Cell-cell contact interactions conditionally determine suppression and selection of the neoplastic phenotype.

Separation of chemical and physical carcinogenesis into the stages of initiation (mutation) and promotion (selection) established that incipient neoplastic cells could persist in the organism indefinitely without expression. Spontaneous mutations associated with cancer also lie dormant in untreated normal tissue. Without selection, there is no tumor development. Experiments in cell culture showed that confluent normal fibroblasts suppress growth of contacting transformed fibroblasts, and that normal keratinocytes similarly suppress tumor formation by adjacent papilloma cells. With cells that are generally more susceptible to transformation, however, prolonged contact inhibition progressively selects mutants that favor neoplastic growth. Selection of individual mutant cells allows them to become a significant fraction of the population and creates an enlarged target for additional genetic hits. Crucially, this enrichment step, not the initial mutation step, is the numerically limiting factor in tumor development. Unexpectedly, variants that are resistant to spontaneous transformation are selected in vitro by growing cells for many low density passages at maximal exponential rate. Confluent cultures of resistant variants suppress the growth and normalize the morphology of contacting transformed cells. Varying the conditions for selection shows that tumorigenic transformation is preceded by intermediate steps of progressively higher saturation density that are increasingly permissive for the expression of the more neoplastic cells in the population. There is also evidence of increasing permissiveness with age of normal tissues in vivo for solitary cancer cells transplanted in their midst. Spontaneous transformation in culture can be used to identify dietary components that are required for promotion and may therefore be applicable in prevention of human cancer.