[Show abstract][Hide abstract] ABSTRACT: In mammalian cultured cells, the activity of a cystine/glutamate transporter, designated System x(c)(-), has been shown to be essential for maintaining intracellular glutathione levels and the extracellular cystine/cysteine redox balance. The substrate-specific subunit of this transporter, xCT, is strongly induced by various stimuli, including oxidative stress, which suggests that xCT is one of the adaptive cellular defense systems against these types of stress. Embryonic fibroblasts from xCT-deficient mice fail to survive unless a cysteine precursor, N-acetylcysteine, is present. However, it is unclear whether xCT has similar functions in vivo because xCT-deficient mice are apparently normal. In this study, we investigated the phenotypes of the xCT-deficient mice under paraquat-induced oxidative stress. At a paraquat dose of 45mg/kg, the survival rate of the xCT-deficient mice was significantly lower than that of the wild-type mice. Under this condition, total glutathione [the reduced form of glutathione (GSH)+the oxidized form of GSH (GSSG)] levels in the lungs of the xCT-deficient mice were lower than those in the lungs of the wild-type mice. Histopathological examinations showed that paraquat administration worsened the alveolar structures of the xCT-deficient mice compared with the wild-type mice. After paraquat treatment, obvious 8-hydroxy-2'-deoxyguanosine and 4-hydroxy-2- nonenal reactivities were detected in the lung of the xCT-deficient mice. Although xCT expression was slightly detectable in the lungs of the normal wild-type mice, paraquat administration induced xCT mRNA expression in the lung. Constitutive expression of xCT mRNA was detected in alveolar macrophages isolated from the pulmonary lavage fluid of the wild-type mice, and paraquat administration strongly enhanced xCT mRNA expression in these cells. GSH levels in bronchoalveolar lavage fluid were significantly higher in the paraquat-treated wild-type mice than in the paraquat-treated xCT-deficient mice. These results suggest that xCT contributes to the maintenance of glutathione levels in lungs and the glutathione redox state as a protective system against paraquat toxicity in vivo.
No preview · Article · Oct 2012 · Free Radical Biology and Medicine
[Show abstract][Hide abstract] ABSTRACT: System x(c)- exchanges intracellular glutamate for extracellular cystine, giving it a potential role in intracellular glutathione synthesis and nonvesicular glutamate release. We report that mice lacking the specific xCT subunit of system x(c)- (xCT(-/-)) do not have a lower hippocampal glutathione content, increased oxidative stress or brain atrophy, nor exacerbated spatial reference memory deficits with aging. Together these results indicate that loss of system x(c)- does not induce oxidative stress in vivo. Young xCT(-/-) mice did however display a spatial working memory deficit. Interestingly, we observed significantly lower extracellular hippocampal glutamate concentrations in xCT(-/-) mice compared to wild-type littermates. Moreover, intrahippocampal perfusion with system x(c)- inhibitors lowered extracellular glutamate, whereas the system x(c)- activator N-acetylcysteine elevated extracellular glutamate in the rat hippocampus. This indicates that system x(c)- may be an interesting target for pathologies associated with excessive extracellular glutamate release in the hippocampus. Correspondingly, xCT deletion in mice elevated the threshold for limbic seizures and abolished the proconvulsive effects of N-acetylcysteine. These novel findings sustain that system x(c)-) is an important source of extracellular glutamate in the hippocampus. System x(c)(-) is required for optimal spatial working memory, but its inactivation is clearly beneficial to decrease susceptibility for limbic epileptic seizures.
Full-text · Article · Apr 2011 · The Journal of Neuroscience : The Official Journal of the Society for Neuroscience
[Show abstract][Hide abstract] ABSTRACT: Glutathione (GSH) plays an important neuroprotective role, and its synthesis depends on the amount of available cysteine (CSH) in the cells. Various kinds of evidence suggest that astrocytes can provide CSH or GSH to neurons, but the delivery mechanism of the thiol-compounds has not been elucidated. In this study, the dynamics of CSH, GSH and their disulphides in astrocyte culture medium were investigated by following the time-course of concentration changes and by computer simulation and curve fitting to experimental data using a mathematical model. The model consists of seven reactions and three transports, which are grouped into four categories: autoxidation of thiols into disulphides, thiol-disulphide exchange and reactions of thiols with medium components, as well as the cellular influx and efflux of thiols and disulphides. The obtained results are interpreted that cystine (CSSC) after entering astrocyte is reduced to CSH, most of which is released to medium and autoxidized to CSSC. The efflux of GSH was estimated to be considerably slower than that of CSH, and most of the excreted GSH is converted to cysteine-glutathione disulphide principally through the thiol-disulphide exchange. The results seem to indicate that astrocytes provide neurons mainly with CSH, rather than GSH, as the antioxidant material for neuroprotection.
