Interaction of CArG elements and a GC-rich repressor element in transcriptional regulation of the smooth muscle myosin heavy chain gene in vascular smooth muscle cells

FAK Activation Promotes SMC Dedifferentiation via Increased DNA Methylation in Contractile Genes

Rationale: Vascular smooth muscle cells (SMCs) exhibit remarkable plasticity and can undergo dedifferentiation upon pathological stimuli associated with disease and interventions. Objective: Although epigenetic changes are critical in SMC phenotype switching, a fundamental regulator that governs the epigenetic machineries regulating the fate of SMC phenotype has not been elucidated. Methods and Results: Using SMCs, mouse models, and human atherosclerosis specimens, we found that focal adhesion kinase (FAK) activation elicits SMC dedifferentiation by stabilizing DNA methyltransferase 3A (DNMT3A). FAK in SMCs is activated in the cytoplasm upon serum stimulation in vitro or vessel injury and active FAK prevents DNMT3A from nuclear FAK-mediated degradation. However, pharmacological or genetic FAK catalytic inhibition forced FAK nuclear localization, which reduced DNMT3A protein via enhanced ubiquitination and proteasomal degradation. Reduced DNMT3A protein led to DNA hypomethylation in contractile gene promoters, which increased SMC contractile protein expression. RNA sequencing identified SMC contractile genes as a foremost upregulated group by FAK inhibition from injured femoral artery samples compared to vehicle group. DNMT3A knockdown in injured arteries reduced DNA methylation and enhanced contractile gene expression supports the notion that nuclear FAK-mediated DNMT3A degradation via E3 ligase TRAF6 drives differentiation of SMCs. Furthermore, we observed that SMCs of human atherosclerotic lesions exhibited decreased nuclear FAK, which was associated with increased DNMT3A levels and decreased contractile gene expression. Conclusions: This study reveals that nuclear FAK induced by FAK catalytic inhibition specifically suppresses DNMT3A expression in injured vessels resulting in maintaining SMC differentiation by promoting the contractile gene expression. Thus, FAK inhibitors may provide a new treatment option to block SMC phenotypic switching during vascular remodeling and atherosclerosis.

MicroRNA-124 controls human vascular smooth muscle cell phenotypic switch via Sp1

Phenotypic switch of vascular smooth muscle cells (VSMCs) plays an important role in the pathogenesis of atherosclerosis and aortic dissection. However, the mechanisms of phenotypic modulation are still unclear. MicroRNAs have emerged as important regulators of VSMC function. We recently found that microRNA-124 (miR-124) was downregulated in proliferative vascular diseases that were characterized by a VSMC phenotypic switch. Therefore, we speculated that the aberrant expression of miR-124 might play a critical role in human aortic VSMC phenotypic switch. Using quantitative RT-PCR, we found that miR-124 was dramatically downregulated in the aortic media of clinical specimens of the dissected aorta and correlated with molecular markers of the contractile VSMC phenotype. Overexpression of miR-124 by mimicking transfection significantly attenuated platelet-derived growth factor-BB-induced human aortic VSMC proliferation and phenotypic switch. Furthermore, we identified specificity protein 1 (Sp1) as the downstream target of miR-124. A luciferase reporter assay was used to confirm direct miR-124 targeting of the 3'-untranslated region of the Sp1 gene and repression of Sp1 expression in human aortic VSMCs. Furthermore, constitutively active Sp1 in miR-124-overexpressing VSMCs reversed the antiproliferative effects of miR-124. These results demonstrated a novel mechanism of miR-124 modulation of VSMC phenotypic switch by targeting Sp1 expression.NEW & NOTEWORTHY Previous studies have demonstrated that miR-124 is involved in the proliferation of a variety of cell types. However, miRNAs are expressed in a tissue-specific manner. We first identified miR-124 as a critical regulator in human aortic vascular smooth muscle cell differentiation, proliferation, and phenotype switch by targeting the 3'-untranslated region of specificity protein 1.

Noncoding RNAs in smooth muscle cell homeostasis: implications in phenotypic switch and vascular disorders

Vascular smooth muscle cells (SMC) are a highly specialized cell type that exhibit extraordinary plasticity in adult animals in response to a number of environmental cues. Upon vascular injury, SMC undergo phenotypic switch from a contractile-differentiated to a proliferative/migratory-dedifferentiated phenotype. This process plays a major role in vascular lesion formation and during the development of vascular remodeling. Vascular remodeling comprises the accumulation of dedifferentiated SMC in the intima of arteries and is central to a number of vascular diseases such as arteriosclerosis, chronic obstructive pulmonary disease or pulmonary hypertension. Therefore, it is critical to understand the molecular mechanisms that govern SMC phenotype. In the last decade, a number of new classes of noncoding RNAs have been described. These molecules have emerged as key factors controlling tissue homeostasis during physiological and pathological conditions. In this review, we will discuss the role of noncoding RNAs, including microRNAs and long noncoding RNAs, in the regulation of SMC plasticity.

The short and long of noncoding sequences in the control of vascular cell phenotypes

The two principal cell types of importance for normal vessel wall physiology are smooth muscle cells and endothelial cells. Much progress has been made over the past 20 years in the discovery and function of transcription factors that coordinate proper differentiation of these cells and the maintenance of vascular homeostasis. More recently, the converging fields of bioinformatics, genomics, and next generation sequencing have accelerated discoveries in a number of classes of noncoding sequences, including transcription factor binding sites (TFBS), microRNA genes, and long noncoding RNA genes, each of which mediates vascular cell differentiation through a variety of mechanisms. Alterations in the nucleotide sequence of key TFBS or deviations in transcription of noncoding RNA genes likely have adverse effects on normal vascular cell phenotype and function. Here, the subject of noncoding sequences that influence smooth muscle cell or endothelial cell phenotype will be summarized as will future directions to further advance our understanding of the increasingly complex molecular circuitry governing normal vascular cell differentiation and how such information might be harnessed to combat vascular diseases.

