Abstract
Despite their individual key roles in promoting head and neck squamous cell carcinoma (HNSCC) progression and treatment resistance, little is known about the impact of intratumoral hypoxia on the activity of the epidermal growth factor receptor (EGFR) signaling pathway in this cancer type. Here, we show that in highly EGFR-expressing HNSCC cells, hypoxic stress triggers the activation of the EGFR and downstream targets, including Akt and phospholipase C (PLC) gamma1. In support of these findings, we also demonstrate that EGFR activation takes place within hypoxic foci in a subset of human HNSCC tissues. Whereas hypoxia had no major effect on HNSCC cell proliferation, it markedly altered tumor cell shape by inducing morphological changes consistent with a more spindle-shaped, fibroblast-like morphology together with an enhanced migratory capacity. We found that hypoxia-induced EGFR activation and cell migration could be prevented by targeting EGFR signaling with the tyrosine kinase inhibitor tyrphostin, the phospholipase C inhibitor U73122, or by inhibiting the expression of the alpha subunit of hypoxia-inducible factor 2 via RNA interference or the topoisomerase II inhibitor etoposide. Our results position hypoxia-inducible factor-2alpha as a novel regulator of EGFR activation under low oxygen conditions, and suggest that hypoxia-induced EGFR signaling may promote a more aggressive phenotype in a fraction of HNSCC tumors. Because EGFR continues in the forefront as a highly attractive target in clinical oncology, further studies are warranted to define the mechanistic and therapeutic implications of the hypoxic response relative to the EGFR signaling pathway in head and neck cancer.