Investigating the Role of the Chromatin Factor SIRT6 in Metabolism of Squamous Cell Carcinoma

Abstract

Epigenetic alterations and metabolic reprogramming are two critical emerging features of cancer. However, in Head and Neck Squamous Cell Carcinoma (HNSCC), how epigenetic changes or metabolic alterations may influence establishment and progression of squamous cell carcinomas remains basically unknown. Especially, lack of understanding of these alterations in a particular cell type or in a particular stage of SCC prompted us to investigate their roles in squamous cell carcinogenesis. In this PhD thesis, using a combination of human cell lines, a Genetically Engineered Mouse Model (GEMM) of SCC where I conditionally deleted SIRT6, and biochemical studies, I demonstrated that: 1) SIRT6, an histone H3K9 and H3K56 deacetylase, acts as a robust tumor suppressor in both human HNSCC and in an in vivo mouse model of cutaneous SCC. 2) Increased glycolysis by loss of SIRT6 is essential for tumor cell growth and maintenance 3) Using LC-MS metabolic profiling and 13C-labeled glucose, I uncovered a role for SIRT6 in regulating the oxidative Pentose Phosphate Pathway (PPP). The oxidative PPP is important for generation of antioxidant molecules such as NADPH and GSH and de novo nucleotide synthesis. 4) SIRT6 deficiency especially enriches for Tumor Propagating Cells (TPCs) by elevating gene expression involved in glucose metabolism, antioxidant responses, and stemness, contributing to a more aggressive tumor phenotype, and 5) Sorting of TPCs followed by RNA-seq analysis demonstrated that Sirt6-deficient TPCs are more glycolytic with elevated PPP, possess a highly active antioxidant program, and exhibit a more stem-like phenotype compared to WT TPCs. Collectively, this thesis research underscores a novel tumor suppressive role for SIRT6 in HNSCC which specifically affects TPCs by epigenetically regulating glucose metabolism and antioxidant responses, suggesting that both chromatin dynamics and metabolism are critical modulators of these unique cells. Furthermore, my studies provide strong evidence for metabolic and epigenetic vulnerabilities of TPCs in SCC, both features that could be therapeutically exploited in the future.