PXR-mediated organophorous flame retardant tricresyl phosphate effects on lipid homeostasis

Oxidative stress mediates the association of organophosphate flame retardants with metabolic obesity in U.S. adults: A combined epidemiologic and bioinformatic study

Obesity is a global public health issue, with limited epidemiologic studies on the relationship and mechanisms between organophosphate flame retardants (OPFRs) and metabolic obesity phenotypes (MOPs). We aimed to explore the link between OPFRs metabolite (m-OPFRs) and MOPs using a combined epidemiologic and bioinformatic approach. We used cross-sectional survey data from the U.S. National Health and Nutrition Examination Survey (2011-2018) to analyze the relationship between m-OPFRs and metabolic health obesity (MHO), as well as metabolic unhealthy obesity (MUO). The dataset encompasses eligible adults to assess the impact of individual, mixed, and mediated effects on the outcome variables through multivariate logistic regression, Bayesian kernel machine regression (BKMR), and mediation analysis. Multiple logistic regression models, stratified by tertiles of exposure showed that bis(1,3-dichloro-2-propyl) phosphate (BDCIPP) levels in the body significantly increased the risk of MHO, with OR and 95%CI of 1.454 (1.082, 1.953) for the second tertile (T2) and 1.598 (1.126, 2.268) for the third tertile (T3), compared to the first tertile (T1). Increased levels of BDCIPP in T3 (1.452(1.013, 2.081)) are associated with MUO, compared to T1. Mixed m-OPFRs and MHO risk in BMKR were positively correlated, with BDCIPP being the primary contributor. We found that the serum uric acid (SUA) and white blood cell count (WBC) indicators significantly mediated the association between BDCIPP and MHO (P < 0.05). Our study suggests that OPFRs, either individual or mixed, are associated with two distinct MOPs, with oxidative stress playing an important role. In addition, in silico analysis was used to screen for shared genes, and eight shared genes and eleven biological pathways identified during the screening process were used to construct the adverse outcome pathway, which suggests that exposure to OPFRs may activate the peroxisome proliferator-activated receptor (PPAR) pathway, thereby increasing the risk of obesity. Further studies are needed to validate our findings.

Organophosphorus Flame Retardants and Metabolic Disruption: An in Silico, in Vitro, and in Vivo Study Focusing on Adiponectin Receptors

BACKGROUND

Environmental chemical exposures have been associated with metabolic outcomes, and typically, their binding to nuclear hormone receptors is considered the molecular initiating event (MIE) for a number of outcomes. However, more studies are needed to understand the influence of such exposures on cell membrane-bound adiponectin receptors (AdipoRs), which are critical metabolic regulators.

OBJECTIVE

We aimed to clarify the potential interactions between AdipoRs and environmental chemicals, specifically organophosphorus flame retardants (OPFRs), and the resultant effects.

METHODS

Employing in silico simulation, cell thermal shift, and noncompetitive binding assays, we screened eight OPFRs for interactions with AdipoR1 and AdipoR2. We tested two key events underlying AdipoR modulation upon OPFR exposure in a liver cell model. The Toxicological Prioritization Index (ToxPi)scoring scheme was used to rank OPFRs according to their potential to disrupt AdipoR-associated metabolism. We further examined the inhibitory effect of OPFRs on AdipoR signaling activation in mouse models.

RESULTS

Analyses identified pi-pi stacking and pi-sulfur interactions between the aryl-OPFRs 2-ethylhexyl diphenyl phosphate (EHDPP), triphenyl phosphate (TPhP), and tricresyl phosphate (TCP) and the transmembrane cavities of AdipoR1 and AdipoR2. Cell thermal shift assays showed a > 3 ° C rightward shift in the AdipoR proteins' melting curves upon exposure to these three compounds. Although the binding sites differed from adiponectin, results suggest that aryl-OPFRs noncompetitively inhibited the binding of the endogenous peptide ligand ADP355 to the receptors. Analyses of key events underlying AdipoR modulation revealed that glucose uptake was notably lower, whereas lipid content was higher in cells exposed to aryl-OPFRs. EHDPP, TCP, and TPhP were ranked as the top three disruptors according to the ToxPi scores. A noncompetitive binding between these aryl-OPFRs and AdipoRs was also observed in wild-type (WT) mice. In db/db mice, the finding of lower blood glucose levels after ADP355 injection was diminished in the presence of a typical aryl-OPFR (TCP). WT mice exposed to TCP demonstrated lower AdipoR1 signaling, which was marked by lower phosphorylated AMP-activated protein kinase (pAMPK) and a higher expression of gluconeogenesis-related genes. Moreover, WT mice exposed to ADP355 demonstrated higher levels of pAMPK protein and peroxisome proliferator-activated receptor-α messenger RNA. This was accompanied by higher glucose disposal and by lower levels of long-chain fatty acids and hepatic triglycerides; these metabolic improvements were negated upon TCP co-treatment.

