Biphasic Modifying Effect of Indole‐3‐carbinol on Diethylnitrosamine‐induced Preneoplastic Glutathione S‐Transferase Placental Form‐positive Liver Cell Foci in Sprague‐Dawley Rats

The biphasic modifying effects of indole‐3‐carbinol (I3C), a naturally occurring constituent of edible cruciferous vegetables, on the development of glutathione S‐transferase placental form (GST‐P)‐positive liver cell foci were investigated by using a medium‐term liver bioassay system and a newborn rat hepatocarcinogenesis system. In Experiment 1, a total of 65 male Sprague‐Dawley (SD) rats were divided into 5 groups. Animals were given a single intraperitoneal (i.p.) injection of 200 mg/kg diethylnitrosamine (DEN) dissolved in saline for groups 1, 2, and 3 or a single i.p. injection of saline for groups 4 and 5. Group 1 was given the diet containing 0.25% I3C for 2 weeks prior to DEN initiation and then basal diet for 8 weeks. Group 2 was given basal diet for 4 weeks prior to and after DEN initiation and then the diet containing 0.25% I3C for 6 weeks. The rats of group 3 were placed on basal diet during the experiment. Animals of groups 4 and 5 were treated in the same manner as those of groups 1 and 2 except for injection with saline instead of DEN solution. All rats were subjected to two‐thirds partial hepatectomy at week 3 and were killed at week 8 after DEN or saline injection. In Experiment 2, a total of 45 female SD rats were dosed with DEN (100 mg/kg, i.p.) or saline at 24 h after birth. After weaning at week 3, the rats were fed diet containing 0.25% I3C for 9 weeks and then were killed at week 12. In Experiment 1, preinitiation exposure to 0.25% I3C caused a significant decrease in numbers of GST‐P‐positive liver cell foci (P<0.05), while postinitiation exposure to 0.25% I3C caused significant increases in both number (No./cm(2)) and area (mm(2)/cm(2)) of GST‐P‐positive liver cell foci (P<0.05 or 0.01). In Experiment 2, the relative liver weight in the DEN + I3C group was significantly increased (P<0.001). The numbers and areas of GST‐P‐positive liver cell foci in the DEN + I3C group were significantly increased as compared to the values of the DEN‐alone group (P<0.001). These results clearly demonstrated that I3C exerts a promoting effect on the postinitiation stage as well as an inhibitory effect on the preinitiation stage in the medium‐term liver bioassay.