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

Different effects of estrogen and progesterone on experimental atherosclerosis in female versus male rabbits. Quantification of cellular proliferation by bromodeoxyuridine

BACKGROUND

The aim of the present study was to compare the effect of estrogen and progesterone on the development of experimental atherosclerosis in female versus male rabbits to assess possible sex-specific differences.

METHODS AND RESULTS

A total of 32 female and 32 male New Zealand White rabbits were ovariectomized or castrated. In addition to a 0.5% cholesterol diet, the rabbits received estradiol alone (1 mg/kg body wt [BW] per week), progesterone alone (25 mg/kg BW per week), or combined estradiol-progesterone in these dosages during 12 weeks. Ovariectomized female and castrated male rabbits served as control groups without hormone treatment. Before excision of the vessels, bromodeoxyuridine labeling was performed to determine the extent of cellular proliferation in the atherosclerotic lesions. The aortic arch was analyzed immunohistologically and morphometrically. An inhibitory effect of estrogen on intimal plaque size was found in female rabbits compared with the ovariectomized control group (0.7 +/- 0.5 versus 3.7 +/- 2.5 mm2, P < .002; proliferating cells, 3.1 +/- 1.8% versus 8.5 +/- 2.6%, P < .002). In combination with progesterone, however, estrogen was not able to reduce intimal plaque size or cellular proliferation. In contrast, estradiol in castrated male rabbits was not associated with an inhibitory effect on cellular proliferation or intimal thickening compared with controls (estrogen treatment, 7.6 +/- 2.1% proliferating cells and 2.8 +/- 1.0 mm2 neointima; control group, 7.2 +/- 2.1% cellular proliferation and 2.9 +/- 1.2 mm2 intimal thickening).

CONCLUSIONS

Our data suggest that the atheroprotective effect of estrogen is probably due to a mechanism that is present in female rabbits only.

Smooth muscle cells isolated from the neointima after vascular injury exhibit altered responses to platelet-derived growth factor and other stimuli

A variety of evidence suggests that vascular smooth muscle cells (SMC) exhibit a more immature phenotype when stimulated by injury to replicate in the adult. One growth characteristic common to immature (embryonic, fetal, and neonatal) SMC is a markedly reduced responsiveness to platelet-derived growth factor (PDGF) and other mitogenic stimuli. Here we demonstrate that SMC isolated from the 14-day neointima of experimentally injured carotid arteries exhibit a similar growth phenotype. The proliferative responses of neointimal cells to the BB homodimer of PDGF, which interacts with both forms of the PDGF receptor, were up to twenty-fold less (as assessed by BrdU immunocytochemistry) than that of adult control tunica media cells over a wide range of PDGF concentrations. Paradoxically, these cells expressed abundant mRNA for the alpha- and beta-subunits of the PDGF receptor (by RT-PCR) and expressed abundant PDGF receptor protein (by Western blotting). Addition of PDGF-BB to neointimal SMC induced significant autophosphorylation of the PDGF receptor, suggesting that the PDGF receptors were fully functional. The chemotactic responses of neointimal SMC to PDGF, in in vitro migration assays, were identical to that of control medial cells. The data further establish the existence of vascular SMC phenotypes characterized by a refractoriness to growth stimulation by specific mitogens, and provide further evidence for the reiteration of developmentally regulated programs following vascular injury in vivo.

Inhibition of the protective effect of estrogen by progesterone in experimental atherosclerosis

The aim of the present study was to determine the effect of progesterone on the action of estrogen in the development of atherosclerosis. A total of 48 female New Zealand white (NZW) rabbits were ovariectomized. The animals were separated into 6 groups of 8 animals each and received subsequently a 0.5% cholesterol diet for 12 weeks. During this cholesterol feeding period, either estradiol (1 mg/kg body weight (BW)/week), progesterone (25 mg/kg BW/week), or combined estradiol/progesterone (in above dosages) was administered intramuscularly in each group (n = 8 each) of ovariectomized rabbits. One additional group of 8 animals received a combined estrogen/ progesterone regimen, but with progesterone at one third of the above mentioned dosage. In another 8 rabbits, progesterone was reduced to one ninth of the maximum dosage above, whereas estrogen was kept the same, at 1 mg/kg BW/week. Eight ovariectomized animals served as the control group and received no hormone treatment. After 12 weeks, the animals were sacrificed and the proximal aortic arch was removed for further histological examination. An inhibitory effect of estrogen of intimal thickening was found, in comparison to the control group (intimal area: 0.7 +/- 0.5 mm2 vs. 3.7 +/- 2.5 mm2, P < 0.01), whereas progesterone alone did not show a significant effect on intimal plaque size (intimal area: 4.0 +/- 2.3 mm2). In combination with progesterone (high dose), estrogen was not able to reduce intimal atherosclerosis (intimal area: 3.4 +/- 2.4 mm2). However, the beneficial effect of estrogen was not affected by progesterone, when this was reduced respectively to one third (intimal area: 0.8 +/- 0.7 mm2), or to one ninth of the highest dosage (intimal area: 0.6 +/- 0.4 mm2). Interestingly, these differences in atherosclerotic plaque development were observed without significant changes in plasma cholesterol concentrations by the administered hormones. In conclusion, progesterone was dose-dependently able to completely inhibit the beneficial effect of estrogen in experimental atherosclerosis, suggesting that progesterone exerts a direct inhibitory effect on the athero-protective action of estrogen. In the context of recently published data, the present work confirms the importance of the 'non-lipid-mediated', anti-atherosclerotic effect of estrogen, probably due to an interaction with six hormone receptors in vascular smooth muscle cells (VSMC).

H19, a marker of developmental transition, is reexpressed in human atherosclerotic plaques and is regulated by the insulin family of growth factors in cultured rabbit smooth muscle cells

H19 is a developmentally regulated gene with putative tumor suppressor activity, and loss of H19 expression may be involved in Wilms' tumorigenesis. In this report, we have performed in situ hybridization analysis of H19 expression during normal rabbit development and in human atherosclerotic plaques. We have also used cultured smooth muscle cells to identify H19 regulatory factors. Our data indicate that H19 expression in the developing skeletal and smooth muscles correlated with specific differentiation events in these tissues. Expression of H19 in the skeletal muscle correlated with nonproliferative, actin-positive muscle cells. In the prenatal blood vessel, H19 expression was both temporally and spatially regulated with initial loss of expression in the inner smooth muscle layers adjacent to the lumen. We also identified H19-positive cells within the adult atherosclerotic lesion and we suggest that these cells may recapitulate earlier developmental events. These results, along with the identification of the insulin family of growth factors as potent regulatory molecules for H19 expression, provide additional clues toward understanding the physiological regulation and function of H19.

Prevention of intimal hyperplasia by irradiation. An experimental study in rabbits

PURPOSE

This experimental study was designed to investigate the effect of irradiation in prevention of intimal hyperplasia.

MATERIAL AND METHODS

Twenty rabbits were divided into 4 groups, which were irradiated with 2, 5, 10 and 20 Gy, respectively. The intima of both femoral arteries was injured by air-drying, and irradiation was performed on the unilateral side. The contralateral+ femoral artery served as a control. Angiograms as well as histologic specimens were obtained 1 month later.

RESULTS

Marked intimal hyperplasia was observed in all control sites. There were no significant differences in thickness of intimal hyperplasia between irradiated and control sites in groups irradiated with 2 and 5 Gy. However, in the 10-Gy- and 20-Gy-irradiated groups, intimal hyperplasia of the irradiated site was significantly suppressed. Medial thinning and dilation of the lumen were observed in the 20-Gy-irradiated group.

CONCLUSION

Radiation may prevent intimal hyperplasia. Further investigation of the optimal dose, timing of irradiation, and long-term patency of irradiated vessels may be needed.

