Reduction of paraquat and related bipyridylium compounds to free radical metabolites by rat hepatocytes

Possible involvement of transcriptional activation of nuclear factor erythroid 2-related factor 2 (Nrf2) in the protective effect of caffeic acid on paraquat-induced oxidative damage in Drosophila melanogaster

Paraquat (PQ) is a widely used herbicide with no antidote which is implicated in the pathogenesis of the Parkinson's disease. The present study then investigated the potential of caffeic acid (CA), a known antioxidant, cardioprotective and neuroprotective molecule to counteract oxidative stress mediated by PQ. In addition, molecular docking was performed to understand the mechanism underlying the inhibitory effect of CA against PQ poisoning. The fruit fly, Drosophila melanogaster, was exposed to PQ (0.44 mg/g of diet) in the absence or presence of CA (0.25, 0.5, 1 and 2 mg/g of died) for 7 days. Data showed that PQ-fed flies had higher incidence of mortality which was associated with mitochondrial dysfunction, increased free Fe(II) content and lipid peroxidation when compared to the control. Co-exposure with CA reduced mortality and markedly attenuated biochemical changes induced by PQ. The mechanism investigated using molecular docking revealed a strong interaction (-6.2 Kcal/mol) of CA with D. melanogaster transcriptional activation of nuclear factor erythroid 2-related factor 2 (Nrf2). This was characterized by the binding of CA to keap-1 domain of Nrf2. Taking together these results indicate the protective effect of CA against PQ-induced oxidative damage in D. melanogaster was likely through its coordination which hinders Nrf2-keap-1 binding leading to an increase of the antioxidant defense system.

Predictors of acute kidney injury after paraquat intoxication

Paraquat intoxication is characterized by multi-organ failure, causing substantial mortality and morbidity. Many paraquat patients experience acute kidney injury (AKI), sometimes requiring hemodialysis. We observed 222 paraquat-intoxicated patients between 2000 and 2012, and divided them into AKI (n = 103) and non-AKI (n = 119) groups. The mortality rate was higher for AKI than non-AKI patients (70.1% vs. 40.0%, P < 0.001). Patients with AKI had a longer time to hospital arrival (P = 0.003), lower PaO₂ (P = 0.006) and higher alveolar-arterial O₂ difference (P < 0.001) 48 h after admission, higher sequential organ failure assessment 48-h score (P < 0.001), higher severity index of paraquat poisoning (SIPP) score (P = 0.016), lower PaCO₂ at admission (P = 0.031), higher PaO₂ at admission (P = 0.015), lower nadir PaCO₂ (P = 0.001) and lower nadir HCO₃ (P = 0.004) than non-AKI patients. Multivariate logistic regression indicated that acute hepatitis (P < 0.001), a longer time to hospital arrival (P < 0.001), higher SIPP score (P = 0.026) and higher PaO₂ at admission (P = 0.014) were predictors of AKI. The area under the receiver operating characteristic curve confirmed that an Acute Kidney Injury Network 48-hour score ≥ 2 predicted AKI necessitating hemodialysis with a sensitivity of 0.6 and specificity of 0.832. AKI is common (46.4%) following paraquat ingestion, and acute hepatitis, the time to hospital arrival, SIPP score and PaO₂ at admission were powerful predictors of AKI. Larger studies with longer follow-up durations are warranted.

Diquat-induced cellular pyridine nucleotide redox changes and alteration of metabolic enzyme activities in colonic carcinoma cells

Previously we have shown that the redox cycler menadione (MQ) induced cellular pyridine nucleotide redox imbalance that was linked to a decrease in aerobic glycolysis and perturbation of the mitochondrial respiratory activity due to the redox cycling of the compound; these processes were potentiated by low glucose. In this study, we investigated how colonic epithelial cells maintained pyridine nucleotide (NAD⁺/NADH and NADP⁺/NADPH) redox homeostasis upon acute metabolic variation and exposure to the redox cycling diquat (DQ). Our results show that DQ challenge disrupted cellular NADH/NAD⁺ redox status and enhanced cellular NADPH generation. Notably, DQ-induced NADH decrease was associated with enhanced lactate production, a process that was potentiated by glucose availability, but not by the mitochondrial substrates, succinate or malate/glutamate. In addition, DQ increased glucose 6-phoshate dehydrogenase (G6PDH) activity consistent with glucose diversion towards pentose phosphate pathway. As a consequence, steady-state NADPH levels were maintained during MQ challenge at normal glucose. In contrast and despite increased G6PDH and malic enzyme (ME) activities, DQ induced cellular NADPH-to-NADP⁺ shift at low glucose, a situation that was reversed by mitochondrial substrates. Collectively, these results are consistent with increased aerobic glycolysis by DQ and specific metabolic changes leading to enhanced NADPH generation upon oxidative challenge.

