Uncoupling of protein kinase D from suppression of EGF-dependent c-Jun phosphorylation in cancer cells.

Protein kinase D (PKD) has been established as a negative modulator of the c-Jun N-terminal kinase (JNK) signaling pathway. We previously demonstrated that induced expression of constitutively active PKD (PKD-S744/748E) that mimics phosphorylation by PKC is sufficient to attenuate epidermal growth factor (EGF) stimulated c-Jun Ser 63 phosphorylation, a natural substrate of JNK, in HEK 293 cells. Because the JNK pathway has been implicated in sustaining both lung and pancreatic cancerous phenotypes, we have utilized stable inducible expression of PKD-S744/748E in clones of A549 non-small cell lung cancer (NSCLC) and Panc1, pancreatic cancer cells to determine its effects on JNK signaling in the context of the cancerous phenotype. In contrast to HEK 293 cells, induced expression of PKD-S744/748E in either A549 NSCLC or Panc1 cells failed to attenuate EGF dependent phosphorylation of c-Jun, indicating that EGF stimulated JNK phosphorylation of c-Jun is uncoupled from PKD suppression in these cancer cells.