Stimulation by paraquat of microsomal and cytochrome P-450-dependent oxidation of glycerol to formaldehyde

Age-related formaldehyde interferes with DNA methyltransferase function, causing memory loss in Alzheimer's disease

Hippocampus-related topographic amnesia is the most common symptom of memory disorders in Alzheimer's disease (AD) patients. Recent studies have revealed that experience-mediated DNA methylation, which is regulated by enzymes with DNA methyltransferase (DNMT) activity, is required for the formation of recent memory as well as the maintenance of remote memory. Notably, overexpression of DNMT3a in the hippocampus can reverse spatial memory deficits in aged mice. However, a decline in global DNA methylation was found in the autopsied hippocampi of patients with AD. Exactly, what endogenous factors that affect DNA methylation still remain to be elucidated. Here, we report a marked increase in endogenous formaldehyde levels is associated with a decline in global DNA methylation in the autopsied hippocampus from AD patients. In vitro and in vivo results show that formaldehyde in excess of normal physiological levels reduced global DNA methylation by interfering DNMTs. Interestingly, intrahippocampal injection of excess formaldehyde before spatial learning in healthy adult rats can mimic the learning difficulty of early stage of AD. Moreover, injection of excess formaldehyde after spatial learning can mimic the loss of remote spatial memory observed in late stage of AD. These findings suggest that aging-associated formaldehyde contributes to topographic amnesia in AD patients.

Betanin attenuates paraquat-induced liver toxicity through a mitochondrial pathway

We attempted to determine whether betanin (from natural pigments) that has anti-oxidant properties would be protective against paraquat-induced liver injury in Sprague-Dawley rats. Paraquat was injected intraperitoneally into rats to induce liver toxicity. The rats were randomly divided into four groups: a control group, a paraquat group, and two groups that received betanin at doses of 25 and 100mg/kg/day three days before and two days after they were administered paraquat. We evaluated liver histopathology, serum liver enzymatic activities, oxidative stress, cytochrome P450 (CYP) 3A2 mRNA expression, and mitochondrial damage. The rats that were injected with paraquat incurred liver injury, evidenced by histological changes and elevated serum aspartate aminotransferase and alanine aminotransferase levels; paraquat also led to oxidative stress, an increase of cytochrome P450 3A2 mRNA expression, and mitochondrial damage, indicated by mitochondrial membrane swelling, reduced mitochondrial cytochrome C, and apoptosis-inducing factor protein levels. Pathological damage and all of the above mentioned markers were lesser in the animals treated with betanin than in those who received paraquat alone. Betanin had a protective effect against paraquat-induced liver damage in rats. The mechanism of the protection appears to be the inhibition of CYP 3A2 expression and protection of mitochondria.

Proteomic identification of toxic volatile organic compound-responsive proteins in Arabidopsis thaliana

The proteins that are responsive to toxic volatile organic compounds (VOC) such as formaldehyde and toluene were analyzed with proteome analysis using two-dimensional difference image gel electrophoresis (DIGE) technology. Twenty-one days after germination (DAG) seedlings of Arabidopsis thaliana were exposed either to the gaseous formaldehyde or toluene in an airtight box installed in a plant growth chamber maintained at 24 degrees C under the long day condition with relatively low light condition. Comparative expression analysis revealed 14 and 22 protein spots as proteins displaying at least 1.5-fold differences in expression upon formaldehyde and toluene treatment, respectively, compared to those of untreated control. Most of the isolated spots were successfully identified by peptide analysis using LC-MS-MS. The VOC-responsive proteins contain ATP synthase CF1, ribulose-1,5-bisphosphate carboxylase/oxygenase, photosystem II light harvesting complex, and enolase, which are components of photosynthesis and carbohydrate metabolism. Despite the relatively low light intensity was applied, many identified VOC-induced proteins were previously known to be up-regulated upon high light stimulus. In addition, proteins involved in the toxin catabolic process and stress hormone-related proteins were identified as toluene-induced proteins. Although the exact function of most of the VOC-responsive proteins identified in these experiments had not been characterized, the protein expression analysis using DIGE was clearly demonstrated that plants are capable of responding actively to VOCs at translational level, and identified proteins may provide valuable tools to account for the effects of abiotic stress caused by air pollutants such as VOCs in plant.

