Triphenyl phosphate-induced developmental toxicity in zebrafish: potential role of the retinoic acid receptor

Aryl phosphate ester-induced pericardial edema in zebrafish embryos is influenced by the ionic composition of exposure media

Pericardial edema - fluid accumulation within the pericardium - is a frequently observed malformation in zebrafish embryo-based chemical toxicity screens. We recently discovered that the severity of triphenyl phosphate (TPHP)-induced pericardial edema was dependent on the ionic strength of exposure media. TPHP is an aryl phosphate ester (APE) widely used as a plasticizer and flame retardant. APEs are characterized by having one or more aryl groups bound to a phosphate center, with TPHP containing only unsubstituted aryl groups. Therefore, the objective of this study was to begin investigating whether, similar to TPHP, pericardial edema induced by other structurally related APEs is dependent on the ionic composition of exposure media. We first mined the peer-reviewed literature to identify other APEs that 1) induced pericardial edema in zebrafish embryos within a minimum of three peer-reviewed publications, and 2) demonstrated a statistically significant induction of pericardial edema in at least 70 % of the studies evaluated. Based on this meta-analysis, we identified four other APEs that caused pericardial edema in zebrafish embryos: isopropylated triphenyl phosphate (IPTPP), cresyl diphenyl phosphate (CDP), tricresyl phosphate (TMPP), and 2-ethylhexyl diphenyl phosphate (EDHPHP). Using TPHP as a positive control and pericardial edema as a readout, we developed concentration-response curves for all four APEs based on static exposure from 24 to 72 h post-fertilization (hpf). We then conducted co-exposures with D-Mannitol (an osmotic diuretic) and exposures within reverse osmosis (RO) water determine whether the ionic composition of exposure media mitigated APE-induced pericardial edema at 72 hpf. Using pericardial edema as an endpoint, the approximate EC50s for TPHP (positive control), IPTPP, CDP, TMPP, and EDHPHP were 6.25, 3.125, 3.125, 25, and 100 µM, respectively, based on exposure from 24 to 72 hpf. Interestingly, similar to our findings with TPHP, co-exposure with D-Mannitol and exposure within ion-deficient water significantly mitigated IPTPP- CDP-, TMPP-, and EDHPHP-induced pericardial edema in zebrafish embryos, suggesting that chemically-induced pericardial edema may be 1) dependent on the ionic composition of exposure media and 2) driven by a disruption in osmoregulation across the embryonic epidermis. Therefore, similar to other assay parameters, our findings underscore the need to standardize the osmolarity of exposure media in order to minimize the potential for false positive/negative hits in zebrafish embryo-based chemical toxicity screens conducted around the world.

Embryonic Exposure to Organophosphate Flame Retardants (OPFRs) Differentially Induces Cardiotoxicity in Rare Minnow (Gobiocypris rarus)

Organophosphorus flame retardants (OPFRs) such as triphenyl phosphate (TPHP) and tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) were reported to impair cardiac function in fish. However, limited information is available regarding their cardiotoxic mechanisms. Using rare minnow (Gobiocypris rarus) as a model, we found that both TPHP and TDCIPP exposures decreased heart rate at 96 h postfertilization (hpf) in embryos. Atropine (an mAChR antagonist) can significantly attenuate the bradycardia caused by TPHP, but only marginally attenuated in TDCIPP treatment, suggesting that TDCIPP-induced bradycardia is independent of mAChR. Unlike TDCIPP, although TPHP-induced bradycardia could be reversed by transferring larvae to a clean medium, the inhibitory effect of AChE activity persisted compared to 96 hpf, indicating the existence of other bradycardia regulatory mechanisms. Transcriptome profiling revealed cardiotoxicity-related pathways in treatments at 24 and 72 hpf in embryos/larvae. Similar transcriptional alterations were also confirmed in the hearts of adult fish. Further studies verified that TPHP and TDCIPP can interfere with Na+/Ca2+ transport and lead to disorders of cardiac excitation-contraction coupling in larvae. Our findings provide useful clues for unveiling the differential cardiotoxic mechanisms of OPFRs and identifying abnormal Na+/Ca2+ transport as one of a select few known factors sufficient to impair fish cardiac function.

Environmentally Relevant Concentrations of Triphenyl Phosphate (TPhP) Impact Development in Zebrafish

A common flame-retardant and plasticizer, triphenyl phosphate (TPhP) is an aryl phosphate ester found in many aquatic environments at nM concentrations. Yet, most studies interrogating its toxicity have used µM concentrations. In this study, we used the model organism zebrafish (Danio rerio) to uncover the developmental impact of nM exposures to TPhP at the phenotypic and molecular levels. At concentrations of 1.5-15 nM (0.5 µg/L-5 µg/L), chronically dosed 5dpf larvae were shorter in length and had pericardial edema phenotypes that had been previously reported for exposures in the µM range. Cardiotoxicity was observed but did not present as cardiac looping defects as previously reported for µM concentrations. The RXR pathway does not seem to be involved at nM concentrations, but the tbx5a transcription factor cascade including natriuretic peptides (nppa and nppb) and bone morphogenetic protein 4 (bmp4) were dysregulated and could be contributing to the cardiac phenotypes. We also demonstrate that TPhP is a weak pro-oxidant, as it increases the oxidative stress response within hours of exposure. Overall, our data indicate that TPhP can affect animal development at environmentally relevant concentrations and its mode of action involves multiple pathways.

Biomarkers of Organophosphate and Polybrominated Diphenyl Ether (PBDE) Flame Retardants of American Workers and Associations with Inhalation and Dermal Exposures

This study evaluated workers' exposures to flame retardants, including polybrominated diphenyl ethers (PBDEs), organophosphate esters (OPEs), and other brominated flame retardants (BFRs), in various industries. The study aimed to characterize OPE metabolite urinary concentrations and PBDE serum concentrations among workers from different industries, compare these concentrations between industries and the general population, and evaluate the likely route of exposure (dermal or inhalation). The results showed that workers from chemical manufacturing had significantly higher (p <0.05) urinary concentrations of OPE metabolites compared to other industries. Spray polyurethane foam workers had significantly higher (p <0.05) urinary concentrations of bis(1-chloro-2-propyl) phosphate (BCPP) compared to other industries. Electronic scrap workers had higher serum concentrations of certain PBDE congeners compared to the general population. Correlations were observed between hand wipe samples and air samples containing specific flame-retardant parent chemicals and urinary metabolite concentrations for some industries, suggesting both dermal absorption and inhalation as primary routes of exposure for OPEs. Overall, this study provides insights into occupational exposure to flame retardants in different industries and highlights the need for further research on emerging flame retardants and exposure reduction interventions.

Leaching kinetics and bioaccumulation potential of additive-derived organophosphate esters in microplastics

Considerable research has been conducted to evaluate microplastics (MPs) as vehicles for the transfer of hazardous pollutants in organisms. However, little effort has been devoted to the chemical release of hazardous additive-derived pollutants from MPs in gut simulations. This study looked at the leaching kinetics of organophosphate esters (OPFRs) from polypropylene (PP) and polystyrene (PS) MPs in the presence of gut surfactants, specifically sodium taurocholate, at two biologically relevant temperatures for marine organisms. Diffusion coefficients of OPFRs ranged from 1.71 × 10-20 to 4.04 × 10-18 m2 s-1 in PP and 2.91 × 10-18 to 1.51 × 10-15 m2 s-1 in PS. The accumulation factors for OPFRs in biota-plastic and biota-sediment interactions ranged from 1.52 × 10-3-69.1 and 0.02-0.7, respectively. Based on B3LYP/6-31G (d,p) calculations, the biodynamic model analysis revealed a slight increase in the bioaccumulation of OPFRs at a minor dose of 0.05% MPs. However, at higher concentrations (0.5% and 5% MPs), there was a decrease in bioaccumulation compared to the lower concentration for most OPFR compounds. In general, the ingestion of PE MPs notably contributed to the bioaccumulation of OPFRs in lugworms, whereas the contribution of PP and PS MPs was minimal. This could vary among sites exhibiting varying levels of MP concentrations or MPs displaying stronger affinities towards chemicals.

