Relationships among metallothionein, cadmium accumulation, and cadmium tolerance in three species of fish

Chemical and biochemical responses to sub-lethal doses of mercury and cadmium in gilthead seabream (Sparus aurata)

Specimens of Sparus aurata were exposed to sub-lethal concentrations of Hg and Cd for 25 days and the levels of both metals were investigated in organs and tissues. Bioaccumulation of Hg decreased as follow: gills > kidney > liver > skin > muscle, while the order of Cd bioaccumulation was: liver > kidney > gills > skin > muscle. Immediately after exposure, both metals showed the highest bioaccumulation in gills and skin indicating that these organs are reliable targets for biomonitoring studies after short term exposure. Metals introduction caused a significant time-dependent concentrations increase in kidney and liver, while in the muscle a significant increase of Hg was recorded only at the end of the experimentation. The effects of exposure were also investigated, at biochemical level, in the liver, which represents the main target of xenobiotics biotransformation and metabolism in fish. Exposed fishes exhibited a reduction of total lipid level, a decrease of polyunsaturated fatty acids (PUFA), together with a MDA increase. This suggests a direct effect of contaminants on oxidative stress induction that, through the MDA increase, altered the membrane fatty acids composition decreasing the PUFA content. As it regards molecular markers related to oxidative stress and lipid metanolism, a significant increase of Nrf2, Hif-1α and Ampk and a decrease of Fas were observed after exposure to both metals, while an Nf-kB increase was recorded in specimens exposed to Hg, docuemnting a correlation with oxidative stress and consequent metabolism adaptation. Finally, these results suggest the possibility to adopt these biomarkers to explore fish metabolic responses to environmental pollution.

Duration of Induced Metallothionein is a Critical Factor in the Cadmium-Resistance Among Goldfish (Carassius auratus) and Common Carp (Cyprinus carpio)

According to the Environmental Protection Agency in Taiwan, the common carp (Cyprinus carpio) is one species of fish for acute toxic test. It has been found to be extremely sensitive to the toxicity of Cd²⁺; Furthermore, the goldfish (Carassius auratus) has a higher resistance than common carp upon Cd²⁺ exposure, but both fish are the same family. The aim of the study was to compare the physiological and histo-pathological responses between goldfish and common carp under exposure to sublethal concentrations of Cd²⁺ in order to understand the reasons behind the Cd²⁺-resistance. Results showed that metallothionein (MT) protein levels in visceral tissues were exceptionally increased and elevated at an earlier time in goldfish than in common carp. Meanwhile, the amount of Cd²⁺ accumulation in goldfish was higher than common carp after Cd²⁺ exposure. The histo-pathological results revealed that the density of gill mucus cells and the thickness of gill epithelium in common carp were raised earlier than in goldfish, but the histo-pathological findings resemble each other. According to the data, we suggested the efficient response of MT proteins may contribute to goldfish with a higher Cd²⁺ tolerance.

Differential regulation of endoplasmic reticulum stress-induced autophagy and apoptosis in two strains of gibel carp (Carassius gibelio) exposed to acute waterborne cadmium

Previous studies illustrated that gibel carp F strain displays better lipid mobilization and antioxidant ability and compared to the A strain. We therefore hypothesized that the F strain would exhibit superior defense to cadmium exposure. Comparative studies were conducted between A and F strains using plasma stress biomarkers, histological observations, and analysis of hepatic molecular events to examine exposure to waterborne Cd (11.9 mg L^-1) for 48 h and 96 h. Waterborne Cd exposure stimulated stress response and hepatic metallothionein mRNA induction in both gibel carp strains confirming exposure. Antioxidant responses were stimulated to counteract Cd toxicity, suggested by the upregulation of mRNA levels of genes associated with nuclear factor erythroid 2-related factor 2 (nrf2) signaling. Cd exposure induced endoplasmic reticulum (ER) stress, meanwhile, branches of genes in unfolded protein response (UPR) were activated. Slight time-dependent effects were implied by greater ER stress, UPR, and apoptosis signals with the duration of Cd exposure. Genotype-specific effects were identified, revealing that the F strain showed greater stress at 96 h exposure and higher antioxidant response compared to the A strain, as indicated by the mRNA levels of genes in nrf2 signaling. ER stress and UPR were also stronger in the F strain after Cd exposure. In contrast, the A strain showed higher autophagy and apoptosis response compared to the F strain. Collectively, combined autophagy and apoptosis were triggered under ER stress, which might serve as defense strategies in both gibel carp strains. The F strain showed greater antioxidant detoxification response and UPR to mitigate Cd toxicity, whereas excessive ER stress contributed to higher autophagy and apoptosis in the A strain. The present study uncovered the differential regulation and defense strategies in fish strains exposed to metal exposure.

