Cutaneous Changes during Prolonged Application of 12-0-Tetradecanoylphorbol-13-acetate on Mouse Skin and Residual Effects after Cessation of Treatment

The epidermal and dermal effects of protracted 12-O-tetradec-anoylphorbol-13-acetate (TPA) treatment (2 μg TPA twice weekly) of Senear mouse skin were studied using cell kinetics and morphometry techniques. In addition, regression of TPA-induced changes was evaluated after cessation of 56 topical applications. During the first week of treatment a reactional hyperplasia, characterized by cell damage, edema, and acute inflammation in both epidermis and dermis, occurred. This picture changed gradually during the following 3 weeks: an epidermal hyperplasia devoid of involutional or inflammatory features was accompanied by a moderate to mild chronic inflammation of the dermis and a hyperplasia of the hair follicles. This remained throughout the experimental period until the topical TPA treatment ceased. Although TPA induced papillomas in only 5% of the animals (maximum = 2 papillomas/animal and no carcinomas), all sustained marked epidermal hyperplasia of approximately 4 to 5 times the normal thickness, and increased the number and volume of hair follicles. The [³H]thymidine pulse-labeling index of the basal layer was approximately 32% (normal 6%). The level of dark keratinocytes remained constant; i.e., 8% of the basal cells were identified as dark cells during the entire experiment. At the subepidermal level the dermal thickness and total cellularity increased, although the proportion of the different cell types changed during the treatment. The mast cell population increased remarkably. After TPA treatment ceased, most of these parameters regressed abruptly during the first 2 weeks. Two to 4 months later, the epidermis was slightly thinner, and the labeling index was 50% lower than normal (2.8%). This study shows that prolonged repetitive TPA applications induced a steady-state hyperplasia without tachyphylaxis, and that this alteration regressed rapidly after treatment ceased. In addition, labeling-index values lower than normal were reached soon after normalization, suggesting that a possible selection of keratinocytes, dependent on TPA for proliferation, took place during the chronic administration of topical TPA. The number of hair follicles, capillary vessels, mast cells, and the dermal thickness never reached normal values after treatment. These important changes in the dermis and hair follicles indicate that the target cells for tumor promoters are not confined to the epidermis alone, and that these tissues could participate actively in carcinogenesis directly, either as tumor-originating tissues (hair follicles), or as inducers or helpers of neoplastic growth (connective tissue cells).