High dietary retinoic acid prevents malignant conversion of skin papillomas induced by a two-stage carcinogenesis protocol in female SENCAR mice.

We have previously reported that high dietary retinoic acid (RA; 30 micrograms/g diet) inhibits carcinoma formation in a two-stage skin carcinogenesis protocol, using 7,12-dimethylbenz[a]anthracene (DMBA) as the initiator and 12-O-tetradecanoyl phorbol-13-acetate (TPA) as the tumor-promoter in female SENCAR mice. We next asked whether switching the diets from high to control levels of RA and vice versa would influence carcinoma formation. Mice at 3 weeks of age were initiated with DMBA (20 micrograms) once, followed by 20 weekly applications of TPA (2 micrograms). At 3 weeks of age mice were weaned onto a diet containing either 3 (control) or 30 (high) micrograms RA/g diet. Half of the mice from either dietary group were switched to the other diet at 20 weeks of age, when papilloma formation was at its peak. These four groups are designated RA 3 micrograms, RA 30 micrograms, RA 3/30 micrograms and RA 30/3 micrograms groups. As previously found, papilloma formation (including incidence and yield) was not significantly affected by dietary treatment. However, high dietary RA inhibited carcinoma formation; specifically cumulative carcinoma incidence (18.5-23.1% versus 50%) and yield (0.19-0.23 versus 0.68) were significantly lower (P < 0.05) in the high dietary RA treatment groups than the RA 3 micrograms control group, as was the carcinoma conversion efficiency (2.1-3.8% versus 9.4%). The beneficial effect on carcinoma formation was still evident when excess RA was given late during the carcinogenesis process (i.e. the RA 3/30 micrograms group). Moreover, a residual effect of excess RA was also seen after the dietary RA was switched to the control level at 20 weeks of age, when papilloma yield was highest (i.e. the RA 30/3 micrograms group). It is therefore concluded that the chemopreventive effect of high dietary RA on skin carcinogenesis induced by a two-stage carcinogenesis protocol with DMBA and TPA resides mainly at the step of conversion from benign papillomas to malignant carcinomas.