Abstract: Epithelial–mesenchymal transition (EMT) is thought to be a crucial event during the early metastasis of tumor cells. Transforming growth factor (TGF)-β1 is involved in the process of EMT in a variety of human malignancies. Matrix metalloproteinase (MMP)-9 plays an important role in tumor invasion and metastasis, and its expression is regulated by various growth factors, including TGF-β1, in different cell types. To date, the role of MMP-9 in TGF-β1-induced EMT in esophageal squamous cell carcinoma (ESCC) remains unclear. In this study, we aimed to elucidate the mechanism underlying MMP-9-mediated TGF-β1 induction of EMT in ESCC. We analyzed the expression of MMP-9, E-cadherin, and vimentin, in ESCC cells (EC-1), before and after the treatment with exogenous TGF-β1 or a broad spectrum MMP inhibitor, GM6001. Additionally, we analyzed the activity of MMP-9 in these cells and performed MMP-9  knockdown experiments. The results obtained in this study demonstrated that the treatment of EC-1 cells with TGF-β1 can induce EMT, together with the upregulation of vimentin and downregulation of E-cadherin expression in a time-dependent manner. The treatment with GM6001 was shown to attenuate TGF-β1-induced EMT. Furthermore, the exposure of EC-1 cells to TGF-β1 increased the expression and activity of MMP-9, while MMP-9  knockdown blocked TGF-β1-induced EMT and inhibited cell invasiveness and migration. Additionally, treatment with the recombinant human MMP-9 was shown to induce EMT and enhance ESCC cell invasion and metastasis. The obtained data suggest that the regulation of MMP-9 by TGF-β1 may represent a novel mechanism underlying TGF-β1-induced EMT in ESCC.
Keywords: epithelial–mesenchymal transition, esophageal squamous cell carcinoma, matrix metalloproteinase-9, transforming growth factor-β1