Isolation of a morphologically and functionally distinct smooth muscle cell type from the intimal aspect of the normal rat aorta. Evidence for smooth muscle cell heterogeneity

Human Induced Pluripotent Stem Cell-Derived Vascular Cells: Recent Progress and Future Directions

Human induced pluripotent stem cells (hiPSCs) hold great promise for cardiovascular regeneration following ischemic injury. Considerable effort has been made toward the development and optimization of methods to differentiate hiPSCs into vascular cells, such as endothelial and smooth muscle cells (ECs and SMCs). In particular, hiPSC-derived ECs have shown robust potential for promoting neovascularization in animal models of cardiovascular diseases, potentially achieving significant and sustained therapeutic benefits. However, the use of hiPSC-derived SMCs that possess high therapeutic relevance is a relatively new area of investigation, still in the earlier investigational stages. In this review, we first discuss different methodologies to derive vascular cells from hiPSCs with a particular emphasis on the role of key developmental signals. Furthermore, we propose a standardized framework for assessing and defining the EC and SMC identity that might be suitable for inducing tissue repair and regeneration. We then highlight the regenerative effects of hiPSC-derived vascular cells on animal models of myocardial infarction and hindlimb ischemia. Finally, we address several obstacles that need to be overcome to fully implement the use of hiPSC-derived vascular cells for clinical application.

Vildagliptin improves vascular smooth muscle relaxation and decreases cellular senescence in the aorta of doxorubicin-treated rats

INTRODUCTION

Doxorubicin (DOX) is a chemotherapeutic agent used in cancer treatment. Its use is limited by later toxicity to the cardiovascular system (CVS). Cellular senescence has been proposed as one mechanism of DOX toxicity. It has also been suggested that senescence reduction can improve the condition in many pathologies. We hypothesised that vildagliptin treatment can reduce senescence and thus improve the relaxation of vascular smooth muscle (VSM) in the aorta of a rat DOX model.

METHODS

The rats received DOX and were treated with vildagliptin for 6 weeks. Thereafter, the rats were sacrificed, and the aorta prepared for measurements of VSM relaxation and RNA isolation to detect the level of senescence markers. To further prove the antisenescence effect of the main vildagliptin effector glucagon-like peptide 1(GLP-1), VSM cells (VSMCs) were incubated with DOX and treated with GLP-1. Subsequently, senescence was detected by senescence-associated beta-galactosidase (SA-β-gal) and by the presence of senescence markers.

RESULTS

DOX in rats caused diminished relaxation of VSM to sodium nitrate and caused an increase in the senescence mRNA markers p16Ink4a and p27Kip1 and the senescence-associated secretory phenotype (SASP) IL-6 and IL-8. Vildagliptin treatment led to improved relaxation and a reduction in senescence and SASP markers. Furthermore, in VSMCs DOX increased SA-β-gal activity, p16Ink4a, p27Kip1, IL-6 and IL-8, and GLP1 treatment led to a decrease of both senescence and SASP markers.

CONCLUSION

In summary we conclude that vildagliptin can reduce senescence and improve relaxation of vascular smooth muscle in the aorta of DOX-treated rats, and GLP-1 can reduce senescence of DOX-treated VSMCs. These data suggest that incretin-based drugs are promising candidates for patients suffering from late doxorubicin cardiovascular toxicity.

Smooth muscle cell fate and plasticity in atherosclerosis

Current knowledge suggests that intimal smooth muscle cells (SMCs) in native atherosclerotic plaque derive mainly from the medial arterial layer. During this process, SMCs undergo complex structural and functional changes giving rise to a broad spectrum of phenotypes. Classically, intimal SMCs are described as dedifferentiated/synthetic SMCs, a phenotype characterized by reduced expression of contractile proteins. Intimal SMCs are considered to have a beneficial role by contributing to the fibrous cap and thereby stabilizing atherosclerotic plaque. However, intimal SMCs can lose their properties to such an extent that they become hard to identify, contribute significantly to the foam cell population, and acquire inflammatory-like cell features. This review highlights mechanisms of SMC plasticity in different stages of native atherosclerotic plaque formation, their potential for monoclonal or oligoclonal expansion, as well as recent findings demonstrating the underestimated deleterious role of SMCs in this disease.

A role for collagen type IV in cardiovascular disease?

Over the past decade, studies have repeatedly found single-nucleotide polymorphisms located in the collagen ( COL) 4A1 and COL4A2 genes to be associated with cardiovascular disease (CVD), and the 13q34 locus harboring these genes is one of ~160 genome-wide significant risk loci for coronary artery disease. COL4A1 and COL4A2 encode the α₁- and α₂-chains of collagen type IV, a major component of basement membranes in various tissues including arteries. Despite the growing body of evidence indicating a role for collagen type IV in CVD, remarkably few studies have aimed to directly investigate such a role. The purpose of this review is to summarize the clinical reports linking 13q34 to coronary artery disease, atherosclerosis, and artery stiffening and to assemble the scattered pieces of evidence from experimental studies based on vascular cells and tissue collectively supporting a role for collagen type IV in atherosclerosis and other macrovascular disease conditions.

Hypoxia paradoxically inhibits the angiogenic response of isolated vessel explants while inducing overexpression of vascular endothelial growth factor

This study was designed to investigate how changes in O2 levels affected angiogenesis in vascular organ culture. Although hypoxia is a potent inducer of angiogenesis, aortic rings cultured in collagen paradoxically failed to produce an angiogenic response in 1-4 % O2. Additionally, aortic neovessels preformed in atmospheric O2 lost pericytes and regressed at a faster rate than control when exposed to hypoxia. Aortic explants remained viable in hypoxia and produced an angiogenic response when returned to atmospheric O2. Hypoxic aortic rings were unresponsive to VEGF, while increased oxygenation of the system dose-dependently enhanced VEGF-induced angiogenesis. Hypoxia-induced refractoriness to angiogenic stimulation was not restricted to the aorta because similar results were obtained with vena cava explants or isolated endothelial cells. Unlike endothelial cells, aorta-derived mural cells were unaffected by hypoxia. Hypoxia downregulated expression in aortic explants of key signaling molecules including VEGFR2, NRP1 and Prkc-beta while upregulating expression of VEGFR1. Medium conditioned by hypoxic cultures exhibited angiostatic and anti-VEGF activities likely mediated by sVEGFr1. Hypoxia reduced expression of VEGFR1 and VEGFR2 in endothelial cells while upregulating VEGFR1 in macrophages and VEGF in both macrophages and mural cells. Thus, changes in O2 levels profoundly affect the endothelial response to angiogenic stimuli. These results suggest that hypoxia-induced angiogenesis is fine-tuned by complex regulatory mechanisms involving not only production of angiogenic factors including VEGF but also differential regulation of VEGFR expression in different cell types and production of inhibitors of VEGF function such as sVEGFR1.

