Effect of oxygen on induction of the cystine transporter by bacterial lipopolysaccharide in mouse peritoneal macrophages

Vascular protection afforded by zinc supplementation in human coronary artery smooth muscle cells mediated by NRF2 signaling under hypoxia/reoxygenation

Zinc (Zn) has antioxidant, anti-inflammatory and anti-proliferative actions, with Zn dysregulation associated with coronary ischemia/reperfusion injury and smooth muscle cell dysfunction. As the majority of studies concerning Zn have been conducted under non-physiological hyperoxic conditions, we compare the effects of Zn chelation or supplementation on total intracellular Zn content, antioxidant NRF2 targeted gene transcription and hypoxia/reoxygenation-induced reactive oxygen species generation in human coronary artery smooth muscle cells (HCASMC) pre-adapted to hyperoxia (18 kPa O2) or normoxia (5 kPa O2). Expression of the smooth muscle marker SM22-α was unaffected by lowering pericellular O2, whereas calponin-1 was significantly upregulated in cells under 5 kPa O2, indicating a more physiological contractile phenotype under 5 kPa O2. Inductively coupled plasma mass spectrometry established that Zn supplementation (10 μM ZnCl2 + 0.5 μM pyrithione) significantly increased total Zn content in HCASMC under 18 but not 5 kPa O2. Zn supplementation increased metallothionein mRNA expression and NRF2 nuclear accumulation in cells under 18 or 5 kPa O2. Notably, NRF2 regulated HO-1 and NQO1 mRNA expression in response to Zn supplementation was only upregulated in cells under 18 but not 5 kPa. Furthermore, whilst hypoxia increased intracellular glutathione (GSH) in cells pre-adapted to 18 but not 5 kPa O2, reoxygenation had negligible effects on GSH or total Zn content. Reoxygenation-induced superoxide generation in cells under 18 kPa O2 was abrogated by PEG-superoxide dismutase but not by PEG-catalase, and Zn supplementation, but not Zn chelation, attenuated reoxygenation-induced superoxide generation in cells under 18 but not 5kPaO2, consistent with a lower redox stress under physiological normoxia. Our findings highlight that culture of HCASMC under physiological normoxia recapitulates an in vivo contractile phenotype and that effects of Zn on NRF2 signaling are altered by oxygen tension.

Comparative Evaluation of [18F]5-Fluoroaminosuberic Acid and (4S)-4-3-[18F]fluoropropyl)-l-Glutamate as System xC--Targeting Radiopharmaceuticals

System [Formula: see text] is an appealing biomarker for targeting oxidative stress with oncologic PET imaging and can serve as an alternative PET biomarker to other metabolic indicators. In this paper, we report a direct comparison of 2 ¹⁸F-labeled amino acid radiopharmaceuticals targeting system [Formula: see text], [¹⁸F]5-fluoroaminosuberic acid ([¹⁸F]FASu) and (4S)-4-(3-[¹⁸F]fluoropropyl)-l-glutamate ([¹⁸F]FSPG), in terms of their uptake specificity and ability to image glioma and lung cancer xenografts in vivo. Methods: Both tracers were synthesized according to previously published procedures. In vitro uptake specificity assays were conducted using prostate (PC-3), glioblastoma (U-87), colorectal (HT-29), ovarian (SKOV3), breast (MDA-MB-231), and lung cancer (A549) cell lines. PET/CT imaging and biodistribution studies were conducted in immunocompromised mice bearing U-87 or A549 xenografts. Results: In vitro cell uptake assays showed that the tracers accumulated in cancer cells in a time-dependent manner and that the uptake of [¹⁸F]FASu was blocked by the system [Formula: see text] inhibitor sulfasalazine and rose bengal, but not by system L inhibitor 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid, system [Formula: see text] inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid, or l-serine, which is a substrate for transporter systems A, ACS, B⁰, and B^0,+ Conversely, [¹⁸F]FSPG uptake decreased significantly in the presence of an excess of L-trans-pyrrolidine-2,4-dicarboxylic acid in 2 of 3 tested cell lines, indicating some reliance on system [Formula: see text] in these cells. In an in vivo setting, [¹⁸F]FASu and [¹⁸F]FSPG generated good-contrast PET images in U-87 and A549 tumor-bearing mice. Tracer accumulation in A549 tumors was 5.0 ± 0.8 percentage injected dose (%ID)/g ([¹⁸F]FASu, n ≥ 5) and 6.3 ± 1.3 %ID/g ([¹⁸F]FSPG, n ≥ 6, P = 0.7786), whereas U-87 xenografts demonstrated uptake of 6.1 ± 2.4 %ID/g ([¹⁸F]FASu, n ≥ 4) and 11.2 ± 4.1 %ID/g ([¹⁸F]FSPG, n ≥ 4, P = 0.0321) at 1 h after injection. Conclusion: [¹⁸F]FSPG had greater in vitro uptake than [¹⁸F]FASu in all cell lines tested; however, our results indicate that residual uptake differences exist between [¹⁸F]FSPG and [¹⁸F]FASu, suggesting alternative transporter activity in the cell lines tested. In vivo studies demonstrated the ability of both [¹⁸F]FASu and [¹⁸F]FSPG to image glioblastoma (U-87) and non-small cell lung cancer (A549) xenografts.

Innate sensing and cellular metabolism: role in fine tuning antiviral immune responses

Several studies over the last decade have identified intimate links between cellular metabolism and macrophage function. Metabolism has been shown to both drive and regulate macrophage function by producing bioenergetic and biosynthetic precursors as well as metabolites (and other bioactive molecules) that regulate gene expression and signal transduction. Many studies have focused on lipopolysaccharide-induced reprogramming, assuming that it is representative of most inflammatory responses. However, emerging evidence suggests that diverse pathogen-associated molecular patterns (PAMPs) are associated with unique metabolic profiles, which may drive pathogen specific immune responses. Further, these metabolic pathways and processes may act as a rheostat to regulate the magnitude of an inflammatory response based on the biochemical features of the local microenvironment. In this review, we will discuss recent work examining the relationship between cellular metabolism and macrophage responses to viral PAMPs and describe how these processes differ from lipopolysaccharide-associated responses. We will also discuss how an improved understanding of the specificity of these processes may offer new insights to fine-tune macrophage function during viral infections or when using viral PAMPs as therapeutics.

Current understanding of ferroptosis in the progression and treatment of pancreatic cancer

Pancreatic cancer is a highly malignant tumour of the digestive tract. Despite advances in treatment, its 5-year survival rate remains low, and its prognosis is the worst among all cancers; innovative therapeutic methods are needed. Ferroptosis is a form of regulatory cell death driven by iron accumulation and lipid peroxidation. Recent studies have found that ferroptosis plays an important role in the development and treatment response of tumours, particularly pancreatic cancer. This article reviews the current understanding of the mechanism of ferroptosis and ferroptosis-related treatment in pancreatic cancer.

Ferroptosis in the tumor microenvironment: perspectives for immunotherapy

Targeting ferroptosis, which provokes lipid peroxidation in cancer cells, presents potentially new avenues for anticancer therapy. Recent studies have begun to explore how immune cells in the tumor microenvironment (TME) respond and adapt to lethal lipid peroxides (LPOs). A better understanding of this process in the TME is likely to uncover another side of ferroptosis in cancer immunity and promote the development of ferroptosis-targeted therapy. This Opinion article overviews the main metabolic processes in ferroptosis, summarizes the emerging roles of ferroptosis not only in immune cells in the TME but also in the crosstalk between tumor cells and immune cells, and presents a perspective on the targeting of ferroptosis in cancer immunotherapy.

