Integrin alpha3beta1-dependent beta-catenin phosphorylation links epithelial Smad signaling to cell contacts

Regulatory roles of microRNA-21 in fibrosis through interaction with diverse pathways (Review)

MicroRNA-21 (miR-21) is a small, non-coding RNA which can regulate gene expression at the post‑transcriptional level. While the fibrogenic process is vital in tissue repair, proliferation and transition of fibrogenic cells combined with an imbalance of secretion and degradation of the extracellular matrix results in excessive tissue remodeling and fibrosis. Recent studies have indicated that miR‑21 is overexpressed during fibrosis and can regulate the fibrogenic process in a variety of organs and tissues via diverse pathways. The present review summarized the significant roles of miR-21 in fibrosis and discussed the underlying key pathways.

Regulation of TGF-β Receptors

In cells responding to extracellular polypeptide ligands, regulatory mechanisms at the level of cell surface receptors are increasingly seen to define the nature of the ligand-induced signaling responses. Processes that govern the levels of receptors at the plasma membrane, including posttranslational modifications, are crucial to ensure receptor function and specify the downstream signals. Indeed, extracellular posttranslational modifications of the receptors help define stability and ligand binding, while intracellular modifications mediate interactions with signaling mediators and accessory proteins that help define the nature of the signaling response. The use of various molecular biology and biochemistry techniques, based on chemical crosslinking, e.g., biotin or radioactive labeling, immunofluorescence to label membrane receptors and flow cytometry, allows for quantification of changes of cell surface receptor presentation. Here, we discuss recent progress in our understanding of the regulation of TGF-β receptors, i.e., the type I (TβRI) and type II (TβRII) TGF-β receptors, and describe basic methods to identify and quantify TGF-β cell surface receptors.

Direct regulation of transforming growth factor β-induced epithelial-mesenchymal transition by the protein phosphatase activity of unphosphorylated PTEN in lung cancer cells

Transforming growth factor β (TGFβ) causes the acquisition of epithelial-mesenchymal transition (EMT). Although the tumor suppressor gene PTEN (phosphatase and tensin homologue deleted from chromosome 10) can negatively regulate many signaling pathways activated by TGFβ, hyperactivation of these signaling pathways is observed in lung cancer cells. We recently showed that PTEN might be subject to TGFβ-induced phosphorylation of its C-terminus, resulting in a loss of its enzyme activities; PTEN with an unphosphorylated C-terminus (PTEN4A), but not PTEN wild, inhibits TGFβ-induced EMT. Nevertheless, whether or not the blockade of TGFβ-induced EMT by the PTEN phosphatase activity might be attributed to the unphosphorylated PTEN C-terminus itself has not been fully determined. Furthermore, the lipid phosphatase activity of PTEN is well characterized, whereas the protein phosphatase activity has not been determined. By using lung cancer cells carrying PTEN domain deletions or point mutants, we investigated the role of PTEN protein phosphatase activities on TGFβ-induced EMT in lung cancer cells. The unphosphorylated PTEN C-terminus might not directly retain the phosphatase activities and repress TGFβ-induced EMT; the modification that keeps the PTEN C-terminus not phosphorylated might enable PTEN to retain the phosphatase activity. PTEN4A with G129E mutation, which lacks lipid phosphatase activity but retains protein phosphatase activity, repressed TGFβ-induced EMT. Furthermore, the protein phosphatase activity of PTEN4A depended on an essential association between the C2 and phosphatase domains. These data suggest that the protein phosphatase activity of PTEN with an unphosphorylated C-terminus might be a therapeutic target to negatively regulate TGFβ-induced EMT in lung cancer cells.

CD151 promotes α3β1 integrin-dependent organization of carcinoma cell junctions and restrains collective cell invasion

Integrins function in collective migration both as major receptors for extracellular matrix and by crosstalk to adherens junctions. Despite extensive research, important questions remain about how integrin signaling mechanisms are integrated into collective migration programs. Tetraspanins form cell surface complexes with a subset of integrins and thus are good candidates for regulating the balance of integrin functional inputs into cell-matrix and cell-cell interactions. For example, tetraspanin CD151 directly associates with α3β1 integrin in carcinoma cells and promotes rapid α3β1-dependent single cell motility, but CD151 also promotes organized adherens junctions and restrains collective carcinoma cell migration on 2D substrates. However, the individual roles of CD151s integrin partners in CD151s pro-junction activity in carcinoma cells were not well understood. Here we find that CD151 promotes organized carcinoma cell junctions via α3β1 integrin, by a mechanism that requires the a3b1 ligand, laminin-332. Loss of CD151 promotes collective 3D invasion and growth in vitro and in vivo, and the enhanced invasion of CD151-silenced cells is α3 integrin dependent, suggesting that CD151 can regulate the balance between α3β1s pro-junction and pro-migratory activities in collective invasion. An analysis of human cancer cases revealed that changes in CD151 expression can be linked to either better or worse clinical outcomes depending on context, including potentially divergent roles for CD151 in different subsets of breast cancer cases. Thus, the role of the CD151-α3β1 complex in carcinoma progression is context dependent, and may depend on the mode of tumor cell invasion.

Hippo and TGF-β interplay in the lung field

The Hippo pathway is comprised of a kinase cascade that involves mammalian Ste20-like serine/threonine kinases (MST1/2) and large tumor suppressor kinases (LATS1/2) and leads to inactivation of transcriptional coactivator with PDZ-binding motif (TAZ) and yes-associated protein (YAP). Protein stability and subcellular localization of TAZ/YAP determine its ability to regulate a diverse array of biological processes, including proliferation, apoptosis, differentiation, stem/progenitor cell properties, organ size control, and tumorigenesis. These actions are enabled by interactions with various transcription factors or through cross talk with other signaling pathways. Interestingly, mechanical stress has been shown to be an upstream regulator of TAZ/YAP activity, and this finding provides a novel clue for understanding how mechanical forces influence a broad spectrum of biological processes, which involve cytoskeletal structure, cell adhesion, and extracellular matrix (ECM) organization. Transforming growth factor-β (TGF-β) pathway is a critical component of lung development and the progression of lung diseases including emphysema, fibrosis, and cancer. In addition, TGF-β is a key regulator of ECM remodeling and cell differentiation processes such as epithelial-mesenchymal transition. In this review, we summarize the current knowledge of the Hippo pathway regarding lung development and diseases, with an emphasis on its interplay with TGF-β signaling.

Integrin α3β1 Binding to Fibronectin Is Dependent on the Ninth Type III Repeat

Fibronectin (Fn) is a promiscuous ligand for numerous cell adhesion receptors or integrins. The vast majority of Fn-integrin interactions are mediated through the Fn Arg-Gly-Asp (RGD) motif located within the tenth type III repeat. In the case of integrins αIIbβ3 and α5β1, the integrin binds RGD and the synergy site (PHSRN) located within the adjacent ninth type III repeat. Prior work has shown that these synergy-dependent integrins are exquisitely sensitive to perturbations in the Fn integrin binding domain conformation. Our own prior studies of epithelial cell responses to recombinant fragments of the Fn integrin binding domain led us to hypothesize that integrin α3β1 binding may also be modulated by the synergy site. To explore this hypothesis, we created a variety of recombinant variants of the Fn integrin binding domain: (i) a previously reported (Leu → Pro) stabilizing mutant (FnIII9'10), (ii) an Arg to Ala synergy site mutation (FnIII9(R)→(A)10), (iii) a two-Gly (FnIII9(2G)10) insertion, and (iv) a four-Gly (FNIII9(4G)10) insertion in the interdomain linker region and used surface plasmon resonance to determine binding kinetics of integrin α3β1 to the Fn fragments. Integrin α3β1 had the highest affinity for FnIII9'10 and FnIII9(2G)10. Mutation within the synergy site decreased integrin α3β1 binding 17-fold, and the four-Gly insertion decreased binding 39-fold compared with FnIII9'10. Cell attachment studies demonstrate that α3β1-mediated epithelial cell binding is greater on FnIII9'10 compared with the other fragments. These studies suggest that the presence and spacing of the RGD and synergy sites modulate integrin α3β1 binding to Fn.

