Dose-dependent carcinogenicity and frequent Ki-ras proto-oncogene activation by dietary N-nitrosodiethylamine in rainbow trout

Histopathological lesions, P-glycoprotein and PCNA expression in zebrafish (Danio rerio) liver after a single exposure to diethylnitrosamine

The presence of carcinogenic compounds in the aquatic environment is a recognized problem. ABC transporters are well known players in the multidrug-resistance (MDR) phenomenon in mammals associated with resistance to chemotherapy, however little is known in fish species. Thus, the aim of this study was to induce hepatic tumours and evaluate long-term effects on P-glycoprotein (P-gp) and proliferating cell nuclear antigen (PCNA) proteins in Danio rerio liver, after exposure to diethylnitrosamine (DEN). Several hepatic histopathological alterations were observed in zebrafish after exposure to DEN including pre-neoplastic lesions 6 and 9 months post-exposure. After 3, 6 and 9 months of exposure to DEN, P-gp and PCNA proteins expression were up-regulated. In conclusion, this study has shown that zebrafish ABC transporters can play a similar role as in human disease, hence zebrafish can be used also as a biological model to investigate in more deep mechanisms involved in disease processes.

Modulatory effect of environmental endocrine disruptors on N-ras oncogene expression in the hermaphroditic fish, Kryptolebias marmoratus

Kryptolebias marmoratus is the only known internally self-fertilizing vertebrate. It shows high susceptibility to many chemical carcinogens and has been proposed as a potential cancer model species alternative to mammals. Since use of this fish species is expected to rise in cancer research, regulation of oncogenes from K. marmoratus needs proper understanding. We cloned and deduced full-length sequence of cDNA of N-ras oncogene from K. marmoratus. Study of expression profile of N-ras by using quantitative real-time RT-PCR revealed that brain had the highest level of expression compared to other tissues. Some embryonic stages showed more N-ras expression than juveniles and adults. Exposure to two environmental endocrine disrupting chemicals (EDCs), bisphenol A (BPA) and 4-nonylphenyl (NP) caused up-regulation of N-ras in gonad, intestine and liver of hermaphrodite K. marmoratus. It is suggested that K. marmoratus may be a suitable model species for oncogene expression studies. The observed EDC-induced expression of N-ras supports the assumption that EDC exposure may predispose the host to the risk of environmental carcinogenesis.

Molecular markers of cancer in cartilaginous fish: immunocytochemical study of PCNA, p-53, myc and ras expression in neoplastic and hyperplastic tissues from free ranging blue sharks, Prionace glauca (L.)

Archival formalin-fixed tissues from wild-caught adult blue sharks, Prionace glauca (L.), were used for immunocytochemical detection of proliferating cell nuclear antigen (PCNA), two oncoproteins from the oncogenes c-myc and pan-ras, and a protein product from the tumour suppressor gene p-53. All sharks were caught during summer months between 2000 and 2006 by recreational fishermen off the USA coast in the northwestern Atlantic. The sharks were necropsied on landing and selected organ samples were collected into elasmobranch formalin and processed for paraffin embedding and light microscopy. Paraffin-embedded sections from collected tissue were both stained with haematoxylin and eosin and processed by immunocytochemical techniques using antibodies raised against the PCNA, p-ras, c-myc and p-53 proteins. The lesions examined in this study included two well differentiated adenomatous gastric polyps, a testicular capsular mesothelioma, a gingival fibropapilloma with elements of ameloblastoma, three liver tumours, two pericardial fibropapillomas and six cases of proliferative serositis (pericarditis and peritonitis). Normal and hyperplastic tissues from blue sharks, and human neoplastic tissues served as negative and positive controls, respectively. We detected upregulation of PCNA in many neoplastic, one dysplastic and in some hyperplastic lesions, and positive p-ras and c-myc signals in some of the neoplastic lesions. None of the examined tissues showed positive p-53 signalling. This is the first literature report on immunocytochemical detection of molecular markers of cancer in sharks and in fish of the class Chondrichthyes.

