Smooth muscle cell differentiation in vitro: models and underlying molecular mechanisms

Development of in vitro models by which to study smooth muscle cell (SMC) differentiation has been hindered by some peculiarities intrinsic to these cells, namely their different embryological origins and their ability to undergo phenotypic modulation in cell culture. Although many in vitro models are available for studying SMC differentiation, careful consideration should be taken so that the model chosen fits the questions being posed. In this review, we summarize several well-established in vitro models available to study SMC differentiation from stem cells and outline novel mechanisms recently identified as underlying SMC differentiation programs.     

Smooth muscle cell hypertrophy versus hyperplasia in hypertension.

Arteries of hypertensive animals have a greater mass of smooth muscle than those of normotensive controls. We examined the contribution of smooth muscle cell hypertrophy and hyperplasia to this increase in mass. Cell size measurements obtained by (i) image analysis of enzyme-dispersed cells, (ii) morphometric evaluation of tissue sections, and (iii) biochemical measures of protein/cell and actin/cell ratios on isolated cells showed that average cell size was greater in spontaneously hypertensive rats than in normotensive Wistar-Kyoto and Sprague-Dawley controls. Average DNA/cell ratios were also increased in spontaneously hypertensive rats while protein/DNA ratios were not different. Analysis of nuclear DNA content of individual cells by flow microfluorimetry and Feulgen-DNA microdensitometry measurements showed that greater than 20% of spontaneously hypertensive rats cells were polyploid while less than 10% of control cells were polyploid. Estimates of cell number per centimeter of aortic length, based on ploidy and DNA content, show no difference between control and hypertensive rats. Thus, smooth muscle hypertrophy alone accounts for the increased mass of smooth muscle in aortas of spontaneously hypertensive rats. Furthermore, this cellular hypertrophy is accompanied by a change in nuclear ploidy. This nuclear response in hypertension may represent a fixed change related to the establishment of a chronic hypertensive state.     

Gremlin promotes vascular smooth muscle cell proliferation and migration

Recent reports highlight the importance of BMP in the vasculature. We investigated the expression pattern and role of the BMP antagonist gremlin in VSMC. We detected gremlin mRNA constitutive expression in adult and embryonic rat aortic VSMC, and in rat carotids. In vitro analysis demonstrated that angiotensin II, TGF-β1 and PDGF induced significant changes in gremlin mRNA expression. Gremlin stable overexpression in A7r5 cells blocked BMP signaling. BMP-induced reduction in VSMC DNA synthesis was markedly inhibited by gremlin overexpression. In fact, gremlin overexpression increased DNA synthesis and cell counts, and accelerated cell cycle progression of VSMC, through mechanisms that include p27^kip1 down-regulation. Gremlin also led to marked increments in VSMC migration. In addition, gremlin gene silencing promoted a significant blockade on cell proliferation and migration. In vivo studies disclosed increased gremlin protein expression in the neointima of balloon-injured carotid arteries. In summary, the BMP antagonist gremlin is constitutively expressed in the normal vasculature. Gremlin induces VSMC proliferation and migration and is significantly regulated by growth factors and injury. We postulate that gremlin plays a part in the development of pathological phenotypic changes of adult VSMC.     

血管平滑肌细胞增殖、迁移的相关信号转导机制

血管平滑肌细胞(VSMC)向内膜增殖和迁移是动脉粥样硬化斑快形成、高血压、血管再狭窄等疾病的共同发病基础之一,多种信号转导通路参与了VSMC增殖、迁移,因此研究相关信号转导机制对上述疾病的防治有重要意义.     

Smooth muscle cell matrix metalloproteinases in idiopathic pulmonary arterial hypertension.

