Phenotypic expression of transformation: induction in cell culture by a phorbol ester.

A biologically potent, tumor-promoting phorbol ester from Croton tiglium L. induces the appearance of transformed clones in a population of contact-inhibited fibroblasts of the mouse cell line 3T3. Mixed populations of cells with 10,000 3T3 cells and 100 virus-transformed cells were exposed to the phorbol ester and, in comparison with untreated cells, showed a marked increase in the numbers of transformed clones that grew. Exposure of 3T3 cultures to the carcinogen 7,12-dimethylbenz(a)anthracene alone or prior to exposure to the phorbol ester did not cause any increase in the number of transformed clones.