Effects of BHA, d-alpha-tocopherol and retinol acetate on TCDD-mediated changes in lipid peroxidation, glutathione peroxidase activity and survival

Selenium-binding protein 1: its physiological function, dependence on aryl hydrocarbon receptors, and role in wasting syndrome by 2,3,7,8-tetrachlorodibenzo-p-dioxin

BACKGROUND

Selenium-binding protein 1 (Selenbp1) is suggested to play a role in tumor suppression, and may be involved in the toxicity produced by dioxin, an activator of aryl hydrocarbon receptors (AhR). However, the mechanism or likelihood is largely unknown because of the limited information available about the physiological role of Selenbp1.

METHODS

To address this issue, we generated Selenbp1-null [Selenbp1 (-/-)] mice, and examined the toxic effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in this mouse model.

RESULTS

Selenbp1 (-/-) mice exhibited only a few differences from wild-type mice in their apparent phenotypes. However, a DNA microarray experiment showed that many genes including Notch1 and Cdk1, which are known to be enhanced in ovarian carcinoma, are also increased in the ovaries of Selenbp1 (-/-) mice. Based on the different responses to TCDD between C57BL/6J and DBA/2J strains of mice, the expression of Selenbp1 is suggested to be under the control of AhR. However, wasting syndrome by TCDD occurred equally in Selenbp1 (-/-) and (+/+) mice.

CONCLUSIONS

The above pieces of evidence suggest that 1) Selenbp1 suppresses the expression of tumor-promoting genes although a reduction in Selenbp1 alone is not very serious as far as the animals are concerned; and 2) Selenbp1 induction by TCDD is neither a pre-requisite for toxicity nor a protective response for combating TCDD toxicity.

GENERAL SIGNIFICANCE

Selenbp1 (-/-) mice exhibit little difference in their apparent phenotype and responsiveness to dioxin compared with the wild-type. This may be due to the compensation of Selenbp1 function by a closely-related protein, Selenbp2.

Analysis of plasma indices of redox homeostasis in dairy cows reared in polluted areas of Piedmont (northern Italy)

Steel manufacturing is responsible for the emission of pollutants, including dioxins and transition metals, inducing reactive oxygen species generation and DNA damage. Dioxin pollution represents the major cause of milk and dairy product contamination, in Italy, and is associated with oxidative stress-related processes, that may impair health and performance of cows. We evaluated the effect of exposure to different concentrations of pollutants derived from steel manufacturing on blood redox homeostasis of bovine cows. We analyzed two groups of dairy cows (A, B), reared in two different polluted areas, and a control group of cows bred in an industry free area. The extent of exposure to contaminants was defined by measuring dioxin level in bulk milk samples collected from animals of each farm. This level was lower in milk of group A than in group B. Plasma concentrations of retinol, alpha-tocopherol and ascorbate, the total antioxidant capacity, and the activities of superoxide dismutase and glutathione peroxidase were higher in control group than in exposed groups. In particular, retinol and tocopherol levels were higher in the group with lower milk dioxin level. Plasma titers of protein-bound carbonyls (PC), nitro-tyrosine, and hydroperoxides were lower in control group than in A or B. Hydroperoxides and PC plasma concentrations were increased in the group with higher milk concentration of dioxin. Our results demonstrate that, irrespective of the nature of chemicals inducing oxidative modifications, the extent of damage to plasma protein and lipid, is correlated with the concentration of dioxin in milk. So, the characterization of blood redox status might be a useful tool for identifying animals exposed to environmental pollutants. Plasma concentrations of retinol, alpha-tocopherol, PC and hydroperoxides could therefore represent good indices of the extent of animal exposure, as they significantly change in groups with different milk concentrations of dioxin.

Restoration of dioxin-induced damage to fetal steroidogenesis and gonadotropin formation by maternal co-treatment with α-lipoic acid

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), an endocrine disruptor, causes reproductive and developmental toxic effects in pups following maternal exposure in a number of animal models. Our previous studies have demonstrated that TCDD imprints sexual immaturity by suppressing the expression of fetal pituitary gonadotropins, the regulators of gonadal steroidogenesis. In the present study, we discovered that all TCDD-produced damage to fetal production of pituitary gonadotropins as well as testicular steroidogenesis can be repaired by co-treating pregnant rats with α-lipoic acid (LA), an obligate co-factor for intermediary metabolism including energy production. While LA also acts as an anti-oxidant, other anti-oxidants; i.e., ascorbic acid, butylated hydroxyanisole and edaravone, failed to exhibit any beneficial effects. Neither wasting syndrome nor CYP1A1 induction in the fetal brain caused through the activation of aryl hydrocarbon receptor (AhR) could be attenuated by LA. These lines of evidence suggest that oxidative stress makes only a minor contribution to the TCDD-induced disorder of fetal steroidogenesis, and LA has a restorative effect by targeting on mechanism(s) other than AhR activation. Following a metabolomic analysis, it was found that TCDD caused a more marked change in the hypothalamus, a pituitary regulator, than in the pituitary itself. Although the components of the tricarboxylic acid cycle and the ATP content of the fetal hypothalamus were significantly changed by TCDD, all these changes were again rectified by exogenous LA. We also provided evidence that the fetal hypothalamic content of endogenous LA is significantly reduced following maternal exposure to TCDD. Thus, the data obtained strongly suggest that TCDD reduces the expression of fetal pituitary gonadotropins to imprint sexual immaturity or disturb development by suppressing the level of LA, one of the key players serving energy production.

