Actin expression in smooth muscle cells of rat aortic intimal thickening, human atheromatous plaque, and cultured rat aortic media

Human smooth muscle VLA-1 integrin: purification, substrate specificity, localization in aorta, and expression during development

A membrane glycoprotein complex was isolated and purified from human smooth muscle by detergent solubilization and affinity chromatography on collagen-Sepharose. The complex was identified as VLA-1 integrin and consisted of two subunits of 195 and 130 kD in SDS-PAGE. Liposomes containing the VLA-1 integrin adhered to surfaces coated with type I, II, III, and IV collagens, Clq subcomponent of the first component of the complement, and laminin. The liposomes specifically adhered to these proteins in a Ca2+, Mg2(+)-dependent manner, but did not bind to gelatin, fibronectin, and thrombospondin substrates. The expression of VLA-1 integrin in different human tissues and cell types, and during aorta smooth muscle development was studied by SDS-PAGE, and subsequent quantitative immunoblotting was performed with antibodies recognizing alpha 1 and beta 1 subunits of the VLA-1 integrin. A high level of VLA-1 integrin expression was an exceptional feature of smooth muscles. Fibroblasts, endothelial cells, keratinocytes, striated muscles, and platelets contained trace amounts of VLA-1 integrin. In the 10-wk-old human fetal aorta, VLA-1 integrin was found only in smooth muscle cells whereas mesenchymal cells, surrounding aortic smooth muscle cells, were VLA-1 integrin negative. By the 24th wk of gestation, the amount of VLA-1 integrin was significantly reduced in the aortic media (4.3-fold for alpha 1 subunit and 2.5-fold for beta 1 subunit) compared with that in the 10-wk-old aortic smooth muscle cells. After birth, the expression of VLA-1 integrin increased and in the 1.5-yr-old child aorta the VLA-1 integrin level was almost the same as in adult aortic media. Smooth muscle cells from intimal thickening of adult aorta express five times less alpha 1 subunit of VLA integrin that smooth muscle cells from adult aortic media. In primary culture of aortic smooth muscle cells, the content of the VLA-1 integrin was dramatically reduced and subcultured cells did not contain VLA-1 integrin at all.

Changes in expression and organization of smooth-muscle-specific alpha-actin during fibronectin-mediated modulation of arterial smooth muscle cell phenotype

The spreading of freshly isolated arterial smooth muscle cells on a substrate of fibronectin is mediated by an integrin receptor on the cell surface. It is associated with organization of actin filaments in stress fibers and marked changes in cell morphology and function, collectively referred to as a transition from a contractile to a synthetic phenotype. To study further how extracellular matrix components affect smooth muscle phenotype, we have analyzed the expression and organization of smooth-muscle-specific alpha-actin in freshly isolated rat aortic smooth muscle cells cultured on a substrate of fibronectin under serum-free conditions. Northern-blot analysis showed that the expression of mRNA for smooth muscle alpha-actin, but not for nonmuscle actin, was strongly repressed during primary culture. On the other hand, the cellular content of alpha-actin was only moderately changed during the same period. Indirect immunofluorescence staining revealed that nonmuscle actin was rapidly organized in stress fibers, which did not stain with a monoclonal antibody against smooth muscle alpha-actin. Filament bundles containing alpha-actin were most prominent in the central parts of the cytoplasm and gradually disappeared as the spreading of the cells progressed. In contrast to the situation with nonmuscle actin, there was no apparent overlap in the staining for alpha-actin and the fibronectin receptor (alpha 5 beta 1), indicating that this receptor interacted with nonmuscle actin during the initial spreading process. Taken together, the results show that the expression and organization of smooth muscle alpha-actin are changed during interaction of the cells with fibronectin early in primary culture. They support the notion that integrin-mediated interactions between extracellular matrix components and arterial smooth muscle cells take part in the control of smooth muscle phenotype.

Changes in the composition of cytoskeletal and cytocontractile proteins of rat aortic smooth muscle cells during aging

Cytoskeletal proteins are used as differentiation markers of vascular smooth muscle cells (SMC). To study possible changes in SMC phenotype during aging, cytoskeletal and cytocontractile proteins were quantified in the aortic intima-medias of 4-, 12-, 30-, and 36-month-old rats by one- and two-dimensional gel electrophoresis. The percentages of myosin and desmin in total protein decreased with age, while those of actin and vimentin remained unchanged. Immunohistochemical comparison of the aortas from 4- and 30-month-old rats showed that the reduction of desmin reflected a selective disappearance of desmin in some cells. There was an age-related increase in the proportion of beta-actin at the expense of the alpha-isoform. Our results suggest an age-dependent modulation of the phenotype of vascular SMC towards the synthetic state, which is opposite to that observed during developmental differentiation.

Time course of smooth muscle cell proliferation in the intima and media of arteries following experimental angioplasty

Smooth muscle cell (SMC) proliferation is known to be an important factor for the development of restenosis after percutaneous transluminal coronary angioplasty. To determine the time course of intimal and medial SMC proliferation and morphological changes after experimental angioplasty, an intimal atheroma was produced with repeated weak electrical stimulations in the right carotid artery of 45 male New Zealand White rabbits. Angioplasty was subsequently performed in 35 rabbits, and the proliferative responses were analyzed with histomorphological and immunohistological criteria at 3, 7, 14, 21, 28, and 42 days after intervention. A hemodynamic relevant stenosis after angioplasty was found in eight (23%) of 35 dilated arteries. In five rabbits the stenosis was due to a mural thrombus, and in three animals restenosis was caused by intimal SMC proliferation. In all dilated arteries the intimal wall thickness increased from 13 +/- 5 intimal cell layers (after electrical stimulation) to 33 +/- 14 cell layers during 28 days after angioplasty (p less than 0.05). Later than 4 weeks after angioplasty, no additional increase of intimal thickening occurred. Application of bromodeoxyuridine 18 and 12 hours before excision of the vessels allowed determination of the percent of cells undergoing DNA synthesis in the intima and media using monoclonal antibody against bromodeoxyuridine. SMCs were identified by alpha-actin staining. Immunohistological quantification of intimal SMC proliferation showed a maximum of cells undergoing DNA synthesis within the first 7 days after angioplasty (p less than 0.01). In contrast, medial proliferation of SMCs was delayed and showed a small but significant increase 21 days after dilatation (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of gelsolin content in rat aortic smooth muscle cells during development, experimental intimal thickening and culture. An immunohistochemical and biochemical study

Gelsolin is a Ca2+ and polyphosphoinositol-phospholipid regulated modulator of actin polymerization present in most mammalian cells and in plasma. Cytoplasmic gelsolin was first described in highly motile cells such as leukocytes, where actin polymerization is dynamic; however arterial smooth muscle cells (SMC), despite their stabilized actin bundles, express high levels of gelsolin. We have investigated gelsolin modulation in rat aortic SMC by immunohistochemistry, Western blotting and Northern hybridization using three models which are known to show modulation of actin isoform expression: development, aortic intimal thickening after experimental endothelial injury, and growth in culture. When related to the protein and mRNA content of adult aortic SMC, gelsolin is expressed about 50% in aortic SMC of five-day-old rats, 20-30% in SMC of intimal thickening 15 days after endothelial injury (when SMC are actively replicating) and in SMC growing in culture; in intimal thickening 60 days after injury (when SMC have returned to quiescence), the gelsolin content becomes similar to that of control SMC. The high level of gelsolin in smooth muscle (SM) tissues and the down regulation with proliferation and migration raises the question as to whether gelsolin in these cells has functions other than the dynamic control of actin filament length. The similar modulation patterns of gelsolin and alpha-SM actin suggest a preferential interaction between these two proteins.

