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

Periostin regulates lysyl oxidase through ERK1/2 MAPK-dependent serum response factor in activated cardiac fibroblasts

Collagen crosslinking, mediated by lysyl oxidase, is an adaptive mechanism of the cardiac repair process initiated by cardiac fibroblasts postmyocardial injury. However, excessive crosslinking leads to cardiac wall stiffening, which impairs the contractile properties of the left ventricle and leads to heart failure. In this study, we investigated the role of periostin, a matricellular protein, in the regulation of lysyl oxidase in cardiac fibroblasts in response to angiotensin II and TGFβ1. Our results indicated that periostin silencing abolished the angiotensin II and TGFβ1-mediated upregulation of lysyl oxidase. Furthermore, the attenuation of periostin expression resulted in a notable reduction in the activity of lysyl oxidase. Downstream of periostin, ERK1/2 MAPK signaling was found to be activated, which in turn transcriptionally upregulates the serum response factor to facilitate the enhanced expression of lysyl oxidase. The periostin-lysyl oxidase association was also positively correlated in an in vivo rat model of myocardial infarction. The expression of periostin and lysyl oxidase was upregulated in the collagen-rich fibrotic scar tissue of the left ventricle. Remarkably, echocardiography data showed a reduction in the left ventricular wall movement, ejection fraction, and fractional shortening, indicative of enhanced stiffening of the cardiac wall. These findings shed light on the mechanistic role of periostin in the collagen crosslinking initiated by activated cardiac fibroblasts. Our findings signify periostin as a possible therapeutic target to reduce excessive collagen crosslinking that contributes to the structural remodeling associated with heart failure.

Human umbilical cord/placenta mesenchymal stem cell conditioned medium attenuates intestinal fibrosis in vivo and in vitro

BACKGROUND

A significant unmet need in inflammatory bowel disease is the lack of anti-fibrotic agents targeting intestinal fibrosis. This study aimed to investigate the anti-fibrogenic properties and mechanisms of the conditioned medium (CM) from human umbilical cord/placenta-derived mesenchymal stem cells (UC/PL-MSC-CM) in a murine intestinal fibrosis model and human primary intestinal myofibroblasts (HIMFs).

METHODS

UC/PL-MSC-CM was concentrated 15-fold using a 3 kDa cut-off filter. C57BL/6 mice aged 7 weeks old were randomly assigned to one of four groups: (1) control, (2) dextran sulfate sodium (DSS), (3) DSS + CM (late-phase treatment), and (4) DSS + CM (early-phase treatment). Chronic DSS colitis and intestinal fibrosis was induced by three cycles of DSS administration. One DSS cycle consisted of 7 days of oral DSS administration (1.75%, 2%, and 2.5% DSS), followed by 14 days of drinking water. UC/PL-MSC-CM was intraperitoneally administered in the late phase (from day 50, 10 times) or early phase (from day 29, 10 times) of DSS cycles. HIMFs were treated with TGF-β1 and co-treated with UC/PL-MSC-CM (10% of culture media) in the cellular model.

RESULTS

In the animal study, UC/PL-MSC-CM reduced submucosa/muscularis propria thickness and collagen deposition, which improved intestinal fibrosis in chronic DSS colitis. The UC/PL-MSC-CM significantly reduced the expressions of procollagen1A1 and α-smooth muscle actin, which DSS significantly elevated. The anti-fibrogenic effect was more apparent in the UC-MSC-CM or early-phase treatment model. The UC/PL-MSC-CM reduced procollagen1A1, fibronectin, and α-smooth muscle actin expression in HIMFs in the cellular model. The UC/PL-MSC-CM downregulated fibrogenesis by suppressing RhoA, MRTF-A, and SRF expression.

CONCLUSIONS

Human UC/PL-MSC-CM inhibits TGF-β1-induced fibrogenic activation in HIMFs by blocking the Rho/MRTF/SRF pathway and chronic DSS colitis-induced intestinal fibrosis. Thus, it may be regarded as a novel candidate for stem cell-based therapy of intestinal fibrosis.

A defective mechanosensing pathway affects fibroblast-to-myofibroblast transition in the old male mouse heart

The cardiac fibroblast interacts with an extracellular matrix (ECM), enabling myofibroblast maturation via a process called mechanosensing. Although in the aging male heart, ECM is stiffer than in the young mouse, myofibroblast development is impaired, as demonstrated in 2-D and 3-D experiments. In old male cardiac fibroblasts, we found a decrease in actin polymerization, α-smooth muscle actin (α-SMA), and Kindlin-2 expressions, the latter an effector of the mechanosensing. When Kindlin-2 levels were manipulated via siRNA interference, young fibroblasts developed an old-like fibroblast phenotype, whereas Kindlin-2 overexpression in old fibroblasts reversed the defective phenotype. Finally, inhibition of overactivated extracellular regulated kinases 1 and 2 (ERK1/2) in the old male fibroblasts rescued actin polymerization and α-SMA expression. Pathological ERK1/2 overactivation was also attenuated by Kindlin-2 overexpression. In contrast, old female cardiac fibroblasts retained an operant mechanosensing pathway. In conclusion, we identified defective components of the Kindlin/ERK/actin/α-SMA mechanosensing axis in aged male fibroblasts.

Grb2-related adaptor protein GRAP is a novel regulator of liver fibrosis

AIMS

Excessive liver fibrosis is frequently observed in chronic liver diseases and associated with decline of liver functions. Hepatic stellate cells (HSCs) are considered the principal mediator of liver fibrosis by trans-differentiating into myofibroblasts. In the present study we investigated the role of Grb2-related adaptor protein (GRAP) in HSC activation and liver fibrosis.

METHODS AND MATERIALS

Liver fibrosis was induced by carbon tetrachloride (CCl4) injection. Gene expression was examined by quantitative PCR. Cell proliferation was evaluated by EdU incorporation. DNA-protein interaction was examined by chromatin immunoprecipitation (ChIP).

KEY FINDINGS

GRAP expression was up-regulated during HSC-myofibroblast transition both in vivo and in vitro. Mechanistically, serum response factor (SRF) and myocardin-related transcription factor A (MRTF-A) formed a complex to bind to the GRAP promoter and activate GRAP transcription. Small interfering RNA (siRNA) mediated GRAP silencing blocked HSC-myofibroblast transition in vitro. Importantly, adeno-associated virus 6 (AAV6) mediated GRAP knockdown in myofibroblasts attenuated liver fibrosis in mice. Of note, inhibition of ERK signaling abrogated enhancement of HSC-myofibroblast transition by GRAP over-expression.

SIGNIFICANCE

Our data suggest that GRAP, possibly via ERK activation, regulates HSC-myofibroblast transition and contributes to liver fibrosis. Screening for small-molecule GRAP inhibitors may yield novel therapeutic solutions against liver fibrosis.

Pharmacogenomic Analyses Implicate B Cell Developmental Status and MKL1 as Determinants of Sensitivity toward Anti-CD20 Monoclonal Antibody Therapy

Monoclonal antibody (mAb) therapy directed against CD20 is an important tool in the treatment of B cell disorders. However, variable patient response and acquired resistance remain important clinical challenges. To identify genetic factors that may influence sensitivity to treatment, the cytotoxic activity of three CD20 mAbs: rituximab; ofatumumab; and obinutuzumab, were screened in high-throughput assays using 680 ethnically diverse lymphoblastoid cell lines (LCLs) followed by a pharmacogenomic assessment. GWAS analysis identified several novel gene candidates. The most significant SNP, rs58600101, in the gene MKL1 displayed ethnic stratification, with the variant being significantly more prevalent in the African cohort and resulting in reduced transcript levels as measured by qPCR. Functional validation of MKL1 by shRNA-mediated knockdown of MKL1 resulted in a more resistant phenotype. Gene expression analysis identified the developmentally associated TGFB1I1 as the most significant gene associated with sensitivity. qPCR among a panel of sensitive and resistant LCLs revealed immunoglobulin class-switching as well as differences in the expression of B cell activation markers. Flow cytometry showed heterogeneity within some cell lines relative to surface Ig isotype with a shift to more IgG⁺ cells among the resistant lines. Pretreatment with prednisolone could partly reverse the resistant phenotype. Results suggest that the efficacy of anti-CD20 mAb therapy may be influenced by B cell developmental status as well as polymorphism in the MKL1 gene. A clinical benefit may be achieved by pretreatment with corticosteroids such as prednisolone followed by mAb therapy.

Serum response factor activates peroxidasin transcription to block senescence of hepatic stellate cells

AIMS

Aberrant liver fibrosis is a hallmark event in end-stage liver diseases. Hepatic stellate cells (HSCs) are considered the major source of myofibroblasts in the liver that produce extracellular matrix proteins to promote liver fibrosis. HSCs undergo senescence in response to various stimuli, a process that can be exploited to dampen liver fibrosis. We investigated the role of serum response factor (SRF) in this process.

