Lipid peroxidation as a possible cause of TCDD toxicity

Epoxides: an underestimated lipid oxidation product

Immense gains in understanding of mechanisms and effects of lipid oxidation have been achieved in the nearly 90 years over which lipid oxidation has been an active research focus. Even so, the substantial questions still being raised about lipid oxidation in this special issue show clearly that missing pieces remain and must be considered for full accounting of this important reaction in any system. In this context, epoxides are spotlighted as a critical overlooked product of lipid autoxidation - underestimated in analysis, underestimated in presence as a functionally active and competitive intermediate and product of lipid oxidation, and underestimated in potential contributions to impact of lipid oxidation on other molecules and cell functions. Logical reasons for ignoring or not finding epoxides are offered in historical development of lipid oxidation knowledge. Reactions generating lipid epoxides in autoxidation are reviewed, limitations in detecting and tracking epoxides are outlined to explain why epoxides may not be detected when they should be present, and justifications for increased research and analysis of epoxides are argued. The main goal is to provide a context for recognizing epoxides as critical products that must be accounted for in determining the state rather than extent of lipid oxidation and in tracking its consequences in oils, foods, personal care products, and tissues. A secondary goal is to stimulate new research using contemporary analyses to fill in the gaps of knowledge about epoxide formation, structure, and reactions in lipid autoxidation.

Cholangiocarcinoma

Cholangiocarcinoma (CCA) is a highly lethal adenocarcinoma of the hepatobiliary system, which can be classified as intrahepatic, perihilar and distal. Each anatomic subtype has distinct genetic aberrations, clinical presentations and therapeutic approaches. In endemic regions, liver fluke infection is associated with CCA, owing to the oncogenic effect of the associated chronic biliary tract inflammation. In other regions, CCA can be associated with chronic biliary tract inflammation owing to choledocholithiasis, cholelithiasis, or primary sclerosing cholangitis, but most CCAs have no identifiable cause. Administration of the anthelmintic drug praziquantel decreases the risk of CCA from liver flukes, but reinfection is common and future vaccination strategies may be more effective. Some patients with CCA are eligible for potentially curative surgical options, such as resection or liver transplantation. Genetic studies have provided new insights into the pathogenesis of CCA, and two aberrations that drive the pathogenesis of non-fluke-associated intrahepatic CCA, fibroblast growth factor receptor 2 fusions and isocitrate dehydrogenase gain-of-function mutations, can be therapeutically targeted. CCA is a highly desmoplastic cancer and targeting the tumour immune microenvironment might be a promising therapeutic approach. CCA remains a highly lethal disease and further scientific and clinical insights are needed to improve patient outcomes.

DNA Product Formation in Female Sprague-Dawley Rats Following Polyhalogenated Aromatic Hydrocarbon (PHAH) Exposure

DNA oxidation damage has been regarded as one of the possible mechanisms for the hepatic carcinogenesis of dioxin-like compounds (DLCs). In this study, we evaluated the toxic equivalency factor (TEF) from the standpoint of induced DNA oxidation products and their relationship to toxicity and carcinogenicity. Nine DNA oxidation products were analyzed in the liver of female Sprague-Dawley rats exposed to 2,3,7,8-tetrachlorodibenzo-pdioxin (TCDD) alone or the tertiary mixture of TCDD, 3,3',4,4',5-pentachlorobiphenyl (PCB 126), and 2,3,4,7,8-pentachlorodibenzofuran (PeCDF) by gavage for 14, 31, and 53 weeks (5 days/week) by LC-MS/MS: 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dGuo); 1,N6-etheno-2'-deoxyadenosine (1,N6-εdAdo); N2,3-ethenoguanine (N2,3-εG); 7-(2-oxoethly)guanine (7-OEG); 1,N2-etheno-2'-deoxyguanosine (1,N2-εdGuo); malondialdehyde (M1dGuo); acrolein (AcrdGuo); crotonaldehyde (CrdGuo); and 4-hydroxynonenal (HNEdGuo) derived 2'-deoxyguanosine adducts. Exposure to TCDD (100 ng/kg/day) significantly induced 1,N6-εdAdo at 31 and 53 weeks, while no increase of 8-oxo-dGuo was observed. Significant increases were observed for 8-oxo-dGuo and 1,N6-εdAdo at all time points following exposure to the tertiary mixture (TEQ 100 ng/kg/day). Exposure to TCDD for 53 weeks only significantly increased 1,N6-εdAdo, while increases of N2,3-εG and 7-OEG were only found in the highest dose group (100 ng/kg/day). Exposure to the tertiary mixture for 53 weeks had no effect on N2,3-εG in any exposure group (TEQ 0, 22, 46, or 100 ng/kg/day), while significant increases were observed for 1,N6-εdAdo (all dose groups), 8-oxo-dGuo (46 and 100 ng/kg/day), and 7-OEG (100 ng/kg/day). While no significant increase was observed at 53 weeks for 1,N2-εdGuo, M1dGuo, AcrdGuo, or CrdGuo following exposure to TCDD (100 ng/kg/day), all of them were significantly induced in animals exposed to the tertiary mixture (TEQ 100 ng/kg/day). This oxidation DNA product data suggest that the simple TEF methodology cannot be applied to evaluate the diverse patterns of toxic effects induced by DLCs.

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.

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.