Cystine-glutamate antiporter deletion accelerates motor recovery and improves histological outcomes following spinal cord injury in mice

xCT is the specific subunit of System xc-, an antiporter importing cystine while releasing glutamate. Although xCT expression has been found in the spinal cord, its expression and role after spinal cord injury (SCI) remain unknown. The aim of this study was to characterize the role of xCT on functional and histological outcomes following SCI induced in wild-type (xCT+/+) and in xCT-deficient mice (xCT−/−). In the normal mouse spinal cord, slc7a11/xCT mRNA was detected in meningeal fibroblasts, vascular mural cells, astrocytes, motor neurons and to a lesser extent in microglia. slc7a11/xCT gene and protein were upregulated within two weeks post-SCI. xCT−/− mice recovered muscular grip strength as well as pre-SCI weight faster than xCT+/+ mice. Histology of xCT−/− spinal cords revealed significantly more spared motor neurons and a higher number of quiescent microglia. In xCT−/− mice, inflammatory polarization shifted towards higher mRNA expression of ym1 and igf1 (anti-inflammatory) while lower levels of nox2 and tnf-a (pro-inflammatory). Although astrocyte polarization did not differ, we quantified an increased expression of lcn2 mRNA. Our results show that slc7a11/xCT is overexpressed early following SCI and is detrimental to motor neuron survival. xCT deletion modulates intraspinal glial activation by shifting towards an anti-inflammatory profile.

An Overview of the Ferroptosis Hallmarks in Friedreich's Ataxia

BACKGROUND

Friedreich's ataxia (FRDA) is a neurodegenerative disease characterized by early mortality due to hypertrophic cardiomyopathy. FRDA is caused by reduced levels of frataxin (FXN), a mitochondrial protein involved in the synthesis of iron-sulphur clusters, leading to iron accumulation at the mitochondrial level, uncontrolled production of reactive oxygen species and lipid peroxidation. These features are also common to ferroptosis, an iron-mediated type of cell death triggered by accumulation of lipoperoxides with distinct morphological and molecular characteristics with respect to other known cell deaths.

SCOPE OF REVIEW

Even though ferroptosis has been associated with various neurodegenerative diseases including FRDA, the mechanisms leading to disease onset/progression have not been demonstrated yet. We describe the molecular alterations occurring in FRDA that overlap with those characterizing ferroptosis.

MAJOR CONCLUSIONS

The study of ferroptotic pathways is necessary for the understanding of FRDA pathogenesis, and anti-ferroptotic drugs could be envisaged as therapeutic strategies to cure FRDA.

Tissue distribution, hormonal regulation, ontogeny, diurnal expression, and induction of mouse cystine transporters Slc3a1 and Slc7a9

Slc7a11 (xCT) and Slc3a1 (rBAT) are cystine uptake transporters that maintain intracellular concentrations of cysteine, the rate-limiting amino acid in glutathione synthesis. This study was conducted to first determine the tissue distribution of the two transporters in male and female mice. Because Slc3a1 was the primary cystine transporter in liver, its sex-divergent expression, ontogeny, diurnal rhythm and whether its mRNA expression is altered by transcription factors (AhR, CAR, PXR, PPARα, and Nrf2) was also investigated. Slc7a11 was expressed highest in brain and gonads. Slc3a1 was expressed highest in kidney and intestine, followed by liver. Duodenal and hepatic Slc3a1 was higher in females than males. Hepatic Slc3a1 was high during darkness and low during daytime. Hepatic Scl3a1 was lowest pre-birth, increased to near maximal levels at birth, decreased back to pre-birth levels between Days 3-10, and then returned to peak levels by Day 45. Except for CAR, activation of transcription factors did not increase hepatic mRNA expression of Slc3a1. Chemical activation of CAR significantly induced Slc3a1 1.4-fold in wild-type but not CAR-null mice. Slc3a1 mRNA was higher in livers of AhR- and Nrf2-null mice compared to wild-type mice. High doses of diquat but not acetaminophen induced Slc3a1, suggesting Slc3a1 may respond to oxidative stress but not necessarily to GSH depletion. Overall, Slc7a11 is mainly expressed in brain and gonads, whereas Slc3a1 is mainly expressed in kidney, small intestine and liver, and its hepatic expression is regulated by diurnal rhythm and certain xenobiotic treatments.

Going the Extra (Synaptic) Mile: Excitotoxicity as the Road Toward Neurodegenerative Diseases

Excitotoxicity is a phenomenon that describes the toxic actions of excitatory neurotransmitters, primarily glutamate, where the exacerbated or prolonged activation of glutamate receptors starts a cascade of neurotoxicity that ultimately leads to the loss of neuronal function and cell death. In this process, the shift between normal physiological function and excitotoxicity is largely controlled by astrocytes since they can control the levels of glutamate on the synaptic cleft. This control is achieved through glutamate clearance from the synaptic cleft and its underlying recycling through the glutamate-glutamine cycle. The molecular mechanism that triggers excitotoxicity involves alterations in glutamate and calcium metabolism, dysfunction of glutamate transporters, and malfunction of glutamate receptors, particularly N-methyl-D-aspartic acid receptors (NMDAR). On the other hand, excitotoxicity can be regarded as a consequence of other cellular phenomena, such as mitochondrial dysfunction, physical neuronal damage, and oxidative stress. Regardless, it is known that the excessive activation of NMDAR results in the sustained influx of calcium into neurons and leads to several deleterious consequences, including mitochondrial dysfunction, reactive oxygen species (ROS) overproduction, impairment of calcium buffering, the release of pro-apoptotic factors, among others, that inevitably contribute to neuronal loss. A large body of evidence implicates NMDAR-mediated excitotoxicity as a central mechanism in the pathogenesis of many neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), and epilepsy. In this review article, we explore different causes and consequences of excitotoxicity, discuss the involvement of NMDAR-mediated excitotoxicity and its downstream effects on several neurodegenerative disorders, and identify possible strategies to study new aspects of these diseases that may lead to the discovery of new therapeutic approaches. With the understanding that excitotoxicity is a common denominator in neurodegenerative diseases and other disorders, a new perspective on therapy can be considered, where the targets are not specific symptoms, but the underlying cellular phenomena of the disease.

Early Neurotoxic Effects of Inorganic Arsenic Modulate Cortical GSH Levels Associated With the Activation of the Nrf2 and NFκB Pathways, Expression of Amino Acid Transporters and NMDA Receptors and the Production of Hydrogen Sulfide

Exposure to toxic metals and metalloids is an important cause of preventable diseases worldwide. Inorganic arsenic (iAs) affects several organs and tissues, causing neurobehavioral alterations in the central nervous system (CNS) that might lead to neurodegeneration. In this work, we wanted to explore the time- and dose-related changes on glutathione (GSH) levels in several regions of the CNS, such as the cortex, striatum, hippocampus, and cerebellum, to identify the initial cellular changes associated to GSH depletion due to iAs exposure. Mice received a single intraperitoneal injection containing 5 or 14 mg/kg sodium arsenite. Animals were killed at 2, 6, and 24 h. Significant depletion of GSH levels was observed in the cortex at 2 and 6 h, while on the striatum, hippocampus, or cerebellum regions, no significant changes were observed. GSH depletion in the cortex was associated with the activation of the nuclear factor erythroid 2-related factor 2 (Nrf2) and nuclear factor kappa B (NFκB) pathways, which led to the upregulation of xCT, excitatory amino acid carrier 1 (EAAC1), glutamate/aspartate transporter (GLAST), and glial glutamate transporter 1 (GLT-1), and the activation of the transsulfuration pathways, which led to the overproduction of H₂S in the cortex and increased levels of GSH in the cortex and cerebellum at 24 h. In the cortex, the N-methyl-D-aspartate (NMDA) receptor subunits NR2A and NR2B were also altered at 24 h. These early effects were not homogeneous among different brain regions and indicate early neurotoxic alterations in the cortex and cerebellum.

Hydrogen peroxide triggers an increase in cell surface expression of system xc- in cultured human glioma cells

System x_c^- exchanges extracellular cystine for intracellular glutamate across the plasma membrane of many cell types. One of the physiological roles of System x_c^- is to provide cystine for synthesis of the antioxidant glutathione. Here we report that hydrogen peroxide (H₂O₂) triggers the translocation of System x_c^- to the plasma membrane within 10 min of the initial exposure. Specifically, we observed a three-fold increase in ³⁵S-l-cystine uptake following a 10 min exposure to 0.3 mM H₂O₂. This effect was dose-dependent with an EC₅₀ for H₂O₂ of 65 μM. We then used cell surface biotinylation analysis to test the hypothesis that the increase in activity is due to an increased number of transporters on the plasma membrane. We demonstrated that the amount of transporter protein, xCT, localized to the plasma membrane doubles within 10 min of H₂O₂ exposure as a result of an increase in its delivery rate and a reduction in its internalization rate. In addition, we demonstrated that H₂O₂ triggered a rapid decrease in total cellular glutathione which recovered within 2 h of the oxidative insult. The kinetics of glutathione recovery matched the time course for the recovery of xCT cell surface expression and System x_c^- activity following removal of the oxidative insult. Collectively, these results suggest that oxidants acutely modulate the activity of System x_c^- by increasing its cell surface expression, and that this process may serve as an important mechanism to increase de novo glutathione synthesis during periods of oxidative stress.

