c‐Src, Insulin‐Like Growth Factor I Receptor, G‐Protein‐Coupled Receptor Kinases and Focal Adhesion Kinase are Enriched Into Prostate Cancer Cell Exosomes

It is well known that Src tyrosine kinase, insulin‐like growth factor 1 receptor (IGF‐IR), and focal adhesion kinase (FAK) play important roles in prostate cancer (PrCa) development and progression. Src, which signals through FAK in response to integrin activation, has been implicated in many aspects of tumor biology, such as cell proliferation, metastasis, and angiogenesis. Furthermore, Src signaling is known to crosstalk with IGF‐IR, which also promotes angiogenesis. In this study, we demonstrate that c‐Src, IGF‐IR, and FAK are packaged into exosomes (Exo), c‐Src in particular being highly enriched in Exo from the androgen receptor (AR)‐positive cell line C4‐2B and AR‐negative cell lines PC3 and DU145. Furthermore, we show that the active phosphorylated form of Src (SrcpY416) is co‐expressed in Exo with phosphorylated FAK (FAKpY861), a known target site of Src, which enhances proliferation and migration. We further demonstrate for the first time exosomal enrichment of G‐protein‐coupled receptor kinase (GRK) 5 and GRK6, both of which regulate Src and IGF‐IR signaling and have been implicated in cancer. Finally, SrcpY416 and c‐Src are both expressed in Exo isolated from the plasma of prostate tumor‐bearing TRAMP mice, and those same mice have higher levels of exosomal c‐Src than their wild‐type counterparts. In summary, we provide new evidence that active signaling molecules relevant to PrCa are enriched in Exo, and this suggests that the Src signaling network may provide useful biomarkers detectable by liquid biopsy, and may contribute to PrCa progression via Exo. J. Cell. Biochem. 118: 66–73, 2017. © 2016 Wiley Periodicals, Inc.