Analysis of ras gene mutations in rainbow trout liver tumors initiated by aflatoxin B1

DEB-FAPy-dG Adducts of 1,3-Butadiene: Synthesis, Structural Characterization, and Formation in 1,2,3,4-Diepoxybutane Treated DNA

Metabolic activation of the human carcinogen 1,3-butadiene (BD) by cytochrome 450 monooxygenases gives rise to a genotoxic diepoxide, 1,2,3,4-diepoxybutane (DEB). This reactive electrophile alkylates guanine bases in DNA to produce N7-(2-hydroxy-3,4-epoxy-1-yl)-dG (N7-DE-dG) adducts. Because of the positive charge at the N7 position of the purine heterocycle, N7-DEB-dG adducts are inherently unstable and can undergo spontaneous depurination or base-catalyzed imidazole ring opening to give N6 -[2-deoxy-D-erythro-pentofuranosyl]-2,6-diamino-3,4-dihydro-4-oxo-5-N-1-(oxiran-2-yl)propan-1-ol-formamidopyrimidine (DEB-FAPy-dG) adducts. Here we report the first synthesis and structural characterization of DEB-FAPy-dG adducts. Authentic standards of DEB-FAPy-dG and its 15 N3 -labeled analogue were used for the development of a quantitative nanoLC-ESI+ -HRMS/MS method, allowing for adduct detection in DEB-treated calf thymus DNA. DEB-FAPy-dG formation in DNA was dependent on DEB concentration and pH, with higher numbers observed under alkaline conditions.

Chemical Biology of N5-Substituted Formamidopyrimidine DNA Adducts

DNA nucleobases are the prime targets for chemical modifications by endogenous and exogenous electrophiles. Alkylation of the N7 position of guanine and adenine in DNA triggers base-catalyzed imidazole ring opening and the formation of N5-substituted formamidopyrimidine (N5-R-FAPy) lesions. Me-FAPy-dG adducts induced by exposure to methylating agents and AFB-FAPy-dG lesions formed by aflatoxin B1 have been shown to persist in cells and to contribute to toxicity and mutagenicity. In contrast, the biological outcomes of other N5-substituted FAPy lesions have not been fully elucidated. To enable their structural and biological evaluation, N5-R-FAPy adducts must be site-specifically incorporated into synthetic DNA strands using phosphoramidite building blocks, which can be complicated by their unusual structural complexity. N5-R-FAPy exist as a mixture of rotamers and can undergo isomerization between α, β anomers and furanose-pyranose forms. In this Perspective, we will discuss the main types of N5-R-FAPy adducts and summarize the strategies for their synthesis and structural elucidation. We will also summarize the chemical biology studies conducted with N5-R-FAPy-containing DNA to elucidate their effects on DNA replication and to identify the mechanisms of N5-R-FAPy repair.

Error-prone replication bypass of the primary aflatoxin B1 DNA adduct, AFB1-N7-Gua

Hepatocellular carcinomas (HCCs) are the third leading cause of cancer deaths worldwide. The highest rates of early onset HCCs occur in geographical regions with high aflatoxin B1 (AFB1) exposure, concomitant with hepatitis B infection. Although the carcinogenic basis of AFB1 has been ascribed to its mutagenic effects, the mutagenic property of the primary AFB1-DNA adduct, AFB1-N7-Gua, in mammalian cells has not been studied extensively. Taking advantage of the ability to create vectors containing a site-specific DNA adduct, the mutagenic potential was determined in primate cells. This adduct was highly mutagenic following replication in COS-7 cells, with a mutation frequency of 45%. The spectrum of mutations was predominantly G to T base substitutions, a result that is consistent with previous mutation data derived from aflatoxin-associated HCCs. To assess which DNA polymerases (pol) might contribute to the mutational outcome, in vitro replication studies were performed. Unexpectedly, replicative pol δ and the error-prone translesion synthesis pol ζ were able to accurately bypass AFB1-N7-Gua. In contrast, replication bypass using pol κ was shown to occur with low fidelity and could account for the commonly detected G to T transversions.

