Exposure to monomethylarsonous acid (MMA(III)) leads to altered selenoprotein synthesis in a primary human lung cell model

Unraveling the genetics of arsenic toxicity with cellular morphology QTL

The health risks that arise from environmental exposures vary widely within and across human populations, and these differences are largely determined by genetic variation and gene-by-environment (gene-environment) interactions. However, risk assessment in laboratory mice typically involves isogenic strains and therefore, does not account for these known genetic effects. In this context, genetically heterogenous cell lines from laboratory mice are promising tools for population-based screening because they provide a way to introduce genetic variation in risk assessment without increasing animal use. Cell lines from genetic reference populations of laboratory mice offer genetic diversity, power for genetic mapping, and potentially, predictive value for in vivo experimentation in genetically matched individuals. To explore this further, we derived a panel of fibroblast lines from a genetic reference population of laboratory mice (the Diversity Outbred, DO). We then used high-content imaging to capture hundreds of cell morphology traits in cells exposed to the oxidative stress-inducing arsenic metabolite monomethylarsonous acid (MMAIII). We employed dose-response modeling to capture latent parameters of response and we then used these parameters to identify several hundred cell morphology quantitative trait loci (cmQTL). Response cmQTL encompass genes with established associations with cellular responses to arsenic exposure, including Abcc4 and Txnrd1, as well as novel gene candidates like Xrcc2. Moreover, baseline trait cmQTL highlight the influence of natural variation on fundamental aspects of nuclear morphology. We show that the natural variants influencing response include both coding and non-coding variation, and that cmQTL haplotypes can be used to predict response in orthogonal cell lines. Our study sheds light on the major molecular initiating events of oxidative stress that are under genetic regulation, including the NRF2-mediated antioxidant response, cellular detoxification pathways, DNA damage repair response, and cell death trajectories.

Cell morphology QTL reveal gene by environment interactions in a genetically diverse cell population

Genetically heterogenous cell lines from laboratory mice are promising tools for population-based screening as they offer power for genetic mapping, and potentially, predictive value for in vivo experimentation in genetically matched individuals. To explore this further, we derived a panel of fibroblast lines from a genetic reference population of laboratory mice (the Diversity Outbred, DO). We then used high-content imaging to capture hundreds of cell morphology traits in cells exposed to the oxidative stress-inducing arsenic metabolite monomethylarsonous acid (MMAIII). We employed dose-response modeling to capture latent parameters of response and we then used these parameters to identify several hundred cell morphology quantitative trait loci (cmQTL). Response cmQTL encompass genes with established associations with cellular responses to arsenic exposure, including Abcc4 and Txnrd1, as well as novel gene candidates like Xrcc2. Moreover, baseline trait cmQTL highlight the influence of natural variation on fundamental aspects of nuclear morphology. We show that the natural variants influencing response include both coding and non-coding variation, and that cmQTL haplotypes can be used to predict response in orthogonal cell lines. Our study sheds light on the major molecular initiating events of oxidative stress that are under genetic regulation, including the NRF2-mediated antioxidant response, cellular detoxification pathways, DNA damage repair response, and cell death trajectories.

Monomethylarsonous acid binds to Cys-104α and Cys-112β of hemoglobin in acute promyelocytic leukemia patients treated with arsenic trioxide

Arsenic trioxide (As₂O₃) has prominent effect in treating acute promyelocytic leukemia (APL). Identification of arsenic-binding proteins has gained attention for their important biological functions. However, none has been published concerning the binding mechanism of arsenic with hemoglobin (Hb) in APL patients after treatment of As₂O₃. The present study discloses the binding sites of arsenic on Hb in APL patients. Concentrations of inorganic arsenic (iAs), monomethyl arsenic (MMA), and dimethyl arsenic (DMA) in erythrocytes of APL patients were quantified using HPLC-inductively coupled plasma-mass spectroscopy (HPLC-ICP-MS). Hb-bound arsenic was identified by size-exclusion chromatography ICP-MS. The binding sites of arsenic on Hb were determined by mass spectrometry (MS). The concentration trend of arsenic species in erythrocytes of 9 APL patients treated with As₂O₃ was iAs>MMA>DMA, and MMA was the predominant methylated arsenic metabolite. Size-exclusion chromatography separation of free and protein-bound arsenic by simultaneous monitoring of ⁵⁷Fe and ⁷⁵As demonstrated the presence of Hb-bound arsenic. MS information suggested monomethylarsonous (MMA^III) was the dominant arsenic bound to Hb, and further identified that Cys-104α and Cys-112β were two binding sites of MMA^III in Hb. MMA^III binding to Cys-104α and Cys-112β was responsible for arsenic accumulation in erythrocytes of APL patients. This interaction may contribute to understand the therapeutic effect of As₂O₃ as an anticancer drug and its toxicity on APL patients.

