Integrity of cell-cell contacts is a critical regulator of TGF-beta 1-induced epithelial-to-myofibroblast transition: role for beta-catenin

Activation of Smad-mediated TGF-β signaling triggers epithelial-mesenchymal transitions in murine cloned corneal progenitor cells

Epithelial-mesenchymal transition (EMT), via activation of Wnt signaling, is prevailing in embryogenesis, but postnatally it only occurs in pathological processes, such as in tissue fibrosis and tumor metastasis. Our prior studies led us to speculate that EMT might be involved in the loss of limbal epithelial stem cells in explant cultures. To examine this hypothesis, we successfully grew murine corneal/limbal epithelial progenitors by prolonging the culture time and by seeding at a low density in a serum-free medium. Single cell-derived clonal growth was accompanied by a gradient of Wnt signaling activity, from the center to the periphery, marked by a centrifugal loss of E-cadherin and β-catenin from intercellular junctions, coupled with nuclear translocation of β-catenin and LEF-1. Large-colony-forming efficiency at central location of colony was higher than peripheral location. Importantly, there was also progressive centrifugal differentiation, with positive K14 keratin expression and the loss of p63 and PCNA nuclear staining, and irreversible EMT, evidenced by cytoplasmic expression of α-SMA and nuclear localization of S100A4; and by nuclear translocation of Smad4. Furthermore, cytoplasmic expression of α-SMA was promoted by high-density cultures and their conditioned media, which contained cell density-dependent levels of TGF-β1, TGF-β2, GM-CSF, and IL-1α. Exogenous TGF-β1 induced α-SMA positive cells in a low-density culture, while TGF-β1 neutralizing antibody partially inhibited α-SMA expression in a high-density culture. Collectively, these results indicate that irreversible EMT emerges in the periphery of clonal expansion where differentiation and senescence of murine corneal/limbal epithelial progenitors occurs as a result of Smad-mediated TGF-β-signaling.

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.

How does TGF-β mediate tubulointerstitial fibrosis?

Tubulointerstitial fibrosis mediates the development of end-stage renal disease from renal injuries of all etiologies and is considered an important predictor of renal survival. Transforming growth factor-β (TGF-β) is one of the most important growth factors that promotes tubulointerstitial fibrosis, but the mechanisms whereby this occurs are not well defined. This is because TGF-β has pleiotropic effects that depend on the target cell type. This review discusses how TGF-β signaling in each of the relevant cell types (eg, tubular epithelium, fibroblasts) may contribute to tubulointerstitial fibrosis progression and suggests ways in which future research can improve our understanding of TGF-β-mediated tubulointerstitial fibrosis.

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.

Nicotine-induced epithelial-mesenchymal transition via Wnt/β-catenin signaling in human airway epithelial cells

Epithelial-mesenchymal transition (EMT) has been proposed to be a mechanism in airway remodeling, which is a characteristic of chronic obstructive pulmonary disease (COPD). Studies have shown that cigarette smoke and nicotine are factors that induce Wnt/β-catenin activation, which is a pathway that has also been implicated in EMT. The main aim of this study was to test whether human bronchial epithelial cells are able to undergo EMT in vitro following nicotine stimulation via the Wnt3a/β-catenin signaling pathway. We show that nicotine activates the Wnt3a signal pathway, which leads to the translocation of β-catenin into the nucleus and activation of β-catenin/Tcf-dependent transcription in the human bronchial epithelial cell (HBEC) line. This accumulation was accompanied by an increase in smooth muscle actin, vimentin, matrix metalloproteinases-9, and type I collagen expression as well as downregulation of E-cadherin, which are typical characteristics of EMT. We also noted that the release of TGF-β(1) from these cells was stimulated by nicotine. Knockdown of Wnt3a with small interfering RNA (siRNA) prevented these effects, implying that β-catenin activation in these responses is Wnt3a dependent. Furthermore, specific knockdown of TGF-β(1) with TGF-β(1) siRNA partially prevented nicotine-induced EMT, suggesting that TGF-β(1) has a role in nicotine-mediated EMT in HBECs. These results suggest that HBECs are able to undergo EMT in vitro upon nicotine stimulation via the Wnt3a/β-catenin signaling pathway.

Hyperosmotic stress regulates the distribution and stability of myocardin-related transcription factor, a key modulator of the cytoskeleton

Hyperosmotic stress initiates several adaptive responses, including the remodeling of the cytoskeleton. Besides maintaining structural integrity, the cytoskeleton has emerged as an important regulator of gene transcription. Myocardin-related transcription factor (MRTF), an actin-regulated coactivator of serum response factor, is a major link between the actin skeleton and transcriptional control. We therefore investigated whether MRTF is regulated by hyperosmotic stress. Here we show that hypertonicity induces robust, rapid, and transient translocation of MRTF from the cytosol to the nucleus in kidney tubular cells. We found that the hyperosmolarity-triggered MRTF translocation is mediated by the RhoA/Rho kinase (ROK) pathway. Moreover, the Rho guanine nucleotide exchange factor GEF-H1 is activated by hyperosmotic stress, and it is a key contributor to the ensuing RhoA activation and MRTF translocation, since siRNA-mediated GEF-H1 downregulation suppresses these responses. While the osmotically induced RhoA activation promotes nuclear MRTF accumulation, the concomitant activation of p38 MAP kinase mitigates this effect. Moderate hyperosmotic stress (600 mosM) drives MRTF-dependent transcription through the cis-element CArG box. Silencing or pharmacological inhibition of MRTF prevents the osmotic stimulation of CArG-dependent transcription and renders the cells susceptible to osmotic shock-induced structural damage. Interestingly, strong hyperosmolarity promotes proteasomal degradation of MRTF, concomitant with apoptosis. Thus, MRTF is an osmosensitive and osmoprotective transcription factor, whose intracellular distribution is regulated by the GEF-H1/RhoA/ROK and p38 pathways. However, strong osmotic stress destabilizes MRTF, concomitant with apoptosis, implying that hyperosmotically induced cell death takes precedence over epithelial-myofibroblast transition, a potential consequence of MRTF-mediated phenotypic reprogramming.

The actin-MRTF-SRF gene regulatory axis and myofibroblast differentiation

Cardiac fibroblasts are responsible for necrotic tissue replacement and scar formation after myocardial infarction (MI) and contribute to remodeling in response to pathological stimuli. This response to insult or injury is largely due to the phenotypic plasticity of fibroblasts. When fibroblasts encounter environmental disturbances, whether biomechanical or humoral, they often transform into smooth muscle-like, contractile cells called "myofibroblasts." The signals that control myofibroblast differentiation include the transforming growth factor (TGF)-β1-Smad pathway and Rho GTPase-dependent actin polymerization. Recent evidence implicates serum response factor (SRF) and the myocardin-related transcription factors (MRTFs) as key mediators of the contractile gene program in response to TGF-β1 or RhoA signaling. This review highlights the function of myofibroblasts in cardiac remodeling and the role of the actin-MRTF-SRF signaling axis in regulating this process.

Wnt/β-Catenin Signaling: a Promising New Target for Fibrosis Diseases

Wnt/β-catenin signaling is involved in virtually every aspect of embryonic development and also controls homeostatic self-renewal in a number of adult tissues. Recently, emerging evidence from researches of organ fibrosis suggest that sustained Wnt/β-catenin pathway reactivation is linked to the pathogenesis of fibrotic disorders. Here we focus on Wnt/β-catenin-related pathogenic effects in different organs, such as lung fibrosis, liver fibrosis, skin fibrosis and renal fibrosis. Additionally, Wnt/β-catenin signaling works in a combinatorial manner with TGF-β signaling in the process of fibrosis, and TGF-β signaling can induce expression of Wnt/β-catenin superfamily members and vice versa. Moreover, network analysis, based on pathway databases, revealed that key factors in the Wnt pathway were targeted by some differentially expressed microRNAs detected in fibrosis diseases. These findings demonstrated the crosstalks between Wnt/β-catenin pathway and TGF-β signalings, and microRNAs, highlighting the role of Wnts in organ fibrogenesis. Most importantly, nowadays there is a variety of Wnt pathway inhibitors which give us the potential therapeutic feasibility, modulation of the Wnt pathway may, therefore, present as a suitable and promising therapeutic strategy in the future.

