Correlation between lipid peroxidation and morphological manifestation of paraquat-induced lung injury in rats

Peroxiredoxin6, a Multitask Antioxidant Enzyme Involved in the Pathophysiology of Chronic Noncommunicable Diseases

SIGNIFICANCE

Chronic noncommunicable diseases (NCDs) are the leading causes of disability and death worldwide. NCDs mainly comprise diabetes mellitus, cardiovascular diseases, chronic obstructive pulmonary disease, cancer, and neurological degenerative diseases, which kill more than 80% of population, especially the elderly, worldwide. Recent Advances: Several recent theories established NCDs as multifactorial diseases, where a combination of genetic, epigenetic, and environmental factors contributes to their pathogenesis. Nevertheless, recent findings suggest that the common factor linking all these pathologies is an increase in oxidative stress and the age-related loss of the antioxidant mechanisms of defense against it. Impairment in mitochondrial homeostasis with consequent deregulation in oxidative stress balance has also been suggested.

CRITICAL ISSUES

Therefore, antioxidant proteins deserve particular attention for their potential role against NCDs. In particular, peroxiredoxin(Prdx)6 is a unique antioxidant enzyme, belonging to the Prdx family, with double properties, peroxidase and phospholipase activities. Through these activities, Prdx6 has been shown to be a powerful antioxidant enzyme, implicated in the pathogenesis of different NCDs. Recently, we described a phenotype of diabetes mellitus in Prdx6 knockout mice, suggesting a pivotal role of Prdx6 in the pathogenesis of cardiometabolic diseases.

FUTURE DIRECTIONS

Increasing awareness on the role of antioxidant defenses in the pathogenesis of NCDs may open novel therapeutic approaches to reduce the burden of this pandemic phenomenon. However, knowledge of the role of Prdx6 in NCD prevention and pathogenesis is still not clarified.

Chronic Treatment with Paraquat Induces Brain Injury, Changes in Antioxidant Defenses System, and Modulates Behavioral Functions in Zebrafish

Paraquat (PQ) administration consists in a chemical model that mimics phenotypes observed in Parkinson's disease (PD), due to its ability to induce changes in dopaminergic system and oxidative stress. The aim of this study was to evaluate the actions of PQ in behavioral functions of adult zebrafish and its influence on oxidative stress biomarkers in brain samples. PQ (20 mg/kg) was administered intraperitoneally with six injections for 16 days (one injection every 3 days). PQ-treated group showed a significant decrease in the time spent in the bottom section and a shorter latency to enter the top area in the novel tank test. Moreover, PQ-exposed fish showed a significant decrease in the number and duration of risk assessment episodes in the light-dark test, as well as an increase in the agonistic behavior in the mirror-induced aggression (MIA) test. PQ induced brain damage by decreasing mitochondrial viability. Concerning the antioxidant defense system, PQ increased catalase (CAT) and glutathione peroxidase (GPx) activities, as well as the non-protein sulfhydryl content (NPSH), but did not change ROS formation and decreased lipid peroxidation. We demonstrate, for the first time, that PQ induces an increase in aggressive behavior, alters non-motor patterns associated to defensive behaviors, and changes redox parameters in zebrafish brain. Overall, our findings may serve as useful tools to investigate the interaction between behavioral and neurochemical impairments triggered by PQ administration in zebrafish.

Maternal levels of endocrine disruptors, polybrominated diphenyl ethers, in early pregnancy are not associated with lower birth weight in the Canadian birth cohort GESTE

BACKGROUND

Polybrominated diphenyl ethers are known endocrine disrupting environmental contaminants used as flame retardants. Their levels have increased in humans over the last ten years, raising concerns about their consequences on human health. Some animal studies suggest that PBDEs can affect fetal growth; however, the results of human studies are contradictory. This study evaluates the association between the most common PBDEs in maternal blood measured in early pregnancy and birth weight.

METHODS

BDE-47, BDE-99, BDE-100 and BDE-153 levels were measured in 349 women during their first prenatal care visit at the University Hospital Center of Sherbrooke (Quebec, Canada). Birth weight and relevant medical information were collected from medical records. In contrast with previous studies, we examined the full range of clinical risk factors known to affect fetal growth as potential confounders, as well as other environmental pollutants that are likely to interact with fetal growth (polychlorinated biphenyls (PCBs), mercury, lead, cadmium and manganese).

RESULTS

There was no statistically significant relationship between PBDE levels in early pregnancy and birth weight in both unadjusted and multivariate regression models.

CONCLUSIONS

Our results suggest that PBDEs in early pregnancy have little or no direct impact on birth weight, at least at the levels of exposure in our population.

Mechanism of cytotoxicity of paraquat

Acute paraquat poisoning seems to be very complex because many possible mechanisms of paraquat cytotoxicity have been reported. Some may not be the cause of paraquat poisoning but the result or an accompanying phenomenon of paraquat action. The mechanism critical for cell damage is still unknown. Paraquat poisoning is probably a combination of several paraquat actions. Arguing which mechanism is more critical may not be important, and these clarified mechanisms should be connected and utilized in the development of treatment for paraquat poisoning. Many people still die of pulmonary fibrosis after paraquat exposure. The next target of study will be to verify the mechanism of pulmonary fibrosis by paraquat on the basis of the outcome of studies such as this review.

