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Caspase-3–dependent cleavage of Bcl-xL in the stroma exosomes is required for their uptake by hematological malignant cells

Ioulia Vardaki, Claire Sanchez, Pedro Fonseca, Magnus Olsson, Dimitrios Chioureas, George Rassidakis, Anders Ullén, Boris Zhivotovsky, Magnus Björkholm and Theocharis Panaretakis

Key Points

  • Bcl-xL is a substrate for active caspase-3 in the exosomes.

  • Molecular or chemical inhibition of exosomal Bcl-xL cleavage attenuates uptake of exosomes by hematological malignant cells.

Abstract

The intercellular crosstalk between hematological malignancies and the tumor microenvironment is mediated by cell-to-cell interactions and soluble factors. One component of the secretome that is gaining increasing attention is the extracellular vesicles and, in particular, the exosomes. Apart from the role as vectors of molecular information, exosomes have been shown to possess intrinsic biological activity. In this study, we found that caspase-3 is activated in L88 bone marrow stroma cell–derived exosomes and identified 1 of the substrates to be the antiapoptotic protein Bcl-xL. The cleaved Bcl-xL is found in a panel of normal and cancer cell–derived exosomes and is localized on the outer leaflet of the exosomal membrane. Incubation of the exosomes with a caspase-3 inhibitor or the pan-caspase inhibitor prevents the cleavage of Bcl-xL. Importantly, MCF-7 cell–derived exosomes that are caspase-3–deficient are enriched in full-length Bcl-xL, whereas ectopic expression of caspase-3 restores the cleavage of Bcl-xL. Chemical inhibition of Bcl-xL with ABT737 or molecular inhibition by using the D61A and D76A Bcl-xL mutant leads to a significant decrease in the uptake of exosomes by hematopoietic malignant cells. These data indicate that the cleaved Bcl-xL is required for the uptake of exosomes by myeloma and lymphoma cells, leading to their increased proliferation. In summary, we demonstrate for the first time that Bcl-xL is an exosomal caspase-3 substrate and that this processing is required for the uptake of exosomes by recipient cells.

  • Submitted May 10, 2016.
  • Accepted October 5, 2016.
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