Differences in metallothionein gene expression in primary cultures of rainbow trout hepatocytes and the RTH-149 cell line

Mercury accumulation and biomarkers of exposure in two popular recreational fishes in Hawaiian waters

Mercury (Hg) exposure has not been examined in many recreational nearshore fish species that are commonly consumed around the Hawaiian Islands. Specific gene transcripts, such as metallothionein (MET) and thioredoxin reductase (TrxR), can be used to examine Hg exposure responses in aquatic organisms. This study measured total mercury (THg) in four species from two groups of Hawaiian nearshore fishes: giant trevally (Caranx ignobilis, n = 13), bluefin trevally (C. melampygus, n = 4), sharp jaw bonefish (Albula virgata, n = 2), and round jaw bonefish (A. glossodonta, n = 19). Total Hg accumulation and abundance profiles of MET and TrxR were evaluated for muscle, liver, and kidney tissues. Total Hg in round jaw bonefish and giant trevally tissues accumulated with length and calculated age. In round jaw bonefish tissues, mean THg was greater in kidney (1156 ng/g wet mass (wm)) than liver (339 ng/g wm) and muscle (330 ng/g wm). Giant trevally muscle (187 ng/g wm) and liver (277 ng/g wm) mean THg did not differ significantly. Fish species in this study were compared to commercial and local fish species with state and federal muscle tissue consumption advisories based on THg benchmarks developed by the U.S. Food and Drug Administration (FDA) and Environmental Protection Agency (EPA). Both bonefishes had mean muscle THg that exceeded benchmarks suggesting consumption advisories should be considered. MET transcript in round jaw bonefish kidney tissue and kidney THg exhibited a marginally significant positive correlation, while TrxR transcript in liver tissue negatively correlated with increasing liver THg. These results contribute to our understanding of Hg exposure associated health effects in fish.

Aquatic Toxicology in Vitro: A Brief Review

There is an urgent need for effective in vitro tests in aquatic toxicology, because only a very small proportion of the chemicals in common use have been adequately tested for their toxicity to aquatic organisms and aquatic ecosystems. Toxicity tests with higher animals, besides being time-consuming and expensive, are ethically questionable, which further increases the importance of developing efficient in vitro toxicity tests. In developing in vitro tests for toxicity assessments, aquatic toxicology lags behind mammalian toxicology. Aqueous environmental chemistry is complex, and the sensitivity of the organisms living in a particular aquatic environment may vary considerably. The predictive value of single-species or cell culture tests is therefore generally considered to be low. Nevertheless, single-species tests, utilising bacteria, algae, protozoans and invertebrates, have frequently been used in in vitro toxicity studies of aquatic pollutants (mainly as screening tests). Attempts at large-scale validations are few. Such attempts seem to be hampered by the complexity of the aquatic ecosystem. Although cells from aquatic organisms have been isolated and cultured for many years, the use of isolated or cultured cells in aquatic toxicology has been limited. However, during the last few years, interest in the use of fish cells in toxicity testing has grown rapidly. For aquatic in vitro toxicology to develop further, a more comparative and mechanistic approach needs to be adopted.

Regulation of hepatic estrogen receptor isoform mRNA expression in rainbow trout (Oncorhynchus mykiss)