Epigallocatechin-3-gallate enhances key enzymatic activities of hepatic thioredoxin and glutathione systems in selenium-optimal mice but activates hepatic Nrf2 responses in selenium-deficient mice

Selenium participates in the antioxidant defense mainly through a class of selenoproteins, including thioredoxin reductase. Epigallocatechin-3-gallate (EGCG) is the most abundant and biologically active catechin in green tea. Depending upon the dose and biological systems, EGCG may function either as an antioxidant or as an inducer of antioxidant defense via its pro-oxidant action or other unidentified mechanisms. By manipulating the selenium status, the present study investigated the interactions of EGCG with antioxidant defense systems including the thioredoxin system comprising of thioredoxin and thioredoxin reductase, the glutathione system comprising of glutathione and glutathione reductase coupled with glutaredoxin, and the Nrf2 system. In selenium-optimal mice, EGCG increased hepatic activities of thioredoxin reductase, glutathione reductase and glutaredoxin. These effects of EGCG appeared to be not due to overt pro-oxidant action because melatonin, a powerful antioxidant, did not influence the increase. However, in selenium-deficient mice, with low basal levels of thioredoxin reductase 1, the same dose of EGCG did not elevate the above-mentioned enzymes; intriguingly EGCG in turn activated hepatic Nrf2 response, leading to increased heme oxygenase 1 and NAD(P)H:quinone oxidoreductase 1 protein levels and thioredoxin activity. Overall, the present work reveals that EGCG is a robust inducer of the Nrf2 system only in selenium-deficient conditions. Under normal physiological conditions, in selenium-optimal mice, thioredoxin and glutathione systems serve as the first line defense systems against the stress induced by high doses of EGCG, sparing the activation of the Nrf2 system.

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EGCG increases hepatic activities of TrxR, GR and Grx in selenium-optimal mice.

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EGCG fails to manipulate the above-mentioned enzymes in selenium-deficient mice.

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EGCG in turn activates hepatic Nrf2 response in selenium-deficient mice.

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Selenium deficiency does not increase EGCG toxicity due to potent Nrf2 response.

TrxR1 as a potent regulator of the Nrf2-Keap1 response system

SIGNIFICANCE

All cells must maintain a balance between oxidants and reductants, while allowing for fluctuations in redox states triggered by signaling, altered metabolic flow, or extracellular stimuli. Furthermore, they must be able to rapidly sense and react to various challenges that would disrupt the redox homeostasis.

RECENT ADVANCES

Many studies have identified Keap1 as a key sensor for oxidative or electrophilic stress, with modification of Keap1 by oxidation or electrophiles triggering Nrf2-mediated transcriptional induction of enzymes supporting reductive and detoxification pathways. However, additional mechanisms for Nrf2 regulation are likely to exist upstream of, or in parallel with, Keap1.

CRITICAL ISSUES

Here, we propose that the mammalian selenoprotein thioredoxin reductase 1 (TrxR1) is a potent regulator of Nrf2. A high chemical reactivity of TrxR1 and its vital role for the thioredoxin (Trx) system distinguishes TrxR1 as a prime target for electrophilic challenges. Chemical modification of the selenocysteine (Sec) in TrxR1 by electrophiles leads to rapid inhibition of thioredoxin disulfide reductase activity, often combined with induction of NADPH oxidase activity of the derivatized enzyme, thereby affecting many downstream redox pathways. The notion of TrxR1 as a regulator of Nrf2 is supported by many publications on effects in human cells of selenium deficiency, oxidative stress or electrophile exposure, as well as the phenotypes of genetic mouse models.

FUTURE DIRECTIONS

Investigation of the role of TrxR1 as a regulator of Nrf2 activation will facilitate further studies of redox control in diverse cells and tissues of mammals, and possibly also in animals of other classes.

Oxidative stress-related liver dysfunction by sodium arsenite: Alleviation by Pistacia lentiscus oil

CONTEXT

Pistacia lentiscus L. (Anacardiaceae) is an evergreen shrub widely distributed throughout the Mediterranean region. Pistacia lentiscus oil (PLo) was particularly known in North African traditional medicine. Thus, people of these regions have used it externally to treat sore throats, burns and wounds, as well as they employed it internally for respiratory allergies. PLo is rich in essential fatty acids, vitamin E and polyphenols. As a very active site of metabolism, liver is reported to be susceptible to arsenic (As) intoxication.

