Autophagy machinery contributes to cell survival and small extracellular vesicle composition in triple-negative breast cancer cells

Macroautophagy (hereafter autophagy) is a catabolic cellular process where double-membraned autophagosomes capture cytoplasmic cargos and fuse with lysosomes for content degradation. Basal autophagy maintains cellular homeostasis by removing long-lived proteins and damaged organelles. Autophagy can also be upregulated to promote cell survival in the presence of stressors such as starvation and oxidative stress. Autophagy can suppress tumorigenesis by maintaining genome stability in normal cells, or enable cancer cell survival during nutrient limitation, hypoxia or chemotherapy treatment. Therefore, inhibiting autophagy may improve chemotherapy efficacy. Triple-negative breast cancers (TNBC) are a subtype of breast cancers that do not over-express hormone receptors. Chemotherapy remains one of the few systemic treatment options for TNBC, making the development of chemotherapy resistance particularly problematic in disease management. This thesis describes cell-intrinsic and cell-extrinsic functions of autophagy machinery in cultured TNBC cells, and explores the potential utility of autophagy inhibition to enhance treatment response. Cytoprotective autophagy was induced in response to epirubicin treatment in TNBC cells. Autophagy inhibition reduced cell viability and improved efficacy of epirubicin in both drug-naïve and drug-resistant cells. Further investigation revealed cell-extrinsic roles of autophagy, in the form of its contribution to the composition of small extracellular vesicles (sEV), nano-sized vesicular entities with known roles in cell-cell communication. Lysosomal inhibition by chloroquine (CQ) induced co-localization of mammalian autophagy-related (ATG) 8 homologs with endolysosomal tetraspanins, and introduced significantly higher levels of ATG8s in TNBC-derived sEV. The concurrent increase in poly-ubiquitinated proteins and autophagy adaptors in sEV suggested a potential mechanism where degradative cargos are loaded into sEV by autophagy machinery and then expelled. CQ-induced enrichment of ATGs was limited to a subpopulation of sEV, highlighting the heterogeneity and context-dependency of sEV composition. Finally, CQ-mediated lysosomal inhibition was found to dampen the growth-promoting effects of sEV in recipient cells. Taken together, this work demonstrated cytoprotective roles of autophagy in TNBC cells, and the dynamic contribution of autophagy machinery to sEV composition, warranting further examination of autophagy inhibition as a potential therapeutic avenue in TNBC.