Normal fibroblasts (KMS-6) derived from a human embryo were transformed in culture into neoplastic cells (KMST-6) by repeated treatment with 60Co gamma ray irradiation. Repeated treatment was necessary to obtain transformation. Control normal cells exhibited normal karyotype (46, XX) and stopped dividing due to cellular ageing at the 40th passage. The transformed cells are presently growing indefinitely (140th passage) and exhibit prominent karyologic aberrations, both numerical and structural. These 2 characteristics, indefinite growth and abnormal karyotype, are thought to be the most important parameters for neoplastic transformation of human fibroblasts. Other indispensable parameters are the presence of active mitotic figures on confluent cell sheets and colony-type morphology. Transformed cells grow into colonies with relatively smooth edges, while normal fibroblasts form colonies with jagged edges, due to the protrusion of growing fibroblasts. Other parameters, such as elevated plating efficiency, enhanced colony formation in soft agar, low serum requirement for growth, high saturation density, and acquisition of transplantability, are not reliable in the early stages of transformation. These parameters probably appear at rather later stages of transformation following several cell divisions. Among other characteristics, the transformed KMST-6 cells exhibit a B-type isozyme pattern of glucose-6-phosphate dehydrogenase, lactate-dehydrogenase isozyme pattern of human origin, no evidence of viral infection and no production of C-type virus particles.

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As reported previously (Namba et al., 1985), normal human fibroblasts were transformed by 60Co gamma-ray irradiation into immortal cells with abnormal karyotypes. These transformed cells (KMST-6), however, showed a low cloning efficiency in soft agar and no transplantability. However, upon treatment with Harvey murine sarcoma virus (Ha-MSV), the cells acquired elevated clonability in soft agar and transplantability in nude mice. Ha-MSV alone, however, did not convert normal human fibroblasts into either immortal or tumorigenic cells. The Ha-MSV-transformed KMST-6 cells showed an enhanced expression of the ras oncogene, but normal and 60Co gamma-ray-transformed cells did not. Our current data suggest that gamma rays worked against normal human cells as an initiator, giving rise to chromosome aberrations and immortality, and that Ha-MSV, probably through its ras oncogene, played a role in the progression of the malignant cell population to a more malignant one showing enhanced colony formation in soft agar and tumorigenicity in nude mice.