MKL1 mediates TGF-β1-induced α-smooth muscle actin expression in human renal epithelial cells

Macrophage MKL1 contributes to cardiac fibrosis in a mouse model of myocardial infarction

AIMS

Cardiac fibrosis is characterized by aberrant collagen deposition in the heart. Macrophage polarization or infiltration is the main reason to accelerate the collagen deposition. We attempted to investigate the involvement of MKL1 in macrophages during the development of cardiac fibrosis.

MATERIALS AND METHODS

Cardiac fibrosis is induced by myocardial infarction (MI). The MKL1f/f mice were crossed to the Lyz2-cre mice to generate macrophage conditional MKL1 knockout mice (MKL1ΔMφ). In addition, macrophage conditional MKL1 overexpression mice (MKL1Mϕ-OE) were constructed by crossing Lyz2-cre mice to MKL1ΔN200-Rosa26 mice.

KEY FINDINGS

MKL1 expression was significantly increased in macrophages of both ischemic cardiomyopathy (ICM) patients and mice induced to develop myocardial infarction. Deletion of MKL1 in macrophages improved the heart function after MI-induced cardiac fibrosis. Consistently, MKL1Mϕ-OE mice displayed more severe cardiac fibrosis and worsened heart function than the control mice after MI. Moreover, administration of a small-molecule MKL1 inhibitor CCG-1423 also decreased the collagen deposition after MI.

SIGNIFICANCE

Our data demonstrate that MKL1 in macrophages contributes to cardiac fibrosis pathogenesis and reinforce the notion that targeting MKL1 may yield effective antifibrotic therapeutics in the heart.

Extracellular matrix viscoelasticity regulates TGFβ1-induced epithelial-mesenchymal transition and apoptosis via integrin linked kinase

Transforming growth factor (TGF)-β1 is a multifunctional cytokine that plays important roles in health and disease. Previous studies have revealed that TGFβ1 activation, signaling, and downstream cell responses including epithelial-mesenchymal transition (EMT) and apoptosis are regulated by the elasticity or stiffness of the extracellular matrix. However, tissues within the body are not purely elastic, rather they are viscoelastic. How matrix viscoelasticity impacts cell fate decisions downstream of TGFβ1 remains unknown. Here, we synthesized polyacrylamide hydrogels that mimic the viscoelastic properties of breast tumor tissue. We found that increasing matrix viscous dissipation reduces TGFβ1-induced cell spreading, F-actin stress fiber formation, and EMT-associated gene expression changes, and promotes TGFβ1-induced apoptosis in mammary epithelial cells. Furthermore, TGFβ1-induced expression of integrin linked kinase (ILK) and colocalization of ILK with vinculin at cell adhesions is attenuated in mammary epithelial cells cultured on viscoelastic substrata in comparison to cells cultured on nearly elastic substrata. Overexpression of ILK promotes TGFβ1-induced EMT and reduces apoptosis in cells cultured on viscoelastic substrata, suggesting that ILK plays an important role in regulating cell fate downstream of TGFβ1 in response to matrix viscoelasticity.

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.

A therapeutic target for CKD: activin A facilitates TGFβ1 profibrotic signaling

BACKGROUND

TGFβ1 is a major profibrotic mediator in chronic kidney disease (CKD). Its direct inhibition, however, is limited by adverse effects. Inhibition of activins, also members of the TGFβ superfamily, blocks TGFβ1 profibrotic effects, but the mechanism underlying this and the specific activin(s) involved are unknown.

METHODS

Cells were treated with TGFβ1 or activins A/B. Activins were inhibited generally with follistatin, or specifically with neutralizing antibodies or type I receptor downregulation. Cytokine levels, signaling and profibrotic responses were assessed with ELISA, immunofluorescence, immunoblotting and promoter luciferase reporters. Wild-type or TGFβ1-overexpressing mice with unilateral ureteral obstruction (UUO) were treated with an activin A neutralizing antibody.

RESULTS

In primary mesangial cells, TGFβ1 induces secretion primarily of activin A, which enables longer-term profibrotic effects by enhancing Smad3 phosphorylation and transcriptional activity. This results from lack of cell refractoriness to activin A, unlike that for TGFβ1, and promotion of TGFβ type II receptor expression. Activin A also supports transcription through regulating non-canonical MRTF-A activation. TGFβ1 additionally induces secretion of activin A, but not B, from tubular cells, and activin A neutralization prevents the TGFβ1 profibrotic response in renal fibroblasts. Fibrosis induced by UUO is inhibited by activin A neutralization in wild-type mice. Worsened fibrosis in TGFβ1-overexpressing mice is associated with increased renal activin A expression and is inhibited to wild-type levels with activin A neutralization.

CONCLUSIONS

Activin A facilitates TGFβ1 profibrotic effects through regulation of both canonical (Smad3) and non-canonical (MRTF-A) signaling, suggesting it may be a novel therapeutic target for preventing fibrosis in CKD.

SIRT6 mediates MRTF-A deacetylation in vascular endothelial cells to antagonize oxLDL-induced ICAM-1 transcription

Oxidized low-density lipoprotein (oxLDL), a known risk factor for atherosclerosis, activates the transcription of adhesion molecules (ICAM-1) in endothelial cells. We previously showed that myocardin-related transcription factor A (MRTF-A) mediates oxLDL-induced ICAM-1 transcription. Here we confirm that ICAM-1 transactivation paralleled dynamic alterations in MRTF-A acetylation. Since treatment with the antioxidant NAC dampened MRTF-A acetylation, MRTF-A acetylation appeared to be sensitive to cellular redox status. Of interest, silencing of SIRT6, a lysine deacetylase, restored MRTF-A acetylation despite the addition of NAC. SIRT6 directly interacted with MRTF-A to modulate MRTF-A acetylation. Deacetylation of MRTF-A by SIRT6 led to its nuclear expulsion thus dampening MRTF-A occupancy on the ICAM-1 promoter. Moreover, SIRT6 expression was downregulated with oxLDL stimulation likely owing to promoter hypermethylation in endothelial cells. DNA methyltransferase 1 (DNMT1) was recruited to the SIRT6 promoter and mediated SIRT6 repression. The ability of DNMT1 to repress SIRT6 promoter partly was dependent on ROS-sensitive serine 154 phosphorylation. In conclusion, our data unveil a novel DNMT1-SIRT6 axis that contributes to the regulation of MRTF-A acetylation and ICAM-1 transactivation in endothelial cells.

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.

MKL1-induced lncRNA SNHG18 drives the growth and metastasis of non-small cell lung cancer via the miR-211-5p/BRD4 axis

Megakaryocytic leukemia 1 (MKL1) is a key transcription factor involved in non-small cell lung cancer (NSCLC) growth and metastasis. Yet, its downstream target genes, especially long non-coding RNA (lncRNA) targets, are poorly investigated. In this study, we employed lncRNA array technology to identify differentially expressed lncRNAs in NSCLC cells with or without overexpression of MKL1. Candidate lncRNAs were further explored for their clinical significance and function in NSCLC. The results showed that MKL1 promoted the expression of lncRNA SNHG18 in NSCLC cells. SNHG18 upregulation in NSCLC specimens correlated with lymph node metastasis and reduced overall survival of NSCLC patients. SNHG18 expression served as an independent prognostic factor for NSCLC. Knockdown of SNHG18 blocked MKL1-induced growth and invasion of NSCLC cells in vitro. Animal studies validated the requirement for SNHG18 in NSCLC growth and metastasis. Moreover, overexpression of SNHG18 promoted NSCLC cell proliferation and invasion. Mechanically, SNHG18 exerted its prometastatic effects on NSCLC cells through repression of miR-211-5p and induction of BRD4. Clinical evidence indicated that SNHG18 expression was negatively correlated with miR-211-5p expression in NSCLC tissues. Altogether, SNHG18 acts as a lncRNA mediator of MKL1 in NSCLC. SNHG18 facilitates NSCLC growth and metastasis by modulating the miR-211-5p/BRD4 axis. Therefore, SNHG18 may be a potential therapeutic target for the treatment of NSCLC.

