1. ¹GSF-Forschungszentrum für Umwelt und Gesundheit, Institut für Klinische Molekularbiologie und Tumorgenetik, München, Germany
2. ²Department of Biochemistry, Institute of Basic Medical Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan
3. ³Medizinische Poliklinik Innenstadt der Ludwig-Maximilians-Universität, München, Germany
4. ⁴Medizinische Klinik III der Ludwig-Maximilians-Universität, GSF-Klinische Kooperationsgruppe Hyperthermie, München, Germany
5. ⁵Medizinische Klinik mit Schwerpunkt Hämatologie und Onkologie, Charité, Humboldt Universität, Berlin, Germany

Correspondence: Dr M Conrad, Institute of Clinical Molecular Biology and Tumor Genetics, GSF-Research Centre, Marchioninistr. 25, Munich, Bavaria 81377, Germany. E-mail: marcus.conrad@gsf.de

⁶Current address: Department of Bioresources, Faculty of Agriculture, Yamagata University, Tsuruoka, Yamagata 997-8555, Japan.

⁷These authors contributed equally to this work.

Received 22 February 2007; Revised 4 July 2007; Accepted 20 August 2007; Published online 10 September 2007.

Abstract

The glutathione-dependent system is one of the key systems regulating cellular redox balance, and thus cell fate. Cysteine, typically present in its oxidized form cystine in the extracellular space, is regarded as the rate-limiting substrate for glutathione (GSH) synthesis. Cystine is transported into cells by the highly specific amino-acid antiporter system x_c^-. Since Burkitt's Lymphoma (BL) cells display limited uptake capacity for cystine, and are thus prone to oxidative stress-induced cell death, we stably expressed the substrate-specific subunit of system x_c^-, xCT, in HH514 BL cells. xCT-overexpressing cells became highly resistant to oxidative stress, particularly upon GSH depletion. Contrary to previous predictions, the increase of intracellular cysteine did not affect the cellular GSH pool, but concomitantly boosted extracellular cysteine concentrations. Even though cells were depleted of bulk GSH, xCT overexpression maintained cellular integrity by protecting against lipid peroxidation, a very early event in cell death progression. Our results show that system x_c^- protects against oxidative stress not by elevating intracellular GSH levels, but rather creates a reducing extracellular environment by driving a highly efficient cystine/cysteine redox cycle. Our findings show that the cystine/cysteine redox cycle by itself must be viewed as a discrete major regulator of cell survival.