Heterogeneity of smooth muscle cells in atheromatous plaque of human aorta

This study was undertaken to investigate the expression of cytoskeletal proteins and the ultrastructure of cells in normal intima and atheromatous plaque of human aorta. It has been established, using double-labeling immunofluorescence, that smooth muscle cells (SMC) in normal aortic intima contain myosin, vimentin, and alpha-actin but do not react with antibodies against desmin. In contrast, 7 of 28 atherosclerotic plaques contained many cells expressing desmin in addition to the other cytoskeletal proteins characteristic of normal intima SMC. These cells were localized predominantly in the plaque cap and had the ultrastructural features of modulated SMC, ie, well-developed endoplasmic reticulum and Golgi apparatus. Besides, some cells in the 13 atherosclerotic plaques proved to be myosin, alpha actin, and desmin negative but contained vimentin and actin as revealed by fluorescent phalloidin. These cells were found in the immediate proximity of atheromatous material and reacted with a monoclonal antibody specific to SMC surface protein (11G10) but not with monoclonal anti-muscle actin (HHF35) and anti-macrophage (HAM56) antibodies. Electron microscopy of this plaque zone revealed that the cytoplasm of these cells was filled with rough endoplasmic reticulum and a developed Golgi complex. At the same time, a certain proportion of cells in this region retained morphologic features of differentiated SMC such as the presence of a basal lamina and myofilament bundles. The revealed peculiarities of cytoskeletal protein expression and the ultrastructure of cells in human aortic atherosclerotic plaques may be explained by a phenotypic modulation of vascular SMC.

Intermediate filament proteins and actin isoforms as markers for soft-tissue tumor differentiation and origin. III. Hemangiopericytomas and glomus tumors

Intermediate filament proteins and actin isoforms of a series of 12 malignant hemangiopericytomas and five glomus tumors were examined by light microscopy, transmission electron microscopy, two-dimensional gel electrophoresis (2D-GE), and by immunohistochemistry, the latter using monoclonal or affinity-purified polyclonal antibodies to desmin, vimentin, cytokeratins, alpha-smooth muscle, and alpha-sarcomeric actins. By light microscopy, all hemangiopericytomas disclosed a predominant vascular pattern with scant storiform, myxoid and spindle cell areas, and with variable degrees of perivascular fibrosis. By ultrastructure, smooth muscle differentiation was observed in each hemangiopericytoma. Immunohistochemically, neoplastic cells of hemangiopericytomas expressed vimentin as the sole intermediate filament protein and lacked alpha-smooth muscle or alpha-sarcomeric actins. 2D-GE revealed only beta and gamma actins, in proportions typical for fibroblastic tissues. Glomus tumors revealed vimentin and alpha-smooth muscle actin within glomus cells by immunohistochemical techniques and disclosed ultrastructurally distinct smooth muscle differentiation. Therefore hemangiopericytomas represent a distinct soft-tissue neoplasm with uniform morphologic, immunohistochemical, and biochemical features most likely related to glomus tumors, the former representing an aggressive and potentially malignant neoplasm of vascular smooth muscle cells and the latter a well-differentiated neoplasm of vascular smooth muscle cells. Because malignant hemangiopericytomas disclose smooth muscle differentiation by ultrastructure, but do not express alpha-smooth muscle actin, as normal pericytes and glomus cells, it is suggested that these neoplasms represent highly vascularized smooth muscle neoplasms, ie, poorly differentiated leiomyosarcomas derived from vascular smooth muscle cells or their equivalent, the pericytes, which have lost alpha-smooth muscle actin as a differentiation marker that is similar to many conventional poorly differentiated leiomyosarcomas.

Platelet-derived growth factor regulates actin isoform expression and growth state in cultured rat aortic smooth muscle cells

The role of platelet-derived growth factor (PDGF) in the control of smooth muscle cell (SMC) differentiation was explored in vitro by examining its effects on expression of the smooth muscle (SM) specific contractile protein SM alpha actin in cultured rat aortic SMC. Quiescent, postconfluent SMC express maximal levels of alpha actin and responded to human platelet-derived growth factor (partially purified from platelets) by entering the cell cycle and undergoing approximately one synchronous round of DNA synthesis. Concomitantly, these cultures exhibited a marked reduction in alpha actin synthesis. Chronic treatment with PDGF (72 hours at 8 or 12 hour intervals) was associated with a transient increase in thymidine labeling index and a decrease in alpha actin expression. Interestingly, between 48 and 72 hours following initial treatment, thymidine labeling indices returned to near control levels while SM alpha actin expression remained depressed. This effect was reversible; fractional alpha actin synthesis increased immediately after PDGF removal. When subsequently stimulated with 10% fetal bovine serum (FBS), cells chronically pretreated with PDGF entered S phase approximately 4 hours earlier than cells pretreated with PDGF vehicle, consistent with the idea that the maintained suppression of alpha actin synthesis in SMC subjected to chronic PDGF treatment was associated with partial cell cycle transit. Chronic treatment with highly purified recombinant PDGF-BB elicited similar effects on alpha actin synthesis and partial cell cycle transit. Flow cytometric analysis of chronic PDGF-treated SMC demonstrated a 25% increase in forward angle light scatter, an index of cell size. These data implicate a possible role for PDGF in regulation of SMC differentiation and suggest a potentially important role for this mitogen in the phenotypic modulation accompanying SMC growth and in mediation of the cellular hypertrophy associated with cell cycle progression.

Action of general and alpha-smooth muscle-specific actin antibody microinjection on stress fibers of cultured smooth muscle cells

Arterial smooth muscle cells express alpha- and gamma-smooth muscle, as well as beta- and gamma-cytoplasmic actins. Two actin antibodies, one recognizing smooth muscle and cytoplasmic actin isoforms, the other recognizing specifically alpha-smooth muscle actin, were microinjected into cultured aortic smooth muscle cells. The effect of these antibodies on stress fiber organization was examined by staining with rhodamine-labeled phalloidin and by immunofluorescence with the same antibodies. Microinjection of the general actin antibody abolished most of the stress fiber staining with all reagents, but did not significantly affect the shape of the injected cells. This suggests that stress fiber integrity is not absolutely necessary for the maintenance of cell shape within the time of observation. Microinjection of the specific alpha-smooth muscle antibody abolished to various extents the staining of stress fibers with this antibody, but left practically intact their staining with rhodamine-labeled phalloidin and with the general actin antibody. This suggests that the incorporation of alpha-smooth muscle actin is not absolutely necessary for the maintenance of stress fiber integrity in cultured smooth muscle cells.

Transformation of rabbit vascular smooth muscle cells by transfection with the early region of SV40 DNA

Rabbit aortic smooth muscle cells (SMC) and Rb-1 cells, a continuous line of the same origin, were transformed by transfection with pSV3-neo DNA, a plasmid containing the SV40 early region linked to the neoR resistance gene. Transformed clones were selected in G418-containing medium at a rate of 10(-4) per cell. All transformed clones were immortalized and contained in the early passages two free recombined plasmids derived from pSV3-neo. At advanced passages pSV3-neo sequences were found integrated in the cellular genome. Transformed cells had an altered morphology and growth pattern that differed among clones. Some clones reached high density in low-serum medium. All the clones stained positively for the intranuclear T antigen. Some clones had distinct transcripts for the large T and small t antigens, while in others only larger or truncated transcripts were found. Alpha-actin filaments were visualized by immunofluorescent staining in all the clones, but Northern blot analysis revealed a significant reduction in transcripts for this actin. All the transformed clones accumulated, to a variable extent, cholesteryl esters after incubation with beta very low-density lipoprotein. Six of the eight transformed clones maintained a diploid chromosome number, but there was an increase in structural chromosome aberrations, predominantly dicentrics. Transfection of pSV3-neo into rabbit vascular SMCs is an efficient model for obtaining transformed clonal populations. These clones show some phenotypic changes that may be relevant to the study of atherogenesis.

