Tris(1,3-dichloro-2-propyl) Phosphate Exposure During the Early-Blastula Stage Alters the Normal Trajectory of Zebrafish Embryogenesis

Molecular insight on the binding of halogenated organic phosphate esters to human serum albumin and its effect on cytotoxicity of halogenated organic phosphate esters

Halogenated Organic Phosphate Esters (OPEs) are commonly found in plasticizers and flame retardants. However, they are one kind of persistent contaminants that can pose a significant threat to human health and ecosystem as new environmental estrogen. In this study, two representative halogenated OPEs, tris(1,3-dichloro-2-propyl) phosphate (TDCP) and tris(2,3-dibromopropyl) phosphate (TDBP), were selected as experimental subjects to investigate their interaction with human serum albumin (HSA). Despite having similar structures, the two ligands exhibited contrasting effects on enzyme activity of HSA, TDCP inhibiting enzyme activity and TDBP activating it. Furthermore, both TDCP and TDBP could bind to HSA at site I, interacted with Arg222 and other residues, and made the conformation of HSA unfolded. Thermodynamic parameters indicated the main driving forces between TDBP and HSA were hydrogen bonding and van der Waals forces, while TDCP was mainly hydrophobic force. Molecular simulations found that more hydrogen bonds of HSA-TDBP formed during the binding process, and the larger charge area of TDBP than TDCP could partially account for the differences observed in their binding abilities to HSA. Notably, the cytotoxicity of TDBP/TDCP was inversely proportional to their binding ability to HSA, implying a new method for determining the cytotoxicity of halogenated OPEs in vitro.

Tris(2-chloroethyl) phosphate (TCEP) exposure inhibits the epithelial-mesenchymal transition (EMT), mesoderm differentiation, and cardiovascular development in early chicken embryos

Tris(2-chloroethyl) phosphate (TCEP) is an organophosphorus flame retardant used worldwide and has been detected in the tissues and eggs of wild birds. Our previous study reported that exposure to TCEP induced developmental delay and cardiovascular dysfunction with attenuated heart rate and vasculogenesis in early chicken embryos. This study aimed to investigate the molecular mechanisms underlying the cardiovascular effects of TCEP on chicken embryos using cardiac transcriptome analysis and to examine whether TCEP exposure affects epithelial-mesenchymal transition (EMT) and mesoderm differentiation during gastrulation. Transcriptome analysis revealed that TCEP exposure decreased the expression of cardiac conduction-related genes and transcription factors on day 5 of incubation. In extraembryonic blood vessels, the expression levels of genes related to fibroblast growth factor (FGF) and vascular endothelial growth factor (VEGF) were significantly reduced by TCEP exposure and vasculogenesis was suppressed. TCEP exposure also attenuated Snail family transcriptional repressor 2 (SNAI2) and T-box transcription factor T (TBXT) signaling in the chicken primitive streak, indicating that TCEP inhibits EMT and mesoderm differentiation during gastrulation at the early developmental stage. These effects on EMT and mesoderm differentiation may be related to subsequent phenotypic defects, including suppression of heart development and blood vessel formation.

Flame retardant tetrabromobisphenol A (TBBPA) disrupts histone acetylation during zebrafish maternal-to-zygotic transition

3,3',5.5'-Tetrabromobisphenol A (TBBPA) is a widely used brominated flame-retardant utilized in the production of electronic devices and plastic paints. The objective of this study is to use zebrafish as a model and determine the effects of TBBPA exposure on early embryogenesis. We initiated TBBPA exposures (0, 10, 20 and 40μM) at 0.75 h post fertilization (hpf) and monitored early developmental events such as cleavage, blastula and epiboly that encompass maternal-to-zygotic transition (MZT) and zygotic genome activation (ZGA). Our data revealed that TBBPA exposures induced onset of developmental delays by 3 hpf (blastula). By 5.5 hpf (epiboly), TBBPA-exposed (10-20 μM) embryos showed concentration-dependent developmental lag by up to 3 stages or 100% mortality at 40 μM. Embryos exposed to sublethal TBBPA concentrations from 0.75-6 hpf and raised in clean water to 120 hpf showed altered larval photomotor response (LPR), suggesting a compromised developmental health. To examine the genetic basis of TBBPA-induced delays, we conducted mRNA-sequencing on embryos exposed to 0 or 40 μM TBBPA from 0.75 hpf to 2, 3.5 or 4.5 hpf. Read count data showed that while TBBPA exposures had no overall impacts on maternal or maternal-zygotic genes, collective read counts for zygotically activated genes were lower in TBBPA treatment at 4.5 hpf compared to time-matched controls, suggesting that TBBPA delays ZGA. Gene ontology assessments for both time- and stage-matched differentially expressed genes revealed TBBPA-induced inhibition of chromatin assembly- a process regulated by histone modifications. Since acetylation is the primary histone modification system operant during early ZGA, we immunostained embryos with an H3K27Ac antibody and demonstrated reduced acetylation in TBBPA-exposed embryos. Leveraging in silico molecular docking studies and in vitro assays, we also showed that TBBPA potentially binds to P300- a protein that catalyzes acetylation- and inhibits P300 activity. Finally, we co-exposed embryos to 20 μM TBBPA and 50 μM n-(4-chloro-3-trifluoromethyl-phenyl)-2-ethoxy-6-pentadecyl-benzamide (CTPB) -a histone acetyltransferase activator that promotes histone acetylation- and showed that TBBPA-CTPB co or pre-exposures significantly reversed TBBPA-only developmental delays, suggesting that TBBPA-induced phenotypes are indeed driven by repression of histone acetylation. Collectively, our work demonstrates that TBBPA disrupts ZGA and early developmental morphology, potentially by inhibiting histone acetylation. Future studies will focus on mechanisms of TBBPA-induced chromatin modifications.

Exposure to tris (1,3-dichloro-2-propyl) phosphate affects the embryonic cardiac development of Oryzias melastigma

Tris (1,3-dichloro-2-propyl) phosphate (TDCPP) is a growing concern and may be a potential risk to marine environmental health due to its widespread usage and distribution. However, the toxic effects of TDCPP on cardiac development in marine fish have not been reported. In this study, Oryzias melastigma embryos were exposed to TDCPP at doses of 0, 0.04, 0.4, 4 and 40 μg/L from early embryogenesis to 10 days postfertilization (dpf). Then, the heart rate and sinus venosus-bulbus arteriosus (SV-BA) distance of the exposed embryos were measured at 5, 6, 8 and 10 dpf. Furthermore, alterations in the mRNA levels of the genes encoding cyclooxygenase-2 (COX-2), bone morphogenetic protein 4 (BMP4), fibroblast growth factor 8 (FGF8), and GATA-binding protein 4 (GATA4) were evaluated at 5, 6, 8 and 10 dpf. We found that the heart rate significantly increased in all TDCPP exposure groups at 10 dpf. The SV-BA distance significantly decreased in all TDCPP exposure groups at all developmental stages (except for the 0.4 μg/L group at 5 dpf and the 4 μg/L group at 10 dpf). The mRNA expression of COX-2 was downregulated at 5 dpf, BMP4 was downregulated at 5 and 6 dpf, FGF8 was downregulated at 5, 6 and 8 dpf, GATA4 was downregulated at 8 dpf, and GATA4 was upregulated at 10 dpf. These results indicate that the changes in heart rate and SV-BA distance might be accompanied by disturbances in the four genes involved in cardiac development. Our findings will help to illustrate the possible cardiac toxic effects of marine fish exposed to TDCPP.

PFOS and F-53B disrupted inner cell mass development in mouse preimplantation embryo

Perfluorooctane sulfonic acid (PFOS) is a perfluoroalkyl and polyfluoroalkyl substance (PFAS) widely used in daily life. As its toxicity was confirmed, it has been gradually substituted by F-53B (chlorinated polyfluoroalkyl sulfonates, Cl-PFESAs) in China. PFOS exposure during prenatal development may hinder the development of preimplantation embryos, as indicated by recent epidemiological research and in vivo assays. However, the embryotoxicity data for F-53B are scarce. Furthermore, knowledge about the toxicity of F-53B and PFOS exposure to internal follicular fluid concentrations on early preimplantation embryo development remains limited. In this study, internal exposure concentrations of PFOS (10 nM) and F-53B (2 nM) in human follicular fluid were chosen to study the effects of PFAS on early mouse preimplantation embryo development. We found that both PFOS and F-53B treated zygotes exhibited higher ROS activity in 8-cell embryos but not in 2-cell stage embryos. PFOS and F-53B significantly affected the proportion and aggregation of the inner cell mass (ICM) in the blastocyst, but not the total cell number. Mouse embryonic stem cells (mESCs, isolated from the ICM) and embryoid body (EB) assays were employed to assess the toxicity of PFOS and F-53B on the development and differentiation of embryonic pluripotent cells. These results suggested that mESCs exhibited more DNA damage and abnormal germ layer differentiation after brief exposure to PFOS or F-53B. Finally, RNA-sequencing revealed that PFOS and F-53B exposure affected mESCs biosynthetic processes and chromatin-nucleosome assembly. Our results indicate that F-53B has potential risks as an alternative to PFOS, which disrupts ICM development and differentiation.

