Effect of dietary selenium on the induction of altered hepatic foci and hepatic tumors by the peroxisome proliferator ciprofibrate

The purpose of this study was to determine if the dietary antioxidant selenium could inhibit hepatocarcinogenesis induced by peroxisome proliferators, which are hypothesized to induce tumors by increased production of hydrogen peroxide or other reactive oxygen species. Rats were fed diets containing the peroxisome proliferator ciprofibrate and one of three concentrations (0.04, 0.2, or 1.0 ppm) of selenium for 6 or 21 months. The incidence of hepatic tumors and the number and volume of γ‐glutamyl transpeptidase‐positive, A TPase‐negative, glucose‐6‐phosphatase‐negative, and gtucose‐6‐phosphatase‐positivefoci at 21 months were lower in rats fed higher levels of selenium (no foci or tumors were seen at 6 mo). Indices of oxidative damage in the liver (thiobarbituric acid reactants, conjugated dienes, and lipid‐soluble fluorescence products), however, were not decreased in rats fed the high‐selenium diet. Therefore, selenium was protective against ciprofibrate‐induced hepatocarcinogenesis, but not by reducing the degree of oxidative damage. The liver selenium and glutathione concentrations, and liver selenium‐dependent glutathione peroxidase activity, increased as dietary selenium increased. Therefore, inhibition of carcinogenesis by selenium was correlated with increased levels of glutathione and glutathione peroxidase, but these did not inhibit the indices of oxidative damage. Peroxisomal ß‐oxidation also increased with the dietary selenium content; it therefore does not appear to be a factor in the inhibition of hepatocarcinogenesis in rats fed higher levels of selenium.     

Lack of effect of dietary alpha-tocopherol on chemically induced hepatocarcinogenesis in rats

We investigated the effects of alpha-tocopherol on diethylnitrosamine (DEN) initiation-phenobarbital (PB) promotion of hepatic foci in female Sprague-Dawley rats. Groups of eight rats were initiated with DEN (15 mg/kg) at 24 hours of age. After weaning, they received diets containing 500 ppm PB and various concentrations of alpha-tocopherol, deficient (0 ppm), adequate (100 ppm), and supplemented (5,000 ppm), for 24 weeks. Rats fed alpha-tocopherol-supplemented diets had significantly greater hepatic alpha-tocopherol levels than those fed alpha-tocopherol-deficient or -adequate diets (p < 0.05). Liver lipid peroxidation (measured as thiobarbituric acid-reactive substances) was significantly greater in rats fed alpha-tocopherol-deficient diets than in those fed alpha-tocopherol-adequate or -supplemented diets (p < 0.05). The dietary alpha-tocopherol level had no significant effect on the ratios of reduced glutathione (GSH) to oxidized GSH or reduced GSH to total GSH in the liver or on the plasma prostaglandin E2 concentration or on the activities of hepatic cytosolic and particulate protein kinase C. Rats fed alpha-tocopherol-adequate or -supplemented diets had significantly greater hepatic glutathione S-transferase, GSH reductase, and GSH peroxidase activities than those fed alpha-tocopherol-deficient diets (p < 0.05). The dietary alpha-tocopherol level did not significantly affect the formation of hepatic gamma-glutamyl transpeptidase- and placental glutathione S-transferase-positive foci. These results suggest that alpha-tocopherol does not influence hepatic foci formation and that reactive oxygen species may not be the underlying mechanism of hepatic foci formation in this DEN initiation-PB promotion model of hepatocarcinogenesis.     

