Investigation of the mechanism for enhancement of radiation transformation in vitro by 12-O-tetradecanoylphorbol-13-acetate.

We have performed experiments designed to investigate the mechanism for the enhancement of radiation transformation in vitro by 12-O-tetradecanoylphorbol-13-acetate (TPA). Two types of experiments, involving C3H 10T1/2 cells and 100 rad X-ray exposures with subsequent TPA treatment, are reported here. In one set of experiments, cultures were initially seeded at differing initial cell densities prior to irradiation. In the other series of experiments, cultures exposed to 100 rads and TPA contained the same initial cell densities (about 300 viable cells per dish); these cultures were allowed to reach confluence and were then reseeded at various cell densities to allow a second cycle of growth to confluence (which involved different numbers of cell divisions). We have observed that the number of transformed foci which ultimately developed per dish treated with 100 rads and TPA remains approximately constant even though the cell density (initial or reseeded) is varied over several orders of magnitude. The yield of transformants per dish which occurred following this treatment was similar to those previously observed in cultures irradiated with higher doses (400-600 rads) of X-rays alone. Our results suggest that a similar mechanism for the induction of malignant transformation may be involved for high doses of radiation alone or for a low dose of radiation followed by TPA treatment.