Cytoskeletal PKC and ERK/MAPK activation in response to N-nitrosamines and phorbol 12-myristate 13-acetate in gastric adenocarcinoma cells.

Phorbol 12-myristate 13-acetate (PMA) exerts its tumor promotional action by targeting cellular PKC. Carcinogenic nitrosamines (NA) induce tumor formation in the absence of a tumor promoter, and have not been implicated as modulators of signal transduction events. We measured the distribution of PKC and ERK/MAPK activities between cytosol, plasma membranes and cytoskeleton of human gastric adenocarcinoma RF-1 cells after 60 minute and 24 hour treatments with 50 microM NA or 100 nM PMA. At 60 minutes, PMA caused translocation of PKC from cytosol to plasma membranes and increased plasma membrane ERK/MAPK activities; 24-hour treatment with PMA resulted in downregulation of membrane PKC and ERK/MAPK, but an increase in cytoskeletal PKC and ERK/MAPK. NA treatment targeted only cytoskeletal activities, producing a sharp increase in PKC at 60 minutes with further increase after 24 hours. Cytoskeletal ERK/MAPK was elevated only after 24 hours with NA. The converging action of NA and PMA on cytoskeletal PKC and ERK/MAPK activities suggests that both agents may focus on a common cellular target.