Regulation of alpha-smooth muscle actin and other polypeptides in proliferating and density-arrested vascular smooth muscle cells

We have examined alpha-smooth muscle actin (alpha-SM actin) protein and mRNA levels in proliferating and density-arrested rabbit vascular smooth muscle cells (SMC) and also studied overall polypeptide synthesis in these cells by two-dimensional (2-D) gel electrophoresis. Of the approximately 1,000 cellular polypeptides resolved by 2-D gel analysis, we consistently detected increased expression of 12 polypeptides in growth-arrested SMC. These polypeptides, with apparent molecular weights of 24,000 to 55,000 exhibited relative increases of between fourfold to greater than tenfold. Three of these polypeptides were expressed at undetectable levels in proliferating SMC. We also detected 12 secreted polypeptides that were expressed at higher levels in growth-arrested SMC. More changes were associated with the secreted polypeptides, since they represented approximately 4% of the total resolved secreted polypeptides, while only 1% of the cellular polypeptides were increased in high-density growth-arrested cells. Under these conditions we observed no change in relative alpha-SM actin protein content as determined by 2-D gel analysis and Western blots. This was corroborated by high levels of alpha-SM actin mRNA levels in both proliferating and high-density growth-arrested SMC. These results indicate rabbit vascular SMC maintain a high level of expression of a smooth muscle differentiation marker (alpha-SM actin) in a proliferation- and density-independent manner. We also examined polypeptide synthesis in SMC isolated by enzymatic digestion of the aorta vs. cells isolated by the explant method. We found that although overall protein patterns were remarkably similar, several differences were observed. These differences were not due to increased contamination by fibroblasts, since both enzymatically- and explant-derived SMC contained high levels of alpha-SM actin as determined by immunofluorescence and by Northern analysis.

Growth characteristics and cytoskeletal organization of cultured smooth muscle cells from human primary stenosing and restenosing lesions

Growth characteristics of human plaque cells selectively extracted from advanced primary stenosing and fresh restenosing lesions by percutaneous transluminal atherectomy were studied in vitro. Cells were isolated either by explant technique or by enzymatic disaggregation, and they were identified as smooth muscle cells (SMC) by positive reaction with antibodies against alpha-smooth muscle actin. Endothelial cells were not found in the atherectomized tissue. The cells of primary stenosing tissue (ps-SMC) exhibited a significantly low growth rate (0.16 +/- 0.04 population doublings per day) in comparison to the cells of restenosing lesions (re-SMC, 0.64 +/- 0.15 population doublings per day). Furthermore, ps-SMC became senescent and remained quiescent after two passages, whereas re-SMC retained a high proliferative activity and became quiescent by passage 8 to 10. Both types of cells responded to increasing serum concentrations in a dose-dependent manner. Ps-SMC failed to respond to purified platelet-derived growth factor (PDGF) and a mitogen mixture isolated from bovine brain (ECGF), but their proliferative activity was increased by the addition of re-SMC-conditioned culture medium. Despite their high basic growth rate, the proliferative activity of re-SMC was significantly stimulated by PDGF and ECGF in a dose-dependent manner. PS-SMC and re-SMC populations consisted of two distinct subpopulations, which could be discriminated by cell size measurements and cell adhesion: 1) relatively small (cell diameter, 18.6 +/- 5 microns), low-adhesive, predominant cells, and 2) enlarged (cell diameter, 27.1 +/- 3 microns), high-adhesive, fibroblast-like cells with abundant microfilaments. Neither ps-SMC or re-SMC stained with antibodies against desmin, but did express vimentin. The organization patterns of vimentin and tubulin were unaltered in comparison to each other and to smooth muscle cells cultured from the media of nonatherosclerotic human arteries.

Cell constitution and characteristics of human atherosclerotic plaques selectively removed by percutaneous atherectomy

The Simpson atherectomy device used for the recanalization of severely stenosed peripheral arteries is able to collect plaque material which can be further characterized. This study reports histological, immunohistochemical and transmission electron microscopic findings on advanced human primary atherosclerotic plaques of peripheral arteries percutaneously removed by a Simpson atherectomy catheter. Material from stenosing plaques consisted of dense connective tissue with abundant amounts of concentrically arranged elastic fibers and lamellae. This meshwork contained numerous cells, often arranged in clusters and oriented with their longer axis parallel to the direction of blood flow. The vast majority of these cells could be easily identified as vimentin-positive and desmin-negative smooth muscle cells containing lipid deposits in the perinuclear region and numerous glycogen particles. Monocytes/macrophages were observed only very infrequently. Plaque tissue contained a range of smooth muscle cell phenotypes. Most of the cells were of an intermediate phenotype, i.e. sparsely filled with myofilament bundles at the cell periphery and a high amount of organelles such as mitochondria, rough endoplasmic reticulum and Golgi cisterns. An intact lining of pieces of intimal tissue with endothelial cells was not observed. Two-dimensional gel electrophoresis of plaque tissue showed the presence of alpha-, beta- and gamma-actin isoforms with a clear predominance of the beta-isoform.

Characterization of a continuous smooth muscle cell line derived from rabbit aorta

A spontaneously arising continuous cell line (Rb-1) derived from collagenase-elastase digested rabbit aorta has been propagated in vitro for over 100 passages. During this period, the Rb-1 cells remained spindle-shaped and formed regularly oriented parallel bundles. After Passage 50, Rb-1 cells were found to be serum-independent in their growth and reached higher saturation density than rabbit aorta smooth muscle cells. Alpha-actin and desmin filaments were detected by immunostaining in Rb-1 cells and early passage of rabbit aorta smooth muscle cells. The proportion of alpha-actin transcripts in Rb-1 cells was lower than that of transcripts for beta- and gamma-actins. The modal chromosome number was maintained at 44 between Passages 11 and 60, and two marker chromosomes were constantly present. Infection of Rb-1 cells with two strains of herpes simplex virus type 1 resulted in high titers of virus, whereas a herpes simplex virus type 2 temperature-sensitive mutant replicated only at the permissive temperature. The Rb-1 cell line could be used for the study of vascular smooth muscle cell proliferation and their interaction with viruses.

Acute and long-term effects of tissue culture on contractile reactivity in renal arteries of the rat

To evaluate long-term effects of contractile and mitogenic stimuli on the contractile reactivity of arterial smooth muscle, we measured the incorporation of the thymidine analogue 5-bromo-2'-deoxyuridine (BrdUrd) and mechanical responses in arterial segments that had been maintained in tissue culture. The experiments were performed on renal arteries that had been isolated from adult rats, chemically sympathectomized, mechanically denuded from endothelium and mounted under distension. Exposure of arterial segments for up to 3 weeks to culture medium supplemented with fetal calf serum resulted in the following consecutive changes: a strong acute contraction, selective pharmacological changes that included decreased contractile responses to phenylephrine and vasopressin and increased relaxing responses to isoproterenol, increased incorporation of BrdUrd, a progressive fall in contractile responses to all vasoconstrictor stimuli, and an increase in excitability. Serum-free medium resulted in a much smaller acute arterial contraction, induced less incorporation of BrdUrd, accelerated the occurrence of hyperexcitability, but did not affect early pharmacological changes or the subsequent fall in overall arterial contractility with tissue culture. Dialysis of the serum or addition of ketanserin abolished the contractile effect of serum but did not affect the incorporation of BrdUrd or the loss of contractility with tissue culture. Addition of serotonin to serum-free culture medium mimicked the contractile response to serum but not the stimulation of BrdUrd incorporation. These data indicate that tissue culture alters the properties of the arterial wall, that contraction does not underlie the proliferative response of arterial smooth muscle to serum-derived mitogens in vitro, and that stimulation of DNA synthesis does in itself not lead to selective changes in arterial contractility.

