Tissue specific regulation of renal N-nitrosodimethylamine-demethylase activity by testosterone in BALB/c mice

Improved redox anti-cancer treatment efficacy through reactive species rhythm manipulation

Rhythms in the pseudo-steady state (PSS) levels of reactive species (RS), particularly superoxide and hydroxyl radicals, exist in cancer cells. The RS rhythm characteristics, particularly frequency and amplitude, are entrained (reset) by the anticancer compounds/drugs. In this work, we show for the first time that the phase of the RS rhythm at which the drug is added is significantly important in determining the cytotoxicity of anticancer compounds/drugs such as menadione and curcumin, in two different cancer cell lines. Curcumin, the more effective of the two drugs (IC₅₀ = 15 µM, SiHa; 6 µM, HCT116) induced reset of superoxide and hydroxyl rhythms from 15.4 h to 9 h, and 25 h to 11 h respectively, as well as caused increases in these radical levels. However, menadione (IC₅₀ = 20 µM, SiHa; 17 µM, HCT116) affected only the superoxide levels. Drug treatment at different time points/phase of the RS rhythm resulted in a maximum of 27% increase in cytotoxicity, which is significant. Further, we report for the first time, an unexpected absence of a correlation between the intracellular PSS RS and antioxidant levels; thus, the practice of using antioxidant enzyme levels as surrogate markers of intracellular oxidative stress levels may need a re-consideration. Therefore, the RS rhythm could be a fundamental/generic target to manipulate for improved cancer therapy.

Application of an updated physiologically based pharmacokinetic model for chloroform to evaluate CYP2E1-mediated renal toxicity in rats and mice

Physiologically based pharmacokinetic (PBPK) models are tools for interpreting toxicological data and extrapolating observations across species and route of exposure. Chloroform (CHCl(3)) is a chemical for which there are PBPK models available in different species and multiple sites of toxicity. Because chloroform induces toxic effects in the liver and kidneys via production of reactive metabolites, proper characterization of metabolism in these tissues is essential for risk assessment. Although hepatic metabolism of chloroform is adequately described by these models, there is higher uncertainty for renal metabolism due to a lack of species-specific data and direct measurements of renal metabolism. Furthermore, models typically fail to account for regional differences in metabolic capacity within the kidney. Mischaracterization of renal metabolism may have a negligible effect on systemic chloroform levels, but it is anticipated to have a significant impact on the estimated site-specific production of reactive metabolites. In this article, rate parameters for chloroform metabolism in the kidney are revised for rats, mice, and humans. New in vitro data were collected in mice and humans for this purpose and are presented here. The revised PBPK model is used to interpret data of chloroform-induced kidney toxicity in rats and mice exposed via inhalation and drinking water. Benchmark dose (BMD) modeling is used to characterize the dose-response relationship of kidney toxicity markers as a function of PBPK-derived internal kidney dose. Applying the PBPK model, it was also possible to characterize the dose response for a recent data set of rats exposed via multiple routes simultaneously. Consistent BMD modeling results were observed regardless of species or route of exposure.

A Framework for Human Relevance Analysis of Information on Carcinogenic Modes of Action

The human relevance framework (HRF) outlines a four-part process, beginning with data on the mode of action (MOA) in laboratory animals, for evaluating the human relevance of animal tumors. Drawing on U.S. EPA and IPCS proposals for animal MOA analysis, the HRF expands those analyses to include a systematic evaluation of comparability, or lack of comparability, between the postulated animal MOA and related information from human data sources. The HRF evolved through a series of case studies representing several different MOAs. HRF analyses produced divergent outcomes, some leading to complete risk assessment and others discontinuing the process, according to the data available from animal and human sources. Two case examples call for complete risk assessments. One is the default: When data are insufficient to confidently postulate a MOA for test animals, the animal tumor data are presumed to be relevant for risk assessment and a complete risk assessment is necessary. The other is the product of a data-based finding that the animal MOA is relevant to humans. For the specific MOA and endpoint combinations studied for this article, full risk assessments are necessary for potentially relevant MOAs involving cytotoxicity and cell proliferation in animals and humans (Case Study 6, chloroform) and formation of urinary-tract calculi (Case Study 7, melamine). In other circumstances, when data-based findings for the chemical and endpoint combination studied indicate that the tumor-related animal MOA is unlikely to have a human counterpart, there is little reason to continue the risk assessment for that combination. Similarly, when qualitative considerations identify MOAs specific to the test species or quantitative considerations indicate that the animal MOA is unlikely to occur in humans, such hazard findings are generally conclusive and further risk assessment is not necessary for the endpoint-MOA combination under study. Case examples include a tumor-related protein specific to test animals (Case Study 3, d-limonene), the tumor consequences of hormone suppression typical of laboratory animals but not humans (Case Study 4, atrazine), and chemical-related enhanced hormone clearance rates in animals relative to humans (Case Study 5, phenobarbital). The human relevance analysis is highly specific for the chemical-MOA-tissue-endpoint combination under analysis in any particular case: different tissues, different endpoints, or alternative MOAs for a given chemical may result in different human relevance findings. By providing a systematic approach to using MOA data, the HRF offers a new tool for the scientific community's overall effort to enhance the predictive power, reliability and transparency of cancer risk assessment.

