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Circulation. 2000;101:524-532

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(Circulation. 2000;101:524.)
© 2000 American Heart Association, Inc.


Basic Science Reports

Characteristics of Coronary Smooth Muscle Cells and Adventitial Fibroblasts

Sachin Patel, MD; Yi Shi, MD, PhD; Rodica Niculescu, DVM; Eugene H. Chung; Jack L. Martin, MD; Andrew Zalewski, MD.

From the Cardiovascular Research Center (S.P., Y.S., R.N., E.H.C., A.Z.), Department of Medicine (Cardiology), Thomas Jefferson University, Philadelphia, Pa; and Bryn Mawr Hospital (J.L.M.), Bryn Mawr, Pa.

Correspondence to Andrew Zalewski, MD, or Yi Shi, MD, PhD, Thomas Jefferson University, Division of Cardiology, Suite 410N, 1025 Walnut St, Philadelphia, PA 19107. E-mail andrew.zalewski@tju.edu or yi.shi{at}tju.edu


*    Abstract
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*Abstract
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Background—Recent findings suggesting the involvement of adventitial cells in coronary repair have raised questions regarding the phenotypic "plasticity" of medial smooth muscle cells (SMCs). Accordingly, the aims of the present study were to examine the characteristics of coronary medial and adventitial cells and to compare the responses of coronary and noncoronary SMCs to stimulation.

Methods and Results—Enzymatically isolated coronary SMCs (human and porcine) were distinct from noncoronary SMCs, showing poor adhesion and spreading, as well as lower proliferation, collagen synthesis, and LDL degradation. Several extracellular matrix components (Matrigel, collagen I and IV, laminin, vitronectin, fibronectin) or growth factors (epidermal growth factor, platelet-derived growth factor-BB, insulin growth factor-1, interleukin-1{alpha}) failed to augment the adhesion or proliferation of coronary SMCs to the levels observed in noncoronary SMCs. Unlike coronary SMCs, coronary fibroblasts demonstrated high adhesion, proliferation, collagen synthesis, and avid LDL metabolism. Limited responses of coronary SMCs were associated with sustained expression of differentiation markers ({alpha}-smooth muscle actin, h-caldesmon, and smooth muscle myosin heavy chain), whereas noncoronary SMCs showed marked phenotypic heterogeneity.

Conclusions—Coronary SMCs appeared to maintain highly differentiated phenotype in response to stimulation, whereas coronary adventitial fibroblasts demonstrated several characteristics that are essential during vascular repair. Coronary SMCs, however, were distinct from noncoronary medial cells, which displayed greater phenotypic heterogeneity and versatility in culture. We postulate that the mechanism of vascular repair may differ among vascular beds, pointing to the importance of coronary artery–specific investigations in vascular biology.


Key Words: remodeling • arteries • muscle, smooth • cells


*    Introduction
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Several mechanical, hemodynamic, or infectious factors contribute to the activation of resident vascular cells, resulting in the formation of intimal lesions.1 2 This process leads to changes in the cellular composition of the vessel wall, which accompany a broad spectrum of cardiovascular disorders.3 4 5 Although intimal cells share some similarities with medial smooth muscle (SM) cells (SMCs), they also display several distinct characteristics regarding morphology, gene expression, and synthetic properties (for a review, see Schwartz et al5 ). Several concepts have emerged regarding the mechanisms underlying the expansion of vascular cells in various disease states.6 7 8 The first paradigm is based on the assumption that the cells involved in vascular repair are primarily of SM origin. The most widely accepted view implies the "plasticity" of virtually all medial SMCs, which modulate their phenotype in response to stimulation. This has been based on the observed changes in cultured SMCs rapidly losing their contractile features and becoming "synthetic" cells.6 A second paradigm, which is based on more recent studies, however, suggests heterogeneity of the arterial media, with only a fraction of SMCs capable of a rapid response to stimulation. These "stem" SMCs, isolated from the rat aorta or carotid artery, differ in regard to morphology, gene expression, and behavior in culture compared with the remaining medial cells.9 10 The third paradigm implies that highly reactive "nonmuscle" cells, being able to proliferate, migrate, and synthesize extracellular matrix proteins, are also involved in the repair of the vessel wall. Although they are sparsely distributed within vascular media (eg, carotid artery, pulmonary artery, and saphenous vein), nonmuscle fibroblasts are abundant in the adventitia.11 12 13 After severe coronary injury (porcine model), nonmuscle cells of adventitial origin proliferate and migrate to the luminal region during neointimal formation.14 15 They also acquire {alpha}-SM actin (myofibroblast formation) and are capable of the synthesis of several extracellular matrix proteins.16 17 18