Edaravone attenuates paraquat-induced lung injury by inhibiting oxidative stress in human type II alveolar epithelial cells

BACKGROUND

Edaravone (3-methyl-1-penyl-2-pyrazolin-5-one) is a potent free-radical scavenger and has the antioxidant ability to inhibit lipid peroxidation. The study aimed to examine the effect of edaravone on protecting the acute injury of human type II alveolar epithelial cells (A549 cells) induced by paraquat (PQ) and the change of production of reactive oxygen species (ROS), malondialdehyde (MDA), superoxide dismutase (SOD).

METHODS

A549 cells were cultured and divided into PQ group (group P), edaravone-treated group (group E) and normal control group (group C). The cells in group P were exposed to paraquat (600 μmol/L), and the cells in group E were treated with edaravone (100 μmol/L) additionally, and no drug intervention was given to the cells in group C. Real-time monitoring by LSCM was used to detect the cell response and the intracellular dynamic change of ROS level in A549 cells after administration of PQ and edaravone. And the levels of SOD and MDA were detected respectively by biochemistry colorimetry. Data were expressed as mean ± standard error of the mean. Statistical analysis was carried out with the soft SPSS 16.0.

RESULTS

The concentration of intracellular ROS significantly increased when PQ was given to A549 cells. But after administration of edaravone, the concentration of intracellular ROS was decreased. Compared to the PQ group, the levels of SOD in the edaravone group were significantly increased while the levels of MDA were markedly decreased.

CONCLUSIONS

Paraquat can increase the oxidative stress, and induce the lipid peroxidation of A549 cells. Edaravone has the effect to scavenge reactive oxygen species, and to protect against the PQ-induced lung toxicity.

Hormetins, antioxidants and prooxidants: defining quercetin-, caffeic acid- and rosmarinic acid-mediated life extension in C. elegans

Quercetin, Caffeic- and Rosmarinic acid exposure extend lifespan in Caenorhabditis elegans. This comparative study uncovers basic common and contrasting underlying mechanisms: For all three compounds, life extension was characterized by hormetic dose response curves, but hsp-level expression was variable. Quercetin and Rosmarinic acid both suppressed bacterial growth; however, antibacterial properties were not the dominant reason for life extension. Exposure to Quercetin, Caffeic- and Rosmarinic acid resulted in reduced body size, altered lipid-metabolism and a tendency towards a delay in reproductive timing; however the total number of offspring was not affected. An indirect dietary restriction effect, provoked by either chemo-repulsion or diminished pharyngeal pumping was rejected. Quercetin and Caffeic acid were shown to increase the antioxidative capacity in vivo and, by means of a lipofuscin assay, reduce the oxidative damage in the nematodes. Finally, it was possible to demonstrate that the life and thermotolerance enhancing properties of Caffeic- and Rosmarinic acid both rely on osr-1, sek-1, sir-2.1 and unc-43 plus daf-16 in the case of Caffeic acid. Taken together, hormesis, in vivo antioxidative/prooxidative properties, modulation of genetic players, as well as the re-allocation of energy all contribute (to some extent and dependent on the polyphenol) to life extension.

Identification of early molecular pathways affected by paraquat in rat lung

We have used global gene expression profiling, combined with pathway analysis tools, to identify in rats the molecular events associated with paraquat toxicity in the lung. Early (2, 8 and 18h) gene expression changes induced following intraperitoneal (i.p.) exposure to paraquat were measured in the caudal lobe of lungs using Affymetrix rat genome GeneChips (31,042 probe sets). A single high dose of paraquat dichloride (20mg/kg) was used that has been shown previously to cause in rats extensive lung fibrosis after 10 days. Hierarchical clustering of 543 paraquat-responsive genes (false discovery rate<0.05) revealed that under these conditions of exposure paraquat induces a staged transcriptional response in the rat lung that precedes the appearance of lung damage. We report here that many of the transcriptional responses to paraquat were rapid (being maximal at 2h post-dose), and that the predominant molecular functions and biological processes associated with these genes include membrane transport, oxidative stress, lung development, epithelial cell differentiation and transforming growth factor beta (TGF-beta) signalling. These data provide novel insights into the molecular pathways that lead to toxicity after exposure of the rat lung to paraquat.