Molecular basis of aflatoxin-induced mutagenesis-role of the aflatoxin B1-formamidopyrimidine adduct

Aflatoxin B1 (AFB1) is a known carcinogen associated with early-onset hepatocellular carcinoma (HCC) and is thought to contribute to over half a million new HCCs per year. Although some of the fundamental risk factors are established, the molecular basis of AFB1-induced mutagenesis in primate cells has not been rigorously investigated. To gain insights into genome instability that is produced as a result of replicating DNAs containing AFB1 adducts, site-specific mutagenesis assays were used to establish the mutagenic potential of the persistent ring-opened AFB1 adduct, AFB1-formamidopyrimidine (AFB1-FAPY). This lesion was highly mutagenic, yielding replication error frequencies of 97%, with the predominant base substitution being a G to T transversion. This transversion is consistent with previous mutational data derived from aflatoxin-associated HCCs. In vitro translesion synthesis assays demonstrated that polymerase (pol) ζ was the most likely candidate polymerase that is responsible for the G to T mutations induced by this adduct.

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.

Unraveling the aflatoxin-FAPY conundrum: structural basis for differential replicative processing of isomeric forms of the formamidopyrimidine-type DNA adduct of aflatoxin B1

Aflatoxin B1 (AFB) epoxide forms an unstable N7 guanine adduct in DNA. The adduct undergoes base-catalyzed ring opening to give a highly persistent formamidopyrimidine (FAPY) adduct which exists as a mixture of forms. Acid hydrolysis of the FAPY adduct gives the FAPY base which exists in two separable but interconvertible forms that have been assigned by various workers as functional, positional, or conformational isomers. Recently, this structural question became important when one of the two major FAPY species in DNA was found to be potently mutagenic and the other a block to replication [Smela, M. E.; Hamm, M. L.; Henderson, P. T.; Harris, C. M.; Harris, T. M.; Essigmann, J. M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 6655-6660]. NMR studies carried out on the AFB-FAPY bases and deoxynucleoside 3',5'-dibutyrates now establish that the separable FAPY bases and nucleosides are diastereomeric N5 formyl derivatives involving axial asymmetry around the congested pyrimidine C5-N5 bond. Anomerization of the protected beta-deoxyriboside was not observed, but in the absence of acyl protection, both anomerization and furanosyl --> pyranosyl ring expansion occurred. In oligodeoxynucleotides, two equilibrating FAPY species, separable by HPLC, are assigned as anomers. The form normally present in duplex DNA is the mutagenic species. It has previously been assigned as the beta anomer by NMR (Mao, H.; Deng, Z. W.; Wang, F.; Harris, T. M.; Stone, M. P. Biochemistry 1998, 37, 4374-4387). In single-stranded environments the dominant species is the beta anomer; it is a block to replication.

Cloning of Ki-ras and Ha-ras cDNAs from the hermaphroditic fish Rivulus marmoratus (Cyprinodontiformes, Rivulidae) and its expression after exposure to 4-nonylphenol

Previous studies on ras proto-oncogene genes in fish have been focused on chemical-associated carcinogenesis, and the expression of fish ras genes was not well-characterized. We investigated Ki- and Ha-ras genes from the hermaphroditic fish Rivulus marmoratus to understand better their expression patterns in specific tissues, as well as their responses to endocrine-disrupting chemicals such as 4-nonylphenol (4-NP). By investigating expression patterns, we found that the R. marmoratus Ki-ras (Rm Ki-ras) gene showed an alternative splicing event between exons 4A and 4B according to tissue types, which is different from the expression pattern of mammalian Ki-ras genes. In the Rm Ki-ras gene, there were two different expressed types, with exons 1-2-3-4A-4B (long form) and with exons 1-2-3-4B (short form). In the Rm Ki-ras gene, the long form was expressed strongly in the gonad and intestine, and the short form was expressed ubiquitously, except for a low level of expression in the liver. Following 4-NP exposure (300 microg/L), the Rm Ki-ras long form in the liver was significantly expressed, while it was expressed moderately in the ovaries. However, the Rm Ha-ras gene was significantly over-expressed in the brain, while its expression in the gonad was down-regulated. In relation to these modulations after 4-NP exposure, we searched the Rm Ha- and Ki-ras promoter regions and found several ERE-half sites, that may be involved in the modulation of ras gene expression following 4-NP exposure. These genes could be applicable as new biomarker genes for assessing exposure to endocrine-disrupting chemicals (EDCs). Further, this implies the disturbance of ras-dependent signal transduction following EDC exposure.