Role of GPX4 in ferroptosis and its pharmacological implication

Ferroptosis is a non-apoptotic form of cell death characterized by iron-dependent lipid peroxidation and metabolic constraints. Dependence on NADPH/H⁺, polyunsaturated fatty acid metabolism, and the mevalonate and glutaminolysis metabolic pathways have been implicated in this novel form of regulated necrotic cell death. Genetic studies performed in cells and mice established the selenoenzyme glutathione peroxidase (GPX4) as the key regulator of this form of cell death. Besides these genetic models, the identification of a series of small molecule ferroptosis-specific inhibitors and inducers have not only helped in the delineation of the molecular underpinnings of ferroptosis but they might also prove highly beneficial when tipping the balance between cell death inhibition and induction in the context of degenerative diseases and cancer, respectively. In the latter, the recent recognition that a subset of cancer cell lines including certain triple negative breast cancer cells and those of therapy-resistant high-mesenchymal cell state present a high dependence on this lipid make-up offers unprecedented opportunities to eradicate difficult to treat cancers. Due to the rapidly growing interest in this form of cell death, we provide an overview herein what we know about this field today and its future translational impact.

Inflammation in stroke: the role of cholinergic, purinergic and glutamatergic signaling

The inflammatory response is a major factor in stroke pathophysiology and contributes to secondary neuronal damage in both acute and chronic stages of the ischemic injury. Recent work in experimental cerebral ischemia has demonstrated the involvement of neurotransmitter signaling in the modulation of neuroinflammation. The present review discusses recent findings on the therapeutic potential and diagnostic perspectives of cholinergic, purinergic and glutamatergic receptors and transporters in experimental stroke. It provides evidence of the role of neurotransmission signaling as a promising inflammatory biomarker in stroke. Finally, recent molecular imaging studies using positron emission tomography of cholinergic receptors and glutamatergic transporters are outlined along with their potential as novel anti-inflammatory therapy to reduce the outcome of cerebral ischemia.

Ferroptosis: bug or feature?

Ferroptosis is an iron-dependent, oxidative form of non-apoptotic cell death. This form of cell death does not share morphological, biochemical, or genetic similarities with classic necrosis, necroptosis, parthanatos, or other forms of non-apoptotic cell death. Ferroptosis can be triggered by depleting the cell of the amino acid cysteine, or by inhibiting the phospholipid hydroperoxidase glutathione peroxidase 4 (GPX4). Why certain stimuli trigger ferroptosis instead of another form of cell death, and whether this process could be adaptive in vivo, are two major unanswered questions concerning this process. Emerging evidence and consideration of related non-apoptotic pathways suggest that ferroptosis could be an adaptive process, albeit one regulated and executed in a manner very different from apoptosis and other forms of cell death.

Why should neuroscientists worry about iron? The emerging role of ferroptosis in the pathophysiology of neuroprogressive diseases

Ferroptosis is a unique form of programmed death, characterised by cytosolic accumulation of iron, lipid hydroperoxides and their metabolites, and effected by the fatal peroxidation of polyunsaturated fatty acids in the plasma membrane. It is a major driver of cell death in neurodegenerative neurological diseases. Moreover, cascades underpinning ferroptosis could be active drivers of neuropathology in major psychiatric disorders. Oxidative and nitrosative stress can adversely affect mechanisms and proteins governing cellular iron homeostasis, such as the iron regulatory protein/iron response element system, and can ultimately be a source of abnormally high levels of iron and a source of lethal levels of lipid membrane peroxidation. Furthermore, neuroinflammation leads to the upregulation of divalent metal transporter1 on the surface of astrocytes, microglia and neurones, making them highly sensitive to iron overload in the presence of high levels of non-transferrin-bound iron, thereby affording such levels a dominant role in respect of the induction of iron-mediated neuropathology. Mechanisms governing systemic and cellular iron homeostasis, and the related roles of ferritin and mitochondria are detailed, as are mechanisms explaining the negative regulation of ferroptosis by glutathione, glutathione peroxidase 4, the cysteine/glutamate antiporter system, heat shock protein 27 and nuclear factor erythroid 2-related factor 2. The potential role of DJ-1 inactivation in the precipitation of ferroptosis and the assessment of lipid peroxidation are described. Finally, a rational approach to therapy is considered, with a discussion on the roles of coenzyme Q₁₀, iron chelation therapy, in the form of deferiprone, deferoxamine (desferrioxamine) and deferasirox, and N-acetylcysteine.

The complex roles of STAT3 and STAT5 in maintaining redox balance: Lessons from STAT-mediated xCT expression in cancer cells

STAT3 and STAT5 mediate diverse cellular processes, transcriptionally regulating gene expression and interacting with cytoplasmic proteins. Their canonical activity is stimulated by cytokines/growth factors through JAK-STAT signaling. As targets of oncogenes with intrinsic tyrosine kinase activity, STAT3 and STAT5 become constitutively active in hematologic neoplasms and solid tumors, promoting cell proliferation and survival and modulating redox homeostasis. This review summarizes reactive oxygen species (ROS)-regulated STAT activation and how STATs influence ROS production. ROS-induced effects on post-translational modifications are presented, and STAT3/5-mediated regulation of xCT, a redox-sensitive target up-regulated in numerous cancers, is discussed with regard to transcriptional cross-talk.

Absence of system xc- on immune cells invading the central nervous system alleviates experimental autoimmune encephalitis

BACKGROUND

Multiple sclerosis (MS) is an autoimmune demyelinating disease that affects the central nervous system (CNS), leading to neurodegeneration and chronic disability. Accumulating evidence points to a key role for neuroinflammation, oxidative stress, and excitotoxicity in this degenerative process. System x_c^- or the cystine/glutamate antiporter could tie these pathological mechanisms together: its activity is enhanced by reactive oxygen species and inflammatory stimuli, and its enhancement might lead to the release of toxic amounts of glutamate, thereby triggering excitotoxicity and neurodegeneration.

METHODS

Semi-quantitative Western blotting served to study protein expression of xCT, the specific subunit of system x_c^-, as well as of regulators of xCT transcription, in the normal appearing white matter (NAWM) of MS patients and in the CNS and spleen of mice exposed to experimental autoimmune encephalomyelitis (EAE), an accepted mouse model of MS. We next compared the clinical course of the EAE disease, the extent of demyelination, the infiltration of immune cells and microglial activation in xCT-knockout (xCT^-/-) mice and irradiated mice reconstituted in xCT^-/- bone marrow (BM), to their proper wild type (xCT^+/+) controls.

RESULTS

xCT protein expression levels were upregulated in the NAWM of MS patients and in the brain, spinal cord, and spleen of EAE mice. The pathways involved in this upregulation in NAWM of MS patients remain unresolved. Compared to xCT^+/+ mice, xCT^-/- mice were equally susceptible to EAE, whereas mice transplanted with xCT^-/- BM, and as such only exhibiting loss of xCT in their immune cells, were less susceptible to EAE. In none of the above-described conditions, demyelination, microglial activation, or infiltration of immune cells were affected.

CONCLUSIONS

Our findings demonstrate enhancement of xCT protein expression in MS pathology and suggest that system x_c^- on immune cells invading the CNS participates to EAE. Since a total loss of system x_c^- had no net beneficial effects, these results have important implications for targeting system x_c^- for treatment of MS.

