Induction of vascular smooth muscle alpha-actin gene transcription in transforming growth factor beta1-activated myofibroblasts mediated by dynamic interplay between the Pur repressor proteins and Sp1/Smad coactivators

Pathology of idiopathic pulmonary fibrosis with particular focus on vascular endothelium and epithelial injury and their therapeutic potential

Idiopathic pulmonary fibrosis (IPF) remains a challenging disease with no drugs available to change the trajectory. It is a condition associated with excessive and highly progressive scarring of the lungs with remodelling and extracellular matrix deposition. It is a highly "destructive" disease of the lungs. The diagnosis of IPF is challenging due to continuous evolution of the disease, which also makes early interventions very difficult. The role of vascular endothelial cells has not been explored in IPF in great detail. We do not know much about their contribution to arterial or vascular remodelling, extracellular matrix changes and contribution to pulmonary hypertension and lung fibrosis in general. Endothelial to mesenchymal transition appears to be central to such changes in IPF. Similarly, for epithelial changes, the process of epithelial to mesenchymal transition seem to be the key both for airway epithelial cells and type-2 pneumocytes. We focus here on endothelial and epithelial cell changes and its contributions to IPF. In this review we revisit the pathology of IPF, mechanistic signalling pathways, clinical definition, update on diagnosis and new advances made in treatment of this disease. We discuss ongoing clinical trials with mode of action. A multidisciplinary collaborative approach is needed to understand this treacherous disease for new therapeutic targets.

Epigenetic Alteration of Smooth Muscle Cells Regulates Endothelin-Dependent Blood Pressure and Hypertensive Arterial Remodeling

Long-standing hypertension (HTN) affects multiple organ systems and leads to pathologic arterial remodeling, which is driven largely by smooth muscle cell (SMC) plasticity. Although genome wide association studies (GWAS) have identified numerous variants associated with changes in blood pressure in humans, only a small percentage of these variants actually cause HTN. In order to identify relevant genes important in SMC function in HTN, we screened three separate human GWAS and Mendelian randomization studies to identify SNPs located within non-coding gene regions, focusing on genes encoding epigenetic enzymes, as these have been recently identified to control SMC fate in cardiovascular disease. We identified SNPs rs62059712 and rs74480102 in the promoter of the human JMJD3 gene and show that the minor C allele increases JMJD3 transcription in SMCs via increased SP1 binding to the JMJD3 promoter. Using our novel SMC-specific Jmjd3-deficient murine model ( Jmjd3 flox/flox Myh11 CreERT ), we show that loss of Jmjd3 in SMCs results in HTN, mechanistically, due to decreased EDNRB expression and a compensatory increase in EDNRA expression. As a translational corollary, through single cell RNA-sequencing (scRNA-seq) of human arteries, we found strong correlation between JMJD3 and EDNRB expression in SMCs. Further, we identified that JMJD3 is required for SMC-specific gene expression, and loss of JMJD3 in SMCs in the setting of HTN results in increased arterial remodeling by promoting the SMC synthetic phenotype. Our findings link a HTN-associated human DNA variant with regulation of SMC plasticity, revealing therapeutic targets that may be used in the screening and/or personalized treatment of HTN.

DUSP8 induces TGF-β-stimulated IL-9 transcription and Th9-mediated allergic inflammation by promoting nuclear export of Pur-α

Dual-specificity phosphatase 8 (DUSP8) is a MAPK phosphatase that dephosphorylates and inactivates the kinase JNK. DUSP8 is highly expressed in T cells; however, the in vivo role of DUSP8 in T cells remains unclear. Using T cell-specific Dusp8 conditional KO (T-Dusp8 cKO) mice, mass spectrometry analysis, ChIP-Seq, and immune analysis, we found that DUSP8 interacted with Pur-α, stimulated interleukin-9 (IL-9) gene expression, and promoted Th9 differentiation. Mechanistically, DUSP8 dephosphorylated the transcriptional repressor Pur-α upon TGF-β signaling, leading to the nuclear export of Pur-α and subsequent IL-9 transcriptional activation. Furthermore, Il-9 mRNA levels were induced in Pur-α-deficient T cells. In addition, T-Dusp8-cKO mice displayed reduction of IL-9 and Th9-mediated immune responses in the allergic asthma model. Reduction of Il-9 mRNA levels in T cells and allergic responses of T-Dusp8-cKO mice was reversed by Pur-α knockout. Remarkably, DUSP8 protein levels and the DUSP8-Pur-α interaction were indeed increased in the cytoplasm of T cells from people with asthma and patients with atopic dermatitis. Collectively, DUSP8 induces TGF-β-stimulated IL-9 transcription and Th9-induced allergic responses by inhibiting the nuclear translocation of the transcriptional repressor Pur-α. DUSP8 may be a T-cell biomarker and therapeutic target for asthma and atopic dermatitis.

LncCMRR Plays an Important Role in Cardiac Differentiation by Regulating the Purb/Flk1 Axis

As crucial epigenetic regulators, long noncoding RNAs (lncRNAs) play critical functions in development processes and various diseases. However, the regulatory mechanism of lncRNAs in early heart development is still limited. In this study, we identified cardiac mesoderm-related lncRNA (LncCMRR). Knockout (KO) of LncCMRR decreased the formation potential of cardiac mesoderm and cardiomyocytes during embryoid body differentiation of mouse embryonic stem (ES) cells. Mechanistic analyses showed that LncCMRR functionally interacted with the transcription suppressor PURB and inhibited its binding potential at the promoter region of Flk1, which safeguarded the transcription of Flk1 during cardiac mesoderm formation. We also carried out gene ontology term and signaling pathway enrichment analyses for the differentially expressed genes after KO of LncCMRR, and found significant correlation of LncCMRR with cardiac muscle contraction, dilated cardiomyopathy, and hypertrophic cardiomyopathy. Consistently, the expression level of Flk1 at E7.75 and the thickness of myocardium at E17.5 were significantly decreased after KO of LncCMRR, and the survival rate and heart function index of LncCMRR-KO mice were also significantly decreased as compared with the wild-type group. These findings indicated that the defects in early heart development led to functional abnormalities in adulthood heart of LncCMRR-KO mice. Conclusively, our findings elucidate the main function and regulatory mechanism of LncCMRR in cardiac mesoderm formation, and provide new insights into lncRNA-mediated regulatory network of mouse ES cell differentiation.

Six Shades of Vascular Smooth Muscle Cells Illuminated by KLF4 (Krüppel-Like Factor 4)

Multiple layers of vascular smooth muscle cells (vSMCs) are present in blood vessels forming the media of the vessel wall. vSMCs provide a vessel wall structure, enabling it to contract and relax, thus modulating blood flow. They also play a crucial role in the development of vascular diseases, such as atherosclerosis and aortic aneurysm formation. vSMCs display a remarkable high degree of plasticity. At present, the number of different vSMC phenotypes has only partially been characterized. By mapping vSMC phenotypes in detail and identifying triggers for phenotype switching, the relevance of the different phenotypes in vascular disease may be identified. Up until recently, vSMCs were classified as either contractile or dedifferentiated (ie, synthetic). However, single-cell RNA sequencing studies revealed such dedifferentiated arterial vSMCs to be highly diverse. Currently, no consensus exist about the number of vSMC phenotypes. Therefore, we reviewed the data from relevant single-cell RNA sequencing studies, and classified a total of 6 vSMC phenotypes. The central dedifferentiated vSMC type that we classified is the mesenchymal-like phenotype. Mesenchymal-like vSMCs subsequently seem to differentiate into fibroblast-like, macrophage-like, osteogenic-like, and adipocyte-like vSMCs, which contribute differentially to vascular disease. This phenotype switching between vSMCs requires the transcription factor KLF4 (Kruppel-like factor 4). Here, we performed an integrated analysis of the data about the recently identified vSMC phenotypes, their associated gene expression profiles, and previous vSMC knowledge to better understand the role of vSMC phenotype transitions in vascular pathology.