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.

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 chemistry and biology of aflatoxin B(1): from mutational spectrometry to carcinogenesis

Dietary exposure to aflatoxin B(1) (AFB(1)) is associated with an increased incidence of hepatocellular carcinoma (HCC), especially in populations in which exposure to hepatitis B virus (HBV) is a common occurrence. Most HCC samples from people living where HBV is prevalent have one striking mutational hotspot: a GC-->TA transversion at the third position of codon 249 of the p53 gene. In this review, the chemical reaction of an electrophilic derivative of aflatoxin with specific DNA sequences is examined, along with the types of mutations caused by AFB(1) and the sequence context dependence of those mutations. An attempt is made to assign the source of these mutations to specific chemical forms of AFB(1)-DNA damage. In addition, epidemiological and experimental data are examined regarding the synergistic effects of AFB(1) and HBV on HCC formation and the predominance of one hotspot GC-->TA transversion in the p53 gene of affected individuals.

Analysis of mutations in the p53 tumor suppressor gene and Ki- and Ha-ras proto-oncogenes in hepatic tumors of European flounder (Platichthys flesus)

DNA from five tumors, three other-injured livers and two normal liver tissue samples from the European flounder were analyzed for mutations in exons 5-8 of the tumor suppressor gene p53 and at codons 12, 13 and 61 of Ha- and Ki-ras proto-oncogenes. No tumor-specific mutations were identified by direct sequencing and single-strand conformation polymorphism of these genes. A number of silent polymorphisms were noted in p53. In addition to a need for more extensive analyses of flounder liver tumor samples for ras or p53 mutations, other cancer-related genes should be investigated.

Evidence of p53 mutation in an early stage of liver cancer in European flounder, Platichthys flesus (L.)

A number of flounders dwelling in highly contaminated coastal areas of Northern Europe develop liver tumours. In order to increase our understanding of the molecular pathogenesis of these sporadic tumours, we examined p53 mutations in eleven hyperplasia and six adenoma. p53 introns 4 to 8 were first sequenced to allow individual amplification of exons 5 to 8. DNA extracted from formalin-fixed livers was amplified and PCR products were directly sequenced. Two major results were obtained. (i) Flounders from different geographical areas displayed a high rate of sequence variation. Base substitutions were identified in both tumour and normal tissues and thus may be considered as polymorphic variations in individuals. (ii) One mutation was detected in two hyperplastic foci from the same flounder. This mutation was a T:A to A:T transversion at codon 147, resulting in the replacement of valine for glutamic acid. This residue took place in the L2 loop of the DNA binding surface. Its substitution by an hydrophilic and charged residue could thus impair p53 (protein) biological activity.

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.

Molecular epizootiology of genotoxic events in marine fish: linking contaminant exposure, DNA damage, and tissue-level alterations

Molecular epizootiological studies are increasingly being used to investigate environmental effects of genotoxic contaminants. The assessment of damage to DNA and linking the damage to subsequent molecular, cellular, or tissue-level alterations is a central component of such studies. Our research has focused on the refinement of the 32P-postlabeling assay for measuring covalent DNA-xenobiotic adducts arising from exposure to polycyclic aromatic compounds, using DNA adducts as molecular dosimeters of genotoxic contaminant exposure in biomonitoring studies, and investigating the relationship of DNA adduct formation to toxicopathic liver disease, including neoplastic lesions. A combination of field and laboratory studies using the 32P-postlabeling assay has shown that DNA adducts in marine fish are effective molecular dosimeters of genotoxic contaminant exposure. Investigations of the relationship of DNA adduct formation to neoplastic liver disease have shown that elevated levels of DNA adducts in certain fish species from contaminated coastal sites are associated with increased prevalences of toxicopathic hepatic lesions, including neoplasms, and that the ability to assess DNA damage has helped to explain, in part, species differences in lesion prevalence. Moreover, in a study of a site in Puget Sound contaminated with polycyclic aromatic compounds, we have shown, for the first time, that elevated levels of hepatic DNA adducts are a significant risk factor for certain degenerative and preneoplastic lesions occurring early in the histogenesis of hepatic neoplasms in feral English sole (Pleuronectes vetulus). These latter findings coupled with our current studies of mutational events in the K-ras proto-oncogene should provide further mechanistic substantiation that mutagenic events resulting from exposure to complex mixtures of genotoxic polycyclic aromatic compounds are involved in the etiology of hepatic neoplasia in English sole.

