Benzo[a]pyrene decreases global and gene specific DNA methylation during zebrafish development

The dual effects of Benzo(a)pyrene/Benzo(a)pyrene-7,8-dihydrodiol-9,10-epoxide on DNA Methylation

Benzo(a)pyrene (BaP) is one of the most thoroughly studied polycyclic aromatic hydrocarbons(PAHs) and a widespread organic pollutant in various areas of human life. Its teratogenic, immunotoxic and carcinogenic effects on organisms are well documented and widely recognized by researchers. In the body, BaP is enzymatically converted to form a more active benzo(a)pyrene-7,8-dihydrodiol-9,10-epoxide (BPDE). BaP/BPDE has the potential to trigger gene mutations, influence epigenetic modifications and cause damage to cellular structures, ultimately contributing to disease onset and progression. However, there are different points of view when studying epigenetics using BaP/BPDE. On the one hand, it is claimed in cancer research that BaP/BPDE contributes to gene hypermethylation and, in particular, induces the hypermethylation of tumor's suppressor gene promoters, leading to gene silencing and subsequent cancer development. Conversely, studies in human and animal populations suggest that exposure to BaP results in genome-wide DNA hypomethylation, potentially leading to adverse outcomes in inflammatory diseases. This apparent contradiction has not been summarized in research for almost four decades. This article presents a comprehensive review of the current literature on the influence of BaP/BPDE on DNA methylation regulation. It demonstrates that BaP/BPDE exerts a dual-phase regulatory effect on methylation, which is influenced by factors such as the concentration and duration of BaP/BPDE exposure, experimental models and detection methods used in various studies. Acute/high concentration exposure to BaP/BPDE often results in global demethylation of DNA, which is associated with inhibition of DNA methyltransferase 1 (DNMT1) after exposure. At certain specific gene loci (e.g., RAR-β), BPDE can form DNA adducts, recruiting DNMT3 and leading to hypermethylation at specific sites. By integrating these different mechanisms, our goal is to unravel the patterns and regulations of BaP/BPDE-induced DNA methylation changes and provide insights into future precision therapies targeting epigenetics.

A Quantitative Adverse Outcome Pathway for Embryonic Activation of the Aryl Hydrocarbon Receptor of Fishes by Polycyclic Aromatic Hydrocarbons Leading to Decreased Fecundity at Adulthood

Quantitative adverse outcome pathways (qAOPs) describe the response-response relationships that link the magnitude and/or duration of chemical interaction with a specific molecular target to the probability and/or severity of the resulting apical-level toxicity of regulatory relevance. The present study developed the first qAOP for latent toxicities showing that early life exposure adversely affects health at adulthood. Specifically, a qAOP for embryonic activation of the aryl hydrocarbon receptor 2 (AHR2) of fishes by polycyclic aromatic hydrocarbons (PAHs) leading to decreased fecundity of females at adulthood was developed by building on existing qAOPs for (1) activation of the AHR leading to early life mortality in birds and fishes, and (2) inhibition of cytochrome P450 aromatase activity leading to decreased fecundity in fishes. Using zebrafish (Danio rerio) as a model species and benzo[a]pyrene as a model PAH, three linked quantitative relationships were developed: (1) plasma estrogen in adult females as a function of embryonic exposure, (2) plasma vitellogenin in adult females as a function of plasma estrogen, and (3) fecundity of adult females as a function of plasma vitellogenin. A fourth quantitative relationship was developed for early life mortality as a function of sensitivity to activation of the AHR2 in a standardized in vitro AHR transactivation assay to integrate toxic equivalence calculations that would allow prediction of effects of exposure to untested PAHs. The accuracy of the predictions from the resulting qAOP were evaluated using experimental data from zebrafish exposed as embryos to another PAH, benzo[k]fluoranthene. The qAOP developed in the present study demonstrates the potential of the AOP framework in enabling consideration of latent toxicities in quantitative ecological risk assessments and regulatory decision-making. Environ Toxicol Chem 2024;43:2145-2156. © 2024 SETAC.

Chronic exposure to environmental concentrations of benzo[a]pyrene causes multifaceted toxic effects of developmental compromise, redox imbalance, and modulated transcriptional profiles in the early life stages of marine medaka (Oryzias melastigma)

Polycyclic aromatic hydrocarbons (PAHs) accumulate and integrate into aquatic environments, raising concerns about the well-being and safety of aquatic ecosystems. Benzo[a]pyrene (BaP), a persistent PAH commonly detected in the environment, has been extensively studied. However, the broader multifaceted toxicity potential of BaP on the early life stages of marine fish during chronic exposure to environmentally relevant concentrations needs further exploration. To fill these knowledge gaps, this study assessed the in vivo biotoxicity of BaP (1, 4, and 8 μg/L) in marine medaka (Oryzias melastigma) during early development over a 30-day exposure period. The investigation included morphological, biochemical, and molecular-level analyses to capture the broader potential of BaP toxicity. Morphological analyses showed that exposure to BaP resulted in skeletal curvatures, heart anomalies, growth retardation, elevated mortality, delayed and reduced hatching rates. Biochemical analyses revealed that BaP exposure not only created oxidative stress but also disrupted the activities of antioxidant enzymes. This disturbance in redox balance was further explored by molecular level investigation. The transcriptional profiles revealed impaired oxidative phosphorylation (OXPHOS) and tricarboxylic acid (TCA) cycle pathways, which potentially inhibited the oxidative respiratory chain in fish following exposure to BaP, and reduced the production of adenosine triphosphate (ATP) and succinate dehydrogenase (SDH). Furthermore, this investigation indicated a potential connection to apoptosis, as demonstrated by fluorescence microscopy and histological analyses, and supported by an increase in the expression levels of related genes via real-time quantitative PCR. This study enhances our understanding of the molecular-level impacts of BaP's multifaceted toxicity in the early life stages of marine medaka, and the associated risks.

Environmental exposure to polycyclic aromatic hydrocarbons: An underestimated risk factor for systemic lupus erythematosus onset and progression

OBJECTIVE

To investigate the link between systemic lupus erythematosus (SLE) incidence and exposure to environmental polycyclic aromatic hydrocarbons (PAH).

METHODS

A case-control study (ChiCTR2000038187) involving 316 SLE patients and 851 healthy controls (HCs) was executed. Environmental exposure was assessed via a questionnaire, stratified by gender and age (females <35 and ≥35 years, males). Blood samples collected from 89 HCs, 85 inactive, and 95 active SLE patients were used to measure serum benzo[a]pyrene diol epoxide -albumin (BPDE-Alb) adducts and PAH concentrations, indicating long-term and short-term exposure respectively. Intergroup comparisons and statistical analyses were conducted using R version 4.3.1.

RESULTS

Diverse patterns were found in how environmental factors affect SLE onset across different demographics. Lifestyle exposure factors were found to be a stronger determinant of SLE onset than occupational exposure factors in women under 35. Indoor air pollution had a significant impact on SLE incidence, potentially comparable to outdoor air pollution. Lifestyle-related PAH exposure had a greater impact on SLE than occupational PAH exposure. PAH exposure levels progressively increase from HCs to inactive and active SLE patients. Active SLE patients show markedly higher BPDE-Alb levels than HCs.

CONCLUSIONS

Environmental PAH, particularly lifestyle-related, are significant, yet under-recognized, risk factors for SLE.

STATEMENT OF ENVIRONMENTAL IMPLICATION

We examined the relationship between exposure to environmental polycyclic aromatic hydrocarbons (PAH) and the incidence of systemic lupus erythematosus (SLE). PAH, prevalent in sources such as cigarette smoke, air pollution, and charred food, pose significant health hazards. This study is the first to investigate specific PAH exposure levels in SLE patients. We determined actual PAH exposure levels in both SLE patients and healthy individuals and indicated that long-term PAH exposure biomarker is more reliable for evaluating exposure in non-occupationally exposed groups like SLE, compared to short-term markers. These findings provide valuable insights for future research on similar non-occupationally exposed populations.

Genome-wide DNA methylation changes in Oryzias melastigma embryos exposed to the water accommodated fraction of crude oil

The water accommodated fraction (WAF) of crude oil exerts considerable impacts on marine fish during embryonic stage. Clarifying changes in epigenetic modifications is helpful for understanding the molecular mechanism underlying the toxicity of embryonic WAF exposure. The aim of this study was to explore genome-wide DNA methylation changes in Oryzias melastigma embryos after exposure to the nominal total petroleum hydrocarbon concentration of 500 μg/L in WAF for 7 days. Whole-genome bisulfite sequencing revealed that 8.47 % and 8.46 % of all the genomic C sites were methylated in the control and WAF-exposed groups, respectively. Among the three sequence contexts, methylated CG site had the largest number in both the two groups. The sequence preferences of nearby methylated cytosines were consistent between the two groups. A total of 4798 differentially methylated regions (DMRs) were identified in the promoter region. Furthermore, Gene Ontology analysis revealed that DMR-related genes were enriched mainly for functions related to development and nervous system. Additionally, the Kyoto Encyclopedia of Genes and Genomes pathways enriched in DMR-related genes were related to nervous system and endocrine system. These novel findings provide comprehensive insights into the genome-wide DNA methylation landscape of O. melastigma following embryonic WAF exposure, shedding light on the epigenetic regulatory mechanisms underlying WAF-induced toxicity.

