Formation and persistence of ethylguanines in liver DNA of rainbow trout (Salmo gairdneri) treated with diethylnitrosamine by water exposure

Time course evaluation of N-nitrosodialkylamines-induced DNA alkylation and oxidation in liver of mosquito fish

Here we simultaneously measured N7-alkylguanines and 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) in liver of small fish, respectively, to assess the time course of the formation and removal of alkylation and oxidative damage to DNA caused by N-nitrosodialkylamines. Mosquito fish (Gambusia affinis) were killed at various times during (4 days) and post-exposure (16 days) to N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) alone or their combination with concentrations of 10 and 50mg/l. The modified guanine adducts were sensitively and selectively quantitated by isotope-dilution LC-MS/MS methods. During exposure, N7-methylguanine (N7-MeG) and N7-ethylguanine (N7-EtG) in liver DNA increased with the duration and dose of N-nitrosodialkylamine exposure, while 8-oxodG was dose-dependently induced within 1 day. It was found that NDMA formed substantially more N7-alkylated guanines and 8-oxodG than NDEA on the basis of adducts formed per micromolar concentration, suggesting that NDMA can be more easily bioactivated than NDEA to form reactive alkylating agents with the concomitant formation of oxygen radicals. After cessation of exposure, N7-alkylguanines remained elevated for 1 day and then gradually decreased over time but still higher than the background levels, even at day 16 (half-lives of 7-8 days). However, 8-oxodG was excised quickly from liver DNA and returned to the background level within 4 days post-exposure (half-lives less than 2 days). Taken together, this study firstly demonstrated that in addition to alkylation, N-nitrosodialkylamines can concurrently cause oxidative damage to DNA in vivo.

MNNG-induced mutations in the adult gill and hepatopancreas and in embryos of rpsL transgenic zebrafish

To evaluate the feasibility of a mutagenicity assay using adult rpsL transgenic zebrafish, 4- to 8-month-old females were exposed to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) (0, 15 or 30 mg/L in a water bath for 2 h). At 2 weeks after exposure, MNNG showed a concentration-dependent significant increase in mutant frequency (MF) of 8 x 10(-5), 18 x 10(-5), and 51 x 10(-5), respectively, in the gill. DNA sequencing revealed that 60-74% of the induced mutations were G:C to A:T transitions, consistent with the known mutagenic effects of MNNG. A marginal but significant increase in MF was observed in the hepatopancreas only in the group exposed to 30 mg/L, with the induction of some G:C to A:T transitions. A time-course of the appearance of mutations was determined in fish treated with 15 mg/L MNNG. In both, the gill and hepatopancreas, a higher MF was observed at 3 weeks than at 2 weeks, suggesting that an expression time of at least 3 weeks is preferable for the assay. When embryos (29 h post-fertilization) were exposed to MNNG (0, 50, and 150 mg/L) for 1 h, MFs increased significantly with an increase in the concentration of MNNG (5 x 10(-5), 40 x 10(-5), and 144 x 10(-5), respectively) at 3 days after exposure. G:C to A:T transitions were the predominant mutations, and these occurred at the same sites in the rpsL gene as in adult tissues. Thus, MNNG induces typical mutations in the gill and hepatopancreas of adult fish, and in embryos, suggesting that the rpsL zebrafish is a useful tool for monitoring genotoxicity caused by water-borne mutagens.

Resolving mechanisms of toxicity while pursuing ecotoxicological relevance?

