Rab20 Is A Stress-Induced Regulator Of Mitochondrial Homeostasis

Mitochondrial homeostasis is tightly regulated via a combination of biogenesis and elimination via a form of selective autophagy called mitophagy. Given the importance of mitochondria for energy production, cellular redox status, and apoptotic signaling, the mitophagy pathway is of particular interest in cancer. While mitophagy has been reported to both assist and hinder tumorigenesis in different studies, the emerging consensus is that mitophagy supports tumor growth. Intriguingly, only a subset of the canonical autophagy machinery is required for mitophagy. Upstream regulators including ULK1, the phosphatidyl-inositol-3-phosphate (PI3P) effectors and components of the omegaosome DFCP1 and WIPI, and ATG9 are recruited to mitochondria independent of LC3. How these components are recruited to mitochondria and the mechanisms involved in engulfment/creation/scaling of the autophagosome around the mitochondrion are unclear. Here we report a novel role for the small GTPase RAB20 in mitochondrial homeostasis. Rab GTPases are master regulators of vesicle trafficking, templating membrane identity, coordinating vesicle docking, fusion, and cargo selection in concert with SNAREs and membrane tethers. Rab20 is normally expressed at low levels but can be induced by in vitro simulated metabolic stress (1% oxygen, glucose deprivation) or secondary to the stress of an actively growing tumor in vivo. Surprisingly, we discovered that silencing RAB20 in multiple Non-Small Cell Lung Cancer (NSCLC) cell lines impairs ischemia-induced mitophagy without impacting autophagic flux measured by LC3 lipidation. This defect is accompanied by an increase in mitochondrial mass. RAB20 silencing is associated with increased Reactive Oxygen Species both under nutrient replete and ischemic conditions. Further, Rab20 appears to influence mitochondrial network organization, manifest as elevated numbers of fragmented mitochondria in the Rab20 silenced cells compared to non-targeting vector isogenic vector control cells. Finally, mitochondrial respiration is impaired in RAB20 silenced cells. Taken together, this work indicates an important role for RAB20 in maintaining mitochondrial fitness. We are actively assessing the functional consequences of Rab20 ablation on lung tumorigenesis in an established mouse model of lung cancer driven by BrafV600E; Trp53−/−. These studies will be important in determining the role of RAB20 and its involvement in mitophagy on tumor progression in vivo. Citation Format: Sunayana Govind Nayak, Samantha Shaw, Colin Stets, Anthony Dent, Anne Marie Strohecker. RAB20 is a stress-induced regulator of mitochondrial homeostasis [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 1242.