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(The FASEB Journal. 1999;13:1169-1183.)
© 1999 FASEB

Regulation of hepatic glutathione synthesis: current concepts and controversies

SHELLY C. LU1

USC Liver Disease Research Center, the Division of Gastrointestinal and Liver Diseases, Department of Medicine, University of Southern California School of Medicine, Los Angeles, California 90033, USA

1Correspondence: HMR Rm. 415, Department of Medicine, USC School of Medicine, 2011 Zonal Ave., Los Angeles, CA, 90033, USA. E-mail: shellylu{at}hsc.usc.edu


   ABSTRACT
TOP
 ABSTRACT
CURRENT CONCEPTS: STRUCTURE AND...
 SYNTHESIS OF HEPATIC GSH
 AREAS OF CONTROVERSY
 SUMMARY AND FUTURE DIRECTIONS
 REFERENCES
 
Glutathione (GSH) is an important intracellular peptide with multiple functions ranging from antioxidant defense to modulation of cell proliferation. GSH is synthesized in the cytosol of all mammalian cells in a tightly regulated manner. The major determinants of GSH synthesis are the availability of cysteine, the sulfur amino acid precursor, and the activity of the rate-limiting enzyme, {gamma}-glutamylcysteine synthetase (GCS). In the liver, major factors that determine the availability of cysteine are diet, membrane transport activities of the three sulfur amino acids cysteine, cystine and methionine, and the conversion of methionine to cysteine via the trans-sulfuration pathway. Many conditions alter GSH level via changes in GCS activity and GCS gene expression. These include oxidative stress, activators of Phase II detoxifying enzymes, antioxidants, drug-resistant tumor cell lines, hormones, cell proliferation, and diabetes mellitus. Since the molecular cloning of GCS, much has been learned about the regulation of this enzyme. Both transcriptional and post-transcriptional mechanisms modulate the activity of this critical cellular enzyme.—Lu, S. C. Regulation of hepatic glutathione synthesis: current concepts and controversies.


Key Words: {gamma}-glutamylcysteine synthetase • cysteine availability • detoxification • antioxidant


   CURRENT CONCEPTS: STRUCTURE AND FUNCTIONS OF GSH
TOP
 ABSTRACT
 CURRENT CONCEPTS: STRUCTURE AND...
SYNTHESIS OF HEPATIC GSH
 AREAS OF CONTROVERSY
 SUMMARY AND FUTURE DIRECTIONS
 REFERENCES
 
GLUTATHIONE IS A UBIQUITOUS TRIPEPTIDE,{gamma}-glutamylcysteinyl glycine, found in most plants, microorganisms, and all mammalian tissues. Glutathione exists in the thiol-reduced (GSH)2 and disulfide-oxidized (GSSG) forms (1) . Eukaryotic cells have three major reservoirs of GSH. Almost 90% of cellular GSH are in the cytosol, 10% in the mitochondria, and a small percentage in the endoplasmic reticulum (2 3 4) . In the endoplasmic reticulum, where GSH is implicated in protein disulfide bond formation, the GSH to GSSG ratio is 3:1 (2) . In the cytoplasm and mitochondria, ratios exceed 10:1 (3 , 4) . Cytosolic GSH in the rat liver turns over rapidly with a half-life of 2–3 h. The {gamma}-glutamyl linkage promotes intracellular stability and the sulfhydryl group is required for GSH's functions (Fig. 1 ). The peptide bond linking the amino-terminal glutamate and the cysteine residue of GSH is through the {gamma}-carboxyl group of glutamate rather than the conventional {alpha}-carboxyl group. This unusual arrangement resists degradation by intracellular peptidases and is subject to hydrolysis by only one known enzyme, {gamma}-glutamyltranspeptidase (GGT), which is on the external surfaces of certain cell types (1 , 4) . Furthermore, the carboxyl-terminal glycine moiety of GSH protects the molecule against cleavage by intracellular {gamma}-glutamylcyclotransferase (4) . As a consequence, GSH resists intracellular degradation and is only metabolized extracellularly.



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  		Figure 1. The structure of glutathione, {gamma}-glutamylcysteinyl glycine. The amino-terminal glutamate and cysteine are linked by the {gamma}-carboxyl group of glutamate.

GSH serves several vital functions, including 1) detoxifying electrophiles; 2) maintaining the essential thiol status of proteins by preventing oxidation of -SH groups or by reducing disulfide bonds induced by oxidant stress; 3) scavenging free radicals; 4) providing a reservoir for cysteine; and 5) modulating critical cellular processes such as DNA synthesis, microtubular-related processes, and immune function (1 , 4 5 6) . Some of the functions of GSH are discussed briefly, as this review focuses on the synthesis of GSH.

Detoxifying functions of GSH
Detoxification of xenobiotics or their metabolites is one of the major functions of GSH. These compounds are electrophiles and form conjugates with GSH either spontaneously or enzymatically in reactions catalyzed by GSH S-transferase (1 , 4) . The conjugates formed are usually excreted from the cell and, in the case of hepatocytes, into bile. The metabolism of GSH conjugates begins with cleavage of the {gamma}-glutamyl moiety by GGT, leaving a cysteinyl-glycine conjugate. The cysteinyl-glycine bond is cleaved by dipeptidase, resulting in a cysteinyl conjugate. This is followed by N-acetylation of the cysteine conjugate, forming a mercapturic acid (Fig. 2 ). The metabolism of GSH conjugates to mercapturic acid begins either in the biliary tree, intestine, or kidney, but the formation of the N-acetylcysteine conjugate usually occurs in the kidney (1) . In addition to exogenous compounds, many endogenously formed compounds also follow similar metabolic pathways. Some examples include estradiol-17-ß, leukotrienes, and prostaglandins (4) . Although the majority of the conjugation reactions to GSH result in detoxification of the compound, occasionally the product itself is highly reactive (1) . One such example is the GSH conjugate of dibromoethane (1) . GSH conjugation irreversibly consumes intracellular GSH.



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  		Figure 2. The mercapturic pathway. X, compounds with an electrophilic center can form GSH conjugate in a reaction catalyzed by GSH S-transferase. The {gamma}-glutamyl moiety is then cleaved by {gamma}-glutamyltranspeptidase, liberating the cysteinyl-glycine conjugate. This is further broken down by dipeptidase, resulting in the formation of the cysteinyl conjugate. This is followed by N-acetylation of the cysteine conjugate catalyzed by N-acetylase, resulting in the formation of a mercapturic acid.

Maintenance of essential thiol status
As the dominant nonprotein thiol in mammalian cells, GSH is essential in maintaining the intracellular redox balance and the essential thiol status of proteins (1 , 4) . To achieve this, GSH undergoes thiol-disulfide exchange in a reaction catalyzed by thiol-transferase, as follows:

Since this reaction is a reversible reaction, the equilibrium is determined by the redox state of the cell, which depends on the concentrations of GSH and GSSG (proportional to the log of [GSH]2/[GSSG]) (6) . Normally, cellular GSSG is kept extremely low (<1% of total GSH pool) so that protein mixed disulfide formation may be limited. The thiol-disulfide equilibrium within the cell is known to regulate a diverse number of metabolic processes including enzyme activity, transport activity, and gene expression via alteration of redox-sensitive trans-activating factors (1 , 6 , 7) .

Antioxidant function of GSH
As a consequence of aerobic metabolism, all aerobic organisms are subject to a certain level of physiological oxidative stress. The intermediates that are formed, such as superoxide (O2-·) and hydrogen peroxide, can lead to the further production of toxic oxygen radicals that can cause lipid peroxidation and cell injury. The endogenously produced hydrogen peroxide is reduced by GSH in the presence of selenium-dependent GSH peroxidase (Fig. 3 ). As a result, GSH is oxidized to GSSG, which in turn is reduced back to GSH by GSSG reductase at the expense of NADPH, forming a redox cycle. Either GSH peroxidase or GSH S-transferase can reduce organic peroxides. Hydrogen peroxide can also be reduced by catalase, which is present only in the peroxisome. In the mitochondria, GSH is particularly important because there is no catalase. Recent studies have shown that mitochondrial GSH is critical in defending against both physiologically and pathologically generated oxidative stress (1 , 7 , 8) . A selective reduction in the mitochondrial GSH pool has been reported in rats fed alcohol and may play an important pathogenetic role in the development of the liver disease (see below) (7 , 8) .



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  		Figure 3. Antioxidant function of GSH. Hydrogen peroxide, which is generated as a result of aerobic metabolism, can be metabolized by GSH peroxidase in the cytosol and by catalase in the peroxisome. To maintain the cellular redox equilibrium, GSSG is reduced back to GSH by GSSG reductase at the expense of NADPH, thereby forming a redox cycle. Organic peroxides can be reduced by either GSH peroxidase or GSH S-transferase. Under severe oxidative stress, the ability of the cell to reduce GSSG to GSH may be overcome, leading to accumulation of GSSG within the cytosol. To avoid a shift in the redox equilibrium, GSSG can either be actively transported out of the cell or react with a protein sulfhydryl (PSH) to form a mixed disulfide (PSSG).

Severe oxidative stress may overcome the ability of the cell to reduce GSSG to GSH, leading to accumulation of GSSG within the cytosol. To protect the cell from a shift in the redox equilibrium, GSSG can be actively exported out of the cell or react with a protein sulfhydryl group, leading to the formation of a mixed disulfide. Thus, severe oxidative stress depletes cellular GSH (1 , 4 , 7) .

GSH as cysteine storage and the {gamma}-glutamyl cycle
One of the most important functions of GSH is to store cysteine because cysteine is extremely unstable extracellularly and rapidly auto-oxidizes to cystine, in a process producing potentially toxic oxygen free radicals (4 , 7) . The {gamma}-glutamyl cycle (Fig. 4 ), first described by Meister in the early 1970s (see ref 4) , allows GSH to serve as a continuous source of cysteine. Here, GSH is released from the cell by carrier-mediated transporter(s) (7) and the ecto-enzyme GGT then transfers the {gamma}-glutamyl moiety of GSH to an amino acid (the best acceptor being cystine), forming {gamma}-glutamyl amino acid and cysteinylglycine. The {gamma}-glutamyl amino acid can then be transported back into the cell to complete the cycle. Once inside the cell, the {gamma}-glutamyl amino acid can be further metabolized to release the amino acid and 5-oxoproline, which can be converted to glutamate and used for resynthesis of GSH. Cysteinylglycine is broken down by dipeptidase to generate cysteine and glycine. Cysteine is readily taken up by most if not all cells. Once inside the cell, the majority of cysteine is incorporated into GSH; some is incorporated into protein, depending on the need of the cell, and some is degraded into sulfate and taurine. For most cells, this mechanism provides a continuous source of cysteine. Thus, the {gamma}-glutamyl cycle allows the efficient utilization of GSH as cysteine storage.



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  		Figure 4. GSH as cysteine storage and the {gamma}-glutamyl cycle. One of the most important functions of the {gamma}-glutamyl cycle is to provide a continuous source of cysteine. Cysteine is taken up readily by most cells; once it enters the cell, most of it is incorporated into GSH. Cysteine is also incorporated into newly synthesized proteins and some is broken down into sulfate and taurine. GSH is exported from the cell [by carrier-mediated transporter(s)] and the ecto-enzyme GGT then transfers the {gamma}-glutamyl moiety of GSH to an amino acid (the best acceptor being cystine), forming {gamma}-glutamyl amino acid and cysteinylglycine (cys-gly). The {gamma}-glutamyl amino acid can then be transported back into the cell to complete the cycle. Once inside the cell, the {gamma}-glutamyl amino acid can be further metabolized to release the amino acid and 5-oxoproline, which can be converted to glutamate. Cysteinylglycine is broken down by dipeptidase (DP) to generate cysteine and glycine, which are then transported back into the cell to be reincorporated into GSH.


   SYNTHESIS OF HEPATIC GSH
TOP
 ABSTRACT
 CURRENT CONCEPTS: STRUCTURE AND...
 SYNTHESIS OF HEPATIC GSH
AREAS OF CONTROVERSY
 SUMMARY AND FUTURE DIRECTIONS
 REFERENCES
 
The liver has one of the highest organ contents of GSH and is unique in two aspects of GSH biosynthesis. First, the hepatocyte has the unique ability to convert methionine to cysteine through the transsulfuration pathway (see below); second, the rate of GSH biosynthesis in the hepatocyte is balanced by its rate of export into plasma, bile, and mitochondria via distinct GSH transport systems (7 , 9) . The mitochondrial GSH transporter maintains the GSH pool in the mitochondria, which cannot biosynthesize GSH (7 , 8) . This transporter is selectively impaired in rats fed alcohol and accounts for the fall in mitochondrial GSH in this liver disease model (7 , 8) . Pathways that consume GSH, such as formation of GSH conjugates and mixed disulfides, account for very little of the hepatocyte GSH utilization under normal physiological conditions. The importance of hepatic GSH to the interorgan GSH homeostasis is underscored by the fact that plasma GSH and cysteine levels are largely determined by the sinusoidal efflux of hepatic GSH (9) . Therefore, a better understanding of the regulation of hepatic GSH synthesis is central to understanding interorgan GSH homeostasis under normal and pathological conditions.

GSH is synthesized from precursor amino acids in the cytosol of virtually all cells (1 , 4 , 7) . The synthesis of GSH from its constituent amino acids, L-glutamate, L-cysteine, and L-glycine, involves two ATP-requiring enzymatic steps:

[1] L-glutamate + L-cysteine + ATP -> {gamma}-glutamyl-L-cysteine + ADP + Pi

[2] {gamma}-glutamyl-L-cysteine + L-glycine + ATP -> GSH + ADP + Pi

The first step of GSH biosynthesis is rate-limiting and catalyzed by {gamma}-glutamylcysteine synthetase (GCS), which exhibits an absolute requirement for either Mg2+ or Mn2+. GCS is composed of a heavy (GCS-HS, Mr ~ 73,000) and a light (GCS-LS, Mr ~ 30,000) subunit, which are encoded by different genes in both rat and human (10 11 12 13) . The enzyme may be dissociated under nondenaturing conditions by treatment with dithiothreitol (14) . The heavy subunit obtained after this treatment exhibits all the catalytic activity of the isolated enzyme as well as feedback inhibition by GSH (14) . Although the heavy subunit is active catalytically, it has a higher Km value for glutamate (18.2 vs. 1.4 mM) and a lower Ki value for GSH (1.8 vs. 8.2 mM) compared with the holoenzyme (15) . Thus, the light subunit plays an important regulatory role for the overall function of the enzyme and allows the holoenzyme to be catalytically more efficient and less subject to inhibition by GSH than the heavy subunit alone. The low affinity of the heavy subunit for glutamate and the high feedback inhibition exerted by GSH (Ki value for GSH is about one-third of the normal physiological liver GSH level) suggest that the heavy subunit alone is not likely to be active physiologically. This remains to be proved.

