The Ski-Zeb2-Meox2 pathway provides a novel mechanism for regulation of the cardiac myofibroblast phenotype

Helicobacter pylori-activated fibroblasts as a silent partner in gastric cancer development

The discovery of Helicobacter pylori (Hp) infection of gastric mucosa leading to active chronic gastritis, gastroduodenal ulcers, and MALT lymphoma laid the groundwork for understanding of the general relationship between chronic infection, inflammation, and cancer. Nevertheless, this sequence of events is still far from full understanding with new players and mediators being constantly identified. Originally, the Hp virulence factors affecting mainly gastric epithelium were proposed to contribute considerably to gastric inflammation, ulceration, and cancer. Furthermore, it has been shown that Hp possesses the ability to penetrate the mucus layer and directly interact with stroma components including fibroblasts and myofibroblasts. These cells, which are the source of biophysical and biochemical signals providing the proper balance between cell proliferation and differentiation within gastric epithelial stem cell compartment, when exposed to Hp, can convert into cancer-associated fibroblast (CAF) phenotype. The crosstalk between fibroblasts and myofibroblasts with gastric epithelial cells including stem/progenitor cell niche involves several pathways mediated by non-coding RNAs, Wnt, BMP, TGF-β, and Notch signaling ligands. The current review concentrates on the consequences of Hp-induced increase in gastric fibroblast and myofibroblast number, and their activation towards CAFs with the emphasis to the altered communication between mesenchymal and epithelial cell compartment, which may lead to inflammation, epithelial stem cell overproliferation, disturbed differentiation, and gradual gastric cancer development. Thus, Hp-activated fibroblasts may constitute the target for anti-cancer treatment and, importantly, for the pharmacotherapies diminishing their activation particularly at the early stages of Hp infection.

Identifying molecular and functional similarities and differences between human primary cardiac valve interstitial cells and ventricular fibroblasts

Introduction: Fibroblasts are mesenchymal cells that predominantly produce and maintain the extracellular matrix (ECM) and are critical mediators of injury response. In the heart, valve interstitial cells (VICs) are a population of fibroblasts responsible for maintaining the structure and function of heart valves. These cells are regionally distinct from myocardial fibroblasts, including left ventricular cardiac fibroblasts (LVCFBs), which are located in the myocardium in close vicinity to cardiomyocytes. Here, we hypothesize these subpopulations of fibroblasts are transcriptionally and functionally distinct. Methods: To compare these fibroblast subtypes, we collected patient-matched samples of human primary VICs and LVCFBs and performed bulk RNA sequencing, extracellular matrix profiling, and functional contraction and calcification assays. Results: Here, we identified combined expression of SUSD2 on a protein-level, and MEOX2, EBF2 and RHOU at a transcript-level to be differentially expressed in VICs compared to LVCFBs and demonstrated that expression of these genes can be used to distinguish between the two subpopulations. We found both VICs and LVCFBs expressed similar activation and contraction potential in vitro, but VICs showed an increase in ALP activity when activated and higher expression in matricellular proteins, including cartilage oligomeric protein and alpha 2-Heremans-Schmid glycoprotein, both of which are reported to be linked to calcification, compared to LVCFBs. Conclusion: These comparative transcriptomic, proteomic, and functional studies shed novel insight into the similarities and differences between valve interstitial cells and left ventricular cardiac fibroblasts and will aid in understanding region-specific cardiac pathologies, distinguishing between primary subpopulations of fibroblasts, and generating region-specific stem-cell derived cardiac fibroblasts.

Canadian Contributions in Fibroblast Biology

Fibroblasts are stromal cells found in virtually every tissue and organ of the body. For many years, these cells were often considered to be secondary in functional importance to parenchymal cells. Over the past 2 decades, focused research into the roles of fibroblasts has revealed important roles for these cells in the homeostasis of healthy tissue, and has demonstrated that activation of fibroblasts to myofibroblasts is a key step in disease initiation and progression in many tissues, with fibrosis now recognized as not only an outcome of disease, but also a central contributor to tissue dysfunction, particularly in the heart and lungs. With a growing understanding of both fibroblast and myofibroblast heterogeneity, and the deciphering of the humoral and mechanical cues that impact the phenotype of these cells, fibroblast biology is rapidly becoming a major focus in biomedical research. In this review, we provide an overview of fibroblast and myofibroblast biology, particularly in the heart, and including a discussion of pathophysiological processes such as fibrosis and scarring. We then discuss the central role of Canadian researchers in moving this field forwards, particularly in cardiac fibrosis, and highlight some of the major contributions of these individuals to our understanding of fibroblast and myofibroblast biology in health and disease.

