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

The mechanisms by which tumor promoters exert their effectson target tissues are not clearly understood. Recent studieshave demonstrated that phorbol ester tumor promoters inducean oxidative burst in phagocytes and DNA single-strand breaks(SSB) in leukocytes. The purpose of the research presented herewas to investigate the clastogenic effects of tumor promotersin the target cell population, primary mouse epidermal cellsco-incubated with leukocytes. Using the alkaline elution assayto detect DNA SSB, it was demonstrated that tumor promotersinduce DNA SSB in primary mouse epidermal cells incubated inthe presence of leukocytes. By increasing the ratio of leukocytesto epidermal cells from 1: 2 to 10: 1, in the presence of 1.6x 10^–6 M 12-O-tetradecanoylphorbol-13-acetate (TPA), aratio dependent increase in DNA SSB was observed (from 9 x 10^–2to 121 DNA SSB per 10⁶ nucleotides). A dose response in DNASSB was seen with TPA over a concentration range of 4 x 10^–9–1.6x 10^–6 M. Mezerein, a second stage tumor promoter, inducedsimilar levels of DNA SSB to that of TPA. 4-O-Methyl TPA, afirst stage tumor promoter, induced significantly fewer DNASSB than either TAP or mezerein at similar concentrations. Theinduction of DNA SSB in epidermal cells treated with TPA andco-incubated with leukocytes was inhibited by catalase but notsuperoxide dismutase. These data indicate that tumor promoterscan act indirectly on target epidermal cells by stimulatingthe release of a clastogenic factor from leukocytes througha mechanism involving H₂O₂.