METHODS AND MATERIALS

Senescence was induced HSCs by serum withdrawal or progressive passage. DNA-protein interaction was evaluated by chromatin immunoprecipitation (ChIP).

RESULTS

SRF expression was down-regulated in HSCs entering into senescence. Coincidently, SRF depletion by RNAi accelerated HSC senescence. Of note, treatment of an anti-oxidant (N-acetylcysteine or NAC) blocked HSC senescence by SRF deficiency suggesting that SRF may antagonize HSC senescence by eliminating excessive reactive oxygen species (ROS). PCR-array based screening identified peroxidasin (PXDN) as a potential target for SRF in HSCs. PXDN expression was inversely correlated with HSC senescence whereas PXDN knockdown accelerated HSC senescence. Further analysis reveals that SRF directly bound to the PXDN promoter and activated PXDN transcription. Consistently, PXDN over-expression protected whereas PXDN depletion amplified HSC senescence. Finally, PXDN knockout mice displayed diminished liver fibrosis compared to wild type mice when subjected to bile duct ligation (BDL).

SIGNIFICANCE

Our data suggest that SRF, via its downstream target PXDN, plays a key role in regulating HSC senescence.

An MRTF-A-ZEB1-IRF9 axis contributes to fibroblast-myofibroblast transition and renal fibrosis

Myofibroblasts, characterized by the expression of the matricellular protein periostin (Postn), mediate the profibrogenic response during tissue repair and remodeling. Previous studies have demonstrated that systemic deficiency in myocardin-related transcription factor A (MRTF-A) attenuates renal fibrosis in mice. In the present study, we investigated the myofibroblast-specific role of MRTF-A in renal fibrosis and the underlying mechanism. We report that myofibroblast-specific deletion of MRTF-A, achieved through crossbreeding Mrtfa-flox mice with Postn-Cre^ERT2 mice, led to amelioration of renal fibrosis. RNA-seq identified zinc finger E-Box binding homeobox 1 (Zeb1) as a downstream target of MRTF-A in renal fibroblasts. MRTF-A interacts with TEA domain transcription factor 1 (TEAD1) to bind to the Zeb1 promoter and activate Zeb1 transcription. Zeb1 knockdown retarded the fibroblast-myofibroblast transition (FMyT) in vitro and dampened renal fibrosis in mice. Transcriptomic assays showed that Zeb1 might contribute to FMyT by repressing the transcription of interferon regulatory factor 9 (IRF9). IRF9 knockdown overcame the effect of Zeb1 depletion and promoted FMyT, whereas IRF9 overexpression antagonized TGF-β-induced FMyT. In conclusion, our data unveil a novel MRTF-A-Zeb1-IRF9 axis that can potentially contribute to fibroblast-myofibroblast transition and renal fibrosis. Screening for small-molecule compounds that target this axis may yield therapeutic options for the mollification of renal fibrosis.

Mechanical stress regulates the mechanotransduction and metabolism of cardiac fibroblasts in fibrotic cardiac diseases

Fibrotic cardiac diseases are characterized by myocardial fibrosis that results in maladaptive cardiac remodeling. Cardiac fibroblasts (CFs) are the main cell type responsible for fibrosis. In response to stress or injury, intrinsic CFs develop into myofibroblasts and produce excess extracellular matrix (ECM) proteins. Myofibroblasts are mechanosensitive cells that can detect changes in tissue stiffness and respond accordingly. Previous studies have revealed that some mechanical stimuli control fibroblast behaviors, including ECM formation, cell migration, and other phenotypic traits. Further, metabolic alteration is reported to regulate fibrotic signaling cascades, such as the transforming growth factor-β pathway and ECM deposition. However, the relationship between metabolic changes and mechanical stress during fibroblast-to-myofibroblast transition remains unclear. This review aims to elaborate on the crosstalk between mechanical stress and metabolic changes during the pathological transition of cardiac fibroblasts.

Transforming Growth Factor-β Receptor-Mediated, p38 Mitogen-Activated Protein Kinase-Dependent Signaling Drives Enhanced Myofibroblast Differentiation during Skin Wound Healing in Mice Lacking Hyaluronan Synthases 1 and 3

Normal myofibroblast differentiation is critical for proper skin wound healing. Neoexpression of α-smooth muscle actin (α-SMA), a marker for myofibroblast differentiation, is driven by transforming growth factor (TGF)-β receptor-mediated signaling. Hyaluronan and its three synthesizing enzymes, hyaluronan synthases (Has 1, 2, and 3), also participate in this process. Closure of skin wounds is significantly accelerated in Has1/3 double-knockout (Has1/3-null) mice. Herein, TGF-β activity and dermal collagen maturation were increased in Has1/3-null healing skin. Cultures of primary skin fibroblasts isolated from Has1/3-null mice had higher levels of TGF-β activity, α-SMA expression, and phosphorylation of p38 mitogen-activated protein kinase at Thr180/Tyr182, compared with wild-type fibroblasts. p38α mitogen-activated protein kinase was a necessary element in a noncanonical TGF-β receptor signaling pathway driving α-SMA expression in Has1/3-null fibroblasts. Myocardin-related transcription factor (MRTF), a cofactor that binds to the transcription factor serum response factor (SRF), was also critical. Nuclear localization of MRTF was increased, and MRTF binding to SRF was enhanced in Has1/3-null fibroblasts. Inhibition of MRTF or SRF expression by RNA interference suppresses α-SMA expression at baseline and diminished its overexpression in Has1/3-null fibroblasts. Interestingly, total matrix metalloproteinase activity was increased in healing skin and fibroblasts from Has1/3-null mice, possibly explaining the increased TGF-β activation.

The actin cytoskeleton: Morphological changes in pre- and fully developed lung cancer

Actin, a primary component of the cell cytoskeleton can have multiple isoforms, each of which can have specific properties uniquely suited for their purpose. These monomers are then bound together to form polymeric filaments utilizing adenosine triphosphate hydrolysis as a source of energy. Proteins, such as Arp2/3, VASP, formin, profilin, and cofilin, serve important roles in the polymerization process. These filaments can further be linked to form stress fibers by proteins called actin-binding proteins, such as α-actinin, myosin, fascin, filamin, zyxin, and epsin. These stress fibers are responsible for mechanotransduction, maintaining cell shape, cell motility, and intracellular cargo transport. Cancer metastasis, specifically epithelial mesenchymal transition (EMT), which is one of the key steps of the process, is accompanied by the formation of thick stress fibers through the Rho-associated protein kinase, MAPK/ERK, and Wnt pathways. Recently, with the advent of "field cancerization," pre-malignant cells have also been demonstrated to possess stress fibers and related cytoskeletal features. Analytical methods ranging from western blot and RNA-sequencing to cryo-EM and fluorescent imaging have been employed to understand the structure and dynamics of actin and related proteins including polymerization/depolymerization. More recent methods involve quantifying properties of the actin cytoskeleton from fluorescent images and utilizing them to study biological processes, such as EMT. These image analysis approaches exploit the fact that filaments have a unique structure (curvilinear) compared to the noise or other artifacts to separate them. Line segments are extracted from these filament images that have assigned lengths and orientations. Coupling such methods with statistical analysis has resulted in development of a new reporter for EMT in lung cancer cells as well as their drug responses.

Prevention of bleomycin-induced lung fibrosis via inhibition of the MRTF/SRF transcription pathway

Bleomycin-induced lung fibrosis is a debilitating disease, linked to high morbidity and mortality in chemotherapy patients. The MRTF/SRF transcription pathway has been proposed as a potential therapeutic target, as it is critical for myofibroblast differentiation, a hallmark of fibrosis. In human lung fibroblasts, the MRTF/SRF pathway inhibitor, CCG-257081, effectively decreased mRNA levels of downstream genes: smooth muscle actin and connective tissue growth factor, with IC₅₀ s of 4 and 15 μM, respectively. The ability of CCG-257081 to prevent inflammation and fibrosis, measured via pulmonary collagen content and histopathology, was tested in a murine model of bleomycin-induced lung fibrosis. Animals were given intraperitoneal bleomycin for 4 weeks and concurrently dosed with CCG-257081 (0, 10, 30, and 100 mg/kg PO), a clinical anti-fibrotic (nintedanib) or the clinical standard of care (prednisolone). Mice treated with 100 mg/kg CCG-257081 gained weight vs. vehicle-treated control mice, while those receiving nintedanib and prednisolone lost significant weight. Hydroxyproline content and histological findings in tissue of animals on 100 mg/kg CCG-257081 were not significantly different from naive tissue, indicating successful prevention. Measures of tissue fibrosis were comparable between CCG-257081 and nintedanib, but only the MRTF/SRF inhibitor decreased plasminogen activator inhibitor-1 (PAI-1), a marker linked to fibrosis, in bronchoalveolar lavage fluid. In contrast, prednisolone led to marked increases in lung fibrosis by all metrics. This study demonstrates the potential use of MRTF/SRF inhibitors to prevent bleomycin-induced lung fibrosis in a clinically relevant model of the disease.