Molecular Mechanisms to Target Cellular Senescence in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) has emerged as a major cause of cancer-related death and is the most common type of liver cancer. Due to the current paucity of drugs for HCC therapy there is a pressing need to develop new therapeutic concepts. In recent years, the role of Serum Response Factor (SRF) and its coactivators, Myocardin-Related Transcription Factors A and B (MRTF-A and -B), in HCC formation and progression has received considerable attention. Targeting MRTFs results in HCC growth arrest provoked by oncogene-induced senescence. The induction of senescence acts as a tumor-suppressive mechanism and therefore gains consideration for pharmacological interventions in cancer therapy. In this article, we describe the key features and the functional role of senescence in light of the development of novel drug targets for HCC therapy with a focus on MRTFs.

Inhibition of Rho kinase suppresses capsular contraction following lens injury in mice

PURPOSE:

We investigated the effect of systemic fasudil hydrochloride and an inhibitor of nuclear translocation of myocardin-related transcription factor-A (MRTF-A) on capsular contraction in a puncture-injured lens in mice.

MATERIALS AND METHODS:

Lens injury of an anterior capsular break was achieved in male adult C57Bl/6 mice under general and topical anesthesia at 1 h after systemic fasudil hydrochloride (intraperitoneal, 10 mg/kg body weight) or vehicle administration. The mice were allowed to heal after instillation of ofloxacin ointment, for 5 and 10 days with daily administration of fasudil hydrochloride or vehicle. In another series of experiment, we examined the effect of systemic administration of an MRTF-A inhibitor (CCG-203971, 100 mg/kg twice a day) on fibrogenic reaction and tissue contraction in an injured lens on day 5 or 10. The eye was processed for histology and immunohistochemistry for SM22, proliferating cell nuclear antigen (PCNA), or MRTF-A. In hematoxylin and eosin - stained samples, the distance between each edge of the break of the anterior capsule was measured and statistically analyzed.

RESULTS:

A cluster of lens cell accumulation was formed adjacent to the edge of the capsular break on day 5. It contained cells labeled for SM22 and PCNA. The size of the cell cluster was larger in fasudil group of mice than in control mice on day 5. Systemic fasudil or CCG-203971 suppressed an excess contraction of the capsular break at certain time points.

CONCLUSION:

Systemic administration of fasudil hydrochloride could be a treatment strategy of postoperative capsular contraction following cataract-intraocular lens surgery.

ICAM-1 and MKL-1 polymorphisms impose considerable impacts on coronary heart disease occurrence

This study was aimed to explore the correlation of intercellular adhesion molecule‐1 (ICAM‐1) K469E and megakaryoblastic leukaemia factor‐1 (MKL‐1) −184C/T polymorphisms with the susceptibility to coronary heart disease (CHD) in the Chinese Han population. 100 CHD patients and 91 healthy people that had no blood connection with each other were enrolled in this case‐control study. ICAM‐1 and MKL‐1 polymorphisms were genotyped by polymerase chain reaction‐restriction fragment length polymorphism (PCR‐RFLP) approach. Multiple logistic regression was used to analyse the correlation between polymorphisms of ICAM‐1 and MKL‐1 and CHD susceptibility. Differences of genotype and allele frequencies of the two SNPs between case and control groups were analysed by chi‐square test. Odds ratios (ORs) and 95% confidence intervals (CIs) were indicated relative susceptibility of CHD. The distributions of ICAM‐1 and MKL‐1 polymorphisms in each group conformed to Hardy‐Weinberg equilibrium (HWE). After adjusting for traditional risk factors, the TT genotype frequency of MKL‐1 −184C/T polymorphism was found significantly higher in case group than in control group (P < .05). Meanwhile, T allele frequency increased in case group compared with control group, and the differences had statistical significance (P = .04, OR = 2.34, 95% CI = 1.34‐5.26). Logistic regression analysis in this study proved that smoking, hypertension, diabetes and triglyceride (TG) were all risk factors for CHD ICAM‐1 K469E polymorphism has no association with the onset of CHD. But MKL‐1 −184C/T polymorphism is associated with the risk of CHD and T allele might be a susceptibility factor for CHD.

MKL1/miR-5100/CAAP1 loop regulates autophagy and apoptosis in gastric cancer cells

PURPOSE

miR-5100 participates in the proliferation of lung cancer and pancreatic cancer cells, and participates in the differentiation of osteoblasts. However, the regulation of gastric cancer cells in gastric cancer cells remains unclear.

EXPERIMENTAL DESIGN

The blood of patients was collected to detect the expression level of miR-5100, and the apoptosis and autophagy levels of cells were detected using western blot, flow cytometry, and confocal. At the same time, in vitro tumor formation experiments in nude mice were used to verify the results of in vitro experiments.

RESULTS

The expression of miR-5100 is related to the prognosis of gastric cancer, miR-5100 can enhance the apoptosis level of gastric cancer cells and inhibit the occurrence of autophagy by targeting CAAP1. MKL1 can inhibit the apoptosis of gastric cancer cells and promote the occurrence of autophagy by targeting CAAP1. At the same time, MKL1 can also increase the expression of miR-5100.

CONCLUSIONS

Our research reveals the mechanism by which the MKL1/miR-5100/CAAP1 loop regulates apoptosis and autophagy levels in gastric cancer cells, and miR-5100 is expected to become a new potential target for gastric cancer treatment.

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.

Cardiac fibrosis: new insights into the pathogenesis

Cardiac fibrosis is defined as the imbalance of extracellular matrix (ECM) production and degradation, thus contributing to cardiac dysfunction in many cardiac pathophysiologic conditions. This review discusses specific markers and origin of cardiac fibroblasts (CFs), and the underlying mechanism involved in the development of cardiac fibrosis. Currently, there are no CFs-specific molecular markers. Most studies use co-labelling with panels of antibodies that can recognize CFs. Origin of fibroblasts is heterogeneous. After fibrotic stimuli, the levels of myocardial pro-fibrotic growth factors and cytokines are increased. These pro-fibrotic growth factors and cytokines bind to its receptors and then trigger the activation of signaling pathway and transcriptional factors via Smad-dependent or Smad independent-manners. These fibrosis-related transcriptional factors regulate gene expression that are involved in the fibrosis to amplify the fibrotic response. Understanding the mechanisms responsible for initiation, progression, and amplification of cardiac fibrosis are of great clinical significance to find drugs that can prevent the progression of cardiac fibrosis.

Isolation, Characterization, And High Throughput Extracellular Flux Analysis of Mouse Primary Renal Tubular Epithelial Cells

Mitochondrial dysfunction in the renal tubular epithelial cells (TECs) can lead to renal fibrosis, a major cause of chronic kidney disease (CKD). Therefore, assessing mitochondrial function in primary TECs may provide valuable insight into the bioenergetic status of the cells, providing insight into the pathophysiology of CKD. While there are a number of complex protocols available for the isolation and purification of proximal tubules in different species, the field lacks a cost-effective method optimized for tubular cell isolation without the need for purification. Here, we provide an isolation protocol that allows for studies focusing on both primary mouse proximal and distal renal TECs. In addition to cost-effective reagents and minimal animal procedures required in this protocol, the isolated cells maintain high energy levels after isolation and can be sub-cultured up to four passages, allowing for continuous studies. Furthermore, using a high throughput extracellular flux analyzer, we assess the mitochondrial respiration directly in the isolated TECs in a 96-well plate for which we provide recommendations for the optimization of cell density and compound concentration. These observations suggest that this protocol can be used for renal tubular ex vivo studies with a consistent, well-standardized production of renal TECs. This protocol may have broader future applications to study mitochondrial dysfunction associated with renal disorders for drug discovery or drug characterization purposes.

