Intracellular cysteine and glutathione delivery systems

Chemically diverse toxicants converge on Fyn and c-Cbl to disrupt precursor cell function

Identification of common mechanistic principles that shed light on the action of the many chemically diverse toxicants to which we are exposed is of central importance in understanding how toxicants disrupt normal cellular function and in developing more effective means of protecting against such effects. Of particular importance is identifying mechanisms operative at environmentally relevant toxicant exposure levels. Chemically diverse toxicants exhibit striking convergence, at environmentally relevant exposure levels, on pathway-specific disruption of receptor tyrosine kinase (RTK) signaling required for cell division in central nervous system (CNS) progenitor cells. Relatively small toxicant-induced increases in oxidative status are associated with Fyn kinase activation, leading to secondary activation of the c-Cbl ubiquitin ligase. Fyn/c-Cbl pathway activation by these pro-oxidative changes causes specific reductions, in vitro and in vivo, in levels of the c-Cbl target platelet-derived growth factor receptor-α and other c-Cbl targets, but not of the TrkC RTK (which is not a c-Cbl target). Sequential Fyn and c-Cbl activation, with consequent pathway-specific suppression of RTK signaling, is induced by levels of methylmercury and lead that affect large segments of the population, as well as by paraquat, an organic herbicide. Our results identify a novel regulatory pathway of oxidant-mediated Fyn/c-Cbl activation as a shared mechanism of action of chemically diverse toxicants at environmentally relevant levels, and as a means by which increased oxidative status may disrupt mitogenic signaling. These results provide one of a small number of general mechanistic principles in toxicology, and the only such principle integrating toxicology, precursor cell biology, redox biology, and signaling pathway analysis in a predictive framework of broad potential relevance to the understanding of pro-oxidant–mediated disruption of normal development.

Chemically different toxins (lead, methylmercury, and paraquat) each cause the intracellular environment to become more oxidized, and thereby activate a common pathway that suppresses signaling from growth factor receptors that may be associated with developmental impairments.

Discovering general principles underlying the effects of toxicant exposure on biological systems is one of the central challenges of toxicological research. We have discovered a previously unrecognized regulatory pathway on which chemically diverse toxicants converge, at environmentally relevant exposure levels, to disrupt the function of progenitor cells of the developing central nervous system. We found that the ability of low levels of methylmercury, lead, and paraquat to make progenitor cells more oxidized causes activation of an enzyme called Fyn kinase. Activated Fyn then activates another enzyme (c-Cbl) that modifies specific proteins—receptors that are required for cell division and survival—to initiate the proteins' degradation. By enhancing degradation of these receptors, their downstream signaling functions are repressed. Analysis of developmental exposure to methylmercury provided evidence that this same pathway is activated in vivo by environmentally relevant toxicant levels. The remarkable sensitivity of progenitor cells to low levels of toxicant exposure, and the discovery of the redox/Fyn/c-Cbl pathway as a mechanism by which small increases in oxidative status can markedly alter cell function, provide a novel and specific means by which exposure to chemically diverse toxicants might perturb normal development. In addition, the principles revealed in our studies appear likely to have broad applicability in understanding the regulation of cell function by alterations in redox balance, regardless of how they might be generated.

Studies on persisting effects of soy-based compared with amino acid-supplemented casein-based diet on protein metabolism and oxidative stress in juvenile pigs

Juvenile growing pigs were studied to explore whether a soy-based diet can induce persistent physiological alterations, especially in protein and energy metabolism, nutrient oxidation and redox homeostasis. In former studies we have shown that in juvenile pigs chronically fed protein diets based on either casein (CAS) or soy protein isolate (SPI), the SPI diet significantly decreases growth rate and increases oxidative stress responsiveness as compared to CAS. In addition, here we show that chronic feeding of SPI vs. CAS diet decreases whole body protein synthesis (WBPS) (p = 0.007) and hepatic gene expression associated with protein synthesis. To study persistent SPI effects, a three-period feeding experiment was designed: In the test group 18 pigs received the CAS diet for 24 days (period 1), followed by 31 days on the SPI diet (period 2) and further 31 days on the CAS diet (period 3). In the control group 18 pigs were fed the CAS diet throughout the three periods (86 days). Temporary consumption of SPI diet results in persistent changes of protein metabolism and oxidative stress responsiveness. After switching back from SPI to CAS diet the decrease of WBPS of the test group vs. control group was of borderline significance (p = 0.061), transcript levels of hepatic gene expressions of leucine aminopeptidase, endopeptidase 24.16, glutathione-S-transferase and peptide methionine sulfoxide reductase were increased. In liver tissue, total glutathione was increased and thiobarbituric acid reactive substances were decreased in the test vs. control group. In conclusion, results suggest that SPI-induced changes in protein and amino acid metabolism as well as in redox homeostasis and antioxidative potential in growing pigs persist 4 weeks after the cessation of SPI feeding.

Increased ROS generation and p53 activation in α-lipoic acid-induced apoptosis of hepatoma cells

Alpha-lipoic acid (alpha-LA) is an antioxidant used for the treatment of a variety of diseases, including liver cirrhosis, heavy metal poisoining, and diabetic polyneuropathy. In addition to its protective effect against oxidative stress, alpha-LA induces apoptosis in different cancer cells types. However, whether alpha-LA acid induces apoptosis of hepatoma cells is unknown. Herein, we investigated whether alpha-LA induces apoptosis in two different hepatoma cell lines FaO and HepG2. The results showed that alpha-LA inhibits the growth of both cell lines as indicated by the reduction in cell number, the reduced expression of cyclin A and the increased levels of the cyclin/CDKs inhibitors, p27(Kip1) and p21(Cip1). Cell cycle arrest was associated with cell loss, and DNA laddering indicative of apoptosis. Apoptosis was preceded by increased generation of reactive oxygen species, and associated with p53 activation, increased expression of Bax, release of cytochrome c from mitochondria, caspases activation, decreased levels of survivin, induction of pro-apoptotic signaling (i.e JNK) and inhibition of anti-apoptotic signaling (i.e. PKB/Akt) pathways. In conclusion, this study provides evidence that alpha-LA induces apoptosis in hepatoma cells, describes a possible sequence of molecular events underlying its lethal effect, and suggests that it may prove useful in liver cancer therapy.

Protective role of l-2-oxothiazolidine-4-carboxylic acid in cisplatin-induced renal injury

BACKGROUND

Oxidative stress and inflammation are implicated in the pathogenesis of cisplatin-induced nephrotoxicity. l-2-oxothiazolidine-4-carboxylic acid (OTC) is a cysteine prodrug, and increases cellular glutathione (GSH). OTC is converted to cysteine by the intracellular enzyme, oxoprolinase. To date, the protective role of OTC on cisplatin-induced renal injury has not been investigated. The purpose of the present study was to examine the protective effect of OTC on cisplatin-induced renal injury and to examine the mechanism of its protection.

METHODS

Mice were treated with cisplatin with or without administration of OTC. The generation of reactive oxygen species (ROS), expression of intercellular adhesion molecule (ICAM)-1 and monocyte chemoattractant protein (MCP)-1 were determined in the kidney using 2',7'-dichlorofluorescein diacetate, immunostaining or western blot analysis. Nuclear factor (NF)-kappaB activity, infiltration of F4/80-positive cells and apoptosis were also investigated in addition to renal function and histology using electrophoretic mobility shift assay, immunostaining, western blot analysis, uridine triphosphate (dUTP) nick-end labelling or periodic acid-Schiff staining. The effect of OTC on superoxide dismutase activity and GSH level in cisplatin-treated normal adult human kidney (HK-2) cells were measured using assay kits.

RESULTS

The administration of OTC resulted in a significant reduction of cisplatin-induced ROS production, the p65 subunit of NF-kappaB translocation into nucleus, expression of ICAM-1, caspase 3 activity, expression of MCP-1 and the infiltration of macrophages into renal tissue. OTC markedly ameliorated renal damage induced by cisplatin through antioxidant and anti-inflammatory effect.

CONCLUSIONS

These results suggest that OTC can be a potential therapeutic agent in cisplatin-induced renal injury through decreasing the ROS levels and activation of NF-kappaB.

Influence of early postnatal nutritional management on oxidative stress and antioxidant defence in extreme prematurity

The increased survival of infants born at mid-gestation in the last decade is associated with significant oxygen free radical-mediated morbidities. Resuscitation with 100% oxygen, oxidant load from parenteral nutrition fluids, and oxidant stress inherent to the systemic inflammatory state subsequent to infection and tissue injury are all contributory.

CONCLUSION

Improving early postnatal protein nutrition and the formulation of parenteral nutrition fluids would potentially reduce the oxidative stress and enhance the antioxidant defence of extremely premature newborns.