Cytodifferentiation and expression of alpha-smooth muscle actin mRNA and protein during primary culture of aortic smooth muscle cells. Correlation with cell density and proliferative state

To investigate a possible correlation between cytodifferentiation, proliferation, and actin expression, smooth muscle cells from the 9-week-old rabbit aortic media were enzyme-dispersed into single cells and were plated in primary culture at different initial seeding densities. The volume fraction of myofilaments (Vv myo) in cells seeded moderately densely fell from 39.5% +/- 1.2% in the intact aortic media to 11.5% +/- 1.6% on Day 5, one day before the onset of logarithmic growth. The Vv myo remained low over the next 3 days, then began to rise as the density of cells increased, returning almost to the original levels after confluency and 1.84 cumulative population doublings (CPD). The expression of alpha-smooth muscle actin mRNA followed a similar time course of change, falling from 84.7% +/- 1.2% of total actin mRNA in freshly isolated cells to 54.0% +/- 6.5% on Day 5, returning to 87.5% +/- 0.5% after confluency. In these cultures, the alpha-smooth muscle actin protein content was 93.7% +/- 2.9% of total actin in freshly isolated cells, 68.7% +/- 3.1% on Day 5, and 73.3% +/- 2.5% 3 days after confluency. In densely seeded cultures, the Vv myo and expression of alpha-smooth muscle actin mRNA fell only slightly on Day 5 and rose to original levels upon confluency after 0.33 CPD. However, at the protein level, alpha-smooth muscle actin decreased on Day 5 and remained low on Day 12. The Vv myo, alpha-smooth muscle actin mRNA, and actin protein of sparsely seeded cells fell on Day 5 and then remained low throughout the culture period, including 5 days after confluency (Day 24), when the cells had undergone 5.37 CPD. Cells that were maintained subconfluent but quiescent on Day 7 in culture had the same low Vv myo, low alpha-actin mRNA expression, and low alpha-actin protein content as actively proliferating cells. The results show that Vv myo and alpha-smooth muscle actin mRNA undergo parallel changes during primary culture according to seeding density, but not to replication, and that alpha-smooth muscle actin protein decreases in culture then remains low irrespective of culture conditions.

Myosin heavy-chain isoforms in adult and developing rabbit vascular smooth muscle

Two monoclonal antibodies specific for smooth muscle myosin (designated SM-E7 and SM-A9) and one monoclonal anti-(human platelet myosin) antibody (designated NM-G2) have been used to study myosin heavy chain composition of smooth muscle cells in adult and in developing rabbit aorta. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis and Western blotting experiments revealed that adult aortic muscle consisted of two myosin heavy chains (MCH) of smooth muscle type, named MHC-1 (205 kDa), and MHC-2 (200 kDa). In the fetal/neonatal stage of development, vascular smooth muscle was found to contain only MHC-1 but not MHC-2. Non-muscle myosin heavy chain, which showed the same electrophoretic mobility as the slower migrating MHC, was expressed in an inverse manner with respect to MHC-2, i.e. it was detectable only in the early stages of development. The distinct pattern of smooth and non-muscle myosin isoform expression during development may be related to the different functional properties of smooth muscle cells during vascular myogenesis.

Cytoskeletal features of immature pulmonary vascular smooth muscle cells: the influence of pulmonary hypertension on normal development

Using an immunohistochemical technique, the development of the cytoskeletal proteins desmin, vimentin, and actin (using alpha isotype and non-isotype specific antibodies) was assessed using a semi-quantitative grading system in the pulmonary vascular smooth muscle of nine normal pigs and 19 normal humans at different ages, and in 13 children with pulmonary hypertensive congenital heart disease. In the normal of both species, immunostaining for vimentin decreased after birth and then increased gradually while immunostaining for desmin and alpha actin increased steadily with age. In pulmonary hypertension, immunostaining for alpha actin and vimentin showed an accelerated increase at between 2 and 8 months. Also, the media showed regional differences in immunostaining which preceded the development of intimal proliferation. The inner media showed less immunoreactivity for all cytoskeletal proteins studied than did the outer media. Within areas of intimal proliferation many cells were immunonegative. These results suggest that the cytoskeletal features of medial smooth muscle cells are remodelled in the normal infant; that this process is altered from at least 2 months in the pulmonary hypertensive infant; and that the smooth muscle cells immediately beneath the internal elastic lamina are remodelled before migrating to form intimal proliferation. Changes in cytoskeletal composition can be related to the previously described postnatal maturation of pulmonary vascular smooth muscle cells.

Expression of extra domain A fibronectin sequence in vascular smooth muscle cells is phenotype dependent

Different fibronectin (FN) variants arise from the single gene transcript alternatively spliced in a tissue-specific manner (Hynes, R. O. 1985. Annu. Rev. Cell Biol. 1:67-90; Owens, R. J., A. R. Kornblihtt, and F. E. Baralle. 1986. Oxf. Surv. Eurcaryotic Genes. 3:141-160). We used mAb IST-9, specific for extra domain A (ED-A) FN sequence, and cDNA probe to ED-A exon to determine whether ED-A is present in FN synthesized by vascular smooth muscle cells (SMCs) and, if so, whether expression of ED-A is SMC phenotype dependent. ED-A-containing FN (A-FN) was not revealed in tunica media of human arteries and normal rat aorta by immunofluorescence and immunoblotting techniques. A cDNA probe to ED-A exon did not hybridize with RNA isolated from human aortic media. A positive reaction with IST-9 was observed in (a) diffuse intimal thickening and atherosclerotic plaque from human arteries; (b) experimentally induced intimal thickening in rat aorta; and (c) cultured vascular SMCs. A-FN mRNA was present in the RNA preparation from human aortic intima as judged by hybridization with cDNA probe to ED-A. On the other hand, an mAb interacting with an epitope common for all FN variants revealed FN in both intima and media of human arteries and in the normal rat aorta. A cDNA probe to a sequence shared by all FN variants hybridized with RNA from both intima and media of human aorta, though the level of expression was higher in intima. The data suggest that ED-A exon is omitted during splicing of the FN mRNA precursor in medial SMCs while the expression of A-FN is characteristic of "modulated" SMCs--those of intimal thickenings, of atherosclerotic lesions, and growing in culture.

The development expression of the rat alpha-vascular and gamma-enteric smooth muscle isoactins: isolation and characterization of a rat gamma-enteric actin cDNA

We have isolated and characterized two cDNA clones from whole rat stomach, pRV alpha A-19 and pRE gamma A-11, which are specific for the alpha-vascular and gamma-enteric smooth muscle isoactins, respectively. The rat gamma-enteric smooth muscle actin contains a single amino acid substitution of a proline for a glutamine at position 359 of the mature peptide when compared with the chicken gizzard gamma-actin sequence (J. Vandekerckhove and K. Weber, FEBS Lett. 102:219, 1979). Sequence comparisons of the 5' and 3' untranslated (UT) regions of the two smooth muscle actin cDNAs demonstrate that these regions contain no apparent sequence similarities. Additional comparisons of the 5' UT regions of the two smooth muscle actin cDNAs to all other known actin sequences reveal no apparent sequence similarities for the rat gamma-enteric isoactin within the 15 base pairs of sequence currently available, while the rat alpha-vascular isoactin contains two separate sequences which are similar to sequences within the 5' UT regions of the human and chicken alpha-vascular actin genes. A similar comparison of the 3' UT regions of the two smooth muscle actins demonstrates that the alpha-vascular isoactins do not contain the high degree of cross-species sequence conservation observed for the other isoactins and that the gamma-enteric isoactin contains an inverted sequence of 52 nucleotides which is similar to a sequence found within the 3' UT regions of the human, chicken, and rat beta-cytoplasmic isoactins. These observations complicate the apparent cross-species conservation of isotype specificity of these domains previously observed for the other actin isoforms. Northern blot analysis of day 15 rat embryos and newborn, day 19 postbirth, and adult rats demonstrates that the day 15 rat embryo displays low to undetectable levels of smooth muscle isoactin mRNA expression. By birth, the stomach and small intestine show dramatic increases in alpha-vascular and gamma-enteric actin expression. These initially high levels of expression decrease through day 19 to adulthood. In the adult rat, the uterus and aorta differ in their content of smooth muscle isoactin mRNA. These results demonstrate that the gamma-enteric and alpha-vascular isoactin mRNAs are coexpressed to various degrees in tissues which contain smooth muscle.

