Effects of an Environmentally Relevant Mixture of Organophosphate Esters Derived From House Dust on Endochondral Ossification in Murine Limb Bud Cultures

Association of organophosphate flame retardants exposure with liver function and the contrasting mediating roles of inflammatory and oxidative stress pathways

Organophosphate flame retardants (OPFRs) have been widely used as alternatives to traditional halogenated flame retardants, resulting in their pervasive environmental presence and significant human exposure. While animal studies using high doses of individual OPFRs have demonstrated hepatotoxicity, evidence of hepatic effects from chronic and low-dose exposure to OPFRs in humans is scarce, and the underlying mechanisms remain largely unknown. In this study, we evaluated the association between urinary metabolites of OPFRs (mOPFRs) and liver function biomarkers in a population aged 1-79 years from the Guangzhou Human Biomonitoring Program. Urinary concentrations of diphenyl phosphate exhibited a significant decreasing trend with increasing age across four age groups (≤12, 13-17, 17-59, ≥60 years), while bis(1-chloro-2-propyl) phosphate concentrations showed a significant positive association with age. Urinary mOPFRs were positively associated with alanine aminotransferase, gamma-glutamyl transferase, and aspartate aminotransferase, and negatively associated with indirect bilirubin levels. Urinary mOPFRs were significantly correlated with total bilirubin, which was consistent with that observed in the National Health and Nutrition Examination Survey database, while other associations were inconsistent. Moreover, we evaluated the roles of inflammatory and oxidative stress markers in the association between urinary mOPFRs and liver function biomarkers by mediation analysis. The results demonstrated that the mediation effects of inflammatory markers were consistent with the total and direct effects of mOPFRs on liver function biomarkers, whereas the mediation effects of oxidative stress markers were discrepant. Inflammation likely exacerbates the adverse effects of OPFRs on liver function, while oxidative stress may play a protective role. We further searched the comparative toxicogenomics database for genes associated with OPFRs and liver function biomarkers to understand the potential regulatory mechanisms.

Impact of exposure to a mixture of organophosphate esters on the adrenal glands of Sprague Dawley rats

There is growing evidence that organophosphate esters (OPEs) can act as endocrine-disrupting chemicals. However, only a few studies have assessed the effects of OPE exposure on one of the most important endocrine glands in the body, the adrenal gland. Our aim was to test the effects of a mixture of OPEs detected in Canadian house dust on adrenal function in Sprague Dawley rats. Adult male and female rats (n = 15 per treatment group) were administered either a vehicle or an OPE mixture (0.048, 1.6, or 48 mg/kg bw/d) for 70 to 72 d via their diet. With OPE exposure, adrenal glands from male adult rats were reduced in weight, whereas those of female rats showed an increase in weight. This led us to investigate whether OPEs induce sex-specific effects on adrenal gland function and the mechanisms involved. Serum levels of two adrenal hormones, aldosterone and corticosterone, were decreased only in male serum samples. Serum levels of renin and adrenocorticotropic hormone, which regulate aldosterone and corticosterone synthesis, respectively, were assessed. Exposure to the OPE mixture decreased renin levels only in males. Serum biochemistry analysis revealed that triglycerides and LDL cholesterol levels were increased in males. Transcriptomic analysis revealed that the top affected pathways in male adrenal glands from all three treatment groups were related to potassium channels, which play a role in regulating aldosterone and corticosterone levels. The most affected pathways in female adrenal glands were related to cholesterol biosynthesis and immune functions. These results show that an environmentally relevant mixture of OPEs affects adrenal function and that these effects are sex specific.

Mechanism-based toxicity screening of organophosphate flame retardants using Tox21 assays and molecular docking analysis

As brominated flame retardants are phased out and regulations on their use become stricter, concerns over organophosphate flame retardants (OPFRs) have increased due to their high production. In response, this study aimed to screen the potential toxicity of emerging OPFRs using in vitro Tox21 assays and in silico molecular docking analysis. For 48 OPFRs collected from the literature, we investigated their bioactivity with human nuclear receptors using Tox21 data, focusing on pathways related to endocrine disruption (ERs, AR), stress response (GR), energy homeostasis (PPARs, FXR), and detoxification (PXR, CAR). For OPFRs not tested in Tox21 assays, molecular docking simulations were performed to predict binding potential. Results showed that CAR/PXR and FXR had relatively high reactivity with diverse OPFRs, indicating potential molecular initiating events (MIEs). Among the 48 OPFRs, 28 interacted with one or more receptors, suggesting they may act as potential stressors of adverse outcome pathways (AOPs) leading to various human diseases. Aryl- and halogenated-OPFRs displayed higher bioactivity compared to alkyl-OPFRs. Additionally, as the logKow value and carbon number of OPFRs increased, their interaction with nuclear receptors also increased. These structure- and physicochemistry-dependent bioactivities provide insights for designing safer OPFRs to avoid regrettable substitutions. Of these prioritized OPFRs, 13 showed low oral points-of-departure (POD) values under 100 mg/kg/day. In contrast, the other 15 OPFRs lacked sufficient data or exhibited less severe toxicity, despite being predicted to be of high concern in our analysis. Since several OPFRs are commonly used in consumer products that can lead to daily human exposure, we suggest that these OPFRs have the potential to reveal undisclosed effects and should therefore undergo further assessment.

