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

Role of UV radiation and oxidation on polyethylene micro- and nanoplastics: impacts on cadmium sorption, bioaccumulation, and toxicity in fish intestinal cells

This study investigated the role of ultraviolet (UV) radiation and oxidation in high-density polyethylene microplastics (2-15 μm) and nanoplastics (0.2-9.9 μm) (NMPs) on particle chemistry, morphology, and reactivity with cadmium (Cd). Additionally, toxicity of NMPs alone and with Cd was evaluated using RTgutGC cells, a model of the rainbow trout (Oncorhynchus mykiss) intestine. The role on NMPs on Cd bioaccumulation in RTgutGC cells was also evaluated. Dynamic light scattering indicated that after UV radiation NPs agglomerated size increased from 0.8 to 28 µm, and to 8 µm when Cd was added. Oxidized MPs agglomerated size increased from 11 and 7 to 46 and 27 µm in non-UV- and UV-aged oxidized MPs when adding Cd, respectively. Cd-coated particles exhibited generally significantly higher zeta potential than non-Cd-coated particles, while attenuated total reflectance-Fourier transform infrared spectroscopy showed that the functional chemistry of the particles was oxidized and modified after being exposed to UV radiation. Presence of NMPs resulted in a significant decrease in Cd bioaccumulation in RTgutGC cells (100.5-87.9 ng Cd/mg protein) compared to Cd alone (138.1 ng Cd/mg protein), although this was not quite significant for co-exposures with UV-aged NPs (105.7 ng Cd/mg protein). No toxicity was observed in RTgutGC cells exposed to NMPs alone for 24 h. Moreover, co-exposures with Cd indicated that NMPs reduce the toxicity of Cd. Altogether these results show that UV aging enhances NMP surface reactivity, increasing Cd absorption in solution, which resulted in a reduction in Cd bioavailability and toxicity.

Evaluation of tire tread particle toxicity to fish using rainbow trout cell lines

Tire and road wear particles (TRWP) resulting from tire abrasion while driving raise concerns due to their potential contribution to aquatic toxicity. Our study aimed to assess cryogenically milled tire tread (CMTT) particle toxicity, used as a proxy for TRWP, and associated chemicals to fish using two Rainbow Trout (Oncorhynchus mykiss) cell lines representing the gill (RTgill-W1) and the intestinal (RTgutGC) epithelium. CMTT toxicity was evaluated through several exposure pathways, including direct contact, leaching, and digestion, while also assessing the impact of particle aging. Following OECD TG249, cell viability was assessed after 24 h acute exposure using a multiple-endpoint assay indicative of cell metabolic activity, membrane integrity and lysosome integrity. In vitro EC50 values for the fish cell lines exceeded river TRWP concentrations (2.02 g/L and 4.65 g/L for RTgill-W1 and RTgutGC cell lines, respectively), and were similar to in vivo LC50 values estimated at 6 g/L. Although toxicity was mainly driven by the leaching of tire-associated chemicals, the presence of the particles contributed to the overall toxicity by inducing a continuous leaching, highlighting the importance of considering combined exposure scenarios. Aging and digestion conditions were also found to mediate CMTT toxicity. Thermooxidation resulted in a decreased chemical leaching and toxicity, while in vitro digestion under mimicked gastrointestinal conditions increased leaching and toxicity. Specific chemicals, especially Zn, 2-mercaptobenzothiazole, 1,3-diphenylguanidine, and N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) were identified as contributors to the overall toxicity. Although 6PPD-quinone was detected in CMTT digestate, cytotoxicity assays with RTgill-W1 and RTgutGC cell lines showed no toxicity up to 6 mg/L, supporting the notion of a specific mode of action of this chemical. This study provides insights into the toxicological mechanisms induced by tire particles and their associated chemicals and can help in the evaluation of potential risks to aquatic life associated with TRWP.

