Tumor promotion by phorbol esters in skin: evidence for a memory effect.

By means of a two-stage promotion protocol in mouse epidermis with 12-O-tetradecanoylphorbol-13-acetate as first-stage promoter and 12-O-retinoylphorbol-13-acetate as second-stage promoter, the effects of the former that are critical and obligatory for tumor promotion were shown to be irreversible in nature for at least 8 weeks. The reversibility of tumor promotion was related to the second stage of promotion, reflecting the reversibility of epidermal hyperplasia induced by 12-O-tetradecanoylphorbol-13-acetate.