Myocardin and ternary complex factors compete for SRF to control smooth muscle gene expression

Protein kinase A-regulated assembly of a MEF2{middle dot}HDAC4 repressor complex controls c-Jun expression in vascular smooth muscle cells

Vascular smooth muscle cells (VSMCs) maintain the ability to modulate their phenotype in response to changing environmental stimuli. This phenotype modulation plays a critical role in the development of most vascular disease states. In these studies, stimulation of cultured vascular smooth muscle cells with platelet-derived growth factor resulted in marked induction of c-jun expression, which was attenuated by protein kinase Cdelta and calcium/calmodulin-dependent protein kinase inhibition. Given that these signaling pathways have been shown to relieve the repressive effects of class II histone deacetylases (HDACs) on myocyte enhancer factor (MEF) 2 proteins, we ectopically expressed HDAC4 and observed repression of c-jun expression. Congruently, suppression of HDAC4 by RNA interference resulted in enhanced c-jun expression. Consistent with these findings, mutation of the MEF2 cis-element in the c-jun promoter resulted in promoter activation during quiescent conditions, suggesting that the MEF2 cis-element functions as a repressor in this context. Furthermore, we demonstrate that protein kinase A attenuates c-Jun expression by promoting the formation of a MEF2.HDAC4 repressor complex by inhibiting salt-inducible kinase 1. Finally, we document a physical interaction between c-Jun and myocardin, and we document that forced expression of c-Jun represses the ability of myocardin to activate smooth muscle gene expression. Thus, MEF2 and HDAC4 act to repress c-Jun expression in quiescent VSMCs, protein kinase A enhances this repression, and platelet-derived growth factor derepresses c-Jun expression through calcium/calmodulin-dependent protein kinases and novel protein kinase Cs. Regulation of this molecular "switch" on the c-jun promoter may thus prove critical for toggling between the activated and quiescent VSMC phenotypes.

Smooth muscle cells give rise to osteochondrogenic precursors and chondrocytes in calcifying arteries

Vascular calcification is a major risk factor for cardiovascular morbidity and mortality. To develop appropriate prevention and/or therapeutic strategies for vascular calcification, it is important to understand the origins of the cells that participate in this process. In this report, we used the SM22-Cre recombinase and Rosa26-LacZ alleles to genetically trace cells derived from smooth muscle. We found that smooth muscle cells (SMCs) gave rise to osteochondrogenic precursor- and chondrocyte-like cells in calcified blood vessels of matrix Gla protein deficient (MGP(-/-)) mice. This lineage reprogramming of SMCs occurred before calcium deposition and was associated with an early onset of Runx2/Cbfa1 expression and the downregulation of myocardin and Msx2. There was no change in the constitutive expression of Sox9 or bone morphogenetic protein 2. Osterix, Wnt3a, and Wnt7a mRNAs were not detected in either calcified MGP(-/-) or noncalcified wild-type (MGP(+/+)) vessels. Finally, mechanistic studies in vitro suggest that Erk signaling might be required for SMC transdifferentiation under calcifying conditions. These results provide strong support for the hypothesis that adult SMCs can transdifferentiate and that SMC transdifferentiation is an important process driving vascular calcification and the appearance of skeletal elements in calcified vascular lesions.

Right into the heart of microRNA-133a

MicroRNAs play an essential role in diverse cellular processes, such as proliferation, differentiation, apoptosis, and stress response. Recent studies demonstrate that miRNAs are important for timing developmental decisions and fine-tuning cellular determination in vertebrate heart development. In an elegant set of experiments reported in this issue of Genes & Development, Liu et al. (3242-3254) demonstrate that miR-133a functions as an inhibitor of cardiomyocyte proliferation and a modifier of serum response factor (SRF)-dependent transcriptional signaling in the murine heart. Both targeted deletion and transgenic overexpression of miR-133a can result in the same cardiac phenotype, ventricular septal defect (VSD) and heart failure. The new data add another piece to the puzzle of regulatory networks that are implicated in cardiac disease. It will be interesting to see, if miR-133a is also involved in human heart diseases, especially VSD and dilated cardiomyopathy.

RhoA-ROCK signaling is involved in contraction-mediated inhibition of SERCA2a expression in cardiomyocytes

In neonatal ventricular cardiomyocytes (NVCM), decreased contractile activity stimulates sarco-endoplasmic reticulum Ca(2+)-ATPase2a (SERCA2a), analogous to reduced myocardial load in vivo. This study investigated in contracting NVCM the role of load-dependent RhoA-ROCK signaling in SERCA2a regulation. Contractile arrest of NVCM resulted in low peri-nuclear localized RhoA levels relative to contracting NVCM. In arrested NVCM, ROCK activity was decreased (59%) and paralleled a loss in F-actin levels. Y-27632-induced ROCK inhibition in contracting NVCM increased SERCA2a messenger RNA expression by 150%. This stimulation was transcriptional, as evident from transfections with the SERCA2a promoter. A reciprocal effect of Y-27632 treatment on the promoter activity of atrial natriuretic factor was observed. SERCA2a transcription was not altered by co-transfection of the RhoA-ROCK-dependent serum response factor (SRF) alone or in combination with myocardin. Furthermore, GATA4, another ROCK-dependent transcription factor, induced rather than repressed SERCA2a transcription. This study shows that contractile activity suppresses SERCA2a gene expression via RhoA-ROCK-dependent transcription modulation. This modulation is likely to be accomplished by a transcription factor other than SRF, myocardin, or GATA4.

Strong smooth muscle differentiation is dependent on myocardin gene amplification in most human retroperitoneal leiomyosarcomas

Myocardin (MYOCD), a serum response factor (SRF) transcriptional cofactor, is essential for cardiac and smooth muscle development and differentiation. We show here by array-based comparative genomic hybridization, fluorescence in situ hybridization, and expression analysis approaches that MYOCD gene is highly amplified and overexpressed in human retroperitoneal leiomyosarcomas (LMS), a very aggressive well-differentiated tumor. MYOCD inactivation by shRNA in a human LMS cell line with MYOCD locus amplification leads to a dramatic decrease of smooth muscle differentiation and strongly reduces cell migration. Moreover, forced MYOCD expression in three undifferentiated sarcoma cell lines and in one liposarcoma cell line confers a strong smooth muscle differentiation phenotype and increased migration abilities. Collectively, these results show that human retroperitoneal LMS differentiation is dependent on MYOCD amplification/overexpression, suggesting that in these well-differentiated LMS, differentiation could be a consequence of an acquired genomic alteration. In this hypothesis, these tumors would not necessarily derive from cells initially committed to smooth muscle differentiation. These data also provide new insights on the cellular origin of these sarcomas and on the complex connections between oncogenesis and differentiation in mesenchymal tumors.

