Regulation of Amino Acid Transport in Glioma

INTRODUCTION: Glutamate is an important molecule in the biology of brain tumors. Glutamate reaches toxic concentrations in peritumoral tissue, contributing to tumor growth and peritumoral excitotoxicity. Mediated by the cystine-glutamate exchanger, System xc- (SXC), glutamate uptake allows production of the antioxidant glutathione (GSH), which protects cells from damage and cell death. We have shown SXC is variably expressed among glioma patients, with ˜50% demonstrating elevated expression, associated seizures, and glutamate excitotoxicity. In a clinical pilot study, we found pharmacological inhibition of SXC reduces glutamate release in gliomas with elevated SXC. We now hypothesize that differences in SXC expression is due to transcriptional and co-receptor regulation, specifically, by p53 and CD44. METHODS: This study utilizes in Vivo propagated glioma xenolines to test the consequences of p53 and CD44 loss on SXC function. We use siRNA/shRNA against p53/CD44, and glutamate assays to test SXC function, as well as chromatin immunopreciptiation (ChIP) assay and immunofluorescence to detail the relationship between between p53/CD44 and SXC. Lastly, using electrophysiological recordings in live brain slices, we test peritumoral excitotoxicity due to SXC function in setting of p54/CD44 loss to determine glutamate excitotoxicity on the surrounding brain. RESULTS: SXC activity and glutamate release is altered by p53 and CD44 expression, altering peritumoral excitotoxicity, invasion, and tumor growth. P53 activity transcriptionally suppresses SXC and prevents glutamate release. Furthermore, SXC is also regulated by the hyaluronan receptor CD44. CD44 appears to be a membrane co-receptor of SXC, and loss of CD44 results in decreased SXC function, and consequently the ability of glioma cells to invade and grow. CONCLUSION: These studies explore new strategies to alter the abnormal glutamate biology of gliomas at a transcriptional level, which is advantageous given poor options for pharmacological inhibition of SXC. Furthermore, the expression of p53 and CD44 may have predictive value regarding treatment and prognosis of glioma.