GSH Uptake Research Articles

Novel Withanolides from Tubocapsicum anomalum Suppress Triple-Negative Breast Cancer by Triggering Apoptosis and p53-ASCT2-SLC7A11-Mediated Ferroptosis.

Triple-negative breast cancer (TNBC) is a malignant breast cancer. There is an urgent need for effective drugs to be developed for TNBC. Tubocapsicum anomalum (T. anomalum) has been reported to have an anti-tumor effect, and six novel withanolides were isolated from it and designated as TAMEWs. However, its anti-TNBC effect is still unknown. The results of an MTT assay indicated a higher sensitivity of TNBC cells to TAMEWs compared to other cells. TAMEWs induced apoptosis via mitochondrial dysfunction. They caused increased levels of lipid ROS and Fe2+, with downregulation of GSH and cystine uptake, and it has been confirmed that TAMEWs induced ferroptosis. Additionally, the results of Western blotting indicate that TAMEWs significantly decrease the expressions of ferroptosis-related proteins. Through further investigation, it was found that the knockdown of the p53 gene resulted in a significant reversal of ferroptosis and the expressions of its associated proteins SLC7A11, ASCT2, and GPX4. In vivo, TAMEWs suppressed TNBC growth with no obvious damage. The IHC results also showed that TAMEWs induced apoptosis and ferroptosis in vivo. Our findings provide the first evidence that TAMEWs suppress TNBC growth through apoptosis and ferroptosis.

Autoregulatory control of mitochondrial glutathione homeostasis.

Mitochondria must maintain adequate amounts of metabolites for protective and biosynthetic functions. However, how mitochondria sense the abundance of metabolites and regulate metabolic homeostasis is not well understood. In this work, we focused on glutathione (GSH), a critical redox metabolite in mitochondria, and identified a feedback mechanism that controls its abundance through the mitochondrial GSH transporter, SLC25A39. Under physiological conditions, SLC25A39 is rapidly degraded by mitochondrial protease AFG3L2. Depletion of GSH dissociates AFG3L2 from SLC25A39, causing a compensatory increase in mitochondrial GSH uptake. Genetic and proteomic analyses identified a putative iron-sulfur cluster in the matrix-facing loop of SLC25A39 as essential for this regulation, coupling mitochondrial iron homeostasis to GSH import. Altogether, our work revealed a paradigm for the autoregulatory control of metabolic homeostasis in organelles.

The 2-Oxoglutarate Carrier Is S-Nitrosylated in the Spinal Cord of G93A Mutant hSOD1 Mice Resulting in Disruption of Mitochondrial Glutathione Transport.

Mitochondrial oxidative stress and dysfunction are strongly implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS). Glutathione (GSH) is an endogenous antioxidant that exists as distinct cytosolic and mitochondrial pools. The status of the mitochondrial GSH pool is reliant on transport from the cytosol through the 2-oxoglutarate carrier (OGC), an inner membrane anion carrier. We have previously reported that the outer mitochondrial membrane protein, Bcl-2, directly binds GSH and is a key regulator of OGC-dependent mitochondrial GSH transport. Here, we show that G93A mutant SOD1 (Cu, Zn-superoxide dismutase) reduces the binding of GSH to Bcl-2 and disrupts mitochondrial GSH uptake in vitro. In the G93A mutant hSOD1 mouse model of ALS, mitochondrial GSH is significantly depleted in spinal cord of end-stage mice. Finally, we show that OGC is heavily S-nitrosylated in the spinal cord of end-stage mice and consequently, the GSH uptake capacity of spinal cord mitochondria isolated from these mutant mice is significantly diminished. Collectively, these findings suggest that spinal cord GSH depletion, particularly at the level of the mitochondria, plays a significant role in ALS pathogenesis induced by mutant SOD1. Furthermore, the depletion of mitochondrial GSH in the G93A mutant hSOD1 mouse model may be caused by the S-nitrosylation of OGC and the capacity of mutant SOD1 to disrupt the Bcl-2/GSH interaction, resulting in a disruption of mitochondrial GSH transport.

