Exosomes are small membrane vesicles secreted into the extracellular compartment by exocytosis.
Tumor exosomes may be involved in the sampling of
antigens to antigen presenting cells or as decoys allowing the
tumor to escape immune-directed destruction. The
proteins present in exosomes secreted by
tumor cells have been poorly defined. This study describes the
protein composition of
mesothelioma cell-derived exosomes in more detail. After electrophoresis of exosome preparations, matrix-assisted
laser desorption ionization time-of-flight (MALDI-TOF) was used to characterize the
protein spots. MHC class I was found to be present together with the
heat shock proteins HSC70 and HSP90. In addition, we found
annexins and PV-1,
proteins involved in membrane transport and function. Cytoskeleton
proteins and their associated
proteins ezrin,
moesin, actinin-4,
desmoplakin, and
fascin were also detected. Besides the molecular motor
kinesin-like
protein, many
enzymes were detected revealing the cytoplasmic orientation of exosomes. Most interesting was the detection of developmental endothelial locus-1 (DEL-1), which can act as a strong
angiogenic factor and can increase the vascular development in the neighborhood of the
tumor. In conclusion,
mesothelioma cells release exosomes that express a discrete set of
proteins involved in antigen presentation, signal transduction, migration, and adhesion. Exosomes may play an important role in the interaction between
tumor cells and their environment.