Methylmercury cytotoxicity in PC12 cells is mediated by primary glutathione depletion independent of excess reactive oxygen species generation

Olive Oil Phenols Prevent Mercury-Induced Phosphatidylserine Exposure and Morphological Changes in Human Erythrocytes Regardless of Their Different Scavenging Activity

Phosphatidylserine (PS) translocation to the external membrane leaflet represents a key mechanism in the pathophysiology of human erythrocytes (RBC) acting as an "eat me" signal for the removal of aged/stressed cells. Loss of physiological membrane asymmetry, however, can lead to adverse effects on the cardiovascular system, activating a prothrombotic activity. The data presented indicate that structurally related olive oil phenols prevent cell alterations induced in intact human RBC exposed to HgCl₂ (5-40 µM) or Ca²⁺ ionophore (5 µM), as measured by hallmarks including PS exposure, reactive oxygen species generation, glutathione depletion and microvesicles formation. The protective effect is observed in a concentration range of 1-30 µM, hydroxytyrosol being the most effective; its in vivo metabolite homovanillic alcohol still retains the biological activity of its dietary precursor. Significant protection is also exerted by tyrosol, in spite of its weak scavenging activity, indicating that additional mechanisms are involved in the protective effect. When RBC alterations are mediated by an increase in intracellular calcium, the protective effect is observed at higher concentrations, indicating that the selected phenols mainly act on Ca²⁺-independent mechanisms, identified as protection of glutathione depletion. Our findings strengthen the nutritional relevance of olive oil bioactive compounds in the claimed health-promoting effects of the Mediterranean Diet.

Characterizing the Low-Dose Effects of Methylmercury on the Early Stages of Embryo Development Using Cultured Human Embryonic Stem Cells

BACKGROUND

Global concerns of methylmercury (MeHg) exposure have been raised, especially on its effects on pregnant women. Recent epidemiological studies have revealed associations between maternal blood/hair MeHg concentrations, adverse pregnancy outcomes, and developmental deficits. However, the underlying mechanisms remain unclear.

OBJECTIVES

In this study, we characterized the effects of MeHg exposure on undifferentiated human embryonic stem cells (hESCs) and extrapolated the effects to human embryonic development.

METHODS

hESCs were exposed to 0, 1, 5, 10, 50, 100 or 200nM MeHg for 24 h or 6 d. Cell adherence and colony formation and expansion were examined under the microscope. Cell attachment, viability/proliferation, apoptosis, stress response, cell cycle, autophagy, and expression of cell lineage marker genes and proteins were measured at the end of exposures.

RESULTS

Our results indicated that exposure to nanomolar concentrations of MeHg was associated with a) higher levels of reactive oxygen species (ROS) and hemeoxygenase-1 (HO-1), suggesting increased stress and adaptive responses; b) lower cellular adhesion, viability/proliferation, and colony formation and expansion; c) higher levels of apoptosis, reflected by higher cleaved caspase-3 expression and Annexin V binding; d) higher levels of cytoskeleton protein α-tubulin expression; e) higher rates of G1/S phase cell cycle arrest; and f) autophagy inhibition, as shown by a lower LC3BII/LC3BI ratio and accumulation of SQSTM1 (p62). These outcomes were accompanied by higher expressions of self-renewal genes or proteins or both, including OCT4, SOX2, NANOG, and cytokine receptor IL6ST, as well as pluripotency and the cell fate regulator cyclin D1.

DISCUSSION

These results revealed that under the selection pressure of exposure to low doses of MeHg, some hESCs underwent apoptosis, whereas others adapted and survived with enhanced self-renewal gene expression and specific morphological phenotypes. Findings from the present study provide in vitro evidence that low doses of MeHg adversely affect hESCs when exposed during a period of time that models embryonic pre-, during, and early postimplantation stages. https://doi.org/10.1289/EHP7349.

No Effect of Lifelong Methylmercury Exposure on Oxidative Status in Zebra Finches (Taeniopygia guttata): A Demonstration of Methylmercury-Induced Selection?

Songbirds exposed to methylmercury (MeHg) often exhibit reduced reproductive success and cognitive abilities. To better understand whether oxidative stress plays a role, we dosed zebra finches (Taeniopygia guttata) with a contaminated (1.2 ppm MeHg-cysteine) or control diet for their entire lives, including during development in the egg. Levels of antioxidant enzymes [superoxide dismutase (SOD1 and SOD2)], oxidative damage (4-hydroxynonenal; 4-HNE), and antioxidant transcription factors [nuclear factor (erythroid-derived 2)-like 2; Nrf2] were measured in the liver and pectoralis muscle of adults. MeHg treatment did not affect levels of 4-HNE or liver SOD2 or Nrf2. Birds in the MeHg treatment differed significantly from controls in pectoralis SOD1 and Nrf2, and tended to differ in liver SOD1 and pectoralis SOD2; however, we detected no overall pattern of effect of MeHg on oxidative status in dosed finches. We suspect that this is a consequence of the differential survival of MeHg-tolerant birds.

