Exosomes in blood and cancer
Exosomes are the smallest of the extracellular vesicles (EVs). They have been detected in a variety of body fluids, including serum and plasma. Exosomes from tumor cells that are found in the circulation (and elsewhere) are enriched in certain intracellular components, including miRNAs. While many small studies document the association of miRNAs, DNA, protein or lipid alterations within exosomes with the presence of a given type of cancer, few if any of these findings have been independently validated. Indeed, there are many gaps in our knowledge of the role of extracellular RNA, DNA, proteins or lipids in cancer detection, prognosis or therapy. Some of the important questions which need to be addressed for a given cancer type include assessing which: (I) body fluid(s) to study; (II) molecular components within a given body fluid to study (RNA, DNA, proteins or lipids contained within and/or outside of EVs); and (III) specific markers within the body fluid fraction provide the most information regarding diagnosis, prognosis, and/or treatment of a given cancer. The most common body fluid analyzed to evaluate the role of exosomes in cancer is blood. Herein we review published findings related to studies that have analyzed exosomes obtained from blood or its components serum or plasma. We also review resources that are available to assist the clinician and scientist wishing to learn more about exosomes. Vesiculopedia and Exocarta are websites dedicated to gathering molecular information on EVs (all sizes) and exosomes, respectively. They contain information on EV content by molecular type (DNA, RNA, protein, lipid) and species from which the EV study originated. The International Society of Extracellular Vesicles and the Journal of Extracellular Vesicles are dedicated to advancing knowledge of EVs. The National Institutes of Health has funding 30 research grants which seek to increase our understanding of extracellular RNA, including that contained within EVs.