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Arteriosclerosis, Thrombosis, and Vascular Biology. 1997;17:665-671

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(Arteriosclerosis, Thrombosis, and Vascular Biology. 1997;17:665-671.)
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Articles

Differentiation of Smooth Muscle Cells in Human Blood Vessels as Defined by Smoothelin, a Novel Marker for the Contractile Phenotype

Frank T. L. van der Loop; Giulio Gabbiani; Gaby Kohnen; Frans C. S. Ramaekers; ; Guillaume J. J. M. van Eys

From the Department of Molecular Cell Biology and Genetics, Cardiovascular Research Institute Maastricht, University of Limburg, Maastricht, Netherlands (F.T.L. van der L., F.C.S.R., G.J.J.M. van E.); University of Geneva (Switzerland), Department of Pathology (G.G.); and University of Glasgow, Department of Obstetrics and Gynecology, UK (G.K.).


*    Abstract
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*Abstract
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Abstract Smoothelin is a constituent of the cytoskeleton specific for smooth muscle cells (SMCs) in a broad range of species. It has been postulated that smoothelin represents a marker of highly differentiated, contractile SMCs. Here, we present data on the presence of smoothelin in the human vascular system that support this hypothesis. For this purpose, smoothelin distribution was studied (1) during vasculogenesis of the placenta, (2) in normal adult blood vessels, and (3) in atherosclerotic lesions. Smoothelin was first observed in placental tissue at approximately week 10 to 11 of gestation. In full-term placenta, it was found in the SMCs of vessels in the large stem villi and in the chorionic plate. Furthermore, it was present in the fetal arteries of smaller stem villi, but it was not found in the veins. In adult blood vessels, a small population of aortic ({approx}10%) and large muscular artery ({approx}30% to 50%) SMCs was positive for smoothelin. In general, smoothelin and desmin were coexpressed in the same SMCs, but expression of desmin appeared to be less abundant. However, the majority of SMCs in these blood vessels were smoothelin- and desmin-negative but expressed vimentin, whereas {alpha}-smooth muscle actin ({alpha}-SMA) was present in all SMCs. The SMCs in the media of small muscular arteries were positive for smoothelin and desmin (>95%), whereas the vimentin-positive SMC type was scarce. Smoothelin was absent in capillaries, pericytic venules, and small veins but was occasionally observed in the SMCs of large veins. Thus, the distribution of smoothelin in the SMCs of the vascular system appears to be limited to blood vessels that are capable of pulsatile contraction. In atherosclerotic femoral arteries, smoothelin-positive cells were detected in the media, the atheromatous plaque, and the intimal thickening. Smoothelin-positive cells were present primarily at the luminal portion of advanced lesions. The presence of a considerable number of such smoothelin-positive cells at that location may indicate that these plaques are no longer expanding.


Key Words: smoothelin • smooth muscle cells • human blood vessels • atherosclerotic lesions • differentiation • placenta


*    Introduction
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up arrowAbstract
*Introduction
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Smooth muscle cells are the most abundant cell type in blood vessel walls. They occur in all vessels except capillaries and pericytic venules. In the media of blood vessels, SMCs are intermingled with elastic lamellae and laminae and appear as concentric, helically arranged bundles. Vascular SMCs have been divided into (at least) two distinct states of differentiation, usually referred to as synthetic and contractile phenotypes,1 2 the latter being predominant in the blood vessels of adult organisms. Contractile SMCs have a muscle-like morphology, contract in response to mechanical and biochemical stimuli, and are involved in the control of blood pressure and blood flow. Synthetic-type SMCs, with a fibroblast-like appearance, proliferate and produce extracellular matrix components. Between these two extremes, SMCs can acquire a broad spectrum of different phenotypes in response to changes in a variety of physiological or pathological factors.2 3 This potential is considered to play an important role in angiogenesis, maintenance of blood vessel wall integrity, and the development of atherosclerosis. Intimal thickening with myofilament-rich SMCs and without large accumulations of lipids is considered a physiological response to hemodynamic forces in specific arterial locations.4 During development of the atherosclerotic plaque, SMCs from the media migrate into the intima and thus participate in the formation of intermediate (transitional) and advanced lesions.5 6