Crucial role of posttranslational modifications of integrin α3 in interstitial lung disease and nephrotic syndrome

Interstitial lung disease, nephrotic syndrome and junctional epidermolysis bullosa is an autosomal recessive multiorgan disorder caused by mutations in the gene for the integrin α3 subunit (ITGA3). The full spectrum of manifestations and genotype-phenotype correlations is still poorly characterized. Here, we uncovered the disease-causing role and the molecular mechanisms underlying a homozygous ITGA3 mutation leading to the single amino acid substitution, p.R463W. The patient suffered from respiratory distress and episodes of cyanosis with onset in the first week of life and had a nephrotic syndrome. Although there was no clinical evidence for cutaneous fragility, the analysis of a skin sample and of skin epithelial cells enabled the direct assessment of the authentic mutant protein. We show that the mutation altered the conformation of the extracellular β-propeller domain of the integrin α3 subunit preventing correct processing of N-linked oligosaccharides, heterodimerization with β1 integrin and maturation through cleavage into heavy and light chains in the Golgi. Confocal microscopy demonstrated that the mutant protein accumulated intracellularly, but it was not present in focal adhesions or on the cell membrane as shown by flow cytometry. These findings highlight that single amino acid changes in the integrin α3 subunit may crucially alter the structure and complex processing of this integrin, completely preventing its functionality. The present report also underscores that ITGA3 mutations may account for atypical cases solely with early onset respiratory and renal involvement.

Nuclear signaling from cadherin adhesion complexes

The arrival of multicellularity in evolution facilitated cell-cell signaling in conjunction with adhesion. As the ectodomains of cadherins interact with each other directly in trans (as well as in cis), spanning the plasma membrane and associating with multiple other entities, cadherins enable the transduction of "outside-in" or "inside-out" signals. We focus this review on signals that originate from the larger family of cadherins that are inwardly directed to the nucleus, and thus have roles in gene control or nuclear structure-function. The nature of cadherin complexes varies considerably depending on the type of cadherin and its context, and we will address some of these variables for classical cadherins versus other family members. Substantial but still fragmentary progress has been made in understanding the signaling mediators used by varied cadherin complexes to coordinate the state of cell-cell adhesion with gene expression. Evidence that cadherin intracellular binding partners also localize to the nucleus is a major point of interest. In some models, catenins show reduced binding to cadherin cytoplasmic tails favoring their engagement in gene control. When bound, cadherins may serve as stoichiometric competitors of nuclear signals. Cadherins also directly or indirectly affect numerous signaling pathways (e.g., Wnt, receptor tyrosine kinase, Hippo, NFκB, and JAK/STAT), enabling cell-cell contacts to touch upon multiple biological outcomes in embryonic development and tissue homeostasis.

Reprogramming during epithelial to mesenchymal transition under the control of TGFβ

Epithelial-mesenchymal transition (EMT) refers to plastic changes in epithelial tissue architecture. Breast cancer stromal cells provide secreted molecules, such as transforming growth factor β (TGFβ), that promote EMT on tumor cells to facilitate breast cancer cell invasion, stemness and metastasis. TGFβ signaling is considered to be abnormal in the context of cancer development; however, TGFβ acting on breast cancer EMT resembles physiological signaling during embryonic development, when EMT generates or patterns new tissues. Interestingly, while EMT promotes metastatic fate, successful metastatic colonization seems to require the inverse process of mesenchymal-epithelial transition (MET). EMT and MET are interconnected in a time-dependent and tissue context-dependent manner and are coordinated by TGFβ, other extracellular proteins, intracellular signaling cascades, non-coding RNAs and chromatin-based molecular alterations. Research on breast cancer EMT/MET aims at delivering biomolecules that can be used diagnostically in cancer pathology and possibly provide ideas for how to improve breast cancer therapy.

A novel aminothiazole KY-05009 with potential to inhibit Traf2- and Nck-interacting kinase (TNIK) attenuates TGF-β1-mediated epithelial-to-mesenchymal transition in human lung adenocarcinoma A549 cells

Transforming growth factor (TGF)-β triggers the epithelial-to-mesenchymal transition (EMT) of cancer cells via well-orchestrated crosstalk between Smad and non-Smad signaling pathways, including Wnt/β-catenin. Since EMT-induced motility and invasion play a critical role in cancer metastasis, EMT-related molecules are emerging as novel targets of anti-cancer therapies. Traf2- and Nck-interacting kinase (TNIK) has recently been considered as a first-in-class anti-cancer target molecule to regulate Wnt signaling pathway, but pharmacologic inhibition of its EMT activity has not yet been studied. Here, using 5-(4-methylbenzamido)-2-(phenylamino)thiazole-4-carboxamide (KY-05009) with TNIK-inhibitory activity, its efficacy to inhibit EMT in cancer cells was validated. The molecular docking/binding study revealed the binding of KY-05009 in the hinge region of TNIK, and the inhibitory activity of KY-05009 against TNIK was confirmed by an ATP competition assay (Ki, 100 nM). In A549 cells, KY-05009 significantly and strongly inhibited the TGF-β-activated EMT through the attenuation of Smad and non-Smad signaling pathways, including the Wnt, NF-κB, FAK-Src-paxillin-related focal adhesion, and MAP kinases (ERK and JNK) signaling pathways. Continuing efforts to identify and validate potential therapeutic targets associated with EMT, such as TNIK, provide new and improved therapies for treating and/or preventing EMT-based disorders, such as cancer metastasis and fibrosis.

HER2 activation results in β-catenin-dependent changes in pulmonary epithelial permeability

The receptor tyrosine kinase human epidermal growth factor receptor-2 (HER2) is known to regulate pulmonary epithelial barrier function; however, the mechanisms behind this effect remain unidentified. We hypothesized that HER2 signaling alters the epithelial barrier through an interaction with the adherens junction (AJ) protein β-catenin, leading to dissolution of the AJ. In quiescent pulmonary epithelial cells, HER2 and β-catenin colocalized along the lateral intercellular junction. HER2 activation by the ligand neuregulin-1 was associated with tyrosine phosphorylation of β-catenin, dissociation of β-catenin from E-cadherin, and decreased E-cadherin-mediated cell adhesion. All effects were blocked with the HER2 inhibitor lapatinib. β-Catenin knockdown using shRNA significantly attenuated neuregulin-1-induced decreases in pulmonary epithelial resistance in vitro. Our data indicate that HER2 interacts with β-catenin, leading to dissolution of the AJ, decreased cell-cell adhesion, and disruption of the pulmonary epithelial barrier.

Signaling mechanisms of the epithelial-mesenchymal transition

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.