The "Tuebingen desiccator" system, a tool to study oxidative stress in vivo and inhalation toxicokinetics

The "Tuebingen desiccator," a gas-tight all-glass closed chamber system (CCS), has been established in Herbert Remmer's Institute of Toxicology, University of Tuebingen, to investigate the mechanisms underlying the exhalation of endogenous volatile hydrocarbons in rats under oxidative stress. Remmer and associates confirmed the former view that ethane and n-pentane were derived from polyunsaturated fatty acids, and they demonstrated that propane, n-butane and isobutane were released from amino acids. Hydrocarbons exhaled following acute ethanol treatment of rats resulted predominantly from ethanol-dependent inhibition of their metabolism and partly from oxidation of proteins. Exhalation of alkanes in carbon tetrachloride exposed rats did not reflect liver damage, which was, however, directly linked to the amount of carbon tetrachloride metabolized. As has first been shown in Herbert Remmer's institute by investigating the fate of inhaled vinyl chloride in rats, the CSS proved to be also an excellent tool for studying toxicokinetics of inhaled gaseous xenobiotics by means of gas uptake experiments. Based on results gained by such studies, it was recently demonstrated that knowledge of compound-specific physicochemical and species-specific physiological parameters are often sufficient to predict important toxicokinetic properties of inhaled chemicals such as tissue burdens at steady state. By means of the CCS, not only kinetics of a parent gaseous substance but also of gaseous metabolites can be investigated in vivo, as exemplified for ethylene oxide and 1, 2-epoxy-3-butene, metabolites of ethylene and 1,3-butadiene, respectively. Gas uptake studies in closed chamber systems are now worldwide used for determining toxicokinetic parameters relevant for physiological toxicokinetic modeling.

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.

Role of oxidative stress and antioxidant defense in 3,3',4,4',5-pentachlorobiphenyl-induced toxicity and species-differential sensitivity in chicken and duck embryos

The role of oxidative stress and antioxidant defense in 3,3',4,4',5-pentachlorobiphenyl (PCB 126)-induced toxicity and species-specific sensitivity was examined in White Leghorn chicken (Gallus domesticus) and Pekin duck (Anas platyrhynchos) embryos. Eggs were injected into the air cell with 0.4-1.6 microgram PCB 126/kg egg in corn oil prior to incubation. Lipid peroxidation measured by thiobarbituric acid reactive substances (TBARS), the GSSG:GSH ratio, and glutathione peroxidase (GPox) activities were determined in liver and adipose tissue of day 19 chicken and day 26 duck embryos. In chicken embryos, PCB 126 increased mortality and the incidence of edema and liver lesions, decreased embryo size, increased eye and head malformations, and markedly reduced fat storage. In contrast, no effects on the endpoints were observed in duck embryos even at the highest dose used in chicken embryos. PCB 126 increased hepatic 7-ethoxyresorufin-O-deethylase (EROD) activity in a dose-dependent manner in chicken but not duck embryos. PCB 126 significantly increased TBARS levels in liver and to a greater degree in adipose tissue of chicken embryos, indicating that adipose tissue is a sensitive target for this compound. Increases in lipid peroxidation by PCB 126 were associated with significant decreases in GPox activity in these tissues. These biochemical changes support oxidative stress playing a role in PCB 126-induced embryo toxicity while antioxidant defenses provided protection against oxidative damage induced by this compound. Ducks, the less-sensitive species, showed higher basal levels of hepatic GPox than chickens, suggesting that this antioxidant enzyme may contribute to the differences in sensitivity to this compound between the two species.

Production of superoxide anion, lipid peroxidation and DNA damage in the hepatic and brain tissues of rats after subchronic exposure to mixtures of TCDD and its congeners

In this study the induction of oxidative stress in the hepatic and brain tissues of rats after subchronic exposure to various mixtures of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and two of its congeners, namely 2,3,4,7,8-pentachlorodibenzofuran (PeCDF) and 3,3',4,4',5-pentachlorobiphenyl (PCB 126) was investigated. Four mixtures of TCDD and its congeners, corresponding to 10, 22, 46 and 100 ng of toxic equivalence (TEQ) kg(-1) day(-1), were administered to groups of rats for 13 weeks. The animals were sacrificed at the end of the exposure period and the biomarkers of oxidative stress, including the production of superoxide anion, lipid peroxidation and DNA single-strand breaks (SSBs), were determined in the hepatic and brain tissues. All mixtures caused dose-dependent increases in the production of superoxide anion, lipid peroxidation and DNA SSBs in both tissues, with significantly higher damage in the hepatic compared with the brain tissues. The 22 ng TEQ dose level (TEQ = 22) contains TCDD, PeCDF and PCB 126 at levels that correspond to 7.3, 14.5 and 73.3 ng kg(-1) day(-1), respectively, and it produced effects that correspond to ca. 50% of the maximal production of superoxide anion, lipid peroxidation and DNA SSBs in the hepatic and brain tissues of those animals. Relative to the doses that are required to produce 50% of the maximal production of the biomarkers of oxidative stress by the individual congeners in hepatic and brain tissues of rats, the concentrations of the congeners in TEQ = 22 did result in significant interactivity, probably in the form of additive effects in the hepatic but not in brain tissues.