Although these concepts remain the subject of ongoing debate, they are not mutually exclusive, because regional differences in the mechanisms of arterial repair may arise from the diverse lineages of vascular cells.19 In particular, the coronary vasculature demonstrates unique development, which differs from that of the aorta and its major tributaries.20 21 22 The results of the present study suggest that coronary SMCs are less responsive to stimulation and exhibit lower synthetic capability than other vascular SMCs. When the cellular constituents of the coronary arteries were analyzed, adventitial cells demonstrated particularly dynamic phenotypic characteristics. These findings suggest there are functional differences in the cellular constituents of coronary and noncoronary vascular beds, which may influence the mechanisms of vascular repair and lesion formation.


*    Methods
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*Methods
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Cell Isolation and Culture
Porcine arteries (epicardial coronary arteries, iliac artery, thoracic aorta) were harvested and opened, and their endothelial layers were removed. Human epicardial coronary arteries and samples of thoracic aorta, which were obtained from the recipients of heart transplantation, were prepared in an identical manner. For coronary arteries, the adventitia and media were separated with the assistance of surgical loops (magnification 2x). The separation of adventitial and medial layers was confirmed through immunostaining with {alpha}-SM actin antibody of random samples (not shown). For other vascular beds, the adventitial layer was stripped off, and only the media was used for cell isolation. The tissues were minced into small pieces and repeatedly incubated with collagenase type II (1 mg/mL; Worthington) and elastase (0.5 mg/mL; Sigma) for 30 minutes at 37°C with rocking. The cells were collected after passage of the digestion solution through a filter (pore size 70 µm; Becton Dickinson). After the addition of 10% heat-inactivated FBS and centrifugation, the cells were suspended in DMEM supplemented with 10% FBS, 100 IU/mL penicillin, 100 µg/mL streptomycin, and 2 mmol/L glutamine. The cells from all digestions were pooled, and the viability was estimated in random samples through trypan blue exclusion or MTT assay. Primary cells and early passage cells (between 2 and 7) were used.

Adhesion and Growth Assays
Freshly isolated cells were plated at 20 000 cells/well in 10% FBS with or without coatings (24-well plates). After 24 hours, the wells were rinsed, and attached cells were trypsinized and counted in a Coulter counter. The rate of adhesion was calculated as a percentage of the initially plated cells. The following coatings were used to enhance coronary SMC adhesion: Matrigel (10 µg/well; Becton Dickinson), fibronectin (100 ng/well; Sigma), type I collagen (22 µg/well; Becton Dickinson), type IV collagen (22 µg/well; Becton Dickinson), laminin (10 µg/well; Sigma), and vitronectin (1 µg/well; Sigma). Briefly, the wells (24-well plates) were coated with 200 µL of the solution at 4°C overnight with gentle rocking. The excess of the coating solution was then removed, and the plates were air dried for 10 minutes. In addition, the wells that were coated with collagen, which was dissolved in 0.01 mol/L HCl, were rinsed twice with DMEM. Each experiment was carried out in triplicate and repeated 3 times on different occasions with cells isolated from multiple donors. Data represent mean±SD of 6 to 9 values from 3 separate experiments.

For growth assay, the cells were plated at 20 000 or 100 000 cells/well (24-well plates) in DMEM supplemented with 10% FBS. At 2 days, cells were washed and fresh 10% FBS was added. Cells were then trypsinized at 2, 4, 7, 9, and 14 days after plating and counted in a Coulter counter. Because coronary SMCs did not adhere and grow under these conditions, they were plated onto Matrigel-coated surfaces, and various growth factors or their combinations were tested in addition to 10% FBS: epidermal growth factor (EGF; 75 µg/mL; kindly provided by Dr James San Antonio, Thomas Jefferson University), platelet-derived growth factor (PDGF-BB; 100 ng/mL, Sigma), insulin growth factor-1(IGF-1; 10 ng/mL; Sigma), and interleukin (IL)-1{alpha} (1 ng/mL; Sigma). Each growth experiment was carried out in triplicate for each time point and repeated 3 times on different occasions with cells isolated from multiple donors. Data represent mean±SD of 9 values from 3 experiments.

