Relative in vitro affinity of hepatic metallothionein for metals

Epigenetic alterations in bioaccumulators of cadmium: Lessons from mammalian kidneys and plants

Faced with unpredictable changes in global weather patterns, release and redistribution of metals through land erosion and water movements add to the increasing use of metals in industrial activities causing high levels of environmental pollution and concern to the health of all living organisms. Cadmium is released into the environment by smelting and mining, entering the food chain via contaminated soils, water, and phosphate fertilizers. Bioaccumulation of cadmium in plants represents the first major step into the human food chain and contributes to toxicity of several organs, especially the kidneys, where biomagnification of cadmium occurs over decades of exposure. Even in small amounts, cadmium brings about alterations at the molecular and cellular levels in eukaryotes through mutagenicity, molecular mimicry at metal binding sites and oxidative stress. The epigenome dictates expression of a gene's output through a number of regulatory steps involving chromatin remodeling, nucleosome unwinding, DNA accessibility, or nucleic acid modifications that ultimately impact the transcriptional and translational machinery. Several epigenetic enzymes exhibit zinc-dependence as zinc metalloenzymes and zinc finger proteins thus making them susceptible to deregulation through displacement by cadmium. In this review, we summarize the literature on cadmium-induced epigenetic mechanisms in mammalian kidneys and plants, compare similarities in the epigenetic defense between these bioaccumulators, and explore how future studies could advance our understanding of the cadmium-induced stress response and disruption to biological health.

Deadly excess copper

Higher eukaryotes' life is impossible without copper redox activity and, literally, every breath we take biochemically demonstrates this. However, this dependence comes at a considerable price to ensure target-oriented copper action. Thereto its uptake, distribution but also excretion are executed by specialized proteins with high affinity for the transition metal. Consequently, malfunction of copper enzymes/transporters, as is the case in hereditary Wilson disease that affects the intracellular copper transporter ATP7B, comes with serious cellular damage. One hallmark of this disease is the progressive copper accumulation, primarily in liver but also brain that becomes deadly if left untreated. Such excess copper toxicity may also result from accidental ingestion or attempted suicide. Recent research has shed new light into the cell-toxic mechanisms and primarily affected intracellular targets and processes of such excess copper that may even be exploited with respect to cancer therapy. Moreover, new therapies are currently under development to fight against deadly toxic copper.

Bioaccumulation of trace metals in edible terrestrial snails, Theba pisana and Otala spp., in a dumpsite area in Morocco and assessment of human health risks for consumers

This study assessed the bioaccumulation patterns of five trace metals (Cd, Cr, Co, Cu, and Zn) in two edible snail species, Theba pisana and Otala spp., collected from a dumpsite in Safi City, Morocco. The results indicated that bioaccumulation might be species-specific, as metal concentration profiles varied between the two snail species. Additionally, higher metal levels in the dumpsite snails confirmed their potential as bioindicators of trace metal pollution in terrestrial environments. However, the distribution of trace elements within the edible parts of the snails showed marked unevenness, with the viscera accumulating more metals than the foot. The study also evaluated the potential human health risks associated with consuming these snails. Trace metal levels in the edible parts exceeded most international safety thresholds. The estimated daily intakes (EDIs) of trace metals through snail consumption were below the provisional tolerable daily intakes (PTDIs) for both children and adults, suggesting that daily consumption is generally safe. Nonetheless, the hazard index (HI) indicated that children might face health risks from long-term consumption of contaminated snails (HI > 1), while adults are less likely to experience such complications (HI < 1). The total target carcinogenic risk (TTCR) was below 1E-04 for both children and adults, indicating negligible to acceptable carcinogenic risks for all consumer groups.

Zinc and manganese homeostasis closely interact in mammalian cells

Understanding the homeostatic interactions among essential trace metals is important for explaining their roles in cellular systems. Recent studies in vertebrates suggest that cellular Mn metabolism is related to Zn metabolism in multifarious cellular processes. However, the underlying mechanism remains unclear. In this study, we examined the changes in the expression of proteins involved in cellular Zn and/or Mn homeostatic control and measured the Mn as well as Zn contents and Zn enzyme activities to elucidate the effects of Mn and Zn homeostasis on each other. Mn treatment decreased the expression of the Zn homeostatic proteins metallothionein (MT) and ZNT1 and reduced Zn enzyme activities, which were attributed to the decreased Zn content. Moreover, loss of Mn efflux transport protein decreased MT and ZNT1 expression and Zn enzyme activity without changing extracellular Mn content. This reduction was not observed when supplementing with the same Cu concentrations and in cells lacking Cu efflux proteins. Furthermore, cellular Zn homeostasis was oppositely regulated in cells expressing Zn and Mn importer ZIP8, depending on whether Zn or Mn concentration was elevated in the extracellular milieu. Our results provide novel insights into the intricate interactions between Mn and Zn homeostasis in mammalian cells and facilitate our understanding of the physiopathology of Mn, which may lead to the development of treatment strategies for Mn-related diseases in the future.

Nondestructive Evaluation of Metal Bioaccumulation and Biochemical Biomarkers in Blood of Broad-Snouted Caiman (Caiman latirostris) from Northeastern Brasil

Studies on the bioaccumulation and toxicity of contaminants in Crocodylians are scarce. We evaluated alterations in concentrations of the nondestructive biomarkers butyrylcholinesterase (BChE), glutathione-S-transferase (GST), superoxide dismutase (SOD), and reduced glutathione (GSH), together with bioaccumulation of the metals iron (Fe), copper (Cu), zinc (Zn), manganese (Mn), chronium (Cr), aluminium (Al), and lead (Pb) in Caiman latirostris captured in Tapacurá Reservoir (TR; São Lourenço da Mata, Pernambuco, Brasil), in urbanized areas of Pernambuco State (UA; Brasil) and from the AME Brasil caiman farm (AF; Marechal Deodoro, Alagoas, Brasil); the latter was used as a potential reference with low levels of contamination. For metal analysis, 500 µL of blood was digested in 65% HNO3 and 30% H2 O2 . The samples were analyzed by inductively coupled plasma-optical emission spectrometry. For analysis of biomarkers, an aliquot of blood was centrifuged to obtain plasma in which biochemical assays were performed. Blood concentrations of metals analyzed in animals from AF were lower compared with TR and UA, confirming that animals from the caiman farm could be used as references with low levels of contamination. Iron, Cu, Mn, Al, and Pb exceeded toxic levels for other vertebrates in animals from TR and UA. Butyrylcholinesterase activity showed significant reduction in adults from UA and TR compared with AF. An increase in the activity of GST and GSH, in adults of TR and UA in relation to AF, was verified. Superoxide dismutase activity showed a significant reduction in adults of TR in relation to AF, and the concentrations of Cu and Mn were negatively correlated with SOD activity. Animals from UA and TR showed greater concentrations of the analyzed metals compared with reference animals, and changes in biomarkers were seen, confirming the potential of these nondestructive chemical and biological parameters in blood of C. latirostris for biomonitoring of pollution. Environ Toxicol Chem 2024;00:1-18. © 2024 SETAC.