Modulation of actin mRNAs in cultured vascular cells by matrix components and TGF-beta 1

Alpha-smooth muscle actin is currently considered a marker of smooth muscle cell differentiation. However, during various physiologic and pathologic conditions, it can be expressed, sometimes only transiently, in a variety of other cell types, such as cardiac and skeletal muscle cells, as well as in nonmuscle cells. In this report, the expression of actin mRNAs in cultured rat capillary endothelial cells (RFCs) and aortic smooth muscle cells (SMCs) has been studied by Northern hybridization in two-dimensional cultures seeded on individual extracellular matrix proteins and in three-dimensional type I collagen gels. In two-dimensional cultures, in addition to cytoplasmic actin mRNAs which are normally found in endothelial cell populations, RFCs expressed alpha-smooth muscle (SM) actin mRNA at low levels. alpha-SM actin mRNA expression is dramatically enhanced by TGF-beta 1. In addition, double immunofluorescence staining with anti-vWF and anti-alpha-SM-1 (a monoclonal antibody to alpha-SM actin) shows that RFCs co-express the two proteins. In three dimensional cultures, RFCs still expressed vWF, but lost staining for alpha-SM actin, whereas alpha-SM actin mRNA became barely detectable. In contrast to two-dimensional cultures, the addition of TGF-beta 1 to the culture media did not enhance alpha-SM actin mRNA in three-dimensional cultures, whereas it induced rapid capillary tube formation. Actin mRNA expression was modulated in SMCs by extracellular matrix components and TGF-beta 1 with a pattern very different from that of RFCs. Namely, the comparison of RFCs with other cell types such as bovine aortic endothelial cells shows that co-expression of endothelial and smooth muscle cell markers is very unique to RFCs and occurs only in particular culture conditions. This could be related to the capacity of these microvascular endothelial cells to modulate their phenotype in physiologic and pathologic conditions, particularly during angiogenesis, and could reflect different embryologic origins for endothelial cell populations.

Balloon catheter injury to rabbit carotid artery. II. Selective increase in reactivity to some vasoconstrictor drugs

The present study examined the changes in reactivity to a variety of vasoconstrictor drugs of the rabbit carotid artery during development of an intimal thickening induced by injury with an inflated balloon catheter. The injured and the unoperated contralateral carotid arteries were studied at 2 and 6 weeks after the operation. To differentiate areas of the injured artery lined by modified smooth muscle cells from areas lined by regenerated endothelial cells, each rabbit was injected with Evans blue dye before sacrifice. Ring segments (3 mm length) from the control and injured arteries were mounted in organ baths to record the circumferential isometric force with a technique that ensured that all rings were set to equivalent initial resting conditions of passive transmural stretch. Compared with the controls, the experimental arteries had a significantly decreased maximum contraction (Emax) in response to KCl at both 2 and 6 weeks. The experimental arteries were also significantly less sensitive to the alpha 1-adrenoceptor agonist, methoxamine, at both 2 weeks (approximately sevenfold) and 6 weeks (fourfold), with a marked decrease in Emax at 2 weeks, which returned to control values at 6 weeks. There was no change in Emax to either serotonin or the thromboxane A2-mimetic, U46619, in the experimental arteries at either time. There was, however, a small but significant increase in the sensitivity to both drugs. There was no difference in response to any of the constrictor agents between the white and blue regions of the experimental vessels.(ABSTRACT TRUNCATED AT 250 WORDS)

Vascular myosin expression during cytokinesis, attachment, and hypertrophy

Primary cultures of canine saphenous vein smooth muscle cells downregulate the expression of the two muscle myosin heavy chains (mMHC) and upregulate the expression of a nonmuscle myosin heavy chain (nmMHC) when maintained in medium containing 10% fetal calf serum (FCS). The cellular function and control of these changes in contractile protein expression are not known. The purposes of these experiments were to determine whether the expression of nmMHC was required for cytokinesis, whether cell attachment stimulated nmMHC expression, and whether during changes in total myosin production the relative amounts of the two mMHCs remained constant. Primary cultures were maintained in FCS, in a serum-free defined medium (SFM), or after 2 days in SFM switched to FCS to induce proliferation and changes in myosin expression. nmMHC expression occurred before cytokinesis if cells were placed directly in FCS, whereas it occurred after cytokinesis if growth-arrested cells were exposed to FCS. The position of the cell in the cell cycle was responsible for these differences, and the temporal correlation of cell-cycle progression and nmMHC expression indicated that expression occurred during G1. Cells that remained unattached in the presence of FCS or attached in SFM did not express nmMHC. During net production or loss of mMHC by growth in SFM or FCS, respectively, the relative amounts of the two mMHC remained constant. These results suggest that 1) the expression of nmMHC requires the combined effect of FCS and attachment, 2) it occurs in cells progressing through G1 but is not required for cytokinesis, and 3) during changes in net myosin production, the two mMHC are coregulated.

High frequency of cytokeratin-producing smooth muscle cells in human atherosclerotic plaques

Using immunofluorescence microscopy we show that cells expressing cytokeratins 8 and 18 are frequently enriched in human vascular wall tissue pathologically altered by the appearance of intimal thickenings and atherosclerotic plaques. These cytokeratins occur in cells which also synthesize IFs containing vimentin and/or desmin, and a considerable proportion of the cytokeratin-positive cells has been identified as smooth muscle cells by colocalization of desmin and/or smooth muscle type alpha-actin. The presence of extremely low concentrations of these cytokeratins in such vascular tissues has been confirmed by gel electrophoresis with immunoblotting as well as by Northern blot hybridization using specific cytokeratin cRNA probes. The results are discussed in relation to the recent demonstration that low-level synthesis of cytokeratins 8 and 18 occurs in other muscular tissues and to the specific proliferative activity of these cells.

Characterization of vascular smooth muscle cell phenotype in long-term culture

Studies of bovine carotid artery smooth muscle cells, during long-term in vitro subcultivation (up to 100 population doublings), have revealed phenotypic heterogeneity among cells, as characterized by differences in proliferative behavior, cell morphology, and contractile-cytoskeletal protein profiles. In vivo, smooth muscle cells were spindle-shaped and expressed desmin and alpha-smooth muscle actin (50% of total actin) as their predominant cytoskeletal and contractile proteins. Within 24 h of culture, vimentin rather than desmin was the predominant intermediate filament protein, with little change in alpha-actin content. Upon initial subcultivation, all cells were flattened and fibroblastic in appearance with a concomitant fivefold reduction in alpha-actin content, whereas the beta and gamma nonmuscle actins predominated. In three out of four cell lines studied, fluctuations in proliferative activity were observed during the life span of the culture. These spontaneous fluctuations in proliferation were accompanied by coordinated changes in morphology and contractile-cytoskeletal protein profiles. During periods of enhanced proliferation a significant proportion of cells reverted to their original spindle-shaped morphology with a simultaneous increase in alpha-actin content (20 to 30% of total actin). These results suggest that in long-term culture smooth muscle cells undergo spontaneous modulations in cell phenotype and may serve as a useful model for studying the regulation of intracellular protein expression.

Alpha-smooth muscle actin is transiently expressed in embryonic rat cardiac and skeletal muscles

Actin isoform expression may change during development, and in certain physiological, experimental and pathological situations. It is accepted that during sarcomeric (skeletal and cardiac) muscle development, the alpha-skeletal and alpha-cardiac isoforms of actin accumulate rapidly at the onset of muscle fibre formation, while there is a rapid fall in the expression of nonmuscle (beta and gamma) actin isoforms. Here we show that, before birth, both skeletal and myocardial cells express significant amounts of alpha-smooth muscle actin mRNA and protein. This expression is transient and disappears over the 1-7 days following birth. Our findings show that the program regulating actin isoform expression in sarcomeric muscle development is complex and that alpha-smooth muscle actin participates in this process.