Characterization and Validation of ceRNA-Mediated Pathway–Pathway Crosstalk Networks Across Eight Major Cardiovascular Diseases

Pathway analysis is considered as an important strategy to reveal the underlying mechanisms of diseases. Pathways that are involved in crosstalk can regulate each other and co-regulate downstream biological processes. Furthermore, some genes in the pathways can function with other genes via the relationship of the competing endogenous RNA (ceRNA) mechanism, which has also been demonstrated to play key roles in cellular biology. However, the comprehensive analysis of ceRNA-mediated pathway crosstalk is lacking. Here, we constructed the landscape of the ceRNA-mediated pathway–pathway crosstalk of eight major cardiovascular diseases (CVDs) based on sequencing data from ∼2,800 samples. Some common features shared by numerous CVDs were uncovered. A fraction of the pathway–pathway crosstalk was conserved in multiple CVDs and a core pathway–pathway crosstalk network was identified, suggesting the similarity of pathway–pathway crosstalk among CVDs. Experimental evidence also demonstrated that the pathway crosstalk was functioned in CVDs. We split all hub pathways of each pathway–pathway crosstalk network into three categories, namely, common hubs, differential hubs, and specific hubs, which could highlight the common or specific biological mechanisms. Importantly, after a comparison analysis of the hub pathways of networks, ∼480 hub pathway-induced common modules were identified to exert functions in CVDs broadly. Moreover, we performed a random walk algorithm on the hub pathway-induced sub-network and identified 23 potentially novel CVD-related pathways. In summary, our study revealed the potential molecular regulatory mechanisms of ceRNA crosstalk in pathway–pathway crosstalk levels and provided a novel routine to investigate the pathway–pathway crosstalk in cardiology. All CVD pathway–pathway crosstalks are provided in http://www.licpathway.net/cepathway/index.html.

The miR-214-3p/c-Ski axis modulates endothelial-mesenchymal transition in human coronary artery endothelial cells in vitro and in mice model in vivo

Cardiovascular disease (CVD) is a leading non-communicable disease with a high fatality rate worldwide. Hypertension, a common cardiovascular condition, is a significant risk factor for the development of heart failure because the activation of the renin-angiotensin system (RAS) is considered to be the major promoting reason behind myocardial fibrosis (MF). In this study, Angiotensin II (Ang II) stimulation-induced endothelial to mesenchymal transition (End-MT) in HCAECs, including the decrease of CD31 level, the increase of α-SMA, collagen I, slug, snail, and TGF-β1 levels, and the promotion of Smad2/3 phosphorylation. Meanwhile, the c-Ski level was reduced in Ang II-stimulated HCAECs. In HCAECs, Ang II-induced changes could be partially attenuated by c-Ski overexpression. miR-214-3p directly targeted c-Ski and inhibited c-Ski expression. Moreover, miR-214-3p inhibition reduced Ang II-caused End-MT in HCAECs. miR-214-3p overexpression further enhanced Ang II-induced End-MT, while c-Ski overexpression could markedly reverse the effects of miR-214-3p overexpression. In the Ang II-induced mouse cardiac hypertrophic model, Ang II-caused increase of cellular cross-sectional area and cardiac fibrosis were partially ameliorated by LV-c-Ski; when mice were co-treated with LV-c-Ski and agomir-214-3p, the beneficial effects of LV-c-Ski were reversed. In conclusion, the miR-214-3p/c-Ski axis modulated Ang II-induced End-MT in HCAECs and cardiac hypertrophy and fibrosis in the mice model.

SKI activates the Hippo pathway via LIMD1 to inhibit cardiac fibroblast activation

We have previously shown that overexpression of SKI, an endogenous TGF-β₁ repressor, deactivates the pro-fibrotic myofibroblast phenotype in the heart. We now show that SKI also functions independently of SMAD/TGF-β signaling, by activating the Hippo tumor-suppressor pathway and inhibiting the Transcriptional co-Activator with PDZ-binding motif (TAZ or WWTR1). The mechanism(s) by which SKI targets TAZ to inhibit cardiac fibroblast activation and fibrogenesis remain undefined. A rat model of post-myocardial infarction was used to examine the expression of TAZ during acute fibrogenesis and chronic heart failure. Results were then corroborated with primary rat cardiac fibroblast cell culture performed both on plastic and on inert elastic substrates, along with the use of siRNA and adenoviral expression vectors for active forms of SKI, YAP, and TAZ. Gene expression was examined by qPCR and luciferase assays, while protein expression was examined by immunoblotting and fluorescence microscopy. Cell phenotype was further assessed by functional assays. Finally, to elucidate SKI’s effects on Hippo signaling, the SKI and TAZ interactomes were captured in human cardiac fibroblasts using BioID2 and mass spectrometry. Potential interactors were investigated in vitro to reveal novel mechanisms of action for SKI. In vitro assays on elastic substrates revealed the ability of TAZ to overcome environmental stimuli and induce the activation of hypersynthetic cardiac myofibroblasts. Further cell-based assays demonstrated that SKI causes specific proteasomal degradation of TAZ, but not YAP, and shifts actin cytoskeleton dynamics to inhibit myofibroblast activation. These findings were supported by identifying the bi-phasic expression of TAZ in vivo during post-MI remodeling and fibrosis. BioID2-based interactomics in human cardiac fibroblasts suggest that SKI interacts with actin-modifying proteins and with LIM Domain-containing protein 1 (LIMD1), a negative regulator of Hippo signaling. Furthermore, we found that LATS2 interacts with TAZ, whereas LATS1 does not, and that LATS2 knockdown prevented TAZ downregulation with SKI overexpression. Our findings indicate that SKI’s capacity to regulate cardiac fibroblast activation is mediated, in part, by Hippo signaling. We postulate that the interaction between SKI and TAZ in cardiac fibroblasts is arbitrated by LIMD1, an important intermediary in focal adhesion-associated signaling pathways. This study contributes to the understanding of the unique physiology of cardiac fibroblasts, and of the relationship between SKI expression and cell phenotype.

