Two MCAT elements of the SM alpha-actin promoter function differentially in SM vs. non-SM cells

The YAP1/TAZ-TEAD transcriptional network regulates gene expression at neuromuscular junctions in skeletal muscle fibers

We examined YAP1/TAZ-TEAD signaling pathway activity at neuromuscular junctions (NMJs) of skeletal muscle fibers in adult mice. Our investigations revealed that muscle-specific knockouts of Yap1 or Taz, or both, demonstrate that these transcriptional coactivators regulate synaptic gene expression, the number and morphology of NMJs, and synaptic nuclei. Yap1 or Taz single knockout mice display reduced grip strength, fragmentation of NMJs, and accumulation of synaptic nuclei. Yap1/Taz muscle-specific double knockout mice do not survive beyond birth and possess almost no NMJs, the few detectable show severely impaired morphology and are organized in widened endplate bands; and with motor nerve endings being mostly absent. Myogenic gene expression is significantly impaired in the denervated muscles of knockout mice. We found that Tead1 and Tead4 transcription rates were increased upon incubation of control primary myotubes with AGRN-conditioned medium. Reduced AGRN-dependent acetylcholine receptor clustering and synaptic gene transcription were observed in differentiated primary Tead1 and Tead4 knockout myotubes. In silico analysis of previously reported genomic occupancy sites of TEAD1/4 revealed evolutionary conserved regions of potential TEAD binding motifs in key synaptic genes, the relevance of which was functionally confirmed by reporter assays. Collectively, our data suggest a role for YAP1/TAZ-TEAD1/TEAD4 signaling, particularly through TAZ-TEAD4, in regulating synaptic gene expression and acetylcholine receptor clustering at NMJs.

Prdm6 drives ductus arteriosus closure by promoting ductus arteriosus smooth muscle cell identity and contractility

Based upon our demonstration that the smooth muscle cell-selective (SMC-selective) putative methyltransferase, Prdm6, interacts with myocardin-related transcription factor-A, we examined Prdm6's role in SMCs in vivo using cell type-specific knockout mouse models. Although SMC-specific depletion of Prdm6 in adult mice was well tolerated, Prdm6 depletion in Wnt1-expressing cells during development resulted in perinatal lethality and a completely penetrant patent ductus arteriosus (DA) phenotype. Lineage tracing experiments in Wnt1Cre2 Prdm6fl/fl ROSA26LacZ mice revealed normal neural crest-derived SMC investment of the outflow tract. In contrast, myography measurements on DA segments isolated from E18.5 embryos indicated that Prdm6 depletion significantly reduced DA tone and contractility. RNA-Seq analyses on DA and ascending aorta samples at E18.5 identified a DA-enriched gene program that included many SMC-selective contractile associated proteins that was downregulated by Prdm6 depletion. Chromatin immunoprecipitation-sequencing experiments in outflow tract SMCs demonstrated that 50% of the genes Prdm6 depletion altered contained Prdm6 binding sites. Finally, using several genome-wide data sets, we identified an SMC-selective enhancer within the Prdm6 third intron that exhibited allele-specific activity, providing evidence that rs17149944 may be the causal SNP for a cardiovascular disease GWAS locus identified within the human PRDM6 gene.

Homeobox A4 suppresses vascular remodeling by repressing YAP/TEAD transcriptional activity

The Hippo signaling pathway is involved in the pathophysiology of various cardiovascular diseases. Yes-associated protein (YAP) and transcriptional enhancer activator domain (TEAD) transcriptional factors, the main transcriptional complex of the Hippo pathway, were recently identified as modulators of phenotypic switching of vascular smooth muscle cells (VSMCs). However, the intrinsic regulator of YAP/TEAD-mediated gene expressions involved in vascular pathophysiology remains to be elucidated. Here, we identified Homeobox A4 (HOXA4) as a potent repressor of YAP/TEAD transcriptional activity using lentiviral shRNA screen. Mechanistically, HOXA4 interacts with TEADs and attenuates YAP/TEAD-mediated transcription by competing with YAP for TEAD binding. We also clarified that the expression of HOXA4 is relatively abundant in the vasculature, especially in VSMCs. In vitro experiments in human VSMCs showed HOXA4 maintains the differentiation state of VSMCs via inhibition of YAP/TEAD-induced phenotypic switching. We generated Hoxa4-deficient mice and confirmed the downregulation of smooth muscle-specific contractile genes and the exacerbation of vascular remodeling after carotid artery ligation in vivo. Our results demonstrate that HOXA4 is a repressor of VSMC phenotypic switching by inhibiting YAP/TEAD-mediated transcription.

