Morphogen pathways as molecular targets for the treatment of fibrosis in systemic sclerosis

Lysine Demethylase 1 Has Demethylase-Dependent and Non-Canonical Functions in Myofibroblast Activation in Systemic Sclerosis

Systemic sclerosis (SSc) is an autoimmune disease of unknown aetiology characterised by vasculopathy with progressive fibrosis of the skin and internal organs. Tissue fibrosis is driven by activated fibroblasts (myofibroblasts) with exacerbated contractile and secretory properties. We previously reported that the long non-coding RNA HOTAIR is a key driver of SSc fibroblast activation. HOTAIR interacts with the chromatin modifiers, the polycomb repressor complex (PRC2) and coREST complex, promoting expression of pro-fibrotic genes. In this study, we show that acute activation of dermal fibroblasts from healthy subjects or SSc patients with transforming growth factor-β and other fibrotic stimuli requires the activity of the lysine-specific demethylase 1 (LSD1) subunit of the co-REST complex. Unexpectedly, LSD1 catalytic activity plays a minor role in fibrotic gene expression in HOTAIR-overexpressing fibroblasts and in maintenance of the stable myofibroblast phenotype of SSc fibroblasts. However, silencing of LSD1 in SSc fibroblasts has a profound effect on pro-fibrotic gene expression, supporting a non-canonical scaffolding function. Our study shows for the first time an essential non-canonical role for LSD1 in pro-fibrotic gene expression in SSc; however, given that this function is insensitive to LSD1 inhibitors, the therapeutic opportunities will depend on future identification of a targetable mediator.

TGF-β1 Drives Integrin-Dependent Pericyte Migration and Microvascular Destabilization in Fibrotic Disease

Interactions between endothelial cells (ECs) and mural pericytes (PCs) are critical in maintaining the stability and function of the microvascular wall. Abnormal interactions between these two cell types are a hallmark of progressive fibrotic diseases such as systemic sclerosis (also known as scleroderma). However, the role of PCs in signaling microvascular dysfunction remains underexplored. We hypothesized that integrin-matrix interactions contribute to PC migration from the vascular wall and conversion into interstitial myofibroblasts. Herein, pro-inflammatory tumor necrosis factor α (TNFα) or a fibrotic growth factor [transforming growth factor β1 (TGF-β1)] were used to evaluate human PC inflammatory and fibrotic phenotypes by assessing their migration, matrix deposition, integrin expression, and subsequent effects on endothelial dysfunction. Both TNFα and TGF-β1 treatment altered integrin expression and matrix protein deposition, but only fibrotic TGF-β1 drove PC migration in an integrin-dependent manner. In addition, integrin-dependent PC migration was correlated to changes in EC angiopoietin-2 levels, a marker of vascular instability. Finally, there was evidence of changes in vascular stability corresponding to disease state in human systemic sclerosis skin. This work shows that TNFα and TGF-β1 induce changes in PC integrin expression and matrix deposition that facilitate migration and reduce vascular stability, providing evidence that microvascular destabilization can be an early indicator of tissue fibrosis.

Notch Signaling in Acute Inflammation and Sepsis

Notch signaling, a highly conserved pathway in mammals, is crucial for differentiation and homeostasis of immune cells. Besides, this pathway is also directly involved in the transmission of immune signals. Notch signaling per se does not have a clear pro- or anti-inflammatory effect, but rather its impact is highly dependent on the immune cell type and the cellular environment, modulating several inflammatory conditions including sepsis, and therefore significantly impacts the course of disease. In this review, we will discuss the contribution of Notch signaling on the clinical picture of systemic inflammatory diseases, especially sepsis. Specifically, we will review its role during immune cell development and its contribution to the modulation of organ-specific immune responses. Finally, we will evaluate to what extent manipulation of the Notch signaling pathway could be a future therapeutic strategy.

The role of the Notch pathway in the pathogenesis of systemic sclerosis: clinical implications

INTRODUCTION

Systemic sclerosis (SSc) is a chronic debilitating disease characterized by vascular insufficiency, widespread fibrosis and immune activation. Current understanding of its pathophysiology remains incomplete, which translates into inefficient therapies. Notch signaling is a central player in the development of physiological and pathological fibrosis not only in general but also in the context of SSc and is most likely involved in the vascular dysfunction that characterizes the disease.