In cellulo monitoring of quinone reductase activity and reactive oxygen species production during the redox cycling of 1,2 and 1,4 quinones

Quinones are highly reactive molecules that readily undergo either one- or two-electron reduction. One-electron reduction of quinones or their derivatives by enzymes such as cytochrome P450 reductase or other flavoproteins generates unstable semiquinones, which undergo redox cycling in the presence of molecular oxygen leading to the formation of highly reactive oxygen species. Quinone reductases 1 and 2 (QR1 and QR2) catalyze the two-electron reduction of quinones to form hydroquinones, which can be removed from the cell by conjugation of the hydroxyl with glucuronide or sulfate thus avoiding its autoxidation and the formation of free radicals and highly reactive oxygen species. This characteristic confers a detoxifying enzyme role to QR1 and QR2, even if this character is strongly linked to the excretion capacity of the cell. Using EPR spectroscopy and confocal microscopy we demonstrated that the amount of reactive oxygen species (ROS) produced by Chinese hamster ovary (CHO) cells overexpressing QR1 or QR2 compared to naive CHO cells was determined by the quinone structural type. Indeed, whereas the amount of ROS produced in the cell was strongly decreased with para-quinones such as menadione in the presence of quinone reductase 1 or 2, a strong increase in ROS was recorded with ortho-quinones such as adrenochrome, aminochrome, dopachrome, or 3,5-di-tert-butyl-o-benzoquinone in cells overexpressing QR, especially QR2. These differences could originate from the excretion process, which is different for para- and ortho-quinones. These results are of particular interest in the case of dopamine considering the association of QR2 with various neurological disorders such as Parkinson disease.

Spectrum of toxic hepatitis following intentional paraquat ingestion: analysis of 187 cases

INTRODUCTION

This retrospective observational study examined the clinical features, the degrees of toxic hepatitis, physiological markers and clinical outcomes after intentional paraquat poisoning and sought to determine what association, if any, might exist between these findings.

METHODS

A total of 187 patients were referred for management of intentional paraquat ingestion between 2000 and 2010. Patients were categorized into two groups according to their hepatic complication, i.e. with (N = 87) or without (N = 100) toxic hepatitis. Demographic, clinical and laboratory data were obtained for analysis. Mortality rates were also analysed.

RESULTS

It was found that patients with toxic hepatitis were younger (39.7 ± 13.7 vs 44.2 ± 16.6 year old, P = 0.046), and suffered from greater incidences of acute respiratory failure (63.2 vs 48.0%, P = 0.037) and acute renal failure (75.9 vs 56.0%, P = 0.004) than patients without hepatitis. The hospitalization period was longer in patients with hepatitis than without hepatitis (16.2 ± 14.6 vs 11.2 ± 12.1 days, P = 0.012), even though there was no difference in mortality rate between both groups (56.3 vs 53.0%, P = 0.649). Notably, the symptoms of toxic hepatitis developed within 6.7 ± 6.3 days of exposure to paraquat with aspartate aminotransferase (AST) 138 ± 156 U/L, alanine aminotransferase (ALT) 127 ± 114 U/L and total bilirubin 2.7 ± 2.6 mg/dL. The hepatitis peaked at 9.5 ± 8.8 days with AST 125 ± 139 U/L, ALT 183 ± 181 U/L and total bilirubin 3.2 ± 3.6 mg/dL. Nevertheless, the symptoms resolved within 17.3 ± 9.8 days of paraquat exposure, and none of the patients died of hepatic complication.

CONCLUSIONS

A substantial proportion of paraquat patients suffered from hepatic complication (46.52%), but the spectrum of hepatitis in these patients seemed mild and transient.

