Adaptation and selection as factors in the spontaneous transformation of NIH-3T3 cells

The effect of calf serum (CS) concentration on the spontaneous transformation of NIH-3T3 cells was investigated. Maintenance of NIH-3T3 cell populations in media containing a low concentration of calf serum (2% CS) increased the proportion of spontaneously transformed cells appearing within a population. Higher CS concentration (10% CS) decreased the proportion of transformed cells in the population. In searching for the reasons for the effect of serum concentration on transformation frequency, three conclusions were reached. (i) Non-focus-forming NIH-3T3 cells exhibit heterogeneity in progression toward transformation. (ii) Low serum concentration can increase the frequency of focus-forming cells within an NIH-3T3 population by decreasing the proliferation of non-focus-forming cells relative to focus-forming cells. (iii) The reproductive selection described in (ii) cannot completely account for the focus-enhancing effects of low serum. Evidence for point (iii) is drawn from focus-forming assays, in 2% CS, for a clone of cells maintained in 10% CS before assay. Cells from this clone made no foci in 50 assays, performed on 5 x 10(6) cells, over a 21 week period. However, maintenance of those same cells in 2% CS for as little as 2 days before assay caused these consistent non-focus formers to make foci. We conclude that the basis for this progression to the focus-forming phenotype is an adaptation to the metabolically constraining 2% CS environment. We propose that this adaptation results from progressive selection of physiological states.