Effects of methyl mercury on protein synthesis in vitro

Toxic Effects of Lead Exposure on Freshwater Climbing Perch, Anabas testudineus, and Bioremediation Using Ocimum sanctum Leaf Powder

The acute and chronic toxicity of lead to Anabas testudineus was determined in this study using static replacement bioassay testing. During the chronic toxicity studies, an experiment on the bioremediation of lead toxicity using Ocimum sanctum leaf powder was conducted. The 96 h LC50 values of lead for Anabas testudineus was 1.08 mg/L. Different biomarkers, such as the hepatosomatic index, gonadosomatic index, and fecundity, were significantly lower in fish subjected to 10% and 20% of the 96 h LC50 values of lead, compared to controls. The 45-day chronic exposure of fish to lead concentrations of 0.2 mg/L and above significantly lowered the number of total RBC, hemoglobin content, HCT (%), plasma protein, and cholesterol while decreasing the level of total WBC, plasma glucose, creatinine, serum AST and serum ALT. The leaf powder of Ocimum sanctum plays a significant role in ameliorating lead toxicity.

Disease-associated metabolic pathways affected by heavy metals and metalloid

Increased exposure to environmental heavy metals and metalloids and their associated toxicities has become a major threat to human health. Hence, the association of these metals and metalloids with chronic, age-related metabolic disorders has gained much interest. The underlying molecular mechanisms that mediate these effects are often complex and incompletely understood. In this review, we summarize the currently known disease-associated metabolic and signaling pathways that are altered following different heavy metals and metalloids exposure, alongside a brief summary of the mechanisms of their impacts. The main focus of this study is to explore how these affected pathways are associated with chronic multifactorial diseases including diabetes, cardiovascular diseases, cancer, neurodegeneration, inflammation, and allergic responses upon exposure to arsenic (As), cadmium (Cd), chromium (Cr), iron (Fe), mercury (Hg), nickel (Ni), and vanadium (V). Although there is considerable overlap among the different heavy metals and metalloids-affected cellular pathways, these affect distinct metabolic pathways as well. The common pathways may be explored further to find common targets for treatment of the associated pathologic conditions.

The toxicology of mercury: Current research and emerging trends

Mercury (Hg) is a persistent bio-accumulative toxic metal with unique physicochemical properties of public health concern since their natural and anthropogenic diffusions still induce high risk to human and environmental health. The goal of this review was to analyze scientific literature evaluating the role of global concerns over Hg exposure due to human exposure to ingestion of contaminated seafood (methyl-Hg) as well as elemental Hg levels of dental amalgam fillings (metallic Hg), vaccines (ethyl-Hg) and contaminated water and air (Hg chloride). Mercury has been recognized as a neurotoxicant as well as immunotoxic and designated by the World Health Organization as one of the ten most dangerous chemicals to public health. It has been shown that the half-life of inorganic Hg in human brains is several years to several decades. Mercury occurs in the environment under different chemical forms as elemental Hg (metallic), inorganic and organic Hg. Despite the raising understanding of the Hg toxicokinetics, there is still fully justified to further explore the emerging theories about its bioavailability and adverse effects in humans. In this review, we describe current research and emerging trends in Hg toxicity with the purpose of providing up-to-date information for a better understanding of the kinetics of this metal, presenting comprehensive knowledge on published data analyzing its metabolism, interaction with other metals, distribution, internal doses and targets, and reservoir organs.

Dysfunctions of the translational machinery in digestive glands of mussels exposed to mercury ions