Modulation of pulmonary vascular smooth muscle cell phenotype in hypoxia: role of cGMP-dependent protein kinase and myocardin

We have previously reported that in ovine fetal pulmonary venous smooth muscle cells (FPVSMC), decreased expression of cGMP-dependent protein kinase (PKG) by hypoxia could explain hypoxia-induced SMC phenotype modulation. In this study, we investigated the role of myocardin, a possible downstream effector of PKG, in SMC phenotype modulation induced by 1 and 24 h of hypoxia. Hypoxia for 1 h induced the phosphorylation of E-26-like protein 1 (Elk-1), indicating a quick activation of Elk-1 after hypoxia. Either hypoxia (1 h) or treatment with DT-3, a PKG inhibitor, increased associations of Elk-1 with myosin heavy chain (MHC) gene and serum response factor (SRF), which was paralleled by a decrease in association of myocardin with MHC gene and SRF. Exposure to hypoxia of FPVSMC for 24 h significantly decreased the promoter activity of multiple SMC marker genes, downregulated protein and mRNA expression of myocardin, and upregulated mRNA expression of Elk-1, but had no significant effects on the phosphorylation of Elk-1. Inhibition of myocardin by siRNA transfection downregulated the expression of SMC marker proteins, while overexpression of myocardin prevented the hypoxia-induced decrease in expression of SMC marker proteins. Inhibition of PKG by siRNA transfection downregulated the expression of myocardin, but upregulated that of Elk-1. Overexpression of PKG prevented hypoxia-induced effects on protein expression of myocardin and Elk-1. These data suggest that PKG induces displacement of myocardin from SRF and upregulates myocardin expression, thus activating the SMC genes transcription. The inhibitory effects of hypoxia on PKG may explain hypoxia-induced SMC phenotype modulation by decreasing the effects of PKG on myocardin.

CHIP represses myocardin-induced smooth muscle cell differentiation via ubiquitin-mediated proteasomal degradation

Myocardin, a coactivator of serum response factor (SRF), plays a critical role in the differentiation of vascular smooth muscle cells (SMCs). However, the molecular mechanisms regulating myocardin stability and activity are not well defined. Here we show that the E3 ligase C terminus of Hsc70-interacting protein (CHIP) represses myocardin-dependent SMC gene expression and transcriptional activity. CHIP interacts with and promotes myocardin ubiquitin-mediated degradation by the proteasome in vivo and in vitro. Furthermore, myocardin ubiquitination by CHIP requires its phosphorylation. Importantly, CHIP overexpression reduces the level of myocardin-dependent SMC contractile gene expression and diminishes arterial contractility ex vivo. These findings for the first time, to our knowledge, demonstrate that CHIP-promoted proteolysis of myocardin plays a key role in the physiological control of SMC phenotype and vessel tone, which may have an important implication for pathophysiological conditions such as atherosclerosis, hypertension, and Alzheimer's disease.

PDGF-DD, a novel mediator of smooth muscle cell phenotypic modulation, is upregulated in endothelial cells exposed to atherosclerosis-prone flow patterns

Platelet-derived growth factor (PDGF)-BB is a well-known smooth muscle (SM) cell (SMC) phenotypic modulator that signals by binding to PDGF alphaalpha-, alphabeta-, and betabeta-membrane receptors. PDGF-DD is a recently identified PDGF family member, and its role in SMC phenotypic modulation is unknown. Here we demonstrate that PDGF-DD inhibited expression of multiple SMC genes, including SM alpha-actin and SM myosin heavy chain, and upregulated expression of the potent SMC differentiation repressor gene Kruppel-like factor-4 at the mRNA and protein levels. On the basis of the results of promoter-reporter assays, changes in SMC gene expression were mediated, at least in part, at the level of transcription. Attenuation of the SMC phenotypic modulatory activity of PDGF-DD by pharmacological inhibitors of ERK phosphorylation and by a small interfering RNA to Kruppel-like factor-4 highlight the role of these two pathways in this process. PDGF-DD failed to repress SM alpha-actin and SM myosin heavy chain in mouse SMCs lacking a functional PDGF beta-receptor. Importantly, PDGF-DD expression was increased in neointimal lesions in the aortic arch region of apolipoprotein C-deficient (ApoE(-/-)) mice. Furthermore, human endothelial cells exposed to an atherosclerosis-prone flow pattern, as in vascular regions susceptible to the development of atherosclerosis, exhibited a significant increase in PDGF-DD expression. These findings demonstrate a novel activity for PDGF-DD in SMC biology and highlight the potential contribution of this molecule to SMC phenotypic modulation in the setting of disturbed blood flow.

Structure, organization and expression of common carp (Cyprinus carpio L.) NKEF-B gene

Natural killer (NK) cell enhancing factor (NKEF) belongs to the newly defined peroxiredoxin (Prx) family. Its functions are to enhance NK cell cytotoxicity and to protect DNA and proteins from oxidative damage. In this study, a partial cDNA sequence of carp NKEF-B was isolated from thymus cDNA library. Subsequently, the full-length cDNA of carp NKEF-B was obtained by means of 3' and 5' RACE, respectively. The full-length cDNA of carp NKEF-B was 1022 bp, consisting of a 73 bp 5'-terminal untranslated region (UTR), a 355 bp 3'-terminal UTR, and a 594 bp open reading frame coding for a protein of 197 amino acids. Carp NKEF-B contained two consensus Val-Cys-Pro (VCP) motifs and three consensus cysteine (Cys-51, Cys-70 and Cys-172) residues. Sequence comparison showed that the deduced amino acid sequence of carp NKEF-B had an overall similarity of 74-96% to that of other species homologues. Phylogenetic analysis revealed that carp NKEF-B forms a cluster with other known teleost NKEF-Bs. Then, by PCR we obtained a 5.1-k long genomic DNA of carp NKEF-B containing six exons and five introns. Real-time RT-PCR results showed that carp NKEF-B gene was predominantly detected in kidney and head kidney under un-infected conditions. Whereas under SVCV-infection condition, the expression of NKEF-B gene was significantly increased in blood cells, gill, intestine and spleen, but maintained in liver, and decreased significantly in kidney and head kidney. Finally, the rNKEF-B was constructed and expressed in Escherichia coli. By using an antibody against carp rNKEF-B, immunohistochemical study further indicated that NKEF-B positive cells are mainly some RBCs and a few epithelial cells in gill and intestine, and that under SVCV-infection condition, these positive cells or positive products in their cytoplasm were mainly increased in gill and spleen sections of carp. The results obtained in the present study will help to understand the function of NKEF-B in teleost innate immunity.

Splicing of HDAC7 modulates the SRF-myocardin complex during stem-cell differentiation towards smooth muscle cells

Histone deacetylases (HDACs) have a central role in the regulation of gene expression. Here we investigated whether HDAC7 has an impact on embryonic stem (ES) cell differentiation into smooth muscle cells (SMCs). ES cells were seeded on collagen-IV-coated flasks and cultured in the absence of leukemia inhibitory factor in differentiation medium to induce SMC differentiation. Western blots and double-immunofluorescence staining demonstrated that HDAC7 has a parallel expression pattern with SMC marker genes. In ex vivo culture of embryonic cells from SM22-LacZ transgenic mice, overexpression of HDAC7 significantly increased beta-galactosidase-positive cell numbers and enzyme activity, indicating its crucial role in SMC differentiation during embryonic development. We found that HDAC7 undergoes alternative splicing during ES cell differentiation. Platelet-derived growth factor enhanced ES cell differentiation into SMCs through upregulation of HDAC7 splicing. Further experiments revealed that HDAC7 splicing induced SMC differentiation through modulation of the SRF-myocardin complex. These findings suggest that HDAC7 splicing is important for SMC differentiation and vessel formation in embryonic development.

