Platelet-derived growth factor-induced destabilization of smooth muscle alpha-actin mRNA

ADAR1-Mediated RNA Editing, A Novel Mechanism Controlling Phenotypic Modulation of Vascular Smooth Muscle Cells

RATIONALE

Vascular smooth muscle cell (SMC) phenotypic modulation is characterized by the downregulation of SMC contractile genes. Platelet-derived growth factor-BB, a well-known stimulator of SMC phenotypic modulation, downregulates SMC genes via posttranscriptional regulation. The underlying mechanisms, however, remain largely unknown.

OBJECTIVE

To establish RNA editing as a novel mechanism controlling SMC phenotypic modulation.

METHODS AND RESULTS

Precursor mRNAs (pre-mRNA) of SMC myosin heavy chain and smooth muscle α-actin were accumulated while their mature mRNAs were downregulated during SMC phenotypic modulation, suggesting an abnormal splicing of the pre-mRNAs. The abnormal splicing resulted from SMC marker pre-mRNA editing that was facilitated by adenosine deaminase acting on RNA 1 (ADAR1), an enzyme converting adenosines to inosines (A→I editing) in RNA sequences. ADAR1 expression inversely correlated with SMC myosin heavy chain and smooth muscle α-actin levels; knockdown of ADAR1 restored SMC myosin heavy chain and smooth muscle α-actin expression in phenotypically modulated SMC, and editase domain mutation diminished the ADAR1-mediated abnormal splicing of SMC marker pre-mRNAs. Moreover, the abnormal splicing/editing of SMC myosin heavy chain and smooth muscle α-actin pre-mRNAs occurred during injury-induced vascular remodeling. Importantly, heterozygous knockout of ADAR1 dramatically inhibited injury-induced neointima formation and restored SMC marker expression, demonstrating a critical role of ADAR1 in SMC phenotypic modulation and vascular remodeling in vivo.

CONCLUSIONS

Our results unraveled a novel molecular mechanism, that is, pre-mRNA editing, governing SMC phenotypic modulation.

PDGFRβ signalling regulates local inflammation and synergizes with hypercholesterolaemia to promote atherosclerosis

Platelet-derived growth factor (PDGF) is a mitogen and chemoattractant for vascular smooth muscle cells (VSMCs). However, the direct effects of PDGF receptor β (PDGFRβ) activation on VSMCs have not been studied in the context of atherosclerosis. Here, we present a new mouse model of atherosclerosis with an activating mutation in PDGFRβ. Increased PDGFRβ signaling induces chemokine secretion and leads to leukocyte accumulation in the adventitia and media of the aorta. Furthermore, PDGFRβ^D849V amplifies and accelerates atherosclerosis in hypercholesterolemic ApoE^−/− or Ldlr^−/− mice. Intriguingly, increased PDGFRβ signaling promotes advanced plaque formation at novel sites in the thoracic aorta and coronary arteries. However, deletion of the PDGFRβ-activated transcription factor STAT1 in VSMCs alleviates inflammation of the arterial wall and reduces plaque burden. These results demonstrate that PDGFRβ pathway activation has a profound effect on vascular disease and support the conclusion that inflammation in the outer arterial layers is a driving process for atherosclerosis.

Cooperative binding of KLF4, pELK-1, and HDAC2 to a G/C repressor element in the SM22α promoter mediates transcriptional silencing during SMC phenotypic switching in vivo

RATIONALE

We previously identified conserved G/C Repressor elements in the promoters of most smooth muscle cell (SMC) marker genes and demonstrated that mutation of this element within the SM22α promoter nearly abrogated repression of this transgene after vascular wire injury or within lesions of ApoE-/- mice. However, the mechanisms regulating the activity of the G/C Repressor are unknown, although we have previously shown that phenotypic switching of cultured SMC is dependent on Krupple-like factor (KLF)4.

OBJECTIVE

The goals of the present studies were to ascertain if (1) injury-induced repression of SM22α gene after vascular injury is mediated through KLF4 binding to the G/C Repressor element and (2) the transcriptional repressor activity of KLF4 on SMC marker genes is dependent on cooperative binding with pELK-1 (downstream activator of the mitogen-activated protein kinase pathway) and subsequent recruitment of histone de-acetylase 2 (HDAC2), which mediates epigenetic gene silencing.

METHODS AND RESULTS

Chromatin immunoprecipitation (ChIP) assays were performed on chromatin derived from carotid arteries of mice having either a wild-type or G/C Repressor mutant SM22α promoter-LacZ transgene. KLF4 and pELK-1 binding to the SM22α promoter was markedly increased after vascular injury and was G/C Repressor dependent. Sequential ChIP assays and proximity ligation analyses in cultured SMC treated with platelet-derived growth factor BB or oxidized phospholipids showed formation of a KLF4, pELK-1, and HDAC2 multiprotein complex dependent on the SM22α G/C Repressor element.

CONCLUSIONS

Silencing of SMC marker genes during phenotypic switching is partially mediated by sequential binding of pELK-1 and KLF4 to G/C Repressor elements. The pELK-1-KLF4 complex in turn recruits HDAC2, leading to reduced histone acetylation and epigenetic silencing.

Smooth muscle cell phenotypic switching in atherosclerosis

Smooth muscle cells (SMCs) possess remarkable phenotypic plasticity that allows rapid adaptation to fluctuating environmental cues, including during development and progression of vascular diseases such as atherosclerosis. Although much is known regarding factors and mechanisms that control SMC phenotypic plasticity in cultured cells, our knowledge of the mechanisms controlling SMC phenotypic switching in vivo is far from complete. Indeed, the lack of definitive SMC lineage-tracing studies in the context of atherosclerosis, and difficulties in identifying phenotypically modulated SMCs within lesions that have down-regulated typical SMC marker genes, and/or activated expression of markers of alternative cell types including macrophages, raise major questions regarding the contributions of SMCs at all stages of atherogenesis. The goal of this review is to rigorously evaluate the current state of our knowledge regarding possible phenotypes exhibited by SMCs within atherosclerotic lesions and the factors and mechanisms that may control these phenotypic transitions.

