Protective effect of all trans retinoic acid against tumor promotion and progression in low- and high-risk protocols of mouse skin chemical carcinogenesis.

In this study, we assessed the protective effects of all trans retinoic acid (RA) against skin tumor promotion and progression in three different protocols of mouse skin multistage carcinogenesis. Under these protocols, where papillomas on the skin of SENCAR mice were induced for their low- and high-probability of conversion to malignant carcinomas, pre-application of RA (10 mu g/mouse/application) to that of tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) or mezerein (MEZ) in 7,12-dimethylbenz(a)anthracene (DMBA)-initiated mouse skin resulted in a highly significant protection against skin tumor promotion. In terms of tumor incidence, multiplicity and volume/mouse, at the termination of the experiment at 20 weeks, RA showed 35-80%, 67-83% and 70-98% protection, respectively. While the effect of RA was profound in all the protocols, maximum affectivity was evident in high-risk papilloma induction protocol where DMBA was used as tumor initiator and one week later TPA was applied once a week for five weeks. In tumor progression studies, at 20 weeks of standard low- and high-risk protocols, when papilloma yield is stabilized, animals which did not receive RA in different protocols were divided into two groups and topically treated twice weekly either with acetone or RA (10 mu g). These treatments were continued for an additional 29 weeks. During these treatments, all suspected carcinoma formation was recorded and carcinomas were verified histologically at the termination of the experiment. In each case, RA showed remarkable protective effects against percentage of mice with carcinomas (35-45% protection), number of carcinomas per mouse (50-63% protection) and the rate of malignant conversion (50-63% protection). The results of the present study clearly demonstrate that irrespective of the risk for skin carcinogenesis, RA is capable of affording protection against the induction of nonmalignant lesions and their subsequent conversion to malignancy.