to determine the effects of INK128 on in vitro radiosensitivity of three pancreatic carcinoma cell lines and a normal fibroblast cell line with mTOR activity defined using immunoblots. DNA double-strand breaks were evaluated according to γH2AX foci. The influence of INK128 on radiation-induced gene translation was determined by microarray analysis of polysome-bound mRNA. Leg tumor xenografts grown from pancreatic carcinoma cells were evaluated for mTOR activity, eIF4F cap complex formation, and tumor growth delay.Results: INK128, while inhibiting mTOR activity in each of the cell lines, enhanced the in vitro radiosensitivity of the pancreatic carcinoma cells but had no effect on normal fibroblasts. The dispersal of radiation-induced γH2AX foci was inhibited in pancreatic carcinoma cells by INK128 as were radiation-induced changes in gene translation. Treatment of mice with INK128 resulted in an inhibition of mTOR activity as well as cap complex formation in tumor xenografts. Whereas INK128 alone had no effect of tumor growth rate, it enhanced the tumor growth delay induced by single and fractionated doses of radiation.Conclusion: These results indicate that mTOR inhibition induced by INK128 enhances the radiosensitivity of pancreatic carcinoma cells and suggest that this effect involves the inhibition of DNA repair. Clin Cancer Res; 20(1); 110–9. ©2013 AACR.

PDF - 99K, Functions associated with the top networks corresponding to the genes whose radiation-induced upregulation was inhibited by INK128

PDF file - 113K, List of 1124 genes whose binding to eIF4E was induced by radiation

Despite the development of new treatment paradigms and improved biologic understanding of head and neck squamous cell carcinoma (HNSCC), therapeutic resistance remains a substantial problem, and novel treatment approaches are needed. Stimulator of interferon genes (STING) is a critical regulator of the antitumor response through regulation of both immune-dependent and tumor-intrinsic mechanisms. As such, the STING pathway has emerged as a rational pharmacologic target leading to the development of multiple STING agonists. These compounds have impressive preclinical efficacy as single agents and with PD-1 (programmed death-1) axis agents. However, clinical evaluation in this context has yet to show substantial efficacy. In contrast to monotherapy approaches, activation of STING in combination with DNA-damaging agents has been shown to enhance the effect of these agents in preclinical models and represents a promising approach to improve outcomes in patients with HNSCC. In this review, we will discuss the preclinical and clinical data supporting the use of STING agonists and highlight potential avenues of exploration to unlock the potential of these agents in HNSCC.

Radiation therapy and systemic therapy are the primary non-surgical treatment modalities for head and neck squamous cell carcinoma (HNSCC). Despite advances in our biologic understanding of this disease and the development of novel therapeutics, treatment resistance remains a significant problem. It has become increasingly evident that the innate and adaptive immune systems play a significant role in the modulation of anti-tumor responses to traditional cancer-directed therapies. By inducing DNA damage and cell death, radiation therapy appears to activate both innate and adaptive immune responses. Immunotherapies targeting programmed cell death protein 1 (PD-1) and programmed cell death ligand 1 (PD-L1) also have yielded promising results, particularly in the recurrent/metastatic setting. In this review, we will discuss the rationale for combining radiotherapy with immunotherapy to harness the immunomodulatory effects of radiation therapy on HNSCC, as well as biomarkers for immune response. We will also review recent preclinical and clinical data exploring these combinations in various contexts, including recurrent/metastatic and locally advanced disease. Among those with locally advanced HNSCC, we will discuss clinical trials employing immunotherapy either concurrently with radiation therapy or as maintenance following chemoradiation in both the definitive and postoperative settings, with or without the use of cisplatin-based or non-cisplatin-based chemotherapy.

