Involvement of glutamate and its transport in the progression of glial tumours

Currently, high grade gliomas are known to release excitotoxic concentrations of glutamate, a process thought to contribute to their malignant phenotype through enhanced autocrine stimulation of their proliferation and destruction of the surrounding nervous tissue. Indeed, most of gliomas show an impaired expression of glial glutamate transporters as well as an overregulation of the cystine/glutamate antiporter that releases glutamate in exchange to cystine. In this study, a tetracycline-dependent inducible system was used to achieve controlled expression of the glutamate transporter 1 (GLT-1) in C6 glioma cells. Non-induced cells show modest glutamate uptake and, in the presence of L-cystine, these cells tend to release substantial amounts of glutamate. Overnight exposure to doxycycline increased D-[3H]-aspartate uptake, reaching similar capacity as observed in cultured astrocytes. Efficient clearance of exogenously applied glutamate was evidenced in these cells, even in the presence of L-cystine. The addition of glutamate (100 ìM) to the medium of non-induced cells significantly increased their proliferation rate, an effect that was blocked when the expression of GLT-1 was induced. These results suggest that impaired glutamate uptake capacity in glioma cells indirectly contributes to their proliferation. For this reason, these cells were used in order to investigate the consequences of increasing glutamate clearance on tumour progression in vivo. Cells were grafted in the striatum of Wistar rats and doxycycline was administered after validation of tumour development by magnetic resonance imaging. Both GLT-1 expression, examined by immunohistochemistry, and glutamate transport activity, measured on synaptosomes, appeared robustly increased in samples from doxycycline-treated animals. Moreover, these rats showed increased survival time as compared to vehicle-treated animals, an effect that was consistent with volumetric data revealing delayed tumour growth. As constitutive deficiency in glutamate clearance at the vicinity of brain tumours is well established, these data illustrate the potential benefit that could be obtained by enhancing glutamate transport by glioma cells in order to reduce their invasive behaviour.