Neuronal glutathione deficiency and age-dependent neurodegeneration in the EAAC1 deficient mouse

Uptake of the neurotransmitter glutamate is effected primarily by transporters expressed on astrocytes, and downregulation of these transporters leads to seizures and neuronal death. Neurons also express a glutamate transporter, termed excitatory amino acid carrier-1 (EAAC1), but the physiological function of this transporter remains uncertain. Here we report that genetically EAAC1-null (Slc1a1(-/-)) mice have reduced neuronal glutathione levels and, with aging, develop brain atrophy and behavioral changes. EAAC1 can also rapidly transport cysteine, an obligate precursor for neuronal glutathione synthesis. Neurons in the hippocampal slices of EAAC1(-/-) mice were found to have reduced glutathione content, increased oxidant levels and increased susceptibility to oxidant injury. These changes were reversed by treating the EAAC1(-/-) mice with N-acetylcysteine, a membrane-permeable cysteine precursor. These findings suggest that EAAC1 is the primary route for neuronal cysteine uptake and that EAAC1 deficiency thereby leads to impaired neuronal glutathione metabolism, oxidative stress and age-dependent neurodegeneration.

Redox regulation of precursor cell function: insights and paradoxes

Studies on oligodendrocytes, the myelin-forming cells of the central nervous system, and on the progenitor cells from which they are derived, have provided several novel insights into the role of intracellular redox state in cell function. This review discusses our findings indicating that intracellular redox state is utilized by the organism as a means of regulating the balance between progenitor cell division and differentiation. This regulation is achieved in part through cell-intrinsic differences that modify the response of cells to extracellular signaling molecules, such that cells that are slightly more reduced are more responsive to inducers of cell survival and division and less responsive to inducers of differentiation or cell death. Cells that are slightly more oxidized, in contrast, show a greater response to inducers of differentiation or cell death, but less response to inducers of proliferation or survival. Regulation is also achieved by the ability of exogenous signaling molecules to modify intracellular redox state in a highly predictable manner, such that signaling molecules that promote self-renewal make progenitor cells more reduced and those that promote differentiation make cells more oxidized. In both cases, the redox changes induced by exposure to exogenous signaling molecules are a necessary component of their mode of action. Paradoxically, the results obtained through studies on the oligodendrocyte lineage are precisely the opposite of what might be predicted from a large number of studies demonstrating the ability of reactive oxidative species to enhance the effects of signaling through receptor tyrosine kinase receptors and to promote cell proliferation. Taken in sum, available data demonstrate clearly the existence of two distinct programs of cellular responses to changes in oxidative status. In one of these, becoming even slightly more oxidized is sufficient to inhibit proliferation and induce differentiation. In the second program, similar changes enhance proliferation. It is not yet clear how cells can interpret putatively identical signals in such opposite manners, but it does already seem clear that resolving this paradox will provide insights of considerable relevance to the understanding of normal development, tissue repair, and tumorigenesis.

Mechanisms underlying the cardioprotective effect of l-cysteine

In many tissues the availability of L-cysteine is a rate-limiting factor in glutathione production, though this has yet to be fully tested in heart. This study aimed to test the hypothesis that supplying hearts with 0.5 mM L-cysteine would preserve glutathione levels leading to an increased resistance to ischaemia reperfusion. Left ventricular function was measured in isolated perfused rat hearts before, during and after exposure to 45 min global normothermic ischaemia. Control hearts received Krebs throughout, whilst in treated hearts 0.5 mM L-cysteine was added to the perfusate 10 min before ischaemia, and was then present throughout ischaemia and for the first 10 min of reperfusion. Reperfusion injury was assessed from the appearance of lactate dehydrogenase (LDH) in the effluent. In two separate groups of control and treated hearts, ATP and glutathione (GSH) contents were measured at the beginning and end of ischaemia. Hearts treated with 0.5 mM L-cysteine showed a significantly higher recovery of rate pressure product (16,256+/- 1288 mmHg bpm vs. 10,324+/- 2102 mmHg bpm, p < 0.05) and a significantly lower release of LDH (0.54+/- 0.16 IU/g wet weight vs. 1.44+/- 0.31 IU/g wet weight, p < 0.05) compared to controls. Also, the L-cysteine treated group showed significantly better preservation of ATP and GSH during ischaemia in comparison to control. These results suggest that the mechanisms underlying the cardioprotective effects of 0.5 mM L-cysteine may include: increased anaerobic energy production either directly or through reduced degradation of adenine nucleotides; direct scavenging of free radicals; and/or improved antioxidant capacity through glutathione preservation.

Dietary lipoic acid supplementation attenuates hypertension in Dahl salt sensitive rats

There is strong evidence that excess dietary salt (NaCl) is a major factor contributing to the development of hypertension. Salt sensitive humans and rats develop hypertension even on a normal salt diet. Salt sensitivity is associated with glucose intolerance and insulin resistance in both humans and animal models, including Dahl salt sensitive (DSS) rats. In insulin resistance, impaired glucose metabolism leads to elevated endogenous aldehydes. These aldehydes bind sulfhydryl groups of membrane proteins, altering calcium channels, increasing cytosolic free calcium ([Ca2+]i) and blood pressure. Treatment with lipoic acid, an endogenous sulfur-containing fatty acid, normalizes insulin resistance and lowers tissue aldehyde conjugates, cytosolic [Ca2+]i, and blood pressure in spontaneously hypertensive rats (SHR). The objective of this study was to investigate the effects of a normal salt diet on tissue aldehyde conjugates, cytosolic [Ca2+]i and blood pressure in DSS rats and to determine whether lipoic acid supplementation prevents the increase in blood pressure and biochemical changes. Starting at 7 weeks of age, DSS rats were divided into three groups of six animals each and treated for 6 weeks with diets as follows: DSS-low salt, 0.4% NaCl; DSS-normal salt, 0.7% NaCl, and; DSS-normal salt + lipoic acid, 0.7% NaCl + lipoic acid 500 mg/kg feed. At completion, animals in the normal salt group had elevated systolic blood pressure, cytosolic [Ca2+]i and tissue aldehyde conjugates as compared to the low salt group. They also showed smooth muscle cell hyperplasia in small arteries and arterioles of the kidney. Dietary lipoic acid supplementation attenuated the increase in systolic blood pressure and associated biochemical and histopathological changes.

Mechanisms of 4-hydroxynonenal-induced neuronal microtubule dysfunction

We have previously demonstrated that neuronal microtubules are exquisitely sensitive to the lipid peroxidation product 4-hydroxynonenal (HNE). The mechanism, however, by which HNE disrupts the microtubules, is not known. Sulfhydryl groups of protein-cysteines constitute main targets of HNE. Indeed, HNE is mainly detoxified by conjugation to glutathione (GSH), a reaction that leads to depletion of cellular GSH. GSH maintains protein sulfhydryl groups in the reduced form and has been implicated in the regulation of cytoskeletal function. Here, we assess what role depletion of cellular GSH plays in the HNE-induced microtubule disruption. We demonstrate that HNE and its intracellularly activated tri-ester analog, HNE(Ac)(3), cause substantial GSH depletion in Neuro2A cells. However, other compounds inducing GSH depletion had no effect on the microtubule network. Therefore, HNE-induced depletion of cellular GSH does not contribute to the HNE-induced microtubule disruption. We previously demonstrated that another main cellular target of HNE is tubulin, the core protein of microtubules containing abundant cysteines. The functional relevance of this adduction, however, had not been evaluated. Here, we demonstrate that exposure of Neuro 2A cells to HNE or HNE(Ac)(3) results in the inhibition of cytosolic taxol-induced tubulin polymerization. These and our previous observations strongly support the hypothesis that HNE-adduction to tubulin is the primary mechanism involved in the HNE-induced loss of the highly dynamic neuronal microtubule network.

Dietary vitamin C supplementation reduces noise-induced hearing loss in guinea pigs

Vitamin C (ascorbate) is a water-soluble, low molecular weight antioxidant that works in conjunction with glutathione and other cellular antioxidants, and is effective against a variety of reactive oxygen species, including superoxide and hydroxyl radicals that have been implicated in the etiology of noise-induced hearing loss (NIHL). Whereas most animals can manufacture their own vitamin C, humans and a few other mammals such as guinea pigs lack the terminal enzyme for vitamin C synthesis and must obtain it from dietary sources. To determine if susceptibility to NIHL could be influenced by manipulating dietary levels of vitamin C, albino guinea pigs were raised for 35 days on a diet with normal, supplemented or deficient levels of ascorbate, then exposed to 4 kHz octave band noise at 114 dB SPL for 6 h to induce permanent threshold shifts (PTS) of the scalp-recorded auditory brainstem response. Animals that received the highest levels of dietary ascorbate developed significantly less PTS for click stimuli and 4, 8, 12, and 16 kHz tones than animals on normal and deficient diets. Outer hair cell loss was minimal in all groups after noise exposure, but permanent damage to stereocilia were observed in noise-exposed ears. The results support the hypothesis that dietary factors influence individual susceptibility to hearing loss, and suggest that high levels of vitamin C may be beneficial in reducing susceptibility to NIHL.