Changes in Biochemical Processes in Cerebellar Granule Cells of Mice Exposed To Methylmercury

At postnatal day 34, male and female C57BL/6J mice were exposed orally once a day to a total of five doses totaling 1.0 or 5.0 mg/kg of methylmercuric chloride or sterile deionized water in moistened rodent chow. Eleven days after the last dose cerebellar granule cells were acutely isolated to measure reactive oxygen species (ROS) levels and mitochondrial membrane potential using CM-H2DCFDA and TMRM dyes, respectively. For visualizing intracellular calcium ion distribution using transmission electron microscopy, mice were perfused 11 days after the last dose of methylmercury (MeHg) using the oxalate-pyroantimonate method. Cytosolic and mitochondrial protein fractions from acutely isolated granule cells were analyzed for cytochrome c content using Western blot analysis. Histochemistry (Fluoro-Jade dye) and immunohistochemistry (activated caspase 3) was performed on frozen serial cerebellar sections to label granule cell death and activation of caspase 3, respectively. Granule cells isolated from MeHg-treated mice showed elevated ROS levels and decreased mitochondrial membrane potential when compared to granule cells from control mice. Electron photomicrographs of MeHg-treated granule cells showed altered intracellular calcium ion homeostasis ([Ca2+]i) when compared to control granule cells. However, in spite of these subcellular changes and moderate relocalization of cytochrome c into the cytosol, the concentrations of MeHg used in this study did not produce significant neuronal cell death/apoptosis at the time point examined, as evidenced by Fluoro-Jade and activated caspase 3 immunostaining, respectively. These results demonstrate that short-term in vivo exposure to total doses of 1.0 and 5.0 mg/kg MeHg through the most common exposure route (oral) can result in significant subcellular changes that are not accompanied by overt neuronal cell death.

Hydroxytyrosol inhibits phosphatidylserine exposure and suicidal death induced by mercury in human erythrocytes: Possible involvement of the glutathione pathway

Hydroxytyrosol (HT) is a phenolic antioxidant naturally occurring in virgin olive oil. In this study, we investigated the possible protective effects of HT on programmed suicidal death (eryptosis) induced by mercury (Hg) treatment in intact human erythrocytes (RBC). Our study confirms that the Hg-eryptosis is characterized by phosphatidylserine (PS) exposure at the cell surface, with cell shrinkage and ATP and glutathione depletion; calcium influx is also a key event that triggers eryptosis. Here we report that cell preconditioning with an optimal dose (1-5 μM) of HT prior to exposure to 2.5 μM HgCl2 causes a noteworthy decrease in PS-exposing RBC, almost restoring ATP and GSH content. Conversely, HT shows no effect against decrease in cell volume nor against influx of extracellular calcium. Taken together our data provide the first experimental evidence of the efficacy of HT in modulating the programmed suicidal death in non nucleated cells; the reported findings also confirm that the prevention of Hg toxicity should be regarded as an additional mechanism responsible for the health-promoting potential of this dietary phenol. Finally, virgin olive oil would appear to be a promising healthy food to reduce the adverse effects of chronic mercury exposure in humans.

The protective role of olive oil hydroxytyrosol against oxidative alterations induced by mercury in human erythrocytes

Hydroxytyrosol (HT) is a phenolic antioxidant naturally occurring in virgin olive oil. In this study, we investigated the possible protective effects of HT on the oxidative and morphological alterations induced by mercury (Hg) in intact human erythrocytes. These cells preferentially accumulate this toxic heavy metal. More importantly, Hg-induced echinocyte formation correlates with increased coagulability of these cells. Our results indicate that HT treatment (10-50 µM) prevents the increase in hemolysis and Reactive Oxygen Species (ROS) generation induced by exposure of cells to micromolar HgCl2 concentrations as well as the decrease in GSH intracellular levels. Moreover, as indicated by scanning electron microscopy, the morphological alterations are also significantly reduced by HT co-treatment. Taken together our data provide the first experimental evidence that HT has the potential to counteract mercury toxicity. The reported effect may be regarded as an additional mechanism underlying the beneficial cardio-protective effects of this dietary antioxidant, also endowed with significant anti-atherogenic and anti-inflammatory properties.