Genomic analysis of xCT-mediated regulatory network: Identification of novel targets against AIDS-associated lymphoma

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiological agent of primary effusion lymphoma (PEL), a rapidly progressing malignancy mostly arising in HIV-infected patients. Even under conventional chemotherapy, PEL continues to portend nearly 100% mortality within several months, which urgently requires novel therapeutic strategies. We have previously demonstrated that targeting xCT, an amino acid transporter for cystine/glutamate exchange, induces significant PEL cell apoptosis through regulation of multiple host and viral factors. More importantly, one of xCT selective inhibitors, Sulfasalazine (SASP), effectively prevents PEL tumor progression in an immune-deficient xenograft model. In the current study, we use Illumina microarray to explore the profile of genes altered by SASP treatment within 3 KSHV⁺ PEL cell-lines, and discover that many genes involved in oxidative stress/antioxidant defense system, apoptosis/anti-apoptosis/cell death, and cellular response to unfolded proteins/topologically incorrect proteins are potentially regulated by xCT. We further validate 2 downstream candidates, OSGIN1 (oxidative stress-induced growth inhibitor 1) and XRCC5 (X-ray repair cross-complementing protein 5), and evaluate their functional relationship with PEL cell survival/proliferation and chemoresistance, respectively. Together, our data indicate that targeting these novel xCT-regulated downstream genes may represent a promising new therapeutic strategy against PEL and/or other AIDS-related lymphoma.

Bach1 differentially regulates distinct Nrf2-dependent genes in human venous and coronary artery endothelial cells adapted to physiological oxygen levels

The effects of physiological oxygen tension on Nuclear Factor-E2-Related Factor 2 (Nrf2)-regulated redox signaling remain poorly understood. We report the first study of Nrf2-regulated signaling in human primary endothelial cells (EC) adapted long-term to physiological O2 (5%). Adaptation of EC to 5% O2 had minimal effects on cell ultrastructure, viability, basal redox status or HIF1-α expression. Affymetrix array profiling and subsequent qPCR/protein validation revealed that induction of select Nrf2 target genes, HO-1 and NQO1, was significantly attenuated in cells adapted to 5% O2, despite nuclear accumulation and DNA binding of Nrf2. Diminished HO-1 induction under 5% O2 was stimulus independent and reversible upon re-adaptation to air or silencing of the Nrf2 repressor Bach1, notably elevated under 5% O2. Induction of GSH-related genes xCT and GCLM were oxygen and Bach1-insensitive during long-term culture under 5% O2, providing the first evidence that genes related to GSH synthesis mediate protection afforded by Nrf2-Keap1 defense pathway in cells adapted to physiological O2 levels encountered in vivo.

Interleukin 1β Regulation of the System xc- Substrate-specific Subunit, xCT, in Primary Mouse Astrocytes Involves the RNA-binding Protein HuR

System xc(-) is a heteromeric amino acid cystine/glutamate antiporter that is constitutively expressed by cells of the CNS, where it functions in the maintenance of intracellular glutathione and extracellular glutamate levels. We recently determined that the cytokine, IL-1β, increases the activity of system xc(-) in CNS astrocytes secondary to an up-regulation of its substrate-specific light chain, xCT, and that this occurs, in part, at the level of transcription. However, an in silico analysis of the murine xCT 3'-UTR identified numerous copies of adenine- and uridine-rich elements, raising the possibility that undefined trans-acting factors governing mRNA stability and translation may also contribute to xCT expression. Here we show that IL-1β increases the level of mRNA encoding xCT in primary cultures of astrocytes isolated from mouse cortex in association with an increase in xCT mRNA half-life. Additionally, IL-1β induces HuR translocation from the nucleus to the cytoplasm. RNA immunoprecipitation analysis reveals that HuR binds directly to the 3'-UTR of xCT in an IL-1β-dependent manner. Knockdown of endogenous HuR protein abrogates the IL-1β-mediated increase in xCT mRNA half-life, whereas overexpression of HuR in unstimulated primary mouse astrocytes doubles the half-life of constitutive xCT mRNA. This latter effect is accompanied by an increase in xCT protein levels, as well as a functional increase in system xc(-) activity. Altogether, these data support a critical role for HuR in mediating the IL-1β-induced stabilization of astrocyte xCT mRNA.

Main path and byways: non-vesicular glutamate release by system xc(-) as an important modifier of glutamatergic neurotransmission

System xc(-) is a cystine/glutamate antiporter that exchanges extracellular cystine for intracellular glutamate. Cystine is intracellularly reduced to cysteine, a building block of GSH. As such, system xc(-) can regulate the antioxidant capacity of cells. Moreover, in several brain regions, system xc(-) is the major source of extracellular glutamate. As such this antiporter is able to fulfill key physiological functions in the CNS, while evidence indicates it also plays a role in certain brain pathologies. Since the transcription of xCT, the specific subunit of system xc(-), is enhanced by the presence of reactive oxygen species and inflammatory cytokines, system xc(-) could be involved in toxic extracellular glutamate release in neurological disorders that are associated with increased oxidative stress and neuroinflammation. System xc(-) has also been reported to contribute to the invasiveness of brain tumors and, as a source of extracellular glutamate, could participate in the induction of peritumoral seizures. Two independent reviews (Pharmacol. Rev. 64, 2012, 780; Antioxid. Redox Signal. 18, 2013, 522), approached from a different perspective, have recently been published on the functions of system xc(-) in the CNS. In this review, we highlight novel achievements and insights covering the regulation of system xc(-) as well as its involvement in emotional behavior, cognition, addiction, neurological disorders and glioblastomas, acquired in the past few years. System xc(-) constitutes an important source of extrasynaptic glutamate in the brain. By modulating the tone of extrasynaptic metabotropic or ionotropic glutamate receptors, it affects excitatory neurotransmission, the threshold for overexcitation and excitotoxicity and, as a consequence, behavior. This review describes the current knowledge of how system xc(-) is regulated and involved in physiological as well as pathophysiological brain functioning.

xCT, not just an amino-acid transporter: a multi-functional regulator of microbial infection and associated diseases

Expression of xCT, a component of the x_c^– amino-acid transporter, is essential for the uptake of cystine required for intracellular glutathione (GSH) synthesis and maintenance of the intracellular redox balance. Therefore, xCT plays an important role not only in the survival of somatic and immune cells, but also in other aspects of tumorigenesis, including the growth and malignant progression of cancer cells, resistance to anticancer drugs, and protection of normal cells against oxidative damage induced by carcinogens. xCT also functions as a factor required for infection by Kaposi’s sarcoma-associated herpesvirus (KSHV), the causative agent of Kaposi’s sarcoma (KS) and other lymphoproliferative diseases associated with HIV/AIDS. In spite of these advances, our understanding of the role of xCT in the pathogenesis of infectious diseases is still limited. Therefore, this review will summarize recent findings about the functions of xCT in diseases associated with microbial (bacterial or viral) infections, in particular KSHV-associated malignancies. We will also discuss the remaining questions, future directions, as well as evidence that supports the potential benefits of exploring system x_c^– as a target for prevention and clinical management of microbial diseases and cancer.

System xC- is a mediator of microglial function and its deletion slows symptoms in amyotrophic lateral sclerosis mice

Amyotrophic lateral sclerosis is the most common adult-onset motor neuron disease and evidence from mice expressing amyotrophic lateral sclerosis-causing SOD1 mutations suggest that neurodegeneration is a non-cell autonomous process where microglial cells influence disease progression. However, microglial-derived neurotoxic factors still remain largely unidentified in amyotrophic lateral sclerosis. With excitotoxicity being a major mechanism proposed to cause motor neuron death in amyotrophic lateral sclerosis, our hypothesis was that excessive glutamate release by activated microglia through their system [Formula: see text] (a cystine/glutamate antiporter with the specific subunit xCT/Slc7a11) could contribute to neurodegeneration. Here we show that xCT expression is enriched in microglia compared to total mouse spinal cord and absent from motor neurons. Activated microglia induced xCT expression and during disease, xCT levels were increased in both spinal cord and isolated microglia from mutant SOD1 amyotrophic lateral sclerosis mice. Expression of xCT was also detectable in spinal cord post-mortem tissues of patients with amyotrophic lateral sclerosis and correlated with increased inflammation. Genetic deletion of xCT in mice demonstrated that activated microglia released glutamate mainly through system [Formula: see text]. Interestingly, xCT deletion also led to decreased production of specific microglial pro-inflammatory/neurotoxic factors including nitric oxide, TNFa and IL6, whereas expression of anti-inflammatory/neuroprotective markers such as Ym1/Chil3 were increased, indicating that xCT regulates microglial functions. In amyotrophic lateral sclerosis mice, xCT deletion surprisingly led to earlier symptom onset but, importantly, this was followed by a significantly slowed progressive disease phase, which resulted in more surviving motor neurons. These results are consistent with a deleterious contribution of microglial-derived glutamate during symptomatic disease. Therefore, we show that system [Formula: see text] participates in microglial reactivity and modulates amyotrophic lateral sclerosis motor neuron degeneration, revealing system [Formula: see text] inactivation, as a potential approach to slow amyotrophic lateral sclerosis disease progression after onset of clinical symptoms.

