MEF2B is a component of a smooth muscle-specific complex that binds an A/T-rich element important for smooth muscle myosin heavy chain gene expression

SWI/SNF Complex in Vascular Smooth Muscle Cells and Its Implications in Cardiovascular Pathologies

Mature vascular smooth muscle cells (VSMC) exhibit a remarkable degree of plasticity, a characteristic that has intrigued cardiovascular researchers for decades. Recently, it has become increasingly evident that the chromatin remodeler SWItch/Sucrose Non-Fermentable (SWI/SNF) complex plays a pivotal role in orchestrating chromatin conformation, which is critical for gene regulation. In this review, we provide a summary of research related to the involvement of the SWI/SNF complexes in VSMC and cardiovascular diseases (CVD), integrating these discoveries into the current landscape of epigenetic and transcriptional regulation in VSMC. These novel discoveries shed light on our understanding of VSMC biology and pave the way for developing innovative therapeutic strategies in CVD treatment.

Reactive Oxygen Species: Modulators of Phenotypic Switch of Vascular Smooth Muscle Cells

Reactive oxygen species (ROS) are natural byproducts of oxygen metabolism in the cell. At physiological levels, they play a vital role in cell signaling. However, high ROS levels cause oxidative stress, which is implicated in cardiovascular diseases (CVD) such as atherosclerosis, hypertension, and restenosis after angioplasty. Despite the great amount of research conducted to identify the role of ROS in CVD, the image is still far from being complete. A common event in CVD pathophysiology is the switch of vascular smooth muscle cells (VSMCs) from a contractile to a synthetic phenotype. Interestingly, oxidative stress is a major contributor to this phenotypic switch. In this review, we focus on the effect of ROS on the hallmarks of VSMC phenotypic switch, particularly proliferation and migration. In addition, we speculate on the underlying molecular mechanisms of these cellular events. Along these lines, the impact of ROS on the expression of contractile markers of VSMCs is discussed in depth. We conclude by commenting on the efficiency of antioxidants as CVD therapies.

Landscape of DNA binding signatures of myocyte enhancer factor-2B reveals a unique interplay of base and shape readout

Myocyte enhancer factor-2B (MEF2B) has the unique capability of binding to its DNA target sites with a degenerate motif, while still functioning as a gene-specific transcriptional regulator. Identifying its DNA targets is crucial given regulatory roles exerted by members of the MEF2 family and MEF2B's involvement in B-cell lymphoma. Analyzing structural data and SELEX-seq experimental results, we deduced the DNA sequence and shape determinants of MEF2B target sites on a high-throughput basis in vitro for wild-type and mutant proteins. Quantitative modeling of MEF2B binding affinities and computational simulations exposed the DNA readout mechanisms of MEF2B. The resulting binding signature of MEF2B revealed distinct intricacies of DNA recognition compared to other transcription factors. MEF2B uses base readout at its half-sites combined with shape readout at the center of its degenerate motif, where A-tract polarity dictates nuances of binding. The predominant role of shape readout at the center of the core motif, with most contacts formed in the minor groove, differs from previously observed protein-DNA readout modes. MEF2B, therefore, represents a unique protein for studies of the role of DNA shape in achieving binding specificity. MEF2B-DNA recognition mechanisms are likely representative for other members of the MEF2 family.

MEF-2 isoforms' (A-D) roles in development and tumorigenesis

Myocyte enhancer factor (MEF)-2 plays a critical role in proliferation, differentiation, and development of various cell types in a tissue specific manner. Four isoforms of MEF-2 (A-D) differentially participate in controlling the cell fate during the developmental phases of cardiac, muscle, vascular, immune and skeletal systems. Through their associations with various cellular factors MEF-2 isoforms can trigger alterations in complex protein networks and modulate various stages of cellular differentiation, proliferation, survival and apoptosis. The role of the MEF-2 family of transcription factors in the development has been investigated in various cell types, and the evolving alterations in this family of transcription factors have resulted in a diverse and wide spectrum of disease phenotypes, ranging from cancer to infection. This review provides a comprehensive account on MEF-2 isoforms (A-D) from their respective localization, signaling, role in development and tumorigenesis as well as their association with histone deacetylases (HDACs), which can be exploited for therapeutic intervention.

MEF2 transcription factors: developmental regulators and emerging cancer genes

The MEF2 transcription factors have roles in muscle, cardiac, skeletal, vascular, neural, blood and immune system cell development through their effects on cell differentiation, proliferation, apoptosis, migration, shape and metabolism. Altered MEF2 activity plays a role in human diseases and has recently been implicated in the development of several cancer types. In particular, MEF2B, the most divergent and least studied protein of the MEF2 family, has a role unique from its paralogs in non-Hodgkin lymphomas. The use of genome-scale technologies has enabled comprehensive MEF2 target gene sets to be identified, contributing to our understanding of MEF2 proteins as nodes in complex regulatory networks. This review surveys the molecular interactions of MEF2 proteins and their effects on cellular and organismal phenotypes. We include a discussion of the emerging roles of MEF2 proteins as oncogenes and tumor suppressors of cancer. Throughout this article we highlight similarities and differences between the MEF2 family proteins, including a focus on functions of MEF2B.

