Signaling mechanisms of the epithelial-mesenchymal transition

Abstract

The epithelial-mesenchymal transition (EMT) is an essential mechanism in embryonic development and tissue repair. EMT also contributes to the progression of disease, including organ fibrosis and cancer. EMT, as well as a similar transition occurring in vascular endothelial cells called endothelial-mesenchymal transition (EndMT), results from the induction of transcription factors that alter gene expression to promote loss of cell-cell adhesion, leading to a shift in cytoskeletal dynamics and a change from epithelial morphology and physiology to the mesenchymal phenotype. Transcription program switching in EMT is induced by signaling pathways mediated by transforming growth factor β (TGF-β) and bone morphogenetic protein (BMP), Wnt-β-catenin, Notch, Hedgehog, and receptor tyrosine kinases. These pathways are activated by various dynamic stimuli from the local microenvironment, including growth factors and cytokines, hypoxia, and contact with the surrounding extracellular matrix (ECM). We discuss how these pathways crosstalk and respond to signals from the microenvironment to regulate the expression and function of EMT-inducing transcription factors in development, physiology, and disease. Understanding these mechanisms will enable the therapeutic control of EMT to promote tissue regeneration, treat fibrosis, and prevent cancer metastasis.

Fig. 1. Cellular changes associated with EMT

Epithelial cells demonstrate apical-basal polarity, show strong cell-cell adhesion through adherens junctions and tight junctions, and have a basal matrix consisting primarily of type IV collagen and laminin (left). Upon induction of EMT, the cells lose their adhesion and change morphology and acquire front end-to-back end polarity (right). These cells cleave and invade the basal lamina and migrate along a newly formed matrix of fibronectin and type I collagen. The abundance of epithelial biomarkers is reduced, whereas that of mesenchymal markers is increased.

Fig. 2. EMT-inducing transcription factors

EMT is triggered by transcription factors that bind and inhibit the expression of genes encoding adherens junction and tight junction molecules, such as E-cadherin, ZO-1, claudins, and occludin. These transcription factors include Snail1/2, ZEB1/2, Twist, and LEF-1, the expression of which is induced by various signaling pathways. Several regulatory molecules can inhibit the function of these transcription factors. GSK-3β can inhibit β-catenin–induced activation of LEF-1 and can also inhibit the stability and nuclear translocation of Snail1/2. The miR-200 family of miRNAs can inhibit the expression of ZEB1/2. GSK-3β and miR-200 can be blocked by the kinase Akt, which is activated by most EMT signaling pathways.

Fig. 3. TGF-β signaling in EMT

TGF-β ligands bind to their type II and type III receptors (TGF-βRII and TGF-βRIII), which causes recruitment and phosphorylation of the type I receptor (TGF-βRI). This activates various signaling pathways, including those mediated by SMAD2/3, Ras, and PI3K (simplified here), which activate transcription factors that induce the expression of genes encoding EMT-inducing transcription factors. At the surface of the cell, TACE cleaves the intracellular domain of TGF-β RI, which can then act as a transcriptional regulator to mediate EMT. Additionally, the Par3–Par6–aPKC (atypical PKC) complex also associates with TGF-βRs at the cell membrane and is involved in cytoskeletal remodeling to promote the mesenchymal phenotype. SMAD-independent pathways (such as through PI3K and ILK) can activate Akt, which in turn can inhibit the function of GSK-3β, a kinase that inhibits nuclear translocation of Snail and β-catenin. Inhibition of SMAD signaling is mediated by SMAD6/7, which prevent the binding and phosphorylation of SMAD2/3 at TGF-βRs, and by Smurf2, which is known to degrade the activated complex of SMAD2/3/4.

Fig. 4. RTK signaling in EMT

Growth factors (GFs), such as FGF, EGF, PDGF, or IGF, stimulate RTKs, which activate various signaling pathways (simplified here), including those mediated by Ras, PI3K, Src, and ILK. These signaling cascades activate transcription factors (TFs) that bind to the promoters of genes that encode EMT-inducing transcription factors, such as Snail1/2, ZEB1/2 and Twist, which induce EMT by inhibiting the expression genes encoding cell adhesion molecules. When present and functional, the tumor suppressor PTEN can suppress PI3K-mediated induction of EMT.

Fig. 5. Wnt, Notch, and Hedgehog signaling in EMT

Wnt ligands bind and activate Frizzled receptors, which promote Dvl-dependent inhibition of GSK-3β, a kinase that causes degradation of cytoplasmic β-catenin. This enables the accumulation and nuclear localization of β-catenin to activate the LEF-1 transcription factor, which promotes the expression of various EMT-associated genes. The intercellular interaction between JAG2 and its receptor Notch induces the g-secretase–mediated cleavage and release of the Notch ICD, which can directly activate target genes associated with EMT signaling. The Notch ICD can also stabilize cytoplasmic β-catenin and activate other pathways, like ERK and NF-κB, that induce the Snail1/2 and LEF-1 transcription factors. Hh signaling induces EMT-associated gene expression through activating Gli transcription factors.

Fig. 6. Matrix signaling in EMT

Type I collagen induces EMT through integrin or DDR1/2 signaling. Both types of receptors activate NF-κB and other transcription factors (TFs) that promote expression of SNAI1/2 and LEF1. Other pathways simplified here, such as those mediated by PI3K, ILK, proline-rich tyrosine kinase 2 (PYK2)–PDK1, and FAK-paxillin, are activated by type I collagen through integrins and DDRs, ultimately promoting the stabilization and activity of the EMT-associated transcription factors Snail1/2 and LEF-1. DDR1 is also known to form complexes with E-cadherin at the cell surface, which are disrupted upon binding to type I collagen.

Fig. 7. Hypoxia signaling in EMT

Under normoxic conditions, prolyl hydroxylases (PHD) promote the degradation of the transcription factor HIF-1α. Under hypoxic conditions, PHDs are inactivated, thus enabling the accumulation and functional activation of HIF-1α, which induces the expression of genes associated with EMT, such as TGF-β, TWIST, and LOX, and the stabilization of the Snail1/2 transcriptional repressors. HIF-1α has also been shown to stabilize β-catenin as well as the Notch ICD after its JAG2-induced cleavage by g-secretase. Both β-catenin and the Notch ICD also promote the expression of EMT-associated genes.