The Cd-Chelex assay: a new sensitive method to determine metallothionein containing zinc and cadmium

Prioritization of contaminated watercourses using an integrated biomarker approach in caged carp

Because of the ever increasing complexity of environmental contamination profiles, there are limitations to the use of analytical pollutant measurements for monitoring and prioritization of watercourses. The potential of biomarkers has been debated for many years, especially in laboratory settings, but there is a need for studies evaluating these approaches in the field. We evaluated the usefulness of a selection of biomarkers, mostly indicators of general physiological status and common stress responses such as oxidative stress, to discriminate among environmental pollution profiles, with the aim of prioritizing contaminated watercourses for targeted remediation efforts. To this end, juvenile common carp (Cyprinus carpio Lin.) were exposed in cages in the field to Flemish watercourses with varying pollution profiles. After six weeks of exposure, the bioaccumulation of key pollutants was measured, and a set of organismal, biochemical and transcriptional endpoints was determined in several tissue types. After data integration a discrete set of 14 parameters was identified, that could successfully distinguish all watercourses from each other. We show that an integrated biomarker approach, mainly targeting common stress responses, can offer the resolving power to discriminate among environmentally relevant exposure scenarios, and a means to prioritize watercourses for targeted remediation.

Toxic and essential elements changed in black-legged kittiwakes (Rissa tridactyla) during their stay in an Arctic breeding area

Seasonal fluctuations in mercury (Hg), cadmium (Cd), zinc (Zn), copper (Cu) and selenium (Se) concentrations were studied in black-legged kittiwakes (Rissa tridactyla) from Kongsfjorden, Svalbard (79°57'N, 12°12'E). Element concentrations were determined in muscle and liver tissue in kittiwakes collected in May, July and October 2007. Stable isotopes of carbon (δ(13)C) and nitrogen (δ(15)N) were analysed in muscle tissue to calculate trophic position (TP) and examine the possible influence of carbon source on element accumulation. Metallothionein (MT) concentrations in liver, as well as Hg and Cd concentration in size-fractionated liver supernatant were determined to evaluate the association between elements and MT. Mercury concentrations declined from May through July to October in both tissues, while concentrations of Cd were similar in May and July and lower in October. A decline in TP between May and July, indicating a shift from fish-based diet towards an invertebrate-based diet explains the declining Hg concentration. The low Hg and Cd concentrations in October may be a result of an increased elimination, probably related to moulting. Selenium decreased in the same manner as Hg in liver and muscle, possibly related to the formation of Se-Hg complexes. Zinc and Cu did not fluctuate in muscle tissue, whereas hepatic Zn concentrations where highest in May. Hepatic Zn concentrations were higher in females compared to males in May, possibly related to egg production. Hepatic MT concentrations were lower in October compared to July, following the same trend as Hg and Cd. Cadmium was predominantly bound to the MT fraction of proteins in liver tissue, whereas Hg was associated with the larger proteins, indicating that MT was not sequestering Hg in the kittiwakes.

Accumulation and detoxification of metals and arsenic in tissues of cattle (Bos taurus), and the risks for human consumption

The aim of this study was to investigate metal accumulation and detoxification processes in cattle from polluted and unpolluted areas. Therefore dairy cows from farms and free ranging Galloway cows from nature reserves were used as study animals. The concentrations of Ag, Cd, Pb, Al, Cr, Mn, Fe, Co, Ni, Cu, Zn and As were determined in muscle, kidney, liver and lungs of cattle from polluted and reference areas in Belgium. In kidney and liver also the metallothionein concentrations were measured. For Ag, Mn, Co, Cu, Zn and As the concentrations in the different tissues were significantly higher in the sampled Galloways than in the sampled dairy cows. On the other hand Cd and Pb were significantly higher in tissues of both cattle breeds from polluted sites. Cadmium seemed to be the most important metal for metallothionein induction in kidneys whereas Zn seemed to be the most important metal for the induction of metallothionein in the liver. This study also suggested that only for Mn and Cd a significant part of the uptake occurs via the lungs. Although in muscle none of the Cd and Pb levels exceeded the European limits for human consumption, 40% of the livers and 85% of the kidneys of all examined cows were above the European limit for cadmium. Based on the existing minimum risk levels (MRLs) for chronic oral exposure, the present results suggested that a person of 70kg should not eat more than 150g cow meat per day because of the Cr levels in the muscles.

Magnesium and calcium deficiencies additively increase zinc concentrations and metallothionein expression in the rat liver

Mg deficiency increases the concentration of Zn in the liver. We investigated the effect of Mg deficiency on the expression of Zn-regulating factors such as Zn transporters and metallothionein (MT) in the rat liver. Because Ca deficiency alleviates some of the effects of Mg deficiency, we also investigated the interactions associated with Ca and Mg deficiencies. Growing male rats were given a control diet, a Mg-deficient diet, a Ca-deficient diet and a Mg- and Ca-deficient diet for 3 weeks. Mg and Ca deficiencies additively increased the mRNA levels of MT-1 and MT-2, the MT protein concentration and the concentration of Zn in the liver. The hepatic mRNA level of Zip14 increased with Mg deficiency but not with Ca deficiency. The dietary treatments did not affect the mRNA levels of other Zn transporters such as Zip1, Zip5, ZnT1, ZnT5 and ZnT6 in the liver. Ca deficiency was found to decrease the amount of femoral Zn and increase serum Zn concentration. This did not occur in the case of Mg deficiency. These results suggest that Mg deficiency enhances hepatic Zn uptake by the up-regulation of Zip14 expression and increases hepatic Zn concentration, leading to the enhancement of MT expression. Ca deficiency causes a transfer of Zn from the bone to the liver, which increases hepatic Zn concentration and, in turn, up-regulates the expression of MT. Because Mg and Ca deficiencies increase hepatic Zn concentration and increase MT expression by different mechanisms, their effects are additive.

Metallothioneins failed to reflect mercury external levels of exposure and bioaccumulation in marine fish--considerations on tissue and species specific responses

The suitability of metallothioneins (MT) in fish as biomarker of exposure to mercury has been questioned. Therefore, this study aimed at investigating the relationship between external levels of exposure, mercury accumulation and MT content, assessing species and tissue specificities. Two ecologically different fish species--Dicentrarchus labrax and Liza aurata--were surveyed in an estuary historically affected by mercury discharges. Total mercury (T-Hg) and MT content were determined in gills, blood, liver, kidney, muscle and brain. All tissues reflected differences in T-Hg accumulation in both species, although D. labrax accumulated higher levels. Regarding MT, D. labrax revealed a depletion in brain MT content and an incapacity to induce MT synthesis in all the other tissues, whereas L. aurata showed the ability to increase MT in liver and muscle. Tissue-specificities were exhibited in the MT inducing potential and in the susceptibility to MT decrease. L. aurata results presented muscle as the most responsive tissue. None of the investigated tissues displayed significant correlations between T-Hg and MT levels. Overall, the applicability of MT content in fish tissues as biomarker of exposure to mercury was uncertain, reporting limitations in reflecting the metal exposure levels and the subsequent accumulation extent.

Selenium moderates mercury toxicity in free-ranging freshwater fish

Due to the extremely high affinity of selenium (Se) to mercury (Hg), Se sequesters Hg and reduces its biological availability in organisms. However the converse is also true. Hg sequesters Se, causing Hg to inhibit the formation of Se dependent enzymes while supplemental Se supports their continued synthesis. Hence, whether or not toxic effects accompany exposure to Hg depends upon the tissue Se:Hg molar ratio of the organism. The main objective of the present study was to investigate how levels of Hg and Se affected metallothionein (MT) induction in free-ranging brown trout, Salmo trutta, from Lake Mjøsa, Norway (a Se depauperate lake). MT is proposed as a sensitive biomarker of potential detrimental effects induced by metals such as Hg. Emphasis was addressed to elucidate if increased tissue Se:Hg molar ratios and Se levels affected the demands for MT in the trout. The Se:Hg molar ratio followed by tissue Se levels were most successful for assessing the relationship between metal exposure and MT levels in the trout. Thus, Hg in molar excess over Se was a stronger inducer of MT synthesis than tissue Hg levels in the trout, supporting the assumption that Se has a prominent protective effect against Hg toxicity. Measuring Hg in animals may therefore provide an inadequate reflection of the potential health risks to humans and wildlife if the protective effects of Se are not considered.