DNA damage induced apoptosis suppressor (DDIAS) is upregulated via ERK5/MEF2B signaling and promotes β-catenin-mediated invasion

DNA damage induced apoptosis suppressor (DDIAS) is an anti-apoptotic protein that promotes cancer cell survival. We previously reported that DDIAS is transcriptionally activated by nuclear factor of activated T cells 2 (NFATc1). However, the upstream regulation of DDIAS expression by growth factors has not been studied. Here, we demonstrate that DDIAS expression is induced by extracellular signal-regulated kinase 5 (ERK5) and myocyte enhancer factor 2B (MEF2B) in response to epidermal growth factor (EGF) and that it positively regulates β-catenin signaling in HeLa cells. The genetic or pharmacological inhibition of ERK5 suppressed DDIAS induction following EGF exposure and the overexpression of constitutively active MEK5 (CA-MEK5) enhanced DDIAS expression. In chromatin immunoprecipitation assays, MEF2B, a downstream target of ERK5, exhibited sequence-specific binding to a MEF2 binding site in the DDIAS promoter following treatment with EGF. The overexpression of MEF2B increased the EGF-mediated induction of DDIAS expression, whereas the knockdown of MEF2B impaired this effect. Furthermore, DDIAS promoted invasion by increasing β-catenin expression at the post-translational level in response to EGF, suggesting that DDIAS plays a crucial role in the metastasis of cancer cells by regulating β-catenin expression. It is unlikely that MEF2B and NFATc1 cooperatively regulate DDIAS transcription in response to EGF. Collectively, EGF activates the ERK5/MEF2 pathway, which in turn induces DDIAS expression to promote cancer cell invasion by activating β-catenin target genes.

MEF2B mutations in non-Hodgkin lymphoma dysregulate cell migration by decreasing MEF2B target gene activation

Myocyte enhancer factor 2B (MEF2B) is a transcription factor with mutation hotspots at K4, Y69 and D83 in diffuse large B-cell lymphoma (DLBCL). To provide insight into the regulatory network of MEF2B, in this study, we analyse global gene expression and DNA-binding patterns. We find that candidate MEF2B direct target genes include RHOB, RHOD, CDH13, ITGA5 and CAV1, and that indirect target genes of MEF2B include MYC, TGFB1, CARD11, MEF2C, NDRG1 and FN1. MEF2B overexpression increases HEK293A cell migration and epithelial-mesenchymal transition, and decreases DLBCL cell chemotaxis. K4E, Y69H and D83V MEF2B mutations decrease the capacity of MEF2B to activate transcription and decrease its' effects on cell migration. The K4E and D83V mutations decrease MEF2B DNA binding. In conclusion, our map of the MEF2B regulome connects MEF2B to drivers of oncogenesis.

MEF2B-Nox1 signaling is critical for stretch-induced phenotypic modulation of vascular smooth muscle cells

OBJECTIVE

Blood vessel hemodynamics have profound influences on function and structure of vascular cells. One of the main mechanical forces influencing vascular smooth muscle cells (VSMC) is cyclic stretch (CS). Increased CS stimulates reactive oxygen species (ROS) production in VSMC, leading to their dedifferentiation, yet the mechanisms involved are poorly understood. This study was designed to test the hypothesis that pathological CS stimulates NADPH oxidase isoform 1 (Nox1)-derived ROS via MEF2B, leading to VSMC dysfunction via a switch from a contractile to a synthetic phenotype.

APPROACH AND RESULTS

Using a newly developed isoform-specific Nox1 inhibitor and gene silencing technology, we demonstrate that a novel pathway, including MEF2B-Nox1-ROS, is upregulated under pathological stretch conditions, and this pathway promotes a VSMC phenotypic switch from a contractile to a synthetic phenotype. We observed that CS (10% at 1 Hz) mimicking systemic hypertension in humans increased Nox1 mRNA, protein levels, and enzymatic activity in a time-dependent manner, and this upregulation was mediated by MEF2B. Furthermore, we show that stretch-induced Nox1-derived ROS upregulated a specific marker for synthetic phenotype (osteopontin), whereas it downregulated classical markers for contractile phenotype (calponin1 and smoothelin B). In addition, our data demonstrated that stretch-induced Nox1 activation decreases actin fiber density and augments matrix metalloproteinase 9 activity, VSMC migration, and vectorial alignment.

