MAL/MRTF-A controls migration of non-invasive cells by upregulation of cytoskeleton-associated proteins

MRTF-A gain-of-function in mice impairs homeostatic renewal of the intestinal epithelium

The actin-regulated transcription factor MRTF-A represents a central relay in mechanotransduction and controls a subset of SRF-dependent target genes. However, gain-of-function studies in vivo are lacking. Here we characterize a conditional MRTF-A transgenic mouse model. While MRTF-A gain-of-function impaired embryonic development, induced expression of constitutively active MRTF-A provoked rapid hepatocyte ballooning and liver failure in adult mice. Specific expression in the intestinal epithelium caused an erosive architectural distortion, villus blunting, cryptal hyperplasia and colonic inflammation, resulting in transient weight loss. Organoids from transgenic mice repeatedly induced in vitro showed impaired self-renewal and defective cryptal compartments. Mechanistically, MRTF-A gain-of-function decreased proliferation and increased apoptosis, but did not induce fibrosis. MRTF-A targets including Acta2 and Pai-1 were induced, whereas markers of stem cells and differentiated cells were reduced. Our results suggest that activated MRTF-A in the intestinal epithelium shifts the balance between proliferation, differentiation and apoptosis.

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

Background

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

Methods

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

Results

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

Conclusion

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

Regulatory roles of fibronectin and integrin α5 in reorganization of the actin cytoskeleton and completion of adipogenesis

Cellular differentiation is characterized by changes in cell morphology that are largely determined by actin dynamics. We previously showed that depolymerization of the actin cytoskeleton triggers the differentiation of preadipocytes into mature adipocytes as a result of inhibition of the transcriptional coactivator activity of megakaryoblastic leukemia 1 (MKL1). The extracellular matrix (ECM) influences cell morphology via interaction with integrins, and reorganization of the ECM is associated with cell differentiation. Here we show that interaction between actin dynamics and ECM rearrangement plays a key role in adipocyte differentiation. We found that depolymerization of the actin cytoskeleton precedes disruption and degradation of fibrillar fibronectin (FN) structures at the cell surface after the induction of adipogenesis in cultured preadipocytes. A FN matrix suppressed both reorganization of the actin cytoskeleton into the pattern characteristic of adipocytes and terminal adipocyte differentiation, and these inhibitory effects were overcome by knockdown of integrin α5 (ITGα5). Peroxisome proliferator–activated receptor γ was required for down-regulation of FN during adipocyte differentiation, and MKL1 was necessary for the expression of ITGα5. Our findings suggest that cell-autonomous down-regulation of FN-ITGα5 interaction contributes to reorganization of the actin cytoskeleton and completion of adipocyte differentiation.

Lamina-associated polypeptide 2α is required for intranuclear MRTF-A activity

Myocardin-related transcription factor A (MRTF-A), a coactivator of serum response factor (SRF), regulates the expression of many cytoskeletal genes in response to cytoplasmic and nuclear actin dynamics. Here we describe a novel mechanism to regulate MRTF-A activity within the nucleus by showing that lamina-associated polypeptide 2α (Lap2α), the nucleoplasmic isoform of Lap2, is a direct binding partner of MRTF-A, and required for the efficient expression of MRTF-A/SRF target genes. Mechanistically, Lap2α is not required for MRTF-A nuclear localization, unlike most other MRTF-A regulators, but is required for efficient recruitment of MRTF-A to its target genes. This regulatory step takes place prior to MRTF-A chromatin binding, because Lap2α neither interacts with, nor specifically influences active histone marks on MRTF-A/SRF target genes. Phenotypically, Lap2α is required for serum-induced cell migration, and deregulated MRTF-A activity may also contribute to muscle and proliferation phenotypes associated with loss of Lap2α. Our studies therefore add another regulatory layer to the control of MRTF-A-SRF-mediated gene expression, and broaden the role of Lap2α in transcriptional regulation.

Pharmacological regulation of tissue fibrosis by targeting the mechanical contraction of myofibroblasts

Fibrosis can occur in almost all tissues and organs and affects normal physiological function, which may have serious consequences, such as organ failure. However, there are currently no effective, broad-spectrum drugs suitable for clinical application. Revealing the process of fibrosis is an important prerequisite for the development of new therapeutic targets and drugs. Studies have shown that the limiting of myofibroblast activation or the promoting of their elimination can ameliorate fibrosis. However, it has not been reported whether a direct decrease in cell contraction can inhibit fibrosis in vivo. Here, we have shown that (-)-blebbistatin (Ble), a non-muscle myosin Ⅱ inhibitor, displayed significant inhibition of liver fibrosis in different chronic injury mouse models in vivo. We found that Ble reduced the stiffness of fibrotic tissues from the early stage, which reduced the extent of myofibroblast activation induced by a stiffer extracellular matrix (ECM). Moreover, Ble also reduced the activation of myofibroblasts induced by TGF-β1, which is the most potent pro-fibrotic cytokine. Mechanistically, Ble reduced mechanical contraction, which inhibited the assembly of stress fibers, decreased the F/G-actin ratio, and led to the exnucleation of YAP1 and MRTF-A. Finally, we verified its broad-spectrum antifibrotic effect in multiple models of organ fibrosis. Our results highlighted the important role of mechanical contraction in myofibroblast activation and maintenance, rather than just a characteristic of activation, suggesting that it may be a potential target to explore broad-spectrum drugs for the treatment of fibrotic diseases.

MICAL2 Promotes Proliferation and Migration of Glioblastoma Cells Through TGF-β/p-Smad2/EMT-Like Signaling Pathway

Recent studies showed that molecule interacting with CasL2 (MICAL2) could be a novel tumor growth factor, and it is closely associated with tumor growth and invasion. However, the role it plays in glioblastoma (GBM) and its potential mechanisms are currently unknown. Our study is designed to identify the effect of MICAL2 on GBM cells and the potential mechanisms behind it. Here, we found that MICAL2 interacts with TGF receptor-type I (TGFRI) and promotes the proliferation and migration of glioblastoma through the TGF-β/p-Smad2/EMT-like signaling pathway. MICAL2-knockdown inhibited the proliferation of glioblastoma cells, which was related to cell cycle arrest and downregulation of DNA replication. The invasion abilities of U87 and U251 cells were reduced after the knockdown of MICAL2. MICAL2 promoted the growth of GBM in nude mice. High MICAL2 predicts poor outcome of GBM patients. MICAL2 could be identified as a novel promising therapeutic target for human GBM.