Anethole Dithiolethione Increases Glutathione in Kidney by Inhibiting γ-Glutamyltranspeptidase: Biochemical Interpretation and Pharmacological Consequences.

Aims Anethole dithiolethione (ADT) is a marketed drug to treat xerostomia. Its mechanism of action is still unknown, but several preclinical studies indicate that it is able to increase intracellular glutathione (GSH) and protect against oxidative stress. Here, we investigated the molecular mechanisms behind these effects. Results Oral treatment of rats confirmed the GSH enhancing properties of ADT; among the different organs examined in this study, only the kidney showed a significant GSH increase that was already observed at low-dose treatments. The increase in GSH correlated with a decrease in γ-glutamyltranspeptidase (γ-GT) activity of the different tissues. In vitro and ex vivo experiments with tubular renal cells and isolated perfused rat kidney showed that the cellular uptake of intact GSH was correlated with the extracellular concentrations of GSH. Conclusions. The prominent in vivopharmacological effect of ADT was a marked increase of GSH concentration in the kidney and a decrease of some systemic and renal biomarkers of oxidative stress. In particular, by inhibition of γ-GT activity, it decreased the production cysteinylglycine, a thiol that has prooxidant effects as the consequence of its autooxidation. The activity of ADT as GSH enhancer in both the circulation and the kidney was long-lasting. All these characteristics make ADT a promising drug to protect the kidney, and in particular proximal tubule cells, from xenobiotic-induced damage.

Age-dependent changes in glutathione metabolism pathways in the lens: New insights into therapeutic strategies to prevent cataract formation-A review.

Ocular tissues possess a robust antioxidant defence system to minimize oxidative stress and preserve tissue structure and function. Glutathione (GSH) is a powerful antioxidant and in the lens exists at unusually high concentrations. However, with advancing age, GSH levels deplete specifically in the lens centre initiating a chain of biochemical events that ultimately result in protein aggregation, light scattering and age-related nuclear cataract. However, antioxidant supplementation has been shown to be ineffective in preventing or delaying cataract indicating that a better understanding of the delivery, uptake and metabolism of GSH in the different regions of the lens is required. This information is essential for the development of scientifically informed approaches that target the delivery of GSH to the lens nucleus, the region of the lens most affected by age-related cataract.

Leishmania LABCG2 transporter is involved in ATP-dependent transport of thiols.

The Leishmania LABCG2 transporter has a key role in the redox metabolism of these protozoan parasites. Recently, the involvement of LABCG2 in virulence, autophagy and oxidative stress has been described. Null mutant parasites for LABCG2 present an increase in the intracellular levels of glutathione (GSH) and trypanothione [T(SH)2]. On the other hand, parasites overexpressing LABCG2 transporter export non-protein thiols to the extracellular medium. To explore if LABCG2 may mediate an active transport of non-protein thiols, the effect of these molecules on ATPase activity of LABCG2 as well as the ability of LABCG2 to transport them was determined using a baculovirus-Sf9 insect cell system. Our results indicate that all thiols tested [GSH, T(SH)2] as well as their oxidized forms GSSG and TS2 (trypanothione disulfide) stimulate LABCG2-ATPase basal activity. We have measured the transport of [3H]-GSH in inside-out Sf9 cell membrane vesicles expressing LABCG2-GFP (green fluorescence protein), finding that LABCG2 was able to mediate a rapid and concentration-dependent uptake of [3H]-GSH in the presence of ATP. Finally, we have analyzed the ability of different thiol species to compete for this uptake, T(SH)2 and TS2 being the best competitors. The IC50 value for [3H]-GSH uptake in the presence of increasing concentrations of T(SH)2 was less than 100 μM, highlighting the affinity of this thiol for LABCG2. These results provide the first direct evidence that LABCG2 is an ABC transporter of reduced and oxidized non-protein thiols in Leishmania, suggesting that this transporter can play a role in the redox metabolism and related processes in this protozoan parasite.