Paraquat-induced gene expression in rat kidney

Paraquat, one of the most widely used herbicides, is highly toxic to humans and animals. There is much information regarding its toxic effects on the lungs, but less is known about its toxicity in other organs. Paraquat is thought to play pivotal roles in the pathophysiology of acute renal failure and the progression of chronic kidney disease. We investigated the effects of paraquat on gene expression in the kidneys of rats treated with paraquat using a DNA array system, and the gene up-regulation observed was confirmed by quantitative real-time RT-PCR. Rats were sacrificed at 3, 24 h after the first injection (20 mg/kg), and at 3 h after the second injection. Expression of six genes had increased significantly by 3 h after the first injection: metallothionein-1 (MT-1), phosphoenolpyruvate carboxykinase, Na/K-transporting ATPase beta1 subunit, glutamate oxaloacetic transaminase, glutathione-S-transferase, and heme oxygenase-1 (HO-1). The transcription levels of MT-1 and HO-1 showed the biggest increases, but the increases did not continue until 24 h after injection, and the second injection had less effect than the first. Up-regulation of MT-1 and HO-1 mRNA levels was confirmed at the protein level. We observed a paraquat-induced increase of these proteins at 3 h post-injection, whereas this level did not continue until 24 h, as observed in RNA levels. The MT-1 protein in kidneys had been consumed. In addition, the protein level due to the second injection did not increase to the same level as that due to the first injection. These results suggest that protection against paraquat injury is mediated by induction of expression of some genes, and suppression on the induction of MT-1 and HO-1 may explain the injury observed due to paraquat intake. This is the first report of inducible pathways of defense against paraquat-induced oxidative stress in the kidney.

Peroxiredoxin 6 gene-targeted mice show increased lung injury with paraquat-induced oxidative stress

Mice with knock-out of peroxiredoxin 6 (Prdx6), a recently described antioxidant enzyme, were evaluated for susceptibility to lung injury with paraquat (PQ) administration. With high dose PQ (30 mg/kg i.p.), all Prdx6-/- mice died (LT50 54 +/- 2.05 h, mean +/- SE) by 4 days, whereas 86% of the wild-type (WT) mice (C57BL/6) survived (n = 14). At 2 days after PQ, lung wet/dry weight ratio increased significantly (p < 0.05) to 7.57 +/- 0.37 in Prdx6-/- mice vs. 5.42 +/- 0.25 in WT mice. Total protein and nucleated cells in bronchoalveolar lavage fluid and TBARS and protein carbonyls in lung homogenate also showed more marked increases in Prdx6-/- mice. At 2.5 days after PQ, light microscopy of WT lungs showed mild injury while Prdx6-/- lungs showed epithelial cell necrosis, perivascular edema, and inflammatory cells. With low dose PQ (12.5 mg/kg), mortality and lung injury were less marked but were significantly greater with Prdx6-/- compared to WT mice. These results show that Prdx6-/- mice have increased susceptibility to lung injury with PQ administration. Thus, Prdx6 protects lungs against PQ toxicity as shown previously for hyperoxia, indicating that it functions as an important lung antioxidant enzyme.

A comparative study of plant and animal mitochondria exposed to paraquat reveals that hydrogen peroxide is not related to the observed toxicity

Rat liver mitochondria are much more susceptible to protein oxidation induced by paraquat than plant mitochondria. The unsaturated index and the peroxidizability index are higher in rat than in potato tuber. The levels of superoxide dismutase and glutathione reductase are concurrent with the different sensitivities to paraquat, with higher activities in plant mitochondria. However, glutathione peroxidase and catalase activities are higher in rat mitochondria. Paraquat (10 mM) inhibited all the enzymatic activities; excluding catalase all the other activities were inhibited to a similar degree. The differential sensitivities of plant and animal mitochondria to paraquat correlate with fatty acid composition of mitochondrial lipids and a similar correlation was also established for some antioxidant enzymes. At the mitochondrial level, H(2)O(2) is not a major factor of paraquat toxicity since rat liver mitochondria which exhibit higher activities of glutathione peroxidase and catalase are however more susceptible to paraquat.

Prion protein protects against DNA damage induced by paraquat in cultured cells

Exposure of cells to paraquat leads to production of superoxide anion (O2*-). This reacts with hydrogen peroxide to give the hydroxyl radical (*OH), leading to lipid peroxidation and cell death. In this study, we investigated the effects of cellular prion protein (PrPC) overexpression on paraquat-induced toxicity by using an established model system, rabbit kidney epithelial A74 cells, which express a doxycycline-inducible murine PrPC gene. PrPC overexpression was found to significantly reduce paraquat-induced cell toxicity, DNA damage, and malondialdehyde acid levels. Superoxide dismutase (total SOD and CuZn-SOD) and glutathione peroxidase activities were higher in doxycycline-stimulated cells. Our findings clearly show that PrPC overexpression plays a protective role against paraquat toxicity, probably by virtue of its superoxide dismutase-like activity.

Consecutive administration of paraquat to rats induces enhanced cholesterol peroxidation and lung injury

It is our hypothesis that as a consequence of increased oxidative stress, rats develop lung injury with increased cholesterol-derived hydroperoxides and oxysterols in lung after consecutive exposure of the rats to paraquat. To test this we administered 10 mg/kg of paraquat i.p. once or seven times (once a day) to Wistar rats. Rats were killed, and lung tissue was collected 24 h after the last paraquat injection. We found that in response to consecutive paraquat doses, there were significant increases in 7alpha- and 7beta-hydroperoxycholest-5-en-3beta-ol (7alpha-OOH and 7beta-OOH; P=0.01) as well as 7alpha- and 7beta-hydroxycholesterol (7alpha-OH and 7beta-OH; P=0.01), and 7-ketocholesterol (7-keto; P=0.03). In addition, pulmonary hemorrhage, thickening of alveolar septum, and inflammatory cell infiltration of macrophages were observed. This is the first report showing enhanced cholesterol peroxidation and lung injury of rats due to consecutive doses of paraquat.

