Chronic hypoxia for the adaptation of extracellular vesicle phenotype

Variations in oxygen level affect the phenotype of cells and extracellular vesicles (EVs). Depending on the metabolic oxygen demand of cells, hypoxic cell culture can produce conditions more like those found in vivo, and with appropriate oxygen levels, mimic hypoxic tumours. However, most previous experiments studying both EVs and the effects of hypoxia on cells use periods of 72 h or less of hypoxia. We hypothesised that this was insufficient time for adaptation to hypoxic conditions both for EVs and cells which may skew the results of such studies. In this study, the effects of acute (72 h) and chronic hypoxia (> 2 weeks) on the phenotype of HepG2 and PC3 cells and their EVs were examined. Cells could be cultured normally under chronic hypoxic conditions and cryopreserved and recovered. The effects of hypoxia on EV phenotype are slow to establish and dependent on cell line. In PC3 cells, the greatest change in phenotype and increase in EV production occurred only with chronic hypoxic culture. In HepG2 cells, the number of EVs produced was insensitive to hypoxic culture and the greatest changes in protein expression were observed after acute hypoxic culture. Nonetheless, biphasic changes in EV phenotype were detected in both cell types in response to either acute or chronic hypoxia. These results indicate that for cells which do not induce consumptive oxygen depletion, prolonged hypoxic culture is required for complete adaptation.