Transcriptional and posttranscriptional regulation of the SMC-selective blood pressure-associated gene, ARHGAP42

We previously showed that ARHGAP42 is a smooth muscle cell (SMC)-selective, RhoA-specific GTPase activating protein that regulates blood pressure and that a minor allele single nucleotide variation within a DNAse hypersensitive regulatory element in intron1 (Int1DHS) increased ARHGAP42 expression by promoting serum response factor binding. The goal of the current study was to identify additional transcriptional and posttranscriptional mechanisms that control ARHGAP42 expression. Using deletion/mutation, gel shift, and chromatin immunoprecipitation experiments, we showed that recombination signal binding protein for immunoglobulin κ-J region (RBPJ) and TEA domain family member 1 (TEAD1) binding to a conserved core region was required for full IntDHS transcriptional activity. Importantly, overexpression of the notch intracellular domain (NICD) or plating SMCs on recombinant jagged-1 increased IntDHS activity and endogenous ARHGAP42 expression while siRNA-mediated knockdown of TEAD1 inhibited ARHGAP42 mRNA levels. Re-chromatin immunoprecipitation experiments indicated that RBPJ and TEAD1 were bound to the Int1DHS enhancer at the same time, and coimmunoprecipitation assays indicated that these factors interacted physically. Our results also suggest TEAD1 and RBPJ bound cooperatively to the Int1DHS and that the presence of TEAD1 promoted the recruitment of NICD by RBPJ. Finally, we showed that ARHGAP42 expression was inhibited by micro-RNA 505 (miR505) which interacted with the ARHGAP42 3'-untranslated region (UTR) to facilitate its degradation and by AK124326, a long noncoding RNA that overlaps with the ARHGAP42 transcription start site on the opposite DNA strand. Since siRNA-mediated depletion of AK124326 was associated with increased H3K9 acetylation and RNA Pol-II binding at the ARHGAP42 gene, it is likely that AK124326 inhibits ARHGAP42 transcription.NEW & NOTEWORTHY First, RBPJ and TEAD1 converge at an intronic enhancer to regulate ARHGAP42 expression in SMCs. Second, TEAD1 and RBPJ interact physically and bind cooperatively to the ARHGAP42 enhancer. Third, miR505 interacts with the ARHGAP42 3'-UTR to facilitate its degradation. Finally, LncRNA, AK124326, inhibits ARHGAP42 transcription.

An evolutionary, structural and functional overview of the mammalian TEAD1 and TEAD2 transcription factors

TEAD proteins constitute a family of highly conserved transcription factors, characterized by a DNA-binding domain called the TEA domain and a protein-binding domain that permits association with transcriptional co-activators. TEAD proteins are unable to induce transcription on their own. They have to interact with transcriptional cofactors to do so. Once TEADs bind their co-activators, the different complexes formed are known to regulate the expression of genes that are crucial for embryonic development, important for organ formation (heart, muscles), and involved in cell death and proliferation. In the first part of this review we describe what is known of the structure of TEAD proteins. We then focus on two members of the family: TEAD1 and TEAD2. First the different transcriptional cofactors are described. These proteins can be classified in three categories: i), cofactors regulating chromatin conformation, ii), cofactors able to bind DNA, and iii), transcriptional cofactors without DNA binding domain. Finally we discuss the recent findings that identified TEAD1 and 2 and its coactivators involved in cancer progression.

Context-dependent switch in chemo/mechanotransduction via multilevel crosstalk among cytoskeleton-regulated MRTF and TAZ and TGFβ-regulated Smad3

Myocardin-related transcription factor (MRTF) and TAZ are major mechanosensitive transcriptional co-activators that link cytoskeleton organization to gene expression. Despite many similarities in their regulation, their physical and/or functional interactions are unknown. Here we show that MRTF and TAZ associate partly through a WW domain-dependent mechanism, and exhibit multilevel crosstalk affecting each other's expression, transport and transcriptional activity. Specifically, MRTF is essential for TAZ expression; TAZ and MRTF inhibit each other's cytosolic mobility and stimulus-induced nuclear accumulation; they antagonize each other's stimulatory effect on the α-smooth muscle actin (SMA) promoter, which harbours nearby cis-elements for both, but synergize on isolated TEAD-elements. Importantly, TAZ confers Smad3 sensitivity to the SMA promoter. Thus, TAZ is a context-dependent switch during mechanical versus mechano/chemical signalling, which inhibits stretch-induced but is indispensable for stretch+TGFβ-induced SMA expression. Crosstalk between these cytoskeleton-regulated factors seems critical for fine-tuning mechanical and mechanochemical transcriptional programmes underlying myofibroblast transition, wound healing and fibrogenesis.