Chloroform: exposure estimation, hazard characterization, and exposure-response analysis

Chloroform has been assessed as a Priority Substance under the Canadian Environmental Protection Act. The general population in Canada is exposed to chloroform principally through inhalation of indoor air, particularly during showering, and through ingestion of tap water. Data on concentrations of chloroform in various media were sufficient to serve as the basis for development of deterministic and probabilistic estimates of exposure for the general population in Canada. On the basis of data acquired principally in studies in experimental animals, chloroform causes hepatic and renal tumors in mice and renal tumors in rats. The weight of evidence indicates that chloroform is likely carcinogenic only at concentrations that induce the obligatory precursor lesions of cytotoxicity and proliferative regenerative response. Since this cytotoxicity is primarily related to rates of formation of reactive, oxidative metabolites, dose response has been characterized in the context of rates of formation of reactive metabolites in the target tissue. Results presented here are from a "hybrid" physiologically based pharmacokinetic (PBPK) animal model that was revised to permit its extension to humans. The relevant measure of exposure response, namely, the mean rate of metabolism in humans associated with a 5% increase in tumor risk (TC05), was estimated on the basis of this PBPK model and compared with tissue dose measures resulting from 24-h multimedia exposure scenarios for Canadians based on midpoint and 95th percentiles for concentrations in outdoor air, indoor air, air in the shower compartment, air in the bathroom after showering, tap water, and food. Nonneoplastic effects observed most consistently at lowest concentrations or doses following repeated exposures of rats and mice to chloroform are cytotoxicity and regenerative proliferation. As for cancer, target organs are the liver and kidney. In addition, chloroform has induced nasal lesions in rats and mice exposed by both inhalation and ingestion at lowest concentrations or doses. The mean rate of metabolism associated with a 5% increase in fatty cysts estimated on the basis of the PBPK model was compared with tissue dose measures resulting from the scenarios already described, and lowest concentrations reported to induce cellular proliferation in the nasal cavities of rats and mice were compared directly with midpoint and 95th percentile estimates of concentrations of chloroform in indoor air in Canada. The degree of confidence in the underlying database and uncertainties in estimates of exposure and in characterization of hazard and dose response are delineated.

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.

Maintenance and activation of Cyp2e-1 gene expression in mouse hepatocytes in primary culture

The expression of Cyp2e-1 mRNA and protein was investigated in the C57BL/6NCrj mouse hepatocytes in primary culture, as well as liver and kidney. The mRNA and protein expression in the liver was in the same range in both sexes and was not affected by orchiectomy or ovariectomy. The mRNA expression was enhanced in the kidney of ovariectomized mice, in which the protein contents were not influenced. Orchiectomy decreased the expression of both mRNA and protein. When the hepatocytes were transferred to primary culture, the amounts of the mRNA were not changed within 24 h and about half remained by day 3. However, the expression was low thereafter. The expression of the protein gradually decreased after the start of culture. Dexamethasone showed a potential as an inducer at more than 10(-8) M. Sex hormones increased the expression of this P-450 species a little in culture, but growth hormone did not. These observations indicated that glucocorticoid hormone plays a role in modifying expression of Cyp2e-1 and that the mouse hepatocyte culture is useful for examining its regulation mechanism.

