3,3',4,4',5-Pentachlorobiphenyl (PCB 126) Decreases Hepatic and Systemic Ratios of Epoxide to Diol Metabolites of Unsaturated Fatty Acids in Male Rats

Abstract

Disruption of the homeostasis of oxygenated regulatory lipid mediators (oxylipins), potential markers of exposure to aryl hydrocarbon receptor (AhR) agonists, such as 3,3',4,4',5-pentachlorobiphenyl (PCB 126), is associated with a range of diseases, including nonalcoholic fatty liver disease and nonalcoholic steatohepatitis. Here we test the hypothesis that PCB 126 exposure alters the levels of oxylipins in rats. Male Sprague-Dawley rats (5-weeks old) were treated over a 3-month period every 2 weeks with intraperitoneal injections of PCB 126 in corn oil (cumulative doses of 0, 19.8, 97.8, and 390 µg/kg b.w.; 6 injections total). PCB 126 treatment caused a reduction in growth rates at the highest dose investigated, a dose-dependent decrease in thymus weights, and a dose-dependent increase in liver weights. Liver PCB 126 levels increased in a dose-dependent manner, while levels in plasma were below or close to the detection limit. The ratios of several epoxides to diol metabolites formed via the cytochrome P450 (P450) monooxygenase/soluble epoxide hydrolase (sEH) pathway from polyunsaturated fatty acids displayed a dose-dependent decrease in the liver and plasma, whereas levels of oxylipins formed by other metabolic pathways were generally not altered by PCB 126 treatment. The effects of PCB 126 on epoxide-to-diol ratios were associated with an increased CYP1A activity in liver microsomes and an increased sEH activity in liver cytosol and peroxisomes. These results suggest that oxylipins are potential biomarkers of exposure to PCB 126 and that the P450/sEH pathway is a therapeutic target for PCB 126-mediated hepatotoxicity that warrants further attention.

Keywords: CYP1A; oxylipin; persistent organic pollutants; polychlorinated biphenyls; soluble epoxide hydrolase.

FIG. 1

PUFAs are metabolized by P450 enzymes with epoxygenase activity/sEH (P450/sEH), LOX, COX, or nonenzymatic autoxidation pathways to a number of oxylipins. Only oxylipins analyzed in this study are shown (see Supplementary Tables S6–S9 for a complete summary). ^aLTB4s include LTB4, 6-trans-LTB4, 20-OH-LTB4 and 20-COOH-LTB4. Abbreviations: DiHDPE, dihydroxy-docosapentaenoic acid; DiHETE, dihydroxy-eicosatetraenoic acid; DiHOME, dihydroxy-octadecenoic acid; DiHODE, dihydroxy-octadecadienoic acid; DiHETrE, dihydroxy-eicosatrienoic acid; EKODE, epoxy-keto-octadecenoic acid; EpDPE, epoxy-docosapentaenoic acid; EpETE, epoxy-eicosatetraenoic acid; EpETrE, epoxy-eicosatrienoic acid; EpODE, epoxy-octadecadienoic acid; EpOME, epoxy-octadecenoic acid; HDoHE, hydroxyl-DHA; HEPE, hydroxy-eicosapentaenoic acid; HETE, hydroxyl-eicosatetraenoic acid; 15(S)-HETrE, hydroxy-trienoic acid; 9-HODE, 9-hydroxy-10,12-octadecadienoic acid; HOTrE, hydroxy-octadecatrienoic acid; 6-keto-PGF1a, 6-keto-prostaglandin F1a; LTB, leukotriene B; LXA4, lipoxin A4; oxo-ETE, oxo-eicosatetraenoic acid; PGE, prostaglandin E; THF-diol, tetrahydrofurandiol; TXB, thromboxane B.

FIG. 2

Chronic PCB 126 treatment of male rats reduced A, body weight gain and increased B, relative liver weights, C, hepatic protein content, and D, extractable lipid content in liver as percent of wet weight. Rats were treated with every 2 weeks with i.p. injections of corn oil (vehicle) or PCB 126 in corn oil for 3 months; see Table 1 for details regarding the doses. Data are presented as the mean ± standard deviation. Dunnett's multiple testing correction procedure was used to compare PCB 126 treatment with the corn oil control. *p < .05; ***P < .001.

FIG. 3

PCB 126 levels in the liver increased with increasing dose. Rats were treated every 2 weeks with i.p. injections of corn oil (vehicle) or PCB 126 in corn oil for 3 months; see Table 1 for details regarding the doses. Data are presented as the mean ± SD.

FIG. 4

Increasing PCB 126 doses resulted in an increase in (A) total hepatic P450 content, (B) EROD activity in liver microsomes as well as sEH activity in (C) cytosol, and (D) peroxisomes relative to controls. Rats were treated every 2 weeks with i.p. injections of corn oil (vehicle) or PCB 126 in corn oil for 3 months; see Table 1 for details regarding the doses. Data are expressed relative to controls because subcellular fractions were generated from frozen tissue samples and are presented as the mean ± SEM. Dunnett’s multiple testing correction procedure was used to compare PCB 126 treatment with the corn oil control. *P < .05; **P < .01; ***P < .001.

FIG. 5

‘Heat Map’ presentation of the levels of epoxides in the P450 monooxygenase/sEH pathway showing some dose-dependent changes in levels of this group of oxylipins in the liver and plasma of PCB 126-treated rats. Dunnett's multiple testing correction procedure was used to compare PCB 126 treatment with the corn oil control. Significantly different from control *P < .05; **P < 01; ***P < 0.001. One-way ANOVA were used to test for a treatment effect.

FIG. 6

‘Heat Map’ presentation of the levels of diols in the P450 monooxygenase/sEH pathway showing dose-dependent changes in levels of this group of oxylipins in the liver and plasma of PCB 126-treated rats. Dunnett’s multiple testing correction procedure was used to compare PCB 126 treatment with the corn oil control. Significantly different from control *P < .05; **P < .01; ***P < .001. One-way ANOVA were used to test for a treatment effect.

FIG. 7

The epoxide-to-diol metabolite ratios of several oxylipins decreased in a dose-dependent manner in the liver from PCB 126-treated male rats, as determined by metabolomics profiling with liquid chromatography-tandem mass spectrometry. Shown are representative examples of epoxide-to-diol ratios of oxylipins derived from (A and B) ARA and (C and D) DHA. See Supplementary Table S10 for a complete summary of epoxide-to-diol ratios. Data are presented as the mean ± SEM (n = 10 for control or n = 8 for PCB treatment groups). Dunnett’s multiple testing correction procedure was used to compare PCB 126 treatment with the corn oil control. Significantly different from control *P < .05; **P < .01; ***P < .001.

FIG. 8

The epoxide-to-diol metabolite ratios decreased in a dose-dependent manner in plasma from PCB 126-treated male rats for several oxylipins, as determined by metabolomics profiling with liquid chromatography-tandem mass spectrometry. Shown are representative examples of epoxide-to-diol ratios of oxylipins derived from (A) ARA, (B) LA, (C) DHA, and (D) EPA. See Supplementary Table S10 for a complete summary of epoxide-to-diol ratios. Data are presented as the mean ± SEM (n = 10 for control or n = 8 for PCB treatment groups). Dunnett’s multiple testing correction procedure was used to compare PCB 126 treatment with the corn oil control. Significantly different from control *P < .05; ***P < .001.