Protein kinase C zeta mediates epidermal growth factor-induced growth of head and neck tumor cells by regulating mitogen-activated protein kinase.

Protein kinase C (PKC) zeta 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 PKCzeta is required for tumor cell proliferation and viability. Examination of total and phosphorylated PKCzeta expression in normal oral mucosa, dysplasia, and carcinoma as well as SCCHN tumor cell lines revealed a significant increase in activated PKCzeta expression from normal to malignant tissue. PKCzeta activity is required for EGF-induced extracellular signal-regulated 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 mitogen-activated protein kinase (MAPK) activity suppressed DNA synthesis. Consistent with this observation, inhibition of PKCzeta using either kinase-dead PKCzeta mutant or peptide inhibitor suppressed autocrine and EGF-induced DNA synthesis. Finally, PKCzeta 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) PKCzeta is associated with SCCHN progression, (b) PKCzeta mediates EGF-stimulated MAPK activation in keratinocytes and SCCHN cell lines, (c) PKCzeta mediates EGFR and MAPK-dependent proliferation in SCCHN cell lines; and (d) PKCzeta inhibitors function additively with other inhibitors that target similar or complementary signaling pathways.

Effect of complementary pathway blockade on efficacy of combination enzastaurin and rapamycin.

BACKGROUND: Rapamycin is an mTOR inhibitor with preclinical efficacy in squamous cell carcinoma of the head and neck (SCCHN). However, mTOR inhibitors also increase Akt activity in SCCHN cell lines, which would promote survival and oncogenesis. Enzastaurin is an AGC kinase inhibitor with nanomolar inhibitory concentrations for Akt and protein kinase C (PKC). Moreover, Akt and PKC inhibitors have demonstrated efficacy in SCCHN. METHODS: We hypothesized that the combination of rapamycin and enzastaurin would be more effective than either agent alone. RESULTS: Rapamycin and enzastaurin generally inhibited putative targets in SCCHN cell lines in culture. In mice xenografted with CAL27 cells, rapamycin and enzastaurin produced growth delay. In contrast, the combination of rapamycin and enzastaurin caused regression of CAL27 tumors with evidence of inhibition of putative targets, survival, angiogenesis and proliferation. CONCLUSION: These data demonstrate that the combination of rapamycin and enzastaurin disrupts critical oncogenic pathways in SCCHN and has efficacy in preclinical models.

Efficacy of the multi-kinase inhibitor enzastaurin is dependent on cellular signaling context.

The number of targeted small molecules being developed in oncology is increasing rapidly. Many of these are designed to inhibit multiple kinases, and thus the mechanisms of responsiveness and predictive biomarkers can be difficult to discern. In fact, with few exceptions, multi-kinase inhibitors are developed with limited mechanism-based patient selection. Enzastaurin is a multi-kinase inhibitor being studied in several malignancies that we hypothesized would be active in squamous cell carcinoma of the head and neck, because it inhibits classic and novel protein kinase C isoforms. Indeed, enzastaurin reduced the growth of SQ-20B and CAL27 tumor xenografts, decreased proliferation in these cell lines, inhibited putative target phosphorylation, and induced cell cycle arrest. Gene expression arrays confirmed that expression of cell cycle genes, including cyclins D and E, were significantly altered by exposure to enzastaurin. However, testing a panel of squamous cell carcinoma of the head and neck cell lines revealed variable sensitivity to enzastaurin, which correlated significantly with baseline cyclin D1 protein expression. Moreover, sensitivity and resistance could be reversed, respectively, by expression or depletion of cyclin D1. Furthermore, analysis of sensitive and resistant cell lines revealed distinct differences in cyclin D1 regulation. Enzastaurin modulated cyclin D1 synthesis through an Akt-regulated pathway in the former, whereas high-level CCND1 gene amplification was present in the latter. These results underscore the critical relevance of cellular signaling context in developing cancer therapies in general and suggest that enzastaurin in particular would be most effective in tumors where baseline cyclin D1 expression is low to moderate and physiologically regulated.

Mechanisms of resistance to EGFR inhibitors in head and neck cancer.

Epidermal growth factor receptor (EGFR), a receptor tyrosine kinase that activates multiple signaling pathways, including phosphatidylinositol-3-kinase/v-AKT murine thymoma viral oncogene homolog protein (Akt), has long been a target of novel therapies. Despite universal EGFR expression in head and neck squamous cell carcinoma (HNSCC), the majority of patients do not respond to EGFR inhibitors. This review focuses on mechanisms of resistance to these agents in HNSCC, and how these may be unique when compared with other malignancies such as non-small cell lung and colorectal cancers. Published studies and abstracts reveal that there are likely several mechanisms underlying resistance, suggesting that different strategies will be required to improve efficacy of EGFR inhibitors in HNSCC.

Factors associated with clinical benefit from epidermal growth factor receptor inhibitors in recurrent and metastatic squamous cell carcinoma of the head and neck.

Single agent epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKI) have demonstrated reproducible response rates of 5-15% in treatment of squamous cell carcinomas of the head and neck (SCCHN). The subset of patients that benefits most from these agents remains unknown. We reviewed individual patient data from five clinical trials of erlotinib, lapatinib, or gefitinib to determine if there are clinical characteristics that are associated with clinical benefit defined as complete response (CR), partial response (PR), and stable disease (SD) >4months. Secondary endpoints included progression-free survival (PFS) and overall survival (OS). Three-hundred and nineteen subjects were included. Observed responses were: 1% CR, 6% PR, 24% SD >4months, 18% SD <4months, 45% progressive disease (PD), 7% not evaluable (NE). The median OS was 6.4months and the median PFS was 2.7months. The most common toxicities observed were rash (grade 1 in 37%, grade 2 in 33%, grade 3+ in 6%) and diarrhea (grade 1 in 30%, grade 2 in 10%, grade 3+ in 5%). Performance status (PS) (p=0.04), older age (p=0.02), and development of rash (p<0.01), diarrhea (p=0.03), or oral side effects (p=0.02) were independently associated with clinical benefit. Older age, better PS, and development of rash were associated with longer PFS and OS. Clinical parameters that appear to predict response to EGFR TKI include PS and age. EGFR mechanistic toxicities that develop during therapy are also highly associated with benefit and suggest a relationship between drug exposure and outcome.