Dietary selenium intake controls rat plasma selenoprotein P concentration

The purpose of this study was to determine the effect of dietary selenium on selenoprotein P concentration. Selenoprotein P was quantitated in plasma by radioimmunoassay. Selenium-dependent glutathione peroxidase activity in plasma and liver 105,000 x g supernatant was measured for comparison. Weanling male rats were fed a selenium-deficient diet or a control diet that contained 0.5 mg selenium/kg as Na2SeO4. The concentration of selenoprotein P fell at approximately the same rate in the rats fed the selenium-deficient diet as did plasma glutathione peroxidase activity. Groups of weanling rats were fed different levels of selenium for 8 wk. Selenoprotein P concentration was proportional to dietary selenium level up to 0.1 mg/kg and was a greater percentage of control values than was glutathione peroxidase activity. No increment in selenoprotein P concentration occurred between 0.1 and 0.5 mg selenium/kg diet. These results indicate that the concentration of selenoprotein P in the plasma is directly dependent on selenium supply in the diet up to 0.1 mg/kg. There is overlap between the dietary selenium ranges in which selenoprotein P concentration and glutathione peroxidase activity increase, but the selenoprotein P range is lower than the glutathione peroxidase range.     

Platelet glutathione peroxidase activity as an index of selenium status in rats

Glutathione peroxidase activity in platelets increased stepwise in selenium-depleted rats that were repleted with graded levels of dietary sodium selenite. In a 3-phase depletion/repletion/depletion feeding study, glutathione peroxidase activity was similar in platelets and liver, which apparently contains the largest labile pool of selenium in the body. The activity of glutathione S-transferase (selenium-independent glutathione peroxidase) in platelets was low and was not affected by selenium deficiency, even though hepatic transferase was markedly elevated in selenium-deficient rats. Vitamin E deficiency did not affect activities of glutathione peroxidase or glutathione S-transferase in platelets or liver. Determination of glutathione peroxidase activity in platelets apparently is a promising technique for assessing selenium status and, possibly, for measuring selenium bioavailability.     

Effect of selenium depletion and repletion on plasma glutathione and glutathione-dependent enzymes in the rat

Selenium deficiency has several known biochemical effects. In the rat, these effects include loss of glutathione peroxidase (GSH-Px) activity, increased plasma glutathione concentration and increased liver glutathione S-transferase (GSH S-Tr) activity. The time course of the development of these changes in rats fed selenium-deficient diets and the time course of reversal of these changes in selenium-deficient rats fed graded levels of selenium were determined. As selenium deficiency was produced, liver cytosolic and plasma GSH-Px activities decreased first and were less than 5% of control when plasma glutathione concentration and liver GSH S-Tr activity began to increase. Elevated liver GSH S-Tr activity in selenium-deficient rats was corrected by refeeding selenium at the lowest level of supplementation (0.015 ppm) for 4 wk. GSH-Px activity required a supplementation of 0.10 ppm selenium for correction to control levels in 4 wk. Based on these studies a classification of the severity of selenium deficiency into mild, moderate and severe categories is proposed. In addition, the effect of dietary sulfur amino acid supplementation on plasma glutathione concentration was studied.     

High dietary intake of sodium selenite induces oxidative DNA damage in rat liver

One mechanism for the cancer-chemopreventive effects of high selenium (Se) intake is hypothesized to be antioxidant protection of DNA. In this work we examine DNA oxidation in whole animals as a function of dietary Se intake and carcinogen administration. Weanling male Sprague-Dawley rats were fed a basal, Torula yeast-based, Se-deficient diet supplemented with 0, 0.15, or 2.0 ppm Se as sodium selenite for 10 wk. They were then injected with 0, 0.1, or 10 mg /kg body weight of the pro-oxidant carcinogen N-nitrosodiethylamine. High levels of carcinogen and high levels of selenite intake each increased concentration of 8-hydroxy-2'-deoxyguanosine in liver DNA. Se-dependent glutathione peroxidase I gene expression and enzyme activity were dramatically reduced by dietary Se deficiency but were unaffected by carcinogen administration. There were no significant main or interactive effects of Se or carcinogen on activity or gene expression of the DNA repair enzyme 8-oxoguanine glycosylase I. These results do not support the hypothesis that high Se intake may be cancer-preventive by inhibiting oxidative DNA damage. Rather than inhibiting oxidative DNA damage, these findings suggest that high dietary intake of inorganic Se may promote in vivo DNA oxidation.     

Effect of dietary selenium and vitamin E on the antioxiant defense systems of rat erythrocytes.