Fibroblast mechanosensing, SKI and Hippo signaling and the cardiac fibroblast phenotype: Looking beyond TGF-β

Cardiac fibroblast activation to hyper-synthetic myofibroblasts following a pathological stimulus or in response to a substrate with increased stiffness may be a key tipping point for the evolution of cardiac fibrosis. Cardiac fibrosis per se is associated with progressive loss of heart pump function and is a primary contributor to heart failure. While TGF-β is a common cytokine stimulus associated with fibroblast activation, a druggable target to quell this driver of fibrosis has remained an elusive therapeutic goal due to its ubiquitous use by different cell types and also in the signaling complexity associated with SMADs and other effector pathways. More recently, mechanical stimulus of fibroblastic cells has been revealed as a major point of activation; this includes cardiac fibroblasts. Further, the complexity of TGF-β signaling has been offset by the discovery of members of the SKI family of proteins and their inherent anti-fibrotic properties. In this respect, SKI is a protein that may bind a number of TGF-β associated proteins including SMADs, as well as signaling proteins from other pathways, including Hippo. As SKI is also known to directly deactivate cardiac myofibroblasts to fibroblasts, this mode of action is a putative candidate for further study into the amelioration of cardiac fibrosis. Herein we provide a synthesis of this topic and highlight novel candidate pathways to explore in the treatment of cardiac fibrosis.

Dynamic Interstitial Cell Response during Myocardial Infarction Predicts Resilience to Rupture in Genetically Diverse Mice

Cardiac ischemia leads to the loss of myocardial tissue and the activation of a repair process that culminates in the formation of a scar whose structural characteristics dictate propensity to favorable healing or detrimental cardiac wall rupture. To elucidate the cellular processes underlying scar formation, here we perform unbiased single-cell mRNA sequencing of interstitial cells isolated from infarcted mouse hearts carrying a genetic tracer that labels epicardial-derived cells. Sixteen interstitial cell clusters are revealed, five of which were of epicardial origin. Focusing on stromal cells, we define 11 sub-clusters, including diverse cell states of epicardial- and endocardial-derived fibroblasts. Comparing transcript profiles from post-infarction hearts in C57BL/6J and 129S1/SvImJ inbred mice, which displays a marked divergence in the frequency of cardiac rupture, uncovers an early increase in activated myofibroblasts, enhanced collagen deposition, and persistent acute phase response in 129S1/SvImJ mouse hearts, defining a crucial time window of pathological remodeling that predicts disease outcome.

SnoN residue (1-366) attenuates hypertrophic scars through resistance to transforming growth factor-β1-induced degradation

Hypertrophic scars (HSs) are characterized by fibroblast hyperproliferation and excessive matrix deposition. During wound healing, transforming growth factor (TGF)-β1/Smad signaling acts as a key regulator. As a transcriptional corepressor of TGF-β1/Smads, SnoN is expressed at low levels in many fibrotic diseases due to TGF-β1/Smad-induced degradation. SnoN residue (1-366; SR) is resistant to TGF-β1-induced degradation. However, the expression and role of SR in HSs are unknown. Here, we inhibited TGF-β1/Smad signaling via overexpression of SR to block fibroblast transdifferentiation, proliferation, and collagen deposition during HS formation. Our results showed that SnoN was downregulated in HS fibroblasts (HSFs) owing to TGF-β1/Smad-induced degradation. Overexpression of SR in normal human dermal fibroblasts (NHDFs) and HSFs successfully blocked phosphorylation of Smad2 and Smad3, thereby inhibiting NHDF transdifferentiation and HSF proliferation and reducing type I collagen (ColI) and type III collagen (ColIII) production and secretion. In addition, we applied overexpressed full-length SnoN (SF) and SR to wound granulation tissue in a rabbit model of HSs. SR reduced wound scarring, improved collagen deposition and arrangement of scar tissue, and decreased mRNA and protein expression of ColI, ColIII, and α-smooth muscle actin (α-SMA) more effectively than SF in vivo. These results suggest that SR could be a promising therapy for the prevention of HS.

Ski drives an acute increase in MMP-9 gene expression and release in primary cardiac myofibroblasts

Many etiologies of heart disease are characterized by expansion and remodeling of the cardiac extracellular matrix (ECM or matrix) which results in cardiac fibrosis. Cardiac fibrosis is mediated in cardiac fibroblasts by TGF‐β₁/R‐Smad2/3 signaling. Matrix component proteins are synthesized by activated resident cardiac fibroblasts known as myofibroblasts (MFB). These events are causal to heart failure with diastolic dysfunction and reduced cardiac filling. We have shown that exogenous Ski, a TGF‐β₁/Smad repressor, localizes in the cellular nucleus and deactivates cardiac myofibroblasts. This deactivation is associated with reduction of myofibroblast marker protein expression in vitro, including alpha smooth muscle actin (α‐SMA) and extracellular domain‐A (ED‐A) fibronectin. We hypothesize that Ski also acutely regulates MMP expression in cardiac MFB. While acute Ski overexpression in cardiac MFB in vitro was not associated with any change in intracellular MMP‐9 protein expression versus LacZ‐treated controls,exogenous Ski caused elevated MMP‐9 mRNA expression and increased MMP‐9 protein secretion versus controls. Zymographic analysis revealed increased MMP‐9‐specific gelatinase activity in myofibroblasts overexpressing Ski versus controls. Moreover, Ski expression was attended by reduced paxillin and focal adhesion kinase phosphorylation (FAK ‐ Tyr 397) versus controls. As myofibroblasts are hypersecretory and less motile relative to fibroblasts, Ski's reduction of paxillin and FAK expression may reflect the relative deactivation of myofibroblasts. Thus, in addition to its known antifibrotic effects, Ski overexpression elevates expression and extracellular secretion/release of MMP‐9 and thus may facilitate internal cytoskeletal remodeling as well as extracellular ECM components. Further, as acute TGF‐β₁ treatment of primary cardiac MFB is known to cause rapid translocation of Ski to the nucleus, our data support an autoregulatory role for Ski in mediating cardiac ECM accumulation.

