The inhibitory effect of retinoic acid (RA) on 12-0-tetradecanoylphorbol-13-acetate- (TPA) induced mouse skin tumors was studied. Two subpopulations of tumors, small (< 2 mm) and large (greater-than-or-equal-to 2 mm) appeared after 12 weeks of cutaneous promotion by TPA (10 nmol), following initiation by application of 2 x 100 nmol of 7,12-dimethylbenz[a]anthracene (DMBA) to the skin. RA in the doses of 17 and 34 nmol, prior to each TPA treatment inhibited (P < 0.05) the formation of small tumors at 12 weeks of promotion. However, RA in either dose did not inhibit the formation of large (greater-than-or-equal-to 2 mm) tumors. Ten weeks following withdrawal of all treatments, the number of large tumors persisted in a significantly (P < 0.05) higher number as compared to small tumors in all groups. Our results provide evidence for the existence of tumor subpopulations with a differential response to RA. In addition, elevated levels of metallothionein (MT) expression were demonstrated in papillomas induced by TPA, 72 h after the last TPA treatment. Comparing papillomas treated with RA prior to each TPA treatment and papillomas treated with TPA only, demonstrated that the elevated MT expression in papillomas was unaffected by RA. This indicated that RA did not affect the expression of a protein that showed elevated level in TPA-induced papillomas.
