Kinetics of naphthalene metabolism in target and non-target tissues of rodents and in nasal and airway microsomes from the Rhesus monkey

Metabolomics of Lung Microdissections Reveals Region- and Sex-Specific Metabolic Effects of Acute Naphthalene Exposure in Mice

Naphthalene is a ubiquitous environmental contaminant produced by combustion of fossil fuels and is a primary constituent of both mainstream and side stream tobacco smoke. Naphthalene elicits region-specific toxicity in airway club cells through cytochrome P450 (P450)-mediated bioactivation, resulting in depletion of glutathione and subsequent cytotoxicity. Although effects of naphthalene in mice have been extensively studied, few experiments have characterized global metabolomic changes in the lung. In individual lung regions, we found metabolomic changes in microdissected mouse lung conducting airways and parenchyma obtained from animals sacrificed at 3 timepoints following naphthalene treatment. Data on 577 unique identified metabolites were acquired by accurate mass spectrometry-based assays focusing on lipidomics and nontargeted metabolomics of hydrophilic compounds. Statistical analyses revealed distinct metabolite profiles between the 2 lung regions. Additionally, the number and magnitude of statistically significant exposure-induced changes in metabolite abundance were different between airways and parenchyma for unsaturated lysophosphatidylcholines, dipeptides, purines, pyrimidines, and amino acids. Importantly, temporal changes were found to be highly distinct for male and female mice with males exhibiting predominant treatment-specific changes only at 2 h postexposure. In females, metabolomic changes persisted until 6 h postnaphthalene treatment, which may explain the previously characterized higher susceptibility of female mice to naphthalene toxicity. In both males and females, treatment-specific changes corresponding to lung remodeling, oxidative stress response, and DNA damage were observed. Overall, this study provides insights into potential mechanisms contributing to naphthalene toxicity and presents a novel approach for lung metabolomic analysis that distinguishes responses of major lung regions.

Incorporating ToxCast™ data into naphthalene human health risk assessment

Chronic inhalation of naphthalene causes nasal olfactory epithelial tumors in rats and benign lung adenomas in mice. The available human data do not establish an association between naphthalene and increased respiratory cancer risk. Therefore, cancer risk assessment of naphthalene in humans depends predominantly on experimental evidence from rodents. The United States Environmental Protection Agency's (US EPA) Toxicity Forecaster (ToxCast™) database contains data from 710 in vitro assays for naphthalene, the majority of which were conducted in human cells. Of these assays, only 18 were active for naphthalene, and all were in human liver cells. No assays were active in human bronchial epithelial cells. In our analysis, all of the active naphthalene ToxCast assay data were reviewed and used to: 1) determine naphthalene human inhalation concentrations corresponding to relevant activity concentrations for all active naphthalene assays, using a physiologically based pharmacokinetic (PBPK) model; and 2) evaluate the transcriptional responses for active assays in the context of consistency with the larger naphthalene data set and proposed modes of action (MoAs) for naphthalene toxicity and carcinogenicity. The transcriptional responses in liver cells largely reflect cellular activities related to oxidative stress and chronic inflammation. Overall, the results from our analysis of the active ToxCast assays for naphthalene are consistent with conclusions from our earlier weight-of-evidence evaluation for naphthalene carcinogenesis.