Exposure of embryos to fenbuconazole causes persistent neurotoxicity in adult zebrafish

In this study, the persistent effects of embryonic exposure to fenbuconazole (FBZ), a triazole fungicide, on neurobehaviour in zebrafish were investigated. After exposure of fertilized eggs to FBZ for 72 h (h), the larvae were cultured to adulthood in clean water. In adult zebrafish embryonically exposed to 50 and 500 ng L-1 FBZ, the ratio of brain weight/body weight was significantly decreased, and the number of apoptotic cells in the brain was significantly increased, accompanied by upregulated protein levels of P53 and downregulated levels of BCL2. The novel tank test showed a significant reduction in the moved distance and speed, and a longer period of adaptation to new environments in the 500 ng L-1 group. The social preference experiment showed impaired social interaction behaviour and reduced time of aggregation in the 500 ng L-1 group. Increased dopamine and norepinephrine levels in the brain might be responsible for this anxiety-like behaviour. In addition to upregulated protein levels of tyrosine hydroxylase and β2-adrenoceptor, the transcription of genes related to dopamine and norepinephrine synthesis in the brain such as th1, th2, ddc, drd1b, dat, and dbh, was increased. The methylation levels of related genes were reduced, which were matched with their increased transcriptional levels. These results demonstrate that embryonic FBZ exposure might cause persistent neurotoxicity in adulthood, which suggests the rational cautious use of FBZ.

Bioconcentration of polycyclic aromatic hydrocarbons in different tissues of zebrafish (Danio rerio) investigated with PBTK model

Due to the lipophilicity, polycyclic aromatic hydrocarbons (PAHs) are easily accumulated in fish. However, the research on PAH bioaccumulation process in different fish tissues and the relevant effect mechanisms are still deficient. The bioconcentration of PAHs (phenanthrene, anthracene, fluoranthene, and pyrene) in different zebrafish tissues (skin, fish muscle, gill, digestive tract, liver, gonad, and residual) was studied. It was found that there was a difference in the PAH concentrations in different zebrafish tissues. Compared with other tissues, the PAH concentration was highest in the skin and lowest in the fish muscle. For example, the steady-state concentration of phenanthrene in the skin was nearly five times higher than that in the muscle. PAH distribution was related with the lipid contents in different zebrafish tissues; however, the correlation was not significant (p > 0.05), indicating that the lipid content was not the determining factor for the PAH distribution. The distribution was also affected by the bioconcentration kinetics of PAHs in different zebrafish tissues, and the PAH hydrophobic properties. In addition, the physiological based toxicokinetic (PBTK) model showed good performance in predicting PAH internal concentrations, and it may be used to predict the concentrations of PAHs in different fish tissues in future.

MosSCI-mediated exogenous gene expression is modulated by genomic positioning

Although the Mos1-mediated single-copy insertion (MosSCI) technique has been widely used to generate stable transgenic Caenorhabditis elegans strains, the link between stability of expression and integration site still needs to be explored. Here, experimental evidence is provided that transgenes are not able to match the level of transcription of their native counterpart, and that insertions at certain locations can result in an external stress-mediated increase in expression. Insertion site ttTi5605 on chromosome II was shown to be a superior location, at least when introducing reproduction related genes. Thus, this study provides a reference for the selection of an optimal site for MosSCI which provides acceptable expression performance whilst minimizing undesirable secondary effects.

Developmental exposure to polycyclic aromatic hydrocarbons (PAHs): Focus on benzo[a]pyrene neurotoxicity

Polycyclic Aromatic Hydrocarbons (PAHs) are a class of ubiquitous organic compounds produced during the incomplete combustion or pyrolysis of organic material. Dietary source is the main route for PAH human exposure by environmental contamination, food industrial processing or domestic cooking methods. The most studied PAH is benzo[a]pyrene (B[a]P), due to its harmful and multiple effects on human health: in addition to its well-known carcinogenic effects, emerging evidence indicates that B[a]P also induces neurotoxicity earlier and at lower doses than B[a]P-induced carcinogenicity making B[a]P neurotoxicity relevant to human health risk assessment. Developmental neurotoxicity of B[a]P has indeed received increasing attention: both human and experimental studies provide evidence of detrimental effects of prenatal or early postnatal B[a]P exposure, even at low doses. Indeed, in some of the multi-dose animal studies, maximal adverse effects were observed at lower B[a]P doses, according to a non-monotonic dose-response curve typical of endocrine-disrupting compounds. In substantial agreement with epidemiological studies, both rodents and zebrafish developmentally exposed to B[a]P exhibit long-term changes in multiple behavioural domains, in the absence of overt toxicological effects at birth (e.g. body weight and morphologic abnormalities). Notably, most targeted behavioural responses converge on locomotor activity and emotional profile, often, but not always, leading to a disinhibitory/hyperactive profile.

Dynamic changes of DNA methylation induced by benzo(a)pyrene in cancer

Benzo(a)pyrene (BaP), the earliest and most significant carcinogen among polycyclic aromatic hydrocarbons (PAHs), has been found in foods, tobacco smoke, and automobiles exhaust, etc. Exposure to BaP induced DNA damage directly, or oxidative stress-related damage, resulting in cell apoptosis and carcinogenesis in human respiratory system, digestive system, reproductive system, etc. Moreover, BaP triggered genome-wide epigenetic alterations by methylation, which might cause disturbances in regulation of gene expression, and thereby induced cancer. It has been proved that BaP reduced genome-wide DNA methylation, and activated proto-oncogene by hypomethylation in the promoter region, but silenced tumor suppressor genes by promoter hypermethylation, resulting in cancer initiation and progression. Here we summarized the changes in DNA methylation in BaP exposure, and revealed the methylation of DNA plays a role in cancer development.

Benzo[a]pyrene-induced multigenerational changes in gene expression, behavior, and DNA methylation are primarily influenced by paternal exposure

Benzo[a]pyrene (BaP), a polycyclic aromatic hydrocarbon (PAH), is implicated in many developmental and behavioral adverse outcomes in offspring of exposed parents. The objective of this study was to investigate sex-dependent multigenerational effects of preconceptional effects of BaP exposure. Adult wild-type (5D) zebrafish were fed 708 μg BaP/g diet (measured) at a rate of 1% body weight twice/day (14 μg BaP/g fish/day) for 21 days. Fish were spawned using a crossover design, and parental (F0) behavior and reproductive indexes were measured. In offspring, behavioral effects were measured at 96 h post fertilization (hpf) in F1 & F2 larvae, and again when F1s were adults. Compared to controls, there was no significant effect on F0 adult behavior immediately following exposure, but locomotor activity was significantly increased in F1 adults of both sexes. Larval behavior (96 hpf, photomotor response assay) was significantly altered in both the F1 and F2 generations. To assess molecular changes associated with BaP exposure, we conducted transcriptome and DNA methylation profiling in F0 gametes (sperm and eggs) and F1 embryos (10 hpf) from all four crosses. Embryos resulting from the BaP male and control female cross had the most differentially expressed genes (DEGs) and differentially methylated regions (DMRs). Some DMRs were associated with genes encoding chromatin modifying enzymes suggesting regulation of chromatin conformation by DNA methylation. Overall, these results suggest that parental dietary BaP exposure significantly contributes to the multigenerational adverse outcomes.

Molecular and phenotypic effects of early exposure to an environmentally relevant pesticide mixture in the Pacific oyster, Crassostrea gigas

Early life stages are crucial for organism development, especially for those displaying external fertilization, whose gametes and early stages face environmental stressors such as xenobiotics. The pacific oyster, Crassostrea gigas, is considered a model species in ecotoxicology because of its ecological characteristics (benthic, sessile, filter feeding). So far studies have investigated the impact of xenobiotics at embryotoxic, genotoxic and physiological endpoints, sometimes at the multigenerational scale, highlighting the role of epigenetic mechanisms in transmitting alterations induced by exposure to single xenobiotics. However, to date, little is known about the impact of environmentally-mimicking contaminants cocktails. Thus, we examined the impact of an early exposure to environmentally relevant mixture on the Pacific oyster life history. We studied transcriptomic, epigenetic and physiological alterations induced in oysters exposed to 18 pesticides and metals at environmental concentration (nominal sum concentration: 2.85 μg.L-1, measured sum concentration: 3.74 ± 0.013 μg.L-1) during embryo-larval stage (0-48 h post fertilization, hpf). No significant differences in embryo-larval abnormalities at 24 hpf were observed during larval and spat rearing; the swimming behaviour of exposed individuals was disturbed, while they were longer and heavier at specific time points, and exhibited a lower epinephrine-induced metamorphosis rate as well as a higher survival rate in the field. In addition, RNA-seq analyses of gastrula embryos revealed the differential expression of development-related genes (e.g. Hox orthologues and cell cycle regulators) between control and exposed oysters. Whole-genome DNA methylation analyses demonstrated a significant modification of DNA methylation in exposed larvae marked by a demethylation trend. Those findings suggest that early exposure to an environmentally relevant pesticide mixture induces multi-scale latent effects possibly affecting life history traits in the Pacific oyster.