Formation of reactive sulfite-derived free radicals by the activation of human neutrophils: an ESR study

The objective of this study was to determine the effect of (bi)sulfite (hydrated sulfur dioxide) on human neutrophils and the ability of these immune cells to produce reactive free radicals due to (bi)sulfite oxidation. Myeloperoxidase (MPO) is an abundant heme protein in neutrophils that catalyzes the formation of cytotoxic oxidants implicated in asthma and inflammatory disorders. In this study sulfite ((•)SO(3)(-)) and sulfate (SO(4)(•-)) anion radicals are characterized with the ESR spin-trapping technique using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) in the reaction of (bi)sulfite oxidation by human MPO and human neutrophils via sulfite radical chain reaction chemistry. After treatment with (bi)sulfite, phorbol 12-myristate 13-acetate-stimulated neutrophils produced DMPO-sulfite anion radical, -superoxide, and -hydroxyl radical adducts. The last adduct probably resulted, in part, from the conversion of DMPO-sulfate to DMPO-hydroxyl radical adduct via a nucleophilic substitution reaction of the radical adduct. This anion radical (SO(4)(•-)) is highly reactive and, presumably, can oxidize target proteins to protein radicals, thereby initiating protein oxidation. Therefore, we propose that the potential toxicity of (bi)sulfite during pulmonary inflammation or lung-associated diseases such as asthma may be related to free radical formation.

Expression of catalase and glutathione S-transferase genes in Chironomus riparius on exposure to cadmium and nonylphenol

Antioxidant enzymes play important roles in the protection against oxidative damage caused by environmental pollutants by scavenging high levels of reactive oxygen species and have been quantified as oxidative stress markers. However, combining mRNA expressions of genes coding for detoxification enzymes along with enzyme activities will be more useful biomarkers of stress. Therefore, in this study the cDNA of the catalase gene from the aquatic midge, Chironomus riparius (CrCAT) was sequenced using 454 pyrosequencing. The 2139 bp CrCAT cDNA included an open reading frame of 1503 bp encoding a putative protein of 500 amino acids with a predicted molecular mass of 56.72 kDa. There was an 18 bp 5' and a long 618 bp 3' untranslated region with a polyadenylation signal site (AATAAA). The deduced amino acid sequence of CrCAT contained several highly conserved motifs including the proximal heme-ligand signature sequence RLFSYNDTX and the proximal active site signature FXRERIPERVVHAKGXGA. A comparative analysis showed the presence of conserved amino acid residues and all of the catalytic amino acids (His(70), Asn(143), and Tyr(353)) were conserved in all species. The CrCAT contained three potential glycosylation sites and a peroxisome targeting signal of 'AKM'. The mRNA was detected using RT-PCR at all developmental stages. The time-course expression of CrCAT was measured using quantitative real-time PCR after exposure to different concentration and durations of Paraquat (PQ), cadmium chloride (Cd) and nonylphenol (NP). The expression of CrCAT was significantly up regulated on exposure to 50 and 100mg/L PQ for 12 and 24h. Among the different concentrations and durations of Cd tested, significantly highest level of expression for CrCAT mRNA and catalase enzyme activity was observed on exposure to 10mg/L for 24h. In the case of NP, the highest level of CrCAT expression was observed after exposure to 100 μg/L for 24h. The expression profiles of three selected C. riparius glutathione S-transferase genes (CrGSTs) viz. CrGSTdelta3, CrGSTsigma4 and CrGSTepsilon1 was also studied on exposure to NP and were up or down regulated at different time points and concentrations. Significantly highest level of expression for CrGSTdelta3 was observed after 48 h and for CrGSTsigma4 and CrGSTepsilon1 after 24h exposure to 100 μg/L of NP. The results show that CrGSTs and CrCAT could be used as potential biomarkers in C. riparius for aquatic ecotoxicological studies.

Characterization and transcriptional regulation of thioredoxin reductase 1 on exposure to oxidative stress inducing environmental pollutants in Chironomus riparius

We characterized thioredoxin reductase 1 (TrxR1) from Chironomus riparius (CrTrxR1) and studied its expression under oxidative stress. The full-length cDNA is 1820bp long and contains an open reading frame (ORF) of 1488bp. The deduced CrTrxR1 protein has 495 amino acids and a calculated molecular mass of 54.41kDa and an isoelectric point of 6.15. There was a 71bp 5' and a 261bp 3' untranslated region with a polyadenylation signal site (AATAAA). Homologous alignments showed the presence of conserved catalytic domain Cys-Val-Asn-Val-Gly-Cys (CVNVGC), the C-terminal amino acids 'CCS' and conserved amino acids required in catalysis. The expression of CrTrxR1 is measured using quantitative real-time PCR after exposure to 50 and 100mg/L of paraquat (PQ) and 2, 10 and 20mg/L of cadmium chloride (Cd). CrTrxR1 mRNA was upregulated after PQ exposure at all conditions tested. The highest level of CrTrxR1 expression was observed after exposure to 10mg/L of Cd for 24h followed by 20mg/L for 48h. Significant downregulation of CrTrxR1 was observed after exposure to 10 and 20mg/L of Cd for 72h. This study shows that the CrTrxR1 could be potentially used as a biomarker of oxidative stress inducing environmental contaminants.