Sp1-dependent activation of KLF4 is required for PDGF-BB-induced phenotypic modulation of smooth muscle

There is clear evidence that the phenotypic modulation of smooth muscle cells (SMCs) contributes to the pathophysiology of vascular disease. Phenotypic modulation refers to the unique ability of SMCs to alter their phenotype in response to extracellular stimuli and is hallmarked by the loss of SMC marker gene expression. The transcription factor Krüppel-like factor 4 (KLF4) is a known powerful negative regulator of SMC marker gene expression that works, in part, by decreasing the expression of the serum response factor (SRF) myocardin. KLF4 is not expressed in healthy adult SMCs but is increased in SMCs in response to vascular injury in vivo or PDGF-BB treatment in vitro. The aim of the present study was to determine the molecular mechanisms that regulate the expression of KLF4 in phenotypically modulated SMCs. The results demonstrated that the transcription factor stimulating protein-1 (Sp1) regulated the expression of KLF4 in SMCs. The KLF4 promoter contains three consensus Sp1 binding sites. Using a series of truncated KLF4 promoters, we showed that only fragments containing these Sp1 sites could be activated by PDGF-BB. In addition, overexpression of Sp1 alone was sufficient to increase the activity of the KLF4 promoter. Moreover, inhibiting Sp1 expression with small-interfering RNA attenuated the effects of PDGF-BB on KLF4 expression. Mutation of the three Sp1 sites within the KLF4 promoter abolished both baseline and PDGF-BB-induced activity. Finally, the results demonstrated enhanced Sp1 binding to the KLF4 promoter in SMCs treated with PDGF-BB in vitro and following vascular injury in vivo. Taken together, the results suggest a novel role for Sp1 in increasing the expression of KLF4 in phenotypically modulated SMCs.

Airway smooth muscle in asthma: phenotype plasticity and function

Clinical asthma is characterized by reversible airway obstruction which is commonly due to an exaggerated airway narrowing referred to as airway hyperresponsiveness (AHR). Although debate exists on the complex etiology of AHR, it is clear that airway smooth muscle (ASM) mediated airway narrowing is a major contributor to airway dysfunction. More importantly, it is now appreciated that smooth muscle is far from being a simple cell with only contractile ability properties. Rather, it is more versatile with the capacity to exhibit numerous cellular functions as it adapts to the microenvironment to which it is exposed. The emerging ability of individual smooth muscle cells to undergo changes in their phenotype (phenotype plasticity) and function (functional plasticity) in response to physiological and pathological cues is an important and active area of research. This article provides a brief review of the current knowledge and emerging concepts in the field of ASM phenotype and function both under healthy and asthmatic conditions.

A rare human sequence variant reveals myocardin autoinhibition

Myocardin (MYOCD) is a transcriptional co-activator that promotes cardiac or smooth muscle gene programs through its interaction with myocyte-enhancing factor (MEF2) or serum-response factor (SRF). Isoforms of MYOCD with a truncated amino terminus show increased activity when compared with those with the full-length amino terminus, but how this is achieved remains unknown. We identified a rare human sequence variation in MYOCD in a patient with congenital heart disease that resulted in a missense mutation at codon 259 (K259R). This variation created a hypomorphic cardiac isoform with impaired SRF binding and transactivation capacity but did not impair the smooth muscle isoform of MYOCD, which lacks the amino terminus. Consistent with differential effects of the amino terminus on the K259R mutation, we found that the cardiac-specific amino terminus acted in an autoinhibitory fashion to bind MYOCD via specific negatively charged residues and thereby repressed SRF-dependent MYOCD activity. This effect was exaggerated in the MYOCD-K259R mutant. The amino terminus was sufficient to impair MYOCD-dependent fibroblast conversion into smooth muscle cells as well as cardiomyocyte hypertrophy. These findings identify a novel mechanism that regulates levels of MYOCD-dependent activation of the SRF genetic program differentially in cardiac and smooth muscle.

Embryonic stem cell differentiation into smooth muscle cells is mediated by Nox4-produced H2O2

NADPH oxidase (Nox4) produces reactive oxygen species (ROS) that are important for vascular smooth muscle cell (SMC) behavior, but the potential impact of Nox4 in stem cell differentiation is unknown. When mouse embryonic stem (ES) cells were plated on collagen IV-coated dishes/flasks, a panel of SMC-specific genes was significantly and consistently upregulated. Nox4 expression was markedly correlated with such a gene induction as confirmed by real-time PCR, immunofluorescence, and Western blot analysis. Overexpression of Nox4 specifically resulted in increased SMC marker production, whereas knockdown of Nox4 induced a decrease. Furthermore, SMC-specific transcription factors, including serum response factor (SRF) and myocardin were activated by Nox4 gene expression. Moreover, Nox4 was demonstrated to drive SMC differentiation through generation of H(2)O(2). Confocal microscopy analysis indicates that SRF was translocated into the nucleus during SMC differentiation in which SRF was phosphorylated. Additionally, autosecreted transforming growth factor (TGF)-beta(1) activated Nox4 and promoted SMC differentiation. Interestingly, cell lines generated from stem cells by Nox4 transfection and G418 selection displayed a characteristic of mature SMCs, including expression of SMC markers and cells with contractile function. Thus we demonstrate for the first time that Nox4 is crucial for SMC differentiation from ES cells, and enforced Nox4 expression can maintain differentiation status and functional features of stem cell-derived SMCs, highlighting its impact on vessel formation in vivo and vascular tissue engineering in the future.

microRNA-133a regulates cardiomyocyte proliferation and suppresses smooth muscle gene expression in the heart

MicroRNAs (miRNAs) modulate gene expression by inhibiting mRNA translation and promoting mRNA degradation, but little is known of their potential roles in organ formation or function. miR-133a-1 and miR-133a-2 are identical, muscle-specific miRNAs that are regulated during muscle development by the SRF transcription factor. We show that mice lacking either miR-133a-1 or miR-133a-2 are normal, whereas deletion of both miRNAs causes lethal ventricular-septal defects in approximately half of double-mutant embryos or neonates; miR-133a double-mutant mice that survive to adulthood succumb to dilated cardiomyopathy and heart failure. The absence of miR-133a expression results in ectopic expression of smooth muscle genes in the heart and aberrant cardiomyocyte proliferation. These abnormalities can be attributed, at least in part, to elevated expression of SRF and cyclin D2, which are targets for repression by miR-133a. These findings reveal essential and redundant roles for miR-133a-1 and miR-133a-2 in orchestrating cardiac development, gene expression, and function and point to these miRNAs as critical components of an SRF-dependent myogenic transcriptional circuit.

Differentiation of bone marrow mesenchymal stem cells into the smooth muscle lineage by blocking ERK/MAPK signaling pathway

Smooth muscle cells (SMCs) are major components of blood vessels and other hollow visceral organs required for tissue engineering of these organs. This study aims to evaluate whether adult bone marrow-derived mesenchymal stem cells (BMMSCs), multipotent cells, can be converted into SMCs. We examined the ERK/MAPK pathway as it exerts anti-myogenic signals in SMCs. Undifferentiated BMMSCs express most SMC marker genes, albeit mainly at low levels, except smooth muscle myosin heavy chain (SMMHC), the most definitive marker of differentiated SMC. The treatment of BMMSC with MEK inhibitor up-regulated the expression of alpha-smooth muscle actin (ASMA), h-caldesmon, and SMMHC in BMMSC in low serum condition. MEK inhibitor-treated BMMSC also contracted a collagen gel in response to endothelin. Interestingly, inhibition of MEK induced myocardin expression in BMMSC. In conclusion, BMMSCs treated MEK inhibitor gain a SMC-like phenotype with ligand-induced cell contractility to endothelin in vitro. This approach has obvious implications for cell therapeutics and tissue engineering of hollow visceral organs such as blood vessels.

