Studies on the mechanism of skin tumor promotion: Evidence for several stages in promotion

Abstract

The effects of nonpromoting and weakly promoting diterpenes on skin tumor promotion by 12-O-tetradecanoylphorbol 13-acetate (TPA) were investigated. When phorbol and phorbol 12,13-diacetate (both nonpromoting) were given simultaneously with TPA after 7,12-dimethylbenz[a]anthracene (DMB) initiation in female mice, they had no effect on TPA promotion. The nonpromoter 4-O-methyl-TPA and the weak promoter mezerein inhibited TPA promotion in a dose-dependent manner when given simultaneously with TPA. Because mezerein was an effective inhibitor of TPA promotion when given simultaneously and because it induces many biological responses similar to those to TPA, the capacity of mezerein to act as an incomplete promoter in a 2-stage promotion protocol was also investigated. Twice-weekly applications of 1, 2 or 5 .mu.g of TPA for 2 wk after DMBA initiation produced 0, 0 and 0.5 papilloma/mouse, respectively, at 20 wk. When the twice-weekly applications of TPA for 2 wk were followed by twice-weekly treatments with 2 .mu.g of mezerein for 18 wk, the number of papillomas per mouse was 2.2, 3.5 and 9.0, respectively. Twice-weekly applications of 2 .mu.g of TPA for 2 wk followed by twice-weekly treatments with 1, 2 or 4 .mu.g of mezerein for 18 wk produced 2.1, 3.5 and 6.8 papillomas/mouse, respectively, in DMBA-treated mice. Twice-weekly doses as high as 40 .mu.g of 4-O-methyl-TPA were not effective in producing tumors when given after a limited treatment with TPA; however, 4-O-methyl-TPA had weak activity as a 1st-stage promoter. Although mezerein by itself is a weak promoter and mimics TPA in many biochemical and morphological effects, it apparently is a potent 2nd-stage promoter in a 2-stage promotion regimen.