Distribution of metals in the queen scallop Aequipecten opercularis during a transplant experiment: Metal rich granules as drivers of Pb bioaccumulation

The queen scallop Aequipecten opercularis accumulates high concentrations of lead (Pb) in its tissues, what has led to the interruption of this fishery in some extraction areas in Galicia (NW Spain). This study follows the dynamics of bioaccumulation of Pb and other metals in this species, the tissue distribution and the subcellular partitioning in selected organs, in order to understand the mechanisms that provoke the high Pb levels reached in its tissues and to increase our knowledge about metal bioaccumulation dynamics in this species. Scallops originating from a clean area were exposed in cages in two places in the Ría de Vigo (one shipyard and a less impacted location) and 10 individuals were collected every month over a three months period. Metal bioaccumulation and metal distribution in several organs, including gills, digestive gland, kidneys, muscle, gonad and remaining tissues, was studied. The results showed that scallops accumulated similar levels of Cd, Pb and Zn at both sites, while Cu and Ni showed an opposite pattern at the shipyard, with Cu concentrations increasing around 10 times and Ni decreasing during the 3 months of exposure. The preferential organs for metal accumulation were the kidneys for Pb and Zn, the digestive gland for Cd, both organs for Cu and Ni, and the muscle for As. Subcellular partitioning of kidney samples additionally showed an extraordinary ability to accumulate Pb and Zn at very high concentrations in kidney granules, a fraction that accounted for 30 to 60 % of Pb in soft-tissues. It is concluded that Pb bioaccumulation in kidney granules is the mechanism responsible for the high levels of Pb observed in this species.

Distribution, bioaccumulation, and trace element transfer among trophic levels in the southeastern Gulf of California

Our understanding of the trophic transfer of pollutants in marine subtropical ecosystems remains limited due to the complexity of their food webs. Thus, we aimed to evaluate Cd, Cu, Mn, Pb, and Zn sources, incorporation, and trophodynamics throughout the food web of the southeastern Gulf of California by stomach content analysis, stable isotope analysis, isotope mixing models, and trace element analysis in biological and environmental matrices. The food web comprised three main trophic guilds (TG1, TG2, and TG3). The bioaccumulation of Cd and Zn from seawater was efficient (> 1000) in TG2 and TG3. Bioaccumulation factor from sediment (BSAF >1) evidenced of Cd in all trophic guilds. In addition, non-trophic Cd relationships were identified in the food web. Based on the trophic magnification factor (TMF), Mn and Pb showed biodilution (TMFMn = 0.38; TMFPb = 0.16), while Cu and Zn exhibited biomagnification (TMFCu = 2.08; TMFZn = 3.31).

Multifunctional Metallothioneins as a Target for Neuroprotection in Parkinson's Disease

Parkinson's disease (PD) is characterized by motor symptoms based on a loss of nigrostriatal dopaminergic neurons and by non-motor symptoms which precede motor symptoms. Neurodegeneration accompanied by an accumulation of α-synuclein is thought to propagate from the enteric nervous system to the central nervous system. The pathogenesis in sporadic PD remains unknown. However, many reports indicate various etiological factors, such as oxidative stress, inflammation, α-synuclein toxicity and mitochondrial impairment, drive neurodegeneration. Exposure to heavy metals contributes to these etiopathogenesis and increases the risk of developing PD. Metallothioneins (MTs) are cysteine-rich metal-binding proteins; MTs chelate metals and inhibit metal-induced oxidative stress, inflammation and mitochondrial dysfunction. In addition, MTs possess antioxidative properties by scavenging free radicals and exert anti-inflammatory effects by suppression of microglial activation. Furthermore, MTs recently received attention as a potential target for attenuating metal-induced α-synuclein aggregation. In this article, we summarize MTs expression in the central and enteric nervous system, and review protective functions of MTs against etiopathogenesis in PD. We also discuss neuroprotective strategies for the prevention of central dopaminergic and enteric neurodegeneration by targeting MTs. This review highlights multifunctional MTs as a target for the development of disease-modifying drugs for PD.

The Impact of Zinc on Manganese Bioavailability and Cytotoxicity in HepG2 Cells

SCOPE

Despite their essentiality, several studies have shown that either manganese (Mn) or zinc (Zn) overexposure may lead to detrimental health effects. Although Mn is transported by some of the SLC family transporters that translocate Zn, the role of Zn in hepatocellular Mn transport and Mn-induced toxicity have yet to be fully characterized.

METHODS AND RESULTS

The human hepatoma cell line, HepG2, is utilized. Total cellular Mn and Zn amounts are determined after cells are treated with Zn 2 or 24 h prior to Mn incubation for additional 24 h with inductively coupled plasma-based spectrometry and labile Zn is assessed with the fluorescent probe FluoZin-3. Furthermore, mRNA expression of genes involved in metal homeostasis, and mechanistic endpoints associated with Mn-induced cytotoxicity are addressed. These results suggest that Zn protects against Mn-induced cytotoxicity and impacts Mn bioavailability to a great extent when cells are preincubated with higher Zn concentrations for longer duration as characterized by decreased activation of caspase-3 as well as lactate dehydrogenase (LDH) release.

CONCLUSIONS

Zn protects against Mn-induced cytotoxicity in HepG2 cells possibly due to decreased Mn bioavailability. Additionally, mRNA expression of metal homeostasis-related genes indicates possible underlying pathways that should to be addressed in future studies.

Transport of toxic metals in the bottom sediments and health risk assessment of Corbicula fluminea (Asiatic clam) collected from Laguna de Bay, Philippines

Laguna de Bay, the 3rd largest lake in Southeast Asia, is the most significant source of freshwater fish in the Philippines. With decades of unregulated discharge of industrial, domestic, and agricultural wastewaters into the lake, this study investigates the apportionment of heavy metals from the bottom sediments and its impact on the toxicity of Corbicula fluminea (Asiatic clam), a popular food item in the markets. The sediment samples from the western part of the lake contained higher Cd, Cu, Pb, and Zn and lower As and Cr concentrations compared to the eastern part. There were positive correlations for As, Cr, Cu, Pb, and Zn and negative correlations for Cd and Ni concentrations noted between sediments and C. fluminea. Human health risk associated with the consumption of C. fluminea collected from Laguna de Bay was attributed to the following heavy metals: Cu > As > Zn > Cd > Pb > Cr > Ni. Interestingly, the observed trend in toxicities of the shellfish was consistent with the transport phenomenon of heavy metals facilitated by the counterclockwise direction of the bottom current. This study strongly suggests the re-evaluation of the waste management plan in the industrial zones and policies regulating the sale of the shellfish harvest.

