Quantitative aspects of metal ion content and toxicity in Drosophila

Biochemical CuSO4 Toxicity in Drosophila melanogaster Depends on Sex and Developmental Stage of Exposure

Copper is a transition metal that exists in different chemical forms (e.g., Cu²⁺,Cu⁺, and Cu⁰) and at high concentrations it is toxic. Here, we investigated the Cu²⁺-induced toxicity in Drosophila melanogaster, evaluating the survival, locomotion, and the activity of acetylcholinesterase (AChE) and glutathione S-transferase (GST) enzymes. Flies were exposed to Cu²⁺(0.1-1 mmol CuSO₄/kg of diet or approximately 0.1-1 mM Cu²⁺) and allowed to mate during 24 h. GST and AChE enzymes were evaluated in the larvae and in the head and the body (thorax + abdomen) of the adult male and females flies. The total number of adult females (0.4-1 mM) and males (0.75 and 1 mM) was decreased by CuSO₄. The climbing ability was hampered in flies exposed to 1 mM Cu²⁺. In larvae, Cu²⁺(0.4-1 mM) increased AChE activity (P < 0.002). In males' heads, 0.4 mM Cu²⁺ increased the AChE activity (P < 0.01). In adults' bodies, Cu²⁺inhibited the activity in both sexes, but with greater effectiveness in males (0.1 to 1 mM) than in females (1 mM). Regarding GST activity, 0.1 mM Cu²⁺increased, but 1 mM decrease GST in larvae. In the head of flies, Cu²⁺decreased the GST activity at intermediate (0.4 mM) and increased GST at the highest concentration (1 mM) in males. In the bodies, the effect of Cu²⁺was similar. In conclusion, Cu²⁺exposure in D. melanogaster disrupted locomotion and enzymatic parameters that can be related to changes in AChE and in the detoxifying GST enzyme.

Oxidative stress, liver biotransformation and genotoxic effects induced by copper in Anguilla anguilla L.--the influence of pre-exposure to beta-naphthoflavone

Fish are exposed in the aquatic ecosystems to different classes of pollutants. Polycyclic aromatic hydrocarbons (PAHs) and heavy metals represent two important classes of aquatic contaminants. Thus, one lot of European eels (Anguilla anguilla L.) was pre-exposed during 24 h to 2.7 microM beta-naphthoflavone (BNF; a PAH-like compound), and subsequently exposed during 24 h to 0, 1 and 2.5 microM copper (Cu). Additionally, another lot not pre-exposed to BNF was exposed to the same Cu concentrations. BNF pre-exposure promoted a significant increase in liver ethoxyresorufin O-deethylase (EROD) activity, but did not change the other responses investigated in eels. On the other hand, both Cu concentrations did not modify the liver EROD activity either in eels pre-exposed to BNF or not. Liver total cytochrome P450 was increased in eels exposed to Cu 2.5 microM, being significantly only in eels not pre-exposed to BNF. Free sulfhydryl group content was decreased by 1 and 2.5 microM in eels pre-exposed to BNF or not pre-exposed, being significant at 2.5 microM Cu in eels not pre-exposed compared to its control. Liver total glutathione (TG), reduced glutathione (GSH) and GSH/oxidized glutathione (GSSG) levels were slightly decreased by 1 and 2.5 microM Cu in eels pre-exposed to BNF, whereas a slight tendency to increase was observed in eels not pre-exposed. Thus, liver TG and GSH significantly decreased in 2.5 microM Cu BNF pre-exposed eels compared to eels not pre-exposed to BNF. Liver glutathione reductase and catalase activities were significantly inhibited by 1 and 2.5 microM Cu in eels pre-exposed to BNF, concomitantly with a slight liver glutathione peroxidase tendency to decrease. Lipid peroxidation was significantly increased by 1 microM Cu in eels either pre-exposed or not pre-exposed to BNF. Liver H(2)O(2) was significantly increased by 1 microM Cu in eels pre-exposed to BNF. Liver DNA integrity was significantly decreased by 1 and 2.5 microM Cu in eels pre-exposed to BNF. The oxidative stress and genotoxic effects induced by Cu in eels pre-exposed to BNF revealed that the metal effects are potentiated by previous exposure to BNF.