Wnt3a/β-catenin increases proliferation in heart valve interstitial cells

BACKGROUND

Valve interstitial cells (VICs), the most prevalent cells in the heart valve, mediate normal valve function and repair in valve injury and disease. The Wnt3a/β-catenin pathway, important for proliferation and endothelial-to-mesenchymal transition in endocardial cushion formation in valve development, is up-regulated in adult valves with calcific aortic stenosis. Therefore, we tested the hypothesis that Wnt3a/β-catenin signaling regulates proliferation in adult VICs.

METHODS

Porcine VICs were treated with 150 ng/ml of exogenous Wnt3a. To measure proliferation, cells were counted on day 4 posttreatment and stained for bromodeoxyuridine (BrdU) at 24 h posttreatment. β-Catenin small interfering RNA (siRNA) was used to knock down β-catenin expression. Apoptosis was measured with terminal deoxynucleotidyl transferase dUTP nick end labeling assay. To assess changes in β-catenin, cells were stained for β-catenin at days 1, 3, 6, and 9 posttreatment. Western blot for β-catenin was performed on whole cell, cytoplasmic, and nuclear extracts at day 4 posttreatment. To measure β-catenin-mediated transcription, TOPFLASH/FOPFLASH reporter assay was performed at 24 h posttreatment.

RESULTS

Wnt3a produced a significant increase in cell number at day 4 posttreatment and in the percentage of BrdU-positive nuclei at 24 h posttreatment. The increase in proliferation was abolished by β-catenin siRNA. Apoptosis was minimal in all conditions. Wnt3a produced progressively greater β-catenin staining as treatment length increased from 1 to 9 days. Wnt3a produced a significant increase in β-catenin protein in both whole cell and nuclear lysates after 4 days of treatment. Wnt3a significantly increased TOPFLASH/FOPFLASH reporter activity after 24 h of treatment.

CONCLUSION

Wnt3a/β-catenin signaling pathway is an important regulator of proliferation in adult VICs.

TDAG51 mediates epithelial-to-mesenchymal transition in human proximal tubular epithelium

Epithelial-to-mesenchymal transition (EMT) contributes to renal fibrosis in chronic kidney disease. Endoplasmic reticulum (ER) stress, a feature of many forms of kidney disease, results from the accumulation of misfolded proteins in the ER and leads to the unfolded protein response (UPR). We hypothesized that ER stress mediates EMT in human renal proximal tubules. ER stress is induced by a variety of stressors differing in their mechanism of action, including tunicamycin, thapsigargin, and the calcineurin inhibitor cyclosporine A. These ER stressors increased the UPR markers GRP78, GRP94, and phospho-eIF2α in human proximal tubular cells. Thapsigargin and cyclosporine A also increased cytosolic Ca2+ concentration and T cell death-associated gene 51 (TDAG51) expression, whereas tunicamycin did not. Thapsigargin was also shown to increase levels of active transforming growth factor (TGF)-β1 in the media of cultured human proximal tubular cells. Thapsigargin induced cytoskeletal rearrangement, β-catenin nuclear translocation, and α-smooth muscle actin and vinculin expression in proximal tubular cells, indicating an EMT response. Subconfluent primary human proximal tubular cells were induced to undergo EMT by TGF-β1 treatment. In contrast, tunicamycin treatment did not produce an EMT response. Plasmid-mediated overexpression of TDAG51 resulted in cell shape change and β-catenin nuclear translocation. These results allowed us to develop a two-hit model of ER stress-induced EMT, where Ca2+ dysregulation-mediated TDAG51 upregulation primes the cell for mesenchymal transformation via Wnt signaling and then TGF-β1 activation leads to a complete EMT response. Thus the release of Ca2+ from ER stores mediates EMT in human proximal tubular epithelium via the induction of TDAG51.

Epithelial-mesenchymal transition in the pathophysiology of airway remodelling in asthma

PURPOSE OF REVIEW

We currently understand little about the mechanisms that lead to airway remodeling in asthma. The origin of the mesenchymal cells that contribute to fibrosis of the airway is poorly understood. However, emerging evidence suggests that the airway epithelium could contribute to airway remodeling through the process of epithelial-mesenchymal transition (EMT) following environmental challenge. In this review, we will discuss the mechanistic features of EMT and highlight recent descriptions of EMT in the airway to further define the role of the airway epithelium in the pathogenesis of asthma.

RECENT FINDINGS

Growth factors, inflammatory mediators, and matricellular proteins expressed following exposure to environmental insults are known to induce downregulation of epithelial cell-cell adhesions and promote mesenchymal gene expression programs both in vitro and in vivo. These results demonstrate that the plastic and dynamic airway epithelium may contribute to airway remodeling via EMT in asthma.

SUMMARY

It is becoming increasingly clear that the airway epithelium orchestrates inflammatory and remodeling responses of the airway. Understanding the regulatory mechanisms involved in epithelial plasticity will be crucial to determine effective therapies to halt the progression of airway remodeling in asthma.

Collagen VIII influences epithelial phenotypic changes in experimental diabetic nephropathy

Epithelial-to-mesenchymal transition (EMT) is an important mechanism of renal tubulo-interstitial fibrosis in diabetic nephropathy (DN). Inducers of EMT, among others, are transforming growth factor-β(1) (TGF-β(1)) as well as extracellular collagens. In renal cells of diabetic mice and in kidneys of patients with DN, the expression of collagen VIII (gene: Col8α1/α2) is enhanced and characteristic features of DN in streptozotocin (STZ)-induced diabetic Col8α1/α2 knockout-(KO) mice are attenuated compared with diabetic wild-type mice. This study aimed to investigate whether collagen type VIII may influence the induction of EMT. DN was induced in wild-type and Col8α1/α2-KO mice using the established and widely accepted low-dose STZ model [treatment for 5 consecutive days (50 mg/kg)]. Healthy and diabetic mice were analyzed for changes in renal function and the expression of EMT-related genes and proteins. Renal morphology, fibrosis, and various EMT markers were studied in kidneys using immunohistological and molecular biological methods. Knockout of Col8α1/α2 attenuated albuminuria, extracellular matrix production, as well as fibrosis. Furthermore, the kidneys of diabetic Col8α1/α2-KO mice showed a marked reduction in interstitial myofibroblasts, and in tubular cells the inhibition of the expression of epithelial markers as well as the expression of typical mesenchymal markers was reduced. The present study demonstrates that in contrast to diabetic wild-type mice EMT-like changes were attenuated in diabetic Col8α1/α2-KO mice, which indicates that either collagen VIII may be one of the major inducers of EMT-like changes in kidneys of diabetic wild-type mice or/possibly the lack of Col8α1/α2 disrupts TGF-β(1)-induced EMT-like changes.

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.

Selective activation of p120ctn-Kaiso signaling to unlock contact inhibition of ARPE-19 cells without epithelial-mesenchymal transition

Contact-inhibition ubiquitously exists in non-transformed cells and explains the poor regenerative capacity of in vivo human retinal pigment epithelial cells (RPE) during aging, injury and diseases. RPE injury or degeneration may unlock mitotic block mediated by contact inhibition but may also promote epithelial-mesenchymal transition (EMT) contributing to retinal blindness. Herein, we confirmed that EMT ensued in post-confluent ARPE-19 cells when contact inhibition was disrupted with EGTA followed by addition of EGF and FGF-2 because of activation of canonical Wnt and Smad/ZEB signaling. In contrast, knockdown of p120-catenin (p120) unlocked such mitotic block by activating p120/Kaiso, but not activating canonical Wnt and Smad/ZEB signaling, thus avoiding EMT. Nuclear BrdU labeling was correlated with nuclear release of Kaiso through p120 nuclear translocation, which was associated with activation of RhoA-ROCK signaling, destabilization of microtubules. Prolonged p120 siRNA knockdown followed by withdrawal further expanded RPE into more compact monolayers with a normal phenotype and a higher density. This new strategy based on selective activation of p120/Kaiso but not Wnt/β-catenin signaling obviates the need of using single cells and the risk of EMT, and may be deployed to engineer surgical grafts containing RPE and other tissues.