The complete nuclear estrogen receptor family in rainbow trout consists of two subtypes (ERalpha and ERbeta) each of which consists of two isoforms (alpha1/alpha2 and beta1/beta2). Transcription rate and mRNA stability of ERalpha1 is affected by 17beta-estradiol (E2) but no information on estrogen regulation exists for the other isoforms. The objective of this study was to compare the mRNA expression patterns of the four ER isoforms in the liver of male trout and in immortalized trout hepatocyte lines (RTH-149 and SOB-15) treated with E2 or 17alpha-ethynylestradiol (EE2) using quantitative RT-PCR. To determine the in vivo dose-response, isogenic male trout were injected intra-peritoneally with 0, 1.5, 15 or 150 microg E2 or an equimolar amount of EE2 and the liver sampled 24 h later. Treatment with either E2 or EE2 significantly (p<0.05) increased the level of ERalpha1 mRNA at all doses tested compared to vehicle, while the response of mRNAs for the other three isoforms did not change. The in vitro dose-response was tested by treating both cell lines with 0, 0.1, 1.0 or 10.0 microM E2 for 48 h. In RTH-149 cells, ERalpha1, ERalpha2 and ERbeta2 mRNAs were significantly higher in cells incubated with 10 microM E2 as compared to cells treated with only vehicle (p<0.05). In SOB-15 cells, ERalpha2 and ERbeta1 mRNAs were significantly higher in cells incubated with 1.0 microM E2 as compared to cells incubated with only vehicle (p<0.05). These results support the conclusion that the mRNAs for the four ER isoforms respond differentially to estrogen regulation.

Metallothioneins in Aquatic Organisms: Fish, Crustaceans, Molluscs, and Echinoderms

Metallothioneins (MTs) have been described in a wide range of organisms, from bacteria to mammals, thus representing an interesting example of evolutionary molecular adaptation. If the moderate variability of MTs across phylogenetically distant organisms reflects their highly conserved function, the specific environmental requirements may explain the multiplicity of isoforms also in the same organism. The MT polymorphism is particularly important in invertebrates with respect to vertebrates. This review is an attempt to summarize the knowledge about MTs from aquatic animals, both vertebrates and invertebrates, to gain new insights into the structure-function relationship of this class of proteins. The large and increasing literature on MTs indicates that MTs from aquatic vertebrates are rather similar to mammalian counterparts, whereas a variety of structures have been described in invertebrates. Although the prototypical αβ-domain organization of vertebrate MTs has been observed in most invertebrate isoforms, some invertebrate MTs display alternative structures in which the canonical organization has been modified, such as the ββ-domain, the αββ-domain, and the multiple α-domain structures of oyster MTs, and the inverted βα-domain organization of sea urchin MTs. In this review we emphasize three major taxa of aquatic invertebrates, the molluscs, the crustaceans and the echinoderms, although some data have been reported for other invertebrates.

Cortisol stimulates the zinc signaling pathway and expression of metallothioneins and ZnT1 in rainbow trout gill epithelial cells

Intracellular zinc signaling is important in the control of a number of cellular processes. Hormonal factors that regulate cellular zinc influx and initiate zinc signals are poorly understood. The present study investigates the possibility for cross talk between the glucocorticoid and zinc signaling pathways in cultured rainbow trout gill epithelial cells. The rainbow trout metallothionein A (MTA) gene possesses a putative glucocorticoid response element and multiple metal response elements 1042 base pairs upstream of the start codon, whereas metallothionein B (MTB) and zinc transporter-1 (ZnT1) have multiple metal response elements but no glucocorticoid response elements in this region. Cortisol increased MTA, MTB, and ZnT1 gene expression, and this stimulation was enhanced if cells were treated with cortisol together with zinc. Cells treated with zinc showed increased zinc accumulation, transepithelial zinc influx (apical to basolateral), and intracellular labile zinc concentrations. These responses were also significantly enhanced in cells pretreated with cortisol and zinc. The cortisol-mediated effects were blocked by the glucocorticoid receptor (GR) antagonist RU-486, indicating mediation via a GR. In reporter gene assays, zinc stimulated MTA promoter activity, whereas cortisol did not. Furthermore, cortisol significantly reduced zinc-stimulated MTA promoter activity in cells expressing exogenous rainbow trout GR. These results demonstrate that cortisol enhances cellular zinc uptake, which in turn stimulates expression of MTA, MTB, and ZnT1 genes.