Prevention of fructose-induced hypertension by dietary vitamins

Essential hypertension in humans may develop through a combination of genetic and environmental factors. Diet has long been under investigation as a potential effector of blood pressure. A diet high in sucrose or fructose can give rise to hyperlipidemia, insulin resistance and hypertension. Insulin resistance, glucose intolerance and oxidative stress are common features of hypertension. If glucose metabolism through the glycolytic pathway is impaired, as in insulin resistance, there will be a build-up of glyceraldehyde, glyceraldehyde-3-phosphate and dihydroxyacetone phosphate with further metabolism to methylglyoxal, a highly reactive ketoaldehyde. Excess aldehydes can bind sulfhydryl groups of membrane proteins, altering membrane calcium channels, increasing cytosolic free calcium, peripheral vascular resistance and blood pressure. The presence of reactive aldehydes can also lead to oxidative stress. Dietary management through lower sucrose or fructose intake and increased consumption of vitamins improves glucose metabolism, lowers tissue aldehydes, increases anti-oxidant capacity and may also prevent hypertension.

Salt-induced hypertension in WKY rats: prevention by alpha-lipoic acid supplementation

There is strong evidence that points to excess dietary salt as a major factor contributing to the development of hypertension. Salt sensitivity is associated with glucose intolerance and insulin resistance in both animal models and humans. In insulin resistance, impaired glucose metabolism leads to elevated endogenous aldehydes which bind to vascular calcium channels, increasing cytosolic [Ca2+]i and blood pressure. In an insulin resistant animal model of hypertension, spontaneously hypertensive rats (SHRs), dietary supplementation with lipoic acid lowers tissue aldehydes and plasma insulin levels and normalizes blood pressure. The objective of this study is to examine the effects of a high salt diet on tissue aldehydes, cytosolic [Ca2+]i and blood pressure in WKY rats and to investigate whether dietary supplementation with lipoic acid can prevent a salt induced increase in blood pressure. Starting at 7 weeks of age, WKY rats were divided into three groups of six animals each and treated for 10 weeks with diets as follows: WKY-normal salt (0.7% NaCl); WKY-high salt (8% NaCl); WKY-high salt + lipoic acid (8% NaCl diet + lipoic acid 500 mg/Kg feed). At completion, animals in the high salt group had elevated systolic blood pressure, platelet [Ca2+]i, and tissue aldehyde conjugates compared with the normal salt group and showed smooth muscle cell hyperplasia in the small arteries and arterioles of the kidneys. Dietary alpha-lipoic acid supplementation in high salt-treated WKY rats normalized systolic blood pressure and cytosolic [Ca2+]i and aldehydes in liver and aorta. Kidney aldehydes and renal vascular changes were attenuated, but not normalized.

The peroxisome proliferator BR931 kills FaO cells by p53-dependent apoptosis

Although suppression of apoptosis has been implicated as a mechanism for the hepatocarcinogenicity of peroxisome proliferators (PPs), they can also induce cell death in rat AH130 and human HepG2 hepatoma cells. To study how PPs induce cell death and to characterize the molecular events involved, we administered the hypolipidemic BR931, a peroxisome proliferator, to rat hepatoma FaO cells. Treatment with increasing concentrations of BR931 (0.015 to 0.6 mM) reduced cell viability in a dose- and time-dependent manner, associated with DNA fragmentation and morphological changes characteristic of apoptosis. BR931 also caused phosphorylation of p53 within 3 hours, translocation of the pro-apoptotic Bax protein to mitochondria, release of cytochrome-c into the cytosol, and activation of caspase-9 and -3. These results indicated that BR931 activated the intrinsic caspase cascade. Pretreatment with three different antioxidants, N-acetylcysteine, Vitamin C and Trolox, reduced apoptosis, suggesting that reactive oxygen species (ROS) plays a role in BR931-induced apoptosis. In support of this hypothesis, BR931 produced increased levels of 8-hydroxy-deoxy-guanosine, a marker of DNA oxidative damage. Antioxidants prevented the p53 phosphorylation, up-regulation of Bax and BR931-induced apoptosis. These results suggest that BR931 can increase generation of ROS, leading to DNA damage and p53 phosphorylation, which, in turn, induces the activation of Bax, release of cytochrome-c from mitochondria and activation of caspases, culminating in cell death.

Redox state as a central modulator of precursor cell function

In our attempts to understand how the balance between self-renewal and differentiation is regulated in dividing precursor cells, we have discovered that intracellular redox state appears to be a critical modulator of this balance in oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells. The intracellular redox state of freshly isolated progenitor cells allows prospective isolation of cells with different self-renewal characteristics, which can be further modulated in opposite directions by prooxidants and antioxidants. Redox state is itself modulated by cell-extrinsic signaling molecules that alter the balance between self-renewal and differentiation: growth factors that promote self-renewal cause progenitors to become more reduced, while exposure to signaling molecules that promote differentiation causes progenitors to become more oxidized. Moreover, pharmacological antagonists of the redox effects of these cell-extrinsic signaling molecules antagonize their effects on self-renewal and differentiation, further suggesting that cell-extrinsic signaling molecules that modulate this balance converge on redox modulation as a critical component of their effector mechanism. A further example of the potential relevance of intracellular redox state to development processes emerges from our attempts to understand why different central nervous system (CNS) regions exhibit different temporal patterns of oligodendrocyte generation and myelinogenesis. Characterization of O-2A progenitor cells (O-2A/OPCs) isolated from different regions indicates that these developmental patterns are consistent with properties of the specific O-2A/OPCs resident in each region. Marked differences were seen in self-renewal and differentiation characteristics of O-2A/OPCs isolated from cortex, optic nerve, and optic chiasm. In conditions where optic nerve-derived O-2A/OPCs generated oligodendrocytes within 2 days, oligodendrocytes arose from chiasm-derived cells after 5 days and from cortical O-2A/OPCs after only 7-10 days. These differences, which appear to be cell intrinsic, were manifested both in reduced percentages of clones producing oligodendrocytes and in a lesser representation of oligodendrocytes in individual clones. In addition, responsiveness of optic nerve-, chiasm-, and cortex-derived O-2A/OPCs to thyroid hormone (TH) and ciliary neurotrophic factor (CNTF), well-characterized inducers of oligodendrocyte generation, was inversely related to the extent of self-renewal observed in basal division conditions. These results demonstrate hitherto unrecognized complexities among the precursor cells thought to be the immediate ancestors of oligodendrocytes and suggest that the properties of these different populations may contribute to the diverse time courses of myelination in different CNS regions. Strikingly, O-2A/OPCs isolated from cortex and analyzed immediately upon isolation were more reduced in their redox state than were optic nerve-derived cells, precisely as would be predicted from our analysis of the role of redox state in modulating the balance between self-renewal and differentiation. Chiasm-derived cells, which exhibited self-renewal properties intermediate between cortex- and optic nerve-derived cells, were more reduced than optic nerve cells but more oxidized that cortical O-2A/OPCs.

Noise-induced hearing loss in chinchillas pre-treated with glutathione monoethylester and R-PIA

The protective effects of glutathione monoethylester (GEE) and GEE in combination with R-N6-phenylisopropyladenosine (R-PIA) were evaluated in the chinchilla when exposed to impulse (145 dB pSPL) or continuous (105 dB SPL, 4 kHz OB) noise. Six groups of 10 chinchillas were used as subjects. Before exposure to noise, the subjects were anesthetized, a 30 microl drop of drug was placed on the round window (GEE [50, 100, 150 mM], GEE 50 mM and R-PIA). Forty minutes later the subject was exposed to either impulse or continuous noise. The 50 mM treatment provided significant protection from impulse noise, but not from continuous noise exposure. The combination provided significant protection from both the continuous and impulse noise. In a separate set of experiments, glutathione (GSH) levels were measured in the perilymph. All the drug treatments elevated GSH levels. The results are discussed in terms of antioxidant treatments as a prophylactic measure against noise-induced hearing loss.

