Comparative study on the selenium- and N-acetylcysteine-related effects on the toxic action of hyperoxia, paraquat and the enzyme reaction hypoxanthine-xanthine oxidase in cultured endothelial cells

The mechanism of action of N-acetylcysteine (NAC): The emerging role of H2S and sulfane sulfur species

Initially adopted as a mucolytic about 60 years ago, the cysteine prodrug N-acetylcysteine (NAC) is the standard of care to treat paracetamol intoxication, and is included on the World Health Organization's list of essential medicines. Additionally, NAC increasingly became the epitome of an "antioxidant". Arguably, it is the most widely used "antioxidant" in experimental cell and animal biology, as well as clinical studies. Most investigators use and test NAC with the idea that it prevents or attenuates oxidative stress. Conventionally, it is assumed that NAC acts as (i) a reductant of disulfide bonds, (ii) a scavenger of reactive oxygen species and/or (iii) a precursor for glutathione biosynthesis. While these mechanisms may apply under specific circumstances, they cannot be generalized to explain the effects of NAC in a majority of settings and situations. In most cases the mechanism of action has remained unclear and untested. In this review, we discuss the validity of conventional assumptions and the scope of a newly discovered mechanism of action, namely the conversion of NAC into hydrogen sulfide and sulfane sulfur species. The antioxidative and cytoprotective activities of per- and polysulfides may explain many of the effects that have previously been ascribed to NAC or NAC-derived glutathione.

Calcium Oxalate Monohydrate is Associated with Endothelial Cell Toxicity But Not with Reactive Oxygen Species Accumulation

One characteristic of ethylene glycol overdose is a cardiopulmonary syndrome including hypertension and pulmonary edema with pathology indicating damage to the endothelium of heart, lung and brain vessels. The mechanism of the cardiopulmonary toxicity is unknown, but has been linked with accumulation of the metabolite calcium oxalate monohydrate (COM) in the endothelium. These studies have evaluated the hypothesis that COM or the oxalate ion produces endothelial damage in vitro and that damage is linked with induction of reactive oxygen species (ROS). In cultured human umbilical vein endothelial cells (HUVEC), COM, but not the oxalate ion, produced cytotoxicity in a dose- and time-dependent manner. Using three ROS-sensitive dyes, HUVEC exposed to COM did not significantly increase ROS production. Additionally, co-treatment with three antioxidants that operate by different mechanisms did not reduce COM cytotoxicity. As such, an increase in ROS production does not explain cell death in endothelial cells. Aluminum citrate, uniquely among citrate compounds, significantly reduced COM cytotoxicity to endothelial cells and thus may act as an adjunct therapy for ethylene glycol poisoning to reduce endothelial damage. These results imply that accumulation of COM in endothelial cells is an important aspect of the cardiopulmonary toxicity from ethylene glycol.

Combination of Selenomethionine and N-Acetylcysteine Alleviates the Joint Toxicities of Aflatoxin B1 and Ochratoxin A by ERK MAPK Signal Pathway in Porcine Alveolar Macrophages

Our previous studies showed that aflatoxin B1 (AFB1) and ochratoxin A (OTA) could trigger joint immune toxicity. Little is known about the combined effects of selenomethionine (SeMet) and N-acetylcysteine (NAC) on the joint toxicities of the two toxins. In this study, results showed that SeMet or NAC alone or in combination significantly alleviated the downswing of cell viability, glutathione production, and phagorytosis induced by AFB1 and OTA in porcine alveolar macrophages. The uptrend of lactate dehydrogenase activities, apoptosis, reactive oxygen species levels, and the relative mRNA of inflammatory cytokines triggered by the two toxins was decreased. Combination of them was more effective than single application. Knockdown of p38, c-JUN N-terminal kinase (JNK), or extracellular signal-regulated kinase (ERK) via use of the corresponding specific siRNA could alleviate the joint toxicities of AFB1 and OTA. However, the ERK but not p38 or JNK pathway was involved in the protection of SeMet and NAC against the immunotoxicity. In conclusion, combination of SeMet and NAC might be a new therapeutic orientation for preventing the joint toxicities induced by AFB1 and OTA.

