Myeloid MKL1 Disseminates Cues to Promote Cardiac Hypertrophy in Mice

The insulin-like growth factor binding protein-microfibrillar associated protein-sterol regulatory element binding protein axis regulates fibroblast-myofibroblast transition and cardiac fibrosis

BACKGROUND AND PURPOSE

Excessive fibrogenesis is associated with adverse cardiac remodelling and heart failure. The myofibroblast, primarily derived from resident fibroblast, is the effector cell type in cardiac fibrosis. Megakaryocytic leukaemia 1 (MKL1) is considered the master regulator of fibroblast-myofibroblast transition (FMyT). The underlying transcriptional mechanism is not completely understood. Our goal was to identify novel transcriptional targets of MKL1 that might regulate FMyT and contribute to cardiac fibrosis.

EXPERIMENTAL APPROACH

RNA sequencing (RNA-seq) performed in primary cardiac fibroblasts identified insulin-like growth factor binding protein 5 (IGFBP5) as one of the genes most significantly up-regulated by constitutively active (CA) MKL1 over-expression. IGFBP5 expression was detected in heart failure tissues using RT-qPCR and western blots.

KEY RESULTS

Once activated, IGFBP5 translocated to the nucleus to elicit a pro-FMyT transcriptional programme. Consistently, IGFBP5 knockdown blocked FMyT in vitro and dampened cardiac fibrosis in mice. Of interest, IGFBP5 interacted with nuclear factor of activated T-cell 4 (NFAT4) to stimulate the transcription of microfibril-associated protein 5 (MFAP5). MFAP5 contributed to FMyT and cardiac fibrosis by enabling sterol response element binding protein 2 (SREBP2)-dependent cholesterol synthesis.

CONCLUSIONS AND IMPLICATIONS

Our data unveil a previously unrecognized transcriptional cascade, initiated by IGFBP5, that promotes FMyT and cardiac fibrosis. Screening for small-molecule compounds that target this axis could yield potential therapeutics against adverse cardiac remodelling.

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.

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.

Epigenetic Repression of Chloride Channel Accessory 2 Transcription in Cardiac Fibroblast: Implication in Cardiac Fibrosis

Cardiac fibrosis is a key pathophysiological process that contributes to heart failure. Cardiac resident fibroblasts, exposed to various stimuli, are able to trans-differentiate into myofibroblasts and mediate the pro-fibrogenic response in the heart. The present study aims to investigate the mechanism whereby transcription of chloride channel accessory 2 (Clca2) is regulated in cardiac fibroblast and its potential implication in fibroblast-myofibroblast transition (FMyT). We report that Clca2 expression was down-regulated in activated cardiac fibroblasts (myofibroblasts) compared to quiescent cardiac fibroblasts in two different animal models of cardiac fibrosis. Clca2 expression was also down-regulated by TGF-β, a potent inducer of FMyT. TGF-β repressed Clca2 expression at the transcriptional level likely via the E-box element between -516 and -224 of the Clca2 promoter. Further analysis revealed that Twist1 bound directly to the E-box element whereas Twist1 depletion abrogated TGF-β induced Clca2 trans-repression. Twist1-mediated Clca2 repression was accompanied by erasure of histone H3/H4 acetylation from the Clca2 promoter. Mechanistically Twist1 interacted with HDAC1 and recruited HDAC1 to the Clca2 promoter to repress Clca2 transcription. Finally, it was observed that Clca2 over-expression attenuated whereas Clca2 knockdown enhanced FMyT. In conclusion, our data demonstrate that a Twist1-HDAC1 complex represses Clca2 transcription in cardiac fibroblasts, which may contribute to FMyT and cardiac fibrosis.

An E2F5-TFDP1-BRG1 Complex Mediates Transcriptional Activation of MYCN in Hepatocytes

Liver regeneration is characterized by cell cycle reentrance of hepatocytes. N-Myc, encoded by MYCN, is a member of the Myc family of transcription factors. Elevation of MYCN expression has been noted in the course of liver regeneration whereas the underlying mechanism remains unclear. Here we describe that up-regulation of MYCN expression, as measured by quantitative PCR, Western blotting, and immunohistochemical staining, paralleled liver regeneration in animal and cell models. MYCN expression was up-regulated as a result of transcriptional activation. Ingenuity pathway analysis (IPA) revealed several up-stream transcriptional regulators for MYCN and RNA interference validated E2F5 and TFDP1 as essential for hepatocyte growth factor (HGF)-induced MYCN trans-activation. Further examination showed that deficiency of BRG1, a chromatin remodeling protein, attenuated MYCN induction during liver regeneration. BRG1 interacted with and was recruited by E2F5/TFDP1 to the MYCN promoter. Mechanistically, BRG1 might play a role regulating histone H3 acetylation and H3K4 trimethylation and facilitating/stabilizing the binding of RNA polymerase II surrounding the MYCN promoter. Over-expression of ectopic MYCN in BRG1-null hepatocytes overcame deficiency of proliferation. Importantly, a positive correlation between MYCN expression and BRG1/E2F5/TFDP1 expression was observed in human liver specimens. In conclusion, our data identify a novel epigenetic pathway where an E2F5-TFDP1-BRG1 complex regulates MYCN transcription to promote liver regeneration.