CONCLUSIONS

These results suggest that CS initiates a signal through MEF2B that potentiates Nox1-mediated ROS production and causes VSMC to switch to a synthetic phenotype. The data also characterize a new Nox1 inhibitor as a potential therapy for treatment of vascular dysfunction in hypertension.

Contribution of myocyte enhancer factor 2 family transcription factors to BZLF1 expression in Epstein-Barr virus reactivation from latency

Reactivation of Epstein-Barr virus (EBV) from latency is dependent on expression of the viral transactivator BZLF1 protein, whose promoter (Zp) normally exhibits only low basal activity but is activated in response to chemical or biological inducers. Using a reporter assay system, we screened for factors that can activate Zp and isolated genes, including those encoding MEF2B, KLF4, and some cellular b-Zip family transcription factors. After confirming their importance and functional binding sites in reporter assays, we prepared recombinant EBV-BAC, in which the binding sites were mutated. Interestingly, the MEF2 mutant virus produced very low levels of BRLF1, another transactivator of EBV, in addition to BZLF1 in HEK293 cells. The virus failed to induce a subset of early genes, such as that encoding BALF5, upon lytic induction, and accordingly, could not replicate to produce progeny viruses in HEK293 cells, but this restriction could be completely lifted by exogenous supply of BRLF1, together with BZLF1. In B cells, induction of BZLF1 by chemical inducers was inhibited by point mutations in the ZII or the three SP1/KLF binding sites of EBV-BAC Zp, while leaky BZLF1 expression was less affected. Mutation of MEF2 sites severely impaired both spontaneous and induced expression of not only BZLF1, but also BRLF1 in comparison to wild-type or revertant virus cases. We also observed that MEF2 mutant EBV featured relatively high repressive histone methylation, such as H3K27me3, but CpG DNA methylation levels were comparable around Zp and the BRLF1 promoter (Rp). These findings shed light on BZLF1 expression and EBV reactivation from latency.

Sumoylation and regulation of cardiac gene expression

Sumoylation is a posttranslational modification process in which SUMO proteins are covalently and reversibly conjugated to their targets via enzymatic cascade reactions. Since the discovery of SUMO-1 in 1996, the SUMO pathway has garnered increased attention due to its role in a number of important biological activities such as cell cycle progression, epigenetic modulation, signal transduction, and DNA replication/repair, as well as its potential implication in human pathogenesis such as in cancer development and metastasis, neurodegenerative disorders and craniofacial defects. The role of the SUMO pathway in regulating cardiogenic gene activity, development and/or disorders is just emerging. Our review is based on recent advances that highlight the regulation of cardiac gene activity in cardiac development and disease by the SUMO conjugation pathway.

Regulatory polymorphism in transcription factor KLF5 at the MEF2 element alters the response to angiotensin II and is associated with human hypertension

Krüppel-like factor 5 (KLF5) is a zinc-finger-type transcription factor that mediates the tissue remodeling in cardiovascular diseases, such as atherosclerosis, restenosis, and cardiac hypertrophy. Our previous studies have shown that KLF5 is induced by angiotensin II (AII), although the precise molecular mechanism is not yet known. Here we analyzed regulatory single nucleotide polymorphisms (SNPs) within the KLF5 locus to identify clinically relevant signaling pathways linking AII and KLF5. One SNP was located at -1282 bp and was associated with an increased risk of hypertension: subjects with the A/A and A/G genotypes at -1282 were at significantly higher risk for hypertension than those with the G/G genotype. Interestingly, a reporter construct corresponding to the -1282G genotype showed much weaker responses to AII than a construct corresponding to -1282A. Electrophoretic mobility shift, chromatin immunoprecipitation, and reporter assays collectively showed that the -1282 SNP is located within a functional myocyte enhancer factor 2 (MEF2) binding site, and that the -1282G genotype disrupts the site and reduces the AII responsiveness of the promoter. Moreover, MEF2 activation via reactive oxygen species and p38 mitogen-activated protein kinase induced KLF5 expression in response to AII, and KLF5 and MEF2 were coexpressed in coronary atherosclerotic plaques. These results suggest that a novel signaling and transcription network involving MEF2A and KLF5 plays an important role in the pathogenesis of cardiovascular diseases such as hypertension.