Platelet resistance to the antiaggregating effect of N-acetyl-L-cysteine in obese, insulin-resistant subjects

INTRODUCTION

We investigated whether the platelets from obese subjects are sensitive as those from controls to the antiaggregating effects of N-acetyl-L-cysteine (NAC)-an antioxidant thiol that increases availability of endogenous nitric oxide (NO)-and of superoxide dismutase (SOD) and amifostine which act as scavengers of superoxide anion.

MATERIALS AND METHODS

In platelets from obese subjects (n=20, body mass index [BMI]=34.2+/-1.9 kg/m(2), homeostasis model assessment [HOMA] index=5.5+/-1.1) and controls (n=20, BMI=21.4+/-0.6 kg/m(2), HOMA index=1.4+/-0.2), we investigated the effects of NAC on aggregation and on 3',5'-cyclic guanosine monophosphate (cGMP) synthesis and the interplay between NAC and the organic nitrates glyceryl trinitrate (GTN) and sodium nitroprusside (SNP). Similar experiments were carried out with SOD and amifostine.

RESULTS

We found that a 3-min platelet exposure to NAC decreased aggregation and increased cGMP in controls, but not in obese subjects. Only more prolonged incubations exerted a small effect also in obese subjects. GTN and SNP increased platelet cGMP in both groups, but their effect was much lower in obese subjects. NAC (3 mmol/l), SOD (150 U/ml), and amifostine (50 micromol/l) enhanced the increase of cGMP elicited by NO donors, but again, the effect was much lower in obese subjects.

CONCLUSIONS

Since antioxidants do not restore the effects of NO in platelets from obese subjects, we hypothesize that oxidative stress is not the unique cause of platelet resistance to NO in obesity and suggest that a resistance to the NO action at the guanylate cyclase level could play a role in this phenomenon, potentially involved in the increased atherothrombotic risk linked to obesity.

The effects of dietary sulfur amino acid deficiency on rat brain glutathione concentration and neural damage in global hemispheric hypoxia-ischemia

Primary brain injury in stroke is followed by an excitotoxic cascade, oxidative stress and further neural damage. Glutathione is critical and depleted in oxidative stress. Since cysteine is limiting in glutathione synthesis, this study investigated the effect of dietary sulfur amino acid (SAA) deficiency on neural damage in a rat model of global hemispheric hypoxia-ischemia (GHHI). Animals were fed with SAA deficient ("deficient") or control diet for 3 days, subjected to right common carotid artery ligation and hypoxia, and diet continued for 3 more days. Histologically evaluated neural damage at 7 days post hypoxia-ischemia was greater in "deficient" rats, shown by mean (+/- SEM) global and hippocampal grid scores of 2.5 +/- 0.7 and 34.9 +/- 9.3%, respectively, vs. controls' scores of 0.1 +/- 0.1 and 0.1 +/- 0.1%, respectively. Mean brain (+/- SEM) reduced glutathione was not different between groups at 6h post hypoxia-ischemia, but was decreased in "deficient" animals 3 days later in neocortex (1.46 micromoles/g wet weight +/- 0.05 vs. 1.67 +/- 0.04 in controls) and thalamus (1.60 micromoles/g wet weight +/- 0.05 vs. 1.78 +/- 0.03 in controls). Administration of a cysteine precursor to "deficient" animals did not ameliorate neural damage. These findings suggest that well-nourished but not "deficient" animals tolerate a mild brain insult. The decline in brain glutathione in the "deficient" animals may be one of several contributing mechanisms.

Dietary vitamin E and C supplementation prevents fructose induced hypertension in rats

In fructose-induced hypertension in Wistar-Kyoto (WKY) rats, excess endogenous aldehydes bind sulfhydryl groups of membrane proteins, altering membrane Ca2+ channels and increasing cytosolic free calcium and blood pressure. The thiol compound N-acetyl cysteine prevents fructose-induced hypertension by binding excess endogenous aldehydes and normalizing membrane Ca2+ channels and cytosolic free calcium. The aim of the present study was to investigate whether dietary supplementation of vitamin E and vitamin C which are known to increase tissue glutathione, a storage form of cysteine, prevents this hypertension and its associated biochemical and histopathological changes. Starting at 7 weeks of age, animals were divided into four groups of six animals each and treated as follows: control group, normal diet and normal drinking water; fructose group, normal diet and 4% fructose in drinking water; fructose + vitamin E group, diet supplemented with vitamin E (34 mg/ kg feed) and 4% fructose in drinking water; fructose + vitamin C group, diet supplemented with vitamin C (1,000 mg/kg feed) and 4% fructose in drinking water. At 14 weeks, systolic blood pressure, platelet [Ca2+]i and kidney and aortic aldehyde conjugates were significantly higher in the fructose group. These animals also displayed smooth muscle cell hyperplasia in the small arteries and arterioles of the kidneys. Dietary vitamin E and C supplementation in fructose-treated WKY rats prevented the increase in systolic blood pressure by normalizing cytosolic [Ca2+]i and kidney and aortic aldehyde conjugates and preventing adverse renal vascular changes.

Dietary supplementation with cysteine prodrugs selectively restores tissue glutathione levels and redox status in protein-malnourished mice(1)

Protein malnutrition (PM) is a major health problem in the world. PM compromises antioxidant defense in the body. In particular, PM decreases tissue glutathione (GSH) levels. A high protein diet was found to restore tissue GSH levels in animal studies, however it is not recommended for the early phase of PM rehabilitation. Therefore, using dietary supplementation to restore tissue GSH without giving a high protein diet may be an adjunct therapy that helps improve antioxidant status during the early rehabilitation of PM. In this study, we systematically compared the efficacy of dietary supplementation of four cysteine prodrugs: N-acetylcysteine, L-2-oxo-4-thiazolidine-carboxylate, methionine, and GSH, on tissue GSH in mice fed a protein-deficient (0.5%) diet. Results showed that dietary supplementation of cysteine prodrugs to PM mice restored GSH levels in liver, lung, heart and spleen, but not in colon. GSH and GSSG levels in brain and kidney were not affected by cysteine prodrug or PM. Supplementation also restored the redox status in liver and heart (based on GSH/GSSG), and in liver and spleen (based on GSSG/2GSH reduction potential). This suggests that the restoration of GSH levels and redox status by cysteine prodrugs are tissue-specific, and that the two indicators of redox status are not always interchangeable. However, all four prodrugs exhibited similar GSH-enhancing capacities, showing no prodrug-specificity as seen in cell culture studies. In conclusion, this study provided information that may be useful in a clinical setting where a short-term oral supplementation of cysteine prodrugs is necessary for the early rehabilitation of PM patients.

Dietary vitamin E supplementation lowers blood pressure in spontaneously hypertensive rats

In spontaneously hypertensive rats (SHRs) excess endogenous aldehydes bind sulfhydryl groups of membrane proteins, altering membrane Ca2+ channels and increasing cytosolic free calcium and blood pressure. The thiol compound, N-acetyl cysteine, normalizes elevated blood pressure in SHRs by binding excess endogenous aldehydes. Vitamin E increases tissue glutathione levels--a storage form of cysteine. The aim of the present study was to investigate whether a dietary supplementation of vitamin E lowers blood pressure and prevents renal vascular changes by normalizing tissue aldehyde conjugates and cytosolic [Ca2+] in SHRs. Starting at 12 weeks of age, animals were divided into three groups of six animals each. Animals in the WKY-control group and SHR-control group were given a normal diet and the SHR-vitamin E group a diet supplemented with vitamin E (34 mg/ kg feed) for the next 9 weeks. After 9 weeks, systolic blood pressure, platelet [Ca2+]i, and liver, kidney and aortic aldehyde conjugates were significantly higher in SHR controls as compared to WKY controls and the SHR-vitamin E group. SHR-controls also showed smooth muscle cell hyperplasia in the small arteries and arterioles of the kidney. Dietary vitamin E supplementation in SHRs lowered the systolic blood pressure, cytosolic [Ca2+], tissue aldehyde conjugates and attenuated adverse renal vascular changes.

Effect of glutathione augmentation on lipid peroxidation after spinal cord injury

Lipid peroxidation (LPO) is considered a major factor in damage spread after spinal cord injury (SCI). Therapies that limit LPO after SCI have demonstrated some utility in clinical trials, but more effective treatments are needed. In the present study the effects of augmenting SC levels of the endogenous antioxidant glutathione (GSH) on LPO after SCI were studied in a rat contusion injury model. A significant decrease in GSH occurred 1h after SCI which was paralleled by increases of 123% in malondialdehyde (MDA) and >500% in the 4-hydroxyalkenals (4-HA's), two LPO products. SC irrigation with gamma-glutamylcysteine (GC) preserved GSH and reduced 4-HA's below naive levels but had no effect on MDA. By 24 h after SCI, MDA returned to naive levels but 4-HA's were still elevated. Once again, GC treatment reduced 4-HA's. 4-HA's are much more reactive than MDA and are considered among the most toxic LPO products. These results suggest that (1) conditions after SCI may favor particular branches of the LPO pathway leading to differential LPO product levels, (2) MDA measurement is not by itself an adequate test for the presence or magnitude of LPO after SCI, (3) binding of GSH to 4-HA's may be an important mechanism by which the GSH system confers protection against LPO after SCI, and (4) SC GSH can be augmented after trauma by local irrigation with GC. These results also suggest that GSH augmentation may be an effective strategy for curtailment of LPO-mediated damage in acute phase SCI.

