Metallothionein induction in early larval stages of tilapia (Oreochromis mossambicus)

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.

Exposure to environmentally relevant cadmium concentrations negatively impacts early life stages of channel catfish (Ictalurus punctatus)

Cadmium is a persistent contaminant of surface waters. The effects of cadmium on early life stages of fish are not well understood, although they are often disproportionately affected by contaminants. The objectives of this study were to examine effects of chronic exposure to environmentally relevant concentrations on growth, development, cellular stress, and glucose metabolism of channel catfish, Ictalurus punctatus. Eggs were wet-fertilized in treatment water at concentrations of 0.4 (control), 2.2 (low), or 8.5 (high) μg L^-1 and monitored through swim-up, black fry stage. Eggs and fry accumulated cadmium dose-dependently. Fertilization rates were unaffected, yet hatch rate was significantly reduced in the high treatment. Survival to black fry and overall size and condition factor were not affected; however, differences in yolk sac size, and presumably energetics of yolk fry, was detected. Physiological pathways were also affected, demonstrated by altered gene expression, most notably in genes related to carbohydrate metabolism. Elevated expression of HK and G6PD, rather than G6P and GADPH, suggests glucose may be shunted towards the pentose-phosphate pathway. Overall, observations indicate cadmium negatively affects development in early life stages of channel catfish, which could lead to shifts in population structure and life history patterns in exposed populations of wild fish.

Development and application of the adverse outcome pathway framework for understanding and predicting chronic toxicity: II. A focus on growth impairment in fish

Adverse outcome pathways (AOPs) organize knowledge on the progression of toxicity through levels of biological organization. By determining the linkages between toxicity events at different levels, AOPs lay the foundation for mechanism-based alternative testing approaches to hazard assessment. Here, we focus on growth impairment in fish to illustrate the initial stages in the process of AOP development for chronic toxicity outcomes. Growth is an apical endpoint commonly assessed in chronic toxicity tests for which a replacement is desirable. Based on several criteria, we identified reduction in food intake to be a suitable key event for initiation of middle-out AOP development. To start exploring the upstream and downstream links of this key event, we developed three AOP case studies, for pyrethroids, selective serotonin reuptake inhibitors (SSRIs) and cadmium. Our analysis showed that the effect of pyrethroids and SSRIs on food intake is strongly linked to growth impairment, while cadmium causes a reduction in growth due to increased metabolic demands rather than changes in food intake. Locomotion impairment by pyrethroids is strongly linked to their effects on food intake and growth, while for SSRIs their direct influence on appetite may play a more important role. We further discuss which alternative tests could be used to inform on the predictive key events identified in the case studies. In conclusion, our work demonstrates how the AOP concept can be used in practice to assess critically the knowledge available for specific chronic toxicity cases and to identify existing knowledge gaps and potential alternative tests.

Effects of maternal cadmium exposure on female reproductive functions, gamete quality, and offspring development in zebrafish (Danio rerio)

Impacts of maternal Cd(2+) exposure on female zebrafish (Danio rerio) were observed in females as well as their offspring. In females, Cd disturbed fecundity and other reproductive functions. In their offspring, it retarded gamete development and growth and influenced gene expression. There was a positive relationship between Cd(2+) contents in ovaries of females and treatment doses of 0-8.9 μM of Cd(2+). The mating rate decreased by 60 % when females were exposed to 8.9-35.6 μM of Cd(2+) for 72 h compared with the control group. It was observed that growth is delayed by one somite stage in maternal-Cd(2+) embryos compared with control embryos, which grew at the sixth-somite stage. The ceratohyal angles of larvae of Cd-exposed adults (maternal Cd(2+)) at 72 h postfertilization (hpf) appeared to have a positive response after doses of maternal Cd. In addition, approximately 30 % of 96-hpf larvae that were treated with a dose of 35.6 μM of maternal Cd(2+) appeared to have pericardial edema. At the 5-hpf stage of maternal Cd(2+) exposure, embryos showed 33 and 37 target genes, respectively, that were significantly downregulated and upregulated as shown by cDNA microarray analysis. A major effect of maternal Cd(2+) exposure on zebrafish embryo genes is that 18.9% of transcription functions were upregulated. In addition, 33.3% of transcripts relative to the function of protein biosynthesis were downregulated. These results showed that maternal Cd(2+) exposure influenced the reproduction ability of females and also caused their embryos to develop with abnormal gene expression.