Hepatic transcriptome response to glucocorticoid receptor activation in rainbow trout

Cortisol, the principal corticosteroid in teleosts, is thought to play a key role in the metabolic adjustments critical for regaining homeostasis. However, the target tissue molecular mechanisms involved in this adaptive response to corticosteroid stimulation are still unclear. Cortisol signaling is mediated predominantly by the glucocorticoid receptor (GR), and previous studies have shown that RU486 (a GR antagonist) offsets corticosteroid signaling in teleosts. To elucidate the molecular basis of GR-mediated metabolic readjustments, we exposed primary culture of trout hepatocytes in vitro to cortisol (to mimic stressed levels seen in fish), RU486, or a combination of both for 24 h. The gene expression was analyzed using a low-density custom-made rainbow trout cDNA array enriched with endocrine-, metabolic-, and stress-related genes. The microarray results for select genes were further validated using quantitative real-time PCR. Cortisol treatment significantly increased glucose production in hepatocytes, and this response was blocked by RU486, confirming GR-mediated corticosteroid signaling. Cortisol also elevated GR transcript levels, and this response was abolished by RU486, whereas both cortisol and RU486, either alone or in combination, reduced GR protein content in trout hepatocytes. Cortisol treatment significantly modulated the expression of several genes known to be involved in intermediary metabolism, cellular stress response, reproduction, and xenobiotic metabolism. Most of these cortisol-mediated transcript changes were abolished in the presence of RU486, suggesting a key role for GR-specific signaling in this adaptive response. Taken together, our results suggest a key role for genomic cortisol signaling in the liver molecular reprogramming that is critical for coping with stress in fish.

Expression of metallothionein gene during embryonic and early larval development in zebrafish

Metallothionein (Mt) has been considered as a molecular marker of metal pollution in aquatic ecosystems. Less is known about the expression of mt gene during embryogenesis. Here, we report the cloning, sequencing, and the expression pattern of mt gene during developmental stages in zebrafish. The zebrafish embryogenesis when takes place in a medium containing a dosage of 1000 microM zinc resulted in high mortality, indicating the deleterious effect of zinc on development. The zebrafish mt gene consists of three exons encoding 60 amino acids with 20 conserved cysteine residues. RT-PCR result indicates the maternal contribution of Mt transcripts. Using digoxigenin (DIG)-labeled anti-sense RNA probe, whole-mount in situ hybridization was performed to observe the expression pattern of zebrafish mt gene during embryonic and early larval stages. Stronger as well as ubiquitous expression of mt gene during early embryonic stages narrowed to specific expression after hatching. The mt promoter region contains seven copies of putative metal-responsive elements (MREs), which are shown to be important for the high level activity by deletion analysis. The expression of mt gene during embryogenesis implies its significant role on development.

Cloning and characterization of metallothionein gene in ayu Plecoglossus altivelis

Metallothionein (MT) has been used widely as a potential molecular marker to detect the deleterious effects of heavy metals in aquatic ecosystem. Here we exposed ayu, Plecoglossus altivelis, to zinc (Zn) and tested the distribution as well as the induction of MT in various tissues such as liver, kidney, intestine and stomach. MT induction was significant in liver tissue, followed by kidney and intestine, whereas no induction was detected in stomach. The gene encoding ayu MT was successfully cloned and characterized. Complete nucleotide sequencing and analysis of the 4.5 kb DNA fragment containing the ayu MT gene revealed that the gene has three exons interrupted by two introns, a 5'-flanking region of about 2.5 kb and about 1.6 kb of 3'-flanking region. In grouper heart and kidney cells, the 2.5 kb promoter containing eight metal responsive elements (MREs), two hepatic nuclear factor 5 responsive elements (HNF5REs) and one cAMP responsive element (CRE) had the highest reporter activity.

