Abstract 2609: Astrocytes decrease the radiosensitivity of glioblastoma stem-like cells.

Abstract Radiotherapy remains a primary treatment modality for glioblastoma (GBM). However, despite their extensive biological heterogeneity, although some GBMs respond better than others, they all essentially fail radiotherapy. This relatively homogeneous clinical response in a background of inter-tumor heterogeneity suggests that the microenvironment may play a significant role in determining their radioresponse. Data now suggest that glioblastoma stem-like cells (GSCs) play a critical role in GBM development and maintenance. Therefore, towards developing a model system for investigating the role of the microenvironment in GBM radioresponse, we have used human GSCs grown in co-culture with human astrocytes. For these studies GSCs were seeded into the lower compartment of Boyden chambers with astrocytes in the upper compartment. GSC numbers were determined at times out to 18 days after seeding according to ATP content. The proliferation rate of GSCs in co-culture was similar to that obtained when astrocytes were not included in the upper chamber, indicating that astrocytes had no effect on the growth rate of GSCs. To determine whether astrocytes influence GSC radiosensitivity, 48h after co-culture initiation GSCs were irradiated with doses up to 6 Gy; ATP content of the cells were defined 11 days later and radiation survival curves generated. The resulting survival curves showed that the GSCs irradiated under co-culture conditions were significantly less sensitive than those irradiated in the absence of astrocytes. To begin to address the mechanisms responsible for the increased radioresistance of co-cultured GSCs, radiation-induced γ-H2AX nuclear foci were defined in GSCs grown with and without astrocyte co-culture. The initial level of γ-H2AX foci in GSCs was reduced significantly in co-cultures as compared to GSCs cultured in the absence of astrocytes suggesting the induction of fewer DNA double strand breaks. In addition, gene expression profiles generated from GSCs grown in co-culture were significantly different from control GSCs suggesting that astrocytes induced phenotypic modifications. Finally, profiling of astrocyte conditioned media revealed increased levels of a variety of chemokines/growth factors, including a number of which that have already been linked to radioresponse such as HGF, IL-6 and EGF. Taken together, these results are consistent with an astrocyte-mediated radioprotection of GBM cells and provide additional support for a critical role of the microenvironment in contributing to GBM radioresistance. Citation Format: Barbara H. Rath, Kevin Camphausen, Philip J. Tofilon. Astrocytes decrease the radiosensitivity of glioblastoma stem-like 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 2609. doi:10.1158/1538-7445.AM2013-2609