High rate of diversification and reversal among subclones of neoplastically transformed NIH 3T3 clones.

NIH 3T3 cells undergo neoplastic transformation when exposed to conditions of moderate physiological growth constraint. One of several characteristics of this transformation that indicates its adaptational nature is its gradual reversibility under conditions of unconstrained growth. We explored the origins of reversibility by isolating cells from each of three highly transformed foci and comparing their focus-forming capacity with that of derivative clones and subclones. A high proportion of the parental cells made dense foci. Six of the nine clones obtained from the three foci produced foci, though the percentage varied widely. The other three clones produced no foci at all. The transformed clones were subcloned and analyzed to evaluate the possibility that the negative clones were genuine revertants, rather than being derived from a small minority of nontransformed cells surrounding or underlying the original foci. In each case the subclones varied widely in the percentage of focus-forming cells and the average was much lower than the parental clone from which they were derived. Indeed, 15 of the 53 subclonal populations produced no foci. The high degree of heterogeneity, including complete reversal of focus-forming capacity, provides additional support for the hypothesis that "spontaneous" transformation is driven by an adaptive response to moderate growth constraint rather than by one or more effectively irreversible mutations.