Competition for H2A.Z underlies the developmental impacts of repetitive element de-repression

The histone variant H2A.Z is central to early embryonic development, determining transcriptional competency through chromatin regulation of gene promoters and enhancers. In addition to genic loci, we find that H2A.Z resides at a subset of evolutionarily young repetitive elements, including DNA transposons, long interspersed nuclear elements and long terminal repeats, during early zebrafish development. Moreover, increases in H2A.Z occur when repetitive elements become transcriptionally active. Acquisition of H2A.Z corresponds with a reduction in the levels of the repressive histone modification H3K9me3 and a moderate increase in chromatin accessibility. Notably, however, de-repression of repetitive elements also leads to a significant reduction in H2A.Z over non-repetitive genic loci. Genic loss of H2A.Z is accompanied by transcriptional silencing at adjacent coding sequences, but remarkably, these impacts are mitigated by augmentation of total H2A.Z protein via transgenic overexpression. Our study reveals that levels of H2A.Z protein determine embryonic sensitivity to de-repression of repetitive elements, that repetitive elements can function as a nuclear sink for epigenetic factors and that competition for H2A.Z greatly influences overall transcriptional output during development. These findings uncover general mechanisms in which counteractive biological processes underlie phenotypic outcomes.

Established and emerging organophosphate esters (OPEs) and the expansion of an environmental contamination issue: A review and future directions

The list of organophosphate esters (OPEs) reported in the environment continues to expand as evidenced by the increasing number of OPE studies in the literature. However, there remains a general dearth of information on more recently produced and used OPEs that are proving to be emerging environmental contaminants. The present review summarizes the available studies in a systematic framework of the current state of knowledge on the analysis, environmental fate, and behavior of emerging OPEs. This review also details future directions to better understand emerging OPEs in the environment. Firstly, we make recommendations that the current structural/practical abbreviations and naming of OPEs be revised and updated. A chemical database (CDB) containing 114 OPEs is presently established based on the suspect list from the current scientific literature. There are 12 established OPEs and a total of 83 emerging OPEs that have been reported in human and/or biota samples. Of the emerging OPEs more than 80% have nearly 100% detection frequencies in samples of certain environmental media including indoor air, wastewater treatment plants, sediment, and fish. In contrast to OPEs considered established contaminants, most emerging OPEs have been identified more recently due to the more pervasive use of high-resolution mass spectrometry (HRMS) based approaches and especially gas or liquid chromatography coupled with HRMS-based non-target analysis (NTA) of environmental sample fractions. Intentional/unintentional industrial use and non-industrial formation are sources of emerging OPEs in the environment. Predicted physical-chemical properties in silico of newer, molecularly larger and more oligomeric OPEs strongly suggest that some compounds such as bisphenol A diphenyl phosphate (BPA-DPP) are highly persistent, bioaccumulative and/or toxic. Limited information on laboratory-based toxicity data has shown that some emerging OPEs elicit harmful effects such as cytotoxicity, development toxicity, hepatotoxicity, and endocrine disruption in exposed humans and mammals. Established, and to a much lesser degree emerging OPEs, have also been shown to transform and degrade in biota and possibly alter their toxicological effects. Research on emerging OPE contaminants is presently limited and more study is warranted on sample analysis methods, source apportionment, transformation processes, environmental behavior, biomarkers of exposure and toxicity.

Developmental toxicity in early chicken embryos on exposure to an organophosphorus flame retardant, tris(2-chloroisopropyl) phosphate

Tris(2-chloroisopropyl) phosphate (TCIPP) is an organophosphate flame retardant detected in the environment and eggs, feathers, and livers. Early-developmental-stage avian embryos are vulnerable to the toxic effects of chemicals. However, studies on the specific effects of TCIPP on avian embryonic development are limited. We aimed to investigate the toxicity of TCIPP in early chicken embryos using a previously developed shell-less incubation system. Fertilized chicken (Gallus gallus domesticus) eggs (n = 220) were exposed to 50 or 500 nmol TCIPP/(g egg) or dimethyl sulfoxide (DMSO) as a vehicle control on Day 0 of incubation. Development of 198 embryos was monitored from Days 3-9 of incubation, and 22 embryos on Day 4 and 74 embryos on Day 9 were dissected. Messenger RNA expression levels for several genes were measured in embryos on Day 4. Both TCIPP-exposed groups showed a significant reduction in survival rate. Imaging analyses revealed significant decreases in body length, head and bill length, eye diameter, and forelimb and hindlimb length in both TCIPP-treated groups. TCIPP exposure significantly impaired the development of extraembryonic blood vessels and the production of red blood cells. A TCIPP-dose-dependent decreasing trend in heart rate was observed on Days 4-7. The somitic angle increased significantly on Days 4-6, and embryos with curved somites showed cleavage in the back and gaps between somites, resulting in asymmetrical somite formation. A significant correlation was found between the somitic angle and FGF8 expression levels, suggesting that TCIPP exposure affects somite formation through an altered FGF-signaling pathway. Embryos with somitic deformities in TCIPP-exposed groups had significantly reduced survival rates, indicating that abnormal segment formation directly increased mortality. Finally, eye weight and ocular luminosity values were significantly reduced, suggesting that TCIPP may also affect eye development. Overall, these findings highlight severe toxic effects of TCIPP on avian embryonic development, including in vascularization, cardiac function, and somite and ocular development.

Tris(1,3-dichloro-2-propyl) phosphate causes female-biased growth inhibition in zebrafish: Linked with gut microbiota dysbiosis

Tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) is ubiquitous in aquatic environment, but its effect on intestinal health of fish has yet not been investigated. In the present study, the AB strain zebrafish embryos were exposed to environmentally realistic concentrations (0, 30, 300, and 3000 ng·L^-1) of TDCIPP for 90 days, after which the fish growth and physiological activities were evaluated, and the intestinal microbes were analyzed by 16S rRNA gene high-throughput sequencing. Our results manifested that the body length and body weight were significantly reduced in the female zebrafish but not in males. Further analyses revealed that TDCIPP resulted in notable histological injury of intestine, which was accompanied by impairment of epithelial barrier integrity (decreased tight junction protein 2), inflammation responses (increased interleukin 1β), and disruption of neurotransmission (increased serotonin) in female intestine. Male intestines maintained intact intestinal structure, and the remarkably increased activity of glutathione peroxidase (GPx) might protect the male zebrafish from inflammation and intestinal damage. Furthermore, 16S rRNA sequencing analysis showed that TDCIPP significantly altered the microbial communities in the intestine in a gender-specific manner, with a remarkable increase in alpha diversity of the gut microbiome in male zebrafish, which might be another mechanism for male fish to protect their intestines from damage by TDCIPP. Correlation analysis revealed that abnormal abundances of pathogenic bacteria (Chryseobacterium, Enterococcus, and Legionella) might be partially responsible for the impaired epithelial barrier integrity and inhibition in female zebrafish growth. Taken together, our study for the first time demonstrates the high susceptibility of intestinal health and gut microbiota of zebrafish to TDCIPP, especially for female zebrafish, which could be partially responsible for the female-biased growth inhibition.

Tris(1,3-dichloro-2-propyl) Phosphate Inhibits Early Embryonic Development by Binding to Gsk-3β Protein in Zebrafish

Recently, several studies have reported that exposure to tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) results in abnormal development of zebrafish embryos in blastocyst and gastrula stages, but molecular mechanisms are still not clear. This lacking strongly affects the interspecific extrapolation of embryonic toxicity induced by TDCIPP and hazard evaluation. In this study, zebrafish embryos were exposed to 100, 500 or 1000 μg/L TDCIPP, and 6-bromoindirubin-3'-oxime (BIO, 35.62 μg/L) was used as a positive control. Results demonstrated that treatment with TDCIPP or BIO caused an abnormal stacking of blastomere cells in mid blastula transition (MBT) stage, and subsequently resulted in epiboly delay of zebrafish embryos. TDCIPP and BIO up-regulated the expression of β-catenin protein and increased its accumulation in nuclei of embryonic cells. This accumulation was considered as a driver for early embryonic developmental toxicity of TDCIPP. Furthermore, TDCIPP and BIO partly shared the same modes of action, and both of them could bind to Gsk-3β protein, and then decreased the phosphorylation level of Gsk-3β in TYR·216 site and lastly inhibited the activity of Gsk-3β kinase, which was responsible for the increased concentrations of β-catenin protein in embryonic cells and accumulation in nuclei. Our findings provide new mechanisms for clarifying the early embryonic developmental toxicity of TDCIPP in zebrafish.

Transcriptomic and metabolomic analyses revealed epiboly delayed mechanisms of 2,5-dichloro-1, 4-benuinone on zebrafish embryos

2,5-Dichloro-1,4-benzenediol (2,5-DCBQ) is a putative disinfection by-product that belongs to the halogenated benzoquinone class. However, its developmental toxicity and related mechanism remained unclarified. In our study, we used zebrafish embryos as the model and exposed them to graded concentrations of 2,5-DCBQ (100, 200, 300, 400 μg/L). We found that the rate of epiboly abnormalities increased significantly in a concentration-dependent manner. The results of whole-mount in situ hybridization (WISH) indicated that the expression patterns and levels of chordin (dorsoventral marker), foxa2 (endodermal marker), eve1 (ventral mesodermal marker), and foxb1a (ectodermal marker) were altered, suggesting that 2,5-DCBQ might affect the germ layer development of zebrafish embryos. Integrated transcriptomic and metabolomic analyses were adopted to explore the molecular mechanisms of embryonic developmental delays. The results showed that 2,5-DCBQ exposure induced 1163 differentially expressed genes (DEGs) and 37 differential metabolites (DEMs). Bioinformatic analysis enriched the most affected molecular pathways (Wnt signaling pathway, cell adhesion molecules, actin cytoskeleton regulation) and metabolic pathways (purine metabolism, aminoacyl-tRNA biosynthesis, arginine and proline metabolism) in zebrafish embryos. To summarize, our findings broadened the molecular mechanisms of 2,5-DCBQ embryotoxicity through multi-omics and bioinformatic analyses.