Forkhead box M1 transcriptional factor is required for smooth muscle cells during embryonic development of blood vessels and esophagus

The forkhead box m1 (Foxm1 or Foxm1b) transcription factor (previously called HFH-11B, Trident, Win, or MPP2) is expressed in a variety of tissues during embryogenesis, including vascular, airway, and intestinal smooth muscle cells (SMCs). Although global deletion of Foxm1 in Foxm1(-/-) mice is lethal in the embryonic period due to multiple abnormalities in the liver, heart, and lung, the specific role of Foxm1 in SMC remains unknown. In the present study, Foxm1 was deleted conditionally in the developing SMC (smFoxm1(-/-) mice). The majority of smFoxm1(-/-) mice died immediately after birth due to severe pulmonary hemorrhage and structural defects in arterial wall and esophagus. Although Foxm1 deletion did not influence SMC differentiation, decreased proliferation of SMC was found in smFoxm1(-/-) blood vessels and esophagus. Depletion of Foxm1 in cultured SMC caused G(2) arrest and decreased numbers of cells undergoing mitosis. Foxm1-deficiency in vitro and in vivo was associated with reduced expression of cell cycle regulatory genes, including cyclin B1, Cdk1-activator Cdc25b phosphatase, Polo-like 1 and JNK1 kinases, and cMyc transcription factor. Foxm1 is critical for proliferation of smooth muscle cells and is required for proper embryonic development of blood vessels and esophagus.

Social stress in mice induces voiding dysfunction and bladder wall remodeling

Several studies have anecdotally reported the occurrence of altered urinary voiding patterns in rodents exposed to social stress. A recent study characterized the urodynamic and central changes in a rat model of social defeat. Here, we describe a similar voiding phenotype induced in mice by social stress and in addition we describe potential molecular mechanisms underlying the resulting bladder wall remodeling. The mechanism leading to the altered voiding habits and underlying bladder phenotype may be relevant to the human syndrome of dysfunctional voiding which is thought to have a psychological component. To better characterize and investigate social stress-induced bladder wall hypertrophy, FVB mice (6 wk old) were randomized to either social stress or control manipulation. The stress involved repeated cycles of a 1-h direct exposure to a larger aggressive C57Bl6 breeder mouse followed by a 23-h period of barrier separation over 4 wk. Social stress resulted in altered urinary voiding patterns suggestive of urinary retention and increased bladder mass. In vivo cystometry revealed an increased volume at micturition with no change in the voiding pressure. Examination of these bladders revealed increased nuclear expression of the transcription factors MEF-2 and NFAT, as well as increased expression of the myosin heavy chain B isoform mRNA. BrdU uptake was increased within the urothelium and lamina propria layers in the social stress group. We conclude that social stress induces urinary retention that ultimately leads to shifts in transcription factors, alterations in myosin heavy chain isoform expression, and increases in DNA synthesis that mediate bladder wall remodeling. Social stress-induced bladder dysfunction in rodents may provide insight into the underlying mechanisms and potential treatment of dysfunctional voiding in humans.

Gene expression programs of human smooth muscle cells: tissue-specific differentiation and prognostic significance in breast cancers

Smooth muscle is present in a wide variety of anatomical locations, such as blood vessels, various visceral organs, and hair follicles. Contraction of smooth muscle is central to functions as diverse as peristalsis, urination, respiration, and the maintenance of vascular tone. Despite the varied physiological roles of smooth muscle cells (SMCs), we possess only a limited knowledge of the heterogeneity underlying their functional and anatomic specializations. As a step toward understanding the intrinsic differences between SMCs from different anatomical locations, we used DNA microarrays to profile global gene expression patterns in 36 SMC samples from various tissues after propagation under defined conditions in cell culture. Significant variations were found between the cells isolated from blood vessels, bronchi, and visceral organs. Furthermore, pervasive differences were noted within the visceral organ subgroups that appear to reflect the distinct molecular pathways essential for organogenesis as well as those involved in organ-specific contractile and physiological properties. Finally, we sought to understand how this diversity may contribute to SMC-involving pathology. We found that a gene expression signature of the responses of vascular SMCs to serum exposure is associated with a significantly poorer prognosis in human cancers, potentially linking vascular injury response to tumor progression.

Myocyte enhancer factor 2B is involved in the inducible expression of NOX1/NADPH oxidase, a vascular superoxide-producing enzyme

NADPH oxidase is a major source of the superoxide produced in cardiovascular tissues. Expression of NOX1, a catalytic subunit of NADPH oxidase, is induced by various vasoactive factors, including angiotensin II, prostaglandin (PG) F(2alpha) and platelet-derived growth factor (PDGF). To clarify the molecular basis of this transcriptional activation, we delineated the promoter region of the NOX1 gene. RT-PCR and 5'-rapid amplification of cDNA ends-based analyses revealed a novel 5'-terminal exon of the rat NOX1 gene located approximately 28 kb upstream of the exon containing the start codon. Both PGF(2alpha) and PDGF enhanced the transcriptional activity of the - 3.6 kb 5'-flanking region of the NOX1 gene in A7r5 cells, a rat vascular smooth muscle cell line. A PGF(2alpha)-response element was located between -146 and -125 in the 5'-flanking region containing a consensus binding site for myocyte enhancer factor 2 (MEF2), to which binding of MEF2 was augmented by PGF(2alpha). Gene silencing of MEF2B by RNA interference significantly suppressed the expression of NOX1, while silencing of activating transcription factor (ATF)-1, previously implicated in up-regulation of NOX1, abolished the PGF(2alpha)- or PDGF-induced expression of MEF2B. These results indicate that superoxide production in vascular smooth muscle cells is regulated by the ATF-1-MEF2B cascade by induction of the expression of the NOX1 gene.