Cytotoxic and genotoxic effects of triphenyl phosphate on root tip cells of Allium cepa L

Triphenyl phosphate (TPhP) is a tri-ester of phosphoric acid and phenol. It is used in products such as polyvinyl chloride, hydraulic fluids, polymers, photographic films and printed circuit cards as a flame retardant. It is also widely used in consumer products like electric and electronic devices, textiles and building necessaries. TPhP can diffuse into its surrounding environment easily, because it does not form a chemical bond with polymeric materials. Therefore, bio-monitoring of such compounds is needed for possible environmental and human health effects. In this study, we evaluated genotoxicity and cytotoxicity of TPhP on Allium cepa on the cells of root tips. A 10 mg/mL stock solution of TPhP was dissolved in DMSO and dilutions of 2, 4, 6, 8 and 10 mg/mL was made in distilled water. Onion bulbs, rooted in test tubes, were exposed to these concentrations of TPhP for 24 h. Distilled water was used as a negative control, 0.7% hydrogen peroxide was used as positive control, and 0.5% DMSO was used as solvent control. Significant inhibition of onion root growth was observed following treatment with the 6, 8 and 10 mg/mL TPhP ranges in comparison with the negative and solvent control groups (p < 0.05). Furthermore, in the TPhP treatment groups, total chromosome aberration ratios were significantly high in comparison with the controls (p < 0.05). These results suggest that TPhP have cytotoxic and genotoxic effects on A. cepa root tips.

Elucidating the toxicity mechanisms of organophosphate esters by adverse outcome pathway network

With the widespread use of organophosphate esters (OPEs), the accumulation and toxicity effect of OPEs in biota are attracting more and more concern. In order to clarify the mechanism of toxicity of OPEs to organisms, this study reviewed the OPEs toxicity and systematically identified the mechanism of OPEs toxicity under the framework of adverse outcome pathway (AOP). OPEs were divided into three groups (alkyl-OPEs, aryl-OPEs, and halogenated-OPEs) and biota was divided into aquatic organism and mammals. The results showed that tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) and triphenyl phosphate (TPHP) mainly caused neurotoxicity, reproductive, and hepatotoxicity in different mechanisms. According to the constructed AOP network, the toxicity mechanism of OPEs on aquatic organisms and mammals is different, which is mainly attributed to the different biological metabolic systems of aquatic organisms and mammals. Interestingly, our results indicate that the toxicity effect of the three kinds of OPEs on aquatic organisms is different, while there was no obvious difference in the mechanism of toxicity of OPEs on mammals. This study provides a theoretical basis for OPEs risk assessment in the future.

Unregistered Hexaphenoxycyclotriphosphazene and Its Metabolite Antagonize Retinoic Acid and Retinoic X Receptors and Cause Early Developmental Damage

Hexaphenoxycyclotriphosphazene (HPCTP), an unregistered chemical, has been used as a substitute for triphenyl phosphate in flame retardants and plasticizers. Here, we identified its metabolite, pentaphenoxycyclotriphosphazene (PPCTP) in the liver of Japanese medaka exposed to HPCTP. When sexually mature female medaka were exposed to HPCTP at 37.0, 90.4, and 465.4 ng/L for 35 days, the HPCTP concentration (642.1-2531.9 ng/g lipid weight [lw]) in the embryos considerably exceeded that (34.7-298.1 ng/g lw) in the maternal muscle, indicating remarkable maternal transfer. During 0-9 days postfertilization, the HPCTP concentration in the embryos decreased continuously, while the PPCTP concentration increased. HPCTP and PPCTP antagonized the retinoic X receptor with 50% inhibitory concentrations (IC50) of 34.8 and 21.2 μM, respectively, and PPCTP also antagonized the retinoic acid receptor with IC50 of 2.79 μM. Such antagonistic activities may contribute to eye deformity (4.7% at 465.4 ng/L), body malformation (2.1% at 90.4 ng/L and 6.8% at 465.4 ng/L), and early developmental mortality (11.6-21.7% in all exposure groups) of the embryos. HPCTP was detected in a main tributary of the Yangtze River Basin. Thus, HPCTP poses a risk to wild fish populations, given the developmental toxicities associated with this chemical and its metabolite.

Upregulation of oxidative stress by triphenyl phosphate (TPhP) exposure causes antioxidant insult and apoptotic process in Epithelioma papulosum cyprini (EPC) cells

Triphenyl phosphate (TPhP) is the predominant compound of organophosphate flame retardants (OPFRs), which can elicit a toxicological effect on physiological response and tissue development of fish. In this study, we investigated the effect of TPhP exposure on cell viability, antioxidant capacities, and apoptosis in EPC cells. Current study revealed that TPhP exposure could decrease cell viability and promote intracellular oxidative stress in EPC cells. In addition, high-dose TPhP exposure could facilitate antioxidant insults and cause mitochondrial collapse in a dose-dependent manner, along with increased gene expressions involved in apoptosis and unfolded protein response (UPR). These results indicated that reactive oxygen species (ROS)-induced cytotoxic stress and cell death were involved in antioxidant insults and apoptotic activation in TPhP-exposed fish cells.

Abnormality of mussel in the early developmental stages induced by graphene and triphenyl phosphate: In silico toxicogenomic data-mining, in vivo, and toxicity pathway-oriented approach

Increasing number of complex mixtures of organic pollutants in coastal area (especially for nanomaterials and micro/nanoplastics associated chemicals) threaten aquatic ecosystems and their joint hazards are complex and demanding tasks. Mussels are the most sensitive marine faunal groups in the world, and their early developmental stages (embryo and larvae) are particularly susceptible to environmental contaminants, which can distinguish the probable mechanisms of mixture-induced growth toxicity. In this study, the potential critical target and biological processes affected by graphene and triphenyl phosphate (TPP) were developed by mining public toxicogenomic data. And their combined toxic effects were verified by toxicological assay at early developmental stages in filter-feeding mussels (embryo and larvae). It showed that interactions among graphene/TPP with 111 genes (ABCB1, TP53, SOD, CAT, HSP, etc.) affected phenotypes along conceptual framework linking these chemicals to developmental abnormality endpoints. The PPAR signaling pathway, monocarboxylic acid metabolic process, regulation of lipid metabolic process, response to oxidative stress, and gonad development were noted as the key molecular pathways that contributed to the developmental abnormality. Enriched phenotype analysis revealed biological processes (cell proliferation, cell apoptosis, inflammatory response, response to oxidative stress, and lipid metabolism) affected by the investigated mixture. Combined, our results supported that adverse effects induced by contaminants/ mixture could not only be mediated by single receptor signaling or be predicted by the simple additive effect of contaminants. The results offer a framework for better comprehending the developmental toxicity of environmental contaminants in mussels and other invertebrate species, which have considerable potential for hazard assessment of coastal mixture.