Changes in arterial PO₂, physiological blood parameters and intracellular antioxidants in free-swimming Atlantic cod (Gadus morhua) exposed to varying levels of hyperoxia

Free-swimming Atlantic cod (Gadus morhua) were exposed to water oxygen pressures (P(w)O₂) ranging from 18.1 to 41.5 kPa and sampled for blood using an indwelling caudal artery cannula. Arterial blood oxygen pressure (P(a)O₂) increased with increasing P(w)O₂, from 12.0 kPa in normoxia (18.1 kPa) to 34.2 kPa in the highest hyperoxic level tested (41.5 kPa). Blood CO₂ pressure and plasma bicarbonate concentration increased with P(w)O₂, indicating reduced ventilation with increased P(w)O₂. Plasma glucose, sodium and potassium were not affected by water oxygen level. Blood oxidative stress biomarkers, reduced glutathione, oxidized glutathione and the oxidative stress index (ratio between oxidized and total glutathione) differed intermittently between normoxia and hyperoxia. The oxidative stress index was higher in the blood of exposed compared to unexposed control cod. Together with elevated P(a)O₂, these findings suggest increased production of reactive oxygen species and increased oxidative stress in Atlantic cod exposed to hyperoxia.

Effects of combined gamma-irradiation and metal (Al+Cd) exposures in Atlantic salmon (Salmo salar L.)

These experiments were designed to investigate transcriptional effects in Atlantic salmon (Salmo salar) after exposure in vivo to ionizing gamma radiation combined with subtoxic levels of aluminum (Al) and cadmium (Cd). Juvenile fish (35 g) in freshwater with or without Al and Cd (255 microg Al/L + 6 microg Cd/L) were exposed to a 75 mGy dose of gamma-irradiation, and induced responses were compared to those of controls. The transcriptional levels of eight genes encoding proteins known to respond to stress in fish were quantified in liver of fish exposed for 5 h to gamma radiation, to Al and Cd or to the combination of Al, Cd and gamma radiation. The studied genes were caspase 3B, caspase 6A, caspase 7, p53 (apoptosis), glutathione reductase (GR), phospholipid hydroperoxide glutathione peroxidase (GSH-Px), (oxidative stress), metallothionein (MT-A) (metal stress) and ubiquitin (Ubi) (protein degradation). The results showed that gamma-irradiation alone induced significant upregulation of caspase 6A, GR, GSH-Px, MT-A and Ubi compared to the control group, while 5 h exposure to Al+Cd alone did not induce any of the studied genes compared to the control. No significant upregulation of the series of investigated genes could be observed in fish exposed to gamma-irradiation in combination with Al+Cl. In conclusion, the results suggest that the presence of Al+Cd in the water counteracted the gamma-irradiation effect by modifying the transcription of genes encoding proteins involved in the defense mechanisms against free radicals in the cells.

Cytosolic distribution of Cd, Cu and Zn, and metallothionein levels in relation to physiological changes in gibel carp (Carassius auratus gibelio) from metal-impacted habitats

In the present study the impact of environmental metal contamination in gibel carp (Carassius auratus gibelio) was investigated coupling disturbances in subcellular metal distribution to effect biomarkers. Gibel carp were caught at six different sampling sites in Flanders (Belgium), characterized by different degrees in environmental metal contamination. Tissue Cd, Cu and Zn concentrations and total metallothioneon (MT) levels were determined in gills, liver and kidney. Cytosolic metal distribution was measured in the main accumulating organs, liver and kidney, revealing tissue- and metal-dependent profiles. The MT pool dominated the cytosolic distribution profile. Although the importance of the MT pool increased with increasing environmental and cytosolic metal concentrations, also an undefined fraction of Cu and Cd fractions (probably free metal ions or metals bound to small organic complexes) increased at the most contaminated sampling sites. Disturbances in serum ion concentrations, serum alanine aminotransferase activity (ALT), hematocrite and condition factor were measured, as indicators of toxic effects. At the sampling site with the highest environmental Cd pollution a significant decrease in serum Ca(2+) concentrations and a significantly increased serum ALT activity were measured suggesting incomplete detoxification of Cd. Increased serum ALT concentrations suggested structural liver damage. The fact that the liver tissue, and probably also the kidney, could not cope with this high Cd burden in combination with the increased uptake of dissolved Cd through the gills most probably contributed to the perturbed serum Ca(2+) homeostasis.

Speciation of lead, copper, zinc and antimony in water draining a shooting range--time dependant metal accumulation and biomarker responses in brown trout (Salmo trutta L.)

The speciation of Pb, Cu, Zn and Sb in a shooting range run-off stream were studied during a period of 23 days. In addition, metal accumulation in gills and liver, red blood cell ALA-D activity, hepatic metallothionine (Cd/Zn-MT) and oxidative stress index (GSSG/ tGSH levels) in brown trout (Salmo trutta L.) exposed to the stream were investigated. Fish, contained in cages, were exposed and sampled after 0, 2, 4, 7, 9, 11 and 23 days of exposure. Trace metals in the water were fractionated in situ according to size (nominal molecular mass) and charge properties. During the experimental period an episode with higher runoff occurred resulting in increased levels of metals in the stream. Pb and Cu were mainly found as high molecular mass species, while Zn and Sb were mostly present as low molecular mass species. Pb, Cu and Sb accumulated on gills, in addition to Al origination from natural sources in the catchment. Pb, Cu and Sb were also detected at elevated concentration in the liver. Blood glucose and plasma Na and Cl levels were significantly altered during the exposure period, and are attributed to elevated concentrations of Pb, Cu and Al. A significant suppression of ALA-D was detected after 11 days. Significant differences were detected in Cd/Zn-MT and oxidative stress (tGSH/GSSG) responses at Day 4. For Pb the results show a clear link between the HMM (high molecular mass) positively charged Pb species, followed by accumulation on gills and liver and a suppression in ALA-D. Thus, high flow episodes can remobilise metals from the catchment, inducing stress to aquatic organisms.

Cadmium is deposited in the gut content of larvae of the beetle Tenebrio molitor and involves a Cd-binding protein of the low cysteine type

Binding of cadmium (Cd) to metallothionein (MT) and non-MT proteins with low contents of cysteine has been observed in terrestrial arthropods. We recently isolated a Cd-binding protein with no cysteine that was induced in Cd-exposed larvae of the beetle Tenebrio molitor. In this study we have examined the molecular distribution of Cd within extracts of different tissues and compartments of Cd-exposed T. molitor larvae. A Cd-peak consistent with the low cysteine Cd-binding protein was induced within the gut content where it could be detected after 4-8 days of exposure. Examination of gut wall tissue revealed no increase in Cd-binding capacity, indicating that no accumulation of MTs was taking place in this tissue. Incorporation of Cd in the gut wall tissue stabilized after 8 days of Cd-exposure at a rather low level compared to the other organs. There was a statistical trend towards Cd being incorporated in the gut content in a manner that was disproportionally high compared to the amount of Cd in the gut wall tissue. The possible role of the low cysteine Cd-binding protein in reducing the uptake of Cd in the tissues is discussed.

