MKL1 is an epigenetic modulator of TGF-β induced fibrogenesis

Resident Fibroblast MKL1 Is Sufficient to Drive Pro-fibrogenic Response in Mice

Fibrosis is an evolutionarily conserved pathophysiological process serving bifurcated purposes. On the one hand, fibrosis is essential for wound healing and contributes to the preservation of organ function. On the other hand, aberrant fibrogenic response may lead to tissue remodeling and precipitate organ failure. Recently lineage tracing studies have shown that resident fibroblasts are the primary mediator of fibrosis taking place in key organs such as the heart, the lungs, and the kidneys. Megakaryocytic leukemia 1 (MKL1) is transcriptional regulator involved in tissue fibrosis. Here we generated resident fibroblast conditional MKL1 knockout (CKO) mice by crossing the Mkl1^f/f mice to the Col1a2-Cre^ERT2 mice. Models of cardiac fibrosis, pulmonary fibrosis, and renal fibrosis were reproduced in the CKO mice and wild type (WT) littermates. Compared to the WT mice, the CKO mice displayed across-the-board attenuation of fibrosis in different models. Our data cement the pivotal role MKL1 plays in tissue fibrosis but point to the cellular origin from which MKL1 exerts its pro-fibrogenic effects.

MiR-210-5p regulates STAT3 activation by targeting STAT5A in the differentiation of dermal fibroblasts

Elucidating the molecular mechanism of the microRNAs in skin fibrosis is critical for identifying a novel therapeutic strategy for hypertrophic scar (HS). In this study, it was shown that miR-210-5p is induced by TGFβ, and that overexpression of miR-210-5p promoted the differentiation of human dermal fibroblasts (HDFs) into myofibroblasts. STAT5A is required for TGFβ-induced STAT3 activity. Here, we show that miR-210-5p attenuated TGFβ-induced STAT3 signaling pathway by suppressing the expression of STAT5A. Taken together, the present study suggests that TGFβ-induced miR-210-5p reduced STAT5A expression, leading to aberrant activation of STAT3, and facilitate skin fibrosis in HDFs.

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.

Alveolar cells under mechanical stressed niche: critical contributors to pulmonary fibrosis

Pulmonary fibrosis arises from the repeated epithelial mild injuries and insufficient repair lead to over activation of fibroblasts and excessive deposition of extracellular matrix, which result in a mechanical stretched niche. However, increasing mechanical stress likely exists before the establishment of fibrosis since early micro injuries increase local vascular permeability and prompt cytoskeletal remodeling which alter cellular mechanical forces. It is noteworthy that COVID-19 patients with severe hypoxemia will receive mechanical ventilation as supportive treatment and subsequent pathology studies indicate lung fibrosis pattern. At advanced stages, mechanical stress originates mainly from the stiff matrix since boundaries between stiff and compliant parts of the tissue could generate mechanical stress. Therefore, mechanical stress has a significant role in the whole development process of pulmonary fibrosis. The alveoli are covered by abundant capillaries and function as the main gas exchange unit. Constantly subject to variety of damages, the alveolar epithelium injuries were recently recognized to play a vital role in the onset and development of idiopathic pulmonary fibrosis. In this review, we summarize the literature regarding the effects of mechanical stress on the fundamental cells constituting the alveoli in the process of pulmonary fibrosis, particularly on epithelial cells, capillary endothelial cells, fibroblasts, mast cells, macrophages and stem cells. Finally, we briefly review this issue from a more comprehensive perspective: the metabolic and epigenetic regulation.

MKL1 Mediates TGF-β Induced RhoJ Transcription to Promote Breast Cancer Cell Migration and Invasion

Differential regulation of gene transcription contributes to cancer metastasis. We investigated the involvement of a Rho GTPase (RhoJ) in breast cancer metastasis focusing on the mechanism underlying RhoJ trans-activation by pro-metastatic cues. We report that expression of RhoJ was up-regulated in malignant breast cancer cells compared to more benign ones. Higher RhoJ expression was also detected in human breast cancer biopsy specimens of advanced stages. RhoJ depletion attenuated breast cancer cell migration and invasion in vitro and metastasis in vivo. The pro-metastatic stimulus TGF-β activated RhoJ via megakaryocytic leukemia 1 (MKL1). MKL1 interacted with and was recruited by ETS-related gene 1 (ERG1) to the RhoJ promoter to activate transcription. In conclusion, our data delineate a novel transcriptional pathway that contributes to breast cancer metastasis. Targeting the ERG1-MKL1-RhoJ axis may be considered as a reasonable approach to treat malignant breast cancer.

An Interplay Between MRTF-A and the Histone Acetyltransferase TIP60 Mediates Hypoxia-Reoxygenation Induced iNOS Transcription in Macrophages

Cardiac ischemia-reperfusion injury (IRI) represents a major pathophysiological event associated with permanent loss of heart function. Several inter-dependent processes contribute to cardiac IRI that include accumulation of reactive oxygen species (ROS), aberrant inflammatory response, and depletion of energy supply. Inducible nitric oxide synthase (iNOS) is a pro-inflammatory mediator and a major catalyst of ROS generation. In the present study we investigated the epigenetic mechanism whereby iNOS transcription is up-regulated in macrophages in the context of cardiac IRI. We report that germline deletion or systemic inhibition of myocardin-related transcription factor A (MRTF-A) in mice attenuated up-regulation of iNOS following cardiac IRI in the heart. In cultured macrophages, depletion or inhibition of MRTF-A suppressed iNOS induction by hypoxia-reoxygenation (HR). In contrast, MRTF-A over-expression potentiated activation of the iNOS promoter by HR. MRTF-A directly binds to the iNOS promoter in response to HR stimulation. MRTF-A binding to the iNOS promoter was synonymous with active histone modifications including trimethylated H3K4, acetylated H3K9, H3K27, and H4K16. Further analysis revealed that MRTF-A interacted with H4K16 acetyltransferase TIP60 to synergistically activate iNOS transcription. TIP60 depletion or inhibition achieved equivalent effects as MRTF-A depletion/inhibition in terms of iNOS repression. Of interest, TIP60 appeared to form a crosstalk with the H3K4 trimethyltransferase complex to promote iNOS trans-activation. In conclusion, we data suggest that the MRTF-A-TIP60 axis may play a critical role in iNOS transcription in macrophages and as such be considered as a potential target for the intervention of cardiac IRI.