Tropoelastin gene expression in individual vascular smooth muscle cells. Relationship to DNA synthesis during vascular development and after arterial injury

After vascular injury, quiescent adult smooth muscle cells (SMCs) revert to a more immature synthetic-state phenotype concomitant with the onset of cell replication. The relationship between SMC proliferation and the reexpression of genes characteristic of immature SMCs (eg,tropoelastin [TE]), on an individual cell basis, has not been determined. Using a combined bromodeoxyuridine (BrdU) immunocytochemistry-TE in situ hybridization technique, we determined the relationship between DNA synthesis and TE gene expression in the rat vascular wall during development of the aortic media (embryonic days 13 to 18), low but detectable levels of TE expression occurred equally in both replicating and nonreplicating SMCs. TE message levels dramatically increased in the late fetal and early postnatal periods (fetal day 19 to 1 month postpartum), after a precipitous drop in SMC replication, and then decreased to undetectable levels by postpartum day 60. After a balloon catheter injury in the adult, a developmental sequence of SMC replication followed by TE gene expression was reiterated in both the media and in the developing neointima. On an individual cell basis, adult SMCs replicating after injury expressed little or no TE message; detectable TE gene expression occurred only in nonreplicating SMCs. The most important implications of these data are that (1) adult SMCs replicating after injury appear to revert to a pre-elastogenic embryonic phenotype; (2) maximal TE expression occurs in SMCs only after the cessation of cell replication; and (3) in both the media and the neointima, adult SMCs responding to injury undergo temporarily sequential changes in phenotype reflective of SMC development.

Pleomorphic soft tissue myogenic sarcomas of adulthood. A reappraisal in the mid-1990s

325 diverse sarcomas, 39 rhabdomyosarcomas (RMS), including all histologic variants, and 135 leiomyosarcomas (LMS) were identified. Within these two groups, 18 (46%) of the RMS and 14 (10%) of the LMS represented pleomorphic variants. These neoplasms were studied by morphology (histology and ultrastructure) and by immunohistochemical methods employing antibodies to intermediate filaments (vimentin and desmin) and actin isoforms [alpha-smooth (sm) and alpha-sarcomeric (sr) actins]. Twenty-four pleomorphic malignant fibrous histiocytomas (MFH) and eight pleomorphic liposarcomas (LS) were examined in a similar fashion. By light microscopy, the pleomorphic RMS, LMS, and MFH were indistinguishable, as each was dominated by pleomorphic cells disposed in a haphazard growth pattern; moreover, many featured fascicular, storiform, and sclerotic zones. The distinction between these neoplasms became apparent only following immunohistochemistry and/or ultrastructural study. All pleomorphic RMS disclosed rudimentary sarcomeres and exhibited the following cytoskeletal profile: vimentin (+) (18 of 18), desmin (+) (14 of 18), alpha-sr actin (+) (18 of 18) and alpha-sm actin (+) (five of 18). All the pleomorphic LMS featured smooth-muscle differentiation of variable degrees in the form of cytoplasmic bundles of microfilaments and associated dense bodies; their cytoskeletal profile was vimentin (+) (14 of 14), desmin (+) (seven of 14), alpha-sr actin (+) (none of 14), and alpha-sm actin (+) (eight of 14). The latter was demonstrated in all moderately differentiated, but absent or only focally expressed in poorly differentiated variants. All pleomorphic MFH and LS were devoid of myogenic (skeletal or smooth) ultrastructural features and expressed vimentin solely. This combined morphological and immunohistochemical study illustrates the following: First, these pleomorphic sarcomas are often indistinguishable by histologic growth pattern alone; thus, an accurate diagnosis requires study with all of these techniques. Second, pleomorphic myogenic sarcomas are restricted to adults and are not uncommon neoplasms among pleomorphic sarcomas: RMS (28%), LMS (21%), MFH (38%), and LS (13%). Third, the study defines desmin-negative and alpha-sm actin-positive pleomorphic RMS, and desmin-negative and alpha-sm-actin-negative pleomorphic LMS.

Adventitial remodeling after coronary arterial injury

BACKGROUND

Intraluminal thrombus formation and medial smooth muscle (SM) cell proliferation are recognized responses of the arterial system to injury. In contrast to these well-characterized processes during vascular repair, changes involving the adventitia have been largely neglected in previous studies. Hence, the goal of this investigation was to assess the response of the adventitia to coronary arterial injury.

METHODS AND RESULTS

Adventitial changes in porcine coronary arteries subjected to medial injury were characterized by immunohistochemistry, histochemistry, and microscopic morphometry. The rapid development of a hypercellular response in the adventitia was evident 3 days after balloon-induced medial injury. Cell proliferation, as assessed by proliferating cell nuclear antigen immunostaining, reached the maximum level in the adventitia at 3 days, whereas at 14 and 28 days, the number of replicating cells reverted toward the baseline. The proliferating activity in the adventitia exceeded that seen in the media at all times after injury. To further define the changes in the phenotype of adventitial cells, the expression of three cytoskeletal proteins (vimentin, alpha-SM actin, and desmin) was characterized. Fibroblasts in normal adventitia expressed vimentin but no alpha-SM actin or desmin. After injury, these cells acquired characteristics of myofibroblasts expressing alpha-SM actin, which peaked at 7 and 14 days. Desmin expression was patchy in the adventitia, as opposed to its homogeneous distribution in medial SM cells. The modulation of fibroblast phenotype was transient, inasmuch as alpha-SM actin immunostaining declined at 28 days after injury, when dense, collagen-rich scar was evident within the adventitia. The above-described changes involving hypercellularity of the adventitia, myofibroblast formation, and fibrosis were associated with a significant focal adventitial thickening at 3, 7, 14, and 28 days after injury (P < .01 versus uninjured coronary arteries).

CONCLUSIONS

This study demonstrates the involvement of the adventitia in the vascular repair process after medial injury. The hypercellularity of the adventitial layer, proliferation of fibroblasts, and modulation of their phenotype to myofibroblasts are associated with the development of the thickened adventitia. It is postulated that these phenomena affect vascular remodeling and may provide an important insight into the mechanisms of vascular disorders.

Differentiated properties and proliferation of arterial smooth muscle cells in culture

The smooth muscle cell is the sole cell type normally found in the media of mammalian arteries. In the adult, it is a terminally differentiated cell that expresses cytoskeletal marker proteins like smooth muscle alpha-actin and smooth muscle myosin heavy chains, and contracts in response to chemical and mechanical stimuli. However, it is able to revert to a proliferative and secretory active state equivalent to that seen during vasculogenesis in the fetus, and this is a prerequisite for the involvement of the smooth muscle cell in the formation of atherosclerotic and restenotic lesions. A similar transition from a contractile to a synthetic phenotype occurs when smooth muscle cells are established in culture. Accordingly, an in vitro system has been used extensively to study the regulation of differentiated properties and proliferation of these cells. During the first few days after seeding, the cells are reorganized structurally with a loss of myofilaments and formation of a widespread endoplasmic reticulum and a prominent Golgi complex. In parallel, they lose their contractility and instead become competent to divide in response to a large variety of mitogens, including platelet-derived growth factor (PDGF) and basic fibroblast growth factor (bFGF). After entering the cell cycle, they start to produce these and other mitogens on their own, and continue to replicate in the absence of exogenous stimuli for a restricted number of generations. Furthermore, they start to secrete extracellular matrix components such as collagen, elastin, and proteoglycans. The mechanisms that control this change in morphology and function of the smooth muscle cells are still poorly understood. Adhesive proteins such as fibronectin and laminin apparently have an important role in determining the basic phenotypic state of the cells and exert their effects via integrin receptors. The proliferative and secretory activities of the cells are influenced by a multitude of growth factors, cytokines, and other molecules. Although much work remains before an integrated view of this regulatory machinery can be achieved, there is no doubt that the cell culture technique has contributed substantially to our knowledge of smooth muscle differentiation and growth. At the same time, it has been crucial in exploring the role of these cells in vascular disease and developing new therapeutic strategies to cope with major causes of human death and disability.