Smad2 is involved in Aggregatibacter actinomycetemcomitans-induced apoptosis

Apoptosis is thought to contribute to the progression of periodontitis. It has been suggested that the apoptosis of epithelial cells may contribute to the loss of epithelial barrier function. Smad2, a downstream signaling molecule of TGF-β receptors (TGF-βRs), is critically involved in apoptosis in several cell types. However, the relationship between smad2 and bacteria-induced apoptosis has not yet been elucidated. It is possible that the regulation of apoptosis induced by periodontopathic bacteria may lead to novel preventive therapies for periodontitis. Therefore, in the present study, we investigated the involvement of smad2 phosphorylation in apoptosis of human gingival epithelial cells induced by Aggregatibacter actinomycetemcomitans (Aa). Aa apparently induced the phosphorylation of smad2 in primary human gingival epithelial cells (HGECs) or the human gingival epithelial cell line, OBA9 cells. In addition, Aa induced phosphorylation of the serine residue of the TGF-β type I receptor (TGF-βRI) in OBA9 cells. SB431542 (a TGF-βRI inhibitor) and siRNA transfection for TGF-βRI, which reduced both TGF-βRI mRNA and protein levels, markedly attenuated the Aa-induced phosphorylation of smad2. Furthermore, the disruption of TGF-βRI signaling cascade by SB431542 and siRNA transfection for TGF-βRI abrogated the activation of cleaved caspase-3 expression and repressed apoptosis in OBA9 cells treated with Aa. Thus, Aa induced apoptosis in gingival epithelial cells by activating the TGF-βRI-smad2-caspase-3 signaling pathway. The results of the present study may suggest that the periodontopathic bacteria, Aa, activates the TGF-βR/smad2 signaling pathway in human gingival epithelial cells and induces apoptosis in epithelial cells, which may lead to new therapeutic strategies that modulate the initiation of periodontitis.

P68 RNA helicase as a molecular target for cancer therapy

The DEAD-box family of RNA helicase is known to be required in virtually all cellular processes involving RNA, and p68 is a prototypic one of the family. Reports have indicated that in addition to ATPase and RNA helicase ability, p68 can also function as a co-activator for transcription factors such as estrogen receptor alpha, tumor suppressor p53 and beta-catenin. More than that, post-translational modification of p68 including phosphorylation, acetylation, sumoylation, and ubiquitylation can regulate the coactivation effect. Furthermore, aberrant expression of p68 in cancers highlights that p68 plays an important role for tumorgenesis and development. In this review, we briefly introduce the function and modulation of p68 in cancer cells, and put forward envisagement about future study about p68.

β-catenin as a regulator and therapeutic target for asthmatic airway remodeling

INTRODUCTION

Pathological alteration in the airway structure, termed as airway remodeling, is a hallmark feature of individuals with asthma and has been described to negatively impact lung function in asthmatics. Recent studies have raised considerable interest in the regulatory role of β-catenin in remodeling asthmatic airways. The WNT/β-catenin signaling pathway is the key to normal lung development and tightly coordinates the maintenance of tissue homeostasis under steady-state conditions. Several studies indicate the crucial role of β-catenin signaling in airway remodeling in asthma and suggest that this pathway may be activated by both the growth factors and mechanical stimuli such as bronchoconstriction.

AREAS COVERED

In this review, we discuss recent literature regarding the mechanisms of β-catenin signaling activation and its mechanistic role in asthmatic airway remodeling. Further, we discuss the possibilities of therapeutic targeting of β-catenin.

EXPERT OPINION

The aberrant activation of β-catenin signaling by both WNT-dependent and -independent mechanisms in asthmatic airways plays a key role in remodeling the airways, including cell proliferation, differentiation, tissue repair and extracellular matrix production. These findings are interesting from both a mechanistic and therapeutic perspective, as several drug classes have now been described that target β-catenin signaling directly.

Cooperation of tyrosine kinase receptor TrkB and epidermal growth factor receptor signaling enhances migration and dispersal of lung tumor cells

TrkB mediates the effects of brain-derived neurotrophic factor (BDNF) in neuronal and nonnneuronal cells. Based on recent reports that TrkB can also be transactivated through epidermal growth-factor receptor (EGFR) signaling and thus regulates migration of early neurons, we investigated the role of TrkB in migration of lung tumor cells. Early metastasis remains a major challenge in the clinical management of non-small cell lung cancer (NSCLC). TrkB receptor signaling is associated with metastasis and poor patient prognosis in NSCLC. Expression of this receptor in A549 cells and in another adenocarcinoma cell line, NCI-H441, promoted enhanced migratory capacity in wound healing assays in the presence of the TrkB ligand BDNF. Furthermore, TrkB expression in A549 cells potentiated the stimulatory effect of EGF in wound healing and in Boyden chamber migration experiments. Consistent with a potential loss of cell polarity upon TrkB expression, cell dispersal and de-clustering was induced in A549 cells independently of exogeneous BDNF. Morphological transformation involved extensive cytoskeletal changes, reduced E-cadherin expression and suppression of E-cadherin expression on the cell surface in TrkB expressing tumor cells. This function depended on MEK and Akt kinase activity but was independent of Src. These data indicate that TrkB expression in lung adenoma cells is an early step in tumor cell dissemination, and thus could represent a target for therapy development.

Integrins and cadherins as therapeutic targets in fibrosis

Fibrosis is the excessive deposition of extracellular matrix proteins into tissues leading to scar formation, disruption of normal tissue architecture and organ failure. Despite the large clinical impact of fibrosis, treatment options are limited. Adhesion molecules, in particular αvβ6 and α3β1 integrins and cadherin-11, have been demonstrated to be important mediators of tissue fibrosis. These data are reviewed here and provide the foundation for these molecules to be potential therapeutic targets for patients with fibrotic diseases.

DA-Raf-dependent inhibition of the Ras-ERK signaling pathway in type 2 alveolar epithelial cells controls alveolar formation

Alveolar formation is coupled to the spatiotemporally regulated differentiation of alveolar myofibroblasts (AMYFs), which contribute to the morphological changes of interalveolar walls. Although the Ras-ERK signaling pathway is one of the key regulators for alveolar formation in developing lungs, the intrinsic molecular and cellular mechanisms underlying its role remain largely unknown. By analyzing the Ras-ERK signaling pathway during postnatal development of lungs, we have identified a critical role of DA-Raf1 (DA-Raf)-a dominant-negative antagonist for the Ras-ERK signaling pathway-in alveolar formation. DA-Raf-deficient mice displayed alveolar dysgenesis as a result of the blockade of AMYF differentiation. DA-Raf is predominantly expressed in type 2 alveolar epithelial cells (AEC2s) in developing lungs, and DA-Raf-dependent MEK1/2 inhibition in AEC2s suppresses expression of tissue inhibitor of matalloprotienase 4 (TIMP4), which prevents a subsequent proteolytic cascade matrix metalloproteinase (MMP)14-MMP2. Furthermore, MMP14-MMP2 proteolytic cascade regulates AMYF differentiation and alveolar formation. Therefore, DA-Raf-dependent inhibition of the Ras-ERK signaling pathway in AEC2s is required for alveolar formation via triggering MMP2 activation followed by AMYF differentiation. These findings reveal a pivotal role of the Ras-ERK signaling pathway in the dynamic regulation of alveolar development.

Inhibition of epithelial-to-mesenchymal transition and pulmonary fibrosis by methacycline

A high-throughput small-molecule screen was conducted to identify inhibitors of epithelial-mesenchymal transition (EMT) that could be used as tool compounds to test the importance of EMT signaling in vivo during fibrogenesis. Transforming growth factor (TGF)-β1-induced fibronectin expression and E-cadherin repression in A549 cells were used as 48-hour endpoints in a cell-based imaging screen. Compounds that directly blocked Smad2/3 phosphorylation were excluded. From 2,100 bioactive compounds, methacycline was identified as an inhibitor of A549 EMT with the half maximal inhibitory concentration (IC50) of roughly 5 μM. In vitro, methacycline inhibited TGF-β1-induced α-smooth muscle actin, Snail1, and collagen I of primary alveolar epithelial cells . Methacycline inhibited TGF-β1-induced non-Smad pathways, including c-Jun N-terminal kinase, p38, and Akt activation, but not Smad or β-catenin transcriptional activity. Methacycline had no effect on baseline c-Jun N-terminal kinase, p38, or Akt activities or lung fibroblast responses to TGF-β1. In vivo, 100 mg/kg intraperitoneal methacycline delivered daily beginning 10 days after intratracheal bleomycin improved survival at Day 17 (P < 0.01). Bleomycin-induced canonical EMT markers, Snail1, Twist1, collagen I, as well as fibronectin protein and mRNA, were attenuated by methacycline (Day 17). Methacycline did not attenuate inflammatory cell accumulation or alter TGF-β1-responsive genes in alveolar macrophages. These studies identify a novel inhibitor of EMT as a potent suppressor of fibrogenesis, further supporting the concept that EMT signaling is important to lung fibrosis. The findings also provide support for testing the impact of methacycline or doxycycline, an active analog, on progression of human pulmonary fibrosis.

