Retinoic acid enhancement of an early step in the transformation of mouse epidermal cells in vitro

Retinoic acid has been reported to act as an inhibitor and as an enhancer of mouse skin carcinogenesis in vivo. However, no in vitro cell transformation model has been reported to be sensitive to both effects. In an attempt to provide such a model, the effect of retinoic acid on an early step in carcinogen-induced transformation of mouse epidermal cell line 271c was measured using a recently described assay. The step observed is altered response to extracellular Ca²⁺ as an epidermal terminal differentiation signal. In six out of twelve experiments retinoic acid increased the frequency of altered colonies resulting from treatment with three chemical carcinogens. The enhancement effect was stronger after DMBA treatment than MNNG or MCA, resulting in up to a 13.7-fold increase in the frequency of colonies exhibiting altered terminal differentiation (TF). On the other hand, up to a 10-fold decrease in TF was observed in other experiments. Both the enhancement and inhibitory effects were greater at the higher doses of retinoic acid tested in the range of 10^-10-10^-7 M. Variations in cloning efficiency or surviving colony density did not account for the effects on TF. Enhancement effects tended to be observed at lower doses of carcinogen, or in experiments in which TF resulting from treatment with carcinogen alone was in the lower range observed. However, the factors determining each effect have yet to be defined. The enhancement effect of retinoic acid was not merely suppression of the phenotypic endpoint of the in vitro assays, because treatment of carcinogen-altered cells with retinoic add or TPA in vitro also enhanced their tumorigenicity in vivo compared to acetone conrols. These findings suggest that studies of the determinants of retinoid activity should be a prerequisite to their use in chemoprevention.