Bioaccumulation of organophosphorus flame retardants in marine organisms in Liaodong Bay and their potential ecological risks based on species sensitivity distribution

Although organophosphorus flame retardants (OPFRs) in aquatic environments have received increasing concern, little information is available on their bioaccumulation and trophic transfer in marine food webs. Consequently, the risks of OPFRs to marine ecosystems are unknown. In this study, seven OPFR compounds in marine biological samples collected from Liaodong Bay, Bohai Sea, were analyzed to evaluate their level and biological amplification effect in the marine food web. The total OPFRs of marine organisms in Liaodong Bay ranged from 2.60 to 776 ng/g ww, and lipids were critical factors affecting the concentration of OPFRs in marine species. Tris (2-ethylhexyl) phosphate (TEHP) and tris(1-chloro-2-propyl) phosphate (TCIPP) were the OPFRs most frequently detected in marine species. Still, tris(2-chloroethyl) phosphate (TCEP) was dominant in most marine species (16/24), and the content of chlorinated OPFRs was highest. At the same time, alkyl OPFRs and aryl OPFRs accounted for the same proportion. No correlation between OPFR concentration and the trophic level was observed in marine organisms from Liaodong Bay. It was shown in the results of the species sensitivity distribution that TCIPP in Chinese seawater does not pose a potential ecological risk to marine species. However, much work remains to be done on accumulating information and the ecological risks of OPFRs in different marine food webs.

Tris(1,3-dichloro-2-propyl) phosphate disrupts the trajectory of cytosine methylation within developing zebrafish embryos

Tris (1,3-dichloro-2-propyl) phosphate (TDCIPP) is an organophosphate ester-based flame retardant widely used within the United States. Within zebrafish, initiation of TDCIPP exposure at 0.75 h post-fertilization (hpf) reliably disrupts cytosine methylation from cleavage (2 hpf) through early-gastrulation (6 hpf). Therefore, the objective of this study was to determine whether TDCIPP-induced effects on cytosine methylation persist beyond 6 hpf. First, we exposed embryos to vehicle or TDCIPP from 0.75 hpf to 6, 24, or 48 hpf, and then conducted bisulfite amplicon sequencing of a target locus (lmo7b) using genomic DNA derived from whole embryos. Within both vehicle- and TDCIPP-treated embryos, CpG methylation was similar at 6 hpf and CHG/CHH methylation were similar at 24 and 48 hpf (relative to 6 hpf). However, relative to 6 hpf within the same treatment, CpG methylation was lower within vehicle-treated embryos at 48 hpf and TDCIPP-treated embryos at 24 and 48 hpf - an effect that was driven by acceleration of CpG hypomethylation. Similar to our previous findings with DNA methyltransferase, we found that, even at high μM concentrations, TDCIPP had no effect on zebrafish and human thymine DNA glycosylase activity (a key enzyme that decreases CpG methylation), suggesting that TDCIPP-induced effects on CpG methylation are not driven by direct interaction with thymine DNA glycosylase. Finally, using 5-methylcytosine (5-mC)-specific whole-mount immunochemistry and automated imaging, we found that exposure to TDCIPP increased 5-mC abundance within the yolk of blastula-stage embryos, suggesting that TDCIPP may impact cytosine methylation of maternally loaded mRNAs during the maternal-to-zygotic transition. Overall, our findings suggest that TDCIPP disrupts the trajectory of cytosine methylation during zebrafish embryogenesis, effects which do not appear to be driven by direct interaction of TDCIPP with key enzymes that regulate cytosine methylation.

Tetrabromobisphenol A perturbs cell fate decisions via BMP signaling in the early embryonic development of zebrafish

Tetrabromobisphenol A (TBBPA) readily accumulates in the egg yolk of aquatic oviparous animals and is transferred to their embryos. Early embryogenesis is vital for organ formation and subsequent development. The developmental toxicity of TBBPA in aquatic animals has been extensively reported. However, few studies have assessed the toxic effects of TBBPA in the early embryonic development. In this work, we found that TBBPA perturbed cell fate decisions along the dorsal-ventral (DV) axis during gastrulation, further disrupting early organogenesis in the entire embryo. TBBPA exposure increased the number of embryonic cells that acquired a ventral cell fate, which formed epidermis, blood and heart tissues. In return, the number of embryonic cells that acquired a dorsal cell fate was greatly decreased, causing the TBBPA-exposed embryos to develop a small brain and small eyes. We revealed that TBBPA elevated the activity gradient of bone morphogenetic protein (BMP) signaling which is responsible for cell fate specification along the DV axis, with up-regulation of BMP ligands (bmp4, bmp7a) and target genes (szl) and promotion signal transduction through phosphorylation of Smad1/5. As the function of BMP signaling in embryogenesis is highly conserved among many vertebrates, these findings highlight the ecological and health risks of TBBPA.

Toxicity of perfluorooctanoic acid on zebrafish early embryonic development determined by single-cell RNA sequencing

The perfluorooctanoic acid (PFOA) poses a high risk for aquatic organisms. Nevertheless, the current toxicity studies rarely report how PFOA affects different cell populations during the embryonic development of fish. Here, the zebrafish embryos at 2-30 hpf were exposed to 1-100 μg/L PFOA. The heartbeat and locomotor behavior were significantly decreased after ≥ 25 μg/L PFOA exposure. The single-cell RNA sequencing showed that PFOA exposure influenced nine cell populations, including heart cells, hatching gland cells, macrophages, lens cells, ionocytes, melanoblasts, optic cup cells, periderm cells, and differentiating neurons cells. Among them, heart cells were the most affected cell population. Functions of cardiac muscle contraction, actin cytoskeleton and oxygen binding were significantly changed in the heart cells, which were involved in the altered expressions of tnni2a.4, acta1a, atp1a1a.2, mylpfa, and so on. Besides, the changes of apoptotic process, innate immune response, and translation in lens cells, hatching gland cells, macrophages and ionocytes should also be of concern. Our study indicates that 2-30 hpf of embryonic development is the sensitivity window for the PFOA exposure. Identification of the target cell population provides clear information of the toxic endpoint of PFOA, which sheds new light on the risk assessment of PFOA on aquatic organisms.

Biological responses of Eisenia fetida towards the exposure and metabolism of tris (2-butoxyethyl) phosphate

The toxicity of various organophosphorus flame retardants (OPFRs) is of increasing concern. However, there is still a lack of research on the toxicity of OPFRs to terrestrial invertebrates and its metabolism in vivo. Herein, earthworms (Eisenia fetida) were exposed to soil spiked with 0, 0.05, 0.5, and 5 mg/kg tris(2-butoxyethyl) phosphate (TBOEP, a typical alkyl OPFRs) for 28 d to study the biological responses to the exposure and metabolism of TBOEP. TBOEP exposure inhibited the activity of acetyl-cholinesterase (64.4-68.6% of that in the control group), increased the energy consumption level, and affected calcium-dependent pathways of E. fetida, which caused a 3.6-12.4% reduction in the weight gain rate (developmental toxicity), a 10.6-69.4% reduction in the number of juveniles (reproduction toxicity), and neurotoxicity to E. fetida. The 5 mg/kg TBOEP exposure caused a significant accumulation of malondialdehyde (1.68 times higher than that in the control group) in E. fetida, which indicated that the balance of oxidation and anti-oxidation of E. fetida was broken. Meanwhile, E. fetida maintained the absorption and metabolic abilities to TBOEP under the environmental condition. The removal rate of soil TBOEP was increased by 25.1-35.5% by the presence of E. fetida. Importantly, TBOEP could accumulate in E. fetida (0.09-76.0 μg/kg) and the activation of cytochrome P450 and glutathione detoxification pathway promoted the metabolism of TBOEP in E. fetida. These findings link the biological responses and metabolic behavior of earthworms under pollution stress and provide fundamental data for the environmental risk assessment and pollution removal of OPFRs in soil.

Tris(1,3-dichloro-2-propyl) phosphate reduces longevity through a specific microRNA-mediated DAF-16/FoxO in an unconventional insulin/insulin-like growth factor‑1 signaling pathway

Tris(1,3-dichloro-2-propyl) phosphate (TDCPP) has received concerns due to its frequent detection in environmental media and biological samples. Our previous study has indicated TDCPP reduced the lifespan of Caenorhabditis elegans (C. elegans) by triggering an unconventional insulin/insulin-like growth factor signaling (IIS) pathway. This study continued to investigate the possible deleterious effects of TDCPP relating to longevity regulation signal pathways and biological processes. Specifically, this study uniquely performed small RNA transcriptome sequencing (RNA-seq), focusing on the underlying mechanisms of TDCPP-reduced the longevity of C. elegans in-depth in microRNAs (miRNAs). Based on Small RNA-seq results and transcript levels of mRNA involved in the unconventional IIS pathway, a small interaction network of miRNAs-mRNAs following TDCPP exposure in C. elegans was preliminarily established. Among them, up-regulated miR-48 and miR-84 (let-7 family members) silence the mRNA of daf-16 (the crucial member of the FoxO family and pivotal regulator in longevity) via post-transcription and translation dampening abilities, further inhibit its downstream target metallothionein-1 (mtl-1), and ultimately contributed to the reduction of nematode longevity and locomotion behaviors. Meanwhile, the high binding affinities of TDCPP with miRNAs cel-miR-48-5p and cel-miR-84-5p strongly support their participation in the regulation of nematode mobility and longevity. These findings provide a comprehensive analysis of TDCPP-reduced longevity from the perspective of miRNAs.

