Modulation of methylmercury uptake by methionine: prevention of mitochondrial dysfunction in rat liver slices by a mimicry mechanism

Amino Acid Transporter LAT1 (SLC7A5) Mediates MeHg-Induced Oxidative Stress Defense in the Human Placental Cell Line HTR-8/SVneo

The placental barrier can protect the fetus from contact with harmful substances. The potent neurotoxin methylmercury (MeHg), however, is very efficiently transported across the placenta. Our previous data suggested that L-type amino acid transporter (LAT)1 is involved in placental MeHg uptake, accepting MeHg-L-cysteine conjugates as substrate due to structural similarity to methionine. The aim of the present study was to investigate the antioxidant defense of placental cells to MeHg exposure and the role of LAT1 in this response. When trophoblast-derived HTR-8/SVneo cells were LAT1 depleted by siRNA-mediated knockdown, they accumulated less MeHg. However, they were more susceptible to MeHg-induced toxicity. This was evidenced in decreased cell viability at a usually noncytotoxic concentration of 0.03 µM MeHg (~6 µg/L). Treatment with ≥0.3 µM MeHg increased cytotoxicity, apoptosis rate, and oxidative stress of HTR-8/SVneo cells. These effects were enhanced under LAT1 knockdown. Reduced cell number was seen when MeHg-exposed cells were cultured in medium low in cysteine, a constituent of the tripeptide glutathione (GSH). Because LAT1-deficient HTR-8/SVneo cells have lower GSH levels than control cells (independent of MeHg treatment), we conclude that LAT1 is essential for de novo synthesis of GSH, required to counteract oxidative stress. Genetic predisposition to decreased LAT1 function combined with MeHg exposure could increase the risk of placental damage.

Dietary exposure to mercury and its relation to cytogenetic instability in populations from "La Mojana" region, northern Colombia

Fish consumption and chronic exposure to low doses of mercury (Hg) seems to activate several molecular mechanisms leading to carcinogenic and/or teratogenic processes. However, Hg genotoxic effects on humans are not completely described. In the present study, we assessed cytogenetic damage in isolated human peripheral lymphocytes using the cytokinesis-block micronucleus cytome assay (CBMN-Cyt), micronucleus formation with anti-kinetochore antibody (CREST staining), levels of total Hg in hair (T-Hg), fish consumption, and estimated Hg dose. The study comprised 39 non-exposed, and 73 residents from La Mojana region, an area with a well-documented Hg contamination. Data showed a significant increase in micronuclei (MNBN), nucleoplasmic bridges (NPB), and necrotic and apoptotic cell frequencies in residents of "La Mojana." The overall mean T-Hg level in hair for exposed residents was 1.12 ± 0.94 mg kg^-1 and 0.15 ± 0.05 in individuals from the reference area. Approximately 40% of analyzed individuals showed T-Hg levels that exceeded US Environmental Protection Agency (USEPA) reference dose. Increased T-Hg levels in hair were related to increased MNBN frequencies and high fish consumption. Other cellular markers, such as necrotic and apoptotic cell frequencies, were also correlated with high fish intake and T-Hg contents. Results of the CREST staining demonstrated that in vivo exposure to Hg induces genetic instability by chromosome fragment loss (clastogenic). Additionally, a high average intake of some fish species, particularly with carnivorous habits like Caquetaia kraussii, Hoplias malabaricus, and Sorubin cuspicaudus, seems to increase MNBN frequencies significantly.

Pollutants and nutrition: Are methylmercury effects on blood pressure and lipoprotein profile comparable to high-fat diet in mice?

Human exposure to methylmercury (MeHg) due to contaminated fish intake as part of a high-fat (HFD), high-carbohydrate diets is a reality today for many populations. HFD is associated with hypertension and hyperlipidemia, primary cardiovascular disease (CVD) risk factors. Some studies suggest that MeHg induces those risk factors. We evaluated the effect of MeHg exposure in mice fed with HFD or control diet for eight weeks. In the last experimental 15 days, the half group received a MeHg solution (20 mg/L) replacing water. Blood pressure (BP), heart rate, lipoprotein concentrations, and paraoxonase activity were evaluated. Liver cholesterol, triacylglycerol, and IBA-1+ cells, as well as transcriptional levels of genes related to lipid metabolism and inflammatory response, were also assessed. HFD and both MeHg groups presented increased BP and total cholesterol (TC). In the liver, HFD but not MeHg was related to an increase in TC. Also, MeHg intoxication reduced paraoxonase activity regardless of diet. MeHg intoxication and HFD increased steatosis and the number of IBA-1+ cells and modified some gene transcripts associated with lipid metabolism. In conclusion, we demonstrated that MeHg effects on CVD risk factors resemble those caused by HFD.

Exposure to Toxic Heavy Metals Can Influence Homocysteine Metabolism?

BACKGROUND

Homocysteine is a sulfur amino acid whose metabolism is activated in two pathways: remethylation to methionine, which requires folate and vitamin B₁₂, and transsulfuration to cystathionine, which needs pyridoxal-5'-phosphate. High homocysteine level increases the risk of developing heart disease, stroke, peripheral vascular diseases, and cognitive impairment. Some evidence showed that exposure to these metals increased plasma homocysteine levels.