GCS is specific for the glutamyl moiety, and Lys-38 of the heavy subunit may be involved in binding to glutamate (16) . GCS in both rat and human is regulated physiologically by:

1) Feedback competitive inhibition by GSH (17 , 18) . Inhibition by GSH is nonallosteric and involves binding of GSH to the glutamate site and another site on the enzyme. This latter binding appears to involve interaction with the thiol moiety of GSH but not with a methyl group (17) .

2) The availability of its precursor, L-cysteine (1 , 4 , 7) . The apparent Km values of GCS for glutamate and cysteine are 1.8 and 0.1–0.3 mM, respectively, in both rat and human (18 , 19) . The intracellular glutamate concentration is several folds higher than the Km value of GCS for glutamate, but the intracellular cysteine concentration approximates the apparent Km value of GCS for cysteine (7 , 19) . Therefore, the availability of intracellular cysteine and the activity of GCS most significantly influence the rate of GSH synthesis.

The second step in the synthesis of GSH is catalyzed by GSH synthetase. This enzyme has not been studied as extensively as GCS. GSH synthetase purified from rat kidney has an Mr of ~118,000 daltons and is composed of two apparently identical subunits (20) . Studies of the mapping of the substrate binding sites of the enzyme with methyl-substituted and other analogs of the substrates indicate that the regions of the active site that bind glycine and the cysteinyl moiety of {gamma}-glutamylcysteine are highly specific. The L-{gamma}-glutamyl moiety, on the other hand, can be replaced by a variety of analogs, indicating that this binding site is not specific (20) . This enzyme has been cloned recently from the rat kidney (21) . The isolated rat kidney enzyme is known to contain a small amount (2%) of carbohydrate and two asparagine residues (residues 124 and 171) in the amino acid sequence, fitting the requirement for N-linked protein glycosylation, and one serine residue, agreeing with the general pattern for an O-linked N-acetylglucosamine addition site (20 , 21) . However, the significance of glycosylation and the overall regulation of this enzyme remain poorly understood. Early studies in hog and pigeon liver suggested that ADP may play a regulatory role (22 , 23) . GSH synthetase is not subject to feedback inhibition by GSH. One recent study in the yeast Saccharomyces cerevisiae showed that this enzyme is dispensable for growth under both normal and oxidative stress conditions due to an accumulation of {gamma}-glutamylcysteine, which was able to protect against oxidative stress (24) . Overexpression of GSH synthetase failed to increase GSH level whereas overexpression of GCS increased the GSH level, consistent with the fact that GCS is the rate-limiting enzyme of GSH synthesis (24) . However, GSH synthetase deficiency in humans can result in dramatic metabolic consequences because the accumulated {gamma}-glutamylcysteine is converted to 5-oxoproline, which can cause severe metabolic acidosis (21) .

Factors that determine the availability of cysteine
One of the major determinants of the rate of GSH synthesis is the availability of cysteine. Cysteine is normally derived from the diet and protein breakdown, and in the liver from methionine via the transsulfuration pathway (7 , 25) . Cysteine differs from other amino acids because its sulfhydryl form, cysteine, is predominant inside the cell whereas its disulfide form, cystine, is predominant outside the cell. Cysteine readily autoxidizes to cystine in the extracellular fluid; once it enters the cell, cystine is rapidly reduced to cysteine (25) . Therefore, the key factors that regulate the hepatocellular level of cysteine other than diet include membrane transport of cysteine, cystine, and methionine as well as the activity of the transsulfuration pathway (25 26 27) . Although glutamate and glycine are also precursors of GSH, there is no evidence to suggest that their transport influences GSH synthesis since they are synthesized via several metabolic pathways within hepatocytes (25) .

Dietary influence
Studies from Tateishi and co-workers have shown that hepatic GSH level is closely related to nutritional conditions, especially the cysteine content of the diet (28 , 29) . Rat liver normally contains 7–8 µmol GSH/g tissue, mostly in the reduced state. Starvation for 48 h results in a significant fall in liver GSH content to between two-thirds and one-half of the normal levels. On refeeding, the GSH level returns to normal within a few hours (28) . Fasting did not affect the activities of GCS and GSH synthetase. The effects of fasting and refeeding on hepatic GSH level were not affected by pretreatment with actinomycin D or cycloheximide, suggesting that the amount of enzymes involved in GSH synthesis was unaffected (28) . This shows the strong dependency of hepatic GSH level on food intake.

Cysteine transport
Cysteine is transported almost entirely as a neutral amino acid, and in rat hepatocytes is transported mainly by the ASC system (26 , 30 , 31) . This system is Na+ dependent and especially reactive with neutral amino acids with short to intermediate length side chains such as serine and alanine. This system has high stereospecificity (it does not take up D-cysteine), is pH sensitive (a change from 7.4 to 6.5 resulted in suppression of L-cysteine transport by 30%), and is not sensitive to adaptive regulation or insulin and glucagon stimulation under conditions producing these effects for System A (26 , 31) . The ASC system mediates both inward and outward flows of its substrate amino acids and is subject to trans-stimulation (32) : a System ASC amino acid at one side of the membrane stimulates the transport of another System ASC amino acid present at the other side of the membrane. Therefore, the intracellular cysteine concentration depends on the intracellular and extracellular levels of not only cysteine, but also other System ASC amino acids. Higher levels of extracellular cysteine will raise its intracellular level, but higher levels of other System ASC amino acids will inhibit competitively the influx of cysteine (cis-inhibition) and stimulate the efflux of cysteine (trans-stimulation).

Cystine transport
The transport of cystine is distinct from that of cysteine (30 , 33 , 34) . Cystine has four ionizable groups and is present mostly as the tetrapolar ion at neutral pH. However, because the pKs of the two amino groups of cystine are ~ 7.9 and 8.8, a considerable part of cystine (~ 20% of total cystine at pH 7.4) is present as the tripolar ion at physiological pH (25) . The system that transports this anionic form of cystine is System Xc-. Glutamate also exists in anionic form at physiological pH and is the only other significant substrate for System Xc-. This system is Na+ independent and mediates one-to-one exchange of cystine for glutamate (34) . The physiologically significant flow via System Xc- is an entry of cystine accompanied by an exit of glutamate, because cells are usually rich in glutamate, which is also transported by systems other than Xc- and synthesized from its precursor, glutamine, but not in cystine (25) . The driving force of this exchange appears to be a steep concentration gradient of glutamate (outside the fibroblast-1.6 mM, inside-24 mM) (25) . Probably this high intracellular glutamate concentration functions to stimulate the influx of cystine to maintain an adequate balance between cysteine and glutamate inside the cells.

Cystine transport in hepatocytes is also mediated by System Xc-. Under normal physiological conditions, cystine is taken up poorly by hepatocytes (27 , 30 , 35) . However, on culturing rat hepatocytes, the activity of System Xc- emerges after a 12 h lag in response to insulin and dexamethasone. The increase in System Xc- activity is dependent on de novo synthesis of both RNA and protein (27) . This may be an adaptive response of cultured cells since there is no detectable cysteine in the culture medium (almost completely autoxidized). The activity of System Xc- can also be induced by treatment with electrophilic agents and O2 (25 , 35) . In both fibroblasts and isolated hepatocytes, treatment with electrophilic agents depletes intracellular GSH levels, followed by enhanced cystine uptake via System Xc- and restoration of cell GSH levels. When cystine uptake was inhibited by glutamate or homocysteate, restoration of cell GSH level was prevented (35) . This suggests that System Xc- is involved in the cell defense mechanism against an electrophilic attack by facilitating increased synthesis of GSH, which in turn may serve for detoxification of the electrophiles. The exact mechanisms of this induction is unclear at present, and it is not known whether it occurs in intact liver. Although normal hepatocytes do not transport cystine, GSH released from the cells can undergo thiol-disulfide exchange with cystine, liberating cysteine, which is then available to the hepatocytes (36) . This phenomenon has also been observed in HepG2 cells (37) . In fact, this was the sole mechanism for providing cysteine for GSH synthesis in HepG2 cells, which are not able to convert methionine to cysteine (see below) (37) .

Methionine transport
Methionine is transported mainly by System L in hepatocytes, as the increase in the intracellular GSH level by methionine was almost completely inhibited by the presence of 2-aminobicyclo-(2, 2, 1)heptane-2-carboxylic acid (BCH, a model substrate of System L), but was scarcely affected by {alpha}-methylamino isobutyrate (MeAIB, a model substrate of System A) (26 , 38) . System L is Na+ independent and not responsive to either adaptive control or hormonal stimulation. System L also exhibits trans-stimulation. Studies from Kilberg and co-workers (26) of the kinetics of substrate uptake by System L revealed two components to this system. Component I has high affinity, low capacity (estimated Km values <200 µM) whereas component II has low affinity and high capacity (estimated Km values between 2 and 5 mM) (26) . In rat hepatocytes, methionine is transported biphasically by both components of system L (39) . In terms of the relative rates of uptake of these three sulfur amino acids by rat hepatocytes cultured for 24 h, the rate of uptake of cysteine is about 3-fold higher than that of methionine and 13-fold higher than that of cystine (38) .

Transsulfuration pathway
The ability for the liver cell to convert methionine to cysteine is important since the liver is the major site of methionine catabolism and the major storage organ for GSH. That cysteine can be synthesized from methionine was first demonstrated by Tarver and co-workers in 1939 (40) . This pathway is termed the transsulfuration or the cystathionine pathway (Fig. 5 ). This pathway is active and unique to the liver cell, as it is absent or insignificant in other GSH-synthesizing systems either in normal or in transformed tissues (41) . The activity of this pathway in the liver is also markedly impaired or absent in the fetus and newborn infant, cirrhotic patients, and patients with homocystinemia (42 , 43) . Liver cancer cells such as HepG2 and HuH-7 cells have a block in the transsulfuration pathway proximal to the formation of homocysteine. These cells are unable to form GSH from methionine but are able to from homocysteine (37) . The mechanism of this block is unknown.



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  		Figure 5. Hepatic methionine metabolism and GSH synthesis. The transsulfuration pathway converts methionine to cysteine, which is then converted to GSH via the GSH synthetic pathway. Methionine can also be resynthesized from homocysteine. , methionine adenosyltransferase; , transmethylation reactions; , S-adenosylhomocysteine hydrolase; , cystathione ß-synthase; , {gamma}-cystathionase; , {gamma}-glutamylcysteine synthetase; , GSH synthetase; , methionine synthase; , betaine-homocysteine methyltransferase.

Characterization of the transsulfuration pathway in the liver has been achieved mainly through the efforts of Reed and co-workers using isolated hepatocytes (44 , 45) . Methionine is sequentially converted to cysteine via several enzymatic steps. The first step requires ATP and involves the activation of methionine to S-adenosylmethionine in a reaction catalyzed by methionine adenosyltransferase. In contrast to the two GSH-synthesizing enzymes, which have a low Km for ATP (~0.1 mM), the Km of hepatic methionine adenosyltransferase for ATP is high (~2 mM) (46) . Therefore, hypoxic depletion of ATP is more likely to affect GSH synthesis from methionine than cysteine. Subsequent demethylation and the removal of the adenosyl moiety yields homocysteine. Homocysteine condenses with serine to form cystathionine in a reaction catalyzed by cystathionine synthase. Cleavage of cystathionine, catalyzed by cystathionase, then releases free cysteine. In this pathway methionine and homocysteine are readily interconvertible, but the subsequent step, the formation of cystathionine, is irreversible (Fig. 5) . Proparglyglycine is a potent irreversible inhibitor of cystathionase; its use has been instrumental in demonstrating the importance of this pathway in the biosynthesis of GSH (44) .

The main regulatory controls of the transsulfuration pathway appear to center on keeping a fine control on the methionine level. The liver methionine level is relatively insensitive to changes in dietary protein intake (47) . The control appears to be exerted at the level of homocysteine: when methionine is needed, homocysteine is remethylated by methionine synthase or betaine-homocysteine methyltransferase; when methionine is in excess, catabolism of homocysteine via the cystathionine synthase reaction is accelerated (48) .

Hepatic conversion of methionine to S-adenosylmethionine represents the major catabolic pathway for methionine, accounting for nearly half of the daily intake of methionine (43) . Patients with liver cirrhosis often exhibit hypermethioninemia and have impaired methionine clearance (49 , 50) because the activity of hepatic methionine adenosyltransferase is significantly impaired (43) . This impairment has been postulated to be one of the mechanisms of a decreased hepatic GSH level (43) . In support of this, administration of S-adenosylmethionine to patients with liver cirrhosis resulted in increased hepatic GSH level (51) . Preliminary results showing an improved 2-year survival in alcoholic liver disease patients treated with S-adenosylmethionine have also been presented (52) . However, the protective mechanisms of S-adenosylmethionine remain ill defined.

Regulation of GCS
The activity of GCS is the other major determinant of the rate of GSH synthesis. GCS is a heterodimer made up of a catalytic (heavy, 73 kDa) and a regulatory (light, 30 kDa) subunit. Changes in GCS activity can result from regulation at multiple levels affecting only the heavy or both the heavy and light subunits. Transcriptional and post-transcriptional regulation of both subunits have been described. Post-transcriptional regulation includes both mRNA stabilization/destabilization and post-translational modification. Most of the studies examined transcriptional regulation of GCS. Table 1 lists conditions known to alter GCS activity and GCS expression (19 , 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96) . Almost all of the reports published before 1997 focused on the regulation of the heavy subunit. These conditions fall into several major categories, including oxidative stress, drug-resistant tumor cells, hormones, and growth related. For simplicity, Table 1 lists cytokines and GSH-conjugating agents as separate categories, but these may also induce oxidative stress. Inducers of Phase II detoxifying enzymes such as ß-naphthoflavone can also induce oxidative stress. Most of these conditions [with the exception of drug-resistant tumor cell lines, heat shock, heavy metals, glucose, cycloheximide, and transforming growth factor ß1 (TGFß1)] altered GCS expression in liver cells.