Purine-rich element binding protein B attenuates the coactivator function of myocardin by a novel molecular mechanism of smooth muscle gene repression

Myocardin is a potent transcriptional coactivator protein, which functions as the master regulator of vascular smooth muscle cell differentiation. The cofactor activity of myocardin is mediated by its physical interaction with serum response factor, a ubiquitously expressed transactivator that binds to CArG boxes in genes encoding smooth muscle-restricted proteins. Purine-rich element binding protein B (Purβ) represses the transcription of the smooth muscle α-actin gene (Acta2) in fibroblasts and smooth muscle cells by interacting with single-stranded DNA sequences flanking two 5' CArG boxes in the Acta2 promoter. In this study, the ability of Purβ to modulate the cofactor activity of myocardin was investigated using a combination of cellular and biochemical approaches. Results of smooth muscle gene promoter-reporter assays indicated that Purβ specifically inhibits the coactivator function of myocardin in a manner requiring the presence of all three single-stranded DNA binding domains in the Purβ homodimer. DNA binding analyses demonstrated that Purβ interacts with CArG-containing DNA elements with a much lower affinity compared to other purine-rich target sequences present in the Acta2 promoter. Co-immunoprecipitation and DNA pull-down assays revealed that Purβ associates with myocardin and serum response factor when free or bound to duplex DNA containing one or more CArG boxes. Functional analysis of engineered Purβ point mutants identified several amino acid residues essential for suppression of myocardin activity. Collectively, these findings suggest an inhibitory mechanism involving direct protein-protein interaction between the homodimeric Purβ repressor and the myocardin-serum response factor-CArG complex.

The Molecular Function of PURA and Its Implications in Neurological Diseases

In recent years, genome-wide analyses of patients have resulted in the identification of a number of neurodevelopmental disorders. Several of them are caused by mutations in genes that encode for RNA-binding proteins. One of these genes is PURA, for which in 2014 mutations have been shown to cause the neurodevelopmental disorder PURA syndrome. Besides intellectual disability (ID), patients develop a variety of symptoms, including hypotonia, metabolic abnormalities as well as epileptic seizures. This review aims to provide a comprehensive assessment of research of the last 30 years on PURA and its recently discovered involvement in neuropathological abnormalities. Being a DNA- and RNA-binding protein, PURA has been implicated in transcriptional control as well as in cytoplasmic RNA localization. Molecular interactions are described and rated according to their validation state as physiological targets. This information will be put into perspective with available structural and biophysical insights on PURA’s molecular functions. Two different knock-out mouse models have been reported with partially contradicting observations. They are compared and put into context with cell biological observations and patient-derived information. In addition to PURA syndrome, the PURA protein has been found in pathological, RNA-containing foci of patients with the RNA-repeat expansion diseases such as fragile X-associated tremor ataxia syndrome (FXTAS) and amyotrophic lateral sclerosis (ALS)/fronto-temporal dementia (FTD) spectrum disorder. We discuss the potential role of PURA in these neurodegenerative disorders and existing evidence that PURA might act as a neuroprotective factor. In summary, this review aims at informing researchers as well as clinicians on our current knowledge of PURA’s molecular and cellular functions as well as its implications in very different neuronal disorders.

Resveratrol and cardiac fibrosis prevention and treatment

Cardiovascular diseases are some of the major causes of morbidity and mortality in developed or developing countries but in developed countries as well. Cardiac fibrosis is one of the most often pathological changes of heart tissues. It occurs as a result of extracellular matrix proteins accumulation at myocardia. Cardiac fibrosis results in impaired cardiac systolic and diastolic functions and is associated with other effects. Therapies with medicines have not been sufficiently successful in treating chronic diseases such as CVD. Therefore, the interest for therapeutic potential of natural compounds and medicinal plants has increased. Plants such as grapes, berries and peanuts contain a polyphenolic compound called "resveratrol" which has been reported to have various therapeutic properties for a variety of diseases. Studies on laboratory models that show that resveratrol has beneficial effects on cardiovascular diseases including myocardial infarction, high blood pressure cardiomyopathy, thrombosis, cardiac fibrosis, and atherosclerosis. In vitro animal models using resveratrol indicated protective effects on the heart by neutralizing reactive oxygen species, preventing inflammation, increasing neoangiogenesis, dilating blood vessels, suppressing apoptosis and delaying atherosclerosis. In this review, we are presenting experimental and clinical results of studies concerning resveratrol effects on cardiac fibrosis as a CVD outcome in humans.

Methylation in pericytes after acute injury promotes chronic kidney disease

The origin and fate of renal myofibroblasts is not clear after acute kidney injury (AKI). Here, we demonstrate that myofibroblasts were activated from quiescent pericytes (qPericytes) and the cell numbers increased after ischemia/reperfusion injury-induced AKI (IRI-AKI). Myofibroblasts underwent apoptosis during renal recovery but one-fifth of them survived in the recovered kidneys on day 28 after IRI-AKI and their cell numbers increased again after day 56. Microarray data showed the distinctive gene expression patterns of qPericytes, activated pericytes (aPericytes, myofibroblasts), and inactivated pericytes (iPericytes) isolated from kidneys before, on day 7, and on day 28 after IRI-AKI. Hypermethylation of the Acta2 repressor Ybx2 during IRI-AKI resulted in epigenetic modification of iPericytes to promote the transition to chronic kidney disease (CKD) and aggravated fibrogenesis induced by a second AKI induced by adenine. Mechanistically, transforming growth factor-β1 decreased the binding of YBX2 to the promoter of Acta2 and induced Ybx2 hypermethylation, thereby increasing α-smooth muscle actin expression in aPericytes. Demethylation by 5-azacytidine recovered the microvascular stabilizing function of aPericytes, reversed the profibrotic property of iPericytes, prevented AKI-CKD transition, and attenuated fibrogenesis induced by a second adenine-AKI. In conclusion, intervention to erase hypermethylation of pericytes after AKI provides a strategy to stop the transition to CKD.

Myofibroblasts and Fibrosis: Mitochondrial and Metabolic Control of Cellular Differentiation

Cardiac fibrosis is mediated by the activation of resident cardiac fibroblasts, which differentiate into myofibroblasts in response to injury or stress. Although myofibroblast formation is a physiological response to acute injury, such as myocardial infarction, myofibroblast persistence, as occurs in heart failure, contributes to maladaptive remodeling and progressive functional decline. Although traditional pathways of activation, such as TGFβ (transforming growth factor β) and AngII (angiotensin II), have been well characterized, less understood are the alterations in mitochondrial function and cellular metabolism that are necessary to initiate and sustain myofibroblast formation and function. In this review, we highlight recent reports detailing the mitochondrial and metabolic mechanisms that contribute to myofibroblast differentiation, persistence, and function with the hope of identifying novel therapeutic targets to treat, and potentially reverse, tissue organ fibrosis.

Structural and functional analysis of single-nucleotide polymorphic variants of purine-rich element-binding protein B

Purine-rich element-binding protein B (Purβ) inhibits myofibroblast differentiation by repressing the expression of the smooth muscle α-actin gene (Acta2). Several reports have identified the structural domains in Purβ that enable its characteristic interaction with purine-rich single-stranded DNA (ssDNA) sequences in the Acta2 promoter. However, little is known about the physical and functional effects of single-nucleotide polymorphisms that alter individual amino acid residues in Purβ. This study evaluated seven rare single amino acid variants of human PURB engineered into the homologous mouse Purβ protein. Mapping the location of variant residues on a homology model of the Purβ homodimer suggested that most of the altered residues are remote from the predicted ssDNA-binding regions of the protein. The repressor activity of each Purβ variant was assessed in transfected fibroblasts and smooth muscle cells via Acta2 promoter-reporter assays. A Q64* nonsense variant was completely inactive while missense variants exhibited repressor activity that ranged from ~1.5-fold greater to ~2-fold less than wild-type Purβ. Lower activity variants P223L and R297Q were expressed in bacteria and purified to homogeneity. Each variant was physically indistinguishable from wild-type Purβ in terms of quaternary structure and thermostability. Results of DNA and protein-binding assays indicated that the P223L and R297Q variants retained high affinity and specificity for purine-rich ssDNA sequences but differed in their interaction with other Acta2 regulatory proteins. These findings suggest that the presence of certain variant residues affects the Acta2 repressor activity of Purβ by altering its interaction with other transcription factors but not with ssDNA.