Quantitative proteomic analysis reveals proteins involved in the neurotoxicity of marine medaka Oryzias melastigma chronically exposed to inorganic mercury

Mercury is a ubiquitous environmental contaminant which exerts neurotoxicity upon animals. Nevertheless, the molecular mechanisms involved in inorganic mercury neurotoxicity are unknown. We investigated protein profiles of marine medaka, chronically exposed to mercuric chloride using two-dimensional difference gel electrophoresis (2D-DIGE) and matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometry (MALDI-TOF-TOF MS) analysis. The mercury accumulation and ultrastructure were also examined in the brain. The results showed that mercury was significantly accumulated in the treated brain, and subsequently caused a noticeable damage. The comparison of 2D-DIGE protein profiles between the control and treatment revealed that 16 protein spots were remarkably altered in abundance, which were further submitted for MALDI-TOF-TOF MS analysis. The identified proteins indicated that inorganic mercury may cause neurotoxicity through the induction of oxidative stress, cytoskeletal assembly dysfunction and metabolic disorders. Thus, this study provided a basis for a better understanding of the molecular mechanisms involved in mercury neurotoxicity.

Modulation of glutathione and its dependent enzymes in gill cells of Anguilla anguilla exposed to silica coated iron oxide nanoparticles with or without mercury co-exposure under in vitro condition

The current study aimed to investigate the modulation of glutathione (GSH) and its dependent enzymes (glutathione reductase, GR; glutathione peroxidase, GPx; glutathione sulfotransferase, GST) from 0 to 72 h in the gill cells of Anguilla anguilla under in vitro condition exposed to silica coated iron oxide nanoparticles functionalized with dithiocarbamate (Fe₃O₄@SiO₂/SiDTC, hereafter called 'IONPs'; 100 nm; 2.5 mgL(-1)) with or without mercury (Hg) coexposure. Significantly decreased TGSH content under IONP alone exposure from 0 to 72 h indicated increased utilization of the TGSH in response to IONP stress. Significant increases in the activity of GR, GPx and GST were depicted when exposed to IONP alone. Lipid peroxidation (LPO), a membrane damage trait also significantly increased under IONP alone exposure indicating the efficient role of antioxidant induction abolishing the IONP-mediated enhanced reactive oxygen species. Under Hg exposure, gill cells displayed significantly increased activity of the studied enzymes (GR, GPx and GST) and LPO which was accompanied by significantly decreased TGSH content. Concomitant (IONPs+Hg) exposure displayed a synergistic response to that of individual responses of either IOPN or Hg which was evident by significant increases in GR, GPx, GST and LPO. Overall, our findings revealed a fine tuning among the GSH and its dependent enzyme modulation under IONP, Hg and its concomitant (IONPs+Hg) exposure in A. anguilla gill cells under in vitro condition.

The role of de novo catecholamine synthesis in mediating methylmercury-induced vesicular dopamine release from rat pheochromocytoma (PC12) cells

The purpose of this study was to characterize methylmercury (MeHg)-induced dopamine (DA) release from undifferentiated pheochromocytoma (PC12) cells and to examine the potential role for DA synthesis in this process. MeHg caused a significant increase in DA release that was both concentration- and time-dependent. DA release was significantly increased by 2µM MeHg at 60min and by 5µM MeHg at 30min; 1µM MeHg was without effect. Because DA release induced by 5µM MeHg was associated with a significant percentage of cell death at 60 and 120min, 2µM MeHg was chosen for further characterization of release mechanisms. MeHg-induced DA release was attenuated but not abolished in the absence of extracellular calcium, whereas the vesicular content depleting drug reserpine (50nM) abolished release. Thus, MeHg-induced DA release requires vesicular exocytosis but not extracellular calcium. MeHg also increased intracellular DA and the rate of DA storage utilization, suggesting a role for DA synthesis in MeHg-induced DA release. The tyrosine hydroxylase inhibitor α-methyltyrosine (300µM, 24h) completely abolished MeHg-induced DA release. MeHg significantly increased DA precursor accumulation in cells treated with 3-hydroxybenzylhydrazine (10µM), revealing that MeHg increases tyrosine hydroxylase activity. Overall, these data demonstrate that MeHg facilitates DA synthesis, increases intracellular DA, and augments vesicular exocytosis.

Mitochondrial electron transport chain in heavy metal-induced neurotoxicity: effects of cadmium, mercury, and copper

To clarify the role of mitochondrial electron transport chain (mtETC) in heavy-metal-induced neurotoxicity, we studied action of Cd²⁺, Hg²⁺, and Cu²⁺ on cell viability, intracellular reactive oxygen species formation, respiratory function, and mitochondrial membrane potential of rat cell line PC12. As found, the metals produced, although in a different way, dose- and time-dependent changes of all these parameters. Importantly, Cd²⁺ beginning from 10 [mu]M and already at short incubation time (3 h) significantly inhibited the FCCP-uncoupled cell respiration; besides, practically the complete inhibition of the respiration was reached after 3 h incubation with 50 [mu]M Hg²⁺ or 500 [mu]M Cd²⁺, whereas even after 48 h exposure with 500 [mu]M Cu²⁺, only a 50% inhibition of the respiration occurred. Against the Cd²⁺-induced cell injury, not only different antioxidants and mitochondrial permeability transition pore inhibitors were protective but also such mtETC effectors as FCCP and stigmatellin (complex III inhibitor). However, all mtETC effectors used did not protect against the Hg²⁺- or Cu²⁺-induced cell damage. Notably, stigmatellin was shown to be one of the strongest protectors against the Cd²⁺-induced cell damage, producing a 15–20% increase in the cell viability. The mechanisms of the mtETC involvement in the heavy-metal-induced mitochondrial membrane permeabilization and cell death are discussed.