The crucial role of ZEB2: From development to epithelial-to-mesenchymal transition and cancer complexity

Zinc finger E-box binding homeobox 2 (ZEB2) is a DNA-binding transcription factor, which is mainly involved in epithelial-to-mesenchymal transition (EMT). EMT is a conserved process during which mature and adherent epithelial-like state is converted into a mobile mesenchymal state. Emerging data indicate that ZEB2 plays a pivotal role in EMT-induced processes such as development, differentiation, and malignant mechanisms, for example, drug resistance, cancer stem cell-like traits, apoptosis, survival, cell cycle arrest, tumor recurrence, and metastasis. In this regard, the understanding of mentioned subjects in the development of normal and cancerous cells could be helpful in cancer complexity of diagnosis and therapy. In this study, we review recent findings about the biological properties of ZEB2 in healthy and cancerous states to find new approaches for cancer treatment.

Cardiac Fibroblast to Myofibroblast Phenotype Conversion-An Unexploited Therapeutic Target

Fibrosis occurs when the synthesis of extracellular matrix outpaces its degradation, and over time can negatively impact tissue and organ function. In the case of cardiac fibrosis, contraction and relaxation of the heart can be impaired to the point of precipitating heart failure, while at the same time fibrosis can result in arrhythmias due to altered electrical properties of the myocardium. The critical event in the evolution of cardiac fibrosis is the phenotype conversion of cardiac fibroblasts to their overly-active counterparts, myofibroblasts: cells demarked by their expression of novel markers such as periostin, by their gain of contractile activity, and by their pronounced and prolonged increase in the production of extracellular matrix components such as collagens. The phenotype change is dramatic, and can be triggered by many stimuli, including mechanical force, inflammatory cytokines, and growth factors. This review will explore fibroblast to myofibroblast transition mechanisms and will consider the therapeutic potential of targeting this process as a means to arrest or even reverse cardiac fibrosis.

Single-cell analysis reveals fibroblast heterogeneity and myeloid-derived adipocyte progenitors in murine skin wounds

During wound healing in adult mouse skin, hair follicles and then adipocytes regenerate. Adipocytes regenerate from myofibroblasts, a specialized contractile wound fibroblast. Here we study wound fibroblast diversity using single-cell RNA-sequencing. On analysis, wound fibroblasts group into twelve clusters. Pseudotime and RNA velocity analyses reveal that some clusters likely represent consecutive differentiation states toward a contractile phenotype, while others appear to represent distinct fibroblast lineages. One subset of fibroblasts expresses hematopoietic markers, suggesting their myeloid origin. We validate this finding using single-cell western blot and single-cell RNA-sequencing on genetically labeled myofibroblasts. Using bone marrow transplantation and Cre recombinase-based lineage tracing experiments, we rule out cell fusion events and confirm that hematopoietic lineage cells give rise to a subset of myofibroblasts and rare regenerated adipocytes. In conclusion, our study reveals that wounding induces a high degree of heterogeneity among fibroblasts and recruits highly plastic myeloid cells that contribute to adipocyte regeneration.

The diversity of fibroblasts contributing to wound healing is unclear. Here, the authors use single-cell RNA-sequencing to identify heterogeneity among murine fibroblasts in the wound and find that recruited myeloid cells contribute to adipocyte regeneration during healing.

MicroRNA-145 induces the senescence of activated hepatic stellate cells through the activation of p53 pathway by ZEB2

Activation of quiescent hepatic stellate cells (HSCs) is the major event in liver fibrosis, along with enhancement of cell proliferation and overproduction of extracellular matrix. Recent findings suggest that senescence of activated HSCs might limit the development of liver fibrosis. The p53, a guardian of the genome is associated with liver fibrosis, has been shown to regulate HSCs senescence. In this study, we report that microRNA-145 (miR-145) and p53 were downregulated in vivo and in vitro, concomitant with the enhanced expression of zinc finger E-box binding homeobox 2 (ZEB2). In addition, overexpression of miR-145 and p53 led to upregulation of the number of senescence-associated β-galactosidase-positive HSCs and the expression of senescence markers p16 and p21, along with the reduced abundance of HSC activation markers α-smooth muscle actin and type I collagen in activated HSCs. Furthermore, silencing of ZEB2 promoted senescence of activated HSCs. Moreover, we also demonstrated that miR-145 specifically targeted the 3'-untranslated regions of ZEB2. In vitro promoter regulation studies show that ZEB2 could bind to the E-box of the p53 promoter as well as inhibit its promoter activity and thus suppress the expression of p53, which in turn repressed activated HSCs senescence. Taken together, our results describe a novel miR-145-ZEB2-p53 regulatory line might participate in the senescence of activated HSCs and might carry potential therapeutic targets for restraining liver fibrosis.