Ascorbic acid promotes a TGFβ1-induced myofibroblast phenotype switch

l‐Ascorbic acid (AA), generally known as vitamin C, is a crucial cofactor for a variety of enzymes, including prolyl‐3‐hydroxylase (P3H), prolyl‐4‐hydroxylase (P4H), and lysyl hydroxylase (LH)‐mediated collagen maturation. Here, we investigated whether AA has additional functions in the regulation of the myofibroblast phenotype, besides its function in collagen biosynthesis. We found that AA positively influences TGFβ1‐induced expression of COL1A1,ACTA2, and COL4A1. Moreover, we demonstrated that AA promotes αSMA stress fiber formation as well as the synthesis and deposition of collagens type I and IV. Additionally, AA amplified the contractile phenotype of the myofibroblasts, as seen by increased contraction of a 3D collagen lattice. Moreover, AA increased the expression of several TGFβ1‐induced genes, including DDR1 and CCN2. Finally, we demonstrated that the mechanism of AA action seems independent of Smad2/3 signaling.

Extracellular matrix remodeling and cardiac fibrosis

Cardiac fibrosis, characterized by excessive deposition of extracellular matrix (ECM) proteins in the myocardium, distorts the architecture of the myocardium, facilitates the progression of arrhythmia and cardiac dysfunction, and influences the clinical course and outcome in patients with heart failure. This review describes the composition and homeostasis in normal cardiac interstitial matrix and introduces cellular and molecular mechanisms involved in cardiac fibrosis. We also characterize the ECM alteration in the fibrotic response under diverse cardiac pathological conditions and depict the role of matricellular proteins in the pathogenesis of cardiac fibrosis. Moreover, the diagnosis of cardiac fibrosis based on imaging and biomarker detection and the therapeutic strategies are addressed. Understanding the comprehensive molecules and pathways involved in ECM homeostasis and remodeling may provide important novel potential targets for preventing and treating cardiac fibrosis.

Electrostatic and Hydrophobic Interactions Mediate Single-Stranded DNA Recognition and Acta2 Repression by Purine-Rich Element-Binding Protein B

Myofibroblast differentiation is characterized by an increased level of expression of cytoskeletal smooth muscle α-actin. In human and murine fibroblasts, the gene encoding smooth muscle α-actin (Acta2) is tightly regulated by a network of transcription factors that either activate or repress the 5' promoter-enhancer in response to environmental cues signaling tissue repair and remodeling. Purine-rich element-binding protein B (Purβ) suppresses the expression of Acta2 by cooperatively interacting with the sense strand of a 5' polypurine sequence containing an inverted MCAT cis element required for gene activation. In this study, we evaluated the chemical basis of nucleoprotein complex formation between the Purβ repressor and the purine-rich strand of the MCAT element in the mouse Acta2 promoter. Quantitative single-stranded DNA (ssDNA) binding assays conducted in the presence of increasing concentrations of monovalent salt or anionic detergent suggested that the assembly of a high-affinity nucleoprotein complex is driven by a combination of electrostatic and hydrophobic interactions. Consistent with the results of pH titration analysis, site-directed mutagenesis revealed several basic amino acid residues in the intermolecular (R267) and intramolecular (K82 and R159) subdomains that are essential for Purβ transcriptional repressor function in Acta2 promoter-reporter assays. In keeping with their diminished Acta2 repressor activity in fibroblasts, purified Purβ variants containing an R267A mutation exhibited reduced binding affinity for purine-rich ssDNA. Moreover, certain double and triple-point mutants were also defective in binding to the Acta2 corepressor protein, Y-box-binding protein 1. Collectively, these findings establish the repertoire of noncovalent interactions that account for the unique structural and functional properties of Purβ.

TGF-beta-induced early gene-1 overexpression promotes oxidative stress protection and actin cytoskeleton rearrangement in human skin fibroblasts

BACKGROUND

Transforming growth factor beta inducible early gene-1 (TIEG-1), a member of the Krüppel-like factor, was identified as a primary response gene for TGF-β. The role of TIEG-1 in skin repair has been mainly addressed in vivo on TIEG-1 null mice model and the mechanism remains unexplored.

METHODS

We investigated the modulation of TIEG-1 expression in normal human skin fibroblasts by either down-expressing or overexpressing the gene. We evaluated reactive oxygen species production and the cell viability of treated cells. The effect of TIEG-1 overexpression was monitored by wound healing assay and immunofluorescence staining of actin fibers organization and alpha-smooth muscle actin (α-SMA). Western blots were carried out to identify the level of expression or phosphorylation of key proteins such as cofilin, Rho GTPases, and p38 mitogen-activated protein kinase (p38 MAPK).

RESULTS

TIEG-1 down-regulation had a deleterious effect on the cell viability. It was significantly reduced (65±5%) and exposure to ultraviolet further increased this effect (47±3%). By contrast, cells overexpressing TIEG-1 had a reduced reactive oxygen species production (75%) compared to control and mock-transfected cells. This overexpression also resulted in formation of actin stress fibers and increased α-SMA expression and an enhanced wound healing feature. RhoB GTPase was upregulated and phosphorylation of cofilin and p38 MAPK was observed.

CONCLUSION

TIEG-1 overexpression in normal human skin fibroblasts results in improved resistance to oxidative stress, myofibroblast-like conversion that involved RhoB signaling pathway with cofilin and p38 MAPK proteins activation.

GENERAL SIGNIFICANCE

This study enlightens the role of TIEG-1 role in skin biology.

Effects of Tongmai oral liquid in femoral ateriovenous fistula

BACKGROUND

This study was conducted to investigate the protective effect of Tongmai oral liquid on arteriovenous fistula function and to provide an effective method to promote fistula maturation.

METHODS

Fifteen female and fifteen male SPF New Zealand rabbits were randomly allocated into 3 groups including control, Aspirin and Tongmai oral liquid groups. A side-to-side femoral arteriovenous fistula was established in each rabbit and then animals were treated with Aspirin or Tongmai oral liquid for 2 weeks. The concentrations of circulating ET-1 and NO were determined before and after operation (on preoperative day, operative day, post-D1, post-D3, post-D7 and post-D15), respectively. Blood flow of the fistula stoma and contralateral artery and vein was determined on the 15th postoperative day. Last, the fistula stoma was dissected to observe patency, thrombosis and adhesion with surrounding tissues.

RESULTS

28 rabbits survived during the surgical process and the following 15-day observational period. Tissue adhesion of arteriovenous fistula with surrounding tissues was improved and fistula thrombosis was reduced by treatment with Tongmai oral liquid. NO concentration decreased to a different extent after vascular surgery. Tongmai oral liquid failed to regulate the equilibrium between NO and ET-1, but it improved blood flow of fistula stoma, as compared to control and Aspirin groups. Blood flow of fistula stoma in the three groups was lower than that of the contralateral femoral artery.

CONCLUSIONS

Tongmai oral liquid improved the function of femoral ateriovenous fistula in the rabbit model by increasing blood flow and reducing thrombosis, probably not by regulating the dynamic equilibrium between NO and ET-1.

Sp1 Mediates a Therapeutic Role of MiR-7a/b in Angiotensin II-Induced Cardiac Fibrosis via Mechanism Involving the TGF-β and MAPKs Pathways in Cardiac Fibroblasts

MicroRNA-7a/b (miR-7a/b) protects cardiac myocytes from apoptosis during ischemia/reperfusion injury; however, its role in angiotensin II (ANG II)-stimulated cardiac fibroblasts (CFs) remains unknown. Therefore, the present study investigated the anti-fibrotic mechanism of miR-7a/b in ANG II-treated CFs. ANG II stimulated the expression of specific protein 1 (Sp1) and collagen I in a dose- and time-dependent manner, and the overexpression of miR-7a/b significantly down-regulated the expression of Sp1 and collagen I stimulated by ANG II (100 nM) for 24 h. miR-7a/b overexpression effectively inhibited MMP-2 expression/activity and MMP-9 expression, as well as CF proliferation and migration. In addition, miR-7a/b also repressed the activation of TGF-β, ERK, JNK and p38 by ANG II. The inhibition of Sp1 binding activity by mithramycin prevented collagen I overproduction; however, miR-7a/b down-regulation reversed this effect. Further studies revealed that Sp1 also mediated miR-7a/b-regulated MMP expression and CF migration, as well as TGF-β and ERK activation. In conclusion, miR-7a/b has an anti-fibrotic role in ANG II-treated CFs that is mediated by Sp1 mechanism involving the TGF-β and MAPKs pathways.