Mercury is an element naturally occurring in the biosphere, but is also released into the environment by human activities, such as mining, smelting, and industrial discharge. Mercury is a biologically harmful element and any exposure of living organisms mainly due to contamination, can cause severe or even lethal side effects. In every form detected, elemental, inorganic, or organic, mercury exhibits toxicity associated with induced oxidative stress. Although the genotoxicity of mercury has been well demonstrated in mussels, little is known about its toxic effects on the translational machinery at the molecular level. To investigate possible effects, we exposed the common mussel Mytilus galloprovincialis in seawater supplemented by 30 μg/L Hg²⁺ for 15 days. We observed that Hg²⁺ was significantly accumulated in the digestive glands of mussels, reaching a level around 80 μg/g tissue (dry weight) at the 15th day of exposure. Exposure of mussels to Hg²⁺ resulted in failure of redox homeostasis, as reflected on lipid peroxidation levels and superoxide dismutase activity in glands, and micronucleus frequency in gills. Extracts from digestive glands after 15-day exposure to Hg²⁺ exhibited decreased tRNA aminoacylation ability and, moreover, a 70% reduction in the ability of 40S ribosomal subunits to form the 48S initiation ribosomal complex. A similar reduction was detected in the ability of ribosomes to translocate peptidyl-tRNA from the A-site to the P-site, an observation coinciding with the notion that regulation of protein synthesis by Hg²⁺ mainly occurs at the initiation and elongation stages of translation. A-site binding, peptidyl transferase activity, and termination of peptide chain synthesis underwent less pronounced but measurable reductions, a finding which explains why poly(Phe)-synthesis in ribosomes isolated from exposed mussels is reduced by 70%. In conclusion, Hg²⁺ apart from being a genotoxic ion acts as a modulator of protein synthesis in mussels, an observation probably related with its ability to induce oxidative stress.

The toxicology of mercury and its compounds

A concentrated review on the toxicology of inorganic mercury together with an extensive review on the neurotoxicology of methylmercury is presented. The challenges of using inorganic mercury in dental amalgam are reviewed both regarding the occupational exposure and the possible health problems for the dental patients. The two remaining "mysteries" of methylmercury neurotoxicology are also being reviewed; the cellular selectivity and the delayed onset of symptoms. The relevant literature on these aspects has been discussed and some suggestions towards explaining these observations have been presented.

Methylmercury tolerance is associated with the humoral stress factor gene Turandot A

Methylmercury (MeHg) is an environmental neurotoxicant that targets the developing nervous system. In an effort to understand mechanisms of MeHg toxicity we have identified candidate genes that confer tolerance to MeHg using a Drosophila model. Whole genome transcript profiling of developing larval brains of MeHg-tolerant and non-tolerant flies has identified Turandot A (TotA) as a potential MeHg tolerance gene. TotA is a secreted humoral stress response factor in Drosophila that is a direct target of conserved innate immunity signaling pathways. Here we characterize TotA expression in newly generated isogenic lines (isolines) of flies derived from our previously established MeHg-tolerant and non-tolerant populations. TotA mRNA transcript and protein expression is seen to be higher in the tolerant isolines than the non-tolerant lines. Elevated TotA expression in the tolerant lines was seen to span all the larval developmental stages pointing toward a difference in the TotA gene regulation between the MeHg tolerant and non-tolerant strains. We show that TotA is most highly expressed in the fat body (liver equivalent) and is selectively upregulated in the fat body of tolerant flies relative to brain and gut tissues. Fat body-specific transgenic expression of TotA invokes MeHg tolerance as seen by enhanced development of flies reared on MeHg food. In addition, cell based assays show that high TotA expressing C6 cells are more tolerant to MeHg than the low TotA expressing S2 cells. Knockdown of TotA in the C6 cells trends toward a reduction in MeHg tolerance. Identification of TotA as a MeHg tolerance gene suggests a role for conserved cytokine/immune signaling pathways in modulating MeHg toxicity.

Hormetic effects of acute methylmercury exposure on grp78 expression in rat brain cortex

This study aims to explore the expression of GRP78, a marker of endoplasmic reticulum (ER) stress, in the cortex of rat brains acutely exposed to methylmercury (MeHg). Thirty Sprague-Dawley (SD) rats were randomly divided into six groups, and decapitated 6 hours (h) after intraperitoneal (i.p.) injection of MeHg (2, 4, 6, 8 or 10 mg/kg body weight) or normal saline. Protein and mRNA expression of Grp78 were detected by western blotting and real-time PCR, respectively. The results showed that a gradual increase in GRP78 protein expression was observed in the cortex of rats acutely exposed to MeHg (2, 4 or 6 mg/kg). Protein levels peaked in the 6 mg/kg group (p < 0.05 vs. controls), decreased in the 8 mg/kg group, and bottomed below the control level in the 10 mg/kg group. Parallel changes were noted for Grp78 mRNA expression. It may be implied that acute exposure to MeHg induced hormetic dose-dependent changes in Grp78 mRNA and protein expression, suggesting that activation of ER stress is involved in MeHg-induced neurotoxicity. Low level MeHg exposure may induce GRP78 protein expression to stimulate endogenous cytoprotective mechanisms.

