Prevention of radiation-induced mammary tumors.

The radiation-induced rat mammary tumor model is useful for studying tumor prevention by treatment in the initiation or promotion stage. In anti-initiation experiments, the administration of radical scavengers or spin-trapping agents before or immediately after irradiation reduced the incidence of mammary tumors, suggesting that free radicals produced by exposure are a potent initiator. To evaluate the role of nitric oxide (NO) in the initiation, NO-specific scavengers or NO synthase inhibitors were administered during the initiation. These agents partially prevented the tumorigenesis, suggesting that radiation-induced NO contributes to tumor initiation. The administration of curcumin during irradiation reduced the incidence of the tumors in the presence of tumor promotor. In anti-promotion experiments on preventing diethylstilbestrol (DES)-dependent tumor development from mammary primodial cells exposed to radiation, tamoxifen decreased the tumor incidence. From the results, estrogen itself or prolactin induced by estrogen may be a promoter for the tumorigenesis. Bezafibrate and simvastatin, agents inducing hypolipidemia and hypocholesterolemia respectively, cause a decrease in the DES-dependent promotion of radiation-induced tumorigenesis. The simultaneous administration of curcumin and DES significantly reduces the development of mammary tumors in irradiated rats. In this review, the endocrinologic and pharmacologic significance of the anti-initiation and anti-promotion is discussed.