Genome-wide microarray analyses identify the protein C receptor as a novel calcineurin/nuclear factor of activated T cells-dependent gene in vascular smooth muscle cell phenotypic modulation

OBJECTIVE

Calcineurin (Cn) and the nuclear factor of activated T cells (NFAT) family of transcription factors are critical in vascular smooth muscle cell (SMC) development and pathology. Here, we used a genomics approach to identify and validate NFAT gene targets activated during platelet-derived growth factor-BB (PDGF-BB)-induced SMC phenotypic modulation.

METHODS AND RESULTS

Genome-wide expression arrays were used to identify genes both (1) differentially activated in response to PDGF-BB and (2) whose differential expression was reduced by both the Cn inhibitor cyclosporin A and the NFAT inhibitor A-285222. The 20 most pharmacologically sensitive genes were validated by quantitative reverse transcription-polymerase chain reaction analysis of PDGF-BB-stimulated SMCs in the presence of Cn/NFAT inhibitors, including the VIVIT peptide. In all experiments, protein C receptor (PROCR) gene activation was reduced. We showed that PROCR expression was virtually absent in untreated, quiescent SMCs. PDGF-BB stimulation, however, induced significant PROCR promoter activation and downstream protein expression in a Cn/NFAT-dependent manner. Mutation of a species-conserved, NFAT binding motif significantly attenuated PDGF-BB-induced PROCR promoter activity, thereby distinguishing NFAT as the first PROCR transcriptional activator to date. Moreover, SMC PROCR expression was upregulated in the neointima as early as 7 days following acute vascular injury in rat carotid arteries.

CONCLUSION

We hereby report PROCR as a novel, NFAT-dependent gene that may be implicated in vascular restenosis and consequent inward remodeling.

Store-operated Ca(2+) entry is not essential for PDGF-BB induced phenotype modulation in rat aortic smooth muscle

Suppression of smooth muscle cell (SMC) differentiation marker genes is central to SMC phenotype modulation during vasculo-proliferative diseases such as atherosclerosis and restenosis. Upregulation of the intermediate-conductance Ca(2+)-activated K(+) channel (K(Ca)3.1) is integral for mitogen-induced suppression of SMC marker genes and post-angioplasty restenosis. Modulation of SMC marker gene expression has been observed following Ca(2+) influx from multiple sources, however, it is unknown whether upregulation of K(Ca)3.1 and/or suppression of SMC differentiation genes is dependent on a Ca(2+) mediated mechanism. The purpose of this study was to determine the dependence of mitogen-induced SMC phenotype modulation on store-operated Ca(2+) entry (SOCE). In growth-arrested, differentiated rat aortic SMCs, platelet-derived growth factor-BB (PDGF-BB) augmented SOCE. However, PDGF-BB induced upregulation of K(Ca)3.1 and downregulation of the SMC marker gene smooth muscle myosin heavy chain (SMMHC) and myocardin was not dependent on SOCE. Co-treatment with the iPLA2 inhibitor bromoenol lactone (BEL) inhibited the effects of PDGF-BB on SMC phenotype modulation and SOCE. Our results indicate SOCE is not required for PDGF-BB induced phenotype modulation in rat aortic SMCs. Rather, we implicate a novel BEL-sensitive mechanism which regulates both SOCE and phenotype modulation, independently.

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

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

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.

Transcriptional and posttranscriptional regulation of angiopoietin-2 expression mediated by IGF and PDGF in vascular smooth muscle cells

Angiopoietins play a significant role in vascular development and angiogenesis. Both angiopoietin-1 (Ang1) and angiopoietin-2 (Ang2) bind the receptor tyrosine kinase Tie2. However, while Ang1 signaling results in the stabilization of vessel structure, Ang2 has been linked to vascular instability. The ratio of these two Tie2 ligands is thus critical for vascular stability and remodeling. This study identifies a mechanism of growth factor-mediated reduction in Ang2 expression in vascular smooth muscle cells (VSMCs). In response to PDGF, VSMCs downregulated Ang2 mRNA levels by 75% within 4 h, with a subsequent decrease in Ang2 protein levels. Quantitation of endogenous transcription rates revealed that PDGF stimulation did not alter Ang2 transcription rates, but instead induced a posttranscriptional mechanism of rapid Ang2 mRNA destabilization. The Ang2 mRNA half-life was reduced by at least 50% after PDGF treatment. The PDGF-induced mRNA turnover mechanism was dependent on several MAPK pathways, including ERK and JNK. In contrast, IGF-I, which did not significantly activate ERK or JNK, stimulated increased Ang2 expression through transcriptional activation. These findings demonstrate that VSMCs adjust Ang2 expression through multiple mechanisms, including changes in transcription as well as posttranscriptional mRNA destabilization.

Enhanced proliferation of cultured human vascular smooth muscle cells linked to increased function of RNA-binding protein HuR

In dividing cells, the RNA-binding protein HuR associates with and stabilizes labile mRNAs encoding proliferative proteins, events that are linked to the increased cytoplasmic presence of HuR. Here, assessment of HuR levels in various vascular pathologies (intimal hyperplasia, atherosclerosis and neointimal proliferation, sclerosis of arterialized saphenous venous graft, and fibromuscular dysplasia) revealed a distinct increase in HuR expression and cytoplasmic abundance within the intima and neointima layers. On the basis of these observations, we postulated a role for HuR in promoting the proliferation of vascular smooth muscle cells. To test this hypothesis directly, we investigated the expression, subcellular localization, and proliferative influence of HuR in human vascular smooth muscle cells (hVSMCs). Treatment of hVSMCs with platelet-derived growth factor increased HuR levels in the cytoplasm, thereby influencing the expression of metabolic, proliferative, and structural genes. Importantly, knockdown of HuR expression by using RNA interference caused a reduction of hVSMC proliferation, both basally and following platelet-derived growth factor treatment. We propose that HuR contributes to regulating hVSMC growth and homeostasis in pathologies associated with vascular smooth muscle proliferation.

