Decreased glutathione peroxidase activity in mice in response to nafenopin is caused by changes in selenium metabolism

Role of Oxidative Stress in Peroxisome Proliferator-Mediated Carcinogenesis

In this review, the evidence about the role of oxidative stress in the induction of hepatocellular carcinomas by peroxisome proliferators is examined. The activation of PPAR-alpha by peroxisome proliferators in rats and mice may produce oxidative stress, due to the induction of enzymes like fatty acyl coenzyme A (CoA) oxidase (AOX) and cytochrome P-450 4A1. The effect of peroxisome proliferators on the antioxidant defense system is reviewed, as is the effect on endpoints resulting from oxidative stress that may be important in carcinogenesis, such as lipid peroxidation, oxidative DNA damage, and transcription factor activation. Peroxisome proliferators clearly inhibit several enzymes in the antioxidant defense system, but studies examining effects on lipid peroxidation and oxidative DNA damage are conflicting. There is a profound species difference in the induction of hepatocellular carcinomas by peroxisome proliferators, with rats and mice being sensitive, whereas species such as nonhuman primates and guinea pigs are not susceptible to the effects of peroxisome proliferators. The possible role of oxidative stress in these species differences is also reviewed. Overall, peroxisome proliferators produce changes in oxidative stress, but whether these changes are important in the carcinogenic process is not clear at this time.

Selenoproteins, Cholesterol-Lowering Drugs, and the Consequences Revisiting of the Mevalonate Pathway

3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) and peroxisome proliferator-activated receptor alpha activators (fibrates) are the backbone of pharmacologic hypercholesterolemia and dyslipidemia treatment. Many of their clinical effects, however, are still enigmatic. This article describes how a side road of the mevalonate pathway, characterized in recent years, can rationalize a major fraction of these unexplained observations. This side road is the enzymatic isopentenylation of selenocysteine-tRNA([Ser]Sec) (Sec-tRNA), the singular tRNA to decode the unusual amino acid selenocysteine. The functionally indispensable isopentenylation of Sec-tRNA requires a unique intermediate from the mevalonate pathway, isopentenyl pyrophosphate, which concomitantly constitutes the central building block for cholesterol biosynthesis, and whose formation is suppressed by statins and fibrates. The resultant inhibition of Sec-tRNA isopentenylation profoundly decreases selenoprotein expression. This effect might seamlessly explain the immunosuppressive, redox, endothelial, sympatholytic, and thyroidal effects of statins and fibrates as well as their common side effects and drug interactions.

Effects of peroxisome proliferators on glutathione and glutathione-related enzymes in rats and hamsters

Peroxisomeproliferators (PPs) cause hepatomegaly, peroxisome proliferation, and hepatocarcinogenesis in rats and mice. Conversely, hamsters are less responsive to these compounds. PPs increase peroxisomal beta-oxidation and P4504A subfamily activity, which has been hypothesized to result in oxidative stress. We hypothesized that differential modulation of glutathione-related defenses could account for the resulting difference in species susceptibility following PP administration. Accordingly, we measured glutathione S-transferase (GST), glutathione peroxidase (GPx), and glutathione reductase (GR) activities, and total glutathione (GSH) in male Sprague-Dawley rats and Syrian hamsters fed two doses of three known peroxisome proliferators [dibutylphthalate (DBP), gemfibrozil, and Wy-14,643] for 6, 34, or 90 days. In rats, decreases in GR, GST, and selenium-dependent GPx were observed following PP treatment at various time points. In hamsters, we observed higher basal levels of activities for GR, GST, and selenium-dependent GPx compared to rats. In addition, hamsters showed decreases in GR and GST activities following PP treatment. Interestingly, selenium-dependent GPx activity was increased in hamsters following treatment with Wy-14,643 and DBP. Treatment for 90 days with Wy-14,643 resulted in no change in GPx1 mRNA in rats and increased GPx1 mRNA in hamsters. Sporadic changes in total GSH and selenium-independent GPx were observed in both species. This divergence in the hydrogen peroxide detoxification ability between rats and hamsters could be a contributing factor in the proposed oxidative stress mechanism of PPs observed in responsive and nonresponsive species.

Mouse liver selenium-binding protein decreased in abundance by peroxisome proliferators

Several studies with two-dimensional gel electrophoresis (2-DE) have shown that the abundance of numerous mouse liver proteins is altered in response to treatment with chemicals known to cause peroxisome proliferation. The peptide masses from tryptic digests of two liver proteins showing dramatic decreases in abundance in response to numerous peroxisome proliferators were used to search sequence databases. The selenium-binding protein 2 (SBP2 formerly 56 kDa acetaminophen-binding protein, AP 56) and selenium-binding protein 1 (SBP1 formerly 56 kDa selenium-binding protein, SP 56) in mouse liver, proteins with a high degree of sequence similarity, were the highest ranked identities obtained. Identity with SBP2 was subsequently confirmed by immunodetection with specific antiserum. Treatment of mice with 0.025% ciprofibrate resulted in the more basic of this pair of proteins being decreased to 30% of control abundance while the acidic protein was decreased to 7% of the control amount. Dexamethasone treatment, in contrast, caused increases of 80% and 20% in the abundance of the acidic and basic forms, respectively. Administration of dexamethasone to mice in combination with ciprofibrate produced expression of the acidic SBP2 at 23% of the control level and the basic SBP2 at 36%, a slightly moderated reduction compared with the decrease that occurred with ciprofibrate alone. These data suggest that peroxisome proliferators such as ciprofibrate cause a decrease in the abundance of the SBP2, which leads to increased cell proliferation, even in the presence of an inhibitor such as dexamethasone. Such a decrease in SBP, thought to serve as cell growth regulation factors, could be central to the nongenotoxic carcinogenicity of the peroxisome proliferators observed in rodents.

