The threshold dose for liver tumor promoting effects of dicyclanil in ICR mice

To determine the threshold dose of dicyclanil (DC) that induces hepatocellular tumor-promoting effects associated with reactive oxygen species (ROS) generation via their metabolic pathways, partial hepatectomized ICR male mice were fed diets containing 0, 187.5, 375 or 750 ppm DC after an intraperitoneal injection of N-diethylnitrosamine (DEN) to initiate hepatocarcinogenesis. Immunohistochemically, the proliferating cell nuclear antigen (PCNA)-positive cell ratio was significantly increased in the DEN + 750 ppm DC group compared with the DEN alone group. However, significant increases in the number of gamma-glutamyltranspeptidase (GGT)-positive cells and formation of microsomal ROS were not observed in the DEN + DC groups compared with the DEN alone group. Real-time polymerase chain reaction (RT-PCR) showed that the expression of Cyp1a1, Cyp1a2, and OGG1genes was significantly up-regulated in mice given diets containing 375 ppm DC or more, 187.5 ppm DC or more, and 750 ppm DC, respectively. These results suggest that the threshold dose of DC that induces ROS-mediated liver tumor promotion in mice is more than 750 ppm, although expression of the Cyp1a2 gene, which is related to ROS generation, was up-regulated in the liver of mice, even at a DC dose of 187.5 ppm.