The development expression of the rat alpha-vascular and gamma-enteric smooth muscle isoactins: isolation and characterization of a rat gamma-enteric actin cDNA

We have isolated and characterized two cDNA clones from whole rat stomach, pRV alpha A-19 and pRE gamma A-11, which are specific for the alpha-vascular and gamma-enteric smooth muscle isoactins, respectively. The rat gamma-enteric smooth muscle actin contains a single amino acid substitution of a proline for a glutamine at position 359 of the mature peptide when compared with the chicken gizzard gamma-actin sequence (J. Vandekerckhove and K. Weber, FEBS Lett. 102:219, 1979). Sequence comparisons of the 5' and 3' untranslated (UT) regions of the two smooth muscle actin cDNAs demonstrate that these regions contain no apparent sequence similarities. Additional comparisons of the 5' UT regions of the two smooth muscle actin cDNAs to all other known actin sequences reveal no apparent sequence similarities for the rat gamma-enteric isoactin within the 15 base pairs of sequence currently available, while the rat alpha-vascular isoactin contains two separate sequences which are similar to sequences within the 5' UT regions of the human and chicken alpha-vascular actin genes. A similar comparison of the 3' UT regions of the two smooth muscle actins demonstrates that the alpha-vascular isoactins do not contain the high degree of cross-species sequence conservation observed for the other isoactins and that the gamma-enteric isoactin contains an inverted sequence of 52 nucleotides which is similar to a sequence found within the 3' UT regions of the human, chicken, and rat beta-cytoplasmic isoactins. These observations complicate the apparent cross-species conservation of isotype specificity of these domains previously observed for the other actin isoforms. Northern blot analysis of day 15 rat embryos and newborn, day 19 postbirth, and adult rats demonstrates that the day 15 rat embryo displays low to undetectable levels of smooth muscle isoactin mRNA expression. By birth, the stomach and small intestine show dramatic increases in alpha-vascular and gamma-enteric actin expression. These initially high levels of expression decrease through day 19 to adulthood. In the adult rat, the uterus and aorta differ in their content of smooth muscle isoactin mRNA. These results demonstrate that the gamma-enteric and alpha-vascular isoactin mRNAs are coexpressed to various degrees in tissues which contain smooth muscle.

Modulation of human aorta smooth muscle cell phenotype: a study of muscle-specific variants of vinculin, caldesmon, and actin expression

Vinculin- and caldesmon-immunoreactive forms and actin isoform patterns were studied in samples of normal and atherosclerotic human aorta. After removal of adventitia and endothelium, the remaining tissue was divided into three layers: media, muscular-elastic (adjacent to media) intima, and subendothelial (juxtaluminal) intima. In media of normal aorta, meta-vinculin accounted for 41.0 +/- 0.9% (mean +/- SEM) of total immunoreactive vinculin (meta-vinculin + vinculin); 150-kDa caldesmon accounted for 78.2 +/- 5.1% of immunoreactive caldesmon (150-kDa + 70-kDa); the fractional contents of alpha-smooth muscle actin, beta-nonmuscle, and gamma-isoactins were 49.0 +/- 0.6%, 30.4 +/- 0.6%, and 20.8 +/- 0.8%, respectively. Muscular-elastic intima was very similar to media by these criteria. In subendothelial intima, the fractional content of meta-vinculin and 150-kDa caldesmon was significantly lower (6.9 +/- 1.5% and 32.7 +/- 7.0%, respectively) than in muscular-elastic intima and media, whereas the isoactin pattern was identical to that in adjacent layers, demonstrating the smooth muscle origin of subendothelial intima cells. In atherosclerotic fibrous plaque, the fractional content of alpha-actin was decreased in subendothelial intima, rather than in media and muscular-elastic intima. Additionally, the proportion of subendothelial intima cells [i.e., the cells that express low amounts of smooth muscle phenotype markers (meta-vinculin, 150-kDa caldesmon, and alpha-actin)] in the total intima cell population increased dramatically in atherosclerotic fibrous plaque. The results suggest that changes in the relative content of meta-vinculin and 150-kDa caldesmon as well as alpha-actin in human aortic intima are associated with atherosclerosis although, in subendothelial intima of normal aorta, a certain smooth muscle cell population exists that expresses reduced amounts of "contractile" phenotype markers, even in the absence of the disease.

Arterial smooth muscle cells in vivo: relationship between actin isoform expression and mitogenesis and their modulation by heparin

Quiescent smooth muscle cells (SMC) in normal artery express a pattern of actin isoforms with alpha-smooth muscle (alpha SM) predominance that switches to beta predominance when the cells are proliferating. We have examined the relationship between the change in actin isoforms and entry of SMC into the growth cycle in an in vivo model of SMC proliferation (balloon injured rat carotid artery). alpha SM actin mRNA declined and cytoplasmic (beta + gamma) actin mRNAs increased in early G0/G1 (between 1 and 8 h after injury). In vivo synthesis and in vitro translation experiments demonstrated that functional alpha SM mRNA is decreased 24 h after injury and is proportional to the amount of mRNA present. At 36 h after injury, SMC prepared by enzymatic digestion were sorted into G0/G1 and S/G2 populations; only the SMC committed to proliferate (S/G2 fraction) showed a relative slight decrease in alpha SM actin and, more importantly, a large decrease in alpha SM actin mRNA. A switch from alpha SM predominance to beta predominance was present in the whole SMC population 5 d after injury. To determine if the change in actin isoforms was associated with proliferation, we inhibited SMC proliferation by approximately 80% with heparin, which has previously been shown to block SMC in late G0/G1 and to reduce the growth fraction. The switch in actin mRNAs and synthesis at 24 h was not prevented; however, alpha SM mRNA and protein were reinduced at 5 d in the heparin-treated animals compared to saline-treated controls. These results suggest that in vivo the synthesis of actin isoforms in arterial SMC depends on the mRNA levels and changes after injury in early G0/G1 whether or not the cells subsequently proliferate. The early changes in actin isoforms are not prevented by heparin, but they are eventually reversed if the SMC are kept in the resting state by the heparin treatment.

Expression of smooth muscle myosin in relation to growth kinetics of cultured aortic smooth muscle cells

The goal of this study is to quantify smooth muscle myosin (SMM) expression at the level of the individual cell and to ascertain whether SMM expression in cultured aortic smooth muscle cells is related to definite growth phases, and whether the initial seeding density affects growth or SMM staining. Rabbit aortic smooth muscle cells (SMCs) were harvested by enzyme digestion of aortic tissue and plated at low (100 cells cm-2), medium (1000 cells cm-2), and high (10,000 cells cm-2) densities. Independent of seeding density, the lag phase lasted 2 to 3 days and, at all three densities, the growth rate during the logarithmic growth phase was almost the same. However, the time, the number of population doubling needed to reach the plateau phase and the cell number in the plateau were influenced by the initial seeding density. Immunofluorescence staining with anti-smooth muscle myosin (ASMM) revealed intensive staining of striated and filamentous patterns in all cells during the lag and early logarithmic growth phases. During the late logarithmic growth phase, two subpopulations of cells appeared, one showing a positive and the other no reaction with SMM antiserum. The lowest relative number of cells which showed positive reactions with SMM antiserum was observed toward the end of the logarithmic growth phase. During the plateau phase, the SMM-positive subpopulation increased, amounting to about 60% of the total number of cells, independent of the seeding density. In terms of absolute numbers, the number of SMM-positive cells increased over the course of 21 days by factors of 13, 72, and 342 for high, medium, and low seeded cultures, respectively. We conclude that a SMC subpopulation can divide without loss of SMM and that some, but not all, cells which lose their SMM may possibly regain it in the postconfluent state.