Myocardin marks the earliest cardiac gene expression and plays an important role in heart development

Myocardin belongs to the SAP domain family of transcription factors and is expressed specifically in cardiac and smooth muscle during embryogenesis and in adulthood. Myocardin functions as a transcriptional coactivator of SRF and is sufficient and necessary for smooth muscle gene expression. However, the in vivo function of myocardin during cardiogenesis is not completely understood. Here we clone myocardin from chick embryonic hearts and show that myocardin protein sequences are highly conserved cross species. Detailed studies of chick myocardin expression reveal that myocardin is expressed in cardiac and smooth muscle lineage during early embryogenesis, similar to that found in mouse. Interestingly, the expression of myocardin in the heart was found enriched in the outflow tract and the sinoatrial segments shortly after the formation of linear heart tube. Such expression pattern is also maintained in later developing embryos, suggesting that myocardin may play a unique role in the formation of those cardiac modules. Similar to its mouse counterpart, chick myocardin is able to activate cardiac and smooth muscle promoter reporter genes and induce smooth muscle gene expression in nonmuscle cells. Ectopic overexpression of myocardin enlarged the embryonic chick heart. Conversely, repression of the endogenous chick myocardin using antisense oligonucleotides or a dominant negative mutant form of myocardin inhibited cardiogenesis. Together, our data place myocardin as one of the earliest cardiac marker genes for cardiogenesis and support the idea that myocardin plays an essential role in cardiac gene expression and cardiogenesis.

The molecular and functional phenotype of glomerular podocytes reveals key features of contractile smooth muscle cells

The glomerular podocyte is a highly specialized cell, with the ability to ultrafilter blood and support glomerular capillary pressures. However, little is known about either the genetic programs leading to this functionality or the final phenotype. We approached this question utilizing a human conditionally immortalized cell line, which differentiates from a proliferating epithelial phenotype to a differentiated form. We profiled gene expression during several time points during differentiation and grouped the regulated genes into major functional categories. A novel category of genes that was upregulated during differentiation was of smooth muscle-related molecules. We further examined the smooth muscle phenotype and showed that podocytes consistently express the differentiated smooth muscle markers smoothelin and calponin and the specific transcription factor myocardin, both in vitro and in vivo. The contractile contribution of the podocyte to the glomerular capillary is controversial. We demonstrated using two novel techniques that podocytes contract vigorously in vitro when differentiated and in real time were able to demonstrate that angiotensin II treatment decreases monolayer resistance, morphologically correlating with enhanced contractility. We conclude that the mature podocyte in vitro possesses functional apparatus of contractile smooth muscle cells, with potential implications for its in vivo ability to regulate glomerular dynamic and permeability characteristics.

Kruppel-like factor 4, Elk-1, and histone deacetylases cooperatively suppress smooth muscle cell differentiation markers in response to oxidized phospholipids

Phenotypic switching of vascular smooth muscle cells (SMCs), such as increased proliferation, enhanced migration, and downregulation of SMC differentiation marker genes, is known to play a key role in the development of atherosclerosis. However, the factors and mechanisms controlling this process are not fully understood. We recently showed that oxidized phospholipids, including 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine (POVPC), which accumulate in atherosclerotic lesions, are potent repressors of expression of SMC differentiation marker genes in cultured SMCs as well as in rat carotid arteries in vivo. Here, we examined the molecular mechanisms whereby POVPC induces suppression of SMC differentiation marker genes in cultured SMCs. Results showed that POVPC induced phosphorylation of ERK1/2 and Elk-1. The MEK inhibitors U-0126 and PD-98059 attenuated POVPC-induced suppression of smooth muscle (SM) alpha-actin and SM-myosin heavy chain. POVPC also induced expression of Krüppel-like factor 4 (Klf4). Chromatin immunoprecipitation assays revealed that POVPC caused simultaneous binding of Elk-1 and Klf4 to the promoter region of the SM alpha-actin gene. Moreover, coimmunoprecipitation assays showed a physical interaction between Elk-1 and Klf4. Results in Klf4-null SMCs showed that blockade of both Klf4 induction and Elk-1 phosphorylation completely abolished POVPC-induced suppression of SMC differentiation marker genes. POVPC-induced suppression of SMC differentiation marker genes was also accompanied by hypoacetylation of histone H4 at the SM alpha-actin promoter, which was mediated by the recruitment of histone deacetylases (HDACs), HDAC2 and HDAC5. Coimmunoprecipitation assays showed that Klf4 interacted with HDAC5. Results provide evidence that Klf4, Elk-1, and HDACs coordinately mediate POVPC-induced suppression of SMC differentiation marker genes.

Serum-induced phosphorylation of the serum response factor coactivator MKL1 by the extracellular signal-regulated kinase 1/2 pathway inhibits its nuclear localization

Megakaryoblastic leukemia 1 (MKL1) is a myocardin-related coactivator of the serum response factor (SRF) transcription factor, which has an integral role in differentiation, migration, and proliferation. Serum induces RhoA-dependent translocation of MKL1 from the cytoplasm to the nucleus and also causes a rapid increase in MKL1 phosphorylation. We have mapped a serum-inducible phosphorylation site and found, surprisingly, that its mutation causes constitutive localization to the nucleus, suggesting that phosphorylation of MKL1 inhibits its serum-induced nuclear localization. The key site, serine 454, resembles a mitogen-activated protein kinase phosphorylation site, and its modification was blocked by the MEK1 inhibitor U0126, implying that extracellular signal-regulated kinase 1/2 (ERK1/2) is the serum-inducible kinase that phosphorylates MKL1. Previous results indicated that G-actin binding to MKL1 promotes its nuclear export, and we found that MKL1 phosphorylation is required for its binding to actin, explaining its effect on localization. We propose a model in which serum induction initially stimulates MKL1 nuclear localization due to a decrease in G-actin levels, but MKL1 is then downregulated by nuclear export due to ERK1/2 phosphorylation.

Regulation of SRF/CArG-dependent gene transcription during chronic partial obstruction of murine small intestine

Intestinal obstructions lead to a variety of motility disorders. Small intestine smooth muscles undergo dramatic phenotypic changes in response to obstruction, but the underlying molecular mechanisms are unknown. Using RT-PCR, ChIP, Re-ChIP, and Western blots, we examined the effect of small bowel mechanical obstruction on smooth muscle gene expression. Obstruction caused a transient hyperplasia, followed by a prolonged hypertrophic response of small intestine smooth muscle cells. Smooth muscle myosin heavy chain (MHC), alpha-actin, and gamma-actin expression decreased initially, and then increased as hypertrophy developed. Myocardin expression decreased initially and then increased, while kruppel-like factors (KLF)4 and KLF5 expression increased initially, and then decreased. Serum response factor (SRF) expression decreased initially, and then recovered to sham-operated levels as hypertrophy developed. SRF binding to smooth muscle MHC and alpha-actin promoters decreased initially, but then increased above sham-operated levels as hypertrophy developed. Elk-1 binding to smooth muscle myosin heavy chain and alpha-actin promoters increased initially, and then decreased to sham-operated levels as hypertrophy developed. c-fos expression increased initially, which was associated with increased SRF/Elk-1 binding to the c-fos promoter. The Elk-1 phosphorylation inhibitor U-0126 inhibited the increase in c-fos expression. These findings indicate a dynamic response of small intestine smooth muscles to bowel obstruction involving switching between differentiated, proliferative, and hypertrophic phenotypes. These results suggest that changes in the expression and interactions between SRF, myocardin, Elk-1, and c-fos play key roles in the phenotypic switching of small intestine smooth muscles in response to mechanical obstruction.