Role of Alpha-Smooth Muscle Actin and Fibroblast Activation Protein Alpha in Ovarian Neoplasms

BACKGROUND/AIMS

Studies show that tumor growth is not just determined by the presence of malignant cells, since interactions between cancer cells and stromal microenvironment have important impacts on the cancer growth and progression. Cancer-associated fibroblasts play a prominent role in this process. The aims of the study were to investigate 2 cancer-associated fibroblasts markers, alpha-smooth muscle actin (α-SMA), and fibroblast activation protein alpha (FAP) in the stromal microenvironment of benign and malignant ovarian epithelial neoplasms, and to relate their tissue expression with prognostic factors in ovarian cancer.

METHODS

α-SMA and FAP were evaluated by immunohistochemistry in malignant (n = 28) and benign (n = 28) ovarian neoplasms. Fisher's exact test was used with a significance level lower than 0.05.

RESULTS

FAP immunostaining was stronger in ovarian cancer when compared to benign neoplasms (p = 0.0366). There was no significant difference in relation to α-SMA expression between malignant and benign ovarian neoplasms as well as prognostic factors. In ovarian cancer, FAP stainings 2/3 was significantly related to histological grades 2 and 3 (p = 0.0183).

CONCLUSION

FAP immunostaining is more intense in malignant neoplasms than in benign ovarian neoplasms, as well as in moderately differentiated and undifferentiated ovarian carcinomas compared to well-differentiated neoplasms, thus indicating that it can be used as a marker of worse prognosis.

Trichostatin A inhibits radiation-induced epithelial-to-mesenchymal transition in the alveolar epithelial cells

Radiation-induced pneumonitis and fibrosis are major complications following thoracic radiotherapy. Epithelial-to-mesenchymal transition (EMT) plays an important role in tissue injury leading to organ fibrosis, including lung. Our previous studies have reported that radiation can induce EMT in the type II alveolar epithelial cells in both in vitro and in vivo. HDAC inhibitors are a new family of anti-cancer agents currently being used in several clinical trials. In addition to their intrinsic anti-tumor properties, HDAC inhibition is also important in other human diseases, including fibrosis and radiation-induced damage. In this study, we evaluated the effect of Trichostatin A (TSA), a HDAC inhibitor, on radiation-induced EMT in type II alveolar epithelial cells (RLE-6TN). Pre-treatment of RLE-6TN cells with TSA inhibited radiation-induced EMT-like morphological alterations including elevated protein level of α-SMA and Snail, reduction of E-cadherin expression, enhanced phosphorylation of GSK3β and ERK1/2, increased generation of ROS. Radiation enhanced the protein level of TGF-β1, which was blocked by N-acetylcysteine, an antioxidant. Treating cells with SB-431542, TGF-β1 type I receptor inhibitor, diminished radiation-induced alterations in the protein levels of p-GSK-3β, Snail-1 and α-SMA, suggesting a regulatory role of TGF-β1 in EMT. Pre-incubation of cells with TSA showed significant decrease in the level of TGF-β1 compared to radiation control. Collectively, these results demonstrate that i] radiation-induced EMT in RLE-6TN cells is mediated by ROS/MEK/ERK and ROS/TGF-β1 signaling pathways and ii] the inhibitory role of TSA in radiation-induced EMT appears to be due, at least in part, to its action of blocking ROS and TGF-β1 signaling.

MiR-93-5p inhibits the EMT of breast cancer cells via targeting MKL-1 and STAT3

Epithelial-mesenchymal transition (EMT) plays an important role in breast cancer cell metastasis. Both (megakaryoblastic leukemia)/myocardin-like 1 (MKL-1) and Signal transducer and activator of transcription 3 (STAT3) have been implicated in the control of cellular metabolism, survival and growth. Our previous study has shown that cooperativity of MKL-1 and STAT3 promoted breast cancer cell migration. Herein, we demonstrate a requirement for MKL-1 and STAT3 in miRNA-mediated cellular EMT to affect breast cancer cell migration. Here we show that cooperativity of MKL-1 and STAT3 promoted the EMT of MCF-7 cells. Importantly, MKL-1 and STAT3 promoted the expression of Vimentin via its promoter CArG box. Interestingly, miR-93-5p inhibits the EMT of breast cancer cells through suppressing the expression of MKL-1 and STAT3 via targeted their 3'UTR. These results demonstrated a novel pathway through which miR-93-5p regulates MKL-1 and STAT3 to affect EMT controlling breast cancer cell migration.

Material Strategies for Modulating Epithelial to Mesenchymal Transitions

Epithelial to mesenchymal transitions (EMT) involve the phenotypic change of epithelial cells into fibroblast-like cells. This process is accompanied by the loss of cell-cell contacts, increased extracellular matrix (ECM) production, stress fiber alignment, and an increase in cell mobility. While essential for development and wound repair, EMT has also been recognized as a contributing factor to fibrotic diseases and cancer. Both chemical and mechanical cues, such as tumor necrosis factor alpha, NF-κB, Wnt, Notch, interleukin-8, metalloproteinase-3, ECM proteins, and ECM stiffness can determine the degree and duration of EMT events. Additionally, transforming growth factor beta is a primary driver of EMT and, interestingly, can be activated through cell-mediated mechanoactivation. In this review, we highlight recent findings demonstrating the contribution of mechanical stimuli, such as tissue and material stiffness, in driving EMT. We then highlight material strategies for controlling EMT events. Finally, we discuss drivers of the similar process of endothelial to mesenchymal transition (EndoMT) and corresponding material strategies for controlling EndoMT.

Local delivery of novel MRTF/SRF inhibitors prevents scar tissue formation in a preclinical model of fibrosis

The myocardin-related transcription factor/serum response factor (MRTF/SRF) pathway represents a promising therapeutic target to prevent fibrosis. We have tested the effects of new pharmacological inhibitors of MRTF/SRF signalling in a preclinical model of fibrosis. CCG-222740, a novel MRTF/SRF inhibitor, markedly decreased SRF reporter gene activity and showed a greater inhibitory effect on MRTF/SRF target genes than the previously described MRTF-A inhibitor CCG-203971. CCG-222740 was also five times more potent, with an IC₅₀ of 5 μM, in a fibroblast-mediated collagen contraction assay, was less cytotoxic, and a more potent inhibitor of alpha-smooth muscle actin protein expression than CCG-203971. Local delivery of CCG-222740 and CCG-203971 in a validated and clinically relevant rabbit model of scar tissue formation after glaucoma filtration surgery increased the long-term success of the surgery by 67% (P < 0.0005) and 33% (P < 0.01), respectively, and significantly decreased fibrosis and scarring histologically. Unlike mitomycin-C, neither CCG-222740 nor CCG-203971 caused any detectable epithelial toxicity or systemic side effects with very low drug levels measured in the aqueous, vitreous, and serum. We conclude that inhibitors of MRTF/SRF-regulated gene transcription such as CCG-222740, potentially represent a new therapeutic strategy to prevent scar tissue formation in the eye and other tissues.