Role of hepcidin in oxidative stress and cell death of cultured mouse renal collecting duct cells: protection against iron and sensitization to cadmium

The liver hormone hepcidin regulates systemic iron homeostasis. Hepcidin is also expressed by the kidney, but exclusively in distal nephron segments. Several studies suggest hepcidin protects against kidney damage involving Fe2+ overload. The nephrotoxic non-essential metal ion Cd2+ can displace Fe2+ from cellular biomolecules, causing oxidative stress and cell death. The role of hepcidin in Fe2+ and Cd2+ toxicity was assessed in mouse renal cortical [mCCD(cl.1)] and inner medullary [mIMCD3] collecting duct cell lines. Cells were exposed to equipotent Cd2+ (0.5-5 μmol/l) and/or Fe2+ (50-100 μmol/l) for 4-24 h. Hepcidin (Hamp1) was transiently silenced by RNAi or overexpressed by plasmid transfection. Hepcidin or catalase expression were evaluated by RT-PCR, qPCR, immunoblotting or immunofluorescence microscopy, and cell fate by MTT, apoptosis and necrosis assays. Reactive oxygen species (ROS) were detected using CellROX™ Green and catalase activity by fluorometry. Hepcidin upregulation protected against Fe2+-induced mIMCD3 cell death by increasing catalase activity and reducing ROS, but exacerbated Cd2+-induced catalase dysfunction, increasing ROS and cell death. Opposite effects were observed with Hamp1 siRNA. Similar to Hamp1 silencing, increased intracellular Fe2+ prevented Cd2+ damage, ROS formation and catalase disruption whereas chelation of intracellular Fe2+ with desferrioxamine augmented Cd2+ damage, corresponding to hepcidin upregulation. Comparable effects were observed in mCCD(cl.1) cells, indicating equivalent functions of renal hepcidin in different collecting duct segments. In conclusion, hepcidin likely binds Fe2+, but not Cd2+. Because Fe2+ and Cd2+ compete for functional binding sites in proteins, hepcidin affects their free metal ion pools and differentially impacts downstream processes and cell fate.

Common carp exposed to binary mixtures of Cd(II) and Zn(II): A study on metal bioaccumulation and ion-homeostasis

The aquatic environment receives a wide variety of contaminants that interact with each other, influencing their mutual toxicity. Therefore, studies of mixtures are needed to fully understand their deleterious effects on aquatic organisms. In the present experiment, we aimed to assess the effects of Cd and Zn mixtures in common carp during a one-week exposure. The used nominal waterborne metal levels were 0.02, 0.05 and 0.10 µM for Cd and 3, 7.5 and 15 µM for Zn. Our results showed on the one hand a fast Cd increase and on the other hand a delayed Zn accumulation. In the mixture scenario an inhibition of Cd accumulation due to Zn was marked in the liver but temporary in the gills. For Zn, the delayed accumulation gives an indication of the efficient homeostasis of this essential metal. Between the different mixtures, a stimulation of Zn accumulation by Cd rather than an inhibition was seen in the highest metal mixtures. However, when compared to an earlier single Zn exposure, a reduced Zn accumulation was observed. Metallothionein gene expression was quickly activated in the analysed tissues suggesting that the organism promptly responded to the stressful situation. Finally, the metal mixture did not alter tissue electrolyte levels.

Role of metal speciation in the exposure medium on the toxicity, bioavailability and bio-reactivity of copper, silver, cadmium and zinc in the rainbow trout gut cell line (RTgutGC)

The role of metal speciation on metal bioavailability, bio-reactivity and toxicity at the fish intestine is poorly understood. To investigate these processes, we used an in vitro model of the rainbow trout (Oncorhynchus mykiss) intestine, the RTgutGC cell line. Cells were exposed to two essential metals (copper and zinc) and two non-essential metals (cadmium and silver) in a medium of well-defined composition, which allowed the determination of metal speciation in solution. Concentrations resulting in a 50% cell viability reduction (EC₅₀) were measured using a viability assay based on two endpoints: metabolic activity and membrane integrity. Metal bioavailability and bio-reactivity was studied at non-toxic (300 nM all metals) and toxic (EC₁₀; Ag-0.6, Cu-0.9, Cd-3, and Zn-9 μM) concentrations. Bioavailability (i.e. intracellular metal accumulation) was determined by ICP-MS, while bio-reactivity (i.e. induction of a metal specific transcriptional response) was determined by measuring the mRNA levels of a known biomarker of metal exposure (i.e. metallothionein) and of copper and zinc transporters (i.e. ATP7A and ZnT1). Dominant metal species in the exposure medium were Zn²⁺, CuHPO₄, CdCl⁺, and AgCl₂^- respectively for Zn, Cu, Cd, and Ag. The EC_50s showed the metal toxicity hierarchy: Ag > Cu > Cd > Zn. In RTgutGC cells, essential metal homeostasis was tightly regulated while non-essential metals accumulated more readily. Non-essential metals were also more bio-reactive inducing higher MT and ZnT1 mRNA levels. Taken together these findings indicate that metal toxicity in RTgutGC cannot solely be explained by extracellular metal speciation but requires the evaluation of metal bioavailability and bio-reactivity.

Metallothionein 2A gene polymorphisms in relation to diseases and trace element levels in humans

Human metallothioneins are a superfamily of low molecular weight intracellular proteins, whose synthesis can be induced by essential elements (primarily Zn and Cu), toxic elements and chemical agents, and stress-producing conditions. Of the four known isoforms in the human body MT2 is the most common. The expression of metallothioneins is encoded by a multigene family of linked genes and can be influenced by single nucleotide polymorphisms (SNPs) in these genes. To date, 24 SNPs in the MT2A gene have been identified with the incidence of about 1 % in various population groups, and three of them were shown to affect physiological and pathophysiological processes. This review summarises current knowledge about these three SNPs in the MT2A gene and their associations with element concentrations in the body of healthy and diseased persons. The most investigated SNP is rs28366003 (MT2A -5 A/G). Reports associate it with longevity, cancer (breast, prostate, laryngeal, and in paranasal sinuses), and chronic renal disease. The second most investigated SNP, rs10636 (MT2A +838G/C), is associated with breast cancer, cardiovascular disease, and type 2 diabetes. Both are also associated with several metal/metalloid concentrations in the organism. The third SNP, rs1610216 (MT2A -209A/G), has been studied for association with type 2 diabetes, cardiomyopathy, hyperglycaemia, and Zn concentrations. Metallothionein concentrations and MT2A polymorphisms have a potential to be used as biomarkers of metal exposure and clinical markers of a number of chronic diseases. This potential needs to be studied and verified in a large number of well-defined groups of participants (several hundreds and thousands) with a focus on particular physiological or pathological condition and taking into consideration other contributing factors, such as environmental exposure and individual genetic and epigenetic makeup.

Subcellular metal distributions and metallothionein associations in rough-toothed dolphins (Steno bredanensis) from Southeastern Brazil

Metals are subject to internal subcellular compartmentalization, altering their bioavailability. Thus, subcellular metal assessments are crucial in biomonitoring efforts. Metal distribution in three subcellular fractions (insoluble - ISF, thermolabile - TLF and thermostable - TSF) were determined by ICP-MS in Steno bredanensis specimens from Southeastern Brazil. Associations between metals, metallothionein (MT) and reduced glutathione (GSH) were also investigated. Differential metal-detoxification mechanisms were observed. MT detoxification was mostly noted for As, Cd, and Pb, while Cu, Cr, Hg, Ni, Se and Ti displayed lower MT-associations. Fe, Zn and Se, on the other hand, were poorly associated to MT, and mostly present in the ISF, indicating low bioavailability. This is the first report on subcellular Sn and Ti distribution in cetaceans and the first in this species in Brazil. Potential protective roles of essential metals against toxic elements are postulated. This study indicates that important biochemical detoxification information is obtained through subcellular fraction analyses in marine mammals.