Carcinogenic effect of N-nitrosodimethylamine on diploid and triploid zebrafish (Danio rerio)

Viability of polyploid organisms in lower vertebrates including fish provides an additional tool to investigate genetic mechanisms of neoplastic transformation caused by carcinogens. Here we present data on differential sensitivity of diploid and triploid zebrafish (Danio rerio) to N-nitrosodimethylamine (NDMA) induced hepatocarcinogenesis. The effect of the carcinogen was studied in 100 diploid and 120 triploid zebrafish. Zebrafish, age 5-6 weeks, were exposed to 50 ppm NDMA for 8 weeks and then were transferred into fresh carcinogen-free water until necropsy. At the necropsy performed 24 weeks after beginning the treatment, cholangiolar tumors (cholangiocarcinomas and cholangiomas) were essentially observed in diploid zebrafish only, while the incidence of hepatocellular tumors (hepatocellular carcinomas and adenomas) was similar in diploid and triploid zebrafish, 7.7% and 9.5%, respectively. By contrast, 36 weeks after beginning the treatment, the incidence of hepatocellular tumors was significantly lower in diploid animals as compared to triploid ones, 10.3% and 33.8%, respectively. The incidence of cholangiolar tumors in diploid and triploid zebrafish was not significantly different, 10.3% and 14.9%, respectively. Therefore, the increase of ploidy appeared to have a differential effect on the induction of these 2 types of liver tumors in zebrafish. This finding suggests a difference in genetic mechanisms of the tumor development revealed by utilization of triploid animals in this study. However, triploid zebrafish demonstrated an overall increase in latency period in the development of both types of hepatic tumors, a finding that can be interpreted as an increased resistance of triploid animals to the carcinogenic effect of NDMA.

ras oncogene mutations in diethylnitrosamine-induced hepatic tumors in medaka (Oryzias latipes), a teleost fish

Medaka fish are an established non-mammalian research model for the study of liver carcinogenesis and exposure to environmental pollutants. Studies have emphasized the development of hepatic neoplasms in medaka following exposure to model carcinogens. To date however, little information is known regarding the mechanisms underlying initiation of hepatic tumors in this species. The aim of this study was to relate our understanding of diethylnitrosamine (DEN)-induced tumor formation to ras gene activation in hepatic neoplasms of exposed medaka. Initial studies were conducted to identify medaka ras exons 1 and 2 by reverse transcriptase polymerase chain reaction (RT-PCR). Amplification of ras exons 1 and 2 from untreated medaka liver resulted in the identification of three polymorphic ras sequence variants exhibiting a high degree of homology to other teleost and mammalian ras genes. Exposure of medaka to 159 ppm of DEN resulted in a wide range of hepatic neoplasms including: hepatocellular adenomas, hepatocellular carcinomas, cholangiomas, and mixed hepatocholangiocellular carcinomas. Individual liver tumors were examined for oncogenically activating ras mutations by probing genomic DNA with probes specific for activating point mutations or by direct cloning and sequencing of ras transcripts using RT-PCR. Using allele-specific oligonucleotide (ASO) analysis, a single point mutation was detected in codon 12 position two in 8/25 (32%) tumors examined. Mutated ras alleles were additionally detected in 12 of 39 (30%) medaka liver tumors by sequence analysis. Ten of the 12 mutations identified contained a single point mutation at codon 12 resulting in a Gly to Asp amino acid substitution. Two unique mutations were identified at codon 16 resulting in either Lys to Asn or Lys to Thr amino acid substitutions. Our results show that ras mutations are induced by DEN and are present in over 30% of the fish that developed tumors. A ras mutation incidence of 30% is similar to that reported in mammalian species exposed to DEN. While mutations at codon 12 have previously been reported, the present study is the first in vivo report of ras point mutations at codon 16.

Cancer genetics and drug discovery in the zebrafish

Fish have a long history of use in cancer toxicology studies, because they develop neoplasms that are histologically similar to human cancers. Because of considerable progress in zebrafish genetics and genomics over the past few years, the zebrafish system has provided many useful tools for studying basic biological processes. These tools include forward genetic screens, transgenic models, specific gene disruptions and small-molecule screens. By combining carcinogenesis assays, genetic analyses and small-molecule screening techniques, the zebrafish is emerging as a powerful system for identifying novel cancer genes and for cancer drug discovery.