Triphenyl phosphate-induced pericardial edema in zebrafish embryos is reversible following depuration in clean water

Triphenyl phosphate (TPHP) - a widely used organophosphate-based flame retardant - blocks cardiac looping during zebrafish development in a concentration-dependent manner, a phenotype that is dependent on disruption of embryonic osmoregulation and pericardial edema formation. However, it's currently unclear whether (1) TPHP-induced effects on osmoregulation are driven by direct TPHP-induced injury to the embryonic epidermis and (2) whether TPHP-induced pericardial edema is reversible or irreversible following cessation of exposure. Therefore, the objectives of this study were to determine whether TPHP-induced pericardial edema is reversible and whether TPHP causes injury to the embryonic epidermis by quantifying the number of DAPI-positive epidermal cells and analyzing the morphology of the yolk sac epithelium using scanning electron microscopy. First, we found that exposure to 5 μM TPHP from 24-72 h post-fertilization (hpf) did not increase prolactin - a hormone that regulates ions and water levels - in embryonic zebrafish, whereas high ionic strength exposure media was associated with elevated levels of prolactin. Second, we found that exposure to 5 μM TPHP from 24-72 hpf did not decrease DAPI-positive epidermal cells within the embryonic epithelium, and that co-exposure with 2.14 μM fenretinide - a synthetic retinoid that promotes epithelial wound repair - from 24-72 hpf did not mitigate the prevalence of TPHP-induced epidermal folds within the yolk sac epithelium when embryos were exposed within high ionic strength exposure media. Finally, we found that the pericardial area and body length of embryos exposed to 5 μM TPHP from 24-72 hpf were similar to vehicle-treated embryos at 120 hpf following transfer to clean water and depuration of TPHP from 72-120 hpf. Overall, our findings suggest that (1) the ionic strength of exposure media may influence the baseline physiology of zebrafish embryos; (2) TPHP does not cause direct injury to the embryonic epidermis; and (3) TPHP-induced effects on pericardial area and body length are reversible 48 h after transferring embryos to clean water.

Exploring the potential of ToxCast™ data for mechanism-based prioritization of chemicals in regulatory context: Case study with priority existing chemicals (PECs) under K-REACH

Recent studies have highlighted the potential of ToxCast™ database to mechanism-based prioritization of chemicals. To explore the applicability of ToxCast data in the context of regulatory inventory chemicals, we screened 510 priority existing chemicals (PECs) regulated under the Act on the Registration and Evaluation of Chemical Substances (K-REACH) using ToxCast bioassays. In our analysis, a hit-call data matrix containing 298984 chemical-gene interactions was computed for 949 bioassays with the intended target genes, which enabled the identification of the putative toxicity mechanisms. Based on the reactivity to the chemicals, we analyzed 412 bioassays whose intended target gene families were cytochrome P450, oxidoreductase, transporter, nuclear receptor, steroid hormone, and DNA-binding. We also identified 141 chemicals based on their reactivity in the bioassays. These chemicals are mainly in consumer products including colorants, preservatives, air fresheners, and detergents. Our analysis revealed that in vitro bioactivities were involved in the relevant mechanisms inducing in vivo toxicity; however, this was not sufficient to predict more hazardous chemicals. Overall, the current results point to a potential and limitation in using ToxCast data for chemical prioritization in regulatory context in the absence of suitable in vivo data.

Direct BMP signaling to chordoblasts is required for the initiation of segmented notochord sheath mineralization in zebrafish vertebral column development

The vertebral column, with the centra as its iteratively arranged building blocks, represents the anatomical key feature of the vertebrate phylum. In contrast to amniotes, where vertebrae are formed from chondrocytes and osteoblasts deriving from the segmentally organized neural crest or paraxial sclerotome, teleost vertebral column development is initiated by chordoblasts of the primarily unsegmented axial notochord, while sclerotomal cells only contribute to later steps of vertebrae formation. Yet, for both mammalian and teleostean model systems, unrestricted signaling by Bone Morphogenetic Proteins (BMPs) or retinoic acid (RA) has been reported to cause fusions of vertebral elements, while the interplay of the two signaling processes and their exact cellular targets remain largely unknown. Here, we address this interplay in zebrafish, identifying BMPs as potent and indispensable factors that, as formerly shown for RA, directly signal to notochord epithelial cells/chordoblasts to promote entpd5a expression and thereby metameric notochord sheath mineralization. In contrast to RA, however, which promotes sheath mineralization at the expense of further collagen secretion and sheath formation, BMP defines an earlier transitory stage of chordoblasts, characterized by sustained matrix production/col2a1 expression and concomitant matrix mineralization/entpd5a expression. BMP-RA epistasis analyses further indicate that RA can only affect chordoblasts and their further progression to merely mineralizing cells after they have received BMP signals to enter the transitory col2a1/entpd5a double-positive stage. This way, both signals ensure consecutively for proper mineralization of the notochord sheath within segmented sections along its anteroposterior axis. Our work sheds further light onto the molecular mechanisms that orchestrate early steps of vertebral column segmentation in teleosts. Similarities and differences to BMP's working mechanisms during mammalian vertebral column formation and the pathomechanisms underlying human bone diseases such as Fibrodysplasia Ossificans Progressiva (FOP) caused by constitutively active BMP signaling are discussed.

Effects of all-trans and 9-cis retinoic acid on differentiating human neural stem cells in vitro

Cyanobacterial blooms are known sources of environmentally-occurring retinoid compounds, including all-trans and 9-cis retinoic acids (RAs). The developmental hazard for aquatic organisms has been described, while the implications for human health hazard assessment are not yet sufficiently characterized. Here, we employ a human neural stem cell model that can differentiate in vitro into a mixed culture of neurons and glia. Cells were exposed to non-cytotoxic 8-1000 nM all-trans or 9-cis RA for 9-18 days (DIV13 and DIV22, respectively). Impact on biomarkers was analyzed on gene expression (RT-qPCR) and protein level (western blot and proteomics) at both time points; network patterning (immunofluorescence) on DIV22. RA exposure significantly concentration-dependently increased gene expression of retinoic acid receptors and the metabolizing enzyme CYP26A1, confirming the chemical-specific response of the model. Expression of thyroid hormone signaling-related genes remained mostly unchanged. Markers of neural progenitors/stem cells (PAX6, SOX1, SOX2, NESTIN) were decreased with increasing RA concentrations, though a basal population remained. Neural markers (DCX, TUJ1, MAP2, NeuN, SYP) remained unchanged or were decreased at high concentrations (200-1000 nM). Conversely, (astro-)glial marker S100β was increased concentration-dependently on DIV22. Together, the biomarker analysis indicates an RA-dependent promotion of glial cell fates over neural differentiation, despite the increased abundance of neural protein biomarkers during differentiation. Interestingly, RA exposure induced substantial changes to the cell culture morphology: while low concentrations resulted in a network-like differentiation pattern, high concentrations (200-1000 nM RA) almost completely prevented such network patterning. After functional confirmation for implications in network function, such morphological features could present a proxy for network formation assessment, an apical key event in (neuro-)developmental Adverse Outcome Pathways. The described application of a human in vitro model for (developmental) neurotoxicity to emerging environmentally-relevant retinoids contributes to the evidence-base for the use of differentiating human in vitro models for human health hazard and risk assessment.

Gestational exposure to triphenyl phosphate induces epigenetic modifications in C57Bl/6 fetal liver

INTRODUCTION

Triphenyl phosphate (TPHP) is a chemical flame retardant and plasticizer which is added to consumer and industrial products. The developmental origins of health and disease hypothesis postulate that in utero exposures can have later-in-life effects on the developing fetus and can alter fetal gene expression. This study aimed to determine whether epigenetic modifications occurred following in utero TPHP exposure in mice and whether these changes were dose and/or sex-dependent.

METHODS

Pregnant C57Bl/6 mice were treated with 0, 5, 25, or 50 mg/kg of TPHP on gestational days (GD) 8, 10, 12, and 14 via intraperitoneal injection and fetal livers were collected on GD 19. Changes in the levels of acetylation of H3 and H4, as well as methylation of H3K9 and global DNA methylation were assessed in the fetal livers by western blot.