Paraquat Increases Cyanide-insensitive Respiration in Murine Lung Epithelial Cells by Activating an NAD(P)H:Paraquat Oxidoreductase

Pulmonary fibrosis is one of the most severe consequences of exposure to paraquat, an herbicide that causes rapid alveolar inflammation and epithelial cell damage. Paraquat is known to induce toxicity in cells by stimulating oxygen utilization via redox cycling and the generation of reactive oxygen intermediates. However, the enzymatic activity mediating this reaction in lung cells is not completely understood. Using self-referencing microsensors, we measured the effects of paraquat on oxygen flux into murine lung epithelial cells. Paraquat (10-100 microm) was found to cause a 2-4-fold increase in cellular oxygen flux. The mitochondrial poisons cyanide, rotenone, and antimycin A prevented mitochondrial- but not paraquat-mediated oxygen flux into cells. In contrast, diphenyleneiodonium (10 microm), an NADPH oxidase inhibitor, blocked the effects of paraquat without altering mitochondrial respiration. NADPH oxidases, enzymes that are highly expressed in lung epithelial cells, utilize molecular oxygen to generate superoxide anion. We discovered that lung epithelial cells possess a distinct cytoplasmic diphenyleneiodonium-sensitive NAD(P)H:paraquat oxidoreductase. This enzyme utilizes oxygen, requires NADH or NADPH, and readily generates the reduced paraquat radical. Purification and sequence analysis identified this enzyme activity as thioredoxin reductase. Purified paraquat reductase from the cells contained thioredoxin reductase activity, and purified rat liver thioredoxin reductase or recombinant enzyme possessed paraquat reductase activity. Reactive oxygen intermediates and subsequent oxidative stress generated from this enzyme are likely to contribute to paraquat-induced lung toxicity.

Modification of intracellular levels of glutathione-dependent formaldehyde dehydrogenase alters glutathione homeostasis and root development

Glutathione (GSH)-dependent formaldehyde dehydrogenase (FALDH) is a highly conserved medium-chain dehydrogenase reductase and the main enzyme that metabolizes intracellular formaldehyde in eukaryotes. It has been recently shown that it exhibits a strong S-nitrosoglutathione (GSNO) reductase activity and could be a candidate to regulate NO-signalling functions. However, there is a lack of knowledge about the tissue distribution of this enzyme in plants. Here, we have studied the localization and developmental expression of the enzyme using immunolocalization and histochemical activity assay methods. We conclude that FALDH is differentially expressed in the organs of Arabidopsis thaliana mature plants, with higher levels in roots and leaves from the first stages of development. Spatial distribution of FALDH in these two organs includes the main cell types [epidermis (Ep) and cortex (Cx) in roots, and mesophyll in leaves] and the vascular system. Arabidopsis thaliana mutants with modified levels of FALDH (both by over- and under-expression of the FALDH-encoding gene) show a significant reduction of root length, and this phenotype correlates with an overall decrease of intracellular GSH levels and alteration of spatial distribution of GSH in the root meristem. Tansgenic roots are partially insensitive to exogenous GSH, suggesting an inability to detect reduction-oxidation (redox) changes of the GSH pool and/or maintain GSH homeostasis.

Enhanced formaldehyde detoxification by overexpression of glutathione-dependent formaldehyde dehydrogenase from Arabidopsis

The ADH2 gene codes for the Arabidopsis glutathione-dependent formaldehyde dehydrogenase (FALDH), an enzyme involved in formaldehyde metabolism in eukaryotes. In the present work, we have investigated the potential role of FALDH in detoxification of exogenous formaldehyde. We have generated a yeast (Saccharomyces cerevisiae) mutant strain (sfa1Delta) by in vivo deletion of the SFA1 gene that codes for the endogenous FALDH. Overexpression of Arabidopsis FALDH in this mutant confers high resistance to formaldehyde added exogenously, which demonstrates the functional conservation of the enzyme through evolution and supports its essential role in formaldehyde metabolism. To investigate the role of the enzyme in plants, we have generated Arabidopsis transgenic lines with modified levels of FALDH. Plants overexpressing the enzyme show a 25% increase in their efficiency to take up exogenous formaldehyde, whereas plants with reduced levels of FALDH (due to either a cosuppression phenotype or to the expression of an antisense construct) show a marked slower rate and reduced ability for formaldehyde detoxification as compared with the wild-type Arabidopsis. These results show that the capacity to take up and detoxify high concentrations of formaldehyde is proportionally related to the FALDH activity in the plant, revealing the essential role of this enzyme in formaldehyde detoxification.