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

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

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

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

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

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

Structural basis of multisite single-stranded DNA recognition and ACTA2 repression by purine-rich element binding protein B (Purβ)

A hallmark of dysfunctional fibroblast to myofibroblast differentiation associated with fibrotic disorders is persistent expression of ACTA2, the gene encoding the cyto-contractile protein smooth muscle α-actin. In this study, a PURB-specific gene knockdown approach was used in conjunction with biochemical analyses of protein subdomain structure and function to reveal the mechanism by which purine-rich element binding protein B (Purβ) restricts ACTA2 expression in mouse embryo fibroblasts (MEFs). Consistent with the hypothesized role of Purβ as a suppressor of myofibroblast differentiation, stable short hairpin RNA-mediated knockdown of Purβ in cultured MEFs promoted changes in cell morphology, actin isoform expression, and cell migration indicative of conversion to a myofibroblast-like phenotype. Promoter-reporter assays in transfected Purβ knockdown MEFs confirmed that these changes were attributable, in part, to derepression of ACTA2 transcription. To map the domains in Purβ responsible for ACTA2 repression, several recombinant truncation mutants were generated and analyzed based on hypothetical, computationally derived models of the tertiary and quaternary structure of Purβ. Discrete subdomains mediating sequence- and strand-specific cis-element binding, protein-protein interaction, and inhibition of a composite ACTA2 enhancer were identified using a combination of biochemical, biophysical, and cell-based assays. Our results indicate that the Purβ homodimer possesses three separate but unequal single-stranded DNA-binding modules formed by subdomain-specific inter- and intramolecular interactions. This structural arrangement suggests that the cooperative assembly of the dimeric Purβ repressor on the sense strand of the ACTA2 enhancer is dictated by the association of each subdomain with distinct purine-rich binding sites within the enhancer.

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

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

Y-box binding protein-1 implicated in translational control of fetal myocardial gene expression after cardiac transplant

Peri-transplant surgical trauma and ischemia/reperfusion injury in accepted murine heterotopic heart grafts has been associated with myofibroblast differentiation, cardiac fibrosis and biomechanical-stress activation of the fetal myocardial smooth muscle α-actin (SMαA) gene. The wound-healing agonists, transforming growth factor β1 and thrombin, are known to coordinate SMαA mRNA transcription and translation in activated myofibroblasts by altering the subcellular localization and mRNA-binding affinity of the Y-box binding protein-1 (YB-1) cold-shock domain (CSD) protein that governs a variety of cellular responses to metabolic stress. YB-1 accumulated in polyribosome-enriched regions of the sarcoplasm proximal to cardiac intercalated discs in accepted heart grafts. YB-1 binding to a purine-rich motif in exon 3 of SMαA mRNA that regulates translational efficiency increased substantially in perfusion-isolated, rod-shaped adult rat cardiomyocytes during phenotypic de-differentiation in the presence of serum-derived growth factors. Cardiomyocyte de-differentiation was accompanied by the loss of a 60 kDa YB-1 variant that was highly expressed in both adult myocardium and freshly isolated myocytes and replacement with the 50 kDa form of YB-1 (p50) typically expressed in myofibroblasts that demonstrated sequence-specific interaction with SMαA mRNA. Accumulation of p50 YB-1 in reprogrammed, de-differentiated myocytes was associated with a 10-fold increase in SMαA protein expression. Endomyocardial biopsies collected from patients up to 14 years after heart transplant showed variable yet coordinately elevated expression of SMαA and p50 YB-1 protein and demonstrable p50 YB-1:SMαA mRNA interaction. The p60 YB-1 variant in human heart graft samples, but neither mouse p60 nor mouse or human p50, reacted with an antibody specific for the phosphoserine 102 modification in the YB-1 CSD. Modulation of YB-1 subcellular compartmentalization and mRNA-binding activity may be linked with reprogramming of contractile protein gene expression in ventricular cardiomyocytes that could contribute to maladaptive remodeling in accepted, long-term heart grafts.

Loss-of-function thrombospondin-1 mutations in familial pulmonary hypertension

Most patients with familial pulmonary arterial hypertension (FPAH) carry mutations in the bone morphogenic protein receptor 2 gene (BMPR2). Yet carriers have only a 20% risk of disease, suggesting that other factors influence penetrance. Thrombospondin-1 (TSP1) regulates activation of TGF-β and inhibits endothelial and smooth muscle cell proliferation, pathways coincidentally altered in pulmonary arterial hypertension (PAH). To determine whether a subset of FPAH patients also have mutations in the TSP1 gene (THBS1) we resequenced the type I repeats of THBS1 encoding the TGF-β regulation and cell growth inhibition domains in 60 FPAH probands, 70 nonfamilial PAH subjects, and in large control groups. We identified THBS1 mutations in three families: a novel missense mutation in two (Asp362Asn), and an intronic mutation in a third (IVS8+255 G/A). Neither mutation was detected in population controls. Mutant 362Asn TSP1 had less than half of the ability of wild-type TSP1 to activate TGF-β. Mutant 362Asn TSP1 also lost the ability to inhibit growth of pulmonary arterial smooth muscle cells and was over threefold less effective at inhibiting endothelial cell growth. The IVS8+255 G/A mutation decreased and/or eliminated local binding of the transcription factors SP1 and MAZ but did not affect RNA splicing. These novel mutations implicate THBS1 as a modifier gene in FPAH. These THBS1 mutations have implications in the genetic evaluation of FPAH patients. However, since FPAH is rare, these data are most relevant as evidence for the importance of TSP1 in pulmonary vascular homeostasis. Further examination of THBS1 in the pathogenesis of PAH is warranted.

Honokiol ameliorates renal fibrosis by inhibiting extracellular matrix and pro-inflammatory factors in vivo and in vitro

BACKGROUND AND PURPOSE

Renal fibrosis acts as the common pathway leading to the development of end-stage renal disease. The present study investigated, in vivo and in vitro, the anti-fibrotic and anti-inflammatory effects, particularly on the epithelial to mesenchymal transition of renal tubular cells, exerted by honokiol, a phytochemical used in traditional medicine, and mechanisms underlying these effects.

EXPERIMENTAL APPROACH

Anti-fibrotic effects in vivo were assayed in a rat model of renal fibrosis [the unilateral ureteral obstruction (UUO) model]. A rat tubular epithelial cell line (NRK-52E) was stimulated by transforming growth factor-β1 (TGF-β1) and treated with honokiol to explore possible mechanisms of these anti-fibrotic effects. Gene or protein expression was analysed by Northern or Western blotting. Transcriptional regulation was investigated using luciferase activity driven by a connective tissue growth factor (CTGF) promoter.

KEY RESULTS

Honokiol slowed development of renal fibrosis both in vivo and in vitro. Honokiol treatment attenuated tubulointerstitial fibrosis and expression of pro-fibrotic factors in the UUO model. Honokiol also decreased expression of the mRNA for the chemokine CCL2 and for the intracellular adhesion molecule-1, as well as accumulation of type I (α1) collagen and fibronectin in UUO kidneys. Phosphorylation of Smad-2/3 induced by TGF-β1 and CTGF luciferase activity in renal tubular cells were also inhibited by honokiol.

CONCLUSIONS AND IMPLICATIONS

Honokiol suppressed expression of pro-fibrotic and pro-inflammatory factors and of extracellular matrix proteins. Honokiol may become a therapeutic agent to prevent renal fibrosis.

Transcriptional networks in epithelial-mesenchymal transition

Backround

Epithelial-mesenchymal transition (EMT) changes polarized epithelial cells into migratory phenotypes associated with loss of cell-cell adhesion molecules and cytoskeletal rearrangements. This form of plasticity is seen in mesodermal development, fibroblast formation, and cancer metastasis.

Methods and Findings

Here we identify prominent transcriptional networks active during three time points of this transitional process, as epithelial cells become fibroblasts. DNA microarray in cultured epithelia undergoing EMT, validated in vivo, were used to detect various patterns of gene expression. In particular, the promoter sequences of differentially expressed genes and their transcription factors were analyzed to identify potential binding sites and partners. The four most frequent cis-regulatory elements (CREs) in up-regulated genes were SRY, FTS-1, Evi-1, and GC-Box, and RNA inhibition of the four transcription factors, Atf2, Klf10, Sox11, and SP1, most frequently binding these CREs, establish their importance in the initiation and propagation of EMT. Oligonucleotides that block the most frequent CREs restrain EMT at early and intermediate stages through apoptosis of the cells.

