Samy Lamouille

Molecular mechanisms of epithelial–mesenchymal transition

The transdifferentiation of epithelial cells into motile mesenchymal cells, a process known as epithelial–mesenchymal transition (EMT), is integral in development, wound healing and stem cell behaviour, and contributes pathologically to fibrosis and cancer progression. This switch in cell differentiation and behaviour is mediated by key transcription factors, including SNAIL, zinc-finger E-box-binding (ZEB) and basic helix–loop–helix transcription factors, the functions of which are finely regulated at the transcriptional, translational and post-translational levels. The reprogramming of gene expression during EMT, as well as non-transcriptional changes, are initiated and controlled by signalling pathways that respond to extracellular cues. Among these, transforming growth factor-β (TGFβ) family signalling has a predominant role; however, the convergence of signalling pathways is essential for EMT.

TGF-β-induced epithelial to mesenchymal transition

During development and in the context of different morphogenetic events, epithelial cells undergo a process called epithelial to mesenchymal transition or transdifferentiation (EMT). In this process, the cells lose their epithelial characteristics, including their polarity and specialized cell-cell contacts, and acquire a migratory behavior, allowing them to move away from their epithelial cell community and to integrate into surrounding tissue, even at remote locations. EMT illustrates the differentiation plasticity during development and is complemented by another process, called mesenchymal to epithelial transition (MET). While being an integral process during development, EMT is also recapitulated under pathological conditions, prominently in fibrosis and in invasion and metastasis of carcinomas. Accordingly, EMT is considered as an important step in tumor progression. TGF-β signaling has been shown to play an important role in EMT. In fact, adding TGF-β to epithelial cells in culture is a convenient way to induce EMT in various epithelial cells. Although much less characterized, epithelial plasticity can also be regulated by TGF-β-related bone morphogenetic proteins (BMPs), and BMPs have been shown to induce EMT or MET depending on the developmental context. In this review, we will discuss the induction of EMT in response to TGF-β, and focus on the underlying signaling and transcription mechanisms.

Multiple targets of miR-302 and miR-372 promote reprogramming of human fibroblasts to induced pluripotent stem cells

The embryonic stem cell–specific cell cycle–regulating (ESCC) family of microRNAs (miRNAs) enhances reprogramming of mouse embryonic fibroblasts to induced pluripotent stem cells. Here we show that the human ESCC miRNA orthologs hsa-miR-302b and hsa-miR-372 promote human somatic cell reprogramming. Furthermore, these miRNAs repress multiple target genes, with downregulation of individual targets only partially recapitulating the total miRNA effects. These targets regulate various cellular processes, including cell cycle, epithelial-mesenchymal transition (EMT), epigenetic regulation and vesicular transport. ESCC miRNAs have a known role in regulating the unique embryonic stem cell cycle. We show that they also increase the kinetics of mesenchymal-epithelial transition during reprogramming and block TGFβ-induced EMT of human epithelial cells. These results demonstrate that the ESCC miRNAs promote dedifferentiation by acting on multiple downstream pathways. We propose that individual miRNAs generally act through numerous pathways that synergize to regulate and enforce cell fate decisions.

Cell size and invasion in TGF-β–induced epithelial to mesenchymal transition is regulated by activation of the mTOR pathway

Epithelial to mesenchymal transition (EMT) occurs during development and cancer progression to metastasis and results in enhanced cell motility and invasion. Transforming growth factor-β (TGF-β) induces EMT through Smads, leading to transcriptional regulation, and through non-Smad pathways. We observe that TGF-β induces increased cell size and protein content during EMT. This translational regulation results from activation by TGF-β of mammalian target of rapamycin (mTOR) through phosphatidylinositol 3-kinase and Akt, leading to the phosphorylation of S6 kinase 1 and eukaryotic initiation factor 4E–binding protein 1, which are direct regulators of translation initiation. Rapamycin, a specific inhibitor of mTOR complex 1, inhibits the TGF-β–induced translation pathway and increase in cell size without affecting the EMT phenotype. Additionally, rapamycin decreases the migratory and invasive behavior of cells that accompany TGF-β–induced EMT. The TGF-β–induced translation pathway through mTOR complements the transcription pathway through Smads. Activation of mTOR by TGF-β, which leads to increased cell size and invasion, adds to the role of TGF-β–induced EMT in cancer progression and may represent a therapeutic opportunity for rapamycin analogues in cancer.

