Critical comparison of ultracentrifugation and a commercial kit for isolation of exosomes derived from glioblastoma and breast cancer cell lines
Exosomes are small extracellular vesicles around 30-100 nm in diameter that are secreted from cells and can be found in most body fluids. Exosomes can be a vital source of biomarkers as they contain various substances (e.g. lipids, RNAs, metabolites and proteins) that can reflect the cell of origin (e.g. cancer cells). For isolation of exosomes present in biological matrices, ultracentrifugation (UC)-based procedures are most common. Other approaches exist, including commercial kits developed for easy and low sample volume isolation. In this study, differential UC and an isolation kit from a major vendor (Total Exosome Isolation Reagent from Thermo Fisher Scientific) were compared. Exosomes were isolated from cell culture media of two different cell sources (patient derived cells from glioblastoma multiforme and the breast cancer cell line MDA-MB-231). For both isolation methods, transmission electron microscopy, dynamic light scattering and western blotting indicated the presence of exosomes. The kit- and UC isolates contained similar amounts of protein measured by the bicinchoninic acid (BCA) assay with absorbance at 562 nm. Using western blot, positive exosome markers were identified in all isolates, and additional exosome markers were identified using MS-based proteomics. For the glioblastoma exosome isolates, the number of proteins identified with liquid chromatography tandem MS (LC-MS/MS) was higher for the UC isolates than the kit isolates when injecting equal protein amounts, contrary to that for the breast cancer exosome isolates. However, negative exosome markers were also found in glioblastoma isolates using LC-MS/MS. Thus, we would not use the term \"exosome isolation\" as impurities may be present with both isolation methods. Notably, potential biomarkers for both diseases were identified in the isolates using LS-MS/MS. In our opinion, the two isolation methods had rather similar performance, although with some minor differences based on cell of origin.