No preview · Article · Mar 2011 · Journal of Biochemistry
[Show abstract][Hide abstract] ABSTRACT: Malfunctioning of system x(c)(-), responsible for exchanging intracellular glutamate for extracellular cystine, can cause oxidative stress and excitotoxicity, both important phenomena in the pathogenesis of Parkinson's disease (PD). We used mice lacking xCT (xCT(-/-) mice), the specific subunit of system x(c)(-), to investigate the involvement of this antiporter in PD. Although cystine that is imported via system x(c)(-) is reduced to cysteine, the rate-limiting substrate in the synthesis of glutathione, deletion of xCT did not result in decreased glutathione levels in striatum. Accordingly, no signs of increased oxidative stress could be observed in striatum or substantia nigra of xCT(-/-) mice. In sharp contrast to expectations, xCT(-/-) mice were less susceptible to 6-hydroxydopamine (6-OHDA)-induced neurodegeneration in the substantia nigra pars compacta compared to their age-matched wild-type littermates. This reduced sensitivity to a PD-inducing toxin might be related to the decrease of 70% in striatal extracellular glutamate levels that was observed in mice lacking xCT. The current data point toward system x(c)(-) as a possible target for the development of new pharmacotherapies for the treatment of PD and emphasize the need to continue the search for specific ligands for system x(c)(-).
Full-text · Article · Dec 2010 · The FASEB Journal
[Show abstract][Hide abstract] ABSTRACT: GSH is the major antioxidant and detoxifier of xenobiotics in mammalian cells. A strong decrease of intracellular GSH has been frequently linked to pathological conditions like ischemia/reperfusion injury and degenerative diseases including diabetes, atherosclerosis, and neurodegeneration. Although GSH is essential for survival, the deleterious effects of GSH deficiency can often be compensated by thiol-containing antioxidants. Using three genetically defined cellular systems, we show here that forced expression of xCT, the substrate-specific subunit of the cystine/glutamate antiporter, in gamma-glutamylcysteine synthetase knock-out cells rescues GSH deficiency by increasing cellular cystine uptake, leading to augmented intracellular and surprisingly high extracellular cysteine levels. Moreover, we provide evidence that under GSH deprivation, the cytosolic thioredoxin/thioredoxin reductase system plays an essential role for the cells to deal with the excess amount of intracellular cystine. Our studies provide first evidence that GSH deficiency can be rescued by an intrinsic genetic mechanism to be considered when designing therapeutic rationales targeting specific redox enzymes to combat diseases linked to GSH deprivation.
Full-text · Article · May 2010 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: We examined the protective effects of N-acetylcysteine (NAC) on the death of glia-free neurons in culture. Under normoxic conditions, the protection by NAC was observed only in cystine-free but not complete medium. When the cells were cultured under hypoxic conditions, NAC much elongated their survival even in the presence of cystine. H2O2 was found to be generated to considerable concentration in the presence of both NAC and cystine, and the administration of catalase prevented the cell death. These results suggest that the harmful effect of NAC is because of H2O2 generated by autoxidation of cysteine, which derives from the reaction between NAC and cystine. The present results raise the possibility that NAC can act as either antioxidant or prooxidant depending on the milieu.
[Show abstract][Hide abstract] ABSTRACT: This study examined the question of whether deficiency of xCT, a cystine-transporter gene, exacerbates ischemia-reperfusion-induced acute renal failure (ARF). Two weeks after the right nephrectomy of male mice at 16-18weeks of age, the left renal vessels were clamped for 45min to induce renal ischemia. After (24h) induction of ischemia, xCT(-/-) mice had elevated concentrations of blood urea nitrogen and creatinine indicative of ARF, while in xCT(+/-) and xCT(+/+) mice, these parameters did not differ from the sham-operated mice. Immunohistochemical analyses of kidneys using antibodies against the oxidative stress markers revealed stronger staining in xCT(-/-) mice compared with xCT(+/+) mice. Induction of xCT mRNA in the kidneys of xCT(+/+) mice was demonstrated using reverse transcriptase (RT)-PCR analysis and was further confirmed using quantitative RT-PCR. These data provide the first in vivo evidence that xCT is induced by oxidative stress and helps prevent ischemia-reperfusion injury to kidneys.