Molecular Determinants of Vascular Smooth Muscle Cell Diversity

Although the primary role of vascular smooth muscle cells (SMCs) is contraction, they exhibit extensive phenotypic diversity and plasticity during normal development, during repair of vascular injury, and in disease states. Results of recent studies indicate that there are unique as well as common transcriptional regulatory mechanisms that control expression of various SMC marker genes in distinct SMC subtypes, and that these mechanisms are complex and dynamic even at the single cell level. This article will review recent progress in our understanding of the transcriptional regulatory mechanisms involved in controlling expression of SMC marker genes with a particular focus on examination of processes that contribute to the phenotypic diversity of SMCs. In addition, because of considerable controversy in the literature regarding the relationship between phenotypically modulated SMCs and myofibroblasts, we will briefly consider both similarities and differences in regulation of gene expression between these cell types.

Kruppel-like factor 4 abrogates myocardin-induced activation of smooth muscle gene expression

Platelet-derived growth factor BB (PDGF-BB) has been shown to be an extremely potent negative regulator of smooth muscle cell (SMC) differentiation. Moreover, previous studies have demonstrated that the Kruppel-like transcription factor (KLF) 4 potently represses the expression of multiple SMC genes. However, the mechanisms whereby KLF4 suppresses SMC gene expression are not known, nor is it clear whether KLF4 contributes to PDGF-BB-induced down-regulation of SMC genes. The goals of the present studies were to determine the molecular mechanisms by which KLF4 represses expression of SMC genes and whether it contributes to PDGF-BB-induced suppression of these genes. Results demonstrated that KLF4 markedly repressed both myocardin-induced activation of SMC genes and expression of myocardin. KLF4 was rapidly up-regulated in PDGF-BB-treated, cultured SMC, and a small interfering RNA to KLF4 partially blocked PDGF-BB-induced SMC gene repression. Both PDGF-BB and KLF4 markedly reduced serum response factor binding to CArG containing regions within intact chromatin. Finally, KLF4, which is normally not expressed in differentiated SMC in vivo, was rapidly up-regulated in vivo in response to vascular injury. Taken together, results indicate that KLF4 represses SMC genes by both down-regulating myocardin expression and preventing serum response factor/myocardin from associating with SMC gene promoters, and suggest that KLF4 may be a key effector of PDGF-BB and injury-induced phenotypic switching of SMC.

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.

Platelet-derived growth factor-BB and Ets-1 transcription factor negatively regulate transcription of multiple smooth muscle cell differentiation marker genes

Platelet-derived growth factor (PDGF)-BB, a potent mitogen for mesenchymal cells, also downregulates expression of multiple smooth muscle (SM) cell (SMC)-specific markers. However, there is conflicting evidence whether PDGF-BB represses SMC marker expression at a transcriptional or posttranscriptional level, and little is known regarding the mechanisms responsible for these effects. Results of the present studies provide clear evidence that PDGF-BB treatment strongly repressed SM alpha-actin, SM myosin heavy chain (MHC), and SM22alpha promoters in SMCs. Of major significance for resolving previous controversies in the field, we found PDGF-BB-induced repression of SMC marker gene promoters in subconfluent, but not postconfluent, cultures. Treatment of postconfluent SMCs with a tyrosine phosphatase inhibitor restored PDGF-BB-induced repression, whereas treatment of subconfluent SMCs with a tyrosine kinase blocker abolished PDGF-BB-induced repression, suggesting that a tyrosine phosphorylation event mediates cell density-dependent effects. On the basis of previous observations that Ets-1 transcription factor is upregulated within phenotypically modulated neointimal SMCs, we tested whether Ets-1 would repress SMC marker expression. Consistent with this hypothesis, results of cotransfection experiments indicated that Ets-1 overexpression reduced transcriptional activity of SMC marker promoter constructs in SMCs, whereas it increased activity of SM alpha-actin promoter in endothelial cells. PDGF-BB treatment increased expression of Ets-1 in cultured SMCs, and SM alpha-actin mRNA expression was reduced in multiple independent clones of SMCs stably transfected with an Ets-1-overexpressing construct. Taken together, results of these experiments provide novel insights regarding possible mechanisms whereby PDGF-BB and Ets-1 may contribute to SMC phenotypic switching associated with vascular injury.

Regulation of the mesangial cell myofibroblast phenotype by actin polymerization

Mesangial cells in diverse glomerular diseases become myofibroblast-like, characterized by activation of smooth muscle alpha-actin (alpha-SMA) expression. In cultured mesangial cells, serum-deprivation markedly increases alpha-SMA expression, cell size, and stress fiber formation. Since stress fibers are assembled from actin monomers, we investigated the hypothesis that alterations in stress fiber formation regulate alpha-SMA expression and hypertrophy. Human mesangial cells were treated with agents that disrupt or stabilize actin stress fibers. Depolymerization of actin stress fibers in serum-deprived cells with actin-depolymerizing agents, cytochalasin B (CytB) and latrunculin B (LatB), or with inhibitors of Rho-kinase, Y-27632 and HA-1077 decreased alpha-SMA mRNA as judged by Northern blot analysis. Western blot analysis showed that CytB also reduced alpha-SMA protein levels. In serum-fed cells, agents that stabilized actin stress fibers, jasplakinolide (Jas) and phalloidin, increased alpha-SMA mRNA and protein. Treatment of human or rat mesangial cells with CytB, LatB, or Y-27632 decreased alpha-SMA promoter activity. In contrast, Jas increased promoter activity 5.6-fold in rat mesangial cells. The presence of an RNA polymerase inhibitor blocked degradation of alpha-SMA mRNA in cells treated with CytB suggesting that destabilization of this message is dependent on a newly transcribed or rapidly degraded factor. Inhibition of actin polymerization by CytB, LatB, Y-27623, and HA-1077 inhibited incorporation of (3)[H]-leucine into newly synthesized protein. Additionally, CytB and LatB decreased cell volume as determined by flow cytometry. Collectively, these results indicate that the state of polymerization of the actin cytoskeleton regulates alpha-SMA expression, hypertrophy, and myofibroblast differentiation in mesangial cells.

