Two-stage malignant transformation in hamster embryo cells

Transformation of primary hamster embryo cells was investigated using 3-methylcholanthrene (MCA), a combination of MCA and 12-O-tetradecanoylphorbol-13-acetate (TPA), and initiation with MCA or dibenz(a,h)anthracene (DBA) followed by promotion with TPA. Evidence for transformation was (a) abnormal cellular morphology, (b) increased lifespan, (c) growth in soft agar, and (d) tumour induction by s.c. inoculation into suckling hamsters. Cells treated with either MCA or MCA+TPA showed the same latent period to morphological transformation, although their tumorigenic potential varied. Cells did not form tumours when TPA was administered 7 days after treatment with either MCA or DBA. However, when administration of TPA was delayed to 27 days after treatment with a transforming dose of MCA or a subthreshold dose of DBA, the cells transformed and produced tumours in hamsters. Our results show that TPA may act as an inhibitor or promoter, depending on the length of time between treatment of the hamster embryo cells with the carcinogen and administration of the TPA. It appears that treatment of cells with TPA before the initiating event is complete inhibits or delays the development of their ability to induce tumours in animals or grow in soft agar. However, with a sufficient interval between the application of the initiating carcinogen and the promoter, transformation occurs, and the ability of cells treated with subthreshold doses of DBA to form tumours is enhanced.