Protein Synthesis
For overall protein synthesis, cells were labeled with 35S-methionine (50 µCi/mL; DuPont-New England Nuclear) for 40 hours (primary cells) or 24 hours (early passages). The conditioned media and 2 washes were collected and subjected to TCA (10%) precipitation. After being washed twice with 5% TCA, radioactivity were determined with a beta scintillation counter (Wallac). Protein synthesis was expressed as dpm per cell number. Data represent mean±SD of 6 values derived from 3 experiments.

For collagen synthesis, subconfluent primary cells (200 000 cells) and confluent cells (passages 2 to 6) were labeled with 14C-proline (10 µCi/mL; DuPont-New England Nuclear) in DMEM containing 10% FBS and ascorbic acid (25 µg/mL) for 40 hours (primary cells) or 24 hours (confluent cells), respectively. The conditioned media and 2 washes were precipitated with 10% TCA at 4°C overnight. After centrifugation, the pellets were washed twice with 5% TCA and once with absolute ethanol. The pellets were dried and then hydrolyzed overnight with 0.45 mL of 0.2 mol/L NaOH. Radiolabeled collagen was quantified according to the collagenase digestion method.23 Briefly, hydrolyzed samples were divided into 2 aliquots (0.2 mL each) and neutralized by the addition of 0.2 mL of 0.08 N HCl. After the addition of N-ethylmaleimide (1.25 µmol/L), CaCl2 (0.25 µmol/L), and bacterial collagenases (25 µg/mL Clostridium histolyticum type III; Calbiochem), samples with a final volume of 0.5 mL were then incubated at 37°C for 5 hours. The control samples were treated without the addition of collagenases. The reaction was stopped by the addition of 0.5 mL of 20% TCA containing 0.5% tannic acid and BSA (100 µg/mL). Newly synthesized collagen was calculated as the difference between the radioactivity values of the samples treated, with or without collagenases (dpm/cell). The data represent mean±SD of 6 to 9 values derived from 3 experiments.

LDL Isolation, Modification, and Labeling
LDL (density, 1.020 to 1.063 g/mL) was isolated from fresh human plasma through sequential ultracentrifugation.24 25 LDL oxidation (oxLDL) was achieved through the incubation of LDL (200 µg/mL) with 5 µmol/L CuSO4 at 37°C for 2 to 3 hours. Oxidation was measured by monitoring the change of the diene absorption (234 nm) and was stopped by the addition of 100 µmol/L EDTA and 40 µmol/L butylated hydroxytoluene. LDL was iodinated with Na-125I (2 mCi; DuPont-New England Nuclear) and eluted through PD-10 columns with 0.15 mol/L NaCl and 1 mmol/L EDTA.26 125I-LDL was dialyzed for 48 hours at 4°C against 0.15 mol/L NaCl and 1 mmol/L EDTA. The activity of 125I-LDL ranged from 200 to 400 cpm/ng LDL protein with >98% of the 125I radioactivity precipitable by TCA and <5% of the 125I radioactivity extractable with chloroform-methanol. 125I-LDL was sterilized by passage through a filter (pore size 0.45 µm), stored at 4°C under argon, and used within 4 weeks.

For assessment of LDL degradation, freshly isolated cells were plated at 100 000 cells/well in DMEM supplemented with 10% human lipoprotein–deficient serum (for native LDL) or 10% FBS (for oxLDL) for 48 hours. Then, 125I-labeled native LDL or oxLDL was added to the cells for an additional 16 hours. The conditioned media were collected and subjected to TCA (10%) precipitation and chloroform extraction. The radioactivity values (125I-tyramine) in the aqueous phase were counted in a gamma counter (Wallac). The cells were lysed (0.1 mol/L NaOH), and cellular protein content was measured. Nonspecific LDL degradation (in the presence of 50-fold excess of unlabeled LDL and in cell-free wells) has been subtracted from all values. Data represent mean±SD of triplicate measurements. The experiments were repeated 3 times on separate occasions in both primary cells and cells at passages 2 to 7.

