In mammalian cultured cells, the cystine/glutamate exchange transport mediated by system x c ؊ is important to maintain intracellular GSH levels. System x c ؊ consists of two protein components, xCT and the heavy chain of 4F2 antigen. The activity of system x c ؊ is induced by various stimuli, including electrophilic agents like diethyl maleate. In the present study, we have investigated the mechanism of the transcriptional regulation of xCT mRNA by diethyl maleate. The xCT gene consisted of twelve exons and sequence analysis identified four electrophile response element (EpRE)-like sequences between ؊230 and ؊1 in the 5-flanking region, designated EpRE-1 to EpRE-4. To identify sequences mediating the constitutive and induced expression of xCT, a series of 5-deletion mutants created from the 5-flanking region were cloned into a luciferase reproter vector and transfected into BHK21 cells. The 5-deletion analysis revealed that the sequence between ؊116 and ؊82 is essential for the basal expression and the sequence between ؊226 and ؊116 containing EpRE-1 is essential in response to diethyl maleate. Mutational analysis demonstrated that EpRE-1 is critically involved in the response to diethyl maleate. Other stress agents like arsenite, cadmium, and hydroquinone seemed to induce system x c ؊ activity via the same sequence. Furthermore, the experiments using the mouse embryonic fibroblasts derived from the Nrf2-deficient mice revealed that the induction of xCT gene by electrophilic agents is mediated by Nrf2. EpRE occurs in a broad spectrum of genes for the proteins that are involved in the defense against xenobiotics and regulates their expression. The present results have demonstrated that xCT is a novel member of this protein family.

Transport system x c À is a member of plasma membrane heterodimeric amino-acid transporters and consists of two protein components, xCT and 4F2hc. This system mediates cystine entry coupled with the exodus of intracellular glutamate and regulates the intracellular glutathione (GSH) levels in most mammalian cultured cells. We studied the activity of system x c À and GSH content in human ovarian cancer cell line (A2780) and its cisplatin (CDDP)-resistant variant (A2780DDP). The rate of cystine uptake was approximately 4.5-fold higher in A2780DDP cells than in A2780 cells and the cystine uptake in A2780DDP cells was mediated by system x c À . Intracellular GSH content was much higher in A2780DDP cells but it fell drastically in the presence of excess glutamate, which inhibited the cystine uptake competitively. xCT and 4F2hc mRNAs were definitely expressed in A2780DDP cells, but far less in A2780 cells. Expression of system x c À activity by transfection with cDNAs for xCT and 4F2hc made A2780 cells more resistant to CDDP. Similar results on the cystine uptake were obtained in human colonic cancer cell lines. These findings suggest that the system x c À plays an important role in maintaining the higher levels of GSH and consequently in CDDP resistance in cancer cell lines.

Transport of system xc- is an exchange agency with high specificity for anionic form of cystine and glutamate. The protein mediating this transport is a disulfide-linked heterodimer of a light chain named xCT and a heavy chain previously known as 4F2hc. We have isolated two cDNAs encoding xCT from the human cDNA library. One clone coded for a protein of 501 amino acids with 12 putative transmembrane domains. When functionally expressed in Xenopus oocytes together with the human 4F2hc, human xCT induced the transport activity whose characteristics are similar to those of system xc-. Another clone seemed to contain a partial human xCT and a long 3' untranslated region. The human xCT gene was localized at chromosome 4q28-31. Analysis of the 5'-flanking region of the human xCT gene revealed several sites for potentially binding of transcriptional factors, including NF-E2 and AP-1. Transport of cystine via system xc- has been known as a regulatory factor for the intracellular glutathione level, and its transport activity is induced in response to the oxygen tension in culture. Northern blot analysis demonstrated that the expression of both xCT and 4F2hc was significantly enhanced by oxygen. The results suggest that oxygen regulates the activity of system xc- by modulating the expression of both xCT and 4F2hc mRNAs.

Amino acid transport in mouse peritoneal macrophages is mediated by several membrane carriers with different substrate specificity and sensitivity to environmental stimuli. We reported previously that transport activities of cystine and arginine in the macrophages were induced markedly by low concentrations of bacterial lipopolysaccharide (LPS). It is known that a variety of macrophage functions are affected by ambient oxygen tension. In this study, we have investigated the effects of oxygen on the induction of amino acid transport activity by LPS and found that the induction of cystine, but not arginine, transport activity was dependent on the ambient oxygen tension. When the macrophages were cultured with 2% O 2 in the presence of 1 ng/ml LPS, induction of cystine transport activity was reduced by ϳ70% compared with cells cultured under normoxic conditions. In macrophages, transport of cystine is mediated by a Na ؉ -independent anionic amino acid transporter named system x c Ϫ . System x c Ϫ is composed of two protein components, xCT and 4F2hc, and the expression of xCT was closely correlated with system x c Ϫ activity. A putative NF-B binding site was found in the 5-flanking region of the xCT gene, but the enhanced expression of xCT by LPS and oxygen was not mediated by NF-B binding. An increase in intracellular GSH in macrophages paralleled induction of xCT, but not ␥-glutamylcysteine synthetase. These results suggest the importance of system x c Ϫ in antioxidant defense in macrophages exposed to LPS and oxidative stress.

The relationship between l-cystine transport and intracellular glutathione (GSH) levels was investigated in cultured pancreatic AR42J acinar and betaTC3 islet cells exposed to diethylmaleate, an electrophilic agent known to activate cellular antioxidant responses. Cystine transport was mediated predominantly by the Na+-independent anionic amino acid transport system x-c, with influx inhibited potently by glutamate and homocysteate but unaffected by cationic or neutral amino acids. Saturable cystine transport was 10-fold higher in AR42J (531 pmol (mg protein)-1 min-1) than in betaTC3 (49 pmol (mg protein)-1 min-1) cells, and GSH levels were higher in AR42J cells. Treatment with 2-mercaptoethanol increased GSH levels in betaTC3 cells from 7.5 to 36 nmol (mg protein)-1, whilst the GSH content in AR42J cells (64 nmol (mg protein)-1) was not altered significantly. Incubation of AR42J or betaTC3 cells with homocysteate (2.5 mM, 0-48 h), a competitive inhibitor of cystine transport via system x-c, reduced intracellular GSH levels and resulted in a time-dependent (6-24 h) induction of system x-c transport activity. Treatment of AR42J cells with diethylmaleate (100 microM, 0-48 h) resulted in a time- (5-10 h) and protein synthesis-dependent induction of cystine transport, with intracellular GSH levels initially decreasing and then increasing 2-fold above control levels after 24 h. Diethylmaleate also depressed GSH levels in betaTC3 cells, but cystine transport was not elevated significantly. In both AR42J and betaTC3 cells, inhibition of gamma-glutamyl cysteine synthetase by buthionine sulphoximine (100 microM, 24 h) reduced GSH levels but had no effect on cystine transport. The present findings establish that induction of system x-c leads to changes in GSH levels in pancreatic AR42J acinar and betaTC3 islet cells, with changes in the intracellular redox state stimulating transporter expression. Induction of activity of system x-c, together with adaptive increases in GSH synthesis in response to oxidative stress, may contribute to cellular antioxidant defences in pancreatic disease.