Use of the comet assay for studying environmental genotoxicity: comparisons between visual and image analyses

In order to evaluate the applicability of different measurement parameters employed in the comet assay for analyzing environmental samples, fish hepatoma (RTH-149) cells were exposed to concentrations of the model genotoxic agent hydrogen peroxide (H(2)O(2); 1, 5, and 10 microM) and to five water samples from sites along the Kishon River, the most polluted river in Israel. DNA damage was scored in parallel by visual and computer-image (Viscomet) analyses using 12 different parameters. Each parameter exhibited a different profile of responses. The four visual parameters were highly sensitive to the lowest (1 microM) H(2)O(2) concentration (1.8-7.0-fold of the control). At 10 microM H(2)O(2) exposure, the visual parameter, percentage severe damage, showed the highest (40.3-fold) response while four other parameters, tail area, tail extent moment (Viscomet), mean actual tail length and cumulative tail length (visual analysis), also had substantially elevated responses (8-11-fold). We found that the DNA damage induced by field samples was similar in magnitude to the damage induced by 1 microM H(2)O(2), with only some of the parameters being highly sensitive to the damage. Only about one-half of the parameters could distinguish four significant levels of genotoxicity among the five sampling sites, while the remaining parameters detected only three levels. It is concluded that the choice of parameters for analyzing genotoxicity in ecotoxicological studies should be made in accordance with the characteristics of each parameter.

In vitro application of the comet assay for aquatic genotoxicity: considering a primary culture versus a cell line

The comet assay, one of the most widely used techniques for the evaluation and detection of DNA strand breaks, is frequently employed in vivo. In vitro assays are usually performed with mammalian cell lines, clearly not the best choice for tests on aquatic genotoxicity. Here we evaluated a fish hepatoma cell line (RTH-149) and a primary blood cell culture from the intertidal colonial tunicate Botryllus schlosseri as possible model targets for comet assays using the genotoxic agent H2O2. We found that DNA strand break levels in RTH-149 fitted dose-dependent responses better than the tunicate cells. Moreover, in B. schlosseri controls, 34% of the cells were already ranked as severely damaged. Assays were then performed on water samples from the polluted Kishon river (Israel) on three different dates, using RTH-149 cells (50% dilutions, 2-h exposures). In all cases, high genotoxicity of the river water was revealed by evaluating comet percentages, average tail lengths and DNA damage levels. This assay was found to be fast and sensitive, appropriate to be employed as a part of a monitoring program. The use of B. schlosseri blood cells should be validated in additional work.

Fish cell lines as a tool in aquatic toxicology

In aquatic toxicology, cytotoxicity tests using continuous fish cell lines have been suggested as a tool for (1) screening or toxicity ranking of anthropogenic chemicals, compound mixtures and environmental samples, (2) establishment of structure-activity relationships, and (3) replacement or supplementation of in vivo animal tests. Due to the small sample volumes necessary for cytotoxicity tests, they appear to be particularly suited for use in chemical fractionation studies. The present contribution reviews the existing literature on cytotoxicity studies with fish cells and considers the influence of cell line and cytotoxicity endpoint selection on the test results. Furthermore, in vitro/in vivo correlations between fish cell lines and intact fish are discussed. During recent years, fish cell lines have been increasingly used for purposes beyond their meanwhile established role for cytotoxicity measurements. They have been successfully introduced for detection of genotoxic effects, and cell lines are now applied for investigations on toxic mechanisms and on biomarkers such as cytochrome P4501A. The development of recombinant fish cell lines may further support their role as a bioanalytical tool in environmental diagnostics.

Induction of two major isoforms of metallothionein in crucian carp (Carassius cuvieri) by air-pumping stress, dexamethasone, and metals