Antidiabetic effect of a prodrug of cysteine, L-2-oxothiazolidine-4-carboxylic acid, through CD38 dimerization and internalization

CD38 is a bifunctional enzyme synthesizing (ADP-ribosyl cyclase) and degrading (cyclic ADP-ribose (cADPR) hydrolase) cADPR, a potent Ca(2+) mobilizer from intracellular pools. CD38 internalization has been proposed as a mechanism by which the ectoenzyme produced intracellular cADPR, and thiol compounds have been shown to induce the internalization of CD38. Here, we show that the disulfide bond between Cys-119 and Cys-201 in CD38 may be involved in CD38 dimerization and internalization. We tested the effect of a reducing agent, l-2-oxothiazolidine-4-carboxylic acid (OTC), a prodrug of cysteine, on CD38 internalization in pancreatic islets. OTC enhanced insulin release from isolated islets as well as CD38 internalization and cytoplasmic Ca(2+) level. Furthermore, islet cells treated with antisense CD38 oligonucleotide showed inhibition of OTC-induced insulin secretion. Intake of OTC in db/db mice ameliorated glucose tolerance, insulin secretion, and morphology of islets when compared with control mice. These data indicate that OTC improves glucose tolerance by enhancing insulin secretion via CD38/cADPR/Ca(2+) signaling machinery. Thus, OTC may represent a novel class of antidiabetic drug.

In vivo reversal of glutathione deficiency and susceptibility to in vivo dexamethasone-induced apoptosis by N-acetylcysteine and L-2-oxothiazolidine-4-carboxylic acid, but not ascorbic acid, in thymocytes from gamma-glutamyltranspeptidase-deficient knockout mice

Cellular glutathione is released during apoptosis and may play a role in the regulation of the mitochondrial permeability transition pore. The question of whether only cytosolic glutathione is important in apoptosis, or whether mitochondrial glutathione also plays a role, was investigated using gamma-glutamyltranspeptidase-deficient knockout mice. Thymocytes from these mice were found to have both glutathione pools diminished and they were more susceptible to dexamethasone (DEX)-induced apoptosis. Supplementation with N-acetylcysteine (NAC) and L-2-oxothiazolidine-4-carboxylic acid replenished both glutathione pools and provided protection from apoptosis. Ascorbate supplementation was beneficial to the mitochondrial glutathione pool, but apoptosis was not prevented. NAC supplementation caused an increase in reactive oxygen species formation and cardiolipin oxidation, but had no adverse affect on the amount of apoptotic cells. Our results suggest that the glutathione status is an important factor in apoptosis and indirect evidence indicates that the cytosolic pool of glutathione may be important in DEX-induced apoptosis, with mitochondrial events being secondary, and may reflect the execution phase.

Endothelium-dependent vasorelaxation in inhibited by in vivo depletion of vascular thiol levels: role of endothelial nitric oxide synthase

Thiols like glutathione may serve as reducing cofactors in the production of nitric oxide (NO) and protect NO from inactivation by radical oxygen species. Depletion of thiol compounds reduces NO-mediated vascular effects in vitro and in vivo. The mechanisms underlying these actions are not clear, but may involve decreased synthesis of NO and/or increased degradation of NO. This study investigates the effect of glutathione depletion on the response to NO-mediated vasodilation induced by acetylcholine (Ach, 10 micrograms/kg), endothelial NO synthase (eNOS) activity and potential markers of vascular superoxide anion (O2.-) production in conscious chronically catheterized rats. Thiol depletion induced by buthionine sulfoximine (BSO, 1 g i.p. within 24 h) decreased the hypotensive effect of Ach by 30% (MAP reduction before BSO 27 +/- 3 mmHg, 19 +/- 3 mmHg after BSO, (mean +/- SEM), p < .05, n = 8). The impaired effect of Ach was associated with a significant reduction in eNOS activity (control: 7.7 +/- 0.8, BSO: 3.9 +/- 0.4 pmol/min/mg protein (p < .05), n = 6). In contrast, neither NADH/NADPH driven membrane-associated oxidases nor lucigenin reductase activity were significantly (p < .05) affected by BSO (BSO: 4415 +/- 123, control: 4105 +/- 455 counts/mg; n = 6) in rat aorta. It is concluded that in vivo thiol depletion results in endothelial dysfunction and a reduced receptor-mediated vascular relaxation. This effect is caused by reduced endothelial NO formation.

N-acetyl-L-cysteine exerts direct anti-aggregating effect on human platelets

BACKGROUND

N-acetyl-L-cysteine, a thiol compound, has been shown to potentiate the inhibition of platelet aggregation exerted by organic nitrates and to increase the anti-aggregating effect of L-arginine, which promotes endogenous synthesis of nitric oxide (NO) acting as substrate of platelet constitutive nitric oxide synthase (NOS). It is not known whether this thiol can exert direct effects on platelet aggregability.

MATERIALS AND METHODS

14 healthy male volunteers provided platelet samples to investigate whether N-acetyl-L-cysteine directly influences platelet function and intraplatelet levels of 3',5' cyclic guanosine monophosphate (cGMP), which represents the second messenger involved in NO-induced antiaggregation. Some experiments were repeated in the presence of NOS inhibitor NG-monomethyl-L-arginine (L-NMMA), of nitric oxide-sensitive guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), of the selective cGMP phosphodiesterase inhibitor zaprinast and of calcium ionophores (A23187, ionomycin).

RESULTS

N-acetyl-L-cysteine at 3000-6000 micromol L-1 decreases the responses of human platelets both in platelet-rich plasma (aggregation induced by adenosine 5-diphosphate) and in whole blood (aggregation induced by collagen). The anti-aggregating effect was prevented by preincubation with L-NMMA and guanylyl cyclase inhibitor ODQ. In resting platelets, N-acetyl-L-cysteine increased the levels of cGMP starting from a concentration of 3000 micromol L-1. Permeabilized platelets exhibited an increased sensitivity to the anti-aggregating effect of N-acetyl-L-cysteine. Also, cGMP phosphodiesterase inhibition or the increase in calcium availability, enhanced N-acetyl-L-cysteine effects on platelets.

CONCLUSION

N-acetyl-L-cysteine exerts direct anti-aggregating effects through an increased bioavailability of platelet nitric oxide.

Dietary vitamin C supplementation lowers blood pressure in spontaneously hypertensive rats

In spontaneously hypertensive rats (SHRs) excess endogenous aldehydes bind sulfhydryl groups of membrane proteins, altering membrane Ca2+ channels and increasing cytosolic free calcium and blood pressure. The thiol compound, N-acetyl cysteine, normalizes elevated blood pressure in SHRs by binding excess endogenous aldehydes. Vitamin C can increase tissue cysteine and glutathione levels. The aim of the present study was to investigate whether a dietary supplementation of vitamin C can lower tissue aldehydes and blood pressure and normalize associated biochemical and histopathological changes in SHRs. Starting at 12 weeks of age, animals were divided into 3 groups of 6 animals each. Animals in the WKY-control group and SHR-control group were given a normal diet and the SHR-vitamin C group a diet supplemented with vitamin C (1000 mg/kg feed) for the next 9 weeks. After nine weeks, systolic blood pressure, platelet [Ca2+]i, plasma insulin and liver, kidney and aortic aldehyde conjugates were significantly higher in SHR controls as compared to WKY controls and the SHR-vitamin C group. SHR-controls also showed smooth muscle cell hyperplasia in the small arteries and arterioles of the kidneys. Dietary vitamin C supplementation in SHRs lowered the systolic blood pressure, tissue aldehyde conjugates and attenuated adverse renal vascular changes.