The transcription factor TEAD1 represses smooth muscle-specific gene expression by abolishing myocardin function

The TEAD (transcriptional enhancer activator domain) proteins share an evolutionarily conserved DNA-binding TEA domain, which binds to the MCAT cis-acting regulatory element. Previous studies have shown that TEAD proteins are involved in regulating the expression of smooth muscle α-actin. However, it remains undetermined whether TEAD proteins play a broader role in regulating expression of other genes in vascular smooth muscle cells. In this study, we show that the expression of TEAD1 is significantly induced during smooth muscle cell phenotypic modulation and negatively correlates with smooth muscle-specific gene expression. We further demonstrate that TEAD1 plays a novel role in suppressing expression of smooth muscle-specific genes, including smooth muscle α-actin, by abolishing the promyogenic function of myocardin, a key mediator of smooth muscle differentiation. Mechanistically, we found that TEAD1 competes with myocardin for binding to serum response factor (SRF), resulting in disruption of myocardin and SRF interactions and thereby attenuating expression of smooth muscle-specific genes. This study provides the first evidence demonstrating that TEAD1 is a novel general repressor of smooth muscle-specific gene expression through interfering with myocardin binding to SRF.

Regulation of myofibroblast differentiation by poly(ADP-ribose) polymerase 1

Poly(ADP-ribosyl)ation (PARylation) is a post-translational protein modification effected by enzymes belonging to the poly(ADP-ribose) polymerase (PARP) superfamily, mainly by PARP-1. The key acceptors of poly(ADP-ribose) include PARP-1 itself, histones, DNA repair proteins, and transcription factors. Because many of these factors are involved in the regulation of myofibroblast differentiation, we examined the role of PARylation on myofibroblast differentiation. Overexpression of PARP-1 with an expression plasmid activated expression of the α-SMA gene (Acta2), a marker of myofibroblast differentiation in lung fibroblasts. Suppression of PARP-1 activity or gene expression with PARP-1 inhibitors or siRNA, respectively, had the opposite effect on these cells. PARP-1-deficient cells also had reduced α-SMA gene expression. DNA pyrosequencing identified hypermethylated regions of the α-SMA gene in PARP-1-deficient cells, relative to wild-type cells. Interestingly, and of potential relevance to human idiopathic pulmonary fibrosis, PARP activity in lung fibroblasts isolated from idiopathic pulmonary fibrosis patients was significantly higher than that in cells isolated from control subjects. Furthermore, PARP-1-deficient mice exhibited reduced pulmonary fibrosis in response to bleomycin-induced lung injury, relative to wild-type controls. These results suggest that PARylation is important for myofibroblast differentiation and the pathogenesis of pulmonary fibrosis.

TEAD1 controls C2C12 cell proliferation and differentiation and regulates three novel target genes

TEAD1 is a transcription factor involved in activation of muscle specific genes, such as the cardiac muscle troponin T gene, skeletal muscle actin, myosin heavy chains genes. Here, we reported that TEAD1 was expressed ubiquitously in different mouse tissues and was up-regulated in differentiation process of the mouse myoblast cell line C2C12. Functional assay revealed that overexpression of TEAD1 gene can arrest the C2C12 cell cycle and promote C2C12 cell differentiation. To understand the physiological role of TEAD1 in muscle development, three new regulated genes of TEAD1, Mrpl21, Ndufa6 and Ccne1, were identified by expression analysis, promoter activity measurement assay. The expression patterns of target genes were detected in the cell differentiation process. The Mrpl21 and Ndufa6 genes were up-regulated in cell differentiation while Ccne1 gene was significantly down-regulated. Overexpression of Mrpl21 and Ndufa6 in C2C12 can up-regulate Myh4 gene expression thus promote C2C12 differentiation, but did not affect cell cycle. Co-overexpression of Ccne1 with Ndufa6 resulted in Myh4 expression decrease and the number of S-phase cells slight increase. Together, our results suggested that TEAD1 may mediate muscle development through its target genes, Mrpl21, Ndufa6 and Ccne1.

The induction of yes-associated protein expression after arterial injury is crucial for smooth muscle phenotypic modulation and neointima formation

OBJECTIVE

Abnormal proliferation and migration of vascular smooth muscle cells (SMCs) are the key events in the progression of neointima formation in response to vascular injury. The goal of this study is to investigate the functional role of a potent oncogene yes-associated protein (YAP) in SM phenotypic modulation in vitro and in vivo.

METHODS AND RESULTS

In vitro cell culture and in vivo in both mouse and rat arterial injury models YAP expression is significantly induced and correlated with the vascular SMC synthetic phenotype. Overexpression of YAP promotes SMC migration and proliferation while attenuating SM contractile gene expression. Conversely, knocking down endogenous YAP in SMCs upregulates SM gene expression but attenuates SMC proliferation and migration. Consistent with this, knocking down YAP expression in a rat carotid balloon injury model and genetic deletion of YAP, specifically, in vascular SMCs in mouse after carotid artery ligation injury attenuates injury-induced SM phenotypic switch and neointima formation.