Osteopontin is not a marker for proliferating human vascular smooth muscle cells

Osteopontin (OP) is a secreted glycoprotein that contains the Arg-Gly-Asp (RGD) cell-binding sequence that binds calcium and is chemotactic and adhesive for rat vascular smooth muscle cells (VSMCs). OP gene expression is upregulated in cultured rat VSMCs in vitro and after balloon carotid injury in vivo, suggesting that OP may be a marker for proliferating VSMCs in vivo and in vitro. Our in situ hybridization studies of human atherosclerotic coronary vessels, however, have shown OP mRNA expression in plaque macrophages but not VSMCs. The current study investigated OP mRNA expression in cultured human VSMCs and macrophages and in an organ culture model of neointima formation in human saphenous vein. OP mRNA expression was not detected by Northern blot analysis of total RNA from subconfluent or confluent cultures of human VSMCs of any passage maintained in normal growth medium or after stimulation with TGF beta 1 (20 ng/mL), angiotensin II (1 mumol/L), or basic fibroblast growth factor (10 mg/mL) but was just detectable after stimulation with activation vitamin D3 (1 mumol/L). In contrast, cultured human macrophages expressed high levels of OP mRNA that were not dependent on lipid loading. OP mRNA was detected in isolated foci in all layers of saphenous veins maintained in organ culture for 14 days, including <2% of neointimal cells, a distribution that paralleled that of tissue macrophages. These results suggest that OP gene expression is not a marker for proliferation of human VSMCs in vitro and highlight a fundamental difference in the biology of human and rodent VSMCs.

Heparin and the phenotype of adult human vascular smooth muscle cells

To study mechanisms controlling growth and phenotype in human vascular smooth muscle cells, we established culture conditions under which these cells proliferate rapidly and achieve life-spans of 50-60 population doublings. In medium containing heparin and heparin-binding growth factors, growth rate and life-span of human vascular smooth muscle cells increased more than 50% relative to cultures with neither supplement, and more than 20% compared to cultures supplemented only with heparin-binding growth factors. In contrast to observations made in rat vascular smooth muscle cells, smooth muscle-specific alpha-actin in the human cells was expressed only in the presence of heparin and colocalized with beta/gamma nonmuscle actins in stress fibers, not in adhesion plaques. Heparin, in the presence of heparin-binding growth factors, also caused more than 170% stimulation of tracer glucosamine incorporation into hyaluronic acid and a 7.5-fold increase in hyaluronic acid accumulation. In comparison, total sulfate incorporation into sulfated glycosaminoglycans increased by less than 40%. In light of our previous findings that heparin suppresses collagen gene expression, we conclude that heparin induces human vascular smooth muscle cells exposed to heparin-binding growth factors to remodel their extracellular matrix by altering the relative rates of hyaluronic acid (HA) and collagen synthesis. The resulting hyaluronic-acid-rich, collagen-poor matrix may enhance infiltration of CD44/hyaluronate-receptor-bearing T-lymphocytes and monocytes into the vascular wall, an early event in atherogenesis.

Oxidized LDL induce hsp70 expression in human smooth muscle cells

Heat shock protein 70 (hsp70) has been detected in atherosclerotic lesions, in which endothelial cells and smooth muscle cells are involved. In a previous report we showed that Ox-LDL, a causal factor in atherosclerosis, could induce hsp70 expression in cultured human endothelial cells [Zhu et al. B.B.R.C. 1994, 200:389]. Here, with immunofluorescence and immunoblotting techniques, we show that Ox-LDL are capable of inducing hsp70 expression also in human smooth muscle cells, and that this induction is dependent on cell density and on the concentration of Ox-LDL. The induced expression of hsp70 was higher in human umbilical vein smooth muscle cells than in a human smooth muscle cell line. Conversely, Ox-LDL was cytotoxic to both types of cells, more so to the human smooth muscle cell line. These observations indicate that Ox-LDL may be a stress responsible for hsp70 expression in atherosclerotic plaques and the presence of hsp70 in plaques may be a useful marker for continuous oxidative damage in the arterial wall.

Smooth muscle cell proliferation is proportional to the degree of balloon injury in a rat model of angioplasty

BACKGROUND

A variable degree of smooth muscle cell (SMC) proliferation after balloon injury has been reported in previous rat studies. It is unknown whether balloon injury induces c-fos expression and whether it is related to the degree of vascular injury in vivo. Therefore, we tested the hypothesis that proportional increases in neointimal formation and c-fos expression might be present after different degrees of balloon dilation.

METHODS AND RESULTS

Angioplasty of the carotid artery was performed with a balloon catheter. Vascular injury was evaluated at 0, 0.5, 1.0, 1.5, and 2 atm (n = 6 for all). In 40 additional rats, total RNA dot blots were performed to assess the effect of various degrees of balloon injury on c-fos expression. SMC proliferation proportional to the increases of inflation pressure was found between 0 and 2 atm with neointimal areas of 0.002 +/- 0.002, 0.069 +/- 0.014, 0.128 +/- 0.043, 0.190 +/- 0.010, and 0.255 +/- 0.041 mm2, respectively. When the degree of SMC proliferation (neointima and neointima/media ratio) was plotted against balloon inflation pressure, a linear relation was observed (r = .733, P < .001 and r = .755, P < .001, respectively). An increase in c-fos expression proportional to the degree of injury was found 30 minutes after injury.

CONCLUSIONS

Neointimal proliferation produced by balloon injury is related to balloon inflation pressure, supporting the concept of an SMC proliferative response proportional to the degree of injury. The increase in SMC proliferation is associated with a proportional increase in the early expression of the c-fos nuclear proto-oncogene.

Structure and expression of a smooth muscle cell-specific gene, SM22 alpha

SM22 alpha is expressed exclusively in smooth muscle-containing tissues of adult animals and is one of the earliest markers of differentiated smooth muscle cells (SMCs). To examine the molecular mechanisms that regulate SMC-specific gene expression, we have isolated and structurally characterized the murine SM22 alpha gene. SM22 alpha is a 6.2-kilobase single copy gene composed of five exons. SM22 alpha mRNA is expressed at high levels in the aorta, uterus, lung, and intestine, and in primary cultures of rat aortic SMCs, and the SMC line, A7r5. In contrast to genes encoding SMC contractile proteins, SM22 alpha gene expression is not decreased in proliferating SMCs. Transient transfection experiments demonstrated that 441 base pairs of SM22 alpha 5'-flanking sequence was necessary and sufficient to program high level transcription of a luciferase reporter gene in both primary rat aortic SMCs and A7r5 cells. DNA sequence analyses revealed that the 441-base pair promoter contains two CArG/SRF boxes, a CACC box, and one potential MEF-2 binding site, cis-acting elements which are each important regulators of striated muscle transcription. Taken together, these studies have identified the murine SM22 alpha promoter as an excellent model system for studies of developmentally regulated, lineage-specific gene expression in SMCs.

Nuclear proteins bind a cis-acting element in the smooth muscle alpha-actin promoter

Identification of the regulators of smooth muscle cell (SMC) gene expression is critical to understanding SMC differentiation and alterations in SMC phenotype in vascular disease. Previous studies revealed positive transcriptional activity within the chicken smooth muscle (SM) alpha-actin promoter region from -209 to -257. In the present study, transient transfections of wild-type and mutant chicken SM alpha-actin promoter/reporter gene constructs into rat aortic SMC demonstrated that the positive transcriptional activity of this region was abolished with a two base pair mutation in a conserved sequence motif at -225 to -233 (TGTTTATC to TACTTATC). Electrophoretic mobility shift assays revealed that nuclear factors bound promoter fragments containing this sequence and that specific mutations in the TGTTTATC motif abolished nuclear factor binding. Studies thus provide evidence for binding of a nuclear factor to a positive cis-acting element within the SM alpha-actin promoter. Further characterization of this factor may contribute to a better understanding of the molecular mechanisms that regulate differentiation of SMC in vascular disease.

