Mouse lymphoma L1210 cells acquire a new cystine transport activity upon adaptation in vitro

System χc- overexpression prevents 2-deoxy-d-ribose-induced β-cell damage

Pancreatic β-cells are vulnerable to oxidative stress, which promotes β-cell failure in type 2 diabetes. System χ_c- is a sodium-independent, cystine/glutamate antiporter that mediates the exchange of extracellular l-cystine and intracellular l-glutamate. The import of l-cystine through this transporter is the rate-limiting step in the glutathione (GSH) biosynthesis pathway that plays a significant role in antioxidative defense. Previously, we reported that 2-deoxy-d-ribose (dRib) induces oxidative damage through GSH depletion in pancreatic β-cells. In the current study, we elucidated the mechanism underlying the oxidative stress-induced β-cell damage. We measured the intracellular l-[¹⁴C]cystine uptake, GSH content, reactive oxygen species (ROS) levels, cytotoxicity, and apoptosis in rat insulinoma cell line, RINm5F. Treatment of dRib decreased the intracellular l-[¹⁴C]cystine uptake and GSH content and increased the intracellular ROS levels, cytotoxicity, and apoptosis in a time- and dose-dependent manner. Conversely, 2-mercaptoethanol (2-ME), a cystine uptake enhancer, recovered the dRib-induced decrease in l-[¹⁴C]cystine uptake, GSH content, and cell viability in a Na⁺-independent manner. In the case of isolated islets, dRib dose-dependently decreased the intracellular l-[¹⁴C]cystine uptake and cell viability; however, dRib-induced cytotoxicity was completely recovered by adding N-acetyl cysteine (NAC). To confirm that system χ_c- mediates the oxidative stress-induced β-cell damage, we overexpressed xCT (the substrate-specific subunit of system χ_c-) using a lentiviral vector in RINm5F cells. Overexpression of xCT fully recovered the dRib-induced decrease in l-[¹⁴C]cystine uptake and GSH content and prevented the dRib-induced increase in ROS levels, cytotoxicity, and apoptosis. The overexpression of xCT showed a protective effect against dRib-induced oxidative damage in RINm5F cells. Our study showed that dRib depletes intracellular GSH content through inhibition of cystine transport via system χ_c- in β-cells.

Redox status in mammalian cells and stem cells during culture in vitro: critical roles of Nrf2 and cystine transporter activity in the maintenance of redox balance

Culturing cells and tissues in vitro has provided valuable insights into the molecular mechanisms regulating redox signaling in cells with implications for medicine. However, standard culture techniques maintain mammalian cells in vitro under an artificial physicochemical environment such as ambient air and 5% CO2. Oxidative stress is caused by the rapid oxidation of cysteine to cystine in culture media catalyzed by transition metals, leading to diminished intracellular cysteine and glutathione (GSH) pools. Some cells, such as fibroblasts and macrophages, express cystine transport activity, designated as system [Formula: see text], which enables cells to maintain these pools to counteract oxidative stress. Additionally, many cells have the ability to activate the redox sensitive transcription factor Nrf2, a master regulator of cellular defenses against oxidative stress, and to upregulate xCT, the subunit of the [Formula: see text] transport system leading to increases in cellular GSH. In contrast, some cells, including lymphoid cells, embryonic stem cells and iPS cells, express relatively low levels of xCT and cannot maintain cellular cysteine and GSH pools. Thus, fibroblasts have been used as feeder cells for the latter cell types based on their ability to supply cysteine. Other key Nrf2 regulated gene products include heme oxygenase 1, peroxiredoxin 1 and sequestosome1. In macrophages, oxidized LDL activates Nrf2 and upregulates the scavenger receptor CD36 forming a positive feedback loop to facilitate removal of the oxidant from the vascular microenvironment. This review describes cell type specific responses to oxygen derived stress, and the key roles that activation of Nrf2 and membrane transport of cystine and cysteine play in the maintenance and proliferation of mammalian cells in culture.