17 - Overexpression of the yeast high affinity glutathione transporter, Hgt1, restores growth in cells lacking γ-glutamylcysteine synthetase

Glutathione (L-γ-glutamyl-L-cysteinylglycine) (GSH) is the most abundant low molecular weight thiol in eukaryotic cells. Its ability to serve as a reducing equivalent in several biological reactions offers protection against reactive oxygen species, xenobiotics, and heavy metals. In addition, GSH serves as an iron ligand for some Fe-S binding proteins involved in both iron regulation and the synthesis and maturation of Fe-S proteins. Hence, GSH is important for iron homeostasis. The biosynthesis of GSH occurs via a two-step ATP-dependent enzymatic process. In the first reaction, γ-glutamylcysteine is formed by γ-glutamylcysteine synthetase (Gsh1). In the second step, glycine is added to the peptide via GSH synthetase (Gsh2). Deletion of the γ-glutamylcysteine synthase gene (GSH1) in yeast leads to growth arrest without GSH supplementation.1 In Saccharomyces cerevisiae, extracellular GSH can be imported via a high affinity GSH transporter, Hgt1.2 The physiological role of GSH depletion on cellular function and subcellular redox status has been well characterized. However, it has not been determined how cells depleted of GSH (∆gsh1) respond to increased GSH uptake. To study this, our lab employed a ∆gsh1 yeast strain engineered to overexpress Hgt1. Yeast strains where Hgt1 is overexpressed have been shown to overaccumulate GSH and glutathione disulfide (GSSG) when GSH is added to the media.3 Our data suggests that the overexpression of Hgt1 alone can partially rescue cells devoid of GSH in a redox-dependent manner. To future elucidate this observation, we will express subcellular targeted redox sensors developed in our lab that specifically equilibrates with GSH:GSSG redox changes.

Regional differences in glutathione accumulation pathways in the rat cornea: Mapping of amino acid transporters involved in glutathione synthesis

In this study we have sought to complete the identification and localisation of uptake pathways involved in accumulating precursor amino acids involved in GSH synthesis in the rat cornea. To do this, we performed reverse transcription PCR (RT-PCR) to identify the Excitatory Amino Acid Transporters (EAAT 1–5) responsible for glutamate uptake, and glycine transporters (GLYT 1–2) at the transcript level. Western blotting was used to verify protein expression, while immunolabelling of sagittal sections was used to localise transporters to the different layers of the cornea. Immunolabelling of en face sections was used to examine the subcellular distribution of proteins in the corneal endothelium. Our findings revealed EAAT 1–5 and GLYT 1–2 to be expressed at the transcript and protein level in the rat cornea. Immunohistochemistry revealed all amino acid transporters to be localised to the epithelium. In the majority of cases, labelling was restricted to the epithelium, and labelling absent from the stroma or endothelium. However, EAAT 4 and GLYT 2 labelling was detected in the stroma with EAAT 4 labelling also present in the endothelium. Overall, the identification of amino acid transporters strongly supports the existence of an intracellular GSH synthesis pathway in the rat corneal epithelium. This suggests that regional differences in GSH accumulation pathways exist, with direct uptake of GSH and intracellular synthesis of GSH restricted to the endothelial and epithelial cell layers, respectively. This information is important in the design of targeted strategies to enhance GSH levels in specific layers of the cornea to prevent against oxidative damage, corneal swelling and loss of corneal transparency.

Study on the glutathione metabolism of the filamentous fungus Aspergillus nidulans.

Yeast protein sequence-based homology search for glutathione (GSH) metabolic enzymes and GSH transporters demonstrated that Aspergillus nidulans has a robust GSH uptake and metabolic system with several paralogous genes. In wet laboratory experiments, two key genes of GSH metabolism, gcsA, and glrA, encoding γ-l-glutamyl-l-cysteine synthetase and glutathione reductase, respectively, were deleted. The gene gcsA was essential, and the ΔgcsA mutant required GSH supplementation at considerably higher concentration than the Saccharomyces cerevisiae gsh1 mutant (8-10 mmol l-1 vs. 0.5 μmol l-1). In addition to some functions known previously, both genes were important in the germination of conidiospores, and both gene deletion strains required the addition of extra GSH to reach wild-type germination rates in liquid cultures. Nevertheless, the supplementation of cultures with 10 mmol l-1 GSH was toxic for the control and ΔglrA strains especially during vegetative growth, which should be considered in future development of high GSH-producer fungal strains. Importantly, the ΔglrA strain was characterized by increased sensitivity toward a wide spectrum of osmotic, cell wall integrity and antimycotic stress conditions in addition to previously reported temperature and oxidative stress sensitivities. These novel phenotypes underline the distinguished functions of GSH and GSH metabolic enzymes in the stress responses of fungi.

