Radiation-induced extracellular vesicle (EV) release of miR-603 promotes IGF1-mediated stem cell state in glioblastomas

Background: Recurrence after radiation therapy is nearly universal for glioblastomas, the most common form of adult brain cancer. The study aims to define clinically pertinent mechanisms underlying this recurrence. Methods: microRNA (miRNA) profiling was performed using matched pre- and post-radiation treatment glioblastoma specimens from the same patients. All specimens harbored unmethylated O-6-methylguanine-DNA methyltransferase promoters (umMGMT) and wild-type isocitrate dehydrogenase (wtIDH). The most altered miRNA, miR-603, was characterized. Findings: While nearly all miRNAs remained unchanged after treatment, decreased levels of few, select miRNAs in the post-treatment specimens were observed, the most notable of which involved miR-603. Unbiased profiling of miR-603 targets revealed insulin-like growth factor 1 (IGF1) and IGF1 receptor (IGF1R). Ionizing radiation (IR) induced cellular export of miR-603 through extracellular vesicle (EV) release, thereby derepressing IGF1 and IGF1R. This de-repression, in turn, promoted cancer stem-cell (CSC) state and acquired radiation resistance in glioblastomas. Export of miR-603 additionally de-repressed MGMT, a DNA repair protein responsible for detoxifying DNA alkylating agents, to promote cross-resistance to these agents. Ectopic miR-603 expression overwhelmed cellular capacity for miR-603 export and synergized with the tumoricidal effects of IR and DNA alkylating agents. Interpretation: Profiling of matched pre- and post-treatment glioblastoma specimens revealed altered homeostasis of select miRNAs in response to radiation. Radiation-induced EV export of miR-603 simultaneously promoted the CSC state and up-regulated DNA repair to promote acquired resistance. These effects were abolished by exogenous miR-603 expression, suggesting potential for clinical translation. (C) 2020 The Authors. Published by Elsevier B.V.