Amyloid formation in rat transthyretin: effect of oxidative stress

BACKGROUND

Transgenic mice carrying a human mutant transthyretin (TTR) gene are too small for in vivo experiments. It is necessary to have rat TTR protein and its antibody to overcome this problem.

METHODS

Posttranslational modification of purified TTR was analyzed by means of matrix-assisted laser desorption ionization/time-of-flight mass spectrometry (MALDI/TOF-MS). Production of amyloid fibrils in vitro was confirmed by thioflavin T test and electron microscopy. Amyloidogenicity of rat TTR from rats with or without challenging paraquat was compared in vitro by thioflavin T test.

RESULTS

MALDI/TOF-MS for rat TTR revealed three major modified forms-sulfate-conjugated, Cys-conjugated and glutathione-conjugated-in addition to the unconjugated (free) form of TTR. Although rat TTR in buffer of pH 7.0 could not make amyloid fibrils, rat TTR at pH 2.0-3.5 significantly formed amyloid fibrils, as confirmed by the thioflavin T test and electron microscopy. TTR purified from rats administered 4 mg/kg of paraquat formed much more amyloid fibrils than that from normal rats at pH 2.0-3.5 and significant amyloid fibrils were confirmed even at pH 7.0.

CONCLUSIONS

Rat TTR may be a valuable experimental tool for examination of the amyloidogenicity of senile systemic amyloidosis (SSA) as well as familial amyloidotic polyneuropathy (FAP) both in vitro and in vivo.

Melatonin: from basic research to cancer treatment clinics

Melatonin, the chief secretory product of the pineal gland, is a direct free radical scavenger, an indirect antioxidant, as well as an important immunomodulatory agent. In both in vitro and in vivo investigations, melatonin protected healthy cells from radiation-induced and chemotherapeutic drug-induced toxicity. Furthermore, several clinical studies have demonstrated the potential of melatonin, either alone or in combination with traditional therapy, to yield a favorable efficacy to toxicity ratio in the treatment of human cancers. This study reviews the literature from laboratory investigations that document the antioxidant and oncostatic actions of melatonin and summarizes the evidence regarding the potential use of melatonin in cancer treatment. This study also provides rationale for the design of larger translational research-based clinical trials.

Role of heat shock protein 60 (HSP60) on paraquat intoxication

The possibility of establishing a new method of treatment against pulmonary fibrosis caused by acute paraquat intoxication, which takes into consideration the role of heat shock protein 60 (HSP60), was investigated in paraquat-exposed rat lung mitochondria. In polyacrylamide electrophoresis, mitochondrial protein bands appeared, especially in the range of molecular weight 60 kDa and higher, whereas protein bands disappeared in the 20-40 kDa range on the 4th day after paraquat exposure. The protein profile was normalized on the 7th day and no remarkable changes were seen thereafter up to the 56th day. The changes seen during the observation period were thought to reflect the course of paraquat-induced dysfunction and subsequent repair. The malondialdehyde concentration in mitochondria decreased until the 7th day but subsequently increased and recovered to normal levels by the 56th day. The relative density of HSP60 increased until the 7th day but subsequently decreased and recovered to normal levels by the 56th day. These two parameters therefore showed symmetrical changes. The change in the malondialdehyde concentration was thought to reflect the course of activation of the antioxidation function in mitochondria and the progression of repair. The change in the relative density of HSP60 was thought to have increased to repair the proteins affected by the paraquat radical and to have normalized with the progression of healing. These results suggest that HSP60 may play an important role in preventing the progression of pulmonary fibrosis induced by paraquat.

7-Hydroperoxycholesterol as a marker of oxidative stress in rat kidney induced by paraquat

The in vivo paraquat-induced oxidative stress in rat tissue was studied by analyzing cholesterol-derived hydroperoxide as an index of lipid peroxidation. Paraquat (10 mg/kg) was administered i.p. to rats. Rats were sacrificed and lung, liver, and kidney were collected 2, 24 h, and 5 d after paraquat injection. Lipids were extracted and analyzed by HPLC with post-column chemiluminescence. We found that two cholesterol-derived hydroperoxides, 7alpha-hydroperoxycholest-5-en-3beta-ol (7alpha-OOH) and 7beta-hydroperoxycholest-5-en-3beta-ol (7beta-OOH) were present in lungs of control animals (0.06 and 0.06 nmol/g, respectively), in livers (6.5 and 15.8 nmol/g, respectively) and in kidneys (3.7 and 8.9 nmol/g, respectively). In liver paraquat increased lipid peroxidation approximately by 60% over the levels of control animals only at 2 h after paraquat treatment. In kidney, augmented lipid peroxidation, 7alpha-OOH and 7beta-OOH (by 70% and 147%, respectively) above levels was found at 2 h after paraquat treatment. Interestingly, these increase remained in kidney of rats 5 d after a single dose of paraquat. In contrast, cholesterol-derived hydroperoxides were not affected in lung of paraquat dosed rats. This is the first report on 7alpha-OOH and 7beta-OOH accumulations in rat liver and kidney, and it seems to reflect greater oxidative stress in the pathology of kidney of rats treated with acute paraquat at low dose.

