Activation by Tyrosine Phosphorylation as a Prerequisite for Protein Kinase Cζ to Mediate Epidermal Growth Factor Receptor Signaling to ERK

The atypical protein kinase C zeta (PKC zeta) was recently shown to mediate epidermal growth factor (EGF)-induced activation of extracellular signal-regulated kinase (ERK) in head and neck squamous carcinoma (HNSCC) cells. Here, it is shown that EGF may induce tyrosine phosphorylation of PKC zeta in several HNSCC cells, breast carcinoma cells, as well as mouse embryonic fibroblasts. In COS-7 cells overexpressing EGF receptor (EGFR) and PKC zeta as a tumor cell model, we show that PKC zeta tyrosine phosphorylation by EGF is induced by catalytic activation. Using a loss-of-function mutant of PKC zeta, we can show that the tyrosine residue 417 in PKC zeta plays an important role in both PKC zeta activation and the ability of PKC zeta to mediate activation of ERK. The importance of PKC zeta in EGF-induced ERK activation can also be shown in several HNSCC and breast carcinoma cell lines as well as in PKC zeta-deficient mouse embryonic fibroblasts. In addition, we present several lines of evidence suggesting the physical association of PKC zeta with EGFR and the importance of the EGFR tyrosine kinase c-Src and the Src-specific phosphorylation site pY845-EGFR in the tyrosine phosphorylation as well as catalytic activation of PKC zeta. This study characterizes PKC zeta as a novel mitogenic downstream mediator of EGFR and indicates PKC zeta as a therapeutic target in some carcinomas. Mol Cancer Res; 8(5); 783-97. (C)2010 AACR.