Toll-like receptor 4 contributes to a myofibroblast phenotype in cardiac fibroblasts and is associated with autophagy after myocardial infarction in a mouse model

BACKGROUND AND AIMS

Cardiac fibrosis after myocardial infarction (MI) is involved in fibroblast transforming and differentiating into myofibroblast phenoconversion, however, the underlying mechanisms are poorly understood. Toll-like receptor 4 (TLR4)-mediated pathogen-associated molecular patterns are key factors that deteriorate cardiac remodelling after MI. Moreover, autophagy has dual roles in cell survival in myocardial tissues after MI. We evaluated the relationship between TLR4 signalling and cardiac myofibroblast transformation-differentiation after MI in vivo and in vitro and analysed the role of autophagy.

METHODS

We reproduced a model of MI by the permanent ligation of the left anterior descending coronary artery of Tlr4-knockout (Tlr4^-/-) and wild-type (WT) male mice. We evaluated scar formation, myofibroblast phenoconversion, LC3 dot formation, autophagy related proteins and α-smooth muscle actin (SMA) in cardiac tissues, 7, 14, and 28 days after myocardial infarction. Cardiac fibroblasts were cultured from Tlr4^-/- or WT mice. Vimentin, α-SMA, bilayer membrane vesicle structures of autophagosomes, and autophagy related proteins were observed after treatment with lipopolysaccharide (LPS) or 3-methyladenine (3-MA) at 24 h.

RESULTS

After MI on 7, 14, and 28 days, Tlr4^-/- mice showed that heart tissue fibrosis and expression of α-SMA, a marker of myofibroblasts, were decreased compared to WT mice. Additionally, levels of LC3II, Atg5, Atg7, and Beclin-1, which are involved in autophagy, were lower than those in WT mice. Further, p62 expression, which is negatively correlated with autophagy levels, was higher in Tlr4^-/- mice. Moreover, LC3-labelled autophagosomes in cardiac tissues were reduced in these animals. In vitro, LPS, a ligand of TLR4, stimulated α-SMA expression in cardiac fibroblasts, enhanced autophagic flux, and increased autophagosome numbers. In contrast, these effects were not obvious in Tlr4^-/- cardiac fibroblasts. LC3II, Atg5, Atg7, and Beclin-1 were upregulated, and p62 was downregulated in cardiac fibroblasts of WT mice stimulated with LPS. However, these effects were blocked by 3-methyladenine, an inhibitor of autophagy.

CONCLUSIONS

These results suggest that TLR4 signalling executes the development of a myofibroblast phenotype after MI via autophagy and could be therapeutically exploited to improve outcome after myocardial injury.

The Functional Role of Zinc Finger E Box-Binding Homeobox 2 (Zeb2) in Promoting Cardiac Fibroblast Activation

Following cardiac injury, fibroblasts are activated and are termed as myofibroblasts, and these cells are key players in extracellular matrix (ECM) remodeling and fibrosis, itself a primary contributor to heart failure. Nutraceuticals have been shown to blunt cardiac fibrosis in both in-vitro and in-vivo studies. However, nutraceuticals have had conflicting results in clinical trials, and there are no effective therapies currently available to specifically target cardiac fibrosis. We have previously shown that expression of the zinc finger E box-binding homeobox 2 (Zeb2) transcription factor increases as fibroblasts are activated. We now show that Zeb2 plays a critical role in fibroblast activation. Zeb2 overexpression in primary rat cardiac fibroblasts is associated with significantly increased expression of embryonic smooth muscle myosin heavy chain (SMemb), ED-A fibronectin and α-smooth muscle actin (α-SMA). We found that Zeb2 was highly expressed in activated myofibroblast nuclei but not in the nuclei of inactive fibroblasts. Moreover, ectopic Zeb2 expression in myofibroblasts resulted in a significantly less migratory phenotype with elevated contractility, which are characteristics of mature myofibroblasts. Knockdown of Zeb2 with siRNA in primary myofibroblasts did not alter the expression of myofibroblast markers, which may indicate that Zeb2 is functionally redundant with other profibrotic transcription factors. These findings add to our understanding of the contribution of Zeb2 to the mechanisms controlling cardiac fibroblast activation.

Signaling Mechanisms of Myofibroblastic Activation: Outside-in and Inside-Out

Myofibroblasts are central mediators of fibrosis. Typically derived from resident fibroblasts, myofibroblasts represent a heterogeneous population of cells that are principally defined by acquired contractile function and high synthetic ability to produce extracellular matrix (ECM). Current literature sheds new light on the critical role of ECM signaling coupled with mechanotransduction in driving myofibroblastic activation. In particular, transforming growth factor β1 (TGF-β1) and extra domain A containing fibronectin (EDA-FN) are thought to be the primary ECM signaling mediators that form and also induce positive feedback loops. The outside-in and inside-out signaling circuits are transmitted and integrated by TGF-β receptors and integrins at the cell membrane, ultimately perpetuating the abundance and activities of TGF-β1 and EDA-FN in the ECM. In this review, we highlight these conceptual advances in understanding myofibroblastic activation, in hope of revealing its therapeutic anti-fibrotic implications.