Activation of Wnt11 by transforming growth factor-β drives mesenchymal gene expression through non-canonical Wnt protein signaling in renal epithelial cells

Transforming growth factor β1 (TGF-β) promotes renal interstitial fibrosis in vivo and the expression of mesenchymal genes in vitro; however, most of its direct targets in epithelial cells are still elusive. In a screen for genes directly activated by TGF-β, we found that components of the Wnt signaling pathway, especially Wnt11, were targets of activation by TGF-β and Smad3 in primary renal epithelial cells. In gain and loss of function experiments, Wnt11 mediated the actions of TGF-β through enhanced activation of mesenchymal marker genes, such as Zeb1, Snail1, Pai1, and αSMA, without affecting Smad3 phosphorylation. Inhibition of Wnt11 by receptor knockdown or treatment with Wnt inhibitors limited the effects of TGF-β on gene expression. We found no evidence that Wnt11 activated the canonical Wnt signaling pathway in renal epithelial cells; rather, the function of Wnt11 was mediated by the c-Jun N-terminal kinase (JNK) pathway. Consistent with the in vitro results, all the TGF-β, Wnt11, and JNK targets were activated in a unilateral ureteral obstruction (UUO) model of renal fibrosis in vivo. Our findings demonstrated cooperativity among the TGF-β, Wnt11, and JNK signaling pathways and suggest new targets for anti-fibrotic therapy in renal tissue.

Wnt signaling induces epithelial-mesenchymal transition with proliferation in ARPE-19 cells upon loss of contact inhibition

Proliferation and epithelial-mesenchymal transition (EMT) of the retinal pigment epithelium (RPE) are hallmarks of proliferative vitreoretinopathy. This study aims at clarifying the role of growth factors, such as epidermal growth factor (EGF), fibroblast growth factor-2 (FGF-2), and transforming growth factor-β1 (TGF-β1), in controlling how RPE proliferates while undergoing EMT. When contact inhibition of post-confluent ARPE-19 cells was disrupted by EGTA, an increase of BrdU labeling was noted only in the presence of EGF and/or FGF-2, and was accompanied by EMT as evidenced by the loss of a normal RPE phenotype (altered cytolocalization of RPE65, N-cadherin, ZO-1, and Na,K-ATPase) and the gain of a mesenchymal phenotype (increased expression of vimentin, S100A4, and α-smooth muscle actin). EMT with proliferation by EGTA+EGF+FGF-2 was accompanied by activation of canonical Wnt signaling (judged by the TCF/LEF promoter activity, increased nuclear levels of and interaction between β-catenin and LEF1 proteins, and the replication by overexpression of β-catenin), abolished by concomitant addition of XAV939, a Wnt inhibitor, but not associated with suppression of Hippo signaling (negative expression of nuclear TAZ or YAP and cytoplasmic p-TAZ or p-YAP). The causative role of Wnt signaling on EMT with proliferation was confirmed by overexpression of stable S33Y β-catenin with EGTA treatment. In addition, contact inhibition disrupted by EGTA in the presence of TGF-β1 also led to EMT, but suppressed proliferation and Wnt signaling. The Wnt signaling triggered by EGF+FGF-2 was sufficient and synergized with TGF-β1 in activating the Smad/ZEB1/2 signaling responsible for EMT. These findings establish a framework for further dissecting how RPE might partake in a number of proliferative vitreoretinopathies characterized by EMT.

Nuclear p120 catenin unlocks mitotic block of contact-inhibited human corneal endothelial monolayers without disrupting adherent junctions

Contact inhibition ubiquitously exists in non-transformed cells that are in contact with neighboring cells. This phenomenon explains the poor regenerative capacity of in vivo human corneal endothelial cells during aging, injury and surgery. This study demonstrated that the conventional approach of expanding human corneal endothelial cells by disrupting contact inhibition with EDTA followed by bFGF activated canonical Wnt signaling and lost the normal phenotype to endothelial-mesenchymal transition, especially if TGFβ1 was added. By contrast, siRNA against p120 catenin (CTNND1) also uniquely promoted proliferation of the endothelial cells by activating trafficking of p120 catenin to the nucleus, thus relieving repression by nuclear Kaiso. This nuclear p120-catenin-Kaiso signaling is associated with activation of RhoA-ROCK signaling, destabilization of microtubules and inhibition of Hippo signaling, but not with activation of Wnt-β-catenin signaling. Consequently, proliferating human corneal endothelial cells maintained a hexagonal shape, with junctional expression of N-cadherin, ZO-1 and Na(+)/K(+)-ATPase. Further expansion of human corneal endothelial monolayers with a normal phenotype and a higher density was possible by prolonging treatment with p120 catenin siRNA followed by its withdrawal. This new strategy of perturbing contact inhibition by selective activation of p120-catenin-Kaiso signaling without disrupting adherent junction could be used to engineer surgical grafts containing normal human corneal endothelial cells to meet a global corneal shortage and for endothelial keratoplasties.

Increase of α-SMA(+) and CK (+) cells as an early sign of epithelial-mesenchymal transition during colorectal carcinogenesis

Our aim was to examine cell transition events by detecting the frequency of intrapithelial α-smooth muscle actin (SMA)(+)/cytokeratin (CK)(+) cells during colorectal adenoma-carcinoma sequence, in relation to E-cadherin expression. Our further aim was to determine the proliferative activity of intraepithelial α-SMA(+) cells. Histologically healthy, adenoma, and colorectal cancer (CRC) biopsy samples were taken during routine colonoscopy and were included into tissue microarrays (TMAs). Slides immunostained for Ki-67, α-SMA, E-cadherin and pan-cytokeratin were digitalized and analyzed by using a digital microscope software. The proportion of α-SMA(+)/CK(+) cells was significantly higher in CRC samples (3.34 ± 1.01%) compared to healthy (1.94 ± 0.69%) or adenoma (1.62 ± 0.78%) samples (p < 0.01). E-cadherin expression negatively correlated with the number of α-SMA(+) cells. The majority of intraepithelial α-SMA(+) cells were in the proliferative phase. During tumor progression, the appearance of dot-like α-SMA staining in CK positive cells may indicate the initial phase of the epithelial-to-mesenchymal transition (EMT). The high proportion of intraepithelial α-SMA(+) proliferating cells may refer to their increased plasticity compared to differentiated cells. The negative correlation between E-cadherin and intraepithelial α-SMA expression suggests that EMT is facilitated by a loss of epithelial cell contact.

Troglitazone induces extracellular matrix and cytoskeleton remodeling in mouse collecting duct cells

Peroxisome proliferator-activated receptor (PPARγ) has been shown to have a protective role in the nephron through its ability to inhibit a transforming growth factor- (TGF-β) mediated fibrotic response. In contrast, PPARγ was also shown to induce a mesenchymal transformation in epithelial intestinal cells. A fibrotic response in the collecting duct has only recently been established; however, the entire collecting duct has not been fully examined. Inner medullary collecting duct cells (IMCD-K2) and mouse cortical collecting duct cells (M1), representing the cortical and medullary collecting duct, were exposed to 5–10 μM troglitazone for 24 hours. Troglitazone resulted in an elongated morphology, 60% decreases in E-cadherin and β-catenin, a 35% decrease in α-catenin, and a 1.5-fold increase in fibronectin. These effects were not reversed with PPARγ antagonists or affected with PPARγ overexpression. Our results indicate that troglitazone induced a mesenchymal-like transformation in M1 and IMCD-K2 epithelial cells independently of PPARγ.

