A qualitative and quantitative assay for cells lacking postconfluence inhibition of cell division: characterization of this phenotype in carcinogen-treated Syrian hamster embryo cells in culture.

We have developed a qualitative and quantitative assay system for detecting cells lacking postconfluence inhibition of cell division (contact insensitivity, CS-) in golden Syrian hamster embryo cells in culture by measuring the number of cells able to form colonies on a lethally irradiated, confluent monolayer of a contact-sensitive established cell line. A subpopulation in normal low-passage cultures of golden Syrian hamster embryo cells temporarily exhibits this CS- phenotype at very low frequency (approximately 4 x 10(-3)) but quickly loses the property within a few passages in vitro. This phenotype is invariably exhibited by various tumorigenic cell lines at very high frequency (7 to 50 x 10(-2)) and appears to correlate with the anchorage-independent growth phenotype. The temporal acquisition of the CS- phenotype by tertiary-passage golden Syrian hamster embryo cells following exposure to N-methyl-N'-nitro-N-nitrosoguanidine was examined. Cells with a stably heritable CS- phenotype are detected after approximately 20 posttreatment population doublings. In contrast, anchorage-independent cells are not detected until 35 to 95 posttreatment population doublings. These CS- cells appear to be preneoplastic cells, since clonally isolated CS- cells did not exhibit anchorage-independent growth until further passaging in vitro. The results suggest that acquisition of the CS- phenotype represents an early stage in neoplastic progression.