A field survey of metal binding to metallothionein and other cytosolic ligands in liver of eels using an on-line isotope dilution method in combination with size exclusion (SE) high pressure liquid chromatography (HPLC) coupled to Inductively Coupled Plasma time-of-flight Mass Spectrometry (ICP-TOFMS)

The effect of metal exposure on the accumulation and cytosolic speciation of metals in livers of wild populations of European eel with special emphasis on metallothioneins (MT) was studied. Four sampling sites in Flanders showing different degrees of heavy metal contamination were selected for this purpose. An on-line isotope dilution method in combination with size exclusion (SE) high pressure liquid chromatography (HPLC) coupled to Inductively Coupled Plasma time-of-flight Mass Spectrometry (ICP-TOFMS) was used to study the cytosolic speciation of the metals. The distribution of the metals Cd, Cu, Ni, Pb and Zn among cytosolic fractions displayed strong differences. The cytosolic concentration of Cd, Ni and Pb increased proportionally with the total liver levels. However, the cytosolic concentrations of Cu and Zn only increased above a certain liver tissue threshold level. Cd, Cu and Zn, but not Pb and Ni, were largely associated with the MT pool in correspondence with the environmental exposure and liver tissue concentrations. Most of the Pb and Ni and a considerable fraction of Cu and Zn, but not Cd, were associated to High Molecular Weight (HMW) fractions. The relative importance of the Cu and Zn in the HMW fraction decreased with increasing contamination levels while the MT pool became progressively more important. The close relationship between the cytosolic metal load and the total MT levels or the metals bound on the MT pool indicates that the metals, rather than other stress factors, are the major factor determining MT induction.

Metal exposure and biological responses in resident and transplanted blue mussels (Mytilus edulis) from the Scheldt estuary

The Western Scheldt river and estuary is known to be highly polluted as it receives industrial, agricultural and domestic effluents from one of the most populated and industrialised zones in Europe. Aquatic organisms are exposed to pollutants, specifically metals that are present in different environmental phases, e.g. dissolved, suspended material or sediment phases. The objective of this study was to study the relationship that exists between environmental metal levels, the degree of metal uptake by aquatic organisms with the concomitant biological responses. For this purpose the bivalve mollusk, Mytilus edulis, was selected as bioaccumulation indicator organism. Environmental (water and sediment) and mussel samples were collected during the late winter (March 2000) from sampling sites in the Scheldt estuary. Sites were selected to represent a salinity gradient from the mouth of the estuary to the furthest distribution area of mussels upstream in the system. Together with standard water quality parameters (e.g. salinity, dissolved oxygen, dissolved organic carbon, etc.) concentrations of twelve metals were analysed in the water (dissolved and suspended matter) and sediments. Levels of these metals were also measured in the soft tissue of M. edulis, together with concomitant biomarker responses in resident mussel populations at each site. The biomarkers that were included in this study were condition index, scope for growth, survival in air, cell membrane stability, DNA damage, and metallothioneins. Data were subjected to multivariate statistical analysis. The physico-chemical parameters and metals in the environmental samples clustered the sites to reflect the distribution based on the salinity gradient. Bioaccumulation results revealed increased metal uptake along a pollution gradient with the highest metal bioaccumulation occurring at the upstream most sites and therefore closest to the in the industrial activities. However, the biomarker responses clustered the sites in a manner that reflected the influence of combination of internal exposure (bioaccumulation) and external exposure (physico-chemical conditions). These differences in biomarker responses clearly demonstrated were attributed to abiotic factors other than metal pollution alone e.g. localized short-term increases in increased suspended sediment concentrations and decreased dissolved oxygen concentrations.

Accumulation and effects of metals in caged carp and resident roach along a metal pollution gradient

Metal accumulation and effects on plasma Ca(2+), alanine transaminase (ALT) and fish condition factor were examined in caged carp (Cyprinus carpio) and resident roach (Rutilus rutilus) at four locations along the Grote Nete River system (Belgium). Cadmium and zinc accumulation were found in carp and roach, with highest concentrations at the most contaminated site (dissolved Cd: 1.82 microg/l, Zn: 967 microg/l). On the tissue level, highest cadmium concentrations were measured in kidneys of carp and roach, followed by gills, intestine and liver, while low concentrations were observed in carcass and muscle. For zinc, a similar pattern was observed (intestine>kidney>gills>liver>carcass>muscle). Comparison between species showed higher cadmium concentrations in feral roach, while zinc levels were lower, owing to the high zinc concentrations in control carp. Furthermore, comparison of metal concentrations between two sampling periods (2005 and 2000-2001) revealed a drastic decrease in cadmium concentration in gills, liver and muscle of roach, similar to the reduction in waterborne cadmium concentrations, while differences for zinc were much less pronounced. In addition to metal accumulation, increased metallothionein concentrations (approximately 2x) were found in carp and roach, while no metal-related effects were found on ALT, Ca(2+)or condition factor. However, negative effects on fish community structure, as assessed by the index of biotic integrity (IBI), were found along the pollution gradient and indicated long-term adverse effects of metal pollution.

Role of Lipoamide Dehydrogenase and Metallothionein on 1-Methyl-4-phenyl-1,2,3,6- tetrahydropyridine-induced Neurotoxicity

In the present study, we investigated the effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on lipoamide dehydrogenase activity and metallothionein content. Lipoamide dehydrogenase is a flavoprotein enzyme, which reduces lipoamide and low molecular weight thiols. This enzyme has also been involved in the conversion of ubiquinone (coenzyme Q-10, oxidized form) to ubiquinol (reduced form). Lipoamide dehydrogenase activity was measured spectrophotometrically following its incubation with different doses of MPTP, MPP+, and divalent metals. MPTP at higher concentrations inhibited the lipoamide dehydrogenase activity, whereas it's potent toxic metabolite 1-methyl-4-phenylpyridinium (MPP+) had a similar effect at lower concentration. Calcium and copper did not affect the enzyme activity at any of the doses tested, whereas, zinc dose dependently enhanced the lipoamide dehydrogenase activity. Additionally, levels of metallothionein in the mouse nigrostriatal system were measured by cadmium affinity method following administration of MPTP. Metallothionein content was significantly reduced in the substantia nigra (SN), and not in the nucleus caudatus putamen (NCP) following a single administration of MPTP (30 mg/kg, i.p.). Our results suggests that both lipoamide dehydrogenase activity and metallothionein levels may be critical for dopaminergic neuronal survival in Parkinson's disease and provides further insights into the neurotoxic mechanisms involved in MPTP-induced neurotoxicity.

Isolation and preliminary characterization of a Cd-binding protein from Tenebrio molitor (Coleoptera)

The effect of cadmium (Cd) exposure on Cd-binding ligands was investigated for the first time in a beetle (Coleoptera), using the mealworm Tenebrio molitor (L) as a model species. Exposure to Cd resulted in an approximate doubling of the Cd-binding capacity of the protein extracts from whole animals. Analysis showed that the increase was mainly explained by the induction of a Cd-binding protein of 7134.5 Da, with non-metallothionein characteristics. Amino acid analysis and de novo sequencing revealed that the protein has an unusually high content of the acidic amino acids aspartic and glutamic acid that may explain how this protein can bind Cd even without cysteine residues. Similarities in the amino acid composition suggest it to belong to a group of little studied proteins often referred to as "Cd-binding proteins without high cysteine content". This is the first report on isolation and peptide sequence determination of such a protein from a coleopteran.

Phenomenon of Hepatic Overload of Copper in Mugil cephalus: Role of Metallothionein and Patterns of Copper Cellular Distribution

In this work we describe a phenomenon of accumulation of copper (Cu) in livers of a teleost fish commonly known as mullet, Mugil cephalus. High levels of Cu, up to 1936 microg/g wet weight were found. The high Cu levels seem not to be associated with environmental Cu contamination, since the fish were collected from widely separated regions with low Cu concentrations. Other fish species sharing the same environment did not show high levels of Cu. The accumulation of Cu in mullet was seen in liver and most of the hepatic Cu was located in the non-cytosolic fraction. The intrahepatic distribution of Cu in mullet seems to depend on the total Cu content in the liver; as the total liver burden of Cu rose, Cu was increasingly recovered from the non-cytosolic fraction. Metallothionein in hepatic cytosols from mullet contained the most Cu. However, the Cu concentration not bound to metallothionein rose when total cytosolic Cu increased; which show that metallothionein, particularly at higher Cu levels, is not the major hepatic Cu-binding protein in cytosols of mullet. This report shows mullet as a very useful model to study the accumulation of Cu in the liver, which may lead to a better understanding of cellular mechanisms which control Cu homeostasis.