Expression of conventional and novel glucose transporters, GLUT1, -9, -10, and -12, in vascular smooth muscle cells

Intimal hyperplasia is characterized by exaggerated proliferation of vascular smooth muscle cells (VSMCs). Enhanced VSMC growth is dependent on increased glucose uptake and metabolism. Facilitative glucose transporters (GLUTs) are comprised of conventional GLUT isoforms (GLUT1-5) and novel GLUT isoforms (GLUT6-14). Previous studies demonstrate that GLUT1 overexpression or GLUT10 downregulation contribute to phenotypic changes in VSMCs. To date, the expression profile of all 14 GLUT isoforms has not been fully examined in VSMCs. Using the proliferative and differentiated phenotypes of human aortic VSMCs, the present study has determined the relative abundance of GLUT1-14 mRNAs by quantitative real-time PCR analysis. Twelve GLUT mRNAs excluding GLUT7 and GLUT14 were detectable in VSMCs. In the proliferative phenotype, the relative abundance of key GLUT mRNAs was GLUT1 (∼43%)>GLUT10 (∼26%)>GLUT9 (∼13%)>GLUT12 (∼4%), whereas in the differentiated phenotype the relative abundance was GLUT10 (∼28%)>GLUT1 (∼25%)>GLUT12 (∼20%)>GLUT9 (∼14%), together constituting 86-87% of total GLUT transcripts. To confirm the expression of key GLUT proteins, immunoblot and immunocytochemical analyses were performed using GLUT isoform-specific primary antibodies. The protein bands characteristic of GLUT1, -9, -10, and -12 were detected in VSMCs in parallel with respective positive controls. In particular, GLUT1 protein expression showed different molecular forms representative of altered glycosylation. While GLUT1 protein displayed a predominant distribution in the plasma membrane, GLUT9, -10, and -12 proteins were mostly distributed in the intracellular compartments. The present study provides the first direct evidence for GLUT9 and GLUT12 expression in VSMCs in conjunction with the previously identified GLUT1 and GLUT10.

CORECLUST: identification of the conserved CRM grammar together with prediction of gene regulation

Identification of transcriptional regulatory regions and tracing their internal organization are important for understanding the eukaryotic cell machinery. Cis-regulatory modules (CRMs) of higher eukaryotes are believed to possess a regulatory ‘grammar’, or preferred arrangement of binding sites, that is crucial for proper regulation and thus tends to be evolutionarily conserved. Here, we present a method CORECLUST (COnservative REgulatory CLUster STructure) that predicts CRMs based on a set of positional weight matrices. Given regulatory regions of orthologous and/or co-regulated genes, CORECLUST constructs a CRM model by revealing the conserved rules that describe the relative location of binding sites. The constructed model may be consequently used for the genome-wide prediction of similar CRMs, and thus detection of co-regulated genes, and for the investigation of the regulatory grammar of the system. Compared with related methods, CORECLUST shows better performance at identification of CRMs conferring muscle-specific gene expression in vertebrates and early-developmental CRMs in Drosophila.

Thrombin induces nestin expression via the transactivation of EGFR signalings in rat vascular smooth muscle cells

Regulation of nestin gene expression is largely unknown despite that it is widely used as a progenitor cell marker. In this study, we showed that nestin expression is regulated by the thrombin-mediated EGFR transactivation in serum-deprived primary cultures of rat vascular smooth muscle cells (VSMCs). This resulted from the direct binding of thrombin to PAR-1 rather than indirectly affecting through the binding to thrombomodulin, as demonstrated by thrombomodulin RNAi. In this process, the PAR-1-induced c-Src plays a critical role through two routes; one was the direct intracellular phosphorylation of EGFR and the other was the extracellular activation of the MMP-2-mediated shedding of HB-EGF. The transactivated EGFR then led to the downstream Ras-Raf-ERK signaling axis, but not the p38 or JNK pathways. In addition, the EMSA experiment showed that the transcriptional factor Sp1 is critical for the thrombin-induced nestin expression in rat VSMCs. Furthermore, RNAi of nestin attenuated the thrombin-induced cell proliferation, indicating that thrombin-induced nestin expression and cell proliferation share the same EGFR transactivation mechanism. This study also suggested that nestin may play an important role in cell proliferation induced by the thrombin-mediated EGFR transactivation.

Genome-wide analysis of transcription factor binding sites based on ChIP-Seq data

Molecular interactions between protein complexes and DNA carry out essential gene regulatory functions. Uncovering such interactions by means of chromatin-immunoprecipitation coupled with massively parallel sequencing (ChIP-Seq) has recently become the focus of intense interest. We here introduce QuEST (Quantitative Enrichment of Sequence Tags), a powerful statistical framework based on the Kernel Density Estimation approach, which utilizes ChIP-Seq data to determine positions where protein complexes come into contact with DNA. Using QuEST, we discovered several thousand binding sites for the human transcription factors SRF, GABP and NRSF at an average resolution of about 20 base-pairs. MEME-based motif analyses on the QuEST-identified sequences revealed DNA binding by cofactors of SRF, providing evidence that cofactor binding specificity can be obtained from ChIP-Seq data. By combining QuEST analyses with gene ontology (GO) annotations and expression data, we illustrate how general functions of transcription factors can be inferred.

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.

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.