Intimal smooth muscle cells are a source but not a sensor of anti-inflammatory CYP450 derived oxylipins

Vascular pathologies are associated with changes in the presence and expression of morphologically distinct vascular smooth muscle cells. In particular, in complex human vascular lesions and models of disease in pigs and rodents, an intimal smooth muscle cell (iSMC) which exhibits a stable epithelioid or rhomboid phenotype in culture is often found to be present in high numbers, and may represent the reemergence of a distinct developmental vascular smooth muscle cell phenotype. The CYP450-oxylipin - soluble epoxide hydrolase (sEH) pathway is currently of great interest in targeting for cardiovascular disease. sEH inhibitors limit the development of hypertension, diabetes, atherosclerosis and aneurysm formation in animal models. We have investigated the expression of CYP450-oxylipin-sEH pathway enzymes and their metabolites in paired intimal (iSMC) and medial (mSMC) cells isolated from rat aorta. iSMC basally released significantly larger amounts of epoxy-oxylipin CYP450 products from eicosapentaenoic acid > docosahexaenoic acid > arachidonic acid > linoleic acid, and expressed higher levels of CYP2C12, CYP2B1, but not CYP2J mRNA compared to mSMC. When stimulated with the pro-inflammatory TLR4 ligand LPS, epoxy-oxylipin production did not change greatly in iSMC. In contrast, LPS induced epoxy-oxylipin products in mSMC and induced CYP2J4. iSMC and mSMC express sEH which metabolizes primary epoxy-oxylipins to their dihydroxy-counterparts. The sEH inhibitors TPPU or AUDA inhibited LPS-induced NFκB activation and iNOS induction in mSMC, but had no effect on NFκB nuclear localization or inducible nitric oxide synthase in iSMC; effects which were recapitulated in part by addition of authentic epoxy-oxylipins. iSMCs are a rich source but not a sensor of anti-inflammatory epoxy-oxylipins. Complex lesions that contain high levels of iSMCs may be more resistant to the protective effects of sEH inhibitors.

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We examined oxylipin production in different SMC phenotypes.

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Intimal SMC produced more oxylipins than medial SMC.

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CYPs were differentially expressed and regulated by LPS in intimal and medial SMC.

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sEH inhibitors reduce inflammation in medial but not intimal SMC.

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Intimal SMC are a source but not sensor of epoxy-oxylipins.

Regulation of angiogenesis, mural cell recruitment and adventitial macrophage behavior by Toll-like receptors

The angiogenic response to injury can be studied by culturing rat or mouse aortic explants in collagen gels. Gene expression studies show that aortic angiogenesis is preceded by an immune reaction with overexpression of Toll-like receptors (TLRs) and TLR-inducible genes. TLR1, 3, and 6 are transiently upregulated at 24 h whereas TLR2, 4, and 8 expression peaks at 24 h but remains elevated during angiogenesis and vascular regression. Expression of TLR5, 7 and 9 steadily increases over time and is highest during vascular regression. Studies with isolated cells show that TLRs are expressed at higher levels in aortic macrophages compared to endothelial or mural cells with the exception of TLR2 and TLR9 which are more abundant in the aortic endothelium. LPS and other TLR ligands dose dependently stimulate angiogenesis and vascular endothelial growth factor production. TLR9 ligands also influence the behavior of nonendothelial cell types by blocking mural cell recruitment and inducing formation of multinucleated giant cells by macrophages. TLR9-induced mural cell depletion is associated with reduced expression of the mural cell recruiting factor PDGFB. The spontaneous angiogenic response of the aortic rings to injury is reduced in cultures from mice deficient in myeloid differentiation primary response 88 (MyD88), a key adapter molecule of TLRs, and following treatment with an inhibitor of the NFκB pathway. These results suggest that the TLR system participates in the angiogenic response of the vessel wall to injury and may play an important role in the regulation of inflammatory angiogenesis in reactive and pathologic processes.

The acute phase reactant orosomucoid-1 is a bimodal regulator of angiogenesis with time- and context-dependent inhibitory and stimulatory properties

Background

Tissues respond to injury by releasing acute phase reaction (APR) proteins which regulate inflammation and angiogenesis. Among the genes upregulated in wounded tissues are tumor necrosis factor-alpha (TNFα) and the acute phase reactant orosomucoid-1 (ORM1). ORM1 has been shown to modulate the response of immune cells to TNFα, but its role on injury- and TNFα-induced angiogenesis has not been investigated. This study was designed to characterize the role of ORM1 in the angiogenic response to injury and TNFα.

Methods and Results

Angiogenesis was studied with in vitro, ex vivo, and in vivo angiogenesis assays. Injured rat aortic rings cultured in collagen gels produced an angiogenic response driven by macrophage-derived TNFα. Microarray analysis and qRT-PCR showed that TNFα and ORM1 were upregulated prior to angiogenic sprouting. Exogenous ORM1 delayed the angiogenic response to injury and inhibited the proangiogenic effect of TNFα in cultures of aortic rings or isolated endothelial cells, but stimulated aortic angiogenesis over time while promoting VEGF production and activity. ORM1 inhibited injury- and TNFα-induced phosphorylation of MEK1/2 and p38 MAPK in aortic rings, but not of NFκB. This effect was injury/TNFα-specific since ORM1 did not inhibit VEGF-induced signaling, and cell-specific since ORM1 inhibited TNFα-induced phosphorylation of MEK1/2 and p38 MAPK in macrophages and endothelial cells, but not mural cells. Experiments with specific inhibitors demonstrated that the MEK/ERK pathway was required for angiogenesis. ORM1 inhibited angiogenesis in a subcutaneous in vivo assay of aortic ring-induced angiogenesis, but stimulated developmental angiogenesis in the chorioallantoic membrane (CAM) assay.

Conclusion

ORM1 regulates injury-induced angiogenesis in a time- and context-dependent manner by sequentially dampening the initial TNFα-induced angiogenic response and promoting the downstream stimulation of the angiogenic process by VEGF. The context-dependent nature of ORM1 angioregulatory function is further demonstrated in the CAM assay where ORM1 stimulates developmental angiogenesis without exerting any inhibitory activity.

Technical Advance: The rat aorta contains resident mononuclear phagocytes with proliferative capacity and proangiogenic properties

Angiogenesis in the aortic ring model is preceded by activation of the immune system and impaired by ablation of adventitial macrophages. Treatment of aortic cultures with M-CSF induced extensive periaortic outgrowth of CD45(+) CD68(+) mononuclear cells with ultrastructural features of macrophages and DCs. Periaortic lysis of collagen caused many CD45(+) CD68(+) cells to attach to the bottom of the culture dish. Lifting the collagen gels left behind patches of CD45(+) CD68(+) cells, which focally organized into branching cords. These cells also expressed CD14, CD169, F4/80, and α-SMA but not CD31, vWF, desmin, or CD163. DNA synthesis studies showed that M-CSF-stimulated cells were actively proliferating. Aortic patch cells showed phagocytic properties and responded to IL-4 and GM-CSF by expressing MHC II, differentiating into DCs, and forming multinucleated giant cells. They also stimulated angiogenesis and VEGF production in aortic ring cultures. This study demonstrates that the rat aorta contains a distinct subset of immature immunocytes capable of proliferating, differentiating into macrophages and DCs, and stimulating angiogenesis. Isolation of these cells in patches from M-CSF-stimulated aortic rings provides a reproducible system to study the biology and angiogenic role of the resident immune system of the aortic wall.

The aortic ring model of angiogenesis: a quarter century of search and discovery

The aortic ring model has become one of the most widely used methods to study angiogenesis and its mechanisms. Many factors have contributed to its popularity including reproducibility, cost effectiveness, ease of use and good correlation with in vivo studies. In this system aortic rings embedded in biomatrix gels and cultured under chemically defined conditions generate arborizing vascular outgrowths which can be stimulated or inhibited with angiogenic regulators. Originally based on the rat aorta, the aortic ring model was later adapted to the mouse for the evaluation of specific molecular alterations in genetically modified animals. Viral transduction of the aortic rings has enabled investigators to overexpress genes of interest in the aortic cultures. Experiments on angiogenic mechanisms have demonstrated that formation of neovessels in aortic cultures is regulated by macrophages, pericytes and fibroblasts through a complex molecular cascade involving growth factors, inflammatory cytokines, axonal guidance cues, extracellular matrix (ECM) molecules and matrix-degrading proteolytic enzymes. These studies have shown that endothelial sprouting can be effectively blocked by depleting the aortic explants of macrophages or by interfering with the angiogenic cascade at multiple levels including growth factor signalling, cell adhesion and proteolytic degradation of the ECM. In this paper, we review the literature in this field and retrace the journey from our first morphological descriptions of the aortic outgrowths to the latest breakthroughs in the cellular and molecular regulation of aortic vessel growth and regression.