OBJECTIVE

The present study evaluates the protective effect of PLo against sodium arsenite-induced hepatic dysfunction and oxidative stress in experimental Wistar rats.

MATERIALS AND METHODS

Twenty-eight rats were equally divided into four groups; the first served as a control, the remaining groups were respectively treated with PLo (3.3 mL/kg body weight), sodium arsenite (5.55 mg/kg body weight) and a combination of sodium arsenite and PLo. After 21 consecutive days, cellular functions were evaluated by hematological, biochemical and oxidative stress markers.

RESULTS

A significant decrease in the levels of red blood cells, haemoglobin (p ≤ 0.001), hematocrit (p ≤ 0.001), reduced glutathione and metallothionein (p ≤ 0.05) associated with a significant increase of malondialdehyde (p ≤ 0.001) were noticed in the arsenic-exposed group when compared to the control. The As-treated group also exhibited an increase in hepatic antioxidant enzymes namely superoxide dismutase, glutathione peroxidase (p ≤ 0.01) and catalase (p ≤ 0.05). However, the co-administration of PLo has relatively reduced arsenic effect.

CONCLUSION

The results showed that arsenic intoxication disturbed the liver pro-oxidant/antioxidant status. PLo co-administration mitigates arsenic-induced oxidative damage in rat.

Monomethylarsonous acid inhibited endogenous cholesterol biosynthesis in human skin fibroblasts

Human exposure to arsenic in drinking water is a widespread public health concern, and such exposure is known to be associated with many human diseases. The detailed molecular mechanisms about how arsenic species contribute to the adverse human health effects, however, remain incompletely understood. Monomethylarsonous acid [MMA(III)] is a highly toxic and stable metabolite of inorganic arsenic. To exploit the mechanisms through which MMA(III) exerts its cytotoxic effect, we adopted a quantitative proteomic approach, by coupling stable isotope labeling by amino acids in cell culture (SILAC) with LC-MS/MS analysis, to examine the variation in the entire proteome of GM00637 human skin fibroblasts following acute MMA(III) exposure. Among the ~6500 unique proteins quantified, ~300 displayed significant changes in expression after exposure with 2 μM MMA(III) for 24 h. Subsequent analysis revealed the perturbation of de novo cholesterol biosynthesis, selenoprotein synthesis and Nrf2 pathways evoked by MMA(III) exposure. Particularly, MMA(III) treatment resulted in considerable down-regulation of several enzymes involved in cholesterol biosynthesis. In addition, real-time PCR analysis showed reduced mRNA levels of select genes in this pathway. Furthermore, MMA(III) exposure contributed to a distinct decline in cellular cholesterol content and significant growth inhibition of multiple cell lines, both of which could be restored by supplementation of cholesterol to the culture media. Collectively, the present study demonstrated that the cytotoxicity of MMA(III) may arise, at least in part, from the down-regulation of cholesterol biosynthesis enzymes and the resultant decrease of cellular cholesterol content.

Cellular interaction and toxicity depend on physicochemical properties and surface modification of redox-active nanomaterials

The study of the chemical and biological properties of CeO2 nanoparticles (CNPs) has expanded recently due to its therapeutic potential, and the methods used to synthesize these materials are diverse. Moreover, conflicting reports exist regarding the toxicity of CNPs. To help resolve these discrepancies, we must first determine whether CNPs made by different methods are similar or different in their physicochemical and catalytic properties. In this paper, we have synthesized several forms of CNPs using identical precursors through a wet chemical process but using different oxidizer/reducer; H2O2 (CNP1), NH4OH (CNP2), or hexamethylenetetramine (HMT-CNP1). Physicochemical properties of these CNPs were extensively studied and found to be different depending on the preparation methods. Unlike CNP1 and CNP2, HMT-CNP1 was readily taken into endothelial cells and the aggregation can be visualized using light microscopy. Exposure to HMT-CNP1 also reduced cell viability at a 10-fold lower concentration than CNP1 or CNP2. Surprisingly, exposure to HMT-CNP1 led to substantial decreases in ATP levels. Mechanistic studies revealed that HMT-CNP1 exhibited substantial ATPase (phosphatase) activity. Though CNP2 also exhibits ATPase activity, CNP1 lacked ATPase activity. The difference in catalytic (ATPase) activity of different CNPs preparation may be due to differences in their morphology and oxygen extraction energy. These results suggest that the combination of increased uptake and ATPase activity of HMT-CNP1 may underlie the biomechanism of the toxicity of this preparation of CNPs and may suggest that ATPase activity should be considered when synthesizing CNPs for use in biomedical applications.