The relative abilities of TCDD and its congeners to induce oxidative stress in the hepatic and brain tissues of rats after subchronic exposure

The abilities of single doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to induce oxidative stress in hepatic and some extra-hepatic tissues of animals, are well documented. In this study we have investigated the induction of oxidative stress in hepatic and brain tissues of rats after subchronic (13 weeks) exposure to TCDD and two of its congeners, namely 2,3,4,7,8-pentachlorodibenzofuran (PeCDF) and 3,3',4,4',5-pentachlorobiphenyl (PCB126). TCDD, PeCDF and PCB126 were administered daily to groups of rats at various doses, for 13 weeks, and biomarkers of oxidative stress, including the production of superoxide anion, lipid peroxidation and DNA-single strand breaks (SSBs), were determined in the hepatic and brain tissues at the end of the exposure period. The three congeners caused dose-dependent increases in the production of superoxide anion, lipid proxidation and DNA-SSBs, with maximal effects achieved at doses ranging between 10-100, 20-92, and 300-550 ng/kg per day for TCDD, PeCDF and PCB126, respectively. The doses that produce 50% of maximal responses by each of the xenobiotics in the hepatic and brain tissues were found to be within the ranges of 7-34, 13-32, and 137-400 ng/kg per day for TCDD, PeCDF and PCB126, respectively. The results of the study suggest that subchronic exposures to TCDD, PeCDF and PCB126 induce significant oxidative damage in the hepatic and brain tissues of rats, with more damage observed in the brain as compared to the hepatic tissues. Also, as inducers of oxidative stress in the hepatic and brain tissues, TCDD is the most potent among the three congeners and PCB126 being the least potent.

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.

Analysis of polychlorinated dioxins and furans in samples of the toxic oil syndrome

1 Polychlorinated dioxins (PCDDs) and furans (PCDFs) are known to produce a wide range of toxic effects. 2 PCDDs and PCDFs are typical contaminants of chlorinated phenols, and pentachlorophenol and related compounds have been shown to be widely distributed among selected oil samples taken from the 1981 Spanish toxic oil epidemic. 3 Six control and eight case oil samples were analysed using GC/MS for PCDDs and PCDFs. Only small concentrations, normally below 1 ng g-1, of the higher chlorinated PCDDs and PCDFs were detected. There were no statistical differences between the case and control oils. 4 These levels seem to be too low to elicit toxic effects, although they could be enough to potentiate the toxicity of other xenobiotics present in the oils. However, it is uncertain whether the levels of these compounds measured in 1990 reflect the levels present when the oils were consumed in 1981, or whether or not the levels measured in crude oils are representative of fried oils.

Increased copper concentrations in rat tissues after acute intoxication with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Recently, acutely toxic doses of the environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) have been reported to affect the hepatic distribution of essential metals in the rat. However, the reduced food intake by TCDD was not taken into account. Therefore, metal concentrations were determined in different rat tissues at the end of a toxicity study with TCDD in which a pair-fed control group was introduced. Male Sprague-Dawley rats received a single i.p. injection of corn oil/acetone with or without TCDD at 125 micrograms/kg. Controls and TCDD-treated rats were fed ad libitum; additionally, pair-fed controls received the amount of food consumed by their TCDD-treated partners 1 day before. Twenty-one days after dosing rats were killed and samples of liver, kidney and jejunum were taken for the analysis of Ca, Cu, Fe, Mg, Mn, and Zn. After acid digestion of the tissues metals were determined by atomic emission spectrometry (AES). The most outstanding effect of TCDD treatment was an increase of the copper levels in the kidney (4-fold, versus pair-fed controls) and in the liver (greater than 2-fold, versus pair-fed controls). Other metals were mainly affected by the reduced food intake only. Since Cu represents a trace metal the homeostasis of which depends on its biliary excretion and since TCDD is known to impair biliary flow and excretion, an impaired biliary excretion of Cu by TCDD is suggested as the causal mechanism.

Polychlorinated biphenyls-induced lipid peroxidation as measured by thiobarbituric acid-reactive substances in liver subcellular fractions of rats

Rats were given a 0.05% polychlorinated biphenyls (PCB) diet supplemented with adequate nutrients for 10 days and not only PCB-induced lipid peroxidation as measured by thiobarbituric acid (TBA)-reactive substances but also variations of lipid peroxides scavengers in liver and its subcellular fractions (nuclei and cell debris, mitochondrial, microsomal and cytosolic fractions) were investigated. The lipid peroxidation in liver and subcellular fractions in the PCB-treated group increased significantly except in the nuclei and cell debris fraction. The increase in lipid peroxidation in the microsomal fraction appeared to be associated in part with the decrease in vitamin E (alpha-tocopherol) content and induction of drug-metabolizing enzymes. In the cytosolic fraction, the total lipid content increased, glutathione peroxidase (GSHPx) activity decreased and the quantity of free radical-reactive substances suppressing lipid peroxidation was low as measured by the 1,1-diphenyl-2-picrylhydrazyl (DPPH) value. From these results, the increase in lipid peroxidation in the cytosolic fraction in the PCB-treated group was ascribed to the abundance and availability of oxidizable substrate attended with fatty liver, to the decline in GSHPx activity, and to the insufficiency in antioxygenic activity as observed by the decrease in the DPPH value.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-induced decrease in the fluidity of rat liver membranes

1. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCCD)-induced lipid peroxidation has previously been demonstrated by assessing the hepatic content of thiobarbituric acid reactive substances (TBARS) as well as the NADPH-dependent microsomal formation of TBARS as well as the NADPH-dependent microsomal formation of TBARS using malondialdehyde as the standard. 2. Changes in membrane fluidity as a result of lipid peroxidation may occur. Therefore the dose- and time-dependent effects of TCDD on lipid peroxidation in mitochondrial, microsomal, and plasma membranes, and changes in membrane fluidity in these subcellular fractions, were examined. Animals were treated with either 50 or 100 micrograms TCDD/kg orally, and killed 3, 6, or 9 days post-treatment. 3. Time-dependent increases occurred in TBARS content and formation following TCDD administration for all three membranes. Similar results were observed after 50 and 100 micrograms TCDD/kg. 4. Following TCDD administration, fluorescence polarization measurements as determined by the fluorescence polarization (r) and anisotropy parameter (a.p.) values demonstrated significant decreases in membrane fluidity in all membrane fractions, indicative of membrane structural alterations. 5. Excellent inverse correlations between lipid peroxidation and membrane fluidity were observed. Thus, decreased membrane fluidity and increased membrane damage may contribute to the toxic manifestations of TCDD as a consequence of an oxidative stress.

