Isolation of methylcholanthrene-"initiated" C3H/10T1/2 cells by inhibiting neoplastic progression with retinyl acetate.

A clone of 3-methylcholanthrene-treated 10T1/2 cells has been isolated which possesses basic characteristics expected of "initiated" cells. In the presence of retinyl acetate, this clone exhibits contact inhibited growth control and is morphologically indistinguishable from the parental 10T1/2 cell line. Removal of retinyl acetate in vitro results in neoplastic transformation after a latent period of 3 weeks. The classical 10T1/2 transformation system was reconstructed by coculturing normal and "initiated" 10T1/2 cells formed either Type II or Type III foci after a latent period of 3-4 weeks, and an additional 22% formed Type I foci. Treatment of "initiated" 10T/2 cells with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate accelerated the formation of transformed foci in the coculture system by reducing the length of the latent period to less than 3 weeks. Injection of 10(6) "initiated" cells/mouse s.c. into nude mice resulted in the appearance of progressively growing fibrosarcomas after a latent period of 5-7 weeks. Dietary supplementation with 4-hydroxyphenyl-retinamide prevented tumor formation; after drug withdrawal, tumors developed in all surviving mice after 6 weeks. We believe this cell line possesses all characteristics expected of "initiated" cells. With this new cell line, designated INIT/10T1/2, we can now study the early biochemical changes in growth control mechanisms resulting in neoplastic transformation and the mechanism(s) of chemoprevention of cancer by vitamin A.