Transport pathways for arsenic and selenium: a minireview

Arsenic and selenium are metalloids found in the environment. Arsenic is considered to pose the most significant potential threat to human health based on frequency of occurrence, toxicity and human exposure. Selenium, on the other hand, ranks only 147th in toxicity but, in contrast to arsenic, is a required micronutrient. Whether a toxin or micronutrient, their metabolism requires that cells to accumulate these metalloids. In this review we discuss the membrane proteins that transport arsenic and selenium into cells, from bacteria to humans, as well as some of the efflux proteins involved in detoxification.

N-acetyl-l-cysteine affords protection against lead-induced cytotoxicity and oxidative stress in human liver carcinoma (HepG2) cells

Although lead exposure has declined in recent years as a result of change to lead-free gasoline, several epidemiological have pointed out that it represents a medical and public health emergency, especially in young children consuming high amounts of lead-contaminated flake paints. A previous study in our laboratory indicated that lead exposure induces cytotoxicity in human liver carcinoma cells. In the present study, we evaluated the role of oxidative stress in lead-induced toxicity, and the protective effect of the anti-oxidant n-acetyl-l-cysteine (NAC). We hypothesized that oxidative stress plays a role in lead-induced cytotoxicity, and that NAC affords protection against this adverse effect. To test this hypothesis, we performed the MTT [3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide] assay and the trypan blue exclusion test for cell viability. We also performed the thiobarbituric acid test for lipid peroxidation. Data obtained from the MTT assay indicated that NAC significantly increased the viability of HepG₂ cells in a dose-dependent manner upon 48 hours of exposure. Similar trend was obtained with the trypan blue exclusion test. Data generated from the thiobarbituric acid test showed a significant (p ≤ 0.05) increase of MDA levels in lead nitrate-treated HepG₂ cells compared to control cells. Interestingly, the addition of NAC to lead nitrate-treated HepG₂ cells significantly decreased cellular content of reactive oxygen species (ROS), as evidenced by the decrease in lipid peroxidation byproducts. Overall, findings from this study suggest that NAC inhibits lead nitrate-induced cytotoxicity and oxidative stress in HepG₂ cells. Hence, NAC may be used as a salvage therapy for lead-induced toxicity in exposed persons.

[Oxidative stress in bronchopulmonary disease: contribution of N-acetylcysteine (NAC)]

Oxidative stress is a frequent mechanism involved in the pathogenesis of bronchopulmonary disease. The cause can be exogenous, in particular related to to atmospheric pollution and tobacco smoke, or endogenous, related to mobilization of inflammatory cells (macrophages and polymorphonuclear neutrophils). In this general review, we present work demonstrating this oxidative stress and activation of inflammatory cells. We discuss the effect of oxidative stress on the bronchial tree and the need to maintain an adequate balance between oxidants and anti-oxidants. This reviews focuses on experimental studies proving the anti-oxidant effect of NAC on glutathione synthesis and on different pharmacological models. We then discuss human trials, initially experimental then in different bronchopulmonary pathologies related to oxidative stress. Acetaminophen intoxication and pulmonary fibrosis are models for use of NAC. Recent work on COPD appears to show a decrease in exacerbations, improvement in symptoms and quality-of-life, and perhaps a reduction in the alteration of ventilatory function.