Transcriptional cofactors Ski and SnoN are major regulators of the TGF-β/Smad signaling pathway in health and disease

The transforming growth factor-β (TGF-β) family plays major pleiotropic roles by regulating many physiological processes in development and tissue homeostasis. The TGF-β signaling pathway outcome relies on the control of the spatial and temporal expression of >500 genes, which depend on the functions of the Smad protein along with those of diverse modulators of this signaling pathway, such as transcriptional factors and cofactors. Ski (Sloan-Kettering Institute) and SnoN (Ski novel) are Smad-interacting proteins that negatively regulate the TGF-β signaling pathway by disrupting the formation of R-Smad/Smad4 complexes, as well as by inhibiting Smad association with the p300/CBP coactivators. The Ski and SnoN transcriptional cofactors recruit diverse corepressors and histone deacetylases to repress gene transcription. The TGF-β/Smad pathway and coregulators Ski and SnoN clearly regulate each other through several positive and negative feedback mechanisms. Thus, these cross-regulatory processes finely modify the TGF-β signaling outcome as they control the magnitude and duration of the TGF-β signals. As a result, any alteration in these regulatory mechanisms may lead to disease development. Therefore, the design of targeted therapies to exert tight control of the levels of negative modulators of the TGF-β pathway, such as Ski and SnoN, is critical to restore cell homeostasis under the specific pathological conditions in which these cofactors are deregulated, such as fibrosis and cancer.

Proteins that repress molecular signaling through the transforming growth factor-beta (TGF-β) pathway offer promising targets for treating cancer and fibrosis. Marina Macías-Silva and colleagues from the National Autonomous University of Mexico in Mexico City review the ways in which a pair of proteins, called Ski and SnoN, interact with downstream mediators of TGF-β to inhibit the effects of this master growth factor. Aberrant levels of Ski and SnoN have been linked to diverse range of diseases involving cell proliferation run amok, and therapies that regulate the expression of these proteins could help normalize TGF-β signaling to healthier physiological levels. For decades, drug companies have tried to target the TGF-β pathway, with limited success. Altering the activity of these repressors instead could provide a roundabout way of remedying pathogenic TGF-β activity in fibrosis and oncology.

Identification of a New Candidate Locus for Ebstein Anomaly in 1p36.2

Ebstein anomaly (EA) is a rare congenital heart defect (CHD) with a poorly characterized genetic etiology. However, some EA patients carry deletions in 1p36, all of which have been reported to carry distal deletions and share loss of the PRDM16 gene, which is currently considered the most likely candidate for EA development in this region. Here, we report a patient with an 11.96-Mb proximal 1p36 deletion, without loss of PRDM16, who presented with EA and a proximal deletion phenotype. This finding suggests that PRDM16 loss is not required for the development of EA in 1p36 deletions and that the loss of an additional proximal locus in 1p36 is also likely associated with EA. Our data suggest that a distal locus containing the SKI gene and a proximal locus containing the CHD-associated genes RERE and UBE4B are the most probable etiological factors for EA in patients with 1p36 deletion syndrome.

Periostin in cardiovascular disease and development: a tale of two distinct roles

Tissue development and homeostasis are dependent upon the concerted synthesis, maintenance, and degradation of extracellular matrix (ECM) molecules. Cardiac fibrosis is now recognized as a primary contributor to incidence of heart failure, particularly heart failure with preserved ejection fraction, wherein cardiac filling in diastole is compromised. Periostin is a cell-associated protein involved in cell fate determination, proliferation, tumorigenesis, and inflammatory responses. As a non-structural component of the ECM, secreted 90 kDa periostin is emerging as an important matricellular factor in cardiac mesenchymal tissue development. In addition, periostin's role as a mediator in cell-matrix crosstalk has also garnered attention for its association with fibroproliferative diseases in the myocardium, and for its association with TGF-β/BMP signaling. This review summarizes the phylogenetic history of periostin, its role in cardiac development, and the major signaling pathways influencing its expression in cardiovascular pathology. Further, we provide a synthesis of the current literature to distinguish the multiple roles of periostin in cardiac health, development and disease. As periostin may be targeted for therapeutic treatment of cardiac fibrosis, these insights may shed light on the putative timing for application of periostin-specific therapies.

Epithelial-to-mesenchymal transition transcription factors in cancer-associated fibroblasts

Beyond inducing epithelial‐to‐mesenchymal transcription (EMT), transcriptional factors of the Snail, ZEB and Twist families (EMT‐TFs) control global plasticity programmes affecting cell stemness and fate. Literature addressing the reactivation of these factors in adult tumour cells is very extensive, as they enable cancer cell plasticity and fuel both tumour initiation and metastatic spread. Incipient data reveal that EMT‐TFs are also expressed in fibroblasts, providing these with additional properties. Here, I will review recent reports on the expression of EMT‐TFs in cancer‐associated fibroblasts (CAFs). The new model suggests that EMT‐TFs can be envisioned as essential metastasis and chemoresistance‐promoting molecules, thereby enabling coordinated plasticity programmes in parenchyma and stroma–tumour compartments.