Resistance to DNA-damaging agents is a significant cause of treatment failure and poor outcomes in oncology. To identify unrecognized regulators of cell survival we performed a whole-genome CRISPR-Cas9 screen using treatment with ionizing radiation as a selective pressure, and identified STING (stimulator of interferon genes) as an intrinsic regulator of tumor cell survival. We show that STING regulates a transcriptional program that controls the generation of reactive oxygen species (ROS), and that STING loss alters ROS homeostasis to reduce DNA damage and to cause therapeutic resistance. In agreement with these data, analysis of tumors from head and neck squamous cell carcinoma patient specimens show that low STING expression is associated with worse outcomes. We also demonstrate that pharmacologic activation of STING enhances the effects of ionizing radiation in vivo, providing a rationale for therapeutic combinations of STING agonists and DNA-damaging agents. These results highlight a role for STING that is beyond its canonical function in cyclic dinucleotide and DNA damage sensing, and identify STING as a regulator of cellular ROS homeostasis and tumor cell susceptibility to reactive oxygen dependent, DNA damaging agents., The endoplasmic reticulum-localized adaptor STING regulates the innate immune response through its ability to sense DNA damage. Here the authors reveal that STING functions as a regulator of cellular ROS homeostasis and tumor cell susceptibility to reactive oxygen dependent, DNA damaging agents.

A new method for detecting N-glycosylation reveals a role for the TRAP complex and a mechanism for cell state–specific regulation., Asparagine (N)–linked glycosylation is required for endoplasmic reticulum (ER) homeostasis, but how this co- and posttranslational modification is maintained during ER stress is unknown. Here, we introduce a fluorescence-based strategy to detect aberrant N-glycosylation in individual cells and identify a regulatory role for the heterotetrameric translocon-associated protein (TRAP) complex. Unexpectedly, cells with knockout of SSR3 or SSR4 subunits restore N-glycosylation over time concurrent with a diminished ER stress transcriptional signature. Activation of ER stress or silencing of the ER chaperone BiP exacerbates or rescues the glycosylation defects, respectively, indicating that SSR3 and SSR4 enable N-glycosylation during ER stress. Protein levels of the SSR3 subunit are ER stress and UBE2J1 dependent, revealing a mechanism that coordinates upstream N-glycosylation proficiency with downstream ER-associated degradation and proteostasis. The fidelity of N-glycosylation is not static in both nontransformed and tumor cells, and the TRAP complex regulates ER glycoprotein quality control under conditions of stress.

The retinoic acid-inducible gene-I (RIG-I) receptor recognizes short 5′-di- and triphosphate base-paired viral RNA and is a critical mediator of the innate immune response against viruses such as influenza A, Ebola, HIV and hepatitis C. This response is reported to require an orchestrated interaction with the tripartite motif 25 (TRIM25) B30.2 protein-interaction domain. Here, we present a novel second RIG-I-binding interface on the TRIM25 B30.2 domain that interacts with CARD1 and CARD2 (caspase activation and recruitment domains) of RIG-I and is revealed by the removal of an N-terminal α-helix that mimics dimerization of the full-length protein. Further characterization of the TRIM25 coiled-coil and B30.2 regions indicated that the B30.2 domains move freely on a flexible tether, facilitating RIG-I CARD recruitment. The identification of a dual binding mode for the TRIM25 B30.2 domain is a first for the SPRY/B30.2 domain family and may be a feature of other SPRY/B30.2 family members.

The innate immune system is our first line of defense against viral pathogens. Host cell pattern recognition receptors sense viral components and initiate immune signaling cascades that result in the production of an array of cytokines to combat infection. Retinoic acid-inducible gene-I (RIG-I) is a pattern recognition receptor that recognizes viral RNA and, when activated, results in the production of type I and III interferons (IFNs) and the upregulation of IFN-stimulated genes. Ubiquitination of RIG-I by the E3 ligases tripartite motif-containing 25 (TRIM25) and Riplet is thought to be requisite for RIG-I activation; however, recent studies have questioned the relative importance of these two enzymes for RIG-I signaling. In this study, we show that deletion of Trim25 does not affect the IFN response to either influenza A virus (IAV), influenza B virus, Sendai virus or several RIG-I agonists. This is in contrast to deletion of either Rig-i or Riplet, which completely abrogated RIG-I-dependent IFN responses. This was consistent in both mouse and human cell lines, as well as in normal human bronchial cells. With most of the current TRIM25 literature based on exogenous expression, these findings provide critical evidence that Riplet, and not TRIM25, is required endogenously for the ubiquitination of RIG-I. Despite this, loss of TRIM25 results in greater susceptibility to IAV infection in vivo, suggesting that it may have an alternative role in host antiviral defense. This study refines our understanding of RIG-I signaling in viral infections and will inform future studies in the field.

