Ring-oxidized metabolites of styrene contribute to styrene-induced Clara-cell toxicity in mice

Transcriptomic re-analyses of human hepatocyte spheroids treated with PFAS reveals chain length and dose-dependent modes of action

To identify pathway perturbations and examine biological modes of action (MOAs) for various perfluoroalkyl substances, we re-analyzed published in vitro gene expression studies from human primary liver spheroids. With treatment times ranging from 10 to 14 days, shorter-chain PFAS (those with 6 or fewer fluorinated carbon atoms in the alkyl chain) showed enrichment for pathways of fatty acid metabolism and fatty acid beta-oxidation with upregulated genes. Longer-chain PFAS compounds, specifically PFOS (perfluorooctane sulfonate), PFDS (perfluorodecane sulfonate), and higher doses of PFOA (perfluorooctanoic acid), had enrichment for pathways involved in steroid metabolism, fatty acid metabolism, and biological oxidation for downregulated genes. Although PFNA (perfluorononanoic acid), PFDA (perfluorodecanoic acid), and PFUnDA (perfluoroundecanoic acid) were more toxic and could only be examined after a 1-day treatment, all three had enrichment patterns similar to those observed with PFOS. With PFOA there were dose-dependent changes in pathway enrichment, shifting from upregulation of fatty acid metabolism and downregulation of steroid metabolism to downregulation of both at higher doses. The response to PFHpS (perfluoroheptanesulfonic acid) was similar to the PFOA pattern at the lower treatment dose. Based on results of transcription factor binding sites analyses, we propose that downregulation of pathways of lipid metabolism by longer chain PFAS may be due to inhibitory interactions of PPARD on genes controlled by PPARA and PPARG. In conclusion, our transcriptomic analysis indicates that the biological MOAs of PFAS compounds differ according to chain length and dose, and that risk assessments for PFAS should consider these differences in biological MOAs when evaluating mixtures of these compounds.

Xenobiotica-metabolizing enzymes in the lung of experimental animals, man and in human lung models

The xenobiotic metabolism in the lung, an organ of first entry of xenobiotics into the organism, is crucial for inhaled compounds entering this organ intentionally (e.g. drugs) and unintentionally (e.g. work place and environmental compounds). Additionally, local metabolism by enzymes preferentially or exclusively occurring in the lung is important for favorable or toxic effects of xenobiotics entering the organism also by routes other than by inhalation. The data collected in this review show that generally activities of cytochromes P450 are low in the lung of all investigated species and in vitro models. Other oxidoreductases may turn out to be more important, but are largely not investigated. Phase II enzymes are generally much higher with the exception of UGT glucuronosyltransferases which are generally very low. Insofar as data are available the xenobiotic metabolism in the lung of monkeys comes closed to that in the human lung; however, very few data are available for this comparison. Second best rate the mouse and rat lung, followed by the rabbit. Of the human in vitro model primary cells in culture, such as alveolar macrophages and alveolar type II cells as well as the A549 cell line appear quite acceptable. However, (1) this generalization represents a temporary oversimplification born from the lack of more comparable data; (2) the relative suitability of individual species/models is different for different enzymes; (3) when more data become available, the conclusions derived from these comparisons quite possibly may change.

Editor's Highlight: Complete Attenuation of Mouse Lung Cell Proliferation and Tumorigenicity in CYP2F2 Knockout and CYP2F1 Humanized Mice Exposed to Inhaled Styrene for up to 2 Years Supports a Lack of Human Relevance

