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 mouse epidermal cells show no differences in PKC isotypes expressed or in PKC translocation or down regulation following activation, suggesting that the basis for differential sensitivity is distal to the receptor. These studies on tumor promoter-inducible genes have focused on those regulated by the transcriptional factor AP-1 (Jun/Fos complex). Phorbol ester and EGF activate 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. New understanding of kinase regulation of gene expression has come from the observation that mitogen-activated protein kinases bind to Jun and Fos family proteins in JB6 cells, thus ruling in the possibility that these kinases regulate the activity of AP-1 in vivo. Inhibition 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. This inquiry has been extended to a mouse keratinocyte model expressing keratin 14 promoter-driven dominant-negative jun. These cells are blocked for expression. The next extension will be to transgenic mice. Overexpression of c-jun is not sufficient for P- to P+ progression, suggesting that another event such as AP-1 protein phosphorylation is limiting. 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.