Oxoguanine glycosylase 1 (OGG1) protects cells from DNA double-strand break damage following methylmercury (MeHg) exposure

Methylmercury (MeHg) is a potent neurotoxin, teratogen, and probable carcinogen, but the underlying mechanisms of its actions remain unclear. Although MeHg causes several types of DNA damage, the toxicological consequences of this macromolecular damage are unknown. MeHg enhances oxidative stress, which can cause various oxidative DNA lesions that are primarily repaired by oxoguanine glycosylase 1 (OGG1). Herein, we compared the response of wild-type and OGG1 null (Ogg1(-/-)) murine embryonic fibroblasts to environmentally relevant, low micromolar concentrations of MeHg by measuring clonogenic efficiency, cell cycle arrest, DNA double-strand breaks (DSBs), and activation of the DNA damage response pathway.Ogg1(-/-) cells exhibited greater sensitivity to MeHg than wild-type controls, as measured by the clonogenic assay, and showed a greater propensity for MeHg-initiated apoptosis. Both wild-type and Ogg1(-/-) cells underwent cell cycle arrest when exposed to micromolar concentrations of MeHg; however, the extent of DSBs was exacerbated in Ogg1(-/-) cells compared with that in wild-type controls. Pretreatment with the antioxidative enzyme catalase reduced levels of DSBs in both wild-type and Ogg1(-/-) cells but failed to block MeHg-initiated apoptosis at micromolar concentrations. Our findings implicate reactive oxygen species mediated DNA damage in the mechanism of MeHg toxicity; and demonstrate for the first time that impaired DNA repair capacity enhances cellular sensitivity to MeHg. Accordingly, the genotoxic properties of MeHg may contribute to its neurotoxic and teratogenic effects, and an individual's response to oxidative stress and DNA damage may constitute an important determinant of risk.

Dietary methylmercury alters the proteome in Atlantic salmon (Salmo salar) kidney

BACKGROUND

Methylmercury (MeHg) is an environmental contaminant most known for its severe neurotoxic effects. Although accumulation of MeHg tends to be several folds higher in kidney compared to other tissues, studies on nephrotoxic effects are almost non-existing. In this study we aim to investigate the toxicity of dietary MeHg in kidney of Atlantic salmon (Salmo salar).

MATERIAL AND METHODS

Atlantic salmon were exposed to dietary MeHg for a period of 8 weeks, before the fish were euthanized and kidney was sampled for proteomic and real time RT-PCR analysis, as well as for mercury determination. Protein separation was done with 2-D PAGE, and differentially regulated spots were picked for analysis using liquid chromatography MS/MS analysis. Moreover, whole blood and liver tissue were sampled for mercury determination and real time RT-PCR (liver).

RESULTS

MeHg exposed fish accumulated significantly more mercury (Hg) than control fish. The proteomic analysis revealed differential abundance of 26 spots in the kidney, and 14 of these protein spots were successfully identified. The proteins identified indicated effects of MeHg on; metabolism, inflammation, oxidative stress, protein-folding, and cell-structural components. Gene expression analysis of selected markers revealed few differentially regulated transcripts in kidney and liver in the exposed fish compared to the control fish. However, the affected transcripts indicated a disruption in the expression of two metabolic markers due to MeHg exposure in liver.

CONCLUSION

This study suggests that dietary MeHg has similar effects in kidney as previously shown for other tissues in fish. The effects observed were in markers for oxidative stress, inflammation and energy metabolism. The identification of proteomic markers in this study provides a basis for a better understanding of MeHg-induced nephrotoxicity in fish.

Heavy metal poisoning and cardiovascular disease

Cardiovascular disease (CVD) is an increasing world health problem. Traditional risk factors fail to account for all deaths from CVD. It is mainly the environmental, dietary and lifestyle behavioral factors that are the control keys in the progress of this disease. The potential association between chronic heavy metal exposure, like arsenic, lead, cadmium, mercury, and CVD has been less well defined. The mechanism through which heavy metals act to increase cardiovascular risk factors may act still remains unknown, although impaired antioxidants metabolism and oxidative stress may play a role. However, the exact mechanism of CVD induced by heavy metals deserves further investigation either through animal experiments or through molecular and cellular studies. Furthermore, large-scale prospective studies with follow up on general populations using appropriate biomarkers and cardiovascular endpoints might be recommended to identify the factors that predispose to heavy metals toxicity in CVD. In this review, we will give a brief summary of heavy metals homeostasis, followed by a description of the available evidence for their link with CVD and the proposed mechanisms of action by which their toxic effects might be explained. Finally, suspected interactions between genetic, nutritional and environmental factors are discussed.