Cardiac Fibroblast Activation Post-Myocardial Infarction: Current Knowledge Gaps

In response to myocardial infarction (MI), the wound healing response of the left ventricle (LV) comprises overlapping inflammatory, proliferative, and maturation phases, and the cardiac fibroblast is a key cell type involved in each phase. It has recently been appreciated that, early post-MI, fibroblasts transform to a proinflammatory phenotype and secrete cytokines and chemokines as well as matrix metalloproteinases (MMPs). Later post-MI, fibroblasts are activated to anti-inflammatory and proreparative phenotypes and generate anti-inflammatory and proangiogenic factors and extracellular matrix (ECM) components that form the infarct scar. Additional studies are needed to systematically examine how fibroblast activation shifts over the timeframe of the MI response and how modulation at different activation stages could alter wound healing and LV remodeling in distinct ways. This review summarizes current fibroblast knowledge as the foundation for a discussion of existing knowledge gaps.

RhoA/ROCK signaling regulates TGFβ-induced epithelial-mesenchymal transition of lens epithelial cells through MRTF-A

Transforming growth factor (TGF)-β-induced epithelial-mesenchymal transition (EMT) leads to the formation of ocular fibrotic pathologies, such as anterior subcapsular cataract and posterior capsule opacification. Remodeling of the actin cytoskeleton, mediated by the Rho family of GTPases, plays a key role in EMT, however, how actin dynamics affect downstream markers of EMT has not been fully determined. Our previous work suggests that myocardin related transcription factor A (MRTF-A), an actin-binding protein, might be an important mediator of TGFβ-induced EMT in lens epithelial cells. The aim of the current study was to determine the requirement of RhoA/ROCK signaling in mediating TGFβ-induced nuclear accumulation of MRTF-A, and ultimate expression of α-smooth muscle actin (αSMA), a marker of a contractile, myofibroblast phenotype. Using rat lens epithelial explants, we demonstrate that ROCK inhibition using Y-27632 prevents TGFβ-induced nuclear accumulation of MRTF-A, E-cadherin/β-catenin complex disassembly, and αSMA expression. Using a novel inhibitor specifically targeting MRTF-A signaling, CCG-203971, we further demonstrate the requirement of MRTF-A nuclear localization and activity in the induction of αSMA expression. Overall, our findings suggest that TGFβ-induced cytoskeletal reorganization through RhoA/ROCK/MRTF-A signaling is critical to EMT of lens epithelial cells.

Cell-cell contact and matrix adhesion promote αSMA expression during TGFβ1-induced epithelial-myofibroblast transition via Notch and MRTF-A

During epithelial-mesenchymal transition (EMT) epithelial cells lose cell-cell adhesion, exhibit morphological changes, and upregulate the expression of cytoskeletal proteins. Previous studies have demonstrated that complete disruption of cell-cell contact can promote transforming growth factor (TGF)-β1-induced EMT and the expression of the myofibroblast marker alpha smooth muscle actin (αSMA). Furthermore, increased cell spreading mediates TGFβ1-induced αSMA expression during EMT. Here, we sought to examine how the presence of partial cell-cell contacts impacts EMT. A microfabrication approach was employed to decouple the effects of cell-cell contact and cell-matrix adhesion in TGFβ1-induced EMT. When cell spreading is controlled, the presence of partial cell-cell contacts enhances expression of αSMA. Moreover, cell spreading and intercellular contacts together control the subcellular localization of activated Notch1 and myocardin related transcription factor (MRTF)-A. Knockdown of Notch1 or MRTF-A as well as pharmacological inhibition of these pathways abates the cell-cell contact mediated expression of αSMA. These data suggest that the interplay between cell-matrix adhesion and intercellular adhesion is an important determinant for some aspects of TGFβ1-induced EMT.

Transcriptional control of cardiac fibroblast plasticity

Cardiac fibroblasts help maintain the normal architecture of the healthy heart and are responsible for scar formation and the healing response to pathological insults. Various genetic, biomechanical, or humoral factors stimulate fibroblasts to become contractile smooth muscle-like cells called myofibroblasts that secrete large amounts of extracellular matrix. Unfortunately, unchecked myofibroblast activation in heart disease leads to pathological fibrosis, which is a major risk factor for the development of cardiac arrhythmias and heart failure. A better understanding of the molecular mechanisms that control fibroblast plasticity and myofibroblast activation is essential to develop novel strategies to specifically target pathological cardiac fibrosis without disrupting the adaptive healing response. This review highlights the major transcriptional mediators of fibroblast origin and function in development and disease. The contribution of the fetal epicardial gene program will be discussed in the context of fibroblast origin in development and following injury, primarily focusing on Tcf21 and C/EBP. We will also highlight the major transcriptional regulatory axes that control fibroblast plasticity in the adult heart, including transforming growth factor β (TGFβ)/Smad signaling, the Rho/myocardin-related transcription factor (MRTF)/serum response factor (SRF) axis, and Calcineurin/transient receptor potential channel (TRP)/nuclear factor of activated T-Cell (NFAT) signaling. Finally, we will discuss recent strategies to divert the fibroblast transcriptional program in an effort to promote cardiomyocyte regeneration. This article is a part of a Special Issue entitled "Fibrosis and Myocardial Remodeling".

Critical role of serum response factor in podocyte epithelial-mesenchymal transition of diabetic nephropathy

PURPOSE

To investigate the expression and function of serum response factor in podocyte epithelial-mesenchymal transition of diabetic nephropathy.

METHODS

The expression of serum response factor, epithelial markers and mesenchymal markers was examined in podocytes or renal cortex tissues following high glucose. Serum response factor was upregulated by its plasmids and downregulated by CCG-1423 to investigate how it influenced podocyte epithelial-mesenchymal transition in diabetic nephropathy. Streptozotocin was used to generate diabetes mellitus in rats.

RESULTS

In podocytes after high glucose treatment, serum response factor and mesenchymal markers increased, while epithelial markers declined. Similar changes were observed in vivo. Serum response factor overexpression in podocytes induced expression of Snail, an important transcription factor mediating epithelial-mesenchymal transition. Blockade of serum response factor reduced Snail induction, protected podocytes from epithelial-mesenchymal transition and ameliorated proteinuria.

CONCLUSION

Together, increased serum response factor activity provokes podocytes' epithelial-mesenchymal transition and dysfunction in diabetic nephropathy. Targeting serum response factor by small-molecule inhibitor may be an attractive therapeutic strategy for diabetic nephropathy.

Cytoskeletal signaling in TGFβ-induced epithelial-mesenchymal transition

Epithelial-mesenchymal transition (EMT) is a physiological process that plays an important role in embryonic development and wound healing and is appropriated during pathological conditions including fibrosis and cancer metastasis. EMT can be initiated by a variety of factors, including transforming growth factor (TGF)-β, and is characterized by loss of epithelial features including cell-cell contacts and apicobasal polarity and acquisition of a motile, mesenchymal phenotype. A key feature of EMT is reorganization of the cytoskeleton and recent studies have elucidated regulation mechanisms governing this process. This review describes changes in gene expression patterns of cytoskeletal associated proteins during TGFβ-induced EMT. It further reports TGFβ-induced intracellular signaling cascades that regulate cytoskeletal reorganization during EMT. Finally, it highlights how changes in cytoskeletal architecture during EMT can regulate gene expression, thus further promoting EMT progression.

