Effects of azocyclotin on gene transcription and steroid metabolome of hypothalamic-pituitary-gonad axis, and their consequences on reproduction in zebrafish (Danio rerio)

Long-term exposure to aryl hydrocarbon receptor agonist neburon induces reproductive toxicity in male zebrafish (Danio rerio)

Neburon is a phenylurea herbicide that is widely used worldwide, but its toxicity is poorly studied. In our previous study, we found that neburon has strong aryl hydrocarbon receptor (AhR) agonist activity, but whether it causes reproductive toxicity is not clear. In the present study, zebrafish were conducted as a model organism to evaluate whether environmental concentrations of neburon (0.1, 1 and 10 µg/L) induce reproductive disorder in males. After exposure to neburon for 150 days from embryo to adult, that the average spawning egg number in high concentration group was 106.40, which was significantly lower than 193.00 in control group. This result was mainly due to the abnormal male reproductive behavior caused by abnormal transcription of genes associated with reproductive behavior in the brain, such as secretogranin-2a. The proportions of spermatozoa in the medium and high concentration groups were 82.40% and 83.84%, respectively, which were significantly lower than 89.45% in control group. This result was mainly caused by hormonal disturbances and an increased proportion of apoptotic cells. The hormonal disruption was due to the significant changes in the transcription levels of key genes in the hypothalamus-pituitary-gonadal axis following neburon treatment. Neburon treatment also significantly activated the AhR signaling pathway, causing oxidative stress damage and eventually leading to a significant increase in apoptosis in the exposed group. Together, these data filled the currently more vacant profile of neburon toxicity and might provide information to assess the ecotoxicity of neburon on male reproduction at environmentally relevant concentrations.

Thyroid and sex hormone disrupting effects of DEHTP at different life stages of zebrafish (Danio rerio)

Di(2-ethylhexyl) terephthalate (DEHTP) is an alternative plasticizer widely used in numerous consumer products, replacing di(2-ethylhexyl) phthalate (DEHP). Hence, DEHTP has been frequently detected in the environment and humans. As a structural isomer and functional analog of DEHP, DEHTP is a suspected endocrine disruptor. Here, we evaluated thyroid-disrupting effects of DEHTP using embryo-larval and adult male zebrafish. We also investigated its sex hormone disruption potential in the adult zebrafish. After 5- and 7-days of exposure to DEHTP, significant increases in whole-body thyroid hormonal levels were observed in the larval fish. Down-regulation of several thyroid-regulating genes, including trh, tshβ, nis, and dio2, was observed, but only after 5-day exposure. Following a 21-day exposure, the adult male zebrafish exhibited a significant decrease in total triiodothyronine and an increase in thyroid-stimulating hormones. Potential changes in the deiodination of thyroid hormones, supported by the up-regulation of two deiodinases, dio1 and dio3a, along with the down-regulation of dio2, could explain the thyroid hormone changes in the adult zebrafish. Moreover, significant trends of decrease in estradiol and 11-ketotestosterone, along with increase of testosterone (T), were observed in the adult zebrafish. Up-regulation of several steroidogenic genes may explain elevated T, while exact mechanisms of action warrant further investigation. Our results demonstrate that DEHTP can cause disruptions of thyroid and sex hormones at different life stages in zebrafish.

Cyhexatin causes developmental toxic effects by disrupting endocrine system and inducing behavioral inhibition, apoptosis and DNA hypomethylation in zebrafish (Danio rerio) larvae

Cyhexatin (CYT), an organotin acaricide, is extensively utilized in developing countries to mitigate plant diseases caused by mites and minimize agricultural crop losses. However, the comprehensive mechanisms underlying the developmental stage of non-target organisms remain largely unexplored. In this study, zebrafish embryos were firstly exposed to CYT (0.06, 0.12, and 0.20 ng/mL, referred to as CYTL, CYTM, and CYTH, respectively) from 2 hpf (hours post fertilization) to 30 dpf (days post fertilization). No developmental toxicity was observed in the CYTL and CYTM groups, except for induced deformed phenotypes in the CYTM group at 120 hpf. However, exposure to CYTH resulted in significant reductions in spontaneous movement (24 hpf), heart rate (48 hpf), hatching rate (48 and 72 hpf), body weight (30 dpf), whole body length (30 dpf), and locomotion (30 dpf). Additionally, CYTH exposure induced morphological malformations, including spinal curvature, pericardial edema, and tail curvature in zebrafish larvae. Moreover, CYTH treatment induced apoptosis, increased reactive oxygen species (ROS) production, and resulted in significant reductions in free T3, cholesterol, estradiol, and testosterone levels in zebrafish larvae, while free T4 levels were increased. RNA-Seq analysis indicated that CYTH exposure led to significant alterations in the genome-wide gene expression profiles of zebrafish, particularly in the thyroid hormone and steroid biosynthesis signaling pathways, indicating endocrine disruption. Furthermore, CYTH exposure induced global DNA hypomethylation, reduced S-adenosylmethionine (SAM) levels and the SAM/S-adenosylhomocysteine (SAH) ratio, elevated SAH levels, and suppressed the mRNA expression of DNA methyltransferases (DNMTs) while also downregulating DNMT1 at both the gene and protein levels in zebrafish larvae. Overall, this study partially elucidated the developmental toxicity and endocrine disruption caused by CYT in zebrafish, providing evidence of the environmental hazards associated with this acaricide.