Growth factor regulation of smooth muscle actin expression and contraction of human articular chondrocytes and meniscal cells in a collagen-GAG matrix

Recent work has demonstrated that human articular chondrocytes and meniscus cells can express the gene for a contractile actin isoform, alpha-smooth muscle actin (SMA), in vivo. The objective of the present study was to evaluate the effects of two growth factors, transforming growth factor (TGF)-beta1 and platelet-derived growth factor (PDGF)-BB, on the SMA content of these cells and their contraction of a collagen-glycosaminoglycan (GAG) analog of extracellular matrix in vitro. TGF-beta1 was found to markedly increase SMA content of the cells and PDGF-BB decreased SMA expression, with both findings achieving statistical significance. A notable finding was the increased contraction of the collagen-GAG matrix induced by TGF-beta1 and the decrease in contraction resulting from PDGF-BB treatment, indicating a causal relationship between expression of SMA and the contractility of the cells. A novel cell force monitor, employed to estimate the force exerted per cell, demonstrated a higher force exerted by the TGF-beta1-treated cells. The findings demonstrate that the expression of SMA by articular chondrocytes and meniscal cells and their associated contractile behavior can be regulated by selected growth factors. This work provides a foundation for the rational investigation of the mechanisms underlying SMA-enabled contraction of these cell types and the control of this behavior in tissue engineering.

Understanding the final stages of wound contraction

Much research has been undertaken to improve our understanding of the processes of wound contraction. This article, the second in a two-part series, focuses on granulation tissue modulation.

Characterization of cultured smooth muscle cells obtained from the palmar digital arteries of horses

OBJECTIVE

To develop methods to isolate, culture, and characterize smooth muscle cells (SMC) from equine palmar digital arteries.

SAMPLE POPULATION

Segments of the medial or lateral palmar digital arteries from the forelimbs of 6 horses.

PROCEDURE

To obtain smooth muscle explants, arterial segments were incised longitudinally. The tunica intima was gently scraped from the underlying tunica media, and explants were obtained from the tunica media. Approximately 18 to 24 explants were obtained from each palmar digital arterial segment. A substrate-attached technique was used to initiate primary culture of SMCCultured cells were identified as SMC, using light microscopy, electron microscopy, reverse transcriptase-polymerase chain reaction (RT-PCR), and northern blot analysis. The replication index and serum dependence of equine SMC in culture was characterized by use of bromodeoxyuridine.

RESULTS

The SMC of equine palmar digital arteries were successfully cultured, as confirmed by RT-PCR and northern blot analysis techniques for smooth muscle alpha-actin and detection of SMC-specific organelles during electron microscopy. When characterized by light and electron microscopy, SMC were found to have undergone phenotypic modulation to a more synthetic phenotype in culture while retaining characteristics of SMC.

CONCLUSIONS AND CLINICAL RELEVANCE

Culture of SMC from equine palmar digital arteries via an explant protocol is a viable technique for studying vascular biological mechanisms in horses. In vitro studies of SMC may aid investigators in determining cellular mechanisms involved in disease processes such as laminitis.

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.

Platelet-derived growth factor-BB stimulates hypertrophy of peritubular smooth muscle cells from rat testis in primary cultures

The tunica propria of seminiferous tubules contains a particular type of smooth muscle cell (myoid cells) arranged in a contractile epithelioid layer that is responsible for sperm and tubular fluid flow. Unlike other types of smooth muscle (SM) cells, highly purified populations of peritubular smooth muscle cells (PSMC) survive and maintain their contractile phenotype in primary cultures in controlled conditions. We used this culture model to investigate the response of the SM contractile phenotype to prolonged exposure to platelet-derived growth factor (PDGF), one of the main factors involved in vascular SM pathologies. We observed that 4-day continuous exposure of PSMC to PDGF-BB at nanomolar concentrations in plain medium enhances contractile phenotype traits and induces cell hypertrophy without inducing proliferation. In Northern and Western blotting experiments, SM-alpha-actin transcript and protein were found to be markedly increased in the PDGF-BB-treated samples, which is in line with the formation of conspicuous SM-alpha-actin-containing stress fibers. Moreover, binding sites for endothelin-1 were increased, and the calcium response to the contractile agonist, determined in single fura-2-loaded cells, was enhanced. In response to PDGF-BB, the cells underwent immediate, transient contraction, as seen in a scanning electron microscope, followed by a gradual increase in size, as evaluated by cytofluorometry, and enhancement of protein synthesis. The observed pattern of response to PDGF-BB was not accompanied by cell proliferation, as assessed by [3H]thymidine incorporation and direct cell counts. Unlike other SM cell types, in which proliferation and loss of contractile traits are induced by PDGF, chronic treatment of PSMC with this growth factor results in hypertrophy rather than hyperplasia.

Differential regulation of p27(Kip1) expression by mitogenic and hypertrophic factors: Involvement of transcriptional and posttranscriptional mechanisms

Platelet-derived growth factor-BB (PDGF-BB) acts as a full mitogen for cultured aortic smooth muscle cells (SMC), promoting DNA synthesis and cell proliferation. In contrast, angiotensin II (Ang II) induces cellular hypertrophy as a result of increased protein synthesis, but is unable to drive cells into S phase. In an effort to understand the molecular basis for this differential growth response, we have examined the downstream effects of PDGF-BB and Ang II on regulators of the cell cycle machinery in rat aortic SMC. Both PDGF-BB and Ang II were found to stimulate the accumulation of G(1) cyclins with similar kinetics. In addition, little difference was observed in the expression level of their catalytic partners, Cdk4 and Cdk2. However, while both factors increased the enzymatic activity of Cdk4, only PDGF-BB stimulated Cdk2 activity in late G(1) phase. The lack of activation of Cdk2 in Ang II-treated cells was causally related to the failure of Ang II to stimulate phosphorylation of the enzyme on threonine and to downregulate p27(Kip1) expression. By contrast, exposure to PDGF-BB resulted in a progressive and dramatic reduction in the level of p27(Kip1) protein. The time course of p27(Kip1) decline was correlated with a reduced rate of synthesis and an increased rate of degradation of the protein. Importantly, the repression of p27(Kip1) synthesis by PDGF-BB was associated with a marked attenuation of Kip1 gene transcription and a corresponding decrease in Kip1 mRNA accumulation. We also show that the failure of Ang II to promote S phase entry is not related to the autocrine production of transforming growth factor-beta1 by aortic SMC. These results identify p27(Kip1) as an important regulator of the phenotypic response of vascular SMC to mitogenic and hypertrophic stimuli.