Multiple repressor pathways contribute to phenotypic switching of vascular smooth muscle cells

Smooth muscle cell (SMC) differentiation is an essential component of vascular development and these cells perform biosynthetic, proliferative, and contractile roles in the vessel wall. SMCs are not terminally differentiated and possess the ability to modulate their phenotype in response to changing local environmental cues. The focus of this review is to provide an overview of the current state of knowledge of molecular mechanisms involved in controlling phenotypic switching of SMC with particular focus on examination of processes that contribute to the repression of SMC marker genes. We discuss the environmental cues which actively regulate SMC phenotypic switching, such as platelet-derived growth factor-BB, as well as several important regulatory mechanisms required for suppressing expression of SMC-specific/selective marker genes in vivo, including those dependent on conserved G/C-repressive elements, and/or highly conserved degenerate CArG elements found in the promoters of many of these marker genes. Finally, we present evidence indicating that SMC phenotypic switching involves multiple active repressor pathways, including Krüppel-like zinc finger type 4, HERP, and ERK-dependent phosphorylation of Elk-1 that act in a complementary fashion.

Loss of Sp1 function via inhibitory phosphorylation in antifolate-resistant human leukemia cells with down-regulation of the reduced folate carrier

The reduced folate carrier (RFC) is the dominant influx transporter for antifolates. A major mechanism of antifolate resistance is loss of RFC (SLC19A1) gene expression due to decreased GC-box-dependent transcription. However, despite the poor GC-box binding in multiple antifolate-resistant cell lines, normal Sp1 levels were retained. Here we explored the post-translational modifications that may disrupt Sp1 function. Phospho-affinity purification of nuclear proteins revealed that resistant cells contained approximately 8-fold more phosphorylated Sp1 than parental cells; treatment of nuclear proteins from these cells with alkaline phosphatase restored GC-box binding. As protein kinase A phosphorylates Sp1, resistant cells were treated with various cAMP-reactive agents, revealing no apparent effect on GC-box binding except for the general phosphodiesterase inhibitor IBMX. As cGMP levels also may be affected by IBMX, resistant cells were treated with 8-pCPT-cGMP, resulting in the complete restoration of GC-box binding, luciferase reporter activity, and RFC mRNA levels. This restoration was abolished in the presence of the protein phosphatase 2A inhibitor (PP2A) okadaic acid. Importantly, whereas resistant cells showed multiple phosphorylated Sp1 forms barely detectable in parental cells, treatment with 8-pCPT-cGMP resulted in their elimination; this disappearance, however, was prevented by the copresence of okadaic acid. These findings provide the first evidence that loss of RFC gene expression in antifolate-resistant cells is associated with an inhibitory Sp1 phosphorylation that can be eliminated by a cGMP-dependent activation of PP2A.

COX-2 inhibitors suppress integrin alpha5 expression in human lung carcinoma cells through activation of Erk: involvement of Sp1 and AP-1 sites

Tumor cell expression of COX-2 has been implicated in the progression of murine and human lung cancer. Inhibition of COX-2 by nonsteroidal antiinflammatory drugs reduces the risk of cancer development in humans and suppresses tumor growth in animal models. However, the underlying mechanisms for this beneficial effect are not fully understood. Here we explore the potential link between the anticancer effects of COX-2 inhibitors and the expression of the integrin alpha5beta1. Expression of this integrin in carcinoma cells is associated with invasiveness and malignant progression. This, together with our studies showing that fibronectin, the ligand of alpha5beta1, stimulates the growth of human lung carcinoma cells, and that this effect is mediated through alpha5beta1-dependent signals, has prompted us to examine the effects of COX-2 inhibitors on alpha5beta1 expression in human non small cell lung carcinoma (NSCLC) cells. We found that the selective COX-2 inhibitors NS398 and Nimesulide decreased mRNA expression and protein production of the integrin alpha5 subunit. This effect was associated with inhibition of NSCLC cell adhesion to fibronectin. The COX-2 inhibitors triggered the phosphorylation of extracellular signal-regulated kinase (Erk) in a time-dependent manner, and the inhibitor of Mek-1/Erk PD98095 prevented their inhibitory effects on integrin alpha5 expression. Transient transfection assays showed that the COX-2 inhibitors affected integrin alpha5 gene transcription by acting between -92 to -41 bp of the human integrin alpha5 gene promoter. Gel mobility shift assays showed that the COX-2 inhibitors increased Sp1 DNA binding, but decreased that of AP-1. These effects were accompanied by an increase in Sp1 protein and a decrease in c-Jun protein expression, as well as inhibition of SAPK/JNK phosphorylation. The Sp1 inhibitor, Mithramycin A, also blocked the inhibitory effect of the COX-2 inhibitors on alpha5 expression and promoter activity. Overall, these findings suggest that COX-2 inhibitors suppress alpha5beta1 integrin expression in NSCLC through effects on integrin alpha5 gene transcription mediated by Erk activation, increased Sp1, decreased AP-1 DNA binding and inactivation of SAPK/JNK signals. Our observations unveil a new mechanism of action against NSCLC for COX-2 inhibitors that relates to regulation of integrin alpha5 gene expression and, consequently, recognition of extracellular matrices (i.e., fibronectin) by tumor cells. (c) 2005 Wiley-Liss, Inc.

5' CArG degeneracy in smooth muscle alpha-actin is required for injury-induced gene suppression in vivo

CC(A/T)6GG-dependent (CArG-dependent) and serum response factor-dependent (SRF-dependent) mechanisms are required for gene expression in smooth muscle cells (SMCs). However, an unusual feature of many SMC-selective promoter CArG elements is that they contain a conserved single G or C substitution in their central A/T-rich region, which reduces binding affinity for ubiquitously expressed SRF. We hypothesized that this CArG degeneracy contributes to cell-specific expression of smooth muscle alpha-actin in vivo, since substitution of c-fos consensus CArGs for the degenerate CArGs resulted in relaxed specificity in cultured cells. Surprisingly, our present results show that these substitutions have no effect on smooth muscle-specific transgene expression during normal development and maturation in transgenic mice. However, these substitutions significantly attenuated injury-induced downregulation of the mutant transgene under conditions where SRF expression was increased but expression of myocardin, a smooth muscle-selective SRF coactivator, was decreased. Finally, chromatin immunoprecipitation analyses, together with cell culture studies, suggested that myocardin selectively enhanced SRF binding to degenerate versus consensus CArG elements. Our results indicate that reductions in myocardin expression and the degeneracy of CArG elements within smooth muscle promoters play a key role in phenotypic switching of smooth muscle cells in vivo, as well as in mediating responses of CArG-dependent smooth muscle genes and growth regulatory genes under conditions in which these 2 classes of genes are differentially expressed.