Organotin Antifouling Compounds and Sex-Steroid Nuclear Receptor Perturbation: Some Structural Insights

Organotin compounds (OTCs) are a commercially important group of organometallic compounds of tin used globally as polyvinyl chloride stabilizers and marine antifouling biocides. Worldwide use of OTCs has resulted in their ubiquitous presence in ecosystems across all the continents. OTCs have metabolic and endocrine disrupting effects in marine and terrestrial organisms. Thus, harmful OTCs (tributyltin) have been banned by the International Convention on the Control of Harmful Antifouling Systems since 2008. However, continued manufacturing by non-member countries poses a substantial risk for animal and human health. In this study, structural binding of common commercial OTCs, tributyltin (TBT), dibutyltin (DBT), monobutyltin (MBT), triphenyltin (TPT), diphenyltin (DPT), monophenyltin (MPT), and azocyclotin (ACT) against sex-steroid nuclear receptors, androgen receptor (AR), and estrogen receptors (ERα, ERβ) was performed using molecular docking and MD simulation. TBT, DBT, DPT, and MPT bound deep within the binding sites of AR, ERα, and Erβ, showing good dock score, binding energy and dissociation constants that were comparable to bound native ligands, testosterone and estradiol. The stability of docking complex was shown by MD simulation of organotin/receptor complex with RMSD, RMSF, Rg, and SASA plots showing stable interaction, low deviation, and compactness of the complex. A high commonality (50-100%) of interacting residues of ERα and ERβ for the docked ligands and bound native ligand (estradiol) indicated that the organotin compounds bound in the same binding site of the receptor as the native ligand. The results suggested that organotins may interfere with the natural steroid/receptor binding and perturb steroid signaling.

Reproductive toxicity of long-term exposure to environmental relevant concentrations of cyprodinil in female zebrafish (Danio rerio)

In recent years, the widespread use of the pesticide cyprodinil has attracted attention due to its harmful effects on aquatic organisms. The purpose of this study was to evaluate the adverse effects of long-term exposure to cyprodinil on the reproductive system of female zebrafish. After the embryos had been treated with 0.1, 1 and 10 μg/L cyprodinil for 180 days, we observed that female fish treated with 1 and 10 μg/L cyprodinil showed decreased sexual attractiveness, a decreased proportion of primordial follicles in the ovary, an increased proportion of mature follicles, and increased egg production. Moreover, exposed females that mated with normal males produced offspring with increased rates of mortality and deformity (the F1 generation). In addition, the levels of gonadotropin and testosterone (T) were increased in females after cyprodinil exposure, especially in the 10 μg/L treated group. After cyprodinil treatment, some key genes in the hypothalamic-pituitary-gonad axis underwent significant changes. For example, gene expression of brain gonadotropin-releasing hormone receptors (gnrhr1, gnrhr2 and gnrhr4) was significantly downregulated after cyprodinil treatment. The study found that expression of the aromatase (cytochrome P450 family 19 subfamily A polypeptide 1a, cyp19a1a) responsible for converting T into estradiol was significantly downregulated after cyprodinil treatment, consistent with elevated T levels in the ovaries and muscles. In summary, these data provide a more comprehensive understanding of the toxicity of cyprodinil and may inform evaluation of the ecotoxicity of cyprodinil to female reproduction at environmentally relevant concentrations.

Mixture effects of imidacloprid and difenconazole on enzymatic activity and gene expression in small yellow croakers (Larimichthys polyactis)

Agrochemicals usually exist as mixtures in aqueous ecosystems and have harmful impacts on the natural environment. Nonetheless, the combined effects and underlying mechanisms of agrochemicals on aqueous organisms remain poorly understood. In the present study, the interactive effects of imidacloprid (IMI) and difenconazole (DIF) on the embryos of small yellow croakers (Larimichthys polyactis) were assessed using various toxicological assays, including acute toxicity, enzymatic activity, and gene expression changes. The results showed that DIF (72-h LC₅₀ value of 0.20 mg L^-1) had higher toxicity than IMI (72-h LC₅₀ value of 12.5 mgL^-1). Simultaneously, combinations of IMI and DIF exerted synergistic acute effects on the embryos of L. polyactis. In addition, the SOD, CAT, GST, and CarE activities were noticeably altered in most single and mixed exposures, relative to the untreated control. The expression of four genes (cyp19a1b, ngln2, klf2a, and socs3a) related to the immune system, endocrine system, and neurodevelopment was also surprisingly altered when the embryos of L. polyactis were subjected to individual and combined exposures relative to the untreated control. Changes in enzymatic activity and gene expression might provide early warning indices for the identification of agrochemical co-exposure. The results of this study provide valuable insights into the comprehensive toxicity of agrochemical mixtures to L. polyactis. Further studies on the long-term effects of agrochemical mixtures on marine fish should be conducted to formulate definitive conclusions concerning hazards.

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.

Full-Length Transcriptome Sequences Provide Insight Into Hermaphroditism of Freshwater Pearl Mussel Hyriopsis schlegelii

The freshwater mussel Hyriopsis schlegelii is a cultured bivalve in China, and the quality of the pearls produced is affected by the type of gonads. However, because of the lack of a published genome and the complexity of sex determination, research on sex reversal and development of this species is limited. In this study, Illumina RNA-seq and PacBio Isoform Sequencing (Iso-Seq) were combined to analyze the gonads of H. schlegelii. A total of 201,481 high-quality transcripts were generated. The study identified 7,922 differentially expressed genes in three comparison group (females versus males, hermaphrodites versus females, and hermaphrodites versus males). Twenty-four genes were identified as potential sex-related genes, including sox9 and wnt4 involved in sex determination, and vtg, cyp17a1 and 17β-hsd2 involved in gonadal development. We also speculated a possible pathways for the formation of hermaphroditism in H. schlegelii. The data provide a clear view of the transcriptome for H. schlegelii gonads and will be valuable in elucidating the mechanisms of gonad development.