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.

Two MCAT elements of the SM alpha-actin promoter function differentially in SM vs. non-SM cells

Transcriptional activity of the smooth muscle (SM) alpha-actin gene is differentially regulated in SM vs. non-SM cells. Contained within the rat SM alpha-actin promoter are two MCAT motifs, binding sites for transcription enhancer factor 1 (TEF-1) transcriptional factors implicated in the regulation of many muscle-specific genes. Transfections of SM alpha-actin promoter-CAT constructs containing wild-type or mutagenized MCAT elements were performed to evaluate their functional significance. Mutation of the MCAT elements resulted in increased transcriptional activity in SM cells, whereas these mutations either had no effect or decreased activity in L6 myotubes or endothelial cells. High-resolution gel shift assays resolved several complexes of different mobilities that were formed between MCAT oligonucleotides and nuclear extracts from the different cell types, although no single band was unique to SM. Western blot analysis of nuclear extracts with polyclonal antibodies to conserved domains of the TEF-1 gene family revealed multiple reactive bands, some that were similar and others that differed between SM and non-SM. Supershift assays with a polyclonal antibody to the TEF-related protein family demonstrated that TEF-1 or TEF-1-related proteins were contained in the shifted complexes. Results suggest that the MCAT elements may contribute to cell type-specific regulation of the SM alpha-actin gene. However, it remains to be determined whether the differential transcriptional activity of MCAT elements in SM vs. non-SM is due to differences in expression of TEF-1 or TEF-1-related proteins or to unique (cell type specific) combinatorial interactions of the MCAT elements with other cis-elements and trans-factors.

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

We have previously shown that maximal expression of the rat smooth muscle myosin heavy chain (SM-MHC) gene in cultured rat aortic smooth muscle cells (SMCs) required the presence of a highly conserved domain (nucleotides -1321 and -1095) that contained two positive-acting serum response factor (SRF) binding elements (CArG boxes 1 and 2) and a negative-acting GC-rich element that was recognized by Sp1 (Madsen, C. S., Hershey, J. C., Hautmann, M. B., White, S. L., and Owens, G. K. (1997) J. Biol. Chem. 272, 6332-6340). In this study, to better understand the functional role of these three cis elements, we created a series of SM-MHC reporter-gene constructs in which each element was mutated either alone or in combination with each other and tested them for activity in transient transfection assays using primary cultured rat aortic SMCs. Results demonstrated that the most proximal SRF binding element (CArG-box1) was active in the absence of CArG-box2, but only upon removal of the GC-rich repressor. In contrast, regardless of sequence context, CArG-box2 was active only when CArG-box1 was present. We further demonstrated using electrophoretic mobility shift assays that Sp1 binding to the GC-rich repressor element did not prevent SRF binding to the adjacent CArG-box2. Thus, unlike other proteins reported to inhibit SRF activity, the repressor activity associated with the GC-rich element does not appear to function through direct inhibition of SRF binding. As a first step toward understanding the importance of these elements in vivo, we performed in vivo footprinting on the intact rat aorta. We demonstrated that both CArG boxes and the GC-rich element were bound by protein within the animal. Additionally, using the rat carotid injury model we showed that Sp1 protein was significantly increased in SMCs located within the myointimal lesion, suggesting that increased expression of this putative repressor factor may contribute to the decreased SM MHC expression within SMCs found in myointimal lesions.

Effects of protein kinase C alpha overexpression on A7r5 smooth muscle cell proliferation and differentiation

Smooth muscle cell differentiation and proliferation are increasingly seen to be intimately tied to the etiology of atherosclerosis and hypertension. To determine the role of PKC alpha in the regulation of smooth muscle cell differentiation and proliferation, the rat embryonic smooth muscle cell line A7r5 was transfected with an expression vector containing the full-length PKC alpha cDNA. Neomycin-resistant clones which exhibited increased PKC alpha levels compared to wild-type cells were selected. The A7r5 cells overexpressing PKC alpha had altered morphology and decreased growth rates compared to wild-type cells and cells transfected only with the neomycin resistance gene. Electrophoretic mobility shift assays showed that nuclear extracts from overexpressing clones gave a different pattern of protein-DNA binding to an AP-1 consensus oligonucleotide compared to wild-type cells. In contrast to the growth characteristics of these clones, their levels of cell differentiation marker proteins such as vinculin and desmin were not affected by PKC alpha overexpression. Moreover, the smooth muscle-specific differentiation marker alpha-actin was markedly reduced, while beta-actin levels were found to remain unchanged. Northern blot analysis confirmed that alpha-actin downregulation occurred at the RNA level. Western blot analysis revealed that A7r5 cells have five different PKC isoforms and that these isoform protein levels were not changed by PKC alpha overexpression. These findings suggest that PKC alpha regulates growth and differentiation of A7r5 smooth muscle cells and that these changes might result from altered expression/function of AP-1 transcription factors.

Angiotensin II-induced stimulation of smooth muscle alpha-actin expression by serum response factor and the homeodomain transcription factor MHox

The objective of the present study was to examine the molecular mechanisms whereby angiotensin II (Ang II) stimulates smooth muscle (SM) alpha-actin expression in rat aortic smooth muscle cells (SMCs). Nuclear run-on analysis and transfection studies indicated that the effects of Ang II on SM alpha-actin were mediated at least in part at the transcriptional level. Transfection of various rat SM alpha-actin promoter/chloramphenicol acetyltransferase (CAT) constructs into SMCs demonstrated that the first 155 bp of the SM alpha-actin promoter was sufficient to confer maximal Ang II responsiveness, conferring an approximately 4-fold increase in reporter activities in these SMCs compared with vehicle-treated SMCs. Mutation of either of two highly conserved CArG elements, designated A (-62) and B (-112), completely abolished Ang II-induced increases in reporter activity, whereas mutation of a homeodomain-like binding sequence at -145 (ATTA) reduced reporter activity by half. Results of EMSAs showed that nuclear extracts from Ang II-treated SMCs exhibited enhanced binding activity of serum response factor (SRF) to the CArG elements and of a homeodomain factor, MHox, to the ATTA element. Northern analyses showed that Ang II also stimulated marked increases in MHox mRNA levels. Western analyses demonstrated that Ang II-induced increases in SRF binding were not due to increased SRF protein expression. Recombinant MHox markedly enhanced binding activity of SRF in EMSAs. Finally, MHox overexpression transactivated a SM alpha-actin promoter/CAT reporter construct by approximately 3.5-fold in transient cotransfection studies. These results provide evidence for involvement of a homeodomain transcription factor, MHox, in Ang II-mediated stimulation of SM alpha-actin via a CArG/SRF-dependent mechanism.