Cultured primate aortic smooth muscle cells express both the PDGF-A and PDGF-B genes but do not secrete mitogenic activity or dimeric platelet-derived growth factor protein

Proliferation of smooth muscle cells (SMC) in the arterial intima of man and experimental animals is important in the pathogenesis of atherosclerosis. Vascular SMC proliferation in vitro is stimulated by a number of agents, including the potent protein mitogen, platelet-derived growth factor (PDGF). Recent studies on rat arterial SMC indicate that these cells may, under certain circumstances, synthesize PDGF protein mitogens, suggesting that the regulation of SMC proliferation in vivo may have an autocrine or paracrine component. In this study we demonstrate that cultured nonhuman primate (baboon) aortic SMC transcribe both the PDGF-A and PDGF-B genes but do not secrete detectable mitogenic activity characteristic of native PDGF. The absence of this activity was not due to the presence in the cell conditioned medium of factors inhibitory for PDGF-mediated mitogenic activity. Metabolic labeling of the cells and immunoprecipitation with specific antibodies to human PDGF did not detect a dimeric (30 kDa) PDGF protein in either the intracellular or extracellular compartments, but instead identified PDGF-related proteins of molecular weight 12 kDa and 100 kDa. These data suggest the presence in vascular SMC of a mechanism regulating the translation of PDGF mRNA that may play an important role in the control of SMC proliferation in vivo.

Localization of T lymphocytes and macrophages in fibrous and complicated human atherosclerotic plaques

The cellular composition of aortic atherosclerotic plaques was analyzed by immunocytochemistry using cell type-specific monoclonal antibodies. T lymphocytes and monocytes/macrophages were detected both in early, fibrous plaques, and in more advanced, complicated ones. Many smooth muscle cells in these plaques expressed the class II MHC antigen, HLA-DR. Since this antigen is inducible by T cell products, our findings suggest that T cell-smooth muscle interactions occur during atherogenesis.

Cell cycle versus density dependence of smooth muscle alpha actin expression in cultured rat aortic smooth muscle cells

Cultured smooth muscle cells (SMC) undergo induction of smooth muscle (SM) alpha actin at confluency. Since confluent cells exhibit contact inhibition of growth, this finding suggests that induction of SM alpha actin may be associated with cell cycle withdrawal. This issue was further examined in the present study using fluorescence-activated cell sorting of SMC undergoing induction at confluency and by examination of the effects of FBS and platelet-derived growth factor (PDGF) on SM alpha actin expression in postconfluent SMC cultures that had already undergone induction. Cell sorting was based on DNA content or differential incorporation of bromodeoxyuridine (Budr). The fractional synthesis of SM alpha actin in confluent cells was increased two- to threefold compared with subconfluent log phase cells, but no differences were observed between confluent cycling (Budr+) and noncycling (Budr-) cells. In cultures not exposed to Budr, confluent cycling S + G2 cells exhibited similar induction. These data indicate that cell cycle withdrawal is not a prerequisite for the induction of SM alpha actin synthesis in SMC at confluency. Growth stimulation of postconfluent cultures with either FBS or PDGF resulted in marked repression of SM alpha actin synthesis but the level of repression was not directly related to entry into S phase in that PDGF was a more potent repressor of SM alpha actin synthesis than was FBS despite a lesser mitogenic effect. This differential effect of FBS versus PDGF did not appear to be due to transforming growth factor-beta present in FBS since addition of transforming growth factor-beta had no effect on PDGF-induced repression. Likewise, FBS (0.1-10.0%) failed to inhibit PDGF-induced repression. Taken together these data demonstrate that factors other than replicative frequency govern differentiation of cultured SMC and suggest that an important function of potent growth factors such as PDGF may be the repression of muscle-specific characteristics.

Identification of intimal subendothelial cells from human aorta in primary culture

Subendothelial cells (SEC) were obtained from the inner intimal layer of adult human aorta by collagenase treatment. SEC were identified in primary culture either as smooth muscle cells by staining with FITC-labeled antisera against human smooth muscle myosin or as macrophages, foam cells and contaminating endothelial cells by their uptake of malondialdehyde treated low density lipoproteins labeled with fluorescent dye 3,3'-dioctadecylindocarbocyanine. Between 1 and 5 days in culture, along with smooth muscle cells (SMC, 38-82%), endothelial cells (0-9%), macrophages and foam cells (2-32%), one more type of cell was found. This cell type resembled SMC in size and shape, but was not stained by antisera to SMC myosin. By ultrastructural criteria these cells were characterized as modulated SMC for they contained prominent rough endoplastic reticulum and Golgi complex together with basement membrane and a large number of plasmalemmal vesicles. Like SMC they reacted with phalloidin and were stained by anti-vimentin but not by anti-desmin monoclonal antibodies. The proportion of such cells varied from 5 to 33% of total cell number and increased in parallel to macrophages and foam cells in vessels with well developed atherosclerotic lesions. We conclude that the applied technique may be used for identification of cultured vascular cells including modulated SMC.

Arterial smooth muscle cells express platelet-derived growth factor (PDGF) A chain mRNA, secrete a PDGF-like mitogen, and bind exogenous PDGF in a phenotype- and growth state-dependent manner

Adult rat arterial smooth muscle cells are shown to express platelet-derived growth factor (PDGF) A chain mRNA, to secrete a PDGF-like mitogen, and to bind exogenous PDGF in a phenotype- and growth state-dependent manner. In the intact aortic media, where the cells are in a contractile phenotype, only minute amounts of PDGF A chain and no B chain (c-sis) RNA were detected. After cultivation and modulation of the cells into a synthetic phenotype, the A chain gene was distinctly expressed, whereas the B chain gene remained unexpressed. Cells kept in serum-free medium on a substrate of plasma fibronectin showed high levels of A chain RNA and high PDGF receptor activity, but did not secrete detectable amounts of PDGF-like mitogen. After exposure to PDGF, which is itself sufficient to initiate DNA synthesis and mitosis in these cells, a PDGF-like mitogen was released into the extracellular medium. Concomitantly, the amount of A chain transcripts per cell and the ability of the cells to bind radioactive PDGF decreased. Similarly, smooth muscle cells initially grown in the presence of serum released more PDGF-like mitogen, contained fewer A chain transcripts, and bound more radioactive PDGF in proliferating than in stationary cultures. The findings confirm the notion that adult rat arterial smooth muscle cells are able to promote their own growth in an autocrine or paracrine manner. Furthermore, they reveal some basic principles in the control of this process.

Quantitative ultrastructural analysis of coronary atherosclerotic involvement in two macaque species

Ultrastructural analyses were employed to observe and to compare in detail lesions of the coronary artery of cynomolgus and rhesus monkeys. Animals were fed individually with the same atherogenic ration under identical conditions for 4, 8, and 12 months, and controls of each species were fed with a low fat, cholesterol-free ration. Transmission electron microscopic studies of coronary arteries from these animals led to the following conclusions: (1) Synthetic smooth muscle cells (SMC) without lipid and macrophages without lipid appeared more frequently in the cynomolgus lesions than in the rhesus lesions. Furthermore, phenotypic expression of synthetic SMCs in the cynomolgus was more active with greater diversity, while the rhesus showed less phenotypic modulation. Macrophages without lipid appeared frequently in the cynomolgus media. (2) Increased percentages of both synthetic SMCs with lipid and macrophages with lipid were demonstrated in the cynomolgus lesions as compared to those in the rhesus. This indicates that foam cells, including SMC- and macrophage-derived foam cells, are more prevalent in cynomolgus than in rhesus. They are considered to play an important role in atherogenesis. (3) Medial disruption, synthetic SMCs, and macrophages containing lipid appeared more often in cynomolgus media than in rhesus media. (4) There were greater percentages of both synthetic SMCs and macrophages in the intima of the myocardial side of coronary arteries in both species. (5) Approximately 42% of all foam cells in the cynomolgus lesions were derived from SMCs. There were fewer macrophages in rhesus lesions. (6) The difference in expression between the two macaque species reflects different responses of macrophages to medial smooth muscle cell (SMC) components. The configuration of the artery wall could be one of the important indicators of these different expressions.