Proteome profiles in medaka (Oryzias melastigma) liver and brain experimentally exposed to acute inorganic mercury

Mercury is a widespread and persistent pollutant occurring in a variety of forms in freshwater and marine ecosystems. Using the proteomic approach, this study examined the protein profiles of the medaka (Oryzias melastigma) liver and brain exposed to an acute mercuric chloride (HgCl(2)) concentration (1000μg/L) for 8h. The results showed that acute exposure of medaka to inorganic mercury enhanced metal accumulation in both the liver and brain, and a higher content of mercury was detected in the latter. Comparison of the two-dimensional electrophoresis protein profiles of HgCl(2)-exposed and non-exposed group revealed that altered protein expression was quantitatively detected in 20 spots in the brain and 27 in the liver. The altered protein spots were subjected to matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometry analysis, with the resultant identification of 46 proteins. The proteins identified were involved in oxidative stress, cytoskeletonal assembly, signal transduction, protein modification, metabolism and other related functions (e.g. immune response, ionoregulation and transporting), highlighting the fact that inorganic mercury toxicity in fish seems to be complex and diverse. This study provided basic information to aid our understanding of the possible molecular mechanisms of acute inorganic mercury toxicity in aquatic organisms, as well as potential protein biomarker candidates for aquatic environmental monitoring.

Evaluation of the cardiovascular effects of methylmercury exposures: current evidence supports development of a dose-response function for regulatory benefits analysis

BACKGROUND

The U.S. Environmental Protection Agency (U.S. EPA) has estimated the neurological benefits of reductions in prenatal methylmercury (MeHg) exposure in past assessments of rules controlling mercury (Hg) emissions. A growing body of evidence suggests that MeHg exposure can also lead to increased risks of adverse cardiovascular impacts in exposed populations.

DATA EXTRACTION

The U.S. EPA assembled the authors of this article to participate in a workshop, where we reviewed the current science concerning cardiovascular health effects of MeHg exposure via fish and seafood consumption and provided recommendations concerning whether cardiovascular health effects should be included in future Hg regulatory impact analyses.

DATA SYNTHESIS

We found the body of evidence exploring the link between MeHg and acute myocardial infarction (MI) to be sufficiently strong to support its inclusion in future benefits analyses, based both on direct epidemiological evidence of an MeHg-MI link and on MeHg's association with intermediary impacts that contribute to MI risk. Although additional research in this area would be beneficial to further clarify key characteristics of this relationship and the biological mechanisms that underlie it, we consider the current epidemiological literature sufficiently robust to support the development of a dose- response function.

CONCLUSIONS

We recommend the development of a dose- response function relating MeHg exposures with MIs for use in regulatory benefits analyses of future rules targeting Hg air emissions.

Responses in the brain proteome of Atlantic cod (Gadus morhua) exposed to methylmercury

The molecular mechanisms underlying the neurotoxicity of methylmercury (MeHg), a ubiquitous environmental contaminant, are not yet fully understood. Furthermore, there is a lack of biomarkers of MeHg neurotoxicity for use in environmental monitoring. We have undertaken a proteomic analysis of brains from Atlantic cod (Gadus morhua) exposed to 0, 0.5 and 2 mg/kg MeHg administered by intraperitoneal injection. The doses were given in two injections, half of the dose on the first day and the second half after 1 week, and the total exposure period lasted 2 weeks. Using 2-DE coupled with MALDI-TOF MS and MS/MS, we observed the level of 71 protein spots to be 20% or more significantly altered following MeHg exposure, and successfully identified 40 of these protein spots. Many of these proteins are associated with main known molecular targets and mechanisms of MeHg-induced neurotoxicity in mammals, such as mitochondrial dysfunction, oxidative stress, altered calcium homeostasis and tubulin/disruption of microtubules. More interestingly, several of the affected proteins, with well-established or recently demonstrated critical functions in nervous system-specific processes, have not previously been associated with MeHg exposure in any species. These proteins include the strongest up-regulated protein, pyridoxal kinase (essential for synthesis of several neurotransmitters), G protein (coupled to neurotransmitter receptors), nicotinamide phosphoribosyl-transferase (protection against axonal degeneration), dihydropyrimidinase-like 5 (or collapsin response mediator protein 5, CRMP-5) (axon guidance and regeneration), septin (dendrite development), phosphatidylethanolamine binding protein (precursor for hippocampal cholinergic neurostimulating peptide) and protein phosphatase 1 (control of brain recovery by synaptic plasticity). The results of the present study aid our understanding of molecular mechanisms underlying MeHg neurotoxicity and defense responses, and provide a large panel of protein biomarker candidates for aquatic environmental monitoring.

