Post-initiation treatment of Indole-3-carbinol did not suppress N-methyl-N-nitrosourea induced mammary carcinogenesis in rats

The consumption of cruciferous vegetables (the Family of Cruciferae) such as cabbage, broccoli and Brussels sprouts has been shown to have cancer chemopreventive effects in humans and experimental animals. Indole-3-carbinol (I3C), one component of cruciferous vegetables, has been shown to exert cancer chemopreventive influence in liver, colon, and mammary tissue when given before or concurrent with exposure to a carcinogen. However in some reports, there has been evidence that consumption of I3C after carcinogen treatment might be associated with tumor promotion in some tissues. There have been no reports, to our knowledge, of post-initiation effects of I3C in the N-methyl-N-nitrosourea (MNU)-induced mammary tumor model in rats. Our studies were performed to examine this question. Ninety-six, 4-week-old female Sprague-Dawley rats were randomly divided into five groups. The animals of groups 1, 2 and 3 received an intraperitoneal injection of MNU at the age of 50 days. The animals of groups 4 and 5 were injected with saline only at the same time. Animals of groups 1 and 2 were given diet containing 100 ppm and 300 ppm I3C from week 1 until week 25 after MNU treatment. The animals of group 4 were given basal diet containing 300 ppm I3C without MNU treatment. All animals were killed at week 25. The incidences of mammary tumors in the groups 1, 2 and 3 were 95.8% (23/24), 83.3% (20/24) and 82.4% (28/34), respectively. The average number of tumors in the tumor bearing rats of the MNU and I3C 300 ppm group (group 2; 3.85 ± 0.63) was higher than that in the MNU alone group (group 3; 2.46 ± 0.31). These results represented that exposure to I3C after carcinogen treatment did not suppress development of mammary tumors.