Effects of selected polychlorinated biphenyl (PCB) congeners on hepatic glutathione, glutathione-related enzymes, and selenium status: implications for oxidative stress.

Polychlorinated biphenyls (PCBs) induce drug metabolism that may lead to the bioactivation of PCBs themselves or alternatively may lead to oxidative events within the cell. The goal of the present study was to determine the influence of congeneric PCBs, selected as substrates for or inducers of drug metabolism, upon hepatic glutathione, glutathione-related enzymes, and selenium status. Male and female Sprague-Dawley rats received two i.p. injections per week of PCB 3 (4-chlorobiphenyl), PCB 28 (2,4,4'-trichlorobiphenyl), PCB 38 (3,4,5-trichlorobiphenyl), PCB 77 (3,3',4,4'-tetrachlorobiphenyl), PCB 153 (2,2',4,4',5,5'-hexachlorobiphenyl), or both PCBs 77 and 153 (100 micromol/kg/injection) and were killed at the end of 1, 2, or 3 weeks. Whole liver homogenates, hepatic cytosol, and microsomes were prepared. Both glutathione reductase and glutathione transferase activities were increased significantly in both male and female rats receiving PCB 77, an aryl hydrocarbon receptor agonist, as well as in those receiving both PCBs 77 and 153. No significant trend was observed in the levels of hepatic total glutathione. PCB 77 treatment decreased hepatic selenium-dependent glutathione peroxidase (SeGPX) activity in both male and female rats significantly. This decrease in activity following PCB 77 treatment was accompanied by a decrease in the cytosolic selenium-dependent glutathione peroxidase gene (GSPx1) transcript, as well as a decrease in hepatic total selenium levels. These data support the concept that exposure to the coplanar PCB 77 suppresses, via gene regulatory mechanisms, the cellular antioxidant enzyme SeGPX and that this decrease involves selenium. Lower halogenated PCBs that may be bioactivated to reactive oxygen species (ROS)-producing metabolites, and higher halogenated PCBs that are not Ah receptor agonists, were inactive.

2,4,4'-trichlorobiphenyl increases STAT5 transcriptional activity.

The promoting effects of polychlorinated biphenyls (PCBs) have been studied extensively in a variety of two-stage carcinogenesis models. However, the molecular mechanisms responsible for the promotion effects of PCBs have not been elucidated. We measured the effect of PCBs on DNA-binding proteins involved in cell proliferation and transformation. Male Sprague-Dawley rats were injected intraperitoneally with mono-, di-, tri-, tetra-, or hexachlorobiphenyls (300 micromol/kg/d) each day for 4 d and killed 4 h after the last injection. To detect alterations in nuclear proteins that could explain the tumor-promoter activity of PCBs, liver nuclear extracts were analyzed by electrophoretic mobility shift assays. Electrophoretic mobility shift assay analysis of signal transducers and activators of transcription (STAT)-binding activity to a consensus gamma-interferon-activated sequence (GAS) element was compared in liver nuclear extracts from treated rats. STAT-binding activity was eightfold to tenfold higher in nuclear extracts from animals treated with 2,4,4'-trichloro- (PCB 28) and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153). Analysis of the protein complex binding to the GAS element, with antibodies specific for STAT3, STAT5, and STAT6, indicated that the protein complex was made up of STAT5 and STAT6 proteins. HepG2 cells transiently transfected with a luciferase reporter gene construct containing many STAT5 binding sites were treated with PCB 28 and PCB 153. PCB 28 stimulated a greater than 25-fold increase in luciferase activity at the highest concentration tested, 1.0 microg/mL. However, enhanced luciferase activity did not occur with PCB 153 treatment. 4-Chlorobiphenyl (PCB 3), PCB 28, and PCB 153 treatment of Sprague-Dawley rats resulted in a large increase in protein binding to a consensus activated protein-1 (AP-1) element. However, 3,4-dichlorobiphenyl (PCB 12) and 3,3',4,4'-tetrachlorobiphenyl (PCB 77) treatments did not increase AP-1 transcription activity. Further analysis of the proteins binding to the AP-1 consensus sequence with antibodies specific for c-fos, junD, and junB indicated that the protein composition consists of junD proteins. These data showed functional differences between noncoplanar and coplanar PCBs with respect to STAT activation and AP-1-DNA binding.

Effects of peroxisome proliferators on antioxidant enzymes and antioxidant vitamins in rats and hamsters.