The IFPs vimentin and desmin have been suggested as markers to discriminate between cells of the synthetic and the contractile phenotypes.1 2 The cellular content of vimentin, desmin, {alpha}-SMA, myosin, and tropomyosin changes when SMCs shift from the synthetic to the contractile phenotype both in vivo and in vitro.7 8 Also, differences of IFP content have been observed among SMCs of different blood vessels.9 10 11

Recently, we identified a SMC protein, smoothelin, that appeared to be useful to monitor SMC differentiation.12 This 59-kD protein is expressed exclusively in fully differentiated (contractile) SMCs. RNA and protein analysis revealed a broad species distribution of this protein. Cells with SMC-like characteristics, such as myofibroblasts and myoepithelial cells, as well as skeletal and cardiac muscle did not contain smoothelin.12 Confocal scanning laser microscopy of sections of human colon, myoma, and arteries, as well as transfection studies, indicated a filamentous organization of smoothelin that is different from those of desmin, vimentin, or {alpha}-SMA. This study elaborates on the distribution of smoothelin in the vascular system, in embryonic and normal blood vessels, and in atherosclerotic lesions. It provides additional evidence that smoothelin expression is specific for contractile SMCs.


*    Methods
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*Methods
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Tissues and Tissue Preparation
Human placenta. Human term placentas (n=8) and 4 term umbilical cords from clinically normal pregnancies (week 38 to 40 of gestation) were collected. In addition, 2 second-trimester placentas (week 25 of gestation) after preterm delivery and 8 first-trimester placentas (week 7 to 11 of gestation) were obtained after legal suction termination of pregnancy for psychosocial or medical reasons that were unlikely to affect placental structure and function. Informed consent was obtained from each patient, and the study was approved by the local ethics committee. Small blocks of placental and cord tissues were snap-frozen in liquid nitrogen.

Normal blood vessels and atherosclerotic lesions. Samples of human arteries, both normal (n=5; 3 patients) and atherosclerotic (n=12; 5 patients); abdominal aortic aneurysm biopsies (immediately after surgery; n=2; 2 patients); and aortas (thorax; 3 to 12 hours postmortem; n=4; 4 individuals) were either frozen in liquid nitrogen, mounted in Tissue-Tek (OCT compound; Miles Inc), and stored at -80°C or routinely embedded in paraffin. The use of the tissues was approved by the local ethics committee. We have adopted the following classification for normal muscular arteries: large muscular arteries (diameter, 2 to 10 mm) containing 10 to 40 concentric layers of SMCs, and small muscular arteries (diameter, 0.3 to 2 mm) containing <10 layers of SMCs.

Tissue sectioning and processing. Serial sections of the frozen placental and umbilical cord material (10 µm) were cut at -21°C, air-dried for 60 minutes, and stored at -70°C. Before use, cryostat sections were fixed in acetone at 4°C for 10 minutes. Sections 4 µm thick were cut from paraffin-embedded normal blood vessels and atherosclerotic plaques. From the frozen blood vessels and atherosclerotic plaques, sections 3 to 5 µm thick were cut at -25°C and immediately fixed in methanol (-20°C; 5 minutes) and acetone (-20°C; 30 seconds), and air-dried for 3 hours. Alternatively, sections were air-dried overnight at 20°C to 22°C (RT) and then treated for 5 minutes with 0.5% Triton X-100 (BDH Chemicals Ltd) in PBS (in mmol/L: sodium chloride 137, disodium hydrogen phosphate dihydrate 13, and potassium dihydrogen phosphate 3, pH 7.4; Merck), followed by a PBS washing step for 5 minutes.