Desmosomes as Signaling Hubs in the Regulation of Cell Behavior

Desmosomes are intercellular junctions, which preserve tissue integrity during homeostatic and stress conditions. These functions rely on their unique structural properties, which enable them to respond to context-dependent signals and transmit them to change cell behavior. Desmosome composition and size vary depending on tissue specific expression and differentiation state. Their constituent proteins are highly regulated by posttranslational modifications that control their function in the desmosome itself and in addition regulate a multitude of desmosome-independent functions. This review will summarize our current knowledge how signaling pathways that control epithelial shape, polarity and function regulate desmosomes and how desmosomal proteins transduce these signals to modulate cell behavior.

Dynamics of m6A RNA Methylome on the Hallmarks of Hepatocellular Carcinoma

Epidemiological data consistently rank hepatocellular carcinoma (HCC) as one of the leading causes of cancer-related deaths worldwide, often posing severe economic burden on health care. While the molecular etiopathogenesis associated with genetic and epigenetic modifications has been extensively explored, the biological influence of the emerging field of epitranscriptomics and its associated aberrant RNA modifications on tumorigenesis is a largely unexplored territory with immense potential for discovering new therapeutic approaches. In particular, the underlying cellular mechanisms of different hallmarks of hepatocarcinogenesis that are governed by the complex dynamics of m6A RNA methylation demand further investigation. In this review, we reveal the up-to-date knowledge on the mechanistic and functional link between m6A RNA methylation and pathogenesis of HCC.

Moesin, an Ezrin/Radixin/Moesin Family Member, Regulates Hepatic Fibrosis

BACKGROUND AND AIMS

Moesin, an ezrin/radixin/moesin family member, is involved in the regulation of cell adhesion, polarity, and migration by cross-linking between the actin cytoskeleton and plasma membrane. The primary effector cell in hepatic fibrosis is the hepatic stellate cell (HSC), which undergoes activation during liver injury leading to increased extracellular matrix production.

APPROACH AND RESULTS

Here, we have hypothesized that moesin plays a critical role in linking the HSC cytoskeleton to the fibrogenic cascade during HSC activation. Moesin phosphorylation was up-regulated during HSC activation and fibrogenesis. Using moesin wild-type (WT) and mutant constructs (phosphomimicking T558D and nonphosphorylatable T558A), we found that cellular motility and contraction were increased in moesin WT-infected and T558D-infected cells, paralleled by an increase in smooth muscle α-actin and collagen 1 expression. In contrast, overexpression of nonphosphorylatable moesin and moesin knockout (KO) decreased cellular motility and contraction. Most importantly, moesin KO led to abrogation of liver fibrosis. The mechanism of moesin's effect was a reduction in myocardin-related transcription factor-A and serum-response factor (SRF)-mediated changes in the actin cytoskeleton, which in turn modulated the expression of matrix genes.

CONCLUSIONS

Taken together, our findings suggest that the linkage between cytoskeletal dynamics and the correlated MRTF/SRF signaling pathway has a pivotal role in HSC activation and fibrogenesis.

The MRTF-A/miR-155/SOX1 pathway mediates gastric cancer migration and invasion

Background

Gastric cancer (GC) is the leading cause of death worldwide and is closely related to metastasis. MRTF-A is one of the most well-characterized genetic markers in cancer. However, the mechanism whereby MRTF-A mediate gastric cancer (GC) tumorigenesis is not fully clear. Increasing evidence has confirmed that miRNA dysregulation is involved in MRTF-A-mediated tumorigenesis, supporting their potential as therapeutic targets for cancer. Although miR-155 has been reported as an upregulated miRNA, the interplay between miR-155 and MRTF-A-mediated gastric cancer progression remain largely elusive.

Methods

Real-time PCR was performed to determine miR-155 expression after transfected with MRTF-A encoding plasmids and siRNA. Potential target genes were identified by Western blot and luciferase reporter assay. Chip assay was proved that MRTF-A binds in the promoter region of miR-155. Transwell assay and Scratch-healing migration assay was used to investigate the role of MRTF-A and SOX1 in gastric cancer cell migration and invasion.

Results

MRTF-A can interact with the miR-155 promoter to promote histone acetylation and RNA polymerase II recruitment via the Wnt-β-catenin pathway. miR-155 promotes gastric cancer cell migration by suppressing SOX1 expressiom by targeting its 3′UTR in vitro and in vivo. MRTF-A inhibited the inhibitory effects of SOX1 on gastric cancer cell migration by promoting the express -ion of miR-155.

Conclusion

Our data therefore provide important and novel insights into how the MRTF-A/miR-155/SOX1 pathway mediates migration and invasion in GC.

Intercalated discs: cellular adhesion and signaling in heart health and diseases

Intercalated discs (ICDs) are highly orchestrated structures that connect neighboring cardiomyocytes in the heart. Three major complexes are distinguished in ICD: desmosome, adherens junction (AJ), and gap junction (GJ). Desmosomes are major cell adhesion junctions that anchor cell membrane to the intermediate filament network; AJs connect the actin cytoskeleton of adjacent cells; and gap junctions metabolically and electrically connect the cytoplasm of adjacent cardiomyocytes. All these complexes work as a single unit, the so-called area composita, interdependently rather than individually. Mutation or altered expression of ICD proteins results in various cardiac diseases, such as ARVC (arrhythmogenic right ventricular cardiomyopathy), dilated cardiomyopathy, and hypotrophy cardiomyopathy, eventually leading to heart failure. In this article, we first review the recent findings on the structural organization of ICD and their functions and then focus on the recent advances in molecular pathogenesis of the ICD-related heart diseases, which include two major areas: i) the ICD gene mutations in cardiac diseases, and ii) the involvement of ICD proteins in signal transduction pathways leading to myocardium remodeling and eventual heart failure. These major ICD-related signaling pathways include Wnt/β-catenin pathway, p38 MAPK cascade, Rho-dependent serum response factor (SRF) signaling, calcineurin/NFAT signaling, Hippo kinase cascade, etc., which are differentially regulated in pathological conditions.

