Identification of a genotoxic mechanism for the carcinogenicity of the environmental pollutant and suspected human carcinogen o ‐anisidine

2‐methoxyaniline ( o ‐anisidine) is an industrial and environmental pollutant and a bladder carcinogen for rodents. The mechanism of its carcinogenicity was investigated with 2 independent methods, 32 P‐postlabeling and 14 C‐labeled o ‐anisidine, to show that o ‐anisidine binds covalently to DNA in vitro after its activation by human hepatic microsomes. We also investigated the capacity of o ‐anisidine to form DNA adducts in vivo . Rats were treated i.p. with o ‐anisidine (0.15 mg/kg daily for 5 days) and DNA from several organs was analyzed by 32 P‐postlabeling. Two o ‐anisidine‐DNA adducts, identical to those found in DNA incubated with o ‐anisidine and human microsomes in vitro , were detected in urinary bladder (4.1 adducts per 10 7 nucleotides), the target organ, and, to a lesser extent, in liver, kidney and spleen. These DNA adducts were identified as deoxyguanosine adducts derived from a metabolite of o ‐anisidine, N ‐(2‐methoxyphenyl)hydroxylamine. This metabolite was identified in incubations with human microsomes. With 9 human hepatic microsomal preparations, we identified the specific CYP catalyzing the formation of the o ‐anisidine metabolites by correlation studies and by examining the effects of CYP inhibitors. On the basis of these analyses, oxidation of o ‐anisidine was attributed mainly to CYP2E1. Using recombinant human CYP (in Supersomes) and purified CYPs, the participation of CYP2E1 in o ‐anisidine oxidation was confirmed. In Supersomes, CYP1A2 was even more efficient in oxidizing o ‐anisidine than CYP2E1, followed by CYP2B6, 1A1, 2A6, 2D6 and 3A4. The results, the first report on the potential of the human microsomal CYP enzymes to activate o ‐anisidine, strongly suggest a carcinogenic potential of this rodent carcinogen for humans. © 2005 Wiley‐Liss, Inc.