Urine Excretion, Organ Distribution, and Placental Transfer of 6PPD and 6PPD-Quinone in Mice and Potential Developmental Toxicity through Nuclear Receptor Pathways

Exposure to 6-PPD quinone causes damage on mitochondrial complex I/II associated with lifespan reduction in Caenorhabditis elegans

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6-PPDQ) is an emerging pollutant transformed from 6-PPD. However, the effect of 6-PPDQ exposure on mitochondrion and underlying mechanism remains largely unclear. Using Caenorhabditis elegans as animal model, exposed to 6-PPDQ at 0.1-10 μg/L was performed form L1 larvae to adult day-1. Exposure to 6-PPDQ (1 and 10 μg/L) could increase oxygen consumption rate and decease adenosine 5'-triphosphate (ATP) content, suggesting induction of mitochondrial dysfunction. Activities of NADH dehydrogenase (complex I) and succinate dehydrogenase (complex II) were inhibited, accompanied by a decrease in expressions of gas-1, nuo-1, and mev-1. RNAi of gas-1 and mev-1 enhanced mitochondrial dysfunction and reduced lifespan of 6-PPDQ exposed nematodes. GAS-1 and MEV-1 functioned in parallel to regulate 6-PPDQ toxicity to reduce the lifespan. Insulin peptides and the insulin signaling pathway acted downstream of GAS-1 and MEV-1 to control the 6-PPDQ toxicity on longevity. Moreover, RNAi of sod-2 and sod-3, targeted genes of daf-16, caused susceptibility to 6-PPDQ toxicity in reducing lifespan and in causing reactive oxygen species (ROS) production. Therefore, 6-PPDQ at environmentally relevant concentrations (ERCs) potentially caused mitochondrial dysfunction by affecting mitochondrial complexes I and II, which was associated with lifespan reduction by affecting insulin signaling in organisms.

Tissue Accumulation and Biotransformation of 6PPD-Quinone in Adult Zebrafish and Its Effects on the Intestinal Microbial Community

The pronounced lethality of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-quinone or 6PPDQ) toward specific salmonids, while sparing other fish species, has received considerable attention. However, the underlying cause of this species-specific toxicity remains unresolved. This study explored 6PPDQ toxicokinetics and intestinal microbiota composition in adult zebrafish during a 14-day exposure to environmentally realistic concentrations, followed by a 7-day recovery phase. Predominant accumulation occurred in the brain, intestine, and eyes, with the lowest levels in the liver. Six metabolites were found to undergo hydroxylation, with two additionally undergoing O-sulfonation. Semiquantitative analyses revealed that the predominant metabolite featured a hydroxy group situated on the phenyl ring adjacent to the quinone. This was further validated by assessing enzyme activity and determining in silico binding interactions. Notably, the binding affinity between 6PPDQ and zebrafish phase I and II enzymes exceeded that with the corresponding coho salmon enzymes by 1.04-1.53 times, suggesting a higher potential for 6PPDQ detoxification in tolerant species. Whole-genome sequencing revealed significant increases in the genera Nocardioides and Rhodococcus after exposure to 6PPDQ. Functional annotation and pathway enrichment analyses predicted that these two genera would be responsible for the biodegradation and metabolism of xenobiotics. These findings offer crucial data for comprehending 6PPDQ-induced species-specific toxicity.

Emerging N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and 6PPD quinone in paired human plasma and urine from Tianjin, China: Preliminary assessment with demographic factors

With increasing concerns about N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and 6PPD-quinone (6PPD-Q), relevant environmental investigations and toxicological research have sprung up in recent years. However, limited information could be found for human body burden assessment. This work collected and analyzed 200 samples consisting of paired urine and plasma samples from participants (50 male and 50 female) in Tianjin, China. Low detection frequencies (DF, <15 %) were found except for urinary 6PPD-Q (86 %), which suggested the poor residue tendency of 6PPD and 6PPD-Q in blood. The low DFs also lead to no substantial association between two chemicals. Data analysis based on urinary 6PPD-Q showed a significant difference between males and females (p < 0.05). No significant correlation was found for other demographic factors (Body Mass Index (BMI), age, drinking, and smoking). The mean values of daily excretion (ng/kg bw/day) calculated using urinary 6PPD-Q for females and males were 7.381 ng/kg bw/day (female) and 3.360 ng/kg bw/day (male), and apparently female suffered higher daily exposure. Further analysis with daily excretion and ALT (alanine aminotransferase)/TSH (thyroid stimulating hormone)/ blood cell analysis indicators found a potential correlation with 6PPD-Q daily excretion and liver/immune functions. Considering this preliminary assessment, systematic research targeting the potential organs at relevant concentrations is required.

