Adaptive changes in spontaneously transformed Balb/3T3 cells during tumor formation and subsequent cultivation

Cells from a clone of Balb/3T3 cells, which underwent spontaneous morphologic transformation and consistently had a high colony-forming efficiency when suspended in agar [colony-forming efficiency in agar (CFEag)], also formed primitive, pleomorphic sarcomas when inoculated into N:NIH(S)II-nu/nu mice. Cells cultured from 1 of the tumors had only 10% of the CFEag of those used to initiate the tumor but drifted back to the original high level during the first 2 months in culture. Twelve clones of the tumor cells differed widely from one another in CFEag in each of seven successive assays over a 10-week period, although the difference was most pronounced in the first assay. The CFEag for some of the clones fluctuated greatly in successive assays. Two of the clones usually scored lower than the others, and their average CFEag for the seven assays was significantly lower than that of the others. The ability of cells from a second tumor to form colonies in agar also increased with successive passaging. All clones from the second tumor increased in CFEag to a similar extent as the parental population in the same time period, suggesting the increase was the result of adaptation rather than selection of preexisting variants. A tumor produced from a different subpopulation of transformed cells yielded cells with the same CFEag as the input cells. Thus some populations of transformed cells change some of their characteristics when passaged in animals and again when passaged in culture.