Mechanisms of the epithelial-mesenchymal transition by TGF-beta

Abstract

The formation of epithelial cell barriers results from the defined spatiotemporal differentiation of stem cells into a specialized and polarized epithelium, a process termed mesenchymal-epithelial transition. The reverse process, epithelial-mesenchymal transition (EMT), is a metastable process that enables polarized epithelial cells to acquire a motile fibroblastoid phenotype. Physiological EMT also plays an essential role in promoting tissue healing, remodeling or repair in response to a variety of pathological insults. On the other hand, pathophysiological EMT is a critical step in mediating the acquisition of metastatic phenotypes by localized carcinomas. Although metastasis clearly is the most lethal aspect of cancer, our knowledge of the molecular events that govern its development, including those underlying EMT, remain relatively undefined. Transforming growth factor-beta (TGF-beta) is a multifunctional cytokine that oversees and directs all aspects of cell development, differentiation and homeostasis, as well as suppresses their uncontrolled proliferation and transformation. Quite dichotomously, tumorigenesis subverts the tumor suppressing function of TGF-beta, and in doing so, converts TGF-beta to a tumor promoter that stimulates pathophysiological EMT and metastasis. It therefore stands to reason that determining how TGF-beta induces EMT in developing neoplasms will enable science and medicine to produce novel pharmacological agents capable of preventing its ability to do so, thereby improving the clinical course of cancer patients. Here we review the cellular, molecular and microenvironmental mechanisms used by TGF-beta to mediate its stimulation of EMT in normal and malignant cells.

Figure 1. Epithelial Cells Transition to Mesenchymal-like Cells in Response to TGF-β

This schematic depicts polarized epithelial cells and their cuboidal structure that is maintained via cell-cell junctions comprised of homotypic E-cadherin molecules that are linked to the cortical actin cytoskeleton by α- and β-catenins. TGF-β stimulation of EMT during wound healing or tumor invasive migration results in the delocalization, degradation, and/or downregulation of cell-cell junctions and, consequently, a loss of epithelial integrity. In addition, the morphologic transition of epithelial cells also is supported by the simultaneous formation of actin stress fibers, the upregulation of integrins, and the activation of focal adhesion complexes. Moreover, the increased production and section of ECM proteins, such as fibronectin and collagen, coupled with the elevated expression and activation of MMPs enables transitioned fibroblastoid-like cells to exhibit invasive and motile phenotypes.

Figure 2. Differential Interactions of TGF-β Receptors with Transmembrane and Membrane Proximal Proteins Complexes Facilitate the Diversity TGF-β Signaling

β1 and β3 integrins interact physically with TβR-II [43-45]. The association of TβR-II with β3 integrin is mediated by FAK, which facilitates the binding of TβR-II to the SH2-binding protein, Grb2. In addition, TβR-II also interacts physically with EGFR (M.K. Wendt and W.P. Schiemann, unpublished observation), which also is activated indirectly by TGF-β through its increased synthesis and secretion of EGFR ligands. The cytoplasmic tails of both TβR-I and TβR-II interact with TRAF6, which ubiquitinates itself and the MAPKKK, TAK1. Additional interactions include the binding of p130Cas to Smad3, as well as that of PAR6 with TβR-I. Importantly, the differential composition of TGF-β receptor and scaffolding complexes directs the coupling of TGF-β to canonical and noncanonical effector activation, as well as underlies the pathophysiological conversion of TGF-β signaling and EMT in malignant epithelial cells. The biological outcomes of these various protein-protein interactions are discussed in the text.

Figure 3. Diverse TGF-β Signaling Pathways Support a Complex Transcriptional Response During EMT

TGF-β stimulates epithelial cells by binding and activating two transmembrane Ser/Thr protein kinase receptors, namely TGF-β type I (TβR-I) and type II (TβR-II). Activation of these ligand:receptor ternary complexes requires TβR-II to transphosphorylate TβR-I, which phosphorylates and activates Smad2/3. Once activated, Smad2/3 form heterocomplexes with Smad4, which collectively translocate to the nucleus to mediate canonical signaling events by TGF-β (left panel). Noncanonical (right panel) TGF-β signaling takes place through its ability to stimulate various alternate signaling pathways discussed in detail herein. Activation of canonical Smad2/3 signaling results in their nuclear translocation with Smad4 and subsequent regulation of gene expression through their numerous interactions with additional transcriptional activators and repressors. Alternatively, activation of noncanonical TGF-β signaling, such as MAP kinases, small GTPases, PI3K/AKT, and NF-κB, also couples TGF-β to its regulation of gene expression profiles operant in mediating EMT. Finally, activation of the transcription factors belonging to the Snail family (e.g., Snail, Twist, or ZEB1), or of Stat3 elicit EMT-gene expression, which ultimately promotes the prolonged induction of EMT and fibroblastoid-like phenotypes of carcinoma cells via DNA methylation-mediated silencing of E-cadherin expression. Altered coupling of TGF-β to its canonical and noncanonical effector pathways leads to differential gene expression patterns that ultimately contribute to the development of oncogenic signaling by TGF-β. Indeed, the initiation of oncogenic signaling by TGF-β converts its regulation of physiological EMT in normal epithelial cells to one of pathophysiologic EMT in their malignant counterparts.