Organohalogens in a whale-blubber-supplemented diet affects hepatic retinol and renal tocopherol concentrations in greenland sled dogs (Canis familiaris)

The aim of this study was to examine the plasma, liver, and kidney status of vitamin A (retinol) and vitamin E (alpha-tocopherol) in two groups of Greenland sled dogs (Canis familiaris), with a total number of 16 bitches and 8 pups. The dogs were fed either minke whale (Balaenoptera acuterostrata) blubber (exposed dogs) or uncontaminated (control group) porcine fat for up to 12 to 21 mo of age. The daily intake of 50-200 g whale blubber (mean: 112 g) constituted between 10.4 and 11.7 microg/kg body weight summation operatororganohalogen contaminants (OHC) (or between 4.6 and 6.1 microg/kg body weight summation operatorpolychlorinated biphenyls [PCB]). Retinol was approximately 18% and alpha-tocopherol 22% higher in the diet of the exposed dogs compared to controls. In adipose tissue, mean of SigmaOHC was 92 ng/g lipid weight (lw) and 5005 ng/g lw for all control (n = 12) and exposed dogs (n = 10), respectively. Hepatic retinol correlated negatively with Sigma-dichlorodiphenyldichloroethane (SigmaDDT) and and Sigma-polybrominated diphenyl ethers (SigmaPBDE) for all exposed animals. A negative correlation between kidney alpha-tocopherol and SigmaPCB concentrations was observed, whereas two positive significant correlations were observed between kidney retinol and Sigma-chlordane-related compounds (SigmaCHL) and dieldrin concentrations. Hepatic alpha-tocopherol concentrations were significantly lower in exposed compared to controls, most likely due to a combination by OHC exposure and high dietary intake of unsaturated fatty acids. These results suggest that dietary exposure from OHC may, even at low concentrations, possibly affect retinol and alpha-tocopherol status in Arctic top predators.

Effect of vitamin E on reproductive function in the mice treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin*

The study was conducted to determine the effects of vitamin E on reproductive performance in mice treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The 75 female and 40 male mice were randomly assigned into five groups. The levels of both TCDD and vitamin E given by gavage were 0 and 0 (Control group), 100 and 0 (experimental group I), 100 and 20 (experimental group II), 100 and 100 (experimental group III), and 100 ng/kg/day and 500 mg/kg/day (experimental group IV), respectively. Males and females were mixed to mate at the ratio of 1:2 after 4-week experiment. The gavage treatments were continued until the end of gestation in female mice after mating. The results showed that the litter number, survival rate, and body weight at birth of offspring in experimental group I had significantly decreased, and the females’ pregnancy rate and pup sex ratio in experimental group I had the decreasing tendencies when compared with the control group. The litter number in experimental group III, survival rate in experimental group II and III, body weight at birth in experimental group III and IV exhibited significant increase compared with experimental group I. The female pregnancy rate in both experimental group III and IV recovered to 100%, but there were no significant differences when compared with experimental group I. The pups’ sex ratio had a gradually increasing tendency with increase of vitamin E level, but there was no significant difference among experimental group I–IV. The results suggest that TCDD could induce reproductive toxicity in mice, whereas vitamin E alleviated adverse effects on reproductive performance in mice caused by TCDD.

Protective Effects of Tea Melanin against 2,3,7,8-Tetrachlorodibenzo-p-dioxin-Induced Toxicity: Antioxidant Activity and Aryl Hydrocarbon Receptor Suppressive Effect

We examined the protective ability of tea melanin against 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced toxicity in C57BL6J mice. Reduced tea melanin (RTM) and non-reduced tea melanin (NRTM) were incorporated to distinguish anti-oxidant activity from alternative pathways. The mice were given a single oral dose of TCDD (100 microg/kg body weight) and then they were administered daily with NRTM or RTM (40 mg/kg, p.o.) for next 14 d. RTM protected the animals against TCDD-induced lipid peroxidation, inhibition of glutathione peroxidase, alteration in reduced and oxidized glutathione concentrations, loss of body weight, and increased relative liver weight. NRTM was less effective as compared to RTM because of its inferior antioxidant activity, but it still displayed a strong protective effect against TCDD toxicity owing to its similar suppression of the activity of the aryl hydrocarbon receptor. Both NRTM and RTM suppressed the expression of CYP1A1 gene and prevented the activation of cytochrome P450 isozyme in the livers of animals exposed to TCDD. These results suggest that tea melanin might be a potential agent offering dual protection against the development of TCDD-induced oxidative stress.