Cd accumulation and Cd-metallothionein as a biomarker in Cepaea hortensis (Helicidae, Pulmonata) from laboratory exposure and metal-polluted habitats

Cepaea hortensis is a widespread terrestrial pulmonate, contributing significantly to element fluxes in soil ecosystems. Due to its capacity of accumulating certain trace elements in its tissues, Cepaea hortensis can serve as a biological indicator of metal accumulation in contaminated areas. In response to Cd exposure this species and related helicid pulmonates are also able to synthesize an inducible, Cd-binding metallothionein (MT) isoform specifically serving in binding and detoxification of this metal. As shown by field-collected garden-snails from a metal-contaminated site near a zinc smelting works in Avonmouth (UK) and an unpolluted reference site in Reutte (Tyrol, Austria), Cd and Cd MT concentrations in midgut gland of C. hortensis from these sites increased with rising Cd concentrations in the soil substrate from the same contaminated sites. By combining the results of these field data with laboratory experiments it appears that midgut gland Cd-MT of Cepaea hortensis seems to fulfil the criteria of a successful biomarker in many respects. First, the synthesis of the protein can rapidly be induced by Cd exposure. Second, the level of Cd MT induction in C. hortensis directly reflects the intensity of metal exposure. Third, the induced signal of increased Cd-MT concentration in C. hortensis is persistent over extended periods of time. Fourth, the Cd-MT signal in C. hortensis seems to be very specific for Cd exposure. Regression analyses demonstrate that tissue levels of Cd and Cd MT in C. hortensis depend on Cd concentrations in the substrate which is represented by either soil or plant material on which snails normally feed. In both cases the best fit for this dependence is exhibited by a semi-logarithmic relationship, with substrate (soil or plant feed) concentrations expressed on a logarithmic scale. It is concluded that C. hortensis and other related pulmonates can successfully be used either as biological indicators of Cd accumulation, or as key species in biomonitoring studies focusing on Cd-MT induction as a biomarker for Cd exposure.

Resistance to water pollution in natural gudgeon (Gobio gobio) populations may be due to genetic adaptation

Anthropogenic disturbances cause the environment to change relatively fast. It is reasonable to assume that it is very unlikely for individuals to develop genetic adaptations to their polluted habitats, since adaptation through natural selection is a relatively slow process. Nevertheless, several studies have shown that such adaptations to changing environmental conditions may develop faster than anticipated. This study investigates the impact of historical metal pollution on a natural population of the gudgeon, Gobio gobio. Specimens from a contaminated site and a reference population were subjected to a series of three exposure experiments to cadmium after an acclimation period to reconstituted fresh water of 36 days. First, we performed an acute toxicity test on a sub-sample of both experimental groups to determine times-to-death (TTD) and lethal body burdens (LBB). The remaining individuals were used in a chronic Cd-exposure experiment, after which total Cd-body concentration, as well as Cd-concentrations and metallothionein-like protein (MTLP) levels in liver and gill tissue were determined. From the specimens that were not sacrificed for these measurements, a random subsample was subjected to a second acute toxicity test to evaluate the effect of chronic Cd-exposure (acclimation) on TTD and LBB. Our results show that, particularly after an extra acclimation period to a sublethal Cd-concentration, specimens originating from the contaminated sample area survived the acute exposure experiments better, despite the fact that neither the average Cd-accumulation rate, nor the lethal body concentrations differed between fishes from both groups. We also find that gudgeons from both populations translocated Cd from the gills (and probably also from other compartments) to the liver, where it can be more efficiently detoxified by MTLPs. Indeed, MTLP levels were found to increase faster in liver and gill tissue of specimens from the contaminated site, resulting in significantly higher MTLP-levels in the organs of these fishes. Although this study does not provide direct evidence for a genetic basis of Cd resistance (i.e. at the gene level), our results indicate that the regulation of MTLP-gene expression may involve a genetic component.

Overexpression of metallothionein in pancreatic beta-cells reduces streptozotocin-induced DNA damage and diabetes

The release of reactive oxygen species (ROS) has been proposed as a cause of streptozotocin (STZ)-induced beta-cell damage. This initiates a destructive cascade, consisting of DNA damage, excess activation of the DNA repair enzyme poly(ADP-ribose) polymerase, and depletion of cellular NAD+. Metallothionein (MT) is an inducible antioxidant protein that has been shown to protect DNA from chemical damage in several cell types. Therefore, we examined whether overexpression of MT could protect beta-cell DNA and thereby prevent STZ-induced diabetes. Two lines of transgenic mice were produced with up to a 30-fold elevation in beta-cell MT. Cultured islets from control mice and MT transgenic mice were exposed to STZ. MT was found to decrease STZ-induced islet disruption, DNA breakage, and depletion of NAD+. To assess in vivo protection, transgenic and control mice were injected with STZ. Transgenic mice had significantly reduced hyperglycemia. Ultrastructural examination of islets from STZ-treated mice showed that MT prevented degranulation and cell death. These results demonstrate that MT can reduce diabetes and confirm the DNA damage mechanism of STZ-induced beta-cell death.

Metallothionein as bioindicator of freshwater metal pollution: European eel and brown trout

The aim of this work was to evaluate the potential of metallothionein (MT) as a bioindicator of heavy metal pollution in brown trout and European eel in the field situation. River Ferrerias (North Spain) provided a good gradient of metal contamination: concentrations of heavy metals were elevated both in water and in sediments at the downstream (polluted) site and were low at the upstream (unpolluted) site. MT levels of brown trout exhibited statistically significant differences between sites. Although European eel at the polluted site had a higher MT content, differences were not significant. It is concluded that MT is a good bioindicator of heavy metal pollution in brown trout.

Dynamics of (Cd,Zn)-metallothioneins in gills, liver and kidney of common carp Cyprinus carpio during cadmium exposure

Cadmium concentrations, (Cd,Zn)-metallothionein (MT) concentrations, MT synthesis and the relative amounts of cadmium bound to (Cd,Zn)-MTs were determined in gills, liver and kidney of common carp Cyprinus carpio exposed to 0, 0.5 microM (0.06 mg.l(-1)), 2.5 microM (0.28 mg.l(-1)) and 7 microM (0.79 mg.l(-1)) Cd for up to 29 days. Cadmium accumulation was in the order kidney > liver > gills. Control levels of hepatic (Cd,Zn)-MT were four times higher compared to those of gills and kidney. No increases in (Cd,Zn)-MT concentrations were observed in liver during the exposure period. In comparison with control carp, (Cd,Zn)-MT concentrations increased up to 4.5 times in kidney and two times in gills. In both these organs, (Cd,Zn)-MT concentrations were linearly related with cadmium tissue levels and with the de novo synthesis of MTs. Hepatic cadmium was almost completely bound to (Cd,Zn)-MT, while percentages of non-MT-bound cadmium were at least 40% in gills and 25% in kidney. This corresponded with a total saturation of (Cd,Zn)-MT by cadmium in kidney and a saturation of approximately 50 and 60% in gills and liver, respectively. The final order of non-MT-bound cadmium was kidney > gills > liver. Our results indicate that cadmium exposure causes toxic effects, which cannot be correlated with the accumulated levels of the metal in tissues. Although cadmium clearly leads to the de novo synthesis of MT and higher (Cd,Zn)-MT concentrations, the role of this protein in the detoxification process is clearly organ-specific and its synthesis does not keep track with cadmium accumulation.