MicroRNA-218 Increases the Sensitivity of Bladder Cancer to Cisplatin by Targeting Glut1

Background/Aims: MicroRNA-218 (miR-218) is down-regulated in many malignancies that have been implicated in the regulation of diverse processes in cancer cells. However, the involvement of miR-218 in chemo-sensitivity to cisplatin and the precise mechanism of this action remained unknown in bladder cancer. Methods: qRT-PCR was used to detect miR-218 and its target Glut1 expression in bladder cancer cell lines T24 and EJ. CCK-8 method was utilized to measure the cell viability. IC 50 was calculated via a probit regression model. Glut1 was detected by western blotting for analysis of potential mechanism. Luciferase reporter assay was utilized to validate Glut1 as a direct target gene of miR-218. The intracellular level of GSH and ROS were determined using a commercial colorimetric assay kit and 2’, 7’-dichlorodihydro-fluorescein diacetate, respectively. Results: Over-expression of miR-218 significantly reduced the rate of glucose uptake and total level of GSH and enhanced the chemo-sensitivity of bladder cancer to cisplatin. Mechanistically, Glut1 was found to be a direct and functional target of miR-218. Up-regulation of Glut1 could restore chemo-resistance in T24 and EJ cells. On the contrary, knockdown of Glut1 could generate a similar effect as up-regulating the expression of miR-218. Conclusions: MiR-218 increases the sensitivity of bladder cancer to cisplatin by targeting Glut1. Restoration of miR-218 and repression of glut1 may provide a potential strategy to restore chemo-sensitivity in bladder cancer.

Genetic and transcriptional study of glutathione metabolism in Oenococcus oeni

Although Oenococcus oeni is the main species that is responsible for malolactic fermentation (MLF), harsh wine conditions can limit its performance. Although several mechanisms underlying the response to stress have been studied in this species, little is known regarding the cellular systems that protect against oxidative stress in other bacteria, such as glutathione (GSH). O. oeni cannot synthesize GSH but contains several genes related to its utilization. In this study, the relative expression (RE) of the seven genes involved in the GSH redox system found in O. oeni PSU-1 (gshR, gpo, three glutaredoxin-like genes and two subunits of an hypothetical transporter) has been measured. The study was performed using three strains, with each exhibiting a different GSH uptake capacity. The strains were grown in a stress-adaptation medium supplemented with 5mM GSH and under different adaptation stress conditions (pH4 and 6% ethanol). The RE showed that only some of these genes, including one for a possible glutaredoxin (OEOE_RS04215) and cydC for a subunit of a putative GSH transporter (OEOE_RS1995), responded to the addition of GSH. The presence of ethanol had a relevant effect on gene expression. Among the studied genes, the one for a NrdH-redoxin (OEOE_RS00645) showed a common response to ethanol in the strains, being over-expressed when grown with GSH. In most cases, the transcriptional changes were more evident for the strain with a higher capacity of GSH uptake. Malolactic performance of the three strains after pre-adaptation was evaluated in wine-like media (12% ethanol and pH3.4). It was observed that the addition of GSH during pre-adaptation growth had a protective role in the cells exposed to low pH and ethanol, resulting in a quicker MLF.

Evidence of Dual Mechanisms of Glutathione Uptake in the Rodent Lens: A Novel Role for Vitreous Humor in Lens Glutathione Homeostasis.