Levels of transaminases, alkaline phosphatase, and protein in tissues of Clarias gariepienus fingerlings exposed to sublethal concentrations of cadmium chloride

The freshwater fish, Clarias gariepienus fingerlings, were exposed to sublethal concentrations (1.7 and 3.4 mg/L) of cadmium chloride for 12 days. Aspartate aminotransferase (AAT), alanine aminotransferase (ALT), alkaline phosphatase (ALP), and total protein levels were assayed in the gill, brain, and muscle of the fish at regular intervals of 6 and 12 days. The activities of AAT, ALT, and ALP of the treated fishes increased significantly in all the tissues compared with the control fish. Protein level in all the tissues showed a significant decrease in comparison to unexposed controls throughout the experimental periods. These results revealed that cadmium chloride effects the intermediary metabolism of C. gariepienus fingerlings and that the assayed enzymes can work as good biomarkers of contamination.

Study the effect of hexavalent chromium on some biochemical, citotoxicological and histopathological aspects of the Orechromis spp. fish

In nature chromium occurs in divalent, trivalent and hexavalent forms. Hexavalent chromium predominates over the trivalent form in natural waters. Knowledge of acute toxicity of a xenobiotic often can be very helpful in predicting and preventing acute damage to aquatic life in receiving waters as well as in regulating toxic waste discharges. The 96 h LC50 tests can be used to obtain toxicity data as rapidly and inexpensively as possible. In the present study, results showed a significant decrease in total glycogen, total lipids and total protein of liver, muscles and gills after 24 and 96 h of exposure to 96 h LC50 of hexavalent chromium (43.7 mg L(-1)). Cytotoxicity of metals is important because some metals are potential mutagens able to induce tumors in humans and experimental animals, the treatment of(Cr VI) at 43.7 mg L(-1) was for 24 and 96 h, respectively. DNA was extracted after treatment from brain and liver of the tested fish. Our results showed appearance of polymorphic bands at the long treatment interval (96 h) ofhexavalent chromium. However, these bands were not appeared when the fish were exposed to the (Cr VI) for the short treatment interval (24 h). Histopathological changes were seen in liver, muscle and gills sections of chromium-exposed fishes. The obtained results were discussed in the study research.

In vitro and in vivo effects of mercuric chloride on thymic endocrine activity, NK and NKT cell cytotoxicity, cytokine profiles (IL-2, IFN-gamma, IL-6): role of the nitric oxide-L-arginine pathway

Mercury (Hg2+) affects cell-mediated immunity, including thymulin production. Thymulin, a zinc-dependent thymic hormone synthesized by thymic epithelial cells (TECs), is involved in NK cell cytotoxicity and Th1 cytokine production (IL-2 and IFN-gamma), which in turn affect both NKT and classic NK spleen cell cytotoxicity. High doses of Hg2+ induce an inflammatory status, increased production of IL-6 and consequent Th1/Th2 imbalance as well as cell-mediated immune depression. The mechanisms by which Hg+ affects the cell-mediated immune response are still unclear. The nitric oxide (NO) pathway may be implicated. The aim of this work was to further explore its noxious role in innate and adaptive immunity and to study the possible role played by the NO pathway. Young Balb/c mice treated in vivo for 1 month with 1.0 mg HgCl2/kg b.w. showed low thymulin activity, depressed NO production (as measured by nitrite and nitrate plasma levels), impaired classic NK spleen cell cytotoxicity, decreased Th1 (IL-2 and IFN-gamma) cytokine profiles, and increased IL-6 production. In vitro, 10(-6) M of HgCl2 inhibited active thymulin kinetics, TEC proliferation, NKT cell cytotoxicity and Th1 cytokine production, whereas IL-6 increased. L-arginine restored thymulin activity, TEC proliferation, NKT cytotoxicity, cytokine profiles and nitrite and nitrate plasma levels both in vivo and in vitro. Since L-arginine is the substrate for NO production, it may compensate for the cell-mediated immune defect induced by HgCl2, via the arginine-NO-pathway. L-arginine is also able to reduce glomerular kidney IgG antibodies deposits induced by higher dose of HgCl2 administration.

