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

Carcinogenesis Models Using Small Fish

Experimental animals are indispensable in life science-related research, including cancer studies. After rats and mice, small fishes, such as zebrafish and medaka, are the second most frequently used model species. Fish models have some advantageous physical characteristics that make them suitable for research, including their small size, some transparency, genetic manipulability, ease of handling, and highly ortholog correspondence with humans. This review introduces technological advances in carcinogenesis model production using small fish. Carcinogenesis model production begins with chemical carcinogenesis, followed by mutagenesis. Gene transfer technology has made it possible to incorporate various mechanisms that act on cancer-related genes in individuals. For example, scientists may now spatiotemporally control gene expression in a single fish through methods including the localization of an expression site via a tissue-specific promoter and expression control using light, heat, or a chemical substance. In addition, genome editing technology is realizing more specific and more efficient gene disruption than conventional mutagenesis, in which the disruption of the gene of interest depends on chance. These technological advances have improved animal models and will soon create carcinogenesis models that better mimic human pathology. We conclude by discussing future expectations for cancer research using small fish.

Involvement of Rev1 in alkylating agent-induced loss of heterozygosity in Oryzias latipes

Translesion synthesis (TLS) polymerases mediate DNA damage bypass during replication. The TLS polymerase Rev1 has two important functions in the TLS pathway, including dCMP transferase activity and acting as a scaffolding protein for other TLS polymerases at the C‐terminus. Because of the former activity, Rev1 bypasses apurinic/apyrimidinic sites by incorporating dCMP, whereas the latter activity mediates assembly of multipolymerase complexes at the DNA lesions. We generated rev1 mutants lacking each of these two activities in Oryzias latipes (medaka) fish and analyzed cytotoxicity and mutagenicity in response to the alkylating agent diethylnitrosamine (DENA). Mutant lacking the C‐terminus was highly sensitive to DENA cytotoxicity, whereas mutant with reduced dCMP transferase activity was slightly sensitive to DENA cytotoxicity, but exhibited a higher tumorigenic rate than wild‐type fish. There was no significant difference in the frequency of DENA‐induced mutations between mutant with reduced dCMP transferase activity and wild‐type cultured cell. However, loss of heterozygosity (LOH) occurred frequently in cells with reduced dCMP transferase activity. LOH is a common genetic event in many cancer types and plays an important role on carcinogenesis. To our knowledge, this is the first report to identify the involvement of the catalytic activity of Rev1 in suppression of LOH.

Fabrication, optimization, and characterization of umbelliferone β-D-galactopyranoside-loaded PLGA nanoparticles in treatment of hepatocellular carcinoma: in vitro and in vivo studies

Umbelliferone β-D-galactopyranoside (UFG), isolated from plants, exhibits promising inhibitory action on numerous diseases. The present research was initiated to develop a suitable delivery system for UFG with an intention to enhance its therapeutic efficacy against diethyl nitrosamine (DEN)-induced hepatocellular carcinoma (HCC) in Wistar rats. UFG-loaded polymeric nanoparticles prepared by sonication were scrutinized for average size, drug loading capacity, zeta potential, and drug release potency in animals. HCC cell lines HuH-7 and Hep G2 were used for in vitro cytotoxic investigation. Several hepatic, nonhepatic, antioxidant, and anti-inflammatory biochemical parameters were estimated to establish the anticancer potential of UFG nanoformulation. Microscopical and histopathological investigations were also undertaken to substantiate the results of our work. Umbelliferone β-D-galactopyranoside-loaded poly(d,l-lactide-co-glycolide) nanoparticles (UFG-PLGA-NP) with particle size of 187.1 nm and polydispersity index 0.16 were uniform in nature with 82.5% release of the total amount of drug after 48 h. Our study successfully established the development and characterization of UFG-PLGA-NP with noticeable effect against both in vivo and in vitro models. The anticancer potential of UFG-PLGA-NP was brought about by the management of DEN-induced reactive oxygen species generation, mitochondrial dysfunction, proinflammatory cytokines alteration, and induction of apoptosis. Positive zeta potential on the surface of UFG-PLGA-NP would have possibly offered higher hepatic accumulation of UFG, particularly in the electron-dense mitochondria organelles, and this was the take-home message from this study. Our results demonstrated that such polymer-loaded delivery systems of UFG can be a better option and can be further explored to improve the clinical outcomes against hepatic cancer.