No significant paraquat-induced oxidative DNA damage in rats

The metabolism of paraquat generates oxygen radicals. Paraquat has thus been suggested as a model compound to induce oxidative damage to DNA, lipids and proteins in different cells and tissues, although experimental data are inconsistent. In order to explore the possibilities for an animal model of oxidative DNA damage in vivo, rats were treated with 20 mg/kg paraquat or vehicle i.p. One and five days later we measured DNA oxidation in terms of 7-hydro-8-oxo-2'-deoxyguanosine (8-oxodG) in the liver and lung as well as the urinary excretion of 8-oxodG. No significant effects on the level of 8-oxodG in the liver, the lung or the urinary excretion, could be distinguished following paraquat treatment. We found, however, a significant correlation (r = 0.69; p<0.0002) between the 8-oxodG level in the lung and the urinary excretion, but no significant correlation between the level in the liver and the urinary excretion or between the levels in the liver and the lung. During the experiment the rats were clearly affected by the paraquat as they were very lethargic compared to the controls. Accordingly, even at toxic doses, paraquat did not cause detectable oxidative damage to DNA. The data do not support the use of paraquat as a model compound in experiments investigating effects or prevention of oxidative damage to DNA.

Changes in mRNAs of inducible nitric oxide synthase and interleukin-1 beta in the liver, kidney and lung tissues of rats acutely exposed to paraquat

Nitric oxide (NO) reacts with superoxide to form the potent oxidant peroxynitrite, which causes serious cell damage. Interleukin-1 beta (IL-1 beta) is known to be a strong activator of NO production via induction of inducible nitric oxide synthase (iNOS). Since paraquat (PQ) undergoes redox cycling in vivo, resulting in a constant generation of superoxide, peroxynitrite may be a pathogenetic factor in the oxidative cell damage. In this study, we have investigated whether mRNAs of iNOS and IL-1 beta are affected in rat liver, kidney and lung tissues by exposure to non-lethal and lethal doses of PQ. Suppression and then marked stimulation of the iNOS mRNA were observed in the liver tissues of rats exposed to a lethal dose of PQ, while the kidney and lung tissues showed little changes. We also detected nitrotyrosine in liver tissues of rats exposed to a lethal dose by immunohistochemistry, suggesting the simultaneous generation of NO and superoxide in liver injury during acute lethal PQ poisoning. On the other hand, the IL-1 beta mRNA in the liver tissues decreased throughout the experiments, suggesting that this cytokine is not responsible for stimulation of the iNOS gene. IL-1 beta mRNA in lung tissues in the non-lethal group showed an increase, with maximum levels at 16-24 h, while little changes were observed in iNOS mRNA in this organ. These data suggest that acute lethal poisoning and non-lethal poisoning by PQ undergo different mechanisms of action of NO and IL-1 beta systems; the former is due, at least in part, to an increase in NO production, while the latter is due to stimulation of IL-1 beta and/or other cytokines.

Involvement of oxidative stress in paraquat-induced metallothionein synthesis under glutathione depletion

The inhibition of glutathione (GSH) synthesis by L-buthionine-SR-sulfoximine (BSO) causes aggravation of hepatotoxicity of paraquat (PQ), an oxidative-stress inducing substance, in mice. On the other hand, synthesis of metallothionein (MT), a cysteine-rich protein having radical scavenging activity, is induced by PQ, and the induction by PQ is significantly enhanced by pretreatment of mice with BSO. The purpose of present study is to examine whether generation of reactive oxygens is involved in the induction of MT synthesis by PQ under inhibition of GSH synthesis. Administration of PQ to BSO-pretreated mice increased hepatic lipid peroxidation and frequency of DNA single strand breakage followed by manifestation of the liver injury and induction of MT synthesis. Both vitamin E and deferoxamine prevented MT induction as well as lipid peroxidation in the liver of mice caused by administration of BSO and PQ. In cultured colon 26 cells, both cytotoxicity and the increase in MT mRNA level caused by PQ were significantly enhanced by pretreatment with BSO. Facilitation of PQ-induced reactive oxygen generation was also observed by BSO treatment. These results suggest that reactive oxygens generated by PQ under inhibition of GSH synthesis may stimulate MT synthesis. GSH depletion markedly increased reactive oxygen generation induced by PQ, probably due to the reduced cellular capability to remove the radical species produced.

Xanthine oxidase mediates paraquat-induced toxicity on cultured endothelial cell

The role of xanthine oxidase in paraquat toxicity was investigated using cultured bovine pulmonary artery endothelial cells. Exposure to paraquat 0.1 mM was done for 24 hr with or without tungsten pretreatment and in the presence or absence of xanthine oxidase inhibitors. Exposure to paraquat significantly increased O2- production and relative xanthine oxidase activity (xanthine oxidase activity divided by total xanthine dehydrogenase plus xanthine oxidase) while depressing cell growth. In contrast, tungsten and allopurinol inhibited the increase of xanthine oxidase activity and decreased O2- release. Cell injury was assessed by leakage of lactate dehydrogenase and by fluorescein diacetate staining; it was found that oxidase inhibitors (both allopurinol and tungsten) reduced paraquat cytotoxicity. Thus the toxicity of paraquat was at least partly due to intracellular O2- production mediated by xanthine oxidase and the subsequent formation of other free radicals.

