Comparative analysis of metabolism of trichloroethylene and tetrachloroethylene among mouse tissues and strains

Abstract

Trichloroethylene (TCE) and tetrachloroethylene (PCE) are structurally similar chemicals that are metabolized through oxidation and glutathione conjugation pathways. Both chemicals have been shown to elicit liver and kidney toxicity in rodents and humans; however, TCE has been studied much more extensively in terms of both metabolism and toxicity. Despite their qualitative similarities, quantitative comparison of tissue- and strain-specific metabolism of TCE and PCE has not been performed. To fill this gap, we conducted a comparative toxicokinetic study where equimolar single oral doses of TCE (800 mg/kg) or PCE (1000 mg/kg) were administered to male mice of C57BL/6J, B6C3F1/J, and NZW/LacJ strains. Samples of liver, kidney, serum, brain, and lung were obtained for up to 36 h after dosing. For each tissue, concentrations of parent compounds, as well as their oxidative and glutathione conjugation metabolites were measured and concentration-time profiles constructed. A multi-compartment toxicokinetic model was developed to quantitatively compare TCE and PCE metabolism. As expected, the flux through oxidation metabolism pathway predominated over that through conjugation across all mouse strains examined, it is 1,200–3,800 fold higher for TCE and 26–34 fold higher for PCE. However, the flux through glutathione conjugation, albeit a minor metabolic pathway, was 21-fold higher for PCE as compared to TCE. The degree of inter-strain variability was greatest for oxidative metabolites in TCE-treated and for glutathione conjugation metabolites in PCE-treated mice. This study provides critical data for quantitative comparisons of TCE and PCE metabolism, and may explain the differences in organ-specific toxicity between these structurally similar chemicals.

Introduction

Trichloroethylene (TCE) and tetrachloroethylene (PCE) are structurally similar chlorinated olefins that are used in chemical manufacture, metal degreasing, and other industrial applications (U.S. EPA, 2011a, U.S. EPA, 2011b). TCE and PCE are high production volume chemicals and are ubiquitous in the environment (IARC, 2014). Humans can be exposed to these chemicals via inhalation and ingestion (ATSDR, 1997; Wu and Schaum, 2000). In a National Health and Nutrition Examination Survey (2013–2014), the rate of detection for TCE and PCE in blood was 0.6% for TCE and 7.4% for PCE (CDC, 2017). TCE and PCE are still prioritized for evaluation of the risks to human health and environment (U.S. EPA, 2017).

There are differences in toxic effects of TCE and PCE in liver, kidney and other tissues (Cichocki et al., 2016). TCE is classified as “carcinogenic to humans” by US EPA (U.S. EPA, 2011b) and IARC (Guha et al., 2012); while PCE is classified as “likely to be a human carcinogen” by US EPA (U.S. EPA, 2011a) and as “probably carcinogenic to humans” by IARC (Guha et al., 2012). A comparative study of toxicodynamics of TCE and PCE in mice showed differences in the effects on liver and kidney, PCE perturbed more molecular pathways in mouse liver and kidney as compared to TCE (Zhou et al., 2017). However, there are no published reports of comparative analysis of toxicokinetics of TCE and PCE.

Upon absorption, TCE and PCE are metabolized through oxidative and glutathione conjugation pathways (Cichocki et al., 2016). Initial oxidation occurs on the double bond by cytochrome P450 s (CYPs) to generate an epoxide, which can be further metabolized. Trichloroacetic acid (TCA) is a major oxidative metabolite of both TCE and PCE, and is a common urinary biomarker of exposure (Forkert et al., 2003; Volkel et al., 1998). The other oxidative metabolite, trichloroethanol (TCOH), is a TCE-specific metabolite that is formed through oxidation of TCE to chloral hydrate (CH), while PCE oxidation occurs through trichloroacetyl chloride (Chiu et al., 2007). Both TCE and PCE can enzymatically conjugate with glutathione to form dichloro- or trichloro-glutathione conjugates (DCVG or TCVG) (Lash et al., 2000). These can be further metabolized via hepatic or renal gamma-glutamyl transferase and di-peptidase to form corresponding cysteine conjugates, DCVC or TCVC, which are then n-acetylated via N-acetyltransferase to generate NAcDCVC or NAcTCVC, respectively. In addition, both NAcDCVC and NAcTCVC can be deacetylated via acylase to yield DCVC or TCVC, respectively. Apart from N-acetylation, DCVC and TCVC can be further bio-activated via cysteine conjugate β lyase to generate reactive thioketenes, or flavin-containing monooxygenase to form corresponding sulfoxides (Lash et al., 2014). These and other reactive species derived from glutathione conjugation are thought to be significant contributors to the nephrotoxicity of TCE and PCE (Lash et al., 2001a, Lash et al., 2003).

Quantitative estimation of inter-individual variability in metabolism is also a critical challenge in human health assessments of TCE and PCE (Cichocki et al., 2017b, Cichocki et al., 2016; Venkatratnam et al., 2017). The inter-strain variability in TCE metabolism has been characterized by using a multi-strain panel of inbred mice (Bradford et al., 2011) and the Collaborative Cross mouse population (Luo et al., 2018a; Venkatratnam et al., 2017, Venkatratnam et al., 2018). Mouse population-derived variability estimates for TCE metabolism closely matched population variability estimates previously derived from human toxicokinetic studies with TCE (Chiu et al., 2014). Likewise, the inter-individual variability in PCE metabolism has also been studied in the Collaborative Cross mouse population (Cichocki et al., 2017b), and a nonalcoholic steatohepatitis mouse model (Cichocki et al., 2017a). However, these studies of inter-strain variability in metabolism and toxicity were conducted separately for TCE or PCE and using doses that were not equivalent. Because of the close structural similarity of these chemicals and paucity of the available toxicokinetic data, a comparative study of equimolar doses was conducted concurrently using three inbred strains, selected based on variability observed across strains with respect to oxidative and glutathione conjugation metabolism for TCE (Bradford et al., 2011). The data from this study fill critical gaps in our understanding of the quantitative differences in TCE and PCE toxicokinetics.