Complexity in interpretation of embryonic epithelial-mesenchymal transition in response to transforming growth factor-beta signaling

Abstract

Epithelial-mesenchymal transition (EMT) is a highly conserved and fundamental process that governs morphogenesis in development and may also contribute to cancer metastasis. Transforming growth factor (TGF-beta) is a potent inducer of EMT in various developmental and tumor systems. The analysis of TGF-beta signal transduction pathways is now considered a critically important area of biology, since many defects occur in these pathways in embryonic development. The complexity of TGF-beta signal transduction networks is overwhelming due to the large numbers of interacting constituents, complicated feedforward, feedback and crosstalk circuitry mechanisms that they involve in addition to the cellular kinetics and enzymatics that contribute to cell signaling. As a result of this complexity, apparently simple but highly important questions remain unanswered, that is, how do epithelial cells respond to such TGF-beta signals? System biology and cellular kinetics play a crucial role in cellular function; omissions of such a critical contributor may lead to inaccurate understanding of embryonic EMT. In this review, we identify and explain why certain conditions need to be considered for a true representation of TGF-beta signaling in vivo to better understand the controlled, yet delicate mechanism of embryonic EMT.

Fig. 1

TGF-β is a multipotent growth factor family: the TGF-β superfamily can exert multiple functions in a cell/tissue-specific manner. Depending upon the isoforms (1, 2, 3, and the BMPs and activins), the TGF-β superfamily can promote cell proliferation, differentiation, cell cycle arrest, apoptosis and/or transformation in a time- and system-dependent manner. However, whether all these roles are directly or indirectly related to the EMT or not still remains to be investigated.

Fig. 2

Can TGF-β signaling complete all stages of EMT? (i) Typical epithelial cells have apical basal polarity, seats on basement membrane, are rich with epithelial markers such as tight junction proteins (e.g. ZO1), adherens junction proteins (e.g. E-cadherin) and desmosomes. (ii) Upon TGF-β signaling these epithelial markers are repressed. Once these proteins are lost, epithelial cells get isolated from their adjacent cells. (iii) Individual epithelial cells begin to transform to mesenchymal cells, interact and degrade basement membrane proteins (basal lamina) while they rearrange their cytoskeletal proteins (e.g. α-smooth muscle actin, microtubles and intermediate filaments) which help in acquiring front-end back-end polarity. (iv) TGF-β signaling also activates the mesenchymal markers vimentin and fibronectin to become full-blown mesenchymal cells for migration through the ECM.

Fig. 3

TGF-β signaling. Smad-dependent: TGF-β ligand binds to receptors I and II at the cell surface. After phosphorylation of TGFβRI with TGFβRII kinase, the receptor forms a complex with Smad2 or Smad3. Smad2 (or Smad3) is phosphorylated and then complexes with Smad4 to enter the nucleus and activate EMT-related gene. Smad-independent: TGF-β can also signal through the MAPK such as ERK and p38 to activate several transcription factors that induce target gene expression as well as via the PI3K and RhoA pathways, which have been shown to be key mediators of EMT. MAPK can also activate Jagged1, which in turn signals via NOTCH to activate bHLH-related transcription factors. At the end, these activated transcription factors (the ‘X’ transcription factors) bind to appropriate binding sites of the EMT genes and either alone (−) or with (+) Smads bound at Smad binding elements (SBE) can regulate EMT-related genes. These activities can be regulated by corepressors and coactivators such as p300/CBP or Ski/SnoN (green arrows are the flow of the signals and red arrows are the repressors).

Fig. 4

TGF-β activated several transcription factors to modulate EMT: upon activation of the TGF-β signaling via Smad or non-Smad pathways (MAPK, RhoA and PI3P), several transcription factors such as basic helix-loop-helix factors (bHLH) as well zinc finger factors can be activated. E-cadherin is commonly repressed by these transcription factors (as shown by round and oval structures bound onto the DNA in the nucleus) by promoting E-cadherin gene silencing, thus activating EMT. These transcription factors can be activated by several external stimulations other than TGF-β, such as Wnt, integrin, stress, BMPs and activins, interferons and number of growth factors [such as epidermal growth factors (EFG), HGF, PDGF and IGF]. It is quite intriguing.