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  		Table 1. Conditions known to alter GCS activity and GCS subunit expression

Oxidative stress
Agents ranging from methyl mercury, quinones, hydrogen peroxide, menadione, cytokines such as tumor necrosis factor (TNF), buthionine sulfoximine (BSO), tert-butylhydroquinone, diethyl maleate (DEM), okadaic acid, 4-hydroxy-2-nonenal, and ionizing radiation, which cause oxidative stress in a variety of cells and organs, increased GCS activity and GCS-HS transcription (60 61 62 63 64 65 , 72 , 76 , 81 82 83 , 87 88 89 90 91 92 93) . In addition to increased gene transcription, DEM and 4-hydroxy-2-nonenal treatments have also been found to stabilize the GCS-HS mRNA (72 , 93) . Recently, increased transcription of GCS-LS has also been found in response to DEM, BSO, tert-butylhydroquinone, ß-naphthoflavone, and 4-hydroxy-2-nonenal (63 , 65 , 82 , 83 , 93 , 97) . 4-Hydroxy-2-nonenal also stabilized the GCS-LS mRNA (93) . Thus, many causes of oxidative stress have similar effects on the expression of both GCS subunits.

Recent efforts have focused on elucidating the molecular mechanism(s) of oxidative stress induced increase in GCS expression. The 5'-flanking regions of both human GCS subunits have been cloned and sequenced (63 , 83 , 97 98 99) . Putative nuclear factor kappa B (NF-{kappa}B), Sp-1, activator protein-1 (AP-1), activator protein-2 (AP-2), metal response (MRE), and antioxidant response (ARE)/electrophile-responsive elements (EpRE) have been identified in the promoter of the heavy subunit (63 , 98) . The promoter of the light subunit also contains AP-1, AP-2, MRE and ARE/EpRE elements (83 , 97) . The region from -817 to +45 nucleotide sequence of the human GCS-HS contains AP-1 or AP-1 like-responsive elements that are critical in mediating the effects of cigarette smoke condensate solution, menadione, hydrogen peroxide, TNF, and ionizing radiation on GCS-HS promoter activity (64 , 76 , 77 , 87 , 88) . In menadione and hydrogen peroxide-mediated increase in GCS-HS gene transcription, the most important AP-1 element was found to be located at -269 to -263 (87) . The same element was also found to be responsible for induction of GCS-HS expression in BSO-resistant cell lines (91) . Recent studies from Sekhar et al. and Morales et al. have further confirmed the importance of AP-1 activity in mediating the effect of oxidative stress on GCS-HS transcription (81 , 88) . On the other hand, Mulcahy et al. described a critical distal ARE element (ARE4), located ~3.1kb upstream of the transcriptional start site, which mediated constitutive and ß-naphthoflavone inducible expression in HepG2 cells (63) . ß-Naphthoflavone is known to activate Phase II enzymes via AREs (63) . AREs are also referred to as electrophile-responsive elements (EpRE), which is the more appropriate term since these elements are clearly induced by pro-oxidants (99) . The ARE4 element of the human GCS-HS has a consensus ARE sequence (5'-GTGACTCAGCG-3') and an embedded PMA-responsive element (TRE, underlined), which can bind AP-1 binding proteins (99) . Thus, AP-1 binding proteins may still be involved in activating ARE4. This possibility was recently investigated by Wild and colleagues (99) . They demonstrated that the AP-1 binding sequence of the AREA4-mediated constitutive expression of the GCS-HS promoter, but not ß-naphthoflavone-induced expression, and that other unknown protein complexes are involved (99) . An adjacent ARE (ARE3), located 34 bases downstream from ARE4, also was not required for ß-naphthoflavone's effect on GCS-HS gene expression (99) .

EpRE and AP-1 have also been found to be important in mediating the effect of ß-naphthoflavone on GCS-LS expression (97) . There is a consensus EpRE site located around 300 base pairs upstream from the translational start site and a consensus AP-1 binding site located 33 base pairs upstream of the EpRE. Both are capable of supporting ß-naphthoflavone inducibility, but the EpRE is more potent. Basal expression is influenced by the AP-1 site whereas EpRE is not required for maximal basal expression (97) . Thus, both of the GCS subunits share in common the presence of functional AP-1 and EpRE, which likely mediate the effect these various pro-oxidants have on GCS subunit gene expression. The nature of the trans-activating protein(s) that bind the ARE or EpRE sites remain to be defined.

Although the effect of oxidative stress and inducers of Phase II enzymes on the GCS subunits expression appear to be similar, different signaling pathways are involved at least in the case of 4-hydroxy-2-nonenal (93) . This agent increased the transcription and stabilized the mRNA of both subunits, but de novo protein synthesis was required only for the light subunit (93) . The molecular mechanisms underlying these differences remain to be elucidated.

Drug-resistant tumor cells
Resistance to commonly used alkylating agents or platinum chemotherapeutic drugs is often multifactorial, involving altered drug transport, biotransformation, and enhanced detoxification capacity (53 54 55 56 57 58 59 , 80 , 84) . GSH has been found to be elevated in a number of drug-resistant tumor cell lines and in tumor cells isolated from patients whose tumors are clinically resistant to drug therapy (53) . The increase in cell GSH is a major contributing factor to drug resistance by binding to or reacting with drugs, interacting with reactive oxygen moieties, preventing damage to proteins or DNA, or by participating in DNA repair processes. Although increased GSH in drug-resistant tumor cells has been documented for over 20 years, the mechanism for the increase was elucidated more recently. Increased GCS activity, steady-state GCS-HS mRNA level, and GCS-HS gene transcription have been found (53 54 55 56 57 58 , 80 , 84) . Whether or not GCS-LS is also altered is controversial. MRP (human multidrug resistance protein), a member of the superfamily of ATP binding cassette membrane transporters, is capable of conferring resistance to multiple classes of chemotherapeutic agents (57 , 58) . Several studies have shown coordinated overexpression of GCS-HS and MRP in drug-resistant tumor cell lines, human colorectal tumors, and human lung cancer specimens after platinum drug exposure (57 , 58) . In contrast, no correlation was found between MRP and GCS-LS expression (57) . Godwin et al. found cisplatin-resistant ovarian tumor cell lines exhibited increased levels of GCS-HS and GCS-LS proteins (56) . However, Yao et al. found no difference in the mRNA level of GCS-LS among different ovarian cancer cell lines with varying cisplatin resistance (84) . The reason for the discrepancy is unclear. In the latter work, AP-1 binding was highly associated with cisplatin resistance and increased GCS-HS expression, whereas AP-2 and NF-{kappa}B binding activities were not (84) . Using a cisplatin-resistant lung cancer cell line SBC-3, Tomonari et al. described the 5'-flanking sequence from -192 to +91 (especially -108 to -28 and +34 to +91 bp) to be involved in cisplatin-induced transcriptional up-regulation of GCS-HS (80) . These regions contain two AP-2 sites, an Sp1 site and a GCF binding site, but no AP-1 site (80) . Different mechanisms and cis-acting elements may be involved in mediating transcriptional changes of GCS-HS in different cells.

Hormonal and cell cycle regulation of GCS
Transcriptional regulation
We and others have shown that GCS activity and GCS-HS expression are under hormonal and growth-related regulation (19 , 65 66 67 68 , 85) . Specifically, in primary cultures of rat hepatocytes the activity of GCS and the GCS-HS mRNA level can be induced by insulin or hydrocortisone treatment or by lowering the initial plating cell density (19 , 65 66 67) . The mechanism was an increase in the transcription of GCS-HS without any change in the stability of the GCS-HS mRNA (66) . The effects of the hormones in cultured cells were confirmed using in vivo models such as insulin-deficient diabetic or adrenalectomized rats. Both exhibited lower hepatic GSH levels and GCS activity, which were prevented with hormone replacement (19) . However, glucocorticoid administration to sham-operated animals did not result in increased hepatic GSH or GCS activity (19) . Thus, it appears these hormones are required for maintenance of normal hepatic GCS activity. Abnormality in GCS activity was observed only when there was hormonal deficiency, not excess. In diabetic rats, the steady-state hepatic GCS-HS mRNA level was decreased compared with controls, but normal if treated with insulin (66) . Thus, in these animal models, a lower hepatic GSH and antioxidant defense may play a pathogenetic role in the associated complications. It should be stressed that these hormones may not exert similar effect in other cell types. In fact, a recent study showed that dexamethasone actually had the opposite effect, namely, lowering of cell GSH and GCS activity, in a type II alveolar epithelial cell line (100) . Thus, results obtained from one cell type may not be applicable to another and results obtained from cell lines may not be comparable to those obtained from primary cultures or in vivo.

In the case of cell density, lowering the initial plating density effectively shifts adult rat hepatocytes from the Go to the G1 phase of the cell cycle, analogous to liver regeneration after partial hepatectomy or after cell death (101) . Consistent with findings in cultured rat hepatocytes, a doubling of the hepatic GSH level, GCS activity, and the steady-state mRNA levels of GCS-HS were noted 12 h after two-thirds partial hepatectomy (68) . The increase in GSH under low cell density and after partial hepatectomy was due to both an increase in GCS activity and cysteine availability (67 , 68) . The increase in GSH occurred before the increase in DNA synthesis during liver regeneration (68) . Some speculations can be made about the significance of the increase in hepatic GSH and cysteine levels prior to DNA synthesis. Previous studies involving lymphocytes and fibroblasts have shown that an increased GSH level was associated with an early proliferative response and was essential for the cell to enter the S phase (102 103 104 105) However, other studies did not find a correlation between GSH levels and cell cycle (106 , 107) . The requirement for increased GSH or thiols prior to DNA synthesis may be related to the fact that proliferating cells require increased amounts of pentoses and thiols. DNA synthesis depends absolutely on the formation of pentoses and on their conversion into deoxyribose by ribonucleotide reductase (108) . The activity of this rate-limiting enzyme in DNA synthesis requires reduced glutaredoxin or thioredoxin, which are maintained by GSH with concomitant oxidation to GSSG via glutathione reductase or oxidation of NADPH via thioredoxin reductase, respectively. Alternatively, an increase in the cellular GSH content may change the thiol-redox status of the cell that activates genes essential for G1 to S transition. Future studies examining the molecular mechanism(s) of the cell-cycle effect on GCS-HS gene expression as well as the significance of the increase in hepatic GSH during liver regeneration will address these possibilities.

In contrast to oxidative stress, which increased the transcription of both heavy and light subunits of GCS, hormones and plating hepatocytes at low cell density had no influence on the expression of GCS-LS (65) . Consistent with findings in cultured cells, GCS-LS mRNA level also remained unchanged after partial hepatectomy (68) . Thus, at least in hepatocytes, there may be more GCS-LS available than GCS-HS, so that a change in the GCS-HS alone changed the GCS activity. That the two subunits are under different regulatory controls have been suggested by others. Gipp et al. found no correlation between the steady-state mRNA levels of the two GCS subunits in different tissues (12) . Huang et al. hinted that liver might synthesize more light subunit than other tissues relative to the amount of heavy subunit, although no data were provided (11) . Using HepG2 cells transfected with antisense directed at GCS-LS, we observed a 80% reduction in the amount of GCS-LS mRNA, accompanied by a significant reduction in the protein level, but no change in the cell GSH level (unpublished observation). This supports the notion that there is more GCS-LS in hepatocytes. Kijima et al. used hammerhead ribozymes designed against the two GCS subunits and showed that in Min-6 mouse islet cells transfected with these ribozymes, this strategy resulted in a decrease of the GCS-HS and GCS-LS expression to 27% and 86% of control, respectively (109) . Despite a minimal change in the GCS-LS expression, cell GSH was depleted even more in the cells treated with ribozyme against GCS-LS. The authors speculated that the low GSH level induced the expression of GCS-LS mRNA level and reversed the decrease in GCS-LS expression by the ribozyme. They concluded that both subunits contributed equally to the enzyme activity. However, whether the ribozyme against GCS-LS also affected the expression of GCS-HS is unknown and the effect of the ribozymes on the GCS subunit protein levels were not examined. Finally, there is likely to be cell-specific differences in the differential regulation of the two GCS subunits.