Transcriptional profiling of the megabladder mouse: a unique model of bladder dysmorphogenesis

Recent studies in our lab identified a mutant mouse model of obstructive nephropathy designated mgb for megabladder. Homozygotic mgb mice (mgb-/-) develop lower urinary tract obstruction in utero due to a lack of bladder smooth muscle differentiation. This defect is the result of a random transgene insertion/translocation into chromosomes 11 and 16. Transcriptional profiling identified a significantly over-expressed cluster of gene products located on the translocated fragment of chromosome 16 including urotensin II-related peptide (Urp), which was shown to be preferentially over-expressed in developing mgb-/- bladders. Pathway analysis of mgb microarray data indicated dysregulation of at least 60 gene products associated with smooth muscle development. In conclusion, the results of this study indicate that the molecular pathways controlling normal smooth muscle development are severely altered in mgb-/- bladders, and provide the first evidence that Urp may play a critical role in bladder smooth muscle development.

FOXO-binding partners: it takes two to tango

Modulation FOXO transcription factor activities can lead to a variety of cellular outputs resulting in changes in proliferation, apoptosis, differentiation and metabolic responses. Although FOXO proteins all contain an identical DNA-binding domain their cellular functions appear to be distinct, as exemplified by differences in the phenotype of Foxo1, Foxo3 and Foxo4 null mutant mice. While some of these differences may be attributable to the differential expression patterns of these transcription factors, many cells and tissues express several FOXO isoforms. Recently it has become clear that FOXO proteins can regulate transcriptional responses independently of direct DNA-binding. It has been demonstrated that FOXOs can associate with a variety of unrelated transcription factors, regulating activation or repression of diverse target genes. The complement of transcription factors expressed in a particular cell type is thus critical in determining the functional end point of FOXO activity. These interactions greatly expand the possibilities for FOXO-mediated regulation of transcriptional programmes. This review details currently described FOXO-binding partners and examines the role of these interactions in regulating cell fate decisions.

Systematic identification of mammalian regulatory motifs' target genes and functions

We developed an algorithm, Lever, that systematically maps metazoan DNA regulatory motifs or motif combinations to sets of genes. Lever assesses whether the motifs are enriched in cis-regulatory modules (CRMs), predicted by our PhylCRM algorithm, in the noncoding sequences surrounding the genes. Lever analysis allows unbiased inference of functional annotations to regulatory motifs and candidate CRMs. We used human myogenic differentiation as a model system to statistically assess greater than 25,000 pairings of gene sets and motifs or motif combinations. We assigned functional annotations to candidate regulatory motifs predicted previously and identified gene sets that are likely to be co-regulated via shared regulatory motifs. Lever allows moving beyond the identification of putative regulatory motifs in mammalian genomes, toward understanding their biological roles. This approach is general and can be applied readily to any cell type, gene expression pattern or organism of interest.

Myocardin regulates expression of contractile genes in smooth muscle cells and is required for closure of the ductus arteriosus in mice

Myocardin (Myocd) is a potent transcriptional coactivator that has been implicated in cardiovascular development and adaptation of the cardiovascular system to hemodynamic stress. To determine the function of myocardin in the developing cardiovascular system, Myocd(F/F)/Wnt1-Cre(+) and Myocd(F/F)/Pax3-Cre(+) mice were generated in which the myocardin gene was selectively ablated in neural crest-derived SMCs populating the cardiac outflow tract and great arteries. Both Myocd(F/F)/Wnt1-Cre(+) and Myocd(F/F)/Pax3-Cre(+) mutant mice survived to birth, but died prior to postnatal day 3 from patent ductus arteriosus (PDA). Neural crest-derived SMCs populating the ductus arteriosus (DA) and great arteries exhibited a cell autonomous block in expression of myocardin-regulated genes encoding SMC-restricted contractile proteins. Moreover, Myocd-deficient vascular SMCs populating the DA exhibited ultrastructural features generally associated with the SMC synthetic, rather than contractile, phenotype. Consistent with these findings, ablation of the Myocd gene in primary aortic SMCs harvested from Myocd conditional mutant mice caused a dramatic decrease in SMC contractile protein expression. Taken together, these data demonstrate that myocardin regulates expression of genes required for the contractile phenotype in neural crest-derived SMCs and provide new insights into the molecular and genetic programs that may underlie PDA.

Myocardin inhibits cellular proliferation by inhibiting NF-kappaB(p65)-dependent cell cycle progression

We previously reported the importance of the serum response factor (SRF) cofactor myocardin in controlling muscle gene expression as well as the fundamental role for the inflammatory transcription factor NF-kappaB in governing cellular fate. Inactivation of myocardin has been implicated in malignant tumor growth. However, the underlying mechanism of myocardin regulation of cellular growth remains unclear. Here we show that NF-kappaB(p65) represses myocardin activation of cardiac and smooth muscle genes in a CArG-box-dependent manner. Consistent with their functional interaction, p65 directly interacts with myocardin and inhibits the formation of the myocardin/SRF/CArG ternary complex in vitro and in vivo. Conversely, myocardin decreases p65-mediated target gene activation by interfering with p65 DNA binding and abrogates LPS-induced TNF-alpha expression. Importantly, myocardin inhibits cellular proliferation by interfering with NF-kappaB-dependent cell-cycle regulation. Cumulatively, these findings identify a function for myocardin as an SRF-independent transcriptional repressor and cell-cycle regulator and provide a molecular mechanism by which interaction between NF-kappaB and myocardin plays a central role in modulating cellular proliferation and differentiation.

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.

Myocardin gene regulatory variants as surrogate markers of cardiac hypertrophy - study in a genetically homogeneous population

Myocardin is thought to contribute to heart hypertrophy as assessed in animal models. The aim of this study was to identify polymorphisms on the myocardin gene and investigate possible relationships with left ventricular structure in human hypertrophic cardiomyopathy (HCM). Eighty-four native Cretan individuals (36 patients with HCM and 48 healthy controls) were examined by direct sequencing and subsequent restriction fragment length polymorphism analysis and six polymorphisms were identified in the promoter region at positions -435T>C (rs758187), -629A>T (rs8071072), -1030C>G (rs1233851), -1069A>G, -1166A>G and -1406G>A (rs976906). Allele and haplotype frequencies were not significantly different between patients and controls. However, patients carrying the [-435C;-629T] allelic variant had decreased left ventricular wall thickness (LVWT, p = 0.020) and left ventricular mass (p = 0.006) as compared with the wild-type genotype. Carrier status of this myocardin promoter allelic variant was also associated with significant lower myocardin mRNA levels in peripheral blood (p = 0.039). Thus, a myocardin promoter allelic variant existing in the normal Cretan population was associated with decreased left ventricular mass in HCM patients and decreased myocardin mRNA levels in peripheral blood. Our results may be limited by the limited sample size, but are strengthened by the genetic homogeneity of the Cretan population. Our data suggest that functional natural myocardin promoter variation might be a genetic factor contributing to inter-individual differences in the development of cardiac hypertrophy.