Purpose: Radiotherapy remains a primary treatment modality for pancreatic carcinoma, a tumor characterized by aberrant mTOR activity. Given the regulatory role of mTOR in gene translation, in this study, we defined the effects of the clinically relevant, ATP-competitive mTOR inhibitor, INK128 on the radiosensitivity of pancreatic carcinoma cell lines. Experimental Design: Clonogenic survival was used to determine the effects of INK128 on in vitro radiosensitivity of three pancreatic carcinoma cell lines and a normal fibroblast cell line with mTOR activity defined using immunoblots. DNA double-strand breaks were evaluated according to γH2AX foci. The influence of INK128 on radiation-induced gene translation was determined by microarray analysis of polysome-bound mRNA. Leg tumor xenografts grown from pancreatic carcinoma cells were evaluated for mTOR activity, eIF4F cap complex formation, and tumor growth delay. Results: INK128, while inhibiting mTOR activity in each of the cell lines, enhanced the in vitro radiosensitivity of the pancreatic carcinoma cells but had no effect on normal fibroblasts. The dispersal of radiation-induced γH2AX foci was inhibited in pancreatic carcinoma cells by INK128 as were radiation-induced changes in gene translation. Treatment of mice with INK128 resulted in an inhibition of mTOR activity as well as cap complex formation in tumor xenografts. Whereas INK128 alone had no effect of tumor growth rate, it enhanced the tumor growth delay induced by single and fractionated doses of radiation. Conclusion: These results indicate that mTOR inhibition induced by INK128 enhances the radiosensitivity of pancreatic carcinoma cells and suggest that this effect involves the inhibition of DNA repair. Clin Cancer Res; 20(1); 110–9. ©2013 AACR.

Background/Purpose: Lung cancer remains the leading cause of cancer-related death. Concurrent chemoradiation is the standard of care for locally advanced non-small cell lung cancer (NSCLC). Despite advancements in treatment, local control remains an issue and survival is suboptimal, highlighting the need for novel therapeutic approaches. Polo-like kinase 4 (PLK4) is a serine/threonine kinase that controls centriole duplication. PLK4 is highly expressed in multiple malignancies including lung cancer. CFI-400945 is a specific PLK4 inhibitor currently undergoing clinical trial evaluation. As radiation causes cell death primarily via a mitotic death, we hypothesized that disruption of the mitotic machinery by inhibition of PLK4 would enhance the effects of radiation. Methods: The effects of CFI-400945 on cell survival and radiation sensitivity were examined using the clonogenic survival assay in two KRAS-mutant lung cancer cell lines (H460 and A549). Confocal microscopy was utilized to determine the effects of PLK4 inhibition on radiation-induced mitotic catastrophe (defined as the presence of 2 or more distinct nuclear lobes within a single cell). Cell cycle distribution by flow cytometry was determined after treatment with CFI-400945 and radiation. Results: Exposure of H460 and A549 lung cancer cell lines to 10nM CFI-400945 for 48h reduced clonogenic survival to 63.9% and 83.1% respectively. CFI-400945 treatment of H460 and A549 cells resulted in radiosensitization of both cell lines with dose enhancement factors (DEF) of 1.60 and 1.31. To understand the mechanism of enhanced radiosensitivity with CFI-400945 treatment the effects on radiation-induced mitotic catastrophe were determined. In both H460 and A549 cells the combination of radiation and PLK4 inhibition enhanced the proportion of cells undergoing mitotic catastrophe. The combination of CFI-400945 and radiation resulted in increased G2/M arrest and appearance of >4N populations by flow cytometric analysis when compared to drug or radiation treatment alone. Conclusion: PLK4 inhibition with CFI-400945 enhances the radiosensitivity of lung cancer cell lines via mitotic catastrophe and G2/M arrest and these data support additional pre-clinical testing. Citation Format: Thomas J. Hayman, Mark R. Bray, Jacqueline M. Mason, Tak W. Mak, Joseph N. Contessa. PLK4 inhibition as a strategy for non-small cell lung cancer radiosensitization [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3935.

