Autocrine Glutamate Signaling Promotes Glioma Cell Invasion

Abstract

Malignant gliomas have been shown to release glutamate, which kills surrounding brain cells, creating room for tumor expansion. This glutamate release occurs primarily via system x C À , a Na + -independent cystine-glutamate exchanger. We show here, in addition, that the released glutamate acts as an essential autocrine/paracrine signal that promotes cell invasion. Specifically, chemotactic invasion and scrape motility assays each show dose-dependent inhibition of cell migration when glutamate release was inhibited using either S-(4)-CPG or sulfasalazine, both potent blockers of system x C À . This inhibition could be overcome by the addition of exogenous glutamate (100 Mmol/L) in the continued presence of the inhibitors. Migration/invasion was also inhibited when Ca 2+ -permeable A-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors (AMPA-R) were blocked using GYKI or Joro spider toxin, whereas CNQX was ineffective. Ca 2+ imaging experiments show that the released glutamate activates Ca 2+ -permeable AMPA-R and induces intracellular Ca 2+ oscillations that are essential for cell migration. Importantly, glioma cells release glutamate in sufficient quantities to activate AMPA-Rs on themselves or neighboring cells, thus acting in an autocrine and/or paracrine fashion. System x C À and the appropriate AMPA-R subunits are expressed in all glioma cell lines, patient-derived glioma cells, and acute patient biopsies investigated. Furthermore, animal studies in which human gliomas were xenographed into scid mice show that chronic inhibition of system x C À -mediated glutamate release leads to smaller and less invasive tumors compared with saline-treated controls. These data suggest that glioma invasion is effectively disrupted by inhibiting an autocrine glutamate signaling loop with a clinically approved candidate drug, sulfasalazine, already in hand.

Disruption of xCT inhibits cancer cell metastasis via the caveolin-1/β-catenin pathway

“…Our finding on the novel function of xCT in tumor metastasis was mechanistically different from a recent work showing that xCT plays a role in glioma cell invasion (Lyons et al, 2007). The underlying mechanism is that glutamate released by system xc-directly promotes cell invasion by inducing intracellular Ca 2 þ oscillations that is essential for cell migration.…”

“…Many studies have shown the higher expression of xCT mRNA in a broad range of cancer cell lines. It was reported that xCT was prominently expressed in all glioma samples acutely derived from five patients (Lyons et al, 2007). Importantly, targeted inhibition of xCT has been demonstrated to suppress the growth of a variety of carcinomas ) p38 is involved in the upregulation of caveolin-1 in xCT À/À melanocyte versus wild-type control.…”

“…Current studies are mostly focused on the role of xCT in cancer cell proliferation. One recent study has shown that xCT affects glioma invasion through the autocrine glutamate signaling loop in the brain (Lyons et al, 2007). Whether xCT is involved in a general control of tumor metastasis remains unaddressed.…”

Dissection of mitogenic and neurodegenerative actions of cystine and glutamate in malignant gliomas

“…Excessive glutamate release by glioma cells has been linked to decreased activities of glial glutamate transporters such as EAAT1 (GLAST, SLC1A3) and EAAT2 (GLT-1, SLC1A2), resulting in decreased glutamate incorporation and consequently reduced extracellular glutamate clearance. In addition, the antiporter system X c À , mediating glutamate release in exchange for cystine, is abundantly expressed in glioblastoma specimens and cell lines Lyons et al, 2007;Savaskan et al, 2008Savaskan et al, , 2009. As a result, reduced EAAT1 and EAAT2 expression and concurrently abundant system X c À activity result in a net balance shifted toward glutamate release, thus promoting glioma progression.…”

Blood glutamate scavengers prolong the survival of rats and mice with brain-implanted gliomas

“…Accordingly, the therapeutic strategies that have been proposed to counteract the Glu-supporting effects on glioma invasiveness which are based on the use of Glu release inhibitors and Glu receptor antagonists [11, 13, 14] are precisely the same as those proposed for the treatment of acute neurodegenerative disorders involving excess Glu in brain such as stroke or head trauma.…”