No preview · Article · Sep 2009 · Archives of Biochemistry and Biophysics
[Show abstract][Hide abstract] ABSTRACT: Altered glutamate signaling is associated with Parkinson's disease. To study the involvement of the cystine/glutamate antiporter in the pathogenesis of Parkinson's disease, we developed new polyclonal antibodies recognizing xCT, the specific subunit of this antiporter. The striatal xCT protein expression level was investigated in a hemi-Parkinson rat model, using semiquantitative western blotting. We observed time-dependent changes after a unilateral 6-hydroxydopamine lesion of the nigrostriatal pathway with increased expression levels in the deafferented striatum after 3 weeks. Twelve weeks postlesion, expression levels returned to normal. These data suggest, for the first time, an involvement of the cystine/glutamate antiporter in determining the aberrant glutamate neurotransmission in the striatum of a parkinsonian brain.
[Show abstract][Hide abstract] ABSTRACT: Because glutathione scavenges reactive oxygen species (ROS) and also donates electrons to antioxidative systems, it may compensate for the oxidative stress caused by SOD1 deficiency. The cystine/glutamate transporter, which consists of two proteins, xCT and 4F2hc, has been designated system x
c−. This transporter system plays a role in the maintenance of glutathione levels in mammalian cells. In the present study, we created SOD1
−/− double-knockout mice by intercrossing xCT-knockout and SOD1-knockout animals. We determined if the double-knockout mice express the phenotypic characteristics unique to SOD1
−/− mice—increased oxidative stress and the production of autoantibodies against erythrocytes. We also compared the phenotype of the double-knockout mice with those of the single-knockout and wild-type mice. Although two major antioxidative systems were found to be defective in the SOD1
−/− mice, relative to the SOD1
−/− mice, no functional deficits were observed. Based on these results, it appears that defects in system x
c− do not exacerbate the phenotypic consequences of SOD1 deficiency in postnatal mice under ordinary breeding conditions.
Full-text · Article · Aug 2008 · Molecular and Cellular Biochemistry
[Show abstract][Hide abstract] 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 xc-. 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 xc-, 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 xc- 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.
[Show abstract][Hide abstract] ABSTRACT: Reactive oxygen species (ROS) produced by neutrophils are essential in the host defense against infections but may be harmful to neutrophils themselves. Glutathione (GSH) plays a pivotal role in protecting cells against ROS-mediated oxidant injury. Cystine/glutamate transporter, designated as system xc- and consisting of two proteins, xCT and 4F2hc, is important to maintain GSH levels in mammalian-cultured cells. In the present paper, we have investigated system xc- in neutrophils. In human peripheral blood neutrophils, neither the activity of system xc- nor xCT mRNA was detected. The activity was induced, and xCT mRNA was expressed when they were cultured in vitro. The mRNA expression was much enhanced in the presence of opsonized zymosan or PMA. In contrast, mouse peritoneal exudate neutrophils, immediately after preparation, exhibited system xc- activity and expressed xCT mRNA. The activity and the expression were heightened further when they were cultured. Peritoneal exudate cells (mostly neutrophils) from xCT-deficient (xCT-/-) mice had lower cysteine content than those from the wild-type mice. GSH levels in the xCT-/-cells decreased rapidly when they were cultured, whereas those in the wild-type cells were maintained during the culture. Apoptosis induced in culture was enhanced in the xCT-/-cells compared with the wild-type cells. These results suggest that system xc- plays an important role in neutrophils when they are activated, and their GSH consumption is accelerated.
No preview · Article · May 2007 · Journal of Leukocyte Biology
[Show abstract][Hide abstract] ABSTRACT: Brain cell suspensions obtained from cerebrum of fetal rats were cultured and after 5 days neurons were separated from the residual cells. These purified neurons, which were replated on the dish, started to die within 24 h in culture. Glutathione content of these neurons decreased rapidly to less than one-tenth of the initial level after 24 h. In the presence of alpha-tocopherol, a well-known antioxidant, the neurons survived for at least 3 days, though glutathione content remained very low. Butylated hydroxyanisol had similar effect, but ascorbic acid and uric acid had no or very little effect. Serotonin, which is assumed to have an antioxidant activity, kept the neurons alive for 3 days. These results suggest that neurons separated from the other types of cells cannot survive due to the oxidative stress, which may otherwise be neutralized by a mechanism involving glutathione, and that antioxidants including serotonin has a beneficial effect on these purified neurons.
[Show abstract][Hide abstract] ABSTRACT: Cystine/glutamate transporter, system xc-, contributes to the maintenance of intracellular glutathione levels and the redox balance in the extracellular space. The main component of the transporter, xCT, is known to be strongly induced by various stimuli like oxidative stress in mammalian cultured cells. We examined the expression of xCT mRNA in vivo in the experimental endotoxemia.