Dysregulation of plasma amino acid levels in HIV-infection and cancer and its relevance for the immune system

T cells have a weak membrane transport actitivity for cystine but strong transport activity for cysteine. Even moderate variations of the cysteine concentration affect T cell functions in spite of the high concentration of cystine in cultures with physiological amino acid concentrations. The IL-2 dependent DNA synthesis and the activation of cytotoxic T cells are positively regulated by cysteine, while the activity of the transcription factor NFkB and the production of IL-2 are stimulated by active oxygen species and inhibited by cysteine or GSH. Macrophages, in contrast to T cells, take up more cystine than they need and release the excess after intracellular reduction as cysteine into the extracellular space. This "cysteine pumping activity" of macrophages raises intracellular GSH levels and DNA synthesis of T cells in the vicinity. The difference between the cystine transport activities of T cells and macrophages, therefore, enables T cells to switch between prooxidant and antioxidant states. The "cysteine pump" favors selectively the antigen-specific T cells that are about to be stimulated by antigen-presenting macrophages. The capacity of macrophages to take up cystine and to release cysteine is inhibited, however, by elevated extracellular glutamate concentrations. Elevated plasma glutamate levels have been found in several pathological conditions including cancer and HIV-infection. In HIV-infected patients, the hyperglutamataemia is aggravated by hypocystinaemia and hypocysteinaemia. Our studies, therefore, suggest that the cysteine supply is impaired in several pathological conditions with immunodeficiencies including AIDS. N-acetyl-cysteine (NAC) is a safe and well established drug that may be considered for the treatment of patients with HIV-infection.

Inhibition of cell division in mouse B-cell hybridomas: an overlooked property of 2-mercaptoethanol and its impact on in vitro antibody production

Thiol 2-mercaptoethanol (2-ME) has been reported to enhance growth in lymphocytes by various investigators. Some have used 50 μM for growing hybridomas in vitro. Concentrations of 50 and 5 μM in 5% FBS supplemented D-MEM were tested to determine their effects on the growth of 5 monoclonal antibody secreting mouse B cell hybridomas and the myeloma Sp2/O-Ag14. Viability after 24 and 48 h exposure was determined by Trypan blue exclusion. Analysis by one-way ANOVA confirmed that 50 μM 2-ME has a significant negative impact (p<0.05) on hybridoma as well as on myeloma growth, whereas no significant difference (p>0.05) between the control and the 5 μM treatment group was observed after 48 h. Also, no significant difference (p>0.05) in the mortality rates between the control and the treatment groups was found. When combined with the observed protracted doubling time in the 50 μM treatment group, these results indicate that the impact of 2-ME is due to inhibition of cell division. The degree of inhibition was observed to vary between the different hybridomas as well as the myeloma. Although the impact of 2-ME on mitosis has been demonstrated in organisms such as the ciliated protozoan Tetrahymena pyriformis, the yeast Saccharomycess cerevisiae, and the egg of the echinoid the sand dollar Dendraster excentricus, this work demonstrates for the first time that 2-ME impedes the growth of mouse B cell hybridomas. We conclude that adding 2-ME to mouse B cell hybridoma growth media may not be beneficial.

The x(c)- cystine/glutamate antiporter: a potential target for therapy of cancer and other diseases

The x(c) (-) cystine/glutamate antiporter is a major plasma membrane transporter for the cellular uptake of cystine in exchange for intracellular glutamate. Its main functions in the body are mediation of cellular cystine uptake for synthesis of glutathione essential for cellular protection from oxidative stress and maintenance of a cystine:cysteine redox balance in the extracellular compartment. In the past decade it has become evident that the x(c) (-) transporter plays an important role in various aspects of cancer, including: (i) growth and progression of cancers that have a critical growth requirement for extracellular cystine/cysteine, (ii) glutathione-based drug resistance, (iii) excitotoxicity due to excessive release of glutamate, and (iv) uptake of herpesvirus 8, a causative agent of Kaposi's sarcoma. The x(c) (-) transporter also plays a role in certain CNS and eye diseases. This review focuses on the expression and function of the x(c) (-) transporter in cells and tissues with particular emphasis on its role in disease pathogenesis. The potential use of x(c) (-) inhibitors (e.g., sulfasalazine) for arresting tumor growth and/or sensitizing cancers is discussed.

Cystine uptake and glutathione level in fetal brain cells in primary culture and in suspension

The glutathione level and the factors affecting this level were investigated in fetal rat brain cells in a primary culture. Early in the culture, the glutathione level of the brain cells decreased, but after 5 h it began to increase. This increase was not observed in a cystine-free medium and was prevented by excess glutamate. Cystine was taken up in freshly isolated brain cell suspensions, and its rate increased during the culture. The cystine uptake was mediated by a Na(+)-independent, glutamate-sensitive route previously found in various types of cells and designated as system X-c. The uptake of cystine is a crucial factor in maintaining the glutathione level of the cells under culture, because it provides cysteine for the cells for glutathione synthesis. Cysteine was undetectable in the medium before the culture, but it appeared, though at a very low level, when the brain cells were cultured there. The source of this cysteine was the cystine in the medium. Presumably the decrease in the glutathione level of the cells in the early stage of the culture resulted from the fact that the medium did not contain cysteine. The enhancement of the cystine uptake during culture may constitute a protective mechanism against the oxidative stress to which the cultured cells are exposed. Regulation of the glutathione level in fetal brain cells in vivo by the transport of cystine and cysteine is discussed.