Interactive Effects of Copper-Silver Mixtures at the Intestinal Epithelium of Rainbow Trout: An In Vitro Approach

While metals are present in mixture in the environment, metal toxicity studies are usually conducted on an individual metal basis. There is a paucity of data in the existing literature regarding specific metal-metal interactions and their effect on metal toxicity and bioavailability. We studied interactions of a silver (Ag)-copper (Cu) mixture at the intestinal epithelium using an intestinal cell line derived from rainbow trout (Oncorhynchus mykiss), the RTgutGC. Exposures were conducted in media containing different chloride concentrations (low chloride, 1 mM; high chloride, 146 mM), thus resulting in different metal speciation. Cytotoxicity was evaluated based on two endpoints, cell metabolic activity and cell membrane integrity. The Ag-Cu mixture toxicity was assessed using two designs: independent action and concentration addition. Metal mixture bioavailability was studied by exposing cells to 500 nM of Ag or Cu as a single metal or a mixture (i.e., 500 nM of Cu plus 500 nM of Ag). We found an antagonistic effect in the low-chloride medium and an additive/synergistic effect in the high-chloride medium. We found that Cu dominates over Ag toxicity and bioavailability, indicating a competitive inhibition when both metals are present as free metal ions in the exposure media, which supports our hypothesis. Our study also suggests different mechanisms of uptake of free metal ions and metal complexes. The study adds valuable information to our understanding of the role of metal speciation on metal mixture toxicity and bioavailability. Environ Toxicol Chem 2024;43:105-114. © 2023 SETAC.

A metallothionein gene from hard clam Meretrix meretrix: Sequence features, expression patterns, and metal tolerance activities

Metallothioneins (MTs) are low-molecular weight cytoplasmic heavy metal binding proteins. MTs can regulate the concentration of essential or non-essential metals in organisms, and have many important biological functions, including detoxification, trace element metabolism, and anti-oxidation. In the present study, we cloned and characterized a metallothionein gene (designated as MmMT) from the hard clam Meretrix meretrix. The complete cDNA sequence of MmMT contained an open reading frame (ORF) of 629 bp, which encoded a protein of 76 amino acids with a predicted molecular mass of 7.66 kDa and a calculated theoretical isoelectric point of 7.24. MmMT is highly similar to previously identified MTs from other species, with typical metallothionein features such as a high cysteine residue content and the absence of histidine and aromatic residues. The mRNA transcripts of MmMT were prevalent in all the tested tissues, and the expression levels of MmMT were highest in the hepatopancreas and hemocytes. During the stimulation of Vibrio splendidus, the mRNA transcripts of MmMT in the hepatopancreas and hemocytes were significantly increased. The Escherichia coli overexpressing MmMT performed strong growth in the media supplemented with CdCl2 and CuSO4 compared to the control strains. These results provide useful information for further investigation of the functions of MmMT in metal detoxification and the innate immune system.

Role of the luminal composition on intestinal metal toxicity, bioavailability and bioreactivity: An in vitro approach based on the cell line RTgutGC

The bioavailability of metal complexes is poorly understood. To evaluate bioavailability and toxicity of neutral and charged complexes as well as free metal ions, Visual Minteq, a chemical equilibrium model, was used to design media containing different metal species. Two non-essential (silver and cadmium) and two essential (copper and zinc) metals were selected. The rainbow trout (Oncorhynchus mykiss) gut cell line (RTgutGC) was used to investigate bioavailability, bioreactivity and toxicity of the different metal species. Toxicity was measured using a multiple endpoint cytotoxicity assay, bioavailability by measuring intracellular metal concentration, and bioreactivity by quantification of mRNA level of the metal responsive genes, metallothionein (MT), glutathione reductase (GR) and zinc transporter 1 (ZnT1). Speciation calculations showed that silver and cadmium preferentially bind chloride, copper phosphate and bicarbonate, and zinc remained primarily as a free ion. Cysteine avidly complexed with all metals reducing toxicity, bioavailability and bioreactivity. Silver and copper toxicity was not affected by inorganic metal speciation, whereas cadmium and zinc toxicity was decreased by chloride complexation. Moreover, reduction of calcium concentration in the medium increased toxicity and bioavailability of cadmium and zinc. Bioavailability of silver and zinc was reduced by low chloride while cadmium bioavailability was increased by low chloride and in presence of bicarbonate. Copper bioavailability was not affected by the medium composition. Cadmium and silver were more bioreactive, independently from the medium composition, in comparison to copper and zinc (i.e., higher induction of MT and GR). Cadmium was the only metal able to induce MT in presence of cysteine. ZnT1 was induced by cadmium in low-chloride, by zinc in low-chloride low-calcium and by cadmium and copper in the bicarbonate media. Overall, this study demonstrates that metal complexation alone is not sufficient to explain metal toxicity, and that anion exchange mechanisms play a role in metal uptake and bioreactivity.

