Oxidative stress and DNA damage in Fischer rats following acute exposure to trichloroethylene or perchloroethylene

Abstract

Oxidative DNA damage is emerging as an biomarker of effect in studies assessing the health risks of occupational chemicals. Trichloroethylene (TCE) and perchloroethylene (PERC) are used in the dry cleaning industry and their metabolism can produce reactive oxygen compounds. The present study examined the potential for TCE and PERC to induce oxidative DNA damage in rats that was detectable as increased urinary excretion of 8-hydroxydeoxyguanosine (8OHdG). Thiobarbaturic acid reactive substances (TBARS) and 8-epi-prostaglandin F_2α (8epiPGF) were also measured as biomarkers of increased oxidative stress. Male Fischer rats were administered a single i.p. injection of 0, 100, 500, or 1000 mg/kg of PERC or TCE. Control rats received only vehicle (1:4 v/v of Alkamuls/water). A positive control group received 100 mg/kg 2-nitropropane (2NP). Rats were sacrificed 24 h after dosing. In rats receiving 2NP or TCE but not PERC, TBARS and the 8OHdG/dG ratios were significantly elevated in liver. Lymphocyte 8OHdG/dG was not affected significantly by 2NP, TCE or PERC. In rats receiving 2NP, urinary excretion of 8OHdG and 8epiPGF2 were significantly increased. In rats receiving TCE or PERC, significant increases in 8epiPGF2 or 8OHdG were not evident. Results indicate that a single high dose of TCE, but not PERC, can induce an increase in oxidative DNA damage in rat liver. However, the usefulness of 8OHdG as a biomarker of TCE-induced oxidative DNA damage is questionable.

Introduction

Trichloroethylene (TCE) and perchloroethylene (PERC) are used in the dry cleaning industry where PERC is the solvent of choice for laundering clothes and TCE is used as a spot remover. TCE is also extensively used as a metal degreaser in other industries (IARC, 1995). The National Institute for Occupational Safety and Health (NIOSH) estimates that in the USA more than 500 000 workers are exposed to PERC and more than 400 000 workers are exposed to TCE (NIOSH, 1994). Both are rodent carcinogens and also produce a wide array of toxicological effects in animals and humans; most notable of which are hepatic-, renal-, and neuro-toxicity (NCI, 1976, NTP, 1990, ATSDR, 1997a, ATSDR, 1997b).

Trichloroacetic acid (TCA) and dichloracetic acid (DCA) are metabolites of TCE and PERC in both man and experimental animals, and are also rodent heptocarcinogens (Bull et al., 1990, DeAngelo et al., 1991, ATSDR, 1997a, ATSDR, 1997b, Volkel et al., 1998). Because TCA and DCA induce lipid peroxidation and oxidative DNA damage, oxidative stress has been implicated in their carcinogenesis (Larson and Bull, 1992, Austin et al., 1996). TCE has recently been reported to induce hepatic lipid peroxidation and elevate oxidative DNA damage in mouse liver (Cojocel et al., 1989, Channel et al., 1998). Although there is no direct evidence that PERC causes oxidative damage, the ability of Vitamin E to protect against PERC toxicity in mice provides indirect evidence (Ebrahim et al., 1996).

Presently, biomonitoring for TCE or PERC exposure in the workplace is accomplished by breath or urine analysis of the parent compound or metabolites (IARC, 1995). These represent biomarkers of exposure, but provide little information on potential activation of these compounds or interactions with target tissues. If TCE and PERC induce oxidative stress, then urinary biomarkers of oxidative damage could serve as biomarkers of biologically effective dose for workers exposed to these compounds in the dry cleaning industry.

DNA is subject to continuous oxidative damage from oxygen radicals generated during normal cellular respiration (Loft et al., 1992, Loft et al., 1994). Unrepaired DNA damage can lead to mutation and possibly cancer (Ames, 1989, Guyton and Kensler, 1993, Hemminki, 1993). Environmental exposures that increase the rate of damage above background levels increase the potential for unrepaired lesions to become permanent mutations (Kuchino et al., 1987, Feig et al., 1994). This concept has led to the use of oxidative DNA damage in the assessment of occupational and environmental exposures to chemicals that are capable of inducing an oxidative stress. Oxygen-based radicals can produce a variety of DNA lesions (Breen and Murphy, 1995), but the 8-hydroxydeoxyguanosine (8OHdG) adduct has been used most extensively as a biomarker of effect (Loft et al., 1993, Tagesson et al., 1993, Lagorio et al., 1994, Schins et al., 1995, Liu et al., 1996, Loft and Poulsen, 1998, Toraason, 1999).

The objective of the present study was to determine if a single exposure to PERC or TCE could induce an oxidative stress in rats that is detectable in urine using a rapid and relatively inexpensive immunoassay for 8OHdG. Malondialdehyde assessed by the Thiobarbituric Acid-Reactive Substances (TBARS) assay is the most widely used marker of lipid peroxidation resulting from oxidative stress. Although the rapid metabolism of malondialdehyde makes it unreliable as a urinary biomarker (DeZwart et al., 1998), it is applicable for assessing oxidative damage in target tissues of experimental animals. In the present study, the TBARS assay was used to determine if TCE or PERC exposures induced oxidative damage in liver or kidney. 8-Epi-prostaglandin F_2α (8epiPGF) is a free radical-catalyzed isomer of arachidonic acid and has proven useful in exploring the role of free radicals in disease processes because it does not undergo metabolism in the body and is stable in excreted urine (Morrow and Roberts, 1997). 8epiPGF was used as a urinary marker of oxidative damage to cell membranes.