TGFβ modulates cell-to-cell communication in early epithelial-to-mesenchymal transition

AIMS/HYPOTHESIS

A key pathology in diabetic nephropathy is tubulointerstitial fibrosis. The condition is characterised by increased deposition of the extracellular matrix, fibrotic scar formation and declining renal function, with the prosclerotic cytokine TGF-β1 mediating many of these catastrophic changes. Here we investigated whether TGF-β1-induced epithelial-to-mesenchymal transition (EMT) plays a role in alterations in cell adhesion, cell coupling and cell communication in the human renal proximal tubule.

METHODS

Whole-cell and cell compartment abundance of E-cadherin, N-cadherin, snail, vimentin, β-catenin and connexin-43 was determined in human kidney cell line (HK)2 and human proximal tubule cells with or without TGF-β1, using western blotting and immunocytochemistry, followed by quantification by densitometry. The contribution of connexin-43 in proximal tubule cell communication was quantified using small interfering RNA knockdown, while dye-transfer was used to assess gap junctional intercellular communication (GJIC). Functional tethering was assessed by single-cell force spectroscopy with or without TGF-β1, or by immunoneutralisation of cadherin ligation.

RESULTS

High glucose (25 mmol/l) increased the secretion of TGF-β1 from HK2 cells. Analysis confirmed early TGF-β1-induced morphological and phenotypical changes of EMT, with altered levels of adhesion and adherens junction proteins. These changes correlated with impaired cell adhesion and decreased tethering between coupled cells. Impaired E-cadherin-mediated adhesion reduced connexin-43 production and GJIC, these effects being mimicked by neutralisation of E-cadherin ligation. Upregulation of N-cadherin failed to restore adhesion or connexin-43-mediated GJIC.

CONCLUSIONS/INTERPRETATION

We provide compelling evidence that TGF-β1-induced EMT instigates a loss of E-cadherin, cell adhesion and ultimately of connexin-mediated cell communication in the proximal tubule under diabetic conditions; these changes occur ahead of overt signs of renal damage.

Interactions between β-catenin and transforming growth factor-β signaling pathways mediate epithelial-mesenchymal transition and are dependent on the transcriptional co-activator cAMP-response element-binding protein (CREB)-binding protein (CBP)

Interactions between transforming growth factor-β (TGF-β) and Wnt are crucial to many biological processes, although specific targets, rationale for divergent outcomes (differentiation versus block of epithelial proliferation versus epithelial-mesenchymal transition (EMT)) and precise mechanisms in many cases remain unknown. We investigated β-catenin-dependent and transforming growth factor-β1 (TGF-β1) interactions in pulmonary alveolar epithelial cells (AEC) in the context of EMT and pulmonary fibrosis. We previously demonstrated that ICG-001, a small molecule specific inhibitor of the β-catenin/CBP (but not β-catenin/p300) interaction, ameliorates and reverses pulmonary fibrosis and inhibits TGF-β1-mediated α-smooth muscle actin (α-SMA) and collagen induction in AEC. We now demonstrate that TGF-β1 induces LEF/TCF TOPFLASH reporter activation and nuclear β-catenin accumulation, while LiCl augments TGF-β-induced α-SMA expression, further confirming co-operation between β-catenin- and TGF-β-dependent signaling pathways. Inhibition and knockdown of Smad3, knockdown of β-catenin and overexpression of ICAT abrogated effects of TGF-β1 on α-SMA transcription/expression, indicating a requirement for β-catenin in these Smad3-dependent effects. Following TGF-β treatment, co-immunoprecipitation demonstrated direct interaction between endogenous Smad3 and β-catenin, while chromatin immunoprecipitation (ChIP)-re-ChIP identified spatial and temporal regulation of α-SMA via complex formation among Smad3, β-catenin, and CBP. ICG-001 inhibited α-SMA expression/transcription in response to TGF-β as well as α-SMA promoter occupancy by β-catenin and CBP, demonstrating a previously unknown requisite TGF-β1/β-catenin/CBP-mediated pro-EMT signaling pathway. Clinical relevance was shown by β-catenin/Smad3 co-localization and CBP expression in AEC of IPF patients. These findings suggest a new therapeutic approach to pulmonary fibrosis by specifically uncoupling CBP/catenin-dependent signaling downstream of TGF-β.

Ligand-independent transforming growth factor-β type I receptor signalling mediates type I collagen-induced epithelial-mesenchymal transition

Evidence suggests epithelial-mesenchymal transition (EMT) as one potential source of fibroblasts in idiopathic pulmonary fibrosis. To assess the contribution of alveolar epithelial cell (AEC) EMT to fibroblast accumulation in vivo following lung injury and the influence of extracellular matrix on AEC phenotype in vitro, Nkx2.1-Cre;mT/mG mice were generated in which AECs permanently express green fluorescent protein (GFP). On days 17-21 following intratracheal bleomycin administration, ~4% of GFP-positive epithelial-derived cells expressed vimentin or α-smooth muscle actin (α-SMA). Primary AECs from Nkx2.1-Cre;mT/mG mice cultured on laminin-5 or fibronectin maintained an epithelial phenotype. In contrast, on type I collagen, cells of epithelial origin displayed nuclear localization of Smad3, acquired spindle-shaped morphology, expressed α-SMA and phospho-Smad3, consistent with activation of the transforming growth factor-β (TGFβ) signalling pathway and EMT. α-SMA induction and Smad3 nuclear localization were blocked by the TGFβ type I receptor (TβRI, otherwise known as Alk5) inhibitor SB431542, while AEC derived from Nkx2.1-Cre;Alk5(flox/KO) mice did not undergo EMT on collagen, consistent with a requirement for signalling via Alk5 in collagen-induced EMT. Inability of a pan-specific TGFβ neutralizing antibody to inhibit effects of collagen together with absence of active TGFβ in culture supernatants is consistent with TGFβ ligand-independent activation of Smad signalling. These results support the notion that AECs can acquire a mesenchymal phenotype following injury in vivo and implicate type I collagen as a key regulator of EMT in AECs through signalling via Alk5, likely in a TGFβ ligand-independent manner.

Explanting is an ex vivo model of renal epithelial-mesenchymal transition

Recognised by their de novo expression of alpha-smooth muscle actin (SMA), recruitment of myofibroblasts is key to the pathogenesis of fibrosis in chronic kidney disease. Increasingly, we realise that epithelial-mesenchymal transition (EMT) may be an important source of these cells. In this study we describe a novel model of renal EMT. Rat kidney explants were finely diced on gelatin-coated Petri dishes and cultured in serum-supplemented media. Morphology and immunocytochemistry were used to identify mesenchymal (vimentin+, α-smooth muscle actin (SMA)+, desmin+), epithelial (cytokeratin+), and endothelial (RECA+) cells at various time points. Cell outgrowths were all epithelial in origin (cytokeratin+) at day 3. By day 10, 50 ± 12% (mean ± SE) of cytokeratin+ cells double-labelled for SMA, indicating EMT. Lectin staining established a proximal tubule origin. By day 17, cultures consisted only of myofibroblasts (SMA+/cytokeratin−). Explanting is a reproducible ex vivo model of EMT. The ability to modify this change in phenotype provides a useful tool to study the regulation and mechanisms of renal tubulointerstitial fibrosis.

Regulation of transforming growth factor-β1-driven lung fibrosis by galectin-3

RATIONALE

Idiopathic pulmonary fibrosis (IPF) is a chronic dysregulated response to alveolar epithelial injury with differentiation of epithelial cells and fibroblasts into matrix-secreting myofibroblasts resulting in lung scaring. The prognosis is poor and there are no effective therapies or reliable biomarkers. Galectin-3 is a β-galactoside binding lectin that is highly expressed in fibrotic tissue of diverse etiologies.