Transforming growth factor-beta 1 modulates myofibroblastic phenotype of rat palatal fibroblasts in vitro

The effects of transforming growth factor-beta 1 (TGF-beta 1) on normal rat palatal fibroblasts in vitro were investigated in the present study in order to unravel the precise mechanisms by which the phenotypic modulation of fibroblasts occurs during the scar formation process. TGF-beta 1 dramatically changed the morphology of normal palatal fibroblasts from polygonal into an elongated shape, which was very similar to that of fibroblasts derived from experimental immature scar tissue in rat palatal mucosa. This morphological transition was concomitant with an increase in the expression of alpha-smooth muscle (alpha-SM) actin protein, a marker for myofibroblasts, when determined by immunocytochemistry. An immunoblot study also revealed that alpha-SM actin expression in palatal fibroblasts became evident after 24 h of TGF-beta 1 treatment and increased time-dependently up to 72 h. Northern blot analysis showed that TGF-beta 1 stimulated endogenous TGF-beta 1 mRNA expression in palatal fibroblasts within 24 h. Neither epidermal growth factor nor basic fibroblast growth factor had any effect on either alpha-SM actin expression or TGF-beta 1 mRNA expression. Pretreatment of palatal fibroblasts with TGF-beta 1 significantly increased the contractile capacity in a three-dimensional collagen gel culture, even when the culture medium was deprived of TGF-beta 1 for 72 h of the experimental period. Moreover, the contractility of scar fibroblasts, which highly expressed alpha-SM actin protein and TGF-beta 1 mRNA, was significantly lowered by a neutralizing antibody to TGF-beta 1. These data strongly suggest that TGF-beta 1 is a potential inducer of phenotypic expression of myofibroblasts in palatal fibroblasts and that auto-induction of TGF-beta 1 mRNA expression may play an important role in the scar formation process in palatal mucosa.

Identification of a potential role for the adventitia in vascular lesion formation after balloon overstretch injury of porcine coronary arteries

BACKGROUND

In the present series of experiments, we examined the onset of cell proliferation and growth factor expression after balloon overstretch injury to porcine coronary arteries.

METHODS AND RESULTS

Domestic juvenile swine underwent balloon overstretch injury to the left anterior descending and circumflex coronary arteries with standard percutaneous transluminal coronary angioplasty balloon catheters. To identify proliferating cells, 5-bromo-2-deoxyuridine (BrDU) was administered over a period of 24 hours before the animals were killed at either 1, 3, 7, or 14 days after injury. Immunohistochemistry was performed with monoclonal antibodies to BrDU and smooth muscle cell markers. Three days after injury, a large number of proliferating cells were located in the adventitia, with significantly fewer positive cells found in the media and lumen. Seven days after injury, proliferating cells were found primarily in the neointima, extending along the luminal surface. In situ hybridization for PDGF A-chain and beta-receptor mRNAs revealed that the expression of these two genes was closely correlated with the sites of proliferation at each time point. Studies in which BrDU was injected between days 2 and 3 and the animals were killed on day 14 suggested that the proliferating adventitial cells may migrate into the neointima.

CONCLUSIONS

These data suggest that adventitial myofibroblasts contribute to the process of vascular lesion formation by proliferating, synthesizing growth factors, and possibly migrating into the neointima. Increased synthesis of alpha-smooth muscle actin observed in the adventitial cells after arterial injury may constrict the injured vessel and contribute to the process of arterial remodeling and late lumen loss after angioplasty.

Differentiated properties and proliferation of arterial smooth muscle cells in culture

The smooth muscle cell is the sole cell type normally found in the media of mammalian arteries. In the adult, it is a terminally differentiated cell that expresses cytoskeletal marker proteins like smooth muscle alpha-actin and smooth muscle myosin heavy chains, and contracts in response to chemical and mechanical stimuli. However, it is able to revert to a proliferative and secretory active state equivalent to that seen during vasculogenesis in the fetus, and this is a prerequisite for the involvement of the smooth muscle cell in the formation of atherosclerotic and restenotic lesions. A similar transition from a contractile to a synthetic phenotype occurs when smooth muscle cells are established in culture. Accordingly, an in vitro system has been used extensively to study the regulation of differentiated properties and proliferation of these cells. During the first few days after seeding, the cells are reorganized structurally with a loss of myofilaments and formation of a widespread endoplasmic reticulum and a prominent Golgi complex. In parallel, they lose their contractility and instead become competent to divide in response to a large variety of mitogens, including platelet-derived growth factor (PDGF) and basic fibroblast growth factor (bFGF). After entering the cell cycle, they start to produce these and other mitogens on their own, and continue to replicate in the absence of exogenous stimuli for a restricted number of generations. Furthermore, they start to secrete extracellular matrix components such as collagen, elastin, and proteoglycans. The mechanisms that control this change in morphology and function of the smooth muscle cells are still poorly understood. Adhesive proteins such as fibronectin and laminin apparently have an important role in determining the basic phenotypic state of the cells and exert their effects via integrin receptors. The proliferative and secretory activities of the cells are influenced by a multitude of growth factors, cytokines, and other molecules. Although much work remains before an integrated view of this regulatory machinery can be achieved, there is no doubt that the cell culture technique has contributed substantially to our knowledge of smooth muscle differentiation and growth. At the same time, it has been crucial in exploring the role of these cells in vascular disease and developing new therapeutic strategies to cope with major causes of human death and disability.