Cipura paludosa attenuates long-term behavioral deficits in rats exposed to methylmercury during early development

In the present study, we evaluated the effects of the ethanolic extract (EE) of Cipura paludosa on locomotor, and anxiety- and depression-like behaviors of adult rats exposed to MeHg during early development. Additionally, the antioxidant enzymes catalase (CAT) and selenium-glutathione peroxidase (Se-GPx) were measured in cortical, hippocampal, and cerebellar tissues. Pregnant Wistar rats were treated by gavage with a single dose of MeHg (8 mg/kg) on gestational day 15, the developmental stage critical for cortical neuron proliferation. Moreover, prenatal MeHg exposure inhibited CAT and Se-GPx in the cortex and cerebellum. Chronic treatment with the EE of C. paludosa attenuated these emotional and antioxidant deficits induced by prenatal MeHg toxic exposure. This study provides novel evidence that developmental exposure to MeHg can affect not only cognitive functions but also locomotor, and anxiety- and depression-like behaviors.

Antioxidant system breakdown in brain of feral golden grey mullet (Liza aurata) as an effect of mercury exposure

Although brain has been recognized as a primary target for mercury toxicity in mammals, the effects of this metal in fish brain are scarcely described. Thus, the main objective of this study was to assess the mercury threat to feral fish (Liza aurata) by estimating the antioxidant defenses and peroxidative damage in brain, keeping in mind the association with mercury accumulation. Sampling was carried out in an estuarine area historically affected by discharges from a chlor-alkali industry-Laranjo Basin (Ria de Aveiro, Portugal). Total mercury (T-Hg) in brain increased towards the contamination source, clearly indicating mercury exposure. An overall antioxidant depletion was verified in brain of fish collected at the mercury-contaminated stations, since total glutathione content and the studied antioxidant enzymes (catalase-CAT, glutathione peroxidase-GPx, glutathione-S-transferase-GST and glutathione reductase-GR) significantly decreased. In addition, this breakdown of the redox-defense system was significantly correlated with the accumulated T-Hg levels. Unexpectedly, fish exhibited unaltered lipid peroxidation levels, pointing out a higher propensity of mercury to inhibit enzymes than to oxidatively damage lipids in the brain. Nevertheless, an increased susceptibility of the fish's brain was identified, leaving the organ more vulnerable to oxidative stress-related challenges. Overall, the current findings provide information to better understand mechanisms of mercury neurotoxicity in fish.

Characterization of antioxidant protection of cultured neural progenitor cells (NPC) against methylmercury (MeHg) toxicity

Methylmercury (MeHg) is an environmental pollutant known to cause neurobehavioral defects and is especially toxic to the developing brain. With recent studies showing that fetal exposure to low-dose MeHg causes developmental abnormalities, it is therefore important to find ways to combat its effects as well as to clarify the mechanism(s) underlying MeHg toxicity. In the present study, the effects of MeHg on cultured neural progenitor cells (NPC) derived from mouse embryonic brain were investigated. We first confirmed the vulnerability of embryonic NPC to MeHg toxicity, NPC from the telencephalon were more sensitive to MeHg compared to those from the diencephalon. Buthionine sulfoximine (BSO) which is known to inhibit glutathione synthesis accelerated MeHg toxicity. Furthermore, antioxidants such as N-acetyl cysteine and alpha-tocopherol dramatically rescued the NPC from MeHg's toxic effects. Interestingly, a 12 hr delay in the addition of either antioxidant was still able to prevent the cells from undergoing cell death. Although it is now difficult to avoid MeHg exposure from our environment and contaminated foods, taking anti-oxidants from foods or supplements may prevent or diminish the toxicological effects of MeHg.

Environmental toxicity, oxidative stress and apoptosis: ménage à trois

Apoptosis is an evolutionary conserved homeostatic process involved in distinct physiological processes including organ and tissue morphogenesis, development and senescence. Its deregulation is also known to participate in the etiology of several human diseases including cancer, neurodegenerative and autoimmune disorders. Environmental stressors (cytotoxic agents, pollutants or toxicants) are well known to induce apoptotic cell death and to contribute to a variety of pathological conditions. Oxidative stress seems to be the central element in the regulation of the apoptotic pathways triggered by environmental stressors. In this work, we review the established mechanisms by which oxidative stress and environmental stressors regulate the apoptotic machinery with the aim to underscore the relevance of apoptosis as a component in environmental toxicity and human disease progression.