Glutamate as a neurotransmitter in the healthy brain

Glutamate is the most abundant free amino acid in the brain and is at the crossroad between multiple metabolic pathways. Considering this, it was a surprise to discover that glutamate has excitatory effects on nerve cells, and that it can excite cells to their death in a process now referred to as "excitotoxicity". This effect is due to glutamate receptors present on the surface of brain cells. Powerful uptake systems (glutamate transporters) prevent excessive activation of these receptors by continuously removing glutamate from the extracellular fluid in the brain. Further, the blood-brain barrier shields the brain from glutamate in the blood. The highest concentrations of glutamate are found in synaptic vesicles in nerve terminals from where it can be released by exocytosis. In fact, glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. It took, however, a long time to realize that. The present review provides a brief historical description, gives a short overview of glutamate as a transmitter in the healthy brain, and comments on the so-called glutamate-glutamine cycle. The glutamate transporters responsible for the glutamate removal are described in some detail.

Glutamate transporters in brain ischemia: to modulate or not?

In this review, we briefly describe glutamate (Glu) metabolism and its specific transports and receptors in the central nervous system (CNS). Thereafter, we focus on excitatory amino acid transporters, cystine/glutamate antiporters (system xc-) and vesicular glutamate transporters, specifically addressing their location and roles in CNS and the molecular mechanisms underlying the regulation of Glu transporters. We provide evidence from in vitro or in vivo studies concerning alterations in Glu transporter expression in response to hypoxia or ischemia, including limited human data that supports the role of Glu transporters in stroke patients. Moreover, the potential to induce brain tolerance to ischemia through modulation of the expression and/or activities of Glu transporters is also discussed. Finally we present strategies involving the application of ischemic preconditioning and pharmacological agents, eg β-lactam antibiotics, amitriptyline, riluzole and N-acetylcysteine, which result in the significant protection of nervous tissues against ischemia.

Redox control of indoleamine 2,3-dioxygenase expression and activity in human monocyte-derived dendritic cells is independent of changes in oxygen tension

Dendritic cells (DCs) initiate adaptive immune responses to pathogens and tumours and maintain tolerance to self and innocuous antigens. These functions occur in organs and tissues exhibiting wide variations in nutrients, growth factors, redox and oxygen tension. Understanding how these microenvironmental factors influence DCs to affect immunological outcomes is of increasing relevance with the emerging success of DC-based cellular vaccines. In a previous study, we examined whether redox, an important environmental cue, could influence DC expression of the immunosuppressive enzyme indoleamine 2,3-dioxygenase (IDO). IDO-competent DCs promote long-term immune homoeostasis by limiting exaggerated inflammatory responses and directing regulatory T-cell effector function. To alter redox, we manipulated the activity of the cystine/glutamate antiporter, which functions to maintain intracellular and extracellular redox. The results of that study showed that redox perturbation strongly induced IDO expression and activity in DCs. While this study was performed using standard cell culture techniques with DCs cultured under 5% CO₂ and 20% O₂, it is clear that DCs capture and present antigens in inflamed tissues and secondary lymphoid organs which exhibit low oxygen tension (1-5% O₂). Therefore, here we investigated whether oxygen tension influences DC expression of IDO in the context of homoeostatic and altered redox.

Plasma amino acids imbalance in cirrhotic patients disturbs the tricarboxylic acid cycle of dendritic cell

An imbalance of plasma amino acids (AA) is observed cirrhotic patients. Here we report that the imbalance suppresses the maturation of dendritic cells (DCs) by reducing the intracellular ATP due to interference with the mitochondrial tricarboxylic acid (TCA) cycle. We used serum-free culture medium consistent with the average concentration of the plasma AA from a healthy volunteer (HCM) and that from patients with advanced cirrhosis (ACM). We compared the function of DCs and the metabolism of glucose-amino acids under each medium. The maturation and intracellular ATP of immature DCs were lower under ACM in spite of the enhancement of mitochondrial respiratory chain complex genes. Metabolomics revealed that the TCA cycle metabolite, fumarate and 2-oxoglutarate were increased in DCs generated under ACM. Consistent with in vitro, In CD1c⁺ or CD14⁺ cells from cirrhotic patients, the gene expression of 2-oxoglutarate-succinate-fumarate transition enzymes were significantly different from the cells of healthy controls.

Alpha-lipoic acid effects on brain glial functions accompanying double-stranded RNA antiviral and inflammatory signaling

Double-stranded RNAs (dsRNA) serve as viral ligands that trigger innate immunity in astrocytes and microglial, as mediated through Toll-like receptor 3 (TLR3) and dsRNA-dependent protein kinase (PKR). Beneficial transient TLR3 and PKR anti-viral signaling can become deleterious when events devolve into inflammation and cytotoxicity. Viral products in the brain cause glial cell dysfunction, and are a putative etiologic factor in neuropsychiatric disorders, notably schizophrenia, bipolar disorder, Parkinson's, and autism spectrum. Alpha-lipoic acid (LA) has been proposed as a possible therapeutic neuroprotectant. The objective of this study was to test our hypothesis that LA can control untoward antiviral mechanisms associated with neural dysfunction. Utilizing rat brain glial cultures (91% astrocytes:9% microglia) treated with PKR- and TLR3-ligand/viral mimetic dsRNA, polyinosinic-polycytidylic acid (polyI:C), we report in vitro glial antiviral signaling and LA reduction of the effects of this signaling. LA blunted the dsRNA-stimulated expression of IFNα/β-inducible genes Mx1, PKR, and TLR3. And in polyI:C treated cells, LA promoted gene expression of rate-limiting steps that benefit healthy neural redox status in glutamateric systems. To this end, LA decreased dsRNA-induced inflammatory signaling by downregulating IL-1β, IL-6, TNFα, iNOS, and CAT2 transcripts. In the presence of polyI:C, LA prevented cultured glial cytotoxicity which was correlated with increased expression of factors known to cooperatively control glutamate/cystine/glutathione redox cycling, namely glutamate uptake transporter GLAST/EAAT1, γ-glutamyl cysteine ligase catalytic and regulatory subunits, and IL-10. Glutamate exporting transporter subunits 4F2hc and xCT were downregulated by LA in dsRNA-stimulated glia. l-Glutamate net uptake was inhibited by dsRNA, and this was relieved by LA. Glutathione synthetase mRNA levels were unchanged by dsRNA or LA. This study demonstrates the protective effects of LA in astroglial/microglial cultures, and suggests the potential for LA efficacy in virus-induced CNS pathologies, with the caveat that antiviral benefits are concomitantly blunted. It is concluded that LA averts key aspects of TLR3- and PKR-provoked glial dysfunction, and provides rationale for exploring LA in whole animal and human clinical studies to blunt or avert neuropsychiatric disorders.

A novel transcription mechanism activated by ethanol: induction of Slc7a11 gene expression via inhibition of the DNA-binding activity of transcriptional repressor octamer-binding transcription factor 1 (OCT-1)

Solute carrier family 7, member 11 (Slc7a11) is a plasma membrane cystine/glutamate exchanger that provides intracellular cystine to produce glutathione, a major cellular antioxidant. Oxidative and endoplasmic reticulum stresses up-regulate Slc7a11 expression by activation of nuclear factor erythroid 2-related factor 2 and transcription factor 4. This study examined the effect of ethanol on Slc7a11 expression and the underlying mechanism involved. Treatment of mouse hepatic stellate cells with ethanol significantly increased Slc7a11 mRNA and protein levels. Deletion of a 20-bp DNA sequence between -2044 to -2024 upstream of the transcription start site significantly increased basal activity and completely abolished the ethanol-induced activity of the Slc7a11 promoter. This deletion did not affect Slc7a11 promoter activity induced by oxidative or endoplasmic reticulum stress. DNA sequence analysis revealed a binding motif for octamer-binding transcription factor 1 (OCT-1) in the deleted fragment. Mutation of this OCT-1 binding motif resulted in a similar effect as the deletion experiment, i.e. it increased the basal promoter activity and abolished the response to ethanol. Ethanol exposure significantly inhibited OCT-1 binding to the Slc7a11 promoter region, although it did not alter OCT-1 mRNA and protein levels. OCT-1 reportedly functions as either a transcriptional enhancer or repressor, depending on the target genes. Results from this study suggest that OCT-1 functions as a repressor on the Slc7a11 promoter and that ethanol inhibits OCT-1 binding to the Slc7a11 promoter, thereby increasing Slc7a11 expression. Taken together, inhibition of the DNA binding activity of transcriptional repressor OCT-1 is a mechanism by which ethanol up-regulates Slc711 expression.

