Programming smooth muscle plasticity with chromatin dynamics

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

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

Molecular mechanisms of collagen isotype-specific modulation of smooth muscle cell phenotype

OBJECTIVE

Smooth muscle cell (SMC) phenotypic modulation, an important component of atherosclerosis progression, is critically regulated by the matrix, with normal components of the healthy SMC matrix limiting modulation and atherosclerosis-associated transitional matrix proteins promoting phenotypic modulation. We sought to determine how collagen IV (which comprises the healthy artery wall) and monomeric collagen I (which comprises atherosclerotic lesions) differentially affect SMC phenotype.

METHODS AND RESULTS

Plating SMCs on collagen IV resulted in elevated expression of SMC contractility proteins compared to collagen I. Concurrent with enhanced contractile gene expression, collagen IV stimulates binding of SRF to CArG boxes in the promoters of smooth muscle actin and smooth muscle myosin heavy chain. Coll IV also stimulated the expression of myocardin, a critical SRF coactivator required to drive expression of SMC specific genes. In contrast to collagen IV, collagen I stimulated enhanced expression of the inflammatory protein vascular cell adhesion molecule (VCAM)-1. NF-kappaB and NFAT-binding sites in the VCAM-1 promoter are critical for collagen I-mediated expression of VCAM-1 promoter activity. However, only inhibitors of NFAT, not NF-kappaB, were able to reduce collagen I-associated VCAM expression, and collagen I but not collagen IV stimulated NFAT transcriptional activity.

CONCLUSIONS

These results show for the first time that collagen IV and collagen I differentially affect smooth muscle phenotypic modulation through multiple pathways.

Notch and Vascular Smooth Muscle Cell Phenotype

The Notch signaling pathway is critical for cell fate determination during embryonic development, including many aspects of vascular development. An emerging paradigm suggests that the Notch gene regulatory network is often recapitulated in the context of phenotypic modulation of vascular smooth muscle cells (VSMC), vascular remodeling, and repair in adult vascular disease following injury. Notch ligand receptor interactions lead to cleavage of receptor, translocation of the intracellular receptor (Notch IC), activation of transcriptional CBF-1/RBP-Jκ–dependent and –independent pathways, and transduction of downstream Notch target gene expression. Hereditary mutations of Notch components are associated with congenital defects of the cardiovascular system in humans such as Alagille syndrome and cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL). Recent loss- or gain-of-function studies have provided insight into novel Notch-mediated CBF-1/RBP-Jκ–dependent and –independent signaling and cross-regulation to other molecules that may play a critical role in VSMC phenotypic switching. Notch receptors are critical for controlling VSMC differentiation and dictating the phenotypic response following vascular injury through interaction with a triad of transcription factors that act synergistically to regulate VSMC differentiation. This review focuses on the role of Notch receptor ligand interactions in dictating VSMC behavior and phenotype and presents recent findings on the molecular interactions between the Notch components and VSMC-specific genes to further understand the function of Notch signaling in vascular tissue and disease.

Distinct phosphodiesterase-4D variants integrate into protein kinase A-based signaling complexes in cardiac and vascular myocytes

Numerous cAMP-elevating agents regulate events required for efficient migration of arterial vascular smooth muscle cells (VSMCs). Interestingly, when the impact of cAMP-elevating agents on individual migration-related events is studied, these agents have been shown to have distinct, and sometimes unexpected, effects. For example, although cAMP-elevating agents inhibit overall migration, they promote VSMC adhesion to extracellular matrix proteins and the formation of membrane extensions, which are both events that are essential for and promote migration. Herein, we extend previous observations that identified phosphodiesterase-4D3 (PDE4D3) as an integral component of a PKA/A kinase-anchoring protein (AKAP) complex in cultured/hypertrophied rat cardiac myocytes to the case for nonhypertrophied cardiac myocytes. Moreover, we show that while rat aortic VSMCs also express PDE4D3, this protein is not detected in PKA/AKAP complexes isolated from these cells. In contrast, we show that another PDE4D splice variant expressed in arterial vascular myocytes, namely, PDE4D8, integrates into PKA/AKAP-based signaling complexes in VSMCs. Consistent with the idea that a PDE4D8/PKA/AKAP complex regulates specific VSMC functions, PKA and PDE4D8 were each recruited to leading-edge structures in migrating VSMCs, and inhibition of PDE4D8 recruitment to pseudopodia of migrating cells caused localized changes in actin dynamics. Our data are presented in the context that cardiac myocytes and arterial VSMCs may use distinct PDE4D variants to regulate selected pools of targeted PKA activity and that disruption of this complex may allow selective regulation of cAMP-dependent events between these two cardiovascular cell types.

