Metabolism and effects of acetoaceto-<i>o</i>-toluidine in the urinary bladder of humanized-liver mice

Occupational exposure to aromatic amines is a major risk factor for urinary bladder cancer. Our previous studies showed that acetoaceto-<i>o</i>-toluidine, which is produced using <i>o</i>-toluidine as a raw material, promotes urinary bladder carcinogenesis in rats. We also found high concentrations of <i>o</i>-toluidine, a human bladder carcinogen, in the urine of acetoaceto-<i>o</i>-toluidine-treated rats, indicating that urinary <i>o</i>-toluidine derived from acetoaceto-<i>o</i>-toluidine may play an important role in bladder carcinogenesis. However, this has not been investigated in humans. In the present study, we used non-humanized (F1-TKm30 mice) and humanized-liver mice established by human hepatocyte transplantation to compare differences in urinary acetoaceto-<i>o</i>-toluidine metabolites produced by human and mouse liver cells. We also examined the changes in acetoaceto-<i>o</i>-toluidine-induced mRNA expression in the liver and the proliferative effects on the bladder epithelium. Urinary <i>o</i>-toluidine was detected in both non-humanized and humanized mice. Acetoaceto-<i>o</i>-toluidine metabolites in the urine, cell proliferation activities, and DNA damage in the bladder urothelium were similar in non-humanized and humanized-liver mice. RNA expression analysis revealed that CYP1A2 expression increased in the livers of humanized-liver mice, and Cyp2c29 expression (equivalent to human CYP2C9/19) increased in the livers of non-humanized mice. These data suggest that acetoaceto-<i>o</i>-toluidine may be a human carcinogen, as evidenced by the detection of urinary <i>o</i>-toluidine in acetoaceto-<i>o</i>-toluidine-treated humanized-liver mice. This animal model is important for extrapolating toxicity data from animals to humans.