Digging in the "soil" of the aorta to understand the growth of abdominal aortic aneurysms

Extensive studies into the etiology of aortic aneurysm disease have focused on the characteristic and unique inflammatory infiltration and elaboration of products of inflammatory cells which can result in matrix degradation. While these changes clearly have a significant impact on the development of aneurysm disease, little attention has been paid to the changes in the parenchymal cells of the aorta. Under normal conditions, the vascular smooth muscle cells which populate the aortic wall are responsible for the maintenance of the matrix components of the media, particularly the elastic fibers. As our understanding of the mechanisms of aneurysm formation and normal arterial anatomy become more sophisticated, it is clear that specific changes to these smooth muscle cells make them active participants in the medial matrix destruction characteristic of aneurysm disease. As others have described for intimal arterial disease, this is the "soil" from which aortic aneurysms grow.

Endothelial-Mesenchymal Transition Occurs during Embryonic Pulmonary Artery Development

Pulmonary vascular remodeling is a process generally associated with pulmonary hypertension that involves intimal thickening, medial hyperthrophy, and plexiform lesions. Morphological studies during pulmonary hypertension have indicated that intimal thickening consists of immature smooth muscle cells (SMCs) associated with determined extracellular matrix components, suggesting an important role for these cells in vascular lesions. Controversy exists regarding the nature and origin of the cells conforming the intimal thickenings. In this study, the authors characterized the in vivo phenotype of the cells located in the pulmonary artery wall during the advanced stages of chicken embryo development and examined whether intimal thickenings are present in such stages. Immunolabeling of cryosections demonstrated presence of intimal thickenings composed of mesenchymal cells that may arise from the endothelium. These cells persist either as nonmuscle throughout the development, or possibly convert to cells expressing alpha -smooth muscle actin (alpha-SM actin). To determine whether pulmonary endothelial cells undergo a transition to mesenchymal cells, the authors used pulmonary artery explants from 10- to 11-day-old chicken embryos and found that explanted endothelial cells detached from the monolayer and acquired mesenchymal characteristics. Some of these cells maintained immunoreactivity for von Willebrand factor (vWF), whereas other jointly lost vWF and gained alpha -SM actin expression (transitional cells), suggesting conversion to SMCs. Therefore, these findings strongly support the authors' in vivo observations and demonstrate that embryonic pulmonary endothelial cells undergo a transition to mesenchymal cells and participate in intimal thickening formation and pulmonary vascular remodeling.

Expression and characterization of the human alpha 2B-adrenoceptor in a vascular smooth muscle cell line

A vascular smooth muscle cell line stably expressing the human alpha 2B-adrenoceptor at a density of 1.5 pmol/mg membrane protein was generated by transfection of rat A7r5 cells. [35S]GTPgammaS binding experiments and [3H]thymidine incorporation experiments indicated that the expressed receptors were functional, had the expected pharmacological characteristics and efficiently stimulated smooth muscle cell proliferation. Confocal fluorescence microscopy was used to visualize alpha2B-adrenoceptors in A7r5-alpha 2B cells and indicated that the receptors were mainly localized in the plasma membrane. The expression of the smooth muscle-specific marker alpha-actin was similar in transfected A7r5-alpha 2B cells and in non-transfected A7r5 wild-type cells. The generated A7r5-alpha 2B cell line will be a useful tool for studying the function and regulation of alpha 2B-adrenoceptors in vascular smooth muscle cells.

Slo1 caveolin-binding motif, a mechanism of caveolin-1-Slo1 interaction regulating Slo1 surface expression

The large conductance, voltage- and Ca2+-activated potassium (MaxiK, BK) channel and caveolin-1 play important roles in regulating vascular contractility. Here, we hypothesized that the MaxiK alpha-subunit (Slo1) and caveolin-1 may interact with each other. Slo1 and caveolin-1 physiological association in native vascular tissue is strongly supported by (i) detergent-free purification of caveolin-1-rich domains demonstrating a pool of aortic Slo1 co-migrating with caveolin-1 to light density sucrose fractions, (ii) reverse co-immunoprecipitation, and (iii) double immunolabeling of freshly isolated myocytes revealing caveolin-1 and Slo1 proximity at the plasmalemma. In HEK293T cells, Slo1-caveolin-1 association was unaffected by the smooth muscle MaxiK beta1-subunit. Sequence analysis revealed two potential caveolin-binding motifs along the Slo1 C terminus, one equivalent, 1007YNMLCFGIY1015, and another mirror image, 537YTEYLSSAF545, to the consensus sequence, varphiXXXXvarphiXXvarphi. Deletion of 1007YNMLCFGIY1015 caused approximately 80% loss of Slo1-caveolin-1 association while preserving channel normal folding and overall Slo1 and caveolin-1 intracellular distribution patterns. 537YTEYLSSAF545 deletion had an insignificant dissociative effect. Interestingly, caveolin-1 coexpression reduced Slo1 surface and functional expression near 70% without affecting channel voltage sensitivity, and deletion of 1007YNMLCFGIY1015 motif obliterated channel surface expression. The results suggest 1007YNMLCFGIY1015 possible participation in Slo1 plasmalemmal targeting and demonstrate its role as a main mechanism for caveolin-1 association with Slo1 potentially serving a dual role: (i) maintaining channels in intracellular compartments downsizing their surface expression and/or (ii) serving as anchor of plasma membrane resident channels to caveolin-1-rich membranes. Because the caveolin-1 scaffolding domain is juxtamembrane, it is tempting to suggest that Slo1-caveolin-1 interaction facilitates the tethering of the Slo1 C-terminal end to the membrane.

Preparation of primary cultured mesenteric artery smooth muscle cells for fluorescent imaging and physiological studies

In this protocol, we describe a method for isolation and culture of smooth muscle cells derived from the adult rat (or mouse) superior mesenteric artery. Arterial myocytes are obtained by enzymatic dissociation and established in primary culture. The cultured cells retain expression of smooth muscle-specific alpha-actin and physiological responses to agonists. Cultured arterial myocytes (prepared from wild-type or transgenic animals) provide a useful model for studying the regulation of a wide range of vascular smooth muscle responses at the cellular and subcellular levels. Plasmids, RNA interference and antisense oligodeoxynucleotides can be readily introduced into the cells to alter protein expression. Fluorescent dyes can also be introduced to visualize a variety of activities, some of which may be specific to vascular smooth muscle cells. This protocol requires about 3 h on each of 2 consecutive days to complete.

Phenotypic heterogeneity influences the behavior of rat aortic smooth muscle cells in collagen lattice

Phenotypic modulation of vascular smooth muscle cells (SMCs) in atherosclerosis and restenosis involves responses to the surrounding microenvironment. SMCs obtained by enzymatic digestion from tunica media of newborn, young adult (YA) and old rats and from the thickened intima (TI) and underlying media of young adult rat aortas 15 days after ballooning were entrapped in floating populated collagen lattice (PCL). TI-SMCs elongated but were poor at PCL contraction and remodeling and expressed less alpha2 integrin compared to other SMCs that appeared more dendritic. During early phases of PCL contraction, SMCs showed a marked decrease in the expression of alpha-smooth muscle actin and myosin. SMCs other than TI-SMCs required 7 days to re-express alpha-smooth muscle actin and myosin. Only TI-SMCs in PCL were able to divide in 48 h, with a greater proportion in S and G2-M cell cycle phases compared to other SMCs. Anti-alpha2 integrin antibody markedly inhibited contraction but not proliferation in YA-SMC-PLCs; anti-alpha1 and anti-alpha2 integrin antibodies induced a similar slight inhibition in TI-SMC-PCLs. Finally, TI-SMCs rapidly migrated from PCL on plastic reacquiring their epithelioid phenotype. Heterogeneity in proliferation and cytoskeleton as well the capacity to remodel the extracellular matrix are maintained, when SMCs are suspended in PCLs.

