| Ciprofibrate (CIP) and phenobarbital (PB) are nongenotoxic hepatic tumor promoters in rodents. Their promoting activities are likely due to alterations on cell proliferation and apoptosis, but the mechanisms have not been elucidated. The initial alteration is hypothesized to be due to an oxidative stress environment, resulting from the induction of H2O2- and superoxide-producing enzymes. In support of the oxidative stress hypothesis, PB and CIP activate the oxidative stress-induced transcription factor, nuclear factor κB (NF-κB). We hypothesized that increased dietary vitamin E in rodents treated with P13 or CIP would decrease NF-κB DNA binding. In the first two studies, male Sprague-Dawley rats were fed a purified diet for 28 days containing 10, 50, or 250 ppm of the antioxidant, vitamin E. At that point, some of the rats were placed on the same purified diet with either 0.05% PB or 0.01% CIP for 10 days. Increased dietary vitamin E decreased PB- and CIP-mediated NF-κB DNA binding activity, which further supports the oxidative stress hypothesis. CIP-treatment also significantly reduced the levels of cytosolic IκBα, IKKα, and IKKβ, with the highest reductions in the low vitamin E group. Our second hypothesis was that dietary vitamin E decreases CIP-induced cell growth by acting through NF-κB. The study design was similar to the previous studies, except that it was performed in mice deficient in the NF-κB p50 subunit or in wild-type mice. In the wild-type mice, CIP-treatment increased p65:p50 NF-κB activity and increased cell proliferation, yet neither endpoint was significantly altered by dietary vitamin E levels. In contrast, the p50 knock-out mice had lower NF-κB activation and higher basal levels of cell proliferation and oxidative stress. The p50 knock-out mice fed higher dietary vitamin E and treated with CIP had 60% lower cell proliferation in comparison to the group fed low vitamin E. Thus, the effects of vitamin E on cell growth parameters are not solely through decreased NF-κB activation. In conclusion, higher dietary vitamin E decreased NF-κB DNA binding, but the effect vitamin E has on cell proliferation is not mediated entirely through NF-κB. |
