Breast cancer is the leading cause of cancer death among females, with tumor metastasis being primarily responsible for breast cancer-associated mortality. Current literatures have shown that microRNAs (miRNAs) are implicated in tumor metastasis. In this study, we found that the expression of miR-720 was significantly downregulated in primary breast cancer, with greater downregulation in metastatic tumors. Statistical analysis of 105 cases of primary human breast cancer demonstrated that decreased expression of miR-720 was correlated with lymph node metastasis. Furthermore, reexpression of miR-720 in breast cancer cells remarkably inhibited cell invasiveness and migration both in vitro and in vivo. Mechanistically, downregulation of TWIST1, a promoter of metastasis that was identified as a direct functional target of miR-720, was attributed to the inhibition of metastasis. Consistent with the reduced TWIST1 levels in breast cancer, reexpression of miR-720 upregulated epithelial markers (E-cadherin and β-catenin) and downregulated mesenchymal markers (N-cadherin, fibronectin, vimentin and matrix metalloproteinase-2). Expression of miR-720 was inversely associated with TWIST1 in human breast cancer tissues. Knockdown of TWIST1 expression by small interfering RNA exhibited similar effects to reintroduction of miR-720, whereas overexpression of TWIST1 (without the 3'-untranslated region) abrogated miR-720-mediated metastasis inhibition. Collectively, our data indicate that miR-720 is frequently decreased in breast cancer and manifests antimetastatic activity by downregulating TWIST1, presenting a novel mechanism of miRNA-mediated regulation of tumor metastasis.
