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

Plasma fibronectin promotes modulation of arterial smooth-muscle cells from contractile to synthetic phenotype

Isolated arterial smooth-muscle cells (SMCs) cultured in medium containing whole blood serum or plasma-derived serum undergo modulation from a contractile to a synthetic phenotype. This process includes the loss of myofilaments and cessation of the ability to contract. Instead, an extensive rough endoplasmic reticulum and a large Golgi complex are formed and, if properly stimulated, the cells start to proliferate actively and to produce extracellular-matrix components. In vivo, a similar change in the differentiated properties of SMCs appears to be an early key event in atherogenesis. The purpose of the present investigation was to try to identify plasma components that promote the modulation of the smooth-muscle phenotype. SMCs were enzymatically isolated from rat aorta and cultured in a defined, serum-free medium. The phenotypic state of the cells was determined by transmission electron microscopy, and their growth status was followed by 3H-thymidine autoradiography and cell counting. Under these conditions, Cohn fractions I (fibrinogen) and V (albumin) were found to partially support cell attachment and transition from the contractile to the synthetic phenotype, whereas fractions II-III and IV (globulins) were inactive in this respect. Analysis on adsorptive columns of gelatin Sepharose 4B indicated that Cohn fraction I, but not fraction V, contained fibronectin, an adhesive protein that is present in plasma and binds to fibrinogen. When seeded on a substrate of plasma fibronectin, the cells attached with high efficiency and modulated into the synthetic phenotype at a rate similar to that observed in serum-containing medium. In the absence of exogenous mitogens, the structural transformation of the cells was not accompanied by a proliferative response.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytocontractile structures and proteins of smooth muscle cells during the formation of experimental lesions

The time course of structural changes in vascular smooth muscle cells (SMC) was investigated during the formation of an experimental lesion in response to balloon injury. We compared the filamentous organization, evaluated by quantitative electron microscopy, with the cellular content of two representative cytocontractile proteins (myosin and tropomyosin) as assessed by immunofluorescence. We found that the changes peak between 7 and 14 days after injury and that they are visible both in the neointima and to a lesser extent in the inner media. While virtually all SMC are of a filament-rich phenotype in the undisturbed media, after balloon injury SMC migrated into the intima and about 90% of these latter cells were either of a organelle-rich or an intermediate phenotype, with the remaining 10% being of the filament-rich phenotype. In the inner media about 40% of cells were either of organelle-rich or intermediate phenotype. In contrast to these profound organizational changes of responding SMC, histochemistry revealed only a slight and probably transient decrease of the cellular content of myosin and tropomyosin at that time point. Twenty-eight days after injury the discrepancies between the content and the organization of cytocontractile proteins became more apparent. While virtually all SMC showed a homogeneous intensive staining with both antibodies, indistinguishable from the media SMC, the organization of cytoplasmic filaments had not totally recovered. Even though this morphological study does not permit conclusions to be drawn on the contractile function of the cells, it shows that both the organization and the content of cytocontractile protein have to be analyzed and compared for SMC changes to be evaluated during the formation of an experimental lesion.

Actin-isoform pattern as a marker of normal or pathological smooth-muscle and fibroblastic tissues

The relative proportions of actin isoforms present in smooth-muscle (SM) and fibroblastic human and non-human tissue extracts were examined by densitometric evaluation of Coomassie-Blue-stained spots in two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) as well as by quantification of radiolabeled actin NH2-terminal peptide spots separated by two-dimensional paper electrophoresis. SM tissues contained alpha- and gamma-SM as well as beta- and gamma-cytoplasmic (CY) actins in different proportions in different organs. Species differences with respect to the ratios of the isoactins were also observed. Moreover, during pregnancy, both human and rat myometrium exhibited a changed actin-isoform pattern, there being an increased proportion of gamma-actin. Analysis of the NH2-terminal peptides showed that, in human myometrium, this was essentially due to an increase in the amount of the gamma-SM isoform. Fibroblastic tissues were found to contain only the beta- and gamma-CY isoforms, the ratio being approximately 2.6:1. Thus, the presence or absence of alpha-actin provides a reliable biochemical criterion for distinguishing between fibroblastic and SM cell populations and/or tissues. This distinction and the evaluation of changes in isoactin ratios may be useful in the study of differentiation as well as physiological and pathological phenomena, and for determining the origin of certain soft-tissue tumours.