Styrene is a mouse-specific lung carcinogen, and short-term mode of action studies have demonstrated that cytotoxicity and/or cell proliferation, and genomic changes are dependent on CYP2F2 metabolism. The current study examined histopathology, cell proliferation, and genomic changes in CD-1, C57BL/6 (WT), CYP2F2(-/-) (KO), and CYP2F2(-/-) (CYP2F1, 2B6, 2A13-transgene) (TG; humanized) mice following exposure for up to 104 weeks to 0- or 120-ppm styrene vapor. Five mice per treatment group were sacrificed at 1, 26, 52, and 78 weeks. Additional 50 mice per treatment group were followed until death or 104 weeks of exposure. Cytotoxicity was present in the terminal bronchioles of some CD-1 and WT mice exposed to styrene, but not in KO or TG mice. Hyperplasia in the terminal bronchioles was present in CD-1 and WT mice exposed to styrene, but not in KO or TG mice. Increased cell proliferation, measured by KI-67 staining, occurred in CD-1 and WT mice exposed to styrene for 1 week, but not after 26, 52, or 78 weeks, nor in KO or TG mice. Styrene increased the incidence of bronchioloalveolar adenomas and carcinomas in CD-1 mice. No increase in lung tumors was found in WT despite clear evidence of lung toxicity, or, KO or TG mice. The absence of preneoplastic lesions and tumorigenicity in KO and TG mice indicates that mouse-specific CYP2F2 metabolism is responsible for both the short-term and chronic toxicity and tumorigenicity of styrene, and activation of styrene by CYP2F2 is a rodent MOA that is neither quantitatively or qualitatively relevant to humans.

Interspecies uncertainty in molecular responses and toxicity of mixtures

Most of the experimental toxicity testing data for chemicals are generated through the use of laboratory animals, namely, rodents such as rats and mice or other species. Interspecies extrapolation is needed to nullify the differences between species so as to use such data for human health/risk assessment. Thus, understanding of interspecies differences is important in extrapolating the laboratory results to humans and conducting human risk assessments based on current credible scientific knowledge. Major causes of interspecies differences in anatomy and physiology, toxicokinetics, injury repair, molecular receptors, and signal transduction pathways responsible for variations in responses to toxic chemicals are outlined. In the risk assessment process, uncertainty associated with data gaps in our knowledge is reflected by application of uncertainty factors for interspecies differences. Refinement of the risk assessment methods is the ultimate goal as we strive to realistically evaluate the impact of toxic chemicals on human populations. Using specific examples from current risk assessment practice, this chapter illustrates the integration of interspecies differences in evaluation of individual chemicals and chemical mixtures.

Studies of styrene, styrene oxide and 4-hydroxystyrene toxicity in CYP2F2 knockout and CYP2F1 humanized mice support lack of human relevance for mouse lung tumors

Styrene (S) is lung tumorigenic in mice but not in rats. S and its alkene-oxidized metabolite styrene oxide (SO) were not lung toxic in CYP2F2(-/-) [knockout] mice, indicating S-induced mouse lung tumors are mediated through mouse-specific CYP2F2-generated ring-oxidized metabolite(s) in lung bronchioles. The human relevance of the CYP2F MOA was assessed by insertion of a human CYP2F1, 2A13, 2B6 transgene into CYP2F2(-/-) mice; CYP2F1 expression and activity were confirmed in the transgenic (TG) mice. No evidence of cytotoxicity or increased cell proliferation (BrdU labeling) was seen in TG mice treated with either S or SO (200mg/kg/day ip for 5days). In contrast to S and SO, 4HS (105mg/kg/day ip for 5days) increased BrdU labeling 5-10-fold in WT mice, <3-fold increase in KO mice and 2-4-fold in TG mice. The limited response of 4HS in KO and TG mice may result from intrinsic toxicity or from further metabolism; regardless of the MOA, these findings indicate that the CYP2F-mediated tumorigenic MOA in WT mice is not operative for S, SO, or for 4HS putatively derived from metabolism of S by CYP2F1 in humans, and thus S-induced mouse lung tumors are unlikely to be relevant to human risk.

