Adaptive evolution of degrees and kinds of neoplastic transformation in cell culture

Human cancers undergo protracted complex development from benign to malignant states, as most thoroughly documented in the mole-to-melanoma sequence. The early stages of the sequence tend to redifferentiate into normal tissues; the later stages progress to ever increasing multiplication and malignancy. When placed under the growth constraint of either crowding or low serum concentrations, the NIH 3T3 line of mouse cells readily undergoes transformation, expressed in the development of foci of cells that continue to multiply at confluence when the rest of the population has stopped. If the nontransformed cells are maintained for 3 months by frequent low-density passages in high concentrations of calf serum, they gradually lose the capacity to undergo transformation when the constraints are applied. The same conditions of passage have been used to reverse the transformation, both processes resembling in principle the reversal of the early stages of the mole-to-melanoma sequence. When the frequent low-density passages are made in high concentrations of fetal bovine serum, which supports a slightly lower growth rate than an equal concentration of calf serum, the degree of transformation is gradually increased, so that the foci become more numerous, broader, and thicker, reaching a maximum in successive assays at about 3 months of passaging. A diversity of focal morphologies is sporadically generated in the calf serum passage by exposing the cells to various concentrations of calf serum for 14 days of growth and confluency before assaying them. The dependence of the number, density, and morphology of foci on the environment in which the cells had been grown before assay reinforces the evidence that the transformation is an epigenetic process. The fact that these effects develop in culture gradually over an extended period of time suggests parallels to the characteristically long-term early regression and later progression, as well as the diversity of the mole-to-melanoma sequence, and may also be representative of other cancers.