Malignant transformation of human fibroblasts by oncogene transfection or carcinogen treatment.

Exposure to chemical carcinogens or radiation is considered to cause most human cancer, but human cells in culture have not been successfully transformed to malignancy by such agents. Malignant transformation is a multi-step process and one explanation for the failure to induce such transformation of human cells in culture could be inability to recognize the phenotypes of carcinogen-treated cells that have undergone intermediate changes, so that these cells can be isolated and exposed a second time to cause further changes. To identify possible intermediates, we transfected diploid human fibroblasts with oncogenes known to be active in cells derived from fibrosarcomas and determined the phenotypes produced. H- or N-ras oncogenes flanked by suitable enhancer and promoter sequences caused the cells to exhibit several characteristics of malignant cells, but not to acquire an infinite life span or form tumors. Transfection of these oncogenes in the same constructions, or a viral K-ras oncogene, into an infinite life span, near-diploid, non-tumorigenic cell strain developed in this laboratory (MSU-1.1 cells) resulted in distinct foci of morphologically-altered, anchorage independent, and growth factor independent cells that formed progressively-growing, invasive malignant sarcomas in athymic mice and expressed the p21s of the transfected ras genes. Transfection of two other infinite life span human cell lines with the H-ras oncogene in the same construction also yielded malignant cells. Recently, we succeeded in inducing the malignant state in MSU-1.1 cells using carcinogen identified that are one step removed from malignant transformation, others that are two-steps removed, etc. Furthermore, we know what new phenotypes these cells need to express to be malignantly transformed and which oncogenes can make such a change. If, as suggested above, proto-oncogenes are the cellular targets for carcinogen attack, it should be possible, by carcinogen treatment to bring about the malignant state. We have recently succeeded in achieving just such transformation by exposing MSU-1.1 cells to chemical carcinogens.