Hepatic microsomal p-nitroanisole O-demethylase. Effects of chlordecone or mirex induction in male and female rats

DDT and Other Chlorinated Insecticides

The use of organochlorine insecticides such as DDT, lindane and cyclodieneshas declined markedly worldwide over the last decades. Most are now banned or not used. At an acute toxicity level they have been relatively safe in use for humans. However, the greatest concerns are their persistence in people, wildlife and the environment due to their slow metabolism. Although their carcinogenicity for humans has not been supported by strong epidemiological evidence, their potential to be modulators of endocrine and immune function at levels remaining in the environment or associated with residual spraying of DDT continue to be of concern. At present, DDT is still allowed by the United Nations for combating malaria, with continual monitoring and assessment where possible. The toxicological consequences of exposure of animals and people to DDT is discussed as well as some analogues and other insecticides such as lindane, dieldrin and chlordecone that, although little used, continue to persist in surroundings and people. Because of circumstances of world health brought about by climate change or human activities that have yet to develop, there may come a time when the importance of some may re-emerge.

Sex-dependent metabolism of xenobiotics

Sex-dependent differences in xenobiotic metabolism have been most extensively studied in the rat. Because sex-dependent differences are most pronounced in rats, this species quickly became the most popular animal model to study sexual dimorphisms in xenobiotic metabolism. Exaggerated sex-dependent variations in metabolism by rats may be the result of extensive inbreeding and/or differential evolution of isoforms of cytochromes P450 in mammals. For example, species-specific gene duplications and gene conversion events in the CYP2 and CYP3 families have produced different isoforms in rats and humans since the species division over 80 million years ago. This observation can help to explain the fact that CYP2C is not found in humans but is a major subfamily in rats (Table 11). Animal studies are used to help determine the metabolism and toxicity of many chemical agents in an attempt to extrapolate the risk of human exposure to these agents. One of the most important concepts in attempting to use rodent studies to identify sensitive individuals in the human population is that human cytochromes P450 differ from rodent cytochromes P450 in both isoform composition and catalytic activities. Xenobiotic metabolism by male rats can reflect human metabolism when the compound of interest is metabolized by CYP1A or CYP2E because there is strong regulatory conservation of these isoforms between rodents and humans. However, problems can arise when rats are used as animal models to predict the potential for sex-dependent differences in xenobiotic handling in humans. Information from countless studies has shown that the identification of sex-dependent differences in metabolism by rats does not translate across other animal species or humans. The major factor contributing to this observation is that CYP2C, a major subfamily in rats, which is expressed in a sex-specific manner, is not found in humans. To date, sex-specific isoforms of cytochromes P450 have not been identified in humans. The lack of expression of sex-dependent isoforms in humans indicates that the male rat is not an accurate model for the prediction of sex-dependent differences in humans. Differences in xenobiotic metabolism among humans are more likely the consequence of intraindividual variations as a result of genetics or environmental exposures rather than from sex-dependent differences in enzyme composition. A major component of the drug discovery and development process is to identify, at as early a stage as possible, the potential for toxicity in humans. Earlier identification of individual differences in xenobiotic metabolism and the potential for toxicity will be facilitated by improving techniques to make better use of human tissue to prepare accurate in vitro systems such as isolated hepatocytes and liver slices to study xenobiotic metabolism and drug-induced toxicities. Accurate systems should possess an array of bioactivation enzymes similar to the in vivo expression of human liver. In addition, the compound concentrations and exposure times used in these in vitro test systems should mimic those achieved in the target tissues of humans. Consideration of such factors will allow the development of compounds with improved efficacy and low toxicity at a more efficient rate. The development of accurate in vitro systems utilizing human tissue will also aid in the investigation of the molecular mechanisms by which the CYP genes are regulated in humans. Such studies will facilitate the study of the basis for differences in expression of isoforms of CYP450 in humans.