Activation of TWIST Transcription by Chromatin Remodeling Protein BRG1 Contributes to Liver Fibrosis in Mice

Liver fibrosis is a complex pathophysiological process to which many different cell types contribute. Endothelial cells play versatile roles in the regulation of liver fibrosis. The underlying epigenetic mechanism is not fully appreciated. In the present study, we investigated the role of BRG1, a chromatin remodeling protein, in the modulation of endothelial cells in response to pro-fibrogenic stimuli in vitro and liver fibrosis in mice. We report that depletion of BRG1 by siRNA abrogated TGF-β or hypoxia induced down-regulation of endothelial marker genes and up-regulation of mesenchymal marker genes in cultured endothelial cells. Importantly, endothelial-specific BRG1 deletion attenuated CCl₄ induced liver fibrosis in mice. BRG1 knockdown in vitro or BRG1 knockout in vivo was accompanied by the down-regulation of TWIST, a key regulator of endothelial phenotype. Mechanistically, BRG1 interacted with and was recruited to the TWIST promoter by HIF-1α to activate TWIST transcription. BRG1 silencing rendered a more repressive chromatin structure surrounding the TWIST promoter likely contributing to TWIST down-regulation. Inhibition of HIF-1α activity dampened liver fibrosis in mice. Similarly, pharmaceutical inhibition of TWIST alleviated liver fibrosis in mice. In conclusion, our data suggest that epigenetic activation of TWIST by BRG1 contributes to the modulation of endothelial phenotype and liver fibrosis. Therefore, targeting the HIF1α-BRG1-TWIST axis may yield novel therapeutic solutions to treat liver fibrosis.

Deacetylation of MRTF-A by SIRT1 defies senescence induced down-regulation of collagen type I in fibroblast cells

Aging provokes both morphological and functional changes in cells, which are accompanied by a fundamental shift in gene expression patterns. One of the characteristic alterations associated with senescence in fibroblast cells is the down-regulation of collagen type I genes. In the present study, we investigated the contribution of myocardin-related transcription factor A, or MRTF-A, in this process. In mouse embryonic fibroblast (MEF) cells and human foreskin fibroblast (HFF) cells, senescence, induced by either progressive passage or treatment with hydrogen peroxide (H₂O₂), led to augmented lysine acetylation of MRTF-A paralleling down-regulation of collagen type I and SIRT1, a lysine deacetylase. SIRT1 interacted with MRTF-A to promote MRTF-A deacetylation. SIRT1 over-expression or activation by selective agonists enhanced trans-activation of the collagen promoters by MRTF-A. On the contrary, SIRT1 depletion or inhibition by specific antagonists suppressed trans-activation of the collagen promoters by MRTF-A. Likewise, mutation of four lysine residues within MRTF-A rendered it more potent in terms of activating the collagen promoters but unresponsive to SIRT1. Importantly, SIRT1 activation in senescent fibroblasts mitigated repression of collagen type I expression whereas SIRT1 inhibition promoted the loss of collagen type I expression prematurely in young fibroblasts. Mechanistically, SIRT1 enhanced the affinity of MRTF-A for the collagen type I promoters. In conclusion, our data unveil a novel mechanism that underscores aging-associated loss of collagen type I in fibroblasts via SIRT1-mediated post-translational modification of MRTF-A.

Mechanotransduction in the Cardiovascular System: From Developmental Origins to Homeostasis and Pathology

With the term ‘mechanotransduction’, it is intended the ability of cells to sense and respond to mechanical forces by activating intracellular signal transduction pathways and the relative phenotypic adaptation. While a known role of mechanical stimuli has been acknowledged for developmental biology processes and morphogenesis in various organs, the response of cells to mechanical cues is now also emerging as a major pathophysiology determinant. Cells of the cardiovascular system are typically exposed to a variety of mechanical stimuli ranging from compression to strain and flow (shear) stress. In addition, these cells can also translate subtle changes in biophysical characteristics of the surrounding matrix, such as the stiffness, into intracellular activation cascades with consequent evolution toward pro-inflammatory/pro-fibrotic phenotypes. Since cellular mechanotransduction has a potential readout on long-lasting modifications of the chromatin, exposure of the cells to mechanically altered environments may have similar persisting consequences to those of metabolic dysfunctions or chronic inflammation. In the present review, we highlight the roles of mechanical forces on the control of cardiovascular formation during embryogenesis, and in the development and pathogenesis of the cardiovascular system.

siRNA- and miRNA-based therapeutics for liver fibrosis

Liver fibrosis is a wound-healing process induced by chronic liver injuries, such as nonalcoholic steatohepatitis, hepatitis, alcohol abuse, and metal poisoning. The accumulation of excessive extracellular matrix (ECM) in the liver is a key characteristic of liver fibrosis. Activated hepatic stellate cells (HSCs) are the major producers of ECM and therefore play irreplaceably important roles during the progression of liver fibrosis. Liver fibrogenesis is highly correlated with the activation of HSCs, which is regulated by numerous profibrotic cytokines. Using RNA interference to downregulate these cytokines in activated HSCs is a promising strategy to reverse liver fibrosis. Meanwhile, microRNAs (miRNAs) have also been exploited for the treatment of liver fibrosis. This review focuses on the current siRNA- and miRNA-based liver fibrosis treatment strategies by targeting activated HSCs in the liver.