The Weight of Evidence Does Not Support the Listing of Styrene as "Reasonably Anticipated to be a Human Carcinogen" in NTP's Twelfth Report on Carcinogens

Styrene was listed as "reasonably anticipated to be a human carcinogen" in the twelfth edition of the National Toxicology Program's Report on Carcinogens based on what we contend are erroneous findings of limited evidence of carcinogenicity in humans, sufficient evidence of carcinogenicity in experimental animals, and supporting mechanistic data. The epidemiology studies show no consistent increased incidence of, or mortality from, any type of cancer. In animal studies, increased incidence rates of mostly benign tumors have been observed only in certain strains of one species (mice) and at one tissue site (lung). The lack of concordance of tumor incidence and tumor type among animals (even within the same species) and humans indicates that there has been no particular cancer consistently observed among all available studies. The only plausible mechanism for styrene-induced carcinogenesis-a non-genotoxic mode of action that is specific to the mouse lung-is not relevant to humans. As a whole, the evidence does not support the characterization of styrene as "reasonably anticipated to be a human carcinogen," and styrene should not be listed in the Report on Carcinogens.

CYP2F2-generated metabolites, not styrene oxide, are a key event mediating the mode of action of styrene-induced mouse lung tumors

Styrene induces lung tumors in mice but not in rats. Although metabolism of styrene to 7,8-styrene oxide (SO) by CYP2E1 has been suggested as a mediator of styrene toxicity, lung toxicity is not attenuated in CYP2E1 knockout mice. However, styrene and/or SO metabolism by mouse lung Clara cell-localized CYP2F2 to ring-oxidized cytotoxic metabolite(s) has been postulated as a key metabolic gateway responsible for both lung toxicity and possible tumorigenicity. To test this hypothesis, the lung toxicity of styrene and SO was evaluated in C57BL/6 (WT) and CYP2F2⁻/⁻ knockout mice treated with styrene (400 mg/kg/day, gavage, or 200 or 400 mg/kg/day, ip) or S- or R-SO (200 mg/kg/day, ip) for 5 days. Styrene treated WT mice displayed significant necrosis and exfoliation of Clara cells, and cumulative BrdU-labeling index of S-phase cells was markedly increased in terminal bronchioles of WT mice exposed to styrene or S- or RSO. In contrast, Clara and terminal bronchiole cell toxicity was not observed in CYP2F2⁻/⁻ mice exposed to either styrene or SO. This study clearly demonstrates that the mouse lung toxicity of both styrene and SO is critically dependent on metabolism by CYP2F2. Importantly, the human isoform of CYP2F, CYP2F1, is expressed at much lower levels and likely does not catalyze significant styrene metabolism, supporting the hypothesis that styrene-induced mouse lung tumors may not quantitatively, or possibly qualitatively, predict lung tumor potential in humans.

Comparative analysis of binding affinities between styrene and mammalian CYP2E1 by bioinformatics approaches

Cytochrome P450 2E1 (CYP2E1) is a cytochrome P450 enzyme involved in styrene metabolism. This study compared the binding affinities between styrene and 11 mammalian CYP2E1 systems using bioinformatics methods. Firstly, amino acid sequences of CYP2E1s were obtained from the Swiss-Prot database. Then, taking the crystal structure of human CYP2E1 as a template, 3D models of the CYP2E1s of other mammals were constructed using the SWISS-MODEL program. Finally, the generated homology models were applied to calculate their docking capacities against styrene and polystyrene using the Surflex-Dock program, which could automatically dock ligands into a receptor's ligand binding site using a protomol based approach and assess the affinity by an empirically derived scoring function. Docking experiments showed that the studied mammalian CYP2E1s had high binding affinities with styrene. For polystyrene, the dimmer of styrene has high binding affinities with CYP2E1s, however, trimer and other high polymers were found hard to be docked into the CYP2E1s. The results of this study indicated that bioinformatics approaches might be useful tools to predict styrene and polystyrene affinities with mammalian CYP2E1s.