Intermediate filament expression in human vascular smooth muscle and in arteriosclerotic plaques

Different regions of human aorta and of other human arteries obtained at autopsy were analyzed with regard to their topography and to the different stages of arteriosclerosis. Material was studied by immunocytochemical techniques with antibodies specific for either desmin (D) or for vimentin (V), the two types of intermediate filament proteins present in vascular smooth muscle cells. In normal arteries endothelial cells as well as the adjacent intimal cells were D-V+. In the media D+V+ as well as D-V+ cells were present, with the relative numbers of each cell type dependent on the particular blood vessel. When cells in arteriosclerotic plaques at different stages of development were examined an occasional plaque showed cells of the D+V+ type. In the majority of plaques however the cells were V-D+. In plaques where severe ulceration and necrotic material was present D-V+ cells were found at the border of the lesion: foam cells when they could be identified appeared to be D-V+.

Identification of smooth muscle-derived foam cells in the atherosclerotic plaque of human aorta with monoclonal antibody IIG10

We have developed a monoclonal antibody that specifically interacts with a surface antigen of human fibroblasts and smooth muscle cells. The antibody (antibody IIG10) recognizes a polypeptide of molecular mass 330,000, revealed by immunoblotting in fibroblast and smooth muscle cell extract, but not in vascular endothelial cells, peritoneal macrophages, peripheral blood lymphocytes nor hepatocytes. In tissue sections the antibody stained smooth muscle cells of myometrium, aorta and smaller blood vessels, and fibroblasts of connective tissue. Specificity of the antibody was further confirmed by double staining of aorta sections. Antibody IIG10 was used to identify smooth muscle-derived foam cells in the atherosclerotic plaque of human aorta.

A monoclonal antibody against alpha-smooth muscle actin: a new probe for smooth muscle differentiation

A monoclonal antibody (anti-alpha sm-1) recognizing exclusively alpha-smooth muscle actin was selected and characterized after immunization of BALB/c mice with the NH2-terminal synthetic decapeptide of alpha-smooth muscle actin coupled to keyhole limpet hemocyanin. Anti-alpha sm-1 helped in distinguishing smooth muscle cells from fibroblasts in mixed cultures such as rat dermal fibroblasts and chicken embryo fibroblasts. In the aortic media, it recognized a hitherto unknown population of cells negative for alpha-smooth muscle actin and for desmin. In 5-d-old rats, this population is about half of the medial cells and becomes only 8 +/- 5% in 6-wk-old animals. In cultures of rat aortic media SMCs, there is a progressive increase of this cell population together with a progressive decrease in the number of alpha-smooth muscle actin-containing stress fibers per cell. Double immunofluorescent studies carried out with anti-alpha sm-1 and anti-desmin antibodies in several organs revealed a heterogeneity of stromal cells. Desmin-negative, alpha-smooth muscle actin-positive cells were found in the rat intestinal muscularis mucosae and in the dermis around hair follicles. Moreover, desmin-positive, alpha-smooth muscle actin-negative cells were identified in the intestinal submucosa, rat testis interstitium, and uterine stroma. alpha-Smooth muscle actin was also found in myoepithelial cells of mammary and salivary glands, which are known to express cytokeratins. Finally, alpha-smooth muscle actin is present in stromal cells of mammary carcinomas, previously considered fibroblastic in nature. Thus, anti-alpha sm-1 antibody appears to be a powerful probe in the study of smooth muscle differentiation in normal and pathological conditions.

Rat hair follicle dermal papillae have an extracellular matrix containing basement membrane components

Dermal papillae are small mesenchymally derived zones at the bases of hair follicles which have an important role in hair morphogenesis in the embryo and control of the hair growth cycle in postnatal mammals. The cells of the papilla are enmeshed in a dense extracellular matrix which undergoes extensive changes in concert with the hair cycle. Here it is shown that this matrix in anagen pelage follicles of postnatal rats contains an abundance of basement membrane components rather than dermal components such as interstitial collagens. In particular, type IV collagen, laminin, and basement membrane types of chondroitin sulfate and heparan sulfate proteoglycans are present as matrix components. Dermal papilla cells, when initially isolated from adult rat vibrissae and cultured in vitro, retained the potential to synthesize this spectrum of matrix components, but this was gradually lost, to be replaced by synthesis of other components including type I and III collagens. It seems likely therefore that the dermal papilla cells in vivo synthesize a basement membrane type of extracellular matrix, although a contribution from epithelial, and in some cases capillary endothelial, cells cannot be ruled out.