Altered glutathione anti-oxidant metabolism during tumor progression in human renal-cell carcinoma

It has been proposed that oxidative stress develops in tumors, with important consequences for growth and progression. To investigate this hypothesis, we measured low m.w. thiols, disulfides, protein-mixed disulfides and a pool of major anti-oxidant enzymes in renal-cortex as well as renal-cell carcinoma (RCC) specimens at stages I-II and III. Our data showed (i) a significant increase in the levels of total intracellular glutathione at both tumor stages (levels were 2.6-2.8 fold higher than those in the normal renal cortex), (ii) a marked lowering of the GSH/GSSG ratio in stage I-II accompanied by a significant decrease of many GSH-dependent enzymes (i.e., GPX, GST, GGT, GR) and (iii) unchanged GSH/GSSG ratio and GSH-dependent enzyme activity in stage III with respect to normal renal cortex. These results indicate that relevant variations exist in the glutathione antioxidant system in the different stages of RCC and support the hypothesis that oxidative stress plays an important role in RCC growth and progression.

Altered gene expression in the liver of gamma-glutamyl transpeptidase-deficient mice

We used mice deficient in gamma-glutamyl transpeptidase (GGT) to analyze the effects of GGT deficiency and altered thiol levels on gene expression in liver. GGT-deficient mice have markedly reduced levels of glutathione (GSH), cysteine, methionine, and cysteinylglycine in liver. Steady-state RNA levels of the catalytic subunit of gamma-glutamylcysteine synthetase (gamma-GCS), the rate-limiting enzyme in GSH synthesis, are elevated 4-fold in these mice, while those for glutathione synthetase (GSH syn) are elevated 2-fold. RNA levels of cystathionase (cystathionine gamma-lyase), a key enzyme in the synthesis of cysteine from methionine, are elevated approximately 3.5-fold. In contrast, levels of RNA coding for multidrug resistance protein 2 (MRP2), which transports GSH into bile, are half wild-type values. We found no change in RNA levels of enzymes related to oxidative injury (CuZn and Mn superoxide dismutases [SOD], catalase, and glutathione peroxidase). Similarly, RNA levels of glutathione reductase and ribonucleotide reductase were unchanged. Furthermore, in contrast to previous in vitro results, methyl methanesulfonate did not induce stress-activated signal transduction as measured by c-jun phosphorylation in livers of GGT-deficient mice, despite further depletion of GSH by buthionine sulfoximine. Our findings indicate that GGT deficiency itself and/or altered thiol levels regulate expression of genes involved in GSH metabolism, but have no effect on the expression of other antioxidant genes.

Redox state is a central modulator of the balance between self-renewal and differentiation in a dividing glial precursor cell

We have discovered that intracellular redox state appears to be a necessary and sufficient modulator of the balance between self-renewal and differentiation in dividing oligodendrocyte-type-2 astrocyte progenitor cells. The intracellular redox state of freshly isolated progenitors allows prospective isolation of cells with different self-renewal characteristics. Redox state is itself modulated by cell-extrinsic signaling molecules that alter the balance between self-renewal and differentiation: growth factors that promote self-renewal cause progenitors to become more reduced, while signaling molecules that promote differentiation cause progenitors to become more oxidized. Moreover, pharmacological antagonists of the redox effects of these cell-extrinsic signaling molecules antagonize their effects on self-renewal and differentiation, indicating that cell-extrinsic signaling molecules that modulate this balance converge on redox modulation as a critical component of their effector mechanism.

Blood glutathione synthesis rates in healthy adults receiving a sulfur amino acid-free diet

The availability of cysteine is thought to be the rate limiting factor for synthesis of the tripeptide glutathione (GSH), based on studies in rodents. GSH status is compromised in various disease states and by certain medications leading to increased morbidity and poor survival. To determine the possible importance of dietary cyst(e)ine availability for whole blood glutathione synthesis in humans, we developed a convenient mass spectrometric method for measurement of the isotopic enrichment of intact GSH and then applied it in a controlled metabolic study. Seven healthy male subjects received during two separate 10-day periods an L-amino acid based diet supplying an adequate amino acid intake or a sulfur amino acid (SAA) (methionine and cysteine) free mixture (SAA-free). On day 10, L-[1-(13)C]cysteine was given as a primed, constant i.v. infusion (3 micromol x kg(-1) x h(-1)) for 6 h, and incorporation of label into whole blood GSH determined by GC/MS selected ion monitoring. The fractional synthesis rate (mean +/- SD; day(-1)) of whole blood GSH was 0.65 +/- 0.13 for the adequate diet and 0.49 +/- 0.13 for the SAA-free diet (P < 0.01). Whole blood GSH was 1,142 +/- 243 and 1,216 +/- 162 microM for the adequate and SAA-free periods (P > 0.05), and the absolute rate of GSH synthesis was 747 +/- 216 and 579 +/- 135 micromol x liter(-1) x day(-1), respectively (P < 0.05). Thus, a restricted dietary supply of SAA slows the rate of whole blood GSH synthesis and diminishes turnover, with maintenance of the GSH concentration in healthy subjects.

Dietary alpha-lipoic acid supplementation lowers blood pressure in spontaneously hypertensive rats

BACKGROUND AND OBJECTIVES

In spontaneously hypertensive rats (SHRs), excess endogenous aldehydes bind sulfhydryl groups of membrane proteins, altering membrane Ca2+ channels and increasing cytosolic free calcium and blood pressure. The thiol compound, N-acetyl cysteine, normalizes elevated blood pressure in SHRs by binding excess endogenous aldehydes and normalizing membrane Ca2+ channels and cytosolic free calcium. The aim of the present study was to investigate whether a dietary supplementation of an endogenous fatty acid, alpha-lipoic acid, another thiol compound that is known to increase tissue cysteine and glutathione, can lower blood pressure and normalize associated biochemical and histopathological changes in SHRs.

METHODS AND RESULTS

Starting at 12 weeks of age, animals were divided into three groups of six animals each. Animals in the Wistar- Kyoto (WKY) rat control group and the SHR control group were given a normal diet, and the SHR-lipoic acid group was given a diet supplemented with lipoic acid (500 mg/kg feed) for the next 9 weeks. After 9 weeks, systolic blood pressure, platelet [Ca2+]i, plasma insulin and liver, kidney and aortic aldehyde conjugates were significantly higher in SHR controls as compared with WKY rat controls and the SHR lipoic acid group. SHR controls also showed smooth muscle cell hyperplasia in the small arteries and arterioles of the kidneys.

CONCLUSIONS

Dietary alpha-lipoic acid supplementation in SHRs lowered the systolic blood pressure, cytosolic [Ca2+]i, blood glucose and insulin levels, and tissue aldehyde conjugates, and attenuated adverse renal vascular changes.

The glutathione precursor L-2-oxothiazolidine-4-carboxylic acid protects against liver injury due to chronic enteral ethanol exposure in the rat

L-2-oxothiazolidine-4-carboxylic acid (OTC) is a cysteine prodrug that maintains glutathione in tissues. Here, its effect on alcohol-induced liver injury in an enteral alcohol feeding model was investigated. Male Wistar rats were given control high-fat or ethanol containing diets enterally for 4 weeks. Treated rats received 500 mg/kg/d of dietary OTC. Ethanol delivery, weight gain, and the cyclic pattern of ethanol in the urine were not different between the OTC-ethanol and ethanol groups. After 4 weeks, serum aspartate transaminase (AST), necrosis and inflammation were elevated significantly by ethanol compared with appropriate high-fat controls, effects blocked by OTC. Moreover, ethanol elevated hepatic tumor necrosis factor alpha (TNF-alpha) messenger RNA (mRNA) and the nuclear transcription factor nuclear factor kappaB (NFkappaB) 2-3 fold. NFkappaB in isolated Kupffer cells was also increased by ethanol. These effects were all blocked by OTC treatment. Additionally, superoxide production was higher in Kupffer cells isolated from ethanol-treated rats, an effect blunted by OTC. OTC also increased circulating glutathione (GSH) levels about 2-fold; however, GSH levels were not affected by ethanol or OTC in livers from the groups studied. Surprisingly, GSH was elevated by ethanol and OTC treatment in isolated Kupffer cells about 2-fold. Moreover, GSH (Ki-10 micromol/L) and cysteinyl-glycine, but not oxidized glutathione (GSSG) or OTC, blunted the LPS-induced increase in calcium in isolated Kupffer cells, possibly by activating a glycine-gated chloride channel due to their structural similarity with glycine. Collectively, it is concluded that GSH is protective, in part, by increasing circulating GSH, which blunts activation of Kupffer cells via the glycine-gated chloride channel.