RESULTS

Results showed that there was a significant decrease in fetal DNA methylation following in utero exposure to 50 mg/kg TPHP compared to the control (0 mg/kg) independent of the sex of the fetus. While there were no significant alterations compared to controls in any histone modifications at any dose or sex following in utero TPHP exposure, we did note a decrease (t test, p = .025) in the levels of acetylated H3 in males versus females following a maternal dose of 25 mg/kg. The monomethylated H3K9 levels were also increased in females versus males following exposure to TPHP at 5 mg/kg (p = .018) and 25 mg/kg (p = .027) when analyzed via unpaired t tests, although not significantly different from controls.

DISCUSSION

The results suggest that gestational TPHP exposure can induce epigenetic modifications in murine fetal tissue. Specifically, global DNA methylation levels were downregulated in response to TPHP. Additionally, males appear to be more sensitive to TPHP-induced histone modifications than females. These data support the need for further studies investigating the impacts of gestational TPHP exposure on the developing fetus.

Triphenyl phosphate-induced pericardial edema in zebrafish embryos is dependent on the ionic strength of exposure media

Pericardial edema is commonly observed in zebrafish embryo-based chemical toxicity screens, and a mechanism underlying edema may be disruption of embryonic osmoregulation. Therefore, the objective of this study was to identify whether triphenyl phosphate (TPHP) - a widely used aryl phosphate ester-based flame retardant - induces pericardial edema via impacts on osmoregulation within embryonic zebrafish. In addition to an increase in TPHP-induced microridges in the embryonic yolk sac epithelium, an increase in ionic strength of exposure media exacerbated TPHP-induced pericardial edema when embryos were exposed from 24 to 72 h post-fertilization (hpf). However, there was no difference in embryonic sodium concentrations in situ within TPHP-exposed embryos relative to embryos exposed to vehicle (0.1% DMSO) from 24 to 72 hpf. Interestingly, increasing the osmolarity of exposure media with mannitol (an osmotic diuretic which mitigates TPHP-induced pericardial edema) and increasing the ionic strength of the exposure media (which exacerbates TPHP-induced pericardial edema) did not affect embryonic doses of TPHP, suggesting that TPHP uptake was not altered under these varying experimental conditions. Overall, our findings suggest that TPHP-induced pericardial edema within zebrafish embryos is dependent on the ionic strength of exposure media, underscoring the importance of further standardization of exposure media and embryo rearing protocols in zebrafish-based chemical toxicity screening assays.

Association of organophosphate ester exposure with cardiovascular disease among US adults: Cross-sectional findings from the 2011-2018 National Health and Nutrition Examination Survey

Organophosphate esters (OPEs) are widely used as flame retardants and plasticizers worldwide. Therefore, the potentially deleterious effect of OPE on human beings deserves extensive attention. The primary objective of this present study was to untangle the relationship between OPE exposure and cardiovascular disease (CVD) among general population. Detailed information about participants' baseline characteristics, involving socioeconomic data, demographic data and key covariates was obtained from National Health and Nutrition Examination Survey (NHANES) 2011-2018. Multivariate logistic regression models with adjustment for prior-determined covariates were utilized to examine the relationship between various OPEs and CVD among US adults and calculate odd ratios (ORs) and corresponding confidence intervals (CIs). Two multi-pollutant statistical strategies (weighted quantile sum regression and Bayesian kernel machine regression) were employed to investigate the joint effect of OPE mixture on CVD. A total of 5067 participants were included in this study. In completely-adjusted logistic model, the highest tertiles of OPE metabolites were positively associated with CVD risk, while the relationships did not reach statistical significance. The weighted quantile sum (WQS) index was significantly correlated with increased prevalence of CVD (adjusted OR: 1.25; CI: 1.02, 1.53, p value = 0.032) and Diphenyl phosphate (DPHP) was the greatest contributor (31.38%). The BKMR also indicated that mixed OPE exposure associated with an increased risk of CVD. Taken together, the present study demonstrated that there were possible links between OPE exposures and increased risk of CVD, while the relationships did not reach statistical significance. Our study provided the suggestive evidence that cumulative effect of OPE mixtures on CVD. DPHP may be a major driver of this positive association. Given the limitation of cross-sectional design and relatively limited kinds of OPE metabolites, further studies are warranted to longitudinally evaluate the potential effect of a wider range of OPEs on CVD or cardiac metabolism.

Comprehensive assessment of the ecological risk of exposure to triphenyl phosphate in a bioindicator tadpole

The toxicity of triphenyl phosphate (TPhP) to aquatic organisms in surface waters has been demonstrated; However, an understanding of toxicity profiles of TPhP in amphibians is limited. Therefore, the adverse effects and threshold concentrations of TPhP on metamorphosis, growth, locomotion, and hepatic antioxidants of Gosner stage 25 Polypedates megacephalus tadpoles under long-term (35 d) exposure to six TPhP concentrations until complete metamorphosis were assessed. Additionally, the overall effect of using integrated multiple biomarkers were determined to demonstrate the potential ecological risks of waterborne TPhP at environmentally relevant concentrations in amphibian tadpoles. With increasing TPhP concentrations, physical parameters (snout-vent length, body mass, condition factor, and hepatic somatic index), jumping distance, hepatic catalase, and superoxide dismutase activities decreased, whereas metamorphosis time and malondialdehyde content increased. The threshold concentration of TPhP that affected the tadpole biomarker, except for metamorphosis rate and jumping distance, was 50-400 μg/L. Furthermore, the standardized scores of the examined integrated biomarkers in the six TPhP concentrations were visualized using radar plots and calculated as the integrated biomarker responses (IBRs). The varying TPhP concentrations had different scores in the radar plots, and the threshold for affecting the IBR value was 10 μg/L, which was close to the TPhP concentration in surface waters. Additionally, IBR values were strongly positively correlated with the TPhP concentrations. These findings indicate that environmentally relevant exposure to waterborne TPhP can pose an ecological risk to amphibian tadpoles. This study can serve as a reference and assist in the formulation of relevant policies and strategies to control TPhP pollution in water bodies.

Toxic effect of triphenyl phosphate (TPHP) on Cyprinus carpio and the intestinal microbial community response

Triphenyl phosphate (TPHP) is a kind of organophosphorus flame retardants, and its use is increasing annually. However, the toxic effect associated with exposure to it has not been adequately investigated. Therefore, in this study, we determined the toxic dose of TPHP in the economic fish species, Cyprinus carpio. Acute and subacute toxicity tests were conducted, and the enrichment of TPHP in the gills, brain, intestines, and liver were determined by Liquid Chromatography-Mass Spectrometry, and the response of carp gut microbial community to TPHP stress was determined using 16 S rRNA gene high-throughput sequencing. Results showed that the 96-h LC₅₀ of TPHP in carp was 7 mg/L. At the 7 d, the order of TPHP absorption was as follows (from highest to lowest): gills > intestine > liver > brain, but at the 28 d and the purification period, the order of TPHP absorption was brain > gills > intestine > liver. TPHP exposure at 3.5 mg/L decreased α-diversity of the intestinal microbial community (p < 0.05), and altered community composition, in particular the relative abundance of dominant microbial populations. Functional profiles of the microbial communities predicted based on 16 S rRNA gene data showed upregulation in the degradation of exogenous substances and energy metabolism of the TPHP-treated groups (p < 0.05), suggesting that intestinal microbial taxa play a role in reducing TPHP toxicity. The results provide insights that could facilitate risk assessments of TPHP pollutants in aquatic environments and the management of associated water pollution.