Benzo(a)pyrene exposure in early life suppresses spermatogenesis in adult male zebrafish and association with the methylation of germ cell-specific genes

Polycyclic aromatic hydrocarbons (PAHs) are environmental contaminants that are widely present in aquatic ecosystems. To assess the impact of early-life exposure to benzo[a]pyrene (BaP), a representative PAH, on reproductive ability in adult male zebrafish (Danio rerio), fertilized embryos were exposed to 0.05, 0.5, 5 and 50 nM of BaP for 96 h, and then the hatched larvae were raised to adulthood in clean water. In one-year-old male fish, the percentage of spermatozoa in testis was significantly reduced in the 0.5, 5 and 50 nM treatments. When the treated fish were mated with untreated fish, significantly decreased rate of egg fertilization and hatching success and significantly elevated malformation rate the F1 larvae were observed in the 0.5, 5 and 50 nM treatments. The transcriptional levels of genes along the brain-pituitary-gonadal axis, involving gnrh3, gnrhr3, fshβ, lhβ, lhγ, lhrγ and ar, were downregulated. In addition, embryonic BaP exposure upregulated the promotor methylation of germ cell-specific genes in the testis of adult fish. The upregulated methylation of ddx4, dnd1, nanos2 in the testis might be associated with the downregulated mRNA levels of these genes, which could be another reason for the inhibition of spermatogenesis. These results indicate that early-life exposure to BaP suppress the reproductive capability of adult male fish, which would cause a decrease in fish population.

Exposure of embryos to phenanthrene impacts the cardiac development in F1 zebrafish larvae and potential reasons

To explore the impact of embryonic exposure to phenanthrene (Phe), a typical tricyclic polycyclic aromatic hydrocarbon, on cardiac development in next generation, fertilized zebrafish embryos were exposed to 0.05, 0.5, 5 and 50 nM Phe for 96 h, and then transferred to clear water and raised to adulthood. The cardiac development in F1 larvae generated by adult females or males mated with unexposed zebrafish was assessed. Malformation and dysfunction of the heart, such as increased heart rate, arrhythmia, enlarged heart and abnormal contraction, were shown in both paternal and maternal F1 larvae. A greater impact on the distance between the sinus venosus and bulbus arteriosus was exhibited in maternal F1 larvae, while paternal F1 larvae displayed a more severe impact on heart rate and arrhythmia. The transcription of genes related to cardiac development was disturbed in F1 larvae. DNA methylation levels in the promoter of some genes were associated with their transcription. The expression of acetylated histone H3K9Ac and H3K14Ac in maternal F1 larvae was no significantly changed, but was significantly downregulated in paternal F1 larvae, which might be associated with the downregulated transcription of tbx5. These results indicate that exposure to Phe during embryogenesis adversely affects cardiac development in F1 generation, and the effects and toxic mechanisms showed sex-linked hereditary differences, highlighting the risk of Phe exposure in early life to heart health in next generation.

Oil and hypoxia alter DNA methylation and transcription of genes related to neurological function in larval Cyprinodon variegatus

DNA methylation is an important epigenetic mark involved in modulating transcription. While multiple studies document the ability of environmental stressors to alter methylation patterns, there is little information regarding the effects of oil and hypoxia on the methylome. Oil and hypoxic stress are threats in coastal ecosystems, which act as nursery habitats for developing fish. To explore the methylation altering effects of oil and hypoxia on developing fish, we exposed larval Cyprinodon variegatus to oil, hypoxia, or both for 48 h followed by 48 h of depuration in clean, normoxic conditions. We then used immunoprecipitation coupled with high-throughput sequencing (MeDIP seq) to evaluate genome-wide methylation changes. We also performed RNA seq to associate methylation and altered transcription. Oil and hypoxia together elicited greater impacts to methylation than either stressor individually. Additionally, the oil+hypoxia treatment exhibited an overlap between differentially methylated regions and differential gene expression at 20 loci. Functional analyses of these loci revealed enrichment of processes related to neurological function and development. Two neurological genes (slc1a2, asxl2) showed altered methylation of promoter CpG islands and transcriptional changes, suggesting epigenetic modulation of gene expression. Our results suggest a possible mechanism explaining altered behavior patterns noted in fish following oil exposure.

Prolonged exposure of azocyclotin induced inter- and transgenerational endocrine disruption on Danio rerio linked to transcriptomic and DNA methylomic alterations

The transgenerational effect assessment linked to epigenetic analysis of environmental pollutants on eco (toxico)logical relevant species is regarded as a potential future risk-assessment tool. As an organotin acaricide widely used in China, azocyclotin can lead to endocrine disrupting effect on directly exposed environmental organisms, but whether it has transgenerational negative impact remains unknown. In order to illustrate this issue, in the present study, zebrafish, an aquatic model animal, was exposed to azocyclotin at less than μg/L level in a time span of embryonic stage to adult stage. Subsequently, the developmental and reproductive endocrine disrupting effects of azocyclotin on exposed F0 and unexposed offspring (F1 and F2) were evaluated. Result indicated that parentally exposed to 0.36 μg/L azocyclotin induced embryonic toxicity to unexposed offspring, and significantly (p < 0.05) reduced body weight (by 8.5%-13.9%), whole body length (by 4.8%-14.3%), hepatosomatic index (by 15.6%-24.3%), gonadosomatic index (by 5.3%-17.1%), egg production (by 19.5%-25.4%), estradiol content (47.0%-65.0%) and proportion of mature germ cells (by 29.3%-41.0% and 39.2%-47.7% for late oocytes and spermatozoa, respectively) in adults of F0 and offspring. Additionally, azocyclotin decreased the contents of 5-methycytosine in gonads of unexposed offspring (by 9.9%-38.6%, p < 0.05), led to genome-wide gene up-regulated expression bias and genomic DNA hypomethylation tendency in unexposed offspring. Moreover, based on the level of differentially methylated cytosine in promoter regions/gene body regions, it was found totally 5331/11,170 (in F1) and 3808/7507 (in F2) differentially expressed genes were closely related with differentially methylated genes (r > 0.6). The present study provided a primary evidence that prolonged exposure to low dose azocyclotin induced inter- and transgenerational endocrine disrupting effects on zebrafish probably linked to transcriptomic and DNA methylomic alterations.

Assessment of parental benzo[a]pyrene exposure-induced cross-generational neurotoxicity and changes in offspring sperm DNA methylome in medaka fish

Previous studies have revealed that DNA methylation changes could serve as potential genomic markers for environmental benzo[a]pyrene (BaP) exposure and intergenerational inheritance of various physiological impairments (e.g. obesity and reproductive pathologies). As a typical aromatic hydrocarbon pollutant, direct BaP exposure has been shown to induce neurotoxicity. To unravel the inheritance mechanisms of the BaP-induced bone phenotype in freshwater medaka, we conducted whole-genome bisulfite sequencing of F1 sperm and identified 776 differentially methylated genes (DMGs). Ingenuity pathway analysis revealed that DMGs were significantly enriched in pathways associated with neuronal development and function. Therefore, it was hypothesized that parental BaP exposure (1 μg/l, 21 days) causes offspring neurotoxicity. Furthermore, the possibility for sperm methylation as an indicator for a neurotoxic phenotype was investigated. The F0 adult brains and F1 larvae were analyzed for BaP-induced direct and inherited toxicity. Acetylcholinesterase activity was significantly reduced in the larvae, together with decreased swimming velocity. Molecular analysis revealed that the marker genes associated with neuron development and growth (alpha1-tubulin, mbp, syn2a, shh, and gap43) as well as brain development (dlx2, otx2, and krox-20) were universally downregulated in the F1 larvae (3 days post-hatching). While parental BaP exposure at an environmentally relevant concentration could induce neurotoxicity in the developing larvae, the brain function of the exposed F0 adults was unaffected. This indicates that developmental neurotoxicity in larvae may result from impaired neuronal development and differentiation, causing delayed brain growth. The present study demonstrates that the possible adverse health effects of BaP in the environment are more extensive than currently understood. Thus, the possibility of multigenerational BaP toxicity should be included in environmental risk assessments.