Fish Embryo Acute Toxicity Testing and the RTgill-W1 Cell Line as In Vitro Models for Whole-Effluent Toxicity (WET) Testing: An In Vitro/In Vivo Comparison of Chemicals Relevant for WET Testing

The fathead minnow (Pimephales promelas) fish embryo acute toxicity (FET) test was compared to the fish gill cells (RTgill-W1) in vitro assay and to the fish larvae acute toxicity test to evaluate their sensitivity for whole-effluent toxicity (WET) testing. The toxicity of 12 chemicals relevant for WET testing was compared as proof of principle. The concentrations lethal to 50% of a population (LC50) of embryos were compared to those in fish larvae and to the 50% effect concentration (EC50) in RTgill-W1 cells from previous literature. Along with traditional FET endpoints (coagulation, somite development, tail detachment, and heartbeat), cardiotoxicity was evaluated for WET applicability. Heart rate was measured at LC20 and LC50 values of six subselected chemicals (Cd, Cu, Ni, ammonia, 3,4-dichloraniline, and benzalkonium chloride). In addition, the toxicity of Cd and Ni was evaluated in RTgill-W1 cells exposed in a hypoosmotic medium to evaluate the effect that osmolarity may have on metal toxicity. A significant correlation was found between the FET and larvae LC50 values but not between the RTgill-W1 EC50 and FET LC50 values. Although sensitivity to Ni and Cd was found to increase with hypoosmotic conditions for FET and RTgill-W1 cells, a correlation was only found with removal of Ni from the analysis. Hypoosmotic conditions increased sensitivity with a significant correlation between RTgill-W1 cells and larvae. Cardiotoxicity was shown in three of the five subselected chemicals (Cd, Cu, and 3,4-dichloroaniline). Overall, both in vitro alternative models have shown good predictability of toxicity in fish in vivo for WET chemicals of interest. Environ Toxicol Chem 2022;41:2721-2731. © 2022 SETAC.

Effect of chloride concentration on the cytotoxicity, bioavailability, and bioreactivity of copper and silver in the rainbow trout gut cell line, RTgutGC

Chloride (Cl^-) influences the bioavailability and toxicity of metals in fish, but the mechanisms by which it influences these processes is poorly understood. Here, we investigated the effect of chloride on the cytotoxicity, bioavailability (i.e., accumulation) and bioreactivity (i.e., induction of mRNA levels of metal responsive genes) of copper (Cu) and silver (Ag) in the rainbow trout gut cell line (RTgutGC). Cells were exposed to metals in media with varying Cl^- concentrations (0, 1, 5 and 146 mM). Metal speciation in exposure medium was analyzed using Visual MINTEQ software. Cytotoxicity of AgNO₃ and CuSO₄ was measured based on two endpoints: metabolic activity and membrane integrity. Cells were exposed to 500 nM of AgNO₃ and CuSO₄ for 24 h in respective media to determine metal bioavailability and bioreactivity. Ag speciation changes from free ionic (Ag⁺) to neutral (AgCl), to negatively charged chloride complexes (AgCl₂^-, AgCl₃^-) with increasing Cl^- concentration in exposure media whereas Cu speciation remains in two forms (Cu²⁺ and CuHPO₄) across all media. Chloride does not affect Ag bioavailability but decreases metal toxicity and bioreactivity. Cells exposed to Ag expressed significantly higher metallothionein mRNA levels in low Cl^- media (0, 1, and 5 mM) than in high Cl^- medium (146 mM). This suggests that chloride complexation reduces silver bioreactivity and toxicity. Conversely, Cu bioavailability and toxicity were higher in the high chloride medium (146 mM) than in the low Cl^- (0, 1, and 5 mM) media, supporting the hypothesis that Cu uptake may occur via a chloride dependent mechanism. CLINICAL TRIALS REGISTRATION: This study did not require clinical trial registration.