Estrogenic and growth inhibitory responses to organophosphorus flame retardant metabolites in zebrafish embryos

Recent evidence has revealed that organophosphorus flame retardants (OPFRs) elicit a variety of toxic effects, including endocrine disruption. The present study examined estrogenic and growth inhibitory responses to OPFR metabolites in comparison to their parent compounds using zebrafish eleutheroembryos.¹ Exposure to 4-hydroxylphenyl diphenyl phosphate (HO-p-TPHP) but not its parent compound triphenyl phosphate (TPHP) elicited upregulation of a marker gene of estrogenic responses, cytochrome P450 19A1b (CYP19A1b), and this upregulation was reversed by co-exposure to an estrogen receptor antagonist. Tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) and bis(1,3-dichloro-2-propyl) phosphate (BDCIPP), as well as 3-hydroxylphenyl diphenyl phosphate (HO-m-TPHP) and diphenyl phosphate (DPHP), did not elicit significant changes in the CYP19A1b expression. Reduction in body length was induced by TPHP and to a lesser extent by its hydroxylated metabolites. Altered expression of genes involved in the synthesis and action of thyroid hormones, including iodothyronine deiodinases 1 and 2, thyroid hormone receptor alpha, and transthyretin, were commonly observed for TPHP and its hydroxylated metabolites. Reduction in the body length was also seen in embryos exposed to TDCIPP but not BDCIPP. The transcriptional effect of TDCIPP was largely different from that of TPHP, with decreased expression of growth hormone and prolactin observed only in TDCIPP-exposed embryos. Considering the concentration-response relationships for the growth retardation and gene expression changes, together with existing evidence from other researchers, it is likely that prolactin is in part involved in the growth inhibition caused by TDCIPP. The present study showed similarities and differences in the endocrine disruptive effects of OPFRs and their metabolites.

Linking biochemical and individual-level effects of chlorpyrifos, triphenyl phosphate, and bisphenol A on sea urchin (Paracentrotus lividus) larvae

The effects of three relevant organic pollutants: chlorpyrifos (CPF), a widely used insecticide, triphenyl phosphate (TPHP), employed as flame retardant and as plastic additive, and bisphenol A (BPA), used primarily as plastic additive, on sea urchin (Paracentrotus lividus) larvae, were investigated. Experiments consisted of exposing sea urchin fertilized eggs throughout their development to the 4-arm pluteus larval stage. The antioxidant enzymes glutathione reductase (GR) and catalase (CAT), the phase II detoxification enzyme glutathione S-transferase (GST), and the neurotransmitter catabolism enzyme acetylcholinesterase (AChE) were assessed in combination with responses at the individual level (larval growth). CPF was the most toxic compound with 10 and 50% effective concentrations (EC₁₀ and EC₅₀) values of 60 and 279 μg/l (0.17 and 0.80 μM), followed by TPHP with EC₁₀ and EC₅₀ values of 224 and 1213 μg/l (0.68 and 3.7 μM), and by BPA with EC₁₀ and EC₅₀ values of 885 and 1549 μg/l (3.9 and 6.8 μM). The toxicity of the three compounds was attributed to oxidative stress, to the modulation of the AChE response, and/or to the reduction of the detoxification efficacy. Increasing trends in CAT activity were observed for BPA and, to a lower extent, for CPF. GR activity showed a bell-shaped response in larvae exposed to CPF, whereas BPA caused an increasing trend in GR. GST also displayed a bell-shaped response to CPF exposure and a decreasing trend was observed for TPHP. An inhibition pattern in AChE activity was observed at increasing BPA concentrations. A potential role of the GST in the metabolism of CPF was proposed, but not for TPHP or BPA, and a significant increase of AChE activity associated with oxidative stress was observed in TPHP-exposed larvae. Among the biochemical responses, the GR activity was found to be a reliable biomarker of exposure for sea urchin early-life stages, providing a first sign of damage. These results show that the integration of responses at the biochemical level with fitness-related responses (e.g., growth) may help to improve knowledge about the impact of toxic substances on marine ecosystems.

Discovery of contaminants with antagonistic activity against retinoic acid receptor in house dust

Retinoic acid receptors (RARs) control reproduction and development in vertebrates, but little attention has been paid to anthropogenic chemicals exhibiting RAR agoniztic/antagonistic activity. Here we applied a His-RARα pull-down assay combined with high-resolution mass spectrometry to identify chemicals with RARα activity in house dust. After screening, a total of 540 peaks were retained as potential RARα ligands. The mass spectra of 14 chemicals matched with those in the database, of which triphenyl phosphate, galaxolidone, di(2-ethylhexyl) phthalate (DEHP), tris(2-ethylhexyl) phosphate (TEHP), and tris(2-butoxyethyl) phosphate were confirmed by their standards. While one chemical in the sample matched with monophenyl phosphate in the MS/MS database, its retention time was much higher than that of monophenyl phosphate standard, suggesting that it may be an in-source fragment. Its parent ion was finally identified to be m/z 399.2663 using a similarity analysis among chromatographic peaks of hundreds of ions at the same retention time in MS¹ spectrum, and bis(2-ethylhexyl) phenyl phosphate (BEHPP) was identified. BEHPP, DEHP, and TEHP were for the first time identified to be RARα antagonists with IC₅₀ values of 6556, 6600, and 2538 nM, respectively. This study improved structural annotation and filled the knowledge gap regarding widespread environmental contaminants with RAR antagonistic activity.

Metabolomic analysis and oxidative stress response reveals the toxicity in Escherichia coli induced by organophosphate flame retardants tris(2-chloroethyl) phosphate and triphenyl phosphate

Organophosphate flame retardants (OPFRs) are emerging environmental pollutants that are increasingly being used in consumer commodities. The adverse effects on biota induced by tris(2-chloroethyl) phosphate (TCEP) and triphenyl phosphate (TPHP) have become a growing concern. Unfortunately, toxic mechanisms at the molecular level for OPFRs in organisms are still lacking. Herein, Escherichia coli (E.coli) was exposed to TCEP and TPHP for 24 and 48 h to reveal oxidative stress response and molecular toxicity mechanisms. The results indicated that promotion of ROS overload occurred at higher dosages groups. The levels of SOD and CAT were significantly elevated along with the increase of MDA attributed to lipid peroxidation. Additionally, apoptosis rates increased, accompanied by a decline in membrane potential and Na⁺/K⁺-ATPase and Ca²⁺/Mg²⁺-ATPase contents, signifying that E. coli cytotoxicity induced by TCEP and TPHP was mediated by oxidative stress. Based on metabolomic analysis, different metabolic pathways were disrupted, including glycolysis/gluconeogenesis, pentose phosphate metabolism, purine metabolism, glutathione metabolism, amino acid biosynthesis, butanoate metabolism, alanine and aspartate metabolism. Most differentially expressed metabolites were downregulated, indicating an inhibitory effect on metabolic functions and key metabolic pathways. These findings generated new insights into the potential environmental risks of OPFRs in aquatic organisms.

Specificity of time- and dose-dependent morphological endpoints in the fish embryo acute toxicity (FET) test for substances with diverse modes of action: the search for a "fingerprint"

The fish embryo acute toxicity (FET) test with the zebrafish (Danio rerio) embryo according to OECD TG 236 was originally developed as an alternative test method for acute fish toxicity testing according to, e.g., OECD TG 203. Given the versatility of the protocol, however, the FET test has found application beyond acute toxicity testing as a common tool in environmental hazard and risk assessment. Whereas the standard OECD guideline is restricted to four core endpoints (coagulation as well as lack of somite formation, heartbeat, and tail detachment) for simple, rapid assessment of acute toxicity, further endpoints can easily be integrated into the FET test protocol. This has led to the hypothesis that an extended FET test might allow for the identification of different classes of toxicants via a "fingerprint" of morphological observations. To test this hypothesis, the present study investigated a set of 18 compounds with highly diverse modes of action with respect to acute and sublethal endpoints. Especially at higher concentrations, most observations proved toxicant-unspecific. With decreasing concentrations, however, observations declined in number, but gained in specificity. Specific observations may at best be made at test concentrations ≤ EC10. The existence of a "fingerprint" based on morphological observations in the FET is, therefore, highly unlikely in the range of acute toxicity, but cannot be excluded for experiments at sublethal concentrations.