CONCLUSIONS

YAP plays a novel integrative role in SM phenotypic modulation by inhibiting SM-specific gene expression while promoting SM proliferation and migration in vitro and in vivo. Blocking the induction of YAP would be a potential therapeutic approach for ameliorating vascular occlusive diseases.

Yap1 protein regulates vascular smooth muscle cell phenotypic switch by interaction with myocardin

The Hippo-Yap (Yes-associated protein) signaling pathway has emerged as one of the critical pathways regulating cell proliferation, differentiation, and apoptosis in response to environmental and developmental cues. However, Yap1 roles in vascular smooth muscle cell (VSMC) biology have not been investigated. VSMCs undergo phenotypic switch, a process characterized by decreased gene expression of VSMC contractile markers and increased proliferation, migration, and matrix synthesis. The goals of the present studies were to investigate the relationship between Yap1 and VSMC phenotypic switch and to determine the molecular mechanisms by which Yap1 affects this essential process in VSMC biology. Results demonstrated that the expression of Yap1 was rapidly up-regulated by stimulation with PDGF-BB (a known inducer of phenotypic switch in VSMCs) and in the injured vessel wall. Knockdown of Yap1 impaired VSMC proliferation in vitro and enhanced the expression of VSMC contractile genes as well by increasing serum response factor binding to CArG-containing regions of VSMC-specific contractile genes within intact chromatin. Conversely, the interaction between serum response factor and its co-activator myocardin was reduced by overexpression of Yap1 in a dose-dependent manner. Taken together, these results indicate that down-regulation of Yap1 promotes VSMC contractile phenotype by both up-regulating myocardin expression and promoting the association of the serum response factor-myocardin complex with VSMC contractile gene promoters and suggest that the Yap1 signaling pathway is a central regulator of phenotypic switch of VSMCs.

Smooth muscle cell phenotypic switching in atherosclerosis

Smooth muscle cells (SMCs) possess remarkable phenotypic plasticity that allows rapid adaptation to fluctuating environmental cues, including during development and progression of vascular diseases such as atherosclerosis. Although much is known regarding factors and mechanisms that control SMC phenotypic plasticity in cultured cells, our knowledge of the mechanisms controlling SMC phenotypic switching in vivo is far from complete. Indeed, the lack of definitive SMC lineage-tracing studies in the context of atherosclerosis, and difficulties in identifying phenotypically modulated SMCs within lesions that have down-regulated typical SMC marker genes, and/or activated expression of markers of alternative cell types including macrophages, raise major questions regarding the contributions of SMCs at all stages of atherogenesis. The goal of this review is to rigorously evaluate the current state of our knowledge regarding possible phenotypes exhibited by SMCs within atherosclerotic lesions and the factors and mechanisms that may control these phenotypic transitions.

Novel cis-regulatory modules control expression of the Hairy and Enhancer of Split-1 (HES1) transcription factor in myoblasts

The expression profile of a gene is controlled by DNA sequences called cis-regulatory modules (CRMs). CRMs can function over large genomic distances and can be located many kilobases away from their target promoters. hes1 is a key developmental gene that is overexpressed in certain cancers and is a primary target of NOTCH signaling. Despite this, analysis of hes1 transcriptional control has been limited solely to its promoter. Here, we identify seven conserved DNA sequence blocks, representing the hes1 promoter and six novel CRMs, within 57 kb upstream of the mouse hes1 gene. We identify 12 binding sites for the RBP-Jκ NOTCH effector and a single M-CAT motif within these regions. We validate RBP-Jκ and TEAD family occupancy in cells in culture and test the response of each of these CRMs to active NOTCH. We show that two regions, CRM5 and CRM7, function as enhancers, and four can repress transcription. A pair of RBP-Jκ motifs arranged in a tail-tail configuration in CRM5 and the M-CAT motif in CRM7 are necessary for enhancer function. Furthermore, these enhancers are occupied by transcriptional co-activators and loop onto the hes1 promoter within the endogenous hes1 locus. This work demonstrates the power of combining computational genomics and experimental methodologies to identify novel CRMs and characterize their function.