MKL1 promotes endothelial-to-mesenchymal transition and liver fibrosis by activating TWIST1 transcription

Excessive fibrogenic response in the liver disrupts normal hepatic anatomy and function heralding such end-stage liver diseases as hepatocellular carcinoma and cirrhosis. Sinusoidal endothelial cells contribute to myofibroblast activation and liver fibrosis by undergoing endothelial-mesenchymal transition (EndMT). The underlying mechanism remains poorly defined. Here we report that inhibition or endothelial-specific deletion of MKL1, a transcriptional modulator, attenuated liver fibrosis in mice. MKL1 inhibition or deletion suppressed EndMT induced by TGF-β. Mechanistically, MKL1 was recruited to the promoter region of TWIST1, a master regulator of EndMT, and activated TWIST1 transcription in a STAT3-dependent manner. A small-molecule STAT3 inhibitor (C188-9) alleviated EndMT in cultured cells and bile duct ligation (BDL) induced liver fibrosis in mice. Finally, direct inhibition of TWIST1 by a small-molecule compound harmine was paralleled by blockade of EndMT in cultured cells and liver fibrosis in mice. In conclusion, our data unveil a novel mechanism underlying EndMT and liver fibrosis and highlight the possibility of targeting the STAT3-MKL1-TWIST1 axis in the intervention of aberrant liver fibrogenesis.

TGF-β in Hepatic Stellate Cell Activation and Liver Fibrogenesis-Updated 2019

Liver fibrosis is an advanced liver disease condition, which could progress to cirrhosis and hepatocellular carcinoma. To date, there is no direct approved antifibrotic therapy, and current treatment is mainly the removal of the causative factor. Transforming growth factor (TGF)-β is a master profibrogenic cytokine and a promising target to treat fibrosis. However, TGF-β has broad biological functions and its inhibition induces non-desirable side effects, which override therapeutic benefits. Therefore, understanding the pleiotropic effects of TGF-β and its upstream and downstream regulatory mechanisms will help to design better TGF-β based therapeutics. Here, we summarize recent discoveries and milestones on the TGF-β signaling pathway related to liver fibrosis and hepatic stellate cell (HSC) activation, emphasizing research of the last five years. This comprises impact of TGF-β on liver fibrogenesis related biological processes, such as senescence, metabolism, reactive oxygen species generation, epigenetics, circadian rhythm, epithelial mesenchymal transition, and endothelial-mesenchymal transition. We also describe the influence of the microenvironment on the response of HSC to TGF-β. Finally, we discuss new approaches to target the TGF-β pathway, name current clinical trials, and explain promises and drawbacks that deserve to be adequately addressed.

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.

A cAbl-MRTF-A Feedback Loop Contributes to Hepatic Stellate Cell Activation

Trans-differentiation of quiescent hepatic stellate cells (HSC) to myofibroblasts is a hallmark event in liver fibrosis. Previous studies have led to the discovery that myocardin-related transcription factor A (MRTF-A) is a key regulator of HSC trans-differentiation or, activation. In the present study we investigated the interplay between MRTF-A and c-Abl (encoded by Abl1), a tyrosine kinase, in this process. We report that hepatic expression levels of c-Abl were down-regulated in MRTF-A knockout (KO) mice compared to wild type (WT) littermates in several different models of liver fibrosis. MRTF-A deficiency also resulted in c-Abl down-regulation in freshly isolated HSCs from the fibrotic livers of mice. MRTF-A knockdown or inhibition repressed c-Abl in cultured HSCs in vitro. Further analyses revealed that MRTF-A directly bound to the Abl1 promoter to activate transcription by interacting with Sp1. Reciprocally, pharmaceutical inhibition of c-Abl suppressed MRTF-A activity. Mechanistically, c-Abl activated extracellular signal-regulated kinase (ERK), which in turn phosphorylated MRTF-A and promoted MRTF-A nuclear trans-localization. In conclusion, our data suggest that a c-Abl-MRTF-A positive feedback loop contributes to HSC activation and liver fibrosis.

Serum response factor (SRF) promotes ROS generation and hepatic stellate cell activation by epigenetically stimulating NCF1/2 transcription

Activation of hepatic stellate cells (HSC) is a hallmark event in liver fibrosis. Accumulation of reactive oxygen species (ROS) serves as a driving force for HSC activation. The regulatory subunits of the NOX complex, NCF1 (p47phox) and NCF2 (p67phox), are up-regulated during HSC activation contributing to ROS production and liver fibrosis. The transcriptional mechanism underlying NCF1/2 up-regulation is not clear. In the present study we investigated the role of serum response factor (SRF) in HSC activation focusing on the transcriptional regulation of NCF1/2. We report that compared to wild type littermates HSC-conditional SRF knockout (CKO) mice exhibited a mortified phenotype of liver fibrosis induced by thioacetamide (TAA) injection or feeding with a methionine-and-choline deficient diet (MCD). More importantly, SRF deletion attenuated ROS levels in HSCs in vivo. Similarly, SRF knockdown in cultured HSCs suppressed ROS production in vitro. Further analysis revealed that SRF deficiency resulted in repression of NCF1/NCF2 expression. Mechanistically, SRF regulated epigenetic transcriptional activation of NCF1/NCF2 by interacting with and recruiting the histone acetyltransferase KAT8 during HSC activation. In conclusion, we propose that SRF integrates transcriptional activation of NCF1/NCF2 and ROS production to promote liver fibrosis.

MKL1 overexpression predicts poor prognosis in patients with papillary thyroid cancer and promotes nodal metastasis

Papillary thyroid cancer (PTC), the most common thyroid malignancy, has a strong propensity for cervical lymph node metastasis (LNM), which increases the risk of locoregional recurrence and decreases survival probability in some high-risk groups. Hence, there is a pressing requirement for a reliable biomarker to predict LNM in thyroid cancer. In the present study, MKL1 (also known as MRTFA) expression was significantly increased in PTC patients with LNM compared with those without. Further receiver operating characteristic (ROC) analysis showed that MKL1 expression had a diagnostic value in the differentiation of LNM in PTC. Furthermore, Kaplan-Meier analysis revealed that high MKL1 expression was associated with significantly decreased survival in PTC. Additionally, our study indicated that MKL1 promoted the migration and invasion of PTC cells. MKL1 interacted with and recruited Smad3 to the promoter of MMP2 to activate MMP2 transcription upon treatment with TGF-β. Moreover, there was significant correlation between expression of TGF-β, MKL1 and MMP2 in our clinical cohort of specimens from individuals with PTC. Our results suggest that the detection of MKL1 expression could be used to predict cervical LNM and inform post-operative follow-up in individuals with PTC.