Transgenerational effects of parental bisphenol S exposure on zebrafish (Danio rerio) reproduction

Bisphenol S (BPS) is extensively used for production of polycarbonates and other commodities, and is often detected in environment and biota. Parental BPS exposure has been reported to interfere with reproductive development of offspring, but limited information is available on its multigenerational reproductive toxicity. In our present study, zebrafish (Danio rerio) were exposed to BPS (1 and 100 μg/L) from 3 hpf to 120 dpf, and the effects on reproduction, sex steroid hormones, DNA methylation levels and gene transcription involved in steroidogenesis and DNA methylation were investigated in unexposed F1-2 offspring. The results showed that 100 μg/L BPS exposure increased DNA methylation in F1 testes, and 1 μg/L BPS led to DNA methylation in F2 ovaries. The increased DNA methylation levels led to decreased expression of steroidogenic enzymes, including cyp11a, cyp17 and 3βhsd, which might be a main reason for the elevated plasma 17β-estradiol and decreased testosterone levels. In addition, sex ratio indicated a female dominance trend, and reproductive capacity of male fish was severely impaired. Overall, these findings suggest that parental BPS exposure impairs reproductive development of unexposed offspring via DNA methylation and BPS-induced epigenetic modification inheritance has a long-term effect on the fitness and sustainability of fish populations.

Epigenetic mechanisms of DNA methylation in the transgenerational effect of ethylhexyl salicylate on zebrafish

In this study, a 120-day whole-life cycle exposure and oviposition experiment on zebrafish with maternal and paternal mixed mating strategy was conducted to investigate the epigenetic mechanism of DNA methylation in ethylhexyl salicylate (EHS, 1, 10, 100 μg/L)-induced transgenerational effects. Results showed that EHS could induce the decrease of DNA methyltransferase 1 (DNMT1) activity and average global DNA methylation level in maternal parents and the increase of the above indexes in paternal parents, while the change of glycine N-methyltransferase activity was opposite to DNMT1. The average global DNA methylation levels were significantly increased in the offsprings of both parents exposed and father-only exposed to EHS, suggesting that EHS-induced epigenetic modifications may be stable and heritable. Hierarchical clustering analysis of promoter at different methylation sites showed that the DNA methylation pattern of offsprings were similar to that of the paternal parents, meaning that the offsprings may have inherited paternal DNA methylation pattern with eya2, pcdh2g5 and pcdh2g1 as key genes and lead to high locomotor activity in offsprings. KEGG pathway analysis showed that parental exposure to EHS may interfere with the central nervous system, insulin function system, melanogenesis system and the normal development of somatic axis of offsprings.

Embryonic exposure to phenanthrene caused developmental defects of craniofacial cartilage in F1 larvae

As a representative polycyclic aromatic hydrocarbon with low ring numbers, phenanthrene (Phe) is ubiquitously present in the environment. In this study, zebrafish embryos were exposed to Phe at 0.05, 0.5, 5 and 50 nmol/L for 96 h, and then cultured to adulthood in clean water, the developmental defects of craniofacial cartilage were observed in F1 larvae produced by adult males and females mated with untreated fish. Delayed development of craniofacial cartilage, including a shorter and wider Meckel's cartilage and mandibular arch were observed in F1 larvae from adult fish of both sexes. Maternal F1 larvae showed a greater impact on the lower jaw than paternal F1 larvae, this may be connected with greater downregulation of the transcription of genes related to the development of craniofacial cartilage such as runt-related transcription factor 2 (runx2), fibroblast growth factor 8 (fgf8), sonic hedgehog (shh), Indian hedgehog (ihh). Further results indicated that the modification DNA methylation levels in the promotors of gene runx2 and shh in maternal and paternal F1 larvae were inherited from embryonic F0 larvae, and might be linked with the toxicity of craniofacial cartilage in F1 larvae. This study illustrated that embryonic exposure to Phe could induce adverse effects on craniofacial development in F1 offspring, emphasizing the importance of transgenerational toxicology studies in risk assessment.

Influence of Benzo(a)pyrene on Different Epigenetic Processes

Epigenetic changes constitute one of the processes that is involved in the mechanisms of carcinogenicity. They include dysregulation of DNA methylation processes, disruption of post-translational patterns of histone modifications, and changes in the composition and/or organization of chromatin. Benzo(a)pyrene (BaP) influences DNA methylation and, depending on its concentrations, as well as the type of cell, tissue and organism it causes hypomethylation or hypermethylation. Moreover, the exposure to polyaromatic hydrocarbons (PAHs), including BaP in tobacco smoke results in an altered methylation status of the offsprings. Researches have indicated a potential relationship between toxicity of BaP and deregulation of the biotin homeostasis pathway that plays an important role in the process of carcinogenesis. Animal studies have shown that parental-induced BaP toxicity can be passed on to the F1 generation as studied on marine medaka (Oryzias melastigma), and the underlying mechanism is likely related to a disturbance in the circadian rhythm. In addition, ancestral exposure of fish to BaP may cause intergenerational osteotoxicity in non-exposed F3 offsprings. Epidemiological studies of lung cancer have indicated that exposure to BaP is associated with changes in methylation levels at 15 CpG; therefore, changes in DNA methylation may be considered as potential mediators of BaP-induced lung cancer. The mechanism of epigenetic changes induced by BaP are mainly due to the formation of CpG-BPDE adducts, between metabolite of BaP-BPDE and CpG, which leads to changes in the level of 5-methylcytosine. BaP also acts through inhibition of DNA methyltransferases activity, as well as by increasing histone deacetylases HDACs, i.e., HDAC2 and HDAC3 activity. The aim of this review is to discuss the mechanism of the epigenetic action of BaP on the basis of the latest publications.

The effect of benzo[a]pyrene on the gut microbiota of Nile tilapia (Oreochromis niloticus)

Benzo[a]pyrene (BaP) is a highly toxic and carcinogenic polycyclic aromatic hydrocarbon (PAH) whose toxicological effects in the gut microbiota of aquatic organisms have not yet been fully revealed. Therefore, in this study, we used high-throughput 16S rRNA gene sequencing to evaluate the effects of BaP in the gut microbiome of Oreochromis niloticus, including its possible participation in the process of detoxification and its ability to recover. The fish were injected with a single intraperitoneal dose of 20 mg kg^-1 of BaP, and the effects in the microbiome were evaluated at 24, 72, and 120 h post-injection. The results indicate a clear dysbiosis (in composition, relative abundance, diversity, and interaction networks) of the gut microbiota during 24 h post-injection, dominated by Fusobacteria and Bacteroidetes and a decrease in Proteobacteria and Spirochaetae. Interestingly, a slight recovery of the microbiome begins at 72 h and stabilises at 120 h post-injection. Pathway analysis revealed the participation of the gut microbiome in PAH degradation mainly at 24 h post-injection. This study provides new insights in the toxicology of BaP in O. niloticus and the first evidence of the ability of the gut microbiome to recovery after a chemical disturbance. KEY POINTS: • Benzo[a]pyrene caused a dysbiosis in the gut microbiota of Oreochromis niloticus. • We observed an enrichment of bacteria involved in the metabolism of xenobiotics. • The gut microbiota was recovered after exposure to benzo[a]pyrene.

Acute benzo[a]pyrene exposure induced oxidative stress, neurotoxicity and epigenetic change in blood clam Tegillarca granosa

The blood clam (Tegillarca granosa) is being developed into a model bivalve mollusc for assessing and monitoring marine pollution on the offshore seabed. However, the information on the response of blood clam to PAHs, an organic pollutant usually deposited in submarine sediment, remains limited. Herein, we employed multiple biomarkers, including histological changes, oxidative stress, neurotoxicity and global DNA methylation, to investigate the effects of 10 and 100 μg/L Bap exposure on the blood clams under laboratory conditions, as well as the potential mechanisms. Acute Bap exposure can induce significant morphological abnormalities in gills as shown through hematoxylin–eosin (H.E) staining, providing an intuitive understanding on the effects of Bap on the structural organization of the blood clams. Meanwhile, the oxidative stress was significantly elevated as manifested by the increase of antioxidants activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) and glutathione-s-transferase (GST), lipid peroxidation (LPO) level and 8-hydroxy-2′-deoxyguanosine (8-OHdG) content. The neurotoxicity was also strengthened by Bap toxicity manifested as inhibited acetylcholinesterase (AChE) and choline acetyltransferase (ChAT) activities. In addition, the global DNA methylation level was investigated, and a significant DNA hypomethylation was observed in Bap exposed the blood clam. The correlation analysis showed that the global DNA methylation was negatively correlated with antioxidants (SOD, CAT and POD) activities, but positively correlated choline enzymes (AChE and ChAT) activities. These results collectively suggested that acute Bap exposure can cause damage in gills structures in the blood clam possibly by generating oxidative stress and neurotoxicity, and the global DNA methylation was inhibited to increase the transcriptional expression level of antioxidants genes and consequently elevate antioxidants activities against Bap toxicity. These results are hoped to shed some new light on the study of ecotoxicology effect of PAHs on marine bivalves.