Sublethal psychotropic pharmaceutical effects on the model organism Danio rerio: Oxidative stress and metal dishomeostasis

Contaminants of emerging concern have become an important environmental problem, especially pharmaceutically active compounds (PhACs), since, after use, these drugs return to the environment, contaminating aquatic ecosystems. Some may display the ability to bioaccumulate and biomagnify throughout the food chain, leading to potential environmental and human deleterious effects which are, however, still largely unknown. In this context, the aim of the present study was to evaluate the effect of two psychotropic drugs commonly found in the environment, carbamazepine (CBZ) and clonazepam (CZP), both isolated and co-administrated, on oxidative stress biomarkers and essential metal homeostasis in Danio rerio fish specimens. No studies are available to data in this regard concerning CZP effects on fish. Reduced Glutathione (GSH), Metallothionein (MT), Catalase (CAT) and Glutathione S-Transferase (GST) were determined, as well as essential metals in fish liver, kidney and brains. Significant oxidative stress effects were observed for several biomarkers, where brain GST activity was the most affected, mainly with regard to CBZ exposure, while GST and CAT activity in the liver were downregulated in the co-administration mixture. In addition, dishomeostasis of several essential elements was detected in all analyzed organs, with a synergistic action of CBZ and CZP in brain, decreasing basal Mg, Al, K, Fe, Co, Ni and Cu levels in this organ, the target site for these drugs in humans. The brain organ was the most affected as observed by altered GST activity and metal dyshomeostasis concerning exposure to both compounds. These compounds, thus, present health risks to the aquatic biota, due to the evidenced deleterious effects and their constant release into the environment due to widespread use. Steps are needed to implement adequate legislation for risk analysis and decision-making in order to mitigate the effects of these emerging contaminants on aquatic ecosystem health.

Altered zinc balance in the Atp7b-/- mouse reveals a mechanism of copper toxicity in Wilson disease

Wilson disease (WD) is an autosomal recessive disorder caused by mutation in the ATP7B gene that affects copper transport in the body. ATP7B mutation damages copper transporter function, ultimately resulting in excessive copper accumulation and subsequent toxicity in both the liver and brain. Mechanisms of copper toxicity, however, are not well defined. The Atp7b-/- mouse model is well-characterized and presents a hepatic phenotype consistent with WD. In this study, we found that the untreated Atp7b-/- mice accumulate approximately 2-fold excess hepatic zinc compared to the wild type. We used targeted transcriptomics and proteomics to analyze the molecular events associated with zinc and copper accumulation in the Atp7b-/- mouse liver. Altered gene expression of Zip5 and ZnT1 zinc transporters indicated a transcriptional homeostatic response, while increased copper/zinc ratios associated with high levels of metallothioneins 1 and 2, indicated altered Zn availability in cells. These data suggest that copper toxicity in Wilson disease includes effects on zinc-dependent proteins. Transcriptional network analysis of RNA-seq data reveals an interconnected network of transcriptional activators with over-representation of zinc-dependent and zinc-responsive transcription factors. In the context of previous research, these observations support the hypothesis that mechanisms of copper toxicity include disruption of intracellular zinc distribution in liver cells. The translational significance of this work lies in oral zinc supplementation in treatment for WD, which is thought to mediate protective effects through the induction of metallothionein synthesis in the intestine. This work indicates broader impacts of altered zinc-copper balance in WD, including global transcriptional responses and altered zinc balance in the liver.

Molecular Neuroprotection Induced by Zinc-Dependent Expression of Hepatitis C-Derived Protein NS5A Targeting Kv2.1 Potassium Channels

We present the design of an innovative molecular neuroprotective strategy and provide proof-of-concept for its implementation, relying on the injury-mediated activation of an ectopic gene construct. As oxidative injury leads to the intracellular liberation of zinc, we hypothesize that tapping onto the zinc-activated metal regulatory element (MRE) transcription factor 1 system to drive expression of the Kv2.1-targeted hepatitis C protein NS5A (hepatitis C nonstructural protein 5A) will provide neuroprotection by preventing cell death-enabling cellular potassium loss in rat cortical neurons in vitro. Indeed, using biochemical and morphologic assays, we demonstrate rapid expression of MRE-driven products in neurons. Further, we report that MRE-driven NS5A expression, induced by a slowly evolving excitotoxic stimulus, functionally blocks injurious, enhanced Kv2.1 potassium whole-cell currents and improves neuronal viability. We suggest this form of "on-demand" neuroprotection could provide the basis for a tenable therapeutic strategy to prevent neuronal cell death in neurodegeneration.

Synergistic cellular responses to heavy metal exposure: A minireview

BACKGROUND

Metal-responsive transcription factor 1 (MTF-1) induces the expression of metallothioneins (MTs) which bind and sequester labile metal ions. While MTF-1 primarily responds to excess metal exposure, additional stress response mechanisms are activated by excess metals. Evidence suggests potential crosstalk between responses mediated by MTF-1 and stress signaling enhances cellular tolerance to metal exposure.

SCOPE OF REVIEW

This review aims to summarize the current understanding of interaction between the stress response mediated by MTF-1 and other cellular mechanisms, notably the nuclear factor κB (NF-κB) and heat shock response (HSR).

MAJOR CONCLUSIONS

Crosstalk between MTF-1 mediated metal response and NF-κB signaling or HSR can modulate expression of stress proteins in response to metal exposure via effects on precursor signals or direct interaction of transcriptional activators. The interaction between stress signaling pathways can enhance cell survival and tolerance through a unified response system.

GENERAL SIGNIFICANCE

Elucidating the interactions between MTF-1 and cell stress response mechanisms is critical to a comprehensive understanding of metal-based cellular effects. Co-activation of HSR and NF-κB signaling allows the cell to detect metal contamination in the environment and improve survival outcomes.

Thermodynamics of Pb(ii) and Zn(ii) binding to MT-3, a neurologically important metallothionein

Isothermal titration calorimetry (ITC) was used to quantify the thermodynamics of Pb(2+) and Zn(2+) binding to metallothionein-3 (MT-3). Pb(2+) binds to zinc-replete Zn7MT-3 displacing each zinc ion with a similar change in free energy (ΔG) and enthalpy (ΔH). EDTA chelation measurements of Zn7MT-3 and Pb7MT-3 reveal that both metal ions are extracted in a tri-phasic process, indicating that they bind to the protein in three populations with different binding thermodynamics. Metal binding is entropically favoured, with an enthalpic penalty that reflects the enthalpic cost of cysteine deprotonation accompanying thiolate ligation of the metal ions. These data indicate that Pb(2+) binding to both apo MT-3 and Zn7MT-3 is thermodynamically favourable, and implicate MT-3 in neuronal lead biochemistry.

Regulatory effects of zinc on cadmium-induced cytotoxicity in chronic inflammation

OBJECTIVES

Zinc (Zn) has major effects on immune system activation while Cadmium (Cd) has anti-inflammatory and anti-proliferative effects in several chronic inflammatory contexts. The aim of this work was to investigate by which mechanisms Zn could compete with Cd and eventually counteract its deleterious effects. Rheumatoid arthritis (RA) synoviocytes exposed to cytokines were used as a model of chronic inflammation; osteoarthritis (OA) synoviocytes were used as control.