Ablation of serum response factor in hepatic stellate cells attenuates liver fibrosis

Trans-differentiation, or activation, of hepatic stellate cells (HSCs) is a hallmark event in liver fibrosis although the underlying mechanism is not fully appreciated. Serum response factor (SRF) is a pleiotropic sequence-specific transcription factor with a ubiquitous expression pattern. In the present study, we investigated the effect of HSC-specific ablation of SRF on liver fibrosis in vivo and the underlying mechanism. We report that SRF bound to the promoter regions of pro-fibrogenic genes, including collagen type I (Col1a1/Col1a2) and alpha smooth muscle actin (Acta2), with greater affinity in activated HSCs compared to quiescent HSCs. Ablation of SRF in HSCs in vitro downregulated the expression of fibrogenic genes by dampening the accumulation of active histone marks. SRF also interacted with MRTF-A, a well-documented co-factor involved in liver fibrosis, on the pro-fibrogenic gene promoters during HSC activation. In addition, SRF directly regulated MRTF-A transcription in activated HSCs. More importantly, HSC conditional SRF knockout (CKO) mice developed a less robust pro-fibrogenic response in the liver in response to CCl₄ injection and BDL compared to wild-type littermates. In conclusion, our data demonstrate that SRF may play an essential role in HSC activation and liver fibrosis. KEY MESSAGES: • SRF deficiency decelerates activation of hepatic stellate cells (HSCs) in vitro. • SRF epigenetically activates pro-fibrogenic transcription to promote HSC maturation. • SRF interacts with MRTF-A and contributes to MRTF-A transcription. • Conditional SRF deletion in HSCs attenuates BDL-induced liver fibrosis in mice. • Conditional SRF ablation in HSCs attenuates CCl₄-induced liver fibrosis in mice.

New insights into the genetics and epigenetics of systemic sclerosis

Systemic sclerosis (SSc) is a severe autoimmune disease that is characterized by vascular abnormalities, immunological alterations and fibrosis of the skin and internal organs. The results of genetic studies in patients with SSc have revealed statistically significant genetic associations with disease manifestations and progression. Nevertheless, genetic susceptibility to SSc is moderate, and the functional consequences of genetic associations remain only partially characterized. A current hypothesis is that, in genetically susceptible individuals, epigenetic modifications constitute the driving force for disease initiation. As epigenetic alterations can occur years before fibrosis appears, these changes could represent a potential link between inflammation and tissue fibrosis. Epigenetics is a fast-growing discipline, and a considerable number of important epigenetic studies in SSc have been published in the past few years that span histone post-translational modifications, DNA methylation, microRNAs and long non-coding RNAs. This Review describes the latest insights into genetic and epigenetic contributions to the pathogenesis of SSc and aims to provide an improved understanding of the molecular pathways that link inflammation and fibrosis. This knowledge will be of paramount importance for the development of medicines that are effective in treating or even reversing tissue fibrosis.

MKL1 mediates TNF-α induced pro-inflammatory transcription by bridging the crosstalk between BRG1 and WDR5

Tumor necrosis factor alpha (TNF-α) is a cytokine that can potently stimulate the synthesis of a range of pro-inflammatory mediators in macrophages. The underlying epigenetic mechanism, however, is underexplored. Here we report that the transcriptional modulator megakaryocytic leukemia 1 (MKL1) is associated with a histone H3K4 methyltransferase activity. Re-ChIP assay suggests that MKL1 interacts with and recruits WDR5, a component of the COMPASS complex responsible for H3K4 methylation, to the promoter regions of pro-inflammatory genes in macrophages treated with TNF-α. WDR5 enhances the ability of MKL1 to stimulate the promoter activities of pro-inflammatory genes. In contrast, silencing of WDR5 attenuates TNF-α induced production of pro-inflammatory mediators and erases the H3K4 methylation from the gene promoters. Of interest, the chromatin remodeling protein BRG1 also plays an essential role in maintaining H3K4 methylation on MKL1 target promoters by interacting with WDR5. MKL1 knockdown disrupts the interaction between BRG1 and WDR5. Together, our data illustrate a role for MKL1 in moderating the crosstalk between BRG1 and WDR5 to activate TNF-α induced pro-inflammatory transcription in macrophages.

The histone demethylase Kdm4 suppresses activation of hepatic stellate cell by inducing MiR-29 transcription

One of the hallmark events during liver fibrosis is the transition of quiescent hepatic stellate cells (HSC) into activated myofibroblasts, which are responsible for the production and deposition of pro-fibrogenic proteins. The epigenetic mechanism underlying HSC trans-differentiation is not fully understood. In the present study we investigated the contribution of histone H3K9 demethylase KDM4 in this process. We report that expression levels of KDM4 were down-regulated during HSC activation paralleling the up-regulation of alpha smooth muscle cell actin (Acta2), a marker of mature myofibroblast. Furthermore, HSCs isolated from mice induced to develop liver fibrosis exhibit lowered KDM4 expression compared to the control mice. In accordance, KDM4 depletion with siRNA accelerated HSC activation. Of interest, the loss of KDM4 was mirrored by the repression of miR-29, an antagonist of liver fibrosis, during HSC activation both in vitro and in vivo. KDM4 knockdown resulted in the down-regulation of miR-29 expression. Mechanistically, the sequence-specific transcription factor SREBP2 interacted with KDM4 to activate miR-29 transcription. In conclusion, our data delineate a novel epigenetic mechanism underlying HSC activation. Targeting this axis may yield potential therapeutics against liver fibrosis.