The effect of dietary selenium and vitamin E on the important cellular antioxidant defense systems was studied in rat erythrocytes. Weanling male Sprague-Dawley rats were fed a basal selenium and vitamin E deficient diet and supplemented with either none or 0.5 ppm selenium and either none or 45 ppm vitamin E for 35 or 40 days. Depletion of dietary selenium resulted in marked decrease of glutathione (GSH) peroxidase in the red cells, but the levels of GSH, catalase and superoxide dismutase were not significantly altered. The red cells of rats fed the basal diet deficient in both selenium and vitamin E had significantly lower levels of GSH and GSH peroxidase, but not of catalase and superoxide dismutase, than in those fed the basal diet and supplemented with either selenium, vitamin E or both. The results suggest that depletion of dietary selenium and vitamin may have a precipitate effect on lowering the levels of GSH and GSH peroxidase in rat erytyrocytes.     

Selenium, vitamin E and the response to swimming stress in the rat.

Experiments were conducted to determine the effects of exercise on rat glutathione peroxidase system enzymes and lipid peroxidation among animals supplemented and unsupplemented with selenium (Se) and vitamin E (E). Liver, muscle and blood were taken before, immediately after and 24 hours after exercising to exhaustion by swimming. No effect of exercise was found on muscle or liver enzymes, although exercise resulted in depressed glutathione reductase (GR) and glucose-6-phosphate dehydrogenase (G6PD) activities in erythrocytes immediately after exercise. Dietary Se supplementation did result in increased hepatic muscle and erythrocyte glutathione peroxidase activity, and decreased hepatic GR, G6PD and "malic enzyme" activities. Thiobarbituric acid reactive substances, and indicator of lipid peroxidation, increased in liver and muscle subsequent to exercise. This increase was reduced in liver, but not eliminated, by dietary E supplementation. The increase was not affected by dietary E in muscle, nor by dietary Se in either tissue.     

Effect of dietary restriction on the age-dependent changes in the expression of antioxidant enzymes in rat liver

The effects of aging and dietary restriction on the expression of several enzymes (superoxide dismutase, catalase and glutathione peroxidase) that are involved in free radical detoxification were studied in liver tissue from male Fischer F344 rats. The expression (i.e., activities and mRNA levels) of superoxide dismutase (Cu-Zn) and catalase decreased with age in liver. Dietary restriction (40% restriction of energy intake) increased the activities of superoxide dismutase (24 to 38%) and catalase (64 to 75%) in liver at 21 and 28 mo of age. Glutathione peroxidase activity in liver of diet-restricted rats was significantly higher (37%) at 28 mo of age than that of rats fed ad libitum. The age-related changes in the relative levels of mRNA for superoxide dismutase, catalase and glutathione peroxidase paralleled the changes in the activities of these enzymes in rats fed ad libitum or rats fed the restricted diet. Thus, the changes in the activities of superoxide dismutase, catalase and glutathione peroxidase with age and dietary restriction appear to arise from changes in the levels of mRNAs coding for these enzymes. Free radical damage, as measured by thiobarbituric acid-reactive material and lipofuscin accumulation, was lower in diet-restricted rats than in rats fed ad libitum.     

Glutathione peroxidase acitvity in intestinal and liver tissues of rats fed various levels of selenium, sulfur and alpha-tocopherol.

The effects of various levels of selenium, alpha-tocopherol and sulfur on glutathione peroxidase (GSH-Px) activity in intestinal and liver tissues were determined in male rats fed corn-soybean or Torula yeast diets. Rats fed corn-soybean diets had greater GSH-Px activity in the small intestine, colon and liver tissues, catalase activity and selenium in the liver, and body weight gains than those fed Torula yeast diets. GSH-Px activity in the small intestine, colon, and liver tissues as well as concentration of selenium in the liver increased with increasing levels of selenium in Torula yeast diets but not with corn-soybean diets. Tocopherol supplementation had no significant effect on GSH-Px activity in rats fed Torula yeast or corn-soybean diets supplemented with selenium. Supplemental sulfur decreased GSH-Px activity in the small intestine tissues and increased activity in colon tissues.     