METHODS

A systematic review was carried out to clarify the relationship between homocysteine blood levels and exposure to toxic heavy metals (Lead, Cadmium, Mercury, and Chromium).

RESULTS

The results of this systematic review indicate that exposure to Pb, Cr, Cd, and Hg is connected with nonphysiological homocysteine levels or vitamin B₁₂ and folate serum concentrations.

CONCLUSIONS

These findings reinforce the importance of involvement in exposure to heavy metals in homocysteine metabolism. This supports the role of blood metals as potential upstream modifiable risk factors to prevent the development of other established risk factors as hyperhomocysteinemia.

Primary amino acids affect the distribution of methylmercury rather than inorganic mercury among tissues of two farmed-raised fish species

The distributions of primary amino acids, MeHg and IHg in body tissues of two commonly farm-raised fish species (common carp: Cyprinus carpio; grass carp: Ctenopharyngodon idellus) in Guizhou Province, SW China, were investigated to understand the effects of primary amino acids on MeHg and IHg metabolism in farm-raised fish. The primary amino acids were classified into four groups: (1) essential and polar amino acids; (2) essential and non-polar amino acids; (3) non-essential and polar amino acids; and (4) non-essential and non-polar amino acids. For both fish species, groups (1, 2 and 3) were enriched in muscle and kidney, whereas group (4) was enriched in scale. The two fish species showed low MeHg concentrations (grass carp: 0.5-3.9 ng/g; common carp:1.0-7.4 ng/g) and low MeHg proportions (grass carp: 2-45%; common carp: 6-37%) in their tissues, which are mainly due to the simple food web structures and the fast growth of the farm-raised fish. Positive correlations (r = 0.342 to 0.472; p < 0.01; n = 78) were observed between MeHg and several primary amino acids (cysteine, threonine, phenylalanine, leucine, valine, glutamate serine and tyrosine) in fish tissues, which may be driven by the formation of MeHg-Cys complexes within fish body. However, no significant correlations were observed between IHg and any primary amino acids, indicating the metabolic processes of IHg and MeHg are different. This study advances our understanding that cysteine and its related/derived amino acids may be an important driving force for MeHg distribution and translocation in fish.

Methylmercury's chemistry: From the environment to the mammalian brain

Methylmercury is a neurotoxicant that is found in fish and rice. MeHg's toxicity is mediated by blockage of -SH and -SeH groups of proteins. However, the identification of MeHg's targets is elusive. Here we focus on the chemistry of MeHg in the abiotic and biotic environment. The toxicological chemistry of MeHg is complex in metazoans, but at the atomic level it can be explained by exchange reactions of MeHg bound to -S(e)H with another free -S(e)H group (R¹S(e)-HgMe + R²-S(e)H ↔ R¹S(e)H + R²-S(e)-HgMe). This reaction was first studied by professor Rabenstein and here it is referred as the "Rabenstein's Reaction". The absorption, distribution, and excretion of MeHg in the environment and in the body of animals will be dictated by Rabenstein's reactions. The affinity of MeHg by thiol and selenol groups and the exchange of MeHg by Rabenstein's Reaction (which is a diffusion controlled reaction) dictates MeHg's neurotoxicity. However, it is important to emphasize that the MeHg exchange reaction velocity with different types of thiol- and selenol-containing proteins will also depend on protein-specific structural and thermodynamical factors. New experimental approaches and detailed studies about the Rabenstein's reaction between MeHg with low molecular mass thiol (LMM-SH) molecules (cysteine, GSH, acetyl-CoA, lipoate, homocysteine) with abundant high molecular mass thiol (HMM-SH) molecules (albumin, hemoglobin) and HMM-SeH (GPxs, Selenoprotein P, TrxR1-3) are needed. The study of MeHg migration from -S(e)-Hg- bonds to free -S(e)H groups (Rabenstein's Reaction) in pure chemical systems and neural cells (with special emphasis to the LMM-SH and HMM-S(e)H molecules cited above) will be critical to developing realistic constants to be used in silico models that will predict the distribution of MeHg in humans.

