Investigation of the mechanisms and therapeutic implications of crosstalk between G-protein-coupled receptors and the Epidermal Growth Factor Receptor in HNSCC

Head and neck squamous cell carcinoma (HNSCC) is characterized by the overexpression of the epidermal growth factor receptor. However, molecular targeting strategies against EGFR have not improved the 5-year survival rates of HNSCC patients. EGFR tyrosine kinase inhibitors displayed limited clinical responses in Phase II trials and the FDA-approved monoclonal antibody cetuximab (C225) did not prevent the occurrence of secondary tumors and distant metastases. G-protein-coupled receptor ligands; gastrin-releasing peptide (GRP), prostaglandin E2 (PGE2) and bradykinin (BK) have all been reported to activate EGFR in HNSCC via extracellular release of EGFR ligands TGF-á and AR. To improve the efficacy of EGFR inhibition in HNSCC, we investigated the efficacy of targeting common signaling intermediates involved in GPCR-EGFR crosstalk. We previously reported that GRP mediated release of EGFR ligands via phosphoinositide-dependent kinase 1 (PDK1) - dependent phosphorylation of a disintegrin and metalloprotease 17 (ADAM17). We subsequently investigated whether PDK1 mediates EGFR activation downstream of PGE2, BK and LPA pathways and the efficacy of different PDK1 targeting strategies in HNSCC. PGE2, BK and LPA-mediated EGFR phosphorylation was abrogated in PDK1 siRNA-transfected HNSCC cells. PDK1 siRNA also decreased PGE2 and BK-mediated HNSCC growth in vitro. Expression of kinase-dead PDK1 (PDK1M) decreased PGE2 -mediated HNSCC growth. PDK1M HNSCC cells demonstrated reduced proliferation compared to control HNSCC cells. HNSCC cells displayed nanomolar sensitivity to the PDK1 inhibitor OSU-03012 compared to normal mucosal cells. Combined treatment with the EGFR TKIs erlotinib or AG1478, plus OSU-03012 enhanced anti-proliferative effects. We have reported that PGE2 and BK mediated MAPK phosphorylation in the presence of EGFR inhibition, and combined GPCR and EGFR demonstrated additive to synergistic anti-tumor effects. To elucidate the EGFR-independent signaling mediated by GPCRs, we used a forward phase phosphoprotein array to identify potential molecular targets that can potentiate EGFR inhibition. We observed that p70S6K phosphorylation was induced in EGFR siRNA-transfected cells and sustained in cetuximab (C225)-treated cells following PGE2 or BK stimulation. Further investigation showed that p70S6K phosphorylation mediated by EGFR downmodulation was dependent on PDK1 and PKCä expression. Combined targeting of EGFR with cetuximab and p70S6K with the mTOR inhibitor RAD001 decreased GPCR-mediated growth in vitro and in vivo. The results from this study have indicated that targeting the GPCR signaling intermediates PDK1 and p70S6K in conjunction with EGFR may be beneficial therapeutic strategies for the subset of HNSCC patients that respond poorly to cetuximab treatment