AREAS COVERED

This review explores the role of the Notch pathway in the pathophysiology of SSc and the potential implications for the diagnosis, evaluation, and management of this yet incurable disease.

EXPERT OPINION

Although major issues still exist about the comprehension of SSc and the design of effective treatments, the knowledge of the role of the Notch pathway in fibrogenesis and vascular biology has shed light and enthusiasm over the field. Drugs that target components of Notch signaling are currently in development including already some in clinical trials. As such, Notch may become a very important topic in the near future (considering both the pathophysiology and treatment perspectives), not only in the context of SSc but also in the vascular-dependent fibrotic processes present in a multitude of diseases.

Targeting the Notch Signaling Pathway in Chronic Inflammatory Diseases

The Notch signaling pathway regulates developmental cell-fate decisions and has recently also been linked to inflammatory diseases. Although therapies targeting Notch signaling in inflammation in theory are attractive, their design and implementation have proven difficult, at least partly due to the broad involvement of Notch signaling in regenerative and homeostatic processes. In this review, we summarize the supporting role of Notch signaling in various inflammation-driven diseases, and highlight efforts to intervene with this pathway by targeting Notch ligands and/or receptors with distinct therapeutic strategies, including antibody designs. We discuss this in light of lessons learned from Notch targeting in cancer treatment. Finally, we elaborate on the impact of individual Notch members in inflammation, which may lay the foundation for development of therapeutic strategies in chronic inflammatory diseases.

Differential DNA Methylation Landscape in Skin Fibroblasts from African Americans with Systemic Sclerosis

The etiology and reasons underlying the ethnic disparities in systemic sclerosis (SSc) remain unknown. African Americans are disproportionally affected by SSc and yet are underrepresented in research. The aim of this study was to comprehensively investigate the association of DNA methylation levels with SSc in dermal fibroblasts from patients of African ancestry. Reduced representation bisulfite sequencing (RRBS) was performed on primary dermal fibroblasts from 15 SSc patients and 15 controls of African ancestry, and over 3.8 million CpG sites were tested for differential methylation patterns between cases and controls. The dermal fibroblasts from African American patients exhibited widespread reduced DNA methylation. Differentially methylated CpG sites were most enriched in introns and intergenic regions while depleted in 5' UTR, promoters, and CpG islands. Seventeen genes and eleven promoters showed significant differential methylation, mostly in non-coding RNA genes and pseudogenes. Gene set enrichment analysis (GSEA) and gene ontology (GO) analyses revealed an enrichment of pathways related to interferon signaling and mesenchymal differentiation. The hypomethylation of DLX5 and TMEM140 was accompanied by these genes' overexpression in patients but underexpression for lncRNA MGC12916. These data show that differential methylation occurs in dermal fibroblasts from African American patients with SSc and identifies novel coding and non-coding genes.

Cellular and molecular mechanisms in fibrosis

The activation of fibroblasts is required for physiological tissue remodelling such as wound healing. However, when the regulatory mechanisms are disrupted and fibroblasts remain persistently activated, the progressive deposition of extracellular matrix proteins leads to tissue fibrosis, which results in dysfunction or even loss of function of the affected organ. Although fibrosis has been recognized as a major cause of morbidity and mortality in modern societies, there are only few treatment options available that directly disrupt the release of extracellular matrix from fibroblasts. Intensive research in recent years, however, identified several pathways as core fibrotic mechanisms that are shared across different fibrotic diseases and organs. We discuss herein selection of those core pathways, especially downstream of the profibrotic TGF-β pathway, which are druggable and which may be transferable from bench to bedside.