Combined lethal toxicity, biochemical responses, and gene expression variations induced by tebuconazole, bifenthrin and their mixture in zebrafish (Danio rerio)

Pesticides commonly occur as mixtures in an aqueous environment, causing deleterious effects on human health and the environment. However, the mechanism underlying the combined effects on aqueous organisms remains largely unknown, especially at low concentrations. In the current study, we inspected the interactive toxicity of tebuconazole (TEB), a triazole fungicide, and bifenthrin (BIF), a pyrethroid insecticide, to zebrafish (Danio rerio) using various toxicological assays. Our data revealed that the 96 h-LC50 (lethal concentration 50) values of BIF to fish at different life periods (embryonic, larval, juvenile, and adult periods) ranged from 0.013 (0.011-0.016) to 0.41 (0.35-0.48) mg a.i. L-1, which were lower than that of TEB ranging from 1.1 (0.88-1.3) to 4.8 (4.1-5.7) mg a.i. L-1. Combination of TEB and BIF induced synergetic acute toxicity to embryonic fish. Activities of T-SOD, POD, and GST were distinctly altered in most individual and joint administrations. Expressions of 16 genes associated with oxidative stress, cellular apoptosis, immune system, and endocrine system at the mRNA level were evaluated, and the information revealed that embryonic zebrafish were impacted by both individual compounds and their combinations. Six genes (cas9, P53, gr, TRα, IL-8, and cxcl-clc) exhibited greater changes when exposed to pesticide mixtures. Therefore, the joint effects induced by the pesticides at low concentrations should be considered in the risk assessment of mixtures and regulated as priorities for mixture risk management in the aqueous ecosystem. More research is needed to identify the threshold concentrations of the realistic pesticide mixtures above which synergistic interactions occur.

Insights into the sex-dependent reproductive toxicity of 2-ethylhexyl diphenyl phosphate on zebrafish (Danio rerio)

As a frequently detected organophosphate ester in various environmental media, the toxic effects of 2-ethylhexyl diphenyl phosphate (EHDPHP) on aquatic organisms of different sexes remain unclear. In this study, adult zebrafish were exposed to 2.5, 50, 250 µg/L of EHDPHP for 21 days to investigate its sex-dependent reproductive toxicity and related mechanisms. EHDPHP exposure significantly inhibited the reproduction of zebrafish, evidenced by the reduced spawning of females, depressed growth and development of their offspring. EHDPHP induced greater impacts on the changes of sex hormones and vitellogenin (VTG) in the males than females. For females, the synthesis of testosterone (T) was inhibited because of the down-regulated gnrhr2, gnrhr3, gnrhr4, gnrh3, gnrh2 and er2β in the brain, while 17β-estradiol (E2) increased in 250 µg/L due to up-regulated cyp19a. For males, the promotion of T was directly related to the up-regulation of fshr, 3βhsd, star, cyp11 and cyp17 in the gonad, and eventually led to the increase of E2. The decrease of plasma 11-KT in both sexes could be mainly attributed to the down-regulation of cyp11b and hsd11b. The plasma VTG decreased in females but increased in males, which was in accordance with the down and up regulation of erα and er2β in the females and males, respectively. All these indicated EHDPHP displayed reproductive toxicity on zebrafish in a sex dependent manner. Molecular docking analysis indicated stronger interaction of EHDPHP with the antagonisms of estrogen receptor (ER) and androgen receptor (AR), as well as the agonism of CYP19A1, which further revealed the sex-dependent reproductive toxicity mechanism of EHDPHP. This study highlights the importance of distinguishing males and females in toxicity evaluation of endocrine disruption chemicals.

Mixture toxicity of thiophanate-methyl and fenvalerate to embryonic zebrafish (Danio rerio) and its underlying mechanism

Though pesticide mixtures can reflect the real-life situation in the water ecosystem, the quantification of their toxicity is still not fully understood. Combined effects of thiophanate-methyl (THM) and fenvalerate (FEN) on embryonic zebrafish (Danio rerio) and underlying mechanism were conducted in this study. Results showed that the 96-h LC₅₀ values of THM to D. rerio at different growth periods ranged from 12.1 to 26.1 mg L^-1, which were lower in comparison with those of FVR ranging from 0.025 to 2.8 mg L^-1. Mixture of THM and FVR exhibited a synergetic response to zebrafish embryos. Activities of Cu/Zn-SOD, POD, caspase 3 and caspase 9 were significantly different in most of single and mixture administrations compared with the control group. In addition, five genes (P53, Cu/Zn-sod, crh, ERα and IL-8) associated with oxidative stress, cellular apoptosis, immune system and endocrine system showed greater variations of expressions when administrated to pesticide mixtures compared with single chemicals. Our experimental results exhibited that mixtures of thiophanate-methyl and fenvalerate produced higher toxicity towards aqueous vertebrates than when determined singly. Collectively, upcoming environmental risk assessments established according to single administrations might not be enough to protect the water ecosystem.