Conclusions

Our results identify new transcriptional interactions with high frequency CREs that modulate the stability of cellular plasticity, and may serve as targets for modulating these transitional states in fibroblasts.

Delayed stress fiber formation mediates pulmonary myofibroblast differentiation in response to TGF-β

Myofibroblast differentiation induced by transforming growth factor-β (TGF-β) and characterized by de novo expression of smooth muscle (SM)-specific proteins is a key process in wound healing and in the pathogenesis of fibrosis. We have previously shown that TGF-β-induced expression and activation of serum response factor (SRF) is required for this process. In this study, we examined the signaling mechanism for SRF activation by TGF-β as it relates to pulmonary myofibroblast differentiation. TGF-β stimulated a profound, but delayed (18-24 h), activation of Rho kinase and formation of actin stress fibers, which paralleled SM α-actin expression. The translational inhibitor cycloheximide blocked these processes without affecting Smad-dependent gene transcription. Inhibition of Rho kinase by Y-27632 or depolymerization of actin by latrunculin B resulted in inhibition TGF-β-induced SRF activation and SM α-actin expression, having no effect on Smad signaling. Conversely, stabilization of actin stress fibers by jasplakinolide was sufficient to drive these processes in the absence of TGF-β. TGF-β promoted a delayed nuclear accumulation of the SRF coactivator megakaryoblastic leukemia-1 (MKL1)/myocardin-related transcription factor-A, which was inhibited by latrunculin B. Furthermore, TGF-β also induced MKL1 expression, which was inhibited by latrunculin B, by SRF inhibitor CCG-1423, or by SRF knockdown. Together, these data suggest a triphasic model for myofibroblast differentiation in response to TGF-β that involves 1) initial Smad-dependent expression of intermediate signaling molecules driving Rho activation and stress fiber formation, 2) nuclear accumulation of MKL1 and activation of SRF as a result of actin polymerization, and 3) SRF-dependent expression of MKL1, driving further myofibroblast differentiation.

Myocardin-related transcription factors A and B are key regulators of TGF-β1-induced fibroblast to myofibroblast differentiation

Myofibroblasts are contractile, smooth muscle-like cells that are characterized by the de novo expression of smooth muscle α-actin (SMαA) and normally function to assist in wound closure, but have been implicated in pathological contractures. Transforming growth factor β-1 (TGF-β1) helps facilitate the differentiation of fibroblasts into myofibroblasts, but the exact mechanism by which this differentiation occurs, in response to TGF-β1, remains unclear. Myocardin-related transcription factors A and B (MRTFs, MRTF-A/B) are transcriptional co-activators that regulate the expression of smooth muscle-specific cytoskeletal proteins, including SMαA, in smooth muscle cells and fibroblasts. In this study, we demonstrate that TGF-β1 mediates myofibroblast differentiation and the expression of a contractile gene program through the actions of the MRTFs. Transient transfection of a constitutively active MRTF-A induced an increase in the expression of SMαA and other smooth muscle-specific cytoskeletal proteins, and an increase in myofibroblast contractility, even in the absence of TGF-β1. MRTF-A/B knockdown, in TGF-β1-differentiated myofibroblasts, resulted in decreased smooth muscle-specific cytoskeletal protein expression levels and reduced contractile force generation, as well as a decrease in focal adhesion size and number. These results provide direct evidence that the MRTFs are mediators of myofibroblast differentiation in response to TGF-β1.

Novel therapeutic approaches for pulmonary fibrosis

Pulmonary fibrosis represents the end stage of a number of heterogeneous conditions and is, to a greater or lesser degree, the hallmark of the interstitial lung diseases. It is characterized by the excessive deposition of extracellular matrix proteins within the pulmonary interstitium leading to the obliteration of functional alveolar units and in many cases, respiratory failure. While a small number of interstitial lung diseases have known aetiologies, most are idiopathic in nature, and of these, idiopathic pulmonary fibrosis is the most common and carries with it an appalling prognosis - median survival from the time of diagnosis is less than 3 years. This reflects the lack of any effective therapy to modify the course of the disease, which in turn is indicative of our incomplete understanding of the pathogenesis of this condition. Current prevailing hypotheses focus on dysregulated epithelial-mesenchymal interactions promoting a cycle of continued epithelial cell injury and fibroblast activation leading to progressive fibrosis. However, it is likely that multiple abnormalities in a myriad of biological pathways affecting inflammation and wound repair - including matrix regulation, epithelial reconstitution, the coagulation cascade, neovascularization and antioxidant pathways - modulate this defective crosstalk and promote fibrogenesis. This review aims to offer a pathogenetic rationale behind current therapies, briefly outlining previous and ongoing clinical trials, but will focus on recent and exciting advancements in our understanding of the pathogenesis of idiopathic pulmonary fibrosis, which may ultimately lead to the development of novel and effective therapeutic interventions for this devastating condition.

Proteomic analysis of lung tissues from patients with pulmonary arterial hypertension

BACKGROUND

Pulmonary arterial hypertension is a disorder of vascular remodeling causing increased resistance to pulmonary blood flow. The expression of proteins in lungs from pulmonary arterial hypertension patients was investigated in an unbiased approach to further understand the pathobiology of this disease.

METHODS AND RESULTS

Label-free liquid chromatography tandem mass spectrometry was used to compare protein profiles in surgical samples of lungs from 8 patients with pulmonary arterial hypertension and 8 control subjects. More than 300 proteins were detected. On the basis of robust criteria, the levels of 25 proteins varied between the 2 groups. The majority of upregulated proteins were associated with cell growth, proliferation, and cell metabolism. Novel findings included an increased expression of chloride intracellular channel 4, receptor for advanced glycation end products, and periostin. Increased expression of chloride intracellular channel 4, a multifunctional protein involved in angiogenesis, and several signaling pathways implicated in pulmonary arterial hypertension--transforming growth factor-β, vascular endothelial growth factor, and bone morphogenetic protein--was confirmed by Western blotting and localized predominantly to endothelial cells in occlusive and plexiform vascular lesions.

CONCLUSIONS

Label-free proteomics identified differences in the expression of several proteins in the pulmonary arterial hypertension lung, many of which are relevant to the disease process. Increased expression of chloride intracellular channel 4 may be pertinent to the disorganized angiogenesis of plexiform lesions.

Isolation and characterization of the core single-stranded DNA-binding domain of purine-rich element binding protein B (Purβ)

Purβ is a single-stranded nucleic acid-binding protein implicated in the injury-induced repression of genes encoding certain muscle-restricted isoforms of actin and myosin expressed in the heart, skeletal muscle, and vasculature. To better understand how the modular arrangement of the primary sequence of Purβ affects the higher order structure and function of the protein, purified recombinant Purβ was subjected to partial proteolysis in an attempt to identify a well-folded truncation protein that retained purine-rich single-stranded DNA-binding activity. Limited tryptic digestion of Purβ liberated a core ∼30kDa fragment corresponding to residues 29-305 as determined by epitope mapping and mass spectrometry. Size exclusion chromatography indicated that the isolated core fragment retains the ability to self-associate while circular dichroism analysis confirmed that the Purβ core domain is stably folded in the absence of glycine-rich N- and C-terminal sequences. Comparative DNA-binding assays revealed that the isolated core domain interacts with purine-rich cis-elements from the smooth muscle α-actin gene with similar specificity but increased affinity compared to full-length Purβ. These findings suggest that the highly conserved modular repeats of Purβ fold to form a core functional domain, which mediates the specific and high affinity binding of the protein to single-stranded DNA.

Myocardin-related transcription factor-a controls myofibroblast activation and fibrosis in response to myocardial infarction

RATIONALE

Myocardial infarction (MI) results in loss of cardiac myocytes in the ischemic zone of the heart, followed by fibrosis and scar formation, which diminish cardiac contractility and impede angiogenesis and repair. Myofibroblasts, a specialized cell type that switches from a fibroblast-like state to a contractile, smooth muscle-like state, are believed to be primarily responsible for fibrosis of the injured heart and other tissues, although the transcriptional mediators of fibrosis and myofibroblast activation remain poorly defined. Myocardin-related transcription factors (MRTFs) are serum response factor (SRF) cofactors that promote a smooth muscle phenotype and are emerging as components of stress-responsive signaling.