Endothelial cells and endothelial progenitor cells in the pathogenesis of systemic sclerosis

Systemic sclerosis (SSc) is a connective tissue disease characterized by excessive fibrosis, microvasculopathy, and autoimmunity. Endothelial cell (EC) injury and subsequent endothelial cell dysfunction is believed to be an initial event that eventually leads to a vicious pathogenic cycle. This process is further enhanced by defective angiogenesis and vasculogenesis, as the vascular repair machinery does not work properly. Endothelial progenitor cells (EPCs) are functionally and quantitatively insufficient to recover the endothelium in SSc patients. The dysfunctional ECs and EPCs not only trigger the formation of typical vascular lesions, such as progressive intimal fibrosis in small arteries and the loss of capillaries, but also promote a series of inflammatory and profibrotic processes, such as endothelial-mesenchymal transition and recruitment and accumulation of monocytic EPCs with profibrotic properties. These processes together contribute to the accumulation of extracellular matrix in the affected tissue. This review features current insights into the roles of ECs and EPCs in the pathogenesis of SSc.

Role of microRNA in the pathogenesis of systemic sclerosis tissue fibrosis and vasculopathy

Systemic Sclerosis (SSc) pathogenesis involves multiple immunological, vascular and fibroproliferative abnormalities that contribute to a severe and complex clinical picture. Vasculopathy and fibroproliferative alterations are two hallmark pathological processes in SSc that are responsible for the most severe clinical manifestations of the disease and determine its clinical outcome and mortality. However, the pathogenesis of SSc vasculopathy and of the uncontrolled SSc fibrotic process remain incompletely understood. Recent investigations into the molecular pathways involved in these processes have identified an important role for epigenetic processes that contribute to overall disease progression and have emphasized microRNAs (miRNAs) as crucial epigenetic regulators. MiRNAs hold unique potential for elucidating SSc pathogenesis, improving diagnosis and developing effective targeted therapies for the disease. This review examines the important role that miRNAs play in the development and regulation of vascular and fibroproliferative alterations associated with SSc pathogenesis and their possible participation in the establishment of pathogenetic connections between these two processes. This review also emphasizes that further understanding of the involvement of miRNA in SSc fibrosis and vasculopathy will very likely provide novel future research directions and allow for the identification of groundbreaking therapeutic interventions within these processes. MiR-21, miR- 31, and miR-155 are of particular interest owing to their important involvement in both SSc vasculopathy and fibroproliferative alterations.

Endothelial to mesenchymal transition (EndoMT) in the pathogenesis of Systemic Sclerosis-associated pulmonary fibrosis and pulmonary arterial hypertension. Myth or reality?

Systemic Sclerosis (SSc) is a systemic autoimmune disease characterized by progressive fibrosis of skin and multiple internal organs and severe functional and structural microvascular alterations. SSc is considered to be the prototypic systemic fibrotic disorder. Despite currently available therapeutic approaches SSc has a high mortality rate owing to the development of SSc-associated interstitial lung disease (ILD) and pulmonary arterial hypertension (PAH), complications that have emerged as the most frequent causes of disability and mortality in SSc. The pathogenesis of the fibrotic process in SSc is complex and despite extensive investigation the exact mechanisms have remained elusive. Myofibroblasts are the cells ultimately responsible for tissue fibrosis and fibroproliferative vasculopathy in SSc. Tissue myofibroblasts in SSc originate from several sources including expansion of quiescent tissue fibroblasts and tissue accumulation of CD34+ fibrocytes. Besides these sources, myofibroblasts in SSc may result from the phenotypic conversion of endothelial cells into activated myofibroblasts, a process known as endothelial to mesenchymal transition (EndoMT). Recently, it has been postulated that EndoMT may play a role in the development of SSc-associated ILD and PAH. However, although several studies have described the occurrence of EndoMT in experimentally induced cardiac, renal, and pulmonary fibrosis and in several human disorders, the contribution of EndoMT to SSc-associated ILD and PAH has not been generally accepted. Here, the experimental evidence supporting the concept that EndoMT plays a role in the pathogenesis of SSc-associated ILD and PAH will be reviewed.