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

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

Integrin-linked kinase regulates smooth muscle differentiation marker gene expression in airway tissue

Phenotypic changes in airway smooth muscle occur with airway inflammation and asthma. These changes may be induced by alterations in the extracellular matrix that initiate signaling pathways mediated by integrin receptors. We hypothesized that integrin-linked kinase (ILK), a multidomain protein kinase that binds to the cytoplasmic tail of beta-integrins, may be an important mediator of signaling pathways that regulate the growth and differentiation state of airway smooth muscle. We disrupted signaling pathways mediated by ILK in intact differentiated tracheal muscle tissues by depleting ILK protein using ILK antisense. The depletion of ILK protein increased the expression of the smooth muscle differentiation marker genes myosin heavy chain (SmMHC), SM22alpha, and calponin and increased the expression of SmMHC protein. Conversely, the overexpression of ILK protein reduced the mRNA levels of SmMHC, SM22alpha, and calponin and SmMHC protein. Analysis by chromatin immunoprecipitation showed that the binding of the transcriptional regulator serum response factor (SRF) to the promoters of SmMHC, SM22alpha, and calponin genes was increased in ILK-depleted tissues and decreased in tissues overexpressing ILK. ILK depletion also increased the amount of SRF that localized within the nucleus. ILK depletion and overexpression, respectively, decreased and increased the activation of its downstream substrate protein kinase B (PKB/Akt). The pharmacological inhibition of Akt activity also increased SRF binding to the promoters of smooth muscle-specific genes and increased expression of smooth muscle proteins, suggesting that ILK may exert its effects by regulating the activity of Akt. We conclude that ILK is a critical regulator of airway smooth muscle differentiation. ILK may mediate signals from integrin receptors that control airway smooth muscle differentiation in response to alterations in the extracellular matrix.

Role of NonO-histone interaction in TNFalpha-suppressed prolyl-4-hydroxylase alpha1

Inflammation is a key process in cardiovascular diseases. The extracellular matrix (ECM) of the vasculature is a major target of inflammatory cytokines, and TNFalpha regulates ECM metabolism by affecting collagen production. In this study, we have examined the pathways mediating TNFalpha-induced suppression of prolyl-4 hydroxylase alpha1 (P4Halpha1), the rate-limiting isoform of P4H responsible for procollagen hydroxylation, maturation, and organization. Using human aortic smooth muscle cells, we found that TNFalpha activated the MKK4-JNK1 pathway, which induced histone (H) 4 lysine 12 acetylation within the TNFalpha response element in the P4Halpha1 promoter. The acetylated-H4 then recruited a transcription factor, NonO, which, in turn, recruited HDACs and induced H3 lysine 9 deacetylation, thereby inhibiting transcription of the P4Halpha1 promoter. Furthermore, we found that TNFalpha oxidized DJ-1, which may be essential for the NonO-P4Halpha1 interaction because treatment with gene specific siRNA to knockout DJ-1 eliminated the TNFalpha-induced NonO-P4Halpha1 interaction and its suppression. Our findings may be relevant to aortic aneurysm and dissection and the stability of the fibrous cap of atherosclerotic plaque in which collagen metabolism is important in arterial remodeling. Defining this cytokine-mediated regulatory pathway may provide novel molecular targets for therapeutic intervention in preventing plaque rupture and acute coronary occlusion.

Epigenetic regulation of vascular endothelial gene expression

Epigenetics refers to chromatin-based pathways important in the regulation of gene expression and includes 3 distinct, but highly interrelated, mechanisms: DNA methylation, histone density and posttranslational modifications, and RNA-based mechanisms. Together, they offer a newer perspective on transcriptional control paradigms in vascular endothelial cells and provide a molecular basis for how the environment impacts the genome to modify disease susceptibility. This review provides an introduction to epigenetic concepts for vascular biologists. Using endothelial nitric oxide synthase (NOS3) as an example, we examine the growing body of evidence implicating epigenetic pathways in the control of vascular endothelial gene expression in health and disease.