Mechanistic understanding of the toxicity of triphenyl phosphate (TPhP) to the marine diatom Phaeodactylum tricornutum: Targeting chloroplast and mitochondrial dysfunction

Triphenyl phosphate (TPhP) has been widely detected in various environmental media, including seawater, threatening the survival of marine organisms, especially marine planktic algae that are directly exposed to contamination. However, the hazard potential of TPhP on marine algae has not been studied thoroughly and systematically. In this study, a marine diatom, Phaeodactylum tricornutum, was treated with three concentrations of TPhP (0.08, 0.4 and 0.8 mg/L), and after 24 h of exposure, population growth, ultrastructure, physiology and transcriptome changes were investigated. The results reflected that TPhP suppressed the population growth of algae in a concentration-dependent manner, and the 24-h EC₅₀ value was 1.27 mg/L. At all test concentrations, P. tricornutum could absorb more than 70% of TPhP from seawater over 24 h. Ultrastructural observations suggested a distorted lamellar structure with higher TPhP treatments, and the contents of chlorophyll and its precursors were also altered, as were photosynthetic activities. Moreover, 0.8 mg/L TPhP decreased the mitochondrial membrane potential, induced ROS overproduction and disrupted the cell membrane permeability of algal cells. At the transcriptomic level, some differentially expressed genes were enriched in photosynthetic electron transport, carbon fixation, chlorophyll biosynthesis, the TCA cycle and mitochondrial glycolysis. Additionally, 0.8 mg/L TPhP inhibited lipid de novo biosynthesis, suggesting that it may target organelle membranes, thereby contributing to functional defects. Chloroplasts and mitochondria were interpreted to be the subcellular targets of TPhP in P. tricornutum. These data promote the understanding of the toxic action mode of TPhP toward marine diatoms.

Bioaccumulation of organophosphorus flame retardants in the marine mussel Mytilus galloprovincialis

The bioaccumulation and depuration of seven organophosphorus flame retardants (OPFRs) in marine mussel Mytilus galloprovincialis were studied. OPFRs showed to be bioavailable in aquatic environments. When mussels are exposed to environmentally relevant concentrations of OPFRs, uptake kinetics fit well to a first-order model with a single compartment; in contrast depuration rates were generally underestimated by that model, most likely because it does not take into account the biotransformation of OPFRs by the organisms. The highest bioaccumulation rates were observed for tricresyl phosphate (TCrP), triphenyl phosphate (TPhP) and 2-ethylhexyldiphenylphosphate (EHDPP). This could be due to the presence of aryl groups in these compounds, their low solubility in water, and their affinity for fat tissues. According to these findings TCrP, with a BCF value of 4042 L kg^-1 wet weight, should be classified in environmental regulations as an accumulative chemical.

Degradation of pesticides diazinon and diazoxon by phosphotriesterase: insight into divergent mechanisms from QM/MM and MD simulations

Enzymatic hydrolysis by phosphotriesterase (PTE) is one of the most effective ways of degrading organophosphorus pesticides, but the catalytic efficiency depends on the structural features of substrates. Here the enzymatic degradation of diazinon (DIN) and diazoxon (DON), characterized by PS and PO, respectively, have been investigated by QM/MM calculations and MM MD simulations. Our calculations demonstrate that the hydrolysis of DON (with PO) is inevitably initiated by the nucleophilic attack of the bridging-OH^- on the phosphorus center, while for DIN (with PS), we proposed a new degradation mechanism, initiated by the nucleophilic attack of the Zn_α-bound water molecule, for its low-energy pathway. For both DIN and DON, the hydrolytic reaction is predicted to be the rate-limiting step, with energy barriers of 18.5 and 17.7 kcal mol^-1, respectively. The transportation of substrates to the active site, the release of the leaving group and the degraded product are generally verified to be favorable by MD simulations via umbrella sampling, both thermodynamically and dynamically. The side-chain residues Phe132, Leu271 and Tyr309 play the gate-switching role to manipulate substrate delivery and product release. In comparison with the DON-enzyme system, the degraded product of DIN is more easily released from the active site. These new findings will contribute to the comprehensive understanding of the enzymatic degradation of toxic organophosphorus compounds by PTE.

Triphenyl phosphate-induced pericardial edema is associated with elevated epidermal ionocytes within zebrafish embryos

Triphenyl phosphate (TPHP) is an organophosphate ester-based plasticizer and flame retardant. The objective of this study was to identify the potential role of epidermal ionocytes in mediating TPHP-induced pericardial edema within zebrafish embryos. Exposure to TPHP from 24 to 72 h post fertilization (hpf) resulted in a significant increase in pericardial edema and the number of ionocytes at 72 hpf relative to time-matched embryos treated with vehicle. In addition, co-exposure of embryos to mannitol (an osmotic diuretic) blocked TPHP-induced pericardial edema and effects on ionocyte abundance. However, knockdown of ATPase1a1.4 - an abundant Na⁺/K⁺-ATPase localized to epidermal ionocytes - mitigated TPHP-induced effects on ionocyte abundance but not pericardial edema, whereas co-exposure of embryos to ouabain - a Na⁺/K⁺-ATPase inhibitor - enhanced TPHP-induced pericardial edema but not ionocyte abundance. Overall, our findings suggest that TPHP may have multiple mechanisms of toxicity leading to an increase in ionocyte abundance and pericardial edema within developing zebrafish embryos.

Molecular neural crest cell markers enable discrimination of organophosphates in the murine cardiac embryonic stem cell test

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Organophosphates induced distinctive effects on neural crest cells within the ESTc.

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Neural crest gene transcripts were of added value to the original ESTc read-out.

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Mechanistic information adds value to the applicability of the ESTc.

The cardiac embryonic stem cell test (ESTc) originally used the differentiation of beating cardiomyocytes for embryotoxicity screenings of compounds. However, the ESTc consists of a heterogeneous cell population, including neural crest (NC) cells, which are important contributors to heart development in vivo. Molecular markers for NC cells were investigated to explore if this approach improved discrimination between structurally related chemicals, using the three organophosphates (OP): chlorpyrifos (CPF), malathion (MLT), and triphenyl phosphate (TPP). To decrease the test duration and to improve the objective quantification of the assay read-out, gene transcript biomarkers were measured on study day 4 instead of the traditional cardiomyocyte beating assessment at day 10. Gene expression profiling and immunocytochemistry were performed using markers for pluripotency, proliferation and cardiomyocyte and NC differentiation. Cell proliferation was also assessed by measurements of embryoid body (EB) size and total protein quantification (day 7). Exposure to the OPs resulted in similar patterns of inhibition of beating cardiomyocyte differentiation and of myosin protein expression on day 10. However, these three chemically related compounds induced distinctive effects on NC cell differentiation, indicated by changes in expression levels of the NC precursor (Msx2), NC marker (Ap2α), and epithelial to mesenchymal transition (EMT; Snai2) gene transcripts. This study shows that investigating NC markers can provide added value for ESTc outcome profiling and may enhance the applicability of this assay for the screening of structurally related test chemicals.

