12‐ O ‐tetradecanoylphorbol‐13‐acetate—induced levels of ap‐1 proteins: a 46‐kda protein immunoprecipitated by anti‐fra‐1 and induced in promotion‐resistant but not promotion‐sensitive JB6 cells

Neoplastic transformation and transcriptional activation by activator protein‐1 (AP‐1) complex are stimulated by tumor‐promoting agents in promotion‐sensitive (P + ) but not promotion‐resistant (P ‐ ) mouse epidermal JB6 cells in culture. This implicates AP‐1 as a specific regulator of signal transduction pathways in the promotion phase of neoplastic transformation. We therefore hypothesized that the defective P ‐ responsiveness may be due to limiting levels of AP‐1 protein components in those cells. In this investigation, steady‐state levels of AP‐1 protein components were measured by immunoprecipitating proteins from 12‐ O ‐tetradecanoylphorbol‐13‐acetate (TPA)‐treated P + and P ‐ cells to discern what may limit the AP‐1 response. Whereas the AP‐1 proteins junB, junD, and fosB did not show differential basal or TPA‐inducible levels in P + and P ‐ cells, a 46‐kDa species precipitated by anti‐fra‐1 antibody was TPA‐inducible in P ‐ cells but not in P + cells, and c‐jun protein was present at higher levels in TPA‐treated and untreated P + cells than in P ‐ cells. These data raise the possibility that the 46‐kDa fra‐1‐related protein may be a negative modulator of AP‐1 activity and suggest that elevated levels of this 46‐kDa species and limiting levels of c‐jun may significantly impair AP‐1 function or transformation response in P ‐ cells or both. © 1992 Wiley‐Liss, Inc.