Short-term exposure of Nile Tilapia (Oreochromis niloticus) to mercury: histopathological changes, mercury bioaccumulation, and protective role of metallothioneins in different exposure routes

To investigate effects of short-term mercury (Hg) exposure in tilapia (Oreochromis niloticus) including histopathological changes, Hg bioaccumulation, and protective role of metallothionein (MT) in different exposure routes, adult tilapias were intraperitoneally injected, orally intubated, or semistatically exposed to 0.5, 1, 2, 5 µg/g mercuric chloride. Histopathology, autometallography (AMG), inductive coupled plasma-atomic emission spectrometry (ICP-AES), and MT immunohistochemistry were determined at 0, 3, 6, 9, 12, and 15 days postexposure. Microscopic lesions were observed in the kidney, hepatopancreas, spleen, and intestine. AMG positive grains were found in renal tubule epithelium, melanomacrophage centers (MMCs), and intestinal epithelium of treated tilapias. Hg concentrations measured by ICP-AES in abdominal visceral organs were significantly higher than in other organs. All exposure routes caused lesions of increasing severity and Hg accumulations in a dose-dependent manner. Semistatic groups produced the highest intensity of lesions, AMG positive staining, as well as total Hg concentrations. Positive MT expression in renal tubule epithelium, pancreatic acini, and splenic MMCs was observed only in semistatic groups. The semistatic exposure route demonstrated the most significant microscopic lesions, Hg bioaccumulation, and MT expression.

Effects of ambient cadmium with calcium on mRNA expressions of calcium uptake related transporters in zebrafish (Danio rerio) larvae

The mRNA expression levels of Ca²⁺ transporter genes including the epithelial calcium channel (ECaC), sodium/calcium exchanger 1b (NCX1b), and plasma membrane calcium ATPase 2 (PMCA2) were measured in zebrafish larvae after exposure to 0.08 μM Cd²⁺ in either water mixed with 0.2 mM Ca²⁺ (lCa) or 2 mM Ca²⁺ (hCa). The ECaC and NCX1b expression decreased at the 48 and 72 h mark, respectively; however, PMCA2 transcripts decreased at 96 h after exposure to Cd²⁺ in lCa environment. On the other hand, the ECaC transcripts were not affected; however, the PMCA2 transcripts were increased at 72 h, while the NCX1b transcripts significantly decreased at 48 and 96 h after exposure to Cd²⁺ in a hCa environment. The Ca²⁺ contents of larvae significantly decreased after Cd²⁺ exposure in a lCa environment; however, the Ca²⁺ contents were evidently higher after exposure to Cd²⁺ in a hCa environment, except for 48th h mark. In addition, ECaC morphants showed lower Ca²⁺ contents of whole-body, and there were higher levels of mortality after exposure to the same condition compared to the wild-type groups. In contrast, injection of ECaC cRNA resulted in an increase in Ca²⁺ content and the rate of Ca²⁺ influx in zebrafish embryos. Summary, the results showed that the Ca²⁺ transporters of zebrafish larvae were impacted after exposures of 0.08 μM Cd. However, in the exposure condition, the ECaC and PMCA2 transcripts could be restored to control levels after the fish were treated in an environment with hCa.

Maternal cadmium exposure induces mt2 and smtB mRNA expression in zebrafish (Danio rerio) females and their offspring

The present study aimed to identify the effects of maternal cadmium (Cd(2+)) exposure on the mRNA expression of mt2 (metallothionein-2) and smtB (similar to metallothionein-B) in female zebrafish (Danio rerio) and their offspring (F1 larvae). Zebrafish females were exposed to 0, 8.9, 17.8, and 35.6 μM Cd(2+) for 72 h, and their ovaries and F1 larvae were collected to measure their Cd(2+) contents and their smtB and mt2 mRNA expression. Cd(2+) contents and the mRNA expression of smtB and mt2 in F1 larvae all showed positive correlations with the maternal Cd(2+) treatment dose. The mt2 was 1.9- to 3.4-fold higher than smtB in F1 larvae. Furthermore, F1 larvae had noticeably enhanced Cd(2+) tolerance after maternal Cd(2+) treatment. These results demonstrate that maternal Cd(2+) was transferred to larval fish and induced mt2 and smtB mRNA expression to protect larva against the impacts of Cd(2+). In female ovaries, mt2 expression showed a noticeable increase after exposure to a metal environment, while smtB did not show exactly the same effect. The study can only conclude that smtB might have a much different role other than just protecting against the impacts of metals.