Long-term consumption of a methionine-supplemented diet increases iron and lipid peroxide levels in rat liver

Methionine is a protective factor against various types of liver damage, but excessive dietary methionine is hepatotoxic. Because the mechanisms of L-methionine-related hepatotoxicity are poorly understood, the effect of long-term excessive L-methionine intake on the metabolism of iron and antioxidants was studied in rat liver to determine whether oxidative stress is involved. Wistar male rats were fed either an L-methionine-supplemented (16.0 g/kg) diet or a control diet for 1, 3, 6 and 9 mo. The growth rate of L-methionine-supplemented rats was significantly slower than that of controls. Iron, ferritin and thiobarbituric acid-reactive substances (TBARS) levels in the liver were greater in supplemented rats than in controls. Serum iron and transferrin levels were significantly lower in L-methionine-treated rats compared with controls. Serum ferritin did not differ between the two groups. Hepatic glutathione peroxidase activity, catalase activity and total glutathione concentrations were higher in rats fed the L-methionine-supplemented diet at 1 and 3 mo, but not at 6 and 9 mo. These results indicate that long-term consumption of excess L-methionine by rats may affect primarily iron metabolism rather than the antioxidant defense system and, consequently, induce an accumulation of iron.     

Effect of dietary cholesterol and fat levels on lipid peroxidation and the activities of antioxidant enzymes in rats

The aim of this study was to investigate the effect of dietary fat levels, with or without cholesterol, on lipid peroxidation and the activities of antioxidant enzymes in rats. Thirty-two Wistar rats aged 4 weeks were divided into 4 groups and fed high (20%; HF) or low (5%; LF) fat, with or without 1% cholesterol, for 6 weeks. Cholesterol feeding resulted in significantly higher concentrations of serum cholesterol, but lowered serum triacylglycerol levels. Cholesterol feeding also led to markedly decreased levels of hepatic thiobarbituric acid reactive substances (TBARS) and lower activities of hepatic superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), catalase, and glucose-6-phosphate dehydrogenase (G6PDH) when compared with cholesterol-free counterparts in both HF and LF diets. On the other hand, rats fed high-fat diets showed increased serum and liver TBARS, but decreased hepatic GSH-Px, SOD, and G6PDH activities. Hepatic catalase activity was lower in rats fed cholesterol-containing diets, but higher in rats fed high-fat diets, and interaction existed between cholesterol and fat feeding. These results suggested that dietary cholesterol might delay lipid peroxidation and decrease the activities of the hepatic antioxidant enzymes. The degree of lipid peroxidation was also influenced by dietary fat levels.     

膳食蛋白质水平对硒利用的影响 Ⅰ、膳食蛋白质水平对大鼠血及组织中硒含量及谷胱甘肽过氧化物酶活力的影响

用低硒(0.03ppm)及高硒(0.30ppm)两类饲料喂养大鼠共三月,每类饲料蛋白质水平从5.3—13.4％,饲料基本成分为玉米和黄豆,实验结果发现,在不同硒水平下,饲料蛋白质水平对机体利用硒的影响大不相同。在低硒(0.03ppm)饲料下,低蛋白组动物血及血、心、肾及胰织组中的含硒量最高,饲养蛋白质水平愈高,各组织中硒含量则显著降低。及肝组织中的谷胱甘肽过氧化物酶活力有相同趋势。在高硒(0.30ppm)饲料组则无此种趋势,相反,蛋白质水平愈高,在肝脏中的贮存愈好。文中并讨论了在我国克山病区单纯补充硒的意义。     

Effects of dietary selenium and fish oil (MaxEPA) on arachidonic acid metabolism and hemostatic function in rats

This study investigated whether hemostatic function can be modified by both the consumption of fish oil and the level of dietary selenium. Male Sprague-Dawley rats were fed for 8 wk semipurified diets containing 7% corn oil (by wt) or 5.5% fish oil (MaxEPA) plus 1.5% corn oil with or without selenium supplementation. Consumption of the four diets caused no difference in weight gain, food intake or plasma malondialdehyde content. The selenium-supplemented rats had significantly higher levels of selenium and glutathione peroxidase activity in plasma. Fish oil feeding decreased ADP-induced platelet aggregation and increased bleeding time. The level of dietary selenium and type of oil interacted to influence the production of 6-keto-prostaglandin F1 alpha: more was produced when corn oil was fed in the selenium-deficient diets. These data suggest that the effect of dietary selenium on hemostatic function and the production of eicosanoids is minor.     