Studies on the development of potential biomarkers for rapid assessment of copper toxicity to freshwater fish using Esomus danricus as model

Living in an environment that has been altered considerably by anthropogenic activities, fish are often exposed to a multitude of stressors including heavy metals. Copper ions are quite toxic to fish when concentrations are increased in environmental exposures often resulting in physiological, histological, biochemical and enzymatic alterations in fish, which have a great potential to serve as biomarkers. Esomus danricus was chosen as model in the present study and the metabolic rate, gill morphology, total glycogen, total protein, superoxide dismutase and catalase were critically evaluated. The 96h LC50 value was found to be 5.5mg/L (Cu as 1.402 mg/L). Fish groups were separately exposed to lethal (5.5 mg/L) and sub lethal concentrations (0.55 mg/L) of copper sulphate over a period of 96h to examine the subtle effects caused at various functional levels. Controls were also maintained simultaneously. Significant decrease in the metabolic rate (p<0.001) of the fish was observed in both the concentrations studied. Studies employing Automated Video Tracking System revealed gross changes in the architecture of gill morphology like loss, fusion, clubbing of secondary gill lamellae, and detachment of gill rakers following softening of gill shaft in fish under lethal exposures indicating reduced respiratory surface area. Biochemical profiles like total glycogen and total protein in gills and muscle of fish exposed to 5.5 mg/L showed appreciable decrease (p<0.05 to 0.001) from control. Significant inhibition of superoxide dismutase (60.83%), catalase (71.57%) from control was observed in fish exposed to 5.5 mg/L at the end of 96h exposure only. Interestingly, in fish exposed to 0.55 mg/L enzyme activity is not affected except for catalase. Toxic responses evaluated at various functional levels are more pronounced in fish exposed to 5.5 mg/L and these can serve as potential biomarkers for rapid assessment of acute copper toxicity in environmental biomonitoring.

Inhibition of amino acid transport and protein synthesis by HgCl2 and methylmercury in astrocytes: selectivity and reversibility

The previously reported observation that submicromolar concentrations of HgCl2 inhibit glutamate uptake reversibly in astrocytes, without effect on 2-deoxyglucose uptake, suggested that elemental mercury vapor, which is oxidized to mercuric mercury in the brain, might cause neurodegenerative change through the mediation of glutamate excitotoxicity. Here, selectivity is explored further by measuring the inhibition of other amino acid transporters and protein synthesis as a function of HgCl2 concentration. The properties of MeHgCl were compared under identical conditions, and some morphological correlates of function were examined. Inhibition of amino acid transport by HgCl2 was selective, whereas MeHgCl was nonselective. The 50% inhibitory concentrations of HgCl2 for uptake of alpha-aminoisobutyric acid by system A, uptake of alpha-aminoisobutyric acid or kynurenine by a system L variant, and uptake of gamma-aminobutyric acid were all two- to fourfold greater than that for uptake of glutamate. The submicromolar concentrations of HgCl2 that inhibited glutamate transport also inhibited protein synthesis, but in a rapidly reversible fashion, and elicited only discrete ultrastructural changes (heterochromatin, increased numbers of lysosomal bodies, and increased complexity of cell surface). In contrast, inhibition of protein synthesis by MeHgCl was acutely (1-h) irreversible and became marked only at concentrations higher than those that elicited gross morphologic change in the form of "bleb"-like swellings. The results lend support to the proposed excitotoxic mediation of mercury vapor neurotoxicity and reveal a sharp contrast between the effects of HgCl2 and MeHgCl on astrocytes.