Silver nanocolloids disrupt medaka embryogenesis through vital gene expressions

Silver nanomaterials are the major components of healthcare products largely because of their antimicrobial effects. However, their unintended toxicity to biological organisms and its mechanism are not well understood. Using medaka fish embryo model, the toxic effects and corresponding mechanisms of silver nanocolloids (SNC, particle size 3.8 ± 1.0-diameter nm) were investigated. SNC caused morphological changes in embryos including cardiovascular malformations, ischemia, underdeveloped central nervous system and eyes, and kyphosis at exposures of 0.5 mg/L. Interestingly, SNC were observed inside the eggs at a level of 786.1 ± 32.5 pg/mg egg weight, and TEM analysis showed that SNC adhered to the surface and inside of the chorion. Meanwhile, medaka oligo DNA microarray and qRT-PCR were used for gene expression analysis in the embryos exposed to 0.05 mg/L SNC for 48 h. As a result, expressions of six of the oxidative stress-, embryogenesis- and morphogenesis-related genes, ctsL, tpm1, rbp, mt, atp2a1, and hox6b6, were affected by the SNC exposure, and these genes' involvement in those malformations was implied. Thus, SNC could potentially cause malformations in the cardiovascular and central nervous systems in developing medaka embryo through SNC-induced differential expression of the genes related to oxidative stress, embryonic cellular proliferation, and morphological development.

Two farnesoid X receptor alpha isoforms in Japanese medaka (Oryzias latipes) are differentially activated in vitro

The nuclear receptor farnesoid X receptor alpha (FXRalpha, NR1H4) is activated by bile acids in multiple species including mouse, rat, and human and in this study we have identified two isoforms of Fxralpha in Japanese medaka (Oryzias latipes), a small freshwater teleost. Both isoforms share a high amino acid sequence identity to mammalian FXRalpha (approximately 70% in the ligand-binding domain). Fxralpha1 and Fxralpha2 differ within the AF1 domain due to alternative splicing at the fourth intron-exon boundary. This process results in Fxralpha1 having an extended N-terminus compared to Fxralpha2. A Gal4DBD-FxralphaLBD fusion construct was activated by chenodeoxycholic, cholic, deoxycholic and lithocholic acids, and the synthetic agonist GW4064 in transient transactivation assays. Activation of the Gal4DBD-FxralphaLBD fusion construct was enhanced by addition of PGC-1alpha, as demonstrated through titration assays. Surprisingly, when the full-length versions of the two Fxralpha isoforms were compared in transient transfection assays, Fxralpha2 was activated by C(24) bile acids and GW4064, while Fxralpha1 was not significantly activated by any of the compounds tested. Since the only significant difference between the full-length constructs was sequence in the AF1 domain, these experiments highlight a key functional region in the Fxralpha AF1 domain. Furthermore, mammalian two-hybrid studies demonstrated the ability of Fxralpha2, but not Fxralpha1, to interact with PGC-1alpha and SRC-1, and supported our results from the transient transfection reporter gene activation assays. These data demonstrate that both mammalian and teleost FXR (Fxralpha2 isoform) are activated by primary and secondary bile acids.

Anchoring novel molecular biomarker responses to traditional responses in fish exposed to environmental contamination

The responses of Dicentrarchus labrax and Liza aurata to aquatic pollution were assessed in a contaminated coastal lagoon, using both traditional and novel biomarkers combined. DNA damage, assessed by comet assay, was higher in both fish species from the contaminated sites, whereas levels of cytochrome P450 1A1 gene expression were not significantly altered. The liver histopathological analysis also revealed significant lesions in fish from contaminated sites. Alterations in ras and xpf genes were analysed and additional pollutant-responsive genes were identified. While no alterations were found in ras gene, a downregulation of xpf gene was observed in D. labrax from a contaminated site. Suppression subtractive hybridization applied to D. labrax collected at a contaminated site, revealed altered expression in genes involved in energy metabolism, immune system activity and antioxidant response. The approach and results reported herein demonstrate the utility of anchoring traditional biomarker responses alongside novel biomarker responses.

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.

Ras gene in marine mussels: a molecular level response to petrochemical exposure

Mussels are susceptible to numerous toxicants and are often employed as bioindicators. This study investigated the status of the ras proto-oncogene in Mytilus galloprovincialis following petrochemical exposure. A M. galloprovincialis homologue of the vertebrate ras gene was isolated, showing conserved sequence in regions of functional importance and a high incidence of polymorphic variation. Mutational damage was investigated in mussels chronically exposed to the water-accommodated fraction of #4 fuel-oil (WAF), and in mussels collected along the NW coast of Portugal in sites with different levels of petrochemical contamination. A ras gene point mutation was identified in the codon 35 of one individual exposed to 12.5% WAF. No mutations were detected in mussels from the WAF control or environmental samples. This represents the first report of a ras gene mutation, experimentally-induced by petrochemical exposure, in an invertebrate species.

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.