Structure, expression and activation of fish ras genes

Ras genes encode proteins that play a central role in cell growth signaling cascades. The fish ras genes characterized to date, have a high degree of nucleotide sequence and deduced amino acid similarity with the mammalian ras gene counterparts. A large proportion and wide variety of mammalian tumors possess mutant forms of ras. In such cases, the localization of ras mutations has been restricted to exons I and II, and to codons 12, 13 and 61. Experimental exposure of fish to a range of genotoxic compounds has similarly led to the production of a ras mutational profile for selected species. The inducing compound, tissue investigated and the fish species studied affect the ras mutational spectrum and incidence observed, despite the apparent conserved sequence homology. Furthermore, the fish ras mutational profile differs from that observed in rodent models, including a novel codon (16) mutation. The role of ras genes in tumor formation in feral fish has been investigated using several species collected from areas of high hydrocarbon contamination. Tomcod (Microgadus tomcod), winter flounder (Pseudopleuronectes americanus) and dragonet (Callionymus lyra) liver samples display evidence of ras gene mutations, though for the latter species the codon affected is not characteristic of ras gene mutational profiles. English sole (Pleuronectes vetulus) and European flounder (Platichthys flesus) liver tumor samples so far examined, on the other hand, do not display ras gene mutations. Thus, the pattern and incidence of ras gene mutations in environmentally-induced tumors also appear to be species specific. In determining the basis of both the species susceptibility observed in the field and species differences in effects of laboratory controlled exposures, the interaction of fish ras genes with other components of the cell growth signaling cascade (such as protein kinase C, additional oncogenes and tumor suppressor genes) are discussed. The effect of promoting agents following contaminant-induced initiation could similarly provide answers in unraveling the question of species susceptibility.

The importance of distinct metabolites of N-nitrosodiethylamine for its in vivo mutagenic specificity

Although N-nitrosodiethylamine (NDEA) is a potent carcinogen in rodents and a probable human carcinogen, little attempts were made to characterize its mutation spectrum in higher eukaryotes. We have compared forward mutation frequencies at multiple (700) loci with the mutational spectrum induced at the vermilion gene of Drosophila, after exposure of post- and pre-meiotic male germ cells to NDEA. Among 30 vermilion mutants collected from post-meiotic stages were 12 G:C-->A:T transitions (40%), 8 A:T-->T:A transversions (27%), and 4 structural rearrangements (13%). The remainder were three A:T-->G:C transitions, two G:C-->C:G transversions and one G:C-->T:A transversion. The results show that although NDEA induces predominantly transitions (40% G:C-->A:T and 10% A:T-->G:C), the frequencies of transversions (37%, of which 27% of A:T-->T:A transversions) and especially of rearrangements (13%) are remarkably high. This mutation spectrum differs significantly from that produced by the direct-ethylating agent N-ethylnitrosourea (ENU), although the relative distribution of ethylated DNA adducts is similar for both carcinogens. These differences, in particular the occurrence of rearrangements, are most likely the result of the requirement of NDEA for bioactivation. Since all four rearrangements were collected from non-metabolizing spermatozoa (or late spermatids), it is hypothesized that they derived from acetaldehyde, a stable metabolite of NDEA. Due to its cytotoxicity, attempts to isolate vermilion mutants from NDEA-exposed pre-meiotic cells were largely unsuccessful, because only two mutants (one A:T-->G:C transition and one 1bp insertion) were collected from those stages. Our results show that NDEA is capable of generating carcinogenic lesions other than base pair substitutions.

Enhanced xenobiotic transporter expression in normal teleost hepatocytes: response to environmental and chemotherapeutic toxins

Many aquatic organisms are resistant to environmental pollutants, probably because their inherent multi-drug-resistant protein extrusion pump (pgp) can be co-opted to handle man-made pollutants. This mechanism of multixenobiotic resistance is similar to the mechanism of multidrug resistance exhibited in chemotherapy-resistant human tumor cells. In the present study, a variety of techniques were used to characterize this toxin defense system in killifish (Fundulus heteroclitus) hepatocytes. The cellular localization and activity of the putative drug efflux system were evaluated. In addition, in vitro and in vivo studies were used to examine the range of expression of this putative drug transporter in the presence of environmental and chemotherapeutic toxins. The broad range of pgp expression generally observed in transformed mammalian cells was found in normal cells of our teleost model. Our findings suggest that the expression of the pgp gene in the killifish could be an excellent indicator of toxin levels or stressors in the environment.