Isoforms of myosin and actin in human, monkey and rat myometrium. Comparison of pregnant and non-pregnant uterus proteins

Using several electrophoretic procedures, we have compared the forms of myosin and actin in pregnant and non-pregnant uterus of woman, monkey (Macaca fascicularis) and rat. On non-dissociating gels, native myosin of the three species migrates as a single band, of identical mobility independently of the physiological state. Remigration of this band in dissociating conditions shows that it is constituted of two heavy chains of respectively 201 kDa and 205 kDa; the relative proportions of these two bands are different for the three animal species but do not vary during pregnancy. Using two-dimensional gel electrophoresis, we found that the 17-kDa light chain of purified uterus myosin exists under two isoelectric forms, the more acidic one becoming progressively predominant at the end of pregnancy in the human as in the monkey uterus, while we observed no changes in the rat. In two-dimensional gel electrophoresis, actin of human, monkey and rat uterus is present under three isoforms, the most basic one (the gamma form) increasing early in pregnancy in the two primate species but being always the most abundant form in the rat. The ATPase activity of human uterus myosin was found to be similar for the protein extracted from both pregnant and non-pregnant uterus. The changes observed in the 17-kDa light chain and in the actin isoforms might nevertheless participate in the modifications of contractility of the uterus during pregnancy of the primates.

Expression of actin mRNAs in rat aortic smooth muscle cells during development, experimental intimal thickening, and culture

The expression of actin-isoform mRNAs in the smooth muscle cells (SMC) of the aortic media in rats has been studied by Northern-blot hybridization, using a general actin-cRNA probe, and two cRNA probes specific for beta- and gamma-cytoplasmic actins, during: (1) development, (2) intimal thickening after endothelial injury induced by balloon catheterization, and (3) growth in culture. In 5-day-old rats, the ratio between alpha-smooth-muscle-actin mRNA and beta- and gamma-cytoplasmic-actin mRNAs was close to 1. It increased to about 4 in 6-week-old rats. Replicating SMC from regions of intimal thickening 15 days after endothelial injury, and SMC growing in culture contained a predominance of cytoplasmic actin mRNAs. Intimal SMC 60 days after endothelial injury (at which time the endothelium had fully regenerated) demonstrated a pattern of actin mRNAs similar to that of normal media. Functional mRNA measured by translation in a reticulocyte lysate showed increases in the level of alpha-actin and decreases in beta-actin in rats from 5 days to 6 weeks of age. These results suggest that during development, under pathological conditions, and in cell culture, the expression of actin isoforms in arterial SMC depends on many factors, including the amount and translation efficiency of mRNAs, and the relative stabilities of the proteins involved.

Rat cerebral microvascular smooth muscle cells in culture

This report describes the development and establishment of long-term serial cultures of adult rat vascular smooth muscle cells (SMC) derived from cerebrocortical resistance vessels (small arteries and arterioles). Electron microscopic examination of microvessels isolated off a 150 microns nylon mesh sieve clearly demonstrated the predominance of these vessel types. Initial outgrowth from collagenase-elastase-treated microvessel fragments yielded both endothelium and smooth muscle cells. However, at confluency (2-3 weeks) these cultures consisted of a homogeneous population of broad, polygonal cells that grew in a multilayered "hill and valley" pattern typical of SMC in vitro. For comparative morphological and functional studies, SMC cultures were also initiated from rat thoracic aortas utilizing ring segments as explants. The smooth muscle origin of cultures derived from both resistance vessel (RV) and aorta (RA) was further demonstrated by positive immunofluorescent staining by the specific smooth muscle alpha-actin and myosin antibodies. Ultrastructural examination of these SMC cultures revealed similar morphologic features consisting of typical cytoplasmic myofilament bundles with associated dense bodies and numerous pinocytotic vesicles. Cell growth studies on early (less than P 15)- and late (greater than P 15)-passage RV- and RA-SMC populations revealed markedly different cell growth responses. Representative growth curves of early- and late-passage RA-SMC showed a significantly higher growth rate (two- to fourfold) than RV-SMC cultures. Both cultures, however, exhibited a marked increase in growth potential at higher passage levels. Heparin, at a concentration of 100 micrograms/ml inhibited the growth of RV-SMC during the first 3 days after addition in both exponential and growth-arrested culture states, whereas RA-SMC cultures showed no inhibitory response. These studies indicate that long-term RV-SMC cultures can serve as a useful model system to study functional and metabolic properties of this cell type and provide the means to explore further the heterogeneity of SMC derived from different vasculatures in normal as well as various disease states.

