Induction of transformation in NIH3T3 cells by moderate growth constraint: evidence that neoplasia is driven by adaptational change.

The NIH3T3 cell line undergoes transformation when the cells are maintained at high densities for extended periods. In an effort to define the physiological status of the cells before and during the transformation process, they were seeded in medium containing 2, 5 or 10% calf serum (CS) and growth was monitored by measurements of cell numbers and by incorporation of [3H]thymidine into DNA. Transformed foci began to appear by 7 days in 2% CS, at least 3 days after the onset of density-dependent growth inhibition, and by 10 days in 5% CS, 6 days after the onset of growth inhibition. The rate of DNA synthesis in both serum concentrations had declined by 7 days to less than 1/15th of the initial rate measured by day 2. Transformation did not occur in cells exposed to 10% CS which, though supporting even higher saturation densities, underwent a 100-fold decline in DNA synthesis after reaching confluence. Transfer and regrowth in 2% CS of cells from all three serum concentration revealed a steep increase in capacity for focus formation of cells previously cultured in 2 and 5% CS after they had reached confluence, but no such increase in cells previously cultured in 10% CS, indicating that the lack of foci in the original 10% CS incubation was due to suppression of transformation and not to masking of foci at very high cell densities. Determination of the fraction of dividing cells by [3H]thymidine autoradiography showed a similar decline prior to the appearance of transformed cells in 2% CS. In addition, a reactivation of mitosis in non-focal cells occurred by day 24, indicating that a much higher proportion of cells escaped the moderate growth constraint than might have been expected by the total number of foci formed on transfer. Our results are consistent with the premise that transformation is an adaptive response to moderate growth constraints, but not with a mutational origin.