Cancer stem cell-vascular endothelial cell interactions in glioblastoma

Glioblastoma (GBM), a higher grade glial tumor, is highly aggressive, therapy resistant and often shows poor patient prognosis due to frequent recurrence. These features of GBM are attributed to presence of a significantly smaller proportion of glioma stem cells (GSCs) that are endowed with self-renewal ability, multi-potent nature and show resistance to therapy in patients. GSCs preferably take shelter close to tumor vasculature due to paracrine need of soluble factors secreted by endothelial cells (ECs) of vasculature. The physical proximity of GSCs to ECs creates a localized perivascular niche where mutual GSC-EC interactions regulate GSC stemness, migration, therapy resistance, and cellular kinetics during tumor growth. Together, perivascular niche presents a therapeutically targetable tumor structure for clinical management of GBM. Thus, understanding cellular and non-cellular components in perivascular niche is vital for designing in vitro and in vivo GBM tumor models. Here, we discuss the components and structure of tumor vascular niche and its impact on tumor progression.

Non-Canonical Wnt Predominates in Activated Rat Hepatic Stellate Cells, Influencing HSC Survival and Paracrine Stimulation of Kupffer Cells

The Wnt system is highly complex and is comprised of canonical and non-canonical pathways leading to the activation of gene expression. Our aim was to examine changes in the expression of Wnt ligands and regulators during hepatic stellate cell (HSC) transdifferentiation and assess the relative contributions of the canonical and non-canonical Wnt pathways in fibrogenic activated HSC. The expression profile of Wnt ligands and regulators in HSC was not supportive for a major role for β-catenin-dependent canonical Wnt signalling, this verified by inability to induce Topflash reporter activity in HSC even when expressing a constitutive active β-catenin. We detected expression of Wnt5a in activated HSC which can signal via non-canonical mechanisms and showed evidence for non-canonical signalling in these cells involving phosphorylation of Dvl2 and pJNK. Stimulation of HSC or Kupffer cells with Wnt5a regulated HSC apoptosis and expression of TGF-β1 and MCP1 respectively. We were unable to confirm a role for β-catenin-dependent canonical Wnt in HSC and instead propose autocrine and paracrine functions for Wnts expressed by activated HSC via non-canonical pathways. The data warrant detailed investigation of Wnt5a in liver fibrosis.

Fibrogenesis, novel lessons from animal models

Systemic sclerosis (SSc) is a devastating chronic autoimmune connective tissue disease characterized by vasculopathy, autoimmunity with inflammation, and progressive fibrogenesis. The current paradigm of the pathogenesis of SSc is that of an unknown initial trigger, leading to a complex interaction of immune cells, endothelial cells, and fibroblasts, producing cytokines, growth and angiogenic factors, and resulting in uncontrolled and persistent tissue fibrogenesis by an altered mesenchymal cell compartment. Animal models are of utmost importance to investigate the different steps in the pathogenesis. This review will elaborate on recent findings in established and more recently developed animal models, presenting data on compounds that are in or ready to be translated into clinical trials, or provide interesting new findings in the understanding of the pathophysiology of SSc. We focus on recent findings concerning the vessel-extracellular matrix interaction, the initial triggering aggressor, the concept of autoimmunity and inflammatory changes, the effector cells and their origins, and the complex interaction of the different signaling pathways in fibrogenesis.

Results of the 4th scientific workshop of the ECCO (I): pathophysiology of intestinal fibrosis in IBD

The fourth scientific workshop of the European Crohn's and Colitis Organization (ECCO) focused on the relevance of intestinal fibrosis in the disease course of inflammatory bowel disease (IBD). The objective was to better understand the pathophysiological mechanisms of intestinal fibrosis, to identify useful markers and imaging modalities of fibrosis in order to assess its presence and progression, and, finally, to point out possible approaches for the prevention and the treatment of fibrosis. The results of this workshop are presented in three separate manuscripts. This first section describes the most important mechanisms that contribute to the initiation and progression of intestinal fibrosis in IBD including the cellular and molecular mediators, the extracellular matrix molecules and matrix metalloproteinases/tissue inhibitors of metalloproteinases-system, the microbiota products, the role of fat, genetic and epigenetic factors, as well as the currently available experimental models. Furthermore, it identifies unanswered questions in the field of intestinal fibrosis and provides a framework for future research.