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.

Desferrioxamine-induced alterations in hepatic iron distribution, DNA damage and lipid peroxidation in control and 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated rats

Desferrioxamine (DFX), an iron chelator, has been used to inhibit in vitro redox cycling of the transition iron irons that may interfere with free-radical formation, and is used clinically to treat conditions associated with iron overload. The effect of DFX on hepatic iron distribution has not been examined. Therefore, we have examined the effect of DFX on iron content and distribution, lipid peroxidation and DNA single-strand breaks in liver. Since 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) enhances hepatic lipid peroxidation and induces DNA damage, the ability of DFX to modulate these effects was also examined. Female Sprague-Dawley rats received 200 mg DFX kg-1 i.p. every 12 h for 10 days and 50 micrograms TCDD kg-1 p.o. on Day 4 as a single dose. All animals were killed 7 days after TCDD administration. The results demonstrate that DFX alone did not decrease hepatic iron content, but enhanced the iron distribution in mitochondria, microsomes and nuclei. A 10-day administration of DFX resulted in enhanced hepatic lipid peroxidation and DNA single-strand breaks, consistent with the alterations in iron distribution. TCDD administration produced large increases in lipid peroxidation in microsomes, nuclei and whole-liver homogenates, as well as enhanced DNA damage. However, the marked increases in lipid peroxidation produced by TCDD were attenuated by DFX, while the DNA damaging effects of TCDD and DFX were additive.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipid peroxidation of rat liver microsomes membranes related to a protein deficiency and/or a PCB treatment

In this study, we investigated the influence of protein deficiency on lipid peroxidation (LP) and cellular defense systems against oxidative damage in control or polychlorinated biphenyl (PCB) treated rats. Rats were fed either a standard diet (22% casein) or a low protein diet (3.5% casein) for 1, 2 and 6 weeks. Five days prior to killing, one half of the animals were given a single i.p. injection of Phenoclor DP6 (50 mg/kg body weight). In protein deficient rats, liver vitamin E was depressed and ascorbate level was lowered. Total and selenium-dependent glutathione peroxidases (GSH-Px) activities were decreased whereas glutathione reductase (GSH-red) was enhanced. Enzymatic and non enzymatic LP ('spontaneous' or with ADP-Fe2+) were increased. Phenoclor DP6 treatment enhanced liver ascorbate concentration. Microsomal LP was increased. Total and selenium-GSHPx remained unmodified while GSH-red was increased. Liver glutathione and alpha-tocopherol contents appeared to be independent of the PCB injection. Our data suggest that low protein intake and PCB exposure may reduce liver defensive protection against electrophilic species.

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.

Carcinogenicity of polyhalogenated biphenyls: PCBs and PBBs

Polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs) are compounds whose physical/chemical properties led to their widespread commercial use. Although their production has been banned or severely limited in most countries since the 1970s, the persistence and stability of these compounds have resulted in a worldwide distribution, especially of PCBs. PBB contamination is limited principally to the state of Michigan, where a series of tragic errors eventually resulted in the accumulation of residues in livestock and the general human population. Long-term exposure to PCBs and PBBs in animals has been associated with the induction of neoplastic nodules in the liver and in some cases hepatocellular carcinoma. This review discusses the role of PCBs and PBBs in the process of carcinogenesis. The mutagenicity/genotoxicity of these compounds, as well as their initiation/promotion potential is discussed. The epidemiology of PCB and PBB exposure is reported along with an estimation of the risk of cancer to humans. Finally, possible molecular mechanisms of action are suggested for polyhalogenated biphenyls in cancer development.

The central nervous system may be involved in TCDD toxicity

In the present study, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was administered to both the most TCDD-susceptible (Long-Evans) and the most TCDD-resistant rat strain (Han/Wistar) as a constant 1-week infusion either centrally (intracerebroventricularly; i.c.v.) or peripherally (s.c.). Lethality, feed and water consumption as well as weight gain were observed. For both strains of rat, feed intake was most severely affected in the groups given TCDD i.c.v., while the s.c. infusion of TCDD did not markedly depress eating. The same pattern of responsiveness was discernible in the reduction of water consumption and of weight gain. Two out of 7 i.c.v.-treated rats of the TCDD-susceptible strain died after TCDD exposure, whereas all s.c.-dosed animals survived. A statistically significant strain difference was manifest in the magnitude of response between the i.c.v.-TCDD groups in feed intake and body weight change. Moreover, no deaths occurred among the TCDD-resistant Han/Wistar rats. An additional experiment did not disclose any difference in TCDD toxicity between 2 peripheral routes (s.c. and i.p.). Further, lethality tended to have a shorter latent period with the readily absorbable dimethyl sulphoxide (DMSO) as the solvent than with the potentially slowly absorbed corn oil. These findings suggest an important role for the central nervous system in TCDD toxicity.

Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans: the risks to human health. A review

1 PCDDs and PCDFs are ubiquitous and persistent in the environment. They are to be found in body tissues of both humans and animals. 2 The most extensively studied PCDD is 2,3,7,8-TCDD. It has been shown to produce a wide range of effects and is considered to be a (non-genotoxic) carcinogen in animals. 3 Studies into the mechanisms of toxicity so far reveal that there is involvement of a specific receptor (Ah), however further work is required to elucidate the mechanisms of the various effects. 4 Reports on a number of human exposures to PCDDs and PCDFs are described. Results from human epidemiological studies are difficult to interpret: there have been problems in methodology; there has been inadequate information on intake, and exposures have often been to mixtures of PCDDs and/or PCDFs together with other related compounds. 5 Many regulatory authorities faced with the problem of providing an index of risk from exposure to mixtures of PCDDs and PCDFs have employed the concept of 'TCDD equivalents'. 6 Whether or not PCDDs and PCDFs pose a significant human health risk at current levels of exposure they remain of considerable interest to the toxicologist.

Effects of commercial chlorophenolate, 2,3,7,8-TCDD, and pure phenoxyacetic acids on hepatic peroxisome proliferation, xenobiotic metabolism and sister chromatid exchange in the rat

The induction of hepatic peroxisome proliferation and drug metabolizing enzymes and of sister chromatid exchange (SCE) in lymphocytes was studied in male Han/Wistar rats after exposing them for 2 weeks to a commercial chlorophenolate formulation (Ky-5) (100 mg/kg/day), to 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD; 0.05-5 micrograms/kg/wk) and to the pure phenoxyacetic acids, 2,4-dichlorophenoxyacetic acid (2,4-D; 100 mg/kg/day) and 2-chloro-4-methylphenoxyacetic acid (MCPA; 100 mg/kg/day). The chlorophenolate formulation and pure 2,4-D and MCPA caused significant increases in the number of peroxisomes in liver cells, although the average size of peroxisomes was not affected, whereas the effect of even the highest dose of 2,3,7,8-TCDD remained small. This finding indicates that dioxin impurities do not account for the peroxisome proliferation induced by chlorophenolate. The relative weight of the liver increased significantly in rats treated with the chlorophenolate formulation and with 2,3,7,8-TCDD (5.0 and 0.5 micrograms/kg). The pattern of induction of xenobiotic metabolizing enzymes showed some differences between chlorophenolate treatment and 2,3,7,8-TCDD treatment. Furthermore, the effects of pure phenoxyacetic acids were different from that seen with chlorophenolate and 2,3,7,8-TCDD. The highest dose of 2,3,7,8-TCDD increased the frequency of SCE in circulating lymphocytes slightly, but significantly.

2,3,7,8, tetrachlorodibenzo-p-dioxin induces oxygen activation associated with cell respiration

We have investigated the influence of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on bioenergetic functions of isolated heart-mitochondria. Electron transfer and energy conservation activities were found to be decreased in the presence of very low amounts of the polychlorinated biphenyl compound (1.5 nmol/mg mitochondrial protein). The effect was greatest when substrates for complex I were used. In this case coupling of oxidative phosphorylation to respiration was drastically diminished, essentially at the expense of state 3 respiration, and P/O values were found around 2 instead of 3. Succinate-related energy conservation remained practically unaffected in the presence of TCDD, suggesting an interference of the toxic compound at coupling site I. SOD plus catalase were found to protect energy-linked respiration from the effect of dioxin indicating the involvement of superoxide radicals and H2O2 in the development of the observed phenomena. The present contribution provides experimental evidence on the formation of these oxygen species in the presence of TCDD. Furthermore, the site of action of TCDD is demonstrated and discussed in relation to the oxygen radical formation observed.

Long-term effects of commercial and congeneric polychlorinated biphenyls on ethane production and malondialdehyde levels, indicators of in vivo lipid peroxidation

Ethane exhalation was increased in male Sprague-Dawley rats following a single intraperitoneal (IP) injection of Aroclor 1254 (500 mg/kg). In the first 2 weeks following Aroclor 1254 treatment, the increase in ethane exhalation was due to an inhibition of metabolism of endogenous ethane rather than to an increase in ethane production. In weeks 3 and 4 following Aroclor 1254 administration, metabolic clearance of ethane returned to and exceeded control levels, while ethane production increased to approximately twice the control rates (day 30). The HPLC determination of in situ hepatic malondialdehyde levels revealed a 2-fold increase in malondialdehyde content on day 30 following the Aroclor 1254 injection. Further, parallel increases in in situ malondialdehyde levels and ethane production rates were also found 30 days following a single IP injection of 3,3',4,4'-tetrachlorobiphenyl, 2,3,4,4',5-pentachlorobiphenyl and 2,2',4,4',5,5'-hexachlorobiphenyl (300 mumol/kg). These effects were not reflected in increased diene conjugation. Redox state of the liver was largely unaffected, as evidenced by the relative concentrations of reduced and oxidized NADPH. However, minor changes in reduced and oxidized glutathione were noted.