Synergistic effect of fenpropathrin and paclobutrazol on early life stages of zebrafish (Danio rerio)

Aquatic organisms are usually exposed to various co-existing pollutants. However, toxic effects of pesticide mixtures on aquatic organisms and its potential underlying mechanism still remain unclear. The joint effects of fenpropathrin (FEN) and paclobutrazol (PAC) on zebrafish (Danio rerio) using diverse toxicological endpoints were investigated in the current work. Our data exhibited that the 96-h LC₅₀ values of FEN to zebrafish at multiple life phases ranged from 0.0029 (0.0013-0.0042) to 0.16 (0.082-0.23) mg a.i. L^-1, which were lower by comparison to PAC ranging from 13.16 (8.564-21.03) to 23.43 (17.94-29.91) mg a.i. L^-1. Combination of FEN and PAC displayed synergistic effect on embryonic zebrafish. Activities of T-SOD, Cu/Zn-SOD and CYP450 were remarkably changed in the majority of single and mixture treatments by comparison to the untreated group. The mRNA levels of 17 genes related to oxidative stress, cellular apoptosis, immune system and endocrine system were assessed, and the data suggested that embryonic zebrafish were affected by both single pesticides and their mixtures. Five genes (P53, tsh, ERα, crh and cxcl-clc) showed greater alterations when exposed to pesticide mixtures by comparison to their individual chemicals. Therefore, it is urgently necessary to conduct more studies on mixture toxicities of different pesticides to explore the chemical mixtures with synergistic interactions.

The reproductive toxicity and potential mechanisms of combined exposure to dibutyl phthalate and diisobutyl phthalate in male zebrafish (Danio rerio)

Dibutyl phthalate (DBP) and diisobutyl phthalate (DiBP) are phthalate compounds frequently detected in the environment. Despite increasing awareness of their toxicity in human and animals, the male reproductive toxicity of their combined exposure remains elusive. The purposes of this study were to investigate whether combined exposure to DBP and DiBP could induce male reproductive toxicity, and to explore the potential toxicological mechanisms. Adult male zebrafish were exposed to DBP (11, 113 and 1133 μg L^-1), DiBP (10, 103 and 1038 μg L^-1) and their mixtures (Mix) (11 + 10, 113 + 103, 1133 + 1038 μg L^-1) for 30 days, and their effects on plasma hormone secretion, testis histology and transcriptomics were examined. Highest concentrations of Mix exposure caused greater imbalance ratio of T/E2 and more severe structural damage to testis than single exposure. These effects were consistent with the testis transcriptome analysis for which 4570 genes were differentially expressed in Mix exposure, while 2795 and 1613 genes were differentially expressed in DBP and DiBP, respectively. KEGG pathway analysis showed that both single and combined exposure of DBP and DiBP could affect cytokine-cytokine receptor interaction. The difference was that combined exposure could also affect steroid hormone synthesis, extracellular matrix receptor interaction, retinol metabolism, and PPAR signaling pathways. These results demonstrated that combined exposure to DBP and DiBP could disrupt spermatogenesis and elicit male reproductive toxicity in zebrafish.

Pyriproxyfen induced impairment of reproductive endocrine homeostasis and gonadal histopathology in zebrafish (Danio rerio) by altered expression of hypothalamus-pituitary-gonadal (HPG) axis genes

Pyriproxyfen (PPF), a broad-spectrum insecticide known to cause reproductive and endocrine disruption in invertebrates, while the data is scarce in aquatic vertebrates. The goal of this study is to investigate the impact of PPF on reproductive endocrine system of male and female zebrafish along hypothalamus-pituitary-gonadal (HPG) axis. In brain, PPF caused significant alteration in the transcripts of erα, lhβ, and cyp19b genes in male and fshβ, lhβ, and cyp19b genes in female zebrafish. The downstream genes of steroidogenic pathway like, star, 3βhsd, 17βhsd, and cyp19a expression were significantly altered in gonad of both sexes. Subsequent changes in circulatory steroid hormone levels lead to imbalance in hormone homeostasis as revealed from estradiol/testosterone (E₂/T) ratio. Further, the vitellogenin transcript level was enhanced in hepatic tissues and their blood plasma content was increased in male (16.21%) and declined in female (21.69%). PPF also induced histopathological changes in gonads such as, reduction of mature spermatocytes in male and vitellogenic oocytes in female zebrafish. The altered E₂/T ratio and gonadal histopathology were supported by the altered transcript levels of HPG axis genes. Overall, these findings provide new insights of PPF in zebrafish reproductive system and highlights for further investigations on its potential risks in aquatic environment.

Parental exposure to cadmium chloride causes developmental toxicity and thyroid endocrine disruption in zebrafish offspring

Cadmium is a common heavy metal pollutant. Previous studies have found that long-term cadmium exposure can cause damage to multiple organs/systems in humans and experimental animals; however, there are few studies that elucidate its effects on offspring development, discuss whether it can be transmitted to offspring from the parent, and debate whether it affects the functional development of the thyroid hormone system in offsprings. In this study, sexually mature zebrafish were exposed to different concentrations of cadmium chloride (0.01 μmol/L, 0.1 μmol/L, and 1 μmol/L) to study reproductive toxicity. It was found that parental zebrafish exposed to 1 μmol/L of cadmium chloride produced offsprings with different degrees of malformation. At 5 days post-fertilization (dpf), the levels of 3,5,3'-triiododenosine (T3) and thyroxine (T4) in the zebrafish were decreased. At 10 dpf, the T4 and T3 levels in the zebrafish of the offspring were significantly reduced. At the same time, the expression of thyroid receptor (trα and trβ) genes in five dpf larvae was significantly up-regulated in the 1 μmol/L treatment group relative to the control group. The mRNAs of thyroid hormone synthesis and metabolism-related genes (tshβ, dio1, dio2, ugt1ab, and ttr) were significantly up-regulated in the 0.1 μmol/L and 1 μmol/L treatment groups. This study demonstrates that parental cadmium chloride exposure produces reproductive toxicity in zebrafish and that the effects can be transferred from the parent to the offspring, resulting in developmental toxicity in the thyroid endocrine system.

