Sex-related difference in hepatic glutathione conjugation of hexachlorobenzene in the rat

Glutathione Conjugation and Protein Adduction by Environmental Pollutant 2,4-Dichlorophenol In Vitro and In Vivo

2,4-Dichlorophenol (2,4-DCP), an environmental pollutant, was reported to cause hepatotoxicity. The biochemical mechanisms of 2,4-DCP induced liver injury remain unknown. The present study showed that 2,4-DCP is chemically reactive and spontaneously reacts with GSH and bovine serum albumin to form GSH conjugates and BSA adducts. The observed conjugation/adduction apparently involved the addition of GSH and departure of chloride via the ipso substitution pathway. Two biliary GSH conjugates and one urinary N-acetyl cysteine conjugate were observed in rats given 2,4-DCP. The N-acetyl cysteine conjugate was chemically synthesized and characterized by mass spectrometry and NMR. As expected, 2,4-DCP was found to modify hepatic protein at cysteine residues in vivo by the same chemistry. The observed protein adduction reached its peak at 15 min and revealed dose dependency. The new findings allowed us to better understand the mechanisms of the toxic action of 2,4-DCP.

The effect of nutrition on chemical toxicity

The toxicity of chemicals, and of reactive oxygen species (ROS), are both affected by nutrition and diet. Calorific excess (continuous feeding), or deficiency (fasting), may increase production of ROS, which are also formed by interaction of toxic chemicals with cytochromes P450 (CYP2E or futile cycling). Both ROS (GSH reductase and peroxidase) and toxic chemicals (S-transferases) are detoxified by GSH enzymes; ROS are scavenged by a system comprising GSH, ascorbic acid and tocopherols, which may be regenerated by NADPH. Dietary protein is necessary for GSH or enzyme replacement, lipids are required for polyunsaturated fatty acids (PUFAs) and prostanoid biosynthesis, lipotropes and phospholipids for synthesis of endoplasmic reticulum, and folate is needed for dug metabolizing activity. Among required minerals, Se is necessary as the essential component of the antioxidant enzyme, glutathione peroxidase. Other dietary factors considered are the natural toxicants, gossypol, lathyrogens, glucosinolates, and saponins, and toxicants from food spoilage, food intoxication and food processing.

Failure of the standard battery of short-term tests in detecting some rodent and human genotoxic carcinogens

Theoretical reasons and experimental evidence indicate that a no-effect level generally cannot be expected for genotoxic carcinogens; as a consequence, in quantitative risk assessment the capability of distinguishing genotoxic from non-genotoxic carcinogens is of fundamental importance in order to identify relevant levels of human exposure. According to generally accepted guidelines, the standard three-test battery for the detection of genotoxic compounds consists of: (i) an in vitro test for gene mutation in bacteria; (ii) an in vitro test in mammalian cells with cytogenetic evaluation of chromosomal damage and/or a test that detects gene mutations; (iii) an in vivo test for chromosomal damage using rodent hematopoietic cells. This test battery is designed to avoid the risk of false negative results for compounds with genotoxic potential, but it cannot be taken for granted that the risk is completely eliminated. As a matter of fact there are some chemicals, classified by the International Agency for Research on Cancer (IARC) as probably or possibly carcinogenic to humans, which gave consistent negative results in this test battery, and in contrast provided positive results in other not routinely employed genotoxicity assays. The failure of the standard test battery in detecting some genotoxic carcinogens is attributable to several causes, but the principal of them are the following ones: in vitro, the artificial metabolic activity of the liver S9-mix, and the different biotransformation of chemicals in cells of different type and from different animal species; in vivo, the pharmacokinetic behaviour of the test compound, and its possible species-, sex- and tissue-specificity.

Effect of estradiol on the induction of porphyria by hexachlorobenzene in the rat