Stereospecific Inhibitory Effects of CCG-1423 on the Cellular Events Mediated by Myocardin-Related Transcription Factor A

CCG-1423 suppresses several pathological processes including cancer cell migration, tissue fibrosis, and the development of atherosclerotic lesions. These suppressions are caused by inhibition of myocardin-related transcription factor A (MRTF-A), which is a critical factor for epithelial–mesenchymal transition (EMT). CCG-1423 can therefore be a potent inhibitor for EMT. CCG-1423 and related compounds, CCG-100602 and CCG-203971 possess similar biological activities. Although these compounds are comprised of two stereoisomers, the differences in their biological activities remain to be assessed. To address this issue, we stereoselectively synthesized optically pure isomers of these compounds and validated their biological activities. The S-isomer of CCG-1423 rather than the R-isomer exhibited modestly but significantly higher inhibitory effects on the cellular events triggered by MRTF-A activation including serum response factor-mediated gene expression and cell migration of fibroblasts and B16F10 melanoma cells. Accordingly, the S-isomer of CCG-1423 more potently blocked the serum-induced nuclear import of MRTF-A than the R-isomer. No such difference was observed in cells treated with each of two stereoisomers of CCG-100602 or CCG-203971. We previously reported that the N-terminal basic domain (NB), which functions as a nuclear localization signal of MRTF-A, is a binding site for CCG-1423. Consistent with the biological activities of two stereoisomers of CCG-1423, docking simulation demonstrated that the S-isomer of CCG-1423 was more likely to bind to NB than the R-isomer. This is a first report demonstrating the stereospecific biological activities of CCG-1423.

Matrix Rigidity Mediates TGFβ1-Induced Epithelial-Myofibroblast Transition by Controlling Cytoskeletal Organization and MRTF-A Localization

Myofibroblasts mediate normal wound healing and upon chronic activation can contribute to the development of pathological conditions including organ fibrosis and cancer. Myofibroblasts can develop from epithelial cells through an epithelial-mesenchymal transition (EMT) during which epithelial cells exhibit drastic morphological changes and upregulate cytoskeletal associated proteins that enable exertion of large contractile forces and remodeling of the surrounding microenvironment. Increased matrix rigidity is a hallmark of fibrosis and tumor progression and mechanical tension has been identified as a regulator of EMT; however, the mechanisms governing the mechanical regulation of EMT are not completely understood. Here, we find that matrix rigidity regulates transforming growth factor (TGF)-β1-induced EMT, with rigid substrata enabling increased myofibroblast marker expression, cell morphology changes, and cytoskeletal reorganization while soft matrices block these changes. Furthermore, we find that matrix rigidity controls the subcellular localization of myocardin related transcription factor (MRTF)-A, a regulator of cytoskeletal protein expression that contributes to the acquisition of myogenic features during EMT. Results from these studies provide insight into how biophysical cues contribute to myofibroblast development from epithelial cells and may suggest ways to enhance wound healing or to engineer therapeutic solutions for fibrosis and cancer.

Aspidin PB, a novel natural anti-fibrotic compound, inhibited fibrogenesis in TGF-β1-stimulated keloid fibroblasts via PI-3K/Akt and Smad signaling pathways

Keloid is an overgrowth of scar tissue that develops around a wound. The mechanisms of keloid formation and development still remain unknown, and no effective treatment is available. Searching for active natural resources may develop better prevention and treatment approaches for keloids. Aspidin PB is a natural resource with lower toxicity. We explored its effect on the regulation of TGF-β1-induced expression of type I collagen, CTGF, and α-SMA in keloid fibroblasts (KFs). Western blotting was used to detect the expression levels of type I collagen, CTGF, α-SMA, PI-3K/Akt and Smad-dependent and Smad-independent signaling pathway. The effect of aspidin PB on cell viability in human keloid fibroblasts was measured by MTT (3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide). The percentage of the apoptotic cells was studied by flow cytometry. Based on our results, we revealed that aspidin PB inhibited the production of type I collagen, CTGF, and α-SMA in TGF-β1-induced KFs by blocking PI-3K/Akt signaling pathway. The TGF-β1-mediated phosphorylated levels of Smad2/3 were inhibited by aspidin PB pretreatment. Conclusively, our study suggests that aspidin PB has an inhibitory effect on fibrogenesis in TGF-β1-induced KFs. Our findings imply that aspidin PB has a therapeutic potential to intervene and prevent keloids and other fibrotic diseases.

A novel inhibitory mechanism of MRTF-A/B on the ICAM-1 gene expression in vascular endothelial cells

The roles of myocardin-related transcription factor A (MRTF-A) and MRTF-B in vascular endothelial cells are not completely understood. Here, we found a novel regulatory mechanism for MRTF-A/B function. MRTF-A/B tend to accumulate in the nucleus in arterial endothelial cells in vivo and human aortic endothelial cells (HAoECs) in vitro. In HAoECs, nuclear localization of MRTF-A/B was not significantly affected by Y27632 or latrunculin B, primarily due to the reduced binding of MRTF-A/B to G-actin and in part, to the low level of MRTF-A phosphorylation by ERK. MRTF-A/B downregulation by serum depletion or transfection of siRNA against MRTF-A and/or MRTF-B induced ICAM-1 expression in HAoECs. It is known that nuclear import of nuclear factor−κB (NF−κB) plays a key role in ICAM-1 gene transcription. However, nuclear accumulation of NF−κB p65 was not observed in MRTF-A/B-depleted HAoECs. Our present findings suggest that MRTF-A/B inhibit ICAM-1 mRNA expression by forming a complex with NF−κB p65 in the nucleus. Conversely, downregulation of MRTF-A/B alleviates this negative regulation without further translocation of NF−κB p65 into the nucleus. These results reveal the novel roles of MRTF-A/B in the homeostasis of vascular endothelium.

Mechanobiology of myofibroblast adhesion in fibrotic cardiac disease

Fibrotic cardiac disease, a leading cause of death worldwide, manifests as substantial loss of function following maladaptive tissue remodeling. Fibrosis can affect both the heart valves and the myocardium and is characterized by the activation of fibroblasts and accumulation of extracellular matrix. Valvular interstitial cells and cardiac fibroblasts, the cell types responsible for maintenance of cardiac extracellular matrix, are sensitive to changing mechanical environments, and their ability to sense and respond to mechanical forces determines both normal development and the progression of disease. Recent studies have uncovered specific adhesion proteins and mechano-sensitive signaling pathways that contribute to the progression of fibrosis. Integrins form adhesions with the extracellular matrix, and respond to changes in substrate stiffness and extracellular matrix composition. Cadherins mechanically link neighboring cells and are likely to contribute to fibrotic disease propagation. Finally, transition to the active myofibroblast phenotype leads to maladaptive tissue remodeling and enhanced mechanotransductive signaling, forming a positive feedback loop that contributes to heart failure. This Commentary summarizes recent findings on the role of mechanotransduction through integrins and cadherins to perpetuate mechanically induced differentiation and fibrosis in the context of cardiac disease.

MKL1 inhibits cell cycle progression through p21 in podocytes

BACKGROUND

The glomerular podocyte is a highly specialized cell type with the ability to ultrafilter blood and support glomerular capillary pressure. However, little is known about the genetic programs leading to this functionality or the final phenotype.

RESULTS

In the current study, we found that the expression of a myocardin/MKL family member, MKL1, was significantly upregulated during cell cycle arrest induced by a temperature switch in murine podocyte clone 5 (MPC5) cells. Further investigation demonstrated that overexpression of MKL1 led to inhibition of cell proliferation by decreasing the number of cells in S phase of the cell cycle. In contrast, MKL1 knockdown by RNA interference had the opposite effect, highlighting a potential role of MKL1 in blocking G1/S transition of the cell cycle in MPC5 cells. Additionally, using an RT(2) Profiler PCR Array, p21 was identified as a direct target of MKL1. We further revealed that MKL1 activated p21 transcription by recruitment to the CArG element in its promoter, thus resulting in cell cycle arrest. In addition, the expression of MKL1 is positively correlated with that of p21 in podocytes in postnatal mouse kidney and significantly upregulated during the morphological switch of podocytes from proliferation to differentiation.

CONCLUSIONS

Our observations demonstrate that MKL1 has physiological roles in the maturation and development of podocytes, and thus its misregulation might lead to glomerular and renal dysfunction.