Hexavalent chromium-induced alteration of proteomic landscape in human skin fibroblast cells

Hexavalent chromium [Cr(VI)] generated during industrial processes is carcinogenic. Although much is known about the deleterious effects caused by reactive oxygen species generated during the reduction of Cr(VI) after its absorption by biological systems, the precise mechanisms underlying Cr(VI) cytotoxicity remain poorly defined. Here, we analyzed, at the global proteome scale, the perturbation of protein expression in GM00637 human skin fibroblast cells upon exposure to potassium dichromate (K₂Cr₂O₇). We were able to quantify ∼4600 unique proteins, among which ∼400 exhibited significant alterations in expression levels upon a 24-h treatment with 0.5 μM K₂Cr₂O₇. Pathway analysis revealed the Cr(VI)-induced perturbation of cholesterol biosynthesis, G-protein signaling, inflammatory response, and selenoprotein pathways. In particular, we discovered that the K₂Cr₂O₇ treatment led to pronouncedly elevated expression of a large number of enzymes involved in de novo cholesterol biosynthesis. Real-time PCR analysis revealed the increased mRNA expression of selected genes involved in cholesterol biosynthesis. Consistently, K₂Cr₂O₇ treatment resulted in marked increases in cellular cholesterol level in multiple cell lines. Moreover, the Cr(VI)-induced growth inhibition of cultured human cells could be rescued by a cholesterol-lowering drug, lovastatin. Together, we demonstrated, for the first time, that Cr(VI) may exert its cytotoxic effect, at least partly, through the up-regulation of enzymes involved in de novo cholesterol biosynthesis and the resultant increase of cholesterol level in cells.

Methylation capacity of arsenic and skin lesions in smelter plant workers

Potential occupational arsenic exposure is a significant problem in smelting plants. The metabolites containing arsenic with an oxidation of +3 have been considered more cytotoxic and genotoxic than their parent inorganic species. The current study examined the capacity of arsenic methylation and its risk on skin lesions. The primary aim of this study is to determine if methylation capacity, as measured by urinary arsenic metabolites, differed in workers with skin lesions compared to workers without skin lesions. Hydride generation-atomic absorption spectrometry was used to determine three arsenic species in urine of workers who had been working in arsenic plants, and primary and secondary methylation indexes were calculated. Skin lesions were examined at the same time. Many workers had obvious skin lesions (36/91). The mean concentrations of inorganic arsenic (iAs), monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) in urine of workers are obviously higher than those of the control group. There are more iAs, MMA, and DMA in urine, higher MMA%, lower iAs% for workers with skin lesions compared with those without skin lesions. Workers with skin lesions have the lowest SMI (3.50±1.21), and they may be in danger. Our results support the viewpoint that individuals who metabolize inorganic arsenic to MMA easily, but metabolize MMA to DMA difficulty have more risk of skin lesions.

The endoplasmic reticulum is a target organelle for trivalent dimethylarsinic acid (DMAIII)-induced cytotoxicity

The purpose of present study was to characterize the endoplasmic reticulum stress and generation of ROS in rat liver RLC-16 cells by exposing to trivalent dimethylarsinous acid (DMAIII) and compared with that of trivalent arsenite (iAsIII) and monomethylarsonous acid (MMAIII). Protein kinase-like endoplasmic reticulum kinase (PERK) phosphorylation was significantly induced in cells exposed to DMAIII, while there was no change in phosphorylated PERK (P-PERK) detected in cells after exposure to iAsIII or MMAIII. The generation of reactive oxygen species (ROS) after DMAIII exposure was found to take place specifically in the endoplasmic reticulum (ER), while previous reports showed that ROS was generated in mitochondria following exposure to MMAIII. Meanwhile, cycloheximide (CHX) which is an inhibitor of protein biosynthesis strongly inhibited the DMAIII-induced intracellular ROS generation in the ER and the phosphorylation of PERK, suggesting the induction of ER stress probably occurs through the inhibition of the protein folding process. Activating transcription factor 4 (ATF4) and C/EBP homologous protein (CHOP) mRNA were induced by all three arsenic species, however, evidence suggested that they might be induced by different pathways in the case of iAsIII and MMAIII. In addition, ER resident molecular chaperone glucose-regulated protein78 (GRP78) was not affected by trivalent arsenicals, while it was induced in positive control only at high concentration (Thapsigargin;Tg), suggesting the GRP78 is less sensitive to low levels of ER stress. In summary, our findings demonstrate that the endoplasmic reticulum is a target organelle for DMAIII-induced cytotoxicity.