Modulation of steroid metabolism and xenobiotic biotransformation responses in zebrafish (Danio rerio) exposed to triadimefon

The widely used fungicide triadimefon (TDF) has been detected in aquatic environments, and appears to disrupt steroid homeostasis; however, the toxic effects on fish reproduction triggered by TDF via the key receptor signaling pathways remain largely unknown. The present study showed that TDF (0.069, 0.138, 0.690 mg/L) exposure not only caused disordered germ cell maturation, but also decreased spawned egg production. In order to better understand this reproductive inhibition, we investigated the effects of TDF based on quantitative PCR, Western blot and mass spectrometry methodology in zebrafish. Due to the preferential accumulation of TDF in the liver, a general pattern of up-regulation of genes involved in biotransformation pathway was observed. A significant increase in abcb4 expression appeared to be responsible for TDF excretion. TDF-induced receptors (AhR2 and PXR) changed many genes involved in steroid metabolism, and subsequent disruptions in steroid homeostasis, which might be the key biological pathway in TDF reproductive toxicity. However, due to the different metabolic demands, the transcript profiles involved in steroid metabolism in zebrafish exhibited a sex-specific expression pattern. For example, the increase in gene expression of ahr2 was accompanied by a reduction in the rate of E2 biosynthesis resulting from the diminished cyp19a1a expression, and in turn led to down-regulation of esr1 and vtg1 in the liver, supporting the anti-estrogenic effect of TDF in male fish. In contrast, the increase in E2 production was accompanied by an increase in Esr1 protein expression caused by TDF and paralleled the increase in ahrr1 expression, suggesting that TDF may induce estrogenic activity through AhR-ER interactions in females. In addition, over-induction of cyp3a65 activity mediated through pxr, which helped to accelerate the transformation from TDF to triadimenol in the liver, appeared to elevate T metabolite rate in females. The down-regulation of fshβ transcript in males further suggested that TDF might adversely affect normal gametogenesis and induce reproductive toxicity.

Tebuconazole induced oxidative stress related hepatotoxicity in adult and larval zebrafish (Danio rerio)

Tebuconazole is widely used as fungicide and has frequently been detected at elevated concentrations in environmental media. To characterize the potential toxicity of tebuconazole on vertebrate and humans. Using zebrafish as a vertebrate model, the toxic effects in liver that produced by low-toxic concentrations of tebuconazole were assessed in adult zebrafish. We further focused on tebuconazole-induced toxicity and its possible mechanism in larval zebrafish using a hepatotoxicity assay. The induction of oxidative stress in adult fish was evaluated by superoxide dismutase (T-SOD), catalase (CAT), peroxidase (POD), glutathione S-transferase (GST) activity, and the increased aspartate aminotransferase (AST)/alanine aminotransferase (ALT) ratio. Significantly increased enzyme activity was observed in the liver of male and female fish at both exposure and depuration stage. Exposure to maximum non-lethal (MNLC) concentration of tebuconazole from 72 to 120 h post-fertilization (hpf) affected the liver size and yolk retention in larval zebrafish. Decreased fluorescence intensity was observed in larval Tg(Apo14:GFP) zebrafish, indicating liver degeneration after tebuconazole treated. Histopathological examination confirmed the alterations in liver histoarchitecture in exposed zebrafish. Significant 1.28-fold and 1.65-fold increases in reactive oxygen species levels were observed in juveniles exposed to MNLC and lethal concentration 10 (LC₁₀) group, respectively. The acridine orange staining assay showed that apoptotic cells occurred in the liver regions. These results indicated that tebuconazole exposure resulted in impacts on the ecological risk in fish and vertebrate. Overall, the present study suggested further research in needed to better understand the tebuconazole-induced toxicity mechanism that associated with oxidative stress.

Disruptive effects of two organotin pesticides on the thyroid signaling pathway in Xenopus laevis during metamorphosis

Organotin compounds are the ubiquitous environmental pollutants due to their wide industrial and agricultural applications and unexpected releasing into the environment, which show characteristic of endocrine disruptors to interfere with the synthesis, receptor binding or action of endogenous-hormones. Organotin pesticides (OTPs) are used in agriculture and may impact endocrine functions on organisms. Thyroid hormones (THs) play fundamental roles in regulating the basal metabolism and energy balance, while thyroid function can be impaired by environmental contaminants. Therefore, it is crucial to clarify the effects and mechanisms of OTPs on hypothalamus-pituitary-thyroid (HPT) axis. In this study, Xenopus laevis tadpoles at stage 51 were exposed to fentin hydroxide and fenbutatin oxide (0.04, 0.20 and 1.00 μg·L^-1) for 21 days. It was found that both compounds caused inhibitory effects on metamorphic development of tadpoles (e.g., significant decrease in hindlimb length and retarding development). Triiodothyronine (T₃) significantly decreased in tadpoles exposed to 0.20 μg/L and 1.00 μg/L of the two OTPs for 14 days or 21 days. The expressions of TH responsive genes trβ, bteb and dio2 were down-regulated, while tshβ and slc5a5 were up-regulated. Surface plasmon resonance (SPR) binding assays showed that fentin hydroxide had a moderate affinity to recombinant human thyroid hormone receptor β but fenbutatin oxide did not have. Result of the SPR assay was highly consistent with the luciferase reporter gene assays that fentin hydroxide suppressed the relative luciferase activity in the presence of T₃ while fenbutatin oxide did not, demonstrating fentin hydroxide but not fenbutatin oxide displayed an antagonistic activity against T₃-TR complex mediated transcriptional activation. Overall, the findings elucidated the mechanisms induced by OTPs along HPT axis. These results highlighted the adverse influences of organotin pesticides on thyroid hormone- dependent development in vertebrates and the need for more comprehensive investigations of their potential ecological risks.