Posttranslational regulation of GCS
GCS is also regulated post-translationally. We and others reported that GSH synthesis and GCS activity were acutely inhibited by hormone-mediated activation of various signal transduction pathways (110 , 111) . These hormones are secreted under stressful conditions, many of which have associated lower hepatic GSH levels (7 , 112 , 113) . The fall in hepatic GSH level can be explained by both an increase in sinusoidal GSH efflux and an inhibition of GSH synthesis (110 , 114) . This may represent the hepatic stress response by increasing the systemic delivery of GSH and cysteine to where they are needed and channeling cysteine away from GSH synthesis to synthesis of stress proteins, many of which are rich in cysteine (115) . Since the liver GSH level is normally very high (5–10 mM), a slight fall in the GSH level would not jeopardize its defense capacity. To explain the inhibition of GCS activity, we showed that GCS-HS is phosphorylated directly by activation of protein kinase A, protein kinase C or Ca2+-calmodulin kinase II (CMK) (116) . The degree of phosphorylation correlated with loss of GCS activity (20% under physiological Mg2+ concentration) and no additional inhibition was observed when GCS was phosphorylated in the presence of all three kinases. In addition, 2-dimensional phosphopeptide mapping studies showed that the same five phosphopeptides were phosphorylated by these kinases, suggesting that these kinases were acting on the same site(s). Phosphorylation of GCS-HS was also demonstrated in cultured hepatocytes after treatment with dibutyryl cAMP (DBcAMP) or phenylephrine using specific antibodies that immunoprecipitated the phosphorylated GCS-HS. There was basal GCS phosphorylation, which increased after treatment with DBcAMP or phenylephrine, suggesting that GCS may be under a basal inhibitory tone. The demonstration of phosphorylation of GCS by using cultured hepatocytes suggests that phosphorylation-dephosphorylation may be an important physiological regulator of GCS. The inhibition of GCS activity after phosphorylation of purified GCS was 20% at 1 mM, a physiologically relevant Mg2+ concentration, which is similar to what we observed when cultured rat hepatocytes or perfused rat livers were treated with DBcAMP, glucagon, or phenylephrine (110 , 116) . Although the degree of inhibition in GCS activity is small, the effect on GSH synthesis and hepatic GSH turnover may be significant. The turnover of normal hepatocyte GSH is estimated at 20%/h or 12 nmol/106 cells/h for a repleted cell, which has 60 nmol GSH/106 cells (36) . This is the amount of GSH that is lost per hour, which is normally balanced by synthesis of GSH. An adult rat liver effluxes 12 µmol of GSH per hour (assuming 108 cells/g and 10 g=liver weight). Thus, even a 20% inhibition in GCS activity would cause a significant reduction of GSH synthesis by the liver (2.4 µmol per hour). Since many pathological and toxic conditions result in sustained increase in cytosolic free Ca2+, which can lead to activation of CMK and phosphorylation of GCS, inhibition of GCS may theoretically further contribute to toxicity under these conditions. The report by Lauterburg and Mitchell that toxic doses of acetaminophen administered in vivo suppressed hepatic GSH synthesis in rats raises this possibility (117) .

Post-translational regulation of the light subunit has not been well studied. In our work on phosphorylation of the heavy subunit, no phosphorylated light subunit was immunoprecipitated when using anti-GCS-HS antibodies at the end of the reaction (116) . Thus, the light subunit is probably not phosphorylated.

Other conditions that alter GCS expression
In addition to oxidative stress, drug-resistance, hormones, and plating density, several other conditions are also known to influence the GCS activity and the steady-state GCS-HS mRNA level. These include treatments with several antioxidants such as butylated hydroxyanisole, GSH-conjugating agents such as DEM and phorone, heavy metals, heat shock, copper deficiency, cycloheximide, TGFß1, high glucose, and nitric oxide (NO) (60 , 70 71 72 73 74 75 , 79 , 86 , 95 , 96) . Several of these treatments such as DEM, phorone and heavy metals can also induce oxidative stress. All of these treatments, with the exception of TGFß1 and high glucose concentration, increased steady-state GCS-HS mRNA level. The mechanism for the increase in most of these treatments was increased transcription. In the case of cycloheximide, the increase in GCS-HS mRNA level occurred by stabilization of the mRNA (79) . Finally, cellular GSH level itself may also modulate GCS expression (see Areas of Controversy).

In addition to hormone (insulin and glucocorticoids) deficiency (19 , 66) , two other treatments that decreased GCS-HS transcription are TGFß1 and high glucose concentration (28 mM) (73 , 86) . Arsalne et al. showed that TGFß1 decreased GCS-HS transcription in a lung epithelial cell line (86) . They speculated whether overexpression of TGFß1 in the alveolar epithelial cells may contribute to low levels of GSH in the alveolar epithelial lining fluid of patients with fibrotic lung disorders (86) . The molecular mechanism remains to be elucidated. Regarding high glucose concentration, Urata et al. showed that GCS-HS mRNA level and transcriptional rates in mouse endothelial cells were increased by treatment with cytokines such as TNF and IL-1ß (73) . The basal GCS-HS mRNA level was decreased and the effect of the cytokines disappeared when cells were grown in media containing high glucose concentration (28 mM). This raises the question of whether the decreased GCS activity and GCS-HS mRNA level observed in diabetic rat livers is due to lack of insulin or hyperglycemia or both. In our work, cultured hepatocytes were grown in medium containing 17.5 mM glucose ± insulin (19 , 66) . Thus, it is likely that insulin was involved in increasing GCS-HS gene transcription in cultured hepatocytes. However, glucose can influence gene expression by itself and the effect of insulin may or may not require glucose (118 , 119) . More work is needed to elucidate the exact mechanism of how insulin and glucose influence GCS-HS gene expression.

The effect of NO on GCS expression is dependent on the cell type (95 , 96) . In rat hepatocytes, NO exerted no influence on the basal GSH level or GCS expression (95) . However, when these cells were treated with interleukin-1 (IL-1), which stimulated the expression of inducible nitric oxide synthase, the GSH level was depleted and GCS expression decreased if NO synthesis was blocked (95) . This would suggest that in response to oxidative stress induced by IL-1, rat hepatocytes up-regulate GCS expression by a mechanism that depends on NO synthesis. In contrast, treatment of rat aortic vascular smooth muscle cells with NO at physiological concentrations resulted in increased cell GSH levels and expression of both GCS subunits (96) . This illustrates the cell type-specific complex interactions between NO and GSH.

Finally, we showed recently that the GCS-HS mRNA level was increased in mutant Eisai Hyperbilirubinemic rats (78) . This along with increased cysteine availability contributed to the high GSH levels found in liver and many other organs (78) . The mechanism for the increased GCS-HS mRNA is unknown and it is speculative whether retained GSH conjugates may have contributed to the increase.


   AREAS OF CONTROVERSY
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 ABSTRACT
 CURRENT CONCEPTS: STRUCTURE AND...
 SYNTHESIS OF HEPATIC GSH
 AREAS OF CONTROVERSY
SUMMARY AND FUTURE DIRECTIONS
 REFERENCES
 
Some of the controversial areas have been discussed, but three others deserve special mention: the role of GSH in apoptosis; the role of NF-{kappa}B in modulation of the GCS-HS gene expression, and the role of cell GSH itself in the modulation of GCS expression.

GSH in apoptosis
Apoptotic cell death is one of the most intensely studied research areas. Whether GSH and/or the intracellular redox state modulate apoptosis is controversial. Numerous studies have shown that apoptotic cells accumulate oxidized proteins and lipids and are presumably exposed to some degree of oxidative stress; antioxidants, including GSH, have been shown to protect against or delay apoptosis triggered by many different stimuli (120 121 122 123 124) . However, apoptosis has been shown to occur under very low oxygen environments indicating that reactive oxygen species are unlikely to be essential mediators under these conditions (121) . Another study showed that the protective effect of thiol agents may be related to down-regulation of Fas expression on T lymphocytes rather than their antioxidative properties (123) . What is clear is that there is accelerated GSH efflux from the cell stimulated to undergo apoptosis. This was shown by several groups in several cell types (mostly lymphocytes) with different proapoptotic stimuli (120 , 121 , 124) . GSH loss occurred prior to chromatin fragmentation or development of oxidative stress, required protease activation, and was blocked by agents known to inhibit GSH transport (121) . Thus, increased export of GSH represents an oxygen-independent mechanism for a fall in the cellular redox state. However, apoptotic degradation of nuclear DNA still occurred with the same kinetics even when GSH efflux was blocked (121) . Similar results were obtained by Ghibelli et al. (120) . GSH loss occurred concomitant with the onset of apoptosis, and modulation of intracellular GSH did not influence the overall extent of apoptosis. These findings seem to dispute the importance of the redox state in modulating apoptosis. What is also unclear is the significance of the GSH efflux during apoptosis and whether this occurs in other cell types. One speculation is that depletion of cell GSH will facilitate apoptosis to occur, provide antioxidants extracellularly, and possibly stimulate phagocytic cells to engulf the apoptotic cell (121) .

Role of NF-{kappa}B
Another area of controversy is the role of NF-{kappa}B in modulation of the GCS-HS gene expression. There is a NF-{kappa}B consensus element ~1kb upstream of the transcriptional start site of the human GCS-HS (63) . Oxidative stress is known to induce NF-{kappa}B activity, and Urata et al. showed that blocking activation of NF-{kappa}B by antisense strategies prevented the cytokine-induced increase in GCS-HS transcription in mouse endothelial cells (73) . We also showed blocking the activation of NF-{kappa}B prevented the increase in the GCS-HS mRNA level induced by BSO and tert-butylhydroquinone in rat hepatocytes (65) . Iwanaga et al. also implicated the NF-{kappa}B consensus element, but not AP1 or ARE, in mediating the effect of ionizing radiation on GCS-HS expression in a human glioblastoma cell line (90) . However, the NF-{kappa}B consensus element was shown not to be involved in mediating the effect of TNF, okadaic acid, and ionizing radiation on GCS-HS expression in HepG2 cells (64 , 81 , 88) . The explanations for these discrepant results are unclear, and whether there is species and/or cell type differences in mediating the effect of these agents at the GCS-HS promoter level remains to be examined.

Role of cell GSH in GCS expression
Several lines of evidence suggest that the expression of GCS-HS is modulated by cell GSH itself. The first is from the work of Urata and colleagues (73) . When mouse endothelial cell GSH was increased by GSH ester, the steady-state mRNA level of GCS-HS increased, regardless of the glucose concentration in the medium (73) . We have also observed that the expression of both subunits of GCS was induced by BSO only after the cell's GSH level fell below 10% (73) . However, this may have been related to oxidative stress. More recently, Yamane et al. showed that an increasing GSH level of ~50% in a rat hepatoma cell line by overexpression of human GCS-HS cDNA resulted in a decrease in the endogenous rat GCS-HS mRNA level by ~50% (92) . Thus, in addition to exerting negative feedback on the GCS enzyme itself, increased GSH may also down-regulate GCS-HS expression. These results conflict directly with those of Urata et al. (73) . It is unclear whether the discrepancy is related to the cell type studied, the method used to increase GSH, or other unknown factors. Whether the light subunit is also affected by an increase in cell GSH is unknown. These unresolved issues deserve further investigation.


   SUMMARY AND FUTURE DIRECTIONS
TOP
 ABSTRACT
 CURRENT CONCEPTS: STRUCTURE AND...
 SYNTHESIS OF HEPATIC GSH
 AREAS OF CONTROVERSY
 SUMMARY AND FUTURE DIRECTIONS
REFERENCES
 
Since the importance of GSH was recognized and the enzymes of GSH synthesis characterized almost half a century ago, we have come a long way in understanding how the synthesis of GSH is regulated. Advances in molecular biology have led to an explosion of knowledge in understanding how the rate-limiting enzyme GCS is regulated at the molecular level. Areas of future investigation should include gaining a better understanding of the role of GSH in cell proliferation, apoptosis, and GCS expression, the pathogenetic importance of impaired GSH synthesis in certain disease states such as diabetes mellitus, cell type-specific differential regulation of the two GCS subunits, post-translational regulation of GCS, and characterization of the signal transduction pathways and trans-activating factors that regulate GCS subunit gene expression. Results from these studies should improve the treatment and prevention of complications that may result from altered GSH synthesis.


   ACKNOWLEDGMENTS
 
This work was supported by National Institutes of Health grant DK45334.


   FOOTNOTES
 
2 Abbreviations: ARE, antioxidant response element; AP-1, activator protein-1; AP-2, activator protein-2; BSO, buthionine sulfoximine; CMK, Ca2+/calmodulin kinase II; DBcAMP, dibutyryl cyclic AMP; DEM, diethyl maleate; EpRE, electrophile response element; GCS, {gamma}-glutamylcysteine synthetase; GCS-HS, GCS heavy subunit; GCS-LS, GCS light subunit; GGT, {gamma}-glutamyltranspeptidase; GSH, reduced glutathione; GSSG, oxidized glutathione; IL-1, interleukin-1; IL-1ß, interleukin-1 beta; MRE, metal response element; MRP, human multidrug resistance protein; NF-{kappa}B, nuclear factor kappa B; NO, nitric oxide; TGF-ß1, transforming growth factor ß1; TNF, tumor necrosis factor.


   REFERENCES
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 ABSTRACT
 CURRENT CONCEPTS: STRUCTURE AND...
 SYNTHESIS OF HEPATIC GSH
 AREAS OF CONTROVERSY
 SUMMARY AND FUTURE DIRECTIONS
 REFERENCES
 

