Abstract 5815: Sox2 elevation downregulates a broad spectrum of cell cycle machinery and induces a reversible state of tumor growth arrest
Authors: Metz Ethan P. , Wuebben Erin L. , Wilder Phillip J. , Rizzino Angie 
Published: September 2020

Abstract
Purpose: The purpose of these studies was to understand how elevated SOX2 affects tumor cell proliferation. The stem cell transcription factor SOX2 has been implicated in the progression and chemoresistance of ~20 different tumor types. Importantly, several studies have demonstrated that the effects of SOX2 on proliferation are dependent on its dosage. Significantly, elevated SOX2 expression is associated with restricted cell proliferation during development and tumor progression. SOX2high, quiescent tumor cells present a significant clinical challenge as these cells are largely resistant to conventional chemotherapies and thus, provide a reservoir to drive disease recurrence. Therefore, understanding the mechanisms through which elevated SOX2 inhibits tumor cell proliferation will provide insights into the biology of drug-resistant, quiescent tumor cells.
Methods: A diverse group of tumor cell lines representative of medulloblastoma (MB), prostate cancer (PCa), neuroblastoma (NB), and pancreatic ductal adenocarcinoma (PDAC) were engineered to express SOX2 from a doxycycline (Dox)-inducible promoter. The effect of SOX2 elevation on proliferation was examined both in vitro and in vivo. Cell cycle analysis was performed by flow cytometry. Kinetics of tumor growth were determined by the subcutaneous engraftment of SOX2-inducible tumor cells in nude mice. SOX2 expression was induced in vivo by addition of Dox to the water of the mice.
Results: Elevating SOX2 in MB, PCa, NB, and PDAC cells inhibited the proliferation of each cell line in vitro. Intriguingly, SOX2 elevation in six different tumor cell lines did not significantly alter their cell cycle distribution, despite strongly inhibiting proliferation. In agreement with these results, western blot analysis of whole cell extracts determined that elevating SOX2 leads to the downregulation of a broad spectrum of cyclins and cyclin-dependent kinases (CDKs) governing progression through multiple phases of the cell cycle. Additionally, SOX2 elevation increased the expression of p27Kip1, which inhibits CDKs regulating each phase of the cell cycle. Importantly, we also determined that elevating SOX2 inhibits the growth of MB, PCa, and PDAC xenografts. Significantly, when Dox was withdrawn and SOX2 returned to endogenous levels, tumor growth rapidly resumed and paralleled the growth kinetics of control tumors.
Conclusion: Our findings demonstrate that elevating SOX2 significantly inhibits the growth of a diverse group of tumor types. Significantly, SOX2, when elevated, appears to function as a master regulator of the cell cycle machinery by downregulating a broad spectrum of cyclins and CDKs. Additionally, our inducible model of tumor growth arrest provides a novel platform to investigate the biology of quiescent tumor cells and the testing of clinically used drugs to target quiescent/dormant tumor cells in vivo.
Citation Format: Ethan P. Metz, Erin L. Wuebben, Phillip J. Wilder, Angie Rizzino. Sox2 elevation downregulates a broad spectrum of cell cycle machinery and induces a reversible state of tumor growth arrest [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5815.