Cadmium exposure in living organisms: A short review

Cadmium (Cd) is a toxic heavy metal that accumulates in living systems. Exposure can occur occupationally or environmentally. Workers within the electroplating, battery production, and pigment industries are at the highest risk for exposure and have been reported to have increased levels of Cd in their blood and urine. Environmental exposure can be the result of anthropogenic activities or smoking. Cd has a long half-life and bio accumulates in plants, invertebrates, and vertebrates. The toxic effects following exposure include growth retardation and organ system toxicity, with kidney and liver toxicity most reported with in higher organisms. At the molecular level, Cd leads to the production of reactive oxygen species, DNA damage, and inhibition of DNA repair. This article gives a brief overview of the correlations between exposure to cadmium occupationally and environmentally and levels measured in blood and urine. It also examines the bioaccumulation of cadmium in aquatic invertebrates and vertebrates indicating that accumulation varies not only by location but also within and between various species.

Survey of heavy metals in internal tissues of Great cormorant collected from southern wetlands of Caspian Sea, Iran

The level of mercury, iron, copper, and zinc was measured in 18 Great cormorants (Phalacrocorax carbo) collected from Anzali and Gomishan wetlands in the south of the Caspian Sea. The mean level of metals in dried tissues of the muscle, liver, and kidney was 2.26, 5.71, 3.79-Hg; 943.54, 379.97, 348.05-Fe; 42.64, 14.78, 60.79-Cu, and 71.97, 134.63, 77.82-Zn, respectively (mg/kg). There was no significant different between genders in terms of accumulation of metals, except for copper in the kidney. The results of Pearson correlation showed a positive and strong relationship between the fat in the liver and mercury (r = 0.95, R² = 0.90). Also, there was a significant difference between the values of all metals with the allowable limits presented in EPA, WHO, and CCME, where all of values were above standard levels. Thus, as the muscles of the bird are sometimes eaten by humans, this result is a serious warning. Nevertheless, the relatively high levels of heavy metals accumulated in different tissues of Great cormorant at that time are a result of their high weight and nourishment they have at the terminal days of their migration due to lack of natural physical activity. Regarding to the importance of heavy metals in birds, we suggest the same study to be conducted on the species in other seasons and wetlands.

Anti-oxidative functions of mt2 and smtB mRNA expression in the gills and brain of zebrafish (Danio rerio) upon cadmium exposure

There were not any past studies about metallothionein isoforms (smtB and mt2) having anti-oxidative functions on zebrafish after Cd²⁺ exposure. On the other hand, the anti-oxidative enzymatic factors such as superoxide dismutase (sod), glutathione peroxidase (gpx1a), and catalase (cat) are used as references to investigate whether the smtB and mt2 have anti-oxidative responses on the gills and brain of zebrafish after 1-6 h of 0 and 1.78 μM Cd²⁺ exposure. The anti-oxidative system such as sod, cat, and gpx1a mRNA expressions demonstrated a cascade response upon Cd²⁺-induced oxidative stress in the present study. Interestingly, the smtB mRNA expression levels increased by 3.2- to 6.1-fold, and mt2 raised by 4.1- to 11.3-fold in gills at 1 and 3 h after exposure to Cd²⁺, respectively. On the other hand, the smtB mRNA levels increased by 10.6- to 58.6-fold, but mt2 mRNA levels increased by 2.3- to 11.1-fold in brain at 1 and 3 h after exposure to Cd²⁺, respectively. In addition, both tissues showed increased apoptosis levels at 3 h, and recovery after 6 h of Cd²⁺ exposure. From the results, we suggest that both mt2 and smtB play a role in anti-oxidation responses within 6 h after exposure to Cd²⁺. In conclusion, the smtB mRNA levels have a higher response than mt2 in the brain, but both mRNA expressions appear to have a similar pattern in the gill. We suggest that smtB plays an important role to defend oxidative stress in the brain of adult zebrafish upon acute Cd²⁺ exposure.

