Quantitative neoplastic transformation of C3H/10T1/2 fibroblasts: dependence upon the size of the initiated cell colony at confluence.

The efficiency of transformation of C3H/10T1/2 fibroblasts by chemical or physical carcinogens varies inversely with the seeding density of the cells. Colony size at confluence, accumulated cell generations, and epigenetic events independent of seeding density have been proposed to explain this variability. By controlling the total number and colony size (i.e., cells/colony) of initiated cells at confluence and their accumulated cell generations, we have determined that (a) the colony size of the initiated cells at confluence is directly related to the expression of the transformed phenotype, (b) the probability that an initiated cell will form a colony of transforming cells does not vary with the accumulation of large numbers of cell generations, and (c) the total number of initiated cells in a confluent culture does not determine the number of transformed foci formed. We have developed a mathematical representation which, when applied to our results and the published results from other laboratories, clearly describes the relationship between the size of the initiated cell colony at confluence and the expression of the transformed phenotype. These results have significant implications in the quantification of the transformation of C3H/10T1/2 cells by chemical and physical carcinogens.