Tehran University of Medical Sciences Iranian Journal of Public Health 2251-6085 50 10 2021 10 03 1973 1982 Yang Kang Department of Pediatrics, the Third Affiliated Hospital of Qiqihar Medical University, Qiqihar 161000, China Minghui Bai Department of Pediatrics, the Third Affiliated Hospital of Qiqihar Medical University, Qiqihar 161000, China Liling Deng Department of Pediatrics, the Third Affiliated Hospital of Qiqihar Medical University, Qiqihar 161000, China Linbo Fan Department of Pediatrics, the Third Affiliated Hospital of Qiqihar Medical University, Qiqihar 161000, China Xing Wang Department of Pediatrics, the Third Affiliated Hospital of Qiqihar Medical University, Qiqihar 161000, China 2021 02 25 2021 06 21 Background: This research was designed to probe into the role of miRNA-21 in the pathogenesis of childhood asthma and its correlation with the severity. Methods: Fifty-four children with bronchial asthma admitted to the Third Affiliated Hospital of Qiqihar Medical University from Jun 2018 to Dec 2019 were included. Forty nine healthy children underwent physical examination at this time period were also enrolled. The miR-21 expression in peripheral blood serum was analyzed by qRT-PCR. The relationship between the expression and severity of asthma in children was explored by Spearman correlation analysis and ROC curve. Bronchial epithelial cell lines were cultured in vitro and divided into blank control group, negative control group and miR-21 inhibition and activation group. The changes of cell proliferation after treatment were detected by CCK-8 test in different groups. The expression of TGF-β1/Smad signaling pathway protein in cells was assessed by Western blot (WB). Results: Compared with that of healthy children, the miR-21 expression in peripheral blood serum of asthmatic children was higher (P<0.001). MiR-21 expression was positively correlated with the severity of illness (r=0.853, P<0.001). The results of cell experiments in vitro signified that miR-21 can promote the proliferation of bronchial epithelial cells, and may be involved in regulating the expression of TGF-β1/ Smad3 signaling pathway, thus affecting cell proliferation. Conclusion: miRNA-21 regulates the proliferation of bronchial epithelial cells by activating TGFβ1/Smad signaling pathway. And it is positively correlated with the severity of asthma in children. https://ijph.tums.ac.ir/index.php/ijph/article/view/23996 https://ijph.tums.ac.ir/index.php/ijph/article/download/23996/7374