Abstract 2129: Induction of autophagy as a mechanism of therapeutic resistance in head and neck cancer

Abstract Background and purpose: Despite decades of research into improved treatments for head and neck cancer, therapeutic resistance remains a major challenge for this malignancy, with roughly 40% of patients developing recurrent disease. Among the plethora of intrinsic and acquired mechanisms of resistance, recent evidence has suggested the cellular process of autophagy may be an additional contributor. This stress response, which normally functions during periods of nutrient deprivation to maintain cellular homeostasis, may have the unintended consequence of protecting tumor cells from the cytotoxic effects of chemotherapy and radiotherapy. We show that autophagy mediates resistance to chemotherapies targeting the EGFR and mTORC pathways in addition to radiotherapy in several head and neck cancer models, begin to elucidate the mechanism of this resistance, and demonstrate that the addition of a specific autophagy inhibitor can block cytoprotective autophagy. Methods and results: Building on well established evidence that mTORC inhibition can directly induce autophagy, recent work has established a novel mechanism by which kinase inactive EGFR resulting from either growth factor starvation or anti-EGFR antibody (cetuximab) can upregulate this response via an interaction with LAMPT4B. Treatment of a panel of HNC cell lines with either serum starvation or cetuximab produced increased formation of LC3-B puncta, a marker for autophagasome formation, by IF staining, and was confirmed by Western blot for increased LC3-B flux and decreased p62. SiRNA knockdown of either EGFR or LAMPT4B in HNC cells abrogated the induction of autophagy in response to either serum starvation or cetuximab treatment. We have also shown that radiation induces autophagy in a time and dose dependent manner. The mechanism of radiation-induced autophagy remains a subject of study; preliminary data suggest that both EGFR and LAMPT4B play a role as siRNA knockdown of those mRNAs reduces the autophagic response. To determine whether blockade of cytoprotective autophagy can help overcome therapeutic resistance, we have tested specific autophagy inhibitors in combination with either chemotherapy or radiation. The novel ULK1 inhibitor SBI-0206965 successfully blocks the induction of autophagy in a panel of HNC lines. More compellingly, while neither SBI-0206965 nor the NEDD4 inhibitor Heclin, impacted cell growth in untreated cells, both were effective in reducing cell survival following treatment with radiation, cetuximab, or mTORC inhibitor (AZD8055). Summary: These pre-clinical studies have established the proof of concept for the cytoprotective effect of autophagy in response to anti-cancer treatments including EGFR or mTORC inhibition and radiotherapy. Further we have identified the addition of specific autophagy inhibitors to standard treatments as a potential strategy to overcome this mechanism of resistance. Citation Format: Adam D. Swick, Tan Xiaojun, Tyler Fowler, Kwangok Nickel, Richard A. Anderson, Randall J. Kimple. Induction of autophagy as a mechanism of therapeutic resistance in head and neck cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2129.