Mechanisms of N-acetylcysteine in the prevention of DNA damage and cancer, with special reference to smoking-related end-points

Although smoking cessation is the primary goal for the control of cancer and other smoking-related diseases, chemoprevention provides a complementary approach applicable to high risk individuals such as current smokers and ex-smokers. The thiol N-acetylcysteine (NAC) works per se in the extracellular environment, and is a precursor of intracellular cysteine and glutathione (GSH). Almost 40 years of experience in the prophylaxis and therapy of a variety of clinical conditions, mostly involving GSH depletion and alterations of the redox status, have established the safety of this drug, even at very high doses and for long-term treatments. A number of studies performed since 1984 have indicated that NAC has the potential to prevent cancer and other mutation-related diseases. N-Acetylcysteine has an impressive array of mechanisms and protective effects towards DNA damage and carcinogenesis, which are related to its nucleophilicity, antioxidant activity, modulation of metabolism, effects in mitochondria, decrease of the biologically effective dose of carcinogens, modulation of DNA repair, inhibition of genotoxicity and cell transformation, modulation of gene expression and signal transduction pathways, regulation of cell survival and apoptosis, anti-inflammatory activity, anti-angiogenetic activity, immunological effects, inhibition of progression to malignancy, influence on cell cycle progression, inhibition of pre-neoplastic and neoplastic lesions, inhibition of invasion and metastasis, and protection towards adverse effects of other chemopreventive agents or chemotherapeutical agents. These mechanisms are herein reviewed and commented on with special reference to smoking-related end-points, as evaluated in in vitro test systems, experimental animals and clinical trials. It is important that all protective effects of NAC were observed under a range of conditions produced by a variety of treatments or imbalances of homeostasis. However, our recent data show that, at least in mouse lung, under physiological conditions NAC does not alter per se the expression of multiple genes detected by cDNA array technology. On the whole, there is overwhelming evidence that NAC has the ability to modulate a variety of DNA damage- and cancer-related end-points.

Effect of N-acetylcysteine on colitis induced by acetic acid in rats

(1) To verify the proposed role of reactive oxygen species (ROS) in ulcerative colitis, the effect of an antioxidant N-acetylcysteine (NAC) was studied in acetic acid (AA)-induced colonic inflammation. (2) Depending on the dose used, NAC administered intracolonically was found to reduce the extent of colonic damage, along with a decrease in myeloperoxidase (MPO) activity, colonic wet weight and wet/dry weight ratio. (3) NAC attenuated the enhanced vascular permeability and prevented the depletion of colonic reduced glutathione (GSH) caused by AA administration. (4) The findings indicate that NAC may prove beneficial in the treatment of colitis.

Synergistic anticancer activity of 1,25-dihydroxyvitamin D(3) and immune cytokines: the involvement of reactive oxygen species

It was previously shown that 1,25-dihydroxyvitamin D(3) (1, 25(OH)(2)D(3)) enhances the cytotoxic activity of tumor necrosis factor alpha (TNFalpha), doxorubicin and menadione. A feature shared by these anticancer agents is the involvement of reactive oxygen species (ROS) in their action. In this work we found that 1, 25(OH)(2)D(3) acted synergistically with interleukin 1 beta (IL-1beta) or interleukin 6 (IL-6) to inhibit the proliferation of MCF-7 breast cancer cells. The extent of the synergism was maximal at 1 nM, a concentration at which 1,25(OH)(2)D(3), acting singly, only marginally reduced the cell number. The thiol antioxidant, N-acetylcysteine (NAC) abolished the synergism between IL-1beta or IL-6 and 1,25(OH)(2)D(3), but had only a small protective effect when the cytokines acted alone. NAC and reduced glutathione (GSH) protected MCF-7 cells from cytotoxicity induced both by TNFalpha alone and by TNFalpha and 1,25(OH)(2)D(3). A two-day exposure to TNFalpha caused a 27.7+/-3.1% (mean +/- SEM) reduction in GSH content. This effect increased to 46.4+/-5.5% by co-treatment with 1, 25(OH)(2)D(3) which did not affect GSH levels on it own. We conclude that 1,25(OH)(2)D(3) can act synergistically with anticancer cytokines present in the tumor milieu and that ROS plays a mediatory role in this interaction.

