Abstract
The effects of dose and duration of treatment with the potent tumor-promoting agent 12-O-tetradecanoylphorbol-13-acetate (TPA) on the formation of skin tumors in Charles River CD-1 mice was studied. Mice were initiated with a single application of 0.2 micromol of 7,12-dimethylbenz[a]anthracene (DMBA) in 0.2 ml acetone. Beginning two weeks after initiation, mice were treated twice weekly with various doses (0.01 - 20 nmol) of TPA in 0.2 ml acetone. Application of either 0.01 or 0.1 nmol of TPA did not elicit tumors during the 50 weeks duration of treatment. A dose-dependent increase in the number of papillomas was observed through the range of 1 to 10 nmol of TPA. Twice weekly applications of 20 nmol of TPA did not further enhance the papilloma incidence. A good correlation was observed between the induction of ornithine decarboxylase (ODC) activity and the formation of skin tumors by various doses of TPA. To determine the effect of promotion duration on the incidence of papillomas and carcinomas, mice were treated with 10 nmol of TPA for various durations (6, 12, 18, 24, 30, or 36 weeks) beginning 2 weeks after initiation with 0.2 micromol of DMBA. Mice promoted for only 6 weeks developed papillomas and carcinomas after promotion had been discontinued. There was an intermediate incidence of tumors in the group treated for 12 weeks. Promotion for 18, 24, 30, or 36 weeks elicited virtually identical yields of papillomas. The incidence of carcinomas was proportional to promotion duration times of 6, 12, and 18 weeks, but carcinoma incidence was less than maximal in mice promoted for 24 weeks or longer. The results indicate that a) the incidence of papillomas serves as a rapid (18 weeks) index for subsequent appearance of carcinomas, b) twice weekly applications of 10 nmol of TPA for 18 weeks following initiation of female CD-1 mice with 0.2 micromol of DMBA is an appropriate protocol for maximum tumor yield in initiation-promotion experiments, and c) ODC induction may be an important component of the mechanism of skin tumor promotion by TPA.
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