The induction of metallothionein (MT) by physical and chemical stress was assessed using the fresh-water fish, crucian carp (Carassius cuvieri Temminck et Schlegel). The fish exposed to violent air-pumping stress for 6 days revealed time-dependent induction of MT-like metal-binding proteins in both their livers and kidneys. Their hepatic contents after exposure to stress were elevated to twice the basal level with 24 h, resulting in more than a 3-fold increase at 144 h, whereas their renal contents gradually increased after 24 h and reached the same level as that in the liver around 96 h. Two major inducible proteins were purified from livers of fish exposed to stress and were shown to be MT based upon their chromatographic behavior, UV absorption spectra and their molecular weights. Consequently, they were termed ccMT-1 and ccMT-2, according to their elution sequence upon anion-exchange chromatography. Both proteins mainly bound zinc in their endogenous forms and showed different immunogenicity to rat and rabbit MTs. Dexamethasone, a potent inducer for MT synthesis in mammals, induced the production of both isoforms in crucian carp, whereas cadmium and zinc ions prominently induced the synthesis of ccMT-2. These results indicate that crucian carp have the ability to produce MTs in response to various kinds of environmental stress and that violent air-pumping stress in crucian carp may induce MT synthesis, in part, via the release of endogenous factor(s), such as glucocorticoids.

Molecular analyses of metallothionein gene regulation

Metallothionein (MT) genes encode small proteins that chelate metal ions through metal-thiolate bonds with cysteine residues. MTs may have a role in cellular zinc homeostasis and metal detoxification. Congruent with these putative functions, MT gene transcription is induced by metals via multiple metal-responsive elements (MREs) present in the MT gene 5'-regulatory regions. This chapter mainly is focused on studies of the functional and physical interactions of MRE binding proteins with MT promoters from human and rainbow trout. In addition to mediating zinc induction, MREs may make important contributions to nonmetal induced promoter activity. In part, differential basal activity of MREs appears to be determined by sequence and position in the promoter. During zinc induction, increased functional MRE activity correlates with increased activity of mammalian MRE binding proteins by zinc treatment in vivo or in vitro, as detected by electrophoretic mobility shift assays. Interestingly, the addition of cadmium in vitro or in vivo has no detectable effect even though it strongly induces MT gene expression in the same time course. This raises questions about how the effects of cadmium are mediated by MREs. The molecular masses and MRE complex migration of the zinc-responsive factors we detect are consistent with mouse and human metal-responsive transcription factor (MTF) and expression of the MTF cDNAs increases co-transfected MT promoter activity in both mammalian and trout cell lines underlining the conservation of MRE binding factor function among diverse species.

Interaction of cadmium and oestradiol-17 beta on metallothionein and vitellogenin synthesis in rainbow trout (Oncorhynchus mykiss)

The induction of metallothionein and vitellogenin synthesis in rainbow trout liver was studied after injection of oestradiol-17 beta alone or in combination with cadmium or zinc. Intraperitoneal injection of oestradiol-17 beta increased the liver somatic index, with subsequent induction of vitellogenin synthesis. Oestradiol-17 beta did not induce metallothionein synthesis. Injection of cadmium induced the synthesis of metallothionein mRNA and metallothionein. Injection of oestradiol-17 beta in combination with cadmium resulted in inhibition of transcription and translation of both vitellogenin and metallothionein. Chromatography of liver cytosols revealed that cadmium, when co-injected with oestradiol-17 beta, did not bind to metallothionein but would initially bind to high-molecular-mass (HMr) cytosolic proteins. In fish injected with cadmium in combination with oestradiol-17 beta, cadmium was gradually redistributed from HMr proteins to metallothionein. This resulted in induction of metallothionein synthesis and in binding of most of the cadmium to metallothionein. Induction of vitellogenin mRNA was observed 15 days after injection, as cadmium was being redistributed to newly synthesized metallothionein. These findings indicate that cadmium inhibits the transcription of vitellogenin. The binding of cadmium to these non-metallothionein proteins represses the induction of metallothionein and results in increased toxicity of the metal. Preinduction of metallothionein by zinc injections resulted in decreased cadmium sensitivity of the fish and a decrease in the repression of vitellogenin mRNA. Furthermore, a role for metallothionein in the detoxification of cadmium is indicated by the induction of vitellogenin synthesis that occurs once metallothionein has begun sequestering cadmium.