Integrated thyroid endocrine disrupting effect on zebrafish (Danio rario) larvae via simultaneously repressing type II iodothyronine deiodinase and activating thyroid receptor-mediated signaling following waterborne exposure to trace azocyclotin

As a widely used organotin acaricide nowadays, azocyclotin (ACT) could induce thyroidal endocrine disruption in fishes and amphibians, but its dominant disrupting mode remains unknown. In this study, zebrafish were firstly exposed to ACT (0.18-0.36 ng/mL) from 2 hpf (hours post fertilization) to 30 dpf (days post fertilization), and a series of developmental toxicological endpoints and thyroid hormones were measured. Result showed that no developmental toxicity to zebrafish was found in 0.18 and 0.24 ng/mL groups except decreased body weight (30 dpf, 0.24 ng/mL). However, exposed to 0.36 ng/mL ACT led to reductions in heartbeat (48 hpf), hatching rate (72 hpf) and bodyweight (30 dpf). General tendencies of decreases in free T3 but increases in free T4 and reductions in ratio of free T3/T4 were also found, inferring that type II deiodinase (Dio2) was repressed. This inference was confirmed by Western analysis that Dio2 expression reduced by 42.7% after 0.36 ng/mL ACT treatment. Moreover, RNA-Seq analysis implied that exposed to 0.36 ng/mL ACT altered the genome-wide gene expression profiles of zebrafish. Totally 5660 genes (involving 3154 down-regulated and 2596 up-regulated genes) were differentially expressed, and 13 deferentially expressed genes including down-regulated dio2 were significantly enriched in thyroid hormone signaling pathway. Subsequently, an in vitro thyroid receptor-reporter gene assay using GH3 cells was performed to further explore the potential disrupting mechanism. Result showed that luciferase activity slightly increased after exposure to ACT alone or ACT combined with low level T3, but was suppressed when combined with high level T3. It indicted there probably existed a competitive relationship in some extent between ACT and T3 in vivo. Overall, the present study provided preliminary evidences that long-term exposure to trace ACT repressed Dio2 expression, declined T3 and then activated thyroid receptor-mediated signaling, thereby leading to integrated thyroid endocrine disruption in zebrafish larvae.

Transcriptomic analysis reveals potential mechanisms of toxicity in a combined exposure to dibutyl phthalate and diisobutyl phthalate in zebrafish (Danio rerio) ovary

Phthalate esters (PAEs), which are notable plasticizers, can be prolific contaminants in aquatic environments, and have been shown to induce reproductive toxicity. However, the studies concerning their toxicity towards aquatic species are based on individual chemicals, and the combined toxicity of PAEs to aquatic organisms remains unclear. The aim of this study was to explore the potential toxicity mechanisms associated with combined exposure to dibutyl phthalate (DBP) and diisobutyl phthalate (DiBP) in adult female zebrafish ovaries. Zebrafish were exposed to DBP, DiBP and their mixtures for 30 days, and their effects on ovarian histology, plasma sex hormones and ovarian transcriptomics were investigated. Plasma estradiol (E2) levels were significantly decreased by 38.9% in the DBP-1133 exposure group and 41.0% in the DiBP-1038 exposure group. The percentage of late/mature oocytes was also significantly decreased by 17.3% under DBP-1133 exposure and 16.2% under DiBP-1038 exposure, while that under combined exposure was not significantly affected. Nevertheless, transcriptome sequencing revealed 2564 differentially expressed genes (DEGs) in zebrafish ovaries after exposure to the mixtures. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that the DEGs were involved in the neuroactive ligand-receptor interaction, GnRH, progesterone-mediated oocyte maturation, oocyte meiosis and steroid hormone biosynthesis signaling pathways. These results revealed that combined exposure exerts potential reproductive toxicity at the molecular level.

Endocrine disrupting effects of tebuconazole on different life stages of zebrafish (Danio rerio)