Nucleotide sequence of exon 2 to 4 of the R-ras gene in the hermaphroditic fish Rivulus marmoratus

We have cloned the ras homologue from the mangrove rivulus (Rivulus marmoratus) after low-stringency plaque hybridization of the mangrove rivulus genomic DNA library. This mangrove rivulus ras homologue showed a 76% amino acid homology to the human R(related)-ras gene with identical exon/intron boundary and was named the mangrove rivulus R-ras gene. The mangrove rivulus R-ras gene spanned 1.8 kb and consisted of at least 5 exons. The exon/intron boundaries coincided with the rule of GT/AG of consensus splice acceptor and donor sequences. To our knowledge this is the first report that fish also have the R-ras gene with identical exon/intron boundaries and extensive homology with human.

Mutations of the p53 tumor suppressor gene and ras oncogenes in aflatoxin hepatocarcinogenesis

Aflatoxin B1 (AFB1) is classified as a group I carcinogen in humans by IARC. However, the exact mechanisms of AFB1 hepatocarcinogenesis have not been fully elucidated. Recent studies have suggested that oncogenes are critical molecular targets for AFB1, and AFB1 causes characteristic genetic changes in the p53 tumor suppressor gene and ras protooncogenes. Up to date, more than 1500 human hepatocellular carcinoma (HCC) samples have been examined for p53 mutations with respect to different AFB1 exposure levels. The most significant finding is that more than 50% of HCC patients from high aflatoxin exposure areas such as southern Africa and Qidong, China harboured a codon 249 G to T transversion in the p53 tumor suppressor gene, which is found to be consistent with the mutagenic specificity of AFB1 observed in vitro. In contrast, this mutational pattern is not found in HCC samples from moderate or low aflatoxin exposure countries or regions. Therefore, this hot-spot mutation is believed to be a molecular fingerprint linking the initial event of AFB1-DNA adduct formation with the ultimate development and progress of human HCC. However, some important points still remain to be explicated. First, in many of these studies, the systematic evaluation of AFB1 exposure is rather limited and the classification of AFB1 exposure level is speculative and confusing, without the definite evidence for the actual aflatoxin exposure level. Second, the role of hepadnaviral infection has to be considered in the induction of this unique mutational spectrum. On the other hand, ras oncogene mutations are frequently found in AFB1-induced HCC samples in experimental animals, while the frequency of ras mutation in human HCC in contrast is much lower than that of p53. Recent studies have provided additional evidence that reactive oxygen species (ROS) and oxidative DNA damage may be involved in AFB1-induced p53 and ras mutations. In future, follow-up cohorts exposed to different levels of AFB1 combined with the determination of putative gene markers are much needed.

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.

Early biochemical markers of effects: enzyme induction, oncogene activation and markers of oxidative damage