Frequent activation of CArG binding factor-A expression and binding in N-methyl-N-nitrosourea-induced rat mammary carcinomas

We previously identified a positive transcriptional element identical to human Ha-ras response element (HRE) within the promoter of the rat Ha-ras gene. We further identified CArG binding factor A (CBF-A), a member of heterogeneous nuclear ribonuclear protein (hnRNP) gene family, as a trans-acting factor that binds the HRE sequence with high affinity in rat mammary carcinoma cells. To determine if activation of CBF-A plays a role in tumor development in vivo , we investigated CBF-A expression and binding activity in rat mammary tumors induced by N-methyl-N-nitrosourea. We found that approximately 82% of tumors expressed CBF-A at levels that were 3-20 fold higher than detected in normal mammary gland. Moreover, elevated CBF-A protein levels were invariably associated with increased binding activity to the HRE. CBF-A mRNA levels in tumors were on average elevated only two fold as compared to normal mammary gland, indicating that increased CBF-A protein levels in tumors resulted from both translational and/or post-translational regulation. The level of CBF-A expression in mammary tumors was independent of Ha-ras mutational status. Together, these findings indicated that deregulation of CBF-A contributes to mammary carcinogenesis via a mechanism that is distinct from its hnRNP functions in binding and post-transcriptional regulation of RNA.

A G/C element mediates repression of the SM22alpha promoter within phenotypically modulated smooth muscle cells in experimental atherosclerosis

A hallmark of smooth muscle cell (SMC) phenotypic switching in atherosclerotic lesions is suppression of SMC differentiation marker gene expression. Yet little is known regarding the molecular mechanisms that control this process. Here we show that transcription of the SMC differentiation marker gene SM22alpha is reduced in atherosclerotic lesions and identify a cis regulatory element in the SM22alpha promoter required for this process. Transgenic mice carrying the SM22alpha promoter-beta-galactosidase (beta-gal) reporter transgene were crossed to apolipoprotein E (ApoE)-/- mice. Cells of the fibrous cap, intima, and underlying media showed complete loss of beta-gal activity in advanced atherosclerotic lesions. Of major significance, mutation of a G/C-rich cis element in the SM22alpha promoter prevented the decrease in SM22alpha promoter-beta-gal reporter transgene expression, including in cells that compose the fibrous cap of the lesion and in medial cells in proximity to the lesion. To begin to assess mechanisms whereby the G/C repressor element mediates suppression of SM22alpha in atherosclerosis, we tested the hypothesis that effects may be mediated by platelet-derived growth factor (PDGF)-BB-induced increases in the G/C binding transcription factor Sp1. Consistent with this hypothesis, results of studies in cultured SMCs showed that: (1) PDGF-BB increased expression of Sp1; (2) PDGF-BB and Sp1 profoundly suppressed SM22alpha promoter activity as well as smooth muscle myosin heavy chain promoter activity through mechanisms that were at least partially dependent on the G/C cis element; and (3) a short interfering RNA to Sp1 increased basal expression and attenuated PDGF-BB induced suppression of SM22alpha. Together, these results support a model whereby a G/C repressor element within the SM22alpha promoter mediates transcriptional repression of this gene within phenotypically modulated SMCs in experimental atherosclerosis and provide indirect evidence implicating PDGF-BB and Sp1 as possible mediators of these effects.

Molecular regulation of vascular smooth muscle cell differentiation in development and disease

The focus of this review is to provide an overview of the current state of knowledge of molecular mechanisms/processes that control differentiation of vascular smooth muscle cells (SMC) during normal development and maturation of the vasculature, as well as how these mechanisms/processes are altered in vascular injury or disease. A major challenge in understanding differentiation of the vascular SMC is that this cell can exhibit a wide range of different phenotypes at different stages of development, and even in adult organisms the cell is not terminally differentiated. Indeed, the SMC is capable of major changes in its phenotype in response to changes in local environmental cues including growth factors/inhibitors, mechanical influences, cell-cell and cell-matrix interactions, and various inflammatory mediators. There has been much progress in recent years to identify mechanisms that control expression of the repertoire of genes that are specific or selective for the vascular SMC and required for its differentiated function. One of the most exciting recent discoveries was the identification of the serum response factor (SRF) coactivator gene myocardin that appears to be required for expression of many SMC differentiation marker genes, and for initial differentiation of SMC during development. However, it is critical to recognize that overall control of SMC differentiation/maturation, and regulation of its responses to changing environmental cues, is extremely complex and involves the cooperative interaction of many factors and signaling pathways that are just beginning to be understood. There is also relatively recent evidence that circulating stem cell populations can give rise to smooth muscle-like cells in association with vascular injury and atherosclerotic lesion development, although the exact role and properties of these cells remain to be clearly elucidated. The goal of this review is to summarize the current state of our knowledge in this area and to attempt to identify some of the key unresolved challenges and questions that require further study.

Basic fibroblast growth factor antagonizes transforming growth factor-beta1-induced smooth muscle gene expression through extracellular signal-regulated kinase 1/2 signaling pathway activation

OBJECTIVE

Transforming growth factor-beta1 (TGFbeta1) and fibroblast growth factor (FGF) families play a pivotal role during vascular development and in the pathogenesis of vascular disease. However, the interaction of intracellular signaling evoked by each of these growth factors is not well understood. The present study was undertaken to examine the molecular mechanisms that mediate the effects of TGFbeta1 and basic FGF (bFGF) on smooth muscle cell (SMC) gene expression.