Prevention of methylmercury-induced mitochondrial depolarization, glutathione depletion and cell death by 15-deoxy-delta-12,14-prostaglandin J(2)

Methylmercury (MeHg) is an environmental toxin that causes severe neurological complications in humans and experimental animals. In addition to neurons, glia in the central nervous system are very susceptible to MeHg toxicity. Pretreatment of glia with the prostaglandin derivative, 15-deoxy-delta-12,14-prostaglandin J(2) (15d-PGJ(2)), caused a significant protection against MeHg cytotoxicity. Results with the C6 glioma cells demonstrated that the protection was dependent on the duration of pretreatment, suggesting that time was required for the up-regulation of cellular defenses. Subsequent experiments indicated that 15d-PGJ(2) prevented MeHg induced mitochondrial depolarization. Similar protection against MeHg cytotoxicity was observed in primary cultures of mouse glia. Analysis of cellular glutathione (GSH) levels indicated that 15d-PGJ(2) caused an up-regulation of GSH and prevented MeHg-induced GSH depletion. Buthionine sulfoximine (BSO), a GSH synthesis inhibitor, completely inhibited the GSH induction by 15d-PGJ(2). However, BSO did not prevent the stabilization of mitochondrial potential and only partially prevented the protection caused by 15d-PGJ(2). While induction of heme oxygenase-1 was implicated in the cytoprotection by 15d-PGJ(2) under some experimental conditions, additional experiments indicated that this enzyme was not involved in the cytoprotection observed in this system. Together, these results suggested that while up-regulation of GSH by 15d-PGJ(2) might help cells to defend against MeHg toxicity, there may be other yet unidentified mechanism(s) initiated by 15d-PGJ(2) treatment that contributed to its protection against MeHg cytotoxicity.

Neuroprotective effects of chlorogenic acid against apoptosis of PC12 cells induced by methylmercury

Chlorogenic acid (CGA) widely exists in edible and medicinal plants. We aimed to evaluate the effect of CGA on the protection from apoptosis by methylmercury (MeHg) in PC12 cells. Cell viability was evaluated by MTT assay. Apoptosis was assayed by flow cytometry detection. Caspase-3 activity was measured by confocal microscopy. Intracellular GSH levels were determined by bicinchoninic acid protein assay. Intracellular reactive oxygen species (ROS) was assessed by means of chloromethyl-dihydrodichlorofluorescein diacetate. Glutathione peroxidase (GPx) activity was determined by UV. In order to elucidate the action of CGA, the protective effects of CGA were compared to Vit.E. CGA was effective at protecting PC12 cells against MeHg-induced damage in dose-dependent manner. CGA not only suppressed the generation of ROS, the decrease of activity in GPx and the decrease of GSH, but also attenuated caspase-3 activation in PC12 cells by MeHg. CGA eventually protected PC12 cells against MeHg-induced apoptosis. The results highlighted that CGA may exert neuroprotective effects through its antioxidant actions.

Neurobehavioural and molecular changes induced by methylmercury exposure during development

There is an increasing body of evidence on the possible environmental influence on neurodevelopmental and neurodegenerative disorders. Both experimental and epidemiological studies have demonstrated the distinctive susceptibility of the developing brain to environmental factors such as lead, mercury and polychlorinated biphenyls at levels of exposure that have no detectable effects in adults. Methylmercury (MeHg) has long been known to affect neurodevelopment in both humans and experimental animals. Neurobehavioural effects reported include altered motoric function and memory and learning disabilities. In addition, there is evidence from recent experimental neurodevelopmental studies that MeHg can induce depression-like behaviour. Several mechanisms have been suggested from in vivo- and in vitro-studies, such as effects on neurotransmitter systems, induction of oxidative stress and disruption of microtubules and intracellular calcium homeostasis. Recent in vitro data show that very low levels of MeHg can inhibit neuronal differentiation of neural stem cells. This review summarises what is currently known about the neurodevelopmental effects of MeHg and consider the strength of different experimental approaches to study the effects of environmentally relevant exposure in vivo and in vitro.

Methylmercury-induced changes in mitochondrial function in striatal synaptosomes are calcium-dependent and ROS-independent

The brain is the main target organ for methylmercury (MeHg), a highly toxic compound that bioaccumulates in aquatic systems, leading to high exposure in humans who consume large amounts of fish. The mechanisms responsible for MeHg-induced changes in neuronal function are, however, not yet fully understood. In the present study we investigated whether MeHg-induced elevations in reactive oxygen species (ROS) or intracellular calcium are responsible for altering mitochondrial metabolic function in rat striatal synaptosomes. MeHg decreased mitochondrial function (measured by the conversion of MTT to formazan) and increased ROS levels in striatal synaptosomes after 30 min exposure. Although co-incubation with the antioxidant Trolox significantly reduced MeHg-induced ROS levels, it failed to restore mitochondrial function. MeHg also increased cytosolic and mitochondrial calcium levels in striatal synaptosomes. These elevations were largely independent of extrasynaptosomal calcium, given that nominal calcium-free buffer with 20 microM EGTA did not prevent MeHg-induced increases in cytosolic calcium. In conclusion, we suggest that ROS are not the cause of mitochondrial dysfunction in striatal synaptosomes after MeHg exposure; rather, we propose that ROS formation is a downstream event that reflects MeHg-induced mitochondrial dysfunction due to increased mitochondrial calcium levels.