Metallothionein biosynthesis as a detoxification mechanism in mercury exposure in fish, spotted scat (Scatophagus argus)

It is of crucial importance to study on the biomarkers types to assess the specification of the pollutants and health status of marine ecosystems in environmental evaluation projects. In this respect, total metallothionein biosynthesis and mercury bioaccumulation in the liver and gills under acute mercury exposure were investigated in fish, Scat (Scatophagus argus). Spotted scat was exposed to different mercury concentrations (0, 10, 20, 30) for 24, 48, 72 h. Total MT levels were determined by enzyme-linked immunosorbent assay (ELISA) method. Mercury contents were determined through cold vapor atomic absorption spectrometry (CVAAS). Induction of MT during exposure was tissue specific, displaying different response pattern in gills and liver. Mercury accumulated in liver much higher than in gills and the latter also showed lower MT level (P<0.05). MT biosynthesis in liver showed a significant (P<0.05) increase after exposure to different mercury concentration with increase in exposure time, whereas total MT content did not significantly (P>0.05) change in gills except for 72 h exposure at 30 μg l(-1). Nonetheless, the relationship between MT biosynthesis and Mercury bioaccumulation in both tissues was significant (P<0.05). The results suggest that this form of MT in S. argus was Hg inducible and could be extended as a biomarker of mercury pollution in marine ecosystems.

Cadmium concentration and metallothionein expression in prostate cancer and benign prostatic hyperplasia of humans

BACKGROUND/PURPOSE

Cadmium (Cd) causes various genitourinary disorders and is a carcinogen for prostate cancer. Metallothionein (MT) is a protein that detoxifies heavy metals. We evaluated changes in Cd concentration and MT expression in human prostate cancer (CaP) and benign prostatic hyperplasia (BPH). Our goal was to clarify the relationship between Cd concentration and MT expression in prostatic diseases.

METHODS

The experimental group consisted of 18 patients who underwent radical prostatectomy for CaP. The control group consisted of 35 patients who underwent transurethral resection of the prostate for BPH. Tissue samples were acquired from the gross tumor site and from resected chips. We determined Cd concentration by atomic absorption, MT expression by immunoblotting, and immunohistochemical staining. The significance of between-group differences for these outcomes was analyzed using Student's t tests.

RESULTS

There was no statistically significant difference in Cd concentration between the CaP and BPH groups. Immunoblots from both groups revealed a single band. The relative intensity of the MT band was 0.58 +/- 0.09 in the BPH group and 0.17 +/- 0.03 in the CaP group. MT expression in patients with BPH was 3.4-fold higher than in those with CaP.

CONCLUSION

MT may bind heavy metals and protect patients from CaP. Additional studies are needed to reveal the factors that influence the expression of MT in prostate epithelial cells, and to analyze the free and compound forms of Cd at the same time.

Cadmium accumulation and metallothionein overexpression in internal spermatic vein of patients with varicocele

OBJECTIVES

To determine the possible molecular mechanism for the thickened wall in the internal spermatic vein (ISV) of patients with varicocele, we examined the cadmium (Cd) content and metallothionein (MT) expression in these diseased vessels. Previous studies have shown that Cd might play a role in the etiology of varicocele-associated infertility. MT, a metal-binding protein, protects against cell apoptosis during hypoxia.

METHODS

The study group consisted of 20 patients with grade 3 left varicocele. The control group consisted of 15 volunteers with left-sided indirect inguinal hernia. Through a left inguinal incision, a 1-cm section of the ISV was resected from each patient to measure the Cd and MT levels. The results were analyzed using Student's t test.

RESULTS

The Cd content in the ISV was 59.84 +/- 5.7 ng/g in the control group and 192.1 +/- 24.2 ng/g in the varicocele group. The relative intensity of the MT band was 40.52 +/- 3.74 in the control group and 78.26 +/- 5.61 in the varicocele group. MT expression was greater in the varicocele group than in the control group, and its deposition in the vascular endothelial layer was predominant using immunohistochemistry staining and confocal laser scanning.

CONCLUSIONS

The results of the present study have demonstrated a greater accumulation of Cd in the ISV of the varicocele group than in the control group. The high Cd content and hypoxic conditions would induce overexpression of MT in the diseased vessels to protect the vascular cells from apoptosis. This might be a mechanism for the thickened wall of the ISV in patients with varicocele.