Water extracts of Brassica oleracea var. costata potentiate paraquat toxicity to rat hepatocytes in vitro

Tronchuda cabbage extracts have been proven to have antioxidant potential against various oxidative species in cell free systems, though its antioxidant potential in cellular models remained to be demonstrated. In the present study, we used primary cultures of rat hepatocytes for the cellular assay system and paraquat PQ exposure as a pro-oxidant model agent, to test whether tronchuda cabbage hydrolysed water extracts provide protective or aggravating effects towards PQ-induced oxidative stress and cell death. For this purpose cellular parameters related to oxidative stress were measured, namely the generation of superoxide anion, glutathione oxidation, lipid peroxidation, intracellular ATP levels, activation of nuclear factor-kappaB (NF-kappaB), activity of antioxidant enzymes, and cell death. The obtained results demonstrated that the studied hydrolysed water extracts of tronchuda cabbage, especially rich in kaempferol (84%) and other polyphenols, namely hydroxycinnamic acids and traces of quercetin, can potentiate the toxicity of PQ in primary cultures of rat hepatocytes. These results highlight that prospective antioxidant effects of plant extracts, observed in vitro, using non-cellular systems, are not always confirmed in cellular models, in which the concentrations required to scavenge pro-oxidant species may be highly detrimental to the cells.

Comparative effects of the herbicides dicamba, 2,4-D and paraquat on non-green potato tuber calli

The effects of the herbicides 1,1'-dimethyl-4,4'-bipyridylium dichloride (paraquat), 3,6-dichloro-2-metoxybenzoic acid (dicamba) and 2,4-dichlorophenoxyacetic acid (2,4-D) on cell growth of non-green potato tuber calli are described. We attempted to relate the effects with toxicity, in particular the enzymes committed to the cellular antioxidant system. Cell cultures were exposed to the herbicides for a period of 4 weeks. Cellular integrity on the basis of fluorescein release was strongly affected by 2,4-D, followed by dicamba, and was not affected by paraquat. However, the three herbicides decreased the energy charge, with paraquat and 2,4-D being very efficient. Paraquat induced catalase (CAT) activity at low concentrations (1 microM), whereas at higher concentrations, inhibition was observed. Dicamba and 2,4-D stimulated CAT as a function of concentration. Superoxide dismutase (SOD) activity was strongly stimulated by paraquat, whereas dicamba and 2,4-D were efficient only at higher concentrations. Glutathione reductase (GR) activity was induced by all the herbicides, suggesting that glutathione and glutathione-dependent enzymes are putatively involved in the detoxification of these herbicides. Paraquat slightly inhibited glutathione S-transferase (GST), whereas 2,4-D and dicamba promoted significant activation. These results indicate that the detoxifying mechanisms for 2,4-D and dicamba may be different from the mechanisms of paraquat detoxification. However, the main cause of cell death induced by paraquat and 2,4-D is putatively related with the cell energy charge decrease.

Decrease in NADPH-Cytochrome P450 Reductase Activity of the Human Heart, Liver and Lungs in the Presence of Alpha-Lipoic Acid

BACKGROUND

NADPH-cytochrome P450 reductase (CPR) is the electron donor protein for several oxygenase enzymes located in the endoplasmic reticulum. These oxygenases include P450 family enzymes involved in the metabolism of endogenous and exogenous substances. The enzyme is involved in adriamycin (anticancer drug) and paraquat (herbicide) toxicity. CPR is a flavoprotein containing both flavine-adenine dinucleotide and flavine mononucleotide. A structural study showed the presence of several sulfhydryl (SH) groups in the CPR molecule. Some of them play a key role in catalytic activity. As alpha-lipoic acid contains a disulfide bond, it may react with the SH group of CPR. The aim of the study was to evaluate the effect of alpha-lipoic acid on human P450 reductase activity.

METHODS

The activity of the enzyme was determined by measuring the rate of cytochrome c reduction at 550 nm, in vitro, using heart, liver and lung microsomes.

RESULTS

The activity of CPR was decreased in all organs after addition of alpha-lipoic acid to the reaction mixture at concentrations of 0.01, 0.10 and 1.00 mM. The decreases in CPR activity were concentration-dependent and the sequence of relative inhibition was as follows: heart >lung >liver. However, the statistical significance of CPR activity vs. control was observed in the heart in the presence of 1.00 mM alpha-lipoic acid and in the lung at 0.10 and 1.00 mM alpha-lipoic acid.