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.

Biologically-active laminin-111 fragment that modulates the epithelial-to-mesenchymal transition in embryonic stem cells

The dynamic interplay between the extracellular matrix and embryonic stem cells (ESCs) constitutes one of the key steps in understanding stem cell differentiation in vitro. Here we report a biologically-active laminin-111 fragment generated by matrix metalloproteinase 2 (MMP2) processing, which is highly up-regulated during differentiation. We show that the β1-chain-derived fragment interacts via α3β1-integrins, thereby triggering the down-regulation of MMP2 in mouse and human ESCs. Additionally, the expression of MMP9 and E-cadherin is up-regulated in mouse ESCs--key players in the epithelial-to-mesenchymal transition. We also demonstrate that the fragment acts through the α3β1-integrin/extracellular matrix metalloproteinase inducer complex. This study reveals a previously unidentified role of laminin-111 in early stem cell differentiation that goes far beyond basement membrane assembly and a mechanism by which an MMP2-cleaved laminin fragment regulates the expression of E-cadherin, MMP2, and MMP9.

How do your contacts (or their absence) shape your fate?

Tissue accumulation of contractile myofibroblasts is a key feature of a multitude of fibrotic diseases. Myofibroblast generation either from epithelial or mesenchymal precursors involves the activation of a myogenic program, hallmarked by the expression of α-smooth muscle actin (SMA). Recent research suggests that this robust phenotypic reprogramming requires two critical inputs: the fibrogenic cytokine transforming growth factor-β1 (TGFβ) and an injury (or absence) of intercellular junctions. This two-hit paradigm of epithelial-myofibroblast transition (EMyT) postulates that the injured (contact-deprived) epithelium is locally and selectively sensitive (topically susceptible) to the transforming effect of TGFβ, while the intact areas are quite resistant to the phenotype-changing effect of this cytokine. Searching for molecular mechanisms underlying the synergy between contact injury and TGFβ, we found that an interplay among three multifunctional transcriptional (co)activators, the junction component β-catenin, the TGFβ receptor target Smad3, and the actin cytoskeleton-regulated myocardin-related transcription factor (MRTF) controls the magnitude and timing of SMA expression.¹ Moreover, this regulation is realized not only at the transcriptional level. Notably, these factors form a pretranscriptional circuit, in which they impact each other’s activity and stability. Based on this recent paper we ponder about the mechanisms of cellular plasticity in the context of EMyT. We propose that topical susceptibility to TGFβ, triggered by cell contact-modulated pretranscriptional and transcriptional control is realized through the crosstalk of a few master regulators, whose coordinated action tailors SMA expression and contributes to the major decision of whether injury leads to healing or fibrosis.

A mechanism enhancing macromolecule transport through paracellular spaces induced by Poly-L-Arginine: Poly-L-Arginine induces the internalization of tight junction proteins via clathrin-mediated endocytosis

PURPOSE

Poly-L-arginine (PLA) enhances the paracellular permeability of the Caco-2 cell monolayer to hydrophilic macromolecules by disappearance of tight junction (TJ) proteins from cell-cell junctions. However, the mechanism of the disappearance of TJ proteins in response to PLA has been unclear. In this study, we investigated the mechanism of disappearance of TJ proteins from cell-cell junctions after the application of PLA to Caco-2 cell monolayers.

METHODS

The membrane conductance (Gt), FITC-dextran (FD-4) permeability, and localization of TJ proteins were examined after the treatment of Caco-2 cell monolayers with PLA in the presence of various endocytosis inhibitors. In addition, the localization of endosome marker proteins was also observed.

RESULTS

Clathrin-mediated endocytosis inhibitors suppressed the increase in Gt and Papp of FD-4 induced by PLA, and also significantly suppressed the disappearance of TJ proteins induced by PLA. Furthermore, occludin, one of the TJ proteins, colocalized with early endosome and recycling endosomes after the internalization of occludin induced by PLA, and then was recycled to the cell-cell junctions.

CONCLUSION

PLA induced the transient internalization of TJ proteins in cell-cell junctions via clathrin-mediated endocytosis, subsequently increasing the permeability of the Caco-2 cell monolayer to FD-4 via a paracellular route.

Integrin Regulation of Epidermal Functions in Wounds

Significance: Integrins are bidirectional signaling receptors for extracellular matrix that regulate both inside-out signaling that controls keratinocyte-mediated changes to the wound microenvironment and outside-in signaling that controls keratinocyte responses to microenvironmental changes. As such, integrins represent attractive therapeutic targets for treatment of chronic wounds or general promotion of wound healing. Advances in wound management are particularly important as the elderly and diabetic populations within the United States continue to grow. Recent Advances: Although integrins are best known for mediating cell adhesion and migration, integrins in wound epidermis also control cell survival, proliferation, matrix remodeling, and paracrine crosstalk to other cellular compartments of the wound. Importantly, the concept of targeting integrins in the clinic has been established for treatment of certain cancers and other diseases, laying the groundwork for similar exploitation of integrins as targets to treat chronic wounds. Critical Issues: Despite their attractiveness as therapeutic targets, integrins have complex roles in wound healing that are impacted by both their own expression and a highly dynamic wound microenvironment that determines ligand availability. Therefore, identifying relevant integrin ligands in the wound and understanding both distinct and overlapping functions that different integrins play in the epidermis will be critical to determine their precise roles in wound healing. Future Directions: Future research should focus on gaining a thorough understanding of the highly coordinated functions of different integrins in wound epidermis, and on determining which of these functions go awry in pathological wounds. This focus should facilitate development of integrin-targeting therapeutics for treating chronic wounds.

The increase of microRNA-21 during lung fibrosis and its contribution to epithelial-mesenchymal transition in pulmonary epithelial cells

Background

The excess and persistent accumulation of fibroblasts due to aberrant tissue repair results in fibrotic diseases such as idiopathic pulmonary fibrosis. Recent reports have revealed significant changes in microRNAs during idiopathic pulmonary fibrosis and evidence in support of a role for microRNAs in myofibroblast differentiation and the epithelial-mesenchymal transition in the context of fibrosis. It has been reported that microRNA-21 is up-regulated in myofibroblasts during fibrosis and promotes transforming growth factor-beta signaling by inhibiting Smad7. However, expression changes in microRNA-21 and the role of microRNA-21 in epithelial-mesenchymal transition during lung fibrosis have not yet been defined.

Methods

Lungs from saline- or bleomycin-treated C57BL/6 J mice and lung specimens from patients with idiopathic pulmonary fibrosis were analyzed. Enzymatic digestions were performed to isolate single lung cells. Lung epithelial cells were isolated by flow cytometric cell sorting. The expression of microRNA-21 was analyzed using both quantitative PCR and in situ hybridization. To induce epithelial-mesenchymal transition in culture, isolated mouse lung alveolar type II cells were cultured on fibronectin-coated chamber slides in the presence of transforming growth factor-β, thus generating conditions that enhance epithelial-mesenchymal transition. To investigate the role of microRNA-21 in epithelial-mesenchymal transition, we transfected cells with a microRNA-21 inhibitor. Total RNA was isolated from the freshly isolated and cultured cells. MicroRNA-21, as well as mRNAs of genes that are markers of alveolar epithelial or mesenchymal cell differentiation, were quantified using quantitative PCR.