Northern blot analysis and in situ hybridization were used to investigate the expression of xCT mRNA in the tissues of the mice exposed to bacterial lipopolysaccharide (LPS).
Northern blot analysis revealed that xCT mRNA was constitutively expressed in the brain, thymus, and spleen, and that the expression of xCT mRNA was strongly up-regulated in thymus and spleen by the administration of a sublethal dose of LPS. In addition to brain, thymus, and spleen, xCT mRNA was detected also in the bronchiolar epithelium of the lung by the administration of the lethal dose of LPS.
xCT is induced in some specific tissues by the administration of LPS. The results suggest that cystine/glutamate transporter plays an important role under the inflammatory conditions.
Preview · Article · Feb 2007 · Journal of Inflammation
[Show abstract][Hide abstract] ABSTRACT: The uptake of glutamate in rat glioma C-6 cells and cultured astrocytes derived from rat cerebral hemispheres was found to be mediated by a Na+-dependent and a Na+-independent system. The Na+-dependent system was inhibited by aspartate and was consistent with the commonly occurring system designated system X−ag. The Na+-independent system was inhibited by cystine and was consistent with system x−c described in various types of cells in the periphery. It was also found that quisqualate selectively and competitively interfered with the Na+-independent glutamate uptake. In C-6 cells, the glutamate uptake via systems X−ag and x−c accounted for approximately 35% and 55% of the total uptake, respectively, at 0.05 mM glutamate. In cultured astrocytes, the glutamate uptake via system X−ag was very potent, whereas the uptake via system x−c was relatively weak and its contribution to the total uptake of glutamate seemed almost negligible. However, in both C-6 cells and astrocytes, system x−c was necessary for the uptake of cystine, another substrate of system x−c. Cystine in the culture medium was an essential precursor of glutathione, and the inhibition of the cystine uptake by excess glutamate as a competitor led to a severe deficiency in glutathione, followed by cell degeneration.
No preview · Article · Oct 2006 · Journal of Neurochemistry
[Show abstract][Hide abstract] ABSTRACT: Cystine/glutamate transporter, designated as system x(-)(c), mediates cystine entry in exchange for intracellular glutamate in mammalian cells. This transporter consists of two protein components, xCT and 4F2 heavy chain, and the former is predicted to mediate the transport activity. This transporter plays a pivotal role for maintaining the intracellular GSH levels and extracellular cystine/cysteine redox balance in cultured cells. To clarify the physiological roles of this transporter in vivo, we generated and characterized mice lacking xCT. The xCT(-/-) mice were healthy in appearance and fertile. However, cystine concentration in plasma was significantly higher in these mice, compared with that in the littermate xCT(-/-) mice, while there was no significant difference in plasma cysteine concentration. Plasma GSH level in xCT(-/-) mice was lower than that in the xCT(-/-) mice. The embryonic fibroblasts derived from xCT(-/-) mice failed to survive in routine culture medium, and 2-mercaptoethanol was required for survival and growth. When 2-mercaptoethanol was removed from the culture medium, cysteine and GSH in these cells dramatically decreased, and cells started to die within 24 h. N-Acetyl cysteine also rescued xCT(-/-)-derived cells and permitted growth. These results demonstrate that system x(-)(c) contributes to maintaining the plasma redox balance in vivo but is dispensable in mammalian development, although it is vitally important to cells in vitro.
Preview · Article · Dec 2005 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: Recent studies have demonstrated that depletion of amino acids results in the induction of several genes and that a genomic cis-element termed amino acid response element (AARE) is required for the induction. System x(c)(-) is an anionic amino acid transport system highly specific for cystine and glutamate, and its activity is known to be induced by cystine deprivation. This transporter is composed of two protein components, xCT and 4F2 heavy chain, and xCT is thought to mediate the transport activity. In the present study, the molecular mechanism for the induction of xCT by amino acid deprivation has been investigated. In mouse NIH3T3 cells, the activity of system x(c)(-) and xCT mRNA is induced not only by deprivation of cystine but also by deprivation of other amino acids. Two AAREs, each located in the opposite direction with an intervening sequence, were found in the 5'-flanking region of the mouse xCT gene. Promoter analysis revealed that both AAREs were necessary for the maximal induction of xCT mRNA in response to the amino acid deprivation. Glucose deprivation had no effect on the induction of the activity of system x(c)(-). Electrophoretic mobility shift assay showed that ATF4, but not ATF2, is involved in the amino acid control of xCT expression. These results demonstrate that xCT is a new member of the proteins whose transcriptional control by the amino acid deprivation is mediated by AARE.
No preview · Article · Jan 2005 · Biochemical and Biophysical Research Communications