Exosomes in Tumor Angiogenesis—Multifunctional Messengers With Mixed Intentions

Tumor angiogenesis has been a primary target as well as a key challenge in cancer treatment. A central factor underlying tumor angiogenesis is the balance between pro- and antiangiogenic signals, which is the result of the dynamic communication between cancer cells and the tumor microenvironment. Exosomes have emerged as a robust platform that mediates cell–cell communication in many biological contexts. Here, we review the research that have revealed the important roles of exosomes in tumor angiogenesis. We emphasize that exosomes are heterogeneous in terms of their angiogenic activities, depending on their cellular origin, molecular contents, and functional contexts. We suggest that innovative approaches to develop exosome-based targeting or delivery tools, harnessing the unique properties of the exosomes in regulating tumor angiogenesis, will foster progress in cancer therapy.