Molecular mechanisms of environmental pollutant-induced cartilage damage: from developmental disorders to osteoarthritis

The objective of the present study was to review the molecular mechanisms of the adverse effects of environmental pollutants on chondrocytes and extracellular matrix (ECM). Existing data demonstrate that both heavy metals, including cadmium (Cd), lead (Pb), and arsenic (As), as well as organic pollutants, including polychlorinated dioxins and furans (PCDD/Fs) and polychlorinated biphenyls (PCB), bisphenol A, phthalates, polycyclic aromatic hydrocarbons (PAH), pesticides, and certain other organic pollutants that target cartilage ontogeny and functioning. Overall, environmental pollutants reduce chondrocyte viability through the induction apoptosis, senescence, and inflammatory response, resulting in cell death and impaired ECM production. The effects of organic pollutants on chondrocyte development and viability were shown to be mediated by binding to the aryl hydrocarbon receptor (AhR) signaling and modulation of non-coding RNA expression. Adverse effects of pollutant exposures were observed in articular and growth plate chondrocytes. These mechanisms also damage chondrocyte precursors and subsequently hinder cartilage development. In addition, pollutant exposure was shown to impair chondrogenesis by inhibiting the expression of Sox9 and other regulators. Along with altered Runx2 signaling, these effects also contribute to impaired chondrocyte hypertrophy and chondrocyte-to-osteoblast trans-differentiation, resulting in altered endochondral ossification. Several organic pollutants including PCDD/Fs, PCBs and PAHs, were shown to induce transgenerational adverse effects on cartilage development and the resulting skeletal deformities. Despite of epidemiological evidence linking human environmental pollutant exposure to osteoarthritis or other cartilage pathologies, the data on the molecular mechanisms of adverse effects of environmental pollutant exposure on cartilage tissue were obtained from studies in laboratory rodents, fish, or cell cultures and should be carefully extrapolated to humans, although they clearly demonstrate that cartilage should be considered a putative target for environmental pollutant toxicity.

Phenotypic and Functional Outcomes in Macrophages Exposed to an Environmentally Relevant Mixture of Organophosphate Esters in Vitro

BACKGROUND

Organophosphate esters (OPEs) are flame retardants and plasticizers used in consumer products. OPEs are found ubiquitously throughout the environment with high concentrations in indoor house dust. Exposure to individual OPEs is associated with immune dysfunction, particularly in macrophages. However, OPEs exist as complex mixtures and the effects of environmentally relevant mixtures on the immune system have not been investigated.

OBJECTIVES

The objectives of this study were to evaluate the toxicity of an environmentally relevant mixture of OPEs that models Canadian house dust on macrophages using phenotypic and functional assessments in vitro.

METHODS

High-content live-cell fluorescent imaging for phenotypic biomarkers of toxicity in THP-1 macrophages treated with the OPE mixture was undertaken. We used confocal microscopy and cholesterol analysis to validate and expand on the observed OPE-induced lipid phenotype. Then, we used flow cytometry and live-cell imaging to conduct functional tests and uncover mechanisms of OPE-induced phagocytic suppression. Finally, we validated our THP-1 findings in human primary peripheral blood mononuclear cells (hPBMC) derived macrophages.

RESULTS

Exposure to non-cytotoxic dilutions of the OPE mixture resulted in higher oxidative stress and disrupted lysosome and lipid homeostasis in THP-1 and primary macrophages. We further observed that phagocytosis of apoptotic cells in THP-1 and primary macrophages was lower in OPE-exposed cells vs. controls. In THP-1 macrophages, phagocytosis of both Gram-positive and Gram-negative bacteria was also lower in OPE-exposed cells vs. controls. Additionally, the OPE mixture altered the expression of phagocytic receptors linked to the recognition of phosphatidylserine and pathogen-associated molecular patterns.

DISCUSSION

The results of this in vitro study suggested that exposure to an environmentally relevant mixture of OPEs resulted in higher lipid retention in macrophages and poor efferocytic response. These effects could translate to enhanced foam cell generation resulting in higher cardiovascular mortality. Furthermore, bacterial phagocytosis was lower in OPE-exposed macrophages in an in vitro setting, which may indicate the potential for reduced bacterial clearance in models of infections. Taken together, our data provide strong evidence that mixtures of OPEs can influence the biology of macrophages and offer new mechanistic insights into the impact of OPE mixtures on the immune system. https://doi.org/10.1289/EHP13869.