Transcriptional regulation of tissue-specific genes by the ERK5 mitogen-activated protein kinase

The ERK5 mitogen-activated protein kinase (MAPK) differs from other MAPKs in possessing a potent transcriptional activation domain. ERK5-/- embryos die from angiogenic defects, but the precise physiological role of ERK5 remains poorly understood. To elucidate molecular functions of ERK5 in the development of vasculature and other tissues, we performed gene profile analyses of erk5-/- mouse embryos and erk5-/- fibroblast cells reconstituted with ERK5 or ERK5(1-740), which lacks the transactivation domain. These experiments revealed several potential ERK5 target genes, including a proapoptotic gene bnip3, known angiogenic genes flt1 and lklf (lung Krüppel-like factor), and genes that regulate cardiovascular development. Among these, LKLF, known for its roles in angiogenesis, T-cell quiescence, and survival, was found to be absolutely dependent on ERK5 for expression in endothelial and T cells. We show that ERK5 drives lklf transcription by activating MEF2 transcription factors. Expression of erk5 short hairpin or a dominant-negative form of the ERK5 upstream activator, MEK5, in T cells led to downregulation of LKLF, increased cell size and upregulation of activation markers. Thus, through its kinase and transcriptional activation domains, ERK5 regulates transcriptional responses of cell survival and quiescence critical for angiogenesis and T-cell function.

PIAS1 activates the expression of smooth muscle cell differentiation marker genes by interacting with serum response factor and class I basic helix-loop-helix proteins

Although a critical component of vascular disease is modulation of the differentiated state of vascular smooth muscle cells (SMC), the mechanisms governing SMC differentiation are relatively poorly understood. We have previously shown that E-boxes and the ubiquitously expressed class I basic helix-loop-helix (bHLH) proteins, including E2-2 and E12, are important in regulation of the SMC differentiation marker gene, the SM alpha-actin gene. The aim of the present study was to identify proteins that bind to class I bHLH proteins in SMC and modulate transcriptional regulation of SMC differentiation marker genes. Herein we report that members of the protein inhibitor of activated STAT (PIAS) family interact with class I bHLH factors as well as serum response factor (SRF). PIAS1 interacted with E2-2 and E12 based on yeast two-hybrid screens, mammalian two-hybrid assays, and/or coimmunoprecipitation assays. Overexpression of PIAS1 significantly activated the SM alpha-actin promoter and mRNA expression, as well as SM myosin heavy chain and SM22alpha, whereas a small interfering RNA for PIAS1 decreased activity of these promoters, as well as endogenous mRNA expression, and SRF binding to SM alpha-actin promoter within intact chromatin in cultured SMC. Of significance, PIAS1 bound to SRF and activated SM alpha-actin promoter expression in wild-type but not SRF(-/-) embryonic stem cells. These results provide novel evidence that PIAS1 modulates transcriptional activation of SMC marker genes through cooperative interactions with both SRF and class I bHLH proteins.

Transforming growth factor-beta1-induced expression of smooth muscle marker genes involves activation of PKN and p38 MAPK

Differentiated vascular smooth muscle cells (SMCs) exhibit a work phenotype characterized by expression of several well documented contractile apparatus-associated proteins. However, SMCs retain the ability to de-differentiate into a proliferative phenotype, which is involved in the progression of vascular diseases such as atherosclerosis and restenosis. Understanding the mechanisms involved in maintaining SMC differentiation is critical for preventing proliferation associated with vascular disease. In this study, the molecular mechanisms through which transforming growth factor-beta1 (TGF-beta1) induces differentiation of SMCs were examined. TGF-beta1 stimulated actin re-organization, inhibited cell proliferation, and up-regulated SMC marker gene expression in PAC-1 SMCs. These effects were blocked by pretreatment of cells with either HA1077 or Y-27632, which inhibit the kinases downstream of RhoA. Moreover, TGF-beta1 activated RhoA and its downstream target PKN. Overexpression of active PKN alone was sufficient to increase the transcriptional activity of the promoters that control expression of smooth muscle (SM) alpha-actin, SM-myosin heavy chain, and SM22alpha. In addition, PKN increased the activities of serum-response factor (SRF), GATA, and MEF2-dependent enhancer-reporters. RNA interference-mediated inhibition of PKN abolished TGF-beta1-induced activation of SMC marker gene promoters. Finally, examination of MAPK signaling demonstrated that TGF-beta1 increased the activity of p38 MAPK, which was required for activation of the SMC marker gene promoters. Co-expression of dominant negative p38 MAPK was sufficient to block PKN-mediated activation of the SMC marker gene promoters as well as the serum-response factor, GATA, and MEF2 enhancers. Taken together, these results identify components of an important intracellular signaling pathway through which TGF-beta1 activates PKN to promote differentiation of SMCs.