Results

The lung epithelial cells isolated from the bleomycin-induced lung fibrosis model system had decreased expression of epithelial marker genes, whereas the expression of mesenchymal marker genes was increased. MicroRNA-21 was significantly upregulated in isolated lung epithelial cells during bleomycin-induced lung fibrosis and human idiopathic pulmonary fibrosis. MicroRNA-21 was also upregulated in the cultured alveolar epithelial cells under the conditions that enhance epithelial-mesenchymal transition. Exogenous administration of a microRNA-21 inhibitor prevented the increased expression of vimentin and alpha-smooth muscle actin in cultured primary mouse alveolar type II cells under culture conditions that induce epithelial-mesenchymal transition.

Conclusions

Our experiments demonstrate that microRNA-21 is increased in lung epithelial cells during lung fibrosis and that it promotes epithelial-mesenchymal transition.

Activated alveolar epithelial cells initiate fibrosis through secretion of mesenchymal proteins

Fibrosis is characterized by accumulation of activated fibroblasts and pathological deposition of fibrillar collagens. Activated fibroblasts overexpress matrix proteins and release factors that promote further recruitment of activated fibroblasts, leading to progressive fibrosis. The contribution of epithelial cells to this process remains unknown. Epithelium-directed injury may lead to activation of epithelial cells with phenotypes and functions similar to activated fibroblasts. Prior reports that used a reporter gene fate-mapping strategy are limited in their ability to investigate the functional significance of epithelial cell-derived mesenchymal proteins during fibrogenesis. We found that lung epithelial cell-derived collagen I activates fibroblast collagen receptor discoidin domain receptor-2, contributes significantly to fibrogenesis, and promotes resolution of lung inflammation. Alveolar epithelial cells undergoing transforming growth factor-β-mediated mesenchymal transition express several other secreted profibrotic factors and are capable of activating lung fibroblasts. These studies provide direct evidence that activated epithelial cells produce mesenchymal proteins that initiate a cycle of fibrogenic effector cell activation, leading to progressive fibrosis. Therapy targeted at epithelial cell production of type I collagen offers a novel pathway for abrogating this progressive cycle and for limiting tissue fibrosis but may lead to sustained lung injury/inflammation.

Differential topical susceptibility to TGFβ in intact and injured regions of the epithelium: key role in myofibroblast transition

Intact and cell contact–deprived regions of an epithelial monolayer are differentially sensitive to the transforming effect of TGFβ. This topical susceptibility is mediated by the interplay between TGFβ- and cell contact–dependent transcription factors and might play a key role in the cell biology of wound healing and fibrosis.

Induction of epithelial–myofibroblast transition (EMyT), a robust fibrogenic phenotype change hallmarked by α-smooth muscle actin (SMA) expression, requires transforming growth factor-β1 (TGFβ) and the absence/uncoupling of intracellular contacts. This suggests that an “injured” epithelium may be topically susceptible to TGFβ. To explore this concept, we use an epithelial wound model in which intact and contact-deprived regions of the same monolayer can be analyzed simultaneously. We show that TGFβ elicits dramatically different responses at these two loci. SMA expression and initially enhanced nuclear Smad3 accumulation followed by Smad3 mRNA and protein down-regulation occur exclusively at the wound. Mechanistically, three transcriptional coactivators whose localization is regulated by cell contact integrity are critical for these local effects. These are myocardin-related transcription factor (MRTF), the driver of the SMA promoter; β-catenin, which counteracts the known inhibitory effect of Smad3 on MRTF and maintains MRTF protein stability and mRNA expression in the wound; and TAZ, a Hippo effector and Smad3 retention factor. Remarkably, active TAZ stimulates the SMA and suppresses the Smad3 promoter, whereas TAZ silencing prevents wound-restricted expression of SMA and loss of Smad3. Such locus-specific reprogramming might play key roles in wound healing and the susceptibility of the injured epithelium to fibrogenesis.

c-Myc regulates proliferation and Fgf10 expression in airway smooth muscle after airway epithelial injury in mouse

During lung development, Fibroblast growth factor 10 (Fgf10), which is expressed in the distal mesenchyme and regulated by Wnt signaling, acts on the distal epithelial progenitors to maintain them and prevent them from differentiating into proximal (airway) epithelial cells. Fgf10-expressing cells in the distal mesenchyme are progenitors for parabronchial smooth muscle cells (PSMCs). After naphthalene, ozone or bleomycin-induced airway epithelial injury, surviving epithelial cells secrete Wnt7b which then activates the PSMC niche to induce Fgf10 expression. This Fgf10 secreted by the niche then acts on a subset of Clara stem cells to break quiescence, induce proliferation and initiate epithelial repair. Here we show that conditional deletion of the Wnt target gene c-Myc from the lung mesenchyme during development does not affect proper epithelial or mesenchymal differentiation. However, in the adult lung we show that after naphthalene-mediated airway epithelial injury c-Myc is important for the activation of the PSMC niche and as such induces proliferation and Fgf10 expression in PSMCs. Our data indicate that conditional deletion of c-Myc from PSMCs inhibits airway epithelial repair, whereas c-Myc ablation from Clara cells has no effect on airway epithelial regeneration. These findings may have important implications for understanding the misregulation of lung repair in asthma and COPD.

Dynamic Interplay of Smooth Muscle α-Actin Gene-Regulatory Proteins Reflects the Biological Complexity of Myofibroblast Differentiation

Myofibroblasts (MFBs) are smooth muscle-like cells that provide contractile force required for tissue repair during wound healing. The leading agonist for MFB differentiation is transforming growth factor β1 (TGFβ1) that induces transcription of genes encoding smooth muscle α-actin (SMαA) and interstitial collagen that are markers for MFB differentiation. TGFβ1 augments activation of Smad transcription factors, pro-survival Akt kinase, and p38 MAP kinase as well as Wingless/int (Wnt) developmental signaling. These actions conspire to activate β-catenin needed for expression of cyclin D, laminin, fibronectin, and metalloproteinases that aid in repairing epithelial cells and their associated basement membranes. Importantly, β-catenin also provides a feed-forward stimulus that amplifies local TGFβ1 autocrine/paracrine signaling causing transition of mesenchymal stromal cells, pericytes, and epithelial cells into contractile MFBs. Complex, mutually interactive mechanisms have evolved that permit several mammalian cell types to activate the SMαA promoter and undergo MFB differentiation. These molecular controls will be reviewed with an emphasis on the dynamic interplay between serum response factor, TGFβ1-activated Smads, Wnt-activated β-catenin, p38/calcium-activated NFAT protein, and the RNA-binding proteins, Purα, Purβ, and YB-1, in governing transcriptional and translational control of the SMαA gene in injury-activated MFBs.

Physical and chemical microenvironmental cues orthogonally control the degree and duration of fibrosis-associated epithelial-to-mesenchymal transitions

Increased tissue stiffness and epithelial-to-mesenchymal transitions (EMTs) are two seemingly discrete hallmarks of fibrotic diseases. Despite recent findings highlighting the influence of tissue mechanical properties on cell phenotype, it remains unclear what role increased tissue stiffness has in the regulation of previously reported fibronectin-mediated EMTs associated with pulmonary fibrosis. Nano-indentation testing of lung interstitial spaces showed that in vivo cell-level Young's moduli increase with the onset of fibrosis from ∼2 to ∼17 kPa. In vitro, we found that stiff, but not soft, fibronectin substrates induce EMT, a response dependent on cell contraction-mediated integrin activation of TGFβ. Activation or suppression of cell contractility with exogenous factors was sufficient to overcome the effect of substrate stiffness. Pulse-chase experiments indicate that the effect of cell contractility is dose- and time-dependent. In response to low levels of TGFβ on soft surfaces, either added exogenously or produced through thrombin-induced contraction, cells will initiate the EMT programme, but upon removal revert to an epithelial phenotype. These results identify matrix stiffness and/or cell contractility as critical targets for novel therapeutics for fibrotic diseases.

