The cystine/glutamate transporter system xc− consists of the light-chain subunit xCT (SLC7A11) and the heavy-chain subunit CD98 (4F2hc or SLC3A2) and exchanges extracellular cystine for intracellular glutamate at the plasma membrane. The imported cystine is reduced to cysteine and used for synthesis of GSH, one of the most important antioxidants in cancer cells. Because cancer cells have increased levels of reactive oxygen species, xCT, responsible for cystine–glutamate exchange, is overexpressed in many cancers, including glioblastoma. However, under glucose-limited conditions, xCT overexpression induces reactive oxygen species accumulation and cell death. Here we report that cell survival under glucose deprivation depends on cell density. We found that high cell density (HD) down-regulates xCT levels and increases cell viability under glucose deprivation. We also found that growth of glioblastoma cells at HD inactivates mTOR and that treatment of cells grown at low density with the mTOR inhibitor Torin 1 down-regulates xCT and inhibits glucose deprivation-induced cell death. The lysosome inhibitor bafilomycin A1 suppressed xCT down-regulation in HD-cultured glioblastoma cells and in Torin 1–treated cells grown at low density. Additionally, bafilomycin A1 exposure or ectopic xCT expression restored glucose deprivation–induced cell death at HD. These results suggest that HD inactivates mTOR and promotes lysosomal degradation of xCT, leading to improved glioblastoma cell viability under glucose-limited conditions. Our findings provide evidence that control of xCT protein expression via lysosomal degradation is an important mechanism for metabolic adaptation in glioblastoma cells.

We have reported that inhibitors of cystathionine γ-lyase (CSE), an H 2 S-generating enzyme, reverse paclitaxel (PCT)-induced peripheral neuropathy (PIPN) in rats (Neuroscience 2011;188:148-156), and that PIPN in rats and mice involves macrophage-derived HMGB1, a DAMP molecule (Neuropharmacology 2018;141:201-213). Thus, we investigated a possible crosstalk between CSE/H 2 S and HMGB1 pathways in macrophages and its implication for PIPN in mice. Repetitive i.p. administration (days 0, 2, 4 and 6) of PCT caused mechanical allodynia, as assessed by von Frey test, which was prevented by repeated i.p. administration of DL-propargylglycine (PPG), a CSE inhibitor. A single administration of PPG as well as β-cyano-L-alanine (BCA), another CSE inhibitor, reversed the established PIPN. In macrophage-like RAW264.7 cells, PCT at 1 μM produced HMGB1 release, an effect abolished by PPG or BCA. Na 2 S, an H 2 S donor, at 30-100 μM also caused HMGB1 release from RAW264.7 cells, which was blocked by Nacetyl-L-cysteine, an antioxidant. Our data suggest that PCT-induced HMGB1 release from macrophages involves endogenous H 2 S generated by CSE, contributing to PIPN in mice.