Immunostaining
Vascular cells were plated onto coverslips with or without Matrigel coating. Coronary SMCs were grown on coverslips coated with and without Matrigel. At different time points, the coverslips were fixed with HistoChoice for 30 minutes and air dried. The coverslips were stained with Vectastain Elite ABC system (Vector Laboratories). They were incubated with primary antibodies for 1 hour at room temperature, followed by biotinylated secondary horse anti-mouse antibodies (1:2000; Vector Laboratories). Table 1Down lists specific antibodies, the concentrations used, and their sources. Negative controls included either the omission of primary antibody or its replacement with irrelevant mouse IgG (125 ng/mL). The immunostains were visualized with the use of a diaminobenzidine tetrahydrochloride substrate kit (Vector Laboratories) followed by hematoxylin counterstaining.


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Table 1. Antibodies Used in the Study

Statistical Analysis
Data are expressed as mean±SD values. One-way ANOVA was used to compare the multigroup variables. If the F test results were significant, Bonferroni’s analysis was carried out to determine the differences among subgroups. A value of P<0.05 was required to reject the null hypothesis.


*    Results
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*Results
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Cell Morphology, Adhesion, and Growth
Enzymatic digestion yielded >80% viable cells regardless of vascular bed (trypan blue exclusion and MTT assay). As expected, medial cells isolated from the aorta and iliac artery displayed a spindle-shaped morphology, followed by a typical "hill-and-valley" pattern at confluence. In contrast, coronary SMCs demonstrated distinct morphology in culture (Figure 1Down). Coronary adventitial cells isolated from the same vessels, however, were spindle shaped after plating and showed a hill-and-valley configuration at confluence, thus displaying morphology similar to that of noncoronary SMCs (Figure 1Down).



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Figure 1. Photomicrograph showing morphology of vascular cells derived from porcine coronary and noncoronary arteries. Coronary SMCs exhibited a rod-shaped morphology at 2 days, which was maintained for >=10 days in culture. In contrast, coronary adventitial cells and noncoronary medial cells (aorta and iliac artery) displayed a spindle-shaped morphology. They reached confluence at 7 to 10 days, when a typical hill-and-valley pattern was observed (magnification 100x). FIB indicates fibroblasts.

Porcine SMCs derived from noncoronary vascular media showed rapid adhesion to plastic, which was comparable to that of coronary adventitial fibroblasts (Figure 2ADown). This contrasted with coronary medial SMCs, which showed significantly lower adhesion (P<0.01). Because cell attachment is dependent on the surrounding extracellular matrix proteins, we attempted to increase coronary SM adhesion through the coating of culture surfaces with several matrix proteins (Matrigel, collagen types I and IV, laminin, vitronectin, fibronectin). As shown in Figure 2BDown, different extracellular matrix proteins, the use of EGF in the culture medium, and the prolongation of adhesion time to 48 hours produced only minimal improvement in the adhesion of coronary SMCs.



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Figure 2. Adhesion of vascular cells derived from coronary arteries and noncoronary vascular beds. A, Freshly isolated porcine cells were plated at 20 000 cells/well in 10% FBS without coating, and number of attached cells was counted 24 hours later. Cells isolated from coronary media (CA-SMC) exhibited lower adhesion compared with cells derived from aortic media (AO-SMC), iliac artery media (IA-SMC), or coronary adventitia (CA-FIB) (*P<0.01 vs all other groups by ANOVA with Bonferroni’s correction). B, Coronary SMCs were plated at 20 000 cells/well (10% FBS with addition of 75 µg/mL EGF) onto Matrigel (MG), collagen type I (COL I), collagen type IV (COL IV), laminin (LAM), fibronectin (FN), or vitronectin (VN). Number of attached cells was counted 48 hours later. Despite these modifications, coronary medial cells showed only modest improvement in cell adhesion. Each column represents mean±SD (n=6 to 9).

As expected, porcine noncoronary SMCs exhibited rapid growth in the presence of 10% FBS, with exponential growth beginning at 2 days (Figure 3ADown). These cells continued to replicate after being subcultured for >10 passages and exhibited similar growth on plastic or Matrigel-coated surfaces. In contrast, the growth response of porcine coronary SMCs was markedly slower, although coronary adventitial fibroblasts isolated from the same vessels displayed a dynamic growth in culture (Figure 3ADown). These differences were not species dependent, inasmuch as human coronary SMCs also demonstrated lower growth capabilities (Figure 3BDown). To determine whether specific growth factors are required for coronary SMC growth, several growth factors (EGF, PDGF-BB, IGF-1, or IL-1{alpha}) were added to culture medium containing 10% FBS. Notwithstanding this stimulation, coronary SMCs continued to demonstrate limited replication in culture (Figure 3CDown). To exclude the possibility that their slower growth was due to low density, porcine coronary SMCs were also plated at high density (100 000 cells/well), which failed to increase coronary SMC growth or their ability to reach confluence after >30 days in culture (not shown).



