Cytogenetic effects caused by phorbol ester tumor promoters in HeLa cells: mechanistic aspects

The effect of the convertogenic ('first-stage') tumor promoter TPA (12-O-tetradecanoylphorbol-13-acetate) and the non-convertogenic ('second-stage') tumor promoter RPA (12-O-retinoylphorbol-13-acetate) on chromosomes was investigated in HeLa cells which have previously been shown to exhibit a radiomimetic response to TPA. As in the case of mouse keratinocytes, only TPA had a significant clastogenic activity at non-cytotoxic concentrations ranging from 10(-8) to 10(-6) M measured after 24 and 48 h. The values observed with RPA did not differ significantly from values observed in the presence of the solvent (acetone, 0.2%). The response to TPA was saturable with respect to the dose of TPA. The chromosomal aberrations (mostly gaps and breaks) were predominantly of the chromatid type. Isochromatid aberrations were caused by a 24 or 48 h treatment with 10(-6) M TPA. The aberrations appear as early as 6-8 h after TPA application, i.e. as soon as the cells have recovered from TPA-induced inhibition in G2-phase. Even a 30 min exposure to 10(-7) M TPA gives the same yield of aberrations as longer treatment, i.e. the response to TPA is 'saturable' with respect to time. Both TPA and the non-clastogenic RPA cause a temporal G2-delay thus indicating that the G2-inhibition is not related to induction of chromosomal aberrations by TPA. The data are consistent with the hypothesis that TPA induces chromosomal aberrations via a receptor-mediated pathway.