Modulation of smooth muscle cell proliferation and migration: role of smooth muscle cell heterogeneity

Proliferation and migration of smooth muscle cells (SMCs) from the media towards the intima are key events in atherosclerosis and restenosis. During these processes, SMC undergo phenotypic modulations leading to SMC dedifferentiation. The identification and characterization of factors controlling these phenotypic changes are crucial in order to prevent the formation of intimal thickening. One of the questions which presently remains open, is to know whether any SMCs of the media are capable of accumulating into the intima or whether only a predisposed medial SMC subpopulation is involved in this process. The latter hypothesis implies that arterial SMCs are phenotypically heterogenous. In this chapter, we will describe the distinct SMC phenotypes identified in arteries of various species, including humans. Their role in the formation of intimal thickening will be discussed.

Pathological situations characterized by altered actin isoform expression

Modulation of actin isoform expression is a well-established feature of developmental phenomena. As one might expect, it is also characteristic of several pathological situations that are the subject of the present review. alpha-Smooth muscle actin has proven to be a reliable marker for identifying (a) vascular smooth muscle cells during vascular development and vascular diseases, and (b) myofibroblasts during wound healing, fibrocontractive diseases, and stromal reaction to epithelial tumours. The hallmark of a differentiated myofibroblast relies on the acquisition of an organized contractile apparatus characterized by alpha-smooth muscle actin-expressing stress fibres. More and more data suggest that alpha-smooth muscle actin plays a direct role in myofibroblast contractile activity through its N-terminal domain AcEEED. Newly developed antibodies against alpha-skeletal and alpha-cardiac actins have allowed the detection of subpopulations of alpha-skeletal positive cardiomyocytes in adult, hypertrophic, and failing heart. These antibodies have also permitted us to identify the differentiation degree of malignant cells in tumours such as rhabdomyosarcoma. Whether the differential expression of actin isoforms in human diseases is functionally relevant is not yet fully established, although studies on human actin mutations, actin null mice, and the N-terminal end of alpha-smooth muscle actin support this possibility.

Estrogen receptors in atherosclerotic human aorta: inhibition of human vascular smooth muscle cell proliferation by estrogens

Estrogen has been postulated to exert direct anti-atherogenic effects via binding to estrogen receptors (ERs) in vascular smooth muscle cells (VSMCs). Therefore, we believe it is important to examine the status of ER expression in the human cardiovascular system and its disorders. In this study, we first evaluated the relative abundance of messenger RNA (mRNA) of both ER subtypes (ERalpha and ERbeta) in the human aorta using reverse transcription followed by quantitative polymerase chain reaction (RT-qPCR). We then examined the immunolocalization of both ERs in VSMCs of human atherosclerotic lesions. In order to examine which ER subtype was associated with the anti-atherogenic effects of estrogen, we examined the effects of estrogen in two VSMC cell lines, one positive only for ERalpha and the other positive only for ERbeta. The relative abundance of mRNAs for both ERs was higher in female aorta with a mild degree of atherosclerosis than in female aorta with a severe degree of atherosclerosis (P < 0.05). In addition, the number of ERalpha and/or ERbeta double positive cells in the neointima was higher in female aorta with a mild degree of atherosclerosis than in female aorta with severe atherosclerosis (P < 0.05). Our in vitro study found that estradiol was able to significantly inhibit the proliferation of ERalpha positive VSMCs but not ERbeta positive VSMCs (P < 0.05). Moreover, estradiol was found to significantly suppress proliferating cell nuclear antigen (PCNA) mRNA levels in ERalpha positive VSMCs compared to that of ERbeta positive VSMCs, consistent with the findings of cell proliferation. Results from this study suggest that estrogens can inhibit the proliferation of VSMCs through ERalpha, especially in pre-menopausal women. Our study also indicates that decreased levels of ER, especially ERalpha, in the female atherosclerotic neointima may be associated with progression of atherosclerotic changes.

Concerted effect of transforming growth factor-β, cyclin inhibitor p21, and c-mycon smooth muscle cell proliferation

Increased aortic smooth muscle cell (SMC) proliferation is a key event in the pathogenesis of atherosclerosis. Transforming growth factor-β (TGF-β) is one of the potent inhibitors of SMC proliferation. The purpose of this study was 1) to explore the effect of TGF-β inhibition on proliferation of SMC and expression of growth regulatory molecules like p21 and c- myc and 2) to determine whether restoration of cell cycle regulatory molecules normalizes the altered proliferation. To test the role of TGF-β in SMC proliferation, using antisense plasmid DNA, we inhibited TGF-β gene from aortic SMC, which resulted in a significant increase ( P < 0.03) in proliferation (studied by quantifying new DNA synthesis with [3H]thymidine uptake assay). In TGF-β-altered SMC (TASMC), the mRNA expression (studied by RT-PCR) of c- myc was increased whereas that of the cyclin inhibitor p21 was completely inhibited. Using p21 sense plasmid DNA, we transfected p21 gene in TASMC, which restored p21 mRNA and protein expression and decreased proliferation ( P < 0.002) in TASMC. Similar treatment with c- myc antisense oligonucleotides significantly ( P < 0.001) decreased the proliferation of TASMC. TASMC also exhibited alteration in morphological changes in SMC but returned to normal with treatment of p21 and TGF-β sense plasmid DNA. Two-dimensional gel electrophoresis analysis of SMC and TASMC demonstrated differential expression of proteins relevant to cellular proliferation and atherosclerosis. This study uniquely analyzes the effect of TGF-β at the molecular level on proliferation of SMC and on cell cycle regulatory molecules, implicating their potential role in the pathogenesis of atherosclerosis.

Phenotypic diversity of smooth muscle cells isolated from human intracranial basilar artery

The present work examined heterogeneity of vascular smooth muscle cells cultured from human cerebral arteries that has not been previously reported. Primary smooth muscle cell cultures were isolated from human intracranial basilar arteries. Using a ring isolation method, multiple clones were generated from the cell cultures. These clones had two distinctly different morphologies: (1) fusiform; and (2) stellate. At confluence the fusiform-shaped clones grew in compact clusters with overlapping cells while the stellate-shaped clones were contact-inhibited growing in a monolayered pattern. The smooth muscle differentiation markers, alpha-smooth muscle-actin, calponin and smooth muscle-myosin heavy chains were expressed in all these clones. In response to serum stimulation, the stellate-shaped clones had a higher growth rate than the fusiform clones. This study reports that smooth muscle cells derived from human basilar arteries are heterogeneous.

Arterial smooth muscle cell heterogeneity: implications for atherosclerosis and restenosis development

During atheromatous plaque formation or restenosis after angioplasty, smooth muscle cells (SMCs) migrate from the media toward the intima, where they proliferate and undergo phenotypic changes. The mechanisms that regulate these phenomena and, in particular, the phenotypic modulation of intimal SMCs have been the subject of numerous studies and much debate during recent years. One view is that any SMCs present in the media could undergo phenotypic modulation. Alternatively, the seminal observation of Benditt and Benditt that human atheromatous plaques have the features of a monoclonal or an oligoclonal lesion has led to the hypothesis that a predisposed, medial SMC subpopulation could play a crucial role in the production of intimal thickening. The presence of a distinct SMC population in the arterial wall implies that under normal conditions, SMCs are phenotypically heterogeneous. The concept of SMC heterogeneity is gaining wider acceptance, as shown by the increasing number of publications on this subject. In this review, we discuss the in vitro studies that demonstrate the presence of distinct SMC subpopulations in arteries of various species, including humans. Their specific features and their regulation will be highlighted. Finally, the relevance of an atheroma-prone phenotype to intimal thickening formation will be discussed.