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Figure 3. Growth of vascular cells derived from coronary and noncoronary arteries. A, Enzymatically isolated porcine cells from aortic media (AO-SMC), iliac artery media (IA-SMC), and coronary adventitia (CA-FIB) and media (CA-SMC) were plated at 20 000 cells/well in 10% FBS. All cells demonstrated similar growth rates except for coronary SMCs. B, Human aortic and coronary cells were grown in 10% FBS (20 000 cells/well). Human coronary SMCs displayed significantly slower proliferation (hCA-SMC) compared with aortic SMCs (hAO-SMC) and coronary adventitial fibroblasts (hCA-FIB). C, Coronary SMCs were plated onto Matrigel-coated surface and grown in 10% FBS with addition of 75 µg/mL EGF, 100 ng/mL PDGF-BB, 10 ng/mL IGF-1, or 1 ng/mL IL-1{alpha}. Despite different stimuli, coronary medial cells exhibited slow growth rates. Each point represents mean±SD (n=9).

Metabolic Labeling, Collagen Synthesis, and LDL Metabolism
To compare metabolic activity in different vascular cells, overall protein synthesis was assessed by 35S-methionine labeling. No major differences were observed in total protein synthesis, which was consistent with similar viability of primary porcine cultures (Figure 4ADown, P=NS). Because "synthetic" phenotype of SMCs is marked by an increase in extracellular matrix synthesis, de novo collagen synthesis was measured with the use of 14C-proline incorporation, followed by collagenase digestion. Noncoronary SMCs from the aorta and iliac artery showed comparable collagen synthesis, whereas coronary medial SMCs produced significantly less collagen (Figure 4BDown, P<0.001). Coronary adventitial fibroblasts from the same coronary arteries also exhibited a higher ability to synthesize collagen than coronary SMCs (P<0.001).



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Figure 4. Synthetic characteristics of vascular cells derived from coronary arteries and noncoronary vascular beds. A, Overall protein synthesis was examined with 35S-methionine labeling. Comparable protein synthesis was observed in cells derived from aortic media (AO-SMC), iliac artery media (IA-SMC), coronary media (CA-SMC), and adventitia (CA-FIB). B, Collagen synthesis was measured by 14C-proline labeling followed by collagenase digestion. Coronary medial SMCs demonstrated significantly lower collagen synthesis compared with noncoronary medial cells (AO-SMC and IA-SMC) or coronary adventitial cells (CA-FIB). Each column represents mean±SD of 6 to 9 values. *P<0.001 vs all other groups by ANOVA with Bonferroni’s correction.

The differences among vascular cells raised the question regarding regional LDL metabolism. To this end, the ability of vascular cells to degrade LDL was determined. Coronary SMCs demonstrated significantly lower degradation of both native LDL (P<0.01) and oxLDL (Figure 5ADown, P<0.001). Coronary adventitial fibroblasts showed the most avid metabolic processing of modified LDL, which exceeded values observed in the coronary and noncoronary SMCs (P<0.001). Differential LDL degradation was not species dependent, because primary cultures of human vascular cells demonstrated a similar pattern (Figure 5BDown). The avid degradation of LDL by adventitial cells was maintained even in late passages (passages 2 to 7; not shown).



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Figure 5. LDL degradation in vascular cells derived from porcine (A) and human (B) coronary and noncoronary vascular beds. SMCs derived from coronary media (CA-SMC) demonstrated lowest levels of LDL metabolic activities. Noncoronary medial SMCs showed higher degradation of native LDL (nLDL) and oxLDL compared with coronary medial SMCs, although their abilities to degrade oxLDL were significantly lower than those of adventitial cells. Results are representative of 3 separate experiments. Each column represents mean±SD of triplicate measurements. *P<0.01 vs noncoronary medial SMCs and coronary fibroblasts; {dagger}P<0.01 vs noncoronary SMCs by ANOVA with Bonferroni’s correction. Abbreviations as in Figure 3Up.