CONCLUSION

alpha-Lipoic acid decreased NADPH-CPR activity in the lung and heart. The present results are promising for future studies to obtain the most effective antidote for adriamycin and paraquat toxicity.

Agrochemical Poisoning

A general increase in the use of chemicals in agriculture has brought about a concomitant increase in the incidence of agrochemical poisoning. Organophosphates are the most common agrochemical poisons followed closely by herbicides. Many agricultural poisons, such as parathion and paraquat are now mixed with a coloring agent such as indigocarmine to prevent their use criminally. In addition, paraquat is fortified with a “stenching” agent. Organo-chlorines have an entirely different mechanism of action. Whereas organophosphates have an anticholinesterase activity, organochlorines act on nerve cells interfering with the transmission of impulses through them. A kerosene-like smell also emanates from death due to organochlorines. The diagnosis lies in the chemical identification of organochlorines in the stomach contents or viscera. Organochlorines also resist putrefaction and can be detected long after death. Paraquat has been involved in suicidal, accidental, and homicidal poisonings. It is mildly corrosive and ulceration around lips and mouth is common in this poisoning. However, the hallmark of paraquat poisoning, especially when the victim has survived a few days, are the profound changes in lungs. Other agrochemicals such as algicides, aphicides, herbicide safeneres, fertilizers, and so on, are less commonly encountered. Governments in most countries have passed legislations to prevent accidental poisonings with these agents. The US government passed the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA) in 1962 and the Indian government passed The Insecticides Act in 1968. Among other things, these acts require manufacturers to use signal words on the labels of insecticides, so the public is warned of their toxicity and accompanying danger.

Activity of NADPH-Cytochrome P-450 Reductase of the Human Heart, Liver and Lungs in the Presence of (-)-Epigallocatechin Gallate, Quercetin and Resveratrol: An in vitro Study

NADPH-cytochrome P-450 reductase (P-450 reductase) plays a crucial role in the metabolism of many endogenic compounds and xenobiotics detoxication. The enzyme is also involved in the toxicity of some clinically important antitumour drugs (doxorubicin) and pesticides (paraquat). P-450 reductase activates them to their more toxic metabolites via one electron reduction which triggers free radical cascade. In some cases however, such transformation is essential to produce therapeutic effect in anticancer drugs. The main purpose of the paper was to evaluate the effect of three natural compounds found in human diet: (-)-epigallocatechin gallate (EGCG), quercetin and resveratrol on P-450 reductase activity. The activity of the enzyme was determined spectrophotometrically by measurement of the rate of cytochrome c reduction at 550 nm, in vitro, using human heart, liver and lung microsomes. It was found that quercetin increased the P-450 reductase activity in human organs at all tested doses. The activity of microcosms in all organs was enhanced according to the concentrations of quercetin, which increased the activity in the order lung>heart>liver. Addition of EGCG to the reaction mixture enhanced the P-450 reductase activity in the following order: liver>heart>lung. However, no significant effect of resveratrol on P-450 reductase activity was observed. It seems that the presence of quercetin and EGCG in the diet may increase P-450 reductase activity during doxorubicin therapy with subsequent increased risk of toxicity. A beneficial effect may be obtained in anticancer therapy with bioreductive agents like tirapazamine.

The influence of oxidative stress on catalase and MnSOD gene transcription in astrocytes

The brain is particularly vulnerable to oxygen free radicals, which have been implicated in the pathology of several neurological disorders. The antioxidant enzyme (AOE) system of the brain may play an important role in the protection against such oxidative stress. We investigated the influence of oxidative stress on the transcription of catalase and MnSOD mRNA. Primary rat astroglial cell cultures were treated either with hydrogen peroxide (H2O2), as a direct mediator of oxidative stress, or with the redox cycling compound paraquat. Both substances led to an increase of catalase and MnSOD mRNA levels. To further elucidate the mechanisms residing behind this increase, transfection experiments were performed. Transient transfection of primary astroglial cells with a reporter plasmid containing the upstream region of the catalase gene showed a decrease in reporter gene activity after exposure of transfected cells to either H2O2 or paraquat. In contrast, transfection experiments done with reporter plasmids for the MnSOD upstream region resulted in an increase of reporter gene activity after H2O2 as well as after paraquat treatment of transfected cells. These results indicate transcriptional regulation of MnSOD and post-transcriptional regulation of catalase gene expression after oxidative stress in primary rat astrocytes.