The role of N-acetylcysteine in protecting synovial fluid biomolecules against radiolytically-mediated oxidative damage: a high field proton NMR study

High field proton (1H) NMR spectroscopy has been employed to evaluate the abilities of the antioxidant thiol drug N-acetylcysteine and exogenous cysteine to protect metabolites present in intact inflammatory synovial fluid samples against oxidative damage arising from gamma-radiolysis (5.00 kGy) in the presence of atmospheric O2. Although oxidation of urate to allantoin by radiolytically-generated *OH radical was readily circumventable by pre-treatment of synovial fluids with N-acetylcysteine (1.00 or 3.00 x 10(-3) mol x dm(-3)) or cysteine (1.00, 2.00 or 5.00 x 10(-3) mol x dm(-3)), both thiols offered only a limited protective capacity with respect to hyaluronate depolymerisation and the production of formate from carbohydrates in general. Radiolytic products generated from the added thiols (predominantly their corresponding disulphides) were simultaneously detectable in 1H Hahn spin-echo spectra of gamma-irradiated synovial fluids, permitting a quantitative evaluation of the radioprotective capacity of these agents. It is concluded that the multicomponent analytical ability of high field 1H NMR spectroscopy provides much useful molecular information regarding mechanisms associated with the radioprotectant actions of thiols in intact biofluids.

Nitric oxide inhalation for paraquat-induced lung injury

BACKGROUND

When ingested, concentrated paraquat can cause either rapid death from multisystem failure and cardiovascular shock or delayed death from progressive pulmonary fibrosis. Diquat ingestion does not usually cause pulmonary fibrosis, but produces early onset acute renal failure.

CASE REPORT

A 52-year-old male ingested approximately 50 mL of a solution containing 13% paraquat and 7% diquat (about 6650 mg of paraquat and 3500 mg of diquat), and subsequently developed adult respiratory distress syndrome and pulmonary fibrosis. Survival prediction employing the criteria of Hart et al. for paraquat plasma levels was 30%. From the probable amount of paraquat ingested, severe toxicity was expected. The clinical course was not consistent with significant diquat toxicity. Treatment included oral Fuller's earth, forced diuresis, hemofiltration, N-acetylcysteine, methylprednisolone, cyclophosphamide, vitamin E, colchicine, and delayed continuous nitric oxide inhalation. The patient recovered and pulmonary function was subsequently normal.

CONCLUSION

It is unclear which, if any, of the above treatments contributed to recovery, but the encouraging outcome suggests a possible benefit of nitric oxide inhalation in paraquat poisoning which deserves further study.

Chronic inhibition of superoxide dismutase produces apoptotic death of spinal neurons

Mutations in the gene for Cu/Zn superoxide dismutase (SOD1) have been detected in some families with an autosomal dominant form of amyotrophic lateral sclerosis; these mutations appear to reduce the activity of this enzyme. To determine whether decreased SOD activity could contribute to motor neuron loss, SOD1 was inhibited chronically with either antisense oligodeoxynucleotides or diethyldithiocarbamate in spinal cord organotypic cultures. Chronic inhibition of SOD resulted in the apoptotic degeneration of spinal neurons, including motor neurons, over several weeks. Motor neuron loss was markedly potentiated by the inhibition of glutamate transport. In this paradigm, motor neuron toxicity could be entirely prevented by the antioxidant N-acetylcysteine and, to a lesser extent, by the non-N-methyl-D-aspartate glutamate receptor antagonist 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride. These data support the hypothesis that the loss of motor neurons in familial amyotrophic lateral sclerosis could be due to a reduction in SOD1 activity, possibly potentiated by inefficient glutamate transport.