Developmental Reprogramming in Mesenchymal Stromal Cells of Human Subjects with Idiopathic Pulmonary Fibrosis

Cellular plasticity and de-differentiation are hallmarks of tissue/organ regenerative capacity in diverse species. Despite a more restricted capacity for regeneration, humans with age-related chronic diseases, such as cancer and fibrosis, show evidence of a recapitulation of developmental gene programs. We have previously identified a resident population of mesenchymal stromal cells (MSCs) in the terminal airways-alveoli by bronchoalveolar lavage (BAL) of human adult lungs. In this study, we characterized MSCs from BAL of patients with stable and progressive idiopathic pulmonary fibrosis (IPF), defined as <5% and ≥10% decline, respectively, in forced vital capacity over the preceding 6-month period. Gene expression profiles of MSCs from IPF subjects with progressive disease were enriched for genes regulating lung development. Most notably, genes regulating early tissue patterning and branching morphogenesis were differentially regulated. Network interactive modeling of a set of these genes indicated central roles for TGF-β and SHH signaling. Importantly, fibroblast growth factor-10 (FGF-10) was markedly suppressed in IPF subjects with progressive disease, and both TGF-β1 and SHH signaling were identified as critical mediators of this effect in MSCs. These findings support the concept of developmental gene re-activation in IPF, and FGF-10 deficiency as a potentially critical factor in disease progression.

The role of α-smooth muscle actin in fibroblast-mediated matrix contraction and remodeling

Cardiac myofibroblasts play an important role in myocardial remodeling. Although α-smooth muscle actin (α-SMA) expression is the hallmark of mature myofibroblasts, its role in regulating fibroblast function remains poorly understood. We explore the effects of the matrix environment in modulating cardiac fibroblast phenotype, and we investigate the role of α-SMA in fibroblast function using loss- and gain-of-function approaches. In murine myocardial infarction, infiltration of the infarct border zone with abundant α-SMA-positive myofibroblasts was associated with scar contraction. Isolated cardiac fibroblasts cultured in plates showed high α-SMA expression localized in stress fibers, exhibited activation of focal adhesion kinase (FAK), and synthesized large amounts of extracellular matrix proteins. In contrast, when these cells were cultured in collagen lattices, they exhibited marked reduction of α-SMA expression, negligible FAK activation, attenuated collagen synthesis, and increased transcription of genes associated with matrix metabolism. Transforming Growth Factor-β1-mediated contraction of fibroblast-populated collagen pads was associated with accentuated α-SMA synthesis. In contrast, serum- and basic Fibroblast Growth Factor-induced collagen pad contraction was associated with reduced α-SMA expression. α-SMA siRNA knockdown attenuated contraction of collagen pads populated with serum-stimulated cells. Surprisingly, α-SMA overexpression also reduced collagen pad contraction, suggesting that α-SMA is not sufficient to promote contraction of the matrix. Reduced contraction by α-SMA-overexpressing cells was associated with attenuated proliferative activity, in the absence of any effects on apoptosis. α-SMA may be implicated in contraction and remodeling of the extracellular matrix, but is not sufficient to induce contraction. α-SMA expression may modulate cellular functions, beyond its effects on contractility.

MicroRNA-145 inhibits hepatic stellate cell activation and proliferation by targeting ZEB2 through Wnt/β-catenin pathway

The activation of hepatic stellates cells (HSCs) is well believed to play a pivotal role in the development of liver fibrosis. MicroRNA-145 (miR-145) is known to suppress the progression of hepatocellular carcinoma, and is previously reported to be associated with Wnt/β-catenin pathway, but its role in the progression of hepatic fibrosis and activation of HSCs remains unknown and is warranted for investigation. In the present study, we found that the expression of miR-145 is significantly down-regulated in vivo in CCl4-induced mice liver fibrosis as well as in transforming growth factor-β1 (TGF-β1) induced HSC-T6 cell lines and human hepatic stellate cell line LX-2 in vitro. Furthermore, over-expression of miR-145 inhibited TGF-β1-induced the activation and proliferation of HSC-T6 cells in vitro. Mechanistically, we identified that zinc finger E-box-binding homeobox 2 (ZEB2), a key mediator of epithelial-to-mesenchymal transition, acted as a functional downstream target for miR-145. Interestingly, ZEB2 was shown to be involved in the TGF-β1-induced HSCs activation by regulating Wnt/β-catenin signaling pathway. Taken together, our results revealed the critical regulatory role of miR-145 in HSCs activation and implied miR-145 as a potential candidate for therapy of hepatic fibrosis by regulation of Wnt/β-catenin through targeting ZEB2.