Superoxide production from paraquat evoked by exogenous NADPH in pulmonary endothelial cells

Superoxide production from paraquat in a pulmonary microvascular endothelial cell (PMEC) suspension was demonstrated using 2-methyl-6-(p-methoxyphenyl)-3,7-dihydroimidazo[1,2-alpha]pyraz in-3-one (MCLA), a chemiluminescence probe, to detect superoxide anions. Increased rates of superoxide production from paraquat, which were sensitive to superoxide dismutase (SOD), required the presence of reduced nicotinamide adenine dinucleotide phosphate (NADPH) in the reaction medium, and occurred instantaneously after the addition of NADPH, which is impermeable to cell membranes. NADH as an electron donor was not as effective, and xanthine or succinate had no influence. Paraquat was anaerobically reduced in the presence of NADPH and PMECs to yield a one-electron reduced radical, and the reduction was inhibited by NADP+. Diphenyleneiodonium, an inhibitor of flavoprotein reductases, also markedly inhibited both paraquat reduction and superoxide production. These results indicate that NADPH-dependent superoxide production from paraquat probably occurs by a flavoprotein with NADPH-dependent reductase activity in cell membranes. NADPH-dependent superoxide production from paraquat was also reproduced using adherent PMECs on wells. Under these conditions, superoxide production was enhanced with agonists, including interleukin-1beta, A23187, and phorbol 12-myristate 13-acetate. The effect of the former two was blocked with staurosporine, while the latter's effect was suppressed with calyculin A.

Regulation of human apolipoprotein A-I gene expression by gramoxone

To induce oxidative stress, HepG2 cells were exposed to a compound known as gramoxone. This compound undergoes a one-electron reduction to form a stable free radical which is capable of generating reactive oxygen species. We demonstrated that exposure of HepG2 cells to gramoxone (0.1 microM) resulted in a 2-fold decrease in apoA-I mRNA with no significant change in apoB and apoE mRNA levels. To examine if increased rates of mRNA degradation were responsible for the reduction in apoA-I mRNA levels, mRNA half-lives were measured in the presence of actinomycin D with and without gramoxone treatment. These studies revealed a 4-fold increase in the rate of apoA-I mRNA degradation in cells exposed to gramoxone. In similarly treated cells, nuclear run-off assays indicated that the transcription rate of the apoA-I gene was also increased 2-fold. Consistent with nuclear run-off assays, transient transfection experiments using a series of pGL2-derived luciferase reporter plasmids containing the human apoAI proximal promoter demonstrated that gramoxone treatment increased apoA-I promoter activity 2-fold. We have identified a potential "antioxidant response element" (ARE) in the apoA-I promoter region that may be responsible for the increase in apoA-I transcriptional activity by gramoxone. Gel mobility shift assays with an ARE oligonucleotide revealed increased levels of a specific protein-DNA complex that formed with nuclear extracts from gramoxone-treated cells. UV cross-linking experiments with the ARE and nuclear extracts from either untreated or gramoxone-treated cells detected proteins of approximately 100 and 115 kDa. When a single copy of the ARE was inserted upstream of the SV40 promoter in a luciferase reporter plasmid, a significant 2-fold induction in luciferase activity was observed in HepG2 cells in the presence of gramoxone. In contrast, a plasmid containing a mutant apoAI-ARE did not confer responsiveness to gramoxone. Furthermore, pGL2 (apoAI-250 mutant ARE), in which point mutations eliminated the ARE in the apoAI promoter, showed no increase in luciferase activity in response to gramoxone. Taken together, the data suggest that gramoxone affects apoA-I mRNA levels by both transcriptional and post-transcriptional mechanisms.