Mechanosensitive transcriptional coactivators MRTF-A and YAP/TAZ regulate nucleus pulposus cell phenotype through cell shape

Cells of the adult nucleus pulposus (NP) are critically important in maintaining overall disc health and function. NP cells reside in a soft, gelatinous matrix that dehydrates and becomes increasingly fibrotic with age. Such changes result in physical cues of matrix stiffness that may be potent regulators of NP cell phenotype and may contribute to a transition toward a senescent and fibroblastic NP cell with a limited capacity for repair. Here, we investigate the mechanosignaling cues generated from changes in matrix stiffness in directing NP cell phenotype and identify mechanisms that can potentially preserve a biosynthetically active, juvenile NP cell phenotype. Using a laminin-functionalized polyethylene glycol hydrogel, we show that when NP cells form rounded, multicell clusters, they are able to maintain cytosolic localization of myocardin-related transcription factor (MRTF)-A, a coactivator of serum-response factor (SRF), known to promote fibroblast-like behaviors in many cells. Upon preservation of a rounded shape, human NP cells similarly showed cytosolic retention of transcriptional coactivator Yes-associated protein (YAP) and its paralogue PDZ-binding motif (TAZ) with associated decline in activation of its transcription factor TEA domain family member-binding domain (TEAD). When changes in cell shape occur, leading to a more spread, fibrotic morphology associated with stronger F-actin alignment, SRF and TEAD are up-regulated. However, targeted deletion of either cofactor was not sufficient to overcome shape-mediated changes observed in transcriptional activation of SRF or TEAD. Findings show that substrate stiffness-induced promotion of F-actin alignment occurs concomitantly with a flattened, spread morphology, decreased NP marker expression, and reduced biosynthetic activity. This work indicates cell shape is a stronger indicator of SRF and TEAD mechanosignaling pathways than coactivators MRTF-A and YAP/TAZ, respectively, and may play a role in the degeneration-associated loss of NP cellularity and phenotype.-Fearing, B. V., Jing, L., Barcellona, M. N., Witte, S. E., Buchowski, J. M., Zebala, L. P., Kelly, M. P., Luhmann, S., Gupta, M. C., Pathak, A., Setton, L. A. Mechanosensitive transcriptional coactivators MRTF-A and YAP/TAZ regulate nucleus pulposus cell phenotype through cell shape.

Two Isoforms of the Guanine Nucleotide Exchange Factor, Daple/CCDC88C Cooperate as Tumor Suppressors

Previously, Aznar et al., showed that Daple/CCDC88C enables Wnt receptors to transactivate trimeric G-proteins during non-canonical Wnt signaling via a novel G-protein binding and activating (GBA) motif. By doing so, Daple serves two opposing roles; earlier during oncogenesis it suppresses neoplastic transformation and tumor growth, but later it triggers epithelial-to-mesenchymal-transition (EMT). We have identified and characterized two isoforms of the human Daple gene. While both isoforms cooperatively suppress tumor growth via their GBA motif, only the full-length transcript triggers EMT and invasion. Both isoforms are suppressed during colon cancer progression, and their reduced expression carries additive prognostic significance. These findings provide insights into the opposing roles of Daple during cancer progression and define the G-protein regulatory GBA motif as one of the minimal modules essential for Daple’s role as a tumor suppressor.

MRTF-A controls myofibroblastic differentiation of human multipotent stromal cells and their tumour-supporting function in xenograft models

Tumour growth and metastatic colonization is strongly influenced by the tumour stroma, including cancer-associated fibroblasts (CAF). Multipotent mesenchymal stromal cells (MSC) are a possible source of CAF following myofibroblastic differentiation, and we have previously shown that MSC support tumour growth. Triggered by tumour cell-derived factors like transforming growth factor β1 (TGF-β1), myofibroblastic MSC differentiation is associated with the increased expression of markers including alpha smooth muscle actin (α-SMA). Here we show that myocardin-related transcription factor A (MRTF-A) plays an important role in myofibroblastic differentiation of primary human MSC in vitro and their tumour-supporting function in vivo. Recombinant TGF-β1 or tumour cell conditioned medium (TCM) elevated α-SMA, calponin 1 and collagen 1 A1 (COL1A1) amount on mRNA and protein level in MSC. This correlated with increased MRTF-A activity during MSC differentiation. MRTF-A knockdown by siRNA or shRNA impaired TGF-β1 and TCM induction of α-SMA and calponin 1, but not of COL1A1. Mixed xenograft experiments using HCT8 colorectal carcinoma cells and primary MSC of different donors revealed a significant reduction in tumour weight and volume upon MRTF-A knockdown in MSC. Our study suggests that MRTF-A is involved in the functional differentiation of MSC towards a tumour-promoting CAF phenotype in vivo.

Protective transcriptional mechanisms in cardiomyocytes and cardiac fibroblasts

Heart failure is the leading cause of morbidity and mortality worldwide. Several lines of evidence suggest that physical activity and exercise can pre-condition the heart to improve the response to acute cardiac injury such as myocardial infarction or ischemia/reperfusion injury, preventing the progression to heart failure. It is becoming more apparent that cardioprotection is a concerted effort between multiple cell types and converging signaling pathways. However, the molecular mechanisms of cardioprotection are not completely understood. What is clear is that the mechanisms underlying this protection involve acute activation of transcriptional activators and their corresponding gene expression programs. Here, we review the known stress-dependent transcriptional programs that are activated in cardiomyocytes and cardiac fibroblasts to preserve function in the adult heart after injury. Focus is given to prominent transcriptional pathways such as mechanical stress or reactive oxygen species (ROS)-dependent activation of myocardin-related transcription factors (MRTFs) and transforming growth factor beta (TGFβ), and gene expression that positively regulates protective PI3K/Akt signaling. Together, these pathways modulate both beneficial and pathological responses to cardiac injury in a cell-specific manner.

IGF2BP1 promotes SRF-dependent transcription in cancer in a m6A- and miRNA-dependent manner

The oncofetal mRNA-binding protein IGF2BP1 and the transcriptional regulator SRF modulate gene expression in cancer. In cancer cells, we demonstrate that IGF2BP1 promotes the expression of SRF in a conserved and N6-methyladenosine (m6A)-dependent manner by impairing the miRNA-directed decay of the SRF mRNA. This results in enhanced SRF-dependent transcriptional activity and promotes tumor cell growth and invasion. At the post-transcriptional level, IGF2BP1 sustains the expression of various SRF-target genes. The majority of these SRF/IGF2BP1-enhanced genes, including PDLIM7 and FOXK1, show conserved upregulation with SRF and IGF2BP1 synthesis in cancer. PDLIM7 and FOXK1 promote tumor cell growth and were reported to enhance cell invasion. Consistently, 35 SRF/IGF2BP1-dependent genes showing conserved association with SRF and IGF2BP1 expression indicate a poor overall survival probability in ovarian, liver and lung cancer. In conclusion, these findings identify the SRF/IGF2BP1-, miRNome- and m6A-dependent control of gene expression as a conserved oncogenic driver network in cancer.