Metabolism and toxicity of styrene in microsomal epoxide hydrolase-deficient mice

Styrene, which is widely used in manufacturing, is both acutely and chronically toxic to mice. Styrene is metabolized by cytochromes P-450 to the toxic metabolite styrene oxide, which is detoxified via hydrolysis with microsomal epoxide hydrolase (mEH) playing a major role. The purpose of these studies was to characterize the importance of this pathway by determining the hepatotoxicity and pneumotoxicity of styrene in wild-type and mEH-deficient (mEH(-/-)) mice. While the mEH(-/-) mice metabolized styrene to styrene oxide at the same rate as the wild-type mice, as expected there was minimal metabolism of styrene oxide to glycol. mEH(-/-) mice were more susceptible to the lethal effects of styrene. Twenty-four hours following the administration of 200 mg/kg ip styrene, mice demonstrated a greater hepatotoxic response due to styrene, as measured by increased serum sorbitol dehydrogenase activity and greater pneumotoxicity as shown by increased protein levels, cell numbers, and lactate dehydrogenase activity in bronchioalveolar lavage fluid. mEH(-/-) mice were also more susceptible to styrene-induced oxidative stress, as indicated by greater decreases in hepatic glutathione levels 3 h after styrene. Styrene oxide at a dose of 150 mg/kg did not produce hepatotoxicity in either wild-type or mEH(-/-) mice. However, styrene oxide produced pneumotoxicity that was similar in the two strains. Thus, mEH plays an important role in the detoxification of styrene but not for exogenously administered styrene oxide.

Indicators of oxidative stress and apoptosis in mouse whole lung and Clara cells following exposure to styrene and its metabolites

In mice, styrene is hepatotoxic, pneumotoxic, and causes lung tumors. One explanation for the mechanism of toxicity is oxidative stress/damage. Previous studies have shown decreased glutathione levels, linked to increased apoptosis, in lung homogenates and isolated Clara cells 3 h following styrene or styrene oxide (SO) administration or in vitro exposure. The objective of the current studies was to determine what effects styrene and its active metabolites, primarily styrene oxide, had on indicators of oxidative stress and attendant apoptosis in order to understand better the mechanism of styrene-induced toxicity. Three hours following in vitro exposure of Clara cells to styrene or SO there were increases in reactive oxygen species (ROS). Following administration of styrene or styrene oxide ip, increases in ROS, superoxide dismutase (SOD), and 8-hydroxydeoxyguanosine (8-OHdG) formation were observed. Since increases in ROS have been linked to increases in apoptosis ratios of bax/bcl-2, mRNA and protein expression were determined 3-240 h following the administration of styrene and R-styrene oxide (RSO). The bax/bcl-2 mRNA ratio increased 12 and 24 h following R-SO and 120 h following styrene administration. However, the bax/bcl-2 protein ratio was not increased until 240 h following R-SO, and 24 and 240 h following styrene administration. However, only a slight increase in caspase 3 was observed. These results indicated that oxidative stress occurred 3h following styrene or styrene oxide as evidenced by increased ROS and SOD. This increased ROS may be responsible for the increased 8-OHdG formation. Our findings of limited apoptosis in Clara cells following acute exposure to styrene or SO are in agreement with others and may reflect the minimal extent to which apoptosis plays a role in acute styrene toxicity. It is clear, however, that oxidative stress and oxidative effects on DNA are increased following exposure to styrene or styrene oxide, and these may play a role in the lung tumorigenesis in mice.

Pulmonary toxicity and environmental contamination: radicals, electron transfer, and protection by antioxidants

The atmosphere is replete with a mixture of toxic substances, both natural and man-made. Inhalation of toxic substances produces a variety of insults to the pulmonary system. Lung poisons include industrial materials, particulates from mining and combustion, agricultural chemicals, cigarette smoke, ozone, and nitrogen oxides, among a large number of other chemicals and environmental contaminants. Many proposals have been advanced to explain the mode of action of pulmonary toxicants. In this review we focus on mechanisms of pulmonary toxicity that involve ET, ROS, and OS. The vast majority of toxicants or their metabolites possess chemical ET functionalities that can undergo redox cycling. Such recycling may generate ROS that can injure various cellular constituents in the lung and in other tissues. ET agents include quinones, metal complexes, aromatic nitro compounds, and conjugated iminium ions. Often, these agents are formed metabolically from parent toxicants. Such metabolic reactions are often catalytic and require only small amounts of the offending material. Oxidative attack is commonly associated with lipid peroxidation and oxidation of DNA, and it may result in strand cleavage and 8-OH-DG production. Toxicity is often accompanied by depletion of natural AOs, which further exacerbates the toxic effect. It is not surprising that the use of AOs, both natural in fruits and vegetables, as well as synthetic, may provide protection from the adverse effects of toxicant exposure. The mechanistic framework described earlier is also applicable to some of the more prominent pulmonary illnesses, such as asthma, COPD, and cancer.