  1. DeLeve, L., Kaplowitz, N. (1991) Glutathione metabolism and its role in hepatotoxicity. Pharmacol. Ther. 52,287-305[Medline]
  2. Hwang, C., Sinsky, A. J., Lodish, H. F. (1992) Oxidized redox state of glutathione in the endoplasmic reticulum. Science 257,1496-1502[Abstract/Free Full Text]
  3. Meredith, M. J., Reed, D. J. (1982) Status of the mitochondrial pool of glutathione in the isolated hepatocyte. J. Biol. Chem. 257,3747-3753[Abstract/Free Full Text]
  4. Meister, A. (1988) Glutathione. Aria, I. M. Jakoby, W. B. Popper, H. Schachter, D. Shafritz, D. A. eds. The Liver: Biology and Pathobiology Second Edition ,401-417 Raven Press New York.
  5. Suthanthiran, M., Anderson, M. E., Sharma, V. K., Meister, A. (1990) Glutathione regulates activation-dependent DNA synthesis in highly purified normal human T lymphocytes stimulated via the CD2 and CD3 antigens. Proc. Natl. Acad. Sci. USA 87,3343-3347[Abstract/Free Full Text]
  6. Hutter, D. E., Till, B. G., Greene, J. J. (1997) Redox state changes in density-dependent regulation of proliferation. Exp. Cell Res. 232,435-438[Medline]
  7. Fernández-Checa, J., Lu, S. C., Ookhtens, M., DeLeve, L., Runnegar, M., Yoshida, H., Saiki, H., Kannan, R., Garcia-Ruiz, C., Kuhlenkamp, J. F., Kaplowitz, N. (1992) The Regulation of Hepatic Glutathione. Tavoloni, N. Berk, P. D. eds. Hepatic Anion Transport and Bile Secretion: Physiology and Pathophysiology ,363-395 Marcel Dekker New York.
  8. Fernández-Checa, J., Kaplowitz, N., Garcia-Ruiz, C., Colell, A., Miranda, M., Mai, M., Ardite, E., Morales, A. (1997) GSH transport in mitochondria: defense against TNF-induced oxidative stress and alcohol-induced defect. Am. J. Physiol. 273,G7-G17[Abstract/Free Full Text]
  9. Ookhtens, M., Kaplowitz, N. (1998) Role of the liver in interorgan homeostasis of glutathione and cyst(e)ine. Sem. Liver Dis. 18,313-329[Medline]
  10. Yan, N., Meister, A. (1990) Amino acid sequence of rat kidney {gamma}-glutamylcysteine synthetase. J. Biol. Chem. 265,1588-1593[Abstract/Free Full Text]
  11. Huang, C., Anderson, M. E., Meister, A. (1993) Amino acid sequence and function of the light subunit of rat kidney {gamma}-glutamylcysteine synthetase. J. Biol. Chem. 268,20578-20583[Abstract/Free Full Text]
  12. Gipp, J. J., Chang, C., Mulcahy, R. T. (1992) Cloning and nucleotide sequence of a full-length cDNA for human liver {gamma}-glutamylcysteine synthetase. Biochem. Biophys. Res. Commun. 185,29-35[Medline]
  13. Gipp, J. J., Bailey, H. H., Mulcahy, R. T. (1995) Cloning and sequence of the cDNA for the light subunit of human liver {gamma}-glutamylcysteine synthetase and relative mRNA levels for heavy and light subunits in human normal tissues. Biochem. Biophys Res. Commun. 206,584-589[Medline]
  14. Seelig, G. F., Simondsen, R. P., Meister, A. (1984) Reversible dissociation of {gamma}-glutamylcysteine synthetase into two subunits. J. Biol. Chem. 259,9345-9347[Abstract/Free Full Text]
  15. Huang, C., Chang, L., Anderson, M. E., Meister, A. (1993) Catalytic and regulatory properties of the heavy subunit of rat kidney {gamma}-glutamylcysteine synthetase. J. Biol. Chem. 268,19675-19680[Abstract/Free Full Text]
  16. Chang, L. S. (1996) The functional involvement of Lys-38 in the heavy subunit of rat kidney {gamma}-glutamylcysteine synthetase: chemical modification and mutagenesis studies. J. Prot. Chem. 15,321-326[Medline]
  17. Huang, C. S., Moore, W. R., Meister, A. (1988) On the active site thiol of {gamma}-glutamylcysteine synthetase: relationship to catalysis, inhibition, and regulation. Proc. Natl. Acad. Sci. USA 85,2464-2468[Abstract/Free Full Text]
  18. Misra, I., Griffith, O. W. (1998) Expression and purification of human {gamma}-glutamylcysteine synthetase. Protein Exp. Purif. 13,268-276[Medline]
  19. Lu, S. C., Ge, J., Kuhlenkamp, J., Kaplowitz, N. (1992) Insulin and glucocorticoid dependence of hepatic {gamma}-glutamylcysteine synthetase and GSH synthesis in the rat: studies in cultured hepatocytes and in vivo. J. Clin. Invest. 90,524-532
  20. Oppenheimer, L., Wellner, V. P., Griffith, O. W., Meister, A. (1979) Glutathione synthetase. Purification from rat kidney and mapping of the substrate binding sites. J. Biol. Chem. 254,5184-5190[Free Full Text]
  21. Huang, C. S., He, W., Meister, A., Anderson, M. E. (1995) Amino acid sequence of rat kidney glutathione synthetase. Proc. Natl. Acad. Sci. USA 92,1232-1236[Abstract/Free Full Text]
  22. Snoke, J. E., Yanan, S., Bloch, K. (1953) Synthesis of glutathione from {gamma}-glutamylcysteine. J. Biol. Chem. 201,573-586[Free Full Text]
  23. Yanari, S., Snoke, J. E., Bloch, K. (1953) Energy source in glutathione biosynthesis. J. Biol. Chem. 201,561-571[Free Full Text]
  24. Grant, C. M., MacIver, F. H., Dawes, I. W. (1997) Glutathione synthetase is dispensable for growth under both normal and oxidative stress conditions in the yeast Saccharomyces cerevisiae due to an accumulation of the dipeptide {gamma}-glutamylcysteine. Mol. Biol. Cell 8,1699-1707[Abstract]
  25. Bannai, S., Ishii, T., Takada, A., Noriko, T. (1989) Regulation of glutathione level by amino acid transport. Taniguchi, N. Higashi, T. Sakamoto, Y. Meister, A. eds. Glutathione Centennial ,407-421 Academic Press San Diego.
  26. Kilberg, M. S. (1982) Amino acid transport in isolated rat hepatocytes. J. Membr. Biol. 69,1-12[Medline]
  27. Takada, A., Bannai, S. (1984) Transport of cystine in isolated rat hepatocytes in primary culture. J. Biol. Chem. 259,2441-2445[Abstract/Free Full Text]
  28. Tateishi, N., Higashi, T., Shinya, S., Naruse, A., Sakamoto, Y. (1974) Studies on the regulation of glutathione level in rat liver. J. Biochem. 75,93-103[Abstract/Free Full Text]
  29. Tateishi, N., Sakamoto, Y. (1983) Nutritional significance of glutathione in rat liver. Sakamoto, Y.et al eds. Glutathione: Storage, Transport and Turnover in Mammals ,13-38 Scientific Socoetu Press Tokyo.
  30. Bannai, S. (1984) Transport of cystine and cysteine in mammalian cells. Biochim. Biophys. Acta 779,289-306[Medline]
  31. Kilberg, M. S., Christensen, H. N., Handlogten, M. E. (1979) Cysteine as a system-specific substrate for transport system ASC in rat hepatocytes. Biochem. Biophys. Res. Commun. 88,744-751[Medline]
  32. Gazzola, G. C., Dall'Asta, V., Guidotti, G. G. (1980) The transport of neutral amino acids in cultured human fibroblasts. J. Biol. Chem. 255,929-396[Abstract/Free Full Text]
  33. Bannai, S., Kitamura, E. (1981) Role of proton dissociation in the transport of cystine and glutamate in human diploid fibroblasts in culture. J. Biol. Chem. 256,5770-5772[Abstract/Free Full Text]
  34. Bannai, S. (1986) Exchange of cystine and glutamate across plasma membrane of human fibroblasts. J. Biol. Chem. 261,2256-2263[Abstract/Free Full Text]
  35. Bannai, S., Takada, A., Kasuga, H., Tateishi, N. (1986) Induction of cystine transport activity in isolated rat hepatocytes by sulfobromophthalein and other electrophilic agents. Hepatology 6,1361-1368[Medline]
  36. Aw, T. Y., Ookhtens, M., Kaplowitz, N. (1986) Mechanism of inhibition of glutathione efflux by methionine from isolated rat hepatocytes. Am. J. Physiol. 251,G354-G361[Abstract/Free Full Text]
  37. Lu, S. C., Huang, H. Y. (1994) Comparison of sulfur amino acid utilization for GSH synthesis between HepG2 cells and cultured rat hepatocytes. Biochem. Pharmacol. 47,859-869[Medline]
  38. Tateishi, N., Sakamoto, Y. (1989) Regulation of glutathione level in primary cultured hepatocytes. Taniguchi, N. Higashi, T. Sakamoto, Y.et al eds. Glutathione Centennial ,57-71 Academic Press San Diego.
  39. Weissbach, L., Handlogten, M. E., Christensen, H. N., Kilberg, M. S. (1982) Evidence for two Na+-independent neutral amino acid transport systems in primary cultures of rat hepatocytes. J. Biol. Chem. 257,12006-12011[Abstract/Free Full Text]
  40. Tarver, H., Schmidt, C. L. A. (1939) The conversion of methionine to cystine: experiments with radioactive sulfur. J. Biol. Chem. 130,67-80[Free Full Text]
  41. Finkelstein, J. D. (1990) Methionine metabolism in mammals. J. Nutr. Biochem. 1,228-237[Medline]
  42. Neuhauser-Berthold, M., Kuhfus, A., Bassler, K. H. (1988) Utilization of glutathione disulfide as cysteine source during long-term parenteral nutrition in the growing rat. Metabolism 37,796-801[Medline]
  43. Mato, J. M., Alvarez, L., Corrales, F. J., Pajares, M. A. (1994) S-adenosylmethionine and the liver. Arias, I. M. Boyer, J. L. Fausto, N. Jakoby, W. B. Schachter, D. A. Shafritz, D. A. eds. The Liver: Biology and Pathobiology Third edition ,461-470 Raven Press New York.
  44. Beatty, P. W., Reed, D. J. (1980) Involvement of the cystathionine pathway in the biosynthesis of glutathione by isolated rat hepatocytes. Arch. Biochem. Biophys. 204,80-87[Medline]
  45. Reed, D. J., Beatty, P. W. (1980) Biosynthesis and regulation of glutathione: toxicological implications. Hodgson, E Bend, J. R. Philpot, R. N. eds. Review of Biochemical Toxicology ,213-241 Elsevier North Holland New York.
  46. Shan, X., Aw, T. Y., Shapira, R., Jones, D. (1989) Oxygen dependence of glutathione synthesis in hepatocytes. Toxicol. Appl. Pharmacol. 101,261-270[Medline]
  47. Finkelstein, J. D., Kyle, W. E., Harris, B. J., Martin, J. J. (1982) Methionine metabolism in mammals: concentration of metabolites in rat tissues. J. Nutr. 112,1011-1018
  48. Cooper, A. J. L. (1983) Biochemistry of sulfur-containing amino acids. Annu. Rev. Biochem. 52,187-222[Medline]
  49. Horowitz, J. H., Rypins, E. B., Henderson, J. M., Heymsfield, S. B., Moffitt, S. D., Bain, R. P., Chawla, R. K., Bleier, J. C., Rudman, D. (1981) Evidence for impairment of transsulfuration pathway in cirrhosis. Gastroenterology 81,668-675[Medline]
  50. Lu, S. C. (1998) Methionine adenosyltransferase and liver disease: it's all about SAM. Gastroenterology 114,403-407[Medline]
  51. Vendemiale, G., Altomare, E., Trizio, T., Le Grazie, C., Di Padova, C., Salerna, M. T., Carrieri, V., Albano, O. (1989) Effect of oral S-adenosyl-L-methionine on hepatic glutathione in patients with liver disease. Scand. J. Gastroenterol. 24,407-415[Medline]
  52. Mato, J. M., Camara, J., Ortiz, P., Rodés, J., and Spanish Collaborative Group for the Study of Alcoholic Liver Cirrhosis. (1997) S-Adenosylmethionine in the treatment of alcoholic liver cirrhosis: results from a multicentric, placebo-controlled, randomized, double-blind clinical trial. Hepatology 26, A489 (abstr.)
  53. Mulcahy, R. T., Bailey, H. H., Gipp, J. J. (1994) Up-regulation of {gamma}-glutamylcysteine synthetase activity in melphalan-resistant human multiple myeloma cells expressing increased glutathione levels. Cancer Chemother. Pharmacol. 34,67-71[Medline]
  54. Mulcahy, R. T., Untawale, S., Gipp, J. J. (1994) Transcriptional up-regulation of {gamma}-glutamylcysteine synthetase gene expression in melphalan-resistant human prostate carcinoma cells. Mol. Pharmacol. 46,909-914[Abstract]
  55. Mulcahy, R. T., Bailey, H. H., Gipp, J. J. (1995) Transfection of complementary DNAs for the heavy and light subunits of human {gamma}-glutamylcysteine synthetase results in an elevation of intracellular glutathione and resistance to melphalan. Cancer Res 55,4771-4775[Abstract/Free Full Text]
  56. Godwin, A. K., Meister, A., O'Dwyer, P. J., Huang, C. S., Hamilton, T. C., Anderson, M. E. (1992) High resistance to cisplatin in human ovarian cancer cell lines is associated with marked increase of glutathione synthesis. Proc. Natl. Acad. Sci. USA 89,3070-3074[Abstract/Free Full Text]
  57. Kuo, M. T., Bao, J. J., Furuichi, M., Yamane, Y., Gomi, A., Savaraj, N., Masuzawa, T., Ishikawa, T. (1998) Frequent coexpression of MRP/GS-X pump and {gamma}-glutamylcysteine synthetase mRNA in drug-resistant cells, untreated tumor cells and normal mouse tissues. Biochem. Pharmacol. 55,605-615[Medline]
  58. Oguri, T., Fujiwara, Y., Isobe, T., Katoh, O., Watanabe, H., Yamakido, M. (1998) Expression of {gamma}-glutamylcysteine synthetase ({gamma}-GCS) and multidrug resistance-associated protein (MRP), but not human canalicular multispecific organic anion transporter (cMOAT), genes correlates with exposure of human lung cancers to platinum drugs. Br. J. Cancer 77,1089-1096[Medline]
  59. Ishikawa, T., Bao, J. J., Yamane, Y., Akimaru, K., Frindrich, K., Wright, C. D., Kuo, M. T. (1996) Coordinated induction of MRP/GS-X pump and {gamma}-glutamylcysteine synthetase by heavy metals in human leukemia cells. J. Biol. Chem. 271,14981-14988[Abstract/Free Full Text]
  60. Woods, J. S., Ellis, M. E. (1995) Up-regulation of glutathione synthesis in rat kidney by methyl mercury. Relationship to mercury-induced oxidative stress. Biochem. Pharmacol. 50,1719-1724[Medline]
  61. Shi, M. M., Kugelman, A., Iwamoto, T., Tian, L., Forman, H. J. (1994) Quinone-induced oxidative stress elevates glutathione and induces {gamma}-glutamylcysteine synthetase activity in rat lung epithelial L2 cells. J. Biol. Chem. 269,26512-26317[Abstract/Free Full Text]
  62. Liu, R. M., Hu, H., Robison, T. W., Forman, H. J. (1996) Differential enhancement of {gamma}-glutamyl transpeptidase and {gamma}-glutamylcysteine synthetase by tert-butylhydroquinone in rat lung epithelial L2 cells. Am. J. Resp. Cell Mol. Biol. 14,186-191[Abstract]
  63. Mulcahy, R. T., Wartman, M. A., Bailey, H. H., Gipp, J. J. (1997) Constitutive and ß-naphthoflavone-induced expression of the human {gamma}-glutamylcysteine synthetase heavy subunit gene is regulated by a distal antioxidant response element/TRE sequence. J. Biol. Chem. 272,7445-7454[Abstract/Free Full Text]
  64. Morales, A., García-Ruiz, C., Miranda, M., Marí, M., Colell, A., Ardite, E., Fernández-Checa, J. C. (1997) Tumor necrosis factor increases hepatocellular glutathione by transcriptional regulation of the heavy subunit chain of {gamma}-glutamylcysteine synthetase. J. Biol. Chem. 272,30371-30379[Abstract/Free Full Text]
  65. Cai, J., Huang, Z., Lu, S. C. (1997) Differential regulation of {gamma}-glutamylcysteine synthetase heavy and light subunit gene expression. Biochem. J. 326,167-172
  66. Cai, J., Sun, W., Lu, S. C. (1995) Hormonal and cell density regulation of hepatic {gamma}-glutamylcysteine synthetase gene expression. Mol. Pharmacol. 48,212-218[Abstract]
