HUMAN HEMATOPOIETIC STEM CELLS GENERATE EXOSOMES CONTAINING ACTIVE PROTEASOMES BUT NOT CASPASES

Abstract

Activities of two classes of neutral proteases were studied: caspases and proteasomes, which are contained in extracellular vesicles generated by mesenchymal and hematopoietic human stem cells (MSCs and HSCs, respectively). The formation of apoptosomes induced by the cytochrome C and dATP did not occurred in these cells. The presence of TNFa in the culture medium of HSCs causes the appearance of caspase-3 intracellular activity. Herewith caspase activity was detected in exosomes also. This activity was completely inhibited by a non-substrate caspase inhibitor, emricasane, and it was not sensitive to proteasome inhibitors. It is assumed that the caspases’ activity from the cultural medium is the sum of membrane (outer side of exosome membrane), intracellular and extracellular activities. Apparently, this activity refers to apoptosomes as it was revealed by gel filtration of cultural medium with TNFa free from exosomes. Thus, TNFa induces the appearance of neutrophils in the culture of HSCs or the generation of other differentiated cells that are capable of apoptosis, in contrast to HSCs or MSCs. Proteasomal activity increased noticeably in exosomes from HSCs after the addition of TNFa. In contrast to exosomes from MSCs, all activity of proteasomes in HSCs-produced exosomes had a membrane localization.