Bioactivation of halogenated hydrocarbons by cytochrome P4502E1

Numerous halogenated hydrocarbons of the alkane, alkene, and alkyne classes are metabolized by P450 enzymes to products that elicit cytotoxic and/or carcinogenic effects. Such halogenated hydrocarbons include anesthetics (e.g., halothane and enflurane) and industrial solvents (e.g., carbon tetrachloride, chloroform, and vinylidine chloride). Formation of reaction intermediates from these compounds occurs via P450-promoted dehalogenation, reduction, or reductive oxygenation, with certain hydrocarbons undergoing all three reaction types. Of the multiple forms of P450 present in liver microsomes, P4502E1 has been identified as the primary catalyst of hydrocarbon bioactivation in animals and, most likely, in humans as well. As hepatic concentrations of this P450 enzyme are highly inducible by ethanol and similar agents, prior exposure to 2E1-inducing compounds can play a pivotal role in halogenated hydrocarbon toxicity. Considering that metabolism governs the cytotoxicity and carcinogenicity of halogenated hydrocarbons, an understanding of the mechanism(s) underlying 2E1 induction in man becomes all the more important.

Hormonal regulation of rat renal cytochrome P450s by androgen and the pituitary

The hormonal regulation of rat renal cytochrome P450s, P450 4A2 (K-5) and K-2, was investigated. The level of P450 4A2 in male rats was five times that in female rats and accounted for some 90% of total cytochrome P450, measured photometrically. Lauric acid omega- and (omega-1)-hydroxylation activities of renal microsomes of male rats were also higher than those of female rats. The sex differences in lauric acid hydroxylation activity seemed to arise from the differences in P450 4A2 concentrations, according to an immunochemical study. P450 K-2 was a female-dominant form in rat kidneys. The level of P450 K-2 in renal microsomes of male rats was one-tenth that of P450 4A2. Castration of male rats decreased the levels of P450 4A2 and treatment of castrated male rats with testosterone reversed the decrease. The castration of male rats decreased the lauric acid hydroxylation of the renal microsomes to the level of female rats. The administration of testosterone to castrated male rats reversed the decrease. Hypophysectomy of male rats decreased the level of P450 4A2 and the administration of growth hormone reversed the decrease when intermittent injections mimicking the male secretory pattern were given, although continuous administration mimicking the female secretory pattern did not. Castration of male rats did not affect the level of P450 K-2, but testosterone decreased its level. Hypophysectomy of male rats increased the level of P450 K-2 and growth hormone decreased its level in hypophysectomized rats. These results suggested that the expression of P450 4A2 was regulated by androgen or growth hormone and regulation of P450 4A2 was different from that of P450 K-2. To explore the regulation of renal cytochrome P450 further, testosterone was given to control (intact) or hypophysectomized adult female rats. P450 4A2 was induced in the kidneys of both control and hypophysectomized female rats to close to the level of male rats. Thus, P450 4A2 was directly regulated by testosterone as well as growth hormone, and the regulation of the male-dominant form in rat kidneys was different from that of the male-specific form in the rat liver, which is regulated mostly by growth hormone.

Mouse renal cytochrome P450IIE1: immunocytochemical localization, sex-related difference and regulation by testosterone

Cytochrome P450IIE1 is responsible for the metabolic activation of N-nitrosodimethylamine and a variety of other chemicals. Renal P450IIE1 was shown previously to be regulated by testosterone in C3H/HeJ and BALB/c mice. The present study investigated the distribution of cytochrome P450IIE1 in the kidneys of C3H/HeJ and BALB/c mice. The amount of P450IIE1 was immunotitrated by immunohistochemistry using polyclonal antibodies against rat P450IIE1. Strong immunoreactivity was identified mainly in the cortical tubules, including proximal tubules and some tubules. Weak immunoreactivity was also observed in the outer medulla when higher concentrations of antibodies were used. Much higher immunostaining was observed in male mice than in female mice when identical antibody dilutions were used. The renal P450IIE1 level in females was elevated to the same level as that in males 24 hr after administration of testosterone. The results showed a specific cellular localization of cytochrome P450IIE1 in mouse kidney. The findings may lead to a better understanding of the site-specific renal toxicity and carcinogenesis due to the activation of chemicals by cytochrome P450IIE1.