OBJECTIVES

To examine the role of galectin-3 in pulmonary fibrosis.

METHODS

We used genetic deletion and pharmacologic inhibition in well-characterized murine models of lung fibrosis. Further mechanistic studies were performed in vitro and on samples from patients with IPF.

MEASUREMENTS AND MAIN RESULTS

Transforming growth factor (TGF)-β and bleomycin-induced lung fibrosis was dramatically reduced in mice deficient in galectin-3, manifest by reduced TGF-β1-induced EMT and myofibroblast activation and collagen production. Galectin-3 reduced phosphorylation and nuclear translocation of β-catenin but had no effect on Smad2/3 phosphorylation. A novel inhibitor of galectin-3, TD139, blocked TGF-β-induced β-catenin activation in vitro and in vivo and attenuated the late-stage progression of lung fibrosis after bleomycin. There was increased expression of galectin-3 in the bronchoalveolar lavage fluid and serum from patients with stable IPF compared with nonspecific interstitial pneumonitis and controls, which rose sharply during an acute exacerbation suggesting that galectin-3 may be a marker of active fibrosis in IPF and that strategies that block galectin-3 may be effective in treating acute fibrotic exacerbations of IPF.

CONCLUSIONS

This study identifies galectin-3 as an important regulator of lung fibrosis and provides a proof of principle for galectin-3 inhibition as a potential novel therapeutic strategy for IPF.

Crosstalk between TGF-beta and MAPK signaling during corneal wound healing

PURPOSE

The aim of this study was to elucidate the mechanisms governing epithelial cell migration and proliferation during wound healing.

METHODS

The authors used wound healing of mouse corneal epithelium to examine the role TGF-β signaling plays during the healing process. To achieve this goal, they used transgenic mice in which the TGF-β receptor type II (Tbr2) was conditionally ablated from the corneal epithelium. Epithelium debridement wounds were made, followed by the assessment of cell migration, proliferation, and immunostaining of various signaling pathway components.

RESULTS

The authors showed that in the absence of TGF-β signaling corneal epithelial wound healing is delayed by 48 hours; this corresponds to a delay in p38MAPK activation. Despite the delayed p38MAPK activation, ATF2, a substrate of p38MAPK, is still phosphorylated, leading to the suppression of cell proliferation at the leading edge of the wound. These data provide evidence that in the absence of TGF-β signaling, the suppression of cell proliferation during the early stages of wound healing is maintained through the JNK activation of ATF2. CONCLUSIONS; Together the data presented here demonstrate the importance of the TGF-β and MAPK signaling pathways in corneal epithelial wound healing.

E-cadherin/β-catenin complex and the epithelial barrier

E-Cadherin/β-catenin complex plays an important role in maintaining epithelial integrity and disrupting this complex affect not only the adhesive repertoire of a cell, but also the Wnt-signaling pathway. Aberrant expression of the complex is associated with a wide variety of human malignancies and disorders of fibrosis resulting from epithelial-mesenchymal transition. These associations provide insights into the complexity that is likely responsible for the fibrosis/tumor suppressive action of E-cadherin/β-catenin.

β-catenin and Smad3 regulate the activity and stability of myocardin-related transcription factor during epithelial-myofibroblast transition

Two novel mechanisms are shown by which injury of intercellular junctions via β-catenin promotes epithelial–myofibroblast transition. β-Catenin interacts with Smad3, thereby preventing the inhibitory effect of the latter on myocardin-related transcription factor (MRTF), and maintains MRTF stability by inhibiting Smad3-mediated, GSK-3β–dependent degradation of MRTF.

Injury to the adherens junctions (AJs) synergizes with transforming growth factor-β1 (TGFβ) to activate a myogenic program (α-smooth muscle actin [SMA] expression) in the epithelium during epithelial–myofibroblast transition (EMyT). Although this synergy plays a key role in organ fibrosis, the underlying mechanisms have not been fully defined. Because we recently showed that Smad3 inhibits myocardin-related transcription factor (MRTF), the driver of the SMA promoter and many other CC(A/T)-rich GG element (CArG) box–dependent cytoskeletal genes, we asked whether AJ components might affect SMA expression through interfering with Smad3. We demonstrate that E-cadherin down-regulation potentiates, whereas β-catenin knockdown inhibits, SMA expression. Contact injury and TGFβ enhance the binding of β-catenin to Smad3, and this interaction facilitates MRTF signaling by two novel mechanisms. First, it inhibits the Smad3/MRTF association and thereby allows the binding of MRTF to its myogenic partner, serum response factor (SRF). Accordingly, β-catenin down-regulation disrupts the SRF/MRTF complex. Second, β-catenin maintains the stability of MRTF by suppressing the Smad3-mediated recruitment of glycogen synthase kinase-3β to MRTF, an event that otherwise leads to MRTF ubiquitination and degradation and the consequent loss of SRF/MRTF–dependent proteins. Thus β-catenin controls MRTF-dependent transcription and emerges as a critical regulator of an array of cytoskeletal genes, the “CArGome.”

Snail involves in the transforming growth factor β1-mediated epithelial-mesenchymal transition of retinal pigment epithelial cells

Background

The proliferation of retinal pigment epithelium (RPE) cells resulting from an epithelial-mesenchymal transition (EMT) plays a key role in proliferative vitreoretinopathy (PVR), which leads to complex retinal detachment and the loss of vision. Genes of Snail family encode the zinc finger transcription factors that have been reported to be essential in EMT during embryonic development and cancer metastasis. However, the function of Snail in RPE cells undergoing EMT is largely unknown.

Principal Findings

Transforming growth factor beta(TGF-β)-1 resulted in EMT in human RPE cells (ARPE-19), which was characterized by the expected decrease in E-cadherin and Zona occludin-1(ZO-1) expression, and the increase in fibronectin and α-smooth muscle actin (α-SMA) expression, as well as the associated increase of Snail expression at both mRNA and protein levels. Furthermore, TGF-β1 treatment caused a significant change in ARPE-19 cells morphology, with transition from a typical epithelial morphology to mesenchymal spindle-shaped. More interestingly, Snail silencing significantly attenuated TGF-β1-induced EMT in ARPE-19 cells by decreasing the mesenchymal markers fibronectin and a-SMA and increasing the epithelial marker E-cadherin and ZO-1. Snail knockdown could effectively suppress ARPE-19 cell migration. Finally, Snail was activated in epiretinal membranes from PVR patients. Taken together, Snail plays very important roles in TGF-β-1-induced EMT in human RPE cells and may contribute to the development of PVR.

Significance

Snail transcription factor plays a critical role in TGF-β1-induced EMT in human RPE cells, which provides deep insight into the pathogenesis of human PVR disease. The specific inhibition of Snail may provide a new approach to treat and prevent PVR.

Transition of asthmatic bronchial fibroblasts to myofibroblasts is inhibited by cell-cell contacts

The role of airway wall remodelling in bronchial asthma is well established. Myofibroblasts, the cells displaying features intermediate between fibroblasts and smooth muscle cells, are involved in this process but the mechanism of myofibroblasts activation in the onset of the disease remains obscure. Myofibroblasts can differentiate from various cell types, including resident fibroblasts, and the fibroblasts to myofibroblasts transition (FMT) can be reproduced in vitro. We aimed to investigate the process of FMT in human bronchial fibroblasts (HBF) derived from non-asthmatic (n = 7) and asthmatic (n = 7) subjects. We also tested whether cell-cell contacts affect FMT by using N-cadherin blocking antibody. HBF plated in low or high cell density were treated with TGF-β(1) up to one week to induce FMT. The percentage of myofibroblsts was counted and expression of α-smooth muscle actin was evaluated by cytoimmunofluorescence, flow cytometry and immunobloting. We demonstrated that the intensity of FMT induced by TGF-β(1)in vitro was strongly enhanced in asthmatic as compared to non-asthmatic HBF populations. This process was facilitated by low cell plating density in both groups of cultures. Furthermore, we proved that neither HBF-conditioned medium nor growth arrest in G(0)/G(1) phase of cell cycle could stop the TGF-β(1)-induced FMT in asthmatic cell populations. However, even in sparse asthmatic HBF, the blocking of N-cadherin resulted in the inhibition of FMT. Our findings show for the first time that the initial absence or an induced loss of cell-cell adhesions in asthmatic HBF populations is important for the completion of FMT.

