Proteomic Analysis of Epithelial to Mesenchymal Transition (EMT) Reveals Cross-talk between SNAIL and HDAC1 Proteins in Breast Cancer Cells

Epithelial to mesenchymal transition (EMT)(1) occurs naturally during embryogenesis, tissue repair, cancer progression, and metastasis. EMT induces cellular and microenvironmental changes resulting in loss of epithelial and acquisition of mesenchymal phenotypes, which promotes cellular invasive and migratory capabilities. EMT can be triggered by extracellular factors, including TGF-, HGF, and EGF. Overexpression of transcription factors, such as SNAIL, SLUG, ZEB1/2, and TWIST1, also induces EMT and is correlated to cancer aggressiveness. Here, the breast adenocarcinoma cell line MCF7 was transduced with SNAIL to identify specific mechanisms controlled by this transcription factor during EMT. Overexpression of SNAIL led to EMT, which was thoroughly validated by molecular, morphological, and functional experiments. Subcellular proteome enrichment followed by GEL-LC-MS/MS was performed to provide extensive protein fractionation and in-depth proteomic analysis. Quantitative analysis relied on a SILAC strategy, using the invasive breast cancer cell line MDA-MB-231 as a reference for quantitation. Subsets of proteins enriched in each subcellular compartment led to a complementary list of 4289 proteins identified with high confidence. A subset of differentially expressed proteins was validated by Western blot, including regulation in specific cellular compartments, potentially caused by protein translocation. Protein network analysis highlighted complexes involved in cell cycle control and epigenetic regulation. Flow cytometry analysis indicated that SNAIL overexpression led to cell cycle arrest in G(0)/G(1) phases. Furthermore, down-regulation of HDAC1 was observed, supporting the involvement of epigenetic processes in SNAIL-induced EMT. When HDAC1 activity was inhibited, MCF7 not only apparently initiated EMT but also up-regulated SNAIL, indicating the cross-talk between these two proteins. Both HDAC1 inhibition and SNAIL overexpression activated the AKT pathway. These molecular mechanisms appear to be essential to EMT and therefore for cancer metastasis. Specific control of such epigenetic processes might then represent effective approaches for clinical management of metastatic cancer. (AU)