The goal of these studies is to determine the required biochemical events that occur between tumor promoter or growth factor-receptor interaction and the activation of effectors of neoplastic transformation. Candidate second messengers include protein phosphorylation dependent on protein kinase C (PKC) or epidermal growth factor (EGF) receptor kinase and kinase-regulated transactivation of gene expression. Phorbol ester- sensitive and -resistant JB6 cells show no differences in PKC isotypes expressed or in PKC translocation or degradation following activation, suggesting that the basis for differential sensitivity is distal to the receptor. Recent studies on 12-0-tetradecanoylphorbol-13-acetate (TPA)- inducible genes have focused on those regulated by the transacting transcriptional factor AP-1 (Jun/Fos complex). The tumor promoters TPA and EGF induce AP-1-regulated gene expression in promotion-sensitive (P+) but not promotion-resistant (P-) JB6 cells, suggesting that AP-1-regulated gene expression may be required for tumor promoter-induced transformation. The mechanism of differential transactivation and transformation by TPA appears to involve differential basal and induced levels of c-Jun and a novel Fra-1-related protein, but does not involve differential induction of c-fos, fosB, junD, or junB. In addition, the phosphorylation of c-Jun and Fra-1 is differentially regulated in P- and P+ cells, suggesting a second promotion-relevant mode of regulation. Knockout of induced c-jun- dependent AP-1 activity by overexpression of a dominant negative c-jun mutant, suggests that AP-1 is required for tumor promoter-induced transformation, i.e., for P+ response. This presents an opportunity for protection of cells and people against carcinogenesis by targeting transcription factors such as AP-1. Overexpression of c-jun, while not sufficient for P- to P+ progression, is, however, sufficient for P+ to tumor cell progression. Examination of a second set of tumor promoter inducible genes, namely TIS genes, reveals that some of them are induced preferentially in P- cells, thus suggesting their possible roles as inhibitors of transformation.