Regional heterogeneity of arterial structural changes

Arterial structural changes in experimental models of hypertension and restenosis differ between vessel types and within vessels. Inspired by the diversity of short-term functional responses to vasoactive agents, hypotheses are presented with respect to the heterogeneity of structural alterations. Considered are the multifactorial nature of smooth muscle cell growth control and the possibility that vascular smooth muscle is not homogeneous but composed of different smooth muscle cell populations. These hypotheses may help explain the origin of both intervascular and intravascular heterogeneity of vascular structural responses.

Effects of a single administration of fibroblast growth factor on vascular wall reaction to injury

Expansion of the vascular wall through formation of neointimal fibromuscular lesions is the basic mechanism underlying stenosis of vascular grafts, restenosis of arteries treated by balloon angioplasty, and other major cardiovascular problems. This study examined the effect of a single, systemic, low dose of basic fibroblast growth factor (bFGF) on formation of neointimal fibromuscular lesions in response to injury. New Zealand white rabbits (n = 76) were subjected to balloon injury of the abdominal aorta. Forty-five rabbits were given a single intravenous dose of bFGF (0.5 microgram/kg) immediately after injury, and 31 rabbits were given only the vehicle solution as controls. After 2 (n = 15), 5 (n = 21), 14 (n = 29), or 28 (n = 11) days the rabbits were sacrificed. Those rabbits receiving the single administration of bFGF exhibited significantly greater intimal thickening (intima/media ratio) than the control group at 5 days (mean +/- standard error of the mean, 0.091 +/- 0.009 versus 0.058 +/- 0.006; p < 0.002), but not at 14 or 28 days. These results were achieved without any significant differences in mitotic indices, as determined by a mitostatic method, between the two groups at any postinjury interval examined. The findings suggest that a single systemic dose of exogenous bFGF has a relatively long term effect on enhancing the neointimal response to vascular injury. Therefore, local control of endogenous bFGF may be useful in limiting formation of vascular neointimal fibromuscular lesions, thus improving the long-term results of vascular grafts, balloon angioplasty, and other cardiovascular procedures.

The smooth muscle alpha-actin gene promoter is differentially regulated in smooth muscle versus non-smooth muscle cells

To identify potential regulators of smooth muscle cell (SMC) differentiation, we studied the molecular mechanisms that control the tissue-specific transcriptional expression of SM alpha-actin, the most abundant protein in fully differentiated SMCs. A construct containing the region from -1 to -125 of the promoter (p125CAT) had high transcriptional activity in SMCs (57-fold > promoterless) and endothelial cells (ECs) (18-fold) but not in skeletal myoblasts or myotubes. Mutation of either of two highly conserved CC(AT-rich)6GG (CArG) motifs at -62 and -112 abolished the activity of p125CAT in SMCs but had no effect in ECs. In contrast, high transcriptional activity in skeletal myotubes, which also express SM alpha-actin, required at least 271 base pairs of the promoter (-1 to > or = -271). Constructs containing 547 base pairs or more of the promoter were transcriptionally active in SMCs and skeletal myotubes but had no activity in skeletal myoblasts or ECs, cell types that do not express SM alpha-actin. Electrophoretic mobility shift assays provided evidence for binding of a unique serum response factor-containing complex of factors to the CArG box elements in SMCs. Results indicate that: 1) transcriptional expression of SM alpha-actin in SMCs requires the interaction of the CArG boxes with SMC nucleoprotein(s); 2) expression of SM alpha-actin in skeletal myotubes requires different cis-elements and trans-factors than in SMCs; and 3) negative-acting cis-elements are important in restricting transcription in cells that do not express SM alpha-actin.

Stretch affects phenotype and proliferation of vascular smooth muscle cells

The exertion of periodic dynamic strain on the arterial wall is hypothesized to be relevant to smooth muscle cell morphology and function. This study has investigated the effect of cyclic mechanical stretching on rabbit aortic smooth muscle cell proliferation and expression of contractile phenotype protein markers. Cells were cultured on flexible-bottomed dishes and cyclic stretch was applied (frequency 30 cycles/min, 15% elongation) using a Flexercell Strain unit. Cyclic stretch potentiated smooth muscle cell proliferation in serum-activated cultures but not in cultures maintained in 0.5% fetal calf serum. Stretching induced a serum-independent increase of h-caldesmon expression and this effect was reversible following termination of mechanical stimulation. Strain was without effect on smooth muscle myosin or calponin expression. In cells grown on laminin stretch-induced h-caldesmon expression was more prominent than in cells cultured on collagen types I and IV, poly-L-lysine and gelatin. These data suggest that cyclic mechanical stimulation possesses dual effect on vascular smooth muscle cell phenotype characteristics since it: 1) potentiates proliferation, an attribute of a dedifferentiated phenotype; and 2) increases expression of h-caldesmon considered a marker of a differentiated smooth muscle cell state.

Expression of gax, a growth arrest homeobox gene, is rapidly down-regulated in the rat carotid artery during the proliferative response to balloon injury

gax is a recently described homeobox gene whose expression in the adult is largely confined to cardiovascular tissues, gax has been shown to be rapidly down-regulated in cultured vascular smooth muscle cells (VSMC) upon stimulation by serum or platelet-derived growth factor. The temporal profile of gax expression in vitro matches that of two families of growth arrest genes: the gas genes and the gadd genes. All of these genes are expressed at their highest levels in quiescent cells and are down-regulated following mitogen activation. Here we report that gax is also down-regulated in vivo in the vascular wall in response to endothelial denudation by balloon angioplasty. The reduction in steady state levels of gax mRNA is transient and occurs with a similar time course to that seen in vitro. The down-regulation of gax in response to balloon injury mirrors the up-regulation seen in a number of early response genes such as c-myc and c-fos. This report is the first to document the in vivo expression of a growth arrest gene which regulates proliferation of vascular smooth muscle cells. In addition, in contrast with previous reports which have demonstrated up-regulation of several genes following balloon injury and/or angioplasty, the present report demonstrates the down-regulation of a regulatory gene within hours of balloon injury. The characteristics of gax suggest it may be required to maintain the gene expression of proteins in VSMC that are associated with the nonproliferative or contractile phenotype in smooth muscle cells.

Extinction of autonomous growth potential in embryonic: adult vascular smooth muscle cell heterokaryons

Vascular smooth muscle cells (SMC) isolated from embyronic and early fetal (e13-e18) rat aortas exhibit an "embryonic growth phenotype" in culture (Cook, C. L., M. C. M. Weiser, P. E. Schwartz, C. L. Jones, and R. A. Majack. 1994. Circ. Res. 74:189-196). Cells in this growth phenotype exhibit autonomous, serum-independent replication, in contrast to SMC in the "adult" growth phenotype, whose proliferation in culture is dependent on exogenous mitogens. To determine which of these two phenotypes is genetically dominant, heterokaryons were constructed between adult and embryonic (day e17) rat aortic SMC. The fused cells were maintained in serum-free medium for 3 d, then were labeled with bromodeoxyuridine (BrdU) for an additional 24 h. Under these conditions, parental e17 SMC exhibited a high rate of self-driven DNA synthesis (73-85% BrdU-positive cells), while parental adult SMC showed minimal replication (13-21% BrdU-positive cells). Homokaryons of parental cells exhibited parental growth phenotypes and showed the expected mitogenic response when stimulated with serum. Heterokaryons between e17 and adult SMC exhibited a nonautonomous growth phenotype; the "adult" growth phenotype was calculated to be dominant in > 89% of all true heterokaryons. The data suggest that adult SMC express molecules capable of genetically extinguishing or otherwise inhibiting the autonomous replication of embryonic SMC.