Knockdown of DNA methyltransferase 1 reduces DNA methylation and alters expression patterns of cardiac genes in embryonic cardiomyocytes

We previously found that DNA methyltransferase 3a (DNMT3a) plays an important role in regulating embryonic cardiomyocyte gene expression, morphology, and function. In this study, we investigated the role of the most abundant DNMT in mammalian cells, DNMT1, in these processes. It is known that DNMT1 is essential for embryonic development, during which it is involved in regulating cardiomyocyte DNA methylation and gene expression. We used siRNA to knock down DNMT1 expression in primary cultures of mouse embryonic cardiomyocytes. Immunofluorescence staining and multielectrode array were, respectively, utilized to evaluate cardiomyocyte growth and electrophysiology. RNA sequencing (RNA‐Seq) and multiplex bisulfite sequencing were, respectively, performed to examine gene expression and promoter methylation. At 72 h post‐transfection, reduction of DNMT1 expression decreased the number and increased the size of embryonic cardiomyocytes. Beat frequency and the amplitude of field action potentials were decreased by DNMT1 siRNA. RNA‐Seq analysis identified 801 up‐regulated genes and 494 down‐regulated genes in the DNMT1 knockdown cells when compared to controls. Pathway analysis of the differentially expressed genes revealed pathways that were associated with cell death and survival, cell morphology, cardiac function, and cardiac disease. Alternative splicing analysis identified 929 differentially expressed exons, including 583 up‐regulated exons and 308 down‐regulated exons. Moreover, decreased methylation levels were found in the promoters of cardiac genes Myh6, Myh7, Myh7b, Tnnc1, Tnni3, Tnnt2, Nppa, Nppb, mef2c, mef2d, Camta2, Cdkn1A, and Cdkn1C. Of these 13 genes, 6 (Myh6, Tnnc1, Tnni3, Tnnt2, Nppa, Nppb) and 1 (Cdkn1C) had increased or decreased gene expression, respectively. Altogether, these data show that DNMT1 is important in embryonic cardiomyocytes by regulating DNA methylation, gene expression, gene splicing, and cell function.

Environmental modulators of diluted bitumen effects in juvenile pink salmon (Oncorhynchus gorbuscha)

Recent and potential expansions in the transportation of diluted bitumen (dilbit) through marine terminals in coastal regions of British Columbia require the examination of potential risks to estuarine species such as Pacific salmon. The estuarine habitat of out-migrated pink salmon (Oncorhynchus gorbuscha) exhibits dynamic temperature and salinity regimes, possibly modifying dilbit exposure, bioavailability and/or its effects. To examine dilbit toxicity and its modification by environmental stressors, juvenile pinks were subchronically exposed for 3 months to the water-accommodated fraction (WAF) of Cold Lake Blend dilbit (winter) in seawater at three salinities (7, 14, and 28‰ [temperature 12.5 °C]) and three temperatures (8.5, 12.5, and 16.5 °C [salinity of 28‰]). Temperature and salinity alone did not affect any measured endpoints in control fish. Dilbit exposure induced higher mortality at high (16.5 °C) and low temperatures (8.5 °C) as well as at higher salinity (28‰) in fish exposed to the highest dilution of WAF [total polycyclic aromatic compounds (TPAC) = 128.9 μg/L]. A concentration-dependent reduction of growth was evident in fish exposed to the medium (TPAC = 97.3 μg/L) and high dilution of WAF at higher temperatures (12.5 and 16.5 °C) and high salinity (28‰). At 28‰, swimming performance (U_burst) was decreased in fish exposed to the highest concentration of dilbit at all 3 temperatures. Gill Na⁺-K⁺-ATPase activity, white muscle lactate, glycogen, and triglyceride concentrations were altered by dilbit exposure and modified by temperature and salinity. In addition, gene expression associated with phase I biotransformation, energy metabolism, mitochondrial activity, and inflammation showed significant upregulation with exposure and temperature stress. Dilbit exposure at PAC concentrations in the ppb range, affected pink salmon at the molecular, biochemical, and whole organism level; effects that were exacerbated by environmental temperature and salinity.

Early-life phenanthrene exposure inhibits reproductive ability in adult zebrafish and the mechanism of action

Phenanthrene (Phe) is a representative polycyclic aromatic hydrocarbon, and its ubiquity makes the risk assessment of Phe in aquatic ecosystems important. To assess the long-term effects of early-life Phe exposure on fish, the embryos of the model organism, zrbrafish (Danio rerio) were exposed to Phe at 0.05, 0.5, 5 and 50 nmol/L for 96 h and then raised to adulthood in clean water. Gonad development and reproductive functions were investigated in 120 day-old fish. The results showed that the percentage of spermatozoa in males and mature oocytes in females were decreased. The spawned egg numbers and the fertilization rate were reduced when the treated fish were mated with untreated fish. The transcription of genes involved in the brain-pituitary-gonadal axis was downregulated. The levels of both 17β-estradiol and testosterone were significantly decreased in the 5 and 50 nmol/L groups compared with the control group. The methylation levels in the promotor of gnrh3 (encoding gonadotropin releasing hormone) were significantly elevated in the adult fish in the 5 and 50 nmol/L treatments, which might be associated with the downregulation of gnrh3 transcription. These results suggested that embryonic exposure to Phe can inhibit the reproductive ability of adult fish, which should be adequately emphasized in its risk assessment.

Effects of Nanoplastics and Butyl Methoxydibenzoylmethane on Early Zebrafish Embryos Identified by Single-Cell RNA Sequencing

Nanoplastics with small particle sizes and high surface area/volume ratios easily absorb environmental pollutants and affect their bioavailability. In this study, polystyrene nanoplastic beads (PS-NPBs) with a particle size of 100 nm and butyl methoxydibenzoylmethane (BMDBM) sunscreen in personal-care products were chosen as target pollutants to study their developmental toxicity and interactive effects on zebrafish embryos. The exposure period was set from 2 to 12 h postfertilization (hpf). BMDBM and PS-NPBs significantly upregulated genes related to antioxidant enzymes and downregulated the gene expression of aromatase and DNA methyltransferases, but the influenced genes were not exactly the same. The combined exposure reduced the adverse effects on the expression of all genes. With the help of the single-cell RNA sequencing technology, neural mid cells were identified as the target cells of both pollutants, and brain development, head development, and the notch signaling pathway were the functions they commonly altered. The key genes and functions that are specifically affected by BMDBM and/or PS-NPBs were identified. BMDBM mainly affects the differentiation and fate of neurons in the central nervous system through the regulation of her5, her6, her11, lfng, pax2a, and fgfr4. The PS-NPBs regulate the expression of olig2, foxg1a, fzd8b, six3a, rx1, lhx2b, nkx2.1a, and sfrp5 to alter nervous system development, retinal development, and stem cell differentiation. The phenotypic responses of zebrafish larvae at 120 hpf were tested, and significant inhibition of locomotor activity was found, indicating that early effects on the central nervous system would have a sustained impact on the behavior of zebrafish.

Effects of pyrene and benzo[a]pyrene on the reproduction and newborn morphology and behavior of the freshwater planarian Girardia tigrina

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous pollutants of aquatic ecosystems. Because they are persistent, there is great potential for chronic toxicity to aquatic species, and the evaluation of reproductive effects is fundamental. In this context, planarians are interesting experimental animals, since they can be sensitive to environmental pollutants, and a wide range of reproductive-related endpoints can be assessed. In this work we evaluated fecundity (number of cocoons), fertility (number of newborns), newborn anomalies, adult weight, regenerative abilities and PAH-residues in tissues of the freshwater planarian Girardia tigrina, exposed to either pyrene or benzo[a]pyrene (B[a]P). Pyrene reduced planarian fecundity and fertility at 18.75 μg L^-1 and 75.00 μg L^-1, while B[a]P reduced planarian fecundity at the 37.50 μg L^-1 treatment, which was accompanied by a 33.7% reduction in fertility. Cocoons were kept in clean media and newborns were evaluated for behavioral and morphological anomalies. Many of the newborns resulting from the B[a]P experiment revealed behavioral anomalies, such as spasms and uncoordinated movements. These behavioral anomalies were observed in 12.9% and 38.2% of newborns resulting from the exposure of adult planarians to 9.38 μg L^-1 and 37.50 μg L^-1 of B[a]P, respectively. This study is the first report on the effects of PAHs in freshwater planarians' sexual reproduction and a decreased reproductive output was evidenced. Moreover, the exposure of adults to B[a]P lead to defects in newborns, raising concern on the possible long-term consequences of these compounds for natural planarian populations.

Exposure to benzo[a]pyrene triggers distinct patterns of microRNA transcriptional profiles in aquatic firefly Aquatica wuhana (Coleoptera: Lampyridae)

Larval aquatic fireflies in fresh water are adversely affected by water pollutants such as benzo(a)pyrene (BaP). However, their response to BaP stress at the microRNA (miRNA)-regulatory level remains unknown. Here, transcriptomes containing 31,872 genes and six miRNA transcriptional profiles were obtained for Aquatica wuhana larvae, and comparative analysis was performed between larvae exposed to BaP (0.01 mg/L) and unexposed controls. Fifteen of 114 miRNAs identified via bioinformatics were detected as differentially expressed (DEMs) upon BaP exposure. Analysis results of predicted target genes of DEM suggests that BaP exposure primarily triggered transcriptional changes of miRNA associated with five major regulatory categories: 1) osmotic balance, 2) energy metabolic efficiency, 3) development, 4) xenobiotic metabolism (oxidative stress), and 5) innate immune response. Based on six innate immune- and xenobiotic metabolism-related pathways enriched by the predicted DEM targets, 11 key BaP-responsive DEMs were further screened to investigate dynamic changes of expression in response to BaP stress at five time points, and also to validate the miRNA sequencing data using quantitative real-time PCR. This study provides valuable information for the protection of firefly resources and supplements the understanding of miRNA regulatory mechanisms in response to water deterioration.

