Glioma cells release excitotoxic concentrations of glutamate

Elevated levels of extracellular glutamate ([Glu](o)) cause uncontrolled Ca 2+ increases in most neurons and are believed to mediate excitotoxic brain injury following stroke and other nervous system insults, In the normal bra in, [Glu], is tightly controlled by uptake into astrocytes. Because the vas t majority of primary brain tumors (gliomas) are derived from astrocytes, w e investigated glutamate uptake in glioma cells surgically isolated from gl ioma patients (glioblastoma multiforme) and in seven established human glio ma cell lines, including STTG-1, D-54 MG, D-65 MG, U-373 MG, U-138 MG, U-25 1 MG, and CH-235 MG. All glioma cells studied showed impaired glutamate upt ake, with a V-max < 10% that of normal astrocytes. Moreover, rather than re moving glutamate from the extracellular fluid, glioma cells release large a mounts of glutamate, resulting in elevations of [Glu](o) in excess of 100 m u M within hours in a space that is 1000-fold larger than the cellular volu me, Exposure of cultured hippocampal neurons to glioma-conditioned medium e licited sustained [Ca2+](i) elevations that were followed by widespread neu ronal death. Similarly, coculturing of hippocampal neurons and glioma cells , either with or without direct contact, resulted in neuronal death. Glioma -induced neuronal death could be completely prevented by treating neurons w ith the N-methyl-D-aspartate receptor antagonists MK-801/D(-)-2-amino-5-pho sphonopentanoic acid or by depletion of glutamate from the medium. Interest ingly, several phenylglycine derivatives including the metabotropic glutama te receptor agonist/antagonist (S)-4-carboxyphenylglycine (S-4CPG) potently and selectively inhibited glutamate release from glioma cells and prevente d neurotoxicity. These data suggest that growing glioma tumors may actively kill surrounding neuronal cells through the release of glutamate, This glu tamate release may also be responsible in part for tumor-associated seizure s that occur frequently in conjunction with glioma, These data also suggest that neurotoxic release of glutamate by gliomas may be prevented by phenyl glycine derivatives, which may thus be useful as an adjuvant treatment for brain tumors.