Cytogenetic effects of phorbol ester tumor promoters: possible role in multistep tumorigenesis

The conversion step of multistage tumorigenesis in mouse skin effected by TPA (12-O-tetradecanoylphorbol-13-acetate) but not by RPA (12-O-retinoylphorbol-13-acetate) may be best explained by the clastogenic (and mitogenic) activity of TPA. In mouse keratinocytes in vitro as well as in vivo TPA (but not RPA) was shown earlier to induce a significant degree of chromosomal aberrations. Details of the clastogenic action of TPA were studied in HeLa cells. Only TPA exerted a significant clastogenic activity at non-cytotoxic concentrations (10(-8) to 10(-6) M) measured after 24 and 48 hrs. Values obtained with RPA were close to those obtained in the presence of solvent (acetone 0.2%). The response to TPA was only to some extent correlated with the dose. Chromosomal aberrations including gaps and breaks were predominantly of the chromatid type; they are first measurable after 6-8 hrs, i.e. as soon as the cells recover from TPA-induced G2-inhibition. A 30 min exposure to TPA (10(-7) M) is sufficient to induce aberrations. The data point to an indirect, possibly receptor-mediated action of TPA. Supposing that TPA-induced chromosomal lesions represent a key event required for conversion, it is suggested that the absolute requirement for DNA replication in the conversion step is necessary to "fix" a certain degree of chromosomal damage. Therefore, the clastogenic as well as the mitogenic activity of TPA appear to be necessary for effecting conversion.