CD16+ fibroblasts foster a trastuzumab-refractory microenvironment that is reversed by VAV2 inhibition

The leukocyte Fcγ receptor (FcγR)-mediated response is important for the efficacy of therapeutic antibodies; however, little is known about the role of FcγRs in other cell types. Here we identify a subset of fibroblasts in human breast cancer that express CD16 (FcγRIII). An abundance of these cells in HER2⁺ breast cancer patients is associated with poor prognosis and response to trastuzumab. Functionally, upon trastuzumab stimulation, CD16⁺ fibroblasts reduce drug delivery by enhancing extracellular matrix stiffness. Interaction between trastuzumab and CD16 activates the intracellular SYK-VAV2-RhoA-ROCK-MLC2-MRTF-A pathway, leading to elevated contractile force and matrix production. Targeting of a Rho family guanine nucleotide exchange factor, VAV2, which is indispensable for the function of CD16 in fibroblasts rather than leukocytes, reverses desmoplasia provoked by CD16⁺ fibroblasts. Collectively, our study reveals a role for the fibroblast FcγR in drug resistance, and suggests that VAV2 is an attractive target to augment the effects of antibody treatments.

Interplay of the transcription factor MRTF-A and matrix stiffness controls mammary acinar structure and protrusion formation

Background

Ongoing differentiation processes characterize the mammary gland during sexual development and reproduction. In contrast, defective remodelling is assumed to be causal for breast tumorigenesis. We have shown recently that the myocardin-related transcription factor A (MRTF-A) is essential for forming regular hollow acinar structures. Moreover, MRTF-A activity is known to depend on the biochemical and physical properties of the surrounding extracellular matrix. In this study we analysed the mutual interaction of different matrix stiffnesses and MRTF-A activities on formation and maintenance of mammary acini.

Methods

Human MCF10A acini and primary mature organoids isolated from murine mammary glands were cultivated in 3D on soft and stiff matrices (200–4000 Pa) in conjunction with the Rho/MRTF/SRF pathway inhibitor CCG-203971 and genetic activation of MRTF-A.

Results

Three-dimensional growth on stiff collagen matrices (> 3000 Pa) was accompanied by increased MRTF-A activity and formation of invasive protrusions in acini cultures of human mammary MCF10A cells. Differential coating and synthetic hydrogels indicated that protrusion formation was attributable to stiffness but not the biochemical constitution of the matrix. Stiffness-induced protrusion formation was also observed in preformed acini isolated from murine mammary glands. Acinar outgrowth in both the MCF10A acini and the primary organoids was partially reverted by treatment with the Rho/MRTF/SRF pathway inhibitor CCG-203971. However, genetic activation of MRTF-A in the mature primary acini also reduced protrusion formation on stiff matrices, whilst it strongly promoted luminal filling matrix-independently.

Conclusion

Our results suggest an intricate crosstalk between matrix stiffness and MRTF-A, whose activity is required for protrusion formation and sufficient for luminal filling of mammary acini.

A WISP1 antibody inhibits MRTF signaling to prevent the progression of established liver fibrosis

Fibrosis is the major risk factor associated with morbidity and mortality in patients with non-alcoholic steatohepatitis (NASH)-driven chronic liver disease. Although numerous efforts have been made to identify the mediators of the initiation of liver fibrosis, the molecular underpinnings of fibrosis progression remain poorly understood, and therapies to arrest liver fibrosis progression are elusive. Here, we identify a pathway involving WNT1-inducible signaling pathway protein 1 (WISP1) and myocardin-related transcription factor (MRTF) as a central mechanism driving liver fibrosis progression through the integrin-dependent transcriptional reprogramming of myofibroblast cytoskeleton and motility. In mice, WISP1 deficiency protects against fibrosis progression, but not fibrosis onset. Moreover, the therapeutic administration of a novel antibody blocking WISP1 halted the progression of existing liver fibrosis in NASH models. These findings implicate the WISP1-MRTF axis as a crucial determinant of liver fibrosis progression and support targeting this pathway by antibody-based therapy for the treatment of NASH fibrosis.

Transcription Factors YAP/TAZ and SRF Cooperate To Specify Renal Myofibroblasts in the Developing Mouse Kidney

BACKGROUND

The embryonic renal stroma consists of multiple molecularly distinct cell subpopulations, the functional significance of which is largely unknown. Previous work has demonstrated that the transcription factors YAP and TAZ play roles in the development and morphogenesis of the nephrons, collecting ducts, and nephron progenitor cells.

METHODS

In embryonic mouse kidneys, we identified a subpopulation of stromal cells with enriched activity in YAP and TAZ. To evaluate the function of these cell types, we genetically ablated both Yap and Taz from the stromal progenitor population and examined how gene activity and development of YAP/TAZ mutant kidneys are affected over a developmental time course.

RESULTS

We found that YAP and TAZ are active in a subset of renal interstitium and that stromal-specific coablation of YAP/TAZ disrupts cortical fibroblast, pericyte, and myofibroblast development, with secondary effects on peritubular capillary differentiation. We also demonstrated that the transcription factor SRF cooperates with YAP/TAZ to drive expression of at least a subset of renal myofibroblast target genes and to specify myofibroblasts but not cortical fibroblasts or pericytes.

CONCLUSIONS

These findings reveal a critical role for YAP/TAZ in specific embryonic stromal cells and suggest that interaction with cofactors, such as SRF, influence the expression of cell type-specific target genes, thus driving stromal heterogeneity. Further, this work reveals functional roles for renal stroma heterogeneity in creating unique microenvironments that influence the differentiation and maintenance of the renal parenchyma.

Lens Fibrosis: Understanding the Dynamics of Cell Adhesion Signaling in Lens Epithelial-Mesenchymal Transition

Injury to the ocular lens perturbs cell-cell and cell-capsule/basement membrane interactions leading to a myriad of interconnected signaling events. These events include cell-adhesion and growth factor-mediated signaling pathways that can ultimately result in the induction and progression of epithelial-mesenchymal transition (EMT) of lens epithelial cells and fibrosis. Since the lens is avascular, consisting of a single layer of epithelial cells on its anterior surface and encased in a matrix rich capsule, it is one of the most simple and desired systems to investigate injury-induced signaling pathways that contribute to EMT and fibrosis. In this review, we will discuss the role of key cell-adhesion and mechanotransduction related signaling pathways that regulate EMT and fibrosis in the lens.

Fibrotic Signaling in Cardiac Fibroblasts and Vascular Smooth Muscle Cells: The Dual Roles of Fibrosis in HFpEF and CAD

Patients with heart failure with preserved ejection fraction (HFpEF) and atherosclerosis-driven coronary artery disease (CAD) will have ongoing fibrotic remodeling both in the myocardium and in atherosclerotic plaques. However, the functional consequences of fibrosis differ for each location. Thus, cardiac fibrosis leads to myocardial stiffening, thereby compromising cardiac function, while fibrotic remodeling stabilizes the atherosclerotic plaque, thereby reducing the risk of plaque rupture. Although there are currently no drugs targeting cardiac fibrosis, it is a field under intense investigation, and future drugs must take these considerations into account. To explore similarities and differences of fibrotic remodeling at these two locations of the heart, we review the signaling pathways that are activated in the main extracellular matrix (ECM)-producing cells, namely human cardiac fibroblasts (CFs) and vascular smooth muscle cells (VSMCs). Although these signaling pathways are highly overlapping and context-dependent, effects on ECM remodeling mainly act through two core signaling cascades: TGF-β and Angiotensin II. We complete this by summarizing the knowledge gained from clinical trials targeting these two central fibrotic pathways.

Anti-fibrotic effects of pharmacologic FGF-2: a review of recent literature

Fibrosis is a process of pathological tissue repair that replaces damaged, formerly functional tissue with a non-functional, collagen-rich scar. Complications of fibrotic pathologies, which can arise in numerous organs and from numerous conditions, result in nearly half of deaths in the developed world. Despite this, therapies that target fibrosis at its mechanistic roots are still notably lacking. The ubiquity of the occurrence of fibrosis in myriad organs emphasizes the fact that there are shared mechanisms underlying fibrotic conditions, which may serve as common therapeutic targets for multiple fibrotic diseases of varied organs. Thus, study of the basic science of fibrosis and of anti-fibrotic modalities is critical to therapeutic development and may have potential to translate across organs and disease states. Fibroblast growth factor 2 (FGF-2) is a broadly studied member of the fibroblast growth factors, a family of multipotent cytokines implicated in diverse cellular and tissue processes, which has previously been recognized for its anti-fibrotic potential. However, the mechanisms underlying this potential are not fully understood, nor is the potential for its use to ameliorate fibrosis in diverse pathologies and tissues. Presented here is a review of recent literature that sheds further light on these questions, with the hopes of inspiring further research into the mechanisms underlying the anti-fibrotic activities of FGF-2, as well as the disease conditions for which pharmacologic FGF-2 might be a useful option in the future.

