Tumor exosomes: a double-edged sword in cancer therapy

Tumor cells produce and secrete more nucleic acids, proteins and lipids than normal cells. These molecules are transported in the blood or around the cells in membrane-encapsulated exosomes. Tumor-derived or tumor-associated exosomes (usually 30-100 nm in diameter) contain abundant biological contents resembling those of the parent cells along with signaling messengers for intercellular communication involved in the pathogenesis, development, progression, and metastasis of cancer. As these exosomes can be detected and isolated from various body fluids, they have become attractive new biomarkers for the diagnosis and prognosis of cancer. Furthermore, tumor exosomes have also attracted increasing attention due to their potential as novel therapeutic strategies for the treatment of cancers. On the one hand, the lipid bilayer membrane-encapsulated vesicles are promising carriers of drugs and other therapeutic materials targeting specific cancer cells. On the other hand, tumor exosomes are important mediators for modulation of the microenvironment that orchestrates events critical to the growth and metastasis of cancer cells as well as chemoresistance. Here, we summarize the advances in our understanding of tumor-associated or tumor-derived exosomes in recent years, and discuss their roles in cancer development, progression, invasion, and metastasis of cancers and, more importantly, their potential in strategies for precision therapy of various cancers as well as important caveats.