HES5-mediated repression of LIGHT transcription may contribute to apoptosis in hepatocytes

Non-alcoholic fatty liver disease (NAFLD) is prototypical form of metabolic syndrome and has become a global pandemic. Hepatocytes undergo apoptosis in the pathogenesis of NAFLD. We report that the lymphokine LIGHT/TNFSF14 was upregulated in the murine NAFLD livers and in hepatocytes treated with free fatty acids (palmitate, PA). LIGHT knockdown or neutralization attenuated PA-induced apoptosis of hepatocytes. Similarly, knockdown or blockade of LTβR, the receptor for LIGHT, ameliorated apoptosis in hepatocytes exposed to PA. Ingenuity pathway analysis (IPA) revealed several Notch-related transcription factors as upstream regulators of LIGHT, of which HES5 expression was downregulated paralleling LIGHT induction in the pathogenesis of NAFLD. HES5 knockdown enhanced whereas HES5 over-expression weakened LIGHT induction in hepatocytes. HES5 was found to directly bind to the LIGHT promoter and repress LIGHT transcription. Mechanistically, HES5 interacted with SIRT1 to deacetylate histone H3/H4 on the LIGHT promoter to repress LIGHT transcription. SIRT1 knockdown or inhibition offset the effect of HES5 over-expression on LIGHT transcription and hepatocyte apoptosis. In conclusion, our data unveil a novel mechanism that might contribute to excessive apoptosis in hepatocyte exposed to free fatty acids.

Dual Regulation of Tank Binding Kinase 1 by BRG1 in Hepatocytes Contributes to Reactive Oxygen Species Production

Excessive accumulation of reactive oxygen species (ROS) is considered a major culprit for the pathogenesis of non-alcoholic fatty liver disease (NAFLD). We have previously shown that deletion of Brahma related gene 1 (BRG1) mitigated NAFLD in mice in part by attenuating ROS production in hepatocyte. Here we report that BRG1 deletion led to simultaneous down-regulation in expression and phosphorylation of tank binding kinase 1 (TBK1) in vivo and in vitro. On the one hand, BRG1 interacted with AP-1 to bind to the TBK1 promoter and directly activated TBK1 transcription in hepatocytes. On the other hand, BRG1 interacted with Sp1 to activate the transcription of c-SRC, a tyrosine kinase essential for TBK1 phosphorylation. Over-expression of c-SRC and TBK1 corrected the deficiency in ROS production in BRG1-null hepatocytes whereas depletion of TBK1 or c-SRC attenuated ROS production. In conclusion, our data suggest that dual regulation of TBK1 activity, at the transcription level and the post-transcriptional level, by BRG1 may constitute an important mechanism underlying excessive ROS production in hepatocytes.

A KDM4-DBC1-SIRT1 Axis Contributes to TGF-b Induced Mesenchymal Transition of Intestinal Epithelial Cells

Intestinal fibrosis is one of the common pathophysiological processes in inflammatory bowel diseases (IBDs). Previously it has been demonstrated that epithelial-mesenchymal transition (EMT) can contribute to the development of intestinal fibrosis. Here we report that conditional ablation of SIRT1, a class III lysine deacetylase, in intestinal epithelial cells exacerbated 2, 4, 6-trinitro-benzene sulfonic acid (TNBS) induced intestinal fibrosis in mice. SIRT1 activity, but not SIRT1 expression, was down-regulated during EMT likely due to up-regulation of its inhibitor deleted in breast cancer 1 (DBC1). TGF-β augmented the recruitment of KDM4A, a histone H3K9 demethylase, to the DBC1 promoter in cultured intestinal epithelial cells (IEC-6) leading to DBC1 trans-activation. KDM4A depletion or inhibition abrogated DBC1 induction by TGF-β and normalized SIRT1 activity. In addition, KDM4A deficiency attenuated TGF-β induced EMT in IEC-6 cells. In conclusion, our data identify a KDM4-DBC1-SIRT1 pathway that regulates EMT to contribute to intestinal fibrosis.