Stress responses and changes in protein metabolism in carp Cyprinus carpio during cadmium exposure

Stress responses and changes in protein metabolism were studied in common carp Cyprinus carpio exposed to 0, 0.8, 4, and 20 microM cadmium (Cd) over a 29-day period. Blood and other tissue samples were taken after 4 and 29 days of exposure. The highest Cd concentration proved to be lethal to the fish, resulting in 100% mortality after 21 days of exposure. Cd accumulated in the tissues in the following order: kidney>liver>gills. Blood hematocrit, blood hemoglobin, plasma glucose, plasma lactate, and tissue total protein contents were not significantly altered. The concentrations of Cd and zinc (Zn) binding metallothioneins ((Cd, Zn)-MTs) were in the following order=liver>kidney>gills. An increase in (Cd, Zn)-MTs was observed at all exposure concentrations at days 4 and 29 in kidney and at Day 29 in gills. No significant changes in (Cd, Zn)-MT contents were found in liver. The concentrations of free amino acids and the activities of proteases were increased at Day 4 in gills, liver, and kidney of carp exposed to 4 and 20 microM Cd, and in gills and kidney at Day 29 in carp exposed to 4 microM Cd. The observed increases in the activities of aspartate aminotransferase and alanine aminotransferase suggest that the observed proteolysis is intended to increase the role of proteins in the energy production during Cd stress. However, this increased activity of both aminotransferases was not found in gills during exposure to the lethal Cd concentration, indicating that Cd may also cause an inhibitory effect on the activity of these enzymes above a certain level.

Metal accumulation and metallothionein in brown trout, Salmo trutta, from two Norwegian rivers differently contaminated with Cd, Cu and Zn

In this work we have studied the accumulation of heavy metals in two brown trout (Salmo trutta) populations in their natural environment and the participation of metal binding to metallothionein (MT) in this process. Cd, Cu and Zn concentrations, total MT (including Cu MT) and Cd/Zn MT were measured in the gills, liver and kidney of trout inhabiting two rivers, one Cu-contaminated and the other Cd/Zn-contaminated, located at Røros, Central Norway. In both populations, high levels of Cu were found in the liver, whereas Cd was accumulated in liver and particularly in the kidney. The proportions of Cd/Zn MT and Cu MT in liver and kidney, but not in gills, reflected the accumulated and the environmental concentrations of these metals. The total Cu MT concentrations in the investigated tissues, however, were highest in trout from the river with the lowest ambient Cu concentration. It is suggested that MTs are of less importance in Cu-acclimated trout. The data also suggest that acclimation to a Cu-rich environment involves reduced Cu accumulation or increased Cu elimination. In trout from the Cd-rich environment, this metal was mainly bound to MT, whereas in trout from the Cu-rich environment Cd was also associated with non-MT proteins. These findings emphasize the importance to determine both Cd/Zn MT and Cu MT levels, when the participation of this protein in metal handling in trout tissues is investigated.

Separation and quantitation of metallothionein isoforms from liver of untreated rats by ion-exchange high-performance liquid chromatography and atomic absorption spectrometry

A sensitive method for determination of metallothionein (MT) isoform levels in rat liver by ion-exchange high-performance liquid chromatography and atomic absorption spectrometry was developed. Critical steps in sample preparation, like MT extraction, MT saturation with Cd and protein separation, were optimized. This method is capable of measuring levels of 2.0 microg/g liver for metallothionein-1 (MT-1) and 1.3 microg/g liver for metallothionein-2 (MT-2), respectively, with a high recovery of 103% on average. The method described, thus, proved suitable for analyzing metallothionein isoform concentrations even in untreated animals. The ratio of MT-1 to MT-2 was found to be 1:1 on average. MT decomposition during storage was very high in whole livers, but could be reduced by about 80% when extracted liver samples were used.

Development of a stable isotope approach for the inductively coupled plasma-mass spectrometry determination of oxidized metallothionein in biological materials

The use of isotope dilution analysis (IDA) with inductively coupled plasma-mass spectrometry (ICP-MS) for the determination of oxidized metallothionein (MT) by a Cd-saturation method is investigated. The method developed here is a modification of an earlier methodology which used a radioactive Cd isotope ((109)Cd). While retaining the many advantages of this previous approach, the procedure presented here uses stable isotope ratio measurements ((114)Cd/(111)Cd) for the determination of MT. Experimental parameters governing the instrumental precision and accuracy for isotope ratio measurements of Cd by ICP-MS were characterized. Systematic errors, including mass bias, detector dead time, and spectroscopic interferences, could be easily corrected. The isotope dilution ICP-MS method was validated by the determination of very low levels of cadmium in biological certified reference materials (NIST SRM 2670 freeze-dried urine, IAEA H-8 horse kidney, and BCR TP-25 lichens). Finally, the IDA procedure was evaluated for the determination of oxidized MT by a Cd-saturation method previously developed using radioactive (109)Cd. The final procedure was applied to the quantification of MT in Long-Evans Cinnamon rat liver cytosol samples and the results were compared with data obtained for the same samples using the reference (109)Cd methodology. A good agreement between the analytical values obtained by both methods was observed.

Hormesis: a stress response in cells exposed to low levels of heavy metals

Cytotoxicity studies using a 3-(4,5 dimethylthiazol-2-yl)-2,5 diphenyl tetrazolium bromide (MTT)-based in vitro toxicity assay revealed that McCoy cells exposed to low concentrations of mercuric (0.7 microM), cadmium (1 microM) and cupric chloride (3 microM) exhibited significant increases in cellular activity. This increased activity, previously termed hormesis, coincided with the production of high levels of the stress proteins, heat shock protein 70 (Hsp 70) and metallothionein, while the high constitutive expression of these proteins in cadmium-resistant mutant (CRM) cells corresponded to constitutive hormetic activity. Hormesis was found to obey uniform kinetics allowing for a mathematical description of this increased activity. These results suggest that hormetic activity is a specific cellular response, and most likely, a stress response to low but harmful levels of toxic agents and may therefore provide a rapid test for the presence of toxicants at concentrations associated with chronic toxicity.

Dissolution of copper-rich granules in hepatic lysosomes by D-penicillamine prevents the development of fulminant hepatitis in Long-Evans cinnamon rats

BACKGROUND/AIM

The Long-Evans cinnamon rat has a mutation homologous to the human Wilson disease gene, leading to gross copper accumulation and the development of hepatitis. D-penicillamine, a copper-chelating drug widely and efficiently used in treating Wilson disease, has also been shown to prevent hepatitis in Long-Evans cinnamon rats. The objectives of this study were: i) to investigate the effectiveness of D-penicillamine when administered to the already affected animals, and ii) to elucidate the mechanism of action of the drug.

METHODS

Long-Evans cinnamon rats were divided into groups according to age and treatment with D-penicillamine. The drug was administered orally before and after the onset of hepatitis. Livers were examined by light and electron microscopy. The effect of D-penicillamine on the subcellular distribution and binding of copper was investigated in more detail. Finally, the interaction between D-penicillamine and specific hepatic copper-binding proteins was studied in vitro.

RESULTS

D-penicillamine when given to either healthy or diseased animals prevented or reversed hepatitis, respectively. The drug particularly inhibited the disease-specific accumulation of copper in lysosomes of hepatocytes, tissue macrophages and Kupffer cells. When administered to diseased animals, the drug sequestered copper particularly from insoluble lysosomal particles. According to results obtained in vitro, the mobilization of this copper is likely to proceed through the solubilization of these particles. In contrast and as supported by the in vitro data, D-penicillamine had only a minor effect on copper bound to metallothionein in the cytosol.

CONCLUSION

Our findings on the Long-Evans cinnamon rat provide some conclusions on the mechanism of action of D-penicillamine in Wilson disease therapy. The drug prevents the formation or promotes the solubilization of copper-rich particles which occur in lysosomes of hepatocytes and Kupffer cells in the livers of patients with Wilson disease. Once chelated with D-penicillamine copper might then be excreted into urine. However, the mobilization of copper by D-penicillamine seems to be limited due to the binding of the metal to metallothionein in liver cytosol. This copper, even at relatively high concentrations, apparently may be well tolerated.