SLIT/ROBO1 signaling suppresses mammary branching morphogenesis by limiting basal cell number

In the field of breast biology, there is a growing appreciation for the "gatekeeping function" of basal cells during development and disease processes yet mechanisms regulating the generation of these cells are poorly understood. Here, we report that the proliferation of basal cells is controlled by SLIT/ROBO1 signaling and that production of these cells regulates outgrowth of mammary branches. We identify the negative regulator TGF-β1 upstream of Robo1 and show that it induces Robo1 expression specifically in the basal layer, functioning together with SLIT2 to restrict branch formation. Loss of SLIT/ROBO1 signaling in this layer alone results in precocious branching due to a surplus of basal cells. SLIT2 limits basal cell proliferation by inhibiting canonical WNT signaling, increasing the cytoplasmic and membrane pools of β-catenin at the expense of its nuclear pool. Together, our studies provide mechanistic insight into how specification of basal cell number influences branching morphogenesis.

The role of TGF-β and epithelial-to mesenchymal transition in diabetic nephropathy

Transforming Growth Factor-beta (TGF-β) is a pro-sclerotic cytokine widely associated with the development of fibrosis in diabetic nephropathy. Central to the underlying pathology of tubulointerstitial fibrosis is epithelial-to-mesenchymal transition (EMT), or the trans-differentiation of tubular epithelial cells into myofibroblasts. This process is accompanied by a number of key morphological and phenotypic changes culminating in detachment of cells from the tubular basement membrane and migration into the interstitium. Ultimately these cells reside as activated myofibroblasts and further exacerbate the state of fibrosis. A large body of evidence supports a role for TGF-β and downstream Smad signalling in the development and progression of renal fibrosis. Here we discuss a role for TGF-β as the principle effector in the development of renal fibrosis in diabetic nephropathy, focusing on the role of the TGF-β1 isoform and its downstream signalling intermediates, the Smad proteins. Specifically we review evidence for TGF-β1 induced EMT in both the proximal and distal regions of the nephron and describe potential therapeutic strategies that may target TGF-β1 activity.

Glutamatergic signaling maintains the epithelial phenotype of proximal tubular cells

Epithelial-mesenchymal transition (EMT) contributes to the progression of renal tubulointerstitial fibrosis. The N-methyl-d-aspartate receptor (NMDAR), which is present in proximal tubular epithelium, is a glutamate receptor that acts as a calcium channel. Activation of NMDAR induces actin rearrangement in cells of the central nervous system, but whether it helps maintain the epithelial phenotype of the proximal tubule is unknown. Here, knockdown of NMDAR1 in a proximal tubule cell line (HK-2) induced changes in cell morphology, reduced E-cadherin expression, and increased α-SMA expression. Induction of EMT with TGF-β1 led to downregulation of both E-cadherin and membrane-associated β-catenin, reorganization of F-actin, expression of mesenchymal markers de novo, upregulation of Snail1, and increased cell migration; co-treatment with NMDA attenuated all of these changes. Furthermore, NMDA reduced TGF-β1-induced phosphorylation of Erk1/2 and Akt and the activation of Ras, suggesting that NMDA antagonizes TGF-β1-induced EMT by inhibiting the Ras-MEK pathway. In the unilateral ureteral obstruction model, treatment with NMDA blunted obstruction-induced upregulation of α-SMA, FSP1, and collagen I and downregulation of E-cadherin. Taken together, these results suggest that NMDAR plays a critical role in preserving the normal epithelial phenotype and modulating tubular EMT.

Lineage tracing demonstrates no evidence of cholangiocyte epithelial-to-mesenchymal transition in murine models of hepatic fibrosis

Whether or not cholangiocytes or their hepatic progenitors undergo an epithelial-to-mesenchymal transition (EMT) to become matrix-producing myofibroblasts during biliary fibrosis is a significant ongoing controversy. To assess whether EMT is active during biliary fibrosis, we used Alfp-Cre × Rosa26-YFP mice, in which the epithelial cells of the liver (hepatocytes, cholangiocytes, and their bipotential progenitors) are heritably labeled at high efficiency with yellow fluorescent protein (YFP). Primary cholangiocytes isolated from our reporter strain were able to undergo EMT in vitro when treated with transforming growth factor-β1 alone or in combination with tumor necrosis factor-α, as indicated by adoption of fibroblastoid morphology, intracellular relocalization of E-cadherin, and expression of α-smooth muscle actin (α-SMA). To determine whether EMT occurs in vivo, we induced liver fibrosis in Alfp-Cre × Rosa26-YFP mice using the bile duct ligation (BDL) (2, 4, and 8 weeks), carbon tetrachloride (CCl(4) ) (3 weeks), and 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC; 2 and 3 weeks) models. In no case did we find evidence of colocalization of YFP with the mesenchymal markers S100A4, vimentin, α-SMA, or procollagen 1α2, although these proteins were abundant in the peribiliary regions.

CONCLUSION

Hepatocytes and cholangiocytes do not undergo EMT in murine models of hepatic fibrosis.

E-cadherin: gatekeeper of airway mucosa and allergic sensitization

The airway epithelium plays a role in immune regulation during environmental challenge, which is intertwined with its barrier function and capacity to limit submucosal access of environmental factors. In asthma, mucosal barrier function is often compromised, with disrupted expression of the adhesion molecule E-cadherin. Recent progress suggests that E-cadherin contributes to the structural and immunological function of airway epithelium, through the regulation of epithelial junctions, proliferation, differentiation, and production of growth factors and proinflammatory mediators that can modulate the immune response. Here, we discuss this novel role for E-cadherin in mediating the crucial immunological decision between maintenance of tolerance versus induction of innate and adaptive immunity.

Scar wars: mapping the fate of epithelial-mesenchymal-myofibroblast transition

The hypothesis that epithelial-mesenchymal transition (EMT) might be a contributor to the accumulation of fibroblasts and myofibroblasts (MFs) in the kidney during fibrogenesis was postulated 15 years ago. This paradigm offered an elegant explanation of how the loss of epithelial functions is coupled to the gain of deleterious mesenchymal functions; for example, excessive matrix deposition. Moreover, it interpreted chronic kidney disease in a developmental context: because the tubular epithelium originates from the metanephric mesenchyme, EMT can be viewed as a dedifferentiation process in response to injury, which might serve healing or--if dysregulated--might facilitate fibrosis. Several observations support the role of EMT in renal fibrosis: (1) Tubular cells can transform to fibroblasts and MFs in vitro. (2) Histological 'snapshots' reveal the coexistence of epithelial and mesenchymal markers in transitioning tubular cells in fibrosis models and human kidney diseases. (3) Early lineage-tracing experiments detected mesenchymal markers in the genetically tagged epithelium. However, the paradigm has been recently challenged; new fate-mapping studies found no evidence for the expression of (myo)fibroblast markers in the epithelium during fibrogenesis. This review summarizes the key findings and caveats, aiming at a balanced view, which neither overestimates the role of the epithelium in MF generation nor denies the importance of epithelial plasticity in fibrogenesis.

Cell density regulates in vitro activation of heart valve interstitial cells

BACKGROUND

Valve interstitial cells, the most prominent cell type in the heart valve, are activated and express α-smooth muscle actin in valve repair and in diseased valves. We hypothesize that cell density, time in culture, and the establishment of cell-cell contacts may be involved in regulating valve interstitial cell activation in vitro.