METHODS

Cell/medium fractionation constants were analyzed for different metals by inductively-coupled-plasma mass-spectrometry by comparison to the 70Zn spike. Interleukin-17 (IL-17) and tumor necrosis factor-alpha (TNF-α) were used to mimic inflammation. Gene expression of ZIP-8 importer, metallothioneins-1 (MT-1s) and the ratio between metalloprotease-3 and the tissue inhibitor of metalloproteinases (MMP-3)/TIMP-1) were evaluated after pre-exposure to cytokines and Cd, with or without the addition of exogenous Zn (0.9 ppm). Cell viability was measured by neutral red assay and IL-6 production by ELISA.

RESULTS

Synoviocytes selectively absorbed and retained Cd in comparison to Zn. Metal import increased with IL-17/TNF-α exposure, through the enhanced ZIP-8 expression. Zn did not modify ZIP-8 expression, while Cd reduced it (p<0.05). Zn induced a reduction of Cd-induced MT-1s expression, in particular of MT-1X (3-fold), and subsequently the final intra-cellular content of Cd. By reducing Cd accumulation in cells, Zn reversed Cd anti-proliferative and anti-inflammatory effects but preserved the low MMP-3/TIMP-1 ratio induced by Cd, which was enhanced by inflammatory conditions.

CONCLUSION

Zinc counteracts the deleterious effect of Cd by reducing its import and accumulation in the cell, without the reactivation of destructive pathways such as MMPs.

Trace elements in loggerhead turtles (Caretta caretta) stranded in mainland Portugal: Bioaccumulation and tissue distribution

Pollution is among the most significant threats that endanger sea turtles worldwide. Waters off the Portuguese mainland are acknowledged as important feeding grounds for juvenile loggerheads. However, there is no data on trace element concentrations in marine turtles occurring in these waters. We present the first assessment of trace element concentrations in loggerhead turtles (Caretta caretta) occurring off the coast of mainland Portugal. Also, we compare our results with those from other areas and discuss parameters that may affect element concentrations. Trace element concentrations (As, Cd, Cu, Pb, Mn, Hg, Ni, Se, Zn) were determined in kidney, liver and muscle samples from 38 loggerheads stranded between 2011 and 2013. As was the only element with higher concentrations in muscle (14.78 μg g^-1 ww) than in liver or kidney. Considering non-essential elements, Cd presented the highest concentrations in kidney (34.67 μg g^-1) and liver (5.03 μg g^-1). Only a weak positive link was found between renal Cd and turtle size. Inter-elemental correlations were observed in both liver and kidney tissues. Hepatic Hg values (0.30 ± 0.03 μg g^-1) were higher than values reported in loggerheads in the Canary Islands but lower than in Mediterranean loggerheads. Cd concentrations in the present study were only exceeded by values found in turtles from the Pacific. Although many endogenous and exogenous parameters related with complex life cycle changes and wide geographic range may influence trace element accumulation, the concentrations of Cd are probably related to the importance of crustaceans in loggerhead diet in the Portuguese coast.

A Simple Metallothionein-Based Biosensor for Enhanced Detection of Arsenic and Mercury

Metallothioneins (MTs) are a family of cysteine-rich proteins whose biological roles include the regulation of essential metal ions and protection against the harmful effects of toxic metals. Due to its high affinity for many toxic, soft metals, recombinant human MT isoform 1a was incorporated into an electrochemical-based biosensor for the detection of As³⁺ and Hg²⁺. A simple design was chosen to maximize its potential in environmental monitoring and MT was physically adsorbed onto paper discs placed on screen-printed carbon electrodes (SPCEs). This system was tested with concentrations of arsenic and mercury typical of contaminated water sources ranging from 5 to 1000 ppb. The analytical performance of the MT-adsorbed paper discs on SPCEs demonstrated a greater than three-fold signal enhancement and a lower detection limit compared to blank SPCEs, 13 ppb for As³⁺ and 45 ppb for Hg²⁺. While not being as low as some of the recommended drinking water limits, the sensitivity of the simple MT-biosensor would be potentially useful in monitoring of areas of concern with a known contamination problem. This paper describes the ability of the metal binding protein metallothionein to enhance the effectiveness of a simple, low-cost electrochemical sensor.

Levels of heavy metals and their binding protein metallothionein in type 2 diabetics with kidney disease

Hyperglycemia, a major metabolic disturbance present in diabetes, promotes oxidative stress. Activation of antioxidant defense is an important mechanism to prevent cell damage. Levels of heavy metals and their binding proteins can contribute to oxidative stress. Antiradical capacity and levels of metallothionein (MT), metals (zinc and copper), and selected antioxidants (bilirubin, cysteine, and glutathione) were determined in 70 type 2 diabetes mellitus (T2DM) subjects and 80 healthy subjects of Caucasian origin. Single nucleotide polymorphism (rs28366003) in MT gene was detected. Antiradical capacity, conjugated bilirubin, and copper were significantly increased in diabetics, whereas MT and glutathione were decreased. Genotype AA of rs28366003 was associated with higher zinc levels in the diabetic group. The studied parameters were not influenced by renal function. This is the first study comprehensively investigating differences in MT and metals relevant to oxidative stress in T2DM. Ascertained differences indicate increased oxidative stress in T2DM accompanied by abnormalities in non-enzymatic antioxidant defense systems.

Arsenic Induction of Metallothionein and Metallothionein Induction Against Arsenic Cytotoxicity

Human exposure to arsenic (As) can lead to oxidative stress that can become evident in organs such as the skin, liver, kidneys and lungs. Several intracellular antioxidant defense mechanisms including glutathione (GSH) and metallothionein (MT) have been shown to minimize As cytotoxicity. The current review summarizes the involvement of MT as an intracellular defense mechanism against As cytotoxicity, mostly in blood. Zinc (Zn) and selenium (Se) supplements are also proposed as a possible remediation of As cytotoxicity. In vivo and in vitro studies on As toxicity were reviewed to summarize cytotoxic mechanisms of As. Intracellular antioxidant defense mechanisms of MT are linked in relation to As cytotoxicity. Arsenic uses a different route, compared to major metal MT inducers such as Zn, to enter/exit blood cells. A number of in vivo and in vitro studies showed that upregulated MT biosynthesis in blood components are related to toxic levels of As. Despite the cysteine residues in MT that aid to bind As, MT is not the preferred binding protein for As. Nonetheless, intracellular oxidative stress due to As toxicity can be minimized, if not eliminated, by MT. Thus MT induction by essential metals such as Zn and Se supplementation could be beneficial to fight against As toxicity.

Induction of Metallothionein

Metallothioneins (MTs) are cysteine-rich metal-binding proteins. These proteins play a pivotal role in heavy metal homeostasis and have been widely studied by biochemists, toxicologists, nutritional scientists, and molecular biologists. It is well established that MTs are inducible proteins. They are normally synthesized at low basal levels, but exposure to a wide variety of heavy metals and many organic compounds will dramatically increase synthesis of MTs. This paper summarizes MT induction by reviewing the chemicals that induce these proteins, the molecular mechanism involved in this induction, and the relationship between MT induction and biological function.