METHODS AND RESULTS

TGFbeta1 induction of SMC gene expression, including smooth muscle protein 22-alpha (SM22alpha) and smooth muscle alpha-actin, was examined in the pluripotent 10T1/2 cells. Marked increase in these mRNA levels by TGFbeta1 was inhibited by c-Src-tyrosine kinase inhibitors and protein synthesis inhibitor cycloheximide. Functional studies with deletion and site-directed mutation analysis of the SM22alpha promoter demonstrated that TGFbeta1 activated the SM22alpha promoter through a CC(A/T-rich)6GG (CArG) box, which serves as a serum response factor (SRF)-binding site. TGFbeta1 increased SRF expression through an increase in transcription of the SRF gene. In the presence of bFGF, TGFbeta1 induction of SMC marker gene expression was significantly attenuated. Transient transfection assays showed that bFGF significantly suppressed induction of the SM22alpha promoter-driven luciferase activity by TGFbeta1, whereas bFGF had no effects on the TGFbeta1-mediated increase in SRF expression and SRF:DNA binding activity. Mitogen-activated protein kinase kinase-1 (MEK1) inhibitor PD98059 abrogated the bFGF-mediated suppression of TGFbeta1-induced SMC gene expression.

CONCLUSIONS

Our data suggest that bFGF-induced MEK/extracellular signal-regulated kinase signaling plays an antagonistic role in TGFbeta1-induced SMC gene expression through suppression of the SRF function. These data indicate that opposing effects of bFGF and TGFbeta1 on SMC gene expression control the phenotypic plasticity of SMCs.

The RhoA/Rho kinase pathway regulates nuclear localization of serum response factor

RhoA and its downstream target Rho kinase regulate serum response factor (SRF)-dependent skeletal and smooth muscle gene expression. We previously reported that long-term serum deprivation reduces transcription of smooth muscle contractile apparatus encoding genes, by redistributing SRF out of the nucleus. Because serum components stimulate RhoA activity, these observations suggest the hypothesis that the RhoA/Rho kinase pathway regulates SRF-dependent smooth muscle gene transcription in part by controlling SRF subcellular localization. Our present results support this hypothesis: cotransfection of cultured airway myocytes with a plasmid expressing constitutively active RhoAV14 selectively enhanced transcription from the SM22 and smooth muscle myosin heavy chain promoters and from a purely SRF-dependent promoter, but had no effect on transcription from the MSV-LTR promoter or from an AP2-dependent promoter. Conversely, inhibition of the RhoA/Rho kinase pathway by cotransfection with a plasmid expressing dominant negative RhoAN19, by cotransfection with a plasmid expressing Clostridial C3 toxin, or by incubation with the Rho kinase inhibitor, Y-27632, all selectively reduced SRF-dependent smooth muscle promoter activity. Furthermore, treatment with Y-27632 selectively reduced binding of SRF from nuclear extracts to its consensus DNA target, selectively reduced nuclear SRF protein content, and partially redistributed SRF from nucleus to cytoplasm, as revealed by quantitative immunocytochemistry. Treatment of cultured airway myocytes with latrunculin B, which reduces actin polymerization, also caused partial redistribution of SRF into the cytoplasm. Together, these results demonstrate for the first time that the RhoA/Rho kinase pathway controls smooth muscle gene transcription in differentiated smooth muscle cells, in part by regulating the subcellular localization of SRF. It is conceivable that the RhoA/Rho kinase pathway influences SRF localization through its effect on actin polymerization dynamics.

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.

Dual regulation of ets-activated gene expression by SP1

The human folate receptor (hFR) type gamma gene is driven by a TATA-less promoter that uses a canonical Sp1 element for basal transcription. Using nuclear extract from 293 (human embryonic) cells, we mapped a second (non-canonical) Sp1 element to which Sp1 bound with a comparable affinity and which overlaps a functional ets binding site (EBS). Mutagenesis experiments revealed that the binding of ets to the EBS activates the promoter synergistically with Sp1 bound at the downstream site; however, binding of Sp1 to the EBS does not contribute to promoter activity. A further increase in Sp1 by inducible expression in recombinant 293 cells resulted in a small but significant decrease in the hFR-gamma promoter activity, but the decrease was abolished when the EBS was deleted from the promoter. In 293 cells, which do not express hFR-gamma, the Sp1 level was relatively high whereas in the hFR-gamma-positive HL60 leukemia cells, the Sp1 level was low and the EBS predominantly bound an ets protein. To account for the above observations, we propose a model in which when the Sp1 level is low, ets out competes Sp1 for binding to the EBS and synergistically enhances the hFR-gamma promoter activity by interacting with Sp1 bound at the canonical site whereas at higher levels, Sp1 represses the promoter by competitively inhibiting the binding of ets. As a partial extension of this model to the regulation of other ets activated genes, we show that Sp1 can predictably bind to a variety of ets elements including those responsive to Ets1 and Spi.1/Pu.1. A dual concentration-dependent action of Sp1 as an activator or a repressor offers a potential mechanism contributing to tissue-specific regulation of ets-dependent genes by Sp1.