Oxidative stress and antioxidant status in rat blood, liver and muscle: effect of dietary lipid, carnitine and exercise

The purpose of this study was to determine the effect of dietary fat, carnitine supplementation, and exercise on oxidative damage and antioxidant status. Male Wistar rats (60 days old) were fed diets containing either hydrogenated fat (HF) or peanut oil (PO) with or without 0.5 mg % (of dry diet) carnitine. The rats were given exercise, i.e. swimming for 60 minutes, for 6 days/week for 6 months under each dietary condition. The blood malondialdehyde (MDA) level was higher in PO-fed rats, more so in exercising ones, while the same was not altered in carnitine-supplemented rats irrespective of the dietary fat or physical activity. The MDA level was significantly decreased in muscle, while increased in liver, of carnitine-fed rats. The blood glutathione (GSH) level also significantly increased in exercising rats as compared to sedentary ones, while carnitine supplementation elevated it in all the groups. Exercise and carnitine supplementation significantly lowered GSH levels in liver while increasing it in muscle. The glutathione peroxidase (GPX) activity was significantly increased in blood and muscle from PO-fed exercising rats as compared to sedentary ones, while carnitine supplementation elevated GPX activity in all the groups. The liver and muscle catalase (CAT) activities were significantly increased in PO-fed exercising rats, while carnitine did not have any effect. The pro-oxidative effect of the monounsaturated fatty acid (MUFA)-rich PO diet and prolonged regular exercise was less pronounced due to augmented antioxidant enzymes, GPX and CAT, induced by training to protect against the oxidative stress, while carnitine supplementation could help to counter lipid peroxidation due to exercise through redistribution of GSH from liver to blood and muscle.     

Grape skin improves antioxidant capacity in rats fed a high fat diet

This study was conducted to investigate the effect of dietary grape skin on lipid peroxidation and antioxidant defense system in rats fed high fat diet. The Sprague-Dawley rats were fed either control (5% fat) diet or high fat (25% fat) diet which was based on AIN-93 diet for 2 weeks, and then they were grouped as control group (C), control + 5% grape skin group (CS), high-fat group (HF), high fat + 5% grape skin group (HFS) with 10 rats each and fed corresponding diets for 4 weeks. The hepatic thiobarbituric acid reacting substances (TBARS) were increased in high fat group as compared with control group, but reduced by grape skin. The serum total antioxidant status, and activities of hepatic catalase and superoxide dismutase, xanthine oxidase and glucose-6-phosphatase were increased by supplementation of grape skin. Glutathione peroxidase activity was significantly higher in CS group than in C group. Grape skin feeding tended to increase the concentration of total glutathione, especially in control group. The ratio of reduced glutathione to oxidized glutathione was lower in high fat groups than in control groups. The ratio was increased by dietary supplementation of grape skin in control group. These results suggest that dietary supplementation of grape skin would be effective on protection of oxidative damage by lipid peroxidation through improvement of antioxidant defense system in rats fed high fat diet as well as rats with low fat diet.     

Effect of types and amount of dietary fat during the initiation phase of hepatocarcinogenesis.