CONCLUSIONS

In silico, in vitro, and in vivo assays suggest that aryl-OPFRs act as noncompetitive inhibitors of AdipoRs, preventing their activation by adiponectin, and thus function as antagonists to these receptors. Our study describes a novel MIE for chemical-induced metabolic disturbances and highlights a new pathway for environmental impact on metabolic health. https://doi.org/10.1289/EHP14634.

Isomer-specific cardiotoxicity induced by tricresyl phosphate in zebrafish embryos/larvae

Tricresyl phosphate (TCP) has received extensive attentions due to its potential adverse effects, while the toxicological information of TCP isomers is limited. In this study, 2 h post-fertilization zebrafish embryos were exposed to tri-o-cresyl phosphate (ToCP), tri-m-cresyl phosphate (TmCP) or tri-p-cresyl phosphate (TpCP) at concentrations of 0, 100, 300 and 600 μg/L until 120 hpf, and the cardiotoxicity and mechanism of TCP isomers in zebrafish embryos/larvae were evaluated. The results showed that ToCP or TmCP exposure induced cardiac morphological defects and dysfunction in zebrafish, characterized by increased distance between sinus venosus and bulbus arteriosis, increased atrium and pericardial sac area, trabecular defects, and decreased heart rate and blood flow velocity, while no adverse effects of TpCP on zebrafish heart were found. Transcriptomic results revealed that extracellular matrix (ECM) and motor proteins, as well as PPAR signaling pathways, were included in the cardiac morphological defects and dysfunction induced by ToCP and TmCP. Co-exposure test with D-mannitol indicated that the inhibition of energy metabolism by ToCP and TmCP affected cardiac morphology and function by decreasing osmoregulation. This study is the first to report the cardiotoxicity induced by TCP in zebrafish from an isomer perspective, providing a new insight into the toxicity of TCP isomers and highlighting the importance of evaluating the toxicity of different isomers.

Association between organophosphate esters individual and mixed exposure with the risk of hyperlipidemia and serum lipid levels among adults in Wuhan, China

Toxicologic studies reported that organophosphate esters (OPEs) may disrupt lipid metabolism, thus affecting serum lipid levels. However, epidemiological evidence regarding the association between OPEs and the risk of hyperlipidemia (HPL) as well as serum lipid levels is scarce. In the present study, our aim was to investigate the impact of individual and mixed OPE exposure on HPL. A total of 1981 Chinese adults were involved based on a cross-sectional design. Overall, we found a positive association between bis(1,3-dichloro-2-propyl) phosphate (BDCIPP) and the risk of HPL. Bis(1-chloro-2-propyl) phosphate (BCIPHIPP) showed a positive association with total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-C). BDCIPP, diphenyl phosphate (DPHP), di-ocresyl phosphate and di-p-cresyl phosphate (Docp&Dpcp), and 4-hydroxyphenyl-diphenyl phosphate (4-OH-DPHP) exhibited a negative association with high-density lipoprotein cholesterol (HDL-C). In stratified analyses, BDCIPP and BCIPHIPP were significantly correlated with the increased risk of HPL in the age ≤ 45 group. Bis(2-butoxyethyl) phosphate (BBOEP) was in relationship with an elevated risk of HPL in the subgroup of BMI < 24 kg/m2. BDCIPP was also positively associated with HPL in men. Quantile-based g computation (qgcomp) and generalized weighted quantile sum regression (gWQS) models demonstrated a negative association between OPEs mixed exposure and HDL-c in the total population, as well as a positive effect of them on HPL in the subgroup of age ≤ 45 years, which is consistent with the individual analyses. Furthermore, joint effect analyses revealed that participants with detected BDCIPP urinary levels and unhealthy lifestyles had the highest risk of HPL. Our findings offer evidence supporting the correlation between exposure to OPE and the risk of HPL, necessitating further prospective studies for validation.

