nitrophenols and 4-nitrosodimethylaniline

Pyridine has been shown to cause liver and kidney damage in animals and in humans. In a previous study we examined the effects of pyridine on rabbit liver and lung microsomal drug-metabolizing enzymes. In this study, in vivo i.p. administration of pyridine to rabbits caused a significant 3.4-fold increase in kidney N-nitrosodimethylamine (NDMA) N-demethylase activity as compared to the activity in control rabbits. The same treatment also significantly stimulated the activity of other cytochrome P4502E1-associated enzymes. The activities of p-nitrophenol hydroxylase and aniline 4-hydroxylase in kidney microsomes were increased 4.9-and 4.5-fold, respectively. Pyridine treatment increased the P450 content of the kidney 1.6-fold (P<0.05). SDS-PAGE of both kidney and liver microsomes of pyridine-treated rabbits showed a protein band of enhanced intensity at 51,000 Mr migrating in the region of cytochrome P4502E1. p-Aminophenol, a 4-hydroxylation product of aniline, has been shown to be nephrotoxic and NDMA, a procarcinogen, has been shown to be carcinogenic following bioactivation by NDMA N-demethylase in a number of tissues including the kidney. Since pyridine was shown to be nephrotoxic, it is expected that pyridine potentiates the toxic and/or carcinogenic effects of aniline, p-nitrophenol and NDMA through induction of their metabolism by the cytochrome P450-dependent drug-metabolizing enzymes.

Topics: Aniline Compounds; Aniline Hydroxylase; Animals; Cytochrome P-450 CYP2E1; In Vitro Techniques; Kidney; Liver; Male; Microsomes; Nitrophenols; Nitroso Compounds; Oxidoreductases, N-Demethylating; Pyridines; Rabbits; Stimulation, Chemical

GM0637, a human fibroblast cell line, was transfected with pCMV2E1, an expression vector containing the full length cDNA for rat cytochrome P450 2E1 (P450 2E1), and with pCMVneo, which contained vector alone, and the selected clones were designated GM2E1 and GMneo, respectively. Western blot analysis showed that GM2E1, but not GMneo, expressed a protein that reacted with anti-human P450 2E1 antibody. The 7-ethoxycoumarin O-deethylase,p-nitrophenol hydroxylase, and N-nitrosodimethylamine (NDMA) demethylase activities of the P450 in these cells were measured in monolayer cell cultures without preparing microsomes. Exposure of the GM2E1 cells to NDMA for 4 days caused severe decreases in cell viability, as determined by crystal violet uptake, and showed a sigmoidal dose-response curve with a median lethal dose of 17 microM. In contrast, the viability of GMneo cells was not altered by NDMA even at concentrations up to 10 mM. Time- and concentration-dependent methylation of DNA, RNA and protein by [14C]NDMA was only observed in cells expressing P450 2E1. Inhibitors of P450 2E1 activity such as ethanol, 4-methylpyrazole, and isoniazid caused a 90% decrease in the methylation of cellular macromolecules and also completely protected the cells against NDMA-mediated toxicity. The cytotoxicity due to exposure to NDMA was partially inhibited by antioxidants such as N-acetylcysteine, ascorbic acid, butylated hydroxyanisole and N-t-butyl-alpha-phenylnitrone but was not potentiated upon glutathione depletion. These results document the ability of rat P450 2E1 to metabolize NDMA to toxic reactive intermediates and demonstrate that this cell line provides a useful model for studying the mechanisms of metabolism-mediated toxicity and carcinogenesis.

Topics: Animals; Antioxidants; Blotting, Western; Buthionine Sulfoximine; Cell Line; Cell Survival; Coumarins; Cyclic N-Oxides; Cytochrome P-450 CYP2E1; Dinitrochlorobenzene; Dose-Response Relationship, Drug; Enzyme Inhibitors; Ethanol; Fibroblasts; Fomepizole; Glutathione Transferase; Humans; Indicators and Reagents; Isoniazid; Nitrogen Oxides; Nitrophenols; Nitroso Compounds; Pyrazoles; Rats; Spin Labels; Time Factors; Transfection

N-Nitrosomethyl(2-hydroxyethyl)amine (NMHEA), when administered by gavage, is a strong liver carcinogen in F344 female rats, but a weak liver carcinogen in male rats. After repeated exposure to NMHEA, either in drinking water or by gavage, female rats accumulated higher levels of DNA-guanine adducts than did their male counterparts, suggesting a correlation with the observed disparity in carcinogenicity. NMHEA has been shown to alkylate rat liver DNA in vivo in a dose-dependent manner. Chemical investigations of NMHEA suggest that it becomes a strong electrophile when a good leaving group is substituted on the hydroxyl. We have proposed that NMHEA is activated to its ultimate carcinogenic form by conjugation with sulfate. The sulfate ester was postulated to undergo rapid cyclization to 3-methyl-1,2,3-oxadizolinium ion, which has previously been found to be a potent methylating agent in vitro. The effect of sulfotransferase inhibitors on the DNA alkylation in rats by NMHEA was studied in vivo. Dichloronitrophenol, a powerful inhibitor of phenol sulfotransferase, had little effect on the methylation and O6-hydroxyethylation of DNA guanine in female rats, while depressing the hydroxyethylation of the N-7 position of guanine. Dichloronitrophenol, however, dramatically enhanced the methylation of DNA in male rats. It also slightly inhibited the N-nitrosodimethylamine-induced methylation of DNA. On the other hand, propylene glycol, an alcohol sulfotransferase inhibitor, had a profound inhibitory effect on DNA methylation induced by NMHEA, very little effect on the formation of N7-(2-hydroxyethyl)guanine, but a very strong effect on the O6-hydroxyethylguanine lesions. NMHEA-induced alkylation was also studied in male and female brachymorphic mice, which are deficient in the ability to synthesize the sulfate donor 3'-phosphoadenosine 5'-phosphosulfate required for sulfotransferase activity, and their heterozygous siblings. No significant differences were seen between the heterozygous and brachymorphic mice in overall levels of alkylation, except in the case of 7-hydroxyethylation. In contrast to rats, male mice showed higher levels of formation of all DNA guanine adducts than did the females. However, propylene glycol was found to depress all the levels of alkylation in the brachymorphic mice, except for N7-(2-hydroxyethyl)guanine, as was observed in rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Topics: Alkylation; Animals; Biotransformation; Dimethylnitrosamine; DNA; Female; Guanine; Hydrolysis; Liver; Male; Nitrophenols; Nitroso Compounds; Rats; Rats, Inbred F344; Sex Factors; Sulfotransferases