Effects of 6PPD-Quinone on Human Liver Cell Lines as Revealed with Cell Viability Assay and Metabolomics Analysis

N-(1,3-Dimethyl butyl)-N'-phenyl-phenylenediamine-quinone (6PPD-Q) is a derivative of the widely used rubber tire antioxidant 6PPD, which was first found to be acutely toxic to coho salmon. Subsequent studies showed that 6PPD-Q had species-specific acute toxicity in fishes and potential hepatotoxicity in mice. In addition, 6PPD-Q has been reported in human urine, demonstrating the potential widespread exposure of humans to this chemical. However, whether 6PPD-Q poses a higher risk to humans than its parent compound, 6PPD, and could cause adverse effects in humans is still unclear. In this study, we utilized two human liver cell models (the human proto-hepatocyte model L02 and the human hepatocellular carcinoma cell line HepG2) to investigate the potentially differential effects of these two chemicals. Cell viability curve analysis showed that 6PPD-Q had lower IC50 values than 6PPD for both liver cell lines, suggesting higher toxicity of 6PPD-Q to human liver cells than 6PPD. In addition, L02 cells are more sensitive to 6PPD-Q exposure, which might be derived from its weaker metabolic transformation of 6PPD-Q, since significantly lower levels of phase I and phase II metabolites were detected in 6PPD-Q-exposed L02 cell culture medium. Furthermore, pathway analysis showed that 6PPD-Q exposure induced changes in phenylalanine, tyrosine, and tryptophan biosynthesis and tyrosine metabolism pathways in L02 cells, which might be the mechanism underlying its liver cell toxicity. Gene expression analysis revealed that exposure to 6PPD-Q induced excessive ROS production in L02 cells. Our results further supported the higher risk of 6PPD-Q than 6PPD and provided insights for understanding the effects of 6PPD-Q on human health.

Chronic exposure to tire rubber-derived contaminant 6PPD-quinone impairs sperm quality and induces the damage of reproductive capacity in male mice

It has been reported that N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q), a derivative of the tire antioxidant, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), exhibits acute toxicity towards organisms. However, the possible reproductive toxicity of 6PPD-Q in mammals has rarely been reported. In this study, the effects of 6PPD-Q on the reproductive toxicity of C57Bl/6 male mice were assessed after exposure to 6PPD-Q for 40 days at 4 mg/kg body weight (bw). Exposure to 6PPD-Q not only led to a decrease in testosterone levels but also adversely affected semen quality and in vitro fertilization (IVF) outcomes, thereby indicating impaired male fertility resulting from 6PPD-Q exposure. Additionally, transcriptomic and metabolomic analyses revealed that 6PPD-Q elicited differential expression of genes and metabolites primarily enriched in spermatogenesis, apoptosis, arginine biosynthesis, and sphingolipid metabolism in the testes of mice. In conclusion, our study reveals the toxicity of 6PPD-Q on the reproductive capacity concerning baseline endocrine disorders, sperm quality, germ cell apoptosis, and the sphingolipid signaling pathway in mice. These findings contribute to an enhanced understanding of the health hazards posed by 6PPD-Q to mammals, thereby facilitating the development of more robust safety regulations governing the utilization and disposal of rubber products.

Environmental profiles, hazard identification, and toxicological hallmarks of emerging tire rubber-related contaminants 6PPD and 6PPD-quinone