Metallothionein-1 gene from Exopalaemon carinicauda and its response to heavy metal ions challenge

Metallothioneins (MTs) belong to a conserved low-molecular-weight protein family that participates in heavy metal binding and detoxification. EcMT-1 was amplified by PCR from genomic DNA of Exopalaenon carinicauda. It contained a 180 bp open reading frame and encoded 59 amino acids. A total of 18 cysteine (Cys) residues were found in the deduced amino acid sequence, which was consistent with the Cys-rich characteristics of MTs. EcMT-1 was mainly expressed in hepatopancreas, followed by stomach and gill. The expression profiles of EcMT-1 indicated that EcMT-1 was significantly increased at 24, 48 h and 12, 24, and 48 h under the treatment of 2.5 μmol/L CdCl₂ and 50 μmol/L CuSO₄. The expression of EcMT-1 at gastrula stage was very low; it was detectable until nauplius stage, and the highest expression level appeared in the postlarvae stage.

Integrity and wound healing of rainbow trout intestinal epithelial cell sheets at hypo-, normo-, and hyper-thermic temperatures

How temperature influences fish physiological systems, such as the intestinal barrier, is important for understanding and alleviating the impact of global warming on fish and aquaculture. Monolayers of the rainbow trout cell line, RTgutGC, with or without linear 500 μm wide gaps (wounds) were the in vitro models used to study the integrity and healing of intestinal epithelial sheets at different temperatures. Cultures at hypothermic (4 °C) or hyperthermic (≥ 26 °C) temperatures were compared to normothermic control cultures (18-22 °C). Monolayers remained intact for at least a week at temperatures from 4 to 28 °C, but had lost their integrity after 3 h at 32 °C as the cells pulled away from one another and from the plastic surface. F-actin appeared as prominent stress fibers in cells at 28 °C and as blobs in cells at 32 °C. At normothermia and at 26 °C, cells migrated as sheets into the gaps and closed (healed) the gaps within 5-6 days. By contrast, wounds took 14 days to heal at 4 °C. At 28 °C some cells migrated into the gap in the first few days but mainly as single cells rather than collectively and wounds never healed. When monolayers with wounds were challenged at 32 °C for 3 h and returned to 18-22 °C, cells lost their shape and actin organization and over the next 6 days detached and died. When monolayers were subjected to 26 °C for 24 h and challenged at 32 °C for 3 h prior to being placed at 18-22 °C, cell shape and actin cytoskeleton were maintained, and wounds were healed over 6 days. Thus, intestinal epithelial cells become thermostabilized for shape, cytoskeleton and migration by a prior heat exposure.

Applications of the RTgill-W1 Cell Line for Acute Whole-Effluent Toxicity Testing: In Vitro-In Vivo Correlation and Optimization of Exposure Conditions

The cell line RTgill-W1 was evaluated as an in vitro alternative model for acute fish whole-effluent toxicity (WET) testing. We determined the 50% effective concentration (EC50) that reduces the viability of RTgill-W1 cells for selected toxicants commonly found in effluent samples and correlated those values with the respective 50% lethal concentration (LC50) of freshwater (fathead minnow, Pimephales promelas) and marine (sheepshead minnow, Cyprinodon variegatus) fish species obtained from the literature. Excluding low water-soluble organics and the volatile sodium hypochlorite, significant correlations were measured for metal, metalloids, ammonia, and higher water-soluble organics between in vitro EC50 values and in vivo LC50 values for both species. Typically, toxicity studies with RTgill-W1 cells are conducted by adding salts to the exposure medium, which may affect the bioavailability of toxicants. Osmotic tolerance of RTgill-W1 cells was found between 150 and 450 mOsm/kg, which were set as the hypoosmotic and hyperosmotic limits. A subset of the toxicants were then reexamined in hypoosmotic and hyperosmotic media. Copper toxicity decreased in hyperosmotic medium, and nickel toxicity increased in hypoosmotic and hyperosmotic media. Linear alkylbenzene sulfonate toxicity was not affected by the medium osmolality. Overall, RTgill-W1 cells have shown potential for applications in measuring metal, metalloids, ammonia, and water-soluble organic chemicals in acute WET tests, as well as complementing current toxicity identification and reduction evaluation strategies. In the present study, RTgill-W1 cells have been established as a valid animal alternative for WET testing, and we show that through manipulation of medium osmotic ranges, sensitivity to nickel was enhanced. Environ Toxicol Chem 2021;40:1050-1061. © 2020 SETAC.