Modulation of smooth muscle phenotype in vitro by homologous cell substrate

We have developed a novel cell culture system that supports the shortening of smooth muscle cells. Primary rat airway smooth muscle cells were plated on an ethanol-fixed, confluent monolayer of homologous smooth muscle cells (homologous cell substrate, HCS). Cells grown on HCS exhibited morphological and functional characteristics consistent with a differentiated phenotype. Cells on HCS were spindle shaped with a well-defined long axis, whereas cells grown on glass were larger and irregularly shaped. Smooth muscle-specific alpha-actin immunostained diffusely in cells on HCS, whereas it appeared as stress fibers in cells on glass. Agonists recruited a greater fraction of HCS cells to contract, resulting in greater changes in cell area or length on average, but the maximal capacity of shortening of individual cells was similar between the groups. Unlike cells on glass, cells on HCS shortened to methacholine. HCS was reversible and persisted over several passages. Agonists stimulated intracellular Ca(2+) oscillations in cells on HCS, whereas they elicited biphasic peak and plateau transients in cells on glass. HCS modulates smooth muscle cell phenotype in vitro.

Mechanically altered embryonic chicken endothelial cells change their phenotype to an epithelioid phenotype

Monolayers of retracted endothelial cells exhibiting wounds or zones denuded of cells were obtained from aortic explants from 10- to 12-day-old chicken embryos. Using time-lapse videomicroscopy, we investigated the sequence of events that occurred both during and after closure of the monolayer wounds. Such wound closure (re-endothelialization process) occurred 4-12 hr after removing the explants, depending on wound width and presence of serum. The cells from along the wound edges appeared to move toward one another. We suggest an important role for bFGF and TGFbeta-2 and -3 during this process. Twenty-five hours after removal there were still some areas of retracted cells, and many of the cells displayed a weak von Willebrand's Factor (vWf) immunoreactivity. Surprisingly, after 63-65 hr many of the endothelial cells had become epithelioid in shape and the vWf immunoreactivity appeared increased. This epithelioid phenotype is currently considered typical of cultured vascular non-muscle-like cells and intimal thickening cells. By 5-7 days, the vast majority of cells in the monolayer had acquired an epithelioid morphology, showing a cobblestone appearance. These cells were significantly smaller than polygonal cells. Most importantly, they showed strong vWf immunoreactivity. At the edge of the monolayers we found that the majority of the cells had become epithelioid. Some of them detached from their neighbors and became round in shape and acquired mesenchymal characteristics, some expressing smooth muscle alpha-actin (SM alpha-actin). These findings demonstrate not only that embryonic endothelial cells that are transiently mechanically altered may change their phenotype to an epithelioid phenotype, but also that these cells may eventually transdifferentiate into mesenchymal cells expressing SM alpha-actin. Since some aspects of endothelial cell behavior have been shown to be regulated by locally released growth factors such as TGFbeta and FGF, we also investigated TGFbeta-2 and -3 and bFGF expression. Presence of TGFbeta-2 and -3 and bFGF-immunoreactive epithelioid and mesenchymal cells indicates that these growth factors may be involved in the changes described.

Shortening velocity and myosin heavy- and light-chain isoform mRNA in rabbit arterial smooth muscle cells

In smooth muscle cells (SMCs) isolated from rabbit carotid, femoral, and saphenous arteries, relative myosin isoform mRNA levels were measured in RT-PCR to test for correlations between myosin isoform expression and unloaded shortening velocity. Unloaded shortening velocity and percent smooth muscle myosin heavy chain 2 (SM2) and myosin light chain 17b (MLC(17b)) mRNA levels were not significantly different in single SMCs isolated from the luminal and adluminal regions of the carotid media. Saphenous artery SMCs shortened significantly faster (P < 0.05) than femoral SMCs and had more SM2 mRNA (P < 0.05) than carotid SMCs and less MLC(17b) mRNA (P < 0.001) and higher tissue levels of SMB mRNA (P < 0.05) than carotid and femoral SMCs. No correlations were found between percent SM2 and percent MLC(17b) mRNA levels and unloaded shortening velocity in SMCs from these arteries. We have previously shown that myosin heavy chain (MHC) SM1/SM2 and SMA/SMB and MLC(17a)/MLC(17b) isoform mRNA levels correlate with protein expression for these isoforms in rabbit smooth muscle tissues. Thus we interpret these results to suggest that 1) SMC myosin isoform expression and unloaded shortening velocity do not vary with distance from the lumen of the carotid artery but do vary in arteries located longitudinally within the arterial tree, 2) MHC SM1/SM2 and/or MLC(17a)/MLC(17b) isoform expression does not correlate with unloaded shortening velocity, and 3) intracellular expression of the MHC SM1/SM2 and MLC(17a)/MLC(17b) isoforms is not coregulated.

Advances towards understanding heart valve response to injury

BACKGROUND

Composed of endocardial endothelial, valvular interstitial, cardiac muscle, and smooth muscle cells (SMC), heart valves are prone to various pathologic conditions the morphology of which has been well described. The morphology of diseased valves suggest that the "response to injury" process occurs in these valves, and is associated with an accumulation of interstitial cells and matrix, valvular inflammation and calcification, conditions that lead to dysfunction. The purpose of this study is to describe the current knowledge of the regulation of the valvular "response to injury" process, since we feel that this paradigm is essential to understanding valve disease.

METHODS

The pertinent literature relating to the cell and molecular biology of valvular repair, and specifically interstitial cell function in valve repair, is reviewed.

RESULTS

The cell and molecular biology of valve interstitial cells are poorly understood. Molecules regulating some of the aspects of the "response to injury" process have been studied, however, the signal transduction pathways, gene activation, and interactions of bioactive molecules with each other, with cells, and with the matrix have not been characterized. Initial studies identify the cell and molecular biology of interstitial cells to be an important area of research. Agents that have been studied include nitric oxide (NO) and FGF-2 and several matrix-related proteins including osteopontin. The present review suggests several directions for future study and a working model of valvular repair is presented.

DISCUSSION

The regulation of the "response to injury" process in the human heart valve is still largely unknown. The cell and molecular events and processes that occur in heart valve function and repair remain poorly understood. These events and processes are vital to our understanding of the pathobiology of heart valve disease, and to the successful design of tissue engineered replacement valves.

Increased rate of apoptosis in intimal arterial smooth muscle cells through endogenous activation of TNF receptors

Intimal proliferation of smooth muscle cells (SMCs) is a key event in the vascular response to injury, including the early stages of atherosclerosis and restenosis after angioplasty. Tumor necrosis factor-alpha (TNF-alpha) has been reported to stimulate growth of cultured human SMCs, but activation of TNF receptors is also known to induce cell death by apoptosis. We report here that SMCs isolated from the neointima of injured rat aortas are characterized by increased expression of TNF-alpha in response to interleukin-1beta and gamma-interferon compared with medial SMCs. Basal and serum-stimulated DNA synthesis was higher in intimal than in medial SMCs. In contrast to previous findings on human SMCs, exposure to interleukin-1beta/gamma-interferon or TNF-alpha did not affect the growth of rat medial SMCs, inhibited DNA synthesis, and decreased cell numbers in cultures of intimal SMCs. Incubation of intimal SMCs with these cytokines also resulted in induction of terminal dUTP nick end-labeling positivity and caspase-3 expression, suggesting cell death by apoptosis, whereas medial cells were markedly less sensitive in this respect. Cytokine-induced apoptosis in intimal cells was effectively inhibited by treatment with antibodies against TNF receptors. These findings suggest that endogenous activation of TNF receptors may represent a way to limit accumulation of SMCs in injured arteries. This mechanism may also be important in SMC death in advanced atherosclerotic plaques.