Induction of oxidative stress by chronic administration of sodium dichromate [chromium VI] and cadmium chloride [cadmium II] to rats

Recent studies have demonstrated that both chromium (VI) and cadmium (II) induce an oxidative stress, as determined by increased hepatic lipid peroxidation, hepatic glutathione depletion, hepatic nuclear DNA damage, and excretion of urinary lipid metabolites. However, whether chronic exposure to low levels of Cr(VI) and Cd(II) will produce an oxidative stress is not shown. The effects of oral, low (0.05 LD50) doses of sodium dichromate [Cr(VI); 2.5 mg/kg/d] and cadmium chloride [Cd(II); 4.4 mg/kg/d] in water on hepatic and brain mitochondrial and microsomal lipid peroxidation, excretion of urinary lipid metabolites including malondialdehyde, formaldehyde, acetaldehyde and acetone, and hepatic nuclear DNA-single strand breaks (SSB) were examined in female Sprague-Dawley rats over a period of 120 d. The animals were treated daily using an intragastric feeding needle. Maximum increases in hepatic and brain lipid peroxidation were observed between 60 and 75 d of treatment with both cations. Following Cr(VI) administration for 75 d, maximum increases in the urinary excretion of malondialdehyde, formaldehyde, acetaldehyde, and acetone were 2.1-, 1.8-, 2.1-, and 2.1-fold, respectively, while under the same conditions involving Cd(II) administration approximately 1.8-, 1.5-, 1.9-, and 1.5-fold increases were observed, respectively, as compared to control values. Following administration of Cr(VI) and Cd(II) for 75 d, approximately 2.4- and 3.8-fold increases in hepatic nuclear DNA-SSB were observed, respectively, while approximately 1.3- and 2.0-fold increases in brain nuclear DNA-SSB were observed, respectively. The results clearly indicate that low dose chronic administration of sodium dichromate and cadmium chloride induces an oxidative stress resulting in tissue damaging effects that may contribute to the toxicity and carcinogenicity of these two cations.

Cadmium-induced excretion of urinary lipid metabolites, DNA damage, glutathione depletion, and hepatic lipid peroxidation in Sprague-Dawley rats

Recent studies have described lipid peroxidation to be an early and sensitive consequence of cadmium exposure, and free radical scavengers and antioxidants have been reported to attenuate cadmium-induced toxicity. These observations suggest that cadmium produces reactive oxygen species that may mediate many of the untoward effects of cadmium. Therefore, the effects of cadmium (II) chloride on reactive oxygen species production were examined following a single oral exposure (0.50 LD50) by assessing hepatic mitochondrial and microsomal lipid peroxidation, glutathione content in the liver, excretion of urinary lipid metabolites, and the incidence of hepatic nuclear DNA damage. Increases in lipid peroxidation of 4.0- and 4.2-fold occurred in hepatic mitochondria and microsomes, respectively, 48 h after the oral administration of 44 mg cadmium (II) chloride/kg, while a 65% decrease in glutathione content was observed in the liver. The urinary excretion of malondialdehyde (MDA), formaldehyde (FA), acetaldehyde (ACT), and acetone (ACON) were determined at 0-96 h after Cd administration. Between 48 and 72 h posttreatment maximal excretion of the four urinary lipid metabolites was observed with increases of 2.2- to 3.6-fold in cadmium (II) chloride-treated rats. Increases in DNA single-strand breaks of 1.7-fold were observed 48 h after administration of cadmium. These results support the hypothesis that cadmium induces production of reactive oxygen species, which may contribute to the tissue-damaging effects of this metal ion.