Cytoskeletal features of normal and atheromatous human arterial smooth muscle cells

The distribution of actin, vimentin, desmin, and tropomyosin was studied in the media of the human aorta and femoral and coronary arteries, as well as in atheromatous plaques from the same arteries, by means of immunofluorescence, densitometric analysis of sodium dodecylsulfate-polyacrylamide gel electrophoresis, and bidimensional gel electrophoresis. The proportions of desmin-containing cells varied in the media of different arteries; 4 per cent of the cells in the aorta, 11 per cent in the coronary artery, and 37 per cent in the femoral artery contained desmin. In fibrous atheromatous plaques, independently of the artery, desmin-containing cells were almost absent, but they reappeared in complicated lesions. The content of vimentin per smooth muscle cell increased in fibrous atheromatous plaques, whereas the content of actin and tropomyosin was less than in normal media. Moreover, the alpha-actin predominance observed in the media was transformed to beta-actin predominance in the atheromatous plaques. These cytoskeletal changes provide new, possibly useful, biochemical markers for the characterization of smooth muscle cells during early and advanced phases of atheroma formation.

Replication of smooth muscle cells in vascular disease

Smooth muscle proliferation has been recognized as central to the pathology of both major forms of vascular disease: atherosclerosis and hypertension. Recent advances in our knowledge of mechanisms of control of proliferation suggest that events occurring in adult animals may recapitulate portions of the developmental biology of the smooth muscle cell. This review attempts to consider the current state of knowledge of the mechanisms controlling smooth muscle proliferation in these two diseases, to put that knowledge into the context of what is known about smooth muscle biology, and to offer two hypotheses on the possible roles of smooth muscle developmental biology in manifestations of atherosclerosis and hypertension in adult humans.

Expression of smooth muscle-specific alpha-isoactin in cultured vascular smooth muscle cells: relationship between growth and cytodifferentiation

The relationship between growth and cytodifferentiation was studied in cultured rat aortic smooth muscle cells (SMCs) using expression of the smooth muscle (SM)-specific isoactins (Vanderkerckhove, J., and K. Weber, 1979, Differentiation, 14:123-133) as a marker for differentiation in these cells. Isoactin expression was evaluated by: (a) measurements of fractional isoactin content and synthesis ([35S]methionine incorporation) by densitometric evaluation of two-dimensional isoelectric focusing sodium dodecyl sulfate gels, and (b) immunocytological examination using SM-specific isoactin antibodies. Results showed the following: (a) Loss of alpha-SM isoactin was not a prerequisite for initiation of cellular proliferation in primary cultures of rat aortic SMCs. (b) alpha-SM isoactin synthesis and content were low in subconfluent log phase growth cells but increased nearly threefold in density-arrested postconfluent cells. Conversely, beta-nonmuscle actin synthesis and content were higher in rapidly dividing subconfluent cultures than in quiescent postconfluent cultures. These changes were observed in primary and subpassaged cultures. (c) alpha-SM actin synthesis was increased by growth arrest of sparse cultures in serum-free medium (SFM; Libby, P., and K. V. O'Brien, 1983, J. Cell. Physiol., 115:217-223) but reached levels equivalent to density-arrested cells only after extended periods in SFM (i.e., greater than 5 d). (d) SFM did not further augment alpha-SM actin synthesis in postconfluent SMC cultures. (e) Serum stimulation of cells that had been growth-arrested in SFM resulted in a dramatic decrease in alpha-SM actin synthesis that preceded the onset of cellular proliferation. These findings demonstrate that cultured vascular SMCs undergo differential expression of isoactins in relation to their growth state and indicate that growth arrest promotes cytodifferentiation in these cells.