Experimental carcinogenicity studies focus on identification of single carcinogens. Humans, however, appear exposed to a variety of low doses of carcinogens. Furthermore, few chemical entities are carcinogenic or toxic per se, but require metabolic activation to form ultimate carcinogens or toxins. In contrast to experimental animals, humans show considerable difference in genetic properties. In that situation it is particularly important to estimate individual capability for metabolic activation. To an increasing extent, activation includes formation of toxic oxygen metabolites. Particular targets for activated species are DNA and lipids; in particular low-density lipoproteins (LDL). Modifications of DNA are important for initiating the multistep process of carcinogenesis, in particular if oncogenes are activated or if tumor supressor genes are inactivated. Such DNA modification can be identical regardless of the reactive specimens being a xenobiotic or an oxygen species. Modification of LDL can start the process of atherosclerosis by transforming macrophages into foam cells, deposited as fatty streaks in the arterial wall. Biomarkers for activation capacity of xenobiotics include the use of prototype substrates and molecular techniques to determine genetic polymorphisms. Oxidative DNA modification can be measured from urinary excretion of oxidatively modified deoxynucleosides, particularly guanosine. Future efforts have to include individual measurements in order to improve the 'resolution' of molecular epidemiological approaches.

Assessment of within-group variation in CYP1A mRNA inducibility in environmentally exposed and chemically treated Atlantic tomcod

CYP1A gene expression has been implicated in the processing of environmental procarcinogens and levels of variation in CYP1A mRNA expression are high in both environmentally exposed and chemically treated Atlantic tomcod. The objective of this study was to evaluate the effects of physical and biological parameters such as temperature, sex, and reproductive state on within-group variation in CYP1A mRNA induction. Levels of variation in CYP1A mRNA expression were directly correlated with mean levels of gene induction. Our results indicate that sex and reproductive state, but not temperature, had significant effects on CYP1A mRNA inducibility in tomcod; however, these parameters did not account for all interindividual variation in CYP1A inducibility. Other intrinsic biological factors, such as genetic polymorphisms in molecular pathways leading to CYP1A induction, may contribute to the high levels of interindividual variation in CYP1A inducibility in Atlantic tomcod.

The genetics of human cancer: implications for ecotoxicology

The study of human cancers has provided evidence that malignant progression is associated with genetic change. It has been suggested that some genetic alterations in tumors may be the result of direct or indirect processes related to environmental chemical exposure. This hypothesis has been supported by genetic evidence in liver tumors which has associated aflatoxin B1 exposure with the detection of inactivating DNA mutations within the human p53 tumor suppressor gene. The detection of activating ras oncogene mutations at high frequency in liver tumors of feral fish suggest that the survey of mutations in genes, such as p53 or other genes, might provide a genetic signature for specific chemical exposure in tissues of aquatic animals derived from environmentally damaged sites.

Quantitative carcinogenesis and dosimetry in rainbow trout for aflatoxin B1 and aflatoxicol, two aflatoxins that form the same DNA adduct

Two exposure protocols were used to establish complete dose-response relationships for the hepatic carcinogenicity and DNA adduction in vivo of aflatoxin B1 (AFB1) and aflatoxicol (AFL) in rainbow trout. By passive egg exposure, AFL was taken up less well than AFB1, but was more efficiently sequestered into the embryo itself, to produce an embryonic DNA binding curve that was linear with carcinogen dose and with a DNA binding index three-fold greater than AFB1. Both aflatoxins produced the same phenotypic response, predominantly mixed hepatocellular/cholangiocellular carcinoma. Tumor responses as logit [incidence] vs. In [dose] were parallel-offset, non-linear responses showing a three-fold greater carcinogenic potency for AFL at all doses examined (i.e. 3 times more AFB1 than AFL required to produce an equivalent liver tumor incidence). By molecular dosimetry analysis (logit [incidence] vs. In [DNA adducts]), the two data sets were coincident, indicating that, per DNA adduct formed in vivo in total embryonic DNA, these two aflatoxins were equally efficient in tumor initiation. By dietary fry exposure, both carcinogens produced linear DNA binding dose responses in liver, but with an AFL target organ DNA binding index only 1.14 times that of AFB1 by this exposure route. The tumor dose-response curves also did not exhibit the three-fold difference shown by embryo exposure, but were closely positioned non-linear curves. Since the DNA binding indices differed by only 14%, the resulting molecular dosimetry curves for AFL and AFB1 by dietary exposure were similar to the tumor response curves. These results indicate that differing exposure routes produced differing relative carcinogenicity estimates based on doses applied, as a result of protocol-dependent differences in AFL and AFB1 pharmacokinetic behaviors, but that potency comparisons based on molecular dose received were similar for the two protocols. By comparison with standard DNA adducts produced in vitro using the dimethyloxirane-produced 8,9-epoxides of AFB1 and AFL, we conclude that > 99% of AFL-DNA adducts produced in vivo were identical to those produced by AFB1. Thus similar molecular dosimetry responses should be expected under all exposure protocols in which the two parent carcinogens do not exhibit differing toxicities to the target organ.