Mechanism for the changes in levels of glutathione upon exposure of cultured mammalian cells to tertiary-butylhydroperoxide and diamide

Qualitative and quantitative changes associated with cellular glutatione (GSH) in response to oxidants were investigated in cultured Chinese hamster V79 cells. Incubation of cells with benzoylperoxide (BZP), tert-butylhydroperoxide (t-BuOOH), hydrogen peroxide or diamide for 1 h reduced the level of total GSH (GSH + GSSG). Among the oxidants, t-BuOOH and diamide caused an increase in levels of glutathione disulfide (GSSG) and a resultant increase in the ratio of the level of GSSG to the level of total GSH, suggestive of the induction within the cells of a pro-oxidant state by the oxidants. o-Phenanthroline, a chelator of divalent ion, almost completely suppressed the decrease in levels of total GSH caused by t-BuOOH while it did not suppressed either increases in levels of GSSG or increases in the ratio of the levels of GSSG to that of total GSH caused by the hydroperoxide. These results suggest that reactive oxygen radicals are involved in the decrease in levels of GSH by treatment with t-BuOOH but not in the increase in the level of GSSG. After treatment with either t-BuOOH or diamide for 1 h, the level of GSH rapidly increased to more than twice the control level during 15-45 min of post-treatment incubation. o-Phenanthroline almost completely suppressed the increase in levels of GSH caused by t-BuOOH, while it did not affect the changes caused by diamide, suggesting a difference between the mechanisms by which t-BuOOH and diamide cause increases in levels of GSH. It seems likely that reactive oxygen radicals participate not only in the decrease in levels of GSH caused by t-BuOOH but also in the rapid increase that occurs after such treatment. Hence, the first decrease in levels of GSH by the hydroperoxide may be causally related to the latter increase. The amount of [35S]-cysteine taken up by cells after treatment with t-BuOOH was about one half of that taken up by control cells. By contrast, the rate of incorporation of radioactive cysteine into acid-soluble material increased to more than twice that of the controls after treatment with t-BuOOH. The increase in the rate of incorporation of [35S]cysteine into acid-soluble material caused by t-BuOOH was not a consequence of inhibition by the hydroperoxide of utilization of cysteine for protein synthesis. Inhibition of protein synthesis by cycloheximide caused neither an increase in the incorporation of cysteine into acid-soluble material nor an increase in rate of biosynthesis of GSH.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of increased immunogenicity by thiol-releasing tumor variants

Even moderate variations of the extracellular cysteine concentration were previously shown to affect T cell functions in vitro despite high concentrations of cystine. We therefore analyzed the membrane transport activities of T cells for cysteine and cystine, and the role of low molecular weight thiol in T cell-mediated host responses against a T cell tumor in vivo. A series of T cell clones and tumors including the highly malignant lymphoma L5178Y ESb and its strongly immunogenic variant ESb-D was found to express extremely weak transport activity for cystine but strong transport activity for cysteine. However, not all cells showed the expected requirement for cysteine (or 2-mercaptoethanol (2-ME)) in the culture medium. One group of clones and tumors including the malignant ESb-lymphoma did not respond to changes of extracellular cystine concentrations and was strongly thiol dependent. This group released only little acid soluble thiol (cysteine) if grown in cystine-containing cultures. The other T cell lines, in contrast, were able to maintain high intracellular GSH levels and DNA synthesis activity in cystine-containing culture medium without cystein or 2-ME and released substantial amounts of thiol. This group included the immunogenic ESb-D line. Additional thiol-releasing ESb variants were obtained by culturing large numbers of L5178Y ESb tumor cells in cultures without cysteine or 2-ME. All of these ESb variants showed a significantly decreased tumorigenicity and some of them induced cytotoxic and protective host responses even against the malignant ESb parent tumor. Taken together, our experiments suggest that the host response against a tumor may be limited in certain cases by the failure of the stimulator (i.e., the tumor) cell to deliver sufficient amounts of cysteine to the responding T cells.

Requirement for prooxidant and antioxidant states in T cell mediated immune responses.--Relevance for the pathogenetic mechanisms of AIDS?