A Cross Talk Between BRG1 and Males Absent on the First Contributes to Reactive Oxygen Species Production in a Mouse Model of Nonalcoholic Steatohepatitis

Aims: Accumulation of reactive oxygen species (ROS) in hepatocytes in response to excessive nutrients and the ensuing liver damages caused by ROS constitute a key pathophysiological event in nonalcoholic steatohepatitis (NASH). In the present study, we investigated the epigenetic mechanism underlying ROS production in NASH pathogenesis. Results: NASH was induced by feeding the mice with a methionine-and-choline-deficient (MCD) diet for 4 weeks. Compared with the control mice (wild type [WT]), mice with hepatocyte-specific deletion of Brg1 (HepcKO), a core component of the mammalian chromatin remodeling complex, developed a less severe form of NASH when fed on the MCD diet. Importantly, ROS levels were attenuated in HepcKO mice as opposed to WT mice. Brahma-related gene 1 (Brg1) deficiency downregulated the transcription of NADPH oxidases (NOX1, NOX2, and NOX4) both in vivo and in vitro. Mechanistically, Brg1 deletion rendered a more repressive chromatin structure surrounding the NOX promoters as characterized by reduced levels of acetylated histones. In addition, Brg1 interacted with the histone H4K16 acetyltransferase males absent on the first (MOF) to activate NOX transcription. MOF knockdown by small interfering RNA or pharmaceutical inhibition by MG149 suppressed NOX transcription and ameliorated ROS levels. Innovation: Our data highlight a novel epigenetic mechanism through which Brg1 and MOF cooperate to regulate ROS production in hepatocytes in response to pro-NASH stimuli. Conclusion: A cross talk between Brg1 and MOF epigenetically activates NOX transcription and elevates ROS synthesis contributing to NASH pathogenesis.

Megakaryocytic leukemia 1 (MKL1) mediates high glucose induced epithelial-mesenchymal transition by activating LOX transcription

Diabetic retinopathy (DR) is one of the most devastating complications of diabetes mellitus. When exposed to high glucose (HG), retinal epithelial cells undergo profound alterations both morphologically and functionally in a well-conserved process known as epithelial-to-mesenchymal transition (EMT). The mechanism governing HG-induced EMT in retinal epithelial cells is not completely understood. Here we report that treatment with 25 mM glucose led to EMT in retinal pigmented epithelial cells (RPE) characterized by a simultaneous down-regulation of E-Cadherin (encoded by CDH1) and up-regulation of alpha smooth muscle actin (encoded by ACTA2). HG-induced EMT in RPEs was accompanied by augmented expression and enhanced nuclear enrichment of MKL1, a transcriptional modulator. In contrast, MKL1 knockdown by siRNA or inhibition by CCG-1423 abrogated HG-induced EMT in RPEs. Of interest, MKL1 mediated the transcriptional activation of LOX, a mesenchymal marker, in RPEs in response to HG stimulation. Mechanistically, MKL1 interacted with and was recruited by AP-1 to the proximal LOX promoter to promote LOX trans-activation likely through altering the chromatin structure. Finally, LOX depletion by siRNA or inhibition by aminopropionitrile in RPEs abolished HG-induced EMT. In conclusion, our data support a role for MKL1 in mediating HG-induced EMT in retinal epithelial cells via epigenetic activation of LOX transcription.

Unraveling SSc Pathophysiology; The Myofibroblast

Systemic sclerosis (SSc) is a severe auto-immune disease, characterized by vasculopathy and fibrosis of connective tissues. SSc has a high morbidity and mortality and unfortunately no disease modifying therapy is currently available. A key cell in the pathophysiology of SSc is the myofibroblast. Myofibroblasts are fibroblasts with contractile properties that produce a large amount of pro-fibrotic extracellular matrix molecules such as collagen type I. In this narrative review we will discuss the presence, formation, and role of myofibroblasts in SSc, and how these processes are stimulated and mediated by cells of the (innate) immune system such as mast cells and T helper 2 lymphocytes. Furthermore, current novel therapeutic approaches to target myofibroblasts will be highlighted for future perspective.

Enhancer of Zeste Homologue 2 Inhibition Attenuates TGF-β Dependent Hepatic Stellate Cell Activation and Liver Fibrosis

BACKGROUND & AIMS

Transdifferentiation of hepatic stellate cells (HSCs) into myofibroblasts is a key event in the pathogenesis of liver fibrosis. Transforming growth factor β (TGF-β) and platelet-derived growth factor (PDGF) are canonical HSC activators after liver injury. The aim of this study was to analyze the epigenetic modulators that differentially control TGF-β and PDGF signaling pathways.

METHODS

We performed a transcriptomic comparison of HSCs treated with TGF-β or PDGF-BB using RNA sequencing. Among the targets that distinguish these 2 pathways, we focused on the histone methyltransferase class of epigenetic modulators.

RESULTS

Enhancer of zeste homolog 2 (EZH2) was expressed differentially, showing significant up-regulation in HSCs activated with TGF-β but not with PDGF-BB. Indeed, EZH2 inhibition using either a pharmacologic (GSK-503) or a genetic (small interfering RNA) approach caused a significant attenuation of TGF-β-induced fibronectin, collagen 1α1, and α-smooth muscle actin, both at messenger RNA and protein levels. Conversely, adenoviral overexpression of EZH2 in HSCs resulted in a significant stimulation of fibronectin protein and messenger RNA levels in TGF-β-treated cells. Finally, we conducted in vivo experiments with mice chronically treated with carbon tetrachloride or bile duct ligation. Administration of GSK-503 to mice receiving either carbon tetrachloride or bile duct ligation led to attenuated fibrosis as assessed by Trichrome and Sirius red stains, hydroxyproline, and α-smooth muscle actin/collagen protein assays.

CONCLUSIONS

TGF-β and PDGF share redundant and distinct transcriptomic targets, with the former predominating in HSC activation. The EZH2 histone methyltransferase is preferentially involved in the TGF-β as opposed to the PDGF signaling pathway. Inhibition of EZH2 attenuates fibrogenic gene transcription in TGF-β-treated HSCs and reduces liver fibrosis in vivo. The data discussed in this publication have been deposited in NCBI's Gene Expression Omnibus and are accessible through GEO Series accession number GSE119606 (https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE119606).