Role of glutathione in determining the differential sensitivity between the cortical and cerebellar regions towards mercury-induced oxidative stress

Certain discrete areas of the CNS exhibit enhanced sensitivity towards MeHg. To determine whether GSH is responsible for this particular sensitivity, we investigated its role in MeHg-induced oxidative insult in primary neuronal and astroglial cell cultures of both cerebellar and cortical origins. For this purpose, ROS and GSH were measured with the fluorescent indicators, CMH(2)DCFDA and MCB. Cell associated-MeHg was measured with (14)C-radiolabeled MeHg. The intracellular GSH content was modified by pretreatment with NAC or DEM. For each of the dependent variables (ROS, GSH, and MTT), there was an overall significant effect of cellular origin, MeHg and pretreatment in all the cell cultures. A trend towards significant interaction between originxMeHgxpretreatment was observed only for the dependent variable, ROS (astrocytes p=0.056; neurons p=0.000). For GSH, a significant interaction between originxMeHg was observed only in astrocytes (p=0.030). The cerebellar cell cultures were more vulnerable (astrocytes(mean)=223.77; neurons(mean)=138.06) to ROS than the cortical cell cultures (astrocytes(mean)=125.18; neurons(mean)=107.91) for each of the tested treatments. The cell associated-MeHg increased when treated with DEM, and the cerebellar cultures varied significantly from the cortical cultures. Non-significant interactions between originxMeHgxpretreatment for GSH did not explain the significant interactions responsible for the increased amount of ROS produced in these cultures. In summary, although GSH modulation influences MeHg-induced toxicity, the difference in the content of GSH in cortical and cerebellar cultures fails to account for the increased ROS production in cerebellar cultures. Hence, different approaches for the future studies regarding the mechanisms behind selectivity of MeHg have been discussed.

Mercury as a risk factor for cardiovascular diseases

Mercury is a heavy metal that exists naturally in the environment. Major sources include the burning of fossil fuels (especially coal) and municipal waste incineration. Mercury can exist in several forms, with the most hazardous being organic methylmercury. In waterways (lakes, rivers, reservoirs, etc.), mercury is converted to methylmercury, which then accumulates in fish, especially in large predatory fish. Fish and fish products are the major--if not the only--source of methylmercury in humans. Mercury has long been recognized as a neurotoxin for humans, but in the last 10 years, its potentially harmful effects on cardiovascular diseases (CVD) have raised a cause for concern, mostly due to the proposed role of mercury in oxidative stress propagation. Some epidemiological studies have indeed found an association between increased levels of mercury in the body and risk of CVD. There are several plausible mechanisms to explain the association; these are discussed in this review. We also review the epidemiological studies that have investigated the association between mercury and CVD.

Specific inhibitory action of anisodamine against a staphylococcal superantigenic toxin, toxic shock syndrome toxin 1 (TSST-1), leading to down-regulation of cytokine production and blocking of TSST-1 toxicity in mice

Toxic shock syndrome toxin 1 (TSST-1), produced by Staphylococcus aureus (including methicillin-resistant S. aureus), is a superantigenic toxin responsible for toxic shock syndrome as well as neonatal TSS-like exanthematous disease. TSST-1 exhibits its deleterious effects by leading to the abnormal proliferation of, e.g., Vbeta2+ T cells and overproduction of proinflammatory cytokines. In the present study we examined the inhibitory effect of a Chinese herbal extract, anisodamine, on TSST-1 using human peripheral blood mononuclear cells (PBMCs). Anisodamine inhibited the production of proinflammatory cytokines better than interleukin-10 (an anti-inflammatory cytokine). The inhibitory effect of anisodamine was greater than that of any tropane alkaloid examined. Anisodamine acted directly on both monocytes and T cells in human PBMCs, and the effect was confirmed at the transcriptional level. Inhibition of NF-kappaB activation was also demonstrated. In contrast, no significant inhibition of Vbeta2+ T-cell proliferation was observed. In mice injected with TSST-1, anisodamine treatment significantly decreased serum proinflammatory cytokine levels and prevented TSST-1-induced death. These results suggest that anisodamine specifically acts against the production of cytokines (inflammatory cytokines in particular) and not against Vbeta2+ T-cell proliferation and that anisodamine may have a beneficial effect on TSST-1-associated disease.