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We previously have identified a subpopulation of contact-insensitive (CS-) cells which lacks density-dependent inhibition of cell division in primary and low-passage cultures of Syrian hamster embryonic (SHE) fibroblastic cells. Further, we have shown that the proportion of these CS- cells declines as a result of the stable phenotypic conversion of the CS- cells to contact-sensitive (CS+) cells. To determine whether these transient CS- cells are more sensitive to carcinogenic/mutagenic perturbation, the susceptibility to neoplastic transformation and somatic mutation induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was examined in clonally isolated cell cultures containing various proportions of CS- cells (0.02-4%). The frequencies of morphological transformation, focus formation, and neoplastic transformation showed a positive correlation to the proportion of CS- cells in the treated cultures. In contrast, the frequency of MNNG-induced somatic mutation at the Na+,K+-ATPase locus was similar among cultures varying in their proportion of CS- cells. Thus, there is a transient subpopulation of CS- cells in primary SHE cell cultures that is more susceptible to neoplastic transformation although equally susceptible to induced point mutation. This dissociation between somatic point mutation and neoplastic transformation indicates a fundamental difference in the nature of these two phenomena. A possible relationship between the propensity of CS- cells (versus CS+ cells) to carcinogen-induced neoplastic transformation and the state of differentiation of the CS- cells is discussed.

A contact-insensitive subpopulation in Syrian hamster cell cultures with a greater susceptibility to chemically induced neoplastic transformation.