Expression of SM Differentiation Markers
To examine whether these characteristics of vascular cells are related to their differentiation, several markers of SM differentiation were assessed in primary cultures. There was pronounced heterogeneity of {alpha}-SM actin expression in noncoronary SMCs (Table 2Down). As illustrated in Figure 6Down, {alpha}-SM actin ranged from negative to strongly positive in subconfluent cells. The markers of "late" SM differentiation, such as h-caldesmon and SM-myosin heavy chain (MHC), also displayed heterogenous distribution in noncoronary SMCs (Figure 6Down). Likewise, a similar pattern was observed with SM1- and SM2-MHC antibodies (not shown). In contrast, isolated coronary SMCs demonstrated uniform expression of {alpha}-SM actin (Table 2Down). As shown in Figure 7Down, they lacked heterogenous distribution of cytoskeletal markers ({alpha}-SM actin, h-caldesmon, and SM-MHC) compared with noncoronary SMCs. This highly differentiated phenotype was present in cells cultured in 10% FBS for 1, 3, 5, and 10 days. Figure 7Down also illustrates the defective spreading of coronary SMCs, which did not change in the presence of several components of extracellular matrix (eg, vitronectin, collagen types I and IV, or fibronectin; not shown).


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Table 2. Percentage of {alpha}-SM Actin–Positive Cells in Cultured SMCs Derived From Different Vascular Beds



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Figure 6. Photomicrographs illustrating heterogeneity of cytoskeletal markers in noncoronary SMCs. Enzymatically isolated porcine aortic medial SMCs were grown in 10% FBS. They demonstrated heterogeneous distribution of {alpha}-SM actin, h-caldesmon, and, to a lesser degree, SM-MHC (3 to 5 days after plating) (magnification x490). N/C indicates negative control (no primary antibody).



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Figure 7. Photomicrographs demonstrating uniform expression of cytoskeletal markers in coronary SMCs. Enzymatically isolated porcine coronary SMCs were grown in 10% FBS. They exhibited uniform and sustained expression of all SM cell markers (3 to 5 days after plating) (magnification x490). N/C indicates negative control (mouse IgG instead of primary antibody).

The vast majority of coronary adventitial fibroblasts were devoid of {alpha}-SM actin after isolation (Figure 8Down). This was followed by a dynamic upregulation in {alpha}-SM actin immunoreactivity in coronary adventitial fibroblasts, which became almost uniformly positive for {alpha}-SM actin at 5 days, thereby acquiring the characteristics of myofibroblasts.



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Figure 8. Photomicrographs showing cytoskeletal changes in coronary adventitial fibroblasts. Enzymatically isolated porcine coronary adventitial cells were grown in 10% FBS. They were initially devoid of {alpha}-SM actin but began to acquire {alpha}-SM actin in culture (3 to 5 days) and became uniformly positive for {alpha}-SM actin at 10 days (magnification x490).


*    Discussion
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up arrowAbstract
up arrowIntroduction
up arrowMethods
up arrowResults
*Discussion
down arrowReferences
 
Two major conclusions can be derived from the results of the present study. First, isolated coronary SMCs exhibit a highly differentiated phenotype, whereas noncoronary SMCs display greater phenotypic heterogeneity and more exuberant responses in culture (eg, proliferation, collagen synthesis, LDL degradation). Second, although coronary SMCs are less responsive than other SMCs, coronary adventitial nonmuscle cells demonstrate several phenotypic characteristics that enable them to react to pathophysiological stimuli.

Coronary Medial SMCs
Different morphology and gene expression, as well as distinct proliferative and migratory capabilities, have been previously identified among noncoronary SMCs.10 11 12 27 28 29 Whether the selective expansion of certain subtypes of SMCs or their differential survival in culture accounted for phenotypic versatility of noncoronary SMCs remains to be determined. Nevertheless, coronary SMCs, isolated and cultured in an identical manner, exhibited marked differences. Although differentiation and growth can be dissociated in culture,30 31 coronary SMCs continued to express differentiation markers and failed to replicate in the presence of several growth factors (Figure 3Up). To account for possible species differences,32 33 34 we also examined human coronary SMCs, which showed similar morphology and proliferative activity as porcine coronary SMCs. It should be underscored, however, that our findings do not necessarily contradict previous reports describing the growth of {alpha}-SM actin–positive cells from human coronary arteries.35 36 The particularly great ability of adventitial fibroblasts to migrate, to overgrow medial SMCs, and to become myofibroblasts (Figure 8Up) raises questions as to the origin of cells previously thought to be SMCs.