Disruption of pancreatic stellate cell myofibroblast phenotype promotes pancreatic tumor invasion

In pancreatic ductal adenocarcinoma (PDAC), differentiation of pancreatic stellate cells (PSCs) into myofibroblast-like cancer-associated fibroblasts (CAFs) can both promote and suppress tumor progression. Here, we show that the Rho effector protein kinase N2 (PKN2) is critical for PSC myofibroblast differentiation. Loss of PKN2 is associated with reduced PSC proliferation, contractility, and alpha-smooth muscle actin (α-SMA) stress fibers. In spheroid co-cultures with PDAC cells, loss of PKN2 prevents PSC invasion but, counter-intuitively, promotes invasive cancer cell outgrowth. PKN2 deletion induces a myofibroblast to inflammatory CAF switch in the PSC matrisome signature both in vitro and in vivo. Further, deletion of PKN2 in the pancreatic stroma induces more locally invasive, orthotopic pancreatic tumors. Finally, we demonstrate that a PKN2KO matrisome signature predicts poor outcome in pancreatic and other solid human cancers. Our data indicate that suppressing PSC myofibroblast function can limit important stromal tumor-suppressive mechanisms, while promoting a switch to a cancer-supporting CAF phenotype.

HuR drives lung fibroblast differentiation but not metabolic reprogramming in response to TGF-β and hypoxia

Background

Pulmonary fibrosis is thought to be driven by recurrent alveolar epithelial injury which leads to the differentiation of fibroblasts into α-smooth muscle actin (α-SMA)-expressing myofibroblasts and subsequent deposition of extracellular matrix (ECM). Transforming growth factor beta-1 (TGF-β1) plays a key role in fibroblast differentiation, which we have recently shown involves human antigen R (HuR). HuR is an RNA binding protein that also increases the translation of hypoxia inducible factor (HIF-1α) mRNA, a transcription factor critical for inducing a metabolic shift from oxidative phosphorylation towards glycolysis. This metabolic shift may cause fibroblast differentiation. We hypothesized that under hypoxic conditions, HuR controls myofibroblast differentiation and glycolytic reprogramming in human lung fibroblasts (HLFs).

Methods

Primary HLFs were cultured in the presence (or absence) of TGF-β1 (5 ng/ml) under hypoxic (1% O₂) or normoxic (21% O₂) conditions. Evaluation included mRNA and protein expression of glycolytic and myofibroblast/ECM markers by qRT-PCR and western blot. Metabolic profiling was done by proton nuclear magnetic resonance (¹H- NMR). Separate experiments were conducted to evaluate the effect of HuR on metabolic reprogramming using siRNA-mediated knock-down.

Results

Hypoxia alone had no significant effect on fibroblast differentiation or metabolic reprogramming. While hypoxia- together with TGFβ1- increased mRNA levels of differentiation and glycolysis genes, such as ACTA2, LDHA, and HK2, protein levels of α-SMA and collagen 1 were significantly reduced. Hypoxia induced cytoplasmic translocation of HuR. Knockdown of HuR reduced features of fibroblast differentiation in response to TGF-β1 with and without hypoxia, including α-SMA and the ECM marker collagen I, but had no effect on lactate secretion.

Conclusions

Hypoxia reduced myofibroblasts differentiation and lactate secretion in conjunction with TGF-β. HuR is an important protein in the regulation of myofibroblast differentiation but does not control glycolysis in HLFs in response to hypoxia. More research is needed to understand the functional implications of HuR in IPF pathogenesis.

Hypoxia induces stress fiber formation in adipocytes in the early stage of obesity

In obese adipose tissue (AT), hypertrophic expansion of adipocytes is not matched by new vessel formation, leading to AT hypoxia. As a result, hypoxia inducible factor-1⍺ (HIF-1⍺) accumulates in adipocytes inducing a transcriptional program that upregulates profibrotic genes and biosynthetic enzymes such as lysyl oxidase (LOX) synthesis. This excess synthesis and crosslinking of extracellular matrix (ECM) components cause AT fibrosis. Although fibrosis is a hallmark of obese AT, the role of fibroblasts, cells known to regulate fibrosis in other fibrosis-prone tissues, is not well studied. Here we have developed an in vitro model of AT to study adipocyte-fibroblast crosstalk in a hypoxic environment. Further, this in vitro model was used to investigate the effect of hypoxia on adipocyte mechanical properties via ras homolog gene family member A (RhoA)/Rho-associated coiled-coil kinases (ROCK) signaling pathways. We confirmed that hypoxia creates a diseased phenotype by inhibiting adipocyte maturation and inducing actin stress fiber formation facilitated by myocardin-related transcription factor A (MRTF-A/MKL1) nuclear translocation. This work presents new potential therapeutic targets for obesity by improving adipocyte maturation and limiting mechanical stress in obese AT.

Serine-Threonine Kinase inhibition as antifibrotic therapy: TGF-β and ROCK inhibitors

Serine-threonine kinases mediate the phosphorylation of intracellular protein targets, transferring a phosphorus group from an ATP molecule to the specific amino acid residues within the target proteins. Serine-threonine kinases regulate multiple key cellular functions. From this large group of kinases, transforming growth factor beta (TGF-β) through the serine-threonine activity of its receptors and Rho kinase (ROCK) play an important role in the development and maintenance of fibrosis in various human diseases, including systemic sclerosis. In recent years, multiple drugs targeting and inhibiting these kinases, have been developed, opening the possibility of becoming potential antifibrotic agents of clinical value for treating fibrotic diseases. This review analyzes the contribution of TGF- β and ROCK-mediated serine-threonine kinase molecular pathways to the development and maintenance of pathological fibrosis and the potential clinical use of their inhibition.

Inhibition of the Arp2/3 complex represses human lung myofibroblast differentiation and attenuates bleomycin-induced pulmonary fibrosis

BACKGROUND AND PURPOSE

The Arp2/3 multiprotein complex regulates branched polymerisation of the actin cytoskeleton and may contribute to collagen synthesis and fibrogenesis in the lung.

EXPERIMENTAL APPROACH

Expression of Arp2/3 components was assessed in human lung fibroblasts and in the bleomycin-induced pulmonary fibrosis model in mice. The Arp2/3 complex was repressed with the allosteric inhibitor CK666 and with interfering RNAs targeting the ARP2, ARP3 and ARPC2 subunits (siARP2, siARP3 and siARPC2) in CCD-16Lu human lung fibroblasts in vitro. Mice received daily intraperitoneal injections of CK666 from the 7th to the 14th day after tracheal bleomycin instillation.

KEY RESULTS

Expression of Arp2/3 complex subunits mRNAs was increased in fibroblasts treated with TGF-β1 and in the lungs of bleomycin-treated mice compared with controls. In vitro, CK666 and siARPC2 inhibited cell growth and TGF-β1-induced α-smooth muscle actin (ACTA2) and collagen-1 (COL1) expression. CK666 also decreased ACTA2 and COL1 expression in unstimulated cells. CK666 reduced Akt phosphorylation and repressed phospho-GSK3β, β-catenin and MRTF-A levels in unstimulated fibroblasts. In vivo, CK666 reduced levels of both procollagen-1 and insoluble collagen in bleomycin-treated mice.

CONCLUSION AND IMPLICATIONS

Expression of the Arp2/3 complex was increased in profibrotic environments in vitro and in vivo. Inhibition of the Arp2/3 complex repressed ACTA2 and COL1 expression and repressed an Akt/phospho-GSK3β/β-catenin/MRTF-A pathway in lung fibroblasts. CK666 exerted antifibrotic properties in the lung in vivo. Inhibition of the Arp2/3 complex could represent an interesting new therapy for idiopathic pulmonary fibrosis and other fibrotic interstitial lung diseases.

RhoA/ROCK Signaling Regulates TGF-β1-Induced Fibrotic Effects in Human Pterygium Fibroblasts through MRTF-A

PURPOSE

The overexpression of transforming growth factor-beta1 (TGF-β1) after surgical excision often leads to excessive fibrosis, indicating the recurrence of pterygium. The aims of the present in vitro study were to investigate the role of RhoA/ROCK signaling in regulating fibrotic effects of primary human pterygium fibroblasts (HPFs), as well as to explore the possible mechanisms of these effects.

METHODS

Pterygium samples were obtained from surgery, and profibrotic activation was induced by TGF-β1. Cell proliferation was detected by CCK-8 assay; cell migration was detected by wound healing assay; quantitative real-time PCR and Western blot were used to detect the effects of TGF-β1 and the role of RhoA/ROCK signaling in the synthesis of alpha-smooth muscle actin (a-SMA), type I and III collagen (COL1 and COL3), and matrix metalloproteinase-9 (MMP9) in HPFs. The changes of signaling pathways were detected by Western blot; and pharmaceutical inhibition of RhoA/ROCK signaling and its downstream MRFT-A/SRF transcription pathway were used to assess their possible mechanism in HPFs fibrosis.