Angiotensin II-induced hypertrophy of rat vascular smooth muscle is associated with increased 18 S rRNA synthesis and phosphorylation of the rRNA transcription factor, upstream binding factor

Hypertrophy of vascular smooth muscle cells (VSMC) is an important adaptive response of hypertension. Drug intervention studies have implicated a role for angiotensin II (A-II) in the mediation of VSMC hypertrophy in vivo, and A-II is a potent hypertrophic agent for VSMC in culture. Our laboratory has previously shown that A-II-induced hypertrophy of cultured VSMC is due in part to generalized increases in protein synthesis and increased content of rRNA. The aim of the present study was to determine if A-II stimulates rRNA gene synthesis and whether the rRNA transcription factor, upstream binding factor (UBF), is involved. Nuclear run-on analysis demonstrated that A-II induced a greater than 5-fold increase in rRNA gene synthesis within 6 h of stimulation. A-II also stimulated a rapid increase in UBF phosphorylation as well as nucleolar localization, but no changes in the content of UBF. Phosphoamino acid analysis showed that phosphorylation occurred only on serine residue(s). Results demonstrate that increased transcription of ribosomal DNA contributes to the A-II-induced increase in protein synthesis and VSMC hypertrophy, and suggest that an important regulatory event in this pathway may be the phosphorylation and/or nucleolar localization of UBF.

Heparin and the phenotype of adult human vascular smooth muscle cells

To study mechanisms controlling growth and phenotype in human vascular smooth muscle cells, we established culture conditions under which these cells proliferate rapidly and achieve life-spans of 50-60 population doublings. In medium containing heparin and heparin-binding growth factors, growth rate and life-span of human vascular smooth muscle cells increased more than 50% relative to cultures with neither supplement, and more than 20% compared to cultures supplemented only with heparin-binding growth factors. In contrast to observations made in rat vascular smooth muscle cells, smooth muscle-specific alpha-actin in the human cells was expressed only in the presence of heparin and colocalized with beta/gamma nonmuscle actins in stress fibers, not in adhesion plaques. Heparin, in the presence of heparin-binding growth factors, also caused more than 170% stimulation of tracer glucosamine incorporation into hyaluronic acid and a 7.5-fold increase in hyaluronic acid accumulation. In comparison, total sulfate incorporation into sulfated glycosaminoglycans increased by less than 40%. In light of our previous findings that heparin suppresses collagen gene expression, we conclude that heparin induces human vascular smooth muscle cells exposed to heparin-binding growth factors to remodel their extracellular matrix by altering the relative rates of hyaluronic acid (HA) and collagen synthesis. The resulting hyaluronic-acid-rich, collagen-poor matrix may enhance infiltration of CD44/hyaluronate-receptor-bearing T-lymphocytes and monocytes into the vascular wall, an early event in atherogenesis.

Serum induction of MEF2/RSRF expression in vascular myocytes is mediated at the level of translation

Vascular smooth muscle cells (VSMCs) reversibly coordinate the expression of VSMC-specific genes and the genes required for cell cycle progression. Here we demonstrate that isoforms of the MEF2/RSRF transcription factor are expressed in VSMCs and in vascular tissue. The MEF2A DNA-binding activity was upregulated when quiescent VSMCs were stimulated to proliferate with serum mitogens. The serum-induction of MEF2A DNA-binding activity occurred approximately 4 h following serum activation, and this correlated with an increase in the level of MEF2A protein without changes in the level of MEF2A mRNA or protein stability. These results indicate that MEF2A induction by serum is regulated at the level of translation.

Characterization of rat aortic smooth muscle cells resistant to the antiproliferative activity of heparin following long-term heparin treatment

Vascular smooth muscle cells (SMC) do not represent a homogeneous population (Schwartz et al., 1990, Am. J. Pathol. 136: 1417-1428). Cellular clones resistant to the antiproliferative activity of heparin were isolated from rat aortic SMC cultures (Pukac et al., 1990, Cell Regul., 1:435-443; San Antonio et al., 1993, Arterioscler. Thromb., 13:748-757) and from explant of human arterial restenotic lesions (Chan et al., 1993, Lancet, 341:341-342). We have shown in the present study that long-term treatment (growth medium supplemented with 200 micrograms/ml heparin, from the second to the tenth passage) of rat aortic SMC, without cell cloning, resulted in a significant loss of sensitivity to the growth inhibition by heparin and its derivatives. The heparin resistance was stable after growing cells for two passages in heparin-free medium, suggesting the selection of a particular phenotype. We tried to characterize these cells and to determine the causes of the resistance to the growth inhibition by heparin. Heparin-treated SMC (HT-SMC) were smaller than their control culture at the same passage, expressed less alpha-SM actin, and did not overgrow after reaching confluence. As in the heparin-resistant clones (San Antonio et al., 1993, Cell Regul., 1:435-443) expression of alpha-SM actin could be increased in HT-SMC by heparin addition before Western blotting. Heparin resistance was associated with a tenfold decrease in [3H]-heparin binding capacity (Bmax = 1.9 x 10(6) sites per cell) compared to control cultures (Bmax = 1.7 x 10(7) sites per cell), which was irreversible after growing the cells for two additional passages in heparin-free medium. We also investigated protein kinase C (PKC) in HT-SMC in terms of both enzymatic activity and protein expression (evaluated by [3H]-staurosporine and [3H]-phorbol-12,13-dibutyrate binding). We found that HT-SMC had only half the PKC activity and expression as control SMC. Therefore, long-term treatment of rat aortic SMC with heparin allowed the selection of a less differentiated subpopulation of cells, exhibiting low sensitivity to the growth inhibition by heparin, which could be related to the low capacity of binding heparin and to a lower PKC activity and/or expression.