The chromatin remodeling protein BRG1 regulates APAP-induced liver injury by modulating CYP3A11 transcription in hepatocyte

Acetaminophen (APAP) overdose represents the most frequent cause of acute liver failure. The underlying epigenetic mechanism is not fully understood. In the present study we investigated the mechanism whereby the chromatin remodeling protein brahma related gene 1 (Brg1) regulates APAP induced liver injury in mice. We report that hepatocyte-specific deletion of Brg1 attenuated APAP induced liver injury in mice as evidenced by reduced plasma ALT and AST levels, decreased liver necrosis, amelioration of GSH depletion, and prolonged survival. Brg1 regulated APAP-induced liver injury likely by stimulating the transcription of Cyp3a11, a key cytochrome enzyme involved in APAP metabolism. Immunoprecipitation coupled with DNA affinity microarray identified hepatocyte nuclear factor 4 (HNF4) as a novel binding partner for Brg1. HNF4 recruited Brg1 to the Cyp3a11 promoter and formed a complex with Brg1 to trans-activate Cyp3a11. In contrast, BRG1 deficiency attenuated HNF4 binding to the Cyp3a11 promoter and dampened Cyp3a11 transcription. Therefore, our data suggest that Brg1 might play an essential role mediating APAP induced liver injury in vivo.

Epigenetic activation of PERP transcription by MKL1 contributes to ROS-induced apoptosis in skeletal muscle cells

Excessive reactive oxygen species (ROS) causes irreparable damages to cells and commit cells to programmed cell death or apoptosis. A panel of well-documented pro-apoptotic genes, including p53 apoptosis effector related to PMP-22 (PERP), are up-regulated and collectively mediate ROS induced apoptosis. The epigenetic mechanism whereby ROS stimulates PERP transcription, however, lacks in-depth characterization. Here we report that the transcriptional modulator megakaryocytic leukemia 1 (MKL1) is activated by H2O2 treatment in skeletal muscle cells (C2C12). Small interfering RNA (siRNA) mediated silencing or small-molecule compound (CCG-1423) mediated inhibition of MKL1 attenuated H2O2 induced apoptosis of C2C12 cells. Over-expression of MKL1 potentiated trans-activation of PERP whereas MKL1 ablation/inhibition abrogated the induction of PERP by H2O2 in C2C12 cells. Mechanistically, MKL1 interacted with and was recruited to the PERP promoter by the transcription factor E2F1. Once bound to the PERP promoter, MKL1 engaged the histone demethylase KDM3A to modulate the chromatin structure surrounding the PERP promoter thereby leading to PERP trans-activation. Depletion of either E2F1 or KDM3A blocked the induction of PERP by H2O2. In conclusion, our data illustrate a novel epigenetic pathway that links PERP transcription to ROS-induced apoptosis in skeletal muscle cells.

Brahma related gene 1 (Brg1) contributes to liver regeneration by epigenetically activating the Wnt/β-catenin pathway in mice

Liver regeneration is a complicated pathophysiologic process that is regulated by a myriad of signaling pathways and transcription factors. The interaction among these pathways and factors, either cooperatively or antagonistically, may ultimately lead to recovery and restoration of liver function or permanent loss of liver function and liver failure. In the present study, we investigated the mechanism whereby the chromatin remodeling protein brahma related gene 1 (Brg1) regulates liver regeneration in mice. The Smarca4-Flox strain of mice was crossbred with the Alb-Cre strain to generate hepatocyte-specific Brg1 knockout mice. Liver injury was induced by partial hepatectomy (PHx). We report that Brg1 deletion in hepatocyte compromised liver regeneration and dampened survival after PHx in mice. Brg1 interacted with β-catenin to potentiate Wnt signaling and promote hepatocyte proliferation. Mechanistically, Brg1 recruited lysine demethylase 4 (KDM4) to activate β-catenin target genes. Our data suggest that Brg1 might play an essential role maintaining hepatic homeostasis and contributing to liver repair.-Li, N., Kong, M., Zeng, S., Hao, C., Li, M., Li, L., Xu, Z., Zhu, M., Xu, Y. Brahma related gene 1 (Brg1) contributes to liver regeneration by epigenetically activating the Wnt/β-catenin pathway in mice.

p300/CBP as a Key Nutritional Sensor for Hepatic Energy Homeostasis and Liver Fibrosis

The overwhelming frequency of metabolic diseases such as obesity and diabetes are closely related to liver diseases, which might share common pathogenic signaling processes. These metabolic disorders in the presence of inflammatory response seem to be triggered by and to reside in the liver, which is the central metabolic organ that plays primary roles in regulating lipid and glucose homeostasis upon alterations of metabolic conditions. Recently, abundant emerging researches suggested that p300 and CREB binding protein (CBP) are crucial regulators of energy homeostasis and liver fibrosis through both their acetyltransferase activities and transcriptional coactivators. Plenty of recent findings demonstrated the potential roles of p300/CBP in mammalian metabolic homeostasis in response to nutrients. This review is focused on the different targets and functions of p300/CBP in physiological and pathological processes, including lipogenesis, lipid export, gluconeogenesis, and liver fibrosis, also provided some nutrients as the regulator of p300/CBP for nutritional therapeutic approaches to treat liver diseases.

HDAC4 stimulates MRTF-A expression and drives fibrogenesis in hepatic stellate cells by targeting miR-206

Activation of hepatic stellate cells (HSCs) is a hallmark event during liver fibrogenesis. We have previously shown that the transcriptional modulator MRTF-A contributes to liver fibrosis by programming epigenetic activation of HSCs. In the present study we investigated the mechanism whereby MRTF-A expression is regulated in this process. We report here that MRTF-A protein levels, but not mRNA levels, were up-regulated in vivo in the livers of mice induced to develop hepatic fibrosis. Pro-fibrogenic stimuli (TGF-β and PDGF-BB) also activated MRTF-A expression post-transcriptionally in vitro in cultured HSCs. miR-206 bound to the 3′-UTR of MRTF-A presumably to inhibit translation. miR-206 levels were down-regulated in response to pro-fibrogenic stimuli in vivo and in vitro allowing MRTF-A proteins to accumulate. Mechanistically, histone deacetylase 4 (HDAC4) was induced by pro-fibrogenic stimuli and recruited to the miR-206 promoter to repress miR-206 transcription. HDAC4 stimulated MRTF-A expression and drove fibrogenesis in HSCs in a miR-206 dependent manner. Therefore, our data reveal an HDAC4-miR-206-MRTF-A axis that can play a potentially important role in HSC activation and liver fibrosis.