EMT and interstitial lung disease: a mysterious relationship

PURPOSE OF REVIEW

Pathogenesis of interstitial lung diseases (ILD) has largely been investigated in the context of the most frequent ILD, idiopathic pulmonary fibrosis (IPF). We review studies of epithelial-to-mesenchymal transition (EMT) and discuss its potential contribution to collagen-producing (myo)fibroblasts in IPF.

RECENT FINDINGS

Endoplasmic reticulum (ER) stress leading to epithelial apoptosis has been reported as a potential etiologic factor in fibrosis. Recent studies further suggest EMT as a link between ER stress and fibrosis. Combinatorial interactions among Smad3, β-catenin and other transcriptional co-activators at the α-smooth muscle actin (α-SMA) promoter provide direct evidence for crosstalk between transforming growth factor-β (TGFβ) and β-catenin pathways during EMT. Lineage tracing yielded conflicting results, with two recent studies supporting and one opposing a role for EMT in lung fibrosis.

SUMMARY

Advances have been made in elucidating causes and mechanisms of EMT, potentially leading to new treatment options, although contributions of EMT to lung fibrosis in vivo remain controversial. In addition to EMT providing a direct source of (myo)fibroblasts, expression of mesenchymal markers may reflect epithelial injury, in which case inhibition of EMT might be deleterious. EMT-derived cells may also contribute to aberrant epithelial-mesenchymal crosstalk that promotes fibrogenesis.

Identification of pY654-β-catenin as a critical co-factor in hypoxia-inducible factor-1α signaling and tumor responses to hypoxia

Hypoxia is linked to epithelial-mesenchymal transition (EMT) and tumor progression in numerous carcinomas. Responses to hypoxia are thought to operate via hypoxia-inducible factors (HIFs), but the importance of co-factors that regulate HIF signaling within tumors is not well understood. Here, we elucidate a signaling pathway that physically and functionally couples tyrosine phosphorylation of β-catenin to HIF1α signaling and HIF1α-mediated tumor EMT. Primary human lung adenocarcinomas accumulate pY654-β-catenin and HIF1α. All pY654-β-catenin, and only the tyrosine phosphorylated form, was found complexed with HIF1α and active Src, both within the human tumors and in lung tumor cell lines exposed to hypoxia. Phosphorylation of Y654, generated by hypoxia mediated, reactive oxygen species (ROS)-dependent Src kinase activation, was required for β-catenin to interact with HIF1α and Src, to promote HIF1α transcriptional activity, and for hypoxia-induced EMT. Mice bearing hypoxic pancreatic islet adenomas, generated by treatment with anti-vascular endothelial growth factor antibodies, accumulate HIF1α/pY654-β-catenin complexes and develop an invasive phenotype. Concurrent administration of the ROS inhibitor N-acetylcysteine abrogated β-catenin/HIF pathway activity and restored adenoma architecture. Collectively, the findings implicate accumulation of pY654-β-catenin specifically complexed to HIF1α and Src kinase as critically involved in HIF1α signaling and tumor invasion. The findings also suggest that targeting ROS-dependent aspects of the pY654-β-catenin/ HIF1α pathway may attenuate untoward biological effects of anti-angiogenic agents and tumor hypoxia.

A tale of two collagen receptors, integrin β1 and discoidin domain receptor 1, in epithelial cell differentiation

As increase in collagen deposition is no longer taken as simply a consequence but, rather, an inducer of disease progression; therefore, the understanding of collagen signal transduction is fundamentally important. Cells contain at least two types of collagen receptors: integrins and discoidin domain receptors (DDRs). The integrin heterodimers α1β1, α2β1, α10β1, and α11β1 are recognized as the non-tyrosine kinase collagen receptors. DDR1 and 2, the tyrosine kinase receptors of collagen, are specifically expressed in epithelium and mesenchyme, respectively. While integrin β1 and DDR1 are both required for cell adhesion on collagen, their roles in epithelial cell differentiation during development and disease progression seem to counteract each other, with integrin β1 favoring epithelium mesenchyme transition (EMT) and DDR1 inducing epithelial cell differentiation. The in vitro evidence shows that the integrin β1 and DDR1 exert opposing actions in regulation of membrane stability of E-cadherin, which itself is a critical regulator of epithelial cell differentiation. Here, we review the functional roles of integrin β1 and DDR1 in regulation of epithelial cell differentiation during development and disease progression, and explore the underlining mechanisms regarding to the regulation of membrane stability of E-cadherin.

Epithelial responses to lung injury: role of the extracellular matrix

The key role of extracellular matrices in alveolar epithelial cell (AEC) biology is highlighted by the phenotypes of primary AECs cultured on a soft laminin gel contrasted with that on a stiff, fibronectin matrix. On laminin, AECs maintain an epithelial phenotype, and progenitor cells within this population proliferate. In contrast, on fibronectin, AECs rapidly lose surfactant expression and spread extensively, changes that depend on activation of latent TGF-β1 by engagement of fibronectin-binding integrins. The progenitor subpopulation responding to TGF-β1 undergoes epithelial mesenchymal transition (EMT). Although it remains uncertain to what degree EMT contributes directly to collagen 1 production, signaling pathways critical to EMT are important for repair and fibrosis, implying that EMT is part of the general program of lung repair. EMT reprogramming requires not only Smad signaling but also pY654-β-catenin. Generation of pY654-β-catenin requires assembly of complexes of the integrin α3β1, E-cadherin, and TGF-β1 receptors, and such assembly is a function of cell-cell and cell-matrix contacts. Sequestration of α3β1 or E-cadherin in such contacts prevents complex assembly, TGF-β1 induced pY654-β-catenin generation and EMT. Disruption of these contacts is a signal for the cells to initiate repair. Critical remaining questions center around better definition of direct versus indirect effects of EMT on collagen deposition and the nature of AEC progenitors differentiating during fibrogenesis. Elucidation of specific inhibitors of EMT should further test the question of whether the process is important to fibrosis in vivo and a viable therapeutic target.

Association of β-catenin with P-Smad3 but not LEF-1 dissociates in vitro profibrotic from anti-inflammatory effects of TGF-β1

Transforming growth factor β1 (TGF-β1) is known to be both anti-inflammatory and profibrotic. Cross-talk between TGF-β/Smad and Wnt/β-catenin pathways in epithelial-mesenchymal transition (EMT) suggests a specific role for β-catenin in profibrotic effects of TGF-β1. However, no such mechanistic role has been demonstrated for β-catenin in the anti-inflammatory effects of TGF-β1. In the present study, we explored the role of β-catenin in the profibrotic and anti-inflammatory effects of TGF-β1 by using a cytosolic, but not membrane, β-catenin knockdown chimera (F-TrCP-Ecad) and the β-catenin/CBP inhibitor ICG-001. TGF-β1 induced nuclear Smad3/β-catenin complex, but not β-catenin/LEF-1 complex or TOP-flash activity, during EMT of C1.1 (renal tubular epithelial) cells. F-TrCP-Ecad selectively degraded TGF-β1-induced cytoplasmic β-catenin and blocked EMT of C1.1 cells. Both F-TrCP-Ecad and ICG-001 blocked TGF-β1-induced Smad3/β-catenin and Smad reporter activity in C1.1 cells, suggesting that TGF-β1-induced EMT depends on β-catenin binding to Smad3, but not LEF-1 downstream of Smad3, through canonical Wnt. In contrast, in J774 macrophages, the β-catenin level was low and was not changed by interferon-γ (IFN-γ) or lipopolysaccharide (LPS) with or without TGF-β1. TGF-β1 inhibition of LPS-induced TNF-α and IFN-γ-stimulated inducible NO synthase (iNOS) expression was not affected by F-TrCP-Ecad, ICG-001 or by overexpression of wild-type β-catenin in J774 cells. Inhibition of β-catenin by either F-TrCP-Ecad or ICG-001 abolished LiCl-induced TOP-flash, but not TGF-β1-induced Smad reporter, activity in J774 cells. These results demonstrate for the first time that β-catenin is required as a co-factor of Smad in TGF-β1-induced EMT of C1.1 epithelial cells, but not in TGF-β1 inhibition of macrophage activation. Targeting β-catenin may dissociate the TGF-β1 profibrotic and anti-inflammatory effects.