Mercury risk in poultry in the Wanshan Mercury Mine, China

In this study, total mercury (THg) and methylmercury (MeHg) concentrations in muscles (leg and breast), organs (intestine, heart, stomach, liver) and blood were investigated for backyard chickens, ducks and geese of the Wanshan Mercury Mine, China. THg in poultry meat products range from 7.9 to 3917.1 ng/g, most of which exceeded the Chinese national standard limit for THg in meat (50 ng/g). Elevated MeHg concentrations (0.4-62.8 ng/g) were also observed in meat products, suggesting that poultry meat can be an important human MeHg exposure source. Ducks and geese showed higher Hg levels than chickens. For all poultry species, the highest Hg concentrations were observed in liver (THg: 23.2-3917.1 ng/g; MeHg: 7.1-62.8 ng/g) and blood (THg: 12.3-338.0 ng/g; MeHg: 1.4-17.6 ng/g). We estimated the Hg burdens in chickens (THg: 15.3-238.1 μg; MeHg: 2.2-15.6 μg), ducks (THg: 15.3-238.1 μg; MeHg: 3.5-14.7 μg) and geese (THg: 83.8-93.4 μg; MeHg: 15.4-29.7 μg). To not exceed the daily intake limit for THg (34.2 μg/day) and MeHg (6 μg/day), we suggested that the maximum amount (g) for chicken leg, breast, heart, stomach, intestine, liver, and blood should be 1384, 1498, 2315, 1214, 1081, 257, and 717, respectively; the maximum amount (g) for duck leg, breast, heart, stomach, intestine, liver, and blood should be 750, 1041, 986, 858, 752, 134, and 573, respectively; and the maximum amount (g) for goose leg, breast, heart, stomach, intestine, liver, and blood should be 941, 1051, 1040, 1131, 964, 137, and 562, respectively.

Methylmercury exposure for 14 days (short-term) produces behavioral and biochemical changes in mouse cerebellum, liver, and serum

Various studies on methylmercury (MeHg)-induced toxicity focused on the central nervous system (CNS) as a primary target. However, MeHg-mediated toxicity is related to metallic interaction with electrophilic groups, which are not solely restricted to the CNS, but these reactive groups are present ubiquitously in several systems/organs. The aim of this study was thus to examine MeHg-induced systemic toxicity in mice using a standardized neurotoxicology testing exposure model to measure cerebellar neurotoxicity by determining biochemical and behavioral parameters in the cerebellum. After 2 weeks exposure to MeHg (40 µg/ml; diluted in drinking water; ad libitum), adult male Swiss mice showed a marked motor impairment characteristic of cerebellar toxicity as noted in the following tests: rotarod, beam walking, pole, and hind limb clasping. MeHg treatment resulted in Hg deposition in the cerebellum as well as reduction in cerebellar weight, glutathione peroxidase (GPx) activity, and interleukin (IL)-6 levels. MeHg ingestion increased cerebellar glutathione reductase (GR) activity and brain-derived neurotrophic factor (BDNF) levels. In addition to cerebellar toxicity, MeHg treatment also elevated total and non-high density lipoprotein (non-HDL) cholesterol levels, as well as serum aspartate transaminase (AST) and alanine transaminase (ALT) enzymatic activities, systemic parameters. Increased liver weight and reduced serum urea levels were also noted in MeHg-exposed mice. Taken together, our findings demonstrated that a well-standardized exposure protocol to examine MeHg-induced neurotoxicity also produced systemic toxicity in mice, which was characterized by changes in markers of hepatic function as well as serum lipid homeostasis.

Selenocystine against methyl mercury cytotoxicity in HepG2 cells

Methyl mercury (MeHg) is a highly toxic substance and the effect of selenium against MeHg toxicity is a hot topic. Until now, no related works have been reported from the view of the point of elemental speciation which is promising to study the mechanism at the molecular level. In this work, to reveal the effect of selenocystine (SeCys₂) against MeHg cytotoxicity in HepG2 cells, a comprehensive analytical platform for speciation study of mercury and selenium in MeHg incubated or MeHg and SeCys₂ co-incubated HepG2 cells was developed by integrating liquid chromatography (LC) - inductively coupled plasma mass spectrometry (ICP-MS) hyphenated techniques and chip-based pretreatment method. Interesting phenomenon was found that the co-incubation of MeHg with SeCys₂ promoted the uptake of MeHg in HepG2 cells, but reduced the cytotoxicity of MeHg. Results obtained by ICP-MS based hyphenated techniques revealed a possible pathway for the incorporation and excretion of mercury species with the coexistence of SeCys₂. The formation of MeHg and SeCys₂ aggregation promotes the uptake of MeHg; majority of MeHg transforms into small molecular complexes (MeHg-glutathione (GSH) and MeHg-cysteine (Cys)) in HepG2 cells; and MeHg-GSH is the elimination species which results in reducing the cytotoxicity of MeHg.

Effects of methyl mercury exposure on pancreatic beta cell development and function

Methyl mercury is an environmental contaminant of worldwide concern. Since the discovery of methyl mercury exposure due to eating contaminated fish as the underlying cause of the Minamata disaster, the scientific community has known about the sensitivity of the developing central nervous system to mercury toxicity. Warnings are given to pregnant women and young children to limit consumption of foods containing methyl mercury to protect the embryonic, fetal and postnatally developing central nervous system. However, evidence also suggests that exposure to methyl mercury or various forms of inorganic mercury may also affect development and function of other organs. Numerous reports indicate a worldwide increase in diabetes, particularly type 2 diabetes. Quite recently, methyl mercury has been shown to have adverse effects on pancreatic beta (β) cell development and function, resulting in insulin resistance and hyperglycemia and may even lead to the development of diabetes. This review discusses possible mechanisms by which methyl mercury exposure may adversely affect pancreatic β cell development and function, and the role that methyl mercury exposure may have in the reported worldwide increase in diabetes, particularly type 2 diabetes. While additional information is needed regarding associations between mercury exposure and specific mechanisms of the pathogenesis of diabetes in the human population, methyl mercury's adverse effects on the body's natural sources of antioxidants suggest that one possible therapeutic strategy could involve supplementation with antioxidants. Thus, it is important that additional investigation be undertaken into the role of methyl mercury exposure and reduced pancreatic β cell function. Copyright © 2016 John Wiley & Sons, Ltd.