Identification of biotransformation products of organophosphate ester from various aquatic species by suspect and non-target screening approach

Organic pollutants that are introduced into the aquatic ecosystem can transform by various mechanisms. Biotransformation is an important process for predicting the remaining structures of pollutants in the ecosystem, and their toxicity. This study focused on triphenyl phosphate (TPHP), which is a commonly used organophosphate flame retardant and plasticizer. Since TPHP is particularly toxic to aquatic organisms, it is essential to understand its biotransformation in the aquatic environment. In the aquatic ecosystem, based on consideration of the producer-consumer-decomposer relationship, the biotransformation products of TPHP were identified, and their toxicity was predicted. Liquid chromatography-high resolution mass spectrometry was used for target, suspect, and non-target analysis. The obtained biotransformation products were estimated for toxicity based on the prediction model. As a result, 29 kinds of TPHP biotransformation products were identified in the aquatic ecosystem. Diphenyl phosphate was detected as a common biotransformation product through a hydrolysis reaction. In addition, products were identified by the biotransformation mechanisms of green algae, daphnid, fish, and microorganism. Most of the biotransformation products were observed to be less toxic than the parent compound due to detoxification except some products (hydroquinone, beta-lyase products, palmitoyl/stearyl conjugated products). Since various species exist in a close relationship with each other in an ecosystem, an integrated approach for not only single species but also various connected species is essential.

Maternal Transfer of 2-Ethylhexyl Diphenyl Phosphate Leads to Developmental Toxicity Possibly by Blocking the Retinoic Acid Receptor and Retinoic X Receptor in Japanese Medaka (Oryzias latipes)

2-Ethylhexyl diphenyl phosphate (EHDPP) has been detected in wild fish with high concentrations, which may pose a risk in the embryo development considering its potential maternal transfer. In this study, EHDPP was demonstrated to elicit antagonistic activity to medaka retinoic acid receptor (mRAR) and retinoic X receptor (mRXR) with 50% inhibitory concentration of 18 and 36 μM, respectively. After adult female medaka were exposed to EHDPP at 156, 405, and 1161 ng/L for 35 days, the embryonic EHDPP concentrations (364-4824 ng/g lipid weight (lw)) were higher than those in the maternal tissues (15.0-4166 ng/g lw), showing notable maternal transfer. The embryonic concentration of EHDPP decreased limitedly during 1-2 day post-fertilization (dpf, the main developmental window of eye) but then decreased sharply after 2 dpf. The transcript abundance of cyp26a1 was inhibited and subsequent increasing embryonic all-trans RA level was observed in embryos, showing RAR/RXR antagonistic activity. These results may specifically contribute to the increased eye deformity incidences in all exposure groups (up to 8.0%; 51/637) relative to the control (1.0%, 7/733). The response behavior of the larvae to light stimulation was impaired in a dose-dependent manner, demonstrating a vision disorder. Because such developmental toxicities were observed at the environmental level, EHDPP may pose a threat to the survival of wild larvae and therefore a population risk for wild fish.

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.

The potential connections of adverse outcome pathways with the hazard identifications of typical organophosphate esters based on toxicity mechanisms

Following the world-wide ban of brominated flame retardants (BFRs), organophosphate esters (OPEs), which could potentially affect human health and ecosystem safety, have been frequently detected in various environmental media. However, the knowledge regarding the underlying toxicity effects of OPEs remains limited. In order to address these issues, this study reviewed the related reports which have been published in recent years. This analysis process included 12 OPEs, 10 model organisms, and 15 cell lines, which were used to systematically examine the mechanisms of endocrine disruption, neurotoxicity, hepatotoxicity, and cardiotoxicity, as well as reproductive and developmental toxicity. Subsequently, an adverse outcome pathway (AOP) framework of the toxicological effects of OPEs was built. The results demonstrated that multiple different pathways may lead to a single same adverse outcome (AO), and there was a certain degree of correlation among the different AOs. It was found that among all the 12 OPEs, tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) may potentially be the most toxic. In addition, rather than the parent chemicals, the metabolites of OPEs may also have different degrees of toxicity effects on aquatic organisms and humans. Overall, the results of the present study also suggested that an AOP framework should be built via fully utilizing the existing toxicity data of OPEs based on in vivo-in vitro-in silico to completely and deeply understand the toxic mechanisms of OPEs. This improved knowledge could then provide a theoretical basis for ecological risk assessments and water quality criteria research in the near future.

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.

Making smart drugs smarter: The importance of linker chemistry in targeted drug delivery

Smart drugs, such as antibody-drug conjugates, for targeted therapy rely on the ability to deliver a warhead to the desired location and to achieve activation at the same site. Thus, designing a smart drug often requires proper linker chemistry for tethering the warhead with a vehicle in such a way that either allows the active drug to retain its potency while being tethered or ensures release and thus activation at the desired location. Recent years have seen much progress in the design of new linker activation strategies. Herein, we review the recent development of chemical strategies used to link the warhead with a delivery vehicle for preferential cleavage at the desired sites.

Mitochondrial-related effects of pentabromophenol, tetrabromobisphenol A, and triphenyl phosphate on murine BV-2 microglia cells

The predominant reliance on bromated flame retardants (BFRs) is diminishing with expanded use of alternative organophosphate flame retardants. However, exposure related issues for susceptible populations, the developing, infirmed, or aged, remain given environmental persistence and home-environment detection. In this regard, reports of flame retardant (FR)-related effects on the innate immune system suggest process by which a spectrum of adverse health effects could manifest across the life-span. As representative of the nervous system innate immune system, the current study examined changes in microglia following exposure to representative FRs, pentabromophenol (PBP), tetrabromobisphenol A (2,2',6,6',-tetrabromo-4,4'-isopropylidine diphenol; TBBPA) and triphenyl phosphate (TPP). Following 18hr exposure of murine BV-2 cells, at dose levels resulting in ≥80% viability (10 and 40 μM), limited alterations in pro-inflammatory responses were observed however, changes were observed in mitochondrial respiration. Basal respiration was altered by PBP; ATP-linked respiration by PBP and TBBPA, and maximum respiration by all three FRs. Basal glycolytic rate was altered by PBP and TBBPA and compensatory glycolysis by all three. Phagocytosis was decreased for PBP and TBBPA. NLRP3 inflammasome activation was assessed using BV-2-ASC (apoptosis-associated speck-like protein containing a CARD) reporter cells to visualize aggregate formation. PBP, showed a direct stimulation of aggregate formation and properties as a NLRP3 inflammasome secondary trigger. TBBPA showed indications of possible secondary triggering activity while no changes were seen with TPP. Thus, the data suggests an effect of all three FRs on mitochondria metabolism yet, different functional outcomes including, phagocytic capability and NLRP3 inflammasome activation.

miR-137 and miR-141 regulate tail defects in zebrafish embryos caused by triphenyl phosphate (TPHP)

Exposure to triphenyl phosphate (TPHP), an organophosphate flame retardants (OPFRs), caused developmental toxicity in zebrafish embryos. However, the underlying molecular mechanism at the epigenetic level is largely unknown. Based on developmental toxicity (i.e., mortality and malformation), we measured expression levels of mRNA genes and their targeted miRNA in zebrafish embryos exposed to TPHP. As a result, TPHP caused developmental delay beginning at the 17-somite stage linking to detrimental effects in the tail and even embryonic mortality. Abnormal tail development was found to be associated with down-regulation of mmp9 and sox9b in both qRT-PCR and whole in-situ hybridization analysis. Also, we identified two microRNAs (i.e., miR-137 and miR-141) and observed their differential over-expression in TPHP-exposed zebrafish embryos. In the microinjection of miR-137 and miR-141 inhibitors, the reduced expression of mmp9 and sox9b upon TPHP exposure was compensated, indicating that epigenetic deregulation of miRNAs modulated putative genes involved in phenotypic tail defects triggered by TPHP in developing zebrafish embryos. This study provides insight for future mechanistic research using teleost fish on function of miRNAs in environmental toxicology.