Cultured arterial smooth muscle cells maintain distinct phenotypes when implanted into carotid artery

Cultured arterial smooth muscle cells (SMCs) with distinct phenotypic features have been described by several laboratories; however, it is not presently known whether this phenotypic heterogeneity can be maintained within an in vivo environment. To answer this question, we have seeded into the intima of denuded rat carotid artery 2 SMC populations with well-established distinct biological features, ie, spindle-shaped, not growing in the absence of serum, and well differentiated versus epithelioid, growing in the absence of serum, and relatively undifferentiated, derived from the aortic media of newborn rats (aged 4 days) and old rats (aged >18 months), respectively. We show that these 2 populations maintain their distinct biochemical features (ie, expression of alpha-smooth muscle actin, smooth muscle myosin heavy chains, and cellular retinol binding protein-1) in the in vivo environment. The old rat media-derived SMCs continue to produce cellular retinol binding protein-1 but little alpha-smooth muscle actin and smooth muscle myosin heavy chains, whereas the newborn rat media-derived SMCs continue to express alpha-smooth muscle actin and smooth muscle myosin heavy chains but no cellular retinol binding protein-1. Our results reinforce the notion of arterial SMC phenotypic heterogeneity and suggest that in our model, heterogeneity is controlled genetically and not by the local environment.

Molecular mechanisms of phenotypic plasticity in smooth muscle cells

Morphological, functional, molecular and cell biology studies have revealed a striking multifunctional nature of individual smooth muscle cells (SMC). SMCs manifest phenotypic plasticity in response to changes in environment and functional requirements, acquiring a range of structural and functional properties bounded by two extremes, called "synthetic" and "contractile." Each phenotypic state is characterized by expression of a unique set of structural, contractile, and receptor proteins and isoforms that correlate with differing patterns of gene expression. Recent studies have identified signaling pathways and transcription factors (e.g., RhoA GTPase/ROCK, also known as Rho kinase, and serum response factor) that regulate the transcriptional activities of genes encoding proteins associated with the contractile apparatus. Mechanical plasticity of contractile-state smooth muscle further extends SMC functional diversity. This may also be regulated, in part, by the RhoA GTPase/ROCK pathway, via reorganization of cytoskeletal and contractile proteins. Future studies that define transcriptional and posttranscriptional mechanisms of SMC plasticity are necessary to fully understand the role of SMC in the pathogenesis and morbidity of human diseases of the airways, vasculature, and gastrointestinal tract.

Enhanced H(2)O(2)-induced cytotoxicity in "epithelioid" smooth muscle cells: implications for neointimal regression

Vascular smooth muscle cells (SMCs) are phenotypically diverse. Although most medial SMCs can be classified as "fusiform," others are of the "epithelioid" phenotype. Proliferation and apoptosis of epithelioid SMCs may contribute importantly to neointimal formation and regression, respectively. Because reactive oxygen species (ROS) are increased in vascular injury and can induce apoptosis of SMCs, we compared the effects of ROS on epithelioid and fusiform SMCs. Epithelioid and fusiform SMC lines were clonally isolated from rat aortic media and studied under similar conditions and passage numbers. H(2)O(2) produced dose- and time-dependent cytotoxicity that was enhanced in epithelioid compared with fusiform cells. After 24-hour exposure to 50 micromol/L H(2)O(2), epithelioid cell numbers were reduced by 34+/-5% versus a 3+/-5% (P<0.05) reduction in fusiform cell numbers. Similar results were obtained whether H(2)O(2) was administered to growth-arrested or growing cells or when epithelioid and fusiform cells were exposed to extracellular O(2)(.-). To investigate whether apoptosis contributed to enhanced ROS-induced cytotoxicity in epithelioid SMCs, terminal deoxyribonucleotidyl transferase (TDT)-mediated dUTP-digoxigenin nick-end labeling (TUNEL) staining was performed. The incidence of TUNEL positivity was 5-fold increased in epithelioid versus fusiform SMCs after treatment with 50 micromol/L H(2)O(2) (19+/-1% epithelioid versus 5+/-1% fusiform, P<0.05). Enhanced H(2)O(2)-induced apoptosis in epithelioid SMCs was confirmed by DNA laddering. Furthermore, when balloon-injured aortas were exposed to H(2)O(2) ex vivo, enhanced apoptosis was observed in neointimal compared with medial SMCs. These results suggest that epithelioid SMCs exhibit enhanced sensitivity to ROS-induced apoptosis, which may play an important role in neointimal regression.

HOXB7 overexpression promotes differentiation of C3H10T1/2 cells to smooth muscle cells

The presence of immature smooth muscle cells and ectopic tissues such as fully-formed bone in atherosclerotic lesions, may result from recapitulation of embryonic mechanisms in the artery wall. We hypothesized that expression of homeobox genes is triggered in atherogenesis and that these regulate proliferation and differentiation of multipotential progenitor cells along one or more specific lineages. We identified expression of the homeobox gene HOXB7 in clones of bovine aortic medial cells previously shown to be multipotent. HOXB7 was subsequently detected in human atherosclerotic plaques by RT-PCR and in situ hybridization. Expression was localized to areas adjacent to calcification and scattered in media and neointima, which may be reflective of a role in either osteoblastic or smooth muscle cell differentiation. To differentiate between these possibilities, we overexpressed HOXB7 in C3H10T1/2 cells, a multipotent cell line able to differentiate into vascular smooth muscle cells (SMC), as well as osteogenic and chondrogenic lineages. Results showed that overexpression of HOXB7 increased proliferation 3.5-fold, and induced an SMC-like cell morphology. In addition, expression of the early SMC markers calponin and SM22alpha increased 4-fold and 3-fold respectively by semi-quantitative RT-PCR. Expression of the intermediate SMC marker smooth muscle myosin heavy chain (SM-MHC) did not change. No increase in osteogenic or chondrogenic differentiation was detected, neither in the C3H10T1/2 cells nor in M2 cells, a bone marrow stromal cell line used to confirm this result. These findings suggest that HOXB7 plays a role in expansion of immature cell populations or dedifferentiation of mature cells.

Cell-specific differences in ET-1 system in adjacent layers of main pulmonary artery. A new source of ET-1

Endothelin-1 (ET-1) is a potent vasoconstrictor that causes sustained constriction of the pulmonary artery and modulates normal vascular tone. Endothelial cells were thought to be the major source of ET-1, but recent studies show that vascular smooth muscle cells (SMCs) are also capable of its synthesis. We examined the ET-1 and endothelin-converting enzyme-1 (ECE-1) system in cells cultured from two adjacent layers, subendothelial (L1) and inner medial (L2), of normal sheep main pulmonary artery and the response of this system to exogenous ET-1 and transforming growth factor-beta1 (TGF-beta1). End points include assessment of preproET-1 (ppET-1) and ECE-1 gene coexpression, measurement of intracellular and released ET-1, and ECE-1 activity. RT-PCR analysis revealed that ppET-1 and ECE-1 transcripts were greater in L1 than in L2 cells. The L1 cells also synthesized (L1, 3.2 +/- 0.1; L2, 1.2 +/- 0.1 fmol/10(6) cells) and released (L1, 9.2 +/- 0.5; L2, 2.3 +/-0.1 fmol/ml) greater amounts of ET-1 than L2 cells. The L2 cells internalized exogenous ET-1 in a dose-dependent manner (EC(50) 8 nmol/l) and were more responsive to exogenous ET-1 than L1 cells, showing upregulation of both the ppET-1 and ECE genes. TGF-beta1 downregulated ET-1-stimulated ppET-1 and ECE-1 transcripts but only in L2 cells. In addition, L1 cells showed greater ECE-1 activity than L2 cells, and in both, the activity was sensitive to the metalloprotease inhibitor phosphoramidon. We conclude that the ET-1 system in L1 and L2 cells is distinct. The data suggest that the two cell types have diverse functions in the arterial wall; the L1 cells, like endothelial cells, provide a local source of ET-1; and since the L2 cells are more responsive to exogenous ET-1, they are likely to affect normal pulmonary vascular tone.

