Evidence is accumulating highlighting the importance of extracellular miRNA as a 
novel biomarker for diagnosing various kinds of malignancies. MiR-21 is one of 
the most studied miRNAs and is over-expressed in cancer tissues. To explore the 
clinical implications and secretory mechanisms of extracellular miR-21, we 
firstly meta-analyzed the diagnostic efficiency of extracellular miR-21 in 
different cancer types. Eighty-one studies based on 59 articles were finally 
included. In our study, extracellular miR-21 was observed to exhibit an 
outstanding diagnostic accuracy in detecting brain cancer (area under the 
summary receiver operating characteristic curve or AUC = 0.94), and this 
accuracy was more obvious in glioma diagnosis (AUC = 0.95). Our validation study 
(n = 45) further confirmed the diagnostic and prognostic role of miR-21 in 
cerebrospinal fluid (CSF) for glioma. These findings inspired us to explore the 
biological function of miR-21. We next conducted mechanistic investigations to 
explain the secretory mechanisms of extracellular miR-21 in glioma. TGF-β/Smad3 
signaling was identified to participate in mediating the release of miR-21 from 
glioma cells. Further targeting TGF-β/Smad3 signaling using galunisertib, an 
inhibitor of the TGF-β type I receptor kinase, can attenuate the secretion of 
miR-21 from glioma cells. Taken together, CSF-based miR-21 might serve as a 
potential biomarker for diagnosing brain cancer, especially for patients with 
glioma. Moreover, extracellular levels of miR-21 were affected by exogenous 
TGF-β activity and galunisertib treatment.