Antibodies
Antibodies used in this study were (1) mouse MAb R4A against smoothelin of the IgG1 subclass selected on the basis of its reactivity pattern with SMCs of blood vessels of the human heart and smooth muscle tissue of the human intestine (molecular characterization of smoothelin as well as reactivity patterns, species, and tissue specificity of the antibody have been described elsewhere12 ); (2) polyclonal rabbit antisera to desmin13 14 21 ; (3) MAb RD301 to desmin15 ; (4) MAb anti–{alpha}-Sm-1 to {alpha}-SMA16 ; (5) polyclonal rabbit antisera to vimentin14 17 21 ; (6) MAb RV203 to vimentin18 ; (7) MAb to {gamma}-SMA (clone B4)19 ; and (8) MAb to smooth muscle myosin (clone hSM-V).20

Immunohistochemistry
Immunoperoxidase staining. After fixation in acetone, the fixed cryostat sections were air-dried and rinsed in PBS for 10 minutes. The sections were preincubated for 20 minutes in PBS supplemented with 1.5% BSA (Sigma Chemical Co). Primary and secondary antibodies were diluted in 1.5% BSA/PBS, and washing steps were carried out in PBS. All incubations were performed in a humidified chamber at RT. The primary antibodies were applied for 60 minutes; control sections revealed no immunostaining. After three washings in PBS (5 minutes each), biotinylated secondary antibody (DAKO A/S) was applied for 30 minutes. Sections were then incubated in 1% H2O2 in absolute methanol for 10 minutes to inactivate endogenous peroxidase, followed by incubation with streptavidin-peroxidase conjugate (streptAB complex/HRP, DAKO) for 20 minutes. The streptavidin-biotin complex was visualized with DAB (Serva). Sections were washed in distilled water, counterstained with Harris hematoxylin, and mounted in DPX.21

Immunofluorescence staining. Indirect immunofluorescence assays were performed according to standard procedures.12 15 18 In brief, the methanol/acetone–or Triton X-100–treated sections were incubated with the primary antibody for 30 minutes in a humidified chamber (RT) and washed three times (10 minutes each) with PBS. The appropriate FITC- or Texas Red–conjugated secondary antibodies (Southern Biotechnology Associates Inc) were applied for 30 minutes (RT). After washing with PBS, the immunofluorescence-stained tissues were mounted in Mowiol. For double-labeling studies, the incubation steps were repeated with primary and secondary antibodies of different subclasses or species. Sections were analyzed with a Zeiss Axiophot microscope.

Immunohistochemistry on paraffin-embedded tissue sections. To stain paraffin-embedded arteries with the smoothelin antibody R4A, sections had to be pretreated by microwave radiation. In brief, the sections were deparaffinized, treated with ethanol (100%), and incubated in methanol/hydrogen peroxide (5%) and demineralized water. The sections were placed in a 10 mmol/L citrate buffer (pH 6.0) and heated in a microwave (3 times 5 minutes).22 After cooling in the citrate buffer, the sections were incubated with undiluted culture supernatant of MAb R4A. After washing in PBS, antibody binding was monitored with the streptAB/HRP complex procedure performed according to the manufacturer's instructions (Vectastain ABC Kit, Vector). The peroxidase activity was visualized with DAB and H2O2. Sections were counterstained with Mayer's hematoxylin, dehydrated, and mounted in Eukitt.22

Gel Electrophoresis and Immunoblots
Approximately 40 cryostat sections (20 µm thick) of fresh-frozen umbilical cord, sigmoid artery, aorta (thorax), aortic aneurysm (abdomen), and colon (positive control tissue) were collected, washed with 1 mL PBS, and centrifuged for 5 minutes at 12 000g. After centrifugation, the pellet was subjected to a Triton X-100 extraction step: the pellet was suspended in 1% Triton X-100, 5 mmol/L EDTA/0.4 mmol/L PMSF (Merck) in PBS, pH 7.4, and extracted for 5 minutes on ice.18 After centrifugation for 5 minutes at 12 000g, the pellet was washed in 1 mL PBS. After a final centrifugation step (5 minutes, 12 000g), this cytoskeletal preparation was dissolved by boiling for 4 minutes in sample buffer containing 2.3% SDS and 5% ß-mercaptoethanol (Bio-Rad Laboratories).23