Hypoxia induces cell-specific changes in gene expression in vascular wall cells: implications for pulmonary hypertension

Mammals respond to reduced oxygen concentrations (hypoxia) in many different ways at the systemic, local, cellular and molecular levels. Within the pulmonary circulation, exposure to chronic hypoxia has been demonstrated to illicit increases in pulmonary artery pressure as well as dramatic structural changes in both large and small vessels. It has become increasingly clear that the response to hypoxia in vivo is differentially regulated at the level of specific cell types within the vessel wall. For instance, in large pulmonary blood vessels there is now convincing evidence to suggest that the medial layer is made up of many different subpopulations of smooth muscle cells. In response to hypoxia there are remarkable differences in the proliferative and matrix producing responses of these cells to the hypoxic environment. Some cell populations proliferate and increase matrix protein synthesis, while in other cell populations no apparent change in the proliferative or differentiation state of the cell takes place. In more peripheral vessels, the predominant proliferative changes in response to hypoxia in the pulmonary circulation occur in the adventitial layer rather than in the medial layer. Here again, specific increases in proliferation and matrix protein synthesis take place. Accumulating evidence suggests that the unique responses exhibited by specific cell types of hypoxia in vivo can be modeled in vitro. We have isolated, in culture, specific medial cell populations which demonstrate significant increases in proliferation in response to hypoxia, and others which exhibit no change or, in fact, a decrease in proliferation under hypoxic conditions. We have also isolated and cloned several unique populations of adventitial fibroblasts. There is good evidence that only certain fibroblast populations are capable of responding to hypoxia with an increase in proliferation. We have begun to elucidate the signaling pathways which are activated in those cell populations that exhibit proliferative responses to hypoxia. We show that hypoxia, in the absence of serum or mitogens, specifically activates select members of the protein kinase C isozyme family, as well as members of the mitogen-activated protein kinase (MAPK) family of proteins. This selective activation appears to take place in response to hypoxia only in those cells exhibiting a proliferative response, and antagonists of this pathway inhibit the response. Thus, there appear to be cells within each organ that demonstrate unique responses to hypoxia. A better understanding of why these cells exist and how they specifically transduce hypoxia-mediated signals will lead to a better understanding of how the changes in the pulmonary circulation take place under conditions of chronic hypoxia.

Augmented expression of inducible NO synthase in vascular smooth muscle cells during aging is associated with enhanced NF-kappaB activation

Vascular smooth muscle cells (SMCs) are important targets for endothelium-derived nitric oxide (NO), but this production is attenuated in injured and diseased arteries and during aging. However, SMCs can produce NO themselves by expressing an inducible form of NO synthase (iNOS) under inflammatory conditions and in the repair process after arterial injury. We examined iNOS expression in SMCs derived from the aortic media of newborn, young adult, and old rats. Our results show that SMCs from newborn rats cannot produce significant amounts of NO on stimulation with interferon-gamma plus lipopolysaccharide or interleukin-1beta. In contrast, SMCs from old rats exhibit markedly enhanced iNOS activity. The difference in iNOS activity between the newborn and the old SMCs was closely correlated with levels of iNOS protein, mRNA, and gene promoter activity. Similarly, intercellular adhesion molecule-1 (ICAM-1) was also expressed more abundantly in the old than in the newborn SMCs in response to cytokines. Both iNOS and ICAM-1 are transcriptionally regulated by nuclear factor kappaB (NF-kappaB). Our data demonstrate an intense transactivation of NF-kappaB in old SMCs on tumor necrosis factor-alpha stimulation but only a weak one in newborn SMCs. The difference in the NF-kappaB activation could be explained by a much faster and more extensive IkappaBalpha degradation in old than in newborn SMCs. These data indicate that the capability to respond to proinflammatory stimuli by activating NF-kappaB differs between SMCs at different stages of development. This results in differential capability to express NF-kappaB-dependent genes such as iNOS and ICAM-1, which could have implications for host defense and the pathogenesis of vascular diseases.

Subendothelial cells from normal bovine arteries exhibit autonomous growth and constitutively activated intracellular signaling

The arterial media is comprised of heterogeneous smooth muscle cell (SMC) subpopulations with markedly different growth responses to pathophysiological stimuli. Little information exists regarding the intracellular signaling pathways that contribute to these differences. Therefore, we investigated the growth-related signaling pathways in a unique subset of subendothelial SMCs (L1 cells) from normal, mature, bovine arteries and compared them with those in "traditional" SMCs derived from the middle media (L2 SMCs). Subendothelial L1 cells exhibited serum-independent autonomous growth, not observed in L2 SMCs. Autonomous growth of L1 cells was driven largely by the constitutively activated extracellular signal-regulated kinase (ERK-1/2) cascade. Inhibition of upstream activators of ERKs (MAP kinase kinase-1, p21(ras), receptor tyrosine kinases, and Gi protein-coupled receptors) led to suppression of autonomous growth in these cells. L1 cells also exhibited constitutive activation of important downstream targets of ERKs (cytosolic phospholipase A(2), cyclooxygenase-2) and secreted large amounts of prostaglandins. Importantly, L1 cells secreted potent mitogenic factor(s), which could potentially contribute in an autocrine fashion to the constitutive activation of these cells. Our data suggest that unique arterial cells with autonomous growth potential and constitutively activated signaling pathways exist in normal arteries and may contribute selectively to the pathogenesis of vascular diseases.

Localization of the ANG II type 2 receptor in the microcirculation of skeletal muscle

Only functional studies have suggested the presence of the ANG II type 2 (AT2) receptor in the microcirculation. To determine the distribution of this receptor in the rat skeletal muscle microcirculation, a polyclonal rabbit anti-rat antiserum was developed and used for immunohistochemistry and Western blot analysis. The antiserum was prepared against a highly specific and antigenic AT2-receptor synthetic peptide and was validated by competition and sensitivity assays. Western blot analysis demonstrated a prominent, single band at approximately 40 kDa in cremaster and soleus muscle. Immunohistochemical analysis revealed a wide distribution of AT2 receptors throughout the skeletal muscle microcirculation in large and small microvessels. Microanatomic studies displayed an endothelial localization of the AT2 receptor, whereas dual labeling with smooth muscle alpha-actin also showed colocalization of the AT2 receptor with vascular smooth muscle cells. Other cells associated with the microvessels also stained positive for AT2 receptors. Briefly, this study confirms previous functional data and localizes the AT2 receptor to the microcirculation. These studies demonstrate that the AT2 receptor is present on a variety of vascular cell types and that it is situated in a fashion that would allow it to directly oppose ANG II type 1 receptor actions.

