Combination of Ionizing Radiation with Glutaminase Inhibition Improves Treatment Response in Head and Neck Squamous Cell Carcinoma

Abstract

Head and neck cancer is the 6th most common cancer worldwide. Ionizing radiation (IR) is an important aspect of cancer treatment, however its efficacy is limited by radioresistance, resulting in locoregional failure rates as high as 50%. The pathways used by cells to evade the cellular damage caused by IR are poorly understood. Initial Reverse Phase Protein Array analyses in our laboratory demonstrated that IR induced metabolic pathways, including those involving the TCA cycle. Glutamate is a key substrate of the TCA cycle, and essential for the production of biomolecules necessary for tumor proliferation. Glutaminase is the enzyme that converts glutamine to glutamate. Analysis of The Cancer Genome Atlas's transcriptome database revealed that glutaminase overexpression (upper 25%) in patients with head and neck squamous cell cancer (HNSCC) was associated with significantly reduced patient survival (p<0.03) . In this study, we aimed to identify whether glutaminase inhibition with CB‐839 (Caymen Chemical) could enhance the cellular response to IR. To do this, we used three representative human‐derived HNSCC cell lines: CAL‐27, FaDu, and HN5. We first established that proliferation of all three HNSCC cell lines was glutamine dependent by Dojindo CCK‐8 cell proliferation assay (20–50% increase, n=3 per line, p<0.01). Moreover, a synergistic decrease in cell survival was observed in clonogenic assays with HNSCC cell lines treated with IR and CB‐839 relative to either treatment alone (n=3 per line, p<0.05). In 3D cell culture, which better represents the host microenvironment, combinatorial treatment significantly reduced HNSCC spheroid size (11–26% decrease, n=3 per line, p<0.0001). To further understand the mechanism behind this reduced cell growth and survival, we examined CB‐839's effect on cellular metabolism using Seahorse MitoStress test assays. CB‐839 significantly reduced spare respiratory capacity and OCR/ECAR rate in HNSCC cell lines examined (p<0.001, p< 2.0×10−6,n>3). Mims et al. 2015 previously demonstrated that suppressing spare respiratory capacity enhances radiation sensitivity, which may explain the observed synergistic effects of CB‐839 in combination with IR. In summary, radiation resistance and locoregional failure is a major hurdle in the successful treatment of HNSCC. Combinatorial treatment with IR and the glutaminase inhibitor, CB‐839, led to an enhanced response over IR alone in HNSCC. These results support glutaminase as a possible target to overcome radiation related treatment resistance in patients with HNSCC.Support or Funding InformationPathways to Cancer Therapeutics Training Grant (CMP & SW T32CA117846‐11A1)VA Career Development Award (V.T. IK2 BX004360‐01A1)This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.