OBJECTIVE

We aimed to examine the effect of MRTF-A on cardiac remodeling and fibrosis.

METHODS AND RESULTS

Here, we show that MRTF-A controls the expression of a fibrotic gene program that includes genes involved in extracellular matrix production and smooth muscle cell differentiation in the heart. In MRTF-A-null mice, fibrosis and scar formation following MI or angiotensin II treatment are dramatically diminished compared with wild-type littermates. This protective effect of MRTF-A deletion is associated with a reduction in expression of fibrosis-associated genes, including collagen 1a2, a direct transcriptional target of SRF/MRTF-A.

CONCLUSIONS

We conclude that MRTF-A regulates myofibroblast activation and fibrosis in response to the renin-angiotensin system and post-MI remodeling.

A bifunctional intronic element regulates the expression of the arginine/lysine transporter Cat-1 via mechanisms involving the purine-rich element binding protein A (Pur alpha)

Expression of the arginine/lysine transporter Cat-1 is highly induced in proliferating and stressed cells via mechanisms that include transcriptional activation. A bifunctional INE (intronic element) within the first intron of the Cat-1 gene was identified and characterized in this study. The INE had high sequence homology to an amino acid response element and was shown to act as a transcriptional enhancer in unstressed cells by binding the transcription factor, purine-rich element binding protein A (Pur alpha). During endoplasmic reticulum stress, binding of Pur alpha to the INE decreased; the element acted as a positive regulator in early stress by binding of the transcription factor ATF4 and as a negative regulator in prolonged stress by binding the stress-induced C/EBP family member, CHOP. We conclude that transcriptional control of the Cat-1 gene is tightly controlled by multiple cis-DNA elements, contributing to regulation of cationic amino acid transport for cell growth and proliferation. In addition, we propose that genes may use stress-response elements such as the INE to support basal expression in the absence of stress.

Specificity protein-1 and -3 trans-activate the ovine placental lactogen gene promoter

The proximal promoter (-383/+16) of the ovine placental lactogen (oPL) gene provides trophoblast-specific expression in vitro. Footprint 6 (FP6; -319/-349) lies within this region, and transfection of two-base pair mutations across FP6 into BeWo cells identified potential binding sites for CCAAT-enhancer binding protein (CEBP) and specificity proteins (Sp). Transfection of CEBP dominant negative or over-expression constructs did not impact transactivation of the proximal promoter. However, Sp1 and Sp3 over-expression constructs increased (p<or=0.05) transactivation. Additionally, Sp1 and Sp3 short-hairpin RNA constructs reduced (p<or=0.01) transactivation of the proximal promoter. In EMSA supershift assays, Sp1 and Sp3 antibodies were able to inhibit migration of the complexes formed with nuclear extracts from BeWo cells and ovine chorionic binucleate cells (oBNC). Furthermore, Southwestern analysis of oBNC nuclear extracts identified a nuclear protein corresponding with Sp3, identified by Western analysis. In conclusion, these results indicate that Sp1 and Sp3 are capable of interacting with FP6 of the oPL gene proximal promoter and function to enhance its transactivation.

Mechanism of strand-specific smooth muscle alpha-actin enhancer interaction by purine-rich element binding protein B (Purbeta)

Expression of the smooth muscle alpha-actin gene in growth-activated vascular smooth muscle cells and stromal fibroblasts is negatively regulated by members of the Pur family of single-stranded DNA/RNA-binding proteins. In particular, Puralpha and Purbeta are postulated to repress transcription by forming helix-destabilizing complexes with the sense strand of an asymmetric polypurine-polypyrimidine tract containing a canonical MCAT enhancer motif in the 5' region of the gene. Herein, we establish the mechanism of Purbeta binding to the purine-rich strand of the enhancer using quantitative methods and purified components. Initial evaluation of DNA-binding specificity and equilibrium stoichiometry via colorimetric-, autoradiographic-, and fluorescence-based assays suggested that Purbeta interacts with two distinct G/A-rich sites within the nominal single-stranded enhancer element to form a high-affinity 2:1 protein:DNA complex. Statistical mechanical analyses of band shift titrations of the nominal element in conjunction with DNase I footprint titrations of the extended smooth muscle alpha-actin 5'-flanking region demonstrated that assembly of the nucleoprotein complex likely occurs in a sequential, cooperative, and monomer-dependent fashion. Resolution of the microscopic energetics of the system indicated that monomer association with two nonidentical sites flanking the core MCAT motif accounts for the majority of the intrinsic binding affinity of Purbeta with intersite cooperativity contributing an approximately 12-fold increase to the stability of the nucleoprotein complex. These findings offer new insights into the mechanism, energetics, and sequence determinants of Purbeta repressor binding to a biologically relevant, contractile phenotype-regulating cis-element while also revealing the thermodynamic confines of putative Purbeta-mediated effects on DNA structure.

Transforming growth factor beta1-mediated activation of the smooth muscle alpha-actin gene in human pulmonary myofibroblasts is inhibited by tumor necrosis factor-alpha via mitogen-activated protein kinase kinase 1-dependent induction of the Egr-1 transcriptional repressor

Transforming growth factor (TGF) beta1 is a mediator of myofibroblast differentiation in healing wounds in which it activates transcription of the smooth muscle alpha-actin (SMalphaA) gene via dynamic interplay of nuclear activators and repressors. Targeting components of TGFbeta1 signaling may be an effective strategy for controlling myofibroblasts in chronic fibrotic diseases. We examined the ability of proinflammatory tumor necrosis factor (TNF)-alpha to antagonize TGFbeta1-mediated human pulmonary myofibroblast differentiation. TNF-alpha abrogated TGFbeta1-induced SMalphaA gene expression at the level of transcription without disrupting phosphorylation of regulatory Smads. Intact mitogen-activated protein kinase kinase (Mek)-extracellular signal-regulated kinase (Erk) kinase signaling was required for myofibroblast repression by TNF-alpha via induction of the early growth response factor-1 (Egr-1) DNA-binding protein. Egr-1 bound to the GC-rich SPUR activation element in the SMalphaA promoter and potently suppressed Smad3- and TGFbeta1-mediated transcription. Reduction in Smad binding to the SMalphaA promoter in TNF-alpha-treated myofibroblasts was accompanied by an increase in Egr-1 and YB-1 repressor binding, suggesting that the molecular mechanism underlying repression may involve competitive interplay between Egr-1, YB-1, and Smads. The ability of TNF-alpha to attenuate myofibroblast differentiation via modulation of a Mek1/Erk/Egr-1 regulatory axis may be useful in designing new therapeutic targets to offset destructive tissue remodeling in chronic fibrotic disease.

Multiple roles for Puralpha in cellular and viral regulation

Pur-alpha is a ubiquitous multifunctional protein that is strongly conserved throughout evolution, binds to both DNA and RNA and functions in the initiation of DNA replication, control of transcription and mRNA translation. In addition, it binds to several cellular regulatory proteins including the retinoblastoma protein, E2F-1, Sp1, YB-1, cyclin T1/Cdk9 and cyclin A/Cdk2. These observations and functional studies provide evidence that Puralpha is a major player in the regulation of the cell cycle and oncogenic transformation. Puralpha also binds to viral proteins such as the large T-antigen of JC virus (JCV) and the Tat protein of human immunodeficiency virus-1 (HIV-1) and plays a role in the cross-communication of these viruses in the opportunistic polyomavirus JC (JCV) brain infection, progressive multifocal leukoencephalopathy (PML). The creation of transgenic mice with inactivation of the PURA gene that encodes Puralpha has revealed that Puralpha is critical for postnatal brain development and has unraveled an essential role of Puralpha in the transport of specific mRNAs to the dendrites and the establishment of the postsynaptic compartment in the developing neurons. Finally, the availability of cell cultures from the PURA knockout mice has allowed studies that have unraveled a role for Puralpha in DNA repair.