Regulation of smooth muscle phenotype

Differentiated smooth muscle cells (SMCs) remain highly plastic, enabling them to alter their phenotype in response to environmental and pathologic stimuli. SMCs in vascular pathologies such as atherosclerosis exhibit phenotypes clearly different from those of the mature cells in normal blood vessels. These phenotypically modulated SMCs play an integral role in the development of vascular diseases. This review addresses recent progress in our understanding of the mechanisms that control SMC phenotype during vascular development and in vascular disease. A particular focus is on the transcriptional control programs of the differentiated state of SMCs.

Combinatorial control of smooth muscle-specific gene expression

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

Characterization of the mouse aortic carboxypeptidase-like protein promoter reveals activity in differentiated and dedifferentiated vascular smooth muscle cells

The dedifferentiation and proliferation of vascular smooth muscle cells (VSMCs) contribute to the formation of vascular lesions. In this study, the regulation of aortic carboxypeptidase-like protein (ACLP) expression in VSMCs was investigated. After mouse carotid injury, the expression of ACLP increases in the dedifferentiated VSMCs of the neointima in a pattern that differs from that of smooth muscle alpha-actin. To better understand the regulation of ACLP in VSMCs, we characterized the 21-exon mouse ACLP gene and 5'-flanking region and examined its promoter activity. In transient transfection assays, 2.5 kb of the ACLP 5'-flanking sequence directed high levels of luciferase reporter activity in primary cultured rat aortic smooth muscle cells, and this activity was not dependent on serum response factor. We identified a positive element between base pairs -156 and -122 by analysis of 5' deletion and mutant constructs. By use of electrophoretic mobility shift assays with rat aortic smooth muscle cell nuclear extracts, Sp1 and Sp3 transcription factors bound to this region, and transfection assays in D.Mel.2 cells revealed that both Sp1 and Sp3 transactivated the ACLP promoter. Transgenic mice harboring the -2.5-kb ACLP promoter upstream from a nuclear-targeted LacZ gene were generated, and expression was detected in the VSMCs of large blood vessels, arterioles, and veins. Interestingly, ACLP promoter-LacZ reporter activity increased within the neointimal VSMCs of injured carotid vessels, consistent with the expression of the endogenous ACLP protein. The ACLP promoter may provide a novel tool to target gene expression to dedifferentiated VSMCs.

Differential binding of an SRF/NK-2/MEF2 transcription factor complex in normal versus neoplastic smooth muscle tissues

The malignant potential of smooth muscle tumors correlates strongly with the disappearance of gamma-smooth muscle isoactin, a lineage-specific marker of smooth muscle development. In this paper, we identify a 36-base pair regulatory motif containing an AT-rich domain, CArG box, and a non-canonical NK-2 homeodomain-binding site that has the capacity to regulate smooth muscle-specific gene expression in cultured intestinal smooth muscle cells. Serum-response factor associates with an NK-2 transcription factor via protein-protein interactions and binds to the core CArG box element. Our studies suggest that the NK-2 transcription factor that associates with serum-response factor during smooth muscle differentiation is Nkx2-3. Myocyte-specific enhancer factor 2 binding to this regulatory complex was also observed but limited to uterine smooth muscle tissues. Smooth muscle neoplasms displayed altered transcription factor binding when compared with normal myometrium. Differential nuclear accessibility of serum-response factor protein during smooth muscle differentiation and neoplastic transformation was also observed. Thus, we have identified a unique regulatory complex whose differential binding properties and nuclear accessibility are associated with modulating gamma-smooth muscle isoactin-specific gene expression in both normal and neoplastic tissues.

Identification of myogenesis-dependent transcriptional enhancers in promoter region of mouse gamma-sarcoglycan gene

Four sarcoglycan subunit proteins, alpha-, beta-, gamma- and delta-sarcoglycans, form a complex on the skeletal muscle cell surface membrane and a gene defect in any one of them causes the loss or marked decrease of whole sarcoglycan complex, resulting in an autosomal recessive muscular dystrophy, sarcoglycanopathy. To characterize the regulation of sarcoglycan transcription during myocyte differentiation, we isolated the promoter regions for all sarcoglycan transcripts and measured the level of transcriptional activity of these promoter regions in the C2C12 skeletal muscle cell line. The promoters of gamma-sarcoglycan and one of two promoters of alpha-sarcoglycan exhibited marked transcriptional activation following differentiation to myotubes. Then, we characterized the 1.5-kb region of the gamma-sarcoglycan promoter by generating reporter-constructs having various deletions and measuring their transcriptional activities. In this promoter, we identified a basal promoter region and two enhancer regions dependent on differentiation. We also showed that A/T-rich and E box elements in the upstream enhancer region are essential for the activation of gamma-sarcoglycan transcription following myotube formation. Furthermore, from the identification of binding proteins to these elements together with the cotransfection experiments with the gamma-sarcoglycan promoter reporter construct and cDNAs encoding these binding factors to 10T1/2 fibroblast cell line, it was suggested that MyoD directs the transcription of gamma-sarcoglycan gene as one of the trans activators.