The mechanistic target of rapamycin (mTOR) is a critical kinase in the regulation of gene translation and has been suggested as a potential target for radiosensitization. The goal of this study was to compare the radiosensitizing activities of the allosteric mTOR inhibitor rapamycin with that of the competitive mTOR inhibitor PP242. On the basis of immunoblot analyses, whereas rapamycin only partially inhibited mTOR complex 1 (mTORC1) activity and had no effect on mTOR complex 2 (mTORC2), PP242 inhibited the activity of both mTOR-containing complexes. Irradiation alone had no effect on mTORC1 or mTORC2 activity. Clonogenic survival was used to define the effects of the mTOR inhibitors on in vitro radiosensitivity. In the two tumor cell lines evaluated, PP242 treatment 1 hour before irradiation increased radiosensitivity, whereas rapamycin had no effect. Addition of PP242 after irradiation also enhanced the radiosensitivity of both tumor lines. To investigate the mechanism of radiosensitization, the induction and repair of DNA double-strand breaks were evaluated according yH2AX foci. PP242 exposure did not influence the initial level of yH2AX foci after irradiation but did significantly delay the dispersal of radiationinduced yH2AX foci. In contrast to the tumor cell lines, the radiosensitivity of a normal human fibroblast cell line was not influenced by PP242. Finally, PP242 administration to mice bearing U251 xenografts enhanced radiationinduced tumor growth delay. These results indicate that in a preclinical tumor model PP242 enhances tumor cell radiosensitivity both in vitro and in vivo and suggest that this effect involves an inhibition of DNA repair.

The advent of the modern era of molecularly targeted therapies in oncology has generated considerable excitement in the field of oncology. While there have been successes with molecularly targeted agents as monotherapies, most solid tumors display only a transient and modest response to single-targeted agents. As such, there has been significant effort in combining molecularly targeted agents with radiotherapy. Receptor tyrosine kinases (RTKs) play central roles in oncogenesis, stress sensitivity, tumor maintenance/progression, and clinical prognosis. Secondary to these roles, receptor tyrosine kinases are attractive targets for cancer therapy and specifically in combination with radiation therapy to enhance tumor radiosensitivity. Significant preclinical and clinical investigations have been performed to understand their roles in regulating the cellular response to radiation. A number of RTKs with relevance to radiation oncology have been identified including EGFR, VEGFR, IGF-1R, c-MET, and HER2. This chapter will highlight the preclinical and clinical findings associated with the combination of radiotherapy and inhibitors of the aforementioned receptors.

Cell surface innate immune receptors can directly detect a variety of extracellular pathogens to which cytoplasmic innate immune sensors are rarely exposed. Instead, within the cytoplasm, the environment is rife with cellular machinery and signaling pathways that are indirectly perturbed by pathogenic microbes to activate intracellular sensors, such as pyrin, NLRP1, NLRP3, or NLRC4. Therefore, subtle changes in key intracellular processes such as phosphorylation, ubiquitination, and other pathways leading to posttranslational protein modification are key determinants of innate immune recognition in the cytoplasm. This concept is critical to establish the “guard hypothesis” whereby otherwise homeostatic pathways that keep innate immune sensors at bay are released in response to alterations in their posttranslational modification status. Originally identified in plants, evidence that a similar guardlike mechanism exists in humans has recently been identified, whereby a mutation that prevents phosphorylation of the innate immune sensor pyrin triggers a dominantly inherited autoinflammatory disease. It is also noteworthy that even when a cytoplasmic innate immune sensor has a direct ligand, such as bacterial peptidoglycan (NOD1 or NOD2), RNA (RIG-I or MDA5), or DNA (cGAS or IFI16), it can still be influenced by posttranslational modification to dramatically alter its response. Therefore, due to their existence in the cytoplasmic milieu, posttranslational modification is a key determinant of intracellular innate immune receptor functionality.

A core component in the cellular response to radiation occurs at the level of translational control of gene expression. Because a critical element in translation control is the availability of the initiation factor eIF4E, which selectively enhances the cap-dependent translation of mRNAs, we investigated a regulatory role for eIF4E in cellular radiosensitivity. eIF4E silencing enhanced the radiosensitivity of tumor cell lines but not normal cells. Similarly, pharmacologic inhibition of eIF4E with ribavirin also enhanced tumor cell radiosensitivity. eIF4E attenuation did not affect cell-cycle phase distribution or radiation-induced apoptosis, but it delayed the dispersion of radiation-induced γH2AX foci and increased the frequency of radiation-induced mitotic catastrophe. Radiation did not affect 4E-BP1 phosphorylation or cap-complex formation but it increased eIF4E binding to more than 1,000 unique transcripts including many implicated in DNA replication, recombination, and repair. Taken together, our findings suggest that eIF4E represents a logical therapeutic target to increase tumor cell radiosensitivity. Cancer Res; 72(9); 2362–72. ©2012 AACR.