Regulation of smooth muscle cell scavenger receptor expression in vivo by atherogenic diets and in vitro by cytokines

Scavenger receptor (ScR)-mediated uptake of modified lipoproteins may contribute to the transformation of smooth muscle cells into lipid-laden foam cells during atherogenesis. This study examined the in vivo expression of ScRs in aortas, with or without balloon injury, taken from hypercholesterolemic or normocholesterolemic rabbits. Numerous intimal cells in the rabbit aortic lesions expressed ScRs as detected by immunocytochemical staining with a goat anti-rabbit ScR antibody. Single immunostaining for cell identification markers in serial sections, as well as double staining, confirmed the expression of ScRs by both intimal smooth muscle cells and macrophages. To explore potential inducers of ScR expression by smooth muscle cells in vivo, we studied the regulation of ScR expression in vitro by cytokines known to be present in atherosclerotic lesions. Tumor necrosis factor-alpha (TNF-alpha) or interferon-gamma (IFN-gamma) increased ScR mRNA levels, protein expression, and AcLDL degradative activity in cultured rabbit aortic smooth muscle cells. The induction of ScR expression in intimal smooth muscle cells in vivo could be a useful marker of smooth muscle cell activation during atherogenesis and may contribute to foam cell formation by this cell type following balloon injury and/or hypercholesterolemia. Cytokines, such as TNF-alpha or IFN-gamma, may stimulate some of the phenotypic changes that characterize the alteration in gene expression of intimal smooth muscle cells in rabbit atherosclerotic lesions.

Changes in the caldesmon isoform content and intimal thickening in the rabbit carotid artery induced by a silicone elastomer collar

The presence of a silicone elastomer collar around one carotid artery of a rabbit induces thickening of the tunica intima. We used immunoblotting to study quantitatively changes in the isoforms of caldesmon, a protein implicated in the regulation of contractility in smooth muscle, while also monitoring the histological changes during 28 days after collaring. Control rabbit carotid arteries (n = 28) contained 245 +/- 6.4 nmol/g protein of the larger isoform of caldesmon (CDh) and 68.3 +/- 3.6 nmol/g protein of the smaller isoform (CD1). Four days after collaring, intimal thickening was slight, but 44% of arterial CDh had been lost; this loss of CDh was therefore from the tunica media. At 10 days, CDh fell to 37% of the control level. Immunofluorescence using CDh-specific antibodies showed that the CDh level was diminished but remained uniform across the wall of collared arteries. At 14 days, when intimal thickening was maximal, there was 30% more CD1 than in controls. At 28 days, the neointima had thinned, and CD1 had fallen to below control levels. Thus, CD1 levels reflected the development and regression of neointima. Changes in caldesmon isoforms showed that smooth muscle cell phenotypic changes occurred throughout the arterial wall.

Expression of meta-vinculin in human coronary arteriosclerosis is related to the histological grade of plaque formation

In the present study we demonstrate that the quantitative reduction of meta-vinculin expression parallels histological changes during the course of coronary arteriosclerosis. Immunofluorescence stainings of coronary arteries revealed that vinculin distribution resembled that of other smooth muscle-specific cytoskeletal proteins like alpha-actin, caldesmon or myosin light chain kinase in labeling smooth muscle cells brightly. Although close to arteriosclerotic plaques, the cellularity as measured by the density of nuclei was often not significantly altered. Cells of this location expressed markedly reduced amounts of vinculin, suggesting that they are smooth muscle cells of a synthetic phenotype. To determine the fractional meta-vinculin content in arteriosclerotic lesions, we performed densitometric scanning of immunoblots incubated with anti-vinculin monoclonal antibodies reacting with both meta-vinculin (150 kDa) and vinculin (130 kDa). In parallel, each tissue sample was evaluated histologically for the degree of arteriosclerotic alterations according to the morphometric atheroma score of Stratford et al. (n = 13). In type 1 lesions covering slight intimal thickening, meta-vinculin represented 36% (mean, range 35%-39%) of the total vinculin immunoreactivity. In type 2 lesions consisting of fibrous plaques of up to twice the original artery wall thickness, meta-vinculin accounted for 28% (mean, range 22%-35%) of the total vinculin content. Meta-vinculin was substantially reduced in type 3 lesions (mean 13%, range 8%-18%) which are characterized by extensive atheromatous plaques. Thus, the meta-vinculin/vinculin ratio differed significantly between early, intermediate and advanced phases of coronary arteriosclerotic plaque formation.

Regulation of actomyosin and contraction in smooth muscle

Unlike striated muscle cells, smooth muscle cells do not have an organized sarcomeric structure. However, all smooth muscle cells contain the contractile proteins, myosin, actin, and tropomyosin. Polymorphism of the myosin heavy chain exists in smooth muscle cells. Two myosin heavy chain (MHC) isoforms, SM1 (204 kDa) and SM2 (200 kDa), are present in smooth muscle cells; however, their ratios vary in smooth muscles from different sources. The hypertrophy of the urinary bladder induced by partial outlet obstruction in rabbits is associated with an alteration of the SM1-to-SM2 ratio from 1:3 to 1:1. Both heavy chains react with polyclonal antibody against smooth muscle myosin; however, antibody prepared against a peptide from the C-terminal region of the SM2 heavy chain cross-reacts only with the SM2 heavy chain. Removal of the obstruction reverses the bladder to normal mass with a concomitant change in the SM1-to-SM2 ratio back to 1:3. The expression of the SM1 mRNA is increased in response to obstruction-induced hypertrophy, and it also returns to normal upon removal of the obstruction. Urinary bladder smooth muscle contains predominantly gamma-actin. Obstruction-induced hypertrophy of the bladder smooth muscle is associated with an increase in the gamma-actin at both protein and mRNA levels. The beta-non-muscle actin is decreased and the alpha-smooth muscle actin is unchanged in response to obstruction-induced bladder hypertrophy. Contraction of all smooth muscles involves similar mechanisms. This review describes our current understanding of the mechanisms regulating contraction of the smooth muscle of the urinary bladder.

Pathobiology of intimal hyperplasia

In the current vascular interventional environment, high restenosis rates have increased awareness of the significance of intimal hyperplasia, a chronic structural lesion that develops after vessel wall injury, and which can lead to luminal stenosis and occlusion. Intimal hyperplasia may be defined as the abnormal migration and proliferation of vascular smooth muscle cells with associated deposition of extracellular connective tissue matrix. The pathology of intimal hyperplasia is reviewed with particular attention to its physiology, pharmacology, cell biology and molecular biology.

Characterization of rat aortic smooth muscle cells resistant to the antiproliferative activity of heparin following long-term heparin treatment

Vascular smooth muscle cells (SMC) do not represent a homogeneous population (Schwartz et al., 1990, Am. J. Pathol. 136: 1417-1428). Cellular clones resistant to the antiproliferative activity of heparin were isolated from rat aortic SMC cultures (Pukac et al., 1990, Cell Regul., 1:435-443; San Antonio et al., 1993, Arterioscler. Thromb., 13:748-757) and from explant of human arterial restenotic lesions (Chan et al., 1993, Lancet, 341:341-342). We have shown in the present study that long-term treatment (growth medium supplemented with 200 micrograms/ml heparin, from the second to the tenth passage) of rat aortic SMC, without cell cloning, resulted in a significant loss of sensitivity to the growth inhibition by heparin and its derivatives. The heparin resistance was stable after growing cells for two passages in heparin-free medium, suggesting the selection of a particular phenotype. We tried to characterize these cells and to determine the causes of the resistance to the growth inhibition by heparin. Heparin-treated SMC (HT-SMC) were smaller than their control culture at the same passage, expressed less alpha-SM actin, and did not overgrow after reaching confluence. As in the heparin-resistant clones (San Antonio et al., 1993, Cell Regul., 1:435-443) expression of alpha-SM actin could be increased in HT-SMC by heparin addition before Western blotting. Heparin resistance was associated with a tenfold decrease in [3H]-heparin binding capacity (Bmax = 1.9 x 10(6) sites per cell) compared to control cultures (Bmax = 1.7 x 10(7) sites per cell), which was irreversible after growing the cells for two additional passages in heparin-free medium. We also investigated protein kinase C (PKC) in HT-SMC in terms of both enzymatic activity and protein expression (evaluated by [3H]-staurosporine and [3H]-phorbol-12,13-dibutyrate binding). We found that HT-SMC had only half the PKC activity and expression as control SMC. Therefore, long-term treatment of rat aortic SMC with heparin allowed the selection of a less differentiated subpopulation of cells, exhibiting low sensitivity to the growth inhibition by heparin, which could be related to the low capacity of binding heparin and to a lower PKC activity and/or expression.

