Tumor progression in nude mice and its representation in cell culture

Varying dilutions containing from 10(6) to 10(3) spontaneously transformed Balb/3T3 cells were inoculated into nude mice [N:NIH(S)II]. Less than half the mice inoculated with 10(3) cells developed tumors. The higher concentrations of cells produced visible tumors in all mice within 2-3 weeks, and these tumors grew rapidly to large sizes. Some tumors initiated by the lower concentrations of cells arose quickly, but others were greatly delayed in onset, then grew slowly, if at all, for several weeks before a rapid acceleration. The delayed acceleration can be considered a form of tumor progression. When first explanted into culture, cells from the early tumors multiplied somewhat more slowly than the parental cells that initiated the tumors, but narrowed the gap in a few weekly passages. By contrast, only a small fraction (less than or equal to 0.001) of cells from the longest delayed tumors could sustain multiplication in culture, although flow cytometry revealed them to have been a rapidly multiplying population when explanted. A relatively large fraction of these explanted tumor cells incorporated a 1-hour pulse of [3H]thymidine into DNA, although at a low rate. The shift to culture apparently slowed progress through the S-period of the cell cycle. The multiplication rate of cell populations from the delayed tumors increased in successive passages in culture. There was great heterogeneity in growth capacity among clones of the tumor cells. The growth rates of some clones declined to the point of extinction, those of others remained constant for several weeks, while those of still others steadily increased in growth rate. The low initial cloning efficiency of cells from the delayed tumors and the heterogeneity of growth rates among the clonable cells indicate that selection plays a major role in the increase of the growth capacity of the cell population. The steady increase in growth rates within clones suggests that physiological adaptation also contributes to the progressive growth of the tumor populations in culture. The results constitute a rationale for using the progressive growth of cells in culture as a model system for discriminating the types of cellular changes that underlie tumor progression.