RESULTS

ROCK inhibitor Y-27632 decreased TGF-β1-induced cell proliferation and migration, reduced the TGF-β1-induced expression of profibrotic markers in HPFs, and suppressed TGF-β1-induced nuclear accumulation of Myocardin-related transcription factor A (MRTF-A) as well as accompanied elevation of F/G-actin ratio in HPFs. MRTF-A/Serum response factor (SRF) inhibitor CCG-100602 attenuated the TGF-β1-induced α-SMA expression and reduced myofibroblast activation in HPFs.

CONCLUSIONS

RhoA/ROCK signaling played a pivotal role in TGF-β1-induced fibrosis and myofibroblast activation in HPFs at least in part by inactivating the downstream MRTF-A/SRF transcriptional pathway.

Nucleocytoplasmic Shuttling of the Mechanosensitive Transcription Factors MRTF and YAP /TAZ

Myocardin-related transcription factor (MRTF) and the paralogous Hippo pathway effectors Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) are transcriptional co-activators that play pivotal roles in myofibroblast generation and activation, and thus the pathogenesis of organ fibrosis. They are regulated by a variety of chemical and mechanical fibrogenic stimuli, primarily at the level of their nucleocytoplasmic shuttling. In this chapter we describe the tools and protocols that allow for exact, quantitative, and automated determination and analysis of the nucleocytoplasmic distribution of endogenous or heterologously expressed MRTF and YAP/TAZ, measured in large cell populations. Dynamic monitoring of nucleocytoplasmic ratios of transcription factors is a novel and important approach, suitable to address both the structural requirements and the regulatory mechanisms underlying transcription factor traffic and the consequent reprogramming of gene expression during fibrogenesis.

Myocardin-related transcription factor and serum response factor regulate cilium turnover by both transcriptional and local mechanisms

Turnover of the primary cilium (PC) is critical for proliferation and tissue homeostasis. Each key component of the PC resorption machinery, the HEF1/Aurora kinase A (AurA)/HDAC6 pathway harbors cis-elements potentially targeted by the transcriptional co-activator myocardin-related transcription factor (MRTF) and/or its partner serum response factor (SRF). Thus we investigated if MRTF and/or SRF regulate PC turnover. Here we show that (1) both MRTF and SRF are indispensable for serum-induced PC resorption, and (2) they act via both transcriptional and local mechanisms. Intriguingly, MRTF and SRF are present in the basal body and/or the PC, and serum facilitates ciliary MRTF recruitment. MRTF promotes the stability and ciliary accumulation of AurA and facilitates SRF phosphorylation. Ciliary SRF interacts with AurA and HDAC6. MRTF also inhibits ciliogenesis. It interacts with and is required for the correct localization of the ciliogenesis modulator CEP290. Thus, MRTF and SRF are critical regulators of PC assembly and/or disassembly, acting both as transcription factors and as PC constituents.

The Genomic Response to TGF-β1 Dictates Failed Repair and Progression of Fibrotic Disease in the Obstructed Kidney

Tubulointerstitial fibrosis is a common and diagnostic hallmark of a spectrum of chronic renal disorders. While the etiology varies as to the causative nature of the underlying pathology, persistent TGF-β1 signaling drives the relentless progression of renal fibrotic disease. TGF-β1 orchestrates the multifaceted program of kidney fibrogenesis involving proximal tubular dysfunction, failed epithelial recovery or re-differentiation, capillary collapse and subsequent interstitial fibrosis eventually leading to chronic and ultimately end-stage disease. An increasing complement of non-canonical elements function as co-factors in TGF-β1 signaling. p53 is a particularly prominent transcriptional co-regulator of several TGF-β1 fibrotic-response genes by complexing with TGF-β1 receptor-activated SMADs. This cooperative p53/TGF-β1 genomic cluster includes genes involved in cellular proliferative control, survival, apoptosis, senescence, and ECM remodeling. While the molecular basis for this co-dependency remains to be determined, a subset of TGF-β1-regulated genes possess both p53- and SMAD-binding motifs. Increases in p53 expression and phosphorylation, moreover, are evident in various forms of renal injury as well as kidney allograft rejection. Targeted reduction of p53 levels by pharmacologic and genetic approaches attenuates expression of the involved genes and mitigates the fibrotic response confirming a key role for p53 in renal disorders. This review focuses on mechanisms underlying TGF-β1-induced renal fibrosis largely in the context of ureteral obstruction, which mimics the pathophysiology of pediatric unilateral ureteropelvic junction obstruction, and the role of p53 as a transcriptional regulator within the TGF-β1 repertoire of fibrosis-promoting genes.

Decreased Substrate Stiffness Promotes a Hypofibrotic Phenotype in Cardiac Fibroblasts

A hypofibrotic phenotype has been observed in cardiac fibroblasts (CFs) isolated from a volume overload heart failure model, aortocaval fistula (ACF). This paradoxical phenotype results in decreased ECM synthesis despite increased TGF-β presence. Since ACF results in decreased tissue stiffness relative to control (sham) hearts, this study investigates whether the effects of substrate stiffness could account for the observed hypofibrotic phenotype in CFs isolated from ACF. CFs isolated from ACF and sham hearts were plated on polyacrylamide gels of a range of stiffness (2 kPa to 50 kPa). Markers related to cytoskeletal and fibrotic proteins were measured. Aspects of the hypofibrotic phenotype observed in ACF CFs were recapitulated by sham CFs on soft substrates. For instance, sham CFs on the softest gels compared to ACF CFs on the stiffest gels results in similar CTGF (0.80 vs. 0.76) and transgelin (0.44 vs. 0.57) mRNA expression. The changes due to stiffness may be explained by the observed decreased nuclear translocation of transcriptional regulators, MRTF-A and YAP. ACF CFs appear to have a mechanical memory of a softer environment, supported by a hypofibrotic phenotype overall compared to sham with less YAP detected in the nucleus, and less CTGF and transgelin on all stiffnesses.

MRTF: Basic Biology and Role in Kidney Disease

A lesser known but crucially important downstream effect of Rho family GTPases is the regulation of gene expression. This major role is mediated via the cytoskeleton, the organization of which dictates the nucleocytoplasmic shuttling of a set of transcription factors. Central among these is myocardin-related transcription factor (MRTF), which upon actin polymerization translocates to the nucleus and binds to its cognate partner, serum response factor (SRF). The MRTF/SRF complex then drives a large cohort of genes involved in cytoskeleton remodeling, contractility, extracellular matrix organization and many other processes. Accordingly, MRTF, activated by a variety of mechanical and chemical stimuli, affects a plethora of functions with physiological and pathological relevance. These include cell motility, development, metabolism and thus metastasis formation, inflammatory responses and—predominantly-organ fibrosis. The aim of this review is twofold: to provide an up-to-date summary about the basic biology and regulation of this versatile transcriptional coactivator; and to highlight its principal involvement in the pathobiology of kidney disease. Acting through both direct transcriptional and epigenetic mechanisms, MRTF plays a key (yet not fully appreciated) role in the induction of a profibrotic epithelial phenotype (PEP) as well as in fibroblast-myofibroblast transition, prime pathomechanisms in chronic kidney disease and renal fibrosis.

In vitro assessment of anti-fibrotic drug activity does not predict in vivo efficacy in murine models of Duchenne muscular dystrophy

AIM

Fibrosis is the most common complication from chronic diseases, and yet no therapy capable of mitigating its effects is available. Our goal is to unveil specific signaling regulating the fibrogenic process and to identify potential small molecule candidates that block fibrogenic differentiation of fibro/adipogenic progenitors.

METHOD

We performed a large-scale drug screen using muscle-resident fibro/adipogenic progenitors from a mouse model expressing EGFP under the Collagen1a1 promotor. We first confirmed that the EGFP was expressed in response to TGFβ1 stimulation in vitro. Then we treated cells with TGFβ1 alone or with drugs from two libraries of known compounds. The drugs ability to block the fibrogenic differentiation was quantified by imaging and flow cytometry. From a two-rounds screening, positive hits were tested in vivo in the mice model for the Duchenne Muscular Dystrophy (mdx mice). The histopathology of the muscles was assessed with picrosirius red (fibrosis) and laminin staining (myofiber size).

KEY FINDINGS

From the in vitro drug screening, we identified 21 drugs and tested 3 in vivo on the mdx mice. None of the three drugs significantly improved muscle histopathology.

SIGNIFICANCE

The in vitro drug screen identified various efficient compounds, none of them strongly inhibited fibrosis in skeletal muscle of mdx mice. To explain these observations, we hypothesize that in Duchenne Muscular Dystrophy, in which fibrosis is a secondary event due to chronic degeneration and inflammation, the drugs tested could have adverse effect on regeneration or inflammation, balancing off any positive effects and leading to the absence of significant results.