In this age of modern biology, aquatic toxicological research has pursued mechanisms of action of toxicants. This has provided potential tools for ecotoxicologic investigations. However, problems of biocomplexity and issues at higher levels of biological organization remain a challenge. In the 1980s and 1990s and continuing to a lesser extent today, organisms residing in highly contaminated field sites or exposed in the laboratory to calibrated concentrations of individual compounds were carefully analyzed for their responses to priority pollutants. Correlation of biochemical and structural analyses in cultured cells and tissues, as well as the in vivo exposures led to the production and application of biomarkers of exposure and effect and to our awareness of genotoxicity and its chronic manifestations, such as neoplasms, in wild fishes. To gain acceptance of these findings in the greater environmental toxicology community, "validation of the model" versus other, better-established often rodent models, was necessary and became a major focus. Resultant biomarkers were applied to heavily contaminated and reference field sites as part of effects assessment and with investigations following large-scale disasters such as oil spills or industrial accidents. Over the past 15 years, in the laboratory, small aquarium fish models such as medaka (Oryzias latipes), zebrafish (Danio rerio), platyfish (Xiphophorus species), fathead minnow (Pimephales promelas), and sheepshead minnow (Cyprinodon variegatus) were increasingly used establishing mechanisms of toxicants. Today, the same organisms provide reliable information at higher levels of biological organization relevant to ecotoxicology. We review studies resolving mechanisms of toxicity and discuss ways to address biocomplexity, mixtures of contaminants, and the need to relate individual level responses to populations and communities.

Mechanistic considerations in small fish carcinogenicity testing

Historically, small fish species have proven useful both as environmental sentinels and as versatile test animals in toxicity and carcinogenicity bioassays. They can be bred in large numbers, have low maintenance and bioassay costs, and have a low background incidence of tumors. However, more mechanistic information is needed to help validate the information garnered from these models and to keep pace with other more fully developed animal models. This paper focuses on mechanistic considerations when using small fish models for carcinogenicity testing. Several small aquarium fish species have proven useful. The Japanese medaka is perhaps the best characterized small fish model for carcinogenicity testing; however, the zebrafish is emerging as an important model because it is well characterized genetically. Both route and methodology of exposure may affect the outcome of the study. Most studies have been conducted by introducing the test compound into the ambient water, but dietary exposures and embryo microinjection have also been used. Other considerations in study design include use of an initiating carcinogen, such as diethlynitrosamine, and differences in xenobiotic metabolism, such as the fact that fish CYP2B is refractory to phenobarbital induction. The small size of these models has perhaps limited some types of mechanistic studies, such as formation and repair of DNA adducts in response to carcinogen exposure. However, improved analytical methods are allowing greater resolution and should be applied to small fish species. Slide-based methods such as immunohistochemistry are an important adjunct to routine histopathology and should be included in study design. However, there is a need for development of more species-specific antibodies for fish research. There is also a need for more fish-specific data on cytokines, serum biochemistry, and oncogenes to strengthen the use of these important test models.

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.

Molecular dosimetry in fish: quantitative target organ DNA adduction and hepatocarcinogenicity for four aflatoxins by two exposure routes in rainbow trout

Rainbow trout, a species highly sensitive to aflatoxins, was used to investigate the relative carcinogenicities of four structurally related aflatoxins in terms of their target organ DNA binding characteristics. Tritiated syntheses were carried out, DNA binding dose-response curves were established, and liver DNA binding indices were calculated for the four aflatoxins following a 2-week dietary fry exposure protocol. The results indicated that adduct levels increased linearly with dietary dose concentration, with relative DNA binding indices of 20.7, 20.3, 2.35, and 2.22 x 10(3) (pmoles aflatoxin mg-1 DNA)/(pmoles aflatoxin g-1 diet) for aflatoxin B1 (AFB1), aflatoxicol (AFL), aflatoxin M1 (AFM1), and aflatoxicol M1 (AFLM1), respectively. A similar protocol used over 7200 trout fry averaging 1.2 g initial body weight to establish full carcinogen dose-response curves for each aflatoxin, along with a single-dose estimate of DNA binding index within the tumor study animals. Owing to trout sensitivity a total of 180 micrograms or less of each aflatoxin was required. Data analyzed on logit incidence vs. Ln dose coordinates generated four curves which were modeled as parallel in slope over most or all dose ranges studied. By this analysis, relative tumorigenic potencies were: AFB1 1.00; AFL 0.936; AFM1 0.086; and AFLM1 0.041. When data were plotted as logit incidence vs. Ln adducts (effective dose received), all aflatoxin adducts described the same dose-response curve; that is, they were equally tumorigenic, except those from AFLM1, which were 2-3 fold less potent. Therefore, by these molecular dose studies, differences in tumorigenicity among the four dietary aflatoxins are largely or entirely accounted for by differences in uptake and metabolism leading to DNA adduction, rather than any inherent differences in tumor initiating potency per DNA adduct.