Evaluation of metallothionein expression as a biomarker of mercury exposure in Scatophagus argus

The effect of mercury exposure to total Metallothionein (MT) response and bioaccumulation under control and acute mercury exposure were investigated in scats (Scatophagus argus). Scats were exposed to different mercury concentrations (10, 20 and 30 microg Hg L(-1)) for 24, 48 and 72 h. Total MT levels were determined by Enzyme-Linked Immunosorbent Assay (ELISA) method. Mercury contents were determined through cold Vapour atomic Absorption spectrometry. Induction of MT during exposure was tissue specific, displaying different response pattern in gill and liver. Mercury accumulated much stronger in liver than gill and the latter also showed lower MT level. MT biosynthesis in liver showed a significant increase after exposure to different mercury concentration during different times. This increase was significantly correlated with mercury bioaccumulation. In contrast, presence of different mercury concentration during different times did not significantly modify total MT except for 72 h exposure at 30 microg L(-1) in gills. The results suggest that this form of MT existing in S. argus was Hg-inducible and could be extended the as a biomarker of mercury pollution in ecosystems.

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.

Quantitative immunochemical evaluation of fish metallothionein upon exposure to cadmium

Efficient implementation of an environmental biomarker requires multi-annual comparability over a wide geographical range. The present study improved the comparability of a quantitative competitive metallothionein (MT) enzyme-linked-immuno-sorbent-assay (ELISA) in the sentinel fish Lithognathus mormyrus by introducing to the assay recombinant MT and beta-actin standards. Commercial antibodies for cod MT and mammalian actin were implemented. In addition, a sensitive anti L. mormyrus MT antibody was produced, adequate only for solid phase immunochemical assays. Cadmium was applied to the fish through injection and feeding to serve as a testing platform of the ELISA. The results demonstrated high potential protective capacity of the liver against toxic levels of transition metals through increasing MT levels. MT transcript levels were evaluated also from fish sampled at polluted and relatively clean natural sites, indicating applicability of MT as biomarker of exposure to a multi-factorial pollution, in comparison to its low revealed sensitivity to controlled cadmium exposure.

Cloning and characterization of a tilapia (Oreochromis aureus) metallothionein gene promoter in Hepa-T1 cells following the administration of various heavy metal ions

Metallothioneins (MTs) are highly conserved intracellular metal-binding proteins that contribute to the homeostasis of essential metals and the detoxification of non-essential heavy metals. MT gene expression is induced by various heavy metal ions, and Zn(2+) is able to bind and activate a transcription factor associated with the MT gene that is known as the metal responsive element (MRE) binding transcription factor-1 (MTF-1). Heavy metals other than Zn(2+), such as Cd(2+) and Cu(2+), fail to activate the binding of MTF-1 to MREs despite their ability to induce the transcription of the MT gene. To study how different metal ions regulate MT gene expression, a tilapia (ti)-MT gene promoter was cloned and its responses to activation by various metal ions measured using a Hepa T1 cell culture model. The tiMT gene promoter contains six functional MREs within 2118bp 5' of the translational start site. A transient gene expression study showed the tiMT gene promoter fragment to be responsive to Cd(2+), Cu(2+), Hg(2+), Pb(2+), and Zn(2+). Deletions from the 5' end and the site-directed mutagenesis of individual MREs in the tiMT gene promoter confirmed that both proximal and distal clusters of MREs were required for the maximal metal induction of the tiMT gene. The distal cluster of MREs greatly enhanced the induction of tiMT gene expression by several of the heavy metal ions, and especially the non-Zn(2+) ions. Individual MREs showed a different responsiveness to metal ions, with MREe being the most potent, MREb being responsive to Zn(2+) but not to other metal ions, and MREa being mainly for the basal expression of the tiMT gene. Electrophoretic mobility shift assay (EMSA) identified a transcription factor that was able to bind most of the MREs, with the exception of MREd, but the binding was only activated by the in vivo administration of Zn(2+), not the administration of Cd(2+) or Cu(2+). In conclusion, the results of this study on a Hepa T1 cell model suggest that the mechanism of MT gene activation by non-Zn(2+) metal ions is different from that of activation by Zn(2+), and that different MREs may be involved in the activation of the tiMT gene by different metal ions without enhancing the binding of MTF-1 to MREs.