Substrate preference of a cytosolic aldehyde dehydrogenase inducible in rat liver by treatment with 3-methylcholanthrene

The substrate preference of an aldehyde dehydrogenase induced in rat liver cytosol by 3-methylcholanthrene was examined. This enzyme, T-ALDH, is identical to the aldehyde dehydrogenase inducible in rat liver by 2,3,7,8-tetrachloro-dibenzo-p-dioxin and the tumor-associated aldehyde dehydrogenase found in rat hepatocellular neoplasms. With either NAD or NADP as coenzyme, the preferred substrates were the aliphatic aldehydes n-hexanal, n-nonanal, and isobutyraldehyde and the aromatic aldehydes 2,5-dihydroxybenzaldehyde, benzaldehyde, and 3-hydroxybenzaldehyde. The results indicate that T-ALDH may play a role in oxidizing a variety of aldehydes produced in physiological lipid metabolism. On the contrary, this isozyme does not seem to participate in the oxidation of small aliphatic aldehydes generated during lipid peroxidation. Similarly, no significant activity could be detected when the enzyme was tested with aldehydes produced in carbohydrate, amino acid, polyamine, steroid, and vitamin metabolism.

Investigations on the role of free radical processes in hexachlorobenzene-induced porphyria in mice

Male C57Bl/10 mice were chronically fed hexachlorobenzene (HCB) (0.02% of the diet) alone or in combination with a single subcutaneous dose of iron (12.5 mg iron per mouse). After eight weeks the group of mice pretreated with the iron overload was highly sensitized to the porphyrogenic effect of HCB, as shown by liver porphyrin accumulation. A synergistic effect of iron was evident on other parameters too, such as HCB-induced hepatic damage, activation of type O of xanthine oxidase, and decreased activity of copper zinc superoxide dismutase and glutathione peroxidase(s). None of these parameters was affected by iron alone. Iron alone and in association with HCB markedly raised the level of lipid peroxides, the increase in the HCB group being smaller. The combined treatment resulted in a significant reduction of HCB's inductive effects on microsomal heme and cytochromes P-450 and b5 and on the activity of aryl hydrocarbon hydroxylase. The content of nonprotein sulfhydryl groups was reduced to the same extent in mice treated with HCB or HCB plus iron. The results suggest that reactive intermediates such as are formed by lipid peroxidation are not sufficient on their own to create the conditions for uroporphyrinogen decarboxylase impairment, as evident in the group of mice receiving iron overload alone. Conversely, HCB administration induced a specific condition of imbalance in the liver between formation and inactivation of reactive intermediates which was associated with hepatic porphyrin accumulation and was potentiated by concomitant administration of iron.

Dietary iron and 2,3,7,8-tetrachlorodibenzo-p-dioxin induced alterations in hepatic lipid peroxidation, glutathione content and body weight

The effects of various levels of dietary iron on hepatic lipid peroxidation (malondialdehyde [MDA] content), reduced glutathione (GSH) and GSH peroxidase (GSH-PX) activity as well as liver and body weights of female rats following TCDD administration were examined. Rats were fed diets containing deficient (6 ppm), normal (35 ppm) and supplemented (120 ppm) iron for 17, 24 and 31 days. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD, 40 micrograms/kg/day P.O.) in corn oil or the vehicle was given on days 9, 8 and 7 prior to sacrifice. TCDD treatment produced a 3-fold increase in hepatic MDA content in animals on normal iron diet. TCDD administration failed to increased MDA content in iron deficient animals. In the iron supplemented groups, TCDD resulted in 2.5 fold increases in lipid peroxidation. Dietary iron had no effect on hepatic GSH-PX activity. Animals on the iron deficient diet had 12-21% decreases in hepatic GSH content. TCDD administration resulted in 15-22% decreases in GSH content in animals on the control and iron supplemented diets. TCDD treatment resulted in significant decreases in body weights of animals on all 3 diets. TCDD induced lipid peroxidation appears to be iron dependent. However, the loss in body weight due to TCDD toxicity may not be dependent on lipid peroxidation.

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.

Effects of chlorinated organics on intermediates in the heme pathway and on uroporphyrinogen decarboxylase

Experimental porphyria induced by PHAHs is characterized by a progressive reduction in the activity of UROD. After intoxication with TCDD, the most porphyrogenic compound known to date, the liver was the principal site of action, as regards both porphyrin accumulation (mostly uroporphyrin) and the degree of enzyme impairment; the kidney was the site of the second greatest accumulation; the brain and erythrocytes were unaffected. Additional modifications of the heme pathway involved induction of the activity of ALAS and, at least in HCB-induced porphyria after iron pretreatment, may have involved reduced activity of uroporphyrinogen III cosynthetase. These changes can alter the amount and the isomeric composition of uroporphyrinogens and uroporphyrins present in the liver in a way that is likely to help reduce formation of coproporphyrinogen III in porphyric animals. As in the human syndrome porphyria cutanea tarda, iron administration increased porphyrin accumulation and the degree of reduction of UROD activity in mice fed HCB. Mice fed HCB also presented an activation of the type O form of XO. This activation was independent of tissue injury derived from the lipid peroxidation that was concomitant with iron administration. The increase in activity of the type O form of XO may be a characteristic feature of the liver damage found in PHAH intoxication and, in intoxicated animals, could be a source in the liver of oxidant species involved in the mechanism of UROD inactivation--if this inactivation is in fact due to an oxidative reaction.

In vivo and in vitro interactions of TCDD and other ligands of the Ah-receptor: effect on embryonic and fetal tissues

Interactions of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and other ligands of the Ah-receptor, had been studied in vivo, in pregnant NMRI mice, and in vitro, in the fetal thymus organ culture system. Benzo(a)pyrene (BP) increased TCDD-induced fetolethality, whereas it did not affect the rate of cleft palate formation. This may indicate that the mechanism of TCDD-induced fetal death is different from that of TCDD-induced cleft palate. 2,3,7,8-Tetrachlorodibenzofuran (TCDBF) and 3,3',4,4'-tetrachloroazoxybenzene (TCAOB) were added together to the thymus organ culture, each at a concentration that caused about 25-50% lymphoid development inhibition. Such treatment resulted in an additive effect of about 75%. Similarly, when the slightly toxic beta-naphthoflavone (BN) was added together with TCDD to the same culture system, it caused a significant increase in the lymphoid inhibitory effect of the latter compound. These may all suggest a common mechanism of action for TCDD and other ligands, which may involve a direct interaction with the receptors present in the thymus.

