Role of spontaneous transformation in carcinogenesis: development of preneoplastic rat tracheal epithelial cells at a constant rate.

The rate of spontaneous transformation of normal rat tracheal epithelial (RTE) cells to preneoplastic enhanced growth (EG) variants was estimated in serum-free culture. Spontaneous transformation of RTE cells has previously been observed, but an accurate estimation of the rate of change has not been possible due to the use of serum and feeder cells in the cultures which prevents both unlimited RTE cell proliferation and an accurate determination of the number of cells at risk. RTE cells were plated in serum-free medium and were switched to serum-containing medium at various times during the first 23 days of culture. In serum-containing medium, normal RTE cells cease proliferation, while EG variants continue to proliferate. The fraction of RTE cell colonies which developed into EG variants increased with time to a maximum of 15% when selection was imposed 5 to 23 days after plating. The number of cells per culture also increased during the same time, suggesting a role for cell proliferation in the spontaneous generation of EG variants. In contrast to the time-dependent increases in cell number and the frequency of EG variants, the rate of development of spontaneous EG variants remained constant with time and was estimated to be 7.5 +/- 4.1 X 10(6) variants/cell generation. The rate of spontaneous preneoplastic transformation of normal epithelial cells reported here, the rates of spontaneous progression of preneoplastic and neoplastic cells reported elsewhere, and the association between cell proliferation in vivo and increased cancer risk are consistent with the hypothesis that spontaneous changes play a role in the multistep progression of cells to cancer.