SRF'ing and SAP'ing - the role of MRTF proteins in cell migration

Actin-based cell migration is a fundamental cellular activity that plays a crucial role in a wide range of physiological and pathological processes. An essential feature of the remodeling of actin cytoskeleton during cell motility is the de novo synthesis of factors involved in the regulation of the actin cytoskeleton and cell adhesion in response to growth-factor signaling, and this aspect of cell migration is critically regulated by serum-response factor (SRF)-mediated gene transcription. Myocardin-related transcription factors (MRTFs) are key coactivators of SRF that link actin dynamics to SRF-mediated gene transcription. In this Review, we provide a comprehensive overview of the role of MRTF in both normal and cancer cell migration by discussing its canonical SRF-dependent as well as its recently emerged SRF-independent functions, exerted through its SAP domain, in the context of cell migration. We conclude by highlighting outstanding questions for future research in this field.

Myocardin-related transcription factor A (MRTF-A) activity-dependent cell adhesion is correlated to focal adhesion kinase (FAK) activity

The regulation of cell-substrate adhesion is tightly linked to the malignant phenotype of tumor cells and plays a role in their migration, invasion, and metastasis. Focal adhesions (FAs) are dynamic adhesion structures that anchor the cell to the extracellular matrix. Myocardin-related transcription factors (MRTFs), co-regulators of the serum response factor (SRF), regulate expression of a set of genes encoding actin cytoskeletal/FA-related proteins. Here we demonstrated that the forced expression of a constitutively active MRTF-A (CA-MRTF-A) in B16F10 melanoma cells induced the up-regulation of actin cytoskeletal and FA proteins, resulting in FA reorganization and the suppression of cell migration. Expression of CA-MRTF-A markedly increased phosphorylation of focal adhesion kinase (FAK) and paxillin, which are important components for FA dynamics. Notably, FAK activation was triggered by the clustering of up-regulated integrins. Our results revealed that the MRTF-SRF-dependent regulation of cell migration requires both the up-regulation of actin cytoskeletal/FA proteins and the integrin-mediated regulation of FA components via the FAK/Src pathway. We also demonstrated that activation of the MRTF-dependent transcription correlates FAK activation in various tumor cells. The elucidation of the correlation between MRTF and FAK activities would be an effective therapeutic target in focus of tumor cell migration.

Overexpression of MICAL2, a novel tumor-promoting factor, accelerates tumor progression through regulating cell proliferation and EMT

Molecule interacting with CasL 2 (MICAL2), a microtubule associated monooxygenase, is involved in cell growth, axon guidance, vesicle trafficking and apoptosis. Recent studies have demonstrated that MICAL2 is highly expressed in tumor and accelerates tumor progression and it is deemed to be a novel tumor-promoting factor. MICAL2 overexpression increases cell proliferation to accelerate tumor growth, and MICAL2 also promotes epithelial-mesenchymal transition (EMT)-related proteins to increase cancer cell metastasis. On mechanism, MICAL2 induces EMT by regulating SRF (serum response factor)/MRTF-A (myocardin related transcription factor A) signaling, Semaphorin/Plexin pathway and inducing ROS (Reactive oxygen species) production. In the present review, we introduced MICAL family, expatiated the structure and functions of MICALs, and summarized the mechanisms of MICAL2 involving tumor progression. The challenges and perspectives for MICAL2 in tumor are also discussed.

Tightly controlled MRTF-A activity regulates epithelial differentiation during formation of mammary acini

Background

Myocardin-related transcription factors (MRTF) A and B link actin dynamics and mechanotransduction to gene expression. In mice, MRTF-A is involved in mammary gland differentiation, but its role in human mammary epithelial cells remains unclear.

Methods

Three-dimensional cultures of human mammary epithelial MCF10A cells were used to model acinar morphogenesis. Stable MRTF-A knockdown, MRTF-A/B rescue and MRTF-A/B overexpression was established to characterize the functional role during morphogenesis using confocal microscopy and expression analysis. Breast cancer patient databases were analyzed for MRTF-A expression.

Results

We showed that a precise temporal control of MRTFs is required for normal morphogenesis of MCF10A mammary acini. MRTF transcriptional activity, but not their protein amounts, is transiently induced during 3D acini formation. MRTF-A knockdown dramatically reduces acini size and prevents lumen formation. These effects are rescued by re-expression of MRTF-A, and partially by MRTF-B. Conversely, overexpression of MRTF-A and MRTF-B increases acini size, resulting in irregular spheroids without lumen and defective apico-basal polarity. These phenotypes correlate with deregulated expression of cell cycle inhibitors p21/Waf1, p27/Kip1 and altered phosphorylation of retinoblastoma protein. In MRTF overexpressing spheroids, proliferation and apoptosis are simultaneously increased at late stages, whilst neither occurs in control acini. MRTFs interfere with anoikis of the inner cells and cause an integrin switch from α6 to α5, repression of E-cadherin and induction of mesenchymal markers vimentin, Snai2 and Zeb1. Moreover, MRTF-overexpressing spheroids are insensitive to alteration in matrix stiffness. In two breast cancer cohorts, high expression of MRTF-A and known target genes was associated with decreased patient survival.

Conclusion

MRTF-A is required for proliferation and formation of mammary acini from luminal epithelial cells. Conversely, elevated MRTF activity results in pre-malignant spheroid formation due to defective proliferation, polarity loss and epithelial-mesenchymal transition.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-017-0860-3) contains supplementary material, which is available to authorized users.

Desmosomes and Intermediate Filaments: Their Consequences for Tissue Mechanics

Adherens junctions (AJs) and desmosomes connect the actin and keratin filament networks of adjacent cells into a mechanical unit. Whereas AJs function in mechanosensing and in transducing mechanical forces between the plasma membrane and the actomyosin cytoskeleton, desmosomes and intermediate filaments (IFs) provide mechanical stability required to maintain tissue architecture and integrity when the tissues are exposed to mechanical stress. Desmosomes are essential for stable intercellular cohesion, whereas keratins determine cell mechanics but are not involved in generating tension. Here, we summarize the current knowledge of the role of IFs and desmosomes in tissue mechanics and discuss whether the desmosome-keratin scaffold might be actively involved in mechanosensing and in the conversion of chemical signals into mechanical strength.

The myocardin-related transcription factor MKL co-regulates the cellular levels of two profilin isoforms

Megakaryoblastic leukemia (MKL)/serum-response factor (SRF)-mediated gene transcription is a highly conserved mechanism that connects dynamic reorganization of the actin cytoskeleton to regulation of expression of a wide range of genes, including SRF itself and many important structural and regulatory components of the actin cytoskeleton. In this study, we examined the possible role of MKL/SRF in the context of regulation of profilin (Pfn), a major controller of actin dynamics and actin cytoskeletal remodeling in cells. We demonstrated that despite being located on different genomic loci, two major isoforms of Pfn (Pfn1 and Pfn2) are co-regulated by a common mechanism involving the action of MKL that is independent of its SRF-related activity. We found that MKL co-regulates the expression of Pfn isoforms indirectly by modulating signal transducer and activator of transcription 1 (STAT1) and utilizing its SAP-domain function. Unexpectedly, our studies revealed that cellular externalization, rather than transcription of Pfn1, is affected by the perturbations of MKL. We further demonstrated that MKL can influence cell migration by modulating Pfn1 expression, indicating a functional connection between MKL and Pfn1 in actin-dependent cellular processes. Finally, we provide initial evidence supporting the ability of Pfn to influence MKL and SRF expression. Collectively, these findings suggest that Pfn may play a role in a possible feedback loop of the actin/MKL/SRF signaling circuit.