The cystine/glutamate antiporter system x(c)(-) in health and disease: from molecular mechanisms to novel therapeutic opportunities

The antiporter system x(c)(-) imports the amino acid cystine, the oxidized form of cysteine, into cells with a 1:1 counter-transport of glutamate. It is composed of a light chain, xCT, and a heavy chain, 4F2 heavy chain (4F2hc), and, thus, belongs to the family of heterodimeric amino acid transporters. Cysteine is the rate-limiting substrate for the important antioxidant glutathione (GSH) and, along with cystine, it also forms a key redox couple on its own. Glutamate is a major neurotransmitter in the central nervous system (CNS). By phylogenetic analysis, we show that system x(c)(-) is a rather evolutionarily new amino acid transport system. In addition, we summarize the current knowledge regarding the molecular mechanisms that regulate system x(c)(-), including the transcriptional regulation of the xCT light chain, posttranscriptional mechanisms, and pharmacological inhibitors of system x(c)(-). Moreover, the roles of system x(c)(-) in regulating GSH levels, the redox state of the extracellular cystine/cysteine redox couple, and extracellular glutamate levels are discussed. In vitro, glutamate-mediated system x(c)(-) inhibition leads to neuronal cell death, a paradigm called oxidative glutamate toxicity, which has successfully been used to identify neuroprotective compounds. In vivo, xCT has a rather restricted expression pattern with the highest levels in the CNS and parts of the immune system. System x(c)(-) is also present in the eye. Moreover, an elevated expression of xCT has been reported in cancer. We highlight the diverse roles of system x(c)(-) in the regulation of the immune response, in various aspects of cancer and in the eye and the CNS.

Enhanced expression of cystine/glutamate transporter in the lung caused by the oxidative-stress-inducing agent paraquat

In mammalian cultured cells, the activity of a cystine/glutamate transporter, designated System xc(-), 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 phenotype 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) 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 structure of the xCT-deficient mice compared with the wild-type mice. After paraquat treatment, obvious 8-hydroxy-2'-deoxyguanosine and 4-hydroxy-2-nonenal reactivity was detected in the lungs 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.

Functional expression of system x(c)- is upregulated by asbestos but not crystalline silica in murine macrophages

CONTEXT

Inhalation of asbestos or silica is associated with chronic and progressive diseases, including fibrosis, cancer, and increased risk of systemic autoimmunity. Because there is a need for treatment options for these diseases, a better understanding of their mechanistic etiologies is essential. While oxidative stress in macrophages is an early consequence of these exposures, it may also serve as a signaling mechanism involved in downstream immune dysregulation. The system x(c)(-) exchange protein is induced by oxidative stress, and exchanges equimolor levels of extracellular cystine for intracellular glutamate. Cystine is subsequently reduced to cysteine, the rate-limiting precursor for glutathione synthesis.

OBJECTIVE

As the primary transporter responsible for cystine/glutamate exchange on macrophages, system x(c)- was hypothesized to be inducible in response to asbestos and silica, and to increase viability through protection from oxidative stress.

RESULTS

When challenged with amphibole asbestos, but not crystalline silica, RAW 264.7 macrophages increased expression of xCT and the rate of cystine/glutamate exchange in sodium-free conditions. This upregulation was prevented with N-acetylcysteine, implicating oxidative stress. Cystine protected the macrophages from asbestos-induced oxidative stress and cell death, supporting the hypothesis that imported cystine was used for synthesis of cellular antioxidants. System x(c)(-) inhibitors, glutamate and S-4-carboxyphenylglycine ((S)-4-CPG), significantly increased oxidative stress and cell death of asbestos-treated macrophages.

CONCLUSION

System x(c)(-) plays a critical role in survival of macrophages exposed to asbestos, but not silica. These data demonstrate a very early difference in the cellular response to these silicates that may have important downstream implications in the pathologic outcome of exposure.

The role of amino acid transporters in GSH synthesis in the blood-brain barrier and central nervous system

Glutathione (GSH) plays a critical role in protecting cells from oxidative stress and xenobiotics, as well as maintaining the thiol redox state, most notably in the central nervous system (CNS). GSH concentration and synthesis are highly regulated within the CNS and are limited by availability of the sulfhydryl amino acid (AA) l-cys, which is mainly transported from the blood, through the blood-brain barrier (BBB), and into neurons. Several antiporter transport systems (e.g., x(c)(-), x(-)(AG), and L) with clearly different luminal and abluminal distribution, Na(+), and pH dependency have been described in brain endothelial cells (BEC) of the BBB, as well as in neurons, astrocytes, microglia and oligodendrocytes from different brain structures. The purpose of this review is to summarize information regarding the different AA transport systems for l-cys and its oxidized form l-cys(2) in the CNS, such as expression and activity in blood-brain barrier endothelial cells, astrocytes and neurons and environmental factors that modulate transport kinetics.

Regulation of NADPH oxidase gene expression with PKA and cytokine IL-4 in neurons and microglia

Neuronal excitation is mediated by the activation of NMDA receptor and associated with the formation of reactive oxygen species due to the activation of NADPH oxidase complex proteins. The activation of Gs protein coupled receptors (GPCRs) induces neuronal activation in the cAMP-dependent protein kinase A (PKA)-mediated signal cascade and regulates NADPH oxidase activity. However, it is unknown whether PKA regulates NADPH oxidase gene expression in neurons and microglia. In the present research, the NADPH oxidase gene expression was studied in rat cortical neurons and microglia in vitro. Purified microglial cells were identified with OX-42 antibody and they also expressed apolipoprotein E (ApoE). The time-dependent effect of cytokine interleukin-4 (IL-4) (20 ng/ml) in NADPH oxidase gene expression was studied in microglial cells. The levels of mRNA were determined by quantitative RT-PCR. The expression of NOX1, NOX2, and NCF2 was upregulated after IL-4 treatment for 4 h, but it was downregulated after 8-24 h. The expression of NCF1 was suppressed during any time of cytokine effect. IL-4 upregulated arginase1 (Arg1) and serine racemase1 (SRR1) gene expressions in microglia. Amyloid beta (Ab) suppressed NOX2, NCF1, and NCF2 gene expressions and upregulated glutamate cystine transporter (xCT), although IL-4 attenuated the effect of Ab (500 μM) in the upregulation of xCT gene expression. The activation of PKA with agonist dibutyryl cAMP (dbcAMP) (100 μM) induced the upregulation of Arg1 gene expression in microglia involving in the process of microglial activation. The transcription of NOX1, NOX2, and NCF1 was suppressed in microglial cells after dbcAMP treatment within 24 h. Neurons were identified with the microtubule-associated protein tau. The uniform distribution of tau along axons was established in normal neurons. Tau protein was redistributed after PKA agonist dbcAMP treatment for 24 h. L-glutamate (50 μM) caused the apoptotic processes and the accumulation of tau in the soma of neurons and along axons. The activation of PKA for 24 h induced the transcriptional upregulation of NOX1 and NCF1 in cortical neurons. However, L-glutamate suppressed NOX1 gene expression in neurons. These data demonstrate that the effects of IL-4 and dbcAMP are similar in the regulation of SRR1, Arg1, and NADPH oxidase complex gene expressions in neurons and microglia. IL-4 prevents glutamate release from microglia suppressing xCT expression induced by Ab. These findings suggest that the activation of GPCR in PKA-mediated pathway leads to transcriptional regulation of NADPH oxidase complex. The modulation of GPCR activation may inhibit the oxidative stress in neurons.