ERK1/2-dependent contractile protein expression in vascular smooth muscle cells

In vivo, vascular smooth muscle (VSM) cells change their contractile phenotype toward a more proliferative phenotype during the pathogenesis of vascular diseases. Because these dedifferentiated VSM cells may gradually regain contractile functions, we aimed to identify signaling pathways that result in an increased expression of contractile proteins in VSM cells. In vitro, serum and thrombin induced a reversible upregulation of smooth muscle myosin heavy-chain (SM-MHC) in cultured neonatal rat VSM cells. Cotransfection of a SM-MHC-promoter chloramphenicol acetyltransferase-construct with dominant-negative N17Ras or N17Raf or treatment with the mitogen-activated/ERK-activating kinase (MEK) inhibitor PD 98059 concentration dependently decreased the serum- or thrombin-induced SM-MHC promoter activity. Consistently, the serum- or thrombin-induced phosphorylation of MEK and extracellular signal-regulated kinase 1/2 (ERK1/2) coincided with a MEK-dependent nuclear accumulation of phosphorylated ERK1/2 and subsequent nuclear phosphorylation of the transcription factors c-myc and Elk-1. A 5'-deletion analysis of cis-elements within the SM-MHC promoter demonstrated that a conserved region (nucleotide -1346 to -1102) was required for both cell type-specific expression and serum- or thrombin-induced upregulation of the SM-MHC promoter in VSM cells. Within this region, 2 CArG-boxes, a GC-rich element, and a CTF/NF-1 site are critical positively acting cis-elements for the serum- or thrombin-induced upregulation of SM-MHC. We conclude that the serum- or thrombin-induced differentiation requires an intact Ras/Raf/MEK/ERK signaling cascade, nuclear translocation of activated ERK1/2, phosphorylation of transcription factors, and several cis-elements within the SM-MHC promoter.

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.

The smooth muscle myosin heavy chain gene exhibits smooth muscle subtype-selective modular regulation in vivo

Previous studies in our laboratory demonstrated that the transgene consisting of the -4.2 to +11.6 kilobase (kb) region of the smooth muscle (SM) myosin heavy chain (MHC) gene was expressed in virtually all SM tissue types in vivo in transgenic mice and that the multiple CArG elements within this region were differentially required in SMC subtypes, implying that the SM-MHC gene was controlled by multiple transcriptional regulatory modules. To investigate this hypothesis, we analyzed specific regulatory regions within the SM-MHC -4.2 to +11.6 kb region by a combination of deletion analyses of various SM-MHC transgenes as well as by DNaseI hypersensitivity assays and in vivo footprinting in intact SMC tissues. The results showed that SM-MHC transgene expression depended on a large number of required regulatory modules that were widely spread over the -4.2 to +11.6 region. Moreover, the results revealed several unexpected novel features of regulation of the SM-MHC gene including: 1) unique combinations of regulatory modules were required for SM-MHC expression in different SMC-subtypes; 2) repressor modules as well as activator modules were both critical for SMC specificity of the gene; 3) certain modules were required in certain contexts but were dispensable in others within a given SMC-subtype (i.e. the net activity of the module was determined by interaction between modules not simply by the sum of module activities); and 4) we identified a highly conserved 200-base pair transcriptional regulatory module at +8 kb that was required in the large arteries but dispensable in the coronary arteries and airways in transgenic mice and contained multiple potential cis-elements that were occupied by nuclear proteins in the intact aorta based on in vivo footprinting. Taken together, the results suggest a model of complex modular control of expression of the SM-MHC gene that varies between SMC subtypes. Moreover, the studies establish the possibility of designing derivatives of the SM-MHC promoter that might be used for targeting gene expression to specific SMC subtypes in vivo.

Gbeta gamma mediate differentiation of vascular smooth muscle cells

Proliferation and subsequent dedifferentiation of vascular smooth muscle (VSM) cells contribute to the pathogenesis of atherosclerosis and postangioplastic restenosis. The dedifferentiation of VSM cells in vivo or in cell culture is characterized by a loss of contractile proteins such as smooth muscle-specific alpha-actin and myosin heavy chain (SM-MHC). Serum increased the expression of contractile proteins in neonatal rat VSM cells, indicating a redifferentiation process. RNase protection assays defined thrombin as a serum component that increases the abundance of SM-MHC transcripts. Additionally, serum and thrombin transiently elevated cytosolic Ca(2+) concentrations, led to a biphasic extracellular signal-regulated kinase (ERK) phosphorylation, up-regulated a transfected SM-MHC promoter construct, and induced expression of the contractile proteins SM-MHC and alpha-actin. Pertussis toxin, N17-Ras/Raf, and PD98059 prevented both the serum- and thrombin-induced second phase ERK phosphorylation and SM-MHC promoter activation. Constitutively active Galpha(q), Galpha(i), Galpha(12), and Galpha(13) failed to up-regulate SM-MHC transcription, whereas Gbetagamma concentration-dependently increased the SM-MHC promoter activity. Furthermore, the Gbetagamma scavenger beta-adrenergic receptor kinase 1 C-terminal peptide abolished the serum-mediated differentiation. We conclude that receptor-mediated differentiation of VSM cells requires Gbetagamma and an intact Ras/Raf/MEK/ERK signaling.

CArG elements control smooth muscle subtype-specific expression of smooth muscle myosin in vivo

Expression of smooth muscle myosin heavy chain (SM-MHC) is tightly controlled depending on the differentiated state of smooth muscle cells (SMCs). To better understand the mechanisms that regulate transcription of the SM-MHC gene in vivo, we tested the function of several conserved CArG elements contained within the -4200 to +11,600 region of this gene that we had previously shown to drive SMC-specific expression in transgenic mice. CArG1 in the 5'-flanking sequence was required for all SMCs, while CArG2 and a novel intronic CArG element were differentially required in SMC subtypes. Of particular note, mutation of the intronic CArG selectively abolished expression in large arteries. A promoter construct containing three repeats of a conserved 227-bp intronic CArG-containing region was sufficient to direct transcription in vascular SMCs in transgenic mice, although this construct was also expressed in skeletal and cardiac muscle. These results support a model in which transcriptional regulation of SM-MHC is controlled by multiple positive and negative modular control regions that differ between SMCs and non-SMCs and among SMC subtypes. We also demonstrated that the CArG elements of the endogenous SM-MHC gene were bound by SRF in chromatin.

