TANK Binding Kinase 1 (TBK1) is a non-canonical IkB kinase that contributes to KRAS-driven lung cancer. It is activated by phosphorylation of Serine-172 by TLR and RIG1 signaling, and this circuit triggers phosphorylation of IRF3 and IRF7, activation of NFκB and the expression of proinflammatory genes and interferons. In addition to its role role in regulating innate immunity, TBK1 also promotes oncogenesis by phosphorylating Akt and enhancing cell survival and by promoting autophagy and mitophagy. TBK1 is also induced under hypoxic conditions and expressed at significant levels in many solid tumors. TBK1 also contributes to prostate cancer dormancy and drug resistance by inhibiting mTOR and to tamoxifen resistance of breast cancer cells by enhancing transcriptional activity of ERα.
Recent studies from our lab revealed a novel role for TBK1 in regulating mitosis. It was found that levels of phospho-TBK1 increases and localizes to centrosomes and the mitotic spindles during mitosis. Depletion of TBK1 was shown to trigger defects in spindle apparatus and prevents mitotic progression (Pillai et al., Nature Communications, 2015). TBK1 physically interacts and phosphorylates centrosomal protein CEP170 and mitotic apparatus protein NuMA. At the same time, it is not clear whether TBK1 regulates mitosis my modulating the Spindle Assembly Checkpoint (SAC).
Our results show that TBK1 colocalizes with Cdc20 on the centrosomes. Additionally, TBK1-inhibited cells showed an increase in the colocalization of BUBR1 and Cdc20, with enhanced recruitment of BUBR1 to kinetochores. To further study how TBK1 affects SAC, lung and breast cancer cells were depleted of TBK1 by CRISPR/Cas9 mediated genome editing. Cells depleted of TBK1 showed very few cells in the mitotic stage; those that entered mitosis had residual levels of TBK1 and showed multipolarity and unusually stable and bundled microtubules. TBK1 knockout cells not only showed aberrant mitotic structures and had elevated levels of SAC components including BUBR1 and Cdc20. Surprisingly, level of mitotic Cyclin B1 remained unchanged in spite of elevated levels of Cdc20 indicating a possible inactivation of Anaphase Promoting complex (APC/C). Also, percentage of Cyclin B1 positive cells was significantly high in mitotic cells enriched using double thymidine block in the presence of TBK1 inhibitor BX795 (R9+BX) and MRT67307 (R9+MRT) as compared to untreated mitotic cells (R9). Further, double thymidine blocked released cells displayed elevated levels of SAC components upon treatment with TBK1 inhibitors. All these findings suggest that TBK1 facilitates mitotic progression through satisfying SAC.
Citation Format: Neha Jaiswal, Smitha Pillai, Namrata BoraSinghal, Srikumar Chellappan. TBK1 regulates mitotic progression by modulating spindle assembly checkpoint in cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3460. doi:10.1158/1538-7445.AM2017-3460