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is commonly used in rubber compounds as antioxidants to protect against degradation from heat, oxygen, and ozone exposure. This practice extends the lifespan of rubber products, including tires, by preventing cracking, aging, and deterioration. However, the environmental consequences of waste generated during rubber product use, particularly the formation of 6PPD-quinone (6PPD-Q) through the reaction of 6PPD with ozone, have raised significant concerns due to their detrimental effects on ecosystems. Extensive research has revealed the widespread occurrence of 6PPD and its derivate 6PPD-Q in various environmental compartments, including air, water, and soil. The emerging substance of 6PPD-Q has been shown to pose acute mortality and long-term hazards to aquatic and terrestrial organisms at concentrations below environmentally relevant levels. Studies have demonstrated toxic effects of 6PPD-Q on a range of organisms, including zebrafish, nematodes, and mammals. These effects include neurobehavioral changes, reproductive dysfunction, and digestive damage through various exposure pathways. Mechanistic insights suggest that mitochondrial stress, DNA adduct formation, and disruption of lipid metabolism contribute to the toxicity induced by 6PPD-Q. Recent findings of 6PPD-Q in human samples, such as blood, urine, and cerebrospinal fluid, underscore the importance of further research on the public health and toxicological implications of these compounds. The distribution, fate, biological effects, and underlying mechanisms of 6PPD-Q in the environment highlight the urgent need for additional research to understand and address the environmental and health impacts of these compounds.

Long-term exposure to 6-PPD quinone at environmentally relevant concentrations causes neurotoxicity by affecting dopaminergic, serotonergic, glutamatergic, and GABAergic neuronal systems in Caenorhabditis elegans

6-PPD quinone (6-PPDQ), an emerging environmental pollutant, is converted based on 6-PPD via ozonation. However, a systematic evaluation on possible neurotoxicity of long-term and low-dose 6-PPDQ exposure and the underlying mechanism remain unknown. In the present work, 0.1-10 μg/L 6-PPDQ was added to treat Caenorhabditis elegans for 4.5 days, with locomotion behavior, neuronal development, sensory perception behavior, neurotransmitter content, and levels of neurotransmission-related genes being the endpoints. 6-PPDQ exposure at 0.1-10 μg/L significantly reduced locomotion behavior, and that at 1-10 μg/L decreased sensory perception behavior in nematodes. Moreover, 6-PPDQ exposure at 10 μg/L notably induced damage to the development of dopaminergic, glutamatergic, serotonergic, and GABAergic neurons. Importantly, nematodes with chronic 6-PPDQ exposure at 10 μg/L were confirmed to suffer obviously decreased dopamine, serotonin, glutamate, dopamine, and GABA contents and altered neurotransmission-related gene expression. Meanwhile, the potential binding sites of 6-PPDQ and neurotransmitter synthesis-related proteins were further shown by molecular docking method. Lastly, Pearson's correlation analysis showed that locomotion behavior and sensory perception behavior were positively correlated with the dopaminergic, serotonergic, glutamatergic, and GABAergic neurotransmission. Consequently, 6-PPDQ exposure disturbed neurotransmitter transmission, while such changed molecular foundation for neurotransmitter transmission was related to 6-PPDQ toxicity induction. The present work sheds new lights on the mechanisms of 6-PPDQ and its possible neurotoxicity to organisms at environmentally relevant concentrations.

Aromatic amine antioxidants (AAs) and p-phenylenediamines-quinones (PPD-Qs) in e-waste recycling industry park: Occupational exposure and liver X receptors (LXRs) disruption potential

Recently, evidence of aromatic amine antioxidants (AAs) existence in the dust of the electronic waste (e-waste) dismantling area has been exposed. However, there are limited studies investigating occupational exposure and toxicity associated with AAs and their transformation products (p-phenylenediamines-quinones, i.e., PPD-Qs). In this study, 115 dust and 42 hand wipe samples collected from an e-waste recycling industrial park in central China were analyzed for 19 AAs and 6 PPD-Qs. Notably, the median concentration of ∑6PPD-Qs (1,110 ng/g and 1,970 ng/m2) was significantly higher (p < 0.05, Mann-Whitney U test) than that of ∑6PPDs (147 ng/g and 34.0 ng/m2) in dust and hand wipes. Among the detected analytes, 4-phenylaminodiphenylamine quinone (DPPD-Q) (median: 781 ng/g) and 1,4-Bis(2-naphthylamino) benzene quinone (DNPD-Q) (median: 156 ng/g), were particularly prominent, which were first detected in the e-waste dismantling area. Occupational exposure assessments and nuclear receptor interference ability, conducted through estimated daily intake (EDI) and molecular docking analysis, respectively, indicated significant occupational exposure to PPD-Qs and suggested prioritized Liver X receptors (LXRs) disruption potential of PPDs and PPD-Qs. The study provides the first evidence of considerable levels of AAs and PPD-Qs in the e-waste-related hand wipe samples and underscores the importance of assessing occupational exposure and associated toxicity effects.

