Abstract 5864: Targeting brain cancer stem cells by potentiating radiation-induced ER stress

Abstract Background. Glioblastoma (GBM) is one of the most malignant brain tumors occurring in both children and adults. Despite an aggressive treatment regimen consisting of surgical resection, radiation therapy and chemotherapy the five-year survival rate is less than 5%. This poor outcome has been attributed to the existence of therapy-resistant GBM stem-like cells (GSCs), which are believed to be responsible for tumor recurrence and patient relapse. Understanding the resistance mechanisms employed by brain tumor stem cells and developing novel methods to target these cells is necessary for prolonged patient survival. Recent evidence suggests that the endoplasmic reticulum stress response pathway may mediate therapeutic resistance in cancer. Here we sought to examine the ER stress response pathway of GSCs in response to ionizing radiation and to increase the extent of ER stress in an effort to promote cell death using the glycolytic inhibitor, 2-deoxy glucose (2-DG). Methods. GSC lines were derived from resected tumor sections. GSCs were irradiated using the Rad Source 2000 Series Biological Irradiator. Transmission Electron Microscopy (TEM) was used to investigate the ultra-structural alterations of GSCs following exposure to 8 Gy radiation. Viability was determined using trypan blue exclusion, MTS and LDH assays. Cell signaling pathways were investigated by western blot analysis. Results. Consistent with previous findings, minimal cell death was observed in GSCs exposed to 2-20 Gy radiation. TEM analysis revealed that exposure to 8 Gy significantly increased ER lumen dilation, suggestive of ER stress. Western blot analysis indicated an increase in ER stress markers GRP78, GRP94 and CHOP, confirming that radiation induces ER stress in GSCs. Treatment with 2-DG induced an increase in ER lumen diameter and ER stress markers. Combined 2-DG (0.5mM and 2mM) and radiotherapy (8 Gy) significantly increased ER luminal diameter and ER stress marker expression over radiation or 2-DG alone. In addition, combined radiation and 2-DG (0.1-2mM) significantly reduced GSC viability compared to radiation or 2-DG alone. Conclusion. The ER stress response pathway is an adaptive mechanism and thought to mediate therapeutic resistance. Here we demonstrate that radiation induces ER stress response pathway including ER lumen dilation and an increase in molecular chaperone expression in GSCs. Potentiating ER stress can switch the pathway from one of adaptation to cell death. 2-DG increased radiation-induced ER stress and promoted cell death. Our data suggests that targeting this adaptive response could increase the efficacy of radiotherapy and prolong patient survival Note: This abstract was not presented at the meeting. Citation Format: Regina M. Graham, Sumedh S. Shah, Alexis J. Musick, Winston Walters, Ricardo J. Komotar, Jeffery S. Prince, Steven Vanni. Targeting brain cancer stem cells by potentiating radiation-induced ER stress [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5864. doi:10.1158/1538-7445.AM2017-5864</jats:p

Targeting Glioblastoma Stem Cells with 2-Deoxy-D-Glucose (2-DG) Potentiates Radiation-Induced Unfolded Protein Response (UPR)

Glioblastoma (GBM) is the most common and aggressive primary brain tumor in adults, and despite optimized treatment options, median survival remains dismal. Contemporary evidence suggests disease recurrence results from expansion of a robustly radioresistant subset of GBM progenitor cells, termed GBM stem cells (GSCs). In this study, we utilized transmission electron microscopy to uncover ultrastructural effects on patient-derived GSC lines exposed to supratherapeutic radiotherapy levels. Elevated autophagosome formation and increased endoplasmic reticulum (ER) internal diameter, a surrogate for ER stress and activation of unfolded protein response (UPR), was uncovered. These observations were confirmed via protein expression through Western blot. Upon interrogating genomic data from an open-access GBM patient database, overexpression of UPR-related chaperone protein genes was inversely correlated with patient survival. This indicated controlled UPR may play a role in promoting radioresistance. To determine if potentiating UPR further can induce apoptosis, we exposed GSCs to radiation with an ER stress-inducing drug, 2-deoxy-D-glucose (2-DG), and found dose-dependent decreases in viability and increased apoptotic marker expression. Taken together, our results indicate GSC radioresistance is, in part, achieved by overexpression and overactivation of ER stress-related pathways, and this effect can be overcome via potentiation of UPR, leading to loss of GSC viability