Tea Polyphenols Protect Against Methylmercury-Induced Cell Injury in Rat Primary Cultured Astrocytes, Involvement of Oxidative Stress and Glutamate Uptake/Metabolism Disorders

Methylmercury (MeHg) is an extremely dangerous environmental contaminant, accumulating preferentially in CNS and causing a series of cytotoxic effects. However, the precise mechanisms are still incompletely understood. The current study explored the mechanisms that contribute to MeHg-induced cell injury focusing on the oxidative stress and Glu uptake/metabolism disorders in rat primary cultured astrocytes. Moreover, the neuroprotective effects of tea polyphenols (TP), a natural antioxidant, against MeHg cytotoxicity were also investigated. Astrocytes were exposed to 0, 2.5, 5, 10, and 20 μM MeHgCl for 6-30 h, or pretreated with 50, 100, 200, and 400 μM TP for 1-12 h; cell viability and LDH release were then determined. For further experiments, 50, 100, and 200 μM of TP pretreatment for 6 h followed by 10 μM MeHgCl for 24 h were performed for the examination of the responses of astrocytes, specifically addressing NPSH levels, ROS generation, ATPase activity, the expressions of Nrf2 pathway as well as Glu metabolism enzyme GS and Glu transporters (GLAST and GLT-1). Exposure of MeHg resulted in damages of astrocytes, which were shown by a loss of cell viability, and supported by high levels of LDH release, morphological changes, apoptosis rates, and NPSH depletion. In addition, astrocytes were sensitive to MeHg-mediated oxidative stress, a finding that is consistent with ROS overproduction; Nrf2 as well as its downstream genes HO-1 and γ-GCSh were markedly upregulated. Moreover, MeHg significantly inhibited GS activity, as well as expressions of GS, GLAST, and GLT-1. On the contrary, pretreatment with TP presented a concentration-dependent prevention against MeHg-mediated cytotoxic effects of astrocytes. In conclusion, the findings clearly indicated that MeHg aggravated oxidative stress and Glu uptake/metabolism dysfunction in astrocytes. TP possesses some abilities to prevent MeHg cytotoxicity through its antioxidative properties.

Heavy Metals and Human Health: Mechanistic Insight into Toxicity and Counter Defense System of Antioxidants

Heavy metals, which have widespread environmental distribution and originate from natural and anthropogenic sources, are common environmental pollutants. In recent decades, their contamination has increased dramatically because of continuous discharge in sewage and untreated industrial effluents. Because they are non-degradable, they persist in the environment; accordingly, they have received a great deal of attention owing to their potential health and environmental risks. Although the toxic effects of metals depend on the forms and routes of exposure, interruptions of intracellular homeostasis include damage to lipids, proteins, enzymes and DNA via the production of free radicals. Following exposure to heavy metals, their metabolism and subsequent excretion from the body depends on the presence of antioxidants (glutathione, α-tocopherol, ascorbate, etc.) associated with the quenching of free radicals by suspending the activity of enzymes (catalase, peroxidase, and superoxide dismutase). Therefore, this review was written to provide a deep understanding of the mechanisms involved in eliciting their toxicity in order to highlight the necessity for development of strategies to decrease exposure to these metals, as well as to identify substances that contribute significantly to overcome their hazardous effects within the body of living organisms.

Methylmercury upregulates RE-1 silencing transcription factor (REST) in SH-SY5Y cells and mouse cerebellum

Methylmercury (MeHg) is a highly neurotoxic compound that, in adequate doses, can cause damage to the brain, including developmental defects and in severe cases cell death. The RE-1-silencing transcription factor (REST) has been found to be involved in the neurotoxic effects of environmental pollutants such as polychlorinated biphenyls (PCBs). In this study, we investigated the effects of MeHg treatment on REST expression and its role in MeHg-induced neurotoxicity in neuroblastoma SH-SY5Y cells. We found that MeHg exposure caused a dose- and time- dependent apoptotic cell death, as evidenced by the appearance of apoptotic hallmarks including caspase-3 processing and annexin V uptake. Moreover, MeHg increased REST gene and gene product expression. MeHg-induced apoptotic cell death was completely abolished by REST knockdown. Interestingly, MeHg (1μM/24h) increased the expression of REST Corepressor (Co-REST) and its binding with REST whereas the other REST cofactor mammalian SIN3 homolog A transcription regulator (mSin3A) was not modified. In addition, we demonstrated that the acetylation of histone protein H4 was reduced after MeHg treatment and was critical for MeHg-induced apoptosis. Accordingly, the pan-histone deacetylase inhibitor trichostatin-A (TSA) prevented MeHg-induced histone protein H4 deacetylation, thereby reverting MeHg-induced neurotoxic effect. Male mice subcutaneously injected with 10mg/kg of MeHg for 10 days showed an increase in REST expression in the granule cell layer of the cerebellum together with a decrease in histone H4 acetylation. Collectively, we demonstrated that methylmercury exposure can cause neurotoxicity by activating REST gene expression and H4 deacetylation.