  67. Lu, S. C., Ge, J. (1992) Loss of suppression of GSH synthesis under low cell density in primary cultures of rat hepatocytes. Am. J. Physiol. 263,C1181-C1189[Abstract/Free Full Text]
  68. Huang, Z., Li, H., Cai, J., Kuhlenkamp, J., Kaplowitz, N., Lu, S. C. (1998) Changes in glutathione homeostasis during liver regeneration in the rat. Hepatology 27,147-153[Medline]
  69. Liu, R. M., Vasiliou, V., Zhu, H., Duh, J. L., Tabor, M. W., Puga, A., Nebert, D. W., Sainsbury, M., Shertzer, H. G. (1994) Regulation of [Ah]gene battery enzymes and glutathione levels by 5,10-dihydroindeno[1,2-b]indole in mouse hepatoma cell lines. Carcinogenesis 15,2347-2352[Abstract/Free Full Text]
  70. Kondo, T., Yoshida, K., Urata, Y., Goto, S., Gasa, S., Taniguchi, N. (1993) {gamma}-Glutamylcysteine synthetase and active transport of glutathione S-conjugate are responsive to heat shock in K562 erythroid cells. J. Biol. Chem. 268,20366-20372[Abstract/Free Full Text]
  71. Borroz, K. I., Buetler, T. M., Eaton, D. L. (1994) Modulation of {gamma}-glutamylcysteine synthetase large subunit mRNA expression by butylated hydroxyanisole. Toxicol. Appl. Pharmacol. 126,150-155[Medline]
  72. Sekhar, K. R., Long, M., Long, J., Xu, Z. Q., Summar, M. L., Freeman, M. L. (1997) Alteration of transcriptional and post-transcriptional expression of gamma-glutamylcysteine synthetase by diethyl maleate. Radiation Res 147,592-297[Medline]
  73. Urata, Y., Yamamoto, H., Goto, S., Tsushima, H., Akazawa, S., Yamashita, S., Nagataki, S., Kondo, T. (1996) Long exposure to high glucose concentration impairs the responsive expression of {gamma}-glutamylcysteine synthetase by interleukin-1ß and tumor necrosis factor-{alpha} in mouse endothelial cells. J. Biol. Chem. 271,15146-15152[Abstract/Free Full Text]
  74. Kuo, P. C., Abe, K. Y., Schroeder, R. A. (1996) Interleukin-1-induced nitric oxide production modulates glutathione synthesis in cultured rat hepatocytes. Am. J. Physiol. 271,C851-C862[Abstract/Free Full Text]
  75. Chen, Y., Saari, J. T., Kang, Y. J. (1995) Expression of {gamma}-glutamylcysteine synthetase in the liver of copper-deficient rats. Proc. Soc. Exp. Biol. Med. 210,102-106[Medline]
  76. Rahman, I., Bel, A., Mulier, B., Lawson, M. F., Harrison, D. J., MacNee, W., Smith, C. A. (1996) Transcriptional regulation of {gamma}-glutamylcysteine synthetase heavy subunit by oxidants in human alveolar epithelial cells. Biochem. Biophys. Res. Commun. 229,832-837[Medline]
  77. Rahman, I., Smith, C. A., Lawson, M. F., Harrison, D. J., MacNee, W. (1996) Induction of {gamma}-glutamylcysteine synthetase by cigarette smoke is associated with AP-1 in human alveolar epithelial cells. FEBS Lett 396,21-25[Medline]
  78. Lu, S. C., Cai, J., Kuhlenkamp, J., Sun, W., Takikawa, H., Takenaka, O., Horie, T., Yi, J., Kaplowitz, N. (1996) Alterations in glutathione homeostasis in mutant Eisai hyperbilirubinemic rats. Hepatology 24,253-258[Medline]
  79. Gomi, A., Masuzawa, T., Ishikawa, T., Kuo, M. T. (1997) Post-transcriptional regulation of MRP/GS-Xpump and {gamma}-glutamylcysteine synthetase expression by 1-(4-amino-2-methyl-5-pyrimidinyl) methyl-3-(2-chloroethyl)-3-nitrosourea and by cycloheximide in human glioma cells. Biochem. Biophys. Res. Commun. 239,51-56[Medline]
  80. Tomonari, A., Nishio, K., Kurokawa, H., Fukumoto, H., Fukuoka, K., Iwamoto, Y., Usuda, J., Suzuki, T., Itakura, M., Saijo, N. (1997) Proximal 5-flanking sequence of the human {gamma}-glutamylcysteine synthetase heavy subunit gene is involved in cisplatin-induced transcriptional up-regulation in a lung cancer cell line SBC-3. Biochem. Biophys. Res. Commun. 236,616-621[Medline]
  81. Sekhar, K. R., Meredith, M. J., Kerr, L. D., Soltaninassab, S. R., Spitz, D. R., Xu, Z. Q., Freeman, M. L. (1997) Expression of glutathione and {gamma}-glutamylcysteine synthetase mRNA is Jun dependent. Biochem. Biophys. Res. Commun. 234,588-593[Medline]
  82. Tian, L., Shi, M. M., Forman, H. J. (1997) Increased transcription of the regulatory subunit of {gamma}-glutamylcysteine synthetase in rat lung epithelial L2 cells exposed to oxidative stress or glutathione depletion. Arch. Biochem. Biophys. 342,126-133[Medline]
  83. Galloway, D. C., Blake, D. G., Shepherd, A. G., McLellan, L. I. (1997) Regulation of human {gamma}-glutamylcysteine synthetase: co-ordinate induction of the catalytic and regulatory subunits in HepG2 cells. Biochem J 328,99-104
  84. Yao, K. S., Godwin, A. K., Johnson, S. W., Ozols, R. F., O'Dwyer, P. J., Hamilton, T. C. (1995) Evidence for altered regulation of {gamma}-glutamylcysteine synthetase gene expression among cisplatin-sensitive and cisplatin-resistant human ovarian cancer cell lines. Cancer Res 55,4367-4374[Abstract/Free Full Text]
  85. Teshigawara, M., Matsumoto, S., Tsuboi, S., Ohmori, S. (1995) Changes in levels of glutathione and related compounds and activities of glutathione-related enzymes during liver regeneration. Res. Exp. Med. 195,55-60[Medline]
  86. Arsalane, K., Dubois, C. M., Muanza, T., Bégin, R., Boudreau, F., Asselin, C., Cantin, A. M. (1997) Transforming growth factor-ß1 is a potent inhibitor of glutathione synthesis in the lung epithelial cell line A549: transcriptional effect on the GSH rate-limiting enzyme, {gamma}-glutamylcysteine synthetase. Am. J. Resp. Cell. Mol. Biol. 17,599-607[Abstract/Free Full Text]
  87. Rahman, I., Smith, C. A. D., Antonicelli, F., MacNee, W. (1998) Characterization of {gamma}-glutamylcysteine synthetase-heavy subunit promoter: a critical role for AP-1. FEBS Lett 427,129-133[Medline]
  88. Morales, A., Miranda, M., Sanchez-Reyes, A., Colell, A., Biete, A., Fernández-Checa, J. C. (1998) Transcriptional regulation of the heavy subunit chain of {gamma}-glutamylcysteine synthetase by ionizing radiation. FEBS Lett 427,15-20[Medline]
  89. Shimizu, T., Iwanaga, M., Yasunaga, A., Urata, Y., Goto, S., Shibata, S., Kondo, T. (1998) Protective role of glutathione synthesis on radiation-induced DNA damage in rabbit brain. Cell. Mol. Neurobiol. 18,299-310[Medline]
  90. Iwanaga, M., Mori, K., Iida, T., Urata, Y., Matsuo, T., Wasunaga, A., Shibata, S., Kondo, T. (1998) Nuclear factor kappa B dependent induction of gamma-glutamylcysteine synthetase by ionizing radiation in T98G human glioblastoma cells. Free Rad. Biol. Med. 24,1256-1268[Medline]
  91. Tanaka, T., Uchiumi, T., Kohno, K., Tomonari, A., Nishio, K., Saijo, N., Kondo, T., Kuwano, M. (1998) Glutathione homeostasis in human hepatic cells: overexpression of {gamma}-glutamylcysteine synthetase gene in cell lines resistant to buthionine sulfoximine, an inhibitor of glutathione synthesis. Biochem. Biophys. Res. Commun. 246,398-403[Medline]
  92. Yamane, Y., Furuichi, M., Song, R., Van, N. T., Mulcahy, R. T., Ishikawa, T., Kuo, M. T. (1998) Expression of multidrug resistance protein/GS-X pump and {gamma}-glutamylcysteine synthetase genes is regulated by oxidative stress. J. Biol. Chem. 273,31075-31085[Abstract/Free Full Text]
  93. Liu, R. M., Gao, L., Choi, J., Forman, H. J. (1998) {gamma}-Glutamylcysteine synthetase: mRNA stabilization and independent subunit transcription by 4-hydroxy-2-nonenal. Am. J. Physiol. 275,L861-L869[Abstract/Free Full Text]
  94. Tipnis, S. R., Blake, D. G., Shepherd, G., McLellan, L. I. (1999) Overexpression of the regulatory subunit of {gamma}-glutamylcysteine synthetase in HeLa cells increases {gamma}-glutamylcysteine synthetase activity and confers drug resistance. Biochem. J. 337,559-566
  95. Kuo, P. C., Abe, K. Y., Schroeder, R. A. (1996) Interleukin-1-induced nitric oxide production modulates glutathione synthesis in cultured rat hepatocytes. Am. J. Physiol. 40,C851-C862
  96. Moellering, D., McAndrew, J., Patel, R. P., Cornwell, T., Lincoln, T., Cao, X., Messina, J. L., Forman, H. J., Jo, H., Darley-Usmar, V. M. (1998) Nitric oxide-dependent induction of glutathione synthesis through increased expression of {gamma}-glutamylcysteine synthetase. Arch. Biochem. Biophys. 358,74-82[Medline]
  97. Moinova, H. R., Mulcahy, R. T. (1998) An electrophile responsive element (EpRE) regulates ß-naphthoflavone induction of the human {gamma}-glutamylcysteine synthetase regulatory subunit gene. J. Biol. Chem. 273,14683-14689[Abstract/Free Full Text]
  98. Mulcahy, R. T., Gipp, J. J. (1995) Identification of a putative antioxidant response element in the 5'-flanking region of the human {gamma}-glutamylcysteine synthetase heavy subunit gene. Biochem. Biophys. Res. Commun. 209,227-233[Medline]
  99. Wild, A. C., Gipp, J. J., Mulcahy, R. T. (1998) Overlapping antioxidant response element and PMA response element sequences mediate basal and ß-naphthoflavone-induced expression of the human {gamma}-glutamylcysteine synthetase catalytic subunit gene. Biochem. J. 332,373-381
  100. Rahman, I., Bel, A., Mulier, B., Donaldson, K., MacNee, W. (1998) Differential regulation of glutathione by oxidants and dexamethasone in alveolar epithelial cells. Am. J. Physiol. 275,L80-L86[Abstract/Free Full Text]
  101. Nakamura, T., Yoshimoto, K., Nakayama, Y., Tomita, Y., Ichihara, A. (1993) Reciprocal modulation of growth and differentiated functions of mature rat hepatocytes in primary culture by cell-cell contact and cell membranes. Proc. Natl. Acad. Sci. USA 80,7229-7233
  102. Shaw, J. P., Chou, I. (1986) Elevation of intracellular glutathione content associated with mitogenic stimulation of quiescent fibroblasts. J. Cell. Physiol. 129,193-198[Medline]
  103. Messina, J. P., Lawrence, D. A. (1989) Cell cycle progression of glutathione-depleted human peripheral blood mononuclear cells is inhibited at S phase. J. Immunol. 143,1974-1981[Abstract]
  104. Iwata, S., Hori, T., Sato, N., Ueda-Taniguchi, Y., Yamabe, T., Nakamura, H., Masutani, H., Yodoi, Y. (1994) Thiol-mediated redox regulation of lymphocyte proliferation. Possible involvement of adult T cell leukemia-derived factor and glutathione in transferrin receptor expression. J. Immunol. 152,5633-5642[Abstract]
  105. Poot, M., Teubert, H., Rabinovitch, P. S., Kavanagh, T. J. (1995) De novo synthesis of glutathione is required for both entry into and progression through the cell cycle. J. Cell. Physiol. 163,555-560[Medline]
  106. Harris, J. W., Teng, S. S. (1972) Sulfhydryl groups during the S phase: comparison of cells from G1, plateau-phase G1, and Go. J. Cell. Physiol. 81,91-96
  107. Lee, F. Y. F., Siemann, D. W., Allalunis-Turner, M. J., Keng, P. C. (1988) Glutathione contents in human and rodent tumor cells in various phases of the cell cycle. Cancer Res 48,3661-3665[Abstract/Free Full Text]
  108. Holmgren, A. (1981) Regulation of ribonucleotide reductase. Curr. Top. Cell. Regul. 19,47-76[Medline]
  109. Kijima, H., Tsuchida, T., Kondo, H., Iida, T., Oshika, Y., Nakamura, M., Scanlon, K. J., Kondo, T., Tamaoki, N. (1998) Hammerhead ribozymes against {gamma}-glutamylcysteine synthetase mRNA down-regulate intracellular glutathione concentration of mouse islet cells. Biochem. Biophys. Res. Commun. 247,697-703[Medline]
  110. Lu, S. C., Kuhlenkamp, J., Garcia-Ruiz, C., Kaplowitz, N. (1991) Hormone-mediated down-regulation of hepatic GSH synthesis in the rat. J. Clin. Invest. 88,260-269
  111. Estrela, J. M., Gil, F., Vila, J. M., Viña, J. (1988) {alpha}-Adrenergic modulation of glutathione metabolism in isolated rat hepatocytes. Am. J. Physiol. 255,E801-E805[Abstract/Free Full Text]
  112. Hirota, M., Inoue, M., Ando, Y., Hirayama, K., Morino, Y., Sakamoto, K., Mori, K., Agaki, M. (1989) Inhibition of stress-induced gastric injury in the rat by glutathione. Gastroenterology 97,853-859[Medline]
  113. Lew, H., Pyke, S., Quintanilha, A. (1985) Changes in the glutathione status of plasma, liver and muscle following exhaustive exercise in rats. Fed. Eur. Biochem. Soc. 185,262-266
  114. Lu, S. C., Garcia-Ruiz, C., Kuhlenkamp, J., Ookhtens, M., Salas-Prato, M., Kaplowitz, N. (1990) Hormonal regulation of GSH efflux. J. Biol. Chem. 265,16088-16095[Abstract/Free Full Text]
  115. Hidalgo, J., Garvey, J. S., Armario, A. (1990) On the metallothionein, glutathione and cysteine relationship in rat liver. J. Pharm. Exp. Ther. 255,554-564[Abstract/Free Full Text]
  116. Sun, W., Huang, Z., Lu, S. C. (1996) Regulation of {gamma}-glutamylcysteine synthetase by protein phosphorylation. Biochem. J. 320,321-328
  117. Lauterburg, B. H., Mitchell, J. R. (1982) Toxic doses of acetaminophen suppress hepatic glutathione synthesis in rats. Hepatology 2,8-12[Medline]
  118. Lefrançois-Martinez, A., Diaz-Guerra, M. J., Vallet, V., Kahn, A., Antoine, B. (1994) Glucose-dependent regulation of the L-pyruvate kinase gene in a hepatoma cell line is independent of insulin and cyclic AMP. FASEB J 8,89-96[Abstract]
  119. Vaulont, S., Kahn, A. (1994) Transcriptional control of metabolic regulation genes by carbohydrates. FASEB J 8,28-35[Abstract]
  120. Ghibelli, L., Coppola, S., Rotilio, G., Lafavia, E., Maresca, V., Ciriolo, M. R. (1995) Non-oxidative loss of glutathione in apoptosis via GSH extrusion. Biochem. Biophys. Res. Commun. 216,313-320[Medline]
  121. Van den Dobbelsteen, D. J., Nobel, C. S. I., Schlegel, J., Cotgreave, I. A., Orrenius, S., Slater, A. F. G. (1996) Rapid and specific efflux of reduced glutathione during apoptosis induced by anti-Fas/APO-1 antibody. J. Biol. Chem. 271,15420-15427[Abstract/Free Full Text]
  122. Kohno, T., Yamada, Y., Hata, T., Mori, H., Yamamura, M., Tomonaga, M., Urata, Y., Goto, S., Kondo, T. (1996) Relation of oxidative stress and glutathione synthesis to CD95 (Fas/APO-1)-mediated apoptosis of adult T cell leukemia cells. J. Immunol. 156,4722-4728[Abstract]
  123. Delneste, Y., Jeannin, P., Sebille, E., Aubry, J. P., Bonnefoy, J. Y. (1996) Thiols prevent Fas (CD95)-mediated T cell apoptosis by down-regulating membrane Fas expression. Eur. J. Immunol. 26,2981-2988[Medline]
  124. Cotgreave, I. A., Gerdes, R. C. (1998) Recent trends in glutathione biochemistry - glutathione-protein interactions: a molecular link between oxidative stress and cell proliferation?. Biochem. Biophys. Res. Commun. 242,1-9[Medline]