Accumulation of dietary organochlorines and vitamins in Baltic seals

We studied the accumulation of polychlorinated biphenyls (PCBs), 1,1,1.trichloro-2,2-bis[p-chlorophenyl]ethane (DDT) and its metabolites, and vitamins A and E in the top levels of the Baltic Sea food web. The investigation focused on the transfer of contaminants and vitamins to the ringed seal (Phoca hispida baltica) and the grey seal (Halichoerus grypus) from their main prey species. The trophic level of the seals was investigated using stable isotopes of nitrogen and the results indicated that both species of Baltic seal feed at approximately the same level. PCBs accumulated to a greater extent in the grey seal than in the ringed seal. Biomagnification factors for DDT compounds were similar for both species of Baltic seal (85-140). Differences in observed DDT levels were due to different prey selection by the two species, while differences in PCB levels indicated a species-specific metabolic system. There was a clearly greater accumulation of DDT compounds than of PCBs in both species of seal. The supply of dietary vitamin A was normally above the recommended level in all the seal populations studied. Low levels of hepatic vitamin A in the Baltic seals could therefore indicate the toxic effects of a high level of persistent organic pollutants on vitamin A dynamics, at least in the ringed seal. In the grey seals, low hepatic vitamin A levels could also be explained by lower levels of dietary vitamin A, compared to the reference grey seals, as it is not known if seals can store unlimited amounts of vitamin A. The greater uptake of vitamin E by Baltic seals, compared to seals in the reference areas, demonstrated by elevated levels of vitamin E in the blubber, could be a response to oxidative stress caused by the high contaminant load. These results further support our previous hypothesis that the toxic effects of environmental contaminants could be causing the observed divergence in vitamin levels between the Baltic seals and the reference seal populations.

Inhibitory effects of vitamin A on TCDD-induced cytochrome P-450 1A1 enzyme activity and expression

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an extremely potent environmental contaminant that produces a wide range of adverse biological effects, including the induction of cytochrome P450 1A1(CYP1A1) that may enhance the toxic effects of TCDD. Several studies indicated that concurrent supplementation of vitamin A could reduce the toxicity, and potentially inhibit CYP1A1 activity (measured as ethoxyresorufin-O-deethylase [EROD] activity). In the present study, we investigated the in vivo effects of vitamin A on EROD activities and the expression of CYP1A1 in the liver of TCDD-treated mice. In Experiment I, the mice were given a single oral dose of 40 mug TCDD/kg body weight with or without the continuous administration of 2500 IU vitamin A/kg body weight/day, and were killed on day 1, 3, 7, 14, or 28. In Experiment II, the mice were given daily an oral dose of 0.1 mug TCDD/kg body weight with or without supplement of 2000 IU vitamin A/kg body weight, and were killed on day 14, 28, or 42. In both experiments, TCDD caused liver damage and increase in relative liver weights, augmented the EROD activities and CYP1A1 expression, and increased the aryl hydrocarbon receptor (AhR) mRNA expression, but did not alter the AhR nuclear translocator (ARNT) mRNA expression. CYP1A1 mRNA expression and AhR mRNA expression showed a similar time course. The liver damage in TCDD + vitamin A-treated mice was less severe than that in TCDD-treated mice. EROD activities, CYP1A1 expression, and AhR mRNA expression in vitamin A + TCDD-treated mice were lower than those in TCDD-treated mice, indicating that supplementation of vitamin A might attenuate the liver damage caused by TCDD.

Protective Effects of Vitamin A and Vitamin E Succinate against 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-Induced Body Wasting, Hepatomegaly, Thymic Atrophy, Production of Reactive Oxygen Species and DNA Damage in C57BL/6J Mice

The protective effect of vitamin A and vitamin E succinate against 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced acute toxicity and measures of oxidative stress was studied. Ten mice were treated with either vitamin A (50 mg/kg every other day for eight days) or vitamin E succiante (150 mg/kg/day followed by a dose of 40 mg/kg/day for five additional days). Half of each of the above groups of animals received TCDD on day 4. Five mice received corn oil or TCDD alone. After five days of TCDD treatment, antioxidant combination treatment with vitamin A and TCDD or vitamin E succinate and TCDD resulted in a significant reduction in indicators of acute toxicity including the decrease in total body and thymus weight as compared to TCDD alone (P<0.05). The combination treatment produced also a significant reduction in the increase in liver weight as compared to TCDD only (P<0.05). Following one day of treatment with 50 microg TCDD/kg, vitamin A and vitamin E succinate produced a significant decrease in the production of superoxide anion by peritoneal lavage cells (P<0.05) and in DNA-single strand breaks in the same cells (P<0.05) as assessed by the reduction of cytochrome c and the alkaline elution technique, respectively. A significant decrease in DNA-single strand breaks in peritoneal lavage cells was observed following 5 days treatment with 50 microg TCDD/kg (P<0.05). The results indicate a potential role for oxidative stress in the acute toxicity of TCDD and a protective effect for vitamin A and vitamin E succinate in the overall toxicity of TCDD including measures of oxidative stress.