Mitochondrial Redox Dysfunction and Environmental Exposures

SIGNIFICANCE

Mitochondria are structurally and biochemically diverse, even within a single type of cell. Protein complexes localized to the inner mitochondrial membrane synthesize ATP by coupling electron transport and oxidative phosphorylation. The organelles produce reactive oxygen species (ROS) from mitochondrial oxygen and ROS can, in turn, alter the function and expression of proteins used for aerobic respiration by post-translational and transcriptional regulation.

RECENT ADVANCES

New interest is emerging not only into the roles of mitochondria in disease development and progression but also as a target for environmental toxicants.

CRITICAL ISSUES

Dysregulation of respiration has been linked to cell death and is a major contributor to acute neuronal trauma, peripheral diseases, as well as chronic neurodegenerative diseases, such as Parkinson's disease and Alzheimer's disease.

FUTURE DIRECTIONS

Here, we discuss the mechanisms underlying the sensitivity of the mitochondrial respiratory complexes to redox modulation, as well as examine the effects of environmental contaminants that have well-characterized mitochondrial toxicity. The contaminants discussed in this review are some of the most prevalent and potent environmental contaminants that have been linked to neurological dysfunction, altered cellular respiration, and oxidation.

Impact of methylmercury exposure on mitochondrial energetics in AC16 and H9C2 cardiomyocytes

It has been reported that chronic low dose exposures of methylmercury (MeHg) is associated with cardiovascular diseases in many populations worldwide. The toxic mechanisms through which these adverse effects occur are currently unknown. The objective of this study was to determine the bioenergetic and cytotoxic effects of MeHg on AC16 and H9C2 cardiomyocyte cell lines. Both cell lines exhibit significantly decreased mitochondrial function, cell viability and increased reactive oxygen species (ROS) production. Decreases in maximal respiration and reserve capacity was observed in both cell lines at 1μM. Bioenergetic profile experiments were also performed in tandem with cells exposed to diamide or menadione, compounds which accumulate in mitochondria and disrupt oxidative phosphorylation. AC16 cells show MeHg dose dependant sensitivities with Stateapparent and ATP production values, but H9C2 cells do not show these trends. H9C2 cells may be more resistant to MeHg toxicity than AC16 cells as reflected in the increases of proton leak and Stateapparent. No changes in expression of respiratory complexes were observed. Results suggest that MeHg has the potential to induce cytotoxicity. Furthermore, MeHg may have differential effects on AC16 and H9C2 cells, derived from human and rat cardiac tissue respectively, suggesting that differences in MeHg toxicity may be species-dependent.

Methionine stimulates motor impairment and cerebellar mercury deposition in methylmercury-exposed mice

Methylmercury (MeHg) is a highly toxic environmental contaminant that produces neurological and developmental impairments in animals and humans. Although its neurotoxic properties have been widely reported, the molecular mechanisms by which MeHg enters the cells and exerts toxicity are not yet completely understood. Taking into account that MeHg is found mostly bound to sulfhydryl-containing molecules such as cysteine in the environment and based on the fact that the MeHg-cysteine complex (MeHg-S-Cys) can be transported via the L-type neutral amino acid carrier transport (LAT) system, the potential beneficial effects of L-methionine (L-Met, a well known LAT substrate) against MeHg (administrated as MeHg-S-Cys)-induced neurotoxicity in mice were investigated. Mice were exposed to MeHg (daily subcutaneous injections of MeHg-S-Cys, 10 mg Hg/kg) and/or L-Met (daily intraperitoneal injections, 250 mg/kg) for 10 consecutive days. After treatments, the measured hallmarks of toxicity were mostly based on behavioral parameters related to motor performance, as well as biochemical parameters related to the cerebellar antioxidant glutathione (GSH) system. MeHg significantly decreased motor activity (open-field test) and impaired motor performance (rota-rod task) compared with controls, as well as producing disturbances in the cerebellar antioxidant GSH system. Interestingly, L-Met administration did not protect against MeHg-induced behavioral and cerebellar changes, but rather increased motor impairments in animals exposed to MeHg. In agreement with this observation, cerebellar levels of mercury (Hg) were higher in animals exposed to MeHg plus L-Met compared to those only exposed to MeHg. However, this event was not observed in kidney and liver. These results are the first to demonstrate that L-Met enhances cerebellar deposition of Hg in mice exposed to MeHg and that this higher deposition may be responsible for the greater motor impairment observed in mice simultaneously exposed to MeHg and L-Met.

