The expression of Smad interacting protein-1 (SIP1; ZEB2) and the de novo expression of vimentin are frequently involved in epithelial-to-mesenchynial transitions (EMTs) under both normal and pathological conditions. In the present study, we investigated the potential role of SIP1 in the regulation of vimentin during the EMT associated with breast tumor cell migration and invasion. Examining several breast tumor cell Unes displaying various degrees of invasiveness, we found SIP1 and vimentin expression only in invasive cell Unes. Also, using a model of cell migration with human mammary MCF10A cells, we showed that SIP1 is induced specifically in vimentin-positive migratory cells. Furthermore, transfection of SIP1 cDNA in MCF10A cells increased their vimentin expression both at the mRNA and protein levels and enhanced their migratory abilities in Boyden Chamber assays. Inversely, inhibition of SIP1 expression by RNAi strategies in BT-549 cells and MCF10A cells decreased vimentin expression. We also showed that SIP1 transfection did not activate the TOP-FLASH reporter system, suggesting that the β-catenin/ TCF pathway is not impUcated in the regulation of vimentin by SIP1. Our results therefore impUcate SIP1 in the regulation of vimentin observed in the EMT associated with breast tumor cell migration, a pathway that may contribute to the metastatic progression of breast cancer.

The cytoplasmic/nuclear relocalization of β-catenin and ZO-1 from the adherens and tight junctions are common processes of the epithelial-mesenchymal transition (EMT) associated with tumor invasion. Data are now accumulating to demonstrate that these molecules, which shuttle between the plasma membrane and the nucleus or the cytosol, are involved in signaling pathways, and contribute to the regulation of genes such as vimentin or matrix metalloproteinase-14 which are turned on during EMT.

In many types of cancers, the fragile histidine triad (Fhit) gene is frequently targeted by genomic alterations leading to a decrease or loss of gene and protein expression. Fhit has been described as a tumor suppressor gene because of its ability to induce apoptosis and to inhibit proliferation of tumor cells. Moreover, several studies have shown a correlation between the lack of Fhit expression and tumor aggressiveness, thus suggesting that Fhit could be involved in tumor progression. In this study, we explored the potential role of Fhit during tumor cell invasion. We first showed that a low Fhit expression is associated with in vivo and in vitro invasiveness of tumor cells. Then, we showed that Fhit overexpression in Fhit-negative highly invasive NCI-H1299 cells by transfection of Fhit cDNA and Fhit inhibition in Fhit-positive poorly invasive HBE4-E6/E7 cells by transfection of Fhit small interfering RNA induce, respectively, a decrease and an increase in migratory/invasive capacities. These changes in cell behavior were associated with a reorganization of tight and adherens junction molecules and a regulation of matrix metalloproteinase and vimentin expression. These results show that Fhit controls the invasive phenotype of lung tumor cells by regulating the expression of genes associated with epithelial-mesenchymal transition.