Metal-responsive elements of the rainbow trout metallothionein-B gene function for basal and metal-induced activity

In this study, the contributions of the two metal-responsive elements (MREs) of the rainbow trout (Salmo gairdnerii) metallothionein (tMT)-B gene promoter (-137 to +5) were analyzed. The effect of MRE mutations on the basal and zinc-induced activities of tMT-B promoter-reporter gene fusions were determined by transfection of a rainbow trout hepatoma (RTH-149) cell line. Together, MREa and MREb cooperate to elicit a significant response to zinc but exhibit differential basal and metal-induced activity. The MREa sequence (-62 to -51) is important for basal promoter activity and can function independently, whereas the more distal MREb (-89 to -100) mainly contributes to metal induction through cooperative interactions with MREa. The degree of basal character of the MREs is partially determined by nucleotide differences at the flexible position N of the MRE consensus TGC(G/A)CNC. In mouse L and HepG2 cells, MREa activity is conserved, but the contributions of the MREb region differ, including reduced cooperativity with MREa. There are also differences in the apparent molecular masses of the rainbow trout and mammalian nuclear factors that bind to the tMT-B promoter and MREa sequence.

Metal-specific induction of metallothionein isoforms in the blue crab Callinectes sapidus in response to single- and mixed-metal exposure

Metallothioneins (MTs) play an important role in the metabolism of copper and zinc during the molt cycle of the blue crab. In this study we examined the hypothesis that MT expression in crabs is metal specific. Anion-exchange chromatography showed one major ZnCuMT (ZnMT-I) in control crabs, two MT isoforms in cadmium-treated crabs (CdMT-I, CdMT-II), and three forms (CuMT-I, CuMT-II, CuMT-III) in copper-treated animals. Amino acid analysis of the carboxymethylated apo-MTs, purified by reversed-phase HPLC, showed minor differences between ZnMT-I, CdMT-I, CdMT-II, CuMT-I, and CuMT-II, while CuMT-III was unique. After combined exposure to cadmium and copper, four MTs with differing copper/cadmium ratios were observed, equivalent to CdMT-II and the three CuMTs. We conclude that the blue crab has four genes that encode different MTs. Transcription of the CdMT-I gene is induced by cadmium, but inhibited by copper. CuMT-I, CdMT-II, and ZnMT-I may be the products of a single gene responsive to copper, cadmium, and zinc. Expression of the CuMT-II and CuMT-III genes is initiated by copper and not by cadmium. We believe that CdMT-I and CuMT-III are important in detoxification, whereas Zn/CuMT-I and CuMT-II are involved in regulatory functions. These results show the importance of the use of mixed-metal exposures in the study of the molecular mechanisms of metal regulation and function.

Developmental regulation of metallothionein mRNA, zinc and copper levels in rainbow trout, Salmo gairdneri

The metallothionein (MT) gene expression profile was followed in rainbow trout during early embryo development and in liver and gonads during the period of sexual maturation. The hepatic MT mRNA levels increase at the end of sexual maturation in both male and female rainbow trout. Although both isoforms of MT mRNA accumulate in the liver, there is a preferential increase in MT-A in the female liver. Concomitantly with this increase in MT there is a redistribution of zinc and copper to MT. In the juvenile female there is an abundance of MT mRNA in the ovaries. This is correlated to high levels of zinc in the MT fraction upon Sephadex G-75 chromatography. During ovary development the MT mRNA levels and the MT-bound zinc levels drop, with an increase in zinc being bound to high-molecular-mass proteins. At ovulation most of the zinc is found in the membrane portion upon centrifugation. In contrast to the ovaries, there are no apparent changes in either trace metal distribution or MT mRNA levels during testis development. In the developing embryo there is an increase in MT-bound copper at gastrulation. This is accompanied by an increase in both isoforms of MT mRNA. At hatch both the copper and zinc levels increase in the MT fraction, with a concomitant increase in mainly MT-A mRNA. These findings indicate that the variations in MT mRNA levels during development are closely associated with metal regulation.