The protease inhibitor gabexate mesylate targets Raf kinase inhibitor protein and reverses epithelial–mesenchymal transition in triple-negative breast cancer cells

The prognosis of triple-negative breast cancer (TNBC) still remains poor, mainly due to the occurrence of early metastases and the lack of effective treatments. The Raf kinase inhibitor protein (RKIP) is a tumor metastasis suppressor frequently downregulated in human cancers. Here, we report that RKIP expression is lost in the tumor tissue of TNBC patients. Treatment with the protease inhibitor gabexate mesylate (GM) increased RKIP expression and reverted the epithelial-mesenchymal transition (EMT) phenotype in MDA-MB-231 cells, a well-established <i>in vitro</i> TNBC model. This phenotypic change was concomitant with the upregulation of the epithelial markers E-Cadherin and Claudin-1, and the downregulation of the mesenchymal markers N-Cadherin, Vimentin, and nuclear β-catenin. Furthermore, GM significantly decreased TNBC cell motility and invasiveness, along with matrix metalloproteinase (MMP)-2 and MMP-9 protein expression and activity. At the mechanistic level, RKIP upregulation inhibited p42/44 mitogen-activated protein kinase (MAPK) and NF-κB signaling that, in turn, downregulated the expression of the two EMT transcription factors Snail and Slug. Despite these preliminary findings needing to be confirmed in more representative <i>ex vivo</i> (such as patient-derived primary cells or patient-derived organoids) and <i>in vivo</i> TNBC models, they provide evidence that the EMT process could be pharmacologically reverted by the protease inhibitor GM in a highly metastatic TNBC <i>in vitro</i> model, deserving further investigation in future studies.