EPIDERMAL GROWTH FACTOR RECEPTOR (EGFR) ACTIVATION BY GASTRIN RELEASING PEPTIDE (GRP) IN HEAD AND NECK CANCER: MECHANISMS AND CLINICAL IMPLICATIONS

Head and neck squamous cell carcinomas (HNSCC) are characterized by upregulation of the epidermal growth factor receptor (EGFR). We previously reported that a gastrin-releasing peptide/gastrin-releasing peptide receptor (GRP/GRPR) autocrine growth pathway is activated early in HNSCC carcinogenesis. GRP can induce rapid phosphorylation of EGFR as well as p42/44 MAPK activation, in part via extracellular release of transforming growth factor alpha(TGF-alpha) by matrix metalloproteinases (MMP). Src family kinases have been reported to be activated by G-protein-coupled receptors (GPCRs) followed by downstream EGFR and MAPK activation. To further elucidate the mechanism of activation of EGFR by GRP in HNSCC, we investigated the role of Src family kinases. Blockade of Src family kinases using three different Src-specific tyrosine kinase inhibitors (A-419259, PP2 or PD0180970) decreased GRP-induced EGFR phosphorylation as well as MAPK activation. GRP also failed to induce MAPK activation in dominant-negative c-Src transfected HNSCC cells. Invasion and growth assays demonstrated that c-Src was required for GRP-induced proliferation or invasion of HNSCC cells. In addition to TGF-alpha release, GRP induced amphiregulin, but not EGF, secretion into HNSCC cell culture medium, an effect that was blocked by the MMP inhibitor, Marimastat. TGF-alpha and amphiregulin secretion by GRP stimulation was also inhibited by blockade of Src family kinases. Further investigation showed that TNF-alpha converting enzyme (TACE) underwent Src-dependent phosphorylation and translocation to the plasma membrane in a complex with c-Src and the p85 subunit of PI-3 kinase, where it regulated amphiregulin release. In addition, we identified that PDK1 kinase, a downstream target of PI-3 kinase, directly phosphorylated TACE. Knockdown of PDK1 augmented the anti-tumor effects of the EGFR inhibitor erlotinib. These findings implicate PDK1 as a new target in HNSCC and suggest that therapeutic strategies that block PDK1 may improve the clinical response to EGFR inhibitors.Combined targeting of GRPR and EGFR pathway also showed enhanced anti-tumor efficacy by inhibiting cancer cell proliferation, invasion and promoting apoptosis. Overall, these findings show the promises and benefits of combination therapy when targeting EGFR and GRPR pathways in head and neck cancer