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.

Organophosphate ester flame retardants and plasticizers affect the phenotype and function of HepG2 liver cells

Exposure to the organophosphate esters (OPEs), used as flame retardants and plasticizers, is associated with a variety of adverse health effects including an increase in the incidence of fatty liver diseases. The goal of this study was to investigate the effects of six OPEs, all detected in Canadian house dust, on the phenotype and function of HepG2 liver cells. We used high-content imaging to investigate the effects of these OPEs on cell survival, mitochondria, oxidative stress, lipid droplets, and lysosomes. Effects on the autophagy/lipophagy pathway were evaluated using confocal microscopy. The triaryl OPEs (isopropylated triphenylphosphate [IPPP], tris(methylphenyl) phosphate [TMPP], and triphenyl phosphate [TPHP]) were more cytotoxic than non-triaryl OPEs (tris(2-butoxyethyl) phosphate [TBOEP], tris(1-chloro-2-propyl) phosphate [TCIPP], and tris(1,3-dichloro-2-propyl) phosphate [TDCIPP]). Exposure to most OPEs increased total mitochondria, reduced reactive oxygen species, and increased total lipid droplet areas and lysosomal intensity. Potency ranking was done using the lowest benchmark concentration/administered equivalent dose method and toxicological prioritization index analyses to integrate all phenotypic endpoints. IPPP, TBOEP, and TPHP ranked as the most potent OPEs, whereas TMPP, TCIPP, and TDCIPP were relatively less bioactive. Confocal microscopic analysis demonstrated that IPPP reduced the colocalization of lipid droplets (PLIN2), lysosomes (LAMP1), and autophagosomes (p62), disrupting autophagy. In contrast, TBOEP rescued cells from bafilomycin A1-induced inhibition of autophagy and/or increased autophagic flux. Together, these data demonstrate that OPEs have adverse effects on HepG2 cells. Further, OPE-induced dysregulation of autophagy may contribute to the association between OPE exposure and adverse effects on liver lipid homeostasis.

Bioactivity assessment of organophosphate flame retardants via a dose-dependent yeast functional genomics approach

Organophosphate flame retardants (OPFRs) have been widely detected in multiple environment media and have many adverse effects with complex toxicity mechanisms. However, the early molecular responses to OPFRs have not been fully elucidated, thereby making it difficult to assess their risks accurately. In this work, we systematically explored the point of departure (POD) of biological pathways at genome-wide level perturbed by 14 OPFRs with three substituents (alkyl, halogen, and aryl) using a dose-dependent functional genomics approach in Saccharomyces cerevisiae at 24 h exposure. Firstly, our results demonstrated that the overall biological potency at gene level (PODDRG20) ranged from 0.013 to 35.079 μM for 14 OPFRs, especially the tributyl phosphate (TnBP) exhibited the strongest biological potency with the least PODDRG20. Secondly, we found that structural characteristics of carbon number and logKow were significantly negatively correlated with POD, and carbon number and logKow also significantly affected lipid metabolism associated processes. Thirdly, these early biological pathways of OPFRs toxification were found to be involved in lipid metabolism, oxidative stress, DNA damage, MAPK signaling pathway, and amino acid and carbohydrate metabolism, among which the lipid metabolism was the most sensitive molecular response perturbed by most OPFRs. More importantly, we identified one resistant mutant strain with knockout of ERG2 (YMR202W) gene participated in steroid biosynthesis pathway, which can serve as a key yeast strain of OPFRs toxification. Overall, our study demonstrated an effective platform for accurately assessing OPFRs risks and provided a basis for further green OPFRs development.