Organophosphorus Flame Retardant TDCPP Displays Genotoxic and Carcinogenic Risks in Human Liver Cells

Tris(1,3-Dichloro-2-propyl)phosphate (TDCPP) is an organophosphorus flame retardant (OPFR) widely used in a variety of consumer products (plastics, furniture, paints, foams, and electronics). Scientific evidence has affirmed the toxicological effects of TDCPP in in vitro and in vivo test models; however, its genotoxicity and carcinogenic effects in human cells are still obscure. Herein, we present genotoxic and carcinogenic properties of TDCPP in human liver cells (HepG2). 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) and neutral red uptake (NRU) assays demonstrated survival reduction in HepG2 cells after 3 days of exposure at higher concentrations (100-400 μM) of TDCPP. Comet assay and flow cytometric cell cycle experiments showed DNA damage and apoptosis in HepG2 cells after 3 days of TDCPP exposure. TDCPP treatment incremented the intracellular reactive oxygen species (ROS), nitric oxide (NO), Ca2+ influx, and esterase level in exposed cells. HepG2 mitochondrial membrane potential (ΔΨm) significantly declined and cytoplasmic localization of P53, caspase 3, and caspase 9 increased after TDCPP exposure. qPCR array quantification of the human cancer pathway revealed the upregulation of 11 genes and downregulation of two genes in TDCPP-exposed HepG2 cells. Overall, this is the first study to explicitly validate the fact that TDCPP bears the genotoxic, hepatotoxic, and carcinogenic potential, which may jeopardize human health.

A review on organophosphate flame retardants in the environment: Occurrence, accumulation, metabolism and toxicity

Organophosphate flame retardants (OPFRs), as a substitute for brominated flame retardants (BFRs), are widely used in industrial production and life. The presence of OPFRs in the environment has an adverse effect on the ecological environment system. This review provides comprehensive data for the occurrence of OPFRs and their diester metabolites (OP diesters) in wastewater treatment plants, surface water, drinking water, sediment, soil, air and dust in the environment. In particular, the accumulation and metabolism of OPFRs in organisms and the types of metabolites and metabolic pathways are discussed for animals and plants. In addition, the toxicity of OP triesters and OP diesters in organisms is discussed. Although research on OPFRs has gradually increased in recent years, there are still many gaps to be filled, especially for metabolic and toxicity mechanisms that need in-depth study. This review also highlights the shortcomings of current research and provides suggestions for a basis for future research on OPFRs.

Precision Biotransformation of Emerging Pollutants by Human Cytochrome P450 Using Computational-Experimental Synergy: A Case Study of Tris(1,3-dichloro-2-propyl) Phosphate

Precision biotransformation is an envisioned strategy offering detailed insights into biotransformation pathways in real environmental settings using experimentally guided high-accuracy quantum chemistry. Emerging pollutants, whose metabolites are easily overlooked but may cause idiosyncratic toxicity, are important targets of such a strategy. We demonstrate here that complex metabolic reactions of tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) catalyzed by human CYP450 enzymes can be mapped via a three-step synergy strategy: (i) screening the possible metabolites via high-throughout (moderate-accuracy) computations; (ii) analyzing the proposed metabolites in vitro by human liver microsomes and recombinant human CYP450 enzymes; and (iii) rationalizing the experimental data via precise mechanisms using high-level targeted computations. Through the bilateral dialogues from qualitative to semi-quantitative to quantitative levels, we show how TDCIPP metabolism especially by CYP3A4 generates bis(1,3-dichloro-2-propyl) phosphate (BDCIPP) as an O-dealkylation metabolite and bis(1,3-dichloro-2-propyl) 3-chloro-1-hydroxy-2-propyl phosphate (alcohol_β-dehalogen) as a dehalogenation/reduction metabolite via the initial rate-determining H-abstraction from αC- and βC-positions. The relative yield ratio [dehalogenation/reduction]/[O-dealkylation] is derived from the relative barriers of H-abstraction at the βC- and αC-positions by CYP3A4, estimated as 0.002 to 0.23, viz., an in vitro measured ratio of 0.04. Importantly, alcohol_β-dehalogen formation points to a new mechanism involving successive oxidation and reduction functions of CYP450, with its precursor aldehyde_β-dehalogen being a key intermediate detected by trapping assays and rationalized by computations. We conclude that the proposed three-step synergy strategy may meet the increasing challenge of elucidating biotransformation mechanisms of substantial synthesized organic compounds in the future.

Gene co-expression network analysis in zebrafish reveals chemical class specific modules

BACKGROUND

Zebrafish is a popular animal model used for high-throughput screening of chemical hazards, however, investigations of transcriptomic mechanisms of toxicity are still needed. Here, our goal was to identify genes and biological pathways that Aryl Hydrocarbon Receptor 2 (AHR2) Activators and flame retardant chemicals (FRCs) alter in developing zebrafish. Taking advantage of a compendium of phenotypically-anchored RNA sequencing data collected from 48-h post fertilization (hpf) zebrafish, we inferred a co-expression network that grouped genes based on their transcriptional response.

RESULTS

Genes responding to the FRCs and AHR2 Activators localized to distinct regions of the network, with FRCs inducing a broader response related to neurobehavior. AHR2 Activators centered in one region related to chemical stress responses. We also discovered several highly co-expressed genes in this module, including cyp1a, and we subsequently show that these genes are definitively within the AHR2 signaling pathway. Systematic removal of the two chemical types from the data, and analysis of network changes identified neurogenesis associated with FRCs, and regulation of vascular development associated with both chemical classes. We also identified highly connected genes responding specifically to each class that are potential biomarkers of exposure.

CONCLUSIONS

Overall, we created the first zebrafish chemical-specific gene co-expression network illuminating how chemicals alter the transcriptome relative to each other. In addition to our conclusions regarding FRCs and AHR2 Activators, our network can be leveraged by other studies investigating chemical mechanisms of toxicity.

Occurrence, distribution and risk assessment of organophosphate ester flame retardants and plasticizers in surface seawater of the West Pacific

Twenty-eight samples of surface seawater were collected from the West Pacific Ocean during 2019 using a high-volume solid-phase extraction with high-throughput organic analysis (Hi-throat/Hi-volume SPE) method, and concentrations of 10 organophosphate ester flame retardants and plasticizers (OPEs) were determined. The total OPE concentration in the samples was 3.02-48.4 ng L^-1 (mean 25.0 ± 10.5 ng L^-1), with tris(2-chloroethyl) phosphate (TCEP) being the largest contributor. Cluster analysis results showed off-shore input from the coast of East and Southeast Asia was an important source of these chemicals. Tri-p-tolyl phosphate (TpTP) should also be considered for long-term monitoring, because of its high detection frequency. Results of a risk assessment indicated low ecological risk to species in the West Pacific Ocean for TPhP and ΣOPEs. Hazard quotients (HQs) were all <1, indicating that the health risk to humans from these chemicals was at acceptable levels.

Associations between the exposure to organophosphate flame retardants during early pregnancy and the risk of spontaneous abortion based on metabolomics combined with tandem mass spectrometry

BACKGROUND

As potential substitutes for polybrominated diphenyl ethers (PBDEs), organophosphate flame retardants (OPFRs) have been frequently detected in the environment. They have been suggested to impair fetal growth and development in toxicological studies. However, there are few studies on their maternal effects before or during early pregnancy.

METHODS

This study was designed to investigate whether exposure to OPFRs before or during early pregnancy is associated with the risk of spontaneous abortion (SAB), using a nested case-control design based on the case data from clinical examinations in Shanghai, China. A total of 110 cases from this cohort project in 2019-2020 were included. The concentrations of OPFRs in maternal urine samples collected in early pregnancy were determined using Ultra high performance liquid chromatography- triple quadrupole mass spectrometer (UHPLC-MS/MS), and pregnancy outcomes were extracted from the medical records. Meanwhile, ultra high performance liquid chromatography time-of-flight mass spectrometer (UHPLC-Q-TOF/MS)-based metabonomics was used to obtain urine metabolic profiles of 110 women in early pregnancy.

RESULTS

According to the quantitative results, the content of bis(1-chloro-2-propyl)phosphate (BCIPP) in urine was significantly different between the SAB patients and the healthy pregnant women. Besides, metabolic profile analysis showed a significant difference in the urine metabolism profile in early pregnancy between SAB cases and controls. Twenty-five different metabolites were screened out, which showed different degrees of correlation with the urinary BCIPP concentration.

CONCLUSIONS

Based on these results, it suggests that there may be a certain correlation between BCIPP concentration in the urine and the risk of SAB from a metabolomics perspective, and its effect may be related to the metabolism of tryptophan and fatty acids.

Utilizing Zebrafish Embryos to Reveal Disruptions in Dorsoventral Patterning

Dorsoventral (DV) patterning is a key landmark of embryonic development that is primarily regulated by bone morphogenetic protein (BMP) signaling. Disruption of DV patterning can result in downstream effects on cell specification and organogenesis. Zebrafish embryos have been extensively used to understand signaling pathways that regulate DV patterning because zebrafish embryos develop ex utero and, in contrast to mammalian embryos, which develop in utero, can be observed in real time using brightfield and fluorescence microscopy. Embryos with disrupted DV patterning are either dorsalized or ventralized, with lack of development of head or trunk/tail structures, respectively. Although these phenotypes are typically accompanied by effects on BMP signaling, exceptions exist where some drugs or environmental chemicals can disrupt DV patterning in the absence of effects on BMP signaling. Therefore, assessments of DV patterning should be accompanied by BMP signaling-specific readouts to confirm the role of BMP disruption. Here, we describe an exposure paradigm and steps for phenotyping zebrafish embryos for two types of DV defects, dorsalization and ventralization, with a range of severities. In addition, we describe a strategy for whole-mount immunohistochemistry of zebrafish embryos with an antibody specific for phospho-SMAD 1/5/9 (pSMAD 1/5/9), as disruption in pSMAD 1/5/9 localization is indicative of an effect on BMP signaling. Taken together, these protocols describe an initial strategy for evaluating DV patterning defects under various experimental conditions and confirming BMP-mediated DV patterning disruptions, which can be followed by additional studies that aim to uncover mechanisms leading to these adverse phenotypes. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Phenotyping for dorsalization and ventralization Basic Protocol 2: Whole-mount immunohistochemistry with antibody to phospho-SMAD 1/5/9.