Relationships between risk factors and morphological patterns of human carotid atherosclerotic plaques. A multivariate discriminant analysis

The histological characterization of the fibroatheromatous plaques and their histogenesis are still to be defined. Factors responsible for the evolution of intimal components and the mechanisms and stages of fibroatheromatous plaque formation are still largely obscure. Focusing on symptomatic plaques, the aim of this study is to determine whether plaque heterogeneity is the result of a haphazard clustering of various components or an organized pattern in response to risk factors. To this end, 180 carotid plaques from patients affected by transient ischemic attacks (TIA) or by stroke, with angiographic stenosis greater than 50%, were studied after endoarterectomy. Clinical and morphological data were collected by means of a pre-defined protocol, quantified and correlated, by using the discriminant analysis, with age, sex, hypertension, diabetes, hypercholesterolemia and smoking habit. Our results show that the relationships between plaque components are non-random and consistent with the knowledge derived from studies on human and experimental plaques. Moreover, some plaque patterns can be significantly correlated with single risk factors. The fibrous plaque was correlated with aging and diabetes; the granulomatous plaque, rich in giant cells, with the female sex and hypertension; the xanthomatous plaque, rich in foam cells and with extensive alcianophilia, with hypercholesterolemia. In the smokers, finally, the plaques were frequently complicated by mural thrombosis.

Rat aortic smooth muscle cells isolated from different layers and at different times after endothelial denudation show distinct biological features in vitro

Endothelial denudation by balloon injury of the rat aorta induces the development of a neointima as a consequence of the migration and proliferation of smooth muscle cells (SMCs). Initially, intimal SMCs show a dedifferentiated phenotype, which reverts to a normal differentiated phenotype after endothelial cells have resurfaced the vessel lumen. We investigated in vitro the proliferative and phenotypic features of SMCs from different layers of rat aorta isolated 15 and 60 days after endothelial denudation. Freshly isolated intimal cells 15 days after balloon injury (IT-15) appeared rounded and showed a decreased content of alpha-smooth muscle actin, smooth muscle myosin, and desmin compared with intimal cells isolated 60 days after balloon injury (IT-60). No morphological and cytoskeletal differences were observed among freshly isolated IT-60 cells and other medial populations, which included medial SMCs that underlie the intimal thickening. In culture, IT-15 cells showed increased proliferative activity both in monolayers and in free-floating collagen lattices. Decreased expression of alpha-smooth muscle actin and smooth muscle myosin was documented in IT-15 cells compared with IT-60 cells and other medial SMC populations in monolayer. Moreover, IT-15 cells suspended in collagen lattices were poor at contracting these collagen lattices compared with IT-60 and control SMCs. IT-60 cells were equivalent to control SMCs at lattice contraction except for a temporary delay at day 1.(ABSTRACT TRUNCATED AT 250 WORDS)

Deregulated expression of the c-myc oncogene abolishes inhibition of proliferation of rat vascular smooth muscle cells by serum reduction, interferon-gamma, heparin, and cyclic nucleotide analogues and induces apoptosis

We have investigated the requirement for c-myc downregulation in the growth arrest of vascular smooth muscle cells (VSMCs). Rat VSMCs were infected with a retrovirus vector directing constitutive expression of either the complete human c-Myc protein (VSM-myc cells) or the c-Myc deletion mutant D106-143, which is inactive in cotransformation and autosuppression assays (VSM-D106-143 myc cells). Clones of transfected VSM-myc cells were isolated that constitutively expressed a range of levels of c-Myc protein from that observed in normal proliferating VSMCs to approximately seven times normal. The growth rates of these clones and their responses to growth inhibitors were then assessed. VSM-myc clones possessed a shorter mean intermitotic time than normal cells, which was inversely correlated (P < .05) with the level of c-Myc protein expressed. VSM-myc cells also expressed lower levels of alpha-smooth muscle actin mRNA and protein and exhibited an altered morphology. The proliferation of normal VSMCs and VSM-D106-143 myc cells was inhibited by serum reduction (0.5% fetal calf serum) and also by treatment with interferon-gamma (100 IU/mL), heparin (50 micrograms/mL), 8-bromo-cAMP (0.1 mmol/L), or 8-bromo-cGMP (0.1 mmol/L). In contrast, proliferation of VSM-myc cells was not inhibited by any of these agents, even if present at 10-fold higher concentrations. However, approximately 75% of VSM-myc cells expressing levels of c-Myc protein seen in normal proliferating VSMCs underwent apoptosis after 4 days of serum reduction or treatment with interferon-gamma. The results show that constitutive c-myc expression induces continuous cell proliferation, reduction in alpha-smooth muscle actin expression and apoptosis in VSMCs. We conclude that downregulation of c-myc is a prerequisite for growth arrest and subsequent survival of VSMCs. Conversely, deregulated c-myc expression may be important in the proliferation and death of VSMCs--characteristics of the pathogenesis of atherosclerosis.

Phenotypic stability and variation in cells of the porcine aorta: collagen and elastin production

The extracellular matrix of the developing vasculature varies in composition as a function of time and position. Cellular models of vascular biology and pathology depend on the assumption that stable phenotypic characteristics of vascular cells can be propagated through several generations of in vitro cultivation. We show that the positional and developmental heterogeneity of matrix phenotypes in the porcine aorta are expressed by explanted vascular smooth muscle cell (SMC) and adventitial cell populations for a limited number of passages. Elastin was expressed most highly by thoracic SMC while interstitial collagen production was usually maximal in abdominal segments. Parallel gradients of collagen types I, III and V, detected by specific ELISA assays, were expressed in early-passage SMC. Adventitial cell populations from the abdominal aorta of the neonatal pig accumulated significant levels of collagen, while these fibroblasts produced less than 10% of the elastin made by SMC. All cell populations expressed alpha-smooth muscle actin in vitro. Gradients of collagen and elastin expression were evident for no more than three passages, and direct outgrowth of cells without limited digestion of the matrix further reduced phenotypic stability. Variation and decline of the elastin phenotype could be due to hypermethylation of regulatory sequences in the elastin gene or trans-acting factors, but elastin production was dose-dependently stimulated to a similar extent (100%; 10 microM 5-azacytidine) in all segmental SMC populations at early (p1) and late (p3) passage. These data indicated that faithful reflection of in vivo SMC behavior was limited to a few population doublings, at least under standard culture conditions. Modification of the cellular environment by reducing serum factors, changing matrix, or adding mechanical stimulation may increase phenotypic stability.