HeliCis: a DNA motif discovery tool for colocalized motif pairs with periodic spacing

Background

Correct temporal and spatial gene expression during metazoan development relies on combinatorial interactions between different transcription factors. As a consequence, cis-regulatory elements often colocalize in clusters termed cis-regulatory modules. These may have requirements on organizational features such as spacing, order and helical phasing (periodic spacing) between binding sites. Due to the turning of the DNA helix, a small modification of the distance between a pair of sites may sometimes drastically disrupt function, while insertion of a full helical turn of DNA (10–11 bp) between cis elements may cause functionality to be restored. Recently, de novo motif discovery methods which incorporate organizational properties such as colocalization and order preferences have been developed, but there are no tools which incorporate periodic spacing into the model.

Results

We have developed a web based motif discovery tool, HeliCis, which features a flexible model which allows de novo detection of motifs with periodic spacing. Depending on the parameter settings it may also be used for discovering colocalized motifs without periodicity or motifs separated by a fixed gap of known or unknown length. We show on simulated data that it can efficiently capture the synergistic effects of colocalization and periodic spacing to improve detection of weak DNA motifs. It provides a simple to use web interface which interactively visualizes the current settings and thereby makes it easy to understand the parameters and the model structure.

Conclusion

HeliCis provides simple and efficient de novo discovery of colocalized DNA motif pairs, with or without periodic spacing. Our evaluations show that it can detect weak periodic patterns which are not easily discovered using a sequential approach, i.e. first finding the binding sites and second analyzing the properties of their pairwise distances.

MEF2: a central regulator of diverse developmental programs

The myocyte enhancer factor 2 (MEF2) transcription factor acts as a lynchpin in the transcriptional circuits that control cell differentiation and organogenesis. The spectrum of genes activated by MEF2 in different cell types depends on extracellular signaling and on co-factor interactions that modulate MEF2 activity. Recent studies have revealed MEF2 to form an intimate partnership with class IIa histone deacetylases, which together function as a point of convergence of multiple epigenetic regulatory mechanisms. We review the myriad roles of MEF2 in development and the mechanisms through which it couples developmental, physiological and pathological signals with programs of cell-specific transcription.

Modulation of SRF-dependent gene expression by association of SPT16 with MKL1

MKL1 (MRTF-A/MAL) is a member of the myocardin-related transcription factor family that plays a key role in the development and differentiation of smooth muscle cells (SMCs) via activation of serum response factor (SRF)-dependent SMC gene expression. MKL1 associates with SRF and stimulates its transcriptional activity. Here, by performing matrix-assisted laser desorption/ionization-time of flight mass spectrometric analysis combined with in vitro glutathione S-transferase pull-down assay, we identified 4 candidate proteins that associate with MKL1 through the N-terminus region of MKL1. SPT16, ATP citrate lyase, nucleolin and radixin were identified, and the physical and functional interactions between MKL1 and SPT16 were examined. SPT16 is a component of the FACT (facilitating chromatin transcription) complex that allows RNA polymerase II to traverse the nucleosomes. SPT16 associates with MKL1 in vitro and in vivo; moreover, SSRP1, another component of the FACT complex, associates with the N-terminus region of MKL1 in vitro. SPT16 synergistically activates the transcriptional activity of MKL1. These results show that the expression of nucleosomal SRF-dependent genes, including the SMC gene, is activated by MKL1 via activation of SRF and recruitment of the FACT complex.

Oxidized Phospholipids Induce Phenotypic Switching of Vascular Smooth Muscle Cells In Vivo and In Vitro

Atherosclerosis is a vascular disease characterized by lipid deposition and inflammation within the arterial wall. Oxidized phospholipids (oxPLs), such as 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (oxPAPC) and its constituents 1-palmytoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine (POVPC) and 1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphocholine (PGPC) are concentrated within atherosclerotic lesions and are known to be potent proinflammatory mediators. Phenotypic switching of smooth muscle cells (SMCs) plays a critical role in the development, progression, and end-stage clinical consequences of atherosclerosis, yet little is known regarding the effects of specific oxPLs on SMC phenotype. The present studies were focused on determining whether oxPLs regulate expression of SMC differentiation marker genes and the molecular mechanisms involved. Results showed that POVPC and PGPC induced profound suppression of smooth muscle (SM) α-actin and SM myosin heavy chain expression while simultaneously increasing expression of MCP-1, MCP-3, and cytolysin. OxPLs also induced nuclear translocation of Krüppel-like transcription factor 4 (KLF4), a known repressor of SMC marker genes. siRNA targeting of KLF4 nearly blocked POVPC-induced suppression of SMC marker genes, and myocardin. POVPC-induced repression of SMC marker genes was also significantly attenuated in KLF4 knockout SMCs. Taken together, these results suggest a novel role for oxPLs in phenotypic modulation of SMCs and indicate that these effects are dependent on the transcription factor, KLF4. These results may have important novel implications for the mechanisms by which oxPLs contribute to the pathogenesis of atherosclerosis.

Prostatic stromal cells derived from benign prostatic hyperplasia specimens possess stem cell like property

INTRODUCTION

The hyper-proliferative activity of stromal smooth muscle (SM) cells is believed to be responsible for the pathogenesis of benign prostatic hyperplasia (BPH). We have observed that those stromal cells can differentiate into unrelated specialized cells. We thus hypothesize that stromal cells derived from adults prostate specimens may contain adult stem cells. To test this hypothesis, human prostate stromal primary cultures were established and used for characterization of their stem cell properties.

METHODS

Immunoblotting, immunohistochemistry, RT-PCR, and tissue culture techniques were used to characterize the primary cultured human prostate-derived stromal cells for their stem cell and differentiation properties. The plasticity of these stromal cells was analyzed using cell culture and histology techniques.

RESULTS

Primary cultured prostate stromal cells from BPH patient possess polygonal and elongated fibroblast/myofibroblast cellular morphology. They are positive in CD30, CD34, CD44, NSE, CD133, Flt-1, stem cell factor (SCF), and neuron-specific enolase (NSE), but negative in C-Kit, stem cell antigen (SCA), SH2, CD11b. Expression of SM myogenic markers in these cells may be induced by sodium butyrate (NaBu) treatment. Induction to osteogenic and adipogenic differentiation in these cells is also evident.

CONCLUSIONS

Our study on primary stromal cells from BPH patients have yielded many interesting findings that these prostate stroma cells possess: (1) mesenchymal stem cell (MSC) markers; (2) strong proliferative potential; and (3) ability to differentiate or transdifferentiate to myogenic, adipogenic, and osteogenic lineages. These cell preparations may serve as a potential tool for studies in prostate adult stem cell research and the regulation of benign prostatic hyperplasia.

Wide-scale analysis of human functional transcription factor binding reveals a strong bias towards the transcription start site

BACKGROUND

Transcription factors (TF) regulate expression by binding to specific DNA sequences. A binding event is functional when it affects gene expression. Functionality of a binding site is reflected in conservation of the binding sequence during evolution and in over represented binding in gene groups with coherent biological functions. Functionality is governed by several parameters such as the TF-DNA binding strength, distance of the binding site from the transcription start site (TSS), DNA packing, and more. Understanding how these parameters control functionality of different TFs in different biological contexts is a must for identifying functional TF binding sites and for understanding regulation of transcription.