Mechanosensing and fibrosis

Tissue injury disrupts the mechanical homeostasis that underlies normal tissue architecture and function. The failure to resolve injury and restore homeostasis gives rise to progressive fibrosis that is accompanied by persistent alterations in the mechanical environment as a consequence of pathological matrix deposition and stiffening. This Review focuses on our rapidly growing understanding of the molecular mechanisms linking the altered mechanical environment in injury, repair, and fibrosis to cellular activation. In particular, our focus is on the mechanisms by which cells transduce mechanical signals, leading to transcriptional and epigenetic responses that underlie both transient and persistent alterations in cell state that contribute to fibrosis. Translation of these mechanobiological insights may enable new approaches to promote tissue repair and arrest or reverse fibrotic tissue remodeling.

SIRT1 antagonizes liver fibrosis by blocking hepatic stellate cell activation in mice

Hepatic stellate cells (HSCs) are a major source of fibrogenesis in the liver, contributing to cirrhosis. When activated, HSCs transdifferentiate into myofibroblasts and undergo profound functional alterations paralleling an overhaul of the transcriptome, the mechanism of which remains largely undefined. We investigated the involvement of the class III deacetylase sirtuin [silent information regulator 1 (SIRT1)] in HSC activation and liver fibrosis. SIRT1 levels were down-regulated in the livers in mouse models of liver fibrosis, in patients with cirrhosis, and in activated HSCs as opposed to quiescent HSCs. SIRT1 activation halted, whereas SIRT1 inhibition promoted, HSC transdifferentiation into myofibroblasts. Liver fibrosis was exacerbated in mice with HSC-specific deletion of SIRT1 [conditional knockout (cKO)], receiving CCl₄ (1 mg/kg) injection or subjected to bile duct ligation, compared to wild-type littermates. SIRT1 regulated peroxisome proliferator activated receptor γ (PPARγ) transcription by deacetylating enhancer of zeste homolog 2 (EZH2) in quiescent HSCs. Finally, EZH2 inhibition or PPARγ activation ameliorated fibrogenesis in cKO mice. In summary, our data suggest that SIRT1 plays an essential role guiding the transition of HSC phenotypes.-Li, M., Hong, W., Hao, C., Li, L., Wu, D., Shen, A., Lu, J., Zheng, Y., Li, P., Xu, Y. SIRT1 antagonizes liver fibrosis by blocking hepatic stellate cell activation in mice.

The histone methyltransferase Suv39h2 contributes to nonalcoholic steatohepatitis in mice

Uncontrolled inflammatory response highlights the central theme of nonalcoholic steatohepatitis (NASH), a growing global pandemic. Hepatocytes and macrophages represent two major sources of hepatic inflammation during NASH pathogenesis, contributing to excessive synthesis of proinflammatory mediators. The epigenetic mechanism that accounts for the activation of hepatocytes and macrophages in this process remains obscure. Here, we report that compared to wild-type littermates, mice with a deficiency in the histone H3K9 methyltransferase suppressor of variegation 39 homolog 2 (Suv39h2, knockout) exhibited a less severe form of NASH induced by feeding with a high-fat, high-carbohydrate diet. Pro-NASH stimuli increased Suv39h2 expression in cell culture, in mice, and in human livers. In hepatocytes, Suv39h2 bound to the Sirt1 gene promoter and repressed Sirt1 transcription. Suv39h2 deficiency normalized Sirt1 expression, allowing nuclear factor kappa B/p65 to become hypoacetylated and thus dampening nuclear factor kappa B-dependent transcription of proinflammatory mediators. In macrophages, Suv39h2-mediated repression of peroxisome proliferator-activated receptor gamma transcription favored a proinflammatory M1 phenotype over an anti-inflammatory M2 phenotype, thereby elevating hepatic inflammation.

CONCLUSION

Suv39h2 plays a pivotal role in the regulation of inflammatory response in hepatocytes and macrophages, contributing to NASH pathogenesis. (Hepatology 2017;65:1904-1919).

Suv39h2 deficiency ameliorates diet-induced steatosis in mice

Steatosis is a prototypical metabolic disorder characterized by accumulation of lipid droplets in the liver, extensive hepatic inflammation, and, in advanced stages, accelerated liver fibrogenesis. The molecular mechanism underlying steatosis is not completely understood. In the present study we investigated the involvement of the histone methyltransferase Suv39h2 in the pathogenesis of steatosis. Expression of Suv39h2 was up-regulated in the liver in two different mouse models of steatosis. Suv39h2 knockout (KO) mice developed a less severe form of steatosis fed on a methione-and-choline deficient (MCD) diet, compared to wild type (WT) littermates, as evidenced by reduced levels of plasma ALT, down-regulated expression of pro-inflammatory mediators, and decreased infiltration of macrophages. In addition, Masson's trichrome staining as well as qPCR measurements of fibrogenic genes suggested that liver fibrosis was attenuated in MCD diet-fed KO mice compared to WT mice. Further analysis found that Suv39h2 repressed SIRT1 expression in the liver by stimulating histone H3K9 trimethylation surrounding the SIRT1 promoter and that Suv39h2 deficiency alleviated SIRT1 expression in MCD diet-fed mice. Therefore, our data support a role of Suv39h2 in promoting steatosis in mice likely through contributing to SIRT1 trans-reperssion.