Critical role of serum response factor in pulmonary myofibroblast differentiation induced by TGF-beta

Transforming growth factor-beta (TGF-beta) is a cytokine implicated in wound healing and in the pathogenesis of pulmonary fibrosis. TGF-beta stimulates myofibroblast differentiation characterized by expression of contractile smooth muscle (SM)-specific proteins such as SM-alpha-actin. In the present study, we examined the role of serum response factor (SRF) in the mechanism of TGF-beta-induced pulmonary myofibroblast differentiation of human lung fibroblasts (HLF). TGF-beta stimulated SM-alpha-actin expression in HLF, which paralleled with a profound induction of SRF expression and activity. Inhibition of SRF by the pharmacologic SRF inhibitor (CCG-1423), or via adenovirus-mediated transduction of SRF short hairpin RNA (shSRF), blocked the expression of both SRF and SM-alpha-actin in response to TGF-beta without affecting Smad-mediated signaling of TGF-beta. However, forced expression of SRF on its own did not promote SM-alpha-actin expression, whereas expression of the constitutively transactivated SRF fusion protein (SRF-VP16) was sufficient to induce SM-alpha-actin expression, suggesting that both expression and transactivation of SRF are important. Activation of protein kinase A (PKA) by forskolin or iloprost resulted in a significant inhibition of SM-alpha-actin expression induced by TGF-beta, and this was associated with inhibition of both SRF expression and activity, but not of Smad-mediated gene transcription. In summary, this is the first direct demonstration that TGF-beta-induced pulmonary myofibroblast differentiation is mediated by SRF, and that inhibition of myofibroblast differentiation by PKA occurs through down-regulation of SRF expression levels and SRF activity, independent of Smad signaling.

Serum response factor neutralizes Pur alpha- and Pur beta-mediated repression of the fetal vascular smooth muscle alpha-actin gene in stressed adult cardiomyocytes

Mouse hearts subjected to repeated transplant surgery and ischemia-reperfusion injury develop substantial interstitial and perivascular fibrosis that was spatially associated with dysfunctional activation of fetal smooth muscle alpha-actin (SM alpha A) gene expression in graft ventricular cardiomyocytes. Compared with cardiac fibroblasts in which nuclear levels of the Sp1 and Smad 2/3 transcriptional-activating proteins increased markedly after transplant injury, the most abundant SM alpha A gene-activating protein in cardiomyocyte nuclei was serum response factor (SRF). Additionally, cardiac intercalated discs in heart grafts contained substantial deposits of Pur alpha, an mRNA-binding protein and known negative modulator of SRF-activated SM alpha A gene transcription. Activation of fetal SM alpha A gene expression in perfusion-isolated adult cardiomyocytes was linked to elevated binding of a novel protein complex consisting of SRF and Pur alpha to a purine-rich DNA element in the SM alpha A promoter called SPUR, previously shown to be required for induction of SM alpha A gene transcription in injury-activated myofibroblasts. Increased SRF binding to SPUR DNA plus one of two nearby CArG box consensus elements was observed in SM alpha A-positive cardiomyocytes in parallel with enhanced Pur alpha:SPUR protein:protein interaction. The data suggest that de novo activation of the normally silent SM alpha A gene in reprogrammed adult cardiomyocytes is linked to elevated interaction of SRF with fetal-specific CArG and injury-activated SPUR elements in the SM alpha A promoter as well as the appearance of novel Pur alpha protein complexes in both the nuclear and cytosolic compartments of these cells.

Transforming growth factor-beta (TGF- 1) down-regulates Notch3 in fibroblasts to promote smooth muscle gene expression

Select signaling pathways have emerged as key players in regulating smooth muscle gene expression during myofibroblast and smooth muscle differentiation, an event that is important for wound healing and vascular remodeling. These include the transforming growth factor-beta (TGF-beta1) signaling cascade, which has been assigned multiple roles in these cells, and the Notch pathway. Notch family members have been implicated in governing cell fate in a variety of cells; however, the mechanisms are not well understood. We sought to explore how these prominent signaling mediators regulate differentiation, and in particular, how they might converge to control the transcription of smooth muscle genes. Using TGF-beta1 to induce the differentiation of 10T1/2 fibroblasts, we investigated the specific function of Notch3. Overexpression of activated Notch3 caused repression of TGF-beta1-induced smooth muscle-specific genes, whereas knockdown of Notch3 by small interfering RNA did not convincingly alter their expression. Surprisingly, the addition of TGF-beta1 caused a significant decrease in Notch3 RNA and protein and a reciprocal increase in Hes1 gene transcription. The repression of Notch3 was mediated by SMAD activity and p38 mitogen-activated protein (MAP) kinase, whereas analysis of the Hes1 promoter revealed direct activation by Smad2 but not Smad3. Furthermore, the Hes1 repressor protein augmented Smad3 transactivation of the SM22alpha promoter. These results offer a novel mechanism by which TGF-beta1 promotes the expression of smooth muscle differentiation genes through the inhibition of Notch3 and activation of Hes1.

Structure-function analysis of mouse Pur beta II. Conformation altering mutations disrupt single-stranded DNA and protein interactions crucial to smooth muscle alpha-actin gene repression

Previous studies from our laboratories have implicated two members of the Pur family of single-stranded DNA/RNA-binding proteins, Pur alpha and Pur beta, in transcriptional repression of the smooth muscle alpha-actin gene in vascular cell types. Although Pur alpha and Pur beta share substantial sequence homology and nucleic acid binding properties, genomic promoter and cis-element occupancy studies reported herein suggest that Pur beta is the dominant factor in gene regulation. To dissect the molecular basis of Pur beta repressor activity, site-directed mutagenesis was used to map amino acids critical to the physical and functional interaction of Pur beta with the smooth muscle alpha-actin promoter. Of all the various acidic, basic, and aromatic residues studied, mutation of positionally conserved arginines in the class I or class II repeat modules significantly attenuated Pur beta repressor activity in transfected vascular smooth muscle cells and fibroblasts. DNA binding and protein-protein interaction assays were conducted with purified recombinant Pur beta and selected mutants to reveal the physical basis for loss-of-function. Mutants R57E, R57E/R96E, and R57A/R96A each exhibited reduced single-stranded DNA binding affinity for an essential promoter element and diminished interaction with corepressor YB-1/MSY1. Structural analyses of the R57A/R96A and R57E/R96E double mutants in comparison to the wild-type Pur beta homodimer revealed aberrant self-association into higher order oligomeric complexes, which correlated with decreased alpha-helical content and defective DNA and protein binding in vitro. These findings point to a previously unrecognized structural role for certain core arginine residues in forming a conformationally stable Pur beta protein capable of physical interactions necessary for smooth muscle alpha-actin gene repression.

Smooth muscle alpha-actin expression and myofibroblast differentiation by TGFbeta are dependent upon MK2

Fibroblasts play a major role in processes such as wound repair, scarring, and fibrosis. Differentiation into myofibroblasts, characterized by upregulation of smooth muscle alpha-actin (smalpha) in response to profibrotic agents such as TGFbeta is believed to be an important step in fibrosis. Therefore, elucidating mechanisms of myofibroblast differentiation might reveal novel targets in treating diseases such as idiopathic pulmonary fibrosis (IPF). MK2 is a kinase substrate of p38 MAP kinase that mediates some effects of p38 activation on the actin cytoskeleton. Using mouse embryonic fibroblasts (MEF) from MK2 knockout (MK2(-/-)) mice, we demonstrate that disrupting expression of MK2 expression reduces filamentous actin and stress fibers. It also causes MK2(-/-) MEF to express less smalpha than their corresponding wild-type (WT) MEF at baseline and in response to TGFbeta. Furthermore, TGFbeta causes downregulation of smalpha in MK2(-/-) MEF, instead of upregulation observed in WT MEF. Expression of other fibroblast markers, such as collagen, is not altered in MK2(-/-) MEF. Our results further suggest that downregulation of smalpha in MK2(-/-) MEF is not due to lack of activation of serum responsive promoter elements, but probably due to reduced smalpha message stability in these cells. These results indicate that MK2 plays a key role in regulation of smalpha expression, and that targeting MK2 might present a therapeutic approach in managing conditions such as pulmonary fibrosis.

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.