Tris(1,3-dichloro-2-propyl)phosphate Reduces Growth Hormone Expression via Binding to Growth Hormone Releasing Hormone Receptors and Inhibits the Growth of Crucian Carp

Tris(1,3-dichloro-2-propyl)phosphate (TDCIPP) has commonly been used as an additive flame retardant and frequently detected in the aquatic environment and in biological samples worldwide. Recently, it was found that exposure to TDCIPP inhibited the growth of zebrafish, but the relevant molecular mechanisms remained unclear. In this study, 5 day-old crucian carp (Carassius auratus) larvae were treated with 0.5, 5, or 50 μg/L TDCIPP for 90 days; the effect on growth was evaluated; and related molecular mechanisms were explored. Results demonstrated that 5 or 50 μg/L TDCIPP treatment significantly inhibited the growth of crucian carp and downregulated the expression of growth hormones (ghs), growth hormone receptor (ghr), and insulin-like growth factor 1 (igf1). Molecular docking, dual-luciferase reporter gene assay, and in vitro experiments demonstrated that TDCIPP could bind to the growth hormone releasing hormone receptor protein of crucian carp and disturb the stimulation of growth hormone releasing hormone to the expression of ghs, resulting in the decrease of the mRNA level of gh1 and gh2 in pituitary cells. Our findings provide new perceptions into the molecular mechanisms of developmental toxicity of TDCIPP in fish.

In vitro biolayer interferometry analysis of acetylcholinesterase as a potential target of aryl-organophosphorus flame-retardants

Organophosphorus flame retardants (OPFRs) have been implicated as neurotoxicants, but their potential neurotoxicity and mechanisms remain poorly understood. Herein, we investigated the neurotoxicity of selected OPFRs using zebrafish as a model organism. Environmentally relevant concentrations (3-1500 nM) of three classes of OPFRs (aryl-OPFRs, chlorinated-OPFRs, and alkyl-OPFRs) were tested in zebrafish larvae (2-144 h post-fertilisation) alongside the neurotoxic chemical chlorpyrifos (CPF) that inhibits acetylcholinesterase (AChE). Exposure to aryl-OPFRs and CPF inhibited AChE activities, while chlorinated- and alkyl-OPFRs did not inhibit these enzymes. Biolayer interferometry (BLI) was used to probe interactions between OPFRs and AChE. The association and dissociation response curves showed that, like CPF, all three selected aryl-OPFRs, triphenyl phosphate (TPHP), tricresyl phosphate (TCP) and cresyl diphenyl phosphate (CDP), bound directly to AChE. The affinity constant (K_D) for TPHP, TCP, CDP and CPF was 2.18 × 10^-4, 5.47 × 10^-5, 1.05 × 10^-4 and 1.70 × 10^-5 M, respectively. In addition, molecular docking revealed that TPHP, TCP, CDP and CPF bound to AChE with glide scores of - 7.8, - 8.3, - 8.1 and - 7.3, respectively. Furthermore, the calculated binding affinity between OPFRs and AChE correlated well with the K_D values measured by BLI. The present study revealed that aryl-OPFRs can act as potent AChE inhibitors, and may therefore present a significant ecological risk to aquatic organisms.

Phenotypically Anchored mRNA and miRNA Expression Profiling in Zebrafish Reveals Flame Retardant Chemical Toxicity Networks

The ubiquitous use of flame retardant chemicals (FRCs) in the manufacture of many consumer products leads to inevitable environmental releases and human exposures. Studying toxic effects of FRCs as a group is challenging since they widely differ in physicochemical properties. We previously used zebrafish as a model to screen 61 representative FRCs and showed that many induced behavioral and teratogenic effects, with aryl phosphates identified as the most active. In this study, we selected 10 FRCs belonging to diverse physicochemical classes and zebrafish toxicity profiles to identify the gene expression responses following exposures. For each FRC, we executed paired mRNA-micro-RNA (miR) sequencing, which enabled us to study mRNA expression patterns and investigate the role of miRs as posttranscriptional regulators of gene expression. We found widespread disruption of mRNA and miR expression across several FRCs. Neurodevelopment was a key disrupted biological process across multiple FRCs and was corroborated by behavioral deficits. Several mRNAs (e.g., osbpl2a) and miRs (e.g., mir-125b-5p), showed differential expression common to multiple FRCs (10 and 7 respectively). These common miRs were also predicted to regulate a network of differentially expressed genes with diverse functions, including apoptosis, neurodevelopment, lipid regulation and inflammation. Commonly disrupted transcription factors (TFs) such as retinoic acid receptor, retinoid X receptor, and vitamin D regulator were predicted to regulate a wide network of differentially expressed mRNAs across a majority of the FRCs. Many of the differential mRNA-TF and mRNA-miR pairs were predicted to play important roles in development as well as cancer signaling. Specific comparisons between TBBPA and its derivative TBBPA-DBPE showed contrasting gene expression patterns that corroborated with their phenotypic profiles. The newer generation FRCs such as IPP and TCEP produced distinct gene expression changes compared to the legacy FRC BDE-47. Our study is the first to establish a mRNA-miR-TF regulatory network across a large group of structurally diverse FRCs and diverse phenotypic responses. The purpose was to discover common and unique biological targets that will help us understand mechanisms of action for these important chemicals and establish this approach as an important tool for better understanding toxic effects of environmental contaminants.

Exposure to organophosphate flame esters during early pregnancy and risk of spontaneous abortion: A case-control study

Prenatal exposure to organophosphate flame retardants (OPFRs) has been associated with adverse pregnancy outcomes including low birth weight and preterm birth. However, no study has addressed the impacts of OPFRs exposure on spontaneous abortion (SAB) and fetal chromosome abnormalities. We examined whether prenatal exposure to OPFRs was associated with increased risk of SAB and fetal chromosome abnormalities. A total of 272 pregnant women, including 136 SAB cases and 136 healthy controls, were enrolled in this case-control study. Urinary concentrations of 3 OPFRs metabolites (diphenyl phosphate (DPHP), bis (1,3-dichloro-2-propyl) phosphate (BDCIPP) and bis (1-chloro-2-propyl) phosphate (BCIPP)) were measured using ultra-high-performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS). More than 70% of the urine samples detected quantifiable levels of 3 OPFRs metabolites. Concentrations of BCIPP were significantly higher in SAB cases than in healthy controls. Higher urinary BCIPP levels were associated with increased risk of SAB. Per unit increase in ln-transformed BCIPP concentrations was associated with 103% (OR = 2.03, 95% CI, 1.57, 2.63) increase in the odds of SAB. While higher BDCIPP levels were associated with increased risk of fetal chromosome abnormalities and the corresponding OR associated with a unit increase in ln-transformed BDCIPP concentrations were 2.34 (95% CI, 1.14, 4.81). Our results suggested the potential developmental toxicity and teratogenicity of some OPFRs.

Synergistic growth inhibition effect of TiO2 nanoparticles and tris(1,3-dichloro-2-propyl) phosphate on earthworms in soil

The co-existence of organic pollutants and nanoparticles in the environment may lead to combined biological effects. The joint toxicity of pollutants and nanoparticles has been receiving increasing attention from researchers, but few studies have focused on soil biota due to the complexity of soil matrices. This study investigated the effects of tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) at 0, 5, and 25 mg/kg and nanoparticulate TiO₂ (nTiO₂) at 0, 500, and 2500 mg/kg in a 3 × 3 factorial arrangement of treatments for 28 days (d) on Eisenia fetida (earthworm). Compared with the control group (the 0 mg/kg TDCIPP + 0 mg/kg nTiO₂ treatment), all other single (TDCIPP or nTiO₂) and binary (TDCIPP + nTiO₂) treatments except for the single 500 mg/kg nTiO₂ treatment significantly reduced the weight gain rate of E. fetida. The binary treatments had significantly greater such effect than their corresponding single treatments, exhibiting a synergistic toxicity between TDCIPP and nTiO2 on the growth of E. fetida. Since TDCIPP and nTiO₂ had no significant effect on their concentrations in the soil or in E. fetida during binary exposure, the synergistic toxicity could be a result of the superimposition of the toxicity pathways of TDCIPP and nTiO₂. Transcriptomic analysis of E. fetida intestinal region revealed that exposure to 25 mg/kg TDCIPP or 2500 mg/kg nTiO₂ affected nutrient-related or cell apoptosis and DNA damage related genes, respectively; their co-exposure greatly inhibited genes related to nutrient digestion and absorption, while causing abnormal transcription of genes related to the development and maintenance of E. fetida's muscles, leading to synergistic toxicity. These findings provide new insights into the environmental risks of organophosphorus flame retardants, nanoparticles, and their co-exposure.