Alterations in the expression of the beta-cytoplasmic and the gamma-smooth muscle actins in hypertrophied urinary bladder smooth muscle

The obstruction of the bladder outlet induces a marked increase in bladder mass, and this is accompanied by reduced contractility of bladder smooth muscle and alteration in the cellular architecture. In this study, we show that the composition of various isoforms of actin, a major component of the contractile apparatus and the cytoskeletal structure of smooth muscle, is altered in response to the obstruction-induced bladder hypertrophy. Northern blot analysis of the total RNA isolated from hypertrophied urinary bladder muscle, using a cDNA probe specific for smooth muscle gamma-actin, shows over 200% increase in the gamma-actin mRNA. However, the estimate of the amount of actin from the 2D gel reveals only a 16% increase in gamma-actin, since the 2D gel electrophoresis does not distinguish gamma-smooth muscle actin from gamma-cytoplasmic actin. The bladder smooth muscle alpha-actin and the smooth muscle alpha-actin mRNA are not altered in response to the hypertrophy. The obstructed bladder also reveals a decrease in the beta-cytoplasmic actin (37%) and a concomitant diminution in the beta-cytoplasmic actin mRNA (29%). Hence, the composition of the actin isoforms in bladder smooth muscle is altered in response to the obstruction-induced hypertrophy. This alteration of the actin isoforms is observed at both the protein and mRNA levels.

Long-term biological response of injured rat carotid artery seeded with smooth muscle cells expressing retrovirally introduced human genes

Cultured vascular smooth muscle cells (SMCs) containing retrovirally introduced genes are a potential vehicle for gene replacement therapy. Because the cultured SMCs are selected for their ability to proliferate in vitro, it is possible that the SMCs might be permanently altered and lose their capacity to respond to growth-suppressing conditions after being seeded back into blood vessels. To investigate this possibility we measured SMC proliferation and intimal thickening in balloon-injured Fischer 344 rat carotid arteries seeded with SMCs stained with the fluorescent marker 1,1'-dioctadecyl-3,3,3',3'-tetramethylindo-carbocyanine perchlorate (DiI) and infected with replication-defective retrovirus expressing human adenosine deaminase or human placental alkaline phosphatase. The majority of the seeded SMCs remained in the intima while a few of the cells appeared to migrate into the first layer of the media. Intimal SMC proliferation returned to background levels (< 0.1% thymidine labeling index) by 28 d. At late times (1 and 12 mo) the morphological appearance of the intima was the same for balloon-injured arteries with or without seeded SMC, except that the seeded arteries continued to express human adenosine deaminase or alkaline phosphatase. These results support the conclusion that cultured SMC infected with a replication-defective virus containing human adenosine deaminase or alkaline phosphatase are not phenotypically altered and do not become transformed. After seeding onto the surface of an injured artery, they stop replicating but continue to express the introduced human genes even over the long term.

Developmentally timed expression of an embryonic growth phenotype in vascular smooth muscle cells

Little is known about the phenotypic changes that occur in vascular smooth muscle cells (SMCs) as the developing aorta undergoes the transition from a loosely organized, highly replicative tissue to a morphologically mature, quiescent tissue. In the present study, we have characterized the in vivo pattern of SMC replication during intrauterine and neonatal aortic development in the rat and have cultured and assessed the in vitro growth properties of embryonic, fetal, and neonatal vascular SMCs. Embryonic SMCs, which exhibited a very high in vivo replication rate (75% to 80% per day), demonstrated a significant potential for self-driven replication, as assessed by the ability to proliferate under serum-deprived conditions. Several lines of evidence suggest that the autonomous growth of SMCs in the "embryonic growth phenotype" may be driven by a unique mechanism independent of known adult SMC mitogens: embryonic SMC replication was not associated with the detectable secretion of mitogenic activity capable of stimulating adult SMCs, and embryonic SMCs were mitogenically unresponsive to a variety of known adult SMC growth factors. The capacity for self-driven growth was lost by embryonic day 20, suggesting that important changes in gene expression and phenotype occur in developing SMCs between embryonic days 18 and 20. Taken together, the data describe a unique embryonic growth phenotype of vascular SMCs and suggest that the replication of aortic SMCs during intrauterine development is self driven, self regulated, and controlled by a developmental timing mechanism. The conversion of SMCs from the embryonic to the late fetal/adult growth phenotype will likely be found to be an important component of a developmental system controlling vascular morphogenesis.

An immunohistochemical study of arterial lesions due to pulmonary hypertension in patients with congenital heart defects

In order to understand some of the cellular mechanisms of interaction in secondary pulmonary vaso-occlusive disease, we studied 21 lung biopsies from patients with different types of congenital cardiac defects. Their ages ranged from four to 248 months (mean 71.5 months; median 41 months). Changes in the cytoskeleton and extracellular matrix were assessed in the arterial wall. Immunostaining was applied to formalin-fixed, paraffin- embedded tissue, using antibodies to muscle-specific actin, vimentin and fibronectin in supra-optimal dilution. The staining for muscle-specific actin in the medial layer revealed a heterogenous pattern, with areas exhibiting low or absent labelling, reflecting a process of dedifferentiation of the smooth muscle cells in those segments. Within intimal proliferative lesions, the expression of muscle-specific actin was variable, being weak in some lesions and strong in those showing concentrically arranged intimal smooth muscle cells, suggesting a reversion of the migrated cells to the contractile phenotype. The endothelial cells of arteries from cases presenting severe qualitative lesions exhibited strong expression of vimentin, reflecting their heightened regenerative activity and/or their necessity to maintain their shape. The expression of fibronectin was greater in the predominantly cellular lesions of the intima when compared to the fibrotic lesions, indicating the role of that matrix glycoprotein in cellular migration and in replicative processes.

Molecular and cellular concepts in atherosclerosis

Atherosclerosis is a complex disease of uncertain cause. Its pathobiology is believed to represent an abnormal expression of the processes of vascular healing. Etiologic models derive from a 'response to injury' paradigm and can be divided into three separate disease stages: endothelial dysfunction, smooth muscle proliferation and architectural disruption. The initiating event of endothelial dysfunction is unknown, but is believed to be related to low-density lipoproteins and/or their oxidized derivatives. Endothelial injury is signalled to the smooth muscle cells of the media by three routes: direct cell-cell interaction, secretion of soluble growth factors and monocyte-derived cytokines. Monocytes are recruited by the endothelium and invade the subintimal space by a complex interaction of a variety of adhesion proteins and receptors on both cell types. Smooth muscle cell proliferation is initiated by a change in phenotype expression from 'contractile' to 'synthetic' resulting from the binding of fibronectin to specific integrin receptors. Three functionally distinct activities may represent separate subtypes of the 'synthetic phenotype': migration from the media to the intima, increased proliferation and inappropriate extracellular matrix synthesis. The loss of normal regulatory control and anchorage independence of proliferation suggest a relationship to oncogenic transformation. Both migration and proliferation result from the binding of platelet-derived growth factor-like factors to smooth muscle cell receptors, which initiates a cascade of intracellular molecular events leading either to cytoskeletal locomotory restructuring or cell cycle activation. Both pathways also appear to be coregulated by integrin receptors and both depend upon phosphorylation of cell membrane, cytosolic and nuclear regulatory proteins. Clinical expression of atherosclerosis may follow sudden loss of architectural integrity of the intimal plaque by three different mechanisms: plaque fissuring, intraluminal plaque rupture or intramural hemorrhage related to abnormal vessel wall stress and/or biochemistry.

Smooth muscle differentiation in human myometrium and uterine leiomyoma

Smooth muscle differentiation has been analysed in human myometrium and leiomyoma by Western blotting with antibodies to smooth muscle specific proteins. No differences in the expression of h-caldesmon, metavinculin, desmin, alpha-smooth muscle actin and calponin were observed. The technique of two-dimensional gel electrophoresis was used, therefore, to further analyse differences between normal smooth muscle cells and their neoplastic counterparts. By comparing the protein patterns of normal myometrium and leiomyoma, it was possible to identify a protein with a molecular weight of approximately 27 kD that is selectively expressed in normal uterine smooth muscle cells. This protein proved to be a low molecular weight variant of calponin, a smooth muscle specific protein of as yet unknown function. Its immediate downregulation in tissue culture of normal myometrium points to a possible role in the process of dedifferentiation.

