P09.17.B ROLE OF THE BRAIN MICROENVIRONMENT AND EXTRACELLULAR VESICLES IN DRIVING ENHANCED GBM CELL INVASION IN RESPONSE TO RADIOTHERAPY

Abstract BACKGROUND Glioblastoma multiforme (GBM) is a highly infiltrative primary brain tumour. Standard of care therapy includes surgical excision, followed by radiotherapy (RT) and chemotherapy. However, RT has been demonstrated in multiple studies to induce a pro-invasive phenotype, potentially contributing to GBM therapeutic reoccurrence, and therefore representing an important clinical challenge. The GBM tumour microenvironment is a heterogeneous niche of varying ECM components and different cell types that interact with the brain microenvironment including neuronal cells, astrocytes and endothelial cells. While recent studies have focused on the effect of radiation on GBM cells, we hypothesised that RT influences the brain stroma to release promigratory factors, including extracellular vesicles (EVs), that may promote infiltration of the surrounding brain tissue. MATERIAL AND METHODS In this study, we looked at the potential role of the tumour secretome in defining the role of the brain microenvironment following irradiation of GBM. Naïve GBM cells were treated with conditioned media from irradiated and control GMB cells and their migratory potential assessed. GBM cells and ex vivo murine brain slices were irradiated alone or in combination before confocal timelapse microscopy and single cell tracking of invading GBM cells. Finally, EVs from irradiated or control GBM cells were isolated via ultracentrifugation and incubated with naïve GBM cells before timelapse microscopy and single cell tracking. RESULTS The results indicated that conditioned media from irradiated cells enhanced the ability of naïve cells to migrate in subconfluent migration assays (G7 cells p value = 0.0035). Pre-irradiation of either GBM cells or ex vivo brain slices increased subsequent invasion of brain tissue by GBM cells (p value <0.0001). Finally, preliminary data indicated that RT induced an increase in EV release from GBM cells and that these have increased capacity to promote migration in naïve cells. CONCLUSION The study indicates that RT-driven infiltration of the brain is not only driven by autocrine signaling between GBM cells, but also paracrine signaling to the surrounding normal brain stroma. Future studies to characterise the irradiated GBM secretome (including EVs) and contributions of other stromal components will be undertaken to further understand the mechanisms by which they influence invasion.