Abstract B16: Loss of SIRT6 reactivates the RNA-binding protein Lin28b to drive pancreatic cancer
Authors: Kugel Sita , Sebastián Carlos , Fitamant Julien , Ross Kenneth N. , Saha Supriya K. , Jain Esha , Gladden Adrianne , Arora Kshitij S. , Kato Yasutaka , Rivera Miguel N. , Ramaswamy Sridhar , Sadreyev Ruslan I. , Goren Alon , Deshpande Vikram , Bardeesy Nabeel , Mostoslavsky Raul 
Published: December 2016

Abstract
Chromatin remodeling proteins are frequently dysregulated in human cancer; however, little is known about how they control tumorigenesis. The NAD+ dependent histone deacetylase Sirtuin 6 (SIRT6) is a nutrient sensor that reprograms the epigenome in response to nutrient stress and shows reduced expression in human pancreatic ductal adenocarcinoma (PDAC) relative to normal tissue. Thus, we aimed to determine the role of SIRT6 in PDAC and to determine how the loss of this chromatin modifier could influence PDAC formation, progression and metastasis.
Here, we uncover an epigenetic program mediated by SIRT6 that is critical for the suppression of PDAC. We demonstrate that SIRT6 inactivation cooperates with oncogenic KRAS to greatly accelerate the development of lethal pancreatic tumors regardless of p53 status. In addition to developing PDAC and high-grade pancreatic intraepithelial neoplasia (PanIN) at an earlier age, SIRT6-deficient tumors had a greater propensity to metastasize to the lung, compared to their SIRT6 wild-type (WT) counterparts. Through genome-wide profiling of SIRT6 WT and knock-out (KO) PDAC cell lines, we identified the oncofetal RNA-binding protein Lin28b as a target of Sirt6-mediated transcriptional repression. Loss of SIRT6 in both murine and human PDAC cell lines resulted in hyperacetylation of histone 3 lysine 9 and lysine 56 at the Lin28b promoter, MYC recruitment, and pronounced induction of Lin28b expression. Knocking down Lin28b resulted in potent suppression of cell proliferation and tumor sphere formation in Sirt6null/low murine and human cell lines, while murine and human Sirt6WT/high PDAC lines were completely insensitive to the same treatment. Functionally, Lin28b binds to and induces the degradation of the let-7 family of microRNA, thus resulting in increased expression of let-7 target genes such as the IGF2BPs and HMGA2, which have been correlated with increased agressiveness and metastasis in pancreatic tumors. Furthermore, Gene Set Enrichment Analysis (GSEA) comparing PDAC tumors and cell lines with high versus low expression of LIN28B revealed that LIN28Bhigh tumors were strongly enriched for the expression of let-7 targets. Finally, either loss of SIRT6 or the reactivation of Lin28b or the let-7 target genes IGF2BP3 and HMGA2 correlated with a worse overall survival in human PDAC patients.
Thus, our critical findings define SIRT6 as a tumor suppressor in PDAC, working as a transcriptional repressor of the oncofetal RNA-binding protein Lin28b. We demonstrate that loss of Sirt6 creates a permissive hyperacetylated chromatin state, allowing for MYC-dependent transactivation of Lin28b. This work not only provides new insights into the epigenetic mechanisms governing the reactivation of oncofetal proteins in cancer but also demonstrates that this pathway is well-conserved in human PDAC, where reduced SIRT6 expression defines a specific subset of tumors which may benefit from novel therapeutic approaches aimed at targeting components of the LIN28B/let-7 pathway.
This abstract is being presented as a short talk in the scientific program. A full abstract is printed in the Proffered Abstracts section PR09 of the Conference Proceedings.
Citation Format: Sita Kugel, Carlos Sebastián, Julien Fitamant, Kenneth N. Ross, Supriya K. Saha, Esha Jain, Adrianne Gladden, Kshitij S. Arora, Yasutaka Kato, Miguel N. Rivera, Sridhar Ramaswamy, Ruslan I. Sadreyev, Alon Goren, Vikram Deshpande, Nabeel Bardeesy, Raul Mostoslavsky.{Authors}. Loss of SIRT6 reactivates the RNA-binding protein Lin28b to drive pancreatic cancer. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2016 May 12-15; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(24 Suppl):Abstract nr B16.