Metallothionein and heavy metals in brown trout (Salmo trutta) and European eel (Anguilla anguilla): a comparative study

The levels and the cellular distribution of heavy metals, and the extent by which the metals binds to metallothionein (MT) in brown trout (Salmo trutta) and European eel (Anguilla anguilla), were analyzed in order to assess the natural conditions of MT and heavy metals in these two fish species. There were no differences in heavy metals and MT concentrations between males and females of brown trout in a nonreproductive status and between adult brown trout individuals. Brown trout presented higher Cu content than European eel. The cellular distribution of Cu was also different between the two fish species; while in brown trout most of the Cu was in the noncytosolic fraction, Cu was mainly located in the cytosol in European eel. However, the cellular distribution of Zn, Cd, and Pb was similar in the two fish species. There was also an important difference in the metal content of MT between both species. Whereas, in brown trout, Cu-binding MT represented 75% of total metal-binding MT, this value was 25% in European eel. The between-species differences found in this study are intrinsic characteristics not associated with environmental factors. These results establish the basis to use MT as a bioindicator.

Association of copper to metallothionein in hepatic lysosomes of Long-Evans cinnamon (LEC) rats during the development of hepatitis [se e comments]

BACKGROUND

The Long-Evans cinnamon (LEC) rat has a mutation homologous to the human Wilson's disease gene, leading to copper-induced hepatotoxicity. The mechanism of how excess copper damages the liver or what chemical form of copper is toxic is still unclear.

RESULTS

In liver cytosol, copper levels were highest just before the onset of hepatitis and declined thereafter. In cytosol, total copper was bound to metallothionein (MT). Considerable amounts of both copper and iron accumulated in lysosomes with increasing age and development of liver damage. Lysosomal levels of presumably reactive non-MT-bound copper were increased. In severely affected livers, large amounts of copper were associated with insoluble material of high density which, upon ultrastructural information, was found to be derived from the lysosomes of Kupffer cells. This copper-rich material is considered to consist of polymeric degradation products of copper-MT.

CONCLUSION

We suggest that chronic copper toxicity in LEC rats involves the uptake of copper-loaded MT into lysosomes, where it is incompletely degraded and polymerizes to an insoluble material containing reactive copper. This copper, together with iron, initiates lysosomal lipid peroxidation, leading to hepatocyte necrosis. Subsequent to phagocytosis by Kupffer cells, the reactive copper may amplify liver damage either directly or through stimulation of these cells.

Outputs of hepatic copper and cadmium stimulated by tetrathiomolybdate (TTM) injection in Long-Evans Cinnamon (LEC) rats pretreated with cadmium, and in Fischer rats pretreated with copper and cadmium

The Long-Evans Cinnamon (LEC) rat, an inbred mutant rat derived from the Long-Evans strain, is characterized by spontaneous hepatitis due to gross accumulation of hepatic Cu. The accumulation, accompanied by marked induction of metallothionein (MT), is believed to be due to the inherent lack of output of Cu into the bile duct and blood vessels. In this study, the acute effect of tetrathiomolybdate (TTM), a chelator for output of hepatic Cu and Cd in LEC rats treated with Cd, was investigated. Female LEC rats were injected subcutaneously with Cd (Cd; 1.0 mg/kg) to induce Cd, Cu-MT. Fischer rats were treated with Cd (Cd; 1.0 mg/kg) and Cu (Cu; 3.0 mg/kg). Forty-eight hours after the injections of metals, TTM (5 mg/kg bw) was injected intravenously under anesthesia. The TTM injection rapidly stimulated biliary excretions of Cu (at a microgram/ml level) and Cd (at a ng/ml level). Furthermore, Cu and Cd concentrations were increased in serum sampled 60 min after the TTM injection. The increase of biliary Cu excretion was not accompanied by increased biliary excretion of MT. The TTM injection caused the hepatic Cu concentrations to decrease from 306 +/- 2 to 262 +/- 12 and from 43 +/- 6 to 20 +/- 5 micrograms/g in LEC and Fischer rats, respectively. The hepatic Cd concentration was not decreased by TTM treatment. Hepatic MT and Cu, but not Cd, concentrations in the MT fraction were also reduced by TTM injection. Our results showed that TTM can rapidly remove Cu from MT to increase bile and blood Cu levels. The output of Cd stimulated by TTM injection may be related to MT reduction resulting from removal of MT-bound Cu. Our results indicate that to avoid the toxic effect of Cu, TTM injection is an effective initial treatment, although it remains to be established how metals, including Cu, are finally metabolized.

Comparison of Zn and vitamin E for protection against hyperoxia-induced lung damage

The objective of the present study was to demonstrate an antioxidant function for Zn in vivo by comparing the efficacy of Zn or Vitamin E without additional energy intake for protection of Zn-deficient (ZnDF) or energy-restricted (ER) rats from hyperoxia-induced lung damage. Zn (200 mumol ZnCl2/kg b.wt.) or Vitamin E (100 mg alpha-tocopherol/kg b.wt.) was injected IP before exposure to 85% oxygen or air for 5 d. During the exposure period, all injected ZnDF or ER rats were restricted to 5 g Zn-deficient or Zn-adequate diet/day, respectively, the amount of diet consumed by the untreated ZnDF or ER rats. We clearly demonstrated that injection of Zn without additional energy intake can protect ZnDF and ER rats from hyperoxia-induced lung damage assessed by the histopathological scoring system and magnetic resonance imaging (MRI). Vitamin E was not as effective as Zn in either ZnDF or ER rats for preventing hyperoxia-induced lung damage. Zn injection did not exert its antioxidant effect through increased lung CuZn-superoxide dismutase activity or metallothionein. This difference in the effectiveness of Vitamin E and Zn for hyperoxic protection in lung injury may be due to the specificity of antioxidant function, i.e., vitamin E inhibits oxidation of membrane lipids and Zn protects sulfhydryl groups of proteins.

Mechanism of nephrotoxicity induced by repeated administration of cadmium chloride in rats

To explore the mechanism of Cd nephrotoxicity, CdCl2 was subcutaneously injected to rats, at 3 mg Cd/kg body weight once a day, for 8 d. In the liver, Cd bound to metallothioneins (MTs-Cd) rose from d 1 after the initiation of CdCl2 administration, and reached a plateau after the administration ceased. In the plasma, MTs-Cd rose from d 4, peaked on d 8, and gradually fell thereafter. In the kidneys, leucine aminopeptidase (LAP) and N-acetyl beta-D-glucosaminidase (NAG) fell during d 6-20, and Cd bound to cellular membranes (Mem-Cd) rose from d 1 and reached a plateau during d 6-20. The Mem-Cd levels were significantly correlated with the reduction in the LAP and NAG activity; the values of MTs-Cd plus Mem-Cd were almost equivalent to those of total Cd. These findings showed that the hepatic synthesis of MTs-Cd occurred followed by its release into plasma; the extent of renal injury was aggravated as the plasma level of MTs-Cd rose; and a greater part of the renal Cd distributed intracellularly as the MTs-binding form, while the residual Cd distributed as the cellular membrane-binding form. Also, it was suggested that Cd that occurred as the cellular membrane- binding form in the kidneys was involved in manifestation of renal injury.

Accumulation of orally given cadmium in Long-Evans Cinnamon (LEC) rats with an inherently abnormal copper metabolism

An inherent defect of biliary Cu excretion and subsequent Cu deposition in the liver have been found in Long-Evans Cinnamon (LEC) rats, which are promising models of Wilson disease. LEC and Fischer rats were given water containing Cd (CdCl2) at a level of 5 ppm for 30 days. Regardless of drinking Cd water, LEC rats showed a very high concentration of Cu (200 to 250 microgram/g) and Cu-metallothionein (Cu-MT) (18 mg/g) in the liver. There was no difference of Cd accumulation in the liver between the two strains exposed to Cd (2.6 and 2.7 microgram/g in the Fischer and LEC groups, respectively). However, the renal Cd concentration was slightly but significantly higher in LEC rats (3.5 microgram/g) than in Fischer rats (2.0 microgram/g). The ratio of renal Cd contents to the sum of renal and hepatic Cd contents was significantly higher in LEC rats (0.25) than in Fischer rats (0.15). The serum Cd concentration in Cd-treated LEC rats increased threefold compared to Cd-treated Fischer rats. It seems likely that Cd from the liver is transported into the kidney in the form of Cd, Cu-MT. There was no difference in uptake of Cd in the hepatic MT fraction between the two strains. Although biliary Cu excretion in LEC rats was significantly lower than that in Fischer rats, reduced excretion of Cd into bile was not found in LEC rats. The gross amounts of Cu and Cu-MT influenced the accumulation of Cd in the kidney rather than in the liver when Cd was given orally at a low level to LEC rats. Our results suggest tht Cu and Cd do not share the same sites of hepatobiliary excretion in rats, although the main route of their excretion is via bile.