Pulmonary arterial hypertension (PAH) results from persistent vasoconstriction, smooth muscle growth and extracellular matrix (ECM) remodelling of pulmonary arteries (PAs). Matrix metalloproteinases (MMPs) are matrix-degrading enzymes involved in ECM turnover, and in smooth muscle cell (SMC) and endothelial cell migration and proliferation. MMP expression and activity are increased in experimental PAH. Therefore, this study investigated whether similar changes occur in idiopathic PAH (IPAH; formerly known as primary pulmonary hypertension). Both in situ and in vitro studies were performed on PAs from patients undergoing lung transplantation for IPAH and from patients treated by lobectomy for localised lung cancer, who served as controls. In IPAH, MMP-tissue inhibitor of metalloproteinase (TIMP) imbalance was found in cultured PA-SMCs, with increased TIMP-1 and decreased MMP-3. MMP-2 activity was markedly elevated as a result of increases in both total MMP-2 and proportion of active MMP-2. In situ zymography and immunolocalisation showed that MMP-2 was associated with SMCs and elastic fibres, and also confirmed the MMP-3-TIMP-1 imbalance. In conclusion, the findings of this study were consistent with a role for the matrix metalloproteinase-tissue inhibitor of metalloproteinase system in pulmonary vascular remodelling in idiopathic pulmonary arterial hypertension. The matrix metalloproteinase-tissue inhibitor of metalloproteinase imbalance may lead to matrix accumulation, and increased matrix metalloproteinase-2 activity may contribute to smooth muscle cell migration and proliferation. Whether these abnormalities are potential therapeutic targets deserves further investigation.     

激肽释放酶-激肽系统在血管平滑肌细胞增殖和迁移中的作用

血管平滑肌细胞(vascular smooth muscle cell,VSMC)增殖和迁移在动脉粥样硬化以及血管内支架成形术后再狭窄的发生和发展中起着重要作用.近年来的研究显示,激肽释放酶-激肽系统(kallikrein-kinin system,KKS)在细胞因子和转导通路方面与VSMC增殖和迁移密切相关.因此,探讨KKS在VSMC增殖和迁移过程中的作用,对于临床防治动脉粥样硬化、血管内支架成形术后再狭窄具有十分重要的意义.     

Smooth muscle cell proliferation and restenosis after stand alone coronary excimer laser angioplasty

It has been shown that coronary excimer laser angioplasty can remove atherosclerotic intracoronary tissue. Stand alone coronary excimer laser angioplasty was successfully performed in a 53 year old white man with 90% stenosis of the left anterior descending coronary artery and exertional angina (Canadian Cardiovascular Society class III). The lesion was reduced to a 30% residual stenosis with use of a 1.2 mm and subsequently a 1.8 mm diameter laser catheter. Early follow-up angiography 24 h later revealed persistent patency and unchanged lesion diameter of the target vessel. The patient was free of symptoms during the 2 month follow-up period, but died suddenly while playing in a tennis tournament 63 days after the procedure. Postmortem histologic examination revealed 80% restenosis at the lesion site without plaque disruption or thrombosis. Specific staining of the histologic specimen for smooth muscle cells using alpha-actin revealed significant smooth muscle cell proliferation at the site of coronary excimer laser angioplasty. However, most of the vessel narrowing appeared to be due to underlying fibrotic plaque as a result of insufficient tissue ablation. This was probably related to the size of the currently available catheters, which are too small to create a large channel.     

Smooth muscle cell proliferation in the rat coronary artery induced by vitamin D

Ultrastructural changes induced in rat coronary arteries by oral administration of a large dose of vitamin D2 were examined by transmission and scanning electron microscopy. Our observations were that smooth muscle cell proliferation occurred without endothelial denudation and without exposure of the subendothelium. While the endothelial cells did show evidence of degeneration, the adjoining endothelial cells migrated beneath so that these degenerated cells could become detached without exposing the subendothelium. Therefore, factors other than platelets seem to participate in the phenotypic modulation and proliferation of the smooth muscle cell. Medial degeneration may be a prerequisite for formation of the lesion.     

Quercetin对人气道平滑肌细胞增殖和迁移的影响及机制

目的 体外培养人气道平滑肌细胞(human airway smooth muscle cells,HASMCs),探讨槲皮素(Quercetin,Que)对血小板源性生长因子BB(PDGF-BB)刺激的HASMCs增殖和迁移的影响及其机制.方法体外培养HASMCs,分为六组;对照组、PdGF-BB组、Que与PDGF-BB联合干预组、Que组、U0126组、U0126与PDGF-BB联合干预组.四甲基偶氮唑蓝(MTT)微量比色法测定HASMCs增殖,transwell法观察细胞迁移,Western blot法检测ERK的磷酸化及Cyclin D1表达水平.结果 与对照组相比,PDGF-BB(20μg/L)显著诱导HASMCs增殖和迁移(P＜0.05),Que(20～80μmol/L)呈浓度依赖性抑制PDGF-BB诱导的HASMCs的增殖和迁移(P＜0.05).PDGF-BB组ERK磷酸化及Cyclin D1 表达水平较对照组明显增高(P＜0.05).Que(80μmol/L)及PDGF-BB干预组其表达量低于PDGF-BB组,此抑制作用与ERK特异性拮抗剂U0126作用相当(P＞0.05).结论 Que抑制PDGF-BB诱导的HASMCs的增殖和迁移,可能是通过调节ERK/Cyelin D1通路起作用.     

