Activation of EMT might be triggered by various signaling pathways depending on the tumor microenvironment. Indeed, tumor-associated stroma can increase the expression of EMT-transcription factors (EMT-TFs). Among signaling pathways are included transforming growth factor β (TGF-β), bone morphogenic protein (BMP), Notch, Wnt/β-catenin, sonic hedgehog, epidermal growth factor (EGF), fibroblast growth factor (FGF), and platelet-derived growth factor (PDGF) 
). Activation of these EMT-inducing signaling pathways leads to the expression of transcription factors that governs EMT-associated genes. They simultaneously repressed the expression of epithelial genes, and on the other hand they induce genes associated with the mesenchymal phenotype. EMT-TFs include basic helix-loop-helix (bHLH) factors TWIST1 and TWIST2, the zinc finger E-box-binding homebox ZEB1 and ZEB2, and the zinc finger binding transcription factors SNAI1 and SNAI2.
Figure 1. Signaling pathway involved in the epithelial to mesenchymal transition. Different signaling pathways can activate epithelial to mesenchymal transition (EMT) through the activation of EMT transcription factors ZEB1, SNAIL, and TWIST. Interleukin 6 (IL-6) and interleukin 1β (IL-1β) can bind cytokine receptors. Signaling is conducted through the activation of Janus kinase (JAK) and the recruitment of signal transducer and activator of transcription proteins (STATs); the dimer of STATs translocates into the nucleus to activate the transcription of genes. The transforming growth factor β (TGF-β) signal is conducted by SMADs protein into the nucleus, and the trimer activates the transcription. Tyrosine kinase receptors (RTK), such as epidermal growth factor receptor (EGFR), fibroblast growth factor receptor (FGFR), or platelet-derived growth factor receptor (PDGFR), induce PI3K, AKT, and nuclear factor-κB (NF-κB). The TGF-β pathway and RTK are also able to trigger the RAS-RAF-MEK-ERK signaling pathway. The WNT signaling results in the release of β-Catenin from the glycogen synthase kinase-3β (GSK3β)–axis inhibition protein (AXIN) complex. β-Catenin moves into the nucleus and binds to the transcription factors T cell factor (TCF) and the lymphoid enhancer-binding factor (LEF). Intracellular domain of the notch receptor (Notch ICD) is cleaved after the activation of the receptor, then it can translocate into the nucleus and act as a transcriptional co-activator. Hedgehog signaling induces EMT-associated gene expression through the activation of GLI1.
SNAIL is the first described transcriptional repressor of E-cadherin; it binds to the E box consensus sequence in the promoter of CDH1
, encoding E-cadherin, and directly repress its transcription 
. SNAIL also induces the downregulation of others epithelial molecules such as Claudins, Occludins, and Mucin-1. SNAIL also has the ability to directly induce mesenchymal genes such as Fibronectin and Matrix Metallopeptidase 9 (MMP9) 
. ZEB1 and ZEB2 repress E-cadherin expression by directly binding to the E-Box element of CDH1 
. They also induce the expression of mesenchymal proteins such as N-cadherin and Vimentin 
. Unlike the last two EMT-TFs, TWIST acts as an indirect repressor of E-cadherin partly due to its transcriptional activation of SNAI2 
. TWIST is also able to activate expression of mesenchymal genes such as N-cadherin and Vimentin 
. The functional loss of E-cadherin is considered as a crucial step in EMT; however, many others epithelial proteins are also downregulated: Mucin-1, Occludins, Claudins, and Desmoplakin. On the other side, mesenchymal markers are gained, they include N-cadherin, Vimentin, Smooth Muscle Actin, Fibronectin, Matrix Metalloproteinases, and Vitronectin 
. In addition to its role in transcriptional regulation, EMT might be orchestrated by other regulatory networks including regulation by microRNAs (miRs), differential splicing, translational and posttranslational control.
Molecular changes described above lead to cellular hallmarks of EMT including the loss of apical-basal polarity, disruption of cell-to-cell contacts (including adherent junctions, tight junctions, and desmosomes), cytoskeleton structure and ECM degradation by expressing matrix metalloproteinases. Consequently, cells ongoing EMT acquire a spindle-shape mesenchymal morphology which allows them motility and ability to degrade and invade their basal ECM