Generation of reactive oxygen species by the redox cycling of nitroprusside

The formation of oxygen species during the redox cycling of sodium nitroprusside by rat liver microsomes and by chemical reductants was evaluated. The reduction of sodium nitroprusside by ascorbate and glutathione results in formation of the nitroprusside nitroxide radical which, on freezing at 77 K, results exclusively in the tetracyano [Fe(CN4)NO]2- and pentacyano [Fe(CN5)NO]3- forms of nitroxide radicals, respectively. The role of reducing agents on the inter-conversion of these two forms of nitroxide radical is discussed. The NADH and NADPH dependent microsomal reduction of nitroprusside results in the production of nitroprusside nitroxide radical, which in the presence of oxygen undergoes redox cycling to generate superoxide radical, and eventually hydroxyl radical is formed by a Fenton-type of reaction. Studies on the effect of several biologically or toxicologically relevant iron chelators on NADPH-dependent microsomal reduction of nitroprusside and subsequent formation of hydroxyl radical indicate that certain iron chelators such as isocitrate act as hydroxyl radical scavengers (depending on its concentration), but other chelators such as EDTA and DPTA function as good catalysts for the generation of hydroxyl radicals. The NADH and nitroprusside dependent microsomal production of hydroxyl radical is better in the presence of ATP, or equal in the presence of acetate, or diminished in the presence of DTPA when compared to the NADPH- and nitroprusside-dependent microsomal production of hydroxyl radicals. The effect of these chelates on the redox cycling of iron and nitroprusside by microsomes is discussed. Rat liver sub-mitochondrial particles and human hepatoblastoma cells (HepG2 cell line) also generated superoxide and hydroxyl radicals during the redox cycling of nitroprusside. These results provide direct evidence for the production of reactive oxygen species during the redox cycling of nitroprusside, The use of nitroprusside as a nitric oxide donor in biological systems may be complicated by the necessity to consider the generation of reactive oxygen species due to redox cycling of this compound by cellular reductases and low-molecular weight reductants.

Thiols metabolism is altered by the herbicides paraquat, dinoseb and 2,4-D: a study in isolated hepatocytes

This report is an extension and complement of a previous study reporting the effect of three herbicides (paraquat, dinoseb and 2,4-D) on cell viability, GSH oxidation, NADH and ATP depletion (Arch. Toxicol. 68:24-31, 1994). Here we report additional data and findings aimed at a better understanding of the toxicity mechanisms induced by these herbicides. Biochemical mechanisms of cytotoxicity induced by the herbicides paraquat (1,1'-dimethyl-4,4'-bipyridylium dichloride), dinoseb (2-sec-butyl-4,6-dinitrophenol) and 2,4-D (2,4-dichlorophenoxyacetic acid) were investigated in freshly isolated rat hepatocytes. Herbicide metabolism, especially paraquat and 2,4-D, rapidly depletes GSH and protein thiols. Paraquat and 2,4-D (1-10 mM) decrease the GSH/GSSG ratio, promote loss of protein thiol contents and induce lipid peroxidation. Dinoseb, the most effective cytotoxic compound under study (used in concentrations 1000-fold lower than paraquat and 2,4-D), had moderate effects upon the GSH/GSSG ratio and lipid peroxidation, causing a depletion of protein thiols of about 20%. The results indicate that the herbicides paraquat and 2,4-D are hepatotoxic and may induce cell death by decreasing cellular GSH/GSSG ratio and protein thiols, and by inducing lipid peroxidation. The cytotoxic action of dinoseb is likely to be related with the uncoupling of oxidative phosphorylation in mitochondria. Therefore, it is likely that liver damage observed during the first phase of herbicide-intoxication is related to these metabolic processes.

Mitochondrial bioenergetics is affected by the herbicide paraquat

The potential toxicity of the herbicide paraquat (1,1-dimethyl-4,4'-bipyridylium dichloride) was tested in bioenergetic functions of isolated rat liver mitochondria. Paraquat increases the rate of State 4 respiration, doubling at 10 mM, indicating uncoupling effects. Additionally, State 3 respiration is depressed by about 15%, at 10 mM paraquat, whereas uncoupled respiration in the presence of CCCP is depressed by about 30%. Furthermore, paraquat partially inhibits the ATPase activity through a direct effect on this enzyme complex. However, at high concentrations (5-10 mM), the ATPase activity is stimulated, probably as consequence of the described uncoupling effect. Depression of respiratory activity is mediated through partial inhibitions of mitochondrial complexes III and IV. Paraquat depresses delta psi as a function of herbicide concentration. In addition, the depolarization induced by ADP is decreased and repolarization is biphasic suggesting a double effect. Repolarization resumes at a level consistently higher than the initial level before ADP addition, for paraquat concentrations up to 10 mM. This particular effect is clear at 1 mM paraquat and tends to fade out with increasing concentrations of the herbicide.