A review of the evidence supporting melatonin's role as an antioxidant

This survey summarizes the findings, accumulated within the last 2 years, concerning melatonin's role in defending against toxic free radicals. Free radicals are chemical constituents that have an unpaired electron in their outer orbital and, because of this feature, are highly reactive. Inspired oxygen, which sustains life, also is harmful because up to 5% of the oxygen (O2) taken in is converted to oxygen-free radicals. The addition of a single electron to O2 produces the superoxide anion radical (O2-.); O2-. is catalytic-reduced by superoxide dismutase, to hydrogen peroxide (H2O2). Although H2O2 is not itself a free radical, it can be toxic at high concentrations and, more importantly, it can be reduced to the hydroxyl radical (.OH). The .OH is the most toxic of the oxygen-based radicals and it wreaks havoc within cells, particularly with macromolecules. In recent in vitro studies, melatonin was shown to be a very efficient neutralizer of the .OH; indeed, in the system used to test its free radical scavenging ability it was found to be significantly more effective than the well known antioxidant, glutathione (GSH), in doing so. Likewise, melatonin has been shown to stimulate glutathione peroxidase (GSH-Px) activity in neural tissue; GSH-PX metabolizes reduced glutathione to its oxidized form and in doing so it converts H2O2 to H2O, thereby reducing generation of the .OH by eliminating its precursor. More recent studies have shown that melatonin is also a more efficient scavenger of the peroxyl radical than is vitamin E. The peroxyl radical is generated during lipid peroxidation and propagates the chain reaction that leads to massive lipid destruction in cell membranes. In vivo studies have demonstrated that melatonin is remarkably potent in protecting against free radical damage induced by a variety of means. Thus, DNA damage resulting from either the exposure of animals to the chemical carcinogen safrole or to ionizing radiation is markedly reduced when melatonin is co-administered. Likewise, the induction of cataracts, generally accepted as being a consequence of free radical attack on lenticular macromolecules, in newborn rats injected with a GSH-depleting drug are prevented when the animals are given daily melatonin injections. Also, paraquat-induced lipid peroxidation in the lungs of rats is overcome when they also receive melatonin during the exposure period. Paraquat is a highly toxic herbicide that inflicts at least part of its damage by generating free radicals.(ABSTRACT TRUNCATED AT 400 WORDS)

Acute-phase reactant proteins and antioxidants in rats intoxicated chronically with paraquat

Paraquat dichloride at 250 ppm in the diet was fed continuously to rats. Though no apparent effect of paraquat was observed until 10 d, some rats then began to show several symptoms such as diarrhea, anorexia, epistaxis, and hypokinesia, and in some cases rats died after this period. The biochemical examination of plasma components revealed appreciable changes in the concentrations of an acute-phase reactant protein and some vitamins that act as antioxidants. alpha-Cysteine proteinase inhibitor increased by 5-fold, and vitamin C and its radical increased by 1.5- and 1.7-fold, respectively, whereas alpha 1 proteinase inhibitor decreased slightly. Paraquat enhanced the cysteine proteinase inhibitor levels in lung, liver, and kidney by 6.2-, 6.0-, and 4.5-fold of control, respectively. Among three components of alpha-cysteine proteinase inhibitor, the T kininogen level of treated rat plasma was about eight-fold higher than control, whereas the high-molecular-weight kininogen level was unchanged. The large increment of T kininogen was also seen in lungs of the treated rats.

Mechanism of cytotoxicity of paraquat. I. NADH oxidation and paraquat radical formation via complex I

The mechanism of cytotoxicity by paraquat was studied focusing attention on its effect on the mitochondrial electron transport system. Paraquat inhibited both mitochondrial and cytoplasmic malate dehydrogenase activities. NADH oxidation was verified in NADH: ubiquinone oxidoreductase (complex I) reaction mixture in which paraquat was an only electron acceptor, and paraquat radical formation was observed as turning blue of the reaction mixture. A kinetic characteristic of this enzyme reaction was that Km was so high as 4.1 mM. The maximum reaction velocity was defined in the range over pH 9. NADH autoxidation with complex I, but without paraquat, was not observed in any pH range. The maximum reaction velocity of the NADH autoxidation by paraquat without complex I was observed in pH 8.5, but the figure was so small as to be negligible. With these results, we propose the hypothesis that paraquat does not promote the autoxidation with complex I, but accepts electrons via complex I to induce paraquat radical formation.

Ascorbate radical levels in human sera and rat plasma intoxicated with paraquat and diquat

To clarify the toxicological mechanism of action of paraquat and diquat, the level of ascorbate radical, an oxidized product of ascorbic acid, was examined using the electron spin resonance (ESR) method. The ascorbate radical level increased to twice the normal level in sera from humans acutely intoxicated with a 1:1 mixture of paraquat and diquat, and to more than 1.5 times the normal level in rat plasma containing moderate levels of paraquat. The dosage level for rats was 0.025% paraquat dichloride in the diet. The ascorbate radical in both intoxicated human sera and rat plasma decayed much faster than that in normal samples.