Rearing environment affects the genetic architecture and plasticity of DNA methylation in Chinook salmon

Genetic architecture and phenotypic plasticity are important considerations when studying trait variation within and among populations. Since environmental change can induce shifts in the genetic architecture and plasticity of traits, it is important to consider both genetic and environmental sources of phenotypic variation. While there is overwhelming evidence for environmental effects on phenotype, the underlying mechanisms are less clear. Variation in DNA methylation is a potential mechanism mediating environmental effects on phenotype due to its sensitivity to environmental stimuli, transgenerational inheritance, and influences on transcription. To characterize the effect of environment on methylation, we created two 6 × 6 (North Carolina II) Chinook salmon breeding crosses and reared the offspring in two environments: uniform hatchery tanks and seminatural stream channels. We sampled the fish twice during development, at the alevin (larval) and fry (juvenile) stages. We measured DNA methylation at 13 genes using a PCR-based bisulfite sequencing protocol. The genetic architecture of DNA methylation differed between rearing environments, with greater additive and nonadditive genetic variance in hatchery fish and greater maternal effects in seminatural channel fish, though gene-specific variation was evident. We observed plasticity in methylation across all assayed genes, as well as gene-specific effects at two genes in alevin and six genes in fry, indicating developmental stage-specific effects of rearing environment on methylation. Characterizing genetic and environmental influences on methylation is critical for future studies on DNA methylation as a potential mechanism for acclimation and adaptation.

A Review of the Functional Roles of the Zebrafish Aryl Hydrocarbon Receptors

Over the last 2 decades, the zebrafish (Danio rerio) has emerged as a stellar model for unraveling molecular signaling events mediated by the aryl hydrocarbon receptor (AHR), an important ligand-activated receptor found in all eumetazoan animals. Zebrafish have 3 AHRs-AHR1a, AHR1b, and AHR2, and studies have demonstrated the diversity of both the endogenous and toxicological functions of the zebrafish AHRs. In this contemporary review, we first highlight the evolution of the zebrafish ahr genes, and the characteristics of the receptors including developmental and adult expression, their endogenous and inducible roles, and the predicted ligands from homology modeling studies. We then review the toxicity of a broad spectrum of AHR ligands across multiple life stages (early stage, and adult), discuss their transcriptomic and epigenetic mechanisms of action, and report on any known interactions between the AHRs and other signaling pathways. Through this article, we summarize the promising research that furthers our understanding of the complex AHR pathway through the extensive use of zebrafish as a model, coupled with a large array of molecular techniques. As much of the research has focused on the functions of AHR2 during development and the mechanism of TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) toxicity, we illustrate the need to address the considerable knowledge gap in our understanding of both the mechanistic roles of AHR1a and AHR1b, and the diverse modes of toxicity of the various AHR ligands.

Multigenerational Impacts of Benzo[a]pyrene on Bone Modeling and Remodeling in Medaka (Oryzias latipes)

Ancestral benzo[a]pyrene (BaP) (1 μg/L, 21 days) exposure has previously been shown to cause skeletal deformities in medaka (Oryzias latipes) larvae in the F1-F3 generation. However, when and how this deformity is induced during bone development remain to be elucidated. The col10a1:nlGFP/osx:mCherry double transgenic medaka model was employed to determine the temporal and spatial changes of col10a1:nlGFP- positive osteochondral progenitor cells (OPCs) and osx:mCherry-positive premature osteoblasts (POBs) [8 days postfertilization (dpf)-31 dpf] in combination with changes in bone mineralization at the tissue level. Ancestral BaP exposure delayed the development of col10a1:nlGFP- and osx:mCherry-positive osteoblasts and reduced the abundance of col10a1:nlGFP-positive osteoblast progenitors and col10a1:nlGFP/osx:mCherry double-positive premature osteoblasts during critical windows of early vertebral bone formation, associated with reduced bone mineralization in embryos (14 dpf) and larvae (31 dpf), compressed vertebral segments in larvae (31 dpf), and reduced bone thickness in adult male medaka (6 months old) of the F1-F3 generations. Both Col10a1:nlGFP and osx:mCherry were identified as potential targets of epigenetic modifications underlying the transgenerational inheritance of BaP bone toxicity. The present study provides novel knowledge of the underlying mechanisms of transgenerational toxicity of BaP at the cellular level.

Dicamba elevates concentrations of S-adenosyl methionine but does not induce oxidative stress or alter DNA methylation in rainbow trout (Oncorhynchus mykiss) hepatocytes

Dicamba is a benzoic acid herbicide used to target woody and broadleaf weeds in industrial, domestic, and municipal spheres. Because of its widespread use, dicamba is frequently detected in surface waters near sites of application. However, little is known regarding the effects of dicamba on freshwater fishes. In the present study, primary cultures of hepatocytes from rainbow trout (Oncorhynchus mykiss) were exposed to either an environmentally relevant (0.22 or 2.2 μg L^-1) or supra-environmental (22 μg L^-1) concentration of dicamba for 48 h to investigate if oxidative stress is a mechanism of toxicity. mRNA abundances of genes involved in the response to oxidative stress, levels of lipid peroxidation, and concentrations of glutathione and s-adenosyl methionine (SAM) were quantified. Results indicate that dicamba does not induce oxidative stress. However, exposure to 2.2 μg L^-1 of dicamba did cause a 5.24-fold increase in concentrations of SAM. To investigate the mechanisms of increased SAM, effects of dicamba on global and genome-wide DNA methylation were quantified. Dicamba did not cause changes to DNA methylation. Overall, dicamba was not acutely toxic to hepatocytes and did not cause oxidative stress or changes in DNA methylation at environmentally relevant concentrations.

Parental transgenerational epigenetic inheritance related to dietary crude oil exposure in Danio rerio

Transgenerational inheritance from both parental lines can occur by genetic and epigenetic inheritance. Maternal effects substantially influence offspring survival and fitness. However, investigation of the paternal contribution to offspring success has been somewhat neglected. In the present study, adult zebrafish were separated into female and male groups exposed for 21 days to either a control diet or to a diet containing water accommodated fractions of crude oil. Four F1 offspring groups were obtained: (1) control (non-exposed parents), (2) paternally exposed, (3) maternally exposed and (4) dual-parent-exposed. To determine the maternal and paternal influence on their offspring, we evaluated responses from molecular to whole organismal levels in both generations. Growth rate, hypoxia resistance and heart rate did not differ among parental groups. However, global DNA methylation in heart tissue was decreased in oil-exposed fish compared with control parents. This decrease was accompanied by an upregulation of glycine N-methyltransferase. Unexpectedly, maternal, paternal and dual exposure all enhanced survival of F1 offspring raised in oiled conditions. Regardless of parental exposure, however, F1 offspring exposed to oil exhibited bradycardia. Compared with offspring from control parents, global DNA methylation was decreased in the three offspring groups derived from oil-exposed parents. However, no difference between groups was observed in gene regulation involved in methylation transfer, suggesting that the changes observed in the F1 populations may have been inherited from both parental lines. Phenotypic responses during exposure to persistent environmental stressors in F1 offspring appear to be influenced by maternal and paternal exposure, potentially benefitting offspring populations to survive in challenging environments.

Overview of biological mechanisms of human carcinogens

This review summarizes the carcinogenic mechanisms for 109 Group 1 human carcinogens identified as causes of human cancer through Volume 106 of the IARC Monographs. The International Agency for Research on Cancer (IARC) evaluates human, experimental and mechanistic evidence on agents suspected of inducing cancer in humans, using a well-established weight of evidence approach. The monographs provide detailed mechanistic information about all carcinogens. Carcinogens with closely similar mechanisms of action (e.g. agents emitting alpha particles) were combined into groups for the review. A narrative synopsis of the mechanistic profiles for the 86 carcinogens or carcinogen groups is presented, based primarily on information in the IARC monographs, supplemented with a non-systematic review. Most carcinogens included a genotoxic mechanism.