Metallothionein does not sequester arsenic(III) ions in condition of acute arsenic toxicity

The major cause of toxicity of trivalent arsenicals is due to their interaction with the sulfhydryl groups in proteins. Because of its high content, Metallothionein (MT) provides one of the most favorable conditions for the binding of As(III) ions to it. MT has long been anticipated for providing resistance in case of arsenic (As) toxicity with similar mechanism as in case of cadmium toxicity. The present study investigates whether the sequestration of As ions by MT is one of the mechanisms in providing protection against acute arsenic toxicity. A rat model study on the metal stoichiometric analysis of MT1 isoform isolated from the liver of arsenic treated, untreated and zinc treated animals has been carried out using the combination of particle induced X-ray emission (PIXE) and electrospray ionisation mass spectrometry (ESI-MS). The results revealed the absence of arsenic bound MT1 in the samples isolated from arsenic treated animals. Although, both Cu and Zn ions were present in MT1 samples isolated from all the treatment groups. Moreover, only partially metallated MT1 with varying number of Zn ions were observed in all the groups. These results suggest that the role of MT during acute arsenic toxicity is different from its already established role in case of cadmium toxicity.

The Tetrahymena metallothionein gene family: twenty-one new cDNAs, molecular characterization, phylogenetic study and comparative analysis of the gene expression under different abiotic stressors

Background

Ciliate metallothioneins (MTs) are included in family 7 of the MT superfamily. This family has been divided into two main subfamilies: 7a or CdMTs and 7b or CuMTs. All ciliate MTs reported have been isolated from different Tetrahymena species and present unique features with regard to standard MTs. Likewise, an expression analysis has been carried out on some of MT genes under metal stress, corroborating their classification into two subfamilies.

Results

We isolated 21 new cDNAs from different Tetrahymena species to obtain a wider view of the biodiversity of these conserved genes. Structural analysis (cysteine patterns) and an updated phylogenetic study both corroborated the previous classification into two subfamilies. A new CuMT from a Tetrahymena-related species Ichthyophthirius multifiliis was also included in this general analysis. We detected a certain tendency towards the presentation of a CdMT tri-modular structure in Borealis group species with respect to Australis group. We report for the first time a semi-complete paralog duplication of a CdMT gene originating a new CdMT gene isoform in T. malaccensis. An asymmetry of the codon usage for glutamine residues was detected between Cd- and CuMTs, and the phylogenetic implications are discussed. A comparative gene expression analysis of several MT genes by qRT-PCR revealed differential behavior among them under different abiotic stressors in the same Tetrahymena species.

Conclusions

The Tetrahymena metallothionein family represents a quite conserved protein structure group with unique features with respect to standard MTs. Both Cd- and CuMT subfamilies present very defined and differentiated characteristics at several levels: cysteine patterns, modular structure, glutamine codon usage and gene expression under metal stress, among others. Gene duplication through evolution seems to be the major genetic mechanism for creating new MT gene isoforms and increasing their functional diversity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2658-6) contains supplementary material, which is available to authorized users.

How does the metallothionein induction in bivalves meet the criteria for biomarkers of metal exposure?

Metallothionein (MT) concentrations in the whole soft tissue or in a particular tissue of bivalves have widely been used in ecotoxicological studies and biomonitoring programmes. This approach is based on the reported results on the enhancement of MT induction in bivalves in response to metal exposure. The validity of using MT induction as a biomarker is briefly assessed in the present study. The sensitivity of MT induction in these organisms is questionable due to the high basal MT level as well as the high natural variability related to the effects of a number of biotic and abiotic factors, which are not well described yet. Moreover, the relationship between exposure to metals, the toxic effects of that exposure, and the appearance of MT in soft tissue, is not well characterized. A variety of factors may influence the appearance and distribution of MT: 1) the uneven distribution of metals in particular portions of the soft tissue and in particular subcellular compartments; 2) pre-exposure to metals, perhaps at non-toxic levels; 3) metal-metal competition and metal-protein interactions; and 4) tissue-specific induction, functions, and isoforms of MT. Therefore, attention is required when using MT induction in bivalves for assessment of metal exposure or consequent toxic effects. The MT concentration can be a reliable indicator only when it is considered in relation with metal uptake kinetics and subcellular partitioning while specifying the isoform of MT synthesised and considering various confounding factors. The kinetic turnover of MT may provide useful information on metal exposure and biological effects since it covers both the synthesis and breakdown of MT as well as the chemical species of metals accumulated and MT.

Pollutant exposure at wastewater treatment works affects the detoxification organs of an urban adapter, the Banana Bat

The Banana Bat, Neoromicia nana, exploits pollution-tolerant chironomids at wastewater treatment works (WWTWs). We investigated how pollutant exposure impacts the detoxification organs, namely the liver and kidney of N. nana. (i) We performed SEM-EDS to quantify metal content and mineral nutrients, and found significant differences in essential metal (Fe and Zn) content in the liver, and significant differences in Cu and one mineral nutrient (K) in the kidneys. (ii) We performed histological analysis and found more histopathological lesions in detoxification organs of WWTW bats. (iii) We calculated hepatosomatic/renalsomatic indices (HSI/RSI) to investigate whole organ effects, and found significant increases in organ size at WWTWs. (iv) We quantified metallothionein 1E (MT1E), using Western Blot immunodetection. Contrary to predictions, we found no significant upregulation of MT1E in bats at WWTWs. Ultimately, N. nana exploiting WWTWs may suffer chronic health problems from sub-lethal damage to organs responsible for detoxifying pollutants.

Fluorescence-tagged metallothionein with CdTe quantum dots analyzed by the chip-CE technique

ABSTRACT

Quantum dots (QDs) are fluorescence nanoparticles (NPs) with unique optic properties which allow their use as probes in chemical, biological, immunological, and molecular imaging. QDs linked with target ligands such as peptides or small molecules can be used as tumor biomarkers. These particles are a promising tool for selective, fast, and sensitive tagging and imaging in medicine. In this study, an attempt was made to use QDs as a marker for human metallothionein (MT) isoforms 1 and 2. Four kinds of CdTe QDs of different sizes bioconjugated with MT were analyzed using the chip-CE technique. Based on the results, it can be concluded that MT is willing to interact with QDs, and the chip-CE technique enables the observation of their complexes. It was also observed that changes ranging roughly 6-7 kDa, a value corresponding to the MT monomer, depend on the hydrodynamic diameters of QDs; also, the MT sample without cadmium interacted stronger with QDs than MT saturated with cadmium. Results show that MT is willing to interact with smaller QDs (blue CdTe) rather than larger ones QDs (red CdTe). To our knowledge, chip-CE has not previously been applied in the study of CdTe QDs interaction with MT.

