An Analysis of the Epidermal Hyperplasia Produced by Acetic Acid, a Weak Tumor Promoter, in the Skin of Female Mice Initiated with Dimethylbenzanthracene

Epidermal tumor promoters like TPA (12-0-tetrade-canoyl-phorbol-13-acetate) produce an epidermal hyperplasia which is a major transformation step in tumor promotion. Acetic acid, an agent that produces an epidermal hyperplasia, has, however, been reported to be a weak promoter of tumorigenesis in mouse skin. The purpose of this investigation was to analyze the epidermal hyperplasia produced by acetic acid in order to help understand why acetic acid is a weak tumor promoter. Epidermal hyperplastic growth produced by TPA in initiated mouse skin is characterized by the appearance of an initial small transient hyperplasia followed by the principal hyperplasia. The application of 667 μmol acetic acid on the skin of mice initiated with 200 nmol dime-thylbenzanthracene does not result in a transient epidermal hyperplasia. Instead, the numbers of nucleated cell layers and the numbers of basal and suprabasal cells/mm interfollicular epidermis begin to decrease within 5 h. The epidermis then begins to lose its stainability with hematoxylin and eosin and by 19 h only faint nuclear outlines are seen. These conditions prevail until about day 2 when epidermal cells again stain. In addition, the epidermis becomes hyperplastic. The principal epidermal hyperplasia produced by TPA results in a 4-fold increase in epidermal wet weight and total protein and a 2-fold increase in total DNA. In contrast, after the application of 667 μmol acetic acid to initiated skin, the maximal increase in wet weight and protein is about 2 times that of normal, and the increase in DNA is only 50% above normal. The RNA content of TPA-induced hyperplastic epidermis reaches a maximum of about 2 times that of normal. In contrast, the RNA content of acetic acid-induced hyperplastic epidermis is actually reduced to 70% below normal levels, making it only 35% of the TPA-induced level. This is consistent with the fact that the cells of TPA-induced hyperplastic epidermis are enlarged and intensely basophilic, whereas the cells in acetic acid-induced hyperplastic epidermis tend to show little enlargement and cytoplasmic basophilia. Maximal mitotic activity in the TPA-induced hyperplastic epidermis is 14 times that of normal, whereas that after acetic acid is only 7 times above normal. Multiple applications of TPA result in an increased epidermal wet weight from that seen after 1 application. In contrast, the application of 667 μmol acetic acid once weekly for 7, 12, and 20 weeks does not result in a further increase in epidermal wet weight from that seen after the first application. Indeed, by 20 weeks epidermal wet weight is slightly less than that seen after the first application, and accordingly the epidermis appears in histologic sections to be less hyperplastic. The evidence strongly suggests that one reason acetic acid is a weak promoter of epidermal tumorigenesis is that it does not produce the degree of epidermal hyperplasia that TPA does and the hyperplasia that is produced is barely maintined, if at all, during chronic treatment.