Microenvironmental Regulation of Metastasis by Exosomes

Exosomes and microvesicles are small spherical membrane fragments shed by several cell types during physiological processes. Increasing attention has been focused on the role of exosomes in malignant progression and metastasis. This chapter highlights the functional roles of tumor exosomes in influencing pre-metastatic niche formation and metastatic behavior. We discuss their interaction with cells within the tumor microenvironment (bone marrow-, endothelial-, and mesenchymal-derived cells) and attempt to shed light on the mechanisms underlying these interactions. We address the emerging role of tumor-secreted exosomes as new players that change the tumor microenvironment during tumor progression. Furthermore, we comment on the contribution of exosomes secreted by non-tumor cells such as endothelial, bone marrow, and mesenchymal cells to metastatic progression. Data are presented to highlight the role of exosomes in influencing the metastatic behavior of tumors. We conclude that exosome research has the potential to uncover new targets and lead to new therapies against tumor and metastatic progression.