The role of microRNAs in skin fibrosis

Fibrotic skin disorders may be debilitating and impair quality of life. There are few effective treatment options for cutaneous fibrotic diseases. In this review, we discuss our current understanding of the role of microRNAs (miRNAs) in skin fibrosis. miRNAs are a class of small, non-coding RNAs involved in skin fibrosis. These small RNAs range from 18 to 25 nucleotides in length and modify gene expression by binding to target messenger RNA (mRNA), causing degradation of the target mRNA or inhibiting the translation into proteins. We present an overview of the biogenesis, maturation and function of miRNAs. We highlight miRNA’s role in key skin fibrotic processes including: transforming growth factor-beta signaling, extracellular matrix deposition, and fibroblast proliferation and differentiation. Some miRNAs are profibrotic and their upregulation favors these processes contributing to fibrosis, while anti-fibrotic miRNAs inhibit these processes and may be reduced in fibrosis. Finally, we describe the diagnostic and therapeutic significance of miRNAs in the management of skin fibrosis. The discovery that miRNAs are detectable in serum, plasma, and other bodily fluids, and are relatively stable, suggests that miRNAs may serve as valuable biomarkers to monitor disease progression and response to treatment. In the treatment of skin fibrosis, antifibrotic miRNAs may be upregulated using mimics and viral vectors. Conversely, profibrotic miRNAs may be downregulated by employing anti-miRNAs, sponges, erasers and masks. We believe that miRNA-based therapies hold promise as important treatments and may transform the management of fibrotic skin diseases by physicians.

Combined inhibition of morphogen pathways demonstrates additive antifibrotic effects and improved tolerability

OBJECTIVES

The morphogen pathways Hedgehog, Wnt and Notch are attractive targets for antifibrotic therapies in systemic sclerosis. Interference with stem cell regeneration, however, may complicate the use of morphogen pathway inhibitors. We therefore tested the hypothesis that combination therapies with low doses of Hedgehog, Wnt and Notch inhibitors maybe safe and effective for the treatment of fibrosis.

METHODS

Skin fibrosis was induced by bleomycin and by overexpression of a constitutively active TGF-β receptor type I. Adverse events were assessed by clinical monitoring, pathological evaluation and quantification of Lgr5-positive intestinal stem cells.

RESULTS

Inhibition of Hedgehog, Wnt and Notch signalling dose-dependently ameliorated bleomycin-induced and active TGF-β receptor type I-induced fibrosis. Combination therapies with low doses of Hedgehog/Wnt inhibitors or Hedgehog/Notch inhibitors demonstrated additive antifibrotic effects in preventive as well as in therapeutic regimes. Combination therapies were well tolerated. In contrast with high dose monotherapies, combination therapies did not reduce the number of Lgr5 positive intestinal stem cells.

CONCLUSIONS

Combined inhibition of morphogen pathways exerts additive antifibrotic effects. Combination therapies are well tolerated and, in contrast to high dose monotherapies, may not impair stem cell renewal. Combined targeting of morphogen pathways may thus help to overcome dose-limiting toxicity of Hedgehog, Wnt and Notch signalling.

The Wnt antagonists DKK1 and SFRP1 are downregulated by promoter hypermethylation in systemic sclerosis

OBJECTIVES

Activated Wnt signalling with decreased expression of endogenous inhibitors has recently been characterised as a central pathomechanism in systemic sclerosis (SSc). Aberrant epigenetic modifications also contribute to the persistent activation of SSc fibroblasts. We investigated whether increased Wnt signalling and epigenetic changes in SSc are causally linked via promoter hypermethylation-induced silencing of Wnt antagonists.

METHODS

The methylation status of endogenous Wnt antagonists in leucocytes and fibroblasts was evaluated by methylation-specific PCR. 5-aza-2'-deoxycytidine was used to inhibit DNA methyltransferases (Dnmts) in cultured fibroblasts and in the mouse model of bleomycin-induced skin fibrosis. Activation of Wnt signalling was assessed by analysing Axin2 mRNA levels and by staining for β-catenin.