PurposeLens glutathione synthesis knockout (LEGSKO) mouse lenses lack de novo glutathione (GSH) synthesis but still maintain >1 mM GSH. We sought to determine the source of this residual GSH and the mechanism by which it accumulates in the lens.MethodsLevels of GSH, glutathione disulfide (GSSG), and GSH-related compounds were measured in vitro and in vivo using isotope standards and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis.ResultsWild-type (WT) lenses could accumulate GSH from γ-glutamylcysteine and glycine or from intact GSH, but LEGSKO lenses could only accumulate GSH from intact GSH, indicating that LEGSKO lens GSH content is not due to synthesis by a salvage pathway. Uptake of GSH in cultured lenses occurred at the same rate for LEGSKO and WT lenses, could not be inhibited, and occurred primarily through cortical fiber cells. In contrast, uptake of GSH from aqueous humor could be competitively inhibited and showed an enhanced Km in LEGSKO lenses. Mouse vitreous had >1 mM GSH, whereas aqueous had <20 μM GSH. Testing physiologically relevant GSH concentrations for uptake in vivo, we found that both LEGSKO and WT lenses could obtain GSH from the vitreous but not from the aqueous. Vitreous rapidly accumulated GSH from the circulation, and depletion of circulating GSH reduced vitreous but not aqueous GSH.ConclusionsThe above data provide, for the first time, evidence for the existence of dual mechanisms of GSH uptake into the lens, one mechanism being a passive, high-flux transport through the vitreous exposed side of the lens versus an active, carrier-mediated uptake mechanism at the anterior of the lens.

In vivo inhibition of tumor progression by 5 hydroxy-1,4-naphthoquinone (juglone) and 2-(4-hydroxyanilino)-1,4-naphthoquinone (Q7) in combination with ascorbate

The purpose of the study was to obtain further in vivo data of antitumor effects and mechanisms triggered by juglone and Q7 in combination with ascorbate. The study was done using Ehrlich ascites tumor-bearing mice. Treatments were intraperitoneal every 24 h for 9 days. Control group was treated with excipient. Previous tests selected the doses of juglone and Q7 plus ascorbate (1 and 100 mg/kg, respectively). Samples of ascitic fluid were collected to evaluate carbonyl proteins, GSH and activity of antioxidant enzymes such as catalase, superoxide dismutase, glutathione peroxidase and glutathione reductase. Hypoxia inducible factor HIF-1α, GLUT1, proteins driving cell cycle (p53, p16 and cyclin A) and apoptosis (poly-ADP-polymerase PARP, Bax and Bcl-xL) were assessed by western blot. Tumor cells were categorized by the phase of cell cycle using flow cytometry and type of cell death using acridine orange/ethidium bromide. A glucose uptake assessment was performed by liquid scintillation using Ehrlich tumor cells cultured with 14C-deoxyglucose. Treatments caused increased protein carbonylation and activity of antioxidant enzymes and decreased levels of GSH, HIF-1α, GLUT1 and glucose uptake in tumor cells. They also caused increased number of tumor cells in G1, p53 and p16 activation and decreased cyclin A, but only when combined with ascorbate. Apoptosis was induced mostly when treatments were done with ascorbate, causing PARP and Bax cleavage, and increased Bax/Bcl-xL ratio. Juglone and Q7 in combination with ascorbate caused inhibition of tumor progress in vivo by triggering apoptosis and cell cycle arrest associated with oxidative stress, suppression of HIF-1 and uncoupling of glycolytic metabolism.

Thiol-based antioxidants elicit mitochondrial oxidation via respiratory complex III.

Excessive oxidation is widely accepted as a precursor to deleterious cellular function. On the other hand, an awareness of the role of reductive stress as a similar pathological insult is emerging. Here we report early dynamic changes in compartmentalized glutathione (GSH) redox potentials in living cells in response to exogenously supplied thiol-based antioxidants. Noninvasive monitoring of intracellular thiol-disulfide exchange via a genetically encoded biosensor targeted to cytosol and mitochondria revealed unexpectedly rapid oxidation of the mitochondrial matrix in response to GSH ethyl ester or N-acetyl-l-cysteine. Oxidation of the probe occurred within seconds in a concentration-dependent manner and was attenuated with the membrane-permeable ROS scavenger tiron. In contrast, the cytosolic sensor did not respond to similar treatments. Surprisingly, the immediate mitochondrial oxidation was not abrogated by depolarization of mitochondrial membrane potential or inhibition of mitochondrial GSH uptake. After detection of elevated levels of mitochondrial ROS, we systematically inhibited multisubunit protein complexes of the mitochondrial respiratory chain and determined that respiratory complex III is a downstream target of thiol-based compounds. Disabling complex III with myxothiazol completely blocked matrix oxidation induced with GSH ethyl ester or N-acetyl-l-cysteine. Our findings provide new evidence of a functional link between exogenous thiol-containing antioxidants and mitochondrial respiration.

