Epigenetic regulation of wound healing and fibrosis

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

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

Human gingival fibroblasts display a non-fibrotic phenotype distinct from skin fibroblasts in three-dimensional cultures

Scar formation following skin injury can be a major psychosocial and physiological problem. However, the mechanisms of scar formation are still not completely understood. Previous studies have shown that wound healing in oral mucosa is faster, associates with a reduced inflammatory response and results to significantly reduced scar formation compared with skin wounds. In the present study, we hypothesized that oral mucosal fibroblasts from human gingiva are inherently distinct from fibroblasts from breast and abdominal skin, two areas prone to excessive scar formation, which may contribute to the preferential wound healing outcome in gingiva. To this end, we compared the phenotype of human gingival and skin fibroblasts cultured in in vivo-like three-dimensional (3D) cultures that mimic the cells' natural extracellular matrix (ECM) niche. To establish 3D cultures, five parallel fibroblast lines from human gingiva (GFBLs) and breast skin (SFBLs) were seeded in high density, and cultured for up to 21 days in serum and ascorbic acid containing medium to induce expression of wound-healing transcriptome and ECM deposition. Cell proliferation, morphology, phenotype and expression of wound healing and scar related genes were analyzed by real-time RT-PCR, Western blotting and immunocytochemical methods. The expression of a set of genes was also studied in three parallel lines of human abdominal SFBLs. Findings showed that GFBLs displayed morphologically distinct organization of the 3D cultures and proliferated faster than SFBLs. GFBLs expressed elevated levels of molecules involved in regulation of inflammation and ECM remodeling (MMPs) while SFBLs showed significantly higher expression of TGF-β signaling, ECM and myofibroblast and cell contractility-related genes. Thus, GFBLs display an inherent phenotype conducive for fast resolution of inflammation and ECM remodeling, characteristic for scar-free wound healing, while SFBLs have a profibrotic, scar-prone phenotype.

miR-218 regulates focal adhesion kinase-dependent TGFβ signaling in fibroblasts

Unlike other tissues in the body, the oral cavity does not scar. Myofibroblasts are the essential cell type in scar tissue formation; gingival fibroblasts are relatively unable to differentiate into myofibroblasts in response to TGFβ. This differential response is attributed to the relative lack of miR-218 in gingival fibroblasts.

Scarring, which occurs in essentially all adult tissue, is characterized by the excessive production and remodeling of extracellular matrix by α-smooth muscle actin (SMA)–expressing myofibroblasts located within connective tissue. Excessive scarring can cause organ failure and death. Oral gingivae do not scar. Compared to dermal fibroblasts, gingival fibroblasts are less responsive to transforming growth factor β (TGFβ) due to the reduced expression, due to the reduced expression and activity of focal adhesion kinase (FAK) by this cell type. Here we show that, compared with dermal fibroblasts, gingival fibroblasts show reduced expression of miR-218. Introduction of pre–miR-218 into gingival fibroblasts elevates FAK expression and, via a FAK/src-dependent mechanism, results in the ability of TGFβ to induce α-SMA. The deubiquitinase cezanne is a direct target of miR-218 and has increased expression in gingival fibroblasts compared with dermal fibroblasts. Knockdown of cezanne in gingival fibroblasts increases FAK expression and causes TGFβ to induce α-smooth muscle actin (α-SMA). These results suggest that miR-218 regulates the ability of TGFβ to induce myofibroblast differentiation in fibroblasts via cezanne/FAK.

MicroRNA profiling implicates the insulin-like growth factor pathway in bleomycin-induced pulmonary fibrosis in mice

BACKGROUND

Idiopathic pulmonary fibrosis is a disease characterized by alveolar epithelial cell injury, inflammatory cell infiltration and deposition of extracellular matrix in lung tissue. As mouse models of bleomycin-induced pulmonary fibrosis display many of the same phenotypes observed in patients with idiopathic pulmonary fibrosis, they have been used to study various aspects of the disease, including altered expression of microRNAs.

RESULTS

In this work, microRNA expression profiling of the lungs from treated C57BL/6J mice, relative to that of untreated controls, was undertaken to determine which alterations in microRNAs could in part regulate the fibrosis phenotype induced by bleomycin delivered through mini-osmotic pumps. We identified 11 microRNAs, including miR-21 and miR-34a, to be significantly differentially expressed (P < 0.01) in lungs of bleomycin treated mice and confirmed these data with real time PCR measurements. In situ hybridization of both miR-21 and miR-34a indicated that they were expressed in alveolar macrophages. Using a previously reported gene expression profile, we identified 195 genes to be both predicted targets of the 11 microRNAs and of altered expression in bleomycin-induced lung disease of C57BL/6J mice. Pathway analysis with these 195 genes indicated that altered microRNA expression may be associated with hepatocyte growth factor signaling, cholecystokinin/gastrin-mediated signaling, and insulin-like growth factor (IGF-1) signaling, among others, in fibrotic lung disease. The relevance of the IGF-1 pathway in this model was then demonstrated by showing lung tissue of bleomycin treated C57BL/6J mice had increased expression of Igf1 and that increased numbers of Igf-1 positive cells, predominantly in macrophages, were detected in the lungs.

CONCLUSIONS

We conclude that altered microRNA expression in macrophages is a feature which putatively influences the insulin-like growth factor signaling component of bleomycin-induced pulmonary fibrosis.