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.

Regulation of microRNA expression in vascular smooth muscle by MRTF-A and actin polymerization

The dynamic properties of the actin cytoskeleton in smooth muscle cells play an important role in a number of cardiovascular disease states. The state of actin does not only mediate mechanical stability and contractile function but can also regulate gene expression via myocardin related transcription factors (MRTFs). These transcriptional co-activators regulate genes encoding contractile and cytoskeletal proteins in smooth muscle. Regulation of small non-coding microRNAs (miRNAs) by actin polymerization may mediate some of these effects. MiRNAs are short non-coding RNAs that modulate gene expression by post-transcriptional regulation of target messenger RNA. In this study we aimed to determine a profile of miRNAs that were 1) regulated by actin/MRTF-A, 2) associated with the contractile smooth muscle phenotype and 3) enriched in muscle cells. This analysis was performed using cardiovascular disease-focused miRNA arrays in both mouse and human cells. The potential clinical importance of actin polymerization in aortic aneurysm was evaluated using biopsies from mildly dilated human thoracic aorta in patients with stenotic tricuspid or bicuspid aortic valve. By integrating information from multiple qPCR based miRNA arrays we identified a group of five miRNAs (miR-1, miR-22, miR-143, miR-145 and miR-378a) that were sensitive to actin polymerization and MRTF-A overexpression in both mouse and human vascular smooth muscle. With the exception of miR-22, these miRNAs were also relatively enriched in striated and/or smooth muscle containing tissues. Actin polymerization was found to be dramatically reduced in the aorta from patients with mild aortic dilations. This was associated with a decrease in actin/MRTF-regulated miRNAs. In conclusion, the transcriptional co-activator MRTF-A and actin polymerization regulated a subset of miRNAs in vascular smooth muscle. Identification of novel miRNAs regulated by actin/MRTF-A may provide further insight into the mechanisms underlying vascular disease states, such as aortic aneurysm, as well as novel ideas regarding therapeutic strategies. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Pharmacological Inhibition of Myocardin-related Transcription Factor Pathway Blocks Lung Metastases of RhoC-Overexpressing Melanoma

Melanoma is the most dangerous form of skin cancer with the majority of deaths arising from metastatic disease. Evidence implicates Rho-activated gene transcription in melanoma metastasis mediated by the nuclear localization of the transcriptional coactivator, myocardin-related transcription factor (MRTF). Here, we highlight a role for Rho and MRTF signaling and its reversal by pharmacologic inhibition using in vitro and in vivo models of human melanoma growth and metastasis. Using two cellular models of melanoma, we clearly show that one cell type, SK-Mel-147, is highly metastatic, has high RhoC expression, and MRTF nuclear localization and activity. Conversely, SK-Mel-19 melanoma cells have low RhoC expression, and decreased levels of MRTF-regulated genes. To probe the dependence of melanoma aggressiveness to MRTF transcription, we use a previously developed small-molecule inhibitor, CCG-203971, which at low micromolar concentrations blocks nuclear localization and activity of MRTF-A. In SK-Mel-147 cells, CCG-203971 inhibits cellular migration and invasion, and decreases MRTF target gene expression. In addition, CCG-203971-mediated inhibition of the Rho/MRTF pathway significantly reduces cell growth and clonogenicity and causes G₁ cell-cycle arrest. In an experimental model of melanoma lung metastasis, the RhoC-overexpressing melanoma cells (SK-Mel-147) exhibited pronounced lung colonization compared with the low RhoC-expressing SK-Mel-19. Furthermore, pharmacologic inhibition of the MRTF pathway reduced both the number and size of lung metastasis resulting in a marked reduction of total lung tumor burden. These data link Rho and MRTF-mediated signaling with aggressive phenotypes and support targeting the MRTF transcriptional pathway as a novel approach to melanoma therapeutics. Mol Cancer Ther; 16(1); 193-204. ©2016 AACR.

The cytoskeleton as a novel target for treatment of renal fibrosis

The incidence of chronic kidney disease (CKD) is increasing, with an estimated prevalence of 12% in the United States (Synder et al., 2009). While CKD may progress to end-stage renal disease (ESRD), which necessitates renal replacement therapy, i.e. dialysis or transplantation, most CKD patients never reach ESRD due to the increased risk of death from cardiovascular disease. It is well-established that regardless of the initiating insult - most often diabetes or hypertension - fibrosis is the common pathogenic pathway that leads to progressive injury and organ dysfunction (Eddy, 2014; Duffield, 2014). As such, there has been extensive research into the molecular and cellular mechanisms of renal fibrosis; however, translation to effective therapeutic strategies has been limited. While a role for the disruption of the cytoskeleton, most notably the actin network, has been established in acute kidney injury over the past two decades, a role in regulating renal fibrosis and CKD is only recently emerging. This review will focus on the role of the cytoskeleton in regulating pro-fibrotic pathways in the kidney, as well as data suggesting that these pathways represent novel therapeutic targets to manage fibrosis and ultimately CKD.

Cadherin-11 is a novel regulator of extracellular matrix synthesis and tissue mechanics

We discovered that Cadherin-11 (CDH11) regulates collagen and elastin synthesis, both affecting the mechanical properties and contractile function of animal tissues. Using a Cdh11-null mouse model, we observed a significant reduction in the mechanical properties [Youngs' modulus and ultimate tensile strength (UTS)] of Cdh11(-/-) as compared to wild-type (WT) mouse tissues, such as the aorta, bladder and skin. The deterioration of mechanical properties (Youngs' modulus and UTS) was accompanied by reduced collagen and elastin content in Cdh11(-/-) mouse tissues as well as in cells in culture. Similarly, knocking down CDH11 abolished collagen and elastin synthesis in human cells, and consequently reduced their ability to generate force. Conversely, engagement of CDH11 through homophilic interactions, led to swift activation of the TGF-β and ROCK pathways as evidenced by phosphorylation of downstream effectors. Subsequently, activation of the key transcription factors, MRTF-A (also known as MKL1) and MYOCD led to significant upregulation of collagen and elastin genes. Taken together, our results demonstrate a novel role of adherens junctions in regulating extracellular matrix (ECM) synthesis with implications for many important biological processes, including maintenance of tissue integrity, wound healing and tissue regeneration.