Effects of diphenyl diselenide on methylmercury toxicity in rats

This study investigates the efficacy of diphenyl diselenide [(PhSe)2] in attenuating methylmercury- (MeHg-)induced toxicity in rats. Adult rats were treated with MeHg [5 mg/kg/day, intragastrically (i.g.)] and/ or (PhSe)2 [1 mg/kg/day, intraperitoneally (i.p.)] for 21 days. Body weight gain and motor deficits were evaluated prior to treatment, on treatment days 11 and 21. In addition, hepatic and cerebral mitochondrial function (reactive oxygen species (ROS) formation, total and nonprotein thiol levels, membrane potential (ΔΨm), metabolic function, and swelling), hepatic, cerebral, and muscular mercury levels, and hepatic, cerebral, and renal thioredoxin reductase (TrxR) activity were evaluated. MeHg caused hepatic and cerebral mitochondrial dysfunction and inhibited TrxR activity in liver (38,9%), brain (64,3%), and kidney (73,8%). Cotreatment with (PhSe)2 protected hepatic and cerebral mitochondrial thiols from depletion by MeHg but failed to completely reverse MeHg's effect on hepatic and cerebral mitochondrial dysfunction or hepatic, cerebral, and renal inhibition of TrxR activity. Additionally, the cotreatment with (PhSe)2 increased Hg accumulation in the liver (50,5%) and brain (49,4%) and increased the MeHg-induced motor deficits and body-weight loss. In conclusion, these results indicate that (PhSe)2 can increase Hg body burden as well as the neurotoxic effects induced by MeHg exposure in rats.

Dibenzofuran-induced mitochondrial dysfunction: Interaction with ANT carrier

Exposure to environmental pollutants such as dibenzofurans and furans is linked to the pathophysiology of several diseases. Dibenzofuran (DBF) is listed as a pollutant of concern due to its persistence in the environment, bioaccumulation and toxicity to humans, being associated with the development of lung diseases and cancers, due to its extremely toxic properties such as carcinogenic and teratogenic. Mitochondria play a key role in cellular homeostasis and keeping a proper energy supply for eukaryotic cells is essential in the fulfillment of the tissues energy-demand. Therefore, interference with mitochondrial function leads to cell death and organ failure. In this work, the effects of DBF on isolated rat liver mitochondria were analyzed. DBF exposure caused a markedly increase in the lag phase that follows depolarization induced by ADP, indicating an effect in the phosphorylative system. This was associated with a dose-dependent decrease in ATPase activity. Moreover, DBF also increased the threshold to the induction of the mitochondrial permeability transition (MPT) by calcium. Pretreatment of mitochondria with DBF also increased the concentration of carboxyatractyloside (CAT) necessary to abolish ADP phosphorylation and to induce the MPT, suggesting that DBF may interfere with mitochondria through an effect on the adenine nucleotide translocase (ANT). By co-immunoprecipitating ANT and Cyclophilin D (CypD) following MPT induction, we observed that in the presence of DBF, the ratio CypD/ANT was decreased. This demonstrates that DBF interferes with the ANT and so prevents CypD binding to the ANT, causing decreased phosphorylative capacity and inhibiting the MPT, which is also reflected by an increase in calcium retention capacity. Clarifying the role of pollutants in some mechanisms of toxicity, such as unbalance of bioenergetics status and mitochondrial function, may help to explain the progressive and chronic evolution of diseases derived from exposure to environmental pollutants.

Comparative study on methyl- and ethylmercury-induced toxicity in C6 glioma cells and the potential role of LAT-1 in mediating mercurial-thiol complexes uptake

Various forms of mercury possess different rates of absorption, metabolism and excretion, and consequently, toxicity. Methylmercury (MeHg) is a highly neurotoxic organic mercurial. Human exposure is mostly due to ingestion of contaminated fish. Ethylmercury (EtHg), another organic mercury compound, has received significant toxicological attention due to its presence in thimerosal-containing vaccines. This study was designed to compare the toxicities induced by MeHg and EtHg, as well as by their complexes with cysteine (MeHg-S-Cys and EtHg-S-Cys) in the C6 rat glioma cell line. MeHg and EtHg caused significant (p<0.0001) decreases in cellular viability when cells were treated during 30min with each mercurial following by a washing period of 24h (EC50 values of 4.83 and 5.05μM, respectively). Significant cytotoxicity (p<0.0001) was also observed when cells were treated under the same conditions with MeHg-S-Cys and EtHg-S-Cys, but the respective EC50 values were significantly increased (11.2 and 9.37μM). l-Methionine, a substrate for the l-type neutral amino acid carrier transport (LAT) system, significantly protected against the toxicities induced by both complexes (MeHg-S-Cys and EtHg-S-Cys). However, no protective effects of l-methionine were observed against MeHg and EtHg toxicities. Corroborating these findings, l-methionine significantly decreased mercurial uptake when cells were exposed to MeHg-S-Cys (p=0.028) and EtHg-S-Cys (p=0.023), but not to MeHg and EtHg. These results indicate that the uptake of MeHg-S-Cys and EtHg-S-Cys into C6 cells is mediated, at least in part, through the LAT system, but MeHg and EtHg enter C6 cells by mechanisms other than LAT system.