Peroxisome proliferators (PPs) cause hepatomegaly, peroxisome proliferation, and hepatocarcinogenesis in rats and mice, whereas hamsters are less responsive to PPs. PPs increase the activities of enzymes involved in peroxisomal beta-oxidation and omega-hydroxylation of fatty acids, which has been hypothesized to result in oxidative stress. The hypothesis of this study was that differential modulation of antioxidant enzymes and vitamins might account for differences in species susceptibility to PPs. Accordingly, we measured the activities of DT-diaphorase and superoxide dismutase (SOD) and the hepatic content of ascorbic acid and alpha-tocopherol in male Sprague-Dawley rats and Syrian hamsters fed 2 doses of 3 known peroxisome proliferators (dibutyl phthalate [DBP], gemfibrozil, and [4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio]acetic acid (Wy-14,643) for 6, 34, or 90 days. In untreated animals, the activity of DT-diaphorase was much higher in hamsters than in rats, but the control levels of SOD, ascorbic acid and alpha-tocopherol were similar. In rats and hamsters treated with Wy-14,643, we observed decreases in alpha-tocopherol content and total SOD activity. DT-diaphorase was decreased in activity following Wy-14,643 treatment in rats at all time points and doses, but only sporadically affected in hamsters. Rats and hamsters treated with DBP demonstrated increased SOD activity at 6 days; however, in the rat, DBP decreased SOD activity at 90 days and alpha-tocopherol content was decreased throughout. In gemfibrozil treated rats and hamsters, a decrease in alpha-tocopherol content and an increase in DT-diaphorase activity were observed. In either species, no consistent trend was observed in total ascorbic acid content after treatment with any of the PPs. In conclusion, these data suggest that both rats and hamsters are compromised in antioxidant capabilities following PP treatment and additional hypotheses for species susceptibility should be considered.

Polychlorinated biphenyl-induced effects on metabolic enzymes, AP-1 binding, vitamin E, and oxidative stress in the rat liver.

Environmental pollutants, such as polychlorinated biphenyls (PCBs), may induce drug metabolism and may be substrates for the induced metabolic enzymes. Both processes may lead to oxidative stress. The goal of this study was to determine the influence of polychlorinated biphenyls, selected as inducers and substrates of drug metabolism, on oxidative events within the liver over a 3-week time course. Male and female Sprague-Dawley rats received two ip injections per week of 4-chlorobiphenyl, 2,4,4'-trichlorobiphenyl, 3,4,5-trichlorobiphenyl, 3,3',4,4'-tetrachlorobiphenyl (PCB 77), 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153), or both PCB 77 and 153 (100 micromol/kg/injection) and were euthanized at the end of 1, 2, or 3 weeks. Hepatic cytochrome P450 1A1 (EROD) activity, DT-diaphorase activity, AP-1 DNA-binding activity, conjugated dienes, and alpha-tocopherol (vitamin E) as well as alpha-tocopheryl quinone (oxidized vitamin E) were determined. While the lower chlorinated biphenyls (at these doses and times) showed little or no effect on these oxidative stress parameters, both CYP 1A1 and DT-diaphorase activities were significantly increased in both male and female rats receiving PCB 77, a ligand for the aryl hydrocarbon receptor. In addition, the DNA-binding activity of the transcription factor AP-1 was increased in rats treated with PCB 77 or PCB 153. Within the lipid fraction there was no significant increase observed in conjugated diene concentrations, but there was a significant increase in alpha-tocopheryl quinone upon treatment with all PCBs tested. These data indicate that alpha-tocopheryl quinone may be a sensitive marker for PCB exposure and is possibly increased by a wide range of PCBs.

Redox cycling of 2-(x'-mono, -di, -trichlorophenyl)- 1, 4-benzoquinones, oxidation products of polychlorinated biphenyls.

Polychlorinated biphenyl (PCB) preparations are complete liver carcinogens in rodents and efficacious promoters in two-stage hepatocarcinogenesis. Cytochrome P450 isozymes catalyze the oxidation of PCBs to mono- and dihydroxy metabolites. The potential for further enzymatic or nonenzymatic oxidation of ortho- and para-dihydroxy PCB metabolites to (semi)quinones raises the possibility that redox cycling involving reactive oxygen species may be involved in PCB toxicity. Seven synthetic 2-(x'-chlorophenyl)-1, 4-benzoquinones (containing one to three chlorines) were investigated for their participation in oxidation-reduction reactions by following the oxidation of NADPH. These observations were made: (i) NADPH alone directly reduced all quinones but only 2-(2'-chlorophenyl)- and 2-(4'-chlorophenyl)-1,4-benzoquinone supported NADPH consumption beyond that required to quantitatively reduce the quinone. (ii) For all quinones, superoxide dismutase increased NADPH oxidation in excess of the amount of quinone, demonstrating the participation of the superoxide radical. (iii) The presence of microsomal enzymes from rat liver increased the rate of NADPH consumption, but only 2-(2'-chlorophenyl)- and 2-(4'-chlorophenyl)-1,4-benzoquinone autoxidized. (iv) The combination of superoxide dismutase with microsomal enzymes accelerated autoxidation from 1.6- to 6.8-fold higher than that found in the absence of microsomal protein. These data support the concept that in the absence of microsomal protein, there occurs a two-electron reduction of the quinone by NADPH to the corresponding hydroquinone that comproportionates with the large reservoir of quinone to initiate autoxidation. In the presence of microsomes, enzymatic one-electron reduction generates a semiquinone radical whose autoxidation with oxygen propagates the redox cycle. These results show the potential of some 2-(x'-chlorophenyl)-1, 4-benzoquinones to initiate the wasteful loss of NADPH.