Inhibition of Cathepsin S Restores TGF-β-induced Epithelial-to-mesenchymal Transition and Tight Junction Turnover in Glioblastoma Cells

Background: Invasive growth is one of the most typical features of aggressive types of malignant cancer, including glioblastoma. Lysosomal cysteine protease-cathepsin S (CTSS), has been reported to be involved in invasive growth and distant metastasis of cancer cells. However, the underlying mechanisms remained elusive. Methods: U87 and U251 human glioblastoma cell lines were applied in this study. Cell migration and invasion ability were measured by wound healing assay and transwell assay. Western blot was employed to detect the expression levels of proteins. Immunofluorescence assays of cells and tissues were used to visualize the localization and expression of proteins. The SPSS software was used for statistical analysis. Results: Our results showed that the high expression of CTSS was link with the grades of glioma tissues. The CTSS inhibitor-Z-FL-COCHO (ZFL), could attenuate TGF-β-induced invasive growth as proven by wound healing and transwell assays. Furthermore, inhibition of CTSS could reverse TGF-β-induced epithelial-to-mesenchymal transition (EMT) and restore TGF-β-triggered tight junction proteins turnover, thus decreasing glioblastoma cell mobility. We also observed that TGF-β could change the morphology of glioblastoma cells, redistribute intermediate-filament, vimentin, which was highly relevant to mesenchymal type cells and enhanced mobility. However, inhibition of CTSS could significantly restore this transformation. Our results proved that PI3K/AKT/mTOR pathway was significantly suppressed in the TGF-β+ZFL (CTSS inhibitor) groups, and AKT activator-SC79, could reverse the anti-invasion effect of CTSS, indicating an important role of PI3K/AKT/mTOR pathway in this process. Conclusion: Z-FL-COCHO (ZFL), a CTSS inhibitor, could reverse TGF-β-induced EMT and change of tight junction proteins via PI3K/AKT/mTOR pathway.