The Purα/Purβ single-strand DNA-binding proteins attenuate smooth-muscle actin gene transactivation in myofibroblasts

Expression of smooth muscle alpha-actin (SMαA) is essential for myofibroblast-mediated wound contraction following tissue injury. The Pur α/β and YB-1 transcriptional repressors govern the DNA-binding activity of serum response factor (SRF) and phosphorylated Smad3 (pSmad3) transcriptional activators during induction of SMαA gene expression in human pulmonary myofibroblasts. In quiescent fibroblasts, Pur α exhibited a novel function in enhancing stability of pre-existing SRF complexes with SMαA core promoter DNA, whereas Pur β was more effective in disrupting SRF-DNA interaction. Pur proteins were less efficient competitors of pre-existing, core-promoter complexes containing both SRF and pSmad3 in nuclear extracts from TGFβ1-activated myofibroblasts. TGFβ1 signaling dissociated a SRF/Pur protein complex with concurrent formation of a transient pSmad3/MRTF-A/Pur β complex during early phase myofibroblast differentiation. Pur β was replaced by Pur α in the pSmad3/MRTF-A complex in mature myofibroblasts. Combining all three repressors potently inhibited SRF and pSmad3 binding to promoter DNA in quiescent fibroblasts and TGFβ1-activated myofibroblasts, respectively. The results point to dynamic interplay between transcriptional activators and repressors in regulating SMαA gene output during myofibroblast differentiation. Therapeutic targeting of nucleoprotein complexes regulating the SMαA promoter may prevent excessive myofibroblast accumulation associated with chronic cardiopulmonary fibrosis and dysfunctional tissue remodeling.

Biomechanics of TGFβ-induced epithelial-mesenchymal transition: implications for fibrosis and cancer

Fibrosis, a disease that results in loss of organ function, contributes to a significant number of deaths worldwide and sustained fibrotic activation has been suggested to increase the risk of developing cancer in a variety of tissues. Fibrogenesis and tumor progression are regulated in part through the activation and activity of myofibroblasts. Increasing evidence links myofibroblasts found within fibrotic lesions and the tumor microenvironment to a process termed epithelial-mesenchymal transition (EMT), a phenotypic change in which epithelial cells acquire mesenchymal characteristics. EMT can be stimulated by soluble signals, including transforming growth factor (TGF)-β, and recent studies have identified a role for mechanical cues in directing EMT. In this review, we describe the role that EMT plays in fibrogenesis and in the progression of cancer, with particular emphasis placed on biophysical signaling mechanisms that control the EMT program. We further describe specific TGFβ-induced intracellular signaling cascades that are affected by cell- and tissue-level mechanics. Finally, we highlight the implications of mechanical induction of EMT on the development of treatments and targeted intervention strategies for fibrosis and cancer.

Transglutaminase-2 mediates calcium-regulated crosslinking of the Y-box 1 (YB-1) translation-regulatory protein in TGFβ1-activated myofibroblasts

Myofibroblast differentiation is required for wound healing and accompanied by activation of smooth muscle α-actin (SMαA) gene expression. The stress-response protein, Y-box binding protein-1 (YB-1) binds SMαA mRNA and regulates its translational activity. Activation of SMαA gene expression in human pulmonary myofibroblasts by TGFβ1 was associated with formation of denaturation-resistant YB-1 oligomers with selective affinity for a known translation-silencer sequence in SMαA mRNA. We have determined that YB-1 is a substrate for the protein-crosslinking enzyme transglutaminase 2 (TG2) that catalyzes calcium-dependent formation of covalent γ-glutamyl-isopeptide linkages in response to reactive oxygen signaling. TG2 transamidation reactions using intact cells, cell lysates, and recombinant YB-1 revealed covalent crosslinking of the 50 kDa YB-1 polypeptide into protein oligomers that were distributed during SDS-PAGE over a 75-250 kDa size range. In vitro YB-1 transamidation required nanomolar levels of calcium and was enhanced by the presence of SMαA mRNA. In human pulmonary fibroblasts, YB-1 crosslinking was inhibited by (a) anti-oxidant cystamine, (b) the reactive-oxygen antagonist, diphenyleneiodonium, (c) competitive inhibition of TG2 transamidation using the aminyl-surrogate substrate, monodansylcadaverine, and (d) transfection with small-interfering RNA specific for human TG2 mRNA. YB-1 crosslinking was partially reversible as a function of oligomer-substrate availability and TG2 enzyme concentration. Intracellular calcium accumulation and peroxidative stress in injury-activated myofibroblasts may govern SMαA mRNA translational activity during wound healing via TG2-mediated crosslinking of the YB-1 mRNA-binding protein.

Prostaglandin E2 inhibits α-smooth muscle actin transcription during myofibroblast differentiation via distinct mechanisms of modulation of serum response factor and myocardin-related transcription factor-A

Differentiation of lung fibroblasts into contractile protein-expressing myofibroblasts by transforming growth factor-β1 (TGF-β1) is a critical event in the pathogenesis of pulmonary fibrosis. Transcription of the contractile protein α-smooth muscle actin (α-SMA) is mediated by the transcription factor serum-response factor (SRF) along with its co-activator, myocardin-related transcription factor-A (MRTF-A). The endogenous lipid mediator prostaglandin E2 (PGE2) exerts anti-fibrotic effects, including the inhibition of myofibroblast differentiation. However, the mechanism by which PGE2 inhibits α-SMA expression is incompletely understood. Here, we show in normal lung fibroblasts that PGE2 reduced the nuclear accumulation of MRTF-A·SRF complexes and consequently inhibited α-SMA promoter activation. It did so both by independently inhibiting SRF gene expression and nuclear import of MRTF-A. We identified that p38 MAPK is critical for TGF-β1-induced SRF gene expression and that PGE2 inhibition of SRF expression is associated with its ability to inhibit p38 activation. Its inhibition of MRTF-A import occurs via activation of cofilin 1 and inactivation of vasodilator-stimulated phosphoprotein. Similar effects of PGE2 on SRF gene expression were observed in fibroblasts from the lungs of patients with idiopathic pulmonary fibrosis. Thus, PGE2 is the first substance described to prevent myofibroblast differentiation by disrupting, via distinct mechanisms, the actions of both SRF and MRTF-A.

TGFBR1 is required for mouse myometrial development

The smooth muscle layer of the uterus (ie, myometrium) is critical for a successful pregnancy and labor. We have shown that the conditional deletion of TGFβ type 1 receptor (TGFBR1) in the female reproductive tract leads to remarkable smooth muscle defects. This study was aimed at defining the cellular and molecular basis of the myometrial defects. We found that TGFBR1 is required for myometrial configuration and formation during early postnatal uterine development. Despite the well-established role of TGFβ signaling in vascular smooth muscle cell differentiation, the majority of smooth muscle genes were expressed in Tgfbr1 conditional knockout (cKO) uteri at similar levels as controls during postnatal uterine development, coinciding with the presence but abnormal distribution of proteins for select smooth muscle markers. Importantly, the uteri of these mice had impaired synthesis of key extracellular matrix proteins and dysregulated expression of platelet-derived growth factors. Furthermore, platelet-derived growth factors induced the migration of uterine stromal cells from both control and Tgfbr1 cKO mice in vitro. Our results suggest that the myometrial defects in Tgfbr1 cKO mice may not directly arise from an intrinsic deficiency in uterine smooth muscle cell differentiation but are linked to the impaired production of key extracellular matrix components and abnormal uterine cell migration during a critical time window of postnatal uterine development. These findings will potentially aid in the design of novel therapies for reproductive disorders associated with myometrial defects.