Remodeling of the aortic smooth muscle cell cytoskeleton during developmental and pathological conditions

The remodeling of aortic smooth muscle cell cytoskeleton has been investigated qualitatively and quantitatively during rat aorta development and experimental or human atheromatosis, using immunofluorescent and biochemical techniques. The cytoskeleton of smooth muscle cells in the intimal thickening 15 days after endothelial removal and in human atheromatous plaque is very similar to that of poorly differentiated aortic smooth muscle cells of foetal and newborn rats. Our studies suggest that cytoskeletal changes (a switch in the synthesis of actin isoforms in particular) are reliable markers of proliferative aortic smooth muscle cells, and of atheromatous smooth muscle cells.

Cytoskeletal features of rat aortic cells during development. An electron microscopic, immunohistochemical, and biochemical study

Actin, vimentin, desmin, and tropomyosin distribution in rat aortic endothelial and smooth muscle cells has been studied during development using fetal (18 to 20 days of gestation), and 5- and 14-day-, and 5-, and 12-week-old rats. Endothelial cells of newborn animals actively replicate and contain many actin stress fibers, whereas, in adult animals, replication is minimal and actin stress fibers are rare. The actin, vimentin, desmin, and tropomyosin content of smooth muscle cells increases gradually from fetal to adult animals. The number of desmin-containing cells also increases from 13% in fetal rats to 51% in adult rats. The beta-actin isoform is predominant in fetal and newborn animals, but gradually the alpha-isoform becomes quantitatively the most important, as seen by bidimensional polyacrylamide gels. Several analogies exist between the features of developing smooth muscle and what is known for developing striated muscle cells. The evolution of cytoskeletal features from fetal to adult animals is remarkably the opposite of what takes place in: (1) rat aortic smooth muscle cells proliferating after an endothelial injury, (2) human arterial smooth muscle cells present in atheromas, and (3) actively growing rat aortic smooth muscle cells in vitro. Thus, the assumption that pathological or cultured smooth muscle cells are "dedifferentiated" is supported by our biochemical observations.

Mechanisms of arterial graft failure. 1. Role of cellular proliferation in early healing of PTFE prostheses

Failure of long-term synthetic arterial bypass grafts has been attributed in part to anastomotic stenosis, but the pathologic basis for this has not been determined. Which cells participate in the formation of the stenosis and the relationship between normal healing and the pathologic development of anastomotic narrowing have not been delineated. In this study we have examined early wound healing in 4-mm polytetrafluorethylene arterial bypass grafts placed in baboons. In this primate model, endothelium and smooth muscle cells (SMCs) derived from the cut ends of adjacent artery form the new intima and migrate together along the luminal surface of the graft at approximately 0.2 mm/day. Both cell types proliferate in association with the growing edge. In addition, both endothelium and SMCs located discretely over anastomoses continue to proliferate despite complete endothelial coverage. Intimal cross-sectional area in this region is always greater than over adjacent graft. Fibroblasts are invariably found in graft matrix and adventitia and do not contribute to formation of intima. It is hypothesized that anastomotic narrowing might be due to chronic endothelial injury and turnover associated with continued SMC proliferation and intimal thickening.

Significance of quiescent smooth muscle migration in the injured rat carotid artery

Cellular accumulation in the intima of injured artery has generally been attributed to smooth muscle cell proliferation. Since smooth muscle cells in normal artery are found mainly in the media, migration of smooth muscle cells into the intima has been considered a necessary prerequisite for subsequent myointimal thickening. The nondividing medial cells would appear to have no role in the reparative process. We have investigated in the rat ballooned carotid the possibility that nondividing cells might also contribute to injury-induced intimal thickening. All proliferating smooth muscle cells were labeled by 3H-thymidine given by continuous intraperitoneal infusion. The amounts of 3H-thymidine used were not toxic and did not inhibit smooth muscle cell proliferation. Autoradiograms performed on histological cross-sections showed a progressive decrease in the fraction of unlabeled cells at 3, 7, and 14 days after carotid injury. However, the actual number of nondividing cells remained constant. The calculated growth fraction for the 14-day period was 40%. A substantial number of unlabeled cells was observed in the intima. These data have led us to conclude that only a small fraction of smooth muscle cells in an artery proliferate in response to the injury stimulus, and do so shortly after injury, or not at all. Furthermore, nondividing, as well as proliferating smooth muscle cells, can migrate and contribute, in a substantial way, to the increase in intimal smooth muscle cell number.