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.

Monohydroperoxides of linoleic acid in endoplasmic lipids of rats exposed to tetrachloromethane

An accurate method for the quantitative determination of hydroperoxy and hydroxy fatty acids in liver microsomes is presented which involves the use of 18O-labeled internal standards. The method is employed for the determination of hydroperoxides in rat liver microsomes after aerobic incubation with Fe2+/ADP and in microsomes from animals exposed to 75 mg tetrachloromethane/kg body weight. The amounts found after artificial microsomal "lipid peroxidation" are almost two orders of magnitude larger than those in microsomes from tetrachloromethane-exposed animals.

The role of iron in 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced lipid peroxidation by rat liver microsomes

Treatment of female Sprague-Dawley rats with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) enhances hepatic lipid peroxidation, and the role of iron in TCDD-induced lipid peroxidation was examined. Ferrous and ferric ions, and the chelators adenosine diphosphate (ADP), ethylenediaminetetraacetic acid (EDTA) and desferrioxamine (DFX) were added to an in vitro microsomal lipid peroxidation system using microsomes from control and TCDD-treated animals. Both ferrous and ferric ions enhanced microsomal lipid peroxidation, with the greatest effect being produced by the combination of ferrous and ferric ions. The addition of ADP and EDTA produced modest increases in microsomal lipid peroxidation, suggesting that these chelators facilitated formation of reactive oxygen species by iron. The addition of DFX markedly inhibited microsomal lipid peroxidation, with greatest inhibition occurring with microsomes from TCD-treated animals. The results indicate that iron is involved in TCDD-induced lipid peroxidation.

Lipid peroxidation and mechanisms of toxicity

Aerobic organisms by definition require oxygen, and the importance of iron in aerobic respiration has long been recognized, but despite their beneficial roles, these elements can pose a real threat to the organism. During oxygen reduction, reactive species such as O2-. and H2O2 are formed readily. Iron can combine with these species, or with molecular oxygen itself, to generate free radicals which will attack the polyunsaturated fatty acids of membrane lipids. This oxidative deterioration of membrane lipids is known as lipid peroxidation. To protect itself against this form of attack, the organism possesses several types of defense mechanisms. Under normal conditions, these defenses appear to offer adequate protection for cell membranes, but the possibility exists that certain foreign compounds may interfere with or even overwhelm these defenses, and herein could lie a general mechanism of toxicity. This possible cause of toxicity is discussed in relation to other suggested causes.

Changes in hamster hepatic cytochrome P-450, ethoxycoumarin O-deethylase, and reduced NAD(P): menadione oxidoreductase following treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin. Partial dissociation of temporal and dose-response relationships from elicited toxicity

The temporal and dose-related characteristics of hepatic enzymes induced in the hamster by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were examined. Male Syrian golden hamsters received a single intraperitoneal injection of TCDD at a dose of 0-500 micrograms/kg. At various times up to 35 days, a number of variables were determined and compared: whole body, liver, and thymus weights; hepatic concentrations of cytochrome P-450 (P-450); and activities of 7-ethoxycoumarin O-deethylase (ECOD) and reduced NAD(P): menadione oxidoreductase (NMOR). Increased liver weights and decreased thymus weights were observed to be dose related. At day 7 following treatment, the approximate ED50 values for these responses were 15 and 100 micrograms/kg respectively. The ED50 values for the increase in hepatic P-450 concentrations and activities of ECOD and NMOR ranged from 0.5 to 2.0 micrograms/kg. At 10 and 500 micrograms/kg, NMOR activity remained maximally induced for up to 35 days. This was also the case for P-450 and ECOD activity at a dose of 10 micrograms/kg. At 500 micrograms/kg, both P-450 and ECOD demonstrated an induction up to day 4 followed by a decrease to near control levels by day 14. This decrease appeared to correlate with changes in hepatic morphology. These results demonstrate a dissociation of the induction of these hepatic enzymes from TCDD-induced lethality, in this species.

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.

Porphyria cutanea tarda, or the uroporphyrinogen decarboxylase deficiency diseases

The term "porphyria cutanea tarda" originally described the dermatological manifestations of various chronic porphyrias. Its usage now is usually restricted to disorders associated with a deficiency of uroporphyrinogen decarboxylase (UROD), for which the term "UROD-deficiency" may be more appropriate. Four etiologic agents have been implicated in this condition: alcohol, oral estrogens, halogenated aromatic hydrocarbons, and iron. An inherited deficiency of UROD is also recognized, with increased susceptibility to these agents. Certain halogenated aromatic hydrocarbons can cause UROD-deficiency in animals and synergism with iron is demonstrable in this model. Neither ethanol nor estrogen has been shown to cause UROD-deficiency in animals. Treatment by venesection to reduce total body iron is safe and effective. The 4-aminoquinoline antimalarial drugs also provide effective treatment, possibly by lysis of affected liver cells. Unlike venesection, they may not reverse the biochemical lesion which causes porphyrins to accumulate. The mechanism of acquired UROD-deficiency is not clear but animal studies suggest a role for the hepatic mixed function oxygenases which initiate iron-dependent inactivation of UROD. Diagnosis is simple, often requiring only appropriate clinical data and testing of a random urine sample. Although not common, the disorder is the most frequently diagnosed disturbance of porphyrin metabolism in many countries, and further insight into its unusual pathogenesis may clarify the hepatotoxic effects of the 4 etiologic agents.