MCAT elements and the TEF-1 family of transcription factors in muscle development and disease

MCAT elements are located in the promoter-enhancer regions of cardiac, smooth, and skeletal muscle-specific genes including cardiac troponin T, beta-myosin heavy chain, smooth muscle alpha-actin, and skeletal alpha-actin, and play a key role in the regulation of these genes during muscle development and disease. The binding factors of MCAT elements are members of the transcriptional enhancer factor-1 (TEF-1) family. However, it has not been fully understood how these transcription factors confer cell-specific expression in muscle, because their expression patterns are relatively broad. Results of recent studies revealed multiple mechanisms whereby TEF-1 family members control MCAT element-dependent muscle-specific gene expression, including posttranslational modifications of TEF-1 family members, the presence of muscle-selective TEF-1 cofactors, and cell-selective control of TEF-1 accessibility to MCAT elements. In addition, of particular interest, recent studies regarding MCAT element-dependent transcription of the myocardin gene and the smooth muscle alpha-actin gene in muscle provide evidence for the transcriptional diversity among distinct cell types and subtypes. This article summarizes the role of MCAT elements and the TEF-1 family of transcription factors in muscle development and disease, and reviews recent progress in our understanding of the transcriptional regulatory mechanisms involved in MCAT element-dependent muscle-specific gene expression.

Smooth muscle cells and myofibroblasts use distinct transcriptional mechanisms for smooth muscle alpha-actin expression

There has been considerable controversy regarding the lineage relationship between smooth muscle cells (SMCs) and myofibroblasts, because they express a number of common cell-selective markers including smooth muscle (SM) alpha-actin. We have shown previously that MCAT elements within the SM alpha-actin promoter confer differential activity in cultured SMCs versus myofibroblasts. In the present study, to determine the role of MCAT elements in vivo, we generated transgenic mice harboring an SM alpha-actin promoter-enhancer-LacZ reporter gene containing MCAT element mutations and compared transgene expression patterns with wild-type SM alpha-actin promoter-enhancer-LacZ transgenic mice. Results showed no differences in LacZ expression patterns in adult SMC-containing tissues. However, of interest, mutations of MCAT elements selectively abolished transgene expression in myofibroblasts within granulation tissue of skin wounds. In addition, mutations of MCAT elements caused a delay in the induction of transgene expression in SMCs, as well as loss of expression in cardiac and skeletal muscles during embryogenesis. Results of small interfering RNA-induced knockdown experiments showed that RTEF-1 regulated SM alpha-actin transcription in myofibroblasts, but not in differentiated SMCs. Moreover, quantitative chromatin immunoprecipitation assays revealed that RTEF-1 bound to the MCAT element-containing region within the SM alpha-actin promoter in myofibroblasts, whereas transcriptional enhancer factor (TEF)-1 was bound to the same region in differentiated SMCs. These results provide novel evidence that, although both SMCs and myofibroblasts express SM alpha-actin, they use distinct transcriptional control mechanisms for regulating its expression. Results also indicate that the MCAT element-mutated SM alpha-actin promoter-enhancer is a useful tool to direct gene expression selectively in differentiated SMCs.

Formation and function of the myofibroblast during tissue repair

It is generally accepted that fibroblast-to-myofibroblast differentiation represents a key event during wound healing and tissue repair. The high contractile force generated by myofibroblasts is beneficial for physiological tissue remodeling but detrimental for tissue function when it becomes excessive such as in hypertrophic scars, in virtually all fibrotic diseases and during stroma reaction to tumors. Specific molecular features as well as factors that control myofibroblast differentiation are potential targets to counteract its development, function, and survival. Such targets include alpha-smooth muscle actin and more recently discovered markers of the myofibroblast cytoskeleton, membrane surface proteins, and the extracellular matrix. Moreover, intervening with myofibroblast stress perception and transmission offers novel strategies to reduce tissue contracture; stress release leads to the instant loss of contraction and promotes apoptosis.

Transcription Enhancer Factor-1-dependent Expression of the α-Tropomyosin Gene in the Three Muscle Cell Types

In vertebrates, the actin-binding proteins tropomyosins are encoded by four distinct genes that are expressed in a complex pattern during development and muscle differentiation. In this study, we have characterized the transcriptional machinery of the alpha-tropomyosin (alpha-Tm) gene in muscle cells. Promoter analysis revealed that a 284-bp proximal promoter region of the Xenopus laevis alpha-Tm gene is sufficient for maximal activity in the three muscle cell types. The transcriptional activity of this promoter in the three muscle cell types depends on both distinct and common cis-regulatory sequences. We have identified a 30-bp conserved sequence unique to all vertebrate alpha-Tm genes that contains an MCAT site that is critical for expression of the gene in all muscle cell types. This site can bind transcription enhancer factor-1 (TEF-1) present in muscle cells both in vitro and in vivo. In serum-deprived differentiated smooth muscle cells, TEF-1 was redistributed to the nucleus, and this correlated with increased activity of the alpha-Tm promoter. Overexpression of TEF-1 mRNA in Xenopus embryonic cells led to activation of both the endogenous alpha-Tm gene and the exogenous 284-bp promoter. Finally, we show that, in transgenic embryos and juveniles, an intact MCAT sequence is required for correct temporal and spatial expression of the 284-bp gene promoter. This study represents the first analysis of the transcriptional regulation of the alpha-Tm gene in vivo and highlights a common TEF-1-dependent regulatory mechanism necessary for expression of the gene in the three muscle lineages.