TGF-β signaling and epithelial-mesenchymal transition in cancer progression.

Purpose of review TGF-&bgr; acts as a potent driver of cancer progression through the induction of epithelial–mesenchymal transition (EMT), in which epithelial cells acquire mesenchymal phenotype, leading to enhanced motility and invasion. Recent reports highlight the fundamental roles of TGF-&bgr;-induced EMT in multiple aspects of cancer progression. In this review, we focus on the novel insights into the roles of TGF-&bgr;-induced EMT in cancer progression and the underlying mechanisms that enable TGF-&bgr; to activate this epithelial plasticity response at transcription, translation, and posttranslational levels. Recent findings Smad-mediated transcription regulation is known to activate TGF-&bgr;-induced EMT. More recently, novel mechanisms of epigenetic control, alternative splicing, miRNAs, translation control, and posttranslational modifications have been shown to play key roles in the control of EMT. In addition to initiating carcinoma cell invasion, TGF-&bgr;-induced EMT can guide cancer cells to de-differentiate and gain cancer stem-cell-like properties. EMT also allows the generation of stromal cells that support and instruct cancer progression. Summary The differentiation plasticity of epithelial cells that mediates TGF-&bgr;-induced EMT and reversion from mesenchymal to epithelial phenotype are increasingly seen as integral aspects of cancer progression that contribute to survival and dissemination of cancer cells. Further mechanistic insights under physiological conditions may lead to new therapeutic or prognostic strategies in cancer treatment.

TGF-β-induced activation of mTOR complex 2 drives epithelial–mesenchymal transition and cell invasion

In cancer progression, carcinoma cells gain invasive behavior through a loss of epithelial characteristics and acquisition of mesenchymal properties, a process that can lead to epithelial–mesenchymal transition (EMT). TGF-β is a potent inducer of EMT, and increased TGF-β signaling in cancer cells is thought to drive cancer-associated EMT. Here, we examine the physiological requirement for mTOR complex 2 (mTORC2) in cells undergoing EMT. TGF-β rapidly induces mTORC2 kinase activity in cells undergoing EMT, and controls epithelial cell progression through EMT. By regulating EMT-associated cytoskeletal changes and gene expression, mTORC2 is required for cell migration and invasion. Furthermore, inactivation of mTORC2 prevents cancer cell dissemination in vivo. Our results suggest that the mTORC2 pathway is an essential downstream branch of TGF-β signaling, and represents a responsive target to inhibit EMT and prevent cancer cell invasion and metastasis.

Regulation of epithelial-mesenchymal and mesenchymal-epithelial transitions by microRNAs.

Epithelial-mesenchymal transition (EMT) and the reverse process, mesenchymal-epithelial transition (MET), are essential during development and in the regulation of stem cell pluripotency, yet these processes are also activated in pathological contexts, such as in fibrosis and cancer progression. In EMT and MET, diverse signaling pathways cooperate in the initiation and progression of the EMT and MET programs, through regulation at transcriptional, post-transcriptional, translational, and post-translational levels. MicroRNAs recently emerged as potent regulators of EMT and MET, with their abilities to target multiple components involved in epithelial integrity or mesenchymal traits. By affecting EMT and MET processes, microRNAs are involved in the regulation of stem cell pluripotency and the control of tumor progression.

