Exosome-dependent trafficking of HSP70 - A novel secretory pathway for cellular stress proteins

The heat shock proteins (HSPs) are a family of intracellular proteins found in all eukaryotes and prokaryotes. Their functions are well characterized and are central to maintaining cellular homeostasis and in promoting cell survival in response to stressful cellular conditions. However, several studies provide evidence that specific members of the HSP family might be secreted via an unidentified exocytotic pathway. Here we show that exosomes, small membrane vesicles that are secreted by numerous cell types, contribute to the release of HSP70 from human peripheral blood mononuclear cells (PBMCs) in both basal and stress-induced (heat shock at 40 or 43 C for 1 h) states. HSP70 release from PBMCs is independent of the common secretory pathway because Brefeldin A, an inhibitor of the classical protein transport pathway, did not block HSP70 release. Furthermore, we show that HSP70 release from PBMCs does not occur via a lipid raft- dependent pathway, because treatment with methyl-beta-cyclodextrin, a raft-disrupting drug, had no affect on HSP70 release. To examine whether exosomes contributed to HSP70 release from PBMCs, exosomes were purified from PBMC cultures, and exosomal number and HSP70 content were determined. We demonstrate that although heat shock does not influence the exosomal secretory rate, the HSP70 content of exosomes isolated from heat shocked PBMCs is significantly higher than control. These data identify a novel secretory pathway by which HSP70 can be actively released from cells in both the basal and stress-induced state.