METHODOLOGY/PRINCIPAL FINDINGS

We introduce a novel method to screen the promoters of a set of genes with shared biological function (obtained from the functional Gene Ontology (GO) classification) against a precompiled library of motifs, and find those motifs which are statistically over-represented in the gene set. More than 8,000 human (and 23,000 mouse) genes, were assigned to one of 134 GO sets. Their promoters were searched (from 200 bp downstream to 1,000 bp upstream the TSS) for 414 known DNA motifs. We optimized the sequence similarity score threshold, independently for every location window, taking into account nucleotide heterogeneity along the promoters of the target genes. The method, combined with binding sequence and location conservation between human and mouse, identifies with high probability functional binding sites for groups of functionally-related genes. We found many location-sensitive functional binding events and showed that they clustered close to the TSS. Our method and findings were tested experimentally.

CONCLUSIONS/SIGNIFICANCE

We identified reliably functional TF binding sites. This is an essential step towards constructing regulatory networks. The promoter region proximal to the TSS is of central importance for regulation of transcription in human and mouse, just as it is in bacteria and yeast.

Programming smooth muscle plasticity with chromatin dynamics

Smooth muscle cells (SMCs) possess remarkable phenotypic plasticity that allows rapid adaptation to fluctuating environmental cues. For example, vascular SMCs undergo profound changes in their phenotype during neointimal formation in response to vessel injury or within atherosclerotic plaques. Recent studies have shown that interaction of serum response factor (SRF) and its numerous accessory cofactors with CArG box DNA sequences within promoter chromatin of SMC genes is a nexus for integrating signals that influence SMC differentiation in development and disease. During development, SMC-restricted sets of posttranslational histone modifications are acquired within the CArG box chromatin of SMC genes. These modifications in turn control the chromatin-binding properties of SRF. The histone modifications appear to encode a SMC-specific epigenetic program that is used by extracellular cues to influence SMC differentiation, by regulating binding of SRF and its partners to the chromatin template. Thus, SMC differentiation is dynamically regulated by the interplay between SRF accessory cofactors, the SRF-CArG interaction, and the underlying histone modification program. As such, the inherent plasticity of the SMC lineage offers unique glimpses into how cellular differentiation is dynamically controlled at the level of chromatin within the context of changing microenvironments. Further elucidation of how chromatin regulates SMC differentiation will undoubtedly yield valuable insights into both normal developmental processes and the pathogenesis of several vascular diseases that display detrimental SMC phenotypic behavior.

Smooth muscle cell signal transduction: implications of vascular biology for vascular surgeons

Vascular smooth muscle cells exhibit varied responses after vessel injury and surgical interventions, including phenotypic switching, migration, proliferation, protein synthesis, and apoptosis. Although the source of the smooth muscle cells that accumulate in the vascular wall is controversial, possibly reflecting migration from the adventitia, from the circulating blood, or in situ differentiation, the intracellular signal transduction pathways that control these processes are being defined. Some of these pathways include the Ras-mitogen-activated protein kinase, phosphatidylinositol 3-kinase-Akt, Rho, death receptor-caspase, and nitric oxide pathways. Signal transduction pathways provide amplification, redundancy, and control points within the cell and culminate in biologic responses. We review some of the signaling pathways activated within smooth muscle cells that contribute to smooth muscle cell heterogeneity and development of pathology such as restenosis and neointimal hyperplasia.

Cooperative interactions between CBP and TORC2 confer selectivity to CREB target gene expression

A number of hormones and growth factors stimulate gene expression by promoting the phosphorylation of CREB (P-CREB), thereby enhancing its association with the histone acetylase paralogs p300 and CBP (CBP/p300). Relative to cAMP, stress signals trigger comparable amounts of CREB phosphorylation, but have minimal effects on CRE-dependent transcription. Here, we show that the latent cytoplasmic coactivator TORC2 mediates target gene activation in response to cAMP signaling by associating with CBP/p300 and increasing its recruitment to a subset of CREB target genes. TORC2 is not activated in response to stress signals, however; and in its absence, P-CREB is unable to stimulate CRE-dependent transcription, due to a block in CBP recruitment. The effect of TORC2 on CBP/p300 promoter occupancy appears pivotal because a gain of function mutant CREB polypeptide with increased affinity for CBP restored CRE-mediated transcription in cells exposed to stress signals. Taken together, these results indicate that TORC2 is one of the long sought after cofactors that mediates the differential effects of cAMP and stress pathways on CREB target gene expression.

Myocardin-related transcription factor B is required for normal mouse vascular development and smooth muscle gene expression

Smooth muscle gene expression is required for the proper development and function of multiple organ systems. Expression of smooth muscle genes is critical for contractile function and tissue architectural integrity. One critical transcription factor for smooth muscle gene expression is the Serum Response Factor (SRF). SRF is expressed ubiquitously, but tissue-specific transcriptional regulation is conferred by its binding to cofactors such as myocardin. Myocardin-related transcription factor B (MRTF-B) is a member of a family of genes (Myocardin(Myocd),Myocardin-related transcription factor A(MRTF-A),MRTF-B) that provides tissue-specificity and potentiate SRF-dependent transcription. Unlike myocardin, which is expressed specifically in smooth and cardiac muscle, MRTF-B is expressed in a wide variety of tissues. To examine the function of MRTF-B, we generated mice containing an insertional mutation of MRTF-B. MRTF-B homozygous mutants die in late gestation with vascular defects and liver hemorrhage. At E9.5, MRTF-B is expressed strongly in the septum transversum mesoderm critical for development of the vitelline system that produces the liver sinusoids and portal venous system. MRTF-B deficiency results in defective smooth muscle gene expression in the liver sinusoids, vitelline veins, and yolk sac, which contributes significantly to the lethal phenotype. These data support our hypothesis that MRTF-B has a unique role in regulating smooth muscle genes important for liver, yolk sac, and portal vascular development.

Myocardin-related transcription factors: critical coactivators regulating cardiovascular development and adaptation

The association of transcriptional coactivators with DNA-binding proteins provides an efficient mechanism to expand and modulate genetic information encoded within the genome. Myocardin-related transcription factors (MRTFs), including myocardin, MRTF-A/MKL1/MAL, and MRTF-B/MKL2, comprise a family of related transcriptional coactivators that physically associate with the MADS box transcription factor, serum response factor, and synergistically activate transcription. MRTFs transduce cytoskeletal signals to the nucleus, activating a subset of serum response factor-dependent genes promoting myogenic differentiation and cytoskeletal organization. MRTFs are multifunctional proteins that share evolutionarily conserved domains required for actin-binding, homo- and heterodimerization, high-order chromatin organization, and transcriptional activation. Mice harboring loss-of-function mutations in myocardin, MRTF-A, and MRTF-B, respectively, display distinct phenotypes, including cell autonomous defects in vascular smooth muscle cell and myoepithelial cell differentiation and function. This article reviews the molecular basis of MRTF function with particular focus on the role MRTFs play in regulating cardiovascular patterning, vascular smooth muscle cell and cardiomyocyte differentiation and in the pathogenesis of congenital heart disease and vascular proliferative syndromes.