Effect of peroxisome proliferator on extracellular glutathione peroxidase in rat

Glutathione peroxidase (GPX) activity measured using tert-butyl hydroperoxide as a substrate detects solely cellular/classical GPX (cGPX) in rat liver and kidney, and extracellular/plasma glutathione peroxidase (EC-GPX) in rat serum. To investigate the effect of peroxisome proliferator on EC-GPX, we measured activities of GPX and catalase in rat liver, kidney and serum, and then we performed immunoblot and Northern blot analyses in the kidney. Rats were fed on a diet containing either 2% (w/w) di-2-ethylhexyl phthalate (DEHP) or 0.25% (w/w) clofibrate for two or three weeks, respectively. Catalase activity was increased 1.4-fold (p < 0.001) in the treated liver, but not in the kidney. GPX activity was decreased to 59.2% (DEHP) and 70.4% (clofibrate) of the control (p < 0.001) in the serum but was unaltered in the liver and kidney. The immunoreactivity for EC-GPX was also significantly decreased in the DEHP-treated kidney compared with the control. The mRNA levels of EC-GPX and cGPX were unaltered. The immunostaining for 4-hydroxy-2-nonenal, a maker of lipid peroxide, was more intense in the treated kidney compared with the control. These results suggest that EC-GPX is post-transcriptionally decreased by peroxisome proliferator through the oxidative stress in the renal tubules. This may be a new deleterious effect of an endocrine disruptor DEHP.

Selenium-dependent cellular glutathione peroxidase protects mice against a pro-oxidant-induced oxidation of NADPH, NADH, lipids, and protein

Since our prior work indicated that Se-dependent cellular glutathione peroxidase (GPX1) was necessary for protection against paraquat lethality, the present studies were to elucidate the biochemical mechanisms related to that protection. Four groups of mice [Se-deficient or -adequate GPX1 knockout and wild-type (WT)] were injected (i.p.) with 50 mg paraquat/kg body weight and tissues were collected 0, 0.5, 1, 2, 3, or 4 h after the injection. Whereas the ratios of NADPH/NADP and NADH/NAD in lung were reduced by 50-70% only 0.5 h after the injection in all groups, these two ratios in liver of the Se-adequate WT were significantly higher than those of the three GPX1 knockout or deficient groups 2-4 h after the injection. The paraquat-induced pulmonary lipid peroxidation and hepatic protein oxidation, measured as F(2)-isoprostanes and carbonyl contents, respectively, peaked at 1 h in these three groups. No such oxidative events were shown in any tissue of the Se-adequate WT throughout the time course. Whereas the F(2)-isoprostane formation was accelerated by both GPX1 knockout and Se deficiency in liver, it was not significantly elevated by the paraquat treatment in brain of any group. The paraquat injection also resulted in temporal changes in lung GPX activity and GPX1 protein in the Se-adequate WT, and significant reductions in lung total SOD activity in the GPX1 knockout or deficient groups. In conclusion, GPX1 plays a critical role in maintaining the redox status of mice under acute oxidative stress, and protects against paraquat-induced oxidative destruction of lipids and protein in vivo. These protections of GPX1 seem to be inducible and coordinated with those of other antioxidant enzymes.

Cellular glutathione peroxidase is the mediator of body selenium to protect against paraquat lethality in transgenic mice

The antioxidative role of Se-dependent cellular glutathione peroxidase (EC 1.11.1.9, GPX1) in vivo has not been established. Our objective was to determine the effects of GPX1 knockout or overexpression on the susceptibility of mice to paraquat toxicity and the contributions of GPX1, compared with other selenoproteins and vitamin E, to body defenses against such acute oxidative stress. Four experiments were conducted using 111 GPX1 knockout or overexpressing mice and the respective controls. Mice were fed diets supplemented with Se (as sodium selenite) at 0-0.4 mg/kg and/or all-rac-alpha-tocopheryl acetate at 0-75 mg/kg before intraperitoneal injections of 12.5, 50 or 125 mg paraquat/kg body weight. All mice that received 50 or 125 mg paraquat/kg died spontaneously, and the survival time of mice was (independent of dietary levels of Se per se or alpha-tocopheryl acetate) solely a function of tissue GPX1 activity (P < 0.001). Severe acute pulmonary interstitial necrosis was found only in the GPX1 overexpressing mice and the controls that had extended survival time. Thiobarbituric acid reacting substances in postmortem liver inversely correlated with the tissue GPX1 activity and dietary levels of Se and/or alpha-tocopheryl acetate. In contrast, all mice that received 12.5 mg paraquat/kg survived and were killed 2 wk after the injection for tissue collection. Compared with the saline injection, this low dose of paraquat resulted in greater (P < 0.001) liver and lung F2-isoprostanes in both the GPX1 knockout mice and the controls. However, there was no difference in plasma alanine transaminase (EC 2.6.1.2) activity or overt injuries in liver, lung and kidney in either group. Our data indicate that GPX1 is the major, if not the only, metabolic form of body Se that protects mice against the lethal oxidative stress caused by high levels of paraquat; it seems less important, however, in protecting mice against the moderate oxidative stress by the low level of paraquat.