The Genome Analysis of Methylobacterium populi YC-XJ1 with Diverse Xenobiotics Biodegrading Capacity and Degradation Characteristics of Related Hydrolase

Methylobacterium populi YC-XJ1 isolated from desert soil exhibited a diverse degrading ability towards aromatic oxyphenoxypropionic acid esters (AOPPs) herbicide, phthalate esters (PAEs), organophosphorus flame retardants (OPFRs), chlorpyrifos and phoxim. The genome of YC-XJ1 was sequenced and analyzed systematically. YC-XJ1 contained a large number of exogenous compounds degradation pathways and hydrolase resources. The quizalofop-p-ethyl (QPE) degrading gene qpeh2 and diethyl phthalate (DEP) degrading gene deph1 were cloned and expressed. The characteristics of corresponding hydrolases were investigated. The specific activity of recombinant QPEH2 was 0.1 ± 0.02 U mg^-1 for QPE with k_cat/K_m values of 1.8 ± 0.016 (mM^-1·s^-1). The specific activity of recombinant DEPH1 was 0.1 ± 0.02 U mg^-1 for DEP with k_cat/K_m values of 0.8 ± 0.02 (mM^-1·s^-1). This work systematically illuminated the metabolic versatility of strain YC-XJ1 via the combination of genomics analysis and laboratory experiments. These results suggested that strain YC-XJ1 with diverse xenobiotics biodegrading capacity was a promising candidate for the bioremediation of polluted sites.

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.

Stereoselective Formation of Multiple Reaction Products by the Phosphotriesterase from Sphingobium sp. TCM1

Organophosphate flame retardants are used to inhibit combustion and increase plasticity in plastics and durable foams. While not neurotoxic, these compounds are potential carcinogens, endocrine disrupters, and developmental toxins. The phosphotriesterase from Sphingobium sp. TCM1 (Sb-PTE) is unique among phosphotriesterase enzymes for its ability to hydrolyze these compounds and its ability to hydrolyze any one of the three different ester bonds within a given substrate. In some cases, the extent of hydrolysis of a methyl ester exceeds that of a p-nitrophenyl ester within a single substrate. There is a stereochemical component to this hydrolysis where the two enantiomers of chiral substrates give different product ratios. To investigate the stereoselectivity for the product distribution of Sb-PTE, a series of 24 phosphotriesters were synthesized with all possible combinations of methyl, cyclohexyl, phenyl, and p-nitrophenyl esters. Prochiral compounds were made chiral by differential isotopic labeling using a chemo/enzymatic strategy, which allowed the differentiation of hydrolysis for each ester in all but two compounds. The rate equations for this unique enzymatic mechanism were derived; the product ratios were determined for each substrate, and the individual kinetic constants for hydrolysis of each ester within each substrate were measured. The findings are consistent with the rate-limiting step for substrate hydrolysis catalyzed by Sb-PTE being the formation of a phosphorane-like intermediate and the kinetic constants and product ratios being dictated by a combination of transition state energies, inductive effects, and stereochemical constraints.

Characterizing biotransformation products and pathways of the flame retardant triphenyl phosphate in Daphnia magna using non-target screening

Triphenyl phosphate (TPHP), one of the organophosphate flame retardants, has been widely used in manufacturing, thereby causing a gradual increase in TPHP concentrations in aquatic environments. However, the information on the biotransformation mechanism of TPHP in invertebrates is lacking. The study identified the biotransformation products of TPHP in Daphnia magna, which showed particularly high toxicity in aquatic organisms, and determined the rates of depuration. Daphnia magna, a standard species for toxicity studies, was exposed to triphenyl phosphate and transferred to the pure medium. The biotransformation products of TPHP and its depuration rates were determined by liquid chromatography-high resolution mass spectrometry. Nine biotransformation products (five in the positive mode and four in the negative mode) of triphenyl phosphate were identified in D. magna. Based on the depuration ratio, the major biotransformation mechanism is estimated to be cysteine conjugation and sulfation. Certain biotransformation products (diphenyl phosphate, hydroxylated triphenyl phosphate, and thiol triphenyl phosphate) might induce toxicity in biota. The results could be used to predict main biotransformation processes and toxic products of organophosphate flame retardants in aquatic invertebrates.

Worker exposure to flame retardants in manufacturing, construction and service industries

Workers in several industries are occupationally exposed to flame retardants. This study characterizes flame retardant exposure for nine industries through air and hand wipe measures for 105 workers. Specifically, we analyzed 24 analytes from three chemical classes: organophosphate flame retardants (OFRs), polybrominated diphenyl ethers (PBDEs), and non-PBDE brominated flame retardants (NPBFRs). The industries were: carpet installation, chemical manufacturing, foam manufacturing, electronic scrap, gymnastics, rigid board installation, nail salons, roofing, and spray polyurethane foam. Workers wore personal air samplers for two entire workdays and provided hand wipe samples before and after the second work day. Bulk products were also analyzed. The air, hand wipe and bulk samples were evaluated for relevant flame retardants. Spray polyurethane foam workers' tris(1-chloro-2-propyl) phosphate air (geometric mean = 48,500 ng/m³) and hand wipe (geometric mean = 83,500 ng per sample) concentrations had the highest mean industry concentration of any flame retardant analyzed in this study, followed by triphenyl phosphate air concentration and tris(1,3-dichloro-2-propyl) phosphate hand wipe concentration from chemical manufacturers. Overall, OFR air and hand wipe concentrations were higher and more prevalent than PBDEs or non-PBDE brominated flame retardants. Some industries including spray polyurethane foam application, chemical manufacturing, foam manufacturing, nail salons, roofing, and rigid polyiso board installation had high potential for both air and hand exposure to OFRs. Carpet installers, electronic scrap workers, and gymnastic workers had exposures to all three classes of flame retardants including PBDEs, which were phased out of production in 2013. Air and dermal exposures to OFRs are prevalent in many industries and are replacing PBDEs in some industries.

Characterization and 16S metagenomic analysis of organophosphorus flame retardants degrading consortia

Three bacterial consortia, named YC-SY1, YC-BJ1 and YC-GZ1, were enriched from different areas of China. Bacterial consortia YC-SY1, YC-BJ1 and YC-GZ1 could efficiently degrade triphenyl phosphate (TPhP) (100 mg/L) by approximately 79.4%, 99.8% and 99.6%, tricresyl phosphate (TCrP) by 90.6%, 91.9% and 96.3%, respectively, within 4 days. And they could retain high degrading efficiency under a broad range of temperature (15-40 ℃), pH (6.0-10.0) and salinity (0-4%). A total of 10 bacterial isolates were selected and investigated their degradation capacity. Among these isolates, two were significantly superior to the others. Strain Rhodococcus sp. YC-JH2 could utilize TPhP (50 mg/L) as sole carbon source for growth with 37.36% degradation within 7 days. Strain Sphingopyxis sp. YC-JH3 could efficiently degrade 96.2% of TPhP (50 mg/L) within 7 days, except that no cell growth was observed. Combined with 16S diversity analysis, our results suggest that the effective components of three bacterial consortia responsible for TPhP and TCrP degradation were almost the same, that is, bacteria capable of degrading TPhP and TCrP are limited, in this study, the most efficient component is Sphingopyxis. This study provides abundant microorganism sources for research on organophosphorus flame retardants (OPFRs) metabolism and bioremediation towards OPFRs-contaminated environments.