Smooth muscle cell apoptosis in arteriosclerosis

Arteriosclerosis, a paradigmatic age-related disease, encompasses (spontaneous) atherosclerosis, restenosis after percutaneous transluminal coronary angioplasty, autologous arterial or vein graft arteriosclerosis and transplant arteriosclerosis. In all types of arteriosclerosis, vascular smooth muscle cell (SMC) accumulation in the intima is a key event, but abundant evidence also indicates the importance of SMC apoptosis in the development of arteriosclerosis. Because SMC proliferation and apoptosis coincide in arteriosclerotic lesions, the balance between these two processes could be a determinant during vessel remodeling and disease development. Various stimuli, including oxidized lipoproteins, altered hemodynamic stress and free radicals, can induce SMC apoptosis in vitro. As risk factors for arteriosclerosis, these stimuli may also lead to vascular cell apoptosis in vivo. The presence of apoptotic cells in atherosclerotic and restenotic lesions could have potential clinical implications for atherogenesis and contributes to the instability of the lesion. Based on the progress in this field, this review focuses on the mechanism and impact of SMC apoptosis in the pathogenesis of arteriosclerosis and highlights the role of biomechanical stress in SMC apoptosis.     

Extracellular matrix glycoprotein biglycan enhances vascular smooth muscle cell proliferation and migration

Proteoglycans are produced and secreted by vascular smooth muscle cells, but the pathophysiological role of these glycoproteins in the vasculature is an enigma. Because the small leucine-rich proteoglycan (SLRP) biglycan is overexpressed in arteriosclerotic lesions, we produced mice constitutively overexpressing biglycan in the vascular smooth muscle, in order to examine the effects on vascular pathology. In the aorta and renal vasculature, increased vascular proliferation was seen both in the basal state and after infusion of angiotensin II (Ang II) in the transgenic mice compared with wild-type controls. In addition, the combination of biglycan overexpression and Ang II infusion resulted in marked increases in vascular smooth muscle cell proliferation and migration in the coronary arteries, as well as increases in fibrosis surrounding the vessels. In vitro, biglycan caused an increase in thymidine incorporation and migration of vascular smooth muscle cells, whereas these parameters were unchanged or reduced in endothelial cells. Moreover, addition of biglycan resulted in an increase in cdk2 expression and decrease in p27 levels in the vascular smooth muscle cells. These results suggest that this extracellular matrix SLRP may be involved in the regulation of vascular smooth muscle growth and migration through cdk2- and p27-dependent pathways. Furthermore, changes in biglycan expression could be a factor influencing the susceptibility of arteries to vascular injury, and may play a direct role in the pathogenesis of vascular lesions.     

Smooth muscle cell and endothelial cell growth factors

Blood vessels are composed basically of two cell types, vascular endothelial cells (ECs) and vascular smooth muscle cells (SMCs), whose proliferation in vivo is tightly regulated. A number of growth regulatory polypeptides have been identified that stimulate the proliferation of vascular cells. This article reviews briefly the structural properties and biologic activities of the best-characterized vascular cell growth factors. A fuller understanding of the properties of vascular cell growth modulators is an important element in delineating the proliferative events that are associated with vascular injury; with SMC hyperplasia such as occurs in restenosis following angioplasty, in atherosclerosis, and in hypertension; and with angiogenesis, both physiologic and pathologic.     

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.     

MicroRNA-29a对血管平滑肌细胞增殖和迁移的影响

目的 研究MicroRNA-29 a对血管平滑肌细胞(VSMC)增殖和迁移的影响并探讨其可能机制.方法 通过MTT法和划痕法分别观察microRNA-29a对VSMCs活力和迁移能力的影响;通过Realtime-PCR和Western印迹检测分别观察micro-RNA-29 a对MMP-9、MMP-2和PCNA表达的影响.结果 与阴性对照组和空白对照组相比较,MicroRNA-29 a反义寡核苷酸组VSMC体外增殖能力和迁移能力明显被抑制(P＜0.05);Realtime-PCR和Western印迹检测发现与阴性对照组和空白对照组相比较,MicroRNA-29 a反义寡核苷酸组MMP-9、MMP-2和PCNA表达明显降低.结论 microRNA-29a可以影响VSMC增殖和迁移,其机制可能是通过调节MMP-9、MMP-2和PCNA的表达来得以实现的.     