Glutathione peroxidase and reactive oxygen species in TCDD-induced lipid peroxidation

Previous studies have shown that high doses of TCDD induce hepatic lipid peroxidation and inhibit selenium dependent glutathione peroxidase (GSH-Px) activity. The dose dependent effects of TCDD on hepatic lipid peroxidation (malondialdehyde content) and GSH-Px activity were determined. A dose as low as 1 microgram/kg induced hepatic lipid peroxidation and inhibited GSH-Px. Based on the use of scavengers of reactive oxygen species, lipid peroxidation (malondialdehyde formation) by hepatic microsomes from both control and TCDD-treated rats appears to be due primarily to H2O2. The results indicate that superoxide, hydroxyl radical and singlet oxygen are also involved. The differences in the reactive oxygen species involved in microsomal lipid peroxidation between control and TCDD treated animals appear to be quantitative rather than qualitative. A 5.9-fold greater rate of malondialdehyde (MDA) formation by microsomes from TCDD treated animals occurred as compared to controls, while livers of TCDD rats had an MDA content that was 5.0-fold greater than the controls. These differences may be due in part to an enhanced production of H2O2 as well as a decrease in the activity of selenium dependent glutathione peroxidase which metabolizes H2O2.

Absence of lipid peroxidation as determined by ethane exhalation in rats treated with 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD)

The exhalation of ethane is widely used as an indicator of in vivo lipid peroxidation. To test the hypothesis that lipid peroxidative events are involved in the toxicity of 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD), we administered a lethal dose of TCDD (60 micrograms/kg), IP to male Sprague-Dawley rats (160-180 g) and measured by gas chromatography the exhalation of ethane into the atmosphere of a closed all-glass exposure chamber. TCDD-treated rats exhaled only slightly more ethane than control rats at a single time point 7 days following TCDD administration. Since the exhalation of ethane is the net result of the endogenous production of the gas and its metabolic degradation, the latter was quantified by measuring the clearance of exogenous ethane (initial concentration = 100 ppm) introduced to the atmosphere of the exposure chamber. The clearance of ethane in TCDD-treated rats was markedly decreased, reaching a minimum 7 days following TCDD treatment. Apparently, the slight increase in exhaled ethane was due to an inhibition of ethane metabolism caused by TCDD. However, rats obviously intoxicated and having lost considerable body weight might be impaired in their ability to transport ethane. To bypass this problem we injected ethane (0.2 ml) directly into the rats IP. Here also the metabolic clearance in TCDD-treated rats was diminished. In a further experiment, rats treated with dithiocarb at a dose where ethane metabolism was totally inhibited exhaled more ethane than did TCDD-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Prooxidant states and tumor promotion

There is convincing evidence that cellular prooxidant states--that is, increased concentrations of active oxygen and organic peroxides and radicals--can promote initiated cells to neoplastic growth. Prooxidant states can be caused by different classes of agents, including hyperbaric oxygen, radiation, xenobiotic metabolites and Fenton-type reagents, modulators of the cytochrome P-450 electron-transport chain, peroxisome proliferators, inhibitors of the antioxidant defense, and membrane-active agents. Many of these agents are promoters or complete carcinogens. They cause chromosomal damage by indirect action, but the role of this damage in carcinogenesis remains unclear. Prooxidant states can be prevented or suppressed by the enzymes of the cellular antioxidant defense and low molecular weight scavenger molecules, and many antioxidants are antipromoters and anticarcinogens. Finally, prooxidant states may modulate the expression of a family of prooxidant genes, which are related to cell growth and differentiation, by inducing alterations in DNA structure or by epigenetic mechanisms, for example, by polyadenosine diphosphate-ribosylation of chromosomal proteins.

Dietary selenium, glutathione peroxidase activity, and toxicity of 2,3,7,8-tetrachloro-dibenzo-p-dioxin

TCDD has been shown to inhibit selenium-dependent glutathione peroxidase activity. The role of selenium in TCDD toxicity is not known. We have therefore examined the effect of TCDD administration on hepatic glutathione peroxidase, aryl hydrocarbon hydroxylase, glutathione reductase, and glutathione S-transferase activities, glutathione content, and lipid peroxidation in rats fed 0, 0.10, and 2.0 ppm dietary selenium. TCDD treatment significantly inhibited selenium-dependent glutathione peroxidase in animals on diets containing 0.10 and 2.0 ppm selenium. The selenium-dependent glutathione peroxidase activities in rats on 0.10 and 2.0 ppm dietary selenium were 8.3-and 4.7-fold greater than in animals fed a diet containing 0 ppm selenium. TCDD administration enhanced hepatic microsomal lipid peroxidation by factors of 4.0, 4.9, and 9.8 in animals fed diets containing 0, 0.10, and 2.0 ppm selenium, respectively. The administration of a lethal dose of TCDD to rats fed diets containing 0, 0.10, and 2.0 ppm selenium resulted in 0, 46, and 7% survival, respectively, after 66 d. Aryl hydrocarbon hydroxylase, glutathione S-transferase, and glutathione reductase activities were induced by TCDD. The results indicate that optimum dietary selenium provides partial protection from the toxic effects of TCDD.