Characterization of the aromatic hydrocarbon receptor gene and its expression in Atlantic tomcod

Cytochrome P4501A1 (CYP1A1) mRNA is not inducible in Atlantic tomcod from the Hudson River that are treated with halogenated aromatic hydrocarbons (HAHs). In contrast, CYP1A1 mRNA is inducible in Hudson River tomcod that are treated with polycyclic aromatic hydrocarbons (PAHs) and in tomcod that are collected from cleaner rivers and treated with HAHs or PAHs. We hypothesize that CYP1A1 transcription is inhibited in Hudson River tomcod because of down-regulation of the aromatic hydrocarbon receptor (AhR) pathway and that separate molecular pathways modulate CYP1A1 transcription in fish treated with HAHs and PAHs. We initially evaluated levels of hepatic nuclear protein binding at enhancer elements (DREs) in the regulatory region of tomcod CYP1A1. No difference in levels of protein binding was observed between tomcod from the Hudson and Miramichi (cleaner) rivers that were untreated or were treated with benzo[a]pyrene. In contrast, levels of protein binding were lower in tomcod from the Hudson River that were treated with TCB than in similarly treated fish from the Miramichi River, suggesting differences between the populations in the structure or expression of AhR pathway molecules. To address this possibility, AhR DNA sequences were characterized from tomcod cDNA and genomic DNA libraries. In tomcod and mammals, AhR is represented by 11 exons, overall peptide sizes are similar, and amino acid sequences at basic, helix-loop-helix, PAAS A, and PAAS B domains are highly conserved. In contrast, little similarity was observed between tomcod and mammals in the sizes or sequences of AhR exons 10 and 11, including the absence in tomcod of glutamine-rich domains. No differences in levels of hepatic AhR mRNA were observed between the two populations or treatment groups when tomcod were untreated or were treated with aromatic hydrocarbons. In contrast, variation in levels of AhR mRNA expression was observed among tomcod tissues; however, no relationship was observed between levels of AhR mRNAs and CYP1A1 mRNAs in tissues from chemically or vehicle control-treated fish. RFLP analysis revealed extensive variation in exons 10 and 11 of AhR cDNA among tomcod from different rivers. Our results suggest that variation between tomcod populations in CYP1A1 mRNA inducibility is reflected by differences in levels of inducible hepatic protein binding to DREs. However, levels of hepatic AhR mRNA are not down-regulated in the Hudson River population, are not affected by AH treatments, and levels of AhR mRNA expression are not responsible for the differential inducibility of CYP1A1 transcription.

Ultrastructural alterations in liver of medaka (Oryzias latipes) exposed to diethylnitrosamine

Liver cytotoxic alterations of adult medaka (Oryzias latipes) following short-term bath exposure (48 hr) to 500 mg/L diethylnitrosamine (DEN) were studied (days 3-21) by electron microscopy and cytochemistry. Control medaka displayed hepatic sexual dimorphism as described for other sexually active fish. Following DEN exposure, decreased glycogen stores with loss of cellular compartmentation obscured sexual dimorphism. A spectrum of organelle alterations, previously not reported in livers of fish, was seen. Early changes in hepatocytes included: nuclear lipid inclusions, nucleolar changes, decreased amounts of granular endoplasmic reticulum (GER), increased fractionation and steatosis of GER, proliferation of smooth ER and lysosomes, reduction in number and content of particulate lipoproteins and vitellogenin in Golgi vesicles, and reduction in number and staining intensity of peroxisomes. At day 14 and/or 21, partial to complete reversal of the above alterations indicated hepatic recovery, and fewer necrotic cells were seen at day 21 versus day 14. Lesions that did not resolve during this study were altered mitochondria and areas of spongiosis hepatis that developed at day 8 and continued to increase throughout the study. Infiltration of lymphocytes, granulocytes, and large numbers of macrophages were late changes. The description, timing, and duration of lesions are of value for consideration as biomarkers of exposure and effect in aquatic toxicology.