TACE-Mediated Ectodomain Shedding of the Type I TGF-β Receptor Downregulates TGF-β Signaling

Regulating TGF-beta receptor presentation provides an avenue to alter a cells responsiveness to TGF-beta. We report that activation of the Erk MAP kinase pathway decreases the TGF-beta-induced Smad3 activation due to decreased cell surface levels of the type I receptor TbetaRI, but not the type II receptor. Inhibition of TACE activity or expression enhanced the cell surface TbetaRI levels and TGF-beta-induced Smad3 and Akt activation. Accordingly, silencing TACE expression in cancer cells enhanced the TbetaRI presentation and TGF-beta responsiveness, including the antiproliferative effect of TGF-beta, and epithelial-to-mesenchymal transition. These results establish a mechanism for downregulating TGF-beta signaling through TACE activation by the Erk MAP kinase pathway and a strategy for evasion of tumor suppression and modulation of epithelial-to-mesenchymal transition during cancer progression. The decreased growth inhibition by TGF-beta, due to elevated TACE activity, complements the growth stimulation resulting from increased release of TGF-alpha family ligands.

Emergence of the phosphoinositide 3-kinase-Akt-mammalian target of rapamycin axis in transforming growth factor-β-induced epithelial-mesenchymal transition.

During development and in pathological contexts such as fibrosis and cancer progression, epithelial cells can initiate a complex transcriptional reprogramming, accompanied by dramatic morphological changes, in a process named ‘epithelial-mesenchymal transition’ (EMT). In this transition, epithelial cells lose their epithelial characteristics to acquire mesenchymal properties and increased motile and invasive behavior. Transforming growth factor-β (TGF-β) has emerged as a major inducer of EMT through activation of downstream signaling pathways, including Smad and non-Smad signaling pathways. Among the non-Smad pathways, increasing evidence is emerging that the phosphoinositide 3-kinase-Akt-mammalian target of rapamycin axis plays a major role in TGF-β-induced EMT, notably through the regulation of translation and cell invasion. Pharmacological inhibitors of the phosphoinositide 3-kinase-Akt-mammalian target of rapamycin pathway may therefore represent an opportunity to selectively target essential aspects of TGF-β-induced EMT and provide an approach to prevent cancer cell dissemination toward metastasis, without the need to fully inactivate TGF-β signaling.

A 14-3-3 Mode-1 Binding Motif Initiates Gap Junction Internalization During Acute Cardiac Ischemia

Altered phosphorylation and trafficking of connexin 43 (Cx43) during acute ischemia contributes to arrhythmogenic gap junction remodeling, yet the critical sequence and accessory proteins necessary for Cx43 internalization remain unresolved. 14‐3‐3 proteins can regulate protein trafficking, and a 14‐3‐3 mode‐1 binding motif is activated upon phosphorylation of Ser373 of the Cx43 C‐terminus. We hypothesized that Cx43Ser373 phosphorylation is important to pathological gap junction remodeling. Immunofluorescence in human heart reveals the enrichment of 14‐3‐3 proteins at intercalated discs, suggesting interaction with gap junctions. Knockdown of 14‐3‐3τ in cell lines increases gap junction plaque size at cell–cell borders. Cx43S373A mutation prevents Cx43/14‐3‐3 complexing and stabilizes Cx43 at the cell surface, indicating avoidance of degradation. Using Langendorff‐perfused mouse hearts, we detect phosphorylation of newly internalized Cx43 at Ser373 and Ser368 within 30 min of no‐flow ischemia. Phosphorylation of Cx43 at Ser368 by protein kinase C and Ser255 by mitogen‐activated protein kinase has previously been implicated in Cx43 internalization. The Cx43S373A mutant is resistant to phosphorylation at both these residues and does not undergo ubiquitination, revealing Ser373 phosphorylation as an upstream gatekeeper of a posttranslational modification cascade necessary for Cx43 internalization. Cx43Ser373 phosphorylation is a potent target for therapeutic interventions to preserve gap junction coupling in the stressed myocardium.