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Home page
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J. Jun, V. Savransky, A. Nanayakkara, S. Bevans, J. Li, P. L. Smith, and V. Y. Polotsky
Intermittent hypoxia has organ-specific effects on oxidative stress
Am J Physiol Regulatory Integrative Comp Physiol, October 1, 2008; 295(4): R1274 - R1281.
[Abstract] [Full Text] [PDF]

Home page
 Mol. Pharmacol.Home page
Y. Fu, S. Zheng, S. C. Lu, and A. Chen
Epigallocatechin-3-gallate Inhibits Growth of Activated Hepatic Stellate Cells by Enhancing the Capacity of Glutathione Synthesis
Mol. Pharmacol., May 1, 2008; 73(5): 1465 - 1473.
[Abstract] [Full Text] [PDF]

Home page
 haematolHome page
A. I. Sergueeva, G. Y. Miasnikova, D. J. Okhotin, A. A. Levina, Z. Debebe, T. Ammosova, X. Niu, E. A. Romanova, S. Nekhai, P. M. DiBello, et al.
Elevated homocysteine, glutathione and cysteinylglycine concentrations in patients homozygous for the Chuvash polycythemia VHL mutation
Haematologica, February 1, 2008; 93(2): 279 - 282.
[Abstract] [Full Text] [PDF]

Home page
 J. Nutr.Home page
R. W. Friesen, E. M. Novak, D. Hasman, and S. M. Innis
Relationship of Dimethylglycine, Choline, and Betaine with Oxoproline in Plasma of Pregnant Women and Their Newborn Infants
J. Nutr., December 1, 2007; 137(12): 2641 - 2646.
[Abstract] [Full Text] [PDF]

Home page
 British Journal of Diabetes & Vascular DiseaseHome page
C. Livingstone and J. Davis
Review: Targeting therapeutics against glutathione depletion in diabetes and its complications
The British Journal of Diabetes & Vascular Disease, November 1, 2007; 7(6): 258 - 265.
[Abstract] [PDF]

Home page
 J. Biol. Chem.Home page
J. Panee, Z. R. Stoytcheva, W. Liu, and M. J. Berry
Selenoprotein H Is a Redox-sensing High Mobility Group Family DNA-binding Protein That Up-regulates Genes Involved in Glutathione Synthesis and Phase II Detoxification
J. Biol. Chem., August 17, 2007; 282(33): 23759 - 23765.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
S. Akai, H. Hosomi, K. Minami, K. Tsuneyama, M. Katoh, M. Nakajima, and T. Yokoi
Knock Down of {gamma}-Glutamylcysteine Synthetase in Rat Causes Acetaminophen-induced Hepatotoxicity
J. Biol. Chem., August 17, 2007; 282(33): 23996 - 24003.
[Abstract] [Full Text] [PDF]

Home page
 Toxicol Ind HealthHome page
M. Koyuturk, R. Yanardag, S. Bolkent, and S. Tunali
The potential role of combined anti-oxidants against cadmium toxicity on liver of rats
Toxicology and Industrial Health, August 1, 2007; 23(7): 393 - 401.
[Abstract] [PDF]

Home page
 J. Biol. Chem.Home page
J. Markovic, C. Borras, A. Ortega, J. Sastre, J. Vina, and F. V. Pallardo
Glutathione Is Recruited into the Nucleus in Early Phases of Cell Proliferation
J. Biol. Chem., July 13, 2007; 282(28): 20416 - 20424.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Clin. Nutr.Home page
V. Purohit, M. F Abdelmalek, S. Barve, N. J Benevenga, C. H Halsted, N. Kaplowitz, K. K Kharbanda, Q.-Y. Liu, S. C Lu, C. J McClain, et al.
Role of S-adenosylmethionine, folate, and betaine in the treatment of alcoholic liver disease: summary of a symposium
Am. J. Clinical Nutrition, July 1, 2007; 86(1): 14 - 24.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Physiol. Gastrointest. Liver Physiol.Home page
A. Dey, A. A. Caro, and A. I. Cederbaum
S-adenosyl methionine protects ob/ob mice from CYP2E1-mediated liver injury
Am J Physiol Gastrointest Liver Physiol, July 1, 2007; 293(1): G91 - G103.
[Abstract] [Full Text] [PDF]

Home page
 Toxicol Ind HealthHome page
C.S. Kim, I.A. Ross, R.L. Sprando, W.D. Johnson, S.C. Sahu, T.J. Flynn, P.L. Wiesenfeld, T.F.X. Collins, R.K. O'Neilll, and P. Sapienza
Distribution of androstenedione and its effects on total free fatty acids in pregnant rats
Toxicology and Industrial Health, March 1, 2007; 23(2): 65 - 74.
[Abstract] [PDF]

Home page
 J. Biol. Chem.Home page
D. Botta, S. Shi, C. C. White, M. J. Dabrowski, C. L. Keener, S. L. Srinouanprachanh, F. M. Farin, C. B. Ware, W. C. Ladiges, R. H. Pierce, et al.
Acetaminophen-induced Liver Injury Is Attenuated in Male Glutamate-cysteine Ligase Transgenic Mice
J. Biol. Chem., September 29, 2006; 281(39): 28865 - 28875.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
K. V. Ramana, A. Bhatnagar, S. Srivastava, U. C. Yadav, S. Awasthi, Y. C. Awasthi, and S. K. Srivastava
Mitogenic Responses of Vascular Smooth Muscle Cells to Lipid Peroxidation-derived Aldehyde 4-Hydroxy-trans-2-nonenal (HNE): ROLE OF ALDOSE REDUCTASE-CATALYZED REDUCTION OF THE HNE-GLUTATHIONE CONJUGATES IN REGULATING CELL GROWTH
J. Biol. Chem., June 30, 2006; 281(26): 17652 - 17660.
[Abstract] [Full Text] [PDF]

Home page
 JCOHome page
P. Yang, J. O. Ebbert, Z. Sun, and R. M. Weinshilboum
Role of the Glutathione Metabolic Pathway in Lung Cancer Treatment and Prognosis: A Review
J. Clin. Oncol., April 10, 2006; 24(11): 1761 - 1769.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
J. Bai and A. I. Cederbaum
Overexpression of CYP2E1 in Mitochondria Sensitizes HepG2 Cells to the Toxicity Caused by Depletion of Glutathione
J. Biol. Chem., February 24, 2006; 281(8): 5128 - 5136.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
J. Hirao, S. Arakawa, K. Watanabe, K. Ito, and T. Furukawa
Effects of Restricted Feeding on Daily Fluctuations of Hepatic Functions Including P450 Monooxygenase Activities in Rats
J. Biol. Chem., February 10, 2006; 281(6): 3165 - 3171.
[Abstract] [Full Text] [PDF]

Home page
 MutagenesisHome page
R.M. Samarth, M. Panwar, M. Kumar, and A. Kumar
Protective effects of Mentha piperita Linn on benzo[a]pyrene-induced lung carcinogenicity and mutagenicity in Swiss albino mice
Mutagenesis, January 1, 2006; 21(1): 61 - 66.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
Y. Chen, H. G. Shertzer, S. N. Schneider, D. W. Nebert, and T. P. Dalton
Glutamate Cysteine Ligase Catalysis: DEPENDENCE ON ATP AND MODIFIER SUBUNIT FOR REGULATION OF TISSUE GLUTATHIONE LEVELS
J. Biol. Chem., October 7, 2005; 280(40): 33766 - 33774.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Clin. Nutr.Home page
C. A Hobbs, M. A Cleves, W. Zhao, S. Melnyk, and S J. James
Congenital heart defects and maternal biomarkers of oxidative stress
Am. J. Clinical Nutrition, September 1, 2005; 82(3): 598 - 604.
[Abstract] [Full Text] [PDF]

Home page
 Mol. Cell. Biol.Home page
H. Yang, N. Magilnick, C. Lee, D. Kalmaz, X. Ou, J. Y. Chan, and S. C. Lu
Nrf1 and Nrf2 Regulate Rat Glutamate-Cysteine Ligase Catalytic Subunit Transcription Indirectly via NF-{kappa}B and AP-1
Mol. Cell. Biol., July 15, 2005; 25(14): 5933 - 5946.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
C. Berasain, E. R. Garcia-Trevijano, J. Castillo, E. Erroba, M. Santamaria, D. C. Lee, J. Prieto, and M. A. Avila
Novel Role for Amphiregulin in Protection from Liver Injury
J. Biol. Chem., May 13, 2005; 280(19): 19012 - 19020.
[Abstract] [Full Text] [PDF]

Home page
 IOVSHome page
D. Gherghel, H. R. Griffiths, E. J. Hilton, I. A. Cunliffe, and S. L. Hosking
Systemic Reduction in Glutathione Levels Occurs in Patients with Primary Open-Angle Glaucoma
Invest. Ophthalmol. Vis. Sci., March 1, 2005; 46(3): 877 - 883.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
M. Benlloch, A. Ortega, P. Ferrer, R. Segarra, E. Obrador, M. Asensi, J. Carretero, and J. M. Estrela
Acceleration of Glutathione Efflux and Inhibition of {gamma}-Glutamyltranspeptidase Sensitize Metastatic B16 Melanoma Cells to Endothelium-induced Cytotoxicity
J. Biol. Chem., February 25, 2005; 280(8): 6950 - 6959.
[Abstract] [Full Text] [PDF]

Home page
 Physiol. GenomicsHome page
A. D. Armendariz, M. Gonzalez, A. V. Loguinov, and C. D. Vulpe
Gene expression profiling in chronic copper overload reveals upregulation of Prnp and App
Physiol Genomics, December 15, 2004; 20(1): 45 - 54.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Clin. Nutr.Home page
S J. James, P. Cutler, S. Melnyk, S. Jernigan, L. Janak, D. W Gaylor, and J. A Neubrander
Metabolic biomarkers of increased oxidative stress and impaired methylation capacity in children with autism
Am. J. Clinical Nutrition, December 1, 2004; 80(6): 1611 - 1617.
[Abstract] [Full Text] [PDF]