L-2-[(13)C]oxothiazolidine-4-carboxylic acid: a probe for precursor mobilization for glutathione synthesis

L-2-oxothiazolidine-4-carboxylic acid (OTZ), a 5-oxoproline analog, is metabolized by 5-oxoprolinase and converted to cysteine, the rate-limiting amino acid for GSH synthesis, with the release of CO(2). [(13)C]OTZ (1.5 mg/kg) was used in 12 healthy men and women (ages 23-73 yr) to indirectly assess precursor mobilization for GSH synthesis when stores were reduced by 2 g acetaminophen. Expired breath samples were analyzed for (13)CO(2), and results were analyzed using noncompartmental and two-compartment open minimal models. Results show an increase in (13)C excretion (higher OTZ hydrolysis) when GSH stores were reduced and 5-oxoprolinase substrate utilization patterns, consequently, were altered (P < 0. 01). A metabolic rate index (MRI) of the OTZ probe was found to be significantly higher after reduction of GSH content by acetaminophen (P < 0.05). The difference in adaptive capacity (difference between control and postacetaminophen metabolic rate indexes) was two times as large in the young than the old subjects (P < 0.01). These data support the use of [(13)C]OTZ as a probe to identify individuals who may be at risk for low GSH stores or who have an impaired capacity to synthesize GSH.

The role of antioxidants in protection from impulse noise

The hearing loss from exposure to noise and ototoxic drugs share a number of audiological and pathological similarities. Recent research has shown that reactive oxygen species (ROS) may be a common factor in both noise- and drug-induced hearing loss. This review describes three experiments that point to ROS as a causative factor in both noise- and drug-induced hearing loss and antioxidants as a protective agent. In the first experiment, the ears of chinchillas were treated with R-N6-phenylisopropyladenoisine (R-PIA) and exposed to 150-dB impulse noise. The treated ears developed substantially less permanent hearing loss (PTS) and hair cell loss than the untreated ears. One interpretation of this experiment is that R-PIA increases the availability of glutathione (GSH). In the second experiment, the role of GSH was specifically examined. The ears of chinchillas were treated with glutathione monoethylester (GEE), a pro-GSH drug that has been shown to readily cross cell membranes and increase GSH levels. The GEE-treated ears had significantly less PTS and hair-cell loss than the nontreated ear. Previous research has shown that moderate levels of noise exposure can increase a subject's resistance to noise, and also increase the availability of antioxidant enzymes in the cochlea. In the third experiment, chinchillas were given a series of "toughening" exposures (i.e., 6 h of a 0.5-kHz OB noise at 95 dB for 10 days). After the series of "toughening" exposures, the subjects were treated with carboplatin, a drug that causes massive inner-hair-cell lesions in the chinchilla. The animals receiving the 10-day toughening exposure developed less PTS and hair-cell loss than the control animals.

Oxidant mechanisms in toxic acute renal failure

Over the last decade, there is accumulating evidence for a role of reactive oxygen metabolites in the pathogenesis of a variety of renal diseases, including gentamicin, glycerol, cisplatin, and cyclosporine A models of toxic acute renal failure. Gentamicin has been shown both in in vitro and in vivo studies to enhance the generation of reactive oxygen metabolites. Iron is important in models of tissue injury, presumably because it is capable of catalyzing free-radical formation. Gentamicin has been shown to cause release of iron from renal cortical mitochondria. Scavengers of reactive oxygen metabolites as well as iron chelators provide protection in gentamicin-induced nephrotoxicity. In glycerol-induced acute renal failure, an animal model of rhabdomyolysis, there is enhanced generation of hydrogen peroxide, and scavengers of reactive oxygen metabolites and iron chelators provide protection. Although the dogma is that the myoglobin is the source of iron, recent studies suggest that cytochrome P450 may be an important source of iron in this model. In addition, there are marked alterations in antioxidant defenses, such as glutathione, as well as changes in heme oxygenase. Several recent in vitro and in vivo studies indicate an important role of reactive oxygen metabolites in cisplatin-induced nephrotoxicity. Thus, catalytic iron is increased both in vitro and in vivo by cisplatin, and iron chelators as well as hydroxyl radical scavengers have been shown to be protective. Recent studies indicate that cytochrome P450 may also be an important source of the catalytic iron in cisplatin nephrotoxicity. Cyclosporine A has been shown to enhance generation of hydrogen peroxide in vitro and enhance lipid peroxidation in vitro and in vivo. Antioxidants have been shown to be protective in cyclosporine A nephrotoxicity. This collective body of evidence suggests an important role for reactive oxygen metabolites in toxic acute renal failure and may provide therapeutic opportunities of preventing or treating acute renal failure in humans.

Dietary vitamin B6 supplementation attenuates hypertension in spontaneously hypertensive rats

In spontaneously hypertensive rats (SHRs) excess endogenous aldehydes bind sulfhydryl groups of membrane proteins, altering membrane Ca2+ channels, increasing cytosolic free calcium and blood pressure. N-acetyl cysteine normalizes elevated blood pressure in SHRs by binding excess endogenous aldehydes. It is known that dietary vitamin B6 supplementation can increase the level of endogenous cysteine. Our objective was to investigate whether a dietary supplementation of vitamin B6 can prevent hypertension and associated changes in SHRs. Starting at 7 weeks of age, animals were divided into three groups of six animals each. Animals in WKY-control group and SHR-control group were given a normal vitamin B6 diet; and SHR-vitamin B6 group, a high vitamin B6 diet (20 times the recommended dietary intake; RDA) for the next 14 weeks. After 14 weeks, systolic blood pressure, platelet [Ca2+]i and liver, kidney and aortic aldehyde conjugates were significantly higher in SHR controls compared to WKY controls. These animals also showed smooth muscle cell hyperplasia in the small arteries and arterioles of the kidneys. Dietary vitamin B6 supplementation attenuated the increase in systolic blood pressure, tissue aldehyde conjugates and associated changes. These results further support the hypothesis that aldehydes are involved in increased systolic blood pressure in SHRs and suggest that vitamin B6 supplementation may be an effective antihypertensive.

Net efflux of cysteine, glutathione and related metabolites from rat hippocampal slices during oxygen/glucose deprivation: dependence on gamma-glutamyl transpeptidase

Extracellular metabolism of the protective substance glutathione (gamma-glutamyl-cysteinyl-glycine) may generate cysteine, glycine, several gamma-glutamyl-containing dipeptides and possibly free glutamate, all of which could participate in neurotoxicity. In the present study, we have examined how blockage of gamma-glutamyl transpeptidase, the key enzyme in glutathione degradation, influences the extracellular concentrations of glutathione, cysteine and related metabolites during anoxia/aglycemia of rat hippocampal slices. The net efflux, i.e., the increase in extracellular concentration due to changes in release and/or uptake, of cysteine, cysteine sulfinate, gamma-glutamyl-glutamate, gamma-glutamyl-glutamine, glutathione, gamma-glutamyl-cysteine and glutamate increased as a result of anoxia/aglycemia. These increases in net efflux of cysteine, cysteine sulfinate, gamma-glutamyl-glutamate and gamma-glutamyl-glutamine were reduced or blocked by acivicin, an inhibitor of gamma-glutamyl transpeptidase. In contrast, acivicin caused an increase in both basal and anoxia/aglycemia-induced net efflux of glutathione whereas the basal and anoxia/aglycemia-induced efflux of glutamate was unchanged by acivicin treatment. The effect of acivicin on the efflux of gamma-glutamyl-cysteine was similar to that of glutathione although less pronounced. Addition of beta-mercaptoethanol to the incubation medium during and after 30 min of anoxia/aglycemia decreased the net efflux of cysteine sulfinate specifically, indicating that the increase in cysteine sulfinate during anoxia/aglycemia may be partly derived from the spontaneous oxidation of cysteine. The results suggest that gamma-glutamyl transpeptidase may be involved in the regulation of the extracellular concentrations of cysteine, several gamma-glutamyl-containing dipeptides and glutathione but not glutamate during ischemia.

Nutritional regulation of glutathione in stroke

In contrast to cardiovascular disease, the impact of nutritional status on the prevention and outcome of stroke has received limited investigation. We present a mechanism based on animal studies, clinical data, and epidemiological data by which protein-energy status in the acute stroke and immediate postinjury periods may affect outcome by regulating reduced glutathione (GSH), a key component of antioxidant defense. As cysteine is the limiting amino acid for GSH synthesis, the GSH concentration of a number of nonneural tissues has been shown to be decreased by fasting, low-protein diets, or diets limiting in sulfur amino acids. The mechanism may also be relevant in brain since GSH in some brain regions is responsive to dietary sulfur amino acid supply and to the pro-cysteine drug, L-2-oxothiazolidine-4-carboxylate. The latter is an intracellular cysteine delivery system used to overcome the toxicity associated with cysteine supplementation. These findings may provide the mechanism to explain both the inverse correlation between dietary protein and stroke mortality and the documented association between suboptimal protein-energy status and diminished functional status following a stroke. Future investigations should examine the role of nutritional intervention in neuroprotective strategies aimed at improving stroke outcome. Pharmacological interventions such as L-2-oxothiazolidine-4-carboxylate should be investigated in animal models of stroke, as well as the impact of nutritional status on the response to these agents. Finally, micronutrient deficiencies that may accompany protein-energy malnutrition, such as selenium, should also be investigated for their role in antioxidant defense in cerebral ischemia.