TGFβ1 regulates Scleraxis expression in primary cardiac myofibroblasts by a Smad-independent mechanism

In cardiac wound healing following myocardial infarction (MI), relatively inactive resident cardiac fibroblasts phenoconvert to hypersynthetic/secretory myofibroblasts that produce large quantities of extracellular matrix (ECM) and fibrillar collagen proteins. Our laboratory and others have identified TGFβ1 as being a persistent stimulus in the chronic and inappropriate wound healing phase that is marked by hypertrophic scarring and eventual stiffening of the entire myocardium, ultimately leading to the pathogenesis of heart failure following MI. Ski is a potent negative regulator of TGFβ/Smad signaling with known antifibrotic effects. Conversely, Scleraxis is a potent profibrotic basic helix-loop-helix transcription factor that stimulates fibrillar collagen expression. We hypothesize that TGFβ1 induces Scleraxis expression by a novel Smad-independent pathway. Our data support the hypothesis that Scleraxis expression is induced by TGFβ1 through a Smad-independent pathway in the cardiac myofibroblast. Specifically, we demonstrate that TGFβ1 stimulates p42/44 (Erk1/2) kinases, which leads to increased Scleraxis expression. Inhibition of MEK1/2 using U0126 led to a sequential temporal reduction of phospho-p42/44 and subsequent Scleraxis expression. We also found that adenoviral Ski expression in primary myofibroblasts caused a significant repression of endogenous Scleraxis expression at both the mRNA and protein levels. Thus we have identified a novel TGFβ1-driven, Smad-independent, signaling cascade that may play an important role in regulating the fibrotic response in activated cardiac myofibroblasts following cardiac injury.

Gaining myocytes or losing fibroblasts: Challenges in cardiac fibroblast reprogramming for infarct repair

Unlike most somatic tissues, the heart possesses a very limited inherent ability to repair itself following damage. Attempts to therapeutically salvage the myocardium after infarction, either by sparing surviving myocytes or by injection of exogenous cells of varied provenance, have met with limited success. Cardiac fibroblasts are numerous, resistant to hypoxia, and amenable to phenotype reprogramming to cardiomyocytes - a potential panacea to an intractable problem. However, the long-term effects of mass conversion of fibroblasts are as-yet unknown. Since fibroblasts play key roles in normal cardiac function, treating these cells as a ready source of replacements for myocytes may have the effect of swapping one problem for another. This review briefly examines the roles of cardiac fibroblasts, recaps the strides made so far in their reprogramming to cardiomyocytes both in vitro and in vivo, and discusses the potential ramifications of large-scale cellular identity swapping. While such therapy offers great promise, the potential repercussions require consideration and careful study.

Autophagy is a regulator of TGF-β1-induced fibrogenesis in primary human atrial myofibroblasts

Transforming growth factor-β₁ (TGF-β₁) is an important regulator of fibrogenesis in heart disease. In many other cellular systems, TGF-β₁ may also induce autophagy, but a link between its fibrogenic and autophagic effects is unknown. Thus we tested whether or not TGF-β₁-induced autophagy has a regulatory function on fibrosis in human atrial myofibroblasts (hATMyofbs). Primary hATMyofbs were treated with TGF-β₁ to assess for fibrogenic and autophagic responses. Using immunoblotting, immunofluorescence and transmission electron microscopic analyses, we found that TGF-β₁ promoted collagen type Iα2 and fibronectin synthesis in hATMyofbs and that this was paralleled by an increase in autophagic activation in these cells. Pharmacological inhibition of autophagy by bafilomycin-A1 and 3-methyladenine decreased the fibrotic response in hATMyofb cells. ATG7 knockdown in hATMyofbs and ATG5 knockout (mouse embryonic fibroblast) fibroblasts decreased the fibrotic effect of TGF-β₁ in experimental versus control cells. Furthermore, using a coronary artery ligation model of myocardial infarction in rats, we observed increases in the levels of protein markers of fibrosis, autophagy and Smad2 phosphorylation in whole scar tissue lysates. Immunohistochemistry for LC3β indicated the localization of punctate LC3β with vimentin (a mesenchymal-derived cell marker), ED-A fibronectin and phosphorylated Smad2. These results support the hypothesis that TGF-β₁-induced autophagy is required for the fibrogenic response in hATMyofbs.

Ski diminishes TGF-β1-induced myofibroblast phenotype via up-regulating Meox2 expression

OBJECTIVE

The aim of the present work was to investigate the mechanism of transforming growth factor (TGF)-β1 and Sloan-Kettering Institute (Ski) in the pathogenesis of hypertrophic scars (HS).

BACKGROUND

Wound healing is an inherent process, but the aberrant wound healing of skin injury may lead to HS. There has been growing evidence suggesting a role for TGF-β1 and Ski in the pathogenesis of fibrosis.

MATERIAL AND METHODS

The MTT assay was used to detect the cell proliferation induced by TGF-β1. The Ski gene was transduced into cells with an adenovirus, and then the function of Ski in cell proliferation and differentiation was observed. Ski mRNA levels were measured by RT-PCR. Western blotting was used to detect the protein expression of α-SMA, E-cadherin, Meox1, Meox2, Zeb1 and Zeb2.

RESULTS

TGF-β1 can promote human skin fibroblast (HSF) cell proliferation in a time-dependent manner, but the promoting effect could be suppressed by Ski. TGF-β1 also induces the formation of the myofibroblast phenotype and the effect of TGF-β1 could be diminished by Ski. Also, Ski modulates the cardiac myofibroblast phenotype and function through suppression of Zeb2 by up-regulating the expression of Meox2.

CONCLUSIONS

Ski diminishes the myofibroblast phenotype induced by TGF-β1 through the suppression of Zeb2 by up-regulating the expression of Meox2.