Neoplastic transformation of human epithelial cells in vitro

Efforts to investigate the progression of events that lead human cells of epithelial origin to become neoplastic in response to carcinogenic agents have been aided by the development of tissue culture systems for propagation of epithelial cells. We have recently developed an in vitro multistep model suitable for the study of human epithelial cell carcinogenesis. Primary human epidermal keratinocytes acquired indefinite lifespan in culture but did not undergo malignant conversion in response to infection with Adl2-SV40 virus. Subsequent addition of Ki-MSV, which contains a K-ras oncogene, to these cells induced morphological alterations and the acquisition of neoplastic properties. Nontumorigenic human epidermal keratinocytes immortalized by Adl2-SV40 virus (RHEK-1) were also transformed by treatment with chemical carcinogens (MNNG or 4NQO) and by X-ray irradiation. Such transformants showed morphological alterations and induced carcinomas when transplanted into nude mice. This in vitro system may be useful in assessing environmental carcinogens for human epithelial cells and in detecting new human oncogenes since ras oncogenes were not activated in these chemical--or X-ray--transformed RHEK-1 lines. Subsequently, it was found that this line could be transformed neoplastically by a variety of retroviruses containing H-ras, bas, fes, fms, erbB and src oncogenes. In addition, our recent results indicate that nontumorigenic RHEK-1 cells can be transformed following transfection with an activated human oncogene. Thus, this in vitro system may be useful in studying the interaction of a variety of carcinogenic agents and human epithelial cells. These findings demonstrate the malignant transformation of human primary epithelial cells in culture by the combined action of tumor viruses and chemical carcinogens or X-ray irradiation and support a multistep process for neoplastic conversion. Further, evidence for the multistep nature of neoplastic transformation of human epithelial cells in vitro using other model systems is presented.