The reverse experiment in two-stage skin carcinogenesis. It cannot be genuinely performed, but when approximated, it...

One of the main assumptions of the two-stage theory of skin carcinogenesis is that the reverse experiment (promotion before initiation) is innocuous and that this in itself proves that there is a qualitative difference between the biological effects of initiators and promoters. However, almost all previous experiments in two-stage carcinogenesis have been arranged to justify the theory. Therefore, in order to test the theory scientifically, a series of experiments was set up comparing the tumor crop in classical two-stage and in reverse experiments. In most, a small, single dose of DMBA (200 or 100 nmol) was applied topically before or after a long-term course of 17 or 10 nmol TPA, but in one series, DMBA in larger doses (4,000 or 8,000 nmol) was given intraperitoneally. In another experiment two topical applications of 17 nmol TPA were given before or after a long-term course of very small doses (10 nmol) of DMBA applied twice a week. DMBA and TPA were generally applied in acetone. The results were statistically analyzed either from the start of the experiment, or from the time of DMBA application. A total of 736 mice were used, and they were studied once a week for papilloma and carcinoma development. No difference between classical and reverse experiments could be found as regards the carcinoma crops. For tumorigenesis (mostly papillomas) it was generally found that the reverse experiment was not innocuous, but produced a larger tumor crop than that produced by either DMBA or TPA alone. The two substances showed a reciprocal enhancing effect, which was sometimes weak, sometimes additive, and sometimes even synergistic, and was statistically most significant when the results were assessed from the time of DMBA application. Although the reverse experiment was not in any way innocuous it always resulted in a lower tumor crop than the classical sequence of DMBA followed by a course of TPA treatment. However, the lower tumor crop in the reverse experiment cannot be used to prove a qualitative difference between initiators and promoters. A course of TPA treatment alters the skin considerably, producing epidermal hyperplasia, an increased rate of cell proliferation, chronic inflammation and many other changes. A topical application of DMBA to such an altered skin is likely to have a considerably reduced tumorigenic effect. A 20-week resting period after TPA treatment seems to increase the effect of a final DMBA application, probably because part of the induced resistance is removed again.(ABSTRACT TRUNCATED AT 400 WORDS)