Elevation of glutathione levels in bovine pulmonary artery endothelial cells by N-acetylcysteine

N-Acetylcysteine (NAC), a cysteine derivative with chemoprotective and radioprotective effects, was found to elevate bovine pulmonary artery endothelial cell (EC) glutathione after in vitro incubation. The elevation in glutathione was associated with enhanced uptake of radioactivity of cystine from the medium. Because cystine in medium was converted rapidly to cysteine and cysteinyl-NAC in the presence of NAC and given that cysteine has a higher affinity for uptake by EC than cystine, we conclude that the enhanced uptake of radioactivity was in the form of cysteine and at least part of the stimulatory effect of NAC on EC glutathione was due to a formation of cysteine by a mixed disulfide reaction of NAC with cystine similar to that previously reported for Chinese hamster ovarian cells (R. D. Issels et al. 1988. Biochem. Pharmacol. 37:881-888). However, NAC was more effective than cysteine in elevating cellular glutathione at equimolar concentrations, and at higher concentrations of NAC an elevation of EC glutathione occurred even in the absence of cystine in the medium through a currently unknown mechanism. Thus, at least two mechanisms are operative in the elevation of endothelial cellular glutathione by NAC. NAC may be a useful compound for elevating glutathione of the pulmonary vasculature for protection against oxidant stress.

Alteration of enzymes in ageing human fibroblasts in culture. V. Mechanisms of glutathione peroxidase modification

Ageing of WI-38 fibroblasts in culture was used as a model in order to investigate the evolution and the alteration of the key antioxidant enzyme glutathione peroxidase. The activity of glutathione peroxidase is influenced by the presence of selenium in the culture medium and we have also shown that the specific activity of this enzyme does not decrease during ageing, but rather slightly increases. No alteration could be detected by immunotitration. Also the kinetic parameter Km for tert-butyl hydroperoxide has not changed. However, the heat resistance of the enzyme dramatically decreases with ageing. Dilutions of the enzyme preparations had the same influence on the thermosensitivity of the enzyme. This dilution effect is most probably linked to the dissociation of the enzyme subunits into dimers and monomers. Moreover, the kinetic of thermoinactivation curves are best explained by consecutive reactions of inactivation with an intermediary enzyme form. These observations strongly support the hypothesis that ageing is associated with an increased dissociation constant of the tetrameric glutathione peroxidase leading to an easier dissociation of the enzyme in old cells.

Alteration of glutathione level in human melanoma cells: effect of N-acetyl-L-cysteine and its analogues

The effects of N-acetyl-L-cysteine (L-NAC), N,N-diacetyl-L-cystine (oxidized form of L-NAC) and N-acetyl-D-cysteine on the intracellular glutathione (GSH) level and their toxicity were investigated in the human melanoma cell culture IGR1. L-NAC applied in 3 mM concentration for 24 hr decreased; when applied for 48 hr it did not alter the intracellular GSH level. Treatment with 1 mM L-NAC for 24 hr had no effect on cellular glutathione, whereas the same concentration applied for 48 hr resulted in an increase in the level of GSH. Both concentrations also induced cell injury as determined by protein assay and trypan blue staining. N,N-diacetyl-L-cystine (0.5 and 1.5 mM, 24 hr) induced a decrease in cellular glutathione content without any apparent cell toxicity. D-NAC (1 and 3 mM, 24 hr) did not influence the GSH level of the melanoma cells; however, it had toxic effects resulting in cell loss.

The antioxidant action of N-acetylcysteine: its reaction with hydrogen peroxide, hydroxyl radical, superoxide, and hypochlorous acid

N-acetylcysteine has been widely used as an antioxidant in vivo and in vitro. Its reaction with four oxidant species has therefore been examined. N-acetylcysteine is a powerful scavenger of hypochlorous acid (H--OCl); low concentrations are able to protect alpha 1-antiproteinase against inactivation by HOCl. N-acetylcysteine also reacts with hydroxyl radical with a rate constant of 1.36 X 10(10) M-1s-1, as determined by pulse radiolysis. It also reacts slowly with H2O2, but no reaction of N-acetylcysteine with superoxide (O2-) could be detected within the limits of our assay procedures.