Naphthalene genotoxicity: DNA adducts in primate and mouse airway explants

Naphthalene (NA) is a ubiquitous environmental pollutant and possible human carcinogen that forms tumors in rodents with tissue/regional and species selectivity. This study seeks to determine whether NA is able to directly adduct DNA in an ex vivo culture system. Metabolically active lung tissue was isolated and incubated in explant culture with carbon-14 labeled NA (0, 25, 250 μM) or 1,2-naphthoquinone (NQ), followed by AMS analyses of metabolite binding to DNA. Despite relatively low metabolic bioactivation in the primate airway, dose-dependent NA-DNA adduct formation was detected. More airway adducts were detected in female mice (4.7-fold) and primates (2.1-fold) than in males of the same species. Few adducts were detected in rat airway or nasal epithelium. NQ, which is a metabolic product of NA, proved to be even more potent, with levels of adduct formation 70-80-fold higher than seen when tissues were incubated with the parent compound NA. This is the first study to demonstrate NA-DNA adduct formation at a site of carcinogenesis, the mouse lung. Adducts were also detected in non-human primate lung and with a NQ metabolite of NA. Taken together, this suggests that NA may contribute to in vivo carcinogenesis through a genotoxic mechanism.

Human CYP2A13 and CYP2F1 Mediate Naphthalene Toxicity in the Lung and Nasal Mucosa of CYP2A13/2F1-Humanized Mice

BACKGROUND

The potential carcinogenicity of naphthalene (NA), a ubiquitous environmental pollutant, in human respiratory tract is a subject of intense debate. Chief among the uncertainties in risk assessment for NA is whether human lung CYP2A13 and CYP2F1 can mediate NA's respiratory tract toxicity.

OBJECTIVES

We aimed to assess the in vivo function of CYP2A13 and CYP2F1 in NA bioactivation and NA-induced respiratory tract toxicity in mouse models.

METHODS

Rates of microsomal NA bioactivation and the effects of an anti-CYP2A antibody were determined for lung and nasal olfactory mucosa (OM) from Cyp2abfgs-null, CYP2A13-humanized, and CYP2A13/2F1-humanized mice. The extent of NA respiratory toxicity was compared among wild-type, Cyp2abfgs-null, and CYP2A13/2F1-humanized mice following inhalation exposure at an occupationally relevant dose (10 ppm for 4 hr).

RESULTS

In vitro studies indicated that the NA bioactivation activities in OM and lung of the CYP2A13/2F1-humanized mice were primarily contributed by, respectively, CYP2A13 and CYP2F1. CYP2A13/2F1-humanized mice showed greater sensitivity to NA than Cyp2abfgs-null mice, with greater depletion of nonprotein sulfhydryl and occurrence of cytotoxicity (observable by routine histology) in the OM, at 2 or 20 hr after termination of NA exposure, in humanized mice. Focal, rather than gross, lung toxicity was observed in Cyp2abfgs-null and CYP2A13/2F1-humanized mice; however, the extent of NA-induced lung injury (shown as volume fraction of damaged cells) was significantly greater in the terminal bronchioles of CYP2A13/2F1-humanized mice than in Cyp2abfgs-null mice.

CONCLUSION

CYP2F1 is an active enzyme. Both CYP2A13 and CYP2F1 are active toward NA in the CYP2A13/2F1-humanized mice, where they play significant roles in NA-induced respiratory tract toxicity. https://doi.org/10.1289/EHP844.

Preparation of Specific Compartments of the Lungs for Pathologic and Biochemical Analysis of Toxicologic Responses

This unit focuses on protocols for assessing microenvironment-specific responses in the thoracic lung tissues. Aspects of the entire respiratory system serve as potential targets for candidate toxicants, but each candidate toxicant may impact distinct sites due to differential distribution of either the toxicant or the target cells. Within the conducting airways, the composition of resident cell populations and the metabolic capabilities of the cell populations vary greatly. Thus, studies of this region of the lung require unique, site-selective methods to clearly define the toxic response. Without site-specific sampling, as described in this chapter, the experimental limit of detection for toxicant effects in conducting airways is weakened because differences unrelated to treatment, but related to location, may dominate the response. The protocols included here allow assessment of toxicological responses in the tracheobronchial airways and the gas exchange area of the lung, with specific application to laboratory mammals. © 2017 by John Wiley & Sons, Inc.