The exact mechanism underlying differences between coronary and noncoronary SMCs remains to be determined. The development of coronary vessels in situ from the coelomic mesothelium, rather than from ectodermal or mesodermal portions of the aorta, may confer different cellular properties.19 22 In fact, cell type–specific transcriptional regulation of differentiation was recently emphasized by Owens et al.37 The extrinsic signals to be considered include extracellular matrix components (eg, laminin, heparan sulfate glycosaminoglycan) that modulate SMC differentiation.38 39 It is less likely, however, that regional variations in the composition of vascular extracellular matrix in situ were responsible for the observed differences after enzymatic cell isolation. We also emphasize that the effects of several other factors, which are absent in culture conditions, may promote dedifferentiation of coronary SM in vivo; these factors include age-, gender-, and environment-dependent phenomena (arterial retention of LDL or advanced glycosylation end products), which await clarifications in studies involving the coronary vasculature.40 41 42

Coronary Adventitial Nonmuscle Cells
The stimulation of coronary adventitial nonmuscle cells resulted in phenotypic changes typically attributed to synthetic SMCs.43 The expression of {alpha}-SM actin by adventitial nonmuscle cells exemplified the formation of myofibroblasts (Figure 8Up).44 45 The selective expansion of a small number of SMCs in adventitial samples was unlikely, inasmuch as coronary SMCs demonstrated poor attachment and growth (Figures 2Up and 3Up). As shown in the present study, adventitial nonmuscle cells displayed several properties necessary for arterial repair (ie, high adhesion, proliferation, and collagen synthesis). Recently, they have also been shown to possess increased matrix-degrading activities and migration compared with coronary SMCs, which constitutively express TIMP-1 and -2.46 Notwithstanding these findings, we emphasize that although the conditions for activation of nonmuscle cells likely exist after acute mechanical medial injury in humans,47 the migration of the cells during chronic medial damage is more difficult to determine. The cytotoxic effects of modified LDL during atherogenesis,42 however, often result in medial thinning, which may enable nonmuscle cell translocation and their involvement in lesion formation.48 49 The avid degradation of oxLDL by activated coronary nonmuscle cells (Figure 5Up) suggested a high expression of scavenger receptor and the possibility of their involvement in foam cell formation on reaching the intima.

Clinical Implications
Disappointing results of several clinical studies that target coronary restenosis have exemplified a low predictive value of pharmacological testing in animal models of noncoronary arterial injury. The presented results and previous observations in vivo indicate greater susceptibility of coronary adventitial fibroblasts to mitogenic stimuli compared with coronary SMCs. Whether the suggested involvement of adventitial cells will affect the outcomes of antihyperplastic interventions applied in vivo (eg, brachytherapy or pharmacological approaches) remains to be determined. Interestingly, however, the adventitial delivery of some agents appears to be more effective than endoluminal administration.50 We postulate that a better understanding of the unique characteristics of coronary vascular cells may provide the insight necessary for future therapeutic interventions aimed specifically at a reduction in coronary hyperplastic responses.

In conclusion, in the present study, we examined the characteristics of coronary medial SMCs and adventitial nonmuscle cells. Coronary SMCs, which differed from noncoronary SMCs, showed a highly differentiated phenotype with a limited ability to adhere, proliferate, and synthesize collagen. In contrast, adventitial nonmuscle cells displayed characteristics usually attributed to synthetic SMCs. We postulate that the mechanisms of vascular repair and lesion formation may differ among vascular beds, which points to the importance of future coronary artery–specific investigations in vascular biology.

Note Added in Proof
Since the submission of this manuscript, Christen et al51 have also reported the presence of a highly differentiated phenotype of coronary SMCs and the unique response of these cells in culture.


*    Acknowledgments
 
This study was supported by National Institutes of Health Grants HL-44150 and HL-60672 and the John S. Sharpe Foundation. The authors gratefully acknowledge Kenneth Margulies, MD, and Carol Fisher, BS (Temple University), for their assistance in the collection of human tissues. We thank Mary Ellen Henderson for her assistance in preparation of the manuscript.

Received May 14, 1999; revision received July 30, 1999; accepted August 13, 1999.


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