Oxidative stress in liver and brain of the hatchling chicken (Gallus domesticus) following in ovo injection with TCDD

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was injected into chicken eggs prior to incubation to study possible mechanisms of toxicity and teratogenicity. One of the suggested mechanisms of teratogenicity is oxidative stress. Eggs were injected simultaneously with TCDD and cotreatment compounds in an attempt to prevent oxidative stress or to block cytochrome P450 activity. Indicators of oxidative stress were assessed in livers and brains of hatchling chicks. In ovo, exposure to TCDD caused significant effects on indicators of oxidative stress in liver, but not in the brain of the hatchling chicks. TCDD did not significantly affect superoxide production. In liver, TCDD treatment caused a decrease in glutathione content and glutathione peroxidase activity and an increase in the ratio of oxidized to reduced glutathione. TCDD increased the susceptibility to lipid peroxidation and oxidative DNA damage in liver. Administration of the antioxidants vitamin E and vitamin A provided partial protection against TCDD-induced oxidative stress in liver. The lack of effect of TCDD in chicken brain could be due to the low cytochrome P4501A activity in this tissue and little accumulation of TCDD in brain compared to liver. Phenytoin, a known inducer of oxidative stress, caused a decrease in glutathione content and an increase in susceptibility to lipid peroxidation in both liver and brain and increased oxidative DNA damage in brain. Responsiveness varied among individual animals, but measures of the oxidative stress were correlated.

Mechanisms of TCDD-induced abnormalities and embryo lethality in white leghorn chickens

The toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds in birds has been well-established in laboratory and field studies. Observed effects of TCDD and related chemicals in birds include developmental deformities, reproductive failure, liver damage, wasting syndrome and death. The mechanism of action of TCDD at the cellular level is primarily mediated through the aryl hydrocarbon receptor (AhR). However, the mechanism of toxic action at the organism level is poorly understood. In this study, the role of radical oxygen species and mixed function oxidize (MFO; cytochrome P4501A) in the mechanism of TCDD-induced abnormalities and lethality were examined by co-injecting radical scavengers and an MFO inhibitor (piperonyl butoxide). Egg injection studies were conducted to determine if in ovo TCDD exposure can cause oxidative stress in white leghorn chicken eggs. Test agents were injected into the yolk prior to incubation. Treatments included TCDD (150 ng/kg), triolein (vehicle control), and various co-treatments including MnTBAP (a mimetic of superoxide dismutase), piperonyl butoxide, piroxicam, vitamin A acetate, and vitamin E succinate. Phenytoin, which is known to cause teratogenesis through oxidative stress was used as a positive control. Eggs were incubated until hatch and then the following parameters were assessed: mortality, hatching success, abnormalities, weights for whole body, liver, heart and brain, and biochemical endpoints for oxidative stress. As a measure of exposure, concentrations of TCDD and ethoxyresorufin-O-deethylase (EROD) activities were measured in tissues of hatchlings. While greater mortality and abnormalities were observed in the TCDD treatment groups, the number of the replicates were not great enough to detect statistically significant differences in abnormality rates for the co-treatments. Some of the observed developmental abnormalities included edema, liver necrosis and bill, eye and limb deformities with TCDD treatments, bill and brain deformities with phenytoin treatments, eye abnormalities with Vitamin E treatments, and abnormal feather pigmentation with piperonyl butoxide treatments.

Effect of chitosan oligosaccharide on 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced oxidative stress in mice

The abilities of two types of chitosan oligosaccharides, chitosan oligosaccharide I (1-kDa<MW<3-kDa) and chitosan oligosaccharide II (3-kDa<MW<5-kDa), to prevent 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced oxidative stress were examined in ICR mice. Chitosan oligosaccharide I had no effect on TCDD-induced alterations in lipid peroxidation and glutathione S-transferase activity or liver weight change. However, mice treated with chitosan oligosaccharide II were protected from TCDD-induced lipid peroxidation, inhibition of glutathione peroxidase and glutathione S-transferase activities, and losses in body and liver weights. These results suggest that chitosan oligosaccharide might be a potential agent for combating TCDD-induced pathogenesis.

Effects of vitamin A and insulin on the antioxidative state of diabetic rat heart: a comparison study with combination treatment