A cell line with decreased sensitivity to the methyl mercury-induced stimulation of alpha-amanitin sensitive RNA synthesis in isolated nuclei

1. In nuclei isolated from cells of the B50 rat neuroblastoma line the stimulatory effect of methyl mercury on alpha-amanitin-sensitive RNA synthesis is very much reduced compared to the stimulatory effect in HeLa nuclei (see: Frenkel G. D. and Randles K. (1982) Specific stimulation of alpha-amanitin-sensitive RNA synthesis in isolated HeLa nuclei by methyl mercury. J. biol. Chem. 257, 6275-6279). 2. The stimulatory effect of another mercury compound, p-hydroxymercuribenzoate, was also much less pronounced in the B50 nuclei. 3. Similar results were obtained with nuclei isolated from B50 cells which had been induced to differentiate by exposure to dibutaryl cyclic AMP. 4. Nuclei isolated from cells of another rat neuroblastoma line (B35), and nuclei from cells of a human neuroblastoma line both exhibited levels of stimulation similar to that of HeLa nuclei. 5. The B50 and HeLa cells were also compared as to their sensitivity to other effects of methyl mercury.

The enhanced rate of transcription of methyl mercury-exposed DNA by RNA polymerase is not sufficient to explain the stimulatory effect of methyl mercury on RNA synthesis in isolated nuclei

Previous work demonstrated two stimulatory effects of methyl mercury on nucleic acid synthesis: (1) in isolated nuclei, methyl mercury stimulates RNA synthesis which is catalyzed by RNA polymerase II [Frenkel and Randles, J. Biol. Chem. 257, 6275-6279 (1982)]. (2) Brief exposure of purified DNA to methyl mercury increases the rate of its transcription by purified RNA polymerase II [Frenkel, Cain, and Chao, Biochem. Biophys. Res. Commun. 127, 849-856 (1985)]. The latter effect was considered as a possible mechanism of the former. Two lines of evidence are presented here which demonstrate that the latter effect is not a sufficient explanation for the former. (1) Mercuric perchlorate has been found to increase the rate of DNA transcription by purified polymerase and the template properties of the mercuric perchlorate-exposed DNA have been found to resemble those of methyl mercury-exposed DNA. Nevertheless, mercuric perchlorate has been shown not to stimulate RNA synthesis in isolated HeLa nuclei. (2) In isolated nuclei of the B50 rat neuroblastoma cell line, RNA synthesis has been found to be stimulated only minimally by methyl mercury. Nevertheless, RNA polymerase II purified from the B50 cells has been found to transcribe methyl mercury-exposed DNA at a higher rate than unexposed control DNA.

In vivo incorporation of [14C]leucine into brain protein of mice treated with methylmercury and thiol complexes of methylmercury

The effect of methylmercury and thiol complexes of methylmercury on inhibition of protein synthesis was evaluated. Mice were injected (i.p.) with the following treatments: methylmercuric chloride, methylmercury-glutathione, methylmercury-cysteinylglycine and control (vehicle) for 10 days. Ten animals from each group were injected with [14C]leucine 90 min prior to death. The brains were removed and the extracted protein was subjected to liquid scintillation analysis. Mice receiving the methylmercury and methylmercury-glutathione treatments exhibited significantly greater weight loss than the control while the methylmercury-cysteinylglycine treatment was not significantly different than the control. Incorporation of [14C]leucine into brain protein was significantly depressed in the methylmercury (81% of control) and the methylmercury-glutathione (79% of control) treatments. Protein synthesis in mice receiving the methylmercury-cysteinylglycine complex although not significantly different than the methylmercury treatments was only 92% of the control mice.

