Enhancement of skin tumorigenesis by a single application of croton oil before or soon after initiation by urethan.

The importance of DNA synthesis in the initiation of skin tumorigenesis by urethan was investigated. Treatment of mice with 0.5% croton oil increases the rate of epidermal DNA synthesis 3to 6-fold beginning at 9 to 12 hr, reaching a peak at 18to 24 hr, and remaining elevated for 3 to 4 days. Croton oil treatment times of -24, -6, +1, and +24 hr were tested for their effect on initiation of skin tumor formation by urethan injected at zero time. Controls initiated with urethan alone developed 1.7 papillomas per mouse after 30 weeks of tumor promotion. The latent period was 21 weeks. Similar results were obtained in the group treated with croton oil at +24 hr (1.8 papillomas per mouse, 21-week latent period). In contrast, when croton oil was given at -24, -6, or +1 hr, tumor yields of 5.3, 5.3, and 4.8 papillomas per mouse were found, with the latent period shortened to 16 weeks in all three groups. In the groups treated with croton oil at -6 and + 1hr, the earliest increase in rates of DNA synthesis are likely to have occurred at about 3 and 10 hr, respectively, after urethan injection. Thus, if the enhancement of urethan initiation by croton oil is related to the croton oil-induced stimulation of DNA synthesis in the skin, then only DNA replication at times more than 10 hr after urethan injection could be important in the process of skin tumor initiation. Inhibition of DNA synthesis by hydroxyurea for about 9 hr beginning 1 hr before, 2 hr after, or 8 hr after injection of urethan did not decrease the tumor incidence in initiationpromotion experiments. These results suggest that increased DNA synthesis at the times tested is not necessary for the enhancement of urethan initiation by croton oil.