Because elevated oxidative stress may exacerbate cardiovascular complications of diabetes mellitus, the current study aimed to investigate the effects of treatment with either vitamin A, an antioxidant, or with insulin on lipid peroxidation products and antioxidant enzyme activities of diabetic rat heart. Also to evaluate whether a combination of vitamin A and insulin exerts more beneficial effects than treatment with each agent alone. Rats were made diabetic with a single injection of streptozotocin (STZ, 55 mg kg(-1) i.p.). Two days after STZ-injection, one group of diabetic rats was treated with vitamin A (retinol acetate, 30 mg kg(-1) day(-1) i.o.) for 12 weeks. A second group of diabetic rats was untreated for 6 weeks and then treated for another 6 weeks with insulin (8-10 IU rat(-1) day(-1) s.c.). Both therapies were applied to another group of diabetic rats for assessment of combined therapy with vitamin A plus insulin. Hearts from 12-week untreated diabetic animals showed about a four-fold increase in the level of thiobarbituric acid reactive substances (TBARS), indicative of increased lipid peroxidation. This was accompanied by approximately 100% increase in both catalase and glutathione peroxidase (GSHPx) enzyme activities. Therapy with insulin alone caused a small but significant improvement in plasma TBARS as well as GSHPx activities, but no significant change in plasma catalase in diabetic animals. Diabetes-induced disturbance in TBARS was almost completely prevented by vitamin A therapy. Although, a similar degree of activities for GSHPx was determined in diabetic animals treated with each agent alone, combination therapy was found to be more effective than single therapies in the recovery of GSHPx of diabetic heart. In contrast to insulin single therapy, vitamin A alone significantly prevented an increase in catalase activity of diabetic heart, and a combination of these agents did not supply any further benefit. Superoxide dismutase (SOD) activity was not found significantly different among the experimental groups. STZ-diabetes also resulted in less plasma retinol and retinol-binding protein (RBP), which was significantly improved by insulin single therapy while vitamin A used alone, failed to increase plasma retinol and RBP levels of diabetic animals. Our findings suggest that single therapy with insulin is unable to preclude oxidative reactions in diabetic heart to the same extent as obtained by vitamin A therapy alone, in spite of allowing recovery of normal growth rate and improved vitamin A metabolism in diabetic rats. A combination of insulin with vitamin A may provide more benefits than use of either agent alone in the treatment of general characteristics of diabetes and the maintenance of antioxidant defence of diabetic heart and thus in the reduction of peroxidative stress-induced cardiac injury.

Highly toxic coplanar PCB126 reduces liver peroxisomal enzyme activities in rats

The effect of the highly toxic coplanar PCB congener, 3,4,5,3',4'-pentachlorobiphenyl (PCB126) on hepatic peroxisomes was studied in rats. The aim of this study was to investigate whether a toxic dose of the dioxin-like coplanar PCB modifies enzyme activities in peroxisomes where plays an important role in lipid metabolism. Treatment with PCB126, at a single i.p. administration of 25 mg/kg which evokes clear suppression of body weight gain, resulted in marked reduction (to about 40-50%) of catalase activity and peroxisomal fatty acyl-CoA β-oxidizing system. Immunoblotting showed that expression of catalase was greatly reduced by the treatment in parallel with the activity. Light microscopy revealed a drastic reduction in granules possessing peroxidase activity, while electron microscopy demonstrated that no apparent morphological changes had taken place. Thus the reduction in catalase activity caused by PCB126 could be attributable to suppression of protein expression. The marked reduction of these peroxisomal enzyme activities might be related to hyperlipidemia caused by dioxin-related compounds in rats and humans.

Changes in the hepatic glutathione peroxidase redox system produced by coplanar polychlorinated biphenyls in Ah-responsive and -less-responsive strains of mice: mechanism and implications for toxicity

The alteration in hepatic glutathione peroxidase (GPx) produced by polychlorinated biphenyls (PCBs) was studied in vivo in aryl hydrocarbon (Ah)-responsive C57BL and -less-responsive DBA strains of mice. 3,3',4,4',5-Pentachlorobiphenyl (PCB 126), one of the high-affinity ligands for the Ah receptor, significantly reduced Se-dependent GPx activity in C57BL mice, but not in DBA mice. A reduction in activity in C57BL mice was also observed following treatment with a high dose of 3,3',4,4'-tetrachlorobiphenyl with lesser affinity for the Ah receptor than PCB 126, but not by 2,2',5,5'-tetrachlorobiphenyl, a low-affinity ligand. To assess the effects on GPx in the liver, the content of reduced glutathione (GSH), an obligate co-factor for GPx, and the activity of two enzymes, γ-glutamyl transpeptidase (γ-GTP) and glutathione reductase (GR), which play a role in supplying GSH were determined after PCB treatment. The results showed that although the hepatic activity of γ-GTP and GR was affected differently by PCB 126, the content of GSH was slightly increased rather than reduced in both strains of mice. The activity of non-Se-dependent GPx, which is due to the catalysis by some isozymes of glutathione S-transferase (GST), was significantly increased only in C57BL mice by PCB 126 treatment. Immunoblot analysis demonstrated that the induction of the class θ GST, which is a potent reducer of peroxides (Hiratsuka et al., 1995. Biochem. Biophys. Res. Commun. 212, 743) reflects the enhancement of the above activity. These results suggest that (i) the PCB-induced reduction in Se-dependent GPx activity is mediated by a mechanism involving the Ah receptor; and (ii) a concomitant increase in the class θ GST partially rescues the Ah-responsive mice from coplanar PCB-induced oxidative stress.

