Constitutive or Induced HIF-2 Addiction is Involved in Resistance to Anti-EGFR Treatment and Radiation Therapy in HNSCC

Background: management of head and neck squamous cell carcinomas (HNSCC) include anti-Epidermal Growth Factor Receptor (EGFR) antibodies and radiotherapy, but resistance emerges in most patients. RAS mutations lead to primary resistance to EGFR blockade in metastatic colorectal cancer but are infrequent in HNSCC, suggesting that other mechanisms are implicated. Since hypoxia and Hypoxia Inducible Factor-1 (HIF-1) have been associated with treatment failure and tumor progression, we hypothesized that EGFR/mammalian Target of Rapamycin (mTOR)/HIF-1 axis inhibition could radiosensitize HNSCC. Methods: We treated the radiosensitive Cal27 used as control, and radioresistant SQ20B and UD-SCC1 cells, in vivo and in vitro, with rapamycin and cetuximab before irradiation and evaluated tumor progression and clonogenic survival. Results: Rapamycin and cetuximab inhibited the mTOR/HIF-1 alpha axis, and sensitized the SQ20B cell line to EGFR-inhibition. However, concomitant delivery of radiation to SQ20B xenografts increased tumor relapse frequency, despite effective HIF-1 inhibition. Treatment failure was associated with the induction of HIF-2 alpha expression by cetuximab and radiotherapy. Strikingly, SQ20B and UD-SCC1 cells clonogenic survival dropped <30% after HIF-2 alpha silencing, suggesting a HIF-2-dependent mechanism of oncogenic addiction. Conclusions: altogether, our data suggest that resistance to EGFR inhibition combined with radiotherapy in HNSCC may depend on tumor HIF-2 expression and underline the urgent need to develop novel HIF-2 targeted treatments.