Exosome-Mediated Transfer of Integrins Promotes Cell-Cell Communication in Prostate Cancer

Intercellular communication has been shown to be mediated by exosomes, exocytosed vesicles of endosomal origin, which are thought to be involved in cancer progression. Recent studies have characterized integrin expression in exosomes but have not investigated whether integrins are actually shuttled to recipient cells and whether they modulate the phenotype of these cells. Integrins are transmembrane receptors that are known to be deregulated in cancer progression. We have focused on the αvβ3 and αvβ6 integrins since they are highly up-regulated in prostate cancer and metastasis. We show here that these αv integrins are present in exosomes of several prostate cancer cells and are transferred from donor to recipient cells. The quality of our exosome preparations was tested by electron microscopy, which confirms the size range (30–150 nm) and the typical cup shape of these vesicles, by continuous sucrose gradient, which shows a density range of 1.13–1.19 g/ml characteristic of exosomes, and by biochemical characterization using antibodies to exosomal markers. The αvβ3 and αvβ6 integrins are transferred to recipient cells as shown by immunoblotting and FACS analysis; the latter demonstrates that these transferred integrins are localized to the cell surface. Furthermore, we show that upon exosome uptake, de novo expression of αvβ3 and αvβ6 increases adhesion and migration of recipient cells on the substrates of these integrins indicating they are functional. Due to high abundance of αvβ3 in the blood of mice and prostate cancer patients, we investigated the expression levels of αvβ3 in the exosomes isolated from their blood. Our data demonstrate that αvβ3 expression levels are higher in exosomes purified from the blood of TRAMP mice carrying tumors compared to exosomes from healthy mice as well as exosomes isolated from prostate cancer patients compared to subjects not affected by cancer. The source of αvβ3 is epithelial as shown by EpCAM bead-based immunoisolation. Also, differential expression of CD9 and CD81 in exosomes from cancer compared to non-cancer patients is shown. Overall, the finding that αvβ3 levels are elevated in prostate cancer patients points to its clinical relevance of potentially being a useful blood biomarker for non invasive liquid biopsy. Also, the transfer of integrins through exosomes provides a strong rationale for further research into their role in cancer progression