Production of limbic motor seizures and brain damage by systemic and intracerebral injections of paraquat in rats

The behavioural and neuropathological effects of both systemic and intrahippocampal injections of paraquat dichloride (1,1'-dimethyl 4,4'-bipyridinium dichloride) were studied in rats. Paraquat (0.1-1.0 mumol) injected into the dorsal hippocampus, produced limbic motor seizures within a few minutes of injection followed by neuronal damage in the CA1 and CA3 pyramidal cell layers, pyriform cortex, dentate granule cell layer and in the hilus fascia dentata at 24 hr (n = 9 rats). A smaller dose of paraquat (10 nmol) was ineffective. The effects of intrahippocampal injections of paraquat (1 mumol) were prevented by administering it together with atropine (50 nmol; n = 6 rats) or by giving it 60 min. after MK 801 (0.3 mg.kg-1 intraperitoneally). Systemic injections of paraquat (20-100 mg.kg-1) also produced forelimb clonus and rearing in 10 out of 15 animals. Neuronal cell death was found 24 hr later in 9 of these rats and was restricted to the pyriform cortex, the brain region with the highest concentrations of paraquat. Atropine (150 mg.kg-1 intraperitoneally given 60 min. previously) completely prevented the motor seizures but cell death still occurred in 2 of the 6 animals tested. In conclusion, both systemic and intrahippocampal injections of paraquat produced behavioural excitation accompanied 24 hr later by brain damage and antagonist studies suggested involvement of muscarinic and NMDA receptors in the neurotoxic mechanism.

Protective effect of liposome-associated alpha-tocopherol against paraquat-induced acute lung toxicity

The present study was undertaken to investigate whether alpha-tocopherol, entrapped in liposomes and delivered directly to the lung, could protect against paraquat-induced lung damage in the rat. Plain liposomes (composed of dipalmitoylphosphatidylcholine, DPPC) or DPPC/alpha-tocopherol liposomes were administered intratracheally to animals 24 hr prior to an intraperitoneal injection of paraquat (20 mg/kg); rats were killed 24 or 48 hr after paraquat treatment. Results of this study showed that lungs of animals treated with paraquat were damaged extensively as evidenced by an increase in lung weight and a significant reduction in lung angiotensin-converting enzyme (ACE) activity and cytochrome P450 concentration. Furthermore, paraquat treatment resulted in a significant decrease in reduced glutathione (GSH) concentrations and a marked elevation in microsomal lipid peroxidation levels as measured by the formation of diene conjugates. Pretreatment of rats with DPPC liposomes alone did not alter significantly the paraquat-induced changes of all parameters examined. On the other hand, pretreatment of rats with DPPC/alpha-tocopherol liposomes 24 hr prior to paraquat challenge resulted in a significant increase in pulmonary alpha-tocopherol concentrations and antagonized paraquat-induced changes in lipid peroxidation, GSH/GSSG ratio, lung ACE activity and cytochrome P450 concentrations. Results of this study suggested that alpha-tocopherol, delivered directly to the lung in a liposomal formulation 24 hr prior to paraquat administration, confers protection against paraquat-induced lung damage.

Brown fat thermogenesis and exercise: two examples of physiological oxidative stress?

Both brown fat tissue (BAT) and skeletal muscle experience large increases of oxygen consumption and oxygen radical generation during activation. This, together with the relatively low activities of antioxidant enzymes in these two tissues and the high lipid content and free fatty acid liberation of BAT, can produce a physiological oxidative stress. Increases of in vivo or in vitro (BAT) lipid peroxidation have been described in these tissues after activation. They react to this oxidative stress in an adaptive way after chronic stimulation. Cold acclimation increases antioxidant enzymes, ascorbate, and especially reduced glutathione (GSH) in BAT. There is controversy about the variations of antioxidants in skeletal muscle after acute exercise. Nevertheless, exercise training seems to increase muscle antioxidant enzymes and GSH. Many reports show that vitamin E levels decrease in the muscle and increase in plasma during exercise. Studies of vitamin E deficiency and supplementation strongly suggest that this vitamin is of protective value during exercise.

Antioxidant-dependent inhibition of diquat-induced toxicity in vivo

The abilities of two experimental antioxidants (U-74006F and U-78517G), as well as the model antioxidant, diphenyl-p-phenylenediamine (DPPD), to protect against diquat-induced toxicity in male Fischer-344 rats were examined. Both experimental compounds afforded near complete protection against diquat-induced hepatotoxicity, as measured by clinical chemistry and histopathological indices. When observed, diquat-induced nephrotoxicity was also inhibited. Minimal protection was afforded by the model compound, DPPD. In follow-up studies with U-78517G, no effect on diquat-induced biliary excretion of oxidized glutathione was observed, suggesting that a shift in the thiol:disulfide ratio is not responsible for diquat-induced hepatotoxicity. These data are consistent with those from previous in vitro studies in our laboratory and are in agreement with studies by others which suggest that lipid peroxidation is an important event in diquat-induced hepatotoxicity in vivo. The antioxidant effects were largely route-independent as either oral pre-treatment alone (200 mg/kg, 24 h before diquat), intravenous pre-treatment alone (6 mg/kg, 5 min before diquat) or the combination of both treatments produced a similar degree of protection. While pre-treatment with antioxidants was quite effective, no significant U-78517G-dependent inhibition of toxicity was observed when administration was delayed by as little as 10 min post diquat. These latter data suggest that initiation of diquat-induced hepatotoxicity is rapid and that these compounds would therefore be unlikely to have clinical utility in the treatment of diquat intoxication.