Autoregulation of angiogenesis by cells of the vessel wall

The cells of the vessel wall can regulate angiogenesis by producing growth factors, proteolytic enzymes, extracellular matrix components, cell adhesion molecules, and vasoactive factors. This property enables preexisting blood vessels to generate new vessels in the absence of exogenous angiogenic stimuli. Vascular autoregulation of angiogenesis can be studied by culturing rat aortic or venous explants in collagen gels under serum-free conditions. In this system, the combined effect of injury and exposure of explants to collagen triggers a self-limited angiogenic response. Interactions among endothelial cells, smooth muscle cells, and fibroblasts play a critical role in the regulation of this process. This chapter reviews the literature on angiogenesis, focusing on the vessel wall as a highly specialized and plastic tissue capable of regenerating itself through autocrine, paracrine, and juxtacrine mechanisms.

Plasminogen activator expression in rat arterial smooth muscle cells depends on their phenotype and is modulated by cytokines

Cultured rat aortic smooth muscle cells (SMCs) exhibit at least 2 phenotypic variants: (1) a spindle-shaped phenotype, obtained from normal adult media, and (2) an epithelioid phenotype, obtained from intimal thickening 15 days after endothelial injury. Both phenotypes can be cloned from each location, with normal media yielding a majority of spindle-shaped clones and intimal thickening yielding a majority of epithelioid clones. These findings suggest that intimal thickening develops essentially from a subpopulation of medial SMCs exhibiting epithelioid features in vitro. Using zymographic and Northern blot analyses, we have studied plasminogen activator (PA) expression by these SMCs. Our results show that epithelioid SMCs, cultured as whole SMC populations or as clones, display higher PA activity than do spindle-shaped SMCs, irrespective of their origin. This is mainly due to differences in the expression of tissue PA and, to a lesser extent, urokinase PA and is accompanied by a decrease in PA inhibitor 1. Tissue PA activity is increased by basic fibroblast growth factor and platelet-derived growth factor-BB, particularly in epithelioid SMCs. Taken together, these results indicate that SMCs are heterogeneous with respect to their proteolytic profile, at least as far as the PA system is concerned. Proteolytic activity of the different SMC populations is modulated by cytokines that play a role in intimal thickening. Our results are in agreement with the suggestion that epithelioid SMCs are mainly responsible for intimal thickening.

Smooth muscle cells isolated from discrete compartments of the mature vascular media exhibit unique phenotypes and distinct growth capabilities

Heterogeneity of smooth muscle cell (SMC) phenotype and function is rapidly emerging as an important concept. We have recently described that phenotypically distinct SMC subpopulations in bovine pulmonary arteries exhibit unique proliferative and matrix-producing responses to hypoxic pulmonary hypertension. To provide better understanding of the molecular mechanisms contributing to this phenomenon, experimental studies will require a reliable in vitro model. The purpose of the present study was first to determine if distinct SMC subpopulations, similar to those observed in vivo, could be selectively isolated from the mature arterial media, and then to evaluate whether select SMC subpopulations would exhibit heightened responses to growth-promoting stimuli and hypoxia. We were able to reproducibly isolate at least four phenotypically unique cell subpopulations from the inner, middle, and outer compartments of the arterial media. Differences in cell phenotype were demonstrated by morphological appearance and differential expression of muscle-specific proteins. The isolated cell subpopulations exhibited markedly different growth capabilities. Two SMC subpopulations grew slowly in 10% serum and were quiescent in plasma-based medium. The other two cell subpopulations, exhibiting nonmuscle characteristics, grew rapidly in 10% serum and proliferated in plasma-based medium and in response to hypoxia. Certain colonies of the nonmuscle-like cell subpopulations were found to grow autonomously under serum-deprived conditions and to secrete mitogenic factors. Our data, demonstrating that phenotypically distinct cells with enhanced growth potential exist within the normal arterial media, support the idea that these unique cells could contribute selectively to the pathogenesis of vascular disease.

Increased proliferation of explanted vascular smooth muscle cells: a marker presaging atherogenesis

The JCR:LA-cp homozygous cp/cp corpulent rat is genetically predisposed to develop atherosclerosis evident after 9 and 18 months of age in males and females and to manifest metabolic derangements resembling those seen in type II diabetes in humans (hyperinsulinemia, insulin resistance, hyperglycemia and hypertriglyceridemia). The present study was undertaken to determine whether vascular smooth muscle cells (SMCs) explanted from vessels destined to become atherosclerotic later in life exhibit intrinsic properties ex vivo that presage atherogenesis to provide a means for evaluating promptly intervention designed to modify it. SMCs were cultured from aortic explants of JCR:LA-cp corpulent (cp/cp) and lean control (+/+) rats of 4, 5, 6, and 9 months of age. Compared with SMCs from controls, SMCs from cp/cp rats exhibited increased proliferation, higher saturation density, increased augmentation of proliferation in response to selected mitogens and greater adherence to extracellular matrix proteins. The increased proliferative activity ex vivo anteceded by several months the development of atherosclerotic lesions in vivo. Thus, it is a promising marker in assessments of the efficacy of interventions designed to retard or prevent atherosclerosis.

Embryonic endothelial cells transdifferentiate into mesenchymal cells expressing smooth muscle actins in vivo and in vitro

All blood vessels are lined by endothelium and, except for the capillaries, surrounded by one or more layers of smooth muscle cells. The origin of the embryonic vascular smooth muscle cell has until now been described from neural crest and locally differentiating mesenchyme. In this study, we have substantial evidence that quail embryonic endothelial cells are competent in the dorsal aorta of the embryo to transdifferentiate into subendothelial mesenchymal cells expressing smooth muscle actins in vivo. At the onset of smooth muscle cell differentiation, QH1-positive endothelial cells were experimentally labeled with a wheat germ agglutinin-colloidal gold marker (WGA-Au). No labeled subendothelial cells were observed at this time. However, 19 hours after the endothelial cells had endocytosed, the WGA-Au-labeled subendothelial mesenchymal cells were observed in the aortic wall. Similarly, during the same time period, subendothelial cells that coexpressed the QH1 endothelial marker and a mesenchymal marker, alpha-smooth muscle actin, were present. In such cells, QH1 expression was reduced to a cell membrane localization. A similar antigen switch was also observed during endocardial-mesenchymal transformation in vitro. Our results are the first direct in vivo evidence that embryonic endothelial cells may transdifferentiate into candidate vascular smooth muscle cells. These data arouse new interpretations of the origin and differentiation of the cells of the vascular wall in normal and diseased vessels.

Phenotypic heterogeneity of rat arterial smooth muscle cell clones. Implications for the development of experimental intimal thickening

It is well accepted that smooth muscle cells (SMCs) cultured from normal rat arterial media have different morphological and biological features compared with SMCs cultured from experimental intimal thickening (IT) 15 days after endothelial injury. It is not known, however, whether the phenotypic modulation producing IT cells occurs in any medial SMCs or only in a particular SMC subpopulation. To distinguish among these possibilities, the phenotypic features of SMC clones derived from normal adult media and the IT 15 days after endothelial lesion were analyzed according to morphological appearance, replicative activity in the presence and absence of fetal calf serum, and [3H]thymidine incorporation and motile activity; these features were compared with those of the respective SMC parental populations. Two categories of SMC clones predominated: spindle clones, with morphological features similar to those of the parental population from the normal media, and epithelioid clones, with morphological features similar to those of the IT parental population. Both categories were present among clones produced from normal media and IT; however, spindle was more common among normal media clones, and epithelioid, among IT clones. The behavior in vitro was distinct for each category of clones and did not depend on their origin. Our results are compatible with the possibility that the SMC population of IT in vivo derives mainly from SMCs belonging to the category exhibiting epithelioid features in vitro.