Therapeutic approaches to modulating glutathione levels as a pharmacological strategy in Alzheimer's disease.

Accumulating evidence has suggested the involvement of oxidative stress in the pathogenesis of Alzheimer's disease (AD). The main endogenous antioxidant, glutathione (GSH), has been shown to decline with ageing and in several age-related degenerative diseases, including AD. Potential options for replenishing GSH levels as a therapeutic target to treat these conditions include the administration of GSH itself, and low toxicity forms of the limiting amino acid for GSH synthesis; cysteine. However, passive GSH uptake is limited due to an unfavourable concentration gradient between the plasma and cytosol. Similarly, cysteine prodrugs have demonstrated limited efficacy to elevate depleted GSH levels in several in vivo and in vitro models of disease. It has been suggested that the decline in GSH levels in AD, may be associated with down regulation of GSH homeostasis rather than substrate limitation. Cellular GSH homeostasis is regulated by non-allosteric feedback inhibition exerted by GSH on glutamate cysteine ligase (GCL), which is responsible for the synthesis of the GSH precursor γ-glutamylcysteine (GGC). In conditions involving down regulated GSH homeostasis, GGC serves as a crucialrate-limiting substrate for GSH synthetase, the main enzyme responsible for condensing glycine with GGC to form the final thiol tripeptide, GSH. In this review, we focus on the therapeutic potential of GGC to elevate cellular GSH levels. We also discuss the efficacy of GGC prodrugs which would be taken up and converted by the unregulated GS to GSH, and the administration of modified GSH compounds, such as GSH esters that could potentially overcome the concentration gradient that prohibits passive GSH uptake, in AD.

In vitro and ex vivo uptake of glutathione (GSH) across the intestinal epithelium and fate of oral GSH after in vivo supplementation.

Glutathione (GSH) is the most prevalent low-molecular-weight thiol in mammalian cells and is crucial for antioxidant defense, nutrient metabolism, and the regulation of pathways essential for whole body homeostasis. GSH transport systems have been identified in the membranes of various tissues and organs, including the small intestine. However, little is known regarding GSH transport across intestinal epithelial cells. The current in vitro and ex vivo uptake study of GSH demonstrated that intact GSH can be transported across intestinal epithelial cells, suggesting that GSH uptake is not proton-dependent. It would appear that the initial uptake of GSH into cells is a rapid process. Furthermore, the visualized GSH after 60 min of transport by MALDI-MS imaging showed localization of intact GSH inside the intestinal wall. In vivo study found that ingested (13)C-GSH was rapidly converted to GSSG and accumulated in red blood cells and liver, but was little present in plasma. The ingested GSH has potent nutraceutical benefits for human health to improve oxidative stress and defense in human.

Maintenance of glutathione levels and its importance in epigenetic regulation.

Maintenance of glutathione levels and its importance in epigenetic regulation.

Glutathione Is a Low-Affinity Substrate of the Human Sodium-Dependent Dicarboxylate Transporter

Background/Aims: During a single pass through the kidneys, more than 80% of glutathione (GSH) is excreted, indicating not only glomerular filtration, but also tubular secretion. The first step in tubular secretion is the uptake of a substance across the basolateral membrane of proximal tubule cells by sodium-dependent and -independent transporters. Due to the dicarboxylate-like structure, we postulated that GSH uptake across the basolateral membrane is mediated by the sodium-dependent dicarboxylate transporter 3 (NaDC3). Methods: Tracer uptake and electrophysiologic measurements using a two-electrode voltage clamp device were performed in Xenopus laevis oocytes expressing the human (h)NaDC3. Results: Uptake of succinate, the reference substrate of hNaDC3, was inhibited by GSH in a dose-dependent manner with an IC₅₀ of 1.88 m<smlcap>M</smlcap>. GSH evoked potential-dependent inward currents, which were abolished under sodium-free conditions. At -60 mV, GSH currents showed saturation kinetics with a K_M of 1.65 m<smlcap>M</smlcap>. Conclusion: hNaDC3 present at the basolateral membrane of proximal tubule cells mediates sodium-dependent GSH uptake. The kinetic data show that NaDC3 is a low-affinity GSH transporter.

