Exosomes are small secreted vesicles that can transfer their content to recipient cells. In cancer, exosome secretion has been implicated in tumor growth and metastatic spread. In this study, researchers at Aarhus University Hospital, Denmark explored the possibility that exosomal pathways might discard tumor-suppressor miRNA that restricts metastatic progression. Secreted miRNA characterized from isogenic bladder carcinoma cell lines with differing metastatic potential were uncoupled from binding to target transcripts or the AGO2-miRISC complex.
In metastatic cells, they observed a relative increase in secretion of miRNA with tumor-suppressor functions, including miR23b, miR224, and miR921. Ectopic expression of miR23b inhibited invasion, anoikis, angiogenesis, and pulmonary metastasis. Silencing of the exocytotic RAB family members RAB27A or RAB27B halted miR23b and miR921 secretion and reduced cellular invasion. Clinically, elevated levels of RAB27B expression were linked to poor prognosis in two independent cohorts of patients with bladder cancer. Moreover, highly exocytosed miRNA from metastatic cells, such as miR23b, were reduced in lymph node metastases compared with patient-matched primary tumors and were correlated with increments in miRNA-targeted RNA. Taken together, these results suggested that exosome-mediated secretion of tumor-suppressor miRNA is selected during tumor progression as a mechanism to coordinate activation of a metastatic cascade.