One-dimensional SDS-PAGE was performed with a Mini Protean II Electrophoresis Cell (Bio-Rad Laboratories). Polyacrylamide slab gels containing 0.1% SDS23 were loaded with the cytoskeletal preparations. After electrophoretic separation, the proteins were stained with Coomassie brilliant blue (PAGE blue 83, BDH Chemicals Ltd) or subjected to immunoblotting. The separated polypeptides were transferred from the slab gels to a nitrocellulose membrane (Schleicher and Schüll, Filter BA 85) by blotting for 1 hour at 100 V in a cold (4°C) buffer containing 25 mmol/L Tris (Merck), 192 mmol/L glycine (Merck), 0.02% SDS, and 20% methanol (Merck).18 24 For protein detection, two procedures using HRP-conjugated secondary antibodies (DAKO) were applied. The first procedure, using 4-chloro-1-naphthol (Merck) and 0.12% hydrogen peroxide (Merck) in PBS, was applied when large amounts of protein were expected or when blots had to be reincubated one or two times with other primary antibodies.18 For detection of low concentrations, the procedure using the chemiluminescence ECL-Kit (Amersham International pcl) was applied after an extensive blocking procedure according to the instructions of the manufacturer.


*    Results
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*Results
down arrowDiscussion
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Smoothelin Distribution in Human Placenta and Umbilical Cord
Smoothelin could be detected immunohistochemically in placental tissue around week 10 to 11 of gestation. Earlier developmental stages (weeks 7 to 8 of gestation) did not show any immunoreactivity, although desmin, {gamma}-SMA, and SMC-specific myosin were already present in some developing vessels. Smoothelin expression seemed to be restricted to the SMCs of fetal arteries in smaller stem villi (Fig 1aDown through 1c). No immunoreactivity was found in the veins (Fig 1aDown). In term placenta, the presence of smoothelin was restricted to vessels in the large stem villi and the chorionic plate, which represent the oldest part in placental development (Fig 1dDown and 1eDown). In large stem villi, a few extravascular stromal cells in close vicinity to the fetal vessels coexpressed {gamma}-SMA, smooth muscle myosin, and smoothelin. Embryonic stromal cells have been shown to exhibit a complex cytoskeletal composition and coexpress vimentin, desmin, {alpha}- and {gamma}-SMA, and smooth muscle myosin.25 In term umbilical cords, smoothelin was observed in the SMCs of all three vessels (two arteries, one vein), whereas the surrounding stromal cells in the Wharton's jelly were negative. The results of these immunohistochemical studies on placental material are summarized in Table 1Down.



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Figure 1. Immunostaining of serial sections of blood vessels from smaller stem villus surrounded by some peripheral villi (a, b, and c) and large stem villus (d and e) of human term placenta. Sections were stained with antibodies against smoothelin (a and d), {gamma}-SMA (b and e), or desmin (c). SMCs of fetal artery and vein coexpress {gamma}-SMA (b) and desmin (c); only SMCs of fetal artery coexpress smoothelin (a). Bar=70 µm (a, b, and c) or 170 µm (d and e).


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Table 1. Summary of Immunohistochemicl Data

Smoothelin Distribution in Adult Blood Vessels
Elastic arteries. The tunica media of elastic arteries, eg, the aorta, contains up to 50 layers of elastic fibers. Interposed between the (autofluorescent) elastic layers are SMCs and some collagen. Immunohistochemistry on sections from four human individuals revealed that the SMCs belong to different types, containing either desmin (Fig 2bDown) or vimentin (Fig 2cDown) in addition to {alpha}-SMA. Only a limited number of media SMCs ({approx}5% to 10%) contained smoothelin (Fig 2aDown). Although double staining of aorta sections is hampered by the autofluorescence of the elastic fibers, coexpression of smoothelin and desmin was detected in the SMCs of the media of the aorta. No smoothelin-positive cells were detected in the intima or in the adventitia. In sections of two biopsies of human abdominal aortic aneurysm specimens, neither smoothelin- nor desmin-positive SMCs were detected, but vimentin-containing SMCs were present (not shown). Western blot analysis confirmed the presence of vimentin and {alpha}-SMA in aorta and aneurysm biopsies, but smoothelin and desmin could not be demonstrated (Table 2Down; Fig 4Down).