Myocardin-Related Transcription Factor A Activation by Competition with WH2 Domain Proteins for Actin Binding

The myocardin-related transcription factors (MRTFs) are coactivators of serum response factor (SRF)-mediated gene expression. Activation of MRTF-A occurs in response to alterations in actin dynamics and critically requires the dissociation of repressive G-actin-MRTF-A complexes. However, the mechanism leading to the release of MRTF-A remains unclear. Here we show that WH2 domains compete directly with MRTF-A for actin binding. Actin nucleation-promoting factors, such as N-WASP and WAVE2, as well as isolated WH2 domains, including those of Spire2 and Cobl, activate MRTF-A independently of changes in actin dynamics. Simultaneous inhibition of Arp2-Arp3 or mutation of the CA region only partially reduces MRTF-A activation by N-WASP and WAVE2. Recombinant WH2 domains and the RPEL domain of MRTF-A bind mutually exclusively to cellular and purified G-actin in vitro The competition by different WH2 domains correlates with MRTF-SRF activation. Following serum stimulation, nonpolymerizable actin dissociates from MRTF-A, and de novo formation of the G-actin-RPEL complex is impaired by a transferable factor. Our work demonstrates that WH2 domains activate MRTF-A and contribute to target gene regulation by a competitive mechanism, independently of their role in actin filament formation.

Unravelling a new mechanism linking actin polymerization and gene transcription

In the recent years, the role of actin and actin-binding proteins in gene transcription has received considerable attention. Nuclear monomeric and polymerized actin and several actin binding proteins have been detected in the mammalian cell nucleus, although their roles in transcription are just beginning to emerge. Our group recently reported that the actin-binding protein Filamin A interacts with the transcriptional coactivator MKL1 to link actin polymerization with transcriptional activity of Serum Response Factor. Here we summarize the regulation and function of MKL1, and highlight this novel mechanism of MKL1 regulation through binding to Filamin A and its implications for cell migration.

Integrins synergise to induce expression of the MRTF-A-SRF target gene ISG15 for promoting cancer cell invasion

Integrin-mediated activation of small GTPases induces the polymerisation of G-actin into various actin structures and the release of the transcriptional co-activator MRTF from G-actin. Here we report that pan-integrin-null fibroblasts seeded on fibronectin and expressing β1- and/or αV-class integrin contained different G-actin pools, nuclear MRTF-A (also known as MKL1 or MAL) levels and MRTF-A-SRF activities. The nuclear MRTF-A levels and activities were highest in cells expressing both integrin classes, lower in cells expressing β1 integrins and lowest in cells expressing the αV integrins. Quantitative proteomics and transcriptomics analyses linked the differential MRTF-A activities to the expression of the ubiquitin-like modifier interferon-stimulated gene 15 (ISG15), which is known to modify focal adhesion and cytoskeletal proteins. The malignant breast cancer cell line MDA-MB-231 expressed high levels of β1 integrins, ISG15 and ISGylated proteins, which promoted invasive properties, whereas non-invasive MDA-MB-468 and MCF-7 cell lines expressed low levels of β1 integrins, ISG15 and ISGylated proteins. Our findings suggest that integrin-adhesion-induced MRTF-A-SRF activation and ISG15 expression constitute a newly discovered signalling circuit that promotes cell migration and invasion.

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".

Filamin A interacts with the coactivator MKL1 to promote the activity of the transcription factor SRF and cell migration

Megakaryoblastic leukemia 1 (MKL1) is a coactivator of serum response factor (SRF) that promotes the expression of genes associated with cell proliferation, motility, adhesion, and differentiation-processes that also involve dynamic cytoskeletal changes in the cell. MKL1 is inactive when bound to monomeric globular actin (G-actin), but signals that activate the small guanosine triphosphatase RhoA cause actin polymerization and MKL1 dissociation from G-actin. We found a new mechanism of MKL1 activation that is mediated through its binding to filamin A (FLNA), a protein that binds filamentous actin (F-actin). The interaction of FLNA and MKL1 was required for the expression of MKL1 target genes in primary fibroblasts, melanoma, mammary and hepatocellular carcinoma cells. We identified the regions of interaction between MKL1 and FLNA, and cells expressing an MKL1 mutant that was unable to bind FLNA exhibited impaired cell migration and reduced expression of MKL1-SRF target genes. Induction and repression of MKL1-SRF target genes correlated with increased or decreased MKL1-FLNA interaction, respectively. Lysophosphatidic acid-induced RhoA activation in primary human fibroblasts promoted the association of endogenous MKL1 with FLNA, whereas exposure to an actin polymerization inhibitor dissociated MKL1 from FLNA and decreased MKL1-SRF target gene expression in melanoma cells. Thus, FLNA functions as a positive cellular transducer linking actin polymerization to MKL1-SRF activity, counteracting the known repressive complex of MKL1 and monomeric G-actin.

Ibuprofen and Diclofenac Restrict Migration and Proliferation of Human Glioma Cells by Distinct Molecular Mechanisms

Background

Non-steroidal anti-inflammatory drugs (NSAIDs) have been associated with anti-tumorigenic effects in different tumor entities. For glioma, research has generally focused on diclofenac; however data on other NSAIDs, such as ibuprofen, is limited. Therefore, we performed a comprehensive investigation of the cellular, molecular, and metabolic effects of ibuprofen and diclofenac on human glioblastoma cells.

Methods

Glioma cell lines were treated with ibuprofen or diclofenac to investigate functional effects on proliferation and cell motility. Cell cycle, extracellular lactate levels, lactate dehydrogenase-A (LDH-A) expression and activity, as well as inhibition of the Signal Transducer and Activator of Transcription 3 (STAT-3) signaling pathway, were determined. Specific effects of diclofenac and ibuprofen on STAT-3 were investigated by comparing their effects with those of the specific STAT-3 inhibitor STATTIC.

Results

Ibuprofen treatment led to a stronger inhibition of cell growth and migration than treatment with diclofenac. Proliferation was affected by cell cycle arrest at different checkpoints by both agents. In addition, diclofenac, but not ibuprofen, decreased lactate levels in all concentrations used. Both decreased STAT-3 phosphorylation; however, diclofenac led to decreased c-myc expression and subsequent reduction in LDH-A activity, whereas treatment with ibuprofen in higher doses induced c-myc expression and less LDH-A alteration.

