Characterization of cystine uptake in cultured astrocytes

Abstract

Glutathione is involved in the maintenance of the structural and functional integrity of membrane proteins, in protection against free radicals and oxidative stress, and in the detoxification of xenobiotics. The cellular uptake of cystine is the rate limiting step in the biosynthesis of glutathione. The precise mechanism for such uptake is not clear as some reports indicate that the uptake occurs through a glutamate–cystine antiporter (system X c – ), whereas, others suggest that it is taken up by the glutamate transporter (system X AG ). Our studies in cultured astrocytes derived from neonatal rats showed that glutamate, D- and L-aspartate inhibited cystine uptake; that factors that increased intracellular glutamate levels, which would have enhanced the activity of the antiporter, did not stimulate cystine uptake; that the uptake was sodium dependent and partially chloride dependent; that the b o,+ and ASC systems, which have been shown to carry cystine in some cells, did not mediate cystine uptake in astrocytes; that glutamate uptake blockers such as L-aspartate-β-hydroxamate (AβH) and L- trans-pyrrolidine-2,4-dicarboxylate (PDC), as well as cystine uptake inhibitor L-α-aminoadipate (AAA) potently reduced cystine uptake. Additionally, deferoxamine (100 μM) as well as ammonium chloride (5 mM), both of which inhibit glutamate uptake, also inhibited cystine uptake. Taken together, our findings indicate that astrocytes take up cystine through a similar, if not identical, system used to take up glutamate. Interference of cystine uptake by astrocytes through the glutamate transport system may have profound effects on the redox state and the structural and functional integrity of the CNS.