Effects of tris(2-chloroethyl) phosphate exposure on chicken embryos in a shell-less incubation system

Tris(2-chloroethyl) phosphate (TCEP) is an organophosphate flame retardant that used in textiles, industrial materials, and furniture to delay the spread of fire after ignition. TCEP has been detected in the tissues and eggs of fish and birds. However, there are no studies regarding the effects of TCEP on avian embryos. In the present study, we investigated the developmental toxicity of TCEP exposure on chicken embryos in a shell-less incubation system, which enables in situ observation. Chicken embryos were treated with graded doses of TCEP (50, 250, and 500 nmol/g egg) on incubation day 0. The survival rate, morphological biometrics, heart rate, and length and branch number of extraembryonic blood vessels were measured on incubation days 3-9. Survival rates were reduced from incubation day 3 and were significantly decreased until day 9. Body length, head + bill length and eye diameter were significantly reduced by TCEP exposure. Regarding skeletal effects, spine length was decreased in a dose-dependent manner on day 9. Body weight on day 9 significantly reduced in all TCEP treatment groups. These results suggest that TCEP exposure to >50 nmol/g egg retards development in chicken embryos. TCEP exposure to 500 nmol/g egg significantly increased heart weight to body weight ratio in the embryos. More than 250 nmol/g egg of TCEP significantly reduced the heart rate of embryos in the early developmental stage. The formation of extraembryonic blood vessels and the number of erythrocytes were significantly reduced even with 50 nmol/g egg of TCEP. These findings suggest that TCEP exposure specifically affects the cardiovascular system in chicken embryos, which leads to developmental delay. The results of this study also demonstrate that the shell-less incubation system can be used to continuously monitor the effects of chemicals on developing avian embryos.

Parental exposure to perfluorobutane sulfonate disturbs the transfer of maternal transcripts and offspring embryonic development in zebrafish

Parental exposure to perfluorobutane sulfonate (PFBS), an aquatic pollutant of emerging concern, is previously found to impair the embryonic development of offspring. However, the impairing mechanisms remain to clarify. In the present study, adult zebrafish were exposed to 0, 10 and 100 μg/L PFBS for 28 d, after which disturbances in maternal transcript transfer and offspring embryogenesis were investigated. Prior to zygotic genome activation, high-throughput transcriptomic sequencing revealed that parental PFBS exposure significantly altered the transcript profile of maternal origin in offspring eggs, while toxic actions varied as a function of PFBS concentrations. In offspring eggs derived from 10 μg/L exposure group, differential transcripts were mainly associated with the histone-DNA interaction of nucleosome, which would modify the compacted chromatin configuration and accessibility of transcriptional factors to DNA sequences. In this regard, the timing of zygotic genome activation was presumably disrupted. Parental exposure to 100 μg/L PFBS primarily interrupted the maternal transfer of adherens junction transcripts, which was supposed to dysregulate the cell-cell adhesion during early embryo formation. Development and growth of offspring embryos were significantly compromised by parental PFBS exposure, as exemplified by higher mortality, delayed hatching, slower heart rate, reduced body weight and neurobehavioral disorders. Overall, the present study presented the first toxicological evidence about the disturbances of PFBS in maternal transcript transfer, although the inherent linkage between maternal transcript modifications and offspring development defects still needs future works to construct.

Review of emerging contaminant tris(1,3-dichloro-2-propyl)phosphate: Environmental occurrence, exposure, and risks to organisms and human health

Tris(1,3-dichloro-2-propyl)phosphate (TDCPP) is a halogen-containing organophosphorus chemical that is widely employed in various consumer products with a high production volume. As an additive flame retardant (FR), TDCPP tends to be released into the environment through multiple routes. It is ubiquitous in environmental media, biotic matrixes, and humans, and thus is deemed to be an emerging environmental contaminant. To date, significant levels of TDCPP and its primary diester metabolite, bis(1,3-dichloro-2-propyl)phosphate, have been detected in human samples of seminal plasma, breast milk, blood plasma, placenta, and urine, thereby causing wide concern about the potential human health effects resulting from exposure to this chemical. Despite the progress in research on TDCPP over the past few years, we are still far from fully understanding the environmental behavior and health risks of this emerging contaminant. Thus, this paper critically reviews the environmental occurrence, exposure, and risks posed by TDCPP to organisms and human health among the literature published in the last decade. It has been demonstrated that TDCPP induces acute-, nerve-, developmental-, reproductive-, hepatic-, nephron-, and endocrine-disrupting toxicity in animals, which has caused increasing concern worldwide. Simultaneously, TDCPP induces cytotoxicity by increasing the formation of reactive oxygen species and inducing endoplasmic reticulum stress in multiple human cell lines in vitro, and also causes endocrine-disrupting effects, including reproductive dysfunction and adverse pregnancy outcomes, according to human epidemiology studies. This review not only provides a better understanding of the behavior of this emerging contaminant in the environment, but also enhances the comprehension of the health risks posed by TDCPP exposure to ecosystems and humans.

Tris(1,3-dichloro-2-propyl)phosphate Reduces the Lifespan via Activation of an Unconventional Insulin/Insulin-Like Growth Factor-1 Signaling Pathway

Tris(1,3-dichloro-2-propyl)phosphate (TDCPP) is an environmental contaminant that has attracted increasing concern due to its presence in environmental media and biological samples. Our previous study demonstrated that exposure to TDCPP reduced the lifespan of Caenorhabditis elegans, but the mechanisms, including the relevant signaling pathways, are unclear. The current study found that TDCPP exposure triggers an unconventional insulin/insulin-like growth factor signaling (IIS) pathway, not by disrupting the insulin-like growth factor-1 receptor DAF-2/IGF1R but by inhibiting the downstream tumor-suppressor factor DAF-18/PTEN. This inhibition reduces PI(3,4,5)P₃ (PIP₃) dephosphorylation, causing buildup that increases the activation of the Akt/Protein Kinase B (PKB) family of serine/threonine kinases. This activation induces DAF-16/FoxO phosphorylation and promotes the sequestration of DAF-16/FoxO in the cytoplasm, reducing the lifespan of nematodes. Our results have important diagnostic and therapeutic implications for controlling TDCPP-related diseases, especially those originating with IIS pathway components.

Changes in the circRNA expression profile of PC12 cells induced by TDCIPP exposure may regulate the downstream NF-κB pathway via the Traf2 gene

As a commonly used organophosphorus flame retardant (OPFR), tris (1,3-dichloro-2-propyl) phosphate (TDCIPP) has become an environmental pollutant. Studies have shown that TDCIPP exposure has many toxic effects, such as neurotoxicity, reproductive development toxicity and endocrine disturbance. CircRNAs are circular noncoding RNAs that have been found to play important roles in the occurrence of a variety of diseases. However, it is unclear whether circRNAs play a role in the cytotoxicity induced by TDCIPP exposure. In this study, we analysed the circRNA microarray results of a control group and a TDCIPP exposure group of PC12 cells and detected 3432 differentially expressed circRNAs (P < 0.05), of which 1682 were upregulated and 1750 were downregulated in the TDCIPP-exposure group. The expression levels of 2 upregulated and 3 downregulated circRNAs were verified by real-time quantitative PCR, and the results were consistent with the microarray results. Then, ceRNA analysis was performed on several kinds of circRNAs to predict the possible binding miRNAs and binding sites. The target genes were analysed by Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis. Through predictive analysis, it was found that rno_circRNA_013845, rno-miR-361-3p, and rno-miR-702-3p may be involved in the regulation of Traf2 expression, thereby affecting the expression of the downstream NF-κB signalling pathway and causing apoptosis.

Integrating omics and traditional analyses to profile the synergistic toxicity of graphene oxide and triphenyl phosphate

The increasing production and applications of graphene oxide (GO, a novel carbon nanomaterial) have raised numerous environmental concerns regarding its ecological risks. Triphenyl phosphate (TPhP) disperses in water and poses an increasing hazard to the ecosystem and human health. It is critical to study the environmental responses and molecular mechanisms of GO and TPhP together to assess both chemicals; however, this information is lacking. The present work revealed that GO promoted the bioaccumulation of TPhP in zebrafish larvae by 5.0%-24.3%. The TPhP-induced growth inhibition of embryos (malformation, mortality, heartbeat, and spontaneous movement) at environmentally relevant concentrations was significantly amplified by GO, and these results were supported by the downregulated levels of genes and proteins associated with cytoskeletal construction and cartilage and eye development. TPhP induced negligible alterations in the genes or proteins involved in oxidative stress and apoptosis, but those related proteins were all upregulated by GO. GO and TPhP coexposure activated the mTOR signaling pathway and subsequently promoted apoptosis in zebrafish by potentiating the oxidative stress induced by TPhP, presenting synergistic toxicity. These findings highlight the potential risks and specific molecular mechanisms of combining emerging carbon nanomaterials with coexisting organic contaminants.

Graphene oxide nanosheets mitigate the developmental toxicity of TDCIPP in zebrafish via activating the mitochondrial respiratory chain and energy metabolism

Graphene oxide (GO), a novel two-dimension carbon nanomaterial, has showed tremendous potential for utilization in intelligent manufacturing and environmental protection. In parallel, tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) is widely distributed in the water environment and represents a great threat to ecosystem health. However, the related knowledge remained absent regarding the impact of GO on the biological risks of TDCIPP. Herein, GO significantly reduced the mortality and malformation rates of zebrafish induced by TDCIPP maximumly by 28.6% and 41.8%, respectively. Decreased mitochondrial respiratory chain (MRC) enzyme and ATP activity induced by TDCIPP were mitigated by GO. Integrating proteomics and metabolomics revealed TDCIPP obviously induced the downregulation of the proteins and metabolites involved in the cytoskeleton, mitochondrial function, carbohydrate and amino acid metabolism, and the TCA cycle, but the alterations were attenuated by GO. GO primarily promoted MRC activity, carbohydrate metabolism, and fatty acid β-oxidation, thus activating the energy metabolism of zebrafish and leading to antagonistic effects on the developmental toxicity of TDCIPP. These results provide a novel view on the co-exposure of GO with other pollutants and promote the reconsideration of the environmental risks of GO.

