How is the EMT process triggered? The growth of the primary tumor modifies the ECM and creates a tumor microenvironment (TME) in which there are stromal cells, such as cancer-associated fibroblasts (CAFs), and inflammatory cells (T-lymphocytes, macrophages, and myeloid-derived suppressor cells) that secrete a vast array of chemokines, cytokines, and growth factors that strongly influence cancer cells. Specific signals present in this milieu activate pathways that induce EMT in cancer cells by the activation of EMT-TFs. A central role is played by TGFβ, Wnt, Notch, and growth factor receptors. The TGFβ family (three TGFβ isoforms, two activins, and many bone morphogenetic proteins) has a prominent role in EMT; the binding to the TGFβ family receptors leads to receptor phosphorylation and activation of the SMAD complexes that translocate to the nucleus and bind TFs regulating the expression of a set of genes including those coding for EMT-TFs. The Wnt signaling pathway has a crucial role in the embryonic development of all animal species, in the regeneration of tissues in adult organisms, and in cancer
[25]. The canonical Wnt pathway is activated upon binding of Wnt ligands to the Frizzled family of membrane receptors, leading to the release and stabilization of β-catenin from the GSK3β–AXIN–APC complex. β-catenin, then, moves to the nucleus and becomes part of a transcriptional complex embedding TCF (T cell factor) and LEF (lymphoid enhancer-binding factor) to promote a gene expression program, which includes the activation of EMT-TFs
[26]. The Notch pathway is activated upon binding of the Delta-like or Jagged family of ligands to the four different isoforms of the Notch receptor (Notch1-4). This binding triggers a series of proteolytic cleavage events that culminate in the release of the active, intracellular fragment of the Notch receptor (Notch-ICD), that, with a direct route from the membrane to the nucleus, functions as a transcriptional co-activator in association with different binding partners and TFs
[27]. Several growth factors, such as epidermal growth factor (EGF), fibroblast growth factor (FGF), insulin growth factor (IGF), hepatocyte growth factor (HGF), platelet-derived growth factor (PDGF), and vascular endothelial growth factor (VEGF) act through their cognate tyrosine receptor kinases. The binding triggers receptor dimerization followed by the stimulation of the kinase activity that phosphorylate the receptor and leads to the activation of the PI3K/AKT, ERK/MAPK, p38 MAPK, and JNK pathways, promoting cell growth and proliferation, as well as cell migration and motility via induction of EMT
[11][28]. Inflammation and hypoxia conditions that are present in the TME can activate EMT as well. Several cytokines trigger the phosphorylation and activation of Janus kinases (JAKs) and signal transducer and activator of transcription proteins (STATs) that, following dimerization, foster the transcription of genes encoding EMT-TFs. Hypoxia can promote EMT through HIF1α, which activates the expression of the EMT-TFs TWIST and SNAIL1
[3][29][30]. Evidence coming from in vitro cell cultures and in vivo models suggest the presence of signaling cooperation and the convergence of these pathways on common targets during EMT. Functional crosstalk among the different pathways have been reported and include, for example, the cooperation between the TGFβ pathway with FGF-activated growth factor receptors
[31], and the crosstalk of TGFβ with Wnt and Notch signaling achieved through SMAD complexes
[3][11].