Molecular Determinants of Vascular Smooth Muscle Cell Diversity

Although the primary role of vascular smooth muscle cells (SMCs) is contraction, they exhibit extensive phenotypic diversity and plasticity during normal development, during repair of vascular injury, and in disease states. Results of recent studies indicate that there are unique as well as common transcriptional regulatory mechanisms that control expression of various SMC marker genes in distinct SMC subtypes, and that these mechanisms are complex and dynamic even at the single cell level. This article will review recent progress in our understanding of the transcriptional regulatory mechanisms involved in controlling expression of SMC marker genes with a particular focus on examination of processes that contribute to the phenotypic diversity of SMCs. In addition, because of considerable controversy in the literature regarding the relationship between phenotypically modulated SMCs and myofibroblasts, we will briefly consider both similarities and differences in regulation of gene expression between these cell types.

Combinatorial control of smooth muscle-specific gene expression

Alterations in the differentiated state of vascular smooth muscle cells (SMCs) are known to play a key role in vascular diseases, yet the mechanisms controlling SMC differentiation are still poorly understand. In this review, we discuss our present knowledge of control of SMC differentiation at the transcriptional level, pointing out some common themes, important paradigms, and unresolved issues in SMC-specific gene regulation. We focus primarily on the serum response factor-CArG box-dependent pathway, because it has been shown to play a critical role in regulation of multiple SMC marker genes. However, we also highlight several other important regulatory elements, such as a transforming growth factor beta control element, E-boxes, and MCAT motifs. We present evidence in support of the notion that SMC-specific gene regulation is not controlled by a few SMC-specific transcription factors but rather by complex combinatorial interactions between multiple general and tissue-specific proteins. Finally, we discuss the implications of chromatin remodeling on SMC differentiation.

Myofibroblasts and mechano-regulation of connective tissue remodelling

During the past 20 years, it has become generally accepted that the modulation of fibroblastic cells towards the myofibroblastic phenotype, with acquisition of specialized contractile features, is essential for connective-tissue remodelling during normal and pathological wound healing. Yet the myofibroblast still remains one of the most enigmatic of cells, not least owing to its transient appearance in association with connective-tissue injury and to the difficulties in establishing its role in the production of tissue contracture. It is clear that our understanding of the myofibroblast its origins, functions and molecular regulation will have a profound influence on the future effectiveness not only of tissue engineering but also of regenerative medicine generally.

Cryptic MCAT enhancer regulation in fibroblasts and smooth muscle cells. Suppression of TEF-1 mediated activation by the single-stranded DNA-binding proteins, Pur alpha, Pur beta, and MSY1

An asymmetric polypurine-polypyrimidine cis-element located in the 5' region of the mouse vascular smooth muscle alpha-actin gene serves as a binding site for multiple proteins with specific affinity for either single- or double-stranded DNA. Here, we test the hypothesis that single-stranded DNA-binding proteins are responsible for preventing a cryptic MCAT enhancer centered within this element from cooperating with a nearby serum response factor-interacting CArG motif to trans-activate the minimal promoter in fibroblasts and smooth muscle cells. DNA binding studies revealed that the core MCAT sequence mediates binding of transcription enhancer factor-1 to the double-stranded polypurine-polypyrimidine element while flanking nucleotides account for interaction of Pur alpha and Pur beta with the purine-rich strand and MSY1 with the complementary pyrimidine-rich strand. Mutations that selectively impaired high affinity single-stranded DNA binding by fibroblast or smooth muscle cell-derived Pur alpha, Pur beta, and MSY1 in vitro, released the cryptic MCAT enhancer from repression in transfected cells. Additional experiments indicated that Pur alpha, Pur beta, and MSY1 also interact specifically, albeit weakly, with double-stranded DNA and with transcription enhancer factor-1. These results are consistent with two plausible models of cryptic MCAT enhancer regulation by Pur alpha, Pur beta, and MSY1 involving either competitive single-stranded DNA binding or masking of MCAT-bound transcription enhancer factor-1.