A comparison of hepatocyte cytotoxic mechanisms for Cu2+ and Cd2+

The molecular cytotoxic mechanisms of hepatocyte cell death induced by CuCl2, an essential redox transition metal has been compared with CdCl2, an environmental toxin. The ED50 concentrations found for Cu2+ and Cd2+ (i.e. 50% membrane lysis in 2 h) were 50 and 20 microM respectively. However reactive oxygen species ('ROS') formation, GSH oxidation and lipid peroxidation were induced by Cu2+ at these concentrations much more rapidly than by Cd2+. The decline of mitochondrial membrane potential though occurred at the same time and to the same extent for both metals. Furthermore the cytotoxicity and decline of mitochondrial membrane potential induced by these metals was prevented by the 'ROS' scavengers dimethyl sulfoxide, mannitol, catalase or SOD, as well as by desferoxamine, N,N diphenylphenylenediamine or alpha-tocopherol succinate. Hepatocyte GSH was protective as GSH depleted hepatocytes were much more susceptible to Cu2+ and Cd2+ than normal hepatocytes. It is concluded that Cu2+-induced cytotoxicity occurs as a result of a mitochondrial 'ROS' formation independently of cytosolic 'ROS' formation due to redox cycling.

Mutagenicity of soluble and insoluble nickel compounds at the gpt locus in G12 Chinese hamster cells

Nickel is an established human and animal carcinogen, but efforts to demonstrate its mutagenicity in a number of cell types have not been successful. In this report we describe the mutational response to nickel compounds in the G12 cell line, an hprt deficient V79 cell line containing a single copy of the E. coli gpt gene. This cell line has a low spontaneous background, making it suitable for assessment of mutagenic responses to environmental contaminants. When G12 cells were treated with insoluble particles of crystalline nickel sulfide < 5 microns in diameter, a strong, dose-dependent mutagenic response was observed up to 80 times the spontaneous background. Of 48 mutant gpt(-) clones isolated that were induced by insoluble nickel, all were capable of DNA amplification of the gpt sequences by polymerase chain reaction (PCR). The ability to produce full-length PCR products is an indication that large deletions of gene sequences have not occurred. When G12 cells were treated with soluble nickel sulfate, the mutational response was not significantly increased over the spontaneous background. This difference in mutagenic response reflects a large difference in the mutagenic potential of soluble and insoluble nickel compounds, which reflects the carcinogenic potential of these forms of nickel.

Evaluation of Drosophila melanogaster as an alternative animal for studying the neurotoxicity of heavy metals

Heavy metals cause irreversible neurobehavioral damage in many developing mammals, but the mechanisms of this damage are unknown. The influence of three heavy metal compounds, triethyllead chloride, lead acetate and cadmium chloride, on lethality, development, behavior and learning was studied using the fruit fly, Drosophila melanogaster. This animal was used because it allows hundreds of subjects to be assayed very easily in individual experiments and because it is a system in which toxicological questions might be answered by using the techniques of modern molecular genetics. When triethyllead chloride, lead acetate or cadmium chloride was placed in the medium, the larval LC50 (+/- standard error) was found to be 0.090 +/- 0.004, 6.60 +/- 0.64 and 0.42 +/- 0.04 mM, respectively. Each of the tested compounds produced a dose-related delay in development. In particular, they caused a delay in the development of larvae to pupae. When larvae were reared on medium containing triethyllead chloride (0.06 mM), lead acetate (3.07 mM) or cadmium chloride (0.11 mM), phototaxis, locomotion and learning were not inhibited. Since significant neurobehavioral effects were not observed under the experimental conditions used, Drosophila does not appear to be an appropriate animal for the genetic dissection of such effects of heavy metals during development.