METHODS

To study cell density, valve interstitial cells were plated at passages 3 to 5, at a density of 17,000 cells/22 × 22 mm(2) coverslip, and grown for 1, 2, 4, 7, and 10 days. Valve interstitial cells were stained for α-smooth muscle actin and viewed under confocal microscopy to characterize the intensity of staining. To study time in culture, valve interstitial cells were plated at a 10-fold higher density to achieve similar growth densities over a shorter time period compared with valve interstitial cells plated at low density. α-Smooth muscle actin staining was compared at the same time points between those plated at high and low densities. To confirm valve interstitial cell activation as indicated by α-smooth muscle actin staining, valve interstitial cells were stained for cofilin at days 2, 5, 8, and 14 days postplating. To study the association of transforming growth factor β with valve interstitial cell activation with respect to cell density, valve interstitial cells were stained for α-smooth muscle actin and transforming growth factor β at 2, 4, 6, and 8 days postplating. To study the activation of the transforming growth factor β signaling pathway, valve interstitial cells were stained for pSmad2/3 at days 2, 4, 6, 8, 10, and 12 days postplating. To study cell contacts and activation, subconfluent and confluent cultures of valve interstitial cells were stained for β-catenin, N-cadherin, and α-smooth muscle actin. Also, whole-cell lysates of subconfluent and confluent valve interstitial cell cultures were probed by Western blot analysis for phospho-β-catenin at Ser33/37/Thr41, which is the form of β-catenin targeted for proteosomal degradation.

RESULTS

The percentage valve interstitial cells with high-intensity α-smooth muscle actin staining decreases significantly between days 1 and 4, and at confluency, most cells show absent or low-intensity staining, regardless of time in culture. Similar results are obtained with cofilin staining. Transforming growth factor β and nuclear pSmad2/3 staining in valve interstitial cells decreases concurrently with valve interstitial cell activation as cell density increases. Examining β-catenin and N-cadherin staining, single valve interstitial cells show no cell-cell contact with strong cytoplasmic staining, with some showing nuclear staining of β-catenin, while confluent monolayers show strong staining of fully established cell-cell contacts, weak cytoplasmic staining, and absent nuclear staining. The presence of cell-cell contacts is associated with a decreased α-smooth muscle actin. The level of phospho-β-catenin at Ser33/37/Thr41 is lower in confluent cultures compared with low-density subconfluent valve interstitial cell cultures.

CONCLUSION

Cell-cell contacts may inhibit valve interstitial cell activation, while absence of cell-cell contacts may contribute to activation.

Fibroblasts and myofibroblasts in renal fibrosis

Interstitial fibrosis, associated with extensive accumulation of extracellular matrix constituents in the cortical interstitium, is directly correlated to progression of renal disease. The earliest histological marker of this progression is the accumulation in the interstitium of fibroblasts with the phenotypic appearance of myofibroblasts. These myofibroblasts are contractile cells that express alpha smooth muscle actin and incorporate it into intracellular stress fibres. Although fibroblasts are histologically visible in normal kidneys, there are relatively few of them and proximal tubular epithelial cells predominate. In progressive disease, however, the interstitium becomes filled with myofibroblasts. In this review, we will examine the phenotype and function of fibroblasts and myofibroblasts in the cortical interstitium and the processes that may modulate them.

Tissue geometry patterns epithelial-mesenchymal transition via intercellular mechanotransduction

Epithelial-mesenchymal transition (EMT) is a phenotypic change in which epithelial cells detach from their neighbors and become motile. Whereas soluble signals such as growth factors and cytokines are responsible for stimulating EMT, here we show that gradients of mechanical stress define the spatial locations at which EMT occurs. When treated with transforming growth factor (TGF)-beta, cells at the corners and edges of square mammary epithelial sheets expressed EMT markers, whereas those in the center did not. Changing the shape of the epithelial sheet altered the spatial pattern of EMT. Traction force microscopy and finite element modeling demonstrated that EMT-permissive regions experienced the highest mechanical stress. Myocardin-related transcription factor (MRTF)-A was localized to the nuclei of cells located in high-stress regions, and inhibiting cytoskeletal tension or MRTF-A expression abrogated the spatial patterning of EMT. These data suggest a causal role for tissue geometry and endogenous mechanical stresses in the spatial patterning of EMT.

Serum- and glucocorticoid-regulated kinase 1 is upregulated following unilateral ureteral obstruction causing epithelial-mesenchymal transition

Obstructive nephropathy leads to chronic kidney disease, characterized by a progressive epithelial-to-mesenchymal cell transition (EMT)-driven interstitial fibrosis. To identify the mechanisms causing EMT, we used the mouse model of unilateral ureteral obstruction and found a rapid and significant increase in serum- and glucocorticoid-regulated kinase-1 (SGK1) expression in the kidneys with an obstructed ureter. Knockout of SGK1 significantly suppressed obstruction-induced EMT, kidney fibrosis, increased glycogen synthase kinase-3β activity, and decreased accumulation of the transcriptional repressor Snail. This caused a reduced expression of the mesenchymal marker α-smooth muscle actin, and collagen deposition in this model. In cultured kidney epithelial cells, mechanical stretch or treatment with transforming growth factor-β not only stimulated the transcription of SGK1, but also stimulated EMT in an SGK1-dependent manner. Activated SGK1 stimulated Snail accumulation and downregulation of the epithelial marker E-cadherin. Hence, our study shows that SGK1 is involved in mediating fibrosis associated with obstructive nephropathy.

Cytoskeletal regulation of epithelial barrier function during inflammation

Increased epithelial permeability is a common and important consequence of mucosal inflammation that results in perturbed body homeostasis and enhanced exposure to external pathogens. The integrity and barrier properties of epithelial layers are regulated by specialized adhesive plasma membrane structures known as intercellular junctions. It is generally believed that inflammatory stimuli increase transepithelial permeability by inducing junctional disassembly. This review highlights molecular events that lead to disruption of epithelial junctions during inflammation. We specifically focus on key mechanisms of junctional regulation that are dependent on reorganization of the perijunctional F-actin cytoskeleton. We discuss critical roles of myosin-II-dependent contractility and actin filament turnover in remodeling of the F-actin cytoskeleton that drive disruption of epithelial barriers under different inflammatory conditions. Finally, we highlight signaling pathways induced by inflammatory mediators that regulate reorganization of actin filaments and junctional disassembly in mucosal epithelia.

E-cadherin and β-catenin expression in canine colorectal adenocarcinoma

E-cadherin and its associated cytoplasmic proteins, including β-catenin, have been examined as potential oncogenic markers due to the significant correlation between tumour dedifferentiation and the invasive capacity of epithelial tumours. The purpose of this study was to evaluate the expression of E-cadherin and β-catenin in canine colorectal cancer using immunohistochemistry and to examine the relationship between this expression and various clinicopathological variables. The expression pattern of E-cadherin and β-catenin was investigated in 44 colorectal canine carcinomas. In the intestinal mucosa of noncancerous areas, epithelial cells demonstrated equally strong membranous expression of E-cadherin and β-catenin localised to the cell-cell junctions. Reduced expression of E-cadherin and β-catenin was demonstrated in 75% and 81.8% of the colorectal carcinoma cases, respectively. The down-regulation of both E-cadherin and β-catenin was correlated with decreased differentiation and increased tumour grade. In addition, the expression of β-catenin was correlated with tumour size. These results suggest that dysfunction of the E-cadherin-catenin complex starts in the early stages of carcinogenesis and that the disruption of the tissue architecture is progressively associated with the invasion of the tumour.