Visualization of yellowish-orange luminescence from cuprous metallothioneins in liver of Long-Evans Cinnamon rat

We describe the first use of an emission probe, based on the cuprous thiolate chromophore, for direct microscopical observation of cuprous metallothioneins located in liver of 15-week-old (just before spontaneous hepatitis) Long-Evans Cinnamon rats. The rats show remarkable accumulations of copper and cuprous metallothioneins. In the mildly fixed liver, we visualized the same yellowish-orange luminescence as the specific emission from cuprous metallothioneins, following excitation in 330-385 nm region. In liver from Long-Evans Agouti rat, a counter part of Long-Evans Cinnamon rat, no similar luminescence was found. So, it was thought that cuprous metallothioneins accumulated in the Long-Evans Cinnamon rat liver might emit the yellowish-orange light. To verify this presumption, we tentatively defined three histochemical criteria, quenching tests by oxidation, protonation and mercury treatment, based on the coordination chemical characteristics of metallothioneins. The emission completely satisfied these criteria. Furthermore, the reliability of these criteria was supported by immunocytochemical and biochemical results. Consequently, all results sufficiently indicate that the yellowish-orange luminescence in the Long-Evans Cinnamon rat liver is the emission from cuprous metallothioneins.

Metallothionein research in terrestrial invertebrates: synopsis and perspectives

While most of metallothionein research during the past years has been carried out on mammals or vertebrates, only relatively few studies have been directed towards invertebrates. Even fewer investigations have focussed on terrestrial invertebrates. The best studied metallothioneins and/or metallothionein genes among terrestrial invertebrates are those from an insect species (Drosophila melanogaster), a nematode (Caenorhabditis elegans) and some terrestrial gastropods (Helix pomatia, Arianta arbustorum). From these few examples it already appears that terrestrial invertebrate metallothioneins provide intriguing models to better understand the multiplicity of functions of these proteins and their evolution within the animal kingdom. Like in mammals, metallothioneins in terrestrial invertebrates seem to perform different functions simultaneously. This is exemplified by terrestrial gastropods, which are able to accumulate different metals in different tissues, in which metal-specific metallothionein isoforms or conformation forms are expressed, allowing these organisms to detoxify more efficiently nonessential trace elements such as cadmium, and at the same time to maintain the homeostasis of essential trace elements such as copper. A major proportion of metallothionein research in terrestrial invertebrates addresses the ecophysiological and ecotoxicological significance of these proteins with regard to the increasing risk due to chemical pollution. One promising aspect in this concern is the potential utilization of metallothioneins as biomarkers for risk assessment in terrestrial environments.

Response of hepatic function to hepatic copper deposition in rats fed a diet containing copper

Fischer rats were a fed diet supplied with copper chloride (150-600 ppm) for 60 d from weaning. Serum (glutamic-oxaloacetic transaminase (GOT) and glutamic-pyruvic transaminase (GPT) activities were increased with the increase of Cu concentration in the diet. Biliary excretion of Cu was related to the dietary Cu level. Depositions of hepatic and renal Cu were also related to the dietary Cu level in a dose-dependent manner. In particular, hepatic (155.2 +/- 13.3 micrograms/g) and renal (44.9 +/- 4.4 micrograms/g) Cu concentrations increased abruptly in the Cu-600 ppm group. In the liver, about 60% of Cu was distributed in the soluble fraction (100,000 g supernatant). In the Cu-600 ppm group, 25% of cystosolic Cu was bound to metallothionein (MT). Our results suggest that chronic exposure to Cu appears to have a deleterious effect on the hepatic function, and further, that even in rats with normal biliary Cu excretion, clearance of Cu from the liver may be marginal when dietary Cu is near the 600-ppm level. Although Cu is an essential nutrient, an overload of Cu should be avoided.

Exposure of human proximal tubule cells to cytotoxic levels of CdCl2 induces the additional expression of metallothionein 1A mRNA

Humans, in contrast to animals, have a complex expression of metallothionein (MT) genes which involves many MT isoforms encoded by a family of genes containing an upper limit of 12 possible functional genes. It is unknown if these human isoforms of MT have distinct functions or if they simply represent a non-essential duplication of gene function. In the present study, MT protein and mRNA for the MT-2A, MT-1A, B, E, F, and G genes was determined for 3 isolates of human proximal tubule (HPT) cells having distinct sensitivities to cadmium. For all 3 HPT isolates, the expression of MT protein and mRNA for the MT-2A, MT-1E, MT-1F and MT-1G isoforms was similar among the isolates and demonstrated no correlation to lethality. However, each isoform mRNA was expressed at different levels when compared to one another. In contrast, the expression of MT-1A mRNA differed in expression and correlated with the differing lethalities displayed by each isolate. The finding of different profiles of mRNA expression provides evidence that the MT isoforms may have unique functions and that mRNA for the MT-1A gene could be a potential marker for heavy metal exposure and/or toxicity.

A copper deficient diet prevents hepatic copper accumulation and dysfunction in Long-Evans Cinnamon (LEC) rats with an abnormal copper metabolism and hereditary hepatitis

Long-Evans Cinnamon (LEC) rats that develop spontaneous hepatitis due to an inherently abnormal Cu metabolism have recently been established. This investigation concerns the effects of a Cu-deficient diet on the Cu metabolism linked to hepatic injury in LEC rats. The hepatic Cu concentration at 30 days after birth was 94 +/- 4 Cu micrograms/g liver in LEC rats, whereas that of Fischer rats at the same age was 7 +/- 1 Cu micrograms/g. From 30 days after birth, all rats were fed a semisynthetic diet with two different levels of Cu, 0.5 or 30 micrograms/g food, for 35 days. In LEC rats fed a Cu-deficient diet (0.5 microgram/g), the hepatic Cu concentration was 39 +/- 7 micrograms/g. The Cu-normal diet (30 micrograms/g) LEC group had a concentration of 357 +/- 15 micrograms/g in the hepatic Cu. The group had significantly higher aspartate aminotransferase (ASAT), alanine aminotransferase (ALAT) and gamma-glutamyl transferase (GGT) levels than did the LEC rats given the Cu-deficient diet. These results suggest that the occurrence of acute hepatitis in LEC rats can be prevented by feeding the animals a Cu-deficient diet.

Biliary excretion of copper in Fischer rats treated with copper salt and in Long-Evans cinnamon (LEC) rats with an inherently abnormal copper metabolism

Increased biliary Cu excretion was found in Fischer rats injected with Cu. The biliary Cu was located at the void (large-molecule region) and total (small-molecule region) volume of a Sephadex G-75 column. The most Cu was found in the total volume. The two Cu peaks comigrated with absorbance at 280 nm. Although the bile from Cu-untreated Fischer rats did not show Cu absorbance in the total volume, absorbance at 280 nm was also found in this region. Even though Long-Evans Cinnamon (LEC) rats deposited a gross amount of Cu (194.0 +/- 27.8 micrograms/g liver) in the liver, they conversely showed reduced Cu excretion into the bile. LEC bile did not show Cu absorbance but rather absorbance at 280 nm in the total volume. Therefore, it seems unlikely that the small molecules found in the Sephadex G-75 regulate biliary Cu excretion in Cu-loaded rats, although the molecules bind to Cu. When the bile from Cu-untreated Fischer and LEC rats was incubated with CuCl2 solution, the most Cu was recovered in the total volume of this column. Our results suggest that reduced biliary Cu excretion in LEC rats is not related to the small molecules, and that Cu cannot be excreted in the form of macromolecules in rats to decrease Cu from the Cu-loaded liver.

