The liver tumor-promoting effects of indole-3-carbinol (I3C), a cytochrome P450 (CYP) 1A inducer found in cruciferous vegetables, were investigated using a medium-term hepatocarcinogenesis model in rats. Six-week-old male F344 rats received an intraperitoneal injection of N-diethylnitrosamine (DEN) and were fed a diet containing 0 (DEN-alone), 0.25, 0.50 or 1.0% of I3C for 8 weeks from 2 weeks after DEN-initiation. The number and area of liver cell foci positive for glutathione S-transferase placental form (GST-P) significantly increased in the livers of rats given 0.5% I3C or more, compared to those in the DEN-alone group. The number of GST-P positive foci also increased in the 0.25% I3C group. The number of liver cells positive for proliferating cell nuclear antigen (PCNA) significantly increased in all I3C groups compared to that in the DEN-alone group. Real-time RT-PCR analysis showed that I3C increased transcript levels of not only Cyp1a1 but also aryl hydrocarbon receptor (AhR) and/or nuclear factor (erythroid-derived 2)-like 2 (Nrf2) gene batteries, such as Cyp1a2, Cyp1b1, Ugt1a6, Nrf2, Nqo1, Gsta5, Gstm2, Ggt1and Gpx2. Reactive oxygen species (ROS) in the microsomal fraction significantly increased in all I3C-treated groups compared to the DEN-alone group, and thiobarbituric acid-reactive substances (TBARS) levels and 8-hydroxy-2'-deoxyguanosine (8-OHdG) content significantly increased in all of the I3C-treated groups and 1.0% I3C group, respectively. These results suggest that I3C is an AhR activator and enhances microsomal ROS production resulting in the upregulation of Nrf2 gene batteries, but the oxidative stress generated overcomes the antioxidant effect of Nrf2-related genes. Such 'a redox imbalance' subsequently induces liver tumor-promoting effects by enhancing cellular proliferation in rats.
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