Phenotypic switching induced by damaged matrix is associated with DNA methyltransferase 3A (DNMT3A) activity and nuclear localization in smooth muscle cells (SMC)

Extracellular matrix changes are often crucial inciting events for fibroproliferative disease. Epigenetic changes, specifically DNA methylation, are critical factors underlying differentiated phenotypes. We examined the dependency of matrix-induced fibroproliferation and SMC phenotype on DNA methyltransferases. The cooperativity of matrix with growth factors, cell density and hypoxia was also examined. Primary rat visceral SMC of early passage (0–2) were plated on native collagen or damaged/heat-denatured collagen. Hypoxia was induced with 3% O₂ (balanced 5% CO₂ and 95% N₂) over 48 hours. Inhibitors were applied 2–3 hours after cells were plated on matrix, or immediately before hypoxia. Cells were fixed and stained for DNMT3A and smooth muscle actin (SMA) or smooth muscle myosin heavy chain. Illumina 450 K array of CpG sites was performed on bisulfite-converted DNA from smooth muscle cells on damaged matrix vs native collagen. Matrix exquisitely regulates DNMT3A localization and expression, and influences differentiation in SMCs exposed to denatured matrix +/− hypoxia. Analysis of DNA methylation signatures showed that Matrix caused significant DNA methylation alterations in a discrete number of CpG sites proximal to genes related to SMC differentiation. Matrix has a profound effect on the regulation of SMC phenotype, which is associated with altered expression, localization of DNMTs and discrete changes DNA methylation.

Enhancement of lysine acetylation accelerates wound repair

In physiopathological conditions, such as diabetes, wound healing is significantly compromised and chronic complications, including ulcers, may occur. In a mouse model of skin repair, we recently reported that wound treatment with Sirtuin activators and class I HDAC inhibitors induced keratinocyte proliferation and enhanced healing via a nitric oxide (NO) dependent mechanism. We observed an increase in total protein acetylation in the wound area, as determined by acetylation of α-tubulin and histone H3 Lysine 9. We reasoned that this process activated cell function as well as regulated gene expression to foster tissue repair. We report here that the direct activation of P300/CBP-associated factor (PCAF) by the histone acetylase activator pentadecylidenemalonate 1b (SPV-106) induced Lysine acetylation in the wound area. This intervention was sufficient to enhance repair process by a NO-independent mechanism. Hence, an impairment of PCAF and/or other GCN5 family acetylases may delay skin repair in physiopathological conditions.

The epigenetic feedback loop between DNA methylation and microRNAs in fibrotic disease with an emphasis on DNA methyltransferases

Epigenetic processes play a key regulatory role in many cancers. Recently, it also has been demonstrated to participate in fibrogenesis, especially in fibrotic disease. Fibrotic disease is a pathological response to tissue injury which can occur in any organ. Mechanisms that orchestrate fibrotic disorders in different organs are amazingly generic, involving generation of activated fibroblasts and myofibroblasts by differentiation processes that require extensive alterations in gene expression. Apart from genetic and environmental factors, epigenetic modifications including a combination of microRNAs and DNA methylation are supposed as regulatory mechanisms to control myofibroblast differentiation. It has become obvious that microRNAs, which act as regulators of gene expression at a post-transcriptional level, are differentially expressed in differentiating cells and play important roles in governing DNA methyltransferases (DNMTs) which are enzymes responsible for setting up and maintaining DNA methylation patterns at specific regions of the genome. Some microRNAs targeting DNMT transcripts lead to the demethylation and transcriptional activation of numerous protein coding gene sequences, thereby contributing to gene expression. Moreover, DNMTs also have a critical role in controlling some specific microRNA expression. This cooperative action among DNMTs, microRNAs and DNA methylation indicates that DNMTs may participate in the pathogenesis of myofibroblast differentiation through silencing of certain gene transcription. In this review, we summarize the current knowledge of a potential link between microRNA expression and DNA methylation on how DNMTs work in the process of fibrogenesis.

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

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

Function and fate of myofibroblasts after myocardial infarction

The importance of cardiac fibroblasts in the regulation of myocardial remodelling following myocardial infarction (MI) is becoming increasingly recognised. Studies over the last few decades have reinforced the concept that cardiac fibroblasts are much more than simple homeostatic regulators of extracellular matrix turnover, but are integrally involved in all aspects of the repair and remodelling of the heart that occurs following MI. The plasticity of fibroblasts is due in part to their ability to undergo differentiation into myofibroblasts. Myofibroblasts are specialised cells that possess a more contractile and synthetic phenotype than fibroblasts, enabling them to effectively repair and remodel the cardiac interstitium to manage the local devastation caused by MI. However, in addition to their key role in cardiac restoration and healing, persistence of myofibroblast activation can drive pathological fibrosis, resulting in arrhythmias, myocardial stiffness and progression to heart failure. The aim of this review is to give an appreciation of both the beneficial and detrimental roles of the myofibroblast in the remodelling heart, to describe some of the major regulatory mechanisms controlling myofibroblast differentiation including recent advances in the microRNA field, and to consider how this cell type could be exploited therapeutically.

Transforming growth factor-beta and the glomerular filtration barrier

The increasing burden of chronic kidney disease worldwide and recent advancements in the understanding of pathologic events leading to kidney injury have opened up new potential avenues for therapies to further diminish progression of kidney disease by targeting the glomerular filtration barrier and reducing proteinuria. The glomerular filtration barrier is affected by many different metabolic and immune-mediated injuries. Glomerular endothelial cells, the glomerular basement membrane, and podocytes—the three components of the filtration barrier—work together to prevent the loss of protein and at the same time allow passage of water and smaller molecules. Damage to any of the components of the filtration barrier can initiate proteinuria and renal fibrosis. Transforming growth factor-beta (TGF-β) is a pleiotropic cytokine strongly associated with the fibrogenic response. It has a known role in tubulointerstitial fibrosis. In this review we will highlight what is known about TGF-β and how it interacts with the components of glomerular filtration barrier and causes loss of function and proteinuria.