Abstract 1984: Cell fate reprogramming of liver tumor-initiating stem-like cells via phosphorylated NUMB and TBC1D15

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Abstract Tumor-initiating stem-like cells (TICs) are defective in their control of asymmetric division. Stem cells are maintained through asymmetric self-renewing divisions in which one daughter cell commits to a specific fate while the other retains the multipotent characteristics of its parent. The p53 interacting protein NUMB preserves this intrinsic cellular asymmetry and functions as a vital barrier against unchecked expansion as seen in TICs. How TICs overcome this control of asymmetric division to cause cancer is unknown. The hypothesis is that accentuated TLR4-NANOG-mediated NUMB phosphorylation and TBC1D15 upregulation promote self-renewal of TICs in liver oncogenesis caused by HCV and alcohol. Here, we determined the causal roles of NUMB phosphorylation and TBC1D15 oncoprotein in alcohol-induced liver tumorigenesis in HCV-Tg mice. Liver-specific TBC1D15 deficiency or non-phosphorylatable mutations of NUMB, reduce liver tumor incidence and tumor-associated NANOG+ TICs in HCV NS5A Tg mice fed alcohol Western diet. TICs lost asymmetric division capacity and overexpressed TBC1D15, which associated with NUMB to promote aPKCζ activity and NUMB phosphorylation. Thus, TIC self-renewal is dependent on TBC1D15 and NUMB phosphorylation. Similarly, aPKCζ-mediated NUMB phosphorylation and induced TBC1D15 are mutually required for self-renewal. Furthermore, TBC1D15 activated NOTCH via binding to NOTCH intracellular domain to trigger its proteolytic. TBC1D15 interacts with all NOTCH isoforms, activates NOTCH pathway, and induces Nanog and TIC self-renewal in a NOTCH-dependent manner. TBC1D15 cooperates with the NOTCH pathway to support TIC tumorigenic activity and an inhibitor of this interaction is potentially therapeutic. Candidate small molecule antagonists have been tested for their ability to block the interaction of TBC1D15-NOTCH/NICD. The originality of this findings lies within the discovery of the novel oncoprotein TBC1D15 and its unique oncogenic activities involving p53 degradation and cooperation with the NOTCH pathway in TICs and CD133+ Huh7 cells. These mechanistic findings have laid down the foundation for a new translational path for discovery of new therapeutic targets. Citation Format: Keigo Machida, Hifzur R. Siddique, Mengmei Zheng, Peleg Winer, Dinesh Babu Uthaya Kumar, Ahmed Rokan, Linda Sher, Stanley M. Tahara, Michael Elowitz, Chengyu Liang, Hidekazu Tsukamoto. Cell fate reprogramming of liver tumor-initiating stem-like cells via phosphorylated NUMB and TBC1D15 [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 1984.