Mouse model of arterial injury

In the present study, we established an injury model of the mouse carotid artery. Complete removal of the endothelium was achieved with a flexible wire. A platelet monolayer covered the denuded surface, and damage to underlying medial smooth muscle cells (SMCs) was detected. Injection of [3H]thymidine was used to determine the replication index for medial SMCs, which was found to be 1.6% at 2 days after denudation and 9.8% at 5 days. SMCs were observed in the intima by day 8 (replication index, 66%), and by 2 weeks the intimal lesion had a similar cell content as the media. In most animals, repair of the endothelial lining was complete 3 weeks after injury. The present model will allow us to use transgenic animals to address questions relevant to vascular biology and atherosclerosis.

Cytokeratins 8 and 18 in smooth muscle cells. Detection in human coronary artery, peripheral vascular, and vein graft disease and in transplantation-associated arteriosclerosis

During development of atherosclerotic lesions, vascular smooth muscle cells (SMCs) undergo changes both phenotypically and in their cytoskeleton composition. An expression of cytokeratins 8 and 18 in SMCs in plaques of the human superficial femoral artery and of cytokeratin 8 in lesions of the aorta was recently described. Since cytokeratins are epithelial markers generally not found in normal adult vascular SMCs, we performed a detailed immunofluorescence microscopy study using a large panel of antibodies against the various cytokeratin polypeptides and other elements of the cytoskeleton. We included lesions of carotid, common and superficial femoral, iliac, and popliteal arteries; the abdominal aorta; and saphenous vein bypass grafts, as well as primary, restenotic, and transplantation-associated lesions of coronary arteries (n = 33). Cytokeratins 8 and 18 were present in myointimal cells of all pathological specimens. Colocalization with smooth muscle alpha-actin identified most cytokeratin-positive cells as SMCs. Only very few cells cosynthesized cytokeratin and desmin, whereas the majority of cytokeratin-positive cells were vimentin-positive. This pattern of cytoskeletal protein synthesis is similar to that found in some fetal and/or neonatal SMCs. These findings suggest that the synthesis of cytokeratins in a subset of SMCs of atherosclerotic lesions is a common phenomenon in coronary artery and peripheral vascular disease as well as graft disease and transplantation-associated arteriosclerosis and that the state of these SMCs is of a "dedifferentiated" fetal type.

Molecular biology: insight into the causes and prevention of restenosis after arterial intervention

Very little is known about the development of postatherectomy or postangioplasty restenosis. Morphologically, restenosis lesions are primarily composed of smooth muscle cells with associated matrix proteins and develop within 3-6 months. Although some degree of smooth muscle cell proliferation is a necessary part of the healing process after injury, it is unclear why only some individuals develop clinically significant lesions. Platelet deposition and release of growth factors have been postulated to be important in initiating the cellular growth response after vascular injury. Current data suggest that growth factors synthesized locally in the vessel wall may be very important in controlling smooth muscle proliferation. In addition, atherosclerotic plaques contain many procoagulant proteins that are exposed by angioplasty or atherectomy. These proteins stimulate a coagulation response and the activation of thrombin, resulting in platelet aggregation and thrombus formation. Thrombin mediates several biologic responses that may facilitate vascular lesion formation and can act directly as a smooth muscle mitogen. Vascular lesion formation as a result of percutaneous transluminal coronary angioplasty or atherectomy may be stimulated by a combination of factors, including platelet deposition and thrombin action, ultimately generating an autocrine growth response in the vessel wall.

Age influences the replicative activity and the differentiation features of cultured rat aortic smooth muscle cell populations and clones

The replicative activity and the differentiation features of aortic smooth muscle cells (SMCs) cultured as whole populations or clones from newborn (4-day-old), young adult (6-week-old), and old (18-month-old) rats were studied by means of cell counting, [3H]thymidine incorporation, and measurement of the expression of cytoskeletal proteins and mRNAs. In whole populations at the fifth passage, replicative activity increased and differentiation features (ie, expression of alpha-smooth muscle actin, desmin, and smooth muscle myosin heavy chains) decreased with increasing age of the donor animal. SMC clones derived from newborn or young adult rats showed more differentiated cytoskeletal features than their parental populations; however, most SMC clones from old rats showed dedifferentiated features similar to those observed in their parental populations. Our results suggest that (1) SMCs of the rat aortic media behave as a heterogeneous population; (2) cultured whole SMC populations behave differently from clones as far as their replicative activity and differentiation features are concerned; and (3) SMCs derived from old rats, whether grown as whole populations or as clones, dedifferentiate more substantially and replicate more actively than corresponding cultures from newborn or young adult rats when submitted to the same amount of serum growth factors; these differences may play a role in arterial development as well as in the formation and evolution of the atheromatous plaque.

Downregulation of c-myc expression by antisense oligonucleotides inhibits proliferation of human smooth muscle cells

BACKGROUND

Proliferation of smooth muscle cells (SMCs) plays an important role in vascular pathobiology, being involved in the development of coronary restenosis and atherosclerosis. The activation of nuclear proto-oncogenes appears to be a final common pathway onto which various mitogenic signals coverage. Accordingly, we attempted to determine whether the activation of the c-myc nuclear proto-oncogene is essential for human SMC proliferation and explored the possibility of inhibiting their growth using antisense oligonucleotides directed against c-myc messenger RNA (mRNA).

METHODS AND RESULTS

Proliferation of human SMCs was associated with an increase in c-myc mRNA expression after growth stimulation. Using 15-mer phosphorothioate oligonucleotides (oligomers), we tested their growth-inhibitory effect in SMCs in vitro. Antisense oligomers directed against the translation initiation region of the human c-myc gene exhibited a significant antiproliferative effect, whereas sense and mismatched oligomers did not inhibit the growth. The growth-inhibitory effect of c-myc antisense oligomers was dose dependent and preventable by an excess of sense oligomers. Furthermore, growth inhibition of SMCs treated with c-myc antisense oligomers was associated with a marked decrease in the c-myc mRNA level. Phosphorothioate oligomers remained stable in medium containing 20% serum and were detectable in SMCs as early as 1 hour after cell exposure. Intact oligomers rapidly accumulated intracellularly and persisted within human SMCs for at least 16 hours.

CONCLUSIONS

c-myc antisense oligomers reduced c-myc expression and produced a significant growth inhibition of human SMCs, indicating an important role of c-myc gene activation in the process of SMC proliferation. Furthermore, extracellular stability and rapid cellular uptake provide the basis for future studies assessing the therapeutic role of the c-myc antisense approach in reducing SMC proliferation in the process of vascular restenosis.

Expression of myosin regulatory light-chain isoforms and regulation of phosphorylation in smooth muscle

Our objectives were to 1) determine how growth state and cell density affect the expression of the smooth muscle (SM) and nonmuscle (NM) isoforms of the 20-kDa myosin regulatory light chains (MLC20) in cultured rat aortic smooth muscle cells (SMC) and 2) to determine whether angiotensin II stimulates differential phosphorylation of SM and NM MLC20 isoforms in an effort to assess whether the SM and NM isoforms may subserve different cellular functions. The results demonstrated that changes in the SM MLC20 isoform content were inversely correlated with cell growth but independent of cell density. MLC20 phosphorylation levels were 20.8 +/- 2.9 and 19.4 +/- 3.7% for SM and NM isoforms, respectively, in unstimulated, substrate-attached SMC. Angiotensin II transiently elevated phosphorylation levels of both the SM and NM MLC20 isoforms to 60-70%. No differences in either the magnitude or the kinetics of phosphorylation were observed for the SM vs. NM isoforms. Forskolin, 3-isobutyl-1-methylxanthine, or isoproterenol treatment led to parallel dephosphorylation of the SM- and NM-specific isoforms followed by depolymerization of stress fibers and cell arborization. The studies provide evidence that growth arrest of cultured SMC enhances expression of cell-specific/-selective proteins characteristic of differentiated SM. However, there was no evidence for differential phosphorylation changes of SM and NM MLC20 isoforms in response to activating or relaxing agents as expected if these isoforms subserve different cellular functions.