Increased expression of cystine/glutamate antiporter in multiple sclerosis

Background

Glutamate excitotoxicity contributes to oligodendrocyte and tissue damage in multiple sclerosis (MS). Intriguingly, glutamate level in plasma and cerebrospinal fluid of MS patients is elevated, a feature which may be related to the pathophysiology of this disease. In addition to glutamate transporters, levels of extracellular glutamate are controlled by cystine/glutamate antiporter x_c^-, an exchanger that provides intracellular cystine for production of glutathione, the major cellular antioxidant. The objective of this study was to analyze the role of the system x_c^- in glutamate homeostasis alterations in MS pathology.

Methods

Primary cultures of human monocytes and the cell line U-937 were used to investigate the mechanism of glutamate release. Expression of cystine glutamate exchanger (xCT) was quantified by quantitative PCR, Western blot, flow cytometry and immunohistochemistry in monocytes in vitro, in animals with experimental autoimmune encephalomyelitis (EAE), the animal model of MS, and in samples of MS patients.

Results and discussion

We show here that human activated monocytes release glutamate through cystine/glutamate antiporter x_c^- and that the expression of the catalytic subunit xCT is upregulated as a consequence of monocyte activation. In addition, xCT expression is also increased in EAE and in the disease proper. In the later, high expression of xCT occurs both in the central nervous system (CNS) and in peripheral blood cells. In particular, cells from monocyte-macrophage-microglia lineage have higher xCT expression in MS and in EAE, indicating that immune activation upregulates xCT levels, which may result in higher glutamate release and contribution to excitotoxic damage to oligodendrocytes.

Conclusions

Together, these results reveal that increased expression of the cystine/glutamate antiporter system x_c^- in MS provides a link between inflammation and excitotoxicity in demyelinating diseases.

The oxidative stress-inducible cystine/glutamate antiporter, system x (c) (-) : cystine supplier and beyond

The oxidative stress-inducible cystine/glutamate exchange system, system x (c) (-) , transports one molecule of cystine, the oxidized form of cysteine, into cells and thereby releases one molecule of glutamate into the extracellular space. It consists of two protein components, the 4F2 heavy chain, necessary for membrane location of the heterodimer, and the xCT protein, responsible for transport activity. Previously, system x (c) (-) has been regarded to be a mere supplier of cysteine to cells for the synthesis of proteins and the antioxidant glutathione (GSH). In that sense, oxygen, electrophilic agents, and bacterial lipopolysaccharide trigger xCT expression to accommodate with increased oxidative stress by stimulating GSH biosynthesis. However, emerging evidence established that system x (c) (-) may act on its own as a GSH-independent redox system by sustaining a redox cycle over the plasma membrane. Hallmarks of this cycle are cystine uptake, intracellular reduction to cysteine and secretion of the surplus of cysteine into the extracellular space. Consequently, increased levels of extracellular cysteine provide a reducing microenvironment required for proper cell signaling and communication, e.g. as already shown for the mechanism of T cell activation. By contrast, the enhanced release of glutamate in exchange with cystine may trigger neurodegeneration due to glutamate-induced cytotoxic processes. This review aims to provide a comprehensive picture from the early days of system x (c) (-) research up to now.

Regulation of xCT expression and system x (c) (-) function in neuronal cells

The glutamate/cystine antiporter system x(c)(-) transports cystine into cells in exchange for glutamate at a ratio of 1:1. It is composed of a specific light chain, xCT, and a heavy chain, 4F2, linked by a disulfide bridge. Intracellularly, cystine is reduced into cysteine, the rate-limiting precursor of glutathione (GSH), an important small molecule antioxidant. Several lines of evidence suggest that the expression of xCT and thereby the presence system x(c)(-) activity plays an important role in the brain. First, it regulates extracellular glutamate concentrations. Second, as brain is prone to oxidative stress due to its high oxygen consumption and lipid content, system x(c)(-) by favoring GSH synthesis, may prevent oxidative damage. Thus, to understand how xCT expression and system x(c)(-) activity are regulated in the central nervous system is of utmost importance. In this review, we will summarize the current knowledge about the molecular basis by which xCT expression and system x(c)(-) activity are regulated in neuronal cell lines, especially the hippocampal cell line, HT22. In addition, we will relate these pathways to findings in other cell types, especially those found in the central nervous system. We will focus on the signaling pathways that modulate the transcription of the xCT gene. Furthermore, we describe possible pathways that modify system x(c)(-) activity beyond the level of xCT transcription, including regulation on the level of membrane trafficking and substrate availability, especially the regulation by glutamate transport through excitatory amino acid transporters.

Regulation of system x(c)(-)activity and expression in astrocytes by interleukin-1β: implications for hypoxic neuronal injury

We recently demonstrated that interleukin-1β (IL-1β) increases system x(c)(-) (cystine/glutamate antiporter) activity in mixed cortical cell cultures, resulting in an increase in hypoxic neuronal injury when glutamate clearance is impaired. Herein, we demonstrate that neurons, astrocytes, and microglia all express system x(c)(-) subunits (xCT, 4F2hc, RBAT) and are capable of cystine import. However, IL-1β stimulation increases mRNA for xCT--the light chain that confers substrate specificity--in astrocytes only; an effect blocked by the transcriptional inhibitor actinomycin D. Additionally, only astrocytes show an increase in cystine uptake following IL-1β exposure; an effect associated with a change in xCT protein. The increase in cystine uptake that follows IL-1β is lacking in astrocytes derived from mice harboring a mutation in Slc7a11 (sut gene), which encodes for xCT, and in wild-type astrocytes treated with the protein synthesis inhibitor cycloheximide. IL-1β does not regulate the light chain of the amino acid transporter, LAT2, or the expression and function of astrocytic excitatory amino acid transporters (EAATs), demonstrating some target selectivity. Finally, the enhanced neuronal vulnerability to hypoxia that followed IL-1β treatment in our mixed culture system was not observed in chimeric cultures consisting of wild-type neurons plated on top of sut astrocytes. Nor was it observed in wild-type cultures treated with a system x(c)(-) inhibitor or an NMDA receptor antagonist. Overall, our data demonstrate that IL-1β selectively regulates system x(c)(-) activity in astrocytes and that this change is specifically responsible for the deleterious, excitotoxic effects of IL-1β found under hypoxic conditions.

Hypoxia increases the dependence of glioma cells on glutathione

Glutathione (GSH) is an essential antioxidant responsible for the maintenance of intracellular redox homeostasis. As tumors outgrow their blood supply and become hypoxic, their redox homeostasis is challenged by the production of nitric oxide and reactive oxygen species (ROS). In gliomas, the sustained import of L-cystine via the L-cystine/L-glutamate exchanger, system x(c)(-), is rate-limiting for the synthesis of GSH. We show that hypoxia causes a significant increase in NO and ROS but without affecting glioma cell growth. This is explained by a concomitant increase in the utilization of GSH, which is accompanied by an increase in the cell-surface expression of xCT, the catalytic subunit of system x(c)(-), and L-cystine uptake. Growth was inhibited when GSH synthesis was blocked by buthionine sulfoximine (BSO), an inhibitor of the enzyme required for GSH synthesis, or when cells were deprived of L-cystine. These findings suggest that glioma cells show an increased requirement for GSH to maintain growth under hypoxic conditions. Therefore, approaches that limit GSH synthesis such as blocking system x(c)(-) may be considered as an adjuvant to radiation or chemotherapy.

Dietary isoflavones and vascular protection: activation of cellular antioxidant defenses by SERMs or hormesis?

During the past decade nutrigenomic studies in humans, animal models and cultured cells have provided important and novel insights into the mechanisms by which dietary isoflavones afford protection against vascular dysfunction through the amelioration of oxidative modifications and upregulation of endogenous antioxidant signaling pathways. In this review, we highlight that increased generation of nitric oxide (NO) and reactive oxygen species (ROS) in the vessel wall in response to dietary isoflavones enhance the activity of antioxidant defense enzymes in endothelial and smooth muscle cells. The estrogenic properties of isoflavones are likely to contribute to the molecular mechanisms by which these compounds activate signal transduction pathways involved in sustaining endothelial function and transcriptional activation of antioxidant defense genes in vascular cells. We evaluate the recent literature that estrogenic and hormetic properties of phytoestrogens are of benefit for the maintenance of vascular function, and conclude that dietary isoflavones can protect against cardiovascular diseases by virtue of their ability to activate signaling pathways leading to increased NO bioavailability and regulation of phase II and antioxidant enzyme expression via the redox sensitive transcription factor Nrf2. In context of epigenetics and the developmental origins of adult disease, it is noteworthy that exposure to dietary soy during fetal development reduces the susceptibility to CVD and obesity in adulthood. Thus, the Nrf2/Keap1 defense pathway provides a key mechanism by which isoflavones can act as hormetic agents to modulate intracellular redox signaling in the vasculature to prolong healthspan and reduce the incidence of age-related cardiovascular diseases.

