Dietary retinoic acid inhibits mouse skin carcinogenesis irrespective of age at initiation.

In the two-stage protocol of skin carcinogenesis, the carcinogen 7,12-dimethylbenz[a]anthracene (DMBA) is applied to the skin of mice at around seven weeks of age. We previously performed DMBA initiation at three weeks of age to study the effect of pharmacological (30 micrograms/g diet) dietary retinoic acid (RA) on skin carcinogenesis. In this study we asked whether dietary pharmacological RA is equally effective against skin carcinogenesis when mice are initiated with (DMBA) at 7 weeks of age and then subjected to weekly applications of the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) or mezerein (MEZ) for 20 weeks. Similar to the three-week initiation protocol, high dietary RA inhibited papilloma incidence and yield in MEZ- but not in TPA-promoted female SENCAR mice. In addition, carcinoma incidence and yield were decreased by high dietary RA in TPA- as well as MEZ-treated mice. These data demonstrate that the high dietary RA diet is as effective in inhibiting papilloma and carcinoma formation when the DMBA is applied at seven weeks of age as at three weeks.