MKL1 links epigenetic activation of MMP2 to ovarian cancer cell migration and invasion

Responding to pro-metastatic cues such as low oxygen tension, cancer cells develop several different strategies to facilitate migration and invasion. During this process, expression levels of matrix metalloproteinases (MMPs) are up-regulated so that cancer cells can more easily enter or exit the circulation. In this report we show that message levels of the transcriptional modulator MKL1 were elevated in malignant forms of ovarian cancer tissues in humans when compared to more benign forms accompanying a similar change in MMP2 expression. MKL1 silencing blocked hypoxia-induced migration and invasion of ovarian cancer cells (SKOV-3) in vitro. Over-expression of MKL1 activated while MKL1 depletion repressed MMP2 transcription in SKOV-3 cells. MKL1 was recruited to the MMP2 promoter by NF-κB in response to hypoxia. Mechanistically, MKL1 recruited a histone methyltransferase, SET1, and a chromatin remodeling protein, BRG1, and coordinated their interaction to alter the chromatin structure surrounding the MMP2 promoter leading to transcriptional activation. Both BRG1 and SET1 were essential for hypoxia-induced MMP2 trans-activation. Finally, expression levels of SET1 and BRG1 were positively correlated with ovarian cancer malignancies in humans. Together, our data suggest that MKL1 promotes ovarian cancer cell migration and invasion by epigenetically activating MMP2 transcription.

Envisioning metastasis as a transdifferentiation phenomenon clarifies discordant results on cancer

Cancer is generally conceived as a dedifferentiation process in which quiescent post-mitotic differentiated cells acquire stem-like properties and the capacity to proliferate. This view holds for the initial stages of carcinogenesis but is more questionable for advanced stages when the cells can transdifferentiate into the contractile phenotype associated to migration and metastasis. Singularly from this perspective, the hallmark of the most aggressive cancers would correspond to a genuine differentiation status, even if it is different from the original one. This seeming paradox could help reconciling discrepancies in the literature about the pro- or anti-tumoral functions of candidate molecules involved in cancer and whose actual effects depend on the tumoral grade. These ambiguities which are likely to concern a myriad of molecules and pathways, are illustrated here with the selected examples of chromatin epigenetics and myocardin-related transcription factors, using the human MCF10A and MCF7 breast cancer cells. Self-renewing stem like cells are characterized by a loose chromatin with low levels of the H3K9 trimetylation, but high levels of this mark can also appear in cancer cells acquiring a contractile-type differentiation state associated to metastasis. Similarly, the myocardin-related transcription factor MRTF-A is involved in metastasis and epithelial-mesenchymal transition, whereas this factor is naturally enriched in the quiescent cells which are precisely the most resistant to cancer: cardiomyocytes. These seeming paradoxes reflect the bistable epigenetic landscape of cancer in which dedifferentiated self-renewing and differentiated migrating states are incompatible at the single cell level, though coexisting at the population level.

Protein inhibitor of activated STAT 4 (PIAS4) regulates liver fibrosis through modulating SMAD3 activity

Excessive fibrogenesis disrupts normal liver structure, impairs liver function, and precipitates the development of cirrhosis, an irreversible end-stage liver disease. A host of factors including nutrition surplus contribute to liver fibrosis but the underlying mechanism is not fully understood. In the present study, we investigated the involvement of protein inhibitor for activated stat 4 (PIAS4) in liver fibrosis in a mouse model of non-alcoholic steatohepatitis (NASH). We report that PIAS4 silencing using short hairpin RNA (shRNA) attenuated high-fat high-carbohydrate (HFHC) diet induced liver fibrosis in mice. Quantitative PCR and Western blotting analyses confirmed that PIAS4 knockdown downregulated a panel of pro-fibrogenic genes including type I and type III collagens, smooth muscle actin, and tissue inhibitors of metalloproteinase. Mechanistically, PIAS4 silencing blocked the recruitment of SMAD3, a potent pro-fibrogenic transcription factor, to the promoter regions of pro-fibrogenic genes and dampened SMAD3 acetylation likely by upregulating SIRT1 expression. In conclusion, PIAS4 may contribute to liver fibrosis by modulating SIRT1-dependent SMAD3 acetylation.

Transcriptional repression of SIRT1 by protein inhibitor of activated STAT 4 (PIAS4) in hepatic stellate cells contributes to liver fibrosis

Interstitial fibrosis represents a key pathological process in non-alcoholic steatohepatitis (NASH). In the liver, fibrogenesis is primarily mediated by activated hepatic stellate cells (HSCs) transitioning from a quiescent state in response to a host of stimuli. The molecular mechanism underlying HSC activation is not completely understood. Here we report that there was a simultaneous up-regulation of PIAS4 expression and down-regulation of SIRT1 expression accompanying increased hepatic fibrogenesis in an MCD-diet induced mouse model of NASH. In cultured primary mouse HSCs, stimulation with high glucose activated PIAS4 while at the same time repressed SIRT1. Over-expression of PIAS4 directly repressed SIRT1 promoter activity. In contrast, depletion of PIAS4 restored SIRT1 expression in HSCs treated with high glucose. Estrogen, a known NASH-protective hormone, antagonized HSC activation by targeting PIAS4. Lentivirus-mediated delivery of short hairpin RNA (shRNA) targeting PIAS4 in mice ameliorated MCD diet induced liver fibrosis by normalizing SIRT1 expression in vivo. PIAS4 promoted HSC activation in a SIRT1-dependent manner in vitro. Mechanistically, PIAS4 mediated SIRT1 repression led to SMAD3 hyperacetylation and enhanced SMAD3 binding to fibrogenic gene promoters. Taken together, our data suggest SIRT1 trans-repression by PIAS4 plays an important role in HSC activation and liver fibrosis.

Facile synthesis of SAM-peptide conjugates through alkyl linkers targeting protein N-terminal methyltransferase 1

We report the first chemical synthesis of SAM-peptide conjugates through alkyl linkers to prepare bisubstrate analogs for protein methyltransferases. We demonstrate its application by developing a series of bisubstrate inhibitors for protein N-terminal methyltransferase 1 and the most potent one exhibits a K_i value of 310 ± 55 nM.