nm23-H1 is a negative regulator of TGF-β1-dependent induction of epithelial-mesenchymal transition

Members of transforming growth factor-β(TGF-β) family are the main inducers of epithelial-mesenchymal transition (EMT) during embryogenesis and cancer pathogenesis. However, a significant crosstalk between TGF-β and other signals occurs during the induction of EMT. nm23-H1 was the first metastasis suppressor gene to be identified on the basis of an inverse relationship between nm23-H1 expression and metastasis stage. Despite extensive studies, the mechanism underlying its ability to suppress metastasis is far from elucidated. We demonstrated here that the nm23-H1 negatively regulated TGF-β1-dependent induction of EMT in non-aggressive lung cancer cell line. nm23-H1 knockdown significantly enhanced TGF-β1-induced suppression of epithelial marker E-cadherin and upregulation of mesenchymal markers β-catenin and fibronectin. The invasive and migratory potential of lung cancer cells upon TGF-β1 treatment was also markedly enhanced by nm23-H1 knockdown. On the other hand, the effect of nm23-H1 depletion on TGF-β1-induced EMT was reversed by ectopic re-expression of shRNA-resistant nm23-H1 protein. Furthermore, TGF-β1-induced EMT potentiated by nm23-H1 depletion was partially dependent on transcriptional factor Snail expression. Finally, we found Src kinase is involved in regulation of TGF-β1-induced EMT by nm23-H1. Our results suggest a means of restoring nm23-H1 to suppress TGF-β1-induced EMT that may exploited therapeutically for the management of metastasis diseases.

Autocrine TGF-β induces epithelial to mesenchymal transition in human amniotic epithelial cells

Human amniotic epithelial cells (hAECs) have been the object of intense research due to their potential therapeutic use. In this paper, we present molecular evidence of a bona fide epithelial to mesenchymal transition (EMT) undergone by hAECs. Amniotic membrane (AM)-derived hAECs showed the presence of typical epithelial markers such as E-cadherin and cytokeratins. hAECs in culture, however, underwent morphological changes acquiring a mesenchymal shape. Epithelial cell markers were lost and typical mesenchymal markers, such as vimentin and α-SMA, appeared. Several genes associated with EMT, such as SNAI1, MMP9, PAI1, or ACTA2, increased their expression. The expression of the transcription activators KLF4 or MTA3 was consistent with the downregulation of CDH1. We have shown that hAECs undergo EMT due to the autocrine production of TGF-β. Furthermore, the addition of the TGF-β receptor I (ALK5) inhibitor SB-431542 or TGF-β neutralizing antibody to hAECs prevented EMT and preserved the hAECs' epithelial phenotype. Altogether, these results suggest that cultured hAECs undergo EMT through the autocrine production of TGF-β.

Transforming growth factor-β1 attenuates junctional adhesion molecule-A and contributes to breast cancer cell invasion

Transforming growth factor-β1 (TGF-β1) is a potent regulator in promoting the invasion and proliferation of breast cancer cells. Junctional adhesion molecule-A (JAM-A) is a tight junction protein that displays an inverse relationship to cell invasiveness in breast cancer cells. Whether TGF-β1 signaling induces alteration of JAM-A expression leading to cell invasion has not been investigated. In this study, we report that TGF-β1 down-regulated JAM-A expression via its effect on both transcriptional and post-translational regulations of JAM-A, thus inducing cell invasion. On exploring whether TGF-β1 might be the upstream regulator of JAM-A expression, we found that knockdown of TGF-β receptors and canonical Smad signaling could upregulate JAM-A level and inhibit cell invasion in MDA-MB-231 cells. TGF-β1 treatment of MCF-7 cells caused a significant reduction of JAM-A mRNA and protein and induced cell invasion. Delineating the signal mechanisms involved in TGF-β1-mediated JAM-A repression, we found that TGF-β1 significantly inhibited JAM-A gene transcription via the activation of Smads. In addition to Smad activation, we found that involvement of p54 JNK is crucial for post-translational modification of TGF-β1-mediated JAM-A protein degradation. Blockage of JNK pathway by inhibitor could attenuate TGF-β1-induced cell invasion. We provide evidences for the first time that TGF-β1 induces breast cancer cell invasion via TGF-β1-mediated control on JAM-A expression. Identification of JAM-A as a downstream target of TGF-β1 represents a crucial mechanism in cancer progression.

Directing epithelial to mesenchymal transition through engineered microenvironments displaying orthogonal adhesive and mechanical cues

Cell interactions with their extracellular matrix (ECM) microenvironments play a major role in directing cellular processes that can drive wound healing and tissue regeneration but, if uncontrolled, lead to pathological progression. One such process, epithelial to mesenchymal transition (EMT), if finely controlled could have significant potential in regenerative medicine approaches. Despite recent findings that highlight the influence of biochemical and mechanical properties of the ECM on EMT, it is still unclear how these two orthogonal cues act synergistically to control epithelial cell phenotype. Here, we cultured lung epithelial cells on combinations of different mutants of fibronectin's cell binding domain that preferentially engage specific integrins and substrates of varying stiffness. Our results suggest that while stiff substrates induce spontaneous EMT, this response can be overcome by with fragments of fibronectin that support α3 and α5 integrin engagement. Furthermore, we found that substrate-induced EMT correlates with transforming growth factor beta activation by resident epithelial cells and is dependent on Rho/ROCK signaling. Suppressing cell-contractility was sufficient to maintain an epithelial phenotype. Our results suggest that integrin-specific engagement of fibronectin adhesive domains and the mechanics of the ECM act synergistically to direct EMT.

Crossroads of integrins and cadherins in epithelia and stroma remodeling

Adhesion events mediated by cadherin and integrin adhesion receptors have fundamental roles in the maintenance of the physiological balance of epithelial tissues, and it is well established that perturbations in their normal functional activity and/or changes in their expression are associated with tumorigenesis. Over the last decades, increasing evidence of a dynamic collaborative interaction between these complexes through their shared interactions with cytoskeletal proteins and common signaling pathways has emerged not only as an important regulator of several aspects of epithelial cell behavior, but also as a coordinated adhesion module that senses and transmits signals from and to the epithelia surrounding microenvironment. The tight regulation of their crosstalk is particularly important during epithelial remodeling events that normally take place during morphogenesis and tissue repair, and when defective it leads to cell transformation and aggravated responses of the tumor microenvironment that contribute to tumorigenesis. In this review we highlight some of the interactions that regulate their crosstalk and how this could be implicated in regulating signals across epithelial tissues to sustain homeostasis.