Nitric oxide regulates smooth-muscle-specific myosin heavy chain gene expression at the transcriptional level-possible role of SRF and YY1 through CArG element

Nitric oxide (NO) plays an important role in vascular regulation through its vasodilatory, antiatherogenic, and antithrombotic properties. NO inhibits platelet adhesion and aggregation and modulates smooth muscle cell (SMC) proliferation and migration. In animals with experimentally induced vascular injury, ec-NOS gene transfection not only restored NO production to normal levels but also increased vascular reactivity of the injured vessels. However, it is unclear whether NO regulates smooth-muscle-specific gene expression. We report here that addition of PDGF-BB to vascular smooth muscle cells suppressed SM-MHC expression but treatment with the NO donors FK409 and SNAP restored SM-MHC mRNA/protein expression. In vitro transfection and subsequent CAT assays demonstrated that exogenous NO can restore PDGF-BB-induced suppression of SM-MHC promoter activity. Promoter deletion analysis revealed that a CArG-3 box located at -1276 bp in the SM-MHC promoter was important for NO-dependent promoter regulation and as well as high level promoter activity. Gel mobility shift assays showed that CArG-3 contained the SRF binding site and a binding site for YY1, a nuclear factor which acts as a negative regulator on muscle-specific promoters. Interestingly, NO donor FK409 reduced YY1 binding to the CArG-3 element but increased SRF binding, suggesting that these two factors bind competitively to the overlapping sites. We also found that mutation to the YY1 binding site in the CArG-3 element resulted in a loss of PDGF-BB-induced suppression of the SM-MHC promoter activity. These findings indicate that NO regulates SM-MHC gene expression at the transcriptional level at least partially through the regulation of transcription factor binding activities on the CArG element. Thus we propose that NO plays a positive role in maintaining the differentiated state of VSMCs.

An overlapping CArG/octamer element is required for regulation of desmin gene transcription in arterial smooth muscle cells

The desmin gene encodes an intermediate filament protein that is present in skeletal, cardiac, and smooth muscle cells. This study shows that the 4-kb upstream region of the murine desmin promoter directs expression of a lacZ reporter gene throughout the heart from E7.5 and in skeletal muscle and vascular smooth muscle cells from E9. 5. The distal fragment (-4005/-2495) is active in arterial smooth muscle cells but not in venous smooth muscle cells or in the heart in vivo. It contains a CArG/octamer overlapping element (designated CArG4) that can bind the serum response factor (SRF) and an Oct-like factor. The desmin distal fragment can replace a SM22alpha regulatory region (-445/-126) that contains two CArG boxes, to cis-activate a minimal (-125/+65) SM22alpha promoter fragment in arterial smooth muscle cells of transgenic embryos. lacZ expression was abolished when mutations were introduced into the desmin CArG4 element that abolished the binding of SRF and/or Oct-like factor. These data suggest that a new type of combined CArG/octamer element plays a prominent role in the regulation of the desmin gene in arterial smooth muscle cells, and SRF and Oct-like factor could cooperate to drive specific expression in these cells.

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

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

Fibroblast growth factor plays a critical role in SM22alpha expression during Xenopus embryogenesis

Although smooth muscle cells (SMCs) are critical components of the circulatory system, the regulatory mechanisms of SMC differentiation remain largely unknown. In the present study, we examined the mechanism of SMC differentiation by using Xenopus laevis SM22alpha (XSM22alpha) as an SMC-specific marker. XSM22alpha cDNA contained a 600-bp open reading frame, and the predicted amino acid sequences were highly conserved in evolution. XSM22alpha transcripts were first detected in heart anlage, head mesenchyme, and the dorsal side of the lateral plate mesoderm at the tail-bud stage, possibly representing the precursors of muscle lineage. At the tadpole stage, XSM22alpha transcripts were restricted to the vascular and visceral SMCs. XSM22alpha was strongly induced by basic fibroblast growth factor (FGF) in animal caps. Although expressions of Xenopus cardiac actin were not affected by the expression of a dominant-negative FGF receptor, its injection dramatically suppressed the XSM22alpha expression. These results suggest that XSM22alpha is a useful molecular marker for the SMC lineage in Xenopus and that FGF signaling plays an important role in the induction of XSM22alpha and in the differentiation of SMCs.