Environmental chemicals differentially affect epigenetic-related mechanisms in the zebrafish liver (ZF-L) cell line and in zebrafish embryos

A number of chemicals have been shown to affect epigenetic patterning and functions. Since epigenetic mechanisms regulate transcriptional networks, epigenetic changes induced by chemical exposure can represent early molecular events for long-term adverse physiological effects. Epigenetics has thus appeared as a research field of major interest within (eco)toxicological sciences. The present study aimed at measuring effects on epigenetic-related mechanisms of selected environmental chemicals (bisphenols, perfluorinated chemicals, methoxychlor, permethrin, vinclozolin and coumarin 47) in zebrafish embryos and liver cells (ZFL). Transcription of genes related to DNA methylation and histone modifications was measured and global DNA methylation was assessed in ZFL cells using the LUMA assay. The differences in results gathered from both models suggest that chemicals affect different mechanisms related to epigenetics in embryos and cells. In zebrafish embryos, exposure to bisphenol A, coumarin 47, methoxychlor and permethrin lead to significant transcriptional changes in epigenetic factors suggesting that they can impact early epigenome reprogramming related to embryonic development. In ZFL cells, significant transcriptional changes were observed upon exposure to all chemicals but coumarin 47; however, only perfluorooctane sulfonate induced significant effects on global DNA methylation. Notably, in contrast to the other tested chemicals, perfluorooctane sulfonate affected only the expression of the histone demethylase kdm5ba. In addition, kdm5ba appeared as a sensitive gene in zebrafish embryos as well. Taken together, the present results suggest a role for kdm5ba in regulating epigenetic patterns in response to chemical exposure, even though mechanisms remain unclear. To confirm these findings, further evidence is required regarding changes in site-specific histone marks and DNA methylation together with their long-term effects on physiological outcomes.

Global DNA methylation changes in rock pigeon (Columba livia) as a sentinel species due to polycyclic aromatic hydrocarbons exposure in Tehran (Iran) as a megacity

Global DNA methylation, as an epigenetic modifications, can be a promising genomic marker for monitoring the contaminants and predicting their adverse health effects. The study aims to assess the effects of 16 PAH concentration on the altered DNA methylation levels in the kidney and liver of rock pigeon (Columba livia), as a sentinel species, from Tehran megacity as well as 40 days benzo(a)pyrene in vitro exposure: (0.1, 2.5, 5, 7.5, and 10 mg kg^-1 bw). Data indicated that the total LMW-PAH (low molecular weight PAHs) group (120.22, 121.34, 103.69, and 128.79 ng g^-1 dw in liver, kidney, skin, and muscle, respectively) in the Tehran samples have higher levels than the other PAHs groups. In addition, the DNA methylation level had negative relation with the total amount of PAHs in liver and kidney. A comparatively higher global DNA hypomethylation (by 8.65% in liver and 3.76% in kidney) was observed in birds exposed to B(a)P. Our results lead us to suggest that DNA hypomethylation in liver and kidney associated with the B(a)P may be useful biomarker discovery (more than the amount of PAH concentration in different tissues of C. livia) in urban areas. In conclusion, based on the overall results assessed, DNA methylation changes in pigeon may show a new target pathway for evaluation of environmental health.

Genome-wide somatic mutation analysis via Hawk-Seq™ reveals mutation profiles associated with chemical mutagens

It is difficult to identify mutagen-induced genome-wide somatic mutations using next generation sequencing; hence, mutagenic features of each mutagen and their roles in cancer development require further elucidation. We described Hawk-Seq™, a highly accurate genome sequencing method and the optimal conditions, for using it to construct libraries that would enable the accurate (c.a. 1 error/10⁷-10⁸ bp) and efficient survey of genome-wide mutations. Genomic mutations in gpt delta mice or Salmonella typhimurium TA100 exposed to methylnitrosourea (MNU), ethylnitrosourea (ENU), diethylnitrosamine (DEN), benzo[a]pyrene (BP), and aristolochic acid (AA) were profiled using Hawk-Seq™ to analyse positions, substitution patterns, or frequencies. The resultant vast mutation data provided high-resolution mutational signatures, including for minor mutational fractions (e.g. G:C>A:T by AA), which enabled the clarification of the mutagenic features of all mutagens. The 96-type mutational signatures of MNU, AA, and BP indicate their partial similarity to signature 11, 22, and 4 or 29, respectively. Meanwhile, signatures attributable to ENU and DEN were highly similar to each other, but not to signature 11, suggesting that the mechanisms of these agents differed from those of typical alkylating agents. Thus, Hawk-Seq™ can clarify genome-wide chemical mutagenicity profiles at extraordinary resolutions, thereby providing insight into mutagen mechanisms and their roles in cancer development.

Evaluation of two independent protocols for the extraction of DNA and RNA from different tissues of sea cucumber Isostichopus badionotus

Isostichopus badionotus is a sea cucumber species of great ecological and economic relevance for Mexico and Central American and Caribbean countries; however, the protocols for the extraction of the nucleic acids have not yet been published. In this study, we describe the first protocols to obtain DNA and RNA from different tissues of I. badionotus, which include the respiratory tree, gonad, longitudinal muscle bands, anterior intestine and cloaca. The extraction of high-quality DNA was performed using the DNeasy Blood & Tissue kit (Qiagen, Valencia, CA, USA) with minor modifications in different points of the protocol. Concerning the RNA, the method of TRIzol was used. This method is particularly advantageous in situations where cells or tissues are enriched for endogenous RNases or when the separation of cytoplasmic RNA from nuclear RNA is impractical. The methodologies used in this study allowed us to obtain DNA and RNA of high quality and integrity in the different tissues of I. badionotus, which will be the basis for future genomic and transcriptomic studies. •The successful extraction of DNA and RNA was achieved in the different tissues of I. badionotus.•The concentrations of DNA and RNA obtained were adequate for a diversity of analyses at a molecular level.

Exposure to Aroclor 1254 persistently suppresses the functions of pancreatic β-cells and deteriorates glucose homeostasis in male mice

Polychlorinated biphenyls (PCBs) are a class of persistent organic pollutants that have been shown to be related to the occurrence of type 2 diabetes mellitus (T2DM). Nevertheless, it is necessary to further explore the development of T2DM caused by PCBs and its underlying mechanisms. In the present study, 21-day-old C57BL/6 male mice were orally treated with Aroclor 1254 (0.5, 5, 50 or 500 μg kg^-1) once every three days. After exposure for 66 d, the mice showed impaired glucose tolerance, 13% and 14% increased fasting serum insulin levels (FSIL), and 63% and 69% increases of the pancreatic β-cell mass in the 50 and 500 μg kg^-1 groups, respectively. After stopping exposure for 90 d, treated mice returned to normoglycemia and normal FSIL. After re-exposure of these recovered mice to Aroclor 1254 for 30 d, fasting plasma glucose showed 15%, 28% and 16% increase in the 5, 50 and 500 μg kg^-1 treatments, FSIL exhibited 35%, 27%, 30% and 32% decrease in the 0.5, 5, 50 or 500 μg kg^-1 groups respectively, and there was no change in pancreatic β-cell mass. Transcription of the pancreatic insulin gene (Ins2) was significantly down-regulated in the 50 and 500 μg kg^-1 groups, while DNA-methylation levels were simultaneously increased in the Ins2 promoter during the course of exposure, recovery and re-exposure. Reduced insulin levels were initially rescued by a compensative increase in β-cell mass. However, β-cell mass eventually failed to make sufficient levels of insulin, resulting in significant increases in fasting blood glucose, and indicating the development of T2DM.

Exposure of low-dose fipronil enantioselectively induced anxiety-like behavior associated with DNA methylation changes in embryonic and larval zebrafish

Fipronil, a broad-spectrum chiral insecticide, has been documented to induce significant neurotoxicity to nontarget aquatic species; however, whether its neurotoxicity behaves enantioselectively and what molecular mechanisms correspond to the neurotoxicity remain unanswered. To date, few investigations have focused on the genomic mechanisms responsible for the enantioselective toxicity of chiral pesticides. The epigenetic modifications, especially DNA methylation, caused by the pesticides are also blind spot of the research works. Video tracking showed that R-fipronil exhibited more intense neurotoxicity, as well as the induction of more severe anxiety-like behavior, such as boosted swimming speed and dysregulated photoperiodic locomotion, to embryonic and larval zebrafish compared with S-fipronil. The MeDIP-Seq analysis, combined with Gene Ontology and KEGG, revealed that R-fipronil disrupted five signaling pathways (MAPK, Calcium signaling, Neuroactive ligand-receptor interaction, Purine metabolism, and Endocytosis) to a greater extent than S-fipronil through the hypermethylation of several important neuro-related genes, whereas no significant alterations of global DNA methylation were observed on the two enantiomers. To summarize, our data indicated that the fipronil-conducted enantioselective neurotoxicity likely applied its enantioselectivity by the dysregulation of DNA methylation. Our study also provided novel epigenetic insights into the study of enantioselective biological effects and the relevant underlying mechanisms of chiral insecticide.