RESULTS

The promoters of DKK1 and SFRP1 were hypermethylated in fibroblasts and peripheral blood mononuclear cells of patients with SSc. Promoter hypermethylation resulted in impaired transcription and decreased expression of DKK1 and SFRP1 in SSc. Treatment of SSc fibroblasts or bleomycin-challenged mice with 5-aza prevented promoter methylation-induced silencing and increased the expression of both genes to normal levels. Reactivation of DKK1 and SFRP1 transcription by 5-aza inhibited canonical Wnt signalling in vitro and in vivo and effectively ameliorated experimental fibrosis.

CONCLUSIONS

We demonstrate that hypermethylation of the promoters of DKK1 and SFRP1 contributes to aberrant Wnt signalling in SSc and that Dnmt inhibition effectively reduces Wnt signalling. These data provide a novel link between epigenetic alterations and increased Wnt signalling in SSc and also have translational implications because Dnmt inhibitors are already approved for clinical use.

Cross-talk between Notch and Hedgehog regulates hepatic stellate cell fate in mice

Liver repair involves phenotypic changes in hepatic stellate cells (HSCs) and reactivation of morphogenic signaling pathways that modulate epithelial-to-mesenchymal/mesenchymal-to-epithelial transitions, such as Notch and Hedgehog (Hh). Hh stimulates HSCs to become myofibroblasts (MFs). Recent lineage tracing studies in adult mice with injured livers showed that some MFs became multipotent progenitors to regenerate hepatocytes, cholangiocytes, and HSCs. We studied primary HSC cultures and two different animal models of fibrosis to evaluate the hypothesis that activating the Notch pathway in HSCs stimulates them to become (and remain) MFs through a mechanism that involves an epithelial-to-mesenchymal-like transition and requires cross-talk with the canonical Hh pathway. We found that when cultured HSCs transitioned into MFs, they activated Hh signaling, underwent an epithelial-to-mesenchymal-like transition, and increased Notch signaling. Blocking Notch signaling in MFs/HSCs suppressed Hh activity and caused a mesenchymal-to-epithelial-like transition. Inhibiting the Hh pathway suppressed Notch signaling and also induced a mesenchymal-to-epithelial-like transition. Manipulating Hh and Notch signaling in a mouse multipotent progenitor cell line evoked similar responses. In mice, liver injury increased Notch activity in MFs and Hh-responsive MF progeny (i.e., HSCs and ductular cells). Conditionally disrupting Hh signaling in MFs of bile-duct-ligated mice inhibited Notch signaling and blocked accumulation of both MF and ductular cells.

CONCLUSIONS

The Notch and Hedgehog pathways interact to control the fate of key cell types involved in adult liver repair by modulating epithelial-to-mesenchymal-like/mesenchymal-to-epithelial-like transitions.

Role of endothelial to mesenchymal transition in the pathogenesis of the vascular alterations in systemic sclerosis

The pathogenesis of Systemic Sclerosis (SSc) is extremely complex, and despite extensive studies, the exact mechanisms involved are not well understood. Numerous recent studies of early events in SSc pathogenesis have suggested that unknown etiologic factors in a genetically receptive host trigger structural and functional microvascular endothelial cell abnormalities. These alterations result in the attraction, transmigration, and accumulation of immune and inflammatory cells in the perivascular tissues, which in turn induce the phenotypic conversion of endothelial cells and quiescent fibroblasts into activated myofibroblasts, a process known as endothelial to mesenchymal transition or EndoMT. The activated myofibroblasts are the effector cells responsible for the severe and frequently progressive fibrotic process and the fibroproliferative vasculopathy that are the hallmarks of SSc. Thus, according to this hypothesis the endothelial and vascular alterations, which include the phenotypic conversion of endothelial cells into activated myofibroblasts, play a crucial role in the development of the progressive fibrotic process affecting skin and multiple internal organs. The role of endothelial cell and vascular alterations, the potential contribution of endothelial to mesenchymal cell transition in the pathogenesis of the tissue fibrosis, and fibroproliferative vasculopathy in SSc will be reviewed here.