Fibronectin and the basement membrane components laminin and collagen type IV influence the phenotypic properties of subcultured rat aortic smooth muscle cells differently

A substrate of the extracellular matrix protein fibronectin has previously been found to promote the modulation of freshly isolated rat aortic smooth muscle cells from a contractile to a synthetic phenotype early in primary culture. In contrast, substrates of the basement membrane proteins laminin and collagen type IV were found to retain the cells in a contractile phenotype. Here, we have studied whether rat aortic smooth muscle cells tht have already adopted a synthetic phenotype are also affected differently by these proteins. For this sake, subcultured cells were detached with trypsin, seeded on substrates of either fibronectin or laminin plus collagen type IV, and incubated in a serum-free medium for one to three days. RNA blot and immunoblot analyses indicated that cells grown on laminin plus collagen type IV expressed smooth muscle alpha-actin transcripts and protein at higher levels than cells grown on fibronectin. Moreover, immunocytochemical and electron-microscopic analyses revealed that cells positively stained for smooth muscle alpha-actin and cells with a cytoplasm dominated by large microfilament bundles were more numerous on laminin plus collagen type IV than on fibronectin. Finally, thymidine autoradiography showed that the DNA synthetic response to stimulation with platelet-derived growth factor or serum was weaker in cells grown on laminin plus collagen type IV than in cells grown on fibronectin. These findings confirm the notion that a substrate of laminin and collagen type IV stimulates the in vitro expression of differentiated smooth muscle traits at a higher level than does a substrate of fibronectin.(ABSTRACT TRUNCATED AT 250 WORDS)

Lysophosphatidylcholine upregulates the level of heparin-binding epidermal growth factor-like growth factor mRNA in human monocytes

Lysophosphatidylcholine is increased in the plasma of hypercholesterolemic patients, is a component of oxidatively modified low-density lipoprotein, and, as such, may play an important role in atherosclerosis. Here we demonstrate that in human monocytes, lysophosphatidylcholine increases the level of mRNA encoding the heparin-binding epidermal growth factor-like growth factor (HB-EGF), a potent smooth muscle mitogen. Lysophosphatidylcholine treatment also enhances the release of heparin-binding mitogenic activity by these cells in culture. The anti-inflammatory glucocorticoid dexamethasone inhibits the upregulation of HB-EGF mRNA induced by either lysophosphatidylcholine or bacterial lipopolysaccharide in cultured monocytes. However, the responses induced by lysophosphatidylcholine and by lipopolysaccharide differ in their kinetics. In addition, the response to lysophosphatidylcholine is resistant to the action of cycloheximide, whereas the response to lipopolysaccharide is not, suggesting that the activation mechanisms induced by these two stimuli are different. Since a nuclear run-on assay showed no effect of lysophosphatidylcholine on the transcription of the HB-EGF gene, we speculate that lysophosphatidylcholine may increase the level of HB-EGF mRNA by altering the processing or degradation of primary or mature transcripts. Lysophosphatidylcholine enhancement of monocyte production of HB-EGF may represent an important result of the interactions among oxidized low-density lipoprotein and monocyte-derived macrophages and may play a role in initiation of smooth muscle proliferation in atherogenesis.

Platelet-derived growth factor-BB-induced suppression of smooth muscle cell differentiation

Previously, we demonstrated that treatment of postconfluent quiescent rat aortic smooth muscle cells (SMCs) with platelet-derived growth factor (PDGF)-BB dramatically reduced smooth muscle (SM) alpha-actin synthesis. In the present studies, we focused on the expression of two other SM-specific proteins, SM myosin heavy chain (SM-MHC) and SM alpha-tropomyosin (SM-alpha TM), to determine whether the actions of PDGF-BB were specific to SM alpha-actin or represented a global ability of PDGF-BB to inhibit expression of cell-specific proteins characteristic of differentiated SMCs. SM-MHC and SM-alpha TM expression were assessed by one- or two-dimensional gel electrophoretic analysis of proteins from cells labeled with [35S]methionine, as well as by Northern analysis of mRNA levels. Synthesis of both SM-specific proteins was decreased by 50-70% in PDGF-BB--treated cells as compared with cells treated with PDGF vehicle. Treatment of cells with 10% fetal bovine serum, which produced a mitogenic effect equivalent to that of PDGF-BB, decreased SM-MHC synthesis by 40% but increased SM-alpha TM synthesis. SM-MHC and SM-alpha TM mRNA expression was decreased by 80% at 24 hours in PDGF-BB--treated postconfluent SMCs, whereas treatment with 10% fetal bovine serum did not decrease the expression of SM-alpha TM mRNA but did inhibit SM-MHC mRNA expression by 36%. Consistent with the absence of detectable PDGF alpha-receptors on these cells, PDGF-AA had no effect on either mitogenesis or expression of SM-MHC or SM-alpha TM.(ABSTRACT TRUNCATED AT 250 WORDS)

Estrogen-induced morphological and immunohistochemical changes in stroma and epithelium of rat ventral prostate

Prostatic smooth muscle cells have been regarded to play a major pathogenetic role during the development of benign prostatic hyperplasia (BPH) in elderly men. Altered hormonal signals (increased estrogen) have been made responsible for the "metabolic" transformation of prostatic smooth muscle cells, which were thought to produce increased amounts of connective tissue fibers observed in BPH. In order to find out the role of metabolically "activated" smooth muscle cells, hormone stimulation experiments were performed in male rats. The effects of androgen deprivation and estrogen stimulation were recorded by semiquantitative analysis of intermediate and myofilament proteins in stromal smooth muscle cells. In castrated or estrogen-treated or estrogen-treated and castrated animals, the reduction of the glandular lumen is the most obvious morphological alteration, accompanied by an increase in connective tissue. Regressive changes occurred most rapidly in castrated animals (already within the first week), slower in castrated estrogen-treated animals and still slower in normal estrogen-treated animals. Regression of the epithelium was accompanied by a marked decrease in immunoreactivity for prostatic binding protein (PBP) in castrated animals, while PBP immunoreactivity in estrogenized animals was retained for up to 6 weeks. Smooth muscle cells became atrophic in castrated animals. This effect was attenuated in estrogen-treated animals. There was no indication for enhanced collagen synthesis by smooth muscle cells. Actin and desmin-immunoreactivity were only slightly altered in experimental animals and showed a changed distribution pattern. Prostatic smooth muscle cells respond less markedly to hormonal alterations than do the fibroblasts.