24-Epibrassinolide protects against ethanol-induced behavioural teratogenesis in zebrafish embryo

Embryonic studies have demonstrated the neurotoxic, teratogenic, and neurobehavioral toxicity of ethanol (EtOH). Although multiple mechanisms may contribute to these effects, oxidative stress has been described as the major damage pathway. In this regard, natural antioxidants have the potential to counteract oxidative stress-induced cellular damage. Therefore, the present study aimed to investigate the potential protective role of 24-epibrassinolide (24-EPI), a natural brassinosteroid with proved antioxidant properties, in EtOH-induced teratogenic effects during early zebrafish development. Embryos (~2 h post-fertilization - hpf) were exposed to 1 % EtOH, co-exposed to 24-EPI (0.01, 0.1 and 1 μM) and to 24-EPI alone (1 μM) for 24 h. Following exposure, biochemical evaluations were made at 26 hpf, developmental analysis was made throughout the embryo-larval period, and behavioural responses were evaluated at 120 hpf. Exposure to 1 % EtOH caused an increase in the number of malformations, which were diminished by 24-EPI. In addition, EtOH induced an accumulation of GSSG and consequent reduction of GSH:GSSG ratio, indicating the involvement of oxidative mechanisms in the EtOH-induced effects. These were reverted by 24-EPI as proved by the GSSG levels and GSH:GSSG ratio that returned to control values. Furthermore, exposure to EtOH resulted in behavioural deficits at 120 hpf as observed by the disrupted response to an aversive stimulus, suggesting the involvement of neurotoxic mechanisms. 24-EPI restored the behavioural deficits observed in a dose-dependent manner. The absence of effects in the embryos exposed solely to 24-EPI showed its safety during the exposure period. In conclusion, EtOH caused developmental teratogenicity and behavioural toxicity by inducing glutathione changes, which were prevented by 24-EPI.

Developmental circulatory failure caused by metabolites of organophosphorus flame retardants in zebrafish, Danio rerio

Organophosphate triesters are used worldwide as additives in flame retardants and plasticizer as a replacement of polybrominated diphenyl ethers. Increasing evidence on human exposure to and environmental contamination with organophosphorus flame retardants (OPFRs) requires an adequate toxicity assessment for this class of chemicals. While developmental toxicity of several OPFRs has been reported, developmental effects of OPFR metabolites have still to be understood. The present study aimed at characterizing developmental effects of OPFR metabolites using zebrafish embryos (Danio rerio). Triphenyl phosphate (TPHP) and two of its metabolites, 3-hydroxylphenyl diphenyl phosphate and 4-hydroxylphenyl diphenyl phosphate, were most potent for inducing pericardial edema and reduction in blood flow in trunk vessels. Other TPHP metabolites, such as diphenyl phosphate and 4-hydroxylphenyl phenyl phosphate, showed no substantial increase in circulatory failure at concentrations up to 30 μM. Tris (1,3-dichloro-2-propyl) phosphate showed circulatory failure at 30 μM, but its metabolite bis(1,3-dichloro-2-propyl) phosphate did not. Neither tris(2-chloroethyl) phosphate nor its metabolite bis(2-chloroethyl) phosphate, induced circulatory failure. The circulatory failure appeared to be enhanced with the increase in the octanol-water partition coefficients of OPFRs and their metabolites, suggesting that developmental circulatory failure posed by these chemicals could be estimated by their bioaccumulative potential. The present study demonstrated developmental circulatory failure of hydroxylated TPHP metabolites, which was almost equipotent to TPHP. Diester OPFR metabolites showed no major developmental toxicity at the concentrations used in this study. The current results establish the foundation for further understanding the similarities and differences in the toxic mechanisms between OPFRs and their metabolites.

Longer commutes are associated with increased human exposure to tris(1,3-dichloro-2-propyl) phosphate

Organophosphate esters (OPEs) are a class of semi-volatile organic compounds (SVOCs) used as flame retardants, plasticizers, and anti-foaming agents. Due to stringent flammability standards in vehicles and the ability of OPEs to migrate out of end-use products, elevated concentrations of OPEs have been found in car dust samples around the world. As many residents of Southern California spend a significant amount of time in their vehicles, there is potential for increased exposure to OPEs associated with longer commute times. As approximately 70% of the University of California, Riverside's undergraduate population commutes, the objective of this study was to use silicone wristbands to monitor personal exposure to OPEs and determine if exposure was associated with commute time in a subset of these students. Participants were asked to wear wristbands for five continuous days and complete daily surveys about the amount of time spent commuting. Data were then used to calculate a participant-specific total commute score. Components of Firemaster 550 (triphenyl phosphate, or TPHP, and isopropylated triaryl phosphate isomers) and Firemaster 600 (TPHP and tert-butylated triaryl phosphate isomers) - both widely used commercial flame retardant formulations - were strongly correlated with other OPEs detected within participant wristbands. Moreover, the concentration of tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) was significantly correlated with the concentration of several Firemaster 500 components and tris(2-chloroisopropyl) phosphate (TCIPP). Finally, out of all OPEs measured, TDCIPP was significantly and positively correlated with total commute score, indicating that longer commutes are associated with increased human exposure to TDCIPP. Overall, our findings raise concerns about the potential for chronic TDCIPP exposure within vehicles and other forms of transportation, particularly within densely populated and traffic-congested areas such as Southern California.

Ciglitazone-a human PPARγ agonist-disrupts dorsoventral patterning in zebrafish

Peroxisome proliferator-activated receptor γ (PPARγ) is a ligand-activated transcription factor that regulates lipid/glucose homeostasis and adipocyte differentiation. While the role of PPARγ in adipogenesis and diabetes has been extensively studied, little is known about PPARγ function during early embryonic development. Within zebrafish, maternally-loaded pparγ transcripts are present within the first 6 h post-fertilization (hpf), and de novo transcription of zygotic pparγ commences at ~48 hpf. Since maternal pparγ transcripts are elevated during a critical window of cell fate specification, the objective of this study was to test the hypothesis that PPARγ regulates gastrulation and dorsoventral patterning during zebrafish embryogenesis. To accomplish this objective, we relied on (1) ciglitazone as a potent PPARγ agonist and (2) a splice-blocking, pparγ-specific morpholino to knockdown pparγ. We found that initiation of ciglitazone-a potent human PPARγ agonist-exposure by 4 hpf resulted in concentration-dependent effects on dorsoventral patterning in the absence of epiboly defects during gastrulation, leading to ventralized embryos by 24 hpf. Interestingly, ciglitazone-induced ventralization was reversed by co-exposure with dorsomorphin, a bone morphogenetic protein signaling inhibitor that induces strong dorsalization within zebrafish embryos. Moreover, mRNA-sequencing revealed that lipid- and cholesterol-related processes were affected by exposure to ciglitazone. However, pparγ knockdown did not block ciglitazone-induced ventralization, suggesting that PPARγ is not required for dorsoventral patterning nor involved in ciglitazone-induced toxicity within zebrafish embryos. Our findings point to a novel, PPARγ-independent mechanism of action and phenotype following ciglitazone exposure during early embryonic development.

Sub-chronic exposure to Tris(1,3-dichloro-2-propyl) phosphate induces sex-dependent hepatotoxicity in rats

As the application and environmental release of tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) are being increased rapidly, serious concerns have been raised regarding its adverse effects on human health. Exposure to TDCIPP has been implicated in hepatotoxicity, but the molecular mechanisms remain unclear. Here, both male and female Sprague Dawley rats were administered TDCIPP with 125, 250, or 500 mg/kg/day for 12 weeks. Then the ultrastructure of liver, biochemical indicators in serum and liver, and hepatic gene expression were analyzed to reveal molecular mechanisms of hepatotoxicity induced by TDCIPP. Continuous TDCIPP exposure decreased body weight, particularly in 500 mg/kg/day TDCIPP-exposed males, and dose dependently increased the ratio of liver to body weight in both genders. The decreased levels of triglyceride, cholesterol, and transaminase in the serum and livers were observed in both genders after TDCIPP exposure, which indicated dysfunction in the hepatic metabolism. Liver histopathology revealed hepatocellular damages in males and females after TDCIPP exposure. The transcriptomic analysis indicated that TDCIPP exposure significantly changed pathways of bile acid metabolism, inflammatory response, oxidative phosphorylation and carcinogenicity in 250 and 500 mg/kg/day TDCIPP-exposed males and 500 mg/kg/day TDCIPP-exposed females, and there was no statistical significance in any other TDCIPP-exposed groups. The transcriptional analysis showed that TDCIPP exposure led to oxidative stress in the livers of rats, thereby increasing the inflammatory response and promoting mechanisms of carcinogenesis in both genders. Finally, TDCIPP led to more severe adverse phenotypic effects in male than female rats.

Tris(1,3-dichloro-2-propyl)phosphate (TDCIPP) disrupts zebrafish tail fin development

The flame retardant, tris(1,3-dichloro-2-propyl) phosphate (TDCIPP), is one of the most developmentally toxic organophosphate flame retardants (OPFRs). However, few mechanistic studies on phenotypic malformation caused by TDCIPP have been conducted. This study investigates the molecular mechanism underlying abnormal tail fin development consistently observed in zebrafish embryos exposed to TDCIPP. The results show that the defects in the tail fin (e.g., bent spine, defective caudal fin, and damaged tip) were associated with altered expression of transcription factors. The significant up-regulation of mmp9 and, among insulin-growth factor (IGF) families, igfbp-1a and igfbp1b was observed, whereas alterations in the expression of cdx4, igf1a, ifg1b, igf2b, and vegaa regulating tail development were dependent on time points. In accordance with changes in mRNA gene expression, TDCIPP impaired vessel formation and disorganized muscle in transgenic Tg(fli-GFP) zebrafish larvae. Furthermore, we found that the overexpression of mmp9 caused by TDCIPP was not linked to igfbp-1. Overall, these findings demonstrate that TDCIPP disrupts the progression of tail fin development, accompanied by defects in vessel and muscle formation in developing zebrafish embryos.