Toxicological stress response and cadmium distribution in hybrid tilapia (Oreochromis sp.) upon cadmium exposure

Adult tilapia were exposed to 0 (control) and 4.45 microM Cd for 0 h, 5 h, 5 days, and 15 days, and the physiological responses of fish were described. The physiological responses were first expressed in gill tissue, in which mucus cells secretion increased, Cd accumulated, cortisol secretion was significantly higher, but serum ACH(50) activity (alternative complement hemolytic assay) was significantly lower than in controlled fish. After 5 days of Cd exposure, the ACH(50) activity showed a greater decrease, but lysozyme and cortisol contents showed significant increases over the control. Cd levels significantly increased in intestines, liver, and kidneys, and a significant induction of metallothionein (MT) protein in hepatic tissue was noted. Finally, the Cd accumulation rate still showed significant increases in these organs. However, the MT content was similar at 5 days and at 15 days after Cd exposure, and the cortisol contents had recovered to the pre-exposure level. In addition, Cd accumulation in muscle was higher after Cd exposure than in controls (t-test, p<0.05). Our results demonstrate (1) that tilapia readily regulate their physiological parameters in order to acclimate to a sublethal Cd environment; (2) these changes of physiological parameters may be related with a succession of cortisol levels following Cd exposure; (3) increasing rate of hepatic MT contents and Cd accumulation rate didn't appear identical after 5-15 days of Cd exposure. This was confirmed that hepatic MT was not a good indicator for Cd levels in tilapia.

Using an integrated approach to link biomarker responses and physiological stress to growth impairment of cadmium-exposed larval topsmelt

In this study, we used an integrated approach to determine whether key biochemical, cellular, and physiological responses were related to growth impairment of cadmium (Cd)-exposed larval topsmelt (Atherinops affinis). Food intake (Artemia franciscana nauplii), oxygen consumption rates, apoptotic DNA fragmentation (TUNEL assay), and metallothionein (MT)-like protein levels, were separately measured in relation to growth of larval topsmelt aqueously exposed to sublethal doses of Cd for 14 days. Cadmium accumulation and concentrations of abundant metals were also evaluated in a subset of fish. Fish in the highest Cd treatments (50 and 100 ppb Cd) were smaller in final mean weight and length, and consumed fewer A. franciscana nauplii than control fish. Food intake was positively correlated with final weight of larval topsmelt in Cd and control treatments; food intake increased as final weight of the fish increased. Oxygen consumption rates were positively correlated with Cd concentration and mean oxygen consumption rates were inversely correlated with final mean weight of topsmelt; the smallest fish were found in the highest Cd treatment and were respiring at higher rates than control fish. Apoptotic DNA fragmentation was concentration-dependent and was associated with diminished growth. Apoptotic DNA fragmentation was elevated in the gill of fish exposed to 50 ppb Cd, and in the gut, gill, and liver of fish exposed to 100 ppb Cd. Metallothionein (MT)-like protein levels in fish from 100 ppb Cd treatments were significantly higher than those in other treatments. Oxygen consumption rates may have increased as a compensatory response to Cd exposure. However, it is likely that the energy produced was allocated to an increased metabolic demand due to apoptosis, MT synthesis, and changes in ion regulation. This diversion of energy expenditures could contribute to growth impairment of Cd-exposed fish.

Tilapia metallothionein genes: PCR-cloning and gene expression studies

Genomic PCR reactions were performed to isolate gene sequences of tilapia metallothionein (tiMT) from Oreochromis mossambicus and Oreochromis aureus. Two AP1 binding sites, four metal responsive elements, and a TATA box are the major cis-acting elements identified in the 800-bp 5' flanking region of the tiMTs obtained in this study. The tiMT gene promoter cloned from O. aureus was characterized in vitro using PLHC-1 cell-line, a hepatocellular carcinoma of a desert topminnow (Poecciliopsis lucida), following the administrations of Cd2+, Co2+, Cu2+, Ni2+, Pb2+ and Zn2+. Only Cd2+, Pb2+ and Zn2+ were able to induce the transcription of tiMT gene promoter in PLHC-1 cells in a dose-dependent manner. Zn2+ had the highest fold induction of tiMT gene promoter activity. Deletion mutants were tested for their abilities to drive the transcription of reporter gene following Cd2+ and Zn2+ administrations. However, Cu2+ and Ni2+ also induced the production of hepatic MT mRNA in vivo. Northern blot analysis showed that liver gave the highest fold induction of MT gene expression following the administration of heavy metal ions. These data indicated that hepatic MT mRNA level in tilapia is a potential sensitive biomarker of exposure to various metal ions including Cu2+, Cd2+, Ni2+, Pb2+, Hg2+ and Zn2+ ions.