GRAPHICAL ABSTRACT

Oxidative stress, bioelements and androgen status in testes of rats subacutely exposed to cadmium

The objective of our study was to examine testicular toxicity of cadmium (Cd), focusing on oxidative stress (OS), essential metals and androgenic status and morphological changes. Male Wistar rats [controls and four Cd-subgroups (n = 6) organized according to the exposure (1, 3, 10 and 21 days)] were intraperitoneally (i.p.) treated with 1 mg CdCl2/kg/day. Testicular Cd deposition was noticed from the 1st day. After 10 and 21 days, copper (Cu) and iron (Fe) increased by 60-109% and 43-67%, respectively, while zinc (Zn) decreased by 24-33%. During 1-21 days of the exposure, decrease in testicular total superoxide dismutase (SOD) and total glutathione-s-transferase (GST) activities occurred gradually by 30-78% and 15-84%, respectively, while superoxide anion radical (O2(-)) increased gradually by 114-271%. After 10-21 days, decrease in testicular catalase (CAT) activity appeared by 13-31%. After 21 days, malondialdehyde (MDA) decreased by 44% and the ratio of oxidized glutathione/reduced glutathione (GSSG/GSH) increased by 130% in testes of the rats exposed to Cd. Additionally, decreased testicular testosterone level and the relative testes mass, along with induced microscopic and macroscopic changes were occured, what can be explained as the consequence of instantly developed OS, impaired essential metals status and Cd testicular deposition.

Lead in the soil-mulberry (Morus alba L.)-silkworm (Bombyx mori) food chain: translocation and detoxification

The translocation of lead (Pb) in the soil-mulberry-silkworm food chain and the process of Pb detoxification in the mulberry-silkworm chain were investigated. The amount of Pb in mulberry, silkworm, feces and silk increased in a dose-responsive manner to the Pb contents in the soils. Mulberry roots sequestered most of the Pb, ranging from 230.78 to 1209.25 mg kg(-1). Over 92% of the Pb in the mulberry leaves was deposited in the cell wall, and 95.29-95.57% of the Pb in the mulberry leaves was integrated with oxalic acid, pectates and protein, and it had low bioavailability. The Pb concentrations in the silkworm feces were 4.50-4.64 times higher than those in the leaves. The synthesis of metallothioneins in three tissues of the silkworms was induced to achieve Pb homeostasis under Pb stress. These results indicated the mechanism involved in Pb transfer along the food chain was controlled by the detoxification of Pb in different trophic levels. Planting mulberry and rearing silkworm could be a promising approach for the remediation of Pb-polluted soils due to the Pb tolerance of mulberry and silkworm.

The effects of acute waterborne exposure to sublethal concentrations of molybdenum on the stress response in rainbow trout, Oncorhynchus mykiss

To determine if molybdenum (Mo) is a chemical stressor, fingerling and juvenile rainbow trout (Oncorhynchus mykiss) were exposed to waterborne sodium molybdate (0, 2, 20, or 1,000 mg l^-1 of Mo) and components of the physiological (plasma cortisol, blood glucose, and hematocrit) and cellular (heat shock protein [hsp] 72, hsp73, and hsp90 in the liver, gills, heart, and erythrocytes and metallothionein [MT] in the liver and gills) stress responses were measured prior to initiation of exposure and at 8, 24, and 96 h. During the acute exposure, plasma cortisol, blood glucose, and hematocrit levels remained unchanged in all treatments. Heat shock protein 72 was not induced as a result of exposure and there were no detectable changes in total hsp70 (72 and 73), hsp90, and MT levels in any of the tissues relative to controls. Both fingerling and juvenile fish responded with similar lack of apparent sensitivity to Mo exposure. These experiments demonstrate that exposure to waterborne Mo of up to 1,000 mg l^-1 did not activate a physiological or cellular stress response in fish. Information from this study suggests that Mo water quality guidelines for the protection of aquatic life are highly protective of freshwater fish, namely rainbow trout.

Metallothionein polymorphisms in pathological processes

Metallothioneins (MTs) are a class of metal-binding proteins characterized by a high cysteine content and low molecular weight. MTs play an important role in metal metabolism and protect cells against the toxic effects of radiation, alkylating agents and oxygen free radicals. The evidence that individual genetic characteristics of MTs play an important role in physiological and pathological processes associated with antioxidant defense and detoxification inspired targeted studies of genetic polymorphisms in a clinical context. In recent years, common MT polymorphisms were identified and associated with, particularly, western lifestyle diseases such as cancer, complications of atherosclerosis, and type 2 diabetes mellitus along with related complications. This review summarizes all evidence regarding MT polymorphisms of major human MTs (MT1, MT2, MT3 and MT4), their relation to pathological processes, and outlines specific applications of MTs as a set of genetic markers for certain pathologies.

Arsenic and manganese alter lead deposition in the rat

Lead (Pb) continues to be a major toxic metal in the environment. Pb exposure frequently occurs in the presence of other metals, such as arsenic (As) and manganese (Mn). Continued exposure to low levels of these metals may lead to long-term toxic effects due to their accumulation in several organs. Despite the recognition that metals in a mixture may alter each other's toxicity by affecting deposition, there is dearth of information on their interactions in vivo. In this work, we investigated the effect of As and Mn on Pb tissue deposition, focusing on the kidney, brain, and liver. Wistar rats were treated with eight doses of each single metal, Pb (5 mg/Kg bw), As (60 mg/L), and Mn 10 mg/Kg bw), or the same doses in a triple metal mixture. The kidney, brain, liver, blood, and urine Pb, As, and Mn concentrations were determined by graphite furnace atomic absorption spectrophotometry. The Pb kidney, brain, and liver concentrations in the metal-mixture-treated group were significantly increased compared to the Pb-alone-treated group, being more pronounced in the kidney (5.4-fold), brain (2.5-fold), and liver (1.6-fold). Urinary excretion of Pb in the metal-mixture-treated rats significantly increased compared with the Pb-treated group, although blood Pb concentrations were analogous to the Pb-treated group. Co-treatment with As, Mn, and Pb alters Pb deposition compared to Pb alone treatment, increasing Pb accumulation predominantly in the kidney and brain. Blood Pb levels, unlike urine, do not reflect the increased Pb deposition in the kidney and brain. Taken together, the results suggest that the nephro- and neurotoxicity of "real-life" Pb exposure scenarios should be considered within the context of metal mixture exposures.

Differential influences of Cu and Zn chronic exposure on Cd and Hg bioaccumulation in an estuarine oyster

In this study, the effects of Cu and Zn exposure, alone and in combination, on the bioaccumulation of Cd and Hg were investigated in an estuarine oyster Crassostrea hongkongensis under different salinity gradients. We showed that Zn, but not Cu, exposure significantly enhanced the Cd bioaccumulation. In contrast, both Cu and Zn exposure significantly enhanced the Hg bioaccumulation. Combined exposure and salinity did not affect the metal interactions in oysters. The increased tissue concentrations of Cd or Hg were associated with their increased storage in inducible metal-binding ligands (e.g. metallothionein-like proteins, MTLP) by Cu/Zn exposure. The differential roles of Cu and Zn exposure in Cd and Hg bioaccumulation resulted from their contrasting ligand induction and affinities. Analysis of field collected oysters indicated that Cu/Zn exposure was a significant contributor to tissue concentrations of Cd, Cu and Hg. Overall, biochemical/physiological changes of the animals chronically exposed to metal stressors played a key role in affecting tissue concentrations of other metals. One metal's ability to enhance the bioaccumulation of other metals depended upon the relative affinities of the metals for MTLP.

