Martin L. Wenk

Malignant conversion of mouse skin tumours is increased by tumour initiators and unaffected by tumour promoters.

Multi-stage carcinogenesis (initiation–promotion) was first demonstrated in mouse skin1,2. The first stage, initiation, is accomplished by a low dose of carcinogen that causes no tumours. Promotion by repeated treatment of initiated mice with certain non-carcinogenic hyperplastic agents results in the rapid production of numerous benign papillomas, a few of which progress to squamous cell carcinomas. Although this model system produces mostly benign tumours, many of the concepts concerning carcinogenesis in epithelial tissues have been derived from mouse skin studies. The permanent change in growth potential accomplished by tumour initiators is generally considered to be a mutagenic event3; cell selection and clonal expansion of initiated cells may be involved in promotion4. In initiation–promotion experiments, more than 90% of the squamous cell carcinomas develop from papillomas5,6, but the conversion rate is low. The factors necessary for this conversion of benign to malignant tumours have not been defined but tumour promoters have been assumed to be involved. However, we report here that the tumour promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) is ineffective in the conversion of papillomas to carcinomas whereas three initiators, urethane, N-methyl-N′-nitro-N-nitrosoguanidine ((MNNG) and 4-nitroquinoline-N-oxide (4-NQO) are effective. This suggests that malignant conversion may result from a further genetic change in papilloma cells and that the ineffectiveness of TPA may be due to its inactivity as a mutagen.

Retinoic acid promotion of papilloma formation in mouse skin

Retinoic acid is a weak promotor of skin tumorigenesis in Charles River CD-1 mice. Multiple papillomas were seen in 17% of the mice treated 3 times weekly with 5.1 micrograms retinoic acid for 20 weeks after initiation by a single treatment with 50 micrograms 7,12-dimethylbenz[alpha]anthracene (DMBA). These results suggest the necessity of a more thorough evaluation of retinoids as tumor promoters before their serious consideration as anti-cancer agents in man.

Effects of short-term exposure to the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate on skin carcinogenesis in sencar mice

Skin tumor promotion after a short-term exposure to 12-O-tetradecanoylphorbol-13-acetate (TPA) was studied in female SENCAR mice. Mice were dosed once by the topical application of 20 micrograms of dimethylbenz[a]anthracene (DMBA) in 0.2 ml acetone. A week later, they received topical applications of TPA (2 or 4 micrograms per 0.2 ml acetone) once or twice a week for periods of 1-10 weeks and were killed at 30 weeks. Skin tumors were counted and measured for size weekly. When TPA was applied once a week for 10 weeks or only twice a week for 2 weeks, there was significant promotion of papilloma formation in a large proportion of mice initiated with DMBA. Mice that received one or two applications had a few skin tumors. The total number of papillomas decreased considerably and the majority appeared to regress after 20 weeks in mice that received TPA treatment for 10 weeks. In mice that received only 4 TPA treatments, however, the majority of the papillomas grew progressively in size and did not regress during the entire experimental period. A greater proportion of these tumors progressed to carcinoma than did those in mice receiving TPA for 10 weeks. Thus, a short-term exposure was effective in causing certain changes in skin of SENCAR mice that led to tumor development and progression.