Influence of vitamins A, C, and E and beta-carotene on aflatoxin B1 binding to DNA in woodchuck hepatocytes

BACKGROUND

There is extensive epidemiologic evidence suggesting a protective role for micronutrients in cancer incidence. This evidence comes from studies of fruit and vegetable intake and serum levels of specific micronutrients. There also is limited in vitro evidence demonstrating that micronutrients can influence the first step in carcinogenesis, binding of chemical carcinogens to DNA. These in vitro studies allow the determination of specific effects of individual micronutrients. The influence of micronutrients on DNA binding of aflatoxin B1 (AFB1), a potent hepatocarcinogen, in mammalian cells is unknown. Woodchuck hepatocytes were used as a model to investigate the effects of vitamin A (all-trans retinol), C (ascorbic acid), ascorbyl palmitate (a synthetic lipophilic derivative of ascorbic acid), vitamin E (alpha-tocopherol), and beta-carotene on AFB1-DNA binding.

METHODS

Woodchuck hepatocytes were treated with 4 doses (0.080, 0.40, 2.0, and 10 microM) of [3H]AFB1 or with different combinations of AFB1 and the vitamins for 6 hours, and adduct levels determined. Western blot analysis of protein extracts of treated cells was used to determine the effects of vitamin A and beta-carotene on glutathione-S- transferase M1 levels.

RESULTS

Vitamin A inhibited formation of AFB1-DNA adducts in a dose-dependent manner throughout a concentration range of 34-122 microM by 40-80%. Vitamin C (0.080-10 mM) was much less effective than vitamin A as an inhibitor of AFB1-DNA binding. Treatment with 6.0-48.3 microM ascorbyl palmitate reduced adduct levels at lower AFB1 concentrations but had no significant effect at higher AFB1 concentrations. beta-Carotene and vitamin E enhanced covalent binding of AFB1 to DNA. Enhancement with beta-carotene was observed when both tetrahydrofuran or liposomes were used as the administration vehicle. Western blot analysis indicated that neither the vitamin A nor beta-carotene treatment affected glutathione-S-transferase M1 protein levels.

CONCLUSIONS

These results demonstrate that micronutrients play a complex role in the process of chemical carcinogenesis. Although protective effects were seen with several antioxidant vitamins, increased DNA adduct formation was observed with beta-carotene and vitamin E. This antioxidant activity may be unrelated to the inhibition of DNA adduct formation. Additional studies are needed to understand the mechanism of enhanced adduct formation.

Expression of ras genes in rainbow trout liver

Members of the ras gene family have been studied in a variety of species. Two ras genes expressed in the normal liver of rainbow trout, ras-1 and ras-2, as well as a portion of a genomic ras-1 allele, are described for the first time in this report. Over 500 bp of trout ras-1 and at least 300 bp of trout ras-2 genes expressed in normal liver have been sequenced; DNA homology to the human ras genes ranges from 76.8% to 87.1%. The base changes resulting from over 400 million years of evolutionary divergence between the species were primarily silent, with few changes in protein sequence. The partial DNA sequence of the genomic ras-1 allele has 86.8% homology to the first two exons of human c-Ha-ras, and its intron has several conserved sequences characteristic of vertebrate intron-exon junctions. However, the predicted amino acid sequence of trout ras-1 differs at only one of the first 172 amino acid residues from human c-Ki-ras with the alternate exon 4b. Since trout ras-1 differs at 17 and 18 residues from the human c-N-ras and c-Ha-ras proteins, respectively, over this region, we conclude that trout ras-1 is a c-Ki-ras gene. The highly conserved nature of this gene suggests that the ras p21 protein has identical functions in normal and neoplastic cells among higher and lower vertebrates.