Sirt6 Loss Increases Stemness And Defense Against Oxidative Stress In Tumor-Propagating Cells, Promoting Tumor Growth And Maintenance In Squamous Cell Carcinoma

Head and neck squamous cell carcinoma (HNSCC) is one of the most common cancers in the US. As such, a better understanding in HNSCC carcinogenic process and contributing molecular mechanisms may pinpoint to new therapeutic interventions for the patients. Notably, despite extensive genetic studies, little is known on the metabolic features of HNSCC. Metabolic reprogramming to aerobic glycolysis has recently emerged as a key event during carcinogenesis. Our lab has shown that the histone deacetylase Sirt6 inhibits HIF-1α- and c-Myc-dependent transcription, both of which are critical in metabolic reprogramming of cells, thereby acting as a tumor suppressor in the context of colon cancer. Based on these results, we aimed to assess whether Sirt6 loss facilitates squamous cell carcinogenesis, specifically affecting glucose metabolism and stemness of cancer cells. In an in vivo model of skin squamous cell carcinogenesis with Sirt6 deletion (K14-cre+; Sirt6fl/fl), tumor onset was faster in Sirt6fl/fl mice compared to WT animals, and those tumors from Sirt6fl/fl mice were bigger. Sirt6 loss also positively regulates tumor maintenance without continuous proliferative cues, suggesting that Sirt6 deletion could impact on the tumor propagating cells (TPCs). Strikingly, loss of Sirt6 increased the percentage of TPCs (a6high/CD34+), most of which were highly glycolytic. Administration of a glycolytic inhibitor (dichloroacetate) in vivo effectively inhibited tumor growth and maintenance, resulting in much fewer percentages of TPCs. Our results indicate that enhanced glucose metabolism is crucial in maintaining and expanding TPCs. In human HNSCC cell lines, Sirt6 negatively regulates glucose metabolism and cell growth in vitro and ex vivo, nicely recapitulating the in vivo observations. Glucose isotope tracing experiments in vitro suggest that pentose phosphate pathway and generation of antioxidant molecules are main metabolic pathways affected by Sirt6. Transcriptomic analysis of TPCs from in vivo skin tumors revealed critical gene expression signatures, where TPCs exhibit decreased differentiation, increased stem-like, and enhanced antioxidative/redox markers. Sirt6 loss even further up-regulated stemness genes and the genes that generate antioxidant molecules, providing molecular mechanisms to explain a more aggressive tumorigenic phenotype in Sirt6-deleted tumors. Overall, our studies indicate that high glycolytic flux (which can contribute to the pentose phosphate pathway and antioxidant molecules) is crucial in TPCs. Our studies clearly demonstrate that Sirt6 acts as a tumor suppressor by regulating glucose metabolism and stemness of tumor cells. This study has further therapeutic implication that targeting antioxidant defense pathways could effectively eradicate TPCs, critical population maintaining a whole tumor. Citation Format: Jee-Eun Choi, Carlos Sebastian, Caroline Lewis, Murat Cetinbas, Gregory Wojtkiewicz, Ruslan Sadreyev, Leif W. Ellisen, Salvador Aznar-Benitah, Raul Mostoslavsky. Sirt6 loss increases stemness and defense against oxidative stress in tumor-propagating cells, promoting tumor growth and maintenance in squamous cell carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4992.