Association of resistance to terminal differentiation with initiation of carcinogenesis in adult mouse epidermal cells.

Primary adult mouse epidermal cells were maintained as a monolayer culture with a high proliferation rate in fibroblast-conditioned medium with low Ca2+ concentration (less than 0.1 mM). Terminal differentiation of the cultures was induced by raising the Ca2+ level in the medium above 0.1 mM. Treatment of adult mouse epidermal cells either in vivo or in vitro with 7,12-dimethylbenz(a)anthracene or N-methyl-N'-nitro-N-nitrosoguanidine yielded colonies which were resistant to terminal differentiation induced by Ca2+. The number of resistant colonies was dependent upon the dose of each carcinogen used whether exposure was in vivo or in vitro. Cultures derived from skin initiated in vivo with 7,12-dimethylbenz(a)anthracene, a strong initiator, resulted in more colonies than were derived from skin initiated with N-methyl-N'-nitro-N-nitrosoguanidine, a moderately potent initiator. Two mouse strains, BALB/c and SENCAR, which differ in sensitivity to skin carcinogenesis, yielded similar numbers of Ca2+-resistant colonies following carcinogen exposure. However, colonies developed spontaneously from untreated SENCAR cells (the sensitive strain), but not from BALB/c cells (the resistant strain). These results support the concept that cells resistant to terminal differentiation are initiated cells. The results also suggest that initiation may occur spontaneously in SENCAR skin, a finding consistent with the reported occurrence of tumors in mice of this strain receiving promoters without exogenous initiator.