House dust-derived mixtures of organophosphate esters alter the phenotype, function, transcriptome, and lipidome of KGN human ovarian granulosa cells

Organophosphate esters (OPEs), used as flame retardants and plasticizers, are present ubiquitously in the environment. Previous studies suggest that exposure to OPEs is detrimental to female fertility in humans. However, no experimental information is available on the effects of OPE mixtures on ovarian granulosa cells, which play essential roles in female reproduction. We used high-content imaging to investigate the effects of environmentally relevant OPE mixtures on KGN human granulosa cell phenotypes. Perturbations to steroidogenesis were assessed using ELISA and qRT-PCR. A high-throughput transcriptomic approach, TempO-Seq, was used to identify transcriptional changes in a targeted panel of genes. Effects on lipid homeostasis were explored using a cholesterol assay and global lipidomic profiling. OPE mixtures altered multiple phenotypic features of KGN cells, with triaryl OPEs in the mixture showing higher potencies than other mixture components. The mixtures increased basal production of steroid hormones; this was mediated by significant changes in the expression of critical transcripts involved in steroidogenesis. Further, the total-OPE mixture disrupted cholesterol homeostasis and the composition of intracellular lipid droplets. Exposure to complex mixtures of OPEs, similar to those found in house dust, may adversely affect female reproductive health by altering a multitude of phenotypic and functional endpoints in granulosa cells. This study provides novel insights into the mechanisms of actions underlying the toxicity induced by OPEs and highlights the need to examine the effects of human relevant chemical mixtures.

Toxicological Mechanisms and Potencies of Organophosphate Esters in KGN Human Ovarian Granulosa Cells as Revealed by High-throughput Transcriptomics

Despite the growing number of studies reporting potential risks associated with exposure to organophosphate esters (OPEs), their molecular mechanisms of action remain poorly defined. We used the high-throughput TempO-Seq™ platform to investigate the effects of frequently detected OPEs on the expression of ∼3000 environmentally responsive genes in KGN human ovarian granulosa cells. Cells were exposed for 48 h to one of five OPEs (0.1 to 50 μM): tris(methylphenyl) phosphate (TMPP), isopropylated triphenyl phosphate (IPPP), tert-butylphenyl diphenyl phosphate (BPDP), triphenyl phosphate (TPHP), or tris(2-butoxyethyl) phosphate (TBOEP). The sequencing data indicate that four OPEs induced transcriptional changes, whereas TBOEP had no effect within the concentration range tested. Multiple pathway databases were used to predict alterations in biological processes based on differentially expressed genes. At lower concentrations, inhibition of the cholesterol biosynthetic pathway was the predominant effect of OPEs; this was likely a consequence of intracellular cholesterol accumulation. At higher concentrations, BPDP and TPHP had distinct effects, primarily affecting pathways involved in cell cycle progression and other stress responses. Benchmark concentration (BMC) modelling revealed that BPDP had the lowest transcriptomic point of departure. However, in vitro to in vivo extrapolation modeling indicated that TMPP was bioactive at lower concentrations than the other OPEs. We conclude that these new approach methodologies provide information on the mechanism(s) underlying the effects of data-poor compounds and assist in the derivation of protective points of departure for use in chemical read-across and decision-making.

Organophosphate esters concentrations in human serum and their associations with Sjögren syndrome

Organophosphate esters (OPEs) are widely used as flame retardants and plasticizers due to their excellent properties. The interference of OPEs on immune function has been proven, but the epidemiological data on OPEs exposure to related immune function diseases, such as sjögren syndrome (SjS), is limited. In cross-sectional study, 283 serum samples were collected from healthy individuals (n = 145) and patients with SjS (n = 138) in Hangzhou, China. Eight OPEs, triethyl phosphate (TEP), tributyl phosphate (TBP), tris (2-chloroethyl) phosphine (TCEP), triphenyl phosphate (TPHP), tri (1-chloro-2-propyl) phosphate (TCIPP), 2-ethylhexyldi-phenyl phosphate (EHDPP), tris (1,3-dichloro-2-propyl) phosphate (TDCIPP), and tri (2-butoxyethyl) phosphate (TBOEP), were frequently measured in serum samples. In addition, we explored the associations between the serum OPEs concentration and the risk of SjS. Results showed that TEP (mean controls 2.17 and cases 3.63 ng/mL) was the most abundant OPEs in the serum samples of the control and case groups, followed by TCIPP (mean controls 0.54 and cases 0.78 ng/mL). Serum TEP, TPHP, and EHDPP concentrations were positively correlated with SjS [odds ratio (OR): 1.97, 1.96, and 2.42, respectively; 95% confidence interval (CI):1.34-2.89, 1.34-2.87, and 1.34-2.87, respectively] in the adjusted model, and a negative correlation of TBP concentrations with SjS in the adjusted model (OR: 0.35, 95% CI: 0.17-0.70) was observed. Compared with the lowest quartile concentrations, the ORs of SjS at the highest quartile concentrations of TEP (OR: 4.93, 95% CI: 2.24-10.82) and TPHP (OR: 4.75, 95% CI:1.89-11.94) were significantly higher. This study suggests that human exposure to OPEs may increase the risk of SjS.