Induction of a mesenchymal expression program in lung epithelial cells by wingless protein (Wnt)/β-catenin requires the presence of c-Jun N-terminal kinase-1 (JNK1)

Recent studies suggest the importance of the transition of airway epithelial cells (EMT) in pulmonary fibrosis, and also indicate a role for Wingless protein (Wnt)/β-catenin signaling in idiopathic pulmonary fibrosis. We investigated the possible role of the Wnt signaling pathway in inducing EMT in lung epithelial cells, and sought to unravel the role of c-Jun-N-terminal-kinase-1 (JNK1). The exposure of C10 lung epithelial cells or primary mouse tracheal epithelial cells (MTECs) to Wnt3a resulted in increases in JNK phosphorylation and nuclear β-catenin content. Because the role of β-catenin as a transcriptional coactivator is well established, we investigated T-cell factor/lymphocyte-enhancement factor (TCF/LEF) transcriptional activity in C10 lung epithelial cells after the activation of Wnt. TCF/LEF transcriptional activity was enhanced after the activation of Wnt, and this increase in TCF/LEF transcriptional activity was diminished after the small interfering (si)RNA-mediated ablation of JNK. The activation of the Wnt pathway by Wnt3a, or the expression of either wild-type or constitutively active β-catenin (S37A), led to the activation of an EMT transcriptome, manifested by the increased mRNA expression of CArG box-binding factor-A, fibroblast-specific protein (FSP)-1, α-smooth muscle actin (α-SMA), and vimentin, increases in the content of α-SMA and FSP1, and the concomitant loss of zona occludens-1. The siRNA-mediated ablation of β-catenin substantially decreased Wnt3a-induced EMT. The siRNA ablation of JNK1 largely abolished Wnt3a, β-catenin, and β-catenin S37a-induced EMT. In MTECs lacking Jnk1, Wnt3a-induced increases in nuclear β-catenin, EMT transcriptome, and the content of α-SMA or FSP1 were substantially diminished. These data show that the activation of the Wnt signaling pathway is capable of inducing an EMT program in lung epithelial cells through β-catenin, and that this process is controlled by JNK1.

Roles of N-acetylglucosaminyltransferase III in epithelial-to-mesenchymal transition induced by transforming growth factor β1 (TGF-β1) in epithelial cell lines

The epithelial-to-mesenchymal transition (EMT) plays crucial roles in embryonic development, wound healing, tissue repair, and cancer progression. Results of this study show how transforming growth factor β1 (TGF-β1) down-regulates expression of N-acetylglucosaminyltransferase III (GnT-III) during EMT-like changes. Treatment with TGF-β1 resulted in a decrease in E-cadherin expression and GnT-III expression, as well as its product, the bisected N-glycans, which was confirmed by erythro-agglutinating phytohemagglutinin lectin blot and HPLC analysis in human MCF-10A and mouse GE11 cells. In contrast with GnT-III, the expression of N-acetylglucosaminyltransferase V was slightly enhanced by TGF-β1 treatment. Changes in the N-glycan patterns on α3β1 integrin, one of the target proteins for GnT-III, were also confirmed by lectin blot analysis. To understand the roles of GnT-III expression in EMT-like changes, the MCF-10A cell was stably transfected with GnT-III. It is of particular interest that overexpression of GnT-III influenced EMT-like changes induced by TGF-β1, which was confirmed by cell morphological changes of phase contrast, immunochemical staining patterns of E-cadherin, and actin. In addition, GnT-III modified E-cadherin, which served to prolong E-cadherin turnover on the cell surface examined by biotinylation and pulse-chase experiments. GnT-III expression consistently inhibited β-catenin translocation from cell-cell contact into the cytoplasm and nucleus. Furthermore, the transwell assay showed that GnT-III expression suppressed TGF-β1-induced cell motility. Taken together, these observations are the first to clearly demonstrate that GnT-III affects cell properties, which in turn influence EMT-like changes, and to explain a molecular mechanism for the inhibitory effects of GnT-III on cancer metastasis.

The mechanical memory of lung myofibroblasts

Fibroblasts differentiate into the highly synthetic and contractile myofibroblast phenotype when exposed to substrates with an elastic modulus corresponding to pathologically stiff fibrotic tissue. Cellular responses to changes in substrate stiffness are typically analyzed after hours or days, which does not enable the monitoring of myofibroblast persistence, a hallmark of fibrosis. To determine long-lasting effects on the fibrotic behavior of lung fibroblasts, we followed a novel approach of explanting and repeatedly passaging fibroblasts on silicone substrates with stiffness representing various states of lung health. Fibrotic activity was determined by assaying for myofibroblast proliferation, cell contractility, expression of α-smooth muscle actin, extracellular matrix and active TGFβ1. As predicted, myofibroblast activity was low on healthy soft substrates and increased with increasing substrate stiffness. However, explanting and mechanically priming lung fibroblasts for 3 weeks on pathologically stiff substrates resulted in sustained myofibroblast activity even after the cells were returned to healthy soft cultures for 2 weeks. Such primed cells retained higher fibrotic activity than cells that had been exclusively cultured on soft substrates, and were not statistically different from cells continuously passaged on stiff surfaces. Inversely, priming lung fibroblasts for 3 weeks on soft substrates partially protected from myofibroblast activation after the shift to stiff substrates. Hence, mechano-sensed information relating to physical conditions of the local cellular environment could permanently induce fibrotic behavior of lung fibroblasts. This priming effect has important implications for the progression and persistence of aggressive fibrotic diseases such as idiopathic pulmonary fibrosis.

Integrin α9β1 promotes malignant tumor growth and metastasis by potentiating epithelial-mesenchymal transition

The integrin α9β1 binds a number of extracellular matrix components to mediate cell adhesion, migration and tissue invasion. Although expressed in a variety of normal human cells including endothelium, it is also expressed in cancer cells. We have previously shown that α9β1 binds VEGF-A to facilitate angiogenesis, an important component of the tumor microenvironment. As α9β1 induces accelerated cancer cell migration, we wished to determine what role it played in cancer growth and metastasis. In this study, we show that α9β1 expression induces molecular changes consistent with epithelial-mesenchymal transition. In addition, we found that α9β1 forms a tri-partite protein complex with β-catenin and E-cadherin, which dissociates following integrin activation and subsequent src and β-catenin phosphorylation. These findings were consistent in cells in which: α9β1 was exogenously over-expressed, or when its expression was suppressed in cancer cells endogenously expressing α9β1. These in vitro results are biologically significant as α9β1-expressing cancer cells induce greater tumor growth and metastases in mice as compared to the cells without α9β1 expression or when integrin expression is suppressed. Furthermore, integrin α9β1 is expressed in primary human small cell lung cancer and patients having a high expression of α9β1 demonstrated significantly worse long-term survival compared with patients with low α9β1 expression. These findings highlight a novel mechanism of integrin α9β1 function in human cancer.

Abl regulates planar polarized junctional dynamics through β-catenin tyrosine phosphorylation

Interactions between epithelial cells are mediated by adherens junctions that are dynamically regulated during development. Here we show that the turnover of β-catenin is increased at cell interfaces that are targeted for disassembly during Drosophila axis elongation. The Abl tyrosine kinase is concentrated at specific planar junctions and is necessary for polarized β-catenin localization and dynamics. abl mutant embryos have decreased β-catenin turnover at shrinking edges, and these defects are accompanied by a reduction in multicellular rosette formation and axis elongation. Abl promotes β-catenin phosphorylation on the conserved tyrosine 667 and expression of an unphosphorylatable β-catenin mutant recapitulates the defects of abl mutants. Notably, a phosphomimetic β-catenin(Y667E) mutation is sufficient to increase β-catenin turnover and rescue axis elongation in abl deficient embryos. These results demonstrate that the asymmetrically localized Abl tyrosine kinase directs planar polarized junctional remodeling during Drosophila axis elongation through the tyrosine phosphorylation of β-catenin.