Abstract 57: Survival of dormant breast cancer cells and metastatic tumor recurrence is dependent upon the activation of autophagy

Abstract

Abstract Cancer recurrence resulting from the metastatic outbreak of dormant disseminated tumor cells following the apparent successful treatment of the primary tumor is a major cause of breast cancer mortality. However, little is known regarding the molecular mechanisms governing tumour cell dormancy and the dormant-to-proliferative switch, impeding the development of effective therapeutic strategies. We therefore investigated whether stress-induced autophagy may promote survival of dormant cancer cells and, consequently, inhibition of autophagy could prevent breast cancer recurrence. To address the functional role of autophagy in breast cancer progression and the potential therapeutic impact of its inhibition, we utilized mouse and human 3D in vitro and in vivo preclinical models of dormancy. The analysis of autophagy markers and use of an autophagic flux biosensor allowed direct visualization of autophagic vesicles and their evolution in breast cancer dormant cells over time. In agreement with our hypothesis, pharmacologic or genetic inhibition of autophagy in dormant breast cancer cells resulted in significantly decreased cell survival and metastatic burden in vitro and in vivo. Furthermore, the inhibition of autophagy prevented the spontaneous dormant-to-proliferative switch of highly metastatic cells. In contrast, proliferating disseminated cells were insensitive to autophagy blockade. Indeed, in vivo analysis of the autophagic flux over time confirmed that autophagy is a critical survival process activated and maintained in dormant breast cancer cells, which is shut down after the cells undergo the dormant-to-proliferative switch. Transcriptomic analysis and in vivo metastatic burden assays identified the autophagy-related 7 (ATG7) gene, but not Beclin1 (BECN1), to be essential for autophagy activation, indicating that a non-canonical autophagy pathway is activated in dormant breast cancer cells. Co-localization studies identified mitochondria as the predominant autophagosomal cargo in breast cancer dormant cells. Mechanistically, inhibition of the autophagic flux in dormant breast cancer cells led to the accumulation of depolarized mitochondria and reactive oxygen species (ROS), resulting in cell apoptosis. This study has important implications regarding the role of autophagy in breast cancer progression and suggests that inhibition of autophagy may be of therapeutic value in preventing breast cancer recurrence. Furthermore, it provides novel insights into the molecular mechanisms for survival of breast cancer dormant cells. Citation Format: Laura Vera Ramirez, Suman K. Vodnala, Ryan Nini, Kent W. Hunter, Jeffrey E. Green. Survival of dormant breast cancer cells and metastatic tumor recurrence is dependent upon the activation of autophagy [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 57.