Hexachlorobenzene (HCB) is porphyrinogenic in adult female but not in male rats. This study aimed to assess the role of 17beta-estradiol in the induction of porphyria by HCB in both sexes by adding or removing the hormone. Groups of intact females, ovariectomized females (Ova), castrated males (Cas), and Cas receiving 17beta-estradiol (4 mg/kg, i.m., once a week beginning 2 weeks prior to HCB) were given five consecutive daily doses of HCB (100 mg/kg in corn oil, p.o.). Porphyria was assessed by urinary uroporphyrin excretion measured at days 16, 31, 38, 45, 52, 59, and 87. The percentage of porphyric rats in intact females increased from day 31 (58%) to day 87 (75%), whereas none of the Ova or Cas rats responded. However, administration of estradiol (days 120-169) and another sequence of HCB doses (days 134-138) to the same Ova rats caused porphyria (50% at day 186). Cas rats given estradiol also developed porphyria (43 and 86% on days 31 and 87, respectively). HCB-treated Ova rats given two doses of estradiol at either days 1 and 8 or days 22 and 29 developed a porphyria of similar magnitude (day 52). The role of estradiol cannot be explained by a reduction of pentachlorothiophenol formation, a putative detoxication pathway. Overall, results show that both sexes have the ability to respond to HCB when 17beta-estradiol is present and suggest that the sexual dimorphism in HCB-induced porphyria in the rat is related to the hormonal status.

Comparative biotransformation of hexachlorobenzene and hexafluorobenzene in relation to the induction of porphyria

The porphyrinogenic action of hexafluorobenzene was investigated and compared to that of hexachlorobenzene. Metabolite patterns in the urine of exposed rats were determined to quantify the extent of metabolism through cytochrome P450 catalysed oxidation and glutathione conjugation. Results obtained demonstrate an almost similar extent of formation of phenolic metabolites. However, in the urine of hexachlorobenzene exposed rats significantly higher levels of the N-acetyl-S-(pentahalophenyl)cysteine were observed than in the urine of hexafluorobenzene exposed rats. Hexafluorobenzene exposure did not result in induction of porphyria, whereas exposure to hexachlorobenzene did result in significantly elevated levels of urinary as well as liver porphyrins. Together these results indicate that if the reactive intermediate is indeed formed in the cytochrome P450 catalysed initial oxidative dehalogenation, the extent of its formation as well as its subsequent reactivity and reaction pathways vary with the type of the halogen substituents. Furthermore, the results seem to indicate that the extent of metabolism of hexahalogenated benzenes into urinary metabolites resulting from glutathione conjugation is a better indication of their porphyrinogenic action than their extent of metabolism to phenolic metabolites. Two explanations for this observation are presented.

Protein variability in male and female Wistar rat liver proteins

In the emerging field of mechanistic toxicology, growing attention is being paid to the interpretation of sex-related toxic responses. The Wistar rat outbred stock is a frequently used rodent for toxicity testing. Outbred strains, which display a relatively high degree of genetic variability between the individual animals are often used in risk assessment, as they are considered to best approximate the variability present in a human population. In this study two-dimensional (2-D) protein gel electrophoresis was applied to investigate the liver protein patterns of male and female Wistar rats and to search for (i) sex-related and (ii) interindividual qualitative and quantitative differences in protein expression. Among the sexes, six proteins were detected that were shown to be exclusively present in male rats and one that was present only in females. A male-specific protein was tentatively assigned to alpha 2u globulin. Seven protein spots showed statistically significant abundance changes (p < 0.001) between males and females, one was tentatively assigned to heme oxygenase 1 and another to the 23 kDa morphin-binding protein. Four sets of protein spots were detected that showed positional shifts in the individual patterns and are likely to represent polymorphic proteins inherent in the Wistar rat. These results form a valuable basis for future investigations of drug-induced changes in the male and female Wistar rat liver pattern.

The effect of nicotine and its interaction with carbon tetrachloride in the rat liver

In order to study the effects of nicotine on liver, groups of rats were given nicotine doses that simulated those seen in chronic smoking (54 and 108 mumol/l of nicotine) for 10 days. A subgroup was also given a single subcutaneous injection of 6 g/kg of carbon tetrachloride (CCl4) shortly before the animals of the group were killed. Histology demonstrated a significant hepatotoxic effect in the group receiving 108 mumol/l of nicotine when compared with the control group in the form of fatty change, focal or confluent necrosis and dark-cell change. The effects in pregnant rats were less severe. Carbon tetrachloride alone induced significant fatty change and focal necrosis in non-pregnant rats but not in pregnant rats. Nicotine also aggravated the CCl4 induced pathological changes in livers of both non-pregnant and pregnant animals. Thus nicotine alone, when given at a concentration of 108 mumol/l, exerted hepatotoxic effects; the alkaloid also aggravated the hepatotoxicity of CCl4. Pregnant rats were more resistant to the hepatotoxic effects produced by nicotine and CCl4.