Toxicity of 10 organic micropollutants and their mixture: Implications for aquatic risk assessment

Micropollutants, as a serious water pollution issue, raise considerable toxicological concerns, particularly when present as components of complex mixtures. Due to the interactions of environmental pollution components (contaminant), the micropollutant problem is increasingly complex, thus, water quality of organic chemical contamination assessed substance-by-substance might lead to underestimation in aquatic environmental risk assessment. To assess the aquatic environmental risk of micropollutants mixture, a total of 10 organic micropollutants were selected and analysed by an approach of integration of literature data, laboratory experiments and prediction techniques. The experiment results showed that all 10 micropollutants were capable of causing toxicity in zebrafish embryos, aquatic invertebrates and algae with the LC50 (50% lethal concentration) values from 1.14 mg/L to 14.37 mg/L. Triclosan, carbamazepine, diazinon and diuron were the most hazardous compounds in the Danube River and the Rhine River. The artificial mixture presented a strong antagonistic relationship, which demonstrated an independent action (IA) model of the mixture. Based on the observed toxicity data, the risk quotients (RQs) of environmental mixtures of the Danube River and the Rhine River were extrapolated. It can be concluded that the micropollutant mixture may pose a potential risk for aquatic ecosystems with the present environmentally measured concentrations in the Danube River and Rhine River. Mixture risk assessment results suggested that the toxicity of studied chemicals might be induced by dissimilar actions, which is in agreement with the mixture toxicity prediction of the IA model. The observed findings could be useful to establish an overview of the pressures, vision, measures and expectations for hazardous substances pollution, which can help in making to informed decisions to reduce the concentration and bioactive fraction of pollutants.

Assessment of environmentally contaminated sediment using a contact assay with early life stage zebrafish (Danio rerio)

Lake Saint-Louis, a shallow fluvial lake near the western tip of the island of Montreal, QC, Canada is an important spawning ground for many species of fish. Sediments in certain areas of the lake are known to be contaminated with high levels of metals and legacy organic chemicals. To improve our understanding of risk to native fish populations, we conducted a study evaluating levels of sediment contamination and potential effects on early life stage fish. Concentrations of PAHs, PCBs, PCDDs and PCDFs were several orders of magnitude higher at two industrial sites (B1 and B2) than at a nearby reference site (IP). Concentrations of 32 metals and metalloids were at least 5-fold higher at B1 and B2 than at IP. Moreover, all available interim sediment quality guidelines (ISQGs) were exceeded at the two contaminated sites, while none were exceeded at the reference site. Biological effects were evaluated using a sediment contact assay. Zebrafish (Danio rerio) embryos were exposed to clean water (control), or to sediment from IP, B1, and B2 until 120 h post fertilization (hpf). Mortality was significantly elevated in fish exposed to the B1, but not the B2 sediment. The frequency of deformities increased with increasing contamination, but this trend was not statistically significant (p > 0.05). Genes that are implicated in the response to PAHs, PCBs, dioxins and furans (cyp1a, cyp1b1, ahr2) were significantly elevated in the 120 hpf larvae exposed to the B1 and B2 sediments. Global DNA methylation, and mRNA expression of genes related to oxidative stress (maft, cat, hmox1, sod2), embryonic development (bmp2b, baf60c), metal exposure (mt2), and DNA repair (gadd45b) were unaffected. Our results suggest that the Beauharnois sector of Lake Saint-Louis is poor quality spawning habitat due to high levels of contamination, and the potential for harmful effects on early life stage fish.

Comparative toxicity of two diluted bitumens to developing yellow perch (Perca flavescens)

Increasing demand for diluted bitumen (dilbit) has led to the development of the oil sands industry and the expansion of transcontinental pipelines. Dilbit is an unresolved complex mixture with variable diluent and bitumen composition. Thus, it is important to understand the effects of the two most transported dilbits in Canada, Access Western Blend (AWB) and Cold Lake Blend (CLB) on a North America native and freshwater fish species, the yellow perch (Perca flavescens). Fertilized embryos were exposed to both dilbits for 16 days, from <24 h post-fertilization until hatch. The treatment regime was a static daily renewal of water accommodated fractions (WAF) and chemically-enhanced water accommodated fractions (CEWAF) at concentrations ranging from 0.01 to 21.3 μg/L of total polycyclic aromatic compounds (TPACs). Hatched embryos were assessed for malformations and changes in the expression of genes associated with phase I and II detoxification and oxidative stress. The prevalence of developmental malformations increased significantly at the highest concentrations of all treatments, with WAF treatments yielding a higher prevalence than CEWAF. The EC50s for AWB and CLB WAF and CEWAF solutions ranged from 9.8 to 24 μg/L TPACs, with the CEWAF of AWB being the least toxic. Relative mRNA levels of cyp1a showed induction by up to 18-fold in WAF and 50-fold in CEWAF treatments at similar concentrations of measured dilbit in solution. Complementary DNA methylation analysis was assessed and fish embryos exposed to AWB CEWAF and CLB WAF showed decreased DNA methylation profiles with increasing exposure to dilbit, suggesting that global gene expression is increasing in these treatments. With recent approvals of pipelines in North America, these data will support site-specific risk assessments and monitoring of Canadian ecosystems should a pipeline spill occur.

Examining multi- and transgenerational behavioral and molecular alterations resulting from parental exposure to an environmental PCB and PBDE mixture

Polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) are persistent organic pollutants extensively used during the 20^th century and still present in aquatic environments despite their ban. Effects of exposure to these compounds over generations are poorly documented. Therefore, our aims were to characterize behavioral responses and underlying molecular mechanisms in zebrafish exposed to an environmentally relevant mixture of PCBs and PBDEs as well as in four unexposed offspring generations. Zebrafish (F0) were chronically exposed from the first meal onward to a diet spiked with a mixture containing 22 PCB and 7 PBDE congeners in proportions and concentrations reflecting environmental situations (ΣPCBs = 1991 and ΣPBDEs = 411 ng/g). Four offspring generations (F1 to F4) were obtained from this F0 and were not further exposed. Behavior was assessed at both larval and adult stages. Mechanisms related to behavioral defects (habenula maturation and c-fos transcription) and methylation (dnmts transcription) were monitored in larvae. Exposed adult F0 as well as F1 and F3 adults displayed no behavioral change while F2 expressed anxiety-like behavior. Larval behavior was also disrupted, i.e. hyperactive after light to dark transition in F1 or hypoactive in F2, F3 and F4. Behavioral disruptions may be related to defect in habenula maturation (observed in F1) and change in c-fos transcription (observed in F1 and F2). Transcription of the gene encoding DNA methyltransferase (dnmt3ba) was also modified in all generations. Our results lead us to hypothesize that chronic dietary exposure to an environmentally relevant mixture of PCB and PBDE triggers multigenerational and transgenerational molecular and behavioral disruptions in a vertebrate model.

Developmental Effects of Cannabidiol and Δ9-Tetrahydrocannabinol in Zebrafish

Cannabidiol (CBD) has gained much attention in the past several years for its therapeutic potential in the treatment of drug-resistant epilepsy, such as Dravet syndrome. Although CBD has shown anecdotal efficacy in reducing seizure frequency, little is known regarding the potential adverse side effects of CBD on physiology, development, organogenesis, or behavior. The goal of this project was to compare the relative morphological, behavioral, and gene expression phenotypes resulting after a developmental exposure to Δ9-tetrahydrocannabinol (THC) or CBD. Zebrafish were exposed from blastula through larval stage (96 h postfertilization [hpf]) to 0.3, 0.6, 1.25, 2.5, 5 mg/l (1, 2, 4, 8, 16 µM) THC or 0.07, 0.1, 0.3, 0.6, 1.25 mg/l CBD (0.25, 0.5, 1, 2, 4 µM). Despite the similarity in THC and CBD dysmorphologies, ie, edemas, curved axis, eye/snout/jaw/trunk/fin deformities, swim bladder distention, and behavioral abnormalities, the LC50 for CBD (0.53 mg/l) was nearly 7 times lower than THC (3.65 mg/l). At 96 hpf, c-fos, dazl, and vasa were differentially expressed following THC exposure, but only c-fos expression was significantly increased by CBD. Cannabidiol was more bioconcentrated compared with THC despite higher THC water concentrations. This work supports the potential for persistent developmental impacts of cannabinoid exposure, but more studies are needed to assess latent effects and their molecular mechanisms of toxicity.

Molecular landscape of esophageal cancer: implications for early detection and personalized therapy

Esophageal cancer (EC) is one of the most lethal cancers and a public health concern worldwide, owing to late diagnosis and lack of efficient treatment. Esophageal squamous cell carcinoma (ESCC) and esophageal adenocarcinoma (EAC) are main histopathological subtypes of EC that show striking differences in geographical distribution, possibly due to differences in exposure to risk factors and lifestyles. ESCC and EAC are distinct diseases in terms of cell of origin, epidemiology, and molecular architecture of tumor cells. Past efforts aimed at translating potential molecular candidates into clinical practice proved to be challenging, underscoring the need for identifying novel candidates for early diagnosis and therapy of EC. Several major international efforts have brought about important advances in identifying molecular landscapes of ESCC and EAC toward understanding molecular mechanisms and critical molecular events driving the progression and pathological features of the disease. In our review, we summarize recent advances in the areas of genomics and epigenomics of ESCC and EAC, their mutational signatures and immunotherapy. We also discuss implications of recent advances in characterizing the genome and epigenome of EC for the discovery of diagnostic/prognostic biomarkers and development of new targets for personalized treatment and prevention.