Characterization and function analysis of Hsp60 and Hsp10 under different acute stresses in black tiger shrimp, Penaeus monodon

Heat shock proteins (Hsps) are a class of highly conserved proteins produced in virtually all living organisms from bacteria to humans. Hsp60 and Hsp10, the most important mitochondrial chaperones, participate in environmental stress responses. In this study, the full-length complementary DNAs (cDNAs) of Hsp60 (PmHsp60) and Hsp10 (PmHsp10) were cloned from Penaeus monodon. Sequence analysis showed that PmHsp60 and PmHsp10 encoded polypeptides of 578 and 102 amino acids, respectively. The expression profiles of PmHsp60 and PmHsp10 were detected in the gills and hepatopancreas of the shrimps under pH challenge, osmotic stress, and heavy metal exposure, and results suggested that PmHsp60 and PmHsp10 were involved in the responses to these stimuli. ATPase and chaperone activity assay indicated that PmHsp60 could slow down protein denaturation and that Hsp60/Hsp10 may be combined to produce a chaperone complex with effective chaperone and ATPase activities. Overall, this study provides useful information to help further understand the functional mechanisms of the environmental stress responses of Hsp60 and Hsp10 in shrimp.

Mapping differential elemental accumulation in fish tissues: assessment of metal and trace element concentrations in wels catfish (Silurus glanis) from the Danube River by ICP-MS

Studies of metal accumulation in fish are mainly focused on the muscle tissue, while the metal accumulation patterns in other tissues have been largely neglected. Muscle is not always a good indicator of the whole fish body contamination. Elemental accumulation in many fish tissues and organs and their potential use in monitoring programs have not received proper attention. In the present study, As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Se, and Zn concentrations were assessed by inductively coupled plasma mass spectrometry (ICP-MS) in the following 14 tissues of the wels catfish (Silurus glanis) from the Danube River: muscle, gills, spleen, liver, kidneys, intestine, gizzard, heart, brain, gallbladder, swim bladder, vertebra, operculum, and gonads. A high level of differential elemental accumulation among the studied tissues was observed. The maximum overall metal accumulation was observed in the vertebra, followed by the kidneys and liver, with the metal pollution index (MPI) values of 0.26, 0.25, and 0.24, respectively. The minimum values were observed in the gallbladder, muscle, brain, and swim bladder, with MPI values of 0.03, 0.06, 0.07, and 0.09, respectively. Average metal concentrations in the fish muscle were below the maximum allowed concentrations for human consumption. The mean As, Cd, Pb, Cu, Fe, and Zn concentrations in the muscle were 0.028, 0.001, 0.001, 0.192, 3.966, and 3.969 μg/g wet weight, respectively. We believe that the presented findings could be of interest for the scientific community and freshwater ecosystem managers. There is a need for further research that would assess less studied tissues in different fish species.

Zn subcellular distribution in liver of goldfish (carassius auratus) with exposure to zinc oxide nanoparticles and mechanism of hepatic detoxification

Zinc Oxide Nanoparticles (ZnO NPs) have attracted increasing concerns because of their widespread use and toxic potential. In this study, Zn accumulations in different tissues (gills, liver, muscle, and gut) of goldfish (Carassius auratus) after exposure to ZnO NPs were studied in comparison with bulk ZnO and Zn²⁺. And the technique of subcellular partitioning was firstly used on the liver of goldfish to study the hepatic accumulation of ZnO NPs. The results showed that at sublethal Zn concentration (2 mg/L), bioaccumulation in goldfish was tissue-specific and dependent on the exposure materials. Compared with Zn²⁺, the particles of bulk ZnO and the ZnO NPs appeared to aggregate in the environmentally contacted tissues (gills and gut), rather than transport to the internal tissues (liver and muscle). The subcellular distributions of liver differed for the three exposure treatments. After ZnO NPs exposure, Zn percentage in metal-rich granule (MRG) increased significantly, and after Zn²⁺ exposure, it increased significantly in the organelles. Metallothionein-like proteins (MTLP) were the main target for Zn²⁺, while MRG played dominant role for ZnO NPs. The different results of subcellular distributions revealed that metal detoxification mechanisms of liver for ZnO NPs, bulk ZnO, and Zn²⁺ were different. Overall, subcellular partitioning provided an interesting start to better understanding of the toxicity of nano- and conventional materials.