Novel PEGylated Lipid Nanoparticles Have a High Encapsulation Efficiency and Effectively Deliver MRTF-B siRNA in Conjunctival Fibroblasts

The master regulator of the fibrosis cascade is the myocardin-related transcription factor/serum response factor (MRTF/SRF) pathway, making it a key target for anti-fibrotic therapeutics. In the past, inhibitors and small interfering RNAs (siRNAs) targeting the MRTF-B gene have been deployed to counter fibrosis in the eye, with the latter showing promising results. However, the biggest challenge in implementing siRNA therapeutics is the method of delivery. In this study, we utilised the novel, pH-sensitive, cationic lipid CL4H6, which has previously demonstrated potent targeting of hepatocytes and endosomal escape, to safely and efficiently deliver an MRTF-B siRNA into human conjunctival fibroblasts. We prepared two lipid nanoparticle (LNP) formulations, incorporating targeting cleavable peptide cY in one of them, and measured their physicochemical properties and silencing effect in human conjunctival fibroblasts. Both proved to be non-cytotoxic at a concentration of 50 nM and effectively silenced the MRTF-B gene in vitro, with the targeting cleavable peptide not affecting the silencing efficiency [LNP with cY: 62.1% and 81.5% versus LNP without cY: 77.7% and 80.2%, at siRNA concentrations of 50 nM (p = 0.06) and 100 nM (p = 0.09), respectively]. On the other hand, the addition of the targeting cleavable peptide significantly increased the encapsulation efficiency of the LNPs from 92.5% to 99.3% (p = 0.0005). In a 3D fibroblast-populated collagen matrix model, both LNP formulations significantly decreased fibroblast contraction after a single transfection. We conclude that the novel PEGylated CL4H6-MRTF-B siRNA-loaded LNPs represent a promising therapeutic approach to prevent conjunctival fibrosis after glaucoma filtration surgery.

Aortic carboxypeptidase-like protein regulates vascular adventitial progenitor and fibroblast differentiation through myocardin related transcription factor A

The vascular adventitia contains numerous cell types including fibroblasts, adipocytes, inflammatory cells, and progenitors embedded within a complex extracellular matrix (ECM) network. In response to vascular injury, adventitial progenitors and fibroblasts become activated and exhibit increased proliferative capacity and differentiate into contractile cells that remodel the ECM. These processes can lead to vascular fibrosis and disease progression. Our previous work established that the ECM protein aortic carboxypeptidase-like protein (ACLP) promotes fibrotic remodeling in the lung and is activated by vascular injury. It is currently unknown what controls vascular adventitial cell differentiation and if ACLP has a role in this process. Using purified mouse aortic adventitia Sca1+ progenitors, ACLP repressed stem cell markers (CD34, KLF4) and upregulated smooth muscle actin (SMA) and collagen I expression. ACLP enhanced myocardin-related transcription factor A (MRTFA) activity in adventitial cells by promoting MRTFA nuclear translocation. Sca1 cells from MRTFA-null mice exhibited reduced SMA and collagen expression induced by ACLP, indicating Sca1 cell differentiation is regulated in part by the ACLP-MRTFA axis. We determined that ACLP induced vessel contraction and increased adventitial collagen in an explant model. Collectively these studies identified ACLP as a mediator of adventitial cellular differentiation, which may result in pathological vessel remodeling.

Epiregulin (EREG) and Myocardin Related Transcription Factor A (MRTF-A) Form a Feedforward Loop to Drive Hepatic Stellate Cell Activation

Trans-differentiation of quiescent hepatic stellate cells (HSC) into myofibroblast cells is considered the linchpin of liver fibrosis. A myriad of signaling pathways contribute to HSC activation and consequently liver fibrosis. Epidermal growth factor (EGF) family of cytokines signal through the cognate receptor EGFR to promote HSC activation. In the present study we investigated the transcription regulation of epiregulin (EREG), an EGFR ligand, during HSC activation. We report that EREG expression was significantly up-regulated in activated HSCs compared to quiescent HSCs isolated from mice. In addition, there was an elevation of EREG expression in HSCs undergoing activation in vitro. Of interest, deficiency of myocardin-related transcription factor A (MRTF-A), a well-documented regulator of HSC trans-differentiation, attenuated up-regulation of EREG expression both in vivo and in vitro. Further analysis revealed that MRTF-A interacted with serum response factor (SRF) to bind directly to the EREG promoter and activate EREG transcription. EREG treatment promoted HSC activation in vitro, which was blocked by MRTF-A depletion or inhibition. Mechanistically, EREG stimulated nuclear trans-location of MRTF-A in HSCs. Together, our data portray an EREG-MRTF-A feedforward loop that contributes to HSC activation and suggest that targeting the EREG-MRTF-A axis may yield therapeutic solutions against liver fibrosis.

Fibroblast contributions to ischemic cardiac remodeling

The heart can respond to increased pathophysiological demand through alterations in tissue structure and function 1 . This process, called cardiac remodeling, is particularly evident following myocardial infarction (MI), where the blockage of a coronary artery leads to widespread death of cardiac muscle. Following MI, necrotic tissue is replaced with extracellular matrix (ECM), and the remaining viable cardiomyocytes (CMs) undergo hypertrophic growth. ECM deposition and cardiac hypertrophy are thought to represent an adaptive response to increase structural integrity and prevent cardiac rupture. However, sustained ECM deposition leads to the formation of a fibrotic scar that impedes cardiac compliance and can induce lethal arrhythmias. Resident cardiac fibroblasts (CFs) are considered the primary source of ECM molecules such as collagens and fibronectin, particularly after becoming activated by pathologic signals. CFs contribute to multiple phases of post-MI heart repair and remodeling, including the initial response to CM death, immune cell (IC) recruitment, and fibrotic scar formation. The goal of this review is to describe how resident fibroblasts contribute to the healing and remodeling that occurs after MI, with an emphasis on how fibroblasts communicate with other cell types in the healing infarct scar 1 –6 .

Controlling cardiac fibrosis through fibroblast state space modulation

The transdifferentiation of cardiac fibroblasts into myofibroblasts after cardiac injury has traditionally been defined by a unidirectional continuum from quiescent fibroblasts, through activated fibroblasts, and finally to fibrotic-matrix producing myofibroblasts. However, recent lineage tracing and single cell RNA sequencing experiments have demonstrated that fibroblast transdifferentiation is much more complex. Growing evidence suggests that fibroblasts are more heterogenous than previously thought, and many new cell states have recently been identified. This review reexamines conventional fibroblast transdifferentiation paradigms with a dynamic state space lens, which could enable a more complex understanding of how fibroblast state dynamics alters fibrotic remodeling of the heart. This review will define cellular state space, how it relates to fibroblast state transitions, and how the canonical and non-canonical fibrotic signaling pathways modulate fibroblast cell state and cardiac fibrosis. Finally, this review explores the therapeutic potential of fibroblast state space modulation by p38 inhibition, yes-associated protein (YAP) inhibition, and fibroblast reprogramming.

Cardiac fibrosis

Myocardial fibrosis, the expansion of the cardiac interstitium through deposition of extracellular matrix proteins, is a common pathophysiologic companion of many different myocardial conditions. Fibrosis may reflect activation of reparative or maladaptive processes. Activated fibroblasts and myofibroblasts are the central cellular effectors in cardiac fibrosis, serving as the main source of matrix proteins. Immune cells, vascular cells and cardiomyocytes may also acquire a fibrogenic phenotype under conditions of stress, activating fibroblast populations. Fibrogenic growth factors (such as transforming growth factor-β and platelet-derived growth factors), cytokines [including tumour necrosis factor-α, interleukin (IL)-1, IL-6, IL-10, and IL-4], and neurohumoral pathways trigger fibrogenic signalling cascades through binding to surface receptors, and activation of downstream signalling cascades. In addition, matricellular macromolecules are deposited in the remodelling myocardium and regulate matrix assembly, while modulating signal transduction cascades and protease or growth factor activity. Cardiac fibroblasts can also sense mechanical stress through mechanosensitive receptors, ion channels and integrins, activating intracellular fibrogenic cascades that contribute to fibrosis in response to pressure overload. Although subpopulations of fibroblast-like cells may exert important protective actions in both reparative and interstitial/perivascular fibrosis, ultimately fibrotic changes perturb systolic and diastolic function, and may play an important role in the pathogenesis of arrhythmias. This review article discusses the molecular mechanisms involved in the pathogenesis of cardiac fibrosis in various myocardial diseases, including myocardial infarction, heart failure with reduced or preserved ejection fraction, genetic cardiomyopathies, and diabetic heart disease. Development of fibrosis-targeting therapies for patients with myocardial diseases will require not only understanding of the functional pluralism of cardiac fibroblasts and dissection of the molecular basis for fibrotic remodelling, but also appreciation of the pathophysiologic heterogeneity of fibrosis-associated myocardial disease.