Influence of ellagic acid on antioxidant defense system and lipid peroxidation in mice

Addition of ellagic acid (EA) to liver microsomes of mice resulted in a steady increase in inhibition of NADPH-dependent lipid peroxidation up to 2 mM concentration. The maximum of 70% inhibition of ascorbate-dependent lipid peroxidation was achieved at 1 mM concentration of EA. Feeding of EA significantly increased the levels of reduced glutathione and glutathione reductase in liver and lungs of male and female mice. However, there were no changes in the activities of catalase and superoxide dismutase. On the other hand, microsomes from liver and lungs of EA fed animals showed significantly suppressed NADPH- and ascorbate-dependent lipid peroxidation.

Studies on the role of lipid peroxidation in the acute toxicity of TCDD in rats

Lipid peroxidation has been shown to be enhanced following exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), but its role in TCDD toxicity is unclear. The present study was undertaken to further elucidate the relations between lipid peroxidation and TCDD lethality. A time course and dose-response experiment in Long-Evans (L-E; LD50 ca. 10 micrograms/kg) and Han/Wistar (H/W; LD50 greater than 3000 micrograms/kg) rats showed that hepatic lipid peroxidation, measured as the amount of thiobarbituric acid-reactive substances (TBA-RS), was induced by TCDD dose-dependently in L-E, but not in H/W rats. Hepatic glutathione peroxidase activity was suppressed in much the same manner in both strains. Lipid peroxidation correlated with body weight loss in L-E rats alone. When 500 micrograms/kg of TCDD was given to L-E rats, lipid peroxidation increased about 3-fold on Day 11 in the liver, while no change was seen in cardiac or renal TBA-RS. The pair-fed controls did not survive the 11-day test period and exhibited gastrointestinal hemorrhages. At 6 days, liver atrophy and elevated (over 2-fold) TBA-RS values were recorded in pair-fed controls but not in their TCDD-treated counterparts. TCDD decreased hepatic glutathione peroxidase activity by almost 50% at 6 days, while pair-feeding was without effect. Liver morphology was different between TCDD-treated and pair-fed rats. Moreover, the livers of TCDD-treated L-E rats contained much higher concentrations of probably peripheral fat-derived fatty acids than did the livers of pair-fed or ad libitum control rats. Restricted feeding over 6 days induced hepatic lipid peroxidation more in H/W than in L-E rats. Endotoxin increased liver TBA levels similarly in both strains having an additive effect with high doses of TCDD in H/W rats. Added as a 0.5% concentration in chow, butylated hydroxyanisole (BHA), but not ethoxyquin, tended to increase survival rate and time in L-E rats exposed to 20 micrograms/kg of TCDD; at 50 micrograms/kg the only survivor was again in the BHA group. However, neither antioxidant had any effect on initial body weight loss. It is concluded that lipid peroxidation mainly arises as a secondary phenomenon in TCDD toxicity, is not the cause of the typical histopathological liver lesion, but may contribute to lethality.

Oxidative stress induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is one of the most potent toxins and tumor promoters known to man. It is prototypical of many halogenated polycyclic hydrocarbons that occur as environmental contaminants. Pathologic lesions produced by these compounds are mediated by an intracellular receptor protein called the TCDD (Ah) receptor which functions as a trans-acting effector of gene expression. However, the ultimate posttranslational pathways and mechanisms involved in the expression of the toxic manifestations of TCDD have received little attention and remain unclear, yet constitute an important segment in our understanding of the overall mechanism of action of TCDD. Recent studies have demonstrated that an oxidative stress occurs in various tissues of TCDD-treated animals. Evidence indicating production of an oxidative stress by TCDD in rodents is reviewed and includes:enhanced in vitro and in vivo hepatic and extrahepatic lipid peroxidation; increased hepatic and macrophage DNA damage; increased urinary excretion of malondialdehyde; decreased hepatic membrane fluidity; increased production of superoxide anion by peritoneal macrophage; and decreased glutathione, nonprotein sulfhydryl, and NADPH contents in liver. The potential role of reactive oxygen species in tumor promotion by TCDD is discussed. Possible sources and mechanisms of production of reactive oxygen species in response to TCDD are considered in light of current information. Evidence demonstrating the involvement of iron in TCDD-induced formation of reactive oxygen species and DNA damage is reviewed. Oxidative damage may contribute to many of the toxic responses produced by TCDD and its bioisosteres, and may be common to most of the tissue-damaging effects.

Screening of pharmacological agents given peripherally with respect to TCDD-induced wasting syndrome in Long-Evans rats

