Expression of initiated and promoted stages of irradiation carcinogenesis in vitro.

Evidence obtained utilizing X-irradiation and ultraviolet light irradiation with 12-O-tetradecanol-phorbol-13-acetate (TPA) for the induction of transformation of Syrian hamster embryo cells (HEC) suggests that there is more than 1 type of initiation of carcinogenesis. Initiated HEC are phenotypically transformed in the presence of TPA (15--20-fold enhancement) but are not necessarily committed to be transformed because some colonies reverted to normal morphology when TPA was removed. Both initiated and promoted HEC are also sensitive to hamster lymphotoxin. The frequency of transformation was reduced by at least 70%. The ability to recognize discrete morphologically altered colonies after TPA addition in vitro indicates that promotion is not merely escape from growth control.