Levels of heavy metals in tissues of shingi fish (Heteropneustes fossilis) from Buriganga River, Bangladesh

Heavy metal pollution was reported in commercially valuable freshwater edible fish in the Buriganga River, Bangladesh. The concentrations of As, Pb, Cd, Cr, Zn, and Cu were investigated in the muscle, gill, stomach, intestine, and liver of Heteropneustes fossilis caught at three stations to assess the degree of fish pollution by heavy metals. Significant differences in concentrations of analyzed elements were observed among different tissues, but not among the stations. The ranges of the measured concentrations (μg/g dry weight) in the tissues of H. fossilis were as follows: arsenic concentration was (0.2-0.4), (0.82-0.90), (3.29-3.99), (2.20-2.80), and (2.41-2.90), that of lead was (1.79-2.20), (4.95-6.55), (10.36-13.38), (5.74-9.70), and (18.20-18.79), that of cadmium was (0.3-0.4), (2.87-4.27), (1.03-1.63), (1.55-4.59), and (2.25-5.50), that of chromium was (1.40-1.70), (3.52-3.72), (2.28-5.29), (2.77-3.79), and (4.25-8.65), that of zinc was (24.47-28.82), (16.82-18.80), (20.22-22.20), (22.86-26.68), and (60.82-67.80), and that of copper was (7.80-8.50), (6.22-6.81), (38.21-44.25), (17.07-21.03), and (43.24-47.30) in the muscle, gill, stomach, intestine, and liver, respectively. This research showed that the liver appeared to be the main heavy metal storage tissue, while the muscle had the lowest levels of analyzed metals. The concentrations of metal in the muscles not exceeded the acceptable levels for a food source for human consumption.

Review on methods for determination of metallothioneins in aquatic organisms

One aspect of environmental degradation in coastal areas is pollution from toxic metals, which are persistent and are bioaccumulated by marine organisms, with serious public health implications. A conventional monitoring system of environmental metal pollution includes measuring the level of selected metals in the whole organism or in respective organs. However, measuring only the metal content in particular organs does not give information about its effect at the subcellular level. Therefore, the evaluation of biochemical biomarker metallothionein may be useful in assessing metal exposure and the prediction of potential detrimental effects induced by metal contamination. There are some methods for the determination of metallothioneins including spectrophotometric method, electrochemical methods, chromatography, saturation-based methods, immunological methods, electrophoresis, and RT-PCR. In this paper, different methods are discussed briefly and the comparison between them will be presented.

Ultrastructural and immunohistochemical investigation on the gills of the teleost, Thalassoma pavo L., exposed to cadmium

An investigation was conducted to determine the effects of the heavy metal, cadmium (Cd), on the gills of the teleost fish, Thalassoma pavo Linnaeus, 1758. The fishes were exposed to several sublethal concentrations of cadmium (10, 40, 60 and 120 μM (mg/L)) for a period of 48, 96 and 192 h. The value of the LC50 after 96 h of cadmium exposure, determined using the System of Finney, was equal to 128.3 μM. The gills of the fishes were examined by light and electron microscopy. Toxic, apoptotic and cadmium effects were analyzed using some neuropeptides, metallothioneins (MT), caspase 3, PCNA and calmodulin, as bioindicators, respectively. The results showed that the alterations in the gills were proportional to the exposure periods and concentrations of the metal, which were found to be both dose and time dependent. The biological responses in the gills of the tested animals are discussed in relation to results obtained by analysis of the biomarkers. These data may be used for the planning of a model to determine biological risk in the marine environment and may be particularly useful to investigate organisms exposed to cadmium.

Accumulation and oxidative stress biomarkers in Japanese flounder larvae and juveniles under chronic cadmium exposure

This study investigated how Cd exposure affected oxidative biomarkers in Japanese flounder, Paralichthys olivaceus, at early life stages (ELS). Fish were exposed to waterborne Cd (0-48microg L(-1)) from embryonic to juvenile stages for 80days. Growth, Cd accumulation, activities of superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), glutathione S-transferase (GST, EC 2.5.1.18), and levels of glutathione (GSH) and lipid peroxidation (LPO) were investigated at three developmental stages. Flounder growth decreased and Cd accumulation increased with increasing Cd concentration. In metamorphosing larvae, CAT and SOD activities were inhibited and GSH level was elevated, while LPO was enhanced by increasing Cd concentrations. CAT and GST activities of settling larvae were inhibited but GSH level was elevated at high Cd concentrations. In juveniles, SOD activity and LPO level were increased but GST activity was inhibited as Cd concentration increased. Antioxidants in flounder at ELS were able to develop ductile responses to defend against oxidative stress, but LPO fatally occurred due to Cd exposure. These biochemical parameters could be used as effective oxidative biomarkers for evaluating Cd contamination and toxicity in marine environments: CAT, SOD, GSH, and LPO for metamorphosing stage; CAT, GSH, and GST for settling stage; and SOD, GST, and LPO for juvenile stage.

