Radiation Induced Metabolic Alterations Associate with Tumor Aggressiveness and Poor Outcome in Glioblastoma
Glioblastoma (GBM) is uniformly fatal with a one year median survival rate, despite the best available treatment, including radiotherapy (RT). Impacts of prior RT on tumor recurrence are poorly understood but may increase tumor aggressiveness. Metabolic changes have been investigated in radiation-induced brain injury; however, the tumor-promoting effect following prior radiation is lacking. Since RT is vital to GBM management, we quantified the tumor-promoting effects of prior radiotherapy (RT) on patient-derived intracranial GBM xenografts and characterized the metabolic alterations associated with the protumorigenic microenvironment. Human xenografts (GBM143) were implanted into nude mice 24h following 20Gy cranial radiation vs. sham animals. Tumors in pre-radiated mice were more proliferative and more infiltrative, yielding faster mortality (p<0.0001). Histologic evaluation of tumor associated macrophage/microglia (TAMs) revealed cells with a more fully activated ameboid morphology in pre-radiated animals. Microdialyzates from the radiated brain at the margin of tumor infiltration contralateral to the site of implantation were analyzed by unsupervised liquid chromatography-mass spectrometry (LC-MS). In pre-radiated animals, metabolites known to be associated with tumor progression (like, modified nucleotides and polyols) were identified. Whole-tissue metabolomic analysis of the pre-radiated brain microenvironment for metabolic alterations in a separate cohort of nude mice using 1H-NMR revealed significant decrease in levels of antioxidants (glutathione (GSH) and ascorbate (ASC)), NAD+, TCA intermediates, and increased ATP and GTP. Glutathione and ASC showed highest VIPpred (1.65) in OPLS-DA analysis. Involvement of ASC catabolism was further confirmed by GC-MS. To assess longevity of radiation effects, we compared survival with implantation occuring 2 months vs. 24h following radiation, finding worse survival in animals implanted at 2 months. These radiation-induced alterations are consistent with a chronic disease-like microenvironment characterized by reduced levels of antioxidants and NAD+, as well as elevated extracellular ATP and GTP serving as chemoattractants, promoting cell motility and vesicular secretion with decreased levels of GSH and ASC exacerbating oxidative stress. Taken together, these data suggest that IR induces tumor-permissive changes in the microenvironment with metabolomic alterations that may facilitate tumor aggressiveness with important implications for recurrent glioblastoma. Harnessing these metabolomic insights may provide opportunities to attenuate RT-associated aggressiveness of recurrent GBM.