Potential adverse outcome pathways with hazard identification of organophosphate esters

Data-driven analysis and pathway-based approaches contribute to reasonable arrangements of limited resources and laboratory tests for continuously emerging commercial chemicals, which provides opportunities to save time and effort for toxicity research. With the widespread usage of organophosphate esters (OPEs) on a global scale, the concentrations generally reached up to micromolar range in environmental media and even in organisms. However, potential adverse effects and toxicity pathways of OPEs have not been systematically assessed. Therefore, it is necessary to review the current situation, formulate the future research priorities, and characterize toxicity mechanisms via data-driven analysis. Results showed that the early toxicity studies focused on neurotoxicity, cytotoxicity, and metabolic disorders. Then the main focus shifted to the mechanisms of cardiotoxicity, endocrine disruption, hepatocytes, reproductive and developmental toxicity to vulnerable sub-populations, such as infants and embryos, affected by OPEs. In addition, several novel OPEs have been emerging, such as bis(2-ethylhexyl)-phenyl phosphate (HDEHP) and oxidation derivatives (OPAsO) generated from organophosphite antioxidants (OPAs), leading to multiple potential ecological and human health risks (neurotoxicity, hepatotoxicity, developmental toxicity, etc.). Notably, in-depth statistical analysis was promising in encapsulating toxicological information to develop adverse outcome pathways (AOPs) frameworks. Subsequently, network-centric analysis and quantitative weight-of-evidence (QWOE) approaches were utilized to construct and evaluate the putative AOPs frameworks of OPEs, showing the moderate confidences of the developed AOPs. In addition, frameworks demonstrated that several events, such as nuclear receptor activation, reactive oxygen species (ROS) production, oxidative stress, and DNA damage, were involved in multiple different adverse outcome (AO), and these AOs had certain degree of connectivity. This study brought new insights into facilitating the complement of AOP efficiently, as well as establishing toxicity pathways framework to inform risk assessment of emerging OPEs.

Tri-n-butyl phosphate delays tissue repair by dysregulating neutrophil function in zebrafish

Tri-n-butyl phosphate (TnBP) is a widely used organophosphate ester, but its effects on the regenerative process under damaging circumstances remain unknown. In the present study, zebrafish larvae were exposed to 0, 50, 100, 200 and 1000 μg/L TnBP, and the caudal fins were cut at 72 hours post fertilization (hpf). First, after exposure to TnBP, the number of total neutrophils decreased together with decreased neutrophils in the tail, and TnBP inhibited chemotaxis. Second, reactive oxygen species (ROS) levels in the zebrafish decreased greatly. Following exposure to TnBP, transcription levels of many genes regulating fin regeneration, such as fgf20a, fgfr1a, bmp2a and bmp4, were significantly downregulated, while inflammatory factors such as cxcl8a, cxcl18b, il-6, and tnfa were abnormally upregulated. In addition, TnBP inhibited the regenerative area after caudal fin amputation. The inflammatory state was adverse during the regenerative process. In summary, TnBP exposure is immunotoxic and decreases oxidative stress in injured zebrafish larvae.

Recombinant Limb Assay as in Vivo Organoid Model

Organ formation initiates once cells become committed to one of the three embryonic germ layers. In the early stages of embryogenesis, different gene transcription networks regulate cell fate after each germ layer is established, thereby directing the formation of complex tissues and functional organs. These events can be modeled in vitro by creating organoids from induced pluripotent, embryonic, or adult stem cells to study organ formation. Under these conditions, the induced cells are guided down the developmental pathways as in embryonic development, resulting in an organ of a smaller size that possesses the essential functions of the organ of interest. Although organoids are widely studied, the formation of skeletal elements in an organoid model has not yet been possible. Therefore, we suggest that the formation of skeletal elements using the recombinant limb (RL) assay system can serve as an in vivo organoid model. RLs are formed from undissociated or dissociated-reaggregated undifferentiated mesodermal cells introduced into an ectodermal cover obtained from an early limb bud. Next, this filled ectoderm is grafted into the back of a donor chick embryo. Under these conditions, the cells can receive the nascent embryonic signals and develop complex skeletal elements. We propose that the formation of skeletal elements induced through the RL system may occur from stem cells or other types of progenitors, thus enabling the study of morphogenetic properties in vivo from these cells for the first time.