Abstract 2858: Endoplasmic reticulum stress as possible target for therapy in glioblastoma
Authors: Peñaranda Fajardo Natalia M. , Meijer Coby , Kruyt. Frank A. E. 
Published: July 2016

Introduction: Glioblastoma (GBM) is the most aggressive form of brain cancer with an overall survival of patients of only 1-2 years after diagnosis. The current standard treatment is largely ineffective due to high resistance and infiltrative nature of GBM. Cellular heterogeneity in the tumor and the presence of cancer stem cells in GBM (GSCs) are considered the main contributors to aggressiveness. Recently, different molecular subtypes have been found in GBM. Proneural (PN) and Mesenchymal (MES) GBM are most distinct and MES tumors are more aggressive and infiltrative. Drugs that induce Endoplasmic Reticulum Stress (ER stress)/Unfolded Protein Response (UPR) activation provide a potential novel therapy for cancer treatment. In this study we aim to characterize and explore the potential of ER stress-inducing drugs to target GBM.

Methods: GBM neurospheres PN and MES were tested for sensitivity to the ER stress inducer Thapsigargin (Tg) by MTS. TGFβ was used to induce mesenchymal transition in PN GBMs. Limiting dilution assay was performed to analyze the spheroid formation capacity upon ER stress induction. Activation of UPR was determined by analyzing protein expression of key signals by Western Blotting or XBP1 splicing assay (RT-PCR). Caspase Glo 3/7 assay was performed as read-out for apoptosis.

Results: GBM MES neurospheres cells were 2-fold more sensitive to the ER stress inducing drug Tg compared to PN cells. The reduction of cell viability upon ER stress induction coincided with induction of apoptosis (caspase 3/7 activation). Consistently MES GBM cells showed stronger increase in apoptosis activation. Tg-induced ER stress resulted in 2- 4-fold reduction in spheroid formation capacity of MES GBM cells and higher Tg concentration also reduced spheroid formation capacity in one of the PN GBM cell lines. Interestingly, TGFβ-induced mesenchymal transition sensitized particularly PN GBM cells to Tg. The activation of UPR was confirmed by induction of GRP78 and CHOP expression. In MES GBM cells particularly activation of the ATF6-UPR branch by Tg ER stress induction was detected and XBP1 splicing activation appeared to be stronger in PN cells. No significant differences were observed in PERK activation among the different GBM subtypes.

Conclusion: ER stress induction appears to be effective for eradicating GBM neurospheres, particularly the aggressive MES subtype. ER stress also targets the GSC compartment in GBM. ATF6 is notably activated in MES GBM cells, while XBP1 seems more activated in PN GBM cells upon Tg-induced ER stress. The relationship between an UPR branch activation and the differential sensitivity for the ER stress induction will be further explored. Overall, targeting the ER stress response appears a promising approach to target GBM.

Citation Format: Natalia M. Peñaranda Fajardo, Coby Meijer, Frank A. E. Kruyt. Endoplasmic reticulum stress as possible target for therapy in glioblastoma. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2858.