PKCε acts as negative allosteric modulator of EGF receptor signalling.

Protein kinase C ε (PKCε) is a transforming oncogene and plays a pivotal role in numerous cellular processes including proliferation, invasion and differentiation. Recently, we described a function of PKCε as a scaffold protein linking PLCγ1 to the EGFR module. Here, in the head and neck squamous carcinoma cell line (HNSCC) FaDu we demonstrate that over-expressed PKCε may be associated with the EGFR. This is linked with the consecutive inhibition of the recruitment of PLCγ1 to the EGFR, of the catalytical activation of PLCγ1 by EGF, and of the PLCγ1-mediated effect of EGF on cell proliferation. These effects are independent of the catalytical as well as the scaffold activity of PKCε but are a function of the cellular expression level of PKCε. In contrast to FaDu cells where the PLCγ1 pathway was selectively affected, in three other HNSCC cell lines investigated over-expression of PKCε resulted in association with EGFR and, subsequently, in either partial (ERK and Akt or PLCγ1 and Akt) or complete (ERK, PLCγ1 and Akt) inhibition of the main EGFR signalling pathways. Together, our data suggest that in particular carcinoma cells highly expressed PKCε may act as negative allosteric modulator of EGFR signalling. This novel function of PKCε provides also the first indication that the EGFR may be a target for allosteric modulation by accessory proteins.

Activation by tyrosine phosphorylation as a prerequisite for protein kinase Cζ to mediate epidermal growth factor receptor signaling to ERK.

The atypical protein kinase Czeta (PKCzeta) 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 PKCzeta in several HNSCC cells, breast carcinoma cells, as well as mouse embryonic fibroblasts. In COS-7 cells overexpressing EGF receptor (EGFR) and PKCzeta as a tumor cell model, we show that PKCzeta tyrosine phosphorylation by EGF is induced by catalytic activation. Using a loss-of-function mutant of PKCzeta, we can show that the tyrosine residue 417 in PKCzeta plays an important role in both PKCzeta activation and the ability of PKCzeta to mediate activation of ERK. The importance of PKCzeta in EGF-induced ERK activation can also be shown in several HNSCC and breast carcinoma cell lines as well as in PKCzeta-deficient mouse embryonic fibroblasts. In addition, we present several lines of evidence suggesting the physical association of PKCzeta 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 PKCzeta. This study characterizes PKCzeta as a novel mitogenic downstream mediator of EGFR and indicates PKCzeta as a therapeutic target in some carcinomas.