Proinsulin C-peptide antagonizes the profibrotic effects of TGF-beta1 via up-regulation of retinoic acid and HGF-related signaling pathways

Novel signaling roles for C-peptide have recently been discovered with evidence that it can ameliorate complications of type 1 diabetes. Here we sought to identify new pathways regulated by C-peptide of relevance to the pathophysiology of diabetic nephropathy. Microarray analysis was performed to identify genes regulated by either C-peptide and/or TGF-beta1 in a human proximal tubular cell line, HK-2. Expression of retinoic acid receptor beta (RARbeta), hepatocyte growth factor (HGF), cellular retinoic acid-binding protein II (CRABPII), vimentin, E-cadherin, Snail, and beta-catenin was assessed by immunoblotting. The cellular localization of vimentin and beta-catenin was determined by immunocytochemistry. Changes in cell morphology were assessed by phase contrast microscopy. Gene expression profiling demonstrated differential expression of 953 and 1458 genes after C-peptide exposure for 18 h or 48 h, respectively. From these, members of the antifibrotic retinoic acid (RA)- and HGF-signaling pathways were selected. Immunoblotting demonstrated that C-peptide increased RARbeta, CRABPII, and HGF. We confirmed a role for RA in reversal of TGF-beta1-induced changes associated with epithelial-mesenchymal transition, including expression changes in Snail, E-cadherin, vimetin, and redistribution of beta-catenin. Importantly, these TGF-beta1-induced changes were inhibited by C-peptide. Further, effects of TGF-beta1 on Snail and E-cadherin expression were blocked by HGF, and inhibitory effects of C-peptide were removed by blockade of HGF activity. This study identifies a novel role for HGF as an effector of C-peptide, possibly via an RA-signaling pathway, highlighting C-peptide as a potential therapy for diabetic nephropathy.

[Dilingdan Decoction prevents renal interstitial fibrosis in a rat model of unilateral ureteral obstruction]

OBJECTIVE

To investigate the preventive and therapeutic effects and the possible mechanisms of Dilingdan Decoction (DLDD), a compound Chinese herbal medicine, on rats with renal tubulointerstitial fibrosis induced by unilateral ureteral obstruction (UUO).

METHODS

Sixty SD rats were randomly divided into sham-operated group, untreated group, enalapril-treated group and DLDD-treated group. Blood urea nitrogen (BUN), serum creatinine (SCr), urine protein quantization in 24 hours and pathological changes of the obstructed kidney were observed. The expressions of transforming growth factor-beta1 (TGF-beta1), alpha-smooth muscle actin (alpha-SMA), fibronectin (FN) and laminin (LN) were detected by immunohistochemical method and colored-multimedia pathological image analysis system.

RESULTS

Massive inflammatory infiltrates and collagen expression in renal interstitial in the untreated group were observed on the 7th day. Compared with the sham-operated group, percentages of area of TGF-beta1, alpha-SMA, FN and LN expressions in the untreated group were markedly increased (P<0.05), while the percentage of area of interstitial fibrosis was decreased in the DLDD-treated group as compared with the untreated group (P<0.05). On the 14th day, the percentages of area of TGF-beta1, alpha-SMA, FN and LN expressions were declined in two treated groups as compared with the untreated group (P<0.05), but had no statistical difference in biochemical indicators, including BUN, SCr and 24-hour urinary protein. On the 21st day, the level of SCr and the percentage of area of TGF-beta1 expression in the DLDD-treated group were lower than those of the enalapril-treated group (P<0.05).

CONCLUSION

DLDD can reduce the excretion of urinary protein and the degree of interstitial fibrosis, and significantly inhibit the expressions of TGF-beta1, alpha-SMA, FN and LN. DLDD is superior to enalapril in protecting renal function after long-time application in rats.

An integrative view on the role of TGF-beta in the progressive tubular deletion associated with chronic kidney disease

Transforming growth factor-beta (TGF-beta) is a cytokine known to participate in several processes related to the development of chronic kidney disease (CKD), including tubular degeneration. This is thought to occur mainly through apoptosis and epithelial-to-mesenchymal transition (EMT) of tubule epithelial cells, which give rise to a reduction of the tubular compartment and a scarring-like, fibrotic healing process of the interstitial compartment. In vivo blockade of TGF-beta action has been shown to reduce CKD-associated tubular damage. However, a direct action of TGF-beta on tubule cells is controversial as the underlying mechanism. On the one hand, TGF-beta is known to induce EMT of tubular cells, although its incidence in vivo can hardly explain the extent of the damage. On the other hand, a few publications have reported that TGF-beta induces a mild degree of apoptosis in cultured tubular cells. This most likely reflects the consequence of the cell-cycle arrest rather than a direct pro-apoptotic effect of TGF-beta. The implications of these observations are analyzed in the pathological context, where normal tubular cells do not normally proliferate, but they might divide for repair purposes. Furthermore, renal fibrosis, a TGF-beta-mediated event, is integrated as a potential, indirect effect contributing to tubule deletion.

Aging-associated oxidative stress modulates the acute inflammatory response in skeletal muscle after contusion injury

Inflammation is an integral component of the response of skeletal muscle to a contusion injury that can be modulated by acute oxidative stress. Less is known regarding the effect of aging-associated oxidative stress on the inflammatory response in injured skeletal muscle. The purpose of this project was to assess the level of oxidative stress in skeletal muscles of young, adult, and old rats and determine its effect on the acute inflammatory response to a contusion injury. Inherent oxidative stress in the muscle was determined by measuring the glutathione:glutathione disulfide ratio, and levels of GP91(phox). Elevated oxidative stress was observed in uninjured muscles of adult and old rats and was accompanied by increased levels of lipid peroxidation and neutrophil chemoattractant CINC-1. After injury, the acute inflammatory response (8h, 3 d) was determined from markers of neutrophil (myeloperoxidase) and macrophage (CD68) content and by expression of NFkappaB, CINC-1 and TGF-beta1. Compared to injured muscles of young rats, NFkappaB, myeloperoxidase activity (8h), macrophage content (3 d), and TGF-beta1 (8h and 3 d) were significantly greater in injured muscles of old rats. We conclude that aging-associated oxidative stress in muscles of old rats exacerbated the inflammatory response to contusion injury and leads to increased TGF-beta1-induced collagen content.

Characterization of connective tissue growth factor expression in primary cultures of human tubular epithelial cells: modulation by hypoxia

Tubular epithelial cells secrete connective tissue growth factor (CTGF, CCN2), which contributes to tubulointerstitial fibrosis. However, the molecular regulation of CTGF in human primary tubular epithelial cells (hPTECs) is not well defined. Therefore, CTGF expression was characterized in hPTECs isolated from healthy parts of tumor nephrectomies, with special emphasis on the regulation by transforming growth factor-β (TGF-β) and hypoxia, essential factors in the development of fibrosis. CTGF synthesis was strongly dependent on cell density. High CTGF levels were detected in sparse cells, whereas CTGF expression was reduced in confluent cells. Concomitantly, stimulation of CTGF by TGF-β or the histone deacetylase inhibitor trichostatin was prevented in dense cells. Exposure of hPTECs to low oxygen tension (1% O2) or the hypoxia mimetic dimethyl-oxalylglycine for 24 h reduced CTGF gene expression in most of the 17 preparations analyzed. Preincubation of the cells under hypoxic conditions significantly reduced TGF-β-mediated upregulation of CTGF. In line with these data, CTGF mRNA was only induced in interstitial cells, but not in tubular cells in kidneys of mice exposed to hypoxia. Longer exposure to hypoxia or TGF-β (up to 72 h) did not induce hPTECs to adopt a mesenchymal phenotype characterized by upregulation of α-smooth muscle actin, downregulation of E-cadherin, or increased sensitivity of the cells in terms of CTGF expression. Sensitivity was restored by inhibition of DNA methylation. Taken together, our data provide evidence that exposure to hypoxia decreased CTGF gene expression. Furthermore, hypoxia per se was not sufficient to induce a mesenchymal phenotype in primary tubular epithelial cells.