Conclusions

This study indicates that both ibuprofen and diclofenac strongly inhibit glioma cells, but the subsequent metabolic responses of both agents are distinct. We postulate that ibuprofen may inhibit tumor cells also by COX- and lactate-independent mechanisms after long-term treatment in physiological dosages, whereas diclofenac mainly acts by inhibition of STAT-3 signaling and downstream modulation of glycolysis.

The MRTF-A/B function as oncogenes in pancreatic cancer

Despite evidence that MRTF-A/B, co-activators of serum response factor (SRF), promotes tumor cell invasion and metastasis in cancer, there are no studies describing MRTF-A/B in pancreatic cancer. To clarify involvement of MRTF-A/B expression in pancreatic cancer, we used quantitative reverse transcription-polymerase chain reaction and western blot analysis to detect MRTF-A/B in pancreatic cancer, intraductal papillary mucinous neoplasm (IPMN) and non-neoplastic pancreata. MRTF-A/B expression differs significantly between cancer and non-neoplastic tissues as well as between non-neoplastic tissues and IPMN bulk tissues. Next, we studied the roles of MRTF-A/B in vitro. Overexpression of MRTF-A/B promoted epithelial-mesenchymal transition (EMT) and generated stem cell-like cells in normal pancreatic cells. We performed quantitative reverse transcription-polymerase chain reaction to detect the level of MRTF-A/B in 19 pancreatic cancer cell lines. We found that their expression was associated with gemcitabine resistance. Like in normal pancreatic cells, MRTF-A/B also promoted EMT and promoted formation of stem cell-like cells in pancreatic cancer and they could regulate microRNA expression associated with EMT and CICs. Finally, to further demonstrate the roles of MRTF-A/B in vivo, we performed nude mouse model of s.c. xenograft and found that overexpression of MRTF-A and MRTF-B promoted pancreatic cancer growth. Elucidating the roles of MRTF-A/B will help us to further understand molecular basis of the disease and offer new gene targets for effective therapies.

Dynamic tensile forces drive collective cell migration through three-dimensional extracellular matrices

Collective cell migration drives tissue remodeling during development, wound repair, and metastatic invasion. The physical mechanisms by which cells move cohesively through dense three-dimensional (3D) extracellular matrix (ECM) remain incompletely understood. Here, we show directly that migration of multicellular cohorts through collagenous matrices occurs via a dynamic pulling mechanism, the nature of which had only been inferred previously in 3D. Tensile forces increase at the invasive front of cohorts, serving a physical, propelling role as well as a regulatory one by conditioning the cells and matrix for further extension. These forces elicit mechanosensitive signaling within the leading edge and align the ECM, creating microtracks conducive to further migration. Moreover, cell movements are highly correlated and in phase with ECM deformations. Migrating cohorts use spatially localized, long-range forces and consequent matrix alignment to navigate through the ECM. These results suggest biophysical forces are critical for 3D collective migration.

Daple is a novel non-receptor GEF required for trimeric G protein activation in Wnt signaling

Wnt signaling is essential for tissue homeostasis and its dysregulation causes cancer. Wnt ligands trigger signaling by activating Frizzled receptors (FZDRs), which belong to the G-protein coupled receptor superfamily. However, the mechanisms of G protein activation in Wnt signaling remain controversial. In this study, we demonstrate that FZDRs activate G proteins and trigger non-canonical Wnt signaling via the Dishevelled-binding protein, Daple. Daple contains a Gα-binding and activating (GBA) motif, which activates Gαi proteins and an adjacent domain that directly binds FZDRs, thereby linking Wnt stimulation to G protein activation. This triggers non-canonical Wnt responses, that is, suppresses the β-catenin/TCF/LEF pathway and tumorigenesis, but enhances PI3K-Akt and Rac1 signals and tumor cell invasiveness. In colorectal cancers, Daple is suppressed during adenoma-to-carcinoma transformation and expressed later in metastasized tumor cells. Thus, Daple activates Gαi and enhances non-canonical Wnt signaling by FZDRs, and its dysregulation can impact both tumor initiation and progression to metastasis.

Histone acetyltransferase p300 promotes MRTF-A-mediates transactivation of VE-cadherin gene in human umbilical vein endothelial cells

Vascular endothelial cadherin (VE-cadherin) is the major determinant of endothelial cell contact integrity and is required in vascular development and angiogenesis. Serum response factor (SRF) plays essential roles in postnatal retinal angiogenesis and adult neovascularization. It is unclear whether transcription of VE-cadherin is mediated by a SRF co-activator, myocardin-related transcription factor-A (MRTF-A). Here we have demonstrated that MRTF-A is a key regulatory factor to activate the transcription of VE-cadherin in human umbilical vein endothelial cells (HUVECs). siRNA-mediated knockdown of MRTF-A decreased the level of VE-cadherin in HUVECs. Vascular endothelial growth factor (VEGF) induced MRTF-A binding to the SRF-binding site (CArG box) within VE-cadherin promoter. Histone acetyltransferase p300 and MRTF-A could synergistically augment the expression of VE-cadherin by enhancing acetylation of histone3K9 (H3K9Ac), histone3K14 (H3K14Ac) and histone4 at the SRF-binding site within VE-cadherin promoter. Taken together, these data identified a detailed regulatory mechanism of VE-cadherin gene expression.

Myocardin related transcription factor A programs epigenetic activation of hepatic stellate cells

BACKGROUND & AIMS

Activation of hepatic stellate cells (HSCs) represents a key process in liver injury and, in the absence of intervention, leads to irreversible cirrhosis contributing significantly to the mortality of patients with liver disease. A missing link in the current understanding of HSC activation is the involvement of the epigenetic machinery. We investigated the role of the myocardin related transcription factor A (MRTF-A) in HSC activation.

METHODS

Liver fibrosis was induced in wild type (WT) and MRTF-A deficient (KO) mice by CCl4 injection. Expression of mRNA and protein was measured by real-time PCR, Western blotting, and immunohistochemistry. Protein binding to DNA was assayed by chromatin immunoprecipitation (ChIP). Knockdown of endogenous proteins was mediated by either small interfering RNA (siRNA) or short hairpin RNA (shRNA), carried by lentiviral particles.

RESULTS

KO mice exhibited resistance to CCl4-induced liver fibrosis compared to WT littermates. The expression of activated HSC signature genes was suppressed in the absence of MRTF-A. ChIP assays revealed that MRTF-A deficiency led to the erasure of key histone modifications, associated with transcriptional activation, such as H3K4 di- and tri-methylation, on the promoter regions of fibrogenic genes. Mechanistically, MRTF-A recruited a histone methyltransferase complex (COMPASS) to the promoters of fibrogenic genes to activate transcription. Silencing of individual COMPASS components dampened transactivation of fibrogenic genes in vitro and blocked liver fibrosis in mice. Oestradiol suppressed HSC activation by dampening the expression and binding activity of COMPASS.