The effects of various levels of corn oil and lard fed during the initiation stage of azoxymethane (AOM)-induced hepatocarcinogenesis were studied in male Fischer 344 rats. The animals were fed diets containing 5%, 13.6%, and 23.5% corn oil or lard two weeks before, during, and until one week after injections of AOM (15 mg/kg body wt s.c.) once weekly for two weeks. One week after AOM treatment, groups of animals fed the 13.6% and 23.5% corn oil or lard diet were transferred to their respective 5% corn oil or lard diet and fed these diets until the termination of the study (34 wk). Immunohistochemical staining of glutathione S-transferase placental form was performed in the liver, and the number of glutathione S-transferase placental form-positive foci was determined. Density, average area, and unit area of foci were significantly inhibited in the animals fed the 13.6% and 23.5% lard diets compared with those fed the 13.6% and 23.5% corn oil diets. These results indicate that the effect of dietary fat during the initiation phase of AOM-induced hepatocarcinogenesis depends on the type of fat and its fatty acid composition. Additionally, the enhancing effect of a corn oil diet in hepatocarcinogenesis is mainly present during the initiation phase of carcinogenesis compared with a lard diet.     

膳食维生素B_6对硒的生物利用率影响的研究 Ⅱ．对大鼠组织谷胱甘肽过氧化物酶活性的影响

实验用４周龄断乳雄性大鼠观察膳食维生素Ｂ６（ＶＢ６）对饲亚硒酸钠（ＳｅＬ）或ＤＬ－硒蛋氨酸（ＳｅＭｅｔ）大鼠组织中ＧＳＨ－Ｐｘ活性的影响，实验期为４周。实验证明：与补ＶＢ６各组相比，缺ＶＢ６各组血浆ＧＳＨ－Ｐｘ活性较高，而在红细胞中的结果相反（Ｐ＜０．０５）；缺ＶＢ６各组动物的骨骼肌、心肌和脾脏中ＧＳＨ－Ｐｘ活性都显著低于补Ｂ６各组（Ｐ＜０．０５），上述结果与给硒的化学形式无关。在用ＳｅＭｅｔ的处理组，缺Ｂ６大鼠的肝脏中ＧＳＨ－Ｐｘ活性显著低于补ＶＢ６的大鼠；而在用ＳｅＬ的处理组则没有观察到ＶＢ６的这种影响。本研究结果提示，硒掺入ＧＳＨ－Ｐｘ是通过一个ＶＢ６依存的过程。     

Suppression of altered hepatic foci development by a high fish oil diet compared with a high corn oil diet in rats

Effects of low corn oil, high corn oil, and high fish oil diets on altered hepatic foci development in female Sprague-Dawley rats were investigated. Rats assigned to Groups 1-4 were initiated with saline as the control and those assigned to Groups 5-7 were initiated with diethylnitrosamine (DEN 15 mg/kg) at 24 hours of age. After weaning, all rats, except those in Group 1, received 500 ppm phenobarbital (PB) in their diet as tumor promoter for three months. Altered hepatic foci development was significantly lower in DEN-initiated rats fed the high fish oil + PB diet than in DEN-initiated rats fed the high corn oil + PB diets. Liver weight and relative liver weight were significantly greater in rats fed the high fish oil + PB diet than in rats fed the other diets, and hepatic biotransformation/detoxification enzyme activities were greater in rats fed the fish oil + PB diets than in rats fed the other diets. These results suggest that the effect of a high fish oil diet on altered hepatic foci may occur through regulation of hepatic biotransformation/detoxification enzyme activities, leading to alteration in the tumor-promoting action of PB. Dietary lipid significantly affected the hepatic phospholipid fatty acid composition of rats. n-3 polyunsaturated fatty acids were incorporated into membrane phospholipid at the expense of n-6 polyunsaturated fatty acids. A high fish oil diet caused greater oxidative stress in rats, as measured by plasma vitamin E level, red blood cell glutathione status, liver lipid peroxidation, and hepatic glutathione reductase activity. Pearson's correlation analysis indicated that the foci number was negatively correlated to the liver thiobarbituric acid-reactive substance and 7-pentoxyresorufin O-dealkylase activity, and the foci area was negatively correlated to the liver thiobarbituric acid-reactive substance activity (p < 0.05) in rats of groups that developed foci. These results suggest that the type of dietary lipid is the more important determinant for gamma-glutamyl transpeptidase-positive foci development than the amount of dietary lipid when rats consumed approximately the same amount of calories in all the dietary groups, and the underlying mechanisms may be partially ascribed to the antioxidant/oxidation status and biotransformation/detoxification system of rats.