Nmethyl-N-nitrosourea (MNU) induces papillary thyroid tumours which lack ras gene mutations in the hermaphroditic fish Rivulus marmoratus

To investigate the patterns of alkylating agent-induced tumour formation, 40 fish (Rivulus marmoratus) were exposed to N-methyl-N-nitrosourea (MNU) at 50 ppm in 10 mM Hepes-buffered synthetic seawater for 2 h. Tumour incidence 4 months after exposure was approximately 95%, and mainly papillary thyroid tumours were induced. For elucidating the molecular event in Rivulus papillary thyroid carcinogenesis, we first analysed for ras gene mutations based on the known ability of MNU to induce point mutations. The amplified R. Marmoratus Ha-and Ki-ras gene exon 1 and 2 regions were checked along with confirming the presence and expression patterns of the DNA repair gene O(6)-methylguanine alkyltransferase (O(6)-MT) and other oncogenes (c-src, c-fos, and c-myc). Ha- and Ki-ras genes from 38 tumour samples were tested for point mutations with direct sequencing but were not found to contain mutations. In this report, based on the lack of Ha- and Ki-ras gene mutations in papillary thyroid tumour induction in R. Marmoratus, we suggest that Ha- and Ki-ras gene-independent pathways such as ret/PTC rearrangements and other activations are involved in Rivulus papillary thyroid carcinogenesis. Teratogenesis Carcinog. Mutagen. 20:1-9, 2000.

Short- and intermediate-term carcinogenicity testing--a review. Part 2: available experimental models

Numerous experimental protocols for short- and intermediate-term carcinogenicity assays have been available for many years. This paper surveys various of these test systems in rodents, fish species, non-vertebrates and avian embryos in ovo. The mouse skin tumour assay and the rat liver foci assay were used to introduce the basic concepts of short- and intermediate-term carcinogenicity testing in the previous part of the review. The focus of this second part of the review is on rodent assays for carcinogenicity testing in the lung, kidney, urinary bladder, pancreas, stomach, oral cavity, small intestine, colon, and on the possibility to combine several target organs in multi-organ models. The potential use of various fish species, non-vertebrates and hatching eggs for carcinogenicity testing is outlined and the advantages and limitations are discussed. This review also presents the problem of validation of any carcinogenicity test system and proposes a strategy for contemporary safety assessment of chemicals with regard to the detection and evaluation of carcinogenicity.

Experimental chemical carcinogenesis in fish

Experimental carcinogenesis using fish species as alternative models is a dynamic field of research. The 1940's expansion of synthetic chemical producing industries coincided with a number of pollution-associated fish neoplasia epizootics, with polycyclic aromatic hydrocarbons as significant components of contaminated sediment in several cases. Epizootics of primarily liver and skin neoplasia in benthic species near coastal urban or industrial areas indicated the sensitivity of fish species to known mammalian carcinogens. Stressing a mechanistic approach, investigators have used data compiled from epizootics as the backbone of current research efforts to define carcinogenesis in fish species. With liver as the focus, patterns of neoplastic development similar to those seen in rodent bioassays have been induced in various fish species by genotoxic carcinogens. Similarities between fish and rodent models include chemical and species-specific responses to exposure and the development of predictable preneoplastic and neoplastic lesions. The expression of molecular molecules related to carcinogenesis is currently under investigation, which includes alterations in certain proteins, enzyme activity, and oncogene/tumor suppressor gene function. The potential for the application of research findings to both human and environmental health issues makes fish species attractive and valuable alternative models in carcinogenesis and toxicity research.

Carcinogenicity of dietary dimethylnitrosomorpholine, N-methyl-N'-nitro-N-nitrosoguanidine, and dibromoethane in rainbow trout

Eighteen-mo feeding trials of rainbow trout were used to test the carcinogenicity of 5 chemicals in this species. A single exposure level was used for each substance. The doses and chemicals tested were 1,556 ppm 2,6-dimethylnitrosomorpholine (DMNM), 500 ppm N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), 2,000 ppm 1,2-dibromoethane (DBE), 2,000 ppm 1,1-dichloroethylene (DCE), and 200 ppm cyclophosphamide (CP). Liver and/or glandular stomach neoplasms were produced by DMNM (liver and stomach), MNNG (stomach), and DBE (chiefly, stomach tumors). In addition, DMNM produced a low incidence of swimbladder papillomas and caused testicular atrophy in 50% of treated males. DCE and CP produced no neoplasms at the exposure levels used. No evidence of other chronic toxicity was seen for any of the 5 compounds.