Exposure to silver nanoparticles inhibits selenoprotein synthesis and the activity of thioredoxin reductase

BACKGROUND

Silver nanoparticles (AgNPs) and silver (Ag)-based materials are increasingly being incorporated into consumer products, and although humans have been exposed to colloidal Ag in many forms for decades, this rise in the use of Ag materials has spurred interest into their toxicology. Recent reports have shown that exposure to AgNPs or Ag ions leads to oxidative stress, endoplasmic reticulum stress, and reduced cell proliferation. Previous studies have shown that Ag accumulates in tissues as silver sulfides (Ag2S) and silver selenide (Ag2Se).

OBJECTIVES

In this study we investigated whether exposure of cells in culture to AgNPs or Ag ions at subtoxic doses would alter the effective metabolism of selenium, that is, the incorporation of selenium into selenoproteins.

METHODS

For these studies we used a keratinocyte cell model (HaCat) and a lung cell model (A549). We also tested (in vitro, both cellular and chemical) whether Ag ions could inhibit the activity of a key selenoenzyme, thioredoxin reductase (TrxR).

RESULTS

We found that exposure to AgNPs or far lower levels of Ag ions led to a dose-dependent inhibition of selenium metabolism in both cell models. The synthesis of protein was not altered under these conditions. Exposure to nanomolar levels of Ag ions effectively blocked selenium metabolism, suggesting that Ag ion leaching was likely the mechanism underlying observed changes during AgNP exposure. Exposure likewise inhibited TrxR activity in cultured cells, and Ag ions were potent inhibitors of purified rat TrxR isoform 1 (cytosolic) (TrxR1) enzyme.

CONCLUSIONS

Exposure to AgNPs leads to the inhibition of selenoprotein synthesis and inhibition of TrxR1. Further, we propose these two sites of action comprise the likely mechanism underlying increases in oxidative stress, increases endoplasmic reticulum stress, and reduced cell proliferation during exposure to Ag.

Protective role of sinapic acid against arsenic: induced toxicity in rats

Arsenic compounds are classified as toxicants and human carcinogens. Environmental exposure to arsenic imposes a big health issue worldwide. Sinapic acid is a phenylpropanoid compound and is found in various herbal materials and high-bran cereals. It has been reported that sinapic acid has antioxidant efficacy as metal chelators due to the orientation of functional groups. However, it has not yet been examined in experimental animals. In light of this fact, the purpose of this study was to characterize the protective role of sinapic acid against arsenic induced toxicity in rats. Rats were orally treated with arsenic alone (5mg/kg body weight (bw)/day) plus sinapic acid at different doses (10, 20 and 40mg/kg bw/day) for 30days. Hepatotoxicity was measured by the increased activities of serum hepatospecific enzymes namely aspartate transaminase, alanine transaminase, alkaline phosphatase, gamma glutamyl transferase, lactate dehydrogenase and total bilirubin along with increased elevation of lipid peroxidative markers, thiobarbituric acid reactive substances, lipid hydroperoxides, protein carbonyl content and conjugated dienes. The toxic effect of arsenic was also indicated by significantly decreased activities of enzymatic antioxidants like superoxide dismutase, catalase, glutathione peroxidase, glutathione-S-transferase, glutathione reductase and glucose-6-phosphate dehydrogenase along with non-enzymatic antioxidant like reduced glutathione. Administration of sinapic acid exhibited significant reversal of arsenic induced toxicity in hepatic tissue. The effect at a dose of 40mg/kgbw/day was more pronounced than the other two doses (10 and 20mg/kgbw/day). All these changes were supported by reduction of arsenic concentration and histopathological observations of the liver. These results suggest that sinapic acid has a protective effect over arsenic induced toxicity in rat.

Therapeutic efficacy of silymarin and naringenin in reducing arsenic-induced hepatic damage in young rats

We investigated the effects of silymarin and naringenin in counteracting arsenic-induced hepatic oxidative stress post exposure. Male wistar rats were chronically exposed to sodium arsenite for eight months followed by oral treatment with silymarin and naringenin (50 mg/kg each) for 15 consecutive days to evaluate hepatic damage and antioxidant potential. Our results demonstrate a significant decrease in hepatic GSH levels, SOD and catalase activities and an increase in GST and TBARS levels after arsenic administration. Silymarin or naringenin administration increased GSH levels and was beneficial in the recovery of altered SOD and catalase activity besides significantly reducing blood and tissue arsenic concentration. Our results point to the antioxidant potential of these flavonoids, which might be of benefit in the clinical recovery of subject exposed to arsenic. These flavonoids can be incorporated into the diet or co-supplemented during chelation treatment, and thus may afford a protective effect against arsenite-induced cytotoxicity.