6060 Background: HPV infection is associated with high p16 expression and relatively good prognosis in head and neck cancers. Analysis of CDKN2A, the gene that encodes the p16 tumor suppressor protein, may further elucidate the association between HPV status and prognosis in head and neck squamous cell carcinomas (HNSCCs). We aimed to identify whether CDKN2A copy number loss was associated with poor survival in HNSCCs stratified by HPV status. Methods: We analyzed The Cancer Genome Atlas (TCGA) head and neck cancer data, integrating genomic measurements with clinical metadata. Patients 85 years old or younger with a primary tumor in the oral cavity, oropharynx, hypopharynx, or larynx were included. Defining CDKN2A copy number loss as a relative log2 copy number ratio < −0.6, CDKN2A mRNA and p16 protein expression levels were compared to confirm significant differences in gene transcription and translation between the copy number loss and non-copy number loss patient groups. Overall survival (OS) and disease-free survival (DFS) were evaluated to characterize prognostic differences between genomic groups. Results: 397 patients negative for HPV (HPV−) and 91 patients positive for HPV (HPV+) HNSCC were identified. 139 HPV− patients and 9 HPV+ patients demonstrated CDKN2A copy number loss. The CDKN2A copy number loss group expressed significantly lower levels of CDKN2A mRNA and p16 protein than did the non-copy number loss group in both HPV+ and HPV− disease. Median OS for HPV− patients with and without CDKN2A copy number loss was 21.8 months and 46.0 months (P = 0.02). Median DFS was 12.0 and 19.4 months respectively (P < 0.05). Median OS for HPV+ patients with and without CDKN2A copy number loss was 12.7 months and 57.4 months (P = 0.004) and median DFS was 7.0 and 36.6 months respectively (P = 0.02). Conclusions: CDKN2A copy number loss was associated with low CDKN2A mRNA and p16 protein expression, with poor prognosis in terms of disease-free and overall survival.

The mammalian target of rapamycin (mTOR) has been suggested as a target for radiosensitization. Given that radiotherapy is a primary treatment modality for glioblastoma (GBM) and that mTOR is often dysregulated in GBM, the goal of this study was to determine the effects of AZD2014, a dual mTORC1/2 inhibitor, on the radiosensitivity of GBM stem-like cells (GSCs).mTORC1 and mTORC2 activities were defined by immunoblot analysis. The effects of this mTOR inhibitor on the in vitro radiosensitivity of GSCs were determined using a clonogenic assay. DNA double strand breaks were evaluated according to γH2AX foci. Orthotopic xenografts initiated from GSCs were used to define the in vivo response to AZD2014 and radiation.Exposure of GSCs to AZD2014 resulted in the inhibition of mTORC1 and 2 activities. Based on clonogenic survival analysis, addition of AZD2014 to culture media 1 hour before irradiation enhanced the radiosensitivity of CD133+ and CD15+ GSC cell lines. Whereas AZD2014 treatment had no effect on the initial level of γH2AX foci, the dispersal of radiation-induced γH2AX foci was significantly delayed. Finally, the combination of AZD2014 and radiation delivered to mice bearing GSC-initiated orthotopic xenografts significantly prolonged survival as compared with the individual treatments.These data indicate that AZD2014 enhances the radiosensitivity of GSCs both in vitro and under orthotopic in vivo conditions and suggest that this effect involves an inhibition of DNA repair. Moreover, these results suggest that this dual mTORC1/2 inhibitor may be a radiosensitizer applicable to GBM therapy.

Body mass index (BMI) has been linked with inferior outcomes in gastrointestinal malignancies. The purpose of this study is to evaluate the effect of BMI on survival in patients with esophageal adenocarcinoma. Medical records were analyzed for patients who underwent esophagectomy after neoadjuvant chemoradiotherapy (nCRT) for adenocarcinoma from 2000 to the present. Patients were grouped into BMI ≤25, >25–30, >30–35, and BMI >35. Overall survival (OS) and disease-free survival (DFS) were analyzed using the Kaplan–Meier method. Multivariate analysis (MVA) was performed using Cox proportional hazard regression model. We identified 303 patients for the analysis. The only difference in patient characteristics between groups was gender. We found no difference in OS and DFS associated with BMI (p = 0.3297 for OS; p = 0.5950 for DFS). There were no differences in postoperative complications or mortality between BMI groups. MVA revealed that higher stage and less than a complete response to nCRT were prognostic for worse OS and DFS, while age, gender, type of surgery, year of diagnosis, and BMI were not prognostic. BMI was neither associated with surgical complications nor survival in patients with esophageal adenocarcinoma treated with nCRT. BMI should not be considered a contraindication to surgical resection after nCRT.