Effects of bipyridylium compounds on calcium release from triadic vesicles isolated from rabbit skeletal muscle

1. The effects of 1,1'-diheptyl-4,4'-bipyridinium dibromide (DHBP), a viologen for electrochromic memory display agent, on calcium release and ryanodine binding were studied with triad-rich sarcoplasmic reticulum (SR) vesicles isolated from rabbit skeletal muscle. 2. DHBP inhibited the calcium release induced by 2 mM caffeine and 2 micrograms ml-1 polylysine with an IC50 value of 5 micrograms ml-1 and 4 micrograms ml-1 respectively. 3. DHBP inhibited [3H]-ryanodine binding in a dose-dependent manner with an IC50 of 2.5 micrograms ml-1 and 90-100% inhibition at 20-30 micrograms ml-1. 4. Calcium uptake by SR was inhibited in the presence of caffeine and this inhibition was antagonized by concomitant addition of DHBP. 5. The effect of DHBP on muscle twitches was studied on the mouse diaphragm. Muscle twitches elicited by direct electrical muscle stimulation and contractions induced by either 10 mM caffeine or 1 microM ryanodine were blocked by pretreatment with DHBP. 6. Data from this study provided evidence that DHBP blocked the calcium release from SR by direct interaction with the calcium release channel, also known as the ryanodine receptor. A possible use of this agent as a specific inhibitor for calcium release and as a muscle relaxant was suggested.

Metabolic alterations in hepatocytes promoted by the herbicides paraquat, dinoseb and 2,4-D

The cytotoxic effects of the herbicides paraquat (1,1'-dimethyl-4,4'-bipyridylium dichloride), dinoseb (2-sec-butyl-4,6-dinitrophenol) and 2,4-D (2,4-dichlorophenoxyacetic acid) on freshly isolated rat hepatocytes were investigated. Paraquat and 2,4-D (1-10 mM) caused a dose and time dependent cell death accompanied by depletion of intracellular glutathione (GSH) and mirroring increase of oxidized glutathione (GSSG). Dinoseb, the most effective cytotoxic compound under study (used in concentrations 1000 fold lower than paraquat and 2,4-D), exhibited moderate effects upon the level of GSH and GSSG. These limited effects are at variance with significant effects upon the adenine and pyridine nucleotide contents. ATP and NADH levels are rapidly depleted by herbicide metabolism. This depletion is observed in the millimolar range for paraquat and 2,4-D and in the micromolar range for dinoseb. 2,4-D completely depletes cellular ATP, with subsequent cell death, as detected by LDH leakage. Paraquat rapidly depletes NADH, according to the redox cycling of the herbicide metabolism. The most effective compound is dinoseb since it exerts similar effects as described for paraquat and 2,4-D at concentrations 1000 fold lower. Simultaneously with NADH and ATP depletion, the levels of ADP, AMP and NAD+ increase in hepatocytes incubated in the presence of the herbicides. In contrast to NADH, the time course and extent of ATP depletion and fall in energy charge correlate reasonably with the time of onset and rate of cell death. It is concluded that the herbicides, paraquat and 2,4-D are hepatotoxic and initiate the process of cell death by decreasing cellular GSH.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of hepatic metallothionein by paraquat

Paraquat, a frequently used contact herbicide, produces oxidative stress by undergoing redox cycling and generating reactive oxygen species. Paraquat is also effective at increasing hepatic levels of metallothionein (MT). The mechanism(s) by which agents that induce oxidative stress produce increases in MT concentrations is not yet known. Therefore, the goal of the current study was to characterize the elevation in hepatic MT produced by paraquat administration to mice and to examine potential mechanism(s) of this increase. A dose-response study for increases in MT showed that administration of 0.1 to 0.5 mmol/kg of paraquat, sc, increased hepatic MT with a maximal increase of 36-fold. Subsequent studies were carried out with paraquat at a dose (0.3 mmol/kg, sc) that caused oxidative stress, as shown by a 35-fold increase in the biliary excretion of oxidized glutathione. There were coordinate elevations of both hepatic MT-I and MT-II mRNA of approximately 5-fold with peaks at both 6 and 24 hr after paraquat. The time course for the elevation in hepatic MT protein following paraquat treatment showed that MT levels had a maximal increase of 18-fold obtained at 36 hr. Paraquat appears to be an indirect MT inducer, in that there were no elevations in MT when cultured mouse hepatocytes were exposed to paraquat. No rise in liver Zn was observed prior to the increase in hepatic MT, thus, a Zn redistribution to the liver did not cause the increase in hepatic MT following paraquat administration. Adrenalectomy did not abolish the increase in MT produced by paraquat, suggesting that adrenal gland products are not required for the increase in MT produced by paraquat. In conclusion, the chemical mediator responsible for the increase in hepatic MT after paraquat was not determined, but the elevation in MT concentration appears to be due to increased transcription.