Resistance of Drosophila to cadmium: biochemical factors in resistant and sensitive strains

With regard to cadmium toxicity, Drosophila strains v;bw and Austin represent extremes in resistance and sensitivity, respectively. Both strains produced metallothionein (MT) in response to Cd ions in their diet. Austin produced more metallothionein than v;bw at Cd ion levels below 0.2 mM, when both strains were allowed lifetime development on Cd2+-containing media. When the rate of MT appearance was measured for 4 days in young adults the results showed no clear trend with time within a strain or between strains. The plot of LC50 vs. MT levels for identical developmental conditions revealed that for v;bw small increases in MT corresponded to large increases in resistance whereas for the sensitive Austin even large increases in MT had comparatively little effect on increasing LC50. Results given here suggest that differences in total MT content do not explain the genetically demonstrable difference in Cd2+-resistance between v;bw and Austin. However, since two MT genes are identified in Drosophila, differences in resistance could be reflective of greater relative amounts of one "more important" MT in the resistant fly.

Cadmium resistance in Drosophila: a small cadmium binding substance

A small cadmium-binding substance (CdBS) has been observed in adult Drosophila melanogaster that were raised for their entire growth cycle on a diet that contained 0.15 mM CdCl2. Induction of CdBS was observed in strains that differed widely in their sensitivity to CdCl2. This report describes the induction of CdBS and some of its characteristics.

Genetic variation in the susceptibility to mercury and other metal compounds in Drosophila melanogaster

The tolerance of Drosophila melanogaster to heavy metal compounds was investigated with special emphasis on methylmercury. A pronounced variation in tolerance to CH3HgOH, HgCl2, (C2H5)3PbCl, (CH3)3SnCl, and CdCl2 was recorded between 12 wild-type strains. After ranking the tolerance of the strains with respect to the five compounds rank correlations for experiments within and between compounds were calculated. The results showed a high degree of correlation within compounds but no unequivocal indication of a correlation between compounds, indicating that different mechanisms of genetic control for tolerance were operating for the five compounds. Rank correlations for experiments with 12 different mercury, lead, tin, and cadmium compounds and the same 12 wild-type strains only indicated one significant correlated response, between tripropyltin and tributyltin. A selection experiment for tolerance to methylmercury was performed with a foundation population, synthesized from four wild-type strains, showing a high initial tolerance. One control and two levels of treatment doses were used. A distinct selection response was obtained and a high tolerance was reached particularly for the high-dose selection line after 12 generations, when the experiment ended. Genetic analysis of the tolerance indicated a dominant and polygenic inheritance. Investigation of the uptake and excretion of CH3Hg203OH showed that the level of tolerance to methylmercury was correlated with the uptake of the mercury but apparently not with the rate of excretion. Cystein increased the susceptibility to methylmercury. Inorganic mercury and trimethyl lead exhibited a synergistic toxic effect, evidently as the result of an in vitro transmethylation of mercury. A high somatic susceptibility to methylmercury also applied to the induction of nondisjunction and sex-linked recessive lethals.

Genetic and physiological parameters associated with cadmium toxicity in Drosophila melanogaster

Two strains of Drosophila melanogaster represent the extremes in resistance and sensitivity to the lethal effects of CdCl2. The strain containing the mutations vermilion and brown (v; bw) and the strain Austin had LC50's of 3.3 and 1.3 mM CdCl2, respectively. The three major chromosomes from these two strains were assorted genetically into the six possible combinations. The measured LC50's for CdCl2 for these six genotypes fell into two groups according to the X chromosome; those containing the X chromosome from v; bw had LC50's 0.5-1.0 mM greater than those in which the X chromosome was from Austin. Since the parent strains differed by 2 mM, we suggest that the X chromosome is a major, but not the sole, site of genes to produce resistance to CdCl2. When 109Cd was in the diet the uptake by v; bw and Austin over 2 days was the same. After 4 days of uptake, the Austin strain excreted the 109Cd five times faster than v; bw but the six genotypes did not differ appreciably in excretion rate from one another and resembled the sensitive parent Austin more than the resistant one. Thus a second process is indicated that distinguishes resistance to CdCl2 that apparently is not associated with the X chromosome.