Molecular identification and cellular localisation of GSH synthesis, uptake, efflux and degradation pathways in the rat ciliary body

The aim of this study is to determine the contribution of the ciliary epithelium to glutathione (GSH) levels in the aqueous by mapping GSH metabolism and transport pathways in the rat ciliary body. Using a combination of molecular and immunohistochemical techniques, we screened and localised enzymes and transporters involved in GSH synthesis, uptake, efflux and degradation. Our findings indicate that both the pigmented epithelial (PE) and the non-pigmented epithelial (NPE) cell layers are capable of accumulating precursor amino acids for GSH synthesis, but only the NPE cells appear to be involved in the direct uptake of precursor amino acids from the stroma. The localisation of GSH efflux transporters to the PE cell and PE-NPE interface indicates that GSH and potentially GSH-S conjugates can be removed from the ciliary epithelium into the stroma, while the location of GSH efflux transporters to the basolateral membrane of the NPE indicates that these cells can mediate GSH secretion into the aqueous. GSH secreted by the ciliary into the aqueous would remain largely intact due to the absence of the GSH degradation enzymes γ-glutamyltranspeptidase (γ-GGT) labelling at the basolateral membrane of the NPE. Therefore, it appears that the ciliary epithelium contains the molecular machinery to mediate GSH secretion into the aqueous.

Characterization of Glutathione Uptake, Synthesis, and Efflux Pathways in the Epithelium and Endothelium of the Rat Cornea

To compare and contrast glutathione (GSH) uptake, synthesis, and efflux pathways in the epithelium and endothelium of the rat cornea. Whole eyes were cryosectioned in an equatorial orientation and sections fixed in either 0.75% paraformaldehyde alone or 0.75% paraformaldehyde plus 0.01% glutaraldehyde. Sections were then labeled with GSH, γ-glutamylcysteine synthetase (γ-GCS), cysteine, xCT, or multidrug resistance-associated proteins (MRP1, 2, 4, and 5 isoforms) antibodies and then with secondary antibodies to visualize labeling patterns. Cornea morphology was visualized using propidium iodide. Reverse transcriptase-polymerase chain reaction was used to determine which of the 3 putative GSH transporters, NaDC3, C-terminal organic anion transporter 1 (OAT1), and/or N-terminal organic anion transporter 3 (OAT3), were present at the transcript level in the cornea. Colocalization of OAT3 and sodium dependent dicarboxylate transporter 3 (NaDC3) was performed by labeling with OAT3 and NaDC3 primary antibodies that were visualized with secondary antibodies and then mounted in 4'6-diamidino-2-phenylindole to visualize cell morphology. Although immunohistochemical labeling showed GSH to be localized throughout the cornea, labeling for cysteine, γ-GCS, xCT, MRP4, and MRP5 was strongest in the epithelium. In contrast, although GSH labeling was strong in the endothelium, xCT and MRP labeling was absent and cysteine and γ-GCS labeling was weak relative to the epithelium. Reverse transcriptase-polymerase chain reaction revealed OAT3 and NaDC3, but not OAT1, to be present at the transcript level. Immunohistochemical labeling showed OAT3 and NaDC3 to be localized to the endothelium but absent from the epithelium. The corneal epithelium and endothelium exhibit differences in GSH uptake, synthesis, and efflux pathways. The corneal epithelium seems to be the region where GSH synthesis and GSH efflux occurs. However, in the endothelium, GSH accumulation is likely to be predominantly by direct uptake of GSH from the aqueous humor.