Protein Kinase Cζ Mediates Epidermal Growth Factor–Induced Growth of Head and Neck Tumor Cells by Regulating Mitogen-Activated Protein Kinase

Abstract

Protein kinase C (PKC) Z has been implicated as a mediator of epidermal growth factor (EGF) receptor (EGFR) signaling in certain cell types. Because EGFR is ubiquitously expressed in squamous cell carcinomas of the head and neck (SCCHN) and plays a key role in tumor progression, we determined whether PKCZ is required for tumor cell proliferation and viability. Examination of total and phosphorylated PKCZ expression in normal oral mucosa, dysplasia, and carcinoma as well as SCCHN tumor cell lines revealed a significant increase in activated PKCZ expression from normal to malignant tissue. PKCZ activity is required for EGF-induced extracellular signalregulated kinase (ERK) activation in both normal human adult epidermal keratinocytes and five of seven SCCHN cell lines. SCCHN cells express constitutively activated EGFR family receptors, and inhibition of either EGFR or mitogenactivated protein kinase (MAPK) activity suppressed DNA synthesis. Consistent with this observation, inhibition of PKCZ using either kinase-dead PKCZ mutant or peptide inhibitor suppressed autocrine and EGF-induced DNA synthesis. Finally, PKCZ inhibition enhanced the effects of both MAPK/ERK kinase (U0126) and broad spectrum PKC inhibitor (chelerythrine chloride) and decreased cell proliferation in SCCHN cell lines. The results indicate that (a) PKCZ is associated with SCCHN progression, (b) PKCZ mediates EGF-stimulated MAPK activation in keratinocytes and SCCHN cell lines, (c) PKCZ mediates EGFR and MAPK-dependent proliferation in SCCHN cell lines; and (d) PKCZ inhibitors function additively with other inhibitors that target similar or complementary signaling pathways. (Cancer Res 2006; 66(12): 6296-303)

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

“…Furthermore, in the COS-7 cell model as well as in several carcinoma cell lines, we showed the critical involvement of PKCζ in the activation of ERK by EGF. Our results also confirm the finding of Cohen et al (11) that in HNSCC cells, the requirement of PKCζ for maximal ERK activation by EGF seems to be cell specific. In addition, here we can show that not only in HNSCC but also in breast carcinoma cells, PKCζ may contribute to activation of ERK downstream of EGFR.…”

“…Furthermore, in several breast carcinoma cells (25) and non-small-cell lung cancer cells (35), PKCζ is required for the function of EGF-induced chemotaxis in the metastasis of cancer cells (36,37). More recently, PKCζ was shown to mediate EGF-induced activation of ERK in several HNSCC cell lines (11). However, the molecular mechanism by which PKCζ becomes activated through the EGFR remained completely unclear.…”

“…To investigate the molecular mechanism by which PKCζ might be able to mediate oncogenic EGFR signaling, we considered recently published data suggesting a critical role of PKCζ in EGF-induced activation of MAPK in HNSCC (11) or chemotaxis in breast carcinoma (25) cells. We used these carcinoma cell types to investigate PKCζ tyrosine phosphorylation, and after treating the cells with 10 nmol/L EGF for 5 minutes, we found significant tyrosine phosphorylation of PKCζ in two HNSCC cell lines (FaDu and PE/Ca-PJ-15), three breast carcinoma cell lines (MCF-7, T47D, and MDA-MB-468), and, for comparison, A431 cells.…”

“…In addition, signal transducer and activator of transcription (STAT) proteins, including STAT3 and STAT5b, have been described to contribute to the EGFR-mediated oncogenic process in HNSCC (9,10). Recently, a novel mitogenic pathway including protein kinase Cζ (PKCζ) has been reported that mediates EGF-induced activation of mitogenactivated protein kinase (MAPK) and MAPK-dependent proliferation in several HNSCC cell lines (11). Whereas the Ras-independent activation of ERK by PKCζ as well as by other PKC isoforms has been repeatedly described (12,13), the molecular mechanism of how PKCζ may be activated downstream of the EGFR is completely unknown.…”

cAMP antagonizes ERK-dependent antiapoptotic action of insulin

“…PI3K can stimulate protein kinase C zeta (PKCζ) in response to insulin in rat adipocytes and muscles and insulin-induced activation of PI3K is sensitive to Ptx (22). Moreover, PKCζ can activate ERK upon EGF stimulation in tumor cells (25). Thus, the antiapototic action of insulin can be exerted through the following signaling cascades based on our observations and the results of previous studies.…”