A salient sign of fatal 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) intoxication is dramatic body weight loss accompanied by hypophagia. Yet, the nature of this wasting syndrome is unknown. As all of the current leptogenic (weight reducing) drugs exert their action by affecting aminergic neurotransmission, this study set out to screen the reversibility of TCDD-induced anorexia with the following agents modulating aminergic neurotransmission: amphetamine, amperozide, chlordiazepoxide, clonidine, haloperidol, morphine, PCPA, phenoxybenzamine, reserpine and sotalol. In addition, dexamethasone, indomethacin, and insulin were included in the drug battery. The agents were administered subcutaneously to adult male Long-Evans rats over a period lasting from 3 to 14 days. Half of each drug group was concomitantly exposed to a lethal dose of TCDD (20 micrograms/kg). None of the regimens were able to mitigate the wasting syndrome. TCDD proved to markedly diminish the nocturnal feed intake while practically sparing daytime feed consumption. Insulin increased the daytime feeding of TCDD-exposed rats, and the termination of treatment resulted in almost total aphagia in this group. Amphetamine, dexamethasone, PCPA, and reserpine caused weight loss in drug control rats and aggravated the action of TCDD. However, clonidine had no effect on the weight of control rats but accelerated weight decline in TCDD-cotreated animals. TCDD seemed to have a somewhat minor influence on drinking than on feeding. Clonidine stimulated water intake in controls but not in TCDD-exposed rats. These results suggest that aminergic neurotransmission is not specifically or crucially affected by TCDD, but further studies are needed to confirm this conclusion.

A role for dietary lipids and antioxidants in the activation of carcinogens

The ways in which dietary polyunsaturated fats and antioxidants affect the balance between activation and detoxification of environmental precarcinogens is discussed, with particular reference to the polycyclic aromatic hydrocarbon benzo(a)pyrene. The structure and composition of membranes and their susceptibility to peroxidation is dependent on the polyunsaturated fatty acid (PUFA) content of the cell and its antioxidant status, both of which are determined to a large degree by dietary intake of these compounds. An increase in the PUFA content of membranes stimulates the oxidation of precarcinogens to reactive intermediates by affecting the configuration and induction of membrane-bound enzymes (e.g., the mixed-function oxidase system and epoxide hydratase); providing increased availability of substrates (hydroperoxides) for peroxidases that cooxidise carcinogens (e.g., prostaglandin synthetase and P-450 peroxidase); and increasing the likelihood of direct activation reactions between peroxyl radicals and precarcinogens. Antioxidants, on the other hand, protect against lipid peroxidation, scavenge oxygen-derived free radicals and reactive carcinogenic species. In addition some synthetic antioxidants exert specific effects on enzymes, which results in increased detoxification and reduced rates of activation. The balance between dietary polyunsaturated fats, antioxidants and the initiation of carcinogenesis is discussed in relation to animal models of chemical carcinogenesis and the epidemiology of human cancer.

Comparison of the effects of carbon tetrachloride and of 2,3,7,8-tetrachlorodibenzo-p-dioxin on the disposition of linoleic acid in rat liver in vitro

Both 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and carbon tetrachloride (CCl4) have conspicuous effects on lipid metabolism in rat liver. Although it is generally accepted that CCl4 administration leads to hepatic lipid peroxidation in vivo, conflicting reports from different laboratories make it unclear whether or not lipid peroxidation is involved in the mechanism of toxicity of TCDD. The present study involved pretreating F344 rats with CCl4 or TCDD, then at predetermined times thereafter, giving [U-14C]linoleic acid. A variety of compound classes were monitored in extracts of liver taken 30 min after the label was given. A previously unreported effect of CCl4 was a conspicuous increase in turnover of 1,2-diglycerides. That CCl4 did cause lipid peroxidation was evident from the presence of allylic hydroxyacids not seen in vehicle-treated controls, greatly increased radioactivity in protein-bound material, and decreased levels of arachidonate without decreased synthesis from linolate. Where effects of TCDD pretreatment could be seen, they were much less than the corresponding effects of CCl4. No allylic hydroxyacids were detected in livers of TCDD-treated rats. The concentration of arachidonate was not reduced, and elongation of linolate was not stimulated, indicating that TCDD did not cause extensive-but-repaired peroxidation. It is concluded that while TCDD may slightly increase hepatic lipid peroxidation in rats in vivo, the extent of such stimulation appears to be too slight to account for the toxicity of TCDD.

A survey of chemicals inducing lipid peroxidation in biological systems

A great number of drugs and chemicals are reviewed which have been shown to stimulate lipid peroxidation in any biological system. The underlying mechanisms, as far as known, are also dealt with. Lipid peroxidation induced by iron ions, organic hydroperoxides, halogenated hydrocarbons, redox cycling drugs, glutathione depleting chemicals, ethanol, heavy metals, ozone, nitrogen dioxide and a number of miscellaneous compounds, e.g. hydrazines, pesticides, antibiotics, are mentioned. It is shown that lipid peroxidation is stimulated by many of these compounds. However, quantitative estimates cannot be given yet and it is still impossible to judge the biological relevance of chemical-induced lipid peroxidation.