Stem Cells and Transplant Arteriosclerosis

Stem cells can differentiate into a variety of cells to replace dead cells or to repair damaged tissues. Recent evidence indicates that stem cells are involved in the pathogenesis of transplant arteriosclerosis, an alloimmune initiated vascular stenosis that often results in transplant organ failure. Although the pathogenesis of transplant arteriosclerosis is not yet fully understood, recent developments in stem cell research have suggested novel mechanisms of vascular remodeling in allografts. For example, stem cells derived from the recipient may repair damaged endothelial cells of arteries in transplant organs. Further evidence suggests that stem cells or endothelial progenitor cells may be released from both bone marrow and non–bone marrow tissues. Vascular stem cells appear to replenish cells that died in donor vessels. Concomitantly, stem/progenitor cells may also accumulate in the intima, where they differentiate into smooth muscle cells. However, several issues concerning the contribution of stem cells to the pathogenesis of transplant arteriosclerosis are controversial, eg, whether bone marrow–derived stem cells can differentiate into smooth muscle cells that form neointimal lesions of the vessel wall. This review summarizes recent research on the role of stem cells in transplant arteriosclerosis, discusses the mechanisms of stem cell homing and differentiation into mature endothelial and smooth muscle cells, and highlights the controversial issues in the field.

Constitutive serum response factor activation by the viral chemokine receptor homologue pUS28 is differentially regulated by Galpha(q/11) and Galpha(16)

Expression of the human cytomegalovirus (HCMV)-encoded chemokine receptor homologue pUS28 in mammalian cells results in ligand-dependent and -independent changes in the activity of multiple cellular signal transduction pathways. The ligand-dependent signalling activity of pUS28 has been shown to be predominantly mediated by heterotrimeric G proteins of the G(i/o) and G(12/13) subfamilies. Ligand-independent constitutive activity of pUS28 causing stimulation of inositol phosphate formation has been correlated with the coupling of pUS28 to G proteins of the G(q) family. It is well known that activation of G(q) proteins by cell surface receptors is coupled to activation of the Rho GTPase RhoA. Activated RhoA regulates numerous cellular functions, including the activity of the transcription factor serum response factor (SRF). The marked activation of G(q) proteins by pUS28 in transfected and HCMV-infected cells prompted us to investigate its effect on SRF activity. The results presented herein demonstrate that expression of pUS28 in COS-7 cells caused a vigorous induction of SRF activity. This effect was observed in the absence of chemokines known to interact with pUS28, and was specifically mediated by endogenous G(q) and/or G(11) as well as RhoA and/or a closely related Rho GTPase. The stimulatory effect of pUS28 and Galpha(q/11) was independent of phospholipase C-beta (PLCbeta) activation and was markedly sensitive to inhibition by wild-type, but not by constitutively active Galpha(16), thus identifying Galpha(16) as a modulator of Galpha(q/11) function likely to act by competing with Galpha(q/11) for and thus uncoupling Galpha(q/11) from activation by pUS28.

The non-excitable smooth muscle: calcium signaling and phenotypic switching during vascular disease

Calcium (Ca(2+)) is a highly versatile second messenger that controls vascular smooth muscle cell (VSMC) contraction, proliferation, and migration. By means of Ca(2+) permeable channels, Ca(2+) pumps and channels conducting other ions such as potassium and chloride, VSMC keep intracellular Ca(2+) levels under tight control. In healthy quiescent contractile VSMC, two important components of the Ca(2+) signaling pathways that regulate VSMC contraction are the plasma membrane voltage-operated Ca(2+) channel of the high voltage-activated type (L-type) and the sarcoplasmic reticulum Ca(2+) release channel, Ryanodine Receptor (RyR). Injury to the vessel wall is accompanied by VSMC phenotype switch from a contractile quiescent to a proliferative motile phenotype (synthetic phenotype) and by alteration of many components of VSMC Ca(2+) signaling pathways. Specifically, this switch that culminates in a VSMC phenotype reminiscent of a non-excitable cell is characterized by loss of L-type channels expression and increased expression of the low voltage-activated (T-type) Ca(2+) channels and the canonical transient receptor potential (TRPC) channels. The expression levels of intracellular Ca(2+) release channels, pumps and Ca(2+)-activated proteins are also altered: the proliferative VSMC lose the RyR3 and the sarcoplasmic/endoplasmic reticulum Ca(2+) ATPase isoform 2a pump and reciprocally regulate isoforms of the ca(2+)/calmodulin-dependent protein kinase II. This review focuses on the changes in expression of Ca(2+) signaling proteins associated with VSMC proliferation both in vitro and in vivo. The physiological implications of the altered expression of these Ca(2+) signaling molecules, their contribution to VSMC dysfunction during vascular disease and their potential as targets for drug therapy will be discussed.