Tebuconazole is a widely used fungicide that has been detected in water ecosystems, of which the concentrations may affect the endocrine function of aquatic organisms. At present study, tissue-specific bioaccumulation of tebuconazole was found in ovary of adult zebrafish, indicating a potential risk of endocrine disruption. In order to evaluate the potential endocrine disrupting effects, three life stages (2 hpf (hours post-fertilization) -60 dpf (days post-fertilization), Stage I; 60-120 dpf, Stage II; 180-208 dpf, Stage III) of zebrafish (Danio rerio) were chronically exposed to tebuconazole at the concentrations ranging from 0.05 mg/L to 1.84 mg/L. Result showed that exposed to tebuconazole could lead to a male-biased sex differentiation in juvenile zebrafish and significant decrease of the percentage of germ cells in sexually-mature zebrafish. Egg production was significantly inhibited by 57.8% and 19.2% after Stage II- and Stage III-exposures, respectively. The contents of 17β-estradiol in gonad decreased by 63.5% when exposed to 0.20 mg/L tebuconazole at Stage II and by 49.5% after exposed to 0.18 mg/L tebuconazole at Stage III, respectively. For all stages exposure, reductions in 17β-estradiol/testosterone ratio were observed, indicating an imbalance in steroids synthesis. Additionally, tebuconazole reduced the expression of cyp19a, which was consistent with the decrease of E2 level. In overall, the present findings indicated that, playing as an anti-estrogen-like chemical, tebuconazole inhibited the expression of Cyp19, thereby impairing steroid hormones biosynthesis, leading to a diminished fecundity of zebrafish.

Parental exposure to tebuconazole causes thyroid endocrine disruption in zebrafish and developmental toxicity in offspring

Azole fungicides are one class of the most extensively applied current-use pesticides. Tebuconazole is a common azole fungicide that has been frequently detected in aquatic ecosystems, thus raising concerns about its ecological safety. However, adverse effects of tebuconazole remain largely unknown, especially with regard to endocrine function in aquatic organisms. In the present study, sexually immature zebrafish were exposed to different concentrations of tebuconazole (0.05, 0.20 and 0.50 mg/L) for 60 days in order to test for transgenerational toxicity on the thyroid endocrine system. Thyroid hormone homeostasis, neuronal, and cardiovascular development were investigated in the F1 generation, which were reared in tebuconazole-free water. In the F0 generation, exposure to 0.20 and 0.50 mg/L tebuconazole reduced both thyroxine (T₄) and 3,5,3'-triiodothyronine (T₃) levels in females, while the T₃ levels were unchanged in males. Decreased heart rate was found in F1 larvae, as well as diminished T₄ levels in F1 eggs/larvae. We also observed significantly increased expression of ugt1ab mRNA in two generations of zebrafish. Moreover, expression of mRNA associated with neuronal development (e.g. α1-tubulin, mbp, gap43) and cardiovascular development (e.g. cacna1ab, tnncal) were significantly downregulated in F1 larvae at 5 and 10 dpf. In addition, tebuconazole was detected in F1 eggs following parental exposure, indicating maternal transfer. This study demonstrated that tebuconazole can be transferred to offspring from exposed parents, causing thyroid endocrine disruption and developmental toxicity.

Simultaneous determination of three organotin pesticides in fruits and vegetables by high-performance liquid chromatography/tandem mass spectrometry

RATIONALE

The presence of organotins in the environment affects food safety, making it important to monitor the levels of organotin pesticides (OTPs) in fruit and vegetable samples.

METHODS

In the present study, a simple and low cost method for simultaneous determination of three OTPs (azocyclotin, fenbutatin oxide and triphenyltin hydroxide) in vegetable and fruit samples was developed and validated, based on solid-phase extraction and liquid chromatography/tandem mass spectrometry.

RESULTS

Extraction with acetonitrile containing 0.2% formic acid positively affected the recoveries of the three OTPs. Moreover, the simultaneous purification of the three OTPs was the most efficient using mixed-mode cation-exchange cartridges and 5.0% ammonium hydroxide in methanol as eluent, and, in this case, mild matrix effects (-9.3% to 21.6%) were obtained for the three OTPs monitored. The developed method reached limits of quantification of 1 μg kg^-1 , and linearity was satisfactory, with correlation coefficients >0.995. A fortification study showed that when spiked at 1.0-50.0 μg kg^-1 the mean trueness values were from 72.3 to 110.0% in all matrices (three vegetables and three fruits). The intra-day precision was <14.1%, and the inter-day precision (n = 11) was <18.2%.

CONCLUSIONS

The proposed method was successfully applied to the simultaneous analysis of three OTPs in vegetables and fruits.

Parental transfer of ethylhexyl methoxy cinnamate and induced biochemical responses in zebrafish

Ethylhexyl methoxy cinnamate (EHMC) is one of the major organic ultraviolet (UV) filter pollutants in the environment. The purpose of this study was to investigate the parental transfer of EHMC and induced biochemical responses in zebrafish (Danio rerio). Zebrafish embryos were exposed to EHMC solution (1, 10, and 100 μg/L) for 4 months until sexual maturation. Then male and female parents were paired to lay eggs. F1 generations were divided into 2 categories: with and without continued EHMC exposure. EHMC was detected in both F0 parents and F1 eggs, indicating that EHMC can accumulate in zebrafish and transfer to offspring through reproduction. The hatching rate decreased and malformation rate increased significantly among parents and progeny embryos in the high concentration exposure group. For 40 dpf (days post-fertilisation) F0 generations, estradiol hormone and vitellogenin (Vtg) contents, the expression levels of Vtg1, P450 aromatase (Cyp19a and Cyp19b), 17β-hydroxysteroid dehydrogenase (Hsd17b1, Hsd17b3), estrogen receptor-alpha and progesterone receptor in all concentration groups decreased significantly, while androgen receptor increased significantly in 10 and 100 μg/L exposure groups compared with the corresponding control group, showing anti-estrogen and androgen effects. For 120 dpf F0 generations, acetylcholinesterase activity was significantly decreased and glutathione and malondialdehyde levels, superoxide dismutase, catalase and glutathione reductase activities were significantly increased in all treatment groups compared with the corresponding control group. In addition, F1 offspring with or without continued exposure to EHMC suffered similar or stronger oxidative stress compared with their parents. DNA breakage and apoptosis also occurred in 120 dpf parental liver cells in all treatment groups as a result of oxidative damage. Results suggested that EHMC have transfer effects between parents and offspring, which may cause negative effects on growth and development of zebrafish and induce biochemical responses in both parents and offspring.