The inhibitory mechanisms of amlodipine in human vascular smooth muscle cell proliferation.

The abnormal proliferation of vascular smooth muscle cells (VSMCs) is closely related to vascular diseases. There is growing evidence that calcium antagonists inhibit VSMC growth/proliferation, yet their molecular mechanisms remain to be determined. Recent reports suggest that p42/p44 mitogen-activated protein kinases (MAPKs) play an important role in cell growth and proliferation induced by growth factors. This study was designed to determine whether these MAPKs are involved in VSMC proliferation induced by basic fibroblast growth factor (bFGF) and to examine the inhibitory effect of amlodipine. Human VSMCs were obtained from inner mammary artery. p42/p44 MAPKs activity was measured by immunoblotting assay using anti-p42/p44 phospho-MAPK antibody. 1) bFGF (20 ng/ml) significantly activated p42/p44 MAPKs with a peak time of 5-15 min, which was maintained for 3 h. PD98059 (100 nM-10 microM), a specific inhibitor of MAPK kinase, inhibited bFGF-induced p42/p44 MAPKs activation in a dose-dependent manner. 2) Amlodipine (1-100 nM) dose-dependently inhibited p42/p44 MAPKs activation by bFGF. 3) Amlodipine (10 nM) could inhibit both short-term and long-term p42/p44 MAPKs activation by bFGF. Our results indicate that bFGF could activate p42/p44 MAPKs. Amlodipine, which could inhibit bFGF-induced human VSMC proliferation, inhibited both short-term and sustained p42/p44 MAPKs activation by bFGF, suggesting that bFGF-induced VSMC proliferation may be related to p42/p44 MAPKs activation, and that the antiproliferative effect of amlodipine may be related to its inhibition of p42/p44 MAPKs activation.     

Beta-arrestins regulate atherosclerosis and neointimal hyperplasia by controlling smooth muscle cell proliferation and migration

Atherosclerosis and arterial injury-induced neointimal hyperplasia involve medial smooth muscle cell (SMC) proliferation and migration into the arterial intima. Because many 7-transmembrane and growth factor receptors promote atherosclerosis, we hypothesized that the multifunctional adaptor proteins beta-arrestin1 and -2 might regulate this pathological process. Deficiency of beta-arrestin2 in ldlr(-/-) mice reduced aortic atherosclerosis by 40% and decreased the prevalence of atheroma SMCs by 35%, suggesting that beta-arrestin2 promotes atherosclerosis through effects on SMCs. To test this potential atherogenic mechanism more specifically, we performed carotid endothelial denudation in congenic wild-type, beta-arrestin1(-/-), and beta-arrestin2(-/-) mice. Neointimal hyperplasia was enhanced in beta-arrestin1(-/-) mice, and diminished in beta-arrestin2(-/-) mice. Neointimal cells expressed SMC markers and did not derive from bone marrow progenitors, as demonstrated by bone marrow transplantation with green fluorescent protein-transgenic cells. Moreover, the reduction in neointimal hyperplasia seen in beta-arrestin2(-/-) mice was not altered by transplantation with either wild-type or beta-arrestin2(-/-) bone marrow cells. After carotid injury, medial SMC extracellular signal-regulated kinase activation and proliferation were increased in beta-arrestin1(-/-) and decreased in beta-arrestin2(-/-) mice. Concordantly, thymidine incorporation and extracellular signal-regulated kinase activation and migration evoked by 7-transmembrane receptors were greater than wild type in beta-arrestin1(-/-) SMCs and less in beta-arrestin2(-/-) SMCs. Proliferation was less than wild type in beta-arrestin2(-/-) SMCs but not in beta-arrestin2(-/-) endothelial cells. We conclude that beta-arrestin2 aggravates atherosclerosis through mechanisms involving SMC proliferation and migration and that these SMC activities are regulated reciprocally by beta-arrestin2 and beta-arrestin1. These findings identify inhibition of beta-arrestin2 as a novel therapeutic strategy for combating atherosclerosis and arterial restenosis after angioplasty.