Styrene Metabolism, Genotoxicity, and Potential Carcinogenicity

This report reviews styrene biotransformation, including minor metabolic routes, and relates metabolism to the genotoxic effects and possible styrene-related carcinogenicity. Styrene is shown to require metabolic activation in order to become notably genotoxic and styrene 7,8-oxide is shown to contribute quantitatively by far the most (in humans more than 95%) to the genotoxicity of styrene, while minor ring oxidation products are also shown to contribute to local toxicities, especially in the respiratory system. Individual susceptibility depending on metabolism polymorphisms and individual DNA repair capacity as well as the dependence of the nonlinearity of the dose-response relationships in the species in question and the consequences for risk evaluation are analyzd.

Comparison of styrene and its metabolites styrene oxide and 4-vinylphenol on cytotoxicity and glutathione depletion in Clara cells of mice and rats

Styrene is a widely used compound in the manufacturing industry. In mice and rats, it is both hepatotoxic and pneumotoxic. It causes lung tumors in mice, but not in rats. The Clara cell is the main target for the toxicity of styrene and its metabolites, and it also has the greatest activity for styrene metabolism. Therefore, Clara cells isolated from CD-1 mice and Sprague-Dawley rats were used to compare the cytotoxicities induced by styrene and its metabolites. The cytotoxicity of styrene was greater in vitro than that of its metabolites styrene oxide (racemic, R- and S-) and 4-vinylphenol in contrast with what has been observed in vivo in previous studies on hepatotoxicity and pneumotoxicity. Susceptibility of rats to styrene and its metabolites are 4-fold less than that observed with mice. Glutathione levels were also measured in mice following addition of the chemicals in vitro and treatment of the CD-1 mice in vivo. Decreases in glutathione concentrations were seen even at doses which did not cause the death of mouse Clara cells. Significant decreases in glutathione were observed 3h after treatment with racemic SO and R-SO. At 12h, rebound effects were seen for all compounds, with all but R-SO rebounding above controls. These studies suggest that in vitro cytotoxicity of styrene and its metabolites does not strictly follow in vivo effects and that decreases in mouse glutathione levels may be related to oxidative stress.

Investigation of bioactivation and toxicity of styrene in CYP2E1 transgenic cells

Styrene has been found to be toxic to the respiratory system, and the toxicity of styrene is metabolism-dependent. CYP2E1 is suggested to be one of the cytochrome P450 enzymes responsible for the bioactivation of styrene. Our work focused on the roles of CYP2E1 and epoxide, a metabolite of styrene epoxidation, in the cytotoxicity of styrene. Styrene was found to be more toxic to h2E1 cells than to the wild type, while there was no difference found when styrene oxide was administered. Both soluble and microsomal epoxide hydrolase inhibitors dramatically enhanced styrene toxicity. Glutathione and glutathione ethyl ester showed protection against styrene cytotoxicity. Cytotoxicity of a selection of styrene analogues, such as ethylbenzene, vinylcyclohexane, and ethylcyclohexane, was assessed to determine if unsaturation is required for styrene toxicity. Ethylbenzene and vinylcyclohexane were found to be as toxic as styrene to h2E1 cells, whereas little toxicity of ethylcyclohexane to h2E1 cells was observed. This indicates the importance of vinyl group of styrene in its cytotoxicity, but saturation of the vinyl group does not necessarily eliminate styrene toxicity. An N-acetylcysteine conjugate derived from styrene oxide was identified by LC/MS/MS in the sample obtained from the incubation of h2E1 cell lysate with styrene in the presence of N-acetylcysteine. Formation of the N-acetylcysteine conjugate was found to be NADPH-dependent. These studies provided strong evidence in support of toxic role of styrene epoxide metabolite in styrene toxicity.