Mechanism of paraquat-stimulated lipid peroxidation in mouse brain and pulmonary microsomes

Paraquat-stimulated NADPH-dependent lipid peroxidation in mouse brain and pulmonary microsomes was inhibited by superoxide dismutase and singlet oxygen quenchers, but not by catalase or hydroxyl radical scavengers. MnCl2, which might form a salt with unsaturated lipid, inhibited the lipid peroxidation in brain microsomes, but not that in pulmonary microsomes. These findings suggest that activated oxygen species, especially superoxide and singlet oxygen, may play a major role in the stimulation of microsomal lipid peroxidation by paraquat in both brain and lung, and that the nature of the lipids exposed to peroxidative attack may be different in microsomes of the two organs.

Effects of MPTP, MPP+, and paraquat on NADPH-dependent lipid peroxidation in mouse brain and lung microsomes

Both MPTP and MPP+ inhibited the NADPH-dependent microsomal LPO in mouse brain and lung. On the other hand, PQ significantly stimulated the LPO in brain microsomes in a dose-dependent manner. The herbicide, however, stimulated lung microsomal LPO only in a narrow concentration range, despite much higher P450 reductase activity in lung microsomes than that in brain microsomes. These findings suggest that the effect of PQ on microsomal LPO is different from those of the analogous neurotoxins, MPTP and MPP+, and is not uniform in brain and lung.

Different effects of paraquat on microsomal lipid peroxidation in mouse brain, lung and liver

Paraquat stimulates NADPH-Fe(2+)-dependent microsomal lipid peroxidation in mouse brain and strongly inhibits it in the liver. In lung microsomes, the lipid peroxidation was stimulated by paraquat at 10(-4) M, but not at higher doses. An antioxidant action of paraquat seemed to account, at least in part, for the lack of stimulation in lung microsomes, but it was inappropriate to explain the result in hepatic microsomes. There was no apparent correlation between the effects of paraquat on the lipid peroxidation and on the activity of NADPH-cytochrome P-450 reductase, the enzyme which initiates redox cycling of paraquat, resulting in generation of active oxygen species. In fact, the effect of paraquat on the lipid peroxidation was independent of paraquat radical production, an intermediate in the cycle. However, the inhibitory potency of N-ethylmaleimide on NADPH-cytochrome P-450 reductase activity paralleled that on the lipid peroxidation stimulated by paraquat in brain and lung. These findings indicate that the effect of paraquat on microsomal lipid peroxidation differs among the organs and that other factors, besides NADPH-cytochrome P-450 reductase, might be involved in the stimulation of lipid peroxidation by paraquat.

Vitamin E and oxidative stress

Oxidative stress can result from or be enhanced by a large variety of conditions, including nutritional imbalance, exposure to chemical and physical agents in the environment, strenuous physical activities, injury, and hereditary disorders. While many enzymes and compounds are involved in protecting cells from the adverse effects of oxidative stress, vitamin E occupies an important and unique position in the overall antioxidant defense. The antioxidant function of vitamin E is closely related to the status of many dietary components. Vitamin E-depleted animals are generally more susceptible to the adverse effects of environmental agents than supplemented animals. Also, vitamin E supplementation is beneficial to certain groups of the population. However, supplementing vitamin E in experimental subjects maintained on a nutritionally adequate diet does not always provide additional protection. Differential metabolic responses in various organs and differences in experimental conditions often contribute in the discrepancies in the literature. The lack of clear evidence for the occurrence of lipid peroxidation or antioxidant function of vitamin E in vivo can be attributed partly to the presence of active pathways for metabolizing hydroperoxides, aldehydes, and other oxidation products. Specific and sensitive techniques for measuring lipid peroxidation products in biological systems are essential for understanding the role of free radical-induced lipid peroxidation in tissue damage and antioxidant function of vitamin E in vivo.

Free malondialdehyde levels in the urine of rats intoxicated with paraquat

We examined the excretion of free malondialdehyde (MDA) in the urine of rats to which a herbicide, Gramoxone, had been orally administered. The herbicide was administered for 2 days at a dose of 60 mg paraquat/kg body weight/day. As a result, the concentration of free MDA decreased following the intake of Gramoxone. The total amount of free MDA increased temporarily, but then it decreased significantly to below normal values. Rats that died during this experimental period did not excrete any free MDA. In the surviving animals, the MDA concentration in serum and lung microsomes decreased, while that in liver microsomes increased slightly after intake of the poison. Although the cause of the decrease in the urinary free MDA level remains unclear, the marked changes may provide valuable information regarding a toxic mechanism of paraquat intake.