Inhibition of apoptosis and virus replication in feline immunodeficiency virus-infected cells by N-acetylcysteine and ascorbic acid

Infection of feline immunodeficiency virus (FIV) has been shown to induce apoptosis that might be associated with the lymphocyte depletion in the infected cats. To investigate the inhibitory effect of antioxidants on FIV-induced apoptosis, we examined the effect of N-acetylcysteine (NAC) and ascorbic acid (AA) on apoptosis and virus replication in feline lymphoblastoid (Fel-039) and fibroblastoid (CRFK) cell lines infected with FIV. The treatment with NAC or AA induced a significant inhibition of viral replication and apoptosis in Fel-039 cells and tumor necrosis factor alpha (TNF-alpha)-treated CRFK cells infected with FIV. Both cell lines in the presence of noncytotoxic concentrations of NAC or AA showed in increase of intracellular glutathione (GSH) level, which might protect the cells against oxidative stresses exerted by FIV infection and TNF-alpha treatment. On the basis of these in vitro results, we suggest that antioxidant therapies aimed at restoring depleted GSH level might be effective for inhibition of viral replication and cell death associated with the development of immunodeficiency.

L-2-Oxothiazolidine-4-carboxylic acid prevents endotoxin-induced cardiac dysfunction

We tested the hypothesis that treatment with the glutathione repleting agent, L-2-oxothiazolidine-4-carboxylic acid (OTZ), could prevent endotoxin-induced ventricular dysfunction. Rabbits were treated with OTZ 2.4 g/kg (10% solution subcutaneously), or an equal volume and osmolality of saline, 24 h prior to, and again (intravenously) just prior to, infusion of 1 mg/kg E. coli endotoxin (or vehicle control). Ventricular contractility was measured in isolated hearts perfused by support rabbits. Contractility did not change in control groups (Saline/Control [n = 7] or OTZ/Control [n = 7]) over 6 h. However, Emax decreased in the Saline/Endotoxin group (-16.1 +/- 4.5% from baseline, n = 7, p < 0.05) and this was prevented by pretreatment with OTZ in the OTZ/ Endotoxin group (+6.3 +/- 4.1%, n = 7, p < 0.05 by analysis of variance). To better understand the mechanism of this effect we measured myocardial glutathione concentration and found it to be greater in OTZ/Endotoxin animals (104 +/- 4 ng/g) than in the Saline/Endotoxin animals (80 +/- 3 ng/g, p < 0.05). OTZ did not appreciably alter the endotoxin-induced increase in serum concentration of tumor necrosis factor (TNF) or the endotoxin-induced increase in myocardial leukocyte content. We conclude that oxygen radicals contribute to the early decrease in left ventricular contractility after endotoxin infusion and this decrease may be prevented by OTZ.

Hepatic export of glutathione and uptake of constituent amino acids, glutamate and cysteine, in broilers in vivo

This study was conducted to document the glutathione (GSH) cycle (interorgan circulation of GSH) in broilers in vivo. Two experiments were conducted on 36 anesthetized male broilers (n = 6 per treatment) implanted with cannulae in the carotid artery, hepatic portal, and hepatic veins. Plasma GSH, glutamate, cysteine, cystine, and cysteinylglycine levels in each vessel were monitored following a bolus injection [Experiment (Exp.) 1] or 30 min continuous infusion (Exp. 2) of GSH, or a gamma-glutamyltranspeptidase inhibitor (AT125) into the hepatic portal vein. Controls received saline alone. The GSH and AT125 treatments were used to determine the effect of increasing the prehepatic GSH load and of inhibiting systemic GSH degradation, respectively, on the GSH cycle. Hepatic export of GSH was clearly evident in all three treatment groups in both experiments (Exp.). The GSH and AT125 treatments raised amino acid levels in some or all of the vessels, whereas cysteinylglycine was elevated by AT125 and depressed by the GSH treatment compared to Controls. Hepatic uptake of glutamate, cysteine, and/or cystine was observed in Controls and GSH-treated birds, but not in birds given AT125 (Exp. 2). Neither hepatic export nor uptake of cysteinylglycine was observed in any treatment group. The results clearly demonstrate the ability of the avian liver to export GSH into the general circulation despite alterations that might arise from changes in extra-hepatic ability to utilize GSH or its constituent amino acids.

Age-related changes of plasma glutathione and cysteine in broilers: effect of dithiothreitol reduction vitro on free and bound pools

A study was conducted to determine free and protein-bound pools of glutathione and cysteine in the plasma of male broiler chicks. Birds were brooded in battery cages and provided ad libitum access to a starter diet and water. Plasma was treated with a reducing agent, dithiothreitol (DTT), or left untreated, and analyzed by HPLC to determine free and protein bound pools of reduced (GSH) and oxidized (GSSG) glutathione, cysteine (Cys), and cystine (Cyss). With respect to total plasma pools of GSH and Cys, between 0 and 21 d of age; 1) free GSH increased from 30 to 90% with a reciprocal decrease in protein-bound GSH, but GSSG was not detected; and 2) free Cys decreased from 20 to 10%, free Cyss increased from 24 to 45%, and protein-bound Cys decreased from 55 to 44%. The majority of the GSH plasma pool in this study was present in a free, acid-soluble form, whereas most of the total Cys pool was present as Cyss or bound to protein.

N-acetylcysteine protects from glutathione depletion in rats exposed to hyperoxia

BACKGROUND

N-acetylcysteine (NAC) may protect against oxidative injury by providing cysteine for glutathione (GSH) biosynthesis or by direct reactions with electrophiles. We have recently shown that hyperoxic exposure of rats prior to liver perfusion is associated with significant decreases in hepatic GSH and significant changes in biliary amino acid concentrations. We hypothesized that NAC administration during hyperoxic exposure would prevent depletion of hepatic GSH by providing cysteine for GSH biosynthesis.

METHODS

NAC was administered during two conditions known to induce GSH depletion: hyperoxic exposure and biochemical inhibition of GSH synthesis using buthionine sulfoximine (BSO). After 48 hours, GSH concentrations in bile, liver and perfusate and biliary amino acid concentrations were determined using isolated perfused liver preparations.

RESULTS

Administration of NAC to rats maintained in normoxic or hyperoxic conditions, prior to liver perfusion, resulted in dose-dependent increases in GSH concentrations in bile, liver and perfusate, increases in bile flow rates and changes in biliary amino acid concentrations. When BSO was given concurrently with NAC in normal or hyperoxic conditions, these effects were not observed, and oxidant stress was evident.

CONCLUSIONS

NAC prevents oxidant stress during hyperoxic exposure, most likely by supplying cysteine as a precursor for GSH synthesis.

Glutathione: an overview of biosynthesis and modulation

Glutathione (GSH; gamma-glutamylcysteinylglycine) is ubiquitous in mammalian and other living cells. It has several important functions, including protection against oxidative stress. It is synthesized from its constituent amino acids by the consecutive actions of gamma-glutamylcysteine synthetase and GSH synthetase. gamma-Glutamylcysteine synthetase activity is modulated by its light subunit and by feedback inhibition of the end product, GSH. Treatment with an inhibitor, buthionine sulfoximine (BSO), of gamma-glutamylcysteine synthetase leads to decreased cellular GSH levels, and its application can provide a useful experimental model of GSH deficiency. Cellular levels of GSH may be increased by supplying substrates and GSH delivery compounds. Increasing cellular GSH may be therapeutically useful.

A phase I/II evaluation of oral L-2-oxothiazolidine-4-carboxylic acid in asymptomatic patients infected with human immunodeficiency virus

A randomized double-blind, placebo-controlled study was conducted in 37 asymptomatic HIV-infected individuals (mean CD4 count 707 cells/mm3) to characterize the safety, pharmacokinetics, and effect on blood thiols of three dosage levels of a cysteine prodrug, L-2-oxothiazolidine-4-carboxylic acid (OTC; Procysteine; Clintec Technologies, Deerfield, IL). Single-dose administration of OTC resulted in measurable plasma levels at all dosages, with a mean peak plasma concentration of 734 +/- 234 nmol/mL at the highest dosage studied. After 4 weeks of administration three times daily, a statistically significant increase was seen in whole blood glutathione in the 1,500 mg and 3,000 mg dose groups compared with the placebo group. A significant increase in whole blood cysteine and peripheral blood mononuclear cell (PBMC) glutathione was not seen during the study period.