Correlation between the distribution of smooth muscle or non muscle myosins and alpha-smooth muscle actin in normal and pathological soft tissues

The distribution of smooth muscle (SM) and non muscle myosins was compared with that of alpha-SM actin in various normal and pathological tissues and in cultured cells by means of indirect immunofluorescence using a monoclonal antibody specific for alpha-SM actin [anti-alpha sm-1, Skalli et al., 1986b] and two polyclonal antibodies raised against bovine aortic myosin (ABAM) and human platelet myosin (AHPM), respectively. In normal tissues ABAM stained vascular and parenchymal smooth muscle cells (SMC), myoepithelial cells and myoid cells of the testis in a pattern similar to that reported by other authors with antisera raised against non vascular SM myosin. Cells stained with ABAM were always positive for anti-alpha sm-1. In human and experimental atheromatous plaques, most cells were positive for AHPM; a variable proportion was also stained for ABAM plus anti-alpha sm-1. Myofibroblasts from rat granulation tissue, Dupuytren's nodule and stroma from breast carcinoma were constantly positive for AHPM and negative for ABAM; however, myofibroblasts from Dupuytren's nodule and breast carcinoma were anti-alpha sm-1 positive. Early primary cultures of rat aortic SMC were positive for ABAM and anti-alpha sm-1 and became negative for ABAM and positive for AHPM after a few days in culture. They remained positive for AHPM and anti-alpha sm-1 after passages; the staining of AHPM and anti-alpha sm-1 appeared to be colocalized along the same stress fibers. These results may be relevant for the understanding of SMC function and adaptation, and show that in non malignant SMC proliferation, alpha-SM actin represents a more general marker of SM origin than SM myosin.

Analysis of alpha-smooth-muscle actin mRNA expression in rat aortic smooth-muscle cells using a specific cDNA probe

We constructed two cDNA probes, the first of which hybridizes with all rat actin mRNAs while the second is specific for alpha-smooth muscle (SM) actin mRNA. Northern hybridization using these probes showed that, in normal rat aortic media, the proportion of alpha-SM actin mRNA expression increases during development, reaching about 90% of the total actin mRNA level in adult animals. As compared to the situation in normal aortic media, the proportion of alpha-SM actin mRNA was found to decrease significantly in intimal thickening 15 days after endothelial injury, i.e. when SM cells (SMCs) are actively replicating. At 60 days after injury, the SMCs were observed to have stopped dividing and to have recovered a normal content of alpha-SM actin mRNA. The content of alpha-SM actin mRNA was also selectively decreased (as compared to controls) in the hypotensive abdominal aortic media located below an aortic ligature, while it was not modified in the thoracic hypertensive segment above the same ligature. Primary cultures of rat aortic SMCs synthesize and contain low amounts of alpha-SM actin, but their alpha-SM actin mRNA content is similar to that of SMCs in vivo. As compared to primary cultures, the proportion of alpha-SM actin mRNA was found to be significantly decreased in SMCs at the fifth passage, at which stage it became comparable to the level of synthesized alpha-SM actin. Thus, the synthesis and expression of alpha-SM actin in SMCs appear to be regulated predominantly at the level of gene transcription in certain situations (e.g. aortic ligature in vivo and culture at the fifth passage), and predominantly at a post-transcriptional level in other situations (e.g. primary culture).

Mechanical properties and structure of isolated pulmonary arteries remodeled by chronic hyperoxia

Normobaric hyperoxia is known to cause pulmonary hypertension with major restructuring of the walls of large and small pulmonary arteries. This study reports the effects of 21 days of exposure to 87% oxygen on the resting and active mechanical properties and structure of pulmonary arterial segments. Segments from the hilar region, extrapulmonary and proximal preacinar, and selected distal preacinar regions were studied. Resting and active (KCl-induced) tension:circumference curves were determined for each vessel. Morphometric measures were made of vessels fixed at a standard circumference using computerized planimetry. The areas of the media and adventitia as well as vessel wall thickness were increased in hyperoxic vessels. The walls of segments from the hypertensive rats demonstrated an increased stiffness based upon analysis of vessel resting tension:circumference relationships while the tangent modulus (a measure of stiffness normalized to tissue dimensions) was unchanged. Paradoxically, despite medial hypertrophy in the pulmonary vessels remodeled by hyperoxia, active tension was reduced. This study reveals that the resulting hypertensive state is not readily explained by an inherent increase in the maximal contractile capabilities of the remodeled vessel. Rather, obliteration of vessels in combination with increased resting stiffness appear to be the basis for pulmonary hypertension induced in hyperoxia.

Intermediate filament proteins and actin isoforms as markers for soft tissue tumor differentiation and origin. I. Smooth muscle tumors

A series of 3 benign and 10 malignant smooth muscle (SM) neoplasms and of 2 malignant fibrous histiocytomas was examined by light microscopy, transmission electron microscopy, two-dimensional gel electrophoresis (2D-GE) and indirect immunofluorescence, using polyclonal monospecific or monoclonal antibodies to desmin, vimentin, cytokeratin, alpha-SM and alpha-sarcomeric (alpha-SR) actins. Benign neoplasms displayed typical light-microscopic features of SM, whereas leiomyosarcomas demonstrated variations in their histologic pattern. In 6 sarcomas, light microscopy suggested a SM differentiation, whereas in the other 4, a predominant nondistinctive spindle-cell pattern was observed. By transmission electron microscopy, all 13 neoplasms showed the minimal essential features of SM differentiation. Immunofluorescence disclosed heterogeneity of cytoskeletal protein expression: 5 neoplasms (3 benign and 2 malignant well-differentiated) expressed desmin, vimentin, and alpha-SM-actin; 2 malignant neoplasms expressed desmin and vimentin; 1 malignant neoplasm expressed desmin, vimentin and alpha-SR actin; 1 malignant neoplasm expressed vimentin and alpha-SR actin; and 4 malignant neoplasms expressed vimentin alone. By 2D-GE, 3 benign and 4 malignant SM neoplasms expressed alpha, beta, and gamma actins, and the remaining expressed only beta and gamma actins. The presence of alpha-SM actin in all benign neoplasms and in 2 well-differentiated leiomyosarcomas suggests that this actin isoform reflects a high degree of cellular differentiation. In 2 leiomyosarcomas, alpha-SR actin was detected by immunofluorescence, which suggested a skeletal muscle differentiation of these neoplasms. This study supports the assumption that leiomyosarcomas represent a heterogeneous group of neoplasms and furnishes new criteria for their characterization.

Cloning and characterization of mammalian myosin regulatory light chain (RLC) cDNA: the RLC gene is expressed in smooth, sarcomeric, and nonmuscle tissues

The 20-kD regulatory light chain (RLC) plays a central role in the regulation of smooth muscle contraction. Little is known about the structure or expression of smooth muscle myosin light chain (MLC) genes. A cDNA library was constructed in the expression vector, lambda gt-11, with mRNA derived from cultured rat aortic smooth muscle cells. Using antibody generated against tracheal smooth muscle myosin, three cDNA clones encoding a RLC were isolated, one of which, SmRLC-2, represents a full-length transcript of the RLC mRNA. The derived amino acid sequence shows 94.2% homology with the chicken gizzard RLC, and 70 and 52% homology with the rat skeletal and cardiac muscle MLC-2 proteins, respectively. Thus, the gene encoding the putative smooth muscle RLC appears to have originated by duplication of the same ancestor that gave rise to the sarcomeric MLC-2 genes. Contrary to the stringent tissue-specific expression of sarcomeric MLC-2 genes, RNA blot hybridization and S1 nuclease mapping demonstrates that the putative smooth muscle RLC gene is expressed in smooth, sarcomeric, and nonmuscle tissues at significant levels. Primer extension analysis suggests that the same promoter region is used in these different tissues. Thus the putative smooth muscle RLC gene appears to be a gene that is constitutively expressed in a large variety of cells and has a differentiated function in smooth muscle.