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Figure 2. Immunofluorescence micrographs of human aorta stained for (a) smoothelin, (b) desmin, and (c) vimentin, indicating that few cells contain smoothelin (a) or desmin (b), whereas many cells contain vimentin (c). Bar=40 µm.


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Table 2. Results of Western Blot Analysis of Human Tissue Samples



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Figure 4. Western blot showing immunoreactions of antibodies against smoothelin (S), desmin (D), and vimentin (V) on cytoskeletal preparations of human sigmoid artery, human aorta (thorax), and human abdominal (abd.) aorta aneurysm. CBB indicates Coomassie brilliant blue staining.

Muscular arteries. In the media of five normal human large muscular arteries, smoothelin was detected in a significant number of SMCs ({approx}30% to 50%), either in frozen tissue sections (Fig 3bDown) or in sections of paraffin-embedded tissue (not shown). In immunofluorescence, smoothelin appeared to be more abundant than desmin. Both staining intensity and number of smoothelin-positive cells appeared to be higher (Fig 3cDown). On Western blots, however, the smoothelin signal was lower than the desmin signal, most likely because of a decreased binding efficiency of the MAb R4A on denatured antigen. The majority of the SMCs contained vimentin and did not express smoothelin.



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Figure 3. Immunofluorescence micrographs indicating localization of smoothelin (red) in media of femoral artery (a) and in media and intimal thickening of femoral artery. c, Double-stained section of media of femoral artery indicating coexpression of smoothelin (red) with desmin (green) in some SMCs. Green/yellow signal in all sections is autofluorescence. ta indicates tunica adventitia; tm, tunica media; ti, tunica intima; iel, internal elastic lamina; lum, lumen; and al, atherosclerotic lesion. Bar=50 µm (a) or 25 µm (b, c).

In the media of small human muscular arteries, virtually all SMCs (>95%) contained smoothelin (Fig 3aUp). Coexpression of smoothelin and desmin was also observed in these SMCs. All cells that contained smoothelin also expressed {alpha}-SMA. The SMC type expressing only vimentin was hardly observed (not shown). The smoothelin antibody also stained the SMCs of vasa vasorum, located in the adventitia of the large muscular arteries. The presence of smoothelin, desmin, vimentin, and {alpha}-SMA was confirmed by Western blot analysis of fresh-frozen human sigmoid artery biopsies (Table 2Up; Fig 4Up).

Capillaries, venules and veins. Smoothelin was not detected in capillaries, pericytic venules, and small veins, but the protein was observed in the concentric layers of SMCs of large veins. The thin layer of SMCs in the media of the wall of small veins consists of cells that do not contain smoothelin, whereas the walls of large veins contain SMCs that express this protein.

The data obtained from our immunohistochemical studies on normal blood vessels are summarized in Table 1Up.

Western blot analysis. Immunoblot experiments (summarized in Table 2Up) showed that smoothelin and desmin were present in extracts of sigmoid artery (Fig 4Up), umbilical cord, and colon but were not detected in extracts of either normal aorta (thorax; Fig 4Up) or abdominal aortic aneurysm (Fig 4Up). Vimentin and {alpha}-SMA were detected in all tissues tested (Table 2Up), a result that can be explained by the variety of cell types that are included in the used homogenates.

Smoothelin Distribution in Atherosclerotic Lesions
Atherosclerotic lesions were investigated for smoothelin expression. Serial sections were stained with the antibody R4A in combination with antibodies directed against other cytoskeletal proteins. Smoothelin-positive cells were found in the majority of the lesions. In advanced human atherosclerotic lesions, smoothelin expression was observed in SMCs of demarcated areas. In general, the luminal side of the lesion contained more positive cells than the intimal side (Fig 5aDown and 5cDown). The staining pattern indicated a more linear organization of smoothelin in the SMCs of the luminal side, compared with the more globular organization on the abluminal side. This finding indicates a more elongated morphology of the SMCs in the luminal area of the lesion. Most cells in the atherosclerotic lesions that contained smoothelin also coexpressed desmin (although not as abundantly as smoothelin; Fig 5aDown and 5bDown), vimentin (with an organization pattern resembling that of desmin; Fig 5cDown and 5dDown), and {alpha}-SMA (not shown). In the luminal area of an intimal thickening, 10% to 20% of the lesions consisted of cells that expressed smoothelin abundantly (Fig 3bUp).