Role of inflammatory lipid and fatty acid metabolic abnormalities induced by plastic additives exposure in childhood asthma

Lipid metabolism play an essential role in occurrence and development of asthma, and it can be disturbed by phthalate esters (PAEs) and organophosphate flame retardants (OPFRs). As a chronic inflammatory respiratory disease, the occurrence risk of childhood asthma is increased by PAEs and OPFRs exposure, but it remains not entirely clear how PAEs and OPFRs contribute the onset and progress of the disease. We have profiled the serum levels of PAEs and OPFRs congeners by liquid chromatography coupled with mass spectrometry, and its relationships with the dysregulation of lipid metabolism in asthmatic, bronchitic (acute inflammation) and healthy (non-inflammation) children. Eight PAEs and nine OPFRs congeners were found in the serum of children (1 - 5 years old) from Shenzhen, and their total median levels were 615.16 ng/mL and 17.06 ng/mL, respectively. Moreover, the serum levels of mono-methyl phthalate (MMP), tri-propyl phosphate (TPP) and tri-n-butyl phosphate (TNBP) were significant higher in asthmatic children than in healthy and bronchitic children as control. Thirty-one characteristic lipids and fatty acids of asthma were screened by machine-learning random forest model based on serum lipidome data, and the alterations of inflammatory characteristic lipids and fatty acids including palmitic acids, 12,13-DiHODE, 14,21-DiHDHA, prostaglandin D2 and LysoPA(18:2) showed significant correlated with high serum levels of MMP, TPP and TNBP. These results imply PAEs and OPFRs promote the occurrence of childhood asthma via disrupting inflammatory lipid and fatty acid metabolism, and provide a novel sight for better understanding the effects of plastic additives on childhood asthma.

Exposure to OPFRs Is Associated with Obesity and Dysregulated Serum Lipid Profiles: Data from 2017-2018 NHANES

Widespread exposure to organophosphorus flame retardants (OPFRs) has been observed in the general population. Emerging studies have revealed OPFRs possess endocrine-disturbing properties. The present study aims to assess the association between urinary metabolites of OPFRs, BMI, and serum lipid profiles. Data from the National Health and Nutrition Examination Survey (NHANES) 2017-2018 were obtained, with 1334 adults enrolled in the current study. Urinary concentrations of bis (1-chloro-2-propyl) phosphate (BCIPP), bis(2-chloroethyl) phosphate (BCEP), bis(1,3-dichloro-2-propyl) phosphate (BDCPP), dibutyl phosphate (DBUP), and diphenyl phosphate (DPHP) were quantified to assess OPFR exposure. Covariate-adjusted linear and logistic regression models were conducted to explore the associations between log2-transformed concentrations of OPFR metabolites, BMI, obesity, and serum lipid profiles. Stratified analyses were performed to assess the heterogeneity of associations by age, gender, race, etc. Positive associations were found between OPFR exposure and the risk of obesity. The multivariate linear analysis indicated that a one-unit increase in log2-transformed urinary concentrations of BCEP and BDCPP was associated with 0.27 (95% CI: 0.02-0.52, p = 0.0338) and 0.56 (95% CI: 0.25-0.87, p = 0.0004) higher BMI value, respectively. One log2-unit increase in urinary BCEP and BDCPP concentrations was associated with 1.1-fold (95% CI: 1.02-1.18, p = 0.0096) and 1.19-fold (95% CI: 1.09-1.30, p = 0.0001) risk for developing obesity. Furthermore, the non-linear relationship between exposure to OPFRs and obesity was identified. Additionally, multivariable linear regression showed that urinary DPHP concentrations were inversely correlated with serum triglyceride (TG) levels (β = -7.41, 95% CI: -12.13 to -2.68, p = 0.0022). However, no other OPFR metabolites were found to be significantly statistically associated with serum lipid levels after adjusting for potential confounders. In conclusion, environmental exposure to OPFRs might contribute to obesity and dysregulated TG concentrations in adults. Future prospective research is warranted to confirm the causal relationship between metabolites of OPFRs and obesity.

Effects of Indoor Dust Exposure on Lung Cells: Association of Chemical Composition with Phenotypic and Lipid Changes in a 3D Lung Cancer Cell Model

Indoor dust is a key contributor to the global human exposome in urban areas since the population develops most of its activities in private and public buildings. To gain insight into the health risks associated with this chronic exposure, it is necessary to characterize the chemical composition of dust and understand its biological impacts using reliable physiological models. The present study investigated the biological effects of chemically characterized indoor dust extracts using three-dimensional (3D) lung cancer cell cultures combining phenotypic and lipidomic analyses. Apart from the assessment of cell viability, reactive oxygen species (ROS) induction, and interleukin-8 release, lipidomics was applied to capture the main lipid changes induced as a cellular response to the extracted dust compounds. The application of chemometric tools enabled the finding of associations between chemical compounds present in dust and lipidic and phenotypic profiles in the cells. This study contributes to a better understanding of the toxicity mechanisms associated with exposure to chemical pollutants present in indoor dust.