Insight into chemically reactive metabolites of aliphatic amine pollutants: A de novo prediction strategy and case study of sertraline

The uncommon metabolic pathways of organic pollutants are easily overlooked, potentially leading to idiosyncratic toxicity. Prediction of their biotransformation associated with the toxic effects is the very purpose that this work focuses, to develop a de novo method to mechanistically predict the reactive toxicity pathways of uncommon metabolites from start aliphatic amine molecules, which employed sertraline triggered by CYP450 enzymes as a model system, as there are growing concerns about the effects on human health posed by antidepressants in the aquatic environment. This de novo prediction strategy combines computational and experimental methods, involving DFT calculations upon sequential growth, in vitro and in vivo assays, dissecting chemically reactive mechanism relevant to toxicity, and rationalizing the fundamental factors. Significantly, desaturation and debenzylation-aromatization as the emerging metabolic pathways of sertraline have been elucidated, with the detection of DNA adducts of oxaziridine metabolite in mice, highlighting the potential reactive toxicity. Molecular orbital analysis supports the reactivity preference for toxicological-relevant C-N desaturation over N-hydroxylation of sertraline, possibly extended to several other aliphatic amines based on the Bell-Evans-Polanyi principle. It was further validated toward some other wide-concerned aliphatic amine pollutants involving atrazine, ε-caprolactam, 6PPD via in silico and in vitro assays, thereby constituting a complete path for de novo prediction from case study to general applications.

Environmentally realistic dose of tire-derived metabolite 6PPD-Q exposure causes intestinal jejunum and ileum damage in mice via cannabinoid receptor-activated inflammation

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q) is a quinone derivative of a common tire additive 6PPD, whose occurrence has been widely reported both in the environment and human bodies including in adults, pregnant women and children. Yet, knowledge on the potential intestinal toxicity of 6PPD-Q in mammals at environmentally relevant dose remain unknown. In this study, the effects of 6PPD-Q on the intestines of adult ICR mice were evaluated by orally administering environmentally relevant dose or lower levels of 6PPD-Q (0.1, 1, 10, and 100 μg/kg) for 21 days. We found that 6PPD-Q disrupted the integrity of the intestinal barrier, mostly in the jejunum and ileum, but not in the duodenum or colon, in a dose-dependent manner. Moreover, intestinal inflammation manifested with elevated levels of TNF-α, IL-1, and IL-6 mostly observed in doses at 10 and 100 μg/kg. Using reverse target screening technology combining molecular dynamic simulation modeling we identified key cannabinoid receptors including CNR2 activation to be potentially mediating the intestinal inflammation induced by 6PPD-Q. In summary, this study provides novel insights into the toxic effects of emerging contaminant 6PPD-Q on mammalian intestines and that the chemical may be a cannabinoid receptor agonist to modulate inflammation.

6PPD-quinone exposure induces neuronal mitochondrial dysfunction to exacerbate Lewy neurites formation induced by α-synuclein preformed fibrils seeding

The emerging toxicant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q) is of wide concern due to its ubiquitous occurrence and high toxicity. Despite regular human exposure, limited evidence exists about its presence in the body and potential health risks. Herein, we analyzed cerebrospinal fluid (CSF) samples from Parkinson's disease (PD) patients and controls. The CSF levels of 6PPD-Q were twice as high in PD patients compared to controls. Immunostaining assays performed with primary dopaminergic neurons confirm that 6PPD-Q at environmentally relevant concentrations can exacerbate the formation of Lewy neurites induced by α-synuclein preformed fibrils (α-syn PFF). Assessment of cellular respiration reveals a considerable decrease in neuronal spare respiratory and ATP-linked respiration, potentially due to changes in mitochondrial membrane potential. Moreover, 6PPD-Q-induced mitochondrial impairment correlates with an upsurge in mitochondrial reactive oxygen species (mROS), and Mito-TEMPO-driven scavenging of mROS can lessen the amount of pathologic phospho-serine 129 α-synuclein. Untargeted metabolomics provides supporting evidence for the connection between 6PPD-Q exposure and changes in neuronal metabolite profiles. In-depth targeted metabolomics further unveils an overall reduction in glycolysis metabolite pool and fluctuations in the quantity of TCA cycle intermediates. Given its potentially harmful attributes, the presence of 6PPD-Q in human brain could potentially be a risk factor for PD.

