miR-302b Acts as a Tumor Suppressor in Melanoma by Targeting Smad2

Epithelial-mesenchymal transition (EMT) is a critical event in malignant transformation of tumor. Numerous microRNAs (miRNAs) have been proven to participate in the EMT of malignant tumor. In this study, the effects of miRNA-302b were evaluated in the EMT of melanoma cells by targeting Smad2 through the TGF-β signaling pathway. The results showed that the miRNA-302b expression was reduced significantly in melanoma tissues, while the Smad2 expression was increased, and the same negative correlation of miRNA-302b and Smad2 was found in melanoma metastasis, clinical TNM staging, and patient survival. Bioinformatic analysis and luciferase reporter system were used to predict and confirm that Smad2 was one of the miR-302b targets. Increased miRNA302b expression downregulated mRNA and protein expression of Smad2, whereas miRNA-302b inhibitor had the opposite effects. Functional assays showed that miRNA-302b mimics inhibited melanoma cell reproduction, promoted cell apoptosis, and suppressed cell migration. Real-time polymerase chain reaction (RT-PCR) and western blot (WB) indicated that miRNA-302b mimics downregulated the expression of Smad2, Snail, Slug, Twist, and vimentin, all of which are biomarkers of the EMT process, and upregulated the expression of E-cadherin which is a fundamental event in mesenchymal-epithelial transition (MET). These findings indicated that miR-302b inhibits the melanoma cell proliferation, invasion, migration, and EMT by targeting Smad2 through the TGF-β signaling pathway.