Mechanosensing dysregulation in the fibroblast: A hallmark of the aging heart

The myofibroblast is a specialized fibroblast that expresses α-smooth muscle actin (α-SMA) and participates in wound contraction and fibrosis. The fibroblast to myofibroblast transition depends on chemical and mechanical signals. A fibroblast senses the changes in the environment (extracellular matrix (ECM)) and transduces these changes to the cytoskeleton and the nucleus, resulting in activation or inhibition of α-SMA transcription in a process called mechanosensing. A stiff matrix greatly facilitates the transition from fibroblast to myofibroblast, and although the aging heart is much stiffer than the young one, the aging fibroblast has difficulties in transitioning into the contractile phenotype. This suggests that the events occurring downstream of the matrix, such as activation or changes in expression levels of various proteins participating in mechanotransduction can negatively alter the ability of the aging fibroblast to become a myofibroblast. In this review, we will discuss in detail the changes in ECM, receptors (integrin or non-integrin), focal adhesions, cytoskeleton, and transcription factors involved in mechanosensing that occur with aging.

Maintaining resting cardiac fibroblasts in vitro by disrupting mechanotransduction

Mechanical cues activate cardiac fibroblasts and induce differentiation into myofibroblasts, which are key steps for development of cardiac fibrosis. In vitro, the high stiffness of plastic culturing conditions will also induce these changes. It is therefore challenging to study resting cardiac fibroblasts and their activation in vitro. Here we investigate the extent to which disrupting mechanotransduction by culturing cardiac fibroblasts on soft hydrogels or in the presence of biochemical inhibitors can be used to maintain resting cardiac fibroblasts in vitro. Primary cardiac fibroblasts were isolated from adult mice and cultured on plastic or soft (4.5 kPa) polyacrylamide hydrogels. Myofibroblast marker gene expression and smooth muscle α-actin (SMA) fibers were quantified by real-time PCR and immunostaining, respectively. Myofibroblast differentiation was prevented on soft hydrogels for 9 days, but had occurred after 15 days on hydrogels. Transferring myofibroblasts to soft hydrogels reduced expression of myofibroblast-associated genes, albeit SMA fibers remained present. Inhibitors of transforming growth factor β receptor I (TGFβRI) and Rho-associated protein kinase (ROCK) were effective in preventing and reversing myofibroblast gene expression. SMA fibers were also reduced by blocker treatment although cell morphology did not change. Reversed cardiac fibroblasts maintained the ability to re-differentiate after the removal of blockers, suggesting that these are functionally similar to resting cardiac fibroblasts. However, actin alpha 2 smooth muscle (Acta2), lysyl oxidase (Lox) and periostin (Postn) were no longer sensitive to substrate stiffness, suggesting that transient treatment with mechanotransduction inhibitors changes the mechanosensitivity of some fibrosis-related genes. In summary, our results bring novel insight regarding the relative importance of specific mechanical signaling pathways in regulating different myofibroblast-associated genes. Furthermore, combining blocker treatment with the use of soft hydrogels has not been tested previously and revealed that only some genes remain mechano-sensitive after phenotypic reversion. This is important information for researchers using inhibitors to maintain a "resting" cardiac fibroblast phenotype in vitro as well as for our current understanding of mechanosensitive gene regulation.

An MRTF-A-Sp1-PDE5 Axis Mediates Angiotensin-II-Induced Cardiomyocyte Hypertrophy

Cardiac hypertrophy is a critical intermediate step in the pathogenesis of heart failure. A myriad of signaling networks converge on cardiomyocytes to elicit hypertrophic growth in response to various injurious stimuli. In the present study, we investigated the cardiomyocyte-specific role of myocardin-related transcription factor A (MRTF-A) in angiotensin-II (Ang-II)-induced cardiac hypertrophy and the underlying mechanism. We report that conditional MRTF-A deletion in cardiomyocytes attenuated Ang-II-induced cardiac hypertrophy in mice. Similarly, MRTF-A knockdown or inhibition suppressed Ang-II-induced prohypertrophic response in cultured cardiomyocytes. Of note, Ang II treatment upregulated expression of phosphodiesterase 5 (PDE5), a known mediator of cardiac hypertrophy and heart failure, in cardiomyocytes, which was blocked by MRTF-A depletion or inhibition. Mechanistically, MRTF-A activated expression of specificity protein 1 (Sp1), which in turn bound to the PDE5 promoter and upregulated PDE5 transcription to promote hypertrophy of cardiomyocytes in response to Ang II stimulation. Therefore, our data unveil a novel MRTF-A–Sp1–PDE5 axis that mediates Ang-II-induced hypertrophic response in cardiomyocytes. Targeting this newly identified MRTF-A–Sp1–PDE5 axis may yield novel interventional solutions against heart failure.

Stimulatory effect of gastroesophageal reflux disease (GERD) on pulmonary fibroblast differentiation

Epidemiological studies indicate that prolonged micro-aspiration of gastric fluid is associated in gastroesophageal reflux disease with the development of chronic respiratory diseases, possibly caused by inflammation-related immunomodulation. Therefore, we sought to ascertain the effect of gastric fluid exposure on pulmonary residential cells. The expression of α-smooth muscle actin as a fibrotic marker was increased in both normal human pulmonary fibroblast cells and mouse macrophages. Gastric fluid enhanced the proliferation and migration of HFL-1 cells and stimulated the expression of inflammatory cytokines in an antibody assay. Elevated expression of the Rho signaling pathway was noted in fibroblast cells stimulated with gastric fluid or conditioned media. These results indicate that gastric fluid alone, or the mixture of proinflammatory mediators induced by gastric fluid in the pulmonary context, can stimulate pulmonary fibroblast cell inflammation, migration, and differentiation, suggesting that a wound healing process is initiated. Subsequent aberrant repair in pulmonary residential cells may lead to pulmonary fibroblast differentiation and fibrotic progression. The results point to a stimulatory effect of chronic GERD on pulmonary fibroblast differentiation, and this may promote the development of chronic pulmonary diseases in the long term.

The Roles of Noncardiomyocytes in Cardiac Remodeling

Cardiac remodeling is a common characteristic of almost all forms of heart disease, including cardiac infarction, valvular diseases, hypertension, arrhythmia, dilated cardiomyopathy and other conditions. It is not merely a simple outcome induced by an increase in the workload of cardiomyocytes (CMs). The remodeling process is accompanied by abnormalities of cardiac structure as well as disturbance of cardiac function, and emerging evidence suggests that a wide range of cells in the heart participate in the initiation and development of cardiac remodeling. Other than CMs, there are numerous noncardiomyocytes (non-CMs) that regulate the process of cardiac remodeling, such as cardiac fibroblasts and immune cells (including macrophages, lymphocytes, neutrophils, and mast cells). In this review, we summarize recent knowledge regarding the definition and significant effects of various non-CMs in the pathogenesis of cardiac remodeling, with a particular emphasis on the involved signaling mechanisms. In addition, we discuss the properties of non-CMs, which serve as targets of many cardiovascular drugs that reduce adverse cardiac remodeling.

Coordinated regulation of cell survival and cell cycle pathways by DDR2-dependent SRF transcription factor in cardiac fibroblasts

Relative resistance to apoptosis and the ability to proliferate and produce a collagen-rich scar determine the critical role of cardiac fibroblasts in wound healing and tissue remodeling following myocardial injury. Identification of cardiac fibroblast-specific factors and mechanisms underlying these aspects of cardiac fibroblast function is therefore of considerable scientific and clinical interest. In the present study, gene knockdown and overexpression approaches and promoter binding assays showed that discoidin domain receptor 2 (DDR2), a mesenchymal cell-specific collagen receptor tyrosine kinase localized predominantly in fibroblasts in the heart, acts via ERK1/2 MAPK-activated serum response factor (SRF) transcription factor to enhance the expression of antiapoptotic cIAP2 in cardiac fibroblasts, conferring resistance against oxidative injury. Furthermore, DDR2 was found to act via ERK1/2 MAPK-activated SRF to transcriptionally upregulate Skp2 that in turn facilitated post-translational degradation of p27, the cyclin-dependent kinase inhibitor that causes cell cycle arrest, to promote G₁-S transition, as evidenced by Rb phosphorylation, increased proliferating cell nuclear antigen (PCNA) levels, and flow cytometry. DDR2-dependent ERK1/2 MAPK activation also suppressed forkhead box O 3a (FoxO3a)-mediated transcriptional induction of p27. Inhibition of the binding of collagen type I to DDR2 using WRG-28 indicated the obligate role of collagen type I in the activation of DDR2 and its regulatory role in cell survival and cell cycle protein expression. Notably, DDR2 levels positively correlated with SRF, cIAP2, and PCNA levels in cardiac fibroblasts from spontaneously hypertensive rats. To conclude, DDR2-mediated ERK1/2 MAPK activation facilitates coordinated regulation of cell survival and cell cycle progression in cardiac fibroblasts via SRF.NEW & NOTEWORTHY Relative resistance to apoptosis and the ability to proliferate and produce a collagen-rich scar enable cardiac fibroblasts to play a central role in myocardial response to injury. This study reports novel findings that mitogen-stimulated cardiac fibroblasts exploit a common regulatory mechanism involving collagen receptor (DDR2)-dependent activation of ERK1/2 MAPK and serum response factor to achieve coordinated regulation of apoptosis resistance and cell cycle progression, which could facilitate their survival and function in the injured myocardium.