RPEL proteins are the molecular targets for CCG-1423, an inhibitor of Rho signaling

Epithelial–msenchymal transition (EMT) is closely associated with cancer and tissue fibrosis. The nuclear accumulation of myocardin-related transcription factor A (MRTF-A/MAL/MKL1) plays a vital role in EMT. In various cells treated with CCG-1423, a novel inhibitor of Rho signaling, the nuclear accumulation of MRTF-A is inhibited. However, the molecular target of this inhibitor has not yet been identified. In this study, we investigated the mechanism of this effect of CCG-1423. The interaction between MRTF-A and importin α/β1 was inhibited by CCG-1423, but monomeric G-actin binding to MRTF-A was not inhibited. We coupled Sepharose with CCG-1423 (CCG-1423 Sepharose) to investigate this mechanism. A pull-down assay using CCG-1423 Sepharose revealed the direct binding of CCG-1423 to MRTF-A. Furthermore, we found that the N-terminal basic domain (NB) of MRTF-A, which acts as a functional nuclear localization signal (NLS) of MRTF-A, was the binding site for CCG-1423. G-actin did not bind to CCG-1423 Sepharose, but the interaction between MRTF-A and CCG-1423 Sepharose was reduced in the presence of G-actin. We attribute this result to the high binding affinity of MRTF-A for G-actin and the proximity of NB to G-actin-binding sites (RPEL motifs). Therefore, when MRTF-A forms a complex with G-actin, the binding of CCG-1423 to NB is expected to be blocked. NF-E2 related factor 2, which contains three distinct basic amino acid-rich NLSs, did not bind to CCG-1423 Sepharose, but other RPEL-containing proteins such as MRTF-B, myocardin, and Phactr1 bound to CCG-1423 Sepharose. These results suggest that the specific binding of CCG-1423 to the NLSs of RPEL-containing proteins. Our proposal to explain the inhibitory action of CCG-1423 is as follows: When the G-actin pool is depleted, CCG-1423 binds specifically to the NLS of MRTF-A/B and prevents the interaction between MRTF-A/B and importin α/β1, resulting in inhibition of the nuclear import of MRTF-A/B.

SAP domain-dependent Mkl1 signaling stimulates proliferation and cell migration by induction of a distinct gene set indicative of poor prognosis in breast cancer patients

Background

The main cause of death of breast cancer patients is not the primary tumor itself but the metastatic disease. Identifying breast cancer-specific signatures for metastasis and learning more about the nature of the genes involved in the metastatic process would 1) improve our understanding of the mechanisms of cancer progression and 2) reveal new therapeutic targets. Previous studies showed that the transcriptional regulator megakaryoblastic leukemia-1 (Mkl1) induces tenascin-C expression in normal and transformed mammary epithelial cells. Tenascin-C is known to be expressed in metastatic niches, is highly induced in cancer stroma and promotes breast cancer metastasis to the lung.

Methods

Using HC11 mammary epithelial cells overexpressing different Mkl1 constructs, we devised a subtractive transcript profiling screen to identify the mechanism by which Mkl1 induces a gene set co-regulated with tenascin-C. We performed computational analysis of the Mkl1 target genes and used cell biological experiments to confirm the effect of these gene products on cell behavior. To analyze whether this gene set is prognostic of accelerated cancer progression in human patients, we used the bioinformatics tool GOBO that allowed us to investigate a large breast tumor data set linked to patient data.

Results

We discovered a breast cancer-specific set of genes including tenascin-C, which is regulated by Mkl1 in a SAP domain-dependent, serum response factor-independent manner and is strongly implicated in cell proliferation, cell motility and cancer. Downregulation of this set of transcripts by overexpression of Mkl1 lacking the SAP domain inhibited cell growth and cell migration. Many of these genes are direct Mkl1 targets since their promoter-reporter constructs were induced by Mkl1 in a SAP domain-dependent manner. Transcripts, most strongly reduced in the absence of the SAP domain were mechanoresponsive. Finally, expression of this gene set is associated with high-proliferative poor-outcome classes in human breast cancer and a strongly reduced survival rate for patients independent of tumor grade.

Conclusions

This study highlights a crucial role for the transcriptional regulator Mkl1 and its SAP domain during breast cancer progression. We identified a novel gene set that correlates with bad prognosis and thus may help in deciding the rigor of therapy.

Novel Rho/MRTF/SRF inhibitors block matrix-stiffness and TGF-β-induced fibrogenesis in human colonic myofibroblasts

BACKGROUND

Ras homolog gene family, member A (RhoA)/Rho-associated coiled-coil forming protein kinase signaling is a key pathway in multiple types of solid organ fibrosis, including intestinal fibrosis. However, the pleiotropic effects of RhoA/Rho-associated coiled-coil forming protein kinase signaling have frustrated targeted drug discovery efforts. Recent recognition of the role of Rho-regulated gene transcription by serum response factor (SRF) and its transcriptional cofactor myocardin-related transcription factor A (MRTF-A) suggest a novel locus for pharmacological intervention.

METHODS

Because RhoA signaling is mediated by both physical and biochemical stimuli, we examined whether pharmacological inhibition of RhoA or the downstream transcription pathway of MRTF-A/SRF could block intestinal fibrogenesis in 2 in vitro models.

RESULTS

In this study, we demonstrate that inhibition of RhoA signaling blocks both matrix-stiffness and transforming growth factor beta-induced fibrogenesis in human colonic myofibroblasts. Repression of alpha-smooth muscle actin and collagen expression was associated with the inhibition of MRTF-A nuclear localization. CCG-1423, a first-generation Rho/MRTF/SRF pathway inhibitor, repressed fibrogenesis in both models, yet has unacceptable cytotoxicity. Novel second-generation inhibitors (CCG-100602 and CCG-203971) repressed both matrix-stiffness and transforming growth factor beta-mediated fibrogenesis as determined by protein and gene expression in a dose-dependent manner.

CONCLUSIONS

Targeting the Rho/MRTF/SRF mechanism with second-generation Rho/MRTF/SRF inhibitors may represent a novel approach to antifibrotic therapeutics.

Sharp-1 regulates TGF-β signaling and skeletal muscle regeneration

Sharp-1 is a basic helix-loop-helix (bHLH) transcriptional repressor that is involved in a number of cellular processes. Our previous studies have demonstrated that Sharp-1 is a negative regulator of skeletal myogenesis and it blocks differentiation of muscle precursor cells by modulating the activity of MyoD. In order to understand its role in pre- and post-natal myogenesis, we assessed skeletal muscle development and freeze-injury-induced regeneration in Sharp-1-deficient mice. We show that embryonic skeletal muscle development is not impaired in the absence of Sharp-1; however, post-natally, the regenerative capacity is compromised. Although the initial phases of injury-induced regeneration proceed normally in Sharp-1(-/-) mice, during late stages, the mutant muscle exhibits necrotic fibers, calcium deposits and fibrosis. TGF-β expression, as well as levels of phosphorylated Smad2 and Smad3, are sustained in the mutant tissue and treatment with decorin, which blocks TGF-β signaling, improves the histopathology of Sharp-1(-/-) injured muscles. In vitro, Sharp-1 associates with Smad3, and its overexpression inhibits TGF-β- and Smad3-mediated expression of extracellular matrix genes in myofibroblasts. These results demonstrate that Sharp-1 regulates muscle regenerative capacity, at least in part, by modulation of TGF-β signaling.

