Clinicopathological significance of microRNA‐21 in extracellular vesicles of pleural lavage fluid of lung adenocarcinoma and its functions inducing the mesothelial to mesenchymal transition

BACKGROUND: Pre‐resection pleural lavage cytology is useful to predict tumor recurrence and the prognosis of lung cancer patients. Recently, extracellular vesicles (EVs) isolated from effusion specimens have come under the spotlight, and several studies showed that microRNA in EVs is associated with prognosis. MicroRNA‐21 (miR‐21) is a representative onco‐microRNA, and miR‐21 in EVs (EV‐miR‐21) promotes cancer dissemination by inducing mesothelial to mesenchymal transition (MMT) in the peritoneal cavity. In this study, we isolated EVs from pleural lavage fluid and focused on EV‐miR‐21 as a diagnostic factor with a relationship to pleural dissemination. METHODS: The Cancer Genome Atlas dataset comprising of 448 cases of lung adenocarcinoma, tissue microarray of 144 cases of lung adenocarcinoma, and pleural lavage fluid of 41 cases was used to examine miR‐21 expression levels. The function of EV‐miR‐21 was investigated in vitro. RESULTS: The miR‐21 expression level in primary sites was associated with a poor prognosis and correlated with pleural invasion of adenocarcinoma. EV‐miR‐21 levels in pleural lavage fluid were associated with positive cytology and pleural invasion in the primary sites, even in cytology‐negative cases. In vitro studies demonstrated that EV‐miR‐21 induces the MMT. Mesothelial cells in the MMT showed functions similar to cancer‐associated fibroblasts, which are an important stromal component in primary sites and disseminated pleural lesions. CONCLUSIONS: EV‐miR‐21 in pleural lavage fluid is important as a diagnostic and prognostic factor. Moreover, EV‐miR‐21 induces the MMT, which can form premetastatic niches of dissemination in the pleural cavity.