Effect of metallothionein 2A gene polymorphism on allele-specific gene expression and metal content in prostate cancer

Metallothioneins (MTs) are highly conserved, small molecular weight, cysteine rich proteins. The major physiological functions of metallothioneins include homeostasis of essential metals Zn and Cu and protection against cytotoxicity of heavy metals. The aim of this study was to determine whether there is an association between the -5 A/G single nucleotide polymorphism (SNP; rs28366003) in core promoter region and expression of metallothionein 2A (MT2A) gene and metal concentration in prostate cancer tissues. MT2A polymorphism was determined by the polymerase chain reaction-restriction fragment length polymorphism technique (PCR-RFLP) using 412 prostate cancer tissue samples. MT2A gene expression analysis was performed by real-time RT-PCR method. A significant association between rs28366003 genotype and MT2A expression level was found. The average mRNA level was found to be lower among minor allele carriers (the risk allele) than average expression among homozygotes for the major allele. Metal levels were analyzed by flamed atomic absorption spectrometer system. Highly statistically significant associations were detected between the SNP and Cd, Zn, Cu and Pb levels. The results of Spearman's rank correlation showed that the expressions of MT2A and Cu, Pb and Ni concentrations were negatively correlated. On the basis of the results obtained in this study, we suggest that SNP polymorphism may affect the MT2A gene expression in prostate and this is associated with some metal accumulation.

Facilitated bioaccumulation of cadmium and copper in the oyster Crassostrea hongkongensis solely exposed to zinc

Exposure to one metal might have significant effects on the bioaccumulation of other metals. In the present study, we examined the possible effects of Zn exposure on the bioaccumulation of Cd and Cu in three populations of the oyster Crassostrea hongkongensis. We found that Zn exposure significantly enhanced the tissue concentrations of Cd and Cu in all populations, and the tissue concentrations of Cd and Cu were highly and positively related to the tissue Zn concentration. Furthermore, the enhanced bioaccumulation of Cd and Cu resulted mainly from their increasing accumulation and distribution in two subcellular fractions (i.e., metallothionein-like proteins and metal-rich granules). Tissue concentrations of Cd and Cu in the natural Zn-contaminated oysters also covaried with tissue Zn concentration, and prediction analyses revealed that Zn exposure was a significant contributor to tissue Cd and Cu concentrations. Therefore, we concluded that the increased Zn bioavailability in ambient waters not only increased the tissue Zn concentration but also enhanced the overall bioaccumulation of Cd and Cu. This study strongly demonstrates that contamination of metals in oysters may result from concurrent exposure to other metals. Thus, environmental managers should consider the possible exposure to other metals such as Zn in order to interpret/predict the tissue concentrations of toxic metals in oysters.

Accumulation of cadmium in insulin-producing β cells

Evidence suggests that chronic low level cadmium exposure impairs the function of insulin-producing β cells and may be associated with type-2 diabetes mellitus. Herein, we describe the cadmium content in primary human islets and define the uptake kinetics and effects of environmentally relevant cadmium concentrations in cultured β cells. The average cadmium content in islets from 10 non-diabetic human subjects was 29 ± 7 nmol/g protein (range 7 to 72 nmol/g protein). Exposure of the β-cell line MIN6 to CdCl 2 concentrations between 0.1 and 1.0 µmol/L resulted in a dose- and time-dependent uptake of cadmium over 72 h. This uptake resulted in an induction of metallthionein expression, likely enhancing cellular cadmium accumulation. Furthermore, cadmium accumulation resulted in an inhibition of glucose stimulated insulin secretion in MIN6 cells and primary mouse islets. Our results indicate that this impairment in β-cell function is not due to an increase in cell death or due to an increase in oxidative stress. We conclude that mouse β cells accumulate cadmium in a dose- and time-dependent manner over a prolonged time course at environmentally relevant concentrations. This uptake leads to a functional impairment of β-cell function without significant alterations in cell viability, expression of genes important for β-cell function or increase in oxidative stress.

The effects of metallothionein 2A polymorphism on lead metabolism: are pregnant women with a heterozygote genotype for metallothionein 2A polymorphism and their newborns at risk of having higher blood lead levels?

OBJECTIVES

Numerous studies indicate that certain genetic polymorphisms modify lead toxicokinetics. Metallothioneins are protective against the toxicity of many metals, including lead. The aim of this study was to determine whether the maternal metallothionein 2A (MT2A) -5 A/G single-nucleotide polymorphism is related to the lead levels in maternal blood, placental tissue and cord blood in 91 pregnant women and their newborns.

METHODS

Venous blood from the mother was collected to investigate lead levels and MT2A polymorphism. Cord blood and placenta were collected for lead levels. Analyses were made using an Atomic Absorption Graphite Furnace Spectrophotometer. Standard PCR-RFLP technique was used to determine MT2A polymorphism.

RESULTS

Blood lead levels of heterozygote genotype (AG) mothers were statistically higher than those of homozygote genotype (AA) (P < 0.05). Maternal lead levels were significantly associated with cord blood lead levels for pregnant women with AA genotype (P < 0.001). This association was not statistically significant for pregnant women with AG. In contrast, the mean value of cord blood lead level for newborns with mothers of AG genotype was slightly higher than others, though the difference was not significant. No significant difference existed in placenta lead levels between the groups.

CONCLUSION

This study suggests that pregnant women with AG genotype for MT2A polymorphism might have high blood lead levels and their newborns may be at risk of low-level cord blood lead variation.

The potential effect of metallothionein 2A -5A/G single nucleotide polymorphism on blood cadmium, lead, zinc and copper levels

Metallothioneins (MTs) are low molecular weight, cysteine-rich, metal-binding proteins. Because of their rich thiol groups, MTs bind to the biologically essential metals and perform these metals' homeostatic regulations; absorb the heavy metals and assist with their transportation and extraction. The aim of this study was to investigate the association between the metallothionein 2A (MT2A) core promoter region -5 A/G single nucleotide polymorphism (SNP) and Cd, Pb, Zn and Cu levels in the blood samples. MT2A polymorphism was determined by the standard polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) technique using the 616 blood samples and the genotype frequencies were found as 86.6% homozygote typical (AA), 12.8% heterozygote (AG) and 0.6% homozygote atypical (GG). Metal levels were analyzed by dual atomic absorption spectrophotometer system and the average levels of Cd, Pb, Zn and Cu in the blood samples were 1.69±1.57 ppb, 30.62±14.13 ppb, 0.98±0.49 ppm and 1.04±0.45 ppm, respectively. As a result; highly statistically significant associations were detected between the -5 A/G core promoter region SNP in the MT2A gene and Cd, Pb and Zn levels (p=0.004, p=0.012 and p=0.002, respectively), but no association was found with Cu level (p=0.595). Individuals with the GG genotype had statistically lower Zn level and higher Cd and Pb levels in the blood samples than individuals with AA and AG genotypes. This study suggests that having the GG genotype individuals may be more sensitive for the metal toxicity and they should be more careful about protecting their health against the toxic effects of the heavy metals.