Cell adhesion and shape regulate TGF-beta1-induced epithelial-myofibroblast transition via MRTF-A signaling

Myofibroblasts, specialized cells that play important roles in wound healing and fibrosis, can develop from epithelial cells through an epithelial-mesenchymal transition (EMT). During EMT, epithelial cells detach from neighboring cells and acquire an elongated, mesenchymal-like morphology. These phenotypic changes are accompanied by changes in gene expression patterns including upregulation of a variety of cytoskeletal associated proteins which contribute to the ability of myofibroblasts to exert large contractile forces. Here, the relationship between cell shape and cytoskeletal tension and the expression of cytoskeletal proteins in transforming growth factor (TGF)-β1-induced EMT is determined. We find that culturing cells in conditions which permit cell spreading and increased contractility promotes the increased expression of myofibroblast markers and cytoskeletal associated proteins. In contrast, blocking cell spreading prevents transdifferentiation to the myofibroblast phenotype. Furthermore, we find that cell shape regulates the expression of cytoskeletal proteins by controlling the subcellular localization of myocardin related transcription factor (MRTF)-A. Pharmacological inhibition of cytoskeletal tension or MRTF-A signaling blocks the acquisition of a myofibroblast phenotype in spread cells while overexpression of MRTF-A promotes the expression of cytoskeletal proteins for all cell shapes. These data suggest that cell shape is a critical determinant of myofibroblast development from epithelial cells.

From tissue mechanics to transcription factors

Changes in tissue stiffness are frequently associated with diseases such as cancer, fibrosis, and atherosclerosis. Several recent studies suggest that, in addition to resulting from pathology, mechanical changes may play a role akin to soluble factors in causing the progression of disease, and similar mechanical control might be essential for normal tissue development and homeostasis. Many cell types alter their structure and function in response to exogenous forces or as a function of the mechanical properties of the materials to which they adhere. This review summarizes recent progress in identifying intracellular signaling pathways, and especially transcriptional programs, that are differentially activated when cells adhere to materials with different mechanical properties or when they are subject to tension arising from external forces. Several cytoplasmic or cytoskeletal signaling pathways involving small GTPases, focal adhesion kinase and transforming growth factor beta as well as the transcriptional regulators MRTF-A, NFκB, and Yap/Taz have emerged as important mediators of mechanical signaling.

Activation of MRTF-A-dependent gene expression with a small molecule promotes myofibroblast differentiation and wound healing

Myocardin-related transcription factors (MRTFs) regulate cellular contractility and motility by associating with serum response factor (SRF) and activating genes involved in cytoskeletal dynamics. We reported previously that MRTF-A contributes to pathological cardiac remodeling by promoting differentiation of fibroblasts to myofibroblasts following myocardial infarction. Here, we show that forced expression of MRTF-A in dermal fibroblasts stimulates contraction of a collagen matrix, whereas contractility of MRTF-A null fibroblasts is impaired under basal conditions and in response to TGF-β1 stimulation. We also identify an isoxazole ring-containing small molecule, previously shown to induce smooth muscle α-actin gene expression in cardiac progenitor cells, as an agonist of myofibroblast differentiation. Isoxazole stimulates myofibroblast differentiation via induction of MRTF-A-dependent gene expression. The MRTF-SRF signaling axis is activated in response to skin injury, and treatment of dermal wounds with isoxazole accelerates wound closure and suppresses the inflammatory response. These results reveal an important role for MRTF-SRF signaling in dermal myofibroblast differentiation and wound healing and suggest that targeting MRTFs pharmacologically may prove useful in treating diseases associated with inappropriate myofibroblast activity.

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

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

Characterization and role of SCAI during renal fibrosis and epithelial-to-mesenchymal transition

During progressive tubulointerstitial fibrosis, renal tubular epithelial cells transform into α-smooth muscle actin (SMA)-expressing myofibroblasts via epithelial-to-mesenchymal transition (EMT). SMA expression is regulated by transforming growth factor (TGF)-β1 and cell contact disruption, through signaling events targeting the serum response factor-myocardin-related transcription factor (MRTF) complex. MRTFs are important regulators of fibrosis, tumor cell invasion, and metastasis. Consistent with the role of MRTFs in tumor progression, suppressor of cancer cell invasion (SCAI) was recently identified as a negative regulator of MRTF. Herein, we studied the role of SCAI in a fibrotic EMT model established on LLC-PK1 cells. SCAI overexpression prevented SMA promoter activation induced by TGF-β1. When co-expressed, it inhibited the stimulatory effects of MRTF-A, MRTF-B or the constitutive active forms of RhoA, Rac1, or Cdc42 on the SMA promoter. SCAI interfered with TGF-β1-induced SMA, connective tissue growth factor, and calponin protein expression; it rescued TGF-β1-induced E-cadherin down-regulation. IHC studies on human kidneys showed that SCAI expression is reduced during fibrosis. Kidneys of diabetic rats and mice with unilateral ureteral obstruction depicted significant loss of SCAI expression. In parallel with the decrease of SCAI protein expression, diabetic rat and mouse kidneys with unilateral ureteral obstruction showed SMA expression, as evidenced by using Western blot analysis. Finally, TGF-β1 treatment of LLC-PK1 cells attenuated SCAI protein expression. These data suggest that SCAI is a novel transcriptional cofactor that regulates EMT and renal fibrosis.

Distinct mesenchymal alterations in N-cadherin and E-cadherin positive primary renal epithelial cells

Background

Renal tubular epithelial cells of proximal and distal origin differ markedly in their physiological functions. Therefore, we hypothesized that they also differ in their capacity to undergo epithelial to mesenchymal alterations.

Results

We used cultures of freshly isolated primary human tubular cells. To distinguish cells of different tubular origin we took advantage of the fact that human proximal epithelial cells uniquely express N-cadherin instead of E-cadherin as major cell-cell adhesion molecule. To provoke mesenchymal alteration we treated these cocultures with TGF-β for up to 6 days. Within this time period, the morphology of distal tubular cells was barely altered. In contrast to tubular cell lines, E-cadherin was not down-regulated by TGF-β, even though TGF-β signal transduction was initiated as demonstrated by nuclear localization of Smad2/3. Analysis of transcription factors and miRNAs possibly involved in E-cadherin regulation revealed high levels of miRNAs of the miR200-family, which may contribute to the stability of E-cadherin expression in human distal tubular epithelial cells. By contrast, proximal tubular epithelial cells altered their phenotype when treated with TGF-β. They became elongated and formed three-dimensional structures. Rho-kinases were identified as modulators of TGF-β-induced morphological alterations. Non-specific inhibition of Rho-kinases resulted in stabilization of the epithelial phenotype, while partial effects were observed upon downregulation of Rho-kinase isoforms ROCK1 and ROCK2. The distinct reactivity of proximal and distal cells was retained when the cells were cultured as polarized cells.

Conclusions

Interference with Rho-kinase signaling provides a target to counteract TGF-β-mediated mesenchymal alterations of epithelial cells, particularly in proximal tubular epithelial cells. Furthermore, primary distal tubular cells differed from cell lines by their high phenotypic stability which included constant expression of E-cadherin. Our cell culture system of primary epithelial cells is thus suitable to understand and modulate cellular remodeling processes of distinct tubular cells relevant for human renal disease.

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

Cardiac fibroblasts are responsible for necrotic tissue replacement and scar formation after myocardial infarction (MI) and contribute to remodeling in response to pathological stimuli. This response to insult or injury is largely due to the phenotypic plasticity of fibroblasts. When fibroblasts encounter environmental disturbances, whether biomechanical or humoral, they often transform into smooth muscle-like, contractile cells called "myofibroblasts." The signals that control myofibroblast differentiation include the transforming growth factor (TGF)-β1-Smad pathway and Rho GTPase-dependent actin polymerization. Recent evidence implicates serum response factor (SRF) and the myocardin-related transcription factors (MRTFs) as key mediators of the contractile gene program in response to TGF-β1 or RhoA signaling. This review highlights the function of myofibroblasts in cardiac remodeling and the role of the actin-MRTF-SRF signaling axis in regulating this process.