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.

Mitochondrial impairment but not peripheral inflammation predicts greater Gulf War illness severity

Gulf War illness (GWI) is an important exemplar of environmentally-triggered chronic multisymptom illness, and a potential model for accelerated aging. Inflammation is the main hypothesized mechanism for GWI, with mitochondrial impairment also proposed. No study has directly assessed mitochondrial respiratory chain function (MRCF) on muscle biopsy in veterans with GWI (VGWI). We recruited 42 participants, half VGWI, with biopsy material successfully secured in 36. Impaired MRCF indexed by complex I and II oxidative phosphorylation with glucose as a fuel source (CI&CIIOXPHOS) related significantly or borderline significantly in the predicted direction to 17 of 20 symptoms in the combined sample. Lower CI&CIIOXPHOS significantly predicted GWI severity in the combined sample and in VGWI separately, with or without adjustment for hsCRP. Higher-hsCRP (peripheral inflammation) related strongly to lower-MRCF (particularly fatty acid oxidation (FAO) indices) in VGWI, but not in controls. Despite this, whereas greater MRCF-impairment predicted greater GWI symptoms and severity, greater inflammation did not. Surprisingly, adjusted for MRCF, higher hsCRP significantly predicted lesser symptom severity in VGWI selectively. Findings comport with a hypothesis in which the increased inflammation observed in GWI is driven by FAO-defect-induced mitochondrial apoptosis. In conclusion, impaired mitochondrial function-but not peripheral inflammation-predicts greater GWI symptoms and severity.

The Function of Xenobiotic Receptors in Metabolic Diseases

Metabolic diseases are a series of metabolic disorders that include obesity, diabetes, insulin resistance, hypertension, and hyperlipidemia. The increased prevalence of metabolic diseases has resulted in higher mortality and mobility rates over the past decades, and this has led to extensive research focusing on the underlying mechanisms. Xenobiotic receptors (XRs) are a series of xenobiotic-sensing nuclear receptors that regulate their downstream target genes expression, thus defending the body from xenobiotic and endotoxin attacks. XR activation is associated with the development of a number of metabolic diseases such as obesity, nonalcoholic fatty liver disease, type 2 diabetes, and cardiovascular diseases, thus suggesting an important role for XRs in modulating metabolic diseases. However, the regulatory mechanism of XRs in the context of metabolic disorders under different nutrient conditions is complex and remains controversial. This review summarizes the effects of XRs on different metabolic components (cholesterol, lipids, glucose, and bile acids) in different tissues during metabolic diseases. As chronic inflammation plays a critical role in the initiation and progression of metabolic diseases, we also discuss the impact of XRs on inflammation to comprehensively recognize the role of XRs in metabolic diseases. This will provide new ideas for treating metabolic diseases by targeting XRs. SIGNIFICANCE STATEMENT: This review outlines the current understanding of xenobiotic receptors on nutrient metabolism and inflammation during metabolic diseases. This work also highlights the gaps in this field, which can be used to direct the future investigations on metabolic diseases treatment by targeting xenobiotic receptors.

Regulation of PXR Function by Coactivator and Corepressor Proteins: Ligand Binding Is Just the Beginning

The pregnane X receptor (PXR, NR1I2) is a nuclear receptor which exerts its regulatory function by heterodimerization with the retinoid-X-receptor α (RXRα, NR2B1) and binding to the promoter and enhancer regions of diverse target genes. PXR is involved in the regulation of drug metabolism and excretion, metabolic and immunological functions and cancer pathogenesis. PXR activity is strongly regulated by the association with coactivator and corepressor proteins. Coactivator proteins exhibit histone acetyltransferase or histone methyltransferase activity or associate with proteins having one of these activities, thus promoting chromatin decondensation and activation of the gene expression. On the contrary, corepressor proteins promote histone deacetylation and therefore favor chromatin condensation and repression of the gene expression. Several studies pointed to clear cell- and ligand-specific differences in the activation of PXR. In this article, we will review the critical role of coactivator and corepressor proteins as molecular determinants of the specificity of PXR-mediated effects. As already known for other nuclear receptors, understanding the complex mechanism of PXR activation in each cell type and under particular physiological and pathophysiological conditions may lead to the development of selective modulators with therapeutic potential.