Mechanistic studies on the role of extracellular vesicles in the tumor microenvironment

Sammanfattning: The transition from normal into malignant cells with acquired uncontrolled growth is a process where cancer cells viciously develop adaptive strategies to overcome microenvironmental and metabolic stress. Hypoxia, or deficient oxygen supply, is a common feature of the expanding tumor which alters cell metabolism and consecutively causes lowered intracellular pH. To alleviate hypoxic and acidotic stress, cancer cells respond by promoting blood vessel formation, angiogenesis, and by activating mechanisms for increased acid-base control. These adaptive processes involve complex modes of cell communication in the tumor microenvironment. The aim of this thesis was to investigate the role of extracellular vesicles (EVs) in the hypoxic tumor microenvironment and the mechanisms by which EVs mediate cell-cell communication as well as regulatory aspects of hypoxia-induced acidosis. In the first part (papers I and II), we provide new insights into the mechanisms of EV transfer between cells. It is demonstrated that a significant proportion of cancer cell-derived EVs is transferred into recipient cells by binding to the glycan chains of the cell surface receptors, heparan sulfate proteoglycans (PGs). We further show that EVs are internalized through a lipid raft-mediated endocytosis pathway negatively regulated by caveolin-1. In the second part (papers III and IV), we show that EVs originating from hypoxic cancer cells exhibit a specific molecular profile and induce a more pronounced pro-angiogenic response in endothelial recipient cells and in an in vivo tumor model as compared to normoxia-derived EVs. EVs may thus serve as biomarkers wherein tumor specific conditions, such as hypoxia, are reflected. Finally, we found that EVs may constitute a functionally active route for the release of the major pH regulatory enzyme, carbonic anhydrase IX (CAIX). Molecular studies revealed that CAIX in cells and in EVs can be substituted with specific glycosaminoglycan chains, defining CAIX as a part-time PG. These findings advance our understanding of the role of EV-dependent signaling in the tumor microenvironment. Data presented in this thesis identify molecular mechanisms by which EVs mediate cell-cell communication that constitute potential targets for therapeutic interventions.