Changes in malignant phenotype of a human carcinoma conditioned by growth environment

The human epidermoid carcinoma HEp3 exhibits highly malignant growth in chicken embryos that disappears progressively in cell culture. When studied by clonal analysis, loss of tumorigenicity was apparent within 20 generations and essentially complete after 40 generations in culture; affected all clones; and occurred under conditions which excluded the selection of variants based on superior growth rate in culture. Once ostensibly lost, the malignant phenotype reappeared after prolonged exposure of nontumorigenic cells to in vivo conditions. This similarly affected all clones assayed over a wide range of inoculum sizes. There was no evidence that HEp3 populations were heterogeneous with respect to tumorigenicity, or that the results were due to preferential selection of genetically determined subpopulations. It is suggested that the malignant phenotype of HEp3 is expressed in response to conditions in the physiological environment.