The assessment of natural causes of metallothionein variability in the gills of European chub (Squaliuscephalus L.)

The possible causes of the variability of gill metallothionein (MT) levels were studied on 182 specimens of 2- and 3-year old European chub (Squaliuscephalus L.) from the Sava River in Croatia. The most pronounced differences in MT levels were obtained between three sampling campaigns, and especially between periods with presumably different metabolic activities (April/May 2006 vs. September 2005 and 2006). Next to the probable influence of metabolic activity on MT level, the correlation analysis indicated a significant association between MTs and the fish size. Differences between males and females, as well as between mature and non-mature fish, were not observed in the young group of studied chub even in the spring reproductive season. Based on the analysis of the site-specific MT variability, it could be concluded that, under the conditions of low dissolved metal concentrations in the river water (as reported for the Sava River), MTs seem to be more affected by different biotic factors than by metal exposure. Therefore, MTs measured in this study were considered as the constitutive gill MT levels of young European chub. The constitutive MT ranges were defined separately for the season of lower metabolic rate (autumn: 1.31-2.00 mg g(-1)) and the season of higher metabolic activity (spring: 2.15-2.95 mg g(-1)).

Expression of mt2 and smt-B upon cadmium exposure and cold shock in zebrafish (Danio rerio)

Metallothionein-2 (mt2) and similar to metallothionein-B (smt-B) are included in the MT gene family. The objective of this study was to compare mt2 and smt-B messenger (m)RNA expressions after cadmium exposure and cold shock with whole-mount in situ hybridization in immature zebrafish (Danio rerio) and with a semi-quantitative RT-PCR in mature zebrafish. Three-day post-fertilization (dpf) larvae were treated with 0, 0.08, 0.26, and 0.89 microM cadmium for 24 and 48 h, and some larvae were challenged with a normal (28.5 degrees C) or low temperature (12 degrees C) for 12, 24, and 48 h. Results were obtained. (1) During embryonic and larval development, mt2 mRNA existed at 6 h post-fertilization (hpf), and the level rapidly increased to 24 hpf, then it gradually increased with further larval growth. smt-B was found at 12 hpf, and it also rapidly increased to 24 hpf, but remained constant during further larval development. (2) The mt2 mRNA signals and whole-body Cd contents displayed dose- and time-dependent responses after Cd exposure. After cold shock, mt2 mRNA signals also showed time-dependent expression. But smt-B mRNA signals were not appeared by either challenge. Besides, mature zebrafish were treated with 1.78 microM Cd and found that the highest levels of smt-B mRNA (smt-B/beta-actin) appeared in brain, and seems a reverse expression between smt-B mRNA and mt2 in brain after Cd exposure. Apparently, mt2 is possibly more relevant to Cd detoxification and cold shock adaptation in zebrafish larvae compared to smt-B, but smt-B might be related to certain physiological functions in neural (or brain) of mature zebrafish.

Influence of Cadmium(II) Ions and Brewery Sludge on Metallothionein Level in Earthworms (Eisenia fetida) - Bio- transforming of Toxic Wastes

Metallothioneins belong to a group of intracellular, high molecular and cysteine-rich proteins whose content in an organism increase with increasing concentration of a heavy metal. The aim of this work was to apply the electrochemical analysis for the analysis of metallothioneins in earthworms exposed to cadmium ions and brewery sludge. Here we utilized adsorptive transfer technique coupled with differential pulse voltammetry Brdicka reaction to determine metallothionein in different biological samples. By means this very sensitive technique it was possible to analyze metallothionein in concentrations below 1 μmol.l⁻¹ with the standard deviation of 4-5%. We found out that the average MT level in the non-treated earthworms oscillated between 19 and 48 μmol.l⁻¹. When we analysed samples of earthworms treated by cadmium, we observed that the MT content increased with the exposition length and increase dose of cadmium ions. Finally, we attempted to study and compare the toxicity of the raw sludge and its leach by using of earthworms. The raw brewery sludge caused the death of the earthworms quickly. Earthworms held in the presence of leach from brewery sludge increased their weight of 147 % of their original weight because they ingested the nutrients from the sludge. The metallothionein level changes markedly with increasing time of exposition and applied dose of toxic compound. It clearly follows from the obtained results that the MT synthesis is insufficient in the first hours of the exposition and increases after more than 24 h.