Occurrence of p-phenylenediamine antioxidants in human urine

General populations are widely exposed to various p-phenylenediamine antioxidants (PPDs). N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), a typical p-phenylenediamine antioxidant, has been detected in human urine samples. However, the occurrence of other widely used PPDs in human urine is still unclear. This study comprehensively characterized the occurrence of 9 PPDs in human urine from 151 Chinese adults. Our results showed that all target PPDs were detected in human urine samples, with the total concentrations of PPDs ranging from 0.41 to 38 ng/mL. PPDs in human urine was dominated by 6PPD (mean 1.2 ng/mL, range < LOD - 3.8 ng/mL), followed by N-phenyl-N'-cyclohexyl-p-phenylenediamine (CPPD; 0.85 ng/mL, Collapse

Key Words

• 6PPD
• Human exposure
• Human urine
• PPDs
• Synthetic antioxidants

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MESH Headings

• Adult
• Humans
• Male
• Female
• Antioxidants
• Phenylenediamines
• Nitro Compounds

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Grants Collapse

Affiliation(s)

Weili Mao Department of Pharmacy, Quzhou Affiliated Hospital of Wenzhou Medical University, Quzhou People's Hospital, Quzhou, Zhejiang 324000, PR China
Hangbiao Jin Key Laboratory of Microbial Technology for Industrial Pollution Control of Zhejiang Province, College of Environment, Zhejiang University of Technology, Hangzhou, Zhejiang 310032, PR China; Innovation Research Center of Advanced Environmental Technology, Eco-Industrial Innovation Institute ZJUT, Quzhou, Zhejiang 324400, PR China
Ruyue Guo Key Laboratory of Microbial Technology for Industrial Pollution Control of Zhejiang Province, College of Environment, Zhejiang University of Technology, Hangzhou, Zhejiang 310032, PR China
Ping Chen Department of Pharmacy, Quzhou Affiliated Hospital of Wenzhou Medical University, Quzhou People's Hospital, Quzhou, Zhejiang 324000, PR China
Songyang Zhong Department of Pharmacy, Quzhou Affiliated Hospital of Wenzhou Medical University, Quzhou People's Hospital, Quzhou, Zhejiang 324000, PR China
Xilin Wu Department of Pharmacy, Quzhou Affiliated Hospital of Wenzhou Medical University, Quzhou People's Hospital, Quzhou, Zhejiang 324000, PR China.

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Stable isotope-assisted mass spectrometry reveals in vivo distribution, metabolism, and excretion of tire rubber-derived 6PPD-quinone in mice

6PPD-quinone (6PPD-Q) has been identified as a ubiquitous contaminant in the surrounding locality, including air particles, roadside soils, dust, and water. Recently, the prevalence of 6PPD-Q in human urine has accentuated the urgency for investigating its biological fate. To address this, we conducted a stable isotope-assisted high-resolution mass spectrometry (HRMS) assay to unveil the distribution, metabolism, excretion, and toxicokinetic properties of this contaminant in a mouse model. Mice were fed with a single dose of deuterated 6PPD-Q-d5 at human-relevant exposure levels. Results indicated that 6PPD-Q was quickly assimilated and distributed into bloodstream and main organs of mice, with the concentrations reaching peaks under 1 h following administration. Notably, 6PPD-Q was primarily distributed in the adipose tissue, marked by a significant Cmax (p < 0.05), followed by the kidney, lung, testis, liver, spleen, heart, and muscle. In addition, our measurement demonstrated that 6PPD-Q can penetrate the blood-brain barrier of mice within 0.5 h after exposure. The half-lives (t1/2) of 6PPD-Q in serum, lung, kidney, and spleen of mice were measured at 12.7 ± 0.3 h, 20.7 ± 1.4 h, 21.6 ± 5.3 h, and 20.6 ± 2.8 h, respectively. Using HRMS combined with isotope tracing techniques, two novel hydroxylated metabolites of 6PPD-Q in the mice liver were identified for the first time, which provides new insights into its rapid elimination in-vivo. Meanwhile, fecal excretion was identified as the main excretory pathway for 6PPD-Q and its hydroxylated metabolites. Collectively, our findings extend the current knowledge on the biological fate and exposure status of 6PPD-Q in a mouse model, which has the potential to be extrapolated to humans.