Home page
 J. Pharmacol. Exp. Ther.Home page
S. K. Kim, K. J. Woodcroft, S. S. Khodadadeh, and R. F. Novak
Insulin Signaling Regulates {gamma}-Glutamylcysteine Ligase Catalytic Subunit Expression in Primary Cultured Rat Hepatocytes
J. Pharmacol. Exp. Ther., October 1, 2004; 311(1): 99 - 108.
[Abstract] [Full Text] [PDF]

Home page
 J. Appl. Physiol.Home page
K. D. Mansfield, M. C. Simon, and B. Keith
Hypoxic reduction in cellular glutathione levels requires mitochondrial reactive oxygen species
J Appl Physiol, October 1, 2004; 97(4): 1358 - 1366.
[Abstract] [Full Text] [PDF]

Home page
 Mol. Pharmacol.Home page
D. Wu and A. Cederbaum
Glutathione Depletion in CYP2E1-Expressing Liver Cells Induces Toxicity Due to the Activation of p38 Mitogen-Activated Protein Kinase and Reduction of Nuclear Factor-{kappa}B DNA Binding Activity
Mol. Pharmacol., September 1, 2004; 66(3): 749 - 760.
[Abstract] [Full Text] [PDF]

Home page
 Proc. Natl. Acad. Sci. USAHome page
J. H. Suh, S. V. Shenvi, B. M. Dixon, H. Liu, A. K. Jaiswal, R.-M. Liu, and T. M. Hagen
Decline in transcriptional activity of Nrf2 causes age-related loss of glutathione synthesis, which is reversible with lipoic acid
PNAS, March 9, 2004; 101(10): 3381 - 3386.
[Abstract] [Full Text] [PDF]

Home page
 Mol. Pharmacol.Home page
Y. Wu, X. Zhang, F. Bardag-Gorce, R. C. V. Robel, J. Aguilo, L. Chen, Y. Zeng, K. Hwang, S. W. French, S. C. Lu, et al.
Retinoid X Receptor {alpha} Regulates Glutathione Homeostasis and Xenobiotic Detoxification Processes in Mouse Liver
Mol. Pharmacol., March 1, 2004; 65(3): 550 - 557.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Clin. Nutr.Home page
M. C. van de Poll, P. B Soeters, N. E. Deutz, K. C. Fearon, and C. H. Dejong
Renal metabolism of amino acids: its role in interorgan amino acid exchange
Am. J. Clinical Nutrition, February 1, 2004; 79(2): 185 - 197.
[Abstract] [Full Text] [PDF]

Home page
 Plant Cell PhysiolHome page
K.'i. Ogawa, A. Hatano-Iwasaki, M. Yanagida, and M. Iwabuchi
Level of Glutathione is Regulated by ATP-Dependent Ligation of Glutamate and Cysteine through Photosynthesis in Arabidopsis thaliana: Mechanism of Strong Interaction of Light Intensity with Flowering
Plant Cell Physiol., January 15, 2004; 45(1): 1 - 8.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
J. A. Fraser, P. Kansagra, C. Kotecki, R. D. C. Saunders, and L. I. McLellan
The Modifier Subunit of Drosophila Glutamate-Cysteine Ligase Regulates Catalytic Activity by Covalent and Noncovalent Interactions and Influences Glutathione Homeostasis in Vivo
J. Biol. Chem., November 21, 2003; 278(47): 46369 - 46377.
[Abstract] [Full Text] [PDF]

Home page
 Toxicol SciHome page
J. Chen, M. Delannoy, S. Odwin, P. He, M. A. Trush, and J. D. Yager
Enhanced Mitochondrial Gene Transcript, ATP, Bcl-2 Protein Levels, and Altered Glutathione Distribution in Ethinyl Estradiol-Treated Cultured Female Rat Hepatocytes
Toxicol. Sci., October 1, 2003; 75(2): 271 - 278.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
A. L. Ortega, J. Carretero, E. Obrador, J. Gambini, M. Asensi, V. Rodilla, and J. M. Estrela
Tumor Cytotoxicity by Endothelial Cells. IMPAIRMENT OF THE MITOCHONDRIAL SYSTEM FOR GLUTATHIONE UPTAKE IN MOUSE B16 MELANOMA CELLS THAT SURVIVE AFTER IN VITRO INTERACTION WITH THE HEPATIC SINUSOIDAL ENDOTHELIUM
J. Biol. Chem., April 11, 2003; 278(16): 13888 - 13897.
[Abstract] [Full Text] [PDF]

Home page
 J. Am. Coll. Nutr.Home page
I. Margaritis, S. Palazzetti, A.-S. Rousseau, M.-J. Richard, and A. Favier
Antioxidant Supplementation and Tapering Exercise Improve Exercise-Induced Antioxidant Response
J. Am. Coll. Nutr., April 1, 2003; 22(2): 147 - 156.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
Y. Yang, M. Z. Dieter, Y. Chen, H. G. Shertzer, D. W. Nebert, and T. P. Dalton
Initial Characterization of the Glutamate-Cysteine Ligase Modifier Subunit Gclm(-/-) Knockout Mouse. NOVEL MODEL SYSTEM FOR A SEVERELY COMPROMISED OXIDATIVE STRESS RESPONSE
J. Biol. Chem., December 13, 2002; 277(51): 49446 - 49452.
[Abstract] [Full Text] [PDF]

Home page
 J. Nutr.Home page
K. G. Calfee-Mason, B. T. Spear, and H. P. Glauert
Vitamin E Inhibits Hepatic NF-{kappa}B Activation in Rats Administered the Hepatic Tumor Promoter, Phenobarbital
J. Nutr., October 1, 2002; 132(10): 3178 - 3185.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
Y. Sagara, K. Ishige, C. Tsai, and P. Maher
Tyrphostins Protect Neuronal Cells from Oxidative Stress
J. Biol. Chem., September 20, 2002; 277(39): 36204 - 36215.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
H. Yang, Y. Zeng, T. D. Lee, Y. Yang, X. Ou, L. Chen, M. Haque, R. Rippe, and S. C. Lu
Role of AP-1 in the Coordinate Induction of Rat Glutamate-cysteine Ligase and Glutathione Synthetase by tert-Butylhydroquinone
J. Biol. Chem., September 13, 2002; 277(38): 35232 - 35239.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Clin. Nutr.Home page
A. Badaloo, M. Reid, T. Forrester, W. C Heird, and F. Jahoor
Cysteine supplementation improves the erythrocyte glutathione synthesis rate in children with severe edematous malnutrition
Am. J. Clinical Nutrition, September 1, 2002; 76(3): 646 - 652.
[Abstract] [Full Text] [PDF]

Home page
 Toxicol PatholHome page
K. T. Morgan, Hong Ni, H. R. Brown, L. Yoon, C. W. Qualls JR, L. M. Crosby, R. Reynolds, B. Gaskill, S. P. Anderson, T. B. Kepler, et al.
Application of cDNA Microarray Technology to In Vitro Toxicology and the Selection of Genes for a Real-Time RT-PCR-Based Screen for Oxidative Stress in Hep-G2 Cells
Toxicol Pathol, June 1, 2002; 30(4): 435 - 451.
[Abstract] [PDF]

Home page
 J. Biol. Chem.Home page
J. M. Pullar and M. B. Hampton
Diphenyleneiodonium Triggers the Efflux of Glutathione from Cultured Cells
J. Biol. Chem., May 24, 2002; 277(22): 19402 - 19407.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
D.-w. Jeong, M.-H. Yoo, T. S. Kim, J.-H. Kim, and I. Y. Kim
Protection of Mice from Allergen-induced Asthma by Selenite. PREVENTION OF EOSINOPHIL INFILTRATION BY INHIBITION OF NF-kappa B ACTIVATION
J. Biol. Chem., May 10, 2002; 277(20): 17871 - 17876.
[Abstract] [Full Text] [PDF]

Home page
 J. Pharmacol. Exp. Ther.Home page
M. Mari, J. Bai, and A. I. Cederbaum
Adenovirus-Mediated Overexpression of Catalase in the Cytosolic or Mitochondrial Compartment Protects against Toxicity Caused by Glutathione Depletion in HepG2 Cells Expressing CYP2E1
J. Pharmacol. Exp. Ther., April 1, 2002; 301(1): 111 - 118.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
M.-K. Han, S.-J. Kim, Y.-R. Park, Y.-M. Shin, H.-J. Park, K.-J. Park, K.-H. Park, H.-K. Kim, S.-I. Jang, N.-H. An, et al.
Antidiabetic Effect of a Prodrug of Cysteine, L-2-Oxothiazolidine-4-carboxylic Acid, through CD38 Dimerization and Internalization
J. Biol. Chem., February 8, 2002; 277(7): 5315 - 5321.
[Abstract] [Full Text] [PDF]

Home page
 FASEB J.Home page
J. M. MATO, F. J. CORRALES, S. C. LU, and M. A. AVILA
S-Adenosylmethionine: a control switch that regulates liver function
FASEB J, January 1, 2002; 16(1): 15 - 26.
[Abstract] [Full Text] [PDF]

Home page
 StrokeHome page
R. M. Adibhatla, J.F. Hatcher, and R.J. Dempsey
Effects of Citicoline on Phospholipid and Glutathione Levels in Transient Cerebral Ischemia
Stroke, October 1, 2001; 32(10): 2376 - 2381.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Physiol. Renal Physiol.Home page
D. Laouari, R. Yang, C. Veau, I. Blanke, and G. Friedlander
Two apical multidrug transporters, P-gp and MRP2, are differently altered in chronic renal failure
Am J Physiol Renal Physiol, April 1, 2001; 280(4): F636 - F645.
[Abstract] [Full Text] [PDF]

Home page
 Proc. Natl. Acad. Sci. USAHome page
K. Chan, X.-D. Han, and Y. W. Kan
An important function of Nrf2 in combating oxidative stress: Detoxification of acetaminophen
PNAS, March 29, 2001; (2001) 81082098.
[Abstract] [Full Text]

Home page
 Mol. Cell. Biol.Home page
K. D. McCullough, J. L. Martindale, L.-O. Klotz, T.-Y. Aw, and N. J. Holbrook
Gadd153 Sensitizes Cells to Endoplasmic Reticulum Stress by Down-Regulating Bcl2 and Perturbing the Cellular Redox State
Mol. Cell. Biol., February 15, 2001; 21(4): 1249 - 1259.
[Abstract] [Full Text]

Home page
 IOVSHome page
R. Kannan, B. Ouyang, E. Wawrousek, N. Kaplowitz, and U. P. Andley
Regulation of GSH in {{alpha}}A-Expressing Human Lens Epithelial Cell Lines and in {{alpha}}A Knockout Mouse Lenses
Invest. Ophthalmol. Vis. Sci., February 1, 2001; 42(2): 409 - 416.
[Abstract] [Full Text]

Home page
 Am. J. Physiol. Gastrointest. Liver Physiol.Home page
H. Yang, Z.-Z. Huang, Z. Zeng, C. Chen, R. R. Selby, and S. C. Lu
Role of promoter methylation in increased methionine adenosyltransferase 2A expression in human liver cancer
Am J Physiol Gastrointest Liver Physiol, February 1, 2001; 280(2): G184 - G190.
[Abstract] [Full Text] [PDF]

Home page
 J. Immunol.Home page
A. O. T. Lau, J. B. Sacci Jr., and A. F. Azad
Host Responses to Plasmodium yoelii Hepatic Stages: A Paradigm in Host-Parasite Interaction
J. Immunol., February 1, 2001; 166(3): 1945 - 1950.
[Abstract] [Full Text] [PDF]

Home page
 J. Nutr.Home page
K. J. Petzke, A. Elsner, J. Proll, F. Thielecke, and C. C. Metges
Long-Term High Protein Intake Does Not Increase Oxidative Stress in Rats
J. Nutr., December 1, 2000; 130(12): 2889 - 2896.
[Abstract] [Full Text] [PDF]

Home page
 CarcinogenesisHome page
A.G. Shepherd, M. M. Manson, H. W.L. Ball, and L. I. McLellan
Regulation of rat glutamate-cysteine ligase ({gamma}-glutamylcysteine synthetase) subunits by chemopreventive agents and in aflatoxin B1-induced preneoplasia
Carcinogenesis, October 1, 2000; 21(10): 1827 - 1834.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Physiol. Cell Physiol.Home page
B. J. Buckley and A. R. Whorton
Adaptive responses to peroxynitrite: increased glutathione levels and cystine uptake in vascular cells
Am J Physiol Cell Physiol, October 1, 2000; 279(4): C1168 - C1176.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Physiol. Gastrointest. Liver Physiol.Home page
S. C. Lu, Z.-Z. Huang, H. Yang, J. M. Mato, M. A. Avila, and H. Tsukamoto
Changes in methionine adenosyltransferase and S-adenosylmethionine homeostasis in alcoholic rat liver
Am J Physiol Gastrointest Liver Physiol, July 1, 2000; 279(1): G178 - G185.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Pathol.Home page
R. H. Pierce, J. S. Campbell, A. B. Stephenson, C. C. Franklin, M. Chaisson, M. Poot, T. J. Kavanagh, P. S. Rabinovitch, and N. Fausto
Disruption of Redox Homeostasis in Tumor Necrosis Factor-Induced Apoptosis in a Murine Hepatocyte Cell Line
Am. J. Pathol., July 1, 2000; 157(1): 221 - 236.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
S. Kotamraju, E. A. Konorev, J. Joseph, and B. Kalyanaraman
Doxorubicin-induced Apoptosis in Endothelial Cells and Cardiomyocytes Is Ameliorated by Nitrone Spin Traps and Ebselen. ROLE OF REACTIVE OXYGEN AND NITROGEN SPECIES
J. Biol. Chem., October 20, 2000; 275(43): 33585 - 33592.
[Abstract] [Full Text] [PDF]

Home page
 Proc. Natl. Acad. Sci. USAHome page
K. Chan, X.-D. Han, and Y. W. Kan
An important function of Nrf2 in combating oxidative stress: Detoxification of acetaminophen
PNAS, April 10, 2001; 98(8): 4611 - 4616.
[Abstract] [Full Text] [PDF]

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