Cytoprotection against mechanical forces delivered through beta 1 integrins requires induction of filamin A

Cells in mechanically active environments can activate cytoprotective mechanisms to maintain membrane integrity in the face of potentially lethal applied forces. Cytoprotection may be mediated by expression of membrane-associated cytoskeletal proteins including filamin A, an actin-binding protein that increases the rigidity of the subcortical actin cytoskeleton. In this study, we tested the hypotheses that applied forces induce the expression of filamin A specifically and that this putative protective response inhibits cell death. Magnetically generated forces were applied to protein-coated magnetite beads bound to human gingival fibroblasts, cells with constitutively low basal levels of filamin A mRNA and protein. Forces applied through collagen or fibronectin, but not bovine serum albumin or poly-l-lysine-coated beads, increased mRNA and protein content of filamin A by 3-7-fold. Forces had no effect on the expression of other filamin isotypes or other cytoskeletal proteins. This effect was dependent on the duration of force and was blocked by anti-beta(1) integrin antibodies. Force also stimulated a 60% increase in expression of luciferase under the control of a filamin A promoter in transiently transfected Rat2 fibroblasts and was dependent on Sp1 transcription factor binding sites located immediately upstream of the transcription start site. Experiments with actinomycin D-treated cells showed that the increased filamin A expression after force application was due in part to prolongation of mRNA half-life. Antisense filamin oligonucleotides blocked force-induced filamin A expression and increased cell death by >2-fold above controls. The force-induced regulation of filamin A was dependent on intact actin filaments. We conclude that cells from mechanically active environments can couple diverse signals from forces applied through beta-integrins to up-regulate the production of cytoprotective cytoskeletal proteins, typified by filamin A.

Hepatocyte nuclear factor-3 homologue 1 (HFH-1) represses transcription of smooth muscle-specific genes

Results show that smooth muscle-specific promoters represent novel downstream targets of the winged helix factor hepatocyte nuclear factor-3 homologue 1 (HFH-1). HFH-1 strongly represses telokin promoter activity when overexpressed in A10 vascular smooth muscle cells. HFH-1 was also found to repress transcription of several other smooth muscle-specific promoters, including the SM22alpha promoter. HFH-1 inhibits telokin promoter activity, by binding to a forkhead consensus site located within an AT-rich region of the telokin promoter. The DNA-binding domain alone was sufficient to mediate inhibition, suggesting that binding of HFH-1 blocks the binding of other positive-acting factors. HFH-1 does not disrupt serum response factor binding to an adjacent CArG box within the telokin promoter, implying that HFH-1 must compete with other unidentified trans-activators to mediate repression. The localization of HFH-1 mRNA to the epithelial cell layer of mouse bladder and stomach implicates HFH-1 in repressing telokin expression in epithelial cells. This suggests that cell-specific expression of telokin is likely mediated by both positive-acting factors in smooth muscle cells and negative-acting factors in nonmuscle cell types. We propose a model in which the smooth muscle specificity of the telokin promoter is regulated by interactions between positive- and negative-acting members of the hepatocyte nuclear factor-3/forkhead family of transcription factors.

Altered sensitivity to single-strand-specific reagents associated with the genomic vascular smooth muscle alpha-actin promoter during myofibroblast differentiation

Stimulation of quiescent AKR-2B mouse fibroblasts with transforming growth factor beta1 results in uniform conversion to a myofibroblast-like phenotype as judged by a rapid accumulation of smooth muscle alpha-actin mRNA and protein. Because transcriptional regulation of the smooth muscle alpha-actin gene in these cells might be mediated by single-stranded DNA-binding proteins, we have examined the sensitivity of genomic DNA to chemical reagents with specificity for unpaired bases in a region of the promoter previously implicated in Puralpha, Purbeta, and MSY1 binding in vitro (Kelm, R. J., Jr., Cogan, J. G., Elder, P. K., Strauch, A. R., and Getz, M. J. (1999) J. Biol. Chem. 274, 14238-14245). Our data reveal specific differences between purified DNA treated in vitro and nucleoprotein complexes treated in living cells. Although some differences were observed in quiescent cells, treatment with transforming growth factor beta1 resulted in the development of additional sensitivity within 1 h. This enhancement was most pronounced in bases immediately upstream of an MCAT enhancer element-containing polypurine-polypyrimidine tract. A TATA-proximal element of similar base distribution showed no such hyperreactivities. These results suggest that activation of the endogenous smooth muscle alpha-actin gene during myofibroblast conversion is accompanied by specific structural changes in the promoter that are consistent with a decline in single-stranded DNA repressor protein binding.

c-Myb modulates transcription of the alpha-smooth muscle actin gene in activated hepatic stellate cells