Age-dependent in situ hepatic and gill CYP1A activity in the see-through medaka (Oryzias latipes)

We used a recently introduced strain of medaka, the see-through medaka, whose internal organs can be seen through the skin, to develop an in situ toxicity assay of ethoxyresorufin-O-deethylase (EROD) activity that detected fluorescence from resorufin, a metabolite of ethoxyresorufin and thus an indicator of CYP1A activity. EROD activity in the liver and gills of 2-week post-hatch see-through medaka exposed simultaneously to various concentrations of 3-methylcholanthrene and 200 microg/L ethoxyresorufin for 24 h was proportional to the 3-methylcholanthrene dose. Activities in the liver and gills peaked at 40 microg/L of 3-methylcholanthrene and then decreased at higher doses, possibly because of 3-methylcholanthrene toxicity. At 1-week post-hatch stage, however, constant high EROD activity was observed in controls and at all 3-methylcholanthrene doses. Four-week post-hatch see-through medaka exhibited less EROD activity than 2-week post-hatch see-through medaka, and activity in the liver peaked at 100 microg/L of 3-methylcholanthrene. Adult see-through medaka were not suitable for fluorescence detection owing to their thick skin, muscle and/or tissue. In tests of oxidative activity response to ethoxyresorufin, 1-day and 1-week post-hatch see-through medaka exhibited high intrinsic EROD activity in the liver, gills, and other organs in the absence of 3-methylcholanthrene. This intrinsic activity declined with growth and explained the high constant EROD activity at 1-week post-hatch stage.

Mutation analysis of ras gene in the liver of European eel (Anguilla anguilla L.) exposed to benzo[a]pyrene

Ras is regarded as one of the most important genes involved in carcinogenesis. Such genes have been characterised in several fish species and the presence of ras mutations have already been described in fish populations from hydrocarbon contaminated areas and following experimental exposure to specific contaminants. The aims of this study were to evaluate the DNA integrity by comet assay, to isolate the normal ras gene of Anguilla anguilla and analyse for the presence of ras gene mutations or changes in gene expression levels induced after one month of benzo[a]pyrene (BaP) experimental exposure. The A. anguilla ras cDNA isolated revealed a 189 amino acid protein and alignment with other vertebrate ras proteins revealed conservation of functionally important regions. Following experimental exposure to BaP, an increase in DNA damage was found by comet assay. However, no point mutations or changes in ras gene expression levels were detected when compared to control samples. In contrast to the majority of fish ras gene sequences, a high degree of polymorphic variation was detected in the A. anguilla ras gene.

Distribution of nanoparticles in the see-through medaka (Oryzias latipes)

OBJECTIVE

Because the environmental fate of manufactured nanoparticles is considered an emerging environmental concern, I used water-suspended fluorescent nanoparticles (solid latex solution) to investigate the distribution of nanoparticles in the eggs and bodies of see-through medaka (Oryzias latipes).

RESULTS

Particles 39.4-42,000 nm in diameter were adsorbed to the chorion of medaka eggs and accumulated in the oil droplets; 474-nm particles had the highest bioavailability to eggs. Particles 39.4 nm in diameter shifted into the yolk and gallbladder during embryonic development. Adult medaka accumulated 39.4-nm nanoparticles mainly in the gills and intestine when exposed to a 10-mg/L nanoparticle solution. Nanoparticles were also detected in the brain, testis, liver, and blood. Concentrations of nanoparticles in the blood of male and female medaka were 16.5 and 10.5 ng/mg blood protein, respectively. These results suggest that nanoparticles are capable of penetrating the blood-brain barrier and that they eventually reach the brain. Salinity-dependent acute toxicity was observed in medaka eggs exposed for 24 hr to nanoparticles.

CONCLUSION

The bioavailability and toxicity of nanoparticles depend on environmental factors and multiple physicochemical properties. Further studies on the toxic effects of nanoparticles used in commercial products and their environmental relevance, are necessary to define the risks and benefit of nanomaterial applications.

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.

The Utility of the Guppy (Poecilia reticulata) and Medaka (Oryzias latipes) in Evaluation of Chemicals for Carcinogenicity

There has been considerable interest in the use of small fish models for detecting potential environmental carcinogens. In this study, both guppies (Poecilia reticulata) and medaka (Oryzias latipes) were exposed in the aquaria water to three known rodent carcinogens for up to 16 months. Nitromethane, which caused mammary gland tumors by inhalation exposure in female rats, harderian gland and lung tumors in male and female mice, and liver tumors in female mice by inhalation, failed to increase tumors in either guppies or medaka. Propanediol, which when given in the feed was a multisite carcinogen in both sexes of rats and mice, caused increased liver tumors in male guppies and male medaka. There was reduced survival in female guppies and no increased tumors in female medaka. 1,2,3-Trichloropropane, which when administered by oral gavage was a multisite carcinogen in both sexes of rats and mice, caused an increased incidence of tumors in the liver of both male and female guppies and medaka and in the gallbladder of male and female medaka. The results of this study demonstrate that for these three chemicals, under these specific exposure conditions, the fish appear less sensitive and have a narrower spectrum of tissues affected than rodents. These results suggest that fish models are of limited utility in screening unknown chemicals for potential carcinogenicity.

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.

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.