Decrease in protein phosphorylation in central and peripheral nervous tissues of methylmercury-treated rat

The protein phosphorylation in extracts of nervous tissues of rats acutely exposed to methylmercury chloride (seven daily injections of 10 mg methylmercury chloride/kg body weight) was examined. In the brain, the phosphorylating activity was dependent on cAMP and Mg2+. The effect of methylmercury on the phosphorylation of brain proteins, including tubulin and MAP-2, was hardly discernible. In peripheral nervous tissues such as the dorsal and ventral roots, sciatic nerves and dorsal root ganglia, the phosphorylating activity was dependent on Ca2+, and the maximal activity was obtained when the tissues were extracted in the presence of 1% Triton X-100. SDS-Polyacrylamide gel electrophoresis revealed that the major phosphorylated proteins in the peripheral tissues were myelin proteins. The effects of methylmercury were not uniform regarding protein species and tissues. The most marked changes were observed in sciatic nerves, in which phosphorylation of the 33 kDa, 28 kDa, 19 kDa, 18 kDa and 15 kDa proteins was significantly decreased in the symptomatic phase of intoxication.

Methylmercury effects on cell cycle kinetics

Methylmercury (MeHg) effects on cell cycle kinetics were investigated to help identify its mechanisms of action. Flow cytometric analysis of normal human fibroblasts grown in vitro in the presence of BrdU allowed quantitation of the proportion of cells in G1, S, G2 and the next G1 phase. This technique provides a rapid and easily performed method of characterizing phase lengths and transition rates for the complete cell cycle. After first exposure to MeHg the cell cycle time was lengthened due to a prolonged G1. At 3 microM MeHg the G1 phase length was 25% longer than the control. The G1/S transition rate was also decreased in a dose-related manner. Confluent cells exposed to MeHg and replated with MeHg respond in the same way as cells which have not been exposed to MeHg before replating. Cells exposed for long times to MeHg lost a detectable G1 effect, and instead showed an increase in the G2 percentage, which was directly related to MeHg concentration and length of exposure. After 8 days at 5 microM MeHg, 45% of the population was in G2. The G2 accumulation was reversible up to 3 days, but at 6 days the cells remained in G2 when the MeHg was removed. Cell counts and viability indicated that there was not a selective loss of cells from the MeHg. MeHg has multiple effects on the cell cycle which include a lengthened G1 and decreased transition probability after short term exposure of cycling cells, and a G2 accumulation after a longer term exposure. There were no detectable S phase effects. It appears that mitosis (the G2 accumulation) and probably synthesis of some macromolecules in G1 (the lengthened G1 and lowered transition probability) are particularly susceptible to MeHg.

Differential responses to methylmercury exposure and recovery in neuroblastoma and glioma cells and fibroblasts

The effects of methylmercury (MeHg) on cytoplasmic microtubules in cultured neuroblastoma cells, glioma cells, and fibroblasts were compared. Neuroblastoma cells appeared to be more sensitive to disruption of microtubules by MeHg than the glioma cells or fibroblasts; cellular concentrations of mercury after MeHg were also higher in neuroblastoma cells. Recovery of microtubule structure was monitored in cells after removal of MeHg; addition of the chelating dimercaptosuccinic acid (DMSA) increased reassembly of microtubules. During MeHg treatment and early recovery, microtubule integrity was dependent on cellular mercury concentrations. However, after prolonged DMSA exposure, mercury appeared to reenter the cell, without causing dissociation of microtubules.

Mercury chloride--and methyl mercury chloride--induced inhibition in NOR activity

Previous work has raised the question of whether mercury compounds inhibit the activity of the nucleolar organizing genes. For that reason we performed a cytophotodensitometric analysis on silver-stained interphase nuclei from human peripheral blood lymphocytes exposed to various concentrations of HgCl2 (0-150 X 10(-6) M), CH3HgCl (0-50 X 10(-6) M), and actinomycin D (0-0.1 microgram/ml). The last compound was used as a positive control. The cells were exposed to the compounds either during G1-early S phase, allowing recovery after the exposure, or from G1 until harvest; thus no recovery was allowed in this case. Like actinomycin D, both mercury compounds were shown to inhibit the NOR activity. This inhibition was furthermore more evident during the short exposure experiment. Our results do confirm that mercury compounds may, to a certain extent, influence some factors regulating the nucleolus-organizing activity.