Inhibition of Cystine Uptake Disrupts the Growth of Primary Brain Tumors

Abstract

Glial cells play an important role in sequestering neuronally released glutamate via Naϩ -dependent transporters. Surprisingly, these transporters are not operational in glial-derived tumors (gliomas). Instead, gliomas release glutamate, causing excitotoxic death of neurons in the vicinity of the tumor. We now show that glutamate release from glioma cells is an obligatory by-product of cellular cystine uptake via system x c Ϫ , an electroneutral cystine-glutamate exchanger. Cystine is an essential precursor for the biosynthesis of glutathione, a major redox regulatory molecule that protects cells from endogenously produced reactive oxygen species (ROS). Glioma cells, but not neurons or astrocytes, rely primarily on cystine uptake via system x c Ϫ for their glutathione synthesis. Inhibition of system x c Ϫ causes a rapid depletion of glutathione, and the resulting loss of ROS defense causes caspase-mediated apoptosis. Glioma cells can be rescued if glutathione status is experimentally restored or if glutathione is substituted by alternate cellular antioxidants, confirming that ROS are indeed mediators of cell death. We describe two potent drugs that permit pharmacological inhibition of system x c Ϫ . One of these drugs, sulfasalazine, is clinically used to treat inflammatory bowel disease and rheumatoid arthritis. Sulfasalazine was able to reduce glutathione levels in tumor tissue and slow tumor growth in vivo in a commonly used intracranial xenograft animal model for human gliomas when administered by intraperitoneal injection. These data suggest that inhibition of cystine uptake into glioma cells through the pharmacological inhibition of system x c Ϫ may be a viable therapeutic strategy with a Food and Drug Administration-approved drug already in hand.

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

“…In line with these findings, enhancing EAAT2 expression in grafted C6 rat glioma cells reduced tumor size and elevated the life span of tumor-bearing animals (Vanhoutte et al, 2009). Moreover, inhibition of glutamate release via system X c À , consisting of xCT (SLC7A11) and CD98 (SLC3A2, 4F2HC), profoundly decelerates the glioma phenotype in vivo (Chung et al, 2005;Lyons et al, 2007;Savaskan et al, 2008) and in addition mitigates tumor-induced brain swelling (Savaskan et al, 2008). Even though the glioma-promoting properties of glutamate release by glioma cells appear to be without doubt, the underlying mechanisms are still poorly understood.…”

“…As to potential glioma therapy, system X c À emerges as a promising target (Chung et al, 2005;Savaskan et al, 2008;Chung and Sontheimer, 2009;Pham et al, 2010). Both components of system X c À (xCT and CD98) have shown to be transcriptionally upregulated in glioma specimens and interference with system X c À may disrupt two major pathophysiological properties of glioma cells: (1) the ability to induce neurodegeneration and (2) the ability to incorporate high levels of cystine required for rapid proliferation.…”

“…Both components of system X c À (xCT and CD98) have shown to be transcriptionally upregulated in glioma specimens and interference with system X c À may disrupt two major pathophysiological properties of glioma cells: (1) the ability to induce neurodegeneration and (2) the ability to incorporate high levels of cystine required for rapid proliferation. Although glioma cells appear to be highly dependent on system X c À -mediated cystine incorporation required for glutathione production and thus the scavenging of reactive oxygen species (Chung et al, 2005;Chung and Sontheimer, 2009;Pham et al, 2010), normal cells might be less vulnerable. This notion is supported by genetic deletion studies, that is, xCT-deficient mice are healthy in appearance and fertile (Sato et al, 2005;Shih et al, 2006;Liu et al, 2007).…”

“…We show that neither glutamate nor AMPA possesses any mitogenic effects on glioma cells even under growth factor-reduced or low-cystine conditions. However, disrupting glutamate secretion by interfering with system X c À activity attenuates glioma cell proliferation, which is because of impaired cystine incorporation required for glutathione synthesis (Jones et al, 2004;Chung et al, 2005) and not because of decreased extracellular levels of glutamate. All glioma cell lines investigated appear to be insensitive to AMPA receptormediated signaling either by solid downregulation of all AMPA receptor subunits or disproportionately high expression of the fully edited GluR2 subunit, which blocks Ca 2 þ permeability of the AMPA receptor complex.…”

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

“…; Narang et al, 2003;Chung et al, 2005;Doxsee et al, 2007). The underlying mechanism was attributed to the inhibition of system xc-, which causes rapid depletion of GSH, and thereby an increase in reactive oxygen species (ROS) and ultimately leads to caspasedependent apoptosis (Doxsee et al, 2007).…”