Non-Small Cell Lung Cancer Utilizes SIRT6 to Initiate Tumorigenesis and Overcome Cellular Senescence

Lung cancer is one of the three most common cancers among men and women and causes more cancer-related deaths than any other type of carcinoma. Of the two subtypes, Non-small cell lung cancer (NSCLC) is the most common. These tumors are characteristically high grade, PET-positive, indicating heightened glucose uptake and altered cancer metabolism. Sirtuin 6 (SIRT6) is a member of the sirtuin family of histone deacetylases and has previously been shown to be a key regulator of glucose metabolism, DNA damage repair, and aging. SIRT6 has been demonstrated to act as a tumor suppressor in colon, pancreatic and liver cancer. However, its role in NSCLC remains elusive. Here, we have developed a mouse model to study the role of SIRT6 in NSCLC, utilizing a KRASG12D driver mutation and p53flox/flox to drive lung tumorigenesis in response to intranasal deliver of adenovirus expressing Cre recombinase. Work described here indicates that SIRT6 expression is necessary for lung tumor initiation and progression. Loss of SIRT6 expression resulted in a delay of tumor growth leading to decreased tumor burden and prolonged survival compared to animals expressing both wild-type alleles of SIRT6. Utilizing NSCLC developed mouse cell lines, we were able to show that cells lacking SIRT6 exhibited decreased proliferation, decreased formation of colonies of soft agar and increased senescence, providing a possible mechanism for the delay in tumor initiation and formation in vivo. Since NSCLC maintain SIRT6 mRNA expression in primary tumors, we sought to identify a potential mechanism by which cancer cells post-translationally regulate SIRT6 activity. In this study, we propose a mechanism where by SIRT6 can be lost in response to the metabolic state of the cell. Data presented here shows that SIRT6 undergoes a glucose-responsive N-terminal cleavage that is predicted to result in loss of its chromatin-localization sequence (CLS), causing it to dissociate from chromatin. More specifically, we found that SIRT6 is directly O-GlcNAcylated and that the O-GlcNAc transferase (OGT) is necessary for this cleavage to occur. Our lab has previously shown that OGT also O-GlcNAcylates the NF-κB subunit, p65, to allow for its full transcriptional activation. As SIRT6 and NF-κB have been previously shown to physically interact and antagonize one another in transcription of key metabolic genes, loss of the CLS of SIRT6 and transcriptional activation of p65 in response to O-GlcNAcylation may serve as a mechanism for p65-induced transcription of metabolic target genes. Work described in this thesis underscores the importance of SIRT6 as an essential enzyme required for NSCLC development, yet once carcinoma is established it becomes dynamically misregulated by altered metabolic flux of the cancer cell.