Radiation-induced translational control of gene expression has been shown to be a component of the cellular radioresponse. Furthermore, recent data suggest strategies targeting eIF4E may be effective for tumor cell radiosensitization. The mechanistic target of rapamycin (mTOR) is a critical kinase in the regulation of eIF4E and mRNA translation. The newly developed ATP-competitive mTOR inhibitor, INK128, inhibits both mTORC1 and mTORC2 activity, and is in currently undergoing evaluation in Phase 1 clinical trials. In the study described here we evaluated the effects of INK128 treatment on tumor cell radiosensitivity in vitro using a clonogenic survival assay and in vivo according to tumor growth delay. For tumor cell lines initiated from different histologic origins in vitro exposure to INK128 resulted in the complete inhibition of both mTORC1 and 2. Treatment with the same concentration of INK128 immediately after irradiation increased tumor cell radiosensitivity with DEFs greater than 1.3. To determine whether the INK128 induced radiosensitization could be extended to an in vivo model we used subcutaneous xenografts initiated from U251 or PSN1 tumor cells. INK128 treatment (oral gavage) inhibited mTOR activity in tumors established from both cell lines as defined by loss of 4E-BP1 phosphorylation. Furthermore, m7-GTP batch chromatography showed that INK128 treatment decreased cap-binding activity in xenografts. To evaluate the effects of INK128 on in vivo tumor radiosensitivity a tumor growth delay assay was performed using mice bearing U251 tumor xenografts. When tumors reached approximately 210mm3 mice were randomized into 4 groups: vehicle, radiation, INK128, and INK128 plus radiation. INK128 was delivered once a day (3 mg/kg, oral gavage) for four days with the tumor locally irradiated (2 Gy) 2h after each of the four drug treatments. Absolute growth delays were calculated as the time in days for tumors in treated mice to grow from 210 to 1000mm3 minus the time in days for tumors to reach the same size in vehicle treated mice. The absolute growth delays for INK128 alone and radiation alone groups were 1.7 and 12.7 days respectively. The growth delay in mice treated with the combination of INK128 and radiation was 22.2 days, which is greater than the sum of the growth delays caused by INK128 and radiation alone. The DEF, obtained by dividing the normalized tumor growth delay in mice treated with the radiation/PP242 combination (20.5) by the absolute growth delay in mice treated with radiation only (12.7), was 1.6. These results suggest that mTOR inhibition by the clinically relevant ATP-competitive mTOR inhibitor, INK128, enhances tumor cell radiosensitivity both in vitro and in vivo. Citation Format: Thomas J. Hayman, Tamalee Kramp, Kevin Camphausen, Philip J. Tofilon. In vitro and in vivo tumor cell radiosensitization by the competitive mTOR inhibitor INK128. [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 69. doi:10.1158/1538-7445.AM2013-69

