Indirect induction of a clastogenic effect in epidermal cells by a tumor promoter

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Abstract
The mechanisms by which tumor promoters exert their effects on target tissues are not clearly understood. Recent studies have demonstrated that phorbol ester tumor promoters induce an oxidative burst in phagocytes and DNA single-strand breaks (SSB) in leukocytes. The purpose of the research presented here was to investigate the clastogenic effects of tumor promoters in the target cell population, primary mouse epidermal cells co-incubated with leukocytes. Using the alkaline elution assay to detect DNA SSB, it was demonstrated that tumor promoters induce DNA SSB in primary mouse epidermal cells incubated in the presence of leukocytes. By increasing the ratio of leukocytes to epidermal cells from 1: 2 to 10: 1, in the presence of 1.6 × 10 −6 M 12-O-tetradecanoylphorbol-13-acetate (TPA), a ratio dependent increase in DNA SSB was observed (from 9 × 10 −2 to 121 DNA SSB per 10 6 nucleotides). A dose response in DNA SSB was seen with TPA over a concentration range of 4 × 10 −9 –1.6 × 10 −6 M. Mezerein, a second stage tumor promoter, induced similar levels of DNA SSB to that of TPA. 4-O-Methyl TPA, a first stage tumor promoter, induced significantly fewer DNA SSB than either TAP or mezerein at similar concentrations. The induction of DNA SSB in epidermal cells treated with TPA and co-incubated with leukocytes was inhibited by catalase but not superoxide dismutase. These data indicate that tumor promoters can act indirectly on target epidermal cells by stimulating the release of a clastogenic factor from leukocytes through a mechanism involving H 2 O 2 .