Molecular mechanisms of decreased smooth muscle differentiation marker expression after vascular injury

While it is well established that phenotypic modulation of vascular smooth muscle cells (VSMCs) contributes to the development and progression of vascular lesions, little is known regarding the molecular mechanisms of phenotypic modulation in vivo. Here we show that vascular injury reduces transcription of VSMC differentiation marker genes, and we identify cis regulatory elements that may mediate this decrease. Using a carotid wire-injury model in mice carrying transgenes for smooth muscle alpha-actin, smooth muscle myosin heavy chain, or a SM22alpha promoter-beta-gal reporter, we collected arteries 7 and 14 days after injury and assessed changes in endogenous protein and mRNA levels and in beta-gal activity. Endogenous levels for all markers were decreased 7 days after injury and returned to nearly control levels by 14 days. beta-gal staining in all lines followed a similar pattern, suggesting that transcriptional downregulation contributed to the injury-induced decreases. To begin to dissect this response, we mutated a putative G/C-rich repressor in the SM22alpha promoter transgene and found that this mutation significantly attenuated injury-induced downregulation. Hence, transcriptional downregulation contributes to injury-induced decreases in VSMC differentiation markers, an effect that may be partially mediated through a G/C-rich repressor element.

CArG binding factor A (CBF-A) is involved in transcriptional regulation of the rat Ha-ras promoter

In the present study we identified a positive transcriptional element within the rat Ha-ras promoter previously known as Ha-ras response element (HRE) and identified a trans-acting factor that binds HRE sequences in rat mammary cells. To identify the binding protein we employed sequence specific DNA affinity chromatography. Amino acid sequence analysis of the affinity-purified proteins was performed by tandem mass spectroscopy. The results unexpectedly demonstrated that in rat mammary cells CArG box-binding factor A (CBF-A) is the major protein species that bind specifically to the rat and human HRE sequences with high affinity. The affinity of CBF-A binding to HRE was significantly higher than to the CArG box described as a recognition sequence for CBF-A protein. Transient transfection assays using reporter plasmids verified that mutations within the HRE that disrupt binding of CBF-A also reduced the activity of the rat Ha-ras promoter. Despite the fact that the HRE within the Ha-ras promoter resembles a binding site for Ets transcription factors, we did not detect the binding of Ets-related proteins to the rat HRE in BICR-M1Rk cells. We further demonstrated a correlation between the presence of HRE binding activity and induction of Ha-ras mRNA expression following serum stimulation in the mammary carcinoma cell line. Taken together, our results suggest that CBF-A may play an important role in transcriptional regulation of Ha-ras promoter activity during normal mammary cell growth and carcinogenesis.

Physiological control of smooth muscle-specific gene expression through regulated nuclear translocation of serum response factor

Prolonged serum deprivation induces a structurally and functionally contractile phenotype in about 1/6 of cultured airway myocytes, which exhibit morphological elongation and accumulate abundant contractile apparatus-associated proteins. We tested the hypothesis that transcriptional activation of genes encoding these proteins accounts for their accumulation during this phenotypic transition by measuring the transcriptional activities of the murine SM22 and human smooth muscle myosin heavy chain promoters during transient transfection in subconfluent, serum fed or 7 day serum-deprived cultured canine tracheal smooth muscle cells. Contrary to our expectation, SM22 and smooth muscle myosin heavy chain promoter activities (but not viral murine sarcoma virus-long terminal repeat promoter activity) were decreased in long term serum-deprived myocytes by at least 8-fold. Because serum response factor (SRF) is a required transcriptional activator of these and other smooth muscle-specific promoters, we evaluated the expression and function of SRF in subconfluent and long term serum-deprived cells. Whole cell SRF mRNA and protein were maintained at high levels in serum-deprived myocytes, but SRF transcription-promoting activity, nuclear SRF binding to consensus CArG sequences, and nuclear SRF protein were reduced. Furthermore, immunocytochemistry revealed extranuclear redistribution of SRF in serum-deprived myocytes; nuclear localization of SRF was restored after serum refeeding. These results uncover a novel mechanism for physiological control of smooth muscle-specific gene expression through extranuclear redistribution of SRF and consequent down-regulation of its transcription-promoting activity.

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.

Upstream stimulatory factor 1 regulates osteopontin expression in smooth muscle cells

Vascular smooth muscle cells (SMCs) undergo a dramatic phenotypic transition in response to injury and ex vivo culture that includes enhanced proliferation, migration, matrix deposition, and alterations in gene expression. Osteopontin is a good marker for the injury-induced SMC phenotypic state in vivo and in vitro. To identify transcription factors that might control the regulation of osteopontin expression, we investigated cultured vascular SMCs that express high and low levels of osteopontin. Using nuclear run-on assays, mRNA stability studies, and deletion analysis, we demonstrate that regulation of osteopontin steady-state mRNA levels in SMCs occurs at the transcriptional level. Transient transfection and gel-shift analyses of osteopontin promoter indicated that a region between -123 and +66 was involved in the expression of osteopontin. Supershift EMSAs identified the bHLH-leucine zipper transcription factor upstream stimulatory factor-1 (USF1) as the protein binding to this sequence. Finally, we show that USF1 protein is induced in vivo within 24 h of balloon angioplasty of rat carotids coordinately with osteopontin induction. These data suggest that USF1 governs expression of osteopontin in cultured vascular SMCs and might contribute to initial osteopontin expression observed post carotid injury and in vascular pathologies in vivo.

