Elsevier

Toxicology Letters

Volume 209, Issue 3, 25 March 2012, Pages 291-298
Toxicology Letters

Regulation of paraoxonase 1 (PON1) in PCB 126-exposed male Sprague Dawley rats

https://doi.org/10.1016/j.toxlet.2012.01.003Get rights and content

Abstract

3,3′,4,4′,5-Pentachlorobiphenyl (PCB 126), an aryl hydrocarbon receptor (AhR) agonist and most potent dioxin-like PCB congener, significantly alters gene expression, lipid metabolism, and oxidative stress in the liver. PON1, an antioxidant and anti-atherogenic enzyme, is produced in the liver and secreted into the blood where it is incorporated into high density lipoprotein (HDL) and protects LDL and cellular membranes against lipid peroxidation. To explore the regulation of PON1, male Sprague-Dawley rats were treated with ip injections of corn oil or 1 μmol/kg or 5 μmol/kg PCB 126 and euthanized up to two weeks afterwards. Serum total and HDL-cholesterol were increased by low dose and decreased by high dose exposure, while LDL-cholesterol was unchanged. PCB 126 significantly increased hepatic PON1 gene expression and liver and serum PON1 activities. Liver and serum thiobarbituric acid reactive substances levels were not elevated except for high dose and long exposure times. Serum antioxidant capacity was unchanged across all exposure doses and time points. This study, the first describing the regulation of gene expression of PON1 by a PCB congener, raises interesting questions whether elevated PON1 is able to ameliorate PCB 126-induced lipid peroxidation and whether serum PON1 levels may serve as a new biomarker of exposure to dioxin-like compounds.

Highlights

PCB126 is a potent AhR agonist and reported to increase oxidative stress. ► PON1 is an important serum antioxidant, produced in the liver and attached to HDL. ► In rats PCB126 increased hepatic PON1 mRNA and hepatic and serum PON1 activity. ► No changes in lipid peroxides (TBARS) or total serum antioxidant capacity were seen. ► Increased PON1 activity may ameliorate oxidative stress produced by PCB126.

Introduction

Polychlorinated biphenyls (PCBs) are persistent organic pollutants and ubiquitous in the environment even though their commercial production has been banned for decades. Most of the 209 individual PCB congeners are highly resistant to biotransformation and bioaccumulate. Certain congeners with non-ortho or mono-ortho chlorine substitution are called “dioxin-like PCBs” because their binding to the arylhydrocarbon receptor (AhR) and biochemical and toxic activities are similar to those of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (Bandiera et al., 1982, Hestermann et al., 2000). PCB 126 is the most potent AhR agonist among all PCB congeners tested (Bandiera et al., 1982) and has a toxic equivalency factor (TEF) of 0.1 compared to TCDD, which qualifies it as the most toxic PCB in this family (Van den Berg et al., 2006). Like TCDD, PCB 126 significantly alters the expression of numerous genes and increases enzyme activities, particularly CYP 1A1/2 (Kopec et al., 2010). PCBs also increase intracellular oxidative stress which has been associated with a variety of disease processes, including carcinogenesis and cardiovascular damage (Hennig et al., 2002, Hennig et al., 2005, Ludewig et al., 2008). Several studies have shown that the dioxin-like PCBs (e.g., PCB77 and PCB 126) induce ROS and cause lipid oxidation in endothelial cells which is associated with atherosclerosis (Hassoun et al., 2002, Choi et al., 2010).

Paraoxonase 1 (PON1), a member of a three gene family which includes PON2 and PON3, is a calcium-dependent hydrolase with substrate specificity towards esters, phosphotriester lactones, carbonates and other compounds (Costa et al., 2005a, Costa et al., 2005b). PON1 is primarily synthesized in the liver and secreted into the blood as a major high density lipoprotein (HDL)-associated enzyme (La Du et al., 1999). Besides chemoprotection through detoxification of organophosphorus (OP) pesticides, it displays preventive properties against cardiovascular disease, mainly through removal of oxidized lipids in low density lipoproteins and cell membranes (Furlong et al., 2005, Furlong et al., 2010).

Recently, XRE (xenobiotic response element)-like sequences were described in the promoter region of PON1 and these sequences were identified as possible binding sites for the activated AhR (Gouedard et al., 2004). In this publication dietary polyphenols and 3-methylcholanthrene (3-MC) were shown to induce PON1 through the AhR-dependent pathway. We therefore hypothesized that PCB 126, as an AhR agonist, may influence PON1 expression and thereby hepatic and serum PON1 activities, a possible protective mechanism against PCB-induced ROS. Here we show that PCB 126 increased hepatic PON1 expression and liver and serum PON1 activity as well as serum HDL levels and hepatic CYP1A1 and AhR transcription in male Sprague Dawley rats in vivo, while no consistent change in lipid peroxides (TBARS) and serum total antioxidant activity was observed.

Section snippets

Chemicals and reagents

PCB 126 was synthesized and purified using a modified Suzuki-coupling method of 3,4,5-trichlorobromobenzene with 3,4-dichlorophenyl boronic acid utilizing a palladium-catalyzed cross coupling reaction as previously reported (Luthe et al., 2009). All reagents were obtained from Fisher Scientific (Pittsburgh, PA) and were the highest purity available, if not stated otherwise.

Animals and treatment protocol

Male Sprague-Dawley rats (75–100 g) from Harlan Laboratories Inc. (Indianapolis, IN) were housed in a controlled environment

Biometrical parameters

In the dose–response study (Table 1, upper half) the growth of rats treated with the high dose (5 μmol/kg) of PCB 126 was significantly decreased (P < 0.05) and the final body weights were decreased by 31% (P < 0.05). Liver weights were significantly increased by the lower dose of 1 μmol/kg PCB 126 (46%, P < 0.05), and both doses significantly increased the relative liver weight (51% and 54%, respectively). In the time–response study (Table 1, lower half) weight gain of PCB 126-treated rats was

Discussion

Recently, researchers reported that PCB 126, the most potent dioxin-like PCB congener and a conventional food chain persistent organic pollutant, was found in Chicago air (Zhao et al., 2010). This suggests a new exposure source besides the food chain. PCB 126 was shown to induce oxidative stress and cause lipid peroxidation (Hassoun et al., 2002). PON1 is believed to protect against lipid peroxidation and to lower the risk of developing coronary artery disease and atherosclerosis (Shih et al.,

Funding

This study is supported by NIEHS P42 ES013661 and DOD DAMD17-02-1-0241.

Conflicts of interest

The authors declare that there are no conflicts of interest.

Acknowledgement

The authors would like to thank the members of the Robertson and Ludewig laboratories for help with animal treatments and necropsy.

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