DNA adducts as early bioindicators of chemical exposure

Sensitive analytical methods indicate the presence of hundreds of chemical contaminants in our environment. However, concentration of these pollutants is usually at the low parts per million or parts per billion level. At such low levels, toxicants induce long-term (chronic) rather than short-term (acute) toxicities. Experiments are designed to evaluate chronic toxicity using early bioindicators. Recently, fish have been used as experimental animals because some species show early (weeks as opposed to years in other research animals), sensitive responses. Thus, medaka (Oryzias latipes), following exposure to diethylnitrosamine (DEN), exhibited liver tumors in several weeks; more interestingly its DNA was modified (to a 'DNA-adduct') after only 24-48 hours exposure. Such adducts show promise as early bioindicators because they are formed within hours of exposure. The formation of DNA-adducts were monitored in medaka and rainbow trout (Salmo gairdneri) exposed to DEN by aqueous or intra-peritoneal routes. Using HPLC-Fluorimetry, O (6)-ethylguanine (O (6)-EtGua) was detected and monitored in acid thermal hydrolysates of DNA isolates. (Detection limit for O (6)-EtGua was as low as 3 ng, 1.7×10(-11) moles). Fourier transform cyclotron resonance mass spectrometry with IR laser desorption/ionization was used advantageously to establish the structure of nucleotides, bases and nucleosides directly, without further cumbersome derivatizations. Other aspects of DNA-adducts are discussed.

Modulation of diethylnitrosamine-induced hepatocarcinogenesis and O6-ethylguanine formation in rainbow trout by indole-3-carbinol, beta-naphthoflavone, and Aroclor 1254

Rainbow trout were fed a diet containing indole-3-carbinol (2000 ppm), beta-naphthoflavone (500 ppm), or Aroclor 1254 (100 ppm) for 6 weeks before a single 24-hr exposure to an aqueous solution of 250 ppm diethylnitrosamine (DEN). The fish were killed 42 weeks later to determine the carcinogenic response. DEN exposure produced an 80.2% incidence of liver tumors and an average of 3.47 tumors per tumor-bearing fish, whereas no tumors were detected in the sham-treated control fish. Tumor induction was inhibited by indole-3-carbinol (27.5% incidence, 1.89 tumors per tumor-bearing fish) but enhanced by beta-naphthoflavone (91.8% incidence, 3.60 tumors per tumor-bearing fish). Aroclor 1254 had no effect on DEN-induced hepatocarcinogenesis (80.0% incidence, 3.03 tumors per tumor-bearing fish). The effects of these modulators on O6-ethylguanine and 7-ethylguanine formation (measured by HPLC and fluorescence spectrophotometry) were examined. Liver DNA ethylguanine levels were reduced in indole-3-carbinol-pretreated fish and increased in beta-naphthoflavone-pretreated fish compared to untreated controls after DEN exposure. Aroclor 1254 pretreatment had no significant effect on DNA ethylguanine formation. Similar O6-ethylguanine to 7-ethylguanine ratios were found among the control and treated groups. The results of this study indicate that modulation of DEN hepatocarcinogenesis by indole-3-carbinol and beta-naphthoflavone may be mediated by their effects on O6-ethylguanine formation and, therefore, on the initiation phase of carcinogenesis.