Induction of cytochrome P-450 in the Norway rat, Rattus norvegicus, following exposure to potential environmental contaminants

Cytochrome P-450 (CYP) induction (consisting of increases in cellular RNA and protein content and associated catalytic activities) occurs predominantly in the liver, but also in small intestine, lung, kidney, and placenta, of Norway rats (Rattus norvegicus) exposed to certain types of potential environmental contaminants. The specific isoform(s) induced in the rat and the magnitudes of the increases observed depend upon the chemical nature of the xenobiotic. For instance, the predominant isoforms induced by nonhalogenated polycyclic aromatic hydrocarbons, such as petroleum derivatives and coal-tar constituents such as the benzopyrenes and the anthracenes, are those of the CYP1A subfamily. Polyhalogenated aromatic hydrocarbons, such as the halogenated dibenzodioxins, dibenzofurans, and biphenyls, may cause the induction of predominantly the CYP1A subfamily, predominantly the CYP2B subfamily, or mixed CYP1A- and CYP2B-type induction, depending upon the halogen substitution pattern. In contrast, the chlorinated hydrocarbon pesticides, such as DDT, dieldrin, chlordane, and mirex, cause almost exclusively the induction of isoforms of the CYP2B (and to a lesser extent the CYP3A) subfamilies. The commonly employed plasticizing agent di-(2-ethylhexyl)phthalate elicits predominantly induction of the CYP4A subfamily. Those xenobiotics that would be expected to be the most pervasive environmental contaminants are typically those that have also been found to cause the most profound CYP induction responses. Such chemicals are extremely lipophilic and tend to accumulate in animal tissues, especially fatty tissues such as the liver. The hepatic CYP induction response to such potential environmental contaminants is typical of the animals' response to lipophilic xenobiotics in general, and serves as a mechanism by which the excretion of such compounds from the body is facilitated.

Influence of chlordecone and mirex exposure on benzo[a]pyrene metabolism of rat-liver microsomes

The metabolism of benzo[a]pyrene (BP) in hepatic microsomes isolated from rats exposed to chlordecone or mirex was compared to that of untreated rats and rats treated with 3-methylcholanthrene (3-MC) or phenobarbital (PB). Treatment with chlordecone resulted in a two- to three-fold increase in cytochrome P-450 content but the BP-hydroxylase activity per mg microsomal protein was unaffected. Addition of alpha-naphthoflavone (alpha-NF) or chlordecone caused changes in BP-hydroxylase activity indicating that chlordecone-induced cytochromes P-450 were similar to control. H.p.l.c. analyses of BP metabolites confirmed this similarity. Treatment with mirex caused a two-fold induction of cytochrome P-450, and BP-hydroxylase activity expressed per mg microsomal protein was increased 1.3-fold. Addition of chlordecone or alpha-NF caused changes in BP-hydroxylase activity, indicating differences between control and mirex-induced cytochromes P-450. H.p.l.c. analyses of BP metabolites confirmed this difference. Treatment with chlordecone or mirex increased microsomal epoxide hydrolase activity three-fold. Chlordecone accumulated in hepatic nuclei.

Sex differences in the effects of microsomal enzyme inducers on hepatic phase I drug metabolism in the rat

This study investigates the sex-dependence of the effects of microsomal enzyme inducers (phenobarbitone, isosafrole and ethanol) on the hepatic phase I metabolism of lignocaine and imipramine. It is shown that all of the inducers exert sex-dependent effects on the enzymes activities known to be sex related in the rat, e.g. lignocaine N-deethylase activity is decreased by phenobarbitone pretreatment in the male but increased by the same treatment in the female. The inducers tend to decrease the sex differences seen in untreated animals. Ethanol may give this effect by its action of decreasing serum testosterone levels but the mechanism of action of the other compounds is uncertain. It is possible that the sex-dependent cytochrome P-450 species are selectively sensitive to the action of the compounds in terms of induction, repression or inhibition. It is clear, however, that the effects of the pretreatments are related to the sex differences in phase I metabolism in the rat.