Platelet oxygen consumption as a peripheral blood marker of brain energetics in a mouse model of severe neurotoxicity

Interactions of chemicals with cerebral cellular systems are often accompanied by similar changes involving components in non-neural tissues. On this basis, indirect strategies have been developed to investigate neural cell function parameters by methods using accessible cells, including platelets and/or peripheral blood lymphocytes. Therefore, here it was investigated whether peripheral blood markers may be useful for assessing the central toxic effects of methylmercury (MeHg). For this purpose, we investigated platelet mitochondrial physiology in a well-established mouse model of MeHg-induced neurotoxicity, and correlated this peripheral activity with behavioural and central biochemical parameters. In order to characterize the cortical toxicity induced by MeHg (20 and 40 mg/L in drinking water, 21 days), the behavioral parameter namely, short-term object recognition, and the central mitochondrial impairment assessed by measuring respiratory complexes I-IV enzyme activities were determined in MeHg-poisoned animals. Neurotoxicity induced by MeHg exposure provoked compromised cortical activity (memory impairment) and reduced NADH dehydrogenase, complex II and II-III activities in the cerebral cortex. These alterations correlated with impaired systemic platelet oxygen consumption of intoxicated mice, which was characterized by reduced electron transfer activity and uncoupled mitochondria. The data brought here demonstrated that impaired systemic platelet oxygen consumption is a sensitive and non-invasive marker of the brain energy deficits induced by MeHg poisoning. Finally, brain and platelets biochemical alterations significantly correlated with cognitive behavior in poisoned mice. Therefore, it could be proposed the use of platelet oxygen consumption as a peripheral blood marker of brain function in a mouse model MeHg-induced neurotoxicity.

Toxicity of ethylmercury (and Thimerosal): a comparison with methylmercury

Ethylmercury (etHg) is derived from the metabolism of thimerosal (o-carboxyphenyl-thio-ethyl-sodium salt), which is the most widely used form of organic mercury. Because of its application as a vaccine preservative, almost every human and animal (domestic and farmed) that has been immunized with thimerosal-containing vaccines has been exposed to etHg. Although methylmercury (meHg) is considered a hazardous substance that is to be avoided even at small levels when consumed in foods such as seafood and rice (in Asia), the World Health Organization considers small doses of thimerosal safe regardless of multiple/repetitive exposures to vaccines that are predominantly taken during pregnancy or infancy. We have reviewed in vitro and in vivo studies that compare the toxicological parameters among etHg and other forms of mercury (predominantly meHg) to assess their relative toxicities and potential to cause cumulative insults. In vitro studies comparing etHg with meHg demonstrate equivalent measured outcomes for cardiovascular, neural and immune cells. However, under in vivo conditions, evidence indicates a distinct toxicokinetic profile between meHg and etHg, favoring a shorter blood half-life, attendant compartment distribution and the elimination of etHg compared with meHg. EtHg's toxicity profile is different from that of meHg, leading to different exposure and toxicity risks. Therefore, in real-life scenarios, a simultaneous exposure to both etHg and meHg might result in enhanced neurotoxic effects in developing mammals. However, our knowledge on this subject is still incomplete, and studies are required to address the predictability of the additive or synergic toxicological effects of etHg and meHg (or other neurotoxicants).

Role of calcium and mitochondria in MeHg-mediated cytotoxicity

Methylmercury (MeHg) mediated cytotoxicity is associated with loss of intracellular calcium (Ca²⁺) homeostasis. The imbalance in Ca²⁺ physiology is believed to be associated with dysregulation of Ca²⁺ intracellular stores and/or increased permeability of the biomembranes to this ion. In this paper we summarize the contribution of glutamate dyshomeostasis in intracellular Ca²⁺ overload and highlight the mitochondrial dysfunctions induced by MeHg via Ca²⁺ overload. Mitochondrial disturbances elicited by Ca²⁺ may involve several molecular events (i.e., alterations in the activity of the mitochondrial electron transport chain complexes, mitochondrial proton gradient dissipation, mitochondrial permeability transition pore (MPTP) opening, thiol depletion, failure of energy metabolism, reactive oxygen species overproduction) that could culminate in cell death. Here we will focus on the role of oxidative stress in these phenomena. Additionally, possible antioxidant therapies that could be effective in the treatment of MeHg intoxication are briefly discussed.

Antioxidant activity of β-selenoamines and their capacity to mimic different enzymes

The antioxidant properties of organoselenium compounds have been extensively investigated because oxidative stress is a hallmark of a variety of chronic human diseases. Here, we reported the influence of substituent groups in the antioxidant activity of β-selenoamines. We have investigated whether they exhibited glutathione peroxidase-like (GPx-like) activity and whether they could be substrate of thioredoxin reductase (TrxR). In the DPPH assay, the β-selenium amines did not exhibit antioxidant activity. However, the β-selenium amines with p-methoxy and tosyl groups prevented the lipid peroxidation. The β-selenium amine compound with p-methoxy substituent group exhibited thiol-peroxidase-like activity (GPx-like activity) and was reduced by the hepatic TrxR. These results contribute to understand the influence of structural alteration of non-conventional selenium compounds as synthetic mimetic of antioxidant enzymes of mammalian organisms.