Tributyltin impaired reproductive success in female zebrafish through disrupting oogenesis, reproductive behaviors and serotonin synthesis

Tributyltin (TBT), an organotin acting as aromatase (Cyp19a1) inhibitor, has been found to disrupt gametogenesis and reproductive behaviors in several fish species. However, few studies addressing the mechanisms underlying the impaired gametogenesis and reproduction have been reported. In this study, female adults of zebrafish (Danio rerio) were continuously exposed to two nominal concentrations of TBT (100 and 500 ng/L, actual concentrations: 90.8 ± 1.3 ng/L and 470.3 ± 2.7 ng/L, respectively) for 28 days. After exposures, TBT decreased the total egg number, reduced the hatchability and elevated the mortality of the larvae. Decreased gonadosomatic index (GSI) and altered percentages of follicles in different developmental stages (increased early-stage follicles and reduced mid/late-stage follicles) were also observed in the ovary of TBT-treated fish. TBT also lowered the plasma level of 17β-estradiol and suppressed the expressions of cyp19a1a in the ovary. In treated fish, up-regulated expressions of aldhla2, sycp3 and dmc1 were present in the ovary, indicating an enhanced level of meiosis. The mRNA level of vtg1 was dramatically suppressed in the liver of TBT-treated fish, suggesting an insufficient synthesis of Vtg protein, consistent with the decreased percentage of mid/late-stage follicles in the ovaries. Moreover, TBT significantly suppressed the reproductive behaviors of the female fish (duration of both sexes simultaneously in spawning area, the frequency of meeting and the visit in spawning area) and down-regulated the mRNA levels of genes involved in the regulation of reproductive behaviors (cyp19a1b, gnrh-3 and kiss 2) in the brain. In addition, TBT significantly suppressed the expressions of serotonin-related genes, such as tph2 (encoding serotonin synthase), pet1 (marker of serotonin neuron) and kiss 1 (the modulator of serotonin synthesis), suggesting that TBT might disrupt the non-reproductive behaviors of zebrafish. The present study demonstrated that TBT may impair the reproductive success of zebrafish females probably through disrupting oogenesis, disturbing reproductive behaviors and altering serotonin synthesis. The present study greatly extends our understanding on the reproductive toxicity of TBT on fish.

The anti-estrogenicity of chronic exposure to semicarbazide in female Japanese flounders (Paralichthys olivaceus), and its potential mechanisms

This study investigated the anti-estrogenic effects of chronic exposure to a new marine pollutant, semicarbazide (SMC; 1, 10, and 100μg/L), in female Paralichthys olivaceus, as well as the associated mechanism. After 130days of exposure, plasma 17β-estradiol and testosterone concentrations, and hepatic estrogen receptors, vitellogenin, and choriogenin mRNA levels decreased significantly in SMC-exposed groups. Moreover, down-regulation of genes in the hypothalamic-pituitary-gonadal axis, including gonadotropin-releasing hormone, gonadotropic hormones and their receptors, the steroidogenic acute regulatory protein, 17α-hydroxylase, 17β-hydroxysteroid dehydrogenase, and cytochrome P450 19A, was observed after SMC exposure. Furthermore, the kisspeptin/g protein-coupled receptor 54 (kiss/gpr54) system and gamma-aminobutyric acid-ergic (GABAergic) system were also affected by SMC: SMC significantly down-regulated mRNA expression of kiss2, gpr54, and the GABA synthesis enzyme gad67. Our results demonstrated for the first time that environmentally relevant concentrations of SMC exerted anti-estrogenicity in female flounders, providing theoretical support for ecological risk assessments of SMC in marine environments.

Organotin Exposure and Vertebrate Reproduction: A Review

Organotin (OTs) compounds are organometallic compounds that are widely used in industry, such as in the manufacture of plastics, pesticides, paints, and others. OTs are released into the environment by anthropogenic actions, leading to contact with aquatic and terrestrial organisms that occur in animal feeding. Although OTs are degraded environmentally, reports have shown the effects of this contamination over the years because it can affect organisms of different trophic levels. OTs act as endocrine-disrupting chemicals (EDCs), which can lead to several abnormalities in organisms. In male animals, OTs decrease the weights of the testis and epididymis and reduce the spermatid count, among other dysfunctions. In female animals, OTs alter the weights of the ovaries and uteri and induce damage to the ovaries. In addition, OTs prevent fetal implantation and reduce mammalian pregnancy rates. OTs cross the placental barrier and accumulate in the placental and fetal tissues. Exposure to OTs in utero leads to the accumulation of lipid droplets in the Sertoli cells and gonocytes of male offspring in addition to inducing early puberty in females. In both genders, this damage is associated with the imbalance of sex hormones and the modulation of the hypothalamic-pituitary-gonadal axis. Here, we report that OTs act as reproductive disruptors in vertebrate studies; among the compounds are tetrabutyltin, tributyltin chloride, tributyltin acetate, triphenyltin chloride, triphenyltin hydroxide, dibutyltin chloride, dibutyltin dichloride, diphenyltin dichloride, monobutyltin, and azocyclotin.