Role of glutamate transporters in the regulation of glutathione levels in human macrophages

Cysteine is the limiting precursor forglutathione synthesis. Because of its low bioavailability, cysteine isgenerally produced from cystine, which may be taken up through twodifferent transporters. The cystine/glutamate antiporter(x system) transports extracellular cystine inexchange for intracellular glutamate. The X_AG transportsystem takes up extracellular cystine, glutamate, and aspartate. Bothare sensitive to competition between cystine and glutamate, and excessextracellular glutamate thus inhibits glutathione synthesis, anonexcitotoxic mechanism for glutamate toxicity. We demonstratedpreviously that human macrophages express the glutamate transportersexcitatory amino acid transporter (EAAT)1 and EAAT2 (which do nottransport cystine, X system) and overcomecompetition for the use of cystine transporters. We now showthat macrophages take up cystine through the x andnot the X_AG system. We also found that glutamate, although competing with cystine uptake, dose-dependently increases glutathione synthesis. We used inhibitors to demonstrate that this increase ismediated by EAATs. EAAT expression in macrophages thus leads toglutamate-dependent enhancement of glutathione synthesis by providingintracellular glutamate for direct insertion in glutathione and alsofor fueling the intracellular pool of glutamate andtrans-stimulating the cystine/glutamate antiporter.