Pulmonary Silicosis Alters MicroRNA Expression in Rat Lung and miR-411-3p Exerts Anti-fibrotic Effects by Inhibiting MRTF-A/SRF Signaling

To identify potential therapeutic targets for pulmonary fibrosis induced by silica, we studied the effects of this disease on the expression of microRNAs (miRNAs) in the lung. Rattus norvegicus pulmonary silicosis models were used in conjunction with high-throughput screening of lung specimens to compare the expression of miRNAs in control and pulmonary silicosis tissues. A total of 70 miRNAs were found to be differentially expressed between control and pulmonary silicosis tissues. This included 41 miRNAs that were upregulated and 29 that were downregulated relative to controls. Among them, miR-292-5p, miR-155-3p, miR-1193-3p, miR-411-3p, miR-370-3p, and miR-409a-5p were found to be similarly altered in rat lung and transforming growth factor (TGF)-β1-induced cultured fibroblasts. Using miRNA mimics and inhibitors, we found that miR-1193-3p, miR-411-3p, and miR-370-3p exhibited potent anti-fibrotic effects, while miR-292-5p demonstrated pro-fibrotic effects in TGF-β1-stimulated lung fibroblasts. Moreover, we also found that miR-411-3p effectively reduced pulmonary silicosis in the mouse lung by regulating Mrtfa expression, as demonstrated using biochemical and histological assays. In conclusion, our findings indicate that miRNA expression is perturbed in pulmonary silicosis and suggest that therapeutic interventions targeting specific miRNAs might be effective in the treatment of this occupational disease.

Fibrostenotic strictures in Crohn's disease

The use of biologic agents including anti-tumor necrosis factor monoclonal antibodies followed by anti-integrins and anti-interleukins has drastically changed the treatment paradigm of Crohn’s disease (CD) by improving clinical symptoms and mucosal healing. However, up to 70% of CD patients still eventually undergo surgery mainly due to fibrostenotic strictures. There are no specific anti-fibrotic drugs yet. This review comprehensively addresses the mechanism, prediction, diagnosis and treatment of the fibrostenotic strictures in CD. We also introduce promising anti-fibrotic agents which may be available in the near future and summarize challenges in developing novel therapies to treat fibrostenotic strictures in CD.

Sacubitril/Valsartan Decreases Cardiac Fibrosis in Left Ventricle Pressure Overload by Restoring PKG Signaling in Cardiac Fibroblasts

Background Heart failure (HF) is invariably accompanied by development of cardiac fibrosis, a form of scarring that increases muscular tissue rigidity and decreases cardiac contractility. Cardiac fibrosis arises from a pathological attempt to repair tissue damaged during maladaptive remodeling. Treatment options to block or reverse fibrosis have proven elusive. Neprilysin is an endopeptidase that degrades vasoactive peptides, including atrial natriuretic peptide. Thus, neprilysin inhibition reduces hypertension, ultimately limiting maladaptive cardiac remodeling. LCZ696, which consists of an angiotensin receptor blocker (valsartan [VAL]) and a neprilysin inhibitor (sacubitril [SAC]), was shown to be well tolerated and significantly reduced the risk of death and hospitalization in HF patients with reduced ejection fraction. We hypothesized that SAC/VAL directly inhibits fibroblast activation and development of pathological fibrosis. Methods and Results We used a mouse model of left ventricle pressure overload coupled to in vitro studies in primary mouse and human cardiac fibroblasts (CFs) to study the impact of SAC/VAL on CF activation and cardiac fibrosis. SAC/VAL significantly ameliorated pressure overload-induced cardiac fibrosis by blocking CF activation and proliferation, leading to functional improvement. Mechanistically, the beneficial impact of SAC/VAL at least partially stemmed from restoration of PKG (protein kinase G) signaling in HF patient-derived CF, which inhibited Rho activation associated with myofibroblast transition. Conclusions This study reveals that SAC/VAL acts directly on CF to prevent maladaptive cardiac fibrosis and dysfunction during pressure overload-induced hypertrophy and suggests that SAC/VAL should be evaluated as a direct antifibrotic therapeutic for conditions such as HF with preserved ejection fraction.

The Adipocyte Acquires a Fibroblast-Like Transcriptional Signature in Response to a High Fat Diet

Visceral white adipose tissue (vWAT) expands and undergoes extensive remodeling during diet-induced obesity. Much is known about the contribution of various stromal vascular cells to the remodeling process, but less is known of the changes that occur within the adipocyte as it becomes progressively dysfunctional. Here, we performed a transcriptome analysis of isolated vWAT adipocytes to assess global pathway changes occurring in response to a chronic high fat diet (HFD). The data demonstrate that the adipocyte responds to the HFD by adopting a fibroblast-like phenotype, characterized by enhanced expression of ECM, focal adhesion and cytoskeletal genes and suppression of many adipocyte programs most notably those associated with mitochondria. This study reveals that during obesity the adipocyte progressively becomes metabolically dysfunctional due to its acquisition of fibrogenic functions. We propose that mechano-responsive transcription factors such as MRTFA and SRF contribute to both upregulation of morphological genes as well as suppression of mitochondrial programs.

The Role of the Epicardium During Heart Development and Repair

The heart is lined by a single layer of mesothelial cells called the epicardium that provides important cellular contributions for embryonic heart formation. The epicardium harbors a population of progenitor cells that undergo epithelial-to-mesenchymal transition displaying characteristic conversion of planar epithelial cells into multipolar and invasive mesenchymal cells before differentiating into nonmyocyte cardiac lineages, such as vascular smooth muscle cells, pericytes, and fibroblasts. The epicardium is also a source of paracrine cues that are essential for fetal cardiac growth, coronary vessel patterning, and regenerative heart repair. Although the epicardium becomes dormant after birth, cardiac injury reactivates developmental gene programs that stimulate epithelial-to-mesenchymal transition; however, it is not clear how the epicardium contributes to disease progression or repair in the adult. In this review, we will summarize the molecular mechanisms that control epicardium-derived progenitor cell migration, and the functional contributions of the epicardium to heart formation and cardiomyopathy. Future perspectives will be presented to highlight emerging therapeutic strategies aimed at harnessing the regenerative potential of the fetal epicardium for cardiac repair.

Mkl1-dependent gene activation is sufficient to induce actin cap assembly

Actin-dependent forces mechanically control both the position and shape of the nucleus. While the mechanisms that establish nuclear position are well defined, less understood is how actin filaments determine nuclear shape. We recently showed that nuclear envelope-spanning LINC complexes promote stress fiber assembly by activating the small GTPase RhoA and Mkl1-dependent gene activation. We now report that a subset of these stress fibers associate with the apical face of the nuclear envelope through LINC complexes that contain the inner nuclear membrane protein Sun2. Apical stress fibers have previously been shown to specifically couple cell and nuclear morphology, suggesting that LINC complexes influence nuclear shape in part by regulating the small GTPase RhoA.

MKL1 mediates TGF-β-induced CTGF transcription to promote renal fibrosis

Aberrant fibrogenesis impairs the architectural and functional homeostasis of the kidneys. It also predicts poor diagnosis in patients with end-stage renal disease (ESRD). Renal tubular epithelial cells (RTEC) can trans-differentiate into myofibroblasts to produce extracellular matrix proteins and contribute to renal fibrosis. Connective tissue growth factor (CTGF) is a cytokine upregulated in RTECs during renal fibrosis. In the present study, we investigated the regulation of CTGF transcription by megakaryocytic leukemia 1 (MKL1). Genetic deletion or pharmaceutical inhibition of MKL1 in mice mitigated renal fibrosis following the unilateral ureteral obstruction procedure. Notably, MKL1 deficiency in mice downregulated CTGF expression in the kidneys. Likewise, MKL1 knockdown or inhibition in RTEs blunted TGF-β induced CTGF expression. Further, it was discovered that MKL1 bound directly to the CTGF promoter by interacting with SMAD3 to activate CTGF transcription. In addition, MKL1 mediated the interplay between p300 and WDR5 to regulate CTGF transcription. CTGF knockdown dampened TGF-β induced pro-fibrogenic response in RTEs. MKL1 activity was reciprocally regulated by CTGF. In conclusion, we propose that targeting the MKL1-CTGF axis may generate novel therapeutic solutions against aberrant renal fibrogenesis.