Crystal structure of MEF2A core bound to DNA at 1.5 A resolution

Members of the myocyte enhancer factor-2 (MEF2) family of transcription factors bind to and activate transcription through A+T-rich DNA sequences found primarily, but not exclusively, in the promoters of muscle-specific genes. Their importance has been established for myogenic development and in activation of the immediate-early gene, c-jun, and recently further functional roles in the immune system have emerged. The MEF2 factors belong to the MADS-box superfamily, sharing homology in a 58 amino acid domain that mediates DNA binding and dimerization. The structures of two MADS-box proteins, SRF and MCM1, bound to their cognate DNA have been previously reported and shown to share extensive similarity in their mode of DNA binding. We have solved the structure of MEF2A 2-78 bound to its DNA consensus sequence at 1.5 A resolution. It reveals how the absence of amino acids N-terminal to the MADS-box contributes to the DNA binding properties of MEF2 proteins and shows that the MEF domain C-terminal to the MADS-box adopts a conformation considerably different from the same region in SRF and MCM1.

Age dependence of smooth muscle myosin expression by cultured rat aortic smooth muscle cells

Vascular smooth muscle cells (SMC) in vivo are highly heterogeneous phenotypically, particularly during development and in the adult during periods of remodeling. Much remains to be learned, however, regarding regulation of the SMC phenotype at the gene level. Here, we studied smooth muscle myosin heavy chain (SMMHC) expression at the transcriptional and mRNA levels in SMC cultured from newborn, adult, and old animals, which express different patterns of differentiation markers. We also examined regulation of SMMHC gene expression by TGF-beta, a cytokine known to be involved in the differentiation process. The activity of SMMHC promoter constructs, the expression of which is smooth-muscle-specific, was greatest in SMC from newborn animals and least in cells from old animals. Thus, differences in the degree of differentiation of SMC from these three sources may at least in part be due to transcriptional events. SMC from the three animal sources each contained mRNAs for the SM-1A and SM-2A tail but not those for the SM-1B and SM-2B head isoforms. Total SMMHC mRNA levels reflected similar differences as found at the transcriptional level. SM-2A mRNA as a proportion of total SMMHC mRNA was greatest in SMC from newborn animals, consistent with their higher degree of differentiation. TGF-beta up-regulated both transcription and mRNA levels but did not change the proportions of SMMHC mRNAs. Though the levels of transcriptional activity and mRNA were widely different in untreated cells, the degree of TGF-beta stimulation was approximately the same in all cases.

Vascular cell adhesion molecule-1 gene expression during human smooth muscle cell differentiation is independent of NF-kappaB activation

Vascular cell adhesion molecule-1 (VCAM-1) gene expression in cytokine-activated cells depends on two kappaB elements. Since VCAM-1 expression appears developmentally regulated and cytokine-inducible in smooth muscle cells (SMCs), we have studied the role of NF-kappaB in differentiated SMC VCAM-1 expression. Confluent SMCs were cultured either in a serum-free medium in order to induce differentiation, or in medium with serum, stimulated or not by tumor necrosis factor alpha (TNF-alpha). The expression of smooth muscle myosin heavy chain, a SMC marker, and VCAM-1 was induced concomitantly in serum-free medium, whereas only VCAM-1 expression was induced by cytokine-treatment. We showed that the p50 and p65 subunits of NF-kappaB were localized in the cytoplasm of differentiating SMCs, whereas they were translocated into the nucleus of TNF-alpha-activated SMCs. Electrophoretic mobility shift assays with VCAM-1 gene kappaB elements failed to detect any induction of DNA-protein complex with nuclear extracts of differentiating SMCs in contrast to the cytokine-activated SMC nuclear extracts. Furthermore, VCAM-1 mRNA induction was inhibited in TNF-alpha-stimulated SMCs, but not in differentiating SMCs, by pyrrolidine dithiocarbamate, an inhibitor of NF-kappaB protein activation. Taken together, these findings suggest that in contrast to TNF-alpha activation, NF-kappaB is not involved in VCAM-1 gene expression during SMC differentiation.

Transcriptional regulation of smooth muscle contractile apparatus expression

The transcriptional regulatory mechanisms that control gene expression during differentiation and contractile protein accumulation are becoming well understood in skeletal and cardiac muscle lineages. Current understanding of smooth muscle-specific gene transcription is much more limited, though recent studies have begun to shed light on this topic. In this review, we summarize some of the themes emerging from these studies and identify transcriptional regulatory elements common to several smooth muscle genes. These include potential binding sites for serum response factor, Sp1, AP2, Mhox, and YY1, as well as a potential transforming growth factor-beta control element. We speculate that it may be possible to manipulate smooth muscle-specific gene expression in asthma or pulmonary arterial hypertension as an eventual therapy.