Steroid receptor coactivator 3 is a coactivator for myocardin, the regulator of smooth muscle transcription and differentiation

Abnormal proliferation of vascular smooth muscle cells (VSMCs) constitutes a key event in atherosclerosis, neointimal hyperplasia, and the response to vascular injury. Estrogen receptor alpha (ERalpha) mediates the protective effects of estrogen in injured blood vessels and regulates ligand-dependent gene expression in vascular cells. However, the molecular mechanisms mediating ERalpha-dependent VSMC gene expression and VSMC proliferation after vascular injury are not well defined. Here, we report that the ER coactivator steroid receptor coactivator 3 (SRC3) is also a coactivator for the major VSMC transcription factor myocardin, which is required for VSMC differentiation to the nonproliferative, contractile state. The N terminus of SRC3, which contains a basic helix-loop-helix/Per-ARNT-Sim protein-protein interaction domain, binds the C-terminal activation domain of myocardin and enhances myocardin-mediated transcriptional activation of VSMC-specific, CArG-containing promoters, including the VSMC-specific genes SM22 and myosin heavy chain. Suppression of endogenous SRC3 expression by specific small interfering RNA attenuates myocardin transcriptional activation in cultured cells. The SRC3-myocardin interaction identifies a site of convergence for nuclear hormone receptor-mediated and VSMC-specific gene regulation and suggests a possible mechanism for the vascular protective effects of estrogen on vascular injury.

Transcription factor CHF1/Hey2 regulates the global transcriptional response to platelet-derived growth factor in vascular smooth muscle cells

The cardiovascular restricted transcription factor CHF1/Hey2 has been previously shown to regulate the smooth muscle response to growth factors. To determine how CHF1/Hey2 affects the smooth muscle response to growth factors, we performed a genomic screen for transcripts that are differentially expressed in wild-type and knockout smooth muscle cells after stimulation with platelet-derived growth factor. We screened 45,101 probes representing >39,000 transcripts derived from at least 34,000 genes, at eight different time points. We analyzed the expression data utilizing an algorithm based on Bayesian statistics to derive the best polynomial clustering model to fit the expression data. We found that in a total of 9,827 transcripts the normalized ratio of knockout to wild-type expression diverged more than threefold from baseline in at least one time point, and these transcripts separated into 17 distinct clusters. Further analysis of each cluster revealed distinct alterations in gene expression patterns for immediate early genes, transcription factors, matrix metalloproteinases, signaling molecules, and other molecules important in vascular biology. Our findings demonstrate that CHF1/Hey2 profoundly affects vascular smooth muscle phenotype by altering both the absolute expression level of a variety of genes and the kinetics of growth factor-induced gene expression.

Smooth muscle cell-specific transcription is regulated by nuclear localization of the myocardin-related transcription factors

On the basis of our previous studies on RhoA signaling in smooth muscle cells (SMC), we hypothesized that RhoA-mediated nuclear translocalization of the myocardin-related transcription factors (MRTFs) was important for regulating SMC phenotype. MRTF-A protein and MRTF-B message were detected in aortic SMC and in many adult mouse organs that contain a large SMC component. Both MRTFs upregulated SMC-specific promoter activity as well as endogenous SM22α expression in multipotential 10T1/2 cells, although to a lesser extent than myocardin. We used enhanced green fluorescent protein (EGFP) fusion proteins to demonstrate that the myocardin factors have dramatically different localization patterns and that the stimulation of SMC-specific transcription by certain RhoA-dependent agonists was likely mediated by increased nuclear translocation of the MRTFs. Importantly, a dominant-negative form of MRTF-A (ΔB1/B2) that traps endogenous MRTFs in the cytoplasm inhibited the SM α-actin, SM22α, and SM myosin heavy chain promoters in SMC and attenuated the effects of sphingosine 1-phosphate and transforming growth factor (TGF)-β on SMC-specific transcription. Our data confirmed the importance of the NH2-terminal RPEL domains for regulating MRTF localization, but our analysis of MRTF-A/myocardin chimeras and myocardin RPEL2 mutations indicated that the myocardin B1/B2 region can override this signal. Gel shift assays demonstrated that myocardin factor activity correlated well with ternary complex formation at the SM α-actin CArGs and that MRTF-serum response factor interactions were partially dependent on CArG sequence. Taken together, our results indicate that the MRTFs regulate SMC-specific gene expression in at least some SMC subtypes and that regulation of MRTF nuclear localization may be important for the effects of selected agonists on SMC phenotype.

Identification of a Unique Transgenic Mouse Line That Develops Megabladder, Obstructive Uropathy, and Renal Dysfunction

Urinary tract malformations, obstructive uropathy, and hypoplasia/dysplasia are extremely important in terms of pediatric health care costs, with end-stage renal failure in children estimated to cost >$15 billion annually in the United States alone. Even so, little is known regarding the mechanisms that control these processes. Identified was a unique mutant mouse model that develops in utero megabladder, resulting in variable hydroureteronephrosis and chronic renal failure secondary to obstructive uropathy. These animals, designated mgb for megabladder, possess a primary defect in bladder smooth muscle development that is apparent by embryonic day 15. The mgb mouse represents an excellent model for the study of normal and pathogenic bladder development, including the postnatal progression of chronic renal failure that results from the development of in utero obstructive uropathy.

Evolutionary-conserved gene expression response profiles across mammalian tissues

Gene expression responses are complex and frequently involve the actions of many genes to effect coordinated patterns. We hypothesized these coordinated responses are evolutionarily conserved and used a comparison of human and mouse gene expression profiles to identify the most prominent conserved features across a set of normal mammalian tissues. Based on data from multiple studies across multiple tissues in human and mouse, 13 gene expression modes across multiple tissues were identified in each of these species using principal component analysis. Strikingly, 1-to-1 pairing of human and mouse modes was observed in 12 out of 13 modes obtained from the two species independently. These paired modes define evolutionarily conserved gene expression response modes (CGEMs). Notably, in this study we were able to extract biological responses that are not overwhelmed by laboratory-to-laboratory or species-to-species variation. Of the variation in our gene expression dataset, 84% can be explained using these CGEMs. Functional annotation was performed using Gene Ontology, pathway, and transcription factor binding site over representation. Our conclusion is that we found an unbiased way of obtaining conserved gene response modes that accounts for a considerable portion of gene expression variation in a given dataset, as well as validates the conservation of major gene expression response modes across the mammals.

Overexpression of Ets-like protein 1 in human esophageal squamous cell carcinoma

AIM: To study the expression pattern of Ets-like protein 1 (Elk-1) in human esophageal squamous cell carcinoma (ESCC) and to analyze its relationship with clinicopathologic parameters.

METHODS: The expression of Elk-1 in fresh esophageal cancer tissues and their corresponding normal mucosae was detected immunohistochemically (IHC) by means of tissue microarray (TMA). Its correlation with clinical characteristics was evaluated and analyzed by univariate analysis. All statistical analyses were performed by SPSS version 13.0.

RESULTS: Expression level of transcription factor Elk-1 increased in 78.5% (84/107) ESCC tissues compared with their matched normal esophageal epithelium. However, the expression of Elk-1 did not show any obvious correlation with degree of differentiation of esophageal carcinoma (in well-differentiated, moderately-differentiated and poorly-differentiated tumors, the increased expression was 7/8, 60/74, and 19/25, respectively, P > 0.05). Moreover, no obvious correlation was found with lymph node metastasis and depth of invasion.

CONCLUSION: Increased expression of transcription factor Elk-1 may play an important role in esophageal carcinogenesis.