Structure-Function Studies of Naphthalene, Phenanthrene, Biphenyl, and Their Derivatives in Interaction with and Oxidation by Cytochromes P450 2A13 and 2A6

Naphthalene, phenanthrene, biphenyl, and their derivatives having different ethynyl, propynyl, butynyl, and propargyl ether substitutions were examined for their interaction with and oxidation by cytochromes P450 (P450) 2A13 and 2A6. Spectral interaction studies suggested that most of these chemicals interacted with P450 2A13 to induce Type I binding spectra more readily than with P450 2A6. Among the various substituted derivatives examined, 2-ethynylnaphthalene, 2-naphthalene propargyl ether, 3-ethynylphenanthrene, and 4-biphenyl propargyl ether had larger ΔAmax/Ks values in inducing Type I binding spectra with P450 2A13 than their parent compounds. P450 2A13 was found to oxidize naphthalene, phenanthrene, and biphenyl to 1-naphthol, 9-hydroxyphenanthrene, and 2- and/or 4-hydroxybiphenyl, respectively, at much higher rates than P450 2A6. Other human P450 enzymes including P450s 1A1, 1A2, 1B1, 2C9, and 3A4 had lower rates of oxidation of naphthalene, phenanthrene, and biphenyl than P450s 2A13 and 2A6. Those alkynylated derivatives that strongly induced Type I binding spectra with P450s 2A13 and 2A6 were extensively oxidized by these enzymes upon analysis with HPLC. Molecular docking studies supported the hypothesis that ligand-interaction energies (U values) obtained with reported crystal structures of P450 2A13 and 2A6 bound to 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, indole, pilocarpine, nicotine, and coumarin are of use in understanding the basis of possible molecular interactions of these xenobiotic chemicals with the active sites of P450 2A13 and 2A6 enzymes. In fact, the ligand-interaction energies with P450 2A13 4EJG bound to these chemicals were found to relate to their induction of Type I binding spectra.

Protective effect of diallyl trisulfide against naphthalene-induced oxidative stress and inflammatory damage in mice

The aim of this study was to investigate the possible protective effects of diallyl trisulfide (DATS) against naphthalene-induced oxidative and inflammatory damage in the livers and lungs of mice. Elevated serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels showed significant hepatic damage after the challenge with 100 mg/kg naphthalene. Hepatic malondialdehyde (MDA) contents and the activity of myeloperoxidase (MPO) increased significantly, accompanying a decrease in the hepatic activity of total superoxide dismutase (SOD) and glutathione (GSH) levels after the naphthalene damage. In addition, the serum levels of nitric oxide (NO), tumor necrosis factor α (TNF-α), and interleukin 8 (IL-8) increased significantly in the groups damaged with naphthalene. The main parameters related to oxidative stress and inflammatory responses in the lungs, including the NO, MPO, and GSH contents, were determined, and the histopathological and immunohistochemical changes in the lung and liver tissues were also observed. In the DATS-treated groups, all of the oxidative and inflammatory damage in the serum, liver, and lung tissues were significantly prevented.

Hypothesis-based weight-of-evidence evaluation and risk assessment for naphthalene carcinogenesis

Inhalation of naphthalene causes olfactory epithelial nasal tumors in rats (but not in mice) and benign lung adenomas in mice (but not in rats). The limited available human data have not identified an association between naphthalene exposure and increased respiratory cancer risk. Assessing naphthalene's carcinogenicity in humans, therefore, depends entirely on experimental evidence from rodents. We evaluated the respiratory carcinogenicity of naphthalene in rodents, and its potential relevance to humans, using our Hypothesis-Based Weight-of-Evidence (HBWoE) approach. We systematically and comparatively reviewed data relevant to key elements in the hypothesized modes of action (MoA) to determine which is best supported by the available data, allowing all of the data from each realm of investigation to inform interpretation of one another. Our analysis supports a mechanism that involves initial metabolism of naphthalene to the epoxide, followed by GSH depletion, cytotoxicity, chronic inflammation, regenerative hyperplasia, and tumor formation, with possible weak genotoxicity from downstream metabolites occurring only at high cytotoxic doses, strongly supporting a non-mutagenic threshold MoA in the rat nose. We also conducted a dose-response analysis, based on the likely MoA, which suggests that the rat nasal MoA is not relevant in human respiratory tissues at typical environmental exposures. Our analysis illustrates how a thorough WoE evaluation can be used to support a MoA, even when a mechanism of action cannot be fully elucidated. A non-mutagenic threshold MoA for naphthalene-induced rat nasal tumors should be considered as a basis to determine human relevance and to guide regulatory and risk-management decisions.