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Figure 5. Immunostained sections of an atherosclerotic lesion double-stained with MAb against smoothelin (a and c) and polyclonal antibodies against desmin (b) and vimentin (d), respectively. *Cells that are smoothelin-negative and desmin-positive; arrows indicate cells that are smoothelin-positive and desmin-negative. lu.s. indicates luminal side. Bar=30 µm.


*    Discussion
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up arrowIntroduction
up arrowMethods
up arrowResults
*Discussion
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Smoothelin Distribution in Placental and Adult Blood Vessels
Vascular SMCs can be subdivided into different classes on the basis of (1) their morphology, (2) their expression patterns of structural proteins (such as IFP or contractile proteins),9 10 11 26 or (3) their degree of differentiation.1 2 Cytoskeletal markers have been used to discriminate between synthetic and contractile arterial SMCs.1 2 Switches in actin and myosin isoforms2 27 or an increase of desmin and tropomyosin content in combination with a decrease in vimentin7 8 are indicative for SMC differentiation toward the contractile phenotype. Previously, we suggested that the presence of smoothelin is limited to the contractile phenotype of the SMCs because cultured, synthetic SMCs did not contain this constituent, and we hypothesized that smoothelin may be involved in the contractile activity of SMCs.12 Our present findings in the different types of vascular SMCs corroborate such a hypothesis.

The presence of smoothelin in term placenta was restricted to the fully developed vessels in the large stem villi and the chorionic plate, which represent the oldest part in placental development. In smaller stem villi, with smaller and less muscularized blood vessels, smoothelin-positive SMCs were present only in the fetal arteries. The vessels of placental stem villi, in particular the fetal arteries, are considered to play a major role in the regulation of the fetal blood flow in the placenta.

For adult elastic and large-diameter muscular arteries, blood vessel type–related differences in SMC types, as differentiated by their vimentin or desmin content, were described previously.9 10 In arteries of this type, which have to withstand tonically the high pressure generated by the heart, a SMC type predominates that contains vimentin but neither smoothelin nor desmin. SMCs of small muscular arteries generate contractions to continue the pulsatile movement of the blood flow and to regulate the blood pressure. These cells are mainly of the SMC type that contains smoothelin and desmin. Smoothelin is absent in small veins that have no continuous layers of SMCs and that do not generate contractions. In large veins that contain concentric layers of SMCs and that do slowly contract, smoothelin-positive SMCs are present. Also, the presence of smoothelin in vascular SMCs of placental and adult blood vessels appears to be correlated to isometric contractile activity. Filamentous smoothelin organization and interaction of smoothelin with actin filaments or stress fiberlike structures 12 may contribute to the contractile capacity or to the structural integrity of SMCs.

Smoothelin Distribution in Atherosclerotic Lesions
The development of experimental intimal thickening, as described for the rat model, is characterized by a migration of SMCs from the media into the intima and their modulation to a synthetic phenotype.28 29 30 31 Some of these cells remain quiescent, while others proliferate within the intima, thus increasing the overall cellular mass of the lesion.31 32 The increase in the number of SMCs takes place mainly at the luminal surface. The number of SMCs decreases after regeneration of the endothelium.14 30 In human atherosclerotic plaque, the SMCs not only proliferate but also secrete extracellular matrix components, thus forming a cap on the luminal side of the lipid core.6 33 Babaev et al34 have shown that the plaque cap of advanced lesions contains many cells that possess ultrastructural features of contractile SMCs. These cells express desmin and other cytoskeletal proteins characteristic of differentiated SMCs. Our findings show that smoothelin is synthesized primarily in cells at the luminal side of atherosclerotic lesions. The presence of smoothelin-containing SMCs may be indicative of a reduced growth of the atherosclerotic plaque. The capping of the atherosclerotic lesion by nonproliferating cells containing desmin, {alpha}-SMA, and smoothelin suggests that growth of such intimal thickenings has ceased. Furthermore, proteins such as desmin and smoothelin may contribute to the integrity of the atherosclerotic cap, which prevents damage by the shearing force of the blood flow.