Dynamic changes in chromatin acetylation and the expression of histone acetyltransferases and histone deacetylases regulate the SM22alpha transcription in response to Smad3-mediated TGFbeta1 signaling

TGFbeta1 plays critical roles in stimulating smooth muscle gene transcription during myofibroblast and smooth muscle cell (SMC) differentiation. Increasing evidence demonstrates that histone modification plays important roles in regulating gene transcription. Here, we investigated the effect of changes in the expression of histone acetyltransferases (HAT) or histone deacetylases (HDAC) on TGFbeta1-induced SM22 promoter activities. We found that overexpressing HAT proteins such as p300 and CBP enhances TGFbeta1-induced SM22 promoter activities; conversely, overexpressing HAT inhibitor such as Twist1 (but not Twist2/Dermo-1) and E1A suppresses this effect of TGFbeta1. We also found that TSA, a HDAC inhibitor that stimulates histone acetylation of the SM22alpha locus, further enhances the transactivational activity of Smad2, Smad3 and Smad4, and relieves the inhibitory effect of Smad6, Smad7, and the dominant negative mutants of Smads. TGFbeta1 also stimulates the association of Smad3 (a potent transactivator for the SM22 promoter) and p300 by co-immunoprecipitation assay. In contrast, overexpressing HDAC 1-6 inhibits TGFbeta1-induced as well as Smad3 and myocardin-activated SM22 promoter. Moreover, chromatin immunoprecipitation (ChIP) assays show that TGFbeta1 induces histone acetylation at the SM22alpha locus. This study demonstrates that the balance of HAT and HDAC expression affects TGFbeta1-induced SM22alpha transcription; TGFbeta1-induced SM22alpha transcription is accompanied by histone hyperacetylation at the SM22alpha locus. This study provides the first evidence showing that histone hyperacetylation of the SM22 promoter is a target of TGFbeta1 signaling, suggesting that modulation of histone acetylation is involved in the molecular mechanisms of TGFbeta1-regulated SMC gene transcription.

Nucleoprotein interactions governing cell type-dependent repression of the mouse smooth muscle alpha-actin promoter by single-stranded DNA-binding proteins Pur alpha and Pur beta

Pur alpha and Pur beta are structurally related single-stranded DNA/RNA-binding proteins implicated in the control of cell growth and differentiation. The goal of this study was to determine whether Pur alpha and Pur beta function in a redundant, distinct, or collaborative manner to suppress smooth muscle alpha-actin gene expression in cell types relevant to wound repair and vascular remodeling. RNA interference-mediated loss-of-function analyses revealed that, although Pur beta was the dominant repressor, the combined action of endogenous Pur alpha and Pur beta was necessary to fully repress the full-length smooth muscle alpha-actin promoter in cultured fibroblasts but to a lesser extent in vascular smooth muscle cells. The activity of a minimal core enhancer containing a truncated 5' Pur repressor binding site was unaffected by knockdown of Pur alpha and/or Pur beta in fibroblasts. Conversely, gain-of-function studies indicated that Pur alpha or Pur beta could each independently repress core smooth muscle alpha-actin enhancer activity albeit in a cell type-dependent fashion. Biochemical analyses indicated that purified recombinant Pur alpha and Pur beta were essentially identical in terms of their binding affinity and specificity for GGN repeat-containing strands of several cis-elements comprising the core enhancer. However, Pur alpha and Pur beta exhibited more distinctive protein interaction profiles when evaluated for binding to enhancer-associated transcription factors in extracts from fibroblasts and vascular smooth muscle cells. These findings support the hypothesis that Pur alpha and Pur beta repress smooth muscle alpha-actin gene transcription by means of DNA strand-selective cis-element binding and cell type-dependent protein-protein interactions.

Contribution of activin receptor–like kinase 5 (transforming growth factor β receptor type I) signaling to the fibrotic phenotype of scleroderma fibroblasts

OBJECTIVE

To use a specific transforming growth factor beta receptor type I (TGFbetaRI; activin receptor-like kinase 5 [ALK-5]) kinase inhibitor (SD208) to determine the role of activation of the TGFbetaRI kinase (ALK-5) in maintaining the profibrotic phenotype of dermal fibroblasts in systemic sclerosis (SSc).

METHODS

The effect of SD208 on the expression of key biochemical markers of the fibrotic phenotype was compared in fibroblasts cultured from clinically involved (lesional) and clinically uninvolved skin of patients with diffuse cutaneous SSc (dcSSc) and in fibroblasts from healthy controls matched for age, sex, and anatomic site. Protein expression was compared together with the ability of fibroblasts to adhere to the extracellular matrix and to remodel and contract a free-floating fibroblast-populated type I collagen lattice.

RESULTS

Inhibiting TGFbetaRI kinase reduced the expression of a cohort of fibrotic markers by dermal fibroblasts from patients with dcSSc, including type I collagen and beta1 integrin. Moreover, inhibition also attenuated the elevated adhesive and contractile abilities of dcSSc fibroblasts.

CONCLUSION

Our data suggest that some of the key profibrotic features of lesional SSc fibroblasts are dependent upon ALK-5 activity. Thus, TGFbetaRI kinase-mediated signaling may contribute to dermal fibrosis in dcSSc.

Sp1-like transcription factors are regulators of embryonic development in vertebrates

Sp1-like family is an expanding transcription factor family. Members of this family bind to the GC-box or GT-box elements in the promoter/enhancers and regulate the expression of the target genes. Currently, this family consists of at least nine members, which may act as a transactivator or a repressor on target promoters. Sp1-like transcription factors are expressed during development of vertebrate embryos in ubiquitous or tissue-specific manners and play various roles in embryonic development. This review mainly summarises their expression patterns and functions during vertebrate embryogenesis.

YB-1 coordinates vascular smooth muscle alpha-actin gene activation by transforming growth factor beta1 and thrombin during differentiation of human pulmonary myofibroblasts

Profibrotic regulatory mechanisms for tissue repair after traumatic injury have developed under strong evolutionary pressure to rapidly stanch blood loss and close open wounds. We have examined the roles played by two profibrotic mediators, transforming growth factor beta1 (TGFbeta1) and thrombin, in directing expression of the vascular smooth muscle alpha-actin (SMalphaA) gene, an important determinant of myofibroblast differentiation and early protein marker for stromal cell response to tissue injury. TGFbeta1 is a well known transcriptional activator of the SMalphaA gene in myofibroblasts. In contrast, thrombin independently elevates SMalphaA expression in human pulmonary myofibroblasts at the posttranscriptional level. A common feature of SMalphaA up-regulation mediated by thrombin and TGFbeta1 is the involvement of the cold shock domain protein YB-1, a potent repressor of SMalphaA gene transcription in human fibroblasts that also binds mRNA and regulates translational efficiency. YB-1 dissociates from SMalphaA enhancer DNA in the presence of TGFbeta1 or its Smad 2, 3, and 4 coregulatory mediators. Thrombin does not effect SMalphaA gene transcription but rather displaces YB-1 from SMalphaA exon 3 coding sequences previously shown to be required for mRNA translational silencing. The release of YB-1 from promoter DNA coupled with its ability to bind RNA and shuttle between the nucleus and cytoplasm is suggestive of a regulatory loop for coordinating SMalphaA gene output in human pulmonary myofibroblasts at both the transcriptional and translational levels. This loop may help restrict organ-destructive remodeling due to excessive myofibroblast differentiation.

SB-431542 inhibits TGF-β-induced contraction of collagen gel by normal and keloid fibroblasts

BACKGROUND

Transforming growth factor (TGF)-beta induces fibroblast contraction, which is implicated in wound healing and keloid formation. SB-431542 is a novel specific inhibitor of TGF-beta type I receptor kinase activity.

OBJECTIVE

We sought to determine whether SB-431542 inhibited TGF-beta-induced fibroblast contraction.

METHODS

We used an in vitro type I collagen gel contraction assay with normal or keloid dermal fibroblasts incorporated.

RESULTS

TGF-beta induced contraction of collagen gels with normal dermal fibroblasts incorporated, which was efficiently suppressed by SB-431542. Keloid fibroblasts showed higher basal contraction of collagen gels in the absence of TGF-beta than normal fibroblasts, which was enhanced by addition of TGF-beta. SB-431542 suppressed both the basal and TGF-beta-enhanced contraction of collagen gels by keloid fibroblasts. These inhibitory effects of SB-431542 were associated with suppression of TGF-beta-induced alpha-smooth muscle actin (alpha-SMA) expression and phosphorylation of Smad2 in normal and keloid fibroblasts.

CONCLUSION

SB-431542 can suppress TGF-beta-induced contraction of collagen gel by normal and keloid dermal fibroblasts. Importantly, SB-431542 can inhibit basal contraction of collagen gel by keloid fibroblasts. These results suggest that an inhibitor of TGF-beta type I receptor kinase activity may have therapeutic potential for excessive skin contraction as observed in keloid.