Simultaneous quantification of multiple urinary naphthalene metabolites by liquid chromatography tandem mass spectrometry

Naphthalene is an environmental toxicant to which humans are exposed. Naphthalene causes dose-dependent cytotoxicity to murine airway epithelial cells but a link between exposure and human pulmonary disease has not been established. Naphthalene toxicity in rodents depends on P450 metabolism. Subsequent biotransformation results in urinary elimination of several conjugated metabolites. Glucuronide and sulfate conjugates of naphthols have been used as markers of naphthalene exposure but, as the current studies demonstrate, these assays provide a limited view of the range of metabolites generated from the parent hydrocarbon. Here, we present a liquid chromatography tandem mass spectrometry method for measurement of the glucuronide and sulfate conjugates of 1-naphthol as well as the mercapturic acids and N-acetyl glutathione conjugates from naphthalene epoxide. Standard curves were linear over 2 log orders. On column detection limits varied from 0.91 to 3.4 ng; limits of quantitation from 1.8 to 6.4 ng. The accuracy of measurement of spiked urine standards was -13.1 to + 5.2% of target and intra-day and inter-day variability averaged 7.2 (± 4.5) and 6.8 (± 5.0) %, respectively. Application of the method to urine collected from mice exposed to naphthalene at 15 ppm (4 hrs) showed that glutathione-derived metabolites accounted for 60-70% of the total measured metabolites and sulfate and glucuronide conjugates were eliminated in equal amounts. The method is robust and directly measures several major naphthalene metabolites including those derived from glutathione conjugation of naphthalene epoxide. The assays do not require enzymatic deconjugation, extraction or derivatization thus simplifying sample work up.

A hybrid CFD-PBPK model for naphthalene in rat and human with IVIVE for nasal tissue metabolism and cross-species dosimetry

A PBPK model for naphthalene in the rat and human that incorporates a hybrid CFD-PBPK description of the upper respiratory tract was developed to support cross-species dosimetry comparisons of naphthalene concentrations and tissue normalized rate of metabolism in the nasal respiratory and olfactory epithelium, lung and liver. In vitro measurements of metabolic rates from microsomal incubations published for rat and monkey (surrogate for human) were scaled to the specific tissue based on the tissue microsomal content and volume of tissue. The model reproduces time courses for naphthalene blood concentrations from intravenous and inhalation exposures in rats and upper respiratory tract extraction data in both naïve rats and rats pre-treated to inhibit nasal metabolism. This naphthalene model was applied to estimate human equivalent inhalation concentrations (HECs) corresponding to several NOAELs or LOAELs for the non-cancer effects of naphthalene in rats. Two approaches for cross-species extrapolation were compared: (1) equivalence based on tissue naphthalene concentration and (2) equivalence based on amount metabolized per minute (normalized to tissue volume). At the NOAEL of 0.1 ppm, the regional gas dosimetry ratio (RGDR) based on naphthalene concentration was 0.18 for the dorsal olfactory region; however, the RGDR rises to 5.4 when based on the normalized amount metabolized due to the lower of expression of CYP isozymes in the nasal epithelium of primates and humans. The resulting HEC is 0.12 ppm (0.63 mg/m(3)) continuous exposure at the rat NOAEL of 0.1 ppm (6 h/day, 5 days/week).