A role for serum response factor in coronary smooth muscle differentiation from proepicardial cells

Coronary artery smooth muscle (SM) cells originate from proepicardial cells that migrate over the surface of the heart, undergo epithelial to mesenchymal transformation and invade the subepicardial and cardiac matrix. Prior to contact with the heart, proepicardial cells exhibit no expression of smooth muscle markers including SMalphaactin, SM22alpha, calponin, SMgammaactin or SM-myosin heavy chain detectable by RT-PCR or by immunostaining. To identify factors required for coronary smooth muscle differentiation, we excised proepicardial cells from Hamburger-Hamilton stage-17 quail embryos and examined them ex vivo. Proepicardial cells initially formed an epithelial colony that was uniformly positive for cytokeratin, an epicardial marker. Transcripts for flk-1, Nkx 2.5, GATA4 or smooth muscle markers were undetectable, indicating an absence of endothelial, myocardial or preformed smooth muscle cells. By 24 hours, cytokeratin-positive cells became SMalphaactin-positive. Moreover, serum response factor, undetectable in freshly isolated proepicardial cells, became strongly expressed in virtually all epicardial cells. By 72 hours, a subset of epicardial cells exhibited a rearrangement of cytoskeletal actin, focal adhesion formation and acquisition of a motile phenotype. Coordinately with mesenchymal transformation, calponin, SM22alpha and SMgammaactin became expressed. By 5–10 days, SM-myosin heavy chain mRNA was found, by which time nearly all cells had become mesenchymal. RT-PCR showed that large increases in serum response factor expression coincide with smooth muscle differentiation in vitro. Two different dominant-negative serum response factor constructs prevented the appearance of calponin-, SM22alpha- and SMgammaactin-positive cells. By contrast, dominant-negative serum response factor did not block mesenchymal transformation nor significantly reduce the number of cytokeratin-positive cells. These results indicate that the stepwise differentiation of coronary smooth muscle cells from proepicardial cells requires transcriptionally active serum response factor.

Developmental biology of the vascular smooth muscle cell: building a multilayered vessel wall

The assembly of the vessel wall from its cellular and extracellular matrix components is a critical process in the development and maturation of the cardiovascular system. However, fundamental questions concerning the origin of vessel wall cells and the mechanisms that regulate their development and differentiation remain unanswered. The initial step of vessel wall morphogenesis is formation of a primary vascular network, comprised of nascent endothelial cell tubes, via the processes of vasculogenesis and angiogenesis. Subsequently, primordial vascular smooth muscle cells (VSMCs) are recruited to the endothelium to form a multilayered vessel wall. During the course of development and maturation, the VSMC plays diverse roles: it is a biosynthetic, proliferative, and contractile component of the vessel wall. Although the field of vascular development has blossomed in the past decade, the molecules and mechanisms that regulate this developmental pathway are not well defined. The focus of this review is on those facets of VSMC development important for transforming a nascent endothelial tube into a multilayered structure. We discuss the primordial VSMC with particular attention to its purported origins, the components of the extracellular milieu that contribute to its development, and the contribution of embryonic hemodynamics to vessel wall assembly.

Molecular control of vascular smooth muscle cell differentiation

Changes in the differentiated state of the vascular smooth muscle cell (SMC) including enhanced growth responsiveness, altered lipid metabolism, and increased matrix production are known to play a key role in development of atherosclerotic disease. As such, there has been extensive interest in understanding the molecular mechanisms and factors that regulate differentiation of vascular SMC, and how this regulation might be disrupted in vascular disease. Key questions include determination of mechanisms that control the coordinate expression of genes required for the differentiated function of the smooth muscle cell, and determination as to how these regulatory processes are influenced by local environmental cues known to be important to control of smooth muscle differentiation. Of particular interest, a number of common cis regulatory elements including highly conserved CArG [CC(A/T)6GG] motifs or CArG-like motifs and a TGF beta control element have been identified in the promoters of virtually all smooth muscle differentiation marker genes characterized to date including smooth muscle alpha-actin, smooth muscle myosin heavy chain, telokin, and SM22 alpha and shown to be required for expression of these genes both in vivo and in vitro. In addition, studies have identified a number of trans factors that interact with these cis elements, and shown how the expression or activity of these factors is modified by local environmental cues such as contractile agonists that are known to influence differentiation of smooth muscle.

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.

Smooth muscle-specific expression of the smooth muscle myosin heavy chain gene in transgenic mice requires 5'-flanking and first intronic DNA sequence

The smooth muscle myosin heavy chain (SM-MHC) gene encodes a major contractile protein whose expression exclusively marks the smooth muscle cell (SMC) lineage. To better understand smooth muscle differentiation at the transcriptional level, we have initiated studies to identify those DNA sequences critical for expression of the SM-MHC gene. Here we report the identification of an SM-MHC promoter-intronic DNA fragment that directs smooth muscle-specific expression in transgenic mice. Transgenic mice harboring an SM-MHC-lacZ reporter construct containing approximately 16 kb of the SM-MHC genomic region from -4.2 to + 11.6 kb (within the first intron) expressed the lacZ transgene in all smooth muscle tissue types. The inclusion of the intronic sequence was required for transgene expression, since 4.2 kb of the 5'-flanking region alone was not sufficient for expression. In the adult mouse, transgene expression was observed in both arterial and venous smooth muscle, in airway smooth muscle of the trachea and bronchi, and in the smooth muscle layers of all abdominal organs, including the stomach, intestine, ureters, and bladder. During development, transgene expression was first detected in airway SMCs at embryonic day 12.5 and in vascular and visceral SMC tissues by embryonic day 14.5. Of interest, expression of the SM-MHC transgene was markedly reduced or absent in some SMC tissues, including the pulmonary circulation. Moreover, the transgene exhibited a heterogeneous pattern between individual SMCs within a given tissue, suggesting the possibility of the existence of different SM-MHC gene regulatory programs between SMC subpopulations and/or of episodic rather than continuous expression of the SM-MHC gene. To our knowledge, results of these studies are the first to identify a promoter region that confers complete SMC specificity in vivo, thus providing a system with which to define SMC-specific transcriptional regulatory mechanisms and to design vectors for SMC-specific gene targeting.