Smoothelin as a Marker for Differentiated SMCs
Smoothelin appears to be a useful tool to discriminate between the different SMC phenotypes, because immunohistological and biochemical data indicate that this cytoskeletal constituent is a specific marker for differentiated SMCs. Smoothelin is absent in primary cultures (bovine aorta allowed to settle on coverslips for 3 to 4 hours), and long-term cultures of vascular SMCs (human uterine and mammary artery cultured for at least five passages), whereas desmin and {alpha}-SMA are present in such cultures. The relatively late start of smoothelin expression in chick embryos that already express {alpha}-SMA in combination with the IFPs vimentin and/or desmin is also in agreement with these observations: preliminary results indicate that smoothelin expression in chick embryo gizzard and femoral artery starts at Hamburger-Hamilton stage 35/3635 (ED 10; unpublished data). The expression of other SMC differentiation markers, such as {alpha}-SMA (ED 2.5), SM-22 (ED 4), calponin (ED 6), h-caldesmon (ED 6), and smooth muscle {alpha}-tropomyosin (ED 6), starts earlier during development.2 A similar finding was observed in the human placenta, in which expression of smoothelin was observed at about week 10 to 11 of gestation in the developing SMCs of the fetal blood vessels. In earlier material (week 7 to 8 of gestation), smoothelin was not expressed, although the developing SMCs of the fetal vessel already coexpressed vimentin, desmin, {alpha}- and {gamma}-SMA, and smooth muscle myosin (Table 1Up).

The inverse relationship between proliferation and SMC-specific contractile protein expression was demonstrated in cultured human arterial muscle cells by Fager et al.36 The short time required to replace desmin by vimentin as the dominant IFP (<24 hours), as well as the relatively fast shift from the {alpha}- to the ß-isoform of actin in vascular SMCs was reported previously.37 Smoothelin expression and organization is lost even faster than these constituents. Thus, smoothelin may be useful to discriminate fully differentiated, contractile SMCs from proliferative SMCs. Also, smoothelin provides a possibility of discriminating SMCs from cells with smooth muscle–like characteristics, such as myofibroblasts and myoepithelial cells. The distribution of smoothelin-positive cells indicates a correlation between smoothelin expression and smooth muscle tissue contractility. Smoothelin may be a useful tool in the evaluation of atherosclerotic lesions, because it can monitor the proliferative nature of such lesions.


*    Selected Abbreviations and Acronyms
 
DAB = 3,3'-diaminobenzidine tetrahydrochloride
ED = embryonic day
HRP = horseradish peroxidase
IFP = intermediate filament protein
MAb = monoclonal antibody
RT = room temperature
SMA = smooth muscle actin
SMC = smooth muscle cell


*    Acknowledgments
 
This work was supported in part by the Swiss National Science Foundation (Drs van der Loop and Gabbiani, grant 31-40372.94) and the Dutch Heart Foundation (Dr van der Loop, grant D90.002). Dr Kohnen was a recipient of a European Community Human Capital and Mobility Fellowship (9030364). The contribution of the Obstetric and Midwifery colleagues at Glasgow Royal Infirmary and The Queen Mother's Hospital (Glasgow) in collecting placental and umbilical cord specimens is gratefully acknowledged. The authors would like to thank Prof Dr Peter Kaufmann (University of Aachen) for helpful discussions and are also indebted to M. Redard (University of Geneva) for the staining of paraffin-embedded tissues.


*    Footnotes
 
Reprint requests to G.J.J.M van Eys, Department of Molecular Cell Biology and Genetics, Cardiovascular Research Institute Maastricht (CARIM), University of Limburg, PO Box 616, 6200 MD Maastricht, The Netherlands.

The Guest Editor for this article was Prof Göran K. Hansson.

Received June 11, 1996; accepted August 21, 1996.


*    References
up arrowTop
up arrowAbstract
up arrowIntroduction
up arrowMethods
up arrowResults
up arrowDiscussion
*References
 

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