Hairy-related transcription factors inhibit Notch-induced smooth muscle alpha-actin expression by interfering with Notch intracellular domain/CBF-1 complex interaction with the CBF-1-binding site

Notch signaling regulates smooth muscle cell phenotype and is critical for vascular development. One Notch target is smooth muscle alpha-actin (SMA), a differentiated smooth muscle cell marker. The Notch intracellular domain (NotchICD) forms a complex with CBF-1 (C-promoter-binding factor-1) and directly induces SMA expression. Using primary human smooth muscle cells, we show that expression of the constitutive active ICD of human Notch1, Notch2, or Notch4 receptors increase SMA levels. NotchICD also induce expression of the transcriptional repressors HRT1 (Hairy-related transcription factor 1) and HRT2, in a CBF-1-dependent manner. However, unlike the activating effects of NotchICD, HRT1 or HRT2 represses basal SMA expression, and both are strong antagonists of NotchICD-induced SMA upregulation. This antagonism does not depend on histone deacetylase activity and occurs at the transcriptional level. Competitive coimmunoprecipitation experiments demonstrate that HRT does not disrupt the association of NotchICD and CBF-1, which form a complex in the presence or absence of HRTs. However, HRT suppresses NotchICD/CBF-1 binding to the SMA promoter, as measured by chromatin immunoprecipitation, and transactivation of an SMA promoter reporter spanning sequences -124/+32. SMA expression was regulated similarly following endogenous Notch activation in smooth muscle cells by coculture with endothelial cells, and this effect was also sensitive to HRT inhibition. Temporally defined HRT activity may constitute a negative feedback mechanism of Notch signaling. Our study presents a novel mechanism by which a balance between Notch signaling and HRT activity determines the expression of smooth muscle differentiation markers including SMA.

The histone demethylase, Jmjd1a, interacts with the myocardin factors to regulate SMC differentiation marker gene expression

We and others have previously shown that the myocardin transcription factors play critical roles in the regulation of smooth muscle cell (SMC) differentiation marker gene expression. In a yeast 2-hybrid screen for proteins that interact with myocardin-related transcription factor-A (MRTF-A), we identified the histone 3 lysine 9 (H3K9)-specific demethylase, Jmjd1a. GST pull-down assays demonstrated that Jmjd1a bound all 3 myocardin family members, and further mapping studies showed that the jumonjiC domain of Jmjd1a was sufficient to mediate this interaction. Overexpression of Jmjd1a in multipotential 10T1/2 cells decreased global levels of di-methyl H3K9, stimulated the SM alpha-actin and SM22 promoters, and synergistically enhanced MRTF-A- and myocardin-dependent transactivation. Using chromatin immunoprecipitation assays, we also demonstrated that TGF-beta-mediated upregulation of SMC differentiation marker gene expression in 10T1/2 cells was associated with decreased H3K9 dimethylation at the CArG-containing regions of the SMC differentiation marker gene promoters. Importantly, knockdown of Jmjd1a in 10T1/2 cells and primary rat aortic SMCs by retroviral delivery of siRNA attenuated TGF-beta-induced upregulation of endogenous SM myosin heavy chain expression. These effects were concomitant with increased H3K9 dimethylation at the SMC differentiation marker gene promoters and with inhibition of MRTF-A-dependent transactivation of the SMC-specific transcription. These results suggest, for the first time, that SMC differentiation marker gene expression is regulated by H3K9 methylation and that the effects of the myocardin factors on SMC-specific transcription may involve the recruitment of Jmjd1a to the SMC-specific promoters.

Muscle LIM protein promotes expression of the acetylcholine receptor gamma-subunit gene cooperatively with the myogenin-E12 complex

Muscle LIM protein (MLP, also referred to as CRP3) is a muscle-specific LIM-only protein, which consists of two LIM motifs. MLP functions as a positive regulator during myogenesis. Here we report that MLP serves as a cofactor regulating the expression of the nicotinic acetylcholine receptor (AChR) gamma-subunit gene in skeletal muscle cells. We found that MLP promoted the expression of the AChR gamma-subunit gene in C2C12 myotubes, but not in C2C12 myoblasts or NIH3T3 fibroblasts. Furthermore, we showed that MLP interacted with myogenin in vivo and enhanced the binding ability of the myogenin-E12 heterodimer to the E boxes in the AChR gamma-subunit gene promoter. Together, these results suggest that MLP promotes the specific expression of the AChR gamma-subunit gene cooperatively with the myogenin-E12 complex during myogenesis.