Differential induction of c-fos and c-jun proto-oncogenes and AP-1 activity by tumor promoter 12-O-tetradecanoyl phorbol 13-acetate in cells at different stages of tumor promotion in vitro.

A two-stage (initiation and promotion) model of chemical transformation of mouse embryonic fibroblasts was used to elucidate the molecular mechanisms of tumor promotion in vitro. C3H10T1/2 cells which had been initiated with a subcarcinogenic dose (0.5 micrograms ml-1) of benzo[a]pyrene (B[a]P) were isolated after 12-O-tetradecanoyl phorbol 13-acetate (TPA) treatment lasting 12, 24 or 36 days. These series of partially promoted cells were designated T-12, T-24 and T-36 cells. T-12 and T-24 cells exhibited higher anchorage-independent growth on soft agar in the presence of TPA than did the initiated or T-36 cells. Cytosolic protein kinase C (PKC) in the resting state was slightly depleted in T-24 and T-36 cells. Proteolytic down-regulation of membrane-bound PKC was enhanced in T-12 cells compared with the initiated cells after 3 h of TPA treatment. Induction of c-fos and c-jun proto-oncogene expression increased two- to threefold in T-12 cells. Moreover, the basal level of c-fos mRNA progressively increased in T-12, T-24 and T-36 cells. As compared with other cell types, T-12 cells had the highest AP-1 DNA-binding activity at both the basal level and at 30 min after TPA treatment. These results indicate that deregulation of the TPA-induced cellular responses occurs in the cells at various stages of tumor promotion, and might be associated with transforming processes.