Effects of vitamin A and/or thyroidectomy on liver microsomal enzymes and their induction in 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated rats

Vitamin A and thyroid hormone status have been shown previously to alter the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in rats. In the present study, we have examined the effects of a vitamin A-excess and a vitamin A-deficient diet on thyroid hormone levels, on selected drug-metabolizing enzymes in liver microsomes, and on their inducibility by TCDD in male Sprague-Dawley rats. Except for a slight increase in serum T3 levels, none of these end points was affected by feeding rats the vitamin A-deficient diet. In contrast, excess dietary vitamin A caused a decrease in serum thyroxine (T4) and triiodothyronine (T3) levels, although the levels of T3 remained in the euthyroid range (60-80 ng/dl). The concentration of liver microsomal cytochromes P-450 and b5 and the basal activity of benzo[a]pyrene hydroxylase and 7-ethoxyresorufin O-de-ethylase were unaffected by excess dietary vitamin A. This result is consistent with our previous observation that the basal activity of these enzymes is dependent more on T3 than on T4 levels. Vitamin A excess markedly suppressed the activity of liver microsomal UDP-glucuronosyl transferase toward 1-naphthol. However, no such enzyme suppression was observed in thyroidectomized rats. This suggests that the suppressive effect of vitamin A on UDP-glucuronosyl transferase activity may be dependent on T3. Neither vitamin A nor thyroid status had any major effect on the inducibility of UDP-glucuronosyl transferase and cytochrome P-450-dependent enzyme activities by TCDD. However, vitamin A and TCDD had a nearly additive effect on suppression of serum T4. It is concluded that liver microsomal enzyme induction is not associated with the modulatory effect of vitamin A and thyroid hormones on the toxicity of TCDD.

Overdose toxicity studies versus threshold: elements of biology must be incorporated into risk assessment

The estimation of risk and evaluation of risk/benefit are traditionally and by necessity oriented at the current state of science and technology to ensure contemporary rights of safety. This demands a careful development in the methodology of hazard identification and risk assessment and its continuous updating and involvement. Compounds, e.g. erythrosine, 2,4,5-T, TCDD, and antioxidants, have still to be judged case-by-case, taking into account all available information on dosage, effect, kinetics, and mechanism of action, i.e., matters of biology rather than of mathematics alone. Considerations of mechanism of action and kinetics, especially recognition of low-dose/high-affinity assumptions in vitro, are necessary. This might lead to a new view upon thresholds which appears to apply for promoters. As accepted in pharmacology the dose-dependent magnitude of response observed in vivo is often a composite effect. Composite effects in toxicology can be viewed as having similar characteristics in vivo as in vitro in terms of potency, slope, maximal efficay and variability. Composite vectors can be antagonistic, leading to toxic and carcinogenic results, but also to protection. A continuous updating of scientific expertise supported by own experimental work is required for the regulator.

Evidence for lipid peroxidation in endotoxin-poisoned mice

Ethane has been identified and quantitated in air exhaled by mice following intraperitoneal injection of 20, 40, or 200 mg of Escherichia coli O111:B4 lipopolysaccharide (LPS) per kg. Significant increases in ethane concentration occurred within 1 to 5 h after LPS administration. In addition, increased concentrations of malondialdehyde were found in crude homogenates of livers obtained from mice 16 h after administration of 20 mg of LPS per kg. These results suggest that lipid peroxidation may be an important mechanism responsible for LPS toxicity.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on lipid peroxidation in microsomal systems in vitro

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) when added to suspensions of rat hepatic microsomes in the presence of NADPH has little influence on the peroxidation of microsomal lipids unless the system also contains complexed ferric ion, in which case TCDD stimulates. This stimulation does not appear to require metabolism of the TCDD. Peroxidation was monitored by production of thiobarbiturate-reactive substances (malondialdehyde and dienals), production of conjugated dienes, and disappearance of polyunsaturated fatty acids. Stimulation of lipid peroxidation by TCDD in a mixed lysosome-microsome preparation resulted in significantly decreased 'leakage' of acid phosphatase into the medium, implying an effect on lysosomal membranes. Consideration both of the present results and data in the literature leads to the conclusion that it is premature to attempt to define the relationship between enzyme induction, lipid peroxidation and TCDD lethality.

Differential regulation of hepatic glutathione transferase and glutathione peroxidase activities in the rat

The effects of the xenobiotics, i.e. butylated hydroxytoluene, beta-naphthoflavone, isosafrole, pregnenolone-16 alpha-carbonitrile, trans-stilbene oxide, 3-methylcholanthrene, phenobarbital, 3,3',4,4'-tetrachlorobiphenyl, 2,2',4,4',5,5'-hexachlorobiphenyl, on rat liver cytosolic glutathione transferase and glutathione peroxidase activities have been investigated. Although the glutathione transferase isozymes (measured by the specific substrates ethacrynic acid and delta 5-androstene-3,17-dione) which have been shown to possess peroxidase activity were significantly increased, little or no increase in peroxidase activity (toward cumene hydroperoxide, tert-butyl hydroperoxide or hydrogen peroxide) was observed. Likewise during a 16-day time course following the administration of Aroclor 1254 or fireMaster BP-6 (each 500 mg/kg, i.p.), potent induction of glutathione transferase activities was seen without any significant increases in peroxidase activities. In fact during the second week of the time course, there were significant decreases in selenium-dependent glutathione peroxidase activity (toward hydrogen peroxide). The inverse regulation of these activities, i.e. the depression of selenium-dependent glutathione peroxidase activity following sustained induction of glutathione transferases, may have direct implications for the toxicity of the polyhalogenated aromatic hydrocarbons.