Redox-sensitive activation of CCL7 by BRG1 in hepatocytes during liver injury

Liver injuries induced by various stimuli share in common an acute inflammatory response, in which circulating macrophages home to the liver parenchyma to participate in the regulation of repair, regeneration, and fibrosis. In the present study we investigated the role of hepatocyte-derived C-C motif ligand 7 (CCL7) in macrophage migration during liver injury focusing on its transcriptional regulation. We report that CCL7 expression was up-regulated in the liver by lipopolysaccharide (LPS) injection (acute liver injury) or methionine-and-choline-deficient (MCD) diet feeding (chronic liver injury) paralleling increased macrophage infiltration. CCL7 expression was also inducible in hepatocytes, but not in hepatic stellate cells or in Kupffer cells, by LPS treatment or exposure to palmitate in vitro. Hepatocyte-specific deletion of Brahma-related gene 1 (BRG1), a chromatin remodeling protein, resulted in a concomitant loss of CCL7 induction and macrophage infiltration in the murine livers. Of interest, BRG1-induced CCL7 transcription and macrophage migration was completely blocked by the antioxidant N-acetylcystine. Further analyses revealed that BRG1 interacted with activator protein 1 (AP-1) to regulate CCL7 transcription in hepatocytes in a redox-sensitive manner mediated in part by casein kinase 2 (CK2)-catalyzed phosphorylation of BRG1. Importantly, a positive correlation between BRG1/CCL7 expression and macrophage infiltration was identified in human liver biopsy specimens. In conclusion, our data unveil a novel role for BRG1 as a redox-sensitive activator of CCL7 transcription.

Choline Kinase Alpha Is a Novel Transcriptional Target of the Brg1 in Hepatocyte: Implication in Liver Regeneration

Liver regeneration is a key compensatory process in response to liver injury serving to contain damages and to rescue liver functions. Hepatocytes, having temporarily exited the cell cycle after embryogenesis, resume proliferation to regenerate the injured liver parenchyma. In the present study we investigated the transcriptional regulation of choline kinase alpha (Chka) in hepatocytes in the context of liver regeneration. We report that Chka expression was significantly up-regulated in the regenerating livers in the partial hepatectomy (PHx) model and the acetaminophen (APAP) injection model. In addition, treatment with hepatocyte growth factor (HGF), a strong pro-proliferative cue, stimulated Chka expression in primary hepatocytes. Chka depletion attenuated HGF-induced proliferation of hepatocytes as evidenced by quantitative PCR and Western blotting measurements of pro-proliferative genes as well as EdU incorporation into replicating DNA. Of interest, deletion of Brahma-related gene 1 (Brg1), a chromatin remodeling protein, attenuated Chka induction in the regenerating livers in mice and in cultured hepatocytes. Further analysis revealed that Brg1 interacted with hypoxia-inducible factor 1 alpha (HIF-1α) to directly bind to the Chka promoter and activate Chka transcription. Finally, examination of human acute liver failure (ALF) specimens identified a positive correlation between Chka expression and Brg1 expression. In conclusion, our data suggest that Brg1-dependent trans-activation of Chka expression may contribute to liver regeneration.

A GSK3-SRF Axis Mediates Angiotensin II Induced Endothelin Transcription in Vascular Endothelial Cells

Endothelin, encoded by ET1, is a vasoactive substance primarily synthesized in vascular endothelial cells (VECs). Elevation of endothelin levels, due to transcriptional hyperactivation, has been observed in a host of cardiovascular diseases. We have previously shown that serum response factor (SRF) is a regulator of ET1 transcription in VECs. Here we report that angiotensin II (Ang II) induced ET1 transcription paralleled activation of glycogen synthase kinase 3 (GSK3) in cultured VECs. GSK3 knockdown or pharmaceutical inhibition attenuated Ang II induced endothelin expression. Of interest, the effect of GSK3 on endothelin transcription relied on the conserved SRF motif within the ET1 promoter. Further analysis revealed that GSK3 interacted with and phosphorylated SRF at serine 224. Phosphorylation of SRF by GSK3 did not influence its recruitment to the ET1 promoter. Instead, GSK3-mediated SRF phosphorylation potentiated its interaction with MRTF-A, a key co-factor for SRF, which helped recruit the chromatin remodeling protein BRG1 to the ET1 promoter resulting in augmented histone H3 acetylation/H3K4 trimethylation. Consistently, over-expression of a constitutively active GSK enhanced Ang II-induced ET1 transcription and knockdown of either MRTF-A or BRG1 abrogated the enhancement of ET1 transcription. In conclusion, our data highlight a previously unrecognized mechanism that contributes to the transcriptional regulation of endothelin. Targeting this GSK3-SRF axis may yield novel approaches in the intervention of cardiovascular diseases.