Optimization of experimental conditions and measurement of oxygen consumption rate (OCR) in zebrafish embryos exposed to organophosphate flame retardants (OPFRs)

The measurement of oxygen consumption rate (OCR) provides a comprehensive understanding of mitochondrial metabolism. However, no study has been conducted to investigate the mitochondrial dysfunction caused by organophosphate flame retardants (OPFRs). The objectives of this study were to optimize the experimental conditions to measure OCR in zebrafish embryos using the Seahorse XFe 24 Extracellular Flux Analyzer, and to investigate the changes of OCR in zebrafish embryos exposed to OPFRs. We first optimized the experimental conditions such as the number of embryos, concentrations of inhibitors, and time points. We determined the factors, i.e., three embryos, 12.5 μM of oligomycin, 8 μM of carbonyl cyanaide 4-(trifluoromethoxy) phenylhydrazone (FCCP), and 24 hpf (hours post-fertilization) time point, for obtaining the typical pattern of OCR in dechorinated zebrafish embryos. After confirming the determinants upon exposure of triclosan, the inhibition of OCR was measured in zebrafish embryos exposed to two major OPFRs, triphenyl phosphate (TPHP) and tris (1,3-dichloro-2-propyl) phosphate (TDCIPP). We found that significant inhibition of OCR was observed in basal respiration for TPHP, and in basal and maximal respiration for TDCIPP exposure, respectively. We suggest the optimum conditions of the Seahorse XFe 24 analyzer to better evaluate OCR in zebrafish embryos, and demonstrate the potential of TPHP and TDCIPP to cause the disruption of energy metabolism associated with mitochondrial dysfunction.

Complex Interplay Among Nuclear Receptor Ligands, Cytosine Methylation, and the Metabolome in Driving Tris(1,3-dichloro-2-propyl)phosphate-Induced Epiboly Defects in Zebrafish

Tris(1,3-dichloro-2-propyl)phosphate (TDCIPP) is a high-production-volume organophosphate flame retardant (OPFR) that induces epiboly defects during zebrafish embryogenesis, leading to the disruption of dorsoventral patterning. Therefore, the objectives of this study were to (1) identify the potential mechanisms involved in TDCIPP-induced epiboly defects and (2) determine whether coexposure to triphenyl phosphate (TPHP)-an OPFR commonly detected with TDCIPP-enhances or mitigates epiboly defects. Although TDCIPP-induced epiboly defects were not associated with adverse impacts on cytoskeletal protein abundance in situ, the coexposure of embryos to TPHP partially blocked TDCIPP-induced epiboly defects. As nuclear receptors are targets for both TPHP and TDCIPP, we exposed the embryos to TDCIPP in the presence or absence of 69 nuclear receptor ligands and, similar to TPHP, found that ciglitazone (a peroxisome proliferator-activated receptor γ agonist) and 17β-estradiol (E2; an estrogen receptor α agonist) nearly abolished TDCIPP-induced epiboly defects. Moreover, E2 and ciglitazone mitigated TDCIPP-induced effects on CpG hypomethylation within the target loci prior to epiboly, and ciglitazone altered TDCIPP-induced effects on the abundance of two polar metabolites (acetylcarnitine and cytidine-5-diphosphocholine) during epiboly. Overall, our results point to a complex interplay among nuclear receptor ligands, cytosine methylation, and the metabolome in both the induction and mitigation of epiboly defects induced by TDCIPP.

A review of organophosphorus flame retardants (OPFRs): occurrence, bioaccumulation, toxicity, and organism exposure

Organophosphorus flame retardants (OPFRs) are increasingly being applied as flame retardants due to their unique properties. OPFRs are commonly detected in various environmental matrices, and organisms are extensively exposed to them. Considering the adverse effects of OPFRs, many researchers have devoted their attention to environmental risk assessments. This review outlines the current knowledge regarding the toxicity of OPFRs based on both in vitro and in vivo experiments in various environmentally relevant test species. The production, absorption, bioaccumulation, and biomagnification of OPFRs in animals and humans are also described. The joint effects of OPFRs and their coexisting characteristics are also discussed based on the limited available data and results. Finally, knowledge gaps and perspectives for future exposure studies of OPFRs in animals and humans are identified.

Effect of Propiconazole on the Lipid Metabolism of Zebrafish Embryos ( Danio rerio)

Propiconazole is a triazole fungicide that has been widely used in agriculture and has been detected in the aquatic environment. This study aimed to investigate the effects of propiconazole exposure on lipid metabolism in the early life stages of zebrafish for 120 h postfertilization (hpf). Using the early life stages of zebrafish to address scientific questions is lower in cost, more efficient, and suitable to meeting current legislation than those in other traditional fish species. Exposure to propiconazole significantly inhibited the development of zebrafish embryos and larvae. This exposure also caused reduced locomotor activities in zebrafish. Furthermore, total cholesterol levels, lipoprotein lipase, and fatty acid synthase activities were significantly decreased. The expression levels of genes involved in lipid metabolism were significantly up-regulated in response to propiconazole exposure. GC-MS/MS analysis revealed that fatty acids were significantly decreased. Together, the findings indicate the potential environmental risks of propiconazole exposure in the aquatic ecosystem.

Exposure to tris(1,3-dichloro-2-propyl) phosphate (TDCPP) induces vascular toxicity through Nrf2-VEGF pathway in zebrafish and human umbilical vein endothelial cells

The growing production and extensive use of organophosphate flame retardants (OPFRs) have led to an increase in their environmental distribution and human exposure. Developmental toxicity is a major concern of OPFRs' adverse health effects. However, the impact of OPFRs exposure on vascular development and the toxicity pathway for developmental defects are poorly understood. In this study, we investigated the effects of exposure to tris(1,3-dichloro-2-propyl) phosphate (TDCPP), a frequently detected OPFR, on early vascular development, and the possible role of nuclear factor erythroid 2-related factor (Nrf2)-dependent angiogenic pathway in TDCPP's vascular toxicity. TDCPP exposure at 300 and 500 μg/L impeded the growth of intersegmental vessels (ISV), a type of microvessels, as early as 30 hpf. Consistently, a similar pattern of decreased extension and remodeling of common cardinal vein (CCV), a typical macrovessel, was observed in zebrafish at 48 hpf and 72 hpf. Developing vasculature in zebrafish was more sensitive than general developmental parameters to TDCPP exposure. The expression of genes related to VEGF signaling pathway dose-dependently decreased in TDCPP-treated larvae. In in vitro experiments using human umbilical vein endothelial cells (HUVECs), the increased cell proliferation induced by VEGF was suppressed by TDCPP exposure in a dose-dependent fashion. In addition, we found a repression of Nrf2 expression and activity in TDCPP-treated larvae and HUVECs. Strikingly, the application of CDDO-Im, a potent Nrf2 activator, enhanced VEGF and protected against defective vascular development in zebrafish. Our results reveal that vascular impairment is a sensitive index for early exposure to TDCPP, which could be considered in the environmental risk assessment of OPFRs. The identification of Nrf2-mediating VEGF pathway provides new insight into the adverse outcome pathway (AOP) of OPFRs.

Toxicity profiling of flame retardants in zebrafish embryos using a battery of assays for developmental toxicity, neurotoxicity, cardiotoxicity and hepatotoxicity toward human relevance

Following the voluntary phase-out of brominated flame retardants (BFRs) due to their environmental persistence and toxicity, the organophosphorus flame retardants (OPFRs) are emerging replacements. However, there is limited information on the potential human health effects of the OPFRs. Zebrafish embryos are a viable vertebrate model organism with many advantages for high throughput testing toward human hazard assessment. We utilized zebrafish embryos to assess developmental toxicity, neurotoxicity, cardiotoxicity and hepatotoxicity, of eight replacement OPFRs: (triphenyl phosphate [TPHP], isopropylated phenyl phosphate [IPP], 2-ethylhexyl diphenyl phosphate [EHDP], tert-butylated phenyl diphenyl phosphate [BPDP], trimethyl phenyl phosphate [TMPP], isodecyl diphenyl phosphate [IDDP], tris(1,3-dichloroisopropyl) phosphate [TDCIPP], and tris(2-chloroethyl) phosphate [TCEP]) and two BFRs (3,3',5,5'- tetrabromobisphenol A [TBBPA] and 2,2'4,4'-brominated diphenyl ether [BDE-47]). To determine potential effects on teratogenicity, embryos were exposed to flame retardants (FRs) at 4 h post fertilization (hpf) to 4 days post fertilization (dpf) and morphological alterations and corresponding survival were evaluated at 2 and 4 dpf. Internal concentrations were measured in larvae used in this assay by liquid chromatography-mass spectrometry. Locomotor activity was assessed in larvae treated for 48 h (from 3 dpf to 5 dpf), followed by hepatotoxicity evaluation. Finally, alterations in heart rate and rhythmicity were assessed to determine cardiotoxicity in 48 hpf embryos exposed to compounds for 3 h. Results suggest that several OPFRs (BPDP, EHDP; IPP, TMPP; TPHP and TDCIPP) produced adverse effects in multiple target organs at concentrations comparable to the two BFRs. As these OPFRs have the capacity to disrupt an integrated vertebrate model, they potentially have the capacity to affect mammalian biology. Then, we compared the lowest effective levels (LEL) in zebrafish with estimated or measured human plasma concentrations using biomonitoring data (human plasma, breast milk, handwipe samples and house dust) and a high throughput toxicokinetic (HTTK) model. Results indicate that for some compounds, the nominal LELs were within the range of human exposures, while internal LELs in zebrafish are above internal exposures in humans. These findings demonstrate the value of the zebrafish model as a relevant screening tool and support the need for further hazard characterization of the OPFRs.