The discovery of decreased plasma cysteine and cystine levels and elevated plasma glutamate levels in HIV-infected patients has led to intense investigations into the role of cysteine in T cell-mediated immune responses. A large body of evidence indicates that certain aspects of the T cell response require the action of active oxygen derivatives while other aspects of the response require the action of antioxidants such as cysteine and glutathione (GSH). The prooxidant and antioxidant states may be required sequentially at different times during T cell activation. The extremely weak cystine transport activity of T cells together with oxidizing metabolites from inflammatory microenvironments appear to be important factors that support the prooxidant state. The relatively high cystine transport activity of the antigen-presenting macrophages, in contrast, provides these cells with a "cysteine pumping" function that allows the antigen binding T cells in their vicinity to shift to the antioxidant state. The difference between the membrane transport activities for cysteine of T cells and macrophages thus appears to be the key element of a mechanism that facilitates both, the prooxidant state of T cells and their regulated shift to the antioxidant state. When T cells do not receive sufficient amounts of cysteine, the intracellular GSH levels and rates of DNA synthesis activity decrease, and the cells may suffer from various manifestations of oxidative damage.(ABSTRACT TRUNCATED AT 250 WORDS)

Low membrane transport activity for cystine in resting and mitogenically stimulated human lymphocyte preparations and human T cell clones

In order to determine whether the cysteine requirement of human T lineage cells is met primarily by extracellular cysteine or by cystine, amino-acid-transport activities were measured in resting and mitogenically stimulated human peripheral blood lymphocytes (PBL) and several human T cell clones and T cell tumors. The transport activity of the small neutral amino acids cysteine and alanine (ASC system) and the transport of the cationic amino acid arginine (y+ system) were found to be markedly increased after stimulation of PBL by the T cell mitogen phytohemagglutinin from Phaseolus vulgaris. The anionic transport activity for cystine and glutamate (Xc- system), in contrast, was extremely weak in both resting and activated human PBL and also in all human T cell lines under test. The weak system Xc- activity of human T lineage cells was further confirmed by an independent line of experiments showing that an increase of the extracellular concentration of glutamate, i.e. a competitive inhibitor of cystine transport, causes a decrease in the intracellular cystine levels in cells of the promonocytic line U937, but not in T lineage cells (Molt-4). A third set of experiments showed that the rate of DNA synthesis in mitogenically stimulated human PBL is strongly influenced by variations of the extracellular cysteine level, even in cultures with relatively high and approximately physiological concentrations of cystine. Cysteine cannot be replaced in this case by the addition of corresponding amounts of cystine or methionine. This demonstrates an important functional consequence of the weak cystine transport activity of human lymphocytes. The results may be relevant for the pathogenetic mechanism of the acquired immunodeficiency syndrome, since the mean plasma cysteine concentration of human-immunodeficiency-virus-1-seropositive persons was found to be strongly decreased in comparison with that of healthy blood donors, and since the cysteine level even of healthy persons is extremely low in comparison with all other protein-forming amino acids.

Uptake of glutamate and cysteine in C-6 glioma cells and in cultured astrocytes

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 xc- was relatively weak and its contribution to the total uptake of glutamate seemed almost negligible. However, in both C-6 cells and astrocytes, system xc- was necessary for the uptake of cystine, another substrate of system xc-. 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.

Induction of cystine transport activity in mouse peritoneal macrophages

Uptake of cystine was investigated in mouse peritoneal macrophages. The rates of the uptake of cystine in resident macrophages or macrophages elicited by some irritants were very low, but a drastic increase was observed when the cells were cultured in vitro. This increase was time-dependent and required protein synthesis. In macrophages elicited by thioglycollate broth, the rate of the uptake of cystine increased by about 40-fold after 16 h in culture. Contrary to the uptake of cystine, the rates of uptake of some neutral amino acids did not change markedly during culture. We characterized the induced activity of the cystine uptake in macrophages elicited by thioglycollate broth. Cystine was taken up in an Na+-independent and pH-sensitive manner, and the uptake was potently inhibited by extracellular glutamate and the analogous anionic amino acids, but not by aspartate. The activity of the glutamate uptake was also induced during the culture in a way similar to that of cystine uptake, and the uptake of glutamate was potently inhibited by cystine. From these results we concluded that the uptake of cystine and glutamate in macrophages was mostly mediated by a single transport system similar to the ones previously reported in human fibroblasts and some other cells. As a consequence of the induction of the activity of the cystine uptake, glutathione levels in macrophages doubled during culture, and a thiol compound, presumably cysteine, was released into the culture medium and accumulated there. When the macrophages were cultured hypoxically, the induction of the cystine uptake activity was markedly depressed, suggesting an involvement of oxygen in the induction.