Mechanisms of toxicity and carcinogenicity of three alkylanilines

Abstract

Alkyl-substituted anilines have been implicated as important etiological agents in human carcinogenesis. Specifically, 2,6-dimethylaniline (2,6-DMA), 3,5-dimethylaniline (3,5-DMA), and 3-ethylaniline (3EA) have been associated with an increased risk of human bladder cancer, independent of cigarette smoking, in a published case-control study. Understanding the metabolic activation of and DNA adduct formation by these chemicals is an important first step in elucidating their mechanisms of carcinogenesis and toxicity. Cytochrome P450-mediated metabolism was profiled based on the hypothesis that N-hydroxylated metabolites are critical intermediates in the formation of DNA adducts. This work was extended to assess in vitro DNA adduct formation with the cell-free and cell-based assays. Accelerator Mass Spectrometry (AMS) was used for detection and semi-quantification of DNA adducts formed by 14C-labeled alkylanilines. Data indicated 3,5-DMA formed high levels of DNA adducts, suggesting that it is a potent carcinogen. Additionally, the levels of adducts exhibited inter-species variation. The effects of phase II metabolism on adduct formation were evaluated by comparing the results obtained from the two types of assays and by assessing the effects of phase II enzyme cofactors on the results of cell-free assay.

(cont.) Results implied that sulfotransferase-mediated metabolism promotes cytotoxicity and mutagenicity of all three alkylanilines; however, glucuronidation may provide a protective mechanism. The effects of N-acetyltransferase-mediated metabolism on DNA adduct formation differed for the three alkylanilines; acetyl-CoA enhanced adduct formation by 3-EA and 2,6-DMA, but it reduced 3,5-DMA adduct formation. Human CYP2A6 universally catalyzed the oxidation of all structural isomers of dimethylanilines and ethylanilines, except 3-EA. In the present work, the hypothesis that 3-EA is a mechanism-based inactivator toward human P450 2A6 through covalent binding was examined by using AMS. 3-EA was characterized as a mechanism-based inactivator with a Ki of 34 !iM and a kinact of 0.055 min'. Results suggest that 3-EA might be involved in more than one biological effect in the human body through multiple pathways. Adduct formation and inhibition of CYP 2A6 by 3-EA might shift the biological effects of other compounds activated by CYP 2A6 dynamically and kinetically while appearing in the biological systems simultaneously.