Translational control of gene expression has been implicated as a component of the cellular response to ionizing radiation (IR). The mammalian target of rapamycin (mTOR) is a critical kinase in the regulation of translation. mTOR exists in two distinct complexes: mTORC1 and mTORC2. mTORC1 regulates 4E-BP1 and p70-s6k, whereas mTORC2 regulates several AGC kinases including AKT. Most studies of mTOR have focused on the use of the allosteric inhibitor rapamycin and its analogs, which incompletely inhibit mTORC1 and do not inhibit mTORC2. In contrast, newly developed competitive mTOR inhibitors (e.g. PP242) inhibit mTORC1 output more completely and inhibit mTORC2. In the study described here, we compared the effects of rapamycin and PP242 on cellular radiosensitivity using a clonogenic assay. In two tumor cell lines originating from different histologies, U251 and MDA-MB-231, PP242 treatment 1h before irradiation increased radiosensitivity with DEFs (dose enhancement factors at 0.1 surviving fraction) of 1.52 and 1.34 respectively, whereas rapamycin had no effect. However, PP242 had no effect on the radiosensitivity of the normal human fibroblast cell line, MRC9. PP242 treatment of U251 cells resulted in a dose-dependent increase in radiosensitivity. Furthermore, addition of PP242 immediately, 1, or 6h after IR resulted in increased radiosensitivity with DEFs of 1.60, 1.50, and 1.26, respectively. To investigate the mechanism of this radiosensitization U251 cells were treated with PP242, irradiated (2Gy) and stained for γH2AX, a marker of DNA double strand breaks (DSBs). PP242 exposure significantly delayed the dispersal of radiation-induced γH2AX foci, consistent with an inhibition of DSB repair. Under conditions of clonogenic survival, exposure of U251 or MDA-MB-231 cells to 2Gy resulted in no detectable increase in mTOR output. PP242 treatment of both tumor lines resulted in complete inhibition of mTORC1/2 output whereas rapamycin only partially inhibited mTORC1 and did not inhibit mTORC2. m7-GTP batch chromatography showed PP242 induced a decrease in cap-bound eIF4G and an increase in cap-bound 4E-BP1, while rapamycin slightly increased 4E-BP1 binding and had no effect on eIF4G binding. Consistent with the effects on cap-complex formation, PP242 treatment caused a considerably greater decrease in polysome abundance than rapamycin. These results suggest that competitive inhibitors of mTOR, in contrast to rapalogs, induce tumor selective radiosensitization. Moreover, the mechanism appears to involve the inhibition of DSB repair and the suppression of translation. 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 5727. doi:1538-7445.AM2012-5727

The translational control of gene expression has been implicated as a component of the cellular response to ionizing radiation (IR). However, the mechanisms mediating this process and its specific role in radiosensitivity have not been defined. eIF4E, an mRNA 5’ cap-binding protein, is a critical protein involved in regulating gene translation in response to a variety of environmental signals. In the study described here, we have determined the effects of IR on eIF4E activity. Moreover, using 3 tumor cell lines (MDA-MB-231, DU145 and A549) and 2 normal cell lines (MRC9 and HMEC), the potential role of eIF4E as a molecular determinant of radiosensitivity was investigated. eIF4E activity is regulated by eIF4E binding protein 1 (4E-BP1); 4E-BP1 phosphorylation releases eIF4E to enhance eIF4F complex formation and cap-dependent translation. Exposure of MDA-MB-231 or MRC9 cells to IR (2 Gy, 1h) resulted in an increase in 4E-BP1 phosphorylation. Consistent with this effect, m7-GTP batch chromatography showed that IR induced an increase in cap-bound eIF4G and a decrease in cap-bound 4E-BP1 proteins. Thus, these results indicate that IR, via 4E-BP1 phosphorylation, enhances eIF4E activity and cap-dependent translation. Whereas phosphorylation of eIF4E has also been shown to influence its translational activity, IR had no effect on eIF4E phosphorylation in any of the cell lines evaluated. To determine the significance of eIF4E as a regulator of radiosensitivity, tumor and normal cell lines were treated with siRNA to eIF4E and radiation survival curves were generated based on the colony formation assay. Knockdown of eIF4E in each of the 3 tumor cell lines resulted in an increase in radiosensitivity with DEFs (dose enhancement factors at 0.1 surviving fraction) ranging from 1.24 to 1.44. In contrast, eIF4E knockdown had no effect on the radiosensitivity of normal fibroblasts (MRC9) or normal mammary epithelial cells (HMEC). To investigate the mechanism through which eIF4E influences tumor cell radiosensitivity, we initially focused on MDA-MB-231 cells. Knockdown of eIF4E had no significant effect on MDA-MB-231 cell cycle distribution indicating that a loss of S-phase cells does not account for the observed radiosensitization. In addition, eIF4E knockdown had no effect on apoptotic cell death after IR (6 Gy, 24 and 48h). However, knockdown of eIF4E significantly enhanced the level of mitotic catastrophe induced by IR (2 Gy). These initial results suggest that eIF4E may influence the ability of tumor cells to respond to genotoxic injury. Whereas the specific mechanisms remain to be determined, these data suggest that eIF4E selectively regulates the radiosensitivity of tumor cells over normal cells. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2510. doi:10.1158/1538-7445.AM2011-2510