Role of cytosolic copper, metallothionein and glutathione in copper toxicity in rat hepatoma tissue culture cells

Effects of metallothionein (MT) synthesis inhibiting compounds (actinomycin D, cycloheximide), MT synthesis stimulating compounds (dexamethasone, dibu-cAMP) and interfering metals (Cd, Zn) on copper accumulation were investigated in rat hepatoma tissue culture cells. Copper-metallothionein (Cu-MT) and MT-associated copper levels were determined to find a possible correlation between cytosolic copper concentrations and MT as a Cu-detoxifying protein. Further, intracellular non-MT associated copper levels and levels of GSH and SOD were determined. Cell viability was tested under all experimental conditions by measuring LDH-release, K+ uptake and total cell protein. Administration of dexamethasone and dibu-cAMP showed no effect on MT levels (compared with controls), and only a marginal effect on 64Cu and total Cu accumulation. Administration of actinomycin D resulted in increased copper accumulation in the particulate fraction, possibly due to inhibition of copper secretion processes and/or protein synthesis. Presence of zinc had no effect on MT levels nor on total Cu and 64Cu levels, in contrast with cadmium which drastically enhanced copper accumulation and MT levels in the cells. Cu/MT ratios varied from 1.0 +/- 0.3 to 3.3 +/- 1.2, which is far below the assumed maximum molar ratio of 8-12 mol Cu per mol MT. SOD levels appeared to be enhanced up to 2- or 3-fold in the presence of Cd2+, relative to control values. The role of GSH as Cu-intermediate in intracellular Cu distribution plus its role in copper defence mechanism(s) was tested by application of BSO, an inhibitor of GSH synthesis. It was found that BSO had no effect on intracellular MT level; it was found however that MT-bound copper levels were markedly decreased. The results presented support a model for copper metabolism in hepatoma tissue culture (HTC) cells, where Cu(I) is complexed by GSH immediately after entering the cell. GSH is capable of transferring copper to MT where it is stored. Depletion of GSH (by administration of Cd2+, actinomycin D, cycloheximide) almost instantaneously results in enhanced cellular toxicity. When also MT is depleted (by actinomycin D) non-MT associated, 'free' cytosolic Cu2+ is elevated, and HTC cells rapidly loose their resistance to copper toxicity, as also reflected in loss of cell viability (LDH, K+ and total cell protein).

Biliary excretion of exogenous cadmium and manganese in Long-Evans Cinnamon (LEC) rats characterized by an inherently gross amount of copper-metallothionein in the liver

Long-Evans Cinnamon (LEC) rats are characterized by the sudden onset of hepatitis around 4 months after birth and the gross accumulation of hepatic copper (Cu) accompanied by metallothionein (MT). The biliary excretion of manganese (Mn) and cadmium (Cd) injected intravenously was studied in 3-month-old LEC rats without signs of hepatitis. Injected Mn was excreted into the bile in LEC and Fischer rats used for comparison. However, increased biliary excretion of Cd was found not in the LEC rat but in the Fischer rat. Excretion of horseradish peroxidase (HRP) injected along with the metal mixture was significantly lower in the LEC group than in the Fischer group. Our results suggest that Mn excretion is not related to the existence of a gross amount of Cu-MT. Reduced excretion of Cd may be partly due to binding to Cu-MT in the liver. Decreased excretion of HRP implies the existence of an inherent defect in the bile excretion route for endo- and exogenous substances.

Effects of copper on rat hepatoma HTC cells and primary cultured rat hepatocytes

Cellular responses to copper, applied in concentrations varying from 0.5 to 200 microM Cu2+, were investigated in two different cell types: rat hepatoma cells (HTC) and primary cultured rat hepatocytes. Accumulation of 64Cu, copper (AAS) levels, cellular viability parameters (cell growth and proliferation, LDH leakage, total cell protein, K+ uptake, and ATP levels), and cell toxicity parameters (metallothionein (MT), glutathione (GSH) and superoxide dismutase (SOD)) were examined over 24 hr incubation periods. Accumulation of radiolabeled copper (applied copper concentrations: 15-200 microM Cu2+) showed a four-fold increase in HTC cells (0.88-3.45 nmol Cu/mg cell protein) and a three-fold increase in hepatocytes (4.94-14.66 nmol Cu/mg cell protein), although quantitative uptake in HTC cells was five times lower. Most of the copper accumulated in the hepatoma cells and hepatocytes was found predominantly in the particulate fraction (i.e., cell membranes and organelles), while only a small quantity was present in the soluble fraction (cell cytosol). Metallothionein concentrations in HTC cells were increased from 43 pmol/mg cell protein (0.5 microM Cu2+ application) up to 223 pmol/mg cell protein (200 microM Cu2+ application), whereas MT in rat hepatocytes were elevated from 139 pmol/mg cell protein to 546 pmol/mg cell protein over the same range of administered Cu2+. Metallothionein synthesis rendered both cell types well equipped to deal with increasing intracellular copper levels. In hepatocytes however, MT synthesis resulted in decreasing non-MT-associated copper levels in the cytosol for Cu administrations up to 100 microM. Above that point however, MT failed to stay in line with increasing cytosolic Cu levels, resulting in cytotoxic effects shown by changes in cell viability and GSH/SOD levels. In HTC cells MT synthesis suppressed the free Cu levels in the cytosol to below 0.1 nmol Cu/mg cell protein over the total range of copper concentrations applied. The results presented indicate that hepatoma HTC cells are more capable of dealing with high accumulated Cu levels than the better established rat hepatocytes. Furthermore, it is clear that comparison of these two cell types regarding their ability to respond on (sub)toxic Cu should be discussed with full consideration for the copper applications involved.

Determination of zinc-65, copper-64 and sulphur-35 labelled rat hepatoma tissue culture metallothioneins by high-performance liquid chromatography with on-line radioactivity detection

Molecular size exclusion (MSE), reversed-phase (RP), and anion-exchange (AE) high-performance liquid chromatography (HPLC) techniques were employed in combination with on-line radioactivity detection, in a study on the kinetic behaviour of 65Zn-, 64Cu- and [35S]cysteine-labelled metallothionein (MT) in rat hepatoma tissue culture (HTC) cells. MSE-HPLC of [35S]cysteine-labelled HTC cell cytosol resulted in co-eluting MT-I and MT-II isoforms (tR 19.80 min; Ve/Vo: 1.85). AE-HPLC of 65Zn-treated HTC cell cytosol yielded separated 65Zn MT-I (tR 11.5 min; I = 64 mM) and 65Zn MT-II (tR 14.5 min; I = 104 mM). RP-HPLC of 64Cu-treated HTC cytosol resulted in separated 64Cu MT-I (tR 26.4 min) and 64Cu MT-II (tR 23.4 min). Determination of the amino acid composition, apparent molecular mass and cysteine content of HTC MT-I and MT-II isoforms showed the characteristics of class I metallothioneins. The rate of dissociation of Zn2+ from Zn-MT could be determined from the losses of 65Zn from MT during a single AE-HPLC run, showing a Zn-MT dissociation half-life of 0.66 h. RP-HPLC showed a delay in incorporation of newly accumulated 64Cu into MT, possibly owing to the appearance of reduced glutathione as an intracellular copper-transfer compound. Application of compartmental analysis in [35S]-cysteine accumulation experiments permitted the determination of the actual rate of MT degradation; when 200 microM of Zn were applied, the MT degradation half-life was 2.0 +/- 0.8 h. These results indicate the potential of combined HPLC techniques and application of radionuclides in studies on the synthesis and degradation of MT and metal-MT complexes.