Naphthalene-induced oxidative stress in rats and the protective effects of vitamin E succinate

Quinone metabolites of naphthalene (NAP) are known to produce lipid peroxidation. However, the ability of naphthalene to induce oxidative stress in experimental animals has not been extensively investigated. Furthermore, the effects of vitamin E succinate [(+)-alpha-tocopherol acid succinate; VES] on naphthalene-induced oxidative stress and tissue damage were assessed. Female Sprague-Dawley rats were treated with a single oral dose of 1100 mg naphthalene/kg (0.50 LD50) in corn oil. Vitamin E succinate-treated rats received 100 mg VES/kg/day orally for 3 d before naphthalene treatment, and 40 mg VES/kg/d after NAP administration. Hepatic and brain tissues and urine samples were collected 0, 12, 24, 48, and 72 h after NAP treatment. Naphthalene treatment resulted in a 2.1-fold increase in lipid peroxidation in liver and brain mitochondria at the 24-h time point. Increases in hepatic and brain mitochondrial lipid peroxidation in VES plus NAP-treated rats were 39-46% less than NAP treated rats at 24 h. DNA-single strand breaks increased 3.0-fold in hepatic tissues in NAP treated rats, and increased only 1.6-fold in VES protected rats at the 24-h time point. Glutathione (GSH) decreased by 83 and 49% in hepatic and brain tissues, respectively, in NAP-treated rats at the 24-h time point, while GSH content in VES plus NAP-treated rats decreased 47 and 21% in hepatic and brain tissues, respectively, at this same time point. Microsomal membrane fluidity, a measurement of membrane damage, increased 1.9- and 1.7-fold in liver and brain tissues, respectively, in NAP-treated rats, and only 1.3- and 1.2-fold in NAP plus VES-treated rats at the 24-h time point. The urinary excretion of malondialdehyde (MDA), formaldehyde (FA), acetaldehyde (ACT), and acetone (ACON) was determined at 0-96 h after NAP administration. Between 12-24 h after NAP administration maximal excretion of the four urinary lipid metabolites was observed, with increases of 4.5-, 2.7-, 2.3-, and 2.8-fold for MDA, FA, ACT, and ACON, respectively, at the 24-h time point. VES reduced the NAP-induced excretion of these urinary metabolites by 28-49% 24 h after NAP administration. These results support the hypothesis that NAP induces oxidative stress and tissue damage, and that vitamin E succinate provides significant protection.

Increased oxidation of ethylene glycol to formaldehyde by microsomes after ethanol treatment: role of oxygen radicals and cytochrome P450

The production of ferryl-type oxidants by microsomes from ethanol-fed rats and pair-fed controls was determined by assaying for the production of formaldehyde from ethylene glycol. Microsomes from the ethanol-fed rats were more reactive than controls in oxidizing ethylene glycol. Catalase was a powerful inhibitor for this reaction, superoxide dismutase was slightly inhibitory and hydroxyl radical scavengers had no effect. These results suggest an important role for H2O2, but not O2-. or .OH in the overall pathway for oxidizing ethylene glycol to formaldehyde. The production of H2O2 by microsomes was increased after ethanol treatment, the extent of increase corresponding to the increase in oxidation of ethylene glycol. A variety of inhibitors and ligands of cytochrome P450, including miconazole, diethyldithiocarbamate, tryptamine, and 4-methylpyrazole, inhibited formaldehyde production by both microsomal preparations. Anti-cytochrome P4502E1 IgG also inhibited the reaction with both microsomal preparations and prevented the increase caused by ethanol treatment. These results indicate that microsomes from ethanol-treated rats are more reactive than pair-fed controls in generating ferryl-type oxidants and that increased production of H2O2 by cytochrome P4502E1 plays a role in the elevated oxidation of ethylene glycol to formaldehyde.

Oxidative stress induced by chronic administration of sodium dichromate [Cr(VI)] to rats

Chromium occurs in the workplace primarily in the valence forms Cr(III) and Cr(VI). Recent studies have demonstrated that sodium dichromate [Cr(VI)] induces greater oxidative stress as compared with Cr(III), as indicated by the production of reactive oxygen species by peritoneal macrophages and hepatic mitochondria and microsomes, and enhanced excretion of urinary lipid metabolites and hepatic DNA-single strand breaks (SSB) following acute oral administration of Cr(III) and Cr(VI). We have therefore examined the chronic effects of sodium dichromate dihydrate [Cr(VI); 10 mg (33.56 mumol)/kg/day] on hepatic mitochondrial and microsomal lipid peroxidation, enhanced excretion of urinary lipid metabolites including malondialdehyde (MDA), formaldehyde (FA), acetaldehyde (ACT), acetone (ACON) and propionaldehyde (PROP), and hepatic DNA damage over a period of 90 days. The maximal increases in hepatic lipid peroxidation and DNA damage were observed at approximately 45 days of treatment. Maximum increases in the urinary excretion of MDA, FA, ACT, ACON and PROP were 3.2-, 2.6-, 4.1-, 3.3- and 2.1-fold, respectively, while a 5.2-fold increase in DNA-SSB was observed. The results clearly indicate that chronic sodium dichromate administration induces oxidative stress resulting in tissue damaging effects which may contribute to the toxicity and carcinogenicity of hexavalent chromium.

