Proteomic analysis reveals microvesicles containing NAMPT as mediators of radiation resistance in glioma

Abstract

Glioma is a malignant brain tumor that is highly resistant to radiation and chemotherapy, where patients survive on average only 15 months after diagnosis. Furthering the understanding of mechanisms leading to radiation resistance of glioma is paramount to identify novel therapeutic targets. Previous studies have shown that glioma stem cells (GSCs) play an important role in promoting radiation resistance and disease recurrence. Herein we analyze the proteomic alterations occurring in patient-derived GSCs upon radiation treatment in order to identify molecular drivers of resistance. We show that proteome changes upon radiation accurately predict the resistance status of the cells, whereas resistance to radiation does not correlate with glioma transcriptional subtypes. We further show that the radio-resistant GSC-267 cell line sheds microvesicles (MVs) enriched in the metabolic enzyme nicotinamide phosphoribosyltransferase (NAMPT). These MVs can be transferred to recipient fibroblasts and radio-sensitive GSCs, enhancing their intracellular total NAD+ and NADH level, and their ability to proliferate when cultured in low serum, treated with a radio-mimetic drug or irradiated. The NAMPT enzymatic inhibitor FK-866 blocked the ability of MVs from GSC-267 cells to mediate these effects. Similarly, GSC-267 cells where NAMPT was knocked-down using shRNA, which produced MVs depleted of this enzyme, were unable to promote cell proliferation. Collectively, our findings demonstrates that proteome-level regulation can accurately predict the radio-resistance status of GSCs, and identifies NAMPT transfer via MVs as a mechanism for spreading radiation resistance within the glioma tumor microenvironment. SignificanceThe highly aggressive and deadly brain cancer glioma is commonly resistant to standard chemo- and radio-therapy. We used systems biology approaches to study patient-derived glioma stem cells (GSCs), which are known to be responsible for therapeutic resistance, and cell-to-cell communication mediated by extracellular vesicles (EVs), which plays an important role in tumor progression. Analysis of the proteome of GSCs and of the EVs they release led us to determine that the EV-mediated transfer of the metabolic enzyme nicotinamide phosphorybosyltransferase (NAMPT) from radio-resistant to less aggressive cells confers resistance to radiation. Our findings identify a mechanism of therapy resistance in glioma, and suggest that NAMPT inhibition could enhance the efficacy of radiation for the treatment of glioma.