Protective effect of a novel peptide against methylmercury-induced toxicity in rat primary astrocytes

Methylmercury (MeHg) is an environmental neurotoxicant associated with aberrant central nervous system (CNS) functions. In this study, we examined the protective effect of a novel anti-inflammatory and cytoprotective nonapeptide, termed IIIM1, against MeHg-induced toxicity in cultured rat neonatal primary astrocytes. Astrocytes were pretreated for 66 h with 5 μg/ml IIIM1 (4.95 μM) followed by 6 h exposure to MeHg (5 μM). MeHg significantly increased F(2)-isoprostane generation, a lipid peroxidation biomarker of oxidative injury and this effect was significantly reduced upon pre-treatment with IIIM1. The MeHg-induced increase in levels of prostaglandin E(2) (PGE(2)), biomarkers of inflammatory responses, was also decreased in the peptide-treated cells. Mass spectrometry analysis revealed no chemical or binding interaction between MeHg and IIIM1, indicating that intracellular cytoprotective mechanism of action accounts for the neuroprotection rather than direct intracellular neutralization of the neurotoxicant with the peptide. These findings point to therapeutic potential for IIIM1 in a plethora of conditions associated with reactive oxygen species (ROS) generation. The implication of these findings may prove beneficial in designing new treatment modalities that efficiently suppress neurotoxicity, triggered not only by MeHg, but also by other metals and environmental agents, as well as chronic disease conditions that inherently increase reactive radical production and inflammatory signaling.

New insights into the metabolism of organomercury compounds: mercury-containing cysteine S-conjugates are substrates of human glutamine transaminase K and potent inactivators of cystathionine γ-lyase

Anthropogenic practices and recycling in the environment through natural processes result in release of potentially harmful levels of mercury into the biosphere. Mercury, especially organic forms, accumulates in the food chain. Mercury reacts readily with sulfur-containing compounds and often exists as a thiol S-conjugate, such as the l-cysteine (Cys)-S-conjugate of methylmercury (CH(3)Hg-S-Cys) or inorganic mercury (Cys-S-Hg-S-Cys). These S-conjugates are structurally similar to l-methionine and l-cystine/l-cystathionine, respectively. Bovine and rat glutamine transaminase K (GTK) catalyze transamination of sulfur-containing amino acids. Recombinant human GTK (rhGTK) has a relatively open catalytic active site, and we report here that this enzyme, like the rat and bovine enzymes, can also utilize sulfur-containing l-amino acids, including l-methionine, l-cystine, and l-cystathionine as substrates. The current study extends this list to include mercuric S-conjugates, and shows that CH(3)Hg-S-Cys and Cys-S-Hg-S-Cys are substrates and reversible inhibitors of rhGTK. The homocysteine S-conjugates, Hcy-S-Hg-S-Hcy and CH(3)Hg-S-Hcy, are also inhibitors. Finally, we show that HgCl(2), CH(3)Hg-S-Cys and Cys-S-Hg-S-Cys are potent irreversible inhibitors of rat cystathionine γ-lyase. The present study broadens our knowledge of the biochemistry of mercury compounds by showing that Cys S-conjugates of mercury interact with enzymes that catalyze transformations of biologically important sulfur-containing amino acids.

Oxidative stress in MeHg-induced neurotoxicity

Methylmercury (MeHg) is an environmental toxicant that leads to long-lasting neurological and developmental deficits in animals and humans. Although the molecular mechanisms mediating MeHg-induced neurotoxicity are not completely understood, several lines of evidence indicate that oxidative stress represents a critical event related to the neurotoxic effects elicited by this toxicant. The objective of this review is to summarize and discuss data from experimental and epidemiological studies that have been important in clarifying the molecular events which mediate MeHg-induced oxidative damage and, consequently, toxicity. Although unanswered questions remain, the electrophilic properties of MeHg and its ability to oxidize thiols have been reported to play decisive roles to the oxidative consequences observed after MeHg exposure. However, a close examination of the relationship between low levels of MeHg necessary to induce oxidative stress and the high amounts of sulfhydryl-containing antioxidants in mammalian cells (e.g., glutathione) have led to the hypothesis that nucleophilic groups with extremely high affinities for MeHg (e.g., selenols) might represent primary targets in MeHg-induced oxidative stress. Indeed, the inhibition of antioxidant selenoproteins during MeHg poisoning in experimental animals has corroborated this hypothesis. The levels of different reactive species (superoxide anion, hydrogen peroxide and nitric oxide) have been reported to be increased in MeHg-exposed systems, and the mechanisms concerning these increments seem to involve a complex sequence of cascading molecular events, such as mitochondrial dysfunction, excitotoxicity, intracellular calcium dyshomeostasis and decreased antioxidant capacity. This review also discusses potential therapeutic strategies to counteract MeHg-induced toxicity and oxidative stress, emphasizing the use of organic selenocompounds, which generally present higher affinity for MeHg when compared to the classically studied agents.