Chromium-induced excretion of urinary lipid metabolites, DNA damage, nitric oxide production, and generation of reactive oxygen species in Sprague-Dawley rats

Chromium and its salts induce cytotoxicity and mutagenesis, and vitamin E has been reported to attenuate chromate-induced cytotoxicity. These observations suggest that chromium produces reactive oxygen species which may mediate many of the untoward effects of chromium. We have therefore examined and compared the effects of Cr(III) (chromium chloride hexahydrate) and Cr(VI) (sodium dichromate) following single oral doses (0.50 LD50) on the production of reactive oxygen species by peritoneal macrophages, and hepatic mitochondria and microsomes in rats. The effects of Cr(III) and Cr(VI) on hepatic mitochondrial and microsomal lipid peroxidation and enhanced excretion of urinary lipid metabolites as well as the incidence of hepatic nuclear DNA damage and nitric oxide (NO) production were also examined. Increases in lipid peroxidation of 1.8- and 2.2-fold occurred in hepatic mitochondria and microsomes, respectively, 48 hr after the oral administration of 25 mg Cr(VI)/kg, while increases of 1.2- and 1.4-fold, respectively, were observed after 895 mg Cr(III)/kg. The urinary excretion of malondialdehyde (MDA), formaldehyde (FA), acetaldehyde (ACT) and acetone (ACON) were determined at 0-96 hr after Cr administration. Between 48 and 72 hr post-treatment, maximal excretion of the four urinary lipid metabolites was observed with increases of 1.5- to 5.4-fold in Cr(VI) treated rats. Peritoneal macrophages from Cr(VI) treated animals 48 hr after treatment resulted in 1.4- and 3.6-fold increases in chemiluminescence and iodonitrotetrazolium reduction, indicating enhanced production of superoxide anion, while macrophages from Cr(III) treated animals showed negligible increases. Increases in DNA single strand breaks of 1.7-fold and 1.5-fold were observed following administration of Cr(VI) and Cr(III), respectively, at 48 hr post-treatment. Enhanced production of NO by peritoneal exudate cells (primarily macrophages) was monitored following Cr(VI) administration at both 24 and 48 hr post-treatment with enhanced production of NO being observed at both timepoints. The results indicate that both Cr(VI) and Cr(III) induce an oxidative stress at equitoxic doses, while Cr(VI) induces greater oxidative stress in rats as compared with Cr(III) treated animals.

Smokeless tobacco induced increases in hepatic lipid peroxidation, DNA damage and excretion of urinary lipid metabolites

The possible role of reactive oxygen species in the toxicity of smokeless tobacco (ST) was explored. The effects of an aqueous smokeless tobacco extract (STE) at doses of 125, 250 and 500 mg STE/kg in rats on the induction of hepatic mitochondrial and microsomal lipid peroxidation and the incidence of hepatic nuclear DNA damage 24 hours post treatment were examined. Dose-dependent increases of 1.8, 2.3 and 4.4-fold in mitochondrial and 1.5, 2.1 and 3.6-fold in microsomal lipid peroxidation occurred at 125, 250 and 500 mg STE/kg, respectively, relative to control values. At these same three doses of STE, 1.3, 1.4 and 2.7-fold increases in hepatic DNA single-strand breaks occurred relative to control values. STE administration also resulted in significant increases in excretion of urinary metabolites. Urinary excretion of the four lipid metabolites malondialdehyde (MDA), formaldehyde (FA), acetaldehyde (ACT) and acetone (ACON) was monitored by HPLC for 72 hours after treatment of rats with 125 and 250 mg STE/kg. Increases occurred in the excretion of the four lipid metabolites at every dose and time point with maximum increases in the excretion of all lipid metabolites being observed between 12 and 24 hours post treatment. The results suggest the involvement of an oxidative stress in the toxicity of STE.