Oxidation of extracellular cysteine/cystine redox state in bleomycin-induced lung fibrosis

Several lines of evidence indicate that depletion of glutathione (GSH), a critical thiol antioxidant, is associated with the pathogenesis of idiopathic pulmonary fibrosis (IPF). However, GSH synthesis depends on the amino acid cysteine (Cys), and relatively little is known about the regulation of Cys in fibrosis. Cys and its disulfide, cystine (CySS), constitute the most abundant low-molecular weight thiol/disulfide redox couple in the plasma, and the Cys/CySS redox state (E(h) Cys/CySS) is oxidized in association with age and smoking, known risk factors for IPF. Furthermore, oxidized E(h) Cys/CySS in the culture media of lung fibroblasts stimulates proliferation and expression of transitional matrix components. The present study was undertaken to determine whether bleomycin-induced lung fibrosis is associated with a decrease in Cys and/or an oxidation of the Cys/CySS redox state and to determine whether these changes were associated with changes in E(h) GSH/glutathione disulfide (GSSG). We observed distinct effects on plasma GSH and Cys redox systems during the progression of bleomycin-induced lung injury. Plasma E(h) GSH/GSSG was selectively oxidized during the proinflammatory phase, whereas oxidation of E(h) Cys/CySS occurred at the fibrotic phase. In the epithelial lining fluid, oxidation of E(h) Cys/CySS was due to decreased food intake. Thus the data show that decreased precursor availability and enhanced oxidation of Cys each contribute to the oxidation of extracellular Cys/CySS redox state in bleomycin-induced lung fibrosis.

Activation of microglia with zymosan promotes excitatory amino acid release via volume-regulated anion channels: the role of NADPH oxidases

Microglia are the resident immune cells of the CNS, which are important for preserving neural tissue functions, but may also contribute to neurodegeneration. Activation of these cells in infection, inflammation, or trauma leads to the release of various toxic molecules, including reactive oxygen species (ROS) and the excitatory amino acid glutamate. In this study, we used an electrophysiologic approach and a D-[(3)H]aspartate (glutamate) release assay to explore the ROS-dependent regulation of glutamate-permeable volume-regulated anion channels (VRACs). Exposure of rat microglia to hypo-osmotic media stimulated Cl(-) currents and D-[(3)H]aspartate release, both of which were inhibited by the selective VRAC blocker, DCPIB. Exogenously applied H(2)O(2) potently increased swelling-activated glutamate release. Stimulation of microglia with zymosan triggered production of endogenous ROS and strongly enhanced glutamate release via VRAC in swollen cells. The effects of zymosan were attenuated by the ROS scavenger, MnTMPyP, and by two inhibitors of NADPH oxidase (NOX), diphenyliodonium and thioridazine. However, zymosan-stimulated glutamate release was insensitive to other NOX blockers, apocynin and HEBSF. This pharmacologic profile pointed to the potential involvement of apocynin-insensitive NOX4. Using RT-PCR we confirmed that NOX4 is expressed in rat microglial cells along with NOX1 and NOX2. To check for potential involvement of phagocytic NOX2, we stimulated this isoform using protein kinase C (PKC) activator, phorbol 12-myristate 13-acetate or inhibited it with the broad spectrum PKC blocker, Gö6983. Both agents potently modulated endogenous ROS production by NOX2 but not VRAC activity. Taken together, these data suggest that the anion channel VRAC may contribute to microglial glutamate release and that its activity is regulated by endogenous ROS originating from NOX4.

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.

System xc- and glutamate transporter inhibition mediates microglial toxicity to oligodendrocytes

Elevated levels of extracellular glutamate cause excitotoxic oligodendrocyte cell death and contribute to progressive oligodendrocyte loss and demyelination in white matter disorders such as multiple sclerosis and periventricular leukomalacia. However, the mechanism by which glutamate homeostasis is altered in such conditions remains elusive. We show here that microglial cells, in their activated state, compromise glutamate homeostasis in cultured oligodendrocytes. Both activated and resting microglial cells release glutamate by the cystine-glutamate antiporter system xc-. In addition, activated microglial cells act to block glutamate transporters in oligodendrocytes, leading to a net increase in extracellular glutamate and subsequent oligodendrocyte death. The blocking of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors or the system xc- antiporter prevented the oligodendrocyte injury produced by exposure to LPS-activated microglial cells in mixed glial cultures. In a whole-mount rat optic nerve, LPS exposure produced wide-spread oligodendrocyte injury that was prevented by AMPA/kainate receptor block and greatly reduced by a system xc- antiporter block. The cell death was typified by swelling and disruption of mitochondria, a feature that was not found in closely associated axonal mitochondria. Our results reveal a novel mechanism by which reactive microglia can contribute to altering glutamate homeostasis and to the pathogenesis of white matter disorders.

Glutamate release from activated microglia requires the oxidative burst and lipid peroxidation

When activated by proinflammatory stimuli, microglia release substantial levels of glutamate, and mounting evidence suggests this contributes to neuronal damage during neuroinflammation. Prior studies indicated a role for the Xc exchange system, an amino acid transporter that antiports glutamate for cystine. Because cystine is used for synthesis of glutathione (GSH) synthesis, we hypothesized that glutamate release is an indirect consequence of GSH depletion by the respiratory burst, which produces superoxide from NADPH oxidase. Microglial glutamate release triggered by lipopolysaccharide was blocked by diphenylene iodonium chloride and apocynin, inhibitors of NADPH oxidase. This glutamate release was also blocked by vitamin E and elicited by lipid peroxidation products 4-hydroxynonenal and acrolein, suggesting that lipid peroxidation makes crucial demands on GSH. Although NADPH oxidase inhibitors also suppressed nitrite accumulation, vitamin E did not; moreover, glutamate release was largely unaffected by nitric oxide donors, inhibitors of nitric oxide synthase, or changes in gene expression. These findings indicate that a considerable degree of the neurodegenerative consequences of neuroinflammation may result from conversion of oxidative stress to excitotoxic stress. This phenomenon entails a biochemical chain of events initiated by a programmed oxidative stress and resultant mass-action amino acid transport. Indeed, some of the neuroprotective effects of antioxidants may be due to interference with these events rather than direct protection against neuronal oxidation.

Monocyte Differentiation, Activation, and Mycobacterial Killing Are Linked to Transsulfuration-dependent Redox Metabolism

Modulation of the ambient redox status by mononuclear phagocytes is central to their role in health and disease. However, little is known about the mechanism of redox regulation during mononuclear phagocyte differentiation and activation, critical cellular steps in innate immunity, and microbial clearance. An important intermediate in GSH-based redox metabolism is homocysteine, which can undergo transmethylation via methionine synthase (MS) or transsulfuration via cystathionine beta-synthase (CBS). The transsulfuration pathway generates cysteine, the limiting reagent in GSH biosynthesis. We now demonstrate that expression of CBS and MS are strongly induced during differentiation of human monocytes and are regulated at the transcriptional and posttranscriptional levels, respectively. The changes in enzyme expression are paralleled by an approximately 150% increase in S-adenosylmethionine (accompanied by a corresponding increase in phospholipid methylation) and a similar increase in GSH. Activation with lipopolysachharide or infection with Mycobacterium smegmatis diminished expression of both enzymes to a significant extent and decreased S-adenosylmethionine concentration by approximately 30% of the control value while GSH and cysteine concentrations increased approximately 100 and 300%, respectively. Blockade of the transsulfuration pathway with propargylglycine suppressed clearance of M. smegmatis by macrophages and inhibited phagolysosomal fusion, whereas N-acetylcysteine promoted phagolysosomal fusion and enhanced mycobacterial clearance 3-fold compared with untreated cells. We posit that regulation of the transsulfuration pathway during monocyte differentiation, activation, and infection can boost host defense against invading pathogens and may represent a heretofore unrecognized antimicrobial therapeutic target.