CONCLUSIONS

Our data illustrate a novel mechanism that connects MRTF-A dependent histone H3K4 methylation to HSC activation.

Inhibition of arginyltransferase 1 induces transcriptional activity of myocardin-related transcription factor A (MRTF-A) and promotes directional migration

Myocardin-related transcription factor A (MRTF-A/MAL/MKL1/BSAC) regulates the expression of serum-response factor (SRF)-dependent target genes in response to the Rho-actin signaling pathway. Overexpression or activation of MRTF-A affects shape, migration, and invasion of cells and contributes to human malignancies, including cancer. In this study, we report that inhibition of arginyltransferase 1 (ATE1), an enzyme mediating post-transcriptional protein arginylation, is sufficient to increase MRTF-A activity in MCF-7 human breast carcinoma cells independently of external growth factor stimuli. In addition, silencing or inhibiting ATE1 disrupted E-cadherin-mediated cell-cell contacts, enhanced formation of actin-rich protrusions, and increased the number of focal adhesions, subsequently leading to elevated chemotactic migration. Although arginylated actin did not differentially affect MRTF-A, a rapid loss of E-cadherin and F-actin reorganization preceded MRTF-A activation upon ATE1 inhibition. Conversely, ectopic ATE1 expression was sufficient to render MRTF-A inactive, both in resting cells and in cells with exogenously activated RhoA-actin pathways. In this study, we provide a critical link between protein arginylation and MRTF-A activity and place ATE1 upstream of myocardin-related transcription factor.

Desmosomes: regulators of cellular signaling and adhesion in epidermal health and disease

Desmosomes are intercellular junctions that mediate cell-cell adhesion and anchor the intermediate filament network to the plasma membrane, providing mechanical resilience to tissues such as the epidermis and heart. In addition to their critical roles in adhesion, desmosomal proteins are emerging as mediators of cell signaling important for proper cell and tissue functions. In this review we highlight what is known about desmosomal proteins regulating adhesion and signaling in healthy skin-in morphogenesis, differentiation and homeostasis, wound healing, and protection against environmental damage. We also discuss how human diseases that target desmosome molecules directly or interfere indirectly with these mechanical and signaling functions to contribute to pathogenesis.

SRF is required for neutrophil migration in response to inflammation

Serum response factor (SRF) is a ubiquitously expressed transcription factor and master regulator of the actin cytoskeleton. We have previously shown that SRF is essential for megakaryocyte maturation and platelet formation and function. Here we elucidate the role of SRF in neutrophils, the primary defense against infections. To study the effect of SRF loss in neutrophils, we crossed Srf(fl/fl) mice with select Cre-expressing mice and studied neutrophil function in vitro and in vivo. Despite normal neutrophil numbers, neutrophil function is severely impaired in Srf knockout (KO) neutrophils. Srf KO neutrophils fail to polymerize globular actin to filamentous actin in response to N-formyl-methionine-leucine-phenylalanine, resulting in significantly disrupted cytoskeletal remodeling. Srf KO neutrophils fail to migrate to sites of inflammation in vivo and along chemokine gradients in vitro. Polarization in response to cytokine stimuli is absent and Srf KO neutrophils show markedly reduced adhesion. Integrins play an essential role in cellular adhesion, and although integrin expression levels are maintained with loss of SRF, integrin activation and trafficking are disrupted. Migration and cellular adhesion are essential for normal cell function, but also for malignant processes such as metastasis, underscoring an essential function for SRF and its pathway in health and disease.

Rho-actin signaling to the MRTF coactivators dominates the immediate transcriptional response to serum in fibroblasts

The transcription factor SRF (serum response factor) recruits two families of coactivators, the MRTFs (myocardin-related transcription factors) and the TCFs (ternary complex factors), to couple gene transcription to growth factor signaling. Here we investigated the role of the SRF network in the immediate transcriptional response of fibroblasts to serum stimulation. SRF recruited its cofactors in a gene-specific manner, and virtually all MRTF binding was directed by SRF. Much of SRF DNA binding was serum-inducible, reflecting a requirement for MRTF-SRF complex formation in nucleosome displacement. We identified 960 serum-responsive SRF target genes, which were mostly MRTF-controlled, as assessed by MRTF chromatin immunoprecipitation (ChIP) combined with deep sequencing (ChIP-seq) and/or sensitivity to MRTF-linked signals. MRTF activation facilitates RNA polymerase II (Pol II) recruitment or promoter escape according to gene context. MRTF targets encode regulators of the cytoskeleton, transcription, and cell growth, underpinning the role of SRF in cytoskeletal dynamics and mechanosensing. Finally, we show that specific activation of either MRTFs or TCFs can reset the circadian clock.

Orchestration of cell surface proteins by Rab11

The organization of cells into interconnected structures such as animal tissues requires a sophisticated system directing receptors and adhesion proteins to the cell surface. The Rab11 small G proteins (Rab11a, b, and Rab25) of the Ras superfamily are master regulators of the surface expression of receptors and adhesion proteins. Acting as a molecular switch, Rab11 builds distinct molecular machinery such as motor protein complexes and the exocyst to transport proteins to the cell surface. Recent evidence reveals Rab11 localization at the trans-Golgi network (TGN), post-Golgi vesicles, and the recycling endosome, placing it at the intersection between the endocytic and exocytic trafficking pathways. We review Rab11 in various cellular contexts, and discuss its regulation and mechanisms by which Rab11 couples with effector proteins.

Redox modification of nuclear actin by MICAL-2 regulates SRF signaling

The serum response factor (SRF) binds to coactivators, such as myocardin-related transcription factor-A (MRTF-A), and mediates gene transcription elicited by diverse signaling pathways. SRF/MRTF-A-dependent gene transcription is activated when nuclear MRTF-A levels increase, enabling the formation of transcriptionally active SRF/MRTF-A complexes. The level of nuclear MRTF-A is regulated by nuclear G-actin, which binds to MRTF-A and promotes its nuclear export. However, pathways that regulate nuclear actin levels are poorly understood. Here, we show that MICAL-2, an atypical actin-regulatory protein, mediates SRF/MRTF-A-dependent gene transcription elicited by nerve growth factor and serum. MICAL-2 induces redox-dependent depolymerization of nuclear actin, which decreases nuclear G-actin and increases MRTF-A in the nucleus. Furthermore, we show that MICAL-2 is a target of CCG-1423, a small molecule inhibitor of SRF/MRTF-A-dependent transcription that exhibits efficacy in various preclinical disease models. These data identify redox modification of nuclear actin as a regulatory switch that mediates SRF/MRTF-A-dependent gene transcription.