Expression of alpha-smooth muscle actin (alpha-SMA) defines the phenotype of activated (myofibroblastic) hepatic stellate cells. These cells, but not quiescent stellate cells, have a high level of alpha-SMA and c-Myb expression, as well as increased c-Myb-binding activities to the proximal alpha-SMA E box. Therefore, we analyzed the role of c-Myb in alpha-SMA transcription and stellate cell activation. Activated primary rat stellate cells displayed a high expression of the -724 and -271 alpha-SMA/luciferase (LUC) chimeric genes, which contain c-Myb binding sites (-223/-216 bp). Alpha-SMA/LUC minigenes with mutation (-219/-217 bp), truncation (-224 bp), or deletion (-191 bp) of the c-Myb binding site were not efficiently transcribed. Transfection of wild-type c-Myb into quiescent stellate cells, which do not express endogenous c-Myb, induced a approximately 10-fold stimulation of -724 alpha-SMA/LUC expression. Conversely, expression of either a dominant-negative c-Myb basic domain mutant (Cys(43) --> Asp) or a c-Myb antisense RNA blocked transcription from the -724 alpha-SMA/LUC or -271 alpha-SMA/LUC in activated cells. Moreover, transfection of c-myb antisense, but not sense, RNA inhibited both expression of the endogenous alpha-SMA gene and stellate cell activation, whereas transfection of c-myb stimulated alpha-SMA expression in quiescent stellate cells. These findings suggest that c-Myb modulates the activation of stellate cells and that integrity of the redox sensor Cys(43) in c-Myb is required for this effect.

Similarities and differences in smooth muscle alpha-actin induction by TGF-beta in smooth muscle versus non-smooth muscle cells

Transforming growth factor-beta (TGF-beta) has been shown to stimulate smooth muscle (SM) alpha-actin expression in smooth muscle cells (SMCs) and non-SMCs. We previously demonstrated that the 2 CArG boxes A and B and a novel TGF-beta control element (TCE) located within the first 125 bp of the SM alpha-actin promoter were required for TGF-beta inducibility of SM alpha-actin in SMCs. The aims of the present study were (1) to determine whether the TCE exhibits SMC specificity or contributes to TGF-beta induction of SM alpha-actin expression in non-SMCs (ie, endothelial cells and fibroblasts) and (2) to determine whether TGF-beta can induce expression of multiple TCE-containing SMC differentiation marker genes, such as SM22alpha, h(1) calponin, and SM myosin heavy chain (SM MHC) in non-SMCs. Results of transient transfection assays demonstrated that mutation of CArG A, CArG B, or the TCE within a 125-bp promoter context completely abolished TGF-beta inducibility of SM alpha-actin in endothelial cells and fibroblasts. However, in contrast to observations in SMCs, inclusion of regions upstream from (-155) completely repressed TGF-beta responsiveness in non-SMCs. Electrophoretic mobility shift assays showed that TGF-beta enhanced binding of a serum response factor to the CArG elements and the binding of an as-yet-unidentified factor to the TCE in endothelial cells and fibroblasts, but to a much lesser extent compared with SMCs. TGF-beta also stimulated expression of the SMC differentiation marker SM22alpha in non-SMCs. However, in contrast to SMCs, TGF-beta did not induce expression of h(1) calponin and SM MHC in non-SMCs. In summary, these results suggest a conserved role for CArG A, CArG B, and the TCE in TGF-beta-induced expression of SM alpha-actin in SMCs and non-SMCs that is modified by a complex interplay of positive- and negative-acting cis elements in a cell-specific manner. Furthermore, observations that TGF-beta stimulated expression of several early but not late differentiation markers in non-SMCs indicate that TGF-beta alone is not sufficient to induce transdifferentiation of non-SMCs into SMCs.

Regulation of smooth muscle alpha-actin expression in vivo is dependent on CArG elements within the 5' and first intron promoter regions

The aims of the present studies were to define sufficient promoter sequences required to drive endogenous expression of smooth muscle (SM) alpha-actin and to determine whether regulation of SM alpha-actin expression in vivo is dependent on CArG (CC(A/T)6GG) cis elements. Promoter deletions and site directed mutagenesis techniques were used to study gene regulation in transgenic mice as well as in smooth muscle cell (SMC) cultures. Results demonstrated that a Lac Z transgene that contained 547 bp of the 5' rat SM alpha-actin promoter was sufficient to drive embryonic expression of SM alpha-actin in the heart and in skeletal muscle but not in SMCs. Transient transfections into SMC cultures demonstrated that the conserved CArG element in the first intron had significant positive activity, and gel shift analyses demonstrated that the intronic CArG bound serum response factor. A transgene construct from -2600 through the first intron (p2600Int/Lac Z) was expressed in embryos and adults in a pattern that closely mimicked endogenous SM alpha-actin expression. Expression in adult mice was completely restricted to SMCs and was detected in esophagus, stomach, intestine, lung, and nearly all blood vessels, including coronary, mesenteric, and renal vascular beds. Mutation of CArG B completely inhibited expression in all cell types, whereas mutation of the intronic CArG selectively abolished expression in SMCs, which suggests that it may act as an SMC-specific enhancer-like element. Taken together, these results provide the first in vivo evidence for the importance of multiple CArG cis elements in the regulation of SM alpha-actin expression.